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So I've got two VPN subscriptions, let's call them VPN A and VPN B. And I like going to a somewhat popular website, which I'll call Site C.

I'm a new customer to VPN B, and I've noticed that I can't access Site C while using VPN B. I can access most other sites with VPN B, but not Site C. While I'm using VPN A, I can access Site C. While I'm not using any VPN, I can access Site C as well.

So my question is: is there any reliable way I can tell whether VPN B is blocking Site C, or Site C is blocking VPN B, or both? I understand that there are certain ways I can make intelligent guesses, but I'm looking for something more like a "smoking gun".

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    Do a traceroute to see how far you can go when accessing the site. Compare this was traces to sites you can access and then you'll see if the block is after the VPN exit (thus not caused by the VPN) or before (caused by the VPN). Commented Aug 13, 2023 at 12:32
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    That works if the block has been made on the routing layer & if traceroute isn't blocked. A VPN could block access to a site based on the SNI, too (transparent HTTPs proxy), which this would not detect. Commented Aug 13, 2023 at 12:35
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    @EsaJokinen: right, this would need a custom payload (TLS ClientHello) for a TCP traceroute, which isn't usually supported by traceroute programs. Commented Aug 13, 2023 at 12:47

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Some VPN providers allow switching the exit points, e.g., for changing the country. If a VPN provider is blocking a site it's likely blocked on every exit point, whereas the site may or may not block all the IP addresses the VPN could be having as an exit point. Therefore, if you can access the site by switching exit points it is likely blocked by the site rather than the VPN.

Otherwise, it would be hard to tell unless the VPN provider used some mechanism (like a placeholder page) for telling that it has blocked content, possibly including a reason for the decision.

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