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Raúl Kripalani
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discovery/mdns.md
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# Multicast DNS
# Multicast DNS (mDNS)
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.
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.
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-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](https://github.com/ipfs/in-web-browsers/issues/89#issue-341357014) workaround can be used.
- `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. 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](https://github.com/ipfs/in-web-browsers/issues/89#issue-341357014) 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.
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.
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**
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
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.
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.
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`.
This allows discovery of all services. The question is `_services._dns-sd._udp.local PTR`.
A peer responds with the answer
@@ -69,7 +65,7 @@ On receipt of a `find all peers` query, the following **additional records** sho
### 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:
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`
@@ -78,7 +74,7 @@ Many existing tools ignore the Additional Records and always send individual que
## Issues
MDNS requires link local addresses. Loopback and "nat busting" addresses should not sent and must be ignored on receipt?
[ ] mDNS requires link-local addresses. Loopback and "NAT busting" addresses should not sent and must be ignored on receipt?
## References
@@ -87,35 +83,43 @@ MDNS requires link local addresses. Loopback and "nat busting" addresses should
- [RFC 6763 - DNS-Based Service Discovery](https://tools.ietf.org/html/rfc6763)
- [Multiaddr](https://github.com/multiformats/multiaddr)
## Worked Example
## Worked Examples
Asumming that `peer-id` is `QmQusTXc1Z9C1mzxsqC9ZTFXCgSkpBRGgW4Jk2QYHxKE22`. Then the `peer-name` is `ciqcmoputolsfsigvm7nx5fwkko2eq26h46qhbj6o4co7uyn2f2srdy`.
Asumming that `peer-id` is `QmQusTXc1Z9C1mzxsqC9ZTFXCgSkpBRGgW4Jk2QYHxKE22`, then the `peer-name` is `ciqcmoputolsfsigvm7nx5fwkko2eq26h46qhbj6o4co7uyn2f2srdy` (base32 encoding of the peer ID).
To make the examples more readable `id` and `name` are used.
### Meta Query
Find all services on the local network.
Goal: find all services on the local network.
#### Question
`_services._dns-sd._udp.local PTR`
```
_services._dns-sd._udp.local PTR
```
#### Answer
```
_services._dns-sd._udp.local IN PTR _p2p._udp.local
```
### Find All Peers
Find all peers on the local network
Goal: find all peers on the local network.
#### Question
```
_p2p._udp.local PTR
```
#### Answer
```
_p2p._udp.local IN PTR `name`._p2p._udp.local
```
#### Additional Records