NAME¶
ssh-agent
—
authentication agent
SYNOPSIS¶
ssh-agent |
[ -c |
-s ]
[-d ]
[-a
bind_address ]
[-t
life ]
[] |
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-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
- Debug mode. When this option is specified
ssh-agent
will not fork.
-k
- Kill the current agent (given by the
SSH_AGENT_PID
environment
variable).
-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 commandline 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.
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.
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.