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Re: outgoing SMTP and dynamic DNS

From: Rob Browning <rlb@cs.utexas.edu>
> I can see that tcpd gets the IP
> address from the initial connection, but where does it get the name it
> uses to reverse resolve for the comparison to the IP?

Get IP
Do a reverse lookup on, get a name foo.bar.org.
Look up the name foo.bar.org. Get a number.
Is the number If not, twist to smtp-refuser or block.

> 2. Configure dynamic DNS.
> How does this help?  From reading the ml FAQ, I thought that dynamic
> DNS still couldn't do anything about reverse resolution, or is this a
> different issue?

Reverse resolution happens through your internet provider. I think the
important point is that the name in your SMTP hello (which will set the
envelope-from) resolve to the IP number you are using. Dynamic DNS makes
that work. TCPD would not care, but some SMTP servers do.

For IRC and so on, the name you get through reverse lookup will not
be the same as your name in Dynamic DNS, but the IP numbers of both will



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