|SSH-AGENT(1)||General Commands Manual||SSH-AGENT(1)|
ssh-agent is a program to hold private
keys used for public key authentication (RSA, DSA, ECDSA). The idea is that
ssh-agent is 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 options are as follows:
stdout. This is the default if
SHELLlooks like it's a csh style of shell.
ssh-agentwill not fork.
stdout. This is the default if
SHELLdoes not look like it's a csh style of shell.
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/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 authentication
will fail. It then sends the identity to the agent. Several identities can
be stored in the agent; the agent can automatically use any of these
ssh-add -l displays the identities
currently held by 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
csh(1) syntax can be generated)
which can be evaluated in the calling shell, eg
`ssh-agent -s` for Bourne-type shells such as
`ssh-agent -c` for csh(1)
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.
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.
|November 21, 2010||OpenBSD-5.1|