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1 .\" $OpenBSD: ssh-agent.1,v 1.73 2022/03/31 17:27:27 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: March 31 2022 $
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 \&Dd
47 .Op Fl a Ar bind_address
48 .Op Fl E Ar fingerprint_hash
49 .Op Fl P Ar allowed_providers
50 .Op Fl t Ar life
51 .Nm ssh-agent
52 .Op Fl a Ar bind_address
53 .Op Fl E Ar fingerprint_hash
54 .Op Fl P Ar allowed_providers
55 .Op Fl t Ar life
56 .Ar command Op Ar arg ...
57 .Nm ssh-agent
58 .Op Fl c | s
59 .Fl k
60 .Sh DESCRIPTION
61 .Nm
62 is a program to hold private keys used for public key authentication.
63 Through use of environment variables the agent can be located
64 and automatically used for authentication when logging in to other
65 machines using
66 .Xr ssh 1 .
67 .Pp
68 The options are as follows:
69 .Bl -tag -width Ds
70 .It Fl a Ar bind_address
71 Bind the agent to the
72 .Ux Ns -domain
73 socket
74 .Ar bind_address .
75 The default is
76 .Pa $TMPDIR/ssh-XXXXXXXXXX/agent.\*(Ltppid\*(Gt .
77 .It Fl c
78 Generate C-shell commands on
79 .Dv stdout .
80 This is the default if
81 .Ev SHELL
82 looks like it's a csh style of shell.
83 .It Fl D
84 Foreground mode.
85 When this option is specified,
86 .Nm
87 will not fork.
88 .It Fl d
89 Debug mode.
90 When this option is specified,
91 .Nm
92 will not fork and will write debug information to standard error.
93 .It Fl E Ar fingerprint_hash
94 Specifies the hash algorithm used when displaying key fingerprints.
95 Valid options are:
96 .Dq md5
97 and
98 .Dq sha256 .
99 The default is
100 .Dq sha256 .
101 .It Fl k
102 Kill the current agent (given by the
103 .Ev SSH_AGENT_PID
104 environment variable).
105 .It Fl P Ar allowed_providers
106 Specify a pattern-list of acceptable paths for PKCS#11 provider and FIDO
107 authenticator middleware shared libraries that may be used with the
108 .Fl S
109 or
110 .Fl s
111 options to
112 .Xr ssh-add 1 .
113 Libraries that do not match the pattern list will be refused.
114 See PATTERNS in
115 .Xr ssh_config 5
116 for a description of pattern-list syntax.
117 The default list is
118 .Dq /usr/lib/*,/usr/local/lib/* .
119 .It Fl s
120 Generate Bourne shell commands on
121 .Dv stdout .
122 This is the default if
123 .Ev SHELL
124 does not look like it's a csh style of shell.
125 .It Fl t Ar life
126 Set a default value for the maximum lifetime of identities added to the agent.
127 The lifetime may be specified in seconds or in a time format specified in
128 .Xr sshd_config 5 .
129 A lifetime specified for an identity with
130 .Xr ssh-add 1
131 overrides this value.
132 Without this option the default maximum lifetime is forever.
133 .It Ar command Op Ar arg ...
134 If a command (and optional arguments) is given,
135 this is executed as a subprocess of the agent.
136 The agent exits automatically when the command given on the command
137 line terminates.
138 .El
139 .Pp
140 There are two main ways to get an agent set up.
141 The first is at the start of an X session,
142 where all other windows or programs are started as children of the
143 .Nm
144 program.
145 The agent starts a command under which its environment
146 variables are exported, for example
147 .Cm ssh-agent xterm & .
148 When the command terminates, so does the agent.
149 .Pp
150 The second method is used for a login session.
151 When
152 .Nm
153 is started,
154 it prints the shell commands required to set its environment variables,
155 which in turn can be evaluated in the calling shell, for example
156 .Cm eval `ssh-agent -s` .
157 .Pp
158 In both cases,
159 .Xr ssh 1
160 looks at these environment variables
161 and uses them to establish a connection to the agent.
162 .Pp
163 The agent initially does not have any private keys.
164 Keys are added using
165 .Xr ssh-add 1
166 or by
167 .Xr ssh 1
168 when
169 .Cm AddKeysToAgent
170 is set in
171 .Xr ssh_config 5 .
172 Multiple identities may be stored in
173 .Nm
174 concurrently and
175 .Xr ssh 1
176 will automatically use them if present.
177 .Xr ssh-add 1
178 is also used to remove keys from
179 .Nm
180 and to query the keys that are held in one.
181 .Pp
182 Connections to
183 .Nm
184 may be forwarded from further remote hosts using the
185 .Fl A
186 option to
187 .Xr ssh 1
188 (but see the caveats documented therein),
189 avoiding the need for authentication data to be stored on other machines.
190 Authentication passphrases and private keys never go over the network:
191 the connection to the agent is forwarded over SSH remote connections
192 and the result is returned to the requester,
193 allowing the user access to their identities anywhere in the network
194 in a secure fashion.
195 .Sh ENVIRONMENT
196 .Bl -tag -width "SSH_AGENT_PID"
197 .It Ev SSH_AGENT_PID
198 When
199 .Nm
200 starts, it stores the name of the agent's process ID (PID) in this variable.
201 .It Ev SSH_AUTH_SOCK
202 When
203 .Nm
204 starts, it creates a
205 .Ux Ns -domain
206 socket and stores its pathname in this variable.
207 It is accessible only to the current user,
208 but is easily abused by root or another instance of the same user.
209 .El
210 .Sh FILES
211 .Bl -tag -width Ds
212 .It Pa $TMPDIR/ssh-XXXXXXXXXX/agent.<ppid>
213 .Ux Ns -domain
214 sockets used to contain the connection to the authentication agent.
215 These sockets should only be readable by the owner.
216 The sockets should get automatically removed when the agent exits.
217 .El
218 .Sh SEE ALSO
219 .Xr ssh 1 ,
220 .Xr ssh-add 1 ,
221 .Xr ssh-keygen 1 ,
222 .Xr ssh_config 5 ,
223 .Xr sshd 8
224 .Sh AUTHORS
225 .An -nosplit
226 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
227 .An Tatu Ylonen .
228 .An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , Theo de Raadt
229 and
230 .An Dug Song
231 removed many bugs, re-added newer features and created OpenSSH.
232 .An Markus Friedl
233 contributed the support for SSH protocol versions 1.5 and 2.0.