1.\" $OpenBSD: ssh-agent.1,v 1.82 2025/02/09 18:24:08 schwarze 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: February 9 2025 $ 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 standard output. 81This is the default if 82.Ev SHELL 83looks like it's a csh style of shell. 84.It Fl D 85Foreground mode. 86When this option is specified, 87.Nm 88will not fork. 89.It Fl d 90Debug mode. 91When this option is specified, 92.Nm 93will not fork and will write debug information to standard error. 94.It Fl E Ar fingerprint_hash 95Specifies the hash algorithm used when displaying key fingerprints. 96Valid options are: 97.Dq md5 98and 99.Dq sha256 . 100The default is 101.Dq sha256 . 102.It Fl k 103Kill the current agent (given by the 104.Ev SSH_AGENT_PID 105environment variable). 106.It Fl O Ar option 107Specify an option when starting 108.Nm . 109The supported options are: 110.Cm allow-remote-pkcs11 , 111.Cm no-restrict-websafe 112and 113.Cm websafe-allow . 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.Pp 147Alternately the 148.Cm websafe-allow 149option allows specifying a pattern-list of key application strings to 150replace the default application allow-list, for example: 151.Dq websafe-allow=ssh:*,example.org,*.example.com 152.Pp 153See PATTERNS in 154.Xr ssh_config 5 155for a description of pattern-list syntax. 156.It Fl P Ar allowed_providers 157Specify a pattern-list of acceptable paths for PKCS#11 provider and FIDO 158authenticator middleware shared libraries that may be used with the 159.Fl S 160or 161.Fl s 162options to 163.Xr ssh-add 1 . 164Libraries that do not match the pattern list will be refused. 165The default list is 166.Dq usr/lib*/*,/usr/local/lib*/* . 167.Pp 168See PATTERNS in 169.Xr ssh_config 5 170for a description of pattern-list syntax. 171.It Fl s 172Generate Bourne shell commands on standard output. 173This is the default if 174.Ev SHELL 175does not look like it's a csh style of shell. 176.It Fl t Ar life 177Set a default value for the maximum lifetime of identities added to the agent. 178The lifetime may be specified in seconds or in a time format specified in 179.Xr sshd_config 5 . 180A lifetime specified for an identity with 181.Xr ssh-add 1 182overrides this value. 183Without this option the default maximum lifetime is forever. 184.It Fl x 185Exit after the last client has disconnected. 186.It Ar command Op Ar arg ... 187If a command (and optional arguments) is given, 188this is executed as a subprocess of the agent. 189The agent exits automatically when the command given on the command 190line terminates. 191.El 192.Pp 193There are three main ways to get an agent set up. 194The first is at the start of an X session, 195where all other windows or programs are started as children of the 196.Nm 197program. 198The agent starts a command under which its environment 199variables are exported, for example 200.Cm ssh-agent xterm & . 201When the command terminates, so does the agent. 202.Pp 203The second method is used for a login session. 204When 205.Nm 206is started, 207it prints the shell commands required to set its environment variables, 208which in turn can be evaluated in the calling shell, for example 209.Cm eval `ssh-agent -s` . 210.Pp 211In both of these cases, 212.Xr ssh 1 213looks at these environment variables 214and uses them to establish a connection to the agent. 215.Pp 216The third way to run 217.Nm 218is via socket activation from a supervising process, such as systemd. 219In this mode, the supervising process creates the listening socket and 220is responsible for starting 221.Nm 222as needed, and also for communicating the location of the socket listener 223to other programs in the user's session. 224Socket activation is used when 225.Nm 226is started with either of the 227.Fl d 228or 229.Fl D 230flags, no socket listening address specified by the 231.Fl a 232flag, and both the 233.Ev LISTEN_FDS 234and 235.Ev LISTEN_PID 236environment variables correctly supplied by the supervising process. 237.Pp 238The agent initially does not have any private keys. 239Keys are added using 240.Xr ssh-add 1 241or by 242.Xr ssh 1 243when 244.Cm AddKeysToAgent 245is set in 246.Xr ssh_config 5 . 247Multiple identities may be stored in 248.Nm 249concurrently and 250.Xr ssh 1 251will automatically use them if present. 252.Xr ssh-add 1 253is also used to remove keys from 254.Nm 255and to query the keys that are held in one. 256.Pp 257Connections to 258.Nm 259may be forwarded from further remote hosts using the 260.Fl A 261option to 262.Xr ssh 1 263(but see the caveats documented therein), 264avoiding the need for authentication data to be stored on other machines. 265Authentication passphrases and private keys never go over the network: 266the connection to the agent is forwarded over SSH remote connections 267and the result is returned to the requester, 268allowing the user access to their identities anywhere in the network 269in a secure fashion. 270.Pp 271.Nm 272will delete all keys it has loaded upon receiving 273.Dv SIGUSR1 . 274.Sh ENVIRONMENT 275.Bl -tag -width "SSH_AGENT_PID" 276.It Ev SSH_AGENT_PID 277When 278.Nm 279starts, it stores the name of the agent's process ID (PID) in this variable. 280.It Ev SSH_AUTH_SOCK 281When 282.Nm 283starts, it creates a 284.Ux Ns -domain 285socket and stores its pathname in this variable. 286It is accessible only to the current user, 287but is easily abused by root or another instance of the same user. 288.El 289.Sh FILES 290.Bl -tag -width Ds 291.It Pa $TMPDIR/ssh-XXXXXXXXXX/agent.<ppid> 292.Ux Ns -domain 293sockets used to contain the connection to the authentication agent. 294These sockets should only be readable by the owner. 295The sockets should get automatically removed when the agent exits. 296.El 297.Sh SEE ALSO 298.Xr ssh 1 , 299.Xr ssh-add 1 , 300.Xr ssh-keygen 1 , 301.Xr ssh_config 5 , 302.Xr sshd 8 303.Sh AUTHORS 304.An -nosplit 305OpenSSH is a derivative of the original and free ssh 1.2.12 release by 306.An Tatu Ylonen . 307.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , Theo de Raadt 308and 309.An Dug Song 310removed many bugs, re-added newer features and created OpenSSH. 311.An Markus Friedl 312contributed the support for SSH protocol versions 1.5 and 2.0. 313