How to trace the route of packets between VPNs?

Question

UDP

I study flows and here the question has ripened.
Let's say a VPN (UDP) is configured on the local machine, and even a triple, for example, then on the same machine there is a virtual machine in which all traffic passes through the first VPN. In this virtual machine we launch VPN (TCP). How will the traffic go?
How can I trace the route of the packets. Do I understand correctly that traffic from the provider in encrypted form goes via UDP to the first VPN, then from the first VPN goes via TCP to the second VPN in encrypted form and on the end machine (VPN TCP will be decrypted and delivered to the recipient).
So why does VPN TCP over VPN TCP not work when connected? Just a pair of UDP with TCP.

Answer 1

When double tunneling, be sure to take into account the MTU and lower the MTU on the nested tunnel. For TCP, the MTU issue is more critical, because usually the don't fragment flag is set in TCP. If at the same time you block ICMP need frag packets, then you get the classic black hole problem.
You can trace the route of a UDP packet to host:port 1.2.3.4:500 using the
traceroute -U -p 500 1.2.3.4 command

Answer 2

Perhaps the window.opener.location property will help you

Answer 3

Redirect can be done using the meta tag:
Where " my.address " is the address of the page to which the transition will be made.
If you look from the side of PHP, then you can make headers, i.e. :
Or, as you wrote above, see the windows.opener.location property.

Answer 4

To find out whether this is a direct entry or not, you can look at the REFFERER field in $_SERVER on the back. This is most likely the best option. Not a 100% guarantee though.

Answer 5

There are a lot of options in PHP, you can use header() if there is no output like echo before, you can also use meta and refresh (even with a delay).
I recommend using:
window.onload = function(){document.getElementById('url_id').click();}
Where you specify the id with the corresponding url, just put it in echo first.
The script that itself clicks on the link in this case when loading the page.