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1 .\" $OpenBSD: ssh-agent.1,v 1.62 2015/11/15 23:54:15 jmc 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: November 15 2015 $
38 .Dt SSH-AGENT 1
39 .Os
40 .Sh NAME
41 .Nm ssh-agent
42 .Nd 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 t Ar life
50 .Op Ar command Op Ar arg ...
51 .Nm ssh-agent
52 .Op Fl c | s
53 .Fl k
54 .Sh DESCRIPTION
55 .Nm
56 is a program to hold private keys used for public key authentication
57 (RSA, DSA, ECDSA, Ed25519).
58 .Nm
59 is usually started in the beginning of an X-session or a login session, and
60 all other windows or programs are started as clients to the ssh-agent
61 program.
62 Through use of environment variables the agent can be located
63 and automatically used for authentication when logging in to other
64 machines using
65 .Xr ssh 1 .
66 .Pp
67 The agent initially does not have any private keys.
68 Keys are added using
69 .Xr ssh 1
70 (see
71 .Cm AddKeysToAgent
72 in
73 .Xr ssh_config 5
74 for details)
75 or
76 .Xr ssh-add 1 .
77 Multiple identities may be stored in
78 .Nm
79 concurrently and
80 .Xr ssh 1
81 will automatically use them if present.
82 .Xr ssh-add 1
83 is also used to remove keys from
84 .Nm
85 and to query the keys that are held in one.
86 .Pp
87 The options are as follows:
88 .Bl -tag -width Ds
89 .It Fl a Ar bind_address
90 Bind the agent to the
91 .Ux Ns -domain
92 socket
93 .Ar bind_address .
94 The default is
95 .Pa $TMPDIR/ssh-XXXXXXXXXX/agent.\*(Ltppid\*(Gt .
96 .It Fl c
97 Generate C-shell commands on
98 .Dv stdout .
99 This is the default if
100 .Ev SHELL
101 looks like it's a csh style of shell.
102 .It Fl D
103 Foreground mode.
104 When this option is specified
105 .Nm
106 will not fork.
107 .It Fl d
108 Debug mode.
109 When this option is specified
110 .Nm
111 will not fork and will write debug information to standard error.
112 .It Fl E Ar fingerprint_hash
113 Specifies the hash algorithm used when displaying key fingerprints.
114 Valid options are:
115 .Dq md5
116 and
117 .Dq sha256 .
118 The default is
119 .Dq sha256 .
120 .It Fl k
121 Kill the current agent (given by the
122 .Ev SSH_AGENT_PID
123 environment variable).
124 .It Fl s
125 Generate Bourne shell commands on
126 .Dv stdout .
127 This is the default if
128 .Ev SHELL
129 does not look like it's a csh style of shell.
130 .It Fl t Ar life
131 Set a default value for the maximum lifetime of identities added to the agent.
132 The lifetime may be specified in seconds or in a time format specified in
133 .Xr sshd_config 5 .
134 A lifetime specified for an identity with
135 .Xr ssh-add 1
136 overrides this value.
137 Without this option the default maximum lifetime is forever.
138 .El
139 .Pp
140 If a command line is given, this is executed as a subprocess of the agent.
141 When the command dies, so does the agent.
142 .Pp
143 The idea is that the agent is run in the user's local PC, laptop, or
144 terminal.
145 Authentication data need not be stored on any other
146 machine, and authentication passphrases never go over the network.
147 However, the connection to the agent is forwarded over SSH
148 remote logins, and the user can thus use the privileges given by the
149 identities anywhere in the network in a secure way.
150 .Pp
151 There are two main ways to get an agent set up:
152 The first is that the agent starts a new subcommand into which some environment
153 variables are exported, eg
154 .Cm ssh-agent xterm & .
155 The second is that the agent prints the needed shell commands (either
156 .Xr sh 1
157 or
158 .Xr csh 1
159 syntax can be generated) which can be evaluated in the calling shell, eg
160 .Cm eval `ssh-agent -s`
161 for Bourne-type shells such as
162 .Xr sh 1
163 or
164 .Xr ksh 1
165 and
166 .Cm eval `ssh-agent -c`
167 for
168 .Xr csh 1
169 and derivatives.
170 .Pp
171 Later
172 .Xr ssh 1
173 looks at these variables and uses them to establish a connection to the agent.
174 .Pp
175 The agent will never send a private key over its request channel.
176 Instead, operations that require a private key will be performed
177 by the agent, and the result will be returned to the requester.
178 This way, private keys are not exposed to clients using the agent.
179 .Pp
180 A
181 .Ux Ns -domain
182 socket is created and the name of this socket is stored in the
183 .Ev SSH_AUTH_SOCK
184 environment
185 variable.
186 The socket is made accessible only to the current user.
187 This method is easily abused by root or another instance of the same
188 user.
189 .Pp
190 The
191 .Ev SSH_AGENT_PID
192 environment variable holds the agent's process ID.
193 .Pp
194 The agent exits automatically when the command given on the command
195 line terminates.
196 .Sh FILES
197 .Bl -tag -width Ds
198 .It Pa $TMPDIR/ssh-XXXXXXXXXX/agent.\*(Ltppid\*(Gt
199 .Ux Ns -domain
200 sockets used to contain the connection to the authentication agent.
201 These sockets should only be readable by the owner.
202 The sockets should get automatically removed when the agent exits.
203 .El
204 .Sh SEE ALSO
205 .Xr ssh 1 ,
206 .Xr ssh-add 1 ,
207 .Xr ssh-keygen 1 ,
208 .Xr sshd 8
209 .Sh AUTHORS
210 OpenSSH is a derivative of the original and free
211 ssh 1.2.12 release by Tatu Ylonen.
212 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
213 Theo de Raadt and Dug Song
214 removed many bugs, re-added newer features and
215 created OpenSSH.
216 Markus Friedl contributed the support for SSH
217 protocol versions 1.5 and 2.0.