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Before the sendmail program reads its configuration file, it calls gethostbyname(3) or getipnodebyname(3) to find all the known aliases for the local machine. The argument given to gethostbyname(3) or getipnodebyname(3) is the value of the $w macro that was derived from a call to gethostname(3) ($w on page 850).

Depending on the version of sendmail you are running, the aliases that are found will be either those from your /etc/hosts file or those found as additional A or AAAA records in a DNS lookup. Then, depending on the DontProbeInterfaces option (DontProbeInterfaces on page 1023), sendmail will round out that picture by examining (probing) each network interface and extracting from it the associated IP address or hostname.

To see the aliases that sendmail found, or to see what it missed and should have found, use the -d0.4 debugging switch (-d0.4 on page 542). Any aliases that are found are printed as:

aka: alias

Depending on your version of sendmail, each alias is either a hostname (such as rog.stan.edu) or an IPv4 address (such as []), or an IPv6 address (such as [IPv6:2002:c0a8:51d2::23f4]).

Prior to V8.13, sendmail would also add leading name components to the list of host names in $=w (for example, for the hostname a.b.c.d, it would add a and a.b). Also prior to V8.13, each such name found (if not duplicated) would be reverse-looked-up to find its IP number and that IP number would be added to the list. Beginning with V8.13, ...

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