[server6] Server should join multicast address

The previous logic was wrong - there's no "listening" on multicast
address, the server should listen on the given address, and join the
multicast group. This PR fixes it.
Also moved the multicast addresses to a common package.

Tested with unit/integ tests, and with coredhcp.

Signed-off-by: Andrea Barberio <insomniac@slackware.it>

7 files changed, 40 insertions, 13 deletions

diff --git a/dhcpv6/async/client.go b/dhcpv6/async/client.go
index 4749e70..b2f977f 100644
--- a/dhcpv6/async/client.go
+++ b/dhcpv6/async/client.go
@@ -197,7 +197,7 @@ func (c *Client) receive(_ dhcpv6.DHCPv6) {
 
 func (c *Client) remoteAddr() (*net.UDPAddr, error) {
 	if c.RemoteAddr == nil {
-		return &net.UDPAddr{IP: client6.AllDHCPRelayAgentsAndServers, Port: dhcpv6.DefaultServerPort}, nil
+		return &net.UDPAddr{IP: dhcpv6.AllDHCPRelayAgentsAndServers, Port: dhcpv6.DefaultServerPort}, nil
 	}
 
 	if addr, ok := c.RemoteAddr.(*net.UDPAddr); ok {
diff --git a/dhcpv6/client6/client.go b/dhcpv6/client6/client.go
index c7bd318..3722579 100644
--- a/dhcpv6/client6/client.go
+++ b/dhcpv6/client6/client.go
@@ -17,12 +17,6 @@ const (
 	MaxUDPReceivedPacketSize  = 8192            // arbitrary size. Theoretically could be up to 65kb
 )
 
-// Broadcast destination IP addresses as defined by RFC 3315
-var (
-	AllDHCPRelayAgentsAndServers = net.ParseIP("ff02::1:2")
-	AllDHCPServers               = net.ParseIP("ff05::1:3")
-)
-
 // Client implements a DHCPv6 client
 type Client struct {
 	ReadTimeout   time.Duration
@@ -119,7 +113,7 @@ func (c *Client) sendReceive(ifname string, packet dhcpv6.DHCPv6, expectedType d
 	// if no RemoteAddr is specified, use AllDHCPRelayAgentsAndServers
 	var raddr net.UDPAddr
 	if c.RemoteAddr == nil {
-		raddr = net.UDPAddr{IP: AllDHCPRelayAgentsAndServers, Port: dhcpv6.DefaultServerPort}
+		raddr = net.UDPAddr{IP: dhcpv6.AllDHCPRelayAgentsAndServers, Port: dhcpv6.DefaultServerPort}
 	} else {
 		if addr, ok := c.RemoteAddr.(*net.UDPAddr); ok {
 			raddr = *addr
diff --git a/dhcpv6/defaults.go b/dhcpv6/defaults.go
index ae14bba..ba25437 100644
--- a/dhcpv6/defaults.go
+++ b/dhcpv6/defaults.go
@@ -1,6 +1,15 @@
 package dhcpv6
 
+import "net"
+
+// Default ports
 const (
 	DefaultClientPort = 546
 	DefaultServerPort = 547
 )
+
+// Default multicast groups
+var (
+	AllDHCPRelayAgentsAndServers = net.ParseIP("ff02::1:2")
+	AllDHCPServers               = net.ParseIP("ff05::1:3")
+)
diff --git a/dhcpv6/nclient6/client_test.go b/dhcpv6/nclient6/client_test.go
index 49b914e..902c7c5 100644
--- a/dhcpv6/nclient6/client_test.go
+++ b/dhcpv6/nclient6/client_test.go
@@ -65,7 +65,7 @@ func serveAndClient(ctx context.Context, responses [][]*dhcpv6.Message, opt ...C
 	h := &handler{
 		responses: responses,
 	}
-	s, err := server6.NewServer(nil, h.handle, server6.WithConn(serverRawConn))
+	s, err := server6.NewServer("", nil, h.handle, server6.WithConn(serverRawConn))
 	if err != nil {
 		panic(err)
 	}
diff --git a/dhcpv6/server6/server.go b/dhcpv6/server6/server.go
index e4fcf91..ab9cb32 100644
--- a/dhcpv6/server6/server.go
+++ b/dhcpv6/server6/server.go
@@ -58,6 +58,7 @@ import (
 	"net"
 
 	"github.com/insomniacslk/dhcp/dhcpv6"
+	"golang.org/x/net/ipv6"
 )
 
 // Handler is a type that defines the handler function to be called every time a
@@ -111,8 +112,12 @@ func WithConn(conn net.PacketConn) ServerOpt {
 	}
 }
 
-// NewServer initializes and returns a new Server object
-func NewServer(addr *net.UDPAddr, handler Handler, opt ...ServerOpt) (*Server, error) {
+// 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
+// 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) {
 	s := &Server{
 		handler: handler,
 	}
@@ -122,10 +127,29 @@ func NewServer(addr *net.UDPAddr, handler Handler, opt ...ServerOpt) (*Server, e
 	}
 
 	if s.conn == nil {
+		// no connection provided by the user, create a new one
 		conn, err := net.ListenUDP("udp6", addr)
 		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 {
+				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
diff --git a/dhcpv6/server6/server_test.go b/dhcpv6/server6/server_test.go
index 3b204f1..c81525a 100644
--- a/dhcpv6/server6/server_test.go
+++ b/dhcpv6/server6/server_test.go
@@ -33,7 +33,7 @@ func setUpClientAndServer(handler Handler) (*nclient6.Client, *Server) {
 		IP:   net.ParseIP("::1"),
 		Port: 0,
 	}
-	s, err := NewServer(laddr, handler)
+	s, err := NewServer("lo", laddr, handler)
 	if err != nil {
 		panic(err)
 	}
diff --git a/examples/server6/main.go b/examples/server6/main.go
index 318937f..363a3ae 100644
--- a/examples/server6/main.go
+++ b/examples/server6/main.go
@@ -18,7 +18,7 @@ func main() {
 		IP:   net.ParseIP("::1"),
 		Port: dhcpv6.DefaultServerPort,
 	}
-	server, err := server6.NewServer(laddr, handler)
+	server, err := server6.NewServer("", laddr, handler)
 	if err != nil {
 		log.Fatal(err)
 	}