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

  • From: william(at)elan.net
  • Date: Tue Sep 13 19:34:32 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@elan.net




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