F=k
Don’t check for loops in EHLO/HELO V8.7 and later
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 later
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', ...
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