In message <[EMAIL PROTECTED]>, 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
