1.\" $OpenBSD: ssh-agent.1,v 1.79 2023/08/10 14:37:32 naddy 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: August 10 2023 $ 38.Dt SSH-AGENT 1 39.Os 40.Sh NAME 41.Nm ssh-agent 42.Nd OpenSSH authentication agent 43.Sh SYNOPSIS 44.Nm ssh-agent 45.Op Fl c | s 46.Op Fl \&Ddx 47.Op Fl a Ar bind_address 48.Op Fl E Ar fingerprint_hash 49.Op Fl O Ar option 50.Op Fl P Ar allowed_providers 51.Op Fl t Ar life 52.Nm ssh-agent 53.Op Fl a Ar bind_address 54.Op Fl E Ar fingerprint_hash 55.Op Fl O Ar option 56.Op Fl P Ar allowed_providers 57.Op Fl t Ar life 58.Ar command Op Ar arg ... 59.Nm ssh-agent 60.Op Fl c | s 61.Fl k 62.Sh DESCRIPTION 63.Nm 64is a program to hold private keys used for public key authentication. 65Through use of environment variables the agent can be located 66and automatically used for authentication when logging in to other 67machines using 68.Xr ssh 1 . 69.Pp 70The options are as follows: 71.Bl -tag -width Ds 72.It Fl a Ar bind_address 73Bind the agent to the 74.Ux Ns -domain 75socket 76.Ar bind_address . 77The default is 78.Pa $TMPDIR/ssh-XXXXXXXXXX/agent.\*(Ltppid\*(Gt . 79.It Fl c 80Generate C-shell commands on 81.Dv stdout . 82This is the default if 83.Ev SHELL 84looks like it's a csh style of shell. 85.It Fl D 86Foreground mode. 87When this option is specified, 88.Nm 89will not fork. 90.It Fl d 91Debug mode. 92When this option is specified, 93.Nm 94will not fork and will write debug information to standard error. 95.It Fl E Ar fingerprint_hash 96Specifies the hash algorithm used when displaying key fingerprints. 97Valid options are: 98.Dq md5 99and 100.Dq sha256 . 101The default is 102.Dq sha256 . 103.It Fl k 104Kill the current agent (given by the 105.Ev SSH_AGENT_PID 106environment variable). 107.It Fl O Ar option 108Specify an option when starting 109.Nm . 110Currently two options are supported: 111.Cm allow-remote-pkcs11 112and 113.Cm no-restrict-websafe . 114.Pp 115The 116.Cm allow-remote-pkcs11 117option allows clients of a forwarded 118.Nm 119to load PKCS#11 or FIDO provider libraries. 120By default only local clients may perform this operation. 121Note that signalling that an 122.Nm 123client is remote is performed by 124.Xr ssh 1 , 125and use of other tools to forward access to the agent socket may circumvent 126this restriction. 127.Pp 128The 129.Cm no-restrict-websafe 130option instructs 131.Nm 132to permit signatures using FIDO keys that might be web authentication 133requests. 134By default, 135.Nm 136refuses signature requests for FIDO keys where the key application string 137does not start with 138.Dq ssh: 139and when the data to be signed does not appear to be a 140.Xr ssh 1 141user authentication request or a 142.Xr ssh-keygen 1 143signature. 144The default behaviour prevents forwarded access to a FIDO key from also 145implicitly forwarding the ability to authenticate to websites. 146.It Fl P Ar allowed_providers 147Specify a pattern-list of acceptable paths for PKCS#11 provider and FIDO 148authenticator middleware shared libraries that may be used with the 149.Fl S 150or 151.Fl s 152options to 153.Xr ssh-add 1 . 154Libraries that do not match the pattern list will be refused. 155See PATTERNS in 156.Xr ssh_config 5 157for a description of pattern-list syntax. 158The default list is 159.Dq usr/lib*/*,/usr/local/lib*/* . 160.It Fl s 161Generate Bourne shell commands on 162.Dv stdout . 163This is the default if 164.Ev SHELL 165does not look like it's a csh style of shell. 166.It Fl t Ar life 167Set a default value for the maximum lifetime of identities added to the agent. 168The lifetime may be specified in seconds or in a time format specified in 169.Xr sshd_config 5 . 170A lifetime specified for an identity with 171.Xr ssh-add 1 172overrides this value. 173Without this option the default maximum lifetime is forever. 174.It Fl x 175Exit after the last client has disconnected. 176.It Ar command Op Ar arg ... 177If a command (and optional arguments) is given, 178this is executed as a subprocess of the agent. 179The agent exits automatically when the command given on the command 180line terminates. 181.El 182.Pp 183There are two main ways to get an agent set up. 184The first is at the start of an X session, 185where all other windows or programs are started as children of the 186.Nm 187program. 188The agent starts a command under which its environment 189variables are exported, for example 190.Cm ssh-agent xterm & . 191When the command terminates, so does the agent. 192.Pp 193The second method is used for a login session. 194When 195.Nm 196is started, 197it prints the shell commands required to set its environment variables, 198which in turn can be evaluated in the calling shell, for example 199.Cm eval `ssh-agent -s` . 200.Pp 201In both cases, 202.Xr ssh 1 203looks at these environment variables 204and uses them to establish a connection to the agent. 205.Pp 206The agent initially does not have any private keys. 207Keys are added using 208.Xr ssh-add 1 209or by 210.Xr ssh 1 211when 212.Cm AddKeysToAgent 213is set in 214.Xr ssh_config 5 . 215Multiple identities may be stored in 216.Nm 217concurrently and 218.Xr ssh 1 219will automatically use them if present. 220.Xr ssh-add 1 221is also used to remove keys from 222.Nm 223and to query the keys that are held in one. 224.Pp 225Connections to 226.Nm 227may be forwarded from further remote hosts using the 228.Fl A 229option to 230.Xr ssh 1 231(but see the caveats documented therein), 232avoiding the need for authentication data to be stored on other machines. 233Authentication passphrases and private keys never go over the network: 234the connection to the agent is forwarded over SSH remote connections 235and the result is returned to the requester, 236allowing the user access to their identities anywhere in the network 237in a secure fashion. 238.Sh ENVIRONMENT 239.Bl -tag -width "SSH_AGENT_PID" 240.It Ev SSH_AGENT_PID 241When 242.Nm 243starts, it stores the name of the agent's process ID (PID) in this variable. 244.It Ev SSH_AUTH_SOCK 245When 246.Nm 247starts, it creates a 248.Ux Ns -domain 249socket and stores its pathname in this variable. 250It is accessible only to the current user, 251but is easily abused by root or another instance of the same user. 252.El 253.Sh FILES 254.Bl -tag -width Ds 255.It Pa $TMPDIR/ssh-XXXXXXXXXX/agent.<ppid> 256.Ux Ns -domain 257sockets used to contain the connection to the authentication agent. 258These sockets should only be readable by the owner. 259The sockets should get automatically removed when the agent exits. 260.El 261.Sh SEE ALSO 262.Xr ssh 1 , 263.Xr ssh-add 1 , 264.Xr ssh-keygen 1 , 265.Xr ssh_config 5 , 266.Xr sshd 8 267.Sh AUTHORS 268.An -nosplit 269OpenSSH is a derivative of the original and free ssh 1.2.12 release by 270.An Tatu Ylonen . 271.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , Theo de Raadt 272and 273.An Dug Song 274removed many bugs, re-added newer features and created OpenSSH. 275.An Markus Friedl 276contributed the support for SSH protocol versions 1.5 and 2.0. 277