xref: /freebsd/crypto/openssh/ssh.1 (revision 3332f1b444d4a73238e9f59cca27bfc95fe936bd)
1.\"
2.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4.\"                    All rights reserved
5.\"
6.\" As far as I am concerned, the code I have written for this software
7.\" can be used freely for any purpose.  Any derived versions of this
8.\" software must be clearly marked as such, and if the derived work is
9.\" incompatible with the protocol description in the RFC file, it must be
10.\" called by a name other than "ssh" or "Secure Shell".
11.\"
12.\" Copyright (c) 1999,2000 Markus Friedl.  All rights reserved.
13.\" Copyright (c) 1999 Aaron Campbell.  All rights reserved.
14.\" Copyright (c) 1999 Theo de Raadt.  All rights reserved.
15.\"
16.\" Redistribution and use in source and binary forms, with or without
17.\" modification, are permitted provided that the following conditions
18.\" are met:
19.\" 1. Redistributions of source code must retain the above copyright
20.\"    notice, this list of conditions and the following disclaimer.
21.\" 2. Redistributions in binary form must reproduce the above copyright
22.\"    notice, this list of conditions and the following disclaimer in the
23.\"    documentation and/or other materials provided with the distribution.
24.\"
25.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
35.\"
36.\" $OpenBSD: ssh.1,v 1.425 2021/07/28 05:57:42 jmc Exp $
37.\" $FreeBSD$
38.Dd $Mdocdate: July 28 2021 $
39.Dt SSH 1
40.Os
41.Sh NAME
42.Nm ssh
43.Nd OpenSSH remote login client
44.Sh SYNOPSIS
45.Nm ssh
46.Op Fl 46AaCfGgKkMNnqsTtVvXxYy
47.Op Fl B Ar bind_interface
48.Op Fl b Ar bind_address
49.Op Fl c Ar cipher_spec
50.Op Fl D Oo Ar bind_address : Oc Ns Ar port
51.Op Fl E Ar log_file
52.Op Fl e Ar escape_char
53.Op Fl F Ar configfile
54.Op Fl I Ar pkcs11
55.Op Fl i Ar identity_file
56.Op Fl J Ar destination
57.Op Fl L Ar address
58.Op Fl l Ar login_name
59.Op Fl m Ar mac_spec
60.Op Fl O Ar ctl_cmd
61.Op Fl o Ar option
62.Op Fl p Ar port
63.Op Fl Q Ar query_option
64.Op Fl R Ar address
65.Op Fl S Ar ctl_path
66.Op Fl W Ar host : Ns Ar port
67.Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
68.Ar destination
69.Op Ar command
70.Sh DESCRIPTION
71.Nm
72(SSH client) is a program for logging into a remote machine and for
73executing commands on a remote machine.
74It is intended to provide secure encrypted communications between
75two untrusted hosts over an insecure network.
76X11 connections, arbitrary TCP ports and
77.Ux Ns -domain
78sockets can also be forwarded over the secure channel.
79.Pp
80.Nm
81connects and logs into the specified
82.Ar destination ,
83which may be specified as either
84.Sm off
85.Oo user @ Oc hostname
86.Sm on
87or a URI of the form
88.Sm off
89.No ssh:// Oo user @ Oc hostname Op : port .
90.Sm on
91The user must prove
92their identity to the remote machine using one of several methods
93(see below).
94.Pp
95If a
96.Ar command
97is specified,
98it is executed on the remote host instead of a login shell.
99.Pp
100The options are as follows:
101.Pp
102.Bl -tag -width Ds -compact
103.It Fl 4
104Forces
105.Nm
106to use IPv4 addresses only.
107.Pp
108.It Fl 6
109Forces
110.Nm
111to use IPv6 addresses only.
112.Pp
113.It Fl A
114Enables forwarding of connections from an authentication agent such as
115.Xr ssh-agent 1 .
116This can also be specified on a per-host basis in a configuration file.
117.Pp
118Agent forwarding should be enabled with caution.
119Users with the ability to bypass file permissions on the remote host
120(for the agent's
121.Ux Ns -domain
122socket) can access the local agent through the forwarded connection.
123An attacker cannot obtain key material from the agent,
124however they can perform operations on the keys that enable them to
125authenticate using the identities loaded into the agent.
126A safer alternative may be to use a jump host
127(see
128.Fl J ) .
129.Pp
130.It Fl a
131Disables forwarding of the authentication agent connection.
132.Pp
133.It Fl B Ar bind_interface
134Bind to the address of
135.Ar bind_interface
136before attempting to connect to the destination host.
137This is only useful on systems with more than one address.
138.Pp
139.It Fl b Ar bind_address
140Use
141.Ar bind_address
142on the local machine as the source address
143of the connection.
144Only useful on systems with more than one address.
145.Pp
146.It Fl C
147Requests compression of all data (including stdin, stdout, stderr, and
148data for forwarded X11, TCP and
149.Ux Ns -domain
150connections).
151The compression algorithm is the same used by
152.Xr gzip 1 .
153Compression is desirable on modem lines and other
154slow connections, but will only slow down things on fast networks.
155The default value can be set on a host-by-host basis in the
156configuration files; see the
157.Cm Compression
158option.
159.Pp
160.It Fl c Ar cipher_spec
161Selects the cipher specification for encrypting the session.
162.Ar cipher_spec
163is a comma-separated list of ciphers
164listed in order of preference.
165See the
166.Cm Ciphers
167keyword in
168.Xr ssh_config 5
169for more information.
170.Pp
171.It Fl D Xo
172.Sm off
173.Oo Ar bind_address : Oc
174.Ar port
175.Sm on
176.Xc
177Specifies a local
178.Dq dynamic
179application-level port forwarding.
180This works by allocating a socket to listen to
181.Ar port
182on the local side, optionally bound to the specified
183.Ar bind_address .
184Whenever a connection is made to this port, the
185connection is forwarded over the secure channel, and the application
186protocol is then used to determine where to connect to from the
187remote machine.
188Currently the SOCKS4 and SOCKS5 protocols are supported, and
189.Nm
190will act as a SOCKS server.
191Only root can forward privileged ports.
192Dynamic port forwardings can also be specified in the configuration file.
193.Pp
194IPv6 addresses can be specified by enclosing the address in square brackets.
195Only the superuser can forward privileged ports.
196By default, the local port is bound in accordance with the
197.Cm GatewayPorts
198setting.
199However, an explicit
200.Ar bind_address
201may be used to bind the connection to a specific address.
202The
203.Ar bind_address
204of
205.Dq localhost
206indicates that the listening port be bound for local use only, while an
207empty address or
208.Sq *
209indicates that the port should be available from all interfaces.
210.Pp
211.It Fl E Ar log_file
212Append debug logs to
213.Ar log_file
214instead of standard error.
215.Pp
216.It Fl e Ar escape_char
217Sets the escape character for sessions with a pty (default:
218.Ql ~ ) .
219The escape character is only recognized at the beginning of a line.
220The escape character followed by a dot
221.Pq Ql \&.
222closes the connection;
223followed by control-Z suspends the connection;
224and followed by itself sends the escape character once.
225Setting the character to
226.Dq none
227disables any escapes and makes the session fully transparent.
228.Pp
229.It Fl F Ar configfile
230Specifies an alternative per-user configuration file.
231If a configuration file is given on the command line,
232the system-wide configuration file
233.Pq Pa /etc/ssh/ssh_config
234will be ignored.
235The default for the per-user configuration file is
236.Pa ~/.ssh/config .
237If set to
238.Dq none ,
239no configuration files will be read.
240.Pp
241.It Fl f
242Requests
243.Nm
244to go to background just before command execution.
245This is useful if
246.Nm
247is going to ask for passwords or passphrases, but the user
248wants it in the background.
249This implies
250.Fl n .
251The recommended way to start X11 programs at a remote site is with
252something like
253.Ic ssh -f host xterm .
254.Pp
255If the
256.Cm ExitOnForwardFailure
257configuration option is set to
258.Dq yes ,
259then a client started with
260.Fl f
261will wait for all remote port forwards to be successfully established
262before placing itself in the background.
263Refer to the description of
264.Cm ForkAfterAuthentication
265in
266.Xr ssh_config 5
267for details.
268.Pp
269.It Fl G
270Causes
271.Nm
272to print its configuration after evaluating
273.Cm Host
274and
275.Cm Match
276blocks and exit.
277.Pp
278.It Fl g
279Allows remote hosts to connect to local forwarded ports.
280If used on a multiplexed connection, then this option must be specified
281on the master process.
282.Pp
283.It Fl I Ar pkcs11
284Specify the PKCS#11 shared library
285.Nm
286should use to communicate with a PKCS#11 token providing keys for user
287authentication.
288.Pp
289.It Fl i Ar identity_file
290Selects a file from which the identity (private key) for
291public key authentication is read.
292The default is
293.Pa ~/.ssh/id_dsa ,
294.Pa ~/.ssh/id_ecdsa ,
295.Pa ~/.ssh/id_ecdsa_sk ,
296.Pa ~/.ssh/id_ed25519 ,
297.Pa ~/.ssh/id_ed25519_sk
298and
299.Pa ~/.ssh/id_rsa .
300Identity files may also be specified on
301a per-host basis in the configuration file.
302It is possible to have multiple
303.Fl i
304options (and multiple identities specified in
305configuration files).
306If no certificates have been explicitly specified by the
307.Cm CertificateFile
308directive,
309.Nm
310will also try to load certificate information from the filename obtained
311by appending
312.Pa -cert.pub
313to identity filenames.
314.Pp
315.It Fl J Ar destination
316Connect to the target host by first making a
317.Nm
318connection to the jump host described by
319.Ar destination
320and then establishing a TCP forwarding to the ultimate destination from
321there.
322Multiple jump hops may be specified separated by comma characters.
323This is a shortcut to specify a
324.Cm ProxyJump
325configuration directive.
326Note that configuration directives supplied on the command-line generally
327apply to the destination host and not any specified jump hosts.
328Use
329.Pa ~/.ssh/config
330to specify configuration for jump hosts.
331.Pp
332.It Fl K
333Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
334credentials to the server.
335.Pp
336.It Fl k
337Disables forwarding (delegation) of GSSAPI credentials to the server.
338.Pp
339.It Fl L Xo
340.Sm off
341.Oo Ar bind_address : Oc
342.Ar port : host : hostport
343.Sm on
344.Xc
345.It Fl L Xo
346.Sm off
347.Oo Ar bind_address : Oc
348.Ar port : remote_socket
349.Sm on
350.Xc
351.It Fl L Xo
352.Sm off
353.Ar local_socket : host : hostport
354.Sm on
355.Xc
356.It Fl L Xo
357.Sm off
358.Ar local_socket : remote_socket
359.Sm on
360.Xc
361Specifies that connections to the given TCP port or Unix socket on the local
362(client) host are to be forwarded to the given host and port, or Unix socket,
363on the remote side.
364This works by allocating a socket to listen to either a TCP
365.Ar port
366on the local side, optionally bound to the specified
367.Ar bind_address ,
368or to a Unix socket.
369Whenever a connection is made to the local port or socket, the
370connection is forwarded over the secure channel, and a connection is
371made to either
372.Ar host
373port
374.Ar hostport ,
375or the Unix socket
376.Ar remote_socket ,
377from the remote machine.
378.Pp
379Port forwardings can also be specified in the configuration file.
380Only the superuser can forward privileged ports.
381IPv6 addresses can be specified by enclosing the address in square brackets.
382.Pp
383By default, the local port is bound in accordance with the
384.Cm GatewayPorts
385setting.
386However, an explicit
387.Ar bind_address
388may be used to bind the connection to a specific address.
389The
390.Ar bind_address
391of
392.Dq localhost
393indicates that the listening port be bound for local use only, while an
394empty address or
395.Sq *
396indicates that the port should be available from all interfaces.
397.Pp
398.It Fl l Ar login_name
399Specifies the user to log in as on the remote machine.
400This also may be specified on a per-host basis in the configuration file.
401.Pp
402.It Fl M
403Places the
404.Nm
405client into
406.Dq master
407mode for connection sharing.
408Multiple
409.Fl M
410options places
411.Nm
412into
413.Dq master
414mode but with confirmation required using
415.Xr ssh-askpass 1
416before each operation that changes the multiplexing state
417(e.g. opening a new session).
418Refer to the description of
419.Cm ControlMaster
420in
421.Xr ssh_config 5
422for details.
423.Pp
424.It Fl m Ar mac_spec
425A comma-separated list of MAC (message authentication code) algorithms,
426specified in order of preference.
427See the
428.Cm MACs
429keyword for more information.
430.Pp
431.It Fl N
432Do not execute a remote command.
433This is useful for just forwarding ports.
434Refer to the description of
435.Cm SessionType
436in
437.Xr ssh_config 5
438for details.
439.Pp
440.It Fl n
441Redirects stdin from
442.Pa /dev/null
443(actually, prevents reading from stdin).
444This must be used when
445.Nm
446is run in the background.
447A common trick is to use this to run X11 programs on a remote machine.
448For example,
449.Ic ssh -n shadows.cs.hut.fi emacs &
450will start an emacs on shadows.cs.hut.fi, and the X11
451connection will be automatically forwarded over an encrypted channel.
452The
453.Nm
454program will be put in the background.
455(This does not work if
456.Nm
457needs to ask for a password or passphrase; see also the
458.Fl f
459option.)
460Refer to the description of
461.Cm StdinNull
462in
463.Xr ssh_config 5
464for details.
465.Pp
466.It Fl O Ar ctl_cmd
467Control an active connection multiplexing master process.
468When the
469.Fl O
470option is specified, the
471.Ar ctl_cmd
472argument is interpreted and passed to the master process.
473Valid commands are:
474.Dq check
475(check that the master process is running),
476.Dq forward
477(request forwardings without command execution),
478.Dq cancel
479(cancel forwardings),
480.Dq exit
481(request the master to exit), and
482.Dq stop
483(request the master to stop accepting further multiplexing requests).
484.Pp
485.It Fl o Ar option
486Can be used to give options in the format used in the configuration file.
487This is useful for specifying options for which there is no separate
488command-line flag.
489For full details of the options listed below, and their possible values, see
490.Xr ssh_config 5 .
491.Pp
492.Bl -tag -width Ds -offset indent -compact
493.It AddKeysToAgent
494.It AddressFamily
495.It BatchMode
496.It BindAddress
497.It CanonicalDomains
498.It CanonicalizeFallbackLocal
499.It CanonicalizeHostname
500.It CanonicalizeMaxDots
501.It CanonicalizePermittedCNAMEs
502.It CASignatureAlgorithms
503.It CertificateFile
504.It CheckHostIP
505.It Ciphers
506.It ClearAllForwardings
507.It Compression
508.It ConnectionAttempts
509.It ConnectTimeout
510.It ControlMaster
511.It ControlPath
512.It ControlPersist
513.It DynamicForward
514.It EscapeChar
515.It ExitOnForwardFailure
516.It FingerprintHash
517.It ForkAfterAuthentication
518.It ForwardAgent
519.It ForwardX11
520.It ForwardX11Timeout
521.It ForwardX11Trusted
522.It GatewayPorts
523.It GlobalKnownHostsFile
524.It GSSAPIAuthentication
525.It GSSAPIDelegateCredentials
526.It HashKnownHosts
527.It Host
528.It HostbasedAcceptedAlgorithms
529.It HostbasedAuthentication
530.It HostKeyAlgorithms
531.It HostKeyAlias
532.It Hostname
533.It IdentitiesOnly
534.It IdentityAgent
535.It IdentityFile
536.It IPQoS
537.It KbdInteractiveAuthentication
538.It KbdInteractiveDevices
539.It KexAlgorithms
540.It KnownHostsCommand
541.It LocalCommand
542.It LocalForward
543.It LogLevel
544.It MACs
545.It Match
546.It NoHostAuthenticationForLocalhost
547.It NumberOfPasswordPrompts
548.It PasswordAuthentication
549.It PermitLocalCommand
550.It PermitRemoteOpen
551.It PKCS11Provider
552.It Port
553.It PreferredAuthentications
554.It ProxyCommand
555.It ProxyJump
556.It ProxyUseFdpass
557.It PubkeyAcceptedAlgorithms
558.It PubkeyAuthentication
559.It RekeyLimit
560.It RemoteCommand
561.It RemoteForward
562.It RequestTTY
563.It SendEnv
564.It ServerAliveInterval
565.It ServerAliveCountMax
566.It SessionType
567.It SetEnv
568.It StdinNull
569.It StreamLocalBindMask
570.It StreamLocalBindUnlink
571.It StrictHostKeyChecking
572.It TCPKeepAlive
573.It Tunnel
574.It TunnelDevice
575.It UpdateHostKeys
576.It User
577.It UserKnownHostsFile
578.It VerifyHostKeyDNS
579.It VersionAddendum
580.It VisualHostKey
581.It XAuthLocation
582.El
583.Pp
584.It Fl p Ar port
585Port to connect to on the remote host.
586This can be specified on a
587per-host basis in the configuration file.
588.Pp
589.It Fl Q Ar query_option
590Queries for the algorithms supported by one of the following features:
591.Ar cipher
592(supported symmetric ciphers),
593.Ar cipher-auth
594(supported symmetric ciphers that support authenticated encryption),
595.Ar help
596(supported query terms for use with the
597.Fl Q
598flag),
599.Ar mac
600(supported message integrity codes),
601.Ar kex
602(key exchange algorithms),
603.Ar key
604(key types),
605.Ar key-cert
606(certificate key types),
607.Ar key-plain
608(non-certificate key types),
609.Ar key-sig
610(all key types and signature algorithms),
611.Ar protocol-version
612(supported SSH protocol versions), and
613.Ar sig
614(supported signature algorithms).
615Alternatively, any keyword from
616.Xr ssh_config 5
617or
618.Xr sshd_config 5
619that takes an algorithm list may be used as an alias for the corresponding
620query_option.
621.Pp
622.It Fl q
623Quiet mode.
624Causes most warning and diagnostic messages to be suppressed.
625.Pp
626.It Fl R Xo
627.Sm off
628.Oo Ar bind_address : Oc
629.Ar port : host : hostport
630.Sm on
631.Xc
632.It Fl R Xo
633.Sm off
634.Oo Ar bind_address : Oc
635.Ar port : local_socket
636.Sm on
637.Xc
638.It Fl R Xo
639.Sm off
640.Ar remote_socket : host : hostport
641.Sm on
642.Xc
643.It Fl R Xo
644.Sm off
645.Ar remote_socket : local_socket
646.Sm on
647.Xc
648.It Fl R Xo
649.Sm off
650.Oo Ar bind_address : Oc
651.Ar port
652.Sm on
653.Xc
654Specifies that connections to the given TCP port or Unix socket on the remote
655(server) host are to be forwarded to the local side.
656.Pp
657This works by allocating a socket to listen to either a TCP
658.Ar port
659or to a Unix socket on the remote side.
660Whenever a connection is made to this port or Unix socket, the
661connection is forwarded over the secure channel, and a connection
662is made from the local machine to either an explicit destination specified by
663.Ar host
664port
665.Ar hostport ,
666or
667.Ar local_socket ,
668or, if no explicit destination was specified,
669.Nm
670will act as a SOCKS 4/5 proxy and forward connections to the destinations
671requested by the remote SOCKS client.
672.Pp
673Port forwardings can also be specified in the configuration file.
674Privileged ports can be forwarded only when
675logging in as root on the remote machine.
676IPv6 addresses can be specified by enclosing the address in square brackets.
677.Pp
678By default, TCP listening sockets on the server will be bound to the loopback
679interface only.
680This may be overridden by specifying a
681.Ar bind_address .
682An empty
683.Ar bind_address ,
684or the address
685.Ql * ,
686indicates that the remote socket should listen on all interfaces.
687Specifying a remote
688.Ar bind_address
689will only succeed if the server's
690.Cm GatewayPorts
691option is enabled (see
692.Xr sshd_config 5 ) .
693.Pp
694If the
695.Ar port
696argument is
697.Ql 0 ,
698the listen port will be dynamically allocated on the server and reported
699to the client at run time.
700When used together with
701.Ic -O forward
702the allocated port will be printed to the standard output.
703.Pp
704.It Fl S Ar ctl_path
705Specifies the location of a control socket for connection sharing,
706or the string
707.Dq none
708to disable connection sharing.
709Refer to the description of
710.Cm ControlPath
711and
712.Cm ControlMaster
713in
714.Xr ssh_config 5
715for details.
716.Pp
717.It Fl s
718May be used to request invocation of a subsystem on the remote system.
719Subsystems facilitate the use of SSH
720as a secure transport for other applications (e.g.\&
721.Xr sftp 1 ) .
722The subsystem is specified as the remote command.
723Refer to the description of
724.Cm SessionType
725in
726.Xr ssh_config 5
727for details.
728.Pp
729.It Fl T
730Disable pseudo-terminal allocation.
731.Pp
732.It Fl t
733Force pseudo-terminal allocation.
734This can be used to execute arbitrary
735screen-based programs on a remote machine, which can be very useful,
736e.g. when implementing menu services.
737Multiple
738.Fl t
739options force tty allocation, even if
740.Nm
741has no local tty.
742.Pp
743.It Fl V
744Display the version number and exit.
745.Pp
746.It Fl v
747Verbose mode.
748Causes
749.Nm
750to print debugging messages about its progress.
751This is helpful in
752debugging connection, authentication, and configuration problems.
753Multiple
754.Fl v
755options increase the verbosity.
756The maximum is 3.
757.Pp
758.It Fl W Ar host : Ns Ar port
759Requests that standard input and output on the client be forwarded to
760.Ar host
761on
762.Ar port
763over the secure channel.
764Implies
765.Fl N ,
766.Fl T ,
767.Cm ExitOnForwardFailure
768and
769.Cm ClearAllForwardings ,
770though these can be overridden in the configuration file or using
771.Fl o
772command line options.
773.Pp
774.It Fl w Xo
775.Ar local_tun Ns Op : Ns Ar remote_tun
776.Xc
777Requests
778tunnel
779device forwarding with the specified
780.Xr tun 4
781devices between the client
782.Pq Ar local_tun
783and the server
784.Pq Ar remote_tun .
785.Pp
786The devices may be specified by numerical ID or the keyword
787.Dq any ,
788which uses the next available tunnel device.
789If
790.Ar remote_tun
791is not specified, it defaults to
792.Dq any .
793See also the
794.Cm Tunnel
795and
796.Cm TunnelDevice
797directives in
798.Xr ssh_config 5 .
799.Pp
800If the
801.Cm Tunnel
802directive is unset, it will be set to the default tunnel mode, which is
803.Dq point-to-point .
804If a different
805.Cm Tunnel
806forwarding mode it desired, then it should be specified before
807.Fl w .
808.Pp
809.It Fl X
810Enables X11 forwarding.
811This can also be specified on a per-host basis in a configuration file.
812.Pp
813X11 forwarding should be enabled with caution.
814Users with the ability to bypass file permissions on the remote host
815(for the user's X authorization database)
816can access the local X11 display through the forwarded connection.
817An attacker may then be able to perform activities such as keystroke monitoring.
818.Pp
819For this reason, X11 forwarding is subjected to X11 SECURITY extension
820restrictions by default.
821Please refer to the
822.Nm
823.Fl Y
824option and the
825.Cm ForwardX11Trusted
826directive in
827.Xr ssh_config 5
828for more information.
829.Pp
830.It Fl x
831Disables X11 forwarding.
832.Pp
833.It Fl Y
834Enables trusted X11 forwarding.
835Trusted X11 forwardings are not subjected to the X11 SECURITY extension
836controls.
837.Pp
838.It Fl y
839Send log information using the
840.Xr syslog 3
841system module.
842By default this information is sent to stderr.
843.El
844.Pp
845.Nm
846may additionally obtain configuration data from
847a per-user configuration file and a system-wide configuration file.
848The file format and configuration options are described in
849.Xr ssh_config 5 .
850.Sh AUTHENTICATION
851The OpenSSH SSH client supports SSH protocol 2.
852.Pp
853The methods available for authentication are:
854GSSAPI-based authentication,
855host-based authentication,
856public key authentication,
857keyboard-interactive authentication,
858and password authentication.
859Authentication methods are tried in the order specified above,
860though
861.Cm PreferredAuthentications
862can be used to change the default order.
863.Pp
864Host-based authentication works as follows:
865If the machine the user logs in from is listed in
866.Pa /etc/hosts.equiv
867or
868.Pa /etc/shosts.equiv
869on the remote machine, the user is non-root and the user names are
870the same on both sides, or if the files
871.Pa ~/.rhosts
872or
873.Pa ~/.shosts
874exist in the user's home directory on the
875remote machine and contain a line containing the name of the client
876machine and the name of the user on that machine, the user is
877considered for login.
878Additionally, the server
879.Em must
880be able to verify the client's
881host key (see the description of
882.Pa /etc/ssh/ssh_known_hosts
883and
884.Pa ~/.ssh/known_hosts ,
885below)
886for login to be permitted.
887This authentication method closes security holes due to IP
888spoofing, DNS spoofing, and routing spoofing.
889[Note to the administrator:
890.Pa /etc/hosts.equiv ,
891.Pa ~/.rhosts ,
892and the rlogin/rsh protocol in general, are inherently insecure and should be
893disabled if security is desired.]
894.Pp
895Public key authentication works as follows:
896The scheme is based on public-key cryptography,
897using cryptosystems
898where encryption and decryption are done using separate keys,
899and it is unfeasible to derive the decryption key from the encryption key.
900The idea is that each user creates a public/private
901key pair for authentication purposes.
902The server knows the public key, and only the user knows the private key.
903.Nm
904implements public key authentication protocol automatically,
905using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
906The HISTORY section of
907.Xr ssl 8
908contains a brief discussion of the DSA and RSA algorithms.
909.Pp
910The file
911.Pa ~/.ssh/authorized_keys
912lists the public keys that are permitted for logging in.
913When the user logs in, the
914.Nm
915program tells the server which key pair it would like to use for
916authentication.
917The client proves that it has access to the private key
918and the server checks that the corresponding public key
919is authorized to accept the account.
920.Pp
921The server may inform the client of errors that prevented public key
922authentication from succeeding after authentication completes using a
923different method.
924These may be viewed by increasing the
925.Cm LogLevel
926to
927.Cm DEBUG
928or higher (e.g. by using the
929.Fl v
930flag).
931.Pp
932The user creates their key pair by running
933.Xr ssh-keygen 1 .
934This stores the private key in
935.Pa ~/.ssh/id_dsa
936(DSA),
937.Pa ~/.ssh/id_ecdsa
938(ECDSA),
939.Pa ~/.ssh/id_ecdsa_sk
940(authenticator-hosted ECDSA),
941.Pa ~/.ssh/id_ed25519
942(Ed25519),
943.Pa ~/.ssh/id_ed25519_sk
944(authenticator-hosted Ed25519),
945or
946.Pa ~/.ssh/id_rsa
947(RSA)
948and stores the public key in
949.Pa ~/.ssh/id_dsa.pub
950(DSA),
951.Pa ~/.ssh/id_ecdsa.pub
952(ECDSA),
953.Pa ~/.ssh/id_ecdsa_sk.pub
954(authenticator-hosted ECDSA),
955.Pa ~/.ssh/id_ed25519.pub
956(Ed25519),
957.Pa ~/.ssh/id_ed25519_sk.pub
958(authenticator-hosted Ed25519),
959or
960.Pa ~/.ssh/id_rsa.pub
961(RSA)
962in the user's home directory.
963The user should then copy the public key
964to
965.Pa ~/.ssh/authorized_keys
966in their home directory on the remote machine.
967The
968.Pa authorized_keys
969file corresponds to the conventional
970.Pa ~/.rhosts
971file, and has one key
972per line, though the lines can be very long.
973After this, the user can log in without giving the password.
974.Pp
975A variation on public key authentication
976is available in the form of certificate authentication:
977instead of a set of public/private keys,
978signed certificates are used.
979This has the advantage that a single trusted certification authority
980can be used in place of many public/private keys.
981See the CERTIFICATES section of
982.Xr ssh-keygen 1
983for more information.
984.Pp
985The most convenient way to use public key or certificate authentication
986may be with an authentication agent.
987See
988.Xr ssh-agent 1
989and (optionally) the
990.Cm AddKeysToAgent
991directive in
992.Xr ssh_config 5
993for more information.
994.Pp
995Keyboard-interactive authentication works as follows:
996The server sends an arbitrary
997.Qq challenge
998text and prompts for a response, possibly multiple times.
999Examples of keyboard-interactive authentication include
1000.Bx
1001Authentication (see
1002.Xr login.conf 5 )
1003and PAM (some
1004.Pf non- Ox
1005systems).
1006.Pp
1007Finally, if other authentication methods fail,
1008.Nm
1009prompts the user for a password.
1010The password is sent to the remote
1011host for checking; however, since all communications are encrypted,
1012the password cannot be seen by someone listening on the network.
1013.Pp
1014.Nm
1015automatically maintains and checks a database containing
1016identification for all hosts it has ever been used with.
1017Host keys are stored in
1018.Pa ~/.ssh/known_hosts
1019in the user's home directory.
1020Additionally, the file
1021.Pa /etc/ssh/ssh_known_hosts
1022is automatically checked for known hosts.
1023Any new hosts are automatically added to the user's file.
1024If a host's identification ever changes,
1025.Nm
1026warns about this and disables password authentication to prevent
1027server spoofing or man-in-the-middle attacks,
1028which could otherwise be used to circumvent the encryption.
1029The
1030.Cm StrictHostKeyChecking
1031option can be used to control logins to machines whose
1032host key is not known or has changed.
1033.Pp
1034When the user's identity has been accepted by the server, the server
1035either executes the given command in a non-interactive session or,
1036if no command has been specified, logs into the machine and gives
1037the user a normal shell as an interactive session.
1038All communication with
1039the remote command or shell will be automatically encrypted.
1040.Pp
1041If an interactive session is requested
1042.Nm
1043by default will only request a pseudo-terminal (pty) for interactive
1044sessions when the client has one.
1045The flags
1046.Fl T
1047and
1048.Fl t
1049can be used to override this behaviour.
1050.Pp
1051If a pseudo-terminal has been allocated the
1052user may use the escape characters noted below.
1053.Pp
1054If no pseudo-terminal has been allocated,
1055the session is transparent and can be used to reliably transfer binary data.
1056On most systems, setting the escape character to
1057.Dq none
1058will also make the session transparent even if a tty is used.
1059.Pp
1060The session terminates when the command or shell on the remote
1061machine exits and all X11 and TCP connections have been closed.
1062.Sh ESCAPE CHARACTERS
1063When a pseudo-terminal has been requested,
1064.Nm
1065supports a number of functions through the use of an escape character.
1066.Pp
1067A single tilde character can be sent as
1068.Ic ~~
1069or by following the tilde by a character other than those described below.
1070The escape character must always follow a newline to be interpreted as
1071special.
1072The escape character can be changed in configuration files using the
1073.Cm EscapeChar
1074configuration directive or on the command line by the
1075.Fl e
1076option.
1077.Pp
1078The supported escapes (assuming the default
1079.Ql ~ )
1080are:
1081.Bl -tag -width Ds
1082.It Cm ~.
1083Disconnect.
1084.It Cm ~^Z
1085Background
1086.Nm .
1087.It Cm ~#
1088List forwarded connections.
1089.It Cm ~&
1090Background
1091.Nm
1092at logout when waiting for forwarded connection / X11 sessions to terminate.
1093.It Cm ~?
1094Display a list of escape characters.
1095.It Cm ~B
1096Send a BREAK to the remote system
1097(only useful if the peer supports it).
1098.It Cm ~C
1099Open command line.
1100Currently this allows the addition of port forwardings using the
1101.Fl L ,
1102.Fl R
1103and
1104.Fl D
1105options (see above).
1106It also allows the cancellation of existing port-forwardings
1107with
1108.Sm off
1109.Fl KL Oo Ar bind_address : Oc Ar port
1110.Sm on
1111for local,
1112.Sm off
1113.Fl KR Oo Ar bind_address : Oc Ar port
1114.Sm on
1115for remote and
1116.Sm off
1117.Fl KD Oo Ar bind_address : Oc Ar port
1118.Sm on
1119for dynamic port-forwardings.
1120.Ic !\& Ns Ar command
1121allows the user to execute a local command if the
1122.Ic PermitLocalCommand
1123option is enabled in
1124.Xr ssh_config 5 .
1125Basic help is available, using the
1126.Fl h
1127option.
1128.It Cm ~R
1129Request rekeying of the connection
1130(only useful if the peer supports it).
1131.It Cm ~V
1132Decrease the verbosity
1133.Pq Ic LogLevel
1134when errors are being written to stderr.
1135.It Cm ~v
1136Increase the verbosity
1137.Pq Ic LogLevel
1138when errors are being written to stderr.
1139.El
1140.Sh TCP FORWARDING
1141Forwarding of arbitrary TCP connections over a secure channel
1142can be specified either on the command line or in a configuration file.
1143One possible application of TCP forwarding is a secure connection to a
1144mail server; another is going through firewalls.
1145.Pp
1146In the example below, we look at encrypting communication for an IRC client,
1147even though the IRC server it connects to does not directly
1148support encrypted communication.
1149This works as follows:
1150the user connects to the remote host using
1151.Nm ,
1152specifying the ports to be used to forward the connection.
1153After that it is possible to start the program locally,
1154and
1155.Nm
1156will encrypt and forward the connection to the remote server.
1157.Pp
1158The following example tunnels an IRC session from the client
1159to an IRC server at
1160.Dq server.example.com ,
1161joining channel
1162.Dq #users ,
1163nickname
1164.Dq pinky ,
1165using the standard IRC port, 6667:
1166.Bd -literal -offset 4n
1167$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
1168$ irc -c '#users' pinky IRC/127.0.0.1
1169.Ed
1170.Pp
1171The
1172.Fl f
1173option backgrounds
1174.Nm
1175and the remote command
1176.Dq sleep 10
1177is specified to allow an amount of time
1178(10 seconds, in the example)
1179to start the program which is going to use the tunnel.
1180If no connections are made within the time specified,
1181.Nm
1182will exit.
1183.Sh X11 FORWARDING
1184If the
1185.Cm ForwardX11
1186variable is set to
1187.Dq yes
1188(or see the description of the
1189.Fl X ,
1190.Fl x ,
1191and
1192.Fl Y
1193options above)
1194and the user is using X11 (the
1195.Ev DISPLAY
1196environment variable is set), the connection to the X11 display is
1197automatically forwarded to the remote side in such a way that any X11
1198programs started from the shell (or command) will go through the
1199encrypted channel, and the connection to the real X server will be made
1200from the local machine.
1201The user should not manually set
1202.Ev DISPLAY .
1203Forwarding of X11 connections can be
1204configured on the command line or in configuration files.
1205.Pp
1206The
1207.Ev DISPLAY
1208value set by
1209.Nm
1210will point to the server machine, but with a display number greater than zero.
1211This is normal, and happens because
1212.Nm
1213creates a
1214.Dq proxy
1215X server on the server machine for forwarding the
1216connections over the encrypted channel.
1217.Pp
1218.Nm
1219will also automatically set up Xauthority data on the server machine.
1220For this purpose, it will generate a random authorization cookie,
1221store it in Xauthority on the server, and verify that any forwarded
1222connections carry this cookie and replace it by the real cookie when
1223the connection is opened.
1224The real authentication cookie is never
1225sent to the server machine (and no cookies are sent in the plain).
1226.Pp
1227If the
1228.Cm ForwardAgent
1229variable is set to
1230.Dq yes
1231(or see the description of the
1232.Fl A
1233and
1234.Fl a
1235options above) and
1236the user is using an authentication agent, the connection to the agent
1237is automatically forwarded to the remote side.
1238.Sh VERIFYING HOST KEYS
1239When connecting to a server for the first time,
1240a fingerprint of the server's public key is presented to the user
1241(unless the option
1242.Cm StrictHostKeyChecking
1243has been disabled).
1244Fingerprints can be determined using
1245.Xr ssh-keygen 1 :
1246.Pp
1247.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1248.Pp
1249If the fingerprint is already known, it can be matched
1250and the key can be accepted or rejected.
1251If only legacy (MD5) fingerprints for the server are available, the
1252.Xr ssh-keygen 1
1253.Fl E
1254option may be used to downgrade the fingerprint algorithm to match.
1255.Pp
1256Because of the difficulty of comparing host keys
1257just by looking at fingerprint strings,
1258there is also support to compare host keys visually,
1259using
1260.Em random art .
1261By setting the
1262.Cm VisualHostKey
1263option to
1264.Dq yes ,
1265a small ASCII graphic gets displayed on every login to a server, no matter
1266if the session itself is interactive or not.
1267By learning the pattern a known server produces, a user can easily
1268find out that the host key has changed when a completely different pattern
1269is displayed.
1270Because these patterns are not unambiguous however, a pattern that looks
1271similar to the pattern remembered only gives a good probability that the
1272host key is the same, not guaranteed proof.
1273.Pp
1274To get a listing of the fingerprints along with their random art for
1275all known hosts, the following command line can be used:
1276.Pp
1277.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1278.Pp
1279If the fingerprint is unknown,
1280an alternative method of verification is available:
1281SSH fingerprints verified by DNS.
1282An additional resource record (RR),
1283SSHFP,
1284is added to a zonefile
1285and the connecting client is able to match the fingerprint
1286with that of the key presented.
1287.Pp
1288In this example, we are connecting a client to a server,
1289.Dq host.example.com .
1290The SSHFP resource records should first be added to the zonefile for
1291host.example.com:
1292.Bd -literal -offset indent
1293$ ssh-keygen -r host.example.com.
1294.Ed
1295.Pp
1296The output lines will have to be added to the zonefile.
1297To check that the zone is answering fingerprint queries:
1298.Pp
1299.Dl $ dig -t SSHFP host.example.com
1300.Pp
1301Finally the client connects:
1302.Bd -literal -offset indent
1303$ ssh -o "VerifyHostKeyDNS ask" host.example.com
1304[...]
1305Matching host key fingerprint found in DNS.
1306Are you sure you want to continue connecting (yes/no)?
1307.Ed
1308.Pp
1309See the
1310.Cm VerifyHostKeyDNS
1311option in
1312.Xr ssh_config 5
1313for more information.
1314.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1315.Nm
1316contains support for Virtual Private Network (VPN) tunnelling
1317using the
1318.Xr tun 4
1319network pseudo-device,
1320allowing two networks to be joined securely.
1321The
1322.Xr sshd_config 5
1323configuration option
1324.Cm PermitTunnel
1325controls whether the server supports this,
1326and at what level (layer 2 or 3 traffic).
1327.Pp
1328The following example would connect client network 10.0.50.0/24
1329with remote network 10.0.99.0/24 using a point-to-point connection
1330from 10.1.1.1 to 10.1.1.2,
1331provided that the SSH server running on the gateway to the remote network,
1332at 192.168.1.15, allows it.
1333.Pp
1334On the client:
1335.Bd -literal -offset indent
1336# ssh -f -w 0:1 192.168.1.15 true
1337# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1338# route add 10.0.99.0/24 10.1.1.2
1339.Ed
1340.Pp
1341On the server:
1342.Bd -literal -offset indent
1343# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1344# route add 10.0.50.0/24 10.1.1.1
1345.Ed
1346.Pp
1347Client access may be more finely tuned via the
1348.Pa /root/.ssh/authorized_keys
1349file (see below) and the
1350.Cm PermitRootLogin
1351server option.
1352The following entry would permit connections on
1353.Xr tun 4
1354device 1 from user
1355.Dq jane
1356and on tun device 2 from user
1357.Dq john ,
1358if
1359.Cm PermitRootLogin
1360is set to
1361.Dq forced-commands-only :
1362.Bd -literal -offset 2n
1363tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1364tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1365.Ed
1366.Pp
1367Since an SSH-based setup entails a fair amount of overhead,
1368it may be more suited to temporary setups,
1369such as for wireless VPNs.
1370More permanent VPNs are better provided by tools such as
1371.Xr ipsecctl 8
1372and
1373.Xr isakmpd 8 .
1374.Sh ENVIRONMENT
1375.Nm
1376will normally set the following environment variables:
1377.Bl -tag -width "SSH_ORIGINAL_COMMAND"
1378.It Ev DISPLAY
1379The
1380.Ev DISPLAY
1381variable indicates the location of the X11 server.
1382It is automatically set by
1383.Nm
1384to point to a value of the form
1385.Dq hostname:n ,
1386where
1387.Dq hostname
1388indicates the host where the shell runs, and
1389.Sq n
1390is an integer \*(Ge 1.
1391.Nm
1392uses this special value to forward X11 connections over the secure
1393channel.
1394The user should normally not set
1395.Ev DISPLAY
1396explicitly, as that
1397will render the X11 connection insecure (and will require the user to
1398manually copy any required authorization cookies).
1399.It Ev HOME
1400Set to the path of the user's home directory.
1401.It Ev LOGNAME
1402Synonym for
1403.Ev USER ;
1404set for compatibility with systems that use this variable.
1405.It Ev MAIL
1406Set to the path of the user's mailbox.
1407.It Ev PATH
1408Set to the default
1409.Ev PATH ,
1410as specified when compiling
1411.Nm .
1412.It Ev SSH_ASKPASS
1413If
1414.Nm
1415needs a passphrase, it will read the passphrase from the current
1416terminal if it was run from a terminal.
1417If
1418.Nm
1419does not have a terminal associated with it but
1420.Ev DISPLAY
1421and
1422.Ev SSH_ASKPASS
1423are set, it will execute the program specified by
1424.Ev SSH_ASKPASS
1425and open an X11 window to read the passphrase.
1426This is particularly useful when calling
1427.Nm
1428from a
1429.Pa .xsession
1430or related script.
1431(Note that on some machines it
1432may be necessary to redirect the input from
1433.Pa /dev/null
1434to make this work.)
1435.It Ev SSH_ASKPASS_REQUIRE
1436Allows further control over the use of an askpass program.
1437If this variable is set to
1438.Dq never
1439then
1440.Nm
1441will never attempt to use one.
1442If it is set to
1443.Dq prefer ,
1444then
1445.Nm
1446will prefer to use the askpass program instead of the TTY when requesting
1447passwords.
1448Finally, if the variable is set to
1449.Dq force ,
1450then the askpass program will be used for all passphrase input regardless
1451of whether
1452.Ev DISPLAY
1453is set.
1454.It Ev SSH_AUTH_SOCK
1455Identifies the path of a
1456.Ux Ns -domain
1457socket used to communicate with the agent.
1458.It Ev SSH_CONNECTION
1459Identifies the client and server ends of the connection.
1460The variable contains
1461four space-separated values: client IP address, client port number,
1462server IP address, and server port number.
1463.It Ev SSH_ORIGINAL_COMMAND
1464This variable contains the original command line if a forced command
1465is executed.
1466It can be used to extract the original arguments.
1467.It Ev SSH_TTY
1468This is set to the name of the tty (path to the device) associated
1469with the current shell or command.
1470If the current session has no tty,
1471this variable is not set.
1472.It Ev SSH_TUNNEL
1473Optionally set by
1474.Xr sshd 8
1475to contain the interface names assigned if tunnel forwarding was
1476requested by the client.
1477.It Ev SSH_USER_AUTH
1478Optionally set by
1479.Xr sshd 8 ,
1480this variable may contain a pathname to a file that lists the authentication
1481methods successfully used when the session was established, including any
1482public keys that were used.
1483.It Ev TZ
1484This variable is set to indicate the present time zone if it
1485was set when the daemon was started (i.e. the daemon passes the value
1486on to new connections).
1487.It Ev USER
1488Set to the name of the user logging in.
1489.El
1490.Pp
1491Additionally,
1492.Nm
1493reads
1494.Pa ~/.ssh/environment ,
1495and adds lines of the format
1496.Dq VARNAME=value
1497to the environment if the file exists and users are allowed to
1498change their environment.
1499For more information, see the
1500.Cm PermitUserEnvironment
1501option in
1502.Xr sshd_config 5 .
1503.Sh FILES
1504.Bl -tag -width Ds -compact
1505.It Pa ~/.rhosts
1506This file is used for host-based authentication (see above).
1507On some machines this file may need to be
1508world-readable if the user's home directory is on an NFS partition,
1509because
1510.Xr sshd 8
1511reads it as root.
1512Additionally, this file must be owned by the user,
1513and must not have write permissions for anyone else.
1514The recommended
1515permission for most machines is read/write for the user, and not
1516accessible by others.
1517.Pp
1518.It Pa ~/.shosts
1519This file is used in exactly the same way as
1520.Pa .rhosts ,
1521but allows host-based authentication without permitting login with
1522rlogin/rsh.
1523.Pp
1524.It Pa ~/.ssh/
1525This directory is the default location for all user-specific configuration
1526and authentication information.
1527There is no general requirement to keep the entire contents of this directory
1528secret, but the recommended permissions are read/write/execute for the user,
1529and not accessible by others.
1530.Pp
1531.It Pa ~/.ssh/authorized_keys
1532Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1533that can be used for logging in as this user.
1534The format of this file is described in the
1535.Xr sshd 8
1536manual page.
1537This file is not highly sensitive, but the recommended
1538permissions are read/write for the user, and not accessible by others.
1539.Pp
1540.It Pa ~/.ssh/config
1541This is the per-user configuration file.
1542The file format and configuration options are described in
1543.Xr ssh_config 5 .
1544Because of the potential for abuse, this file must have strict permissions:
1545read/write for the user, and not writable by others.
1546.Pp
1547.It Pa ~/.ssh/environment
1548Contains additional definitions for environment variables; see
1549.Sx ENVIRONMENT ,
1550above.
1551.Pp
1552.It Pa ~/.ssh/id_dsa
1553.It Pa ~/.ssh/id_ecdsa
1554.It Pa ~/.ssh/id_ecdsa_sk
1555.It Pa ~/.ssh/id_ed25519
1556.It Pa ~/.ssh/id_ed25519_sk
1557.It Pa ~/.ssh/id_rsa
1558Contains the private key for authentication.
1559These files
1560contain sensitive data and should be readable by the user but not
1561accessible by others (read/write/execute).
1562.Nm
1563will simply ignore a private key file if it is accessible by others.
1564It is possible to specify a passphrase when
1565generating the key which will be used to encrypt the
1566sensitive part of this file using AES-128.
1567.Pp
1568.It Pa ~/.ssh/id_dsa.pub
1569.It Pa ~/.ssh/id_ecdsa.pub
1570.It Pa ~/.ssh/id_ecdsa_sk.pub
1571.It Pa ~/.ssh/id_ed25519.pub
1572.It Pa ~/.ssh/id_ed25519_sk.pub
1573.It Pa ~/.ssh/id_rsa.pub
1574Contains the public key for authentication.
1575These files are not
1576sensitive and can (but need not) be readable by anyone.
1577.Pp
1578.It Pa ~/.ssh/known_hosts
1579Contains a list of host keys for all hosts the user has logged into
1580that are not already in the systemwide list of known host keys.
1581See
1582.Xr sshd 8
1583for further details of the format of this file.
1584.Pp
1585.It Pa ~/.ssh/rc
1586Commands in this file are executed by
1587.Nm
1588when the user logs in, just before the user's shell (or command) is
1589started.
1590See the
1591.Xr sshd 8
1592manual page for more information.
1593.Pp
1594.It Pa /etc/hosts.equiv
1595This file is for host-based authentication (see above).
1596It should only be writable by root.
1597.Pp
1598.It Pa /etc/shosts.equiv
1599This file is used in exactly the same way as
1600.Pa hosts.equiv ,
1601but allows host-based authentication without permitting login with
1602rlogin/rsh.
1603.Pp
1604.It Pa /etc/ssh/ssh_config
1605Systemwide configuration file.
1606The file format and configuration options are described in
1607.Xr ssh_config 5 .
1608.Pp
1609.It Pa /etc/ssh/ssh_host_key
1610.It Pa /etc/ssh/ssh_host_dsa_key
1611.It Pa /etc/ssh/ssh_host_ecdsa_key
1612.It Pa /etc/ssh/ssh_host_ed25519_key
1613.It Pa /etc/ssh/ssh_host_rsa_key
1614These files contain the private parts of the host keys
1615and are used for host-based authentication.
1616.Pp
1617.It Pa /etc/ssh/ssh_known_hosts
1618Systemwide list of known host keys.
1619This file should be prepared by the
1620system administrator to contain the public host keys of all machines in the
1621organization.
1622It should be world-readable.
1623See
1624.Xr sshd 8
1625for further details of the format of this file.
1626.Pp
1627.It Pa /etc/ssh/sshrc
1628Commands in this file are executed by
1629.Nm
1630when the user logs in, just before the user's shell (or command) is started.
1631See the
1632.Xr sshd 8
1633manual page for more information.
1634.El
1635.Sh EXIT STATUS
1636.Nm
1637exits with the exit status of the remote command or with 255
1638if an error occurred.
1639.Sh SEE ALSO
1640.Xr scp 1 ,
1641.Xr sftp 1 ,
1642.Xr ssh-add 1 ,
1643.Xr ssh-agent 1 ,
1644.Xr ssh-keygen 1 ,
1645.Xr ssh-keyscan 1 ,
1646.Xr tun 4 ,
1647.Xr ssh_config 5 ,
1648.Xr ssh-keysign 8 ,
1649.Xr sshd 8
1650.Sh STANDARDS
1651.Rs
1652.%A S. Lehtinen
1653.%A C. Lonvick
1654.%D January 2006
1655.%R RFC 4250
1656.%T The Secure Shell (SSH) Protocol Assigned Numbers
1657.Re
1658.Pp
1659.Rs
1660.%A T. Ylonen
1661.%A C. Lonvick
1662.%D January 2006
1663.%R RFC 4251
1664.%T The Secure Shell (SSH) Protocol Architecture
1665.Re
1666.Pp
1667.Rs
1668.%A T. Ylonen
1669.%A C. Lonvick
1670.%D January 2006
1671.%R RFC 4252
1672.%T The Secure Shell (SSH) Authentication Protocol
1673.Re
1674.Pp
1675.Rs
1676.%A T. Ylonen
1677.%A C. Lonvick
1678.%D January 2006
1679.%R RFC 4253
1680.%T The Secure Shell (SSH) Transport Layer Protocol
1681.Re
1682.Pp
1683.Rs
1684.%A T. Ylonen
1685.%A C. Lonvick
1686.%D January 2006
1687.%R RFC 4254
1688.%T The Secure Shell (SSH) Connection Protocol
1689.Re
1690.Pp
1691.Rs
1692.%A J. Schlyter
1693.%A W. Griffin
1694.%D January 2006
1695.%R RFC 4255
1696.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1697.Re
1698.Pp
1699.Rs
1700.%A F. Cusack
1701.%A M. Forssen
1702.%D January 2006
1703.%R RFC 4256
1704.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1705.Re
1706.Pp
1707.Rs
1708.%A J. Galbraith
1709.%A P. Remaker
1710.%D January 2006
1711.%R RFC 4335
1712.%T The Secure Shell (SSH) Session Channel Break Extension
1713.Re
1714.Pp
1715.Rs
1716.%A M. Bellare
1717.%A T. Kohno
1718.%A C. Namprempre
1719.%D January 2006
1720.%R RFC 4344
1721.%T The Secure Shell (SSH) Transport Layer Encryption Modes
1722.Re
1723.Pp
1724.Rs
1725.%A B. Harris
1726.%D January 2006
1727.%R RFC 4345
1728.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1729.Re
1730.Pp
1731.Rs
1732.%A M. Friedl
1733.%A N. Provos
1734.%A W. Simpson
1735.%D March 2006
1736.%R RFC 4419
1737.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1738.Re
1739.Pp
1740.Rs
1741.%A J. Galbraith
1742.%A R. Thayer
1743.%D November 2006
1744.%R RFC 4716
1745.%T The Secure Shell (SSH) Public Key File Format
1746.Re
1747.Pp
1748.Rs
1749.%A D. Stebila
1750.%A J. Green
1751.%D December 2009
1752.%R RFC 5656
1753.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1754.Re
1755.Pp
1756.Rs
1757.%A A. Perrig
1758.%A D. Song
1759.%D 1999
1760.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1761.%T Hash Visualization: a New Technique to improve Real-World Security
1762.Re
1763.Sh AUTHORS
1764OpenSSH is a derivative of the original and free
1765ssh 1.2.12 release by Tatu Ylonen.
1766Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1767Theo de Raadt and Dug Song
1768removed many bugs, re-added newer features and
1769created OpenSSH.
1770Markus Friedl contributed the support for SSH
1771protocol versions 1.5 and 2.0.
1772