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 containers. The effect would emulate, as far as the containers 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 containers, thus allowing individual containers 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+ containers 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.
Contents
A little background
On our systems, we use the HN to provide privileged services which are not appropriate for access by the containers. 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 containers 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 containers) 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 containers and also to host DNS for a few customer domain.
Setting up a HN-based firewall
This setup emulates (to the containers anyway) an external hardware firewall. It protects the HN from any access and then defines what services and ports are allowed/banned for individual containers. This leaves the firewall controlled by the site administrator, not be individual containers 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 containers' 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 containers are configured in /etc/firewall.d/*
OKPORTS="53"
# hosts allowed full access through the firewall,
# to all containers 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 $ip
success ; echo
done
CTSETUPS=`echo /etc/firewall.d/*`
if [ "$CTSETUPS" != "/etc/firewall.d/*" ] ; then
echo "Firewall: Setting up container firewalls"
for i in $CTSETUPS ; do
. $i
echo -n " $CTNAME CT$CTID"
if [ -n "$BANNED" ]; then
for source in $BANNED ; do iptables -I FORWARD -j DROP --destination $CTIP --source $source ; done
fi
if [ -n "$OPENPORTS" ]; then
for port in $OPENPORTS ; do iptables -I FORWARD -j ACCEPT --protocol tcp --destination $CTIP --destination-port $port ; done
for port in $OPENPORTS ; do iptables -I FORWARD -j ACCEPT --protocol udp --destination $CTIP --destination-port $port ; done
fi
if [ -n "$DMZS" ]; then
for source in $DMZS ; do iptables -I FORWARD -j ACCEPT --protocol tcp --destination $CTIP --source $source ; done
for source in $DMZS ; do iptables -I FORWARD -j ACCEPT --protocol udp --destination $CTIP --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 CTID="1" # the container's ID# CTNAME="Customer1" # A human-friendly label for the container CTIP="192.168.1.34" # the IP address for this container 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 container's services BANNED="" # IPs and blocks that should be entirely # blocked from the container'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 containers very easily. This method proves a lot easier to manage than Fedora's iptables-config mechamism!
Setting up a firewall that allows per-container configuration
This setup configures iptables on the HN to disallow access to all hosts, including the containers. However, it allows all traffic into the containers 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. :)
If you want to use a firewall inside a container, please load these modules BEFORE starting the container:
modprobe xt_tcpudp modprobe ip_conntrack
If you do not, you will get an error like this: "iptables: No chain/target/match by that name"