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Multicast DNS (mDNS)

Local peer discovery with zero configuration using multicast DNS.

Lifecycle Stage Maturity Status Latest Revision
1A Working Draft Active r2, 2021-10-12

Authors: @richardschneider

Interest Group: @yusefnapora, @raulk, @daviddias, @jacobheun

See the lifecycle document for context about the maturity level and spec status.

Table of Contents

Overview

The goal is to allow peers to discover each other when on the same local network with zero configuration. mDNS uses a multicast system of DNS records; this allows all peers on the local network to see all query responses.

Conceptually, it is very simple. When a peer starts (or detects a network change), it sends a query for all peers. As responses come in, the peer adds the other peers' information into its local database of peers.

Definitions

  • service-name is the DNS Service Discovery (DNS-SD) service name for all peers. It is defined as _p2p._udp.local.

  • host-name is the fully qualified name of the peer. It is derived from the peer's name and p2p.local.

  • peer-name is the case-insensitive unique identifier of the peer, and is less than 64 characters.

    As the this field doesn't carry any meaning, it is sufficient to ensure the uniqueness of this identifier. Peers SHOULD generate a random, lower-case alphanumeric string of least 32 characters in length when booting up their node. Peers SHOULD NOT use their Peer ID here because a future Peer ID could exceed the DNS label limit of 63 characters.

If a private network is in use, then the service-name contains the base-16 encoding of the network's fingerprint as in _p2p-X._udp.local. This prevents public and private networks from discovering each other's peers.

Peer Discovery

Request

To find all peers, a DNS message is sent with the question _p2p._udp.local PTR. Peers will then start responding with their details.

Note that a peer must respond to its own query. This allows other peers to passively discover it.

Response

On receipt of a find all peers query, a peer sends a DNS response message (QR = 1) that contains the answer

<service-name> PTR <peer-name>.<service-name>

The additional records of the response contain the peer's discovery details:

<peer-name>.<service-name> TXT "dnsaddr=..."

The TXT record contains the multiaddresses that the peer is listening on. Each multiaddress is a TXT attribute with the form dnsaddr=/.../p2p/QmId. Multiple dnsaddr attributes and/or TXT records are allowed.

DNS Service Discovery

DNS-SD support is not needed for peers to discover each other. However, it is extremely useful for network administrators to discover what is running on the network.

Meta Query

This allows discovery of all services. The question is _services._dns-sd._udp.local PTR.

A peer responds with the answer

    _services._dns-sd._udp.local PTR <service-name>

Find All Response

On receipt of a find all peers query, the following additional records should be included

    <peer-name>.<service-name> SRV ... <host-name>
    <host-name>              A <ipv4 address>
    <host-name>              AAAA <ipv6 address>

Gotchas

Many existing tools ignore the Additional Records, and always send individual queries for the peer's discovery details. To accomodate this, a peer should respond to the following queries:

  • <peer-name>.<service-name> SRV
  • <peer-name>.<service-name> TXT
  • <host-name> A
  • <host-name> AAAA

Issues

[ ] mDNS requires link-local addresses. Loopback and "NAT busting" addresses should not sent and must be ignored on receipt?

References

Meta Query

Goal: find all services on the local network.

Question

_services._dns-sd._udp.local PTR

Answer

_services._dns-sd._udp.local IN PTR _p2p._udp.local

Find All Peers

Goal: find all peers on the local network.

Question

_p2p._udp.local PTR

Answer

_p2p._udp.local IN PTR `<peer-name>`._p2p._udp.local

Additional Records

  • <peer-name>._p2p._udp.local IN TXT dnsaddr=/ip6/2001:DB8::7573:b0a8:46b0:bfea/tcp/4001/p2p/id
  • <peer-name>._p2p._udp.local IN TXT dnsaddr=/ip4/192.0.2.0/tcp/4001/p2p/id