SSH-AGENT(1) | General Commands Manual | SSH-AGENT(1) |
ssh-agent
—
authentication agent
ssh-agent |
[-c | -s ]
[-Dd ] [-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 using
ssh(1).
The agent initially does not have any private keys. Keys are added
using ssh(1) (see
AddKeysToAgent
in
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 one.
The options are as follows:
-a
bind_address-c
stdout
. This is the
default if SHELL
looks like it's a csh style of
shell.-D
ssh-agent
will not fork.-d
ssh-agent
will not fork and will write debug
information to standard error.-E
fingerprint_hash-k
SSH_AGENT_PID
environment variable).-s
stdout
. This is
the default if SHELL
does not look like it's a csh
style of shell.-t
lifeIf 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
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 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 15, 2015 | OpenBSD-5.9 |