ssh-agent is a program to hold private keys used for public key
authentication. 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 $TMPDIR/ssh-XXXXXXXXXX/agent.<ppid>.
-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
Specifies the hash algorithm used when displaying key
fingerprints. Valid options are: “md5” and “sha256”. The
default is “sha256”.
-k Kill the current agent (given by the SSH_AGENT_PID
Specify a pattern-list of acceptable paths for PKCS#11
provider and FIDO authenticator middleware shared libraries
that may be used with the -S or -s options 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
-s 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.
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
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
example ssh-agent xterm &. When the command terminates, so does
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 eval `ssh-agent -s`.
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.
Connections to 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_PID When ssh-agent starts, it stores the name of the
agent's process ID (PID) in this variable.
SSH_AUTH_SOCK When 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
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.
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.
This page is part of the openssh (Portable OpenSSH) project.
Information about the project can be found at
http://www.openssh.com/portable.html. If you have a bug report for
this manual page, see ⟨http://www.openssh.com/report.html⟩. This
page was obtained from the tarball openssh-8.5p1.tar.gz fetched
from ⟨http://ftp.eu.openbsd.org/pub/OpenBSD/OpenSSH/portable/⟩ on
2021-04-01. If you discover any rendering problems in this HTML
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BSD June 22, 2020 BSD