NAME
ssh-agent —
authentication agent
SYNOPSIS
ssh-agent |
[-c | -s]
[-Dd]
[-a
bind_address]
[-E
fingerprint_hash]
[-P
pkcs11_whitelist]
[-t life]
[command
[arg ...]] |
DESCRIPTION
ssh-agent is a program to hold private keys used for public
key authentication (RSA, DSA, ECDSA, Ed25519).
ssh-agent is
usually started in the beginning of an X-session or a login session, and all
other windows or programs are started as clients to the ssh-agent program.
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 agent initially does not have any private keys. Keys are added using
ssh(1) (see
AddKeysToAgent in
ssh_config(5) for details)
or
ssh-add(1). 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.
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).
-
-
- -P
pkcs11_whitelist
- Specify a pattern-list of acceptable paths for PKCS#11
shared libraries that may be added using the -s option
to ssh-add(1). The default
is to allow loading PKCS#11 libraries from
“/usr/lib/*,/usr/pkg/lib/*”. PKCS#11 libraries that do not
match the whitelist will be refused. See PATTERNS in
ssh_config(5) for a
description of pattern-list syntax.
-
-
- -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.
If a command line is given, this is executed as a subprocess of the agent. When
the command dies, so does the agent.
The idea is that the agent is run in the user's local PC, laptop, or terminal.
Authentication data need not be stored on any other machine, and
authentication passphrases never go over the network. However, the connection
to the agent is forwarded over SSH remote logins, and the user can thus use
the privileges given by the identities anywhere in the network in a secure
way.
There are two main ways to get an agent set up: The first is that the agent
starts a new subcommand into which some environment variables are exported, eg
ssh-agent xterm &. The second is that the agent prints
the needed shell commands (either
sh(1) or
csh(1) syntax can be generated)
which can be evaluated in the calling shell, eg
eval `ssh-agent
-s` for Bourne-type shells such as
sh(1) or
ksh(1) and
eval
`ssh-agent -c` for
csh(1) and
derivatives.
Later
ssh(1) looks at these variables
and uses them to establish a connection to the agent.
The agent will never send a private key over its request channel. Instead,
operations that require a private key will be performed by the agent, and the
result will be returned to the requester. This way, private keys are not
exposed to clients using the agent.
A
UNIX-domain socket is created and the name of this
socket is stored in the
SSH_AUTH_SOCK
environment
variable. The socket is made accessible only to the current user. This method
is easily abused by root or another instance of the same user.
The
SSH_AGENT_PID
environment variable holds the agent's
process ID.
The agent exits automatically when the command given on the command line
terminates.
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
ssh(1),
ssh-add(1),
ssh-keygen(1),
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 protocol versions 1.5 and 2.0.