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English | 中文版

MDNS Protocol

[TOC]

Overview

MDNS is used for local network service and host discovery. It is based on UDP, with domain names ending in .local, using multicast: IPv4 address 224.0.0.251 or IPv6 address ff02::fb, port 5353.

Every host that joins the LAN, if mDNS service is enabled, will multicast a message containing its domain name and IP to all hosts in the LAN. Other hosts with mDNS service will respond.

When a device needs a service, it uses mDNS to query the IP address corresponding to a domain name. The device receiving the query will also respond via multicast; other hosts can also receive this response, and will record the domain name, IP, TTL, etc., updating their cache.

RR Types

Value RR Type Reference Description and Purpose
1 A [RFC1035] IPv4 address record (32-bit IPv4 address)
2 NS [RFC1035] Name server; provides the name of an authoritative name server for the zone
5 CNAME [RFC1035] Canonical name; maps one name to another (provides a form of alias)
6 SOA [RFC1035] Start of authority; provides authoritative info for the zone (name server, contact email, serial, timers)
12 PTR [RFC1035] Pointer; provides address-to-(canonical) name mapping; used for reverse lookups in in-addr.arpa and ip6.arpa domains for IPv4 and IPv6
15 MX [RFC1035] Mail exchanger; provides the name of a mail host for a domain
16 TXT [RFC1035]
[RFC1464]		 Text; provides various info (e.g., used with SPF to identify authorized mail servers)
28 AAAA [RFC3596] IPv6 address record (128-bit IPv6 address)
33 SRV [RFC2782] Server selection; transport endpoint for generic services
35 NAPTR [RFC3403] Naming authority pointer; supports alternate namespaces
41 OPT [RFC2671] Pseudo-RR; supports larger datagrams, labels, return codes in ENDS0
251 IXFR [RFC1995] Incremental zone transfer
252 AXFR [RFC1035]
[RFC5936]		 Full zone transfer; carried over TCP
255 (ANY) [RFC1035] Request any record

Structure

mDNS is based on DNS, and the protocol structure includes two parts: header and message;

dns protocol structure

  • Header

    mdns header

    The header is 12 bytes long. Each field is explained below:

    • ID: Identifier, 16 bits
    • QR: Query/Response flag, 1 bit
      • 0: Query
      • 1: Response
    • OPCODE: Query type, 4 bits
      • 0: Standard query
      • 1: Inverse query
      • 2: Status query
      • 3: Reserved
      • 4: Notify
      • 5: Update
      • 6~15: Reserved
    • AR: Set to 1 only in responses, 1 bit
    • TC: Truncation flag, 1 bit
    • RD: Optional, recursion desired, 1 bit
    • RA: Recursion available, 1 bit
    • Z: Reserved, must be 0, 3 bits
    • AD: Authentic data, 1 bit
    • CD: Checking disabled, 1 bit
    • RCODE: Set only in DNS responses, 4 bits
      • 0: No error
      • 1: Format error
      • 2: Server failure
      • 3: Name error
      • 4: Not implemented
      • 5: Refused
      • 6~15: Reserved
    • QDCOUNT: Number of questions, 16 bits
    • ANCOUNT: Number of answers, 16 bits
    • NSCOUNT: Number of authority records, 16 bits
    • ARCOUNT: Number of additional records, 16 bits

  • Message

    • Question section
Variable Description Length (bits)
QNAME Queried domain name variable
QTYPE Query resource record type 16
QCLASS Query class (1=IN, etc.) 16

Example:

mdns query request

- Answer section
Variable Description Length (bits)
Name Queried domain name variable
Type Resource record type (RR) 16
Class Query class (1=IN, etc.) 16
Time to live (TTL) RR cache time in seconds 32
RDLength Length of DATA field 16
RDATA
Address		 Data field
Address		 variable

Example:

mdns query response

- Authority section
Variable Description Length (bits)
Name Domain:
Service
Protocol
Name		 variable
Type Resource record type (RR) 16
Class Query class (1=IN, etc.) 16
Time to live (TTL) RR cache time in seconds 32
Data length Length of DATA field 16
Data:
Priority
Weight
Port
Target		 DATA field:
Priority
Weight
Port
Target		 variable
16
16
16
variable

authority section example

  • Additional section

Structure is the same as the answer section.

additional section example

MDNS Process

  • Name discovery:
Querying host->Group 224.0.0.251: Send mDNS multicast message, providing its own name
Group 224.0.0.251->Queried host: Multicast message
Queried host->Querying host: Send mDNS response, including its domain name
  • Name query:
Querying host->Group 224.0.0.251: Send mDNS multicast query for domain ending with .local
Group 224.0.0.251->Queried host: Multicast message
Queried host->Group 224.0.0.251: Send mDNS response, including its hostname and IP address
Group 224.0.0.251->Querying host: Receive result
  • Register (announce) service:

Related Commands

dns-sd

  • -A Test multicast DNS registration service, and test adding, updating, and deleting DNS records using multicast DNS.
  • -U Test multicast DNS registration service, and test updating DNS TXT records for services registered with multicast DNS.
  • -N Test adding large NULL records for services registered with multicast DNS.
  • -T Test adding large TXT records for services registered with multicast DNS.
  • -M Test creating registrations with multiple TXT records.
  • -I Test registration and immediate TXT record update.
  • -R Register (announce) a service with the given name and type in the specified domain, listening on the specified port of the current machine. Format: dns-sd -R name type domain port <TXT>...
    • name: Service name, valid unicode string (including dots, spaces, slashes, colons, etc.), max length 63 UTF-8 bytes.
    • type: Must be in the format application-protocol._tcp or application-protocol._udp.
    • domain: Domain to register the service in, ending with .local
    • port: Port number the service listens on
    • : Text content, key-value type Example:
     dns-sd -R "my test" _http._tcp han.local 10086 path=/main.html
  • -B Browse for instances of type services in the domain. Example: 
     dns-sd -B _http.tcp
  • ...

References