Why do you feel that way? Are you aware that the spam problem has forced many to block _all_ mail coming from dynamic IP addresses - regardless of
Trying to avoid the pitched ideological battle and get back to a technical discussion.
I think I understand dynamic vs. static IP addresses, and I understand why some people feel compelled to filter on them, and why others consider that an ill-conceived idea. But how is the filtering actually performed?
Is this simply recognizing the well-known non-routable IP addresses (10..., 192.168.., etc.) in the source address of an incoming connection to port 25? I don't see how that could ever happen. Is it scanning email headers for those addresses? I don't see that this would filter much of anything. One of the "ideological battle" messages blamed some part of this on NAT; I don't see where NAT has anything to do with this, as NAT is not the only source of dynamic addresses.
I'm sending this email via a dial-up PPP connection. My ISP has assigned me IP address 209.248.81.177, derived from where I landed in the modem pool. If I disconnect and dial-in again I will likely get a different IP address. 209.248.81.177 is what I think is meant by "dynamic address" in this discussion.
When I send email, it goes from my PC to a server at my ISP. The mail server has the static IP address 209.248.82.245. It is registered under the DNS name mail.ophidian.com.
The above two IP addresses appear as the first "Received:" headers in email that I send.
From just the IP addresses themselves, I don't see any way to distinguish the one address as dynamic and the other as static. One could perform a reverse DNS lookup, which should succeed on the static address and fail on the dynamic.
Is this what it means to "block mail from dynamic addresses"? To block incoming connections to port 25 unless a reverse DNS lookup succeeds? Is any other checking done with the results of the lookup?
I don't really understand so-called dynamic DNS, other that what seems self evident from the name. But wouldn't that provide a way to get around such blocking and send mail from dynamic addresses?
A more succinct way of asking the above might be to ask how one would write a pf filter (or an email filter) to recognize and block email from dynamic addresses.
Edward A. Gardner eag at ophidian dot com Ophidian Designs 719 593-8866 voice 1262 Hofstead Terrace 719 210-7200 cell Colorado Springs, CO 80907
