Annotation of src/usr.bin/ssh/ssh.1, Revision 1.177
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: .\"
1.177 ! jmc 37: .\" $OpenBSD: ssh.1,v 1.176 2003/09/29 11:40:51 jmc 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
1.176 jmc 46: .Op Fl 1246AaCfgkNnqsTtVvXx
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
305: Open command line (only useful for adding port forwardings using the
306: .Fl L
307: and
308: .Fl R
1.176 jmc 309: options).
1.104 djm 310: .It Cm ~R
1.176 jmc 311: Request rekeying of the connection
312: (only useful for SSH protocol version 2 and if the peer supports it).
1.104 djm 313: .El
1.49 markus 314: .Ss X11 and TCP forwarding
1.110 deraadt 315: If the
316: .Cm ForwardX11
317: variable is set to
318: .Dq yes
1.176 jmc 319: (or see the description of the
1.110 deraadt 320: .Fl X
321: and
322: .Fl x
323: options described later)
324: and the user is using X11 (the
1.2 deraadt 325: .Ev DISPLAY
1.1 deraadt 326: environment variable is set), the connection to the X11 display is
327: automatically forwarded to the remote side in such a way that any X11
328: programs started from the shell (or command) will go through the
329: encrypted channel, and the connection to the real X server will be made
1.40 aaron 330: from the local machine.
331: The user should not manually set
1.2 deraadt 332: .Ev DISPLAY .
1.1 deraadt 333: Forwarding of X11 connections can be
334: configured on the command line or in configuration files.
1.2 deraadt 335: .Pp
336: The
1.44 aaron 337: .Ev DISPLAY
1.2 deraadt 338: value set by
339: .Nm
1.176 jmc 340: will point to the server machine, but with a display number greater than zero.
1.40 aaron 341: This is normal, and happens because
1.2 deraadt 342: .Nm
343: creates a
344: .Dq proxy
345: X server on the server machine for forwarding the
1.1 deraadt 346: connections over the encrypted channel.
1.2 deraadt 347: .Pp
348: .Nm
1.1 deraadt 349: will also automatically set up Xauthority data on the server machine.
350: For this purpose, it will generate a random authorization cookie,
351: store it in Xauthority on the server, and verify that any forwarded
352: connections carry this cookie and replace it by the real cookie when
1.40 aaron 353: the connection is opened.
354: The real authentication cookie is never
1.1 deraadt 355: sent to the server machine (and no cookies are sent in the plain).
1.2 deraadt 356: .Pp
1.163 stevesk 357: If the
358: .Cm ForwardAgent
359: variable is set to
360: .Dq yes
1.176 jmc 361: (or see the description of the
1.163 stevesk 362: .Fl A
363: and
364: .Fl a
1.168 jmc 365: options described later) and
1.163 stevesk 366: the user is using an authentication agent, the connection to the agent
367: is automatically forwarded to the remote side.
1.2 deraadt 368: .Pp
1.1 deraadt 369: Forwarding of arbitrary TCP/IP connections over the secure channel can
1.120 stevesk 370: be specified either on the command line or in a configuration file.
1.40 aaron 371: One possible application of TCP/IP forwarding is a secure connection to an
1.92 markus 372: electronic purse; another is going through firewalls.
1.49 markus 373: .Ss Server authentication
1.2 deraadt 374: .Nm
1.49 markus 375: automatically maintains and checks a database containing
1.40 aaron 376: identifications for all hosts it has ever been used with.
1.116 markus 377: Host keys are stored in
1.49 markus 378: .Pa $HOME/.ssh/known_hosts
1.40 aaron 379: in the user's home directory.
1.116 markus 380: Additionally, the file
1.147 deraadt 381: .Pa /etc/ssh/ssh_known_hosts
1.116 markus 382: is automatically checked for known hosts.
1.40 aaron 383: Any new hosts are automatically added to the user's file.
1.176 jmc 384: If a host's identification ever changes,
1.2 deraadt 385: .Nm
1.1 deraadt 386: warns about this and disables password authentication to prevent a
1.40 aaron 387: trojan horse from getting the user's password.
1.176 jmc 388: Another purpose of this mechanism is to prevent man-in-the-middle attacks
389: which could otherwise be used to circumvent the encryption.
1.40 aaron 390: The
1.2 deraadt 391: .Cm StrictHostKeyChecking
1.158 stevesk 392: option can be used to prevent logins to machines whose
1.1 deraadt 393: host key is not known or has changed.
1.65 aaron 394: .Pp
395: The options are as follows:
1.2 deraadt 396: .Bl -tag -width Ds
1.176 jmc 397: .It Fl 1
398: Forces
399: .Nm
400: to try protocol version 1 only.
401: .It Fl 2
402: Forces
403: .Nm
404: to try protocol version 2 only.
405: .It Fl 4
406: Forces
407: .Nm
408: to use IPv4 addresses only.
409: .It Fl 6
410: Forces
411: .Nm
412: to use IPv6 addresses only.
1.54 markus 413: .It Fl A
414: Enables forwarding of the authentication agent connection.
415: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 416: .Pp
1.168 jmc 417: Agent forwarding should be enabled with caution.
418: Users with the ability to bypass file permissions on the remote host
419: (for the agent's Unix-domain socket)
420: can access the local agent through the forwarded connection.
421: An attacker cannot obtain key material from the agent,
1.165 stevesk 422: however they can perform operations on the keys that enable them to
423: authenticate using the identities loaded into the agent.
1.176 jmc 424: .It Fl a
425: Disables forwarding of the authentication agent connection.
1.108 markus 426: .It Fl b Ar bind_address
427: Specify the interface to transmit from on machines with multiple
428: interfaces or aliased addresses.
1.176 jmc 429: .It Fl C
430: Requests compression of all data (including stdin, stdout, stderr, and
431: data for forwarded X11 and TCP/IP connections).
432: The compression algorithm is the same used by
433: .Xr gzip 1 ,
434: and the
435: .Dq level
436: can be controlled by the
437: .Cm CompressionLevel
438: option for protocol version 1.
439: Compression is desirable on modem lines and other
440: slow connections, but will only slow down things on fast networks.
441: The default value can be set on a host-by-host basis in the
442: configuration files; see the
443: .Cm Compression
444: option.
445: .It Fl c Ar blowfish | 3des | des
1.44 aaron 446: Selects the cipher to use for encrypting the session.
1.2 deraadt 447: .Ar 3des
1.40 aaron 448: is used by default.
1.44 aaron 449: It is believed to be secure.
1.5 deraadt 450: .Ar 3des
451: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
452: .Ar blowfish
1.176 jmc 453: is a fast block cipher; it appears very secure and is much faster than
1.40 aaron 454: .Ar 3des .
1.131 stevesk 455: .Ar des
456: is only supported in the
457: .Nm
458: client for interoperability with legacy protocol 1 implementations
459: that do not support the
460: .Ar 3des
1.168 jmc 461: cipher.
462: Its use is strongly discouraged due to cryptographic weaknesses.
1.90 markus 463: .It Fl c Ar cipher_spec
1.51 markus 464: Additionally, for protocol version 2 a comma-separated list of ciphers can
1.61 aaron 465: be specified in order of preference.
1.90 markus 466: See
467: .Cm Ciphers
468: for more information.
1.176 jmc 469: .It Fl D Ar port
470: Specifies a local
471: .Dq dynamic
472: application-level port forwarding.
473: This works by allocating a socket to listen to
474: .Ar port
475: on the local side, and whenever a connection is made to this port, the
476: connection is forwarded over the secure channel, and the application
477: protocol is then used to determine where to connect to from the
478: remote machine.
479: Currently the SOCKS4 and SOCKS5 protocols are supported, and
480: .Nm
481: will act as a SOCKS server.
482: Only root can forward privileged ports.
483: Dynamic port forwardings can also be specified in the configuration file.
484: .It Fl e Ar ch | ^ch | none
1.2 deraadt 485: Sets the escape character for sessions with a pty (default:
486: .Ql ~ ) .
1.40 aaron 487: The escape character is only recognized at the beginning of a line.
488: The escape character followed by a dot
1.2 deraadt 489: .Pq Ql \&.
1.176 jmc 490: closes the connection;
491: followed by control-Z suspends the connection;
492: and followed by itself sends the escape character once.
1.40 aaron 493: Setting the character to
1.2 deraadt 494: .Dq none
495: disables any escapes and makes the session fully transparent.
1.176 jmc 496: .It Fl F Ar configfile
497: Specifies an alternative per-user configuration file.
498: If a configuration file is given on the command line,
499: the system-wide configuration file
500: .Pq Pa /etc/ssh/ssh_config
501: will be ignored.
502: The default for the per-user configuration file is
503: .Pa $HOME/.ssh/config .
1.2 deraadt 504: .It Fl f
505: Requests
506: .Nm
1.40 aaron 507: to go to background just before command execution.
508: This is useful if
1.2 deraadt 509: .Nm
510: is going to ask for passwords or passphrases, but the user
1.40 aaron 511: wants it in the background.
1.44 aaron 512: This implies
1.2 deraadt 513: .Fl n .
1.1 deraadt 514: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 515: something like
516: .Ic ssh -f host xterm .
1.34 markus 517: .It Fl g
518: Allows remote hosts to connect to local forwarded ports.
1.176 jmc 519: .It Fl I Ar smartcard_device
520: Specifies which smartcard device to use.
521: The argument is the device
522: .Nm
523: should use to communicate with a smartcard used for storing the user's
524: private RSA key.
1.2 deraadt 525: .It Fl i Ar identity_file
1.144 stevesk 526: Selects a file from which the identity (private key) for
1.68 markus 527: RSA or DSA authentication is read.
1.144 stevesk 528: The default is
1.49 markus 529: .Pa $HOME/.ssh/identity
1.144 stevesk 530: for protocol version 1, and
531: .Pa $HOME/.ssh/id_rsa
532: and
533: .Pa $HOME/.ssh/id_dsa
534: for protocol version 2.
1.40 aaron 535: Identity files may also be specified on
536: a per-host basis in the configuration file.
537: It is possible to have multiple
1.2 deraadt 538: .Fl i
539: options (and multiple identities specified in
1.1 deraadt 540: configuration files).
1.2 deraadt 541: .It Fl k
1.175 markus 542: Disables forwarding of Kerberos tickets.
1.42 aaron 543: This may also be specified on a per-host basis in the configuration file.
1.176 jmc 544: .It Fl L Xo
545: .Sm off
546: .Ar port : host : hostport
547: .Sm on
548: .Xc
549: Specifies that the given port on the local (client) host is to be
550: forwarded to the given host and port on the remote side.
551: This works by allocating a socket to listen to
552: .Ar port
553: on the local side, and whenever a connection is made to this port, the
554: connection is forwarded over the secure channel, and a connection is
555: made to
556: .Ar host
557: port
558: .Ar hostport
559: from the remote machine.
560: Port forwardings can also be specified in the configuration file.
561: Only root can forward privileged ports.
562: IPv6 addresses can be specified with an alternative syntax:
563: .Sm off
564: .Xo
565: .Ar port No / Ar host No /
566: .Ar hostport .
567: .Xc
568: .Sm on
1.2 deraadt 569: .It Fl l Ar login_name
1.40 aaron 570: Specifies the user to log in as on the remote machine.
571: This also may be specified on a per-host basis in the configuration file.
1.91 markus 572: .It Fl m Ar mac_spec
573: Additionally, for protocol version 2 a comma-separated list of MAC
574: (message authentication code) algorithms can
575: be specified in order of preference.
576: See the
577: .Cm MACs
578: keyword for more information.
1.176 jmc 579: .It Fl N
580: Do not execute a remote command.
581: This is useful for just forwarding ports
582: (protocol version 2 only).
1.2 deraadt 583: .It Fl n
584: Redirects stdin from
585: .Pa /dev/null
586: (actually, prevents reading from stdin).
1.1 deraadt 587: This must be used when
1.2 deraadt 588: .Nm
1.40 aaron 589: is run in the background.
590: A common trick is to use this to run X11 programs on a remote machine.
591: For example,
1.2 deraadt 592: .Ic ssh -n shadows.cs.hut.fi emacs &
593: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 594: connection will be automatically forwarded over an encrypted channel.
595: The
1.2 deraadt 596: .Nm
1.1 deraadt 597: program will be put in the background.
598: (This does not work if
1.2 deraadt 599: .Nm
600: needs to ask for a password or passphrase; see also the
601: .Fl f
602: option.)
603: .It Fl o Ar option
1.127 stevesk 604: Can be used to give options in the format used in the configuration file.
1.1 deraadt 605: This is useful for specifying options for which there is no separate
1.40 aaron 606: command-line flag.
1.176 jmc 607: For full details of the options listed below, and their possible values, see
608: .Xr ssh_config 5 .
609: .Pp
610: .Bl -tag -width Ds -offset indent -compact
611: .It AddressFamily
612: .It BatchMode
613: .It BindAddress
614: .It ChallengeResponseAuthentication
615: .It CheckHostIP
616: .It Cipher
617: .It Ciphers
618: .It ClearAllForwardings
619: .It Compression
620: .It CompressionLevel
621: .It ConnectionAttempts
622: .It ConnectionTimeout
623: .It DynamicForward
624: .It EscapeChar
625: .It ForwardAgent
626: .It ForwardX11
627: .It GatewayPorts
628: .It GlobalKnownHostsFile
629: .It GSSAPIAuthentication
630: .It GSSAPIDelegateCredentials
631: .It Host
632: .It HostbasedAuthentication
633: .It HostKeyAlgorithms
634: .It HostKeyAlias
635: .It HostName
636: .It IdentityFile
637: .It KeepAlive
638: .It LocalForward
639: .It LogLevel
640: .It MACs
641: .It NoHostAuthenticationForLocalhost
642: .It NumberOfPasswordPrompts
643: .It PasswordAuthentication
644: .It Port
645: .It PreferredAuthentications
646: .It Protocol
647: .It ProxyCommand
648: .It PubkeyAuthentication
649: .It RemoteForward
650: .It RhostsRSAAuthentication
651: .It RSAAuthentication
652: .It SmartcardDevice
653: .It StrictHostKeyChecking
654: .It UsePrivilegedPort
655: .It User
656: .It UserKnownHostsFile
657: .It VerifyHostKeyDNS
658: .It XAuthLocation
659: .El
1.2 deraadt 660: .It Fl p Ar port
1.40 aaron 661: Port to connect to on the remote host.
662: This can be specified on a
1.1 deraadt 663: per-host basis in the configuration file.
1.2 deraadt 664: .It Fl q
1.40 aaron 665: Quiet mode.
666: Causes all warning and diagnostic messages to be suppressed.
1.176 jmc 667: .It Fl R Xo
668: .Sm off
669: .Ar port : host : hostport
670: .Sm on
671: .Xc
672: Specifies that the given port on the remote (server) host is to be
673: forwarded to the given host and port on the local side.
674: This works by allocating a socket to listen to
675: .Ar port
676: on the remote side, and whenever a connection is made to this port, the
677: connection is forwarded over the secure channel, and a connection is
678: made to
679: .Ar host
680: port
681: .Ar hostport
682: from the local machine.
683: Port forwardings can also be specified in the configuration file.
684: Privileged ports can be forwarded only when
685: logging in as root on the remote machine.
686: IPv6 addresses can be specified with an alternative syntax:
687: .Sm off
688: .Xo
689: .Ar port No / Ar host No /
690: .Ar hostport .
691: .Xc
692: .Sm on
1.80 djm 693: .It Fl s
1.172 jmc 694: May be used to request invocation of a subsystem on the remote system.
695: Subsystems are a feature of the SSH2 protocol which facilitate the use
1.176 jmc 696: of SSH as a secure transport for other applications (eg.\&
697: .Xr sftp 1 ) .
1.172 jmc 698: The subsystem is specified as the remote command.
1.176 jmc 699: .It Fl T
700: Disable pseudo-tty allocation.
1.2 deraadt 701: .It Fl t
1.40 aaron 702: Force pseudo-tty allocation.
1.43 brad 703: This can be used to execute arbitrary
1.40 aaron 704: screen-based programs on a remote machine, which can be very useful,
705: e.g., when implementing menu services.
1.73 markus 706: Multiple
707: .Fl t
708: options force tty allocation, even if
709: .Nm
710: has no local tty.
1.176 jmc 711: .It Fl V
712: Display the version number and exit.
1.2 deraadt 713: .It Fl v
1.40 aaron 714: Verbose mode.
715: Causes
1.2 deraadt 716: .Nm
1.40 aaron 717: to print debugging messages about its progress.
718: This is helpful in
1.1 deraadt 719: debugging connection, authentication, and configuration problems.
1.73 markus 720: Multiple
721: .Fl v
1.169 naddy 722: options increase the verbosity.
723: The maximum is 3.
1.2 deraadt 724: .It Fl X
1.1 deraadt 725: Enables X11 forwarding.
1.54 markus 726: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 727: .Pp
1.168 jmc 728: X11 forwarding should be enabled with caution.
729: Users with the ability to bypass file permissions on the remote host
730: (for the user's X authorization database)
731: can access the local X11 display through the forwarded connection.
732: An attacker may then be able to perform activities such as keystroke monitoring.
1.176 jmc 733: .It Fl x
734: Disables X11 forwarding.
1.2 deraadt 735: .El
736: .Sh CONFIGURATION FILES
737: .Nm
1.158 stevesk 738: may additionally obtain configuration data from
739: a per-user configuration file and a system-wide configuration file.
740: The file format and configuration options are described in
741: .Xr ssh_config 5 .
1.2 deraadt 742: .Sh ENVIRONMENT
743: .Nm
1.1 deraadt 744: will normally set the following environment variables:
1.176 jmc 745: .Bl -tag -width LOGNAME
1.2 deraadt 746: .It Ev DISPLAY
747: The
748: .Ev DISPLAY
1.40 aaron 749: variable indicates the location of the X11 server.
1.44 aaron 750: It is automatically set by
1.2 deraadt 751: .Nm
752: to point to a value of the form
753: .Dq hostname:n
754: where hostname indicates
1.176 jmc 755: the host where the shell runs, and n is an integer \*(Ge 1.
1.40 aaron 756: .Nm
757: uses this special value to forward X11 connections over the secure
758: channel.
1.107 markus 759: The user should normally not set
760: .Ev DISPLAY
761: explicitly, as that
1.1 deraadt 762: will render the X11 connection insecure (and will require the user to
763: manually copy any required authorization cookies).
1.2 deraadt 764: .It Ev HOME
1.1 deraadt 765: Set to the path of the user's home directory.
1.2 deraadt 766: .It Ev LOGNAME
767: Synonym for
1.12 aaron 768: .Ev USER ;
769: set for compatibility with systems that use this variable.
1.2 deraadt 770: .It Ev MAIL
1.129 stevesk 771: Set to the path of the user's mailbox.
1.40 aaron 772: .It Ev PATH
1.2 deraadt 773: Set to the default
774: .Ev PATH ,
775: as specified when compiling
1.12 aaron 776: .Nm ssh .
1.118 markus 777: .It Ev SSH_ASKPASS
778: If
779: .Nm
780: needs a passphrase, it will read the passphrase from the current
781: terminal if it was run from a terminal.
782: If
783: .Nm
784: does not have a terminal associated with it but
785: .Ev DISPLAY
786: and
787: .Ev SSH_ASKPASS
788: are set, it will execute the program specified by
789: .Ev SSH_ASKPASS
790: and open an X11 window to read the passphrase.
791: This is particularly useful when calling
792: .Nm
793: from a
794: .Pa .Xsession
795: or related script.
796: (Note that on some machines it
797: may be necessary to redirect the input from
798: .Pa /dev/null
799: to make this work.)
1.18 markus 800: .It Ev SSH_AUTH_SOCK
1.129 stevesk 801: Identifies the path of a unix-domain socket used to communicate with the
1.17 markus 802: agent.
1.166 stevesk 803: .It Ev SSH_CONNECTION
804: Identifies the client and server ends of the connection.
1.40 aaron 805: The variable contains
1.166 stevesk 806: four space-separated values: client ip-address, client port number,
807: server ip-address and server port number.
1.73 markus 808: .It Ev SSH_ORIGINAL_COMMAND
809: The variable contains the original command line if a forced command
810: is executed.
811: It can be used to extract the original arguments.
1.2 deraadt 812: .It Ev SSH_TTY
1.1 deraadt 813: This is set to the name of the tty (path to the device) associated
1.40 aaron 814: with the current shell or command.
815: If the current session has no tty,
1.1 deraadt 816: this variable is not set.
1.2 deraadt 817: .It Ev TZ
1.1 deraadt 818: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 819: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 820: on to new connections).
1.2 deraadt 821: .It Ev USER
1.1 deraadt 822: Set to the name of the user logging in.
1.2 deraadt 823: .El
824: .Pp
1.44 aaron 825: Additionally,
1.2 deraadt 826: .Nm
1.44 aaron 827: reads
828: .Pa $HOME/.ssh/environment ,
1.2 deraadt 829: and adds lines of the format
830: .Dq VARNAME=value
1.161 marc 831: to the environment if the file exists and if users are allowed to
832: change their environment.
1.176 jmc 833: For more information, see the
1.161 marc 834: .Cm PermitUserEnvironment
1.162 stevesk 835: option in
1.161 marc 836: .Xr sshd_config 5 .
1.2 deraadt 837: .Sh FILES
1.36 markus 838: .Bl -tag -width Ds
1.116 markus 839: .It Pa $HOME/.ssh/known_hosts
1.129 stevesk 840: Records host keys for all hosts the user has logged into that are not
1.2 deraadt 841: in
1.147 deraadt 842: .Pa /etc/ssh/ssh_known_hosts .
1.2 deraadt 843: See
844: .Xr sshd 8 .
1.102 itojun 845: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
846: Contains the authentication identity of the user.
847: They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1.48 markus 848: These files
849: contain sensitive data and should be readable by the user but not
1.15 markus 850: accessible by others (read/write/execute).
851: Note that
852: .Nm
1.48 markus 853: ignores a private key file if it is accessible by others.
1.15 markus 854: It is possible to specify a passphrase when
1.1 deraadt 855: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 856: sensitive part of this file using 3DES.
1.102 itojun 857: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1.1 deraadt 858: Contains the public key for authentication (public part of the
1.40 aaron 859: identity file in human-readable form).
1.48 markus 860: The contents of the
861: .Pa $HOME/.ssh/identity.pub
1.176 jmc 862: file should be added to the file
1.2 deraadt 863: .Pa $HOME/.ssh/authorized_keys
864: on all machines
1.137 deraadt 865: where the user wishes to log in using protocol version 1 RSA authentication.
1.48 markus 866: The contents of the
867: .Pa $HOME/.ssh/id_dsa.pub
1.102 itojun 868: and
869: .Pa $HOME/.ssh/id_rsa.pub
1.48 markus 870: file should be added to
1.115 markus 871: .Pa $HOME/.ssh/authorized_keys
1.48 markus 872: on all machines
1.137 deraadt 873: where the user wishes to log in using protocol version 2 DSA/RSA authentication.
1.48 markus 874: These files are not
1.40 aaron 875: sensitive and can (but need not) be readable by anyone.
1.48 markus 876: These files are
1.84 markus 877: never used automatically and are not necessary; they are only provided for
1.1 deraadt 878: the convenience of the user.
1.2 deraadt 879: .It Pa $HOME/.ssh/config
1.40 aaron 880: This is the per-user configuration file.
1.158 stevesk 881: The file format and configuration options are described in
882: .Xr ssh_config 5 .
1.2 deraadt 883: .It Pa $HOME/.ssh/authorized_keys
1.115 markus 884: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.40 aaron 885: The format of this file is described in the
1.2 deraadt 886: .Xr sshd 8
1.40 aaron 887: manual page.
1.176 jmc 888: In the simplest form the format is the same as the
889: .Pa .pub
1.116 markus 890: identity files.
1.48 markus 891: This file is not highly sensitive, but the recommended
892: permissions are read/write for the user, and not accessible by others.
1.147 deraadt 893: .It Pa /etc/ssh/ssh_known_hosts
1.40 aaron 894: Systemwide list of known host keys.
1.116 markus 895: This file should be prepared by the
1.1 deraadt 896: system administrator to contain the public host keys of all machines in the
1.40 aaron 897: organization.
898: This file should be world-readable.
899: This file contains
1.1 deraadt 900: public keys, one per line, in the following format (fields separated
1.116 markus 901: by spaces): system name, public key and optional comment field.
1.40 aaron 902: When different names are used
1.1 deraadt 903: for the same machine, all such names should be listed, separated by
1.40 aaron 904: commas.
1.176 jmc 905: The format is described in the
1.2 deraadt 906: .Xr sshd 8
1.1 deraadt 907: manual page.
1.2 deraadt 908: .Pp
1.1 deraadt 909: The canonical system name (as returned by name servers) is used by
1.2 deraadt 910: .Xr sshd 8
1.1 deraadt 911: to verify the client host when logging in; other names are needed because
1.2 deraadt 912: .Nm
1.1 deraadt 913: does not convert the user-supplied name to a canonical name before
914: checking the key, because someone with access to the name servers
915: would then be able to fool host authentication.
1.147 deraadt 916: .It Pa /etc/ssh/ssh_config
1.40 aaron 917: Systemwide configuration file.
1.158 stevesk 918: The file format and configuration options are described in
919: .Xr ssh_config 5 .
1.147 deraadt 920: .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1.141 markus 921: These three files contain the private parts of the host keys
922: and are used for
923: .Cm RhostsRSAAuthentication
924: and
925: .Cm HostbasedAuthentication .
1.155 stevesk 926: If the protocol version 1
927: .Cm RhostsRSAAuthentication
1.157 deraadt 928: method is used,
1.155 stevesk 929: .Nm
930: must be setuid root, since the host key is readable only by root.
931: For protocol version 2,
932: .Nm
933: uses
934: .Xr ssh-keysign 8
935: to access the host keys for
936: .Cm HostbasedAuthentication .
937: This eliminates the requirement that
938: .Nm
939: be setuid root when that authentication method is used.
940: By default
1.141 markus 941: .Nm
1.155 stevesk 942: is not setuid root.
1.2 deraadt 943: .It Pa $HOME/.rhosts
944: This file is used in
1.176 jmc 945: .Em rhosts
1.2 deraadt 946: authentication to list the
1.40 aaron 947: host/user pairs that are permitted to log in.
948: (Note that this file is
1.1 deraadt 949: also used by rlogin and rsh, which makes using this file insecure.)
950: Each line of the file contains a host name (in the canonical form
951: returned by name servers), and then a user name on that host,
1.40 aaron 952: separated by a space.
1.92 markus 953: On some machines this file may need to be
1.1 deraadt 954: world-readable if the user's home directory is on a NFS partition,
955: because
1.2 deraadt 956: .Xr sshd 8
1.40 aaron 957: reads it as root.
958: Additionally, this file must be owned by the user,
959: and must not have write permissions for anyone else.
960: The recommended
1.1 deraadt 961: permission for most machines is read/write for the user, and not
962: accessible by others.
1.2 deraadt 963: .Pp
1.1 deraadt 964: Note that by default
1.2 deraadt 965: .Xr sshd 8
1.1 deraadt 966: will be installed so that it requires successful RSA host
1.176 jmc 967: authentication before permitting
968: .Em rhosts
969: authentication.
1.137 deraadt 970: If the server machine does not have the client's host key in
1.147 deraadt 971: .Pa /etc/ssh/ssh_known_hosts ,
1.137 deraadt 972: it can be stored in
1.2 deraadt 973: .Pa $HOME/.ssh/known_hosts .
974: The easiest way to do this is to
1.1 deraadt 975: connect back to the client from the server machine using ssh; this
1.48 markus 976: will automatically add the host key to
1.2 deraadt 977: .Pa $HOME/.ssh/known_hosts .
978: .It Pa $HOME/.shosts
979: This file is used exactly the same way as
1.176 jmc 980: .Pa .rhosts .
1.2 deraadt 981: The purpose for
1.1 deraadt 982: having this file is to be able to use rhosts authentication with
1.2 deraadt 983: .Nm
984: without permitting login with
1.176 jmc 985: .Xr rlogin
1.2 deraadt 986: or
987: .Xr rsh 1 .
988: .It Pa /etc/hosts.equiv
989: This file is used during
1.176 jmc 990: .Em rhosts
1.173 jmc 991: authentication.
1.40 aaron 992: It contains
1.176 jmc 993: canonical hosts names, one per line (the full format is described in the
1.2 deraadt 994: .Xr sshd 8
1.40 aaron 995: manual page).
996: If the client host is found in this file, login is
1.1 deraadt 997: automatically permitted provided client and server user names are the
1.40 aaron 998: same.
999: Additionally, successful RSA host authentication is normally
1000: required.
1001: This file should only be writable by root.
1.2 deraadt 1002: .It Pa /etc/shosts.equiv
1.44 aaron 1003: This file is processed exactly as
1.2 deraadt 1004: .Pa /etc/hosts.equiv .
1.1 deraadt 1005: This file may be useful to permit logins using
1.2 deraadt 1006: .Nm
1.1 deraadt 1007: but not using rsh/rlogin.
1.147 deraadt 1008: .It Pa /etc/ssh/sshrc
1.1 deraadt 1009: Commands in this file are executed by
1.2 deraadt 1010: .Nm
1.1 deraadt 1011: when the user logs in just before the user's shell (or command) is started.
1012: See the
1.2 deraadt 1013: .Xr sshd 8
1.1 deraadt 1014: manual page for more information.
1.2 deraadt 1015: .It Pa $HOME/.ssh/rc
1.1 deraadt 1016: Commands in this file are executed by
1.2 deraadt 1017: .Nm
1.1 deraadt 1018: when the user logs in just before the user's shell (or command) is
1019: started.
1.44 aaron 1020: See the
1.2 deraadt 1021: .Xr sshd 8
1.1 deraadt 1022: manual page for more information.
1.31 markus 1023: .It Pa $HOME/.ssh/environment
1024: Contains additional definitions for environment variables, see section
1025: .Sx ENVIRONMENT
1026: above.
1.58 itojun 1027: .El
1.145 markus 1028: .Sh DIAGNOSTICS
1029: .Nm
1030: exits with the exit status of the remote command or with 255
1031: if an error occurred.
1.2 deraadt 1032: .Sh SEE ALSO
1.176 jmc 1033: .Xr gzip 1 ,
1.2 deraadt 1034: .Xr rsh 1 ,
1035: .Xr scp 1 ,
1.83 djm 1036: .Xr sftp 1 ,
1.2 deraadt 1037: .Xr ssh-add 1 ,
1038: .Xr ssh-agent 1 ,
1039: .Xr ssh-keygen 1 ,
1040: .Xr telnet 1 ,
1.176 jmc 1041: .Xr hosts.equiv 5 ,
1.159 stevesk 1042: .Xr ssh_config 5 ,
1.160 naddy 1043: .Xr ssh-keysign 8 ,
1.87 itojun 1044: .Xr sshd 8
1.106 markus 1045: .Rs
1046: .%A T. Ylonen
1047: .%A T. Kivinen
1048: .%A M. Saarinen
1049: .%A T. Rinne
1050: .%A S. Lehtinen
1051: .%T "SSH Protocol Architecture"
1.150 markus 1052: .%N draft-ietf-secsh-architecture-12.txt
1053: .%D January 2002
1.106 markus 1054: .%O work in progress material
1055: .Re
1.173 jmc 1056: .Sh AUTHORS
1057: OpenSSH is a derivative of the original and free
1058: ssh 1.2.12 release by Tatu Ylonen.
1059: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1060: Theo de Raadt and Dug Song
1061: removed many bugs, re-added newer features and
1062: created OpenSSH.
1063: Markus Friedl contributed the support for SSH
1064: protocol versions 1.5 and 2.0.