SSH-AGENT(1) General Commands Manual SSH-AGENT(1)
ssh-agent - authentication agent
ssh-agent [-c | -s] [-d] [-a bind_address] [-E fingerprint_hash]
[-t life] [command [arg ...]]
ssh-agent [-c | -s] -k
ssh-agent is a program to hold private keys used for public key
authentication (RSA, DSA, ECDSA, Ed25519). ssh-agent is usually started
in the beginning of an X-session or a login session, and all other
windows or programs are started as clients to the ssh-agent program.
Through use of environment variables the agent can be located and
automatically used for authentication when logging in to other machines
The agent initially does not have any private keys. Keys are added using
ssh-add(1). 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
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
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
-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 idea is that the agent is run in the user's local PC, laptop, or
terminal. Authentication data need not be stored on any other machine,
and authentication passphrases never go over the network. However, the
connection to the agent is forwarded over SSH remote logins, and the user
can thus use the privileges given by the identities anywhere in the
network in a secure way.
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
connection 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,
private 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
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
ssh(1), ssh-add(1), ssh-keygen(1), sshd(8)
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.
OpenBSD 5.7 December 21, 2014 OpenBSD 5.7
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