– authentication agent
[-c | -s
[command [arg ...
[-c | -s
is a program to hold private keys used for public key authentication
The idea is that
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
Through use of environment variables the agent can be located
and automatically used for authentication when logging in to other
The options are as follows:
The default is
This is the default if
looks like it’s a csh style of shell.
will not fork.
This is the default if
does not look like it’s a csh style of shell.
A lifetime specified for an identity with
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
When executed without arguments,
adds the files
If the identity has a passphrase,
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.
displays the identities currently held by the agent.
The idea is that the agent is run in the user’s local PC, laptop, or
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
syntax can be generated) which can be evaluated in the calling shell, eg
eval `ssh-agent -s`
for Bourne-type shells such as
eval `ssh-agent -c`
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
The socket is made accessible only to the current user.
This method is easily abused by root or another instance of the same
environment variable holds the agent’s process ID.
environment variable is set to value other than
the OpenSSL random generator is reseeded from
The number of bytes read is defined by the SSH_USE_STRONG_RNG value.
Minimum is 14 bytes.
This setting is not recommended on the computers without the hardware
random generator because insufficient entropy causes the connection to
be blocked until enough entropy is available.
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
Markus Friedl contributed the support for SSH
protocol versions 1.5 and 2.0.