Connect to a server by using SSH on Linux or Mac OS X

  • Last updated on: 2018-11-13
  • Authored by: Rackspace Support

This article provides steps for connecting to a cloud server from a computer running Linux® or MacOS® X by using Secure Shell (SSH). It also discusses generating an SSH key and adding a public key to the server.

Introduction

SSH is a protocol through which you can access your cloud server and run shell commands. You can use SSH keys to identify trusted computers without the need for passwords and to interact with your servers.

SSH is encrypted with Secure Sockets Layer (SSL), which makes it difficult for these communications to be intercepted and read.

Note: Many of the commands in this article must be run on your local computer. The default commands listed are for the Linux command line or MacOS X Terminal. To make SSH connections from Windows®, you can use a client similar to the free program, PuTTY. To generate keys, you can use a related program, PuTTYGen.

Log in

Using the Internet Protocol (IP) address and password for your cloud server, log in by running the following ssh command with username@ipaddress as the argument:

ssh root@123.45.67.890

The system prompts you to enter the password for the account to which you’re connecting.

Remote host identification

If you rebuilt your cloud server, you might get the following message:

WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!

One of the security features of SSH is that when you log in to a cloud server, the remote host has its own key that identifies it. When you try to connect, your SSH client checks the server’s key against any keys that it has saved from previous connections to that IP address. After you rebuild a cloud server, that remote host key changes, so your computer warns you of possibly suspicious activity.

To ensure the security of your server, you can use the web console in the Cloud Control Panel to verify your server’s new key. If you’re confident that you aren’t being spoofed, you can skip that step and delete the record of the old SSH host key as follows:

On your local computer, edit the SSH known_hosts file and remove any lines that start with your cloud server’s IP address.

Note: Use the editor of your choice, such as nano on Debian or Ubuntu servers or vi on RPM or CENTOS servers. For simplicity, this article just uses nano. If you prefer to use vi, substitute vi for nano in the edit commands. For more on using vi, see https://community.rackspace.com/general/f/general-discussion-forum/6676/basics-of-the-vi-text-editor. For more on using nano, see https://support.rackspace.com/how-to/modify-your-hosts-file/.

nano ~/.ssh/known_hosts

If you are not using Linux or MacOS X on your local computer, the location of the known_hosts file might differ. Refer to your OS for information about the file location. PuTTY on Windows gives you the option to replace the saved host key.

Generate a new SSH key pair

You can secure SSH access to your cloud server against brute force password attacks by using a public-private key pair. A public key is placed on the server and a matching private key is placed on your local computer. If you configure SSH on your server to accept only connections using keys, then no one can log in by using just a password. Connecting clients are required to use a private key that has a public key registered on the server. For more on security, review Linux server security best practices.

Use the following steps to generate an SSH key pair:

  1. Run the following command using your email address as a label. Substitute your email address for your_email@example.com in the command.

    ssh-keygen -t rsa -C "your_email@example.com"
    

    A message indicates that your public-private RSA key pair is being generated.

    At the prompt, press Enter to use the default location or enter a file in which to save the key and press Enter.

  2. If you want the additional security of a password for the key pair, enter a passphraseand press Enter. If you don’t want to use a password with the key pair, press Enter to continue without setting one.

    Your key pair is generated, and the output looks similar to the following example:

    Your identification has been saved in /LocalFileLocation/id_rsa.
    Your public key has been saved in /LocalFileLocation/id_rsa.pub.
    The key fingerprint is: 01:0f:f4:3b:ca:85:d6:17:a1:7d:f0:68:9d:f0:a2:db your_email@example.com
    
  3. Optionally, add your new key to the local ssh-agent file to enable SSH to find your key without the need to specify its location every time that you connect:

    ssh-add ~/.ssh/id_rsa
    

    You can use an SSH configuration shortcut instead of the ssh-agent file by following the instructions in the Shortcut configuration section later in this article.

Add the public key to your cloud account

To make it easy to add your key to new cloud servers that you create, upload the public key to your cloud account by following these steps:

  1. Log in to the Cloud Control Panel.
  2. In the top navigation bar, click Select a Product > Rackspace Cloud.
  3. Select Servers > SSH Keys.
  4. Click Add Public Key.
  5. Enter a key name, such as Work Laptop, to remind you which computer this key is for.
  6. Select the region for which you want to store the public key. To store your key in multiple regions, repeat these steps for each region. The key must reside in the same region as the server.
  7. Paste the contents of the id_rsa.pub file that you created into the Public Key field. You can get the file contents by either opening the file in a text editor or by running the following command:

    cat ~/.ssh/id_rsa.pub
    
  8. Click Add Public Key.

If you want to add the key manually, instead of by using the Control Panel, review Linux server security best practices and use the following command:

 ssh-copy-id -i ~/.ssh/id_rsa.pub {username}@{remotePublicIPAddress}

Create a new server by using a stored key

When you create a new cloud server, you can add a stored key to the new server.

  1. On the Create Server page, expand the Advanced Options section.

  2. From the SSH Key menu, select your key from the list.

  3. If you don’t see a stored key in the list, you can perform one of the following actions:

    • Switch the region for the new server to the region where you have stored the SSH key.
    • Repeat the steps in the preceding section, Add the public key to your cloud account, to add the key to the region in which you want to create the new server.

Add the key to an existing server

You can’t use the Cloud Control Panel to add a public key to an existing server. Follow these steps to add the key manually:

  1. On your cloud server, create a directory named .ssh in the home folder of the user that you connect to by using SSH.

    mkdir -p ~/.ssh
    
  2. Create or edit the authorized_keys file and add your public key to the list of authorized keys by using the following command:

    nano ~/.ssh/authorized_keys
    

    A key is all on one line, so ensure that the key isn’t broken by line breaks. You can have multiple keys in the authorized_keys file, with one key per line.

  3. Set the correct permissions on the key by using the following commands:

    chmod 700 ~/.ssh
    chmod 600 ~/.ssh/authorized_keys
    
  4. If you have any issues and need to fix permissions issues, run the following comand:

    restorecon -R -v /root/.ssh 
    

After you have added the public key to the authorized_keys, you can make an SSH connection by using your key pair instead of the account password.

Shortcut configuration

Use the following instructions to set up a connection shortcut by creating a ~/.ssh/config file on your local computer and adding your server and key details to it.

  1. Using a text editor, add the following text to the ~/.ssh/config file, changing the values to match your server information:

    Host shortcutName
    HostName serverAddressOrIPAddress
    User remoteUsername
    IdentityFile /path/to/appropriate/ssh/rsa/private/key
    

    Each of the following entries describes a feature of the server:

    • Host: A shortcut name that you use to tell SSH to use this connection.
    • HostName: The address of the server to which you connect.
    • User: The name of the user account to connect to on the server.
    • IdentityFile: The location of the private key file (id_rsa).
  2. After you set up the config file, connect to the server by using the following command with your shortcut name:

    ssh shortcutName
    

Troubleshooting

If you have trouble making a new connection after you restart the server, use the following steps to help you resolve the issue:

  • The best way to troubleshoot SSH or SFTP login issues is to attempt to login through SSH while logged into the Emergency Console and to watch the log, which typically includes the reason for a failure. If no reason is given, it could be a firewall issue. For RPM servers, run the following command to watch the log:

      tail -f /var/log/secure
    

    For Debian servers, run the following command to watch the log:

      tail -f /var/log/auth.log
    
  • If you get a connection timeout error, check the IP address that you used to ensure that it’s correct. You might also check the server’s iptables to ensure that it isn’t blocking the port used by SSH.
  • If you get a connection refused error, you might be trying to use SSH with the wrong port. If you changed your server to listen to a port other than 22, use the -p option with SSH to specify the port.
  • If your login is rejected, then you might have an issue with your key. Change the sshd configuration to allow password connections by setting PasswordAuthentication to yes. Restart the server and try again. If you connect after these changes, then the issue is with the key and you must verify that the key is in the right place on the server.
  • If all else fails, review your changes and restart the SSH daemon on the server by running the following command:

    sudo service ssh restart
    

    If you get a message that the SSH service is unknown, run the command with sshd as the service name instead.

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