SSH-AGENT(1) General Commands Manual SSH-AGENT(1)
NAME
ssh-agent — OpenSSH authentication agent
SYNOPSIS
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
DESCRIPTION
ssh-agent is a program to hold private keys used for public key authen-
tication. Through use of environment variables the agent can be located
and automatically used for authentication when logging in to other ma-
chines using ssh(1).
The options are as follows:
-a bind_address
Bind the agent to the Unix-domain socket bind_address. The de-
fault is $TMPDIR/ssh-XXXXXXXXXX/agent.<ppid>.
-c Generate C-shell commands on standard output. This is the de-
fault 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 finger-
prints. 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. The supported op-
tions are: allow-remote-pkcs11, no-restrict-websafe and
websafe-allow.
The allow-remote-pkcs11 option allows clients of a forwarded
ssh-agent to load PKCS#11 or FIDO provider libraries. By de-
fault 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 re-
quests. 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.
Alternately the websafe-allow option allows specifying a pat-
tern-list of key application strings to replace the default ap-
plication allow-list, for example:
“websafe-allow=ssh:*,example.org,*.example.com”
See PATTERNS in ssh_config(5) for a description of pattern-list
syntax.
-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. The default list is
“usr/lib*/*,/usr/local/lib*/*”.
See PATTERNS in ssh_config(5) for a description of pattern-list
syntax.
-s Generate Bourne shell commands on standard output. 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 three 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 ex-
ample eval `ssh-agent -s`.
In both of these cases, ssh(1) looks at these environment variables and
uses them to establish a connection to the agent.
The third way to run ssh-agent is via socket activation from a supervis-
ing process, such as systemd. In this mode, the supervising process
creates the listening socket and is responsible for starting ssh-agent
as needed, and also for communicating the location of the socket lis-
tener to other programs in the user's session. Socket activation is
used when ssh-agent is started with either of the -d or -D flags, no
socket listening address specified by the -a flag, and both the
LISTEN_FDS and LISTEN_PID environment variables correctly supplied by
the supervising process.
The agent initially does not have any private keys. Keys are added us-
ing 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 re-
move 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 us-
ing the -A option to ssh(1) (but see the caveats documented therein),
avoiding the need for authentication data to be stored on other ma-
chines. Authentication passphrases and private keys never go over the
network: the connection to the agent is forwarded over SSH remote con-
nections 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 will delete all keys it has loaded upon receiving SIGUSR1.
ENVIRONMENT
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 accessi-
ble only to the current user, but is easily abused by
root or another instance of the same user.
In Debian, ssh-agent is installed with the set-group-id bit set, to pre-
vent ptrace(2) attacks retrieving private key material. This has the
side-effect of causing the run-time linker to remove certain environment
variables which might have security implications for set-id programs,
including LD_PRELOAD, LD_LIBRARY_PATH, and TMPDIR. If you need to set
any of these environment variables, you will need to do so in the pro-
gram executed by ssh-agent.
FILES
$TMPDIR/ssh-XXXXXXXXXX/agent.<ppid>
Unix-domain sockets used to contain the connection to the au-
thentication agent. These sockets should only be readable by
the owner. The sockets should get automatically removed when
the agent exits.
SEE ALSO
ssh(1), ssh-add(1), ssh-keygen(1), ssh_config(5), sshd(8)
AUTHORS
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 pro-
tocol versions 1.5 and 2.0.
Debian February 9, 2025 SSH-AGENT(1)
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