Linux Security HOWTO
Kevin Fenzi,
[email protected] & Dave Wreski,
[email protected]
v0.9.11, 1 May 1998
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. NOTE: This is a beta version of this document.
Improvements, constructive criticism, additions and corrections are
gratefully accepted. Please mail your feedback to both authors. Be
sure and include "Linux", "security" or "HOWTO" in the subject line of
your mail to avoid spam filters and to bring your mail to the quick
attention of the authors.
______________________________________________________________________
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 Your 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 & Encryption
6.1 PGP and Public Key Cryptography
6.2 SSL, S-HTTP, HTTPS and S/MIME
6.3 Linux x-kernel IPSEC Implementation
6.4 SSH (Secure Shell), stelnet
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 Kernel Compile Options
7.2 Kernel Devices
8. Network Security
8.1 Packet Sniffers
8.2 System services and tcp_wrappers
8.3 Verify Your DNS Information
8.4 identd
8.5 SATAN , ISS, and Other Network Scanners
8.6 Sendmail, qmail and MTA's.
8.7 Denial of Service Attacks
8.8 NFS (Network File System) Security.
8.9 NIS (Network Information Service) (formerly YP).
8.10 Firewalls
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 under way.
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. Thanks to
______________________________________________________________________
1. Introduction
This document covers some of the main security 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 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 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 who archive such information, including:
ftp://sunsite.unc.edu/pub/Linux/docs/HOWTO
In addition, you should generally be able to find this document on the
Linux Worldwide Web home page via:
http://sunsite.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 documents can be accepted. Use
the concepts, examples and other content at your own risk.
Additionally, this is an early version, with many possibilities for
inaccuracies and 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-like terms.
1.4. Copyright Information
This document is copyrighted (c)1998 Kevin Fenzi and Dave Wreski, and
distributed under the following terms:
o 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.
o 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.
o 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 foundatation 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
becomming 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, your data. 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 still 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 is 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 makes
it harder to surf the web.
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.
o Risk is the possibility that an intruder may be successful in
attempting to access your computer. Can an intruder read, write
files, or execute programs that could cause damage? Can they
delete critical data? 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. A single user that is
allowed to login using an rhosts file, or allowing the use of an
insecure service, such as tftp, you risk an intruder using this to
'get 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.
o Threat is typically from someone with motivation to gain
unathorized access to your network, or computer. You must decide
who you trust to have access to your system, and what threat they
could impose.
There are several types of intruders, and it is useful to keep the
different characteristics in mind as you are securing your systems.
o The Curious - This type of intruder is basically interested in
finding out what type of system and data, you have.
o The Malicious - This type of intruder is out to either bring down
your systems, or deface your web page, or otherwise cause you time
and money to recover.
o 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.
o 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.
o Vulnerability describes how well protected your computer is from
another network, and the potential for someone gaining unathorized
access.
What's at stake if someone breaks into your system? Of course the
concerns of a dynamic PPP home user will be different than 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.
2.5. Means of Securing Your Site
This document will discuss various means in which you can secure the
assets you have worked hard for: your local machine, data, users,
network, even your reputation. What would happen to your reputation
if an intruder deleted some of your user's 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 several reasons an intruder may be interested in your
systems, which we will discuss later.
2.5.1. Host Security
Perhaps the area of most concentration on security is done with 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 of machines
becomes larger.
2.5.2. Your Network Security
Network security is also as necessary as local host security. With
your single system, or a distributed computing network, the Internet,
or hundreds, if not thousands or more computers on the same network,
you can't rely on each one of those systems being secure. Making sure
authorized users are the only ones permitted to use your network
resources, building firewalls, using strong encryption, and ensuring
there are no rogue, or 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 that by doing something like changing the login
name from 'root' to 'toor', for example, to try and obscure someone
from breaking into your system as root is only a false sense of
security, and will result in very unpleasant consequences. Rest
assured that any system attacker will quickly see through such empty
security measures. Simply because you may have a small site, or
relatively low profile does not mean an intruder won't be interested
in what you have. We'll discuss what your protecting in the next
sections.
2.6. Organization of This Document
This document has been segregated into a number of sections. They
cover several broad kinds of security issues. The first, physical
security, covers how you need to protect your physical machine from
tampering. The second describes how to protect your system from
tampering by local users. The third, files and filesystem security
show 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 machine. network security, describes how to better secure your
Linux system from network attacks. Security preperation discusses how
to prepair your machine(s) before bringing the on-line. The next
discusses what to do when you detect a system compromise in progress
or detect one that has recently happened. Then links to other security
resources are enumerated, and finally some questions and answers and a
few closing words.
The two main points to realize when reading this document are:
o Be aware of your system. Check system logs such as
/var/log/messages and keep an eye on your system, and
o Two, 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 environment, 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 a 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 survailance are all a good idea, but beyond
the scope of this document. :)
3.1. Computer locks
Many more 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.
Under Linux/x86 many PC BIOSs let you set a boot password. This
doesn't provide all that much security (bios can be reset, or removed
if someone can get into the case), but might be a good deterant (ie it
will take time and leave traces of tampering).
Many x86 bioses also allow you to specify various other good security
settings. Check your bios manual or look at it the next time you boot
up. Some examples are: disallow booting from floppy drives and
passwords to access some bios features.
On Linux/Sparc, your SPARC EEPROM can be set to require a boot-up
password. This might slow attackers down.
NOTE: If you have a server machine, and you setup 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 even of a power
failure. ;(
3.3. Boot Loader Security
The various Linux boot loaders also can have a boot password set.
Using lilo, take a look at the "restricted" and "password" settings.
"password" allows you to set a bootup password. "restricted" will let
the machine boot _unless_ someone specifies options at the lilo:
prompt (like 'single').
Keep in mind when setting all these passwords that you need to
remember them. :) Also remember that these passwords will mearly slow
the determined attacker. This 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, as well as password-protecting
your computer's 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 setup 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 even 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
vty 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 does not 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.
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, a trouble light should go on. 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.
Some things to check for in your logs:
o Short or incomplete logs.
o Logs containing strange timestamps.
o Logs with incorrect permissions or ownership.
o Records of reboots or restarting of services.
o missing logs.
o 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 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 them 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:
o Give them the minimal amount of privileges they need.
o Be aware when/where they login from, or should be logging in from.
o Make sure and remove their account when they no longer need the
access.
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 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. Running as root
all the time is a very very very bad idea.
Several tricks to avoid messing up your own box as root:
o When doing some complex command, try running it first in a non
destructive way...especially commands that use globbing: ie, you
are going to do a "rm foo*.bak", instead, 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.
o Some people find it helpfull to do a "touch /-i" on their systems.
This will make commands like: "rm -rf *" ask you if you really want
to delete all the files. (It does this by your shell resolving the
"-i" file first, and treating it as the -i option to rm.) This will
not help with rm statements with no * in them. ;(
o 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.
o The command path for the root user is very important. The command
path, or the PATH environment variable, defines the location the
shell searches for programs. Try and limit the command path for
the root user as much as possible, and never use '.', meaning 'the
current directory', in your PATH statement. 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.
o Never use the rlogin/rsh/rexec (called the r-utilities) suite of
tools as root. They are subject to many sorts of attacks, and are
downright dangerous run as root. Never create a .rhosts file for
root.
o 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.
o 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 superuser 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 successfull 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,
but use sudo so you can 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
the user root access. This includes most editors, for example. Also,
a program as innocous 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 yet be
secure.
5. Files and Filesystem Security
A few minutes of preparation and planning ahead before putting your
systems online can help to protect your system, and the data that is
stored on them.
o There should never be a reason for user's 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 user's home
partitions, as well as /var, which prohibit execution of programs,
and creation of character or block devices, which should never be
necessary anyway.
o 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 mounting
read-only wherever possible.
o Configure your user's file-creation umask to be as restrictive as
possible. Commonly used settings are 022, 033, and the most
restrictive 077, and are added to /etc/profile.
o 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.
o The /var/log/wtmp and /var/run/utmp files contain the login records
for all users on your system. Its 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.
o 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 this file, which has been
the source of attacks involving deleting /etc/passwd or
/etc/shadow. See the chattr(1) man page for information on the
immutable bit.
o 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 \( -perm -04000 -o -perm -02000 \)
You can descriminitely remove the SUID or SGID permissions on a suspi-
cious program with chmod(1), then change it back if you absolutely
feel it is necessary.
o 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 -print
and be sure you know why those files are writable. In the normal
course of operation, several files will be writable, including some
from /dev, and symbolic links.
o
Unowned files may also be an indication an intruder has accessed
your system. You can locate files on your system that do not have
an owner, or belong to a group with the command:
root# find / -nouser -o -nogroup -print
o 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
o
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 inadvertantly 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. 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(1).
If you are using Red Hat, and adhered 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 insure that your system files are not open for
casual editing by users and groups who shouldn't be doing such system
maintance.
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:
o To be able to view contents of a file
o To be able to read a directory
Write:
o To be able to add to or change a file
o To be able to delete or move files in a directory
Execute:
o To be able to run a binary program or shell script
o 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. If the sticky bit is set on a directory, then a
user may only delete files that the user 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 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 also 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 set of the 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 files do:
-r-------- Allow read access to the file by owner
--w------- Allows the owner to modify or delete the file
---x------ The owner can execute this program, but not shell scripts,
which still need read permission
---s------ Will execute with effective user ID = owner
-------s-- Will execute with effective user ID = 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 set of the 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 now be read by owner
d-wx------ Files can now 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
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.
5.4. Trojan Horses
A Trojan Horse is named after the fabled ploy in Homers great literary
work. The idea is that you put up a program or binary that sounds
great, and get other people to download it and run it as root. Then,
you 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 signs, RPM files so you can
verify you are installing the real thing. Other distributions have
similar methods. You should never run any binary you don't have the
source for or a well known binary 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
some program from it's 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 & 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 a introduction is in order. Encryption is very useful,
possibly even nessessary in this day and age. There are all sorts of
methods of encrypting data, each with their 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,
whatever 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 below)
can often guess passwords unless your password is sufficently random.
PAM modules (see below) allow you to use a different encryption
routine with your passwords (MD5 or the like).
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 which is used for PGP, involves
cryptography that uses one key for encryption, and one key for
decryption. Traditionally, cryptography involves using the same key
for encryption that is used for decryption. This "private key" must
be known to both parties, and somehow transferred from one another
securely.
Public key encryption alleviates the need to securely transmit the key
that is used for encryption by using two seperate 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,
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 a the PGP FAQ.
http://www.pgp.com/service/export/faq/55faq.cgi Be sure to use the
version that is applicable to your country, as due to export
restrictions by the US Government, strong-encryption is considered a
military weapon, and prohibited from being transferred in electronic
form outside the country.
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, which is available at
ftp://sunsite.unc.edu/pub/Linux/apps/crypto.
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 times 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 are, and where to find more information.
o 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.
o S-HTTP: - S-HTTP is another protocol that provides security
services across the Internet. It was designed to provide
confidentiality, authenticity, 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]
o S/MIME: - S/MIME, or Secure Multipurpose Internet Mail Extension,
is an encryption standard used to encrypt electronic mail, or other
types of messages on the Internet. It is an open standard
developed by RSA, so it is hopefully 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 x-kernel IPSEC Implementation
Along with CIPE, and other forms of data encryption, there is also an
implemention of IPSEC for Linux. IPSEC is an effort by the IETF to
create cryptographically secure communications at the IP network
level, which also provides 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 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.
As with other forms of cryptography, it is not distributed with the
kernel by default due to export restrictions.
6.4. SSH (Secure Shell), 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 for user authentication.
This 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.
SSLeay is a free implmentation of Netscape's Secure Sockets Layer
protocol, including 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/
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 on the
fly your authentication methods, requirements, and encapsulate all
local authentication methods without re-compiling any of your
binaries. Configuration of PAM is beyond the scope of this document,
but be suer 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:
o Use a non DES encryption for your passwords. (Making them harder to
brute force decode)
o Set resource limits on all your users so they can't perform denial
of service attacks (number of processes, amount of memory, etc)
o Enable shadow passwords (see below) on the fly
o 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 dot-rhosts files in user's home
directories by adding these lines to /etc/pam.d/login:
#
# 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 travelling
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 management) 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). The authentication is 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.
The overall effect of installing Kerberos and the numerous other
programs that go with it is to virtually eliminate the ability of
users to "spoof" the system into believing they are someone else.
Unfortunately, installing Kerberos is very intrusive, requiring the
modification or replacement of numerous stanard programs.
You can find more information on kerberos at
http://www.veritas.com/common/f/97042301.htm 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.]
6.8. Shadow Passwords.
Shadow passwords are a means of keeping your encrypted password
information secret from normal users. Normally this encrypted password
is stored in your /etc/passwd file for all to read. They can then run
password guesser programs on it and attempt to determine what it is.
Shadow passwords save this information to a /etc/shadow file that only
privileged users can read. In order to run shadow passwords you need
to make sure all your utilities that need access to password
information are recompiled to support it. PAM (above) also allows you
to just plug in a shadow module and 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://sunsite.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 non easily
guessable 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. They encrypt
each one and check it against your encrypted password. If they get a
match they are in.
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 get your passwd (unix /etc/passwd)
file, but these are more common than you might think.
6.10. CFS - Cryptographic File System and TCFS - transparent crypto-
graphic File System
CFS is a way of encrypting an entire file system and allow users to
store encrypted files on them. It uses a NFS server running on the
local machine. rpms are avail 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, adding more integration with the file system, so
that it's transparent to any users using the file system that it's
encrypted. more information at:
http://edu-gw.dia.unisa.it/tcfs/
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 doing things like: grabbing your passwords as you type
them without you knowing it, reading documents or information you are
reading on your screen, or even using a hole to gain superuser 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 can use xhost to specify what hosts are allowed access
to your display. This is not very secure at all. 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 login, you get a much better
access method: MIT-MAGIC-COOKIE-1. A 128bit cookie is generated and
stored in your .Xauthorty 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 only that connection
access. See the Remote-X-Apps mini-howto, available at
http://sunsite.unc.edu/LDP/HOWTO/mini/Remote-X-Apps.html.
You can also use ssh (see ssh, above) to allow secure X connections.
This has the advantage of also being transparent to the end user, and
means that no un-encrypted 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 you wish to run X programs off of.
6.11.2. SVGA
SVGAlib programs are typically SUID-root in order to access all your
Linux machines 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 the kernel is very secure, and the kernel itself won't be
compromised. To prevent some of the latest networkworking attacks, you
should try and keep your kernel version current. You can find new
kernels at
ftp://ftp.kernel.org.
7.1. Kernel Compile Options
o 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 the packet goes thru does not need to inspect the
packet, and just forwards it on. This could lead to data entering
your system that may be a potential exploit.
o 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.
o 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 and disable 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
This file (and many other files in /proc) will always appear to be
zero length, but in fact aren't. This is a newly introduced kernel
feature, so be sure your using a kernel 2.0.33 or later.
o IP: firewall packet logging (CONFIG_IP_FIREWALL_VERBOSE)
This option gives you information about packets your firewall
received, like sender, receipient, port, etc.
o 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 for your connection
that aren't supposed to be there.
o IP: syn cookies (CONFIG_SYN_COOKIES)
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.
o 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.
o 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 userspace program, to determine if
you would like to accept or deny the packet, based on its validity.
7.2. 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 retrieve 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 random.
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 | uuencode -
This will print six random characters on the console, suitable for
password generation.
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 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
"Password" 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
admins 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 wire.)
8.2. System services and tcp_wrappers
As soon as 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 (ie, 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 tho 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 will want to leave enabled are:
o ftp
o telnet
o mail, such as pop-3 or imap
o identd
o time
If you know you are not going to use some particular package, you can
also delete it entirely. rpm -e under the Red Hat distribution will
erase an entire package. Under debian dpkg likely 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 runlevel, rename the appropriate file with a lower-case
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 machines 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 you an idea that
you are under attack. If you add new services, you should be sure to
configure it to use tcp_wrappers TCP based. For example, a normal
dial-up user can prevent outsiders from connecting to your 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.
8.3. Verify Your DNS Information
Keeping up-to-date DNS information about all hosts on your network can
help to increase security. In the event of 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. 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 identd for specific users
(they can make a .noident file), you can log all identd requests (I
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 and ISS
are two 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 it can, and tries to determine what service is running there.
Based on this information, you could find out the machine is
vulnerable to a specific exploit on that server.
SATAN (Security Administrators 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 sun-site
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
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-Sentry is a commercial port scanner from www.psionic.com. Look
at it's home page on the web for more information.
http://www.psionic.com
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
making 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.
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.
http://www.sendmail.org
If you are tired of upgrading your version of sendmail every week, you
might consider switching over to qmail. qmail was designed with
security in mind from the ground up. It's fast and stable and secure.
http://www.qmail.org
8.7. Denial of Service Attacks
A Denial of service 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.
o 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 the section on
kernel security for proper kernel protection options.
o Pentium "F00F" Bug - It was recently discovered that a series of
assembly codes send 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 kernel 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!
o 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_ machines 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.
o Ping o' Death - The Ping o' Death attack is a result of incoming
ICMP ECHO REQUEST packets being larger than the kernel data
structures that store this information can hold. 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.
o 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 most exploit code, 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 with nfs filesystem support builtin to their kernels (or some
other client support if they are non Linux machines). Mountd keeps
track of mounted filesystems in /etc/mtab, and can display them with
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 superuser 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 export only. Never export your entire root
directory, export only directories you need to export.
See the NFS HOWTO for more information on NFS: NFS HOWTO
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 affect 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 usefull. 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://sunsite.unc.edu/mdw/HOWTO/NIS-HOWTO.html
8.10. Firewalls
Firewalls are a means of restricting 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 and methods of setting up firewalls. Linux
machines make pretty good low cost firewalls. Firewall code can be
built right into 2.0 and higher kernels. The ipfwadm user space tool
allows you to change what types of network traffic you allow on the
fly. You can also log particular types of network traffic.
Firewalls are a very usefull and important technique in securing your
network. It is important to realize that you should 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 sunsite archive for more information on firewalls
and Linux.
http://sunsite.unc.edu/mdw/HOWTO/Firewall-HOWTO.html
More information can also be found in the IP-Masquerade mini-howto:
http://sunsite.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/
9. Security Preparation (before you go on-line)
Ok, so you have checked over your system, and determined its as secure
as feasible, and are ready to put it online. There are a few things
you should now do in order to be prepared in case an intrusion
actually does happen, 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 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 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 an easy one to maintain. This includes four tapes
for during the week, one tape for even Friday's, and one tape for odd
Friday's. Perform an incremental backup every day, and a full backup
on the appropriate Friday tape. If you make some particular important
changes or add some important data to your system, a 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.
Specifically, the files /var/lib/rpm/fileindex.rpm and
/var/lib/rpm/packages.rpm most likely won't fit on a single floppy.
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.
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 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 un 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 they appeared to
tamper 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.
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 the 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 CDROM. 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 (ftp.redhat.com for example) and get
all the updated packages since you received your distribution CDROM.
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 breakin? The first thing to do is to remain calm. Hasty
actions can cause more harm than the attacker would have.
10.1. Security Compromise under way.
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 setting 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 are
normally 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 intruders site.
If you can't deny all people from the same site as the intruder,
locking the users 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 backdoors).
After you have done one of the above (disconnected 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 and 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 and 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
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 here
http://www.caldera.com/tech-
ref/security/. Red Hat has not yet seperated their security fixes from
bugfixes, but their distribution errata is available at
http://www.redhat.com/errata It is very likely that if one vendor has
released a security update, that most other Linux vendors will as
well.
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 make a
tripwire run and it should 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 insure 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.
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 to, 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 hackers 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 or similar).
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. cert.org
Replay has archives of many security programs. Since they are outside
the US, they don't need to obey US crypto restrictions. replay.com
Matt Blaze is the author of CFS and a great security advocate. Matt
Blaze's stuff
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
Rootshell.com is a great site for seeing what exploits are currently
being used by crackers: rootshell.com exploits
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: Dan Farmers trouble.org
The Linux security WWW is a good site for Linux security information:
Linux Security WWW
Reptile has lots of good Linux security information on his site:
Reptiles Linux Security Page
Infilsec has a vulnerability engine that can tell you what
vunerabilities affect a specific platform: Infilsec vunerability
engine
CIAC sends out periodic security bulitins on common exploits: CIAC
bulitins
A good starting point for Linux Pluggable Authentication modules can
be found at
http://www.kernel.org/pub/linux/libs/pam/.
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
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 specify 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
o Host: A computer system attached to a network
o Firewall: A component or set of components that restricts access
between a protected network and the Internet, or between other sets
of networks.
o 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 strongs walls, room for extra
troops, and the occasional useful tub of boiling hot oil for
discouraging attackers.
o Dual-homed Host: A general-purpose computer system that has at
least two network interfaces.
o Packet: The fundamental unit of communication on the Internet.
o 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). accomplish packet filtering, you set up a set of
rules that specifiy 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.
o 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.
o 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 on to real servers, and relay
answers back to clients.
o 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 to legimite purposes.
o Buffer Overflow: Common coding style is never to allocate buffers
"large enough" and not checking for overflows. When such buffers
are overflows, 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.
o 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.
o 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.
o 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 desire to deny ever having sent that data.
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 himself load 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. Logging in as root from a remote machine always fails.
Answer: See the section on 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 5.0 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 your 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.
o The pwconv and unpwconv programs can be used to convert back and
forth between shadow and non-shadowed passwords
o The pwck and grpck programs can be used to verify proper
organization of the passwd and group files.
o The programs useradd, usermod, and userdel can be used to add,
delete and modify user accounts. The programs groupadd, groupmod,
and groupdel will do the same for groups.
o Group passwords can be created using gpasswd.
All these programs are 'shadow-aware' -- that is; if you enable
shadow it 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. Thanks to
Information here is collected from many sources. Thanks to the
following that 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]>
