server6: Fix listening on multicast addresses

Joining a multicast group with an address that can't be received on a
socket is ineffective, at least on linux.
This updates the logic of NewServer in a mostly backwards-compatible
way, to enable listening on arbitrary multicast addresses:

 * Unicast addresses see no user-visible change, but don't join a
   multicast group for which they don't receive traffic anyway
 * Multicast addresses start actually receiving traffic for the group
   they represent, and don't join the default group. **this is a
   behaviour change**: previously they would receive traffic for the
   default group if it was on the same port and **not** for the group
   they represent. I consider that previous behaviour a bug
 * Wildcard addresses, if on the proper port, will join both
   AllDHCPRelayAgentsAndServers and AllDHCPServers **this is a behaviour
   change**: previously only AllDHCPRelayAgentsAndServers was joined
 * Wildcard addresses on another port: no visible change, same as
   unicast case

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

1 files changed, 34 insertions, 19 deletions

diff --git a/dhcpv6/server6/server.go b/dhcpv6/server6/server.go
index ab9cb32..4506195 100644
--- a/dhcpv6/server6/server.go
+++ b/dhcpv6/server6/server.go
@@ -126,31 +126,46 @@ func NewServer(ifname string, addr *net.UDPAddr, handler Handler, opt ...ServerO
 		o(s)
 	}
 
-	if s.conn == nil {
-		// no connection provided by the user, create a new one
-		conn, err := net.ListenUDP("udp6", addr)
+	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
+	}
+
+	var iface *net.Interface
+	if ifname == "" {
+		iface = nil
+	} else {
+		iface, err = net.InterfaceByName(ifname)
 		if err != nil {
 			return nil, err
 		}
-		// join multicast group on the specified interface
-		var iface *net.Interface
-		if ifname == "" {
-			iface = nil
-		} else {
-			iface, err = net.InterfaceByName(ifname)
-			if err != nil {
+	}
+
+	p := ipv6.NewPacketConn(s.conn)
+	if addr.IP.IsMulticast() {
+		if err := p.JoinGroup(iface, addr); err != nil {
+			return nil, err
+		}
+	} else if addr.IP.IsUnspecified() && addr.Port == dhcpv6.DefaultServerPort {
+		// For wildcard addresses on the correct port, listen on both multicast
+		// addresses defined in the RFC as a "default" behaviour
+		for _, g := range []net.IP{dhcpv6.AllDHCPRelayAgentsAndServers, dhcpv6.AllDHCPServers} {
+			group := net.UDPAddr{
+				IP:   g,
+				Port: dhcpv6.DefaultServerPort,
+			}
+			if err := p.JoinGroup(iface, &group); err != nil {
 				return nil, err
 			}
+
 		}
-		group := net.UDPAddr{
-			IP:   dhcpv6.AllDHCPRelayAgentsAndServers,
-			Port: dhcpv6.DefaultServerPort,
-		}
-		p := ipv6.NewPacketConn(conn)
-		if err := p.JoinGroup(iface, &group); err != nil {
-			return nil, err
-		}
-		s.conn = conn
 	}
+
 	return s, nil
 }