Annotation of src/usr.bin/ssh/ssh.1, Revision 1.362
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: .\"
1.362 ! djm 36: .\" $OpenBSD: ssh.1,v 1.361 2015/07/20 18:44:12 millert Exp $
1.361 millert 37: .Dd $Mdocdate: July 20 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).
1.302 djm 307: .Nm
308: will also try to load certificate information from the filename obtained
309: by appending
310: .Pa -cert.pub
311: to identity filenames.
1.360 millert 312: .Pp
1.269 djm 313: .It Fl K
314: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
315: credentials to the server.
1.360 millert 316: .Pp
1.218 jmc 317: .It Fl k
318: Disables forwarding (delegation) of GSSAPI credentials to the server.
1.360 millert 319: .Pp
1.218 jmc 320: .It Fl L Xo
321: .Sm off
322: .Oo Ar bind_address : Oc
323: .Ar port : host : hostport
324: .Sm on
325: .Xc
1.360 millert 326: .It Fl L Xo
327: .Sm off
328: .Oo Ar bind_address : Oc
329: .Ar port : remote_socket
330: .Sm on
331: .Xc
332: .It Fl L Xo
333: .Sm off
334: .Ar local_socket : host : hostport
335: .Sm on
336: .Xc
337: .It Fl L Xo
338: .Sm off
339: .Ar local_socket : remote_socket
340: .Sm on
341: .Xc
342: Specifies that connections to the given TCP port or Unix socket on the local
343: (client) host are to be forwarded to the given host and port, or Unix socket,
344: on the remote side.
345: This works by allocating a socket to listen to either a TCP
1.218 jmc 346: .Ar port
347: on the local side, optionally bound to the specified
1.360 millert 348: .Ar bind_address ,
349: or to a Unix socket.
350: Whenever a connection is made to the local port or socket, the
1.218 jmc 351: connection is forwarded over the secure channel, and a connection is
1.360 millert 352: made to either
1.218 jmc 353: .Ar host
354: port
1.360 millert 355: .Ar hostport ,
356: or the Unix socket
357: .Ar remote_socket ,
1.218 jmc 358: from the remote machine.
1.360 millert 359: .Pp
1.218 jmc 360: Port forwardings can also be specified in the configuration file.
1.360 millert 361: Only the superuser can forward privileged ports.
1.308 djm 362: IPv6 addresses can be specified by enclosing the address in square brackets.
1.360 millert 363: .Pp
1.218 jmc 364: By default, the local port is bound in accordance with the
365: .Cm GatewayPorts
366: setting.
367: However, an explicit
368: .Ar bind_address
369: may be used to bind the connection to a specific address.
1.2 deraadt 370: The
1.218 jmc 371: .Ar bind_address
372: of
373: .Dq localhost
374: indicates that the listening port be bound for local use only, while an
375: empty address or
376: .Sq *
377: indicates that the port should be available from all interfaces.
1.360 millert 378: .Pp
1.218 jmc 379: .It Fl l Ar login_name
380: Specifies the user to log in as on the remote machine.
381: This also may be specified on a per-host basis in the configuration file.
1.360 millert 382: .Pp
1.218 jmc 383: .It Fl M
384: Places the
385: .Nm
386: client into
387: .Dq master
388: mode for connection sharing.
1.231 stevesk 389: Multiple
390: .Fl M
391: options places
392: .Nm
393: into
394: .Dq master
395: mode with confirmation required before slave connections are accepted.
1.218 jmc 396: Refer to the description of
397: .Cm ControlMaster
398: in
399: .Xr ssh_config 5
400: for details.
1.360 millert 401: .Pp
1.218 jmc 402: .It Fl m Ar mac_spec
403: Additionally, for protocol version 2 a comma-separated list of MAC
404: (message authentication code) algorithms can
405: be specified in order of preference.
406: See the
407: .Cm MACs
408: keyword for more information.
1.360 millert 409: .Pp
1.218 jmc 410: .It Fl N
411: Do not execute a remote command.
412: This is useful for just forwarding ports
413: (protocol version 2 only).
1.360 millert 414: .Pp
1.218 jmc 415: .It Fl n
416: Redirects stdin from
417: .Pa /dev/null
418: (actually, prevents reading from stdin).
419: This must be used when
1.2 deraadt 420: .Nm
1.218 jmc 421: is run in the background.
422: A common trick is to use this to run X11 programs on a remote machine.
423: For example,
424: .Ic ssh -n shadows.cs.hut.fi emacs &
425: will start an emacs on shadows.cs.hut.fi, and the X11
426: connection will be automatically forwarded over an encrypted channel.
427: The
1.2 deraadt 428: .Nm
1.218 jmc 429: program will be put in the background.
430: (This does not work if
1.2 deraadt 431: .Nm
1.218 jmc 432: needs to ask for a password or passphrase; see also the
433: .Fl f
434: option.)
1.360 millert 435: .Pp
1.218 jmc 436: .It Fl O Ar ctl_cmd
437: Control an active connection multiplexing master process.
438: When the
439: .Fl O
440: option is specified, the
441: .Ar ctl_cmd
442: argument is interpreted and passed to the master process.
443: Valid commands are:
444: .Dq check
1.305 markus 445: (check that the master process is running),
446: .Dq forward
1.318 jmc 447: (request forwardings without command execution),
1.323 okan 448: .Dq cancel
449: (cancel forwardings),
1.218 jmc 450: .Dq exit
1.318 jmc 451: (request the master to exit), and
1.317 djm 452: .Dq stop
453: (request the master to stop accepting further multiplexing requests).
1.360 millert 454: .Pp
1.218 jmc 455: .It Fl o Ar option
456: Can be used to give options in the format used in the configuration file.
457: This is useful for specifying options for which there is no separate
458: command-line flag.
459: For full details of the options listed below, and their possible values, see
460: .Xr ssh_config 5 .
1.2 deraadt 461: .Pp
1.218 jmc 462: .Bl -tag -width Ds -offset indent -compact
463: .It AddressFamily
464: .It BatchMode
465: .It BindAddress
1.338 djm 466: .It CanonicalDomains
1.339 djm 467: .It CanonicalizeFallbackLocal
468: .It CanonicalizeHostname
469: .It CanonicalizeMaxDots
470: .It CanonicalizePermittedCNAMEs
1.218 jmc 471: .It ChallengeResponseAuthentication
472: .It CheckHostIP
473: .It Cipher
474: .It Ciphers
475: .It ClearAllForwardings
476: .It Compression
477: .It CompressionLevel
478: .It ConnectionAttempts
479: .It ConnectTimeout
480: .It ControlMaster
481: .It ControlPath
1.321 djm 482: .It ControlPersist
1.218 jmc 483: .It DynamicForward
484: .It EscapeChar
1.263 markus 485: .It ExitOnForwardFailure
1.353 jmc 486: .It FingerprintHash
1.218 jmc 487: .It ForwardAgent
488: .It ForwardX11
1.321 djm 489: .It ForwardX11Timeout
1.218 jmc 490: .It ForwardX11Trusted
491: .It GatewayPorts
492: .It GlobalKnownHostsFile
493: .It GSSAPIAuthentication
494: .It GSSAPIDelegateCredentials
495: .It HashKnownHosts
496: .It Host
497: .It HostbasedAuthentication
1.355 djm 498: .It HostbasedKeyTypes
1.218 jmc 499: .It HostKeyAlgorithms
500: .It HostKeyAlias
501: .It HostName
502: .It IdentityFile
503: .It IdentitiesOnly
1.316 jmc 504: .It IPQoS
1.321 djm 505: .It KbdInteractiveAuthentication
1.218 jmc 506: .It KbdInteractiveDevices
1.314 jmc 507: .It KexAlgorithms
1.218 jmc 508: .It LocalCommand
509: .It LocalForward
510: .It LogLevel
511: .It MACs
1.337 jmc 512: .It Match
1.218 jmc 513: .It NoHostAuthenticationForLocalhost
514: .It NumberOfPasswordPrompts
515: .It PasswordAuthentication
516: .It PermitLocalCommand
1.291 markus 517: .It PKCS11Provider
1.218 jmc 518: .It Port
519: .It PreferredAuthentications
520: .It Protocol
521: .It ProxyCommand
1.336 jmc 522: .It ProxyUseFdpass
1.359 markus 523: .It PubkeyAcceptedKeyTypes
1.218 jmc 524: .It PubkeyAuthentication
1.251 dtucker 525: .It RekeyLimit
1.218 jmc 526: .It RemoteForward
1.319 jmc 527: .It RequestTTY
1.218 jmc 528: .It RhostsRSAAuthentication
529: .It RSAAuthentication
530: .It SendEnv
531: .It ServerAliveInterval
532: .It ServerAliveCountMax
1.347 jmc 533: .It StreamLocalBindMask
534: .It StreamLocalBindUnlink
1.218 jmc 535: .It StrictHostKeyChecking
536: .It TCPKeepAlive
537: .It Tunnel
538: .It TunnelDevice
1.354 djm 539: .It UpdateHostKeys
1.218 jmc 540: .It UsePrivilegedPort
541: .It User
542: .It UserKnownHostsFile
543: .It VerifyHostKeyDNS
1.276 jmc 544: .It VisualHostKey
1.218 jmc 545: .It XAuthLocation
546: .El
1.360 millert 547: .Pp
1.218 jmc 548: .It Fl p Ar port
549: Port to connect to on the remote host.
550: This can be specified on a
551: per-host basis in the configuration file.
1.360 millert 552: .Pp
1.362 ! djm 553: .It Fl Q Ar query_option
1.332 djm 554: Queries
555: .Nm
1.341 deraadt 556: for the algorithms supported for the specified version 2.
557: The available features are:
558: .Ar cipher
1.332 djm 559: (supported symmetric ciphers),
1.341 deraadt 560: .Ar cipher-auth
1.340 djm 561: (supported symmetric ciphers that support authenticated encryption),
1.341 deraadt 562: .Ar mac
1.332 djm 563: (supported message integrity codes),
1.341 deraadt 564: .Ar kex
1.332 djm 565: (key exchange algorithms),
1.341 deraadt 566: .Ar key
1.362 ! djm 567: (key types),
! 568: .Ar key-cert
! 569: (certificate key types),
! 570: .Ar key-plain
! 571: (non-certificate key types), and
1.356 djm 572: .Ar protocol-version
573: (supported SSH protocol versions).
1.360 millert 574: .Pp
1.218 jmc 575: .It Fl q
576: Quiet mode.
1.271 djm 577: Causes most warning and diagnostic messages to be suppressed.
1.360 millert 578: .Pp
1.218 jmc 579: .It Fl R Xo
580: .Sm off
581: .Oo Ar bind_address : Oc
582: .Ar port : host : hostport
583: .Sm on
584: .Xc
1.360 millert 585: .It Fl R Xo
586: .Sm off
587: .Oo Ar bind_address : Oc
588: .Ar port : local_socket
589: .Sm on
590: .Xc
591: .It Fl R Xo
592: .Sm off
593: .Ar remote_socket : host : hostport
594: .Sm on
595: .Xc
596: .It Fl R Xo
597: .Sm off
598: .Ar remote_socket : local_socket
599: .Sm on
600: .Xc
601: Specifies that connections to the given TCP port or Unix socket on the remote
602: (server) host are to be forwarded to the given host and port, or Unix socket,
603: on the local side.
604: This works by allocating a socket to listen to either a TCP
1.218 jmc 605: .Ar port
1.360 millert 606: or to a Unix socket on the remote side.
607: Whenever a connection is made to this port or Unix socket, the
608: connection is forwarded over the secure channel, and a connection
609: is made to either
1.218 jmc 610: .Ar host
611: port
1.360 millert 612: .Ar hostport ,
613: or
614: .Ar local_socket ,
1.218 jmc 615: from the local machine.
1.2 deraadt 616: .Pp
1.218 jmc 617: Port forwardings can also be specified in the configuration file.
618: Privileged ports can be forwarded only when
619: logging in as root on the remote machine.
1.324 jmc 620: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 621: .Pp
1.360 millert 622: By default, TCP listening sockets on the server will be bound to the loopback
1.218 jmc 623: interface only.
1.280 tobias 624: This may be overridden by specifying a
1.218 jmc 625: .Ar bind_address .
626: An empty
627: .Ar bind_address ,
628: or the address
629: .Ql * ,
630: indicates that the remote socket should listen on all interfaces.
631: Specifying a remote
632: .Ar bind_address
633: will only succeed if the server's
634: .Cm GatewayPorts
635: option is enabled (see
636: .Xr sshd_config 5 ) .
1.281 djm 637: .Pp
638: If the
639: .Ar port
640: argument is
1.282 djm 641: .Ql 0 ,
1.281 djm 642: the listen port will be dynamically allocated on the server and reported
643: to the client at run time.
1.305 markus 644: When used together with
645: .Ic -O forward
646: the allocated port will be printed to the standard output.
1.360 millert 647: .Pp
1.218 jmc 648: .It Fl S Ar ctl_path
1.304 jmc 649: Specifies the location of a control socket for connection sharing,
1.303 djm 650: or the string
651: .Dq none
652: to disable connection sharing.
1.218 jmc 653: Refer to the description of
654: .Cm ControlPath
655: and
656: .Cm ControlMaster
657: in
658: .Xr ssh_config 5
659: for details.
1.360 millert 660: .Pp
1.218 jmc 661: .It Fl s
662: May be used to request invocation of a subsystem on the remote system.
663: Subsystems are a feature of the SSH2 protocol which facilitate the use
664: of SSH as a secure transport for other applications (eg.\&
665: .Xr sftp 1 ) .
666: The subsystem is specified as the remote command.
1.360 millert 667: .Pp
1.218 jmc 668: .It Fl T
1.357 dtucker 669: Disable pseudo-terminal allocation.
1.360 millert 670: .Pp
1.218 jmc 671: .It Fl t
1.357 dtucker 672: Force pseudo-terminal allocation.
1.218 jmc 673: This can be used to execute arbitrary
674: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 675: e.g. when implementing menu services.
1.218 jmc 676: Multiple
677: .Fl t
678: options force tty allocation, even if
1.194 jakob 679: .Nm
1.218 jmc 680: has no local tty.
1.360 millert 681: .Pp
1.218 jmc 682: .It Fl V
683: Display the version number and exit.
1.360 millert 684: .Pp
1.218 jmc 685: .It Fl v
686: Verbose mode.
687: Causes
1.176 jmc 688: .Nm
1.218 jmc 689: to print debugging messages about its progress.
690: This is helpful in
691: debugging connection, authentication, and configuration problems.
692: Multiple
693: .Fl v
694: options increase the verbosity.
695: The maximum is 3.
1.360 millert 696: .Pp
1.290 dtucker 697: .It Fl W Ar host : Ns Ar port
698: Requests that standard input and output on the client be forwarded to
699: .Ar host
700: on
701: .Ar port
702: over the secure channel.
703: Implies
704: .Fl N ,
705: .Fl T ,
706: .Cm ExitOnForwardFailure
707: and
1.326 dtucker 708: .Cm ClearAllForwardings .
709: Works with Protocol version 2 only.
1.360 millert 710: .Pp
1.261 stevesk 711: .It Fl w Xo
712: .Ar local_tun Ns Op : Ns Ar remote_tun
713: .Xc
714: Requests
715: tunnel
716: device forwarding with the specified
1.218 jmc 717: .Xr tun 4
1.261 stevesk 718: devices between the client
719: .Pq Ar local_tun
720: and the server
721: .Pq Ar remote_tun .
722: .Pp
1.228 jmc 723: The devices may be specified by numerical ID or the keyword
724: .Dq any ,
725: which uses the next available tunnel device.
1.261 stevesk 726: If
727: .Ar remote_tun
728: is not specified, it defaults to
729: .Dq any .
1.228 jmc 730: See also the
1.218 jmc 731: .Cm Tunnel
1.261 stevesk 732: and
733: .Cm TunnelDevice
734: directives in
1.218 jmc 735: .Xr ssh_config 5 .
1.261 stevesk 736: If the
737: .Cm Tunnel
738: directive is unset, it is set to the default tunnel mode, which is
739: .Dq point-to-point .
1.360 millert 740: .Pp
1.218 jmc 741: .It Fl X
742: Enables X11 forwarding.
1.54 markus 743: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 744: .Pp
1.218 jmc 745: X11 forwarding should be enabled with caution.
1.168 jmc 746: Users with the ability to bypass file permissions on the remote host
1.218 jmc 747: (for the user's X authorization database)
748: can access the local X11 display through the forwarded connection.
749: An attacker may then be able to perform activities such as keystroke monitoring.
750: .Pp
751: For this reason, X11 forwarding is subjected to X11 SECURITY extension
752: restrictions by default.
753: Please refer to the
754: .Nm
755: .Fl Y
756: option and the
757: .Cm ForwardX11Trusted
758: directive in
759: .Xr ssh_config 5
760: for more information.
1.360 millert 761: .Pp
1.218 jmc 762: .It Fl x
763: Disables X11 forwarding.
1.360 millert 764: .Pp
1.218 jmc 765: .It Fl Y
766: Enables trusted X11 forwarding.
767: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
768: controls.
1.360 millert 769: .Pp
1.278 djm 770: .It Fl y
771: Send log information using the
772: .Xr syslog 3
773: system module.
774: By default this information is sent to stderr.
1.218 jmc 775: .El
1.224 jmc 776: .Pp
777: .Nm
778: may additionally obtain configuration data from
779: a per-user configuration file and a system-wide configuration file.
780: The file format and configuration options are described in
781: .Xr ssh_config 5 .
1.222 jmc 782: .Sh AUTHENTICATION
1.249 jmc 783: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 784: The default is to use protocol 2 only,
785: though this can be changed via the
1.222 jmc 786: .Cm Protocol
787: option in
1.284 jmc 788: .Xr ssh_config 5
789: or the
1.222 jmc 790: .Fl 1
791: and
792: .Fl 2
793: options (see above).
794: Both protocols support similar authentication methods,
1.284 jmc 795: but protocol 2 is the default since
1.222 jmc 796: it provides additional mechanisms for confidentiality
797: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.320 djm 798: and integrity (hmac-md5, hmac-sha1,
799: hmac-sha2-256, hmac-sha2-512,
1.330 markus 800: umac-64, umac-128, hmac-ripemd160).
1.222 jmc 801: Protocol 1 lacks a strong mechanism for ensuring the
802: integrity of the connection.
803: .Pp
804: The methods available for authentication are:
1.260 jmc 805: GSSAPI-based authentication,
1.222 jmc 806: host-based authentication,
807: public key authentication,
808: challenge-response authentication,
809: and password authentication.
810: Authentication methods are tried in the order specified above,
811: though protocol 2 has a configuration option to change the default order:
812: .Cm PreferredAuthentications .
813: .Pp
814: Host-based authentication works as follows:
1.218 jmc 815: If the machine the user logs in from is listed in
816: .Pa /etc/hosts.equiv
817: or
818: .Pa /etc/shosts.equiv
819: on the remote machine, and the user names are
820: the same on both sides, or if the files
821: .Pa ~/.rhosts
822: or
823: .Pa ~/.shosts
824: exist in the user's home directory on the
825: remote machine and contain a line containing the name of the client
826: machine and the name of the user on that machine, the user is
1.222 jmc 827: considered for login.
828: Additionally, the server
829: .Em must
830: be able to verify the client's
831: host key (see the description of
1.218 jmc 832: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 833: and
1.222 jmc 834: .Pa ~/.ssh/known_hosts ,
835: below)
836: for login to be permitted.
1.218 jmc 837: This authentication method closes security holes due to IP
1.222 jmc 838: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 839: [Note to the administrator:
840: .Pa /etc/hosts.equiv ,
841: .Pa ~/.rhosts ,
842: and the rlogin/rsh protocol in general, are inherently insecure and should be
843: disabled if security is desired.]
1.189 dtucker 844: .Pp
1.222 jmc 845: Public key authentication works as follows:
846: The scheme is based on public-key cryptography,
847: using cryptosystems
848: where encryption and decryption are done using separate keys,
849: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 850: The idea is that each user creates a public/private
851: key pair for authentication purposes.
852: The server knows the public key, and only the user knows the private key.
1.222 jmc 853: .Nm
854: implements public key authentication protocol automatically,
1.349 sobrado 855: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.222 jmc 856: Protocol 1 is restricted to using only RSA keys,
1.310 djm 857: but protocol 2 may use any.
1.333 jmc 858: The HISTORY section of
1.222 jmc 859: .Xr ssl 8
1.311 jmc 860: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 861: .Pp
1.218 jmc 862: The file
863: .Pa ~/.ssh/authorized_keys
864: lists the public keys that are permitted for logging in.
865: When the user logs in, the
1.2 deraadt 866: .Nm
1.218 jmc 867: program tells the server which key pair it would like to use for
868: authentication.
1.222 jmc 869: The client proves that it has access to the private key
870: and the server checks that the corresponding public key
871: is authorized to accept the account.
1.218 jmc 872: .Pp
1.222 jmc 873: The user creates his/her key pair by running
1.218 jmc 874: .Xr ssh-keygen 1 .
875: This stores the private key in
1.207 djm 876: .Pa ~/.ssh/identity
1.222 jmc 877: (protocol 1),
878: .Pa ~/.ssh/id_dsa
879: (protocol 2 DSA),
1.310 djm 880: .Pa ~/.ssh/id_ecdsa
881: (protocol 2 ECDSA),
1.343 naddy 882: .Pa ~/.ssh/id_ed25519
1.349 sobrado 883: (protocol 2 Ed25519),
1.222 jmc 884: or
885: .Pa ~/.ssh/id_rsa
886: (protocol 2 RSA)
1.218 jmc 887: and stores the public key in
888: .Pa ~/.ssh/identity.pub
1.222 jmc 889: (protocol 1),
890: .Pa ~/.ssh/id_dsa.pub
891: (protocol 2 DSA),
1.310 djm 892: .Pa ~/.ssh/id_ecdsa.pub
893: (protocol 2 ECDSA),
1.343 naddy 894: .Pa ~/.ssh/id_ed25519.pub
1.349 sobrado 895: (protocol 2 Ed25519),
1.222 jmc 896: or
897: .Pa ~/.ssh/id_rsa.pub
898: (protocol 2 RSA)
1.218 jmc 899: in the user's home directory.
1.222 jmc 900: The user should then copy the public key
1.218 jmc 901: to
902: .Pa ~/.ssh/authorized_keys
1.222 jmc 903: in his/her home directory on the remote machine.
904: The
1.218 jmc 905: .Pa authorized_keys
906: file corresponds to the conventional
907: .Pa ~/.rhosts
908: file, and has one key
1.222 jmc 909: per line, though the lines can be very long.
1.218 jmc 910: After this, the user can log in without giving the password.
911: .Pp
1.301 jmc 912: A variation on public key authentication
913: is available in the form of certificate authentication:
914: instead of a set of public/private keys,
915: signed certificates are used.
916: This has the advantage that a single trusted certification authority
917: can be used in place of many public/private keys.
1.333 jmc 918: See the CERTIFICATES section of
1.301 jmc 919: .Xr ssh-keygen 1
920: for more information.
921: .Pp
922: The most convenient way to use public key or certificate authentication
923: may be with an authentication agent.
1.218 jmc 924: See
925: .Xr ssh-agent 1
926: for more information.
927: .Pp
1.222 jmc 928: Challenge-response authentication works as follows:
929: The server sends an arbitrary
930: .Qq challenge
931: text, and prompts for a response.
932: Protocol 2 allows multiple challenges and responses;
933: protocol 1 is restricted to just one challenge/response.
934: Examples of challenge-response authentication include
1.335 jmc 935: .Bx
936: Authentication (see
1.222 jmc 937: .Xr login.conf 5 )
1.335 jmc 938: and PAM (some
939: .Pf non- Ox
940: systems).
1.222 jmc 941: .Pp
942: Finally, if other authentication methods fail,
1.218 jmc 943: .Nm
944: prompts the user for a password.
945: The password is sent to the remote
946: host for checking; however, since all communications are encrypted,
947: the password cannot be seen by someone listening on the network.
1.232 jmc 948: .Pp
949: .Nm
950: automatically maintains and checks a database containing
951: identification for all hosts it has ever been used with.
952: Host keys are stored in
953: .Pa ~/.ssh/known_hosts
954: in the user's home directory.
955: Additionally, the file
956: .Pa /etc/ssh/ssh_known_hosts
957: is automatically checked for known hosts.
958: Any new hosts are automatically added to the user's file.
959: If a host's identification ever changes,
960: .Nm
961: warns about this and disables password authentication to prevent
962: server spoofing or man-in-the-middle attacks,
963: which could otherwise be used to circumvent the encryption.
964: The
965: .Cm StrictHostKeyChecking
966: option can be used to control logins to machines whose
967: host key is not known or has changed.
968: .Pp
1.218 jmc 969: When the user's identity has been accepted by the server, the server
1.357 dtucker 970: either executes the given command in a non-interactive session or,
971: if no command has been specified, logs into the machine and gives
972: the user a normal shell as an interactive session.
1.218 jmc 973: All communication with
974: the remote command or shell will be automatically encrypted.
975: .Pp
1.357 dtucker 976: If an interactive session is requested
977: .Nm
978: by default will only request a pseudo-terminal (pty) for interactive
979: sessions when the client has one.
980: The flags
981: .Fl T
982: and
983: .Fl t
984: can be used to override this behaviour.
985: .Pp
986: If a pseudo-terminal has been allocated the
1.218 jmc 987: user may use the escape characters noted below.
988: .Pp
1.357 dtucker 989: If no pseudo-terminal has been allocated,
1.218 jmc 990: the session is transparent and can be used to reliably transfer binary data.
991: On most systems, setting the escape character to
992: .Dq none
993: will also make the session transparent even if a tty is used.
994: .Pp
995: The session terminates when the command or shell on the remote
1.247 jmc 996: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 997: .Sh ESCAPE CHARACTERS
1.218 jmc 998: When a pseudo-terminal has been requested,
1.2 deraadt 999: .Nm
1.218 jmc 1000: supports a number of functions through the use of an escape character.
1001: .Pp
1002: A single tilde character can be sent as
1003: .Ic ~~
1004: or by following the tilde by a character other than those described below.
1005: The escape character must always follow a newline to be interpreted as
1006: special.
1007: The escape character can be changed in configuration files using the
1008: .Cm EscapeChar
1009: configuration directive or on the command line by the
1010: .Fl e
1011: option.
1012: .Pp
1013: The supported escapes (assuming the default
1014: .Ql ~ )
1015: are:
1016: .Bl -tag -width Ds
1017: .It Cm ~.
1018: Disconnect.
1019: .It Cm ~^Z
1020: Background
1.234 jmc 1021: .Nm .
1.218 jmc 1022: .It Cm ~#
1023: List forwarded connections.
1024: .It Cm ~&
1025: Background
1.2 deraadt 1026: .Nm
1.218 jmc 1027: at logout when waiting for forwarded connection / X11 sessions to terminate.
1028: .It Cm ~?
1029: Display a list of escape characters.
1030: .It Cm ~B
1031: Send a BREAK to the remote system
1032: (only useful for SSH protocol version 2 and if the peer supports it).
1033: .It Cm ~C
1034: Open command line.
1035: Currently this allows the addition of port forwardings using the
1.279 stevesk 1036: .Fl L ,
1037: .Fl R
1.218 jmc 1038: and
1.279 stevesk 1039: .Fl D
1.225 jmc 1040: options (see above).
1.322 markus 1041: It also allows the cancellation of existing port-forwardings
1042: with
1.262 stevesk 1043: .Sm off
1.322 markus 1044: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1045: .Sm on
1.322 markus 1046: for local,
1047: .Sm off
1048: .Fl KR Oo Ar bind_address : Oc Ar port
1049: .Sm on
1050: for remote and
1051: .Sm off
1052: .Fl KD Oo Ar bind_address : Oc Ar port
1053: .Sm on
1054: for dynamic port-forwardings.
1.218 jmc 1055: .Ic !\& Ns Ar command
1056: allows the user to execute a local command if the
1057: .Ic PermitLocalCommand
1058: option is enabled in
1.176 jmc 1059: .Xr ssh_config 5 .
1.218 jmc 1060: Basic help is available, using the
1061: .Fl h
1062: option.
1063: .It Cm ~R
1064: Request rekeying of the connection
1065: (only useful for SSH protocol version 2 and if the peer supports it).
1.327 dtucker 1066: .It Cm ~V
1067: Decrease the verbosity
1068: .Pq Ic LogLevel
1069: when errors are being written to stderr.
1070: .It Cm ~v
1.328 jmc 1071: Increase the verbosity
1.327 dtucker 1072: .Pq Ic LogLevel
1073: when errors are being written to stderr.
1.176 jmc 1074: .El
1.246 jmc 1075: .Sh TCP FORWARDING
1076: Forwarding of arbitrary TCP connections over the secure channel can
1077: be specified either on the command line or in a configuration file.
1078: One possible application of TCP forwarding is a secure connection to a
1079: mail server; another is going through firewalls.
1080: .Pp
1081: In the example below, we look at encrypting communication between
1082: an IRC client and server, even though the IRC server does not directly
1083: support encrypted communications.
1084: This works as follows:
1085: the user connects to the remote host using
1086: .Nm ,
1087: specifying a port to be used to forward connections
1088: to the remote server.
1089: After that it is possible to start the service which is to be encrypted
1090: on the client machine,
1091: connecting to the same local port,
1092: and
1093: .Nm
1094: will encrypt and forward the connection.
1095: .Pp
1096: The following example tunnels an IRC session from client machine
1097: .Dq 127.0.0.1
1098: (localhost)
1099: to remote server
1100: .Dq server.example.com :
1101: .Bd -literal -offset 4n
1102: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1103: $ irc -c '#users' -p 1234 pinky 127.0.0.1
1104: .Ed
1105: .Pp
1106: This tunnels a connection to IRC server
1107: .Dq server.example.com ,
1108: joining channel
1109: .Dq #users ,
1110: nickname
1111: .Dq pinky ,
1112: using port 1234.
1113: It doesn't matter which port is used,
1114: as long as it's greater than 1023
1115: (remember, only root can open sockets on privileged ports)
1116: and doesn't conflict with any ports already in use.
1117: The connection is forwarded to port 6667 on the remote server,
1118: since that's the standard port for IRC services.
1119: .Pp
1120: The
1121: .Fl f
1122: option backgrounds
1123: .Nm
1124: and the remote command
1125: .Dq sleep 10
1126: is specified to allow an amount of time
1127: (10 seconds, in the example)
1128: to start the service which is to be tunnelled.
1129: If no connections are made within the time specified,
1130: .Nm
1131: will exit.
1132: .Sh X11 FORWARDING
1.218 jmc 1133: If the
1134: .Cm ForwardX11
1135: variable is set to
1136: .Dq yes
1137: (or see the description of the
1.227 jmc 1138: .Fl X ,
1139: .Fl x ,
1.218 jmc 1140: and
1.227 jmc 1141: .Fl Y
1.226 jmc 1142: options above)
1.218 jmc 1143: and the user is using X11 (the
1144: .Ev DISPLAY
1145: environment variable is set), the connection to the X11 display is
1146: automatically forwarded to the remote side in such a way that any X11
1147: programs started from the shell (or command) will go through the
1148: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1149: from the local machine.
1.218 jmc 1150: The user should not manually set
1151: .Ev DISPLAY .
1152: Forwarding of X11 connections can be
1153: configured on the command line or in configuration files.
1154: .Pp
1155: The
1156: .Ev DISPLAY
1157: value set by
1158: .Nm
1159: will point to the server machine, but with a display number greater than zero.
1160: This is normal, and happens because
1161: .Nm
1162: creates a
1163: .Dq proxy
1164: X server on the server machine for forwarding the
1165: connections over the encrypted channel.
1.200 djm 1166: .Pp
1.218 jmc 1167: .Nm
1168: will also automatically set up Xauthority data on the server machine.
1169: For this purpose, it will generate a random authorization cookie,
1170: store it in Xauthority on the server, and verify that any forwarded
1171: connections carry this cookie and replace it by the real cookie when
1172: the connection is opened.
1173: The real authentication cookie is never
1174: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1175: .Pp
1.218 jmc 1176: If the
1177: .Cm ForwardAgent
1178: variable is set to
1179: .Dq yes
1180: (or see the description of the
1181: .Fl A
1.191 djm 1182: and
1.218 jmc 1183: .Fl a
1.226 jmc 1184: options above) and
1.218 jmc 1185: the user is using an authentication agent, the connection to the agent
1186: is automatically forwarded to the remote side.
1.252 jmc 1187: .Sh VERIFYING HOST KEYS
1188: When connecting to a server for the first time,
1189: a fingerprint of the server's public key is presented to the user
1190: (unless the option
1191: .Cm StrictHostKeyChecking
1192: has been disabled).
1193: Fingerprints can be determined using
1194: .Xr ssh-keygen 1 :
1195: .Pp
1196: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1197: .Pp
1.274 grunk 1198: If the fingerprint is already known, it can be matched
1199: and the key can be accepted or rejected.
1.358 djm 1200: If only legacy (MD5) fingerprints for the server are available, the
1201: .Xr ssh-keygen 1
1202: .Fl E
1203: option may be used to downgrade the fingerprint algorithm to match.
1204: .Pp
1.274 grunk 1205: Because of the difficulty of comparing host keys
1.352 djm 1206: just by looking at fingerprint strings,
1.274 grunk 1207: there is also support to compare host keys visually,
1208: using
1209: .Em random art .
1210: By setting the
1.275 grunk 1211: .Cm VisualHostKey
1.274 grunk 1212: option to
1.275 grunk 1213: .Dq yes ,
1.274 grunk 1214: a small ASCII graphic gets displayed on every login to a server, no matter
1215: if the session itself is interactive or not.
1216: By learning the pattern a known server produces, a user can easily
1217: find out that the host key has changed when a completely different pattern
1218: is displayed.
1219: Because these patterns are not unambiguous however, a pattern that looks
1220: similar to the pattern remembered only gives a good probability that the
1221: host key is the same, not guaranteed proof.
1222: .Pp
1223: To get a listing of the fingerprints along with their random art for
1224: all known hosts, the following command line can be used:
1225: .Pp
1226: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1227: .Pp
1.252 jmc 1228: If the fingerprint is unknown,
1229: an alternative method of verification is available:
1230: SSH fingerprints verified by DNS.
1231: An additional resource record (RR),
1232: SSHFP,
1233: is added to a zonefile
1234: and the connecting client is able to match the fingerprint
1235: with that of the key presented.
1236: .Pp
1237: In this example, we are connecting a client to a server,
1238: .Dq host.example.com .
1239: The SSHFP resource records should first be added to the zonefile for
1240: host.example.com:
1241: .Bd -literal -offset indent
1.259 jakob 1242: $ ssh-keygen -r host.example.com.
1.252 jmc 1243: .Ed
1244: .Pp
1245: The output lines will have to be added to the zonefile.
1246: To check that the zone is answering fingerprint queries:
1247: .Pp
1248: .Dl $ dig -t SSHFP host.example.com
1249: .Pp
1250: Finally the client connects:
1251: .Bd -literal -offset indent
1252: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1253: [...]
1254: Matching host key fingerprint found in DNS.
1255: Are you sure you want to continue connecting (yes/no)?
1256: .Ed
1257: .Pp
1258: See the
1259: .Cm VerifyHostKeyDNS
1260: option in
1261: .Xr ssh_config 5
1262: for more information.
1.250 jmc 1263: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1264: .Nm
1265: contains support for Virtual Private Network (VPN) tunnelling
1266: using the
1267: .Xr tun 4
1268: network pseudo-device,
1269: allowing two networks to be joined securely.
1270: The
1271: .Xr sshd_config 5
1272: configuration option
1273: .Cm PermitTunnel
1274: controls whether the server supports this,
1275: and at what level (layer 2 or 3 traffic).
1276: .Pp
1277: The following example would connect client network 10.0.50.0/24
1.265 otto 1278: with remote network 10.0.99.0/24 using a point-to-point connection
1279: from 10.1.1.1 to 10.1.1.2,
1280: provided that the SSH server running on the gateway to the remote network,
1281: at 192.168.1.15, allows it.
1282: .Pp
1283: On the client:
1.250 jmc 1284: .Bd -literal -offset indent
1285: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1286: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1287: # route add 10.0.99.0/24 10.1.1.2
1288: .Ed
1289: .Pp
1290: On the server:
1291: .Bd -literal -offset indent
1292: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1293: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1294: .Ed
1295: .Pp
1296: Client access may be more finely tuned via the
1297: .Pa /root/.ssh/authorized_keys
1298: file (see below) and the
1299: .Cm PermitRootLogin
1300: server option.
1.255 jmc 1301: The following entry would permit connections on
1.250 jmc 1302: .Xr tun 4
1.255 jmc 1303: device 1 from user
1.250 jmc 1304: .Dq jane
1.255 jmc 1305: and on tun device 2 from user
1.250 jmc 1306: .Dq john ,
1307: if
1308: .Cm PermitRootLogin
1309: is set to
1310: .Dq forced-commands-only :
1311: .Bd -literal -offset 2n
1312: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1313: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1314: .Ed
1315: .Pp
1.264 ray 1316: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1317: it may be more suited to temporary setups,
1318: such as for wireless VPNs.
1319: More permanent VPNs are better provided by tools such as
1320: .Xr ipsecctl 8
1321: and
1322: .Xr isakmpd 8 .
1.2 deraadt 1323: .Sh ENVIRONMENT
1324: .Nm
1.1 deraadt 1325: will normally set the following environment variables:
1.237 jmc 1326: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1327: .It Ev DISPLAY
1328: The
1329: .Ev DISPLAY
1.40 aaron 1330: variable indicates the location of the X11 server.
1.44 aaron 1331: It is automatically set by
1.2 deraadt 1332: .Nm
1333: to point to a value of the form
1.233 jmc 1334: .Dq hostname:n ,
1335: where
1336: .Dq hostname
1337: indicates the host where the shell runs, and
1338: .Sq n
1339: is an integer \*(Ge 1.
1.40 aaron 1340: .Nm
1341: uses this special value to forward X11 connections over the secure
1342: channel.
1.107 markus 1343: The user should normally not set
1344: .Ev DISPLAY
1345: explicitly, as that
1.1 deraadt 1346: will render the X11 connection insecure (and will require the user to
1347: manually copy any required authorization cookies).
1.2 deraadt 1348: .It Ev HOME
1.1 deraadt 1349: Set to the path of the user's home directory.
1.2 deraadt 1350: .It Ev LOGNAME
1351: Synonym for
1.12 aaron 1352: .Ev USER ;
1353: set for compatibility with systems that use this variable.
1.2 deraadt 1354: .It Ev MAIL
1.129 stevesk 1355: Set to the path of the user's mailbox.
1.40 aaron 1356: .It Ev PATH
1.2 deraadt 1357: Set to the default
1358: .Ev PATH ,
1359: as specified when compiling
1.234 jmc 1360: .Nm .
1.118 markus 1361: .It Ev SSH_ASKPASS
1362: If
1363: .Nm
1364: needs a passphrase, it will read the passphrase from the current
1365: terminal if it was run from a terminal.
1366: If
1367: .Nm
1368: does not have a terminal associated with it but
1369: .Ev DISPLAY
1370: and
1371: .Ev SSH_ASKPASS
1372: are set, it will execute the program specified by
1373: .Ev SSH_ASKPASS
1374: and open an X11 window to read the passphrase.
1375: This is particularly useful when calling
1376: .Nm
1377: from a
1.196 jmc 1378: .Pa .xsession
1.118 markus 1379: or related script.
1380: (Note that on some machines it
1381: may be necessary to redirect the input from
1382: .Pa /dev/null
1383: to make this work.)
1.18 markus 1384: .It Ev SSH_AUTH_SOCK
1.233 jmc 1385: Identifies the path of a
1386: .Ux Ns -domain
1387: socket used to communicate with the agent.
1.166 stevesk 1388: .It Ev SSH_CONNECTION
1389: Identifies the client and server ends of the connection.
1.40 aaron 1390: The variable contains
1.233 jmc 1391: four space-separated values: client IP address, client port number,
1392: server IP address, and server port number.
1.73 markus 1393: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1394: This variable contains the original command line if a forced command
1.73 markus 1395: is executed.
1396: It can be used to extract the original arguments.
1.2 deraadt 1397: .It Ev SSH_TTY
1.1 deraadt 1398: This is set to the name of the tty (path to the device) associated
1.40 aaron 1399: with the current shell or command.
1400: If the current session has no tty,
1.1 deraadt 1401: this variable is not set.
1.2 deraadt 1402: .It Ev TZ
1.214 jmc 1403: This variable is set to indicate the present time zone if it
1.257 jmc 1404: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1405: on to new connections).
1.2 deraadt 1406: .It Ev USER
1.1 deraadt 1407: Set to the name of the user logging in.
1.2 deraadt 1408: .El
1409: .Pp
1.44 aaron 1410: Additionally,
1.2 deraadt 1411: .Nm
1.44 aaron 1412: reads
1.207 djm 1413: .Pa ~/.ssh/environment ,
1.2 deraadt 1414: and adds lines of the format
1415: .Dq VARNAME=value
1.233 jmc 1416: to the environment if the file exists and users are allowed to
1.161 marc 1417: change their environment.
1.176 jmc 1418: For more information, see the
1.161 marc 1419: .Cm PermitUserEnvironment
1.162 stevesk 1420: option in
1.161 marc 1421: .Xr sshd_config 5 .
1.2 deraadt 1422: .Sh FILES
1.236 jmc 1423: .Bl -tag -width Ds -compact
1.309 jmc 1424: .It Pa ~/.rhosts
1.240 jmc 1425: This file is used for host-based authentication (see above).
1.92 markus 1426: On some machines this file may need to be
1.240 jmc 1427: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1428: because
1.2 deraadt 1429: .Xr sshd 8
1.40 aaron 1430: reads it as root.
1431: Additionally, this file must be owned by the user,
1432: and must not have write permissions for anyone else.
1433: The recommended
1.1 deraadt 1434: permission for most machines is read/write for the user, and not
1435: accessible by others.
1.2 deraadt 1436: .Pp
1.309 jmc 1437: .It Pa ~/.shosts
1.240 jmc 1438: This file is used in exactly the same way as
1439: .Pa .rhosts ,
1440: but allows host-based authentication without permitting login with
1441: rlogin/rsh.
1.272 mcbride 1442: .Pp
1.309 jmc 1443: .It Pa ~/.ssh/
1.272 mcbride 1444: This directory is the default location for all user-specific configuration
1445: and authentication information.
1446: There is no general requirement to keep the entire contents of this directory
1447: secret, but the recommended permissions are read/write/execute for the user,
1448: and not accessible by others.
1.236 jmc 1449: .Pp
1.309 jmc 1450: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1451: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1452: that can be used for logging in as this user.
1.238 jmc 1453: The format of this file is described in the
1454: .Xr sshd 8
1455: manual page.
1456: This file is not highly sensitive, but the recommended
1457: permissions are read/write for the user, and not accessible by others.
1458: .Pp
1.309 jmc 1459: .It Pa ~/.ssh/config
1.238 jmc 1460: This is the per-user configuration file.
1461: The file format and configuration options are described in
1462: .Xr ssh_config 5 .
1463: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1464: read/write for the user, and not writable by others.
1.238 jmc 1465: .Pp
1.309 jmc 1466: .It Pa ~/.ssh/environment
1.239 jmc 1467: Contains additional definitions for environment variables; see
1468: .Sx ENVIRONMENT ,
1.238 jmc 1469: above.
1470: .Pp
1.309 jmc 1471: .It Pa ~/.ssh/identity
1472: .It Pa ~/.ssh/id_dsa
1.310 djm 1473: .It Pa ~/.ssh/id_ecdsa
1.343 naddy 1474: .It Pa ~/.ssh/id_ed25519
1.309 jmc 1475: .It Pa ~/.ssh/id_rsa
1.238 jmc 1476: Contains the private key for authentication.
1477: These files
1478: contain sensitive data and should be readable by the user but not
1479: accessible by others (read/write/execute).
1480: .Nm
1481: will simply ignore a private key file if it is accessible by others.
1482: It is possible to specify a passphrase when
1483: generating the key which will be used to encrypt the
1484: sensitive part of this file using 3DES.
1485: .Pp
1.309 jmc 1486: .It Pa ~/.ssh/identity.pub
1487: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1488: .It Pa ~/.ssh/id_ecdsa.pub
1.343 naddy 1489: .It Pa ~/.ssh/id_ed25519.pub
1.309 jmc 1490: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1491: Contains the public key for authentication.
1492: These files are not
1493: sensitive and can (but need not) be readable by anyone.
1494: .Pp
1.309 jmc 1495: .It Pa ~/.ssh/known_hosts
1.244 jmc 1496: Contains a list of host keys for all hosts the user has logged into
1497: that are not already in the systemwide list of known host keys.
1.238 jmc 1498: See
1.244 jmc 1499: .Xr sshd 8
1500: for further details of the format of this file.
1.238 jmc 1501: .Pp
1.309 jmc 1502: .It Pa ~/.ssh/rc
1.238 jmc 1503: Commands in this file are executed by
1504: .Nm
1.245 jmc 1505: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1506: started.
1507: See the
1508: .Xr sshd 8
1509: manual page for more information.
1510: .Pp
1.309 jmc 1511: .It Pa /etc/hosts.equiv
1.240 jmc 1512: This file is for host-based authentication (see above).
1513: It should only be writable by root.
1.236 jmc 1514: .Pp
1.309 jmc 1515: .It Pa /etc/shosts.equiv
1.240 jmc 1516: This file is used in exactly the same way as
1517: .Pa hosts.equiv ,
1518: but allows host-based authentication without permitting login with
1519: rlogin/rsh.
1.236 jmc 1520: .Pp
1.238 jmc 1521: .It Pa /etc/ssh/ssh_config
1522: Systemwide configuration file.
1523: The file format and configuration options are described in
1524: .Xr ssh_config 5 .
1525: .Pp
1.309 jmc 1526: .It Pa /etc/ssh/ssh_host_key
1527: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1528: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1529: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1530: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1531: These files contain the private parts of the host keys
1.245 jmc 1532: and are used for host-based authentication.
1533: If protocol version 1 is used,
1.238 jmc 1534: .Nm
1535: must be setuid root, since the host key is readable only by root.
1536: For protocol version 2,
1537: .Nm
1538: uses
1539: .Xr ssh-keysign 8
1.245 jmc 1540: to access the host keys,
1541: eliminating the requirement that
1.238 jmc 1542: .Nm
1.245 jmc 1543: be setuid root when host-based authentication is used.
1.238 jmc 1544: By default
1.2 deraadt 1545: .Nm
1.238 jmc 1546: is not setuid root.
1547: .Pp
1.309 jmc 1548: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1549: Systemwide list of known host keys.
1550: This file should be prepared by the
1551: system administrator to contain the public host keys of all machines in the
1552: organization.
1.244 jmc 1553: It should be world-readable.
1554: See
1.238 jmc 1555: .Xr sshd 8
1.244 jmc 1556: for further details of the format of this file.
1.236 jmc 1557: .Pp
1.309 jmc 1558: .It Pa /etc/ssh/sshrc
1.1 deraadt 1559: Commands in this file are executed by
1.2 deraadt 1560: .Nm
1.245 jmc 1561: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1562: See the
1.2 deraadt 1563: .Xr sshd 8
1.1 deraadt 1564: manual page for more information.
1.58 itojun 1565: .El
1.312 jmc 1566: .Sh EXIT STATUS
1567: .Nm
1568: exits with the exit status of the remote command or with 255
1569: if an error occurred.
1.2 deraadt 1570: .Sh SEE ALSO
1571: .Xr scp 1 ,
1.83 djm 1572: .Xr sftp 1 ,
1.2 deraadt 1573: .Xr ssh-add 1 ,
1574: .Xr ssh-agent 1 ,
1575: .Xr ssh-keygen 1 ,
1.242 jmc 1576: .Xr ssh-keyscan 1 ,
1.250 jmc 1577: .Xr tun 4 ,
1.159 stevesk 1578: .Xr ssh_config 5 ,
1.160 naddy 1579: .Xr ssh-keysign 8 ,
1.87 itojun 1580: .Xr sshd 8
1.329 jmc 1581: .Sh STANDARDS
1.106 markus 1582: .Rs
1.329 jmc 1583: .%A S. Lehtinen
1584: .%A C. Lonvick
1585: .%D January 2006
1.256 jmc 1586: .%R RFC 4250
1.329 jmc 1587: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1588: .Re
1.329 jmc 1589: .Pp
1.256 jmc 1590: .Rs
1.329 jmc 1591: .%A T. Ylonen
1592: .%A C. Lonvick
1593: .%D January 2006
1.256 jmc 1594: .%R RFC 4251
1.329 jmc 1595: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1596: .Re
1.329 jmc 1597: .Pp
1.256 jmc 1598: .Rs
1.329 jmc 1599: .%A T. Ylonen
1600: .%A C. Lonvick
1601: .%D January 2006
1.256 jmc 1602: .%R RFC 4252
1.329 jmc 1603: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1604: .Re
1.329 jmc 1605: .Pp
1.256 jmc 1606: .Rs
1.329 jmc 1607: .%A T. Ylonen
1608: .%A C. Lonvick
1609: .%D January 2006
1.256 jmc 1610: .%R RFC 4253
1.329 jmc 1611: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1612: .Re
1.329 jmc 1613: .Pp
1.256 jmc 1614: .Rs
1.329 jmc 1615: .%A T. Ylonen
1616: .%A C. Lonvick
1617: .%D January 2006
1.256 jmc 1618: .%R RFC 4254
1.329 jmc 1619: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1620: .Re
1.329 jmc 1621: .Pp
1.256 jmc 1622: .Rs
1.329 jmc 1623: .%A J. Schlyter
1624: .%A W. Griffin
1625: .%D January 2006
1.256 jmc 1626: .%R RFC 4255
1.329 jmc 1627: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1628: .Re
1.329 jmc 1629: .Pp
1.256 jmc 1630: .Rs
1.329 jmc 1631: .%A F. Cusack
1632: .%A M. Forssen
1633: .%D January 2006
1.256 jmc 1634: .%R RFC 4256
1.329 jmc 1635: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1636: .Re
1.329 jmc 1637: .Pp
1.256 jmc 1638: .Rs
1.329 jmc 1639: .%A J. Galbraith
1640: .%A P. Remaker
1641: .%D January 2006
1.256 jmc 1642: .%R RFC 4335
1.329 jmc 1643: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1644: .Re
1.329 jmc 1645: .Pp
1.256 jmc 1646: .Rs
1.329 jmc 1647: .%A M. Bellare
1648: .%A T. Kohno
1649: .%A C. Namprempre
1650: .%D January 2006
1.256 jmc 1651: .%R RFC 4344
1.329 jmc 1652: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1653: .Re
1.329 jmc 1654: .Pp
1.256 jmc 1655: .Rs
1.329 jmc 1656: .%A B. Harris
1657: .%D January 2006
1.256 jmc 1658: .%R RFC 4345
1.329 jmc 1659: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1660: .Re
1.329 jmc 1661: .Pp
1.258 djm 1662: .Rs
1.329 jmc 1663: .%A M. Friedl
1664: .%A N. Provos
1665: .%A W. Simpson
1666: .%D March 2006
1.258 djm 1667: .%R RFC 4419
1.329 jmc 1668: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1669: .Re
1.329 jmc 1670: .Pp
1.266 markus 1671: .Rs
1.329 jmc 1672: .%A J. Galbraith
1673: .%A R. Thayer
1674: .%D November 2006
1.266 markus 1675: .%R RFC 4716
1.329 jmc 1676: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1677: .Re
1.329 jmc 1678: .Pp
1.313 djm 1679: .Rs
1.329 jmc 1680: .%A D. Stebila
1681: .%A J. Green
1682: .%D December 2009
1.313 djm 1683: .%R RFC 5656
1.329 jmc 1684: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1685: .Re
1.329 jmc 1686: .Pp
1.274 grunk 1687: .Rs
1688: .%A A. Perrig
1689: .%A D. Song
1690: .%D 1999
1.329 jmc 1691: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1692: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1693: .Re
1.173 jmc 1694: .Sh AUTHORS
1695: OpenSSH is a derivative of the original and free
1696: ssh 1.2.12 release by Tatu Ylonen.
1697: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1698: Theo de Raadt and Dug Song
1699: removed many bugs, re-added newer features and
1700: created OpenSSH.
1701: Markus Friedl contributed the support for SSH
1702: protocol versions 1.5 and 2.0.
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