I have several http sensors that I want to monitor the page load both in front and behind a load balancer. I want to use a simple HTTP sensor ever minute and a full page HTTP sensor every 10 minutes. The web servers will not respond if you dont specify a valid host header in the request.

I created a device for the IP address at the load balancer and three more devices (one per web server) with each of their IP addresses.

For each device, I then created a simple HTTP sensor with the full URL and it never loads. I get a "Read Timeout" error message on the status page.

For each device, I then created a full page load sensor with the full URL and it works!

What the heck is going on?

For web servers that dont require the host header, the simple HTTP sensor works fine.

Article Comments

HTTP and HTTP Full Web Page sensors use completely different technologies. For detailed information, please see What is the difference between "HTTP" and "HTTP Full Web Page" Web Server sensors?


Oct, 2010 - Permalink

Dear Terry,

PRTG of course uses a 'Host Header' with all its HTTP Sensors, otherwise they would practically almost always fail. "Read Timeout" means that the connection to the webserver is established, but PRTG does not receive an answer, or an answer that is too short. Please take a look at the webservers logs, maybe you can see there why the sensors go into "Read Timeout".

Best Regards.


Oct, 2010 - Permalink

Ok a little more data on the problem:

I do a packet capture on the probe and I never see any http packets to the IP address on the device. Note: the host header will resolve a different IP address because we typically want people to look at the site thru the load balancer - not directly, but I want PRTG to use the IP address as defined in the device, along with the host header as defined in the sensor.


Oct, 2010 - Permalink

The device and the IP of the device are not relevant in the case of an HTTP-Sensor, if this one has a full URL (ex. http://server/page.htm) in its own settings. Then the sensor will use this URL and resolve it via DNS to query 'server'. The host-header then will refer to 'server' (without DNS resolution).


Oct, 2010 - Permalink

Hi,

I would like some help with HTTP sensor issue. I had all my HTTP sensors down with the following error: "empty response", but all my HTTP services are ok.

Could you please help me?

Thanks in advance!


Dec, 2010 - Permalink

Dear Bruno,

which exact HTTP Sensors do you use?

Best Regards.


Dec, 2010 - Permalink

Torsten,

all of them with this issue are HTTPFULLSENSOR. I have one HTTPSENSOR, but this one is ok.

Thanks in advance!


Dec, 2010 - Permalink

This would be then a problem most likely with the installed IE on the PRTG PC/Server, as the HTTP Full Page Sensor uses an OCX-Instance of the IE. If this fails, there is unfortunately very little we can do. We recommend to have IE8 installed, please check if it is an older version which is installed.


Dec, 2010 - Permalink

Let me know if I understood.

This is an issue related to an OCX instance of the IE that all http full sensors use, and it doesn't matter if I access my web interface useing another browser. Is it?

Or it just happens if I use IE to access my web interface?

Thanks a lot.


Dec, 2010 - Permalink

Another thing. I have checked and my IE is version 8. Is there another thing to do/check?

Thanks


Dec, 2010 - Permalink

The only browser we can be sure of being installed on a PRTG machine is IE, which is why an OCX-instance is used for the HTTP Fullpage Sensor. Potentially other installed browser are ignored.
Another thing to check is if the user account under which PRTGs Probe Service runs is allowed to access this page with the IE, or if any security settings prevent this.


Dec, 2010 - Permalink