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.