Message from <zabbix-IP> is missing header. Message ignored.

[phil570]

Hello,

Yesterday, I don't know why, one of our zabbix agent became unreachable for more than 5 minutes, and this generated an alert in Zabbix. This happened just after linking a template to the host in Zabbix, but now, even if I unlink the template from the host, it doesn't go back to normal, so I don't know if it's related.

When I checked the server, it was up and running without any problem, so I checked that the zabbix agent was running. Then I looked at the logs of the agent and found this message "Message from <zabbix-IP> is missing header. Message ignored."

I started to search online for the reason, but couldn't find why I have this problem.

My Zabbix server version is 3.4.6 and the agent is 4.0.0alpha8.

Here are the troubleshooting steps that I did :

I already checked that the server IP address is added in zabbix_agentd.conf.
I tried to run the command zabbix_get -k system.uname -s 10.20.48.10 and go the response zabbix_get [95149]: Check access restrictions in Zabbix agent configuration
I also tried to telnet from the server to the host on the port 10050, I'm able to connect, but after 2-3 seconds, the connection is closed by the host :

[root@h916d1zab01p ansible]# telnet 10.20.48.10 10050
Trying 10.20.48.10...
Connected to 10.20.48.10.
Escape character is '^]'.
Connection closed by foreign host.

I don't know what to do anymore. Did someone already have this problem?

Thanks in advance,

Philippe

[ingus.vilnis]

Hi Philippe,

The reason for your problem is because 4.0 agents don't accept server requests without headers anymore.

https://www.zabbix.com/documentation/4.0/manual/installation/upgrade_notes_400#plain_text_protocol_dropped

In other words - 4.0 agents can't work with older servers using passive checks. Active checks still are working though.

Your options:
1) Downgrade agent to 3.4 (safest)
2) Upgrade server to 4.0 (prepare well for the upgrade, count downtime, have backups etc)
3) Switch to Active checks if that is applicable. (make sure active checks are configured, hostnames matching etc)

[phil570]

Hello Ingus.Vilnis,

Thank you very much for your fast answer.

I will try the solutions you suggested and I'll let you know

[thesubmitter]

Can you update the documentation to reflect this incompatbility please.

[ingus.vilnis]

You probably could create a feature request for the compatibility. However it has always been so that newer agents are not guaranteed to work with older servers and this is exactly the case.

As alternative switch your checks to Zabbix agent (active) and carry on without any problems. The only downside that with active checks you can't have custom scheduled polling intervals, and you can't force the "Check now' function.

[phil570]

Hello Ingus.vilnis,

Sorry for the (very) late answer.

Just to let you know that the suggested solution of downgrading the agent to 3.4 worked perfectly.

Many thanks for your help

[ingus.vilnis]

Hi,

Glad you got this resolved!