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, will not
-d Debug mode. When this option is specified, 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”.
-k Kill the current agent (given by the SSH_AGENT_PID
Specify an option when starting . Currently two options
are supported: allow-remote-pkcs11 and
The allow-remote-pkcs11 option allows clients of a
forwarded to load PKCS#11 or FIDO provider libraries. By
default only local clients may perform this operation.
Note that signalling that an client is remote is
performed by ssh(1), and use of other tools to forward
access to the agent socket may circumvent this
The no-restrict-websafe option instructs to permit
signatures using FIDO keys that might be web
authentication requests. By default, refuses signature
requests for FIDO keys where the key application string
does not start with “ssh:” and when the data to be signed
does not appear to be a ssh(1) user authentication
request or a ssh-keygen(1) signature. The default
behaviour prevents forwarded access to a FIDO key from
also implicitly forwarding the ability to authenticate to
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 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 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 concurrently
and ssh(1) will automatically use them if present. ssh-add(1) is
also used to remove keys from and to query the keys that are held
Connections to 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 starts, it stores the name of the agent's
process ID (PID) in this variable.
SSH_AUTH_SOCK When 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
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-9.6p1.tar.gz
2023-12-22. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
GNU August 10, 2023 SSH-AGENT(1)