jcs.org
jcs's openbsd hax
openbsd
1.\" $OpenBSD: ssh-agent.1,v 1.86 2025/10/04 21:41:35 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: October 4 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 \&DdTU
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 TU
54.Op Fl a Ar bind_address
55.Op Fl E Ar fingerprint_hash
56.Op Fl O Ar option
57.Op Fl P Ar allowed_providers
58.Op Fl t Ar life
59.Ar command Op Ar arg ...
60.Nm ssh-agent
61.Op Fl c | s
62.Fl k
63.Nm ssh-agent
64.Fl u
65.Sh DESCRIPTION
66.Nm
67is a program to hold private keys used for public key authentication.
68Through use of environment variables the agent can be located
69and automatically used for authentication when logging in to other
70machines using
71.Xr ssh 1 .
72.Pp
73The options are as follows:
74.Bl -tag -width Ds
75.It Fl a Ar bind_address
76Bind the agent to the
77.Ux Ns -domain
78socket
79.Ar bind_address .
80The default is to create a socket at a random path matching
81.Pa $HOME/.ssh/agent/s.* .
82.It Fl c
83Generate C-shell commands on standard output.
84This is the default if
85.Ev SHELL
86looks like it's a csh style of shell.
87.It Fl D
88Foreground mode.
89When this option is specified,
90.Nm
91will not fork.
92.It Fl d
93Debug mode.
94When this option is specified,
95.Nm
96will not fork and will write debug information to standard error.
97.It Fl E Ar fingerprint_hash
98Specifies the hash algorithm used when displaying key fingerprints.
99Valid options are:
100.Dq md5
101and
102.Dq sha256 .
103The default is
104.Dq sha256 .
105.It Fl k
106Kill the current agent (given by the
107.Ev SSH_AGENT_PID
108environment variable).
109.It Fl O Ar option
110Specify an option when starting
111.Nm .
112The supported options are:
113.Cm allow-remote-pkcs11 ,
114.Cm no-restrict-websafe
115and
116.Cm websafe-allow .
117.Pp
118The
119.Cm allow-remote-pkcs11
120option allows clients of a forwarded
121.Nm
122to load PKCS#11 or FIDO provider libraries.
123By default only local clients may perform this operation.
124Note that signalling that an
125.Nm
126client is remote is performed by
127.Xr ssh 1 ,
128and use of other tools to forward access to the agent socket may circumvent
129this restriction.
130.Pp
131The
132.Cm no-restrict-websafe
133option instructs
134.Nm
135to permit signatures using FIDO keys that might be web authentication
136requests.
137By default,
138.Nm
139refuses signature requests for FIDO keys where the key application string
140does not start with
141.Dq ssh:
142and when the data to be signed does not appear to be an
143.Xr ssh 1
144user authentication request or an
145.Xr ssh-keygen 1
146signature.
147The default behaviour prevents forwarded access to a FIDO key from also
148implicitly forwarding the ability to authenticate to websites.
149.Pp
150Alternately the
151.Cm websafe-allow
152option allows specifying a pattern-list of key application strings to
153replace the default application allow-list, for example:
154.Dq websafe-allow=ssh:*,example.org,*.example.com
155.Pp
156See PATTERNS in
157.Xr ssh_config 5
158for a description of pattern-list syntax.
159.It Fl P Ar allowed_providers
160Specify a pattern-list of acceptable paths for PKCS#11 provider and FIDO
161authenticator middleware shared libraries that may be used with the
162.Fl S
163or
164.Fl s
165options to
166.Xr ssh-add 1 .
167Libraries that do not match the pattern list will be refused.
168The default list is
169.Dq /usr/lib/*,/usr/local/lib/* .
170.Pp
171See PATTERNS in
172.Xr ssh_config 5
173for a description of pattern-list syntax.
174.It Fl s
175Generate Bourne shell commands on standard output.
176This is the default if
177.Ev SHELL
178does not look like it's a csh style of shell.
179.It Fl T
180Bind the agent socket in a randomised subdirectory of the form
181.Pa $TMPDIR/ssh-XXXXXXXXXX/agent.\*(Ltppid\*(Gt ,
182instead of the default behaviour of using a randomised name matching
183.Pa $HOME/.ssh/agent/s.* .
184.It Fl t Ar life
185Set a default value for the maximum lifetime of identities added to the agent.
186The lifetime may be specified in seconds or in a time format specified in
187.Xr sshd_config 5 .
188A lifetime specified for an identity with
189.Xr ssh-add 1
190overrides this value.
191Without this option the default maximum lifetime is forever.
192.It Fl U
193Instructs
194.Nm
195not to clean up stale agent sockets under
196.Pa $HOME/.ssh/agent/ .
197.It Fl u
198Instructs
199.Nm
200to only clean up stale agent sockets under
201.Pa $HOME/.ssh/agent/
202and then exit immediately.
203If this option is given twice,
204.Nm
205will delete stale agent sockets regardless of the host name that created them.
206.It Ar command Op Ar arg ...
207If a command (and optional arguments) is given,
208this is executed as a subprocess of the agent.
209The agent exits automatically when the command given on the command
210line terminates.
211.El
212.Pp
213There are two main ways to get an agent set up.
214The first is at the start of an X session,
215where all other windows or programs are started as children of the
216.Nm
217program.
218The agent starts a command under which its environment
219variables are exported, for example
220.Cm ssh-agent xterm & .
221When the command terminates, so does the agent.
222.Pp
223The second method is used for a login session.
224When
225.Nm
226is started,
227it prints the shell commands required to set its environment variables,
228which in turn can be evaluated in the calling shell, for example
229.Cm eval `ssh-agent -s` .
230.Pp
231In both cases,
232.Xr ssh 1
233looks at these environment variables
234and uses them to establish a connection to the agent.
235.Pp
236The agent initially does not have any private keys.
237Keys are added using
238.Xr ssh-add 1
239or by
240.Xr ssh 1
241when
242.Cm AddKeysToAgent
243is set in
244.Xr ssh_config 5 .
245Multiple identities may be stored in
246.Nm
247concurrently and
248.Xr ssh 1
249will automatically use them if present.
250.Xr ssh-add 1
251is also used to remove keys from
252.Nm
253and to query the keys that are held in one.
254.Pp
255Connections to
256.Nm
257may be forwarded from further remote hosts using the
258.Fl A
259option to
260.Xr ssh 1
261(but see the caveats documented therein),
262avoiding the need for authentication data to be stored on other machines.
263Authentication passphrases and private keys never go over the network:
264the connection to the agent is forwarded over SSH remote connections
265and the result is returned to the requester,
266allowing the user access to their identities anywhere in the network
267in a secure fashion.
268.Pp
269.Nm
270will delete all keys it has loaded upon receiving
271.Dv SIGUSR1 .
272.Sh ENVIRONMENT
273.Bl -tag -width "SSH_AGENT_PID"
274.It Ev SSH_AGENT_PID
275When
276.Nm
277starts, it stores the name of the agent's process ID (PID) in this variable.
278.It Ev SSH_AUTH_SOCK
279When
280.Nm
281starts, it creates a
282.Ux Ns -domain
283socket and stores its pathname in this variable.
284It is accessible only to the current user,
285but is easily abused by root or another instance of the same user.
286.El
287.Sh FILES
288.Bl -tag -width Ds
289.It Pa $HOME/.ssh/agent/s.*
290.Ux Ns -domain
291sockets used to contain the connection to the authentication agent.
292These sockets should only be readable by the owner.
293The sockets should get automatically removed when the agent exits.
294.El
295.Sh SEE ALSO
296.Xr ssh 1 ,
297.Xr ssh-add 1 ,
298.Xr ssh-keygen 1 ,
299.Xr ssh_config 5 ,
300.Xr sshd 8
301.Sh AUTHORS
302.An -nosplit
303OpenSSH is a derivative of the original and free ssh 1.2.12 release by
304.An Tatu Ylonen .
305.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , Theo de Raadt
306and
307.An Dug Song
308removed many bugs, re-added newer features and created OpenSSH.
309.An Markus Friedl
310contributed the support for SSH protocol versions 1.5 and 2.0.