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When accessing sites through my ISP, certain sites don't load. The DNS resolves fine (via nslookup; also, the hosts file is fine), but then accessing them through any port times out (eg. tracert or ping or ftp). After investigating which sites are blocked, I believe my ISP is blocking all traffic that is going through certain countries. My ISP is unaware of any such block, and told me that it is not its job to investigate why sites on my computers don't load. How can I diagnose more precisely where the block is, so I can tell my ISP where to look for the problem?

Alex
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    It might be that other ISPs are blocking traffic, or the host ISP themselves are. If your ISP is not blocking the traffic, there's little they can do. – Ron Trunk Sep 05 '19 at 19:38
  • Other ISP's can block my ISP's traffic? Can you give a reference for how this can happen, so I can find out who these other ISP's are? Maybe I can contact the other ISP's so that, if they can't unblock it, at least I can find out why it's being blocked. – Alex Sep 07 '19 at 20:16
  • ISPs exchange traffic according to business agreements. Those agreements may restrict what traffic they route. If you’re not their customer you have no influence. – Ron Trunk Sep 07 '19 at 20:18

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If this is a website, you can use tools like https://downforeveryoneorjustme.com/ to make sure that it's not just you.

Try a VPN. Many VPNs offer you a free trial and make it look like you're from another country.

If all else fails, do a deeper dive with tracert to see where in the leg your packets are getting lost.

If it's within the first 5 or so hops, odds are its local to your ISP or your immediate service area.

It's important to remember that computers see blocks as network outages and will try to route around the problem. So if VPN, etc., all work but it doesn't work without them, chances are good that your ISP is at fault.

scuba_mike
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  • I already said in my post that I believe my ISP was at fault -- precisely *because* all VPN's work. I said I was interested in *resolving* this problem (since the block by my ISP is unintentional): so a VPN won't help there. I also said in my original post that `tracert` times out (i.e., starting with the first hop): so it doesn't help my ISP figuring out *where* the blockage might be. – Alex Sep 07 '19 at 20:14
  • Why do you believe it’s your ISP – Ron Trunk Sep 07 '19 at 20:20
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    `tracert` timing out is usually because it's trying to resolve the IP address. Try running with the `-d` option (assuming Windows since the linux command is `traceroute`) and see what you get. You may need to play around with the options in tracert (for example, try port 80 or 443 since those are rarely blocked). If those still don't help, you're left with port scanning to see which ports are being blocked. Without more information (e.g. your location, ISP, which servers you're trying to hit), it will be difficult to help you. – scuba_mike Sep 16 '19 at 18:29