add option for binding to interface

this effectively still bind to an address, but resolves it based on the
interface instead of explicitly.

README.md

@@ -132,7 +132,8 @@ All options can be passed either as command-line flags or environment variables:
 | `--cluster-key KEY` | WESHER_CLUSTER_KEY | shared key for cluster membership; must be 32 bytes base64 encoded; will be generated if not provided | autogenerated/loaded |
 | `--join HOST,...` | WESHER_JOIN | comma separated list of hostnames or IP addresses to existing cluster members; if not provided, will attempt resuming any known state or otherwise wait for further members |  |
 | `--init` | WESHER_INIT | whether to explicitly (re)initialize the cluster; any known state from previous runs will be forgotten | `false` |
-| `--bind-addr ADDR` | WESHER_BIND_ADDR | IP address to bind to for cluster membership | autodetected |
+| `--bind-addr ADDR` | WESHER_BIND_ADDR | IP address to bind to for cluster membership (cannot be used with --bind-iface) | autodetected |
+| `--bind-iface IFACE` | WESHER_BIND_IFACE | Interface to bind to for cluster membership (cannot be used with --bind-addr)|  |
 | `--cluster-port PORT` | WESHER_CLUSTER_PORT | port used for membership gossip traffic (both TCP and UDP); must be the same across cluster | `7946` |
 | `--wireguard-port PORT` | WESHER_WIREGUARD_PORT | port used for wireguard traffic (UDP); must be the same across cluster | `51820` |
 | `--overlay-net ADDR/MASK` | WESHER_OVERLAY_NET | the network in which to allocate addresses for the overlay mesh network (CIDR format); smaller networks increase the chance of IP collision | `10.0.0.0/8` |

cluster.go

@@ -64,10 +64,28 @@ func newCluster(config *config, wg *wgState) (*cluster, error) {
 	}
 	state.ClusterKey = clusterKey
 
+	// we check for mutual exclusion in config.go
+	bindAddr := config.BindAddr
+	if config.BindIface != "" {
+		iface, err := net.InterfaceByName(config.BindIface)
+		if err != nil {
+			return nil, err
+		}
+		addrs, err := iface.Addrs()
+		if err != nil {
+			return nil, err
+		}
+		if len(addrs) > 0 {
+			if addr, ok := addrs[0].(*net.IPNet); ok {
+				bindAddr = addr.IP.String()
+			}
+		}
+	}
+
 	mlConfig := memberlist.DefaultWANConfig()
 	mlConfig.LogOutput = logrus.StandardLogger().WriterLevel(logrus.DebugLevel)
 	mlConfig.SecretKey = clusterKey
-	mlConfig.BindAddr = config.BindAddr
+	mlConfig.BindAddr = bindAddr
 	mlConfig.BindPort = config.ClusterPort
 	mlConfig.AdvertisePort = config.ClusterPort
 	if config.UseIPAsName && config.BindAddr != "0.0.0.0" {

config.go

@@ -14,7 +14,8 @@ type config struct {
 	ClusterKey    []byte   `id:"cluster-key" desc:"shared key for cluster membership; must be 32 bytes base64 encoded; will be generated if not provided"`
 	Join          []string `desc:"comma separated list of hostnames or IP addresses to existing cluster members; if not provided, will attempt resuming any known state or otherwise wait for further members."`
 	Init          bool     `desc:"whether to explicitly (re)initialize the cluster; any known state from previous runs will be forgotten"`
-	BindAddr      string   `id:"bind-addr" desc:"IP address to bind to for cluster membership"`
+	BindAddr      string   `id:"bind-addr" desc:"IP address to bind to for cluster membership traffic (cannot be used with --bind-iface)"`
+	BindIface     string   `id:"bind-iface" desc:"Interface to bind to for cluster membership traffic (cannot be used with --bind-addr)"`
 	ClusterPort   int      `id:"cluster-port" desc:"port used for membership gossip traffic (both TCP and UDP); must be the same across cluster" default:"7946"`
 	WireguardPort int      `id:"wireguard-port" desc:"port used for wireguard traffic (UDP); must be the same across cluster" default:"51820"`
 	OverlayNet    *network `id:"overlay-net" desc:"the network in which to allocate addresses for the overlay mesh network (CIDR format); smaller networks increase the chance of IP collision" default:"10.0.0.0/8"`
@@ -44,7 +45,9 @@ func loadConfig() (*config, error) {
 	}
 
 	// FIXME: this is a workaround for memberlist refusing to listen on public IPs if BindAddr==0.0.0.0
-	if config.BindAddr == "" {
+	if config.BindAddr != "" && config.BindIface != "" {
+		return nil, fmt.Errorf("setting both bind address and bind interface is not supported")
+	} else if config.BindAddr == "" && config.BindIface == "" {
 		detectedBindAddr, err := sockaddr.GetPublicIP()
 		if err != nil {
 			return nil, err