p2p/server: add DNS resolved candidates to distributed hash table

Currently, DNS resolved candidates of eth protocol are added as candidates for
dialing but not added to DHT. The DHT's seed nodes are still configured via
hardcoded nodes in bootnodes flag. As we want to replace the hardcoded bootnodes
entirely with DNS resolved nodes, we resolve some nodes from the DNS discovery
URL and put them into DHT when setting up the p2p discovery.

p2p/server.go

@@ -62,6 +62,12 @@ const (
 
 	// Maximum amount of time allowed for writing a complete message.
 	frameWriteTimeout = 20 * time.Second
+
+	// The number of DHT seed nodes got from DNS resolve
+	numSeedNodesFromDns = 5
+
+	// Name of eth protocol
+	ethProtocolName = "eth"
 )
 
 var errServerStopped = errors.New("server stopped")
@@ -539,6 +545,21 @@ func (srv *Server) setupDiscovery() error {
 	added := make(map[string]bool)
 	for _, proto := range srv.Protocols {
 		if proto.DialCandidates != nil && !added[proto.Name] {
+			// If the protocol is eth, the dial candidates are retrieved from
+			// DNS URL. We want to use the DNS discovery for bootstrap nodes
+			// too, so resolve some nodes from DNS and add to DHT.
+			if proto.Name == ethProtocolName {
+				for i := 0; i < numSeedNodesFromDns; i++ {
+					if !proto.DialCandidates.Next() {
+						break
+					}
+
+					node := proto.DialCandidates.Node()
+					srv.BootstrapNodes = append(srv.BootstrapNodes, node)
+					srv.BootstrapNodesV5 = append(srv.BootstrapNodesV5, node)
+				}
+			}
+
 			srv.discmix.AddSource(proto.DialCandidates)
 			added[proto.Name] = true
 		}