In here, you can set host-specific configuration options. To specify your new port, use a format like this:. If you have a passphrase on your private SSH key, you will be prompted to enter the passphrase every time you use it to connect to a remote host. To avoid having to repeatedly do this, you can run an SSH agent. This small utility stores your private key after you have entered the passphrase for the first time.
It will be available for the duration of your terminal session, allowing you to connect in the future without re-entering the passphrase. This will start the agent program and place it into the background. Now, you need to add your private key to the agent, so that it can manage your key:.
You will have to enter your passphrase if one is set. Afterwards, your identity file is added to the agent, allowing you to use your key to sign in without having to re-enter the passphrase again. If you wish to be able to connect without a password to one server from within another server, you will need to forward your SSH key information. This will allow you to authenticate to another server through the server you are connected to, using the credentials on your local computer. After this is done, you need to connect to your first server using the -A option.
This forwards your credentials to the server for this session:. You will connect as if your private SSH key were located on this server.
This section contains some common server-side configuration options that can shape the way that your server responds and what types of connections are allowed. If you have SSH keys configured, tested, and working properly, it is probably a good idea to disable password authentication. This will prevent any user from signing in with SSH using a password. Inside of the file, search for the PasswordAuthentication directive.
If it is commented out, uncomment it. Set it to no to disable password logins:. After you have made the change, save and close the file. To implement the changes, you should restart the SSH service. Some administrators suggest that you change the default port that SSH runs on. This can help decrease the number of authentication attempts your server is subjected to from automated bots. To change the port that the SSH daemon listens on, you will have to log in to your remote server.
Once you are inside, you can change the port that SSH runs on by finding the Port 22 specification and modifying it to reflect the port you wish to use. For instance, to change the port to , put this in your file:. Save and close the file when you are finished. To implement the changes, you must restart the SSH daemon.
After the daemon restarts, you will need to authenticate by specifying the port number demonstrated in an earlier section. To explicitly limit the user accounts who are able to log in through SSH, you can take a few different approaches, each of which involve editing the SSH daemon config file.
The first method of specifying the accounts that are allowed to login is using the AllowUsers directive. Search for the AllowUsers directive in the file. If one does not exist, create it anywhere.
After the directive, list the user accounts that should be allowed to login through SSH:. If you are more comfortable with group management, you can use the AllowGroups directive instead.
If this is the case, just add a single group that should be allowed SSH access we will create this group and add members momentarily :. Now, you can create a system group without a home directory matching the group you specified by typing:. Make sure that you add whatever user accounts you need to this group.
This can be done by typing:. It is often advisable to completely disable root login through SSH after you have set up an SSH user account that has sudo privileges. Inside, search for a directive called PermitRootLogin. If it is commented, uncomment it. There are some cases where you might want to disable root access generally, but enable it in order to allow certain applications to run correctly.
An example of this might be a backup routine. We will demonstrate with the ssh-copy-id command here, but you can use any of the methods of copying keys we discuss in other sections:. Now, log into the remote server. This should include the full path to the executable, plus any arguments:. Find the directive PermitRootLogin , and change the value to forced-commands-only. This will only allow SSH key logins to use root when a command has been specified for the key:.
The SSH daemon can be configured to automatically forward the display of X applications on the server to the client machine. For this to function correctly, the client must have an X windows system configured and enabled.
Search for the X11Forwarding directive. Graphical applications started on the server through this session should be displayed on the local computer. The performance might be a bit slow, but it is very helpful in a pinch. On your local computer, you can define individual configurations for some or all of the servers you connect to. Inside, you can define individual configuration options by introducing each with a Host keyword, followed by an alias.
You can also use wildcards to match more than one host. Keep in mind that later matches can override earlier ones. Because of this, you should put your most general matches at the top. SSH keys come in many sizes, but a popular choice is an RSA bit encryption , which is comparable to a digit long password.
SSH keys always come in pairs, and every pair is made up of a private key and a public key. Who or what possesses these keys determines the type of SSH key pair. If the private key and the public key remain with the user, this set of SSH keys is referred to as user keys.
If the private and public keys are on a remote system, then this key pair is referred to as host keys. Another type of SSH key is a session key. When a large amount of data is being transmitted, session keys are used to encrypt this information.
To keep things simple, we will focus on how user keys work. In a user key set, the private key remains on the system being used to access the remote system i. Private keys should never be shared with anyone and should be secured on a system — i. A public key is used to encrypt information, can be shared, and is used by the user and the remote server. On the server end, the public key is saved in a file that contains a list of authorized public keys. Before you can start using SSH keys, first you need to generate your own SSH key pair on the system you would like to use to access a remote system.
Please see the instructions below. The SSH protocol supports many authentication methods. Arguably one the most important of these is Public Key authentication for interactive and automated connections. The motivation for using public key authentication over simple passwords is security. Public key authentication provides cryptographic strength that even extremely long passwords can not offer.
With SSH , public key authentication improves security considerably as it frees the users from remembering complicated passwords or worse yet, writing them down. In addition to security public key authentication also offers usability benefits - it allows users to implement single sign-on across the SSH servers they connect to. Public key authentication also allows automated, passwordless login that is a key enabler for the countless secure automation processes that execute within enterprise networks globally.
Public key cryptography revolves around a couple of key concepts. The sections below explain these briefly. As with any encryption scheme, public key authentication is based on an algorithm. That feature should be used with care, as it allows a compromised server to use the user's credentials from the original agent. Agent forwarding can, however, be a major convenience feature for power users in less security critical environments. The default key sizes used by the ssh-keygen tool are generally of acceptable strength.
In fact, since the protocol never reveals the public keys that are acceptable for user authentication, the algorithms used for the keys are not as critical as they are in, for example, PKI certificates. For RSA keys, bits is probably a good choice today There is just no practical benefit from using smaller keys. Identity keys are usually stored in a user's. However, it is possible to specify any file name and any location when creating a private key, and provide the path name with the -i option to the SSH client.
When a user tries to log in using key-based authentication, the OpenSSH server looks for authorized keys from a directory specifies in the server configuration using the AuthorizedKeysFile option. The default is. This is convenient, but the user can then give these keys to friends or colleagues, or even sell them for Bitcoins this has actually happened.
SSH keys are furthermore permanent and remain valid until expressly removed. If authorized keys are added for root or service accounts, they easily remain valid even after the person who installed them has left the organization. They are also a convenient way for hackers to establish permanent presence on a system if there is no detection and alerts about unauthorized new keys.
For these reasons, most larger organizations want to move authorized keys to a root-owned location and established a controlled provisioning and termination process for them. Create a suitable root-owned directory, e. In practice, however, this is not always so simple, especially in larger environments. User names may come from directories e. Many organizations have varying OpenSSH versions, including very old systems or custom SSH builds that have non-standard built-in paths.
We recommend using key management tools such as Universal SSH Key Manager to hide this complexity in larger environments. These tools can also implement a provisioning, termination, and approval workflow for keys and alerts about unauthorized changes made by root users.
The OpenSSH server has a feature I would call it a bug that it counts testing whether a particular key can be used for authentication as an authentication attempt. This has the consequence that if the user has more than five keys in. This often causes key-based authentication to fail and is often difficult for users to figure out.
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