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jcs's openbsd hax
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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 1.447 2025/12/22 01:17:31 djm Exp $
37.Dd $Mdocdate: December 22 2025 $
38.Dt SSH 1
39.Os
40.Sh NAME
41.Nm ssh
42.Nd OpenSSH remote login client
43.Sh SYNOPSIS
44.Nm ssh
45.Op Fl 46AaCfGgKkMNnqsTtVvXxYy
46.Op Fl B Ar bind_interface
47.Op Fl b Ar bind_address
48.Op Fl c Ar cipher_spec
49.Op Fl D Oo Ar bind_address : Oc Ns Ar port
50.Op Fl E Ar log_file
51.Op Fl e Ar escape_char
52.Op Fl F Ar configfile
53.Op Fl I Ar pkcs11
54.Op Fl i Ar identity_file
55.Op Fl J Ar destination
56.Op Fl L Ar address
57.Op Fl l Ar login_name
58.Op Fl m Ar mac_spec
59.Op Fl O Ar ctl_cmd
60.Op Fl o Ar option
61.Op Fl P Ar tag
62.Op Fl p Ar port
63.Op Fl R Ar address
64.Op Fl S Ar ctl_path
65.Op Fl W Ar host : Ns Ar port
66.Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
67.Ar destination
68.Op Ar command Op Ar argument ...
69.Nm
70.Op Fl Q Ar query_option
71.Sh DESCRIPTION
72.Nm
73(SSH client) is a program for logging into a remote machine and for
74executing commands on a remote machine.
75It is intended to provide secure encrypted communications between
76two untrusted hosts over an insecure network.
77X11 connections, arbitrary TCP ports and
78.Ux Ns -domain
79sockets can also be forwarded over the secure channel.
80.Pp
81.Nm
82connects and logs into the specified
83.Ar destination ,
84which may be specified as either
85.Sm off
86.Oo user @ Oc hostname
87.Sm on
88or a URI of the form
89.Sm off
90.No ssh:// Oo user @ Oc hostname Op : port .
91.Sm on
92The user must prove
93their identity to the remote machine using one of several methods
94(see below).
95.Pp
96If a
97.Ar command
98is specified,
99it will be executed on the remote host instead of a login shell.
100A complete command line may be specified as
101.Ar command ,
102or it may have additional arguments.
103If supplied, the arguments will be appended to the command, separated by
104spaces, before it is sent to the server to be executed.
105.Pp
106The options are as follows:
107.Pp
108.Bl -tag -width Ds -compact
109.It Fl 4
110Forces
111.Nm
112to use IPv4 addresses only.
113.Pp
114.It Fl 6
115Forces
116.Nm
117to use IPv6 addresses only.
118.Pp
119.It Fl A
120Enables forwarding of connections from an authentication agent such as
121.Xr ssh-agent 1 .
122This can also be specified on a per-host basis in a configuration file.
123.Pp
124Agent forwarding should be enabled with caution.
125Users with the ability to bypass file permissions on the remote host
126(for the agent's
127.Ux Ns -domain
128socket) can access the local agent through the forwarded connection.
129An attacker cannot obtain key material from the agent,
130however they can perform operations on the keys that enable them to
131authenticate using the identities loaded into the agent.
132A safer alternative may be to use a jump host
133(see
134.Fl J ) .
135.Pp
136.It Fl a
137Disables forwarding of the authentication agent connection.
138.Pp
139.It Fl B Ar bind_interface
140Bind to the address of
141.Ar bind_interface
142before attempting to connect to the destination host.
143This is only useful on systems with more than one address.
144.Pp
145.It Fl b Ar bind_address
146Use
147.Ar bind_address
148on the local machine as the source address
149of the connection.
150Only useful on systems with more than one address.
151.Pp
152.It Fl C
153Requests compression of all data (including stdin, stdout, stderr, and
154data for forwarded X11, TCP and
155.Ux Ns -domain
156connections).
157The compression algorithm is the same used by
158.Xr gzip 1 .
159Compression is desirable on modem lines and other
160slow connections, but will only slow down things on fast networks.
161The default value can be set on a host-by-host basis in the
162configuration files; see the
163.Cm Compression
164option in
165.Xr ssh_config 5 .
166.Pp
167.It Fl c Ar cipher_spec
168Selects the cipher specification for encrypting the session.
169.Ar cipher_spec
170is a comma-separated list of ciphers
171listed in order of preference.
172See the
173.Cm Ciphers
174keyword in
175.Xr ssh_config 5
176for more information.
177.Pp
178.It Fl D Xo
179.Sm off
180.Oo Ar bind_address : Oc
181.Ar port
182.Sm on
183.Xc
184Specifies a local
185.Dq dynamic
186application-level port forwarding.
187This works by allocating a socket to listen to
188.Ar port
189on the local side, optionally bound to the specified
190.Ar bind_address .
191Whenever a connection is made to this port, the
192connection is forwarded over the secure channel, and the application
193protocol is then used to determine where to connect to from the
194remote machine.
195Currently the SOCKS4 and SOCKS5 protocols are supported, and
196.Nm
197will act as a SOCKS server.
198Only root can forward privileged ports.
199Dynamic port forwardings can also be specified in the configuration file.
200.Pp
201IPv6 addresses can be specified by enclosing the address in square brackets.
202Only the superuser can forward privileged ports.
203By default, the local port is bound in accordance with the
204.Cm GatewayPorts
205setting.
206However, an explicit
207.Ar bind_address
208may be used to bind the connection to a specific address.
209The
210.Ar bind_address
211of
212.Dq localhost
213indicates that the listening port be bound for local use only, while an
214empty address or
215.Sq *
216indicates that the port should be available from all interfaces.
217.Pp
218.It Fl E Ar log_file
219Append debug logs to
220.Ar log_file
221instead of standard error.
222.Pp
223.It Fl e Ar escape_char
224Sets the escape character for sessions with a pty (default:
225.Ql ~ ) .
226The escape character is only recognized at the beginning of a line.
227The escape character followed by a dot
228.Pq Ql \&.
229closes the connection;
230followed by control-Z suspends the connection;
231and followed by itself sends the escape character once.
232Setting the character to
233.Dq none
234disables any escapes and makes the session fully transparent.
235.Pp
236.It Fl F Ar configfile
237Specifies an alternative per-user configuration file.
238If a configuration file is given on the command line,
239the system-wide configuration file
240.Pq Pa /etc/ssh/ssh_config
241will be ignored.
242The default for the per-user configuration file is
243.Pa ~/.ssh/config .
244If set to
245.Dq none ,
246no configuration files will be read.
247.Pp
248.It Fl f
249Requests
250.Nm
251to go to background just before command execution.
252This is useful if
253.Nm
254is going to ask for passwords or passphrases, but the user
255wants it in the background.
256This implies
257.Fl n .
258The recommended way to start X11 programs at a remote site is with
259something like
260.Ic ssh -f host xterm .
261.Pp
262If the
263.Cm ExitOnForwardFailure
264configuration option is set to
265.Dq yes ,
266then a client started with
267.Fl f
268will wait for all remote port forwards to be successfully established
269before placing itself in the background.
270Refer to the description of
271.Cm ForkAfterAuthentication
272in
273.Xr ssh_config 5
274for details.
275.Pp
276.It Fl G
277Causes
278.Nm
279to print its configuration after evaluating
280.Cm Host
281and
282.Cm Match
283blocks and exit.
284.Pp
285.It Fl g
286Allows remote hosts to connect to local forwarded ports.
287If used on a multiplexed connection, then this option must be specified
288on the master process.
289.Pp
290.It Fl I Ar pkcs11
291Specify the PKCS#11 shared library
292.Nm
293should use to communicate with a PKCS#11 token providing keys for user
294authentication.
295.Pp
296.It Fl i Ar identity_file
297Selects a file from which the identity (private key) for
298public key authentication is read.
299You can also specify a public key file to use the corresponding
300private key that is loaded in
301.Xr ssh-agent 1
302when the private key file is not present locally.
303The default is
304.Pa ~/.ssh/id_rsa ,
305.Pa ~/.ssh/id_ecdsa ,
306.Pa ~/.ssh/id_ecdsa_sk ,
307.Pa ~/.ssh/id_ed25519
308and
309.Pa ~/.ssh/id_ed25519_sk .
310Identity files may also be specified on
311a per-host basis in the configuration file.
312It is possible to have multiple
313.Fl i
314options (and multiple identities specified in
315configuration files).
316If no certificates have been explicitly specified by the
317.Cm CertificateFile
318directive,
319.Nm
320will also try to load certificate information from the filename obtained
321by appending
322.Pa -cert.pub
323to identity filenames.
324.Pp
325.It Fl J Ar destination
326Connect to the target host by first making an
327.Nm
328connection to the jump host described by
329.Ar destination
330and then establishing a TCP forwarding to the ultimate destination from
331there.
332Multiple jump hops may be specified separated by comma characters.
333IPv6 addresses can be specified by enclosing the address in square brackets.
334This is a shortcut to specify a
335.Cm ProxyJump
336configuration directive.
337Note that configuration directives supplied on the command-line generally
338apply to the destination host and not any specified jump hosts.
339Use
340.Pa ~/.ssh/config
341to specify configuration for jump hosts.
342.Pp
343.It Fl K
344Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
345credentials to the server.
346.Pp
347.It Fl k
348Disables forwarding (delegation) of GSSAPI credentials to the server.
349.Pp
350.It Fl L Xo
351.Sm off
352.Oo Ar bind_address : Oc
353.Ar port : host : hostport
354.Sm on
355.Xc
356.It Fl L Xo
357.Sm off
358.Oo Ar bind_address : Oc
359.Ar port : remote_socket
360.Sm on
361.Xc
362.It Fl L Xo
363.Sm off
364.Ar local_socket : host : hostport
365.Sm on
366.Xc
367.It Fl L Xo
368.Sm off
369.Ar local_socket : remote_socket
370.Sm on
371.Xc
372Specifies that connections to the given TCP port or Unix socket on the local
373(client) host are to be forwarded to the given host and port, or Unix socket,
374on the remote side.
375This works by allocating a socket to listen to either a TCP
376.Ar port
377on the local side, optionally bound to the specified
378.Ar bind_address ,
379or to a Unix socket.
380Whenever a connection is made to the local port or socket, the
381connection is forwarded over the secure channel, and a connection is
382made to either
383.Ar host
384port
385.Ar hostport ,
386or the Unix socket
387.Ar remote_socket ,
388from the remote machine.
389.Pp
390Port forwardings can also be specified in the configuration file.
391Only the superuser can forward privileged ports.
392IPv6 addresses can be specified by enclosing the address in square brackets.
393.Pp
394By default, the local port is bound in accordance with the
395.Cm GatewayPorts
396setting.
397However, an explicit
398.Ar bind_address
399may be used to bind the connection to a specific address.
400The
401.Ar bind_address
402of
403.Dq localhost
404indicates that the listening port be bound for local use only, while an
405empty address or
406.Sq *
407indicates that the port should be available from all interfaces.
408.Pp
409.It Fl l Ar login_name
410Specifies the user to log in as on the remote machine.
411This also may be specified on a per-host basis in the configuration file.
412.Pp
413.It Fl M
414Places the
415.Nm
416client into
417.Dq master
418mode for connection sharing.
419Multiple
420.Fl M
421options places
422.Nm
423into
424.Dq master
425mode but with confirmation required using
426.Xr ssh-askpass 1
427before each operation that changes the multiplexing state
428(e.g. opening a new session).
429Refer to the description of
430.Cm ControlMaster
431in
432.Xr ssh_config 5
433for details.
434.Pp
435.It Fl m Ar mac_spec
436A comma-separated list of MAC (message authentication code) algorithms,
437specified in order of preference.
438See the
439.Cm MACs
440keyword in
441.Xr ssh_config 5
442for more information.
443.Pp
444.It Fl N
445Do not execute a remote command.
446This is useful for just forwarding ports.
447Refer to the description of
448.Cm SessionType
449in
450.Xr ssh_config 5
451for details.
452.Pp
453.It Fl n
454Redirects stdin from
455.Pa /dev/null
456(actually, prevents reading from stdin).
457This must be used when
458.Nm
459is run in the background.
460A common trick is to use this to run X11 programs on a remote machine.
461For example,
462.Ic ssh -n shadows.cs.hut.fi emacs &
463will start an emacs on shadows.cs.hut.fi, and the X11
464connection will be automatically forwarded over an encrypted channel.
465The
466.Nm
467program will be put in the background.
468(This does not work if
469.Nm
470needs to ask for a password or passphrase; see also the
471.Fl f
472option.)
473Refer to the description of
474.Cm StdinNull
475in
476.Xr ssh_config 5
477for details.
478.Pp
479.It Fl O Ar ctl_cmd
480Control an active connection multiplexing master process.
481When the
482.Fl O
483option is specified, the
484.Ar ctl_cmd
485argument is interpreted and passed to the master process.
486Valid commands are:
487.Dq check
488(check that the master process is running),
489.Dq conninfo
490(report information about the master connection),
491.Dq channels
492(report information about open channels),
493.Dq forward
494(request forwardings without command execution),
495.Dq cancel
496(cancel forwardings),
497.Dq proxy
498(connect to a running multiplexing master in proxy mode),
499.Dq exit
500(request the master to exit), and
501.Dq stop
502(request the master to stop accepting further multiplexing requests).
503.Pp
504.It Fl o Ar option
505Can be used to give options in the format used in the configuration file.
506This is useful for specifying options for which there is no separate
507command-line flag.
508For full details of the options listed below, and their possible values, see
509.Xr ssh_config 5 .
510.Pp
511.Bl -tag -width Ds -offset indent -compact
512.It AddKeysToAgent
513.It AddressFamily
514.It BatchMode
515.It BindAddress
516.It BindInterface
517.It CASignatureAlgorithms
518.It CanonicalDomains
519.It CanonicalizeFallbackLocal
520.It CanonicalizeHostname
521.It CanonicalizeMaxDots
522.It CanonicalizePermittedCNAMEs
523.It CertificateFile
524.It ChannelTimeout
525.It CheckHostIP
526.It Ciphers
527.It ClearAllForwardings
528.It Compression
529.It ConnectTimeout
530.It ConnectionAttempts
531.It ControlMaster
532.It ControlPath
533.It ControlPersist
534.It DynamicForward
535.It EnableEscapeCommandline
536.It EnableSSHKeysign
537.It EscapeChar
538.It ExitOnForwardFailure
539.It FingerprintHash
540.It ForkAfterAuthentication
541.It ForwardAgent
542.It ForwardX11
543.It ForwardX11Timeout
544.It ForwardX11Trusted
545.It GSSAPIAuthentication
546.It GSSAPIDelegateCredentials
547.It GatewayPorts
548.It GlobalKnownHostsFile
549.It HashKnownHosts
550.It Host
551.It HostKeyAlgorithms
552.It HostKeyAlias
553.It HostbasedAcceptedAlgorithms
554.It HostbasedAuthentication
555.It Hostname
556.It IPQoS
557.It IdentitiesOnly
558.It IdentityAgent
559.It IdentityFile
560.It IgnoreUnknown
561.It Include
562.It KbdInteractiveAuthentication
563.It KbdInteractiveDevices
564.It KexAlgorithms
565.It KnownHostsCommand
566.It LocalCommand
567.It LocalForward
568.It LogLevel
569.It LogVerbose
570.It MACs
571.It NoHostAuthenticationForLocalhost
572.It NumberOfPasswordPrompts
573.It ObscureKeystrokeTiming
574.It PKCS11Provider
575.It PasswordAuthentication
576.It PermitLocalCommand
577.It PermitRemoteOpen
578.It Port
579.It PreferredAuthentications
580.It ProxyCommand
581.It ProxyJump
582.It ProxyUseFdpass
583.It PubkeyAcceptedAlgorithms
584.It PubkeyAuthentication
585.It RekeyLimit
586.It RemoteCommand
587.It RemoteForward
588.It RequestTTY
589.It RequiredRSASize
590.It RevokedHostKeys
591.It SecurityKeyProvider
592.It SendEnv
593.It ServerAliveCountMax
594.It ServerAliveInterval
595.It SessionType
596.It SetEnv
597.It StdinNull
598.It StreamLocalBindMask
599.It StreamLocalBindUnlink
600.It StrictHostKeyChecking
601.It SyslogFacility
602.It TCPKeepAlive
603.It Tag
604.It Tunnel
605.It TunnelDevice
606.It UpdateHostKeys
607.It User
608.It UserKnownHostsFile
609.It VerifyHostKeyDNS
610.It VisualHostKey
611.It XAuthLocation
612.El
613.Pp
614.It Fl P Ar tag
615Specify a tag name that may be used to select configuration in
616.Xr ssh_config 5 .
617Refer to the
618.Cm Tag
619and
620.Cm Match
621keywords in
622.Xr ssh_config 5
623for more information.
624.It Fl p Ar port
625Port to connect to on the remote host.
626This can be specified on a
627per-host basis in the configuration file.
628.Pp
629.It Fl Q Ar query_option
630Queries for the algorithms supported by one of the following features:
631.Ar cipher
632(supported symmetric ciphers),
633.Ar cipher-auth
634(supported symmetric ciphers that support authenticated encryption),
635.Ar help
636(supported query terms for use with the
637.Fl Q
638flag),
639.Ar mac
640(supported message integrity codes),
641.Ar kex
642(key exchange algorithms),
643.Ar key
644(key types),
645.Ar key-ca-sign
646(valid CA signature algorithms for certificates),
647.Ar key-cert
648(certificate key types),
649.Ar key-plain
650(non-certificate key types),
651.Ar key-sig
652(all key types and signature algorithms),
653.Ar protocol-version
654(supported SSH protocol versions), and
655.Ar sig
656(supported signature algorithms).
657Alternatively, any keyword from
658.Xr ssh_config 5
659or
660.Xr sshd_config 5
661that takes an algorithm list may be used as an alias for the corresponding
662query_option.
663.Pp
664.It Fl q
665Quiet mode.
666Causes most warning and diagnostic messages to be suppressed.
667.Pp
668.It Fl R Xo
669.Sm off
670.Oo Ar bind_address : Oc
671.Ar port : host : hostport
672.Sm on
673.Xc
674.It Fl R Xo
675.Sm off
676.Oo Ar bind_address : Oc
677.Ar port : local_socket
678.Sm on
679.Xc
680.It Fl R Xo
681.Sm off
682.Ar remote_socket : host : hostport
683.Sm on
684.Xc
685.It Fl R Xo
686.Sm off
687.Ar remote_socket : local_socket
688.Sm on
689.Xc
690.It Fl R Xo
691.Sm off
692.Oo Ar bind_address : Oc
693.Ar port
694.Sm on
695.Xc
696Specifies that connections to the given TCP port or Unix socket on the remote
697(server) host are to be forwarded to the local side.
698.Pp
699This works by allocating a socket to listen to either a TCP
700.Ar port
701or to a Unix socket on the remote side.
702Whenever a connection is made to this port or Unix socket, the
703connection is forwarded over the secure channel, and a connection
704is made from the local machine to either an explicit destination specified by
705.Ar host
706port
707.Ar hostport ,
708or
709.Ar local_socket ,
710or, if no explicit destination was specified,
711.Nm
712will act as a SOCKS 4/5 proxy and forward connections to the destinations
713requested by the remote SOCKS client.
714.Pp
715Port forwardings can also be specified in the configuration file.
716Privileged ports can be forwarded only when
717logging in as root on the remote machine.
718IPv6 addresses can be specified by enclosing the address in square brackets.
719.Pp
720By default, TCP listening sockets on the server will be bound to the loopback
721interface only.
722This may be overridden by specifying a
723.Ar bind_address .
724An empty
725.Ar bind_address ,
726or the address
727.Ql * ,
728indicates that the remote socket should listen on all interfaces.
729Specifying a remote
730.Ar bind_address
731will only succeed if the server's
732.Cm GatewayPorts
733option is enabled (see
734.Xr sshd_config 5 ) .
735.Pp
736If the
737.Ar port
738argument is
739.Ql 0 ,
740the listen port will be dynamically allocated on the server and reported
741to the client at run time.
742When used together with
743.Ic -O forward ,
744the allocated port will be printed to the standard output.
745.Pp
746.It Fl S Ar ctl_path
747Specifies the location of a control socket for connection sharing,
748or the string
749.Dq none
750to disable connection sharing.
751Refer to the description of
752.Cm ControlPath
753and
754.Cm ControlMaster
755in
756.Xr ssh_config 5
757for details.
758.Pp
759.It Fl s
760May be used to request invocation of a subsystem on the remote system.
761Subsystems facilitate the use of SSH
762as a secure transport for other applications (e.g.\&
763.Xr sftp 1 ) .
764The subsystem is specified as the remote command.
765Refer to the description of
766.Cm SessionType
767in
768.Xr ssh_config 5
769for details.
770.Pp
771.It Fl T
772Disable pseudo-terminal allocation.
773.Pp
774.It Fl t
775Force pseudo-terminal allocation.
776This can be used to execute arbitrary
777screen-based programs on a remote machine, which can be very useful,
778e.g. when implementing menu services.
779Multiple
780.Fl t
781options force tty allocation, even if
782.Nm
783has no local tty.
784.Pp
785.It Fl V
786Display the version number and exit.
787.Pp
788.It Fl v
789Verbose mode.
790Causes
791.Nm
792to print debugging messages about its progress.
793This is helpful in
794debugging connection, authentication, and configuration problems.
795Multiple
796.Fl v
797options increase the verbosity.
798The maximum is 3.
799.Pp
800.It Fl W Ar host : Ns Ar port
801Requests that standard input and output on the client be forwarded to
802.Ar host
803on
804.Ar port
805over the secure channel.
806Implies
807.Fl N ,
808.Fl T ,
809.Cm ExitOnForwardFailure
810and
811.Cm ClearAllForwardings ,
812though these can be overridden in the configuration file or using
813.Fl o
814command line options.
815.Pp
816.It Fl w Xo
817.Ar local_tun Ns Op : Ns Ar remote_tun
818.Xc
819Requests
820tunnel
821device forwarding with the specified
822.Xr tun 4
823devices between the client
824.Pq Ar local_tun
825and the server
826.Pq Ar remote_tun .
827.Pp
828The devices may be specified by numerical ID or the keyword
829.Dq any ,
830which uses the next available tunnel device.
831If
832.Ar remote_tun
833is not specified, it defaults to
834.Dq any .
835See also the
836.Cm Tunnel
837and
838.Cm TunnelDevice
839directives in
840.Xr ssh_config 5 .
841.Pp
842If the
843.Cm Tunnel
844directive is unset, it will be set to the default tunnel mode, which is
845.Dq point-to-point .
846If a different
847.Cm Tunnel
848forwarding mode it desired, then it should be specified before
849.Fl w .
850.Pp
851.It Fl X
852Enables X11 forwarding.
853This can also be specified on a per-host basis in a configuration file.
854.Pp
855X11 forwarding should be enabled with caution.
856Users with the ability to bypass file permissions on the remote host
857(for the user's X authorization database)
858can access the local X11 display through the forwarded connection.
859An attacker may then be able to perform activities such as keystroke monitoring.
860.Pp
861For this reason, X11 forwarding is subjected to X11 SECURITY extension
862restrictions by default.
863Refer to the
864.Nm
865.Fl Y
866option and the
867.Cm ForwardX11Trusted
868directive in
869.Xr ssh_config 5
870for more information.
871.Pp
872.It Fl x
873Disables X11 forwarding.
874.Pp
875.It Fl Y
876Enables trusted X11 forwarding.
877Trusted X11 forwardings are not subjected to the X11 SECURITY extension
878controls.
879.Pp
880.It Fl y
881Send log information using the
882.Xr syslog 3
883system module.
884By default this information is sent to stderr.
885.El
886.Pp
887.Nm
888may additionally obtain configuration data from
889a per-user configuration file and a system-wide configuration file.
890The file format and configuration options are described in
891.Xr ssh_config 5 .
892.Sh AUTHENTICATION
893The OpenSSH SSH client supports SSH protocol 2.
894.Pp
895The methods available for authentication are:
896GSSAPI-based authentication,
897host-based authentication,
898public key authentication,
899keyboard-interactive authentication,
900and password authentication.
901Authentication methods are tried in the order specified above,
902though
903.Cm PreferredAuthentications
904can be used to change the default order.
905.Pp
906Host-based authentication works as follows:
907If the machine the user logs in from is listed in
908.Pa /etc/hosts.equiv
909or
910.Pa /etc/shosts.equiv
911on the remote machine, the user is non-root and the user names are
912the same on both sides, or if the files
913.Pa ~/.rhosts
914or
915.Pa ~/.shosts
916exist in the user's home directory on the
917remote machine and contain a line containing the name of the client
918machine and the name of the user on that machine, the user is
919considered for login.
920Additionally, the server
921.Em must
922be able to verify the client's
923host key (see the description of
924.Pa /etc/ssh/ssh_known_hosts
925and
926.Pa ~/.ssh/known_hosts ,
927below)
928for login to be permitted.
929This authentication method closes security holes due to IP
930spoofing, DNS spoofing, and routing spoofing.
931[Note to the administrator:
932.Pa /etc/hosts.equiv ,
933.Pa ~/.rhosts ,
934and the rlogin/rsh protocol in general, are inherently insecure and should be
935disabled if security is desired.]
936.Pp
937Public key authentication works as follows:
938The scheme is based on public-key cryptography,
939using cryptosystems
940where encryption and decryption are done using separate keys,
941and it is unfeasible to derive the decryption key from the encryption key.
942The idea is that each user creates a public/private
943key pair for authentication purposes.
944The server knows the public key, and only the user knows the private key.
945.Nm
946implements public key authentication protocol automatically,
947using one of the ECDSA, Ed25519 or RSA algorithms.
948.Pp
949The file
950.Pa ~/.ssh/authorized_keys
951lists the public keys that are permitted for logging in.
952When the user logs in, the
953.Nm
954program tells the server which key pair it would like to use for
955authentication.
956The client proves that it has access to the private key
957and the server checks that the corresponding public key
958is authorized to accept the account.
959.Pp
960The server may inform the client of errors that prevented public key
961authentication from succeeding after authentication completes using a
962different method.
963These may be viewed by increasing the
964.Cm LogLevel
965to
966.Cm DEBUG
967or higher (e.g. by using the
968.Fl v
969flag).
970.Pp
971The user creates their key pair by running
972.Xr ssh-keygen 1 .
973This stores the private key in
974.Pa ~/.ssh/id_ecdsa
975(ECDSA),
976.Pa ~/.ssh/id_ecdsa_sk
977(authenticator-hosted ECDSA),
978.Pa ~/.ssh/id_ed25519
979(Ed25519),
980.Pa ~/.ssh/id_ed25519_sk
981(authenticator-hosted Ed25519),
982or
983.Pa ~/.ssh/id_rsa
984(RSA)
985and stores the public key in
986.Pa ~/.ssh/id_ecdsa.pub
987(ECDSA),
988.Pa ~/.ssh/id_ecdsa_sk.pub
989(authenticator-hosted ECDSA),
990.Pa ~/.ssh/id_ed25519.pub
991(Ed25519),
992.Pa ~/.ssh/id_ed25519_sk.pub
993(authenticator-hosted Ed25519),
994or
995.Pa ~/.ssh/id_rsa.pub
996(RSA)
997in the user's home directory.
998The user should then copy the public key
999to
1000.Pa ~/.ssh/authorized_keys
1001in their home directory on the remote machine.
1002The
1003.Pa authorized_keys
1004file corresponds to the conventional
1005.Pa ~/.rhosts
1006file, and has one key
1007per line, though the lines can be very long.
1008After this, the user can log in without giving the password.
1009.Pp
1010A variation on public key authentication
1011is available in the form of certificate authentication:
1012instead of a set of public/private keys,
1013signed certificates are used.
1014This has the advantage that a single trusted certification authority
1015can be used in place of many public/private keys.
1016See the CERTIFICATES section of
1017.Xr ssh-keygen 1
1018for more information.
1019.Pp
1020The most convenient way to use public key or certificate authentication
1021may be with an authentication agent.
1022See
1023.Xr ssh-agent 1
1024and (optionally) the
1025.Cm AddKeysToAgent
1026directive in
1027.Xr ssh_config 5
1028for more information.
1029.Pp
1030Keyboard-interactive authentication works as follows:
1031The server sends an arbitrary
1032.Qq challenge
1033text and prompts for a response, possibly multiple times.
1034Examples of keyboard-interactive authentication include
1035.Bx
1036Authentication (see
1037.Xr login.conf 5 )
1038and PAM (some
1039.Pf non- Ox
1040systems).
1041.Pp
1042Finally, if other authentication methods fail,
1043.Nm
1044prompts the user for a password.
1045The password is sent to the remote
1046host for checking; however, since all communications are encrypted,
1047the password cannot be seen by someone listening on the network.
1048.Pp
1049.Nm
1050automatically maintains and checks a database containing
1051identification for all hosts it has ever been used with.
1052Host keys are stored in
1053.Pa ~/.ssh/known_hosts
1054in the user's home directory.
1055Additionally, the file
1056.Pa /etc/ssh/ssh_known_hosts
1057is automatically checked for known hosts.
1058Any new hosts are automatically added to the user's file.
1059If a host's identification ever changes,
1060.Nm
1061warns about this and disables password authentication to prevent
1062server spoofing or man-in-the-middle attacks,
1063which could otherwise be used to circumvent the encryption.
1064The
1065.Cm StrictHostKeyChecking
1066option can be used to control logins to machines whose
1067host key is not known or has changed.
1068.Pp
1069When the user's identity has been accepted by the server, the server
1070either executes the given command in a non-interactive session or,
1071if no command has been specified, logs into the machine and gives
1072the user a normal shell as an interactive session.
1073All communication with
1074the remote command or shell will be automatically encrypted.
1075.Pp
1076If an interactive session is requested,
1077.Nm
1078by default will only request a pseudo-terminal (pty) for interactive
1079sessions when the client has one.
1080The flags
1081.Fl T
1082and
1083.Fl t
1084can be used to override this behaviour.
1085.Pp
1086If a pseudo-terminal has been allocated, the
1087user may use the escape characters noted below.
1088.Pp
1089If no pseudo-terminal has been allocated,
1090the session is transparent and can be used to reliably transfer binary data.
1091On most systems, setting the escape character to
1092.Dq none
1093will also make the session transparent even if a tty is used.
1094.Pp
1095The session terminates when the command or shell on the remote
1096machine exits and all X11 and TCP connections have been closed.
1097.Sh ESCAPE CHARACTERS
1098When a pseudo-terminal has been requested,
1099.Nm
1100supports a number of functions through the use of an escape character.
1101.Pp
1102A single tilde character can be sent as
1103.Ic ~~
1104or by following the tilde by a character other than those described below.
1105The escape character must always follow a newline to be interpreted as
1106special.
1107The escape character can be changed in configuration files using the
1108.Cm EscapeChar
1109configuration directive or on the command line by the
1110.Fl e
1111option.
1112.Pp
1113The supported escapes (assuming the default
1114.Ql ~ )
1115are:
1116.Bl -tag -width Ds
1117.It Cm ~.
1118Disconnect.
1119.It Cm ~^Z
1120Background
1121.Nm .
1122.It Cm ~#
1123List forwarded connections.
1124.It Cm ~&
1125Background
1126.Nm
1127at logout when waiting for forwarded connection / X11 sessions to terminate.
1128.It Cm ~?
1129Display a list of escape characters.
1130.It Cm ~B
1131Send a BREAK to the remote system
1132(only useful if the peer supports it).
1133.It Cm ~C
1134Open command line.
1135Currently this allows the addition of port forwardings using the
1136.Fl L ,
1137.Fl R
1138and
1139.Fl D
1140options (see above).
1141It also allows the cancellation of existing port-forwardings
1142with
1143.Sm off
1144.Fl KL Oo Ar bind_address : Oc Ar port
1145.Sm on
1146for local,
1147.Sm off
1148.Fl KR Oo Ar bind_address : Oc Ar port
1149.Sm on
1150for remote and
1151.Sm off
1152.Fl KD Oo Ar bind_address : Oc Ar port
1153.Sm on
1154for dynamic port-forwardings.
1155.Ic !\& Ns Ar command
1156allows the user to execute a local command if the
1157.Ic PermitLocalCommand
1158option is enabled in
1159.Xr ssh_config 5 .
1160Basic help is available, using the
1161.Fl h
1162option.
1163.It Cm ~I
1164Show information about the current SSH connection.
1165.It Cm ~R
1166Request rekeying of the connection
1167(only useful if the peer supports it).
1168.It Cm ~V
1169Decrease the verbosity
1170.Pq Ic LogLevel
1171when errors are being written to stderr.
1172.It Cm ~v
1173Increase the verbosity
1174.Pq Ic LogLevel
1175when errors are being written to stderr.
1176.El
1177.Sh TCP FORWARDING
1178Forwarding of arbitrary TCP connections over a secure channel
1179can be specified either on the command line or in a configuration file.
1180One possible application of TCP forwarding is a secure connection to a
1181mail server; another is going through firewalls.
1182.Pp
1183In the example below, we look at encrypting communication for an IRC client,
1184even though the IRC server it connects to does not directly
1185support encrypted communication.
1186This works as follows:
1187the user connects to the remote host using
1188.Nm ,
1189specifying the ports to be used to forward the connection.
1190After that it is possible to start the program locally,
1191and
1192.Nm
1193will encrypt and forward the connection to the remote server.
1194.Pp
1195The following example tunnels an IRC session from the client
1196to an IRC server at
1197.Dq server.example.com ,
1198joining channel
1199.Dq #users ,
1200nickname
1201.Dq pinky ,
1202using the standard IRC port, 6667:
1203.Bd -literal -offset 4n
1204$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
1205$ irc -c '#users' pinky IRC/127.0.0.1
1206.Ed
1207.Pp
1208The
1209.Fl f
1210option backgrounds
1211.Nm
1212and the remote command
1213.Dq sleep 10
1214is specified to allow an amount of time
1215(10 seconds, in the example)
1216to start the program which is going to use the tunnel.
1217If no connections are made within the time specified,
1218.Nm
1219will exit.
1220.Sh X11 FORWARDING
1221If the
1222.Cm ForwardX11
1223variable is set to
1224.Dq yes
1225(or see the description of the
1226.Fl X ,
1227.Fl x ,
1228and
1229.Fl Y
1230options above)
1231and the user is using X11 (the
1232.Ev DISPLAY
1233environment variable is set), the connection to the X11 display is
1234automatically forwarded to the remote side in such a way that any X11
1235programs started from the shell (or command) will go through the
1236encrypted channel, and the connection to the real X server will be made
1237from the local machine.
1238The user should not manually set
1239.Ev DISPLAY .
1240Forwarding of X11 connections can be
1241configured on the command line or in configuration files.
1242.Pp
1243The
1244.Ev DISPLAY
1245value set by
1246.Nm
1247will point to the server machine, but with a display number greater than zero.
1248This is normal, and happens because
1249.Nm
1250creates a
1251.Dq proxy
1252X server on the server machine for forwarding the
1253connections over the encrypted channel.
1254.Pp
1255.Nm
1256will also automatically set up Xauthority data on the server machine.
1257For this purpose, it will generate a random authorization cookie,
1258store it in Xauthority on the server, and verify that any forwarded
1259connections carry this cookie and replace it by the real cookie when
1260the connection is opened.
1261The real authentication cookie is never
1262sent to the server machine (and no cookies are sent in the plain).
1263.Pp
1264If the
1265.Cm ForwardAgent
1266variable is set to
1267.Dq yes
1268(or see the description of the
1269.Fl A
1270and
1271.Fl a
1272options above) and
1273the user is using an authentication agent, the connection to the agent
1274is automatically forwarded to the remote side.
1275.Sh VERIFYING HOST KEYS
1276When connecting to a server for the first time,
1277a fingerprint of the server's public key is presented to the user
1278(unless the option
1279.Cm StrictHostKeyChecking
1280has been disabled).
1281Fingerprints can be determined using
1282.Xr ssh-keygen 1 :
1283.Pp
1284.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1285.Pp
1286If the fingerprint is already known, it can be matched
1287and the key can be accepted or rejected.
1288If only legacy (MD5) fingerprints for the server are available, the
1289.Xr ssh-keygen 1
1290.Fl E
1291option may be used to downgrade the fingerprint algorithm to match.
1292.Pp
1293Because of the difficulty of comparing host keys
1294just by looking at fingerprint strings,
1295there is also support to compare host keys visually,
1296using
1297.Em random art .
1298By setting the
1299.Cm VisualHostKey
1300option to
1301.Dq yes ,
1302a small ASCII graphic gets displayed on every login to a server, no matter
1303if the session itself is interactive or not.
1304By learning the pattern a known server produces, a user can easily
1305find out that the host key has changed when a completely different pattern
1306is displayed.
1307Because these patterns are not unambiguous however, a pattern that looks
1308similar to the pattern remembered only gives a good probability that the
1309host key is the same, not guaranteed proof.
1310.Pp
1311To get a listing of the fingerprints along with their random art for
1312all known hosts, the following command line can be used:
1313.Pp
1314.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1315.Pp
1316If the fingerprint is unknown,
1317an alternative method of verification is available:
1318SSH fingerprints verified by DNS.
1319An additional resource record (RR),
1320SSHFP,
1321is added to a zonefile
1322and the connecting client is able to match the fingerprint
1323with that of the key presented.
1324.Pp
1325In this example, we are connecting a client to a server,
1326.Dq host.example.com .
1327The SSHFP resource records should first be added to the zonefile for
1328host.example.com:
1329.Bd -literal -offset indent
1330$ ssh-keygen -r host.example.com.
1331.Ed
1332.Pp
1333The output lines will have to be added to the zonefile.
1334To check that the zone is answering fingerprint queries:
1335.Pp
1336.Dl $ dig -t SSHFP host.example.com
1337.Pp
1338Finally the client connects:
1339.Bd -literal -offset indent
1340$ ssh -o "VerifyHostKeyDNS ask" host.example.com
1341[...]
1342Matching host key fingerprint found in DNS.
1343Are you sure you want to continue connecting (yes/no)?
1344.Ed
1345.Pp
1346See the
1347.Cm VerifyHostKeyDNS
1348option in
1349.Xr ssh_config 5
1350for more information.
1351.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1352.Nm
1353contains support for Virtual Private Network (VPN) tunnelling
1354using the
1355.Xr tun 4
1356network pseudo-device,
1357allowing two networks to be joined securely.
1358The
1359.Xr sshd_config 5
1360configuration option
1361.Cm PermitTunnel
1362controls whether the server supports this,
1363and at what level (layer 2 or 3 traffic).
1364.Pp
1365The following example would connect client network 10.0.50.0/24
1366with remote network 10.0.99.0/24 using a point-to-point connection
1367from 10.1.1.1 to 10.1.1.2,
1368provided that the SSH server running on the gateway to the remote network,
1369at 192.168.1.15, allows it.
1370.Pp
1371On the client:
1372.Bd -literal -offset indent
1373# ssh -f -w 0:1 192.168.1.15 true
1374# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1375# route add 10.0.99.0/24 10.1.1.2
1376.Ed
1377.Pp
1378On the server:
1379.Bd -literal -offset indent
1380# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1381# route add 10.0.50.0/24 10.1.1.1
1382.Ed
1383.Pp
1384Client access may be more finely tuned via the
1385.Pa /root/.ssh/authorized_keys
1386file (see below) and the
1387.Cm PermitRootLogin
1388server option.
1389The following entry would permit connections on
1390.Xr tun 4
1391device 1 from user
1392.Dq jane
1393and on tun device 2 from user
1394.Dq john ,
1395if
1396.Cm PermitRootLogin
1397is set to
1398.Dq forced-commands-only :
1399.Bd -literal -offset 2n
1400tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1401tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1402.Ed
1403.Pp
1404Since an SSH-based setup entails a fair amount of overhead,
1405it may be more suited to temporary setups,
1406such as for wireless VPNs.
1407More permanent VPNs are better provided by tools such as
1408.Xr ipsecctl 8
1409and
1410.Xr isakmpd 8 .
1411.Sh ENVIRONMENT
1412.Nm
1413will normally set the following environment variables:
1414.Bl -tag -width "SSH_ORIGINAL_COMMAND"
1415.It Ev DISPLAY
1416The
1417.Ev DISPLAY
1418variable indicates the location of the X11 server.
1419It is automatically set by
1420.Nm
1421to point to a value of the form
1422.Dq hostname:n ,
1423where
1424.Dq hostname
1425indicates the host where the shell runs, and
1426.Sq n
1427is an integer \*(Ge 1.
1428.Nm
1429uses this special value to forward X11 connections over the secure
1430channel.
1431The user should normally not set
1432.Ev DISPLAY
1433explicitly, as that
1434will render the X11 connection insecure (and will require the user to
1435manually copy any required authorization cookies).
1436.It Ev HOME
1437Set to the path of the user's home directory.
1438.It Ev LOGNAME
1439Synonym for
1440.Ev USER ;
1441set for compatibility with systems that use this variable.
1442.It Ev MAIL
1443Set to the path of the user's mailbox.
1444.It Ev PATH
1445Set to the default
1446.Ev PATH ,
1447as specified when compiling
1448.Nm .
1449.It Ev SSH_ASKPASS
1450If
1451.Nm
1452needs a passphrase, it will read the passphrase from the current
1453terminal if it was run from a terminal.
1454If
1455.Nm
1456does not have a terminal associated with it but
1457.Ev DISPLAY
1458and
1459.Ev SSH_ASKPASS
1460are set, it will execute the program specified by
1461.Ev SSH_ASKPASS
1462and open an X11 window to read the passphrase.
1463This is particularly useful when calling
1464.Nm
1465from a
1466.Pa .xsession
1467or related script.
1468(Note that on some machines it
1469may be necessary to redirect the input from
1470.Pa /dev/null
1471to make this work.)
1472.It Ev SSH_ASKPASS_REQUIRE
1473Allows further control over the use of an askpass program.
1474If this variable is set to
1475.Dq never
1476then
1477.Nm
1478will never attempt to use one.
1479If it is set to
1480.Dq prefer ,
1481then
1482.Nm
1483will prefer to use the askpass program instead of the TTY when requesting
1484passwords.
1485Finally, if the variable is set to
1486.Dq force ,
1487then the askpass program will be used for all passphrase input regardless
1488of whether
1489.Ev DISPLAY
1490is set.
1491.It Ev SSH_AUTH_SOCK
1492Identifies the path of a
1493.Ux Ns -domain
1494socket used to communicate with the agent.
1495.It Ev SSH_CONNECTION
1496Identifies the client and server ends of the connection.
1497The variable contains
1498four space-separated values: client IP address, client port number,
1499server IP address, and server port number.
1500.It Ev SSH_ORIGINAL_COMMAND
1501This variable contains the original command line if a forced command
1502is executed.
1503It can be used to extract the original arguments.
1504.It Ev SSH_TTY
1505This is set to the name of the tty (path to the device) associated
1506with the current shell or command.
1507If the current session has no tty,
1508this variable is not set.
1509.It Ev SSH_TUNNEL
1510Optionally set by
1511.Xr sshd 8
1512to contain the interface names assigned if tunnel forwarding was
1513requested by the client.
1514.It Ev SSH_USER_AUTH
1515Optionally set by
1516.Xr sshd 8 ,
1517this variable may contain a pathname to a file that lists the authentication
1518methods successfully used when the session was established, including any
1519public keys that were used.
1520.It Ev TZ
1521This variable is set to indicate the present time zone if it
1522was set when the daemon was started (i.e. the daemon passes the value
1523on to new connections).
1524.It Ev USER
1525Set to the name of the user logging in.
1526.El
1527.Pp
1528Additionally,
1529.Nm
1530reads
1531.Pa ~/.ssh/environment ,
1532and adds lines of the format
1533.Dq VARNAME=value
1534to the environment if the file exists and users are allowed to
1535change their environment.
1536For more information, see the
1537.Cm PermitUserEnvironment
1538option in
1539.Xr sshd_config 5 .
1540.Sh FILES
1541.Bl -tag -width Ds -compact
1542.It Pa ~/.rhosts
1543This file is used for host-based authentication (see above).
1544On some machines this file may need to be
1545world-readable if the user's home directory is on an NFS partition,
1546because
1547.Xr sshd 8
1548reads it as root.
1549Additionally, this file must be owned by the user,
1550and must not have write permissions for anyone else.
1551The recommended
1552permission for most machines is read/write for the user, and not
1553accessible by others.
1554.Pp
1555.It Pa ~/.shosts
1556This file is used in exactly the same way as
1557.Pa .rhosts ,
1558but allows host-based authentication without permitting login with
1559rlogin/rsh.
1560.Pp
1561.It Pa ~/.ssh/
1562This directory is the default location for all user-specific configuration
1563and authentication information.
1564There is no general requirement to keep the entire contents of this directory
1565secret, but the recommended permissions are read/write/execute for the user,
1566and not accessible by others.
1567.Pp
1568.It Pa ~/.ssh/authorized_keys
1569Lists the public keys (ECDSA, Ed25519, RSA)
1570that can be used for logging in as this user.
1571The format of this file is described in the
1572.Xr sshd 8
1573manual page.
1574This file is not highly sensitive, but the recommended
1575permissions are read/write for the user, and not accessible by others.
1576.Pp
1577.It Pa ~/.ssh/config
1578This is the per-user configuration file.
1579The file format and configuration options are described in
1580.Xr ssh_config 5 .
1581Because of the potential for abuse, this file must have strict permissions:
1582read/write for the user, and not writable by others.
1583.Pp
1584.It Pa ~/.ssh/environment
1585Contains additional definitions for environment variables; see
1586.Sx ENVIRONMENT ,
1587above.
1588.Pp
1589.It Pa ~/.ssh/id_ecdsa
1590.It Pa ~/.ssh/id_ecdsa_sk
1591.It Pa ~/.ssh/id_ed25519
1592.It Pa ~/.ssh/id_ed25519_sk
1593.It Pa ~/.ssh/id_rsa
1594Contains the private key for authentication.
1595These files
1596contain sensitive data and should be readable by the user but not
1597accessible by others (read/write/execute).
1598.Nm
1599will simply ignore a private key file if it is accessible by others.
1600It is possible to specify a passphrase when
1601generating the key which will be used to encrypt the
1602sensitive part of this file using AES-128.
1603.Pp
1604.It Pa ~/.ssh/id_ecdsa.pub
1605.It Pa ~/.ssh/id_ecdsa_sk.pub
1606.It Pa ~/.ssh/id_ed25519.pub
1607.It Pa ~/.ssh/id_ed25519_sk.pub
1608.It Pa ~/.ssh/id_rsa.pub
1609Contains the public key for authentication.
1610These files are not
1611sensitive and can (but need not) be readable by anyone.
1612.Pp
1613.It Pa ~/.ssh/known_hosts
1614Contains a list of host keys for all hosts the user has logged into
1615that are not already in the systemwide list of known host keys.
1616See
1617.Xr sshd 8
1618for further details of the format of this file.
1619.Pp
1620.It Pa ~/.ssh/rc
1621Commands in this file are executed by
1622.Nm
1623when the user logs in, just before the user's shell (or command) is
1624started.
1625See the
1626.Xr sshd 8
1627manual page for more information.
1628.Pp
1629.It Pa /etc/hosts.equiv
1630This file is for host-based authentication (see above).
1631It should only be writable by root.
1632.Pp
1633.It Pa /etc/shosts.equiv
1634This file is used in exactly the same way as
1635.Pa hosts.equiv ,
1636but allows host-based authentication without permitting login with
1637rlogin/rsh.
1638.Pp
1639.It Pa /etc/ssh/ssh_config
1640Systemwide configuration file.
1641The file format and configuration options are described in
1642.Xr ssh_config 5 .
1643.Pp
1644.It Pa /etc/ssh/ssh_host_ecdsa_key
1645.It Pa /etc/ssh/ssh_host_ed25519_key
1646.It Pa /etc/ssh/ssh_host_rsa_key
1647These files contain the private parts of the host keys
1648and are used for host-based authentication.
1649.Pp
1650.It Pa /etc/ssh/ssh_known_hosts
1651Systemwide list of known host keys.
1652This file should be prepared by the
1653system administrator to contain the public host keys of all machines in the
1654organization.
1655It should be world-readable.
1656See
1657.Xr sshd 8
1658for further details of the format of this file.
1659.Pp
1660.It Pa /etc/ssh/sshrc
1661Commands in this file are executed by
1662.Nm
1663when the user logs in, just before the user's shell (or command) is started.
1664See the
1665.Xr sshd 8
1666manual page for more information.
1667.El
1668.Sh EXIT STATUS
1669.Nm
1670exits with the exit status of the remote command or with 255
1671if an error occurred.
1672.Sh SEE ALSO
1673.Xr scp 1 ,
1674.Xr sftp 1 ,
1675.Xr ssh-add 1 ,
1676.Xr ssh-agent 1 ,
1677.Xr ssh-keygen 1 ,
1678.Xr ssh-keyscan 1 ,
1679.Xr tun 4 ,
1680.Xr ssh_config 5 ,
1681.Xr ssh-keysign 8 ,
1682.Xr sshd 8
1683.Sh STANDARDS
1684.Rs
1685.%A S. Lehtinen
1686.%A C. Lonvick
1687.%D January 2006
1688.%R RFC 4250
1689.%T The Secure Shell (SSH) Protocol Assigned Numbers
1690.Re
1691.Pp
1692.Rs
1693.%A T. Ylonen
1694.%A C. Lonvick
1695.%D January 2006
1696.%R RFC 4251
1697.%T The Secure Shell (SSH) Protocol Architecture
1698.Re
1699.Pp
1700.Rs
1701.%A T. Ylonen
1702.%A C. Lonvick
1703.%D January 2006
1704.%R RFC 4252
1705.%T The Secure Shell (SSH) Authentication Protocol
1706.Re
1707.Pp
1708.Rs
1709.%A T. Ylonen
1710.%A C. Lonvick
1711.%D January 2006
1712.%R RFC 4253
1713.%T The Secure Shell (SSH) Transport Layer Protocol
1714.Re
1715.Pp
1716.Rs
1717.%A T. Ylonen
1718.%A C. Lonvick
1719.%D January 2006
1720.%R RFC 4254
1721.%T The Secure Shell (SSH) Connection Protocol
1722.Re
1723.Pp
1724.Rs
1725.%A J. Schlyter
1726.%A W. Griffin
1727.%D January 2006
1728.%R RFC 4255
1729.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1730.Re
1731.Pp
1732.Rs
1733.%A F. Cusack
1734.%A M. Forssen
1735.%D January 2006
1736.%R RFC 4256
1737.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1738.Re
1739.Pp
1740.Rs
1741.%A J. Galbraith
1742.%A P. Remaker
1743.%D January 2006
1744.%R RFC 4335
1745.%T The Secure Shell (SSH) Session Channel Break Extension
1746.Re
1747.Pp
1748.Rs
1749.%A M. Bellare
1750.%A T. Kohno
1751.%A C. Namprempre
1752.%D January 2006
1753.%R RFC 4344
1754.%T The Secure Shell (SSH) Transport Layer Encryption Modes
1755.Re
1756.Pp
1757.Rs
1758.%A B. Harris
1759.%D January 2006
1760.%R RFC 4345
1761.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1762.Re
1763.Pp
1764.Rs
1765.%A M. Friedl
1766.%A N. Provos
1767.%A W. Simpson
1768.%D March 2006
1769.%R RFC 4419
1770.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1771.Re
1772.Pp
1773.Rs
1774.%A J. Galbraith
1775.%A R. Thayer
1776.%D November 2006
1777.%R RFC 4716
1778.%T The Secure Shell (SSH) Public Key File Format
1779.Re
1780.Pp
1781.Rs
1782.%A D. Stebila
1783.%A J. Green
1784.%D December 2009
1785.%R RFC 5656
1786.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1787.Re
1788.Pp
1789.Rs
1790.%A A. Perrig
1791.%A D. Song
1792.%D 1999
1793.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1794.%T Hash Visualization: a New Technique to improve Real-World Security
1795.Re
1796.Sh AUTHORS
1797OpenSSH is a derivative of the original and free
1798ssh 1.2.12 release by Tatu Ylonen.
1799Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1800Theo de Raadt and Dug Song
1801removed many bugs, re-added newer features and
1802created OpenSSH.
1803Markus Friedl contributed the support for SSH
1804protocol versions 1.5 and 2.0.