Add config options for ListenAddr & Interfaces

These options are helpful when the bridge is running on a multi-homed
device like a wireless router.

README.md

@@ -28,6 +28,25 @@ To compile `hap-z2m`, use:
 
     go build -v -trimpath -ldflags="-s -w" ./cmd/...
 
+Configuration
+==============
+
+The MQTT broker that Zigbee2MQTT connects to is specified using the following:
+
+- Server
+- Username
+- Password
+
+Additionally, these options control networking aspects for the bridge:
+
+- `ListenAddr` can be used to bind to a specific port and/or address, 
+  useful for selecting a fixed port for firewall rules.
+
+- `Interfaces` limit which interfaces mDNS broadcasts appear on, 
+  and which addresses to use when broadcasting.
+
+These settings are optional and can be left blank.
+
 License
 ========
 

bridge.go

@@ -51,6 +51,10 @@ type Bridge struct {
 	Username string
 	Password string
 
+	// address and interfaces to bind to
+	ListenAddr string
+	Interfaces []string
+
 	DebugMode bool
 
 	ctx       context.Context
@@ -118,6 +122,9 @@ func (br *Bridge) StartHAP() error {
 		return err
 	}
 
+	br.server.Addr = br.ListenAddr
+	br.server.Ifaces = br.Interfaces
+
 	if br.DebugMode {
 		haplog.Debug.Enable()
 	}

cmd/hapz2m/main.go

@@ -8,6 +8,7 @@ import (
 	"flag"
 	"fmt"
 	"log"
+	"net"
 	"net/http"
 	"os"
 	"os/signal"
@@ -29,7 +30,11 @@ var (
 	debugMode  = flag.Bool("debug", false, "enable debug messages")
 )
 
+// config struct
 type config struct {
+	ListenAddr string
+	Interfaces []string
+
 	Server, Username, Password string
 }
 
@@ -73,6 +78,17 @@ func main() {
 	br.Password = cfg.Password
 	br.DebugMode = *debugMode
 
+	// validate ListenAddr if specified
+	if cfg.ListenAddr != "" {
+		_, _, err := net.SplitHostPort(cfg.ListenAddr)
+		if err != nil {
+			log.Fatalf("invalid ListenAddr: %v", err)
+		}
+		br.ListenAddr = cfg.ListenAddr
+	}
+
+	br.Interfaces = cfg.Interfaces
+
 	err = br.ConnectMQTT()
 	if err != nil {
 		log.Printf("cannot connect to MQTT: %s", err)

hapz2m.conf

@@ -1,4 +1,12 @@
 {
+	// specify a host:port to listen on
+	// if not specified, a random port will be used
+	//"ListenAddr": ":8585",
+
+	// limit interfaces to use for DNS-SD (mDNS)
+	// leave blank for all interfaces
+	//"Interfaces": ["eth0", "eth1"],
+
 	// MQTT server credentials
 	"Server": "tcp://localhost:1883",
 	"Username": "hapz2m",