server6: Create UDP conn manually for more control

Similar to server4 where the UDP connection is manually created using
the socket interfaces, this creates a connection with adequate options:

 * SO_BINDTODEVICE or equivalent if an interface is requested
 * V6ONLY when supported by the operating system
 * Allows binding to a multicast address specifically instead of
   falling back to wildcard

Signed-off-by: Anatole Denis <natolumin@unverle.fr>

1 files changed, 22 insertions, 10 deletions

diff --git a/dhcpv6/server6/server.go b/dhcpv6/server6/server.go
index 4506195..ac25076 100644
--- a/dhcpv6/server6/server.go
+++ b/dhcpv6/server6/server.go
@@ -112,9 +112,14 @@ func WithConn(conn net.PacketConn) ServerOpt {
 	}
 }
 
-// NewServer initializes and returns a new Server object, listening on `addr`,
-// and joining the multicast group ff02::1:2 . If `addr` is nil, IPv6 unspec is
-// used. If `WithConn` is used with a non-nil address, `addr` and `ifname` have
+// NewServer initializes and returns a new Server object, listening on `addr`.
+// * If `addr` is a multicast group, the group will be additionally joined
+// * If `addr` is the wildcard address on the DHCPv6 server port (`[::]:547), the
+//   multicast groups All_DHCP_Relay_Agents_and_Servers(`[ff02::1:2]`) and
+//   All_DHCP_Servers(`[ff05::1:3]:547`) will be joined.
+// * If `addr` is nil, IPv6 unspec on the DHCP server port is used and the above
+//   behaviour applies
+// If `WithConn` is used with a non-nil address, `addr` and `ifname` have
 // no effect. In such case, joining the multicast group is the caller's
 // responsibility.
 func NewServer(ifname string, addr *net.UDPAddr, handler Handler, opt ...ServerOpt) (*Server, error) {
@@ -125,19 +130,21 @@ func NewServer(ifname string, addr *net.UDPAddr, handler Handler, opt ...ServerO
 	for _, o := range opt {
 		o(s)
 	}
-
 	if s.conn != nil {
 		return s, nil
 	}
 
-	var err error
-	// no connection provided by the user, create a new one
-	s.conn, err = net.ListenUDP("udp6", addr)
-	if err != nil {
-		return nil, err
+	if addr == nil {
+		addr = &net.UDPAddr{
+			IP:   net.IPv6unspecified,
+			Port: dhcpv6.DefaultServerPort,
+		}
 	}
 
-	var iface *net.Interface
+	var (
+		err   error
+		iface *net.Interface
+	)
 	if ifname == "" {
 		iface = nil
 	} else {
@@ -146,6 +153,11 @@ func NewServer(ifname string, addr *net.UDPAddr, handler Handler, opt ...ServerO
 			return nil, err
 		}
 	}
+	// no connection provided by the user, create a new one
+	s.conn, err = NewIPv6UDPConn(ifname, addr)
+	if err != nil {
+		return nil, err
+	}
 
 	p := ipv6.NewPacketConn(s.conn)
 	if addr.IP.IsMulticast() {