This document tries to answer some of the frequently asked questions about the check_* rulesets in sendmail 8.8.
The DNS check in check_mail contains the comment:
# no DNS entry? this is dangerous!for the rule:
R$*<@$*$~P>$* $#error $@ 4.1.8 $: "451 unresolvable host name, check your configuration." no real nameWhy is this dangerous?
Some MTAs may react on the reply code, which indicates a temporary error, with very fast retries. This may tie up some of your resources and quickly fill your logfile.
sendmail 8.9 can distinguish between temporary and permanent DNS failures, so this isn't a problem in the new version.
Let's take an example:The sender is A@extern.site the recipient is B@local.site and B has a .forward installed with C@external.site (here we naturally assume that only local.site is local to your site.) In this case, the e-mail will be accepted when sent from A to B, but it can't be fowarded to C, since check_compat is called with A@extern.site $| C@external.site, which will be rejected in our scenario.
As explained elsewhere, it is not well advised to base the decision about a relay attempt on the envelope MAIL FROM address. However, this is what the proposed simple check_compat ruleset does.
If {client_addr} can't be resolved (by the name services available to sendmail) or the forward and reverse lookup do not match, then {client_name} will be [${client_addr}] i.e., the IP address in square brackets.