Here are a few title options, playing with the question format and emphasizing the core issue and solution: Option 1 (Direct and Clear): * Connecting from EC2 to RDS: How do I troubleshoot \'Ca

Connecting from EC2 Instance to RDS: Addressing Firewalls and DNS

When attempting to connect to an RDS instance from an EC2 instance, users may encounter a "Can't connect to MySQL server" error. This issue stems from a combination of firewall configurations and DNS settings.

Firewall Configuration

To establish connectivity between the EC2 and RDS instances, the EC2 instance's security group must allow inbound traffic to the RDS instance's port 3306. Additionally, the RDS instance's security group must allow inbound connections from the EC2 instance's IP address or IPv4 CIDR block.

To configure the RDS security group, navigate to the RDS instance in the AWS Management Console and select "Security Group Rules." Click "Edit" and add an inbound rule allowing traffic from the EC2 instance's IP address or CIDR block.

DNS Resolution

For connectivity to succeed, the EC2 instance must be able to resolve the RDS instance's fully qualified domain name (FQDN) to its IP address. This can be achieved by ensuring that the EC2 instance's DNS settings are properly configured to use Amazon DNS, using either the following methods:

• Updating Resolv.conf: Add the following line to the "/etc/resolv.conf" file:

  nameserver 169.254.169.253

• Using Metadata Server: Query the metadata server for DNS information:

  echo $$ "cat /opt/aws/bin/cfn-init -s $$AWS::StackName -r Ecinstance -q ip"

Once these configurations are in place, connectivity should be established between the EC2 and RDS instances, allowing the successful execution of the MySQL command from the EC2 instance.

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