D. J. Bernstein
Internet publication
djbdns

User's guide to name qualification

Qualification means conversion of a short host name that you type, such as cheetah, into a complete (``fully qualified'') domain name, such as cheetah.heaven.af.mil.

This page explains the djbdns qualification procedure. These rules are followed by the dns_ip4_qualify library routine in djbdns, and by programs that use the dns_ip4_qualify routine.

Rewriting instructions

Normally the djbdns qualification procedure follows instructions listed in /etc/dnsrewrite, a file created by your system administrator. You can override /etc/dnsrewrite by creating your own file and setting the $DNSREWRITEFILE environment variable to the name of that file.

Sample instructions:

     # anything.local -> me
     -.local:me
     # me -> 127.0.0.1
     =me:127.0.0.1
     # any.name.a -> any.name.af.mil
     *.a:.af.mil
     # any-name-without-dots -> any-name-without-dots.heaven.af.mil
     ?:.heaven.af.mil
     # remove trailing dot
     *.:

Instructions are followed in order, each at most once. There are four types of instructions:

=post:new means that the host name post is replaced by new.
*post:new means that any name of the form prepost is replaced by prenew.
?post:new means that any name of the form prepost, where pre does not contain dots or brackets, is replaced by prenew.
-post:new means that any name of the form prepost is replaced by new.

Searching

The djbdns qualification procedure can search through DNS for several possible qualifications of a name. For example, the name

     cheetah+.heaven.af.mil+.af.mil

is qualified as cheetah.heaven.af.mil if that name has IP addresses listed in DNS, or cheetah.af.mil otherwise.

In general, x+y1+y2+y3 is qualified as xy1 if xy1 has IP addresses listed in DNS; otherwise, as xy2 if xy2 has IP addresses listed in DNS; otherwise, as xy3. You can list any number of +'s.

Searching is applied after rewriting, so you can use a rewriting instruction such as

     ?:+.heaven.af.mil+.af.mil

to have lion qualified as lion.heaven.af.mil or lion.af.mil, and tiger qualified as tiger.heaven.af.mil or tiger.af.mil, and so on.

Searching is generally not a recommended feature. If you rely on gw being qualified as gw.af.mil, and someone suddenly adds a new gw.heaven.af.mil, you'll end up talking to the wrong host. It's better to rely on syntactic rules that you control.

Compatibility mechanisms

If the rewriting-instructions file does not exist, the djbdns qualification procedure looks for a local domain name in three places:

the $LOCALDOMAIN environment variable, if it is set; or
the first domain or search line in /etc/resolv.conf, if /etc/resolv.conf exists and has such a line; or
everything after the first dot in the system's hostname.

It then creates rewriting instructions of the form

     ?:.domain
     *.:

so that .domain is added to any name without dots or brackets.

You can specify searching in $LOCALDOMAIN by using several domain names separated by spaces. Your system administrator can specify searching in /etc/resolv.conf by putting several domains on a search line.

Compatibility notes

Different DNS client programs use different qualification procedures. Two major differences between the djbdns qualification procedure and other qualification procedures:

Most programs use only /etc/resolv.conf. They don't know anything about /etc/dnsrewrite and $DNSREWRITEFILE.
Most long-running programs don't notice changes in /etc/resolv.conf; they read /etc/resolv.conf when they start, and they don't reread it until they are restarted. In contrast, the djbdns qualification procedure checks for changes every 10 minutes or 10000 uses.

Two minor differences:

Some programs interpret a domain line in /etc/resolv.conf as specifying a search list consisting of various suffixes of the domain.
Many programs will search the local domain for names with dots.

If you want the local domain searched for names with dots, you can set it up with rewriting:

     # aol.com -> aol.com or aol.com.heaven.af.mil
     *:++.heaven.af.mil
     # but skip directly to heaven.af.mil if no dots
     ?++.heaven.af.mil:.heaven.af.mil