xref: /freebsd/crypto/openssh/ssh-agent.1 (revision cfd6422a5217410fbd66f7a7a8a64d9d85e61229)
1.\" $OpenBSD: ssh-agent.1,v 1.64 2016/11/30 06:54:26 jmc Exp $
2.\" $FreeBSD$
3.\"
4.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
5.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
6.\"                    All rights reserved
7.\"
8.\" As far as I am concerned, the code I have written for this software
9.\" can be used freely for any purpose.  Any derived versions of this
10.\" software must be clearly marked as such, and if the derived work is
11.\" incompatible with the protocol description in the RFC file, it must be
12.\" called by a name other than "ssh" or "Secure Shell".
13.\"
14.\" Copyright (c) 1999,2000 Markus Friedl.  All rights reserved.
15.\" Copyright (c) 1999 Aaron Campbell.  All rights reserved.
16.\" Copyright (c) 1999 Theo de Raadt.  All rights reserved.
17.\"
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19.\" modification, are permitted provided that the following conditions
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38.Dd $Mdocdate: November 30 2016 $
39.Dt SSH-AGENT 1
40.Os
41.Sh NAME
42.Nm ssh-agent
43.Nd authentication agent
44.Sh SYNOPSIS
45.Nm ssh-agent
46.Op Fl c | s
47.Op Fl \&Ddx
48.Op Fl a Ar bind_address
49.Op Fl E Ar fingerprint_hash
50.Op Fl P Ar pkcs11_whitelist
51.Op Fl t Ar life
52.Op Ar command Op Ar arg ...
53.Nm ssh-agent
54.Op Fl c | s
55.Fl k
56.Sh DESCRIPTION
57.Nm
58is a program to hold private keys used for public key authentication
59(RSA, DSA, ECDSA, Ed25519).
60.Nm
61is usually started in the beginning of an X-session or a login session, and
62all other windows or programs are started as clients to the ssh-agent
63program.
64Through use of environment variables the agent can be located
65and automatically used for authentication when logging in to other
66machines using
67.Xr ssh 1 .
68.Pp
69The agent initially does not have any private keys.
70Keys are added using
71.Xr ssh 1
72(see
73.Cm AddKeysToAgent
74in
75.Xr ssh_config 5
76for details)
77or
78.Xr ssh-add 1 .
79Multiple identities may be stored in
80.Nm
81concurrently and
82.Xr ssh 1
83will automatically use them if present.
84.Xr ssh-add 1
85is also used to remove keys from
86.Nm
87and to query the keys that are held in one.
88.Pp
89The options are as follows:
90.Bl -tag -width Ds
91.It Fl a Ar bind_address
92Bind the agent to the
93.Ux Ns -domain
94socket
95.Ar bind_address .
96The default is
97.Pa $TMPDIR/ssh-XXXXXXXXXX/agent.\*(Ltppid\*(Gt .
98.It Fl c
99Generate C-shell commands on
100.Dv stdout .
101This is the default if
102.Ev SHELL
103looks like it's a csh style of shell.
104.It Fl D
105Foreground mode.
106When this option is specified
107.Nm
108will not fork.
109.It Fl d
110Debug mode.
111When this option is specified
112.Nm
113will not fork and will write debug information to standard error.
114.It Fl E Ar fingerprint_hash
115Specifies the hash algorithm used when displaying key fingerprints.
116Valid options are:
117.Dq md5
118and
119.Dq sha256 .
120The default is
121.Dq sha256 .
122.It Fl k
123Kill the current agent (given by the
124.Ev SSH_AGENT_PID
125environment variable).
126.It Fl P Ar pkcs11_whitelist
127Specify a pattern-list of acceptable paths for PKCS#11 shared libraries
128that may be added using the
129.Fl s
130option to
131.Xr ssh-add 1 .
132The default is to allow loading PKCS#11 libraries from
133.Dq /usr/lib/*,/usr/local/lib/* .
134PKCS#11 libraries that do not match the whitelist will be refused.
135See PATTERNS in
136.Xr ssh_config 5
137for a description of pattern-list syntax.
138.It Fl s
139Generate Bourne shell commands on
140.Dv stdout .
141This is the default if
142.Ev SHELL
143does not look like it's a csh style of shell.
144.It Fl t Ar life
145Set a default value for the maximum lifetime of identities added to the agent.
146The lifetime may be specified in seconds or in a time format specified in
147.Xr sshd_config 5 .
148A lifetime specified for an identity with
149.Xr ssh-add 1
150overrides this value.
151Without this option the default maximum lifetime is forever.
152.It Fl x
153Exit after the last client has disconnected.
154.El
155.Pp
156If a command line is given, this is executed as a subprocess of the agent.
157When the command dies, so does the agent.
158.Pp
159The idea is that the agent is run in the user's local PC, laptop, or
160terminal.
161Authentication data need not be stored on any other
162machine, and authentication passphrases never go over the network.
163However, the connection to the agent is forwarded over SSH
164remote logins, and the user can thus use the privileges given by the
165identities anywhere in the network in a secure way.
166.Pp
167There are two main ways to get an agent set up:
168The first is that the agent starts a new subcommand into which some environment
169variables are exported, eg
170.Cm ssh-agent xterm & .
171The second is that the agent prints the needed shell commands (either
172.Xr sh 1
173or
174.Xr csh 1
175syntax can be generated) which can be evaluated in the calling shell, eg
176.Cm eval `ssh-agent -s`
177for Bourne-type shells such as
178.Xr sh 1
179or
180.Xr ksh 1
181and
182.Cm eval `ssh-agent -c`
183for
184.Xr csh 1
185and derivatives.
186.Pp
187Later
188.Xr ssh 1
189looks at these variables and uses them to establish a connection to the agent.
190.Pp
191The agent will never send a private key over its request channel.
192Instead, operations that require a private key will be performed
193by the agent, and the result will be returned to the requester.
194This way, private keys are not exposed to clients using the agent.
195.Pp
196A
197.Ux Ns -domain
198socket is created and the name of this socket is stored in the
199.Ev SSH_AUTH_SOCK
200environment
201variable.
202The socket is made accessible only to the current user.
203This method is easily abused by root or another instance of the same
204user.
205.Pp
206The
207.Ev SSH_AGENT_PID
208environment variable holds the agent's process ID.
209.Pp
210The agent exits automatically when the command given on the command
211line terminates.
212.Sh FILES
213.Bl -tag -width Ds
214.It Pa $TMPDIR/ssh-XXXXXXXXXX/agent.<ppid>
215.Ux Ns -domain
216sockets used to contain the connection to the authentication agent.
217These sockets should only be readable by the owner.
218The sockets should get automatically removed when the agent exits.
219.El
220.Sh SEE ALSO
221.Xr ssh 1 ,
222.Xr ssh-add 1 ,
223.Xr ssh-keygen 1 ,
224.Xr sshd 8
225.Sh AUTHORS
226.An -nosplit
227OpenSSH is a derivative of the original and free ssh 1.2.12 release by
228.An Tatu Ylonen .
229.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , Theo de Raadt
230and
231.An Dug Song
232removed many bugs, re-added newer features and created OpenSSH.
233.An Markus Friedl
234contributed the support for SSH protocol versions 1.5 and 2.0.
235