3.1 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 IPFS peers. It is defined as _ipfs._udp.local.
host-name is the name of the peer. It is derived from the peer's ID and ipfs.local, for example
Qmid.ipfs.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 _ipfs._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
<serice-name> PTR <peer-id>.<service-name>
The additional records of the response contain the peer's discovery details
<peer-id>.<serice-name> SRV ... <port> <host-name>
<peer-id>.<serice-name> TXT ...
<host-name> A <ipv4 address>
<host-name> AAAA <ipv6 address>
Multiple A and AAAA records are expected. The TXT is not needed for IPFS peer discovery, but is required by DNS-SD.
Multiported Peers
If a peer is listening on multiple ports, it must respond with multiple SRV records for each
port it is listening on. If the ports do not listen on the same IP addresses, then each 'SRV' record
must have a different host-name.
Multiaddress
TODO from DNS records
TODO to DNS records
DNS Service Discovery
DNS-SD support is not needed for IPFS peers to discover each other. However, it is extremely usefull for network admistrators 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>
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>.<serice-name> SRV<peer-id>.<serice-name> TXT<host-name> A<host-name> AAAA
References
Worked Example
TODO