command [arg ...]
ssh-agent is a program to hold private keys used for
public key authentication. 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.
Through use of environment variables the agent can be located and
automatically used for authentication when logging in to other machines
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
SHELL looks like it's a csh style of
- 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
- Kill the current agent (given by the
- Specify a pattern-list of acceptable paths for PKCS#11 provider shared
libraries that may be used with the
-s option to
ssh-add(1). Libraries that do not match the pattern list
will be refused. See PATTERNS in ssh_config(5) for a
description of pattern-list syntax. The default list is
- Generate Bourne shell commands on
stdout. This is
the default if
SHELL does not look like it's a csh
style of shell.
- 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.
command [arg ...]
- If a command (and optional arguments) is given, this is executed as a
subprocess of the agent. The agent exits automatically when the command
given on the command line terminates.
There are two main ways to get an agent set up. The first is at
the start of an X session, where all other windows or programs are started
as children of the
ssh-agent program. The agent
starts a command under which its environment variables are exported, for
ssh-agent xterm &. When the command
terminates, so does the agent.
The second method is used for a login session. When
ssh-agent is started, it prints the shell commands
required to set its environment variables, which in turn can be evaluated in
the calling shell, for example
In both cases, ssh(1) looks at these environment
variables and uses them to establish a connection to the agent.
The agent initially does not have any private keys. Keys are added
using ssh-add(1) or by ssh(1) when
AddKeysToAgent is set in
ssh_config(5). 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.
ssh-agent may be forwarded
from further remote hosts using the
-A option to
ssh(1) (but see the caveats documented therein), avoiding
the need for authentication data to be stored on other machines.
Authentication passphrases and private keys never go over the network: the
connection to the agent is forwarded over SSH remote connections and the
result is returned to the requester, allowing the user access to their
identities anywhere in the network in a secure fashion.
ssh-agent starts, it stores the name of the
agent's process ID (PID) in this variable.
- If key operation needs a confirmation or passphrase, and
DISPLAY is set
will use ssh-askpass(1) to interact with user. By
default will be executed program
located in “libexec” directory but if
SSH_ASKPASS is set, will be executed program
SSH_ASKPASS. Program may open a X11
window to ask user for confirmation or passphrase.
Remark: On Android
ssh-agent starts, it creates a
UNIX-domain socket and stores its pathname in this
variable. It is accessible only to the current user, but is easily abused
by root or another instance of the same user.
PKIX-SSH 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.
Roumen Petrov contributed support for X.509
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