How Amazon ELB Health check Works?

Question

I am having problems configuring ELB for my servers.

I start 2 micro instances with the exact same conf and try to do Load Balancing. However they never pass the health check (HTTP port 80 path:"/").

• Ping is ok on the website. So is telnet on 80.

How did the health check works? Am I doing anything really wrong?

EDIT:

• Both Direct browser access and GET (via curl) works correctly (status 200)

Solution 1

I have the same problem. I just have it checking TCP:80 as an interim solution (which works fine).

Solution 2

I've come to the conclusion that the response, apart from being a HTTP 200 response, must contain certain headers. I've had a HTTP 200 returned from my tomcat server running on the instance not work, but a static html page served by httpd (also returning a 200 code) work fine. Looking at the headers, one of the few differences was that the tomcat one didn't contain the content-type. Not sure why that would make a difference, though.

Solution 3

I assume it performs an HTTP GET request to the specified path "/", of each of the servers and looks for a successful HTTP response code (200). Can you successfully make a GET request to http://<backend_server_IP>:80/ via a browser (or CLI tool such as wget or curl).

If the request is served successfully, the second thing to ask is: have you configured your servers' security group so that access to port 80 is limited to a particular source address or subnet? If so, you need to add ELB's security group to the filter. The group is always called:

amazon-elb/amazon-elb-sg

So you can just add this under the 'Source' field in the Security Groups section of the AWS Console.

Solution 4

You must be using a specific domain name on your web server. Or you set up your web server to meet any request (binding *) or follow the documentation.

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Comments

[Amar AttilaZz]

I have the same problem. I just have it checking TCP:80 as an interim solution (which works fine).

[sharon]

It worked! Thanks! But I still dont know how the health check works .. :( Can you write an answer so I can check it as accepted?

[Wonil]

Are you using Django? Are you redirect HTTP to HTTPS? I summarized my experience how to solve AWS ELB health check with HTTP to my blog: androidkr.blogspot.kr/2014/03/…

[Giraffe]

They both don't pass the health check?

[sharon]

The port 80 is open to the world (0.0.0.0/0). Both direct browser access and curl works correctly (200)

[sharon]

It was not accessible working with the internal IP. I changed it, its now working, but the health check still not passing.