ssh-agent Man page

Resume Wikipedia de Ssh-agent

ssh-agent est l’agent d’authentification inclus dans la suite logicielle OpenSSH.
Un agent SSH stocke en mémoire les clés privées utilisées lors de l’authentification par clef publique (RSA, DSA, ECDSA) pendant toute la durée de la session. L’utilisation d’un agent, évite donc d’avoir à retaper la phrase secrète à chaque fois que l’on sollicite l’utilisation de la clé privée. L’agent se lance au début d’une session (graphique ou non) et tous les appels (fenêtres ou autres programmes) sont réalisés en tant que client du programme de l’agent SSH. Grâce à des variables d’environnement, l’agent peut être trouvé et être utilisé pour l’authentification lors de la connexion à d’autres machines en SSH.

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


ssh-agent — authentication agent


ssh-agent [-c | -s] [-Dd] [-a bind_address] [-E fingerprint_hash] [-t life] [command [arg …]] ssh-agent [-c | -s] -k


ssh-agent is a program to hold private keys used for public key authenti‐
cation (RSA, DSA, ECDSA, Ed25519). ssh-agent is usually 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.

The agent initially does not have any private keys. Keys are added using
ssh (see AddKeysToAgent in ssh_config(5) for details) or ssh-add.
Multiple identities may be stored in ssh-agent concurrently and ssh
will automatically use them if present. ssh-add is also used to
remove keys from ssh-agent and to query the keys that are held in one.

The options are as follows:

-a bind_address
Bind the agent to the UNIX-domain socket bind_address. The
default is $TMPDIR/ssh-XXXXXXXXXX/agent..

-c Generate C-shell commands on stdout. This is the default if
SHELL looks like it’s a csh style of shell.

-D Foreground mode. When this option is specified ssh-agent will
not fork.

-d Debug mode. When this option is specified ssh-agent will not
fork and will write debug information to standard error.

-E fingerprint_hash
Specifies the hash algorithm used when displaying key finger‐
prints. Valid options are: “md5” and “sha256”. The default is

-k Kill the current agent (given by the SSH_AGENT_PID environment

-s Generate Bourne shell commands on stdout. This is the default if
SHELL does not look like it’s a csh style of shell.

-t life
Set a default value for the maximum lifetime of identities added
to the agent. The lifetime may be specified in seconds or in a
time format specified in sshd_config(5). A lifetime specified
for an identity with ssh-add overrides this value. Without
this option the default maximum lifetime is forever.

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

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

There are two main ways to get an agent set up: The first is that the
agent starts a new subcommand into which some environment variables are
exported, eg ssh-agent xterm &. The second is that the agent prints the
needed shell commands (either sh or csh syntax can be generated)
which can be evaluated in the calling shell, eg eval `ssh-agent -s` for
Bourne-type shells such as sh or ksh and eval `ssh-agent -c` for
csh and derivatives.

Later ssh looks at these variables and uses them to establish a con‐
nection to the agent.

The agent will never send a private key over its request channel.
Instead, operations that require a private key will be performed by the
agent, and the result will be returned to the requester. This way, pri‐
vate keys are not exposed to clients using the agent.

A UNIX-domain socket is created 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 process ID.

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

In Debian, ssh-agent is installed with the set-group-id bit set, to pre‐
vent ptrace(2) attacks retrieving private key material. This has the
side-effect of causing the run-time linker to remove certain environment
variables which might have security implications for set-id programs,
including LD_PRELOAD, LD_LIBRARY_PATH, and TMPDIR. If you need to set
any of these environment variables, you will need to do so in the program
executed by ssh-agent.

$TMPDIR/ssh-XXXXXXXXXX/agent. UNIX-domain sockets used to contain the connection to the authen‐
tication agent. These sockets should only be readable by the
owner. The sockets should get automatically removed when the
agent exits.


ssh, ssh-add, ssh-keygen, sshd(8)

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 cre‐
ated OpenSSH. Markus Friedl contributed the support for SSH protocol
versions 1.5 and 2.0.

BSD November 15, 2015 BSD