Restricting output to only allow localhost using iptables

Posted by Dave Forgac on Server Fault See other posts from Server Fault or by Dave Forgac
Published on 2012-03-22T16:29:26Z Indexed on 2012/03/22 17:33 UTC
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firewall

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iptables

I would like to restrict outbound traffic to only localhost using iptables. I already have a default DROP policy on OUTPUT and a rule REJECTing all traffic. I need to add a rule above that in the OUTPUT chain.

I have seen a couple different examples for this type of rule, the most common being:

-A OUTPUT -o lo -j ACCEPT

and

-A OUTPUT -o lo -s 127.0.0.1 -d 127.0.0.1 -j ACCEPT

Is there any reason to use the latter rather than the former? Can packets on lo have an address other than 127.0.0.1?

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