Performing SCP to an EC2 Instance Without a Password Prompt

Transferring files securely to an Amazon EC2 instance is a common need when working on AWS. If you’ve ever tried using scp to transfer files and encountered a password prompt, even though you’ve configured your SSH keys correctly, you’re not alone. Let’s walk through ensuring that your scp command for copying files to your EC2 instance is seamless and password-free.

The Importance of Argument Order in SCP

The key to using scp without a password prompt lies in the correct usage of command-line arguments. When using scp with a private key, the -i option must be specified at the very beginning, before any other arguments like the source and destination paths.

Here is the correct syntax for your command:

scp -i mykey.pem somefile.txt root@my.ec2.id.amazonaws.com:/

Explanation

• -i mykey.pem: This specifies the identity file, your private key, which is essential for authenticating without a password.
• somefile.txt: The file you wish to copy. Replace this with the actual path to your file.
• root@my.ec2.id.amazonaws.com:/: The destination where you are copying the file. Make sure to replace with your actual username and EC2 instance address.

If you omit the -i option or place it incorrectly, scp defaults to traditional username/password authentication, hence the password prompt.

Common Mistakes to Avoid

Incorrect Argument Order

Ensure -i is placed before the file and destination arguments. For instance, switching the order to place somefile.txt before -i mykey.pem can thwart the passwordless authentication:

scp somefile.txt -i mykey.pem root@my.ec2.id.amazonaws.com:/

Using the Wrong Username

The username root might not always be correct for your instance. Certain AMIs use different default usernames such as ec2-user, ubuntu, or admin. Checking the AMI documentation to determine the correct default username is crucial.

File Permissions

Ensure your private key file (mykey.pem) has the correct permissions. SSH requires private key files to be read-only by the owner.

chmod 400 mykey.pem

This command sets the file to be readable only by you, which is a security requirement SSH enforces.

Security Group and Network Issues

Make sure your EC2 instance’s security group allows SSH access from your IP address.

Security Groups function as virtual firewalls in AWS, controlling the inbound and outbound traffic to your instances. If your IP isn’t whitelisted, SCP will not be able to establish a connection, irrespective of key configuration.

For further details on configuring your instance’s security group, refer to AWS Security Groups documentation.

By aligning your command syntax accurately and ensuring network settings are properly configured, you should find your scp file transfers to EC2 instances both smooth and secure, without any unexpected password prompts.