NAME¶
ssh-agent —
authentication agent
SYNOPSIS¶
ssh-agent |
[-c | -s]
[-d]
[-a
bind_address]
[-t life]
[command
[arg ...]] |
DESCRIPTION¶
ssh-agent is a program to hold private keys used for public
key authentication (RSA, DSA, ECDSA). The idea is that
ssh-agent is 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 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 agent initially does not have any private keys. Keys are added using
ssh-add(1). When executed without arguments,
ssh-add(1) adds the files
~/.ssh/id_rsa,
~/.ssh/id_dsa,
~/.ssh/id_ecdsa and
~/.ssh/identity. If the identity has a passphrase,
ssh-add(1) asks for the passphrase on the terminal if it has
one or from a small X11 program if running under X11. If neither of these is
the case then the authentication will fail. It then sends the identity to the
agent. Several identities can be stored in the agent; the agent can
automatically use any of these identities.
ssh-add -l
displays the identities currently held by 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¶
- ~/.ssh/identity
- Contains the protocol version 1 RSA authentication identity
of the user.
- ~/.ssh/id_dsa
- Contains the protocol version 2 DSA authentication identity
of the user.
- ~/.ssh/id_ecdsa
- Contains the protocol version 2 ECDSA authentication
identity of the user.
- ~/.ssh/id_rsa
- Contains the protocol version 2 RSA authentication identity
of the user.
- $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.