Multicast DNS (mDNS) is a

computer networking protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any variation of a physical quantity. The protocol defines the rules, syntax, semantics (computer science), sem ...

that resolves hostnames to IP addresses within small networks that do not include a local

name server A name server is a computer application that implements a network service for providing responses to queries against a directory service. It translates an often humanly meaningful, text-based identifier to a system-internal, often numeric identi ...

. It is a zero-configuration service, using essentially the same programming interfaces, packet formats and operating semantics as unicast

Domain Name System The Domain Name System (DNS) is a hierarchical and distributed name service that provides a naming system for computers, services, and other resources on the Internet or other Internet Protocol (IP) networks. It associates various information ...

(DNS). It was designed to work as either a stand-alone protocol or compatible 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 Network packet, packets) to other hosts on an Internet Protoco ...

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

History

Multicast DNS was first proposed by Bill Woodcock and Bill Manning in the

IETF The Internet Engineering Task Force (IETF) is a standards organization for the Internet standard, Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster ...

in 2000, and was eventually published as standards-track by Stuart Cheshire and Marc Krochmal thirteen years later.

Protocol overview

When an mDNS client needs to resolve a hostname, it sends an

IP multicast IP multicast is a method of sending Internet Protocol (IP) datagrams to a group of interested receivers in a single transmission. It is the IP-specific form of multicast and is used for streaming media and other network applications. It uses speci ...

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 (digital), counter or timestamp attached to or embedded in the data. Once the prescribed ev ...

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

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 Node (networking), network node participating in a computer network using IPv6. IP addresses are ...

* UDP port 5353 * When using

Ethernet frame In computer networking, an Ethernet frame is a data link layer protocol data unit and uses the underlying Ethernet physical layer transport mechanisms. In other words, a data unit on an Ethernet link transports an Ethernet frame as its paylo ...

s, the standard IP multicast MAC address (for

IPv4 Internet Protocol version 4 (IPv4) is the first version of the Internet Protocol (IP) as a standalone specification. It is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. ...

) or (for

IPv6 Internet Protocol version 6 (IPv6) is the most recent version of the Internet Protocol (IP), the communication protocol, communications protocol that provides an identification and location system for computers on networks and routes traffic ...

) 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 called 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 the top-level ...

(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 Service Discovery (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