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Logging Into User On Server: Permission Denied (publickey)

November 14, 2019

I recently spun up a Digital Ocean droplet and almost immediately ran into problems logging into it.

For context - the steps I took were:

  1. Log in as root:1

    $ ssh -i ~/.ssh/mykey root@host
  2. Create a new user2

    $ adduser $USERNAME
  3. Add that user to the super user group

    $ usermod -aG sudo $USERNAME
  4. I switched to the user to confirm that the user was created properly

    $ su $USERNAME
  5. Exit to get back to my local computer

    $ exit
    $ exit

    Needed to exit twice because the first one exited the user to return to root, the second exit is from root.

  6. Now, I tried to log into my user directly

    $ ssh $USERNAME@host

    This was when things started not working correctly.

    $ ssh $USERNAME@host
    $USERNAME@host: Permission denied (publickey).

I was expecting that I should be able to type in my password, my server seemed to be configured to only accept login via a public/private key exchange.

Allowing Direct Login Via SSH

To enable logging into my server directly as a user (rather than root - since my goal is to disable root eventually), I needed to be able to configure the user’s SSH key.

To do that I followed the following steps:

  1. Log into the server as root
  2. Modify (temporarily) the sshd_config for the server:

    • vim /etc/ssh/sshd_config
    • Find the PasswordAuthentication section and set it to yes

      # Change to no to disable tunnelled clear text passwords
      PasswordAuthentication yes
  3. Reload the sshd

    $ service sshd reload
  4. Exit the server and return to your local machine.
  5. Use ssh-copy-id, a tool from OpenSSH, to copy your public key to the server for the user.3

    $ ssh-copy-id -i ~/.ssh/mykey $USERNAME@host

If successful, you should see a printout that ends like:

Number of key(s) added:        1

Now try logging into the machine, with:   "ssh ‘$USERNAME@host’”
and check to make sure that only the key(s) you wanted were added.
  1. Give it a go - you should see something like:

    ssh -i ~/.ssh/mykey $USERNAME@host
    Welcome to Ubuntu 16.04.6 LTS (GNU/Linux 4.4.0-166-generic x86_64)
  2. Documentation: https://help.ubuntu.com
  3. Management: https://landscape.canonical.com
  4. Support: https://ubuntu.com/advantage

0 packages can be updated. 0 updates are security updates.

New release ‘18.04.3 LTS’ available. Run ‘do-release-upgrade’ to upgrade to it.

Last login: Sun Nov 3 15:55:34 2019 from xxx.xxx.xxx.xxx

## Quick Summary
The reason we couldn’t log into our user via `ssh` was because we:
1. Hadn’t configured the ssh public key for the user yet
2. The server did not allow password authentication for logging in.

The solution was:
1. Allow password authentication (temporarily) (in `sshd_config`)
2. Copy the public key to the server with `ssh-copy-id`

As a final step, I logged back into root for a moment to modify `sshd_config` to once again not accept password authentication, but that’s a user preference decision.

## Footnotes
* <sup>1</sup> The `host` is the IP address for the server. I also am specifying _which_ key to use because I decided to _not_ use the default key for my droplet. 
*  <sup>2</sup> Throughout this post, I refer to `$USERNAME` - this is a placeholder. In my case, for example, I typed `stephen` as the name of my user.
* <sup>3</sup> Learn more about `ssh-copy-id` here:  [Ssh-copy-id for copying SSH keys to servers | ssh.com ](https://www.ssh.com/ssh/copy-id). Digital Ocean also documented these approaches, including alternatives to `ssh-copy-id` here: [How to Upload an SSH Public Key to an Existing Droplet | DigitalOcean](https://www.digitalocean.com/docs/droplets/how-to/add-ssh-keys/to-existing-droplet/). 

## Resources
* [How to Upload an SSH Public Key to an Existing Droplet :: DigitalOcean Product Documentation](https://www.digitalocean.com/docs/droplets/how-to/add-ssh-keys/to-existing-droplet/)

Stephen Weiss

Thanks for reading! My name's Stephen Weiss. I live in Chicago with my wife, Kate, and dog, Finn.
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