Search domain explained

A search domain is a domain used as part of a domain search list. The search list, as well as the local domain name, is used by a resolver to create a fully qualified domain name (FQDN) from a relative name.^[1] For this purpose, the local domain name functions as a single-item search list.

IPv4

In an IPv4 environment, search domains are often set via DHCPv4, along with the local domain name. The domain search list is configured by the Domain Search Option (DHCPv4 option number 119), while the local domain name is configured by Domain Name (DHCPv4 option number 15).^{[2] [3]}

The Windows operating system, however, understands the Domain Search Option only since Windows 10 April 2018 Update.^[4] As a workaround for previous versions, many network administrators use Group Policy Objects to set the domain search list for Windows machines.^[5]

IPv6

In an IPv6 environment, the domain search list is called a DNS Search List (DNSSL) and can be configured by Router Advertisement and DHCP.^{[6] [7]}

Manually configuring domain search lists

In CentOS Linux search domain can be defined by editing the ifcfg file corresponding to the network.^[8] In Mac OS X the setting is located under the DNS tab, next to DNS server settings. A similar setting in Microsoft Windows is the Connection-specific DNS Suffix.

Functionality

When looking up a bare name in DNS, the network stack will add the search domains to it to form fully qualified domain names, and look up those as well.^[9] For example, if the domain search list contains "wikipedia.org", typing "en" in the browser will direct the user to "en.wikipedia.org". Some ISPs add their own search domains via DHCP settings, similar to how they add DNS servers and other networking information; if this is undesired, the user can change this setting to ".local" or a Top level domain.

See also

• Domain Name System
• DHCP Options

Notes and References

1. Mockapetris. P.V.. Domain names - concepts and facilities. IETF Documents. IETF. 24 July 2017. en. HTML. 10.17487/RFC1034. November 1987. Relative names are either taken relative to a well known origin, or to a list of domains used as a search list. Relative names appear mostly at the user interface, where their interpretation varies from implementation to implementation, and in master files, where they are relative to a single origin domain name. The most common interpretation uses the root "." as either the single origin or as one of the members of the search list, so a multi-label relative name is often one where the trailing dot has been omitted to save typing..
2. 10.17487/RFC3397. Bernard. Aboba. Stuart. Cheshire. RFC 3397 - Dynamic Host Configuration Protocol (DHCP) Domain Search Option. IETF Documents. IETF. November 2002. 22 July 2017. en. HTML.
3. Alexander. Steve. Droms. Ralph. DHCP Options and BOOTP Vendor Extensions. IETF Documents. IETF. 24 July 2017. section 3.17. 10. en. HTML. 10.17487/RFC2132. March 1997. 3.17. Domain Name
   This option specifies the domain name that client should use when resolving hostnames via the Domain Name System.
   The code for this option is 15.. free.
4. Web site: What's New in DHCP . Microsoft Learn . 2024-01-13 . 2021-07-29.
5. Web site: Zuba. Matt. Windows Server 2008 RC2 DHCP Server Option 119 - Matt Zuba. Matt Zuba. 22 July 2017. 16 March 2011.
6. Jeong. Jaehoon. Park. Soohong. Beloeil. Luc. Madanapalli. Syam. RFC 8106 - IPv6 Router Advertisement Options for DNS Configuration. IETF Documents. 2017 . IETF. 4 December 2019. en. HTML. 10.17487/RFC8106.
7. Droms. Ralph. R. . Droms . RFC 3646 - DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6). IETF Documents. 2003 . IETF. 24 July 2017. en. HTML. 10.17487/RFC3646. free.
8. Web site: How can I add additional search domains to the resolv.conf created by dhclient in CentOS. Super User. 2016-12-18.
9. Web site: OS X Mountain Lion: Edit DNS and search domain settings. Apple Support. 22 July 2017. 4 September 2015.