Linux Security HOWTO
Kevin Fenzi,
[email protected] & Dave Wreski,
[email protected]
v1.0.2, 25 April 1999
This document is a general overview of security issues that face the
administrator of Linux systems. It covers general security philosophy
and a number of specific examples of how to better secure your Linux
system from intruders. Also included are pointers to security-related
material and programs. Improvements, constructive criticism, additions
and corrections are gratefully accepted. Please mail your feedback to
both authors, with "Security HOWTO" in the subject.
______________________________________________________________________
Table of Contents
1. Introduction
1.1 New Versions of this Document
1.2 Feedback
1.3 Disclaimer
1.4 Copyright Information
2. Overview
2.1 Why Do We Need Security?
2.2 How Secure Is Secure?
2.3 What Are You Trying to Protect?
2.4 Developing A Security Policy
2.5 Means of Securing Your Site
2.5.1 Host Security
2.5.2 Network Security
2.5.3 Security Through Obscurity
2.6 Organization of This Document
3. Physical Security
3.1 Computer locks
3.2 BIOS Security
3.3 Boot Loader Security
3.4 xlock and vlock
3.5 Detecting Physical Security Compromises
4. Local Security
4.1 Creating New Accounts
4.2 Root Security
5. Files and Filesystem Security
5.1 Umask Settings
5.2 File Permissions
5.3 Integrity Checking with Tripwire
5.4 Trojan Horses
6. Password Security and Encryption
6.1 PGP and Public-Key Cryptography
6.2 SSL, S-HTTP, HTTPS and S/MIME
6.3 Linux IPSEC Implementations
6.4 (TT
6.5 PAM - Pluggable Authentication Modules
6.6 Cryptographic IP Encapsulation (CIPE)
6.7 Kerberos
6.8 Shadow Passwords.
6.9 "Crack" and "John the Ripper"
6.10 CFS - Cryptographic File System and TCFS - Transparent Cryptographic File System
6.11 X11, SVGA and display security
6.11.1 X11
6.11.2 SVGA
6.11.3 GGI (Generic Graphics Interface project)
7. Kernel Security
7.1 2.0 Kernel Compile Options
7.2 2.2 Kernel Compile Options
7.3 Kernel Devices
8. Network Security
8.1 Packet Sniffers
8.2 System services and tcp_wrappers
8.3 Verify Your DNS Information
8.4 (TT
8.5 SATAN, ISS, and Other Network Scanners
8.5.1 Detecting Port Scans
8.6 (TT
8.7 Denial of Service Attacks
8.8 NFS (Network File System) Security.
8.9 NIS (Network Information Service) (formerly YP).
8.10 Firewalls
8.11 IP Chains - Linux Kernel 2.2.x Firewalling
8.12 VPN's - Virtual Private Networks
9. Security Preparation (before you go on-line)
9.1 Make a Full Backup of Your Machine
9.2 Choosing a Good Backup Schedule
9.3 Backup Your RPM or Debian File Database
9.4 Keep Track of Your System Accounting Data
9.5 Apply All New System Updates.
10. What To Do During and After a Breakin
10.1 Security Compromise Underway.
10.2 Security Compromise has already happened
10.2.1 Closing the Hole
10.2.2 Assessing the Damage
10.2.3 Backups, Backups, Backups!
10.2.4 Tracking Down the Intruder.
11. Security Sources
11.1 FTP Sites
11.2 Web Sites
11.3 Mailing Lists
11.4 Books - Printed Reading Material
12. Glossary
13. Frequently Asked Questions
14. Conclusion
15. Acknowledgements
______________________________________________________________________
1. Introduction
This document covers some of the main issues that affect Linux
security. General philosophy and net-born resources are discussed.
A number of other HOWTO documents overlap with security issues, and
those documents have been pointed to wherever appropriate.
This document is not meant to be a up to date exploits document. Large
numbers of new exploits happen all the time. This document will tell
you where to look for such up to date information, and will give some
general methods to prevent such exploits from taking place.
1.1. New Versions of this Document
New versions of this document will be periodically posted to
comp.os.linux.answers. They will also be added to the various
anonymous FTP sites that archive such information, including:
ftp://metalab.unc.edu/pub/Linux/docs/HOWTO
In addition, you should generally be able to find this document on the
Linux World Wide Web home page via:
http://metalab.unc.edu/mdw/linux.html
Finally, the very latest version of this document should also be
available in various formats from:
http://scrye.com/~kevin/lsh/
1.2. Feedback
All comments, error reports, additional information and criticism of
all sorts should be directed to:
[email protected]
and
[email protected]
Note: Please send your feedback to both authors. Also, be sure and
include "Linux" "security", or "HOWTO" in your subject to avoid
Kevin's spam filter.
1.3. Disclaimer
No liability for the contents of this document can be accepted. Use
the concepts, examples and other content at your own risk.
Additionally, this is an early version, possibly with many
inaccuracies or errors.
A number of the examples and descriptions use the RedHat(tm) package
layout and system setup. Your mileage may vary.
As far as we know, only programs that, under certain terms may be used
or evaluated for personal purposes will be described. Most of the
programs will be available, complete with source, under GNU
<
http://www.gnu.org/copyleft/gpl.html> terms.
1.4. Copyright Information
This document is copyrighted (c)1998,1999 Kevin Fenzi and Dave Wreski,
and distributed under the following terms:
� Linux HOWTO documents may be reproduced and distributed in whole or
in part, in any medium, physical or electronic, as long as this
copyright notice is retained on all copies. Commercial
redistribution is allowed and encouraged; however, the authors
would like to be notified of any such distributions.
� All translations, derivative works, or aggregate works
incorporating any Linux HOWTO documents must be covered under this
copyright notice. That is, you may not produce a derivative work
from a HOWTO and impose additional restrictions on its
distribution. Exceptions to these rules may be granted under
certain conditions; please contact the Linux HOWTO coordinator at
the address given below.
� If you have questions, please contact Tim Bynum, the Linux HOWTO
coordinator, at
[email protected]
2. Overview
This document will attempt to explain some procedures and commonly-
used software to help your Linux system be more secure. It is
important to discuss some of the basic concepts first, and create a
security foundation, before we get started.
2.1. Why Do We Need Security?
In the ever-changing world of global data communications, inexpensive
Internet connections, and fast-paced software development, security is
becoming more and more of an issue. Security is now a basic
requirement because global computing is inherently insecure. As your
data goes from point A to point B on the Internet, for example, it may
pass through several other points along the way, giving other users
the opportunity to intercept, and even alter, it. Even other users on
your system may maliciously transform your data into something you did
not intend. Unauthorized access to your system may be obtained by
intruders, also known as "crackers", who then use advanced knowledge
to impersonate you, steal information from you, or even deny you
access to your own resources. If you're wondering what the difference
is between a "Hacker" and a "Cracker", see Eric Raymond's document,
"How to Become A Hacker", available at
http://sagan.earthspace.net/~esr/faqs/hacker-howto.html.
2.2. How Secure Is Secure?
First, keep in mind that no computer system can ever be "completely
secure". All you can do is make it increasingly difficult for someone
to compromise your system. For the average home Linux user, not much
is required to keep the casual cracker at bay. For high profile Linux
users (banks, telecommunications companies, etc), much more work is
required.
Another factor to take into account is that the more secure your
system is, the more intrusive your security becomes. You need to
decide where in this balancing act your system will still usable, and
yet secure for your purposes. For instance, you could require everyone
dialing into your system to use a call-back modem to call them back at
their home number. This is more secure, but if someone is not at home,
it makes it difficult for them to login. You could also setup your
Linux system with no network or connection to the Internet, but this
limits it's usefulness.
If you are a large to medium-sized site, you should establish a
security policy stating how much security is required by your site and
what auditing is in place to check it. You can find a well-known
security policy example at
http://ds.internic.net/rfc/rfc2196.txt. It
has been recently updated, and contains a great framework for
establishing a security policy for your company.
2.3. What Are You Trying to Protect?
Before you attempt to secure your system, you should determine what
level of threat you have to protect against, what risks you should or
should not take, and how vulnerable your system is as a result. You
should analyze your system to know what you're protecting, why you're
protecting it, what value it has, and who has responsibility for your
data and other assets.
� Risk is the possibility that an intruder may be successful in
attempting to access your computer. Can an intruder read or write
files, or execute programs that could cause damage? Can they
delete critical data? Can they prevent you or your company from
getting important work done? Don't forget: someone gaining access
to your account, or your system, can also impersonate you.
Additionally, having one insecure account on your system can result
in your entire network being compromised. If you allow a single
user to login using a .rhosts file, or to use an insecure service,
such as tftp, you risk an intruder getting 'his foot in the door'.
Once the intruder has a user account on your system, or someone
else's system, it can be used to gain access to another system, or
another account.
� Threat is typically from someone with motivation to gain
unauthorized access to your network or computer. You must decide
who you trust to have access to your system, and what threat they
could pose.
There are several types of intruders, and it is useful to keep
their different characteristics in mind as you are securing your
systems.
� The Curious - This type of intruder is basically interested in
finding out what type of system and data you have.
� The Malicious - This type of intruder is out to either bring down
your systems, or deface your web page, or otherwise force you to
spend time and money recovering from the damage he has caused.
� The High-Profile Intruder - This type of intruder is trying to use
your system to gain popularity and infamy. He might use your high-
profile system to advertise his abilities.
� The Competition - This type of intruder is interested in what data
you have on your system. It might be someone who thinks you have
something that could benefit him, financially or otherwise.
� The Borrowers - This type of intruder is interested in setting up
shop on your system and using it's resources for their own
purposes. They typically will run chat or irc servers, porn archive
sites, or even DNS servers.
� The Leapfrogger - This type of intruder is only interested in your
system to use it to get into other systems. If your system is well
connected or a gateway to a number of internal hosts, you may well
see this type trying to compromise your system.
� Vulnerability describes how well-protected your computer is from
another network, and the potential for someone to gain unauthorized
access.
What's at stake if someone breaks into your system? Of course the
concerns of a dynamic PPP home user will be different from those of
a company connecting their machine to the Internet, or another
large network.
How much time would it take to retrieve/recreate any data that was
lost? An initial time investment now can save ten times more time
later if you have to recreate data that was lost. Have you checked
your backup strategy, and verified your data lately?
2.4. Developing A Security Policy
Create a simple, generic policy for your system that your users can
readily understand and follow. It should protect the data you're
safeguarding as well as the privacy of the users. Some things to
consider adding are: who has access to the system (Can my friend use
my account?), who's allowed to install software on the system, who
owns what data, disaster recovery, and appropriate use of the system.
A generally accepted security policy starts with the phrase
That which is not permitted is prohibited
This means that unless you grant access to a service for a user, that
user shouldn't be using that service until you do grant access. Make
sure the policies work on your regular user account. Saying, "Ah, I
can't figure this permissions problem out, I'll just do it as root"
can lead to security holes that are very obvious, and even ones that
haven't been exploited yet.
rfc1244 is a document that describes how to create your own network
security polity.
rfc1281 is a document that shows an example security policy with
detailed descriptions of each step.
Finally, you might want to look at the COAST policy archive at
ftp://coast.cs.purdue.edu/pub/doc/policy to see what some real life
security policies look like.
2.5. Means of Securing Your Site
This document will discuss various means with which you can secure the
assets you have worked hard for: your local machine, your data, your
users, your network, even your reputation. What would happen to your
reputation if an intruder deleted some of your users' data? Or
defaced your web site? Or published your company's corporate project
plan for next quarter? If you are planning a network installation,
there are many factors you must take into account before adding a
single machine to your network.
Even if you have a single dialup PPP account, or just a small site,
this does not mean intruders won't be interested in your systems.
Large, high profile sites are not the only targets -- many intruders
simply want to exploit as many sites as possible, regardless of their
size. Additionally, they may use a security hole in your site to gain
access to other sites you're connected to.
Intruders have a lot of time on their hands, and can avoid guessing
how you've obscured your system just by trying all the possibilities.
There are also a number of reasons an intruder may be interested in
your systems, which we will discuss later.
2.5.1. Host Security
Perhaps the area of security on which administrators concentrate most
is host-based security. This typically involves making sure your own
system is secure, and hoping everyone else on your network does the
same. Choosing good passwords, securing your host's local network
services, keeping good accounting records, and upgrading programs with
known security exploits are among the things the local security
administrator is responsible for doing. Although this is absolutely
necessary, it can become a daunting task once your network becomes
larger than a few machines.
2.5.2. Network Security
Network security is also as necessary as local host security. With
hundreds, thousands, or more computers on the same network, you can't
rely on each one of those systems being secure. Ensuring that only
authorized users can use your network, building firewalls, using
strong encryption, and ensuring there are no "rogue" (that is,
unsecured) machines on your network are all part of the network
security administrator's duties.
This document will discuss some of the techniques used to secure your
site, and hopefully show you some of the ways to prevent an intruder
from gaining access to what you are trying to protect.
2.5.3. Security Through Obscurity
One type of security that must be discussed is "security through
obscurity". This means, for example, moving a service that has known
security vunerabilities to a non standard port in hopes that attackers
won't notice it's there and thus won't exploit it. Rest assured that
they can determine that its there and will exploit it. Security
through obscurity is no security at all. Simply because you may have a
small site, or a relatively low profile, does not mean an intruder
won't be interested in what you have. We'll discuss what you're
protecting in the next sections.
2.6. Organization of This Document
This document has been divided into a number of sections. They cover
several broad security issues. The first, ``Physical Security'',
covers how you need to protect your physical machine from tampering.
The second, ``Local Security'', describes how to protect your system
from tampering by local users. The third, ``Files and Filesystem
Security'', shows you how to setup your filesystems and premissions on
your files. The next, ``Password Security and Encryption'', discusses
how to use encryption to better secure your machine and network.
``Kernel Security'' discusses what kernel options you should set or be
aware of for a more secure system. ``Network Security'', describes
how to better secure your Linux system from network attacks.
``Security Preparation'', discusses how to prepare your machine(s)
before bringing them on-line. Next, ``What To Do During and After a
Break-in'', discusses what to do when you detect a system compromise
in progress or detect one that has recently happened. In ``Security
Resources'', some primary security resources are enumerated. The Q
and A section ``Frequently Asked Questions'', answers some frequently
asked questions, and finally a conclusion in ``Conclusion'' section.
The two main points to realize when reading this document are:
� Be aware of your system. Check system logs such as
/var/log/messages and keep an eye on your system, and
� Keep your system up to date by making sure you have installed the
current versions of software and have upgraded per security alerts.
Just doing this will help make your system markedly more secure.
3. Physical Security
The first layer of security you need to take into account is the
physical security of your computer systems. Who has direct physical
access to your machine? Should they? Can you protect your machine from
their tampering? Should you?
How much physical security you need on your system is very dependent
on your situation, and/or budget.
If you are a home user, you probably don't need a lot (although you
might need to protect your machine from tampering by children or
annoying relatives). If you are in a Lab, you need considerably more,
but users will still need to be able to get work done on the machines.
Many of the following sections will help out. If you are in an office,
you may or may not need to secure your machine off hours or while you
are away. At some companies, leaving your console unsecured is a
termination offense.
Obvious physical security methods such as locks on doors, cables,
locked cabinets, and video surveillance are all good ideas, but beyond
the scope of this document. :)
3.1. Computer locks
Many modern PC cases include a "locking" feature. Usually this will be
a socket on the front of the case that allows you to turn an included
key to a locked or unlocked position. Case locks can help prevent
someone from stealing your PC, or opening up the case and directly
manipulating/stealing your hardware. They can also sometimes prevent
someone from rebooting your computer on their own floppy or other
hardware.
These case locks do different things according to the support in the
motherboard and how the case is constructed. On many PC's they make it
so you have to break the case to get the case open. On some others,
they make it so that it will not let you plug in new keyboards and
mice. Check your motherboard or case instructions for more
information. This can sometimes be a very useful feature, even though
the locks are usually very low quality and can easily be defeated by
attackers with locksmithing.
Some cases (most notably SPARCs and macs) have a dongle on the back
that, if you put a cable through attackers would have to cut the cable
or break the case to get into it. Just putting a padlock or combo lock
through these can be a good deterrent to someone stealing your
machine.
3.2. BIOS Security
The BIOS is the lowest level of software that configures or
manipulates your x86-based hardware. LILO and other Linux boot methods
access the BIOS to determine how to boot up your Linux machine. Other
hardware that Linux runs on has similar software (OpenFirmware on Macs
and new Suns, Sun boot PROM, etc...). You can use your BIOS to prevent
attackers from rebooting your machine and manipulating your Linux
system.
Many PC BIOSs let you set a boot password. This doesn't provide all
that much security (the BIOS can be reset, or removed if someone can
get into the case), but might be a good deterrent (i.e. it will take
time and leave traces of tampering). Similarly, on S/Linux (Linux for
SPARC(tm) porcessor machines), your EEPROM can be set to require a
boot-up password. This might slow attackers down.
Many x86 BIOSs also allow you to specify various other good security
settings. Check your BIOS manual or look at it the next time you boot
up. For example, some BIOSs disallow booting from floppy drives and
some require passwords to access some BIOS features.
Note: If you have a server machine, and you set up a boot password,
your machine will not boot up unattended. Keep in mind that you will
need to come in and supply the password in the event of a power
failure. ;(
3.3. Boot Loader Security
The various Linux boot loaders also can have a boot password set.
LILO, for example, has password and restricted settings; password
always requires password at boot time, whereas restricted requires a
boot-time password only if you specify options (such as single) at the
LILO prompt.
Keep in mind when setting all these passwords that you need to
remember them. :) Also remember that these passwords will merely slow
the determined attacker. They won't prevent someone from booting from
a floppy, and mounting your root partition. If you are using security
in conjunction with a boot loader, you might as well disable booting
from a floppy in your computer's BIOS, and password-protect the BIOS.
If anyone has security-related information from a different boot
loader, we would love to hear it. (grub, silo, milo, linload, etc).
Note: If you have a server machine, and you set up a boot password,
your machine will not boot up unattended. Keep in mind that you will
need to come in and supply the password in the event of a power
failure. ;(
3.4. xlock and vlock
If you wander away from your machine from time to time, it is nice to
be able to "lock" your console so that no one tampers with or looks at
your work. Two programs that do this are: xlock and vlock.
xlock is a X display locker. It should be included in any Linux
distributions that support X. Check out the man page for it for more
options, but in general you can run xlock from any xterm on your
console and it will lock the display and require your password to
unlock.
vlock is a simple little program that allows you to lock some or all
of the virtual consoles on your Linux box. You can lock just the one
you are working in or all of them. If you just lock one, others can
come in and use the console; they will just not be able to use your
virtual console until you unlock it. vlock ships with redhat Linux,
but your mileage may vary.
Of course locking your console will prevent someone from tampering
with your work, but won't prevent them from rebooting your machine or
otherwise disrupting your work. It also does not prevent them from
accessing your machine from another machine on the network and causing
problems.
More importantly, it does not prevent someone from switching out of
the X Window System entirely, and going to a normal virtual console
login prompt, or to the VC that X11 was started from, and suspending
it, thus obtaining your priviledges. For this reason, you might
consider only using it while under control of xdm.
3.5. Detecting Physical Security Compromises
The first thing to always note is when your machine was rebooted.
Since Linux is a robust and stable OS, the only times your machine
should reboot is when you take it down for OS upgrades, hardware
swapping, or the like. If your machine has rebooted without you doing
it, that may be a sign that an intruder has compromised it. Many of
the ways that your machine can be compromised require the intruder to
reboot or power off your machine.
Check for signs of tampering on the case and computer area. Although
many intruders clean traces of their presence out of logs, it's a good
idea to check through them all and note any discrepancy.
It is also a good idea to store log data at a secure location, such as
a dedicated log server within your well-protected network. Once a
machine has been compromised, log data becomes of little use as it
most likely has also been modified by the intruder.
The syslog daemon can be configured to automatically send log data to
a central syslog server, but this is typically sent in cleartext data,
allowing an intruder to view data as it is being transferred. This
may reveal information about your network that is not intended to be
public. There are syslog daemons available that encrypt the data as
it is being sent.
Also be aware that faking syslog messages is easy - with an exploit
program having been published. Syslog even accepts net log entries
claiming to come from the local host without indicating their true
origin.
Some things to check for in your logs:
� Short or incomplete logs.
� Logs containing strange timestamps.
� Logs with incorrect permissions or ownership.
� Records of reboots or restarting of services.
� missing logs.
� su entries or logins from strange places.
We will discuss system log data ``later'' in the HOWTO.
4. Local Security
The next thing to take a look at is the security in your system
against attacks from local users. Did we just say local users? Yes!
Getting access to a local user account is one of the first things that
system intruders attempt while on their way to exploiting the root
account. With lax local security, they can then "upgrade" their normal
user access to root access using a variety of bugs and poorly setup
local services. If you make sure your local security is tight, then
the intruder will have another hurdle to jump.
Local users can also cause a lot of havoc with your system even
(especially) if they really are who they say they are. Providing
accounts to people you don't know or have no contact information for
is a very bad idea.
4.1. Creating New Accounts
You should make sure to provide user accounts with only the minimal
requirements for the task they need to do. If you provide your son
(age 10) with an account, you might want him to only have access to a
word processor or drawing program, but be unable to delete data that
is not his.
Several good rules of thumb when allowing other people legitimate
access to your Linux machine:
� Give them the minimal amount of privileges they need.
� Be aware when/where they login from, or should be logging in from.
� Make sure to remove inactive accounts
� The use of the same user-ID on all computers and networks is
advisable to ease account maintence, as well as permit easier
analysis of log data.
� The creation of group user-IDs should be absolutely prohibited.
User accounts also provide accountability, and this is not possible
with group accounts.
Many local user accounts that are used in security compromises are
ones that have not been used in months or years. Since no one is using
them they, provide the ideal attack vehicle.
4.2. Root Security
The most sought-after account on your machine is the root (superuser)
account. This account has authority over the entire machine, which
may also include authority over other machines on the network.
Remember that you should only use the root account for very short,
specific tasks, and should mostly run as a normal user. Even small
mistakes made while logged in as the root user can cause problems. The
less time you are on with root privledges, the safer you will be.
Several tricks to avoid messing up your own box as root:
� When doing some complex command, try running it first in a non-
destructive way...especially commands that use globbing: e.g., if
you want to do "rm foo*.bak", first do "ls foo*.bak" and make sure
you are going to delete the files you think you are. Using echo in
place of destructive commands also sometimes works.
� Provide your users with a default alias to the rm command to ask
for confirmation for deletion of files.
� Only become root to do single specific tasks. If you find yourself
trying to figure out how to do something, go back to a normal user
shell until you are sure what needs to be done by root.
� The command path for the root user is very important. The command
path (that is, the PATH environment variable) specifies the
directories in which the shell searches for programs. Try to limit
the command path for the root user as much as possible, and never
include . (which means "the current directory") in your PATH.
Additionally, never have writable directories in your search path,
as this can allow attackers to modify or place new binaries in your
search path, allowing them to run as root the next time you run
that command.
� Never use the rlogin/rsh/rexec suite of tools (called the r-
utilities) as root. They are subject to many sorts of attacks, and
are downright dangerous run as root. Never create a .rhosts file
for root.
� The /etc/securetty file contains a list of terminals that root can
login from. By default (on Red Hat Linux) this is set to only the
local virtual consoles(vtys). Be very careful of adding anything
else to this file. You should be able to login remotely as your
regular user account and then su if you need to (hopefully over
``ssh'' or other encrypted channel), so there is no need to be able
to login directly as root.
� Always be slow and deliberate running as root. Your actions could
affect a lot of things. Think before you type!
If you absolutely positively need to allow someone (hopefully very
trusted) to have root access to your machine, there are a few tools
that can help. sudo allows users to use their password to access a
limited set of commands as root. This would allow you to, for
instance, let a user be able to eject and mount removable media on
your Linux box, but have no other root privileges. sudo also keeps a
log of all successful and unsuccessful sudo attempts, allowing you to
track down who used what command to do what. For this reason sudo
works well even in places where a number of people have root access,
because it helps you keep track of changes made.
Although sudo can be used to give specific users specific privileges
for specific tasks, it does have several shortcomings. It should be
used only for a limited set of tasks, like restarting a server, or
adding new users. Any program that offers a shell escape will give
root access to a user invoking it via sudo. This includes most
editors, for example. Also, a program as innocuous as /bin/cat can be
used to overwrite files, which could allow root to be exploited.
Consider sudo as a means for accountability, and don't expect it to
replace the root user and still be secure.
5. Files and Filesystem Security
A few minutes of preparation and planning ahead before putting your
systems online can help to protect them and the data stored on them.
� There should never be a reason for users' home directories to allow
SUID/SGID programs to be run from there. Use the nosuid option in
/etc/fstab for partitions that are writable by others than root.
You may also wish to use nodev and noexec on users' home
partitions, as well as /var, thus prohibiting execution of
programs, and creation of character or block devices, which should
never be necessary anyway.
� If you are exporting filesystems using NFS, be sure to configure
/etc/exports with the most restrictive access possible. This means
not using wildcards, not allowing root write access, and exporting
read-only wherever possible.
� Configure your users' file-creation umask to be as restrictive as
possible. See ``umask settings''.
� If you are mounting filesystems using a network filesystem such as
NFS, be sure to configure /etc/exports with suitable restrictions.
Typically, using `nodev', `nosuid', and perhaps `noexec', are
desireable.
� Set filesystem limits instead of allowing unlimited as is the
default. You can control the per-user limits using the resource-
limits PAM module and /etc/pam.d/limits.conf. For example, limits
for group users might look like this:
@users hard core 0
@users hard nproc 50
@users hard rss 5000
This says to prohibit the creation of core files, restrict the number
of processes to 50, and restrict memory usage per user to 5M.
� The /var/log/wtmp and /var/run/utmp files contain the login records
for all users on your system. Their integrity must be maintained
because it can be used to determine when and from where a user (or
potential intruder) has entered your system. These files should
also have 644 permissions, without affecting normal system
operation.
� The immutable bit can be used to prevent accidentally deleting or
overwriting a file that must be protected. It also prevents
someone from creating a symbolic link to the file (such symbolic
links have been the source of attacks involving deleting
/etc/passwd or /etc/shadow). See the chattr(1) man page for
information on the immutable bit.
� SUID and SGID files on your system are a potential security risk,
and should be monitored closely. Because these programs grant
special privileges to the user who is executing them, it is
necessary to ensure that insecure programs are not installed. A
favorite trick of crackers is to exploit SUID-root programs, then
leave a SUID program as a backdoor to get in the next time, even if
the original hole is plugged.
Find all SUID/SGID programs on your system, and keep track of what
they are, so you are aware of any changes which could indicate a
potential intruder. Use the following command to find all
SUID/SGID programs on your system:
root# find / -type f \( -perm -04000 -o -perm -02000 \)
The Debian distribution runs a job each night to determine what SUID
files exist. It then compairs this to the previous nights run. You can
look in /var/log/suid* for this log.
You can remove the SUID or SGID permissions on a suspicious program
with chmod, then change it back if you absolutely feel it is
necessary.
� World-writable files, particularly system files, can be a security
hole if a cracker gains access to your system and modifies them.
Additionally, world-writable directories are dangerous, since they
allow a cracker to add or delete files as he wishes. To locate all
world-writable files on your system, use the following command:
root# find / -perm -2 ! -type l -ls
and be sure you know why those files are writable. In the normal
course of operation, several files will be world-writable, including
some from /dev, and symbolic links, thus the ! -type l which excludes
these from the previous find command.
�
Unowned files may also be an indication an intruder has accessed
your system. You can locate files on your system that have no
owner, or belong to no group with the command:
root# find / -nouser -o -nogroup -print
� Finding .rhosts files should be a part of your regular system
administration duties, as these files should not be permitted on
your system. Remember, a cracker only needs one insecure account
to potentially gain access to your entire network. You can locate
all .rhosts files on your system with the following command:
root# find /home -name .rhosts -print
�
Finally, before changing permissions on any system files, make sure
you understand what you are doing. Never change permissions on a
file because it seems like the easy way to get things working.
Always determine why the file has that permission before changing
it.
5.1. Umask Settings
The umask command can be used to determine the default file creation
mode on your system. It is the octal complement of the desired file
mode. If files are created without any regard to their permissions
settings, the user could inadvertently give read or write permission
to someone that should not have this permission. Typically umask
settings include 022, 027, and 077 (which is the most restrictive).
Normally the umask is set in /etc/profile, so it applies to all users
on the system. The file creation mask can be calculated by
subtracting the desired value from 777. In other words, a umask of
777 would cause newly-created files to contain no read, write or
execute permission for anyone. A mask of 666 would cause newly-
created files to have a mask of 111. For example, you may have a line
that looks like this:
# Set the user's default umask
umask 033
Be sure to make root's umask 077, which will disable read, write, and
execute permission for other users, unless explicitly changed using
chmod. In this case, newly-created directories would have 744 permis�
sions, obtained by subtracting 033 from 777. Newly-created files
using the 033 umask would have permissions of 644.
If you are using Red Hat, and adhere to their user and group ID
creation scheme (User Private Groups), it is only necessary to use 002
for a umask. This is due to the fact that the default configuration
is one user per group.
5.2. File Permissions
It's important to ensure that your system files are not open for
casual editing by users and groups who shouldn't be doing such system
maintenance.
Unix seperates access control on files and directories according to
three characteristics: owner, group, and other. There is always
exactly one owner, any number of members of the group, and everyone
else.
A quick explanation of Unix permissions:
Ownership - Which user(s) and group(s) retain(s) control of the
permission settings of the node and parent of the node
Permissions - Bits capable of being set or reset to allow certain
types of access to it. Permissions for directories may have a
different meaning than the same set of permissions on files.
Read:
� To be able to view contents of a file
� To be able to read a directory
Write:
� To be able to add to or change a file
� To be able to delete or move files in a directory
Execute:
� To be able to run a binary program or shell script
� To be able to search in a directory, combined with read permission
Save Text Attribute: (For directories)
The "sticky bit" also has a different meaning when applied to
directories than when applied to files. If the sticky bit is
set on a directory, then a user may only delete files that the
he owns or for which he has explicit write permission granted,
even when he has write access to the directory. This is
designed for directories like /tmp, which are world-writable,
but where it may not be desirable to allow any user to delete
files at will. The sticky bit is seen as a t in a long
directory listing.
SUID Attribute: (For Files)
This describes set-user-id permissions on the file. When the
set user ID access mode is set in the owner permissions, and the
file is executable, processes which run it are granted access to
system resources based on user who owns the file, as opposed to
the user who created the process. This is the cause of many
"buffer overflow" exploits.
SGID Attribute: (For Files)
If set in the group permissions, this bit controls the "set
group id" status of a file. This behaves the same way as SUID,
except the group is affected instead. The file must be
executable for this to have any effect.
SGID Attribute: (For directories)
If you set the SGID bit on a directory (with chmod g+s
directory), files created in that directory will have their
group set to the directory's group.
You - The owner of the file
Group - The group you belong to
Everyone - Anyone on the system that is not the owner or a member
of the group
File Example:
-rw-r--r-- 1 kevin users 114 Aug 28 1997 .zlogin
1st bit - directory? (no)
2nd bit - read by owner? (yes, by kevin)
3rd bit - write by owner? (yes, by kevin)
4th bit - execute by owner? (no)
5th bit - read by group? (yes, by users)
6th bit - write by group? (no)
7th bit - execute by group? (no)
8th bit - read by everyone? (yes, by everyone)
9th bit - write by everyone? (no)
10th bit - execute by everyone? (no)
The following lines are examples of the minimum sets of permissions
that are required to perform the access described. You may want to
give more permission than what's listed here, but this should describe
what these minimum permissions on files do:
-r-------- Allow read access to the file by owner
--w------- Allows the owner to modify or delete the file
(Note that anyone with write permission to the directory
the file is in can overwrite it and thus delete it)
---x------ The owner can execute this program, but not shell scripts,
which still need read permission
---s------ Will execute with effective User ID = to owner
--------s- Will execute with effective Group ID = to group
-rw------T No update of "last modified time". Usually used for swap
files
---t------ No effect. (formerly sticky bit)
Directory Example:
drwxr-xr-x 3 kevin users 512 Sep 19 13:47 .public_html/
1st bit - directory? (yes, it contains many files)
2nd bit - read by owner? (yes, by kevin)
3rd bit - write by owner? (yes, by kevin)
4th bit - execute by owner? (yes, by kevin)
5th bit - read by group? (yes, by users
6th bit - write by group? (no)
7th bit - execute by group? (yes, by users)
8th bit - read by everyone? (yes, by everyone)
9th bit - write by everyone? (no)
10th bit - execute by everyone? (yes, by everyone)
The following lines are examples of the minimum sets of permissions
that are required to perform the access described. You may want to
give more permission than what's listed, but this should describe what
these minimum permissions on directories do:
dr-------- The contents can be listed, but file attributes can't be read
d--x------ The directory can be entered, and used in full execution
paths
dr-x------ File attributes can be read by owner
d-wx------ Files can be created/deleted, even if the directory
isn't the current one
d------x-t Prevents files from deletion by others with write
access. Used on /tmp
d---s--s-- No effect
System configuration files (usually in /etc) are usually mode 640
(-rw-r-----), and owned by root. Depending on your sites security
requirements, you might adjust this. Never leave any system files
writable by a group or everyone. Some configuration files, including
/etc/shadow, should only be readable by root, and directories in /etc
should at least not be accessible by others.
SUID Shell Scripts
SUID shell scripts are a serious security risk, and for this
reason the kernel will not honor them. Regardless of how secure
you think the shell script is, it can be exploited to give the
cracker a root shell.
5.3. Integrity Checking with Tripwire Tripwire
Another very good way to detect local (and also network) attacks on
your system is to run an integrity checker like Tripwire. Tripwire
runs a number of checksums on all your important binaries and config
files and compares them against a database of former, known-good
values as a reference. Thus, any changes in the files will be flagged.
It's a good idea to install Tripwire onto a floppy, and then
physically set the write protect on the floppy. This way intruders
can't tamper with Tripwire itself or change the database. Once you
have Tripwire setup, it's a good idea to run it as part of your normal
security administration duties to see if anything has changed.
You can even add a crontab entry to run Tripwire from your floppy
every night and mail you the results in the morning. Something like:
# set mailto
MAILTO=kevin
# run Tripwire
15 05 * * * root /usr/local/adm/tcheck/tripwire
will mail you a report each morning at 5:15am.
Tripwire can be a godsend to detecting intruders before you would
otherwise notice them. Since a lot of files change on the average
system, you have to be careful what is cracker activity and what is
your own doing.
You can find Tripwire at
http://www.tripwiresecurity.com, free of
charge. Manuals and support can be purchased.
5.4. Trojan Horses
"Trojan Horses" are named after the fabled ploy in Homer's "Iliad".
The idea is that a cracker distributes a program or binary that sounds
great, and encourages other people to download it and run it as root.
Then the program can compromise their system while they are not paying
attention. While they think the binary they just pulled down does one
thing (and it might very well), it also compromises their security.
You should take care of what programs you install on your machine.
Redhat provides MD5 checksums and PGP signatures on it's RPM files so
you can verify you are installing the real thing. Other distributions
have similar methods. You should never run any unfamiliar binary, for
which you don't have the source, as root! Few attackers are willing to
release source code to public scrutiny.
Although it can be complex, make sure you are getting the source for a
program from its real distribution site. If the program is going to
run as root, make sure either you or someone you trust has looked over
the source and verified it.
6. Password Security and Encryption
One of the most important security features used today are passwords.
It is important for both you and all your users to have secure,
unguessable passwords. Most of the more recent Linux distributions
include passwd programs that do not allow you to set a easily
guessable password. Make sure your passwd program is up to date and
has these features.
In-depth discussion of encryption is beyond the scope of this
document, but an introduction is in order. Encryption is very useful,
possibly even necessary in this day and age. There are all sorts of
methods of encrypting data, each with its own set of characteristics.
Most Unicies (and Linux is no exception) primarily use a one-way
encryption algorithm, called DES (Data Encryption Standard) to encrypt
your passwords. This encrypted password is then stored in (typically)
/etc/passwd (or less commonly) /etc/shadow. When you attempt to login,
the password you type in is encrypted again and compared with the
entry in the file that stores your passwords. If they match, it must
be the same password, and you are allowed access. Although DES is a
two-way encryption algorithm (you can code and then decode a message,
given the right keys), the variant that most unices use is one-way.
This means that it should not be possible to reverse the encryption to
get the password from the contents of /etc/passwd (or /etc/shadow).
Brute force attacks, such as "Crack" or "John the Ripper" (see Section
``'') can often guess passwords unless your password is sufficiently
random. PAM modules (see below) allow you to use a different
encryption routine with your passwords (MD5 or the like). You can use
Crack to your advantage, as well. Consider periodically running Crack
against your own password database, to find insecure passwords. Then
contact the offending user, and instruct him to change his password.
You can go to
http://consult.cern.ch/writeup/security/security_3.html
for information on how to choose a good password.
6.1. PGP and Public-Key Cryptography
Public-key cryptography, such as that used for PGP, uses one key for
encryption, and one key for decryption. Traditional cryptography,
however, uses the same key for encryption and decryption; this key
must be known to both parties, and thus somehow transferred from one
to the other securely.
To alleviate the need to securely transmit the encryption key, public-
key encryption uses two separate keys: a public key and a private key.
Each person's public key is available by anyone to do the encryption,
while at the same time each person keeps his or her private key to
decrypt messages encrypted with the correct public key.
There are advantages to both public key and private key cryptography,
and you can read about those differences in the RSA Cryptography FAQ
<
http://www.rsa.com/rsalabs/newfaq/>, listed at the end of this
section.
PGP (Pretty Good Privacy) is well-supported on Linux. Versions 2.6.2
and 5.0 are known to work well. For a good primer on PGP and how to
use it, take a look at the PGP FAQ:
http://www.pgp.com/service/export/faq/55faq.cgi
Be sure to use the version that is applicable to your country. Due to
export restrictions by the US Government, strong-encryption is
prohibited from being transferred in electronic form outside the
country.
US export controls are now managed by EAR (Export Administration
Regulations). They are no longer governed by ITAR.
There is also a step-by-step guide for configuring PGP on Linux
available at
http://mercury.chem.pitt.edu/~angel/LinuxFocus/English/November1997/article7.html.
It was written for the international version of PGP, but is easily
adaptable to the United States version. You may also need a patch for
some of the latest versions of Linux; the patch is available at
ftp://metalab.unc.edu/pub/Linux/apps/crypto.
There is a project working on a free re-implementation of pgp with
open source. GnuPG is a complete and free replacement for PGP. Because
it does not use IDEA or RSA it can be used without any restrictions.
GnuPG is nearly in compliance with RFC2440 (OpenPGP). See the GNU
Privacy Guard web page for more information:
http://www.gpg.org/.
More information on cryptography can be found in the RSA cryptography
FAQ, available at
http://www.rsa.com/rsalabs/newfaq/. Here you will
find information on such terms as "Diffie-Hellman", "public-key
cryptography", "digital certificates", etc.
6.2. SSL, S-HTTP, HTTPS and S/MIME
Often users ask about the differences between the various security and
encryption protocols, and how to use them. While this isn't an
encryption document, it is a good idea to explain briefly what each
protocol is, and where to find more information.
� SSL: - SSL, or Secure Sockets Layer, is an encryption method
developed by Netscape to provide security over the Internet. It
supports several different encryption protocols, and provides
client and server authentication. SSL operates at the transport
layer, creates a secure encrypted channel of data, and thus can
seamlessly encrypt data of many types. This is most commonly seen
when going to a secure site to view a secure online document with
Communicator, and serves as the basis for secure communications
with Communicator, as well as many other Netscape Communications
data encryption. More information can be found at
http://www.consensus.com/security/ssl-talk-faq.html. Information
on Netscape's other security implementations, and a good starting
point for these protocols is available at
http://home.netscape.com/info/security-doc.html.
� S-HTTP: - S-HTTP is another protocol that provides security
services across the Internet. It was designed to provide
confidentiality, authentication, integrity, and non-repudiability
[cannot be mistaken for someone else] while supporting multiple
key-management mechanisms and cryptographic algorithms via option
negotiation between the parties involved in each transaction. S-
HTTP is limited to the specific software that is implementing it,
and encrypts each message individually. [ From RSA Cryptography
FAQ, page 138]
� S/MIME: - S/MIME, or Secure Multipurpose Internet Mail Extension,
is an encryption standard used to encrypt electronic mail and other
types of messages on the Internet. It is an open standard
developed by RSA, so it is likely we will see it on Linux one day
soon. More information on S/MIME can be found at
http://home.netscape.com/assist/security/smime/overview.html.
6.3. Linux IPSEC Implementations
Along with CIPE, and other forms of data encryption, there is also
several other implementations of IPSEC for Linux. IPSEC is an effort
by the IETF to create cryptographically-secure communications at the
IP network level, and to provide authentication, integrity, access
control, and confidentiality. Information on IPSEC and Internet draft
can be found at
http://www.ietf.org/html.charters/ipsec-charter.html.
You can also find links to other protocols involving key management,
and an IPSEC mailing list and archives.
The x-kernel Linux implementation, which is being developed at the
University of Arizona, uses an object-based framework for implementing
network protocols called x-kernel, and can be found at
http://www.cs.arizona.edu/xkernel/hpcc-blue/linux.html. Most simply,
the x-kernel is a method of passing messages at the kernel level,
which makes for an easier implementation.
Another freely-available IPSEC implementation is the Linux FreeS/WAN
IPSEC. Their web page states,
"These services allow you to build secure tunnels through
untrusted networks. Everything passing through the untrusted
net is encrypted by the IPSEC gateway machine and decrypted
by the gateway at the other end. The result is Virtual Pri�
vate Network or VPN. This is a network which is effectively
private even though it includes machines at several differ�
ent sites connected by the insecure Internet."
It's available for download from
http://www.xs4all.nl/~freeswan/, and
has just reached 1.0 at the time of this writing.
As with other forms of cryptography, it is not distributed with the
kernel by default due to export restrictions.
6.4. ssh (Secure Shell) and stelnet
ssh and stelnet are programs that allow you to login to remote systems
and have a encrypted connection.
ssh is a suite of programs used as a secure replacement for rlogin,
rsh and rcp. It uses public-key cryptography to encrypt
communications between two hosts, as well as to authenticate users.
It can be used to securely login to a remote host or copy data between
hosts, while preventing man-in-the-middle attacks (session hijacking)
and DNS spoofing. It will perform data compression on your
connections, and secure X11 communications between hosts. The ssh
home page can be found at
http://www.cs.hut.fi/ssh/
You can also use ssh from your Windows workstation to your Linux ssh
server. There are several freely available Windows client
implementations, including the one at
http://guardian.htu.tuwien.ac.at/therapy/ssh/ as well as a commercial
implementation from DataFellows, at
http://www.datafellows.com. There
is also a open source project to re-implement ssh called "psst...".
For more information see:
http://www.net.lut.ac.uk/psst/
SSLeay is a free implementation of Netscape's Secure Sockets Layer
protocol, developed by Eric Young. It includes several applications,
such as Secure telnet, a module for Apache, several databases, as well
as several algorithms including DES, IDEA and Blowfish.
Using this library, a secure telnet replacement has been created that
does encryption over a telnet connection. Unlike SSH, stelnet uses
SSL, the Secure Sockets Layer protocol developed by Netscape. You can
find Secure telnet and Secure FTP by starting with the SSLeay FAQ,
available at
http://www.psy.uq.oz.au/~ftp/Crypto/.
SRP is another secure telnet/ftp implementation. From their web page:
"The SRP project is developing secure Internet software for
free worldwide use. Starting with a fully-secure Telnet and
FTP distribution, we hope to supplant weak networked authen�
tication systems with strong replacements that do not sacri�
fice user-friendliness for security. Security should be the
default, not an option!"
For more information, go to
http://srp.stanford.edu/srp.
6.5. PAM - Pluggable Authentication Modules
Newer versions of the Red Hat Linux distribution ship with a unified
authentication scheme called "PAM". PAM allows you to change your
authentication methods and requirements on the fly, and encapsulate
all local authentication methods without recompiling any of your
binaries. Configuration of PAM is beyond the scope of this document,
but be sure to take a look at the PAM web site for more information.
http://www.kernel.org/pub/linux/libs/pam/index.html.
Just a few of the things you can do with PAM:
� Use encryption other than DES for your passwords. (Making them
harder to brute-force decode)
� Set resource limits on all your users so they can't perform denial-
of-service attacks (number of processes, amount of memory, etc)
� Enable shadow passwords (see below) on the fly
� allow specific users to login only at specific times from specific
places
Within a few hours of installing and configuring your system, you can
prevent many attacks before they even occur. For example, use PAM to
disable the system-wide usage of .rhosts files in user's home
directories by adding these lines to /etc/pam.d/rlogin:
#
# Disable rsh/rlogin/rexec for users
#
login auth required pam_rhosts_auth.so no_rhosts
6.6. Cryptographic IP Encapsulation (CIPE)
The primary goal of this software is to provide a facility for secure
(against eavesdropping, including traffic analysis, and faked message
injection) subnetwork interconnection across an insecure packet
network such as the Internet.
CIPE encrypts the data at the network level. Packets traveling
between hosts on the network are encrypted. The encryption engine is
placed near the driver which sends and receives packets.
This is unlike SSH, which encrypts the data by connection, at the
socket level. A logical connection between programs running on
different hosts is encrypted.
CIPE can be used in tunnelling, in order to create a Virtual Private
Network. Low-level encryption has the advantage that it can be made
to work transparently between the two networks connected in the VPN,
without any change to application software.
Summarized from the CIPE documentation:
The IPSEC standards define a set of protocols which can be
used (among other things) to build encrypted VPNs. However,
IPSEC is a rather heavyweight and complicated protocol set
with a lot of options, implementations of the full protocol
set are still rarely used and some issues (such as key man�
agement) are still not fully resolved. CIPE uses a simpler
approach, in which many things which can be parameterized
(such as the choice of the actual encryption algorithm used)
are an install-time fixed choice. This limits flexibility,
but allows for a simple (and therefore efficient, easy to
debug...) implementation.
Further information can be found at
http://www.inka.de/~bigred/devel/cipe.html
As with other forms of cryptography, it is not distributed with the
kernel by default due to export restrictions.
6.7. Kerberos
Kerberos is an authentication system developed by the Athena Project
at MIT. When a user logs in, Kerberos authenticates that user (using a
password), and provides the user with a way to prove her identity to
other servers and hosts scattered around the network.
This authentication is then used by programs such as rlogin to allow
the user to login to other hosts without a password (in place of the
.rhosts file). This authentication method can also used by the mail
system in order to guarantee that mail is delivered to the correct
person, as well as to guarantee that the sender is who he claims to
be.
Kerberos and the other programs that come with it, prevent users from
"spoofing" the system into believing they are someone else.
Unfortunately, installing Kerberos is very intrusive, requiring the
modification or replacement of numerous standard programs.
You can find more information about kerberos by looking at the
kerberos FAQ, and the code can be found at
http://nii.isi.edu/info/kerberos/.
[From: Stein, Jennifer G., Clifford Neuman, and Jeffrey L. Schiller.
"Kerberos: An Authentication Service for Open Network Systems." USENIX
Conference Proceedings, Dallas, Texas, Winter 1998.]
Kerberos should not be your first step in improving security of your
host. It is quite involved, and not as widely used as, say, SSH.
6.8. Shadow Passwords.
Shadow passwords are a means of keeping your encrypted password
information secret from normal users. Normally, this encrypted
passwords are stored in /etc/passwd file for all to read. Anyone can
then run password guesser programs on them and attempt to determine
what they are. Shadow passwords, by contrast, are saved in
/etc/shadow, which only privileged users can read. In order to use
shadow passwords, you need to make sure all your utilities that need
access to password information are recompiled to support them. PAM
(above) also allows you to just plug in a shadow module; it doesn't
require re-compilation of executables. You can refer to the Shadow-
Password HOWTO for further information if necessary. It is available
at
http://metalab.unc.edu/LDP/HOWTO/Shadow-Password-HOWTO.html It is
rather dated now, and will not be required for distributions
supporting PAM.
6.9. "Crack" and "John the Ripper"
If for some reason your passwd program is not enforcing hard-to-guess
passwords, you might want to run a password-cracking program and make
sure your users' passwords are secure.
Password cracking programs work on a simple idea: they try every word
in the dictionary, and then variations on those words, encrypting each
one and checking it against your encrypted password. If they get a
match they know what your password is.
There are a number of programs out there...the two most notable of
which are "Crack" and "John the Ripper"
(
http://www.false.com/security/john/index.html) . They will take up a
lot of your cpu time, but you should be able to tell if an attacker
could get in using them by running them first yourself and notifying
users with weak passwords. Note that an attacker would have to use
some other hole first in order to read your /etc/passwd file, but such
holes are more common than you might think.
Because security is only as strong as the most insecure host, it is
worth mentioning that if you have any Windows machines on your
network, you should check out L0phtCrack, a Crack implementation for
Windows. It's available from
http://www.l0pht.com
6.10. CFS - Cryptographic File System and TCFS - Transparent Crypto�
graphic File System
CFS is a way of encrypting an entire directory trees and allowing
users to store encrypted files on them. It uses a NFS server running
on the local machine. RPMS are available at
http://www.replay.com/redhat/, and more information on how it all
works is at
ftp://ftp.research.att.com/dist/mab/.
TCFS improves on CFS by adding more integration with the file system,
so that it's transparent to users that the file system that is
encrypted. more information at:
http://edu-gw.dia.unisa.it/tcfs/.
It also need not be used on entire filesystems. It works on
directories trees as well.
6.11. X11, SVGA and display security
6.11.1. X11
It's important for you to secure your graphical display to prevent
attackers from grabbing your passwords as you type them, reading
documents or information you are reading on your screen, or even using
a hole to gain root access. Running remote X applications over a
network also can be fraught with peril, allowing sniffers to see all
your interaction with the remote system.
X has a number of access-control mechanisms. The simplest of them is
host-based: you use xhost to specify what hosts are allowed access to
your display. This is not very secure at all, because if someone has
access to your machine, they can xhost + their machine and get in
easily. Also, if you have to allow access from an untrusted machine,
anyone there can compromise your display.
When using xdm (X Display Manager) to log in, you get a much better
access method: MIT-MAGIC-COOKIE-1. A 128-bit "cookie" is generated and
stored in your .Xauthority file. If you need to allow a remote machine
access to your display, you can use the xauth command and the
information in your .Xauthority file to provide access to only that
connection. See the Remote-X-Apps mini-howto, available at
http://metalab.unc.edu/LDP/HOWTO/mini/Remote-X-Apps.html.
You can also use ssh (see ``'', above) to allow secure X connections.
This has the advantage of also being transparent to the end user, and
means that no unencrypted data flows across the network.
Take a look at the Xsecurity man page for more information on X
security. The safe bet is to use xdm to login to your console and then
use ssh to go to remote sites on which you with to run X programs.
6.11.2. SVGA
SVGAlib programs are typically SUID-root in order to access all your
Linux machine's video hardware. This makes them very dangerous. If
they crash, you typically need to reboot your machine to get a usable
console back. Make sure any SVGA programs you are running are
authentic, and can at least be somewhat trusted. Even better, don't
run them at all.
6.11.3. GGI (Generic Graphics Interface project)
The Linux GGI project is trying to solve several of the problems with
video interfaces on Linux. GGI will move a small piece of the video
code into the Linux kernel, and then control access to the video
system. This means GGI will be able to restore your console at any
time to a known good state. They will also allow a secure attention
key, so you can be sure that there is no Trojan horse login program
running on your console.
http://synergy.caltech.edu/~ggi/
7. Kernel Security
This is a description of the kernel configuration options that relate
to security, and an explanation of what they do, and how to use them.
As the kernel controls your computer's networking, it is important
that it be very secure, and not be compromised. To prevent some of the
latest networking attacks, you should try to keep your kernel version
current. You can find new kernels at <
ftp://ftp.kernel.org> or from
your distribution vendor.
There is also a international group providing a single unified crypto
patch to the mainstream linux kernel. This patch provides support for
a number of cyrptographic subsystems and things that cannot be
included in the mainstream kernel due to export restrictions. For more
information, visit their web page at:
http://www.kerneli.org
7.1. 2.0 Kernel Compile Options
For 2.0.x kernels, the following options apply. You should see these
options during the kernel configuration process. Many of the comments
here are from ./linux/Documentation/Configure.help, which is the same
document that is referenced while using the Help facility during the
make config stage of compiling the kernel.
� Network Firewalls (CONFIG_FIREWALL)
This option should be on if you intend to run any firewalling or
masquerading on your linux machine. If it's just going to be a
regular client machine, it's safe to say no.
� IP: forwarding/gatewaying (CONFIG_IP_FORWARD)
If you enable IP forwarding, your Linux box essentially becomes a
router. If your machine is on a network, you could be forwarding
data from one network to another, and perhaps subverting a firewall
that was put there to prevent this from happening. Normal dial-up
users will want to disable this, and other users should concentrate
on the security implications of doing this. Firewall machines will
want this enabled, and used in conjunction with firewall software.
You can enable IP forwarding dynamically using the following
command:
root# echo 1 > /proc/sys/net/ipv4/ip_forward
and disable it with the command:
root# echo 0 > /proc/sys/net/ipv4/ip_forward
Keep in mind the files, and their sizes, do not reflect their actual
sizes, and despite being zero-length, may or may not be.
� IP: syn cookies (CONFIG_SYN_COOKIES)
a "SYN Attack" is a denial of service (DoS) attack that consumes
all the resources on your machine, forcing you to reboot. We can't
think of a reason you wouldn't normally enable this. In the 2.1
kernel series this config option mearly allows syn cookies, but
does not enable them. To enable them, you have to do:
root# echo 1 > /proc/sys/net/ipv4/tcp_syncookies <P>
� IP: Firewalling (CONFIG_IP_FIREWALL)
This option is necessary if you are going to configure your machine
as a firewall, do masquerading, or wish to protect your dial-up
workstation from someone entering via your PPP dial-up interface.
� IP: firewall packet logging (CONFIG_IP_FIREWALL_VERBOSE)
This option gives you information about packets your firewall
received, like sender, recipient, port, etc.
� IP: Drop source routed frames (CONFIG_IP_NOSR)
This option should be enabled. Source routed frames contain the
entire path to their destination inside of the packet. This means
that routers through which the packet goes do not need to inspect
it, and just forward it on. This could lead to data entering your
system that may be a potential exploit.
� IP: masquerading (CONFIG_IP_MASQUERADE) If one of the computers on
your local network for which your Linux box acts as a firewall
wants to send something to the outside, your box can "masquerade"
as that host, i.e., it forwards the traffice to the intended
destination, but makes it look like it came from the firewall box
itself. See
http://www.indyramp.com/masq for more information.
� IP: ICMP masquerading (CONFIG_IP_MASQUERADE_ICMP) This option adds
ICMP masquerading to the previous option of only masquerading TCP
or UDP traffic.
� IP: transparent proxy support (CONFIG_IP_TRANSPARENT_PROXY) This
enables your Linux firewall to transparently redirect any network
traffice originating from the local network and destined for a
remote host to a local server, called a "transparent proxy server".
This makes the local computers think they are talking to the remote
end, while in fact they are connected to the local proxy. See the
IP-Masquerading HOWTO and
http://www.indyramp.com/masq for more
information.
� IP: always defragment (CONFIG_IP_ALWAYS_DEFRAG)
Generally this option is disabled, but if you are building a
firewall or a masquerading host, you will want to enable it. When
data is sent from one host to another, it does not always get sent
as a single packet of data, but rather it is fragmented into
several pieces. The problem with this is that the port numbers are
only stored in the first fragment. This means that someone can
insert information into the remaining packets that isn't supposed
to be there. It could also prevent a teardrop attack against an
internal host that is not yet itself patched against it.
� Packet Signatures (CONFIG_NCPFS_PACKET_SIGNING)
This is an option that is available in the 2.1 kernel series that
will sign NCP packets for stronger security. Normally you can
leave it off, but it is there if you do need it.
� IP: Firewall packet netlink device (CONFIG_IP_FIREWALL_NETLINK)
This is a really neat option that allows you to analyze the first
128 bytes of the packets in a user-space program, to determine if
you would like to accept or deny the packet, based on its validity.
7.2. 2.2 Kernel Compile Options
For 2.2.x kernels, many of the options are the same, but a few new
ones have been developed. Many of the comments here are from
./linux/Documentation/Configure.help, which is the same document that
is referenced while using the Help facility during the make config
stage of compiling the kernel. Only the newly- added options are
listed below. Consult the 2.0 description for a list of other
necessary options. The most signficant change in the 2.2 kernel
series is the IP firewalling code. The ipchains program is now used
to install IP firewalling, instead of the ipfwadm program used in the
2.0 kernel.
� Socket Filtering (CONFIG_FILTER)
For most people, it's safe to say no to this option. This option
allows you to connect a userspace filter to any socket and
determine if packets should be allowed or denied. Unless you have a
very specific need and are capable of programming such a filter,
you should say no. Also note that as of this writing, all protocols
were supported except TCP.
� Port Forwarding Port Forwarding is an addition to IP Masquerading
which allows some forwarding of packets from outside to inside a
firewall on given ports. This could be useful if, for example, you
want to run a web server behind the firewall or masquerading host
and that web server should be accessible from the outside world. An
external client sends a request to port 80 of the firewall, the
firewall forwards this request to the web server, the web server
handles the request and the results are sent through the firewall
to the original client. The client thinks that the firewall machine
itself is running the web server. This can also be used for load
balancing if you have a farm of identical web servers behind the
firewall.
Information about this feature is available from
http://www.monmouth.demon.co.uk/ipsubs/portforwarding.html (to
browse the WWW, you need to have access to a machine on the
Internet that has a program like lynx or netscape). For general
info, please see
ftp://ftp.compsoc.net/users/steve/ipportfw/linux21/
� Socket Filtering (CONFIG_FILTER) Using this option, user-space
programs can attach a filter to any socket and thereby tell the
kernel that it should allow or disallow certain types of data to
get through the socket. Linux socket filtering works on all socket
types except TCP for now. See the text file
./linux/Documentation/networking/filter.txt for more information.
� IP: Masquerading The 2.2 kernel masquerading has been improved. It
provides additional support for masquerading special protocols,
etc. Be sure to read the IP Chains HOWTO for more information.
7.3. Kernel Devices
There are a few block and character devices available on Linux that
will also help you with security.
The two devices /dev/random and /dev/urandom are provided by the
kernel to provide random data at any time.
Both /dev/random and /dev/urandom should be secure enough to use in
generating PGP keys, ssh challenges, and other applications where
secure random numbers are requisite. Attackers should be unable to
predict the next number given any initial sequence of numbers from
these sources. There has been a lot of effort put in to ensuring that
the numbers you get from these sources are random in every sense of
the word.
The only difference is that /dev/random runs out of random bytes and
it makes you wait for more to be accumulated. Note that on some
systems, it can block for a long time waiting for new user-generated
entry to be entered into the system. So you have to use care before
using /dev/random. (Perhaps the best thing to do is to use it when
you're generating sensitive keying information, and you tell the user
to pound on the keyboard repeatedly until you print out "OK, enough".)
/dev/random is high quality entropy, generated from measuring the
inter-interrupt times etc. It blocks until enough bits of random data
are available.
/dev/urandom is similar, but when the store of entropy is running low,
it'll return a cryptographically strong hash of what there is. This
isn't as secure, but it's enough for most applications.
You might read from the devices using something like:
root# head -c 6 /dev/urandom | mmencode
This will print six random characters on the console, suitable for
password generation. You can find mmencode in the metamail package.
See /usr/src/linux/drivers/char/random.c for a description of the
algorithm.
Thanks to Theodore Y. Ts'o, Jon Lewis, and others from Linux-kernel
for helping me (Dave) with this.
8. Network Security
Network security is becoming more and more important as people spend
more and more time connected. Compromising network security is often
much easier than compromising physical or local, and is much more
common.
There are a number of good tools to assist with network security, and
more and more of them are shipping with Linux distributions.
8.1. Packet Sniffers
One of the most common ways intruders gain access to more systems on
your network is by employing a packet sniffer on a already compromised
host. This "sniffer" just listens on the Ethernet port for things like
passwd and login and su in the packet stream and then logs the traffic
after that. This way, attackers gain passwords for systems they are
not even attempting to break into. Clear-text passwords are very
vulnerable to this attack.
Example: Host A has been compromised. Attacker installs a sniffer.
Sniffer picks up admin logging into Host B from Host C. It gets the
admin's personal password as they login to B. Then, the admin does a
su to fix a problem. They now have the root password for Host B. Later
the admin lets someone telnet from his account to Host Z on another
site. Now the attacker has a password/login on Host Z.
In this day and age, the attacker doesn't even need to compromise a
system to do this: they could also bring a laptop or pc into a
building and tap into your net.
Using ssh or other encrypted password methods thwarts this attack.
Things like APOP for POP accounts also prevents this attack. (Normal
POP logins are very vulnerable to this, as is anything that sends
clear-text passwords over the network.)
8.2. System services and tcp_wrappers
Before you put your Linux system on ANY network the first thing to
look at is what services you need to offer. Services that you do not
need to offer should be disabled so that you have one less thing to
worry about and attackers have one less place to look for a hole.
There are a number of ways to disable services under Linux. You can
look at your /etc/inetd.conf file and see what services are being
offered by your inetd. Disable any that you do not need by commenting
them out (# at the beginning of the line), and then sending your inetd
process a SIGHUP.
You can also remove (or comment out) services in your /etc/services
file. This will mean that local clients will also be unable to find
the service (i.e., if you remove ftp, and try and ftp to a remote site
from that machine it will fail with an "unknown service" message).
It's usually not worth the trouble to remove services, since it
provides no additional security. If a local person wanted to use ftp
even though you had commented it out, they would make their own client
that use the common FTP port and would still work fine.
Some of the services you might want to leave enabled are:
� ftp
� telnet (or ssh)
� mail, such as pop-3 or imap
� identd
If you know you are not going to use some particular package, you can
also delete it entirely. rpm -e packagename under the Red Hat
distribution will erase an entire package. Under debian dpkg --remove
does the same thing.
Additionally, you really want to disable the rsh/rlogin/rcp utilities,
including login (used by rlogin), shell (used by rcp), and exec (used
by rsh) from being started in /etc/inetd.conf. These protocols are
extremely insecure and have been the cause of exploits in the past.
You should check your /etc/rc.d/rcN.d, (where N is your systems run
level) and see if any of the servers started in that directory are not
needed. The files in /etc/rc.d/rcN.d are actually symbolic links to
the directory /etc/rc.d/init.d. Renaming the files in the init.d
directory has the effect of disabling all the symbolic links in
/etc/rc.d/rcN.d. If you only wish to disable a service for a
particular run level, rename the appropriate file by replacing the
upper-case S with a lower-case s, like this:
root# cd /etc/rc6.d
root# mv S45dhcpd s45dhcpd
If you have BSD style rc files, you will want to check /etc/rc* for
programs you don't need.
Most Linux distributions ship with tcp_wrappers "wrapping" all your
TCP services. A tcp_wrapper (tcpd) is invoked from inetd instead of
the real server. tcpd then checks the host that is requesting the
service, and either executes the real server, or denies access from
that host. tcpd allows you to restrict access to your TCP services.
You should make a /etc/hosts.allow and add in only those hosts that
need to have access to your machine's services.
If you are a home dialup user, we suggest you deny ALL. tcpd also logs
failed attempts to access services, so this can give alert you if you
are under attack. If you add new services, you should be sure to
configure them to use tcp_wrappers if they are TCP based. For
example, a normal dial-up user can prevent outsiders from connecting
to his machine, yet still have the ability to retrieve mail, and make
network connections to the Internet. To do this, you might add the
following to your /etc/hosts.allow:
ALL: 127.
And of course /etc/hosts.deny would contain:
ALL: ALL
which will prevent external connections to your machine, yet still
allow you from the inside to connect to servers on the Internet.
Keep in mind that tcp_wrappers only protect services executed from
inetd, and a select few others. There very well may be other services
running on your machine. You can use netstat -ta to find a list of
all the services your machine is offering.
8.3. Verify Your DNS Information
Keeping up-to-date DNS information about all hosts on your network can
help to increase security. If an unauthorized host becomes connected
to your network, you can recognize it by its lack of a DNS entry.
Many services can be configured to not accept connections from hosts
that do not have valid DNS entries.
8.4. identd
identd is a small program that typically runs out of your inetd
server. It keeps track of what user is running what TCP service, and
then reports this to whoever requests it.
Many people misunderstand the usefulness of identd, and so disable it
or block all off site requests for it. identd is not there to help out
remote sites. There is no way of knowing if the data you get from the
remote identd is correct or not. There is no authentication in identd
requests.
Why would you want to run it then? Because it helps you out, and is
another data-point in tracking. If your identd is un compromised, then
you know it's telling remote sites the user-name or uid of people
using TCP services. If the admin at a remote site comes back to you
and tells you user so-and-so was trying to hack into their site, you
can easily take action against that user. If you are not running
identd, you will have to look at lots and lots of logs, figure out who
was on at the time, and in general take a lot more time to track down
the user.
The identd that ships with most distributions is more configurable
than many people think. You can disable it for specific users (they
can make a .noident file), you can log all identd requests (We
recommend it), you can even have identd return a uid instead of a user
name or even NO-USER.
8.5. SATAN, ISS, and Other Network Scanners
There are a number of different software packages out there that do
port and service based scanning of machines or networks. SATAN, ISS,
SAINT, and Nessus are some of the more well-known ones. This software
connects to the target machine (or all the target machines on a
network) on all the ports they can, and try to determine what service
is running there. Based on this information, you can tell if the
machine is vulnerable to a specific exploit on that server.
SATAN (Security Administrator's Tool for Analyzing Networks) is a port
scanner with a web interface. It can be configured to do light,
medium, or strong checks on a machine or a network of machines. It's a
good idea to get SATAN and scan your machine or network, and fix the
problems it finds. Make sure you get the copy of SATAN from metalab
<
http://metalab.unc.edu/pub/packages/security/Satan-for-Linux/> or a
reputable FTP or web site. There was a Trojan copy of SATAN that was
distributed out on the net.
http://www.trouble.org/~zen/satan/satan.html. Note that SATAN has not
been updated in quite a while, and some of the other tools below might
do a better job.
ISS (Internet Security Scanner) is another port-based scanner. It is
faster than Satan, and thus might be better for large networks.
However, SATAN tends to provide more information.
Abacus is a suite of tools to provide host based security and
intrusion detection. look at it's home page on the web for more
information.
http://www.psionic.com/abacus/
SAINT is a updated version of SATAN. It is web based and has many more
up to date tests than SATAN. You can find out more about it at:
http://www.wwdsi.com/~saint
Nessus is a free security scanner. It has a GTK graphical interface
for ease of use. It is also designed with a very nice plugin setup for
new port scanning tests. For more information, take a look at:
http://www.nessus.org
8.5.1. Detecting Port Scans
There are some tools designed to alert you to probes by SATAN and ISS
and other scanning software. However, liberal use of tcp_wrappers, and
make sure to look over your log files regularly, you should be able to
notice such probes. Even on the lowest setting, SATAN still leaves
traces in the logs on a stock Red Hat system.
There are also "stealth" port scanners. A packet with the TCP ACK bit
set (as is done with established connections) will likely get through
a packet-filtering firewall. The returned RST packet from a port that
_had no established session_ can be taken as proof of life on that
port. I don't think TCP wrappers will detect this.
8.6. sendmail , qmail and MTA's
One of the most important services you can provide is a mail server.
Unfortunately, it is also one of the most vulnerable to attack, simply
due to the number of tasks it must perform and the privileges it
typically needs.
If you are using sendmail it is very important to keep up on current
versions. sendmail has a long long history of security exploits.
Always make sure you are running the most recent version from
http://www.sendmail.org.
Keep in mind that sendmail does not have to be running in order for
you to send mail. If you are a home user, you can disable sendmail
entirely, and simply use your mail client to send mail. You might
also choose to remove the "-bd" flag from the sendmail startup file,
thereby disabling incoming requests for mail. In other words, you can
execute sendmail from your startup script using the following instead:
# /usr/lib/sendmail -q15m
This will cause sendmail to flush the mail queue every fifteen minutes
for any messages that could not be successfully delivered on the first
attempt.
Many administrators choose not to use sendmail, and instead choose one
of the other mail transport agents. You might consider switching over
to qmail. qmail was designed with security in mind from the ground up.
It's fast, stable, and secure. Qmail can be found at
http://www.qmail.org
In direct competition to qmail is "postfix", written by Wietse Venema,
the author of tcp_wrappers and other security tools. Formerly called
vmailer, and sponsored by IBM, this is also a mail transport agent
written from the ground up with security in mind. You can find more
information about vmailer at
http://www.postfix.org
8.7. Denial of Service Attacks
A "Denial of Service" (DoS) attack is one where the attacker tries to
make some resource too busy to answer legitimate requests, or to deny
legitimate users access to your machine.
Denial of service attacks have increased greatly in recent years. Some
of the more popular and recent ones are listed below. Note that new
ones show up all the time, so this is just a few examples. Read the
Linux security lists and the bugtraq list and archives for more
current information.
� SYN Flooding - SYN flooding is a network denial of service attack.
It takes advantage of a "loophole" in the way TCP connections are
created. The newer Linux kernels (2.0.30 and up) have several
configurable options to prevent SYN flood attacks from denying
people access to your machine or services. See ``Kernel Security''
for proper kernel protection options.
� Pentium "F00F" Bug - It was recently discovered that a series of
assembly codes sent to a genuine Intel Pentium processor would
reboot the machine. This affects every machine with a Pentium
processor (not clones, not Pentium Pro or PII), no matter what
operating system it's running. Linux kernels 2.0.32 and up contain
a work around for this bug, preventing it from locking your
machine. Kernel 2.0.33 has an improved version of the kernel fix,
and is suggested over 2.0.32. If you are running on a Pentium, you
should upgrade now!
� Ping Flooding - Ping flooding is a simple brute-force denial of
service attack. The attacker sends a "flood" of ICMP packets to
your machine. If they are doing this from a host with better
bandwidth than yours, your machine will be unable to send anything
on the network. A variation on this attack, called "smurfing",
sends ICMP packets to a host with your machine's return IP,
allowing them to flood you less detectably. You can find more
information about the "smurf" attack at
http://www.quadrunner.com/~chuegen/smurf.txt
If you are ever under a ping flood attack, use a tool like tcpdump
to determine where the packets are coming from (or appear to be
coming from), then contact your provider with this information.
Ping floods can most easily be stopped at the router level or by
using a firewall.
� Ping o' Death - The Ping o' Death attack sends ICMP ECHO REQUEST
packets that are too large to fit in the kernel data structures
intended to store them. Because sending a single, large (65,510
bytes) "ping" packet to many systems will cause them to hang or
even crash, this problem was quickly dubbed the "Ping o' Death."
This one has long been fixed, and is no longer anything to worry
about.
� Teardrop / New Tear - One of the most recent exploits involves a
bug present in the IP fragmentation code on Linux and Windows
platforms. It is fixed in kernel version 2.0.33, and does not
require selecting any kernel compile-time options to utilize the
fix. Linux is apparently not vulnerable to the "newtear" exploit.
You can find code for most exploits, and a more in-depth
description of how they work, at
http://www.rootshell.com using
their search engine.
8.8. NFS (Network File System) Security.
NFS is a very widely-used file sharing protocol. It allows servers
running nfsd and mountd to "export" entire filesystems to other
machines using NFS filesystem support built in to their kernels (or
some other client support if they are not Linux machines). mountd
keeps track of mounted filesystems in /etc/mtab, and can display them
with showmount.
Many sites use NFS to serve home directories to users, so that no
matter what machine in the cluster they login to, they will have all
their home files.
There is some small amount of security allowed in exporting
filesystems. You can make your nfsd map the remote root user (uid=0)
to the nobody user, denying them total access to the files exported.
However, since individual users have access to their own (or at least
the same uid) files, the remote root user can login or su to their
account and have total access to their files. This is only a small
hindrance to an attacker that has access to mount your remote
filesystems.
If you must use NFS, make sure you export to only those machines that
you really need to. Never export your entire root directory; export
only directories you need to export.
See the NFS HOWTO for more information on NFS, available at
http://metalab.unc.edu/mdw/HOWTO/NFS-HOWTO.html
8.9. NIS (Network Information Service) (formerly YP).
Network Information service (formerly YP) is a means of distributing
information to a group of machines. The NIS master holds the
information tables and converts them into NIS map files. These maps
are then served over the network, allowing NIS client machines to get
login, password, home directory and shell information (all the
information in a standard /etc/passwd file). This allows users to
change their password once and have it take effect on all the machines
in the NIS domain.
NIS is not at all secure. It was never meant to be. It was meant to be
handy and useful. Anyone that can guess the name of your NIS domain
(anywhere on the net) can get a copy of your passwd file, and use
"crack" and "John the Ripper" against your users' passwords. Also, it
is possible to spoof NIS and do all sorts of nasty tricks. If you must
use NIS, make sure you are aware of the dangers.
There is a much more secure replacement for NIS, called NIS+. Check
out the NIS HOWTO for more information:
http://metalab.unc.edu/mdw/HOWTO/NIS-HOWTO.html
8.10. Firewalls
Firewalls are a means of controlling what information is allowed into
and out of your local network. Typically the firewall host is
connected to the Internet and your local LAN, and the only access from
your LAN to the Internet is through the firewall. This way the
firewall can control what passes back and forth from the Internet and
your lan.
There are a number of types of firewalls and methods of setting them
up. Linux machines make pretty good firewalls. Firewall code can be
built right into 2.0 and higher kernels. The ipfwadm for 2.0 kernels,
or ipchains for 2.2 kernels, user-space tools allows you to change, on
the fly, the types of network traffic you allow. You can also log
particular types of network traffic.
Firewalls are a very useful and important technique in securing your
network. However, never think that because you have a firewall, you
don't need to secure the machines behind it. This is a fatal mistake.
Check out the very good Firewall-HOWTO at your latest metalab archive
for more information on firewalls and Linux.
http://metalab.unc.edu/mdw/HOWTO/Firewall-HOWTO.html
More information can also be found in the IP-Masquerade mini-howto:
http://metalab.unc.edu/mdw/HOWTO/mini/IP-Masquerade.html
More information on ipfwadm (The tool that lets you change settings on
your firewall, can be found at it's home page:
http://www.xos.nl/linux/ipfwadm/
If you have no experience with firewalls, and plan to set up one for
more than just a simple security policy, the Firewalls book by
O'Reilly and Associates or other online firewall document is mandatory
reading. Check out
http://www.ora.com for more information. The
National Institute of Standards and Technology have put together an
excellent document on firewalls. Although dated 1995, it is still
quite good. You can find it at
http://csrc.nist.gov/nistpubs/800-10/main.html. Also of interest
includes:
� The Freefire Project -- a list of freely-available firewall tools,
available at
http://sites.inka.de/sites/lina/freefire-
l/index_en.html
� SunWorld Firewall Design -- written by the authors of the O'Reilly
book, this provides a rough introduction to the different firewall
types. It's available at
http://www.sunworld.com/swol-01-1996/swol-01-firewall.html
8.11. IP Chains - Linux Kernel 2.2.x Firewalling
Linux IP Firewalling Chains is an update to the 2.0 Linux firewalling
code for the 2.2 kernel. It has a great deal more features than
previous implementations, including:
� More flexible packet manipulations
� More complex accounting
� Simple policy changes possible atomically
� Fragments can be explicitly blocked, denied, etc.
� Logs suspicious packets.
� Can handle protocols other than ICMP/TCP/UDP.
If you are currently using ipfwadm on your 2.0 kernel, there are
scripts available to convert the ipfwadm command format to the format
ipchains uses.
Be sure to read the IP Chains HOWTO for further information. It is
avilable at
http://www.rustcorp.com/linux/ipchains/HOWTO.html
8.12. VPN's - Virtual Private Networks
VPN's are a way to establish a "virtual" network on top of some
already existing network. This virtual network often is encrypted and
passes traffic only to and from some known entities that have joined
the network. VPN's are often used to connect someone working at home
over the public internet to a internal company network by using a
encrypted virtual network.
If you are running a linux masquerading firewall and need to pass MS
PPTP (Microsoft's VPN point to point product) packets, there is a
linux kernel patch out to do just that. See: ip-masq-vpn.
There are several linux VPN solutions available:
� vpnd. See the
http://www.crosswinds.net/nuremberg/~anstein/unix/vpnd.html.
� Free S/Wan, available at
http://www.xs4all.nl/~freeswan/
� ssh can be used to construct a VPN. See the VPN mini-howto for
more information.
� vps (virtual private server) at
http://www.strongcrypto.com.
See also the section on IPSEC for pointers and more information.
9. Security Preparation (before you go on-line)
Ok, so you have checked over your system, and determined it's as
secure as feasible, and you're ready to put it online. There are a
few things you should now do in order to prepare for an intrusion, so
you can quickly disable the intruder, and get back up and running.
9.1. Make a Full Backup of Your Machine
Discussion of backup methods and storage is beyond the scope of this
document, but here are a few words relating to backups and security:
If you have less than 650mb of data to store on a partition, a CD-R
copy of your data is a good way to go (as it's hard to tamper with
later, and if stored properly can last a long time). Tapes and other
re-writable media should be write-protected as soon as your backup is
complete, and then verified to prevent tampering. Make sure you store
your backups in a secure off-line area. A good backup will ensure that
you have a known good point to restore your system from.
9.2. Choosing a Good Backup Schedule
A six-tape cycle is easy to maintain. This includes four tapes for
during the week, one tape for even Fridays, and one tape for odd
Fridays. Perform an incremental backup every day, and a full backup
on the appropriate Friday tape. If you make some particularly
important changes or add some important data to your system, a full
backup might well be in order.
9.3. Backup Your RPM or Debian File Database
In the event of an intrusion, you can use your RPM database like you
would use tripwire, but only if you can be sure it too hasn't been
modified. You should copy the RPM database to a floppy, and keep this
copy off-line at all times. The Debian distribution likely has
something similar.
The files /var/lib/rpm/fileindex.rpm and /var/lib/rpm/packages.rpm
most likely won't fit on a single floppy. But if Compressed, each
should fit on a seperate floppy.
Now, when your system is compromised, you can use the command:
root# rpm -Va
to verify each file on the system. See the rpm man page, as there are
a few other options that can be included to make it less verbose.
Keep in mind you must also be sure your RPM binary has not been com�
promised.
This means that every time a new RPM is added to the system, the RPM
database will need to be rearchived. You will have to decide the
advantages versus drawbacks.
9.4. Keep Track of Your System Accounting Data
It is very important that the information that comes from syslog has
not been compromised. Making the files in /var/log readable and
writable by only a limited number of users is a good start.
Be sure to keep an eye on what gets written there, especially under
the auth facility. Multiple login failures, for example, can indicate
an attempted break-in.
Where to look for your log file will depend on your distribution. In a
Linux system that conforms to the "Linux Filesystem Standard", such as
Red Hat, you will want to look in /var/log and check messages,
mail.log, and others.
You can find out where your distribution is logging to by looking at
your /etc/syslog.conf file. This is the file that tells syslogd (the
system logging daemon) where to log various messages.
You might also want to configure your log-rotating script or daemon to
keep logs around longer so you have time to examine them. Take a look
at the logrotate package on recent Red Hat distributions. Other
distributions likely have a similar process.
If your log files have been tampered with, see if you can determine
when the tampering started, and what sort of things appeared to be
tampered with. Are there large periods of time that cannot be
accounted for? Checking backup tapes (if you have any) for untampered
log files is a good idea.
Log files are typically modified by the intruder in order to cover his
tracks, but they should still be checked for strange happenings. You
may notice the intruder attempting to gain entrance, or exploit a
program in order to obtain the root account. You might see log entries
before the intruder has time to modify them.
You should also be sure to seperate the auth facility from other log
data, including attempts to switch users using su, login attempts, and
other user accounting information.
If possible, configure syslog to send a copy of the most important
data to a secure system. This will prevent an intruder from covering
his tracks by deleting his login/su/ftp/etc attempts. See the
syslog.conf man page, and refer to the @ option.
There are several more advanced syslogd programs out there. Take a
look at
http://www.core-sdi.com/ssyslog/ for Secure Syslog. Secure
Syslog allows you to encrypt your syslog entries and make sure no one
has tampered with them.
Another syslogd with more features is syslog-ng. It allows you a lot
more flexability in your logging and also can has your remote syslog
streams to prevent tampering.
Finally, log files are much less useful when no one is reading them.
Take some time out every once in a while to look over your log files,
and get a feeling for what they look like on a normal day. Knowing
this can help make unusual things stand out.
9.5. Apply All New System Updates.
Most Linux users install from a CD-ROM. Due to the fast-paced nature
of security fixes, new (fixed) programs are always being released.
Before you connect your machine to the network, it's a good idea to
check with your distribution's ftp site and get all the updated
packages since you received your distribution CD-ROM. Many times these
packages contain important security fixes, so it's a good idea to get
them installed.
10. What To Do During and After a Breakin
So you have followed some of the advice here (or elsewhere) and have
detected a break-in? The first thing to do is to remain calm. Hasty
actions can cause more harm than the attacker would have.
10.1. Security Compromise Underway.
Spotting a security compromise under way can be a tense undertaking.
How you react can have large consequences.
If the compromise you are seeing is a physical one, odds are you have
spotted someone who has broken into your home, office or lab. You
should notify your local authorities. In a lab, you might have spotted
someone trying to open a case or reboot a machine. Depending on your
authority and procedures, you might ask them to stop, or contact your
local security people.
If you have detected a local user trying to compromise your security,
the first thing to do is confirm they are in fact who you think they
are. Check the site they are logging in from. Is it the site they
normally log in from? No? Then use a non-electronic means of getting
in touch. For instance, call them on the phone or walk over to their
office/house and talk to them. If they agree that they are on, you can
ask them to explain what they were doing or tell them to cease doing
it. If they are not on, and have no idea what you are talking about,
odds are this incident requires further investigation. Look into such
incidents , and have lots of information before making any
accusations.
If you have detected a network compromise, the first thing to do (if
you are able) is to disconnect your network. If they are connected via
modem, unplug the modem cable; if they are connected via ethernet,
unplug the Ethernet cable. This will prevent them from doing any
further damage, and they will probably see it as a network problem
rather than detection.
If you are unable to disconnect the network (if you have a busy site,
or you do not have physical control of your machines), the next best
step is to use something like tcp_wrappers or ipfwadm to deny access
from the intruder's site.
If you can't deny all people from the same site as the intruder,
locking the user's account will have to do. Note that locking an
account is not an easy thing. You have to keep in mind .rhosts files,
FTP access, and a host of possible backdoors).
After you have done one of the above (disconnected the network, denied
access from their site, and/or disabled their account), you need to
kill all their user processes and log them off.
You should monitor your site well for the next few minutes, as the
attacker will try to get back in. Perhaps using a different account,
and/or from a different network address.
10.2. Security Compromise has already happened
So you have either detected a compromise that has already happened or
you have detected it and locked (hopefully) the offending attacker out
of your system. Now what?
10.2.1. Closing the Hole
If you are able to determine what means the attacker used to get into
your system, you should try to close that hole. For instance, perhaps
you see several FTP entries just before the user logged in. Disable
the FTP service and check and see if there is an updated version, or
if any of the lists know of a fix.
Check all your log files, and make a visit to your security lists and
pages and see if there are any new common exploits you can fix. You
can find Caldera security fixes at
http://www.caldera.com/tech-
ref/security/. Red Hat has not yet seperated their security fixes from
bug fixes, but their distribution errata is available at
http://www.redhat.com/errata
Debian now has a security mailing list and web page. See:
http://www.debian.com/security/ for more information.
It is very likely that if one vendor has released a security update,
that most other Linux vendors will as well.
There is now a linux security auditing project. They are methodically
going through all the user space utilities and looking for possible
security exploits and overflows. From their announcement:
"We are attempting a systematic audit of Linux sources with
a view to being as secure as OpenBSD. We have already uncov�
ered (and fixed) some problems, but more help is welcome.
The list is unmoderated and also a useful resource for gen�
eral security discussions. The list address is: security-
[email protected] To subscribe, send a mail to:
[email protected]"
If you don't lock the attacker out, they will likely be back. Not just
back on your machine, but back somewhere on your network. If they were
running a packet sniffer, odds are good they have access to other
local machines.
10.2.2. Assessing the Damage
The first thing is to assess the damage. What has been compromised?
If you are running an Integrity Checker like Tripwire, you can use it
to perform an integrity check, and should help to tell you. If not,
you will have to look around at all your important data.
Since Linux systems are getting easier and easier to install, you
might consider saving your config files and then wiping your disk(s)
and reinstalling, then restoring your user files from backups and your
config files. This will ensure that you have a new, clean system. If
you have to backup files from the compromised system, be especially
cautious of any binaries that you restore, as they may be Trojan
horses placed there by the intruder.
Re-installation should be considered mandatory upon an intruder
obtaining root access. Additionally, you'd like to keep any evidence
there is, so having a spare disk in the safe may make sense.
Then you have to worry about how long ago the compromise happened, and
whether the backups hold any damaged work. More on backups later.
10.2.3. Backups, Backups, Backups!
Having regular backups is a godsend for security matters. If your
system is compromised, you can restore the data you need from backups.
Of course, some data is valuable to the attacker too, and they will
not only destroy it, they will steal it and have their own copies; but
at least you will still have the data.
You should check several backups back into the past before restoring a
file that has been tampered with. The intruder could have compromised
your files long ago, and you could have made many successful backups
of the compromised file!!!
Of course, there are also a raft of security concerns with backups.
Make sure you are storing them in a secure place. Know who has access
to them. (If an attacker can get your backups, they can have access to
all your data without you ever knowing it.)
10.2.4. Tracking Down the Intruder.
Ok, you have locked the intruder out, and recovered your system, but
you're not quite done yet. While it is unlikely that most intruders
will ever be caught, you should report the attack.
You should report the attack to the admin contact at the site where
the attacker attacked your system. You can look up this contact with
whois or the Internic database. You might send them an email with all
applicable log entries and dates and times. If you spotted anything
else distinctive about your intruder, you might mention that too.
After sending the email, you should (if you are so inclined) follow up
with a phone call. If that admin in turn spots your attacker, they
might be able to talk to the admin of the site where they are coming
from and so on.
Good crackers often use many intermediate systems, some (or many) of
which may not even know they have been compromised. Trying to track a
cracker back to their home system can be difficult. Being polite to
the admins you talk to can go a long way to getting help from them.
You should also notify any security organizations you are a part of
(CERT <
http://www.cert.org/> or similar), as well as your Linux system
vendor.
11. Security Sources
There are a LOT of good sites out there for Unix security in general
and Linux security specifically. It's very important to subscribe to
one (or more) of the security mailing lists and keep current on
security fixes. Most of these lists are very low volume, and very
informative.
11.1. FTP Sites
CERT is the Computer Emergency Response Team. They often send out
alerts of current attacks and fixes. See
ftp://ftp.cert.org for more
information.
Replay (
http://www.replay.com) has archives of many security programs.
Since they are outside the US, they don't need to obey US crypto
restrictions.
Matt Blaze is the author of CFS and a great security advocate. Matt's
archive is available at
ftp://ftp.research.att.com/pub/mab
<
ftp://ftp.research.att.com/pub/mab>
tue.nl is a great security FTP site in the Netherlands.
ftp.win.tue.nl
11.2. Web Sites
� The Hacker FAQ is a FAQ about hackers: The Hacker FAQ
� The COAST archive has a large number of Unix security programs and
information: COAST
� SuSe Security Page:
http://www.suse.de/security/
� Rootshell.com is a great site for seeing what exploits are
currently being used by crackers:
http://www.rootshell.com/
� BUGTRAQ puts out advisories on security issues: BUGTRAQ archives
� CERT, the Computer Emergency Response Team, puts out advisories on
common attacks on unix platforms: CERT home
� Dan Farmer is the author of SATAN and many other security tools.
His home site has some interesting security survey information, as
well as security tools:
http://www.trouble.org
� The Linux security WWW is a good site for Linux security
information: Linux Security WWW
� Infilsec has a vulnerability engine that can tell you what
vunerabilities affect a specific platform:
http://www.infilsec.com/vulnerabilities/
� CIAC sends out periodic security bulletins on common exploits:
http://ciac.llnl.gov/cgi-bin/index/bulletins
� A good starting point for Linux Pluggable Authentication modules
can be found at
http://www.kernel.org/pub/linux/libs/pam/.
� The debian project has a web page for their security fixes and
information. It is at
http://www.debian.com/security/.
� WWW Security FAQ, written by Lincoln Stein, is a great web security
reference. Find it at
http://www.w3.org/Security/Faq/www-security-
faq.html
11.3. Mailing Lists
Bugtraq: To subscribe to bugtraq, send mail to
[email protected]
containing the message body subscribe bugtraq. (see links above for
archives).
CIAC: Send e-mail to
[email protected]. In the BODY (not
subject) of the message put (either or both): subscribe ciac-bulletin
Red Hat has a number of mailing lists, the most important of which is
the redhat-announce list. You can read about security (and other)
fixes as soon as they come out. Send email to
[email protected] and
put subscribe redhat-announce.
The Debian project has a security mailing list that covers their
security fixes. see
http://www.debian.com/security/ for more
information.
11.4. Books - Printed Reading Material
There are a number of good security books out there. This section
lists a few of them. In addition to the security specific books,
security is covered in a number of other books on system
administration.
Building Internet Firewalls By D. Brent Chapman & Elizabeth D. Zwicky
1st Edition September 1995
ISBN: 1-56592-124-0
Practical UNIX & Internet Security, 2nd Edition By Simson Garfinkel &
Gene Spafford
2nd Edition April 1996
ISBN: 1-56592-148-8
Computer Security Basics By Deborah Russell & G.T. Gangemi, Sr.
1st Edition July 1991
ISBN: 0-937175-71-4
Linux Network Administrator's Guide By Olaf Kirch
1st Edition January 1995
ISBN: 1-56592-087-2
PGP: Pretty Good Privacy By Simson Garfinkel
1st Edition December 1994
ISBN: 1-56592-098-8
Computer Crime A Crimefighter's Handbook By David Icove, Karl Seger &
William VonStorch (Consulting Editor Eugene H. Spafford)
1st Edition August 1995
ISBN: 1-56592-086-4
12. Glossary
� authentication: The property of knowing that the data received is
the same as the data that was sent, and that the claimed sender is
in fact the actual sender.
� bastion Host: A computer system that must be highly secured because
it is vulnerable to attack, usually because it is exposed to the
Internet and is a main point of contact for users of internal
networks. It gets its name from the highly fortified projects on
the outer walls of medieval castles. Bastions overlook critical
areas of defense, usually having strong walls, room for extra
troops, and the occasional useful tub of boiling hot oil for
discouraging attackers.
� buffer overflow: Common coding style is to never allocate large
enough buffers, and to not check for overflows. When such buffers
overflow, the executing program (daemon or set-uid program) can be
tricked in doing some other things. Generally this works by
overwriting a function's return address on the stack to point to
another location.
� denial of service: A denial of service attack is when an attacker
consumes the resources on your computer for things it was not
intended to be doing, thus preventing normal use of your network
resources for legimite purposes.
� dual-homed Host: A general-purpose computer system that has at
least two network interfaces.
� firewall: A component or set of components that restricts access
between a protected network and the Internet, or between other sets
of networks.
� host: A computer system attached to a network.
� IP spoofing: IP Spoofing is a complex technical attack that is made
up of several components. It is a security exploit that works by
tricking computers in a trust-relationship that you are someone
that you really aren't. There is an extensive paper written by
daemon9, route, and infinity in the Volume Seven, Issue fourty-
Eight issue of Phrack Magazine.
� non-repudiation: The property of a receiver being able to prove
that the sender of some data did in fact send the data even though
the sender might later deny ever having sent it.
� packet: The fundamental unit of communication on the Internet.
� packet filtering: The action a device takes to selectively control
the flow of data to and from a network. Packet filters allow or
block packets, usually while routing them from one network to
another (most often from the Internet to an internal network, and
vice-versa). To accomplish packet filtering, you set up rules that
specify what types of packets (those to or from a particular IP
address or port) are to be allowed and what types are to be
blocked.
� perimeter network: A network added between a protected network and
an external network, in order to provide an additional layer of
security. A perimeter network is sometimes called a DMZ.
� proxy server: A program that deals with external servers on behalf
of internal clients. Proxy clients talk to proxy servers, which
relay approved client requests to real servers, and relay answers
back to clients.
� superuser: An informal name for root.
13. Frequently Asked Questions
1. Is it more secure to compile driver support directly into the
kernel, instead of making it a module?
Answer: Some people think it is better to disable the ability to
load device drivers using modules, because an intruder could load a
Trojan module or a module that could affect system security.
However, in order to load modules, you must be root. The module
object files are also only writable by root. This means the
intruder would need root access to insert a module. If the
intruder gains root access, there are more serious things to worry
about than whether he will load a module.
Modules are for dynamically loading support for a particular device
that may be infrequently used. On server machines, or firewalls
for instance, this is very unlikely to happen. For this reason, it
would make more sense to compile support directly into the kernel
for machines acting as a server. Modules are also slower than
support compiled directly in the kernel.
2. Why does logging in as root from a remote machine always fail?
Answer: See ``Root Security''. This is done intentionally to
prevent remote users from attempting to connect via telnet to your
machine as root, which is a serious security vulnerability. Don't
forget: potential intruders have time on their side, and can run
automated programs to find your password.
3. How do I enable shadow passwords on my Red Hat 4.2 or 5.x Linux
box?
Answer: Shadow passwords is a mechanism for storing your password
in a file other than the normal /etc/passwd file. This has several
advantages. The first one is that the shadow file, /etc/shadow, is
only readable by root, unlike /etc/passwd, which must remain
readable by everyone. The other advantage is that as the
administrator, you can enable or disable accounts without everyone
knowing the status of other users' accounts.
The /etc/passwd file is then used to store user and group names,
used by programs like /bin/ls to map the user ID to the proper
username in a directory listing.
The /etc/shadow file then only contains the username and his/her
password, and perhaps accounting information, like when the account
expires, etc.
To enable shadow passwords, run pwconv as root, and /etc/shadow
should now exist, and be used by applications. Since you are using
RH 4.2 or above, the PAM modules will automatically adapt to the
change from using normal /etc/passwd to shadow passwords without
any other change.
Since you're interested in securing your passwords, perhaps you
would also be interested in generating good passwords to begin
with. For this you can use the pam_cracklib module, which is part
of PAM. It runs your password against the Crack libraries to help
you decide if it is too easily guessable by password cracking
programs.
4. How can I enable the Apache SSL extensions?
Answer:
1.Get SSLeay 0.8.0 or later from
<
ftp://ftp.psy.uq.oz.au/pub/Crypto/SSL>
2.Build and test and install it!
3.Get Apache 1.2.5 source
4.Get Apache SSLeay extensions from here
<
ftp://ftp.ox.ac.uk/pub/crypto/SSL/apache_1.2.5+ssl_1.13.tar.gz>
5.Unpack it in the apache-1.2.5 source directory and patch Apache
as per the README.
6.Configure and build it.
You might also try Replay Associates which has many pre-built
packages, and is located outside of the United States.
5. How can I manipulate user accounts, and still retain security?
Answer: The Red Hat distribution, especially RH5.0, contains a
great number of tools to change the properties of user accounts.
� The pwconv and unpwconv programs can be used to convert between
shadow and non-shadowed passwords.
� The pwck and grpck programs can be used to verify proper
organization of the passwd and group files.
� The useradd, usermod, and userdel programs can be used to add,
delete and modify user accounts. The groupadd, groupmod, and
groupdel programs will do the same for groups.
� Group passwords can be created using gpasswd.
All these programs are "shadow-aware" -- that is, if you enable
shadow they will use /etc/shadow for password information,
otherwise it won't.
See the respective man pages for further information.
6. How can I password protect specific HTML documents using Apache?
I bet you didn't know about
http://www.apacheweek.org, did you?
You can find information on user Authentication at
http://www.apacheweek.com/features/userauth as well as other web
server security tips from
http://www.apache.org/docs/misc/security_tips.html
14. Conclusion
By subscribing to the security alert mailing lists, and keeping
current, you can do a lot towards securing your machine. If you pay
attention to your log files and run something like tripwire regularly,
you can do even more.
A reasonable level of computer security is not difficult to maintain
on a home machine. More effort is required on business machines, but
Linux can indeed be a secure platform. Due to the nature of Linux
development, security fixes often come out much faster than they do on
commercial operating systems, making Linux an ideal platform when
security is a requirement.
15. Acknowledgements
Information here is collected from many sources. Thanks to the
following that either indirectly or directly have contributed:
following who either indirectly or directly have contributed:
Rob Riggs
[email protected]
S. Coffin
[email protected]
Viktor Przebinda
[email protected]
Roelof Osinga
[email protected]
Kyle Hasselbacher
[email protected]
David S. Jackson
[email protected]
Todd G. Ruskell
[email protected]
Rogier Wolff
[email protected]
Antonomasia
[email protected]
Nic Bellamy
[email protected]
Eric Hanchrow
[email protected]
Robert J.
[email protected]
Ulrich Alpers
[email protected]
David Noha
[email protected]
The following have translated this HOWTO into various other languages!
A special thank you to all of them for help spreading the linux
word...
Polish: Ziemek Borowski
[email protected]
Japanese: FUJIWARA Teruyoshi
[email protected]
Indonesian: Tedi Heriyanto
[email protected]
