In <20031212002615(_dot_)3378(_dot_)qmail(_at_)earth(_dot_)serverbox(_dot_)net>
"Ned Harvey" <spf(_at_)nedharvey(_dot_)com> writes:
How does the receiver's mailserver know what IP address the message
came from? Just by looking in the message headers? Message headers
are trivial to spoof. There has to be something better.
The receiving MTA has to look at the IP address contained in the
TCP/IP packets used to communicate with the sending MTA. There are
random sequence numbers contained in the packet headers that are
checked by the OS which makes it very hard to spoof the IP address.
1- Base the verification on the *last* IP address of the *last*
relay that talks to the receiver.
Right, this is what the SPF system does. Only the *last* relay can be
checked. The domain owner must authorize the use IP address of that
relay in order for the SPF system to give an OK. (It might still give
an "unknown" as well as a "denied".)
-wayne
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