internal/radvd: add support for DNSSL (#47)

Signed-off-by: Matt Layher <mdlayher@gmail.com>

integration/radvd/radvd_test.go

@@ -114,6 +114,8 @@ Retransmit time           :  unspecified (0x00000000)
   Autonomous address conf.:          Yes
   Valid time              :         7200 (0x00001c20) seconds
   Pref. time              :         1800 (0x00000708) seconds
+ DNS search list          : lan 
+  DNS search list lifetime:         1200 (0x000004b0) seconds
  MTU                      :         1500 bytes (valid)
  Source link-layer address: 02:73:53:00:CA:FE
  from fe80::73:53ff:fe00:cafe

internal/radvd/radvd.go

@@ -181,6 +181,12 @@ func (s *Server) sendAdvertisement(addr net.Addr) error {
 	}
 
 	options = append(options,
+		&ndp.DNSSearchList{
+			// TODO: audit all lifetimes and express them in relation to each other
+			Lifetime: 20 * time.Minute,
+			// TODO: single source of truth for search domain name
+			DomainNames: []string{"lan"},
+		},
 		ndp.NewMTU(uint32(s.iface.MTU)),
 		&ndp.LinkLayerAddress{
 			Direction: ndp.Source,