netdev - Re: Disable tcp MSS option in three way handshake?

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Message-ID: <1351204948.6537.263.camel@edumazet-glaptop>
Date:	Fri, 26 Oct 2012 00:42:28 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Vincent Li <vincent.mc.li@...il.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Disable tcp MSS option in three way handshake?

On Thu, 2012-10-25 at 15:27 -0700, Vincent Li wrote:
> Hi,
> 
> this sounds crazy, we have a weird situation that an unknown tcp
> implementation not putting tcp MSS option in the SYN/ACK which caused
> us some issue. I am tasked to mimic the unknown tcp immplementation on
> not sending MSS in tcp SYN/ACK, I am wondering if I can achieve that
> by modifying linux kernel tcp code,  there is socket option
> TCP_MAXSEG, but that seems only affecting the size of MSS, not
> removing the MSS option. do you have any pointer on how to do that in
> kernel tcp code?

You'll have to patch the code.

Or else, you could add a new feature to net/netfilter/xt_TCPMSS.c

(We already have 

#define XT_TCPMSS_CLAMP_PMTU 0xffff

You could add

#define XT_TCPMSS_REMOVE 0xfffe

and replace MSS option by NOP



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