I setup an Elastic Beanstalk with load balancer forwarding port 80 to port 5000 on EC2 instance. My EC2 instance listens on port 5000, not port 80. The EC2 instance has a private ip 172.31.14.151. On another EC2 which is in the same subnet as the EC2 running the Springboot web server, I got http responses for the two following http request:
curl 172.31.14.151:5000
curl 172.31.14.151:80
I do not understand why I got http response from 172.31.14.15:80. The EC2 I am running the curl command is on the same subnet as the EC2 running webserver. The http request should not go through any router and not through load balancer. But the webserver is running on port 5000, not port 80.
Is there a Nginx instance running on the EC2 instance with webserver?
If I configure the webserver to listen on port 80 and let the Elastic loadbalancer forward port 80 to port 80 on EC2 instance, I got Nginx 502 bad gateway response from doing the curl request
curl 172.31.14.151:80
I don't know which Elastic Beanstalk Solution Stack you are using, but most of the AWS Solution Stacks come coupled with Proxy Servers by default. For example, if you're running Java SE the proxy server is NGINX, but if you're running Java with Tomcat the proxy server is Apache.
By default these proxies accept HTTP Traffic on the default HTTP port (80), manage the connections, then proxy the requests from the backing application server (In your case, port 5000). This helps manage the connection to the backing application, as well as serve static content, or if you configure them correctly, customized Error messages based on the HTTP Status code. I'd suggest that if you can, you send the load balancer traffic to port 80 because Apache or NGINX can usually handle connection load better than most custom applications.