The SSH connection to your server is one of the most important way of accessing and managing your server, because of this, it is a target for any attacker that wants access to your server. Securing SSH is therefore very important. As I have already described in Secure servers ssh access, there are a couple of precautions you can take to prevent attackers from guessing your password.

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SSH host keys are stored in /etc/ssh/, which you generally do not need to choose. These keys were generated when the openssh-server package was installed. You can list the fingerprint of the keys by ssh-keygen -l -f /etc/ssh/sshhostkey.pub though you will need to repeat this for each public key. To be able to use ssh, scp, or sftp to connect to the server from a client machine, generate an authorization key pair by following the steps below. Note that keys must be.

Since, if nothing else, it’s quite annoying to have to type passwords all the time, you might have already considered creating SSH-keys, and configured SSH passwordless login with SSH-key for your server.

If the SSH-key is compromised

If your ssh-key fell into the wrong hands, anybody could access your server from anywhere on the internet. Restricting the ssh-key is one possibility to reduce the risk of an attacker hacking into your server by obtaining the ssh-key. Of course, if an ssh-key has been compromised, it should be removed from all affected systems and replaced by a new one as soon as possible, but still, restricting the SSH-key in some way could avoid the worst scenario.

Aug 19, 2019  Steps to Creating SSH keys on CentOS Step 1: Create SSH Key Pair. Generating public/private rsa key pair. Step 2: Copy Public Key to CentOS Server. The fastest and easiest method is by utilizing. Step 3: Connect to Remote Server Using SSH Keys. Step 4: Disable Password Authentication.

With the following commands, you can generate ssh key. Run: ssh-keygen -t rsa. For a more secure 4096-bit key, run: ssh-keygen -t rsa -b 4096 Press enter when asked where you want to save the key (this will use the default location). Enter a passphrase for your key. This will re-generate the dsa keys for your host. This will generate a key pair for your system. Note: Administrators that have other users connecting to their sshd2 daemon should notify the users of the host-key change. If you do not, the users will receive a warning the next time they connect. SSH Keys and Public Key Authentication. The SSH protocol uses public key cryptography for authenticating hosts and users. The authentication keys, called SSH keys, are created using the keygen program. SSH introduced public key authentication as a more secure alternative to the older.rhosts authentication. I used the 'ssh-keygen -t rsa' command to generate keys and copied conte. Stack Exchange Network. Stack Exchange network consists of 175 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Restricting an SSH-Key

When you add a SSH-key to the server, you add it to the users .ssh/authorized_keys file. This file allows you to add the keys that are allowed to authenticate for this user, but the authorized_keys file allows much more then just adding the key. As I have already described in Subversion via ssh using authorized_keys, the authorized_keys file can be used to restrict the possibilities of the user when he is logging in with an SSH-Key.

The format of the .ssh/authorized_keys file is as follows:

The SSH-key you add when you set up the SSH passwordless login with SSH-key already contains the “keytype”, “base64-encoded-key” and “comment”. That leaves the “options” where you can define a lot of ssh options for the key. As shown in SSH passwordless login with SSH-key the options can set a specific command to be run after login instead of a shell or the user can be restricted so he cannot perform certain actions.

When you use the following option, you can restrict where a user is allowed to login from using the SSH-key. To do so, add the option shown below to the line of the SSH-key. If you need multiple options, they should be separated by a “,”.

The IP addresses or hostnames can be specified as a list in the ‘from’ pattern-list. The list is comma-separated and allows you to use the wildcards ‘*’ (a wildcard that matches zero or more characters) or ‘?’ (a wildcard that matches exactly one character). Such an authorized_keys line might then look like this:

The example line above would only allow the login from a client if it comes from an IP address in the range from 10.0.0.0 to 10.0.0.9 or with a host resolveable in the example.com domain. As the example shows, other options are separated with a comma.

Check the “AUTHORIZED_KEYS FILE FORMAT”section of the sshd(8) manpage for a full list of options you can define in the authorized_keys file.

Disable password login

You probably want to disable password login to avoid someone logging in from somewhere else then the hosts you have configured in the authorized_keys file. To disable password authentication globally for all users, the following 3 settings need to be changed to “no” in the /etc/ssh/sshd_config file:

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If you want to disable the password authentication only for one user you have two options. The first is to configure the “PasswordAuthentication no” parameter in the /etc/ssh/sshd_config inside a “Match directive” to apply it only for a specific user, a list of users or a group.

This does not work with sshd versions before 4.4 (See the OpenSSH 4.4 release notes). At the time of writing CentOS 5.10 uses OpenSSH 4.3 and so does not support the “Match directive”. CentOS 6.x includes OpenSSH 5.3 which supports “Match directive”. To determine the version of the SSH server running execute the following command.

The following Lines need to be added to the /etc/ssh/sshd_config to disable password login for one or more users. In the example below you can see a list of users provided in the Match directive:

If you need to disable password authentication for a group of users, the Match directive allows the following syntax to do this: Mediafire download key doesn't generate.

If the ssh server version does not allow the use of the Match directive you can still disable password login for certain users by locking the user account. What the –lock option really does is that it changes the password (which is stored encrypted) to a value that will never match any encrypted password (The value “!”):

With this locked user, you will still see the password prompt when you try to logon with them (with no SSH key), but whatever password you enter will never pass authentication. To unlock such an account, you would need to set a new password with this command:

As with any configuration of the ssh server, the daemon needs to be notified of configuration changes. This can be done in two ways: you can notify the ssh daemon via the “reload” option, or you can restart the ssh server via the “restart” option. In both cases the open ssh session will stay connected.

Test your setup

As with any setting, you should test it to verify you did not miss any configuration. While configuring sshd of a remote server I’ve always found it handy to stay online with one session where you configure the server and, test access to the server with another local terminal. The ssh server will not kick you out of a connected session while reloading the configuration or restarting it. This can be useful in cases where you lock yourself out by misconfiguration. As long as the session stays open, you can still correct the configuration ,even if you can not login any more with a new session.

To verify that the ssh-key is limited to only your IP address, it can be handy to have another location from where you can try to login to verify the server is really rejecting the key:

With this command you will try to login using the ssh-key provided in the command line. The -v option will help us to see if the server properly rejects the ssh-key. When you execute the command above from a client that is allowed to use the ssh-key, you should be directly logged in and see the shell, but from any other location you should find lines similar to the following:

Those lines indicate that the key was rejected by the ssh server as they are not allowed to be used from this IP address or hostname.

To verify that the password authentication is not allowed, use a user that does not have an ssh-key in its ssh configuration and try to login to the server:

The error message here shows the allowed authentication methods. You can see that the “password” method is not listed in the message and indicates that you have successfully disabled the password login.

Read more of my posts on my blog at http://blog.tinned-software.net/.

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Introduction

Secure Shell (SSH) is an encrypted protocol used by Linux users to connect to their remote servers.

Generally, there are two ways for clients to access their servers – using password based authentication or public key based authentication.

Using SSH keys for authentication is highly recommended, as a safer alternative to passwords.

This tutorial will guide you through the steps on how to generate and set up SSH keys on CentOS 7. We also cover connecting to a remote server using the keys and disabling password authentication.

1. Check for Existing Keys

Prior to any installation, it is wise to check whether there are any existing keys on the client machines.

Open the terminal and list all public keys stored with the following command:

The output informs you about any generated keys currently on the system. If there aren’t any, the message tells you it cannot access /.ssh/id_*.pub , as there is no such file or directory.

2. Verify SSH is Installed

To check if thw package is installed, run the command:

If you already have SSH, the output tells you which version it is running. Currently, the latest version is OpenSSH 8.0/8.0p1.

Note: Refer to our guide If you need to install and enable SSH on your CentOS system.

Steps to Creating SSH keys on CentOS

1. Start by logging into the source machine (local server) and creating a 2048-bit RSA key pair using the command:

If you want to tighten up security measures, you can create a 4096-bit key by adding the -b 4096 flag:

2. After entering the command, you should see the following prompt:

3. To save the file in the suggested directory, press Enter. Alternatively, you can specify another location.

Note: If you already have a key pair in the proposed location, it is advisable to pick another directory. Otherwise it will overwrite existing SSH keys.

4. Next, the prompt will continue with:

Although creating a passphrase isn’t mandatory, it is highly advisable.

5. Finally, the output will end by specifying the following information:

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Now you need to add the public key to the remote CentOS server.

You can copy the public SSH key on the remote server using several different methods:

1. using the ssh-copy-id script
2. using Secure Copy (scp)
3. manually copying the key

The fastest and easiest method is by utilizing ssh-copy-id. If the option is available, we recommend using it. Otherwise, try any of the other two noted.

1. Start by typing the following command, specifying the SSH user account, and the IP address of the remote host:

If it is the first time your local computer is accessing this specific remote server you will receive the following output:

2. Confirm the connection – type yes and hit Enter.

3. Once it locates the id_rsa.pub key created on the local machine, it will ask you to provide the password for the remote account. Type in the password and hit Enter.

4. Once the connection has been established, it adds the public key on the remote server. This is done by copying the ~/.ssh/id_rsa.pub file to the remote server’s ~/.ssh directory. You can locate it under the name authorized_keys.

5. Lastly, the output tells you the number of keys added, along with clear instructions on what to do next:

1. First, set up an SSH connection with the remote user:

2. Next, create the ~/.ssh directory as well as the authorized_keys file:

3. Use the chmod command to change the file permission:

chmod 700 makes the file executable, while chmod 600 allows the user to read and write the file.

4. Now, open a new terminal session, on the local computer.

5. Copy the content from id_rsa.pub (the SSH public key) to the previously created authorized_keys file on the remote CentOS server by typing the command:

With this, the public key has been safely stored on the remote account.

1. To manually add the public SSH key to the remote machine, you first need to open the content from the ~/.ssh/id_rsa.pub file:

2. As in the image below, the key starts with ssh-rsa and ends with the username of the local computer and hostname of the remote machine:

3. Copy the content of the file, as you will need later.

4. Then, in the terminal window, connect to the remote server on which you wish to copy the public key. Use the following command to establish the connection:

5. Create a ~/.ssh directory and authorized_keys file on the CentOS server with the following command:

6. Change their file permission by typing:

7. Next, open the authorized_keys file with an editor of your preference. For example, to open it with Nano, type:

8. Add the public key, previously copied in step 2 of this section, in a new line in (under the existing content).

9. Save the changes and close the file.

10. Finally, log into the server to verify that everything is set up correctly.

Once you have completed the previous steps (creating an RSA Key Pair and copying the Public Key to the CentOS server), you will be able to connect to the remote host without typing the password for the remote account.

All you need to do is type in the following command:

If you didn’t specify a passphrase while creating the SSH key pair, you will automatically log in the remote server.

Otherwise, type in the passphrase you supplied in the initial steps and press Enter.

Once the shell confirms the key match, it will open a new session for direct communication with the server.

Although you managed to access the CentOS server without having to provide a password, it still has a password-based authentication system running on the machine. This makes it a potential target for brute force attacks.

You should disable password authentication entirely by following the outlined steps.

Note: Consider performing the following steps through a non-root account with sudo privileges, as an additional safety layer.

1. Using the SSH keys, log into the remote CentOS server which has administrative privileges:

2. Next, open the SSH daemon configuration file using a text editor of your choice:

3. Look for the following line in the file:

4. Edit the configuration by changing the yes value to no. Thus, the directive should be as following:

5. Save the file and exit the text editor.
6. To enable the changes, restart the sshdservice using the command:

7. Verify the SSH connection to the server is still functioning correctly. Open a new terminal window and type in the command:

In this article, you learned how to generate SSH key pairs and set up an SSH key-based authentication. We also covered copying keys to your remote CentOS server, and disabling SSH password authentication.

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