sendmail, 4th Edition

$j

Our official canonical name All versions

The $j macro is used to hold the fully qualified domain name of the local machine. V8 sendmail automatically defines $j to be the fully qualified canonical name of the local host.^[319] However, you can still redefine $j if necessary—for example, if sendmail cannot figure out your fully qualified canonical name, or if your machine has multiple network interfaces and sendmail chooses the name associated with the wrong interface.

A fully qualified domain name is one that begins with the local hostname, which is followed by a dot and all the components of the local domain.

The hostname part is the name of the local machine. That name is defined at boot time in ways that vary with the version of Unix you are using.

The local domain refers to the DNS domain, not to the NIS domain. If DNS is running, the domain is defined in the /etc/resolv.conf file. For example:

domain wash.dc.gov

At many sites, the local hostname is already fully qualified. To tell whether your site uses just the local hostname, run sendmail with a -d0.4 switch:

% /usr/sbin/sendmail -d0.4 -bt < /dev/null
canonical name: wash            ← not fully qualified (and wrong!)
canonical name: wash.dc.gov     ← fully qualified (correct)

The $j macro is used in two ways by sendmail. Because $j holds the fully qualified domain name, sendmail uses that name to avoid making SMTP connections to itself. It also uses that name in all phases of SMTP conversations that require the local machine’s identity. One indication ...