EC2 instances won't respond when accessed by IPv4

Question

I have been creating simple EC2 instances, which include code to draw content from a DynamoDB database. At first they can be accessed normally by entering their IP address into a browser, but once the database is implemented any attempt to access them results in a "refused to connect" error. This persists after the database instance is deleted. What do I do? Below is the user data script for the EC2 instances:

#!/bin/bash -ex
wget https://aws-tc-largeobjects.s3-us-west-2.amazonaws.com/DEV-AWS-MO-GCNv2/FlaskApp.zip
unzip FlaskApp.zip
cd FlaskApp/
yum -y install python3-pip
pip install -r requirements.txt
yum -y install stress
export PHOTOS_BUCKET=${SUB_PHOTOS_BUCKET}
export AWS_DEFAULT_REGION=
export DYNAMO_MODE=on
FLASK_APP=application.py /usr/local/bin/flask run --host=0.0.0.0 --port=80

secondabhi_aws · Answer

It seems to be connection issue and firewall might be blocking it. Can you please make sure of following:

1. Network access control list (network ACL) associated with the instance's subnet has rules allowing both outbound and inbound traffic to the internet.
2. Route table that's associated with your instance’s subnet has a default route to an internet gateway (0.0.0.0/0).
3. Internet gateway exists and is associated with the route 
4. Security group attached to the instance’s elastic network interface has rules allowing outbound internet traffic (0.0.0.0/0) for required ports and protocols. Also make sure, inbound rule of the security group allows traffic for required protocl and ports

Also make sure, if the instance is in a public subnet then it has public ip address, as otherwise it won't be accessible outside the virtual private cloud (VPC)  in which it resides.

If none of this helps, then please go through this [re:Post Knowledge Center Article](https://repost.aws/knowledge-center/ec2-linux-resolve-ssh-connection-errors), this would certainly help you.

Hope you find this useful.

Abhishek

EC2 instances won't respond when accessed by IPv4

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