1.\" $OpenBSD: ssh-agent.1,v 1.62 2015/11/15 23:54:15 jmc Exp $ 2.\" $FreeBSD$ 3.\" 4.\" Author: Tatu Ylonen <ylo@cs.hut.fi> 5.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland 6.\" All rights reserved 7.\" 8.\" As far as I am concerned, the code I have written for this software 9.\" can be used freely for any purpose. Any derived versions of this 10.\" software must be clearly marked as such, and if the derived work is 11.\" incompatible with the protocol description in the RFC file, it must be 12.\" called by a name other than "ssh" or "Secure Shell". 13.\" 14.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. 15.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. 16.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. 17.\" 18.\" Redistribution and use in source and binary forms, with or without 19.\" modification, are permitted provided that the following conditions 20.\" are met: 21.\" 1. Redistributions of source code must retain the above copyright 22.\" notice, this list of conditions and the following disclaimer. 23.\" 2. Redistributions in binary form must reproduce the above copyright 24.\" notice, this list of conditions and the following disclaimer in the 25.\" documentation and/or other materials provided with the distribution. 26.\" 27.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 28.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 29.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 30.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 31.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 32.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 33.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 34.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 35.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 36.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 37.\" 38.Dd $Mdocdate: November 15 2015 $ 39.Dt SSH-AGENT 1 40.Os 41.Sh NAME 42.Nm ssh-agent 43.Nd authentication agent 44.Sh SYNOPSIS 45.Nm ssh-agent 46.Op Fl c | s 47.Op Fl \&Ddx 48.Op Fl a Ar bind_address 49.Op Fl E Ar fingerprint_hash 50.Op Fl t Ar life 51.Op Ar command Op Ar arg ... 52.Nm ssh-agent 53.Op Fl c | s 54.Fl k 55.Sh DESCRIPTION 56.Nm 57is a program to hold private keys used for public key authentication 58(RSA, DSA, ECDSA, Ed25519). 59.Nm 60is usually started in the beginning of an X-session or a login session, and 61all other windows or programs are started as clients to the ssh-agent 62program. 63Through use of environment variables the agent can be located 64and automatically used for authentication when logging in to other 65machines using 66.Xr ssh 1 . 67.Pp 68The agent initially does not have any private keys. 69Keys are added using 70.Xr ssh 1 71(see 72.Cm AddKeysToAgent 73in 74.Xr ssh_config 5 75for details) 76or 77.Xr ssh-add 1 . 78Multiple identities may be stored in 79.Nm 80concurrently and 81.Xr ssh 1 82will automatically use them if present. 83.Xr ssh-add 1 84is also used to remove keys from 85.Nm 86and to query the keys that are held in one. 87.Pp 88The options are as follows: 89.Bl -tag -width Ds 90.It Fl a Ar bind_address 91Bind the agent to the 92.Ux Ns -domain 93socket 94.Ar bind_address . 95The default is 96.Pa $TMPDIR/ssh-XXXXXXXXXX/agent.\*(Ltppid\*(Gt . 97.It Fl c 98Generate C-shell commands on 99.Dv stdout . 100This is the default if 101.Ev SHELL 102looks like it's a csh style of shell. 103.It Fl D 104Foreground mode. 105When this option is specified 106.Nm 107will not fork. 108.It Fl d 109Debug mode. 110When this option is specified 111.Nm 112will not fork and will write debug information to standard error. 113.It Fl E Ar fingerprint_hash 114Specifies the hash algorithm used when displaying key fingerprints. 115Valid options are: 116.Dq md5 117and 118.Dq sha256 . 119The default is 120.Dq sha256 . 121.It Fl k 122Kill the current agent (given by the 123.Ev SSH_AGENT_PID 124environment variable). 125.It Fl s 126Generate Bourne shell commands on 127.Dv stdout . 128This is the default if 129.Ev SHELL 130does not look like it's a csh style of shell. 131.It Fl t Ar life 132Set a default value for the maximum lifetime of identities added to the agent. 133The lifetime may be specified in seconds or in a time format specified in 134.Xr sshd_config 5 . 135A lifetime specified for an identity with 136.Xr ssh-add 1 137overrides this value. 138Without this option the default maximum lifetime is forever. 139.It Fl x 140Exit after the last client has disconnected. 141.El 142.Pp 143If a command line is given, this is executed as a subprocess of the agent. 144When the command dies, so does the agent. 145.Pp 146The idea is that the agent is run in the user's local PC, laptop, or 147terminal. 148Authentication data need not be stored on any other 149machine, and authentication passphrases never go over the network. 150However, the connection to the agent is forwarded over SSH 151remote logins, and the user can thus use the privileges given by the 152identities anywhere in the network in a secure way. 153.Pp 154There are two main ways to get an agent set up: 155The first is that the agent starts a new subcommand into which some environment 156variables are exported, eg 157.Cm ssh-agent xterm & . 158The second is that the agent prints the needed shell commands (either 159.Xr sh 1 160or 161.Xr csh 1 162syntax can be generated) which can be evaluated in the calling shell, eg 163.Cm eval `ssh-agent -s` 164for Bourne-type shells such as 165.Xr sh 1 166or 167.Xr ksh 1 168and 169.Cm eval `ssh-agent -c` 170for 171.Xr csh 1 172and derivatives. 173.Pp 174Later 175.Xr ssh 1 176looks at these variables and uses them to establish a connection to the agent. 177.Pp 178The agent will never send a private key over its request channel. 179Instead, operations that require a private key will be performed 180by the agent, and the result will be returned to the requester. 181This way, private keys are not exposed to clients using the agent. 182.Pp 183A 184.Ux Ns -domain 185socket is created and the name of this socket is stored in the 186.Ev SSH_AUTH_SOCK 187environment 188variable. 189The socket is made accessible only to the current user. 190This method is easily abused by root or another instance of the same 191user. 192.Pp 193The 194.Ev SSH_AGENT_PID 195environment variable holds the agent's process ID. 196.Pp 197The agent exits automatically when the command given on the command 198line terminates. 199.Sh FILES 200.Bl -tag -width Ds 201.It Pa $TMPDIR/ssh-XXXXXXXXXX/agent.\*(Ltppid\*(Gt 202.Ux Ns -domain 203sockets used to contain the connection to the authentication agent. 204These sockets should only be readable by the owner. 205The sockets should get automatically removed when the agent exits. 206.El 207.Sh SEE ALSO 208.Xr ssh 1 , 209.Xr ssh-add 1 , 210.Xr ssh-keygen 1 , 211.Xr sshd 8 212.Sh AUTHORS 213OpenSSH is a derivative of the original and free 214ssh 1.2.12 release by Tatu Ylonen. 215Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 216Theo de Raadt and Dug Song 217removed many bugs, re-added newer features and 218created OpenSSH. 219Markus Friedl contributed the support for SSH 220protocol versions 1.5 and 2.0. 221