Tribblix: manual page: ssh-agent.1

SSH-AGENT(1) User Commands SSH-AGENT(1)

NAME

ssh-agent - OpenSSH authentication agent

SYNOPSIS

ssh-agent [-c | -s] [-DdTU] [-a bind_address] [-E fingerprint_hash]
[-O option] [-P allowed_providers] [-t life]
ssh-agent [-TU] [-a bind_address] [-E fingerprint_hash] [-O option]
[-P allowed_providers] [-t life] command [arg ...]
ssh-agent [-c | -s] -k
ssh-agent -u

DESCRIPTION

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 to create a socket at a random path matching
$HOME/.ssh/agent/s.*.

-c Generate C-shell commands on standard output. 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. The supported
options 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
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 an
ssh(1) user authentication request or an 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
pattern-list of key application strings to replace the default
application 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 Bind the agent socket in a randomised subdirectory of the form
$TMPDIR/ssh-XXXXXXXXXX/agent.<ppid>, instead of the default
behaviour of using a randomised name matching
$HOME/.ssh/agent/s.*.

-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.

-U Instructs ssh-agent not to clean up stale agent sockets under
$HOME/.ssh/agent/.

-u Instructs ssh-agent to only clean up stale agent sockets under
$HOME/.ssh/agent/ and then exit immediately. If this option is
given twice, ssh-agent will delete stale agent sockets
regardless of the host name that created them.

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
example 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
supervising 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 listener 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
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 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
accessible only to the current user, but is easily
abused by root or another instance of the same user.

FILES

$HOME/.ssh/agent/s.*
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.

AUTHORS