.\" $OpenBSD: ssh-agent.1,v 1.62 2015/11/15 23:54:15 jmc Exp $
.\"
.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
.\"                    All rights reserved
.\"
.\" As far as I am concerned, the code I have written for this software
.\" can be used freely for any purpose.  Any derived versions of this
.\" software must be clearly marked as such, and if the derived work is
.\" incompatible with the protocol description in the RFC file, it must be
.\" called by a name other than "ssh" or "Secure Shell".
.\"
.\" Copyright (c) 1999,2000 Markus Friedl.  All rights reserved.
.\" Copyright (c) 1999 Aaron Campbell.  All rights reserved.
.\" Copyright (c) 1999 Theo de Raadt.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
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.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
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.\"
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.\"
.Dd $Mdocdate: November 15 2015 $
.Dt SSH-AGENT 1
.Os
.Sh NAME
.Nm ssh-agent
.Nd authentication agent
.Sh SYNOPSIS
.Nm ssh-agent
.Op Fl c | s
.Op Fl \&Dd
.Op Fl a Ar bind_address
.Op Fl E Ar fingerprint_hash
.Op Fl t Ar life
.Op Ar command Op Ar arg ...
.Nm ssh-agent
.Op Fl c | s
.Fl k
.Sh DESCRIPTION
.Nm
is a program to hold private keys used for public key authentication
(RSA, DSA, ECDSA, Ed25519).
.Nm
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
.Xr ssh 1 .
.Pp
The agent initially does not have any private keys.
Keys are added using
.Xr ssh 1
(see
.Cm AddKeysToAgent
in
.Xr ssh_config 5
for details)
or
.Xr ssh-add 1 .
Multiple identities may be stored in
.Nm
concurrently and
.Xr ssh 1
will automatically use them if present.
.Xr ssh-add 1
is also used to remove keys from
.Nm
and to query the keys that are held in one.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl a Ar bind_address
Bind the agent to the
.Ux Ns -domain
socket
.Ar bind_address .
The default is
.Pa $TMPDIR/ssh-XXXXXXXXXX/agent.\*(Ltppid\*(Gt .
.It Fl c
Generate C-shell commands on
.Dv stdout .
This is the default if
.Ev SHELL
looks like it's a csh style of shell.
.It Fl D
Foreground mode.
When this option is specified
.Nm
will not fork.
.It Fl d
Debug mode.
When this option is specified
.Nm
will not fork and will write debug information to standard error.
.It Fl E Ar fingerprint_hash
Specifies the hash algorithm used when displaying key fingerprints.
Valid options are:
.Dq md5
and
.Dq sha256 .
The default is
.Dq sha256 .
.It Fl k
Kill the current agent (given by the
.Ev SSH_AGENT_PID
environment variable).
.It Fl s
Generate Bourne shell commands on
.Dv stdout .
This is the default if
.Ev SHELL
does not look like it's a csh style of shell.
.It Fl t Ar 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
.Xr sshd_config 5 .
A lifetime specified for an identity with
.Xr ssh-add 1
overrides this value.
Without this option the default maximum lifetime is forever.
.El
.Pp
If a command line is given, this is executed as a subprocess of the agent.
When the command dies, so does the agent.
.Pp
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.
.Pp
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
.Cm ssh-agent xterm & .
The second is that the agent prints the needed shell commands (either
.Xr sh 1
or
.Xr csh 1
syntax can be generated) which can be evaluated in the calling shell, eg
.Cm eval `ssh-agent -s`
for Bourne-type shells such as
.Xr sh 1
or
.Xr ksh 1
and
.Cm eval `ssh-agent -c`
for
.Xr csh 1
and derivatives.
.Pp
Later
.Xr ssh 1
looks at these variables and uses them to establish a connection to the agent.
.Pp
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.
.Pp
A
.Ux Ns -domain
socket is created and the name of this socket is stored in the
.Ev 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.
.Pp
The
.Ev SSH_AGENT_PID
environment variable holds the agent's process ID.
.Pp
The agent exits automatically when the command given on the command
line terminates.
.Sh FILES
.Bl -tag -width Ds
.It Pa $TMPDIR/ssh-XXXXXXXXXX/agent.\*(Ltppid\*(Gt
.Ux Ns -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.
.El
.Sh SEE ALSO
.Xr ssh 1 ,
.Xr ssh-add 1 ,
.Xr ssh-keygen 1 ,
.Xr sshd 8
.Sh 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.