Date: Tue, 16 Jan 2007 10:58:55 -0800 From: Chuck Swiger <cswiger@mac.com> To: Randall Stewart <rrs@cisco.com> Cc: freebsd-net <freebsd-net@freebsd.org> Subject: Re: Problem with port 0 Message-ID: <DD2642C7-61D8-40A2-8036-493743B9678A@mac.com> In-Reply-To: <45AD1C0F.7000701@cisco.com> References: <f9876c510701120903r65543ef4nafc7eeead2becb42@mail.gmail.com> <20070112163057.2a3ec8f0.rnsanchez@wait4.org> <45A807F8.7080603@FreeBSD.org> <45ACCFF4.4040709@cisco.com> <E7D65849-3A24-4A84-B1B4-D888BDB6E18E@mac.com> <45AD1C0F.7000701@cisco.com>
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On Jan 16, 2007, at 10:40 AM, Randall Stewart wrote: >> "reserved" means one "SHOULD NOT" use that port, where the phrase >> in caps is defined in RFC-2119 (http://www.ietf.org/rfc/rfc2119.txt). [ ...format-flowed quoting trimmed... ] > So let me see if I understand the statements above.. > > We are using the SHOULD NOT.. which is YOU REALLY REALLY REALLY > REALLY REALLY REALLY should not do this unless you have some > very dramatic demonstrative need to do so and know the FULL > consequences of the action. Pretty much. :-) If you have a choice about solving the problem in a way that avoids doing something described as "SHOULD NOT", then one really ought to prefer that choice. > The bit from wikipedia, while not authoritative in my mind, says you > can send FROM the port, but don't expect an answer back.. which > implies > you cannot bind it and/or cannot read from it if your source > port is 0... Of course in TCP this is totally useless since you > have to get something back in order to setup the handshake. I don't consider wikipedia to be authoritative either, but it can be a useful reference or guideline in conjunction with other sources. Note that there are some uncommonly used TCP variants (T/TCP comes to mind) where you can short-circuit the 3-way handshake and put actual data in the initial SYN packet. I would agree that one SHOULD NOT listen on port 0. > In UDP I guess one could get a packet if the other O/S did not > have any bind restrictions.. or one were to use a raw socket. Sure, or use BPF/PCAP to generate the packets directly. > But why all this for something you SHOULD NOT DO.. one of the > consequences in my mind of this is that not all O/S's may be > able to read your data .. nor respond to it. > > Seems to me a lot of hassle when one can just use a different > port :-0 Well, yes-- there are another 65500+ ports available. -- -Chuck
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