Re: TCP ports and SMTP clients

[Miguel Delapaz]

The IBM z/VM Operating System <IBMVM@LISTSERV.UARK.EDU>
wrote on 10/23/2006 09:54:21 AM:

> 
> Are you saying that any VM user can open a connection to port 25 of

> the VM IP Stack, or do you mean external (to VM) users?
> 
> Regards,
> Richard Schuh

Provided you have not specified FREELOWPORTS
in your ASSORTEDPARMS statement, the only users who may have a TCP Listen
on PORT 25 are those that are specified with that privlidge on the PORT
statement.

If someone does have such a listen (the
SMTP server, for example would), then any user can connect to that port
to initiate a dialog with the server.  The port that is used for this
connection on the client's TCP/IP stack (which may or may not be the same
as the server's), will generally be an ephemeral port (http://en.wikipedia.org/wiki/Ephemeral_port).

Regards,
Miguel Delapaz
z/VM TCP/IP Development