.local

The domain name .local is a special-use domain name reserved by the Internet Engineering Task Force (IETF) so that it may not be installed as a top-level domain in the Domain Name System (DNS) of the Internet. As such it is similar to the other special domain names, such as .localhost. However, ''.local'' has since been designated for use in link-local networking, in applications of multicast DNS (mDNS) and zero-configuration networking (zeroconf) so that DNS service may be established without local installations of conventional DNS infrastructure on local area networks.

Multicast DNS standard

The Internet Engineering Task Force (IETF) reserves the use of the domain name label ''.local'' as a special-use domain name for hostnames in local area networks that can be resolved via the Multicast DNS name resolution protocol. Any DNS query for a name ending with the label ''local'' must be sent to the mDNS IPv4 link-local multicast address , or its IPv6 equivalent . A domain name ending in ''.local'' may be resolved concurrently via other mechanisms, for example, unicast DNS.

mDNS implementations

RFC 6762 was authored by Apple Inc. employees Stuart Cheshire and Marc Krochmal, and Apple's Bonjour zeroconf networking software implements mDNS. That service will automatically resolve the private IP addresses of link-local Macintosh computers running MacOS 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 zeroconf. 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.

Windows has some support for mDNS, which can be improved 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 implement zeroconf.

Microsoft recommendations

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:

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:

which would of course preclude using that or any other domain ending with ''.local''.

Microsoft TechNet article 708159 suggested ''.local'' for the exact opposite reason:

but later recommended against it:

TechNet article 726016 cautioned against using ''.local'':

By default, a freshly installed Windows Server 2016 Essentials also adds .local as the default dns-prefix, when a user doesn't select the advanced option. Thus resulting in a domain with .local extension.

Linux

Linux distributions use the Name Service Switch configuration file /etc/nsswitch.conf in which mDNS name resolution was added via the mdns4_minimal plugin to nsswitch. In this configuration, where mdns4_minimal precedes the standard dns option, which uses /etc/resolv.conf, the mDNS resolution will block subsequent DNS resolution on the local network.

hosts:    files mdns_minimal  OTFOUND=returndns # for ipv4 and ipv6

or

hosts:    files mdns4_minimal  OTFOUND=returndns # for ipv4 only

This is a configuration choice made by distributions such as Ubuntu/Debian and SuSE and Red Hat, each of which have their own package configuration script that will install the mdns_minimal module as above.

In this way ''.local'' requests are being prevented from leaking to the internet, but also block legitimate ''.local'' requests for configured DNS servers.

Global DNS queries

As ''local'' is an officially reserved special-use domain name host names with this top level label are not resolvable in the global Domain Name System.

References

{{DEFAULTSORT:LOCAL
Domain Name System
Pseudo-top-level domains

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