As noted earlier, the email to fax gateway is universally available. Mail protocol has been designed to go through. Unless one pay attention, any mail to fax gateway is reachable from anywhere in the world.
To make it worst, this is generally exactly what most people need. To cope with that the Linuxconf email to fax gateway supports various scheme to limit access to this facility. Here they are.
Whenever a mail message goes through a mail gateway, a new line is added in the enveloppe of the mail. This line tell us which gateway managed the message and when each received it.
By counting the number of "Received" lines in the enveloppe, the fax gateway can tell if the mail originated from the local network or not. Linuxconf proposes this as a strategy to differentiate faxes from the inside from faxes coming from the internet.
Beware that this is not 100% fool proof. If the email to fax gateway
is visible from the internet, anyone can telnet
to it and
inject a message in it. This will look like a local fax. To make
sure the concept of "local faxes" is meaningful, you need
some firewalling mechanism isolating the fax gateway from the internet.
This way, the only way to reach it from outside will be by using
at least another mail gateway and this will be detected as a non
local fax.
The email to fax gateway is not always the mail server of the organisation. It is simpler if it is though.
If this is the case, the following solution may be used:
You can define a list of users who can fax to various location. A user is identified by his email address. You can also enter his PGP public signature. With this, the email to fax gateway will be able to certify that a fax message really came from the fax user.
You can selectivly tell if PGP signatures are needed for different fax destinations.