Re: [Exim] SMTP Spoofing - Preventing Local Relaying

Hi Mail, on Mon, 03 Mar 2003 20:51:50 -0500 you wrote:

> I don't think I explained the problem well, it's not easy to describe..

I don't think you understand the problem well.

> Basically there should be no user/domain (besides local) with SMTP
> access-

Right. This statement suggests to me that you are a bit confused. SMTP
could be used for both receiving inbound mail (to your users) and sending
outbound mail, either from your local machine (SquirrelMail/whatever) or
via relaying from an external machine. What I think you're trying to say
is that you don't want anyone, except local users logged in to
SquirrelMail on your machine, to be able to use your server as an SMTP
smarthost (i.e. use your server as a relay to arbitrary domains). That's
fine. However, your server isn't doing that, so I think the problem may be
in your understanding.

> But my problem is, if someone knows of two domains housed on the box
> (domainA and domainB), then they can setup an MTU client and list
> domainA.com as the SMTP server; then my mail server will act as the SMTP
> server for domainA.com and it'll accept mail from the MTU and pass it
> off to domainB.com -or any other domain on the box if they have the name
> of any of the domains housed on the machine..

And what's the problem? Is the machine an MX for domainB.com? If so, then
what's the problem? If not, then how is mail getting into your spools for
SquirrelMail to read? If it's getting in via some other method and in fact
your machine doesn't need to accept "inbound" mail at all, then you could
just firewall off Exim, or not run the daemon at all if SquirrelMail can
send using a command line.

In summary, if your machine is an MX for domainB.com, then it will of
course be possible for anyone to set your machine as an "outgoing" SMTP
server in their config and send mail to domainB.com. It won't be
particularly useful for the end-user, as the "outgoing mail server"
setting is really intended to have a server which will relay to arbitrary
domains in it, but as long as your server is only accepting mail for
domains it's configured for and not arbitrary domains - what's the
problem? That's how e-mail works!



Tim