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I have some Ubuntu servers deployed in VULTR Global Cloud and I have about 300 network ports each server.

However, every server will be filtered in China suddenly (I can ping from China). I use nmap to scan a port like 22, It will show "open" at first most times, but when I tried to connect to the port, it will be filtered. After a few minutes, if I use nmap to scan the port, it will "open" again.

What strange is, if I reinstall the server, or even if I take a snapshot of the filtered server, then I restore the snapshot to the server, every ports will be nornal for several days. This confuses me.

I also have two servers, I don't wanted to reinstall to restore from snapshot, so I disable ufw, enable ufw, remove ufw... disable iptables, reboot server or do something similar like these, it will be ok suddenly for several days. (don't know how and why). Every time it took me 2 or 3 hours to make this fine.

I think this is not just because the GFW, and I don't know how to solve the problem. Can anyone help me to solve it or explain the reason ?

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