Annotation of src/usr.bin/ssh/ssh.1, Revision 1.167
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: .\"
1.167 ! stevesk 37: .\" $OpenBSD: ssh.1,v 1.166 2002/09/12 19:50:36 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
1.164 stevesk 51: .Op Fl afgknqstvxACNTX1246
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.
1.165 stevesk 405: .Pp
406: Agent forwarding should be enabled with caution. Users with the
407: ability to bypass file permissions on the remote host (for the agent's
408: Unix-domain socket) can access the local agent through the forwarded
409: connection. An attacker cannot obtain key material from the agent,
410: however they can perform operations on the keys that enable them to
411: authenticate using the identities loaded into the agent.
1.108 markus 412: .It Fl b Ar bind_address
413: Specify the interface to transmit from on machines with multiple
414: interfaces or aliased addresses.
1.131 stevesk 415: .It Fl c Ar blowfish|3des|des
1.44 aaron 416: Selects the cipher to use for encrypting the session.
1.2 deraadt 417: .Ar 3des
1.40 aaron 418: is used by default.
1.44 aaron 419: It is believed to be secure.
1.5 deraadt 420: .Ar 3des
421: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
422: .Ar blowfish
423: is a fast block cipher, it appears very secure and is much faster than
1.40 aaron 424: .Ar 3des .
1.131 stevesk 425: .Ar des
426: is only supported in the
427: .Nm
428: client for interoperability with legacy protocol 1 implementations
429: that do not support the
430: .Ar 3des
431: cipher. Its use is strongly discouraged due to cryptographic
432: weaknesses.
1.90 markus 433: .It Fl c Ar cipher_spec
1.51 markus 434: Additionally, for protocol version 2 a comma-separated list of ciphers can
1.61 aaron 435: be specified in order of preference.
1.90 markus 436: See
437: .Cm Ciphers
438: for more information.
1.2 deraadt 439: .It Fl e Ar ch|^ch|none
440: Sets the escape character for sessions with a pty (default:
441: .Ql ~ ) .
1.40 aaron 442: The escape character is only recognized at the beginning of a line.
443: The escape character followed by a dot
1.2 deraadt 444: .Pq Ql \&.
445: closes the connection, followed
1.1 deraadt 446: by control-Z suspends the connection, and followed by itself sends the
1.40 aaron 447: escape character once.
448: Setting the character to
1.2 deraadt 449: .Dq none
450: disables any escapes and makes the session fully transparent.
451: .It Fl f
452: Requests
453: .Nm
1.40 aaron 454: to go to background just before command execution.
455: This is useful if
1.2 deraadt 456: .Nm
457: is going to ask for passwords or passphrases, but the user
1.40 aaron 458: wants it in the background.
1.44 aaron 459: This implies
1.2 deraadt 460: .Fl n .
1.1 deraadt 461: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 462: something like
463: .Ic ssh -f host xterm .
1.34 markus 464: .It Fl g
465: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 466: .It Fl i Ar identity_file
1.144 stevesk 467: Selects a file from which the identity (private key) for
1.68 markus 468: RSA or DSA authentication is read.
1.144 stevesk 469: The default is
1.49 markus 470: .Pa $HOME/.ssh/identity
1.144 stevesk 471: for protocol version 1, and
472: .Pa $HOME/.ssh/id_rsa
473: and
474: .Pa $HOME/.ssh/id_dsa
475: for protocol version 2.
1.40 aaron 476: Identity files may also be specified on
477: a per-host basis in the configuration file.
478: It is possible to have multiple
1.2 deraadt 479: .Fl i
480: options (and multiple identities specified in
1.1 deraadt 481: configuration files).
1.125 jakob 482: .It Fl I Ar smartcard_device
483: Specifies which smartcard device to use. The argument is
484: the device
485: .Nm
486: should use to communicate with a smartcard used for storing the user's
487: private RSA key.
1.2 deraadt 488: .It Fl k
1.42 aaron 489: Disables forwarding of Kerberos tickets and AFS tokens.
490: This may also be specified on a per-host basis in the configuration file.
1.2 deraadt 491: .It Fl l Ar login_name
1.40 aaron 492: Specifies the user to log in as on the remote machine.
493: This also may be specified on a per-host basis in the configuration file.
1.91 markus 494: .It Fl m Ar mac_spec
495: Additionally, for protocol version 2 a comma-separated list of MAC
496: (message authentication code) algorithms can
497: be specified in order of preference.
498: See the
499: .Cm MACs
500: keyword for more information.
1.2 deraadt 501: .It Fl n
502: Redirects stdin from
503: .Pa /dev/null
504: (actually, prevents reading from stdin).
1.1 deraadt 505: This must be used when
1.2 deraadt 506: .Nm
1.40 aaron 507: is run in the background.
508: A common trick is to use this to run X11 programs on a remote machine.
509: For example,
1.2 deraadt 510: .Ic ssh -n shadows.cs.hut.fi emacs &
511: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 512: connection will be automatically forwarded over an encrypted channel.
513: The
1.2 deraadt 514: .Nm
1.1 deraadt 515: program will be put in the background.
516: (This does not work if
1.2 deraadt 517: .Nm
518: needs to ask for a password or passphrase; see also the
519: .Fl f
520: option.)
1.53 markus 521: .It Fl N
522: Do not execute a remote command.
1.137 deraadt 523: This is useful for just forwarding ports
1.53 markus 524: (protocol version 2 only).
1.2 deraadt 525: .It Fl o Ar option
1.127 stevesk 526: Can be used to give options in the format used in the configuration file.
1.1 deraadt 527: This is useful for specifying options for which there is no separate
1.40 aaron 528: command-line flag.
1.2 deraadt 529: .It Fl p Ar port
1.40 aaron 530: Port to connect to on the remote host.
531: This can be specified on a
1.1 deraadt 532: per-host basis in the configuration file.
1.2 deraadt 533: .It Fl q
1.40 aaron 534: Quiet mode.
535: Causes all warning and diagnostic messages to be suppressed.
1.80 djm 536: .It Fl s
1.117 itojun 537: 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
538: of SSH as a secure transport for other applications (eg. sftp). The
1.80 djm 539: subsystem is specified as the remote command.
1.2 deraadt 540: .It Fl t
1.40 aaron 541: Force pseudo-tty allocation.
1.43 brad 542: This can be used to execute arbitrary
1.40 aaron 543: screen-based programs on a remote machine, which can be very useful,
544: e.g., when implementing menu services.
1.73 markus 545: Multiple
546: .Fl t
547: options force tty allocation, even if
548: .Nm
549: has no local tty.
1.53 markus 550: .It Fl T
1.69 markus 551: Disable pseudo-tty allocation.
1.2 deraadt 552: .It Fl v
1.40 aaron 553: Verbose mode.
554: Causes
1.2 deraadt 555: .Nm
1.40 aaron 556: to print debugging messages about its progress.
557: This is helpful in
1.1 deraadt 558: debugging connection, authentication, and configuration problems.
1.73 markus 559: Multiple
560: .Fl v
561: options increases the verbosity.
1.61 aaron 562: Maximum is 3.
1.2 deraadt 563: .It Fl x
1.40 aaron 564: Disables X11 forwarding.
1.2 deraadt 565: .It Fl X
1.1 deraadt 566: Enables X11 forwarding.
1.54 markus 567: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 568: .Pp
569: X11 forwarding should be enabled with caution. Users with the ability
570: to bypass file permissions on the remote host (for the user's X
571: authorization database) can access the local X11 display through the
572: forwarded connection. An attacker may then be able to perform
573: activities such as keystroke monitoring.
1.2 deraadt 574: .It Fl C
1.1 deraadt 575: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 576: data for forwarded X11 and TCP/IP connections).
577: The compression algorithm is the same used by
1.34 markus 578: .Xr gzip 1 ,
579: and the
1.2 deraadt 580: .Dq level
581: can be controlled by the
582: .Cm CompressionLevel
1.167 ! stevesk 583: option for protocol version 1.
1.40 aaron 584: Compression is desirable on modem lines and other
1.1 deraadt 585: slow connections, but will only slow down things on fast networks.
586: The default value can be set on a host-by-host basis in the
587: configuration files; see the
1.121 pvalchev 588: .Cm Compression
1.158 stevesk 589: option.
1.132 markus 590: .It Fl F Ar configfile
591: Specifies an alternative per-user configuration file.
592: If a configuration file is given on the command line,
593: the system-wide configuration file
1.147 deraadt 594: .Pq Pa /etc/ssh/ssh_config
1.132 markus 595: will be ignored.
596: The default for the per-user configuration file is
597: .Pa $HOME/.ssh/config .
1.2 deraadt 598: .It Fl L Ar port:host:hostport
1.1 deraadt 599: Specifies that the given port on the local (client) host is to be
1.40 aaron 600: forwarded to the given host and port on the remote side.
601: This works by allocating a socket to listen to
1.2 deraadt 602: .Ar port
1.1 deraadt 603: on the local side, and whenever a connection is made to this port, the
604: connection is forwarded over the secure channel, and a connection is
605: made to
1.32 markus 606: .Ar host
607: port
608: .Ar hostport
1.40 aaron 609: from the remote machine.
610: Port forwardings can also be specified in the configuration file.
611: Only root can forward privileged ports.
1.32 markus 612: IPv6 addresses can be specified with an alternative syntax:
613: .Ar port/host/hostport
1.2 deraadt 614: .It Fl R Ar port:host:hostport
1.1 deraadt 615: Specifies that the given port on the remote (server) host is to be
1.40 aaron 616: forwarded to the given host and port on the local side.
617: This works by allocating a socket to listen to
1.2 deraadt 618: .Ar port
1.1 deraadt 619: on the remote side, and whenever a connection is made to this port, the
620: connection is forwarded over the secure channel, and a connection is
621: made to
1.32 markus 622: .Ar host
623: port
624: .Ar hostport
1.40 aaron 625: from the local machine.
626: Port forwardings can also be specified in the configuration file.
627: Privileged ports can be forwarded only when
1.1 deraadt 628: logging in as root on the remote machine.
1.107 markus 629: IPv6 addresses can be specified with an alternative syntax:
630: .Ar port/host/hostport
1.133 stevesk 631: .It Fl D Ar port
632: Specifies a local
633: .Dq dynamic
634: application-level port forwarding.
635: This works by allocating a socket to listen to
636: .Ar port
637: on the local side, and whenever a connection is made to this port, the
638: connection is forwarded over the secure channel, and the application
639: protocol is then used to determine where to connect to from the
640: remote machine. Currently the SOCKS4 protocol is supported, and
641: .Nm
642: will act as a SOCKS4 server.
643: Only root can forward privileged ports.
644: Dynamic port forwardings can also be specified in the configuration file.
1.85 jakob 645: .It Fl 1
646: Forces
647: .Nm
648: to try protocol version 1 only.
1.46 markus 649: .It Fl 2
650: Forces
651: .Nm
1.50 markus 652: to try protocol version 2 only.
1.32 markus 653: .It Fl 4
654: Forces
655: .Nm
656: to use IPv4 addresses only.
657: .It Fl 6
658: Forces
659: .Nm
660: to use IPv6 addresses only.
1.2 deraadt 661: .El
662: .Sh CONFIGURATION FILES
663: .Nm
1.158 stevesk 664: may additionally obtain configuration data from
665: a per-user configuration file and a system-wide configuration file.
666: The file format and configuration options are described in
667: .Xr ssh_config 5 .
1.2 deraadt 668: .Sh ENVIRONMENT
669: .Nm
1.1 deraadt 670: will normally set the following environment variables:
1.2 deraadt 671: .Bl -tag -width Ds
672: .It Ev DISPLAY
673: The
674: .Ev DISPLAY
1.40 aaron 675: variable indicates the location of the X11 server.
1.44 aaron 676: It is automatically set by
1.2 deraadt 677: .Nm
678: to point to a value of the form
679: .Dq hostname:n
680: where hostname indicates
1.40 aaron 681: the host where the shell runs, and n is an integer >= 1.
682: .Nm
683: uses this special value to forward X11 connections over the secure
684: channel.
1.107 markus 685: The user should normally not set
686: .Ev DISPLAY
687: explicitly, as that
1.1 deraadt 688: will render the X11 connection insecure (and will require the user to
689: manually copy any required authorization cookies).
1.2 deraadt 690: .It Ev HOME
1.1 deraadt 691: Set to the path of the user's home directory.
1.2 deraadt 692: .It Ev LOGNAME
693: Synonym for
1.12 aaron 694: .Ev USER ;
695: set for compatibility with systems that use this variable.
1.2 deraadt 696: .It Ev MAIL
1.129 stevesk 697: Set to the path of the user's mailbox.
1.40 aaron 698: .It Ev PATH
1.2 deraadt 699: Set to the default
700: .Ev PATH ,
701: as specified when compiling
1.12 aaron 702: .Nm ssh .
1.118 markus 703: .It Ev SSH_ASKPASS
704: If
705: .Nm
706: needs a passphrase, it will read the passphrase from the current
707: terminal if it was run from a terminal.
708: If
709: .Nm
710: does not have a terminal associated with it but
711: .Ev DISPLAY
712: and
713: .Ev SSH_ASKPASS
714: are set, it will execute the program specified by
715: .Ev SSH_ASKPASS
716: and open an X11 window to read the passphrase.
717: This is particularly useful when calling
718: .Nm
719: from a
720: .Pa .Xsession
721: or related script.
722: (Note that on some machines it
723: may be necessary to redirect the input from
724: .Pa /dev/null
725: to make this work.)
1.18 markus 726: .It Ev SSH_AUTH_SOCK
1.129 stevesk 727: Identifies the path of a unix-domain socket used to communicate with the
1.17 markus 728: agent.
1.166 stevesk 729: .It Ev SSH_CONNECTION
730: Identifies the client and server ends of the connection.
1.40 aaron 731: The variable contains
1.166 stevesk 732: four space-separated values: client ip-address, client port number,
733: server ip-address and server port number.
1.73 markus 734: .It Ev SSH_ORIGINAL_COMMAND
735: The variable contains the original command line if a forced command
736: is executed.
737: It can be used to extract the original arguments.
1.2 deraadt 738: .It Ev SSH_TTY
1.1 deraadt 739: This is set to the name of the tty (path to the device) associated
1.40 aaron 740: with the current shell or command.
741: If the current session has no tty,
1.1 deraadt 742: this variable is not set.
1.2 deraadt 743: .It Ev TZ
1.1 deraadt 744: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 745: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 746: on to new connections).
1.2 deraadt 747: .It Ev USER
1.1 deraadt 748: Set to the name of the user logging in.
1.2 deraadt 749: .El
750: .Pp
1.44 aaron 751: Additionally,
1.2 deraadt 752: .Nm
1.44 aaron 753: reads
754: .Pa $HOME/.ssh/environment ,
1.2 deraadt 755: and adds lines of the format
756: .Dq VARNAME=value
1.161 marc 757: to the environment if the file exists and if users are allowed to
758: change their environment.
1.162 stevesk 759: See the
1.161 marc 760: .Cm PermitUserEnvironment
1.162 stevesk 761: option in
1.161 marc 762: .Xr sshd_config 5 .
1.2 deraadt 763: .Sh FILES
1.36 markus 764: .Bl -tag -width Ds
1.116 markus 765: .It Pa $HOME/.ssh/known_hosts
1.129 stevesk 766: Records host keys for all hosts the user has logged into that are not
1.2 deraadt 767: in
1.147 deraadt 768: .Pa /etc/ssh/ssh_known_hosts .
1.2 deraadt 769: See
770: .Xr sshd 8 .
1.102 itojun 771: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
772: Contains the authentication identity of the user.
773: They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1.48 markus 774: These files
775: contain sensitive data and should be readable by the user but not
1.15 markus 776: accessible by others (read/write/execute).
777: Note that
778: .Nm
1.48 markus 779: ignores a private key file if it is accessible by others.
1.15 markus 780: It is possible to specify a passphrase when
1.1 deraadt 781: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 782: sensitive part of this file using 3DES.
1.102 itojun 783: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1.1 deraadt 784: Contains the public key for authentication (public part of the
1.40 aaron 785: identity file in human-readable form).
1.48 markus 786: The contents of the
787: .Pa $HOME/.ssh/identity.pub
788: file should be added to
1.2 deraadt 789: .Pa $HOME/.ssh/authorized_keys
790: on all machines
1.137 deraadt 791: where the user wishes to log in using protocol version 1 RSA authentication.
1.48 markus 792: The contents of the
793: .Pa $HOME/.ssh/id_dsa.pub
1.102 itojun 794: and
795: .Pa $HOME/.ssh/id_rsa.pub
1.48 markus 796: file should be added to
1.115 markus 797: .Pa $HOME/.ssh/authorized_keys
1.48 markus 798: on all machines
1.137 deraadt 799: where the user wishes to log in using protocol version 2 DSA/RSA authentication.
1.48 markus 800: These files are not
1.40 aaron 801: sensitive and can (but need not) be readable by anyone.
1.48 markus 802: These files are
1.84 markus 803: never used automatically and are not necessary; they are only provided for
1.1 deraadt 804: the convenience of the user.
1.2 deraadt 805: .It Pa $HOME/.ssh/config
1.40 aaron 806: This is the per-user configuration file.
1.158 stevesk 807: The file format and configuration options are described in
808: .Xr ssh_config 5 .
1.2 deraadt 809: .It Pa $HOME/.ssh/authorized_keys
1.115 markus 810: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.40 aaron 811: The format of this file is described in the
1.2 deraadt 812: .Xr sshd 8
1.40 aaron 813: manual page.
1.116 markus 814: In the simplest form the format is the same as the .pub
815: identity files.
1.48 markus 816: This file is not highly sensitive, but the recommended
817: permissions are read/write for the user, and not accessible by others.
1.147 deraadt 818: .It Pa /etc/ssh/ssh_known_hosts
1.40 aaron 819: Systemwide list of known host keys.
1.116 markus 820: This file should be prepared by the
1.1 deraadt 821: system administrator to contain the public host keys of all machines in the
1.40 aaron 822: organization.
823: This file should be world-readable.
824: This file contains
1.1 deraadt 825: public keys, one per line, in the following format (fields separated
1.116 markus 826: by spaces): system name, public key and optional comment field.
1.40 aaron 827: When different names are used
1.1 deraadt 828: for the same machine, all such names should be listed, separated by
1.40 aaron 829: commas.
830: The format is described on the
1.2 deraadt 831: .Xr sshd 8
1.1 deraadt 832: manual page.
1.2 deraadt 833: .Pp
1.1 deraadt 834: The canonical system name (as returned by name servers) is used by
1.2 deraadt 835: .Xr sshd 8
1.1 deraadt 836: to verify the client host when logging in; other names are needed because
1.2 deraadt 837: .Nm
1.1 deraadt 838: does not convert the user-supplied name to a canonical name before
839: checking the key, because someone with access to the name servers
840: would then be able to fool host authentication.
1.147 deraadt 841: .It Pa /etc/ssh/ssh_config
1.40 aaron 842: Systemwide configuration file.
1.158 stevesk 843: The file format and configuration options are described in
844: .Xr ssh_config 5 .
1.147 deraadt 845: .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1.141 markus 846: These three files contain the private parts of the host keys
847: and are used for
848: .Cm RhostsRSAAuthentication
849: and
850: .Cm HostbasedAuthentication .
1.155 stevesk 851: If the protocol version 1
852: .Cm RhostsRSAAuthentication
1.157 deraadt 853: method is used,
1.155 stevesk 854: .Nm
855: must be setuid root, since the host key is readable only by root.
856: For protocol version 2,
857: .Nm
858: uses
859: .Xr ssh-keysign 8
860: to access the host keys for
861: .Cm HostbasedAuthentication .
862: This eliminates the requirement that
863: .Nm
864: be setuid root when that authentication method is used.
865: By default
1.141 markus 866: .Nm
1.155 stevesk 867: is not setuid root.
1.2 deraadt 868: .It Pa $HOME/.rhosts
869: This file is used in
870: .Pa \&.rhosts
871: authentication to list the
1.40 aaron 872: host/user pairs that are permitted to log in.
873: (Note that this file is
1.1 deraadt 874: also used by rlogin and rsh, which makes using this file insecure.)
875: Each line of the file contains a host name (in the canonical form
876: returned by name servers), and then a user name on that host,
1.40 aaron 877: separated by a space.
1.92 markus 878: On some machines this file may need to be
1.1 deraadt 879: world-readable if the user's home directory is on a NFS partition,
880: because
1.2 deraadt 881: .Xr sshd 8
1.40 aaron 882: reads it as root.
883: Additionally, this file must be owned by the user,
884: and must not have write permissions for anyone else.
885: The recommended
1.1 deraadt 886: permission for most machines is read/write for the user, and not
887: accessible by others.
1.2 deraadt 888: .Pp
1.1 deraadt 889: Note that by default
1.2 deraadt 890: .Xr sshd 8
1.1 deraadt 891: will be installed so that it requires successful RSA host
1.40 aaron 892: authentication before permitting \s+2.\s0rhosts authentication.
1.137 deraadt 893: If the server machine does not have the client's host key in
1.147 deraadt 894: .Pa /etc/ssh/ssh_known_hosts ,
1.137 deraadt 895: it can be stored in
1.2 deraadt 896: .Pa $HOME/.ssh/known_hosts .
897: The easiest way to do this is to
1.1 deraadt 898: connect back to the client from the server machine using ssh; this
1.48 markus 899: will automatically add the host key to
1.2 deraadt 900: .Pa $HOME/.ssh/known_hosts .
901: .It Pa $HOME/.shosts
902: This file is used exactly the same way as
903: .Pa \&.rhosts .
904: The purpose for
1.1 deraadt 905: having this file is to be able to use rhosts authentication with
1.2 deraadt 906: .Nm
907: without permitting login with
1.151 millert 908: .Nm rlogin
1.2 deraadt 909: or
910: .Xr rsh 1 .
911: .It Pa /etc/hosts.equiv
912: This file is used during
1.40 aaron 913: .Pa \&.rhosts authentication.
914: It contains
1.1 deraadt 915: canonical hosts names, one per line (the full format is described on
916: the
1.2 deraadt 917: .Xr sshd 8
1.40 aaron 918: manual page).
919: If the client host is found in this file, login is
1.1 deraadt 920: automatically permitted provided client and server user names are the
1.40 aaron 921: same.
922: Additionally, successful RSA host authentication is normally
923: required.
924: This file should only be writable by root.
1.2 deraadt 925: .It Pa /etc/shosts.equiv
1.44 aaron 926: This file is processed exactly as
1.2 deraadt 927: .Pa /etc/hosts.equiv .
1.1 deraadt 928: This file may be useful to permit logins using
1.2 deraadt 929: .Nm
1.1 deraadt 930: but not using rsh/rlogin.
1.147 deraadt 931: .It Pa /etc/ssh/sshrc
1.1 deraadt 932: Commands in this file are executed by
1.2 deraadt 933: .Nm
1.1 deraadt 934: when the user logs in just before the user's shell (or command) is started.
935: See the
1.2 deraadt 936: .Xr sshd 8
1.1 deraadt 937: manual page for more information.
1.2 deraadt 938: .It Pa $HOME/.ssh/rc
1.1 deraadt 939: Commands in this file are executed by
1.2 deraadt 940: .Nm
1.1 deraadt 941: when the user logs in just before the user's shell (or command) is
942: started.
1.44 aaron 943: See the
1.2 deraadt 944: .Xr sshd 8
1.1 deraadt 945: manual page for more information.
1.31 markus 946: .It Pa $HOME/.ssh/environment
947: Contains additional definitions for environment variables, see section
948: .Sx ENVIRONMENT
949: above.
1.58 itojun 950: .El
1.145 markus 951: .Sh DIAGNOSTICS
952: .Nm
953: exits with the exit status of the remote command or with 255
954: if an error occurred.
1.67 aaron 955: .Sh AUTHORS
1.78 markus 956: OpenSSH is a derivative of the original and free
957: ssh 1.2.12 release by Tatu Ylonen.
958: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
959: Theo de Raadt and Dug Song
960: removed many bugs, re-added newer features and
961: created OpenSSH.
962: Markus Friedl contributed the support for SSH
963: protocol versions 1.5 and 2.0.
1.2 deraadt 964: .Sh SEE ALSO
965: .Xr rsh 1 ,
966: .Xr scp 1 ,
1.83 djm 967: .Xr sftp 1 ,
1.2 deraadt 968: .Xr ssh-add 1 ,
969: .Xr ssh-agent 1 ,
970: .Xr ssh-keygen 1 ,
971: .Xr telnet 1 ,
1.159 stevesk 972: .Xr ssh_config 5 ,
1.160 naddy 973: .Xr ssh-keysign 8 ,
1.87 itojun 974: .Xr sshd 8
1.106 markus 975: .Rs
976: .%A T. Ylonen
977: .%A T. Kivinen
978: .%A M. Saarinen
979: .%A T. Rinne
980: .%A S. Lehtinen
981: .%T "SSH Protocol Architecture"
1.150 markus 982: .%N draft-ietf-secsh-architecture-12.txt
983: .%D January 2002
1.106 markus 984: .%O work in progress material
985: .Re
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