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, resolving such names via unicast DNS servers has fallen into disfavor as computers, printers and other devices implementing zero-configuration networking (zeroconf) have become increasingly common.
Multicast DNS (mDNS) standard
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The Internet Engineering Task Force (IETF) standards-track RFC 6762 (February 20, 2013) reserves the use of the domain name label local as a pseudo-top-level domain 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 224.0.0.251, or its IPv6 equivalent FF02::FB. Domain name ending in local, may be resolved concurrently via other mechanisms, e.g., 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 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 Windows operating systems often do not have built-in mDNS function, it can be added by installing zeroconf software available from Apple and other third parties, and it is beginning to be added in Windows 10.
Finally, many printers and other peripheral devices also implement the mDNS protocol in order to provide simplified connections to them from computers that implement zero configuration networking.
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:
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.
Microsoft 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 [1].
TechNet article 726016 cautioned against using .local:
…we do not recommend using unregistered suffixes, such as .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
Computers running modern Linux distributions and using the common nsswitch system may have /etc/nsswitch.conf configuration files in which mDNS name resolution was added via the mdns5_minimal plugin to nsswitch. In this configuration, where mdns5_minimal precedes the standard dns option, which uses /etc/resolv.conf, the mDNS resolution will block subsequent DNS resolution on the local network.
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hosts: files mdns5_minimal [NOTFOUND=return] dns
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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 legit .local requests for configured DNS servers.
Global DNS queries
As local is an officially reserved special-use domain name (RFC 6762 of February 2013) host names with this top level label are not resolvable in the global Domain Name System. However, a considerable proportion of the queries submitted to it do specify the domain. Statistics for the L root name server operated by ICANN show that on August 14, 2015, the server received approximately 1331 local queries per second, third in frequency after .com (4355 queries/s), and .net (2481 queries/s), or sixth including the invalid gTLDs www (First, with 9416 queries/s), html (third, with 2727 queries/s), and home (fifth, with 1692 queries/s). On April 12, 2013, the server received approximately 2300 local queries/s, fourth in frequency after .com, .net, and .org.
Historical data from the site are available via the Wayback Machine. In June 2009, for example, the L server received an average of 400 such queries/s, fourth after .com, .arpa, and .net.
See also
- .localhost
- Multicast DNS
- Zero-configuration networking
References
