NAME

     ssh-agent — authentication agent

SYNOPSIS

     ssh-agent [-c | -s] [-Dd] [-a bind_address] [-E fingerprint_hash] [-t life]
               [-P pkcs11_whitelist] [command [arg ...]]
     ssh-agent [-c | -s] -k

DESCRIPTION

     ssh-agent is a program to hold private keys used for public key authentication (RSA, DSA,
     ECDSA, Ed25519).  ssh-agent is usually 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 agent initially does not have any private keys.  Keys are added using ssh(1) (see
     AddKeysToAgent in ssh_config(5) for details) or ssh-add(1).  Multiple identities may be
     stored in ssh-agent concurrently and ssh(1) will automatically use them if present.
     ssh-add(1) is also used to remove keys from ssh-agent and to query the keys that are held in
     one.

     The options are as follows:

     -a bind_address
             Bind the agent to the UNIX-domain socket bind_address.  The default is
             $TMPDIR/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      Foreground mode.  When this option is specified ssh-agent will not fork.

     -d      Debug mode.  When this option is specified ssh-agent will not fork and will write
             debug information to standard error.

     -E fingerprint_hash
             Specifies the hash algorithm used when displaying key fingerprints.  Valid options
             are: “md5” and “sha256”.  The default is “sha256”.

     -k      Kill the current agent (given by the SSH_AGENT_PID environment variable).

     -P      Specify a pattern-list of acceptable paths for PKCS#11 shared libraries that may be
             added using the -s option to ssh-add(1).  The default is to allow loading PKCS#11
             libraries from “/usr/lib/*,/usr/local/lib/*”.  PKCS#11 libraries that do not match
             the whitelist will be refused.  See PATTERNS in ssh_config(5) for a description of
             pattern-list syntax.

     -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 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 command line is given, this is executed as a subprocess of the agent.  When the command
     dies, so does 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 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 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.

     In Debian, ssh-agent is installed with the set-group-id bit set, to prevent ptrace(2)
     attacks retrieving private key material.  This has the side-effect of causing the run-time
     linker to remove certain environment variables which might have security implications for
     set-id programs, including LD_PRELOAD, LD_LIBRARY_PATH, and TMPDIR.  If you need to set any
     of these environment variables, you will need to do so in the program executed by ssh-agent.

FILES

     $TMPDIR/ssh-XXXXXXXXXX/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.