Annotation of src/usr.bin/ssh/ssh.1, Revision 1.90
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: .\"
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.
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: .\"
1.90 ! markus 37: .\" $OpenBSD: ssh.1,v 1.89 2001/02/10 15:48:53 stevesk Exp $
1.2 deraadt 38: .Dd September 25, 1999
39: .Dt SSH 1
40: .Os
41: .Sh NAME
42: .Nm ssh
1.20 provos 43: .Nd OpenSSH secure shell client (remote login program)
1.2 deraadt 44: .Sh SYNOPSIS
45: .Nm ssh
46: .Op Fl l Ar login_name
1.5 deraadt 47: .Op Ar hostname | user@hostname
1.2 deraadt 48: .Op Ar command
49: .Pp
50: .Nm ssh
1.86 jakob 51: .Op Fl afgknqstvxACNPTX1246
1.51 markus 52: .Op Fl c Ar cipher_spec
1.2 deraadt 53: .Op Fl e Ar escape_char
54: .Op Fl i Ar identity_file
55: .Op Fl l Ar login_name
56: .Op Fl o Ar option
57: .Op Fl p Ar port
1.12 aaron 58: .Oo Fl L Xo
59: .Sm off
1.33 markus 60: .Ar port :
1.12 aaron 61: .Ar host :
62: .Ar hostport
63: .Sm on
64: .Xc
65: .Oc
66: .Oo Fl R Xo
67: .Sm off
1.33 markus 68: .Ar port :
1.12 aaron 69: .Ar host :
70: .Ar hostport
71: .Sm on
72: .Xc
73: .Oc
1.5 deraadt 74: .Op Ar hostname | user@hostname
1.2 deraadt 75: .Op Ar command
1.44 aaron 76: .Sh DESCRIPTION
1.2 deraadt 77: .Nm
1.5 deraadt 78: (Secure Shell) is a program for logging into a remote machine and for
1.40 aaron 79: executing commands on a remote machine.
80: It is intended to replace
1.1 deraadt 81: rlogin and rsh, and provide secure encrypted communications between
1.40 aaron 82: two untrusted hosts over an insecure network.
83: X11 connections and
1.1 deraadt 84: arbitrary TCP/IP ports can also be forwarded over the secure channel.
1.2 deraadt 85: .Pp
86: .Nm
1.44 aaron 87: connects and logs into the specified
1.2 deraadt 88: .Ar hostname .
1.1 deraadt 89: The user must prove
1.49 markus 90: his/her identity to the remote machine using one of several methods
91: depending on the protocol version used:
92: .Pp
93: .Ss SSH protocol version 1
1.2 deraadt 94: .Pp
1.1 deraadt 95: First, if the machine the user logs in from is listed in
1.2 deraadt 96: .Pa /etc/hosts.equiv
1.1 deraadt 97: or
1.2 deraadt 98: .Pa /etc/shosts.equiv
1.1 deraadt 99: on the remote machine, and the user names are
100: the same on both sides, the user is immediately permitted to log in.
1.44 aaron 101: Second, if
1.2 deraadt 102: .Pa \&.rhosts
1.1 deraadt 103: or
1.2 deraadt 104: .Pa \&.shosts
1.1 deraadt 105: exists in the user's home directory on the
106: remote machine and contains a line containing the name of the client
107: machine and the name of the user on that machine, the user is
1.40 aaron 108: permitted to log in.
109: This form of authentication alone is normally not
1.1 deraadt 110: allowed by the server because it is not secure.
1.2 deraadt 111: .Pp
1.1 deraadt 112: The second (and primary) authentication method is the
1.2 deraadt 113: .Pa rhosts
1.1 deraadt 114: or
1.2 deraadt 115: .Pa hosts.equiv
1.40 aaron 116: method combined with RSA-based host authentication.
117: It means that if the login would be permitted by
1.49 markus 118: .Pa $HOME/.rhosts ,
119: .Pa $HOME/.shosts ,
1.2 deraadt 120: .Pa /etc/hosts.equiv ,
1.1 deraadt 121: or
1.2 deraadt 122: .Pa /etc/shosts.equiv ,
1.11 deraadt 123: and if additionally the server can verify the client's
1.44 aaron 124: host key (see
1.2 deraadt 125: .Pa /etc/ssh_known_hosts
1.23 markus 126: and
127: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 128: in the
1.2 deraadt 129: .Sx FILES
1.40 aaron 130: section), only then login is permitted.
131: This authentication method closes security holes due to IP
132: spoofing, DNS spoofing and routing spoofing.
133: [Note to the administrator:
1.2 deraadt 134: .Pa /etc/hosts.equiv ,
1.49 markus 135: .Pa $HOME/.rhosts ,
1.1 deraadt 136: and the rlogin/rsh protocol in general, are inherently insecure and should be
137: disabled if security is desired.]
1.2 deraadt 138: .Pp
1.44 aaron 139: As a third authentication method,
1.2 deraadt 140: .Nm
1.1 deraadt 141: supports RSA based authentication.
142: The scheme is based on public-key cryptography: there are cryptosystems
143: where encryption and decryption are done using separate keys, and it
144: is not possible to derive the decryption key from the encryption key.
1.40 aaron 145: RSA is one such system.
1.44 aaron 146: The idea is that each user creates a public/private
1.40 aaron 147: key pair for authentication purposes.
148: The server knows the public key, and only the user knows the private key.
1.44 aaron 149: The file
1.2 deraadt 150: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 151: lists the public keys that are permitted for logging
1.40 aaron 152: in.
153: When the user logs in, the
1.2 deraadt 154: .Nm
1.1 deraadt 155: program tells the server which key pair it would like to use for
1.40 aaron 156: authentication.
157: The server checks if this key is permitted, and if
1.1 deraadt 158: so, sends the user (actually the
1.2 deraadt 159: .Nm
1.1 deraadt 160: program running on behalf of the user) a challenge, a random number,
1.40 aaron 161: encrypted by the user's public key.
162: The challenge can only be
163: decrypted using the proper private key.
164: The user's client then decrypts the
1.1 deraadt 165: challenge using the private key, proving that he/she knows the private
166: key but without disclosing it to the server.
1.2 deraadt 167: .Pp
168: .Nm
1.40 aaron 169: implements the RSA authentication protocol automatically.
170: The user creates his/her RSA key pair by running
1.2 deraadt 171: .Xr ssh-keygen 1 .
1.44 aaron 172: This stores the private key in
1.49 markus 173: .Pa $HOME/.ssh/identity
1.1 deraadt 174: and the public key in
1.49 markus 175: .Pa $HOME/.ssh/identity.pub
1.40 aaron 176: in the user's home directory.
177: The user should then copy the
1.2 deraadt 178: .Pa identity.pub
1.44 aaron 179: to
1.49 markus 180: .Pa $HOME/.ssh/authorized_keys
1.44 aaron 181: in his/her home directory on the remote machine (the
1.2 deraadt 182: .Pa authorized_keys
1.44 aaron 183: file corresponds to the conventional
1.49 markus 184: .Pa $HOME/.rhosts
1.1 deraadt 185: file, and has one key
1.40 aaron 186: per line, though the lines can be very long).
187: After this, the user can log in without giving the password.
188: RSA authentication is much
1.1 deraadt 189: more secure than rhosts authentication.
1.2 deraadt 190: .Pp
1.1 deraadt 191: The most convenient way to use RSA authentication may be with an
1.40 aaron 192: authentication agent.
193: See
1.2 deraadt 194: .Xr ssh-agent 1
1.1 deraadt 195: for more information.
1.2 deraadt 196: .Pp
1.44 aaron 197: If other authentication methods fail,
1.2 deraadt 198: .Nm
1.40 aaron 199: prompts the user for a password.
200: The password is sent to the remote
1.1 deraadt 201: host for checking; however, since all communications are encrypted,
202: the password cannot be seen by someone listening on the network.
1.2 deraadt 203: .Pp
1.49 markus 204: .Ss SSH protocol version 2
205: .Pp
206: When a user connects using the protocol version 2
207: different authentication methods are available:
208: At first, the client attempts to authenticate using the public key method.
209: If this method fails password authentication is tried.
210: .Pp
211: The public key method is similar to RSA authentication described
1.68 markus 212: in the previous section except that the DSA or RSA algorithm is used
213: instead.
214: The client uses his private key
1.49 markus 215: .Pa $HOME/.ssh/id_dsa
216: to sign the session identifier and sends the result to the server.
217: The server checks whether the matching public key is listed in
218: .Pa $HOME/.ssh/authorized_keys2
219: and grants access if both the key is found and the signature is correct.
220: The session identifier is derived from a shared Diffie-Hellman value
221: and is only known to the client and the server.
222: .Pp
223: If public key authentication fails or is not available a password
224: can be sent encrypted to the remote host for proving the user's identity.
225: This protocol 2 implementation does not yet support Kerberos or
226: S/Key authentication.
227: .Pp
228: Protocol 2 provides additional mechanisms for confidentiality
1.51 markus 229: (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
1.49 markus 230: and integrity (hmac-sha1, hmac-md5).
231: Note that protocol 1 lacks a strong mechanism for ensuring the
232: integrity of the connection.
233: .Pp
234: .Ss Login session and remote execution
235: .Pp
1.1 deraadt 236: When the user's identity has been accepted by the server, the server
237: either executes the given command, or logs into the machine and gives
1.40 aaron 238: the user a normal shell on the remote machine.
239: All communication with
1.1 deraadt 240: the remote command or shell will be automatically encrypted.
1.2 deraadt 241: .Pp
1.1 deraadt 242: If a pseudo-terminal has been allocated (normal login session), the
1.2 deraadt 243: user can disconnect with
244: .Ic ~. ,
245: and suspend
246: .Nm
247: with
248: .Ic ~^Z .
249: All forwarded connections can be listed with
1.44 aaron 250: .Ic ~#
1.2 deraadt 251: and if
1.1 deraadt 252: the session blocks waiting for forwarded X11 or TCP/IP
1.2 deraadt 253: connections to terminate, it can be backgrounded with
254: .Ic ~&
255: (this should not be used while the user shell is active, as it can cause the
1.40 aaron 256: shell to hang).
257: All available escapes can be listed with
1.2 deraadt 258: .Ic ~? .
259: .Pp
260: A single tilde character can be sent as
261: .Ic ~~
262: (or by following the tilde by a character other than those described above).
1.1 deraadt 263: The escape character must always follow a newline to be interpreted as
1.40 aaron 264: special.
265: The escape character can be changed in configuration files
266: or on the command line.
1.2 deraadt 267: .Pp
1.1 deraadt 268: If no pseudo tty has been allocated, the
269: session is transparent and can be used to reliably transfer binary
1.40 aaron 270: data.
271: On most systems, setting the escape character to
1.2 deraadt 272: .Dq none
273: will also make the session transparent even if a tty is used.
274: .Pp
1.71 djm 275: The session terminates when the command or shell on the remote
1.1 deraadt 276: machine exists and all X11 and TCP/IP connections have been closed.
277: The exit status of the remote program is returned as the exit status
278: of
1.2 deraadt 279: .Nm ssh .
280: .Pp
1.49 markus 281: .Ss X11 and TCP forwarding
282: .Pp
1.1 deraadt 283: If the user is using X11 (the
1.2 deraadt 284: .Ev DISPLAY
1.1 deraadt 285: environment variable is set), the connection to the X11 display is
286: automatically forwarded to the remote side in such a way that any X11
287: programs started from the shell (or command) will go through the
288: encrypted channel, and the connection to the real X server will be made
1.40 aaron 289: from the local machine.
290: The user should not manually set
1.2 deraadt 291: .Ev DISPLAY .
1.1 deraadt 292: Forwarding of X11 connections can be
293: configured on the command line or in configuration files.
1.2 deraadt 294: .Pp
295: The
1.44 aaron 296: .Ev DISPLAY
1.2 deraadt 297: value set by
298: .Nm
1.1 deraadt 299: will point to the server machine, but with a display number greater
1.40 aaron 300: than zero.
301: This is normal, and happens because
1.2 deraadt 302: .Nm
303: creates a
304: .Dq proxy
305: X server on the server machine for forwarding the
1.1 deraadt 306: connections over the encrypted channel.
1.2 deraadt 307: .Pp
308: .Nm
1.1 deraadt 309: will also automatically set up Xauthority data on the server machine.
310: For this purpose, it will generate a random authorization cookie,
311: store it in Xauthority on the server, and verify that any forwarded
312: connections carry this cookie and replace it by the real cookie when
1.40 aaron 313: the connection is opened.
314: The real authentication cookie is never
1.1 deraadt 315: sent to the server machine (and no cookies are sent in the plain).
1.2 deraadt 316: .Pp
1.1 deraadt 317: If the user is using an authentication agent, the connection to the agent
318: is automatically forwarded to the remote side unless disabled on
319: command line or in a configuration file.
1.2 deraadt 320: .Pp
1.1 deraadt 321: Forwarding of arbitrary TCP/IP connections over the secure channel can
1.40 aaron 322: be specified either on command line or in a configuration file.
323: One possible application of TCP/IP forwarding is a secure connection to an
1.1 deraadt 324: electronic purse; another is going trough firewalls.
1.2 deraadt 325: .Pp
1.49 markus 326: .Ss Server authentication
327: .Pp
1.2 deraadt 328: .Nm
1.49 markus 329: automatically maintains and checks a database containing
1.40 aaron 330: identifications for all hosts it has ever been used with.
1.49 markus 331: RSA host keys are stored in
332: .Pa $HOME/.ssh/known_hosts
333: and
1.68 markus 334: host keys used in the protocol version 2 are stored in
1.49 markus 335: .Pa $HOME/.ssh/known_hosts2
1.40 aaron 336: in the user's home directory.
1.49 markus 337: Additionally, the files
1.2 deraadt 338: .Pa /etc/ssh_known_hosts
1.49 markus 339: and
340: .Pa /etc/ssh_known_hosts2
341: are automatically checked for known hosts.
1.40 aaron 342: Any new hosts are automatically added to the user's file.
343: If a host's identification
1.1 deraadt 344: ever changes,
1.2 deraadt 345: .Nm
1.1 deraadt 346: warns about this and disables password authentication to prevent a
1.40 aaron 347: trojan horse from getting the user's password.
348: Another purpose of
1.1 deraadt 349: this mechanism is to prevent man-in-the-middle attacks which could
1.40 aaron 350: otherwise be used to circumvent the encryption.
351: The
1.2 deraadt 352: .Cm StrictHostKeyChecking
1.1 deraadt 353: option (see below) can be used to prevent logins to machines whose
354: host key is not known or has changed.
1.65 aaron 355: .Pp
356: The options are as follows:
1.2 deraadt 357: .Bl -tag -width Ds
1.4 dugsong 358: .It Fl a
1.42 aaron 359: Disables forwarding of the authentication agent connection.
1.54 markus 360: .It Fl A
361: Enables forwarding of the authentication agent connection.
362: This can also be specified on a per-host basis in a configuration file.
1.6 deraadt 363: .It Fl c Ar blowfish|3des
1.44 aaron 364: Selects the cipher to use for encrypting the session.
1.2 deraadt 365: .Ar 3des
1.40 aaron 366: is used by default.
1.44 aaron 367: It is believed to be secure.
1.5 deraadt 368: .Ar 3des
369: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
370: It is presumably more secure than the
1.2 deraadt 371: .Ar des
1.64 markus 372: cipher which is no longer fully supported in
1.51 markus 373: .Nm ssh .
1.5 deraadt 374: .Ar blowfish
375: is a fast block cipher, it appears very secure and is much faster than
1.40 aaron 376: .Ar 3des .
1.90 ! markus 377: .It Fl c Ar cipher_spec
1.51 markus 378: Additionally, for protocol version 2 a comma-separated list of ciphers can
1.61 aaron 379: be specified in order of preference.
1.90 ! markus 380: See
! 381: .Cm Ciphers
! 382: for more information.
1.2 deraadt 383: .It Fl e Ar ch|^ch|none
384: Sets the escape character for sessions with a pty (default:
385: .Ql ~ ) .
1.40 aaron 386: The escape character is only recognized at the beginning of a line.
387: The escape character followed by a dot
1.2 deraadt 388: .Pq Ql \&.
389: closes the connection, followed
1.1 deraadt 390: by control-Z suspends the connection, and followed by itself sends the
1.40 aaron 391: escape character once.
392: Setting the character to
1.2 deraadt 393: .Dq none
394: disables any escapes and makes the session fully transparent.
395: .It Fl f
396: Requests
397: .Nm
1.40 aaron 398: to go to background just before command execution.
399: This is useful if
1.2 deraadt 400: .Nm
401: is going to ask for passwords or passphrases, but the user
1.40 aaron 402: wants it in the background.
1.44 aaron 403: This implies
1.2 deraadt 404: .Fl n .
1.1 deraadt 405: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 406: something like
407: .Ic ssh -f host xterm .
1.34 markus 408: .It Fl g
409: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 410: .It Fl i Ar identity_file
1.44 aaron 411: Selects the file from which the identity (private key) for
1.68 markus 412: RSA or DSA authentication is read.
1.44 aaron 413: Default is
1.49 markus 414: .Pa $HOME/.ssh/identity
1.40 aaron 415: in the user's home directory.
416: Identity files may also be specified on
417: a per-host basis in the configuration file.
418: It is possible to have multiple
1.2 deraadt 419: .Fl i
420: options (and multiple identities specified in
1.1 deraadt 421: configuration files).
1.2 deraadt 422: .It Fl k
1.42 aaron 423: Disables forwarding of Kerberos tickets and AFS tokens.
424: This may also be specified on a per-host basis in the configuration file.
1.2 deraadt 425: .It Fl l Ar login_name
1.40 aaron 426: Specifies the user to log in as on the remote machine.
427: This also may be specified on a per-host basis in the configuration file.
1.2 deraadt 428: .It Fl n
429: Redirects stdin from
430: .Pa /dev/null
431: (actually, prevents reading from stdin).
1.1 deraadt 432: This must be used when
1.2 deraadt 433: .Nm
1.40 aaron 434: is run in the background.
435: A common trick is to use this to run X11 programs on a remote machine.
436: For example,
1.2 deraadt 437: .Ic ssh -n shadows.cs.hut.fi emacs &
438: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 439: connection will be automatically forwarded over an encrypted channel.
440: The
1.2 deraadt 441: .Nm
1.1 deraadt 442: program will be put in the background.
443: (This does not work if
1.2 deraadt 444: .Nm
445: needs to ask for a password or passphrase; see also the
446: .Fl f
447: option.)
1.53 markus 448: .It Fl N
449: Do not execute a remote command.
1.70 markus 450: This is useful if you just want to forward ports
1.53 markus 451: (protocol version 2 only).
1.2 deraadt 452: .It Fl o Ar option
1.1 deraadt 453: Can be used to give options in the format used in the config file.
454: This is useful for specifying options for which there is no separate
1.40 aaron 455: command-line flag.
456: The option has the same format as a line in the configuration file.
1.2 deraadt 457: .It Fl p Ar port
1.40 aaron 458: Port to connect to on the remote host.
459: This can be specified on a
1.1 deraadt 460: per-host basis in the configuration file.
1.16 markus 461: .It Fl P
462: Use a non-privileged port for outgoing connections.
463: This can be used if your firewall does
464: not permit connections from privileged ports.
1.30 provos 465: Note that this option turns off
1.16 markus 466: .Cm RhostsAuthentication
467: and
1.72 markus 468: .Cm RhostsRSAAuthentication
469: for older servers.
1.2 deraadt 470: .It Fl q
1.40 aaron 471: Quiet mode.
472: Causes all warning and diagnostic messages to be suppressed.
473: Only fatal errors are displayed.
1.80 djm 474: .It Fl s
475: 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
476: of SSH as a secure transport for other application (eg. sftp). The
477: subsystem is specified as the remote command.
1.2 deraadt 478: .It Fl t
1.40 aaron 479: Force pseudo-tty allocation.
1.43 brad 480: This can be used to execute arbitrary
1.40 aaron 481: screen-based programs on a remote machine, which can be very useful,
482: e.g., when implementing menu services.
1.73 markus 483: Multiple
484: .Fl t
485: options force tty allocation, even if
486: .Nm
487: has no local tty.
1.53 markus 488: .It Fl T
1.69 markus 489: Disable pseudo-tty allocation.
1.2 deraadt 490: .It Fl v
1.40 aaron 491: Verbose mode.
492: Causes
1.2 deraadt 493: .Nm
1.40 aaron 494: to print debugging messages about its progress.
495: This is helpful in
1.1 deraadt 496: debugging connection, authentication, and configuration problems.
1.73 markus 497: Multiple
498: .Fl v
499: options increases the verbosity.
1.61 aaron 500: Maximum is 3.
1.2 deraadt 501: .It Fl x
1.40 aaron 502: Disables X11 forwarding.
1.2 deraadt 503: .It Fl X
1.1 deraadt 504: Enables X11 forwarding.
1.54 markus 505: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 506: .It Fl C
1.1 deraadt 507: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 508: data for forwarded X11 and TCP/IP connections).
509: The compression algorithm is the same used by
1.34 markus 510: .Xr gzip 1 ,
511: and the
1.2 deraadt 512: .Dq level
513: can be controlled by the
514: .Cm CompressionLevel
1.40 aaron 515: option (see below).
516: Compression is desirable on modem lines and other
1.1 deraadt 517: slow connections, but will only slow down things on fast networks.
518: The default value can be set on a host-by-host basis in the
519: configuration files; see the
1.2 deraadt 520: .Cm Compress
1.1 deraadt 521: option below.
1.2 deraadt 522: .It Fl L Ar port:host:hostport
1.1 deraadt 523: Specifies that the given port on the local (client) host is to be
1.40 aaron 524: forwarded to the given host and port on the remote side.
525: This works by allocating a socket to listen to
1.2 deraadt 526: .Ar port
1.1 deraadt 527: on the local side, and whenever a connection is made to this port, the
528: connection is forwarded over the secure channel, and a connection is
529: made to
1.32 markus 530: .Ar host
531: port
532: .Ar hostport
1.40 aaron 533: from the remote machine.
534: Port forwardings can also be specified in the configuration file.
535: Only root can forward privileged ports.
1.32 markus 536: IPv6 addresses can be specified with an alternative syntax:
537: .Ar port/host/hostport
1.2 deraadt 538: .It Fl R Ar port:host:hostport
1.1 deraadt 539: Specifies that the given port on the remote (server) host is to be
1.40 aaron 540: forwarded to the given host and port on the local side.
541: This works by allocating a socket to listen to
1.2 deraadt 542: .Ar port
1.1 deraadt 543: on the remote side, and whenever a connection is made to this port, the
544: connection is forwarded over the secure channel, and a connection is
545: made to
1.32 markus 546: .Ar host
547: port
548: .Ar hostport
1.40 aaron 549: from the local machine.
550: Port forwardings can also be specified in the configuration file.
551: Privileged ports can be forwarded only when
1.1 deraadt 552: logging in as root on the remote machine.
1.85 jakob 553: .It Fl 1
554: Forces
555: .Nm
556: to try protocol version 1 only.
1.46 markus 557: .It Fl 2
558: Forces
559: .Nm
1.50 markus 560: to try protocol version 2 only.
1.32 markus 561: .It Fl 4
562: Forces
563: .Nm
564: to use IPv4 addresses only.
565: .It Fl 6
566: Forces
567: .Nm
568: to use IPv6 addresses only.
1.2 deraadt 569: .El
570: .Sh CONFIGURATION FILES
571: .Nm
1.1 deraadt 572: obtains configuration data from the following sources (in this order):
573: command line options, user's configuration file
1.2 deraadt 574: .Pq Pa $HOME/.ssh/config ,
575: and system-wide configuration file
576: .Pq Pa /etc/ssh_config .
577: For each parameter, the first obtained value
1.40 aaron 578: will be used.
579: The configuration files contain sections bracketed by
580: .Dq Host
581: specifications, and that section is only applied for hosts that
582: match one of the patterns given in the specification.
583: The matched host name is the one given on the command line.
1.2 deraadt 584: .Pp
1.1 deraadt 585: Since the first obtained value for each parameter is used, more
586: host-specific declarations should be given near the beginning of the
587: file, and general defaults at the end.
1.2 deraadt 588: .Pp
1.1 deraadt 589: The configuration file has the following format:
1.2 deraadt 590: .Pp
591: Empty lines and lines starting with
592: .Ql #
593: are comments.
594: .Pp
595: Otherwise a line is of the format
596: .Dq keyword arguments .
597: The possible
1.1 deraadt 598: keywords and their meanings are as follows (note that the
599: configuration files are case-sensitive):
1.2 deraadt 600: .Bl -tag -width Ds
601: .It Cm Host
1.1 deraadt 602: Restricts the following declarations (up to the next
1.2 deraadt 603: .Cm Host
1.1 deraadt 604: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 605: given after the keyword.
606: .Ql \&*
607: and
608: .Ql ?
609: can be used as wildcards in the
1.40 aaron 610: patterns.
611: A single
1.2 deraadt 612: .Ql \&*
613: as a pattern can be used to provide global
1.40 aaron 614: defaults for all hosts.
615: The host is the
1.2 deraadt 616: .Ar hostname
1.1 deraadt 617: argument given on the command line (i.e., the name is not converted to
618: a canonicalized host name before matching).
1.2 deraadt 619: .It Cm AFSTokenPassing
1.42 aaron 620: Specifies whether to pass AFS tokens to remote host.
621: The argument to this keyword must be
1.2 deraadt 622: .Dq yes
623: or
624: .Dq no .
625: .It Cm BatchMode
626: If set to
627: .Dq yes ,
1.40 aaron 628: passphrase/password querying will be disabled.
629: This option is useful in scripts and other batch jobs where you have no
630: user to supply the password.
631: The argument must be
1.2 deraadt 632: .Dq yes
633: or
634: .Dq no .
1.34 markus 635: .It Cm CheckHostIP
636: If this flag is set to
637: .Dq yes ,
638: ssh will additionally check the host ip address in the
639: .Pa known_hosts
1.42 aaron 640: file.
641: This allows ssh to detect if a host key changed due to DNS spoofing.
1.34 markus 642: If the option is set to
643: .Dq no ,
644: the check will not be executed.
1.2 deraadt 645: .It Cm Cipher
1.62 markus 646: Specifies the cipher to use for encrypting the session
1.64 markus 647: in protocol version 1.
1.40 aaron 648: Currently,
1.64 markus 649: .Dq blowfish
1.1 deraadt 650: and
1.10 provos 651: .Dq 3des
1.40 aaron 652: are supported.
653: The default is
1.2 deraadt 654: .Dq 3des .
1.45 markus 655: .It Cm Ciphers
656: Specifies the ciphers allowed for protocol version 2
657: in order of preference.
658: Multiple ciphers must be comma-separated.
659: The default is
1.88 provos 660: .Pp
661: .Bd -literal
662: ``3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes128-cbc,
663: aes192-cbc,aes256-cbc,rijndael128-cbc,rijndael192-cbc,
664: rijndael256-cbc,rijndael-cbc@lysator.liu.se''
665: .Ed
1.2 deraadt 666: .It Cm Compression
1.40 aaron 667: Specifies whether to use compression.
668: The argument must be
1.2 deraadt 669: .Dq yes
670: or
671: .Dq no .
672: .It Cm CompressionLevel
1.40 aaron 673: Specifies the compression level to use if compression is enable.
674: The argument must be an integer from 1 (fast) to 9 (slow, best).
675: The default level is 6, which is good for most applications.
676: The meaning of the values is the same as in
1.34 markus 677: .Xr gzip 1 .
1.2 deraadt 678: .It Cm ConnectionAttempts
1.1 deraadt 679: Specifies the number of tries (one per second) to make before falling
1.40 aaron 680: back to rsh or exiting.
681: The argument must be an integer.
682: This may be useful in scripts if the connection sometimes fails.
1.68 markus 683: .It Cm PubkeyAuthentication
684: Specifies whether to try public key authentication.
1.50 markus 685: The argument to this keyword must be
686: .Dq yes
687: or
688: .Dq no .
689: Note that this option applies to protocol version 2 only.
1.2 deraadt 690: .It Cm EscapeChar
691: Sets the escape character (default:
692: .Ql ~ ) .
693: The escape character can also
1.40 aaron 694: be set on the command line.
695: The argument should be a single character,
1.2 deraadt 696: .Ql ^
697: followed by a letter, or
698: .Dq none
699: to disable the escape
1.1 deraadt 700: character entirely (making the connection transparent for binary
701: data).
1.44 aaron 702: .It Cm FallBackToRsh
1.1 deraadt 703: Specifies that if connecting via
1.2 deraadt 704: .Nm
1.1 deraadt 705: fails due to a connection refused error (there is no
1.2 deraadt 706: .Xr sshd 8
1.44 aaron 707: listening on the remote host),
1.2 deraadt 708: .Xr rsh 1
1.1 deraadt 709: should automatically be used instead (after a suitable warning about
1.40 aaron 710: the session being unencrypted).
711: The argument must be
1.2 deraadt 712: .Dq yes
713: or
714: .Dq no .
715: .It Cm ForwardAgent
1.1 deraadt 716: Specifies whether the connection to the authentication agent (if any)
1.40 aaron 717: will be forwarded to the remote machine.
718: The argument must be
1.2 deraadt 719: .Dq yes
720: or
1.54 markus 721: .Dq no .
722: The default is
1.2 deraadt 723: .Dq no .
724: .It Cm ForwardX11
1.1 deraadt 725: Specifies whether X11 connections will be automatically redirected
1.44 aaron 726: over the secure channel and
1.2 deraadt 727: .Ev DISPLAY
1.40 aaron 728: set.
1.44 aaron 729: The argument must be
1.2 deraadt 730: .Dq yes
731: or
1.38 markus 732: .Dq no .
733: The default is
1.3 deraadt 734: .Dq no .
735: .It Cm GatewayPorts
736: Specifies whether remote hosts are allowed to connect to local
737: forwarded ports.
738: The argument must be
739: .Dq yes
740: or
741: .Dq no .
742: The default is
1.2 deraadt 743: .Dq no .
744: .It Cm GlobalKnownHostsFile
1.44 aaron 745: Specifies a file to use instead of
1.2 deraadt 746: .Pa /etc/ssh_known_hosts .
1.74 markus 747: .It Cm HostKeyAlias
748: Specifies an alias that should be used instead of the
749: real host name when looking up or saving the host key
1.82 stevesk 750: in the known_hosts files.
751: This option is useful for tunneling ssh connections
1.74 markus 752: or if you have multiple servers running on a single host.
1.2 deraadt 753: .It Cm HostName
1.40 aaron 754: Specifies the real host name to log into.
755: This can be used to specify nicknames or abbreviations for hosts.
756: Default is the name given on the command line.
757: Numeric IP addresses are also permitted (both on the command line and in
1.2 deraadt 758: .Cm HostName
1.1 deraadt 759: specifications).
1.2 deraadt 760: .It Cm IdentityFile
1.1 deraadt 761: Specifies the file from which the user's RSA authentication identity
1.2 deraadt 762: is read (default
1.49 markus 763: .Pa $HOME/.ssh/identity
1.2 deraadt 764: in the user's home directory).
1.1 deraadt 765: Additionally, any identities represented by the authentication agent
1.40 aaron 766: will be used for authentication.
767: The file name may use the tilde
768: syntax to refer to a user's home directory.
769: It is possible to have
1.1 deraadt 770: multiple identity files specified in configuration files; all these
771: identities will be tried in sequence.
1.2 deraadt 772: .It Cm KeepAlive
1.1 deraadt 773: Specifies whether the system should send keepalive messages to the
1.40 aaron 774: other side.
775: If they are sent, death of the connection or crash of one
776: of the machines will be properly noticed.
777: However, this means that
1.1 deraadt 778: connections will die if the route is down temporarily, and some people
1.41 aaron 779: find it annoying.
1.2 deraadt 780: .Pp
781: The default is
782: .Dq yes
783: (to send keepalives), and the client will notice
1.40 aaron 784: if the network goes down or the remote host dies.
785: This is important in scripts, and many users want it too.
1.2 deraadt 786: .Pp
787: To disable keepalives, the value should be set to
788: .Dq no
789: in both the server and the client configuration files.
790: .It Cm KerberosAuthentication
1.42 aaron 791: Specifies whether Kerberos authentication will be used.
792: The argument to this keyword must be
1.4 dugsong 793: .Dq yes
794: or
795: .Dq no .
1.2 deraadt 796: .It Cm KerberosTgtPassing
1.42 aaron 797: Specifies whether a Kerberos TGT will be forwarded to the server.
798: This will only work if the Kerberos server is actually an AFS kaserver.
799: The argument to this keyword must be
1.4 dugsong 800: .Dq yes
801: or
802: .Dq no .
1.2 deraadt 803: .It Cm LocalForward
1.1 deraadt 804: Specifies that a TCP/IP port on the local machine be forwarded over
1.40 aaron 805: the secure channel to given host:port from the remote machine.
806: The first argument must be a port number, and the second must be
807: host:port.
808: Multiple forwardings may be specified, and additional
809: forwardings can be given on the command line.
810: Only the superuser can forward privileged ports.
1.24 markus 811: .It Cm LogLevel
812: Gives the verbosity level that is used when logging messages from
813: .Nm ssh .
814: The possible values are:
1.77 markus 815: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
816: The default is INFO.
1.14 dugsong 817: .It Cm NumberOfPasswordPrompts
1.42 aaron 818: Specifies the number of password prompts before giving up.
819: The argument to this keyword must be an integer.
820: Default is 3.
1.34 markus 821: .It Cm PasswordAuthentication
1.40 aaron 822: Specifies whether to use password authentication.
823: The argument to this keyword must be
1.34 markus 824: .Dq yes
825: or
826: .Dq no .
1.50 markus 827: Note that this option applies to both protocol version 1 and 2.
1.2 deraadt 828: .It Cm Port
1.40 aaron 829: Specifies the port number to connect on the remote host.
830: Default is 22.
1.45 markus 831: .It Cm Protocol
832: Specifies the protocol versions
833: .Nm
834: should support in order of preference.
835: The possible values are
836: .Dq 1
837: and
838: .Dq 2 .
839: Multiple versions must be comma-separated.
840: The default is
1.49 markus 841: .Dq 1,2 .
842: This means that
843: .Nm
844: tries version 1 and falls back to version 2
1.52 hugh 845: if version 1 is not available.
1.2 deraadt 846: .It Cm ProxyCommand
1.40 aaron 847: Specifies the command to use to connect to the server.
848: The command
849: string extends to the end of the line, and is executed with
850: .Pa /bin/sh .
851: In the command string,
852: .Ql %h
853: will be substituted by the host name to
854: connect and
855: .Ql %p
856: by the port.
857: The command can be basically anything,
858: and should read from its standard input and write to its standard output.
859: It should eventually connect an
1.2 deraadt 860: .Xr sshd 8
1.1 deraadt 861: server running on some machine, or execute
1.2 deraadt 862: .Ic sshd -i
1.40 aaron 863: somewhere.
864: Host key management will be done using the
1.1 deraadt 865: HostName of the host being connected (defaulting to the name typed by
866: the user).
1.29 markus 867: Note that
868: .Cm CheckHostIP
869: is not available for connects with a proxy command.
1.2 deraadt 870: .Pp
871: .It Cm RemoteForward
1.1 deraadt 872: Specifies that a TCP/IP port on the remote machine be forwarded over
1.40 aaron 873: the secure channel to given host:port from the local machine.
874: The first argument must be a port number, and the second must be
875: host:port.
876: Multiple forwardings may be specified, and additional
877: forwardings can be given on the command line.
878: Only the superuser can forward privileged ports.
1.2 deraadt 879: .It Cm RhostsAuthentication
1.40 aaron 880: Specifies whether to try rhosts based authentication.
881: Note that this
1.1 deraadt 882: declaration only affects the client side and has no effect whatsoever
1.40 aaron 883: on security.
884: Disabling rhosts authentication may reduce
1.1 deraadt 885: authentication time on slow connections when rhosts authentication is
1.40 aaron 886: not used.
887: Most servers do not permit RhostsAuthentication because it
888: is not secure (see RhostsRSAAuthentication).
889: The argument to this keyword must be
1.2 deraadt 890: .Dq yes
891: or
892: .Dq no .
893: .It Cm RhostsRSAAuthentication
1.1 deraadt 894: Specifies whether to try rhosts based authentication with RSA host
1.40 aaron 895: authentication.
896: This is the primary authentication method for most sites.
897: The argument must be
1.2 deraadt 898: .Dq yes
899: or
900: .Dq no .
901: .It Cm RSAAuthentication
1.40 aaron 902: Specifies whether to try RSA authentication.
903: The argument to this keyword must be
1.2 deraadt 904: .Dq yes
905: or
906: .Dq no .
1.1 deraadt 907: RSA authentication will only be
908: attempted if the identity file exists, or an authentication agent is
909: running.
1.50 markus 910: Note that this option applies to protocol version 1 only.
1.81 markus 911: .It Cm ChallengeResponseAuthentication
912: Specifies whether to use challenge response authentication.
913: Currently there is only support for
1.27 markus 914: .Xr skey 1
1.40 aaron 915: authentication.
916: The argument to this keyword must be
1.27 markus 917: .Dq yes
918: or
919: .Dq no .
920: The default is
921: .Dq no .
1.2 deraadt 922: .It Cm StrictHostKeyChecking
923: If this flag is set to
1.44 aaron 924: .Dq yes ,
1.2 deraadt 925: .Nm
1.79 stevesk 926: will never automatically add host keys to the
1.2 deraadt 927: .Pa $HOME/.ssh/known_hosts
1.48 markus 928: and
929: .Pa $HOME/.ssh/known_hosts2
1.79 stevesk 930: files, and refuses to connect to hosts whose host key has changed.
1.40 aaron 931: This provides maximum protection against trojan horse attacks.
932: However, it can be somewhat annoying if you don't have good
1.2 deraadt 933: .Pa /etc/ssh_known_hosts
1.48 markus 934: and
935: .Pa /etc/ssh_known_hosts2
1.1 deraadt 936: files installed and frequently
1.79 stevesk 937: connect to new hosts.
938: This option forces the user to manually
939: add all new hosts.
940: If this flag is set to
941: .Dq no ,
942: .Nm
943: will automatically add new host keys to the
944: user known hosts files.
945: If this flag is set to
946: .Dq ask ,
947: new host keys
948: will be added to the user known host files only after the user
949: has confirmed that is what they really want to do, and
950: .Nm
951: will refuse to connect to hosts whose host key has changed.
1.40 aaron 952: The host keys of
1.79 stevesk 953: known hosts will be verified automatically in all cases.
1.40 aaron 954: The argument must be
1.79 stevesk 955: .Dq yes ,
956: .Dq no
1.2 deraadt 957: or
1.79 stevesk 958: .Dq ask .
959: The default is
960: .Dq ask .
1.16 markus 961: .It Cm UsePrivilegedPort
962: Specifies whether to use a privileged port for outgoing connections.
963: The argument must be
964: .Dq yes
965: or
966: .Dq no .
967: The default is
968: .Dq yes .
969: Note that setting this option to
970: .Dq no
1.30 provos 971: turns off
1.16 markus 972: .Cm RhostsAuthentication
973: and
1.72 markus 974: .Cm RhostsRSAAuthentication
975: for older servers.
1.34 markus 976: .It Cm User
1.40 aaron 977: Specifies the user to log in as.
978: This can be useful if you have a different user name on different machines.
979: This saves the trouble of
1.34 markus 980: having to remember to give the user name on the command line.
981: .It Cm UserKnownHostsFile
982: Specifies a file to use instead of
983: .Pa $HOME/.ssh/known_hosts .
1.2 deraadt 984: .It Cm UseRsh
1.40 aaron 985: Specifies that rlogin/rsh should be used for this host.
986: It is possible that the host does not at all support the
1.2 deraadt 987: .Nm
1.40 aaron 988: protocol.
989: This causes
1.2 deraadt 990: .Nm
1.40 aaron 991: to immediately execute
1.2 deraadt 992: .Xr rsh 1 .
1.1 deraadt 993: All other options (except
1.2 deraadt 994: .Cm HostName )
1.40 aaron 995: are ignored if this has been specified.
996: The argument must be
1.2 deraadt 997: .Dq yes
998: or
999: .Dq no .
1.55 markus 1000: .It Cm XAuthLocation
1001: Specifies the location of the
1002: .Xr xauth 1
1003: program.
1004: The default is
1005: .Pa /usr/X11R6/bin/xauth .
1.58 itojun 1006: .El
1.2 deraadt 1007: .Sh ENVIRONMENT
1008: .Nm
1.1 deraadt 1009: will normally set the following environment variables:
1.2 deraadt 1010: .Bl -tag -width Ds
1011: .It Ev DISPLAY
1012: The
1013: .Ev DISPLAY
1.40 aaron 1014: variable indicates the location of the X11 server.
1.44 aaron 1015: It is automatically set by
1.2 deraadt 1016: .Nm
1017: to point to a value of the form
1018: .Dq hostname:n
1019: where hostname indicates
1.40 aaron 1020: the host where the shell runs, and n is an integer >= 1.
1021: .Nm
1022: uses this special value to forward X11 connections over the secure
1023: channel.
1024: The user should normally not set DISPLAY explicitly, as that
1.1 deraadt 1025: will render the X11 connection insecure (and will require the user to
1026: manually copy any required authorization cookies).
1.2 deraadt 1027: .It Ev HOME
1.1 deraadt 1028: Set to the path of the user's home directory.
1.2 deraadt 1029: .It Ev LOGNAME
1030: Synonym for
1.12 aaron 1031: .Ev USER ;
1032: set for compatibility with systems that use this variable.
1.2 deraadt 1033: .It Ev MAIL
1.1 deraadt 1034: Set to point the user's mailbox.
1.40 aaron 1035: .It Ev PATH
1.2 deraadt 1036: Set to the default
1037: .Ev PATH ,
1038: as specified when compiling
1.12 aaron 1039: .Nm ssh .
1.18 markus 1040: .It Ev SSH_AUTH_SOCK
1.17 markus 1041: indicates the path of a unix-domain socket used to communicate with the
1042: agent.
1.2 deraadt 1043: .It Ev SSH_CLIENT
1.40 aaron 1044: Identifies the client end of the connection.
1045: The variable contains
1.1 deraadt 1046: three space-separated values: client ip-address, client port number,
1047: and server port number.
1.73 markus 1048: .It Ev SSH_ORIGINAL_COMMAND
1049: The variable contains the original command line if a forced command
1050: is executed.
1051: It can be used to extract the original arguments.
1.2 deraadt 1052: .It Ev SSH_TTY
1.1 deraadt 1053: This is set to the name of the tty (path to the device) associated
1.40 aaron 1054: with the current shell or command.
1055: If the current session has no tty,
1.1 deraadt 1056: this variable is not set.
1.2 deraadt 1057: .It Ev TZ
1.1 deraadt 1058: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 1059: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 1060: on to new connections).
1.2 deraadt 1061: .It Ev USER
1.1 deraadt 1062: Set to the name of the user logging in.
1.2 deraadt 1063: .El
1064: .Pp
1.44 aaron 1065: Additionally,
1.2 deraadt 1066: .Nm
1.44 aaron 1067: reads
1068: .Pa $HOME/.ssh/environment ,
1.2 deraadt 1069: and adds lines of the format
1070: .Dq VARNAME=value
1.12 aaron 1071: to the environment.
1.2 deraadt 1072: .Sh FILES
1.36 markus 1073: .Bl -tag -width Ds
1.2 deraadt 1074: .It Pa $HOME/.ssh/known_hosts
1.1 deraadt 1075: Records host keys for all hosts the user has logged into (that are not
1.2 deraadt 1076: in
1077: .Pa /etc/ssh_known_hosts ) .
1078: See
1079: .Xr sshd 8 .
1.48 markus 1080: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa
1081: Contains the RSA and the DSA authentication identity of the user.
1082: These files
1083: contain sensitive data and should be readable by the user but not
1.15 markus 1084: accessible by others (read/write/execute).
1085: Note that
1086: .Nm
1.48 markus 1087: ignores a private key file if it is accessible by others.
1.15 markus 1088: It is possible to specify a passphrase when
1.1 deraadt 1089: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 1090: sensitive part of this file using 3DES.
1.48 markus 1091: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub
1.1 deraadt 1092: Contains the public key for authentication (public part of the
1.40 aaron 1093: identity file in human-readable form).
1.48 markus 1094: The contents of the
1095: .Pa $HOME/.ssh/identity.pub
1096: file should be added to
1.2 deraadt 1097: .Pa $HOME/.ssh/authorized_keys
1098: on all machines
1.40 aaron 1099: where you wish to log in using RSA authentication.
1.48 markus 1100: The contents of the
1101: .Pa $HOME/.ssh/id_dsa.pub
1102: file should be added to
1103: .Pa $HOME/.ssh/authorized_keys2
1104: on all machines
1105: where you wish to log in using DSA authentication.
1106: These files are not
1.40 aaron 1107: sensitive and can (but need not) be readable by anyone.
1.48 markus 1108: These files are
1.84 markus 1109: never used automatically and are not necessary; they are only provided for
1.1 deraadt 1110: the convenience of the user.
1.2 deraadt 1111: .It Pa $HOME/.ssh/config
1.40 aaron 1112: This is the per-user configuration file.
1113: The format of this file is described above.
1114: This file is used by the
1.2 deraadt 1115: .Nm
1.40 aaron 1116: client.
1117: This file does not usually contain any sensitive information,
1.1 deraadt 1118: but the recommended permissions are read/write for the user, and not
1119: accessible by others.
1.2 deraadt 1120: .It Pa $HOME/.ssh/authorized_keys
1.40 aaron 1121: Lists the RSA keys that can be used for logging in as this user.
1122: The format of this file is described in the
1.2 deraadt 1123: .Xr sshd 8
1.40 aaron 1124: manual page.
1125: In the simplest form the format is the same as the .pub
1.1 deraadt 1126: identity files (that is, each line contains the number of bits in
1127: modulus, public exponent, modulus, and comment fields, separated by
1.40 aaron 1128: spaces).
1129: This file is not highly sensitive, but the recommended
1.1 deraadt 1130: permissions are read/write for the user, and not accessible by others.
1.48 markus 1131: .It Pa $HOME/.ssh/authorized_keys2
1.68 markus 1132: Lists the public keys (DSA/RSA) that can be used for logging in as this user.
1.48 markus 1133: This file is not highly sensitive, but the recommended
1134: permissions are read/write for the user, and not accessible by others.
1135: .It Pa /etc/ssh_known_hosts, /etc/ssh_known_hosts2
1.40 aaron 1136: Systemwide list of known host keys.
1.48 markus 1137: .Pa /etc/ssh_known_hosts
1138: contains RSA and
1139: .Pa /etc/ssh_known_hosts2
1.68 markus 1140: contains DSA or RSA keys for protocol version 2.
1.48 markus 1141: These files should be prepared by the
1.1 deraadt 1142: system administrator to contain the public host keys of all machines in the
1.40 aaron 1143: organization.
1144: This file should be world-readable.
1145: This file contains
1.1 deraadt 1146: public keys, one per line, in the following format (fields separated
1147: by spaces): system name, number of bits in modulus, public exponent,
1.40 aaron 1148: modulus, and optional comment field.
1149: When different names are used
1.1 deraadt 1150: for the same machine, all such names should be listed, separated by
1.40 aaron 1151: commas.
1152: The format is described on the
1.2 deraadt 1153: .Xr sshd 8
1.1 deraadt 1154: manual page.
1.2 deraadt 1155: .Pp
1.1 deraadt 1156: The canonical system name (as returned by name servers) is used by
1.2 deraadt 1157: .Xr sshd 8
1.1 deraadt 1158: to verify the client host when logging in; other names are needed because
1.2 deraadt 1159: .Nm
1.1 deraadt 1160: does not convert the user-supplied name to a canonical name before
1161: checking the key, because someone with access to the name servers
1162: would then be able to fool host authentication.
1.2 deraadt 1163: .It Pa /etc/ssh_config
1.40 aaron 1164: Systemwide configuration file.
1165: This file provides defaults for those
1.1 deraadt 1166: values that are not specified in the user's configuration file, and
1.40 aaron 1167: for those users who do not have a configuration file.
1168: This file must be world-readable.
1.2 deraadt 1169: .It Pa $HOME/.rhosts
1170: This file is used in
1171: .Pa \&.rhosts
1172: authentication to list the
1.40 aaron 1173: host/user pairs that are permitted to log in.
1174: (Note that this file is
1.1 deraadt 1175: also used by rlogin and rsh, which makes using this file insecure.)
1176: Each line of the file contains a host name (in the canonical form
1177: returned by name servers), and then a user name on that host,
1.40 aaron 1178: separated by a space.
1179: One some machines this file may need to be
1.1 deraadt 1180: world-readable if the user's home directory is on a NFS partition,
1181: because
1.2 deraadt 1182: .Xr sshd 8
1.40 aaron 1183: reads it as root.
1184: Additionally, this file must be owned by the user,
1185: and must not have write permissions for anyone else.
1186: The recommended
1.1 deraadt 1187: permission for most machines is read/write for the user, and not
1188: accessible by others.
1.2 deraadt 1189: .Pp
1.1 deraadt 1190: Note that by default
1.2 deraadt 1191: .Xr sshd 8
1.1 deraadt 1192: will be installed so that it requires successful RSA host
1.40 aaron 1193: authentication before permitting \s+2.\s0rhosts authentication.
1194: If your server machine does not have the client's host key in
1.2 deraadt 1195: .Pa /etc/ssh_known_hosts ,
1196: you can store it in
1197: .Pa $HOME/.ssh/known_hosts .
1198: The easiest way to do this is to
1.1 deraadt 1199: connect back to the client from the server machine using ssh; this
1.48 markus 1200: will automatically add the host key to
1.2 deraadt 1201: .Pa $HOME/.ssh/known_hosts .
1202: .It Pa $HOME/.shosts
1203: This file is used exactly the same way as
1204: .Pa \&.rhosts .
1205: The purpose for
1.1 deraadt 1206: having this file is to be able to use rhosts authentication with
1.2 deraadt 1207: .Nm
1208: without permitting login with
1209: .Xr rlogin 1
1210: or
1211: .Xr rsh 1 .
1212: .It Pa /etc/hosts.equiv
1213: This file is used during
1.40 aaron 1214: .Pa \&.rhosts authentication.
1215: It contains
1.1 deraadt 1216: canonical hosts names, one per line (the full format is described on
1217: the
1.2 deraadt 1218: .Xr sshd 8
1.40 aaron 1219: manual page).
1220: If the client host is found in this file, login is
1.1 deraadt 1221: automatically permitted provided client and server user names are the
1.40 aaron 1222: same.
1223: Additionally, successful RSA host authentication is normally
1224: required.
1225: This file should only be writable by root.
1.2 deraadt 1226: .It Pa /etc/shosts.equiv
1.44 aaron 1227: This file is processed exactly as
1.2 deraadt 1228: .Pa /etc/hosts.equiv .
1.1 deraadt 1229: This file may be useful to permit logins using
1.2 deraadt 1230: .Nm
1.1 deraadt 1231: but not using rsh/rlogin.
1.2 deraadt 1232: .It Pa /etc/sshrc
1.1 deraadt 1233: Commands in this file are executed by
1.2 deraadt 1234: .Nm
1.1 deraadt 1235: when the user logs in just before the user's shell (or command) is started.
1236: See the
1.2 deraadt 1237: .Xr sshd 8
1.1 deraadt 1238: manual page for more information.
1.2 deraadt 1239: .It Pa $HOME/.ssh/rc
1.1 deraadt 1240: Commands in this file are executed by
1.2 deraadt 1241: .Nm
1.1 deraadt 1242: when the user logs in just before the user's shell (or command) is
1243: started.
1.44 aaron 1244: See the
1.2 deraadt 1245: .Xr sshd 8
1.1 deraadt 1246: manual page for more information.
1.31 markus 1247: .It Pa $HOME/.ssh/environment
1248: Contains additional definitions for environment variables, see section
1249: .Sx ENVIRONMENT
1250: above.
1.5 deraadt 1251: .It Pa libcrypto.so.X.1
1252: A version of this library which includes support for the RSA algorithm
1253: is required for proper operation.
1.58 itojun 1254: .El
1.67 aaron 1255: .Sh AUTHORS
1.78 markus 1256: OpenSSH is a derivative of the original and free
1257: ssh 1.2.12 release by Tatu Ylonen.
1258: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1259: Theo de Raadt and Dug Song
1260: removed many bugs, re-added newer features and
1261: created OpenSSH.
1262: Markus Friedl contributed the support for SSH
1263: protocol versions 1.5 and 2.0.
1.2 deraadt 1264: .Sh SEE ALSO
1265: .Xr rlogin 1 ,
1266: .Xr rsh 1 ,
1267: .Xr scp 1 ,
1.83 djm 1268: .Xr sftp 1 ,
1.2 deraadt 1269: .Xr ssh-add 1 ,
1270: .Xr ssh-agent 1 ,
1271: .Xr ssh-keygen 1 ,
1272: .Xr telnet 1 ,
1.87 itojun 1273: .Xr sshd 8