feat: add support for ReuseAddr

listen_no_reuseport.go

@@ -7,16 +7,18 @@ import "net"
 
 const supportsReusePort = false
 
-func listenTCP(network, addr string, reuseport bool) (net.Listener, error) {
-	if reuseport {
+func listenTCP(network, addr string, reuseport, reuseaddr bool) (net.Listener, error) {
+	if reuseport || reuseaddr {
 		// TODO(tmthrgd): return an error?
 	}
 
 	return net.Listen(network, addr)
 }
 
-func listenUDP(network, addr string, reuseport bool) (net.PacketConn, error) {
-	if reuseport {
+const supportsReuseAddr = false
+
+func listenUDP(network, addr string, reuseport, reuseaddr bool) (net.PacketConn, error) {
+	if reuseport || reuseaddr {
 		// TODO(tmthrgd): return an error?
 	}
 

listen_reuseport.go

@@ -25,19 +25,41 @@ func reuseportControl(network, address string, c syscall.RawConn) error {
 	return opErr
 }
 
-func listenTCP(network, addr string, reuseport bool) (net.Listener, error) {
+const supportsReuseAddr = true
+
+func reuseaddrControl(network, address string, c syscall.RawConn) error {
+	var opErr error
+	err := c.Control(func(fd uintptr) {
+		opErr = unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_REUSEADDR, 1)
+	})
+	if err != nil {
+		return err
+	}
+
+	return opErr
+}
+
+func listenTCP(network, addr string, reuseport, reuseaddr bool) (net.Listener, error) {
 	var lc net.ListenConfig
-	if reuseport {
+	switch {
+	case reuseaddr && reuseport:
+	case reuseport:
 		lc.Control = reuseportControl
+	case reuseaddr:
+		lc.Control = reuseaddrControl
 	}
 
 	return lc.Listen(context.Background(), network, addr)
 }
 
-func listenUDP(network, addr string, reuseport bool) (net.PacketConn, error) {
+func listenUDP(network, addr string, reuseport, reuseaddr bool) (net.PacketConn, error) {
 	var lc net.ListenConfig
-	if reuseport {
+	switch {
+	case reuseaddr && reuseport:
+	case reuseport:
 		lc.Control = reuseportControl
+	case reuseaddr:
+		lc.Control = reuseaddrControl
 	}
 
 	return lc.ListenPacket(context.Background(), network, addr)

server.go

@@ -226,6 +226,10 @@ type Server struct {
 	// Whether to set the SO_REUSEPORT socket option, allowing multiple listeners to be bound to a single address.
 	// It is only supported on certain GOOSes and when using ListenAndServe.
 	ReusePort bool
+	// Whether to set the SO_REUSEADDR socket option, allowing multiple listeners to be bound to a single address.
+	// Crucially this allows binding when an existing server is listening on `0.0.0.0` or `::`.
+	// It is only supported on certain GOOSes and when using ListenAndServe.
+	ReuseAddr bool
 	// AcceptMsgFunc will check the incoming message and will reject it early in the process.
 	// By default DefaultMsgAcceptFunc will be used.
 	MsgAcceptFunc MsgAcceptFunc
@@ -304,7 +308,7 @@ func (srv *Server) ListenAndServe() error {
 
 	switch srv.Net {
 	case "tcp", "tcp4", "tcp6":
-		l, err := listenTCP(srv.Net, addr, srv.ReusePort)
+		l, err := listenTCP(srv.Net, addr, srv.ReusePort, srv.ReuseAddr)
 		if err != nil {
 			return err
 		}
@@ -317,7 +321,7 @@ func (srv *Server) ListenAndServe() error {
 			return errors.New("dns: neither Certificates nor GetCertificate set in Config")
 		}
 		network := strings.TrimSuffix(srv.Net, "-tls")
-		l, err := listenTCP(network, addr, srv.ReusePort)
+		l, err := listenTCP(network, addr, srv.ReusePort, srv.ReuseAddr)
 		if err != nil {
 			return err
 		}
@@ -327,7 +331,7 @@ func (srv *Server) ListenAndServe() error {
 		unlock()
 		return srv.serveTCP(l)
 	case "udp", "udp4", "udp6":
-		l, err := listenUDP(srv.Net, addr, srv.ReusePort)
+		l, err := listenUDP(srv.Net, addr, srv.ReusePort, srv.ReuseAddr)
 		if err != nil {
 			return err
 		}

server_test.go

@@ -3,6 +3,7 @@ package dns
 import (
 	"context"
 	"crypto/tls"
+	"errors"
 	"fmt"
 	"io"
 	"net"
@@ -1041,6 +1042,176 @@ func TestServerReuseport(t *testing.T) {
 	}
 }
 
+func TestServerReuseaddr(t *testing.T) {
+	startServerFn := func(t *testing.T, network, addr string, expectSuccess bool) (*Server, chan error) {
+		t.Helper()
+		wait := make(chan struct{})
+		srv := &Server{
+			Net:               network,
+			Addr:              addr,
+			NotifyStartedFunc: func() { close(wait) },
+			ReuseAddr:         true,
+		}
+
+		fin := make(chan error, 1)
+		go func() {
+			fin <- srv.ListenAndServe()
+		}()
+
+		select {
+		case <-wait:
+		case err := <-fin:
+			switch {
+			case expectSuccess:
+				t.Fatalf("%s: failed to start server: %v", t.Name(), err)
+			default:
+				fin <- err
+				return nil, fin
+			}
+		}
+		return srv, fin
+	}
+
+	externalIPFn := func(t *testing.T) (string, error) {
+		t.Helper()
+		ifaces, err := net.Interfaces()
+		if err != nil {
+			return "", err
+		}
+		for _, iface := range ifaces {
+			if iface.Flags&net.FlagUp == 0 {
+				continue // interface down
+			}
+			if iface.Flags&net.FlagLoopback != 0 {
+				continue // loopback interface
+			}
+			addrs, err := iface.Addrs()
+			if err != nil {
+				return "", err
+			}
+			for _, addr := range addrs {
+				var ip net.IP
+				switch v := addr.(type) {
+				case *net.IPNet:
+					ip = v.IP
+				case *net.IPAddr:
+					ip = v.IP
+				}
+				if ip == nil || ip.IsLoopback() {
+					continue
+				}
+				ip = ip.To4()
+				if ip == nil {
+					continue // not an ipv4 address
+				}
+				return ip.String(), nil
+			}
+		}
+		return "", errors.New("are you connected to the network?")
+	}
+
+	freePortFn := func(t *testing.T) int {
+		t.Helper()
+		addr, err := net.ResolveTCPAddr("tcp", "localhost:0")
+		if err != nil {
+			t.Fatalf("unable resolve tcp addr: %s", err)
+		}
+
+		l, err := net.ListenTCP("tcp", addr)
+		if err != nil {
+			t.Fatalf("unable listen tcp: %s", err)
+		}
+		defer l.Close()
+		return l.Addr().(*net.TCPAddr).Port
+	}
+
+	t.Run("should-fail-tcp", func(t *testing.T) {
+		// ReuseAddr should fail if you try to bind to exactly the same
+		// combination of source address and port.
+		// This should fail whether or not ReuseAddr is supported on a
+		// particular OS
+		ip, err := externalIPFn(t)
+		if err != nil {
+			t.Skip("no external IPs found")
+			return
+		}
+		port := freePortFn(t)
+		srv1, fin1 := startServerFn(t, "tcp", fmt.Sprintf("%s:%d", ip, port), true)
+		srv2, fin2 := startServerFn(t, "tcp", fmt.Sprintf("%s:%d", ip, port), false)
+		switch {
+		case srv2 != nil && srv2.started:
+			t.Fatalf("second ListenAndServe should not have started")
+		default:
+			if err := <-fin2; err == nil {
+				t.Fatalf("second ListenAndServe should have returned a startup error: %v", err)
+			}
+		}
+
+		if err := srv1.Shutdown(); err != nil {
+			t.Fatalf("failed to shutdown first server: %v", err)
+		}
+		if err := <-fin1; err != nil {
+			t.Fatalf("first ListenAndServe returned error after Shutdown: %v", err)
+		}
+	})
+	t.Run("should-succeed-tcp", func(t *testing.T) {
+		if !supportsReuseAddr {
+			t.Skip("reuseaddr is not supported")
+		}
+		ip, err := externalIPFn(t)
+		if err != nil {
+			t.Skip("no external IPs found")
+			return
+		}
+		port := freePortFn(t)
+
+		// ReuseAddr should succeed if you try to bind to the same port but a different source address
+		srv1, fin1 := startServerFn(t, "tcp", fmt.Sprintf("localhost:%d", port), true)
+		srv2, fin2 := startServerFn(t, "tcp", fmt.Sprintf("%s:%d", ip, port), true)
+
+		if err := srv1.Shutdown(); err != nil {
+			t.Fatalf("failed to shutdown first server: %v", err)
+		}
+		if err := srv2.Shutdown(); err != nil {
+			t.Fatalf("failed to shutdown second server: %v", err)
+		}
+		if err := <-fin1; err != nil {
+			t.Fatalf("first ListenAndServe returned error after Shutdown: %v", err)
+		}
+		if err := <-fin2; err != nil {
+			t.Fatalf("second ListenAndServe returned error after Shutdown: %v", err)
+		}
+	})
+	t.Run("should-succeed-udp", func(t *testing.T) {
+		if !supportsReuseAddr {
+			t.Skip("reuseaddr is not supported")
+		}
+		ip, err := externalIPFn(t)
+		if err != nil {
+			t.Skip("no external IPs found")
+			return
+		}
+		port := freePortFn(t)
+
+		// ReuseAddr should succeed if you try to bind to the same port but a different source address
+		srv1, fin1 := startServerFn(t, "udp", fmt.Sprintf("localhost:%d", port), true)
+		srv2, fin2 := startServerFn(t, "udp", fmt.Sprintf("%s:%d", ip, port), true)
+
+		if err := srv1.Shutdown(); err != nil {
+			t.Fatalf("failed to shutdown first server: %v", err)
+		}
+		if err := srv2.Shutdown(); err != nil {
+			t.Fatalf("failed to shutdown second server: %v", err)
+		}
+		if err := <-fin1; err != nil {
+			t.Fatalf("first ListenAndServe returned error after Shutdown: %v", err)
+		}
+		if err := <-fin2; err != nil {
+			t.Fatalf("second ListenAndServe returned error after Shutdown: %v", err)
+		}
+	})
+}
+
 func TestServerRoundtripTsig(t *testing.T) {
 	secret := map[string]string{"test.": "so6ZGir4GPAqINNh9U5c3A=="}