North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: mail service with no mx (was - Re: Computer systems blamed for feeble hurricane response?)
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
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