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1 .\" $OpenBSD: ssh-agent.1,v 1.50 2010/01/17 21:49:09 tedu 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: January 17 2010 $
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 Li | Fl s
46 .Op Fl d
47 .Op Fl a Ar bind_address
48 .Op Fl t Ar life
49 .Op Ar command Op Ar arg ...
50 .Nm ssh-agent
51 .Op Fl c Li | Fl s
52 .Fl k
53 .Sh DESCRIPTION
54 .Nm
55 is a program to hold private keys used for public key authentication
56 (RSA, DSA).
57 The idea is that
58 .Nm
59 is 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 options are as follows:
68 .Bl -tag -width Ds
69 .It Fl a Ar bind_address
70 Bind the agent to the
71 .Ux Ns -domain
72 socket
73 .Ar bind_address .
74 The default is
75 .Pa /tmp/ssh-XXXXXXXXXX/agent.\*(Ltppid\*(Gt .
76 .It Fl c
77 Generate C-shell commands on
78 .Dv stdout .
79 This is the default if
80 .Ev SHELL
81 looks like it's a csh style of shell.
82 .It Fl d
83 Debug mode.
84 When this option is specified
85 .Nm
86 will not fork.
87 .It Fl k
88 Kill the current agent (given by the
89 .Ev SSH_AGENT_PID
90 environment variable).
91 .It Fl s
92 Generate Bourne shell commands on
93 .Dv stdout .
94 This is the default if
95 .Ev SHELL
96 does not look like it's a csh style of shell.
97 .It Fl t Ar life
98 Set a default value for the maximum lifetime of identities added to the agent.
99 The lifetime may be specified in seconds or in a time format specified in
100 .Xr sshd_config 5 .
101 A lifetime specified for an identity with
102 .Xr ssh-add 1
103 overrides this value.
104 Without this option the default maximum lifetime is forever.
105 .El
106 .Pp
107 If a commandline is given, this is executed as a subprocess of the agent.
108 When the command dies, so does the agent.
109 .Pp
110 The agent initially does not have any private keys.
111 Keys are added using
112 .Xr ssh-add 1 .
113 When executed without arguments,
114 .Xr ssh-add 1
115 adds the files
116 .Pa ~/.ssh/id_rsa ,
117 .Pa ~/.ssh/id_dsa
118 and
119 .Pa ~/.ssh/identity .
120 If the identity has a passphrase,
121 .Xr ssh-add 1
122 asks for the passphrase on the terminal if it has one or from a small X11
123 program if running under X11.
124 If neither of these is the case then the authentication will fail.
125 It then sends the identity to the agent.
126 Several identities can be stored in the
127 agent; the agent can automatically use any of these identities.
128 .Ic ssh-add -l
129 displays the identities currently held by the agent.
130 .Pp
131 The idea is that the agent is run in the user's local PC, laptop, or
132 terminal.
133 Authentication data need not be stored on any other
134 machine, and authentication passphrases never go over the network.
135 However, the connection to the agent is forwarded over SSH
136 remote logins, and the user can thus use the privileges given by the
137 identities anywhere in the network in a secure way.
138 .Pp
139 There are two main ways to get an agent set up:
140 The first is that the agent starts a new subcommand into which some environment
141 variables are exported, eg
142 .Cm ssh-agent xterm & .
143 The second is that the agent prints the needed shell commands (either
144 .Xr sh 1
145 or
146 .Xr csh 1
147 syntax can be generated) which can be evaluated in the calling shell, eg
148 .Cm eval `ssh-agent -s`
149 for Bourne-type shells such as
150 .Xr sh 1
151 or
152 .Xr ksh 1
153 and
154 .Cm eval `ssh-agent -c`
155 for
156 .Xr csh 1
157 and derivatives.
158 .Pp
159 Later
160 .Xr ssh 1
161 looks at these variables and uses them to establish a connection to the agent.
162 .Pp
163 The agent will never send a private key over its request channel.
164 Instead, operations that require a private key will be performed
165 by the agent, and the result will be returned to the requester.
166 This way, private keys are not exposed to clients using the agent.
167 .Pp
168 A
169 .Ux Ns -domain
170 socket is created and the name of this socket is stored in the
171 .Ev SSH_AUTH_SOCK
172 environment
173 variable.
174 The socket is made accessible only to the current user.
175 This method is easily abused by root or another instance of the same
176 user.
177 .Pp
178 The
179 .Ev SSH_AGENT_PID
180 environment variable holds the agent's process ID.
181 .Pp
182 The agent exits automatically when the command given on the command
183 line terminates.
184 .Sh FILES
185 .Bl -tag -width Ds
186 .It Pa ~/.ssh/identity
187 Contains the protocol version 1 RSA authentication identity of the user.
188 .It Pa ~/.ssh/id_dsa
189 Contains the protocol version 2 DSA authentication identity of the user.
190 .It Pa ~/.ssh/id_rsa
191 Contains the protocol version 2 RSA authentication identity of the user.
192 .It Pa /tmp/ssh-XXXXXXXXXX/agent.\*(Ltppid\*(Gt
193 .Ux Ns -domain
194 sockets used to contain the connection to the authentication agent.
195 These sockets should only be readable by the owner.
196 The sockets should get automatically removed when the agent exits.
197 .El
198 .Sh SEE ALSO
199 .Xr ssh 1 ,
200 .Xr ssh-add 1 ,
201 .Xr ssh-keygen 1 ,
202 .Xr sshd 8
203 .Sh AUTHORS
204 OpenSSH is a derivative of the original and free
205 ssh 1.2.12 release by Tatu Ylonen.
206 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
207 Theo de Raadt and Dug Song
208 removed many bugs, re-added newer features and
209 created OpenSSH.
210 Markus Friedl contributed the support for SSH
211 protocol versions 1.5 and 2.0.