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SSH-AGENT(1) OpenBSD Reference Manual SSH-AGENT(1)
NAME
ssh-agent - authentication agent
SYNOPSIS
ssh-agent [-a bind_address] [-c | -s] [-t life] [-d] [command [args ...]]
ssh-agent [-c | -s] -k
DESCRIPTION
ssh-agent is a program to hold private keys used for public key authenti-
cation (RSA, DSA). The idea is that ssh-agent is started in the begin-
ning of an X-session or a login session, and all other windows or pro-
grams 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
Bind the agent to the unix-domain socket bind_address. The de-
fault is /tmp/ssh-XXXXXXXXXX/agent.<ppid>.
-c Generate C-shell commands on stdout. This is the default if
SHELL looks like it's a csh style of shell.
-s Generate Bourne shell commands on stdout. This is the default if
SHELL does not look like it's a csh style of shell.
-k Kill the current agent (given by the SSH_AGENT_PID environment
variable).
-t life
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.
-d Debug mode. When this option is specified ssh-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 appli-
cation 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 identi-
ties. 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 ter-
minal. Authentication data need not be stored on any other machine, and
authentication passphrases never go over the network. However, the con-
nection to the agent is forwarded over SSH remote logins, and the user
can thus use the privileges given by the identities anywhere in the net-
work 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 con-
nection to the agent.
The agent will never send a private key over its request channel. In-
stead, operations that require a private key will be performed by the
agent, and the result will be returned to the requester. This way, pri-
vate 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 anoth-
er 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.
FILES
~/.ssh/identity
Contains the protocol version 1 RSA authentication identity of
the user.
~/.ssh/id_dsa
Contains the protocol version 2 DSA authentication identity of
the user.
~/.ssh/id_rsa
Contains the protocol version 2 RSA authentication identity of
the user.
/tmp/ssh-XXXXXXXXXX/agent.<ppid>
Unix-domain sockets used to contain the connection to the authen-
tication agent. These sockets should only be readable by the
owner. The sockets should get automatically removed when the
agent exits.
SEE ALSO
ssh(1), ssh-add(1), ssh-keygen(1), sshd(8)
AUTHORS
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 cre-
ated OpenSSH. Markus Friedl contributed the support for SSH protocol
versions 1.5 and 2.0.
OpenBSD 4.2 June 5, 2007 2
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