Re: [PATCH] xt_request_find_match

[Patrick McHardy <kaber@trash.net>]

Jan Engelhardt wrote:
> On Dec 19 2006 12:51, Patrick McHardy wrote:
> 
>>>Reusing code is a good idea, and I would like to do so from my 
>>>match modules. netfilter already provides a xt_request_find_target() but 
>>>an xt_request_find_match() does not yet exist. This patch adds it.
>>
>>Why does your match module needs to lookup other matches?
> 
> 
> To use them?
> 
> I did not want to write
> 
> 
> some_xt_target() {
>     if(skb->nh.iph->protocol == IPPROTO_TCP)
>         do_this();
>     else
>         do_that();
> }

I don't think

xt_request_find_match(match->family, "tcp", 0)->match(lots of arguments)

is better than a simple comparison. Besides that the tcp match itself
expects that the protocol match already checked for IPPROTO_TCP, so
you'd still have to do it.

> since the xt_tcpudp module provides far more checks than just the protocol
> (TCP/UDP), like
> 
>     /* To quote Alan:
> 
>        Don't allow a fragment of TCP 8 bytes in. Nobody normal
>        causes this. Its a cracker trying to break in by doing a
>        flag overwrite to pass the direction checks.
>     */

This check makes sure the flags are not overwritten _after you
matched on them_. It doesn't matter at all if you're only
interested in the protocol since the user didn't tell you to care.