Annotation of src/usr.bin/ssh/ssh.1, Revision 1.170
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: .\"
1.170 ! markus 37: .\" $OpenBSD: ssh.1,v 1.169 2003/04/12 11:40:15 naddy 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
1.168 jmc 51: .Bk -words
1.169 naddy 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
1.168 jmc 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
1.168 jmc 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
1.170 ! markus 304: .It Cm ~B
! 305: Send a BREAK to the remote system.
1.149 jakob 306: .It Cm ~C
307: Open command line (only useful for adding port forwardings using the
308: .Fl L
309: and
310: .Fl R
311: options)
1.104 djm 312: .It Cm ~R
1.105 djm 313: Request rekeying of the connection (only useful for SSH protocol version 2
314: and if the peer supports it)
1.104 djm 315: .El
1.2 deraadt 316: .Pp
1.49 markus 317: .Ss X11 and TCP forwarding
318: .Pp
1.110 deraadt 319: If the
320: .Cm ForwardX11
321: variable is set to
322: .Dq yes
323: (or, see the description of the
324: .Fl X
325: and
326: .Fl x
327: options described later)
328: and the user is using X11 (the
1.2 deraadt 329: .Ev DISPLAY
1.1 deraadt 330: environment variable is set), the connection to the X11 display is
331: automatically forwarded to the remote side in such a way that any X11
332: programs started from the shell (or command) will go through the
333: encrypted channel, and the connection to the real X server will be made
1.40 aaron 334: from the local machine.
335: The user should not manually set
1.2 deraadt 336: .Ev DISPLAY .
1.1 deraadt 337: Forwarding of X11 connections can be
338: configured on the command line or in configuration files.
1.2 deraadt 339: .Pp
340: The
1.44 aaron 341: .Ev DISPLAY
1.2 deraadt 342: value set by
343: .Nm
1.1 deraadt 344: will point to the server machine, but with a display number greater
1.40 aaron 345: than zero.
346: This is normal, and happens because
1.2 deraadt 347: .Nm
348: creates a
349: .Dq proxy
350: X server on the server machine for forwarding the
1.1 deraadt 351: connections over the encrypted channel.
1.2 deraadt 352: .Pp
353: .Nm
1.1 deraadt 354: will also automatically set up Xauthority data on the server machine.
355: For this purpose, it will generate a random authorization cookie,
356: store it in Xauthority on the server, and verify that any forwarded
357: connections carry this cookie and replace it by the real cookie when
1.40 aaron 358: the connection is opened.
359: The real authentication cookie is never
1.1 deraadt 360: sent to the server machine (and no cookies are sent in the plain).
1.2 deraadt 361: .Pp
1.163 stevesk 362: If the
363: .Cm ForwardAgent
364: variable is set to
365: .Dq yes
366: (or, see the description of the
367: .Fl A
368: and
369: .Fl a
1.168 jmc 370: options described later) and
1.163 stevesk 371: the user is using an authentication agent, the connection to the agent
372: is automatically forwarded to the remote side.
1.2 deraadt 373: .Pp
1.1 deraadt 374: Forwarding of arbitrary TCP/IP connections over the secure channel can
1.120 stevesk 375: be specified either on the command line or in a configuration file.
1.40 aaron 376: One possible application of TCP/IP forwarding is a secure connection to an
1.92 markus 377: electronic purse; another is going through firewalls.
1.2 deraadt 378: .Pp
1.49 markus 379: .Ss Server authentication
380: .Pp
1.2 deraadt 381: .Nm
1.49 markus 382: automatically maintains and checks a database containing
1.40 aaron 383: identifications for all hosts it has ever been used with.
1.116 markus 384: Host keys are stored in
1.49 markus 385: .Pa $HOME/.ssh/known_hosts
1.40 aaron 386: in the user's home directory.
1.116 markus 387: Additionally, the file
1.147 deraadt 388: .Pa /etc/ssh/ssh_known_hosts
1.116 markus 389: is automatically checked for known hosts.
1.40 aaron 390: Any new hosts are automatically added to the user's file.
391: If a host's identification
1.1 deraadt 392: ever changes,
1.2 deraadt 393: .Nm
1.1 deraadt 394: warns about this and disables password authentication to prevent a
1.40 aaron 395: trojan horse from getting the user's password.
396: Another purpose of
1.1 deraadt 397: this mechanism is to prevent man-in-the-middle attacks which could
1.40 aaron 398: otherwise be used to circumvent the encryption.
399: The
1.2 deraadt 400: .Cm StrictHostKeyChecking
1.158 stevesk 401: option can be used to prevent logins to machines whose
1.1 deraadt 402: host key is not known or has changed.
1.65 aaron 403: .Pp
404: The options are as follows:
1.2 deraadt 405: .Bl -tag -width Ds
1.4 dugsong 406: .It Fl a
1.42 aaron 407: Disables forwarding of the authentication agent connection.
1.54 markus 408: .It Fl A
409: Enables forwarding of the authentication agent connection.
410: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 411: .Pp
1.168 jmc 412: Agent forwarding should be enabled with caution.
413: Users with the ability to bypass file permissions on the remote host
414: (for the agent's Unix-domain socket)
415: can access the local agent through the forwarded connection.
416: An attacker cannot obtain key material from the agent,
1.165 stevesk 417: however they can perform operations on the keys that enable them to
418: authenticate using the identities loaded into the agent.
1.108 markus 419: .It Fl b Ar bind_address
420: Specify the interface to transmit from on machines with multiple
421: interfaces or aliased addresses.
1.131 stevesk 422: .It Fl c Ar blowfish|3des|des
1.44 aaron 423: Selects the cipher to use for encrypting the session.
1.2 deraadt 424: .Ar 3des
1.40 aaron 425: is used by default.
1.44 aaron 426: It is believed to be secure.
1.5 deraadt 427: .Ar 3des
428: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
429: .Ar blowfish
430: is a fast block cipher, it appears very secure and is much faster than
1.40 aaron 431: .Ar 3des .
1.131 stevesk 432: .Ar des
433: is only supported in the
434: .Nm
435: client for interoperability with legacy protocol 1 implementations
436: that do not support the
437: .Ar 3des
1.168 jmc 438: cipher.
439: Its use is strongly discouraged due to cryptographic weaknesses.
1.90 markus 440: .It Fl c Ar cipher_spec
1.51 markus 441: Additionally, for protocol version 2 a comma-separated list of ciphers can
1.61 aaron 442: be specified in order of preference.
1.90 markus 443: See
444: .Cm Ciphers
445: for more information.
1.2 deraadt 446: .It Fl e Ar ch|^ch|none
447: Sets the escape character for sessions with a pty (default:
448: .Ql ~ ) .
1.40 aaron 449: The escape character is only recognized at the beginning of a line.
450: The escape character followed by a dot
1.2 deraadt 451: .Pq Ql \&.
452: closes the connection, followed
1.1 deraadt 453: by control-Z suspends the connection, and followed by itself sends the
1.40 aaron 454: escape character once.
455: Setting the character to
1.2 deraadt 456: .Dq none
457: disables any escapes and makes the session fully transparent.
458: .It Fl f
459: Requests
460: .Nm
1.40 aaron 461: to go to background just before command execution.
462: This is useful if
1.2 deraadt 463: .Nm
464: is going to ask for passwords or passphrases, but the user
1.40 aaron 465: wants it in the background.
1.44 aaron 466: This implies
1.2 deraadt 467: .Fl n .
1.1 deraadt 468: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 469: something like
470: .Ic ssh -f host xterm .
1.34 markus 471: .It Fl g
472: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 473: .It Fl i Ar identity_file
1.144 stevesk 474: Selects a file from which the identity (private key) for
1.68 markus 475: RSA or DSA authentication is read.
1.144 stevesk 476: The default is
1.49 markus 477: .Pa $HOME/.ssh/identity
1.144 stevesk 478: for protocol version 1, and
479: .Pa $HOME/.ssh/id_rsa
480: and
481: .Pa $HOME/.ssh/id_dsa
482: for protocol version 2.
1.40 aaron 483: Identity files may also be specified on
484: a per-host basis in the configuration file.
485: It is possible to have multiple
1.2 deraadt 486: .Fl i
487: options (and multiple identities specified in
1.1 deraadt 488: configuration files).
1.125 jakob 489: .It Fl I Ar smartcard_device
490: Specifies which smartcard device to use. The argument is
491: the device
492: .Nm
493: should use to communicate with a smartcard used for storing the user's
494: private RSA key.
1.2 deraadt 495: .It Fl k
1.42 aaron 496: Disables forwarding of Kerberos tickets and AFS tokens.
497: This may also be specified on a per-host basis in the configuration file.
1.2 deraadt 498: .It Fl l Ar login_name
1.40 aaron 499: Specifies the user to log in as on the remote machine.
500: This also may be specified on a per-host basis in the configuration file.
1.91 markus 501: .It Fl m Ar mac_spec
502: Additionally, for protocol version 2 a comma-separated list of MAC
503: (message authentication code) algorithms can
504: be specified in order of preference.
505: See the
506: .Cm MACs
507: keyword for more information.
1.2 deraadt 508: .It Fl n
509: Redirects stdin from
510: .Pa /dev/null
511: (actually, prevents reading from stdin).
1.1 deraadt 512: This must be used when
1.2 deraadt 513: .Nm
1.40 aaron 514: is run in the background.
515: A common trick is to use this to run X11 programs on a remote machine.
516: For example,
1.2 deraadt 517: .Ic ssh -n shadows.cs.hut.fi emacs &
518: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 519: connection will be automatically forwarded over an encrypted channel.
520: The
1.2 deraadt 521: .Nm
1.1 deraadt 522: program will be put in the background.
523: (This does not work if
1.2 deraadt 524: .Nm
525: needs to ask for a password or passphrase; see also the
526: .Fl f
527: option.)
1.53 markus 528: .It Fl N
529: Do not execute a remote command.
1.137 deraadt 530: This is useful for just forwarding ports
1.53 markus 531: (protocol version 2 only).
1.2 deraadt 532: .It Fl o Ar option
1.127 stevesk 533: Can be used to give options in the format used in the configuration file.
1.1 deraadt 534: This is useful for specifying options for which there is no separate
1.40 aaron 535: command-line flag.
1.2 deraadt 536: .It Fl p Ar port
1.40 aaron 537: Port to connect to on the remote host.
538: This can be specified on a
1.1 deraadt 539: per-host basis in the configuration file.
1.2 deraadt 540: .It Fl q
1.40 aaron 541: Quiet mode.
542: Causes all warning and diagnostic messages to be suppressed.
1.80 djm 543: .It Fl s
1.117 itojun 544: 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
545: of SSH as a secure transport for other applications (eg. sftp). The
1.80 djm 546: subsystem is specified as the remote command.
1.2 deraadt 547: .It Fl t
1.40 aaron 548: Force pseudo-tty allocation.
1.43 brad 549: This can be used to execute arbitrary
1.40 aaron 550: screen-based programs on a remote machine, which can be very useful,
551: e.g., when implementing menu services.
1.73 markus 552: Multiple
553: .Fl t
554: options force tty allocation, even if
555: .Nm
556: has no local tty.
1.53 markus 557: .It Fl T
1.69 markus 558: Disable pseudo-tty allocation.
1.2 deraadt 559: .It Fl v
1.40 aaron 560: Verbose mode.
561: Causes
1.2 deraadt 562: .Nm
1.40 aaron 563: to print debugging messages about its progress.
564: This is helpful in
1.1 deraadt 565: debugging connection, authentication, and configuration problems.
1.73 markus 566: Multiple
567: .Fl v
1.169 naddy 568: options increase the verbosity.
569: The maximum is 3.
570: .It Fl V
571: Display the version number and exit.
1.2 deraadt 572: .It Fl x
1.40 aaron 573: Disables X11 forwarding.
1.2 deraadt 574: .It Fl X
1.1 deraadt 575: Enables X11 forwarding.
1.54 markus 576: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 577: .Pp
1.168 jmc 578: X11 forwarding should be enabled with caution.
579: Users with the ability to bypass file permissions on the remote host
580: (for the user's X authorization database)
581: can access the local X11 display through the forwarded connection.
582: An attacker may then be able to perform activities such as keystroke monitoring.
1.2 deraadt 583: .It Fl C
1.1 deraadt 584: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 585: data for forwarded X11 and TCP/IP connections).
586: The compression algorithm is the same used by
1.34 markus 587: .Xr gzip 1 ,
588: and the
1.2 deraadt 589: .Dq level
590: can be controlled by the
591: .Cm CompressionLevel
1.167 stevesk 592: option for protocol version 1.
1.40 aaron 593: Compression is desirable on modem lines and other
1.1 deraadt 594: slow connections, but will only slow down things on fast networks.
595: The default value can be set on a host-by-host basis in the
596: configuration files; see the
1.121 pvalchev 597: .Cm Compression
1.158 stevesk 598: option.
1.132 markus 599: .It Fl F Ar configfile
600: Specifies an alternative per-user configuration file.
601: If a configuration file is given on the command line,
602: the system-wide configuration file
1.147 deraadt 603: .Pq Pa /etc/ssh/ssh_config
1.132 markus 604: will be ignored.
605: The default for the per-user configuration file is
606: .Pa $HOME/.ssh/config .
1.2 deraadt 607: .It Fl L Ar port:host:hostport
1.1 deraadt 608: Specifies that the given port on the local (client) host is to be
1.40 aaron 609: forwarded to the given host and port on the remote side.
610: This works by allocating a socket to listen to
1.2 deraadt 611: .Ar port
1.1 deraadt 612: on the local side, and whenever a connection is made to this port, the
613: connection is forwarded over the secure channel, and a connection is
614: made to
1.32 markus 615: .Ar host
616: port
617: .Ar hostport
1.40 aaron 618: from the remote machine.
619: Port forwardings can also be specified in the configuration file.
620: Only root can forward privileged ports.
1.32 markus 621: IPv6 addresses can be specified with an alternative syntax:
622: .Ar port/host/hostport
1.2 deraadt 623: .It Fl R Ar port:host:hostport
1.1 deraadt 624: Specifies that the given port on the remote (server) host is to be
1.40 aaron 625: forwarded to the given host and port on the local side.
626: This works by allocating a socket to listen to
1.2 deraadt 627: .Ar port
1.1 deraadt 628: on the remote side, and whenever a connection is made to this port, the
629: connection is forwarded over the secure channel, and a connection is
630: made to
1.32 markus 631: .Ar host
632: port
633: .Ar hostport
1.40 aaron 634: from the local machine.
635: Port forwardings can also be specified in the configuration file.
636: Privileged ports can be forwarded only when
1.1 deraadt 637: logging in as root on the remote machine.
1.107 markus 638: IPv6 addresses can be specified with an alternative syntax:
639: .Ar port/host/hostport
1.133 stevesk 640: .It Fl D Ar port
641: Specifies a local
642: .Dq dynamic
643: application-level port forwarding.
644: This works by allocating a socket to listen to
645: .Ar port
646: on the local side, and whenever a connection is made to this port, the
647: connection is forwarded over the secure channel, and the application
648: protocol is then used to determine where to connect to from the
1.168 jmc 649: remote machine.
650: Currently the SOCKS4 protocol is supported, and
1.133 stevesk 651: .Nm
652: will act as a SOCKS4 server.
653: Only root can forward privileged ports.
654: Dynamic port forwardings can also be specified in the configuration file.
1.85 jakob 655: .It Fl 1
656: Forces
657: .Nm
658: to try protocol version 1 only.
1.46 markus 659: .It Fl 2
660: Forces
661: .Nm
1.50 markus 662: to try protocol version 2 only.
1.32 markus 663: .It Fl 4
664: Forces
665: .Nm
666: to use IPv4 addresses only.
667: .It Fl 6
668: Forces
669: .Nm
670: to use IPv6 addresses only.
1.2 deraadt 671: .El
672: .Sh CONFIGURATION FILES
673: .Nm
1.158 stevesk 674: may additionally obtain configuration data from
675: a per-user configuration file and a system-wide configuration file.
676: The file format and configuration options are described in
677: .Xr ssh_config 5 .
1.2 deraadt 678: .Sh ENVIRONMENT
679: .Nm
1.1 deraadt 680: will normally set the following environment variables:
1.2 deraadt 681: .Bl -tag -width Ds
682: .It Ev DISPLAY
683: The
684: .Ev DISPLAY
1.40 aaron 685: variable indicates the location of the X11 server.
1.44 aaron 686: It is automatically set by
1.2 deraadt 687: .Nm
688: to point to a value of the form
689: .Dq hostname:n
690: where hostname indicates
1.40 aaron 691: the host where the shell runs, and n is an integer >= 1.
692: .Nm
693: uses this special value to forward X11 connections over the secure
694: channel.
1.107 markus 695: The user should normally not set
696: .Ev DISPLAY
697: explicitly, as that
1.1 deraadt 698: will render the X11 connection insecure (and will require the user to
699: manually copy any required authorization cookies).
1.2 deraadt 700: .It Ev HOME
1.1 deraadt 701: Set to the path of the user's home directory.
1.2 deraadt 702: .It Ev LOGNAME
703: Synonym for
1.12 aaron 704: .Ev USER ;
705: set for compatibility with systems that use this variable.
1.2 deraadt 706: .It Ev MAIL
1.129 stevesk 707: Set to the path of the user's mailbox.
1.40 aaron 708: .It Ev PATH
1.2 deraadt 709: Set to the default
710: .Ev PATH ,
711: as specified when compiling
1.12 aaron 712: .Nm ssh .
1.118 markus 713: .It Ev SSH_ASKPASS
714: If
715: .Nm
716: needs a passphrase, it will read the passphrase from the current
717: terminal if it was run from a terminal.
718: If
719: .Nm
720: does not have a terminal associated with it but
721: .Ev DISPLAY
722: and
723: .Ev SSH_ASKPASS
724: are set, it will execute the program specified by
725: .Ev SSH_ASKPASS
726: and open an X11 window to read the passphrase.
727: This is particularly useful when calling
728: .Nm
729: from a
730: .Pa .Xsession
731: or related script.
732: (Note that on some machines it
733: may be necessary to redirect the input from
734: .Pa /dev/null
735: to make this work.)
1.18 markus 736: .It Ev SSH_AUTH_SOCK
1.129 stevesk 737: Identifies the path of a unix-domain socket used to communicate with the
1.17 markus 738: agent.
1.166 stevesk 739: .It Ev SSH_CONNECTION
740: Identifies the client and server ends of the connection.
1.40 aaron 741: The variable contains
1.166 stevesk 742: four space-separated values: client ip-address, client port number,
743: server ip-address and server port number.
1.73 markus 744: .It Ev SSH_ORIGINAL_COMMAND
745: The variable contains the original command line if a forced command
746: is executed.
747: It can be used to extract the original arguments.
1.2 deraadt 748: .It Ev SSH_TTY
1.1 deraadt 749: This is set to the name of the tty (path to the device) associated
1.40 aaron 750: with the current shell or command.
751: If the current session has no tty,
1.1 deraadt 752: this variable is not set.
1.2 deraadt 753: .It Ev TZ
1.1 deraadt 754: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 755: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 756: on to new connections).
1.2 deraadt 757: .It Ev USER
1.1 deraadt 758: Set to the name of the user logging in.
1.2 deraadt 759: .El
760: .Pp
1.44 aaron 761: Additionally,
1.2 deraadt 762: .Nm
1.44 aaron 763: reads
764: .Pa $HOME/.ssh/environment ,
1.2 deraadt 765: and adds lines of the format
766: .Dq VARNAME=value
1.161 marc 767: to the environment if the file exists and if users are allowed to
768: change their environment.
1.162 stevesk 769: See the
1.161 marc 770: .Cm PermitUserEnvironment
1.162 stevesk 771: option in
1.161 marc 772: .Xr sshd_config 5 .
1.2 deraadt 773: .Sh FILES
1.36 markus 774: .Bl -tag -width Ds
1.116 markus 775: .It Pa $HOME/.ssh/known_hosts
1.129 stevesk 776: Records host keys for all hosts the user has logged into that are not
1.2 deraadt 777: in
1.147 deraadt 778: .Pa /etc/ssh/ssh_known_hosts .
1.2 deraadt 779: See
780: .Xr sshd 8 .
1.102 itojun 781: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
782: Contains the authentication identity of the user.
783: They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1.48 markus 784: These files
785: contain sensitive data and should be readable by the user but not
1.15 markus 786: accessible by others (read/write/execute).
787: Note that
788: .Nm
1.48 markus 789: ignores a private key file if it is accessible by others.
1.15 markus 790: It is possible to specify a passphrase when
1.1 deraadt 791: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 792: sensitive part of this file using 3DES.
1.102 itojun 793: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1.1 deraadt 794: Contains the public key for authentication (public part of the
1.40 aaron 795: identity file in human-readable form).
1.48 markus 796: The contents of the
797: .Pa $HOME/.ssh/identity.pub
798: file should be added to
1.2 deraadt 799: .Pa $HOME/.ssh/authorized_keys
800: on all machines
1.137 deraadt 801: where the user wishes to log in using protocol version 1 RSA authentication.
1.48 markus 802: The contents of the
803: .Pa $HOME/.ssh/id_dsa.pub
1.102 itojun 804: and
805: .Pa $HOME/.ssh/id_rsa.pub
1.48 markus 806: file should be added to
1.115 markus 807: .Pa $HOME/.ssh/authorized_keys
1.48 markus 808: on all machines
1.137 deraadt 809: where the user wishes to log in using protocol version 2 DSA/RSA authentication.
1.48 markus 810: These files are not
1.40 aaron 811: sensitive and can (but need not) be readable by anyone.
1.48 markus 812: These files are
1.84 markus 813: never used automatically and are not necessary; they are only provided for
1.1 deraadt 814: the convenience of the user.
1.2 deraadt 815: .It Pa $HOME/.ssh/config
1.40 aaron 816: This is the per-user configuration file.
1.158 stevesk 817: The file format and configuration options are described in
818: .Xr ssh_config 5 .
1.2 deraadt 819: .It Pa $HOME/.ssh/authorized_keys
1.115 markus 820: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.40 aaron 821: The format of this file is described in the
1.2 deraadt 822: .Xr sshd 8
1.40 aaron 823: manual page.
1.116 markus 824: In the simplest form the format is the same as the .pub
825: identity files.
1.48 markus 826: This file is not highly sensitive, but the recommended
827: permissions are read/write for the user, and not accessible by others.
1.147 deraadt 828: .It Pa /etc/ssh/ssh_known_hosts
1.40 aaron 829: Systemwide list of known host keys.
1.116 markus 830: This file should be prepared by the
1.1 deraadt 831: system administrator to contain the public host keys of all machines in the
1.40 aaron 832: organization.
833: This file should be world-readable.
834: This file contains
1.1 deraadt 835: public keys, one per line, in the following format (fields separated
1.116 markus 836: by spaces): system name, public key and optional comment field.
1.40 aaron 837: When different names are used
1.1 deraadt 838: for the same machine, all such names should be listed, separated by
1.40 aaron 839: commas.
840: The format is described on the
1.2 deraadt 841: .Xr sshd 8
1.1 deraadt 842: manual page.
1.2 deraadt 843: .Pp
1.1 deraadt 844: The canonical system name (as returned by name servers) is used by
1.2 deraadt 845: .Xr sshd 8
1.1 deraadt 846: to verify the client host when logging in; other names are needed because
1.2 deraadt 847: .Nm
1.1 deraadt 848: does not convert the user-supplied name to a canonical name before
849: checking the key, because someone with access to the name servers
850: would then be able to fool host authentication.
1.147 deraadt 851: .It Pa /etc/ssh/ssh_config
1.40 aaron 852: Systemwide configuration file.
1.158 stevesk 853: The file format and configuration options are described in
854: .Xr ssh_config 5 .
1.147 deraadt 855: .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1.141 markus 856: These three files contain the private parts of the host keys
857: and are used for
858: .Cm RhostsRSAAuthentication
859: and
860: .Cm HostbasedAuthentication .
1.155 stevesk 861: If the protocol version 1
862: .Cm RhostsRSAAuthentication
1.157 deraadt 863: method is used,
1.155 stevesk 864: .Nm
865: must be setuid root, since the host key is readable only by root.
866: For protocol version 2,
867: .Nm
868: uses
869: .Xr ssh-keysign 8
870: to access the host keys for
871: .Cm HostbasedAuthentication .
872: This eliminates the requirement that
873: .Nm
874: be setuid root when that authentication method is used.
875: By default
1.141 markus 876: .Nm
1.155 stevesk 877: is not setuid root.
1.2 deraadt 878: .It Pa $HOME/.rhosts
879: This file is used in
880: .Pa \&.rhosts
881: authentication to list the
1.40 aaron 882: host/user pairs that are permitted to log in.
883: (Note that this file is
1.1 deraadt 884: also used by rlogin and rsh, which makes using this file insecure.)
885: Each line of the file contains a host name (in the canonical form
886: returned by name servers), and then a user name on that host,
1.40 aaron 887: separated by a space.
1.92 markus 888: On some machines this file may need to be
1.1 deraadt 889: world-readable if the user's home directory is on a NFS partition,
890: because
1.2 deraadt 891: .Xr sshd 8
1.40 aaron 892: reads it as root.
893: Additionally, this file must be owned by the user,
894: and must not have write permissions for anyone else.
895: The recommended
1.1 deraadt 896: permission for most machines is read/write for the user, and not
897: accessible by others.
1.2 deraadt 898: .Pp
1.1 deraadt 899: Note that by default
1.2 deraadt 900: .Xr sshd 8
1.1 deraadt 901: will be installed so that it requires successful RSA host
1.40 aaron 902: authentication before permitting \s+2.\s0rhosts authentication.
1.137 deraadt 903: If the server machine does not have the client's host key in
1.147 deraadt 904: .Pa /etc/ssh/ssh_known_hosts ,
1.137 deraadt 905: it can be stored in
1.2 deraadt 906: .Pa $HOME/.ssh/known_hosts .
907: The easiest way to do this is to
1.1 deraadt 908: connect back to the client from the server machine using ssh; this
1.48 markus 909: will automatically add the host key to
1.2 deraadt 910: .Pa $HOME/.ssh/known_hosts .
911: .It Pa $HOME/.shosts
912: This file is used exactly the same way as
913: .Pa \&.rhosts .
914: The purpose for
1.1 deraadt 915: having this file is to be able to use rhosts authentication with
1.2 deraadt 916: .Nm
917: without permitting login with
1.151 millert 918: .Nm rlogin
1.2 deraadt 919: or
920: .Xr rsh 1 .
921: .It Pa /etc/hosts.equiv
922: This file is used during
1.40 aaron 923: .Pa \&.rhosts authentication.
924: It contains
1.1 deraadt 925: canonical hosts names, one per line (the full format is described on
926: the
1.2 deraadt 927: .Xr sshd 8
1.40 aaron 928: manual page).
929: If the client host is found in this file, login is
1.1 deraadt 930: automatically permitted provided client and server user names are the
1.40 aaron 931: same.
932: Additionally, successful RSA host authentication is normally
933: required.
934: This file should only be writable by root.
1.2 deraadt 935: .It Pa /etc/shosts.equiv
1.44 aaron 936: This file is processed exactly as
1.2 deraadt 937: .Pa /etc/hosts.equiv .
1.1 deraadt 938: This file may be useful to permit logins using
1.2 deraadt 939: .Nm
1.1 deraadt 940: but not using rsh/rlogin.
1.147 deraadt 941: .It Pa /etc/ssh/sshrc
1.1 deraadt 942: Commands in this file are executed by
1.2 deraadt 943: .Nm
1.1 deraadt 944: when the user logs in just before the user's shell (or command) is started.
945: See the
1.2 deraadt 946: .Xr sshd 8
1.1 deraadt 947: manual page for more information.
1.2 deraadt 948: .It Pa $HOME/.ssh/rc
1.1 deraadt 949: Commands in this file are executed by
1.2 deraadt 950: .Nm
1.1 deraadt 951: when the user logs in just before the user's shell (or command) is
952: started.
1.44 aaron 953: See the
1.2 deraadt 954: .Xr sshd 8
1.1 deraadt 955: manual page for more information.
1.31 markus 956: .It Pa $HOME/.ssh/environment
957: Contains additional definitions for environment variables, see section
958: .Sx ENVIRONMENT
959: above.
1.58 itojun 960: .El
1.145 markus 961: .Sh DIAGNOSTICS
962: .Nm
963: exits with the exit status of the remote command or with 255
964: if an error occurred.
1.67 aaron 965: .Sh AUTHORS
1.78 markus 966: OpenSSH is a derivative of the original and free
967: ssh 1.2.12 release by Tatu Ylonen.
968: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
969: Theo de Raadt and Dug Song
970: removed many bugs, re-added newer features and
971: created OpenSSH.
972: Markus Friedl contributed the support for SSH
973: protocol versions 1.5 and 2.0.
1.2 deraadt 974: .Sh SEE ALSO
975: .Xr rsh 1 ,
976: .Xr scp 1 ,
1.83 djm 977: .Xr sftp 1 ,
1.2 deraadt 978: .Xr ssh-add 1 ,
979: .Xr ssh-agent 1 ,
980: .Xr ssh-keygen 1 ,
981: .Xr telnet 1 ,
1.159 stevesk 982: .Xr ssh_config 5 ,
1.160 naddy 983: .Xr ssh-keysign 8 ,
1.87 itojun 984: .Xr sshd 8
1.106 markus 985: .Rs
986: .%A T. Ylonen
987: .%A T. Kivinen
988: .%A M. Saarinen
989: .%A T. Rinne
990: .%A S. Lehtinen
991: .%T "SSH Protocol Architecture"
1.150 markus 992: .%N draft-ietf-secsh-architecture-12.txt
993: .%D January 2002
1.106 markus 994: .%O work in progress material
995: .Re
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