How to Troubleshoot SSH Authentication Issues

Validated on 4 Mar 2024 • Last edited on 8 Jul 2024

SSH is the primary method available for managing DigitalOcean Droplets. Dealing with SSH errors or failures can be frustrating because the errors themselves often prohibit you from accessing your servers.

There are two prerequisites to troubleshooting SSH issues:

  1. Should I troubleshoot SSH? Determine whether troubleshooting is the right decision or if migration/redeployment is more appropriate.
  2. What should I do before troubleshooting SSH?. Make sure the issue is truly with SSH, then review the information and skills necessary to resolve SSH issues, like having root access to the server and understanding how to access and edit files.

When to Consider Migration or Redeployment

To resolve your issue quickly, first determine whether troubleshooting the connection is the right solution for your problem or if you should instead focus on recovering your data for redeployment.

Some issues, such as an accidental recursive rm or chmod command or incorrect network configuration, can lock you out of a Droplet permanently. Other issues may seem like connection problems, but are actually more complex issues with no clear resolution, like corrupted file systems, incorrect file permissions and ownership, and broken system packages and required libraries.

You can typically identify boot errors through the Droplet console startup output. File system issues and startup failures that prevent a working console login session are signs that troubleshooting your network configuration may not be the better option. In situations like this, the best approach is to salvage what you can. In some cases, a good backup or snapshot strategy is the fastest way back to your previous working environment.

What to Do Before Troubleshooting

If you’ve decided that troubleshooting is right for your situation, go through the following steps:

  1. Check the control panel. Before anything else, make sure there are no ongoing issues, like an outage in the region impacting your Droplet.

  2. Check if Droplet is disabled because of abuse. Droplets are sometimes disabled due to the detection of abusive activity. If your Droplet has been disabled, an email has been sent to the email address linked to your DigitalOcean account with the title Networking Disabled: <your-droplet-name>. You can also log in to the support portal to see if any support tickets have been created for your resources.

    If your Droplet has been disabled due to suspected abuse, contact our support team for further information.

  3. Recover root access. If you do not have the current root password, reset it using the reset root password function in the control panel.

  4. Access the Recovery Console. If you cannot log in to the Droplet, the Recovery Console is another way to gain access (as long as your Droplet is running and you have a working root password).

  5. Reboot your Droplet. Many connectivity problems can be resolved after a reboot. If you’re experiencing connectivity issues, try rebooting the Droplet and see if this resolves the issue.

    Before rebooting your Droplet, we highly recommend taking a snapshot of it. This allows you to redeploy your Droplet in its current configuration if rebooting the Droplet causes more serious problems.

    To reboot your Droplet, log in to it and run the following command:

    sudo reboot

  6. Review file management and permissions. Some of these solutions may require you to review or edit files on the system or manage permissions.

  7. Check logs. Once you can get into the Droplet, check the system’s log files for more information to identify the error so you can then look up a solution.

    You can learn more about the logs on your server with this Linux logging tutorial and this journalctl and systemd logging tutorial.

  1. Use verbose SSH output. The level of detail an SSH client provides about the SSH session is generally quiet by default. It’s helpful to have more information when debugging an issue.

    For the OpenSSH client, you can use the -v option with multiple v entries to increase the verbosity of the output, as in ssh -v user@203.0.113.0. While most issues are revealed with a single v, some issues may benefit from -vvv.

    The PuTTY client supports an Event Log accessible from the context icon in the application window bar. There’s also an option for configuring session logging from the settings page when initiating the connection.

After you decide to troubleshoot an SSH issue instead of migrating or redeploying, you can identify and resolve specific SSH errors based on which phase of a successful SSH connection you need to debug.

Once the SSH connection is established and the protocol is initiated to communicate securely, the system can then verify the user connecting to the system. A wide variety of authentication mechanisms are supported. This walk-through covers the two most common: password and private/public key pair.

Errors

Permission Denied With Password

Note
If you assigned an SSH key when creating your Droplet, PasswordAuthentication is disabled for your Droplet and you need to use your SSH key to log in.

You might see these errors in both PuTTY and OpenSSH clients when attempting to log in to a Droplet with a password:

root@203.0.113.0's password:
Permission denied (publickey,password).

root@203.0.113.0's password:
Access denied

Server sent disconnect message
type 2 (protocol error):
"Too many authentication failures for root"

This indicates that authentication has failed and can be caused by a number of issues. Here are some steps you can take to troubleshoot this issue:

Permission Denied With Key

This login method uses cryptographic keys to authenticate a user. Learn more about how SSH keys work in SSH Essentials.

When you create a Droplet with SSH keys, SSH keys are the only authentication method supported. You can enable password authentication in the SSH service configuration file once you successfully log in with your SSH key.

You might see an error like this:

Permission denied (publickey).

Disconnected: No supported authentication methods available (server sent: publickey)

Many of the most common issues regarding key-based authentication are caused by incorrect file permissions or ownership. Here are some steps you can take to troubleshoot this issue:

Password Does Not Work In Console

If you cannot recover access to the console, this could indicate issues with the file system used for authentication or configuration issues within the PAM subsystem. This would also impact attempts to reset the root password and log in through the console.

From the console, you see this login prompt:

Ubuntu 14.04.4 LTS server tty1
server Login:
Password:

But when you enter the correct password, you might get this error:

Login incorrect

After a password reset, you receive a prompt like this:

You are required to change your password immediately (root enforced)
Changing password for root.
(Current) UNIX Password:

You must re-enter the current password. If your connection closes immediately, then you may have made a mistake re-entering the current password, so try again.

On success, you are then prompted to enter the new password twice:

Enter new UNIX password:
Retype new UNIX password:

However, if the session restarts after entering the same new password twice (meaning you get sent back to the login prompt), it typically means that there is a problem with one of the critical files managing your authentication data.

You can attempt to log in again using the console after a password reset.

If the problem persists after resetting the Droplet’s password, consider using the recovery environment to prepare your data for re-deployment or attempt to resolve the issues with the PAM configuration or file system.

Solutions

Checking Available Authentication Methods

If you use verbose SSH client output or logging, check that the message outlining authentication methods includes password and/or publickey in the list:

debug1: Authentications that can continue: publickey,password

If the message doesn’t include the authentication method you want to use, take a look at the /etc/ssh/sshd_config configuration file. It’s a common error to accidentally set the PasswordAuthentication value to yes but PermitRootLogin to no or without-password when logging in as root.

Ensure that the appropriate configuration for your login method is set, then restart the service.

Fixing Key Permissions And Ownership

The OpenSSH server and client require strict permissions on the key files used. Both the host and the client should have the following permissions and owners:

  • ~/.ssh permissions should be 700
  • ~/.ssh should be owned by your account
  • ~/.ssh/authorized_keys permissions should be 600
  • ~/.ssh/authorized_keys should be owned by your account

Client environments should additionally have the following permissions and owners:

  • ~/.ssh/config permissions should be 600
  • ~/.ssh/id_* permissions should be 600

These changes may need to be made through the Recovery Console.

Checking SSH Public And Private Keys

If you forget which private key matches which public key, OpenSSH tools and the PuTTY suite of applications provide a way to generate a public key from a private key. You can use that to compare the contents of the ~/.ssh/authorized_keys file on your Droplets.

To get a public key from a private key in an OpenSSH environment, use the ssh-keygen command as follows, specifying the path of the private key. By default, it’s ~/.ssh/id_rsa.

ssh-keygen -y -f ~/.ssh/id_rsa

This generates a public key, like this:

ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCfBiMwCU1xoVVp0VbSYV3gTDV/jB57IHdILQ8kJ2622//Lmi4gDPlxA6HXVKq8odkGD/5MjqUw85X2rwEbhoBul74+LCToYJvvvBaDPCgg5z1icCKIJ1m/LJBrGNqPKCgqFWu0EH4/EFP2XIQqWqX1BZtJu/2YWrTr+xFOE/umoYmOd+t3dzQqMsv/2Aw+WmA/x/B9h+41WrobDgCExYNLPYcD0PO7fpsa8CcrZCo+TUWCe7MgQQCSM6WD4+PuYFpUWGw3ILTT51bOxoUhAo19U8B2QqxbMwZomzL1vIBhbUlbzyP/xgePTUhEXROTiTFx8W9yetDYLkfrQI8Q05+f

In PuTTY environments, the PuTTYgen.exe command loads a GUI where you can use the Load action to import the private key file. In PuTTY, this is normally stored in .ppk format, and you need to know the location of the file.

Once you import the key, the window contains a Public key for pasting into OpenSSH authorized_keys file section with a similar-looking sequence. If you select that text and paste it into a file, it collapses the + characters that it shows, and produce the public key.

ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCfBiMwCU1xoVVp0VbSYV3gTDV/jB57IHdILQ8kJ2622//Lmi4gDPlxA6HXVKq8odkGD/5MjqUw85X2rwEbhoBul74+LCToYJvvvBaDPCgg5z1icCKIJ1m/LJBrGNqPKCgqFWu0EH4/EFP2XIQqWqX1BZtJu/2YWrTr+xFOE/umoYmOd+t3dzQqMsv/2Aw+WmA/x/B9h+41WrobDgCExYNLPYcD0PO7fpsa8CcrZCo+TUWCe7MgQQCSM6WD4+PuYFpUWGw3ILTT51bOxoUhAo19U8B2QqxbMwZomzL1vIBhbUlbzyP/xgePTUhEXROTiTFx8W9yetDYLkfrQI8Q05+f imported-openssh-key

You can ignore the comment following the public key (which is imported-openssh-key) as it may differ from your generated key comment.

In both cases, make sure this public key is included as a line in your ~/.ssh/authorized_keys file on the server, and add it if not.

OpenSSH 7 And Deprecated Key Algorithms

On systems with OpenSSH 7 (FreeBSD and CoreOS, by default), any older DSA-based keys are not supported for authentication. The ssh-dss key is considered weak and using more modern key algorithms is strongly recommended.

Consequently, the best solution is to generate more modern keys and update your existing hosts to allow the new keys. However, as a workaround, you can set the PubkeyAcceptedKeyTypes directive to +ssh-dss in your /etc/ssh/sshd_config file.

Conclusion

For steps on successfully setting up key-based authentication, you can learn how to add SSH keys to Droplets or read SSH Essentials: Working with SSH Servers, Clients, and Keys.

If you need further help, you can open a support ticket. Make sure to include the following information:

  • The username, host, and port you are using to connect.
  • The authentication mechanism you expect to use.
  • The full output of the errors linked to the stage of error, including verbose output of the SSH client
  • All of the information you’ve gathered from troubleshooting so far.
  • Anything you were unclear about while referencing this article.

Including all the above diagnostic information and clarifying where you are encountering the issue when trying to connect can help us quickly get up to speed with where your need on the issue is.

Problems with SSH connectivity include hostname resolution errors and connections being refused or timing out.
Problems during SSH protocol initiation include the client suddenly getting dropped or closed, the client returning errors about cipher negotiation, or issues with an unknown or changed remote host.
Problems with SSH shell environments include being unable to fork a process, the system reporting it’s not a valid shell, or issues reaching the home directory.
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