Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!sdd.hp.com!elroy.jpl.nasa.gov!ncar!asuvax!noao!rstevens
From: rstevens@noao.edu (Rich Stevens)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: why UDP send ICMP port unreachable message?
Message-ID: <1991Mar1.181515.12734@noao.edu>
Date: 1 Mar 91 18:15:15 GMT
References: <9103011243.AA10028@ucbvax.Berkeley.EDU>
Organization: National Optical Astronomy Observatories, Tucson AZ
Lines: 33

In article <9103011243.AA10028@ucbvax.Berkeley.EDU> TAYBENGH@NUSDISCS.BITNET writes:
> According to my understanding, this ICMP message
> is never received by UDP, nor is reported to the user if the user send a
> UDP message to a non-existent server (destination port).

UDP does indeed receive this message.  It's not documented well,
so you really need to go to the sources to see what's going on.
The publicly available "BSD Networking Release" suffices.

The sequence of events is:  icmp_input() receives the ICMP error.
The ICMP "type" is ICMP_UNREACH and the ICMP "code" is "port
unreachable".  This code is converted to PRC_UNREACH_PORT and
udp_ctlinput() is called to process the error.  udp_ctlinput()
calls in_pcbnotify(), converting the PRC_UNREACH_PORT error
into the ECONNREFUSED errno value, using the table inetctlerrmap[]
(in the file ip_input.c).  A 32-bit internet address is passed to
udp_ctlinput() and this address is passed again to in_pcbnotify().
This address is the address that the datagram was sent to that
generated the ICMP error.  The routine in_pcbnotify() is interesting
because it looks at this address and finds *every* socket that is
connect()ed to this address and sends those sockets the asynchronous
error ECONNREFUSED.

So there are three caveats: (1) you'll only know about the error if
you've done a connect(); (2) not only will you find out about the
error, but *every* process on your host that is UDP "connect()ed"
to the host that sends the ICMP port unreachable gets a ECONNREFUSED
error; (3) you usually don't find out about the error when you send()
the datagram to the port that no one is reading from, in most cases
you'll find out when you do a recv() on that socket (assuming the
classic send-a-datagram, receive-a-datagram scenario).

	Rich Stevens (rstevens@noao.edu)