Forum Discussion
Klaus_75074
Nimbostratus
NimbostratusBIG-IP does not see string
Hi,
I have a problem with a custom health monitor on the BIG-IP:
I want to look for a string that is more or less at the end of the GET reply. The problem is that the BIG-IP does not see the string, it seems that it checks only the first x bytes of the reply (when I configure a string that is at the beginning, the monitor works). Is there a parameter for the "inspection depth"?
Regards,
Klaus.
hoolioCirrostratus
Hi Klaus,
SOL3451 describes a limit of 5120 bytes that BIG-IP will parse to find the receive string. I don't think this is configurable. The simplest fix might be to rework the page you're monitoring to ensure the string is in the first 5120 bytes. If that's not possible, you can use an EAV to get around the issue.
Aaron
Click here
An HTTP health monitor reads up to 5,120 bytes into HTTP data. If the monitor cannot find a match to the configured search string within the first 5,120 bytes, the BIG-IP LTM marks the node as DOWN.
Important: The standard HTTP health monitor does not send a well-formed HTTP GET statement that includes HTTP headers. Therefore, some servers may refuse to respond or not return response information as expected in the response HTTP headers. For information, refer to SOL3224: HTTP health checks may fail even though the node is responding correctly.
If you require a health monitor that searches for a string deeper than 5,120 bytes into an HTTP response, you must create an External Application Verification (EAV) health monitor. The amount of data an EAV health monitor can read is determined by the program that the EAV uses (such as cURL) to read the data.
For specific instructions about configuring an EAV health monitor, refer to the BIG-IP Reference Guide.
Klaus_75074Hi Aaron,
thanks ... do you know a good tutorial for writing external monitors. I found the chapter in configuration guide not that instructive.
Regards,
Klaus.- hoolioDeb added some handy examples to the codeshare section here:
http://devcentral.f5.com/wiki/default.aspx/AdvDesignConfig.CodeShare
Click here
Check out the cURL examples for something to get you started. Reply with your testing and any issues if you run into problems.
[Edit: SOL7444 has some helpful info as well]
Note: Beginning in BIG-IP LTM version 9.0, you can use an external monitor to monitor the status or health of a node using an application that is separate from the BIG-IP bigd health-checking daemon. The same behavior is achieved in BIG-IP version 4.x using an EAV pinger. For information about the requirements for an EAV output, refer to SOL1561: Requirements for EAV output.
By default, the bigd health-checking daemon assumes that any stdout messages sent from the external monitor to the console equate to a positive response, where the node is marked up.
When a node is up, the external monitor will print data to the stdout and bigd will mark the node up. When a node is down, the external monitor does not print data to the stdout and bigd will mark the node down.
Important: If a node fails to meet the external monitor criteria, the external monitor should not send any data to stdout. If you create an external monitor that displays a node as up, but also displays down, the bigd will interpret both responses as up.
Aaron - Klaus_75074Hi Aaron,
everything works fine now, thanks a lot.
Regards,
Klaus.