39.Dt SSH 1
40.Os
41.Sh NAME
42.Nm ssh
43.Nd OpenSSH SSH client (remote login program)
44.Sh SYNOPSIS
45.Nm ssh
46.Bk -words
47.Op Fl 1246AaCfgKkMNnqsTtVvXxYy
48.Op Fl b Ar bind_address
49.Op Fl c Ar cipher_spec
50.Op Fl D Oo Ar bind_address : Oc Ns Ar port
51.Op Fl E Ar log_file
52.Op Fl e Ar escape_char
53.Op Fl F Ar configfile
54.Op Fl I Ar pkcs11
55.Op Fl i Ar identity_file
56.Op Fl L Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
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 port
62.Op Fl Q Cm cipher | cipher-auth | mac | kex | key
63.Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
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.Oo Ar user Ns @ Oc Ns Ar hostname
68.Op Ar command
69.Ek
70.Sh DESCRIPTION
71.Nm
72(SSH client) is a program for logging into a remote machine and for
73executing commands on a remote machine.
74It is intended to replace rlogin and rsh,
75and provide secure encrypted communications between
76two untrusted hosts over an insecure network.
77X11 connections and arbitrary TCP ports
78can also be forwarded over the secure channel.
79.Pp
80.Nm
81connects and logs into the specified
82.Ar hostname
83(with optional
84.Ar user
85name).
86The user must prove
87his/her identity to the remote machine using one of several methods
88depending on the protocol version used (see below).
89.Pp
90If
91.Ar command
92is specified,
93it is executed on the remote host instead of a login shell.
94.Pp
95The options are as follows:
96.Bl -tag -width Ds
97.It Fl 1
98Forces
99.Nm
100to try protocol version 1 only.
101.It Fl 2
102Forces
103.Nm
104to try protocol version 2 only.
105.It Fl 4
106Forces
107.Nm
108to use IPv4 addresses only.
109.It Fl 6
110Forces
111.Nm
112to use IPv6 addresses only.
113.It Fl A
114Enables forwarding of the authentication agent connection.
115This can also be specified on a per-host basis in a configuration file.
116.Pp
117Agent forwarding should be enabled with caution.
118Users with the ability to bypass file permissions on the remote host
119(for the agent's
120.Ux Ns -domain
121socket) can access the local agent through the forwarded connection.
122An attacker cannot obtain key material from the agent,
123however they can perform operations on the keys that enable them to
124authenticate using the identities loaded into the agent.
125.It Fl a
126Disables forwarding of the authentication agent connection.
127.It Fl b Ar bind_address
128Use
129.Ar bind_address
130on the local machine as the source address
131of the connection.
132Only useful on systems with more than one address.
133.It Fl C
134Requests compression of all data (including stdin, stdout, stderr, and
135data for forwarded X11 and TCP connections).
136The compression algorithm is the same used by
137.Xr gzip 1 ,
138and the
139.Dq level
140can be controlled by the
141.Cm CompressionLevel
142option for protocol version 1.
143Compression is desirable on modem lines and other
144slow connections, but will only slow down things on fast networks.
145The default value can be set on a host-by-host basis in the
146configuration files; see the
147.Cm Compression
148option.
149.It Fl c Ar cipher_spec
150Selects the cipher specification for encrypting the session.
151.Pp
152Protocol version 1 allows specification of a single cipher.
153The supported values are
154.Dq 3des ,
155.Dq blowfish ,
156and
157.Dq des .
158.Ar 3des
159(triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
160It is believed to be secure.
161.Ar blowfish
162is a fast block cipher; it appears very secure and is much faster than
163.Ar 3des .
164.Ar des
165is only supported in the
166.Nm
167client for interoperability with legacy protocol 1 implementations
168that do not support the
169.Ar 3des
170cipher.
171Its use is strongly discouraged due to cryptographic weaknesses.
172The default is
173.Dq 3des .
174.Pp
175For protocol version 2,
176.Ar cipher_spec
177is a comma-separated list of ciphers
178listed in order of preference.
179See the
180.Cm Ciphers
181keyword in
182.Xr ssh_config 5
183for more information.
184.It Fl D Xo
185.Sm off
186.Oo Ar bind_address : Oc
187.Ar port
188.Sm on
189.Xc
190Specifies a local
191.Dq dynamic
192application-level port forwarding.
193This works by allocating a socket to listen to
194.Ar port
195on the local side, optionally bound to the specified
196.Ar bind_address .
197Whenever a connection is made to this port, the
198connection is forwarded over the secure channel, and the application
199protocol is then used to determine where to connect to from the
200remote machine.
201Currently the SOCKS4 and SOCKS5 protocols are supported, and
202.Nm
203will act as a SOCKS server.
204Only root can forward privileged ports.
205Dynamic port forwardings can also be specified in the configuration file.
206.Pp
207IPv6 addresses can be specified by enclosing the address in square brackets.
208Only the superuser can forward privileged ports.
209By default, the local port is bound in accordance with the
210.Cm GatewayPorts
211setting.
212However, an explicit
213.Ar bind_address
214may be used to bind the connection to a specific address.
215The
216.Ar bind_address
217of
218.Dq localhost
219indicates that the listening port be bound for local use only, while an
220empty address or
221.Sq *
222indicates that the port should be available from all interfaces.
223.It Fl E Ar log_file
224Append debug logs to
225.Ar log_file
226instead of standard error.
227.It Fl e Ar escape_char
228Sets the escape character for sessions with a pty (default:
229.Ql ~ ) .
230The escape character is only recognized at the beginning of a line.
231The escape character followed by a dot
232.Pq Ql \&.
233closes the connection;
234followed by control-Z suspends the connection;
235and followed by itself sends the escape character once.
236Setting the character to
237.Dq none
238disables any escapes and makes the session fully transparent.
239.It Fl F Ar configfile
240Specifies an alternative per-user configuration file.
241If a configuration file is given on the command line,
242the system-wide configuration file
243.Pq Pa /etc/ssh/ssh_config
244will be ignored.
245The default for the per-user configuration file is
246.Pa ~/.ssh/config .
247.It Fl f
248Requests
249.Nm
250to go to background just before command execution.
251This is useful if
252.Nm
253is going to ask for passwords or passphrases, but the user
254wants it in the background.
255This implies
256.Fl n .
257The recommended way to start X11 programs at a remote site is with
258something like
259.Ic ssh -f host xterm .
260.Pp
261If the
262.Cm ExitOnForwardFailure
263configuration option is set to
264.Dq yes ,
265then a client started with
266.Fl f
267will wait for all remote port forwards to be successfully established
268before placing itself in the background.
269.It Fl g
270Allows remote hosts to connect to local forwarded ports.
271.It Fl I Ar pkcs11
272Specify the PKCS#11 shared library
273.Nm
274should use to communicate with a PKCS#11 token providing the user's
275private RSA key.
276.It Fl i Ar identity_file
277Selects a file from which the identity (private key) for
278public key authentication is read.
279The default is
280.Pa ~/.ssh/identity
281for protocol version 1, and
282.Pa ~/.ssh/id_dsa ,
283.Pa ~/.ssh/id_ecdsa ,
284.Pa ~/.ssh/id_ed25519
285and
286.Pa ~/.ssh/id_rsa
287for protocol version 2.
288Identity files may also be specified on
289a per-host basis in the configuration file.
290It is possible to have multiple
291.Fl i
292options (and multiple identities specified in
293configuration files).
294.Nm
295will also try to load certificate information from the filename obtained
296by appending
297.Pa -cert.pub
298to identity filenames.
299.It Fl K
300Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
301credentials to the server.
302.It Fl k
303Disables forwarding (delegation) of GSSAPI credentials to the server.
304.It Fl L Xo
305.Sm off
306.Oo Ar bind_address : Oc
307.Ar port : host : hostport
308.Sm on
309.Xc
310Specifies that the given port on the local (client) host is to be
311forwarded to the given host and port on the remote side.
312This works by allocating a socket to listen to
313.Ar port
314on the local side, optionally bound to the specified
315.Ar bind_address .
316Whenever a connection is made to this port, the
317connection is forwarded over the secure channel, and a connection is
318made to
319.Ar host
320port
321.Ar hostport
322from the remote machine.
323Port forwardings can also be specified in the configuration file.
324IPv6 addresses can be specified by enclosing the address in square brackets.
325Only the superuser can forward privileged ports.
326By default, the local port is bound in accordance with the
327.Cm GatewayPorts
328setting.
329However, an explicit
330.Ar bind_address
331may be used to bind the connection to a specific address.
332The
333.Ar bind_address
334of
335.Dq localhost
336indicates that the listening port be bound for local use only, while an
337empty address or
338.Sq *
339indicates that the port should be available from all interfaces.
340.It Fl l Ar login_name
341Specifies the user to log in as on the remote machine.
342This also may be specified on a per-host basis in the configuration file.
343.It Fl M
344Places the
345.Nm
346client into
347.Dq master
348mode for connection sharing.
349Multiple
350.Fl M
351options places
352.Nm
353into
354.Dq master
355mode with confirmation required before slave connections are accepted.
356Refer to the description of
357.Cm ControlMaster
358in
359.Xr ssh_config 5
360for details.
361.It Fl m Ar mac_spec
362Additionally, for protocol version 2 a comma-separated list of MAC
363(message authentication code) algorithms can
364be specified in order of preference.
365See the
366.Cm MACs
367keyword for more information.
368.It Fl N
369Do not execute a remote command.
370This is useful for just forwarding ports
371(protocol version 2 only).
372.It Fl n
373Redirects stdin from
374.Pa /dev/null
375(actually, prevents reading from stdin).
376This must be used when
377.Nm
378is run in the background.
379A common trick is to use this to run X11 programs on a remote machine.
380For example,
381.Ic ssh -n shadows.cs.hut.fi emacs &
382will start an emacs on shadows.cs.hut.fi, and the X11
383connection will be automatically forwarded over an encrypted channel.
384The
385.Nm
386program will be put in the background.
387(This does not work if
388.Nm
389needs to ask for a password or passphrase; see also the
390.Fl f
391option.)
392.It Fl O Ar ctl_cmd
393Control an active connection multiplexing master process.
394When the
395.Fl O
396option is specified, the
397.Ar ctl_cmd
398argument is interpreted and passed to the master process.
399Valid commands are:
400.Dq check
401(check that the master process is running),
402.Dq forward
403(request forwardings without command execution),
404.Dq cancel
405(cancel forwardings),
406.Dq exit
407(request the master to exit), and
408.Dq stop
409(request the master to stop accepting further multiplexing requests).
410.It Fl o Ar option
411Can be used to give options in the format used in the configuration file.
412This is useful for specifying options for which there is no separate
413command-line flag.
414For full details of the options listed below, and their possible values, see
415.Xr ssh_config 5 .
416.Pp
417.Bl -tag -width Ds -offset indent -compact
418.It AddressFamily
419.It BatchMode
420.It BindAddress
421.It CanonicalDomains
422.It CanonicalizeFallbackLocal
423.It CanonicalizeHostname
424.It CanonicalizeMaxDots
425.It CanonicalizePermittedCNAMEs
426.It ChallengeResponseAuthentication
427.It CheckHostIP
428.It Cipher
429.It Ciphers
430.It ClearAllForwardings
431.It Compression
432.It CompressionLevel
433.It ConnectionAttempts
434.It ConnectTimeout
435.It ControlMaster
436.It ControlPath
437.It ControlPersist
438.It DynamicForward
439.It EscapeChar
440.It ExitOnForwardFailure
441.It ForwardAgent
442.It ForwardX11
443.It ForwardX11Timeout
444.It ForwardX11Trusted
445.It GatewayPorts
446.It GlobalKnownHostsFile
447.It GSSAPIAuthentication
448.It GSSAPIDelegateCredentials
449.It HashKnownHosts
450.It Host
451.It HostbasedAuthentication
452.It HostKeyAlgorithms
453.It HostKeyAlias
454.It HostName
455.It IdentityFile
456.It IdentitiesOnly
457.It IPQoS
458.It KbdInteractiveAuthentication
459.It KbdInteractiveDevices
460.It KexAlgorithms
461.It LocalCommand
462.It LocalForward
463.It LogLevel
464.It MACs
465.It Match
466.It NoHostAuthenticationForLocalhost
467.It NumberOfPasswordPrompts
468.It PasswordAuthentication
469.It PermitLocalCommand
470.It PKCS11Provider
471.It Port
472.It PreferredAuthentications
473.It Protocol
474.It ProxyCommand
475.It ProxyUseFdpass
476.It PubkeyAuthentication
477.It RekeyLimit
478.It RemoteForward
479.It RequestTTY
480.It RhostsRSAAuthentication
481.It RSAAuthentication
482.It SendEnv
483.It ServerAliveInterval
484.It ServerAliveCountMax
485.It StrictHostKeyChecking
486.It TCPKeepAlive
487.It Tunnel
488.It TunnelDevice
489.It UsePrivilegedPort
490.It User
491.It UserKnownHostsFile
492.It VerifyHostKeyDNS
493.It VersionAddendum
494.It VisualHostKey
495.It XAuthLocation
496.El
497.It Fl p Ar port
498Port to connect to on the remote host.
499This can be specified on a
500per-host basis in the configuration file.
501.It Fl Q Cm cipher | cipher-auth | mac | kex | key
502Queries
503.Nm
504for the algorithms supported for the specified version 2.
505The available features are:
506.Ar cipher
507(supported symmetric ciphers),
508.Ar cipher-auth
509(supported symmetric ciphers that support authenticated encryption),
510.Ar mac
511(supported message integrity codes),
512.Ar kex
513(key exchange algorithms),
514.Ar key
515(key types).
516.It Fl q
517Quiet mode.
518Causes most warning and diagnostic messages to be suppressed.
519.It Fl R Xo
520.Sm off
521.Oo Ar bind_address : Oc
522.Ar port : host : hostport
523.Sm on
524.Xc
525Specifies that the given port on the remote (server) host is to be
526forwarded to the given host and port on the local side.
527This works by allocating a socket to listen to
528.Ar port
529on the remote side, and whenever a connection is made to this port, the
530connection is forwarded over the secure channel, and a connection is
531made to
532.Ar host
533port
534.Ar hostport
535from the local machine.
536.Pp
537Port forwardings can also be specified in the configuration file.
538Privileged ports can be forwarded only when
539logging in as root on the remote machine.
540IPv6 addresses can be specified by enclosing the address in square brackets.
541.Pp
542By default, the listening socket on the server will be bound to the loopback
543interface only.
544This may be overridden by specifying a
545.Ar bind_address .
546An empty
547.Ar bind_address ,
548or the address
549.Ql * ,
550indicates that the remote socket should listen on all interfaces.
551Specifying a remote
552.Ar bind_address
553will only succeed if the server's
554.Cm GatewayPorts
555option is enabled (see
556.Xr sshd_config 5 ) .
557.Pp
558If the
559.Ar port
560argument is
561.Ql 0 ,
562the listen port will be dynamically allocated on the server and reported
563to the client at run time.
564When used together with
565.Ic -O forward
566the allocated port will be printed to the standard output.
567.It Fl S Ar ctl_path
568Specifies the location of a control socket for connection sharing,
569or the string
570.Dq none
571to disable connection sharing.
572Refer to the description of
573.Cm ControlPath
574and
575.Cm ControlMaster
576in
577.Xr ssh_config 5
578for details.
579.It Fl s
580May be used to request invocation of a subsystem on the remote system.
581Subsystems are a feature of the SSH2 protocol which facilitate the use
582of SSH as a secure transport for other applications (eg.\&
583.Xr sftp 1 ) .
584The subsystem is specified as the remote command.
585.It Fl T
586Disable pseudo-tty allocation.
587.It Fl t
588Force pseudo-tty allocation.
589This can be used to execute arbitrary
590screen-based programs on a remote machine, which can be very useful,
591e.g. when implementing menu services.
592Multiple
593.Fl t
594options force tty allocation, even if
595.Nm
596has no local tty.
597.It Fl V
598Display the version number and exit.
599.It Fl v
600Verbose mode.
601Causes
602.Nm
603to print debugging messages about its progress.
604This is helpful in
605debugging connection, authentication, and configuration problems.
606Multiple
607.Fl v
608options increase the verbosity.
609The maximum is 3.
610.It Fl W Ar host : Ns Ar port
611Requests that standard input and output on the client be forwarded to
612.Ar host
613on
614.Ar port
615over the secure channel.
616Implies
617.Fl N ,
618.Fl T ,
619.Cm ExitOnForwardFailure
620and
621.Cm ClearAllForwardings .
622Works with Protocol version 2 only.
623.It Fl w Xo
624.Ar local_tun Ns Op : Ns Ar remote_tun
625.Xc
626Requests
627tunnel
628device forwarding with the specified
629.Xr tun 4
630devices between the client
631.Pq Ar local_tun
632and the server
633.Pq Ar remote_tun .
634.Pp
635The devices may be specified by numerical ID or the keyword
636.Dq any ,
637which uses the next available tunnel device.
638If
639.Ar remote_tun
640is not specified, it defaults to
641.Dq any .
642See also the
643.Cm Tunnel
644and
645.Cm TunnelDevice
646directives in
647.Xr ssh_config 5 .
648If the
649.Cm Tunnel
650directive is unset, it is set to the default tunnel mode, which is
651.Dq point-to-point .
652.It Fl X
653Enables X11 forwarding.
654This can also be specified on a per-host basis in a configuration file.
655.Pp
656X11 forwarding should be enabled with caution.
657Users with the ability to bypass file permissions on the remote host
658(for the user's X authorization database)
659can access the local X11 display through the forwarded connection.
660An attacker may then be able to perform activities such as keystroke monitoring.
661.Pp
662For this reason, X11 forwarding is subjected to X11 SECURITY extension
663restrictions by default.
664Please refer to the
665.Nm
666.Fl Y
667option and the
668.Cm ForwardX11Trusted
669directive in
670.Xr ssh_config 5
671for more information.
672.It Fl x
673Disables X11 forwarding.
674.It Fl Y
675Enables trusted X11 forwarding.
676Trusted X11 forwardings are not subjected to the X11 SECURITY extension
677controls.
678.It Fl y
679Send log information using the
680.Xr syslog 3
681system module.
682By default this information is sent to stderr.
683.El
684.Pp
685.Nm
686may additionally obtain configuration data from
687a per-user configuration file and a system-wide configuration file.
688The file format and configuration options are described in
689.Xr ssh_config 5 .
690.Sh AUTHENTICATION
691The OpenSSH SSH client supports SSH protocols 1 and 2.
692The default is to use protocol 2 only,
693though this can be changed via the
694.Cm Protocol
695option in
696.Xr ssh_config 5
697or the
698.Fl 1
699and
700.Fl 2
701options (see above).
702Both protocols support similar authentication methods,
703but protocol 2 is the default since
704it provides additional mechanisms for confidentiality
705(the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
706and integrity (hmac-md5, hmac-sha1,
707hmac-sha2-256, hmac-sha2-512,
708umac-64, umac-128, hmac-ripemd160).
709Protocol 1 lacks a strong mechanism for ensuring the
710integrity of the connection.
711.Pp
712The methods available for authentication are:
713GSSAPI-based authentication,
714host-based authentication,
715public key authentication,
716challenge-response authentication,
717and password authentication.
718Authentication methods are tried in the order specified above,
719though protocol 2 has a configuration option to change the default order:
720.Cm PreferredAuthentications .
721.Pp
722Host-based authentication works as follows:
723If the machine the user logs in from is listed in
724.Pa /etc/hosts.equiv
725or
726.Pa /etc/shosts.equiv
727on the remote machine, and the user names are
728the same on both sides, or if the files
729.Pa ~/.rhosts
730or
731.Pa ~/.shosts
732exist in the user's home directory on the
733remote machine and contain a line containing the name of the client
734machine and the name of the user on that machine, the user is
735considered for login.
736Additionally, the server
737.Em must
738be able to verify the client's
739host key (see the description of
740.Pa /etc/ssh/ssh_known_hosts
741and
742.Pa ~/.ssh/known_hosts ,
743below)
744for login to be permitted.
745This authentication method closes security holes due to IP
746spoofing, DNS spoofing, and routing spoofing.
747[Note to the administrator:
748.Pa /etc/hosts.equiv ,
749.Pa ~/.rhosts ,
750and the rlogin/rsh protocol in general, are inherently insecure and should be
751disabled if security is desired.]
752.Pp
753Public key authentication works as follows:
754The scheme is based on public-key cryptography,
755using cryptosystems
756where encryption and decryption are done using separate keys,
757and it is unfeasible to derive the decryption key from the encryption key.
758The idea is that each user creates a public/private
759key pair for authentication purposes.
760The server knows the public key, and only the user knows the private key.
761.Nm
762implements public key authentication protocol automatically,
763using one of the DSA, ECDSA, ED25519 or RSA algorithms.
764Protocol 1 is restricted to using only RSA keys,
765but protocol 2 may use any.
766The HISTORY section of
767.Xr ssl 8
768contains a brief discussion of the DSA and RSA algorithms.
769.Pp
770The file
771.Pa ~/.ssh/authorized_keys
772lists the public keys that are permitted for logging in.
773When the user logs in, the
774.Nm
775program tells the server which key pair it would like to use for
776authentication.
777The client proves that it has access to the private key
778and the server checks that the corresponding public key
779is authorized to accept the account.
780.Pp
781The user creates his/her key pair by running
782.Xr ssh-keygen 1 .
783This stores the private key in
784.Pa ~/.ssh/identity
785(protocol 1),
786.Pa ~/.ssh/id_dsa
787(protocol 2 DSA),
788.Pa ~/.ssh/id_ecdsa
789(protocol 2 ECDSA),
790.Pa ~/.ssh/id_ed25519
791(protocol 2 ED25519),
792or
793.Pa ~/.ssh/id_rsa
794(protocol 2 RSA)
795and stores the public key in
796.Pa ~/.ssh/identity.pub
797(protocol 1),
798.Pa ~/.ssh/id_dsa.pub
799(protocol 2 DSA),
800.Pa ~/.ssh/id_ecdsa.pub
801(protocol 2 ECDSA),
802.Pa ~/.ssh/id_ed25519.pub
803(protocol 2 ED25519),
804or
805.Pa ~/.ssh/id_rsa.pub
806(protocol 2 RSA)
807in the user's home directory.
808The user should then copy the public key
809to
810.Pa ~/.ssh/authorized_keys
811in his/her home directory on the remote machine.
812The
813.Pa authorized_keys
814file corresponds to the conventional
815.Pa ~/.rhosts
816file, and has one key
817per line, though the lines can be very long.
818After this, the user can log in without giving the password.
819.Pp
820A variation on public key authentication
821is available in the form of certificate authentication:
822instead of a set of public/private keys,
823signed certificates are used.
824This has the advantage that a single trusted certification authority
825can be used in place of many public/private keys.
826See the CERTIFICATES section of
827.Xr ssh-keygen 1
828for more information.
829.Pp
830The most convenient way to use public key or certificate authentication
831may be with an authentication agent.
832See
833.Xr ssh-agent 1
834for more information.
835.Pp
836Challenge-response authentication works as follows:
837The server sends an arbitrary
838.Qq challenge
839text, and prompts for a response.
840Protocol 2 allows multiple challenges and responses;
841protocol 1 is restricted to just one challenge/response.
842Examples of challenge-response authentication include
843.Bx
844Authentication (see
845.Xr login.conf 5 )
846and PAM (some
847.Pf non- Ox
848systems).
849.Pp
850Finally, if other authentication methods fail,
851.Nm
852prompts the user for a password.
853The password is sent to the remote
854host for checking; however, since all communications are encrypted,
855the password cannot be seen by someone listening on the network.
856.Pp
857.Nm
858automatically maintains and checks a database containing
859identification for all hosts it has ever been used with.
860Host keys are stored in
861.Pa ~/.ssh/known_hosts
862in the user's home directory.
863Additionally, the file
864.Pa /etc/ssh/ssh_known_hosts
865is automatically checked for known hosts.
866Any new hosts are automatically added to the user's file.
867If a host's identification ever changes,
868.Nm
869warns about this and disables password authentication to prevent
870server spoofing or man-in-the-middle attacks,
871which could otherwise be used to circumvent the encryption.
872The
873.Cm StrictHostKeyChecking
874option can be used to control logins to machines whose
875host key is not known or has changed.
876.Pp
877When the user's identity has been accepted by the server, the server
878either executes the given command, or logs into the machine and gives
879the user a normal shell on the remote machine.
880All communication with
881the remote command or shell will be automatically encrypted.
882.Pp
883If a pseudo-terminal has been allocated (normal login session), the
884user may use the escape characters noted below.
885.Pp
886If no pseudo-tty has been allocated,
887the session is transparent and can be used to reliably transfer binary data.
888On most systems, setting the escape character to
889.Dq none
890will also make the session transparent even if a tty is used.
891.Pp
892The session terminates when the command or shell on the remote
893machine exits and all X11 and TCP connections have been closed.
894.Sh ESCAPE CHARACTERS
895When a pseudo-terminal has been requested,
896.Nm
897supports a number of functions through the use of an escape character.
898.Pp
899A single tilde character can be sent as
900.Ic ~~
901or by following the tilde by a character other than those described below.
902The escape character must always follow a newline to be interpreted as
903special.
904The escape character can be changed in configuration files using the
905.Cm EscapeChar
906configuration directive or on the command line by the
907.Fl e
908option.
909.Pp
910The supported escapes (assuming the default
911.Ql ~ )
912are:
913.Bl -tag -width Ds
914.It Cm ~.
915Disconnect.
916.It Cm ~^Z
917Background
918.Nm .
919.It Cm ~#
920List forwarded connections.
921.It Cm ~&
922Background
923.Nm
924at logout when waiting for forwarded connection / X11 sessions to terminate.
925.It Cm ~?
926Display a list of escape characters.
927.It Cm ~B
928Send a BREAK to the remote system
929(only useful for SSH protocol version 2 and if the peer supports it).
930.It Cm ~C
931Open command line.
932Currently this allows the addition of port forwardings using the
933.Fl L ,
934.Fl R
935and
936.Fl D
937options (see above).
938It also allows the cancellation of existing port-forwardings
939with
940.Sm off
941.Fl KL Oo Ar bind_address : Oc Ar port
942.Sm on
943for local,
944.Sm off
945.Fl KR Oo Ar bind_address : Oc Ar port
946.Sm on
947for remote and
948.Sm off
949.Fl KD Oo Ar bind_address : Oc Ar port
950.Sm on
951for dynamic port-forwardings.
952.Ic !\& Ns Ar command
953allows the user to execute a local command if the
954.Ic PermitLocalCommand
955option is enabled in
956.Xr ssh_config 5 .
957Basic help is available, using the
958.Fl h
959option.
960.It Cm ~R
961Request rekeying of the connection
962(only useful for SSH protocol version 2 and if the peer supports it).
963.It Cm ~V
964Decrease the verbosity
965.Pq Ic LogLevel
966when errors are being written to stderr.
967.It Cm ~v
968Increase the verbosity
969.Pq Ic LogLevel
970when errors are being written to stderr.
971.El
972.Sh TCP FORWARDING
973Forwarding of arbitrary TCP connections over the secure channel can
974be specified either on the command line or in a configuration file.
975One possible application of TCP forwarding is a secure connection to a
976mail server; another is going through firewalls.
977.Pp
978In the example below, we look at encrypting communication between
979an IRC client and server, even though the IRC server does not directly
980support encrypted communications.
981This works as follows:
982the user connects to the remote host using
983.Nm ,
984specifying a port to be used to forward connections
985to the remote server.
986After that it is possible to start the service which is to be encrypted
987on the client machine,
988connecting to the same local port,
989and
990.Nm
991will encrypt and forward the connection.
992.Pp
993The following example tunnels an IRC session from client machine
994.Dq 127.0.0.1
995(localhost)
996to remote server
997.Dq server.example.com :
998.Bd -literal -offset 4n
999$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1000$ irc -c '#users' -p 1234 pinky 127.0.0.1
1001.Ed
1002.Pp
1003This tunnels a connection to IRC server
1004.Dq server.example.com ,
1005joining channel
1006.Dq #users ,
1007nickname
1008.Dq pinky ,
1009using port 1234.
1010It doesn't matter which port is used,
1011as long as it's greater than 1023
1012(remember, only root can open sockets on privileged ports)
1013and doesn't conflict with any ports already in use.
1014The connection is forwarded to port 6667 on the remote server,
1015since that's the standard port for IRC services.
1016.Pp
1017The
1018.Fl f
1019option backgrounds
1020.Nm
1021and the remote command
1022.Dq sleep 10
1023is specified to allow an amount of time
1024(10 seconds, in the example)
1025to start the service which is to be tunnelled.
1026If no connections are made within the time specified,
1027.Nm
1028will exit.
1029.Sh X11 FORWARDING
1030If the
1031.Cm ForwardX11
1032variable is set to
1033.Dq yes
1034(or see the description of the
1035.Fl X ,
1036.Fl x ,
1037and
1038.Fl Y
1039options above)
1040and the user is using X11 (the
1041.Ev DISPLAY
1042environment variable is set), the connection to the X11 display is
1043automatically forwarded to the remote side in such a way that any X11
1044programs started from the shell (or command) will go through the
1045encrypted channel, and the connection to the real X server will be made
1046from the local machine.
1047The user should not manually set
1048.Ev DISPLAY .
1049Forwarding of X11 connections can be
1050configured on the command line or in configuration files.
1051.Pp
1052The
1053.Ev DISPLAY
1054value set by
1055.Nm
1056will point to the server machine, but with a display number greater than zero.
1057This is normal, and happens because
1058.Nm
1059creates a
1060.Dq proxy
1061X server on the server machine for forwarding the
1062connections over the encrypted channel.
1063.Pp
1064.Nm
1065will also automatically set up Xauthority data on the server machine.
1066For this purpose, it will generate a random authorization cookie,
1067store it in Xauthority on the server, and verify that any forwarded
1068connections carry this cookie and replace it by the real cookie when
1069the connection is opened.
1070The real authentication cookie is never
1071sent to the server machine (and no cookies are sent in the plain).
1072.Pp
1073If the
1074.Cm ForwardAgent
1075variable is set to
1076.Dq yes
1077(or see the description of the
1078.Fl A
1079and
1080.Fl a
1081options above) and
1082the user is using an authentication agent, the connection to the agent
1083is automatically forwarded to the remote side.
1084.Sh VERIFYING HOST KEYS
1085When connecting to a server for the first time,
1086a fingerprint of the server's public key is presented to the user
1087(unless the option
1088.Cm StrictHostKeyChecking
1089has been disabled).
1090Fingerprints can be determined using
1091.Xr ssh-keygen 1 :
1092.Pp
1093.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1094.Pp
1095If the fingerprint is already known, it can be matched
1096and the key can be accepted or rejected.
1097Because of the difficulty of comparing host keys
1098just by looking at hex strings,
1099there is also support to compare host keys visually,
1100using
1101.Em random art .
1102By setting the
1103.Cm VisualHostKey
1104option to
1105.Dq yes ,
1106a small ASCII graphic gets displayed on every login to a server, no matter
1107if the session itself is interactive or not.
1108By learning the pattern a known server produces, a user can easily
1109find out that the host key has changed when a completely different pattern
1110is displayed.
1111Because these patterns are not unambiguous however, a pattern that looks
1112similar to the pattern remembered only gives a good probability that the
1113host key is the same, not guaranteed proof.
1114.Pp
1115To get a listing of the fingerprints along with their random art for
1116all known hosts, the following command line can be used:
1117.Pp
1118.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1119.Pp
1120If the fingerprint is unknown,
1121an alternative method of verification is available:
1122SSH fingerprints verified by DNS.
1123An additional resource record (RR),
1124SSHFP,
1125is added to a zonefile
1126and the connecting client is able to match the fingerprint
1127with that of the key presented.
1128.Pp
1129In this example, we are connecting a client to a server,
1130.Dq host.example.com .
1131The SSHFP resource records should first be added to the zonefile for
1132host.example.com:
1133.Bd -literal -offset indent
1134$ ssh-keygen -r host.example.com.
1135.Ed
1136.Pp
1137The output lines will have to be added to the zonefile.
1138To check that the zone is answering fingerprint queries:
1139.Pp
1140.Dl $ dig -t SSHFP host.example.com
1141.Pp
1142Finally the client connects:
1143.Bd -literal -offset indent
1144$ ssh -o "VerifyHostKeyDNS ask" host.example.com
1145[...]
1146Matching host key fingerprint found in DNS.
1147Are you sure you want to continue connecting (yes/no)?
1148.Ed
1149.Pp
1150See the
1151.Cm VerifyHostKeyDNS
1152option in
1153.Xr ssh_config 5
1154for more information.
1155.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1156.Nm
1157contains support for Virtual Private Network (VPN) tunnelling
1158using the
1159.Xr tun 4
1160network pseudo-device,
1161allowing two networks to be joined securely.
1162The
1163.Xr sshd_config 5
1164configuration option
1165.Cm PermitTunnel
1166controls whether the server supports this,
1167and at what level (layer 2 or 3 traffic).
1168.Pp
1169The following example would connect client network 10.0.50.0/24
1170with remote network 10.0.99.0/24 using a point-to-point connection
1171from 10.1.1.1 to 10.1.1.2,
1172provided that the SSH server running on the gateway to the remote network,
1173at 192.168.1.15, allows it.
1174.Pp
1175On the client:
1176.Bd -literal -offset indent
1177# ssh -f -w 0:1 192.168.1.15 true
1178# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1179# route add 10.0.99.0/24 10.1.1.2
1180.Ed
1181.Pp
1182On the server:
1183.Bd -literal -offset indent
1184# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1185# route add 10.0.50.0/24 10.1.1.1
1186.Ed
1187.Pp
1188Client access may be more finely tuned via the
1189.Pa /root/.ssh/authorized_keys
1190file (see below) and the
1191.Cm PermitRootLogin
1192server option.
1193The following entry would permit connections on
1194.Xr tun 4
1195device 1 from user
1196.Dq jane
1197and on tun device 2 from user
1198.Dq john ,
1199if
1200.Cm PermitRootLogin
1201is set to
1202.Dq forced-commands-only :
1203.Bd -literal -offset 2n
1204tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1205tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1206.Ed
1207.Pp
1208Since an SSH-based setup entails a fair amount of overhead,
1209it may be more suited to temporary setups,
1210such as for wireless VPNs.
1211More permanent VPNs are better provided by tools such as
1212.Xr ipsecctl 8
1213and
1214.Xr isakmpd 8 .
1215.Sh ENVIRONMENT
1216.Nm
1217will normally set the following environment variables:
1218.Bl -tag -width "SSH_ORIGINAL_COMMAND"
1219.It Ev DISPLAY
1220The
1221.Ev DISPLAY
1222variable indicates the location of the X11 server.
1223It is automatically set by
1224.Nm
1225to point to a value of the form
1226.Dq hostname:n ,
1227where
1228.Dq hostname
1229indicates the host where the shell runs, and
1230.Sq n
1231is an integer \*(Ge 1.
1232.Nm
1233uses this special value to forward X11 connections over the secure
1234channel.
1235The user should normally not set
1236.Ev DISPLAY
1237explicitly, as that
1238will render the X11 connection insecure (and will require the user to
1239manually copy any required authorization cookies).
1240.It Ev HOME
1241Set to the path of the user's home directory.
1242.It Ev LOGNAME
1243Synonym for
1244.Ev USER ;
1245set for compatibility with systems that use this variable.
1246.It Ev MAIL
1247Set to the path of the user's mailbox.
1248.It Ev PATH
1249Set to the default
1250.Ev PATH ,
1251as specified when compiling
1252.Nm .
1253.It Ev SSH_ASKPASS
1254If
1255.Nm
1256needs a passphrase, it will read the passphrase from the current
1257terminal if it was run from a terminal.
1258If
1259.Nm
1260does not have a terminal associated with it but
1261.Ev DISPLAY
1262and
1263.Ev SSH_ASKPASS
1264are set, it will execute the program specified by
1265.Ev SSH_ASKPASS
1266and open an X11 window to read the passphrase.
1267This is particularly useful when calling
1268.Nm
1269from a
1270.Pa .xsession
1271or related script.
1272(Note that on some machines it
1273may be necessary to redirect the input from
1274.Pa /dev/null
1275to make this work.)
1276.It Ev SSH_AUTH_SOCK
1277Identifies the path of a
1278.Ux Ns -domain
1279socket used to communicate with the agent.
1280.It Ev SSH_CONNECTION
1281Identifies the client and server ends of the connection.
1282The variable contains
1283four space-separated values: client IP address, client port number,
1284server IP address, and server port number.
1285.It Ev SSH_ORIGINAL_COMMAND
1286This variable contains the original command line if a forced command
1287is executed.
1288It can be used to extract the original arguments.
1289.It Ev SSH_TTY
1290This is set to the name of the tty (path to the device) associated
1291with the current shell or command.
1292If the current session has no tty,
1293this variable is not set.
1294.It Ev TZ
1295This variable is set to indicate the present time zone if it
1296was set when the daemon was started (i.e. the daemon passes the value
1297on to new connections).
1298.It Ev USER
1299Set to the name of the user logging in.
1300.El
1301.Pp
1302Additionally,
1303.Nm
1304reads
1305.Pa ~/.ssh/environment ,
1306and adds lines of the format
1307.Dq VARNAME=value
1308to the environment if the file exists and users are allowed to
1309change their environment.
1310For more information, see the
1311.Cm PermitUserEnvironment
1312option in
1313.Xr sshd_config 5 .
1314.Sh FILES
1315.Bl -tag -width Ds -compact
1316.It Pa ~/.rhosts
1317This file is used for host-based authentication (see above).
1318On some machines this file may need to be
1319world-readable if the user's home directory is on an NFS partition,
1320because
1321.Xr sshd 8
1322reads it as root.
1323Additionally, this file must be owned by the user,
1324and must not have write permissions for anyone else.
1325The recommended
1326permission for most machines is read/write for the user, and not
1327accessible by others.
1328.Pp
1329.It Pa ~/.shosts
1330This file is used in exactly the same way as
1331.Pa .rhosts ,
1332but allows host-based authentication without permitting login with
1333rlogin/rsh.
1334.Pp
1335.It Pa ~/.ssh/
1336This directory is the default location for all user-specific configuration
1337and authentication information.
1338There is no general requirement to keep the entire contents of this directory
1339secret, but the recommended permissions are read/write/execute for the user,
1340and not accessible by others.
1341.Pp
1342.It Pa ~/.ssh/authorized_keys
1343Lists the public keys (DSA, ECDSA, ED25519, RSA)
1344that can be used for logging in as this user.
1345The format of this file is described in the
1346.Xr sshd 8
1347manual page.
1348This file is not highly sensitive, but the recommended
1349permissions are read/write for the user, and not accessible by others.
1350.Pp
1351.It Pa ~/.ssh/config
1352This is the per-user configuration file.
1353The file format and configuration options are described in
1354.Xr ssh_config 5 .
1355Because of the potential for abuse, this file must have strict permissions:
1356read/write for the user, and not writable by others.
1357.Pp
1358.It Pa ~/.ssh/environment
1359Contains additional definitions for environment variables; see
1360.Sx ENVIRONMENT ,
1361above.
1362.Pp
1363.It Pa ~/.ssh/identity
1364.It Pa ~/.ssh/id_dsa
1365.It Pa ~/.ssh/id_ecdsa
1366.It Pa ~/.ssh/id_ed25519
1367.It Pa ~/.ssh/id_rsa
1368Contains the private key for authentication.
1369These files
1370contain sensitive data and should be readable by the user but not
1371accessible by others (read/write/execute).
1372.Nm
1373will simply ignore a private key file if it is accessible by others.
1374It is possible to specify a passphrase when
1375generating the key which will be used to encrypt the
1376sensitive part of this file using 3DES.
1377.Pp
1378.It Pa ~/.ssh/identity.pub
1379.It Pa ~/.ssh/id_dsa.pub
1380.It Pa ~/.ssh/id_ecdsa.pub
1381.It Pa ~/.ssh/id_ed25519.pub
1382.It Pa ~/.ssh/id_rsa.pub
1383Contains the public key for authentication.
1384These files are not
1385sensitive and can (but need not) be readable by anyone.
1386.Pp
1387.It Pa ~/.ssh/known_hosts
1388Contains a list of host keys for all hosts the user has logged into
1389that are not already in the systemwide list of known host keys.
1390See
1391.Xr sshd 8
1392for further details of the format of this file.
1393.Pp
1394.It Pa ~/.ssh/rc
1395Commands in this file are executed by
1396.Nm
1397when the user logs in, just before the user's shell (or command) is
1398started.
1399See the
1400.Xr sshd 8
1401manual page for more information.
1402.Pp
1403.It Pa /etc/hosts.equiv
1404This file is for host-based authentication (see above).
1405It should only be writable by root.
1406.Pp
1407.It Pa /etc/shosts.equiv
1408This file is used in exactly the same way as
1409.Pa hosts.equiv ,
1410but allows host-based authentication without permitting login with
1411rlogin/rsh.
1412.Pp
1413.It Pa /etc/ssh/ssh_config
1414Systemwide configuration file.
1415The file format and configuration options are described in
1416.Xr ssh_config 5 .
1417.Pp
1418.It Pa /etc/ssh/ssh_host_key
1419.It Pa /etc/ssh/ssh_host_dsa_key
1420.It Pa /etc/ssh/ssh_host_ecdsa_key
1421.It Pa /etc/ssh/ssh_host_ed25519_key
1422.It Pa /etc/ssh/ssh_host_rsa_key
1423These files contain the private parts of the host keys
1424and are used for host-based authentication.
1425If protocol version 1 is used,
1426.Nm
1427must be setuid root, since the host key is readable only by root.
1428For protocol version 2,
1429.Nm
1430uses
1431.Xr ssh-keysign 8
1432to access the host keys,
1433eliminating the requirement that
1434.Nm
1435be setuid root when host-based authentication is used.
1436By default
1437.Nm
1438is not setuid root.
1439.Pp
1440.It Pa /etc/ssh/ssh_known_hosts
1441Systemwide list of known host keys.
1442This file should be prepared by the
1443system administrator to contain the public host keys of all machines in the
1444organization.
1445It should be world-readable.
1446See
1447.Xr sshd 8
1448for further details of the format of this file.
1449.Pp
1450.It Pa /etc/ssh/sshrc
1451Commands in this file are executed by
1452.Nm
1453when the user logs in, just before the user's shell (or command) is started.
1454See the
1455.Xr sshd 8
1456manual page for more information.
1457.El
1458.Sh EXIT STATUS
1459.Nm
1460exits with the exit status of the remote command or with 255
1461if an error occurred.
1462.Sh SEE ALSO
1463.Xr scp 1 ,
1464.Xr sftp 1 ,
1465.Xr ssh-add 1 ,
1466.Xr ssh-agent 1 ,
1467.Xr ssh-keygen 1 ,
1468.Xr ssh-keyscan 1 ,
1469.Xr tun 4 ,
1470.Xr hosts.equiv 5 ,
1471.Xr ssh_config 5 ,
1472.Xr ssh-keysign 8 ,
1473.Xr sshd 8
1474.Sh STANDARDS
1475.Rs
1476.%A S. Lehtinen
1477.%A C. Lonvick
1478.%D January 2006
1479.%R RFC 4250
1480.%T The Secure Shell (SSH) Protocol Assigned Numbers
1481.Re
1482.Pp
1483.Rs
1484.%A T. Ylonen
1485.%A C. Lonvick
1486.%D January 2006
1487.%R RFC 4251
1488.%T The Secure Shell (SSH) Protocol Architecture
1489.Re
1490.Pp
1491.Rs
1492.%A T. Ylonen
1493.%A C. Lonvick
1494.%D January 2006
1495.%R RFC 4252
1496.%T The Secure Shell (SSH) Authentication Protocol
1497.Re
1498.Pp
1499.Rs
1500.%A T. Ylonen
1501.%A C. Lonvick
1502.%D January 2006
1503.%R RFC 4253
1504.%T The Secure Shell (SSH) Transport Layer Protocol
1505.Re
1506.Pp
1507.Rs
1508.%A T. Ylonen
1509.%A C. Lonvick
1510.%D January 2006
1511.%R RFC 4254
1512.%T The Secure Shell (SSH) Connection Protocol
1513.Re
1514.Pp
1515.Rs
1516.%A J. Schlyter
1517.%A W. Griffin
1518.%D January 2006
1519.%R RFC 4255
1520.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1521.Re
1522.Pp
1523.Rs
1524.%A F. Cusack
1525.%A M. Forssen
1526.%D January 2006
1527.%R RFC 4256
1528.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1529.Re
1530.Pp
1531.Rs
1532.%A J. Galbraith
1533.%A P. Remaker
1534.%D January 2006
1535.%R RFC 4335
1536.%T The Secure Shell (SSH) Session Channel Break Extension
1537.Re
1538.Pp
1539.Rs
1540.%A M. Bellare
1541.%A T. Kohno
1542.%A C. Namprempre
1543.%D January 2006
1544.%R RFC 4344
1545.%T The Secure Shell (SSH) Transport Layer Encryption Modes
1546.Re
1547.Pp
1548.Rs
1549.%A B. Harris
1550.%D January 2006
1551.%R RFC 4345
1552.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1553.Re
1554.Pp
1555.Rs
1556.%A M. Friedl
1557.%A N. Provos
1558.%A W. Simpson
1559.%D March 2006
1560.%R RFC 4419
1561.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1562.Re
1563.Pp
1564.Rs
1565.%A J. Galbraith
1566.%A R. Thayer
1567.%D November 2006
1568.%R RFC 4716
1569.%T The Secure Shell (SSH) Public Key File Format
1570.Re
1571.Pp
1572.Rs
1573.%A D. Stebila
1574.%A J. Green
1575.%D December 2009
1576.%R RFC 5656
1577.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1578.Re
1579.Pp
1580.Rs
1581.%A A. Perrig
1582.%A D. Song
1583.%D 1999
1584.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1585.%T Hash Visualization: a New Technique to improve Real-World Security
1586.Re
1587.Sh AUTHORS
1588OpenSSH is a derivative of the original and free
1589ssh 1.2.12 release by Tatu Ylonen.
1590Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1591Theo de Raadt and Dug Song
1592removed many bugs, re-added newer features and
1593created OpenSSH.
1594Markus Friedl contributed the support for SSH
1595protocol versions 1.5 and 2.0.
| 39.Dt SSH 1
40.Os
41.Sh NAME
42.Nm ssh
43.Nd OpenSSH SSH client (remote login program)
44.Sh SYNOPSIS
45.Nm ssh
46.Bk -words
47.Op Fl 1246AaCfgKkMNnqsTtVvXxYy
48.Op Fl b Ar bind_address
49.Op Fl c Ar cipher_spec
50.Op Fl D Oo Ar bind_address : Oc Ns Ar port
51.Op Fl E Ar log_file
52.Op Fl e Ar escape_char
53.Op Fl F Ar configfile
54.Op Fl I Ar pkcs11
55.Op Fl i Ar identity_file
56.Op Fl L Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
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 port
62.Op Fl Q Cm cipher | cipher-auth | mac | kex | key
63.Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
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.Oo Ar user Ns @ Oc Ns Ar hostname
68.Op Ar command
69.Ek
70.Sh DESCRIPTION
71.Nm
72(SSH client) is a program for logging into a remote machine and for
73executing commands on a remote machine.
74It is intended to replace rlogin and rsh,
75and provide secure encrypted communications between
76two untrusted hosts over an insecure network.
77X11 connections and arbitrary TCP ports
78can also be forwarded over the secure channel.
79.Pp
80.Nm
81connects and logs into the specified
82.Ar hostname
83(with optional
84.Ar user
85name).
86The user must prove
87his/her identity to the remote machine using one of several methods
88depending on the protocol version used (see below).
89.Pp
90If
91.Ar command
92is specified,
93it is executed on the remote host instead of a login shell.
94.Pp
95The options are as follows:
96.Bl -tag -width Ds
97.It Fl 1
98Forces
99.Nm
100to try protocol version 1 only.
101.It Fl 2
102Forces
103.Nm
104to try protocol version 2 only.
105.It Fl 4
106Forces
107.Nm
108to use IPv4 addresses only.
109.It Fl 6
110Forces
111.Nm
112to use IPv6 addresses only.
113.It Fl A
114Enables forwarding of the authentication agent connection.
115This can also be specified on a per-host basis in a configuration file.
116.Pp
117Agent forwarding should be enabled with caution.
118Users with the ability to bypass file permissions on the remote host
119(for the agent's
120.Ux Ns -domain
121socket) can access the local agent through the forwarded connection.
122An attacker cannot obtain key material from the agent,
123however they can perform operations on the keys that enable them to
124authenticate using the identities loaded into the agent.
125.It Fl a
126Disables forwarding of the authentication agent connection.
127.It Fl b Ar bind_address
128Use
129.Ar bind_address
130on the local machine as the source address
131of the connection.
132Only useful on systems with more than one address.
133.It Fl C
134Requests compression of all data (including stdin, stdout, stderr, and
135data for forwarded X11 and TCP connections).
136The compression algorithm is the same used by
137.Xr gzip 1 ,
138and the
139.Dq level
140can be controlled by the
141.Cm CompressionLevel
142option for protocol version 1.
143Compression is desirable on modem lines and other
144slow connections, but will only slow down things on fast networks.
145The default value can be set on a host-by-host basis in the
146configuration files; see the
147.Cm Compression
148option.
149.It Fl c Ar cipher_spec
150Selects the cipher specification for encrypting the session.
151.Pp
152Protocol version 1 allows specification of a single cipher.
153The supported values are
154.Dq 3des ,
155.Dq blowfish ,
156and
157.Dq des .
158.Ar 3des
159(triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
160It is believed to be secure.
161.Ar blowfish
162is a fast block cipher; it appears very secure and is much faster than
163.Ar 3des .
164.Ar des
165is only supported in the
166.Nm
167client for interoperability with legacy protocol 1 implementations
168that do not support the
169.Ar 3des
170cipher.
171Its use is strongly discouraged due to cryptographic weaknesses.
172The default is
173.Dq 3des .
174.Pp
175For protocol version 2,
176.Ar cipher_spec
177is a comma-separated list of ciphers
178listed in order of preference.
179See the
180.Cm Ciphers
181keyword in
182.Xr ssh_config 5
183for more information.
184.It Fl D Xo
185.Sm off
186.Oo Ar bind_address : Oc
187.Ar port
188.Sm on
189.Xc
190Specifies a local
191.Dq dynamic
192application-level port forwarding.
193This works by allocating a socket to listen to
194.Ar port
195on the local side, optionally bound to the specified
196.Ar bind_address .
197Whenever a connection is made to this port, the
198connection is forwarded over the secure channel, and the application
199protocol is then used to determine where to connect to from the
200remote machine.
201Currently the SOCKS4 and SOCKS5 protocols are supported, and
202.Nm
203will act as a SOCKS server.
204Only root can forward privileged ports.
205Dynamic port forwardings can also be specified in the configuration file.
206.Pp
207IPv6 addresses can be specified by enclosing the address in square brackets.
208Only the superuser can forward privileged ports.
209By default, the local port is bound in accordance with the
210.Cm GatewayPorts
211setting.
212However, an explicit
213.Ar bind_address
214may be used to bind the connection to a specific address.
215The
216.Ar bind_address
217of
218.Dq localhost
219indicates that the listening port be bound for local use only, while an
220empty address or
221.Sq *
222indicates that the port should be available from all interfaces.
223.It Fl E Ar log_file
224Append debug logs to
225.Ar log_file
226instead of standard error.
227.It Fl e Ar escape_char
228Sets the escape character for sessions with a pty (default:
229.Ql ~ ) .
230The escape character is only recognized at the beginning of a line.
231The escape character followed by a dot
232.Pq Ql \&.
233closes the connection;
234followed by control-Z suspends the connection;
235and followed by itself sends the escape character once.
236Setting the character to
237.Dq none
238disables any escapes and makes the session fully transparent.
239.It Fl F Ar configfile
240Specifies an alternative per-user configuration file.
241If a configuration file is given on the command line,
242the system-wide configuration file
243.Pq Pa /etc/ssh/ssh_config
244will be ignored.
245The default for the per-user configuration file is
246.Pa ~/.ssh/config .
247.It Fl f
248Requests
249.Nm
250to go to background just before command execution.
251This is useful if
252.Nm
253is going to ask for passwords or passphrases, but the user
254wants it in the background.
255This implies
256.Fl n .
257The recommended way to start X11 programs at a remote site is with
258something like
259.Ic ssh -f host xterm .
260.Pp
261If the
262.Cm ExitOnForwardFailure
263configuration option is set to
264.Dq yes ,
265then a client started with
266.Fl f
267will wait for all remote port forwards to be successfully established
268before placing itself in the background.
269.It Fl g
270Allows remote hosts to connect to local forwarded ports.
271.It Fl I Ar pkcs11
272Specify the PKCS#11 shared library
273.Nm
274should use to communicate with a PKCS#11 token providing the user's
275private RSA key.
276.It Fl i Ar identity_file
277Selects a file from which the identity (private key) for
278public key authentication is read.
279The default is
280.Pa ~/.ssh/identity
281for protocol version 1, and
282.Pa ~/.ssh/id_dsa ,
283.Pa ~/.ssh/id_ecdsa ,
284.Pa ~/.ssh/id_ed25519
285and
286.Pa ~/.ssh/id_rsa
287for protocol version 2.
288Identity files may also be specified on
289a per-host basis in the configuration file.
290It is possible to have multiple
291.Fl i
292options (and multiple identities specified in
293configuration files).
294.Nm
295will also try to load certificate information from the filename obtained
296by appending
297.Pa -cert.pub
298to identity filenames.
299.It Fl K
300Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
301credentials to the server.
302.It Fl k
303Disables forwarding (delegation) of GSSAPI credentials to the server.
304.It Fl L Xo
305.Sm off
306.Oo Ar bind_address : Oc
307.Ar port : host : hostport
308.Sm on
309.Xc
310Specifies that the given port on the local (client) host is to be
311forwarded to the given host and port on the remote side.
312This works by allocating a socket to listen to
313.Ar port
314on the local side, optionally bound to the specified
315.Ar bind_address .
316Whenever a connection is made to this port, the
317connection is forwarded over the secure channel, and a connection is
318made to
319.Ar host
320port
321.Ar hostport
322from the remote machine.
323Port forwardings can also be specified in the configuration file.
324IPv6 addresses can be specified by enclosing the address in square brackets.
325Only the superuser can forward privileged ports.
326By default, the local port is bound in accordance with the
327.Cm GatewayPorts
328setting.
329However, an explicit
330.Ar bind_address
331may be used to bind the connection to a specific address.
332The
333.Ar bind_address
334of
335.Dq localhost
336indicates that the listening port be bound for local use only, while an
337empty address or
338.Sq *
339indicates that the port should be available from all interfaces.
340.It Fl l Ar login_name
341Specifies the user to log in as on the remote machine.
342This also may be specified on a per-host basis in the configuration file.
343.It Fl M
344Places the
345.Nm
346client into
347.Dq master
348mode for connection sharing.
349Multiple
350.Fl M
351options places
352.Nm
353into
354.Dq master
355mode with confirmation required before slave connections are accepted.
356Refer to the description of
357.Cm ControlMaster
358in
359.Xr ssh_config 5
360for details.
361.It Fl m Ar mac_spec
362Additionally, for protocol version 2 a comma-separated list of MAC
363(message authentication code) algorithms can
364be specified in order of preference.
365See the
366.Cm MACs
367keyword for more information.
368.It Fl N
369Do not execute a remote command.
370This is useful for just forwarding ports
371(protocol version 2 only).
372.It Fl n
373Redirects stdin from
374.Pa /dev/null
375(actually, prevents reading from stdin).
376This must be used when
377.Nm
378is run in the background.
379A common trick is to use this to run X11 programs on a remote machine.
380For example,
381.Ic ssh -n shadows.cs.hut.fi emacs &
382will start an emacs on shadows.cs.hut.fi, and the X11
383connection will be automatically forwarded over an encrypted channel.
384The
385.Nm
386program will be put in the background.
387(This does not work if
388.Nm
389needs to ask for a password or passphrase; see also the
390.Fl f
391option.)
392.It Fl O Ar ctl_cmd
393Control an active connection multiplexing master process.
394When the
395.Fl O
396option is specified, the
397.Ar ctl_cmd
398argument is interpreted and passed to the master process.
399Valid commands are:
400.Dq check
401(check that the master process is running),
402.Dq forward
403(request forwardings without command execution),
404.Dq cancel
405(cancel forwardings),
406.Dq exit
407(request the master to exit), and
408.Dq stop
409(request the master to stop accepting further multiplexing requests).
410.It Fl o Ar option
411Can be used to give options in the format used in the configuration file.
412This is useful for specifying options for which there is no separate
413command-line flag.
414For full details of the options listed below, and their possible values, see
415.Xr ssh_config 5 .
416.Pp
417.Bl -tag -width Ds -offset indent -compact
418.It AddressFamily
419.It BatchMode
420.It BindAddress
421.It CanonicalDomains
422.It CanonicalizeFallbackLocal
423.It CanonicalizeHostname
424.It CanonicalizeMaxDots
425.It CanonicalizePermittedCNAMEs
426.It ChallengeResponseAuthentication
427.It CheckHostIP
428.It Cipher
429.It Ciphers
430.It ClearAllForwardings
431.It Compression
432.It CompressionLevel
433.It ConnectionAttempts
434.It ConnectTimeout
435.It ControlMaster
436.It ControlPath
437.It ControlPersist
438.It DynamicForward
439.It EscapeChar
440.It ExitOnForwardFailure
441.It ForwardAgent
442.It ForwardX11
443.It ForwardX11Timeout
444.It ForwardX11Trusted
445.It GatewayPorts
446.It GlobalKnownHostsFile
447.It GSSAPIAuthentication
448.It GSSAPIDelegateCredentials
449.It HashKnownHosts
450.It Host
451.It HostbasedAuthentication
452.It HostKeyAlgorithms
453.It HostKeyAlias
454.It HostName
455.It IdentityFile
456.It IdentitiesOnly
457.It IPQoS
458.It KbdInteractiveAuthentication
459.It KbdInteractiveDevices
460.It KexAlgorithms
461.It LocalCommand
462.It LocalForward
463.It LogLevel
464.It MACs
465.It Match
466.It NoHostAuthenticationForLocalhost
467.It NumberOfPasswordPrompts
468.It PasswordAuthentication
469.It PermitLocalCommand
470.It PKCS11Provider
471.It Port
472.It PreferredAuthentications
473.It Protocol
474.It ProxyCommand
475.It ProxyUseFdpass
476.It PubkeyAuthentication
477.It RekeyLimit
478.It RemoteForward
479.It RequestTTY
480.It RhostsRSAAuthentication
481.It RSAAuthentication
482.It SendEnv
483.It ServerAliveInterval
484.It ServerAliveCountMax
485.It StrictHostKeyChecking
486.It TCPKeepAlive
487.It Tunnel
488.It TunnelDevice
489.It UsePrivilegedPort
490.It User
491.It UserKnownHostsFile
492.It VerifyHostKeyDNS
493.It VersionAddendum
494.It VisualHostKey
495.It XAuthLocation
496.El
497.It Fl p Ar port
498Port to connect to on the remote host.
499This can be specified on a
500per-host basis in the configuration file.
501.It Fl Q Cm cipher | cipher-auth | mac | kex | key
502Queries
503.Nm
504for the algorithms supported for the specified version 2.
505The available features are:
506.Ar cipher
507(supported symmetric ciphers),
508.Ar cipher-auth
509(supported symmetric ciphers that support authenticated encryption),
510.Ar mac
511(supported message integrity codes),
512.Ar kex
513(key exchange algorithms),
514.Ar key
515(key types).
516.It Fl q
517Quiet mode.
518Causes most warning and diagnostic messages to be suppressed.
519.It Fl R Xo
520.Sm off
521.Oo Ar bind_address : Oc
522.Ar port : host : hostport
523.Sm on
524.Xc
525Specifies that the given port on the remote (server) host is to be
526forwarded to the given host and port on the local side.
527This works by allocating a socket to listen to
528.Ar port
529on the remote side, and whenever a connection is made to this port, the
530connection is forwarded over the secure channel, and a connection is
531made to
532.Ar host
533port
534.Ar hostport
535from the local machine.
536.Pp
537Port forwardings can also be specified in the configuration file.
538Privileged ports can be forwarded only when
539logging in as root on the remote machine.
540IPv6 addresses can be specified by enclosing the address in square brackets.
541.Pp
542By default, the listening socket on the server will be bound to the loopback
543interface only.
544This may be overridden by specifying a
545.Ar bind_address .
546An empty
547.Ar bind_address ,
548or the address
549.Ql * ,
550indicates that the remote socket should listen on all interfaces.
551Specifying a remote
552.Ar bind_address
553will only succeed if the server's
554.Cm GatewayPorts
555option is enabled (see
556.Xr sshd_config 5 ) .
557.Pp
558If the
559.Ar port
560argument is
561.Ql 0 ,
562the listen port will be dynamically allocated on the server and reported
563to the client at run time.
564When used together with
565.Ic -O forward
566the allocated port will be printed to the standard output.
567.It Fl S Ar ctl_path
568Specifies the location of a control socket for connection sharing,
569or the string
570.Dq none
571to disable connection sharing.
572Refer to the description of
573.Cm ControlPath
574and
575.Cm ControlMaster
576in
577.Xr ssh_config 5
578for details.
579.It Fl s
580May be used to request invocation of a subsystem on the remote system.
581Subsystems are a feature of the SSH2 protocol which facilitate the use
582of SSH as a secure transport for other applications (eg.\&
583.Xr sftp 1 ) .
584The subsystem is specified as the remote command.
585.It Fl T
586Disable pseudo-tty allocation.
587.It Fl t
588Force pseudo-tty allocation.
589This can be used to execute arbitrary
590screen-based programs on a remote machine, which can be very useful,
591e.g. when implementing menu services.
592Multiple
593.Fl t
594options force tty allocation, even if
595.Nm
596has no local tty.
597.It Fl V
598Display the version number and exit.
599.It Fl v
600Verbose mode.
601Causes
602.Nm
603to print debugging messages about its progress.
604This is helpful in
605debugging connection, authentication, and configuration problems.
606Multiple
607.Fl v
608options increase the verbosity.
609The maximum is 3.
610.It Fl W Ar host : Ns Ar port
611Requests that standard input and output on the client be forwarded to
612.Ar host
613on
614.Ar port
615over the secure channel.
616Implies
617.Fl N ,
618.Fl T ,
619.Cm ExitOnForwardFailure
620and
621.Cm ClearAllForwardings .
622Works with Protocol version 2 only.
623.It Fl w Xo
624.Ar local_tun Ns Op : Ns Ar remote_tun
625.Xc
626Requests
627tunnel
628device forwarding with the specified
629.Xr tun 4
630devices between the client
631.Pq Ar local_tun
632and the server
633.Pq Ar remote_tun .
634.Pp
635The devices may be specified by numerical ID or the keyword
636.Dq any ,
637which uses the next available tunnel device.
638If
639.Ar remote_tun
640is not specified, it defaults to
641.Dq any .
642See also the
643.Cm Tunnel
644and
645.Cm TunnelDevice
646directives in
647.Xr ssh_config 5 .
648If the
649.Cm Tunnel
650directive is unset, it is set to the default tunnel mode, which is
651.Dq point-to-point .
652.It Fl X
653Enables X11 forwarding.
654This can also be specified on a per-host basis in a configuration file.
655.Pp
656X11 forwarding should be enabled with caution.
657Users with the ability to bypass file permissions on the remote host
658(for the user's X authorization database)
659can access the local X11 display through the forwarded connection.
660An attacker may then be able to perform activities such as keystroke monitoring.
661.Pp
662For this reason, X11 forwarding is subjected to X11 SECURITY extension
663restrictions by default.
664Please refer to the
665.Nm
666.Fl Y
667option and the
668.Cm ForwardX11Trusted
669directive in
670.Xr ssh_config 5
671for more information.
672.It Fl x
673Disables X11 forwarding.
674.It Fl Y
675Enables trusted X11 forwarding.
676Trusted X11 forwardings are not subjected to the X11 SECURITY extension
677controls.
678.It Fl y
679Send log information using the
680.Xr syslog 3
681system module.
682By default this information is sent to stderr.
683.El
684.Pp
685.Nm
686may additionally obtain configuration data from
687a per-user configuration file and a system-wide configuration file.
688The file format and configuration options are described in
689.Xr ssh_config 5 .
690.Sh AUTHENTICATION
691The OpenSSH SSH client supports SSH protocols 1 and 2.
692The default is to use protocol 2 only,
693though this can be changed via the
694.Cm Protocol
695option in
696.Xr ssh_config 5
697or the
698.Fl 1
699and
700.Fl 2
701options (see above).
702Both protocols support similar authentication methods,
703but protocol 2 is the default since
704it provides additional mechanisms for confidentiality
705(the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
706and integrity (hmac-md5, hmac-sha1,
707hmac-sha2-256, hmac-sha2-512,
708umac-64, umac-128, hmac-ripemd160).
709Protocol 1 lacks a strong mechanism for ensuring the
710integrity of the connection.
711.Pp
712The methods available for authentication are:
713GSSAPI-based authentication,
714host-based authentication,
715public key authentication,
716challenge-response authentication,
717and password authentication.
718Authentication methods are tried in the order specified above,
719though protocol 2 has a configuration option to change the default order:
720.Cm PreferredAuthentications .
721.Pp
722Host-based authentication works as follows:
723If the machine the user logs in from is listed in
724.Pa /etc/hosts.equiv
725or
726.Pa /etc/shosts.equiv
727on the remote machine, and the user names are
728the same on both sides, or if the files
729.Pa ~/.rhosts
730or
731.Pa ~/.shosts
732exist in the user's home directory on the
733remote machine and contain a line containing the name of the client
734machine and the name of the user on that machine, the user is
735considered for login.
736Additionally, the server
737.Em must
738be able to verify the client's
739host key (see the description of
740.Pa /etc/ssh/ssh_known_hosts
741and
742.Pa ~/.ssh/known_hosts ,
743below)
744for login to be permitted.
745This authentication method closes security holes due to IP
746spoofing, DNS spoofing, and routing spoofing.
747[Note to the administrator:
748.Pa /etc/hosts.equiv ,
749.Pa ~/.rhosts ,
750and the rlogin/rsh protocol in general, are inherently insecure and should be
751disabled if security is desired.]
752.Pp
753Public key authentication works as follows:
754The scheme is based on public-key cryptography,
755using cryptosystems
756where encryption and decryption are done using separate keys,
757and it is unfeasible to derive the decryption key from the encryption key.
758The idea is that each user creates a public/private
759key pair for authentication purposes.
760The server knows the public key, and only the user knows the private key.
761.Nm
762implements public key authentication protocol automatically,
763using one of the DSA, ECDSA, ED25519 or RSA algorithms.
764Protocol 1 is restricted to using only RSA keys,
765but protocol 2 may use any.
766The HISTORY section of
767.Xr ssl 8
768contains a brief discussion of the DSA and RSA algorithms.
769.Pp
770The file
771.Pa ~/.ssh/authorized_keys
772lists the public keys that are permitted for logging in.
773When the user logs in, the
774.Nm
775program tells the server which key pair it would like to use for
776authentication.
777The client proves that it has access to the private key
778and the server checks that the corresponding public key
779is authorized to accept the account.
780.Pp
781The user creates his/her key pair by running
782.Xr ssh-keygen 1 .
783This stores the private key in
784.Pa ~/.ssh/identity
785(protocol 1),
786.Pa ~/.ssh/id_dsa
787(protocol 2 DSA),
788.Pa ~/.ssh/id_ecdsa
789(protocol 2 ECDSA),
790.Pa ~/.ssh/id_ed25519
791(protocol 2 ED25519),
792or
793.Pa ~/.ssh/id_rsa
794(protocol 2 RSA)
795and stores the public key in
796.Pa ~/.ssh/identity.pub
797(protocol 1),
798.Pa ~/.ssh/id_dsa.pub
799(protocol 2 DSA),
800.Pa ~/.ssh/id_ecdsa.pub
801(protocol 2 ECDSA),
802.Pa ~/.ssh/id_ed25519.pub
803(protocol 2 ED25519),
804or
805.Pa ~/.ssh/id_rsa.pub
806(protocol 2 RSA)
807in the user's home directory.
808The user should then copy the public key
809to
810.Pa ~/.ssh/authorized_keys
811in his/her home directory on the remote machine.
812The
813.Pa authorized_keys
814file corresponds to the conventional
815.Pa ~/.rhosts
816file, and has one key
817per line, though the lines can be very long.
818After this, the user can log in without giving the password.
819.Pp
820A variation on public key authentication
821is available in the form of certificate authentication:
822instead of a set of public/private keys,
823signed certificates are used.
824This has the advantage that a single trusted certification authority
825can be used in place of many public/private keys.
826See the CERTIFICATES section of
827.Xr ssh-keygen 1
828for more information.
829.Pp
830The most convenient way to use public key or certificate authentication
831may be with an authentication agent.
832See
833.Xr ssh-agent 1
834for more information.
835.Pp
836Challenge-response authentication works as follows:
837The server sends an arbitrary
838.Qq challenge
839text, and prompts for a response.
840Protocol 2 allows multiple challenges and responses;
841protocol 1 is restricted to just one challenge/response.
842Examples of challenge-response authentication include
843.Bx
844Authentication (see
845.Xr login.conf 5 )
846and PAM (some
847.Pf non- Ox
848systems).
849.Pp
850Finally, if other authentication methods fail,
851.Nm
852prompts the user for a password.
853The password is sent to the remote
854host for checking; however, since all communications are encrypted,
855the password cannot be seen by someone listening on the network.
856.Pp
857.Nm
858automatically maintains and checks a database containing
859identification for all hosts it has ever been used with.
860Host keys are stored in
861.Pa ~/.ssh/known_hosts
862in the user's home directory.
863Additionally, the file
864.Pa /etc/ssh/ssh_known_hosts
865is automatically checked for known hosts.
866Any new hosts are automatically added to the user's file.
867If a host's identification ever changes,
868.Nm
869warns about this and disables password authentication to prevent
870server spoofing or man-in-the-middle attacks,
871which could otherwise be used to circumvent the encryption.
872The
873.Cm StrictHostKeyChecking
874option can be used to control logins to machines whose
875host key is not known or has changed.
876.Pp
877When the user's identity has been accepted by the server, the server
878either executes the given command, or logs into the machine and gives
879the user a normal shell on the remote machine.
880All communication with
881the remote command or shell will be automatically encrypted.
882.Pp
883If a pseudo-terminal has been allocated (normal login session), the
884user may use the escape characters noted below.
885.Pp
886If no pseudo-tty has been allocated,
887the session is transparent and can be used to reliably transfer binary data.
888On most systems, setting the escape character to
889.Dq none
890will also make the session transparent even if a tty is used.
891.Pp
892The session terminates when the command or shell on the remote
893machine exits and all X11 and TCP connections have been closed.
894.Sh ESCAPE CHARACTERS
895When a pseudo-terminal has been requested,
896.Nm
897supports a number of functions through the use of an escape character.
898.Pp
899A single tilde character can be sent as
900.Ic ~~
901or by following the tilde by a character other than those described below.
902The escape character must always follow a newline to be interpreted as
903special.
904The escape character can be changed in configuration files using the
905.Cm EscapeChar
906configuration directive or on the command line by the
907.Fl e
908option.
909.Pp
910The supported escapes (assuming the default
911.Ql ~ )
912are:
913.Bl -tag -width Ds
914.It Cm ~.
915Disconnect.
916.It Cm ~^Z
917Background
918.Nm .
919.It Cm ~#
920List forwarded connections.
921.It Cm ~&
922Background
923.Nm
924at logout when waiting for forwarded connection / X11 sessions to terminate.
925.It Cm ~?
926Display a list of escape characters.
927.It Cm ~B
928Send a BREAK to the remote system
929(only useful for SSH protocol version 2 and if the peer supports it).
930.It Cm ~C
931Open command line.
932Currently this allows the addition of port forwardings using the
933.Fl L ,
934.Fl R
935and
936.Fl D
937options (see above).
938It also allows the cancellation of existing port-forwardings
939with
940.Sm off
941.Fl KL Oo Ar bind_address : Oc Ar port
942.Sm on
943for local,
944.Sm off
945.Fl KR Oo Ar bind_address : Oc Ar port
946.Sm on
947for remote and
948.Sm off
949.Fl KD Oo Ar bind_address : Oc Ar port
950.Sm on
951for dynamic port-forwardings.
952.Ic !\& Ns Ar command
953allows the user to execute a local command if the
954.Ic PermitLocalCommand
955option is enabled in
956.Xr ssh_config 5 .
957Basic help is available, using the
958.Fl h
959option.
960.It Cm ~R
961Request rekeying of the connection
962(only useful for SSH protocol version 2 and if the peer supports it).
963.It Cm ~V
964Decrease the verbosity
965.Pq Ic LogLevel
966when errors are being written to stderr.
967.It Cm ~v
968Increase the verbosity
969.Pq Ic LogLevel
970when errors are being written to stderr.
971.El
972.Sh TCP FORWARDING
973Forwarding of arbitrary TCP connections over the secure channel can
974be specified either on the command line or in a configuration file.
975One possible application of TCP forwarding is a secure connection to a
976mail server; another is going through firewalls.
977.Pp
978In the example below, we look at encrypting communication between
979an IRC client and server, even though the IRC server does not directly
980support encrypted communications.
981This works as follows:
982the user connects to the remote host using
983.Nm ,
984specifying a port to be used to forward connections
985to the remote server.
986After that it is possible to start the service which is to be encrypted
987on the client machine,
988connecting to the same local port,
989and
990.Nm
991will encrypt and forward the connection.
992.Pp
993The following example tunnels an IRC session from client machine
994.Dq 127.0.0.1
995(localhost)
996to remote server
997.Dq server.example.com :
998.Bd -literal -offset 4n
999$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1000$ irc -c '#users' -p 1234 pinky 127.0.0.1
1001.Ed
1002.Pp
1003This tunnels a connection to IRC server
1004.Dq server.example.com ,
1005joining channel
1006.Dq #users ,
1007nickname
1008.Dq pinky ,
1009using port 1234.
1010It doesn't matter which port is used,
1011as long as it's greater than 1023
1012(remember, only root can open sockets on privileged ports)
1013and doesn't conflict with any ports already in use.
1014The connection is forwarded to port 6667 on the remote server,
1015since that's the standard port for IRC services.
1016.Pp
1017The
1018.Fl f
1019option backgrounds
1020.Nm
1021and the remote command
1022.Dq sleep 10
1023is specified to allow an amount of time
1024(10 seconds, in the example)
1025to start the service which is to be tunnelled.
1026If no connections are made within the time specified,
1027.Nm
1028will exit.
1029.Sh X11 FORWARDING
1030If the
1031.Cm ForwardX11
1032variable is set to
1033.Dq yes
1034(or see the description of the
1035.Fl X ,
1036.Fl x ,
1037and
1038.Fl Y
1039options above)
1040and the user is using X11 (the
1041.Ev DISPLAY
1042environment variable is set), the connection to the X11 display is
1043automatically forwarded to the remote side in such a way that any X11
1044programs started from the shell (or command) will go through the
1045encrypted channel, and the connection to the real X server will be made
1046from the local machine.
1047The user should not manually set
1048.Ev DISPLAY .
1049Forwarding of X11 connections can be
1050configured on the command line or in configuration files.
1051.Pp
1052The
1053.Ev DISPLAY
1054value set by
1055.Nm
1056will point to the server machine, but with a display number greater than zero.
1057This is normal, and happens because
1058.Nm
1059creates a
1060.Dq proxy
1061X server on the server machine for forwarding the
1062connections over the encrypted channel.
1063.Pp
1064.Nm
1065will also automatically set up Xauthority data on the server machine.
1066For this purpose, it will generate a random authorization cookie,
1067store it in Xauthority on the server, and verify that any forwarded
1068connections carry this cookie and replace it by the real cookie when
1069the connection is opened.
1070The real authentication cookie is never
1071sent to the server machine (and no cookies are sent in the plain).
1072.Pp
1073If the
1074.Cm ForwardAgent
1075variable is set to
1076.Dq yes
1077(or see the description of the
1078.Fl A
1079and
1080.Fl a
1081options above) and
1082the user is using an authentication agent, the connection to the agent
1083is automatically forwarded to the remote side.
1084.Sh VERIFYING HOST KEYS
1085When connecting to a server for the first time,
1086a fingerprint of the server's public key is presented to the user
1087(unless the option
1088.Cm StrictHostKeyChecking
1089has been disabled).
1090Fingerprints can be determined using
1091.Xr ssh-keygen 1 :
1092.Pp
1093.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1094.Pp
1095If the fingerprint is already known, it can be matched
1096and the key can be accepted or rejected.
1097Because of the difficulty of comparing host keys
1098just by looking at hex strings,
1099there is also support to compare host keys visually,
1100using
1101.Em random art .
1102By setting the
1103.Cm VisualHostKey
1104option to
1105.Dq yes ,
1106a small ASCII graphic gets displayed on every login to a server, no matter
1107if the session itself is interactive or not.
1108By learning the pattern a known server produces, a user can easily
1109find out that the host key has changed when a completely different pattern
1110is displayed.
1111Because these patterns are not unambiguous however, a pattern that looks
1112similar to the pattern remembered only gives a good probability that the
1113host key is the same, not guaranteed proof.
1114.Pp
1115To get a listing of the fingerprints along with their random art for
1116all known hosts, the following command line can be used:
1117.Pp
1118.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1119.Pp
1120If the fingerprint is unknown,
1121an alternative method of verification is available:
1122SSH fingerprints verified by DNS.
1123An additional resource record (RR),
1124SSHFP,
1125is added to a zonefile
1126and the connecting client is able to match the fingerprint
1127with that of the key presented.
1128.Pp
1129In this example, we are connecting a client to a server,
1130.Dq host.example.com .
1131The SSHFP resource records should first be added to the zonefile for
1132host.example.com:
1133.Bd -literal -offset indent
1134$ ssh-keygen -r host.example.com.
1135.Ed
1136.Pp
1137The output lines will have to be added to the zonefile.
1138To check that the zone is answering fingerprint queries:
1139.Pp
1140.Dl $ dig -t SSHFP host.example.com
1141.Pp
1142Finally the client connects:
1143.Bd -literal -offset indent
1144$ ssh -o "VerifyHostKeyDNS ask" host.example.com
1145[...]
1146Matching host key fingerprint found in DNS.
1147Are you sure you want to continue connecting (yes/no)?
1148.Ed
1149.Pp
1150See the
1151.Cm VerifyHostKeyDNS
1152option in
1153.Xr ssh_config 5
1154for more information.
1155.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1156.Nm
1157contains support for Virtual Private Network (VPN) tunnelling
1158using the
1159.Xr tun 4
1160network pseudo-device,
1161allowing two networks to be joined securely.
1162The
1163.Xr sshd_config 5
1164configuration option
1165.Cm PermitTunnel
1166controls whether the server supports this,
1167and at what level (layer 2 or 3 traffic).
1168.Pp
1169The following example would connect client network 10.0.50.0/24
1170with remote network 10.0.99.0/24 using a point-to-point connection
1171from 10.1.1.1 to 10.1.1.2,
1172provided that the SSH server running on the gateway to the remote network,
1173at 192.168.1.15, allows it.
1174.Pp
1175On the client:
1176.Bd -literal -offset indent
1177# ssh -f -w 0:1 192.168.1.15 true
1178# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1179# route add 10.0.99.0/24 10.1.1.2
1180.Ed
1181.Pp
1182On the server:
1183.Bd -literal -offset indent
1184# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1185# route add 10.0.50.0/24 10.1.1.1
1186.Ed
1187.Pp
1188Client access may be more finely tuned via the
1189.Pa /root/.ssh/authorized_keys
1190file (see below) and the
1191.Cm PermitRootLogin
1192server option.
1193The following entry would permit connections on
1194.Xr tun 4
1195device 1 from user
1196.Dq jane
1197and on tun device 2 from user
1198.Dq john ,
1199if
1200.Cm PermitRootLogin
1201is set to
1202.Dq forced-commands-only :
1203.Bd -literal -offset 2n
1204tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1205tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1206.Ed
1207.Pp
1208Since an SSH-based setup entails a fair amount of overhead,
1209it may be more suited to temporary setups,
1210such as for wireless VPNs.
1211More permanent VPNs are better provided by tools such as
1212.Xr ipsecctl 8
1213and
1214.Xr isakmpd 8 .
1215.Sh ENVIRONMENT
1216.Nm
1217will normally set the following environment variables:
1218.Bl -tag -width "SSH_ORIGINAL_COMMAND"
1219.It Ev DISPLAY
1220The
1221.Ev DISPLAY
1222variable indicates the location of the X11 server.
1223It is automatically set by
1224.Nm
1225to point to a value of the form
1226.Dq hostname:n ,
1227where
1228.Dq hostname
1229indicates the host where the shell runs, and
1230.Sq n
1231is an integer \*(Ge 1.
1232.Nm
1233uses this special value to forward X11 connections over the secure
1234channel.
1235The user should normally not set
1236.Ev DISPLAY
1237explicitly, as that
1238will render the X11 connection insecure (and will require the user to
1239manually copy any required authorization cookies).
1240.It Ev HOME
1241Set to the path of the user's home directory.
1242.It Ev LOGNAME
1243Synonym for
1244.Ev USER ;
1245set for compatibility with systems that use this variable.
1246.It Ev MAIL
1247Set to the path of the user's mailbox.
1248.It Ev PATH
1249Set to the default
1250.Ev PATH ,
1251as specified when compiling
1252.Nm .
1253.It Ev SSH_ASKPASS
1254If
1255.Nm
1256needs a passphrase, it will read the passphrase from the current
1257terminal if it was run from a terminal.
1258If
1259.Nm
1260does not have a terminal associated with it but
1261.Ev DISPLAY
1262and
1263.Ev SSH_ASKPASS
1264are set, it will execute the program specified by
1265.Ev SSH_ASKPASS
1266and open an X11 window to read the passphrase.
1267This is particularly useful when calling
1268.Nm
1269from a
1270.Pa .xsession
1271or related script.
1272(Note that on some machines it
1273may be necessary to redirect the input from
1274.Pa /dev/null
1275to make this work.)
1276.It Ev SSH_AUTH_SOCK
1277Identifies the path of a
1278.Ux Ns -domain
1279socket used to communicate with the agent.
1280.It Ev SSH_CONNECTION
1281Identifies the client and server ends of the connection.
1282The variable contains
1283four space-separated values: client IP address, client port number,
1284server IP address, and server port number.
1285.It Ev SSH_ORIGINAL_COMMAND
1286This variable contains the original command line if a forced command
1287is executed.
1288It can be used to extract the original arguments.
1289.It Ev SSH_TTY
1290This is set to the name of the tty (path to the device) associated
1291with the current shell or command.
1292If the current session has no tty,
1293this variable is not set.
1294.It Ev TZ
1295This variable is set to indicate the present time zone if it
1296was set when the daemon was started (i.e. the daemon passes the value
1297on to new connections).
1298.It Ev USER
1299Set to the name of the user logging in.
1300.El
1301.Pp
1302Additionally,
1303.Nm
1304reads
1305.Pa ~/.ssh/environment ,
1306and adds lines of the format
1307.Dq VARNAME=value
1308to the environment if the file exists and users are allowed to
1309change their environment.
1310For more information, see the
1311.Cm PermitUserEnvironment
1312option in
1313.Xr sshd_config 5 .
1314.Sh FILES
1315.Bl -tag -width Ds -compact
1316.It Pa ~/.rhosts
1317This file is used for host-based authentication (see above).
1318On some machines this file may need to be
1319world-readable if the user's home directory is on an NFS partition,
1320because
1321.Xr sshd 8
1322reads it as root.
1323Additionally, this file must be owned by the user,
1324and must not have write permissions for anyone else.
1325The recommended
1326permission for most machines is read/write for the user, and not
1327accessible by others.
1328.Pp
1329.It Pa ~/.shosts
1330This file is used in exactly the same way as
1331.Pa .rhosts ,
1332but allows host-based authentication without permitting login with
1333rlogin/rsh.
1334.Pp
1335.It Pa ~/.ssh/
1336This directory is the default location for all user-specific configuration
1337and authentication information.
1338There is no general requirement to keep the entire contents of this directory
1339secret, but the recommended permissions are read/write/execute for the user,
1340and not accessible by others.
1341.Pp
1342.It Pa ~/.ssh/authorized_keys
1343Lists the public keys (DSA, ECDSA, ED25519, RSA)
1344that can be used for logging in as this user.
1345The format of this file is described in the
1346.Xr sshd 8
1347manual page.
1348This file is not highly sensitive, but the recommended
1349permissions are read/write for the user, and not accessible by others.
1350.Pp
1351.It Pa ~/.ssh/config
1352This is the per-user configuration file.
1353The file format and configuration options are described in
1354.Xr ssh_config 5 .
1355Because of the potential for abuse, this file must have strict permissions:
1356read/write for the user, and not writable by others.
1357.Pp
1358.It Pa ~/.ssh/environment
1359Contains additional definitions for environment variables; see
1360.Sx ENVIRONMENT ,
1361above.
1362.Pp
1363.It Pa ~/.ssh/identity
1364.It Pa ~/.ssh/id_dsa
1365.It Pa ~/.ssh/id_ecdsa
1366.It Pa ~/.ssh/id_ed25519
1367.It Pa ~/.ssh/id_rsa
1368Contains the private key for authentication.
1369These files
1370contain sensitive data and should be readable by the user but not
1371accessible by others (read/write/execute).
1372.Nm
1373will simply ignore a private key file if it is accessible by others.
1374It is possible to specify a passphrase when
1375generating the key which will be used to encrypt the
1376sensitive part of this file using 3DES.
1377.Pp
1378.It Pa ~/.ssh/identity.pub
1379.It Pa ~/.ssh/id_dsa.pub
1380.It Pa ~/.ssh/id_ecdsa.pub
1381.It Pa ~/.ssh/id_ed25519.pub
1382.It Pa ~/.ssh/id_rsa.pub
1383Contains the public key for authentication.
1384These files are not
1385sensitive and can (but need not) be readable by anyone.
1386.Pp
1387.It Pa ~/.ssh/known_hosts
1388Contains a list of host keys for all hosts the user has logged into
1389that are not already in the systemwide list of known host keys.
1390See
1391.Xr sshd 8
1392for further details of the format of this file.
1393.Pp
1394.It Pa ~/.ssh/rc
1395Commands in this file are executed by
1396.Nm
1397when the user logs in, just before the user's shell (or command) is
1398started.
1399See the
1400.Xr sshd 8
1401manual page for more information.
1402.Pp
1403.It Pa /etc/hosts.equiv
1404This file is for host-based authentication (see above).
1405It should only be writable by root.
1406.Pp
1407.It Pa /etc/shosts.equiv
1408This file is used in exactly the same way as
1409.Pa hosts.equiv ,
1410but allows host-based authentication without permitting login with
1411rlogin/rsh.
1412.Pp
1413.It Pa /etc/ssh/ssh_config
1414Systemwide configuration file.
1415The file format and configuration options are described in
1416.Xr ssh_config 5 .
1417.Pp
1418.It Pa /etc/ssh/ssh_host_key
1419.It Pa /etc/ssh/ssh_host_dsa_key
1420.It Pa /etc/ssh/ssh_host_ecdsa_key
1421.It Pa /etc/ssh/ssh_host_ed25519_key
1422.It Pa /etc/ssh/ssh_host_rsa_key
1423These files contain the private parts of the host keys
1424and are used for host-based authentication.
1425If protocol version 1 is used,
1426.Nm
1427must be setuid root, since the host key is readable only by root.
1428For protocol version 2,
1429.Nm
1430uses
1431.Xr ssh-keysign 8
1432to access the host keys,
1433eliminating the requirement that
1434.Nm
1435be setuid root when host-based authentication is used.
1436By default
1437.Nm
1438is not setuid root.
1439.Pp
1440.It Pa /etc/ssh/ssh_known_hosts
1441Systemwide list of known host keys.
1442This file should be prepared by the
1443system administrator to contain the public host keys of all machines in the
1444organization.
1445It should be world-readable.
1446See
1447.Xr sshd 8
1448for further details of the format of this file.
1449.Pp
1450.It Pa /etc/ssh/sshrc
1451Commands in this file are executed by
1452.Nm
1453when the user logs in, just before the user's shell (or command) is started.
1454See the
1455.Xr sshd 8
1456manual page for more information.
1457.El
1458.Sh EXIT STATUS
1459.Nm
1460exits with the exit status of the remote command or with 255
1461if an error occurred.
1462.Sh SEE ALSO
1463.Xr scp 1 ,
1464.Xr sftp 1 ,
1465.Xr ssh-add 1 ,
1466.Xr ssh-agent 1 ,
1467.Xr ssh-keygen 1 ,
1468.Xr ssh-keyscan 1 ,
1469.Xr tun 4 ,
1470.Xr hosts.equiv 5 ,
1471.Xr ssh_config 5 ,
1472.Xr ssh-keysign 8 ,
1473.Xr sshd 8
1474.Sh STANDARDS
1475.Rs
1476.%A S. Lehtinen
1477.%A C. Lonvick
1478.%D January 2006
1479.%R RFC 4250
1480.%T The Secure Shell (SSH) Protocol Assigned Numbers
1481.Re
1482.Pp
1483.Rs
1484.%A T. Ylonen
1485.%A C. Lonvick
1486.%D January 2006
1487.%R RFC 4251
1488.%T The Secure Shell (SSH) Protocol Architecture
1489.Re
1490.Pp
1491.Rs
1492.%A T. Ylonen
1493.%A C. Lonvick
1494.%D January 2006
1495.%R RFC 4252
1496.%T The Secure Shell (SSH) Authentication Protocol
1497.Re
1498.Pp
1499.Rs
1500.%A T. Ylonen
1501.%A C. Lonvick
1502.%D January 2006
1503.%R RFC 4253
1504.%T The Secure Shell (SSH) Transport Layer Protocol
1505.Re
1506.Pp
1507.Rs
1508.%A T. Ylonen
1509.%A C. Lonvick
1510.%D January 2006
1511.%R RFC 4254
1512.%T The Secure Shell (SSH) Connection Protocol
1513.Re
1514.Pp
1515.Rs
1516.%A J. Schlyter
1517.%A W. Griffin
1518.%D January 2006
1519.%R RFC 4255
1520.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1521.Re
1522.Pp
1523.Rs
1524.%A F. Cusack
1525.%A M. Forssen
1526.%D January 2006
1527.%R RFC 4256
1528.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1529.Re
1530.Pp
1531.Rs
1532.%A J. Galbraith
1533.%A P. Remaker
1534.%D January 2006
1535.%R RFC 4335
1536.%T The Secure Shell (SSH) Session Channel Break Extension
1537.Re
1538.Pp
1539.Rs
1540.%A M. Bellare
1541.%A T. Kohno
1542.%A C. Namprempre
1543.%D January 2006
1544.%R RFC 4344
1545.%T The Secure Shell (SSH) Transport Layer Encryption Modes
1546.Re
1547.Pp
1548.Rs
1549.%A B. Harris
1550.%D January 2006
1551.%R RFC 4345
1552.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1553.Re
1554.Pp
1555.Rs
1556.%A M. Friedl
1557.%A N. Provos
1558.%A W. Simpson
1559.%D March 2006
1560.%R RFC 4419
1561.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1562.Re
1563.Pp
1564.Rs
1565.%A J. Galbraith
1566.%A R. Thayer
1567.%D November 2006
1568.%R RFC 4716
1569.%T The Secure Shell (SSH) Public Key File Format
1570.Re
1571.Pp
1572.Rs
1573.%A D. Stebila
1574.%A J. Green
1575.%D December 2009
1576.%R RFC 5656
1577.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1578.Re
1579.Pp
1580.Rs
1581.%A A. Perrig
1582.%A D. Song
1583.%D 1999
1584.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1585.%T Hash Visualization: a New Technique to improve Real-World Security
1586.Re
1587.Sh AUTHORS
1588OpenSSH is a derivative of the original and free
1589ssh 1.2.12 release by Tatu Ylonen.
1590Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1591Theo de Raadt and Dug Song
1592removed many bugs, re-added newer features and
1593created OpenSSH.
1594Markus Friedl contributed the support for SSH
1595protocol versions 1.5 and 2.0.
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