John, > 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?
These are (usually) called ephemeral ports. The logic is OS specific. Your answers are here: http://www.ncftpd.com/ncftpd/doc/misc/ephemeral_ports.html http://www.iana.org/assignments/port-numbers > 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 ... The scenario you describe could happen. A general rule saying "no traffic at all on port X" is probably not such a hot idea. People usually set up their firewall rules with only half of the information. For instance, allow TCP connections from the internet to machine X if the destination port is Y. Any source port would be acceptable. Regards, David
