sendmail, 4th Edition

FEATURE(require_rdns)—V8.14 and Later

When a client machine connects to your sendmail server, sendmail records the IP address of the connecting client in the ${client_addr} macro (${client_addr} on page 810). Next, sendmail looks up that IP address (performs a reverse DNS lookup of that address) to find the client’s hostname. The status of that lookup is stored in the ${client_resolve} macro (${client_resolve} on page 814). This FEATURE(require_rdns) rejects connections from clients for whom the reverse lookup fails.

You declare this FEATURE(require_rdns) like this:

FEATURE(`require_rdns')

If you declare this feature, the following logic will be performed as the last step under basic relay checks:

• If the value in ${client_addr} is also in the RELAY_DOMAIN (The RELAY_DOMAIN mc macro on page 269) or the RELAY_DOMAIN_FILE (The RELAY_DOMAIN_FILE mc macro on page 269) list of domains and hosts for which to relay, the connection is allowed to relay, and no further checking is done by this feature.

• If the result of the lookup (the value in the ${client_resolve} macro) is the literal OK, the address is accepted and any additional relay checks are performed.

• If the result of the lookup (the value in the ${client_resolve} macro) is the literal FAIL, the following error is returned in the SMTP transaction and the connection is disallowed:

  550 5.7.1 Fix reverse DNS for failed IP address here

• If the result of the lookup (the value in the ${client_resolve} macro) is the literal TEMP, the following error ...