Recently I configured some passive checks in Zabbix. Now, I saw that some checks failed for some periods of time, but this is not clear when looking at the chart. It seems that the value was stable for the last week but in real performance went down and nobody recognized it.

Is there a way to send NULL to Zabbix to indicate that fetching of the data failed? And to add a trigger for this case? I'm currently using the command line tool zabbix_sender to upload the results, but can change to an alternative.

In contrast, munin stops graphing 10 minutes after receiving the last value and the missing values are also not influencing the calculation of the average value.

Here you can see how the 'missing data' is displayed:

in the chart (I marked the 'missing data'): enter image description here

in the 'Last data' table (the line is grayed out, but the value 'last check' shows almost the current time) enter image description here

Btw: the same problems is in the icmppingsec[] check which just returns 0 in case of an error. Not very helpful for graphing but at least this avoids false negatives.


2 Answers 2


The fact that you are using zabbix_sender implies that these items are of type "Zabbix trapper". With trapper items, the graphing behavior you described above is expected.

Regular items of types like "Zabbix agent" and "Zabbix agent (active)" have update interval configured, so the frontend knows that data is expected to come, say, every 30 seconds. If there is no data for a period of 4 update intervals or so, the graphing tool does not draw lines for this period. So, in your case, if the items would be of any other type, the marked lines would not be drawn.

However, with trapper items, there is no update interval, so the frontend does not know how frequently data is expected to come. It may come each minute, each day, or at irregular intervals. So the current solution is to connect every two points in the graph, and extend the first and last values to the border of the graph.

The behavior may be improved in future under https://support.zabbix.com/browse/ZBX-3356 .

  • Ok, I see, there is a bug report for this. This should be answer enough. But there is absolutely no way to explicitly send NULL to Zabbix? Feb 26, 2014 at 14:05
  • 1
    @Daniel: Unfortunately, there is no concept of "no value" in Zabbix. You either have to designate a specific numeric value to serve as an indicator of "no value" (like 0 for icmppingsec[]) or make the item go unsupported. In order to achieve the latter, you can send in a special string "ZBX_NOTSUPPORTED". This will not affect the graphing issue, but since Zabbix 2.2 it is possible to notify on internal events, which are generated when an item goes unsupported (see zabbix.com/documentation/2.2/manual/introduction/…).
    – asaveljevs
    Feb 26, 2014 at 14:48

In fact, it is possible. But this can change in a future version, because I think it's a bug in Zabbix:

If you change the item type to 'Zabbix agent (active)' (instead of 'Zabbix trapper'), you can set the 'Update interval (in sec)' and this will work as expected, even when using zabbix_sender instead of the agent.

The reason for this: Zabbix agent and zabbix_sender use the same protocol (there are only minor differences like the string "sender data" vs. "agent data" in the "request" field). Additionally, Zabbix doesn't seem to check the origin of agent data nor does it check the protocol string, so zabbix_sender can even be run on any host to push active zabbix agent results - and after a while (i measured about 2-3 times the update interval), the graph line stops as expected, if there is no data anymore.

The only one disadvantage of this solution: You cannot use 'Allowed hosts' option anymore.

  • Your explanation is absolutely correct. The bug itself is tracked at ZBX-7416.
    – asaveljevs
    Mar 27, 2014 at 8:47

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