Setting up an iptables firewall

This document consists of two parts. The first is setting up a firewall (using iptables) on the HN, which will restrict traffic to the VEs. The effect would emulate, as far as the VEs and their customers are concerned, an external hardware firewall controlled by the sysadmin. The second is setting up a firewall that protects the HN itself but still allows traffic to the VEs, thus allowing individual VEs to define their own iptables.

While the firewalls shown here can be accomplished using iptables manually (or using Fedora core's iptables service), the methods presented here are especially modular and easy to modify. This is important when you have 20+ VEs and a lot of other things to be doing...

The scripts and pathnames given here are for Fedora Core 6, though they can probably be applied to most similar SysV-like systems with little modification.

A little background

On our systems, we use the HN to provide privileged services which are not appropriate for access by the VEs. For example, the HN acts as a backup server, runs Nagios for health monitoring, has a webserver for managing the 3ware RAID controller, etc. The VEs are leased to customers, who can't entirely be trusted, especially if they get hacked. As such, our scenario is one in which the HN must be protected from all access (even from the VEs) except for a few trusted hosts (e.g. my home-office).

The exception to this is the nameserver, which we want open to the world. We use it as a caching nameserver for our VEs and also to host DNS for a few customer domain.

Setting up a HN-based firewall

This setup emulates (to the VEs anyway) an external hardware firewall. It protects the HN from any access and then defines what services and ports are allowed/banned for individual VEs. This leaves the firewall controlled by the site administrator, not be individual VEs and the hackers who've gotten into them. ;)

First off, let's disable Fedora's existing iptables service:

service iptables stop
chkconfig iptables off

Now create the new firewall service. This code should be /etc/init.d/firewall and then should be chmod'd 755.

#!/bin/sh
# firewall      Start iptables firewall
# chkconfig: 2345 08 92
# description:  Starts, stops and saves iptables firewall
# This script sets up the firewall for the INPUT chain (which is for the HN itself)
# and then processes the config files under /etc/firewall.d to set up additional rules
# in the FORWARD chain to allow access to VEs' services.

. /etc/init.d/functions

# the IP block allocated to this server
SEGMENT="192.168.0.0/24"
# the IP used by the hosting server itself
THISHOST="192.168.0.1"
# services that should be allowed to the HN; services for VEs are configured in /etc/firewall.d/*
OKPORTS="53"
# hosts allowed full access through the firewall, to all VEs and to this server
DMZS="12.34.56.78 90.123.45.67"

purge() {
  echo -n "Firewall: Purging and allowing all traffic"
  iptables -P OUTPUT ACCEPT
  iptables -P FORWARD ACCEPT
  iptables -P INPUT ACCEPT
  iptables -F
  success ; echo
}

setup() {
  echo -n "Firewall: Setting default policies to DROP"
  iptables -P INPUT DROP
  iptables -P FORWARD DROP
  iptables -I INPUT   -j ACCEPT -m state --state ESTABLISHED,RELATED
  iptables -I FORWARD -j ACCEPT -m state --state ESTABLISHED,RELATED
  iptables -I INPUT -j ACCEPT -i lo
  iptables -I FORWARD -j ACCEPT --source $SEGMENT
  success ; echo

  echo "Firewall: Allowing access to HN"
  for port in $OKPORTS ; do
    echo -n "          port $port"
    iptables -I INPUT -j ACCEPT -s $SEGMENT -d $THISHOST --protocol tcp --destination-port $port
    iptables -I INPUT -j ACCEPT -s $SEGMENT -d $THISHOST --protocol udp --destination-port $port
    success ; echo
  done
  for ip in $DMZS ; do
    echo -n "          DMZ $ip"
    iptables -I INPUT   -i eth0 -j ACCEPT -s $ip
    iptables -I FORWARD -i eth0 -j ACCEPT -s $ip
    success ; echo
  done

  VESETUPS=`echo /etc/firewall.d/*`
  if [ "$VESETUPS" != "/etc/firewall.d/*" ] ; then
  echo "Firewall: Setting up VE firewalls"
  for i in $VESETUPS ; do
    . $i
    echo -n "          $VENAME VE$VEID"
    if [ -n "$BANNED" ]; then
      for source in $BANNED ;  do iptables -I FORWARD -j DROP --destination $VEIP --source $source ; done
    fi
    if [ -n "$OPENPORTS" ]; then
      for port in $OPENPORTS ; do iptables -I FORWARD -j ACCEPT --protocol tcp --destination $VEIP --destination-port $port ; done
      for port in $OPENPORTS ; do iptables -I FORWARD -j ACCEPT --protocol udp --destination $VEIP --destination-port $port ; done
    fi
    if [ -n "$DMZS" ]; then
      for source in $DMZS ; do iptables -I FORWARD -j ACCEPT --protocol tcp --destination $VEIP --source $source ; done
      for source in $DMZS ; do iptables -I FORWARD -j ACCEPT --protocol udp --destination $VEIP --source $source ; done
    fi
    [ $? -eq 0 ] && success || failure
    echo
  done
  fi
}

case "$1" in
  start)
    echo "Starting firewall..."
    purge
    setup
    ;;
  stop)
    echo "Stopping firewall..."
    purge
    ;;
  restart)
    $0 stop
    $0 start
    ;;
  status)
    iptables -n -L
    ;;
  *)
    echo "Usage: $0 <start|stop|restart|status>"
    ;;
esac

The above script can be called like this:

service firewall start
service firewall stop
service firewall restart
service firewall status

It will set up the firewall for the HN according to the parameters you specified for OKPORTS, DMZs, etc. and then it will call each file under /etc/firewall.d and process its configuration.

So create a file under /etc/firewall.d The exact filename isn't important, as long as it's meaningful to you, e.g. ExampleCompany or ve12 and give it content like this:

# This file is processed by /etc/init.d/firewall
VEID="1"                      # the VE's ID#
VENAME="Customer1"            # A human-friendly label for the VE
VEIP="192.168.1.34"           # the IP address for this VE
OPENPORTS="80 443"            # ports that should be universally opened to the entire Internet
DMZS="1.2.3.0/24 5.6.7.8/32"  # IPs and blocks that should have full access to the VE's services
BANNED=""                     # IPs and blocks that should be entirely blocked from the VE's services

And there you go. Go ahead and start the firewall and check its status:

service firewall restart
service firewall status

As you can see, you can now add and edit the configurations for individual VEs very easily. This method proves a lot easier to manage than Fedora's iptables-config mechamism!

Setting up a firewall that allows per-VE configuration

This setup configures iptables on the HN to disallow access to all hosts, including the VEs. However, it allows all traffic into the VEs so they may define their own iptables rules and therefore manage their own firewall.

This content is missing. You are invited to fill it in, if you get to it before I do. :)

See also

• Traffic accounting with iptables
• More info on iptables