TUN-7021: Fix proxy-dns not starting when cloudflared tunnel is run

This PR starts a separate server for proxy-dns if the configuration is
available. This fixes a problem on cloudflared not starting in proxy-dns
mode if the url flag (which isn't necessary for proxy-dns) is not
provided. Note: This is still being supported for legacy reasons and
since proxy-dns is not a tunnel and should not be part of the
cloudflared tunnel group of commands.

Author: sudarshan-reddy

cmd/cloudflared/tunnel/cmd.go

@@ -195,6 +195,13 @@ func TunnelCommand(c *cli.Context) error {
 		return runClassicTunnel(sc)
 	}
 
+	if c.String("proxy-dns") != "" {
+		// NamedTunnelProperties are nil since proxy dns server does not need it.
+		// This is supported for legacy reasons: dns proxy server is not a tunnel and ideally should
+		// not run as part of cloudflared tunnel.
+		return StartServer(sc.c, buildInfo, nil, sc.log)
+	}
+
 	return errors.New(tunnelCmdErrorMessage)
 }