---
name: "qualification"
section: "5"
platform: "linux"
package: "djbdns-utils"
version: "1:1.05-22"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/qualification"
---

# qualification(5)

User's guide to name qualification

## NAME

qualification - User's guide to name qualification

## OVERVIEW

**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**(3)](/dns_ip4_qualify/3.md) 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:

- 1\.

  the  *$LOCALDOMAIN* environment variable, if it is set; or

- 2\.

  the first  *domain* or  *search* line in  */etc/resolv.conf*, if  */etc/resolv.conf* exists and has such a line; or

- 3\.

  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

## SEE ALSO

[dnsipq(1)](/dnsipq/1.md)

http://cr.yp.to/djbdns.html