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