When executing the Checkmk agent for Linux via ssh you might encounter error messages, when something is configured properly. Usually the service Check_MK will notify you about any connection problems that might occur. Down below we will list a couple of these error messages and try to give some pointers as to what might solve your troubles.
|Error message||one probable cause||one probable solution|
|Agent exited with code 255: Permission denied, please try again.||The public key in the file authorized_keys on the host might contain an error. This can easily happen when - for example - a line break is somehow inserted in the key or you omitted a single character, when copying the key to the host.||Double and triple check, that the public key on the host you are trying to monitor is 100 % the same as on you Checkmk server.|
|Agent exited with code 255: Host key verification failed.CRIT, Got no information from host, execution time 0.0 sec|
The error message here is clear. The "host key verification failed". But what does this mean. It might just mean, that you never introduced your Checkmk server and the host to one another and hence the key fingerprint is not available in the file _~/.ssh/known_hosts_ on your Checkmk server.
This one can be resolved easily. Login to your site and create an ssh connection to your host.
ssh should now ask you, if you want actually want to connect to this machine. You should answer by typing 'yes'. This will add the host to the list of known hosts.