[RESOLVED] String Match Issue with iptables

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PatP
Posts: 4
Joined: 2013/03/11 16:21:44

[RESOLVED] String Match Issue with iptables

Post by PatP » 2016/08/11 17:41:38

Hi all,

I am fairly familiar with Linux and iptables, but I am having an issue
with the "--m -string pattern" matching functionality. I am trying to
use iptables with this extension/option to log DNS requests containing
a specified URL string, but iptables does not seem to match if the search
string contains a '.' (i.e. a period).

As an example of this issue, I first set up a rule to log the traversal
of DNS request packets leaving a single ported computer, that contain
a matching string of "google". The iptables command is:

Code: Select all

iptables -t filter -I OUTPUT 1 -o eth0 -p udp -m udp --dport 53 \
         -m string --algo bm --icase --string "google"          \
         -j LOG --log-level info --log-prefix "iptables-string-match: "
After running this command, I use iptables to check what is configured,
and the following is the (expected) results for the output chain:

Code: Select all

Chain OUTPUT (policy ACCEPT ...)
           target     prot opt in     out     source               destination
           LOG        udp  --  *      eth0    0.0.0.0/0            0.0.0.0/0           udp dpt:53 STRING match "google" ALGO name bm TO 65535 ICASE LOG flags 0 level 6 prefix `iptables-string-match: '
I then perform a "dig google.com" command (which works), followed by a
dmesg command, which prints the following:

Code: Select all

iptables-string-match: IN= OUT=eth0 SRC=192.168.0.106 DST=192.168.0.1 LEN=56 TOS=0x00 PREC=0x00 TTL=64 ID=21838 PROTO=UDP SPT=52372 DPT=53 LEN=36
Thus iptables and this rule obviously work.

However, when I change the rule slightly by adding a ".com" to the end
of the search string, the iptables logging does not work.

Here is the corresponding iptables command:

Code: Select all

iptables -t filter -I OUTPUT 1 -o eth0 -p udp -m udp --dport 53 \
         -m string --algo bm --icase --string "google.com"      \
         -j LOG --log-level info --log-prefix "iptables-string-match: "
After I remove the previous rule, run this new (slightly altered) command,
and then use iptables to check what is configured, I get the following
as the (expected) results for the output chain:

Code: Select all

Chain OUTPUT (policy ACCEPT ...)
           target     prot opt in     out     source               destination
           LOG        udp  --  *      eth0    0.0.0.0/0            0.0.0.0/0           udp dpt:53 STRING match "google.com" ALGO name bm TO 65535 ICASE LOG flags 0 level 6 prefix `iptables-string-match: '
I then perform the "dig google.com" command (which again works), followed
by a dmesg command, which prints nothing for this last DNS request.

I have tried various search strings, and all appear to work, except
when they contain a '.'. I have used wireshark to examine the request
packets going out to the DNS server, and see exacting what I expect.
But for some reason that I don't understand, iptables does not seem to
match with a search string containing a '.'.

I have tried this on CentOS 6.8 (with iptables 1.4.7) and CentOS 7.2.1511
(with iptables 1.4.21).

Can someone explain or help me with this issue?
Last edited by PatP on 2016/08/11 23:13:42, edited 1 time in total.

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TrevorH
Forum Moderator
Posts: 26887
Joined: 2009/09/24 10:40:56
Location: Brighton, UK

Re: String Match Issue with iptables

Post by TrevorH » 2016/08/11 22:35:36

The dig packets going out don't have dots in them, they've been replaced by 0x03.

Code: Select all

	0x0020:  0001 0000 0000 0000 0377 7777 0667 6f6f  .........www.goo
	0x0030:  676c 6503 636f 6d00 0001 0001            gle.com.....
I got it to work using

Code: Select all

iptables -R OUTPUT 1 -p udp -m udp --dport 53 -m string --algo bm --icase --hex-string \|676f6f676c6503636f6d\|   -j LOG
but that's hex based to --icase is not applicable so you'd need buckets of entries to cater for all the possibilities!
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PatP
Posts: 4
Joined: 2013/03/11 16:21:44

Re: String Match Issue with iptables

Post by PatP » 2016/08/11 23:12:26

Thanks. This explains a lot.

I did not realize that the domain names in DNS requests are encoded (i.e. the separating
period characters are replaced by the field length of the next field).

Regards.

Pat

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