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I have struggled throughout the years to get a solid understanding on iptables. Any time I try and read through the man pages my eyes start to glaze over.

I have a service that I only want to allow the localhost to have access to.

What terms (or configuration, if someone is feeling generous) should I Google for to allow only localhost host to have access to a given port?

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What port is this service running on? –  Bart De Vos Mar 14 '11 at 14:44
44344, its a service written in-house –  iptablessuck Mar 14 '11 at 14:46
I would like to change my nick temporarily for 'iptablesrules' but I can't –  Art Shayderov Mar 14 '11 at 14:47
@Art, they only suck because I dont understand them :) –  iptablessuck Mar 14 '11 at 14:48
@iptablessuck actually it looks like I can. But I won't cause I'm not sure I will be able to change it back :) –  Art Shayderov Mar 14 '11 at 14:53
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2 Answers

up vote 11 down vote accepted

If by service you mean a specific port, then the following two lines should work. Change the "25" to whatever port you're trying to restrict.

iptables -A INPUT -p tcp -s localhost --dport 25 -j ACCEPT
iptables -A INPUT -p tcp --dport 25 -j DROP
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"Anything coming from localhost to port 25, accept" and the second rule says "Drop anything coming into port 25". The first line is processed first, allowing localhost, and anything else will get dropped by the second line. Yes? –  iptablessuck Mar 14 '11 at 14:49
That's correct! –  Hyppy Mar 14 '11 at 14:51
TCP should be in lowercase, there... –  pepoluan Mar 14 '11 at 16:45
@Hyppy, how would you "undo" this? –  tester Aug 17 '13 at 2:28
@tester type those commands again, but replace -A with -D –  pepoluan Aug 25 '13 at 17:14
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I'd recommend:

iptables -A INPUT -i lo -p tcp --dport $APP_PORT -j ACCEPT
iptables -A INPUT -p tcp --dport $APP_PORT -j DROP

Because, self-addressed packets do not necessarily have as its source, but they all 'enter' from the lo interface.

Now, if you really want to understand iptables the first thing you should do is to download and print good diagrams explaining the relations of the netfilter tables. Here are two great ones:

Finally, read a lot of iptables HOWTO's. The practical examples would help you get up-to-speed real quick :)

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ty! lo shows up for me after using these commands with the last command from this link cyberciti.biz/faq/howto-display-linux-iptables-loaded-rules iptables -L -v -n --line-numbers –  Gracchus Aug 25 '13 at 3:01
@Gracchus I find it much easier to use iptables-save , save the output to a file, edit it with vim or emacs, and reimport the edited file using iptables-apply –  pepoluan Aug 25 '13 at 17:16
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