Date: Thu, 25 Oct 2001 18:36:08 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: David Raistrick <drais@wow.atlasta.net> Cc: john_wilson100@excite.com, freebsd-stable@FreeBSD.ORG Subject: Re: 4.4-STABLE machine unusable (was Re: Openssh) Message-ID: <200110260136.f9Q1a8917003@apollo.backplane.com> References: <Pine.BSF.4.21.0110251806040.150-100000@wow.atlasta.net>
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:On Thu, 25 Oct 2001 john_wilson100@excite.com wrote:
:
:> Guys,
:>
:> You can say all you like, but something in stable is totally fscked up. As
:> soon as I log in and start doing anything that involves a bit of traffic
:> (e.g. tailing a file), the connection freezes and I have to kill it. sshd
:> doesn't die, so I can log in again. I can reproducibly freeze it by
:> doing... well practically anything:
:>
:> tail /var/log/messages, vi, cat, etc. all freeze the connection. Strangely
:> enough,
:
:I've seen this before, or something that sounds identical. telnet did the
:same thing, and anything over a size i dont remember via http did it as
:well.
:
:The workaround I found was to drop the MTU on the ethernet card (a generic
:ne2k card at the time, no idea what it was plugged into.) down to 512 and
:it was fine. Move it above 512 and the problems came back.
:...
TCP does what is known as MTU discovery to figure out the lowest MTU
in the connection path. TCP then sets the no-frag bit on its packets.
This can break down if you are running through a misconfigured firewall
or an intermediate router or machine does not respond with the correct
ICMP error when an oversized no-frag packet is received. If the
firewall blocks ICMP error #3 (destination unreachable) subcode 4,
your TCP connection will not properly detect the MTU.
Reducing the client machine's interface MTU is a work-around (it sets
a maximum MTU which is hopefully less then the maximum MTU of routers
in between you and the destination), but the best solution is to figure
out where the misconfigured router/machine is and fix it.
-Matt
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