I found that the TUN interface was actually writing back correctly. It seems that Xcode was just displaying incorrect information. Although Xcode showed 0.0.0.0:0 and no packets being read or written on that interface, it was likely just an issue with Xcode not displaying the correct interface properly.

No, this is on iOS. The goal is to drop specific DNS packets on iOS, while forwarding all other packets through a local socket. However, it currently seems impossible to achieve this on iOS. So far, I’ve only managed to forward UDP packets. For other protocols, I haven’t found a way to capture the full IP packet, making it impossible to reconstruct the IP packet correctly.