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

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Date:	Fri, 26 Oct 2012 15:54:17 -0700
From:	Vincent Li <vincent.mc.li@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Disable tcp MSS option in three way handshake?

On Thu, Oct 25, 2012 at 3:42 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> 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
>
>
>

for the sake of complete information, net/netfilter/xt_TCPOPTSTRIP.c
already have this feature to strip tcp option

Vincent
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