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 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 ifSHELL
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
andno-restrict-websafe
.The
allow-remote-pkcs11
option allows clients of a forwardedssh-agent
to load PKCS#11 or FIDO provider libraries. By default only local clients may perform this operation. Note that signalling that anssh-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 instructsssh-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 ifSHELL
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.
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
- $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.
SEE ALSO
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 protocol versions 1.5 and 2.0.