# 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]

[@richardschneider]: https://github.com/richardschneider
[@yusefnapora]: https://github.com/yusefnapora
[@raulk]: https://github.com/raulk
[@daviddias]: https://github.com/daviddias
[@jacobheun]: https://github.com/jacobheun

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

[lifecycle-spec]: https://github.com/libp2p/specs/blob/master/00-framework-01-spec-lifecycle.md

## Table of Contents

- [Multicast DNS (mDNS)](#multicast-dns-mdns)
  - [Table of Contents](#table-of-contents)
  - [Overview](#overview)
  - [Definitions](#definitions)
  - [Peer Discovery](#peer-discovery)
    - [Request](#request)
    - [Response](#response)
  - [DNS Service Discovery](#dns-service-discovery)
    - [Meta Query](#meta-query)
    - [Find All Response](#find-all-response)
    - [Gotchas](#gotchas)
  - [Issues](#issues)
  - [References](#references)
    - [Meta Query](#meta-query-1)
      - [Question](#question)
      - [Answer](#answer)
    - [Find All Peers](#find-all-peers)
      - [Question](#question-1)
      - [Answer](#answer-1)
      - [Additional Records](#additional-records)

## 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](https://github.com/libp2p/specs/blob/master/pnet/Private-Networks-PSK-V1.md) 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

- [RFC 1035 - Domain Names (DNS)](https://tools.ietf.org/html/rfc1035)
- [RFC 6762 - Multicast DNS](https://tools.ietf.org/html/rfc6762)
- [RFC 6763 - DNS-Based Service Discovery](https://tools.ietf.org/html/rfc6763)
- [Multiaddr](https://github.com/multiformats/multiaddr)


### 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`