| SSH-AGENT(1) | General Commands Manual | SSH-AGENT(1) |
ssh-agent —
ssh-agent |
[-a bind_address]
[-c | -s]
[-t life]
[-d] [command
[args ...]] |
ssh-agent |
[-c | -s]
-k |
ssh-agent is a program to hold private keys used for
public key authentication (RSA, DSA). 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.-sstdout. This is
the default if SHELL does not look like it's a csh
style of shell.-kSSH_AGENT_PID
environment variable).-t
life-dssh-agent will not fork.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/id_rsa, ~/.ssh/id_dsa
and ~/.ssh/identity. If the identity has a
passphrase, ssh-add(1) asks for the passphrase (using a
small X11 application if running under X11, or from the terminal if running
without X). 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 evalled 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.
| May 31, 2007 | BSD |