Annotation of src/usr.bin/ssh/ssh.1, Revision 1.171
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: .\"
1.171 ! markus 37: .\" $OpenBSD: ssh.1,v 1.170 2003/05/14 22:24:42 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
1.168 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
1.171 ! markus 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
1.168 jmc 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
1.168 jmc 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
1.168 jmc 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
491: Specifies which smartcard device to use. The argument is
492: the device
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
1.42 aaron 497: Disables forwarding of Kerberos tickets and AFS tokens.
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
1.117 itojun 545: 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
546: of SSH as a secure transport for other applications (eg. sftp). The
1.80 djm 547: subsystem is specified as the remote command.
1.2 deraadt 548: .It Fl t
1.40 aaron 549: Force pseudo-tty allocation.
1.43 brad 550: This can be used to execute arbitrary
1.40 aaron 551: screen-based programs on a remote machine, which can be very useful,
552: e.g., when implementing menu services.
1.73 markus 553: Multiple
554: .Fl t
555: options force tty allocation, even if
556: .Nm
557: has no local tty.
1.53 markus 558: .It Fl T
1.69 markus 559: Disable pseudo-tty allocation.
1.2 deraadt 560: .It Fl v
1.40 aaron 561: Verbose mode.
562: Causes
1.2 deraadt 563: .Nm
1.40 aaron 564: to print debugging messages about its progress.
565: This is helpful in
1.1 deraadt 566: debugging connection, authentication, and configuration problems.
1.73 markus 567: Multiple
568: .Fl v
1.169 naddy 569: options increase the verbosity.
570: The maximum is 3.
571: .It Fl V
572: Display the version number and exit.
1.2 deraadt 573: .It Fl x
1.40 aaron 574: Disables X11 forwarding.
1.2 deraadt 575: .It Fl X
1.1 deraadt 576: Enables X11 forwarding.
1.54 markus 577: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 578: .Pp
1.168 jmc 579: X11 forwarding should be enabled with caution.
580: Users with the ability to bypass file permissions on the remote host
581: (for the user's X authorization database)
582: can access the local X11 display through the forwarded connection.
583: An attacker may then be able to perform activities such as keystroke monitoring.
1.2 deraadt 584: .It Fl C
1.1 deraadt 585: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 586: data for forwarded X11 and TCP/IP connections).
587: The compression algorithm is the same used by
1.34 markus 588: .Xr gzip 1 ,
589: and the
1.2 deraadt 590: .Dq level
591: can be controlled by the
592: .Cm CompressionLevel
1.167 stevesk 593: option for protocol version 1.
1.40 aaron 594: Compression is desirable on modem lines and other
1.1 deraadt 595: slow connections, but will only slow down things on fast networks.
596: The default value can be set on a host-by-host basis in the
597: configuration files; see the
1.121 pvalchev 598: .Cm Compression
1.158 stevesk 599: option.
1.132 markus 600: .It Fl F Ar configfile
601: Specifies an alternative per-user configuration file.
602: If a configuration file is given on the command line,
603: the system-wide configuration file
1.147 deraadt 604: .Pq Pa /etc/ssh/ssh_config
1.132 markus 605: will be ignored.
606: The default for the per-user configuration file is
607: .Pa $HOME/.ssh/config .
1.2 deraadt 608: .It Fl L Ar port:host:hostport
1.1 deraadt 609: Specifies that the given port on the local (client) host is to be
1.40 aaron 610: forwarded to the given host and port on the remote side.
611: This works by allocating a socket to listen to
1.2 deraadt 612: .Ar port
1.1 deraadt 613: on the local side, and whenever a connection is made to this port, the
614: connection is forwarded over the secure channel, and a connection is
615: made to
1.32 markus 616: .Ar host
617: port
618: .Ar hostport
1.40 aaron 619: from the remote machine.
620: Port forwardings can also be specified in the configuration file.
621: Only root can forward privileged ports.
1.32 markus 622: IPv6 addresses can be specified with an alternative syntax:
623: .Ar port/host/hostport
1.2 deraadt 624: .It Fl R Ar port:host:hostport
1.1 deraadt 625: Specifies that the given port on the remote (server) host is to be
1.40 aaron 626: forwarded to the given host and port on the local side.
627: This works by allocating a socket to listen to
1.2 deraadt 628: .Ar port
1.1 deraadt 629: on the remote side, and whenever a connection is made to this port, the
630: connection is forwarded over the secure channel, and a connection is
631: made to
1.32 markus 632: .Ar host
633: port
634: .Ar hostport
1.40 aaron 635: from the local machine.
636: Port forwardings can also be specified in the configuration file.
637: Privileged ports can be forwarded only when
1.1 deraadt 638: logging in as root on the remote machine.
1.107 markus 639: IPv6 addresses can be specified with an alternative syntax:
640: .Ar port/host/hostport
1.133 stevesk 641: .It Fl D Ar port
642: Specifies a local
643: .Dq dynamic
644: application-level port forwarding.
645: This works by allocating a socket to listen to
646: .Ar port
647: on the local side, and whenever a connection is made to this port, the
648: connection is forwarded over the secure channel, and the application
649: protocol is then used to determine where to connect to from the
1.168 jmc 650: remote machine.
651: Currently the SOCKS4 protocol is supported, and
1.133 stevesk 652: .Nm
653: will act as a SOCKS4 server.
654: Only root can forward privileged ports.
655: Dynamic port forwardings can also be specified in the configuration file.
1.85 jakob 656: .It Fl 1
657: Forces
658: .Nm
659: to try protocol version 1 only.
1.46 markus 660: .It Fl 2
661: Forces
662: .Nm
1.50 markus 663: to try protocol version 2 only.
1.32 markus 664: .It Fl 4
665: Forces
666: .Nm
667: to use IPv4 addresses only.
668: .It Fl 6
669: Forces
670: .Nm
671: to use IPv6 addresses only.
1.2 deraadt 672: .El
673: .Sh CONFIGURATION FILES
674: .Nm
1.158 stevesk 675: may additionally obtain configuration data from
676: a per-user configuration file and a system-wide configuration file.
677: The file format and configuration options are described in
678: .Xr ssh_config 5 .
1.2 deraadt 679: .Sh ENVIRONMENT
680: .Nm
1.1 deraadt 681: will normally set the following environment variables:
1.2 deraadt 682: .Bl -tag -width Ds
683: .It Ev DISPLAY
684: The
685: .Ev DISPLAY
1.40 aaron 686: variable indicates the location of the X11 server.
1.44 aaron 687: It is automatically set by
1.2 deraadt 688: .Nm
689: to point to a value of the form
690: .Dq hostname:n
691: where hostname indicates
1.40 aaron 692: the host where the shell runs, and n is an integer >= 1.
693: .Nm
694: uses this special value to forward X11 connections over the secure
695: channel.
1.107 markus 696: The user should normally not set
697: .Ev DISPLAY
698: explicitly, as that
1.1 deraadt 699: will render the X11 connection insecure (and will require the user to
700: manually copy any required authorization cookies).
1.2 deraadt 701: .It Ev HOME
1.1 deraadt 702: Set to the path of the user's home directory.
1.2 deraadt 703: .It Ev LOGNAME
704: Synonym for
1.12 aaron 705: .Ev USER ;
706: set for compatibility with systems that use this variable.
1.2 deraadt 707: .It Ev MAIL
1.129 stevesk 708: Set to the path of the user's mailbox.
1.40 aaron 709: .It Ev PATH
1.2 deraadt 710: Set to the default
711: .Ev PATH ,
712: as specified when compiling
1.12 aaron 713: .Nm ssh .
1.118 markus 714: .It Ev SSH_ASKPASS
715: If
716: .Nm
717: needs a passphrase, it will read the passphrase from the current
718: terminal if it was run from a terminal.
719: If
720: .Nm
721: does not have a terminal associated with it but
722: .Ev DISPLAY
723: and
724: .Ev SSH_ASKPASS
725: are set, it will execute the program specified by
726: .Ev SSH_ASKPASS
727: and open an X11 window to read the passphrase.
728: This is particularly useful when calling
729: .Nm
730: from a
731: .Pa .Xsession
732: or related script.
733: (Note that on some machines it
734: may be necessary to redirect the input from
735: .Pa /dev/null
736: to make this work.)
1.18 markus 737: .It Ev SSH_AUTH_SOCK
1.129 stevesk 738: Identifies the path of a unix-domain socket used to communicate with the
1.17 markus 739: agent.
1.166 stevesk 740: .It Ev SSH_CONNECTION
741: Identifies the client and server ends of the connection.
1.40 aaron 742: The variable contains
1.166 stevesk 743: four space-separated values: client ip-address, client port number,
744: server ip-address and server port number.
1.73 markus 745: .It Ev SSH_ORIGINAL_COMMAND
746: The variable contains the original command line if a forced command
747: is executed.
748: It can be used to extract the original arguments.
1.2 deraadt 749: .It Ev SSH_TTY
1.1 deraadt 750: This is set to the name of the tty (path to the device) associated
1.40 aaron 751: with the current shell or command.
752: If the current session has no tty,
1.1 deraadt 753: this variable is not set.
1.2 deraadt 754: .It Ev TZ
1.1 deraadt 755: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 756: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 757: on to new connections).
1.2 deraadt 758: .It Ev USER
1.1 deraadt 759: Set to the name of the user logging in.
1.2 deraadt 760: .El
761: .Pp
1.44 aaron 762: Additionally,
1.2 deraadt 763: .Nm
1.44 aaron 764: reads
765: .Pa $HOME/.ssh/environment ,
1.2 deraadt 766: and adds lines of the format
767: .Dq VARNAME=value
1.161 marc 768: to the environment if the file exists and if users are allowed to
769: change their environment.
1.162 stevesk 770: See the
1.161 marc 771: .Cm PermitUserEnvironment
1.162 stevesk 772: option in
1.161 marc 773: .Xr sshd_config 5 .
1.2 deraadt 774: .Sh FILES
1.36 markus 775: .Bl -tag -width Ds
1.116 markus 776: .It Pa $HOME/.ssh/known_hosts
1.129 stevesk 777: Records host keys for all hosts the user has logged into that are not
1.2 deraadt 778: in
1.147 deraadt 779: .Pa /etc/ssh/ssh_known_hosts .
1.2 deraadt 780: See
781: .Xr sshd 8 .
1.102 itojun 782: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
783: Contains the authentication identity of the user.
784: They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1.48 markus 785: These files
786: contain sensitive data and should be readable by the user but not
1.15 markus 787: accessible by others (read/write/execute).
788: Note that
789: .Nm
1.48 markus 790: ignores a private key file if it is accessible by others.
1.15 markus 791: It is possible to specify a passphrase when
1.1 deraadt 792: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 793: sensitive part of this file using 3DES.
1.102 itojun 794: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1.1 deraadt 795: Contains the public key for authentication (public part of the
1.40 aaron 796: identity file in human-readable form).
1.48 markus 797: The contents of the
798: .Pa $HOME/.ssh/identity.pub
799: file should be added to
1.2 deraadt 800: .Pa $HOME/.ssh/authorized_keys
801: on all machines
1.137 deraadt 802: where the user wishes to log in using protocol version 1 RSA authentication.
1.48 markus 803: The contents of the
804: .Pa $HOME/.ssh/id_dsa.pub
1.102 itojun 805: and
806: .Pa $HOME/.ssh/id_rsa.pub
1.48 markus 807: file should be added to
1.115 markus 808: .Pa $HOME/.ssh/authorized_keys
1.48 markus 809: on all machines
1.137 deraadt 810: where the user wishes to log in using protocol version 2 DSA/RSA authentication.
1.48 markus 811: These files are not
1.40 aaron 812: sensitive and can (but need not) be readable by anyone.
1.48 markus 813: These files are
1.84 markus 814: never used automatically and are not necessary; they are only provided for
1.1 deraadt 815: the convenience of the user.
1.2 deraadt 816: .It Pa $HOME/.ssh/config
1.40 aaron 817: This is the per-user configuration file.
1.158 stevesk 818: The file format and configuration options are described in
819: .Xr ssh_config 5 .
1.2 deraadt 820: .It Pa $HOME/.ssh/authorized_keys
1.115 markus 821: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.40 aaron 822: The format of this file is described in the
1.2 deraadt 823: .Xr sshd 8
1.40 aaron 824: manual page.
1.116 markus 825: In the simplest form the format is the same as the .pub
826: identity files.
1.48 markus 827: This file is not highly sensitive, but the recommended
828: permissions are read/write for the user, and not accessible by others.
1.147 deraadt 829: .It Pa /etc/ssh/ssh_known_hosts
1.40 aaron 830: Systemwide list of known host keys.
1.116 markus 831: This file should be prepared by the
1.1 deraadt 832: system administrator to contain the public host keys of all machines in the
1.40 aaron 833: organization.
834: This file should be world-readable.
835: This file contains
1.1 deraadt 836: public keys, one per line, in the following format (fields separated
1.116 markus 837: by spaces): system name, public key and optional comment field.
1.40 aaron 838: When different names are used
1.1 deraadt 839: for the same machine, all such names should be listed, separated by
1.40 aaron 840: commas.
841: The format is described on the
1.2 deraadt 842: .Xr sshd 8
1.1 deraadt 843: manual page.
1.2 deraadt 844: .Pp
1.1 deraadt 845: The canonical system name (as returned by name servers) is used by
1.2 deraadt 846: .Xr sshd 8
1.1 deraadt 847: to verify the client host when logging in; other names are needed because
1.2 deraadt 848: .Nm
1.1 deraadt 849: does not convert the user-supplied name to a canonical name before
850: checking the key, because someone with access to the name servers
851: would then be able to fool host authentication.
1.147 deraadt 852: .It Pa /etc/ssh/ssh_config
1.40 aaron 853: Systemwide configuration file.
1.158 stevesk 854: The file format and configuration options are described in
855: .Xr ssh_config 5 .
1.147 deraadt 856: .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1.141 markus 857: These three files contain the private parts of the host keys
858: and are used for
859: .Cm RhostsRSAAuthentication
860: and
861: .Cm HostbasedAuthentication .
1.155 stevesk 862: If the protocol version 1
863: .Cm RhostsRSAAuthentication
1.157 deraadt 864: method is used,
1.155 stevesk 865: .Nm
866: must be setuid root, since the host key is readable only by root.
867: For protocol version 2,
868: .Nm
869: uses
870: .Xr ssh-keysign 8
871: to access the host keys for
872: .Cm HostbasedAuthentication .
873: This eliminates the requirement that
874: .Nm
875: be setuid root when that authentication method is used.
876: By default
1.141 markus 877: .Nm
1.155 stevesk 878: is not setuid root.
1.2 deraadt 879: .It Pa $HOME/.rhosts
880: This file is used in
881: .Pa \&.rhosts
882: authentication to list the
1.40 aaron 883: host/user pairs that are permitted to log in.
884: (Note that this file is
1.1 deraadt 885: also used by rlogin and rsh, which makes using this file insecure.)
886: Each line of the file contains a host name (in the canonical form
887: returned by name servers), and then a user name on that host,
1.40 aaron 888: separated by a space.
1.92 markus 889: On some machines this file may need to be
1.1 deraadt 890: world-readable if the user's home directory is on a NFS partition,
891: because
1.2 deraadt 892: .Xr sshd 8
1.40 aaron 893: reads it as root.
894: Additionally, this file must be owned by the user,
895: and must not have write permissions for anyone else.
896: The recommended
1.1 deraadt 897: permission for most machines is read/write for the user, and not
898: accessible by others.
1.2 deraadt 899: .Pp
1.1 deraadt 900: Note that by default
1.2 deraadt 901: .Xr sshd 8
1.1 deraadt 902: will be installed so that it requires successful RSA host
1.40 aaron 903: authentication before permitting \s+2.\s0rhosts authentication.
1.137 deraadt 904: If the server machine does not have the client's host key in
1.147 deraadt 905: .Pa /etc/ssh/ssh_known_hosts ,
1.137 deraadt 906: it can be stored in
1.2 deraadt 907: .Pa $HOME/.ssh/known_hosts .
908: The easiest way to do this is to
1.1 deraadt 909: connect back to the client from the server machine using ssh; this
1.48 markus 910: will automatically add the host key to
1.2 deraadt 911: .Pa $HOME/.ssh/known_hosts .
912: .It Pa $HOME/.shosts
913: This file is used exactly the same way as
914: .Pa \&.rhosts .
915: The purpose for
1.1 deraadt 916: having this file is to be able to use rhosts authentication with
1.2 deraadt 917: .Nm
918: without permitting login with
1.151 millert 919: .Nm rlogin
1.2 deraadt 920: or
921: .Xr rsh 1 .
922: .It Pa /etc/hosts.equiv
923: This file is used during
1.40 aaron 924: .Pa \&.rhosts authentication.
925: It contains
1.1 deraadt 926: canonical hosts names, one per line (the full format is described on
927: the
1.2 deraadt 928: .Xr sshd 8
1.40 aaron 929: manual page).
930: If the client host is found in this file, login is
1.1 deraadt 931: automatically permitted provided client and server user names are the
1.40 aaron 932: same.
933: Additionally, successful RSA host authentication is normally
934: required.
935: This file should only be writable by root.
1.2 deraadt 936: .It Pa /etc/shosts.equiv
1.44 aaron 937: This file is processed exactly as
1.2 deraadt 938: .Pa /etc/hosts.equiv .
1.1 deraadt 939: This file may be useful to permit logins using
1.2 deraadt 940: .Nm
1.1 deraadt 941: but not using rsh/rlogin.
1.147 deraadt 942: .It Pa /etc/ssh/sshrc
1.1 deraadt 943: Commands in this file are executed by
1.2 deraadt 944: .Nm
1.1 deraadt 945: when the user logs in just before the user's shell (or command) is started.
946: See the
1.2 deraadt 947: .Xr sshd 8
1.1 deraadt 948: manual page for more information.
1.2 deraadt 949: .It Pa $HOME/.ssh/rc
1.1 deraadt 950: Commands in this file are executed by
1.2 deraadt 951: .Nm
1.1 deraadt 952: when the user logs in just before the user's shell (or command) is
953: started.
1.44 aaron 954: See the
1.2 deraadt 955: .Xr sshd 8
1.1 deraadt 956: manual page for more information.
1.31 markus 957: .It Pa $HOME/.ssh/environment
958: Contains additional definitions for environment variables, see section
959: .Sx ENVIRONMENT
960: above.
1.58 itojun 961: .El
1.145 markus 962: .Sh DIAGNOSTICS
963: .Nm
964: exits with the exit status of the remote command or with 255
965: if an error occurred.
1.67 aaron 966: .Sh AUTHORS
1.78 markus 967: OpenSSH is a derivative of the original and free
968: ssh 1.2.12 release by Tatu Ylonen.
969: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
970: Theo de Raadt and Dug Song
971: removed many bugs, re-added newer features and
972: created OpenSSH.
973: Markus Friedl contributed the support for SSH
974: protocol versions 1.5 and 2.0.
1.2 deraadt 975: .Sh SEE ALSO
976: .Xr rsh 1 ,
977: .Xr scp 1 ,
1.83 djm 978: .Xr sftp 1 ,
1.2 deraadt 979: .Xr ssh-add 1 ,
980: .Xr ssh-agent 1 ,
981: .Xr ssh-keygen 1 ,
982: .Xr telnet 1 ,
1.159 stevesk 983: .Xr ssh_config 5 ,
1.160 naddy 984: .Xr ssh-keysign 8 ,
1.87 itojun 985: .Xr sshd 8
1.106 markus 986: .Rs
987: .%A T. Ylonen
988: .%A T. Kivinen
989: .%A M. Saarinen
990: .%A T. Rinne
991: .%A S. Lehtinen
992: .%T "SSH Protocol Architecture"
1.150 markus 993: .%N draft-ietf-secsh-architecture-12.txt
994: .%D January 2002
1.106 markus 995: .%O work in progress material
996: .Re
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