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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 1.320 2011/08/02 01:22:11 djm Exp $
37.Dd $Mdocdate: August 2 2011 $
38.Dt SSH 1
39.Os
40.Sh NAME
41.Nm ssh
42.Nd OpenSSH SSH client (remote login program)
43.Sh SYNOPSIS
44.Nm ssh
45.Bk -words
46.Op Fl 1246AaCfgKkMNnqsTtVvXxYy
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 escape_char
51.Op Fl F Ar configfile
52.Op Fl I Ar pkcs11
53.Op Fl i Ar identity_file
54.Op Fl L Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
55.Op Fl l Ar login_name
56.Op Fl m Ar mac_spec
57.Op Fl O Ar ctl_cmd
58.Op Fl o Ar option
59.Op Fl p Ar port
60.Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
61.Op Fl S Ar ctl_path
62.Op Fl W Ar host : Ns Ar port
63.Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
64.Oo Ar user Ns @ Oc Ns Ar hostname
65.Op Ar command
66.Ek
67.Sh DESCRIPTION
68.Nm
69(SSH client) is a program for logging into a remote machine and for
70executing commands on a remote machine.
71It is intended to replace rlogin and rsh,
72and provide secure encrypted communications between
73two untrusted hosts over an insecure network.
74X11 connections and arbitrary TCP ports
75can also be forwarded over the secure channel.
76.Pp
77.Nm
78connects and logs into the specified
79.Ar hostname
80(with optional
81.Ar user
82name).
83The user must prove
84his/her identity to the remote machine using one of several methods
85depending on the protocol version used (see below).
86.Pp
87If
88.Ar command
89is specified,
90it is executed on the remote host instead of a login shell.
91.Pp
92The options are as follows:
93.Bl -tag -width Ds
94.It Fl 1
95Forces
96.Nm
97to try protocol version 1 only.
98.It Fl 2
99Forces
100.Nm
101to try protocol version 2 only.
102.It Fl 4
103Forces
104.Nm
105to use IPv4 addresses only.
106.It Fl 6
107Forces
108.Nm
109to use IPv6 addresses only.
110.It Fl A
111Enables forwarding of the authentication agent connection.
112This can also be specified on a per-host basis in a configuration file.
113.Pp
114Agent forwarding should be enabled with caution.
115Users with the ability to bypass file permissions on the remote host
116(for the agent's
117.Ux Ns -domain
118socket) can access the local agent through the forwarded connection.
119An attacker cannot obtain key material from the agent,
120however they can perform operations on the keys that enable them to
121authenticate using the identities loaded into the agent.
122.It Fl a
123Disables forwarding of the authentication agent connection.
124.It Fl b Ar bind_address
125Use
126.Ar bind_address
127on the local machine as the source address
128of the connection.
129Only useful on systems with more than one address.
130.It Fl C
131Requests compression of all data (including stdin, stdout, stderr, and
132data for forwarded X11 and TCP connections).
133The compression algorithm is the same used by
134.Xr gzip 1 ,
135and the
136.Dq level
137can be controlled by the
138.Cm CompressionLevel
139option for protocol version 1.
140Compression is desirable on modem lines and other
141slow connections, but will only slow down things on fast networks.
142The default value can be set on a host-by-host basis in the
143configuration files; see the
144.Cm Compression
145option.
146.It Fl c Ar cipher_spec
147Selects the cipher specification for encrypting the session.
148.Pp
149Protocol version 1 allows specification of a single cipher.
150The supported values are
151.Dq 3des ,
152.Dq blowfish ,
153and
154.Dq des .
155.Ar 3des
156(triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
157It is believed to be secure.
158.Ar blowfish
159is a fast block cipher; it appears very secure and is much faster than
160.Ar 3des .
161.Ar des
162is only supported in the
163.Nm
164client for interoperability with legacy protocol 1 implementations
165that do not support the
166.Ar 3des
167cipher.
168Its use is strongly discouraged due to cryptographic weaknesses.
169The default is
170.Dq 3des .
171.Pp
172For protocol version 2,
173.Ar cipher_spec
174is a comma-separated list of ciphers
175listed in order of preference.
176See the
177.Cm Ciphers
178keyword in
179.Xr ssh_config 5
180for more information.
181.It Fl D Xo
182.Sm off
183.Oo Ar bind_address : Oc
184.Ar port
185.Sm on
186.Xc
187Specifies a local
188.Dq dynamic
189application-level port forwarding.
190This works by allocating a socket to listen to
191.Ar port
192on the local side, optionally bound to the specified
193.Ar bind_address .
194Whenever a connection is made to this port, the
195connection is forwarded over the secure channel, and the application
196protocol is then used to determine where to connect to from the
197remote machine.
198Currently the SOCKS4 and SOCKS5 protocols are supported, and
199.Nm
200will act as a SOCKS server.
201Only root can forward privileged ports.
202Dynamic port forwardings can also be specified in the configuration file.
203.Pp
204IPv6 addresses can be specified by enclosing the address in square brackets.
205Only the superuser can forward privileged ports.
206By default, the local port is bound in accordance with the
207.Cm GatewayPorts
208setting.
209However, an explicit
210.Ar bind_address
211may be used to bind the connection to a specific address.
212The
213.Ar bind_address
214of
215.Dq localhost
216indicates that the listening port be bound for local use only, while an
217empty address or
218.Sq *
219indicates that the port should be available from all interfaces.
220.It Fl e Ar escape_char
221Sets the escape character for sessions with a pty (default:
222.Ql ~ ) .
223The escape character is only recognized at the beginning of a line.
224The escape character followed by a dot
225.Pq Ql \&.
226closes the connection;
227followed by control-Z suspends the connection;
228and followed by itself sends the escape character once.
229Setting the character to
230.Dq none
231disables any escapes and makes the session fully transparent.
232.It Fl F Ar configfile
233Specifies an alternative per-user configuration file.
234If a configuration file is given on the command line,
235the system-wide configuration file
236.Pq Pa /etc/ssh/ssh_config
237will be ignored.
238The default for the per-user configuration file is
239.Pa ~/.ssh/config .
240.It Fl f
241Requests
242.Nm
243to go to background just before command execution.
244This is useful if
245.Nm
246is going to ask for passwords or passphrases, but the user
247wants it in the background.
248This implies
249.Fl n .
250The recommended way to start X11 programs at a remote site is with
251something like
252.Ic ssh -f host xterm .
253.Pp
254If the
255.Cm ExitOnForwardFailure
256configuration option is set to
257.Dq yes ,
258then a client started with
259.Fl f
260will wait for all remote port forwards to be successfully established
261before placing itself in the background.
262.It Fl g
263Allows remote hosts to connect to local forwarded ports.
264.It Fl I Ar pkcs11
265Specify the PKCS#11 shared library
266.Nm
267should use to communicate with a PKCS#11 token providing the user's
268private RSA key.
269.It Fl i Ar identity_file
270Selects a file from which the identity (private key) for
271public key authentication is read.
272The default is
273.Pa ~/.ssh/identity
274for protocol version 1, and
275.Pa ~/.ssh/id_dsa ,
276.Pa ~/.ssh/id_ecdsa
277and
278.Pa ~/.ssh/id_rsa
279for protocol version 2.
280Identity files may also be specified on
281a per-host basis in the configuration file.
282It is possible to have multiple
283.Fl i
284options (and multiple identities specified in
285configuration files).
286.Nm
287will also try to load certificate information from the filename obtained
288by appending
289.Pa -cert.pub
290to identity filenames.
291.It Fl K
292Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
293credentials to the server.
294.It Fl k
295Disables forwarding (delegation) of GSSAPI credentials to the server.
296.It Fl L Xo
297.Sm off
298.Oo Ar bind_address : Oc
299.Ar port : host : hostport
300.Sm on
301.Xc
302Specifies that the given port on the local (client) host is to be
303forwarded to the given host and port on the remote side.
304This works by allocating a socket to listen to
305.Ar port
306on the local side, optionally bound to the specified
307.Ar bind_address .
308Whenever a connection is made to this port, the
309connection is forwarded over the secure channel, and a connection is
310made to
311.Ar host
312port
313.Ar hostport
314from the remote machine.
315Port forwardings can also be specified in the configuration file.
316IPv6 addresses can be specified by enclosing the address in square brackets.
317Only the superuser can forward privileged ports.
318By default, the local port is bound in accordance with the
319.Cm GatewayPorts
320setting.
321However, an explicit
322.Ar bind_address
323may be used to bind the connection to a specific address.
324The
325.Ar bind_address
326of
327.Dq localhost
328indicates that the listening port be bound for local use only, while an
329empty address or
330.Sq *
331indicates that the port should be available from all interfaces.
332.It Fl l Ar login_name
333Specifies the user to log in as on the remote machine.
334This also may be specified on a per-host basis in the configuration file.
335.It Fl M
336Places the
337.Nm
338client into
339.Dq master
340mode for connection sharing.
341Multiple
342.Fl M
343options places
344.Nm
345into
346.Dq master
347mode with confirmation required before slave connections are accepted.
348Refer to the description of
349.Cm ControlMaster
350in
351.Xr ssh_config 5
352for details.
353.It Fl m Ar mac_spec
354Additionally, for protocol version 2 a comma-separated list of MAC
355(message authentication code) algorithms can
356be specified in order of preference.
357See the
358.Cm MACs
359keyword for more information.
360.It Fl N
361Do not execute a remote command.
362This is useful for just forwarding ports
363(protocol version 2 only).
364.It Fl n
365Redirects stdin from
366.Pa /dev/null
367(actually, prevents reading from stdin).
368This must be used when
369.Nm
370is run in the background.
371A common trick is to use this to run X11 programs on a remote machine.
372For example,
373.Ic ssh -n shadows.cs.hut.fi emacs &
374will start an emacs on shadows.cs.hut.fi, and the X11
375connection will be automatically forwarded over an encrypted channel.
376The
377.Nm
378program will be put in the background.
379(This does not work if
380.Nm
381needs to ask for a password or passphrase; see also the
382.Fl f
383option.)
384.It Fl O Ar ctl_cmd
385Control an active connection multiplexing master process.
386When the
387.Fl O
388option is specified, the
389.Ar ctl_cmd
390argument is interpreted and passed to the master process.
391Valid commands are:
392.Dq check
393(check that the master process is running),
394.Dq forward
395(request forwardings without command execution),
396.Dq exit
397(request the master to exit), and
398.Dq stop
399(request the master to stop accepting further multiplexing requests).
400.It Fl o Ar option
401Can be used to give options in the format used in the configuration file.
402This is useful for specifying options for which there is no separate
403command-line flag.
404For full details of the options listed below, and their possible values, see
405.Xr ssh_config 5 .
406.Pp
407.Bl -tag -width Ds -offset indent -compact
408.It AddressFamily
409.It BatchMode
410.It BindAddress
411.It ChallengeResponseAuthentication
412.It CheckHostIP
413.It Cipher
414.It Ciphers
415.It ClearAllForwardings
416.It Compression
417.It CompressionLevel
418.It ConnectionAttempts
419.It ConnectTimeout
420.It ControlMaster
421.It ControlPath
422.It DynamicForward
423.It EscapeChar
424.It ExitOnForwardFailure
425.It ForwardAgent
426.It ForwardX11
427.It ForwardX11Trusted
428.It GatewayPorts
429.It GlobalKnownHostsFile
430.It GSSAPIAuthentication
431.It GSSAPIDelegateCredentials
432.It HashKnownHosts
433.It Host
434.It HostbasedAuthentication
435.It HostKeyAlgorithms
436.It HostKeyAlias
437.It HostName
438.It IdentityFile
439.It IdentitiesOnly
440.It IPQoS
441.It KbdInteractiveDevices
442.It KexAlgorithms
443.It LocalCommand
444.It LocalForward
445.It LogLevel
446.It MACs
447.It NoHostAuthenticationForLocalhost
448.It NumberOfPasswordPrompts
449.It PasswordAuthentication
450.It PermitLocalCommand
451.It PKCS11Provider
452.It Port
453.It PreferredAuthentications
454.It Protocol
455.It ProxyCommand
456.It PubkeyAuthentication
457.It RekeyLimit
458.It RemoteForward
459.It RequestTTY
460.It RhostsRSAAuthentication
461.It RSAAuthentication
462.It SendEnv
463.It ServerAliveInterval
464.It ServerAliveCountMax
465.It StrictHostKeyChecking
466.It TCPKeepAlive
467.It Tunnel
468.It TunnelDevice
469.It UsePrivilegedPort
470.It User
471.It UserKnownHostsFile
472.It VerifyHostKeyDNS
473.It VisualHostKey
474.It XAuthLocation
475.El
476.It Fl p Ar port
477Port to connect to on the remote host.
478This can be specified on a
479per-host basis in the configuration file.
480.It Fl q
481Quiet mode.
482Causes most warning and diagnostic messages to be suppressed.
483.It Fl R Xo
484.Sm off
485.Oo Ar bind_address : Oc
486.Ar port : host : hostport
487.Sm on
488.Xc
489Specifies that the given port on the remote (server) host is to be
490forwarded to the given host and port on the local side.
491This works by allocating a socket to listen to
492.Ar port
493on the remote side, and whenever a connection is made to this port, the
494connection is forwarded over the secure channel, and a connection is
495made to
496.Ar host
497port
498.Ar hostport
499from the local machine.
500.Pp
501Port forwardings can also be specified in the configuration file.
502Privileged ports can be forwarded only when
503logging in as root on the remote machine.
504IPv6 addresses can be specified by enclosing the address in square braces.
505.Pp
506By default, the listening socket on the server will be bound to the loopback
507interface only.
508This may be overridden by specifying a
509.Ar bind_address .
510An empty
511.Ar bind_address ,
512or the address
513.Ql * ,
514indicates that the remote socket should listen on all interfaces.
515Specifying a remote
516.Ar bind_address
517will only succeed if the server's
518.Cm GatewayPorts
519option is enabled (see
520.Xr sshd_config 5 ) .
521.Pp
522If the
523.Ar port
524argument is
525.Ql 0 ,
526the listen port will be dynamically allocated on the server and reported
527to the client at run time.
528When used together with
529.Ic -O forward
530the allocated port will be printed to the standard output.
531.It Fl S Ar ctl_path
532Specifies the location of a control socket for connection sharing,
533or the string
534.Dq none
535to disable connection sharing.
536Refer to the description of
537.Cm ControlPath
538and
539.Cm ControlMaster
540in
541.Xr ssh_config 5
542for details.
543.It Fl s
544May be used to request invocation of a subsystem on the remote system.
545Subsystems are a feature of the SSH2 protocol which facilitate the use
546of SSH as a secure transport for other applications (eg.\&
547.Xr sftp 1 ) .
548The subsystem is specified as the remote command.
549.It Fl T
550Disable pseudo-tty allocation.
551.It Fl t
552Force pseudo-tty allocation.
553This can be used to execute arbitrary
554screen-based programs on a remote machine, which can be very useful,
555e.g. when implementing menu services.
556Multiple
557.Fl t
558options force tty allocation, even if
559.Nm
560has no local tty.
561.It Fl V
562Display the version number and exit.
563.It Fl v
564Verbose mode.
565Causes
566.Nm
567to print debugging messages about its progress.
568This is helpful in
569debugging connection, authentication, and configuration problems.
570Multiple
571.Fl v
572options increase the verbosity.
573The maximum is 3.
574.It Fl W Ar host : Ns Ar port
575Requests that standard input and output on the client be forwarded to
576.Ar host
577on
578.Ar port
579over the secure channel.
580Implies
581.Fl N ,
582.Fl T ,
583.Cm ExitOnForwardFailure
584and
585.Cm ClearAllForwardings
586and works with Protocol version 2 only.
587.It Fl w Xo
588.Ar local_tun Ns Op : Ns Ar remote_tun
589.Xc
590Requests
591tunnel
592device forwarding with the specified
593.Xr tun 4
594devices between the client
595.Pq Ar local_tun
596and the server
597.Pq Ar remote_tun .
598.Pp
599The devices may be specified by numerical ID or the keyword
600.Dq any ,
601which uses the next available tunnel device.
602If
603.Ar remote_tun
604is not specified, it defaults to
605.Dq any .
606See also the
607.Cm Tunnel
608and
609.Cm TunnelDevice
610directives in
611.Xr ssh_config 5 .
612If the
613.Cm Tunnel
614directive is unset, it is set to the default tunnel mode, which is
615.Dq point-to-point .
616.It Fl X
617Enables X11 forwarding.
618This can also be specified on a per-host basis in a configuration file.
619.Pp
620X11 forwarding should be enabled with caution.
621Users with the ability to bypass file permissions on the remote host
622(for the user's X authorization database)
623can access the local X11 display through the forwarded connection.
624An attacker may then be able to perform activities such as keystroke monitoring.
625.Pp
626For this reason, X11 forwarding is subjected to X11 SECURITY extension
627restrictions by default.
628Please refer to the
629.Nm
630.Fl Y
631option and the
632.Cm ForwardX11Trusted
633directive in
634.Xr ssh_config 5
635for more information.
636.It Fl x
637Disables X11 forwarding.
638.It Fl Y
639Enables trusted X11 forwarding.
640Trusted X11 forwardings are not subjected to the X11 SECURITY extension
641controls.
642.It Fl y
643Send log information using the
644.Xr syslog 3
645system module.
646By default this information is sent to stderr.
647.El
648.Pp
649.Nm
650may additionally obtain configuration data from
651a per-user configuration file and a system-wide configuration file.
652The file format and configuration options are described in
653.Xr ssh_config 5 .
654.Sh AUTHENTICATION
655The OpenSSH SSH client supports SSH protocols 1 and 2.
656The default is to use protocol 2 only,
657though this can be changed via the
658.Cm Protocol
659option in
660.Xr ssh_config 5
661or the
662.Fl 1
663and
664.Fl 2
665options (see above).
666Both protocols support similar authentication methods,
667but protocol 2 is the default since
668it provides additional mechanisms for confidentiality
669(the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
670and integrity (hmac-md5, hmac-sha1,
671hmac-sha2-256, hmac-sha2-512,
672umac-64, hmac-ripemd160).
673Protocol 1 lacks a strong mechanism for ensuring the
674integrity of the connection.
675.Pp
676The methods available for authentication are:
677GSSAPI-based authentication,
678host-based authentication,
679public key authentication,
680challenge-response authentication,
681and password authentication.
682Authentication methods are tried in the order specified above,
683though protocol 2 has a configuration option to change the default order:
684.Cm PreferredAuthentications .
685.Pp
686Host-based authentication works as follows:
687If the machine the user logs in from is listed in
688.Pa /etc/hosts.equiv
689or
690.Pa /etc/shosts.equiv
691on the remote machine, and the user names are
692the same on both sides, or if the files
693.Pa ~/.rhosts
694or
695.Pa ~/.shosts
696exist in the user's home directory on the
697remote machine and contain a line containing the name of the client
698machine and the name of the user on that machine, the user is
699considered for login.
700Additionally, the server
701.Em must
702be able to verify the client's
703host key (see the description of
704.Pa /etc/ssh/ssh_known_hosts
705and
706.Pa ~/.ssh/known_hosts ,
707below)
708for login to be permitted.
709This authentication method closes security holes due to IP
710spoofing, DNS spoofing, and routing spoofing.
711[Note to the administrator:
712.Pa /etc/hosts.equiv ,
713.Pa ~/.rhosts ,
714and the rlogin/rsh protocol in general, are inherently insecure and should be
715disabled if security is desired.]
716.Pp
717Public key authentication works as follows:
718The scheme is based on public-key cryptography,
719using cryptosystems
720where encryption and decryption are done using separate keys,
721and it is unfeasible to derive the decryption key from the encryption key.
722The idea is that each user creates a public/private
723key pair for authentication purposes.
724The server knows the public key, and only the user knows the private key.
725.Nm
726implements public key authentication protocol automatically,
727using one of the DSA, ECDSA or RSA algorithms.
728Protocol 1 is restricted to using only RSA keys,
729but protocol 2 may use any.
730The
731.Sx HISTORY
732section of
733.Xr ssl 8
734contains a brief discussion of the DSA and RSA algorithms.
735.Pp
736The file
737.Pa ~/.ssh/authorized_keys
738lists the public keys that are permitted for logging in.
739When the user logs in, the
740.Nm
741program tells the server which key pair it would like to use for
742authentication.
743The client proves that it has access to the private key
744and the server checks that the corresponding public key
745is authorized to accept the account.
746.Pp
747The user creates his/her key pair by running
748.Xr ssh-keygen 1 .
749This stores the private key in
750.Pa ~/.ssh/identity
751(protocol 1),
752.Pa ~/.ssh/id_dsa
753(protocol 2 DSA),
754.Pa ~/.ssh/id_ecdsa
755(protocol 2 ECDSA),
756or
757.Pa ~/.ssh/id_rsa
758(protocol 2 RSA)
759and stores the public key in
760.Pa ~/.ssh/identity.pub
761(protocol 1),
762.Pa ~/.ssh/id_dsa.pub
763(protocol 2 DSA),
764.Pa ~/.ssh/id_ecdsa.pub
765(protocol 2 ECDSA),
766or
767.Pa ~/.ssh/id_rsa.pub
768(protocol 2 RSA)
769in the user's home directory.
770The user should then copy the public key
771to
772.Pa ~/.ssh/authorized_keys
773in his/her home directory on the remote machine.
774The
775.Pa authorized_keys
776file corresponds to the conventional
777.Pa ~/.rhosts
778file, and has one key
779per line, though the lines can be very long.
780After this, the user can log in without giving the password.
781.Pp
782A variation on public key authentication
783is available in the form of certificate authentication:
784instead of a set of public/private keys,
785signed certificates are used.
786This has the advantage that a single trusted certification authority
787can be used in place of many public/private keys.
788See the
789.Sx CERTIFICATES
790section of
791.Xr ssh-keygen 1
792for more information.
793.Pp
794The most convenient way to use public key or certificate authentication
795may be with an authentication agent.
796See
797.Xr ssh-agent 1
798for more information.
799.Pp
800Challenge-response authentication works as follows:
801The server sends an arbitrary
802.Qq challenge
803text, and prompts for a response.
804Protocol 2 allows multiple challenges and responses;
805protocol 1 is restricted to just one challenge/response.
806Examples of challenge-response authentication include
807BSD Authentication (see
808.Xr login.conf 5 )
809and PAM (some non-OpenBSD systems).
810.Pp
811Finally, if other authentication methods fail,
812.Nm
813prompts the user for a password.
814The password is sent to the remote
815host for checking; however, since all communications are encrypted,
816the password cannot be seen by someone listening on the network.
817.Pp
818.Nm
819automatically maintains and checks a database containing
820identification for all hosts it has ever been used with.
821Host keys are stored in
822.Pa ~/.ssh/known_hosts
823in the user's home directory.
824Additionally, the file
825.Pa /etc/ssh/ssh_known_hosts
826is automatically checked for known hosts.
827Any new hosts are automatically added to the user's file.
828If a host's identification ever changes,
829.Nm
830warns about this and disables password authentication to prevent
831server spoofing or man-in-the-middle attacks,
832which could otherwise be used to circumvent the encryption.
833The
834.Cm StrictHostKeyChecking
835option can be used to control logins to machines whose
836host key is not known or has changed.
837.Pp
838When the user's identity has been accepted by the server, the server
839either executes the given command, or logs into the machine and gives
840the user a normal shell on the remote machine.
841All communication with
842the remote command or shell will be automatically encrypted.
843.Pp
844If a pseudo-terminal has been allocated (normal login session), the
845user may use the escape characters noted below.
846.Pp
847If no pseudo-tty has been allocated,
848the session is transparent and can be used to reliably transfer binary data.
849On most systems, setting the escape character to
850.Dq none
851will also make the session transparent even if a tty is used.
852.Pp
853The session terminates when the command or shell on the remote
854machine exits and all X11 and TCP connections have been closed.
855.Sh ESCAPE CHARACTERS
856When a pseudo-terminal has been requested,
857.Nm
858supports a number of functions through the use of an escape character.
859.Pp
860A single tilde character can be sent as
861.Ic ~~
862or by following the tilde by a character other than those described below.
863The escape character must always follow a newline to be interpreted as
864special.
865The escape character can be changed in configuration files using the
866.Cm EscapeChar
867configuration directive or on the command line by the
868.Fl e
869option.
870.Pp
871The supported escapes (assuming the default
872.Ql ~ )
873are:
874.Bl -tag -width Ds
875.It Cm ~.
876Disconnect.
877.It Cm ~^Z
878Background
879.Nm .
880.It Cm ~#
881List forwarded connections.
882.It Cm ~&
883Background
884.Nm
885at logout when waiting for forwarded connection / X11 sessions to terminate.
886.It Cm ~?
887Display a list of escape characters.
888.It Cm ~B
889Send a BREAK to the remote system
890(only useful for SSH protocol version 2 and if the peer supports it).
891.It Cm ~C
892Open command line.
893Currently this allows the addition of port forwardings using the
894.Fl L ,
895.Fl R
896and
897.Fl D
898options (see above).
899It also allows the cancellation of existing remote port-forwardings
900using
901.Sm off
902.Fl KR Oo Ar bind_address : Oc Ar port .
903.Sm on
904.Ic !\& Ns Ar command
905allows the user to execute a local command if the
906.Ic PermitLocalCommand
907option is enabled in
908.Xr ssh_config 5 .
909Basic help is available, using the
910.Fl h
911option.
912.It Cm ~R
913Request rekeying of the connection
914(only useful for SSH protocol version 2 and if the peer supports it).
915.El
916.Sh TCP FORWARDING
917Forwarding of arbitrary TCP connections over the secure channel can
918be specified either on the command line or in a configuration file.
919One possible application of TCP forwarding is a secure connection to a
920mail server; another is going through firewalls.
921.Pp
922In the example below, we look at encrypting communication between
923an IRC client and server, even though the IRC server does not directly
924support encrypted communications.
925This works as follows:
926the user connects to the remote host using
927.Nm ,
928specifying a port to be used to forward connections
929to the remote server.
930After that it is possible to start the service which is to be encrypted
931on the client machine,
932connecting to the same local port,
933and
934.Nm
935will encrypt and forward the connection.
936.Pp
937The following example tunnels an IRC session from client machine
938.Dq 127.0.0.1
939(localhost)
940to remote server
941.Dq server.example.com :
942.Bd -literal -offset 4n
943$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
944$ irc -c '#users' -p 1234 pinky 127.0.0.1
945.Ed
946.Pp
947This tunnels a connection to IRC server
948.Dq server.example.com ,
949joining channel
950.Dq #users ,
951nickname
952.Dq pinky ,
953using port 1234.
954It doesn't matter which port is used,
955as long as it's greater than 1023
956(remember, only root can open sockets on privileged ports)
957and doesn't conflict with any ports already in use.
958The connection is forwarded to port 6667 on the remote server,
959since that's the standard port for IRC services.
960.Pp
961The
962.Fl f
963option backgrounds
964.Nm
965and the remote command
966.Dq sleep 10
967is specified to allow an amount of time
968(10 seconds, in the example)
969to start the service which is to be tunnelled.
970If no connections are made within the time specified,
971.Nm
972will exit.
973.Sh X11 FORWARDING
974If the
975.Cm ForwardX11
976variable is set to
977.Dq yes
978(or see the description of the
979.Fl X ,
980.Fl x ,
981and
982.Fl Y
983options above)
984and the user is using X11 (the
985.Ev DISPLAY
986environment variable is set), the connection to the X11 display is
987automatically forwarded to the remote side in such a way that any X11
988programs started from the shell (or command) will go through the
989encrypted channel, and the connection to the real X server will be made
990from the local machine.
991The user should not manually set
992.Ev DISPLAY .
993Forwarding of X11 connections can be
994configured on the command line or in configuration files.
995.Pp
996The
997.Ev DISPLAY
998value set by
999.Nm
1000will point to the server machine, but with a display number greater than zero.
1001This is normal, and happens because
1002.Nm
1003creates a
1004.Dq proxy
1005X server on the server machine for forwarding the
1006connections over the encrypted channel.
1007.Pp
1008.Nm
1009will also automatically set up Xauthority data on the server machine.
1010For this purpose, it will generate a random authorization cookie,
1011store it in Xauthority on the server, and verify that any forwarded
1012connections carry this cookie and replace it by the real cookie when
1013the connection is opened.
1014The real authentication cookie is never
1015sent to the server machine (and no cookies are sent in the plain).
1016.Pp
1017If the
1018.Cm ForwardAgent
1019variable is set to
1020.Dq yes
1021(or see the description of the
1022.Fl A
1023and
1024.Fl a
1025options above) and
1026the user is using an authentication agent, the connection to the agent
1027is automatically forwarded to the remote side.
1028.Sh VERIFYING HOST KEYS
1029When connecting to a server for the first time,
1030a fingerprint of the server's public key is presented to the user
1031(unless the option
1032.Cm StrictHostKeyChecking
1033has been disabled).
1034Fingerprints can be determined using
1035.Xr ssh-keygen 1 :
1036.Pp
1037.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1038.Pp
1039If the fingerprint is already known, it can be matched
1040and the key can be accepted or rejected.
1041Because of the difficulty of comparing host keys
1042just by looking at hex strings,
1043there is also support to compare host keys visually,
1044using
1045.Em random art .
1046By setting the
1047.Cm VisualHostKey
1048option to
1049.Dq yes ,
1050a small ASCII graphic gets displayed on every login to a server, no matter
1051if the session itself is interactive or not.
1052By learning the pattern a known server produces, a user can easily
1053find out that the host key has changed when a completely different pattern
1054is displayed.
1055Because these patterns are not unambiguous however, a pattern that looks
1056similar to the pattern remembered only gives a good probability that the
1057host key is the same, not guaranteed proof.
1058.Pp
1059To get a listing of the fingerprints along with their random art for
1060all known hosts, the following command line can be used:
1061.Pp
1062.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1063.Pp
1064If the fingerprint is unknown,
1065an alternative method of verification is available:
1066SSH fingerprints verified by DNS.
1067An additional resource record (RR),
1068SSHFP,
1069is added to a zonefile
1070and the connecting client is able to match the fingerprint
1071with that of the key presented.
1072.Pp
1073In this example, we are connecting a client to a server,
1074.Dq host.example.com .
1075The SSHFP resource records should first be added to the zonefile for
1076host.example.com:
1077.Bd -literal -offset indent
1078$ ssh-keygen -r host.example.com.
1079.Ed
1080.Pp
1081The output lines will have to be added to the zonefile.
1082To check that the zone is answering fingerprint queries:
1083.Pp
1084.Dl $ dig -t SSHFP host.example.com
1085.Pp
1086Finally the client connects:
1087.Bd -literal -offset indent
1088$ ssh -o "VerifyHostKeyDNS ask" host.example.com
1089[...]
1090Matching host key fingerprint found in DNS.
1091Are you sure you want to continue connecting (yes/no)?
1092.Ed
1093.Pp
1094See the
1095.Cm VerifyHostKeyDNS
1096option in
1097.Xr ssh_config 5
1098for more information.
1099.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1100.Nm
1101contains support for Virtual Private Network (VPN) tunnelling
1102using the
1103.Xr tun 4
1104network pseudo-device,
1105allowing two networks to be joined securely.
1106The
1107.Xr sshd_config 5
1108configuration option
1109.Cm PermitTunnel
1110controls whether the server supports this,
1111and at what level (layer 2 or 3 traffic).
1112.Pp
1113The following example would connect client network 10.0.50.0/24
1114with remote network 10.0.99.0/24 using a point-to-point connection
1115from 10.1.1.1 to 10.1.1.2,
1116provided that the SSH server running on the gateway to the remote network,
1117at 192.168.1.15, allows it.
1118.Pp
1119On the client:
1120.Bd -literal -offset indent
1121# ssh -f -w 0:1 192.168.1.15 true
1122# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1123# route add 10.0.99.0/24 10.1.1.2
1124.Ed
1125.Pp
1126On the server:
1127.Bd -literal -offset indent
1128# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1129# route add 10.0.50.0/24 10.1.1.1
1130.Ed
1131.Pp
1132Client access may be more finely tuned via the
1133.Pa /root/.ssh/authorized_keys
1134file (see below) and the
1135.Cm PermitRootLogin
1136server option.
1137The following entry would permit connections on
1138.Xr tun 4
1139device 1 from user
1140.Dq jane
1141and on tun device 2 from user
1142.Dq john ,
1143if
1144.Cm PermitRootLogin
1145is set to
1146.Dq forced-commands-only :
1147.Bd -literal -offset 2n
1148tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1149tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1150.Ed
1151.Pp
1152Since an SSH-based setup entails a fair amount of overhead,
1153it may be more suited to temporary setups,
1154such as for wireless VPNs.
1155More permanent VPNs are better provided by tools such as
1156.Xr ipsecctl 8
1157and
1158.Xr isakmpd 8 .
1159.Sh ENVIRONMENT
1160.Nm
1161will normally set the following environment variables:
1162.Bl -tag -width "SSH_ORIGINAL_COMMAND"
1163.It Ev DISPLAY
1164The
1165.Ev DISPLAY
1166variable indicates the location of the X11 server.
1167It is automatically set by
1168.Nm
1169to point to a value of the form
1170.Dq hostname:n ,
1171where
1172.Dq hostname
1173indicates the host where the shell runs, and
1174.Sq n
1175is an integer \*(Ge 1.
1176.Nm
1177uses this special value to forward X11 connections over the secure
1178channel.
1179The user should normally not set
1180.Ev DISPLAY
1181explicitly, as that
1182will render the X11 connection insecure (and will require the user to
1183manually copy any required authorization cookies).
1184.It Ev HOME
1185Set to the path of the user's home directory.
1186.It Ev LOGNAME
1187Synonym for
1188.Ev USER ;
1189set for compatibility with systems that use this variable.
1190.It Ev MAIL
1191Set to the path of the user's mailbox.
1192.It Ev PATH
1193Set to the default
1194.Ev PATH ,
1195as specified when compiling
1196.Nm .
1197.It Ev SSH_ASKPASS
1198If
1199.Nm
1200needs a passphrase, it will read the passphrase from the current
1201terminal if it was run from a terminal.
1202If
1203.Nm
1204does not have a terminal associated with it but
1205.Ev DISPLAY
1206and
1207.Ev SSH_ASKPASS
1208are set, it will execute the program specified by
1209.Ev SSH_ASKPASS
1210and open an X11 window to read the passphrase.
1211This is particularly useful when calling
1212.Nm
1213from a
1214.Pa .xsession
1215or related script.
1216(Note that on some machines it
1217may be necessary to redirect the input from
1218.Pa /dev/null
1219to make this work.)
1220.It Ev SSH_AUTH_SOCK
1221Identifies the path of a
1222.Ux Ns -domain
1223socket used to communicate with the agent.
1224.It Ev SSH_CONNECTION
1225Identifies the client and server ends of the connection.
1226The variable contains
1227four space-separated values: client IP address, client port number,
1228server IP address, and server port number.
1229.It Ev SSH_ORIGINAL_COMMAND
1230This variable contains the original command line if a forced command
1231is executed.
1232It can be used to extract the original arguments.
1233.It Ev SSH_TTY
1234This is set to the name of the tty (path to the device) associated
1235with the current shell or command.
1236If the current session has no tty,
1237this variable is not set.
1238.It Ev TZ
1239This variable is set to indicate the present time zone if it
1240was set when the daemon was started (i.e. the daemon passes the value
1241on to new connections).
1242.It Ev USER
1243Set to the name of the user logging in.
1244.El
1245.Pp
1246Additionally,
1247.Nm
1248reads
1249.Pa ~/.ssh/environment ,
1250and adds lines of the format
1251.Dq VARNAME=value
1252to the environment if the file exists and users are allowed to
1253change their environment.
1254For more information, see the
1255.Cm PermitUserEnvironment
1256option in
1257.Xr sshd_config 5 .
1258.Sh FILES
1259.Bl -tag -width Ds -compact
1260.It Pa ~/.rhosts
1261This file is used for host-based authentication (see above).
1262On some machines this file may need to be
1263world-readable if the user's home directory is on an NFS partition,
1264because
1265.Xr sshd 8
1266reads it as root.
1267Additionally, this file must be owned by the user,
1268and must not have write permissions for anyone else.
1269The recommended
1270permission for most machines is read/write for the user, and not
1271accessible by others.
1272.Pp
1273.It Pa ~/.shosts
1274This file is used in exactly the same way as
1275.Pa .rhosts ,
1276but allows host-based authentication without permitting login with
1277rlogin/rsh.
1278.Pp
1279.It Pa ~/.ssh/
1280This directory is the default location for all user-specific configuration
1281and authentication information.
1282There is no general requirement to keep the entire contents of this directory
1283secret, but the recommended permissions are read/write/execute for the user,
1284and not accessible by others.
1285.Pp
1286.It Pa ~/.ssh/authorized_keys
1287Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in as
1288this user.
1289The format of this file is described in the
1290.Xr sshd 8
1291manual page.
1292This file is not highly sensitive, but the recommended
1293permissions are read/write for the user, and not accessible by others.
1294.Pp
1295.It Pa ~/.ssh/config
1296This is the per-user configuration file.
1297The file format and configuration options are described in
1298.Xr ssh_config 5 .
1299Because of the potential for abuse, this file must have strict permissions:
1300read/write for the user, and not accessible by others.
1301.Pp
1302.It Pa ~/.ssh/environment
1303Contains additional definitions for environment variables; see
1304.Sx ENVIRONMENT ,
1305above.
1306.Pp
1307.It Pa ~/.ssh/identity
1308.It Pa ~/.ssh/id_dsa
1309.It Pa ~/.ssh/id_ecdsa
1310.It Pa ~/.ssh/id_rsa
1311Contains the private key for authentication.
1312These files
1313contain sensitive data and should be readable by the user but not
1314accessible by others (read/write/execute).
1315.Nm
1316will simply ignore a private key file if it is accessible by others.
1317It is possible to specify a passphrase when
1318generating the key which will be used to encrypt the
1319sensitive part of this file using 3DES.
1320.Pp
1321.It Pa ~/.ssh/identity.pub
1322.It Pa ~/.ssh/id_dsa.pub
1323.It Pa ~/.ssh/id_ecdsa.pub
1324.It Pa ~/.ssh/id_rsa.pub
1325Contains the public key for authentication.
1326These files are not
1327sensitive and can (but need not) be readable by anyone.
1328.Pp
1329.It Pa ~/.ssh/known_hosts
1330Contains a list of host keys for all hosts the user has logged into
1331that are not already in the systemwide list of known host keys.
1332See
1333.Xr sshd 8
1334for further details of the format of this file.
1335.Pp
1336.It Pa ~/.ssh/rc
1337Commands in this file are executed by
1338.Nm
1339when the user logs in, just before the user's shell (or command) is
1340started.
1341See the
1342.Xr sshd 8
1343manual page for more information.
1344.Pp
1345.It Pa /etc/hosts.equiv
1346This file is for host-based authentication (see above).
1347It should only be writable by root.
1348.Pp
1349.It Pa /etc/shosts.equiv
1350This file is used in exactly the same way as
1351.Pa hosts.equiv ,
1352but allows host-based authentication without permitting login with
1353rlogin/rsh.
1354.Pp
1355.It Pa /etc/ssh/ssh_config
1356Systemwide configuration file.
1357The file format and configuration options are described in
1358.Xr ssh_config 5 .
1359.Pp
1360.It Pa /etc/ssh/ssh_host_key
1361.It Pa /etc/ssh/ssh_host_dsa_key
1362.It Pa /etc/ssh/ssh_host_ecdsa_key
1363.It Pa /etc/ssh/ssh_host_rsa_key
1364These three files contain the private parts of the host keys
1365and are used for host-based authentication.
1366If protocol version 1 is used,
1367.Nm
1368must be setuid root, since the host key is readable only by root.
1369For protocol version 2,
1370.Nm
1371uses
1372.Xr ssh-keysign 8
1373to access the host keys,
1374eliminating the requirement that
1375.Nm
1376be setuid root when host-based authentication is used.
1377By default
1378.Nm
1379is not setuid root.
1380.Pp
1381.It Pa /etc/ssh/ssh_known_hosts
1382Systemwide list of known host keys.
1383This file should be prepared by the
1384system administrator to contain the public host keys of all machines in the
1385organization.
1386It should be world-readable.
1387See
1388.Xr sshd 8
1389for further details of the format of this file.
1390.Pp
1391.It Pa /etc/ssh/sshrc
1392Commands in this file are executed by
1393.Nm
1394when the user logs in, just before the user's shell (or command) is started.
1395See the
1396.Xr sshd 8
1397manual page for more information.
1398.El
1399.Sh EXIT STATUS
1400.Nm
1401exits with the exit status of the remote command or with 255
1402if an error occurred.
1403.Sh SEE ALSO
1404.Xr scp 1 ,
1405.Xr sftp 1 ,
1406.Xr ssh-add 1 ,
1407.Xr ssh-agent 1 ,
1408.Xr ssh-keygen 1 ,
1409.Xr ssh-keyscan 1 ,
1410.Xr tun 4 ,
1411.Xr hosts.equiv 5 ,
1412.Xr ssh_config 5 ,
1413.Xr ssh-keysign 8 ,
1414.Xr sshd 8
1415.Rs
1416.%R RFC 4250
1417.%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1418.%D 2006
1419.Re
1420.Rs
1421.%R RFC 4251
1422.%T "The Secure Shell (SSH) Protocol Architecture"
1423.%D 2006
1424.Re
1425.Rs
1426.%R RFC 4252
1427.%T "The Secure Shell (SSH) Authentication Protocol"
1428.%D 2006
1429.Re
1430.Rs
1431.%R RFC 4253
1432.%T "The Secure Shell (SSH) Transport Layer Protocol"
1433.%D 2006
1434.Re
1435.Rs
1436.%R RFC 4254
1437.%T "The Secure Shell (SSH) Connection Protocol"
1438.%D 2006
1439.Re
1440.Rs
1441.%R RFC 4255
1442.%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1443.%D 2006
1444.Re
1445.Rs
1446.%R RFC 4256
1447.%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1448.%D 2006
1449.Re
1450.Rs
1451.%R RFC 4335
1452.%T "The Secure Shell (SSH) Session Channel Break Extension"
1453.%D 2006
1454.Re
1455.Rs
1456.%R RFC 4344
1457.%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1458.%D 2006
1459.Re
1460.Rs
1461.%R RFC 4345
1462.%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1463.%D 2006
1464.Re
1465.Rs
1466.%R RFC 4419
1467.%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1468.%D 2006
1469.Re
1470.Rs
1471.%R RFC 4716
1472.%T "The Secure Shell (SSH) Public Key File Format"
1473.%D 2006
1474.Re
1475.Rs
1476.%R RFC 5656
1477.%T "Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer"
1478.%D 2009
1479.Re
1480.Rs
1481.%T "Hash Visualization: a New Technique to improve Real-World Security"
1482.%A A. Perrig
1483.%A D. Song
1484.%D 1999
1485.%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1486.Re
1487.Sh AUTHORS
1488OpenSSH is a derivative of the original and free
1489ssh 1.2.12 release by Tatu Ylonen.
1490Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1491Theo de Raadt and Dug Song
1492removed many bugs, re-added newer features and
1493created OpenSSH.
1494Markus Friedl contributed the support for SSH
1495protocol versions 1.5 and 2.0.
1496