* Re: Disable tcp MSS option in three way handshake?
2012-10-25 22:42 ` Eric Dumazet
@ 2012-10-25 22:52 ` Vincent Li
2012-10-25 23:15 ` Vijay Subramanian
2012-10-26 22:54 ` Vincent Li
2012-11-04 0:25 ` Jan Engelhardt
2 siblings, 1 reply; 7+ messages in thread
From: Vincent Li @ 2012-10-25 22:52 UTC (permalink / raw)
To: Eric Dumazet; +Cc: netdev@vger.kernel.org
On Thu, Oct 25, 2012 at 3:42 PM, Eric Dumazet <eric.dumazet@gmail.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
>
>
>
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?
^ permalink raw reply [flat|nested] 7+ messages in thread* Re: Disable tcp MSS option in three way handshake?
2012-10-25 22:52 ` Vincent Li
@ 2012-10-25 23:15 ` Vijay Subramanian
0 siblings, 0 replies; 7+ messages in thread
From: Vijay Subramanian @ 2012-10-25 23:15 UTC (permalink / raw)
To: Vincent Li; +Cc: Eric Dumazet, netdev@vger.kernel.org
> 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
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: Disable tcp MSS option in three way handshake?
2012-10-25 22:42 ` Eric Dumazet
2012-10-25 22:52 ` Vincent Li
@ 2012-10-26 22:54 ` Vincent Li
2012-11-04 0:25 ` Jan Engelhardt
2 siblings, 0 replies; 7+ messages in thread
From: Vincent Li @ 2012-10-26 22:54 UTC (permalink / raw)
To: Eric Dumazet; +Cc: netdev@vger.kernel.org
On Thu, Oct 25, 2012 at 3:42 PM, Eric Dumazet <eric.dumazet@gmail.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
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: Disable tcp MSS option in three way handshake?
2012-10-25 22:42 ` Eric Dumazet
2012-10-25 22:52 ` Vincent Li
2012-10-26 22:54 ` Vincent Li
@ 2012-11-04 0:25 ` Jan Engelhardt
2 siblings, 0 replies; 7+ messages in thread
From: Jan Engelhardt @ 2012-11-04 0:25 UTC (permalink / raw)
To: Eric Dumazet; +Cc: Vincent Li, netdev@vger.kernel.org
On Friday 2012-10-26 00:42, Eric Dumazet wrote:
>On Thu, 2012-10-25 at 15:27 -0700, Vincent Li wrote:
>>
>> 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
Or you could just use -j TCPOPTSTRIP.
^ permalink raw reply [flat|nested] 7+ messages in thread