Solution for unable to SSH into a CAS host.

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The CVK host can be pinged, but cannot be accessed via SSH. The xconsole interface also shows a response.

As the xconsole interface on-site is also unable to login.

It is possible to try switching to tty2 login. After pressing alt + F2 to switch to tty2, you will be able to login, which could prove that the system is able to be accessed but there seems to be a problem with the SSH service. 

When attempting to SSH into the node from other nodes, an error occurred with input/output error. Initially, it was suspected that the root partition of the node was full or had become read-only, but after checking with tty2 login, it was discovered that this was not the case, and that touch files were proceeding normally. 

It is suspected that the SSH connection limit has been reached, thus preventing new connections. By performing netstat -anlp | grep -w 22, the number of connections occupying the SSH port was discovered to be in the thousands. Moreover, the sources of these connections were found to be originating from the CVM nodes. 

Similarly, by performing netstat -anlp | grep -w 22 and checking ps aux | grep for the corresponding process ID, it was discovered that this SSH connection was initiated by the Tomcat service.

Therefore, it can be confirmed that the Tomcat service on the CVM started too many SSH connections, causing the SSH connection limit on the CVK host to be reached, and new connections could not be established.

 Restart the tomcat8 service on the CVM node to resolve the issue.

 service tomcat8 restart