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Monitor External Websites with GTM



The team is looking to set up a WIP to provide DNS failover for some external sites.  These sites are not behind our LTMs in the iQuery mesh.  We just want to peform a simple health check that determines whether the external site is up.  

Anyone have any experience with this?  I am able to curl to the websites just fine from the GTM devices.



@JustCooLpOOLe If you can reach it you can perform a health check to it. You should be able to configure this as a generic host with no health monitor and then at the pool level you would use a health monitor for the respective website that will return the information you would like to have to classify the site as working.

Thanks @paulj !  

We have that configuration but still shows down even though I can successfully curl to the site from our GTM.  We're looking into the prober pool configuration and routing to determine the source of the health check.

@JustCooLpOOLe You might take this opportunity on the GTM to perform a tcpdump to see what you are receiving back from the destination. The following should be a close enough syntax and you can change the port if it's HTTPS rather than HTTP.

tcpdump -nni 0.0:nnp host <gtm_ip> and host <destination_IP> and port 80

It is common that sometimes a curl will function slightly different than the health monitor. A good example of this is I was able to perform a curl from my lab LTM to my lab apache server and the response was the one I expected but when I did a very similar request using the health monitor it would fail because the server said it was not able to find the destination that I requested. I ended up having to add in an entry that allowed the server to respond to any request to itself on 80 which I later narrowed it down once I came up with a lab root domain to use.

Yeah, we're running tcpdumps now to see what's happening.  I'll update the thread if we find out what's going on.


Also be cognizant that F5 health checks don't follow redirects. So, any health check should be for the final URL. A health check only passes if a 200 status code is received, so if you receive a redirect or auth required, you may have issues without using the final URL or setting up authentication for the monitor.