SSH-AGENT(1) User Commands 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

-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

-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.


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


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.

illumos August 10, 2023 illumos