| SSH-AGENT(1) | General Commands Manual | SSH-AGENT(1) |
ssh-agent —
ssh-agent |
[-c | -s]
[-d] [-a
bind_address] [-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). 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 ssh(1).
The options are as follows:
-a
bind_address-cstdout. This is the
default if SHELL looks like it's a csh style of
shell.-dssh-agent will not fork.-kSSH_AGENT_PID
environment variable).-sstdout. This is
the default if SHELL does not look like it's a csh
style of shell.-t
lifeIf 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/id_rsa,
~/.ssh/id_dsa,
~/.ssh/id_ecdsa,
~/.ssh/id_ed25519 and
~/.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 identities.
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
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.
| December 7, 2013 | BSD |