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

       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).

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

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

               The no-restrict-websafe option instructs ssh-agent to
               permit signatures using FIDO keys that might be web
               authentication requests.  By default, ssh-agent 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 websites.

       -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 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         top

       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.

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

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GNU                          August 10, 2023                SSH-AGENT(1)