Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!uwm.edu!rpi!dali.cs.montana.edu!milton!Tomobiki-Cho!mrc
From: mrc@Tomobiki-Cho.CAC.Washington.EDU (Mark Crispin)
Newsgroups: comp.dcom.sys.cisco
Subject: Re: Access restrictions give host unreachable...
Message-ID: <5684@milton.u.washington.edu>
Date: 29 Jul 90 02:43:15 GMT
References: <1990Jul28.234310.27064@mel.dit.csiro.au>
Sender: news@milton.u.washington.edu
Organization: Mendou Zaibatsu, Tomobiki-Cho, Butsumetsu-Shi
Lines: 34

In article <1990Jul28.234310.27064@mel.dit.csiro.au> smart@manta.mel.dit.csiro.au (Robert Smart) writes:
>and my login to the remote machine got blown away! Obviously the returning
>"host unreachable" caused telnet (or maybe it was rlogin) to drop the
>connection.

This is a bug in your system's Unix kernel.  It afflicts many
BSD-based Unix systems.  At some time in the writing of BSD TCP,
someone got the idea that ICMP Unreachables were so important that it
should cause all pending read() or write() system calls on a TCP
socket for that host to fail.  The claim is that the user program
should check the error code, realize it is not fatal, give a message
to the user, and continue.

The fact that no Unix user programs actually do this escaped this
person, as did the real-world situation where ICMP Unreachables
happen for transient conditions.  I'll leave aside the issue of
whether issuing a warning message (Milnet TAC's do this) is really a
good idea in this day and age of file transfers and display editors.

The fix is to make ICMP Unreachables only nuke open() calls, and let
TCP (or the user's own impatience) deal with lost connections on a
read() or write().  Nag your vendor to do this; I managed to get NeXT
to do so and it was a big improvement!

There are a couple of patches floating around on the network that
disable ICMP Unreachable processing entirely.

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