3 ms·
When an ISP blocks certain ports by dropping the SYN packet, the client sees a time out. There is nothing to "time" that can prove it's the ISP dropping it.
by mrb 9y ago
When an ISP blocks certain ports by dropping the SYN packet, the client sees a time out. There is nothing to "time" that can prove it's the ISP dropping it.
- jiveturkey 9y agoyes there is. When you don't get a RST back at the expected time (say *2), you know SYN was dropped. Are you arguing that it could be packet loss? You address that by taking multiple samples, and by comparing against loss to ports that you get ACK back from.
- mrb 9y ago«you know SYN was dropped» But you don't know who dropped it: the ISP or the remote server. In order to show it's the network between the client and server dropping it, you need a server that behaves in a known way, hence open.zorinaq.com I used to work in the InfoSec industry, running port scans from various locations, and open.zorinaq.com was incredibly useful to ensure there was no random firewall preventing us from finding certain open ports. That was the primary motivation why I built the service.