Annotation of src/usr.bin/ssh/sshd.8, Revision 1.294
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: .\"
1.294 ! djm 36: .\" $OpenBSD: sshd.8,v 1.293 2017/11/03 03:18:53 dtucker Exp $
! 37: .Dd $Mdocdate: November 3 2017 $
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
1.293 dtucker 103: directives in the configuration file that would apply are applied before the
104: configuration is written to standard output.
105: The connection parameters are supplied as keyword=value pairs and may be
106: supplied in any order, either with multiple
107: .Fl C
108: options or as a comma-separated list.
1.243 jmc 109: The keywords are
1.293 dtucker 110: .Dq addr,
1.243 jmc 111: .Dq user ,
112: .Dq host ,
1.265 dtucker 113: .Dq laddr ,
114: .Dq lport ,
1.293 dtucker 115: and
1.292 djm 116: .Dq rdomain
1.293 dtucker 117: and correspond to source address, user, resolved source host name,
118: local address, local port number and routing domain respectively.
1.252 jmc 119: .It Fl c Ar host_certificate_file
120: Specifies a path to a certificate file to identify
121: .Nm
122: during key exchange.
123: The certificate file must match a host key file specified using the
124: .Fl h
125: option or the
126: .Cm HostKey
127: configuration directive.
1.200 jmc 128: .It Fl D
129: When this option is specified,
130: .Nm
131: will not detach and does not become a daemon.
132: This allows easy monitoring of
133: .Nm sshd .
1.2 deraadt 134: .It Fl d
1.36 aaron 135: Debug mode.
1.250 djm 136: The server sends verbose debug output to standard error,
137: and does not put itself in the background.
1.36 aaron 138: The server also will not fork and will only process one connection.
139: This option is only intended for debugging for the server.
1.194 jmc 140: Multiple
141: .Fl d
142: options increase the debugging level.
1.67 aaron 143: Maximum is 3.
1.268 dtucker 144: .It Fl E Ar log_file
145: Append debug logs to
146: .Ar log_file
147: instead of the system log.
1.120 markus 148: .It Fl e
1.269 dtucker 149: Write debug logs to standard error instead of the system log.
1.235 jmc 150: .It Fl f Ar config_file
1.36 aaron 151: Specifies the name of the configuration file.
152: The default is
1.167 deraadt 153: .Pa /etc/ssh/sshd_config .
1.16 markus 154: .Nm
155: refuses to start if there is no configuration file.
1.2 deraadt 156: .It Fl g Ar login_grace_time
1.1 deraadt 157: Gives the grace time for clients to authenticate themselves (default
1.191 stevesk 158: 120 seconds).
1.36 aaron 159: If the client fails to authenticate the user within
160: this many seconds, the server disconnects and exits.
161: A value of zero indicates no limit.
1.2 deraadt 162: .It Fl h Ar host_key_file
1.160 stevesk 163: Specifies a file from which a host key is read.
1.7 markus 164: This option must be given if
165: .Nm
166: is not run as root (as the normal
1.160 stevesk 167: host key files are normally not readable by anyone but root).
168: The default is
1.259 djm 169: .Pa /etc/ssh/ssh_host_dsa_key ,
1.285 naddy 170: .Pa /etc/ssh/ssh_host_ecdsa_key ,
1.273 naddy 171: .Pa /etc/ssh/ssh_host_ed25519_key
1.259 djm 172: and
1.285 naddy 173: .Pa /etc/ssh/ssh_host_rsa_key .
1.75 markus 174: It is possible to have multiple host key files for
1.285 naddy 175: the different host key algorithms.
1.2 deraadt 176: .It Fl i
1.7 markus 177: Specifies that
178: .Nm
1.194 jmc 179: is being run from
180: .Xr inetd 8 .
1.156 markus 181: .It Fl o Ar option
182: Can be used to give options in the format used in the configuration file.
183: This is useful for specifying options for which there is no separate
184: command-line flag.
1.200 jmc 185: For full details of the options, and their values, see
186: .Xr sshd_config 5 .
1.2 deraadt 187: .It Fl p Ar port
1.1 deraadt 188: Specifies the port on which the server listens for connections
189: (default 22).
1.158 stevesk 190: Multiple port options are permitted.
1.210 stevesk 191: Ports specified in the configuration file with the
192: .Cm Port
193: option are ignored when a command-line port is specified.
194: Ports specified using the
195: .Cm ListenAddress
196: option override command-line ports.
1.2 deraadt 197: .It Fl q
1.36 aaron 198: Quiet mode.
199: Nothing is sent to the system log.
200: Normally the beginning,
1.1 deraadt 201: authentication, and termination of each connection is logged.
1.242 dtucker 202: .It Fl T
203: Extended test mode.
204: Check the validity of the configuration file, output the effective configuration
205: to stdout and then exit.
206: Optionally,
207: .Cm Match
208: rules may be applied by specifying the connection parameters using one or more
209: .Fl C
210: options.
1.243 jmc 211: .It Fl t
212: Test mode.
213: Only check the validity of the configuration file and sanity of the keys.
214: This is useful for updating
215: .Nm
216: reliably as configuration options may change.
1.61 markus 217: .It Fl u Ar len
218: This option is used to specify the size of the field
219: in the
220: .Li utmp
221: structure that holds the remote host name.
222: If the resolved host name is longer than
223: .Ar len ,
224: the dotted decimal value will be used instead.
225: This allows hosts with very long host names that
226: overflow this field to still be uniquely identified.
227: Specifying
228: .Fl u0
229: indicates that only dotted decimal addresses
230: should be put into the
231: .Pa utmp
232: file.
1.144 stevesk 233: .Fl u0
1.194 jmc 234: may also be used to prevent
1.144 stevesk 235: .Nm
236: from making DNS requests unless the authentication
237: mechanism or configuration requires it.
238: Authentication mechanisms that may require DNS include
1.285 naddy 239: .Cm HostbasedAuthentication
1.144 stevesk 240: and using a
241: .Cm from="pattern-list"
242: option in a key file.
1.170 stevesk 243: Configuration options that require DNS include using a
244: USER@HOST pattern in
245: .Cm AllowUsers
246: or
247: .Cm DenyUsers .
1.2 deraadt 248: .El
1.214 jmc 249: .Sh AUTHENTICATION
1.285 naddy 250: The OpenSSH SSH daemon supports SSH protocol 2 only.
1.284 jmc 251: Each host has a host-specific key,
1.214 jmc 252: used to identify the host.
1.212 jmc 253: Whenever a client connects, the daemon responds with its public
1.285 naddy 254: host key.
1.212 jmc 255: The client compares the
1.285 naddy 256: host key against its own database to verify that it has not changed.
257: Forward security is provided through a Diffie-Hellman key agreement.
1.214 jmc 258: This key agreement results in a shared session key.
259: The rest of the session is encrypted using a symmetric cipher, currently
260: 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
261: The client selects the encryption algorithm
262: to use from those offered by the server.
263: Additionally, session integrity is provided
264: through a cryptographic message authentication code
1.289 djm 265: (hmac-md5, hmac-sha1, umac-64, umac-128,
1.263 djm 266: hmac-sha2-256 or hmac-sha2-512).
1.214 jmc 267: .Pp
268: Finally, the server and the client enter an authentication dialog.
1.212 jmc 269: The client tries to authenticate itself using
1.214 jmc 270: host-based authentication,
271: public key authentication,
272: challenge-response authentication,
273: or password authentication.
1.212 jmc 274: .Pp
275: If the client successfully authenticates itself, a dialog for
276: preparing the session is entered.
277: At this time the client may request
278: things like allocating a pseudo-tty, forwarding X11 connections,
279: forwarding TCP connections, or forwarding the authentication agent
280: connection over the secure channel.
281: .Pp
1.216 jmc 282: After this, the client either requests a shell or execution of a command.
1.212 jmc 283: The sides then enter session mode.
284: In this mode, either side may send
285: data at any time, and such data is forwarded to/from the shell or
286: command on the server side, and the user terminal in the client side.
287: .Pp
288: When the user program terminates and all forwarded X11 and other
289: connections have been closed, the server sends command exit status to
290: the client, and both sides exit.
1.2 deraadt 291: .Sh LOGIN PROCESS
1.1 deraadt 292: When a user successfully logs in,
1.2 deraadt 293: .Nm
1.1 deraadt 294: does the following:
1.2 deraadt 295: .Bl -enum -offset indent
296: .It
1.1 deraadt 297: If the login is on a tty, and no command has been specified,
1.40 aaron 298: prints last login time and
1.2 deraadt 299: .Pa /etc/motd
1.1 deraadt 300: (unless prevented in the configuration file or by
1.207 djm 301: .Pa ~/.hushlogin ;
1.2 deraadt 302: see the
1.40 aaron 303: .Sx FILES
1.2 deraadt 304: section).
305: .It
1.1 deraadt 306: If the login is on a tty, records login time.
1.2 deraadt 307: .It
308: Checks
309: .Pa /etc/nologin ;
310: if it exists, prints contents and quits
1.1 deraadt 311: (unless root).
1.2 deraadt 312: .It
1.1 deraadt 313: Changes to run with normal user privileges.
1.2 deraadt 314: .It
1.1 deraadt 315: Sets up basic environment.
1.2 deraadt 316: .It
1.200 jmc 317: Reads the file
1.207 djm 318: .Pa ~/.ssh/environment ,
1.200 jmc 319: if it exists, and users are allowed to change their environment.
1.188 stevesk 320: See the
1.187 marc 321: .Cm PermitUserEnvironment
1.188 stevesk 322: option in
1.187 marc 323: .Xr sshd_config 5 .
1.2 deraadt 324: .It
1.1 deraadt 325: Changes to user's home directory.
1.2 deraadt 326: .It
327: If
1.207 djm 328: .Pa ~/.ssh/rc
1.276 djm 329: exists and the
330: .Xr sshd_config 5
331: .Cm PermitUserRC
332: option is set, runs it; else if
1.168 deraadt 333: .Pa /etc/ssh/sshrc
1.2 deraadt 334: exists, runs
1.36 aaron 335: it; otherwise runs xauth.
336: The
1.2 deraadt 337: .Dq rc
338: files are given the X11
1.1 deraadt 339: authentication protocol and cookie in standard input.
1.226 jmc 340: See
341: .Sx SSHRC ,
342: below.
1.2 deraadt 343: .It
1.1 deraadt 344: Runs user's shell or command.
1.279 djm 345: All commands are run under the user's login shell as specified in the
346: system password database.
1.2 deraadt 347: .El
1.226 jmc 348: .Sh SSHRC
349: If the file
350: .Pa ~/.ssh/rc
351: exists,
352: .Xr sh 1
353: runs it after reading the
354: environment files but before starting the user's shell or command.
355: It must not produce any output on stdout; stderr must be used
356: instead.
357: If X11 forwarding is in use, it will receive the "proto cookie" pair in
358: its standard input (and
359: .Ev DISPLAY
360: in its environment).
361: The script must call
362: .Xr xauth 1
363: because
364: .Nm
365: will not run xauth automatically to add X11 cookies.
366: .Pp
367: The primary purpose of this file is to run any initialization routines
368: which may be needed before the user's home directory becomes
369: accessible; AFS is a particular example of such an environment.
370: .Pp
371: This file will probably contain some initialization code followed by
372: something similar to:
373: .Bd -literal -offset 3n
374: if read proto cookie && [ -n "$DISPLAY" ]; then
375: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
376: # X11UseLocalhost=yes
377: echo add unix:`echo $DISPLAY |
378: cut -c11-` $proto $cookie
379: else
380: # X11UseLocalhost=no
381: echo add $DISPLAY $proto $cookie
382: fi | xauth -q -
383: fi
384: .Ed
385: .Pp
386: If this file does not exist,
387: .Pa /etc/ssh/sshrc
388: is run, and if that
389: does not exist either, xauth is used to add the cookie.
1.2 deraadt 390: .Sh AUTHORIZED_KEYS FILE FORMAT
1.125 markus 391: .Cm AuthorizedKeysFile
1.262 jmc 392: specifies the files containing public keys for
1.217 jmc 393: public key authentication;
1.281 djm 394: if this option is not specified, the default is
1.261 djm 395: .Pa ~/.ssh/authorized_keys
396: and
397: .Pa ~/.ssh/authorized_keys2 .
1.36 aaron 398: Each line of the file contains one
1.2 deraadt 399: key (empty lines and lines starting with a
400: .Ql #
401: are ignored as
1.36 aaron 402: comments).
1.285 naddy 403: Public keys consist of the following space-separated fields:
1.217 jmc 404: options, keytype, base64-encoded key, comment.
1.285 naddy 405: The options field is optional.
406: The keytype is
1.259 djm 407: .Dq ecdsa-sha2-nistp256 ,
408: .Dq ecdsa-sha2-nistp384 ,
409: .Dq ecdsa-sha2-nistp521 ,
1.272 jmc 410: .Dq ssh-ed25519 ,
1.75 markus 411: .Dq ssh-dss
412: or
1.285 naddy 413: .Dq ssh-rsa ;
414: the comment field is not used for anything (but may be convenient for the
415: user to identify the key).
1.2 deraadt 416: .Pp
1.285 naddy 417: Note that lines in this file can be several hundred bytes long
1.203 dtucker 418: (because of the size of the public key encoding) up to a limit of
419: 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
420: keys up to 16 kilobits.
1.36 aaron 421: You don't want to type them in; instead, copy the
1.217 jmc 422: .Pa id_dsa.pub ,
1.259 djm 423: .Pa id_ecdsa.pub ,
1.273 naddy 424: .Pa id_ed25519.pub ,
1.75 markus 425: or the
1.113 itojun 426: .Pa id_rsa.pub
1.1 deraadt 427: file and edit it.
1.175 stevesk 428: .Pp
429: .Nm
1.285 naddy 430: enforces a minimum RSA key modulus size of 768 bits.
1.2 deraadt 431: .Pp
1.58 deraadt 432: The options (if present) consist of comma-separated option
1.36 aaron 433: specifications.
434: No spaces are permitted, except within double quotes.
1.141 stevesk 435: The following option specifications are supported (note
436: that option keywords are case-insensitive):
1.2 deraadt 437: .Bl -tag -width Ds
1.282 djm 438: .It Cm agent-forwarding
439: Enable authentication agent forwarding previously disabled by the
440: .Cm restrict
441: option.
1.253 djm 442: .It Cm cert-authority
1.251 djm 443: Specifies that the listed key is a certification authority (CA) that is
444: trusted to validate signed certificates for user authentication.
445: .Pp
446: Certificates may encode access restrictions similar to these key options.
447: If both certificate restrictions and key options are present, the most
448: restrictive union of the two is applied.
1.2 deraadt 449: .It Cm command="command"
1.1 deraadt 450: Specifies that the command is executed whenever this key is used for
1.36 aaron 451: authentication.
452: The command supplied by the user (if any) is ignored.
1.148 markus 453: The command is run on a pty if the client requests a pty;
1.36 aaron 454: otherwise it is run without a tty.
1.194 jmc 455: If an 8-bit clean channel is required,
1.147 deraadt 456: one must not request a pty or should specify
1.89 markus 457: .Cm no-pty .
1.36 aaron 458: A quote may be included in the command by quoting it with a backslash.
1.287 djm 459: .Pp
1.36 aaron 460: This option might be useful
1.189 espie 461: to restrict certain public keys to perform just a specific operation.
1.36 aaron 462: An example might be a key that permits remote backups but nothing else.
1.211 jmc 463: Note that the client may specify TCP and/or X11
1.287 djm 464: forwarding unless they are explicitly prohibited, e.g. using the
465: .Cm restrict
466: key option.
467: .Pp
1.233 dtucker 468: The command originally supplied by the client is available in the
469: .Ev SSH_ORIGINAL_COMMAND
470: environment variable.
1.149 markus 471: Note that this option applies to shell, command or subsystem execution.
1.287 djm 472: Also note that this command may be superseded by a
1.251 djm 473: .Xr sshd_config 5
474: .Cm ForceCommand
1.287 djm 475: directive.
476: .Pp
477: If a command is specified and a forced-command is embedded in a certificate
478: used for authentication, then the certificate will be accepted only if the
479: two commands are identical.
1.2 deraadt 480: .It Cm environment="NAME=value"
1.1 deraadt 481: Specifies that the string is to be added to the environment when
1.36 aaron 482: logging in using this key.
483: Environment variables set this way
484: override other default environment values.
485: Multiple options of this type are permitted.
1.188 stevesk 486: Environment processing is disabled by default and is
487: controlled via the
488: .Cm PermitUserEnvironment
489: option.
1.218 jmc 490: .It Cm from="pattern-list"
1.244 djm 491: Specifies that in addition to public key authentication, either the canonical
1.245 jmc 492: name of the remote host or its IP address must be present in the
1.244 djm 493: comma-separated list of patterns.
1.270 jmc 494: See PATTERNS in
1.229 jmc 495: .Xr ssh_config 5
496: for more information on patterns.
1.244 djm 497: .Pp
498: In addition to the wildcard matching that may be applied to hostnames or
499: addresses, a
500: .Cm from
1.248 sobrado 501: stanza may match IP addresses using CIDR address/masklen notation.
1.244 djm 502: .Pp
503: The purpose of this option is to optionally increase security: public key
504: authentication by itself does not trust the network or name servers or
505: anything (but the key); however, if somebody somehow steals the key, the key
506: permits an intruder to log in from anywhere in the world.
507: This additional option makes using a stolen key more difficult (name
508: servers and/or routers would have to be compromised in addition to
509: just the key).
1.218 jmc 510: .It Cm no-agent-forwarding
511: Forbids authentication agent forwarding when this key is used for
512: authentication.
1.2 deraadt 513: .It Cm no-port-forwarding
1.211 jmc 514: Forbids TCP forwarding when this key is used for authentication.
1.36 aaron 515: Any port forward requests by the client will return an error.
1.230 jmc 516: This might be used, e.g. in connection with the
1.2 deraadt 517: .Cm command
1.1 deraadt 518: option.
1.218 jmc 519: .It Cm no-pty
520: Prevents tty allocation (a request to allocate a pty will fail).
1.240 djm 521: .It Cm no-user-rc
1.241 jmc 522: Disables execution of
1.240 djm 523: .Pa ~/.ssh/rc .
1.2 deraadt 524: .It Cm no-X11-forwarding
1.1 deraadt 525: Forbids X11 forwarding when this key is used for authentication.
526: Any X11 forward requests by the client will return an error.
1.107 djm 527: .It Cm permitopen="host:port"
1.278 bentley 528: Limit local port forwarding with
529: .Xr ssh 1
530: .Fl L
531: such that it may only connect to the specified host and port.
1.257 djm 532: IPv6 addresses can be specified by enclosing the address in square brackets.
1.146 stevesk 533: Multiple
1.107 djm 534: .Cm permitopen
1.197 jmc 535: options may be applied separated by commas.
536: No pattern matching is performed on the specified hostnames,
537: they must be literal domains or addresses.
1.264 dtucker 538: A port specification of
539: .Cm *
540: matches any port.
1.282 djm 541: .It Cm port-forwarding
542: Enable port forwarding previously disabled by the
543: .Cm restrict
1.294 ! djm 544: option.
1.256 djm 545: .It Cm principals="principals"
546: On a
547: .Cm cert-authority
548: line, specifies allowed principals for certificate authentication as a
549: comma-separated list.
550: At least one name from the list must appear in the certificate's
551: list of principals for the certificate to be accepted.
552: This option is ignored for keys that are not marked as trusted certificate
553: signers using the
554: .Cm cert-authority
555: option.
1.282 djm 556: .It Cm pty
557: Permits tty allocation previously disabled by the
558: .Cm restrict
559: option.
560: .It Cm restrict
561: Enable all restrictions, i.e. disable port, agent and X11 forwarding,
562: as well as disabling PTY allocation
563: and execution of
564: .Pa ~/.ssh/rc .
565: If any future restriction capabilities are added to authorized_keys files
566: they will be included in this set.
1.209 reyk 567: .It Cm tunnel="n"
568: Force a
569: .Xr tun 4
570: device on the server.
571: Without this option, the next available device will be used if
572: the client requests a tunnel.
1.282 djm 573: .It Cm user-rc
574: Enables execution of
575: .Pa ~/.ssh/rc
576: previously disabled by the
577: .Cm restrict
578: option.
579: .It Cm X11-forwarding
580: Permits X11 forwarding previously disabled by the
581: .Cm restrict
582: option.
1.2 deraadt 583: .El
584: .Pp
1.219 jmc 585: An example authorized_keys file:
1.222 jmc 586: .Bd -literal -offset 3n
1.219 jmc 587: # Comments allowed at start of line
588: ssh-rsa AAAAB3Nza...LiPk== user@example.net
1.222 jmc 589: from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
590: AAAAB2...19Q== john@example.net
591: command="dump /home",no-pty,no-port-forwarding ssh-dss
592: AAAAC3...51R== example.net
593: permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
594: AAAAB5...21S==
595: tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
596: jane@example.net
1.282 djm 597: restrict,command="uptime" ssh-rsa AAAA1C8...32Tv==
598: user@example.net
599: restrict,pty,command="nethack" ssh-rsa AAAA1f8...IrrC5==
600: user@example.net
1.219 jmc 601: .Ed
1.2 deraadt 602: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 603: The
1.194 jmc 604: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 605: and
1.207 djm 606: .Pa ~/.ssh/known_hosts
1.36 aaron 607: files contain host public keys for all known hosts.
608: The global file should
1.37 brad 609: be prepared by the administrator (optional), and the per-user file is
1.288 dtucker 610: maintained automatically: whenever the user connects to an unknown host,
1.36 aaron 611: its key is added to the per-user file.
1.2 deraadt 612: .Pp
1.254 djm 613: Each line in these files contains the following fields: markers (optional),
1.285 naddy 614: hostnames, keytype, base64-encoded key, comment.
1.36 aaron 615: The fields are separated by spaces.
1.2 deraadt 616: .Pp
1.254 djm 617: The marker is optional, but if it is present then it must be one of
618: .Dq @cert-authority ,
619: to indicate that the line contains a certification authority (CA) key,
620: or
621: .Dq @revoked ,
622: to indicate that the key contained on the line is revoked and must not ever
623: be accepted.
624: Only one marker should be used on a key line.
625: .Pp
1.196 david 626: Hostnames is a comma-separated list of patterns
1.220 jmc 627: .Pf ( Ql *
1.197 jmc 628: and
629: .Ql \&?
1.195 mouring 630: act as
1.290 djm 631: wildcards); each pattern in turn is matched against the host name.
632: When
633: .Nm sshd
634: is authenticating a client, such as when using
635: .Cm HostbasedAuthentication ,
636: this will be the canonical client host name.
637: When
638: .Xr ssh 1
1.291 jmc 639: is authenticating a server, this will be the host name
1.290 djm 640: given by the user, the value of the
641: .Xr ssh 1
642: .Cm HostkeyAlias
643: if it was specified, or the canonical server hostname if the
644: .Xr ssh 1
645: .Cm CanonicalizeHostname
646: option was used.
647: .Pp
1.36 aaron 648: A pattern may also be preceded by
1.195 mouring 649: .Ql \&!
1.2 deraadt 650: to indicate negation: if the host name matches a negated
1.1 deraadt 651: pattern, it is not accepted (by that line) even if it matched another
652: pattern on the line.
1.231 dtucker 653: A hostname or address may optionally be enclosed within
654: .Ql \&[
655: and
656: .Ql \&]
657: brackets then followed by
658: .Ql \&:
1.232 jmc 659: and a non-standard port number.
1.2 deraadt 660: .Pp
1.205 djm 661: Alternately, hostnames may be stored in a hashed form which hides host names
1.206 jmc 662: and addresses should the file's contents be disclosed.
663: Hashed hostnames start with a
664: .Ql |
1.205 djm 665: character.
666: Only one hashed hostname may appear on a single line and none of the above
667: negation or wildcard operators may be applied.
668: .Pp
1.285 naddy 669: The keytype and base64-encoded key are taken directly from the host key; they
1.220 jmc 670: can be obtained, for example, from
1.285 naddy 671: .Pa /etc/ssh/ssh_host_rsa_key.pub .
1.1 deraadt 672: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 673: .Pp
674: Lines starting with
675: .Ql #
676: and empty lines are ignored as comments.
677: .Pp
1.1 deraadt 678: When performing host authentication, authentication is accepted if any
1.254 djm 679: matching line has the proper key; either one that matches exactly or,
680: if the server has presented a certificate for authentication, the key
681: of the certification authority that signed the certificate.
682: For a key to be trusted as a certification authority, it must use the
683: .Dq @cert-authority
684: marker described above.
685: .Pp
686: The known hosts file also provides a facility to mark keys as revoked,
687: for example when it is known that the associated private key has been
688: stolen.
689: Revoked keys are specified by including the
690: .Dq @revoked
691: marker at the beginning of the key line, and are never accepted for
692: authentication or as certification authorities, but instead will
693: produce a warning from
694: .Xr ssh 1
695: when they are encountered.
696: .Pp
697: It is permissible (but not
1.1 deraadt 698: recommended) to have several lines or different host keys for the same
1.36 aaron 699: names.
700: This will inevitably happen when short forms of host names
701: from different domains are put in the file.
702: It is possible
1.1 deraadt 703: that the files contain conflicting information; authentication is
704: accepted if valid information can be found from either file.
1.2 deraadt 705: .Pp
1.1 deraadt 706: Note that the lines in these files are typically hundreds of characters
707: long, and you definitely don't want to type in the host keys by hand.
1.254 djm 708: Rather, generate them by a script,
709: .Xr ssh-keyscan 1
1.285 naddy 710: or by taking, for example,
711: .Pa /etc/ssh/ssh_host_rsa_key.pub
1.1 deraadt 712: and adding the host names at the front.
1.254 djm 713: .Xr ssh-keygen 1
1.255 jmc 714: also offers some basic automated editing for
1.254 djm 715: .Pa ~/.ssh/known_hosts
716: including removing hosts matching a host name and converting all host
717: names to their hashed representations.
1.221 jmc 718: .Pp
719: An example ssh_known_hosts file:
720: .Bd -literal -offset 3n
721: # Comments allowed at start of line
722: closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
723: cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
1.205 djm 724: # A hashed hostname
725: |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
726: AAAA1234.....=
1.254 djm 727: # A revoked key
728: @revoked * ssh-rsa AAAAB5W...
729: # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
730: @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
1.120 markus 731: .Ed
1.2 deraadt 732: .Sh FILES
1.223 jmc 733: .Bl -tag -width Ds -compact
1.258 jmc 734: .It Pa ~/.hushlogin
1.204 jmc 735: This file is used to suppress printing the last login time and
736: .Pa /etc/motd ,
737: if
738: .Cm PrintLastLog
739: and
740: .Cm PrintMotd ,
741: respectively,
742: are enabled.
743: It does not suppress printing of the banner specified by
744: .Cm Banner .
1.223 jmc 745: .Pp
1.258 jmc 746: .It Pa ~/.rhosts
1.225 jmc 747: This file is used for host-based authentication (see
748: .Xr ssh 1
749: for more information).
750: On some machines this file may need to be
751: world-readable if the user's home directory is on an NFS partition,
752: because
753: .Nm
754: reads it as root.
755: Additionally, this file must be owned by the user,
756: and must not have write permissions for anyone else.
757: The recommended
758: permission for most machines is read/write for the user, and not
1.1 deraadt 759: accessible by others.
1.2 deraadt 760: .Pp
1.258 jmc 761: .It Pa ~/.shosts
1.225 jmc 762: This file is used in exactly the same way as
763: .Pa .rhosts ,
764: but allows host-based authentication without permitting login with
765: rlogin/rsh.
1.238 mcbride 766: .Pp
1.258 jmc 767: .It Pa ~/.ssh/
1.238 mcbride 768: This directory is the default location for all user-specific configuration
769: and authentication information.
770: There is no general requirement to keep the entire contents of this directory
771: secret, but the recommended permissions are read/write/execute for the user,
772: and not accessible by others.
1.2 deraadt 773: .Pp
1.258 jmc 774: .It Pa ~/.ssh/authorized_keys
1.277 sobrado 775: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.273 naddy 776: that can be used for logging in as this user.
1.223 jmc 777: The format of this file is described above.
1.234 dtucker 778: The content of the file is not highly sensitive, but the recommended
1.225 jmc 779: permissions are read/write for the user, and not accessible by others.
1.234 dtucker 780: .Pp
781: If this file, the
782: .Pa ~/.ssh
783: directory, or the user's home directory are writable
784: by other users, then the file could be modified or replaced by unauthorized
785: users.
786: In this case,
787: .Nm
788: will not allow it to be used unless the
789: .Cm StrictModes
790: option has been set to
791: .Dq no .
1.2 deraadt 792: .Pp
1.258 jmc 793: .It Pa ~/.ssh/environment
1.36 aaron 794: This file is read into the environment at login (if it exists).
795: It can only contain empty lines, comment lines (that start with
1.2 deraadt 796: .Ql # ) ,
1.36 aaron 797: and assignment lines of the form name=value.
798: The file should be writable
1.6 aaron 799: only by the user; it need not be readable by anyone else.
1.188 stevesk 800: Environment processing is disabled by default and is
801: controlled via the
802: .Cm PermitUserEnvironment
803: option.
1.223 jmc 804: .Pp
1.258 jmc 805: .It Pa ~/.ssh/known_hosts
1.225 jmc 806: Contains a list of host keys for all hosts the user has logged into
807: that are not already in the systemwide list of known host keys.
808: The format of this file is described above.
809: This file should be writable only by root/the owner and
1.223 jmc 810: can, but need not be, world-readable.
811: .Pp
1.258 jmc 812: .It Pa ~/.ssh/rc
1.226 jmc 813: Contains initialization routines to be run before
814: the user's home directory becomes accessible.
1.1 deraadt 815: This file should be writable only by the user, and need not be
816: readable by anyone else.
1.223 jmc 817: .Pp
1.258 jmc 818: .It Pa /etc/hosts.equiv
1.227 jmc 819: This file is for host-based authentication (see
820: .Xr ssh 1 ) .
821: It should only be writable by root.
1.223 jmc 822: .Pp
1.258 jmc 823: .It Pa /etc/moduli
1.283 djm 824: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange"
825: key exchange method.
1.223 jmc 826: The file format is described in
827: .Xr moduli 5 .
1.283 djm 828: If no usable groups are found in this file then fixed internal groups will
829: be used.
1.223 jmc 830: .Pp
1.258 jmc 831: .It Pa /etc/motd
1.223 jmc 832: See
833: .Xr motd 5 .
834: .Pp
1.258 jmc 835: .It Pa /etc/nologin
1.223 jmc 836: If this file exists,
837: .Nm
838: refuses to let anyone except root log in.
839: The contents of the file
840: are displayed to anyone trying to log in, and non-root connections are
841: refused.
842: The file should be world-readable.
843: .Pp
1.258 jmc 844: .It Pa /etc/shosts.equiv
1.227 jmc 845: This file is used in exactly the same way as
846: .Pa hosts.equiv ,
847: but allows host-based authentication without permitting login with
848: rlogin/rsh.
1.225 jmc 849: .Pp
1.258 jmc 850: .It Pa /etc/ssh/ssh_host_dsa_key
1.259 djm 851: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.273 naddy 852: .It Pa /etc/ssh/ssh_host_ed25519_key
1.258 jmc 853: .It Pa /etc/ssh/ssh_host_rsa_key
1.266 dtucker 854: These files contain the private parts of the host keys.
1.223 jmc 855: These files should only be owned by root, readable only by root, and not
856: accessible to others.
857: Note that
858: .Nm
1.228 jmc 859: does not start if these files are group/world-accessible.
1.223 jmc 860: .Pp
1.258 jmc 861: .It Pa /etc/ssh/ssh_host_dsa_key.pub
1.259 djm 862: .It Pa /etc/ssh/ssh_host_ecdsa_key.pub
1.273 naddy 863: .It Pa /etc/ssh/ssh_host_ed25519_key.pub
1.258 jmc 864: .It Pa /etc/ssh/ssh_host_rsa_key.pub
1.266 dtucker 865: These files contain the public parts of the host keys.
1.223 jmc 866: These files should be world-readable but writable only by
867: root.
868: Their contents should match the respective private parts.
869: These files are not
870: really used for anything; they are provided for the convenience of
871: the user so their contents can be copied to known hosts files.
872: These files are created using
873: .Xr ssh-keygen 1 .
874: .Pp
1.258 jmc 875: .It Pa /etc/ssh/ssh_known_hosts
1.235 jmc 876: Systemwide list of known host keys.
877: This file should be prepared by the
878: system administrator to contain the public host keys of all machines in the
879: organization.
880: The format of this file is described above.
881: This file should be writable only by root/the owner and
882: should be world-readable.
883: .Pp
1.258 jmc 884: .It Pa /etc/ssh/sshd_config
1.223 jmc 885: Contains configuration data for
886: .Nm sshd .
887: The file format and configuration options are described in
888: .Xr sshd_config 5 .
889: .Pp
1.258 jmc 890: .It Pa /etc/ssh/sshrc
1.226 jmc 891: Similar to
892: .Pa ~/.ssh/rc ,
893: it can be used to specify
1.36 aaron 894: machine-specific login-time initializations globally.
895: This file should be writable only by root, and should be world-readable.
1.223 jmc 896: .Pp
1.258 jmc 897: .It Pa /var/empty
1.223 jmc 898: .Xr chroot 2
899: directory used by
900: .Nm
901: during privilege separation in the pre-authentication phase.
902: The directory should not contain any files and must be owned by root
903: and not group or world-writable.
904: .Pp
1.258 jmc 905: .It Pa /var/run/sshd.pid
1.223 jmc 906: Contains the process ID of the
907: .Nm
908: listening for connections (if there are several daemons running
909: concurrently for different ports, this contains the process ID of the one
910: started last).
911: The content of this file is not sensitive; it can be world-readable.
1.56 aaron 912: .El
1.2 deraadt 913: .Sh SEE ALSO
914: .Xr scp 1 ,
1.90 djm 915: .Xr sftp 1 ,
1.2 deraadt 916: .Xr ssh 1 ,
1.5 deraadt 917: .Xr ssh-add 1 ,
1.2 deraadt 918: .Xr ssh-agent 1 ,
1.5 deraadt 919: .Xr ssh-keygen 1 ,
1.235 jmc 920: .Xr ssh-keyscan 1 ,
1.200 jmc 921: .Xr chroot 2 ,
1.136 markus 922: .Xr login.conf 5 ,
923: .Xr moduli 5 ,
1.184 stevesk 924: .Xr sshd_config 5 ,
1.200 jmc 925: .Xr inetd 8 ,
1.128 mpech 926: .Xr sftp-server 8
1.198 jmc 927: .Sh AUTHORS
928: OpenSSH is a derivative of the original and free
929: ssh 1.2.12 release by Tatu Ylonen.
930: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
931: Theo de Raadt and Dug Song
932: removed many bugs, re-added newer features and
933: created OpenSSH.
934: Markus Friedl contributed the support for SSH
935: protocol versions 1.5 and 2.0.
936: Niels Provos and Markus Friedl contributed support
937: for privilege separation.