SSH-AGENT linux command manual
SSH-AGENT(1) BSD General Commands Manual SSH-AGENT(1)
NAME
ssh-agent - authentication agent
SYNOPSIS
ssh-agent [-a bind_address] [-c | -s] [-t life] [-d]
[command [args ...]]
ssh-agent [-c | -s] -k
DESCRIPTION
ssh-agent is a program to hold private keys used for public key authen-
tication (RSA, DSA). 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 /tmp/ssh-XXXXXXXX/agent..
-c Generate C-shell commands on stdout. This is the default if
SHELL looks like it's a csh style of shell.
-s Generate Bourne shell commands on stdout. This is the default
if SHELL does not look like it's a csh style of shell.
-k Kill the current agent (given by the SSH_AGENT_PID environment
variable).
-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(8). A lifetime specified for an
identity with ssh-add(1) overrides this value. Without this
option the default maximum lifetime is forever.
-d Debug mode. When this option is specified ssh-agent will not
fork.
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 $HOME/.ssh/id_rsa, $HOME/.ssh/id_dsa and $HOME/.ssh/identity. If
the identity has a passphrase, ssh-add(1) asks for the passphrase (using
a small X11 application if running under X11, or from the terminal if
running without X). 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 setup: Either the agent starts a
new subcommand into which some environment variables are exported, or
the agent prints the needed shell commands (either sh(1) or csh(1) syn-
tax can be generated) which can be evalled in the calling shell. 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, pri-
vate 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
$HOME/.ssh/identity
Contains the protocol version 1 RSA authentication identity of
the user.
$HOME/.ssh/id_dsa
Contains the protocol version 2 DSA authentication identity of
the user.
$HOME/.ssh/id_rsa
Contains the protocol version 2 RSA authentication identity of
the user.
/tmp/ssh-XXXXXXXX/agent.
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.
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 pro-
tocol versions 1.5 and 2.0.
SEE ALSO
ssh(1), ssh-add(1), ssh-keygen(1), sshd(8)
BSD September 25, 1999 BSD