Linux Instance SSH Issues

Common causes for connection issues

Description: General reasons why SSH might fail.

Causes:

• Incorrect security group settings.
• Network ACL blocking traffic.
• Missing public/private key pair.

Error: Connection timed out

Solution:

• Ensure inbound rules allow SSH (port 22).
• Verify the instance has a public IP.
• Check network ACL settings.

Error: unable to load key ... Expecting: ANY PRIVATE KEY

Solution:

• Ensure you're using the correct `.pem` file.
• Convert the key to correct format if needed:
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ssh-keygen -p -m PEM -f key.pem

Error: User key not recognized by server

Solution:

• Ensure the correct key is present in `~/.ssh/authorized_keys`.
• Re-add the key:
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echo "ssh-rsa AAAA..." >> ~/.ssh/authorized_keys

Error: Permission denied or connection closed by [instance] port 22

Solution:

• Set correct permissions for the private key:
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chmod 400 key.pem

• Ensure SSH is enabled on the instance:
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sudo systemctl start sshd

Error: Unprotected private key file

Solution:

• Set strict permissions:
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chmod 400 key.pem

Error: Private key must begin with "-----BEGIN RSA PRIVATE KEY-----"

Solution:

• Ensure the `.pem` file is not corrupted.
• Download the correct key from AWS.

Error: Server refused our key or No supported authentication methods available

Solution:

• Ensure you are using the correct key.
• Check the instance's `sshd_config` file.

Cannot ping instance

Solution:

• Ensure ICMP is allowed in security groups.
• Check if instance is in a private subnet.

Error: Server unexpectedly closed network connection

Solution:

• Check for CPU/Memory exhaustion.
• Restart SSH service:
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sudo systemctl restart sshd

Error: Host key validation failed for EC2 Instance Connect

Solution:

• Remove the old key:
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ssh-keygen -R [instance-ip]

Can't connect to Ubuntu instance using EC2 Instance Connect

Solution:

• Ensure `ec2-instance-connect` package is installed:
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sudo apt update && sudo apt install ec2-instance-connect

I've lost my private key. How can I connect to my instance?

Solution:

• Stop the instance and create a new key pair.
• Detach the root volume and attach it to another instance.
• Manually add the new key to `~/.ssh/authorized_keys`.
• Reattach the volume and restart the instance.