I use Firewall Builder for fast prototyping of my iptables configuration. When a firewall rule matches and logging for that rule is enabled, one line like this is added to /var/log/messages:

Sep  1 09:48:43 server kernel: [9490931.734574] RULE 12 -- DENY IN=eth0 OUT= MAC=a1:b2:c3:d4:e5:f6:00:11:22:33:44:55:66:77 SRC=1.2.3.4 DST=5.6.7.8 LEN=96 TOS=0x00 PREC=0x00 TTL=58 ID=0 DF PROTO=ICMP TYPE=8 CODE=0 ID=27754 SEQ=0 

(sensitive information has been forged, of course 🙂

Depending on the protocol, the same field is not always in the same position, e.g.: destination port (DPT) could be in position 23 or 24. So if you want to list, say, the inward interface, source address, destination address, protocol and destination port you need a smarter matching. This one-liner worked for me:

perl -alne 'if (m{RULE 12}) { my %field ; foreach $token (@F) { next unless $token =~ /=/ ; my ($k,$v) = split(/=/,$token,2) ; $field{$k} = $v } ; print qq{ @field{ qw{IN SRC DST PROTO DPT} } } }' /var/log/messages | sort | uniq -c | sort -nr   

That perl part means: if the line matches "RULE 12" then I initialize the %field hash. Then I go through the tokens and I select those that contain a "=", I split on the equal sign and fill the hash. Finally, when %field is ready, I print the interesting fields.

I don't need to worry about splitting the line and save the "tokens", because perl's autosplit (-a) takes care of it. And I don't need to bother about printing newlines, because -l takes care of it.

And the sort/uniq/sort dance is the old trick to count the occurrences of he same line in the output.