Add xt_ECHO sample target

extensions/libxt_ECHO.c

@@ -0,0 +1,34 @@
+#include <stdio.h>
+#include <getopt.h>
+#include <xtables.h>
+
+static void echo_tg_help(void)
+{
+	printf("ECHO takes no options\n\n");
+}
+
+static int echo_tg_parse(int c, char **argv, int invert, unsigned int *flags,
+                         const void *entry, struct xt_entry_target **target)
+{
+	return 0;
+}
+
+static void echo_tg_check(unsigned int flags)
+{
+}
+
+static struct xtables_target echo_tg_reg = {
+	.version       = XTABLES_VERSION,
+	.name          = "ECHO",
+	.family        = AF_UNSPEC,
+	.size          = XT_ALIGN(0),
+	.userspacesize = XT_ALIGN(0),
+	.help          = echo_tg_help,
+	.parse         = echo_tg_parse,
+	.final_check   = echo_tg_check,
+};
+
+static void _init(void)
+{
+	xtables_register_target(&echo_tg_reg);
+}