(Redirected from SSH Keys)
This article or section needs expansion.
Jan 22, 2019 Today I'll show you how to generate SSH keys. Generate SSH key with ssh-keygen. Ssh-keygen is a standard utility supplied with SSH package. If you have ssh command on your system, you probably have the ssh-keygen command as well. Without any command line options, ssh-keygen will ask you a few questions and create the key with default settings. I want to add a user to Red Hat Linux that will not use a password for logging in, but instead use a public key for ssh. This would be on the command line. Nov 10, 2011 How to Generate A Public/Private SSH Key Linux By Damien – Posted on Nov 10, 2011 Nov 18, 2011 in Linux If you are using SSH frequently to connect to a remote host, one of the way to secure the connection is to use a public/private SSH key so no password is transmitted over the network and it can prevent against brute force attack.
Reason: The intro and Background section ignore the server perspective. (Discuss in Talk:SSH keys#)
SSH keys can serve as a means of identifying yourself to an SSH server using public-key cryptography and challenge-response authentication. The major advantage of key-based authentication is that in contrast to password authentication it is not prone to brute-force attacks and you do not expose valid credentials, if the server has been compromised.[1]
Furthermore SSH key authentication can be more convenient than the more traditional password authentication. When used with a program known as an SSH agent, SSH keys can allow you to connect to a server, or multiple servers, without having to remember or enter your password for each system.
Key-based authentication is not without its drawbacks and may not be appropriate for all environments, but in many circumstances it can offer some strong advantages. A general understanding of how SSH keys work will help you decide how and when to use them to meet your needs.
This article assumes you already have a basic understanding of the Secure Shell protocol and have installed the openssh package.
- 2Generating an SSH key pair
- 2.1Choosing the authentication key type
- 2.2Choosing the key location and passphrase
- 3Copying the public key to the remote server
- 4SSH agents
- 4.1ssh-agent
- 4.3Keychain
- 4.4x11-ssh-askpass
- 4.5pam_ssh
- 5Troubleshooting
Background
SSH keys are always generated in pairs with one known as the private key and the other as the public key. The private key is known only to you and it should be safely guarded. By contrast, the public key can be shared freely with any SSH server to which you wish to connect.
If an SSH server has your public key on file and sees you requesting a connection, it uses your public key to construct and send you a challenge. This challenge is an encrypted message and it must be met with the appropriate response before the server will grant you access. What makes this coded message particularly secure is that it can only be understood by the private key holder. While the public key can be used to encrypt the message, it cannot be used to decrypt that very same message. Only you, the holder of the private key, will be able to correctly understand the challenge and produce the proper response.
This challenge-response phase happens behind the scenes and is invisible to the user. As long as you hold the private key, which is typically stored in the
~/.ssh/
directory, your SSH client should be able to reply with the appropriate response to the server.A private key is a guarded secret and as such it is advisable to store it on disk in an encrypted form. When the encrypted private key is required, a passphrase must first be entered in order to decrypt it. While this might superficially appear as though you are providing a login password to the SSH server, the passphrase is only used to decrypt the private key on the local system. The passphrase is not transmitted over the network.
Generating an SSH key pair
An SSH key pair can be generated by running the
ssh-keygen
command, defaulting to 3072-bit RSA (and SHA256) which the ssh-keygen(1) man page says is 'generally considered sufficient' and should be compatible with virtually all clients and servers:The randomart image was introduced in OpenSSH 5.1 as an easier means of visually identifying the key fingerprint.
Note: You can use the
-a
switch to specify the number of KDF rounds on the password encryption.You can also add an optional comment field to the public key with the
-C
switch, to more easily identify it in places such as ~/.ssh/known_hosts
, ~/.ssh/authorized_keys
and ssh-add -L
output. For example:will add a comment saying which user created the key on which machine and when.
Choosing the authentication key type
OpenSSH supports several signing algorithms (for authentication keys) which can be divided in two groups depending on the mathematical properties they exploit:
- DSA and RSA, which rely on the practical difficulty of factoring the product of two large prime numbers,
- ECDSA and Ed25519, which rely on the elliptic curve discrete logarithm problem. (example)
Elliptic curve cryptography (ECC) algorithms are a more recent addition to public key cryptosystems. One of their main advantages is their ability to provide the same level of security with smaller keys, which makes for less computationally intensive operations (i.e. faster key creation, encryption and decryption) and reduced storage and transmission requirements.
OpenSSH 7.0 deprecated and disabled support for DSA keys due to discovered vulnerabilities, therefore the choice of cryptosystem lies within RSA or one of the two types of ECC.
#RSA keys will give you the greatest portability, while #Ed25519 will give you the best security but requires recent versions of client & server[2][dead link 2020-04-02 ⓘ]. #ECDSA is likely more compatible than Ed25519 (though still less than RSA), but suspicions exist about its security (see below).
Note: These keys are used only to authenticate you; choosing stronger keys will not increase CPU load when transferring data over SSH.
RSA
ssh-keygen
defaults to RSA therefore there is no need to specify it with the -t
option. It provides the best compatibility of all algorithms but requires the key size to be larger to provide sufficient security.Minimum key size is 1024 bits, default is 3072 (see ssh-keygen(1)) and maximum is 16384.
If you wish to generate a stronger RSA key pair (e.g. to guard against cutting-edge or unknown attacks and more sophisticated attackers), simply specify the
-b
option with a higher bit value than the default:Be aware though that there are diminishing returns in using longer keys.[3][4] The GnuPG FAQ reads: 'If you need more security than RSA-2048 offers, the way to go would be to switch to elliptical curve cryptography — not to continue using RSA'.[5]
On the other hand, the latest iteration of the NSA Fact Sheet Suite B Cryptography[dead link 2020-04-02 ⓘ] suggests a minimum 3072-bit modulus for RSA while '[preparing] for the upcoming quantum resistant algorithm transition'.[6]
ECDSA
The Elliptic Curve Digital Signature Algorithm (ECDSA) was introduced as the preferred algorithm for authentication in OpenSSH 5.7. Some vendors also disable the required implementations due to potential patent issues.
There are two sorts of concerns with it:
- Political concerns, the trustworthiness of NIST-produced curves being questioned after revelations that the NSA willingly inserts backdoors into softwares, hardware components and published standards were made; well-known cryptographers haveexpresseddoubts about how the NIST curves were designed, and voluntary tainting has already beenproved in the past.
- Technical concerns, about the difficulty to properly implement the standard and the slowness and design flaws which reduce security in insufficiently precautious implementations.
Unix Generate Ssh Public Key On Mac
Both of those concerns are best summarized in libssh curve25519 introduction. Although the political concerns are still subject to debate, there is a clear consensus that #Ed25519 is technically superior and should therefore be preferred.
Ed25519
Ed25519 was introduced in OpenSSH 6.5 of January 2014: 'Ed25519 is an elliptic curve signature scheme that offers better security than ECDSA and DSA and good performance'. Its main strengths are its speed, its constant-time run time (and resistance against side-channel attacks), and its lack of nebulous hard-coded constants.[7] See also this blog post by a Mozilla developer on how it works.
It is already implemented in many applications and libraries and is the default key exchange algorithm (which is different from key signature) in OpenSSH.
Ed25519 key pairs can be generated with:
There is no need to set the key size, as all Ed25519 keys are 256 bits.
Keep in mind that older SSH clients and servers may not support these keys.
Choosing the key location and passphrase
Upon issuing the
ssh-keygen
command, you will be prompted for the desired name and location of your private key. By default, keys are stored in the ~/.ssh/
directory and named according to the type of encryption used. You are advised to accept the default name and location in order for later code examples in this article to work properly.When prompted for a passphrase, choose something that will be hard to guess if you have the security of your private key in mind. A longer, more random password will generally be stronger and harder to crack should it fall into the wrong hands.
It is also possible to create your private key without a passphrase. While this can be convenient, you need to be aware of the associated risks. Without a passphrase, your private key will be stored on disk in an unencrypted form. Anyone who gains access to your private key file will then be able to assume your identity on any SSH server to which you connect using key-based authentication. Furthermore, without a passphrase, you must also trust the root user, as he can bypass file permissions and will be able to access your unencrypted private key file at any time.
Note: Previously, the private key password was encoded in an insecure way: only a single round of an MD5 hash. OpenSSH 6.5 and later support a new, more secure format to encode your private key. This format is the default since OpenSSH version 7.8. Ed25519 keys have always used the new encoding format. To upgrade to the new format, simply change the key's passphrase, as described in the next section.
Changing the private key's passphrase without changing the key
If the originally chosen SSH key passphrase is undesirable or must be changed, one can use the
ssh-keygen
command to change the passphrase without changing the actual key. This can also be used to change the password encoding format to the new standard.Managing multiple keys
It is possible — although controversial [8][9] — to use the same SSH key pair for multiple hosts.
On the other hand, it is rather easy to maintain distinct keys for multiple hosts by using the
IdentityFile
directive in your openSSH config file:See ssh_config(5) for full description of these options.
Storing SSH keys on hardware tokens
SSH keys can also be stored on a security token like a smart card or a USB token. This has the advantage that the private key is stored securely on the token instead of being stored on disk. When using a security token the sensitive private key is also never present in the RAM of the PC; the cryptographic operations are performed on the token itself. A cryptographic token has the additional advantage that it is not bound to a single computer; it can easily be removed from the computer and carried around to be used on other computers.
Examples are hardware tokens are described in:
- YubiKey#Using a YubiKey with SSH, and
Copying the public key to the remote server
This article or section needs expansion.
Reason: How to do this if you force public key authentication? (Discuss in Talk:SSH keys#)
Once you have generated a key pair, you will need to copy the public key to the remote server so that it will use SSH key authentication. The public key file shares the same name as the private key except that it is appended with a
.pub
extension. Note that the private key is not shared and remains on the local machine.Simple method
Note: This method might fail if the remote server uses a non-
sh
shell such as tcsh
as default and uses OpenSSH older than 6.6.1p1. See this bug report.If your key file is
~/.ssh/id_rsa.pub
you can simply enter the following command.If your username differs on remote machine, be sure to prepend the username followed by
@
to the server name.If your public key filename is anything other than the default of
~/.ssh/id_rsa.pub
you will get an error stating /usr/bin/ssh-copy-id: ERROR: No identities found
. In this case, you must explicitly provide the location of the public key.If the ssh server is listening on a port other than default of 22, be sure to include it within the host argument.
Manual method
By default, for OpenSSH, the public key needs to be concatenated with
~/.ssh/authorized_keys
. Begin by copying the public key to the remote server.The above example copies the public key (
id_ecdsa.pub
) to your home directory on the remote server via scp
. Do not forget to include the :
at the end of the server address. Also note that the name of your public key may differ from the example given.On the remote server, you will need to create the
~/.ssh
directory if it does not yet exist and append your public key to the authorized_keys
file.The last two commands remove the public key file from the server and set the permissions on the
authorized_keys
file such that it is only readable and writable by you, the owner.SSH agents
If your private key is encrypted with a passphrase, this passphrase must be entered every time you attempt to connect to an SSH server using public-key authentication. Each individual invocation of
ssh
or scp
will need the passphrase in order to decrypt your private key before authentication can proceed.An SSH agent is a program which caches your decrypted private keys and provides them to SSH client programs on your behalf. In this arrangement, you must only provide your passphrase once, when adding your private key to the agent's cache. This facility can be of great convenience when making frequent SSH connections.
An agent is typically configured to run automatically upon login and persist for the duration of your login session. A variety of agents, front-ends, and configurations exist to achieve this effect. This section provides an overview of a number of different solutions which can be adapted to meet your specific needs.
ssh-agent
ssh-agent
is the default agent included with OpenSSH. It can be used directly or serve as the back-end to a few of the front-end solutions mentioned later in this section. When ssh-agent
is run, it forks to background and prints necessary environment variables. E.g.To make use of these variables, run the command through the
eval
command.Once
ssh-agent
is running, you will need to add your private key to its cache:If your private key is encrypted,
ssh-add
will prompt you to enter your passphrase. Once your private key has been successfully added to the agent you will be able to make SSH connections without having to enter your passphrase.Tip: To make all
ssh
clients, including git
store keys in the agent on first use, add the configuration setting AddKeysToAgent yes
to ~/.ssh/config
. Other possible values are confirm
, ask
and no
(default).In order to start the agent automatically and make sure that only one
ssh-agent
process runs at a time, add the following to your ~/.bashrc
:This will run a
ssh-agent
process if there is not one already, and save the output thereof. If there is one running already, we retrieve the cached ssh-agent
output and evaluate it which will set the necessary environment variables.There also exist a number of front-ends to
ssh-agent
and alternative agents described later in this section which avoid this problem.Start ssh-agent with systemd user
It is possible to use the systemd/User facilities to start the agent. Use this if you would like your ssh agent to run when you are logged in, regardless of whether x is running.
Add
SSH_AUTH_SOCK DEFAULT='${XDG_RUNTIME_DIR}/ssh-agent.socket'
to ~/.pam_environment
. Then enable or start the service with the --user
flag.Note: If you use GNOME, this environment variable is overridden by default. See GNOME/Keyring#Disable keyring daemon components.
Tip: When starting the agent via systemd as described above, it is possible to automatically enter the passphrase of your default key and add it to the agent. See systemd-user-pam-ssh for details.
ssh-agent as a wrapper program
An alternative way to start ssh-agent (with, say, each X session) is described in this ssh-agent tutorial by UC Berkeley Labs. A basic use case is if you normally begin X with the
startx
command, you can instead prefix it with ssh-agent
like so:And so you do not even need to think about it you can put an alias in your
.bash_aliases
file or equivalent:Doing it this way avoids the problem of having extraneous
ssh-agent
instances floating around between login sessions. Exactly one instance will live and die with the entire X session.Note: As an alternative to calling
ssh-agent startx
, you can add eval $(ssh-agent)
to ~/.xinitrc
.See the below notes on using x11-ssh-askpass with ssh-add for an idea on how to immediately add your key to the agent.
GnuPG Agent
The gpg-agent has OpenSSH agent emulation. See GnuPG#SSH agent for necessary configuration.
Keychain
Keychain is a program designed to help you easily manage your SSH keys with minimal user interaction. It is implemented as a shell script which drives both ssh-agent and ssh-add. A notable feature of Keychain is that it can maintain a single ssh-agent process across multiple login sessions. This means that you only need to enter your passphrase once each time your local machine is booted.
Installation
Install the keychain package.
Configuration
Warning: As of 2015-09-26, the
-Q, --quick
option has the unexpected side-effect of making keychain switch to a newly-spawned ssh-agent upon relogin (at least on systems using GNOME), forcing you to re-add all the previously registered keys.Add a line similar to the following to your shell configuration file, e.g. if using Bash:
Note:
~/.bashrc
is used instead of the upstream suggested ~/.bash_profile
because on Arch it is sourced by both login and non-login shells, making it suitable for textual and graphical environments alike. See Bash#Invocation for more information on the difference between those.In the above example,
- the
--eval
switch outputs lines to be evaluated by the openingeval
command; this sets the necessary environments variables for SSH client to be able to find your agent. --quiet
will limit output to warnings, errors, and user prompts.
Multiple keys can be specified on the command line, as shown in the example. By default keychain will look for key pairs in the
~/.ssh/
directory, but absolute path can be used for keys in non-standard location. You may also use the --confhost
option to inform keychain to look in ~/.ssh/config
for IdentityFile
settings defined for particular hosts, and use these paths to locate keys.See
keychain --help
or keychain(1) for details on setting keychain for other shells.To test Keychain, simply open a new terminal emulator or log out and back in your session. It should prompt you for the passphrase of the specified private key(s) (if applicable), either using the program set in
$SSH_ASKPASS
or on the terminal.Because Keychain reuses the same ssh-agent process on successive logins, you should not have to enter your passphrase the next time you log in or open a new terminal. You will only be prompted for your passphrase once each time the machine is rebooted.
Tips
- keychain expects public key files to exist in the same directory as their private counterparts, with a
.pub
extension. If the private key is a symlink, the public key can be found alongside the symlink or in the same directory as the symlink target (this capability requires thereadlink
command to be available on the system).
- to disable the graphical prompt and always enter your passphrase on the terminal, use the
--nogui
option. This allows to copy-paste long passphrases from a password manager for example.
- if you do not want to be immediately prompted for unlocking the keys but rather wait until they are needed, use the
--noask
option.
Note: Keychain is able to manage GPG keys in the same fashion. By default it attempts to start ssh-agent only, but you can modify this behavior using the
--agents
option, e.g.--agents ssh,gpg
. See keychain(1).x11-ssh-askpass
The x11-ssh-askpass package provides a graphical dialog for entering your passhrase when running an X session. x11-ssh-askpass depends only on the libx11 and libxt libraries, and the appearance of x11-ssh-askpass is customizable. While it can be invoked by the ssh-add program, which will then load your decrypted keys into ssh-agent, the following instructions will, instead, configure x11-ssh-askpass to be invoked by the aforementioned Keychain script.
Install the keychain and x11-ssh-askpass packages.
Edit your
~/.xinitrc
file to include the following lines, replacing the name and location of your private key if necessary. Be sure to place these commands before the line which invokes your window manager.In the above example, the first line invokes keychain and passes the name and location of your private key. If this is not the first time keychain was invoked, the following two lines load the contents of
$HOSTNAME-sh
and $HOSTNAME-sh-gpg
, if they exist. These files store the environment variables of the previous instance of keychain.Calling x11-ssh-askpass with ssh-add
The ssh-add manual page specifies that, in addition to needing the
DISPLAY
variable defined, you also need SSH_ASKPASS
set to the name of your askpass program (in this case x11-ssh-askpass). It bears keeping in mind that the default Arch Linux installation places the x11-ssh-askpass binary in /usr/lib/ssh/
, which will not be in most people's PATH
. This is a little annoying, not only when declaring the SSH_ASKPASS
variable, but also when theming. You have to specify the full path everywhere. Both inconveniences can be solved simultaneously by symlinking:This is assuming that
~/bin
is in your PATH
. So now in your .xinitrc
, before calling your window manager, one just needs to export the SSH_ASKPASS
environment variable:and your X resources will contain something like:
Doing it this way works well with the above method on using ssh-agent as a wrapper program. You start X with
ssh-agent startx
and then add ssh-add to your window manager's list of start-up programs.Theming
The appearance of the x11-ssh-askpass dialog can be customized by setting its associated X resources. Some examples are the .ad files at https://github.com/sigmavirus24/x11-ssh-askpass. See x11-ssh-askpass(1)[dead link 2019-05-05] for full details.
Alternative passphrase dialogs
There are other passphrase dialog programs which can be used instead of x11-ssh-askpass. The following list provides some alternative solutions.
- ksshaskpass uses the KDE Wallet.
- openssh-askpass uses the Qt library.
pam_ssh
The pam_ssh project exists to provide a Pluggable Authentication Module (PAM) for SSH private keys. This module can provide single sign-on behavior for your SSH connections. On login, your SSH private key passphrase can be entered in place of, or in addition to, your traditional system password. Once you have been authenticated, the pam_ssh module spawns ssh-agent to store your decrypted private key for the duration of the session.
To enable single sign-on behavior at the tty login prompt, install the unofficial pam_sshAUR package.
Note: pam_ssh 2.0 now requires that all private keys used in the authentication process be located under
~/.ssh/login-keys.d/
.Create a symlink to your private key file and place it in
~/.ssh/login-keys.d/
. Replace the id_rsa
in the example below with the name of your own private key file.Edit the
/etc/pam.d/login
configuration file to include the text highlighted in bold in the example below. The order in which these lines appear is significiant and can affect login behavior.Warning: Misconfiguring PAM can leave the system in a state where all users become locked out. Before making any changes, you should have an understanding of how PAM configuration works as well as a backup means of accessing the PAM configuration files, such as an Arch Live CD, in case you become locked out and need to revert any changes. An IBM developerWorks article is available which explains PAM configuration in further detail.
In the above example, login authentication initially proceeds as it normally would, with the user being prompted to enter his user password. The additional
auth
authentication rule added to the end of the authentication stack then instructs the pam_ssh module to try to decrypt any private keys found in the ~/.ssh/login-keys.d
directory. The try_first_pass
option is passed to the pam_ssh module, instructing it to first try to decrypt any SSH private keys using the previously entered user password. If the user's private key passphrase and user password are the same, this should succeed and the user will not be prompted to enter the same password twice. In the case where the user's private key passphrase user password differ, the pam_ssh module will prompt the user to enter the SSH passphrase after the user password has been entered. The optional
control value ensures that users without an SSH private key are still able to log in. In this way, the use of pam_ssh will be transparent to users without an SSH private key.If you use another means of logging in, such as an X11 display manager like SLiM or XDM and you would like it to provide similar functionality, you must edit its associated PAM configuration file in a similar fashion. Packages providing support for PAM typically place a default configuration file in the
/etc/pam.d/
directory.Further details on how to use pam_ssh and a list of its options can be found in the pam_ssh(8) man page.
Using a different password to unlock the SSH key
If you want to unlock the SSH keys or not depending on whether you use your key's passphrase or the (different!) login password, you can modify
/etc/pam.d/system-auth
toFor an explanation, see [10].
Known issues with pam_ssh
Work on the pam_ssh project is infrequent and the documentation provided is sparse. You should be aware of some of its limitations which are not mentioned in the package itself.
- Versions of pam_ssh prior to version 2.0 do not support SSH keys employing the newer option of ECDSA (elliptic curve) cryptography. If you are using earlier versions of pam_ssh you must use either RSA or DSA keys.
- The
ssh-agent
process spawned by pam_ssh does not persist between user logins. If you like to keep a GNU Screen session active between logins you may notice when reattaching to your screen session that it can no longer communicate with ssh-agent. This is because the GNU Screen environment and those of its children will still reference the instance of ssh-agent which existed when GNU Screen was invoked but was subsequently killed in a previous logout. The Keychain front-end avoids this problem by keeping the ssh-agent process alive between logins.
pam_exec-ssh
As an alternative to pam_ssh you can use pam_exec-sshAUR. It is a shell script that uses pam_exec. Help for configuration can be found upstream.
GNOME Keyring
If you use the GNOME desktop, the GNOME Keyring tool can be used as an SSH agent. See the GNOME Keyring article for further details.
Store SSH keys with Kwallet
For instructions on how to use kwallet to store your SSH keys, see KDE Wallet#Using the KDE Wallet to store ssh key passphrases.
KeePass2 with KeeAgent plugin
KeeAgent is a plugin for KeePass that allows SSH keys stored in a KeePass database to be used for SSH authentication by other programs.
- Supports both PuTTY and OpenSSH private key formats.
- Works with native SSH agent on Linux/Mac and with PuTTY on Windows.
See KeePass#Plugin Installation in KeePass or install the keepass-plugin-keeagent package.
This agent can be used directly, by matching KeeAgent socket:
KeePass -> Tools -> Options -> KeeAgent -> Agent mode socket file -> %XDG_RUNTIME_DIR%/keeagent.socket
-and environment variable:export SSH_AUTH_SOCK='$XDG_RUNTIME_DIR'/keeagent.socket'
.KeePassXC
The KeePassXC fork of KeePass supports being used as an SSH agent by default. It is also compatible with KeeAgent's database format.
Troubleshooting
Key ignored by the server
- If it appears that the SSH server is ignoring your keys, ensure that you have the proper permissions set on all relevant files.
- For the local machine:
- For the remote machine:
- If that does not solve the problem you may try temporarily setting
StrictModes
tono
in/etc/ssh/sshd_config
. If authentication withStrictModes off
is successful, it is likely an issue with file permissions persists.
- Make sure keys in
~/.ssh/authorized_keys
are entered correctly and only use one single line. - Make sure the remote machine supports the type of keys you are using: some servers do not support ECDSA keys, try using RSA or DSA keys instead, see #Generating an SSH key pair.
- You may want to use debug mode and monitor the output while connecting:
See also
- OpenSSH key management: Part 1, Part 2, Part 3
Retrieved from 'https://wiki.archlinux.org/index.php?title=SSH_keys&oldid=604264'
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Password authentication is the default method most SSH (Secure Shell) clients use to authenticate with remote servers, but it suffers from potential security vulnerabilities, like brute-force login attempts. An alternative to password authentication is public key authentication, in which you generate and store on your computer a pair of cryptographic keys and then configure your server to recognize and accept your keys. Using key-based authentication offers a range of benefits:
- Key-based login is not a major target for brute-force hacking attacks.
- If a server that uses SSH keys is compromised by a hacker, no authorization credentials are at risk of being exposed.
- Because a password isn’t required at login, you are able to able to log in to servers from within scripts or automation tools that you need to run unattended. For example, you can set up periodic updates for your servers with a configuration management tool like Ansible, and you can run those updates without having to be physically present.
This guide will explain how the SSH key login scheme works, how to generate an SSH key, and how to use those keys with your Linode.
NoteIf you’re unfamiliar with SSH connections, review the Getting Started with Linode guide.
How SSH Keys Work
SSH keys are generated in pairs and stored in plain-text files. The key pair (or keypair) consists of two parts:
- A private key, usually named
id_rsa
. The private key is stored on your local computer and should be kept secure, with permissions set so that no other users on your computer can read the file.Caution
- A public key, usually named
id_rsa.pub
. The public key is placed on the server you intend to log in to. You can freely share your public key with others. If someone else adds your public key to their server, you will be able to log in to that server.
When a site or service asks for your SSH key, they are referring to your SSH public key (
id_rsa.pub
). For instance, services like GitHub and Gitlab allow you to place your SSH public key on their servers to streamline the process of pushing code changes to remote repositories.The authorized_keys File
In order for your Linode to recognize and accept your key pair, you will need to upload your public key to your server. More specifically, you will need to upload your public key to the home directory of the user you would like to log in as. If you would like to log in to more than one user on the server using your key pair, you will need to add your public key to each of those users.
To set up SSH key authentication for one of your server’s users, add your public key to a new line inside the user’s
authorized_keys
file. This file is stored inside a directory named .ssh/
under the user’s home folder. A user’s authorized_keys
file can store more than one public key, and each public key is listed on its own line. If your file contains more than one public key, then the owner of each key listed will be able to log in as that user.Granting Someone Else Access to your Server
To give someone else access to your server’s user, simply add their public key on a new line in your
authorized_keys
file, just as you would add your own. To revoke access for that person, remove that same line and save the changes.Challenge-Response
When logging in to a server using SSH, if there is a public key on file on that server, the server will create a challenge. This challenge will be crafted in such a way that only the holder of the private SSH key will be able to decipher it.
This challenge-response action happens without any user interaction. If the person attempting to log in has the corresponding private key, then they will be safely logged in. If not, the login will either fail or fall back to a password-based authentication scheme.
SSH Key Passphrases
You can optionally provide an additional level of security for your SSH keys by encrypting them with a passphrase at the time of creation. When you attempt to log in using an encrypted SSH key, you will be prompted to enter its passphrase. This is not to be confused with a password, as this passphrase only decrypts the key file locally and is not transferred over the Internet as a password might be.
If you’d like to set up your logins so that they require no user input, then creating a passphrase might not be desirable, but it is strongly recommended nevertheless.
Linux and macOS
Generate a Key Pair
Perform the steps in this section on your local machine.
- Create a new key pair.
Caution
This command will overwrite an existing RSA key pair, potentially locking you out of other systems.If you’ve already created a key pair, skip this step. To check for existing keys, runls ~/.ssh/id_rsa*
.If you accidentally lock yourself out of the SSH service on your Linode, you can still use the Lish console to login to your server. After you’ve logged in via Lish, update yourauthorized_keys
file to use your new public key. This should re-establish normal SSH access.The-b
flag instructsssh-keygen
to increase the number of bits used to generate the key pair, and is suggested for additional security. - Press Enter to use the default names
id_rsa
andid_rsa.pub
in the/home/your_username/.ssh
directory before entering your passphrase. - While creating the key pair, you will be given the option to encrypt the private key with a passphrase. This means that the key pair cannot be used without entering the passphrase (unless you save that passphrase to your local machine’s keychain manager). We suggest that you use the key pair with a passphrase, but you can leave this field blank if you don’t want to use one.
Upload your Public Key
There are a few different ways to upload your public key to your Linode from Linux and macOS client systems:
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Using ssh-copy-id
ssh-copy-id
is a utility available on some operating systems that can copy a SSH public key to a remote server over SSH.- To use
ssh-copy-id
, pass your username and the IP address of the server you would like to access: - You’ll see output like the following, and a prompt to enter your user’s password:
- Verify that you can log in to the server with your key.
Using Secure Copy (scp)
Secure Copy (
Cautionscp
) is a tool that copies files from a local computer to a remote server over SSH:These instructions will overwrite any existing contents of the
authorized_keys
file on your server. If you have already set up other public keys on your server, use the ssh-copy-id
command or enter your key manually.- Connect to your server via SSH with the user you would like to add your key to:
- Create the
~/.ssh
directory andauthorized_keys
file if they don’t already exist: - Give the
~/.ssh
directory andauthorized_keys
files appropriate file permissions: - In another terminal on your local machine, use
scp
to copy the contents of your SSH public key (id_rsa.pub
) into theauthorized_keys
file on your server. Substitute in your own username and your server’s IP address: - Verify that you can log in to the server with your key.
Manually Copy an SSH Key
You can also manually add an SSH key to a server:
- Begin by copying the contents of your public SSH key on your local computer. You can use the following command to output the contents of the file:You should see output similar to the following:Note that the public key begins with
ssh-rsa
and ends with[email protected]
. - Once you have copied that text, connect to your server via SSH with the user you would like to add your key to:
- Create the
~/.ssh
directory andauthorized_keys
file if they don’t already exist: - Give the
~/.ssh
directory andauthorized_keys
files appropriate file permissions: - Open the
authorized_keys
file with the text editor of your choice (nano
, for example). Then, paste the contents of your public key that you copied in step one on a new line at the end of the file. - Save and close the file.
Note
If you initially logged into the server asroot
but edited theauthorized_keys
file of another user, then the.ssh/
folder andauthorized_keys
file of that user may be owned byroot
. Set that other user as the files’ owner: - Verify that you can log in to the server with your key.
Connect to the Remote Server
- SSH into the server from your local machine:
- If you chose to use a passphrase when creating your SSH key, you will be prompted to enter it when you attempt to log in. Depending on your desktop environment, a window may appear:
Caution
Do not allow the local machine to remember the passphrase in its keychain unless you are on a private computer which you trust.You may also see the passphrase prompt at your command line: - Enter your password. You should see the connection establish in the local terminal.
Windows
The following instructions use the PuTTY software to connect over SSH, but other options are available on Windows too.
Generate a Key Pair with PuTTY
- Download PuTTYgen (
puttygen.exe
) and PuTTY (putty.exe
) from the official site. - Launch
puttygen.exe
. TheRSA
key type at the bottom of the window is selected by default for an RSA key pair butED25519
(EdDSA using Curve25519) is a comparable option if your remote machine’s SSH server supports DSA signatures. Do not use theSSH-1(RSA)
key type unless you know what you’re doing. - Increase the RSA key size from
2048
bits4096
and click Generate: - PuTTY uses the random input from your mouse to generate a unique key. Once key generation begins, keep moving your mouse until the progress bar is filled:
- When finished, PuTTY will display the new public key. Right-click on it and select Select All, then copy the public key into a Notepad file.
- Save the public key as a
.txt
file or some other plaintext format. This is important–a rich text format such as.rtf
or.doc
can add extra formatting characters and then your private key won’t work: - Enter a passphrase for the private key in the Key passphrase and Confirm passphrase text fields. Important: Make a note of your passphrase, you’ll need it later:
- Click Save private key. Choose a file name and location in Explorer while keeping the
ppk
file extension. If you plan to create multiple key pairs for different servers, be sure to give them different names so that you don’t overwrite old keys with new:
Manually Copy the SSH Key with PuTTY
- Launch
putty.exe
. Find the Connection tree in the Category window, expand SSH and select Auth. Click Browse and navigate to the private key you created above: - Scroll back to the top of the Category window and click Session. Enter the hostname or IP address of your Linode. PuTTY’s default TCP port is
22
, the IANA assigned port for for SSH traffic. Change it if your server is listening on a different port. Name the session in the Saved Sessions text bar and click Save: - Click the Open button to establish a connection. You will be prompted to enter a login name and password for the remote server.
- Once you’re logged in to the remote server, configure it to authenticate with your SSH key pair instead of a user’s password. Create an
.ssh
directory in your home directory on your Linode, create a blankauthorized_keys
file inside, and set their access permissions: - Open the
authorized_keys
file with the text editor of your choice (nano
, for example). Then, paste the contents of your public key that you copied in step one on a new line at the end of the file. - Save, close the file, and exit PuTTY.
- Verify that you can log in to the server with your key.
Using WinSCP
Unix Generate Ssh Key
Uploading a public key from Windows can also be done using WinSCP:
CautionThese instructions will overwrite any existing contents of the
authorized_keys
file on your server. If you have already set up other public keys on your server, use the PuTTY instructions instead.- In the login window, enter your Linode’s public IP address as the hostname, the user you would like to add your key to, and your user’s password. Click Login to connect.
- Once connected, WinSCP will show two file tree sections. The left shows files on your local computer and the right shows files on your Linode. Using the file explorer on the left, navigate to the file where you saved your public key in Windows. Select the public key file and click Upload in the toolbar above.
- You’ll be prompted to enter a path on your Linode where you want to upload the file. Upload the file to
/home/your_username/.ssh/authorized_keys
. - Verify that you can log in to the server with your key.
Connect to the Remote Server with PuTTY
Start PuTTY and Load your saved session. You’ll be prompted to enter your server user’s login name as before. However, this time you will be prompted for your private SSH key’s passphrase rather than the password for your server’s user. Enter the passphrase and press Enter.
Troubleshooting
If your SSH connections are not working as expected, or if you have locked yourself out of your system, review the Troubleshooting SSH guide for troubleshooting help.
Upload your SSH Key to the Cloud Manager
It is possible to provision each new Linode you create with an SSH public key automatically through the Cloud Manager.
- Log in to the Cloud Manager.
- Click on your username at the top right hand side of the page. Then click on My Profile in the dropdown menu that appears:
Note
If you are viewing the Cloud Manager in a smaller browser window or on a smaller device, then the My Profile link will appear in the sidebar links. To view the sidebar links, click on the disclosure button to the left of the blue Create button at the top of the page. - From the My Profile page, select the SSH Keys tab, and then click Add a SSH Key:
- Create a label for your key, then paste in the contents of your public SSH key (
id_rsa.pub
): - Click Add Key.
- When you next create a Linode you’ll be given the opportunity to include your SSH key in the Linode’s creation. This key will be added to the root user of the new Linode.In the Create Linode form, select the SSH key you’d like to include. This field will appear below the Root Password field:
Next Steps
After you set up your SSH keys and confirm they are working as expected, review the How to Secure Your Server guide for instructions on disabling password authentication for your server.
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Generate Ssh Key Linux Azure
This guide is published under a CC BY-ND 4.0 license.