Re: PMTU discovery is broken on kernel 3.7.1 for UDP sockets

On 12/19/12 17:35, Ben Hutchings wrote:

On Wed, 2012-12-19 at 17:10 +0400, Yurij M. Plotnikov wrote:
   

quoted

On kernel 3.7.1 I get strange behaviour of IP_MTU_DISCOVER socket
option. The behaviour in case of IP_PMTUDISC_DO and IP_PMTUDISC_WANT
values of IP_MTU_DISCOVER socket option on SOCK_DGRAM socket are the
same and packet is always sent with "Don't Fragment" bit in case of
IP_PMTUDISC_WANT. Also, the value of IP_MTU socket option is not updated.
     

You could try reverting:

commit ee9a8f7ab2edf801b8b514c310455c94acc232f6
Author: Steffen Klassert[off-list ref]
Date:   Mon Oct 8 00:56:54 2012 +0000

     ipv4: Don't report stale pmtu values to userspace

     We report cached pmtu values even if they are already expired.
     Change this to not report these values after they are expired
     and fix a race in the expire time calculation, as suggested by
     Eric Dumazet.

Still, PMTU information is not supposed to expire for 10 minutes...

   

With reverted commit there is no such problem on 3.7.1: IP_MTU is 
updated and DF is set only for the first packet in case of 
IP_PMTUDISC_WANT.

[...]
   

quoted

On eth2 on host_B and on eth1 on host_C change MTU from 1500 to 750.
Wait for a while.

9. send(6, lenght=1400) ->  1400 // the packet is sent with "Don't
Fragment" bit, tcpdump on eth1 on host_B shows it
10. sleep(5);
11. send(6, length=1400) ->  -1 with EMSGSIZE
12. sleep(5);
13. getsockopt(6,IP_MTU) ->  0 // Returns that MTU is 1500 once again. So
value is not updated.
     

[...]

What if you read this option immediately before the sleep(5)?
   

It still returns that MTU is 1500.

Yurij.