Firewall and Proxy Server HOWTO
Mark Grennan,
[email protected]
v0.67, 26 September 1999
This document is designed to teach the basics of firewall systems and
give you some detail on setting up both a filtering and proxy firewall
on a Linux based PC. An HTML version of this document is available at
_h_t_t_p_:_/_/_w_w_w_._g_r_e_n_n_a_n_._c_o_m_/_F_i_r_e_w_a_l_l_-_H_O_W_T_O_._h_t_m_l
______________________________________________________________________
Table of Contents
1. Introduction
1.1 Feedback
1.2 Disclaimer
1.3 Copyright
1.4 My Reasons for Writing this
1.5 Further Readings
2. Understanding Firewalls
2.1 Firewall Politics
2.2 Types of Firewalls
2.2.1 Packet Filtering Firewalls
2.2.2 Proxy Servers
2.2.3 Application Proxy
2.2.4 SOCKS Proxy
3. Firewall Architecture
4. Setting up the Linux Filtering Firewall
4.1 Hardware requirements
5. Software requirements
5.1 Available packages
6. Preparing the Linux system
6.1 Compiling the Kernel
6.2 Configuring two network cards
6.3 Configuring the Network Addresses
6.4 Testing your network
6.5 Securing the Firewall
7. IP filtering setup (IPFWADM)
8. IP filtering setup (IPCHAINS)
9. Making it easy-er
10. Installing a Transparent SQUID proxy
11. Installing the TIS Proxy server
11.1 Getting the software
11.2 Compiling the TIS FWTK
11.3 Installing the TIS FWTK
11.4 Configuring the TIS FWTK
11.4.1 The netperm-table file
11.4.2 The /etc/services file
12. The SOCKS Proxy Server
12.1 Setting up the Proxy Server
12.2 Configuring the Proxy Server
12.2.1 The Access File
12.2.2 The Routing File
12.3 Working With a Proxy Server
12.3.1 Unix
12.3.2 MS Windows with Trumpet Winsock
12.3.3 Getting the Proxy Server to work with UDP Packets
12.4 Drawbacks with Proxy Servers
13. Advanced Configurations
13.1 A large network with emphasis on security
13.1.1 The Network Setup
13.1.2 The Proxy Setup
14. Making Management Easy
15. Defeating a Proxy Firewall
______________________________________________________________________
11.. IInnttrroodduuccttiioonn
David Rudder wrote this original version of this Firewall-HOWTO,
these many moons ago, and I'd still like to thank him for allowing me
to update his work.
Firewalls have gained great popularity as the ultimate in Internet
Security. Like most hot subject they are also often misunderstood.
This HOWTO will go over the basics of what a firewall is and how to
set one up.
I am using kernel 2.2.15 and RedHat 6.0 to develop this howto so the
examples here are based on this distribution. If you find differences
in your distribution, please email me and I'll update this howto.
11..11.. FFeeeeddbbaacckk
Any feedback is very welcome. PPLLEEAASSEE RREEPPOORRTT AANNYY IINNAACCCCUURRAACCIIEESS IINN TTHHIISS
PPAAPPEERR!!!!!! I am human, and prone to making mistakes. If you find a fix
Anything please send it to me. I will try to answer all e-mail, but I
am busy, so don't get insulted if I don't.
_M_y _e_m_a_i_l _a_d_d_r_e_s_s _i_s mmaarrkk@@ggrreennnnaann..ccoomm <<mailto:
[email protected]>
11..22.. DDiissccllaaiimmeerr
II AAMM NNOOTT RREESSPPOONNSSIIBBLLEE FFOORR AANNYY DDAAMMAAGGEESS IINNCCUURRRREEDD DDUUEE TTOO AACCTTIIOONNSS TTAAKKEENN
BBAASSEEDD OONN TTHHIISS DDOOCCUUMMEENNTT. This document is meant as an introduction to
how firewalls and proxy servers work. I am not, nor do I pretend to
be, a security expert. ;-) I am just some guy who has read to much and
likes computers more than most people. Please, I am writing this to
help get people acquainted with this subject, and I am not ready to
stake my life on the accuracy of what is in here.
11..33.. CCooppyyrriigghhtt
Unless otherwise stated, Linux HOWTO documents are copyrighted by
their respective authors. 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 author would
like to be notified of any such distributions.
All translations, derivative works, or aggregate works incorporating
any Linux HOWTO documents must be covered under this copyright notice.
That is, you may not produce a derivative work from a HOWTO and impose
additional restrictions on its distribution. Exceptions to these rules
may be granted under certain conditions; please contact the Linux
HOWTO coordinator.
In short, we wish to promote dissemination of this information through
as many channels as possible. However, we do wish to retain copyright
on the HOWTO documents, and would like to be notified of any plans to
redistribute the HOWTOs.
If you have any questions, please email me. (See Above)
11..44.. MMyy RReeaassoonnss ffoorr WWrriittiinngg tthhiiss
Several years ago, while working for the State of Oklahoma as their
"Internet Administrator" I was ask to "put the State on the Internet",
with no budget. (Note: There was no such title at the time. I was
just the guy doing all the work.) The best way to make this happen was
to use as much free software and junk hardware as I could. Linux and
a bunch of old 486s where all I had to work with.
Commercial firewalls are VERY over priced and the documentation on how
they work is considered almost top secret. I found creating a firewall
of my own was almost impossible.
At my next job, I was asked to put in a firewall. Linux had just
added firewall code. So again with no budget I started building a
firewall with Linux. Six months later my firewall was in place and
this document was updated.
11..55.. FFuurrtthheerr RReeaaddiinnggss
+o The The Linux Networking Overview HOWTO
<
http://sunsite.unc.edu/mdw/HOWTO/Networking-Overview-HOWTO.html>
+o The Ethernet HOWTO <
http://sunsite.unc.edu/mdw/HOWTO/Ethernet-
HOWTO.html>
+o IPchains Firewalling made Easy! <
http://www.nerdherd.net/ipchains/>
+o Linux Network Address Translation
<
http://www.linas.org/linux/load.html>
+o The Net-3 HOWTO <
http://sunsite.unc.edu/mdw/HOWTO/NET-3-HOWTO.html>
+o The NET-PPP HOWTO <
http://sunsite.unc.edu/mdw/HOWTO/PPP-HOWTO.html>
+o TCP/IP Network Administrator's Guide by O'Reilly and Associates
<
http://www.oreilly.com/catalog/tcp2/>
+o The Documentation for the TIS Firewall Toolkit
<
http://www.tis.com/research/software/>
For security issues in general you should check out these web sites.
+o _S_e_c_u_r_e _L_i_n_u_x _<
http://okcforum.org/~markg/Secure_Linux/>
This is my own security site where I have collected white papers,
documentation and programs about securing Unix systems.
[ More URLS go here ]
22.. UUnnddeerrssttaannddiinngg FFiirreewwaallllss
A firewall is a structure intended to keep a fire from spreading. In
a building a firewall is a brick wall completely dividing sections of
the building. In a car a firewall is the metal wall separating the
engine and passenger compartments.
Internet firewalls are intended to keep the flames of Internet hell
out of your private LAN. Or, to keep the members of your LAN pure and
chased by denying them access the all the evil Internet temptations.
;-)
The first computer firewall was a non-routing Unix host with
connections to two different networks. One network card connected to
the Internet and the other to the private LAN. To reach the Internet
from the private network, you had to logon to the firewall (Unix)
server. You then used the resources of the system to access the
Internet. For example, you could use X-windows to run Netscape's
browser on the firewall system and have the display on your work
station. With the browser running on the firewall it has access to
both networks.
This sort of dual homed system (a system with two network connections)
is great if you can TRUST ALL of your users. You can simple setup a
Linux system and give everyone needing Internet access accounts on it.
With this setup, the only computer on your private network that knows
anything about the outside world is the firewall. No one can download
to their personal workstations. They must first download a file to the
firewall and then download the file from the firewall to their
workstation.
BIG NOTE: 99% of all break-ins start with gaining account level access
on the system being attacked. Because of this I don't recommend this
type of firewall. It is also very limiting.
22..11.. FFiirreewwaallll PPoolliittiiccss
Don't believe a firewall is all you need. _S_e_t _p_o_l_i_c_i_e_s _f_i_r_s_t.
Firewalls are used for two prepossess.
1. to keep people (worms / crackers) out.
2. to keep people (employees / children) in.
When I started working on firewalls I was surprised to learn the
company I worked for were more interested in "spying" on their
employees then keeping crackers out of their networks.
At least in my state (Oklahoma) employers have the right to monitor
phone calls and Internet activity as long as they inform the employees
they are do it.
Big Brother is not government. Big Brother = Big Business.
Don't get me wrong. People should work, not play at work. And I feel
the work ethic has been eroding. However, I have also observed
management types are the biggest abusers of the rules they set. I have
seen hourly workers reprimanded for using the Internet to looking for
bus roughs to get to work while the same manager used hours of work
time looking for fine restaurants and nightclubs to take prospective
customers.
My fix for this type of abuse is to publish the firewall logs on a Web
page for everyone to see.
The security business can be scary. If you are the firewall manager,
watch your back.
22..22.. TTyyppeess ooff FFiirreewwaallllss
There are two types of firewalls.
1. Filtering Firewalls - that block selected network packets.
2. Proxy Servers (sometimes called firewalls) - that make network
connections for you.
22..22..11.. PPaacckkeett FFiilltteerriinngg FFiirreewwaallllss
Packet Filtering is the type of firewall built into the Linux kernel.
A filtering firewall works at the network level. Data is only allowed
to leave the system if the firewall rules allow it. As packets arrive
they are filtered by their type, source address, destination address,
and port information contained in each packet.
Many network routers have the ability perform some firewall services.
Filtering firewalls can be thought of as type of router. Because of
this you need a deep understanding of IP packet structure to work with
one.
Because very little data is analyzed and logged filtering firewalls
take less CPU and create less latency in your network.
Filtering firewalls do not provide for password controls. User can not
identify themselves. The only identity a user has is the IP number
assigned to their workstation. This can be a problem if you are going
to use DHCP (Dynamic IP assignments). Because rules are based on IP
numbers you will have to adjust the rules as new IP numbers are
assigned. I don't know how to automate this process.
Filtering firewalls are more transparent to the user. The user does
not have to setup rules in their applications to use the Internet.
With most proxy servers this is not true.
22..22..22.. PPrrooxxyy SSeerrvveerrss
Proxies are mostly used to control, or monitor, outbound traffic. Some
application proxies cache the data requested. This lowers bandwidth
requirements and decreases the access time for the next user how
access the same data. It also gives unquestionable evidence of what
was transferred.
There are to types of proxy servers.
1. Application Proxies - that do the work for you.
2. SOCKS Proxies - that cross wire ports.
22..22..33.. AApppplliiccaattiioonn PPrrooxxyy
The best example is a person telneting to a another computer and then
telneting from there to the outside world. Only with a application
proxy server the process is automated. As you telnet to the outside
world the client send you to the proxy first. The proxy then connects
to the server you requested (the outside world) and returns the data
to you.
Because proxy servers are handling all the communications, they can
log every thing they (you) do. For HTTP (web) proxies this would
include very URL they you see. For FTP proxies this includes every
file you download. They can even filter out "inappropriate" words from
the sites you visit or scan for viruses.
Application proxy servers can authenticate users. Before a connection
to the outside is made, the server can ask the user to login first. To
a web user this would make every site look like it required a login.
22..22..44.. SSOOCCKKSS PPrrooxxyy
A SOCKS server is a lot like an old switch board. It simply cross
wires your connect through the system to another outside connection.
Most SOCKS server only work with TCP type connections. And like
filtering firewalls they don't provide for user authentication. They
can however record where each user connected to.
33.. FFiirreewwaallll AArrcchhiitteeccttuurree
There are lots of ways to structure your network to protect your
systems using a firewall.
If you have a dedicated connections to the Internet through a router,
you could plug the router directly into your firewall system. Or, you
could go through a hub to provide for full access servers outside your
firewall.
You could setup some hard filtering rules in the router. However, this
router may be owned by your ISP so you may not have control. You can
ask your ISP to put in filters.
________ __________
_/\__/\_ | Router | | | _______________
| | | No | (DMZ) | Firewall | (LAN) | |
/ Internet \----|Filters |--(HUB)--| System |--(HUB)--| Workstation/s |
\_ _ _ _/ |________| | |__________| |_______________|
\/ \/ \/ |
(Outside)
(Server)
You may be using a dialup service like an ISDN line. In this case you
might use a third network card to provide provide a filtered DMZ. This
gives you full control over your Internet services and still separates
them from your regular network.
__________
_/\__/\_ | | _______________
| | | Firewall | (LAN) | |
/ Internet \----| System |--(HUB)--| Workstation/s |
\_ _ _ _/ |__________| |_______________|
\/ \/ \/ |
(DMZ)
(HUB)
If you are not providing Internet services yourself but you do want to
monitor where your users are going, you will want to use a proxy
server. This can be integrated with the firewall.
__________
_/\__/\_ | Proxy / | _______________
| | | Firewall | (LAN) | |
/ Internet \----| System |--(HUB)--| Workstation/s |
\_ _ _ _/ |__________| |_______________|
\/ \/ \/
You can put the proxy server on your LAN as will. In this case the
firewall should have rules to only allow the proxy server to connect
to the Internet for the services it is providing. This way the users
can get to the Internet only through the proxy.
__________
_/\__/\_ | | _______________
| | | Firewall | (LAN) | |
/ Internet \----| System |--(HUB)--| Workstation/s |
\_ _ _ _/ |__________| | |_______________|
\/ \/ \/ | ______________
| | |
+----| Proxy Server |
|______________|
If you are going to run a service like YAHOO or maybe SlashDot you may
want to make your system by using redundant routers and firewalls.
(Check out the High Availability HowTo.)
By using a round-robin DNS techniques or using load-balancing
application servers, you can create a 100% uptime service.
_/\__/\_ _/\__/\_
| | | |
/ ISP #1 \______ (WAN)_____/ Partners \
\_ _ _ _/ | (HUB) \_ _ _ _/
\/ \/ \/ | ___|____ \/ \/ \/
__|___ |_______ |
_/\__/\_ |_____ | | || ______
| | | || (DMZ) |Firewall|| (LAN) | |
/ ISP #2 \--|Router||--(HUB)--| System ||--(HUB)--| WS/s |
\_ _ _ _/ |______| | |________| | |______|
\/ \/ \/ | | | ______
(Outside) (Shared) | | |
(Server) (Server) +----|Proxy |
|______|
It is easy to let your network get out of hand. Keep control of every
connection. It only takes a user with a modem to compromise your LAN.
44.. SSeettttiinngg uupp tthhee LLiinnuuxx FFiilltteerriinngg FFiirreewwaallll
44..11.. HHaarrddwwaarree rreeqquuiirreemmeennttss
Filtering firewalls don't require fancy hardware. They are little more
then simple routers.
All you need is:
1. a 486-DX66 with 16 meg of memory
2. a 200m hard disk (500 recommended)
3. network connections (LAN Cards, Serial Ports, Wireless?)
4. monitor and keyboard
With some systems by using a serial port console, you can even
eliminate the monitor and keyboard.
If you need a proxy server that will handle lots of traffic, you
should get the largest system you can afford. This is because for
every user that connects to the system it will be creating another
process. If you will have 50 or more concurrent users I'm guessing you
will need:
1. a Pentium II with 64meg of memory
2. a two gig hard disk to store all the logs
3. two network connections
4. monitor and keyboard
The network connections can be any type (NIC cards, ISDN, even
modems).
55.. SSooffttwwaarree rreeqquuiirreemmeennttss
55..11.. AAvvaaiillaabbllee ppaacckkaaggeess
To create a filtering firewall, you don't need any special software.
Linux will do.
If you are using an older Linux kernel (1.0.x or older) you will need
a copy of ipfwadm from hhttttpp::////wwwwww..xxooss..nnll//lliinnuuxx//iippffwwaaddmm//
If you are using 2.1.102 or newer you will be using ipchaining as
developed by hhttttpp::////wwwwww..rruussttccoorrpp..ccoomm//lliinnuuxx//iippcchhaaiinnss//
If you want to setup a proxy server you will need one of these
packages.
1. Squid
2. The TIS Firewall Toolkit (FWTK)
3. SOCKS
Squid is a great package and works with Linux's Transparent Proxy
feature. I will be describing how to setup this server.
AT the time of this writing, Network Associates
<
http://www.networkassociates.com/> and Trusted Information System's
(TIS) , have merged. So keep watching their web sites for more
information about changes. Mean while, the Tool Kit can still be had
at. hhttttpp::////wwwwww..ttiiss..ccoomm//rreesseeaarrcchh//ssooffttwwaarree//
<<
http://www.tis.com/research/software/>
Trusted Information System put out a collection of programs designed
to facilitate firewalling. With this toolkit, you set up one daemon
for each service (WWW, telnet ect.) you will be using.
66.. PPrreeppaarriinngg tthhee LLiinnuuxx ssyysstteemm
66..11.. CCoommppiilliinngg tthhee KKeerrnneell
Start with a clean minimal installation of your Linux distribution.
The less software you have loaded the less holes, backdoors and/or
bugs there will be to introduce security problems in your server.
Pick a stable kernel. I am using kernel 2.2.9 or greater kernel for my
system. So this documentation is based on it's settings.
You well need to recompile the Linux kernel with the appropriate
options. If you haven't recompiled your kernel before you should read
the Kernel HOWTO, the Ethernet HOWTO, and the NET-2 HOWTO.
Here are the network related setting I know work. I have marked some
with a ?. If you will be using this feature, turn it on as well.
I use "make menuconfig" to edit my kernel settings.
<*> Packet socket
[ ] Kernel/User netlink socket
[*] Network firewalls
[ ] Socket Filtering
<*> Unix domain sockets
[*] TCP/IP networking
[ ] IP: multicasting
[*] IP: advanced router
[ ] IP: kernel level autoconfiguration
[*] IP: firewalling
[?] IP: always defragment (required for masquerading)
[?] IP: transparent proxy support
[?] IP: masquerading
--- Protocol-specific masquerading support will be built as modules.
[?] IP: ICMP masquerading
--- Protocol-specific masquerading support will be built as modules.
[ ] IP: masquerading special modules support
[*] IP: optimize as router not host
< > IP: tunneling
< > IP: GRE tunnels over IP
[?] IP: aliasing support
[*] IP: TCP syncookie support (not enabled per default)
--- (it is safe to leave these untouched)
< > IP: Reverse ARP
[*] IP: Allow large windows (not recommended if <16Mb of memory)
< > The IPv6 protocol (EXPERIMENTAL)
---
< > The IPX protocol
< > Appletalk DDP
< > CCITT X.25 Packet Layer (EXPERIMENTAL)
< > LAPB Data Link Driver (EXPERIMENTAL)
[ ] Bridging (EXPERIMENTAL)
[ ] 802.2 LLC (EXPERIMENTAL)
< > Acorn Econet/AUN protocols (EXPERIMENTAL)
< > WAN router
[ ] Fast switching (read help!)
[ ] Forwarding between high speed interfaces
[ ] PU is too slow to handle full bandwidth
QoS and/or fair queueing --->
After making all the setting you need you should recompile, reinstall
the kernel and reboot.
I use the command:
make dep;make clean;make bzlilo;make modules;make modules_install;init
6 to accomplish all of this in one step.
66..22.. CCoonnffiigguurriinngg ttwwoo nneettwwoorrkk ccaarrddss
If you have two network cards in your computer, you may need to add an
append statement to your /etc/lilo.conf file to describe the IRQ and
address of both cards. My lilo append statement looks like this:
append="ether=12,0x300,eth0 ether=15,0x340,eth1"
66..33.. CCoonnffiigguurriinngg tthhee NNeettwwoorrkk AAddddrreesssseess
Now we arrive at the fun part of our setup. I'm not going to go deep
into how to setup a LAN. Read the Networking-HOWTO to solve your
problems here.
Your goal is to provide two network connection to your filtering
firewall system. One on the Internet (unsecured side) and one on the
LAN (secure side).
Anyway, you have a few decisions to make.
1. Will you use Real IP number or Make some up for your LAN.
2. Will your ISP assign the number or will you be using static IP
numbers?
Since you don't want the internet to have access to your private
network, you don't need to use "real addresses". You could just
makeup addresses for your private LAN. But this is not recommended. If
data gets routed out of your LAN, it might end up at another systems
port.
There are a number of Internet address ranges set aside for private
networks. Of these, 192.168.2.xxx, is set aside and we will use it in
our examples.
You will need to use IP masquerading to make this happen. With this
process the firewall will forward packets and translate them into
"REAL " " IP address to travel on the Internet.
Using these non-routable IP address makes your network is more secure.
Internet routers will not pass packets with these addresses.
You may want to read the IP Masquerading HOWTO at this point.
199.1.2.10 __________ 192.168.2.1
_/\__/\_ \ | | / _______________
| | \| Firewall |/ | |
/ Internet \--------| System |------------| Workstation/s |
\_ _ _ _/ |__________| |_______________|
\/ \/ \/
You must have a "real" IP address to assign to your Internet network
card. This address can be permanently assigned to you. (A static IP
address) or it can be assigned at network connect time by the PPP
process.
You assign your inside IP numbers. Like 192.168.2.1 to the LAN card.
This will be your gateway IP address. You can assign all the other
machines in the protected network (LAN) a number in the 192.168.2.xxx
range. (192.168.2.2 through 192.168.2.254)
I use RedHat Linux. To configure the network at boot time I added a
ifcfg-eth1 file in the /etc/sysconfig/network-scripts directory. You
may also find a ifcfg-ppp0 or ifcfg-tr0 in this directory. These
'ifcfg-' files are used by RedHat to configure and enable your network
devices at boot time. The are named after the connection type.
Here is the ifcfg-eth1 (second ehternet card) for our example;
DEVICE=eth1
IPADDR=192.168.2.1
NETMASK=255.255.255.0
NETWORK=192.168.2.0
BROADCAST=192.168.2.255
GATEWAY=199.1.2.10
ONBOOT=yes
If you are going to use a dialup connection you will need to look at
the ifcfg-ppp0 and the chat-ppp0 file. These control your PPP
connection.
This ifcfg file might look like;
DEVICE="ppp0"
ONBOOT="yes"
USERCTL="no"
MODEMPORT="/dev/modem"
LINESPEED="115200"
PERSIST="yes"
DEFABORT="yes"
DEBUG="yes"
INITSTRING="ATZ"
DEFROUTE="yes"
HARDFLOWCTL="yes"
ESCAPECHARS="no"
PPPOPTIONS=""
PAPNAME="LoginID"
REMIP=""
NETMASK=""
IPADDR=""
MRU=""
MTU=""
DISCONNECTTIMEOUT=""
RETRYTIMEOUT="5"
BOOTPROTO="none"
66..44.. TTeessttiinngg yyoouurr nneettwwoorrkk
Start by using the ifconfig and route commands. If you have two
network cards ifconfig should look something like:
#ifconfig
lo Link encap:Local Loopback
inet addr:127.0.0.0 Bcast:127.255.255.255 Mask:255.0.0.0
UP BROADCAST LOOPBACK RUNNING MTU:3584 Metric:1
RX packets:1620 errors:0 dropped:0 overruns:0
TX packets:1620 errors:0 dropped:0 overruns:0
eth0 Link encap:10Mbps Ethernet HWaddr 00:00:09:85:AC:55
inet addr:199.1.2.10 Bcast:199.1.2.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:1000 errors:0 dropped:0 overruns:0
TX packets:1100 errors:0 dropped:0 overruns:0
Interrupt:12 Base address:0x310
eth1 Link encap:10Mbps Ethernet HWaddr 00:00:09:80:1E:D7
inet addr:192.168.2.1 Bcast:192.168.2.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:1110 errors:0 dropped:0 overruns:0
TX packets:1111 errors:0 dropped:0 overruns:0
Interrupt:15 Base address:0x350
and your route table should look like:
#route -n
Kernel routing table
Destination Gateway Genmask Flags MSS Window Use Iface
199.1.2.0 * 255.255.255.0 U 1500 0 15 eth0
192.168.2.0 * 255.255.255.0 U 1500 0 0 eth1
127.0.0.0 * 255.0.0.0 U 3584 0 2 lo
default 199.1.2.10 * UG 1500 0 72 eth0
NNoottee:: 199.1.2.0 is the Internet side of this firewall and 192.168.2.0
is the private (LAN) side.
You should start by making sure every computer on your LAN can ping
the inside address of your firewall system. (192.168.2.1 in this
example) If not, go over the NET-2 HOWTO again and work on the network
some more.
Next, from the firewall, try to ping a Internet system. I use
www.internic.net as my test point. If it doesn't work, try a server
at your ISP. If this doesn't work some part of your Internet
connection is wrong. You should be able to connect to the anywhere on
the Internet from the firewall. Try looking at your default gateway
setting. If you are using a dialup connection double check your user
ID and Password. Reread the Net-2 HOWTO, and try again.
Now try to ping the outside address of the firewall (199.1.2.10) from
a computer on your LAN. This shouldn't work. If it does, you have
masquerading or IP Forwarding turned on, or you already have some
packet filtering set. Turn them off and try again. You need to know
the filtering is in place.
For kernels newer then 2.1.102 you can issue the command;
echo "0" > /proc/sys/net/ipv4/ip_forward
If you are using an older kernel (WHY) you will need to re-compile
your kernel with forwarding turned off. (Just upgrade.)
Try pinging the outside address of the firewall (199.1.2.10) again. It
shouldn't work.
Now turn on IP forwarding and/or masquerading. You should be able to
ping the anywhere on the Internet from any system on your LAN.
echo "0" > /proc/sys/net/ipv4/ip_forward
BBIIGG NNOOTTEE:: If you are using "REAL" IP addresses on your LAN (not
192.168.2.*) and you can't ping the internet but you CAN ping the
Internet side of your firewall, make sure your ISP is routing packets
for your private network address.
A test for this problem is to have someone else on the Internet (say a
friend using a local provider) use traceroute to your network. If the
trace stops at your providers router, then they are not forwarding
your traffic.
It works? Great. The hard part is done. :-)
66..55.. SSeeccuurriinngg tthhee FFiirreewwaallll
A firewall isn't any good if the system it is build on is left wide
open to attacks. A "bad guy" could gain access to the through a non
firewall service and modify it for their own needs. You need to
turning off any unneeded services.
Look in your /etc/inetd.conf file. This file configures inetd also
known as the "super server". It controls a bunch of the server
daemons and starts them as they are requested by a packet arriving at
a "well known" port.
You should turn off echo, discard, daytime, chargen, ftp, gopher,
shell, login, exec, talk, ntalk, pop-2, pop-3, netstat, systat, tftp,
bootp, finger, cfinger, time, swat and linuxconfig if you have one.
To turn a service off, put # as the first character of the service
line. When your done, send a SIG-HUP to the process by typing ""kkiillll
--HHUUPP <<ppiidd>>"", where <pid> is the process number of inetd. This will
make inetd re-read its configuration file (inetd.conf) and restart
without taking your system down.
Test this by telneting to port 15 (netstat) on firewall. If you get
any output you have not turned these services off.
telnet localhost 19
You can also create the file /etc/nologin. Put a few line of text in
it like (BUZZ OFF). When this file exists, login will not allow user
to logon. They will see the contents of this file and their logins
refused. Only root can logon.
You can also edit the file /etc/securetty. If the user is root,
then the login must be occurring on a tty listed in
/etc/securetty. Failures will be logged with the syslog facility.
With both of these controls in place the only way to logon to the
firewall will be as root from the console.
77.. IIPP ffiilltteerriinngg sseettuupp ((IIPPFFWWAADDMM))
If you are using kernel 2.1.102 or newer skip to the next section on
IPCHAINS.
In older kernels IP Forwarding is turned on by default in the kernel.
Because of this, your network should start by denying access to
everything and flushing any ipfw rules in place from the last time it
was run. This script fragment should go in your network startup
script. (/etc/rc.d/init.d/network)
#
# setup IP packet Accounting and Forwarding
#
# Forwarding
#
# By default DENY all services
ipfwadm -F -p deny
# Flush all commands
ipfwadm -F -f
ipfwadm -I -f
ipfwadm -O -f
Now we have the ultimate firewall. Nothing can get through.
Now create the file /etc/rc.d/rc.firewall. This script should allow
email, Web and DNS traffic through. ;-)
#! /bin/sh
#
# rc.firewall
#
# Source function library.
. /etc/rc.d/init.d/functions
# Get config.
. /etc/sysconfig/network
# Check that networking is up.
if [ ${NETWORKING} = "no" ]
then
exit 0
fi
case "$1" in
start)
echo -n "Starting Firewall Services: "
# Allow email to got to the server
/sbin/ipfwadm -F -a accept -b -P tcp -S 0.0.0.0/0 1024:65535 -D 192.1.2.10 25
# Allow email connections to outside email servers
/sbin/ipfwadm -F -a accept -b -P tcp -S 192.1.2.10 25 -D 0.0.0.0/0 1024:65535
# Allow Web connections to your Web Server
/sbin/ipfwadm -F -a accept -b -P tcp -S 0.0.0.0/0 1024:65535 -D 192.1.2.11 80
# Allow Web connections to outside Web Server
/sbin/ipfwadm -F -a accept -b -P tcp -S 192.1.2.* 80 -D 0.0.0.0/0 1024:65535
# Allow DNS traffic
/sbin/ipfwadm -F -a accept -b -P udp -S 0.0.0.0/0 53 -D 192.1.2.0/24
;;
stop)
echo -n "Stooping Firewall Services: "
ipfwadm -F -p deny
;;
status)
echo -n "Now do you show firewall stats?"
;;
restart|reload)
$0 stop
$0 start
;;
*)
echo "Usage: firewall {start|stop|status|restart|reload}"
exit 1
esac
NOTE: In this example we have the email (smtp) server running at
192.1.2.10 that must be able to send and receive on port 25. The web
server running at 192.1.2.11. We are allowing anyone on the LAN to get
to outside web and DNS servers.
This is not perfectly secure. Because port 80 doesn't have to used as
a web port, a smart hacker might use this port to create a virtual
private network (VPN) through the firewall. The way around this is to
setup a web proxy. and only allow the proxy through the firewall.
Users on the LAN will have to go through the proxy to get to outside
web servers.
You might also be interested in accounting for traffic going through
your firewall. This script will count ever packet. You could add a
line or two to account for packets going to just a single system.
# Flush the current accounting rules
ipfwadm -A -f
# Accounting
/sbin/ipfwadm -A -f
/sbin/ipfwadm -A out -i -S 192.1.2.0/24 -D 0.0.0.0/0
/sbin/ipfwadm -A out -i -S 0.0.0.0/0 -D 192.1.2.0/24
/sbin/ipfwadm -A in -i -S 192.1.2.0/24 -D 0.0.0.0/0
/sbin/ipfwadm -A in -i -S 0.0.0.0/0 -D 192.1.2.0/24
If all you need is a filtering firewall you can stop here. Test it
and Enjoy.
88.. IIPP ffiilltteerriinngg sseettuupp ((IIPPCCHHAAIINNSS))
Linux ipchains is a rewrite of the Linux IPv4 firewalling code (which
was mainly stolen from BSD) and a rewrite of ipfwadm, which was a
rewrite of BSD's ipfw, I believe. It is required to administer the IP
packet filters in Linux kernel versions 2.1.102 and above.
The older code doesn't deal with fragments, has 32-bit counters (on
Intel at least), doesn't allow specification of protocols other than
TCP, UDP or ICMP, can't make large changes atomically, can't specify
inverse rules, has some quirks, and can be tough to manage (making it
prone to user error). Or so the author says.
I'm not going to get real deep into how to control an IPChains
firewall because there is a GREAT!! HOWTO on it at
http://www.rustcorp.com/linux/ipchains/HOWTO.html
<
http://www.rustcorp.com/linux/ipchains/HOWTO.html>. I'd just end up
duplicating it here. Here are the basics.
You work with chains by name. You start with three built-in chains
input, output and forward which you can't delete. You can create
chains of your own. Rules can then be added and deleted from these
rule sets.
The operations to work on entire chains are;
1. Create a new chain (-N).
2. Delete an empty chain (-X).
3. Change the policy for a built-in chain. (-P).
4. List the rules in a chain (-L).
5. Flush the rules out of a chain (-F).
6. Zero the packet and byte counters on all rules in a chain (-Z).
There are several ways to manipulate rules inside a chain:
1. Append a new rule to a chain (-A).
2. Insert a new rule at some position in a chain (-I).
3. Replace a rule at some position in a chain (-R).
4. Delete a rule at some position in a chain (-D).
5. Delete the first rule that matches in a chain (-D).
There are a few operations for masquerading, which are in ipchains for
want of a good place to put them:
1. List the currently masqueraded connections (-M -L).
2. Set masquerading timeout values (-M -S).
There are some timing issues involved in altering firewall rules. If
you are not careful, you can let packets through while you are half-
way through your changes. A simplistic approach is to do the
following:
# ipchains -I input 1 -j DENY
# ipchains -I output 1 -j DENY
# ipchains -I forward 1 -j DENY
... make changes ...
# ipchains -D input 1
# ipchains -D output 1
# ipchains -D forward 1
#
This drops all packets for the duration of the changes.
Here a duplicate of the above firewall rules in IPChains.
#!/bin/sh
#
# rc.firewall
#
## Flush everything, start from scratch
/sbin/ipchains -F input
/sbin/ipchains -F output
/sbin/ipchains -F forward
## Redirect for HTTP Transparent Proxy
#$IPCHAINS -A input -p tcp -s 192.1.2.0/24 -d 0/0 80 -j REDIRECT 8080
## Create your own chain
/sbin/ipchains -N my-chain
# Allow email to got to the server
/sbin/ipchains -A my-chain -s 0.0.0.0/0 smtp -d 192.1.2.10 1024:-j ACCEPT
# Allow email connections to outside email servers
/sbin/ipchains -A my-chain -s 192.1.2.10 -d 0.0.0.0/0 smtp -j ACCEPT
# Allow Web connections to your Web Server
/sbin/ipchains -A my-chains -s 0.0.0.0/0 www -d 192.1.2.11 1024: -j ACCEPT
# Allow Web connections to outside Web Server
/sbin/ipchains -A my-chains -s 192.1.2.0/24 1024: -d 0.0.0.0/0 www -j ACCEPT
# Allow DNS traffic
/sbin/ipchains -A my-chains -p UDP -s 0.0.0.0/0 dns -d 192.1.2.0/24 -j ACCEPT
## If you are using masquerading
# don't masq internal-internal traffic
/sbin/ipchains -A forward -s 192.1.2.0/24 -d 192.1.2.0/24 -j ACCEPT
# don't masq external interface direct
/sbin/ipchains -A forward -s 199.1.2.0/24 -d 0/0 -j ACCEPT
# masquerade all internal IP's going outside
/sbin/ipchains -A forward -s 192.1.2.0/24 -d 0/0 -j MASQ
## Deny everything else
/sbin/ipchains -P my-chains input DENY
Don't stop here. This is not a great firewall and I'm sure you have
other services you will be providing. Again, read the IPCHAINS-HOWTO.
99.. MMaakkiinngg iitt eeaassyy--eerr
There are graphical and web based interfaces being developed to work
with the Linux filtering rules. Some companies have even create
commercial firewalls based on Linux by putting it in their own box
with their own management code. (nice)
gfcc (GTK+ Firewall Control Center) is a GTK+ application which can
control Linux firewall policies and rules, based on ipchains package.
Go to
http://icarus.autostock.co.kr <
http://icarus.autostock.co.kr/>
and get your copy.
kfirewall is a GUI frontend for ipchains or ipfwadm (depending on your
kernel version).
http://megaman.ypsilonia.net/kfirewall/
<
http://megaman.ypsilonia.net/kfirewall/>
FCT is an HTML based tool for the configuration of a firewall. It
features automatic script-generation for IP-filtering commands
(ipfwadm) on a firewall for multiple interfaces and any internet
services.
http://www.fen.baynet.de/~ft114/FCT/firewall.htm
<
http://www.fen.baynet.de/~ft114/FCT/firewall.htm>
1100.. IInnssttaalllliinngg aa TTrraannssppaarreenntt SSQQUUIIDD pprrooxxyy
The squid proxy is available at hhttttpp::////ssqquuiidd..nnllaannrr..nneett//
<<
http://squid.nlanr.net/>.
The SQUID developers provide RedHat and Debian packages. If you can,
use one of these.
1111.. IInnssttaalllliinngg tthhee TTIISS PPrrooxxyy sseerrvveerr
1111..11.. GGeettttiinngg tthhee ssooffttwwaarree
The TIS FWTK is available at hhttttpp::////wwwwww..ttiiss..ccoomm//rreesseeaarrcchh//ssooffttwwaarree//
<<
http://www.tis.com/research/software/ >.
DDoonn''tt mmaakkee tthhee mmiissttaakkee II ddiidd.. When you ftp files from TIS, READ THE
README's. The TIS fwtk is locked up in a hidden directory on their
server.
TIS requires you read their agreement at
http://www.tis.com/research/software/fwtk_readme.html
<
http://www.tis.com/research/software/fwtk_readme.html > and then sseenndd
eemmaaiill ttoo ffwwttkk--rreeqquueesstt@@ttiissllaabbss..ccoomm <<mailto:
[email protected]>
with only the word aacccceepptteedd in the body of the message to learn the
name of this hidden directory. No subject is needed in the message.
Their system will then mails you back the directory name (good for 12
hours) to download the source.
As of this writing, the current version of FWTK is 2.1.
1111..22.. CCoommppiilliinngg tthhee TTIISS FFWWTTKK
Version 2.1 of the FWTK compiles much easier then any of the older
versions.
EXPLAIN HERE!!!
Now run mmaakkee.
1111..33.. IInnssttaalllliinngg tthhee TTIISS FFWWTTKK
Run mmaakkee iinnssttaallll.
The default installation directory is /usr/local/etc. You could change
this (I didn't) to a more secure directory. I chose to change the
access to this directory to 'chmod 700'.
All last is left now is to configure the firewall.
1111..44.. CCoonnffiigguurriinngg tthhee TTIISS FFWWTTKK
Now the fun really begins. We must teach the system to call theses new
services and create the tables to control them.
I'm not going to try to re-write the TIS FWTK manual here. I will show
you the setting I found worked and explain the problems I ran into and
how I got around them.
There are three files that make up these controls.
+o /etc/services
+o Tells the system what ports a services is on.
+o /etc/inetd.conf
+o Tells inetd what program to call when someone knocks on a service
port.
+o /usr/local/etc/netperm-table
+o Tells the FWTK services who to allow and deny service to.
To get the FWTK functioning, you should edit these files from the
bottom up. Editing the services file without the inetd.conf or
netperm-table file set correctly could make your system inaccessible.
1111..44..11.. TThhee nneettppeerrmm--ttaabbllee ffiillee
This file controls who can access the services of the TIS FWTK. You
should think about the traffic using the firewall from both sides.
People outside your network should identify themselves before gaining
access, but the people inside your network might be allowed to just
pass through.
So people can identify themselves, the firewall uses a program called
aauutthhssrrvv to keep a database of user IDs and passwords. The
authentication section of the netperm-table controls where the
database is keep and who can access it.
I had some trouble closing the access to this service. Note the
premit-hosts line I show uses a '*' to give everyone access. The
correct setting for this line is '' authsrv: premit-hosts localhost if
you can get it working.
#
# Proxy configuration table
#
# Authentication server and client rules
authsrv: database /usr/local/etc/fw-authdb
authsrv: permit-hosts *
authsrv: badsleep 1200
authsrv: nobogus true
# Client Applications using the Authentication server
*: authserver 127.0.0.1 114
To initialize the database, su to root, and run ..//aauutthhssrrvv in the
/var/local/etc directory to create the administrative user record.
Here is a sample session.
Read the FWTK documentation to learn how to add users and groups.
#
# authsrv
authsrv# list
authsrv# adduser admin "Auth DB admin"
ok - user added initially disabled
authsrv# ena admin
enabled
authsrv# proto admin pass
changed
authsrv# pass admin "plugh"
Password changed.
authsrv# superwiz admin
set wizard
authsrv# list
Report for users in database
user group longname ok? proto last
------ ------ ------------------ ----- ------ -----
admin Auth DB admin ena passw never
authsrv# display admin
Report for user admin (Auth DB admin)
Authentication protocol: password
Flags: WIZARD
authsrv# ^D
EOT
#
The telnet gateway (tn-gw) controls are straight forward and the first
you should set up.
In my example, I permit host from inside the private network to pass
through without authenticating themselves. (permit-hosts 19961.2.*
-passok) But, any other user must enter their user ID and password to
use the proxy. (permit-hosts * -auth)
I also allow one other system (192.1.2.202) to access the firewall
directly without going through the firewall at all. The two inetacl-
in.telnetd lines do this. I will explain how these lines are called
latter.
The Telnet timeout should be keep short.
# telnet gateway rules:
tn-gw: denial-msg /usr/local/etc/tn-deny.txt
tn-gw: welcome-msg /usr/local/etc/tn-welcome.txt
tn-gw: help-msg /usr/local/etc/tn-help.txt
tn-gw: timeout 90
tn-gw: permit-hosts 192.1.2.* -passok -xok
tn-gw: permit-hosts * -auth
# Only the Administrator can telnet directly to the Firewall via Port 24
netacl-in.telnetd: permit-hosts 192.1.2.202 -exec /usr/sbin/in.telnetd
The r-commands work the same way as telnet.
# rlogin gateway rules:
rlogin-gw: denial-msg /usr/local/etc/rlogin-deny.txt
rlogin-gw: welcome-msg /usr/local/etc/rlogin-welcome.txt
rlogin-gw: help-msg /usr/local/etc/rlogin-help.txt
rlogin-gw: timeout 90
rlogin-gw: permit-hosts 192.1.2.* -passok -xok
rlogin-gw: permit-hosts * -auth -xok
# Only the Administrator can telnet directly to the Firewall via Port
netacl-rlogind: permit-hosts 192.1.2.202 -exec /usr/libexec/rlogind -a
You shouldn't have anyone accessing your firewall directly and that
includes FTP so don't put an FTP, server on you firewall.
Again, the permit-hosts line allows anyone in the protected network
free access to the Internet and all others must authenticate
themselves. I included logging of every file sent and received to my
controls. (-log { retr stor })
The ftp timeout controls how long it will take to drop a bad
connections as well as how long a connection will stay open with out
activity.
# ftp gateway rules:
ftp-gw: denial-msg /usr/local/etc/ftp-deny.txt
ftp-gw: welcome-msg /usr/local/etc/ftp-welcome.txt
ftp-gw: help-msg /usr/local/etc/ftp-help.txt
ftp-gw: timeout 300
ftp-gw: permit-hosts 192.1.2.* -log { retr stor }
ftp-gw: permit-hosts * -authall -log { retr stor }
Web, gopher and browser based ftp are contorted by the http-gw. The
first two lines create a directory to store ftp and web documents as
they are passing through the firewall. I make these files owned by
root and put the in a directory accessible only by root.
The Web connection should be kept short. It controls how long the user
will wait on a bad connections.
# www and gopher gateway rules:
http-gw: userid root
http-gw: directory /jail
http-gw: timeout 90
http-gw: default-httpd www.afs.net
http-gw: hosts 192.1.2.* -log { read write ftp }
http-gw: deny-hosts *
The ssl-gw is really just a pass anything gateway. Be carefully with
it. In this example I allow anyone inside the protected network to
connect to any server outside the network except the addresses
127.0.0.* and 192.1.1.* and then only on ports 443 through 563. Ports
443 through 563 are known SSL ports.
# ssl gateway rules:
ssl-gw: timeout 300
ssl-gw: hosts 192.1.2.* -dest { !127.0.0.* !192.1.1.* *:443:563 }
ssl-gw: deny-hosts *
Here is an example of how to use the plug-gw to allow connections to a
news server. In this example I allow anyone inside the protected
network to connect to only one system and only to it's news port.
The seconded line allows the news server to pass its data back to the
protected network.
Because most clients expect to stay connected while the user read
news, the timeout for a news server should be long.
# NetNews Pluged gateway
plug-gw: timeout 3600
plug-gw: port nntp 192.1.2.* -plug-to 199.5.175.22 -port nntp
plug-gw: port nntp 199.5.175.22 -plug-to 192.1.2.* -port nntp
The finger gateway is simple. Anyone inside the protected network must
login first and then we allow them to use the finger program on the
firewall. Anyone else just gets a message.
# Enable finger service
netacl-fingerd: permit-hosts 192.1.2.* -exec /usr/libexec/fingerd
netacl-fingerd: permit-hosts * -exec /bin/cat /usr/local/etc/finger.txt
I haven't setup the Mail and X-windows services so I'm not including
examples. If anyone has a working example, please send me email.
1111..44..22.. TThhee //eettcc//sseerrvviicceess ffiillee
This is where it all begins. When a client connects to the firewall it
connects on a known port (less then 1024). For example telnet
connects on port 23. The inetd deamon hears this connection and looks
up the name of these service in the /etc/services file. It then calls
the program assigned to the name in the /etc/inetd.conf file.
Some of the services we are creating are not normally in the
/etc/services file. You can assign some of them to any port you want.
For example, I have assigned the administrator's telnet port (telnet-
a) to port 24. You could assign it to port 2323 if you wished. For the
administrator (YOU) to connect directly to the firewall you will need
to telnet to port 24 not 23 and if you setup your netperm-table file,
like I did, you will only be able to this from one system inside your
protected network.
telnet-a 24/tcp
ftp-gw 21/tcp # this named changed
auth 113/tcp ident # User Verification
ssl-gw 443/tcp
1122.. TThhee SSOOCCKKSS PPrrooxxyy SSeerrvveerr
1122..11.. SSeettttiinngg uupp tthhee PPrrooxxyy SSeerrvveerr
The SOCKS proxy server available from hhttttpp::////wwwwww..ssoocckkss..nneecc..ccoomm//.
Uncompressed and untar the files into a directory on your system, and
follow the instructions on how to make it. I had a couple problems
when I made it. Make sure that your Makefiles are correct.
One important thing to note is that the proxy server needs to be added
to /etc/inetd.conf. You must add a line:
socks stream tcp nowait nobody /usr/local/etc/sockd sockd
to tell the server to run when requested.
1122..22.. CCoonnffiigguurriinngg tthhee PPrrooxxyy SSeerrvveerr
The SOCKS program needs two separate configuration files. One to tell
the access allowed, and one to route the requests to the appropriate
proxy server. The access file should be housed on the server. The
routing file should be housed on every UNIX machine. The DOS and,
presumably, Macintosh computers will do their own routing.
1122..22..11.. TThhee AAcccceessss FFiillee
With socks4.2 Beta, the access file is called "sockd.conf".It should
contain 2 lines, a permit and a deny line. Each line will have three
entries:
+o The Identifier (permit/deny)
+o The IP address
+o The address modifier
The identifier is either permit or deny. You should have both a
permit and a deny line.
The IP address holds a four byte address in typical IP dot notation.
I.E. 192.168.2.0.
The address modifier is also a typical IP address four byte number. It
works like a netmask. Envision this number to be 32 bits (1s or 0s).
If the bit is a 1, the corresponding bit of the address that it is
checking must match the corresponding bit in the IP address field. For
instance, if the line is:
permit 192.168.2.23 255.255.255.255
it will permit only the IP address that matches every bit in
192.168.2.23, eg, only 192.168.2.3. The line:
permit 192.168.2.0 255.255.255.0
will permit every number within group 192.168.2.0 through
192.168.2.255, the whole C Class domain. One should not have the
line:
permit 192.168.2.0 0.0.0.0
as this will permit every address, regardless.
So, first permit every address you want to permit, and then deny the
rest. To allow everyone in the domain 192.168.2.xxx, the lines:
permit 192.168.2.0 255.255.255.0
deny 0.0.0.0 0.0.0.0
will work nicely. Notice the first "0.0.0.0" in the deny line. With
a modifier of 0.0.0.0, the IP address field does not matter. All 0's
is the norm because it is easy to type.
More than one entry of each is allowed.
Specific users can also be granted or denied access. This is done via
ident authentication. Not all systems support ident, including
Trumpet Winsock, so I will not go into it here. The documentation
with socks is quite adequate on this subject.
1122..22..22.. TThhee RRoouuttiinngg FFiillee
The routing file in SOCKS is poorly named "socks.conf". I say "poorly
named" because it is so close to the name of the access file that it
is easy to get the two confused.
The routing file is there to tell the SOCKS clients when to use socks
and when not to. For instance, in our network, 192.168.2.3 will not
need to use socks to talk with 192.168.2.1, firewall. It has a direct
connection in via Ethernet. It defines 127.0.0.1, the loopback,
automatically. Of course you do not need SOCKS to talk to yourself.
There are three entries:
+o deny
+o direct
+o sockd
Deny tells SOCKS when to reject a request. This entry has the same
three fields as in sockd.conf, identifier, address and modifier.
Generally, since this is also handled by sockd.conf, the access file,
the modifier field is set to 0.0.0.0. If you want to preclude
yourself from calling any place, you can do it here.
The direct entry tells which addresses to not use socks for. These
are all the addresses that can be reached without the proxy server.
Again we have the three fields, identifier, address and modifier. Our
example would have
direct 192.168.2.0 255.255.255.0
Thus going direct for any on our protected network.
The sockd entry tells the computer which host has the socks server
daemon on it. The syntax is:
sockd @=<serverlist> <IP address> <modifier>
Notice the @= entry. This allows you to set the IP addresses of a
list of proxy servers. In our example, we only use one proxy server.
But, you can have many to allow a greater load and for redundancy in
case of failure.
The IP address and modifier fields work just like in the other
examples. You specify which addresses go where through these. 6.2.3.
DNS from behind a Firewall
Setting up Domain Name service from behind a firewall is a relatively
simple task. You need merely to set up the DNS on the firewalling
machine. Then, set each machine behind the firewall to use this DNS.
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To have your applications work with the proxy server, they need to be
"sockified". You will need two different telnets, one for direct
communication, one for communication via the proxy server. SOCKS
comes with instructions on how to SOCKify a program, as well as a
couple pre-SOCKified programs. If you use the SOCKified version to go
somewhere direct, SOCKS will automatically switch over to the direct
version for you. Because of this, we want to rename all the programs
on our protected network and replace them with the SOCKified programs.
"Finger" becomes "finger.orig", "telnet" becomes "telnet.orig", etc.
You must tell SOCKS about each of these via the include/socks.h file.
Certain programs will handle routing and sockifying itself. Netscape
is one of these. You can use a proxy server under Netscape by
entering the server's address (192.168.2.1 in our case) in the SOCKs
field under Proxies. Each application will need at least a little
messing with, regardless of how it handles a proxy server.
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Trumpet Winsock comes with built in proxy server capabilities. In the
"setup" menu, enter the IP address of the server, and the addresses of
all the computers reachable directly. Trumpet will then handle all
outgoing packets.
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The SOCKS package works only with TCP packets, not UDP. This makes it
quite a bit less useful. Many useful programs, such as talk and
Archie, use UDP. There is a package designed to be used as a proxy
server for UDP packets called UDPrelay, by Tom Fitzgerald
<
[email protected]>. Unfortunately, at the time of this writing, it is
not compatible with Linux.
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The proxy server is, above all, a security device. Using it to
increase internet access with limited IP addresses will have many
drawbacks. A proxy server will allow greater access from inside the
protected network to the outside, but will keep the inside completely
inaccessible from the outside. This means no servers, talk or archive
connections, or direct mailing to the inside computers. These
drawbacks might seem slight, but think of it this way:
+o You have left a report you are doing on your computer inside a
firewall protected network. You are at home, and decide that you
would like to go over it. You can not. You can not reach your
computer because it is behind the firewall. You try to log into
firewall first, but since everyone has proxy server access, no one
has set up an account for you on it.
+o Your daughter goes to college. You want to email her. You have
some private things to talk about, and would rather have your mail
sent directly to your machine. You trust your systems
administrator completely, but still, this is private mail.
+o The inability to use UDP packets represents a big drawback with the
proxy servers. I imagine UDP capabilities will be coming shortly.
FTP causes another problem with a proxy server. When getting or doing
an ls, the FTP server opens a socket on the client machine and sends
the information through it. A proxy server will not allow this, so
FTP doesn't particularly work.
And, proxy servers run slow. Because of the greater overhead, almost
any other means of getting this access will be faster.
Basically, if you have the IP addresses, and you are not worried about
security, do not use a firewall and/or proxy servers. If you do not
have the IP addresses, but you are also not worried about security,
you might also want to look into using an IP emulator, like Term,
Slirp or TIA. Term is available from ffttpp::////ssuunnssiittee..uunncc..eedduu, Slirp is
available from ffttpp::////bblliittzzeenn..ccaannbbeerrrraa..eedduu..aauu//ppuubb//sslliirrpp, and TIA is
available from marketplace.com. These packages will run faster, allow
better connections, and provide a greater level of access to the
inside network from the internet. Proxy servers are good for those
networks which have a lot of hosts that will want to connect to the
internet on the fly, with one setup and little work after that.
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There is one configuration I would like to go over before wrapping
this document up. The one I have just outlined will probably suffice
for most people. However, I think the next outline will show a more
advanced configuration that can clear up some questions. If you have
questions beyond what I have just covered, or are just interested in
the versatility of proxy servers and firewalls, read on.
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Say, for instance, you are the leader of millisha and you wish to
network your site. You have 50 computers and a subnet of 32 (5 bits)
IP numbers. You need various levels of access within your network
because you tell your followers different things. Therefore, you'll
need to protect certain parts of the network from the rest.
The levels are:
1. The external level. This is the level that gets shown to
everybody. This is where you rant and rave to get new volunteers.
2. TTrroooopp This is the level of people who have gotten beyond the
external level. Here is where you teach them about the evail
government and how to make bombs.
3. MMeerrcceennaarryy Here is where the _r_e_a_l plans are keep. In this level is
stored all the information on how the 3rd world government is going
to take over the world, your plans involving Newt Gingrich,
Oklahoma City, lown care products and what really is stored in that
hangers at area 51.
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The IP numbers are arranged as:
+o 1 number is 192.168.2.255, which is the broadcast address and is
not usable.
+o 23 of the 32 IP addresses are allocated to 23 machines that will be
accessible to the internet.
+o 1 extra IP goes to a Linux box on that network
+o 1 extra goes to a different Linux box on that network.
+o 2 IP #'s go to the router
+o 4 are left over, but given domain names paul, ringo, john, and
george, just to confuse things a bit.
+o The protected networks both have the addresses 192.168.2.xxx
Then, two separate networks are built, each in different rooms. They
are routed via infrared Ethernet so that they are completely invisible
to the outside room. Luckily, infrared ethernet works just like
normal ethernet.
These networks are each connected to one of the Linux boxes with an
extra IP address.
There is a file server connecting the two protected networks. This is
because the plans for taking over the world involves some of the
higher Troops. The file server holds the address 192.168.2.17 for the
Troop network and 192.168.2.23 for the Mercenary network. It has to
have different IP addresses because it has to have different Ethernet
cards. IP Forwarding on it is turned off.
IP Forwarding on both Linux boxes is also turned off. The router will
not forward packets destined for 192.168.2.xxx unless explicitly told
to do so, so the internet will not be able to get in. The reason for
turning off IP Forwarding here is so that packets from the Troop's
network will not be able to reach the Mercenary network, and vica
versa.
The NFS server can also be set to offer different files to the
different networks. This can come in handy, and a little trickery
with symbolic links can make it so that the common files can be shared
with all. Using this setup and another ethernet card can offer this
one file server for all three networks.
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Now, since all three levels want to be able to monitor the network for
their own devious purposes, all three need to have net access. The
external network is connected directly into the internet, so we don't
have to mess with proxy servers here. The Mercenary and Troop
networks are behind firewalls, so it is necessary to set up proxy
servers here.
Both networks will be setup very similarly. They both have the same
IP addresses assigned to them. I will throw in a couple of
parameters, just to make things more interesting though.
1. No one can use the file server for internet access. This exposes
the file server to viruses and other nasty things, and it is rather
important, so its off limits.
2. We will not allow troop access to the World Wide Web. They are in
training, and this kind of information retrieval power might prove
to be damaging.
So, the sockd.conf file on the Troop's Linux box will have this line:
deny 192.168.2.17 255.255.255.255
and on the Mercenary machine:
deny 192.168.2.23 255.255.255.255
And, the Troop's Linux box will have this line
deny 0.0.0.0 0.0.0.0 eq 80
This says to deny access to all machines trying to access the port
equal (eq) to 80, the http port. This will still allow all other
services, just deny Web access.
Then, both files will have:
permit 192.168.2.0 255.255.255.0
to allow all the computers on the 192.168.2.xxx network to use this
proxy server except for those that have already been denied (ie. The
file server and Web access from the Troop network).
The Troop's sockd.conf file will look like:
deny 192.168.2.17 255.255.255.255
deny 0.0.0.0 0.0.0.0 eq 80
permit 192.168.2.0 255.255.255.0
and the Mercenary file will look like:
deny 192.168.2.23 255.255.255.255
permit 192.168.2.0 255.255.255.0
This should configure everything correctly. Each network is isolated
accordingly, with the proper amount of interaction. Everyone should
be happy.
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There are several packages that will make managing your firewall
easier.
WebMin
http://www.webmin.com/ <
http://www.webmin.com/>
[ List URLs HERE ]
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Just to spoil your day, and keep you on your toes about security, I'll
describe how easy it is to defeat a proxy firewall.
Lets say you have done everything in this document and have a very
secure server and network. You have a DMZ and no one can get into
your network and you are logging every connection made to the outside
world. You make all your users go through a proxy and the only service
you allow to go direct to the outside is DNS (port 53).
One port, that is all it takes to make a firewall worthless. Here is
how it is done.
Start by setting up a Linux box somewhere outside your LAN. A good
choice would be a box at home connected to the Internet through a
cable modem.
Ask your ISP for three IP numbers. Most cable companies will provide
up to three.
On this box you need to install the client part of a Virtual Private
Network (vpn). See:
http://sunsite.auc.dk/vpnd/
<
http://sunsite.auc.dk/vpnd/>
Now setup the server side on the VPN with another Linux box. Connect
this server to it's client through port 53. Turn on routing and
forwarding and put an unused IP number you got from your ISP on it's
LAN port.
Finally, on a workstation on the private LAN, change the default
gateway to point to the vpn servers and add the third IP number to
it's LAN port.
Now, from this workstation, you can go anywhere. The only thing the
firewall admin will see is a really long DNS lookup.
Now, take over the world!
