Quick notes on the Linux iptables 'ipset' extension

October 29, 2014

For a long time Linux's iptables firewall had an annoying lack in that it had no way to do efficient matching against a set of IP addresses. If you had a lot of IP addresses to match things against (for example if you were firewalling hundreds or thousands of IP addresses and IP address ranges off from your SMTP port), you needed one iptables rule for each entry and then they were all checked sequentially. This didn't make your life happy, to put it one way. In modern Linuxes, ipsets are finally the answer to this; they give you support for efficient sets of various things, including random CIDR netblocks.

(This entry suggests that ipsets only appeared in mainline Linux kernels as of 2.6.39. Ubuntu 12.04, 14.04, Fedora 20, and RHEL/CentOS 7 all have them while RHEL 5 appears to be too old.)

To work with ipsets, the first thing you need is the user level tool for creating and manipulating them. For no particularly sensible reason your Linux distribution probably doesn't install this when you install the standard iptables stuff; instead you'll need to install an additional package, usually called ipset. Iptables itself contains the code to use ipsets, but without ipset to create the sets you can't actually install any rules that use them.

(I wish I was kidding about this but I'm not.)

The basic use of ipsets is to make a set, populate it, and match against it. Let's take an example:

ipset create smtpblocks hash:net counters
ipset add smtpblocks
ipset add smtpblocks
iptables -A INPUT -p tcp --dport 25 -m set --match-set smtpblocks src -j DROP

(Both entries are currently on the Spamhaus EDROP list.)

Note that the set must exist before you can add iptables rules that refer to it. The ipset manpage has a long discussion of the various types of sets that you can use and the iptables-extensions manpage has a discussion of --match-set and the SET target for adding entries to sets from iptables rules. The hash:net I'm using here holds random CIDR netblocks (including /32s, ie single hosts) and is set to have counters.

It would be nice if there was a simple command to get just a listing of the members of an ipset. Unfortunately there isn't, as plain 'ipset list' insists on outputting a few lines of summary information before it lists the members. Since I don't know if these are constant I'm using 'ipset list -t save | grep "^add "', which seems ugly but seems likely to keep working forever.

Unfortunately I don't think there's an officially supported and documented ipset command for adding multiple entries into a set at once in a single command invocation; instead you're apparently expected to run 'ipset add ...' repeatedly. You can abuse the 'ipset restore' command for this if you want to by creating appropriately formatted input; check the output of 'ipset save' to see what it needs to look like. This may even be considered a stable interface by the ipset authors.

Ipset syntax and usage appears to have changed over time, so old discussions of it that you find online may not work quite as written (and someday these notes may be out of date that way as well).

PS: I can sort of see a lot of clever uses for ipsets, but I've only started exploring them right now and my iptables usage is fairly basic in general. I encourage you to read the ipset manpage and go wild.

Sidebar: how I think you're supposed to use list sets

As an illustrated example:

ipset create spamhaus-drop hash:net counters
ipset create spamhaus-edrop hash:net counters
[... populate both from spamhaus ...]

ipset create spamhaus list:set
ipset add spamhaus spamhaus-drop
ipset add spamhaus spamhaus-edrop

iptables -A INPUT -p tcp --dport 25 -m set --match-set spamhaus src -j DROP

This way your iptables rules can be indifferent about exactly what goes into the 'spamhaus' ipset, although of course this will be slightly less efficient than checking a single merged set.

Written on 29 October 2014.
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Last modified: Wed Oct 29 23:30:13 2014
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