iptables evilness

Matt came to me with an interesting problem at Bulletproof this week. We have a dedicated hosting customer who talks to an external application for e-commerce. The IP for this was going to change but they needed to do to some testing before the switch. As usual with most enterprise applications, the hostname was hard coded. 🙁

Matt suggested we do some DNS poisoning or do some transparent proxying using squid or similar. While these would have worked they required firewall changes through three levels of firewalls and extra infrastructure.

So I turned to an evil solution, iptables. 🙂 Most people use DNAT on the inbound connection from the Internet to their internal private network to port forward to internal servers, or perform one-to-one NAT mappings. There is nothing stopping you using it the other way around.

Lets say that every time someone browses to http://bulletproof.net we want them to hit http://inodes.org instead. All you need to do is use DNAT to translate one IP address into the other.
animal:~ johnf$ host bulletproof.net
bulletproof.net has address
animal:~ johnf$ host inodes.org
inodes.org has address
animal:~ johnf$ sudo iptables -t nat -A PREROUTING -d -j DNAT –to


Now for some testing, a ping looks normal


animal:~johnf$ ping www.bulletproof.net
PING www.bulletproof.net.au ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=241 time=198 ms


but a tcpdump looks like


animal:~johnf$ sudo tcpdump -ni eth0
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes
16:35:25.099510 IP > icmp 64: echo request seq 1
16:35:25.301712 IP > icmp 64: echo reply seq 1


Of course if anyone needs to try and debug this they are going to have a really fun time working out what is going on. 🙂

If you want to test this yourself you can do it on your own machine using the OUTPUT chain instead of PREROUTING.

2 thoughts on “iptables evilness

  1. John,
    You sound like somebody who understands about iptables,
    so I thought you could advice on the following.
    I am running iptables firewall for my home network and I know how to do all the normal things: forwarding external http requests to my internal webserver, etc. Yet I can’t manage to do the following non-standard trick, which I planned as a joke. Here is what I need to happen: when my friend tries to use my internal host to reach his favorite website, I forward the IP to my own internal webserver and serve him my fake version of the page. Sounds like an easy thing for iptables – use PREROUTING chain to forward the IP of the targeted website to the IP of my dedicated webserver on the inside. So far this did not work for me: internal hosts do loose connection with the target website, but my internal webserver is not getting any requests and browser request times out. Here is what I have added to my rc.firewall script:

    $IPTABLES -t nat -A PREROUTING -i eth1 -p tcp -d –dport 80 -j DNAT –to-destination
    $IPTABLES -A FORWARD -i eth1 -p tcp -d –dport 80 -j ACCEPT

    where eth1 connects to my internal LAN and is the web address that I need forwarded to my fake server. What am I missing?

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