SSH-AGENT(1) BSD General Commands Manual SSH-AGENT(1)
ssh-agent - authentication agent
ssh-agent [-a bind_address] [-c | -s] [-d] [command [args ...]]
ssh-agent [-c | -s] -k
ssh-agent is a program to hold private keys used for public key authenti-
cation (RSA, DSA). The idea is that ssh-agent is started in the begin-
ning of an X-session or a login session, and all other windows or pro-
grams 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:
Bind the agent to the unix-domain socket bind_address. The
default is /tmp/ssh-XXXXXXXX/agent.<ppid>.
-c Generate C-shell commands on stdout. This is the default if
SHELL looks like it's a csh style of shell.
-s Generate Bourne shell commands on stdout. This is the default if
SHELL does not look like it's a csh style of shell.
-k Kill the current agent (given by the SSH_AGENT_PID environment
-d Debug mode. When this option is specified ssh-agent will not
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 files
$HOME/.ssh/id_rsa, $HOME/.ssh/id_dsa and $HOME/.ssh/identity. 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 run-
ning without X). It then sends the identity to the agent. Several iden-
tities 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 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 setup: Either the agent starts a
new subcommand into which some environment variables are exported, or the
agent prints 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
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
Contains the protocol version 1 RSA authentication identity of
Contains the protocol version 2 DSA authentication identity of
Contains the protocol version 2 RSA authentication identity of
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
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.
ssh(1), ssh-add(1), ssh-keygen(1), sshd(8)
BSD September 25, 1999 BSD