Auteur: James P. Roberts Datum: Aan: Giuliano Gavazzi CC: Exim users list Onderwerp: Re: [Exim] Dealing with spam from outblaze.com
<snip> > aren't DNS-based RBLs quite expensive, even more than callbacks? <snip>
No, I think DNS RBLs are much cheaper. It's a single DNS lookup request
(UDP, not TCP, if done right) to the DNS RBL server, and a single
(should be UDP) reply.
A callback is the establishment (or at least attempt to establish) a TCP
connection to the MX server of the claimed sender, right? That has tons
more overhead per event than a DNS lookup. I mean, the first thing that
happens in a TCP connection sequence is at least one DNS query to
determine who to connect to! Actually, there are usually more than
than, with one request to determine the MX of the sender's domain,
followed by another to get the A record (IP) of the MX. Then there's
the TCP traffic itself.
Of course, if the DNS RBL server is a slow dog, the actual time spent
waiting for it could be longer than a callback to a fast MX server. It
all depends. But the total amount of internet traffic generated should
be much less for a DNS RBL lookup, than for a callback.
