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I created a classic loadbalancer on AWS with two EC2 instances running a simple flask application.

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The security group on the ELB is set to allow all inbound/outbound. The security groups for the EC2 instances are set to HTTP 0.0.0.0/0 port 5000. The health of instances is good - I can see that in the ELB dashboard and also can see the 200 ping responses from my applications in debug. I can access the webapplications directly when using the EC2's ip addresses with no problem. However, when using ELB's public DSN, the request times out:

enter image description here

This timeout occurs even if the applications are not running. So I assume that the problem is at the front end (with ELB). I have gone through tons of aws troubleshoot suggestions (which are not very straightforward for me as a starter) but did not manage to get it working. Any ideas?

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Security Group allowing all traffic from all sourcesCheck your ELB's security group that it's allowing 0.0.0.0/0 to access port 80.

You say 'set to default' but by default there are no inbound rules on a security group.

  • Yes, firewalls are set "All traffic" - I assume that covers 0.0.0.0/0. – Nazar Mar 13 '18 at 12:12
  • "All traffic" just means all protocols/ports, the source still needs to be set to allow traffic from all IPs. Added image to illustrate. – Jason Floyd Mar 14 '18 at 7:11
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    Yep, explicitly specifying the rules solved the problem. Thank you. – Nazar Mar 16 '18 at 14:03

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