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Inbound DNS access is restricted

ec2

Classification:

compliance

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Description

Reduce the possibility of a breach by checking EC2 security groups for inbound rules that allow unfettered access to TCP and UDP port 53, commonly used during DNS resolution when a request is sent from DNS clients to DNS servers or between DNS servers.

Rationale

Malicious activity, such as distributed denial-of-service (DDoS) attacks, can occur when permitting unfettered DNS access.

Remediation

Console

Follow the Security group rules docs to learn how to add a security group rule that will restrict access to a specific port.

CLI

  1. Run describe-security-groups with a filter to expose security groups that allow access to port 53.

    describe-security-groups.sh

        aws ec2 revoke-security-group-egress
        --group-id your-group-id
        --ip-permissions '[{"IpProtocol": "tcp", "FromPort": 53, "ToPort": 53, "IpRanges": [{"CidrIp": "192.0.2.0/24"}]}]'