Annotation of src/usr.bin/ssh/ssh.1, Revision 1.364
1.1 deraadt 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
1.59 deraadt 6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
1.93 deraadt 12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
1.59 deraadt 15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
18: .\" are met:
19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
23: .\" documentation and/or other materials provided with the distribution.
1.1 deraadt 24: .\"
1.59 deraadt 25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31: .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32: .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 35: .\"
1.364 ! jmc 36: .\" $OpenBSD: ssh.1,v 1.363 2015/09/24 06:15:11 djm Exp $
! 37: .Dd $Mdocdate: September 24 2015 $
1.2 deraadt 38: .Dt SSH 1
39: .Os
40: .Sh NAME
41: .Nm ssh
1.96 deraadt 42: .Nd OpenSSH SSH client (remote login program)
1.2 deraadt 43: .Sh SYNOPSIS
44: .Nm ssh
1.306 jmc 45: .Bk -words
1.351 jmc 46: .Op Fl 1246AaCfGgKkMNnqsTtVvXxYy
1.108 markus 47: .Op Fl b Ar bind_address
1.51 markus 48: .Op Fl c Ar cipher_spec
1.306 jmc 49: .Op Fl D Oo Ar bind_address : Oc Ns Ar port
1.331 dtucker 50: .Op Fl E Ar log_file
1.2 deraadt 51: .Op Fl e Ar escape_char
1.176 jmc 52: .Op Fl F Ar configfile
1.292 jmc 53: .Op Fl I Ar pkcs11
1.2 deraadt 54: .Op Fl i Ar identity_file
1.360 millert 55: .Op Fl L Ar address
1.176 jmc 56: .Op Fl l Ar login_name
57: .Op Fl m Ar mac_spec
1.198 djm 58: .Op Fl O Ar ctl_cmd
1.176 jmc 59: .Op Fl o Ar option
60: .Op Fl p Ar port
1.362 djm 61: .Op Fl Q Ar query_option
1.360 millert 62: .Op Fl R Ar address
1.198 djm 63: .Op Fl S Ar ctl_path
1.290 dtucker 64: .Op Fl W Ar host : Ns Ar port
1.306 jmc 65: .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
1.176 jmc 66: .Oo Ar user Ns @ Oc Ns Ar hostname
1.2 deraadt 67: .Op Ar command
1.306 jmc 68: .Ek
1.44 aaron 69: .Sh DESCRIPTION
1.2 deraadt 70: .Nm
1.96 deraadt 71: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 72: executing commands on a remote machine.
1.176 jmc 73: It is intended to replace rlogin and rsh,
74: and provide secure encrypted communications between
1.40 aaron 75: two untrusted hosts over an insecure network.
1.348 millert 76: X11 connections, arbitrary TCP ports and
77: .Ux Ns -domain
78: sockets can also be forwarded over the secure channel.
1.2 deraadt 79: .Pp
80: .Nm
1.44 aaron 81: connects and logs into the specified
1.176 jmc 82: .Ar hostname
83: (with optional
84: .Ar user
85: name).
1.1 deraadt 86: The user must prove
1.49 markus 87: his/her identity to the remote machine using one of several methods
1.221 jmc 88: depending on the protocol version used (see below).
1.49 markus 89: .Pp
1.176 jmc 90: If
91: .Ar command
92: is specified,
1.219 jmc 93: it is executed on the remote host instead of a login shell.
1.2 deraadt 94: .Pp
1.218 jmc 95: The options are as follows:
1.361 millert 96: .Pp
1.360 millert 97: .Bl -tag -width Ds -compact
1.218 jmc 98: .It Fl 1
99: Forces
1.2 deraadt 100: .Nm
1.218 jmc 101: to try protocol version 1 only.
1.360 millert 102: .Pp
1.218 jmc 103: .It Fl 2
104: Forces
1.2 deraadt 105: .Nm
1.218 jmc 106: to try protocol version 2 only.
1.360 millert 107: .Pp
1.218 jmc 108: .It Fl 4
109: Forces
1.2 deraadt 110: .Nm
1.218 jmc 111: to use IPv4 addresses only.
1.360 millert 112: .Pp
1.218 jmc 113: .It Fl 6
114: Forces
1.2 deraadt 115: .Nm
1.218 jmc 116: to use IPv6 addresses only.
1.360 millert 117: .Pp
1.218 jmc 118: .It Fl A
119: Enables forwarding of the authentication agent connection.
120: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 121: .Pp
1.218 jmc 122: Agent forwarding should be enabled with caution.
123: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 124: (for the agent's
1.286 sobrado 125: .Ux Ns -domain
126: socket) can access the local agent through the forwarded connection.
1.218 jmc 127: An attacker cannot obtain key material from the agent,
128: however they can perform operations on the keys that enable them to
129: authenticate using the identities loaded into the agent.
1.360 millert 130: .Pp
1.218 jmc 131: .It Fl a
132: Disables forwarding of the authentication agent connection.
1.360 millert 133: .Pp
1.218 jmc 134: .It Fl b Ar bind_address
135: Use
136: .Ar bind_address
137: on the local machine as the source address
138: of the connection.
139: Only useful on systems with more than one address.
1.360 millert 140: .Pp
1.218 jmc 141: .It Fl C
142: Requests compression of all data (including stdin, stdout, stderr, and
1.348 millert 143: data for forwarded X11, TCP and
144: .Ux Ns -domain
145: connections).
1.218 jmc 146: The compression algorithm is the same used by
147: .Xr gzip 1 ,
148: and the
149: .Dq level
150: can be controlled by the
151: .Cm CompressionLevel
152: option for protocol version 1.
153: Compression is desirable on modem lines and other
154: slow connections, but will only slow down things on fast networks.
155: The default value can be set on a host-by-host basis in the
156: configuration files; see the
157: .Cm Compression
158: option.
1.360 millert 159: .Pp
1.218 jmc 160: .It Fl c Ar cipher_spec
161: Selects the cipher specification for encrypting the session.
1.2 deraadt 162: .Pp
1.218 jmc 163: Protocol version 1 allows specification of a single cipher.
164: The supported values are
165: .Dq 3des ,
1.220 jmc 166: .Dq blowfish ,
1.218 jmc 167: and
168: .Dq des .
1.230 jmc 169: For protocol version 2,
1.218 jmc 170: .Ar cipher_spec
171: is a comma-separated list of ciphers
172: listed in order of preference.
1.283 jmc 173: See the
174: .Cm Ciphers
1.307 dtucker 175: keyword in
176: .Xr ssh_config 5
177: for more information.
1.360 millert 178: .Pp
1.218 jmc 179: .It Fl D Xo
180: .Sm off
181: .Oo Ar bind_address : Oc
182: .Ar port
183: .Sm on
184: .Xc
185: Specifies a local
186: .Dq dynamic
187: application-level port forwarding.
188: This works by allocating a socket to listen to
189: .Ar port
190: on the local side, optionally bound to the specified
191: .Ar bind_address .
192: Whenever a connection is made to this port, the
193: connection is forwarded over the secure channel, and the application
194: protocol is then used to determine where to connect to from the
195: remote machine.
196: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 197: .Nm
1.218 jmc 198: will act as a SOCKS server.
199: Only root can forward privileged ports.
200: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 201: .Pp
1.308 djm 202: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 203: Only the superuser can forward privileged ports.
204: By default, the local port is bound in accordance with the
205: .Cm GatewayPorts
206: setting.
207: However, an explicit
208: .Ar bind_address
209: may be used to bind the connection to a specific address.
210: The
211: .Ar bind_address
212: of
213: .Dq localhost
214: indicates that the listening port be bound for local use only, while an
215: empty address or
216: .Sq *
217: indicates that the port should be available from all interfaces.
1.360 millert 218: .Pp
1.331 dtucker 219: .It Fl E Ar log_file
220: Append debug logs to
221: .Ar log_file
222: instead of standard error.
1.360 millert 223: .Pp
1.229 jmc 224: .It Fl e Ar escape_char
1.218 jmc 225: Sets the escape character for sessions with a pty (default:
226: .Ql ~ ) .
227: The escape character is only recognized at the beginning of a line.
228: The escape character followed by a dot
229: .Pq Ql \&.
230: closes the connection;
231: followed by control-Z suspends the connection;
232: and followed by itself sends the escape character once.
233: Setting the character to
1.2 deraadt 234: .Dq none
1.218 jmc 235: disables any escapes and makes the session fully transparent.
1.360 millert 236: .Pp
1.218 jmc 237: .It Fl F Ar configfile
238: Specifies an alternative per-user configuration file.
239: If a configuration file is given on the command line,
240: the system-wide configuration file
241: .Pq Pa /etc/ssh/ssh_config
242: will be ignored.
243: The default for the per-user configuration file is
244: .Pa ~/.ssh/config .
1.360 millert 245: .Pp
1.218 jmc 246: .It Fl f
247: Requests
248: .Nm
249: to go to background just before command execution.
250: This is useful if
1.176 jmc 251: .Nm
1.218 jmc 252: is going to ask for passwords or passphrases, but the user
253: wants it in the background.
254: This implies
255: .Fl n .
256: The recommended way to start X11 programs at a remote site is with
257: something like
258: .Ic ssh -f host xterm .
1.277 djm 259: .Pp
260: If the
261: .Cm ExitOnForwardFailure
262: configuration option is set to
263: .Dq yes ,
264: then a client started with
265: .Fl f
266: will wait for all remote port forwards to be successfully established
267: before placing itself in the background.
1.360 millert 268: .Pp
1.350 djm 269: .It Fl G
270: Causes
271: .Nm
272: to print its configuration after evaluating
273: .Cm Host
274: and
275: .Cm Match
276: blocks and exit.
1.360 millert 277: .Pp
1.218 jmc 278: .It Fl g
279: Allows remote hosts to connect to local forwarded ports.
1.346 djm 280: If used on a multiplexed connection, then this option must be specified
281: on the master process.
1.360 millert 282: .Pp
1.291 markus 283: .It Fl I Ar pkcs11
1.294 jmc 284: Specify the PKCS#11 shared library
1.176 jmc 285: .Nm
1.293 markus 286: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 287: private RSA key.
1.360 millert 288: .Pp
1.218 jmc 289: .It Fl i Ar identity_file
290: Selects a file from which the identity (private key) for
1.310 djm 291: public key authentication is read.
1.218 jmc 292: The default is
293: .Pa ~/.ssh/identity
294: for protocol version 1, and
1.310 djm 295: .Pa ~/.ssh/id_dsa ,
1.343 naddy 296: .Pa ~/.ssh/id_ecdsa ,
297: .Pa ~/.ssh/id_ed25519
1.310 djm 298: and
1.218 jmc 299: .Pa ~/.ssh/id_rsa
300: for protocol version 2.
301: Identity files may also be specified on
302: a per-host basis in the configuration file.
303: It is possible to have multiple
304: .Fl i
305: options (and multiple identities specified in
306: configuration files).
1.364 ! jmc 307: If no certificates have been explicitly specified by the
1.363 djm 308: .Cm CertificateFile
309: directive,
1.302 djm 310: .Nm
311: will also try to load certificate information from the filename obtained
312: by appending
313: .Pa -cert.pub
314: to identity filenames.
1.360 millert 315: .Pp
1.269 djm 316: .It Fl K
317: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
318: credentials to the server.
1.360 millert 319: .Pp
1.218 jmc 320: .It Fl k
321: Disables forwarding (delegation) of GSSAPI credentials to the server.
1.360 millert 322: .Pp
1.218 jmc 323: .It Fl L Xo
324: .Sm off
325: .Oo Ar bind_address : Oc
326: .Ar port : host : hostport
327: .Sm on
328: .Xc
1.360 millert 329: .It Fl L Xo
330: .Sm off
331: .Oo Ar bind_address : Oc
332: .Ar port : remote_socket
333: .Sm on
334: .Xc
335: .It Fl L Xo
336: .Sm off
337: .Ar local_socket : host : hostport
338: .Sm on
339: .Xc
340: .It Fl L Xo
341: .Sm off
342: .Ar local_socket : remote_socket
343: .Sm on
344: .Xc
345: Specifies that connections to the given TCP port or Unix socket on the local
346: (client) host are to be forwarded to the given host and port, or Unix socket,
347: on the remote side.
348: This works by allocating a socket to listen to either a TCP
1.218 jmc 349: .Ar port
350: on the local side, optionally bound to the specified
1.360 millert 351: .Ar bind_address ,
352: or to a Unix socket.
353: Whenever a connection is made to the local port or socket, the
1.218 jmc 354: connection is forwarded over the secure channel, and a connection is
1.360 millert 355: made to either
1.218 jmc 356: .Ar host
357: port
1.360 millert 358: .Ar hostport ,
359: or the Unix socket
360: .Ar remote_socket ,
1.218 jmc 361: from the remote machine.
1.360 millert 362: .Pp
1.218 jmc 363: Port forwardings can also be specified in the configuration file.
1.360 millert 364: Only the superuser can forward privileged ports.
1.308 djm 365: IPv6 addresses can be specified by enclosing the address in square brackets.
1.360 millert 366: .Pp
1.218 jmc 367: By default, the local port is bound in accordance with the
368: .Cm GatewayPorts
369: setting.
370: However, an explicit
371: .Ar bind_address
372: may be used to bind the connection to a specific address.
1.2 deraadt 373: The
1.218 jmc 374: .Ar bind_address
375: of
376: .Dq localhost
377: indicates that the listening port be bound for local use only, while an
378: empty address or
379: .Sq *
380: indicates that the port should be available from all interfaces.
1.360 millert 381: .Pp
1.218 jmc 382: .It Fl l Ar login_name
383: Specifies the user to log in as on the remote machine.
384: This also may be specified on a per-host basis in the configuration file.
1.360 millert 385: .Pp
1.218 jmc 386: .It Fl M
387: Places the
388: .Nm
389: client into
390: .Dq master
391: mode for connection sharing.
1.231 stevesk 392: Multiple
393: .Fl M
394: options places
395: .Nm
396: into
397: .Dq master
398: mode with confirmation required before slave connections are accepted.
1.218 jmc 399: Refer to the description of
400: .Cm ControlMaster
401: in
402: .Xr ssh_config 5
403: for details.
1.360 millert 404: .Pp
1.218 jmc 405: .It Fl m Ar mac_spec
406: Additionally, for protocol version 2 a comma-separated list of MAC
407: (message authentication code) algorithms can
408: be specified in order of preference.
409: See the
410: .Cm MACs
411: keyword for more information.
1.360 millert 412: .Pp
1.218 jmc 413: .It Fl N
414: Do not execute a remote command.
415: This is useful for just forwarding ports
416: (protocol version 2 only).
1.360 millert 417: .Pp
1.218 jmc 418: .It Fl n
419: Redirects stdin from
420: .Pa /dev/null
421: (actually, prevents reading from stdin).
422: This must be used when
1.2 deraadt 423: .Nm
1.218 jmc 424: is run in the background.
425: A common trick is to use this to run X11 programs on a remote machine.
426: For example,
427: .Ic ssh -n shadows.cs.hut.fi emacs &
428: will start an emacs on shadows.cs.hut.fi, and the X11
429: connection will be automatically forwarded over an encrypted channel.
430: The
1.2 deraadt 431: .Nm
1.218 jmc 432: program will be put in the background.
433: (This does not work if
1.2 deraadt 434: .Nm
1.218 jmc 435: needs to ask for a password or passphrase; see also the
436: .Fl f
437: option.)
1.360 millert 438: .Pp
1.218 jmc 439: .It Fl O Ar ctl_cmd
440: Control an active connection multiplexing master process.
441: When the
442: .Fl O
443: option is specified, the
444: .Ar ctl_cmd
445: argument is interpreted and passed to the master process.
446: Valid commands are:
447: .Dq check
1.305 markus 448: (check that the master process is running),
449: .Dq forward
1.318 jmc 450: (request forwardings without command execution),
1.323 okan 451: .Dq cancel
452: (cancel forwardings),
1.218 jmc 453: .Dq exit
1.318 jmc 454: (request the master to exit), and
1.317 djm 455: .Dq stop
456: (request the master to stop accepting further multiplexing requests).
1.360 millert 457: .Pp
1.218 jmc 458: .It Fl o Ar option
459: Can be used to give options in the format used in the configuration file.
460: This is useful for specifying options for which there is no separate
461: command-line flag.
462: For full details of the options listed below, and their possible values, see
463: .Xr ssh_config 5 .
1.2 deraadt 464: .Pp
1.218 jmc 465: .Bl -tag -width Ds -offset indent -compact
466: .It AddressFamily
467: .It BatchMode
468: .It BindAddress
1.338 djm 469: .It CanonicalDomains
1.339 djm 470: .It CanonicalizeFallbackLocal
471: .It CanonicalizeHostname
472: .It CanonicalizeMaxDots
473: .It CanonicalizePermittedCNAMEs
1.363 djm 474: .It CertificateFile
1.218 jmc 475: .It ChallengeResponseAuthentication
476: .It CheckHostIP
477: .It Cipher
478: .It Ciphers
479: .It ClearAllForwardings
480: .It Compression
481: .It CompressionLevel
482: .It ConnectionAttempts
483: .It ConnectTimeout
484: .It ControlMaster
485: .It ControlPath
1.321 djm 486: .It ControlPersist
1.218 jmc 487: .It DynamicForward
488: .It EscapeChar
1.263 markus 489: .It ExitOnForwardFailure
1.353 jmc 490: .It FingerprintHash
1.218 jmc 491: .It ForwardAgent
492: .It ForwardX11
1.321 djm 493: .It ForwardX11Timeout
1.218 jmc 494: .It ForwardX11Trusted
495: .It GatewayPorts
496: .It GlobalKnownHostsFile
497: .It GSSAPIAuthentication
498: .It GSSAPIDelegateCredentials
499: .It HashKnownHosts
500: .It Host
501: .It HostbasedAuthentication
1.355 djm 502: .It HostbasedKeyTypes
1.218 jmc 503: .It HostKeyAlgorithms
504: .It HostKeyAlias
505: .It HostName
506: .It IdentityFile
507: .It IdentitiesOnly
1.316 jmc 508: .It IPQoS
1.321 djm 509: .It KbdInteractiveAuthentication
1.218 jmc 510: .It KbdInteractiveDevices
1.314 jmc 511: .It KexAlgorithms
1.218 jmc 512: .It LocalCommand
513: .It LocalForward
514: .It LogLevel
515: .It MACs
1.337 jmc 516: .It Match
1.218 jmc 517: .It NoHostAuthenticationForLocalhost
518: .It NumberOfPasswordPrompts
519: .It PasswordAuthentication
520: .It PermitLocalCommand
1.291 markus 521: .It PKCS11Provider
1.218 jmc 522: .It Port
523: .It PreferredAuthentications
524: .It Protocol
525: .It ProxyCommand
1.336 jmc 526: .It ProxyUseFdpass
1.359 markus 527: .It PubkeyAcceptedKeyTypes
1.218 jmc 528: .It PubkeyAuthentication
1.251 dtucker 529: .It RekeyLimit
1.218 jmc 530: .It RemoteForward
1.319 jmc 531: .It RequestTTY
1.218 jmc 532: .It RhostsRSAAuthentication
533: .It RSAAuthentication
534: .It SendEnv
535: .It ServerAliveInterval
536: .It ServerAliveCountMax
1.347 jmc 537: .It StreamLocalBindMask
538: .It StreamLocalBindUnlink
1.218 jmc 539: .It StrictHostKeyChecking
540: .It TCPKeepAlive
541: .It Tunnel
542: .It TunnelDevice
1.354 djm 543: .It UpdateHostKeys
1.218 jmc 544: .It UsePrivilegedPort
545: .It User
546: .It UserKnownHostsFile
547: .It VerifyHostKeyDNS
1.276 jmc 548: .It VisualHostKey
1.218 jmc 549: .It XAuthLocation
550: .El
1.360 millert 551: .Pp
1.218 jmc 552: .It Fl p Ar port
553: Port to connect to on the remote host.
554: This can be specified on a
555: per-host basis in the configuration file.
1.360 millert 556: .Pp
1.362 djm 557: .It Fl Q Ar query_option
1.332 djm 558: Queries
559: .Nm
1.341 deraadt 560: for the algorithms supported for the specified version 2.
561: The available features are:
562: .Ar cipher
1.332 djm 563: (supported symmetric ciphers),
1.341 deraadt 564: .Ar cipher-auth
1.340 djm 565: (supported symmetric ciphers that support authenticated encryption),
1.341 deraadt 566: .Ar mac
1.332 djm 567: (supported message integrity codes),
1.341 deraadt 568: .Ar kex
1.332 djm 569: (key exchange algorithms),
1.341 deraadt 570: .Ar key
1.362 djm 571: (key types),
572: .Ar key-cert
573: (certificate key types),
574: .Ar key-plain
575: (non-certificate key types), and
1.356 djm 576: .Ar protocol-version
577: (supported SSH protocol versions).
1.360 millert 578: .Pp
1.218 jmc 579: .It Fl q
580: Quiet mode.
1.271 djm 581: Causes most warning and diagnostic messages to be suppressed.
1.360 millert 582: .Pp
1.218 jmc 583: .It Fl R Xo
584: .Sm off
585: .Oo Ar bind_address : Oc
586: .Ar port : host : hostport
587: .Sm on
588: .Xc
1.360 millert 589: .It Fl R Xo
590: .Sm off
591: .Oo Ar bind_address : Oc
592: .Ar port : local_socket
593: .Sm on
594: .Xc
595: .It Fl R Xo
596: .Sm off
597: .Ar remote_socket : host : hostport
598: .Sm on
599: .Xc
600: .It Fl R Xo
601: .Sm off
602: .Ar remote_socket : local_socket
603: .Sm on
604: .Xc
605: Specifies that connections to the given TCP port or Unix socket on the remote
606: (server) host are to be forwarded to the given host and port, or Unix socket,
607: on the local side.
608: This works by allocating a socket to listen to either a TCP
1.218 jmc 609: .Ar port
1.360 millert 610: or to a Unix socket on the remote side.
611: Whenever a connection is made to this port or Unix socket, the
612: connection is forwarded over the secure channel, and a connection
613: is made to either
1.218 jmc 614: .Ar host
615: port
1.360 millert 616: .Ar hostport ,
617: or
618: .Ar local_socket ,
1.218 jmc 619: from the local machine.
1.2 deraadt 620: .Pp
1.218 jmc 621: Port forwardings can also be specified in the configuration file.
622: Privileged ports can be forwarded only when
623: logging in as root on the remote machine.
1.324 jmc 624: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 625: .Pp
1.360 millert 626: By default, TCP listening sockets on the server will be bound to the loopback
1.218 jmc 627: interface only.
1.280 tobias 628: This may be overridden by specifying a
1.218 jmc 629: .Ar bind_address .
630: An empty
631: .Ar bind_address ,
632: or the address
633: .Ql * ,
634: indicates that the remote socket should listen on all interfaces.
635: Specifying a remote
636: .Ar bind_address
637: will only succeed if the server's
638: .Cm GatewayPorts
639: option is enabled (see
640: .Xr sshd_config 5 ) .
1.281 djm 641: .Pp
642: If the
643: .Ar port
644: argument is
1.282 djm 645: .Ql 0 ,
1.281 djm 646: the listen port will be dynamically allocated on the server and reported
647: to the client at run time.
1.305 markus 648: When used together with
649: .Ic -O forward
650: the allocated port will be printed to the standard output.
1.360 millert 651: .Pp
1.218 jmc 652: .It Fl S Ar ctl_path
1.304 jmc 653: Specifies the location of a control socket for connection sharing,
1.303 djm 654: or the string
655: .Dq none
656: to disable connection sharing.
1.218 jmc 657: Refer to the description of
658: .Cm ControlPath
659: and
660: .Cm ControlMaster
661: in
662: .Xr ssh_config 5
663: for details.
1.360 millert 664: .Pp
1.218 jmc 665: .It Fl s
666: May be used to request invocation of a subsystem on the remote system.
667: Subsystems are a feature of the SSH2 protocol which facilitate the use
668: of SSH as a secure transport for other applications (eg.\&
669: .Xr sftp 1 ) .
670: The subsystem is specified as the remote command.
1.360 millert 671: .Pp
1.218 jmc 672: .It Fl T
1.357 dtucker 673: Disable pseudo-terminal allocation.
1.360 millert 674: .Pp
1.218 jmc 675: .It Fl t
1.357 dtucker 676: Force pseudo-terminal allocation.
1.218 jmc 677: This can be used to execute arbitrary
678: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 679: e.g. when implementing menu services.
1.218 jmc 680: Multiple
681: .Fl t
682: options force tty allocation, even if
1.194 jakob 683: .Nm
1.218 jmc 684: has no local tty.
1.360 millert 685: .Pp
1.218 jmc 686: .It Fl V
687: Display the version number and exit.
1.360 millert 688: .Pp
1.218 jmc 689: .It Fl v
690: Verbose mode.
691: Causes
1.176 jmc 692: .Nm
1.218 jmc 693: to print debugging messages about its progress.
694: This is helpful in
695: debugging connection, authentication, and configuration problems.
696: Multiple
697: .Fl v
698: options increase the verbosity.
699: The maximum is 3.
1.360 millert 700: .Pp
1.290 dtucker 701: .It Fl W Ar host : Ns Ar port
702: Requests that standard input and output on the client be forwarded to
703: .Ar host
704: on
705: .Ar port
706: over the secure channel.
707: Implies
708: .Fl N ,
709: .Fl T ,
710: .Cm ExitOnForwardFailure
711: and
1.326 dtucker 712: .Cm ClearAllForwardings .
713: Works with Protocol version 2 only.
1.360 millert 714: .Pp
1.261 stevesk 715: .It Fl w Xo
716: .Ar local_tun Ns Op : Ns Ar remote_tun
717: .Xc
718: Requests
719: tunnel
720: device forwarding with the specified
1.218 jmc 721: .Xr tun 4
1.261 stevesk 722: devices between the client
723: .Pq Ar local_tun
724: and the server
725: .Pq Ar remote_tun .
726: .Pp
1.228 jmc 727: The devices may be specified by numerical ID or the keyword
728: .Dq any ,
729: which uses the next available tunnel device.
1.261 stevesk 730: If
731: .Ar remote_tun
732: is not specified, it defaults to
733: .Dq any .
1.228 jmc 734: See also the
1.218 jmc 735: .Cm Tunnel
1.261 stevesk 736: and
737: .Cm TunnelDevice
738: directives in
1.218 jmc 739: .Xr ssh_config 5 .
1.261 stevesk 740: If the
741: .Cm Tunnel
742: directive is unset, it is set to the default tunnel mode, which is
743: .Dq point-to-point .
1.360 millert 744: .Pp
1.218 jmc 745: .It Fl X
746: Enables X11 forwarding.
1.54 markus 747: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 748: .Pp
1.218 jmc 749: X11 forwarding should be enabled with caution.
1.168 jmc 750: Users with the ability to bypass file permissions on the remote host
1.218 jmc 751: (for the user's X authorization database)
752: can access the local X11 display through the forwarded connection.
753: An attacker may then be able to perform activities such as keystroke monitoring.
754: .Pp
755: For this reason, X11 forwarding is subjected to X11 SECURITY extension
756: restrictions by default.
757: Please refer to the
758: .Nm
759: .Fl Y
760: option and the
761: .Cm ForwardX11Trusted
762: directive in
763: .Xr ssh_config 5
764: for more information.
1.360 millert 765: .Pp
1.218 jmc 766: .It Fl x
767: Disables X11 forwarding.
1.360 millert 768: .Pp
1.218 jmc 769: .It Fl Y
770: Enables trusted X11 forwarding.
771: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
772: controls.
1.360 millert 773: .Pp
1.278 djm 774: .It Fl y
775: Send log information using the
776: .Xr syslog 3
777: system module.
778: By default this information is sent to stderr.
1.218 jmc 779: .El
1.224 jmc 780: .Pp
781: .Nm
782: may additionally obtain configuration data from
783: a per-user configuration file and a system-wide configuration file.
784: The file format and configuration options are described in
785: .Xr ssh_config 5 .
1.222 jmc 786: .Sh AUTHENTICATION
1.249 jmc 787: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 788: The default is to use protocol 2 only,
789: though this can be changed via the
1.222 jmc 790: .Cm Protocol
791: option in
1.284 jmc 792: .Xr ssh_config 5
793: or the
1.222 jmc 794: .Fl 1
795: and
796: .Fl 2
797: options (see above).
798: Both protocols support similar authentication methods,
1.284 jmc 799: but protocol 2 is the default since
1.222 jmc 800: it provides additional mechanisms for confidentiality
801: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.320 djm 802: and integrity (hmac-md5, hmac-sha1,
803: hmac-sha2-256, hmac-sha2-512,
1.330 markus 804: umac-64, umac-128, hmac-ripemd160).
1.222 jmc 805: Protocol 1 lacks a strong mechanism for ensuring the
806: integrity of the connection.
807: .Pp
808: The methods available for authentication are:
1.260 jmc 809: GSSAPI-based authentication,
1.222 jmc 810: host-based authentication,
811: public key authentication,
812: challenge-response authentication,
813: and password authentication.
814: Authentication methods are tried in the order specified above,
815: though protocol 2 has a configuration option to change the default order:
816: .Cm PreferredAuthentications .
817: .Pp
818: Host-based authentication works as follows:
1.218 jmc 819: If the machine the user logs in from is listed in
820: .Pa /etc/hosts.equiv
821: or
822: .Pa /etc/shosts.equiv
823: on the remote machine, and the user names are
824: the same on both sides, or if the files
825: .Pa ~/.rhosts
826: or
827: .Pa ~/.shosts
828: exist in the user's home directory on the
829: remote machine and contain a line containing the name of the client
830: machine and the name of the user on that machine, the user is
1.222 jmc 831: considered for login.
832: Additionally, the server
833: .Em must
834: be able to verify the client's
835: host key (see the description of
1.218 jmc 836: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 837: and
1.222 jmc 838: .Pa ~/.ssh/known_hosts ,
839: below)
840: for login to be permitted.
1.218 jmc 841: This authentication method closes security holes due to IP
1.222 jmc 842: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 843: [Note to the administrator:
844: .Pa /etc/hosts.equiv ,
845: .Pa ~/.rhosts ,
846: and the rlogin/rsh protocol in general, are inherently insecure and should be
847: disabled if security is desired.]
1.189 dtucker 848: .Pp
1.222 jmc 849: Public key authentication works as follows:
850: The scheme is based on public-key cryptography,
851: using cryptosystems
852: where encryption and decryption are done using separate keys,
853: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 854: The idea is that each user creates a public/private
855: key pair for authentication purposes.
856: The server knows the public key, and only the user knows the private key.
1.222 jmc 857: .Nm
858: implements public key authentication protocol automatically,
1.349 sobrado 859: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.222 jmc 860: Protocol 1 is restricted to using only RSA keys,
1.310 djm 861: but protocol 2 may use any.
1.333 jmc 862: The HISTORY section of
1.222 jmc 863: .Xr ssl 8
1.311 jmc 864: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 865: .Pp
1.218 jmc 866: The file
867: .Pa ~/.ssh/authorized_keys
868: lists the public keys that are permitted for logging in.
869: When the user logs in, the
1.2 deraadt 870: .Nm
1.218 jmc 871: program tells the server which key pair it would like to use for
872: authentication.
1.222 jmc 873: The client proves that it has access to the private key
874: and the server checks that the corresponding public key
875: is authorized to accept the account.
1.218 jmc 876: .Pp
1.222 jmc 877: The user creates his/her key pair by running
1.218 jmc 878: .Xr ssh-keygen 1 .
879: This stores the private key in
1.207 djm 880: .Pa ~/.ssh/identity
1.222 jmc 881: (protocol 1),
882: .Pa ~/.ssh/id_dsa
883: (protocol 2 DSA),
1.310 djm 884: .Pa ~/.ssh/id_ecdsa
885: (protocol 2 ECDSA),
1.343 naddy 886: .Pa ~/.ssh/id_ed25519
1.349 sobrado 887: (protocol 2 Ed25519),
1.222 jmc 888: or
889: .Pa ~/.ssh/id_rsa
890: (protocol 2 RSA)
1.218 jmc 891: and stores the public key in
892: .Pa ~/.ssh/identity.pub
1.222 jmc 893: (protocol 1),
894: .Pa ~/.ssh/id_dsa.pub
895: (protocol 2 DSA),
1.310 djm 896: .Pa ~/.ssh/id_ecdsa.pub
897: (protocol 2 ECDSA),
1.343 naddy 898: .Pa ~/.ssh/id_ed25519.pub
1.349 sobrado 899: (protocol 2 Ed25519),
1.222 jmc 900: or
901: .Pa ~/.ssh/id_rsa.pub
902: (protocol 2 RSA)
1.218 jmc 903: in the user's home directory.
1.222 jmc 904: The user should then copy the public key
1.218 jmc 905: to
906: .Pa ~/.ssh/authorized_keys
1.222 jmc 907: in his/her home directory on the remote machine.
908: The
1.218 jmc 909: .Pa authorized_keys
910: file corresponds to the conventional
911: .Pa ~/.rhosts
912: file, and has one key
1.222 jmc 913: per line, though the lines can be very long.
1.218 jmc 914: After this, the user can log in without giving the password.
915: .Pp
1.301 jmc 916: A variation on public key authentication
917: is available in the form of certificate authentication:
918: instead of a set of public/private keys,
919: signed certificates are used.
920: This has the advantage that a single trusted certification authority
921: can be used in place of many public/private keys.
1.333 jmc 922: See the CERTIFICATES section of
1.301 jmc 923: .Xr ssh-keygen 1
924: for more information.
925: .Pp
926: The most convenient way to use public key or certificate authentication
927: may be with an authentication agent.
1.218 jmc 928: See
929: .Xr ssh-agent 1
930: for more information.
931: .Pp
1.222 jmc 932: Challenge-response authentication works as follows:
933: The server sends an arbitrary
934: .Qq challenge
935: text, and prompts for a response.
936: Protocol 2 allows multiple challenges and responses;
937: protocol 1 is restricted to just one challenge/response.
938: Examples of challenge-response authentication include
1.335 jmc 939: .Bx
940: Authentication (see
1.222 jmc 941: .Xr login.conf 5 )
1.335 jmc 942: and PAM (some
943: .Pf non- Ox
944: systems).
1.222 jmc 945: .Pp
946: Finally, if other authentication methods fail,
1.218 jmc 947: .Nm
948: prompts the user for a password.
949: The password is sent to the remote
950: host for checking; however, since all communications are encrypted,
951: the password cannot be seen by someone listening on the network.
1.232 jmc 952: .Pp
953: .Nm
954: automatically maintains and checks a database containing
955: identification for all hosts it has ever been used with.
956: Host keys are stored in
957: .Pa ~/.ssh/known_hosts
958: in the user's home directory.
959: Additionally, the file
960: .Pa /etc/ssh/ssh_known_hosts
961: is automatically checked for known hosts.
962: Any new hosts are automatically added to the user's file.
963: If a host's identification ever changes,
964: .Nm
965: warns about this and disables password authentication to prevent
966: server spoofing or man-in-the-middle attacks,
967: which could otherwise be used to circumvent the encryption.
968: The
969: .Cm StrictHostKeyChecking
970: option can be used to control logins to machines whose
971: host key is not known or has changed.
972: .Pp
1.218 jmc 973: When the user's identity has been accepted by the server, the server
1.357 dtucker 974: either executes the given command in a non-interactive session or,
975: if no command has been specified, logs into the machine and gives
976: the user a normal shell as an interactive session.
1.218 jmc 977: All communication with
978: the remote command or shell will be automatically encrypted.
979: .Pp
1.357 dtucker 980: If an interactive session is requested
981: .Nm
982: by default will only request a pseudo-terminal (pty) for interactive
983: sessions when the client has one.
984: The flags
985: .Fl T
986: and
987: .Fl t
988: can be used to override this behaviour.
989: .Pp
990: If a pseudo-terminal has been allocated the
1.218 jmc 991: user may use the escape characters noted below.
992: .Pp
1.357 dtucker 993: If no pseudo-terminal has been allocated,
1.218 jmc 994: the session is transparent and can be used to reliably transfer binary data.
995: On most systems, setting the escape character to
996: .Dq none
997: will also make the session transparent even if a tty is used.
998: .Pp
999: The session terminates when the command or shell on the remote
1.247 jmc 1000: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 1001: .Sh ESCAPE CHARACTERS
1.218 jmc 1002: When a pseudo-terminal has been requested,
1.2 deraadt 1003: .Nm
1.218 jmc 1004: supports a number of functions through the use of an escape character.
1005: .Pp
1006: A single tilde character can be sent as
1007: .Ic ~~
1008: or by following the tilde by a character other than those described below.
1009: The escape character must always follow a newline to be interpreted as
1010: special.
1011: The escape character can be changed in configuration files using the
1012: .Cm EscapeChar
1013: configuration directive or on the command line by the
1014: .Fl e
1015: option.
1016: .Pp
1017: The supported escapes (assuming the default
1018: .Ql ~ )
1019: are:
1020: .Bl -tag -width Ds
1021: .It Cm ~.
1022: Disconnect.
1023: .It Cm ~^Z
1024: Background
1.234 jmc 1025: .Nm .
1.218 jmc 1026: .It Cm ~#
1027: List forwarded connections.
1028: .It Cm ~&
1029: Background
1.2 deraadt 1030: .Nm
1.218 jmc 1031: at logout when waiting for forwarded connection / X11 sessions to terminate.
1032: .It Cm ~?
1033: Display a list of escape characters.
1034: .It Cm ~B
1035: Send a BREAK to the remote system
1036: (only useful for SSH protocol version 2 and if the peer supports it).
1037: .It Cm ~C
1038: Open command line.
1039: Currently this allows the addition of port forwardings using the
1.279 stevesk 1040: .Fl L ,
1041: .Fl R
1.218 jmc 1042: and
1.279 stevesk 1043: .Fl D
1.225 jmc 1044: options (see above).
1.322 markus 1045: It also allows the cancellation of existing port-forwardings
1046: with
1.262 stevesk 1047: .Sm off
1.322 markus 1048: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1049: .Sm on
1.322 markus 1050: for local,
1051: .Sm off
1052: .Fl KR Oo Ar bind_address : Oc Ar port
1053: .Sm on
1054: for remote and
1055: .Sm off
1056: .Fl KD Oo Ar bind_address : Oc Ar port
1057: .Sm on
1058: for dynamic port-forwardings.
1.218 jmc 1059: .Ic !\& Ns Ar command
1060: allows the user to execute a local command if the
1061: .Ic PermitLocalCommand
1062: option is enabled in
1.176 jmc 1063: .Xr ssh_config 5 .
1.218 jmc 1064: Basic help is available, using the
1065: .Fl h
1066: option.
1067: .It Cm ~R
1068: Request rekeying of the connection
1069: (only useful for SSH protocol version 2 and if the peer supports it).
1.327 dtucker 1070: .It Cm ~V
1071: Decrease the verbosity
1072: .Pq Ic LogLevel
1073: when errors are being written to stderr.
1074: .It Cm ~v
1.328 jmc 1075: Increase the verbosity
1.327 dtucker 1076: .Pq Ic LogLevel
1077: when errors are being written to stderr.
1.176 jmc 1078: .El
1.246 jmc 1079: .Sh TCP FORWARDING
1080: Forwarding of arbitrary TCP connections over the secure channel can
1081: be specified either on the command line or in a configuration file.
1082: One possible application of TCP forwarding is a secure connection to a
1083: mail server; another is going through firewalls.
1084: .Pp
1085: In the example below, we look at encrypting communication between
1086: an IRC client and server, even though the IRC server does not directly
1087: support encrypted communications.
1088: This works as follows:
1089: the user connects to the remote host using
1090: .Nm ,
1091: specifying a port to be used to forward connections
1092: to the remote server.
1093: After that it is possible to start the service which is to be encrypted
1094: on the client machine,
1095: connecting to the same local port,
1096: and
1097: .Nm
1098: will encrypt and forward the connection.
1099: .Pp
1100: The following example tunnels an IRC session from client machine
1101: .Dq 127.0.0.1
1102: (localhost)
1103: to remote server
1104: .Dq server.example.com :
1105: .Bd -literal -offset 4n
1106: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1107: $ irc -c '#users' -p 1234 pinky 127.0.0.1
1108: .Ed
1109: .Pp
1110: This tunnels a connection to IRC server
1111: .Dq server.example.com ,
1112: joining channel
1113: .Dq #users ,
1114: nickname
1115: .Dq pinky ,
1116: using port 1234.
1117: It doesn't matter which port is used,
1118: as long as it's greater than 1023
1119: (remember, only root can open sockets on privileged ports)
1120: and doesn't conflict with any ports already in use.
1121: The connection is forwarded to port 6667 on the remote server,
1122: since that's the standard port for IRC services.
1123: .Pp
1124: The
1125: .Fl f
1126: option backgrounds
1127: .Nm
1128: and the remote command
1129: .Dq sleep 10
1130: is specified to allow an amount of time
1131: (10 seconds, in the example)
1132: to start the service which is to be tunnelled.
1133: If no connections are made within the time specified,
1134: .Nm
1135: will exit.
1136: .Sh X11 FORWARDING
1.218 jmc 1137: If the
1138: .Cm ForwardX11
1139: variable is set to
1140: .Dq yes
1141: (or see the description of the
1.227 jmc 1142: .Fl X ,
1143: .Fl x ,
1.218 jmc 1144: and
1.227 jmc 1145: .Fl Y
1.226 jmc 1146: options above)
1.218 jmc 1147: and the user is using X11 (the
1148: .Ev DISPLAY
1149: environment variable is set), the connection to the X11 display is
1150: automatically forwarded to the remote side in such a way that any X11
1151: programs started from the shell (or command) will go through the
1152: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1153: from the local machine.
1.218 jmc 1154: The user should not manually set
1155: .Ev DISPLAY .
1156: Forwarding of X11 connections can be
1157: configured on the command line or in configuration files.
1158: .Pp
1159: The
1160: .Ev DISPLAY
1161: value set by
1162: .Nm
1163: will point to the server machine, but with a display number greater than zero.
1164: This is normal, and happens because
1165: .Nm
1166: creates a
1167: .Dq proxy
1168: X server on the server machine for forwarding the
1169: connections over the encrypted channel.
1.200 djm 1170: .Pp
1.218 jmc 1171: .Nm
1172: will also automatically set up Xauthority data on the server machine.
1173: For this purpose, it will generate a random authorization cookie,
1174: store it in Xauthority on the server, and verify that any forwarded
1175: connections carry this cookie and replace it by the real cookie when
1176: the connection is opened.
1177: The real authentication cookie is never
1178: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1179: .Pp
1.218 jmc 1180: If the
1181: .Cm ForwardAgent
1182: variable is set to
1183: .Dq yes
1184: (or see the description of the
1185: .Fl A
1.191 djm 1186: and
1.218 jmc 1187: .Fl a
1.226 jmc 1188: options above) and
1.218 jmc 1189: the user is using an authentication agent, the connection to the agent
1190: is automatically forwarded to the remote side.
1.252 jmc 1191: .Sh VERIFYING HOST KEYS
1192: When connecting to a server for the first time,
1193: a fingerprint of the server's public key is presented to the user
1194: (unless the option
1195: .Cm StrictHostKeyChecking
1196: has been disabled).
1197: Fingerprints can be determined using
1198: .Xr ssh-keygen 1 :
1199: .Pp
1200: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1201: .Pp
1.274 grunk 1202: If the fingerprint is already known, it can be matched
1203: and the key can be accepted or rejected.
1.358 djm 1204: If only legacy (MD5) fingerprints for the server are available, the
1205: .Xr ssh-keygen 1
1206: .Fl E
1207: option may be used to downgrade the fingerprint algorithm to match.
1208: .Pp
1.274 grunk 1209: Because of the difficulty of comparing host keys
1.352 djm 1210: just by looking at fingerprint strings,
1.274 grunk 1211: there is also support to compare host keys visually,
1212: using
1213: .Em random art .
1214: By setting the
1.275 grunk 1215: .Cm VisualHostKey
1.274 grunk 1216: option to
1.275 grunk 1217: .Dq yes ,
1.274 grunk 1218: a small ASCII graphic gets displayed on every login to a server, no matter
1219: if the session itself is interactive or not.
1220: By learning the pattern a known server produces, a user can easily
1221: find out that the host key has changed when a completely different pattern
1222: is displayed.
1223: Because these patterns are not unambiguous however, a pattern that looks
1224: similar to the pattern remembered only gives a good probability that the
1225: host key is the same, not guaranteed proof.
1226: .Pp
1227: To get a listing of the fingerprints along with their random art for
1228: all known hosts, the following command line can be used:
1229: .Pp
1230: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1231: .Pp
1.252 jmc 1232: If the fingerprint is unknown,
1233: an alternative method of verification is available:
1234: SSH fingerprints verified by DNS.
1235: An additional resource record (RR),
1236: SSHFP,
1237: is added to a zonefile
1238: and the connecting client is able to match the fingerprint
1239: with that of the key presented.
1240: .Pp
1241: In this example, we are connecting a client to a server,
1242: .Dq host.example.com .
1243: The SSHFP resource records should first be added to the zonefile for
1244: host.example.com:
1245: .Bd -literal -offset indent
1.259 jakob 1246: $ ssh-keygen -r host.example.com.
1.252 jmc 1247: .Ed
1248: .Pp
1249: The output lines will have to be added to the zonefile.
1250: To check that the zone is answering fingerprint queries:
1251: .Pp
1252: .Dl $ dig -t SSHFP host.example.com
1253: .Pp
1254: Finally the client connects:
1255: .Bd -literal -offset indent
1256: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1257: [...]
1258: Matching host key fingerprint found in DNS.
1259: Are you sure you want to continue connecting (yes/no)?
1260: .Ed
1261: .Pp
1262: See the
1263: .Cm VerifyHostKeyDNS
1264: option in
1265: .Xr ssh_config 5
1266: for more information.
1.250 jmc 1267: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1268: .Nm
1269: contains support for Virtual Private Network (VPN) tunnelling
1270: using the
1271: .Xr tun 4
1272: network pseudo-device,
1273: allowing two networks to be joined securely.
1274: The
1275: .Xr sshd_config 5
1276: configuration option
1277: .Cm PermitTunnel
1278: controls whether the server supports this,
1279: and at what level (layer 2 or 3 traffic).
1280: .Pp
1281: The following example would connect client network 10.0.50.0/24
1.265 otto 1282: with remote network 10.0.99.0/24 using a point-to-point connection
1283: from 10.1.1.1 to 10.1.1.2,
1284: provided that the SSH server running on the gateway to the remote network,
1285: at 192.168.1.15, allows it.
1286: .Pp
1287: On the client:
1.250 jmc 1288: .Bd -literal -offset indent
1289: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1290: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1291: # route add 10.0.99.0/24 10.1.1.2
1292: .Ed
1293: .Pp
1294: On the server:
1295: .Bd -literal -offset indent
1296: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1297: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1298: .Ed
1299: .Pp
1300: Client access may be more finely tuned via the
1301: .Pa /root/.ssh/authorized_keys
1302: file (see below) and the
1303: .Cm PermitRootLogin
1304: server option.
1.255 jmc 1305: The following entry would permit connections on
1.250 jmc 1306: .Xr tun 4
1.255 jmc 1307: device 1 from user
1.250 jmc 1308: .Dq jane
1.255 jmc 1309: and on tun device 2 from user
1.250 jmc 1310: .Dq john ,
1311: if
1312: .Cm PermitRootLogin
1313: is set to
1314: .Dq forced-commands-only :
1315: .Bd -literal -offset 2n
1316: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1317: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1318: .Ed
1319: .Pp
1.264 ray 1320: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1321: it may be more suited to temporary setups,
1322: such as for wireless VPNs.
1323: More permanent VPNs are better provided by tools such as
1324: .Xr ipsecctl 8
1325: and
1326: .Xr isakmpd 8 .
1.2 deraadt 1327: .Sh ENVIRONMENT
1328: .Nm
1.1 deraadt 1329: will normally set the following environment variables:
1.237 jmc 1330: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1331: .It Ev DISPLAY
1332: The
1333: .Ev DISPLAY
1.40 aaron 1334: variable indicates the location of the X11 server.
1.44 aaron 1335: It is automatically set by
1.2 deraadt 1336: .Nm
1337: to point to a value of the form
1.233 jmc 1338: .Dq hostname:n ,
1339: where
1340: .Dq hostname
1341: indicates the host where the shell runs, and
1342: .Sq n
1343: is an integer \*(Ge 1.
1.40 aaron 1344: .Nm
1345: uses this special value to forward X11 connections over the secure
1346: channel.
1.107 markus 1347: The user should normally not set
1348: .Ev DISPLAY
1349: explicitly, as that
1.1 deraadt 1350: will render the X11 connection insecure (and will require the user to
1351: manually copy any required authorization cookies).
1.2 deraadt 1352: .It Ev HOME
1.1 deraadt 1353: Set to the path of the user's home directory.
1.2 deraadt 1354: .It Ev LOGNAME
1355: Synonym for
1.12 aaron 1356: .Ev USER ;
1357: set for compatibility with systems that use this variable.
1.2 deraadt 1358: .It Ev MAIL
1.129 stevesk 1359: Set to the path of the user's mailbox.
1.40 aaron 1360: .It Ev PATH
1.2 deraadt 1361: Set to the default
1362: .Ev PATH ,
1363: as specified when compiling
1.234 jmc 1364: .Nm .
1.118 markus 1365: .It Ev SSH_ASKPASS
1366: If
1367: .Nm
1368: needs a passphrase, it will read the passphrase from the current
1369: terminal if it was run from a terminal.
1370: If
1371: .Nm
1372: does not have a terminal associated with it but
1373: .Ev DISPLAY
1374: and
1375: .Ev SSH_ASKPASS
1376: are set, it will execute the program specified by
1377: .Ev SSH_ASKPASS
1378: and open an X11 window to read the passphrase.
1379: This is particularly useful when calling
1380: .Nm
1381: from a
1.196 jmc 1382: .Pa .xsession
1.118 markus 1383: or related script.
1384: (Note that on some machines it
1385: may be necessary to redirect the input from
1386: .Pa /dev/null
1387: to make this work.)
1.18 markus 1388: .It Ev SSH_AUTH_SOCK
1.233 jmc 1389: Identifies the path of a
1390: .Ux Ns -domain
1391: socket used to communicate with the agent.
1.166 stevesk 1392: .It Ev SSH_CONNECTION
1393: Identifies the client and server ends of the connection.
1.40 aaron 1394: The variable contains
1.233 jmc 1395: four space-separated values: client IP address, client port number,
1396: server IP address, and server port number.
1.73 markus 1397: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1398: This variable contains the original command line if a forced command
1.73 markus 1399: is executed.
1400: It can be used to extract the original arguments.
1.2 deraadt 1401: .It Ev SSH_TTY
1.1 deraadt 1402: This is set to the name of the tty (path to the device) associated
1.40 aaron 1403: with the current shell or command.
1404: If the current session has no tty,
1.1 deraadt 1405: this variable is not set.
1.2 deraadt 1406: .It Ev TZ
1.214 jmc 1407: This variable is set to indicate the present time zone if it
1.257 jmc 1408: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1409: on to new connections).
1.2 deraadt 1410: .It Ev USER
1.1 deraadt 1411: Set to the name of the user logging in.
1.2 deraadt 1412: .El
1413: .Pp
1.44 aaron 1414: Additionally,
1.2 deraadt 1415: .Nm
1.44 aaron 1416: reads
1.207 djm 1417: .Pa ~/.ssh/environment ,
1.2 deraadt 1418: and adds lines of the format
1419: .Dq VARNAME=value
1.233 jmc 1420: to the environment if the file exists and users are allowed to
1.161 marc 1421: change their environment.
1.176 jmc 1422: For more information, see the
1.161 marc 1423: .Cm PermitUserEnvironment
1.162 stevesk 1424: option in
1.161 marc 1425: .Xr sshd_config 5 .
1.2 deraadt 1426: .Sh FILES
1.236 jmc 1427: .Bl -tag -width Ds -compact
1.309 jmc 1428: .It Pa ~/.rhosts
1.240 jmc 1429: This file is used for host-based authentication (see above).
1.92 markus 1430: On some machines this file may need to be
1.240 jmc 1431: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1432: because
1.2 deraadt 1433: .Xr sshd 8
1.40 aaron 1434: reads it as root.
1435: Additionally, this file must be owned by the user,
1436: and must not have write permissions for anyone else.
1437: The recommended
1.1 deraadt 1438: permission for most machines is read/write for the user, and not
1439: accessible by others.
1.2 deraadt 1440: .Pp
1.309 jmc 1441: .It Pa ~/.shosts
1.240 jmc 1442: This file is used in exactly the same way as
1443: .Pa .rhosts ,
1444: but allows host-based authentication without permitting login with
1445: rlogin/rsh.
1.272 mcbride 1446: .Pp
1.309 jmc 1447: .It Pa ~/.ssh/
1.272 mcbride 1448: This directory is the default location for all user-specific configuration
1449: and authentication information.
1450: There is no general requirement to keep the entire contents of this directory
1451: secret, but the recommended permissions are read/write/execute for the user,
1452: and not accessible by others.
1.236 jmc 1453: .Pp
1.309 jmc 1454: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1455: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1456: that can be used for logging in as this user.
1.238 jmc 1457: The format of this file is described in the
1458: .Xr sshd 8
1459: manual page.
1460: This file is not highly sensitive, but the recommended
1461: permissions are read/write for the user, and not accessible by others.
1462: .Pp
1.309 jmc 1463: .It Pa ~/.ssh/config
1.238 jmc 1464: This is the per-user configuration file.
1465: The file format and configuration options are described in
1466: .Xr ssh_config 5 .
1467: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1468: read/write for the user, and not writable by others.
1.238 jmc 1469: .Pp
1.309 jmc 1470: .It Pa ~/.ssh/environment
1.239 jmc 1471: Contains additional definitions for environment variables; see
1472: .Sx ENVIRONMENT ,
1.238 jmc 1473: above.
1474: .Pp
1.309 jmc 1475: .It Pa ~/.ssh/identity
1476: .It Pa ~/.ssh/id_dsa
1.310 djm 1477: .It Pa ~/.ssh/id_ecdsa
1.343 naddy 1478: .It Pa ~/.ssh/id_ed25519
1.309 jmc 1479: .It Pa ~/.ssh/id_rsa
1.238 jmc 1480: Contains the private key for authentication.
1481: These files
1482: contain sensitive data and should be readable by the user but not
1483: accessible by others (read/write/execute).
1484: .Nm
1485: will simply ignore a private key file if it is accessible by others.
1486: It is possible to specify a passphrase when
1487: generating the key which will be used to encrypt the
1488: sensitive part of this file using 3DES.
1489: .Pp
1.309 jmc 1490: .It Pa ~/.ssh/identity.pub
1491: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1492: .It Pa ~/.ssh/id_ecdsa.pub
1.343 naddy 1493: .It Pa ~/.ssh/id_ed25519.pub
1.309 jmc 1494: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1495: Contains the public key for authentication.
1496: These files are not
1497: sensitive and can (but need not) be readable by anyone.
1498: .Pp
1.309 jmc 1499: .It Pa ~/.ssh/known_hosts
1.244 jmc 1500: Contains a list of host keys for all hosts the user has logged into
1501: that are not already in the systemwide list of known host keys.
1.238 jmc 1502: See
1.244 jmc 1503: .Xr sshd 8
1504: for further details of the format of this file.
1.238 jmc 1505: .Pp
1.309 jmc 1506: .It Pa ~/.ssh/rc
1.238 jmc 1507: Commands in this file are executed by
1508: .Nm
1.245 jmc 1509: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1510: started.
1511: See the
1512: .Xr sshd 8
1513: manual page for more information.
1514: .Pp
1.309 jmc 1515: .It Pa /etc/hosts.equiv
1.240 jmc 1516: This file is for host-based authentication (see above).
1517: It should only be writable by root.
1.236 jmc 1518: .Pp
1.309 jmc 1519: .It Pa /etc/shosts.equiv
1.240 jmc 1520: This file is used in exactly the same way as
1521: .Pa hosts.equiv ,
1522: but allows host-based authentication without permitting login with
1523: rlogin/rsh.
1.236 jmc 1524: .Pp
1.238 jmc 1525: .It Pa /etc/ssh/ssh_config
1526: Systemwide configuration file.
1527: The file format and configuration options are described in
1528: .Xr ssh_config 5 .
1529: .Pp
1.309 jmc 1530: .It Pa /etc/ssh/ssh_host_key
1531: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1532: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1533: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1534: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1535: These files contain the private parts of the host keys
1.245 jmc 1536: and are used for host-based authentication.
1537: If protocol version 1 is used,
1.238 jmc 1538: .Nm
1539: must be setuid root, since the host key is readable only by root.
1540: For protocol version 2,
1541: .Nm
1542: uses
1543: .Xr ssh-keysign 8
1.245 jmc 1544: to access the host keys,
1545: eliminating the requirement that
1.238 jmc 1546: .Nm
1.245 jmc 1547: be setuid root when host-based authentication is used.
1.238 jmc 1548: By default
1.2 deraadt 1549: .Nm
1.238 jmc 1550: is not setuid root.
1551: .Pp
1.309 jmc 1552: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1553: Systemwide list of known host keys.
1554: This file should be prepared by the
1555: system administrator to contain the public host keys of all machines in the
1556: organization.
1.244 jmc 1557: It should be world-readable.
1558: See
1.238 jmc 1559: .Xr sshd 8
1.244 jmc 1560: for further details of the format of this file.
1.236 jmc 1561: .Pp
1.309 jmc 1562: .It Pa /etc/ssh/sshrc
1.1 deraadt 1563: Commands in this file are executed by
1.2 deraadt 1564: .Nm
1.245 jmc 1565: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1566: See the
1.2 deraadt 1567: .Xr sshd 8
1.1 deraadt 1568: manual page for more information.
1.58 itojun 1569: .El
1.312 jmc 1570: .Sh EXIT STATUS
1571: .Nm
1572: exits with the exit status of the remote command or with 255
1573: if an error occurred.
1.2 deraadt 1574: .Sh SEE ALSO
1575: .Xr scp 1 ,
1.83 djm 1576: .Xr sftp 1 ,
1.2 deraadt 1577: .Xr ssh-add 1 ,
1578: .Xr ssh-agent 1 ,
1579: .Xr ssh-keygen 1 ,
1.242 jmc 1580: .Xr ssh-keyscan 1 ,
1.250 jmc 1581: .Xr tun 4 ,
1.159 stevesk 1582: .Xr ssh_config 5 ,
1.160 naddy 1583: .Xr ssh-keysign 8 ,
1.87 itojun 1584: .Xr sshd 8
1.329 jmc 1585: .Sh STANDARDS
1.106 markus 1586: .Rs
1.329 jmc 1587: .%A S. Lehtinen
1588: .%A C. Lonvick
1589: .%D January 2006
1.256 jmc 1590: .%R RFC 4250
1.329 jmc 1591: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1592: .Re
1.329 jmc 1593: .Pp
1.256 jmc 1594: .Rs
1.329 jmc 1595: .%A T. Ylonen
1596: .%A C. Lonvick
1597: .%D January 2006
1.256 jmc 1598: .%R RFC 4251
1.329 jmc 1599: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1600: .Re
1.329 jmc 1601: .Pp
1.256 jmc 1602: .Rs
1.329 jmc 1603: .%A T. Ylonen
1604: .%A C. Lonvick
1605: .%D January 2006
1.256 jmc 1606: .%R RFC 4252
1.329 jmc 1607: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1608: .Re
1.329 jmc 1609: .Pp
1.256 jmc 1610: .Rs
1.329 jmc 1611: .%A T. Ylonen
1612: .%A C. Lonvick
1613: .%D January 2006
1.256 jmc 1614: .%R RFC 4253
1.329 jmc 1615: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1616: .Re
1.329 jmc 1617: .Pp
1.256 jmc 1618: .Rs
1.329 jmc 1619: .%A T. Ylonen
1620: .%A C. Lonvick
1621: .%D January 2006
1.256 jmc 1622: .%R RFC 4254
1.329 jmc 1623: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1624: .Re
1.329 jmc 1625: .Pp
1.256 jmc 1626: .Rs
1.329 jmc 1627: .%A J. Schlyter
1628: .%A W. Griffin
1629: .%D January 2006
1.256 jmc 1630: .%R RFC 4255
1.329 jmc 1631: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1632: .Re
1.329 jmc 1633: .Pp
1.256 jmc 1634: .Rs
1.329 jmc 1635: .%A F. Cusack
1636: .%A M. Forssen
1637: .%D January 2006
1.256 jmc 1638: .%R RFC 4256
1.329 jmc 1639: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1640: .Re
1.329 jmc 1641: .Pp
1.256 jmc 1642: .Rs
1.329 jmc 1643: .%A J. Galbraith
1644: .%A P. Remaker
1645: .%D January 2006
1.256 jmc 1646: .%R RFC 4335
1.329 jmc 1647: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1648: .Re
1.329 jmc 1649: .Pp
1.256 jmc 1650: .Rs
1.329 jmc 1651: .%A M. Bellare
1652: .%A T. Kohno
1653: .%A C. Namprempre
1654: .%D January 2006
1.256 jmc 1655: .%R RFC 4344
1.329 jmc 1656: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1657: .Re
1.329 jmc 1658: .Pp
1.256 jmc 1659: .Rs
1.329 jmc 1660: .%A B. Harris
1661: .%D January 2006
1.256 jmc 1662: .%R RFC 4345
1.329 jmc 1663: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1664: .Re
1.329 jmc 1665: .Pp
1.258 djm 1666: .Rs
1.329 jmc 1667: .%A M. Friedl
1668: .%A N. Provos
1669: .%A W. Simpson
1670: .%D March 2006
1.258 djm 1671: .%R RFC 4419
1.329 jmc 1672: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1673: .Re
1.329 jmc 1674: .Pp
1.266 markus 1675: .Rs
1.329 jmc 1676: .%A J. Galbraith
1677: .%A R. Thayer
1678: .%D November 2006
1.266 markus 1679: .%R RFC 4716
1.329 jmc 1680: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1681: .Re
1.329 jmc 1682: .Pp
1.313 djm 1683: .Rs
1.329 jmc 1684: .%A D. Stebila
1685: .%A J. Green
1686: .%D December 2009
1.313 djm 1687: .%R RFC 5656
1.329 jmc 1688: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1689: .Re
1.329 jmc 1690: .Pp
1.274 grunk 1691: .Rs
1692: .%A A. Perrig
1693: .%A D. Song
1694: .%D 1999
1.329 jmc 1695: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1696: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1697: .Re
1.173 jmc 1698: .Sh AUTHORS
1699: OpenSSH is a derivative of the original and free
1700: ssh 1.2.12 release by Tatu Ylonen.
1701: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1702: Theo de Raadt and Dug Song
1703: removed many bugs, re-added newer features and
1704: created OpenSSH.
1705: Markus Friedl contributed the support for SSH
1706: protocol versions 1.5 and 2.0.
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