mail service with no mx (was - Re: Computer systems blamed for feeble hurricane response?)

Tue Sep 13 23:31:05 UTC 2005

On Wed, 14 Sep 2005, Roy Badami wrote:

>    william(at)elan> Could you elaborate on how firewall will
>    william(at)elan> determine if the connection is from mail server
>    william(at)elan> or from telnet on port 25?
>
> Perhaps because most telnet clients will attempt telnet option
> negotiation?  If so one could avoid this by using a client such as
> netcat...

Telnet option negotiation is at Layer 7 after TCP connection has been
established. Firewalls typically don't operate at this level (TCP session
is Layer 4 if I remember right) and would refuse or reject (difference
type of ICMP response) based solely on attempt to connect to certain
ip or certain TCP/UDP port.

-- 
William Leibzon
Elan Networks
william at elan.net