diff --git a/src/common.c b/src/common.c
index 7465eb5..37ae48d 100644
--- a/src/common.c
+++ b/src/common.c
@@ -404,6 +404,52 @@ check_topdomain(char *str, int allow_wildcard, char **errormsg)
 	return 0;
 }
 
+int
+query_datalen(const char *qname, const char *topdomain)
+{
+	/* Return number of data bytes embedded in DNS query name,
+	 * or -1 if domains do not match.
+	 */
+	int qpos = strlen(qname);
+	int tpos = strlen(topdomain);
+	if (tpos < 3 || qpos < tpos) {
+		/* Domain or query name too short */
+		return -1;
+	}
+	/* Backward string compare */
+	qpos--;
+	tpos--;
+	while (qpos >= 0) {
+		if (topdomain[tpos] == '*') {
+			/* Wild match, is first in topdomain */
+			if (qname[qpos] == '*') {
+				/* Don't match against stars in query name */
+				return -1;
+			} else if (qpos == 0 || qname[qpos-1] == '.') {
+				/* Reached start of query name or chunk separator */
+				return qpos;
+			}
+			qpos--;
+		} else if (tolower(qname[qpos]) == tolower(topdomain[tpos])) {
+			/* Matching char, exclude wildcard in query name */
+			if (tpos == 0) {
+				/* Fully matched domain */
+				if (qpos == 0 || qname[qpos-1] == '.') {
+					/* Start of name or has dot before matching topdomain */
+					return qpos;
+				}
+				/* Query name has longer chunk than topdomain */
+				return -1;
+			}
+			tpos--;
+			qpos--;
+		} else {
+			return -1;
+		}
+	}
+	return -1;
+}
+
 #if defined(WINDOWS32) || defined(ANDROID)
 #ifndef ANDROID
 int
diff --git a/src/common.h b/src/common.h
index 87800ed..0f990c1 100644
--- a/src/common.h
+++ b/src/common.h
@@ -128,6 +128,8 @@ void read_password(char*, size_t);
 
 int check_topdomain(char *, int, char **);
 
+int query_datalen(const char *qname, const char *topdomain);
+
 #if defined(WINDOWS32) || defined(ANDROID)
 #ifndef ANDROID
 int inet_aton(const char *cp, struct in_addr *inp);
diff --git a/tests/common.c b/tests/common.c
index 277fd5a..0b1fcab 100644
--- a/tests/common.c
+++ b/tests/common.c
@@ -143,6 +143,52 @@ START_TEST(test_topdomain_wild)
 }
 END_TEST
 
+START_TEST(test_query_datalen)
+{
+	char *topdomain = "r.foo.com";
+	/* With data */
+	ck_assert(query_datalen("foobar.r.foo.com", topdomain) == 7);
+	ck_assert(query_datalen("foobar.r.FoO.Com", topdomain) == 7);
+	ck_assert(query_datalen("foo.bar.r.FoO.Com", topdomain) == 8);
+	ck_assert(query_datalen(".r.foo.com", topdomain) == 1);
+	/* Without data */
+	ck_assert(query_datalen("r.foo.com", topdomain) == 0);
+	ck_assert(query_datalen("R.foo.com", topdomain) == 0);
+	/* Shorter query name */
+	ck_assert(query_datalen("foo.com", topdomain) == -1);
+	/* Mismatched query name */
+	ck_assert(query_datalen("b.foo.com", topdomain) == -1);
+	ck_assert(query_datalen("*.foo.com", topdomain) == -1);
+	/* Query name overlaps topdomain, but is longer */
+	ck_assert(query_datalen("bar.foo.com", topdomain) == -1);
+}
+END_TEST
+
+START_TEST(test_query_datalen_wild)
+{
+	char *topdomain = "*.foo.com";
+	/* With data */
+	ck_assert(query_datalen("foobar.a.foo.com", topdomain) == 7);
+	ck_assert(query_datalen("foobar.r.FoO.Com", topdomain) == 7);
+	ck_assert(query_datalen("foo.bar.r.FoO.Com", topdomain) == 8);
+	ck_assert(query_datalen("foo.Ab.foo.cOm", topdomain) == 4);
+	ck_assert(query_datalen("foo.Abcd.Foo.com", topdomain) == 4);
+	ck_assert(query_datalen("***.STARs.foo.com", topdomain) == 4);
+	ck_assert(query_datalen(".a.foo.com", topdomain) == 1);
+	ck_assert(query_datalen(".ab.foo.com", topdomain) == 1);
+	/* Without data */
+	ck_assert(query_datalen("rr.foo.com", topdomain) == 0);
+	ck_assert(query_datalen("b.foo.com", topdomain) == 0);
+	ck_assert(query_datalen("B.foo.com", topdomain) == 0);
+	/* Shorter query name */
+	ck_assert(query_datalen("foo.com", topdomain) == -1);
+	/* Wildcard part of query name matching topdomain */
+	ck_assert(query_datalen("aa.*.foo.com", topdomain) == -1);
+	/* Mismatched query name */
+	ck_assert(query_datalen("bar.r.boo.com", topdomain) == -1);
+}
+END_TEST
+
 START_TEST(test_parse_format_ipv4)
 {
 	char *host = "192.168.2.10";
@@ -245,6 +291,8 @@ test_common_create_tests()
 	tcase_add_test(tc, test_topdomain_length);
 	tcase_add_test(tc, test_topdomain_chunks);
 	tcase_add_test(tc, test_topdomain_wild);
+	tcase_add_test(tc, test_query_datalen);
+	tcase_add_test(tc, test_query_datalen_wild);
 	tcase_add_test(tc, test_parse_format_ipv4);
 	tcase_add_test(tc, test_parse_format_ipv4_listen_all);