Annotation of src/usr.bin/ssh/sshd.8, Revision 1.279
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: .\"
1.279 ! djm 36: .\" $OpenBSD: sshd.8,v 1.278 2014/11/15 14:41:03 bentley Exp $
! 37: .Dd $Mdocdate: November 15 2014 $
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
1.2 deraadt 47: .Op Fl b Ar bits
1.242 dtucker 48: .Op Fl C Ar connection_spec
1.251 djm 49: .Op Fl c Ar host_certificate_file
1.268 dtucker 50: .Op Fl E Ar log_file
1.2 deraadt 51: .Op Fl f Ar config_file
52: .Op Fl g Ar login_grace_time
53: .Op Fl h Ar host_key_file
54: .Op Fl k Ar key_gen_time
1.156 markus 55: .Op Fl o Ar option
1.2 deraadt 56: .Op Fl p Ar port
1.61 markus 57: .Op Fl u Ar len
1.194 jmc 58: .Ek
1.40 aaron 59: .Sh DESCRIPTION
1.2 deraadt 60: .Nm
1.212 jmc 61: (OpenSSH Daemon) is the daemon program for
1.2 deraadt 62: .Xr ssh 1 .
1.275 tedu 63: Together these programs replace rlogin and rsh,
1.235 jmc 64: and provide secure encrypted communications between two untrusted hosts
1.36 aaron 65: over an insecure network.
1.2 deraadt 66: .Pp
67: .Nm
1.212 jmc 68: listens for connections from clients.
1.40 aaron 69: It is normally started at boot from
1.2 deraadt 70: .Pa /etc/rc .
71: It forks a new
1.36 aaron 72: daemon for each incoming connection.
73: The forked daemons handle
1.1 deraadt 74: key exchange, encryption, authentication, command execution,
75: and data exchange.
1.2 deraadt 76: .Pp
77: .Nm
1.200 jmc 78: can be configured using command-line options or a configuration file
79: (by default
1.212 jmc 80: .Xr sshd_config 5 ) ;
81: command-line options override values specified in the
1.1 deraadt 82: configuration file.
1.25 markus 83: .Nm
84: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 85: .Dv SIGHUP ,
1.230 jmc 86: by executing itself with the name and options it was started with, e.g.\&
1.97 deraadt 87: .Pa /usr/sbin/sshd .
1.18 aaron 88: .Pp
89: The options are as follows:
1.2 deraadt 90: .Bl -tag -width Ds
1.200 jmc 91: .It Fl 4
92: Forces
93: .Nm
94: to use IPv4 addresses only.
95: .It Fl 6
96: Forces
97: .Nm
98: to use IPv6 addresses only.
1.2 deraadt 99: .It Fl b Ar bits
1.120 markus 100: Specifies the number of bits in the ephemeral protocol version 1
1.246 djm 101: server key (default 1024).
1.243 jmc 102: .It Fl C Ar connection_spec
103: Specify the connection parameters to use for the
104: .Fl T
105: extended test mode.
106: If provided, any
107: .Cm Match
108: directives in the configuration file
109: that would apply to the specified user, host, and address will be set before
110: the configuration is written to standard output.
111: The connection parameters are supplied as keyword=value pairs.
112: The keywords are
113: .Dq user ,
114: .Dq host ,
1.265 dtucker 115: .Dq laddr ,
116: .Dq lport ,
1.243 jmc 117: and
118: .Dq addr .
119: All are required and may be supplied in any order, either with multiple
120: .Fl C
121: options or as a comma-separated list.
1.252 jmc 122: .It Fl c Ar host_certificate_file
123: Specifies a path to a certificate file to identify
124: .Nm
125: during key exchange.
126: The certificate file must match a host key file specified using the
127: .Fl h
128: option or the
129: .Cm HostKey
130: configuration directive.
1.200 jmc 131: .It Fl D
132: When this option is specified,
133: .Nm
134: will not detach and does not become a daemon.
135: This allows easy monitoring of
136: .Nm sshd .
1.2 deraadt 137: .It Fl d
1.36 aaron 138: Debug mode.
1.250 djm 139: The server sends verbose debug output to standard error,
140: and does not put itself in the background.
1.36 aaron 141: The server also will not fork and will only process one connection.
142: This option is only intended for debugging for the server.
1.194 jmc 143: Multiple
144: .Fl d
145: options increase the debugging level.
1.67 aaron 146: Maximum is 3.
1.268 dtucker 147: .It Fl E Ar log_file
148: Append debug logs to
149: .Ar log_file
150: instead of the system log.
1.120 markus 151: .It Fl e
1.269 dtucker 152: Write debug logs to standard error instead of the system log.
1.235 jmc 153: .It Fl f Ar config_file
1.36 aaron 154: Specifies the name of the configuration file.
155: The default is
1.167 deraadt 156: .Pa /etc/ssh/sshd_config .
1.16 markus 157: .Nm
158: refuses to start if there is no configuration file.
1.2 deraadt 159: .It Fl g Ar login_grace_time
1.1 deraadt 160: Gives the grace time for clients to authenticate themselves (default
1.191 stevesk 161: 120 seconds).
1.36 aaron 162: If the client fails to authenticate the user within
163: this many seconds, the server disconnects and exits.
164: A value of zero indicates no limit.
1.2 deraadt 165: .It Fl h Ar host_key_file
1.160 stevesk 166: Specifies a file from which a host key is read.
1.7 markus 167: This option must be given if
168: .Nm
169: is not run as root (as the normal
1.160 stevesk 170: host key files are normally not readable by anyone but root).
171: The default is
1.167 deraadt 172: .Pa /etc/ssh/ssh_host_key
1.160 stevesk 173: for protocol version 1, and
1.259 djm 174: .Pa /etc/ssh/ssh_host_dsa_key ,
1.273 naddy 175: .Pa /etc/ssh/ssh_host_ecdsa_key .
176: .Pa /etc/ssh/ssh_host_ed25519_key
1.259 djm 177: and
1.167 deraadt 178: .Pa /etc/ssh/ssh_host_rsa_key
1.160 stevesk 179: for protocol version 2.
1.75 markus 180: It is possible to have multiple host key files for
1.120 markus 181: the different protocol versions and host key algorithms.
1.2 deraadt 182: .It Fl i
1.7 markus 183: Specifies that
184: .Nm
1.194 jmc 185: is being run from
186: .Xr inetd 8 .
1.7 markus 187: .Nm
188: is normally not run
1.1 deraadt 189: from inetd because it needs to generate the server key before it can
1.36 aaron 190: respond to the client, and this may take tens of seconds.
191: Clients would have to wait too long if the key was regenerated every time.
1.230 jmc 192: However, with small key sizes (e.g. 512) using
1.7 markus 193: .Nm
194: from inetd may
1.1 deraadt 195: be feasible.
1.2 deraadt 196: .It Fl k Ar key_gen_time
1.120 markus 197: Specifies how often the ephemeral protocol version 1 server key is
198: regenerated (default 3600 seconds, or one hour).
1.36 aaron 199: The motivation for regenerating the key fairly
1.200 jmc 200: often is that the key is not stored anywhere, and after about an hour
1.1 deraadt 201: it becomes impossible to recover the key for decrypting intercepted
202: communications even if the machine is cracked into or physically
1.36 aaron 203: seized.
204: A value of zero indicates that the key will never be regenerated.
1.156 markus 205: .It Fl o Ar option
206: Can be used to give options in the format used in the configuration file.
207: This is useful for specifying options for which there is no separate
208: command-line flag.
1.200 jmc 209: For full details of the options, and their values, see
210: .Xr sshd_config 5 .
1.2 deraadt 211: .It Fl p Ar port
1.1 deraadt 212: Specifies the port on which the server listens for connections
213: (default 22).
1.158 stevesk 214: Multiple port options are permitted.
1.210 stevesk 215: Ports specified in the configuration file with the
216: .Cm Port
217: option are ignored when a command-line port is specified.
218: Ports specified using the
219: .Cm ListenAddress
220: option override command-line ports.
1.2 deraadt 221: .It Fl q
1.36 aaron 222: Quiet mode.
223: Nothing is sent to the system log.
224: Normally the beginning,
1.1 deraadt 225: authentication, and termination of each connection is logged.
1.242 dtucker 226: .It Fl T
227: Extended test mode.
228: Check the validity of the configuration file, output the effective configuration
229: to stdout and then exit.
230: Optionally,
231: .Cm Match
232: rules may be applied by specifying the connection parameters using one or more
233: .Fl C
234: options.
1.243 jmc 235: .It Fl t
236: Test mode.
237: Only check the validity of the configuration file and sanity of the keys.
238: This is useful for updating
239: .Nm
240: reliably as configuration options may change.
1.61 markus 241: .It Fl u Ar len
242: This option is used to specify the size of the field
243: in the
244: .Li utmp
245: structure that holds the remote host name.
246: If the resolved host name is longer than
247: .Ar len ,
248: the dotted decimal value will be used instead.
249: This allows hosts with very long host names that
250: overflow this field to still be uniquely identified.
251: Specifying
252: .Fl u0
253: indicates that only dotted decimal addresses
254: should be put into the
255: .Pa utmp
256: file.
1.144 stevesk 257: .Fl u0
1.194 jmc 258: may also be used to prevent
1.144 stevesk 259: .Nm
260: from making DNS requests unless the authentication
261: mechanism or configuration requires it.
262: Authentication mechanisms that may require DNS include
263: .Cm RhostsRSAAuthentication ,
1.215 jmc 264: .Cm HostbasedAuthentication ,
1.144 stevesk 265: and using a
266: .Cm from="pattern-list"
267: option in a key file.
1.170 stevesk 268: Configuration options that require DNS include using a
269: USER@HOST pattern in
270: .Cm AllowUsers
271: or
272: .Cm DenyUsers .
1.2 deraadt 273: .El
1.214 jmc 274: .Sh AUTHENTICATION
275: The OpenSSH SSH daemon supports SSH protocols 1 and 2.
1.249 jmc 276: The default is to use protocol 2 only,
1.214 jmc 277: though this can be changed via the
278: .Cm Protocol
279: option in
280: .Xr sshd_config 5 .
1.277 sobrado 281: Protocol 2 supports DSA, ECDSA, Ed25519 and RSA keys;
1.214 jmc 282: protocol 1 only supports RSA keys.
283: For both protocols,
284: each host has a host-specific key,
285: normally 2048 bits,
286: used to identify the host.
287: .Pp
288: Forward security for protocol 1 is provided through
289: an additional server key,
290: normally 768 bits,
291: generated when the server starts.
1.212 jmc 292: This key is normally regenerated every hour if it has been used, and
293: is never stored on disk.
294: Whenever a client connects, the daemon responds with its public
295: host and server keys.
296: The client compares the
297: RSA host key against its own database to verify that it has not changed.
298: The client then generates a 256-bit random number.
299: It encrypts this
300: random number using both the host key and the server key, and sends
301: the encrypted number to the server.
302: Both sides then use this
303: random number as a session key which is used to encrypt all further
304: communications in the session.
305: The rest of the session is encrypted
306: using a conventional cipher, currently Blowfish or 3DES, with 3DES
307: being used by default.
308: The client selects the encryption algorithm
309: to use from those offered by the server.
310: .Pp
1.214 jmc 311: For protocol 2,
312: forward security is provided through a Diffie-Hellman key agreement.
313: This key agreement results in a shared session key.
314: The rest of the session is encrypted using a symmetric cipher, currently
315: 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
316: The client selects the encryption algorithm
317: to use from those offered by the server.
318: Additionally, session integrity is provided
319: through a cryptographic message authentication code
1.267 markus 320: (hmac-md5, hmac-sha1, umac-64, umac-128, hmac-ripemd160,
1.263 djm 321: hmac-sha2-256 or hmac-sha2-512).
1.214 jmc 322: .Pp
323: Finally, the server and the client enter an authentication dialog.
1.212 jmc 324: The client tries to authenticate itself using
1.214 jmc 325: host-based authentication,
326: public key authentication,
327: challenge-response authentication,
328: or password authentication.
1.212 jmc 329: .Pp
330: If the client successfully authenticates itself, a dialog for
331: preparing the session is entered.
332: At this time the client may request
333: things like allocating a pseudo-tty, forwarding X11 connections,
334: forwarding TCP connections, or forwarding the authentication agent
335: connection over the secure channel.
336: .Pp
1.216 jmc 337: After this, the client either requests a shell or execution of a command.
1.212 jmc 338: The sides then enter session mode.
339: In this mode, either side may send
340: data at any time, and such data is forwarded to/from the shell or
341: command on the server side, and the user terminal in the client side.
342: .Pp
343: When the user program terminates and all forwarded X11 and other
344: connections have been closed, the server sends command exit status to
345: the client, and both sides exit.
1.2 deraadt 346: .Sh LOGIN PROCESS
1.1 deraadt 347: When a user successfully logs in,
1.2 deraadt 348: .Nm
1.1 deraadt 349: does the following:
1.2 deraadt 350: .Bl -enum -offset indent
351: .It
1.1 deraadt 352: If the login is on a tty, and no command has been specified,
1.40 aaron 353: prints last login time and
1.2 deraadt 354: .Pa /etc/motd
1.1 deraadt 355: (unless prevented in the configuration file or by
1.207 djm 356: .Pa ~/.hushlogin ;
1.2 deraadt 357: see the
1.40 aaron 358: .Sx FILES
1.2 deraadt 359: section).
360: .It
1.1 deraadt 361: If the login is on a tty, records login time.
1.2 deraadt 362: .It
363: Checks
364: .Pa /etc/nologin ;
365: if it exists, prints contents and quits
1.1 deraadt 366: (unless root).
1.2 deraadt 367: .It
1.1 deraadt 368: Changes to run with normal user privileges.
1.2 deraadt 369: .It
1.1 deraadt 370: Sets up basic environment.
1.2 deraadt 371: .It
1.200 jmc 372: Reads the file
1.207 djm 373: .Pa ~/.ssh/environment ,
1.200 jmc 374: if it exists, and users are allowed to change their environment.
1.188 stevesk 375: See the
1.187 marc 376: .Cm PermitUserEnvironment
1.188 stevesk 377: option in
1.187 marc 378: .Xr sshd_config 5 .
1.2 deraadt 379: .It
1.1 deraadt 380: Changes to user's home directory.
1.2 deraadt 381: .It
382: If
1.207 djm 383: .Pa ~/.ssh/rc
1.276 djm 384: exists and the
385: .Xr sshd_config 5
386: .Cm PermitUserRC
387: option is set, runs it; else if
1.168 deraadt 388: .Pa /etc/ssh/sshrc
1.2 deraadt 389: exists, runs
1.36 aaron 390: it; otherwise runs xauth.
391: The
1.2 deraadt 392: .Dq rc
393: files are given the X11
1.1 deraadt 394: authentication protocol and cookie in standard input.
1.226 jmc 395: See
396: .Sx SSHRC ,
397: below.
1.2 deraadt 398: .It
1.1 deraadt 399: Runs user's shell or command.
1.279 ! djm 400: All commands are run under the user's login shell as specified in the
! 401: system password database.
1.2 deraadt 402: .El
1.226 jmc 403: .Sh SSHRC
404: If the file
405: .Pa ~/.ssh/rc
406: exists,
407: .Xr sh 1
408: runs it after reading the
409: environment files but before starting the user's shell or command.
410: It must not produce any output on stdout; stderr must be used
411: instead.
412: If X11 forwarding is in use, it will receive the "proto cookie" pair in
413: its standard input (and
414: .Ev DISPLAY
415: in its environment).
416: The script must call
417: .Xr xauth 1
418: because
419: .Nm
420: will not run xauth automatically to add X11 cookies.
421: .Pp
422: The primary purpose of this file is to run any initialization routines
423: which may be needed before the user's home directory becomes
424: accessible; AFS is a particular example of such an environment.
425: .Pp
426: This file will probably contain some initialization code followed by
427: something similar to:
428: .Bd -literal -offset 3n
429: if read proto cookie && [ -n "$DISPLAY" ]; then
430: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
431: # X11UseLocalhost=yes
432: echo add unix:`echo $DISPLAY |
433: cut -c11-` $proto $cookie
434: else
435: # X11UseLocalhost=no
436: echo add $DISPLAY $proto $cookie
437: fi | xauth -q -
438: fi
439: .Ed
440: .Pp
441: If this file does not exist,
442: .Pa /etc/ssh/sshrc
443: is run, and if that
444: does not exist either, xauth is used to add the cookie.
1.2 deraadt 445: .Sh AUTHORIZED_KEYS FILE FORMAT
1.125 markus 446: .Cm AuthorizedKeysFile
1.262 jmc 447: specifies the files containing public keys for
1.217 jmc 448: public key authentication;
1.262 jmc 449: if none is specified, the default is
1.261 djm 450: .Pa ~/.ssh/authorized_keys
451: and
452: .Pa ~/.ssh/authorized_keys2 .
1.36 aaron 453: Each line of the file contains one
1.2 deraadt 454: key (empty lines and lines starting with a
455: .Ql #
456: are ignored as
1.36 aaron 457: comments).
1.217 jmc 458: Protocol 1 public keys consist of the following space-separated fields:
459: options, bits, exponent, modulus, comment.
460: Protocol 2 public key consist of:
461: options, keytype, base64-encoded key, comment.
462: The options field is optional;
463: its presence is determined by whether the line starts
1.189 espie 464: with a number or not (the options field never starts with a number).
1.217 jmc 465: The bits, exponent, modulus, and comment fields give the RSA key for
1.75 markus 466: protocol version 1; the
1.1 deraadt 467: comment field is not used for anything (but may be convenient for the
468: user to identify the key).
1.75 markus 469: For protocol version 2 the keytype is
1.259 djm 470: .Dq ecdsa-sha2-nistp256 ,
471: .Dq ecdsa-sha2-nistp384 ,
472: .Dq ecdsa-sha2-nistp521 ,
1.272 jmc 473: .Dq ssh-ed25519 ,
1.75 markus 474: .Dq ssh-dss
475: or
476: .Dq ssh-rsa .
1.2 deraadt 477: .Pp
1.1 deraadt 478: Note that lines in this file are usually several hundred bytes long
1.203 dtucker 479: (because of the size of the public key encoding) up to a limit of
480: 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
481: keys up to 16 kilobits.
1.36 aaron 482: You don't want to type them in; instead, copy the
1.113 itojun 483: .Pa identity.pub ,
1.217 jmc 484: .Pa id_dsa.pub ,
1.259 djm 485: .Pa id_ecdsa.pub ,
1.273 naddy 486: .Pa id_ed25519.pub ,
1.75 markus 487: or the
1.113 itojun 488: .Pa id_rsa.pub
1.1 deraadt 489: file and edit it.
1.175 stevesk 490: .Pp
491: .Nm
492: enforces a minimum RSA key modulus size for protocol 1
493: and protocol 2 keys of 768 bits.
1.2 deraadt 494: .Pp
1.58 deraadt 495: The options (if present) consist of comma-separated option
1.36 aaron 496: specifications.
497: No spaces are permitted, except within double quotes.
1.141 stevesk 498: The following option specifications are supported (note
499: that option keywords are case-insensitive):
1.2 deraadt 500: .Bl -tag -width Ds
1.253 djm 501: .It Cm cert-authority
1.251 djm 502: Specifies that the listed key is a certification authority (CA) that is
503: trusted to validate signed certificates for user authentication.
504: .Pp
505: Certificates may encode access restrictions similar to these key options.
506: If both certificate restrictions and key options are present, the most
507: restrictive union of the two is applied.
1.2 deraadt 508: .It Cm command="command"
1.1 deraadt 509: Specifies that the command is executed whenever this key is used for
1.36 aaron 510: authentication.
511: The command supplied by the user (if any) is ignored.
1.148 markus 512: The command is run on a pty if the client requests a pty;
1.36 aaron 513: otherwise it is run without a tty.
1.194 jmc 514: If an 8-bit clean channel is required,
1.147 deraadt 515: one must not request a pty or should specify
1.89 markus 516: .Cm no-pty .
1.36 aaron 517: A quote may be included in the command by quoting it with a backslash.
518: This option might be useful
1.189 espie 519: to restrict certain public keys to perform just a specific operation.
1.36 aaron 520: An example might be a key that permits remote backups but nothing else.
1.211 jmc 521: Note that the client may specify TCP and/or X11
1.51 hugh 522: forwarding unless they are explicitly prohibited.
1.233 dtucker 523: The command originally supplied by the client is available in the
524: .Ev SSH_ORIGINAL_COMMAND
525: environment variable.
1.149 markus 526: Note that this option applies to shell, command or subsystem execution.
1.251 djm 527: Also note that this command may be superseded by either a
528: .Xr sshd_config 5
529: .Cm ForceCommand
530: directive or a command embedded in a certificate.
1.2 deraadt 531: .It Cm environment="NAME=value"
1.1 deraadt 532: Specifies that the string is to be added to the environment when
1.36 aaron 533: logging in using this key.
534: Environment variables set this way
535: override other default environment values.
536: Multiple options of this type are permitted.
1.188 stevesk 537: Environment processing is disabled by default and is
538: controlled via the
539: .Cm PermitUserEnvironment
540: option.
1.155 markus 541: This option is automatically disabled if
542: .Cm UseLogin
543: is enabled.
1.218 jmc 544: .It Cm from="pattern-list"
1.244 djm 545: Specifies that in addition to public key authentication, either the canonical
1.245 jmc 546: name of the remote host or its IP address must be present in the
1.244 djm 547: comma-separated list of patterns.
1.270 jmc 548: See PATTERNS in
1.229 jmc 549: .Xr ssh_config 5
550: for more information on patterns.
1.244 djm 551: .Pp
552: In addition to the wildcard matching that may be applied to hostnames or
553: addresses, a
554: .Cm from
1.248 sobrado 555: stanza may match IP addresses using CIDR address/masklen notation.
1.244 djm 556: .Pp
557: The purpose of this option is to optionally increase security: public key
558: authentication by itself does not trust the network or name servers or
559: anything (but the key); however, if somebody somehow steals the key, the key
560: permits an intruder to log in from anywhere in the world.
561: This additional option makes using a stolen key more difficult (name
562: servers and/or routers would have to be compromised in addition to
563: just the key).
1.218 jmc 564: .It Cm no-agent-forwarding
565: Forbids authentication agent forwarding when this key is used for
566: authentication.
1.2 deraadt 567: .It Cm no-port-forwarding
1.211 jmc 568: Forbids TCP forwarding when this key is used for authentication.
1.36 aaron 569: Any port forward requests by the client will return an error.
1.230 jmc 570: This might be used, e.g. in connection with the
1.2 deraadt 571: .Cm command
1.1 deraadt 572: option.
1.218 jmc 573: .It Cm no-pty
574: Prevents tty allocation (a request to allocate a pty will fail).
1.240 djm 575: .It Cm no-user-rc
1.241 jmc 576: Disables execution of
1.240 djm 577: .Pa ~/.ssh/rc .
1.2 deraadt 578: .It Cm no-X11-forwarding
1.1 deraadt 579: Forbids X11 forwarding when this key is used for authentication.
580: Any X11 forward requests by the client will return an error.
1.107 djm 581: .It Cm permitopen="host:port"
1.278 bentley 582: Limit local port forwarding with
583: .Xr ssh 1
584: .Fl L
585: such that it may only connect to the specified host and port.
1.257 djm 586: IPv6 addresses can be specified by enclosing the address in square brackets.
1.146 stevesk 587: Multiple
1.107 djm 588: .Cm permitopen
1.197 jmc 589: options may be applied separated by commas.
590: No pattern matching is performed on the specified hostnames,
591: they must be literal domains or addresses.
1.264 dtucker 592: A port specification of
593: .Cm *
594: matches any port.
1.256 djm 595: .It Cm principals="principals"
596: On a
597: .Cm cert-authority
598: line, specifies allowed principals for certificate authentication as a
599: comma-separated list.
600: At least one name from the list must appear in the certificate's
601: list of principals for the certificate to be accepted.
602: This option is ignored for keys that are not marked as trusted certificate
603: signers using the
604: .Cm cert-authority
605: option.
1.209 reyk 606: .It Cm tunnel="n"
607: Force a
608: .Xr tun 4
609: device on the server.
610: Without this option, the next available device will be used if
611: the client requests a tunnel.
1.2 deraadt 612: .El
613: .Pp
1.219 jmc 614: An example authorized_keys file:
1.222 jmc 615: .Bd -literal -offset 3n
1.219 jmc 616: # Comments allowed at start of line
617: ssh-rsa AAAAB3Nza...LiPk== user@example.net
1.222 jmc 618: from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
619: AAAAB2...19Q== john@example.net
620: command="dump /home",no-pty,no-port-forwarding ssh-dss
621: AAAAC3...51R== example.net
622: permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
623: AAAAB5...21S==
624: tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
625: jane@example.net
1.219 jmc 626: .Ed
1.2 deraadt 627: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 628: The
1.194 jmc 629: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 630: and
1.207 djm 631: .Pa ~/.ssh/known_hosts
1.36 aaron 632: files contain host public keys for all known hosts.
633: The global file should
1.37 brad 634: be prepared by the administrator (optional), and the per-user file is
1.220 jmc 635: maintained automatically: whenever the user connects from an unknown host,
1.36 aaron 636: its key is added to the per-user file.
1.2 deraadt 637: .Pp
1.254 djm 638: Each line in these files contains the following fields: markers (optional),
639: hostnames, bits, exponent, modulus, comment.
1.36 aaron 640: The fields are separated by spaces.
1.2 deraadt 641: .Pp
1.254 djm 642: The marker is optional, but if it is present then it must be one of
643: .Dq @cert-authority ,
644: to indicate that the line contains a certification authority (CA) key,
645: or
646: .Dq @revoked ,
647: to indicate that the key contained on the line is revoked and must not ever
648: be accepted.
649: Only one marker should be used on a key line.
650: .Pp
1.196 david 651: Hostnames is a comma-separated list of patterns
1.220 jmc 652: .Pf ( Ql *
1.197 jmc 653: and
654: .Ql \&?
1.195 mouring 655: act as
1.1 deraadt 656: wildcards); each pattern in turn is matched against the canonical host
657: name (when authenticating a client) or against the user-supplied
1.36 aaron 658: name (when authenticating a server).
659: A pattern may also be preceded by
1.195 mouring 660: .Ql \&!
1.2 deraadt 661: to indicate negation: if the host name matches a negated
1.1 deraadt 662: pattern, it is not accepted (by that line) even if it matched another
663: pattern on the line.
1.231 dtucker 664: A hostname or address may optionally be enclosed within
665: .Ql \&[
666: and
667: .Ql \&]
668: brackets then followed by
669: .Ql \&:
1.232 jmc 670: and a non-standard port number.
1.2 deraadt 671: .Pp
1.205 djm 672: Alternately, hostnames may be stored in a hashed form which hides host names
1.206 jmc 673: and addresses should the file's contents be disclosed.
674: Hashed hostnames start with a
675: .Ql |
1.205 djm 676: character.
677: Only one hashed hostname may appear on a single line and none of the above
678: negation or wildcard operators may be applied.
679: .Pp
1.49 markus 680: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.220 jmc 681: can be obtained, for example, from
1.167 deraadt 682: .Pa /etc/ssh/ssh_host_key.pub .
1.1 deraadt 683: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 684: .Pp
685: Lines starting with
686: .Ql #
687: and empty lines are ignored as comments.
688: .Pp
1.1 deraadt 689: When performing host authentication, authentication is accepted if any
1.254 djm 690: matching line has the proper key; either one that matches exactly or,
691: if the server has presented a certificate for authentication, the key
692: of the certification authority that signed the certificate.
693: For a key to be trusted as a certification authority, it must use the
694: .Dq @cert-authority
695: marker described above.
696: .Pp
697: The known hosts file also provides a facility to mark keys as revoked,
698: for example when it is known that the associated private key has been
699: stolen.
700: Revoked keys are specified by including the
701: .Dq @revoked
702: marker at the beginning of the key line, and are never accepted for
703: authentication or as certification authorities, but instead will
704: produce a warning from
705: .Xr ssh 1
706: when they are encountered.
707: .Pp
708: It is permissible (but not
1.1 deraadt 709: recommended) to have several lines or different host keys for the same
1.36 aaron 710: names.
711: This will inevitably happen when short forms of host names
712: from different domains are put in the file.
713: It is possible
1.1 deraadt 714: that the files contain conflicting information; authentication is
715: accepted if valid information can be found from either file.
1.2 deraadt 716: .Pp
1.1 deraadt 717: Note that the lines in these files are typically hundreds of characters
718: long, and you definitely don't want to type in the host keys by hand.
1.254 djm 719: Rather, generate them by a script,
720: .Xr ssh-keyscan 1
1.40 aaron 721: or by taking
1.167 deraadt 722: .Pa /etc/ssh/ssh_host_key.pub
1.1 deraadt 723: and adding the host names at the front.
1.254 djm 724: .Xr ssh-keygen 1
1.255 jmc 725: also offers some basic automated editing for
1.254 djm 726: .Pa ~/.ssh/known_hosts
727: including removing hosts matching a host name and converting all host
728: names to their hashed representations.
1.221 jmc 729: .Pp
730: An example ssh_known_hosts file:
731: .Bd -literal -offset 3n
732: # Comments allowed at start of line
733: closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
734: cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
1.205 djm 735: # A hashed hostname
736: |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
737: AAAA1234.....=
1.254 djm 738: # A revoked key
739: @revoked * ssh-rsa AAAAB5W...
740: # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
741: @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
1.120 markus 742: .Ed
1.2 deraadt 743: .Sh FILES
1.223 jmc 744: .Bl -tag -width Ds -compact
1.258 jmc 745: .It Pa ~/.hushlogin
1.204 jmc 746: This file is used to suppress printing the last login time and
747: .Pa /etc/motd ,
748: if
749: .Cm PrintLastLog
750: and
751: .Cm PrintMotd ,
752: respectively,
753: are enabled.
754: It does not suppress printing of the banner specified by
755: .Cm Banner .
1.223 jmc 756: .Pp
1.258 jmc 757: .It Pa ~/.rhosts
1.225 jmc 758: This file is used for host-based authentication (see
759: .Xr ssh 1
760: for more information).
761: On some machines this file may need to be
762: world-readable if the user's home directory is on an NFS partition,
763: because
764: .Nm
765: reads it as root.
766: Additionally, this file must be owned by the user,
767: and must not have write permissions for anyone else.
768: The recommended
769: permission for most machines is read/write for the user, and not
1.1 deraadt 770: accessible by others.
1.2 deraadt 771: .Pp
1.258 jmc 772: .It Pa ~/.shosts
1.225 jmc 773: This file is used in exactly the same way as
774: .Pa .rhosts ,
775: but allows host-based authentication without permitting login with
776: rlogin/rsh.
1.238 mcbride 777: .Pp
1.258 jmc 778: .It Pa ~/.ssh/
1.238 mcbride 779: This directory is the default location for all user-specific configuration
780: and authentication information.
781: There is no general requirement to keep the entire contents of this directory
782: secret, but the recommended permissions are read/write/execute for the user,
783: and not accessible by others.
1.2 deraadt 784: .Pp
1.258 jmc 785: .It Pa ~/.ssh/authorized_keys
1.277 sobrado 786: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.273 naddy 787: that can be used for logging in as this user.
1.223 jmc 788: The format of this file is described above.
1.234 dtucker 789: The content of the file is not highly sensitive, but the recommended
1.225 jmc 790: permissions are read/write for the user, and not accessible by others.
1.234 dtucker 791: .Pp
792: If this file, the
793: .Pa ~/.ssh
794: directory, or the user's home directory are writable
795: by other users, then the file could be modified or replaced by unauthorized
796: users.
797: In this case,
798: .Nm
799: will not allow it to be used unless the
800: .Cm StrictModes
801: option has been set to
802: .Dq no .
1.2 deraadt 803: .Pp
1.258 jmc 804: .It Pa ~/.ssh/environment
1.36 aaron 805: This file is read into the environment at login (if it exists).
806: It can only contain empty lines, comment lines (that start with
1.2 deraadt 807: .Ql # ) ,
1.36 aaron 808: and assignment lines of the form name=value.
809: The file should be writable
1.6 aaron 810: only by the user; it need not be readable by anyone else.
1.188 stevesk 811: Environment processing is disabled by default and is
812: controlled via the
813: .Cm PermitUserEnvironment
814: option.
1.223 jmc 815: .Pp
1.258 jmc 816: .It Pa ~/.ssh/known_hosts
1.225 jmc 817: Contains a list of host keys for all hosts the user has logged into
818: that are not already in the systemwide list of known host keys.
819: The format of this file is described above.
820: This file should be writable only by root/the owner and
1.223 jmc 821: can, but need not be, world-readable.
822: .Pp
1.258 jmc 823: .It Pa ~/.ssh/rc
1.226 jmc 824: Contains initialization routines to be run before
825: the user's home directory becomes accessible.
1.1 deraadt 826: This file should be writable only by the user, and need not be
827: readable by anyone else.
1.223 jmc 828: .Pp
1.258 jmc 829: .It Pa /etc/hosts.equiv
1.227 jmc 830: This file is for host-based authentication (see
831: .Xr ssh 1 ) .
832: It should only be writable by root.
1.223 jmc 833: .Pp
1.258 jmc 834: .It Pa /etc/moduli
1.223 jmc 835: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
836: The file format is described in
837: .Xr moduli 5 .
838: .Pp
1.258 jmc 839: .It Pa /etc/motd
1.223 jmc 840: See
841: .Xr motd 5 .
842: .Pp
1.258 jmc 843: .It Pa /etc/nologin
1.223 jmc 844: If this file exists,
845: .Nm
846: refuses to let anyone except root log in.
847: The contents of the file
848: are displayed to anyone trying to log in, and non-root connections are
849: refused.
850: The file should be world-readable.
851: .Pp
1.258 jmc 852: .It Pa /etc/shosts.equiv
1.227 jmc 853: This file is used in exactly the same way as
854: .Pa hosts.equiv ,
855: but allows host-based authentication without permitting login with
856: rlogin/rsh.
1.225 jmc 857: .Pp
1.258 jmc 858: .It Pa /etc/ssh/ssh_host_key
859: .It Pa /etc/ssh/ssh_host_dsa_key
1.259 djm 860: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.273 naddy 861: .It Pa /etc/ssh/ssh_host_ed25519_key
1.258 jmc 862: .It Pa /etc/ssh/ssh_host_rsa_key
1.266 dtucker 863: These files contain the private parts of the host keys.
1.223 jmc 864: These files should only be owned by root, readable only by root, and not
865: accessible to others.
866: Note that
867: .Nm
1.228 jmc 868: does not start if these files are group/world-accessible.
1.223 jmc 869: .Pp
1.258 jmc 870: .It Pa /etc/ssh/ssh_host_key.pub
871: .It Pa /etc/ssh/ssh_host_dsa_key.pub
1.259 djm 872: .It Pa /etc/ssh/ssh_host_ecdsa_key.pub
1.273 naddy 873: .It Pa /etc/ssh/ssh_host_ed25519_key.pub
1.258 jmc 874: .It Pa /etc/ssh/ssh_host_rsa_key.pub
1.266 dtucker 875: These files contain the public parts of the host keys.
1.223 jmc 876: These files should be world-readable but writable only by
877: root.
878: Their contents should match the respective private parts.
879: These files are not
880: really used for anything; they are provided for the convenience of
881: the user so their contents can be copied to known hosts files.
882: These files are created using
883: .Xr ssh-keygen 1 .
884: .Pp
1.258 jmc 885: .It Pa /etc/ssh/ssh_known_hosts
1.235 jmc 886: Systemwide list of known host keys.
887: This file should be prepared by the
888: system administrator to contain the public host keys of all machines in the
889: organization.
890: The format of this file is described above.
891: This file should be writable only by root/the owner and
892: should be world-readable.
893: .Pp
1.258 jmc 894: .It Pa /etc/ssh/sshd_config
1.223 jmc 895: Contains configuration data for
896: .Nm sshd .
897: The file format and configuration options are described in
898: .Xr sshd_config 5 .
899: .Pp
1.258 jmc 900: .It Pa /etc/ssh/sshrc
1.226 jmc 901: Similar to
902: .Pa ~/.ssh/rc ,
903: it can be used to specify
1.36 aaron 904: machine-specific login-time initializations globally.
905: This file should be writable only by root, and should be world-readable.
1.223 jmc 906: .Pp
1.258 jmc 907: .It Pa /var/empty
1.223 jmc 908: .Xr chroot 2
909: directory used by
910: .Nm
911: during privilege separation in the pre-authentication phase.
912: The directory should not contain any files and must be owned by root
913: and not group or world-writable.
914: .Pp
1.258 jmc 915: .It Pa /var/run/sshd.pid
1.223 jmc 916: Contains the process ID of the
917: .Nm
918: listening for connections (if there are several daemons running
919: concurrently for different ports, this contains the process ID of the one
920: started last).
921: The content of this file is not sensitive; it can be world-readable.
1.56 aaron 922: .El
1.2 deraadt 923: .Sh SEE ALSO
924: .Xr scp 1 ,
1.90 djm 925: .Xr sftp 1 ,
1.2 deraadt 926: .Xr ssh 1 ,
1.5 deraadt 927: .Xr ssh-add 1 ,
1.2 deraadt 928: .Xr ssh-agent 1 ,
1.5 deraadt 929: .Xr ssh-keygen 1 ,
1.235 jmc 930: .Xr ssh-keyscan 1 ,
1.200 jmc 931: .Xr chroot 2 ,
1.136 markus 932: .Xr login.conf 5 ,
933: .Xr moduli 5 ,
1.184 stevesk 934: .Xr sshd_config 5 ,
1.200 jmc 935: .Xr inetd 8 ,
1.128 mpech 936: .Xr sftp-server 8
1.198 jmc 937: .Sh AUTHORS
938: OpenSSH is a derivative of the original and free
939: ssh 1.2.12 release by Tatu Ylonen.
940: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
941: Theo de Raadt and Dug Song
942: removed many bugs, re-added newer features and
943: created OpenSSH.
944: Markus Friedl contributed the support for SSH
945: protocol versions 1.5 and 2.0.
946: Niels Provos and Markus Friedl contributed support
947: for privilege separation.
![[BACK]](/icons/back.gif){kind=icon}
Return to sshd.8 CVS log ![[TXT]](/icons/text.gif){kind=icon}
![[DIR]](/icons/dir.gif){kind=icon}
Up to [local] / src / usr.bin / ssh