Add mDNS query construction

src/mdns.zig

@@ -41,11 +41,33 @@ fn get_mdns_socket(ip_info: util.IPInfo) !socket {
 }
 
 fn construct_mdns_query(domain: util.Domain, ip_info: util.IPInfo, buff: []u8) !usize {
-    _ = domain;
-    _ = ip_info;
     var n: usize = 0;
     var buff_writer = std.io.fixedBufferStream(buff);
-    n += try buff_writer.write("test");
+
+    const header =
+        [_]u8{0x00} ** 2 ++ // transaction ID
+        [_]u8{ 0x00, 0x00 } ++ // Standard Query flags
+        [_]u8{ 0x00, 0x01 } ++ // 1 question
+        [_]u8{0x00} ** 6 // Response only
+    ;
+    n += try buff_writer.write(&header);
+
+    for (domain.labels) |label| {
+        if (label.len == 0) break;
+        n += try buff_writer.write(&[_]u8{@truncate(label.len)});
+        n += try buff_writer.write(label);
+    }
+    n += try buff_writer.write(&[_]u8{0x00}); // End null byte
+
+    const question_footer =
+        [_]u8{ 0x00, switch (ip_info.version) {
+        util.IP_VER_ENUM.IPv4 => 0x01,
+        util.IP_VER_ENUM.IPv6 => 0x1c,
+    } } ++ // A or AAAA record
+        [_]u8{ 0x00, 0x01 } // IN query
+    ;
+    n += try buff_writer.write(&question_footer);
+
     return n;
 }