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11f790bbb1
[ssh-agent.1]
clarify/state that private keys are not exposed to clients using the
agent; ok markus@
188 lines
6.4 KiB
Groff
188 lines
6.4 KiB
Groff
.\" $OpenBSD: ssh-agent.1,v 1.29 2001/11/19 18:40:46 stevesk Exp $
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.\"
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.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
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.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
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.\" All rights reserved
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.\"
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.\" As far as I am concerned, the code I have written for this software
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.\" can be used freely for any purpose. Any derived versions of this
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.\" software must be clearly marked as such, and if the derived work is
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.\" incompatible with the protocol description in the RFC file, it must be
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.\" called by a name other than "ssh" or "Secure Shell".
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.\"
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.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
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.\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
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.\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.Dd September 25, 1999
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.Dt SSH-AGENT 1
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.Os
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.Sh NAME
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.Nm ssh-agent
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.Nd authentication agent
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.Sh SYNOPSIS
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.Nm ssh-agent
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.Op Fl c Li | Fl s
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.Op Fl d
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.Op Ar command Op Ar args ...
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.Nm ssh-agent
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.Op Fl c Li | Fl s
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.Fl k
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.Sh DESCRIPTION
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.Nm
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is a program to hold private keys used for public key authentication
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(RSA, DSA).
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The idea is that
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.Nm
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is started in the beginning of an X-session or a login session, and
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all other windows or programs are started as clients to the ssh-agent
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program.
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Through use of environment variables the agent can be located
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and automatically used for authentication when logging in to other
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machines using
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.Xr ssh 1 .
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.Pp
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The options are as follows:
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.Bl -tag -width Ds
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.It Fl c
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Generate C-shell commands on
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.Dv stdout .
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This is the default if
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.Ev SHELL
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looks like it's a csh style of shell.
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.It Fl s
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Generate Bourne shell commands on
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.Dv stdout .
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This is the default if
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.Ev SHELL
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does not look like it's a csh style of shell.
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.It Fl k
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Kill the current agent (given by the
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.Ev SSH_AGENT_PID
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environment variable).
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.It Fl d
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Debug mode. When this option is specified
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.Nm
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will not fork.
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.El
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.Pp
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If a commandline is given, this is executed as a subprocess of the agent.
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When the command dies, so does the agent.
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.Pp
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The agent initially does not have any private keys.
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Keys are added using
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.Xr ssh-add 1 .
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When executed without arguments,
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.Xr ssh-add 1
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adds the
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.Pa $HOME/.ssh/identity
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file.
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If the identity has a passphrase,
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.Xr ssh-add 1
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asks for the passphrase (using a small X11 application if running
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under X11, or from the terminal if running without X).
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It then sends the identity to the agent.
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Several identities can be stored in the
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agent; the agent can automatically use any of these identities.
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.Ic ssh-add -l
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displays the identities currently held by the agent.
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.Pp
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The idea is that the agent is run in the user's local PC, laptop, or
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terminal.
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Authentication data need not be stored on any other
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machine, and authentication passphrases never go over the network.
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However, the connection to the agent is forwarded over SSH
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remote logins, and the user can thus use the privileges given by the
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identities anywhere in the network in a secure way.
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.Pp
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There are two main ways to get an agent setup:
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Either the agent starts a new subcommand into which some environment
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variables are exported, or the agent prints the needed shell commands
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(either
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.Xr sh 1
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or
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.Xr csh 1
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syntax can be generated) which can be evalled in the calling shell.
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Later
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.Xr ssh 1
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looks at these variables and uses them to establish a connection to the agent.
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.Pp
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The agent will never send a private key over its request channel.
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Instead, operations that require a private key will be performed
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by the agent, and the result will be returned to the requester.
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This way, private keys are not exposed to clients using the agent.
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.Pp
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A unix-domain socket is created
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.Pq Pa /tmp/ssh-XXXXXXXX/agent.<pid> ,
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and the name of this socket is stored in the
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.Ev SSH_AUTH_SOCK
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environment
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variable.
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The socket is made accessible only to the current user.
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This method is easily abused by root or another instance of the same
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user.
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.Pp
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The
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.Ev SSH_AGENT_PID
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environment variable holds the agent's PID.
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.Pp
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The agent exits automatically when the command given on the command
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line terminates.
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.Sh FILES
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.Bl -tag -width Ds
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.It Pa $HOME/.ssh/identity
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Contains the protocol version 1 RSA authentication identity of the user.
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This file should not be readable by anyone but the user.
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It is possible to
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specify a passphrase when generating the key; that passphrase will be
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used to encrypt the private part of this file.
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This file is not used by
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.Nm
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but is normally added to the agent using
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.Xr ssh-add 1
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at login time.
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.It Pa $HOME/.ssh/id_dsa
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Contains the protocol version 2 DSA authentication identity of the user.
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.It Pa $HOME/.ssh/id_rsa
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Contains the protocol version 2 RSA authentication identity of the user.
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.It Pa /tmp/ssh-XXXXXXXX/agent.<pid>
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Unix-domain sockets used to contain the connection to the
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authentication agent.
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These sockets should only be readable by the owner.
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The sockets should get automatically removed when the agent exits.
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.El
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.Sh AUTHORS
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OpenSSH is a derivative of the original and free
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ssh 1.2.12 release by Tatu Ylonen.
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Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
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Theo de Raadt and Dug Song
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removed many bugs, re-added newer features and
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created OpenSSH.
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Markus Friedl contributed the support for SSH
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protocol versions 1.5 and 2.0.
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.Sh SEE ALSO
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.Xr ssh 1 ,
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.Xr ssh-add 1 ,
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.Xr ssh-keygen 1 ,
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.Xr sshd 8
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