How to create an Iptables rule using domain name

If it is all possible please consider setting up a transparent application-level proxy. An application proxy will probably be much easier to do this type of filtering with.

If you must do it with iptables a kludgy option would be to create a chain, create a command line script that periodically updates that particular chain with the results from DNS for the name you need to use.

If someone was going to attempt to create such a thing that truly did packet-filtering based DNS it would almost certainly have to be done via userspace. Specifically you would use something like libnetfilter_queue. I have never used it, but Packetbl may be close to do this, but it doesn't appear to be very well maintained.

Another alternative might be to setup Layer 7 filtering. You might be able to filter a portion of the request if DNS name is being transmuted as part of the payload of the packet.

I needed iptables to allow ssh access based on domain name from my home ip but wanted to keep it closed for all other addresses. Since I have a dynamic ip that changes occasionally, I wrote a script to update the rules based on the ip of my dynamic dns entry. I'm new at this so I'm sure there is a better way. Replace "yourname" with your dynamic dns hostname.

#!/bin/sh

/usr/bin/nslookup yourname.dynalias.org > temp
found=0
address=""
while read LINE
do
 if [[ "$LINE" == Address* ]]; then
        let found++
        if [[ $found == 2 ]]; then
                address=${LINE:8};
                /sbin/iptables-save > /root/rulesdump
                while read LINE2
                do
                        if [[ "$LINE2" == *$address* ]]; then
                                ruleexists=1;
                        fi
                done < /root/rulesdump;
                if [[ "$ruleexists" != 1 ]]; then
                        /sbin/iptables -D INPUT -j LOG_DROP
                        /sbin/iptables -A INPUT -s $address -p tcp -m tcp --dport 22 -j LOG_ACCEPT
                        /sbin/iptables -A INPUT -j LOG_DROP
                fi
        fi
 fi
done < /root/temp;

Put the above script in crontab to run every so often.

Short answer, if you are sure the IP is static forever:

iptables -A INPUT -s `dig host.your-domain-name.com +short`/32 -p tcp -m tcp --dport 22 -j ACCEPT

Same thing in ruby and shorter:

#!/usr/bin/env ruby
require 'resolv'
ip = Resolv.getaddress "mydomain.com"
`iptables -A INPUT -p tcp --dport 22 -s #{ip} -j ACCEPT` if `iptables --list` !~ /#{ip}/