Annotation of src/usr.bin/ssh/sshd.8, Revision 1.246
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: .\"
1.64 deraadt 7: .\" As far as I am concerned, the code I have written for this software
8: .\" can be used freely for any purpose. Any derived versions of this
9: .\" software must be clearly marked as such, and if the derived work is
10: .\" incompatible with the protocol description in the RFC file, it must be
11: .\" called by a name other than "ssh" or "Secure Shell".
12: .\"
1.99 deraadt 13: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
1.64 deraadt 16: .\"
17: .\" Redistribution and use in source and binary forms, with or without
18: .\" modification, are permitted provided that the following conditions
19: .\" are met:
20: .\" 1. Redistributions of source code must retain the above copyright
21: .\" notice, this list of conditions and the following disclaimer.
22: .\" 2. Redistributions in binary form must reproduce the above copyright
23: .\" notice, this list of conditions and the following disclaimer in the
24: .\" documentation and/or other materials provided with the distribution.
1.1 deraadt 25: .\"
1.64 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: .\"
1.246 ! djm 37: .\" $OpenBSD: sshd.8,v 1.245 2008/06/11 07:30:37 jmc Exp $
! 38: .Dd $Mdocdate: June 11 2008 $
1.2 deraadt 39: .Dt SSHD 8
40: .Os
41: .Sh NAME
42: .Nm sshd
1.120 markus 43: .Nd OpenSSH SSH daemon
1.2 deraadt 44: .Sh SYNOPSIS
45: .Nm sshd
1.194 jmc 46: .Bk -words
1.243 jmc 47: .Op Fl 46DdeiqTt
1.2 deraadt 48: .Op Fl b Ar bits
1.242 dtucker 49: .Op Fl C Ar connection_spec
1.2 deraadt 50: .Op Fl f Ar config_file
51: .Op Fl g Ar login_grace_time
52: .Op Fl h Ar host_key_file
53: .Op Fl k Ar key_gen_time
1.156 markus 54: .Op Fl o Ar option
1.2 deraadt 55: .Op Fl p Ar port
1.61 markus 56: .Op Fl u Ar len
1.194 jmc 57: .Ek
1.40 aaron 58: .Sh DESCRIPTION
1.2 deraadt 59: .Nm
1.212 jmc 60: (OpenSSH Daemon) is the daemon program for
1.2 deraadt 61: .Xr ssh 1 .
1.235 jmc 62: Together these programs replace
63: .Xr rlogin 1
64: and
65: .Xr rsh 1 ,
66: and provide secure encrypted communications between two untrusted hosts
1.36 aaron 67: over an insecure network.
1.2 deraadt 68: .Pp
69: .Nm
1.212 jmc 70: listens for connections from clients.
1.40 aaron 71: It is normally started at boot from
1.2 deraadt 72: .Pa /etc/rc .
73: It forks a new
1.36 aaron 74: daemon for each incoming connection.
75: The forked daemons handle
1.1 deraadt 76: key exchange, encryption, authentication, command execution,
77: and data exchange.
1.2 deraadt 78: .Pp
79: .Nm
1.200 jmc 80: can be configured using command-line options or a configuration file
81: (by default
1.212 jmc 82: .Xr sshd_config 5 ) ;
83: command-line options override values specified in the
1.1 deraadt 84: configuration file.
1.25 markus 85: .Nm
86: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 87: .Dv SIGHUP ,
1.230 jmc 88: by executing itself with the name and options it was started with, e.g.\&
1.97 deraadt 89: .Pa /usr/sbin/sshd .
1.18 aaron 90: .Pp
91: The options are as follows:
1.2 deraadt 92: .Bl -tag -width Ds
1.200 jmc 93: .It Fl 4
94: Forces
95: .Nm
96: to use IPv4 addresses only.
97: .It Fl 6
98: Forces
99: .Nm
100: to use IPv6 addresses only.
1.2 deraadt 101: .It Fl b Ar bits
1.120 markus 102: Specifies the number of bits in the ephemeral protocol version 1
1.246 ! djm 103: server key (default 1024).
1.243 jmc 104: .It Fl C Ar connection_spec
105: Specify the connection parameters to use for the
106: .Fl T
107: extended test mode.
108: If provided, any
109: .Cm Match
110: directives in the configuration file
111: that would apply to the specified user, host, and address will be set before
112: the configuration is written to standard output.
113: The connection parameters are supplied as keyword=value pairs.
114: The keywords are
115: .Dq user ,
116: .Dq host ,
117: and
118: .Dq addr .
119: All are required and may be supplied in any order, either with multiple
120: .Fl C
121: options or as a comma-separated list.
1.200 jmc 122: .It Fl D
123: When this option is specified,
124: .Nm
125: will not detach and does not become a daemon.
126: This allows easy monitoring of
127: .Nm sshd .
1.2 deraadt 128: .It Fl d
1.36 aaron 129: Debug mode.
130: The server sends verbose debug output to the system
131: log, and does not put itself in the background.
132: The server also will not fork and will only process one connection.
133: This option is only intended for debugging for the server.
1.194 jmc 134: Multiple
135: .Fl d
136: options increase the debugging level.
1.67 aaron 137: Maximum is 3.
1.120 markus 138: .It Fl e
139: When this option is specified,
140: .Nm
141: will send the output to the standard error instead of the system log.
1.235 jmc 142: .It Fl f Ar config_file
1.36 aaron 143: Specifies the name of the configuration file.
144: The default is
1.167 deraadt 145: .Pa /etc/ssh/sshd_config .
1.16 markus 146: .Nm
147: refuses to start if there is no configuration file.
1.2 deraadt 148: .It Fl g Ar login_grace_time
1.1 deraadt 149: Gives the grace time for clients to authenticate themselves (default
1.191 stevesk 150: 120 seconds).
1.36 aaron 151: If the client fails to authenticate the user within
152: this many seconds, the server disconnects and exits.
153: A value of zero indicates no limit.
1.2 deraadt 154: .It Fl h Ar host_key_file
1.160 stevesk 155: Specifies a file from which a host key is read.
1.7 markus 156: This option must be given if
157: .Nm
158: is not run as root (as the normal
1.160 stevesk 159: host key files are normally not readable by anyone but root).
160: The default is
1.167 deraadt 161: .Pa /etc/ssh/ssh_host_key
1.160 stevesk 162: for protocol version 1, and
1.167 deraadt 163: .Pa /etc/ssh/ssh_host_rsa_key
1.160 stevesk 164: and
1.167 deraadt 165: .Pa /etc/ssh/ssh_host_dsa_key
1.160 stevesk 166: for protocol version 2.
1.75 markus 167: It is possible to have multiple host key files for
1.120 markus 168: the different protocol versions and host key algorithms.
1.2 deraadt 169: .It Fl i
1.7 markus 170: Specifies that
171: .Nm
1.194 jmc 172: is being run from
173: .Xr inetd 8 .
1.7 markus 174: .Nm
175: is normally not run
1.1 deraadt 176: from inetd because it needs to generate the server key before it can
1.36 aaron 177: respond to the client, and this may take tens of seconds.
178: Clients would have to wait too long if the key was regenerated every time.
1.230 jmc 179: However, with small key sizes (e.g. 512) using
1.7 markus 180: .Nm
181: from inetd may
1.1 deraadt 182: be feasible.
1.2 deraadt 183: .It Fl k Ar key_gen_time
1.120 markus 184: Specifies how often the ephemeral protocol version 1 server key is
185: regenerated (default 3600 seconds, or one hour).
1.36 aaron 186: The motivation for regenerating the key fairly
1.200 jmc 187: often is that the key is not stored anywhere, and after about an hour
1.1 deraadt 188: it becomes impossible to recover the key for decrypting intercepted
189: communications even if the machine is cracked into or physically
1.36 aaron 190: seized.
191: A value of zero indicates that the key will never be regenerated.
1.156 markus 192: .It Fl o Ar option
193: Can be used to give options in the format used in the configuration file.
194: This is useful for specifying options for which there is no separate
195: command-line flag.
1.200 jmc 196: For full details of the options, and their values, see
197: .Xr sshd_config 5 .
1.2 deraadt 198: .It Fl p Ar port
1.1 deraadt 199: Specifies the port on which the server listens for connections
200: (default 22).
1.158 stevesk 201: Multiple port options are permitted.
1.210 stevesk 202: Ports specified in the configuration file with the
203: .Cm Port
204: option are ignored when a command-line port is specified.
205: Ports specified using the
206: .Cm ListenAddress
207: option override command-line ports.
1.2 deraadt 208: .It Fl q
1.36 aaron 209: Quiet mode.
210: Nothing is sent to the system log.
211: Normally the beginning,
1.1 deraadt 212: authentication, and termination of each connection is logged.
1.242 dtucker 213: .It Fl T
214: Extended test mode.
215: Check the validity of the configuration file, output the effective configuration
216: to stdout and then exit.
217: Optionally,
218: .Cm Match
219: rules may be applied by specifying the connection parameters using one or more
220: .Fl C
221: options.
1.243 jmc 222: .It Fl t
223: Test mode.
224: Only check the validity of the configuration file and sanity of the keys.
225: This is useful for updating
226: .Nm
227: reliably as configuration options may change.
1.61 markus 228: .It Fl u Ar len
229: This option is used to specify the size of the field
230: in the
231: .Li utmp
232: structure that holds the remote host name.
233: If the resolved host name is longer than
234: .Ar len ,
235: the dotted decimal value will be used instead.
236: This allows hosts with very long host names that
237: overflow this field to still be uniquely identified.
238: Specifying
239: .Fl u0
240: indicates that only dotted decimal addresses
241: should be put into the
242: .Pa utmp
243: file.
1.144 stevesk 244: .Fl u0
1.194 jmc 245: may also be used to prevent
1.144 stevesk 246: .Nm
247: from making DNS requests unless the authentication
248: mechanism or configuration requires it.
249: Authentication mechanisms that may require DNS include
250: .Cm RhostsRSAAuthentication ,
1.215 jmc 251: .Cm HostbasedAuthentication ,
1.144 stevesk 252: and using a
253: .Cm from="pattern-list"
254: option in a key file.
1.170 stevesk 255: Configuration options that require DNS include using a
256: USER@HOST pattern in
257: .Cm AllowUsers
258: or
259: .Cm DenyUsers .
1.2 deraadt 260: .El
1.214 jmc 261: .Sh AUTHENTICATION
262: The OpenSSH SSH daemon supports SSH protocols 1 and 2.
263: Both protocols are supported by default,
264: though this can be changed via the
265: .Cm Protocol
266: option in
267: .Xr sshd_config 5 .
268: Protocol 2 supports both RSA and DSA keys;
269: protocol 1 only supports RSA keys.
270: For both protocols,
271: each host has a host-specific key,
272: normally 2048 bits,
273: used to identify the host.
274: .Pp
275: Forward security for protocol 1 is provided through
276: an additional server key,
277: normally 768 bits,
278: generated when the server starts.
1.212 jmc 279: This key is normally regenerated every hour if it has been used, and
280: is never stored on disk.
281: Whenever a client connects, the daemon responds with its public
282: host and server keys.
283: The client compares the
284: RSA host key against its own database to verify that it has not changed.
285: The client then generates a 256-bit random number.
286: It encrypts this
287: random number using both the host key and the server key, and sends
288: the encrypted number to the server.
289: Both sides then use this
290: random number as a session key which is used to encrypt all further
291: communications in the session.
292: The rest of the session is encrypted
293: using a conventional cipher, currently Blowfish or 3DES, with 3DES
294: being used by default.
295: The client selects the encryption algorithm
296: to use from those offered by the server.
297: .Pp
1.214 jmc 298: For protocol 2,
299: forward security is provided through a Diffie-Hellman key agreement.
300: This key agreement results in a shared session key.
301: The rest of the session is encrypted using a symmetric cipher, currently
302: 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
303: The client selects the encryption algorithm
304: to use from those offered by the server.
305: Additionally, session integrity is provided
306: through a cryptographic message authentication code
1.237 pvalchev 307: (hmac-md5, hmac-sha1, umac-64 or hmac-ripemd160).
1.214 jmc 308: .Pp
309: Finally, the server and the client enter an authentication dialog.
1.212 jmc 310: The client tries to authenticate itself using
1.214 jmc 311: host-based authentication,
312: public key authentication,
313: challenge-response authentication,
314: or password authentication.
1.212 jmc 315: .Pp
316: If the client successfully authenticates itself, a dialog for
317: preparing the session is entered.
318: At this time the client may request
319: things like allocating a pseudo-tty, forwarding X11 connections,
320: forwarding TCP connections, or forwarding the authentication agent
321: connection over the secure channel.
322: .Pp
1.216 jmc 323: After this, the client either requests a shell or execution of a command.
1.212 jmc 324: The sides then enter session mode.
325: In this mode, either side may send
326: data at any time, and such data is forwarded to/from the shell or
327: command on the server side, and the user terminal in the client side.
328: .Pp
329: When the user program terminates and all forwarded X11 and other
330: connections have been closed, the server sends command exit status to
331: the client, and both sides exit.
1.2 deraadt 332: .Sh LOGIN PROCESS
1.1 deraadt 333: When a user successfully logs in,
1.2 deraadt 334: .Nm
1.1 deraadt 335: does the following:
1.2 deraadt 336: .Bl -enum -offset indent
337: .It
1.1 deraadt 338: If the login is on a tty, and no command has been specified,
1.40 aaron 339: prints last login time and
1.2 deraadt 340: .Pa /etc/motd
1.1 deraadt 341: (unless prevented in the configuration file or by
1.207 djm 342: .Pa ~/.hushlogin ;
1.2 deraadt 343: see the
1.40 aaron 344: .Sx FILES
1.2 deraadt 345: section).
346: .It
1.1 deraadt 347: If the login is on a tty, records login time.
1.2 deraadt 348: .It
349: Checks
350: .Pa /etc/nologin ;
351: if it exists, prints contents and quits
1.1 deraadt 352: (unless root).
1.2 deraadt 353: .It
1.1 deraadt 354: Changes to run with normal user privileges.
1.2 deraadt 355: .It
1.1 deraadt 356: Sets up basic environment.
1.2 deraadt 357: .It
1.200 jmc 358: Reads the file
1.207 djm 359: .Pa ~/.ssh/environment ,
1.200 jmc 360: if it exists, and users are allowed to change their environment.
1.188 stevesk 361: See the
1.187 marc 362: .Cm PermitUserEnvironment
1.188 stevesk 363: option in
1.187 marc 364: .Xr sshd_config 5 .
1.2 deraadt 365: .It
1.1 deraadt 366: Changes to user's home directory.
1.2 deraadt 367: .It
368: If
1.207 djm 369: .Pa ~/.ssh/rc
1.2 deraadt 370: exists, runs it; else if
1.168 deraadt 371: .Pa /etc/ssh/sshrc
1.2 deraadt 372: exists, runs
1.36 aaron 373: it; otherwise runs xauth.
374: The
1.2 deraadt 375: .Dq rc
376: files are given the X11
1.1 deraadt 377: authentication protocol and cookie in standard input.
1.226 jmc 378: See
379: .Sx SSHRC ,
380: below.
1.2 deraadt 381: .It
1.1 deraadt 382: Runs user's shell or command.
1.2 deraadt 383: .El
1.226 jmc 384: .Sh SSHRC
385: If the file
386: .Pa ~/.ssh/rc
387: exists,
388: .Xr sh 1
389: runs it after reading the
390: environment files but before starting the user's shell or command.
391: It must not produce any output on stdout; stderr must be used
392: instead.
393: If X11 forwarding is in use, it will receive the "proto cookie" pair in
394: its standard input (and
395: .Ev DISPLAY
396: in its environment).
397: The script must call
398: .Xr xauth 1
399: because
400: .Nm
401: will not run xauth automatically to add X11 cookies.
402: .Pp
403: The primary purpose of this file is to run any initialization routines
404: which may be needed before the user's home directory becomes
405: accessible; AFS is a particular example of such an environment.
406: .Pp
407: This file will probably contain some initialization code followed by
408: something similar to:
409: .Bd -literal -offset 3n
410: if read proto cookie && [ -n "$DISPLAY" ]; then
411: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
412: # X11UseLocalhost=yes
413: echo add unix:`echo $DISPLAY |
414: cut -c11-` $proto $cookie
415: else
416: # X11UseLocalhost=no
417: echo add $DISPLAY $proto $cookie
418: fi | xauth -q -
419: fi
420: .Ed
421: .Pp
422: If this file does not exist,
423: .Pa /etc/ssh/sshrc
424: is run, and if that
425: does not exist either, xauth is used to add the cookie.
1.2 deraadt 426: .Sh AUTHORIZED_KEYS FILE FORMAT
1.125 markus 427: .Cm AuthorizedKeysFile
1.217 jmc 428: specifies the file containing public keys for
429: public key authentication;
430: if none is specified, the default is
431: .Pa ~/.ssh/authorized_keys .
1.36 aaron 432: Each line of the file contains one
1.2 deraadt 433: key (empty lines and lines starting with a
434: .Ql #
435: are ignored as
1.36 aaron 436: comments).
1.217 jmc 437: Protocol 1 public keys consist of the following space-separated fields:
438: options, bits, exponent, modulus, comment.
439: Protocol 2 public key consist of:
440: options, keytype, base64-encoded key, comment.
441: The options field is optional;
442: its presence is determined by whether the line starts
1.189 espie 443: with a number or not (the options field never starts with a number).
1.217 jmc 444: The bits, exponent, modulus, and comment fields give the RSA key for
1.75 markus 445: protocol version 1; the
1.1 deraadt 446: comment field is not used for anything (but may be convenient for the
447: user to identify the key).
1.75 markus 448: For protocol version 2 the keytype is
449: .Dq ssh-dss
450: or
451: .Dq ssh-rsa .
1.2 deraadt 452: .Pp
1.1 deraadt 453: Note that lines in this file are usually several hundred bytes long
1.203 dtucker 454: (because of the size of the public key encoding) up to a limit of
455: 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
456: keys up to 16 kilobits.
1.36 aaron 457: You don't want to type them in; instead, copy the
1.113 itojun 458: .Pa identity.pub ,
1.217 jmc 459: .Pa id_dsa.pub ,
1.75 markus 460: or the
1.113 itojun 461: .Pa id_rsa.pub
1.1 deraadt 462: file and edit it.
1.175 stevesk 463: .Pp
464: .Nm
465: enforces a minimum RSA key modulus size for protocol 1
466: and protocol 2 keys of 768 bits.
1.2 deraadt 467: .Pp
1.58 deraadt 468: The options (if present) consist of comma-separated option
1.36 aaron 469: specifications.
470: No spaces are permitted, except within double quotes.
1.141 stevesk 471: The following option specifications are supported (note
472: that option keywords are case-insensitive):
1.2 deraadt 473: .Bl -tag -width Ds
474: .It Cm command="command"
1.1 deraadt 475: Specifies that the command is executed whenever this key is used for
1.36 aaron 476: authentication.
477: The command supplied by the user (if any) is ignored.
1.148 markus 478: The command is run on a pty if the client requests a pty;
1.36 aaron 479: otherwise it is run without a tty.
1.194 jmc 480: If an 8-bit clean channel is required,
1.147 deraadt 481: one must not request a pty or should specify
1.89 markus 482: .Cm no-pty .
1.36 aaron 483: A quote may be included in the command by quoting it with a backslash.
484: This option might be useful
1.189 espie 485: to restrict certain public keys to perform just a specific operation.
1.36 aaron 486: An example might be a key that permits remote backups but nothing else.
1.211 jmc 487: Note that the client may specify TCP and/or X11
1.51 hugh 488: forwarding unless they are explicitly prohibited.
1.233 dtucker 489: The command originally supplied by the client is available in the
490: .Ev SSH_ORIGINAL_COMMAND
491: environment variable.
1.149 markus 492: Note that this option applies to shell, command or subsystem execution.
1.2 deraadt 493: .It Cm environment="NAME=value"
1.1 deraadt 494: Specifies that the string is to be added to the environment when
1.36 aaron 495: logging in using this key.
496: Environment variables set this way
497: override other default environment values.
498: Multiple options of this type are permitted.
1.188 stevesk 499: Environment processing is disabled by default and is
500: controlled via the
501: .Cm PermitUserEnvironment
502: option.
1.155 markus 503: This option is automatically disabled if
504: .Cm UseLogin
505: is enabled.
1.218 jmc 506: .It Cm from="pattern-list"
1.244 djm 507: Specifies that in addition to public key authentication, either the canonical
1.245 jmc 508: name of the remote host or its IP address must be present in the
1.244 djm 509: comma-separated list of patterns.
1.229 jmc 510: See
511: .Sx PATTERNS
512: in
513: .Xr ssh_config 5
514: for more information on patterns.
1.244 djm 515: .Pp
516: In addition to the wildcard matching that may be applied to hostnames or
517: addresses, a
518: .Cm from
519: stanza may match IP addressess using CIDR address/masklen notation.
520: .Pp
521: The purpose of this option is to optionally increase security: public key
522: authentication by itself does not trust the network or name servers or
523: anything (but the key); however, if somebody somehow steals the key, the key
524: permits an intruder to log in from anywhere in the world.
525: This additional option makes using a stolen key more difficult (name
526: servers and/or routers would have to be compromised in addition to
527: just the key).
1.218 jmc 528: .It Cm no-agent-forwarding
529: Forbids authentication agent forwarding when this key is used for
530: authentication.
1.2 deraadt 531: .It Cm no-port-forwarding
1.211 jmc 532: Forbids TCP forwarding when this key is used for authentication.
1.36 aaron 533: Any port forward requests by the client will return an error.
1.230 jmc 534: This might be used, e.g. in connection with the
1.2 deraadt 535: .Cm command
1.1 deraadt 536: option.
1.218 jmc 537: .It Cm no-pty
538: Prevents tty allocation (a request to allocate a pty will fail).
1.240 djm 539: .It Cm no-user-rc
1.241 jmc 540: Disables execution of
1.240 djm 541: .Pa ~/.ssh/rc .
1.2 deraadt 542: .It Cm no-X11-forwarding
1.1 deraadt 543: Forbids X11 forwarding when this key is used for authentication.
544: Any X11 forward requests by the client will return an error.
1.107 djm 545: .It Cm permitopen="host:port"
1.133 itojun 546: Limit local
1.107 djm 547: .Li ``ssh -L''
1.111 stevesk 548: port forwarding such that it may only connect to the specified host and
1.146 stevesk 549: port.
550: IPv6 addresses can be specified with an alternative syntax:
1.200 jmc 551: .Ar host Ns / Ns Ar port .
1.146 stevesk 552: Multiple
1.107 djm 553: .Cm permitopen
1.197 jmc 554: options may be applied separated by commas.
555: No pattern matching is performed on the specified hostnames,
556: they must be literal domains or addresses.
1.209 reyk 557: .It Cm tunnel="n"
558: Force a
559: .Xr tun 4
560: device on the server.
561: Without this option, the next available device will be used if
562: the client requests a tunnel.
1.2 deraadt 563: .El
564: .Pp
1.219 jmc 565: An example authorized_keys file:
1.222 jmc 566: .Bd -literal -offset 3n
1.219 jmc 567: # Comments allowed at start of line
568: ssh-rsa AAAAB3Nza...LiPk== user@example.net
1.222 jmc 569: from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
570: AAAAB2...19Q== john@example.net
571: command="dump /home",no-pty,no-port-forwarding ssh-dss
572: AAAAC3...51R== example.net
573: permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
574: AAAAB5...21S==
575: tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
576: jane@example.net
1.219 jmc 577: .Ed
1.2 deraadt 578: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 579: The
1.194 jmc 580: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 581: and
1.207 djm 582: .Pa ~/.ssh/known_hosts
1.36 aaron 583: files contain host public keys for all known hosts.
584: The global file should
1.37 brad 585: be prepared by the administrator (optional), and the per-user file is
1.220 jmc 586: maintained automatically: whenever the user connects from an unknown host,
1.36 aaron 587: its key is added to the per-user file.
1.2 deraadt 588: .Pp
1.1 deraadt 589: Each line in these files contains the following fields: hostnames,
1.36 aaron 590: bits, exponent, modulus, comment.
591: The fields are separated by spaces.
1.2 deraadt 592: .Pp
1.196 david 593: Hostnames is a comma-separated list of patterns
1.220 jmc 594: .Pf ( Ql *
1.197 jmc 595: and
596: .Ql \&?
1.195 mouring 597: act as
1.1 deraadt 598: wildcards); each pattern in turn is matched against the canonical host
599: name (when authenticating a client) or against the user-supplied
1.36 aaron 600: name (when authenticating a server).
601: A pattern may also be preceded by
1.195 mouring 602: .Ql \&!
1.2 deraadt 603: to indicate negation: if the host name matches a negated
1.1 deraadt 604: pattern, it is not accepted (by that line) even if it matched another
605: pattern on the line.
1.231 dtucker 606: A hostname or address may optionally be enclosed within
607: .Ql \&[
608: and
609: .Ql \&]
610: brackets then followed by
611: .Ql \&:
1.232 jmc 612: and a non-standard port number.
1.2 deraadt 613: .Pp
1.205 djm 614: Alternately, hostnames may be stored in a hashed form which hides host names
1.206 jmc 615: and addresses should the file's contents be disclosed.
616: Hashed hostnames start with a
617: .Ql |
1.205 djm 618: character.
619: Only one hashed hostname may appear on a single line and none of the above
620: negation or wildcard operators may be applied.
621: .Pp
1.49 markus 622: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.220 jmc 623: can be obtained, for example, from
1.167 deraadt 624: .Pa /etc/ssh/ssh_host_key.pub .
1.1 deraadt 625: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 626: .Pp
627: Lines starting with
628: .Ql #
629: and empty lines are ignored as comments.
630: .Pp
1.1 deraadt 631: When performing host authentication, authentication is accepted if any
1.36 aaron 632: matching line has the proper key.
633: It is thus permissible (but not
1.1 deraadt 634: recommended) to have several lines or different host keys for the same
1.36 aaron 635: names.
636: This will inevitably happen when short forms of host names
637: from different domains are put in the file.
638: It is possible
1.1 deraadt 639: that the files contain conflicting information; authentication is
640: accepted if valid information can be found from either file.
1.2 deraadt 641: .Pp
1.1 deraadt 642: Note that the lines in these files are typically hundreds of characters
643: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 644: Rather, generate them by a script
1.40 aaron 645: or by taking
1.167 deraadt 646: .Pa /etc/ssh/ssh_host_key.pub
1.1 deraadt 647: and adding the host names at the front.
1.221 jmc 648: .Pp
649: An example ssh_known_hosts file:
650: .Bd -literal -offset 3n
651: # Comments allowed at start of line
652: closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
653: cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
1.205 djm 654: # A hashed hostname
655: |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
656: AAAA1234.....=
1.120 markus 657: .Ed
1.2 deraadt 658: .Sh FILES
1.223 jmc 659: .Bl -tag -width Ds -compact
660: .It ~/.hushlogin
1.204 jmc 661: This file is used to suppress printing the last login time and
662: .Pa /etc/motd ,
663: if
664: .Cm PrintLastLog
665: and
666: .Cm PrintMotd ,
667: respectively,
668: are enabled.
669: It does not suppress printing of the banner specified by
670: .Cm Banner .
1.223 jmc 671: .Pp
672: .It ~/.rhosts
1.225 jmc 673: This file is used for host-based authentication (see
674: .Xr ssh 1
675: for more information).
676: On some machines this file may need to be
677: world-readable if the user's home directory is on an NFS partition,
678: because
679: .Nm
680: reads it as root.
681: Additionally, this file must be owned by the user,
682: and must not have write permissions for anyone else.
683: The recommended
684: permission for most machines is read/write for the user, and not
1.1 deraadt 685: accessible by others.
1.2 deraadt 686: .Pp
1.223 jmc 687: .It ~/.shosts
1.225 jmc 688: This file is used in exactly the same way as
689: .Pa .rhosts ,
690: but allows host-based authentication without permitting login with
691: rlogin/rsh.
1.238 mcbride 692: .Pp
693: .It ~/.ssh/
694: This directory is the default location for all user-specific configuration
695: and authentication information.
696: There is no general requirement to keep the entire contents of this directory
697: secret, but the recommended permissions are read/write/execute for the user,
698: and not accessible by others.
1.2 deraadt 699: .Pp
1.223 jmc 700: .It ~/.ssh/authorized_keys
1.225 jmc 701: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.223 jmc 702: The format of this file is described above.
1.234 dtucker 703: The content of the file is not highly sensitive, but the recommended
1.225 jmc 704: permissions are read/write for the user, and not accessible by others.
1.234 dtucker 705: .Pp
706: If this file, the
707: .Pa ~/.ssh
708: directory, or the user's home directory are writable
709: by other users, then the file could be modified or replaced by unauthorized
710: users.
711: In this case,
712: .Nm
713: will not allow it to be used unless the
714: .Cm StrictModes
715: option has been set to
716: .Dq no .
717: The recommended permissions can be set by executing
718: .Dq chmod go-w ~/ ~/.ssh ~/.ssh/authorized_keys .
1.2 deraadt 719: .Pp
1.223 jmc 720: .It ~/.ssh/environment
1.36 aaron 721: This file is read into the environment at login (if it exists).
722: It can only contain empty lines, comment lines (that start with
1.2 deraadt 723: .Ql # ) ,
1.36 aaron 724: and assignment lines of the form name=value.
725: The file should be writable
1.6 aaron 726: only by the user; it need not be readable by anyone else.
1.188 stevesk 727: Environment processing is disabled by default and is
728: controlled via the
729: .Cm PermitUserEnvironment
730: option.
1.223 jmc 731: .Pp
732: .It ~/.ssh/known_hosts
1.225 jmc 733: Contains a list of host keys for all hosts the user has logged into
734: that are not already in the systemwide list of known host keys.
735: The format of this file is described above.
736: This file should be writable only by root/the owner and
1.223 jmc 737: can, but need not be, world-readable.
738: .Pp
739: .It ~/.ssh/rc
1.226 jmc 740: Contains initialization routines to be run before
741: the user's home directory becomes accessible.
1.1 deraadt 742: This file should be writable only by the user, and need not be
743: readable by anyone else.
1.223 jmc 744: .Pp
745: .It /etc/hosts.allow
746: .It /etc/hosts.deny
747: Access controls that should be enforced by tcp-wrappers are defined here.
748: Further details are described in
749: .Xr hosts_access 5 .
750: .Pp
751: .It /etc/hosts.equiv
1.227 jmc 752: This file is for host-based authentication (see
753: .Xr ssh 1 ) .
754: It should only be writable by root.
1.223 jmc 755: .Pp
756: .It /etc/moduli
757: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
758: The file format is described in
759: .Xr moduli 5 .
760: .Pp
761: .It /etc/motd
762: See
763: .Xr motd 5 .
764: .Pp
765: .It /etc/nologin
766: If this file exists,
767: .Nm
768: refuses to let anyone except root log in.
769: The contents of the file
770: are displayed to anyone trying to log in, and non-root connections are
771: refused.
772: The file should be world-readable.
773: .Pp
774: .It /etc/shosts.equiv
1.227 jmc 775: This file is used in exactly the same way as
776: .Pa hosts.equiv ,
777: but allows host-based authentication without permitting login with
778: rlogin/rsh.
1.225 jmc 779: .Pp
1.223 jmc 780: .It /etc/ssh/ssh_host_key
781: .It /etc/ssh/ssh_host_dsa_key
782: .It /etc/ssh/ssh_host_rsa_key
783: These three files contain the private parts of the host keys.
784: These files should only be owned by root, readable only by root, and not
785: accessible to others.
786: Note that
787: .Nm
1.228 jmc 788: does not start if these files are group/world-accessible.
1.223 jmc 789: .Pp
790: .It /etc/ssh/ssh_host_key.pub
791: .It /etc/ssh/ssh_host_dsa_key.pub
792: .It /etc/ssh/ssh_host_rsa_key.pub
793: These three files contain the public parts of the host keys.
794: These files should be world-readable but writable only by
795: root.
796: Their contents should match the respective private parts.
797: These files are not
798: really used for anything; they are provided for the convenience of
799: the user so their contents can be copied to known hosts files.
800: These files are created using
801: .Xr ssh-keygen 1 .
802: .Pp
1.235 jmc 803: .It /etc/ssh/ssh_known_hosts
804: Systemwide list of known host keys.
805: This file should be prepared by the
806: system administrator to contain the public host keys of all machines in the
807: organization.
808: The format of this file is described above.
809: This file should be writable only by root/the owner and
810: should be world-readable.
811: .Pp
1.223 jmc 812: .It /etc/ssh/sshd_config
813: Contains configuration data for
814: .Nm sshd .
815: The file format and configuration options are described in
816: .Xr sshd_config 5 .
817: .Pp
818: .It /etc/ssh/sshrc
1.226 jmc 819: Similar to
820: .Pa ~/.ssh/rc ,
821: it can be used to specify
1.36 aaron 822: machine-specific login-time initializations globally.
823: This file should be writable only by root, and should be world-readable.
1.223 jmc 824: .Pp
825: .It /var/empty
826: .Xr chroot 2
827: directory used by
828: .Nm
829: during privilege separation in the pre-authentication phase.
830: The directory should not contain any files and must be owned by root
831: and not group or world-writable.
832: .Pp
833: .It /var/run/sshd.pid
834: Contains the process ID of the
835: .Nm
836: listening for connections (if there are several daemons running
837: concurrently for different ports, this contains the process ID of the one
838: started last).
839: The content of this file is not sensitive; it can be world-readable.
1.56 aaron 840: .El
1.2 deraadt 841: .Sh SEE ALSO
842: .Xr scp 1 ,
1.90 djm 843: .Xr sftp 1 ,
1.2 deraadt 844: .Xr ssh 1 ,
1.5 deraadt 845: .Xr ssh-add 1 ,
1.2 deraadt 846: .Xr ssh-agent 1 ,
1.5 deraadt 847: .Xr ssh-keygen 1 ,
1.235 jmc 848: .Xr ssh-keyscan 1 ,
1.200 jmc 849: .Xr chroot 2 ,
850: .Xr hosts_access 5 ,
1.136 markus 851: .Xr login.conf 5 ,
852: .Xr moduli 5 ,
1.184 stevesk 853: .Xr sshd_config 5 ,
1.200 jmc 854: .Xr inetd 8 ,
1.128 mpech 855: .Xr sftp-server 8
1.198 jmc 856: .Sh AUTHORS
857: OpenSSH is a derivative of the original and free
858: ssh 1.2.12 release by Tatu Ylonen.
859: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
860: Theo de Raadt and Dug Song
861: removed many bugs, re-added newer features and
862: created OpenSSH.
863: Markus Friedl contributed the support for SSH
864: protocol versions 1.5 and 2.0.
865: Niels Provos and Markus Friedl contributed support
866: for privilege separation.
1.216 jmc 867: .Sh CAVEATS
868: System security is not improved unless
869: .Nm rshd ,
870: .Nm rlogind ,
871: and
872: .Nm rexecd
873: are disabled (thus completely disabling
874: .Xr rlogin
875: and
876: .Xr rsh
877: into the machine).
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