Load testing sites behind an EC2 load balancer with ApacheBench

When ever I try to load test a site that is load balanced on EC2 I get the following message:

ab -n 20 -c 20 http://www.somesite.com/

Benchmarking www.somesite.com (be patient)...Send request failed!
apr_socket_recv: Connection reset by peer (54)

Any sites that are hosted on EC2 without the load balancer can be load tested successfully, so I figure it's not related to the security group I have configured.

Does AWS EC2 load balancer block this type of request by default? If so how does it differentiate between an AB and normal browser based http request?

Does AWS EC2 load balancer block this type of request by default?

Not that I know of - are you by chance running Mac OS X Lion? There seem to be a commonly encountered bug running ab on this particular operating system release - according to Fixing ApacheBench bug on Mac OS X Lion a patch is available (see there for details), though it's probably worthwhile and preferable to check for an official update first, insofar this patch (or an equivalent fix) has supposedly been incorporated in the meantime as of version httpd-2.3.15-beta at least (see Fixing Apachebench on OS X Lion).

Obviously, you could verify this first (and apply it as a workaround as well in case) by running the same test from a respectively different operating system (release).

Good luck!

Amazon ELB's are scaled up when load increases. When you have 5 requests per second you are on a different load balancer than the people running 1000 requests per second. What happens is that when you're running tests on the loadbalancer it goes from 5 to 1000 requests per second and the loadbalancer cannot handle the load so it will deny requests. If you wait 10-15 minutes for the loadbalancer to scale up while the test is running you should notice that all the requests will complete.

Amazon can manually upgrade your loadbalancer if you expect this is going to cause problems with your production environment. I have a loadbalancer / auto scaling group set up in the cloud and our traffic can go from a few hundred per second to a few thousand in a matter of 1-2 hours on the weekends and we don't actually see this behavior while running in the actual production environment, we only see this when we run tests with jmeter.

Load balancers are often configured to rate-limit or block requests that don't look like they came from "real browsers" as a security measure. I suspect that might be the case here; I'm sure Amazon has some docs.

Consider capturing a real request from your browser (using say Fiddler or Firebug) and add the headers you might need with ab's -H parameter. You often need a "Host", "Accept", and "Accept-Encoding" for example, perhaps with a user-agent string that looks realistic.