Fix UDP listener on IPv4-only Linux (#787)

On some systems, IPv6 is disabled (for example, CIS benchmark recommends to disable it when not used), but currently all UDP connections are using AF_INET6 sockets.
When we are binding AF_INET6 socket to an address like ::ffff:1.2.3.4 (IPv4 addresses are parsed by net.ParseIP this way), we can't send or receive IPv6 packets anyway, so this will not break any scenarios.

---------

Co-authored-by: Wade Simmons <wsimmons@slack-corp.com>

udp/udp_linux.go

@@ -22,6 +22,7 @@ import (
 
 type StdConn struct {
 	sysFd int
+	isV4  bool
 	l     *logrus.Logger
 	batch int
 }
@@ -45,9 +46,22 @@ const (
 
 type _SK_MEMINFO [_SK_MEMINFO_VARS]uint32
 
+func maybeIPV4(ip net.IP) (net.IP, bool) {
+	ip4 := ip.To4()
+	if ip4 != nil {
+		return ip4, true
+	}
+	return ip, false
+}
+
 func NewListener(l *logrus.Logger, ip net.IP, port int, multi bool, batch int) (Conn, error) {
+	ipV4, isV4 := maybeIPV4(ip)
+	af := unix.AF_INET6
+	if isV4 {
+		af = unix.AF_INET
+	}
 	syscall.ForkLock.RLock()
-	fd, err := unix.Socket(unix.AF_INET6, unix.SOCK_DGRAM, unix.IPPROTO_UDP)
+	fd, err := unix.Socket(af, unix.SOCK_DGRAM, unix.IPPROTO_UDP)
 	if err == nil {
 		unix.CloseOnExec(fd)
 	}
@@ -58,17 +72,24 @@ func NewListener(l *logrus.Logger, ip net.IP, port int, multi bool, batch int) (
 		return nil, fmt.Errorf("unable to open socket: %s", err)
 	}
 
-	var lip [16]byte
-	copy(lip[:], ip.To16())
-
 	if multi {
 		if err = unix.SetsockoptInt(fd, unix.SOL_SOCKET, unix.SO_REUSEPORT, 1); err != nil {
 			return nil, fmt.Errorf("unable to set SO_REUSEPORT: %s", err)
 		}
 	}
 
 	//TODO: support multiple listening IPs (for limiting ipv6)
-	if err = unix.Bind(fd, &unix.SockaddrInet6{Addr: lip, Port: port}); err != nil {
+	var sa unix.Sockaddr
+	if isV4 {
+		sa4 := &unix.SockaddrInet4{Port: port}
+		copy(sa4.Addr[:], ipV4)
+		sa = sa4
+	} else {
+		sa6 := &unix.SockaddrInet6{Port: port}
+		copy(sa6.Addr[:], ip.To16())
+		sa = sa6
+	}
+	if err = unix.Bind(fd, sa); err != nil {
 		return nil, fmt.Errorf("unable to bind to socket: %s", err)
 	}
 
@@ -77,7 +98,7 @@ func NewListener(l *logrus.Logger, ip net.IP, port int, multi bool, batch int) (
 	//v, err := unix.GetsockoptInt(fd, unix.SOL_SOCKET, unix.SO_INCOMING_CPU)
 	//l.Println(v, err)
 
-	return &StdConn{sysFd: fd, l: l, batch: batch}, err
+	return &StdConn{sysFd: fd, isV4: isV4, l: l, batch: batch}, err
 }
 
 func (u *StdConn) Rebind() error {
@@ -143,7 +164,11 @@ func (u *StdConn) ListenOut(r EncReader, lhf LightHouseHandlerFunc, cache *firew
 
 		//metric.Update(int64(n))
 		for i := 0; i < n; i++ {
-			udpAddr.IP = names[i][8:24]
+			if u.isV4 {
+				udpAddr.IP = names[i][4:8]
+			} else {
+				udpAddr.IP = names[i][8:24]
+			}
 			udpAddr.Port = binary.BigEndian.Uint16(names[i][2:4])
 			r(udpAddr, plaintext[:0], buffers[i][:msgs[i].Len], h, fwPacket, lhf, nb, q, cache.Get(u.l))
 		}
@@ -192,13 +217,18 @@ func (u *StdConn) ReadMulti(msgs []rawMessage) (int, error) {
 }
 
 func (u *StdConn) WriteTo(b []byte, addr *Addr) error {
+	if u.isV4 {
+		return u.writeTo4(b, addr)
+	}
+	return u.writeTo6(b, addr)
+}
 
+func (u *StdConn) writeTo6(b []byte, addr *Addr) error {
 	var rsa unix.RawSockaddrInet6
 	rsa.Family = unix.AF_INET6
-	p := (*[2]byte)(unsafe.Pointer(&rsa.Port))
-	p[0] = byte(addr.Port >> 8)
-	p[1] = byte(addr.Port)
-	copy(rsa.Addr[:], addr.IP)
+	// Little Endian -> Network Endian
+	rsa.Port = (addr.Port >> 8) | ((addr.Port & 0xff) << 8)
+	copy(rsa.Addr[:], addr.IP.To16())
 
 	for {
 		_, _, err := unix.Syscall6(
@@ -221,6 +251,39 @@ func (u *StdConn) WriteTo(b []byte, addr *Addr) error {
 	}
 }
 
+func (u *StdConn) writeTo4(b []byte, addr *Addr) error {
+	addrV4, isAddrV4 := maybeIPV4(addr.IP)
+	if !isAddrV4 {
+		return fmt.Errorf("Listener is IPv4, but writing to IPv6 remote")
+	}
+
+	var rsa unix.RawSockaddrInet4
+	rsa.Family = unix.AF_INET
+	// Little Endian -> Network Endian
+	rsa.Port = (addr.Port >> 8) | ((addr.Port & 0xff) << 8)
+	copy(rsa.Addr[:], addrV4)
+
+	for {
+		_, _, err := unix.Syscall6(
+			unix.SYS_SENDTO,
+			uintptr(u.sysFd),
+			uintptr(unsafe.Pointer(&b[0])),
+			uintptr(len(b)),
+			uintptr(0),
+			uintptr(unsafe.Pointer(&rsa)),
+			uintptr(unix.SizeofSockaddrInet4),
+		)
+
+		if err != 0 {
+			return &net.OpError{Op: "sendto", Err: err}
+		}
+
+		//TODO: handle incomplete writes
+
+		return nil
+	}
+}
+
 func (u *StdConn) ReloadConfig(c *config.C) {
 	b := c.GetInt("listen.read_buffer", 0)
 	if b > 0 {