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ssh-agent [-c | -s] -k
ssh-agent is a program to hold private keys used for public key authenti-
cation (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:
Bind the agent to the unix-domain socket bind_address. The
default is /tmp/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
-k Kill the current agent (given by the SSH_AGENT_PID environment
-s Generate Bourne shell commands on stdout. This is the default if
SHELL does not look like it's a csh style of shell.
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 (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 ter-
minal. Authentication data need not be stored on any other machine, and
authentication passphrases never go over the network. However, the con-
nection to the agent is forwarded over SSH remote logins, and the user
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
Contains the protocol version 1 RSA authentication identity of
Contains the protocol version 2 DSA authentication identity of
Contains the protocol version 2 ECDSA authentication identity of
Contains the protocol version 2 RSA authentication identity of
Unix-domain sockets used to contain the connection to the authen-
tication agent. These sockets should only be readable by the
owner. The sockets should get automatically removed when the
The reseeding of the OpenSSL random generator is usually done
from /dev/urandom. If the SSH_USE_STRONG_RNG environment vari-
able is set to value other than 0 the OpenSSL random generator is
reseeded from /dev/random. The number of bytes read is defined
by the SSH_USE_STRONG_RNG value. Minimum is 14 bytes. This set-
ting is not recommended on the computers without the hardware
random generator because insufficient entropy causes the connec-
tion to be blocked until enough entropy is available.
ssh(1), ssh-add(1), ssh-keygen(1), sshd(8)