ssh-agent man page

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 authentication (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(1).

The agent initially does not have any private keys. Keys are added using ssh(1) (see AddKeysToAgent in ssh_config(5) for details) or ssh-add(1). Multiple identities may be stored in ssh-agent concurrently and ssh(1) will automatically use them if present. ssh-add(1) 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.<ppid>.
Generate C-shell commands on stdout. This is the default if SHELL looks like it's a csh style of shell.
Foreground mode. When this option is specified ssh-agent will not fork.
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 fingerprints. Valid options are: “md5” and “sha256”. The default is “sha256”.
Kill the current agent (given by the SSH_AGENT_PID environment variable).
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(1) 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 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 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(1) or csh(1) syntax can be generated) which can be evaluated in the calling shell, eg eval `ssh-agent -s` for Bourne-type shells such as sh(1) or ksh(1) and eval `ssh-agent -c` for csh(1) and derivatives.

Later ssh(1) looks at these variables and uses them to establish a connection 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, private 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 terminates.


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.


The reseeding of the OpenSSL random generator is usually done from /dev/urandom. If the SSH_USE_STRONG_RNG environment variable is set to value other than 0 the OpenSSL random generator is reseeded from /dev/random. The number of bytes read is defined by the SSH_USE_STRONG_RNG value. Minimum is 14 bytes. This setting is not recommended on the computers without the hardware random generator because insufficient entropy causes the connection to be blocked until enough entropy is available.

See Also

ssh(1), ssh-add(1), ssh-keygen(1), 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 created OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0.

Referenced By

autossh(1), cowpoke(1), gsissh(1), gsissh_config(5), gsisshd(8), gsisshd_config(5), gsissh-keygen(1), kdesrc-build(1), keychain(1), mgarepo(8), mpssh(1), mussh(1), openpgp2ssh(1), pam_ssh(8), scp(1), ssh(1), ssh-add(1), ssh_config(5), ssh-copy-id(1), sshd(8), sshd_config(5), ssh-keygen(1), ssh-pkcs11-helper(8), x11-ssh-askpass.1x(1), xlock(1).

Explore man page connections for ssh-agent(1).

November 15 2015