Annotation of src/usr.bin/ssh/sshd.8, Revision 1.286
1.1 deraadt 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
1.64 deraadt 6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
1.99 deraadt 12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
1.64 deraadt 15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
18: .\" are met:
19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
23: .\" documentation and/or other materials provided with the distribution.
1.1 deraadt 24: .\"
1.64 deraadt 25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31: .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32: .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 35: .\"
1.286 ! djm 36: .\" $OpenBSD: sshd.8,v 1.285 2016/08/15 12:32:04 naddy Exp $
! 37: .Dd $Mdocdate: August 15 2016 $
1.2 deraadt 38: .Dt SSHD 8
39: .Os
40: .Sh NAME
41: .Nm sshd
1.120 markus 42: .Nd OpenSSH SSH daemon
1.2 deraadt 43: .Sh SYNOPSIS
44: .Nm sshd
1.194 jmc 45: .Bk -words
1.243 jmc 46: .Op Fl 46DdeiqTt
1.242 dtucker 47: .Op Fl C Ar connection_spec
1.251 djm 48: .Op Fl c Ar host_certificate_file
1.268 dtucker 49: .Op Fl E Ar log_file
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
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 .
1.275 tedu 61: Together these programs replace rlogin and rsh,
1.235 jmc 62: and 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.
1.243 jmc 97: .It Fl C Ar connection_spec
98: Specify the connection parameters to use for the
99: .Fl T
100: extended test mode.
101: If provided, any
102: .Cm Match
103: directives in the configuration file
104: that would apply to the specified user, host, and address will be set before
105: the configuration is written to standard output.
106: The connection parameters are supplied as keyword=value pairs.
107: The keywords are
108: .Dq user ,
109: .Dq host ,
1.265 dtucker 110: .Dq laddr ,
111: .Dq lport ,
1.243 jmc 112: and
113: .Dq addr .
114: All are required and may be supplied in any order, either with multiple
115: .Fl C
116: options or as a comma-separated list.
1.252 jmc 117: .It Fl c Ar host_certificate_file
118: Specifies a path to a certificate file to identify
119: .Nm
120: during key exchange.
121: The certificate file must match a host key file specified using the
122: .Fl h
123: option or the
124: .Cm HostKey
125: configuration directive.
1.200 jmc 126: .It Fl D
127: When this option is specified,
128: .Nm
129: will not detach and does not become a daemon.
130: This allows easy monitoring of
131: .Nm sshd .
1.2 deraadt 132: .It Fl d
1.36 aaron 133: Debug mode.
1.250 djm 134: The server sends verbose debug output to standard error,
135: and does not put itself in the background.
1.36 aaron 136: The server also will not fork and will only process one connection.
137: This option is only intended for debugging for the server.
1.194 jmc 138: Multiple
139: .Fl d
140: options increase the debugging level.
1.67 aaron 141: Maximum is 3.
1.268 dtucker 142: .It Fl E Ar log_file
143: Append debug logs to
144: .Ar log_file
145: instead of the system log.
1.120 markus 146: .It Fl e
1.269 dtucker 147: Write debug logs to standard error instead of the system log.
1.235 jmc 148: .It Fl f Ar config_file
1.36 aaron 149: Specifies the name of the configuration file.
150: The default is
1.167 deraadt 151: .Pa /etc/ssh/sshd_config .
1.16 markus 152: .Nm
153: refuses to start if there is no configuration file.
1.2 deraadt 154: .It Fl g Ar login_grace_time
1.1 deraadt 155: Gives the grace time for clients to authenticate themselves (default
1.191 stevesk 156: 120 seconds).
1.36 aaron 157: If the client fails to authenticate the user within
158: this many seconds, the server disconnects and exits.
159: A value of zero indicates no limit.
1.2 deraadt 160: .It Fl h Ar host_key_file
1.160 stevesk 161: Specifies a file from which a host key is read.
1.7 markus 162: This option must be given if
163: .Nm
164: is not run as root (as the normal
1.160 stevesk 165: host key files are normally not readable by anyone but root).
166: The default is
1.259 djm 167: .Pa /etc/ssh/ssh_host_dsa_key ,
1.285 naddy 168: .Pa /etc/ssh/ssh_host_ecdsa_key ,
1.273 naddy 169: .Pa /etc/ssh/ssh_host_ed25519_key
1.259 djm 170: and
1.285 naddy 171: .Pa /etc/ssh/ssh_host_rsa_key .
1.75 markus 172: It is possible to have multiple host key files for
1.285 naddy 173: the different host key algorithms.
1.2 deraadt 174: .It Fl i
1.7 markus 175: Specifies that
176: .Nm
1.194 jmc 177: is being run from
178: .Xr inetd 8 .
1.156 markus 179: .It Fl o Ar option
180: Can be used to give options in the format used in the configuration file.
181: This is useful for specifying options for which there is no separate
182: command-line flag.
1.200 jmc 183: For full details of the options, and their values, see
184: .Xr sshd_config 5 .
1.2 deraadt 185: .It Fl p Ar port
1.1 deraadt 186: Specifies the port on which the server listens for connections
187: (default 22).
1.158 stevesk 188: Multiple port options are permitted.
1.210 stevesk 189: Ports specified in the configuration file with the
190: .Cm Port
191: option are ignored when a command-line port is specified.
192: Ports specified using the
193: .Cm ListenAddress
194: option override command-line ports.
1.2 deraadt 195: .It Fl q
1.36 aaron 196: Quiet mode.
197: Nothing is sent to the system log.
198: Normally the beginning,
1.1 deraadt 199: authentication, and termination of each connection is logged.
1.242 dtucker 200: .It Fl T
201: Extended test mode.
202: Check the validity of the configuration file, output the effective configuration
203: to stdout and then exit.
204: Optionally,
205: .Cm Match
206: rules may be applied by specifying the connection parameters using one or more
207: .Fl C
208: options.
1.243 jmc 209: .It Fl t
210: Test mode.
211: Only check the validity of the configuration file and sanity of the keys.
212: This is useful for updating
213: .Nm
214: reliably as configuration options may change.
1.61 markus 215: .It Fl u Ar len
216: This option is used to specify the size of the field
217: in the
218: .Li utmp
219: structure that holds the remote host name.
220: If the resolved host name is longer than
221: .Ar len ,
222: the dotted decimal value will be used instead.
223: This allows hosts with very long host names that
224: overflow this field to still be uniquely identified.
225: Specifying
226: .Fl u0
227: indicates that only dotted decimal addresses
228: should be put into the
229: .Pa utmp
230: file.
1.144 stevesk 231: .Fl u0
1.194 jmc 232: may also be used to prevent
1.144 stevesk 233: .Nm
234: from making DNS requests unless the authentication
235: mechanism or configuration requires it.
236: Authentication mechanisms that may require DNS include
1.285 naddy 237: .Cm HostbasedAuthentication
1.144 stevesk 238: and using a
239: .Cm from="pattern-list"
240: option in a key file.
1.170 stevesk 241: Configuration options that require DNS include using a
242: USER@HOST pattern in
243: .Cm AllowUsers
244: or
245: .Cm DenyUsers .
1.2 deraadt 246: .El
1.214 jmc 247: .Sh AUTHENTICATION
1.285 naddy 248: The OpenSSH SSH daemon supports SSH protocol 2 only.
1.284 jmc 249: Each host has a host-specific key,
1.214 jmc 250: used to identify the host.
1.212 jmc 251: Whenever a client connects, the daemon responds with its public
1.285 naddy 252: host key.
1.212 jmc 253: The client compares the
1.285 naddy 254: host key against its own database to verify that it has not changed.
255: Forward security is provided through a Diffie-Hellman key agreement.
1.214 jmc 256: This key agreement results in a shared session key.
257: The rest of the session is encrypted using a symmetric cipher, currently
258: 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
259: The client selects the encryption algorithm
260: to use from those offered by the server.
261: Additionally, session integrity is provided
262: through a cryptographic message authentication code
1.267 markus 263: (hmac-md5, hmac-sha1, umac-64, umac-128, hmac-ripemd160,
1.263 djm 264: hmac-sha2-256 or hmac-sha2-512).
1.214 jmc 265: .Pp
266: Finally, the server and the client enter an authentication dialog.
1.212 jmc 267: The client tries to authenticate itself using
1.214 jmc 268: host-based authentication,
269: public key authentication,
270: challenge-response authentication,
271: or password authentication.
1.212 jmc 272: .Pp
273: If the client successfully authenticates itself, a dialog for
274: preparing the session is entered.
275: At this time the client may request
276: things like allocating a pseudo-tty, forwarding X11 connections,
277: forwarding TCP connections, or forwarding the authentication agent
278: connection over the secure channel.
279: .Pp
1.216 jmc 280: After this, the client either requests a shell or execution of a command.
1.212 jmc 281: The sides then enter session mode.
282: In this mode, either side may send
283: data at any time, and such data is forwarded to/from the shell or
284: command on the server side, and the user terminal in the client side.
285: .Pp
286: When the user program terminates and all forwarded X11 and other
287: connections have been closed, the server sends command exit status to
288: the client, and both sides exit.
1.2 deraadt 289: .Sh LOGIN PROCESS
1.1 deraadt 290: When a user successfully logs in,
1.2 deraadt 291: .Nm
1.1 deraadt 292: does the following:
1.2 deraadt 293: .Bl -enum -offset indent
294: .It
1.1 deraadt 295: If the login is on a tty, and no command has been specified,
1.40 aaron 296: prints last login time and
1.2 deraadt 297: .Pa /etc/motd
1.1 deraadt 298: (unless prevented in the configuration file or by
1.207 djm 299: .Pa ~/.hushlogin ;
1.2 deraadt 300: see the
1.40 aaron 301: .Sx FILES
1.2 deraadt 302: section).
303: .It
1.1 deraadt 304: If the login is on a tty, records login time.
1.2 deraadt 305: .It
306: Checks
307: .Pa /etc/nologin ;
308: if it exists, prints contents and quits
1.1 deraadt 309: (unless root).
1.2 deraadt 310: .It
1.1 deraadt 311: Changes to run with normal user privileges.
1.2 deraadt 312: .It
1.1 deraadt 313: Sets up basic environment.
1.2 deraadt 314: .It
1.200 jmc 315: Reads the file
1.207 djm 316: .Pa ~/.ssh/environment ,
1.200 jmc 317: if it exists, and users are allowed to change their environment.
1.188 stevesk 318: See the
1.187 marc 319: .Cm PermitUserEnvironment
1.188 stevesk 320: option in
1.187 marc 321: .Xr sshd_config 5 .
1.2 deraadt 322: .It
1.1 deraadt 323: Changes to user's home directory.
1.2 deraadt 324: .It
325: If
1.207 djm 326: .Pa ~/.ssh/rc
1.276 djm 327: exists and the
328: .Xr sshd_config 5
329: .Cm PermitUserRC
330: option is set, runs it; else if
1.168 deraadt 331: .Pa /etc/ssh/sshrc
1.2 deraadt 332: exists, runs
1.36 aaron 333: it; otherwise runs xauth.
334: The
1.2 deraadt 335: .Dq rc
336: files are given the X11
1.1 deraadt 337: authentication protocol and cookie in standard input.
1.226 jmc 338: See
339: .Sx SSHRC ,
340: below.
1.2 deraadt 341: .It
1.1 deraadt 342: Runs user's shell or command.
1.279 djm 343: All commands are run under the user's login shell as specified in the
344: system password database.
1.2 deraadt 345: .El
1.226 jmc 346: .Sh SSHRC
347: If the file
348: .Pa ~/.ssh/rc
349: exists,
350: .Xr sh 1
351: runs it after reading the
352: environment files but before starting the user's shell or command.
353: It must not produce any output on stdout; stderr must be used
354: instead.
355: If X11 forwarding is in use, it will receive the "proto cookie" pair in
356: its standard input (and
357: .Ev DISPLAY
358: in its environment).
359: The script must call
360: .Xr xauth 1
361: because
362: .Nm
363: will not run xauth automatically to add X11 cookies.
364: .Pp
365: The primary purpose of this file is to run any initialization routines
366: which may be needed before the user's home directory becomes
367: accessible; AFS is a particular example of such an environment.
368: .Pp
369: This file will probably contain some initialization code followed by
370: something similar to:
371: .Bd -literal -offset 3n
372: if read proto cookie && [ -n "$DISPLAY" ]; then
373: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
374: # X11UseLocalhost=yes
375: echo add unix:`echo $DISPLAY |
376: cut -c11-` $proto $cookie
377: else
378: # X11UseLocalhost=no
379: echo add $DISPLAY $proto $cookie
380: fi | xauth -q -
381: fi
382: .Ed
383: .Pp
384: If this file does not exist,
385: .Pa /etc/ssh/sshrc
386: is run, and if that
387: does not exist either, xauth is used to add the cookie.
1.2 deraadt 388: .Sh AUTHORIZED_KEYS FILE FORMAT
1.125 markus 389: .Cm AuthorizedKeysFile
1.262 jmc 390: specifies the files containing public keys for
1.217 jmc 391: public key authentication;
1.281 djm 392: if this option is not specified, the default is
1.261 djm 393: .Pa ~/.ssh/authorized_keys
394: and
395: .Pa ~/.ssh/authorized_keys2 .
1.36 aaron 396: Each line of the file contains one
1.2 deraadt 397: key (empty lines and lines starting with a
398: .Ql #
399: are ignored as
1.36 aaron 400: comments).
1.285 naddy 401: Public keys consist of the following space-separated fields:
1.217 jmc 402: options, keytype, base64-encoded key, comment.
1.285 naddy 403: The options field is optional.
404: The keytype is
1.259 djm 405: .Dq ecdsa-sha2-nistp256 ,
406: .Dq ecdsa-sha2-nistp384 ,
407: .Dq ecdsa-sha2-nistp521 ,
1.272 jmc 408: .Dq ssh-ed25519 ,
1.75 markus 409: .Dq ssh-dss
410: or
1.285 naddy 411: .Dq ssh-rsa ;
412: the comment field is not used for anything (but may be convenient for the
413: user to identify the key).
1.2 deraadt 414: .Pp
1.285 naddy 415: Note that lines in this file can be several hundred bytes long
1.203 dtucker 416: (because of the size of the public key encoding) up to a limit of
417: 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
418: keys up to 16 kilobits.
1.36 aaron 419: You don't want to type them in; instead, copy the
1.217 jmc 420: .Pa id_dsa.pub ,
1.259 djm 421: .Pa id_ecdsa.pub ,
1.273 naddy 422: .Pa id_ed25519.pub ,
1.75 markus 423: or the
1.113 itojun 424: .Pa id_rsa.pub
1.1 deraadt 425: file and edit it.
1.175 stevesk 426: .Pp
427: .Nm
1.285 naddy 428: enforces a minimum RSA key modulus size of 768 bits.
1.2 deraadt 429: .Pp
1.58 deraadt 430: The options (if present) consist of comma-separated option
1.36 aaron 431: specifications.
432: No spaces are permitted, except within double quotes.
1.141 stevesk 433: The following option specifications are supported (note
434: that option keywords are case-insensitive):
1.2 deraadt 435: .Bl -tag -width Ds
1.282 djm 436: .It Cm agent-forwarding
437: Enable authentication agent forwarding previously disabled by the
438: .Cm restrict
439: option.
1.253 djm 440: .It Cm cert-authority
1.251 djm 441: Specifies that the listed key is a certification authority (CA) that is
442: trusted to validate signed certificates for user authentication.
443: .Pp
444: Certificates may encode access restrictions similar to these key options.
445: If both certificate restrictions and key options are present, the most
446: restrictive union of the two is applied.
1.2 deraadt 447: .It Cm command="command"
1.1 deraadt 448: Specifies that the command is executed whenever this key is used for
1.36 aaron 449: authentication.
450: The command supplied by the user (if any) is ignored.
1.148 markus 451: The command is run on a pty if the client requests a pty;
1.36 aaron 452: otherwise it is run without a tty.
1.194 jmc 453: If an 8-bit clean channel is required,
1.147 deraadt 454: one must not request a pty or should specify
1.89 markus 455: .Cm no-pty .
1.36 aaron 456: A quote may be included in the command by quoting it with a backslash.
457: This option might be useful
1.189 espie 458: to restrict certain public keys to perform just a specific operation.
1.36 aaron 459: An example might be a key that permits remote backups but nothing else.
1.211 jmc 460: Note that the client may specify TCP and/or X11
1.51 hugh 461: forwarding unless they are explicitly prohibited.
1.233 dtucker 462: The command originally supplied by the client is available in the
463: .Ev SSH_ORIGINAL_COMMAND
464: environment variable.
1.149 markus 465: Note that this option applies to shell, command or subsystem execution.
1.251 djm 466: Also note that this command may be superseded by either a
467: .Xr sshd_config 5
468: .Cm ForceCommand
469: directive or a command embedded in a certificate.
1.2 deraadt 470: .It Cm environment="NAME=value"
1.1 deraadt 471: Specifies that the string is to be added to the environment when
1.36 aaron 472: logging in using this key.
473: Environment variables set this way
474: override other default environment values.
475: Multiple options of this type are permitted.
1.188 stevesk 476: Environment processing is disabled by default and is
477: controlled via the
478: .Cm PermitUserEnvironment
479: option.
1.218 jmc 480: .It Cm from="pattern-list"
1.244 djm 481: Specifies that in addition to public key authentication, either the canonical
1.245 jmc 482: name of the remote host or its IP address must be present in the
1.244 djm 483: comma-separated list of patterns.
1.270 jmc 484: See PATTERNS in
1.229 jmc 485: .Xr ssh_config 5
486: for more information on patterns.
1.244 djm 487: .Pp
488: In addition to the wildcard matching that may be applied to hostnames or
489: addresses, a
490: .Cm from
1.248 sobrado 491: stanza may match IP addresses using CIDR address/masklen notation.
1.244 djm 492: .Pp
493: The purpose of this option is to optionally increase security: public key
494: authentication by itself does not trust the network or name servers or
495: anything (but the key); however, if somebody somehow steals the key, the key
496: permits an intruder to log in from anywhere in the world.
497: This additional option makes using a stolen key more difficult (name
498: servers and/or routers would have to be compromised in addition to
499: just the key).
1.218 jmc 500: .It Cm no-agent-forwarding
501: Forbids authentication agent forwarding when this key is used for
502: authentication.
1.2 deraadt 503: .It Cm no-port-forwarding
1.211 jmc 504: Forbids TCP forwarding when this key is used for authentication.
1.36 aaron 505: Any port forward requests by the client will return an error.
1.230 jmc 506: This might be used, e.g. in connection with the
1.2 deraadt 507: .Cm command
1.1 deraadt 508: option.
1.218 jmc 509: .It Cm no-pty
510: Prevents tty allocation (a request to allocate a pty will fail).
1.240 djm 511: .It Cm no-user-rc
1.241 jmc 512: Disables execution of
1.240 djm 513: .Pa ~/.ssh/rc .
1.2 deraadt 514: .It Cm no-X11-forwarding
1.1 deraadt 515: Forbids X11 forwarding when this key is used for authentication.
516: Any X11 forward requests by the client will return an error.
1.107 djm 517: .It Cm permitopen="host:port"
1.278 bentley 518: Limit local port forwarding with
519: .Xr ssh 1
520: .Fl L
521: such that it may only connect to the specified host and port.
1.257 djm 522: IPv6 addresses can be specified by enclosing the address in square brackets.
1.146 stevesk 523: Multiple
1.107 djm 524: .Cm permitopen
1.197 jmc 525: options may be applied separated by commas.
526: No pattern matching is performed on the specified hostnames,
527: they must be literal domains or addresses.
1.264 dtucker 528: A port specification of
529: .Cm *
530: matches any port.
1.282 djm 531: .It Cm port-forwarding
532: Enable port forwarding previously disabled by the
533: .Cm restrict
1.256 djm 534: .It Cm principals="principals"
535: On a
536: .Cm cert-authority
537: line, specifies allowed principals for certificate authentication as a
538: comma-separated list.
539: At least one name from the list must appear in the certificate's
540: list of principals for the certificate to be accepted.
541: This option is ignored for keys that are not marked as trusted certificate
542: signers using the
543: .Cm cert-authority
544: option.
1.282 djm 545: .It Cm pty
546: Permits tty allocation previously disabled by the
547: .Cm restrict
548: option.
549: .It Cm restrict
550: Enable all restrictions, i.e. disable port, agent and X11 forwarding,
551: as well as disabling PTY allocation
552: and execution of
553: .Pa ~/.ssh/rc .
554: If any future restriction capabilities are added to authorized_keys files
555: they will be included in this set.
1.209 reyk 556: .It Cm tunnel="n"
557: Force a
558: .Xr tun 4
559: device on the server.
560: Without this option, the next available device will be used if
561: the client requests a tunnel.
1.282 djm 562: .It Cm user-rc
563: Enables execution of
564: .Pa ~/.ssh/rc
565: previously disabled by the
566: .Cm restrict
567: option.
568: .It Cm X11-forwarding
569: Permits X11 forwarding previously disabled by the
570: .Cm restrict
571: option.
1.2 deraadt 572: .El
573: .Pp
1.219 jmc 574: An example authorized_keys file:
1.222 jmc 575: .Bd -literal -offset 3n
1.219 jmc 576: # Comments allowed at start of line
577: ssh-rsa AAAAB3Nza...LiPk== user@example.net
1.222 jmc 578: from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
579: AAAAB2...19Q== john@example.net
580: command="dump /home",no-pty,no-port-forwarding ssh-dss
581: AAAAC3...51R== example.net
582: permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
583: AAAAB5...21S==
584: tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
585: jane@example.net
1.282 djm 586: restrict,command="uptime" ssh-rsa AAAA1C8...32Tv==
587: user@example.net
588: restrict,pty,command="nethack" ssh-rsa AAAA1f8...IrrC5==
589: user@example.net
1.219 jmc 590: .Ed
1.2 deraadt 591: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 592: The
1.194 jmc 593: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 594: and
1.207 djm 595: .Pa ~/.ssh/known_hosts
1.36 aaron 596: files contain host public keys for all known hosts.
597: The global file should
1.37 brad 598: be prepared by the administrator (optional), and the per-user file is
1.220 jmc 599: maintained automatically: whenever the user connects from an unknown host,
1.36 aaron 600: its key is added to the per-user file.
1.2 deraadt 601: .Pp
1.254 djm 602: Each line in these files contains the following fields: markers (optional),
1.285 naddy 603: hostnames, keytype, base64-encoded key, comment.
1.36 aaron 604: The fields are separated by spaces.
1.2 deraadt 605: .Pp
1.254 djm 606: The marker is optional, but if it is present then it must be one of
607: .Dq @cert-authority ,
608: to indicate that the line contains a certification authority (CA) key,
609: or
610: .Dq @revoked ,
611: to indicate that the key contained on the line is revoked and must not ever
612: be accepted.
613: Only one marker should be used on a key line.
614: .Pp
1.196 david 615: Hostnames is a comma-separated list of patterns
1.220 jmc 616: .Pf ( Ql *
1.197 jmc 617: and
618: .Ql \&?
1.195 mouring 619: act as
1.1 deraadt 620: wildcards); each pattern in turn is matched against the canonical host
621: name (when authenticating a client) or against the user-supplied
1.36 aaron 622: name (when authenticating a server).
623: A pattern may also be preceded by
1.195 mouring 624: .Ql \&!
1.2 deraadt 625: to indicate negation: if the host name matches a negated
1.1 deraadt 626: pattern, it is not accepted (by that line) even if it matched another
627: pattern on the line.
1.231 dtucker 628: A hostname or address may optionally be enclosed within
629: .Ql \&[
630: and
631: .Ql \&]
632: brackets then followed by
633: .Ql \&:
1.232 jmc 634: and a non-standard port number.
1.2 deraadt 635: .Pp
1.205 djm 636: Alternately, hostnames may be stored in a hashed form which hides host names
1.206 jmc 637: and addresses should the file's contents be disclosed.
638: Hashed hostnames start with a
639: .Ql |
1.205 djm 640: character.
641: Only one hashed hostname may appear on a single line and none of the above
642: negation or wildcard operators may be applied.
643: .Pp
1.285 naddy 644: The keytype and base64-encoded key are taken directly from the host key; they
1.220 jmc 645: can be obtained, for example, from
1.285 naddy 646: .Pa /etc/ssh/ssh_host_rsa_key.pub .
1.1 deraadt 647: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 648: .Pp
649: Lines starting with
650: .Ql #
651: and empty lines are ignored as comments.
652: .Pp
1.1 deraadt 653: When performing host authentication, authentication is accepted if any
1.254 djm 654: matching line has the proper key; either one that matches exactly or,
655: if the server has presented a certificate for authentication, the key
656: of the certification authority that signed the certificate.
657: For a key to be trusted as a certification authority, it must use the
658: .Dq @cert-authority
659: marker described above.
660: .Pp
661: The known hosts file also provides a facility to mark keys as revoked,
662: for example when it is known that the associated private key has been
663: stolen.
664: Revoked keys are specified by including the
665: .Dq @revoked
666: marker at the beginning of the key line, and are never accepted for
667: authentication or as certification authorities, but instead will
668: produce a warning from
669: .Xr ssh 1
670: when they are encountered.
671: .Pp
672: It is permissible (but not
1.1 deraadt 673: recommended) to have several lines or different host keys for the same
1.36 aaron 674: names.
675: This will inevitably happen when short forms of host names
676: from different domains are put in the file.
677: It is possible
1.1 deraadt 678: that the files contain conflicting information; authentication is
679: accepted if valid information can be found from either file.
1.2 deraadt 680: .Pp
1.1 deraadt 681: Note that the lines in these files are typically hundreds of characters
682: long, and you definitely don't want to type in the host keys by hand.
1.254 djm 683: Rather, generate them by a script,
684: .Xr ssh-keyscan 1
1.285 naddy 685: or by taking, for example,
686: .Pa /etc/ssh/ssh_host_rsa_key.pub
1.1 deraadt 687: and adding the host names at the front.
1.254 djm 688: .Xr ssh-keygen 1
1.255 jmc 689: also offers some basic automated editing for
1.254 djm 690: .Pa ~/.ssh/known_hosts
691: including removing hosts matching a host name and converting all host
692: names to their hashed representations.
1.221 jmc 693: .Pp
694: An example ssh_known_hosts file:
695: .Bd -literal -offset 3n
696: # Comments allowed at start of line
697: closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
698: cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
1.205 djm 699: # A hashed hostname
700: |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
701: AAAA1234.....=
1.254 djm 702: # A revoked key
703: @revoked * ssh-rsa AAAAB5W...
704: # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
705: @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
1.120 markus 706: .Ed
1.2 deraadt 707: .Sh FILES
1.223 jmc 708: .Bl -tag -width Ds -compact
1.258 jmc 709: .It Pa ~/.hushlogin
1.204 jmc 710: This file is used to suppress printing the last login time and
711: .Pa /etc/motd ,
712: if
713: .Cm PrintLastLog
714: and
715: .Cm PrintMotd ,
716: respectively,
717: are enabled.
718: It does not suppress printing of the banner specified by
719: .Cm Banner .
1.223 jmc 720: .Pp
1.258 jmc 721: .It Pa ~/.rhosts
1.225 jmc 722: This file is used for host-based authentication (see
723: .Xr ssh 1
724: for more information).
725: On some machines this file may need to be
726: world-readable if the user's home directory is on an NFS partition,
727: because
728: .Nm
729: reads it as root.
730: Additionally, this file must be owned by the user,
731: and must not have write permissions for anyone else.
732: The recommended
733: permission for most machines is read/write for the user, and not
1.1 deraadt 734: accessible by others.
1.2 deraadt 735: .Pp
1.258 jmc 736: .It Pa ~/.shosts
1.225 jmc 737: This file is used in exactly the same way as
738: .Pa .rhosts ,
739: but allows host-based authentication without permitting login with
740: rlogin/rsh.
1.238 mcbride 741: .Pp
1.258 jmc 742: .It Pa ~/.ssh/
1.238 mcbride 743: This directory is the default location for all user-specific configuration
744: and authentication information.
745: There is no general requirement to keep the entire contents of this directory
746: secret, but the recommended permissions are read/write/execute for the user,
747: and not accessible by others.
1.2 deraadt 748: .Pp
1.258 jmc 749: .It Pa ~/.ssh/authorized_keys
1.277 sobrado 750: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.273 naddy 751: that can be used for logging in as this user.
1.223 jmc 752: The format of this file is described above.
1.234 dtucker 753: The content of the file is not highly sensitive, but the recommended
1.225 jmc 754: permissions are read/write for the user, and not accessible by others.
1.234 dtucker 755: .Pp
756: If this file, the
757: .Pa ~/.ssh
758: directory, or the user's home directory are writable
759: by other users, then the file could be modified or replaced by unauthorized
760: users.
761: In this case,
762: .Nm
763: will not allow it to be used unless the
764: .Cm StrictModes
765: option has been set to
766: .Dq no .
1.2 deraadt 767: .Pp
1.258 jmc 768: .It Pa ~/.ssh/environment
1.36 aaron 769: This file is read into the environment at login (if it exists).
770: It can only contain empty lines, comment lines (that start with
1.2 deraadt 771: .Ql # ) ,
1.36 aaron 772: and assignment lines of the form name=value.
773: The file should be writable
1.6 aaron 774: only by the user; it need not be readable by anyone else.
1.188 stevesk 775: Environment processing is disabled by default and is
776: controlled via the
777: .Cm PermitUserEnvironment
778: option.
1.223 jmc 779: .Pp
1.258 jmc 780: .It Pa ~/.ssh/known_hosts
1.225 jmc 781: Contains a list of host keys for all hosts the user has logged into
782: that are not already in the systemwide list of known host keys.
783: The format of this file is described above.
784: This file should be writable only by root/the owner and
1.223 jmc 785: can, but need not be, world-readable.
786: .Pp
1.258 jmc 787: .It Pa ~/.ssh/rc
1.226 jmc 788: Contains initialization routines to be run before
789: the user's home directory becomes accessible.
1.1 deraadt 790: This file should be writable only by the user, and need not be
791: readable by anyone else.
1.223 jmc 792: .Pp
1.258 jmc 793: .It Pa /etc/hosts.equiv
1.227 jmc 794: This file is for host-based authentication (see
795: .Xr ssh 1 ) .
796: It should only be writable by root.
1.223 jmc 797: .Pp
1.258 jmc 798: .It Pa /etc/moduli
1.283 djm 799: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange"
800: key exchange method.
1.223 jmc 801: The file format is described in
802: .Xr moduli 5 .
1.283 djm 803: If no usable groups are found in this file then fixed internal groups will
804: be used.
1.223 jmc 805: .Pp
1.258 jmc 806: .It Pa /etc/motd
1.223 jmc 807: See
808: .Xr motd 5 .
809: .Pp
1.258 jmc 810: .It Pa /etc/nologin
1.223 jmc 811: If this file exists,
812: .Nm
813: refuses to let anyone except root log in.
814: The contents of the file
815: are displayed to anyone trying to log in, and non-root connections are
816: refused.
817: The file should be world-readable.
818: .Pp
1.258 jmc 819: .It Pa /etc/shosts.equiv
1.227 jmc 820: This file is used in exactly the same way as
821: .Pa hosts.equiv ,
822: but allows host-based authentication without permitting login with
823: rlogin/rsh.
1.225 jmc 824: .Pp
1.258 jmc 825: .It Pa /etc/ssh/ssh_host_dsa_key
1.259 djm 826: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.273 naddy 827: .It Pa /etc/ssh/ssh_host_ed25519_key
1.258 jmc 828: .It Pa /etc/ssh/ssh_host_rsa_key
1.266 dtucker 829: These files contain the private parts of the host keys.
1.223 jmc 830: These files should only be owned by root, readable only by root, and not
831: accessible to others.
832: Note that
833: .Nm
1.228 jmc 834: does not start if these files are group/world-accessible.
1.223 jmc 835: .Pp
1.258 jmc 836: .It Pa /etc/ssh/ssh_host_dsa_key.pub
1.259 djm 837: .It Pa /etc/ssh/ssh_host_ecdsa_key.pub
1.273 naddy 838: .It Pa /etc/ssh/ssh_host_ed25519_key.pub
1.258 jmc 839: .It Pa /etc/ssh/ssh_host_rsa_key.pub
1.266 dtucker 840: These files contain the public parts of the host keys.
1.223 jmc 841: These files should be world-readable but writable only by
842: root.
843: Their contents should match the respective private parts.
844: These files are not
845: really used for anything; they are provided for the convenience of
846: the user so their contents can be copied to known hosts files.
847: These files are created using
848: .Xr ssh-keygen 1 .
849: .Pp
1.258 jmc 850: .It Pa /etc/ssh/ssh_known_hosts
1.235 jmc 851: Systemwide list of known host keys.
852: This file should be prepared by the
853: system administrator to contain the public host keys of all machines in the
854: organization.
855: The format of this file is described above.
856: This file should be writable only by root/the owner and
857: should be world-readable.
858: .Pp
1.258 jmc 859: .It Pa /etc/ssh/sshd_config
1.223 jmc 860: Contains configuration data for
861: .Nm sshd .
862: The file format and configuration options are described in
863: .Xr sshd_config 5 .
864: .Pp
1.258 jmc 865: .It Pa /etc/ssh/sshrc
1.226 jmc 866: Similar to
867: .Pa ~/.ssh/rc ,
868: it can be used to specify
1.36 aaron 869: machine-specific login-time initializations globally.
870: This file should be writable only by root, and should be world-readable.
1.223 jmc 871: .Pp
1.258 jmc 872: .It Pa /var/empty
1.223 jmc 873: .Xr chroot 2
874: directory used by
875: .Nm
876: during privilege separation in the pre-authentication phase.
877: The directory should not contain any files and must be owned by root
878: and not group or world-writable.
879: .Pp
1.258 jmc 880: .It Pa /var/run/sshd.pid
1.223 jmc 881: Contains the process ID of the
882: .Nm
883: listening for connections (if there are several daemons running
884: concurrently for different ports, this contains the process ID of the one
885: started last).
886: The content of this file is not sensitive; it can be world-readable.
1.56 aaron 887: .El
1.2 deraadt 888: .Sh SEE ALSO
889: .Xr scp 1 ,
1.90 djm 890: .Xr sftp 1 ,
1.2 deraadt 891: .Xr ssh 1 ,
1.5 deraadt 892: .Xr ssh-add 1 ,
1.2 deraadt 893: .Xr ssh-agent 1 ,
1.5 deraadt 894: .Xr ssh-keygen 1 ,
1.235 jmc 895: .Xr ssh-keyscan 1 ,
1.200 jmc 896: .Xr chroot 2 ,
1.136 markus 897: .Xr login.conf 5 ,
898: .Xr moduli 5 ,
1.184 stevesk 899: .Xr sshd_config 5 ,
1.200 jmc 900: .Xr inetd 8 ,
1.128 mpech 901: .Xr sftp-server 8
1.198 jmc 902: .Sh AUTHORS
903: OpenSSH is a derivative of the original and free
904: ssh 1.2.12 release by Tatu Ylonen.
905: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
906: Theo de Raadt and Dug Song
907: removed many bugs, re-added newer features and
908: created OpenSSH.
909: Markus Friedl contributed the support for SSH
910: protocol versions 1.5 and 2.0.
911: Niels Provos and Markus Friedl contributed support
912: for privilege separation.
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