ドメイン名前解決

一般に、ドメイン名は IP アドレスを表し、DNS（Domain Name System）で関連付けられています。ここでは、ドメイン名前解決の設定方法とドメイン名の解決方法について説明します。

Name Service Switch

この記事またはセクションは加筆を必要としています。

理由: Mention nss-mdns, nss-tls-git^AUR and others. (議論: トーク:ドメイン名前解決#)

The Name Service Switch (NSS) facility is part of the GNU C Library (glibc) and backs the getaddrinfo(3) API, used to resolve domain names. NSS allows system databases to be provided by separate services, whose search order can be configured by the administrator in nsswitch.conf(5). The database responsible for domain name resolution is the hosts database, for which glibc offers the following services:

files: reads the /etc/hosts file, see hosts(5)
dns: the glibc resolver which reads /etc/resolv.conf, see resolv.conf(5)

systemd provides three NSS services for hostname resolution:

nss-resolve(8) — a caching DNS stub resolver, described in systemd-resolved
nss-myhostname(8) — provides local hostname resolution without having to edit /etc/hosts, described in Network configuration#Local hostname resolution
nss-mymachines(8) — provides hostname resolution for the names of local systemd-machined(8) containers

Resolve a domain name using NSS

NSS databases can be queried with getent(1). A domain name can be resolved through NSS using:

$ getent hosts domain_name

ノート: While most programs resolve domain names using NSS, some may read /etc/resolv.conf and/or /etc/hosts directly. See Network configuration#Local hostname resolution.

Glibc resolver

The glibc resolver reads /etc/resolv.conf for every resolution to determine the nameservers and options to use.

resolv.conf(5) lists nameservers together with some configuration options. Nameservers listed first are tried first, up to three nameservers may be listed. Lines starting with a number sign (#) are ignored.

ノート: The glibc resolver does not cache queries. To improve query lookup time you can set up a caching resolver. Glibc resolver also can not validate DNSSEC. A DNSSEC capable validator resolver is required for that one. See #DNS servers for more information.

Overwriting of /etc/resolv.conf

Network managers tend to overwrite /etc/resolv.conf, for specifics see the corresponding section:

To prevent programs from overwriting /etc/resolv.conf, it is also possible to write-protect it by setting the immutable file attribute:

# chattr +i /etc/resolv.conf

ヒント: If you want multiple processes to write to /etc/resolv.conf, you can use resolvconf.

Limit lookup time

If you are confronted with a very long hostname lookup (may it be in pacman or while browsing), it often helps to define a small timeout after which an alternative nameserver is used. To do so, put the following in /etc/resolv.conf.

options timeout:1

Hostname lookup delayed with IPv6

If you experience a 5 second delay when resolving hostnames it might be due to a DNS-server/Firewall misbehaving and only giving one reply to a parallel A and AAAA request.[1] You can fix that by setting the following option in /etc/resolv.conf:

options single-request

Local domain names

To be able to use the hostname of local machine names without the fully qualified domain name, add a line to /etc/resolv.conf with the local domain such as:

domain example.org

That way you can refer to local hosts such as mainmachine1.example.org as simply mainmachine1 when using the ssh command, but the drill command still requires the fully qualified domain names in order to perform lookups.

Lookup utilities

To query specific DNS servers and DNS/DNSSEC records you can use dedicated DNS lookup utilities. These tools implement DNS themselves and do not use NSS.

ldns provides drill(1), which is a tool designed to retrieve information out of the DNS.

For example, to query a specific nameserver with drill for the TXT records of a domain:

$ drill @nameserver TXT domain

Unless a DNS server is specified, drill will use the nameservers defined in /etc/resolv.conf.

ヒント: Some DNS servers ship with their own DNS lookup utilities. E.g.

knot provides khost(1) and kdig(1).
Unbound has unbound-host(1).
BIND has dig(1), host(1), nslookup(1) and a bunch of dnssec- tools.

Resolver のパフォーマンス

Glibc resolver は問い合わせをキャッシュしません。ローカルキャッシュを実装するには、 systemd-resolved を使用するか、ローカルキャッシュ DNS server を設定し、 127.0.0.1 と ::1 を /etc/resolv.conf または /etc/resolvconf (openresolv を使用する場合) でネームサーバとして使用します。

ヒント:

drill または dig は、クエリー時間を報告します。
通常、ルータは自身のキャッシングリゾルバをネットワークの DNS サーバとして設定し、ネットワーク全体に DNS キャッシュを提供します。
次の DNS サーバへの切り替えに時間がかかりすぎる場合は、ドメイン名前解決#timeout オプションを使ってホスト名の検索時間を減らすを試してください。

ヒントとテクニック

ローカルドメイン名

完全修飾ドメイン名を使わずにローカルマシンのホストネームを使いたい場合、以下のように resolv.conf にローカルドメインを追加してください:

domain localdomain.com

上記の設定によって ssh コマンドを使用する際に mainmachine1.localdomain.com とローカルホストを指定するところを mainmachine1 だけで参照できるようになります。ただし、drill コマンドを実行するときはルックアップを実行するため完全修飾ドメイン名が必要になります。