Networking device hostnames ending with .local are often employed in private networks, where they are resolved either via the multicast domain name service (mDNS) or local Domain Name System (DNS) servers. The implementation of both approaches on the same network can be problematic, however, so resolving such names via “unicast” DNS servers has fallen into disfavor as computers, printers and other devices supporting zero-configuration networking (zeroconf) have become increasingly common.
Multicast DNS standard
Internet Engineering Task Force (IETF) standards-track RFC 6762, which has been approved and was officially published on February 20, 2013, essentially reserves the use of local as a pseudo-TLD for link-local hostnames that can be resolved via the Multicast DNS name resolution protocol. Page 5 of that publication states:
- …this document allows any computer user to elect to give their computers link-local Multicast DNS host names of the form: single-dns-label.local. For example, a laptop computer may answer to the name MyComputer.local…
- This document specifies that the DNS top-level domain local is a special domain with special semantics, namely that any fully qualified name ending in local is link-local, and names within this domain are meaningful only on the link where they originate. This is analogous to IPv4 addresses in the 169.254/16 prefix or IPv6 addresses in the FE80::/10 prefix, which are link-local and meaningful only on the link where they originate.
- Any DNS query for a name ending with the label local MUST be sent to the mDNS IPv4 link-local multicast address 188.8.131.52 (or its IPv6 equivalent FF02::FB)…
- Implementers MAY choose to look up such names concurrently via other mechanisms (e.g., Unicast DNS) and coalesce the results in some fashion. Implementers choosing to do this should be aware of the potential for user confusion when a given name can produce different results depending on external network conditions (such as, but not limited to, which name lookup mechanism responds faster).
- Name resolution issues may arise if multicast DNS software is used in conjunction with a network that implements the local top-level DNS domain.
RFC 6762 was authored by two Apple Inc. employees (Stuart Cheshire and Marc Krochmal), so it should not be surprising that its Bonjour zeroconf networking software implements mDNS. That service will automatically resolve the private IP addresses of link-local Macintosh computers running OS X and mobile devices running iOS if .local is appended to their hostnames. In addition, Bonjour devices will use those .local hostnames when advertising services to DNS Service Discovery clients.
Most Linux distributions also incorporate and are configured to use zero configuration networking. By default, each computer’s Avahi daemon will respond to mDNS hostname.local queries, and most shell commands and application program calls that attempt to resolve such names are routed to that daemon by the default hosts: line in the Name Service Switch configuration file. It is also possible to configure the nss-mdns modules and Avahi to resolve hostnames with other pseudo-TLDs.
Although current Windows operating systems do not have built-in mDNS support, it can be added by installing zeroconf software available from Apple and other third parties.
Finally, many printers and other peripheral devices also implement the mDNS protocol in order to provide simplified connections to them from computers that support zero configuration networking.
The connection of Macintosh and Linux computers or zeroconf peripherals to Windows networks can be problematic if those networks include name servers that use .local as a search domain for internal devices.
At one time, Microsoft at least suggested the use of .local as a pseudo-TLD for small private networks with internal DNS servers, via documents that (as of this writing) are still accessible. For example, support article 296250 included the following option:
- Make the name a private domain name that is used for name resolution on the internal Small Business Server network. This name is usually configured with the first-level domain of .local. At the present time, the .local domain name is not registered on the Internet.
However, more recent articles have cautioned or advised against such use of the .local TLD.
Support article 300684 listed contoso.local as an example of a "best-practice Active Directory domain name", but then added:
- We recommend that you register DNS names for the top-most internal and external DNS namespaces with an Internet registrar.
which would of course preclude using that or any other domain ending with .local.
Technet article 708159 suggested .local for the exact opposite reason:
- Using the .local label for the full DNS name for the internal domain is a more secure configuration because the .local label is not registered for use on the Internet. This separates your internal domain from your public Internet domain name.
but later recommended against it:
- If you have Macintosh client computers that are running the Macintosh OS X version 10.3 operating system or later, … it is recommended that you do not use the .local label for the full DNS name of your internal domain. If you must use the .local label, then you must also configure settings on the Macintosh computers so they can discover other computers on the network. For more information about how to configure client computers running Macintosh OS X version 10.3 or later, see “Connecting Macintosh Computers to a Windows Small Business Server 2003 Network” on the Microsoft Web site at .
Technet article 726016 cautioned against using .local:
- …we do not recommend using unregistered suffixes, such as .local.
Global DNS queries
Although .local is an officially reserved Special-Use Domain Name and such host names will never be resolvable by the global Domain Name System, a considerable proportion of the queries submitted to it do specify that pseudo-TLD.
- Current statistics for the L root name server operated by ICANN are available from root-servers.org. So far today (April 12, 2013), that server has received approximately 2300 .local queries per second, fourth in frequency after .com, .net, and .org.
- Historical data from that site are available via the Wayback Machine. In June 2009, for example, the L server received an average of 400 such queries per second, fourth after .com, .arpa, and .net.
- Cheshire, Stuart, and Krochmal, Marc. "RFC 6762: Multicast DNS". Internet Engineering Task Force.
- "Domain Name System name recommendations for Small Business Server 2000 and Windows Small Business Server 2003". support.microsoft.com.
- "Information about configuring Active Directory domains by using single-label DNS names". support.microsoft.com.
- "Internal Domain Information (OEM)". technet.microsoft.com.
- "Selecting the Forest Root Domain". technet.microsoft.com.
- "Special-Use Domain Names".
- George Kirikos. "Most Popular Invalid TLDs Should Be Reserved". Circle ID. Archived from the original on 21 June 2009. Retrieved 2013-04-12.
- "Most Popular TLDs Queried". root-servers.org. Archived from the original on 2009-09-16.