In computer networking, the multicast DNS (mDNS) protocol resolves

hostname In computer networking, a hostname (archaically nodename) is a label that is assigned to a device connected to a computer network and that is used to identify the device in various forms of electronic communication, such as the World Wide Web. Hos ...

s to IP addresses within small networks that do not include a local name server. It is a zero-configuration service, using essentially the same programming interfaces, packet formats and operating semantics as unicast Domain Name Service (DNS). It was designed to work as either a stand-alone protocol or compatibly with standard DNS servers. It uses IP multicast

User Datagram Protocol In computer networking, the User Datagram Protocol (UDP) is one of the core communication protocols of the Internet protocol suite used to send messages (transported as datagrams in packets) to other hosts on an Internet Protocol (IP) network. ...

(UDP) packets, and is implemented by the Apple Bonjour and open source Avahi software packages, included in most Linux distributions. Although the Windows 10 implementation was limited to discovering networked printers, subsequent releases resolved hostnames as well. mDNS can work in conjunction with

DNS Service Discovery Zero-configuration networking (zeroconf) is a set of technologies that automatically creates a usable computer network based on the Internet Protocol Suite (TCP/IP) when computers or network peripherals are interconnected. It does not require man ...

(DNS-SD), a companion zero-configuration networking technique specified separately in RFC 6763.

History

Multicast DNS was first proposed by Bill Woodcock and Bill Manning in the IETF in 2000, and was eventually published as standards-track IETF RFC 6762 by

Stuart Cheshire Stuart Cheshire is a Distinguished Engineer, Scientist and Technologist (DEST) at Apple. He pioneered Zeroconf networking while employed at Apple. Zeroconf was originally released by Apple as Rendezvous, but later renamed Bonjour. Subsequently, h ...

and Marc Krochmal thirteen years later.

Protocol overview

When an mDNS client needs to resolve a hostname, it sends an IP multicast query message that asks the host having that name to identify itself. That target machine then multicasts a message that includes its IP address. All machines in that subnet can then use that information to update their mDNS caches. Any host can relinquish its claim to a name by sending a response packet with a

time to live Time to live (TTL) or hop limit is a mechanism which limits the lifespan or lifetime of data in a computer or network. TTL may be implemented as a counter or timestamp attached to or embedded in the data. Once the prescribed event count or timesp ...

(TTL) equal to zero. By default, mDNS exclusively resolves hostnames ending with the

.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 ...

top-level domain. This can cause problems if .local includes hosts that do not implement mDNS but that can be found via a conventional unicast DNS server. Resolving such conflicts requires network-configuration changes that mDNS was designed to avoid.

Packet structure

An mDNS message is a multicast UDP packet sent using the following addressing: *

IPv4 address Internet Protocol version 4 (IPv4) is the fourth version of the Internet Protocol (IP). It is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. IPv4 was the first version de ...

IPv6 address An Internet Protocol Version 6 address (IPv6 address) is a numeric label that is used to identify and locate a network interface of a computer or a network node participating in a computer network using IPv6. IP addresses are included in the pac ...

* UDP port 5353 * When using Ethernet frames, the standard IP multicast MAC address (for IPv4) or (for IPv6) The payload structure is based on the unicast DNS packet format, consisting of two parts—the header and the data.. The header is identical to that found in unicast DNS, as are the sub-sections in the data part: queries, answers, authoritative-nameservers, and additional records. The number of records in each sub-section matches the value of the corresponding *COUNT field in the header.

Queries

The wire format for records in the query section is slightly modified from that in unicast DNS, adding the single-bit UNICAST-RESPONSE field. As in unicast DNS, the QNAME field consists of a series of length/value sub-fields called "labels". Each label represents one of the dot-separated substrings in a

fully qualified domain name A fully qualified domain name (FQDN), sometimes also referred to as an ''absolute domain name'', is a domain name that specifies its exact location in the tree hierarchy of the Domain Name System (DNS). It specifies all domain levels, including th ...

(FQDN). The list is terminated by either a single null-byte representing the "root" of the DNS, or by a byte with the two high-order bits set (value 192) to signal an indirect pointer to another location in the message. This is known as name compression in RFC-6762. The UNICAST-RESPONSE field is used to minimize unnecessary broadcasts on the network: if the bit is set, responders SHOULD send a directed-unicast response directly to the inquiring node rather than broadcasting the response to the entire network. The QCLASS field is identical to that found in unicast DNS.

Resource Records

All records in the answers, authoritative-nameservers, and additional records sections have the same format and are collectively known as Resource Records (RR). Resource Records in mDNS also have a slightly modified general format compared to unicast DNS: The CACHE-FLUSH bit is used to instruct neighbor nodes that the record should overwrite, rather than be appended onto, any existing cached entries for this RRNAME and RRTYPE. The formats of the RDATA fields are the same as those found in unicast DNS. However,

(DNS-SD), the most common use-case for mDNS, specifies slight modifications to some of their formats (notably TXT records).

References

{{Reflist

External links

Multicast DNS
- information site maintained by mDNS designer, Stuart Cheshire
LLMNR, Multicast DNS and names on your LAN
Domain Name System Application layer protocols