tests: net: dns: Add test for invalid DNS answer parsing

Make sure we catch invalid answer during parsing.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>

tests/net/lib/dns_packet/src/main.c

@@ -50,6 +50,13 @@ static uint8_t query_mdns[] = {
 
 static uint16_t tid1 = 0xda0f;
 
+static uint8_t invalid_answer_resp_ipv4[18] = {
+	/* DNS msg header (12 bytes) */
+	0x01, 0x00, 0x80, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00,
+	0x00, 0x00, 0x01, 0x00, 0x01,
+};
+
+
 static int eval_query(const char *dname, uint16_t tid, enum dns_rr_type type,
 		      uint8_t *expected, uint16_t expected_len)
 {
@@ -1260,6 +1267,21 @@ static void test_dns_flags_len(void)
 		      "DNS message length check failed (%d)", ret);
 }
 
+static void test_dns_invalid_answer(void)
+{
+	struct dns_msg_t dns_msg = { 0 };
+	enum dns_rr_type type;
+	uint32_t ttl;
+	int ret;
+
+	dns_msg.msg = invalid_answer_resp_ipv4;
+	dns_msg.msg_size = sizeof(invalid_answer_resp_ipv4);
+	dns_msg.answer_offset = 12;
+
+	ret = dns_unpack_answer(&dns_msg, 0, &ttl, &type);
+	zassert_equal(ret, -EINVAL, "DNS message answer check succeed (%d)", ret);
+}
+
 void test_main(void)
 {
 	ztest_test_suite(dns_tests,
@@ -1271,7 +1293,8 @@ void test_main(void)
 			 ztest_unit_test(test_dns_id_len),
 			 ztest_unit_test(test_dns_flags_len),
 			 ztest_unit_test(test_dns_malformed_responses),
-			 ztest_unit_test(test_dns_valid_responses)
+			 ztest_unit_test(test_dns_valid_responses),
+			 ztest_unit_test(test_dns_invalid_answer)
 		);
 
 	ztest_run_test_suite(dns_tests);