Broadcast discovery with InfoAll, interface broadcast detection, clean output

lib/client/udp.go

@@ -14,36 +14,46 @@ import (
 const PicomapPort = 28781
 
 type udpTransport struct {
-	conn *net.UDPConn
-	addr *net.UDPAddr
-	buf  bytes.Buffer
+	conn      *net.UDPConn
+	addr      *net.UDPAddr
+	broadcast bool
+	buf       bytes.Buffer
 }
 
-func isBroadcast(ip net.IP) bool {
-	if ip.Equal(net.IPv4bcast) {
-		return true
-	}
-	ip4 := ip.To4()
-	return ip4 != nil && ip4[3] == 255
-}
-
-func interfaceIPv4(name string) (net.IP, error) {
+func interfaceIPv4Net(name string) (net.IP, *net.IPNet, error) {
 	ifi, err := net.InterfaceByName(name)
 	if err != nil {
-		return nil, fmt.Errorf("interface %s: %w", name, err)
+		return nil, nil, fmt.Errorf("interface %s: %w", name, err)
 	}
 	addrs, err := ifi.Addrs()
 	if err != nil {
-		return nil, fmt.Errorf("interface %s addrs: %w", name, err)
+		return nil, nil, fmt.Errorf("interface %s addrs: %w", name, err)
 	}
 	for _, a := range addrs {
 		if ipnet, ok := a.(*net.IPNet); ok {
 			if ip4 := ipnet.IP.To4(); ip4 != nil {
-				return ip4, nil
+				return ip4, ipnet, nil
 			}
 		}
 	}
-	return nil, fmt.Errorf("interface %s has no IPv4 address", name)
+	return nil, nil, fmt.Errorf("interface %s has no IPv4 address", name)
+}
+
+func broadcastAddr(ip net.IP, mask net.IPMask) net.IP {
+	bcast := make(net.IP, 4)
+	ip4 := ip.To4()
+	for i := range 4 {
+		bcast[i] = ip4[i] | ^mask[i]
+	}
+	return bcast
+}
+
+func InterfaceBroadcast(name string) (string, error) {
+	ip, ipnet, err := interfaceIPv4Net(name)
+	if err != nil {
+		return "", err
+	}
+	return broadcastAddr(ip, ipnet.Mask).String(), nil
 }
 
 func enableBroadcast(conn *net.UDPConn) error {
@@ -58,6 +68,17 @@ func enableBroadcast(conn *net.UDPConn) error {
 	return serr
 }
 
+func isInterfaceBroadcast(iface string, ip net.IP) bool {
+	if iface == "" {
+		return false
+	}
+	localIP, ipnet, err := interfaceIPv4Net(iface)
+	if err != nil {
+		return false
+	}
+	return ip.Equal(broadcastAddr(localIP, ipnet.Mask)) || ip.Equal(net.IPv4bcast)
+}
+
 func NewUDP(addr string, iface string, timeout time.Duration) (*Client, error) {
 	raddr, err := net.ResolveUDPAddr("udp4", fmt.Sprintf("%s:%d", addr, PicomapPort))
 	if err != nil {
@@ -66,7 +87,7 @@ func NewUDP(addr string, iface string, timeout time.Duration) (*Client, error) {
 
 	var laddr *net.UDPAddr
 	if iface != "" {
-		ip, err := interfaceIPv4(iface)
+		ip, _, err := interfaceIPv4Net(iface)
 		if err != nil {
 			return nil, err
 		}
@@ -78,14 +99,13 @@ func NewUDP(addr string, iface string, timeout time.Duration) (*Client, error) {
 		return nil, fmt.Errorf("listen: %w", err)
 	}
 
-	if isBroadcast(raddr.IP) {
-		if err := enableBroadcast(conn); err != nil {
-			conn.Close()
-			return nil, fmt.Errorf("SO_BROADCAST: %w", err)
-		}
+	if err := enableBroadcast(conn); err != nil {
+		conn.Close()
+		return nil, fmt.Errorf("SO_BROADCAST: %w", err)
 	}
 
-	return &Client{transport: &udpTransport{conn: conn, addr: raddr}, timeout: timeout}, nil
+	bcast := isInterfaceBroadcast(iface, raddr.IP)
+	return &Client{transport: &udpTransport{conn: conn, addr: raddr, broadcast: bcast}, timeout: timeout}, nil
 }
 
 func (t *udpTransport) Send(data []byte) error {
@@ -111,6 +131,8 @@ func (t *udpTransport) Reader() io.Reader {
 	}
 }
 
+func (t *udpTransport) Broadcast() bool { return t.broadcast }
+
 func (t *udpTransport) Close() error {
 	return t.conn.Close()
 }