Ubuntu Manpage: ssh-agent — OpenSSH authentication agent

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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 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 “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.

AUTHORS