Re: 92 Byte ICMP Blocking Problem

["Steven M. Bellovin" <smb () research att com>]

In message <20030912175258.GB616832 () hiwaay net>, Chris Adams writes:
> Once upon a time, Richard J.Sears <rsears () adnc com> said:
> > Since then, we have been hammered with customer complaints concerning
> > the inability to talk to mail servers and ssh to their servers, as well
> > as other weird network issues, all centering around the time we started
> > blocking 92 Byte ICMP packets.
> >
> > Has anyone else seen this, and if so, is the only resolution to stop the
> > blockage of 92 Byte ICMP Packets..?
>
> Yes.  As soon as we put the policy route map in place, we had some
> people unable to talk via SSH, SMTP, or POP3.  It was random: one person
> here in the office couldn't SSH to a particular server.  He could SSH to
> other servers, and the rest of us could SSH to the server he could not.
> We had similar experiences with SMTP and POP3.  When we took the policy
> route map back out, the problems went away.
>
> This is with IOS 12.0(25)S1 on a 7513 doing dCEF.  We put the policy
> route map on the FE interface linking this router to the POP core
> router; this router has MC-T3 interfaces and ethernets to Ascend TNTs
> and such.  The intent was to stop the 92 byte ICMP echos from reaching
> the Ascend TNTs, since several of them were rebooting constantly.

I wonder if it's a Path MTU problem.  Can you turn off Path MTU on some 
of the affected hosts and see if it solves the problem?


                --Steve Bellovin, http://www.research.att.com/~smb