port question

[David A. Bandel]

On Sun, Aug 23, 2009 at 20:13, Tony Alfrey<tonyalfrey at earthlink.net> wrote:
> David A. Bandel wrote:
>>
>> 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.
>
> Yes, I see that; one seems to be the "source port" and the "destination
> port" (which is perhaps where the port 587 from me and port 25 to e-mail
> come in)

Wrong again.  One port will normally be 1024+.  On UNIX systems, only
root can bind a port below 1024.  And the upper port is chosen at
random, much like the starting sequence number.

>
>
>> 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.
>
> That seems very universal.

Deliberately so.

>
>>  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.
>
> Frankly, I wish I didn't have to deal with comcast at all.

I block port 25 as well for all my clients.  They have to use an
alternate port.  Until all Windoze systems are off the internet,
spambots will be around trying to flood inboxes with mail -- all using
port 25.

>
>>
>> Get and use tcptraceroute to find what's blocked where.
>>
>> David A. Bandel
>
>
> Very Good!  Thank you!
> Thanks to everyone!
>


David A. Bandel
-- 
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