SSH-AGENT(1) General Commands Manual SSH-AGENT(1)

ssh-agent —
authentication agent

ssh-agent command args ...

ssh-agent [-c | -s]

ssh-agent -k

ssh-agent is a program to hold private keys used for public key authentication (RSA, DSA). The idea is that ssh-agent is started in the beginning of an X-session or a login session, and all other windows or programs are started as clients to the ssh-agent program. Through use of environment variables the agent can be located and automatically used for authentication when logging in to other machines using ssh(1).

The options are as follows:

Generate C-shell commands on stdout. This is the default if SHELL looks like it's a csh style of shell.
Generate Bourne shell commands on stdout. This is the default if SHELL does not look like it's a csh style of shell.
Kill the current agent (given by the SSH_AGENT_PID environment variable).

If a commandline is given, this is executed as a subprocess of the agent. When the command dies, so does the agent.

The agent initially does not have any private keys. Keys are added using ssh-add(1). When executed without arguments, ssh-add(1) adds the $HOME/.ssh/identity file. If the identity has a passphrase, ssh-add(1) asks for the passphrase (using a small X11 application if running under X11, or from the terminal if running without X). It then sends the identity to the agent. Several identities can be stored in the agent; the agent can automatically use any of these identities. ssh-add -l displays the identities currently held by the agent.

The idea is that the agent is run in the user's local PC, laptop, or terminal. Authentication data need not be stored on any other machine, and authentication passphrases never go over the network. However, the connection to the agent is forwarded over SSH remote logins, and the user can thus use the privileges given by the identities anywhere in the network in a secure way.

There are two main ways to get an agent setup: Either you let the agent start a new subcommand into which some environment variables are exported, or you let the agent print the needed shell commands (either sh(1) or csh(1) syntax can be generated) which can be evalled in the calling shell. Later ssh(1) looks at these variables and uses them to establish a connection to the agent.

A unix-domain socket is created (/tmp/ssh-XXXXXXXX/agent.<pid>), and the name of this socket is stored in the SSH_AUTH_SOCK environment variable. The socket is made accessible only to the current user. This method is easily abused by root or another instance of the same user.

The SSH_AGENT_PID environment variable holds the agent's PID.

The agent exits automatically when the command given on the command line terminates.

$HOME/.ssh/identity
Contains the RSA authentication identity of the user. This file should not be readable by anyone but the user. It is possible to specify a passphrase when generating the key; that passphrase will be used to encrypt the private part of this file. This file is not used by ssh-agent but is normally added to the agent using ssh-add(1) at login time.
$HOME/.ssh/id_dsa
Contains the DSA authentication identity of the user.
/tmp/ssh-XXXXXXXX/agent.<pid>
Unix-domain sockets used to contain the connection to the authentication agent. These sockets should only be readable by the owner. The sockets should get automatically removed when the agent exits.

OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added newer features and created OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0.

ssh(1), ssh-add(1), ssh-keygen(1), sshd(8)
September 25, 1999 BSD