What happened:
My web domain is files.oracle.silentred.club
Backend ip is 3.113.87.179. This backend has 80 port for web page, and 6800 port for websocket service.
I enabled proxy for this domain in DNS configuration on Cloudflare.
Problem:
# Just cannot establish TCP connection by domain
telnet files.oracle.silentred.club 6800
Trying 104.27.175.3...
^C
# Direct connection is ok. I think there is something wrong with the Cloudflare proxy.
telnet 3.113.87.179 6800
Trying 3.113.87.179...
Connected to 3.113.87.179.
Escape character is '^]'.
^]quit
telnet> quit
Connection closed.