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How To Troubleshoot SSH Authentication Issues On Your Droplet

PostedApril 28, 2017 11.4k views Linux Basics Arch Arch Linux CentOS CoreOS Debian FreeBSD RHEL Ubuntu Ubuntu 12.04 Ubuntu 16.04

Introduction

This is a continuation of our SSH troubleshooting article series.

The first article will help you determine when to troubleshoot an issue instead of migrating or redeploying, and provides resources for the information you'll need to have to troubleshoot effectively. The other parts of this series cover how to identify and resolve specific SSH errors, and are broken down by which phase of a successful SSH connection you need to debug:

Once the 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 walkthrough will cover the two most common: password and private/public key pair.

Prerequisites

To troubleshoot SSH issues, you will need to make sure your Droplet is responding normally from the web console with a working network configuration. You can do this by following the How and When To Troubleshoot SSH Issues tutorial.

Before troubleshooting SSH, you should always check your cloud panel for ongoing issues in the region impacting your Droplet, the hypervisor status, and the state of the Droplet through the web console.

Errors

Below are some common SSH authentication errors you might encounter.

Permission Denied With Password

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

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

Error output
root@111.111.111.111's password: Permission denied (publickey,password).
PuTTY Error output
root@111.111.111.111'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. You can learn more about how SSH keys work in our SSH Essentials tutorial.

This is the only authentication method supported when you create a Droplet using SSH keys. You can enable password authentication in the SSH service configuration file once you successfully login with your SSH key.

You might see an error like this:

Error output
Permission denied (publickey).
PuTTY Error output
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 are not able to recover access to the console, this could indicate issues with the file system that impact the authentication mechanism or configuration issues within the PAM subsystem. This would also impact attempts to reset the root password for the Droplet and log in through the console.

From the console, you'll see this login prompt:

Prompt
Ubuntu 14.04.4 LTS server tty1 server Login: Password:

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

Output
Login incorrect

After a password reset, you'll get this prompt like this:

Prompt
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 will then be prompted to enter the new password twice:

Prompt
Enter new UNIX password: Retype new UNIX password:

However, if after entering the same new password twice your session restarts (i.e., you're sent back to the prompt for login again) or you see an error, it typically means that there's a problem with one of the critical files used in managing your authentication data.

In this scenario, you will want to 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

Below are some troubleshooting methods and solutions to common SSH authentication errors.

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:

Output
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 Droplet web 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 will generate a public key, like this:

Output
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 will load 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 will contain 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 will collapse the + characters that it shows, and produce the public key.

Output
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 with OpenSSH 7 (FreeBSD and CoreOS, by default), any older DSA-based keys will not be 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, the following guides are great references:

If you need further help authenticating via SSH, you can open a ticket with our Support Team. Make sure to include:

  • 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.

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