specs/discovery/mdns.md

Multicast DNS

Author: Richard Schneider (makaretu@gmail.com)

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 is local database of peers.

Definitions

service-name is the DNS-SD service name for all peers. It is defined as _p2p._udp.local.

host-name is the name of the peer. It is derived from the peer's ID and p2p.local, for example Qmid.p2p.local.

peer-id is the ID of the peer. It normally is the base-58 enconding of the hash of the peer's public key.

port is a port that the peer listens on. Normally 4001.

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 it's own query. Thus 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-id>.<service-name>

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

<peer-id>.<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=/ip4/.../tcp/.../p2p/QmId. Multiple dnsaddr attributes are expected.

DNS Service Discovery

DNS-SD support is not needed for peers to discover each other. However, it is extremely usefull 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-id>.<service-name> SRV ... <port> <host-name>
<host-name>              A <ipv4 address>
<host-name>              AAAA <ipv6 address>

If a peer is listening on multiple ports, it should respond with multiple SRV records for each port it is listening on.

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

Issues

• like urls, dns names are case insensitive. A peer-id is base58btc encoded and is case sensitive.
• MDNS requires link local addresses. Loopback and "nat busting" addresses should not sent and must be ignored on receipt?

References

• RFC 1035 - Domain Names (DNS)
• RFC 6762 - Multicast DNS
• RFC 6763 - DNS-Based Service Discovery
• Multiaddr

Worked Example

TODO