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 using
The agent initially does not have any private keys. Keys are added
using ssh(1) (see
ssh_config(5) for details) or
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>.
- Generate C-shell commands on
stdout. This is the default if
SHELLlooks like it's a csh style of shell.
- Foreground mode. When this option is specified
ssh-agentwill not fork.
- Debug mode. When this option is specified
ssh-agentwill not fork and will write debug information to standard error.
- Specifies the hash algorithm used when displaying key fingerprints. Valid options are: “md5” and “sha256”. The default is “sha256”.
- Kill the current agent (given by the
- Generate Bourne shell commands on
stdout. This is the default if
SHELLdoes 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 command line 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
ssh-agent xterm &. The second is
that the agent prints the needed shell commands (either
or csh(1) syntax can be generated) which can be evaluated in the calling
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
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
SSH_AGENT_PID environment variable
holds the agent's process ID.
The agent exits automatically when the command given on the command line terminates.
- 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.
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.