Unscrambling Forwards
The traditional use of the
~/.forward file, as its name
implies, is to forward mail to another site.
Unfortunately, as users move from machine to
machine, they can leave behind a series of
~/.forward files, each of
which points to the next machine in a chain. As
machine names change and as old machines are
retired, the links in this chain can be broken. One
common consequence is a bounced mail message (“host
unknown”) with a dozen or so Received: (Received: on page 1162) header
lines.
As the mail administrator, you should beware of the ~/.forward files of users at your site. If any contain offsite addresses, you should periodically use the SMTP expn command[217] to examine them. For example, consider a local user whose ~/.forward contains the following line:
user@remote.domain
This causes all local mail for the user to be
forwarded to the host remote.domain for delivery there. The
validity of that address can be checked with
nslookup and
telnet(1) at port 25[218] and the SMTP expn command:
%ns -q=mx remote.domainAddress: 123.45.67.89 remote.domain preference = 0, mail exchanger = mail.remote.domain remote.domain preference = 10, mail exchanger = mx.another.domain %telnet mail.remote.domain 25Trying 123.45.123.45 ... Connected to mail.remote.domain. Escape character is '^]'. 220 mail.remote.domain Sendmail 8.14.1/8.14.1 ready at Thu, 13 Dec 2007 09:48:09 −0600 (MDT) 220 ESMTP spoken hereexpn user250 <user@another.site>quit221 remote.domain closing connection ...
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