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 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 a
ssh-agent client 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, 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.
In Debian,
ssh-agent is installed with the
set-group-id bit set, to prevent
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 program executed by
ssh-agent.
- $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.