— Authentication agent
||[-c | -s]
is a program to hold private keys used for public
key authentication (RSA, DSA, ECDSA, Ed25519). It may contain X.509
certificates that match private keys and extra certificates used to build
chain of certificates leading to a trusted certificate authority.
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
for details) or ssh-add(1)
Multiple identities may be stored in ssh-agent
will automatically use them if present.
is also used to remove keys from
and to query the keys that are held in one.
The options are as follows:
- Bind the agent to the UNIX-domain
socket bind_address. The default is
- 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.
- 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).
- Specify a pattern-list of acceptable paths for PKCS#11
shared libraries that may be added using the -s option
to ssh-add(1). The default is to allow loading PKCS#11
libraries from “/usr/lib/*,/usr/local/lib/*”. PKCS#11
libraries that do not match the whitelist will be refused. See PATTERNS in
ssh_config(5) for a description of pattern-list
- Generate Bourne shell commands on
stdout. This is the default if
SHELL does not look like it's a csh style of
- 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
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)
syntax can be generated) which can be evaluated in
the calling shell, eg eval `ssh-agent -s`
shells such as sh(1)
eval `ssh-agent -c`
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.
-domain socket is created and the name of this
socket is stored in the
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.
environment variable holds the agent's
The agent exits automatically when the command given on the command line
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.
- If key operation needs a confirmation or passphrase, and
DISPLAY is set ssh-agent will
use ssh-askpass(1) to interact with user. By default
will be executed program ssh-askpass located in
“libexec” directory but if
is set, will be executed program specified by
SSH_ASKPASS. Program may open a X11 window to ask
user for confirmation or passphrase.
PKIX-SSH is a derivative of the original and free ssh 1.2.12 release by
. Aaron Campbell
, Markus Friedl
, 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.
contributed support for X.509