I'm reaching out to the community for some insights and solutions regarding DNS scanning in Iran, particularly because as of April 2026, ICMP ping is largely ineffective due to heavy throttling and blocking by ISPs. Given these challenges, we're looking for ways to identify solid open DNS resolvers for tunneling protocols like DNSTT and Slipstream when the internet is restricted.
The big question I'm tackling is whether using a TCP handshake (specifically a TCP SYN probe to port 53) can effectively replace ICMP ping for host validation in this tricky network environment. Here's what I'm considering:
- Sending a TCP SYN to port 53: If I receive a SYN-ACK (meaning the port is open) or RST (meaning the port is closed but the host is alive), I can mark the IP as live.
- Following that, I would quickly send a lightweight DNS query to check for open resolvers and their performance metrics like latency and hijacking potential.
So, does this approach work well in networks that are heavily censored in Iran? I'm especially interested in feedback from developers and users who have experience with TCP SYN-based discovery in tools like PYDNS-Scanner or any custom setups. What have your real-world experiences shown regarding success rates, false positives, and potential DPI detection issues? Any advice on rate limiting and other creative methods to survive these restrictions would be greatly appreciated!
5 Answers
Consider modifying existing libraries to tweak retries and delays for DNS queries. Also, masscan can help scan networks quickly if you have the right bandwidth. Just proceed with caution due to the strict controls in place!
I recommend checking out tcping for testing TCP connections. It’s pretty useful for validating hosts over a TCP connection, especially using ports like 53. You can check out tcping.org for more info. Just be wary of the load you put on the network!
Using TCP SYN for initial validation works quite well in situations like yours. Common tools like nmap have this built in, and many users have reported that TCP probes are usually faster and more reliable than ICMP. Although DPI can detect these probes too, adding randomness can help evade throttling. It might be a bit tricky, but it's definitely worth experimenting with!
Totally agree, and adding a layer of randomness really makes a difference. Have you tested any specific tools or methods that you found most effective?
If you want to dive deeper into advanced techniques, definitely look into creating a custom setup for probing. This could help you avoid detection while efficiently scanning for open resolvers. Also, remember to maintain a low profile with your scans.
Instead of just relying on typical methods, you might also try some alternate approaches like scanning other ports (e.g., DoH - HTTPS over DNS) when TCP port 53 is giving you trouble. It's all about finding what works best in your specific case.
Definitely! Exploring DoH could open up new avenues especially since some users might have access to those services without restrictions.
Thanks for that tip! I'll definitely look into tcping. Balancing the load is crucial given the current situation.