Fix Autossh Publickey Error On Ubuntu User Service

by Kenji Nakamura 51 views

Introduction

Hey guys! Running into issues with autossh on your Ubuntu 24.04.3 system, especially when trying to set it up as a user service? You're not alone! It’s a fairly common problem, especially when dealing with SSH tunnels and ensuring they persist across reboots. This article will delve deep into the dreaded "publickey error" that crops up when autossh is configured as a user service, why it happens, and, more importantly, how to fix it. We’ll break down the problem, explore the common pitfalls, and arm you with the knowledge to get your SSH tunnels up and running smoothly. Whether you’re a seasoned sysadmin or just getting your feet wet with Linux, this guide is designed to help you troubleshoot and resolve this frustrating issue.

Understanding the Publickey Error

So, what exactly is this "publickey error" that's plaguing your autossh service? In the world of SSH, authentication often relies on a pair of keys: a private key that stays securely on your machine and a public key that gets placed on the server you want to connect to. When you try to SSH into the server, your computer uses the private key to prove its identity, and the server checks it against the corresponding public key. If they match, you're in! However, if there's a mismatch, or if the server can't find your public key, you'll be greeted with the "publickey error." Now, when you're manually starting your autossh service, things might be working perfectly fine. This usually means that your SSH keys are set up correctly for your user account under your normal interactive shell environment. But, the twist comes when you configure autossh to run as a user service. A user service operates in a slightly different environment than your regular shell. It doesn't always have access to the same environment variables, and that's where things can get tricky. The service might not know where to find your SSH keys, or it might be using a different user context altogether. To really nail down the issue, we need to consider things like the user the service is running as, the paths to your SSH keys, and how autossh is being invoked. We'll break this down step-by-step to ensure you understand each component and how they interact.

Diagnosing the Issue

Alright, let's get our hands dirty and figure out what's causing this publickey kerfuffle. The first step is to put on your detective hat and gather some clues. Here’s a systematic approach to diagnosing the issue:

  1. Service Status: Start by checking the status of your autossh service. Use the command systemctl --user status your-autossh-service.service. This command will give you a glimpse into whether the service is running, any error messages, and the logs that can provide valuable insights. Look for lines that mention SSH, authentication failures, or any key-related issues. This will be your first breadcrumb.
  2. Journal Logs: Dig deeper into the logs using journalctl --user -u your-autossh-service.service. This command dives into the system journal specifically for your autossh service. Pay close attention to the timestamps to correlate errors with the service startup. Errors related to authentication, key access, or SSH connection failures are your prime suspects.
  3. User Context: Identify the user context under which autossh is running. If the service is running under a different user than you expect, it might not have access to the correct SSH keys. Check your service file (~/.config/systemd/user/your-autossh-service.service) and look for lines like User= or any directives that might change the user context.
  4. SSH Key Paths: Double-check that autossh knows where to find your SSH keys. The default location is usually ~/.ssh/id_rsa (for the private key) and ~/.ssh/id_rsa.pub (for the public key). However, if you're using a different key or storing them in a non-standard location, you need to make sure autossh is aware. Look for any -i flags in your autossh command that specify the identity file (private key).
  5. Permissions: File permissions can be sneaky culprits. SSH is very picky about the permissions on your .ssh directory and the keys themselves. Your private key should have permissions 600 (readable and writable only by the owner), and the .ssh directory should have permissions 700 (readable, writable, and executable only by the owner). Use ls -l ~/.ssh to check the permissions.
  6. SSH Agent: If you're using an SSH agent, make sure it's running within the user session and that autossh is aware of it. Sometimes, the agent isn't properly initialized when the service starts, leading to authentication failures. You might need to explicitly tell autossh to use the agent by setting the SSH_AUTH_SOCK environment variable.

By methodically going through these steps, you'll start to paint a clearer picture of what's going wrong. Each step is like peeling back a layer of the onion, bringing you closer to the root cause of the publickey error.

Common Causes and Solutions

Okay, now that we've done some detective work, let's zoom in on some of the most common culprits behind the autossh publickey error and, more importantly, how to fix them. Here are a few scenarios and their solutions:

1. Incorrect User Context

Problem: Your autossh service might be running under a different user account than the one you configured your SSH keys for. This often happens if you've inadvertently set the User= directive in your service file to a different user or if the service is being started by the system rather than your user session.

Solution:

  • Verify the User: Double-check your service file (~/.config/systemd/user/your-autossh-service.service) and make sure there's no User= directive or that it's set to your correct username. If you're running the service as a user service, it should ideally run under your user account by default.
  • User Session: Ensure that the service is started within your user session. Use the command systemctl --user start your-autossh-service.service to start the service specifically within your user context. Avoid using system-wide service managers like sudo systemctl start for user services.

2. Incorrect SSH Key Paths

Problem: Autossh might be looking for your SSH keys in the wrong location. This is especially common if you've moved your keys, are using a non-standard key filename, or haven't explicitly told autossh where to find them.

Solution:

  • -i Flag: In your autossh command, use the -i flag to explicitly specify the path to your private key. For example: autossh -i /path/to/your/private_key .... This ensures that autossh uses the correct key.
  • Default Location: If you're using the default key names (id_rsa and id_rsa.pub), make sure they're located in the ~/.ssh/ directory. This is the standard location where SSH clients look for keys.

3. Permission Problems

Problem: SSH is very strict about the permissions on your .ssh directory and your SSH keys. If the permissions are too open, SSH will refuse to use them, leading to the publickey error.

Solution:

  • Restrict Permissions: Use the following commands to set the correct permissions:
    chmod 700 ~/.ssh
chmod 600 ~/.ssh/id_rsa
chmod 644 ~/.ssh/id_rsa.pub
    These commands ensure that your .ssh directory is only accessible by you, your private key is only readable and writable by you, and your public key is readable by anyone but writable only by you.

4. SSH Agent Issues

Problem: If you're using an SSH agent to manage your keys, autossh might not be aware of it or the agent might not be running within your user session. This can happen if the agent isn't properly initialized when the service starts.

Solution:

  • SSH_AUTH_SOCK: Set the SSH_AUTH_SOCK environment variable in your service file. This variable tells autossh where to find the SSH agent's socket. Add the following to your service file:
    [Service]
Environment=SSH_AUTH_SOCK=%t/ssh-agent.socket
  • Agent Forwarding: Ensure that agent forwarding is enabled in your SSH configuration. This allows the SSH agent to be used for connections made through the tunnel. Add ForwardAgent yes to your ~/.ssh/config file for the relevant host.

5. Missing or Incorrect Public Key on the Server

Problem: The public key corresponding to your private key might not be present in the ~/.ssh/authorized_keys file on the server, or it might be incorrect.

Solution:

  • Copy Public Key: Use the ssh-copy-id command to copy your public key to the server:
    ssh-copy-id user@your-server
    This command automates the process of appending your public key to the ~/.ssh/authorized_keys file on the server. If you don't have ssh-copy-id, you can manually copy the contents of your ~/.ssh/id_rsa.pub file to the ~/.ssh/authorized_keys file on the server.
  • Verify Key: Double-check that the public key in ~/.ssh/authorized_keys on the server matches the public key on your client machine. Even a small discrepancy can cause authentication to fail.

By systematically addressing these common causes, you’ll be well on your way to squashing that publickey error and getting your autossh service purring like a kitten.

Example Service File Configuration

To make things crystal clear, let's look at an example of a well-configured autossh service file. This will give you a solid template to work from and highlight the key elements we've discussed.

[Unit]
Description=AutoSSH Tunnel Service
After=network.target

[Service]
User=yourusername
Environment=SSH_AUTH_SOCK=%t/ssh-agent.socket
ExecStart=/usr/bin/autossh -M 0 -o "ServerAliveInterval 30" -o "ServerAliveCountMax 3" -i /home/yourusername/.ssh/id_rsa -N -f -L 2000:localhost:3000 yourusername@yourserver

Restart=on-failure
RestartSec=60

[Install]
WantedBy=default.target

Let's break down this service file:

  • [Unit] Section:
    • Description: A human-readable description of the service.
    • After=network.target: This ensures that the service starts after the network is up.
  • [Service] Section:
    • User=yourusername: This specifies the user account under which the service will run. Make sure this is your username!
    • Environment=SSH_AUTH_SOCK=%t/ssh-agent.socket: This sets the environment variable for the SSH agent, as discussed earlier.
    • ExecStart: This is the heart of the service. It specifies the command that will be executed to start autossh.
      • /usr/bin/autossh: The path to the autossh executable.
      • -M 0: Sets the monitoring port to 0, disabling the built-in monitoring feature (we're assuming you have other monitoring in place).
      • `-o