Re: iptables --string-replace

[Pablo Neira Ayuso <pablo@netfilter.org>]

On 17/01/11 04:41, Jan Engelhardt wrote:
> On Monday 2011-01-17 03:44, Ben K wrote:
> 
>>> Matching across packets would incur unwanted complexity.
>>
>> Just curious, does the current string match implementation match
>> across packets? If not, then surely adding replace functionality (with
>> the same compromise) is not overly complex?
> 
> The string match does indeed not work across packets. I do not know why 
> we have it, it won't have much use for stream protocols either and was 
> probably devised for datagrams.

Could you tell me why is not useful for stream protocols?

> I can't say for sure what the original 
> authors' intentions were. xt_string also works on the entire IP packet, 
> so there is a chance for false positives if one only wants to match 
> actual L7 payload.

It's easy to extend it to make it start after the IP header. I'll send a
patch for this.

I guess that it's going to be hard to find some pattern that matches in
the IP header, so that false positive that you mention has a very low
probability.