dns-domain: add dns_name_to_wire_format()

The function converts a domain name string to the wire format
described in RFC 1035 Section 3.1.

src/shared/dns-domain.c

@@ -715,3 +715,37 @@ int dns_name_single_label(const char *name) {
 
         return r == 0 && *name == 0;
 }
+
+/* Encode a domain name according to RFC 1035 Section 3.1 */
+int dns_name_to_wire_format(const char *domain, uint8_t *buffer, size_t len) {
+        uint8_t *label_length;
+        uint8_t *out;
+        int r;
+
+        assert_return(buffer, -EINVAL);
+        assert_return(domain, -EINVAL);
+        assert_return(domain[0], -EINVAL);
+
+        out = buffer;
+
+        do {
+                /* reserve a byte for label length */
+                if (len == 0)
+                        return -ENOBUFS;
+                len--;
+                label_length = out;
+                out++;
+
+                /* convert and copy a single label */
+                r = dns_label_unescape(&domain, (char *) out, len);
+                if (r < 0)
+                        return r;
+
+                /* fill label length, move forward */
+                *label_length = r;
+                out += r;
+                len -= r;
+        } while (r != 0);
+
+        return out - buffer;
+}

src/shared/dns-domain.h

@@ -67,3 +67,5 @@ int dns_name_address(const char *p, int *family, union in_addr_union *a);
 
 int dns_name_root(const char *name);
 int dns_name_single_label(const char *name);
+
+int dns_name_to_wire_format(const char *domain, uint8_t *buffer, size_t len);