9

Under windows there is a registry setting to enable "Blackhole Router Detection", enabling path MTU discovery to function when there are misconfigured routers / firewalls in the path blocking the ICMP fragmentation needed messages.

http://technet.microsoft.com/en-us/library/cc958871.aspx

Is there an equivalent feature that can be enabled on Linux?

I am running Ubuntu 9.10 Kernel 2.6.31-14-generic and would be very interested in any commands to enable / monitor this behaviour or any links to articles / howtos explaining how it is implemented.

13

Yes, you can.

To quote from the linux kernel mailing list in 2008:

From: John Heffner <johnwheffner@...>
Subject: Re: PMTU Discovery - Does it work?
Date: Thursday, May 22, 2008 - 1:19 pm
Message-ID: <1e41a3230805221019u4c739527u2a9c1217d1c7411c@mail.gmail.com>

[...]

This is a well known issue (RFC 2923), and was the motivation for the development of MTU probing (RFC 4821), enabled with tcp_mtu_probing.

John Heffner wrote the changes to the linux kernel in 2007.

so to turn on tcp_mtu_probing

# echo 2 > /proc/sys/net/ipv4/tcp_mtu_probing 

Possible values

0: disabled
1: enabled when black hole detected
2: always enabled 
1
  • RFC2923 - TCP Problems with Path MTU Discovery, September 2000
  • RFC4821 - Packetization Layer Path MTU, March 2007 (takes into account ICMP blackhole)

I think support to ICMP blackhole recovery (as indicated in RFC4821, but complete RFC ¿is not implemented?) was added to Linux kernel 2.6.17. It can be enabled by means of tcp_mtu_probing option.

More info: http://staff.psc.edu/mathis/MTU/

I haven't found more updated info so far (additional info is welcome).

-1

The last information that I knew was that Linux had no support for PMTU blackhole router discovery.

This is due to the Linux non-compliance of RFC 2923.

  • 1
    RFC 2923 is informational and lists a number of common issues. What do you mean when you claim non-compliance to this? – Oskar Berggren Jul 1 '13 at 9:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.