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

["Steven M. Bellovin" <smb () cs columbia edu>]

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