「ドメイン名前解決」の版間の差分

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* [http://linux-ip.net/pages/diagrams.html#domain-name-system-overview Domain name system overview] - Diagram about DNS
 
* [http://linux-ip.net/pages/diagrams.html#domain-name-system-overview Domain name system overview] - Diagram about DNS
 
* [[Alternative DNS services]]
 
* [[Alternative DNS services]]
 
== 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 {{ic|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 {{ic|mainmachine1.localdomain.com}} とローカルホストを指定するところを {{ic|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。
 

2022年11月11日 (金) 17:16時点における版

関連記事

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-gitAUR 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:

systemd provides three NSS services for hostname resolution:

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.

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::1/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:


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-gitAUR
  • opennic-up — Automates the renewal of the DNS servers with the most responsive OpenNIC servers
https://github.com/kewlfft/opennic-up || opennic-upAUR
  • nextdns — A DNS-over-HTTPS CLI client for NextDNS
https://github.com/nextdns/nextdns || nextdnsAUR

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 servers

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.

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: Fill in the unknowns. (議論: トーク:ドメイン名前解決#)
Name Package Capabilities resolvconf Supported protocols
Authoritative Recursive Cache Validates
DNSSEC
DNS DNSCrypt DNS
over TLS
DNS
over HTTPS
BIND bind Yes Yes Yes Yes Yes Yes No Server1 Server
CoreDNS corednsAUR or coredns-binAUR ? ? ? ? ? ? ? Yes ?
Deadwood (MaraDNS recursor) maradnsAUR 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-gitAUR 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
  1. 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.
  2. From Wikipedia: dnsmasq has limited authoritative support, intended for internal network use rather than public Internet use.
  3. 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 maradnsAUR 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.

See also