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

• Masqueraded internal network (various operating systems), which we call "GOOD".
• Exposed servers in a separate network (called "DMZ" for Demilitarized Zone).
• PPP Connection to the Internet (called "BAD").

   External Network (BAD)
           |
           |
       ppp0|
    ---------------
    | 192.84.219.1|             Server Network (DMZ)
    |             |eth0
    |             |----------------------------------------------
    |             |192.84.219.250 |             |              |
    |             |               |             |              |
    |192.168.1.250|               |             |              |
    ---------------          --------       -------        -------
           | eth1            | SMTP |       | DNS |        | WWW |
           |                 --------       -------        -------
           |              192.84.219.128  192.84.219.129  192.84.218.130
           |
   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

• SMTP to external
• Accept SMTP from internal and external
• Accept POP-3 from internal

Name Server

• Send DNS to external
• Accept DNS from internal, external and packet filter box

Web server

• Accept HTTP from internal and external
• Rsync access from internal

Internal:

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

• Anti-spoofing

  Since we don't have any asymmetric routing, we can simply turn on anti-spoofing for all interfaces.

  # for f in /proc/sys/net/ipv4/conf/*/rp_filter; do echo 1 > $f; done
  # 
  

• Set filtering rules to DENY all:

  We still allow local loopback traffic, but deny anything else.

  # ipchains -A input -i ! lo -j DENY
  # ipchains -A output -i ! lo -j DENY
  # ipchains -A forward -j DENY
  # 
  

• Set Up Interfaces

  This is usually done in the boot scripts. Make sure the above steps are done before the interfaces are configured, to prevent packet leakage before the rules are set up.

• Insert per-protocol masquerading modules.

  We need to insert the masquerading module for FTP, so that active and passive FTP `just work' from the internal network.

  # insmod ip_masq_ftp
  # 
  

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 192.168.1.0/24 -i eth0 -j good-dmz
ipchains -A forward -s 192.168.1.0/24 -i ppp0 -j good-bad
ipchains -A forward -s 192.84.219.0/24 -i ppp0 -j dmz-bad
ipchains -A forward -s 192.84.219.0/24 -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:

• Allow WWW, ftp, traceroute, ssh to external
• Allow SMTP to Mail server
• Allow POP-3 to Mail server
• Allow DNS to Name server
• Allow rsync to Web server
• Allow WWW to Web server
• Allow ping to packet filter box

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 192.84.219.128 smtp -j ACCEPT
ipchains -A good-dmz -p tcp -d 192.84.219.128 pop3 -j ACCEPT
ipchains -A good-dmz -p udp -d 192.84.219.129 domain -j ACCEPT
ipchains -A good-dmz -p tcp -d 192.84.219.129 domain -j ACCEPT
ipchains -A good-dmz -p tcp -d 192.84.218.130 www -j ACCEPT
ipchains -A good-dmz -p tcp -d 192.84.218.130 rsync -j ACCEPT
ipchains -A good-dmz -p icmp -j icmp-acc
ipchains -A good-dmz -j DENY -l

Bad (external) to DMZ (servers).

• DMZ restrictions:
  • Mail server
    • SMTP to external
    • Accept SMTP from internal and external
    • Accept POP-3 from internal
  • Name server
    • Send DNS to external
    • Accept DNS from internal, external and packet filter box
  • Web server
    • Accept HTTP from internal and external
    • Rsync access from internal
• Things we allow from external network to DMZ.
  • Don't log violations, as they may happen.

  ipchains -A bad-dmz -p tcp -d 192.84.219.128 smtp -j ACCEPT
  ipchains -A bad-dmz -p udp -d 192.84.219.129 domain -j ACCEPT
  ipchains -A bad-dmz -p tcp -d 192.84.219.129 domain -j ACCEPT
  ipchains -A bad-dmz -p tcp -d 192.84.218.130 www -j ACCEPT
  ipchains -A bad-dmz -p icmp -j icmp-acc
  ipchains -A bad-dmz -j DENY
  

Good (internal) to Bad (external).

• Internal restrictions:
  • Allow WWW, ftp, traceroute, ssh to external
  • Allow SMTP to Mail server
  • Allow POP-3 to Mail server
  • Allow DNS to Name server
  • Allow rsync to Web server
  • Allow WWW to Web server
  • Allow ping to packet filter box
• Many people allow everything from the internal to external networks, then add restrictions. We're being fascist.
  • Log violations.
  • Passive FTP handled by masq. module.
  • UDP destination ports 33434 and up are used by traceroute.

  ipchains -A good-bad -p tcp --dport www -j MASQ
  ipchains -A good-bad -p tcp --dport ssh -j MASQ
  ipchains -A good-bad -p udp --dport 33434:33500 -j MASQ
  ipchains -A good-bad -p tcp --dport ftp -j MASQ
  ipchains -A good-bad -p icmp --icmp-type ping -j MASQ
  ipchains -A good-bad -j REJECT -l
  

DMZ to Good (internal).

• Internal restrictions:
  • Allow WWW, ftp, traceroute, ssh to external
  • Allow SMTP to Mail server
  • Allow POP-3 to Mail server
  • Allow DNS to Name server
  • Allow rsync to Web server
  • Allow WWW to Web server
  • Allow ping to packet filter box
• If we were masquerading from the internal network to the DMZ, simply refuse any packets coming the other way. As it is, only allow packets which might be part of an established connection.

  ipchains -A dmz-good -p tcp ! -y -s 192.84.219.128 smtp -j ACCEPT
  ipchains -A dmz-good -p udp -s 192.84.219.129 domain -j ACCEPT
  ipchains -A dmz-good -p tcp ! -y -s 192.84.219.129 domain -j ACCEPT
  ipchains -A dmz-good -p tcp ! -y -s 192.84.218.130 www -j ACCEPT
  ipchains -A dmz-good -p tcp ! -y -s 192.84.218.130 rsync -j ACCEPT
  ipchains -A dmz-good -p icmp -j icmp-acc
  ipchains -A dmz-good -j DENY -l
  

DMZ to bad (external).

• DMZ restrictions:
  • Mail server
    • SMTP to external
    • Accept SMTP from internal and external
    • Accept POP-3 from internal
  • Name server
    • Send DNS to external
    • Accept DNS from internal, external and packet filter box
  • Web server
    • Accept HTTP from internal and external
    • Rsync access from internal

• ipchains -A dmz-bad -p tcp -s 192.84.219.128 smtp -j ACCEPT
  ipchains -A dmz-bad -p udp -s 192.84.219.129 domain -j ACCEPT
  ipchains -A dmz-bad -p tcp -s 192.84.219.129 domain -j ACCEPT
  ipchains -A dmz-bad -p tcp ! -y -s 192.84.218.130 www -j ACCEPT
  ipchains -A dmz-bad -p icmp -j icmp-acc
  ipchains -A dmz-bad -j DENY -l
  

Bad (external) to Good (internal).

• We don't allow anything (non-masqueraded) from the external network to the internal network

  ipchains -A bad-good -j REJECT
  

Packet Filtering for the Linux Box Itself

• If we want to use packet filtering on packets coming into the box itself, we need to do filtering in the input chain. We create one chain for each destination interface:

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

• Create jumps to them:

  ipchains -A input -d 192.84.219.1 -j bad-if
  ipchains -A input -d 192.84.219.250 -j dmz-if
  ipchains -A input -d 192.168.1.250 -j good-if
  

Bad (external) interface.

• Packet Filter box:
  • PING any network
  • TRACEROUTE any network
  • Access DNS
• External interface also receives replies to masqueraded packets (masquerading uses source ports 61000 to 65095) and ICMP errors for them and PING replies.

  ipchains -A bad-if -i ! ppp0 -j DENY -l
  ipchains -A bad-if -p TCP --dport 61000:65095 -j ACCEPT
  ipchains -A bad-if -p UDP --dport 61000:65095 -j ACCEPT
  ipchains -A bad-if -p ICMP --icmp-type pong -j ACCEPT
  ipchains -A bad-if -j icmp-acc
  ipchains -A bad-if -j DENY
  

DMZ interface.

• Packet Filter box restrictions:
  • PING any network
  • TRACEROUTE any network
  • Access DNS
• DMZ interface receives DNS replies, ping replies and ICMP errors.

  ipchains -A dmz-if -i ! eth0 -j DENY
  ipchains -A dmz-if -p TCP ! -y -s 192.84.219.129 53 -j ACCEPT
  ipchains -A dmz-if -p UDP -s 192.84.219.129 53 -j ACCEPT
  ipchains -A dmz-if -p ICMP --icmp-type pong -j ACCEPT
  ipchains -A dmz-if -j icmp-acc
  ipchains -A dmz-if -j DENY -l
  

Good (internal) interface.

• Packet Filter box restrictions:
  • PING any network
  • TRACEROUTE any network
  • Access DNS
• Internal restrictions:
  • Allow WWW, ftp, traceroute, ssh to external
  • Allow SMTP to Mail server
  • Allow POP-3 to Mail server
  • Allow DNS to Name server
  • Allow rsync to Web server
  • Allow WWW to Web server
  • Allow ping to packet filter box
• Internal interface receives pings, ping replies and ICMP errors.

  ipchains -A good-if -i ! eth1 -j DENY
  ipchains -A good-if -p ICMP --icmp-type ping -j ACCEPT
  ipchains -A good-if -p ICMP --icmp-type pong -j ACCEPT
  ipchains -A good-if -j icmp-acc
  ipchains -A good-if -j DENY -l
  

7.5 Finally

• Delete blocking rules:

  ipchains -D input 1
  ipchains -D forward 1
  ipchains -D output 1
  

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