John, For a service like SMTP and POP the ports the servers use to talk are actually VERY specific to my knowledge. Either being 25 for SMTP or 110 for POP. Keep in mind that the random numbered ports are on the ORIGIN end not the destination. In other words Your machien/server would connect with a source port of #### but the destination port on the receiving host is always 25 or 110 for email. Some goes with FTP and Telnet. In truth it is pretty much the same for ANY port based service short of filesharing and gaming.
I hope this helps and good luck. Sincerely, Larry Mitchell A+, Netowrk+ Network Administrator I.V. Net L.L.P www.IVNET.com ----- Original Message ----- From: "John Horne" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Wednesday, May 22, 2002 8:28 AM Subject: non-privileged port selection - how is it done? > Hello, > > Perhaps a newbie question, but I'm a bit stumped with this one. > > Given a service such as e-mail which uses a non-privileged port to send mail > out, are there are any specific mechanisms as to which port is selected? > This will no doubt be dependant on the O/S, but is it really a random > numbered port, the first non-privileged port it knows is not in use, or does > the O/S have any other mechanism for selecting the port? > > My problem is that given that a site has a firewall blocking specific > non-privileged ports (e.g. 2222) against all IP traffic (both as a source > port or a destination port), if a genuine site tries to e-mail them a > message and the sending host selects that port (2222) then the mail message > will not be sent. The MTA will probably queue the message and try later with > a different port number, but it seems possible that the message may never be > delivered simply because the port numbers selected are all blocked by the > recipient site. The sending MTA may well just ditch the message and/or mail > the sender that it had a problem. > > Either i am missing something obvious about all this, or does this seem like > a possible scenario? > > > Many thanks, > > John. > > ------------------------------------------------------------------------ > John Horne, University of Plymouth, UK Tel: +44 (0)1752 233914 > E-mail: [EMAIL PROTECTED] > PGP key available from public key servers
