3.3 KiB
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 fully qualified name of the peer. It is derived from the peer's name and p2p.local.
peer-name is the case-insenstive unique identifier of the peer and less than 64 characters.
It is normally the base-32 encoding of peer's ID.
If the encoding of the peer's ID exceeds 63 characters, then the Split at 63rd character workaround can be used.
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. 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 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-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
- RFC 1035 - Domain Names (DNS)
- RFC 6762 - Multicast DNS
- RFC 6763 - DNS-Based Service Discovery
- Multiaddr
Worked Example
TODO