Make mDNS opportunistic

It seems like mDNS was intended to be opportunistic, but right now,
if creating an mDNS client fails, the whole ICE connection fails.
This allows pion to work in more restricted network namespaces,
or on less supported environments like 9front.

This fixes webwormhole on 9front.

README.md

@@ -49,6 +49,7 @@ Check out the **[contributing wiki](https://github.com/pion/webrtc/wiki/Contribu
 * [Nevio Vesic](https://github.com/0x19)
 * [David Hamilton](https://github.com/dihamilton)
 * [adwpc](https://github.com/adwpc)
+* [Ori Bernstein](https://eigenstate.org)
 
 ### License
 MIT License - see [LICENSE](LICENSE) for full text

agent.go

@@ -354,8 +354,10 @@ func NewAgent(config *AgentConfig) (*Agent, error) {
 
 	var mDNSConn *mdns.Conn
 	mDNSConn, mDNSMode, err = createMulticastDNS(mDNSMode, mDNSName, log)
+	// Opportunistic mDNS: If we can't open the connection, that's ok: we
+	// can continue without it.
 	if err != nil {
-		return nil, err
+		log.Warnf("Failed to initialize mDNS %s: %v", mDNSName, err)
 	}
 	closeMDNSConn := func() {
 		if mDNSConn != nil {
@@ -808,6 +810,9 @@ func (a *Agent) AddRemoteCandidate(c Candidate) error {
 }
 
 func (a *Agent) resolveAndAddMulticastCandidate(c *CandidateHost) {
+	if a.mDNSConn == nil {
+		return
+	}
 	_, src, err := a.mDNSConn.Query(context.TODO(), c.Address())
 	if err != nil {
 		a.log.Warnf("Failed to discover mDNS candidate %s: %v", c.Address(), err)