| But just so you know...
IANA (Internet Assigned Numbers Authority) assigns "well known" port numbers,
e.g. port 25 for SMTP, 110 for POP3, etc. Any software written to use these
services (e.g. a POP3 client) knows that, by default, it should connect to its
appropriate port (e.g. port 110) on a server.
Of course you don't *have* to use the standard ports. There's nothing stopping
you starting sendmail on port 119, except that all the other SMTP servers will
still try and connect to port 25, and any NNTP server that tries to connect to
the server will get might confused. That's why the port numbers are "well
known".
Port numbers are essentially assigned at random. There's nothing special about
SMTP using port 25, or NNTP using 119, they're just the ports that those
protocols expect to use because once upon a time somebody pulled the numbers
out of a hat.
In your case, you can't use port 110 for the second POP3 proxy, so you can just
choose another port. It doesn't matter which port you use, as long as it doesn't
conflict with something else (port 25, for instance). It doesn't matter that it
isn't "well known", because the only people connecting to it know that it's
different and can tell their POP3 clients accordingly.
You might want to choose a port less than 1024. Ports less than 1024 can only be
bound to by root, whereas anybody can bind to a port over 1024. (Yes, I know
that it's a firewall and there is only root, but some habits are good to pick
up.)
The only implication is that some clients are so sure that you will only ever
want to connect to port 110 that they don't provide a method to change the port
they connect to. Most POP3 clients I've seen sensibly assume 110 by default and
allow you to choose whatever else you want.
As .1 says, when the transparent generic proxy comes along, the POP3 clients
will be able to specify the real server without worrying about the firewall.
PJDM
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