Quick notes on the Linux iptables 'ipset' extension
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 126.96.36.199/19 ipset add smtpblocks 188.8.131.52/24 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
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 '
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
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 '
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 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.