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7. A Serious Example.

This example was extracted from Michael Neuling and my March 1999 LinuxWorld Tutorial; this is not the only way to solve the given problem, but it is probably the simplest. I hope you will find it informative.

7.1 The Arrangement

   External Network (BAD)
    ||             Server Network (DMZ)
    |             |eth0
    |             |----------------------------------------------
    |             | |             |              |
    |             |               |             |              |
    ||               |             |              |
    ---------------          --------       -------        -------
           | eth1            | SMTP |       | DNS |        | WWW |
           |                 --------       -------        -------
   Internal Network (GOOD)

7.2 Goals

Packet Filter box:

PING any network

This is really useful to tell if a machine is down.

TRACEROUTE any network

Once again, useful for diagnosis.

Access DNS

To make ping and DNS more useful.

Within the DMZ:

Mail server

Name Server

Web server


Allow WWW, ftp, traceroute, ssh to external

These are fairly standard things to allow: some places start by allowing the internal machines to do just about everything, but here we're being restrictive.

Allow SMTP to Mail server

Obviously, we want them to be able to send mail out.

Allow POP-3 to Mail server

This is how they read their mail.

Allow DNS to Name server

They need to be able to look up external names for WWW, ftp, traceroute and ssh.

Allow rsync to Web server

This is how they synchronize the external web server with the internal one.

Allow WWW to Web server

Obviously, they should be able to connect to our external web server.

Allow ping to packet filter box

This is a courteous thing to allow: it means that they can test if the firewall box is down (so we don't get blamed if an external site is broken).

7.3 Before Packet Filtering

7.4 Packet Filtering for Through Packets

With masquerading, it's best to filter in the forward chain.

Split forward chain into various user chains depending on source/dest interfaces; this breaks the problem down into managable chunks.

ipchains -N good-dmz
ipchains -N bad-dmz
ipchains -N good-bad
ipchains -N dmz-good
ipchains -N dmz-bad
ipchains -N bad-good

ACCEPTing standard error ICMPs is a common thing to do, so we create a chain for it.

ipchains -N icmp-acc

Set Up Jumps From forward Chain

Unfortunately, we only know (in the forward chain) the outgoing interface. Thus, to figure out what interface the packet came in on, we use the source address (the anti-spoofing prevents address faking).

Note that we log anything which doesn't match any of these (obviously, this should never happen).

ipchains -A forward -s -i eth0 -j good-dmz
ipchains -A forward -s -i ppp0 -j good-bad
ipchains -A forward -s -i ppp0 -j dmz-bad
ipchains -A forward -s -i eth1 -j dmz-good
ipchains -A forward -i eth0 -j bad-dmz
ipchains -A forward -i eth1 -j bad-good
ipchains -A forward -j DENY -l

Define the icmp-acc Chain

Packets which are one of the error ICMPs get ACCEPTed, otherwise, control will pass back to the calling chain.

ipchains -A icmp-acc -p icmp --icmp-type destination-unreachable -j ACCEPT
ipchains -A icmp-acc -p icmp --icmp-type source-quench -j ACCEPT
ipchains -A icmp-acc -p icmp --icmp-type time-exceeded -j ACCEPT
ipchains -A icmp-acc -p icmp --icmp-type parameter-problem -j ACCEPT

Good (Internal) to DMZ (Servers)

Internal restrictions:

Could do masquerading from internal network into DMZ, but here we don't. Since noone in the internal network should be trying to do evil things, we log any packets that get denied.

Note that old versions of Debian called `pop3' `pop-3' in /etc/services, which disagrees with RFC1700.

ipchains -A good-dmz -p tcp -d smtp -j ACCEPT
ipchains -A good-dmz -p tcp -d pop3 -j ACCEPT
ipchains -A good-dmz -p udp -d domain -j ACCEPT
ipchains -A good-dmz -p tcp -d domain -j ACCEPT
ipchains -A good-dmz -p tcp -d www -j ACCEPT
ipchains -A good-dmz -p tcp -d rsync -j ACCEPT
ipchains -A good-dmz -p icmp -j icmp-acc
ipchains -A good-dmz -j DENY -l

Bad (external) to DMZ (servers).

Good (internal) to Bad (external).

DMZ to Good (internal).

DMZ to bad (external).

Bad (external) to Good (internal).

Packet Filtering for the Linux Box Itself

Bad (external) interface.

DMZ interface.

Good (internal) interface.

7.5 Finally

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