Core: Support REUSE_PORT (#2149)

Automatically submitted.

core/dnsserver/listen_go111.go

+// +build go1.11
+// +build aix darwin dragonfly freebsd linux netbsd openbsd
+
+package dnsserver
+
+import (
+	"context"
+	"net"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+const supportsReusePort = true
+
+func reuseportControl(network, address string, c syscall.RawConn) (opErr error) {
+	err := c.Control(func(fd uintptr) {
+		opErr = unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_REUSEPORT, 1)
+	})
+	if err != nil {
+		return err
+	}
+	return opErr
+}
+
+func listen(network, addr string) (net.Listener, error) {
+	lc := net.ListenConfig{Control: reuseportControl}
+	return lc.Listen(context.Background(), network, addr)
+}
+
+func listenPacket(network, addr string) (net.PacketConn, error) {
+	lc := net.ListenConfig{Control: reuseportControl}
+	return lc.ListenPacket(context.Background(), network, addr)
+}

core/dnsserver/listen_go_not111.go

+// +build !go1.11 !aix,!darwin,!dragonfly,!freebsd,!linux,!netbsd,!openbsd
+
+package dnsserver
+
+import "net"
+
+func listen(network, addr string) (net.Listener, error) { return net.Listen(network, addr) }
+
+func listenPacket(network, addr string) (net.PacketConn, error) {
+	return net.ListenPacket(network, addr)
+}

core/dnsserver/server.go

@@ -135,7 +135,7 @@ func (s *Server) ServePacket(p net.PacketConn) error {
 
 // Listen implements caddy.TCPServer interface.
 func (s *Server) Listen() (net.Listener, error) {
-	l, err := net.Listen("tcp", s.Addr[len(transport.DNS+"://"):])
+	l, err := listen("tcp", s.Addr[len(transport.DNS+"://"):])
 	if err != nil {
 		return nil, err
 	}
@@ -144,7 +144,7 @@ func (s *Server) Listen() (net.Listener, error) {
 
 // ListenPacket implements caddy.UDPServer interface.
 func (s *Server) ListenPacket() (net.PacketConn, error) {
-	p, err := net.ListenPacket("udp", s.Addr[len(transport.DNS+"://"):])
+	p, err := listenPacket("udp", s.Addr[len(transport.DNS+"://"):])
 	if err != nil {
 		return nil, err
 	}