cmd/derper: listen on host of flag server addr for port 80 and 3478 (#2768)

cmd/derper: listen on host of flag server addr for port 80 and 3478

When using custom derp on the server with multiple IP addresses,
we would like to bind derp 80, 443 and stun 3478 to a certain IP.

derp command provides flag `-a` to customize which address to bind
for port 443. But port :80 and :3478 were hard-coded.

Fixes #2767

Signed-off-by: Li Chuangbo <im@chuangbo.li>

cmd/derper/derper.go

@@ -117,6 +117,11 @@ func main() {
 		tsweb.DevMode = true
 	}
 
+	listenHost, _, err := net.SplitHostPort(*addr)
+	if err != nil {
+		log.Fatalf("invalid server address: %v", err)
+	}
+
 	var logPol *logpolicy.Policy
 	if *logCollection != "" {
 		logPol = logpolicy.New(*logCollection)
@@ -181,7 +186,7 @@ func main() {
 	debug.Handle("traffic", "Traffic check", http.HandlerFunc(s.ServeDebugTraffic))
 
 	if *runSTUN {
-		go serveSTUN()
+		go serveSTUN(listenHost)
 	}
 
 	httpsrv := &http.Server{
@@ -199,7 +204,6 @@ func main() {
 		WriteTimeout: 30 * time.Second,
 	}
 
-	var err error
 	if letsEncrypt {
 		if *certDir == "" {
 			log.Fatalf("missing required --certdir flag")
@@ -226,7 +230,7 @@ func main() {
 		}
 		go func() {
 			port80srv := &http.Server{
-				Addr:         ":80", // the default, but to be explicit
+				Addr:         net.JoinHostPort(listenHost, "80"),
 				Handler:      certManager.HTTPHandler(tsweb.Port80Handler{Main: mux}),
 				ReadTimeout:  30 * time.Second,
 				WriteTimeout: 30 * time.Second,
@@ -248,8 +252,9 @@ func main() {
 	}
 }
 
-func serveSTUN() {
-	pc, err := net.ListenPacket("udp", ":3478")
+func serveSTUN(host string) {
+
+	pc, err := net.ListenPacket("udp", net.JoinHostPort(host, "3478"))
 	if err != nil {
 		log.Fatalf("failed to open STUN listener: %v", err)
 	}