Annotation of src/usr.bin/ssh/sshd.8, Revision 1.234
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: .\"
1.234 ! dtucker 37: .\" $OpenBSD: sshd.8,v 1.233 2006/07/19 13:07:10 dtucker Exp $
1.2 deraadt 38: .Dd September 25, 1999
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 .
1.42 hugh 61: Together these programs replace rlogin and rsh, and
1.1 deraadt 62: provide secure encrypted communications between two untrusted hosts
1.36 aaron 63: over an insecure network.
1.2 deraadt 64: .Pp
65: .Nm
1.212 jmc 66: listens for connections from clients.
1.40 aaron 67: It is normally started at boot from
1.2 deraadt 68: .Pa /etc/rc .
69: It forks a new
1.36 aaron 70: daemon for each incoming connection.
71: The forked daemons handle
1.1 deraadt 72: key exchange, encryption, authentication, command execution,
73: and data exchange.
1.2 deraadt 74: .Pp
75: .Nm
1.200 jmc 76: can be configured using command-line options or a configuration file
77: (by default
1.212 jmc 78: .Xr sshd_config 5 ) ;
79: command-line options override values specified in the
1.1 deraadt 80: configuration file.
1.25 markus 81: .Nm
82: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 83: .Dv SIGHUP ,
1.230 jmc 84: by executing itself with the name and options it was started with, e.g.\&
1.97 deraadt 85: .Pa /usr/sbin/sshd .
1.18 aaron 86: .Pp
87: The options are as follows:
1.2 deraadt 88: .Bl -tag -width Ds
1.200 jmc 89: .It Fl 4
90: Forces
91: .Nm
92: to use IPv4 addresses only.
93: .It Fl 6
94: Forces
95: .Nm
96: to use IPv6 addresses only.
1.2 deraadt 97: .It Fl b Ar bits
1.120 markus 98: Specifies the number of bits in the ephemeral protocol version 1
99: server key (default 768).
1.200 jmc 100: .It Fl D
101: When this option is specified,
102: .Nm
103: will not detach and does not become a daemon.
104: This allows easy monitoring of
105: .Nm sshd .
1.2 deraadt 106: .It Fl d
1.36 aaron 107: Debug mode.
108: The server sends verbose debug output to the system
109: log, and does not put itself in the background.
110: The server also will not fork and will only process one connection.
111: This option is only intended for debugging for the server.
1.194 jmc 112: Multiple
113: .Fl d
114: options increase the debugging level.
1.67 aaron 115: Maximum is 3.
1.120 markus 116: .It Fl e
117: When this option is specified,
118: .Nm
119: will send the output to the standard error instead of the system log.
1.2 deraadt 120: .It Fl f Ar configuration_file
1.36 aaron 121: Specifies the name of the configuration file.
122: The default is
1.167 deraadt 123: .Pa /etc/ssh/sshd_config .
1.16 markus 124: .Nm
125: refuses to start if there is no configuration file.
1.2 deraadt 126: .It Fl g Ar login_grace_time
1.1 deraadt 127: Gives the grace time for clients to authenticate themselves (default
1.191 stevesk 128: 120 seconds).
1.36 aaron 129: If the client fails to authenticate the user within
130: this many seconds, the server disconnects and exits.
131: A value of zero indicates no limit.
1.2 deraadt 132: .It Fl h Ar host_key_file
1.160 stevesk 133: Specifies a file from which a host key is read.
1.7 markus 134: This option must be given if
135: .Nm
136: is not run as root (as the normal
1.160 stevesk 137: host key files are normally not readable by anyone but root).
138: The default is
1.167 deraadt 139: .Pa /etc/ssh/ssh_host_key
1.160 stevesk 140: for protocol version 1, and
1.167 deraadt 141: .Pa /etc/ssh/ssh_host_rsa_key
1.160 stevesk 142: and
1.167 deraadt 143: .Pa /etc/ssh/ssh_host_dsa_key
1.160 stevesk 144: for protocol version 2.
1.75 markus 145: It is possible to have multiple host key files for
1.120 markus 146: the different protocol versions and host key algorithms.
1.2 deraadt 147: .It Fl i
1.7 markus 148: Specifies that
149: .Nm
1.194 jmc 150: is being run from
151: .Xr inetd 8 .
1.7 markus 152: .Nm
153: is normally not run
1.1 deraadt 154: from inetd because it needs to generate the server key before it can
1.36 aaron 155: respond to the client, and this may take tens of seconds.
156: Clients would have to wait too long if the key was regenerated every time.
1.230 jmc 157: However, with small key sizes (e.g. 512) using
1.7 markus 158: .Nm
159: from inetd may
1.1 deraadt 160: be feasible.
1.2 deraadt 161: .It Fl k Ar key_gen_time
1.120 markus 162: Specifies how often the ephemeral protocol version 1 server key is
163: regenerated (default 3600 seconds, or one hour).
1.36 aaron 164: The motivation for regenerating the key fairly
1.200 jmc 165: often is that the key is not stored anywhere, and after about an hour
1.1 deraadt 166: it becomes impossible to recover the key for decrypting intercepted
167: communications even if the machine is cracked into or physically
1.36 aaron 168: seized.
169: A value of zero indicates that the key will never be regenerated.
1.156 markus 170: .It Fl o Ar option
171: Can be used to give options in the format used in the configuration file.
172: This is useful for specifying options for which there is no separate
173: command-line flag.
1.200 jmc 174: For full details of the options, and their values, see
175: .Xr sshd_config 5 .
1.2 deraadt 176: .It Fl p Ar port
1.1 deraadt 177: Specifies the port on which the server listens for connections
178: (default 22).
1.158 stevesk 179: Multiple port options are permitted.
1.210 stevesk 180: Ports specified in the configuration file with the
181: .Cm Port
182: option are ignored when a command-line port is specified.
183: Ports specified using the
184: .Cm ListenAddress
185: option override command-line ports.
1.2 deraadt 186: .It Fl q
1.36 aaron 187: Quiet mode.
188: Nothing is sent to the system log.
189: Normally the beginning,
1.1 deraadt 190: authentication, and termination of each connection is logged.
1.137 stevesk 191: .It Fl t
192: Test mode.
193: Only check the validity of the configuration file and sanity of the keys.
1.157 deraadt 194: This is useful for updating
1.137 stevesk 195: .Nm
196: reliably as configuration options may change.
1.61 markus 197: .It Fl u Ar len
198: This option is used to specify the size of the field
199: in the
200: .Li utmp
201: structure that holds the remote host name.
202: If the resolved host name is longer than
203: .Ar len ,
204: the dotted decimal value will be used instead.
205: This allows hosts with very long host names that
206: overflow this field to still be uniquely identified.
207: Specifying
208: .Fl u0
209: indicates that only dotted decimal addresses
210: should be put into the
211: .Pa utmp
212: file.
1.144 stevesk 213: .Fl u0
1.194 jmc 214: may also be used to prevent
1.144 stevesk 215: .Nm
216: from making DNS requests unless the authentication
217: mechanism or configuration requires it.
218: Authentication mechanisms that may require DNS include
219: .Cm RhostsRSAAuthentication ,
1.215 jmc 220: .Cm HostbasedAuthentication ,
1.144 stevesk 221: and using a
222: .Cm from="pattern-list"
223: option in a key file.
1.170 stevesk 224: Configuration options that require DNS include using a
225: USER@HOST pattern in
226: .Cm AllowUsers
227: or
228: .Cm DenyUsers .
1.2 deraadt 229: .El
1.214 jmc 230: .Sh AUTHENTICATION
231: The OpenSSH SSH daemon supports SSH protocols 1 and 2.
232: Both protocols are supported by default,
233: though this can be changed via the
234: .Cm Protocol
235: option in
236: .Xr sshd_config 5 .
237: Protocol 2 supports both RSA and DSA keys;
238: protocol 1 only supports RSA keys.
239: For both protocols,
240: each host has a host-specific key,
241: normally 2048 bits,
242: used to identify the host.
243: .Pp
244: Forward security for protocol 1 is provided through
245: an additional server key,
246: normally 768 bits,
247: generated when the server starts.
1.212 jmc 248: This key is normally regenerated every hour if it has been used, and
249: is never stored on disk.
250: Whenever a client connects, the daemon responds with its public
251: host and server keys.
252: The client compares the
253: RSA host key against its own database to verify that it has not changed.
254: The client then generates a 256-bit random number.
255: It encrypts this
256: random number using both the host key and the server key, and sends
257: the encrypted number to the server.
258: Both sides then use this
259: random number as a session key which is used to encrypt all further
260: communications in the session.
261: The rest of the session is encrypted
262: using a conventional cipher, currently Blowfish or 3DES, with 3DES
263: being used by default.
264: The client selects the encryption algorithm
265: to use from those offered by the server.
266: .Pp
1.214 jmc 267: For protocol 2,
268: forward security is provided through a Diffie-Hellman key agreement.
269: This key agreement results in a shared session key.
270: The rest of the session is encrypted using a symmetric cipher, currently
271: 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
272: The client selects the encryption algorithm
273: to use from those offered by the server.
274: Additionally, session integrity is provided
275: through a cryptographic message authentication code
276: (hmac-sha1 or hmac-md5).
277: .Pp
278: Finally, the server and the client enter an authentication dialog.
1.212 jmc 279: The client tries to authenticate itself using
1.214 jmc 280: host-based authentication,
281: public key authentication,
282: challenge-response authentication,
283: or password authentication.
1.212 jmc 284: .Pp
285: If the client successfully authenticates itself, a dialog for
286: preparing the session is entered.
287: At this time the client may request
288: things like allocating a pseudo-tty, forwarding X11 connections,
289: forwarding TCP connections, or forwarding the authentication agent
290: connection over the secure channel.
291: .Pp
1.216 jmc 292: After this, the client either requests a shell or execution of a command.
1.212 jmc 293: The sides then enter session mode.
294: In this mode, either side may send
295: data at any time, and such data is forwarded to/from the shell or
296: command on the server side, and the user terminal in the client side.
297: .Pp
298: When the user program terminates and all forwarded X11 and other
299: connections have been closed, the server sends command exit status to
300: the client, and both sides exit.
1.2 deraadt 301: .Sh LOGIN PROCESS
1.1 deraadt 302: When a user successfully logs in,
1.2 deraadt 303: .Nm
1.1 deraadt 304: does the following:
1.2 deraadt 305: .Bl -enum -offset indent
306: .It
1.1 deraadt 307: If the login is on a tty, and no command has been specified,
1.40 aaron 308: prints last login time and
1.2 deraadt 309: .Pa /etc/motd
1.1 deraadt 310: (unless prevented in the configuration file or by
1.207 djm 311: .Pa ~/.hushlogin ;
1.2 deraadt 312: see the
1.40 aaron 313: .Sx FILES
1.2 deraadt 314: section).
315: .It
1.1 deraadt 316: If the login is on a tty, records login time.
1.2 deraadt 317: .It
318: Checks
319: .Pa /etc/nologin ;
320: if it exists, prints contents and quits
1.1 deraadt 321: (unless root).
1.2 deraadt 322: .It
1.1 deraadt 323: Changes to run with normal user privileges.
1.2 deraadt 324: .It
1.1 deraadt 325: Sets up basic environment.
1.2 deraadt 326: .It
1.200 jmc 327: Reads the file
1.207 djm 328: .Pa ~/.ssh/environment ,
1.200 jmc 329: if it exists, and users are allowed to change their environment.
1.188 stevesk 330: See the
1.187 marc 331: .Cm PermitUserEnvironment
1.188 stevesk 332: option in
1.187 marc 333: .Xr sshd_config 5 .
1.2 deraadt 334: .It
1.1 deraadt 335: Changes to user's home directory.
1.2 deraadt 336: .It
337: If
1.207 djm 338: .Pa ~/.ssh/rc
1.2 deraadt 339: exists, runs it; else if
1.168 deraadt 340: .Pa /etc/ssh/sshrc
1.2 deraadt 341: exists, runs
1.36 aaron 342: it; otherwise runs xauth.
343: The
1.2 deraadt 344: .Dq rc
345: files are given the X11
1.1 deraadt 346: authentication protocol and cookie in standard input.
1.226 jmc 347: See
348: .Sx SSHRC ,
349: below.
1.2 deraadt 350: .It
1.1 deraadt 351: Runs user's shell or command.
1.2 deraadt 352: .El
1.226 jmc 353: .Sh SSHRC
354: If the file
355: .Pa ~/.ssh/rc
356: exists,
357: .Xr sh 1
358: runs it after reading the
359: environment files but before starting the user's shell or command.
360: It must not produce any output on stdout; stderr must be used
361: instead.
362: If X11 forwarding is in use, it will receive the "proto cookie" pair in
363: its standard input (and
364: .Ev DISPLAY
365: in its environment).
366: The script must call
367: .Xr xauth 1
368: because
369: .Nm
370: will not run xauth automatically to add X11 cookies.
371: .Pp
372: The primary purpose of this file is to run any initialization routines
373: which may be needed before the user's home directory becomes
374: accessible; AFS is a particular example of such an environment.
375: .Pp
376: This file will probably contain some initialization code followed by
377: something similar to:
378: .Bd -literal -offset 3n
379: if read proto cookie && [ -n "$DISPLAY" ]; then
380: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
381: # X11UseLocalhost=yes
382: echo add unix:`echo $DISPLAY |
383: cut -c11-` $proto $cookie
384: else
385: # X11UseLocalhost=no
386: echo add $DISPLAY $proto $cookie
387: fi | xauth -q -
388: fi
389: .Ed
390: .Pp
391: If this file does not exist,
392: .Pa /etc/ssh/sshrc
393: is run, and if that
394: does not exist either, xauth is used to add the cookie.
1.2 deraadt 395: .Sh AUTHORIZED_KEYS FILE FORMAT
1.125 markus 396: .Cm AuthorizedKeysFile
1.217 jmc 397: specifies the file containing public keys for
398: public key authentication;
399: if none is specified, the default is
400: .Pa ~/.ssh/authorized_keys .
1.36 aaron 401: Each line of the file contains one
1.2 deraadt 402: key (empty lines and lines starting with a
403: .Ql #
404: are ignored as
1.36 aaron 405: comments).
1.217 jmc 406: Protocol 1 public keys consist of the following space-separated fields:
407: options, bits, exponent, modulus, comment.
408: Protocol 2 public key consist of:
409: options, keytype, base64-encoded key, comment.
410: The options field is optional;
411: its presence is determined by whether the line starts
1.189 espie 412: with a number or not (the options field never starts with a number).
1.217 jmc 413: The bits, exponent, modulus, and comment fields give the RSA key for
1.75 markus 414: protocol version 1; the
1.1 deraadt 415: comment field is not used for anything (but may be convenient for the
416: user to identify the key).
1.75 markus 417: For protocol version 2 the keytype is
418: .Dq ssh-dss
419: or
420: .Dq ssh-rsa .
1.2 deraadt 421: .Pp
1.1 deraadt 422: Note that lines in this file are usually several hundred bytes long
1.203 dtucker 423: (because of the size of the public key encoding) up to a limit of
424: 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
425: keys up to 16 kilobits.
1.36 aaron 426: You don't want to type them in; instead, copy the
1.113 itojun 427: .Pa identity.pub ,
1.217 jmc 428: .Pa id_dsa.pub ,
1.75 markus 429: or the
1.113 itojun 430: .Pa id_rsa.pub
1.1 deraadt 431: file and edit it.
1.175 stevesk 432: .Pp
433: .Nm
434: enforces a minimum RSA key modulus size for protocol 1
435: and protocol 2 keys of 768 bits.
1.2 deraadt 436: .Pp
1.58 deraadt 437: The options (if present) consist of comma-separated option
1.36 aaron 438: specifications.
439: No spaces are permitted, except within double quotes.
1.141 stevesk 440: The following option specifications are supported (note
441: that option keywords are case-insensitive):
1.2 deraadt 442: .Bl -tag -width Ds
443: .It Cm command="command"
1.1 deraadt 444: Specifies that the command is executed whenever this key is used for
1.36 aaron 445: authentication.
446: The command supplied by the user (if any) is ignored.
1.148 markus 447: The command is run on a pty if the client requests a pty;
1.36 aaron 448: otherwise it is run without a tty.
1.194 jmc 449: If an 8-bit clean channel is required,
1.147 deraadt 450: one must not request a pty or should specify
1.89 markus 451: .Cm no-pty .
1.36 aaron 452: A quote may be included in the command by quoting it with a backslash.
453: This option might be useful
1.189 espie 454: to restrict certain public keys to perform just a specific operation.
1.36 aaron 455: An example might be a key that permits remote backups but nothing else.
1.211 jmc 456: Note that the client may specify TCP and/or X11
1.51 hugh 457: forwarding unless they are explicitly prohibited.
1.233 dtucker 458: The command originally supplied by the client is available in the
459: .Ev SSH_ORIGINAL_COMMAND
460: environment variable.
1.149 markus 461: Note that this option applies to shell, command or subsystem execution.
1.2 deraadt 462: .It Cm environment="NAME=value"
1.1 deraadt 463: Specifies that the string is to be added to the environment when
1.36 aaron 464: logging in using this key.
465: Environment variables set this way
466: override other default environment values.
467: Multiple options of this type are permitted.
1.188 stevesk 468: Environment processing is disabled by default and is
469: controlled via the
470: .Cm PermitUserEnvironment
471: option.
1.155 markus 472: This option is automatically disabled if
473: .Cm UseLogin
474: is enabled.
1.218 jmc 475: .It Cm from="pattern-list"
476: Specifies that in addition to public key authentication, the canonical name
477: of the remote host must be present in the comma-separated list of
1.229 jmc 478: patterns.
1.218 jmc 479: The purpose
480: of this option is to optionally increase security: public key authentication
481: by itself does not trust the network or name servers or anything (but
482: the key); however, if somebody somehow steals the key, the key
483: permits an intruder to log in from anywhere in the world.
484: This additional option makes using a stolen key more difficult (name
485: servers and/or routers would have to be compromised in addition to
486: just the key).
1.229 jmc 487: .Pp
488: See
489: .Sx PATTERNS
490: in
491: .Xr ssh_config 5
492: for more information on patterns.
1.218 jmc 493: .It Cm no-agent-forwarding
494: Forbids authentication agent forwarding when this key is used for
495: authentication.
1.2 deraadt 496: .It Cm no-port-forwarding
1.211 jmc 497: Forbids TCP forwarding when this key is used for authentication.
1.36 aaron 498: Any port forward requests by the client will return an error.
1.230 jmc 499: This might be used, e.g. in connection with the
1.2 deraadt 500: .Cm command
1.1 deraadt 501: option.
1.218 jmc 502: .It Cm no-pty
503: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 504: .It Cm no-X11-forwarding
1.1 deraadt 505: Forbids X11 forwarding when this key is used for authentication.
506: Any X11 forward requests by the client will return an error.
1.107 djm 507: .It Cm permitopen="host:port"
1.133 itojun 508: Limit local
1.107 djm 509: .Li ``ssh -L''
1.111 stevesk 510: port forwarding such that it may only connect to the specified host and
1.146 stevesk 511: port.
512: IPv6 addresses can be specified with an alternative syntax:
1.200 jmc 513: .Ar host Ns / Ns Ar port .
1.146 stevesk 514: Multiple
1.107 djm 515: .Cm permitopen
1.197 jmc 516: options may be applied separated by commas.
517: No pattern matching is performed on the specified hostnames,
518: they must be literal domains or addresses.
1.209 reyk 519: .It Cm tunnel="n"
520: Force a
521: .Xr tun 4
522: device on the server.
523: Without this option, the next available device will be used if
524: the client requests a tunnel.
1.2 deraadt 525: .El
526: .Pp
1.219 jmc 527: An example authorized_keys file:
1.222 jmc 528: .Bd -literal -offset 3n
1.219 jmc 529: # Comments allowed at start of line
530: ssh-rsa AAAAB3Nza...LiPk== user@example.net
1.222 jmc 531: from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
532: AAAAB2...19Q== john@example.net
533: command="dump /home",no-pty,no-port-forwarding ssh-dss
534: AAAAC3...51R== example.net
535: permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
536: AAAAB5...21S==
537: tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
538: jane@example.net
1.219 jmc 539: .Ed
1.2 deraadt 540: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 541: The
1.194 jmc 542: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 543: and
1.207 djm 544: .Pa ~/.ssh/known_hosts
1.36 aaron 545: files contain host public keys for all known hosts.
546: The global file should
1.37 brad 547: be prepared by the administrator (optional), and the per-user file is
1.220 jmc 548: maintained automatically: whenever the user connects from an unknown host,
1.36 aaron 549: its key is added to the per-user file.
1.2 deraadt 550: .Pp
1.1 deraadt 551: Each line in these files contains the following fields: hostnames,
1.36 aaron 552: bits, exponent, modulus, comment.
553: The fields are separated by spaces.
1.2 deraadt 554: .Pp
1.196 david 555: Hostnames is a comma-separated list of patterns
1.220 jmc 556: .Pf ( Ql *
1.197 jmc 557: and
558: .Ql \&?
1.195 mouring 559: act as
1.1 deraadt 560: wildcards); each pattern in turn is matched against the canonical host
561: name (when authenticating a client) or against the user-supplied
1.36 aaron 562: name (when authenticating a server).
563: A pattern may also be preceded by
1.195 mouring 564: .Ql \&!
1.2 deraadt 565: to indicate negation: if the host name matches a negated
1.1 deraadt 566: pattern, it is not accepted (by that line) even if it matched another
567: pattern on the line.
1.231 dtucker 568: A hostname or address may optionally be enclosed within
569: .Ql \&[
570: and
571: .Ql \&]
572: brackets then followed by
573: .Ql \&:
1.232 jmc 574: and a non-standard port number.
1.2 deraadt 575: .Pp
1.205 djm 576: Alternately, hostnames may be stored in a hashed form which hides host names
1.206 jmc 577: and addresses should the file's contents be disclosed.
578: Hashed hostnames start with a
579: .Ql |
1.205 djm 580: character.
581: Only one hashed hostname may appear on a single line and none of the above
582: negation or wildcard operators may be applied.
583: .Pp
1.49 markus 584: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.220 jmc 585: can be obtained, for example, from
1.167 deraadt 586: .Pa /etc/ssh/ssh_host_key.pub .
1.1 deraadt 587: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 588: .Pp
589: Lines starting with
590: .Ql #
591: and empty lines are ignored as comments.
592: .Pp
1.1 deraadt 593: When performing host authentication, authentication is accepted if any
1.36 aaron 594: matching line has the proper key.
595: It is thus permissible (but not
1.1 deraadt 596: recommended) to have several lines or different host keys for the same
1.36 aaron 597: names.
598: This will inevitably happen when short forms of host names
599: from different domains are put in the file.
600: It is possible
1.1 deraadt 601: that the files contain conflicting information; authentication is
602: accepted if valid information can be found from either file.
1.2 deraadt 603: .Pp
1.1 deraadt 604: Note that the lines in these files are typically hundreds of characters
605: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 606: Rather, generate them by a script
1.40 aaron 607: or by taking
1.167 deraadt 608: .Pa /etc/ssh/ssh_host_key.pub
1.1 deraadt 609: and adding the host names at the front.
1.221 jmc 610: .Pp
611: An example ssh_known_hosts file:
612: .Bd -literal -offset 3n
613: # Comments allowed at start of line
614: closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
615: cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
1.205 djm 616: # A hashed hostname
617: |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
618: AAAA1234.....=
1.120 markus 619: .Ed
1.2 deraadt 620: .Sh FILES
1.223 jmc 621: .Bl -tag -width Ds -compact
622: .It ~/.hushlogin
1.204 jmc 623: This file is used to suppress printing the last login time and
624: .Pa /etc/motd ,
625: if
626: .Cm PrintLastLog
627: and
628: .Cm PrintMotd ,
629: respectively,
630: are enabled.
631: It does not suppress printing of the banner specified by
632: .Cm Banner .
1.223 jmc 633: .Pp
634: .It ~/.rhosts
1.225 jmc 635: This file is used for host-based authentication (see
636: .Xr ssh 1
637: for more information).
638: On some machines this file may need to be
639: world-readable if the user's home directory is on an NFS partition,
640: because
641: .Nm
642: reads it as root.
643: Additionally, this file must be owned by the user,
644: and must not have write permissions for anyone else.
645: The recommended
646: permission for most machines is read/write for the user, and not
1.1 deraadt 647: accessible by others.
1.2 deraadt 648: .Pp
1.223 jmc 649: .It ~/.shosts
1.225 jmc 650: This file is used in exactly the same way as
651: .Pa .rhosts ,
652: but allows host-based authentication without permitting login with
653: rlogin/rsh.
1.2 deraadt 654: .Pp
1.223 jmc 655: .It ~/.ssh/authorized_keys
1.225 jmc 656: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.223 jmc 657: The format of this file is described above.
1.234 ! dtucker 658: The content of the file is not highly sensitive, but the recommended
1.225 jmc 659: permissions are read/write for the user, and not accessible by others.
1.234 ! dtucker 660: .Pp
! 661: If this file, the
! 662: .Pa ~/.ssh
! 663: directory, or the user's home directory are writable
! 664: by other users, then the file could be modified or replaced by unauthorized
! 665: users.
! 666: In this case,
! 667: .Nm
! 668: will not allow it to be used unless the
! 669: .Cm StrictModes
! 670: option has been set to
! 671: .Dq no .
! 672: The recommended permissions can be set by executing
! 673: .Dq chmod go-w ~/ ~/.ssh ~/.ssh/authorized_keys .
1.2 deraadt 674: .Pp
1.223 jmc 675: .It ~/.ssh/environment
1.36 aaron 676: This file is read into the environment at login (if it exists).
677: It can only contain empty lines, comment lines (that start with
1.2 deraadt 678: .Ql # ) ,
1.36 aaron 679: and assignment lines of the form name=value.
680: The file should be writable
1.6 aaron 681: only by the user; it need not be readable by anyone else.
1.188 stevesk 682: Environment processing is disabled by default and is
683: controlled via the
684: .Cm PermitUserEnvironment
685: option.
1.223 jmc 686: .Pp
687: .It ~/.ssh/known_hosts
1.225 jmc 688: Contains a list of host keys for all hosts the user has logged into
689: that are not already in the systemwide list of known host keys.
690: The format of this file is described above.
691: This file should be writable only by root/the owner and
1.223 jmc 692: can, but need not be, world-readable.
693: .Pp
694: .It ~/.ssh/rc
1.226 jmc 695: Contains initialization routines to be run before
696: the user's home directory becomes accessible.
1.1 deraadt 697: This file should be writable only by the user, and need not be
698: readable by anyone else.
1.223 jmc 699: .Pp
700: .It /etc/hosts.allow
701: .It /etc/hosts.deny
702: Access controls that should be enforced by tcp-wrappers are defined here.
703: Further details are described in
704: .Xr hosts_access 5 .
705: .Pp
706: .It /etc/hosts.equiv
1.227 jmc 707: This file is for host-based authentication (see
708: .Xr ssh 1 ) .
709: It should only be writable by root.
1.223 jmc 710: .Pp
711: .It /etc/moduli
712: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
713: The file format is described in
714: .Xr moduli 5 .
715: .Pp
716: .It /etc/motd
717: See
718: .Xr motd 5 .
719: .Pp
720: .It /etc/nologin
721: If this file exists,
722: .Nm
723: refuses to let anyone except root log in.
724: The contents of the file
725: are displayed to anyone trying to log in, and non-root connections are
726: refused.
727: The file should be world-readable.
728: .Pp
729: .It /etc/shosts.equiv
1.227 jmc 730: This file is used in exactly the same way as
731: .Pa hosts.equiv ,
732: but allows host-based authentication without permitting login with
733: rlogin/rsh.
1.225 jmc 734: .Pp
735: .It /etc/ssh/ssh_known_hosts
736: Systemwide list of known host keys.
737: This file should be prepared by the
738: system administrator to contain the public host keys of all machines in the
739: organization.
740: The format of this file is described above.
741: This file should be writable only by root/the owner and
742: should be world-readable.
1.223 jmc 743: .Pp
744: .It /etc/ssh/ssh_host_key
745: .It /etc/ssh/ssh_host_dsa_key
746: .It /etc/ssh/ssh_host_rsa_key
747: These three files contain the private parts of the host keys.
748: These files should only be owned by root, readable only by root, and not
749: accessible to others.
750: Note that
751: .Nm
1.228 jmc 752: does not start if these files are group/world-accessible.
1.223 jmc 753: .Pp
754: .It /etc/ssh/ssh_host_key.pub
755: .It /etc/ssh/ssh_host_dsa_key.pub
756: .It /etc/ssh/ssh_host_rsa_key.pub
757: These three files contain the public parts of the host keys.
758: These files should be world-readable but writable only by
759: root.
760: Their contents should match the respective private parts.
761: These files are not
762: really used for anything; they are provided for the convenience of
763: the user so their contents can be copied to known hosts files.
764: These files are created using
765: .Xr ssh-keygen 1 .
766: .Pp
767: .It /etc/ssh/sshd_config
768: Contains configuration data for
769: .Nm sshd .
770: The file format and configuration options are described in
771: .Xr sshd_config 5 .
772: .Pp
773: .It /etc/ssh/sshrc
1.226 jmc 774: Similar to
775: .Pa ~/.ssh/rc ,
776: it can be used to specify
1.36 aaron 777: machine-specific login-time initializations globally.
778: This file should be writable only by root, and should be world-readable.
1.223 jmc 779: .Pp
780: .It /var/empty
781: .Xr chroot 2
782: directory used by
783: .Nm
784: during privilege separation in the pre-authentication phase.
785: The directory should not contain any files and must be owned by root
786: and not group or world-writable.
787: .Pp
788: .It /var/run/sshd.pid
789: Contains the process ID of the
790: .Nm
791: listening for connections (if there are several daemons running
792: concurrently for different ports, this contains the process ID of the one
793: started last).
794: The content of this file is not sensitive; it can be world-readable.
1.56 aaron 795: .El
1.2 deraadt 796: .Sh SEE ALSO
797: .Xr scp 1 ,
1.90 djm 798: .Xr sftp 1 ,
1.2 deraadt 799: .Xr ssh 1 ,
1.5 deraadt 800: .Xr ssh-add 1 ,
1.2 deraadt 801: .Xr ssh-agent 1 ,
1.5 deraadt 802: .Xr ssh-keygen 1 ,
1.200 jmc 803: .Xr chroot 2 ,
804: .Xr hosts_access 5 ,
1.136 markus 805: .Xr login.conf 5 ,
806: .Xr moduli 5 ,
1.184 stevesk 807: .Xr sshd_config 5 ,
1.200 jmc 808: .Xr inetd 8 ,
1.128 mpech 809: .Xr sftp-server 8
1.198 jmc 810: .Sh AUTHORS
811: OpenSSH is a derivative of the original and free
812: ssh 1.2.12 release by Tatu Ylonen.
813: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
814: Theo de Raadt and Dug Song
815: removed many bugs, re-added newer features and
816: created OpenSSH.
817: Markus Friedl contributed the support for SSH
818: protocol versions 1.5 and 2.0.
819: Niels Provos and Markus Friedl contributed support
820: for privilege separation.
1.216 jmc 821: .Sh CAVEATS
822: System security is not improved unless
823: .Nm rshd ,
824: .Nm rlogind ,
825: and
826: .Nm rexecd
827: are disabled (thus completely disabling
828: .Xr rlogin
829: and
830: .Xr rsh
831: into the machine).
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