TL;DR... Write down your Tor server's GATEWAY IP address. if your server goes offline, try tracerouting to your Tor server's gateway IP address as well as your Tor server's IP address! You might be surprised by what you see!
And also install MTR... It is helpful!! Here's the detailed explanation on how you can perform MTR command on your computer and server https://support.gcore.com/hc/en-us/articles/19864311353105-How-to-run-MTR-on...
Okay, here is what happened...
Late last night I tried to log into my VPS where I am running my Tor bridge server. I was unable to connect. It seemed like my server was offline so I filed a ticket with my service provider which is GCore Labs ( https://hosting.gcorelabs.com/ ) (I've been running my Tor bridge on their server for 4 years.)
However, I discovered that my server was not actually down. I was able to login to my server using VNC in the control panel. I did some network diagnostics to diagnose my network connectivity problem. Even though my server was running, it was essentially offline. I could not ping or traceroute from my server to 8.8.8.8 or 1.1.1.1. It looked like there was a network outage. I used some looking glass servers to see what was going on as well.
Here was what I discovered... From the Internet, I could ping and traceroute my VPS server's gateway (which is one hop away from my server) but if I tried to ping or traceroute using my server's actual IP address then this is where it stopped working.
The culprit was Cogent Communications' routers! [AS174] As soon as packets destined for my server's IP address reached Cogent's routers, they were dropped!! However, packets destined for my server's gateway went through!
From what I understand, my server's gateway is the physical machine running
my VPS. How can a packet reach my server's gateway, but not reach my server? It's not like the packet was not making it from my gateway to my server. The packets were being dropped at the edge of Cogent's network [AS174]. I also ran a traceroute on Cogents own looking glass server ( https://cogentco.com/en/looking-glass ) and when I used my server's IP, it showed nothing but when I used my gateway IP it worked! Tell me how that is possible if Cogent was not blocking my server's IP address?
I sent Cogent an email asking why they were blocking my server's IP address in their routers. By the time they got back to me, my server was back online and was not being blocked. They sent me an email saying that packets going to my server were routing through their network, which by that time they were.
What is so interesting about my Tor server that a major network backbone blocks my IP?
If you look at my Tor server's usage graphs, you can see where it is periodically offline. Look at the 6 month graph for the average number of connected clients and you will see places where it is not online. https://metrics.torproject.org/rs.html#details/4A0B065DB3CF807C6910DFEF6D9CC...
I bet the reason that my server periodically goes offline is because it is being blocked and not because it is actually offline!
Have you ever had this happen to your server?
Landon