Annotation of src/usr.bin/ssh/ssh.1, Revision 1.167.2.2
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: .\"
1.167.2.2! brad 37: .\" $OpenBSD: ssh.1,v 1.175 2003/07/22 13:35:22 markus 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
1.167.2.1 margarid 51: .Bk -words
1.167.2.2! brad 52: .Op Fl afgknqstvxACNTVX1246
1.108 markus 53: .Op Fl b Ar bind_address
1.51 markus 54: .Op Fl c Ar cipher_spec
1.2 deraadt 55: .Op Fl e Ar escape_char
56: .Op Fl i Ar identity_file
57: .Op Fl l Ar login_name
1.91 markus 58: .Op Fl m Ar mac_spec
1.2 deraadt 59: .Op Fl o Ar option
60: .Op Fl p Ar port
1.132 markus 61: .Op Fl F Ar configfile
1.12 aaron 62: .Oo Fl L Xo
63: .Sm off
1.33 markus 64: .Ar port :
1.12 aaron 65: .Ar host :
66: .Ar hostport
67: .Sm on
68: .Xc
69: .Oc
1.167.2.1 margarid 70: .Ek
71: .Bk -words
1.12 aaron 72: .Oo Fl R Xo
73: .Sm off
1.33 markus 74: .Ar port :
1.12 aaron 75: .Ar host :
76: .Ar hostport
77: .Sm on
78: .Xc
79: .Oc
1.135 naddy 80: .Op Fl D Ar port
1.130 stevesk 81: .Ar hostname | user@hostname
1.2 deraadt 82: .Op Ar command
1.167.2.1 margarid 83: .Ek
1.44 aaron 84: .Sh DESCRIPTION
1.2 deraadt 85: .Nm
1.96 deraadt 86: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 87: executing commands on a remote machine.
88: It is intended to replace
1.1 deraadt 89: rlogin and rsh, and provide secure encrypted communications between
1.40 aaron 90: two untrusted hosts over an insecure network.
91: X11 connections and
1.1 deraadt 92: arbitrary TCP/IP ports can also be forwarded over the secure channel.
1.2 deraadt 93: .Pp
94: .Nm
1.44 aaron 95: connects and logs into the specified
1.2 deraadt 96: .Ar hostname .
1.1 deraadt 97: The user must prove
1.49 markus 98: his/her identity to the remote machine using one of several methods
99: depending on the protocol version used:
100: .Pp
101: .Ss SSH protocol version 1
1.2 deraadt 102: .Pp
1.1 deraadt 103: First, if the machine the user logs in from is listed in
1.2 deraadt 104: .Pa /etc/hosts.equiv
1.1 deraadt 105: or
1.2 deraadt 106: .Pa /etc/shosts.equiv
1.1 deraadt 107: on the remote machine, and the user names are
108: the same on both sides, the user is immediately permitted to log in.
1.44 aaron 109: Second, if
1.2 deraadt 110: .Pa \&.rhosts
1.1 deraadt 111: or
1.2 deraadt 112: .Pa \&.shosts
1.1 deraadt 113: exists in the user's home directory on the
114: remote machine and contains a line containing the name of the client
115: machine and the name of the user on that machine, the user is
1.40 aaron 116: permitted to log in.
117: This form of authentication alone is normally not
1.1 deraadt 118: allowed by the server because it is not secure.
1.2 deraadt 119: .Pp
1.107 markus 120: The second authentication method is the
1.2 deraadt 121: .Pa rhosts
1.1 deraadt 122: or
1.2 deraadt 123: .Pa hosts.equiv
1.40 aaron 124: method combined with RSA-based host authentication.
125: It means that if the login would be permitted by
1.49 markus 126: .Pa $HOME/.rhosts ,
127: .Pa $HOME/.shosts ,
1.2 deraadt 128: .Pa /etc/hosts.equiv ,
1.1 deraadt 129: or
1.2 deraadt 130: .Pa /etc/shosts.equiv ,
1.11 deraadt 131: and if additionally the server can verify the client's
1.44 aaron 132: host key (see
1.147 deraadt 133: .Pa /etc/ssh/ssh_known_hosts
1.23 markus 134: and
135: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 136: in the
1.2 deraadt 137: .Sx FILES
1.40 aaron 138: section), only then login is permitted.
139: This authentication method closes security holes due to IP
140: spoofing, DNS spoofing and routing spoofing.
141: [Note to the administrator:
1.2 deraadt 142: .Pa /etc/hosts.equiv ,
1.49 markus 143: .Pa $HOME/.rhosts ,
1.1 deraadt 144: and the rlogin/rsh protocol in general, are inherently insecure and should be
145: disabled if security is desired.]
1.2 deraadt 146: .Pp
1.44 aaron 147: As a third authentication method,
1.2 deraadt 148: .Nm
1.1 deraadt 149: supports RSA based authentication.
150: The scheme is based on public-key cryptography: there are cryptosystems
151: where encryption and decryption are done using separate keys, and it
152: is not possible to derive the decryption key from the encryption key.
1.40 aaron 153: RSA is one such system.
1.44 aaron 154: The idea is that each user creates a public/private
1.40 aaron 155: key pair for authentication purposes.
156: The server knows the public key, and only the user knows the private key.
1.44 aaron 157: The file
1.2 deraadt 158: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 159: lists the public keys that are permitted for logging
1.40 aaron 160: in.
161: When the user logs in, the
1.2 deraadt 162: .Nm
1.1 deraadt 163: program tells the server which key pair it would like to use for
1.40 aaron 164: authentication.
165: The server checks if this key is permitted, and if
1.1 deraadt 166: so, sends the user (actually the
1.2 deraadt 167: .Nm
1.1 deraadt 168: program running on behalf of the user) a challenge, a random number,
1.40 aaron 169: encrypted by the user's public key.
170: The challenge can only be
171: decrypted using the proper private key.
172: The user's client then decrypts the
1.1 deraadt 173: challenge using the private key, proving that he/she knows the private
174: key but without disclosing it to the server.
1.2 deraadt 175: .Pp
176: .Nm
1.40 aaron 177: implements the RSA authentication protocol automatically.
178: The user creates his/her RSA key pair by running
1.2 deraadt 179: .Xr ssh-keygen 1 .
1.44 aaron 180: This stores the private key in
1.49 markus 181: .Pa $HOME/.ssh/identity
1.1 deraadt 182: and the public key in
1.49 markus 183: .Pa $HOME/.ssh/identity.pub
1.40 aaron 184: in the user's home directory.
185: The user should then copy the
1.2 deraadt 186: .Pa identity.pub
1.44 aaron 187: to
1.49 markus 188: .Pa $HOME/.ssh/authorized_keys
1.44 aaron 189: in his/her home directory on the remote machine (the
1.2 deraadt 190: .Pa authorized_keys
1.44 aaron 191: file corresponds to the conventional
1.49 markus 192: .Pa $HOME/.rhosts
1.1 deraadt 193: file, and has one key
1.40 aaron 194: per line, though the lines can be very long).
195: After this, the user can log in without giving the password.
196: RSA authentication is much
1.1 deraadt 197: more secure than rhosts authentication.
1.2 deraadt 198: .Pp
1.1 deraadt 199: The most convenient way to use RSA authentication may be with an
1.40 aaron 200: authentication agent.
201: See
1.2 deraadt 202: .Xr ssh-agent 1
1.1 deraadt 203: for more information.
1.2 deraadt 204: .Pp
1.44 aaron 205: If other authentication methods fail,
1.2 deraadt 206: .Nm
1.40 aaron 207: prompts the user for a password.
208: The password is sent to the remote
1.1 deraadt 209: host for checking; however, since all communications are encrypted,
210: the password cannot be seen by someone listening on the network.
1.2 deraadt 211: .Pp
1.49 markus 212: .Ss SSH protocol version 2
213: .Pp
1.145 markus 214: When a user connects using protocol version 2
215: similar authentication methods are available.
1.107 markus 216: Using the default values for
217: .Cm PreferredAuthentications ,
1.123 markus 218: the client will try to authenticate first using the hostbased method;
219: if this method fails public key authentication is attempted,
220: and finally if this method fails keyboard-interactive and
221: password authentication are tried.
1.49 markus 222: .Pp
223: The public key method is similar to RSA authentication described
1.107 markus 224: in the previous section and allows the RSA or DSA algorithm to be used:
1.102 itojun 225: The client uses his private key,
1.49 markus 226: .Pa $HOME/.ssh/id_dsa
1.102 itojun 227: or
228: .Pa $HOME/.ssh/id_rsa ,
1.49 markus 229: to sign the session identifier and sends the result to the server.
230: The server checks whether the matching public key is listed in
1.115 markus 231: .Pa $HOME/.ssh/authorized_keys
1.49 markus 232: and grants access if both the key is found and the signature is correct.
233: The session identifier is derived from a shared Diffie-Hellman value
234: and is only known to the client and the server.
235: .Pp
236: If public key authentication fails or is not available a password
237: can be sent encrypted to the remote host for proving the user's identity.
1.107 markus 238: .Pp
239: Additionally,
240: .Nm
241: supports hostbased or challenge response authentication.
1.49 markus 242: .Pp
243: Protocol 2 provides additional mechanisms for confidentiality
1.51 markus 244: (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
1.94 deraadt 245: and integrity (hmac-md5, hmac-sha1).
1.49 markus 246: Note that protocol 1 lacks a strong mechanism for ensuring the
247: integrity of the connection.
248: .Pp
249: .Ss Login session and remote execution
250: .Pp
1.1 deraadt 251: When the user's identity has been accepted by the server, the server
252: either executes the given command, or logs into the machine and gives
1.40 aaron 253: the user a normal shell on the remote machine.
254: All communication with
1.1 deraadt 255: the remote command or shell will be automatically encrypted.
1.2 deraadt 256: .Pp
1.1 deraadt 257: If a pseudo-terminal has been allocated (normal login session), the
1.104 djm 258: user may use the escape characters noted below.
1.2 deraadt 259: .Pp
1.1 deraadt 260: If no pseudo tty has been allocated, the
261: session is transparent and can be used to reliably transfer binary
1.40 aaron 262: data.
263: On most systems, setting the escape character to
1.2 deraadt 264: .Dq none
265: will also make the session transparent even if a tty is used.
266: .Pp
1.71 djm 267: The session terminates when the command or shell on the remote
1.92 markus 268: machine exits and all X11 and TCP/IP connections have been closed.
1.1 deraadt 269: The exit status of the remote program is returned as the exit status
270: of
1.2 deraadt 271: .Nm ssh .
1.104 djm 272: .Pp
273: .Ss Escape Characters
274: .Pp
275: When a pseudo terminal has been requested, ssh supports a number of functions
1.117 itojun 276: through the use of an escape character.
1.104 djm 277: .Pp
278: A single tilde character can be sent as
279: .Ic ~~
1.119 stevesk 280: or by following the tilde by a character other than those described below.
1.104 djm 281: The escape character must always follow a newline to be interpreted as
282: special.
283: The escape character can be changed in configuration files using the
284: .Cm EscapeChar
1.117 itojun 285: configuration directive or on the command line by the
1.104 djm 286: .Fl e
287: option.
288: .Pp
289: The supported escapes (assuming the default
290: .Ql ~ )
291: are:
292: .Bl -tag -width Ds
293: .It Cm ~.
294: Disconnect
295: .It Cm ~^Z
296: Background ssh
297: .It Cm ~#
298: List forwarded connections
299: .It Cm ~&
300: Background ssh at logout when waiting for forwarded connection / X11 sessions
1.140 markus 301: to terminate
1.104 djm 302: .It Cm ~?
303: Display a list of escape characters
1.167.2.2! brad 304: .It Cm ~B
! 305: Send a BREAK to the remote system (only useful for SSH protocol version 2
! 306: and if the peer supports it)
1.149 jakob 307: .It Cm ~C
308: Open command line (only useful for adding port forwardings using the
309: .Fl L
310: and
311: .Fl R
312: options)
1.104 djm 313: .It Cm ~R
1.105 djm 314: Request rekeying of the connection (only useful for SSH protocol version 2
315: and if the peer supports it)
1.104 djm 316: .El
1.2 deraadt 317: .Pp
1.49 markus 318: .Ss X11 and TCP forwarding
319: .Pp
1.110 deraadt 320: If the
321: .Cm ForwardX11
322: variable is set to
323: .Dq yes
324: (or, see the description of the
325: .Fl X
326: and
327: .Fl x
328: options described later)
329: and the user is using X11 (the
1.2 deraadt 330: .Ev DISPLAY
1.1 deraadt 331: environment variable is set), the connection to the X11 display is
332: automatically forwarded to the remote side in such a way that any X11
333: programs started from the shell (or command) will go through the
334: encrypted channel, and the connection to the real X server will be made
1.40 aaron 335: from the local machine.
336: The user should not manually set
1.2 deraadt 337: .Ev DISPLAY .
1.1 deraadt 338: Forwarding of X11 connections can be
339: configured on the command line or in configuration files.
1.2 deraadt 340: .Pp
341: The
1.44 aaron 342: .Ev DISPLAY
1.2 deraadt 343: value set by
344: .Nm
1.1 deraadt 345: will point to the server machine, but with a display number greater
1.40 aaron 346: than zero.
347: This is normal, and happens because
1.2 deraadt 348: .Nm
349: creates a
350: .Dq proxy
351: X server on the server machine for forwarding the
1.1 deraadt 352: connections over the encrypted channel.
1.2 deraadt 353: .Pp
354: .Nm
1.1 deraadt 355: will also automatically set up Xauthority data on the server machine.
356: For this purpose, it will generate a random authorization cookie,
357: store it in Xauthority on the server, and verify that any forwarded
358: connections carry this cookie and replace it by the real cookie when
1.40 aaron 359: the connection is opened.
360: The real authentication cookie is never
1.1 deraadt 361: sent to the server machine (and no cookies are sent in the plain).
1.2 deraadt 362: .Pp
1.163 stevesk 363: If the
364: .Cm ForwardAgent
365: variable is set to
366: .Dq yes
367: (or, see the description of the
368: .Fl A
369: and
370: .Fl a
1.167.2.1 margarid 371: options described later) and
1.163 stevesk 372: the user is using an authentication agent, the connection to the agent
373: is automatically forwarded to the remote side.
1.2 deraadt 374: .Pp
1.1 deraadt 375: Forwarding of arbitrary TCP/IP connections over the secure channel can
1.120 stevesk 376: be specified either on the command line or in a configuration file.
1.40 aaron 377: One possible application of TCP/IP forwarding is a secure connection to an
1.92 markus 378: electronic purse; another is going through firewalls.
1.2 deraadt 379: .Pp
1.49 markus 380: .Ss Server authentication
381: .Pp
1.2 deraadt 382: .Nm
1.49 markus 383: automatically maintains and checks a database containing
1.40 aaron 384: identifications for all hosts it has ever been used with.
1.116 markus 385: Host keys are stored in
1.49 markus 386: .Pa $HOME/.ssh/known_hosts
1.40 aaron 387: in the user's home directory.
1.116 markus 388: Additionally, the file
1.147 deraadt 389: .Pa /etc/ssh/ssh_known_hosts
1.116 markus 390: is automatically checked for known hosts.
1.40 aaron 391: Any new hosts are automatically added to the user's file.
392: If a host's identification
1.1 deraadt 393: ever changes,
1.2 deraadt 394: .Nm
1.1 deraadt 395: warns about this and disables password authentication to prevent a
1.40 aaron 396: trojan horse from getting the user's password.
397: Another purpose of
1.1 deraadt 398: this mechanism is to prevent man-in-the-middle attacks which could
1.40 aaron 399: otherwise be used to circumvent the encryption.
400: The
1.2 deraadt 401: .Cm StrictHostKeyChecking
1.158 stevesk 402: option can be used to prevent logins to machines whose
1.1 deraadt 403: host key is not known or has changed.
1.65 aaron 404: .Pp
405: The options are as follows:
1.2 deraadt 406: .Bl -tag -width Ds
1.4 dugsong 407: .It Fl a
1.42 aaron 408: Disables forwarding of the authentication agent connection.
1.54 markus 409: .It Fl A
410: Enables forwarding of the authentication agent connection.
411: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 412: .Pp
1.167.2.1 margarid 413: Agent forwarding should be enabled with caution.
414: Users with the ability to bypass file permissions on the remote host
415: (for the agent's Unix-domain socket)
416: can access the local agent through the forwarded connection.
417: An attacker cannot obtain key material from the agent,
1.165 stevesk 418: however they can perform operations on the keys that enable them to
419: authenticate using the identities loaded into the agent.
1.108 markus 420: .It Fl b Ar bind_address
421: Specify the interface to transmit from on machines with multiple
422: interfaces or aliased addresses.
1.131 stevesk 423: .It Fl c Ar blowfish|3des|des
1.44 aaron 424: Selects the cipher to use for encrypting the session.
1.2 deraadt 425: .Ar 3des
1.40 aaron 426: is used by default.
1.44 aaron 427: It is believed to be secure.
1.5 deraadt 428: .Ar 3des
429: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
430: .Ar blowfish
431: is a fast block cipher, it appears very secure and is much faster than
1.40 aaron 432: .Ar 3des .
1.131 stevesk 433: .Ar des
434: is only supported in the
435: .Nm
436: client for interoperability with legacy protocol 1 implementations
437: that do not support the
438: .Ar 3des
1.167.2.1 margarid 439: cipher.
440: Its use is strongly discouraged due to cryptographic weaknesses.
1.90 markus 441: .It Fl c Ar cipher_spec
1.51 markus 442: Additionally, for protocol version 2 a comma-separated list of ciphers can
1.61 aaron 443: be specified in order of preference.
1.90 markus 444: See
445: .Cm Ciphers
446: for more information.
1.2 deraadt 447: .It Fl e Ar ch|^ch|none
448: Sets the escape character for sessions with a pty (default:
449: .Ql ~ ) .
1.40 aaron 450: The escape character is only recognized at the beginning of a line.
451: The escape character followed by a dot
1.2 deraadt 452: .Pq Ql \&.
453: closes the connection, followed
1.1 deraadt 454: by control-Z suspends the connection, and followed by itself sends the
1.40 aaron 455: escape character once.
456: Setting the character to
1.2 deraadt 457: .Dq none
458: disables any escapes and makes the session fully transparent.
459: .It Fl f
460: Requests
461: .Nm
1.40 aaron 462: to go to background just before command execution.
463: This is useful if
1.2 deraadt 464: .Nm
465: is going to ask for passwords or passphrases, but the user
1.40 aaron 466: wants it in the background.
1.44 aaron 467: This implies
1.2 deraadt 468: .Fl n .
1.1 deraadt 469: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 470: something like
471: .Ic ssh -f host xterm .
1.34 markus 472: .It Fl g
473: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 474: .It Fl i Ar identity_file
1.144 stevesk 475: Selects a file from which the identity (private key) for
1.68 markus 476: RSA or DSA authentication is read.
1.144 stevesk 477: The default is
1.49 markus 478: .Pa $HOME/.ssh/identity
1.144 stevesk 479: for protocol version 1, and
480: .Pa $HOME/.ssh/id_rsa
481: and
482: .Pa $HOME/.ssh/id_dsa
483: for protocol version 2.
1.40 aaron 484: Identity files may also be specified on
485: a per-host basis in the configuration file.
486: It is possible to have multiple
1.2 deraadt 487: .Fl i
488: options (and multiple identities specified in
1.1 deraadt 489: configuration files).
1.125 jakob 490: .It Fl I Ar smartcard_device
1.167.2.2! brad 491: Specifies which smartcard device to use.
! 492: The argument is the device
1.125 jakob 493: .Nm
494: should use to communicate with a smartcard used for storing the user's
495: private RSA key.
1.2 deraadt 496: .It Fl k
1.167.2.2! brad 497: Disables forwarding of Kerberos tickets.
1.42 aaron 498: This may also be specified on a per-host basis in the configuration file.
1.2 deraadt 499: .It Fl l Ar login_name
1.40 aaron 500: Specifies the user to log in as on the remote machine.
501: This also may be specified on a per-host basis in the configuration file.
1.91 markus 502: .It Fl m Ar mac_spec
503: Additionally, for protocol version 2 a comma-separated list of MAC
504: (message authentication code) algorithms can
505: be specified in order of preference.
506: See the
507: .Cm MACs
508: keyword for more information.
1.2 deraadt 509: .It Fl n
510: Redirects stdin from
511: .Pa /dev/null
512: (actually, prevents reading from stdin).
1.1 deraadt 513: This must be used when
1.2 deraadt 514: .Nm
1.40 aaron 515: is run in the background.
516: A common trick is to use this to run X11 programs on a remote machine.
517: For example,
1.2 deraadt 518: .Ic ssh -n shadows.cs.hut.fi emacs &
519: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 520: connection will be automatically forwarded over an encrypted channel.
521: The
1.2 deraadt 522: .Nm
1.1 deraadt 523: program will be put in the background.
524: (This does not work if
1.2 deraadt 525: .Nm
526: needs to ask for a password or passphrase; see also the
527: .Fl f
528: option.)
1.53 markus 529: .It Fl N
530: Do not execute a remote command.
1.137 deraadt 531: This is useful for just forwarding ports
1.53 markus 532: (protocol version 2 only).
1.2 deraadt 533: .It Fl o Ar option
1.127 stevesk 534: Can be used to give options in the format used in the configuration file.
1.1 deraadt 535: This is useful for specifying options for which there is no separate
1.40 aaron 536: command-line flag.
1.2 deraadt 537: .It Fl p Ar port
1.40 aaron 538: Port to connect to on the remote host.
539: This can be specified on a
1.1 deraadt 540: per-host basis in the configuration file.
1.2 deraadt 541: .It Fl q
1.40 aaron 542: Quiet mode.
543: Causes all warning and diagnostic messages to be suppressed.
1.80 djm 544: .It Fl s
1.167.2.2! brad 545: May be used to request invocation of a subsystem on the remote system.
! 546: Subsystems are a feature of the SSH2 protocol which facilitate the use
! 547: of SSH as a secure transport for other applications (eg. sftp).
! 548: The subsystem is specified as the remote command.
1.2 deraadt 549: .It Fl t
1.40 aaron 550: Force pseudo-tty allocation.
1.43 brad 551: This can be used to execute arbitrary
1.40 aaron 552: screen-based programs on a remote machine, which can be very useful,
553: e.g., when implementing menu services.
1.73 markus 554: Multiple
555: .Fl t
556: options force tty allocation, even if
557: .Nm
558: has no local tty.
1.53 markus 559: .It Fl T
1.69 markus 560: Disable pseudo-tty allocation.
1.2 deraadt 561: .It Fl v
1.40 aaron 562: Verbose mode.
563: Causes
1.2 deraadt 564: .Nm
1.40 aaron 565: to print debugging messages about its progress.
566: This is helpful in
1.1 deraadt 567: debugging connection, authentication, and configuration problems.
1.73 markus 568: Multiple
569: .Fl v
1.167.2.2! brad 570: options increase the verbosity.
! 571: The maximum is 3.
! 572: .It Fl V
! 573: Display the version number and exit.
1.2 deraadt 574: .It Fl x
1.40 aaron 575: Disables X11 forwarding.
1.2 deraadt 576: .It Fl X
1.1 deraadt 577: Enables X11 forwarding.
1.54 markus 578: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 579: .Pp
1.167.2.1 margarid 580: X11 forwarding should be enabled with caution.
581: Users with the ability to bypass file permissions on the remote host
582: (for the user's X authorization database)
583: can access the local X11 display through the forwarded connection.
584: An attacker may then be able to perform activities such as keystroke monitoring.
1.2 deraadt 585: .It Fl C
1.1 deraadt 586: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 587: data for forwarded X11 and TCP/IP connections).
588: The compression algorithm is the same used by
1.34 markus 589: .Xr gzip 1 ,
590: and the
1.2 deraadt 591: .Dq level
592: can be controlled by the
593: .Cm CompressionLevel
1.167 stevesk 594: option for protocol version 1.
1.40 aaron 595: Compression is desirable on modem lines and other
1.1 deraadt 596: slow connections, but will only slow down things on fast networks.
597: The default value can be set on a host-by-host basis in the
598: configuration files; see the
1.121 pvalchev 599: .Cm Compression
1.158 stevesk 600: option.
1.132 markus 601: .It Fl F Ar configfile
602: Specifies an alternative per-user configuration file.
603: If a configuration file is given on the command line,
604: the system-wide configuration file
1.147 deraadt 605: .Pq Pa /etc/ssh/ssh_config
1.132 markus 606: will be ignored.
607: The default for the per-user configuration file is
608: .Pa $HOME/.ssh/config .
1.2 deraadt 609: .It Fl L Ar port:host:hostport
1.1 deraadt 610: Specifies that the given port on the local (client) host is to be
1.40 aaron 611: forwarded to the given host and port on the remote side.
612: This works by allocating a socket to listen to
1.2 deraadt 613: .Ar port
1.1 deraadt 614: on the local side, and whenever a connection is made to this port, the
615: connection is forwarded over the secure channel, and a connection is
616: made to
1.32 markus 617: .Ar host
618: port
619: .Ar hostport
1.40 aaron 620: from the remote machine.
621: Port forwardings can also be specified in the configuration file.
622: Only root can forward privileged ports.
1.32 markus 623: IPv6 addresses can be specified with an alternative syntax:
624: .Ar port/host/hostport
1.2 deraadt 625: .It Fl R Ar port:host:hostport
1.1 deraadt 626: Specifies that the given port on the remote (server) host is to be
1.40 aaron 627: forwarded to the given host and port on the local side.
628: This works by allocating a socket to listen to
1.2 deraadt 629: .Ar port
1.1 deraadt 630: on the remote side, and whenever a connection is made to this port, the
631: connection is forwarded over the secure channel, and a connection is
632: made to
1.32 markus 633: .Ar host
634: port
635: .Ar hostport
1.40 aaron 636: from the local machine.
637: Port forwardings can also be specified in the configuration file.
638: Privileged ports can be forwarded only when
1.1 deraadt 639: logging in as root on the remote machine.
1.107 markus 640: IPv6 addresses can be specified with an alternative syntax:
641: .Ar port/host/hostport
1.133 stevesk 642: .It Fl D Ar port
643: Specifies a local
644: .Dq dynamic
645: application-level port forwarding.
646: This works by allocating a socket to listen to
647: .Ar port
648: on the local side, and whenever a connection is made to this port, the
649: connection is forwarded over the secure channel, and the application
650: protocol is then used to determine where to connect to from the
1.167.2.1 margarid 651: remote machine.
1.167.2.2! brad 652: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.133 stevesk 653: .Nm
1.167.2.2! brad 654: will act as a SOCKS server.
1.133 stevesk 655: Only root can forward privileged ports.
656: Dynamic port forwardings can also be specified in the configuration file.
1.85 jakob 657: .It Fl 1
658: Forces
659: .Nm
660: to try protocol version 1 only.
1.46 markus 661: .It Fl 2
662: Forces
663: .Nm
1.50 markus 664: to try protocol version 2 only.
1.32 markus 665: .It Fl 4
666: Forces
667: .Nm
668: to use IPv4 addresses only.
669: .It Fl 6
670: Forces
671: .Nm
672: to use IPv6 addresses only.
1.2 deraadt 673: .El
674: .Sh CONFIGURATION FILES
675: .Nm
1.158 stevesk 676: may additionally obtain configuration data from
677: a per-user configuration file and a system-wide configuration file.
678: The file format and configuration options are described in
679: .Xr ssh_config 5 .
1.2 deraadt 680: .Sh ENVIRONMENT
681: .Nm
1.1 deraadt 682: will normally set the following environment variables:
1.2 deraadt 683: .Bl -tag -width Ds
684: .It Ev DISPLAY
685: The
686: .Ev DISPLAY
1.40 aaron 687: variable indicates the location of the X11 server.
1.44 aaron 688: It is automatically set by
1.2 deraadt 689: .Nm
690: to point to a value of the form
691: .Dq hostname:n
692: where hostname indicates
1.40 aaron 693: the host where the shell runs, and n is an integer >= 1.
694: .Nm
695: uses this special value to forward X11 connections over the secure
696: channel.
1.107 markus 697: The user should normally not set
698: .Ev DISPLAY
699: explicitly, as that
1.1 deraadt 700: will render the X11 connection insecure (and will require the user to
701: manually copy any required authorization cookies).
1.2 deraadt 702: .It Ev HOME
1.1 deraadt 703: Set to the path of the user's home directory.
1.2 deraadt 704: .It Ev LOGNAME
705: Synonym for
1.12 aaron 706: .Ev USER ;
707: set for compatibility with systems that use this variable.
1.2 deraadt 708: .It Ev MAIL
1.129 stevesk 709: Set to the path of the user's mailbox.
1.40 aaron 710: .It Ev PATH
1.2 deraadt 711: Set to the default
712: .Ev PATH ,
713: as specified when compiling
1.12 aaron 714: .Nm ssh .
1.118 markus 715: .It Ev SSH_ASKPASS
716: If
717: .Nm
718: needs a passphrase, it will read the passphrase from the current
719: terminal if it was run from a terminal.
720: If
721: .Nm
722: does not have a terminal associated with it but
723: .Ev DISPLAY
724: and
725: .Ev SSH_ASKPASS
726: are set, it will execute the program specified by
727: .Ev SSH_ASKPASS
728: and open an X11 window to read the passphrase.
729: This is particularly useful when calling
730: .Nm
731: from a
732: .Pa .Xsession
733: or related script.
734: (Note that on some machines it
735: may be necessary to redirect the input from
736: .Pa /dev/null
737: to make this work.)
1.18 markus 738: .It Ev SSH_AUTH_SOCK
1.129 stevesk 739: Identifies the path of a unix-domain socket used to communicate with the
1.17 markus 740: agent.
1.166 stevesk 741: .It Ev SSH_CONNECTION
742: Identifies the client and server ends of the connection.
1.40 aaron 743: The variable contains
1.166 stevesk 744: four space-separated values: client ip-address, client port number,
745: server ip-address and server port number.
1.73 markus 746: .It Ev SSH_ORIGINAL_COMMAND
747: The variable contains the original command line if a forced command
748: is executed.
749: It can be used to extract the original arguments.
1.2 deraadt 750: .It Ev SSH_TTY
1.1 deraadt 751: This is set to the name of the tty (path to the device) associated
1.40 aaron 752: with the current shell or command.
753: If the current session has no tty,
1.1 deraadt 754: this variable is not set.
1.2 deraadt 755: .It Ev TZ
1.1 deraadt 756: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 757: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 758: on to new connections).
1.2 deraadt 759: .It Ev USER
1.1 deraadt 760: Set to the name of the user logging in.
1.2 deraadt 761: .El
762: .Pp
1.44 aaron 763: Additionally,
1.2 deraadt 764: .Nm
1.44 aaron 765: reads
766: .Pa $HOME/.ssh/environment ,
1.2 deraadt 767: and adds lines of the format
768: .Dq VARNAME=value
1.161 marc 769: to the environment if the file exists and if users are allowed to
770: change their environment.
1.162 stevesk 771: See the
1.161 marc 772: .Cm PermitUserEnvironment
1.162 stevesk 773: option in
1.161 marc 774: .Xr sshd_config 5 .
1.2 deraadt 775: .Sh FILES
1.36 markus 776: .Bl -tag -width Ds
1.116 markus 777: .It Pa $HOME/.ssh/known_hosts
1.129 stevesk 778: Records host keys for all hosts the user has logged into that are not
1.2 deraadt 779: in
1.147 deraadt 780: .Pa /etc/ssh/ssh_known_hosts .
1.2 deraadt 781: See
782: .Xr sshd 8 .
1.102 itojun 783: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
784: Contains the authentication identity of the user.
785: They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1.48 markus 786: These files
787: contain sensitive data and should be readable by the user but not
1.15 markus 788: accessible by others (read/write/execute).
789: Note that
790: .Nm
1.48 markus 791: ignores a private key file if it is accessible by others.
1.15 markus 792: It is possible to specify a passphrase when
1.1 deraadt 793: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 794: sensitive part of this file using 3DES.
1.102 itojun 795: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1.1 deraadt 796: Contains the public key for authentication (public part of the
1.40 aaron 797: identity file in human-readable form).
1.48 markus 798: The contents of the
799: .Pa $HOME/.ssh/identity.pub
800: file should be added to
1.2 deraadt 801: .Pa $HOME/.ssh/authorized_keys
802: on all machines
1.137 deraadt 803: where the user wishes to log in using protocol version 1 RSA authentication.
1.48 markus 804: The contents of the
805: .Pa $HOME/.ssh/id_dsa.pub
1.102 itojun 806: and
807: .Pa $HOME/.ssh/id_rsa.pub
1.48 markus 808: file should be added to
1.115 markus 809: .Pa $HOME/.ssh/authorized_keys
1.48 markus 810: on all machines
1.137 deraadt 811: where the user wishes to log in using protocol version 2 DSA/RSA authentication.
1.48 markus 812: These files are not
1.40 aaron 813: sensitive and can (but need not) be readable by anyone.
1.48 markus 814: These files are
1.84 markus 815: never used automatically and are not necessary; they are only provided for
1.1 deraadt 816: the convenience of the user.
1.2 deraadt 817: .It Pa $HOME/.ssh/config
1.40 aaron 818: This is the per-user configuration file.
1.158 stevesk 819: The file format and configuration options are described in
820: .Xr ssh_config 5 .
1.2 deraadt 821: .It Pa $HOME/.ssh/authorized_keys
1.115 markus 822: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.40 aaron 823: The format of this file is described in the
1.2 deraadt 824: .Xr sshd 8
1.40 aaron 825: manual page.
1.116 markus 826: In the simplest form the format is the same as the .pub
827: identity files.
1.48 markus 828: This file is not highly sensitive, but the recommended
829: permissions are read/write for the user, and not accessible by others.
1.147 deraadt 830: .It Pa /etc/ssh/ssh_known_hosts
1.40 aaron 831: Systemwide list of known host keys.
1.116 markus 832: This file should be prepared by the
1.1 deraadt 833: system administrator to contain the public host keys of all machines in the
1.40 aaron 834: organization.
835: This file should be world-readable.
836: This file contains
1.1 deraadt 837: public keys, one per line, in the following format (fields separated
1.116 markus 838: by spaces): system name, public key and optional comment field.
1.40 aaron 839: When different names are used
1.1 deraadt 840: for the same machine, all such names should be listed, separated by
1.40 aaron 841: commas.
842: The format is described on the
1.2 deraadt 843: .Xr sshd 8
1.1 deraadt 844: manual page.
1.2 deraadt 845: .Pp
1.1 deraadt 846: The canonical system name (as returned by name servers) is used by
1.2 deraadt 847: .Xr sshd 8
1.1 deraadt 848: to verify the client host when logging in; other names are needed because
1.2 deraadt 849: .Nm
1.1 deraadt 850: does not convert the user-supplied name to a canonical name before
851: checking the key, because someone with access to the name servers
852: would then be able to fool host authentication.
1.147 deraadt 853: .It Pa /etc/ssh/ssh_config
1.40 aaron 854: Systemwide configuration file.
1.158 stevesk 855: The file format and configuration options are described in
856: .Xr ssh_config 5 .
1.147 deraadt 857: .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1.141 markus 858: These three files contain the private parts of the host keys
859: and are used for
860: .Cm RhostsRSAAuthentication
861: and
862: .Cm HostbasedAuthentication .
1.155 stevesk 863: If the protocol version 1
864: .Cm RhostsRSAAuthentication
1.157 deraadt 865: method is used,
1.155 stevesk 866: .Nm
867: must be setuid root, since the host key is readable only by root.
868: For protocol version 2,
869: .Nm
870: uses
871: .Xr ssh-keysign 8
872: to access the host keys for
873: .Cm HostbasedAuthentication .
874: This eliminates the requirement that
875: .Nm
876: be setuid root when that authentication method is used.
877: By default
1.141 markus 878: .Nm
1.155 stevesk 879: is not setuid root.
1.2 deraadt 880: .It Pa $HOME/.rhosts
881: This file is used in
882: .Pa \&.rhosts
883: authentication to list the
1.40 aaron 884: host/user pairs that are permitted to log in.
885: (Note that this file is
1.1 deraadt 886: also used by rlogin and rsh, which makes using this file insecure.)
887: Each line of the file contains a host name (in the canonical form
888: returned by name servers), and then a user name on that host,
1.40 aaron 889: separated by a space.
1.92 markus 890: On some machines this file may need to be
1.1 deraadt 891: world-readable if the user's home directory is on a NFS partition,
892: because
1.2 deraadt 893: .Xr sshd 8
1.40 aaron 894: reads it as root.
895: Additionally, this file must be owned by the user,
896: and must not have write permissions for anyone else.
897: The recommended
1.1 deraadt 898: permission for most machines is read/write for the user, and not
899: accessible by others.
1.2 deraadt 900: .Pp
1.1 deraadt 901: Note that by default
1.2 deraadt 902: .Xr sshd 8
1.1 deraadt 903: will be installed so that it requires successful RSA host
1.40 aaron 904: authentication before permitting \s+2.\s0rhosts authentication.
1.137 deraadt 905: If the server machine does not have the client's host key in
1.147 deraadt 906: .Pa /etc/ssh/ssh_known_hosts ,
1.137 deraadt 907: it can be stored in
1.2 deraadt 908: .Pa $HOME/.ssh/known_hosts .
909: The easiest way to do this is to
1.1 deraadt 910: connect back to the client from the server machine using ssh; this
1.48 markus 911: will automatically add the host key to
1.2 deraadt 912: .Pa $HOME/.ssh/known_hosts .
913: .It Pa $HOME/.shosts
914: This file is used exactly the same way as
915: .Pa \&.rhosts .
916: The purpose for
1.1 deraadt 917: having this file is to be able to use rhosts authentication with
1.2 deraadt 918: .Nm
919: without permitting login with
1.151 millert 920: .Nm rlogin
1.2 deraadt 921: or
922: .Xr rsh 1 .
923: .It Pa /etc/hosts.equiv
924: This file is used during
1.167.2.2! brad 925: .Pa \&.rhosts
! 926: authentication.
1.40 aaron 927: It contains
1.1 deraadt 928: canonical hosts names, one per line (the full format is described on
929: the
1.2 deraadt 930: .Xr sshd 8
1.40 aaron 931: manual page).
932: If the client host is found in this file, login is
1.1 deraadt 933: automatically permitted provided client and server user names are the
1.40 aaron 934: same.
935: Additionally, successful RSA host authentication is normally
936: required.
937: This file should only be writable by root.
1.2 deraadt 938: .It Pa /etc/shosts.equiv
1.44 aaron 939: This file is processed exactly as
1.2 deraadt 940: .Pa /etc/hosts.equiv .
1.1 deraadt 941: This file may be useful to permit logins using
1.2 deraadt 942: .Nm
1.1 deraadt 943: but not using rsh/rlogin.
1.147 deraadt 944: .It Pa /etc/ssh/sshrc
1.1 deraadt 945: Commands in this file are executed by
1.2 deraadt 946: .Nm
1.1 deraadt 947: when the user logs in just before the user's shell (or command) is started.
948: See the
1.2 deraadt 949: .Xr sshd 8
1.1 deraadt 950: manual page for more information.
1.2 deraadt 951: .It Pa $HOME/.ssh/rc
1.1 deraadt 952: Commands in this file are executed by
1.2 deraadt 953: .Nm
1.1 deraadt 954: when the user logs in just before the user's shell (or command) is
955: started.
1.44 aaron 956: See the
1.2 deraadt 957: .Xr sshd 8
1.1 deraadt 958: manual page for more information.
1.31 markus 959: .It Pa $HOME/.ssh/environment
960: Contains additional definitions for environment variables, see section
961: .Sx ENVIRONMENT
962: above.
1.58 itojun 963: .El
1.145 markus 964: .Sh DIAGNOSTICS
965: .Nm
966: exits with the exit status of the remote command or with 255
967: if an error occurred.
1.2 deraadt 968: .Sh SEE ALSO
969: .Xr rsh 1 ,
970: .Xr scp 1 ,
1.83 djm 971: .Xr sftp 1 ,
1.2 deraadt 972: .Xr ssh-add 1 ,
973: .Xr ssh-agent 1 ,
974: .Xr ssh-keygen 1 ,
975: .Xr telnet 1 ,
1.159 stevesk 976: .Xr ssh_config 5 ,
1.160 naddy 977: .Xr ssh-keysign 8 ,
1.87 itojun 978: .Xr sshd 8
1.106 markus 979: .Rs
980: .%A T. Ylonen
981: .%A T. Kivinen
982: .%A M. Saarinen
983: .%A T. Rinne
984: .%A S. Lehtinen
985: .%T "SSH Protocol Architecture"
1.150 markus 986: .%N draft-ietf-secsh-architecture-12.txt
987: .%D January 2002
1.106 markus 988: .%O work in progress material
989: .Re
1.167.2.2! brad 990: .Sh AUTHORS
! 991: OpenSSH is a derivative of the original and free
! 992: ssh 1.2.12 release by Tatu Ylonen.
! 993: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
! 994: Theo de Raadt and Dug Song
! 995: removed many bugs, re-added newer features and
! 996: created OpenSSH.
! 997: Markus Friedl contributed the support for SSH
! 998: protocol versions 1.5 and 2.0.
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