SSH-AGENT(1)SSH-AGENT(1)NAMEssh-agent - authentication agent
SYNOPSISssh-agent [-c | s] [-d] [-a bind_address] [-t life] [command [arg...]
ssh-agent [-c | s] -k
DESCRIPTIONssh-agent is a program to hold private keys used for public key authen‐
tication (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 pro‐
gram. 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
default is $TMPDIR/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.
-d Debug mode. When this option is specified ssh-agent will not
fork.
-k Kill the current agent (given by the SSH_AGENT_PID environment
variable).
-s Generate Bourne shell commands on stdout. This is the default
if SHELL does not look like it's a csh style of shell.
-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.
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,
~/.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 run‐
ning 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 dis‐
plays the identities currently held by the agent.
The idea is that the agent is run in the user's local PC, lap‐
top, 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 environ‐
ment 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 chan‐
nel. 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 com‐
mand 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_ecdsa
Contains the protocol version 2 ECDSA authentication identity of
the user.
~/.ssh/id_ed25519
Contains the protocol version 2 ED25519 authentication identity
of the user.
~/.ssh/id_rsa
Contains the protocol version 2 RSA authentication identity of
the user.
$TMPDIR/ssh-XXXXXXXXXX/agent.ppid
UNIX-domain sockets used to contain the connection to the
authentication agent. These sockets should only be readable by
the owner. The sockets should get automatically removed when
the agent exits.
SEE ALSOssh(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 created OpenSSH. Markus Friedl contributed the support for SSH
protocol versions 1.5 and 2.0.
December 7 2013 SSH-AGENT(1)
