Prefer IPv6 over IPv4-in-IPv6

conn.go

@@ -954,7 +954,7 @@ func (c *Conn) readLoop(name string, pktConn ipPacketConn, inboundBufferSize int
 									c.log.Warnf("[%s] no interface for %d", c.name, ifIndex)
 									return
 								}
-								var selectedAddr *netip.Addr
+								var selectedAddrs []netip.Addr
 								for _, addr := range ifc.ipAddrs {
 									addrCopy := addr
 
@@ -977,13 +977,34 @@ func (c *Conn) readLoop(name string, pktConn ipPacketConn, inboundBufferSize int
 										}
 									}
 
-									selectedAddr = &addrCopy
-									break
+									selectedAddrs = append(selectedAddrs, addrCopy)
 								}
-								if selectedAddr == nil {
+								if len(selectedAddrs) == 0 {
 									c.log.Debugf("[%s] failed to find suitable IP for interface %d; deriving address from source address c.name,instead", ifIndex)
 								} else {
-									localAddress = selectedAddr
+									// choose the best match
+									var choice *netip.Addr
+									for _, option := range selectedAddrs {
+										optCopy := option
+										if option.Is4() {
+											// select first
+											choice = &optCopy
+											break
+										}
+										// we're okay with 4In6 for now but ideally we get a an actual IPv6.
+										// Maybe in the future we never want this but it does look like Docker
+										// can route IPv4 over IPv6.
+										if choice == nil {
+											choice = &optCopy
+										} else if !optCopy.Is4In6() {
+											choice = &optCopy
+										}
+										if !optCopy.Is4In6() {
+											break
+										}
+										// otherwise keep searching for an actual IPv6
+									}
+									localAddress = choice
 								}
 							}
 							if ifIndex == -1 || localAddress == nil {

conn_test.go

@@ -451,6 +451,52 @@ func TestValidCommunicationIPv46MixedLocalAddress(t *testing.T) {
 	}
 }
 
+func TestValidCommunicationIPv66Mixed(t *testing.T) {
+	if runtime.GOARCH == "386" {
+		t.Skip("IPv6 not supported on 386 for some reason")
+	}
+
+	lim := test.TimeOut(time.Second * 10)
+	defer lim.Stop()
+
+	report := test.CheckRoutines(t)
+	defer report()
+
+	aSock6 := createListener6(t)
+	bSock6 := createListener6(t)
+
+	aServer, err := Server(nil, ipv6.NewPacketConn(aSock6), &Config{
+		LocalNames: []string{"pion-mdns-1.local"},
+	})
+	check(err, t)
+
+	bServer, err := Server(nil, ipv6.NewPacketConn(bSock6), &Config{})
+	check(err, t)
+
+	header, addr, err := bServer.QueryAddr(context.TODO(), "pion-mdns-1.local")
+	check(err, t)
+	if header.Type != dnsmessage.TypeAAAA {
+		t.Fatalf("expected AAAA but got %s", header.Type)
+	}
+	if addr.String() == localAddress {
+		t.Fatalf("unexpected local address: %v", addr)
+	}
+	if addr.Is4In6() {
+		t.Fatalf("expected address to not be ipv4-to-ipv6 mapped: %v", addr)
+	}
+	checkIPv6(addr, t)
+
+	check(aServer.Close(), t)
+	check(bServer.Close(), t)
+
+	if len(aServer.queries) > 0 {
+		t.Fatalf("Queries not cleaned up after aServer close")
+	}
+	if len(bServer.queries) > 0 {
+		t.Fatalf("Queries not cleaned up after bServer close")
+	}
+}
+
 func TestValidCommunicationIPv66MixedLocalAddress(t *testing.T) {
 	if runtime.GOARCH == "386" {
 		t.Skip("IPv6 not supported on 386 for some reason")