Annotation of src/usr.bin/ssh/sshd.8, Revision 1.236
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,
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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: .\"
1.236 ! jmc 37: .\" $OpenBSD: sshd.8,v 1.235 2007/03/20 15:57:15 jmc Exp $
! 38: .Dd $Mdocdate$
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
1.200 jmc 47: .Op Fl 46Ddeiqt
1.2 deraadt 48: .Op Fl b Ar bits
49: .Op Fl f Ar config_file
50: .Op Fl g Ar login_grace_time
51: .Op Fl h Ar host_key_file
52: .Op Fl k Ar key_gen_time
1.156 markus 53: .Op Fl o Ar option
1.2 deraadt 54: .Op Fl p Ar port
1.61 markus 55: .Op Fl u Ar len
1.194 jmc 56: .Ek
1.40 aaron 57: .Sh DESCRIPTION
1.2 deraadt 58: .Nm
1.212 jmc 59: (OpenSSH Daemon) is the daemon program for
1.2 deraadt 60: .Xr ssh 1 .
1.235 jmc 61: Together these programs replace
62: .Xr rlogin 1
63: and
64: .Xr rsh 1 ,
65: and provide secure encrypted communications between two untrusted hosts
1.36 aaron 66: over an insecure network.
1.2 deraadt 67: .Pp
68: .Nm
1.212 jmc 69: listens for connections from clients.
1.40 aaron 70: It is normally started at boot from
1.2 deraadt 71: .Pa /etc/rc .
72: It forks a new
1.36 aaron 73: daemon for each incoming connection.
74: The forked daemons handle
1.1 deraadt 75: key exchange, encryption, authentication, command execution,
76: and data exchange.
1.2 deraadt 77: .Pp
78: .Nm
1.200 jmc 79: can be configured using command-line options or a configuration file
80: (by default
1.212 jmc 81: .Xr sshd_config 5 ) ;
82: command-line options override values specified in the
1.1 deraadt 83: configuration file.
1.25 markus 84: .Nm
85: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 86: .Dv SIGHUP ,
1.230 jmc 87: by executing itself with the name and options it was started with, e.g.\&
1.97 deraadt 88: .Pa /usr/sbin/sshd .
1.18 aaron 89: .Pp
90: The options are as follows:
1.2 deraadt 91: .Bl -tag -width Ds
1.200 jmc 92: .It Fl 4
93: Forces
94: .Nm
95: to use IPv4 addresses only.
96: .It Fl 6
97: Forces
98: .Nm
99: to use IPv6 addresses only.
1.2 deraadt 100: .It Fl b Ar bits
1.120 markus 101: Specifies the number of bits in the ephemeral protocol version 1
102: server key (default 768).
1.200 jmc 103: .It Fl D
104: When this option is specified,
105: .Nm
106: will not detach and does not become a daemon.
107: This allows easy monitoring of
108: .Nm sshd .
1.2 deraadt 109: .It Fl d
1.36 aaron 110: Debug mode.
111: The server sends verbose debug output to the system
112: log, and does not put itself in the background.
113: The server also will not fork and will only process one connection.
114: This option is only intended for debugging for the server.
1.194 jmc 115: Multiple
116: .Fl d
117: options increase the debugging level.
1.67 aaron 118: Maximum is 3.
1.120 markus 119: .It Fl e
120: When this option is specified,
121: .Nm
122: will send the output to the standard error instead of the system log.
1.235 jmc 123: .It Fl f Ar config_file
1.36 aaron 124: Specifies the name of the configuration file.
125: The default is
1.167 deraadt 126: .Pa /etc/ssh/sshd_config .
1.16 markus 127: .Nm
128: refuses to start if there is no configuration file.
1.2 deraadt 129: .It Fl g Ar login_grace_time
1.1 deraadt 130: Gives the grace time for clients to authenticate themselves (default
1.191 stevesk 131: 120 seconds).
1.36 aaron 132: If the client fails to authenticate the user within
133: this many seconds, the server disconnects and exits.
134: A value of zero indicates no limit.
1.2 deraadt 135: .It Fl h Ar host_key_file
1.160 stevesk 136: Specifies a file from which a host key is read.
1.7 markus 137: This option must be given if
138: .Nm
139: is not run as root (as the normal
1.160 stevesk 140: host key files are normally not readable by anyone but root).
141: The default is
1.167 deraadt 142: .Pa /etc/ssh/ssh_host_key
1.160 stevesk 143: for protocol version 1, and
1.167 deraadt 144: .Pa /etc/ssh/ssh_host_rsa_key
1.160 stevesk 145: and
1.167 deraadt 146: .Pa /etc/ssh/ssh_host_dsa_key
1.160 stevesk 147: for protocol version 2.
1.75 markus 148: It is possible to have multiple host key files for
1.120 markus 149: the different protocol versions and host key algorithms.
1.2 deraadt 150: .It Fl i
1.7 markus 151: Specifies that
152: .Nm
1.194 jmc 153: is being run from
154: .Xr inetd 8 .
1.7 markus 155: .Nm
156: is normally not run
1.1 deraadt 157: from inetd because it needs to generate the server key before it can
1.36 aaron 158: respond to the client, and this may take tens of seconds.
159: Clients would have to wait too long if the key was regenerated every time.
1.230 jmc 160: However, with small key sizes (e.g. 512) using
1.7 markus 161: .Nm
162: from inetd may
1.1 deraadt 163: be feasible.
1.2 deraadt 164: .It Fl k Ar key_gen_time
1.120 markus 165: Specifies how often the ephemeral protocol version 1 server key is
166: regenerated (default 3600 seconds, or one hour).
1.36 aaron 167: The motivation for regenerating the key fairly
1.200 jmc 168: often is that the key is not stored anywhere, and after about an hour
1.1 deraadt 169: it becomes impossible to recover the key for decrypting intercepted
170: communications even if the machine is cracked into or physically
1.36 aaron 171: seized.
172: A value of zero indicates that the key will never be regenerated.
1.156 markus 173: .It Fl o Ar option
174: Can be used to give options in the format used in the configuration file.
175: This is useful for specifying options for which there is no separate
176: command-line flag.
1.200 jmc 177: For full details of the options, and their values, see
178: .Xr sshd_config 5 .
1.2 deraadt 179: .It Fl p Ar port
1.1 deraadt 180: Specifies the port on which the server listens for connections
181: (default 22).
1.158 stevesk 182: Multiple port options are permitted.
1.210 stevesk 183: Ports specified in the configuration file with the
184: .Cm Port
185: option are ignored when a command-line port is specified.
186: Ports specified using the
187: .Cm ListenAddress
188: option override command-line ports.
1.2 deraadt 189: .It Fl q
1.36 aaron 190: Quiet mode.
191: Nothing is sent to the system log.
192: Normally the beginning,
1.1 deraadt 193: authentication, and termination of each connection is logged.
1.137 stevesk 194: .It Fl t
195: Test mode.
196: Only check the validity of the configuration file and sanity of the keys.
1.157 deraadt 197: This is useful for updating
1.137 stevesk 198: .Nm
199: reliably as configuration options may change.
1.61 markus 200: .It Fl u Ar len
201: This option is used to specify the size of the field
202: in the
203: .Li utmp
204: structure that holds the remote host name.
205: If the resolved host name is longer than
206: .Ar len ,
207: the dotted decimal value will be used instead.
208: This allows hosts with very long host names that
209: overflow this field to still be uniquely identified.
210: Specifying
211: .Fl u0
212: indicates that only dotted decimal addresses
213: should be put into the
214: .Pa utmp
215: file.
1.144 stevesk 216: .Fl u0
1.194 jmc 217: may also be used to prevent
1.144 stevesk 218: .Nm
219: from making DNS requests unless the authentication
220: mechanism or configuration requires it.
221: Authentication mechanisms that may require DNS include
222: .Cm RhostsRSAAuthentication ,
1.215 jmc 223: .Cm HostbasedAuthentication ,
1.144 stevesk 224: and using a
225: .Cm from="pattern-list"
226: option in a key file.
1.170 stevesk 227: Configuration options that require DNS include using a
228: USER@HOST pattern in
229: .Cm AllowUsers
230: or
231: .Cm DenyUsers .
1.2 deraadt 232: .El
1.214 jmc 233: .Sh AUTHENTICATION
234: The OpenSSH SSH daemon supports SSH protocols 1 and 2.
235: Both protocols are supported by default,
236: though this can be changed via the
237: .Cm Protocol
238: option in
239: .Xr sshd_config 5 .
240: Protocol 2 supports both RSA and DSA keys;
241: protocol 1 only supports RSA keys.
242: For both protocols,
243: each host has a host-specific key,
244: normally 2048 bits,
245: used to identify the host.
246: .Pp
247: Forward security for protocol 1 is provided through
248: an additional server key,
249: normally 768 bits,
250: generated when the server starts.
1.212 jmc 251: This key is normally regenerated every hour if it has been used, and
252: is never stored on disk.
253: Whenever a client connects, the daemon responds with its public
254: host and server keys.
255: The client compares the
256: RSA host key against its own database to verify that it has not changed.
257: The client then generates a 256-bit random number.
258: It encrypts this
259: random number using both the host key and the server key, and sends
260: the encrypted number to the server.
261: Both sides then use this
262: random number as a session key which is used to encrypt all further
263: communications in the session.
264: The rest of the session is encrypted
265: using a conventional cipher, currently Blowfish or 3DES, with 3DES
266: being used by default.
267: The client selects the encryption algorithm
268: to use from those offered by the server.
269: .Pp
1.214 jmc 270: For protocol 2,
271: forward security is provided through a Diffie-Hellman key agreement.
272: This key agreement results in a shared session key.
273: The rest of the session is encrypted using a symmetric cipher, currently
274: 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
275: The client selects the encryption algorithm
276: to use from those offered by the server.
277: Additionally, session integrity is provided
278: through a cryptographic message authentication code
279: (hmac-sha1 or hmac-md5).
280: .Pp
281: Finally, the server and the client enter an authentication dialog.
1.212 jmc 282: The client tries to authenticate itself using
1.214 jmc 283: host-based authentication,
284: public key authentication,
285: challenge-response authentication,
286: or password authentication.
1.212 jmc 287: .Pp
288: If the client successfully authenticates itself, a dialog for
289: preparing the session is entered.
290: At this time the client may request
291: things like allocating a pseudo-tty, forwarding X11 connections,
292: forwarding TCP connections, or forwarding the authentication agent
293: connection over the secure channel.
294: .Pp
1.216 jmc 295: After this, the client either requests a shell or execution of a command.
1.212 jmc 296: The sides then enter session mode.
297: In this mode, either side may send
298: data at any time, and such data is forwarded to/from the shell or
299: command on the server side, and the user terminal in the client side.
300: .Pp
301: When the user program terminates and all forwarded X11 and other
302: connections have been closed, the server sends command exit status to
303: the client, and both sides exit.
1.2 deraadt 304: .Sh LOGIN PROCESS
1.1 deraadt 305: When a user successfully logs in,
1.2 deraadt 306: .Nm
1.1 deraadt 307: does the following:
1.2 deraadt 308: .Bl -enum -offset indent
309: .It
1.1 deraadt 310: If the login is on a tty, and no command has been specified,
1.40 aaron 311: prints last login time and
1.2 deraadt 312: .Pa /etc/motd
1.1 deraadt 313: (unless prevented in the configuration file or by
1.207 djm 314: .Pa ~/.hushlogin ;
1.2 deraadt 315: see the
1.40 aaron 316: .Sx FILES
1.2 deraadt 317: section).
318: .It
1.1 deraadt 319: If the login is on a tty, records login time.
1.2 deraadt 320: .It
321: Checks
322: .Pa /etc/nologin ;
323: if it exists, prints contents and quits
1.1 deraadt 324: (unless root).
1.2 deraadt 325: .It
1.1 deraadt 326: Changes to run with normal user privileges.
1.2 deraadt 327: .It
1.1 deraadt 328: Sets up basic environment.
1.2 deraadt 329: .It
1.200 jmc 330: Reads the file
1.207 djm 331: .Pa ~/.ssh/environment ,
1.200 jmc 332: if it exists, and users are allowed to change their environment.
1.188 stevesk 333: See the
1.187 marc 334: .Cm PermitUserEnvironment
1.188 stevesk 335: option in
1.187 marc 336: .Xr sshd_config 5 .
1.2 deraadt 337: .It
1.1 deraadt 338: Changes to user's home directory.
1.2 deraadt 339: .It
340: If
1.207 djm 341: .Pa ~/.ssh/rc
1.2 deraadt 342: exists, runs it; else if
1.168 deraadt 343: .Pa /etc/ssh/sshrc
1.2 deraadt 344: exists, runs
1.36 aaron 345: it; otherwise runs xauth.
346: The
1.2 deraadt 347: .Dq rc
348: files are given the X11
1.1 deraadt 349: authentication protocol and cookie in standard input.
1.226 jmc 350: See
351: .Sx SSHRC ,
352: below.
1.2 deraadt 353: .It
1.1 deraadt 354: Runs user's shell or command.
1.2 deraadt 355: .El
1.226 jmc 356: .Sh SSHRC
357: If the file
358: .Pa ~/.ssh/rc
359: exists,
360: .Xr sh 1
361: runs it after reading the
362: environment files but before starting the user's shell or command.
363: It must not produce any output on stdout; stderr must be used
364: instead.
365: If X11 forwarding is in use, it will receive the "proto cookie" pair in
366: its standard input (and
367: .Ev DISPLAY
368: in its environment).
369: The script must call
370: .Xr xauth 1
371: because
372: .Nm
373: will not run xauth automatically to add X11 cookies.
374: .Pp
375: The primary purpose of this file is to run any initialization routines
376: which may be needed before the user's home directory becomes
377: accessible; AFS is a particular example of such an environment.
378: .Pp
379: This file will probably contain some initialization code followed by
380: something similar to:
381: .Bd -literal -offset 3n
382: if read proto cookie && [ -n "$DISPLAY" ]; then
383: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
384: # X11UseLocalhost=yes
385: echo add unix:`echo $DISPLAY |
386: cut -c11-` $proto $cookie
387: else
388: # X11UseLocalhost=no
389: echo add $DISPLAY $proto $cookie
390: fi | xauth -q -
391: fi
392: .Ed
393: .Pp
394: If this file does not exist,
395: .Pa /etc/ssh/sshrc
396: is run, and if that
397: does not exist either, xauth is used to add the cookie.
1.2 deraadt 398: .Sh AUTHORIZED_KEYS FILE FORMAT
1.125 markus 399: .Cm AuthorizedKeysFile
1.217 jmc 400: specifies the file containing public keys for
401: public key authentication;
402: if none is specified, the default is
403: .Pa ~/.ssh/authorized_keys .
1.36 aaron 404: Each line of the file contains one
1.2 deraadt 405: key (empty lines and lines starting with a
406: .Ql #
407: are ignored as
1.36 aaron 408: comments).
1.217 jmc 409: Protocol 1 public keys consist of the following space-separated fields:
410: options, bits, exponent, modulus, comment.
411: Protocol 2 public key consist of:
412: options, keytype, base64-encoded key, comment.
413: The options field is optional;
414: its presence is determined by whether the line starts
1.189 espie 415: with a number or not (the options field never starts with a number).
1.217 jmc 416: The bits, exponent, modulus, and comment fields give the RSA key for
1.75 markus 417: protocol version 1; the
1.1 deraadt 418: comment field is not used for anything (but may be convenient for the
419: user to identify the key).
1.75 markus 420: For protocol version 2 the keytype is
421: .Dq ssh-dss
422: or
423: .Dq ssh-rsa .
1.2 deraadt 424: .Pp
1.1 deraadt 425: Note that lines in this file are usually several hundred bytes long
1.203 dtucker 426: (because of the size of the public key encoding) up to a limit of
427: 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
428: keys up to 16 kilobits.
1.36 aaron 429: You don't want to type them in; instead, copy the
1.113 itojun 430: .Pa identity.pub ,
1.217 jmc 431: .Pa id_dsa.pub ,
1.75 markus 432: or the
1.113 itojun 433: .Pa id_rsa.pub
1.1 deraadt 434: file and edit it.
1.175 stevesk 435: .Pp
436: .Nm
437: enforces a minimum RSA key modulus size for protocol 1
438: and protocol 2 keys of 768 bits.
1.2 deraadt 439: .Pp
1.58 deraadt 440: The options (if present) consist of comma-separated option
1.36 aaron 441: specifications.
442: No spaces are permitted, except within double quotes.
1.141 stevesk 443: The following option specifications are supported (note
444: that option keywords are case-insensitive):
1.2 deraadt 445: .Bl -tag -width Ds
446: .It Cm command="command"
1.1 deraadt 447: Specifies that the command is executed whenever this key is used for
1.36 aaron 448: authentication.
449: The command supplied by the user (if any) is ignored.
1.148 markus 450: The command is run on a pty if the client requests a pty;
1.36 aaron 451: otherwise it is run without a tty.
1.194 jmc 452: If an 8-bit clean channel is required,
1.147 deraadt 453: one must not request a pty or should specify
1.89 markus 454: .Cm no-pty .
1.36 aaron 455: A quote may be included in the command by quoting it with a backslash.
456: This option might be useful
1.189 espie 457: to restrict certain public keys to perform just a specific operation.
1.36 aaron 458: An example might be a key that permits remote backups but nothing else.
1.211 jmc 459: Note that the client may specify TCP and/or X11
1.51 hugh 460: forwarding unless they are explicitly prohibited.
1.233 dtucker 461: The command originally supplied by the client is available in the
462: .Ev SSH_ORIGINAL_COMMAND
463: environment variable.
1.149 markus 464: Note that this option applies to shell, command or subsystem execution.
1.2 deraadt 465: .It Cm environment="NAME=value"
1.1 deraadt 466: Specifies that the string is to be added to the environment when
1.36 aaron 467: logging in using this key.
468: Environment variables set this way
469: override other default environment values.
470: Multiple options of this type are permitted.
1.188 stevesk 471: Environment processing is disabled by default and is
472: controlled via the
473: .Cm PermitUserEnvironment
474: option.
1.155 markus 475: This option is automatically disabled if
476: .Cm UseLogin
477: is enabled.
1.218 jmc 478: .It Cm from="pattern-list"
479: Specifies that in addition to public key authentication, the canonical name
480: of the remote host must be present in the comma-separated list of
1.229 jmc 481: patterns.
1.218 jmc 482: The purpose
483: of this option is to optionally increase security: public key authentication
484: by itself does not trust the network or name servers or anything (but
485: the key); however, if somebody somehow steals the key, the key
486: permits an intruder to log in from anywhere in the world.
487: This additional option makes using a stolen key more difficult (name
488: servers and/or routers would have to be compromised in addition to
489: just the key).
1.229 jmc 490: .Pp
491: See
492: .Sx PATTERNS
493: in
494: .Xr ssh_config 5
495: for more information on patterns.
1.218 jmc 496: .It Cm no-agent-forwarding
497: Forbids authentication agent forwarding when this key is used for
498: authentication.
1.2 deraadt 499: .It Cm no-port-forwarding
1.211 jmc 500: Forbids TCP forwarding when this key is used for authentication.
1.36 aaron 501: Any port forward requests by the client will return an error.
1.230 jmc 502: This might be used, e.g. in connection with the
1.2 deraadt 503: .Cm command
1.1 deraadt 504: option.
1.218 jmc 505: .It Cm no-pty
506: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 507: .It Cm no-X11-forwarding
1.1 deraadt 508: Forbids X11 forwarding when this key is used for authentication.
509: Any X11 forward requests by the client will return an error.
1.107 djm 510: .It Cm permitopen="host:port"
1.133 itojun 511: Limit local
1.107 djm 512: .Li ``ssh -L''
1.111 stevesk 513: port forwarding such that it may only connect to the specified host and
1.146 stevesk 514: port.
515: IPv6 addresses can be specified with an alternative syntax:
1.200 jmc 516: .Ar host Ns / Ns Ar port .
1.146 stevesk 517: Multiple
1.107 djm 518: .Cm permitopen
1.197 jmc 519: options may be applied separated by commas.
520: No pattern matching is performed on the specified hostnames,
521: they must be literal domains or addresses.
1.209 reyk 522: .It Cm tunnel="n"
523: Force a
524: .Xr tun 4
525: device on the server.
526: Without this option, the next available device will be used if
527: the client requests a tunnel.
1.2 deraadt 528: .El
529: .Pp
1.219 jmc 530: An example authorized_keys file:
1.222 jmc 531: .Bd -literal -offset 3n
1.219 jmc 532: # Comments allowed at start of line
533: ssh-rsa AAAAB3Nza...LiPk== user@example.net
1.222 jmc 534: from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
535: AAAAB2...19Q== john@example.net
536: command="dump /home",no-pty,no-port-forwarding ssh-dss
537: AAAAC3...51R== example.net
538: permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
539: AAAAB5...21S==
540: tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
541: jane@example.net
1.219 jmc 542: .Ed
1.2 deraadt 543: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 544: The
1.194 jmc 545: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 546: and
1.207 djm 547: .Pa ~/.ssh/known_hosts
1.36 aaron 548: files contain host public keys for all known hosts.
549: The global file should
1.37 brad 550: be prepared by the administrator (optional), and the per-user file is
1.220 jmc 551: maintained automatically: whenever the user connects from an unknown host,
1.36 aaron 552: its key is added to the per-user file.
1.2 deraadt 553: .Pp
1.1 deraadt 554: Each line in these files contains the following fields: hostnames,
1.36 aaron 555: bits, exponent, modulus, comment.
556: The fields are separated by spaces.
1.2 deraadt 557: .Pp
1.196 david 558: Hostnames is a comma-separated list of patterns
1.220 jmc 559: .Pf ( Ql *
1.197 jmc 560: and
561: .Ql \&?
1.195 mouring 562: act as
1.1 deraadt 563: wildcards); each pattern in turn is matched against the canonical host
564: name (when authenticating a client) or against the user-supplied
1.36 aaron 565: name (when authenticating a server).
566: A pattern may also be preceded by
1.195 mouring 567: .Ql \&!
1.2 deraadt 568: to indicate negation: if the host name matches a negated
1.1 deraadt 569: pattern, it is not accepted (by that line) even if it matched another
570: pattern on the line.
1.231 dtucker 571: A hostname or address may optionally be enclosed within
572: .Ql \&[
573: and
574: .Ql \&]
575: brackets then followed by
576: .Ql \&:
1.232 jmc 577: and a non-standard port number.
1.2 deraadt 578: .Pp
1.205 djm 579: Alternately, hostnames may be stored in a hashed form which hides host names
1.206 jmc 580: and addresses should the file's contents be disclosed.
581: Hashed hostnames start with a
582: .Ql |
1.205 djm 583: character.
584: Only one hashed hostname may appear on a single line and none of the above
585: negation or wildcard operators may be applied.
586: .Pp
1.49 markus 587: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.220 jmc 588: can be obtained, for example, from
1.167 deraadt 589: .Pa /etc/ssh/ssh_host_key.pub .
1.1 deraadt 590: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 591: .Pp
592: Lines starting with
593: .Ql #
594: and empty lines are ignored as comments.
595: .Pp
1.1 deraadt 596: When performing host authentication, authentication is accepted if any
1.36 aaron 597: matching line has the proper key.
598: It is thus permissible (but not
1.1 deraadt 599: recommended) to have several lines or different host keys for the same
1.36 aaron 600: names.
601: This will inevitably happen when short forms of host names
602: from different domains are put in the file.
603: It is possible
1.1 deraadt 604: that the files contain conflicting information; authentication is
605: accepted if valid information can be found from either file.
1.2 deraadt 606: .Pp
1.1 deraadt 607: Note that the lines in these files are typically hundreds of characters
608: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 609: Rather, generate them by a script
1.40 aaron 610: or by taking
1.167 deraadt 611: .Pa /etc/ssh/ssh_host_key.pub
1.1 deraadt 612: and adding the host names at the front.
1.221 jmc 613: .Pp
614: An example ssh_known_hosts file:
615: .Bd -literal -offset 3n
616: # Comments allowed at start of line
617: closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
618: cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
1.205 djm 619: # A hashed hostname
620: |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
621: AAAA1234.....=
1.120 markus 622: .Ed
1.2 deraadt 623: .Sh FILES
1.223 jmc 624: .Bl -tag -width Ds -compact
625: .It ~/.hushlogin
1.204 jmc 626: This file is used to suppress printing the last login time and
627: .Pa /etc/motd ,
628: if
629: .Cm PrintLastLog
630: and
631: .Cm PrintMotd ,
632: respectively,
633: are enabled.
634: It does not suppress printing of the banner specified by
635: .Cm Banner .
1.223 jmc 636: .Pp
637: .It ~/.rhosts
1.225 jmc 638: This file is used for host-based authentication (see
639: .Xr ssh 1
640: for more information).
641: On some machines this file may need to be
642: world-readable if the user's home directory is on an NFS partition,
643: because
644: .Nm
645: reads it as root.
646: Additionally, this file must be owned by the user,
647: and must not have write permissions for anyone else.
648: The recommended
649: permission for most machines is read/write for the user, and not
1.1 deraadt 650: accessible by others.
1.2 deraadt 651: .Pp
1.223 jmc 652: .It ~/.shosts
1.225 jmc 653: This file is used in exactly the same way as
654: .Pa .rhosts ,
655: but allows host-based authentication without permitting login with
656: rlogin/rsh.
1.2 deraadt 657: .Pp
1.223 jmc 658: .It ~/.ssh/authorized_keys
1.225 jmc 659: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.223 jmc 660: The format of this file is described above.
1.234 dtucker 661: The content of the file is not highly sensitive, but the recommended
1.225 jmc 662: permissions are read/write for the user, and not accessible by others.
1.234 dtucker 663: .Pp
664: If this file, the
665: .Pa ~/.ssh
666: directory, or the user's home directory are writable
667: by other users, then the file could be modified or replaced by unauthorized
668: users.
669: In this case,
670: .Nm
671: will not allow it to be used unless the
672: .Cm StrictModes
673: option has been set to
674: .Dq no .
675: The recommended permissions can be set by executing
676: .Dq chmod go-w ~/ ~/.ssh ~/.ssh/authorized_keys .
1.2 deraadt 677: .Pp
1.223 jmc 678: .It ~/.ssh/environment
1.36 aaron 679: This file is read into the environment at login (if it exists).
680: It can only contain empty lines, comment lines (that start with
1.2 deraadt 681: .Ql # ) ,
1.36 aaron 682: and assignment lines of the form name=value.
683: The file should be writable
1.6 aaron 684: only by the user; it need not be readable by anyone else.
1.188 stevesk 685: Environment processing is disabled by default and is
686: controlled via the
687: .Cm PermitUserEnvironment
688: option.
1.223 jmc 689: .Pp
690: .It ~/.ssh/known_hosts
1.225 jmc 691: Contains a list of host keys for all hosts the user has logged into
692: that are not already in the systemwide list of known host keys.
693: The format of this file is described above.
694: This file should be writable only by root/the owner and
1.223 jmc 695: can, but need not be, world-readable.
696: .Pp
697: .It ~/.ssh/rc
1.226 jmc 698: Contains initialization routines to be run before
699: the user's home directory becomes accessible.
1.1 deraadt 700: This file should be writable only by the user, and need not be
701: readable by anyone else.
1.223 jmc 702: .Pp
703: .It /etc/hosts.allow
704: .It /etc/hosts.deny
705: Access controls that should be enforced by tcp-wrappers are defined here.
706: Further details are described in
707: .Xr hosts_access 5 .
708: .Pp
709: .It /etc/hosts.equiv
1.227 jmc 710: This file is for host-based authentication (see
711: .Xr ssh 1 ) .
712: It should only be writable by root.
1.223 jmc 713: .Pp
714: .It /etc/moduli
715: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
716: The file format is described in
717: .Xr moduli 5 .
718: .Pp
719: .It /etc/motd
720: See
721: .Xr motd 5 .
722: .Pp
723: .It /etc/nologin
724: If this file exists,
725: .Nm
726: refuses to let anyone except root log in.
727: The contents of the file
728: are displayed to anyone trying to log in, and non-root connections are
729: refused.
730: The file should be world-readable.
731: .Pp
732: .It /etc/shosts.equiv
1.227 jmc 733: This file is used in exactly the same way as
734: .Pa hosts.equiv ,
735: but allows host-based authentication without permitting login with
736: rlogin/rsh.
1.225 jmc 737: .Pp
1.223 jmc 738: .It /etc/ssh/ssh_host_key
739: .It /etc/ssh/ssh_host_dsa_key
740: .It /etc/ssh/ssh_host_rsa_key
741: These three files contain the private parts of the host keys.
742: These files should only be owned by root, readable only by root, and not
743: accessible to others.
744: Note that
745: .Nm
1.228 jmc 746: does not start if these files are group/world-accessible.
1.223 jmc 747: .Pp
748: .It /etc/ssh/ssh_host_key.pub
749: .It /etc/ssh/ssh_host_dsa_key.pub
750: .It /etc/ssh/ssh_host_rsa_key.pub
751: These three files contain the public parts of the host keys.
752: These files should be world-readable but writable only by
753: root.
754: Their contents should match the respective private parts.
755: These files are not
756: really used for anything; they are provided for the convenience of
757: the user so their contents can be copied to known hosts files.
758: These files are created using
759: .Xr ssh-keygen 1 .
760: .Pp
1.235 jmc 761: .It /etc/ssh/ssh_known_hosts
762: Systemwide list of known host keys.
763: This file should be prepared by the
764: system administrator to contain the public host keys of all machines in the
765: organization.
766: The format of this file is described above.
767: This file should be writable only by root/the owner and
768: should be world-readable.
769: .Pp
1.223 jmc 770: .It /etc/ssh/sshd_config
771: Contains configuration data for
772: .Nm sshd .
773: The file format and configuration options are described in
774: .Xr sshd_config 5 .
775: .Pp
776: .It /etc/ssh/sshrc
1.226 jmc 777: Similar to
778: .Pa ~/.ssh/rc ,
779: it can be used to specify
1.36 aaron 780: machine-specific login-time initializations globally.
781: This file should be writable only by root, and should be world-readable.
1.223 jmc 782: .Pp
783: .It /var/empty
784: .Xr chroot 2
785: directory used by
786: .Nm
787: during privilege separation in the pre-authentication phase.
788: The directory should not contain any files and must be owned by root
789: and not group or world-writable.
790: .Pp
791: .It /var/run/sshd.pid
792: Contains the process ID of the
793: .Nm
794: listening for connections (if there are several daemons running
795: concurrently for different ports, this contains the process ID of the one
796: started last).
797: The content of this file is not sensitive; it can be world-readable.
1.56 aaron 798: .El
1.2 deraadt 799: .Sh SEE ALSO
800: .Xr scp 1 ,
1.90 djm 801: .Xr sftp 1 ,
1.2 deraadt 802: .Xr ssh 1 ,
1.5 deraadt 803: .Xr ssh-add 1 ,
1.2 deraadt 804: .Xr ssh-agent 1 ,
1.5 deraadt 805: .Xr ssh-keygen 1 ,
1.235 jmc 806: .Xr ssh-keyscan 1 ,
1.200 jmc 807: .Xr chroot 2 ,
808: .Xr hosts_access 5 ,
1.136 markus 809: .Xr login.conf 5 ,
810: .Xr moduli 5 ,
1.184 stevesk 811: .Xr sshd_config 5 ,
1.200 jmc 812: .Xr inetd 8 ,
1.128 mpech 813: .Xr sftp-server 8
1.198 jmc 814: .Sh AUTHORS
815: OpenSSH is a derivative of the original and free
816: ssh 1.2.12 release by Tatu Ylonen.
817: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
818: Theo de Raadt and Dug Song
819: removed many bugs, re-added newer features and
820: created OpenSSH.
821: Markus Friedl contributed the support for SSH
822: protocol versions 1.5 and 2.0.
823: Niels Provos and Markus Friedl contributed support
824: for privilege separation.
1.216 jmc 825: .Sh CAVEATS
826: System security is not improved unless
827: .Nm rshd ,
828: .Nm rlogind ,
829: and
830: .Nm rexecd
831: are disabled (thus completely disabling
832: .Xr rlogin
833: and
834: .Xr rsh
835: into the machine).
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