1.\" $OpenBSD: ssh-agent.1,v 1.55 2014/04/16 23:28:12 djm Exp $ 2.\" 3.\" Author: Tatu Ylonen <ylo@cs.hut.fi> 4.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland 5.\" All rights reserved 6.\" 7.\" As far as I am concerned, the code I have written for this software 8.\" can be used freely for any purpose. Any derived versions of this 9.\" software must be clearly marked as such, and if the derived work is 10.\" incompatible with the protocol description in the RFC file, it must be 11.\" called by a name other than "ssh" or "Secure Shell". 12.\" 13.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. 14.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. 15.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. 16.\" 17.\" Redistribution and use in source and binary forms, with or without 18.\" modification, are permitted provided that the following conditions 19.\" are met: 20.\" 1. Redistributions of source code must retain the above copyright 21.\" notice, this list of conditions and the following disclaimer. 22.\" 2. Redistributions in binary form must reproduce the above copyright 23.\" notice, this list of conditions and the following disclaimer in the 24.\" documentation and/or other materials provided with the distribution. 25.\" 26.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 27.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 28.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 29.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 30.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 31.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 32.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 33.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 34.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 35.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 36.\" 37.Dd $Mdocdate: April 16 2014 $ 38.Dt SSH-AGENT 1 39.Os 40.Sh NAME 41.Nm ssh-agent 42.Nd authentication agent 43.Sh SYNOPSIS 44.Nm ssh-agent 45.Op Fl c | s 46.Op Fl d 47.Op Fl a Ar bind_address 48.Op Fl t Ar life 49.Op Ar command Op Ar arg ... 50.Nm ssh-agent 51.Op Fl c | s 52.Fl k 53.Sh DESCRIPTION 54.Nm 55is a program to hold private keys used for public key authentication 56(RSA, DSA, ECDSA, ED25519). 57.Nm 58is usually started in the beginning of an X-session or a login session, and 59all other windows or programs are started as clients to the ssh-agent 60program. 61Through use of environment variables the agent can be located 62and automatically used for authentication when logging in to other 63machines using 64.Xr ssh 1 . 65.Pp 66The agent initially does not have any private keys. 67Keys are added using 68.Xr ssh-add 1 . 69Multiple identities may be stored in 70.Nm 71concurrently and 72.Xr ssh 1 73will automatically use them if present. 74.Xr ssh-add 1 75is also used to remove keys from 76.Nm 77and to query the keys that are held in one. 78.Pp 79The options are as follows: 80.Bl -tag -width Ds 81.It Fl a Ar bind_address 82Bind the agent to the 83.Ux Ns -domain 84socket 85.Ar bind_address . 86The default is 87.Pa $TMPDIR/ssh-XXXXXXXXXX/agent.\*(Ltppid\*(Gt . 88.It Fl c 89Generate C-shell commands on 90.Dv stdout . 91This is the default if 92.Ev SHELL 93looks like it's a csh style of shell. 94.It Fl d 95Debug mode. 96When this option is specified 97.Nm 98will not fork. 99.It Fl k 100Kill the current agent (given by the 101.Ev SSH_AGENT_PID 102environment variable). 103.It Fl s 104Generate Bourne shell commands on 105.Dv stdout . 106This is the default if 107.Ev SHELL 108does not look like it's a csh style of shell. 109.It Fl t Ar life 110Set a default value for the maximum lifetime of identities added to the agent. 111The lifetime may be specified in seconds or in a time format specified in 112.Xr sshd_config 5 . 113A lifetime specified for an identity with 114.Xr ssh-add 1 115overrides this value. 116Without this option the default maximum lifetime is forever. 117.El 118.Pp 119If a commandline is given, this is executed as a subprocess of the agent. 120When the command dies, so does the agent. 121.Pp 122The idea is that the agent is run in the user's local PC, laptop, or 123terminal. 124Authentication data need not be stored on any other 125machine, and authentication passphrases never go over the network. 126However, the connection to the agent is forwarded over SSH 127remote logins, and the user can thus use the privileges given by the 128identities anywhere in the network in a secure way. 129.Pp 130There are two main ways to get an agent set up: 131The first is that the agent starts a new subcommand into which some environment 132variables are exported, eg 133.Cm ssh-agent xterm & . 134The second is that the agent prints the needed shell commands (either 135.Xr sh 1 136or 137.Xr csh 1 138syntax can be generated) which can be evaluated in the calling shell, eg 139.Cm eval `ssh-agent -s` 140for Bourne-type shells such as 141.Xr sh 1 142or 143.Xr ksh 1 144and 145.Cm eval `ssh-agent -c` 146for 147.Xr csh 1 148and derivatives. 149.Pp 150Later 151.Xr ssh 1 152looks at these variables and uses them to establish a connection to the agent. 153.Pp 154The agent will never send a private key over its request channel. 155Instead, operations that require a private key will be performed 156by the agent, and the result will be returned to the requester. 157This way, private keys are not exposed to clients using the agent. 158.Pp 159A 160.Ux Ns -domain 161socket is created and the name of this socket is stored in the 162.Ev SSH_AUTH_SOCK 163environment 164variable. 165The socket is made accessible only to the current user. 166This method is easily abused by root or another instance of the same 167user. 168.Pp 169The 170.Ev SSH_AGENT_PID 171environment variable holds the agent's process ID. 172.Pp 173The agent exits automatically when the command given on the command 174line terminates. 175.Sh FILES 176.Bl -tag -width Ds 177.It Pa $TMPDIR/ssh-XXXXXXXXXX/agent.\*(Ltppid\*(Gt 178.Ux Ns -domain 179sockets used to contain the connection to the authentication agent. 180These sockets should only be readable by the owner. 181The sockets should get automatically removed when the agent exits. 182.El 183.Sh SEE ALSO 184.Xr ssh 1 , 185.Xr ssh-add 1 , 186.Xr ssh-keygen 1 , 187.Xr sshd 8 188.Sh AUTHORS 189OpenSSH is a derivative of the original and free 190ssh 1.2.12 release by Tatu Ylonen. 191Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 192Theo de Raadt and Dug Song 193removed many bugs, re-added newer features and 194created OpenSSH. 195Markus Friedl contributed the support for SSH 196protocol versions 1.5 and 2.0. 197