feat: add support for custom DNS server IP (#142)

I need to test my app for a custom DNS server (that's not the default
one), and `multicastdns()` does not supports it.

I've added the `ip` option according to [the multicast-dns
dependency](https://github.com/mafintosh/multicast-dns#mdns--multicastdnsoptions).

src/index.ts

@@ -16,6 +16,7 @@ export interface MulticastDNSInit {
   interval?: number
   serviceTag?: string
   port?: number
+  ip?: string
   compat?: boolean
   compatQueryPeriod?: number
   compatQueryInterval?: number
@@ -33,6 +34,7 @@ class MulticastDNS extends EventEmitter<PeerDiscoveryEvents> implements PeerDisc
   private readonly interval: number
   private readonly serviceTag: string
   private readonly port: number
+  private readonly ip: string
   private _queryInterval: ReturnType<typeof setInterval> | null
   private readonly _goMdns?: GoMulticastDNS
   private readonly components: MulticastDNSComponents
@@ -44,6 +46,7 @@ class MulticastDNS extends EventEmitter<PeerDiscoveryEvents> implements PeerDisc
     this.broadcast = init.broadcast !== false
     this.interval = init.interval ?? (1e3 * 10)
     this.serviceTag = init.serviceTag ?? 'ipfs.local'
+    this.ip = init.ip ?? '224.0.0.251'
     this.port = init.port ?? 5353
     this._queryInterval = null
     this._onPeer = this._onPeer.bind(this)
@@ -81,7 +84,7 @@ class MulticastDNS extends EventEmitter<PeerDiscoveryEvents> implements PeerDisc
       return
     }
 
-    this.mdns = multicastDNS({ port: this.port })
+    this.mdns = multicastDNS({ port: this.port, ip: this.ip })
     this.mdns.on('query', this._onMdnsQuery)
     this.mdns.on('response', this._onMdnsResponse)
 

test/multicast-dns.spec.ts

@@ -225,4 +225,31 @@ describe('MulticastDNS', () => {
 
     await stop(mdnsA)
   })
+
+  it('find another peer with different udp4 address', async function () {
+    this.timeout(40 * 1000)
+
+    const mdnsA = mdns({
+      broadcast: false, // do not talk to ourself
+      port: 50005,
+      ip: '224.0.0.252',
+      compat: false
+    })(getComponents(pA, aMultiaddrs))
+
+    const mdnsB = mdns({
+      port: 50005, // port must be the same
+      ip: '224.0.0.252', // ip must be the same
+      compat: false
+    })(getComponents(pB, bMultiaddrs))
+
+    await start(mdnsA, mdnsB)
+
+    const { detail: { id } } = await new Promise<CustomEvent<PeerInfo>>((resolve) => mdnsA.addEventListener('peer', resolve, {
+      once: true
+    }))
+
+    expect(pB.toString()).to.eql(id.toString())
+
+    await stop(mdnsA, mdnsB)
+  })
 })