Annotation of src/usr.bin/ssh/sshd.8, Revision 1.323
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: .\"
1.323 ! djm 36: .\" $OpenBSD: sshd.8,v 1.322 2023/01/18 01:50:21 millert Exp $
! 37: .Dd $Mdocdate: January 18 2023 $
1.2 deraadt 38: .Dt SSHD 8
39: .Os
40: .Sh NAME
41: .Nm sshd
1.308 jmc 42: .Nd OpenSSH daemon
1.2 deraadt 43: .Sh SYNOPSIS
44: .Nm sshd
1.194 jmc 45: .Bk -words
1.323 ! djm 46: .Op Fl 46DGdeiqTtV
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 .
1.316 jmc 61: It provides secure encrypted communications between two untrusted hosts
1.36 aaron 62: over an insecure network.
1.2 deraadt 63: .Pp
64: .Nm
1.212 jmc 65: listens for connections from clients.
1.40 aaron 66: It is normally started at boot from
1.2 deraadt 67: .Pa /etc/rc .
68: It forks a new
1.36 aaron 69: daemon for each incoming connection.
70: The forked daemons handle
1.1 deraadt 71: key exchange, encryption, authentication, command execution,
72: and data exchange.
1.2 deraadt 73: .Pp
74: .Nm
1.200 jmc 75: can be configured using command-line options or a configuration file
76: (by default
1.212 jmc 77: .Xr sshd_config 5 ) ;
78: command-line options override values specified in the
1.1 deraadt 79: configuration file.
1.25 markus 80: .Nm
81: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 82: .Dv SIGHUP ,
1.230 jmc 83: by executing itself with the name and options it was started with, e.g.\&
1.97 deraadt 84: .Pa /usr/sbin/sshd .
1.18 aaron 85: .Pp
86: The options are as follows:
1.2 deraadt 87: .Bl -tag -width Ds
1.200 jmc 88: .It Fl 4
89: Forces
90: .Nm
91: to use IPv4 addresses only.
92: .It Fl 6
93: Forces
94: .Nm
95: to use IPv6 addresses only.
1.243 jmc 96: .It Fl C Ar connection_spec
97: Specify the connection parameters to use for the
98: .Fl T
99: extended test mode.
100: If provided, any
101: .Cm Match
1.293 dtucker 102: directives in the configuration file that would apply are applied before the
103: configuration is written to standard output.
104: The connection parameters are supplied as keyword=value pairs and may be
105: supplied in any order, either with multiple
106: .Fl C
107: options or as a comma-separated list.
1.243 jmc 108: The keywords are
1.316 jmc 109: .Dq addr ,
1.243 jmc 110: .Dq user ,
111: .Dq host ,
1.265 dtucker 112: .Dq laddr ,
113: .Dq lport ,
1.293 dtucker 114: and
1.292 djm 115: .Dq rdomain
1.293 dtucker 116: and correspond to source address, user, resolved source host name,
117: local address, local port number and routing domain respectively.
1.252 jmc 118: .It Fl c Ar host_certificate_file
119: Specifies a path to a certificate file to identify
120: .Nm
121: during key exchange.
122: The certificate file must match a host key file specified using the
123: .Fl h
124: option or the
125: .Cm HostKey
126: configuration directive.
1.200 jmc 127: .It Fl D
128: When this option is specified,
129: .Nm
130: will not detach and does not become a daemon.
131: This allows easy monitoring of
132: .Nm sshd .
1.2 deraadt 133: .It Fl d
1.36 aaron 134: Debug mode.
1.250 djm 135: The server sends verbose debug output to standard error,
136: and does not put itself in the background.
1.316 jmc 137: The server also will not
138: .Xr fork 2
139: and will only process one connection.
1.36 aaron 140: This option is only intended for debugging for the server.
1.194 jmc 141: Multiple
142: .Fl d
143: options increase the debugging level.
1.67 aaron 144: Maximum is 3.
1.268 dtucker 145: .It Fl E Ar log_file
146: Append debug logs to
147: .Ar log_file
148: instead of the system log.
1.120 markus 149: .It Fl e
1.269 dtucker 150: Write debug logs to standard error instead of the system log.
1.235 jmc 151: .It Fl f Ar config_file
1.36 aaron 152: Specifies the name of the configuration file.
153: The default is
1.167 deraadt 154: .Pa /etc/ssh/sshd_config .
1.16 markus 155: .Nm
156: refuses to start if there is no configuration file.
1.323 ! djm 157: .It Fl G
! 158: Parse and print configuration file.
! 159: Check the validity of the configuration file, output the effective configuration
! 160: to stdout and then exit.
! 161: Optionally,
! 162: .Cm Match
! 163: rules may be applied by specifying the connection parameters using one or more
! 164: .Fl C
! 165: options.
1.2 deraadt 166: .It Fl g Ar login_grace_time
1.1 deraadt 167: Gives the grace time for clients to authenticate themselves (default
1.191 stevesk 168: 120 seconds).
1.36 aaron 169: If the client fails to authenticate the user within
170: this many seconds, the server disconnects and exits.
171: A value of zero indicates no limit.
1.2 deraadt 172: .It Fl h Ar host_key_file
1.160 stevesk 173: Specifies a file from which a host key is read.
1.7 markus 174: This option must be given if
175: .Nm
176: is not run as root (as the normal
1.160 stevesk 177: host key files are normally not readable by anyone but root).
178: The default is
1.285 naddy 179: .Pa /etc/ssh/ssh_host_ecdsa_key ,
1.273 naddy 180: .Pa /etc/ssh/ssh_host_ed25519_key
1.259 djm 181: and
1.285 naddy 182: .Pa /etc/ssh/ssh_host_rsa_key .
1.75 markus 183: It is possible to have multiple host key files for
1.285 naddy 184: the different host key algorithms.
1.2 deraadt 185: .It Fl i
1.7 markus 186: Specifies that
187: .Nm
1.194 jmc 188: is being run from
189: .Xr inetd 8 .
1.156 markus 190: .It Fl o Ar option
191: Can be used to give options in the format used in the configuration file.
192: This is useful for specifying options for which there is no separate
193: command-line flag.
1.200 jmc 194: For full details of the options, and their values, see
195: .Xr sshd_config 5 .
1.2 deraadt 196: .It Fl p Ar port
1.1 deraadt 197: Specifies the port on which the server listens for connections
198: (default 22).
1.158 stevesk 199: Multiple port options are permitted.
1.210 stevesk 200: Ports specified in the configuration file with the
201: .Cm Port
202: option are ignored when a command-line port is specified.
203: Ports specified using the
204: .Cm ListenAddress
205: option override command-line ports.
1.2 deraadt 206: .It Fl q
1.36 aaron 207: Quiet mode.
208: Nothing is sent to the system log.
209: Normally the beginning,
1.1 deraadt 210: authentication, and termination of each connection is logged.
1.242 dtucker 211: .It Fl T
212: Extended test mode.
213: Check the validity of the configuration file, output the effective configuration
214: to stdout and then exit.
215: Optionally,
216: .Cm Match
217: rules may be applied by specifying the connection parameters using one or more
218: .Fl C
219: options.
1.323 ! djm 220: This is similar to the
! 221: .Fl G
! 222: flag, but it includes the additional testing performed by the
! 223: .Fl t
! 224: flag.
1.243 jmc 225: .It Fl t
226: Test mode.
227: Only check the validity of the configuration file and sanity of the keys.
228: This is useful for updating
229: .Nm
230: reliably as configuration options may change.
1.61 markus 231: .It Fl u Ar len
232: This option is used to specify the size of the field
233: in the
1.321 jmc 234: .Vt utmp
1.61 markus 235: structure that holds the remote host name.
236: If the resolved host name is longer than
237: .Ar len ,
238: the dotted decimal value will be used instead.
239: This allows hosts with very long host names that
240: overflow this field to still be uniquely identified.
241: Specifying
242: .Fl u0
243: indicates that only dotted decimal addresses
244: should be put into the
245: .Pa utmp
246: file.
1.144 stevesk 247: .Fl u0
1.194 jmc 248: may also be used to prevent
1.144 stevesk 249: .Nm
250: from making DNS requests unless the authentication
251: mechanism or configuration requires it.
252: Authentication mechanisms that may require DNS include
1.285 naddy 253: .Cm HostbasedAuthentication
1.144 stevesk 254: and using a
255: .Cm from="pattern-list"
256: option in a key file.
1.170 stevesk 257: Configuration options that require DNS include using a
258: USER@HOST pattern in
259: .Cm AllowUsers
260: or
261: .Cm DenyUsers .
1.322 millert 262: .It Fl V
263: Display the version number and exit.
1.2 deraadt 264: .El
1.214 jmc 265: .Sh AUTHENTICATION
1.285 naddy 266: The OpenSSH SSH daemon supports SSH protocol 2 only.
1.284 jmc 267: Each host has a host-specific key,
1.214 jmc 268: used to identify the host.
1.212 jmc 269: Whenever a client connects, the daemon responds with its public
1.285 naddy 270: host key.
1.212 jmc 271: The client compares the
1.285 naddy 272: host key against its own database to verify that it has not changed.
1.310 dtucker 273: Forward secrecy is provided through a Diffie-Hellman key agreement.
1.214 jmc 274: This key agreement results in a shared session key.
1.309 naddy 275: The rest of the session is encrypted using a symmetric cipher.
1.214 jmc 276: The client selects the encryption algorithm
277: to use from those offered by the server.
278: Additionally, session integrity is provided
1.310 dtucker 279: through a cryptographic message authentication code (MAC).
1.214 jmc 280: .Pp
281: Finally, the server and the client enter an authentication dialog.
1.212 jmc 282: The client tries to authenticate itself using
1.214 jmc 283: host-based authentication,
284: public key authentication,
285: challenge-response authentication,
286: or password authentication.
1.212 jmc 287: .Pp
288: If the client successfully authenticates itself, a dialog for
289: preparing the session is entered.
290: At this time the client may request
291: things like allocating a pseudo-tty, forwarding X11 connections,
292: forwarding TCP connections, or forwarding the authentication agent
293: connection over the secure channel.
294: .Pp
1.317 dtucker 295: After this, the client either requests an interactive shell or execution
296: or a non-interactive command, which
297: .Nm
298: will execute via the user's shell using its
299: .Fl c
300: option.
1.212 jmc 301: The sides then enter session mode.
302: In this mode, either side may send
303: data at any time, and such data is forwarded to/from the shell or
304: command on the server side, and the user terminal in the client side.
305: .Pp
306: When the user program terminates and all forwarded X11 and other
307: connections have been closed, the server sends command exit status to
308: the client, and both sides exit.
1.2 deraadt 309: .Sh LOGIN PROCESS
1.1 deraadt 310: When a user successfully logs in,
1.2 deraadt 311: .Nm
1.1 deraadt 312: does the following:
1.2 deraadt 313: .Bl -enum -offset indent
314: .It
1.1 deraadt 315: If the login is on a tty, and no command has been specified,
1.40 aaron 316: prints last login time and
1.2 deraadt 317: .Pa /etc/motd
1.1 deraadt 318: (unless prevented in the configuration file or by
1.207 djm 319: .Pa ~/.hushlogin ;
1.2 deraadt 320: see the
1.40 aaron 321: .Sx FILES
1.2 deraadt 322: section).
323: .It
1.1 deraadt 324: If the login is on a tty, records login time.
1.2 deraadt 325: .It
326: Checks
327: .Pa /etc/nologin ;
328: if it exists, prints contents and quits
1.1 deraadt 329: (unless root).
1.2 deraadt 330: .It
1.1 deraadt 331: Changes to run with normal user privileges.
1.2 deraadt 332: .It
1.1 deraadt 333: Sets up basic environment.
1.2 deraadt 334: .It
1.200 jmc 335: Reads the file
1.207 djm 336: .Pa ~/.ssh/environment ,
1.200 jmc 337: if it exists, and users are allowed to change their environment.
1.188 stevesk 338: See the
1.187 marc 339: .Cm PermitUserEnvironment
1.188 stevesk 340: option in
1.187 marc 341: .Xr sshd_config 5 .
1.2 deraadt 342: .It
1.1 deraadt 343: Changes to user's home directory.
1.2 deraadt 344: .It
345: If
1.207 djm 346: .Pa ~/.ssh/rc
1.276 djm 347: exists and the
348: .Xr sshd_config 5
349: .Cm PermitUserRC
350: option is set, runs it; else if
1.168 deraadt 351: .Pa /etc/ssh/sshrc
1.2 deraadt 352: exists, runs
1.316 jmc 353: it; otherwise runs
354: .Xr xauth 1 .
1.36 aaron 355: The
1.2 deraadt 356: .Dq rc
357: files are given the X11
1.1 deraadt 358: authentication protocol and cookie in standard input.
1.226 jmc 359: See
360: .Sx SSHRC ,
361: below.
1.2 deraadt 362: .It
1.1 deraadt 363: Runs user's shell or command.
1.279 djm 364: All commands are run under the user's login shell as specified in the
365: system password database.
1.2 deraadt 366: .El
1.226 jmc 367: .Sh SSHRC
368: If the file
369: .Pa ~/.ssh/rc
370: exists,
371: .Xr sh 1
372: runs it after reading the
373: environment files but before starting the user's shell or command.
374: It must not produce any output on stdout; stderr must be used
375: instead.
376: If X11 forwarding is in use, it will receive the "proto cookie" pair in
377: its standard input (and
378: .Ev DISPLAY
379: in its environment).
380: The script must call
381: .Xr xauth 1
382: because
383: .Nm
384: will not run xauth automatically to add X11 cookies.
385: .Pp
386: The primary purpose of this file is to run any initialization routines
387: which may be needed before the user's home directory becomes
388: accessible; AFS is a particular example of such an environment.
389: .Pp
390: This file will probably contain some initialization code followed by
391: something similar to:
392: .Bd -literal -offset 3n
393: if read proto cookie && [ -n "$DISPLAY" ]; then
394: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
395: # X11UseLocalhost=yes
396: echo add unix:`echo $DISPLAY |
397: cut -c11-` $proto $cookie
398: else
399: # X11UseLocalhost=no
400: echo add $DISPLAY $proto $cookie
401: fi | xauth -q -
402: fi
403: .Ed
404: .Pp
405: If this file does not exist,
406: .Pa /etc/ssh/sshrc
407: is run, and if that
408: does not exist either, xauth is used to add the cookie.
1.2 deraadt 409: .Sh AUTHORIZED_KEYS FILE FORMAT
1.125 markus 410: .Cm AuthorizedKeysFile
1.262 jmc 411: specifies the files containing public keys for
1.217 jmc 412: public key authentication;
1.281 djm 413: if this option is not specified, the default is
1.261 djm 414: .Pa ~/.ssh/authorized_keys
415: and
416: .Pa ~/.ssh/authorized_keys2 .
1.36 aaron 417: Each line of the file contains one
1.2 deraadt 418: key (empty lines and lines starting with a
419: .Ql #
420: are ignored as
1.36 aaron 421: comments).
1.285 naddy 422: Public keys consist of the following space-separated fields:
1.217 jmc 423: options, keytype, base64-encoded key, comment.
1.285 naddy 424: The options field is optional.
1.305 naddy 425: The supported key types are:
426: .Pp
427: .Bl -item -compact -offset indent
428: .It
429: sk-ecdsa-sha2-nistp256@openssh.com
430: .It
431: ecdsa-sha2-nistp256
432: .It
433: ecdsa-sha2-nistp384
434: .It
435: ecdsa-sha2-nistp521
436: .It
1.306 djm 437: sk-ssh-ed25519@openssh.com
438: .It
1.305 naddy 439: ssh-ed25519
440: .It
441: ssh-dss
442: .It
443: ssh-rsa
444: .El
445: .Pp
446: The comment field is not used for anything (but may be convenient for the
1.285 naddy 447: user to identify the key).
1.2 deraadt 448: .Pp
1.285 naddy 449: Note that lines in this file can be several hundred bytes long
1.203 dtucker 450: (because of the size of the public key encoding) up to a limit of
1.305 naddy 451: 8 kilobytes, which permits RSA keys up to 16 kilobits.
1.36 aaron 452: You don't want to type them in; instead, copy the
1.217 jmc 453: .Pa id_dsa.pub ,
1.259 djm 454: .Pa id_ecdsa.pub ,
1.305 naddy 455: .Pa id_ecdsa_sk.pub ,
1.273 naddy 456: .Pa id_ed25519.pub ,
1.306 djm 457: .Pa id_ed25519_sk.pub ,
1.75 markus 458: or the
1.113 itojun 459: .Pa id_rsa.pub
1.1 deraadt 460: file and edit it.
1.175 stevesk 461: .Pp
462: .Nm
1.296 dtucker 463: enforces a minimum RSA key modulus size of 1024 bits.
1.2 deraadt 464: .Pp
1.58 deraadt 465: The options (if present) consist of comma-separated option
1.36 aaron 466: specifications.
467: No spaces are permitted, except within double quotes.
1.141 stevesk 468: The following option specifications are supported (note
469: that option keywords are case-insensitive):
1.2 deraadt 470: .Bl -tag -width Ds
1.282 djm 471: .It Cm agent-forwarding
472: Enable authentication agent forwarding previously disabled by the
473: .Cm restrict
474: option.
1.253 djm 475: .It Cm cert-authority
1.251 djm 476: Specifies that the listed key is a certification authority (CA) that is
477: trusted to validate signed certificates for user authentication.
478: .Pp
479: Certificates may encode access restrictions similar to these key options.
480: If both certificate restrictions and key options are present, the most
481: restrictive union of the two is applied.
1.2 deraadt 482: .It Cm command="command"
1.1 deraadt 483: Specifies that the command is executed whenever this key is used for
1.36 aaron 484: authentication.
485: The command supplied by the user (if any) is ignored.
1.148 markus 486: The command is run on a pty if the client requests a pty;
1.36 aaron 487: otherwise it is run without a tty.
1.194 jmc 488: If an 8-bit clean channel is required,
1.147 deraadt 489: one must not request a pty or should specify
1.89 markus 490: .Cm no-pty .
1.36 aaron 491: A quote may be included in the command by quoting it with a backslash.
1.287 djm 492: .Pp
1.36 aaron 493: This option might be useful
1.189 espie 494: to restrict certain public keys to perform just a specific operation.
1.36 aaron 495: An example might be a key that permits remote backups but nothing else.
1.211 jmc 496: Note that the client may specify TCP and/or X11
1.287 djm 497: forwarding unless they are explicitly prohibited, e.g. using the
498: .Cm restrict
499: key option.
500: .Pp
1.233 dtucker 501: The command originally supplied by the client is available in the
502: .Ev SSH_ORIGINAL_COMMAND
503: environment variable.
1.149 markus 504: Note that this option applies to shell, command or subsystem execution.
1.287 djm 505: Also note that this command may be superseded by a
1.251 djm 506: .Xr sshd_config 5
507: .Cm ForceCommand
1.287 djm 508: directive.
509: .Pp
510: If a command is specified and a forced-command is embedded in a certificate
511: used for authentication, then the certificate will be accepted only if the
512: two commands are identical.
1.2 deraadt 513: .It Cm environment="NAME=value"
1.1 deraadt 514: Specifies that the string is to be added to the environment when
1.36 aaron 515: logging in using this key.
516: Environment variables set this way
517: override other default environment values.
518: Multiple options of this type are permitted.
1.188 stevesk 519: Environment processing is disabled by default and is
520: controlled via the
521: .Cm PermitUserEnvironment
522: option.
1.299 jmc 523: .It Cm expiry-time="timespec"
524: Specifies a time after which the key will not be accepted.
1.320 djm 525: The time may be specified as a YYYYMMDD[Z] date or a YYYYMMDDHHMM[SS][Z] time.
526: Dates and times will be interpreted in the system time zone unless suffixed
527: by a Z character, in which case they will be interpreted in the UTC time zone.
1.218 jmc 528: .It Cm from="pattern-list"
1.244 djm 529: Specifies that in addition to public key authentication, either the canonical
1.245 jmc 530: name of the remote host or its IP address must be present in the
1.244 djm 531: comma-separated list of patterns.
1.270 jmc 532: See PATTERNS in
1.229 jmc 533: .Xr ssh_config 5
534: for more information on patterns.
1.244 djm 535: .Pp
536: In addition to the wildcard matching that may be applied to hostnames or
537: addresses, a
538: .Cm from
1.248 sobrado 539: stanza may match IP addresses using CIDR address/masklen notation.
1.244 djm 540: .Pp
541: The purpose of this option is to optionally increase security: public key
542: authentication by itself does not trust the network or name servers or
543: anything (but the key); however, if somebody somehow steals the key, the key
544: permits an intruder to log in from anywhere in the world.
545: This additional option makes using a stolen key more difficult (name
546: servers and/or routers would have to be compromised in addition to
547: just the key).
1.218 jmc 548: .It Cm no-agent-forwarding
549: Forbids authentication agent forwarding when this key is used for
550: authentication.
1.2 deraadt 551: .It Cm no-port-forwarding
1.211 jmc 552: Forbids TCP forwarding when this key is used for authentication.
1.36 aaron 553: Any port forward requests by the client will return an error.
1.230 jmc 554: This might be used, e.g. in connection with the
1.2 deraadt 555: .Cm command
1.1 deraadt 556: option.
1.218 jmc 557: .It Cm no-pty
558: Prevents tty allocation (a request to allocate a pty will fail).
1.240 djm 559: .It Cm no-user-rc
1.241 jmc 560: Disables execution of
1.240 djm 561: .Pa ~/.ssh/rc .
1.2 deraadt 562: .It Cm no-X11-forwarding
1.1 deraadt 563: Forbids X11 forwarding when this key is used for authentication.
564: Any X11 forward requests by the client will return an error.
1.302 djm 565: .It Cm permitlisten="[host:]port"
1.301 jmc 566: Limit remote port forwarding with the
1.300 djm 567: .Xr ssh 1
568: .Fl R
1.302 djm 569: option such that it may only listen on the specified host (optional) and port.
1.300 djm 570: IPv6 addresses can be specified by enclosing the address in square brackets.
571: Multiple
572: .Cm permitlisten
573: options may be applied separated by commas.
574: Hostnames may include wildcards as described in the PATTERNS section in
575: .Xr ssh_config 5 .
576: A port specification of
577: .Cm *
578: matches any port.
579: Note that the setting of
580: .Cm GatewayPorts
581: may further restrict listen addresses.
1.302 djm 582: Note that
583: .Xr ssh 1
584: will send a hostname of
585: .Dq localhost
586: if a listen host was not specified when the forwarding was requested, and
1.303 jmc 587: that this name is treated differently to the explicit localhost addresses
1.302 djm 588: .Dq 127.0.0.1
589: and
590: .Dq ::1 .
1.107 djm 591: .It Cm permitopen="host:port"
1.301 jmc 592: Limit local port forwarding with the
1.278 bentley 593: .Xr ssh 1
594: .Fl L
1.300 djm 595: option such that it may only connect to the specified host and port.
1.257 djm 596: IPv6 addresses can be specified by enclosing the address in square brackets.
1.146 stevesk 597: Multiple
1.107 djm 598: .Cm permitopen
1.197 jmc 599: options may be applied separated by commas.
1.312 djm 600: No pattern matching or name lookup is performed on the
601: specified hostnames, they must be literal host names and/or addresses.
1.264 dtucker 602: A port specification of
603: .Cm *
604: matches any port.
1.282 djm 605: .It Cm port-forwarding
606: Enable port forwarding previously disabled by the
607: .Cm restrict
1.294 djm 608: option.
1.256 djm 609: .It Cm principals="principals"
610: On a
611: .Cm cert-authority
612: line, specifies allowed principals for certificate authentication as a
613: comma-separated list.
614: At least one name from the list must appear in the certificate's
615: list of principals for the certificate to be accepted.
616: This option is ignored for keys that are not marked as trusted certificate
617: signers using the
618: .Cm cert-authority
619: option.
1.282 djm 620: .It Cm pty
621: Permits tty allocation previously disabled by the
622: .Cm restrict
623: option.
1.307 djm 624: .It Cm no-touch-required
625: Do not require demonstration of user presence
626: for signatures made using this key.
1.313 djm 627: This option only makes sense for the FIDO authenticator algorithms
628: .Cm ecdsa-sk
629: and
630: .Cm ed25519-sk .
631: .It Cm verify-required
632: Require that signatures made using this key attest that they verified
633: the user, e.g. via a PIN.
1.311 naddy 634: This option only makes sense for the FIDO authenticator algorithms
1.307 djm 635: .Cm ecdsa-sk
636: and
637: .Cm ed25519-sk .
1.282 djm 638: .It Cm restrict
639: Enable all restrictions, i.e. disable port, agent and X11 forwarding,
640: as well as disabling PTY allocation
641: and execution of
642: .Pa ~/.ssh/rc .
1.318 naddy 643: If any future restriction capabilities are added to authorized_keys files,
1.282 djm 644: they will be included in this set.
1.209 reyk 645: .It Cm tunnel="n"
646: Force a
647: .Xr tun 4
648: device on the server.
649: Without this option, the next available device will be used if
650: the client requests a tunnel.
1.282 djm 651: .It Cm user-rc
652: Enables execution of
653: .Pa ~/.ssh/rc
654: previously disabled by the
655: .Cm restrict
656: option.
657: .It Cm X11-forwarding
658: Permits X11 forwarding previously disabled by the
659: .Cm restrict
660: option.
1.2 deraadt 661: .El
662: .Pp
1.219 jmc 663: An example authorized_keys file:
1.222 jmc 664: .Bd -literal -offset 3n
1.315 djm 665: # Comments are allowed at start of line. Blank lines are allowed.
666: # Plain key, no restrictions
667: ssh-rsa ...
668: # Forced command, disable PTY and all forwarding
669: restrict,command="dump /home" ssh-rsa ...
670: # Restriction of ssh -L forwarding destinations
671: permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-rsa ...
672: # Restriction of ssh -R forwarding listeners
673: permitlisten="localhost:8080",permitlisten="[::1]:22000" ssh-rsa ...
674: # Configuration for tunnel forwarding
675: tunnel="0",command="sh /etc/netstart tun0" ssh-rsa ...
676: # Override of restriction to allow PTY allocation
677: restrict,pty,command="nethack" ssh-rsa ...
678: # Allow FIDO key without requiring touch
679: no-touch-required sk-ecdsa-sha2-nistp256@openssh.com ...
680: # Require user-verification (e.g. PIN or biometric) for FIDO key
681: verify-required sk-ecdsa-sha2-nistp256@openssh.com ...
682: # Trust CA key, allow touch-less FIDO if requested in certificate
683: cert-authority,no-touch-required,principals="user_a" ssh-rsa ...
1.219 jmc 684: .Ed
1.2 deraadt 685: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 686: The
1.194 jmc 687: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 688: and
1.207 djm 689: .Pa ~/.ssh/known_hosts
1.36 aaron 690: files contain host public keys for all known hosts.
691: The global file should
1.37 brad 692: be prepared by the administrator (optional), and the per-user file is
1.288 dtucker 693: maintained automatically: whenever the user connects to an unknown host,
1.36 aaron 694: its key is added to the per-user file.
1.2 deraadt 695: .Pp
1.314 djm 696: Each line in these files contains the following fields: marker (optional),
1.285 naddy 697: hostnames, keytype, base64-encoded key, comment.
1.36 aaron 698: The fields are separated by spaces.
1.2 deraadt 699: .Pp
1.254 djm 700: The marker is optional, but if it is present then it must be one of
701: .Dq @cert-authority ,
702: to indicate that the line contains a certification authority (CA) key,
703: or
704: .Dq @revoked ,
705: to indicate that the key contained on the line is revoked and must not ever
706: be accepted.
707: Only one marker should be used on a key line.
708: .Pp
1.196 david 709: Hostnames is a comma-separated list of patterns
1.220 jmc 710: .Pf ( Ql *
1.197 jmc 711: and
712: .Ql \&?
1.195 mouring 713: act as
1.290 djm 714: wildcards); each pattern in turn is matched against the host name.
715: When
716: .Nm sshd
717: is authenticating a client, such as when using
718: .Cm HostbasedAuthentication ,
719: this will be the canonical client host name.
720: When
721: .Xr ssh 1
1.291 jmc 722: is authenticating a server, this will be the host name
1.290 djm 723: given by the user, the value of the
724: .Xr ssh 1
725: .Cm HostkeyAlias
726: if it was specified, or the canonical server hostname if the
727: .Xr ssh 1
728: .Cm CanonicalizeHostname
729: option was used.
730: .Pp
1.36 aaron 731: A pattern may also be preceded by
1.195 mouring 732: .Ql \&!
1.2 deraadt 733: to indicate negation: if the host name matches a negated
1.1 deraadt 734: pattern, it is not accepted (by that line) even if it matched another
735: pattern on the line.
1.231 dtucker 736: A hostname or address may optionally be enclosed within
737: .Ql \&[
738: and
739: .Ql \&]
740: brackets then followed by
741: .Ql \&:
1.232 jmc 742: and a non-standard port number.
1.2 deraadt 743: .Pp
1.205 djm 744: Alternately, hostnames may be stored in a hashed form which hides host names
1.206 jmc 745: and addresses should the file's contents be disclosed.
746: Hashed hostnames start with a
747: .Ql |
1.205 djm 748: character.
749: Only one hashed hostname may appear on a single line and none of the above
750: negation or wildcard operators may be applied.
751: .Pp
1.285 naddy 752: The keytype and base64-encoded key are taken directly from the host key; they
1.220 jmc 753: can be obtained, for example, from
1.285 naddy 754: .Pa /etc/ssh/ssh_host_rsa_key.pub .
1.1 deraadt 755: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 756: .Pp
757: Lines starting with
758: .Ql #
759: and empty lines are ignored as comments.
760: .Pp
1.1 deraadt 761: When performing host authentication, authentication is accepted if any
1.254 djm 762: matching line has the proper key; either one that matches exactly or,
763: if the server has presented a certificate for authentication, the key
764: of the certification authority that signed the certificate.
765: For a key to be trusted as a certification authority, it must use the
766: .Dq @cert-authority
767: marker described above.
768: .Pp
769: The known hosts file also provides a facility to mark keys as revoked,
770: for example when it is known that the associated private key has been
771: stolen.
772: Revoked keys are specified by including the
773: .Dq @revoked
774: marker at the beginning of the key line, and are never accepted for
775: authentication or as certification authorities, but instead will
776: produce a warning from
777: .Xr ssh 1
778: when they are encountered.
779: .Pp
780: It is permissible (but not
1.1 deraadt 781: recommended) to have several lines or different host keys for the same
1.36 aaron 782: names.
783: This will inevitably happen when short forms of host names
784: from different domains are put in the file.
785: It is possible
1.1 deraadt 786: that the files contain conflicting information; authentication is
787: accepted if valid information can be found from either file.
1.2 deraadt 788: .Pp
1.1 deraadt 789: Note that the lines in these files are typically hundreds of characters
790: long, and you definitely don't want to type in the host keys by hand.
1.254 djm 791: Rather, generate them by a script,
792: .Xr ssh-keyscan 1
1.285 naddy 793: or by taking, for example,
794: .Pa /etc/ssh/ssh_host_rsa_key.pub
1.1 deraadt 795: and adding the host names at the front.
1.254 djm 796: .Xr ssh-keygen 1
1.255 jmc 797: also offers some basic automated editing for
1.254 djm 798: .Pa ~/.ssh/known_hosts
799: including removing hosts matching a host name and converting all host
800: names to their hashed representations.
1.221 jmc 801: .Pp
802: An example ssh_known_hosts file:
803: .Bd -literal -offset 3n
804: # Comments allowed at start of line
805: cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
1.205 djm 806: # A hashed hostname
807: |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
808: AAAA1234.....=
1.254 djm 809: # A revoked key
810: @revoked * ssh-rsa AAAAB5W...
811: # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
812: @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
1.120 markus 813: .Ed
1.2 deraadt 814: .Sh FILES
1.223 jmc 815: .Bl -tag -width Ds -compact
1.258 jmc 816: .It Pa ~/.hushlogin
1.204 jmc 817: This file is used to suppress printing the last login time and
818: .Pa /etc/motd ,
819: if
820: .Cm PrintLastLog
821: and
822: .Cm PrintMotd ,
823: respectively,
824: are enabled.
825: It does not suppress printing of the banner specified by
826: .Cm Banner .
1.223 jmc 827: .Pp
1.258 jmc 828: .It Pa ~/.rhosts
1.225 jmc 829: This file is used for host-based authentication (see
830: .Xr ssh 1
831: for more information).
832: On some machines this file may need to be
833: world-readable if the user's home directory is on an NFS partition,
834: because
835: .Nm
836: reads it as root.
837: Additionally, this file must be owned by the user,
838: and must not have write permissions for anyone else.
839: The recommended
840: permission for most machines is read/write for the user, and not
1.1 deraadt 841: accessible by others.
1.2 deraadt 842: .Pp
1.258 jmc 843: .It Pa ~/.shosts
1.225 jmc 844: This file is used in exactly the same way as
845: .Pa .rhosts ,
846: but allows host-based authentication without permitting login with
847: rlogin/rsh.
1.238 mcbride 848: .Pp
1.258 jmc 849: .It Pa ~/.ssh/
1.238 mcbride 850: This directory is the default location for all user-specific configuration
851: and authentication information.
852: There is no general requirement to keep the entire contents of this directory
853: secret, but the recommended permissions are read/write/execute for the user,
854: and not accessible by others.
1.2 deraadt 855: .Pp
1.258 jmc 856: .It Pa ~/.ssh/authorized_keys
1.277 sobrado 857: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.273 naddy 858: that can be used for logging in as this user.
1.223 jmc 859: The format of this file is described above.
1.234 dtucker 860: The content of the file is not highly sensitive, but the recommended
1.225 jmc 861: permissions are read/write for the user, and not accessible by others.
1.234 dtucker 862: .Pp
863: If this file, the
864: .Pa ~/.ssh
865: directory, or the user's home directory are writable
866: by other users, then the file could be modified or replaced by unauthorized
867: users.
868: In this case,
869: .Nm
870: will not allow it to be used unless the
871: .Cm StrictModes
872: option has been set to
873: .Dq no .
1.2 deraadt 874: .Pp
1.258 jmc 875: .It Pa ~/.ssh/environment
1.36 aaron 876: This file is read into the environment at login (if it exists).
877: It can only contain empty lines, comment lines (that start with
1.2 deraadt 878: .Ql # ) ,
1.36 aaron 879: and assignment lines of the form name=value.
880: The file should be writable
1.6 aaron 881: only by the user; it need not be readable by anyone else.
1.188 stevesk 882: Environment processing is disabled by default and is
883: controlled via the
884: .Cm PermitUserEnvironment
885: option.
1.223 jmc 886: .Pp
1.258 jmc 887: .It Pa ~/.ssh/known_hosts
1.225 jmc 888: Contains a list of host keys for all hosts the user has logged into
889: that are not already in the systemwide list of known host keys.
890: The format of this file is described above.
891: This file should be writable only by root/the owner and
1.223 jmc 892: can, but need not be, world-readable.
893: .Pp
1.258 jmc 894: .It Pa ~/.ssh/rc
1.226 jmc 895: Contains initialization routines to be run before
896: the user's home directory becomes accessible.
1.1 deraadt 897: This file should be writable only by the user, and need not be
898: readable by anyone else.
1.223 jmc 899: .Pp
1.258 jmc 900: .It Pa /etc/hosts.equiv
1.227 jmc 901: This file is for host-based authentication (see
902: .Xr ssh 1 ) .
903: It should only be writable by root.
1.223 jmc 904: .Pp
1.258 jmc 905: .It Pa /etc/moduli
1.283 djm 906: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange"
907: key exchange method.
1.223 jmc 908: The file format is described in
909: .Xr moduli 5 .
1.283 djm 910: If no usable groups are found in this file then fixed internal groups will
911: be used.
1.223 jmc 912: .Pp
1.258 jmc 913: .It Pa /etc/motd
1.223 jmc 914: See
915: .Xr motd 5 .
916: .Pp
1.258 jmc 917: .It Pa /etc/nologin
1.223 jmc 918: If this file exists,
919: .Nm
920: refuses to let anyone except root log in.
921: The contents of the file
922: are displayed to anyone trying to log in, and non-root connections are
923: refused.
924: The file should be world-readable.
925: .Pp
1.258 jmc 926: .It Pa /etc/shosts.equiv
1.227 jmc 927: This file is used in exactly the same way as
928: .Pa hosts.equiv ,
929: but allows host-based authentication without permitting login with
930: rlogin/rsh.
1.225 jmc 931: .Pp
1.259 djm 932: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.273 naddy 933: .It Pa /etc/ssh/ssh_host_ed25519_key
1.258 jmc 934: .It Pa /etc/ssh/ssh_host_rsa_key
1.266 dtucker 935: These files contain the private parts of the host keys.
1.223 jmc 936: These files should only be owned by root, readable only by root, and not
937: accessible to others.
938: Note that
939: .Nm
1.228 jmc 940: does not start if these files are group/world-accessible.
1.223 jmc 941: .Pp
1.259 djm 942: .It Pa /etc/ssh/ssh_host_ecdsa_key.pub
1.273 naddy 943: .It Pa /etc/ssh/ssh_host_ed25519_key.pub
1.258 jmc 944: .It Pa /etc/ssh/ssh_host_rsa_key.pub
1.266 dtucker 945: These files contain the public parts of the host keys.
1.223 jmc 946: These files should be world-readable but writable only by
947: root.
948: Their contents should match the respective private parts.
949: These files are not
950: really used for anything; they are provided for the convenience of
951: the user so their contents can be copied to known hosts files.
952: These files are created using
953: .Xr ssh-keygen 1 .
954: .Pp
1.258 jmc 955: .It Pa /etc/ssh/ssh_known_hosts
1.235 jmc 956: Systemwide list of known host keys.
957: This file should be prepared by the
958: system administrator to contain the public host keys of all machines in the
959: organization.
960: The format of this file is described above.
961: This file should be writable only by root/the owner and
962: should be world-readable.
963: .Pp
1.258 jmc 964: .It Pa /etc/ssh/sshd_config
1.223 jmc 965: Contains configuration data for
966: .Nm sshd .
967: The file format and configuration options are described in
968: .Xr sshd_config 5 .
969: .Pp
1.258 jmc 970: .It Pa /etc/ssh/sshrc
1.226 jmc 971: Similar to
972: .Pa ~/.ssh/rc ,
973: it can be used to specify
1.36 aaron 974: machine-specific login-time initializations globally.
975: This file should be writable only by root, and should be world-readable.
1.223 jmc 976: .Pp
1.258 jmc 977: .It Pa /var/empty
1.223 jmc 978: .Xr chroot 2
979: directory used by
980: .Nm
981: during privilege separation in the pre-authentication phase.
982: The directory should not contain any files and must be owned by root
983: and not group or world-writable.
984: .Pp
1.258 jmc 985: .It Pa /var/run/sshd.pid
1.223 jmc 986: Contains the process ID of the
987: .Nm
988: listening for connections (if there are several daemons running
989: concurrently for different ports, this contains the process ID of the one
990: started last).
991: The content of this file is not sensitive; it can be world-readable.
1.56 aaron 992: .El
1.2 deraadt 993: .Sh SEE ALSO
994: .Xr scp 1 ,
1.90 djm 995: .Xr sftp 1 ,
1.2 deraadt 996: .Xr ssh 1 ,
1.5 deraadt 997: .Xr ssh-add 1 ,
1.2 deraadt 998: .Xr ssh-agent 1 ,
1.5 deraadt 999: .Xr ssh-keygen 1 ,
1.235 jmc 1000: .Xr ssh-keyscan 1 ,
1.200 jmc 1001: .Xr chroot 2 ,
1.136 markus 1002: .Xr login.conf 5 ,
1003: .Xr moduli 5 ,
1.184 stevesk 1004: .Xr sshd_config 5 ,
1.200 jmc 1005: .Xr inetd 8 ,
1.128 mpech 1006: .Xr sftp-server 8
1.198 jmc 1007: .Sh AUTHORS
1008: OpenSSH is a derivative of the original and free
1009: ssh 1.2.12 release by Tatu Ylonen.
1010: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1011: Theo de Raadt and Dug Song
1012: removed many bugs, re-added newer features and
1013: created OpenSSH.
1014: Markus Friedl contributed the support for SSH
1015: protocol versions 1.5 and 2.0.
1016: Niels Provos and Markus Friedl contributed support
1017: for privilege separation.
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