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SSH-AGENT(1)              BSD General Commands Manual             SSH-AGENT(1)

     ssh-agent - authentication agent

     ssh-agent [-c | -s] [-d] [-a bind_address] [-t life] [command [arg ...]]
     ssh-agent [-c | -s] -k

     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

     -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 speci-
             fied 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

     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 run-
     ning under X11, or from the terminal if running without X).  It then sends the iden-
     tity 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 cur-
     rently 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 for-
     warded 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

     The agent will never send a private key over its request channel.  Instead, opera-
     tions 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 cur-
     rent user.  This method is easily abused by root or another instance of the same

     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.

             Contains the protocol version 1 RSA authentication identity of the user.

             Contains the protocol version 2 DSA authentication identity of the user.

             Contains the protocol version 2 ECDSA authentication identity of the user.

             Contains the protocol version 2 RSA authentication identity of the user.

             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.

             The reseeding of the OpenSSL random generator is usually done from
             /dev/urandom.  If the SSH_USE_STRONG_RNG environment variable 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 setting is not recommended on the computers without the hard-
             ware random generator because insufficient entropy causes the connection to
             be blocked until enough entropy is available.

     ssh(1), ssh-add(1), ssh-keygen(1), sshd(8)

     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 con-
     tributed the support for SSH protocol versions 1.5 and 2.0.

BSD                            December 12, 2017                           BSD

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