NAME

     ssh-agent — OpenSSH authentication agent

SYNOPSIS

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

DESCRIPTION

     ssh-agent is a program to hold private keys used for public key authentication.  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
             $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 allowed_providers
             Specify a pattern-list of acceptable paths for PKCS#11 provider and FIDO authenticator middleware
             shared libraries that may be used with the -S or -s options to ssh-add(1).  Libraries that do not
             match the pattern list will be refused.  See PATTERNS in ssh_config(5) for a description of
             pattern-list syntax.  The default list is “/usr/lib/*,/usr/local/lib/*”.

     -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.

     command [arg ...]
             If a command (and optional arguments) is given, this is executed as a subprocess of the agent.  The
             agent exits automatically when the command given on the command line terminates.

     There are two main ways to get an agent set up.  The first is at the start of an X session, where all other
     windows or programs are started as children of the ssh-agent program.  The agent starts a command under
     which its environment variables are exported, for example ssh-agent xterm &.  When the command terminates,
     so does the agent.

     The second method is used for a login session.  When ssh-agent is started, it prints the shell commands
     required to set its environment variables, which in turn can be evaluated in the calling shell, for example
     eval `ssh-agent -s`.

     In both cases, ssh(1) looks at these environment variables and uses them to establish a connection to the
     agent.

     The agent initially does not have any private keys.  Keys are added using ssh-add(1) or by ssh(1) when
     AddKeysToAgent is set in ssh_config(5).  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.

     Connections to ssh-agent may be forwarded from further remote hosts using the -A option to ssh(1) (but see
     the caveats documented therein), avoiding the need for authentication data to be stored on other machines.
     Authentication passphrases and private keys never go over the network: the connection to the agent is
     forwarded over SSH remote connections and the result is returned to the requester, allowing the user access
     to their identities anywhere in the network in a secure fashion.

ENVIRONMENT

     SSH_AGENT_PID  When ssh-agent starts, it stores the name of the agent's process ID (PID) in this variable.

     SSH_AUTH_SOCK  When ssh-agent starts, it creates a UNIX-domain socket and stores its pathname in this
                    variable.  It is accessible only to the current user, but is easily abused by root or
                    another instance of the same user.

     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), ssh_config(5), 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.