Annotation of src/usr.bin/ssh/ssh.1, Revision 1.59
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,
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! 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: .\"
1.2 deraadt 37: .Dd September 25, 1999
38: .Dt SSH 1
39: .Os
40: .Sh NAME
41: .Nm ssh
1.20 provos 42: .Nd OpenSSH secure shell client (remote login program)
1.2 deraadt 43: .Sh SYNOPSIS
44: .Nm ssh
45: .Op Fl l Ar login_name
1.5 deraadt 46: .Op Ar hostname | user@hostname
1.2 deraadt 47: .Op Ar command
48: .Pp
49: .Nm ssh
1.54 markus 50: .Op Fl afgknqtvxACNPTX246
1.51 markus 51: .Op Fl c Ar cipher_spec
1.2 deraadt 52: .Op Fl e Ar escape_char
53: .Op Fl i Ar identity_file
54: .Op Fl l Ar login_name
55: .Op Fl o Ar option
56: .Op Fl p Ar port
1.12 aaron 57: .Oo Fl L Xo
58: .Sm off
1.33 markus 59: .Ar port :
1.12 aaron 60: .Ar host :
61: .Ar hostport
62: .Sm on
63: .Xc
64: .Oc
65: .Oo Fl R Xo
66: .Sm off
1.33 markus 67: .Ar port :
1.12 aaron 68: .Ar host :
69: .Ar hostport
70: .Sm on
71: .Xc
72: .Oc
1.5 deraadt 73: .Op Ar hostname | user@hostname
1.2 deraadt 74: .Op Ar command
1.44 aaron 75: .Sh DESCRIPTION
1.2 deraadt 76: .Nm
1.5 deraadt 77: (Secure Shell) is a program for logging into a remote machine and for
1.40 aaron 78: executing commands on a remote machine.
79: It is intended to replace
1.1 deraadt 80: rlogin and rsh, and provide secure encrypted communications between
1.40 aaron 81: two untrusted hosts over an insecure network.
82: X11 connections and
1.1 deraadt 83: arbitrary TCP/IP ports can also be forwarded over the secure channel.
1.2 deraadt 84: .Pp
85: .Nm
1.44 aaron 86: connects and logs into the specified
1.2 deraadt 87: .Ar hostname .
1.1 deraadt 88: The user must prove
1.49 markus 89: his/her identity to the remote machine using one of several methods
90: depending on the protocol version used:
91: .Pp
92: .Ss SSH protocol version 1
1.2 deraadt 93: .Pp
1.1 deraadt 94: First, if the machine the user logs in from is listed in
1.2 deraadt 95: .Pa /etc/hosts.equiv
1.1 deraadt 96: or
1.2 deraadt 97: .Pa /etc/shosts.equiv
1.1 deraadt 98: on the remote machine, and the user names are
99: the same on both sides, the user is immediately permitted to log in.
1.44 aaron 100: Second, if
1.2 deraadt 101: .Pa \&.rhosts
1.1 deraadt 102: or
1.2 deraadt 103: .Pa \&.shosts
1.1 deraadt 104: exists in the user's home directory on the
105: remote machine and contains a line containing the name of the client
106: machine and the name of the user on that machine, the user is
1.40 aaron 107: permitted to log in.
108: This form of authentication alone is normally not
1.1 deraadt 109: allowed by the server because it is not secure.
1.2 deraadt 110: .Pp
1.1 deraadt 111: The second (and primary) authentication method is the
1.2 deraadt 112: .Pa rhosts
1.1 deraadt 113: or
1.2 deraadt 114: .Pa hosts.equiv
1.40 aaron 115: method combined with RSA-based host authentication.
116: It means that if the login would be permitted by
1.49 markus 117: .Pa $HOME/.rhosts ,
118: .Pa $HOME/.shosts ,
1.2 deraadt 119: .Pa /etc/hosts.equiv ,
1.1 deraadt 120: or
1.2 deraadt 121: .Pa /etc/shosts.equiv ,
1.11 deraadt 122: and if additionally the server can verify the client's
1.44 aaron 123: host key (see
1.2 deraadt 124: .Pa /etc/ssh_known_hosts
1.23 markus 125: and
126: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 127: in the
1.2 deraadt 128: .Sx FILES
1.40 aaron 129: section), only then login is permitted.
130: This authentication method closes security holes due to IP
131: spoofing, DNS spoofing and routing spoofing.
132: [Note to the administrator:
1.2 deraadt 133: .Pa /etc/hosts.equiv ,
1.49 markus 134: .Pa $HOME/.rhosts ,
1.1 deraadt 135: and the rlogin/rsh protocol in general, are inherently insecure and should be
136: disabled if security is desired.]
1.2 deraadt 137: .Pp
1.44 aaron 138: As a third authentication method,
1.2 deraadt 139: .Nm
1.1 deraadt 140: supports RSA based authentication.
141: The scheme is based on public-key cryptography: there are cryptosystems
142: where encryption and decryption are done using separate keys, and it
143: is not possible to derive the decryption key from the encryption key.
1.40 aaron 144: RSA is one such system.
1.44 aaron 145: The idea is that each user creates a public/private
1.40 aaron 146: key pair for authentication purposes.
147: The server knows the public key, and only the user knows the private key.
1.44 aaron 148: The file
1.2 deraadt 149: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 150: lists the public keys that are permitted for logging
1.40 aaron 151: in.
152: When the user logs in, the
1.2 deraadt 153: .Nm
1.1 deraadt 154: program tells the server which key pair it would like to use for
1.40 aaron 155: authentication.
156: The server checks if this key is permitted, and if
1.1 deraadt 157: so, sends the user (actually the
1.2 deraadt 158: .Nm
1.1 deraadt 159: program running on behalf of the user) a challenge, a random number,
1.40 aaron 160: encrypted by the user's public key.
161: The challenge can only be
162: decrypted using the proper private key.
163: The user's client then decrypts the
1.1 deraadt 164: challenge using the private key, proving that he/she knows the private
165: key but without disclosing it to the server.
1.2 deraadt 166: .Pp
167: .Nm
1.40 aaron 168: implements the RSA authentication protocol automatically.
169: The user creates his/her RSA key pair by running
1.2 deraadt 170: .Xr ssh-keygen 1 .
1.44 aaron 171: This stores the private key in
1.49 markus 172: .Pa $HOME/.ssh/identity
1.1 deraadt 173: and the public key in
1.49 markus 174: .Pa $HOME/.ssh/identity.pub
1.40 aaron 175: in the user's home directory.
176: The user should then copy the
1.2 deraadt 177: .Pa identity.pub
1.44 aaron 178: to
1.49 markus 179: .Pa $HOME/.ssh/authorized_keys
1.44 aaron 180: in his/her home directory on the remote machine (the
1.2 deraadt 181: .Pa authorized_keys
1.44 aaron 182: file corresponds to the conventional
1.49 markus 183: .Pa $HOME/.rhosts
1.1 deraadt 184: file, and has one key
1.40 aaron 185: per line, though the lines can be very long).
186: After this, the user can log in without giving the password.
187: RSA authentication is much
1.1 deraadt 188: more secure than rhosts authentication.
1.2 deraadt 189: .Pp
1.1 deraadt 190: The most convenient way to use RSA authentication may be with an
1.40 aaron 191: authentication agent.
192: See
1.2 deraadt 193: .Xr ssh-agent 1
1.1 deraadt 194: for more information.
1.2 deraadt 195: .Pp
1.44 aaron 196: If other authentication methods fail,
1.2 deraadt 197: .Nm
1.40 aaron 198: prompts the user for a password.
199: The password is sent to the remote
1.1 deraadt 200: host for checking; however, since all communications are encrypted,
201: the password cannot be seen by someone listening on the network.
1.2 deraadt 202: .Pp
1.49 markus 203: .Ss SSH protocol version 2
204: .Pp
205: When a user connects using the protocol version 2
206: different authentication methods are available:
207: At first, the client attempts to authenticate using the public key method.
208: If this method fails password authentication is tried.
209: .Pp
210: The public key method is similar to RSA authentication described
211: in the previous section except that the DSA algorithm is used
212: instead of the patented RSA algorithm.
213: The client uses his private DSA key
214: .Pa $HOME/.ssh/id_dsa
215: to sign the session identifier and sends the result to the server.
216: The server checks whether the matching public key is listed in
217: .Pa $HOME/.ssh/authorized_keys2
218: and grants access if both the key is found and the signature is correct.
219: The session identifier is derived from a shared Diffie-Hellman value
220: and is only known to the client and the server.
221: .Pp
222: If public key authentication fails or is not available a password
223: can be sent encrypted to the remote host for proving the user's identity.
224: This protocol 2 implementation does not yet support Kerberos or
225: S/Key authentication.
226: .Pp
227: Protocol 2 provides additional mechanisms for confidentiality
1.51 markus 228: (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
1.49 markus 229: and integrity (hmac-sha1, hmac-md5).
230: Note that protocol 1 lacks a strong mechanism for ensuring the
231: integrity of the connection.
232: .Pp
233: .Ss Login session and remote execution
234: .Pp
1.1 deraadt 235: When the user's identity has been accepted by the server, the server
236: either executes the given command, or logs into the machine and gives
1.40 aaron 237: the user a normal shell on the remote machine.
238: All communication with
1.1 deraadt 239: the remote command or shell will be automatically encrypted.
1.2 deraadt 240: .Pp
1.1 deraadt 241: If a pseudo-terminal has been allocated (normal login session), the
1.2 deraadt 242: user can disconnect with
243: .Ic ~. ,
244: and suspend
245: .Nm
246: with
247: .Ic ~^Z .
248: All forwarded connections can be listed with
1.44 aaron 249: .Ic ~#
1.2 deraadt 250: and if
1.1 deraadt 251: the session blocks waiting for forwarded X11 or TCP/IP
1.2 deraadt 252: connections to terminate, it can be backgrounded with
253: .Ic ~&
254: (this should not be used while the user shell is active, as it can cause the
1.40 aaron 255: shell to hang).
256: All available escapes can be listed with
1.2 deraadt 257: .Ic ~? .
258: .Pp
259: A single tilde character can be sent as
260: .Ic ~~
261: (or by following the tilde by a character other than those described above).
1.1 deraadt 262: The escape character must always follow a newline to be interpreted as
1.40 aaron 263: special.
264: The escape character can be changed in configuration files
265: or on the command line.
1.2 deraadt 266: .Pp
1.1 deraadt 267: If no pseudo tty has been allocated, the
268: session is transparent and can be used to reliably transfer binary
1.40 aaron 269: data.
270: On most systems, setting the escape character to
1.2 deraadt 271: .Dq none
272: will also make the session transparent even if a tty is used.
273: .Pp
1.1 deraadt 274: The session terminates when the command or shell in on the remote
275: machine exists and all X11 and TCP/IP connections have been closed.
276: The exit status of the remote program is returned as the exit status
277: of
1.2 deraadt 278: .Nm ssh .
279: .Pp
1.49 markus 280: .Ss X11 and TCP forwarding
281: .Pp
1.1 deraadt 282: If the user is using X11 (the
1.2 deraadt 283: .Ev DISPLAY
1.1 deraadt 284: environment variable is set), the connection to the X11 display is
285: automatically forwarded to the remote side in such a way that any X11
286: programs started from the shell (or command) will go through the
287: encrypted channel, and the connection to the real X server will be made
1.40 aaron 288: from the local machine.
289: The user should not manually set
1.2 deraadt 290: .Ev DISPLAY .
1.1 deraadt 291: Forwarding of X11 connections can be
292: configured on the command line or in configuration files.
1.2 deraadt 293: .Pp
294: The
1.44 aaron 295: .Ev DISPLAY
1.2 deraadt 296: value set by
297: .Nm
1.1 deraadt 298: will point to the server machine, but with a display number greater
1.40 aaron 299: than zero.
300: This is normal, and happens because
1.2 deraadt 301: .Nm
302: creates a
303: .Dq proxy
304: X server on the server machine for forwarding the
1.1 deraadt 305: connections over the encrypted channel.
1.2 deraadt 306: .Pp
307: .Nm
1.1 deraadt 308: will also automatically set up Xauthority data on the server machine.
309: For this purpose, it will generate a random authorization cookie,
310: store it in Xauthority on the server, and verify that any forwarded
311: connections carry this cookie and replace it by the real cookie when
1.40 aaron 312: the connection is opened.
313: The real authentication cookie is never
1.1 deraadt 314: sent to the server machine (and no cookies are sent in the plain).
1.2 deraadt 315: .Pp
1.1 deraadt 316: If the user is using an authentication agent, the connection to the agent
317: is automatically forwarded to the remote side unless disabled on
318: command line or in a configuration file.
1.2 deraadt 319: .Pp
1.1 deraadt 320: Forwarding of arbitrary TCP/IP connections over the secure channel can
1.40 aaron 321: be specified either on command line or in a configuration file.
322: One possible application of TCP/IP forwarding is a secure connection to an
1.1 deraadt 323: electronic purse; another is going trough firewalls.
1.2 deraadt 324: .Pp
1.49 markus 325: .Ss Server authentication
326: .Pp
1.2 deraadt 327: .Nm
1.49 markus 328: automatically maintains and checks a database containing
1.40 aaron 329: identifications for all hosts it has ever been used with.
1.49 markus 330: RSA host keys are stored in
331: .Pa $HOME/.ssh/known_hosts
332: and
333: DSA host keys are stored in
334: .Pa $HOME/.ssh/known_hosts2
1.40 aaron 335: in the user's home directory.
1.49 markus 336: Additionally, the files
1.2 deraadt 337: .Pa /etc/ssh_known_hosts
1.49 markus 338: and
339: .Pa /etc/ssh_known_hosts2
340: are automatically checked for known hosts.
1.40 aaron 341: Any new hosts are automatically added to the user's file.
342: If a host's identification
1.1 deraadt 343: ever changes,
1.2 deraadt 344: .Nm
1.1 deraadt 345: warns about this and disables password authentication to prevent a
1.40 aaron 346: trojan horse from getting the user's password.
347: Another purpose of
1.1 deraadt 348: this mechanism is to prevent man-in-the-middle attacks which could
1.40 aaron 349: otherwise be used to circumvent the encryption.
350: The
1.2 deraadt 351: .Cm StrictHostKeyChecking
1.1 deraadt 352: option (see below) can be used to prevent logins to machines whose
353: host key is not known or has changed.
1.2 deraadt 354: .Sh OPTIONS
355: .Bl -tag -width Ds
1.4 dugsong 356: .It Fl a
1.42 aaron 357: Disables forwarding of the authentication agent connection.
1.54 markus 358: .It Fl A
359: Enables forwarding of the authentication agent connection.
360: This can also be specified on a per-host basis in a configuration file.
1.6 deraadt 361: .It Fl c Ar blowfish|3des
1.44 aaron 362: Selects the cipher to use for encrypting the session.
1.2 deraadt 363: .Ar 3des
1.40 aaron 364: is used by default.
1.44 aaron 365: It is believed to be secure.
1.5 deraadt 366: .Ar 3des
367: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
368: It is presumably more secure than the
1.2 deraadt 369: .Ar des
1.51 markus 370: cipher which is no longer supported in
371: .Nm ssh .
1.5 deraadt 372: .Ar blowfish
373: is a fast block cipher, it appears very secure and is much faster than
1.40 aaron 374: .Ar 3des .
1.51 markus 375: .It Fl c Ar "3des-cbc,blowfish-cbc,arcfour,cast128-cbc"
376: Additionally, for protocol version 2 a comma-separated list of ciphers can
377: be specified in order of preference. Protocol version 2 supports
378: 3DES, Blowfish and CAST128 in CBC mode and Arcfour.
1.2 deraadt 379: .It Fl e Ar ch|^ch|none
380: Sets the escape character for sessions with a pty (default:
381: .Ql ~ ) .
1.40 aaron 382: The escape character is only recognized at the beginning of a line.
383: The escape character followed by a dot
1.2 deraadt 384: .Pq Ql \&.
385: closes the connection, followed
1.1 deraadt 386: by control-Z suspends the connection, and followed by itself sends the
1.40 aaron 387: escape character once.
388: Setting the character to
1.2 deraadt 389: .Dq none
390: disables any escapes and makes the session fully transparent.
391: .It Fl f
392: Requests
393: .Nm
1.40 aaron 394: to go to background just before command execution.
395: This is useful if
1.2 deraadt 396: .Nm
397: is going to ask for passwords or passphrases, but the user
1.40 aaron 398: wants it in the background.
1.44 aaron 399: This implies
1.2 deraadt 400: .Fl n .
1.1 deraadt 401: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 402: something like
403: .Ic ssh -f host xterm .
1.34 markus 404: .It Fl g
405: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 406: .It Fl i Ar identity_file
1.44 aaron 407: Selects the file from which the identity (private key) for
1.40 aaron 408: RSA authentication is read.
1.44 aaron 409: Default is
1.49 markus 410: .Pa $HOME/.ssh/identity
1.40 aaron 411: in the user's home directory.
412: Identity files may also be specified on
413: a per-host basis in the configuration file.
414: It is possible to have multiple
1.2 deraadt 415: .Fl i
416: options (and multiple identities specified in
1.1 deraadt 417: configuration files).
1.2 deraadt 418: .It Fl k
1.42 aaron 419: Disables forwarding of Kerberos tickets and AFS tokens.
420: This may also be specified on a per-host basis in the configuration file.
1.2 deraadt 421: .It Fl l Ar login_name
1.40 aaron 422: Specifies the user to log in as on the remote machine.
423: This also may be specified on a per-host basis in the configuration file.
1.2 deraadt 424: .It Fl n
425: Redirects stdin from
426: .Pa /dev/null
427: (actually, prevents reading from stdin).
1.1 deraadt 428: This must be used when
1.2 deraadt 429: .Nm
1.40 aaron 430: is run in the background.
431: A common trick is to use this to run X11 programs on a remote machine.
432: For example,
1.2 deraadt 433: .Ic ssh -n shadows.cs.hut.fi emacs &
434: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 435: connection will be automatically forwarded over an encrypted channel.
436: The
1.2 deraadt 437: .Nm
1.1 deraadt 438: program will be put in the background.
439: (This does not work if
1.2 deraadt 440: .Nm
441: needs to ask for a password or passphrase; see also the
442: .Fl f
443: option.)
1.53 markus 444: .It Fl N
445: Do not execute a remote command.
446: This is usefull if you just want to forward ports
447: (protocol version 2 only).
1.2 deraadt 448: .It Fl o Ar option
1.1 deraadt 449: Can be used to give options in the format used in the config file.
450: This is useful for specifying options for which there is no separate
1.40 aaron 451: command-line flag.
452: The option has the same format as a line in the configuration file.
1.2 deraadt 453: .It Fl p Ar port
1.40 aaron 454: Port to connect to on the remote host.
455: This can be specified on a
1.1 deraadt 456: per-host basis in the configuration file.
1.16 markus 457: .It Fl P
458: Use a non-privileged port for outgoing connections.
459: This can be used if your firewall does
460: not permit connections from privileged ports.
1.30 provos 461: Note that this option turns off
1.16 markus 462: .Cm RhostsAuthentication
463: and
464: .Cm RhostsRSAAuthentication .
1.2 deraadt 465: .It Fl q
1.40 aaron 466: Quiet mode.
467: Causes all warning and diagnostic messages to be suppressed.
468: Only fatal errors are displayed.
1.2 deraadt 469: .It Fl t
1.40 aaron 470: Force pseudo-tty allocation.
1.43 brad 471: This can be used to execute arbitrary
1.40 aaron 472: screen-based programs on a remote machine, which can be very useful,
473: e.g., when implementing menu services.
1.53 markus 474: .It Fl T
475: Disable pseudo-tty allocation (protocol version 2 only).
1.2 deraadt 476: .It Fl v
1.40 aaron 477: Verbose mode.
478: Causes
1.2 deraadt 479: .Nm
1.40 aaron 480: to print debugging messages about its progress.
481: This is helpful in
1.1 deraadt 482: debugging connection, authentication, and configuration problems.
1.22 provos 483: The verbose mode is also used to display
484: .Xr skey 1
485: challenges, if the user entered "s/key" as password.
1.2 deraadt 486: .It Fl x
1.40 aaron 487: Disables X11 forwarding.
1.2 deraadt 488: .It Fl X
1.1 deraadt 489: Enables X11 forwarding.
1.54 markus 490: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 491: .It Fl C
1.1 deraadt 492: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 493: data for forwarded X11 and TCP/IP connections).
494: The compression algorithm is the same used by
1.34 markus 495: .Xr gzip 1 ,
496: and the
1.2 deraadt 497: .Dq level
498: can be controlled by the
499: .Cm CompressionLevel
1.40 aaron 500: option (see below).
501: Compression is desirable on modem lines and other
1.1 deraadt 502: slow connections, but will only slow down things on fast networks.
503: The default value can be set on a host-by-host basis in the
504: configuration files; see the
1.2 deraadt 505: .Cm Compress
1.1 deraadt 506: option below.
1.2 deraadt 507: .It Fl L Ar port:host:hostport
1.1 deraadt 508: Specifies that the given port on the local (client) host is to be
1.40 aaron 509: forwarded to the given host and port on the remote side.
510: This works by allocating a socket to listen to
1.2 deraadt 511: .Ar port
1.1 deraadt 512: on the local side, and whenever a connection is made to this port, the
513: connection is forwarded over the secure channel, and a connection is
514: made to
1.32 markus 515: .Ar host
516: port
517: .Ar hostport
1.40 aaron 518: from the remote machine.
519: Port forwardings can also be specified in the configuration file.
520: Only root can forward privileged ports.
1.32 markus 521: IPv6 addresses can be specified with an alternative syntax:
522: .Ar port/host/hostport
1.2 deraadt 523: .It Fl R Ar port:host:hostport
1.1 deraadt 524: Specifies that the given port on the remote (server) host is to be
1.40 aaron 525: forwarded to the given host and port on the local side.
526: This works by allocating a socket to listen to
1.2 deraadt 527: .Ar port
1.1 deraadt 528: on the remote side, and whenever a connection is made to this port, the
529: connection is forwarded over the secure channel, and a connection is
530: made to
1.32 markus 531: .Ar host
532: port
533: .Ar hostport
1.40 aaron 534: from the local machine.
535: Port forwardings can also be specified in the configuration file.
536: Privileged ports can be forwarded only when
1.1 deraadt 537: logging in as root on the remote machine.
1.46 markus 538: .It Fl 2
539: Forces
540: .Nm
1.50 markus 541: to try protocol version 2 only.
1.32 markus 542: .It Fl 4
543: Forces
544: .Nm
545: to use IPv4 addresses only.
546: .It Fl 6
547: Forces
548: .Nm
549: to use IPv6 addresses only.
1.2 deraadt 550: .El
551: .Sh CONFIGURATION FILES
552: .Nm
1.1 deraadt 553: obtains configuration data from the following sources (in this order):
554: command line options, user's configuration file
1.2 deraadt 555: .Pq Pa $HOME/.ssh/config ,
556: and system-wide configuration file
557: .Pq Pa /etc/ssh_config .
558: For each parameter, the first obtained value
1.40 aaron 559: will be used.
560: The configuration files contain sections bracketed by
561: .Dq Host
562: specifications, and that section is only applied for hosts that
563: match one of the patterns given in the specification.
564: The matched host name is the one given on the command line.
1.2 deraadt 565: .Pp
1.1 deraadt 566: Since the first obtained value for each parameter is used, more
567: host-specific declarations should be given near the beginning of the
568: file, and general defaults at the end.
1.2 deraadt 569: .Pp
1.1 deraadt 570: The configuration file has the following format:
1.2 deraadt 571: .Pp
572: Empty lines and lines starting with
573: .Ql #
574: are comments.
575: .Pp
576: Otherwise a line is of the format
577: .Dq keyword arguments .
578: The possible
1.1 deraadt 579: keywords and their meanings are as follows (note that the
580: configuration files are case-sensitive):
1.2 deraadt 581: .Bl -tag -width Ds
582: .It Cm Host
1.1 deraadt 583: Restricts the following declarations (up to the next
1.2 deraadt 584: .Cm Host
1.1 deraadt 585: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 586: given after the keyword.
587: .Ql \&*
588: and
589: .Ql ?
590: can be used as wildcards in the
1.40 aaron 591: patterns.
592: A single
1.2 deraadt 593: .Ql \&*
594: as a pattern can be used to provide global
1.40 aaron 595: defaults for all hosts.
596: The host is the
1.2 deraadt 597: .Ar hostname
1.1 deraadt 598: argument given on the command line (i.e., the name is not converted to
599: a canonicalized host name before matching).
1.2 deraadt 600: .It Cm AFSTokenPassing
1.42 aaron 601: Specifies whether to pass AFS tokens to remote host.
602: The argument to this keyword must be
1.2 deraadt 603: .Dq yes
604: or
605: .Dq no .
606: .It Cm BatchMode
607: If set to
608: .Dq yes ,
1.40 aaron 609: passphrase/password querying will be disabled.
610: This option is useful in scripts and other batch jobs where you have no
611: user to supply the password.
612: The argument must be
1.2 deraadt 613: .Dq yes
614: or
615: .Dq no .
1.34 markus 616: .It Cm CheckHostIP
617: If this flag is set to
618: .Dq yes ,
619: ssh will additionally check the host ip address in the
620: .Pa known_hosts
1.42 aaron 621: file.
622: This allows ssh to detect if a host key changed due to DNS spoofing.
1.34 markus 623: If the option is set to
624: .Dq no ,
625: the check will not be executed.
1.2 deraadt 626: .It Cm Cipher
1.40 aaron 627: Specifies the cipher to use for encrypting the session.
628: Currently,
1.2 deraadt 629: .Dq blowfish ,
1.1 deraadt 630: and
1.10 provos 631: .Dq 3des
1.40 aaron 632: are supported.
633: The default is
1.2 deraadt 634: .Dq 3des .
1.45 markus 635: .It Cm Ciphers
636: Specifies the ciphers allowed for protocol version 2
637: in order of preference.
638: Multiple ciphers must be comma-separated.
639: The default is
1.51 markus 640: .Dq 3des-cbc,blowfish-cbc,arcfour,cast128-cbc .
1.2 deraadt 641: .It Cm Compression
1.40 aaron 642: Specifies whether to use compression.
643: The argument must be
1.2 deraadt 644: .Dq yes
645: or
646: .Dq no .
647: .It Cm CompressionLevel
1.40 aaron 648: Specifies the compression level to use if compression is enable.
649: The argument must be an integer from 1 (fast) to 9 (slow, best).
650: The default level is 6, which is good for most applications.
651: The meaning of the values is the same as in
1.34 markus 652: .Xr gzip 1 .
1.2 deraadt 653: .It Cm ConnectionAttempts
1.1 deraadt 654: Specifies the number of tries (one per second) to make before falling
1.40 aaron 655: back to rsh or exiting.
656: The argument must be an integer.
657: This may be useful in scripts if the connection sometimes fails.
1.50 markus 658: .It Cm DSAAuthentication
659: Specifies whether to try DSA authentication.
660: The argument to this keyword must be
661: .Dq yes
662: or
663: .Dq no .
664: DSA authentication will only be
665: attempted if a DSA identity file exists.
666: Note that this option applies to protocol version 2 only.
1.2 deraadt 667: .It Cm EscapeChar
668: Sets the escape character (default:
669: .Ql ~ ) .
670: The escape character can also
1.40 aaron 671: be set on the command line.
672: The argument should be a single character,
1.2 deraadt 673: .Ql ^
674: followed by a letter, or
675: .Dq none
676: to disable the escape
1.1 deraadt 677: character entirely (making the connection transparent for binary
678: data).
1.44 aaron 679: .It Cm FallBackToRsh
1.1 deraadt 680: Specifies that if connecting via
1.2 deraadt 681: .Nm
1.1 deraadt 682: fails due to a connection refused error (there is no
1.2 deraadt 683: .Xr sshd 8
1.44 aaron 684: listening on the remote host),
1.2 deraadt 685: .Xr rsh 1
1.1 deraadt 686: should automatically be used instead (after a suitable warning about
1.40 aaron 687: the session being unencrypted).
688: The argument must be
1.2 deraadt 689: .Dq yes
690: or
691: .Dq no .
692: .It Cm ForwardAgent
1.1 deraadt 693: Specifies whether the connection to the authentication agent (if any)
1.40 aaron 694: will be forwarded to the remote machine.
695: The argument must be
1.2 deraadt 696: .Dq yes
697: or
1.54 markus 698: .Dq no .
699: The default is
1.2 deraadt 700: .Dq no .
701: .It Cm ForwardX11
1.1 deraadt 702: Specifies whether X11 connections will be automatically redirected
1.44 aaron 703: over the secure channel and
1.2 deraadt 704: .Ev DISPLAY
1.40 aaron 705: set.
1.44 aaron 706: The argument must be
1.2 deraadt 707: .Dq yes
708: or
1.38 markus 709: .Dq no .
710: The default is
1.3 deraadt 711: .Dq no .
712: .It Cm GatewayPorts
713: Specifies whether remote hosts are allowed to connect to local
714: forwarded ports.
715: The argument must be
716: .Dq yes
717: or
718: .Dq no .
719: The default is
1.2 deraadt 720: .Dq no .
721: .It Cm GlobalKnownHostsFile
1.44 aaron 722: Specifies a file to use instead of
1.2 deraadt 723: .Pa /etc/ssh_known_hosts .
724: .It Cm HostName
1.40 aaron 725: Specifies the real host name to log into.
726: This can be used to specify nicknames or abbreviations for hosts.
727: Default is the name given on the command line.
728: Numeric IP addresses are also permitted (both on the command line and in
1.2 deraadt 729: .Cm HostName
1.1 deraadt 730: specifications).
1.2 deraadt 731: .It Cm IdentityFile
1.1 deraadt 732: Specifies the file from which the user's RSA authentication identity
1.2 deraadt 733: is read (default
1.49 markus 734: .Pa $HOME/.ssh/identity
1.2 deraadt 735: in the user's home directory).
1.1 deraadt 736: Additionally, any identities represented by the authentication agent
1.40 aaron 737: will be used for authentication.
738: The file name may use the tilde
739: syntax to refer to a user's home directory.
740: It is possible to have
1.1 deraadt 741: multiple identity files specified in configuration files; all these
742: identities will be tried in sequence.
1.48 markus 743: .It Cm IdentityFile2
744: Specifies the file from which the user's DSA authentication identity
745: is read (default
1.49 markus 746: .Pa $HOME/.ssh/id_dsa
1.48 markus 747: in the user's home directory).
748: The file name may use the tilde
749: syntax to refer to a user's home directory.
750: It is possible to have
751: multiple identity files specified in configuration files; all these
752: identities will be tried in sequence.
1.2 deraadt 753: .It Cm KeepAlive
1.1 deraadt 754: Specifies whether the system should send keepalive messages to the
1.40 aaron 755: other side.
756: If they are sent, death of the connection or crash of one
757: of the machines will be properly noticed.
758: However, this means that
1.1 deraadt 759: connections will die if the route is down temporarily, and some people
1.41 aaron 760: find it annoying.
1.2 deraadt 761: .Pp
762: The default is
763: .Dq yes
764: (to send keepalives), and the client will notice
1.40 aaron 765: if the network goes down or the remote host dies.
766: This is important in scripts, and many users want it too.
1.2 deraadt 767: .Pp
768: To disable keepalives, the value should be set to
769: .Dq no
770: in both the server and the client configuration files.
771: .It Cm KerberosAuthentication
1.42 aaron 772: Specifies whether Kerberos authentication will be used.
773: The argument to this keyword must be
1.4 dugsong 774: .Dq yes
775: or
776: .Dq no .
1.2 deraadt 777: .It Cm KerberosTgtPassing
1.42 aaron 778: Specifies whether a Kerberos TGT will be forwarded to the server.
779: This will only work if the Kerberos server is actually an AFS kaserver.
780: The argument to this keyword must be
1.4 dugsong 781: .Dq yes
782: or
783: .Dq no .
1.2 deraadt 784: .It Cm LocalForward
1.1 deraadt 785: Specifies that a TCP/IP port on the local machine be forwarded over
1.40 aaron 786: the secure channel to given host:port from the remote machine.
787: The first argument must be a port number, and the second must be
788: host:port.
789: Multiple forwardings may be specified, and additional
790: forwardings can be given on the command line.
791: Only the superuser can forward privileged ports.
1.24 markus 792: .It Cm LogLevel
793: Gives the verbosity level that is used when logging messages from
794: .Nm ssh .
795: The possible values are:
1.39 djm 796: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
1.24 markus 797: The default is INFO.
1.14 dugsong 798: .It Cm NumberOfPasswordPrompts
1.42 aaron 799: Specifies the number of password prompts before giving up.
800: The argument to this keyword must be an integer.
801: Default is 3.
1.34 markus 802: .It Cm PasswordAuthentication
1.40 aaron 803: Specifies whether to use password authentication.
804: The argument to this keyword must be
1.34 markus 805: .Dq yes
806: or
807: .Dq no .
1.50 markus 808: Note that this option applies to both protocol version 1 and 2.
1.2 deraadt 809: .It Cm Port
1.40 aaron 810: Specifies the port number to connect on the remote host.
811: Default is 22.
1.45 markus 812: .It Cm Protocol
813: Specifies the protocol versions
814: .Nm
815: should support in order of preference.
816: The possible values are
817: .Dq 1
818: and
819: .Dq 2 .
820: Multiple versions must be comma-separated.
821: The default is
1.49 markus 822: .Dq 1,2 .
823: This means that
824: .Nm
825: tries version 1 and falls back to version 2
1.52 hugh 826: if version 1 is not available.
1.2 deraadt 827: .It Cm ProxyCommand
1.40 aaron 828: Specifies the command to use to connect to the server.
829: The command
830: string extends to the end of the line, and is executed with
831: .Pa /bin/sh .
832: In the command string,
833: .Ql %h
834: will be substituted by the host name to
835: connect and
836: .Ql %p
837: by the port.
838: The command can be basically anything,
839: and should read from its standard input and write to its standard output.
840: It should eventually connect an
1.2 deraadt 841: .Xr sshd 8
1.1 deraadt 842: server running on some machine, or execute
1.2 deraadt 843: .Ic sshd -i
1.40 aaron 844: somewhere.
845: Host key management will be done using the
1.1 deraadt 846: HostName of the host being connected (defaulting to the name typed by
847: the user).
1.29 markus 848: Note that
849: .Cm CheckHostIP
850: is not available for connects with a proxy command.
1.2 deraadt 851: .Pp
852: .It Cm RemoteForward
1.1 deraadt 853: Specifies that a TCP/IP port on the remote machine be forwarded over
1.40 aaron 854: the secure channel to given host:port from the local machine.
855: The first argument must be a port number, and the second must be
856: host:port.
857: Multiple forwardings may be specified, and additional
858: forwardings can be given on the command line.
859: Only the superuser can forward privileged ports.
1.2 deraadt 860: .It Cm RhostsAuthentication
1.40 aaron 861: Specifies whether to try rhosts based authentication.
862: Note that this
1.1 deraadt 863: declaration only affects the client side and has no effect whatsoever
1.40 aaron 864: on security.
865: Disabling rhosts authentication may reduce
1.1 deraadt 866: authentication time on slow connections when rhosts authentication is
1.40 aaron 867: not used.
868: Most servers do not permit RhostsAuthentication because it
869: is not secure (see RhostsRSAAuthentication).
870: The argument to this keyword must be
1.2 deraadt 871: .Dq yes
872: or
873: .Dq no .
874: .It Cm RhostsRSAAuthentication
1.1 deraadt 875: Specifies whether to try rhosts based authentication with RSA host
1.40 aaron 876: authentication.
877: This is the primary authentication method for most sites.
878: The argument must be
1.2 deraadt 879: .Dq yes
880: or
881: .Dq no .
882: .It Cm RSAAuthentication
1.40 aaron 883: Specifies whether to try RSA authentication.
884: The argument to this keyword must be
1.2 deraadt 885: .Dq yes
886: or
887: .Dq no .
1.1 deraadt 888: RSA authentication will only be
889: attempted if the identity file exists, or an authentication agent is
890: running.
1.50 markus 891: Note that this option applies to protocol version 1 only.
1.27 markus 892: .It Cm SkeyAuthentication
893: Specifies whether to use
894: .Xr skey 1
1.40 aaron 895: authentication.
896: The argument to this keyword must be
1.27 markus 897: .Dq yes
898: or
899: .Dq no .
900: The default is
901: .Dq no .
1.2 deraadt 902: .It Cm StrictHostKeyChecking
903: If this flag is set to
1.44 aaron 904: .Dq yes ,
1.2 deraadt 905: .Nm
1.1 deraadt 906: ssh will never automatically add host keys to the
1.2 deraadt 907: .Pa $HOME/.ssh/known_hosts
1.48 markus 908: and
909: .Pa $HOME/.ssh/known_hosts2
910: files, and refuses to connect hosts whose host key has changed.
1.40 aaron 911: This provides maximum protection against trojan horse attacks.
912: However, it can be somewhat annoying if you don't have good
1.2 deraadt 913: .Pa /etc/ssh_known_hosts
1.48 markus 914: and
915: .Pa /etc/ssh_known_hosts2
1.1 deraadt 916: files installed and frequently
1.40 aaron 917: connect new hosts.
918: Basically this option forces the user to manually
919: add any new hosts.
920: Normally this option is disabled, and new hosts
921: will automatically be added to the known host files.
922: The host keys of
923: known hosts will be verified automatically in either case.
924: The argument must be
1.2 deraadt 925: .Dq yes
926: or
927: .Dq no .
1.16 markus 928: .It Cm UsePrivilegedPort
929: Specifies whether to use a privileged port for outgoing connections.
930: The argument must be
931: .Dq yes
932: or
933: .Dq no .
934: The default is
935: .Dq yes .
936: Note that setting this option to
937: .Dq no
1.30 provos 938: turns off
1.16 markus 939: .Cm RhostsAuthentication
940: and
941: .Cm RhostsRSAAuthentication .
1.34 markus 942: .It Cm User
1.40 aaron 943: Specifies the user to log in as.
944: This can be useful if you have a different user name on different machines.
945: This saves the trouble of
1.34 markus 946: having to remember to give the user name on the command line.
947: .It Cm UserKnownHostsFile
948: Specifies a file to use instead of
949: .Pa $HOME/.ssh/known_hosts .
1.2 deraadt 950: .It Cm UseRsh
1.40 aaron 951: Specifies that rlogin/rsh should be used for this host.
952: It is possible that the host does not at all support the
1.2 deraadt 953: .Nm
1.40 aaron 954: protocol.
955: This causes
1.2 deraadt 956: .Nm
1.40 aaron 957: to immediately execute
1.2 deraadt 958: .Xr rsh 1 .
1.1 deraadt 959: All other options (except
1.2 deraadt 960: .Cm HostName )
1.40 aaron 961: are ignored if this has been specified.
962: The argument must be
1.2 deraadt 963: .Dq yes
964: or
965: .Dq no .
1.55 markus 966: .It Cm XAuthLocation
967: Specifies the location of the
968: .Xr xauth 1
969: program.
970: The default is
971: .Pa /usr/X11R6/bin/xauth .
1.58 itojun 972: .El
1.2 deraadt 973: .Sh ENVIRONMENT
974: .Nm
1.1 deraadt 975: will normally set the following environment variables:
1.2 deraadt 976: .Bl -tag -width Ds
977: .It Ev DISPLAY
978: The
979: .Ev DISPLAY
1.40 aaron 980: variable indicates the location of the X11 server.
1.44 aaron 981: It is automatically set by
1.2 deraadt 982: .Nm
983: to point to a value of the form
984: .Dq hostname:n
985: where hostname indicates
1.40 aaron 986: the host where the shell runs, and n is an integer >= 1.
987: .Nm
988: uses this special value to forward X11 connections over the secure
989: channel.
990: The user should normally not set DISPLAY explicitly, as that
1.1 deraadt 991: will render the X11 connection insecure (and will require the user to
992: manually copy any required authorization cookies).
1.2 deraadt 993: .It Ev HOME
1.1 deraadt 994: Set to the path of the user's home directory.
1.2 deraadt 995: .It Ev LOGNAME
996: Synonym for
1.12 aaron 997: .Ev USER ;
998: set for compatibility with systems that use this variable.
1.2 deraadt 999: .It Ev MAIL
1.1 deraadt 1000: Set to point the user's mailbox.
1.40 aaron 1001: .It Ev PATH
1.2 deraadt 1002: Set to the default
1003: .Ev PATH ,
1004: as specified when compiling
1.12 aaron 1005: .Nm ssh .
1.18 markus 1006: .It Ev SSH_AUTH_SOCK
1.17 markus 1007: indicates the path of a unix-domain socket used to communicate with the
1008: agent.
1.2 deraadt 1009: .It Ev SSH_CLIENT
1.40 aaron 1010: Identifies the client end of the connection.
1011: The variable contains
1.1 deraadt 1012: three space-separated values: client ip-address, client port number,
1013: and server port number.
1.2 deraadt 1014: .It Ev SSH_TTY
1.1 deraadt 1015: This is set to the name of the tty (path to the device) associated
1.40 aaron 1016: with the current shell or command.
1017: If the current session has no tty,
1.1 deraadt 1018: this variable is not set.
1.2 deraadt 1019: .It Ev TZ
1.1 deraadt 1020: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 1021: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 1022: on to new connections).
1.2 deraadt 1023: .It Ev USER
1.1 deraadt 1024: Set to the name of the user logging in.
1.2 deraadt 1025: .El
1026: .Pp
1.44 aaron 1027: Additionally,
1.2 deraadt 1028: .Nm
1.44 aaron 1029: reads
1030: .Pa $HOME/.ssh/environment ,
1.2 deraadt 1031: and adds lines of the format
1032: .Dq VARNAME=value
1.12 aaron 1033: to the environment.
1.2 deraadt 1034: .Sh FILES
1.36 markus 1035: .Bl -tag -width Ds
1.2 deraadt 1036: .It Pa $HOME/.ssh/known_hosts
1.1 deraadt 1037: Records host keys for all hosts the user has logged into (that are not
1.2 deraadt 1038: in
1039: .Pa /etc/ssh_known_hosts ) .
1040: See
1041: .Xr sshd 8 .
1.48 markus 1042: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa
1043: Contains the RSA and the DSA authentication identity of the user.
1044: These files
1045: contain sensitive data and should be readable by the user but not
1.15 markus 1046: accessible by others (read/write/execute).
1047: Note that
1048: .Nm
1.48 markus 1049: ignores a private key file if it is accessible by others.
1.15 markus 1050: It is possible to specify a passphrase when
1.1 deraadt 1051: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 1052: sensitive part of this file using 3DES.
1.48 markus 1053: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub
1.1 deraadt 1054: Contains the public key for authentication (public part of the
1.40 aaron 1055: identity file in human-readable form).
1.48 markus 1056: The contents of the
1057: .Pa $HOME/.ssh/identity.pub
1058: file should be added to
1.2 deraadt 1059: .Pa $HOME/.ssh/authorized_keys
1060: on all machines
1.40 aaron 1061: where you wish to log in using RSA authentication.
1.48 markus 1062: The contents of the
1063: .Pa $HOME/.ssh/id_dsa.pub
1064: file should be added to
1065: .Pa $HOME/.ssh/authorized_keys2
1066: on all machines
1067: where you wish to log in using DSA authentication.
1068: These files are not
1.40 aaron 1069: sensitive and can (but need not) be readable by anyone.
1.48 markus 1070: These files are
1071: never used automatically and are not necessary; they is only provided for
1.1 deraadt 1072: the convenience of the user.
1.2 deraadt 1073: .It Pa $HOME/.ssh/config
1.40 aaron 1074: This is the per-user configuration file.
1075: The format of this file is described above.
1076: This file is used by the
1.2 deraadt 1077: .Nm
1.40 aaron 1078: client.
1079: This file does not usually contain any sensitive information,
1.1 deraadt 1080: but the recommended permissions are read/write for the user, and not
1081: accessible by others.
1.2 deraadt 1082: .It Pa $HOME/.ssh/authorized_keys
1.40 aaron 1083: Lists the RSA keys that can be used for logging in as this user.
1084: The format of this file is described in the
1.2 deraadt 1085: .Xr sshd 8
1.40 aaron 1086: manual page.
1087: In the simplest form the format is the same as the .pub
1.1 deraadt 1088: identity files (that is, each line contains the number of bits in
1089: modulus, public exponent, modulus, and comment fields, separated by
1.40 aaron 1090: spaces).
1091: This file is not highly sensitive, but the recommended
1.1 deraadt 1092: permissions are read/write for the user, and not accessible by others.
1.48 markus 1093: .It Pa $HOME/.ssh/authorized_keys2
1094: Lists the DSA keys that can be used for logging in as this user.
1095: This file is not highly sensitive, but the recommended
1096: permissions are read/write for the user, and not accessible by others.
1097: .It Pa /etc/ssh_known_hosts, /etc/ssh_known_hosts2
1.40 aaron 1098: Systemwide list of known host keys.
1.48 markus 1099: .Pa /etc/ssh_known_hosts
1100: contains RSA and
1101: .Pa /etc/ssh_known_hosts2
1102: contains DSA keys.
1103: These files should be prepared by the
1.1 deraadt 1104: system administrator to contain the public host keys of all machines in the
1.40 aaron 1105: organization.
1106: This file should be world-readable.
1107: This file contains
1.1 deraadt 1108: public keys, one per line, in the following format (fields separated
1109: by spaces): system name, number of bits in modulus, public exponent,
1.40 aaron 1110: modulus, and optional comment field.
1111: When different names are used
1.1 deraadt 1112: for the same machine, all such names should be listed, separated by
1.40 aaron 1113: commas.
1114: The format is described on the
1.2 deraadt 1115: .Xr sshd 8
1.1 deraadt 1116: manual page.
1.2 deraadt 1117: .Pp
1.1 deraadt 1118: The canonical system name (as returned by name servers) is used by
1.2 deraadt 1119: .Xr sshd 8
1.1 deraadt 1120: to verify the client host when logging in; other names are needed because
1.2 deraadt 1121: .Nm
1.1 deraadt 1122: does not convert the user-supplied name to a canonical name before
1123: checking the key, because someone with access to the name servers
1124: would then be able to fool host authentication.
1.2 deraadt 1125: .It Pa /etc/ssh_config
1.40 aaron 1126: Systemwide configuration file.
1127: This file provides defaults for those
1.1 deraadt 1128: values that are not specified in the user's configuration file, and
1.40 aaron 1129: for those users who do not have a configuration file.
1130: This file must be world-readable.
1.2 deraadt 1131: .It Pa $HOME/.rhosts
1132: This file is used in
1133: .Pa \&.rhosts
1134: authentication to list the
1.40 aaron 1135: host/user pairs that are permitted to log in.
1136: (Note that this file is
1.1 deraadt 1137: also used by rlogin and rsh, which makes using this file insecure.)
1138: Each line of the file contains a host name (in the canonical form
1139: returned by name servers), and then a user name on that host,
1.40 aaron 1140: separated by a space.
1141: One some machines this file may need to be
1.1 deraadt 1142: world-readable if the user's home directory is on a NFS partition,
1143: because
1.2 deraadt 1144: .Xr sshd 8
1.40 aaron 1145: reads it as root.
1146: Additionally, this file must be owned by the user,
1147: and must not have write permissions for anyone else.
1148: The recommended
1.1 deraadt 1149: permission for most machines is read/write for the user, and not
1150: accessible by others.
1.2 deraadt 1151: .Pp
1.1 deraadt 1152: Note that by default
1.2 deraadt 1153: .Xr sshd 8
1.1 deraadt 1154: will be installed so that it requires successful RSA host
1.40 aaron 1155: authentication before permitting \s+2.\s0rhosts authentication.
1156: If your server machine does not have the client's host key in
1.2 deraadt 1157: .Pa /etc/ssh_known_hosts ,
1158: you can store it in
1159: .Pa $HOME/.ssh/known_hosts .
1160: The easiest way to do this is to
1.1 deraadt 1161: connect back to the client from the server machine using ssh; this
1.48 markus 1162: will automatically add the host key to
1.2 deraadt 1163: .Pa $HOME/.ssh/known_hosts .
1164: .It Pa $HOME/.shosts
1165: This file is used exactly the same way as
1166: .Pa \&.rhosts .
1167: The purpose for
1.1 deraadt 1168: having this file is to be able to use rhosts authentication with
1.2 deraadt 1169: .Nm
1170: without permitting login with
1171: .Xr rlogin 1
1172: or
1173: .Xr rsh 1 .
1174: .It Pa /etc/hosts.equiv
1175: This file is used during
1.40 aaron 1176: .Pa \&.rhosts authentication.
1177: It contains
1.1 deraadt 1178: canonical hosts names, one per line (the full format is described on
1179: the
1.2 deraadt 1180: .Xr sshd 8
1.40 aaron 1181: manual page).
1182: If the client host is found in this file, login is
1.1 deraadt 1183: automatically permitted provided client and server user names are the
1.40 aaron 1184: same.
1185: Additionally, successful RSA host authentication is normally
1186: required.
1187: This file should only be writable by root.
1.2 deraadt 1188: .It Pa /etc/shosts.equiv
1.44 aaron 1189: This file is processed exactly as
1.2 deraadt 1190: .Pa /etc/hosts.equiv .
1.1 deraadt 1191: This file may be useful to permit logins using
1.2 deraadt 1192: .Nm
1.1 deraadt 1193: but not using rsh/rlogin.
1.2 deraadt 1194: .It Pa /etc/sshrc
1.1 deraadt 1195: Commands in this file are executed by
1.2 deraadt 1196: .Nm
1.1 deraadt 1197: when the user logs in just before the user's shell (or command) is started.
1198: See the
1.2 deraadt 1199: .Xr sshd 8
1.1 deraadt 1200: manual page for more information.
1.2 deraadt 1201: .It Pa $HOME/.ssh/rc
1.1 deraadt 1202: Commands in this file are executed by
1.2 deraadt 1203: .Nm
1.1 deraadt 1204: when the user logs in just before the user's shell (or command) is
1205: started.
1.44 aaron 1206: See the
1.2 deraadt 1207: .Xr sshd 8
1.1 deraadt 1208: manual page for more information.
1.31 markus 1209: .It Pa $HOME/.ssh/environment
1210: Contains additional definitions for environment variables, see section
1211: .Sx ENVIRONMENT
1212: above.
1.5 deraadt 1213: .It Pa libcrypto.so.X.1
1214: A version of this library which includes support for the RSA algorithm
1215: is required for proper operation.
1.58 itojun 1216: .El
1.2 deraadt 1217: .Sh AUTHOR
1.20 provos 1218: OpenSSH
1.37 deraadt 1219: is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
1.40 aaron 1220: but with bugs removed and newer features re-added.
1221: Rapidly after the
1.37 deraadt 1222: 1.2.12 release, newer versions of the original ssh bore successively
1223: more restrictive licenses, and thus demand for a free version was born.
1.47 markus 1224: .Pp
1.37 deraadt 1225: This version of OpenSSH
1.20 provos 1226: .Bl -bullet
1227: .It
1.35 aaron 1228: has all components of a restrictive nature (i.e., patents, see
1.21 deraadt 1229: .Xr ssl 8 )
1230: directly removed from the source code; any licensed or patented components
1231: are chosen from
1232: external libraries.
1.20 provos 1233: .It
1.47 markus 1234: has been updated to support SSH protocol 1.5 and 2, making it compatible with
1235: all other SSH clients and servers.
1.20 provos 1236: .It
1.44 aaron 1237: contains added support for
1.20 provos 1238: .Xr kerberos 8
1239: authentication and ticket passing.
1240: .It
1.21 deraadt 1241: supports one-time password authentication with
1.20 provos 1242: .Xr skey 1 .
1243: .El
1.25 provos 1244: .Pp
1.26 aaron 1245: OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
1.25 provos 1246: Niels Provos, Theo de Raadt, and Dug Song.
1.47 markus 1247: .Pp
1248: The support for SSH protocol 2 was written by Markus Friedl.
1.2 deraadt 1249: .Sh SEE ALSO
1250: .Xr rlogin 1 ,
1251: .Xr rsh 1 ,
1252: .Xr scp 1 ,
1253: .Xr ssh-add 1 ,
1254: .Xr ssh-agent 1 ,
1255: .Xr ssh-keygen 1 ,
1256: .Xr telnet 1 ,
1.5 deraadt 1257: .Xr sshd 8 ,
1258: .Xr ssl 8
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