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。

1 Name Service Switch
- 1.1 NSS を使用してドメイン名を解決する
2 Glibc resolver
3 Lookup utilities
4 Resolver のパフォーマンス
5 Privacy and security
- 5.1 Application-level DNS
- 5.2 Oblivious DNS
6 Third-party DNS services
7 DNS サーバ
- 7.1 Authoritative-only servers
- 7.2 Conditional forwarding
8 See also

Name Service Switch

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

Name Service Switch (NSS) 機能は GNU C ライブラリ (glibc) の一部であり、getaddrinfo(3) API をサポートしており、ドメイン名を解決するために使用されます。NSS では、システムデータベースを別々のサービスで提供することができ、その検索順序は管理者が nsswitch.conf(5) で設定することができます。ドメイン名の解決を行うデータベースは hosts データベースであり、glibc は以下のサービスを提供しています。

files: /etc/hosts ファイルを読み込む。hosts(5) を参照してください。
dns :/etc/resolv.conf を読み込む glibc resolver。 resolv.conf(5) を参照してください。

systemd はホスト名解決のために 3 つの NSS サービスを提供します。

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 ネットワーク設定#ローカルのホストネーム解決
nss-mymachines(8) — provides hostname resolution for the names of local systemd-machined(8) containers

NSS を使用してドメイン名を解決する

NSS データベースは getent(1) でクエリできます。ドメイン名は、NSS を介して解決できます。

$ getent hosts domain_name

ノート: ほとんどのプログラムは NSS を使用してドメイン名を解決しますが、/etc/resolv.conf や /etc/hosts を直接読み取るプログラムもあります。ネットワーク設定#localhost がネットワーク上で解決されるを参照してください。

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。

Privacy and security

The DNS protocol is unencrypted and does not account for confidentiality, integrity or authentication, so if you use an untrusted network or a malicious ISP, your DNS queries can be eavesdropped and the responses manipulated. Furthermore, DNS servers can conduct DNS hijacking.

You need to trust your DNS server to treat your queries confidentially. DNS servers are provided by ISPs and third-parties. Alternatively you can run your own recursive name server, which however takes more effort. If you use a DHCP client in untrusted networks, be sure to set static name servers to avoid using and being subject to arbitrary DNS servers. To secure your communication with a remote DNS server you can use an encrypted protocol, like DNS over TLS (RFC 7858), DNS over HTTPS (RFC 8484), or DNSCrypt, provided that both the upstream server and your resolver support the protocol. An alternative can be a dedicated software to encrypt and decrypt the communication, such as stunnel. To verify that responses are actually from authoritative name servers, you can validate DNSSEC, provided that both the upstream server(s) and your resolver support it.

Application-level DNS

Be aware that some client software, such as major web browsers[2][3], are starting to implement DNS over HTTPS. While the encryption of queries may often be seen as a bonus, it also means the software sidetracks queries around the system resolver configuration.[4]

Firefox provides configuration options to enable or disable DNS over HTTPS and select a DNS server.

Chromium will examine the user's system resolver and enable DNS over HTTPS if the system resolver addresses are known to also provide DNS over HTTPS. See this blog post for more information and how DNS over HTTPS can be disabled.

Mozilla has proposed universally disabling application-level DNS if the system resolver cannot resolve the domain use-application-dns.net. Currently, this is only implemented in Firefox.

Oblivious DNS

Oblivious DNS is a system which addresses a number of DNS privacy concerns. See Cloudflare's article for more information.

Third-party DNS services

ノート: Before using a third-party DNS service, check its privacy policy for information on how user data is handled. User data has value and can be sold to other parties.

There are various third-party DNS services available, some of which also have dedicated software:

cloudflared — A DNS client for Cloudflare DNS over HTTPS

https://developers.cloudflare.com/1.1.1.1/dns-over-https/cloudflared-proxy || cloudflared

dingo — A DNS client for Google DNS over HTTPS

https://github.com/pforemski/dingo || dingo-git^AUR

opennic-up — Automates the renewal of the DNS servers with the most responsive OpenNIC servers

https://github.com/kewlfft/opennic-up || opennic-up^AUR

nextdns — A DNS-over-HTTPS CLI client for NextDNS

https://github.com/nextdns/nextdns || nextdns^AUR

You can use dnsperftest to test the performance of the most popular DNS resolvers from your location. dnsperf.com provides global benchmarks between providers.

DNS サーバ

DNS servers can be authoritative and recursive. If they are neither, they are called stub resolvers and simply forward all queries to another recursive name server. Stub resolvers are typically used to introduce DNS caching on the local host or network. Note that the same can also be achieved with a fully-fledged name server. This section compares the available DNS servers, for a more detailed comparison, refer to Wikipedia:Comparison of DNS server software.

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Name	Package	Capabilities				resolvconf	Supported protocols
Name	Package	Authoritative	Recursive	Cache	Validates DNSSEC	resolvconf	DNS	DNSCrypt	DNS over TLS	DNS over HTTPS
BIND	bind	Yes	Yes	Yes	Yes	Yes	Yes	No	Server¹	Server
CoreDNS	coredns^AUR or coredns-bin^AUR	?	?	?	?	?	?	?	Yes	?
Deadwood (MaraDNS recursor)	maradns^AUR	No	Yes	Yes	No	No	Yes	No	No	No
dnscrypt-proxy	dnscrypt-proxy	No	No	Yes	No	No	Server	Resolver	No	Yes
dnsmasq	dnsmasq	Partial²	No	Yes	Yes	Yes	Yes	No	No	No
Knot Resolver	knot-resolver	No	Yes	Yes	Yes	No	Yes	No	Yes	Server
pdnsd	pdnsd	Yes	Yes	Permanent	No	Yes	Yes	No	No	No
PowerDNS Recursor	powerdns-recursor	No	Yes	Yes	Yes	Yes	Yes	No	Partial	No
Rescached	rescached-git^AUR	No	No	Yes	No	Yes	Yes	No	Yes	Yes
SmartDNS	smartdns	No	No	Yes	No	?	Yes	No	Resolver	Resolver
Stubby	stubby	No	No	No	Yes	No	Server	No	Resolver	No
systemd-resolved	systemd	No	No	Yes	Yes	Yes	Resolver and limited server	No	Resolver	No
Unbound	unbound	Partial	Yes	Yes³	Yes	Yes	Yes	Server	Yes	Server

BIND can serve both DNS over TLS and DNS over HTTPS (see tls{} and listen-on), but cannot yet forward queries to a DNS over TLS/DNS over HTTPS upstream. The dig tool can make queries over DNS over TLS and DNS over HTTPS (using +tls and +https options), though without any certificate checks.
From Wikipedia: dnsmasq has limited authoritative support, intended for internal network use rather than public Internet use.
The Redis backend can be used to provide a persistent cache for Unbound.

Authoritative-only servers

Name	Package	DNSSEC	Geographic balancing
gdnsd	gdnsd	No	Yes
Knot DNS	knot	Yes	Yes
MaraDNS	maradns^AUR	No	?
NSD	nsd	No	No
PowerDNS	powerdns	Yes	Yes

Conditional forwarding

It is possible to use specific DNS resolvers when querying specific domain names. This is particularly useful when connecting to a VPN, so that queries to the VPN network are resolved by the VPN's DNS, while queries to the internet will still be resolved by your standard DNS resolver. It can also be used on local networks.

To implement it, you need to use a local resolver because glibc does not support it.

In a dynamic environment (laptops and to some extents desktops), you need to configure your resolver based on the network(s) you are connected to. The best way to do that is to use openresolv because it supports multiple subscribers. Some network managers support it, either through openresolv, or by configuring the resolver directly. NetworkManager supports conditional forwarding without openresolv.

ノート: Although you could use other conditions for forwarding (for example, source IP address), "conditional forwarding" appears to be the name used for the "domain queried" condition.