xref: /openssh-portable/ssh-agent.1 (revision fc270baf)
1.\" $OpenBSD: ssh-agent.1,v 1.72 2020/06/22 05:52:05 djm 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
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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.
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37.Dd $Mdocdate: June 22 2020 $
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
62is a program to hold private keys used for public key authentication.
63Through use of environment variables the agent can be located
64and automatically used for authentication when logging in to other
65machines using
66.Xr ssh 1 .
67.Pp
68The options are as follows:
69.Bl -tag -width Ds
70.It Fl a Ar bind_address
71Bind the agent to the
72.Ux Ns -domain
73socket
74.Ar bind_address .
75The default is
76.Pa $TMPDIR/ssh-XXXXXXXXXX/agent.\*(Ltppid\*(Gt .
77.It Fl c
78Generate C-shell commands on
79.Dv stdout .
80This is the default if
81.Ev SHELL
82looks like it's a csh style of shell.
83.It Fl D
84Foreground mode.
85When this option is specified
86.Nm
87will not fork.
88.It Fl d
89Debug mode.
90When this option is specified
91.Nm
92will not fork and will write debug information to standard error.
93.It Fl E Ar fingerprint_hash
94Specifies the hash algorithm used when displaying key fingerprints.
95Valid options are:
96.Dq md5
97and
98.Dq sha256 .
99The default is
100.Dq sha256 .
101.It Fl k
102Kill the current agent (given by the
103.Ev SSH_AGENT_PID
104environment variable).
105.It Fl P Ar allowed_providers
106Specify a pattern-list of acceptable paths for PKCS#11 provider and FIDO
107authenticator middleware shared libraries that may be used with the
108.Fl S
109or
110.Fl s
111options to
112.Xr ssh-add 1 .
113Libraries that do not match the pattern list will be refused.
114See PATTERNS in
115.Xr ssh_config 5
116for a description of pattern-list syntax.
117The default list is
118.Dq /usr/lib/*,/usr/local/lib/* .
119.It Fl s
120Generate Bourne shell commands on
121.Dv stdout .
122This is the default if
123.Ev SHELL
124does not look like it's a csh style of shell.
125.It Fl t Ar life
126Set a default value for the maximum lifetime of identities added to the agent.
127The lifetime may be specified in seconds or in a time format specified in
128.Xr sshd_config 5 .
129A lifetime specified for an identity with
130.Xr ssh-add 1
131overrides this value.
132Without this option the default maximum lifetime is forever.
133.It Ar command Op Ar arg ...
134If a command (and optional arguments) is given,
135this is executed as a subprocess of the agent.
136The agent exits automatically when the command given on the command
137line terminates.
138.El
139.Pp
140There are two main ways to get an agent set up.
141The first is at the start of an X session,
142where all other windows or programs are started as children of the
143.Nm
144program.
145The agent starts a command under which its environment
146variables are exported, for example
147.Cm ssh-agent xterm & .
148When the command terminates, so does the agent.
149.Pp
150The second method is used for a login session.
151When
152.Nm
153is started,
154it prints the shell commands required to set its environment variables,
155which in turn can be evaluated in the calling shell, for example
156.Cm eval `ssh-agent -s` .
157.Pp
158In both cases,
159.Xr ssh 1
160looks at these environment variables and uses them to establish a connection to the agent.
161.Pp
162The agent initially does not have any private keys.
163Keys are added using
164.Xr ssh-add 1
165or by
166.Xr ssh 1
167when
168.Cm AddKeysToAgent
169is set in
170.Xr ssh_config 5 .
171Multiple identities may be stored in
172.Nm
173concurrently and
174.Xr ssh 1
175will automatically use them if present.
176.Xr ssh-add 1
177is also used to remove keys from
178.Nm
179and to query the keys that are held in one.
180.Pp
181Connections to
182.Nm
183may be forwarded from further remote hosts using the
184.Fl A
185option to
186.Xr ssh 1
187(but see the caveats documented therein),
188avoiding the need for authentication data to be stored on other machines.
189Authentication passphrases and private keys never go over the network:
190the connection to the agent is forwarded over SSH remote connections
191and the result is returned to the requester,
192allowing the user access to their identities anywhere in the network
193in a secure fashion.
194.Sh ENVIRONMENT
195.Bl -tag -width "SSH_AGENT_PID"
196.It Ev SSH_AGENT_PID
197When
198.Nm
199starts, it stores the name of the agent's process ID (PID) in this variable.
200.It Ev SSH_AUTH_SOCK
201When
202.Nm
203starts, it creates a
204.Ux Ns -domain
205socket and stores its pathname in this variable.
206It is accessible only to the current user,
207but is easily abused by root or another instance of the same user.
208.El
209.Sh FILES
210.Bl -tag -width Ds
211.It Pa $TMPDIR/ssh-XXXXXXXXXX/agent.<ppid>
212.Ux Ns -domain
213sockets used to contain the connection to the authentication agent.
214These sockets should only be readable by the owner.
215The sockets should get automatically removed when the agent exits.
216.El
217.Sh SEE ALSO
218.Xr ssh 1 ,
219.Xr ssh-add 1 ,
220.Xr ssh-keygen 1 ,
221.Xr ssh_config 5 ,
222.Xr sshd 8
223.Sh AUTHORS
224.An -nosplit
225OpenSSH is a derivative of the original and free ssh 1.2.12 release by
226.An Tatu Ylonen .
227.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , Theo de Raadt
228and
229.An Dug Song
230removed many bugs, re-added newer features and created OpenSSH.
231.An Markus Friedl
232contributed the support for SSH protocol versions 1.5 and 2.0.
233