The UUCP suite of software gets the name of the local host from the uname(2) system call, whereas sendmail gets the name of the local host from the gethostbyname(3) or getipnodebyname(3) system call. For sendmail to easily handle UUCP addresses, V8 sendmail also makes use of the uname(2) function.

First the host part of the fully qualified name returned by gethostbyname(3) or getipnodebyname(3) is saved as the first string in the class $=w. Then uname(2) is called. If the call succeeds, the macro $k and the class $=k are both given the nodename value returned. If the call fails, both are given the same hostname value that was given to the $j. If the system does not have uname(2) available (if HASUNAME was not defined when sendmail was compiled; see HAS...), sendmail simulates it. The sendmail program's internal replacement for uname begins by reading /etc/whoami. If that file does not exist or cannot be read, sendmail scans /usr/include/whoami.h for a line beginning with the #define sysname. If that fails and if pre-V8.10 sendmail was compiled with TRUST_POPEN^[18] defined, sendmail executes the following command and reads its output as the UUCP node name:

^[18] TRUST_POPEN was a security risk and was eliminated from V8.10 sendmail. Instead of defining it, consider creating an /etc/whoami file and populating it or defining $k directly in your configuration file.

uname -l

If all these fail, $k is set to be the same as $j.

$k is assigned its value when sendmail first begins to run. It can be given a new value either in the configuration file or from the command line. Because $k does not change once it is defined, you need not prefix it with $& when using it in rules.