is a program to hold private keys used
for public key authentication (RSA, DSA, ECDSA, Ed25519).
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 ssh(1)
The agent initially does not have any private keys. Keys are added using
for details) or
. Multiple identities may be stored in
will automatically use them if present.
is also used to remove keys from
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
- Generate C-shell commands on
stdout. This is the default if
SHELL looks like it's a csh style of
- Foreground mode. When this option is specified
ssh-agent will not fork.
- Debug mode. When this option is specified
ssh-agent will 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
- 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.
- 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 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
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
syntax can be generated) which can be evaluated in the calling shell, eg
eval `ssh-agent -s`
for Bourne-type shells such
and eval `ssh-agent -c`
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.
-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 user.
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 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
. If you need to set any of these
environment variables, you will need to do so in the program executed by
- 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.
OpenSSH is a derivative of the original and free ssh 1.2.12 release by
Theo de Raadt
removed many bugs, re-added newer
features and created OpenSSH. Markus Friedl
contributed the support for SSH protocol versions 1.5 and 2.0.