man > ssh-agent(1)

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

NAME
       ssh-agent — OpenSSH authentication agent

SYNOPSIS
       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
       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 no-restrict-websafe.

               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 signa‐
               ture  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 authentica‐
               tion 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  authentica‐
               tor  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  PAT‐
               TERNS 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  con‐
       nection 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  pro‐
       grams,  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 automati‐
               cally 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.

Debian                                     August 10, 2023                              SSH-AGENT(1)