Jul 26, 2022 by Thibault Debatty | 1013 views
You probably use ssh on a regular basis. It's easy and powerful. But once in a while you might be confronted to ssh utility tools like
ssh-add. What are these, and what are they used for?
But first, some background: ssh keys...
When you connect to a remote device (usually a server) with
scp, you can (and often do) authenticate yourself using a password. You can also authenticate using SSH keys. Here is how it works:
First, on your device, your create a pair of keys: a private key and the corresponding public key. You upload the public key to the remote device.
When you need to authenticate on the remote device:
According to the principle of public key cryptography, this challenge can only be decrypted using the corresponding private key.
As you are (normally) the only owner of the private key, you are the only one able to correctly decrypt the challenge and recover the original random string. Hence this allows to verify your identity.
Using SSH keys has multiple advantages:
SSH keys have one caveat: they are stored on your computer. So if another user or a malware can read your SSH key, he (it) will be able to authenticate on your remote devices (servers). Hence SSH keys must be protected. One way to do so is to define a passphrase when you create the keys.
This way, each time you use ssh or scp, you will have to provide the passphrase to unlock your private key. This provides a nice layer of security, but can be pretty annoying if you have to type this passphrase too often. This is where ssh-agent comes into play...
ssh-agent is a small utility that keeps the cleartext version of your private SSH keys in memory. Hence each time you use ssh or scp, they actually forward the challenge to the
ssh-agent running in the background. This means you only have to provide the passphrase once, when the key is loaded.
On most Linux systems,
ssh-agent is automatically started when you login, and stopped when you terminate (logout from) the local session.
If you must manually start
ssh-agent command outputs environment variable settings that must be used later by
ssh-agent is usually started with something like:
ssh-agent automatically loads SSH keys stored in the
.ssh directory under the user's home directory. You can also load additional keys with
You can also load a key stored in an environment variable. This is a pattern that you can see in a Continuous Delivery (CD) pipeline for example:
ssh-add <(echo "$SSH_PRIVATE_KEY")