A search domain is a domain used as part of a domain search list. The domain 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. For this purpose, the local domain name functions as a single-item search list.
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).
The Windows operating system, however, does not understand the Domain Search Option; as a workaround, many network administrators use Group Policy Objects to set the domain search list for Windows machines.
In an IPv6 environment, the domain search list is called a DNS Search List (DNSSL) and can be configured by Router Advertisement and DHCP.
Manually configuring domain search lists
In Linux it can be defined by editing the ifcfg file corresponding to the network. 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.
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. 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".
- Mockapetris, P.V. (November 1987). "Domain names - concepts and facilities" (HTML). IETF Documents. IETF. doi:10.17487/RFC1034. Retrieved 24 July 2017.
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.
- Bernard, Aboba; Stuart, Cheshire (November 2002). "RFC 3397 - Dynamic Host Configuration Protocol (DHCP) Domain Search Option" (HTML). IETF Documents. IETF. doi:10.17487/RFC3397. Retrieved 22 July 2017.
- Alexander, Steve; Droms, Ralph (March 1997). "DHCP Options and BOOTP Vendor Extensions" (HTML). IETF Documents. section 3.17: IETF. p. 10. doi:10.17487/RFC2132. Retrieved 24 July 2017.
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.
- Zuba, Matt (16 March 2011). "Windows Server 2008 RC2 DHCP Server Option 119 - Matt Zuba". Matt Zuba. Retrieved 22 July 2017.
- Droms, Ralph. "RFC 3646 - DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)" (HTML). IETF Documents. IETF. doi:10.17487/RFC3646. Retrieved 24 July 2017.
- "How can I add additional search domains to the resolv.conf created by dhclient in CentOS". Super User. Retrieved 2016-12-18.
- "OS X Mountain Lion: Edit DNS and search domain settings". Apple Support. 4 September 2015. Retrieved 22 July 2017.
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