netdev - Fw: [Bug 55861] New: PMTU discovery no longer works in Linux 3.6+ with routers that do not send next hop MTU information

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Date:	Wed, 27 Mar 2013 08:31:27 -0700
From:	Stephen Hemminger <stephen@...workplumber.org>
To:	netdev@...r.kernel.org
Subject: Fw: [Bug 55861] New: PMTU discovery no longer works in Linux 3.6+
 with routers that do not send next hop MTU information

Begin forwarded message:

Date: Wed, 27 Mar 2013 08:25:40 -0700
From: "bugzilla-daemon@...zilla.kernel.org" <bugzilla-daemon@...zilla.kernel.org>
To: "stephen@...workplumber.org" <stephen@...workplumber.org>
Subject: [Bug 55861] New: PMTU discovery no longer works in Linux 3.6+ with routers that do not send next hop MTU information

https://bugzilla.kernel.org/show_bug.cgi?id=55861

           Summary: PMTU discovery no longer works in Linux 3.6+ with
                    routers that do not send next hop MTU information
           Product: Networking
           Version: 2.5
    Kernel Version: 3.6 onwards
          Platform: All
        OS/Version: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: IPV4
        AssignedTo: shemminger@...ux-foundation.org
        ReportedBy: _@...b.eu
        Regression: Yes

After upgrading recently, I found that path MTU discovery no longer worked
correctly for accessing some devices on the other side of an IPsec tunnel.

Bisection revealed the problems started with 3.6 and are still present in
3.9-rc4 (latest available at time of reporting).

Some investigation into code changes leads me to the belief that Linux lost
support for handling ICMP destination unreachable fragmentation needed packets
for which the next hop MTU field is zero. This is an expected condition when
dealing with older routers, as RFC792 originally defined ICMP destination
unreachable fragmentation needed without a next hop MTU field, and it was later
added in bytes previously allocated as unused.

The particular router in my case generating such packets is a machine running
OpenBSD 4.6.

A commit that appears to be of particular interest in this bug is
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=46517008e1168dc926cf2c47d529efc07eca85c0

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