Annotation of src/usr.bin/ssh/ssh.1, Revision 1.188
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: .\"
1.188 ! jmc 37: .\" $OpenBSD: ssh.1,v 1.187 2004/05/22 06:32:12 djm 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
1.178 markus 46: .Op Fl 1246AaCfgkNnqsTtVvXxY
1.108 markus 47: .Op Fl b Ar bind_address
1.51 markus 48: .Op Fl c Ar cipher_spec
1.176 jmc 49: .Op Fl D Ar port
1.2 deraadt 50: .Op Fl e Ar escape_char
1.176 jmc 51: .Op Fl F Ar configfile
1.2 deraadt 52: .Op Fl i Ar identity_file
1.176 jmc 53: .Bk -words
1.12 aaron 54: .Oo Fl L Xo
55: .Sm off
1.33 markus 56: .Ar port :
1.12 aaron 57: .Ar host :
58: .Ar hostport
59: .Sm on
60: .Xc
61: .Oc
1.168 jmc 62: .Ek
1.176 jmc 63: .Op Fl l Ar login_name
64: .Op Fl m Ar mac_spec
65: .Op Fl o Ar option
1.168 jmc 66: .Bk -words
1.176 jmc 67: .Op Fl p Ar port
68: .Ek
1.12 aaron 69: .Oo Fl R Xo
70: .Sm off
1.33 markus 71: .Ar port :
1.12 aaron 72: .Ar host :
73: .Ar hostport
74: .Sm on
75: .Xc
76: .Oc
1.176 jmc 77: .Oo Ar user Ns @ Oc Ns Ar hostname
1.2 deraadt 78: .Op Ar command
1.44 aaron 79: .Sh DESCRIPTION
1.2 deraadt 80: .Nm
1.96 deraadt 81: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 82: executing commands on a remote machine.
1.176 jmc 83: It is intended to replace rlogin and rsh,
84: and provide secure encrypted communications between
1.40 aaron 85: two untrusted hosts over an insecure network.
1.176 jmc 86: X11 connections and arbitrary TCP/IP ports
87: can also be forwarded over the secure channel.
1.2 deraadt 88: .Pp
89: .Nm
1.44 aaron 90: connects and logs into the specified
1.176 jmc 91: .Ar hostname
92: (with optional
93: .Ar user
94: name).
1.1 deraadt 95: The user must prove
1.49 markus 96: his/her identity to the remote machine using one of several methods
1.176 jmc 97: depending on the protocol version used.
1.49 markus 98: .Pp
1.176 jmc 99: If
100: .Ar command
101: is specified,
102: .Ar command
103: is executed on the remote host instead of a login shell.
1.49 markus 104: .Ss SSH protocol version 1
1.1 deraadt 105: First, if the machine the user logs in from is listed in
1.2 deraadt 106: .Pa /etc/hosts.equiv
1.1 deraadt 107: or
1.2 deraadt 108: .Pa /etc/shosts.equiv
1.1 deraadt 109: on the remote machine, and the user names are
110: the same on both sides, the user is immediately permitted to log in.
1.44 aaron 111: Second, if
1.176 jmc 112: .Pa .rhosts
1.1 deraadt 113: or
1.176 jmc 114: .Pa .shosts
1.1 deraadt 115: exists in the user's home directory on the
116: remote machine and contains a line containing the name of the client
117: machine and the name of the user on that machine, the user is
1.40 aaron 118: permitted to log in.
119: This form of authentication alone is normally not
1.1 deraadt 120: allowed by the server because it is not secure.
1.2 deraadt 121: .Pp
1.107 markus 122: The second authentication method is the
1.176 jmc 123: .Em rhosts
1.1 deraadt 124: or
1.176 jmc 125: .Em hosts.equiv
1.40 aaron 126: method combined with RSA-based host authentication.
127: It means that if the login would be permitted by
1.49 markus 128: .Pa $HOME/.rhosts ,
129: .Pa $HOME/.shosts ,
1.2 deraadt 130: .Pa /etc/hosts.equiv ,
1.1 deraadt 131: or
1.2 deraadt 132: .Pa /etc/shosts.equiv ,
1.11 deraadt 133: and if additionally the server can verify the client's
1.44 aaron 134: host key (see
1.147 deraadt 135: .Pa /etc/ssh/ssh_known_hosts
1.23 markus 136: and
137: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 138: in the
1.2 deraadt 139: .Sx FILES
1.176 jmc 140: section), only then is login permitted.
1.40 aaron 141: This authentication method closes security holes due to IP
142: spoofing, DNS spoofing and routing spoofing.
143: [Note to the administrator:
1.2 deraadt 144: .Pa /etc/hosts.equiv ,
1.49 markus 145: .Pa $HOME/.rhosts ,
1.1 deraadt 146: and the rlogin/rsh protocol in general, are inherently insecure and should be
147: disabled if security is desired.]
1.2 deraadt 148: .Pp
1.44 aaron 149: As a third authentication method,
1.2 deraadt 150: .Nm
1.1 deraadt 151: supports RSA based authentication.
152: The scheme is based on public-key cryptography: there are cryptosystems
153: where encryption and decryption are done using separate keys, and it
154: is not possible to derive the decryption key from the encryption key.
1.40 aaron 155: RSA is one such system.
1.44 aaron 156: The idea is that each user creates a public/private
1.40 aaron 157: key pair for authentication purposes.
158: The server knows the public key, and only the user knows the private key.
1.176 jmc 159: .Pp
1.44 aaron 160: The file
1.2 deraadt 161: .Pa $HOME/.ssh/authorized_keys
1.176 jmc 162: lists the public keys that are permitted for logging in.
1.40 aaron 163: When the user logs in, the
1.2 deraadt 164: .Nm
1.1 deraadt 165: program tells the server which key pair it would like to use for
1.40 aaron 166: authentication.
1.176 jmc 167: The server checks if this key is permitted, and if so,
168: sends the user (actually the
1.2 deraadt 169: .Nm
1.1 deraadt 170: program running on behalf of the user) a challenge, a random number,
1.40 aaron 171: encrypted by the user's public key.
1.176 jmc 172: The challenge can only be decrypted using the proper private key.
173: The user's client then decrypts the challenge using the private key,
174: proving that he/she knows the private key
175: but without disclosing it to the server.
1.2 deraadt 176: .Pp
177: .Nm
1.40 aaron 178: implements the RSA authentication protocol automatically.
179: The user creates his/her RSA key pair by running
1.2 deraadt 180: .Xr ssh-keygen 1 .
1.44 aaron 181: This stores the private key in
1.49 markus 182: .Pa $HOME/.ssh/identity
1.176 jmc 183: and stores the public key in
1.49 markus 184: .Pa $HOME/.ssh/identity.pub
1.40 aaron 185: in the user's home directory.
186: The user should then copy the
1.2 deraadt 187: .Pa identity.pub
1.44 aaron 188: to
1.49 markus 189: .Pa $HOME/.ssh/authorized_keys
1.44 aaron 190: in his/her home directory on the remote machine (the
1.2 deraadt 191: .Pa authorized_keys
1.44 aaron 192: file corresponds to the conventional
1.49 markus 193: .Pa $HOME/.rhosts
1.1 deraadt 194: file, and has one key
1.40 aaron 195: per line, though the lines can be very long).
196: After this, the user can log in without giving the password.
1.176 jmc 197: RSA authentication is much more secure than
198: .Em rhosts
199: authentication.
1.2 deraadt 200: .Pp
1.1 deraadt 201: The most convenient way to use RSA authentication may be with an
1.40 aaron 202: authentication agent.
203: See
1.2 deraadt 204: .Xr ssh-agent 1
1.1 deraadt 205: for more information.
1.2 deraadt 206: .Pp
1.44 aaron 207: If other authentication methods fail,
1.2 deraadt 208: .Nm
1.40 aaron 209: prompts the user for a password.
210: The password is sent to the remote
1.1 deraadt 211: host for checking; however, since all communications are encrypted,
212: the password cannot be seen by someone listening on the network.
1.49 markus 213: .Ss SSH protocol version 2
1.176 jmc 214: When a user connects using protocol version 2,
1.145 markus 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;
1.176 jmc 219: if this method fails, public key authentication is attempted,
220: and finally if this method fails, keyboard-interactive and
1.123 markus 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
1.176 jmc 236: If public key authentication fails or is not available, a password
237: can be sent encrypted to the remote host to prove 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: .Ss Login session and remote execution
1.1 deraadt 249: When the user's identity has been accepted by the server, the server
250: either executes the given command, or logs into the machine and gives
1.40 aaron 251: the user a normal shell on the remote machine.
252: All communication with
1.1 deraadt 253: the remote command or shell will be automatically encrypted.
1.2 deraadt 254: .Pp
1.1 deraadt 255: If a pseudo-terminal has been allocated (normal login session), the
1.104 djm 256: user may use the escape characters noted below.
1.2 deraadt 257: .Pp
1.176 jmc 258: If no pseudo-tty has been allocated,
259: the session is transparent and can be used to reliably transfer binary data.
1.40 aaron 260: On most systems, setting the escape character to
1.2 deraadt 261: .Dq none
262: will also make the session transparent even if a tty is used.
263: .Pp
1.71 djm 264: The session terminates when the command or shell on the remote
1.92 markus 265: machine exits and all X11 and TCP/IP connections have been closed.
1.176 jmc 266: The exit status of the remote program is returned as the exit status of
1.2 deraadt 267: .Nm ssh .
1.104 djm 268: .Ss Escape Characters
1.176 jmc 269: When a pseudo-terminal has been requested,
270: .Nm
271: supports a number of functions through the use of an escape character.
1.104 djm 272: .Pp
273: A single tilde character can be sent as
274: .Ic ~~
1.119 stevesk 275: or by following the tilde by a character other than those described below.
1.104 djm 276: The escape character must always follow a newline to be interpreted as
277: special.
278: The escape character can be changed in configuration files using the
279: .Cm EscapeChar
1.117 itojun 280: configuration directive or on the command line by the
1.104 djm 281: .Fl e
282: option.
283: .Pp
284: The supported escapes (assuming the default
285: .Ql ~ )
286: are:
287: .Bl -tag -width Ds
288: .It Cm ~.
1.176 jmc 289: Disconnect.
1.104 djm 290: .It Cm ~^Z
1.176 jmc 291: Background
292: .Nm ssh .
1.104 djm 293: .It Cm ~#
1.176 jmc 294: List forwarded connections.
1.104 djm 295: .It Cm ~&
1.176 jmc 296: Background
297: .Nm
298: at logout when waiting for forwarded connection / X11 sessions to terminate.
1.104 djm 299: .It Cm ~?
1.176 jmc 300: Display a list of escape characters.
1.170 markus 301: .It Cm ~B
1.176 jmc 302: Send a BREAK to the remote system
303: (only useful for SSH protocol version 2 and if the peer supports it).
1.149 jakob 304: .It Cm ~C
1.186 djm 305: Open command line.
306: Currently this allows the addition of port forwardings using the
1.149 jakob 307: .Fl L
308: and
309: .Fl R
1.186 djm 310: options (see below).
1.188 ! jmc 311: It also allows the cancellation of existing remote port-forwardings
1.186 djm 312: using
313: .Fl KR Ar hostport .
1.187 djm 314: Basic help is available, using the
315: .Fl h
1.186 djm 316: option.
1.104 djm 317: .It Cm ~R
1.176 jmc 318: Request rekeying of the connection
319: (only useful for SSH protocol version 2 and if the peer supports it).
1.104 djm 320: .El
1.49 markus 321: .Ss X11 and TCP forwarding
1.110 deraadt 322: If the
323: .Cm ForwardX11
324: variable is set to
325: .Dq yes
1.176 jmc 326: (or see the description of the
1.110 deraadt 327: .Fl X
328: and
329: .Fl x
330: options described later)
331: and the user is using X11 (the
1.2 deraadt 332: .Ev DISPLAY
1.1 deraadt 333: environment variable is set), the connection to the X11 display is
334: automatically forwarded to the remote side in such a way that any X11
335: programs started from the shell (or command) will go through the
336: encrypted channel, and the connection to the real X server will be made
1.40 aaron 337: from the local machine.
338: The user should not manually set
1.2 deraadt 339: .Ev DISPLAY .
1.1 deraadt 340: Forwarding of X11 connections can be
341: configured on the command line or in configuration files.
1.2 deraadt 342: .Pp
343: The
1.44 aaron 344: .Ev DISPLAY
1.2 deraadt 345: value set by
346: .Nm
1.176 jmc 347: will point to the server machine, but with a display number greater than zero.
1.40 aaron 348: This is normal, and happens because
1.2 deraadt 349: .Nm
350: creates a
351: .Dq proxy
352: X server on the server machine for forwarding the
1.1 deraadt 353: connections over the encrypted channel.
1.2 deraadt 354: .Pp
355: .Nm
1.1 deraadt 356: will also automatically set up Xauthority data on the server machine.
357: For this purpose, it will generate a random authorization cookie,
358: store it in Xauthority on the server, and verify that any forwarded
359: connections carry this cookie and replace it by the real cookie when
1.40 aaron 360: the connection is opened.
361: The real authentication cookie is never
1.1 deraadt 362: sent to the server machine (and no cookies are sent in the plain).
1.2 deraadt 363: .Pp
1.163 stevesk 364: If the
365: .Cm ForwardAgent
366: variable is set to
367: .Dq yes
1.176 jmc 368: (or see the description of the
1.163 stevesk 369: .Fl A
370: and
371: .Fl a
1.168 jmc 372: options described later) and
1.163 stevesk 373: the user is using an authentication agent, the connection to the agent
374: is automatically forwarded to the remote side.
1.2 deraadt 375: .Pp
1.1 deraadt 376: Forwarding of arbitrary TCP/IP connections over the secure channel can
1.120 stevesk 377: be specified either on the command line or in a configuration file.
1.40 aaron 378: One possible application of TCP/IP forwarding is a secure connection to an
1.92 markus 379: electronic purse; another is going through firewalls.
1.49 markus 380: .Ss Server authentication
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.
1.176 jmc 391: If a host's identification ever changes,
1.2 deraadt 392: .Nm
1.1 deraadt 393: warns about this and disables password authentication to prevent a
1.40 aaron 394: trojan horse from getting the user's password.
1.176 jmc 395: Another purpose of this mechanism is to prevent man-in-the-middle attacks
396: which could otherwise be used to circumvent the encryption.
1.40 aaron 397: The
1.2 deraadt 398: .Cm StrictHostKeyChecking
1.158 stevesk 399: option can be used to prevent logins to machines whose
1.1 deraadt 400: host key is not known or has changed.
1.65 aaron 401: .Pp
402: The options are as follows:
1.2 deraadt 403: .Bl -tag -width Ds
1.176 jmc 404: .It Fl 1
405: Forces
406: .Nm
407: to try protocol version 1 only.
408: .It Fl 2
409: Forces
410: .Nm
411: to try protocol version 2 only.
412: .It Fl 4
413: Forces
414: .Nm
415: to use IPv4 addresses only.
416: .It Fl 6
417: Forces
418: .Nm
419: to use IPv6 addresses only.
1.54 markus 420: .It Fl A
421: Enables forwarding of the authentication agent connection.
422: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 423: .Pp
1.168 jmc 424: Agent forwarding should be enabled with caution.
425: Users with the ability to bypass file permissions on the remote host
426: (for the agent's Unix-domain socket)
427: can access the local agent through the forwarded connection.
428: An attacker cannot obtain key material from the agent,
1.165 stevesk 429: however they can perform operations on the keys that enable them to
430: authenticate using the identities loaded into the agent.
1.176 jmc 431: .It Fl a
432: Disables forwarding of the authentication agent connection.
1.108 markus 433: .It Fl b Ar bind_address
434: Specify the interface to transmit from on machines with multiple
435: interfaces or aliased addresses.
1.176 jmc 436: .It Fl C
437: Requests compression of all data (including stdin, stdout, stderr, and
438: data for forwarded X11 and TCP/IP connections).
439: The compression algorithm is the same used by
440: .Xr gzip 1 ,
441: and the
442: .Dq level
443: can be controlled by the
444: .Cm CompressionLevel
445: option for protocol version 1.
446: Compression is desirable on modem lines and other
447: slow connections, but will only slow down things on fast networks.
448: The default value can be set on a host-by-host basis in the
449: configuration files; see the
450: .Cm Compression
451: option.
452: .It Fl c Ar blowfish | 3des | des
1.44 aaron 453: Selects the cipher to use for encrypting the session.
1.2 deraadt 454: .Ar 3des
1.40 aaron 455: is used by default.
1.44 aaron 456: It is believed to be secure.
1.5 deraadt 457: .Ar 3des
458: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
459: .Ar blowfish
1.176 jmc 460: is a fast block cipher; it appears very secure and is much faster than
1.40 aaron 461: .Ar 3des .
1.131 stevesk 462: .Ar des
463: is only supported in the
464: .Nm
465: client for interoperability with legacy protocol 1 implementations
466: that do not support the
467: .Ar 3des
1.168 jmc 468: cipher.
469: Its use is strongly discouraged due to cryptographic weaknesses.
1.90 markus 470: .It Fl c Ar cipher_spec
1.51 markus 471: Additionally, for protocol version 2 a comma-separated list of ciphers can
1.61 aaron 472: be specified in order of preference.
1.90 markus 473: See
474: .Cm Ciphers
475: for more information.
1.176 jmc 476: .It Fl D Ar port
477: Specifies a local
478: .Dq dynamic
479: application-level port forwarding.
480: This works by allocating a socket to listen to
481: .Ar port
482: on the local side, and whenever a connection is made to this port, the
483: connection is forwarded over the secure channel, and the application
484: protocol is then used to determine where to connect to from the
485: remote machine.
486: Currently the SOCKS4 and SOCKS5 protocols are supported, and
487: .Nm
488: will act as a SOCKS server.
489: Only root can forward privileged ports.
490: Dynamic port forwardings can also be specified in the configuration file.
491: .It Fl e Ar ch | ^ch | none
1.2 deraadt 492: Sets the escape character for sessions with a pty (default:
493: .Ql ~ ) .
1.40 aaron 494: The escape character is only recognized at the beginning of a line.
495: The escape character followed by a dot
1.2 deraadt 496: .Pq Ql \&.
1.176 jmc 497: closes the connection;
498: followed by control-Z suspends the connection;
499: and followed by itself sends the escape character once.
1.40 aaron 500: Setting the character to
1.2 deraadt 501: .Dq none
502: disables any escapes and makes the session fully transparent.
1.176 jmc 503: .It Fl F Ar configfile
504: Specifies an alternative per-user configuration file.
505: If a configuration file is given on the command line,
506: the system-wide configuration file
507: .Pq Pa /etc/ssh/ssh_config
508: will be ignored.
509: The default for the per-user configuration file is
510: .Pa $HOME/.ssh/config .
1.2 deraadt 511: .It Fl f
512: Requests
513: .Nm
1.40 aaron 514: to go to background just before command execution.
515: This is useful if
1.2 deraadt 516: .Nm
517: is going to ask for passwords or passphrases, but the user
1.40 aaron 518: wants it in the background.
1.44 aaron 519: This implies
1.2 deraadt 520: .Fl n .
1.1 deraadt 521: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 522: something like
523: .Ic ssh -f host xterm .
1.34 markus 524: .It Fl g
525: Allows remote hosts to connect to local forwarded ports.
1.176 jmc 526: .It Fl I Ar smartcard_device
527: Specifies which smartcard device to use.
528: The argument is the device
529: .Nm
530: should use to communicate with a smartcard used for storing the user's
531: private RSA key.
1.2 deraadt 532: .It Fl i Ar identity_file
1.144 stevesk 533: Selects a file from which the identity (private key) for
1.68 markus 534: RSA or DSA authentication is read.
1.144 stevesk 535: The default is
1.49 markus 536: .Pa $HOME/.ssh/identity
1.144 stevesk 537: for protocol version 1, and
538: .Pa $HOME/.ssh/id_rsa
539: and
540: .Pa $HOME/.ssh/id_dsa
541: for protocol version 2.
1.40 aaron 542: Identity files may also be specified on
543: a per-host basis in the configuration file.
544: It is possible to have multiple
1.2 deraadt 545: .Fl i
546: options (and multiple identities specified in
1.1 deraadt 547: configuration files).
1.2 deraadt 548: .It Fl k
1.179 dtucker 549: Disables forwarding (delegation) of GSSAPI credentials to the server.
1.176 jmc 550: .It Fl L Xo
551: .Sm off
552: .Ar port : host : hostport
553: .Sm on
554: .Xc
555: Specifies that the given port on the local (client) host is to be
556: forwarded to the given host and port on the remote side.
557: This works by allocating a socket to listen to
558: .Ar port
559: on the local side, and whenever a connection is made to this port, the
560: connection is forwarded over the secure channel, and a connection is
561: made to
562: .Ar host
563: port
564: .Ar hostport
565: from the remote machine.
566: Port forwardings can also be specified in the configuration file.
567: Only root can forward privileged ports.
568: IPv6 addresses can be specified with an alternative syntax:
569: .Sm off
570: .Xo
571: .Ar port No / Ar host No /
572: .Ar hostport .
573: .Xc
574: .Sm on
1.2 deraadt 575: .It Fl l Ar login_name
1.40 aaron 576: Specifies the user to log in as on the remote machine.
577: This also may be specified on a per-host basis in the configuration file.
1.91 markus 578: .It Fl m Ar mac_spec
579: Additionally, for protocol version 2 a comma-separated list of MAC
580: (message authentication code) algorithms can
581: be specified in order of preference.
582: See the
583: .Cm MACs
584: keyword for more information.
1.176 jmc 585: .It Fl N
586: Do not execute a remote command.
587: This is useful for just forwarding ports
588: (protocol version 2 only).
1.2 deraadt 589: .It Fl n
590: Redirects stdin from
591: .Pa /dev/null
592: (actually, prevents reading from stdin).
1.1 deraadt 593: This must be used when
1.2 deraadt 594: .Nm
1.40 aaron 595: is run in the background.
596: A common trick is to use this to run X11 programs on a remote machine.
597: For example,
1.2 deraadt 598: .Ic ssh -n shadows.cs.hut.fi emacs &
599: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 600: connection will be automatically forwarded over an encrypted channel.
601: The
1.2 deraadt 602: .Nm
1.1 deraadt 603: program will be put in the background.
604: (This does not work if
1.2 deraadt 605: .Nm
606: needs to ask for a password or passphrase; see also the
607: .Fl f
608: option.)
609: .It Fl o Ar option
1.127 stevesk 610: Can be used to give options in the format used in the configuration file.
1.1 deraadt 611: This is useful for specifying options for which there is no separate
1.40 aaron 612: command-line flag.
1.176 jmc 613: For full details of the options listed below, and their possible values, see
614: .Xr ssh_config 5 .
615: .Pp
616: .Bl -tag -width Ds -offset indent -compact
617: .It AddressFamily
618: .It BatchMode
619: .It BindAddress
620: .It ChallengeResponseAuthentication
621: .It CheckHostIP
622: .It Cipher
623: .It Ciphers
624: .It ClearAllForwardings
625: .It Compression
626: .It CompressionLevel
627: .It ConnectionAttempts
1.185 dtucker 628: .It ConnectTimeout
1.176 jmc 629: .It DynamicForward
630: .It EscapeChar
631: .It ForwardAgent
632: .It ForwardX11
1.178 markus 633: .It ForwardX11Trusted
1.176 jmc 634: .It GatewayPorts
635: .It GlobalKnownHostsFile
636: .It GSSAPIAuthentication
637: .It GSSAPIDelegateCredentials
638: .It Host
639: .It HostbasedAuthentication
640: .It HostKeyAlgorithms
641: .It HostKeyAlias
642: .It HostName
643: .It IdentityFile
1.182 markus 644: .It IdentitiesOnly
1.176 jmc 645: .It LocalForward
646: .It LogLevel
647: .It MACs
648: .It NoHostAuthenticationForLocalhost
649: .It NumberOfPasswordPrompts
650: .It PasswordAuthentication
651: .It Port
652: .It PreferredAuthentications
653: .It Protocol
654: .It ProxyCommand
655: .It PubkeyAuthentication
656: .It RemoteForward
657: .It RhostsRSAAuthentication
658: .It RSAAuthentication
1.184 jmc 659: .It SendEnv
1.181 markus 660: .It ServerAliveInterval
661: .It ServerAliveCountMax
1.176 jmc 662: .It SmartcardDevice
663: .It StrictHostKeyChecking
1.180 markus 664: .It TCPKeepAlive
1.176 jmc 665: .It UsePrivilegedPort
666: .It User
667: .It UserKnownHostsFile
668: .It VerifyHostKeyDNS
669: .It XAuthLocation
670: .El
1.2 deraadt 671: .It Fl p Ar port
1.40 aaron 672: Port to connect to on the remote host.
673: This can be specified on a
1.1 deraadt 674: per-host basis in the configuration file.
1.2 deraadt 675: .It Fl q
1.40 aaron 676: Quiet mode.
677: Causes all warning and diagnostic messages to be suppressed.
1.176 jmc 678: .It Fl R Xo
679: .Sm off
680: .Ar port : host : hostport
681: .Sm on
682: .Xc
683: Specifies that the given port on the remote (server) host is to be
684: forwarded to the given host and port on the local side.
685: This works by allocating a socket to listen to
686: .Ar port
687: on the remote side, and whenever a connection is made to this port, the
688: connection is forwarded over the secure channel, and a connection is
689: made to
690: .Ar host
691: port
692: .Ar hostport
693: from the local machine.
694: Port forwardings can also be specified in the configuration file.
695: Privileged ports can be forwarded only when
696: logging in as root on the remote machine.
697: IPv6 addresses can be specified with an alternative syntax:
698: .Sm off
699: .Xo
700: .Ar port No / Ar host No /
701: .Ar hostport .
702: .Xc
703: .Sm on
1.80 djm 704: .It Fl s
1.172 jmc 705: May be used to request invocation of a subsystem on the remote system.
706: Subsystems are a feature of the SSH2 protocol which facilitate the use
1.176 jmc 707: of SSH as a secure transport for other applications (eg.\&
708: .Xr sftp 1 ) .
1.172 jmc 709: The subsystem is specified as the remote command.
1.176 jmc 710: .It Fl T
711: Disable pseudo-tty allocation.
1.2 deraadt 712: .It Fl t
1.40 aaron 713: Force pseudo-tty allocation.
1.43 brad 714: This can be used to execute arbitrary
1.40 aaron 715: screen-based programs on a remote machine, which can be very useful,
716: e.g., when implementing menu services.
1.73 markus 717: Multiple
718: .Fl t
719: options force tty allocation, even if
720: .Nm
721: has no local tty.
1.176 jmc 722: .It Fl V
723: Display the version number and exit.
1.2 deraadt 724: .It Fl v
1.40 aaron 725: Verbose mode.
726: Causes
1.2 deraadt 727: .Nm
1.40 aaron 728: to print debugging messages about its progress.
729: This is helpful in
1.1 deraadt 730: debugging connection, authentication, and configuration problems.
1.73 markus 731: Multiple
732: .Fl v
1.169 naddy 733: options increase the verbosity.
734: The maximum is 3.
1.2 deraadt 735: .It Fl X
1.1 deraadt 736: Enables X11 forwarding.
1.54 markus 737: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 738: .Pp
1.168 jmc 739: X11 forwarding should be enabled with caution.
740: Users with the ability to bypass file permissions on the remote host
741: (for the user's X authorization database)
742: can access the local X11 display through the forwarded connection.
743: An attacker may then be able to perform activities such as keystroke monitoring.
1.176 jmc 744: .It Fl x
745: Disables X11 forwarding.
1.178 markus 746: .It Fl Y
747: Enables trusted X11 forwarding.
1.2 deraadt 748: .El
749: .Sh CONFIGURATION FILES
750: .Nm
1.158 stevesk 751: may additionally obtain configuration data from
752: a per-user configuration file and a system-wide configuration file.
753: The file format and configuration options are described in
754: .Xr ssh_config 5 .
1.2 deraadt 755: .Sh ENVIRONMENT
756: .Nm
1.1 deraadt 757: will normally set the following environment variables:
1.176 jmc 758: .Bl -tag -width LOGNAME
1.2 deraadt 759: .It Ev DISPLAY
760: The
761: .Ev DISPLAY
1.40 aaron 762: variable indicates the location of the X11 server.
1.44 aaron 763: It is automatically set by
1.2 deraadt 764: .Nm
765: to point to a value of the form
766: .Dq hostname:n
767: where hostname indicates
1.176 jmc 768: the host where the shell runs, and n is an integer \*(Ge 1.
1.40 aaron 769: .Nm
770: uses this special value to forward X11 connections over the secure
771: channel.
1.107 markus 772: The user should normally not set
773: .Ev DISPLAY
774: explicitly, as that
1.1 deraadt 775: will render the X11 connection insecure (and will require the user to
776: manually copy any required authorization cookies).
1.2 deraadt 777: .It Ev HOME
1.1 deraadt 778: Set to the path of the user's home directory.
1.2 deraadt 779: .It Ev LOGNAME
780: Synonym for
1.12 aaron 781: .Ev USER ;
782: set for compatibility with systems that use this variable.
1.2 deraadt 783: .It Ev MAIL
1.129 stevesk 784: Set to the path of the user's mailbox.
1.40 aaron 785: .It Ev PATH
1.2 deraadt 786: Set to the default
787: .Ev PATH ,
788: as specified when compiling
1.12 aaron 789: .Nm ssh .
1.118 markus 790: .It Ev SSH_ASKPASS
791: If
792: .Nm
793: needs a passphrase, it will read the passphrase from the current
794: terminal if it was run from a terminal.
795: If
796: .Nm
797: does not have a terminal associated with it but
798: .Ev DISPLAY
799: and
800: .Ev SSH_ASKPASS
801: are set, it will execute the program specified by
802: .Ev SSH_ASKPASS
803: and open an X11 window to read the passphrase.
804: This is particularly useful when calling
805: .Nm
806: from a
807: .Pa .Xsession
808: or related script.
809: (Note that on some machines it
810: may be necessary to redirect the input from
811: .Pa /dev/null
812: to make this work.)
1.18 markus 813: .It Ev SSH_AUTH_SOCK
1.129 stevesk 814: Identifies the path of a unix-domain socket used to communicate with the
1.17 markus 815: agent.
1.166 stevesk 816: .It Ev SSH_CONNECTION
817: Identifies the client and server ends of the connection.
1.40 aaron 818: The variable contains
1.166 stevesk 819: four space-separated values: client ip-address, client port number,
820: server ip-address and server port number.
1.73 markus 821: .It Ev SSH_ORIGINAL_COMMAND
822: The variable contains the original command line if a forced command
823: is executed.
824: It can be used to extract the original arguments.
1.2 deraadt 825: .It Ev SSH_TTY
1.1 deraadt 826: This is set to the name of the tty (path to the device) associated
1.40 aaron 827: with the current shell or command.
828: If the current session has no tty,
1.1 deraadt 829: this variable is not set.
1.2 deraadt 830: .It Ev TZ
1.1 deraadt 831: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 832: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 833: on to new connections).
1.2 deraadt 834: .It Ev USER
1.1 deraadt 835: Set to the name of the user logging in.
1.2 deraadt 836: .El
837: .Pp
1.44 aaron 838: Additionally,
1.2 deraadt 839: .Nm
1.44 aaron 840: reads
841: .Pa $HOME/.ssh/environment ,
1.2 deraadt 842: and adds lines of the format
843: .Dq VARNAME=value
1.161 marc 844: to the environment if the file exists and if users are allowed to
845: change their environment.
1.176 jmc 846: For more information, see the
1.161 marc 847: .Cm PermitUserEnvironment
1.162 stevesk 848: option in
1.161 marc 849: .Xr sshd_config 5 .
1.2 deraadt 850: .Sh FILES
1.36 markus 851: .Bl -tag -width Ds
1.116 markus 852: .It Pa $HOME/.ssh/known_hosts
1.129 stevesk 853: Records host keys for all hosts the user has logged into that are not
1.2 deraadt 854: in
1.147 deraadt 855: .Pa /etc/ssh/ssh_known_hosts .
1.2 deraadt 856: See
857: .Xr sshd 8 .
1.102 itojun 858: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
859: Contains the authentication identity of the user.
860: They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1.48 markus 861: These files
862: contain sensitive data and should be readable by the user but not
1.15 markus 863: accessible by others (read/write/execute).
864: Note that
865: .Nm
1.48 markus 866: ignores a private key file if it is accessible by others.
1.15 markus 867: It is possible to specify a passphrase when
1.1 deraadt 868: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 869: sensitive part of this file using 3DES.
1.102 itojun 870: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1.1 deraadt 871: Contains the public key for authentication (public part of the
1.40 aaron 872: identity file in human-readable form).
1.48 markus 873: The contents of the
874: .Pa $HOME/.ssh/identity.pub
1.176 jmc 875: file should be added to the file
1.2 deraadt 876: .Pa $HOME/.ssh/authorized_keys
877: on all machines
1.137 deraadt 878: where the user wishes to log in using protocol version 1 RSA authentication.
1.48 markus 879: The contents of the
880: .Pa $HOME/.ssh/id_dsa.pub
1.102 itojun 881: and
882: .Pa $HOME/.ssh/id_rsa.pub
1.48 markus 883: file should be added to
1.115 markus 884: .Pa $HOME/.ssh/authorized_keys
1.48 markus 885: on all machines
1.137 deraadt 886: where the user wishes to log in using protocol version 2 DSA/RSA authentication.
1.48 markus 887: These files are not
1.40 aaron 888: sensitive and can (but need not) be readable by anyone.
1.48 markus 889: These files are
1.84 markus 890: never used automatically and are not necessary; they are only provided for
1.1 deraadt 891: the convenience of the user.
1.2 deraadt 892: .It Pa $HOME/.ssh/config
1.40 aaron 893: This is the per-user configuration file.
1.158 stevesk 894: The file format and configuration options are described in
895: .Xr ssh_config 5 .
1.183 djm 896: Because of the potential for abuse, this file must have strict permissions:
897: read/write for the user, and not accessible by others.
1.2 deraadt 898: .It Pa $HOME/.ssh/authorized_keys
1.115 markus 899: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.40 aaron 900: The format of this file is described in the
1.2 deraadt 901: .Xr sshd 8
1.40 aaron 902: manual page.
1.176 jmc 903: In the simplest form the format is the same as the
904: .Pa .pub
1.116 markus 905: identity files.
1.48 markus 906: This file is not highly sensitive, but the recommended
907: permissions are read/write for the user, and not accessible by others.
1.147 deraadt 908: .It Pa /etc/ssh/ssh_known_hosts
1.40 aaron 909: Systemwide list of known host keys.
1.116 markus 910: This file should be prepared by the
1.1 deraadt 911: system administrator to contain the public host keys of all machines in the
1.40 aaron 912: organization.
913: This file should be world-readable.
914: This file contains
1.1 deraadt 915: public keys, one per line, in the following format (fields separated
1.116 markus 916: by spaces): system name, public key and optional comment field.
1.40 aaron 917: When different names are used
1.1 deraadt 918: for the same machine, all such names should be listed, separated by
1.40 aaron 919: commas.
1.176 jmc 920: The format is described in the
1.2 deraadt 921: .Xr sshd 8
1.1 deraadt 922: manual page.
1.2 deraadt 923: .Pp
1.1 deraadt 924: The canonical system name (as returned by name servers) is used by
1.2 deraadt 925: .Xr sshd 8
1.1 deraadt 926: to verify the client host when logging in; other names are needed because
1.2 deraadt 927: .Nm
1.1 deraadt 928: does not convert the user-supplied name to a canonical name before
929: checking the key, because someone with access to the name servers
930: would then be able to fool host authentication.
1.147 deraadt 931: .It Pa /etc/ssh/ssh_config
1.40 aaron 932: Systemwide configuration file.
1.158 stevesk 933: The file format and configuration options are described in
934: .Xr ssh_config 5 .
1.147 deraadt 935: .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1.141 markus 936: These three files contain the private parts of the host keys
937: and are used for
938: .Cm RhostsRSAAuthentication
939: and
940: .Cm HostbasedAuthentication .
1.155 stevesk 941: If the protocol version 1
942: .Cm RhostsRSAAuthentication
1.157 deraadt 943: method is used,
1.155 stevesk 944: .Nm
945: must be setuid root, since the host key is readable only by root.
946: For protocol version 2,
947: .Nm
948: uses
949: .Xr ssh-keysign 8
950: to access the host keys for
951: .Cm HostbasedAuthentication .
952: This eliminates the requirement that
953: .Nm
954: be setuid root when that authentication method is used.
955: By default
1.141 markus 956: .Nm
1.155 stevesk 957: is not setuid root.
1.2 deraadt 958: .It Pa $HOME/.rhosts
959: This file is used in
1.176 jmc 960: .Em rhosts
1.2 deraadt 961: authentication to list the
1.40 aaron 962: host/user pairs that are permitted to log in.
963: (Note that this file is
1.1 deraadt 964: also used by rlogin and rsh, which makes using this file insecure.)
965: Each line of the file contains a host name (in the canonical form
966: returned by name servers), and then a user name on that host,
1.40 aaron 967: separated by a space.
1.92 markus 968: On some machines this file may need to be
1.1 deraadt 969: world-readable if the user's home directory is on a NFS partition,
970: because
1.2 deraadt 971: .Xr sshd 8
1.40 aaron 972: reads it as root.
973: Additionally, this file must be owned by the user,
974: and must not have write permissions for anyone else.
975: The recommended
1.1 deraadt 976: permission for most machines is read/write for the user, and not
977: accessible by others.
1.2 deraadt 978: .Pp
1.1 deraadt 979: Note that by default
1.2 deraadt 980: .Xr sshd 8
1.1 deraadt 981: will be installed so that it requires successful RSA host
1.176 jmc 982: authentication before permitting
983: .Em rhosts
984: authentication.
1.137 deraadt 985: If the server machine does not have the client's host key in
1.147 deraadt 986: .Pa /etc/ssh/ssh_known_hosts ,
1.137 deraadt 987: it can be stored in
1.2 deraadt 988: .Pa $HOME/.ssh/known_hosts .
989: The easiest way to do this is to
1.1 deraadt 990: connect back to the client from the server machine using ssh; this
1.48 markus 991: will automatically add the host key to
1.2 deraadt 992: .Pa $HOME/.ssh/known_hosts .
993: .It Pa $HOME/.shosts
994: This file is used exactly the same way as
1.176 jmc 995: .Pa .rhosts .
1.2 deraadt 996: The purpose for
1.1 deraadt 997: having this file is to be able to use rhosts authentication with
1.2 deraadt 998: .Nm
999: without permitting login with
1.176 jmc 1000: .Xr rlogin
1.2 deraadt 1001: or
1002: .Xr rsh 1 .
1003: .It Pa /etc/hosts.equiv
1004: This file is used during
1.176 jmc 1005: .Em rhosts
1.173 jmc 1006: authentication.
1.40 aaron 1007: It contains
1.176 jmc 1008: canonical hosts names, one per line (the full format is described in the
1.2 deraadt 1009: .Xr sshd 8
1.40 aaron 1010: manual page).
1011: If the client host is found in this file, login is
1.1 deraadt 1012: automatically permitted provided client and server user names are the
1.40 aaron 1013: same.
1014: Additionally, successful RSA host authentication is normally
1015: required.
1016: This file should only be writable by root.
1.2 deraadt 1017: .It Pa /etc/shosts.equiv
1.44 aaron 1018: This file is processed exactly as
1.2 deraadt 1019: .Pa /etc/hosts.equiv .
1.1 deraadt 1020: This file may be useful to permit logins using
1.2 deraadt 1021: .Nm
1.1 deraadt 1022: but not using rsh/rlogin.
1.147 deraadt 1023: .It Pa /etc/ssh/sshrc
1.1 deraadt 1024: Commands in this file are executed by
1.2 deraadt 1025: .Nm
1.1 deraadt 1026: when the user logs in just before the user's shell (or command) is started.
1027: See the
1.2 deraadt 1028: .Xr sshd 8
1.1 deraadt 1029: manual page for more information.
1.2 deraadt 1030: .It Pa $HOME/.ssh/rc
1.1 deraadt 1031: Commands in this file are executed by
1.2 deraadt 1032: .Nm
1.1 deraadt 1033: when the user logs in just before the user's shell (or command) is
1034: started.
1.44 aaron 1035: See the
1.2 deraadt 1036: .Xr sshd 8
1.1 deraadt 1037: manual page for more information.
1.31 markus 1038: .It Pa $HOME/.ssh/environment
1039: Contains additional definitions for environment variables, see section
1040: .Sx ENVIRONMENT
1041: above.
1.58 itojun 1042: .El
1.145 markus 1043: .Sh DIAGNOSTICS
1044: .Nm
1045: exits with the exit status of the remote command or with 255
1046: if an error occurred.
1.2 deraadt 1047: .Sh SEE ALSO
1.176 jmc 1048: .Xr gzip 1 ,
1.2 deraadt 1049: .Xr rsh 1 ,
1050: .Xr scp 1 ,
1.83 djm 1051: .Xr sftp 1 ,
1.2 deraadt 1052: .Xr ssh-add 1 ,
1053: .Xr ssh-agent 1 ,
1054: .Xr ssh-keygen 1 ,
1055: .Xr telnet 1 ,
1.176 jmc 1056: .Xr hosts.equiv 5 ,
1.159 stevesk 1057: .Xr ssh_config 5 ,
1.160 naddy 1058: .Xr ssh-keysign 8 ,
1.87 itojun 1059: .Xr sshd 8
1.106 markus 1060: .Rs
1061: .%A T. Ylonen
1062: .%A T. Kivinen
1063: .%A M. Saarinen
1064: .%A T. Rinne
1065: .%A S. Lehtinen
1066: .%T "SSH Protocol Architecture"
1.150 markus 1067: .%N draft-ietf-secsh-architecture-12.txt
1068: .%D January 2002
1.106 markus 1069: .%O work in progress material
1070: .Re
1.173 jmc 1071: .Sh AUTHORS
1072: OpenSSH is a derivative of the original and free
1073: ssh 1.2.12 release by Tatu Ylonen.
1074: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1075: Theo de Raadt and Dug Song
1076: removed many bugs, re-added newer features and
1077: created OpenSSH.
1078: Markus Friedl contributed the support for SSH
1079: protocol versions 1.5 and 2.0.
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