ssh-agent(1) — Linux manual page


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

NAME         top

       ssh-agent — OpenSSH authentication agent

SYNOPSIS         top

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

DESCRIPTION         top

       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, will not

       -d      Debug mode.  When this option is specified, 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).

       -O option
               Specify an option when starting .  Currently two options
               are supported: allow-remote-pkcs11 and

               The allow-remote-pkcs11 option allows clients of a
               forwarded to load PKCS#11 or FIDO provider libraries.  By
               default only local clients may perform this operation.
               Note that signalling that an client is remote is
               performed by ssh(1), and use of other tools to forward
               access to the agent socket may circumvent this

               The no-restrict-websafe option instructs to permit
               signatures using FIDO keys that might be web
               authentication requests.  By default, refuses signature
               requests for FIDO keys where the key application string
               does not start with “ssh:” and when the data to be signed
               does not appear to be a ssh(1) user authentication
               request or a ssh-keygen(1) signature.  The default
               behaviour prevents forwarded access to a FIDO key from
               also implicitly forwarding the ability to authenticate to

       -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

       -s      Generate Bourne shell commands on stdout.  This is the
               default if SHELL does not look like it's a csh style of

       -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

       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 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 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 concurrently
       and ssh(1) will automatically use them if present.  ssh-add(1) is
       also used to remove keys from and to query the keys that are held
       in one.

       Connections to 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         top

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

       SSH_AUTH_SOCK  When 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

FILES         top

               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         top

       ssh(1), ssh-add(1), ssh-keygen(1), ssh_config(5), sshd(8)

AUTHORS         top

       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.

COLOPHON         top

       This page is part of the openssh (Portable OpenSSH) project.
       Information about the project can be found at  If you have a bug report
       for this manual page, see ⟨⟩.
       This page was obtained from the tarball openssh-9.6p1.tar.gz
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       ⟨⟩ on
       2023-12-22.  If you discover any rendering problems in this HTML
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       part of the original manual page), send a mail to

GNU                          August 10, 2023                SSH-AGENT(1)