cmd/geth, p2p: add support for custom discovery UDP port (#24979)

This adds a new flag to set the discovery port to be different from
the TCP listener port.

Co-authored-by: Felix Lange <fjl@twurst.com>

p2p/server.go

@@ -136,6 +136,10 @@ type Config struct {
 	// the server is started.
 	ListenAddr string
 
+	// If DiscAddr is set to a non-nil value, the server will use ListenAddr
+	// for TCP and DiscAddr for the UDP discovery protocol.
+	DiscAddr string
+
 	// If set to a non-nil value, the given NAT port mapper
 	// is used to make the listening port available to the
 	// Internet.
@@ -549,7 +553,15 @@ func (srv *Server) setupDiscovery() error {
 		return nil
 	}
 
-	addr, err := net.ResolveUDPAddr("udp", srv.ListenAddr)
+	listenAddr := srv.ListenAddr
+
+	// Use an alternate listening address for UDP if
+	// a custom discovery address is configured.
+	if srv.DiscAddr != "" {
+		listenAddr = srv.DiscAddr
+	}
+
+	addr, err := net.ResolveUDPAddr("udp", listenAddr)
 	if err != nil {
 		return err
 	}