Re: 2.0.21: TCP hangs solved by disabling Path MTU Discovery

A.N.Kuznetsov (
Wed, 25 Sep 1996 18:28:14 +0400 (MSD)

Chris Fearnley (cjf@netaxs.COM) wrote:
: Greetings,

: I recently reported that HTTP connections were hanging without sending
: any data under 2.0.21. Alan Cox's suggestion of building with Path
: MTU Discovery disabled has resolved that problem. Thanks Alan,
: everything seems to be working fine now. I'll know for sure tonight
: when cron runs the tape backup (which also failed under the old kernel).

: The listing for Path MTU Discovery suggests it's only
: useful for broken PC clients. But many non-local machines including my
: PC running 2.0.20 failed to connect to this machine until I rebuilt it
: as described. The machine is on a TCP/IP network connected via a Cisco
: on a Frame Relay 56k link. Since this is standard technology I'm
: uncertain why we ran into the problem. Should broaden
: it's description of the Path MTU Discovery option or is there still a
: bug hiding somewhere?

Apparently, this option should be turned off
(and moved to experimental status) by default for 2.0.
What a pity! Really, I am sure, if I had at least ONE tcpdump
of failed connection, I'd found it. Alas, nobody sent it :-(

Please, try to make it!

- start tcpdump host your-host.
Please, do not filter by protocol or destination address.
- flush routing cache by adding and deleting any route
(route add -host lo; route del
- try to make connection, that will fail.

Alexey Kuznetsov.