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Reverse DNS Requirements for UNH E-Mail Gateways

As a spam abatement procedure, UNH e-mail "gateway" servers block incoming e-mail connections from Internet (IP) addresses that can't be reliably tied back to a hostname. These rules apply to incoming mail to unh.edu, cisunix.unh.edu, granite.edu, and usnh.edu domains, and any other domain that has the same MX records as these.

Specifically, we require (a) that the connecting IP address have a PTR (reverse-lookup) record in Domain Name Service (DNS) that returns a hostname, and (b) lookup of that hostname must return the same connecting IP address.

Error messages that might be returned to a connecting mail server are:

• If reverse lookup of the connecting IP address fails, the mail will be be rejected with a message like:

  Fix reverse DNS for ipaddr, or use your ISP server

• If a reverse lookup of the connecting IP address times out (for example, because no DNS servers could be reached for that address, or don't respond to a DNS query) the mail is temporarily rejected with a message like:

  Client IP address ipaddr does not resolve

• Finally, if a reverse DNS lookup results in a hostname, but a lookup of the resulting hostname fails, or gives a different IP address than the connecting IP address, the mail is temporarily rejected with a message like:

  Possibly forged hostname for ipaddr

This rejection mechanism is not the only one we use, but it's been relatively effective in denying a large number of spammers access to our mailboxes. Unfortunately, it can also reveal problems with the DNS of innocent mailers.

If you find your non-spam mail has been blocked with one of the above messages, we suggest:

1. Fix, or get your Internet Service Provider (ISP) to fix, the DNS entry for the mail-sending system; or
2. Have your mail-sending system route mail through your ISP's SMTP server instead of connecting directly to UNH machines.

If you would like more information, or you would like us to set up a "bypass" that exempts your mail server from this requirement, please send mail (from Gmail, if necessary) to the System Manager. Please include all relevant information, including any error messages you're receiving, and the IP address of the problematic server.