A Peer Discovery module enables libp2p to find peers to connect to. Think of these mechanisms as ways to join the rest of the network, as railing points.

With these system, a libp2p node can both have a set of nodes to always connect on boot (bootstraper nodes), discover nodes through locality (e.g connected in the same LAN) or through serendipity (random walks on a DHT).

These mechanisms save configuration and enable a node to operate without any explicit dials, it will just work. Once new peers are discovered, their known data is stored in the peer's PeerStore.

1. Bootstrap list of Peers when booting a node

For this demo, we will connect to IPFS default bootstrapper nodes and so, we will need to support the same set of features those nodes have, that are: TCP, mplex and NOISE. You can see the complete example at 1.js.

First, we create our libp2p node.

const Libp2p = require('libp2p')
const Bootstrap = require('libp2p-bootstrap')

const node = Libp2p.create({
  modules: {
    transport: [ TCP ],
    streamMuxer: [ Mplex ],
    connEncryption: [ NOISE ],
    peerDiscovery: [ Bootstrap ]
  },
  config: {
    peerDiscovery: {
      bootstrap: {
        interval: 60e3,
        enabled: true,
        list: bootstrapers
      }
    }
  }
})

In this configuration, we use a bootstrappers array listing peers to connect on boot. Here is the list used by js-ipfs and go-ipfs.

const bootstrapers = [
  '/ip4/104.131.131.82/tcp/4001/p2p/QmaCpDMGvV2BGHeYERUEnRQAwe3N8SzbUtfsmvsqQLuvuJ',
  '/dnsaddr/bootstrap.libp2p.io/p2p/QmNnooDu7bfjPFoTZYxMNLWUQJyrVwtbZg5gBMjTezGAJN',
  '/dnsaddr/bootstrap.libp2p.io/p2p/QmbLHAnMoJPWSCR5Zhtx6BHJX9KiKNN6tpvbUcqanj75Nb',
  '/dnsaddr/bootstrap.libp2p.io/p2p/QmZa1sAxajnQjVM8WjWXoMbmPd7NsWhfKsPkErzpm9wGkp',
  '/dnsaddr/bootstrap.libp2p.io/p2p/QmQCU2EcMqAqQPR2i9bChDtGNJchTbq5TbXJJ16u19uLTa',
  '/dnsaddr/bootstrap.libp2p.io/p2p/QmcZf59bWwK5XFi76CZX8cbJ4BhTzzA3gU1ZjYZcYW3dwt'
]

Now, once we create and start the node, we can listen for events such as peer:discovery and peer:connect, these events tell us when we found a peer, independently of the discovery mechanism used and when we actually dialed to that peer.

const node = await Libp2p.create({
  peerId,
  addresses: {
    listen: ['/ip4/0.0.0.0/tcp/0']
  }
  modules: {
    transport: [ TCP ],
    streamMuxer: [ Mplex ],
    connEncryption: [ NOISE ],
    peerDiscovery: [ Bootstrap ]
  },
  config: {
    peerDiscovery: {
      bootstrap: {
        interval: 60e3,
        enabled: true,
        list: bootstrapers
      }
    }
  }
})

node.connectionManager.on('peer:connect', (connection) => {
  console.log('Connection established to:', connection.remotePeer.toB58String())	// Emitted when a new connection has been created
})

node.on('peer:discovery', (peerId) => {
  // No need to dial, autoDial is on
  console.log('Discovered:', peerId.toB58String())
})

await node.start()

From running 1.js, you should see the following:

> node 1.js
Discovered: QmaCpDMGvV2BGHeYERUEnRQAwe3N8SzbUtfsmvsqQLuvuJ
Discovered: QmNnooDu7bfjPFoTZYxMNLWUQJyrVwtbZg5gBMjTezGAJN
Discovered: QmbLHAnMoJPWSCR5Zhtx6BHJX9KiKNN6tpvbUcqanj75Nb
Discovered: QmZa1sAxajnQjVM8WjWXoMbmPd7NsWhfKsPkErzpm9wGkp
Discovered: QmQCU2EcMqAqQPR2i9bChDtGNJchTbq5TbXJJ16u19uLTa
Discovered: QmcZf59bWwK5XFi76CZX8cbJ4BhTzzA3gU1ZjYZcYW3dwt
Connection established to: QmaCpDMGvV2BGHeYERUEnRQAwe3N8SzbUtfsmvsqQLuvuJ
Connection established to: QmNnooDu7bfjPFoTZYxMNLWUQJyrVwtbZg5gBMjTezGAJN
Connection established to: QmZa1sAxajnQjVM8WjWXoMbmPd7NsWhfKsPkErzpm9wGkp
Connection established to: QmQCU2EcMqAqQPR2i9bChDtGNJchTbq5TbXJJ16u19uLTa
Connection established to: QmcZf59bWwK5XFi76CZX8cbJ4BhTzzA3gU1ZjYZcYW3dwt
Connection established to: QmbLHAnMoJPWSCR5Zhtx6BHJX9KiKNN6tpvbUcqanj75Nb

2. MulticastDNS to find other peers in the network

For this example, we need libp2p-mdns, go ahead and npm install it. You can find the complete solution at 2.js.

Update your libp2p configuration to include MulticastDNS.

const Libp2p = require('libp2p')
const MulticastDNS = require('libp2p-mdns')

const createNode = () => {
  return Libp2p.create({
    addresses: {
      listen: ['/ip4/0.0.0.0/tcp/0']
    }
    modules: {
      transport: [ TCP ],
      streamMuxer: [ Mplex ],
      connEncryption: [ NOISE ],
      peerDiscovery: [ MulticastDNS ]
    },
    config: {
      peerDiscovery: {
        mdns: {
          interval: 20e3,
          enabled: true
        }
      }
    }
  })
}

To observe it working, spawn two nodes.

const [node1, node2] = await Promise.all([
  createNode(),
  createNode()
])

node1.on('peer:discovery', (peer) => console.log('Discovered:', peer.id.toB58String()))
node2.on('peer:discovery', (peer) => console.log('Discovered:', peer.id.toB58String()))

If you run this example, you will see the other peers being discovered.

> node 2.js
Discovered: QmSSbQpuKrxkoXHm1v4Pi35hPN5hUHMZoBoawEs2Nhvi8m
Discovered: QmRcXXhtG8vTqwVBRonKWtV4ovDoC1Fe56WYtcrw694eiJ

3. Where to find other Peer Discovery Mechanisms

There are plenty more Peer Discovery Mechanisms out there, you can:

Find one in libp2p-webrtc-star. Yes, a transport with discovery capabilities! This happens because WebRTC requires a rendezvous point for peers to exchange SDP offer, which means we have one or more points that can introduce peers to each other. Think of it as MulticastDNS for the Web, as in MulticastDNS only works in LAN.
Any DHT will offer you a discovery capability. You can simple random-walk the routing tables to find other peers to connect to. For example libp2p-kad-dht can be used for peer discovery. An example how to configure it to enable random walks can be found here.
You can create your own Discovery service, a registry, a list, a radio beacon, you name it!