shared: dns-name - add dns_name_between()

Given three DNS names this function indicates if the second argument lies
strictly between the first and the third according to the canonical DNS
name order. Note that the order is circular, so the last name is
considered to be before the first.

src/shared/dns-domain.c

@@ -529,6 +529,28 @@ int dns_name_endswith(const char *name, const char *suffix) {
         }
 }
 
+int dns_name_between(const char *a, const char *b, const char *c) {
+        int n;
+
+        /* Determine if b is strictly greater than a and strictly smaller than c.
+           We consider the order of names to be circular, so that if a is
+           strictly greater than c, we consider b to be between them if it is
+           either greater than a or smaller than c. This is how the canonical
+           DNS name order used in NSEC records work. */
+
+        n = dns_name_compare_func(a, c);
+        if (n == 0)
+                return -EINVAL;
+        else if (n < 0)
+                /*       a<---b--->c       */
+                return dns_name_compare_func(a, b) < 0 &&
+                       dns_name_compare_func(b, c) < 0;
+        else
+                /* <--b--c         a--b--> */
+                return dns_name_compare_func(b, c) < 0 ||
+                       dns_name_compare_func(a, b) < 0;
+}
+
 int dns_name_reverse(int family, const union in_addr_union *a, char **ret) {
         const uint8_t *p;
         int r;

src/shared/dns-domain.h

@@ -50,6 +50,7 @@ unsigned long dns_name_hash_func(const void *s, const uint8_t hash_key[HASH_KEY_
 int dns_name_compare_func(const void *a, const void *b);
 extern const struct hash_ops dns_name_hash_ops;
 
+int dns_name_between(const char *a, const char *b, const char *c);
 int dns_name_equal(const char *x, const char *y);
 int dns_name_endswith(const char *name, const char *suffix);
 

src/test/test-dns-domain.c

@@ -160,6 +160,38 @@ static void test_dns_name_equal(void) {
         test_dns_name_equal_one("..", "..", -EINVAL);
 }
 
+static void test_dns_name_between_one(const char *a, const char *b, const char *c, int ret) {
+        int r;
+
+        r = dns_name_between(a, b, c);
+        assert_se(r == ret);
+
+        r = dns_name_between(c, b, a);
+        if (ret >= 0)
+                assert_se(r == 0);
+        else
+                assert_se(r == ret);
+}
+
+static void test_dns_name_between(void) {
+        /* see https://tools.ietf.org/html/rfc4034#section-6.1
+           Note that we use "\033.z.example" in stead of "\001.z.example" as we
+           consider the latter invalid */
+        test_dns_name_between_one("example", "a.example", "yljkjljk.a.example", true);
+        test_dns_name_between_one("a.example", "yljkjljk.a.example", "Z.a.example", true);
+        test_dns_name_between_one("yljkjljk.a.example", "Z.a.example", "zABC.a.EXAMPLE", true);
+        test_dns_name_between_one("Z.a.example", "zABC.a.EXAMPLE", "z.example", true);
+        test_dns_name_between_one("zABC.a.EXAMPLE", "z.example", "\\033.z.example", true);
+        test_dns_name_between_one("z.example", "\\033.z.example", "*.z.example", true);
+        test_dns_name_between_one("\\033.z.example", "*.z.example", "\\200.z.example", true);
+        test_dns_name_between_one("*.z.example", "\\200.z.example", "example", true);
+        test_dns_name_between_one("\\200.z.example", "example", "a.example", true);
+
+        test_dns_name_between_one("example", "a.example", "example", -EINVAL);
+        test_dns_name_between_one("example", "example", "yljkjljk.a.example", false);
+        test_dns_name_between_one("example", "yljkjljk.a.example", "yljkjljk.a.example", false);
+}
+
 static void test_dns_name_endswith_one(const char *a, const char *b, int ret) {
         assert_se(dns_name_endswith(a, b) == ret);
 }
@@ -225,6 +257,7 @@ int main(int argc, char *argv[]) {
         test_dns_name_normalize();
         test_dns_name_equal();
         test_dns_name_endswith();
+        test_dns_name_between();
         test_dns_name_root();
         test_dns_name_single_label();
         test_dns_name_reverse();