SSH-AGENT(1) UNIX System V (September 25, 1999) 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 authentication (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.<ppid>.
-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 $HOME/.ssh/id_rsa,
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$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 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 evalled 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
$HOME/.ssh/identity
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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.<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.
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