Can a custom HTTP monitor work with a JSON response?

Hi Ben,

 

 

You can send any TCP or HTTP request and check for any response with a TCP or HTTP monitor. But both need to be a single static string that's sent and a static string that's checked for in the response. You can use the steps in these two links to troubleshoot a failing monitor:

 

 

http://devcentral.f5.com/wiki/default.aspx/AdvDesignConfig/TroubleshootingLtmMonitors.html

 

http://support.f5.com/kb/en-us/solutions/public/12000/500/sol12531.html

 

 

tcpdump might be the simplest method to start troubleshooting this issue. You can use tcpdump to see the exact request bigd is sending and the response the pool member is sending.

 

 

If you want to look for a specific sequence of bytes in the response binary data, you can use ascii escape sequences (without the spaces between \ x and the two characters):

 

 

\ x 00 - null

 

\ x 01 - SOH

 

...

 

 

sol4186: Entering escape sequences in an ECV health monitor's send and receive strings

 

http://support.f5.com/kb/en-us/solutions/public/4000/100/sol4186.html

 

 

Aaron

Can you try to change the content type from "application/json" to "text/html"?

 

 

Btw, how do you inspect the contents? Any error message?

I'm a new member, so I'm just looking around.

Thanks Aaron and Hum. It seems that it should be possible to change the response to "text/html", I've asked our Devs to look into this. Currently, in order to inspect the results of the JSON repsonse I have to download the file (Firefox/IE) or use wget which dowloads automatically. Worst case I'll have to look at the bits as Aaron suggested. Ben

As an alternative, you might want to use curl command also ... curl can show the contents directly to standard output

Hi Aaron,

 

 

So my Devs say changing the response is not going to happen, I'm going to need to go the HEX route.

 

 

Using Wireshark, I sniffed the health check coming from the LTM on the web server, I see the response going back to the LTM self IP as 10 bytes of data representing:

 

 

{"d":true} , this equates to 7b 22 64 22 3a 74 72 75 65 7d

 

 

To make things easy for my initial test I'm just going to look for "true" which is 74727565

 

 

So I thought my receive string should be:

 

 

\x74\x72\x75\x65

 

 

But that is not matching.

 

 

I'm running :

 

BIG-IP 9.4.5 Build 1049.10 Final

 

 

Any thoughts?

 

 

Ben

 

I got this working using \x74\x72\x75\x65. The reason it was not working is because our Active LTM was all screwed up and needed a reboot, the LTM it was not syncing the configs from the GUI to the internals. Why? That is for us to find out another day... :-)

 

 

Thanks all!

 

 

 

Ben

Hi Ben,

 

 

You could probably simplify this a bit and just use "true" in the receive statement (without the quotes).

 

 

Aaron

For anyone reading this post! There's better ways to do this now.

 

JSON.sh is available now to do this in pure shell, https://github.com/dominictarr/JSON.sh

 

I've written up an example using JSON.sh as the basis; which should be able to handle most JSON responses generically.

 

http://blog.routedlogic.net/?p=555