Q: Zabbix: agent not reachable but still working? I have a zabbix server which monitors two machines: the server itself and another one which collects some logs from other machines and send data to the zabbix server for monitoring.
Everything worked good since a week ago. 
Suddendly, the zabbix server stated that zabbix agent on the collector was unreachable.
The service is on, "netstat -tulpn" shows 10050 in listening mode, the data itself is transmitted to the zabbix server but...that server still think that the collector is unreachable (it even shows the red "ZBX" button in configuration-> host).
I wonder what's the problem here...any idea? 
I should add that there are no firewalls and that the entire process is (and has to be) on public ip and both the machines are CentOS 7.

A: This is a bit more of a troubleshooting process, but the most likely reasons are:


*

*connection from the server to the agent fails - try telnetting from server to port 10050 on the agent

*agent config changed and it does not allow connections from that server anymore (parameter Server); another possible cause would be server's outgoing IP changing


To get more clarity, move mouse cursor over the red "Z" and you will see an error message.

A: What about network congestion ?
I had the same problems with hosts on a remote network passing on an IPsec tunnel and the network load was too heavy at some moments (backups and other processes).
Network load on the computer itself (Zabbix client or Zabbix server).
Sometimes your hostname could be misconfigured on the client or the server.
