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From: Bill Fenner <fenner@research.att.com>
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To: Randall Stewart <rrs@cisco.com>
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	<20070112163057.2a3ec8f0.rnsanchez@wait4.org>
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Date: Tue, 16 Jan 2007 14:00:15 -0800
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Cc: freebsd-net@freebsd.org, "Bruce M. Simpson" <bms@freebsd.org>,
	Ricardo Nabinger Sanchez <rnsanchez@wait4.org>, hugme@hugme.org
Subject: Re: Problem with port 0
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I'd note that RFC 768 explicitly mentions this possibility:

Source Port is an optional field, when meaningful, it indicates the port
of the sending  process,  and may be assumed  to be the port  to which a
reply should  be addressed  in the absence of any other information.  If
not used, a value of zero is inserted.

I think Hug is saying that since the boot loader is the only thing that's
running on the box, it's got no reason to use a port number, so it's
not used.

Hug's error message:
>with default tftp - Jan 12 09:37:55 dukeengi01 tftpd[80898]: connect: Can't
>assign requested address

is consistent with the following code from in_pcbconnect_setup():

        if (sin->sin_port == 0)
                return (EADDRNOTAVAIL);

which dates back to BSD 4.4 and before.

This is probably not completely wrong, but it's not right in this particular
case - tftpd should be able to connect a UDP socket to a remote port 0 in
order to respond to this request.

(FreeBSD definitely can't send from port 0, but that's OK because we don't
want to.)

  Bill