Annotation of src/usr.bin/ssh/ssh.1, Revision 1.163
1.1 deraadt 1: .\" -*- nroff -*-
2: .\"
3: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5: .\" All rights reserved
6: .\"
1.59 deraadt 7: .\" As far as I am concerned, the code I have written for this software
8: .\" can be used freely for any purpose. Any derived versions of this
9: .\" software must be clearly marked as such, and if the derived work is
10: .\" incompatible with the protocol description in the RFC file, it must be
11: .\" called by a name other than "ssh" or "Secure Shell".
12: .\"
1.93 deraadt 13: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
1.59 deraadt 16: .\"
17: .\" Redistribution and use in source and binary forms, with or without
18: .\" modification, are permitted provided that the following conditions
19: .\" are met:
20: .\" 1. Redistributions of source code must retain the above copyright
21: .\" notice, this list of conditions and the following disclaimer.
22: .\" 2. Redistributions in binary form must reproduce the above copyright
23: .\" notice, this list of conditions and the following disclaimer in the
24: .\" documentation and/or other materials provided with the distribution.
1.1 deraadt 25: .\"
1.59 deraadt 26: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
32: .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
33: .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
34: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 36: .\"
1.163 ! stevesk 37: .\" $OpenBSD: ssh.1,v 1.162 2002/08/12 17:30:35 stevesk Exp $
1.2 deraadt 38: .Dd September 25, 1999
39: .Dt SSH 1
40: .Os
41: .Sh NAME
42: .Nm ssh
1.96 deraadt 43: .Nd OpenSSH SSH client (remote login program)
1.2 deraadt 44: .Sh SYNOPSIS
45: .Nm ssh
46: .Op Fl l Ar login_name
1.130 stevesk 47: .Ar hostname | user@hostname
1.2 deraadt 48: .Op Ar command
49: .Pp
50: .Nm ssh
1.86 jakob 51: .Op Fl afgknqstvxACNPTX1246
1.108 markus 52: .Op Fl b Ar bind_address
1.51 markus 53: .Op Fl c Ar cipher_spec
1.2 deraadt 54: .Op Fl e Ar escape_char
55: .Op Fl i Ar identity_file
56: .Op Fl l Ar login_name
1.91 markus 57: .Op Fl m Ar mac_spec
1.2 deraadt 58: .Op Fl o Ar option
59: .Op Fl p Ar port
1.132 markus 60: .Op Fl F Ar configfile
1.12 aaron 61: .Oo Fl L Xo
62: .Sm off
1.33 markus 63: .Ar port :
1.12 aaron 64: .Ar host :
65: .Ar hostport
66: .Sm on
67: .Xc
68: .Oc
69: .Oo Fl R Xo
70: .Sm off
1.33 markus 71: .Ar port :
1.12 aaron 72: .Ar host :
73: .Ar hostport
74: .Sm on
75: .Xc
76: .Oc
1.135 naddy 77: .Op Fl D Ar port
1.130 stevesk 78: .Ar hostname | user@hostname
1.2 deraadt 79: .Op Ar command
1.44 aaron 80: .Sh DESCRIPTION
1.2 deraadt 81: .Nm
1.96 deraadt 82: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 83: executing commands on a remote machine.
84: It is intended to replace
1.1 deraadt 85: rlogin and rsh, and provide secure encrypted communications between
1.40 aaron 86: two untrusted hosts over an insecure network.
87: X11 connections and
1.1 deraadt 88: arbitrary TCP/IP ports can also be forwarded over the secure channel.
1.2 deraadt 89: .Pp
90: .Nm
1.44 aaron 91: connects and logs into the specified
1.2 deraadt 92: .Ar hostname .
1.1 deraadt 93: The user must prove
1.49 markus 94: his/her identity to the remote machine using one of several methods
95: depending on the protocol version used:
96: .Pp
97: .Ss SSH protocol version 1
1.2 deraadt 98: .Pp
1.1 deraadt 99: First, if the machine the user logs in from is listed in
1.2 deraadt 100: .Pa /etc/hosts.equiv
1.1 deraadt 101: or
1.2 deraadt 102: .Pa /etc/shosts.equiv
1.1 deraadt 103: on the remote machine, and the user names are
104: the same on both sides, the user is immediately permitted to log in.
1.44 aaron 105: Second, if
1.2 deraadt 106: .Pa \&.rhosts
1.1 deraadt 107: or
1.2 deraadt 108: .Pa \&.shosts
1.1 deraadt 109: exists in the user's home directory on the
110: remote machine and contains a line containing the name of the client
111: machine and the name of the user on that machine, the user is
1.40 aaron 112: permitted to log in.
113: This form of authentication alone is normally not
1.1 deraadt 114: allowed by the server because it is not secure.
1.2 deraadt 115: .Pp
1.107 markus 116: The second authentication method is the
1.2 deraadt 117: .Pa rhosts
1.1 deraadt 118: or
1.2 deraadt 119: .Pa hosts.equiv
1.40 aaron 120: method combined with RSA-based host authentication.
121: It means that if the login would be permitted by
1.49 markus 122: .Pa $HOME/.rhosts ,
123: .Pa $HOME/.shosts ,
1.2 deraadt 124: .Pa /etc/hosts.equiv ,
1.1 deraadt 125: or
1.2 deraadt 126: .Pa /etc/shosts.equiv ,
1.11 deraadt 127: and if additionally the server can verify the client's
1.44 aaron 128: host key (see
1.147 deraadt 129: .Pa /etc/ssh/ssh_known_hosts
1.23 markus 130: and
131: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 132: in the
1.2 deraadt 133: .Sx FILES
1.40 aaron 134: section), only then login is permitted.
135: This authentication method closes security holes due to IP
136: spoofing, DNS spoofing and routing spoofing.
137: [Note to the administrator:
1.2 deraadt 138: .Pa /etc/hosts.equiv ,
1.49 markus 139: .Pa $HOME/.rhosts ,
1.1 deraadt 140: and the rlogin/rsh protocol in general, are inherently insecure and should be
141: disabled if security is desired.]
1.2 deraadt 142: .Pp
1.44 aaron 143: As a third authentication method,
1.2 deraadt 144: .Nm
1.1 deraadt 145: supports RSA based authentication.
146: The scheme is based on public-key cryptography: there are cryptosystems
147: where encryption and decryption are done using separate keys, and it
148: is not possible to derive the decryption key from the encryption key.
1.40 aaron 149: RSA is one such system.
1.44 aaron 150: The idea is that each user creates a public/private
1.40 aaron 151: key pair for authentication purposes.
152: The server knows the public key, and only the user knows the private key.
1.44 aaron 153: The file
1.2 deraadt 154: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 155: lists the public keys that are permitted for logging
1.40 aaron 156: in.
157: When the user logs in, the
1.2 deraadt 158: .Nm
1.1 deraadt 159: program tells the server which key pair it would like to use for
1.40 aaron 160: authentication.
161: The server checks if this key is permitted, and if
1.1 deraadt 162: so, sends the user (actually the
1.2 deraadt 163: .Nm
1.1 deraadt 164: program running on behalf of the user) a challenge, a random number,
1.40 aaron 165: encrypted by the user's public key.
166: The challenge can only be
167: decrypted using the proper private key.
168: The user's client then decrypts the
1.1 deraadt 169: challenge using the private key, proving that he/she knows the private
170: key but without disclosing it to the server.
1.2 deraadt 171: .Pp
172: .Nm
1.40 aaron 173: implements the RSA authentication protocol automatically.
174: The user creates his/her RSA key pair by running
1.2 deraadt 175: .Xr ssh-keygen 1 .
1.44 aaron 176: This stores the private key in
1.49 markus 177: .Pa $HOME/.ssh/identity
1.1 deraadt 178: and the public key in
1.49 markus 179: .Pa $HOME/.ssh/identity.pub
1.40 aaron 180: in the user's home directory.
181: The user should then copy the
1.2 deraadt 182: .Pa identity.pub
1.44 aaron 183: to
1.49 markus 184: .Pa $HOME/.ssh/authorized_keys
1.44 aaron 185: in his/her home directory on the remote machine (the
1.2 deraadt 186: .Pa authorized_keys
1.44 aaron 187: file corresponds to the conventional
1.49 markus 188: .Pa $HOME/.rhosts
1.1 deraadt 189: file, and has one key
1.40 aaron 190: per line, though the lines can be very long).
191: After this, the user can log in without giving the password.
192: RSA authentication is much
1.1 deraadt 193: more secure than rhosts authentication.
1.2 deraadt 194: .Pp
1.1 deraadt 195: The most convenient way to use RSA authentication may be with an
1.40 aaron 196: authentication agent.
197: See
1.2 deraadt 198: .Xr ssh-agent 1
1.1 deraadt 199: for more information.
1.2 deraadt 200: .Pp
1.44 aaron 201: If other authentication methods fail,
1.2 deraadt 202: .Nm
1.40 aaron 203: prompts the user for a password.
204: The password is sent to the remote
1.1 deraadt 205: host for checking; however, since all communications are encrypted,
206: the password cannot be seen by someone listening on the network.
1.2 deraadt 207: .Pp
1.49 markus 208: .Ss SSH protocol version 2
209: .Pp
1.145 markus 210: When a user connects using protocol version 2
211: similar authentication methods are available.
1.107 markus 212: Using the default values for
213: .Cm PreferredAuthentications ,
1.123 markus 214: the client will try to authenticate first using the hostbased method;
215: if this method fails public key authentication is attempted,
216: and finally if this method fails keyboard-interactive and
217: password authentication are tried.
1.49 markus 218: .Pp
219: The public key method is similar to RSA authentication described
1.107 markus 220: in the previous section and allows the RSA or DSA algorithm to be used:
1.102 itojun 221: The client uses his private key,
1.49 markus 222: .Pa $HOME/.ssh/id_dsa
1.102 itojun 223: or
224: .Pa $HOME/.ssh/id_rsa ,
1.49 markus 225: to sign the session identifier and sends the result to the server.
226: The server checks whether the matching public key is listed in
1.115 markus 227: .Pa $HOME/.ssh/authorized_keys
1.49 markus 228: and grants access if both the key is found and the signature is correct.
229: The session identifier is derived from a shared Diffie-Hellman value
230: and is only known to the client and the server.
231: .Pp
232: If public key authentication fails or is not available a password
233: can be sent encrypted to the remote host for proving the user's identity.
1.107 markus 234: .Pp
235: Additionally,
236: .Nm
237: supports hostbased or challenge response authentication.
1.49 markus 238: .Pp
239: Protocol 2 provides additional mechanisms for confidentiality
1.51 markus 240: (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
1.94 deraadt 241: and integrity (hmac-md5, hmac-sha1).
1.49 markus 242: Note that protocol 1 lacks a strong mechanism for ensuring the
243: integrity of the connection.
244: .Pp
245: .Ss Login session and remote execution
246: .Pp
1.1 deraadt 247: When the user's identity has been accepted by the server, the server
248: either executes the given command, or logs into the machine and gives
1.40 aaron 249: the user a normal shell on the remote machine.
250: All communication with
1.1 deraadt 251: the remote command or shell will be automatically encrypted.
1.2 deraadt 252: .Pp
1.1 deraadt 253: If a pseudo-terminal has been allocated (normal login session), the
1.104 djm 254: user may use the escape characters noted below.
1.2 deraadt 255: .Pp
1.1 deraadt 256: If no pseudo tty has been allocated, the
257: session is transparent and can be used to reliably transfer binary
1.40 aaron 258: data.
259: On most systems, setting the escape character to
1.2 deraadt 260: .Dq none
261: will also make the session transparent even if a tty is used.
262: .Pp
1.71 djm 263: The session terminates when the command or shell on the remote
1.92 markus 264: machine exits and all X11 and TCP/IP connections have been closed.
1.1 deraadt 265: The exit status of the remote program is returned as the exit status
266: of
1.2 deraadt 267: .Nm ssh .
1.104 djm 268: .Pp
269: .Ss Escape Characters
270: .Pp
271: When a pseudo terminal has been requested, ssh supports a number of functions
1.117 itojun 272: through the use of an escape character.
1.104 djm 273: .Pp
274: A single tilde character can be sent as
275: .Ic ~~
1.119 stevesk 276: or by following the tilde by a character other than those described below.
1.104 djm 277: The escape character must always follow a newline to be interpreted as
278: special.
279: The escape character can be changed in configuration files using the
280: .Cm EscapeChar
1.117 itojun 281: configuration directive or on the command line by the
1.104 djm 282: .Fl e
283: option.
284: .Pp
285: The supported escapes (assuming the default
286: .Ql ~ )
287: are:
288: .Bl -tag -width Ds
289: .It Cm ~.
290: Disconnect
291: .It Cm ~^Z
292: Background ssh
293: .It Cm ~#
294: List forwarded connections
295: .It Cm ~&
296: Background ssh at logout when waiting for forwarded connection / X11 sessions
1.140 markus 297: to terminate
1.104 djm 298: .It Cm ~?
299: Display a list of escape characters
1.149 jakob 300: .It Cm ~C
301: Open command line (only useful for adding port forwardings using the
302: .Fl L
303: and
304: .Fl R
305: options)
1.104 djm 306: .It Cm ~R
1.105 djm 307: Request rekeying of the connection (only useful for SSH protocol version 2
308: and if the peer supports it)
1.104 djm 309: .El
1.2 deraadt 310: .Pp
1.49 markus 311: .Ss X11 and TCP forwarding
312: .Pp
1.110 deraadt 313: If the
314: .Cm ForwardX11
315: variable is set to
316: .Dq yes
317: (or, see the description of the
318: .Fl X
319: and
320: .Fl x
321: options described later)
322: and the user is using X11 (the
1.2 deraadt 323: .Ev DISPLAY
1.1 deraadt 324: environment variable is set), the connection to the X11 display is
325: automatically forwarded to the remote side in such a way that any X11
326: programs started from the shell (or command) will go through the
327: encrypted channel, and the connection to the real X server will be made
1.40 aaron 328: from the local machine.
329: The user should not manually set
1.2 deraadt 330: .Ev DISPLAY .
1.1 deraadt 331: Forwarding of X11 connections can be
332: configured on the command line or in configuration files.
1.2 deraadt 333: .Pp
334: The
1.44 aaron 335: .Ev DISPLAY
1.2 deraadt 336: value set by
337: .Nm
1.1 deraadt 338: will point to the server machine, but with a display number greater
1.40 aaron 339: than zero.
340: This is normal, and happens because
1.2 deraadt 341: .Nm
342: creates a
343: .Dq proxy
344: X server on the server machine for forwarding the
1.1 deraadt 345: connections over the encrypted channel.
1.2 deraadt 346: .Pp
347: .Nm
1.1 deraadt 348: will also automatically set up Xauthority data on the server machine.
349: For this purpose, it will generate a random authorization cookie,
350: store it in Xauthority on the server, and verify that any forwarded
351: connections carry this cookie and replace it by the real cookie when
1.40 aaron 352: the connection is opened.
353: The real authentication cookie is never
1.1 deraadt 354: sent to the server machine (and no cookies are sent in the plain).
1.2 deraadt 355: .Pp
1.163 ! stevesk 356: If the
! 357: .Cm ForwardAgent
! 358: variable is set to
! 359: .Dq yes
! 360: (or, see the description of the
! 361: .Fl A
! 362: and
! 363: .Fl a
! 364: options described later) and
! 365: the user is using an authentication agent, the connection to the agent
! 366: is automatically forwarded to the remote side.
1.2 deraadt 367: .Pp
1.1 deraadt 368: Forwarding of arbitrary TCP/IP connections over the secure channel can
1.120 stevesk 369: be specified either on the command line or in a configuration file.
1.40 aaron 370: One possible application of TCP/IP forwarding is a secure connection to an
1.92 markus 371: electronic purse; another is going through firewalls.
1.2 deraadt 372: .Pp
1.49 markus 373: .Ss Server authentication
374: .Pp
1.2 deraadt 375: .Nm
1.49 markus 376: automatically maintains and checks a database containing
1.40 aaron 377: identifications for all hosts it has ever been used with.
1.116 markus 378: Host keys are stored in
1.49 markus 379: .Pa $HOME/.ssh/known_hosts
1.40 aaron 380: in the user's home directory.
1.116 markus 381: Additionally, the file
1.147 deraadt 382: .Pa /etc/ssh/ssh_known_hosts
1.116 markus 383: is automatically checked for known hosts.
1.40 aaron 384: Any new hosts are automatically added to the user's file.
385: If a host's identification
1.1 deraadt 386: ever changes,
1.2 deraadt 387: .Nm
1.1 deraadt 388: warns about this and disables password authentication to prevent a
1.40 aaron 389: trojan horse from getting the user's password.
390: Another purpose of
1.1 deraadt 391: this mechanism is to prevent man-in-the-middle attacks which could
1.40 aaron 392: otherwise be used to circumvent the encryption.
393: The
1.2 deraadt 394: .Cm StrictHostKeyChecking
1.158 stevesk 395: option can be used to prevent logins to machines whose
1.1 deraadt 396: host key is not known or has changed.
1.65 aaron 397: .Pp
398: The options are as follows:
1.2 deraadt 399: .Bl -tag -width Ds
1.4 dugsong 400: .It Fl a
1.42 aaron 401: Disables forwarding of the authentication agent connection.
1.54 markus 402: .It Fl A
403: Enables forwarding of the authentication agent connection.
404: This can also be specified on a per-host basis in a configuration file.
1.108 markus 405: .It Fl b Ar bind_address
406: Specify the interface to transmit from on machines with multiple
407: interfaces or aliased addresses.
1.131 stevesk 408: .It Fl c Ar blowfish|3des|des
1.44 aaron 409: Selects the cipher to use for encrypting the session.
1.2 deraadt 410: .Ar 3des
1.40 aaron 411: is used by default.
1.44 aaron 412: It is believed to be secure.
1.5 deraadt 413: .Ar 3des
414: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
415: .Ar blowfish
416: is a fast block cipher, it appears very secure and is much faster than
1.40 aaron 417: .Ar 3des .
1.131 stevesk 418: .Ar des
419: is only supported in the
420: .Nm
421: client for interoperability with legacy protocol 1 implementations
422: that do not support the
423: .Ar 3des
424: cipher. Its use is strongly discouraged due to cryptographic
425: weaknesses.
1.90 markus 426: .It Fl c Ar cipher_spec
1.51 markus 427: Additionally, for protocol version 2 a comma-separated list of ciphers can
1.61 aaron 428: be specified in order of preference.
1.90 markus 429: See
430: .Cm Ciphers
431: for more information.
1.2 deraadt 432: .It Fl e Ar ch|^ch|none
433: Sets the escape character for sessions with a pty (default:
434: .Ql ~ ) .
1.40 aaron 435: The escape character is only recognized at the beginning of a line.
436: The escape character followed by a dot
1.2 deraadt 437: .Pq Ql \&.
438: closes the connection, followed
1.1 deraadt 439: by control-Z suspends the connection, and followed by itself sends the
1.40 aaron 440: escape character once.
441: Setting the character to
1.2 deraadt 442: .Dq none
443: disables any escapes and makes the session fully transparent.
444: .It Fl f
445: Requests
446: .Nm
1.40 aaron 447: to go to background just before command execution.
448: This is useful if
1.2 deraadt 449: .Nm
450: is going to ask for passwords or passphrases, but the user
1.40 aaron 451: wants it in the background.
1.44 aaron 452: This implies
1.2 deraadt 453: .Fl n .
1.1 deraadt 454: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 455: something like
456: .Ic ssh -f host xterm .
1.34 markus 457: .It Fl g
458: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 459: .It Fl i Ar identity_file
1.144 stevesk 460: Selects a file from which the identity (private key) for
1.68 markus 461: RSA or DSA authentication is read.
1.144 stevesk 462: The default is
1.49 markus 463: .Pa $HOME/.ssh/identity
1.144 stevesk 464: for protocol version 1, and
465: .Pa $HOME/.ssh/id_rsa
466: and
467: .Pa $HOME/.ssh/id_dsa
468: for protocol version 2.
1.40 aaron 469: Identity files may also be specified on
470: a per-host basis in the configuration file.
471: It is possible to have multiple
1.2 deraadt 472: .Fl i
473: options (and multiple identities specified in
1.1 deraadt 474: configuration files).
1.125 jakob 475: .It Fl I Ar smartcard_device
476: Specifies which smartcard device to use. The argument is
477: the device
478: .Nm
479: should use to communicate with a smartcard used for storing the user's
480: private RSA key.
1.2 deraadt 481: .It Fl k
1.42 aaron 482: Disables forwarding of Kerberos tickets and AFS tokens.
483: This may also be specified on a per-host basis in the configuration file.
1.2 deraadt 484: .It Fl l Ar login_name
1.40 aaron 485: Specifies the user to log in as on the remote machine.
486: This also may be specified on a per-host basis in the configuration file.
1.91 markus 487: .It Fl m Ar mac_spec
488: Additionally, for protocol version 2 a comma-separated list of MAC
489: (message authentication code) algorithms can
490: be specified in order of preference.
491: See the
492: .Cm MACs
493: keyword for more information.
1.2 deraadt 494: .It Fl n
495: Redirects stdin from
496: .Pa /dev/null
497: (actually, prevents reading from stdin).
1.1 deraadt 498: This must be used when
1.2 deraadt 499: .Nm
1.40 aaron 500: is run in the background.
501: A common trick is to use this to run X11 programs on a remote machine.
502: For example,
1.2 deraadt 503: .Ic ssh -n shadows.cs.hut.fi emacs &
504: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 505: connection will be automatically forwarded over an encrypted channel.
506: The
1.2 deraadt 507: .Nm
1.1 deraadt 508: program will be put in the background.
509: (This does not work if
1.2 deraadt 510: .Nm
511: needs to ask for a password or passphrase; see also the
512: .Fl f
513: option.)
1.53 markus 514: .It Fl N
515: Do not execute a remote command.
1.137 deraadt 516: This is useful for just forwarding ports
1.53 markus 517: (protocol version 2 only).
1.2 deraadt 518: .It Fl o Ar option
1.127 stevesk 519: Can be used to give options in the format used in the configuration file.
1.1 deraadt 520: This is useful for specifying options for which there is no separate
1.40 aaron 521: command-line flag.
1.2 deraadt 522: .It Fl p Ar port
1.40 aaron 523: Port to connect to on the remote host.
524: This can be specified on a
1.1 deraadt 525: per-host basis in the configuration file.
1.16 markus 526: .It Fl P
527: Use a non-privileged port for outgoing connections.
1.137 deraadt 528: This can be used if a firewall does
1.16 markus 529: not permit connections from privileged ports.
1.30 provos 530: Note that this option turns off
1.16 markus 531: .Cm RhostsAuthentication
532: and
1.72 markus 533: .Cm RhostsRSAAuthentication
534: for older servers.
1.2 deraadt 535: .It Fl q
1.40 aaron 536: Quiet mode.
537: Causes all warning and diagnostic messages to be suppressed.
1.80 djm 538: .It Fl s
1.117 itojun 539: May be used to request invocation of a subsystem on the remote system. Subsystems are a feature of the SSH2 protocol which facilitate the use
540: of SSH as a secure transport for other applications (eg. sftp). The
1.80 djm 541: subsystem is specified as the remote command.
1.2 deraadt 542: .It Fl t
1.40 aaron 543: Force pseudo-tty allocation.
1.43 brad 544: This can be used to execute arbitrary
1.40 aaron 545: screen-based programs on a remote machine, which can be very useful,
546: e.g., when implementing menu services.
1.73 markus 547: Multiple
548: .Fl t
549: options force tty allocation, even if
550: .Nm
551: has no local tty.
1.53 markus 552: .It Fl T
1.69 markus 553: Disable pseudo-tty allocation.
1.2 deraadt 554: .It Fl v
1.40 aaron 555: Verbose mode.
556: Causes
1.2 deraadt 557: .Nm
1.40 aaron 558: to print debugging messages about its progress.
559: This is helpful in
1.1 deraadt 560: debugging connection, authentication, and configuration problems.
1.73 markus 561: Multiple
562: .Fl v
563: options increases the verbosity.
1.61 aaron 564: Maximum is 3.
1.2 deraadt 565: .It Fl x
1.40 aaron 566: Disables X11 forwarding.
1.2 deraadt 567: .It Fl X
1.1 deraadt 568: Enables X11 forwarding.
1.54 markus 569: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 570: .It Fl C
1.1 deraadt 571: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 572: data for forwarded X11 and TCP/IP connections).
573: The compression algorithm is the same used by
1.34 markus 574: .Xr gzip 1 ,
575: and the
1.2 deraadt 576: .Dq level
577: can be controlled by the
578: .Cm CompressionLevel
1.158 stevesk 579: option.
1.40 aaron 580: Compression is desirable on modem lines and other
1.1 deraadt 581: slow connections, but will only slow down things on fast networks.
582: The default value can be set on a host-by-host basis in the
583: configuration files; see the
1.121 pvalchev 584: .Cm Compression
1.158 stevesk 585: option.
1.132 markus 586: .It Fl F Ar configfile
587: Specifies an alternative per-user configuration file.
588: If a configuration file is given on the command line,
589: the system-wide configuration file
1.147 deraadt 590: .Pq Pa /etc/ssh/ssh_config
1.132 markus 591: will be ignored.
592: The default for the per-user configuration file is
593: .Pa $HOME/.ssh/config .
1.2 deraadt 594: .It Fl L Ar port:host:hostport
1.1 deraadt 595: Specifies that the given port on the local (client) host is to be
1.40 aaron 596: forwarded to the given host and port on the remote side.
597: This works by allocating a socket to listen to
1.2 deraadt 598: .Ar port
1.1 deraadt 599: on the local side, and whenever a connection is made to this port, the
600: connection is forwarded over the secure channel, and a connection is
601: made to
1.32 markus 602: .Ar host
603: port
604: .Ar hostport
1.40 aaron 605: from the remote machine.
606: Port forwardings can also be specified in the configuration file.
607: Only root can forward privileged ports.
1.32 markus 608: IPv6 addresses can be specified with an alternative syntax:
609: .Ar port/host/hostport
1.2 deraadt 610: .It Fl R Ar port:host:hostport
1.1 deraadt 611: Specifies that the given port on the remote (server) host is to be
1.40 aaron 612: forwarded to the given host and port on the local side.
613: This works by allocating a socket to listen to
1.2 deraadt 614: .Ar port
1.1 deraadt 615: on the remote side, and whenever a connection is made to this port, the
616: connection is forwarded over the secure channel, and a connection is
617: made to
1.32 markus 618: .Ar host
619: port
620: .Ar hostport
1.40 aaron 621: from the local machine.
622: Port forwardings can also be specified in the configuration file.
623: Privileged ports can be forwarded only when
1.1 deraadt 624: logging in as root on the remote machine.
1.107 markus 625: IPv6 addresses can be specified with an alternative syntax:
626: .Ar port/host/hostport
1.133 stevesk 627: .It Fl D Ar port
628: Specifies a local
629: .Dq dynamic
630: application-level port forwarding.
631: This works by allocating a socket to listen to
632: .Ar port
633: on the local side, and whenever a connection is made to this port, the
634: connection is forwarded over the secure channel, and the application
635: protocol is then used to determine where to connect to from the
636: remote machine. Currently the SOCKS4 protocol is supported, and
637: .Nm
638: will act as a SOCKS4 server.
639: Only root can forward privileged ports.
640: Dynamic port forwardings can also be specified in the configuration file.
1.85 jakob 641: .It Fl 1
642: Forces
643: .Nm
644: to try protocol version 1 only.
1.46 markus 645: .It Fl 2
646: Forces
647: .Nm
1.50 markus 648: to try protocol version 2 only.
1.32 markus 649: .It Fl 4
650: Forces
651: .Nm
652: to use IPv4 addresses only.
653: .It Fl 6
654: Forces
655: .Nm
656: to use IPv6 addresses only.
1.2 deraadt 657: .El
658: .Sh CONFIGURATION FILES
659: .Nm
1.158 stevesk 660: may additionally obtain configuration data from
661: a per-user configuration file and a system-wide configuration file.
662: The file format and configuration options are described in
663: .Xr ssh_config 5 .
1.2 deraadt 664: .Sh ENVIRONMENT
665: .Nm
1.1 deraadt 666: will normally set the following environment variables:
1.2 deraadt 667: .Bl -tag -width Ds
668: .It Ev DISPLAY
669: The
670: .Ev DISPLAY
1.40 aaron 671: variable indicates the location of the X11 server.
1.44 aaron 672: It is automatically set by
1.2 deraadt 673: .Nm
674: to point to a value of the form
675: .Dq hostname:n
676: where hostname indicates
1.40 aaron 677: the host where the shell runs, and n is an integer >= 1.
678: .Nm
679: uses this special value to forward X11 connections over the secure
680: channel.
1.107 markus 681: The user should normally not set
682: .Ev DISPLAY
683: explicitly, as that
1.1 deraadt 684: will render the X11 connection insecure (and will require the user to
685: manually copy any required authorization cookies).
1.2 deraadt 686: .It Ev HOME
1.1 deraadt 687: Set to the path of the user's home directory.
1.2 deraadt 688: .It Ev LOGNAME
689: Synonym for
1.12 aaron 690: .Ev USER ;
691: set for compatibility with systems that use this variable.
1.2 deraadt 692: .It Ev MAIL
1.129 stevesk 693: Set to the path of the user's mailbox.
1.40 aaron 694: .It Ev PATH
1.2 deraadt 695: Set to the default
696: .Ev PATH ,
697: as specified when compiling
1.12 aaron 698: .Nm ssh .
1.118 markus 699: .It Ev SSH_ASKPASS
700: If
701: .Nm
702: needs a passphrase, it will read the passphrase from the current
703: terminal if it was run from a terminal.
704: If
705: .Nm
706: does not have a terminal associated with it but
707: .Ev DISPLAY
708: and
709: .Ev SSH_ASKPASS
710: are set, it will execute the program specified by
711: .Ev SSH_ASKPASS
712: and open an X11 window to read the passphrase.
713: This is particularly useful when calling
714: .Nm
715: from a
716: .Pa .Xsession
717: or related script.
718: (Note that on some machines it
719: may be necessary to redirect the input from
720: .Pa /dev/null
721: to make this work.)
1.18 markus 722: .It Ev SSH_AUTH_SOCK
1.129 stevesk 723: Identifies the path of a unix-domain socket used to communicate with the
1.17 markus 724: agent.
1.2 deraadt 725: .It Ev SSH_CLIENT
1.40 aaron 726: Identifies the client end of the connection.
727: The variable contains
1.1 deraadt 728: three space-separated values: client ip-address, client port number,
729: and server port number.
1.73 markus 730: .It Ev SSH_ORIGINAL_COMMAND
731: The variable contains the original command line if a forced command
732: is executed.
733: It can be used to extract the original arguments.
1.2 deraadt 734: .It Ev SSH_TTY
1.1 deraadt 735: This is set to the name of the tty (path to the device) associated
1.40 aaron 736: with the current shell or command.
737: If the current session has no tty,
1.1 deraadt 738: this variable is not set.
1.2 deraadt 739: .It Ev TZ
1.1 deraadt 740: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 741: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 742: on to new connections).
1.2 deraadt 743: .It Ev USER
1.1 deraadt 744: Set to the name of the user logging in.
1.2 deraadt 745: .El
746: .Pp
1.44 aaron 747: Additionally,
1.2 deraadt 748: .Nm
1.44 aaron 749: reads
750: .Pa $HOME/.ssh/environment ,
1.2 deraadt 751: and adds lines of the format
752: .Dq VARNAME=value
1.161 marc 753: to the environment if the file exists and if users are allowed to
754: change their environment.
1.162 stevesk 755: See the
1.161 marc 756: .Cm PermitUserEnvironment
1.162 stevesk 757: option in
1.161 marc 758: .Xr sshd_config 5 .
1.2 deraadt 759: .Sh FILES
1.36 markus 760: .Bl -tag -width Ds
1.116 markus 761: .It Pa $HOME/.ssh/known_hosts
1.129 stevesk 762: Records host keys for all hosts the user has logged into that are not
1.2 deraadt 763: in
1.147 deraadt 764: .Pa /etc/ssh/ssh_known_hosts .
1.2 deraadt 765: See
766: .Xr sshd 8 .
1.102 itojun 767: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
768: Contains the authentication identity of the user.
769: They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1.48 markus 770: These files
771: contain sensitive data and should be readable by the user but not
1.15 markus 772: accessible by others (read/write/execute).
773: Note that
774: .Nm
1.48 markus 775: ignores a private key file if it is accessible by others.
1.15 markus 776: It is possible to specify a passphrase when
1.1 deraadt 777: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 778: sensitive part of this file using 3DES.
1.102 itojun 779: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1.1 deraadt 780: Contains the public key for authentication (public part of the
1.40 aaron 781: identity file in human-readable form).
1.48 markus 782: The contents of the
783: .Pa $HOME/.ssh/identity.pub
784: file should be added to
1.2 deraadt 785: .Pa $HOME/.ssh/authorized_keys
786: on all machines
1.137 deraadt 787: where the user wishes to log in using protocol version 1 RSA authentication.
1.48 markus 788: The contents of the
789: .Pa $HOME/.ssh/id_dsa.pub
1.102 itojun 790: and
791: .Pa $HOME/.ssh/id_rsa.pub
1.48 markus 792: file should be added to
1.115 markus 793: .Pa $HOME/.ssh/authorized_keys
1.48 markus 794: on all machines
1.137 deraadt 795: where the user wishes to log in using protocol version 2 DSA/RSA authentication.
1.48 markus 796: These files are not
1.40 aaron 797: sensitive and can (but need not) be readable by anyone.
1.48 markus 798: These files are
1.84 markus 799: never used automatically and are not necessary; they are only provided for
1.1 deraadt 800: the convenience of the user.
1.2 deraadt 801: .It Pa $HOME/.ssh/config
1.40 aaron 802: This is the per-user configuration file.
1.158 stevesk 803: The file format and configuration options are described in
804: .Xr ssh_config 5 .
1.2 deraadt 805: .It Pa $HOME/.ssh/authorized_keys
1.115 markus 806: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.40 aaron 807: The format of this file is described in the
1.2 deraadt 808: .Xr sshd 8
1.40 aaron 809: manual page.
1.116 markus 810: In the simplest form the format is the same as the .pub
811: identity files.
1.48 markus 812: This file is not highly sensitive, but the recommended
813: permissions are read/write for the user, and not accessible by others.
1.147 deraadt 814: .It Pa /etc/ssh/ssh_known_hosts
1.40 aaron 815: Systemwide list of known host keys.
1.116 markus 816: This file should be prepared by the
1.1 deraadt 817: system administrator to contain the public host keys of all machines in the
1.40 aaron 818: organization.
819: This file should be world-readable.
820: This file contains
1.1 deraadt 821: public keys, one per line, in the following format (fields separated
1.116 markus 822: by spaces): system name, public key and optional comment field.
1.40 aaron 823: When different names are used
1.1 deraadt 824: for the same machine, all such names should be listed, separated by
1.40 aaron 825: commas.
826: The format is described on the
1.2 deraadt 827: .Xr sshd 8
1.1 deraadt 828: manual page.
1.2 deraadt 829: .Pp
1.1 deraadt 830: The canonical system name (as returned by name servers) is used by
1.2 deraadt 831: .Xr sshd 8
1.1 deraadt 832: to verify the client host when logging in; other names are needed because
1.2 deraadt 833: .Nm
1.1 deraadt 834: does not convert the user-supplied name to a canonical name before
835: checking the key, because someone with access to the name servers
836: would then be able to fool host authentication.
1.147 deraadt 837: .It Pa /etc/ssh/ssh_config
1.40 aaron 838: Systemwide configuration file.
1.158 stevesk 839: The file format and configuration options are described in
840: .Xr ssh_config 5 .
1.147 deraadt 841: .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1.141 markus 842: These three files contain the private parts of the host keys
843: and are used for
844: .Cm RhostsRSAAuthentication
845: and
846: .Cm HostbasedAuthentication .
1.155 stevesk 847: If the protocol version 1
848: .Cm RhostsRSAAuthentication
1.157 deraadt 849: method is used,
1.155 stevesk 850: .Nm
851: must be setuid root, since the host key is readable only by root.
852: For protocol version 2,
853: .Nm
854: uses
855: .Xr ssh-keysign 8
856: to access the host keys for
857: .Cm HostbasedAuthentication .
858: This eliminates the requirement that
859: .Nm
860: be setuid root when that authentication method is used.
861: By default
1.141 markus 862: .Nm
1.155 stevesk 863: is not setuid root.
1.2 deraadt 864: .It Pa $HOME/.rhosts
865: This file is used in
866: .Pa \&.rhosts
867: authentication to list the
1.40 aaron 868: host/user pairs that are permitted to log in.
869: (Note that this file is
1.1 deraadt 870: also used by rlogin and rsh, which makes using this file insecure.)
871: Each line of the file contains a host name (in the canonical form
872: returned by name servers), and then a user name on that host,
1.40 aaron 873: separated by a space.
1.92 markus 874: On some machines this file may need to be
1.1 deraadt 875: world-readable if the user's home directory is on a NFS partition,
876: because
1.2 deraadt 877: .Xr sshd 8
1.40 aaron 878: reads it as root.
879: Additionally, this file must be owned by the user,
880: and must not have write permissions for anyone else.
881: The recommended
1.1 deraadt 882: permission for most machines is read/write for the user, and not
883: accessible by others.
1.2 deraadt 884: .Pp
1.1 deraadt 885: Note that by default
1.2 deraadt 886: .Xr sshd 8
1.1 deraadt 887: will be installed so that it requires successful RSA host
1.40 aaron 888: authentication before permitting \s+2.\s0rhosts authentication.
1.137 deraadt 889: If the server machine does not have the client's host key in
1.147 deraadt 890: .Pa /etc/ssh/ssh_known_hosts ,
1.137 deraadt 891: it can be stored in
1.2 deraadt 892: .Pa $HOME/.ssh/known_hosts .
893: The easiest way to do this is to
1.1 deraadt 894: connect back to the client from the server machine using ssh; this
1.48 markus 895: will automatically add the host key to
1.2 deraadt 896: .Pa $HOME/.ssh/known_hosts .
897: .It Pa $HOME/.shosts
898: This file is used exactly the same way as
899: .Pa \&.rhosts .
900: The purpose for
1.1 deraadt 901: having this file is to be able to use rhosts authentication with
1.2 deraadt 902: .Nm
903: without permitting login with
1.151 millert 904: .Nm rlogin
1.2 deraadt 905: or
906: .Xr rsh 1 .
907: .It Pa /etc/hosts.equiv
908: This file is used during
1.40 aaron 909: .Pa \&.rhosts authentication.
910: It contains
1.1 deraadt 911: canonical hosts names, one per line (the full format is described on
912: the
1.2 deraadt 913: .Xr sshd 8
1.40 aaron 914: manual page).
915: If the client host is found in this file, login is
1.1 deraadt 916: automatically permitted provided client and server user names are the
1.40 aaron 917: same.
918: Additionally, successful RSA host authentication is normally
919: required.
920: This file should only be writable by root.
1.2 deraadt 921: .It Pa /etc/shosts.equiv
1.44 aaron 922: This file is processed exactly as
1.2 deraadt 923: .Pa /etc/hosts.equiv .
1.1 deraadt 924: This file may be useful to permit logins using
1.2 deraadt 925: .Nm
1.1 deraadt 926: but not using rsh/rlogin.
1.147 deraadt 927: .It Pa /etc/ssh/sshrc
1.1 deraadt 928: Commands in this file are executed by
1.2 deraadt 929: .Nm
1.1 deraadt 930: when the user logs in just before the user's shell (or command) is started.
931: See the
1.2 deraadt 932: .Xr sshd 8
1.1 deraadt 933: manual page for more information.
1.2 deraadt 934: .It Pa $HOME/.ssh/rc
1.1 deraadt 935: Commands in this file are executed by
1.2 deraadt 936: .Nm
1.1 deraadt 937: when the user logs in just before the user's shell (or command) is
938: started.
1.44 aaron 939: See the
1.2 deraadt 940: .Xr sshd 8
1.1 deraadt 941: manual page for more information.
1.31 markus 942: .It Pa $HOME/.ssh/environment
943: Contains additional definitions for environment variables, see section
944: .Sx ENVIRONMENT
945: above.
1.58 itojun 946: .El
1.145 markus 947: .Sh DIAGNOSTICS
948: .Nm
949: exits with the exit status of the remote command or with 255
950: if an error occurred.
1.67 aaron 951: .Sh AUTHORS
1.78 markus 952: OpenSSH is a derivative of the original and free
953: ssh 1.2.12 release by Tatu Ylonen.
954: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
955: Theo de Raadt and Dug Song
956: removed many bugs, re-added newer features and
957: created OpenSSH.
958: Markus Friedl contributed the support for SSH
959: protocol versions 1.5 and 2.0.
1.2 deraadt 960: .Sh SEE ALSO
961: .Xr rsh 1 ,
962: .Xr scp 1 ,
1.83 djm 963: .Xr sftp 1 ,
1.2 deraadt 964: .Xr ssh-add 1 ,
965: .Xr ssh-agent 1 ,
966: .Xr ssh-keygen 1 ,
967: .Xr telnet 1 ,
1.159 stevesk 968: .Xr ssh_config 5 ,
1.160 naddy 969: .Xr ssh-keysign 8 ,
1.87 itojun 970: .Xr sshd 8
1.106 markus 971: .Rs
972: .%A T. Ylonen
973: .%A T. Kivinen
974: .%A M. Saarinen
975: .%A T. Rinne
976: .%A S. Lehtinen
977: .%T "SSH Protocol Architecture"
1.150 markus 978: .%N draft-ietf-secsh-architecture-12.txt
979: .%D January 2002
1.106 markus 980: .%O work in progress material
981: .Re