ssh-agent [-c | -s] [-d] [-a bind_address] [-t life]
[-P pkcs11_whitelist] [command [arg ...]]
ssh-agent [-c | -s] -k
ssh-agent is a program to hold private keys used for public key authenti-
cation (RSA, DSA, ECDSA, ED25519). The idea is that ssh-agent is started
in the beginning of an X-session or a login session, and all other win-
dows or programs are started as clients to the ssh-agent program.
Through use of environment variables the agent can be located and auto-
matically used for authentication when logging in to other machines using
The options are as follows:
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 Debug mode. When this option is specified ssh-agent will not
-k Kill the current agent (given by the SSH_AGENT_PID environment
-P Specify a pattern-list of acceptable paths for PKCS#11 shared
libraries that may be added using the -s option to ssh-add(1).
The default is to allow loading PKCS#11 libraries from
``/usr/lib/*,/usr/local/lib/*''. PKCS#11 libraries that do not
match the whitelist will be refused. See PATTERNS in
ssh_config(5) for a description of pattern-list syntax.
-s Generate Bourne shell commands on stdout. This is the default if
SHELL does not look like it's a csh style of shell.
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.
If a commandline is given, this is executed as a subprocess of the agent.
When the command dies, so does the agent.
The agent initially does not have any private keys. Keys are added using
ssh-add(1). When executed without arguments, ssh-add(1) adds the files
~/.ssh/id_rsa, ~/.ssh/id_dsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ed25519 and
~/.ssh/identity. If the identity has a passphrase, ssh-add(1) asks for
the passphrase on the terminal if it has one or from a small X11 program
if running under X11. If neither of these is the case then the authenti-
There are two main ways to get an agent set up: The first is that the
agent starts a new subcommand into which some environment variables are
exported, eg ssh-agent xterm &. The second is that the agent prints the
needed shell commands (either sh(1) or csh(1) syntax can be generated)
which can be evaluated in the calling shell, eg eval `ssh-agent -s` for
Bourne-type shells such as sh(1) or ksh(1) and eval `ssh-agent -c` for
csh(1) and derivatives.
Later ssh(1) looks at these variables and uses them to establish a con-
nection to the agent.
The agent will never send a private key over its request channel.
Instead, operations that require a private key will be performed by the
agent, and the result will be returned to the requester. This way, pri-
vate keys are not exposed to clients using the agent.
A UNIX-domain socket is created and the name of this socket is stored in
the SSH_AUTH_SOCK environment variable. The socket is made accessible
only to the current user. This method is easily abused by root or
another instance of the same user.
The SSH_AGENT_PID environment variable holds the agent's process ID.
The agent exits automatically when the command given on the command line
In Debian, ssh-agent is installed with the set-group-id bit set, to pre-
vent 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.
Contains the protocol version 1 RSA authentication identity of
Contains the protocol version 2 DSA authentication identity of
Contains the protocol version 2 ECDSA authentication identity of
Contains the protocol version 2 ED25519 authentication identity
of the user.
Contains the protocol version 2 RSA authentication identity of
Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
de Raadt and Dug Song removed many bugs, re-added newer features and cre-
ated OpenSSH. Markus Friedl contributed the support for SSH protocol
versions 1.5 and 2.0.
BSD September 21, 2018 BSD
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