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

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Message-ID: <CAGK4HS8J2v=2SxmpwXR9vkyXJ9ibhMrPdMgUFxjTyXGt3g7vxA@mail.gmail.com>
Date:	Thu, 25 Oct 2012 16:15:38 -0700
From:	Vijay Subramanian <subramanian.vijay@...il.com>
To:	Vincent Li <vincent.mc.li@...il.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Disable tcp MSS option in three way handshake?

> thanks for the reply, I did a quick look at the code, as a quick dirty
> hack, can I change the
>
>  699 /* Set up TCP options for SYN-ACKs. */
>  700 static unsigned int tcp_synack_options(struct sock *sk,
>  701                                    struct request_sock *req,
>  702                                    unsigned int mss, struct sk_buff *skb,
>  703                                    struct tcp_out_options *opts,
>  704                                    struct tcp_md5sig_key **md5,
>  705                                    struct tcp_extend_values *xvp)
>  706 {
>
> ..................
>  730         /* We always send an MSS option. */
>  731         opts->mss = mss; <---------here set opts->mss = 0 ?
>
> would that work?


tcp_make_synack() calls tcp_options_write() has this:

        if (unlikely(opts->mss)) {
                *ptr++ = htonl((TCPOPT_MSS << 24) |
                               (TCPOLEN_MSS << 16) |
                               opts->mss);
        }

It will not add the option if opts->mss is zero, so this should work too.

Vijay
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