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

  • From: Steven M. Bellovin
  • Date: Tue Sep 13 21:36:35 2005

In message <20050913235950.GA16550@flounder.net>, Adam McKenna writes:
>
>On Tue, Sep 13, 2005 at 04:31:05PM -0700, william(at)elan.net wrote:
>> 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.
>
>Application layer firewalls have existed for at least 6 years.
>
Make that 15....

		--Steven M. Bellovin, http://www.cs.columbia.edu/~smb






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