Name
F=k
Synopsis
When another host connects
to the local host and that other host claims to have the same
canonical name as the local host, it should be considered an error.
In V8.6 sendmail, setting the
CheckLoopBack
variable in
conf.c determined whether this error was
detected. But beginning with V8.7 sendmail, this
check is based on the delivery agent. If the F=k
delivery agent flag is absent, the check is done. If the
F=k
delivery agent flag is present, the check is
skipped.
The check is performed only for SMTP connections. The literal canonical name given in the connecting host’s HELO or EHLO response is compared to the canonical name for the local host. If they are the same, the following error is printed, and the connection is disallowed:
553 5.3.5 host config error: mail loops back to myself← V8.6 553 5.3.5 host config error: mail loops back to me (MX problem?)← V8.7 and above
Here, host is the name of the offending host.
A problem can arise at sites that run two different invocations of
sendmail (one for SMTP connections and another
for command-line invocation, where each uses a different
configuration file). In this instance, when the latter connects to
the former, this error can occur. Such sites might find it necessary
to set the F=k
delivery agent flag for the
delivery agent that handles SMTP connections (usually
smtp
). With an mc
configuration, the following command does just that:
define(`SMTP_MAILER_FLAGS',`k') ← prior to V8.10 MODIFY_MAILER_FLAGS(`SMTP', `+k') ← V8.10 ...
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