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>

cmd/utils/flags.go

@@ -835,6 +835,12 @@ var (
 		Usage:    "Sets DNS discovery entry points (use \"\" to disable DNS)",
 		Category: flags.NetworkingCategory,
 	}
+	DiscoveryPortFlag = &cli.IntFlag{
+		Name:     "discovery.port",
+		Usage:    "Use a custom UDP port for P2P discovery",
+		Value:    30303,
+		Category: flags.NetworkingCategory,
+	}
 
 	// Console
 	JSpathFlag = &flags.DirectoryFlag{
@@ -1116,12 +1122,15 @@ func setBootstrapNodesV5(ctx *cli.Context, cfg *p2p.Config) {
 	}
 }
 
-// setListenAddress creates a TCP listening address string from set command
-// line flags.
+// setListenAddress creates TCP/UDP listening address strings from set command
+// line flags
 func setListenAddress(ctx *cli.Context, cfg *p2p.Config) {
 	if ctx.IsSet(ListenPortFlag.Name) {
 		cfg.ListenAddr = fmt.Sprintf(":%d", ctx.Int(ListenPortFlag.Name))
 	}
+	if ctx.IsSet(DiscoveryPortFlag.Name) {
+		cfg.DiscAddr = fmt.Sprintf(":%d", ctx.Int(DiscoveryPortFlag.Name))
+	}
 }
 
 // setNAT creates a port mapper from command line flags.