I have written port-forwarding iptables rules that I plan to roll-out to several machines in an automated way using Ansible. However, the POSTROUTING chain rule requires a modification of the source address to be that of the local machine so the machine receiving the forwarded packets knows where to respond. The rule looks like:

iptables -t nat -A POSTROUTING -d -o eth1 -p tcp --dport 22 -j SNAT --to-source

That is, after using DNAT in PREROUTING to change the destination address to (and ACCEPTing it), I must then use SNAT to change the source address in the network header before sending the packet into the network.

Can this rule be written in such a way to specify symbolically that --to-source should be "the local machine's IP"? I accomplished this by compiling the necessary commands into a series of piped commands:

iptables -t nat -A POSTROUTING -d -o eth1 -p tcp --dport 22 -j SNAT --to-source $(ifconfig eth1 | grep -i "inet " | awk '{print $2}')

However, my intent is to generate an iptables configuration file and just roll that out to each machine if possible.


iptables config scripts are just shell scripts. You can add the local ip address to a variable in the 'normal' way:

LOCALIP=`hostname -i`

and then replace any hard coded ip address instances with

  • Isn't $(hostname -i) usually or some other localhost address? – AlexP Jan 4 '17 at 13:56
  • not unless you've specifically added the FQDN to the hosts file – slowko Jan 4 '17 at 13:58
  • hostname -i on my Arch Linux machine returns My /etc/hosts file has two entries; both are localhost.localdomain. – sherrellbc Jan 4 '17 at 14:04
  • You might need to use another method to discern the local ip (parsing the output of ifconfig for example) but the basic point remains. I did test the above on Ubuntu and Centos. – slowko Jan 4 '17 at 14:08
  • 1
    @sherrellbc Since you are using ansible, you should be able to use something like {{ansible_default_ipv4.address}} – user4556274 Jan 4 '17 at 14:38

You could use the MASQUERADE target instead of SNAT.

You might want to read Difference between SNAT and Masquerade however, as MASQUERADE has an overhead.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.