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NAME | SYNOPSIS | DESCRIPTION | ENVIRONMENT | FILES | SEE ALSO | AUTHORS | COLOPHON |
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SSH-AGENT(1) General Commands Manual SSH-AGENT(1)
ssh-agent — OpenSSH authentication agent
ssh-agent [-c | -s] [-Dd] [-a bind_address] [-E fingerprint_hash]
[-O option] [-P allowed_providers] [-t life] ssh-agent [-a
bind_address] [-E fingerprint_hash] [-O option] [-P
allowed_providers] [-t life] command [arg ...] ssh-agent [-c | -s]
-k
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:
-a bind_address
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
error.
-E fingerprint_hash
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
environment variable).
-O option
Specify an option when starting ssh-agent. Currently two
options are supported: allow-remote-pkcs11 and
no-restrict-websafe.
The allow-remote-pkcs11 option allows clients of a
forwarded ssh-agent to load PKCS#11 or FIDO provider
libraries. By default only local clients may perform this
operation. Note that signalling that an ssh-agent client
is remote is performed by ssh(1), and use of other tools
to forward access to the agent socket may circumvent this
restriction.
The no-restrict-websafe option instructs ssh-agent to
permit signatures using FIDO keys that might be web
authentication requests. By default, ssh-agent 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
websites.
-P allowed_providers
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
“usr/lib*/*,/usr/local/lib*/*”.
-s Generate Bourne shell commands on stdout. This is the
default if SHELL does not look like it's a csh style of
shell.
-t life
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
example 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 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
same user.
$TMPDIR/ssh-XXXXXXXXXX/agent.<ppid>
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.
ssh(1), ssh-add(1), ssh-keygen(1), ssh_config(5), sshd(8)
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-9.9p2.tar.gz
fetched from
⟨http://ftp.eu.openbsd.org/pub/OpenBSD/OpenSSH/portable/⟩ on
2025-08-11. 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
[email protected]
GNU August 10, 2023 SSH-AGENT(1)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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