port question

[David A. Bandel]

On Sun, Aug 23, 2009 at 18:57, Tony Alfrey<tonyalfrey at earthlink.net> wrote:
> David A. Bandel wrote:
>
>>
>> I can't make this much clearer.
>
> Yes, thanks, it is very clear! I'm just wondering about the actual packet of
> bits that is assembled at each layer.  My understanding is that the port ID
> is part of the bit stream that makes up the packet.  If this packet starts
> out from ME where I (my mail client) put in an ID of port 587 into the
> header that precedes the data bits, does an additional piece of header get
> added (perhaps at the sending SMTP server end) with an ID of port 25 so that
> it makes it to the receiving SMTP server?

The application, sendmail, only records the IP/hostname of the host to
which it connects/connected to it and inserts that in the header.
This is to identify where the message came from and the path it took
to get where it got.  This is all outlined in the RFCs.  The ports in
use are of no interest in determining where something came from.  Host
172.16.16.1 is host 172.16.16.1 no matter what ports were used for the
communications, and BTW, TCP communications use two ports, not one.
The commonly used port for e-mail is 25.  Has to be or e-mail servers
couldn't find each other, so all e-mail servers bind port 25 so you
can talk to them.  Mine also bind other ports (465 for SSL and 925 as
an alternate).  Comcast blocks port 25 outgoing, but NOT incoming.  So
you can receive mail all day, but not send or relay.

Get and use tcptraceroute to find what's blocked where.

David A. Bandel
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