Annotation of src/usr.bin/ssh/sshd.8, Revision 1.301
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: .\"
1.301 ! jmc 36: .\" $OpenBSD: sshd.8,v 1.300 2018/06/06 18:24:15 djm Exp $
! 37: .Dd $Mdocdate: June 6 2018 $
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
1.285 naddy 169: .Pa /etc/ssh/ssh_host_ecdsa_key ,
1.273 naddy 170: .Pa /etc/ssh/ssh_host_ed25519_key
1.259 djm 171: and
1.285 naddy 172: .Pa /etc/ssh/ssh_host_rsa_key .
1.75 markus 173: It is possible to have multiple host key files for
1.285 naddy 174: the different host key algorithms.
1.2 deraadt 175: .It Fl i
1.7 markus 176: Specifies that
177: .Nm
1.194 jmc 178: is being run from
179: .Xr inetd 8 .
1.156 markus 180: .It Fl o Ar option
181: Can be used to give options in the format used in the configuration file.
182: This is useful for specifying options for which there is no separate
183: command-line flag.
1.200 jmc 184: For full details of the options, and their values, see
185: .Xr sshd_config 5 .
1.2 deraadt 186: .It Fl p Ar port
1.1 deraadt 187: Specifies the port on which the server listens for connections
188: (default 22).
1.158 stevesk 189: Multiple port options are permitted.
1.210 stevesk 190: Ports specified in the configuration file with the
191: .Cm Port
192: option are ignored when a command-line port is specified.
193: Ports specified using the
194: .Cm ListenAddress
195: option override command-line ports.
1.2 deraadt 196: .It Fl q
1.36 aaron 197: Quiet mode.
198: Nothing is sent to the system log.
199: Normally the beginning,
1.1 deraadt 200: authentication, and termination of each connection is logged.
1.242 dtucker 201: .It Fl T
202: Extended test mode.
203: Check the validity of the configuration file, output the effective configuration
204: to stdout and then exit.
205: Optionally,
206: .Cm Match
207: rules may be applied by specifying the connection parameters using one or more
208: .Fl C
209: options.
1.243 jmc 210: .It Fl t
211: Test mode.
212: Only check the validity of the configuration file and sanity of the keys.
213: This is useful for updating
214: .Nm
215: reliably as configuration options may change.
1.61 markus 216: .It Fl u Ar len
217: This option is used to specify the size of the field
218: in the
219: .Li utmp
220: structure that holds the remote host name.
221: If the resolved host name is longer than
222: .Ar len ,
223: the dotted decimal value will be used instead.
224: This allows hosts with very long host names that
225: overflow this field to still be uniquely identified.
226: Specifying
227: .Fl u0
228: indicates that only dotted decimal addresses
229: should be put into the
230: .Pa utmp
231: file.
1.144 stevesk 232: .Fl u0
1.194 jmc 233: may also be used to prevent
1.144 stevesk 234: .Nm
235: from making DNS requests unless the authentication
236: mechanism or configuration requires it.
237: Authentication mechanisms that may require DNS include
1.285 naddy 238: .Cm HostbasedAuthentication
1.144 stevesk 239: and using a
240: .Cm from="pattern-list"
241: option in a key file.
1.170 stevesk 242: Configuration options that require DNS include using a
243: USER@HOST pattern in
244: .Cm AllowUsers
245: or
246: .Cm DenyUsers .
1.2 deraadt 247: .El
1.214 jmc 248: .Sh AUTHENTICATION
1.285 naddy 249: The OpenSSH SSH daemon supports SSH protocol 2 only.
1.284 jmc 250: Each host has a host-specific key,
1.214 jmc 251: used to identify the host.
1.212 jmc 252: Whenever a client connects, the daemon responds with its public
1.285 naddy 253: host key.
1.212 jmc 254: The client compares the
1.285 naddy 255: host key against its own database to verify that it has not changed.
256: Forward security is provided through a Diffie-Hellman key agreement.
1.214 jmc 257: This key agreement results in a shared session key.
258: The rest of the session is encrypted using a symmetric cipher, currently
259: 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
260: The client selects the encryption algorithm
261: to use from those offered by the server.
262: Additionally, session integrity is provided
263: through a cryptographic message authentication code
1.289 djm 264: (hmac-md5, hmac-sha1, umac-64, umac-128,
1.263 djm 265: hmac-sha2-256 or hmac-sha2-512).
1.214 jmc 266: .Pp
267: Finally, the server and the client enter an authentication dialog.
1.212 jmc 268: The client tries to authenticate itself using
1.214 jmc 269: host-based authentication,
270: public key authentication,
271: challenge-response authentication,
272: or password authentication.
1.212 jmc 273: .Pp
274: If the client successfully authenticates itself, a dialog for
275: preparing the session is entered.
276: At this time the client may request
277: things like allocating a pseudo-tty, forwarding X11 connections,
278: forwarding TCP connections, or forwarding the authentication agent
279: connection over the secure channel.
280: .Pp
1.216 jmc 281: After this, the client either requests a shell or execution of a command.
1.212 jmc 282: The sides then enter session mode.
283: In this mode, either side may send
284: data at any time, and such data is forwarded to/from the shell or
285: command on the server side, and the user terminal in the client side.
286: .Pp
287: When the user program terminates and all forwarded X11 and other
288: connections have been closed, the server sends command exit status to
289: the client, and both sides exit.
1.2 deraadt 290: .Sh LOGIN PROCESS
1.1 deraadt 291: When a user successfully logs in,
1.2 deraadt 292: .Nm
1.1 deraadt 293: does the following:
1.2 deraadt 294: .Bl -enum -offset indent
295: .It
1.1 deraadt 296: If the login is on a tty, and no command has been specified,
1.40 aaron 297: prints last login time and
1.2 deraadt 298: .Pa /etc/motd
1.1 deraadt 299: (unless prevented in the configuration file or by
1.207 djm 300: .Pa ~/.hushlogin ;
1.2 deraadt 301: see the
1.40 aaron 302: .Sx FILES
1.2 deraadt 303: section).
304: .It
1.1 deraadt 305: If the login is on a tty, records login time.
1.2 deraadt 306: .It
307: Checks
308: .Pa /etc/nologin ;
309: if it exists, prints contents and quits
1.1 deraadt 310: (unless root).
1.2 deraadt 311: .It
1.1 deraadt 312: Changes to run with normal user privileges.
1.2 deraadt 313: .It
1.1 deraadt 314: Sets up basic environment.
1.2 deraadt 315: .It
1.200 jmc 316: Reads the file
1.207 djm 317: .Pa ~/.ssh/environment ,
1.200 jmc 318: if it exists, and users are allowed to change their environment.
1.188 stevesk 319: See the
1.187 marc 320: .Cm PermitUserEnvironment
1.188 stevesk 321: option in
1.187 marc 322: .Xr sshd_config 5 .
1.2 deraadt 323: .It
1.1 deraadt 324: Changes to user's home directory.
1.2 deraadt 325: .It
326: If
1.207 djm 327: .Pa ~/.ssh/rc
1.276 djm 328: exists and the
329: .Xr sshd_config 5
330: .Cm PermitUserRC
331: option is set, runs it; else if
1.168 deraadt 332: .Pa /etc/ssh/sshrc
1.2 deraadt 333: exists, runs
1.36 aaron 334: it; otherwise runs xauth.
335: The
1.2 deraadt 336: .Dq rc
337: files are given the X11
1.1 deraadt 338: authentication protocol and cookie in standard input.
1.226 jmc 339: See
340: .Sx SSHRC ,
341: below.
1.2 deraadt 342: .It
1.1 deraadt 343: Runs user's shell or command.
1.279 djm 344: All commands are run under the user's login shell as specified in the
345: system password database.
1.2 deraadt 346: .El
1.226 jmc 347: .Sh SSHRC
348: If the file
349: .Pa ~/.ssh/rc
350: exists,
351: .Xr sh 1
352: runs it after reading the
353: environment files but before starting the user's shell or command.
354: It must not produce any output on stdout; stderr must be used
355: instead.
356: If X11 forwarding is in use, it will receive the "proto cookie" pair in
357: its standard input (and
358: .Ev DISPLAY
359: in its environment).
360: The script must call
361: .Xr xauth 1
362: because
363: .Nm
364: will not run xauth automatically to add X11 cookies.
365: .Pp
366: The primary purpose of this file is to run any initialization routines
367: which may be needed before the user's home directory becomes
368: accessible; AFS is a particular example of such an environment.
369: .Pp
370: This file will probably contain some initialization code followed by
371: something similar to:
372: .Bd -literal -offset 3n
373: if read proto cookie && [ -n "$DISPLAY" ]; then
374: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
375: # X11UseLocalhost=yes
376: echo add unix:`echo $DISPLAY |
377: cut -c11-` $proto $cookie
378: else
379: # X11UseLocalhost=no
380: echo add $DISPLAY $proto $cookie
381: fi | xauth -q -
382: fi
383: .Ed
384: .Pp
385: If this file does not exist,
386: .Pa /etc/ssh/sshrc
387: is run, and if that
388: does not exist either, xauth is used to add the cookie.
1.2 deraadt 389: .Sh AUTHORIZED_KEYS FILE FORMAT
1.125 markus 390: .Cm AuthorizedKeysFile
1.262 jmc 391: specifies the files containing public keys for
1.217 jmc 392: public key authentication;
1.281 djm 393: if this option is not specified, the default is
1.261 djm 394: .Pa ~/.ssh/authorized_keys
395: and
396: .Pa ~/.ssh/authorized_keys2 .
1.36 aaron 397: Each line of the file contains one
1.2 deraadt 398: key (empty lines and lines starting with a
399: .Ql #
400: are ignored as
1.36 aaron 401: comments).
1.285 naddy 402: Public keys consist of the following space-separated fields:
1.217 jmc 403: options, keytype, base64-encoded key, comment.
1.285 naddy 404: The options field is optional.
405: The keytype is
1.259 djm 406: .Dq ecdsa-sha2-nistp256 ,
407: .Dq ecdsa-sha2-nistp384 ,
408: .Dq ecdsa-sha2-nistp521 ,
1.272 jmc 409: .Dq ssh-ed25519 ,
1.75 markus 410: .Dq ssh-dss
411: or
1.285 naddy 412: .Dq ssh-rsa ;
413: the comment field is not used for anything (but may be convenient for the
414: user to identify the key).
1.2 deraadt 415: .Pp
1.285 naddy 416: Note that lines in this file can be several hundred bytes long
1.203 dtucker 417: (because of the size of the public key encoding) up to a limit of
418: 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
419: keys up to 16 kilobits.
1.36 aaron 420: You don't want to type them in; instead, copy the
1.217 jmc 421: .Pa id_dsa.pub ,
1.259 djm 422: .Pa id_ecdsa.pub ,
1.273 naddy 423: .Pa id_ed25519.pub ,
1.75 markus 424: or the
1.113 itojun 425: .Pa id_rsa.pub
1.1 deraadt 426: file and edit it.
1.175 stevesk 427: .Pp
428: .Nm
1.296 dtucker 429: enforces a minimum RSA key modulus size of 1024 bits.
1.2 deraadt 430: .Pp
1.58 deraadt 431: The options (if present) consist of comma-separated option
1.36 aaron 432: specifications.
433: No spaces are permitted, except within double quotes.
1.141 stevesk 434: The following option specifications are supported (note
435: that option keywords are case-insensitive):
1.2 deraadt 436: .Bl -tag -width Ds
1.282 djm 437: .It Cm agent-forwarding
438: Enable authentication agent forwarding previously disabled by the
439: .Cm restrict
440: option.
1.253 djm 441: .It Cm cert-authority
1.251 djm 442: Specifies that the listed key is a certification authority (CA) that is
443: trusted to validate signed certificates for user authentication.
444: .Pp
445: Certificates may encode access restrictions similar to these key options.
446: If both certificate restrictions and key options are present, the most
447: restrictive union of the two is applied.
1.2 deraadt 448: .It Cm command="command"
1.1 deraadt 449: Specifies that the command is executed whenever this key is used for
1.36 aaron 450: authentication.
451: The command supplied by the user (if any) is ignored.
1.148 markus 452: The command is run on a pty if the client requests a pty;
1.36 aaron 453: otherwise it is run without a tty.
1.194 jmc 454: If an 8-bit clean channel is required,
1.147 deraadt 455: one must not request a pty or should specify
1.89 markus 456: .Cm no-pty .
1.36 aaron 457: A quote may be included in the command by quoting it with a backslash.
1.287 djm 458: .Pp
1.36 aaron 459: This option might be useful
1.189 espie 460: to restrict certain public keys to perform just a specific operation.
1.36 aaron 461: An example might be a key that permits remote backups but nothing else.
1.211 jmc 462: Note that the client may specify TCP and/or X11
1.287 djm 463: forwarding unless they are explicitly prohibited, e.g. using the
464: .Cm restrict
465: key option.
466: .Pp
1.233 dtucker 467: The command originally supplied by the client is available in the
468: .Ev SSH_ORIGINAL_COMMAND
469: environment variable.
1.149 markus 470: Note that this option applies to shell, command or subsystem execution.
1.287 djm 471: Also note that this command may be superseded by a
1.251 djm 472: .Xr sshd_config 5
473: .Cm ForceCommand
1.287 djm 474: directive.
475: .Pp
476: If a command is specified and a forced-command is embedded in a certificate
477: used for authentication, then the certificate will be accepted only if the
478: two commands are identical.
1.2 deraadt 479: .It Cm environment="NAME=value"
1.1 deraadt 480: Specifies that the string is to be added to the environment when
1.36 aaron 481: logging in using this key.
482: Environment variables set this way
483: override other default environment values.
484: Multiple options of this type are permitted.
1.188 stevesk 485: Environment processing is disabled by default and is
486: controlled via the
487: .Cm PermitUserEnvironment
488: option.
1.299 jmc 489: .It Cm expiry-time="timespec"
490: Specifies a time after which the key will not be accepted.
491: The time may be specified as a YYYYMMDD date or a YYYYMMDDHHMM[SS] time
492: in the system time-zone.
1.218 jmc 493: .It Cm from="pattern-list"
1.244 djm 494: Specifies that in addition to public key authentication, either the canonical
1.245 jmc 495: name of the remote host or its IP address must be present in the
1.244 djm 496: comma-separated list of patterns.
1.270 jmc 497: See PATTERNS in
1.229 jmc 498: .Xr ssh_config 5
499: for more information on patterns.
1.244 djm 500: .Pp
501: In addition to the wildcard matching that may be applied to hostnames or
502: addresses, a
503: .Cm from
1.248 sobrado 504: stanza may match IP addresses using CIDR address/masklen notation.
1.244 djm 505: .Pp
506: The purpose of this option is to optionally increase security: public key
507: authentication by itself does not trust the network or name servers or
508: anything (but the key); however, if somebody somehow steals the key, the key
509: permits an intruder to log in from anywhere in the world.
510: This additional option makes using a stolen key more difficult (name
511: servers and/or routers would have to be compromised in addition to
512: just the key).
1.218 jmc 513: .It Cm no-agent-forwarding
514: Forbids authentication agent forwarding when this key is used for
515: authentication.
1.2 deraadt 516: .It Cm no-port-forwarding
1.211 jmc 517: Forbids TCP forwarding when this key is used for authentication.
1.36 aaron 518: Any port forward requests by the client will return an error.
1.230 jmc 519: This might be used, e.g. in connection with the
1.2 deraadt 520: .Cm command
1.1 deraadt 521: option.
1.218 jmc 522: .It Cm no-pty
523: Prevents tty allocation (a request to allocate a pty will fail).
1.240 djm 524: .It Cm no-user-rc
1.241 jmc 525: Disables execution of
1.240 djm 526: .Pa ~/.ssh/rc .
1.2 deraadt 527: .It Cm no-X11-forwarding
1.1 deraadt 528: Forbids X11 forwarding when this key is used for authentication.
529: Any X11 forward requests by the client will return an error.
1.300 djm 530: .It Cm permitlisten="host:port"
1.301 ! jmc 531: Limit remote port forwarding with the
1.300 djm 532: .Xr ssh 1
533: .Fl R
534: option such that it may only listen on the specified host and port.
535: IPv6 addresses can be specified by enclosing the address in square brackets.
536: Multiple
537: .Cm permitlisten
538: options may be applied separated by commas.
539: Hostnames may include wildcards as described in the PATTERNS section in
540: .Xr ssh_config 5 .
541: A port specification of
542: .Cm *
543: matches any port.
544: Note that the setting of
545: .Cm GatewayPorts
546: may further restrict listen addresses.
1.107 djm 547: .It Cm permitopen="host:port"
1.301 ! jmc 548: Limit local port forwarding with the
1.278 bentley 549: .Xr ssh 1
550: .Fl L
1.300 djm 551: option such that it may only connect to the specified host and port.
1.257 djm 552: IPv6 addresses can be specified by enclosing the address in square brackets.
1.146 stevesk 553: Multiple
1.107 djm 554: .Cm permitopen
1.197 jmc 555: options may be applied separated by commas.
556: No pattern matching is performed on the specified hostnames,
557: they must be literal domains or addresses.
1.264 dtucker 558: A port specification of
559: .Cm *
560: matches any port.
1.282 djm 561: .It Cm port-forwarding
562: Enable port forwarding previously disabled by the
563: .Cm restrict
1.294 djm 564: option.
1.256 djm 565: .It Cm principals="principals"
566: On a
567: .Cm cert-authority
568: line, specifies allowed principals for certificate authentication as a
569: comma-separated list.
570: At least one name from the list must appear in the certificate's
571: list of principals for the certificate to be accepted.
572: This option is ignored for keys that are not marked as trusted certificate
573: signers using the
574: .Cm cert-authority
575: option.
1.282 djm 576: .It Cm pty
577: Permits tty allocation previously disabled by the
578: .Cm restrict
579: option.
580: .It Cm restrict
581: Enable all restrictions, i.e. disable port, agent and X11 forwarding,
582: as well as disabling PTY allocation
583: and execution of
584: .Pa ~/.ssh/rc .
585: If any future restriction capabilities are added to authorized_keys files
586: they will be included in this set.
1.209 reyk 587: .It Cm tunnel="n"
588: Force a
589: .Xr tun 4
590: device on the server.
591: Without this option, the next available device will be used if
592: the client requests a tunnel.
1.282 djm 593: .It Cm user-rc
594: Enables execution of
595: .Pa ~/.ssh/rc
596: previously disabled by the
597: .Cm restrict
598: option.
599: .It Cm X11-forwarding
600: Permits X11 forwarding previously disabled by the
601: .Cm restrict
602: option.
1.2 deraadt 603: .El
604: .Pp
1.219 jmc 605: An example authorized_keys file:
1.222 jmc 606: .Bd -literal -offset 3n
1.219 jmc 607: # Comments allowed at start of line
608: ssh-rsa AAAAB3Nza...LiPk== user@example.net
1.222 jmc 609: from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
610: AAAAB2...19Q== john@example.net
611: command="dump /home",no-pty,no-port-forwarding ssh-dss
612: AAAAC3...51R== example.net
613: permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
614: AAAAB5...21S==
615: tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
616: jane@example.net
1.282 djm 617: restrict,command="uptime" ssh-rsa AAAA1C8...32Tv==
618: user@example.net
619: restrict,pty,command="nethack" ssh-rsa AAAA1f8...IrrC5==
620: user@example.net
1.219 jmc 621: .Ed
1.2 deraadt 622: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 623: The
1.194 jmc 624: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 625: and
1.207 djm 626: .Pa ~/.ssh/known_hosts
1.36 aaron 627: files contain host public keys for all known hosts.
628: The global file should
1.37 brad 629: be prepared by the administrator (optional), and the per-user file is
1.288 dtucker 630: maintained automatically: whenever the user connects to an unknown host,
1.36 aaron 631: its key is added to the per-user file.
1.2 deraadt 632: .Pp
1.254 djm 633: Each line in these files contains the following fields: markers (optional),
1.285 naddy 634: hostnames, keytype, base64-encoded key, comment.
1.36 aaron 635: The fields are separated by spaces.
1.2 deraadt 636: .Pp
1.254 djm 637: The marker is optional, but if it is present then it must be one of
638: .Dq @cert-authority ,
639: to indicate that the line contains a certification authority (CA) key,
640: or
641: .Dq @revoked ,
642: to indicate that the key contained on the line is revoked and must not ever
643: be accepted.
644: Only one marker should be used on a key line.
645: .Pp
1.196 david 646: Hostnames is a comma-separated list of patterns
1.220 jmc 647: .Pf ( Ql *
1.197 jmc 648: and
649: .Ql \&?
1.195 mouring 650: act as
1.290 djm 651: wildcards); each pattern in turn is matched against the host name.
652: When
653: .Nm sshd
654: is authenticating a client, such as when using
655: .Cm HostbasedAuthentication ,
656: this will be the canonical client host name.
657: When
658: .Xr ssh 1
1.291 jmc 659: is authenticating a server, this will be the host name
1.290 djm 660: given by the user, the value of the
661: .Xr ssh 1
662: .Cm HostkeyAlias
663: if it was specified, or the canonical server hostname if the
664: .Xr ssh 1
665: .Cm CanonicalizeHostname
666: option was used.
667: .Pp
1.36 aaron 668: A pattern may also be preceded by
1.195 mouring 669: .Ql \&!
1.2 deraadt 670: to indicate negation: if the host name matches a negated
1.1 deraadt 671: pattern, it is not accepted (by that line) even if it matched another
672: pattern on the line.
1.231 dtucker 673: A hostname or address may optionally be enclosed within
674: .Ql \&[
675: and
676: .Ql \&]
677: brackets then followed by
678: .Ql \&:
1.232 jmc 679: and a non-standard port number.
1.2 deraadt 680: .Pp
1.205 djm 681: Alternately, hostnames may be stored in a hashed form which hides host names
1.206 jmc 682: and addresses should the file's contents be disclosed.
683: Hashed hostnames start with a
684: .Ql |
1.205 djm 685: character.
686: Only one hashed hostname may appear on a single line and none of the above
687: negation or wildcard operators may be applied.
688: .Pp
1.285 naddy 689: The keytype and base64-encoded key are taken directly from the host key; they
1.220 jmc 690: can be obtained, for example, from
1.285 naddy 691: .Pa /etc/ssh/ssh_host_rsa_key.pub .
1.1 deraadt 692: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 693: .Pp
694: Lines starting with
695: .Ql #
696: and empty lines are ignored as comments.
697: .Pp
1.1 deraadt 698: When performing host authentication, authentication is accepted if any
1.254 djm 699: matching line has the proper key; either one that matches exactly or,
700: if the server has presented a certificate for authentication, the key
701: of the certification authority that signed the certificate.
702: For a key to be trusted as a certification authority, it must use the
703: .Dq @cert-authority
704: marker described above.
705: .Pp
706: The known hosts file also provides a facility to mark keys as revoked,
707: for example when it is known that the associated private key has been
708: stolen.
709: Revoked keys are specified by including the
710: .Dq @revoked
711: marker at the beginning of the key line, and are never accepted for
712: authentication or as certification authorities, but instead will
713: produce a warning from
714: .Xr ssh 1
715: when they are encountered.
716: .Pp
717: It is permissible (but not
1.1 deraadt 718: recommended) to have several lines or different host keys for the same
1.36 aaron 719: names.
720: This will inevitably happen when short forms of host names
721: from different domains are put in the file.
722: It is possible
1.1 deraadt 723: that the files contain conflicting information; authentication is
724: accepted if valid information can be found from either file.
1.2 deraadt 725: .Pp
1.1 deraadt 726: Note that the lines in these files are typically hundreds of characters
727: long, and you definitely don't want to type in the host keys by hand.
1.254 djm 728: Rather, generate them by a script,
729: .Xr ssh-keyscan 1
1.285 naddy 730: or by taking, for example,
731: .Pa /etc/ssh/ssh_host_rsa_key.pub
1.1 deraadt 732: and adding the host names at the front.
1.254 djm 733: .Xr ssh-keygen 1
1.255 jmc 734: also offers some basic automated editing for
1.254 djm 735: .Pa ~/.ssh/known_hosts
736: including removing hosts matching a host name and converting all host
737: names to their hashed representations.
1.221 jmc 738: .Pp
739: An example ssh_known_hosts file:
740: .Bd -literal -offset 3n
741: # Comments allowed at start of line
742: closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
743: cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
1.205 djm 744: # A hashed hostname
745: |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
746: AAAA1234.....=
1.254 djm 747: # A revoked key
748: @revoked * ssh-rsa AAAAB5W...
749: # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
750: @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
1.120 markus 751: .Ed
1.2 deraadt 752: .Sh FILES
1.223 jmc 753: .Bl -tag -width Ds -compact
1.258 jmc 754: .It Pa ~/.hushlogin
1.204 jmc 755: This file is used to suppress printing the last login time and
756: .Pa /etc/motd ,
757: if
758: .Cm PrintLastLog
759: and
760: .Cm PrintMotd ,
761: respectively,
762: are enabled.
763: It does not suppress printing of the banner specified by
764: .Cm Banner .
1.223 jmc 765: .Pp
1.258 jmc 766: .It Pa ~/.rhosts
1.225 jmc 767: This file is used for host-based authentication (see
768: .Xr ssh 1
769: for more information).
770: On some machines this file may need to be
771: world-readable if the user's home directory is on an NFS partition,
772: because
773: .Nm
774: reads it as root.
775: Additionally, this file must be owned by the user,
776: and must not have write permissions for anyone else.
777: The recommended
778: permission for most machines is read/write for the user, and not
1.1 deraadt 779: accessible by others.
1.2 deraadt 780: .Pp
1.258 jmc 781: .It Pa ~/.shosts
1.225 jmc 782: This file is used in exactly the same way as
783: .Pa .rhosts ,
784: but allows host-based authentication without permitting login with
785: rlogin/rsh.
1.238 mcbride 786: .Pp
1.258 jmc 787: .It Pa ~/.ssh/
1.238 mcbride 788: This directory is the default location for all user-specific configuration
789: and authentication information.
790: There is no general requirement to keep the entire contents of this directory
791: secret, but the recommended permissions are read/write/execute for the user,
792: and not accessible by others.
1.2 deraadt 793: .Pp
1.258 jmc 794: .It Pa ~/.ssh/authorized_keys
1.277 sobrado 795: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.273 naddy 796: that can be used for logging in as this user.
1.223 jmc 797: The format of this file is described above.
1.234 dtucker 798: The content of the file is not highly sensitive, but the recommended
1.225 jmc 799: permissions are read/write for the user, and not accessible by others.
1.234 dtucker 800: .Pp
801: If this file, the
802: .Pa ~/.ssh
803: directory, or the user's home directory are writable
804: by other users, then the file could be modified or replaced by unauthorized
805: users.
806: In this case,
807: .Nm
808: will not allow it to be used unless the
809: .Cm StrictModes
810: option has been set to
811: .Dq no .
1.2 deraadt 812: .Pp
1.258 jmc 813: .It Pa ~/.ssh/environment
1.36 aaron 814: This file is read into the environment at login (if it exists).
815: It can only contain empty lines, comment lines (that start with
1.2 deraadt 816: .Ql # ) ,
1.36 aaron 817: and assignment lines of the form name=value.
818: The file should be writable
1.6 aaron 819: only by the user; it need not be readable by anyone else.
1.188 stevesk 820: Environment processing is disabled by default and is
821: controlled via the
822: .Cm PermitUserEnvironment
823: option.
1.223 jmc 824: .Pp
1.258 jmc 825: .It Pa ~/.ssh/known_hosts
1.225 jmc 826: Contains a list of host keys for all hosts the user has logged into
827: that are not already in the systemwide list of known host keys.
828: The format of this file is described above.
829: This file should be writable only by root/the owner and
1.223 jmc 830: can, but need not be, world-readable.
831: .Pp
1.258 jmc 832: .It Pa ~/.ssh/rc
1.226 jmc 833: Contains initialization routines to be run before
834: the user's home directory becomes accessible.
1.1 deraadt 835: This file should be writable only by the user, and need not be
836: readable by anyone else.
1.223 jmc 837: .Pp
1.258 jmc 838: .It Pa /etc/hosts.equiv
1.227 jmc 839: This file is for host-based authentication (see
840: .Xr ssh 1 ) .
841: It should only be writable by root.
1.223 jmc 842: .Pp
1.258 jmc 843: .It Pa /etc/moduli
1.283 djm 844: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange"
845: key exchange method.
1.223 jmc 846: The file format is described in
847: .Xr moduli 5 .
1.283 djm 848: If no usable groups are found in this file then fixed internal groups will
849: be used.
1.223 jmc 850: .Pp
1.258 jmc 851: .It Pa /etc/motd
1.223 jmc 852: See
853: .Xr motd 5 .
854: .Pp
1.258 jmc 855: .It Pa /etc/nologin
1.223 jmc 856: If this file exists,
857: .Nm
858: refuses to let anyone except root log in.
859: The contents of the file
860: are displayed to anyone trying to log in, and non-root connections are
861: refused.
862: The file should be world-readable.
863: .Pp
1.258 jmc 864: .It Pa /etc/shosts.equiv
1.227 jmc 865: This file is used in exactly the same way as
866: .Pa hosts.equiv ,
867: but allows host-based authentication without permitting login with
868: rlogin/rsh.
1.225 jmc 869: .Pp
1.259 djm 870: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.273 naddy 871: .It Pa /etc/ssh/ssh_host_ed25519_key
1.258 jmc 872: .It Pa /etc/ssh/ssh_host_rsa_key
1.266 dtucker 873: These files contain the private parts of the host keys.
1.223 jmc 874: These files should only be owned by root, readable only by root, and not
875: accessible to others.
876: Note that
877: .Nm
1.228 jmc 878: does not start if these files are group/world-accessible.
1.223 jmc 879: .Pp
1.259 djm 880: .It Pa /etc/ssh/ssh_host_ecdsa_key.pub
1.273 naddy 881: .It Pa /etc/ssh/ssh_host_ed25519_key.pub
1.258 jmc 882: .It Pa /etc/ssh/ssh_host_rsa_key.pub
1.266 dtucker 883: These files contain the public parts of the host keys.
1.223 jmc 884: These files should be world-readable but writable only by
885: root.
886: Their contents should match the respective private parts.
887: These files are not
888: really used for anything; they are provided for the convenience of
889: the user so their contents can be copied to known hosts files.
890: These files are created using
891: .Xr ssh-keygen 1 .
892: .Pp
1.258 jmc 893: .It Pa /etc/ssh/ssh_known_hosts
1.235 jmc 894: Systemwide list of known host keys.
895: This file should be prepared by the
896: system administrator to contain the public host keys of all machines in the
897: organization.
898: The format of this file is described above.
899: This file should be writable only by root/the owner and
900: should be world-readable.
901: .Pp
1.258 jmc 902: .It Pa /etc/ssh/sshd_config
1.223 jmc 903: Contains configuration data for
904: .Nm sshd .
905: The file format and configuration options are described in
906: .Xr sshd_config 5 .
907: .Pp
1.258 jmc 908: .It Pa /etc/ssh/sshrc
1.226 jmc 909: Similar to
910: .Pa ~/.ssh/rc ,
911: it can be used to specify
1.36 aaron 912: machine-specific login-time initializations globally.
913: This file should be writable only by root, and should be world-readable.
1.223 jmc 914: .Pp
1.258 jmc 915: .It Pa /var/empty
1.223 jmc 916: .Xr chroot 2
917: directory used by
918: .Nm
919: during privilege separation in the pre-authentication phase.
920: The directory should not contain any files and must be owned by root
921: and not group or world-writable.
922: .Pp
1.258 jmc 923: .It Pa /var/run/sshd.pid
1.223 jmc 924: Contains the process ID of the
925: .Nm
926: listening for connections (if there are several daemons running
927: concurrently for different ports, this contains the process ID of the one
928: started last).
929: The content of this file is not sensitive; it can be world-readable.
1.56 aaron 930: .El
1.2 deraadt 931: .Sh SEE ALSO
932: .Xr scp 1 ,
1.90 djm 933: .Xr sftp 1 ,
1.2 deraadt 934: .Xr ssh 1 ,
1.5 deraadt 935: .Xr ssh-add 1 ,
1.2 deraadt 936: .Xr ssh-agent 1 ,
1.5 deraadt 937: .Xr ssh-keygen 1 ,
1.235 jmc 938: .Xr ssh-keyscan 1 ,
1.200 jmc 939: .Xr chroot 2 ,
1.136 markus 940: .Xr login.conf 5 ,
941: .Xr moduli 5 ,
1.184 stevesk 942: .Xr sshd_config 5 ,
1.200 jmc 943: .Xr inetd 8 ,
1.128 mpech 944: .Xr sftp-server 8
1.198 jmc 945: .Sh AUTHORS
946: OpenSSH is a derivative of the original and free
947: ssh 1.2.12 release by Tatu Ylonen.
948: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
949: Theo de Raadt and Dug Song
950: removed many bugs, re-added newer features and
951: created OpenSSH.
952: Markus Friedl contributed the support for SSH
953: protocol versions 1.5 and 2.0.
954: Niels Provos and Markus Friedl contributed support
955: for privilege separation.