sendmail, 4th Edition

Canonify a Host with /canon

The /canon rule-testing command causes sendmail to look up the canonical (official, fully qualified) name of a host and print the result. The form for this command looks like this:

/canon host

If host is missing, the following usage message is printed:

Usage: /canon address

When you correctly supply the hostname as the argument, sendmail looks up the canonical name and returns the result:

> /canon icsic
getcanonname(icsic) returns icsic.icsi.berkeley.edu
>

Here, the hostname icsic was looked up. Because its canonical name was found, that name is printed following the returns. If the hostname had not been found, sendmail would have printed that same name after the returns:

> /canon foo
getcanonname(foo) returns foo

If you wish to watch the actual process of a host being canonified, you can turn on the -d38.20 debugging switch (-d38.20 on page 568) with the rule-testing -d command (Add Debugging for Detail on page 318):

> -d38.20
>

With that setting, the previous lookup of icsic produces a trace of all the steps that sendmail takes:

> /canon icsic
getcanonname(icsic), trying dns
getcanonname(icsic), trying files
text_getcanonname(icsic)
getcanonname(icsic.icsi.berkeley.edu), found
getcanonname(icsic) returns icsic.icsi.berkeley.edu

Here, sendmail first looked up icsic using DNS. That lookup failed, so sendmail fell back to looking it up in the /etc/hosts file, where it was found. The order in which these techniques are tried is defined by your service switch (ServiceSwitchFile ...