qualification(5)

💖 qualification(5) — bestie's guide to name qualification 💖

okay so this is qualification(5) from the File Formats Manual, and it is literally the glow-up guide for your hostnames. strap in 🎀

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✨ NAME ✨

qualification — the user's guide to name qualification, aka how your short lil nickname for a computer becomes its whole entire government name 🫶

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🌸 OVERVIEW 🌸

so like. qualification is when you type a short host name — say cheetah — and it gets converted into a complete ("fully qualified") domain name, like cheetah.heaven.af.mil. it's giving nickname → full legal name. iconic. 💅

this page explains the djbdns qualification procedure. these rules are the ones followed by the dns_ip4_qualify library routine in djbdns, and by programs that use the dns_ip4_qualify(3) routine. no cap. ✨

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🦋 Rewriting instructions 🦋

normally the djbdns qualification procedure follows the instructions listed in /etc/dnsrewrite, a file created by your system administrator (the group chat admin, basically 👑). you can override /etc/dnsrewrite by making your own file and setting the $DNSREWRITEFILE environment variable to the name of that file. main character energy, honestly.

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 get followed in order, each one at most once. there are four types of instructions, and yes bestie we are covering all four 💖

• =post:new — means the host name post is replaced by new. that's it. straightforward. slay.
• *post:new — means any name of the form prepost is replaced by prenew. so whatever comes before stays, and the ending gets swapped.
• ?post:new — means any name of the form prepost is replaced by prenew, but only when pre does not contain dots or brackets. so picky, and honestly? valid.
• -post:new — means any name of the form prepost is replaced by new. note the difference from *: here the pre part gets dropped entirely, not kept. 💀 do not mix these up.

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🔥 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. it's like trying on outfits until one actually has plans. 🎀

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. go off. ✨

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. every cat gets a maybe-domain 😭

⚠️ 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. like texting the wrong number but it's your infrastructure. it's better to rely on syntactic rules that you control. periodt. 💅

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🌈 Compatibility mechanisms 🌈

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

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. cute, right? 🫶

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. 💜

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⭐ 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. not them being oblivious 🙈
• 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. consistency queen 👑

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

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🧚 SEE ALSO 🧚

• dnsipq(1)
• http://cr.yp.to/djbdns.html

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and that's the whole page bestie!! 🎀 the lil footer at the bottom just says qualification(5) — no source/version or date to report, so consider this your sign-off: qualification(5), that's a wrap, mwah 💋💖✨