Using SSH keys in Linux

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Parent page: SSH

Creating a key pair

Before creating a new key pair, check to see if you already have one. If you do, but can't remember where you've used it, it's better to create a fresh one, since you shouldn't install a key of unknown security.

Key pairs are typically located in the .ssh/ directory in your home directory. By default, a key is named with an "id_" prefix, followed by the key type ("rsa", "dsa", "ed25519"), and the public key also has a ".pub" suffix. So a common example is id_rsa and id_rsa.pub.

If you do need a new key, you can generate it with the ssh-keygen command:

[name@server]$ ssh-keygen -b 4096 -t rsa

(this example explicitly asks for a 4-kbit RSA key, which is a reasonable choice.)

The output will be similar to the following:

Generating public/private rsa key pair.
Enter file in which to save the key (/home/username/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/username/.ssh/id_rsa.
Your public key has been saved in /home/username/.ssh/id_rsa.pub.
The key fingerprint is:
ef:87:b5:b1:4d:7e:69:95:3f:62:f5:0d:c0:7b:f1:5e username@hostname
The key's randomart image is:
+--[ RSA 2048]----+
|                 |
|                 |
|           .     |
|            o .  |
|        S    o o.|
|         .  + +oE|
|          .o O.oB|
|         .. +oo+*|
|          ... o..|
+-----------------+

When prompted, enter a passphrase. If you already have key pairs saved with the default names, you may wish to enter a different file name for the new keys to avoid overwriting existing key pairs.

Installing the public part of the key

The simplest, safest way to install a key to a remote system is using the ssh-copy-id command:

ssh-copy-id -i mynewkey graham.computecanada.ca:

This assumes that the new keypair is named "mynewkey" and "mynewkey.pub", and that your username on the remote machine is the same as your local username.

If necessary, you can do this "manually" - in fact, ssh-copy-id isn't doing anything very magic. It's simply connecting to the remote machine, and placing the public key into .ssh/authorized_keys in your home directory there. The main benefit from using ssh-copy-id is that it will create files and directories if necessary, and will ensure that the permissions on them are correct. You can do it entirely yourself by copying the public key file to the remote server, then:

mkdir ~/.ssh
cat id_rsa.pub >> ~/.ssh/authorized_keys
chmod --recursive go-rwx ~/.ssh
chmod go-w ~

SSH is picky about permissions, on both the client and the server. SSH will fail if the following conditions are not met:

  • The private key file must not be accessible to others. chmod go-rwx id_rsa
  • Your remote home directory must not be writable by others chmod go-w ~
  • Same for your remote ~/.ssh and ~/.ssh/authorized_keys chmod --recursive go-rwxw ~/.ssh

Note that debugging the remote conditions may bot be obvious without the help of the remote machine's system administrators.

Connecting using a key pair

  • Finally test the new key by sshing to the remote machine from the local machine with
    [name@server]$ ssh -i /home/ubuntu/.ssh/id_rsa USERNAME@ADDRESS

    where

    • /home/ubuntu/.ssh/id_rsa specifies your private key file;
    • USERNAME is the user name on the remote machine;
    • ADDRESS is the address of the remote machine.

    If you have administrative access on the server and created the account for other users, they should test the connection out themselves and not disclose their private key.