linux-kernel - Re: How would I do this? (expert tricks) OT

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Date:	Tue, 19 Jun 2007 19:28:25 +0200 (CEST)
From:	Jan Engelhardt <jengelh@...putergmbh.de>
To:	Marc Perkel <mperkel@...oo.com>
cc:	linux-kernel@...r.kernel.org
Subject: Re: How would I do this? (expert tricks) OT


On Jun 19 2007 10:14, Marc Perkel wrote:
>> 
>> tcpdump -lni any port 25
>> iptables -p tcp --dport 25 -j NFQUEUE
>> ...
>> 
>
>Thanks Jan, but I'm not sure it answers my question.

There's more than one way to do it.

One is...
	tcpdump -lni eth0 tcp [extra operands to match SYN packets] |
	myprogram

a longer one is to write your own netfilter userspace program
that receives the TCP SYNs (by means of -j NFQUEUE) and does
take action.

Another one is to use -j LOG and let your program parse
down /var/log/firewall. Like

	iptables -A INPUT -p tcp --dport 25 --syn -j LOG --log-prefix "[evil]"
	tail -f /var/log/firewall | grep '^\[evil\]' | myscript

myscript:
#!/usr/bin/perl

while (defined(my $line = <>)) {
	my($ip) = ($line =~ /SRC=(\S+)/);
	# Do something
}

>I want to run a script every time a connection attempt is made in real time

The scripts runs constantly, preferably.

>with the IP address as a parameter to the script. How would I do that? Suppose
>my script is:
>
>iplog <ipaddress>
>
>
>
>
>       
>____________________________________________________________________________________
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>

	Jan
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