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diegodias
Asked: 2011-11-24 13:48:26 +0800 CST

How Amazon ELB Health check Works?

  • 772

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)
load-balancing amazon-ec2 healthcheck

6 Answers

  1. 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.

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  2. 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.

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  3. Best Answer

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

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  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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  5. so i've just got the http check to work by pointing it at a static html file (as opposed to a php page).

    '/' does not work despite it returning a valid http status when using curl, etc. '/file.html' does however pass the health check.

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    • Firstly, Ensure the ELB security traffic has allowed traffic back from the instance, and the instance security group allows traffic from the ELB.
    • It was mentioned response must be 200
    • And finally do you have more than one interface from what I can tell it will choose randomly which interface.
    • ELB Needs to be in a subnet with internet route to IGW
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