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Date:	Thu, 1 Oct 1998 22:43:35 PDT
From:	"Gregory W. Smith" <gsmith@well.com >
Reply-To:	scoug-programming@scoug.com
To:	< "scoug-programming@scoug.com" > scoug-programming@scoug.com >
Subject:	SCOUG-Programming: FTP server - ports

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On Thu, 1 Oct 1998 21:20:00 PDT, Peter Skye wrote:
>You know, I'm reading my own post and even _I_ can't understand it.
>
>Peter Skye wrote:
>>
>> How do you block ports from being assigned?
>> (and a bunch of mumbo jumbo afterwards)
>
>I'll try again.
>
>If we start the FTP server, it listens at some preassigned port. If a
>user logs in, they're given a different port on which to conduct
>business.
>
>Where do we list ports that we don't want the FTP (or other) server to
>assign to a user? My specific reason is that I might have some "other"
>server which only runs occasionally, and this "other" server listens at
>some preassigned (but different) port just like the FTP server. I don't
>want the FTP server to ever assign that particular port to a user, so
>that it's always available in case I start my "other" server.
>
>Example: My "other" server listens at port 2300, but I haven't started
>it yet. The FTP server sees a user log on and (through TCP/IP, I guess)
>assigns a unique port to that user. If port 2300 is, by chance,
>assigned to the FTP user, then I can't start my "other" server.
>
>- Peter Skye

As I understand it (and I might be wrong), ports do not get "used up"
on a system because they are assigned on a per-process basis. You can
find a list of the default assignments in /mptn/etc/services where you
see the default ports for FTP are 20 and 21.
If the FTP server gets a request from joe@abc.net on port 20, it
spawns child process "A" that has the established connection on port
20. The child process then opens a data connection with joe@abc.net on
port 21 and then does all of the file transfers. The parent process
then closes its connection on port 20 and then re-opens port 20 to
listen for the next user. If fred@def.com then connects on port 20,
the server spawns another child process "B". It is up to the tcp stack
to keep track of who gets which data. When a packet comes in for port
20 from joe@abc.net, the OS gives it to child process "A". A packet
for port 20 from fred@def.net goes to process "B".
Think of the ports as file designators. You can be running many
processes that use 'stdin' at the same time. But different processes
will have different file associations with 'stdin'. At least that is
my understanding from setting up communicating processes on Unix. Whe
you fork a process, both processes share the file descriptors and the
parent must close the file so the child process can use it. I would
assume that ports act the same way.
-- Greg Smith
===============================================================
Gregory W. Smith (WD9GAY) gsmith@well.com
finger gsmith@well.com for PGP public key
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