Would you like to learn how to configure OpenSSH to allow SSH login using RSA keys? In this tutorial, we are going to show you all the steps required to configure the OpenSSH service ao allow SSH login using RSA keys on Ubuntu Linux. Ubuntu 18.04. Ubuntu 19.10. Ubuntu 20.04. 2019-9-6 If you interact regularly with SSH commands and remote hosts, you may find that using a key pair instead of passwords can be convenient. Instead of the remote system prompting for a password with each connection, authentication can be automatically negotiated using a public and private key pair. RSA is the most popular asymmetric encryption algorithm. In this tutorial we will look how to create RSA keys with ssh-keygen. RSA algorithm is created by researchers named Ron Rivest, Adi Shamir and Leonard Adleman in the MIT. And named with their names first letters. Ssh-keygen -t rsa -b 4096 ssh-keygen -t dsa ssh-keygen -t ecdsa -b 521 ssh-keygen -t ed25519 Specifying the File Name. Normally, the tool prompts for the file in which to store the key. However, it can also be specified on the command line using the -f option. Ssh-keygen -f /tatu-key-ecdsa -t ecdsa -b 521 Copying the Public Key to.
Did you know you can passwordless SSH? Here's how, and how to decide whether you should.
If you interact regularly with SSH commands and remote hosts, you may find that using a key pair instead of passwords can be convenient. Instead of the remote system prompting for a password with each connection, authentication can be automatically negotiated using a public and private key pair.
The private key remains secure on your own workstation, and the public key gets placed in a specific location on each remote system that you access. Your private key may be secured locally with a passphrase. A local caching program such as
ssh-agent
or gnome-keyring
allows you to enter that passphrase periodically, instead of each time you use the key to access a remote system.[ Free download: Advanced Linux commands cheat sheet. ]
Generating a key pair and propagating the public key
Generating your key pair and propagating your public key is simpler than it sounds. Let’s walk through it.
Generating the key
The minimum effort to generate a key pair involves running the
ssh-keygen
command, and choosing the defaults at all the prompts:The default location to store the keys is in the
~/.ssh
directory, which will be created if it does not exist:Allowing this command to create the directory also ensures that the owner and permissions are set correctly. Some applications will not use keys if the permissions to the private key are too open.
The file ending in
.pub
is the public key that needs to be transferred to the remote systems. It is a file containing a single line: The protocol, the key, and an email used as an identifier. Options for the ssh-keygen
command allow you to specify a different identifier:After generating the key pair, the
ssh-keygen
command also displays the fingerprint and randomart image that are unique to this key. This information can be shared with other people who may need to verify your public key.Later you can view these with:
The
-l
option lists the fingerprint, and the -v
option adds the ASCII art.Propagating the public key to a remote system
If password authentication is currently enabled, then the easiest way to transfer the public key to the remote host is with the
ssh-copy-id
command. If you used the default name for the key all you need to specify is the remote user and host:Following the instructions from the output, verify that you can connect using the key pair. If you implemented a passphrase, you will be prompted for the passphrase to use the private key:
Examine the resulting authorized key file. This is where the public key was appended. If the directory or file did not exist, then it was (or they were) created with the correct ownership and permissions. Each line is a single authorized public key:
To revoke access for this key pair, remove the line for the public key.
There are many other options that can be added to this line in the authorized key file to control access. These options are usually used by administrators placing the public keys on a system with restrictions. These restrictions may include where the connection may originate, what command(s) may be run, and even a date indicating when to stop accepting this key. These and more options are listed in the
sshd
man page.Changing the passphrase
If you need to change a passphrase on your private key or if you initially set an empty passphrase and want that protection at a later time, use the
ssh-keygen
command with the -p
option:You can add additional options to specify the key (
-f
), and the old (-P
) or new (-N
) passphrases on the command line. Remember that any passwords specified on the command line will be saved in your shell history.See the
ssh-keygen
man page for additional options.Rotating keys
While the public key by itself is meant to be shared, keep in mind that if someone obtains your private key, they can then use that to access all systems that have the public key. These key pairs also do not have a period of validity like GNU Privacy Guard (GPG) keys or public key infrastructure (PKI) certificates.
If you have any reason to suspect that a private key has been stolen or otherwise compromised, you should replace that key pair. The old public key has to be removed from all systems, a new key has to be generated with
ssh-keygen
, and the new public key has to be transferred to the desired remote systems.If you are rotating keys as a precaution and without any concern of compromise, you can use the old key pair to authenticate the transfer of the new public key before removing the old key.
Is using empty passphrases ever a good idea?
There are several things to think about when considering an empty passphrase for your SSH private key.
How secure is the private key file?
If you tend to work from multiple client systems and want to either have multiple copies of your key or keep a copy on removable media, then it really is a good idea to have a passphrase on the private key. This practice is in addition to protecting access to the key file with encrypted media.
However, if you have only one copy of the private key and it is kept on a system that is well secured and not shared, then having a passphrase is simply one more level of protection just in case.
Remember that changing the passphrase on one copy does not change the passphrase on other copies. The passphrase is simply locking access to a specific key file.
Why do think you need an empty passphrase?
There are cases for keys with empty passphrases. Some utilities that need to automatically transfer files between systems need a passwordless method to authenticate. The
kdump
utility, when configured to dump the kernel to a remote system using SSH, is one example.Another common use is to generate a key pair for a script that is designed to run unattended, such as from a cron job.
How about a middle ground alternative?
By itself, a passphrase-protected private key requires the passphrase to be entered each time the key is used. This setup does not feel like passwordless SSH. However, there are caching mechanisms that allow you to enter the key passphrase once and then use the key over and over without reentering that passphrase.
More Linux resources
OpenSSH comes with an
ssh-agent
daemon and an ssh-add
utility to cache the unlocked private key. The GNOME desktop also has a keyring daemon that stores passwords and secrets but also implements an SSH agent.The lifetime of the cached key can be configured with each of the agents or when the key is added. In many cases, it defaults to an unlimited lifetime, but the cache is cleared when the user logs out of the system. You will be prompted for the passphrase only once per login session.
If there is a scheduled application that needs to run outside of a user login session, it may be possible to use a secret or other password manager to automate the unlocking of the key. For example, Ansible Tower stores credentials in a secure database. This database includes an SSH private key used to connect to the remote systems (managed nodes), and any passphrases necessary for those private keys. Once those credentials are stored, a job can be scheduled to run a playbook on a regular schedule.
Automating propagation
A centralized identity manager such as FreeIPA can assist with key propagation. Upload the public key to the server as an attribute of a user account, and then propagate it to the hosts in the domain as needed. FreeIPA can also provide additional host-based access control for where a key may be used.
Keys can also be distributed using Ansible modules. The
openssh_keypair
module uses ssh-keygen
to generate keys and the authorized_key
module adds and removes SSH authorized keys for particular user accounts.Wrapping up
SSH key pairs are only one way to automate authentication without passwords. Using the Generic Security Services Application Program Interface (GSSAPI) authentication is also common when trying to reduce the use of passwords on a network with centralized user management. SSH key pairs are the easier option to implement when single sign-on (SSO) is not already available.
Many source code repositories grant access using SSH keys. You can upload a public key to an account in the hosting organization such as the Fedora Account System, GitLab, or GitHub sites and use that key pair to authenticate when pulling and pushing content to repositories.
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Introduction
SSH, or secure shell, is an encrypted protocol used to administer and communicate with servers. When working with an Ubuntu server, chances are you will spend most of your time in a terminal session connected to your server through SSH.
In this guide, we’ll focus on setting up SSH keys for a vanilla Ubuntu 16.04 installation. SSH keys provide an easy, secure way of logging into your server and are recommended for all users.
Step 1 — Create the RSA Key Pair
The first step is to create a key pair on the client machine (usually your computer):
By default
ssh-keygen
will create a 2048-bit RSA key pair, which is secure enough for most use cases (you may optionally pass in the -b 4096
flag to create a larger 4096-bit key).After entering the command, you should see the following output:
Press
ENTER
to save the key pair into the .ssh/
subdirectory in your home directory, or specify an alternate path.If you had previously generated an SSH key pair, you may see the following prompt:
If you choose to overwrite the key on disk, you will not be able to authenticate using the previous key anymore. Be very careful when selecting yes, as this is a destructive process that cannot be reversed.
You should then see the following prompt:
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Here you optionally may enter a secure passphrase, which is highly recommended. A passphrase adds an additional layer of security to prevent unauthorized users from logging in. To learn more about security, consult our tutorial on How To Configure SSH Key-Based Authentication on a Linux Server.
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You should then see the following output:
You now have a public and private key that you can use to authenticate. The next step is to place the public key on your server so that you can use SSH-key-based authentication to log in.
Step 2 — Copy the Public Key to Ubuntu Server
The quickest way to copy your public key to the Ubuntu host is to use a utility called
ssh-copy-id
. Due to its simplicity, this method is highly recommended if available. If you do not have ssh-copy-id
available to you on your client machine, you may use one of the two alternate methods provided in this section (copying via password-based SSH, or manually copying the key).Copying Public Key Using ssh-copy-id
The
ssh-copy-id
tool is included by default in many operating systems, so you may have it available on your local system. For this method to work, you must already have password-based SSH access to your server.To use the utility, you simply need to specify the remote host that you would like to connect to and the user account that you have password SSH access to. This is the account to which your public SSH key will be copied.
The syntax is:
You may see the following message:
This means that your local computer does not recognize the remote host. This will happen the first time you connect to a new host. Type “yes” and press
ENTER
to continue.Next, the utility will scan your local account for the
id_rsa.pub
key that we created earlier. When it finds the key, it will prompt you for the password of the remote user’s account:Type in the password (your typing will not be displayed for security purposes) and press
ENTER
. The utility will connect to the account on the remote host using the password you provided. It will then copy the contents of your ~/.ssh/id_rsa.pub
key into a file in the remote account’s home ~/.ssh
directory called authorized_keys
.Coles hydra crane parts manual. You should see the following output:
At this point, your
id_rsa.pub
key has been uploaded to the remote account. You can continue on to Step 3.Copying Public Key Using SSH
If you do not have
ssh-copy-id
available, but you have password-based SSH access to an account on your server, you can upload your keys using a conventional SSH method.We can do this by using the
cat
command to read the contents of the public SSH key on our local computer and piping that through an SSH connection to the remote server. ![4096 4096](https://s3.amazonaws.com/cdn.freshdesk.com/data/helpdesk/attachments/production/6024258901/original/blob1464941842510.png?1464941840)
On the other side, we can make sure that the
~/.ssh
directory exists and has the correct permissions under the account we’re using.We can then output the content we piped over into a file called
authorized_keys
within this directory. We’ll use the >>
redirect symbol to append the content instead of overwriting it. This will let us add keys without destroying previously added keys.The full command looks like this:
You may see the following message:
This means that your local computer does not recognize the remote host. This will happen the first time you connect to a new host. Type “yes” and press
ENTER
to continue.Afterwards, you should be prompted to enter the remote user account password:
After entering your password, the content of your
id_rsa.pub
key will be copied to the end of the authorized_keys
file of the remote user’s account. Continue on to Step 3 if this was successful.Copying Public Key Manually
If you do not have password-based SSH access to your server available, you will have to complete the above process manually.
We will manually append the content of your
id_rsa.pub
file to the ~/.ssh/authorized_keys
file on your remote machine.To display the content of your
id_rsa.pub
key, type this into your local computer:You will see the key’s content, which should look something like this:
Access your remote host using whichever method you have available.
Once you have access to your account on the remote server, you should make sure the
~/.ssh
directory exists. This command will create the directory if necessary, or do nothing if it already exists:Now, you can create or modify the
authorized_keys
file within this directory. You can add the contents of your id_rsa.pub
file to the end of the authorized_keys
file, creating it if necessary, using this command:In the above command, substitute the
public_key_string
with the output from the cat ~/.ssh/id_rsa.pub
command that you executed on your local system. It should start with ssh-rsa AAAA.
.Finally, we’ll ensure that the
~/.ssh
directory and authorized_keys
file have the appropriate permissions set:This recursively removes all “group” and “other” permissions for the
~/.ssh/
directory.If you’re using the
root
account to set up keys for a user account, it’s also important that the ~/.ssh
directory belongs to the user and not to root
:In this tutorial our user is named sammy but you should substitute the appropriate username into the above command.
We can now attempt passwordless authentication with our Ubuntu server.
Step 3 — Authenticate to Ubuntu Server Using SSH Keys
If you have successfully completed one of the procedures above, you should be able to log into the remote host without the remote account’s password.
The basic process is the same:
If this is your first time connecting to this host (if you used the last method above), you may see something like this:
This means that your local computer does not recognize the remote host. Type “yes” and then press
ENTER
to continue.If you did not supply a passphrase for your private key, you will be logged in immediately. If you supplied a passphrase for the private key when you created the key, you will be prompted to enter it now (note that your keystrokes will not display in the terminal session for security). After authenticating, a new shell session should open for you with the configured account on the Ubuntu server.
If key-based authentication was successful, continue on to learn how to further secure your system by disabling password authentication.
Ssh Keygen Rsa 2048
Step 4 — Disable Password Authentication on your Server
If you were able to log into your account using SSH without a password, you have successfully configured SSH-key-based authentication to your account. However, your password-based authentication mechanism is still active, meaning that your server is still exposed to brute-force attacks.
Before completing the steps in this section, make sure that you either have SSH-key-based authentication configured for the root account on this server, or preferably, that you have SSH-key-based authentication configured for a non-root account on this server with
sudo
privileges. This step will lock down password-based logins, so ensuring that you will still be able to get administrative access is crucial.Once you’ve confirmed that your remote account has administrative privileges, log into your remote server with SSH keys, either as root or with an account with
sudo
privileges. Then, open up the SSH daemon’s configuration file:Inside the file, search for a directive called
PasswordAuthentication
. This may be commented out. Uncomment the line and set the value to “no”. This will disable your ability to log in via SSH using account passwords:Save and close the file when you are finished by pressing
CTRL
+ X
, then Y
to confirm saving the file, and finally ENTER
to exit nano. To actually implement these changes, we need to restart the sshd
service:As a precaution, open up a new terminal window and test that the SSH service is functioning correctly before closing this session:
Watch gintama episode 1 english dubbed. Once you have verified your SSH service, you can safely close all current server sessions.
The SSH daemon on your Ubuntu server now only responds to SSH keys. Password-based authentication has successfully been disabled.
Create Ssh Key Rsa 4096
Conclusion
You should now have SSH-key-based authentication configured on your server, allowing you to sign in without providing an account password.
Ssh Keygen Arguments
If you’d like to learn more about working with SSH, take a look at our SSH Essentials Guide.