Annotation of src/usr.bin/ssh/sshd.8, Revision 1.266
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: .\"
1.266 ! dtucker 36: .\" $OpenBSD: sshd.8,v 1.265 2012/05/13 01:42:32 dtucker Exp $
! 37: .Dd $Mdocdate: May 13 2012 $
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
1.2 deraadt 50: .Op Fl f Ar config_file
51: .Op Fl g Ar login_grace_time
52: .Op Fl h Ar host_key_file
53: .Op Fl k Ar key_gen_time
1.156 markus 54: .Op Fl o Ar option
1.2 deraadt 55: .Op Fl p Ar port
1.61 markus 56: .Op Fl u Ar len
1.194 jmc 57: .Ek
1.40 aaron 58: .Sh DESCRIPTION
1.2 deraadt 59: .Nm
1.212 jmc 60: (OpenSSH Daemon) is the daemon program for
1.2 deraadt 61: .Xr ssh 1 .
1.235 jmc 62: Together these programs replace
63: .Xr rlogin 1
64: and
65: .Xr rsh 1 ,
66: and provide secure encrypted communications between two untrusted hosts
1.36 aaron 67: over an insecure network.
1.2 deraadt 68: .Pp
69: .Nm
1.212 jmc 70: listens for connections from clients.
1.40 aaron 71: It is normally started at boot from
1.2 deraadt 72: .Pa /etc/rc .
73: It forks a new
1.36 aaron 74: daemon for each incoming connection.
75: The forked daemons handle
1.1 deraadt 76: key exchange, encryption, authentication, command execution,
77: and data exchange.
1.2 deraadt 78: .Pp
79: .Nm
1.200 jmc 80: can be configured using command-line options or a configuration file
81: (by default
1.212 jmc 82: .Xr sshd_config 5 ) ;
83: command-line options override values specified in the
1.1 deraadt 84: configuration file.
1.25 markus 85: .Nm
86: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 87: .Dv SIGHUP ,
1.230 jmc 88: by executing itself with the name and options it was started with, e.g.\&
1.97 deraadt 89: .Pa /usr/sbin/sshd .
1.18 aaron 90: .Pp
91: The options are as follows:
1.2 deraadt 92: .Bl -tag -width Ds
1.200 jmc 93: .It Fl 4
94: Forces
95: .Nm
96: to use IPv4 addresses only.
97: .It Fl 6
98: Forces
99: .Nm
100: to use IPv6 addresses only.
1.2 deraadt 101: .It Fl b Ar bits
1.120 markus 102: Specifies the number of bits in the ephemeral protocol version 1
1.246 djm 103: server key (default 1024).
1.243 jmc 104: .It Fl C Ar connection_spec
105: Specify the connection parameters to use for the
106: .Fl T
107: extended test mode.
108: If provided, any
109: .Cm Match
110: directives in the configuration file
111: that would apply to the specified user, host, and address will be set before
112: the configuration is written to standard output.
113: The connection parameters are supplied as keyword=value pairs.
114: The keywords are
115: .Dq user ,
116: .Dq host ,
1.265 dtucker 117: .Dq laddr ,
118: .Dq lport ,
1.243 jmc 119: and
120: .Dq addr .
121: All are required and may be supplied in any order, either with multiple
122: .Fl C
123: options or as a comma-separated list.
1.252 jmc 124: .It Fl c Ar host_certificate_file
125: Specifies a path to a certificate file to identify
126: .Nm
127: during key exchange.
128: The certificate file must match a host key file specified using the
129: .Fl h
130: option or the
131: .Cm HostKey
132: configuration directive.
1.200 jmc 133: .It Fl D
134: When this option is specified,
135: .Nm
136: will not detach and does not become a daemon.
137: This allows easy monitoring of
138: .Nm sshd .
1.2 deraadt 139: .It Fl d
1.36 aaron 140: Debug mode.
1.250 djm 141: The server sends verbose debug output to standard error,
142: and does not put itself in the background.
1.36 aaron 143: The server also will not fork and will only process one connection.
144: This option is only intended for debugging for the server.
1.194 jmc 145: Multiple
146: .Fl d
147: options increase the debugging level.
1.67 aaron 148: Maximum is 3.
1.120 markus 149: .It Fl e
150: When this option is specified,
151: .Nm
152: will send the output to the 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 ,
175: .Pa /etc/ssh/ssh_host_ecdsa_key
176: and
1.167 deraadt 177: .Pa /etc/ssh/ssh_host_rsa_key
1.160 stevesk 178: for protocol version 2.
1.75 markus 179: It is possible to have multiple host key files for
1.120 markus 180: the different protocol versions and host key algorithms.
1.2 deraadt 181: .It Fl i
1.7 markus 182: Specifies that
183: .Nm
1.194 jmc 184: is being run from
185: .Xr inetd 8 .
1.7 markus 186: .Nm
187: is normally not run
1.1 deraadt 188: from inetd because it needs to generate the server key before it can
1.36 aaron 189: respond to the client, and this may take tens of seconds.
190: Clients would have to wait too long if the key was regenerated every time.
1.230 jmc 191: However, with small key sizes (e.g. 512) using
1.7 markus 192: .Nm
193: from inetd may
1.1 deraadt 194: be feasible.
1.2 deraadt 195: .It Fl k Ar key_gen_time
1.120 markus 196: Specifies how often the ephemeral protocol version 1 server key is
197: regenerated (default 3600 seconds, or one hour).
1.36 aaron 198: The motivation for regenerating the key fairly
1.200 jmc 199: often is that the key is not stored anywhere, and after about an hour
1.1 deraadt 200: it becomes impossible to recover the key for decrypting intercepted
201: communications even if the machine is cracked into or physically
1.36 aaron 202: seized.
203: A value of zero indicates that the key will never be regenerated.
1.156 markus 204: .It Fl o Ar option
205: Can be used to give options in the format used in the configuration file.
206: This is useful for specifying options for which there is no separate
207: command-line flag.
1.200 jmc 208: For full details of the options, and their values, see
209: .Xr sshd_config 5 .
1.2 deraadt 210: .It Fl p Ar port
1.1 deraadt 211: Specifies the port on which the server listens for connections
212: (default 22).
1.158 stevesk 213: Multiple port options are permitted.
1.210 stevesk 214: Ports specified in the configuration file with the
215: .Cm Port
216: option are ignored when a command-line port is specified.
217: Ports specified using the
218: .Cm ListenAddress
219: option override command-line ports.
1.2 deraadt 220: .It Fl q
1.36 aaron 221: Quiet mode.
222: Nothing is sent to the system log.
223: Normally the beginning,
1.1 deraadt 224: authentication, and termination of each connection is logged.
1.242 dtucker 225: .It Fl T
226: Extended test mode.
227: Check the validity of the configuration file, output the effective configuration
228: to stdout and then exit.
229: Optionally,
230: .Cm Match
231: rules may be applied by specifying the connection parameters using one or more
232: .Fl C
233: options.
1.243 jmc 234: .It Fl t
235: Test mode.
236: Only check the validity of the configuration file and sanity of the keys.
237: This is useful for updating
238: .Nm
239: reliably as configuration options may change.
1.61 markus 240: .It Fl u Ar len
241: This option is used to specify the size of the field
242: in the
243: .Li utmp
244: structure that holds the remote host name.
245: If the resolved host name is longer than
246: .Ar len ,
247: the dotted decimal value will be used instead.
248: This allows hosts with very long host names that
249: overflow this field to still be uniquely identified.
250: Specifying
251: .Fl u0
252: indicates that only dotted decimal addresses
253: should be put into the
254: .Pa utmp
255: file.
1.144 stevesk 256: .Fl u0
1.194 jmc 257: may also be used to prevent
1.144 stevesk 258: .Nm
259: from making DNS requests unless the authentication
260: mechanism or configuration requires it.
261: Authentication mechanisms that may require DNS include
262: .Cm RhostsRSAAuthentication ,
1.215 jmc 263: .Cm HostbasedAuthentication ,
1.144 stevesk 264: and using a
265: .Cm from="pattern-list"
266: option in a key file.
1.170 stevesk 267: Configuration options that require DNS include using a
268: USER@HOST pattern in
269: .Cm AllowUsers
270: or
271: .Cm DenyUsers .
1.2 deraadt 272: .El
1.214 jmc 273: .Sh AUTHENTICATION
274: The OpenSSH SSH daemon supports SSH protocols 1 and 2.
1.249 jmc 275: The default is to use protocol 2 only,
1.214 jmc 276: though this can be changed via the
277: .Cm Protocol
278: option in
279: .Xr sshd_config 5 .
1.259 djm 280: Protocol 2 supports DSA, ECDSA and RSA keys;
1.214 jmc 281: protocol 1 only supports RSA keys.
282: For both protocols,
283: each host has a host-specific key,
284: normally 2048 bits,
285: used to identify the host.
286: .Pp
287: Forward security for protocol 1 is provided through
288: an additional server key,
289: normally 768 bits,
290: generated when the server starts.
1.212 jmc 291: This key is normally regenerated every hour if it has been used, and
292: is never stored on disk.
293: Whenever a client connects, the daemon responds with its public
294: host and server keys.
295: The client compares the
296: RSA host key against its own database to verify that it has not changed.
297: The client then generates a 256-bit random number.
298: It encrypts this
299: random number using both the host key and the server key, and sends
300: the encrypted number to the server.
301: Both sides then use this
302: random number as a session key which is used to encrypt all further
303: communications in the session.
304: The rest of the session is encrypted
305: using a conventional cipher, currently Blowfish or 3DES, with 3DES
306: being used by default.
307: The client selects the encryption algorithm
308: to use from those offered by the server.
309: .Pp
1.214 jmc 310: For protocol 2,
311: forward security is provided through a Diffie-Hellman key agreement.
312: This key agreement results in a shared session key.
313: The rest of the session is encrypted using a symmetric cipher, currently
314: 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
315: The client selects the encryption algorithm
316: to use from those offered by the server.
317: Additionally, session integrity is provided
318: through a cryptographic message authentication code
1.263 djm 319: (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160,
320: hmac-sha2-256 or hmac-sha2-512).
1.214 jmc 321: .Pp
322: Finally, the server and the client enter an authentication dialog.
1.212 jmc 323: The client tries to authenticate itself using
1.214 jmc 324: host-based authentication,
325: public key authentication,
326: challenge-response authentication,
327: or password authentication.
1.212 jmc 328: .Pp
329: If the client successfully authenticates itself, a dialog for
330: preparing the session is entered.
331: At this time the client may request
332: things like allocating a pseudo-tty, forwarding X11 connections,
333: forwarding TCP connections, or forwarding the authentication agent
334: connection over the secure channel.
335: .Pp
1.216 jmc 336: After this, the client either requests a shell or execution of a command.
1.212 jmc 337: The sides then enter session mode.
338: In this mode, either side may send
339: data at any time, and such data is forwarded to/from the shell or
340: command on the server side, and the user terminal in the client side.
341: .Pp
342: When the user program terminates and all forwarded X11 and other
343: connections have been closed, the server sends command exit status to
344: the client, and both sides exit.
1.2 deraadt 345: .Sh LOGIN PROCESS
1.1 deraadt 346: When a user successfully logs in,
1.2 deraadt 347: .Nm
1.1 deraadt 348: does the following:
1.2 deraadt 349: .Bl -enum -offset indent
350: .It
1.1 deraadt 351: If the login is on a tty, and no command has been specified,
1.40 aaron 352: prints last login time and
1.2 deraadt 353: .Pa /etc/motd
1.1 deraadt 354: (unless prevented in the configuration file or by
1.207 djm 355: .Pa ~/.hushlogin ;
1.2 deraadt 356: see the
1.40 aaron 357: .Sx FILES
1.2 deraadt 358: section).
359: .It
1.1 deraadt 360: If the login is on a tty, records login time.
1.2 deraadt 361: .It
362: Checks
363: .Pa /etc/nologin ;
364: if it exists, prints contents and quits
1.1 deraadt 365: (unless root).
1.2 deraadt 366: .It
1.1 deraadt 367: Changes to run with normal user privileges.
1.2 deraadt 368: .It
1.1 deraadt 369: Sets up basic environment.
1.2 deraadt 370: .It
1.200 jmc 371: Reads the file
1.207 djm 372: .Pa ~/.ssh/environment ,
1.200 jmc 373: if it exists, and users are allowed to change their environment.
1.188 stevesk 374: See the
1.187 marc 375: .Cm PermitUserEnvironment
1.188 stevesk 376: option in
1.187 marc 377: .Xr sshd_config 5 .
1.2 deraadt 378: .It
1.1 deraadt 379: Changes to user's home directory.
1.2 deraadt 380: .It
381: If
1.207 djm 382: .Pa ~/.ssh/rc
1.2 deraadt 383: exists, runs it; else if
1.168 deraadt 384: .Pa /etc/ssh/sshrc
1.2 deraadt 385: exists, runs
1.36 aaron 386: it; otherwise runs xauth.
387: The
1.2 deraadt 388: .Dq rc
389: files are given the X11
1.1 deraadt 390: authentication protocol and cookie in standard input.
1.226 jmc 391: See
392: .Sx SSHRC ,
393: below.
1.2 deraadt 394: .It
1.1 deraadt 395: Runs user's shell or command.
1.2 deraadt 396: .El
1.226 jmc 397: .Sh SSHRC
398: If the file
399: .Pa ~/.ssh/rc
400: exists,
401: .Xr sh 1
402: runs it after reading the
403: environment files but before starting the user's shell or command.
404: It must not produce any output on stdout; stderr must be used
405: instead.
406: If X11 forwarding is in use, it will receive the "proto cookie" pair in
407: its standard input (and
408: .Ev DISPLAY
409: in its environment).
410: The script must call
411: .Xr xauth 1
412: because
413: .Nm
414: will not run xauth automatically to add X11 cookies.
415: .Pp
416: The primary purpose of this file is to run any initialization routines
417: which may be needed before the user's home directory becomes
418: accessible; AFS is a particular example of such an environment.
419: .Pp
420: This file will probably contain some initialization code followed by
421: something similar to:
422: .Bd -literal -offset 3n
423: if read proto cookie && [ -n "$DISPLAY" ]; then
424: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
425: # X11UseLocalhost=yes
426: echo add unix:`echo $DISPLAY |
427: cut -c11-` $proto $cookie
428: else
429: # X11UseLocalhost=no
430: echo add $DISPLAY $proto $cookie
431: fi | xauth -q -
432: fi
433: .Ed
434: .Pp
435: If this file does not exist,
436: .Pa /etc/ssh/sshrc
437: is run, and if that
438: does not exist either, xauth is used to add the cookie.
1.2 deraadt 439: .Sh AUTHORIZED_KEYS FILE FORMAT
1.125 markus 440: .Cm AuthorizedKeysFile
1.262 jmc 441: specifies the files containing public keys for
1.217 jmc 442: public key authentication;
1.262 jmc 443: if none is specified, the default is
1.261 djm 444: .Pa ~/.ssh/authorized_keys
445: and
446: .Pa ~/.ssh/authorized_keys2 .
1.36 aaron 447: Each line of the file contains one
1.2 deraadt 448: key (empty lines and lines starting with a
449: .Ql #
450: are ignored as
1.36 aaron 451: comments).
1.217 jmc 452: Protocol 1 public keys consist of the following space-separated fields:
453: options, bits, exponent, modulus, comment.
454: Protocol 2 public key consist of:
455: options, keytype, base64-encoded key, comment.
456: The options field is optional;
457: its presence is determined by whether the line starts
1.189 espie 458: with a number or not (the options field never starts with a number).
1.217 jmc 459: The bits, exponent, modulus, and comment fields give the RSA key for
1.75 markus 460: protocol version 1; the
1.1 deraadt 461: comment field is not used for anything (but may be convenient for the
462: user to identify the key).
1.75 markus 463: For protocol version 2 the keytype is
1.259 djm 464: .Dq ecdsa-sha2-nistp256 ,
465: .Dq ecdsa-sha2-nistp384 ,
466: .Dq ecdsa-sha2-nistp521 ,
1.75 markus 467: .Dq ssh-dss
468: or
469: .Dq ssh-rsa .
1.2 deraadt 470: .Pp
1.1 deraadt 471: Note that lines in this file are usually several hundred bytes long
1.203 dtucker 472: (because of the size of the public key encoding) up to a limit of
473: 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
474: keys up to 16 kilobits.
1.36 aaron 475: You don't want to type them in; instead, copy the
1.113 itojun 476: .Pa identity.pub ,
1.217 jmc 477: .Pa id_dsa.pub ,
1.259 djm 478: .Pa id_ecdsa.pub ,
1.75 markus 479: or the
1.113 itojun 480: .Pa id_rsa.pub
1.1 deraadt 481: file and edit it.
1.175 stevesk 482: .Pp
483: .Nm
484: enforces a minimum RSA key modulus size for protocol 1
485: and protocol 2 keys of 768 bits.
1.2 deraadt 486: .Pp
1.58 deraadt 487: The options (if present) consist of comma-separated option
1.36 aaron 488: specifications.
489: No spaces are permitted, except within double quotes.
1.141 stevesk 490: The following option specifications are supported (note
491: that option keywords are case-insensitive):
1.2 deraadt 492: .Bl -tag -width Ds
1.253 djm 493: .It Cm cert-authority
1.251 djm 494: Specifies that the listed key is a certification authority (CA) that is
495: trusted to validate signed certificates for user authentication.
496: .Pp
497: Certificates may encode access restrictions similar to these key options.
498: If both certificate restrictions and key options are present, the most
499: restrictive union of the two is applied.
1.2 deraadt 500: .It Cm command="command"
1.1 deraadt 501: Specifies that the command is executed whenever this key is used for
1.36 aaron 502: authentication.
503: The command supplied by the user (if any) is ignored.
1.148 markus 504: The command is run on a pty if the client requests a pty;
1.36 aaron 505: otherwise it is run without a tty.
1.194 jmc 506: If an 8-bit clean channel is required,
1.147 deraadt 507: one must not request a pty or should specify
1.89 markus 508: .Cm no-pty .
1.36 aaron 509: A quote may be included in the command by quoting it with a backslash.
510: This option might be useful
1.189 espie 511: to restrict certain public keys to perform just a specific operation.
1.36 aaron 512: An example might be a key that permits remote backups but nothing else.
1.211 jmc 513: Note that the client may specify TCP and/or X11
1.51 hugh 514: forwarding unless they are explicitly prohibited.
1.233 dtucker 515: The command originally supplied by the client is available in the
516: .Ev SSH_ORIGINAL_COMMAND
517: environment variable.
1.149 markus 518: Note that this option applies to shell, command or subsystem execution.
1.251 djm 519: Also note that this command may be superseded by either a
520: .Xr sshd_config 5
521: .Cm ForceCommand
522: directive or a command embedded in a certificate.
1.2 deraadt 523: .It Cm environment="NAME=value"
1.1 deraadt 524: Specifies that the string is to be added to the environment when
1.36 aaron 525: logging in using this key.
526: Environment variables set this way
527: override other default environment values.
528: Multiple options of this type are permitted.
1.188 stevesk 529: Environment processing is disabled by default and is
530: controlled via the
531: .Cm PermitUserEnvironment
532: option.
1.155 markus 533: This option is automatically disabled if
534: .Cm UseLogin
535: is enabled.
1.218 jmc 536: .It Cm from="pattern-list"
1.244 djm 537: Specifies that in addition to public key authentication, either the canonical
1.245 jmc 538: name of the remote host or its IP address must be present in the
1.244 djm 539: comma-separated list of patterns.
1.229 jmc 540: See
541: .Sx PATTERNS
542: in
543: .Xr ssh_config 5
544: for more information on patterns.
1.244 djm 545: .Pp
546: In addition to the wildcard matching that may be applied to hostnames or
547: addresses, a
548: .Cm from
1.248 sobrado 549: stanza may match IP addresses using CIDR address/masklen notation.
1.244 djm 550: .Pp
551: The purpose of this option is to optionally increase security: public key
552: authentication by itself does not trust the network or name servers or
553: anything (but the key); however, if somebody somehow steals the key, the key
554: permits an intruder to log in from anywhere in the world.
555: This additional option makes using a stolen key more difficult (name
556: servers and/or routers would have to be compromised in addition to
557: just the key).
1.218 jmc 558: .It Cm no-agent-forwarding
559: Forbids authentication agent forwarding when this key is used for
560: authentication.
1.2 deraadt 561: .It Cm no-port-forwarding
1.211 jmc 562: Forbids TCP forwarding when this key is used for authentication.
1.36 aaron 563: Any port forward requests by the client will return an error.
1.230 jmc 564: This might be used, e.g. in connection with the
1.2 deraadt 565: .Cm command
1.1 deraadt 566: option.
1.218 jmc 567: .It Cm no-pty
568: Prevents tty allocation (a request to allocate a pty will fail).
1.240 djm 569: .It Cm no-user-rc
1.241 jmc 570: Disables execution of
1.240 djm 571: .Pa ~/.ssh/rc .
1.2 deraadt 572: .It Cm no-X11-forwarding
1.1 deraadt 573: Forbids X11 forwarding when this key is used for authentication.
574: Any X11 forward requests by the client will return an error.
1.107 djm 575: .It Cm permitopen="host:port"
1.133 itojun 576: Limit local
1.107 djm 577: .Li ``ssh -L''
1.111 stevesk 578: port forwarding such that it may only connect to the specified host and
1.146 stevesk 579: port.
1.257 djm 580: IPv6 addresses can be specified by enclosing the address in square brackets.
1.146 stevesk 581: Multiple
1.107 djm 582: .Cm permitopen
1.197 jmc 583: options may be applied separated by commas.
584: No pattern matching is performed on the specified hostnames,
585: they must be literal domains or addresses.
1.264 dtucker 586: A port specification of
587: .Cm *
588: matches any port.
1.256 djm 589: .It Cm principals="principals"
590: On a
591: .Cm cert-authority
592: line, specifies allowed principals for certificate authentication as a
593: comma-separated list.
594: At least one name from the list must appear in the certificate's
595: list of principals for the certificate to be accepted.
596: This option is ignored for keys that are not marked as trusted certificate
597: signers using the
598: .Cm cert-authority
599: option.
1.209 reyk 600: .It Cm tunnel="n"
601: Force a
602: .Xr tun 4
603: device on the server.
604: Without this option, the next available device will be used if
605: the client requests a tunnel.
1.2 deraadt 606: .El
607: .Pp
1.219 jmc 608: An example authorized_keys file:
1.222 jmc 609: .Bd -literal -offset 3n
1.219 jmc 610: # Comments allowed at start of line
611: ssh-rsa AAAAB3Nza...LiPk== user@example.net
1.222 jmc 612: from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
613: AAAAB2...19Q== john@example.net
614: command="dump /home",no-pty,no-port-forwarding ssh-dss
615: AAAAC3...51R== example.net
616: permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
617: AAAAB5...21S==
618: tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
619: jane@example.net
1.219 jmc 620: .Ed
1.2 deraadt 621: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 622: The
1.194 jmc 623: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 624: and
1.207 djm 625: .Pa ~/.ssh/known_hosts
1.36 aaron 626: files contain host public keys for all known hosts.
627: The global file should
1.37 brad 628: be prepared by the administrator (optional), and the per-user file is
1.220 jmc 629: maintained automatically: whenever the user connects from an unknown host,
1.36 aaron 630: its key is added to the per-user file.
1.2 deraadt 631: .Pp
1.254 djm 632: Each line in these files contains the following fields: markers (optional),
633: hostnames, bits, exponent, modulus, comment.
1.36 aaron 634: The fields are separated by spaces.
1.2 deraadt 635: .Pp
1.254 djm 636: The marker is optional, but if it is present then it must be one of
637: .Dq @cert-authority ,
638: to indicate that the line contains a certification authority (CA) key,
639: or
640: .Dq @revoked ,
641: to indicate that the key contained on the line is revoked and must not ever
642: be accepted.
643: Only one marker should be used on a key line.
644: .Pp
1.196 david 645: Hostnames is a comma-separated list of patterns
1.220 jmc 646: .Pf ( Ql *
1.197 jmc 647: and
648: .Ql \&?
1.195 mouring 649: act as
1.1 deraadt 650: wildcards); each pattern in turn is matched against the canonical host
651: name (when authenticating a client) or against the user-supplied
1.36 aaron 652: name (when authenticating a server).
653: A pattern may also be preceded by
1.195 mouring 654: .Ql \&!
1.2 deraadt 655: to indicate negation: if the host name matches a negated
1.1 deraadt 656: pattern, it is not accepted (by that line) even if it matched another
657: pattern on the line.
1.231 dtucker 658: A hostname or address may optionally be enclosed within
659: .Ql \&[
660: and
661: .Ql \&]
662: brackets then followed by
663: .Ql \&:
1.232 jmc 664: and a non-standard port number.
1.2 deraadt 665: .Pp
1.205 djm 666: Alternately, hostnames may be stored in a hashed form which hides host names
1.206 jmc 667: and addresses should the file's contents be disclosed.
668: Hashed hostnames start with a
669: .Ql |
1.205 djm 670: character.
671: Only one hashed hostname may appear on a single line and none of the above
672: negation or wildcard operators may be applied.
673: .Pp
1.49 markus 674: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.220 jmc 675: can be obtained, for example, from
1.167 deraadt 676: .Pa /etc/ssh/ssh_host_key.pub .
1.1 deraadt 677: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 678: .Pp
679: Lines starting with
680: .Ql #
681: and empty lines are ignored as comments.
682: .Pp
1.1 deraadt 683: When performing host authentication, authentication is accepted if any
1.254 djm 684: matching line has the proper key; either one that matches exactly or,
685: if the server has presented a certificate for authentication, the key
686: of the certification authority that signed the certificate.
687: For a key to be trusted as a certification authority, it must use the
688: .Dq @cert-authority
689: marker described above.
690: .Pp
691: The known hosts file also provides a facility to mark keys as revoked,
692: for example when it is known that the associated private key has been
693: stolen.
694: Revoked keys are specified by including the
695: .Dq @revoked
696: marker at the beginning of the key line, and are never accepted for
697: authentication or as certification authorities, but instead will
698: produce a warning from
699: .Xr ssh 1
700: when they are encountered.
701: .Pp
702: It is permissible (but not
1.1 deraadt 703: recommended) to have several lines or different host keys for the same
1.36 aaron 704: names.
705: This will inevitably happen when short forms of host names
706: from different domains are put in the file.
707: It is possible
1.1 deraadt 708: that the files contain conflicting information; authentication is
709: accepted if valid information can be found from either file.
1.2 deraadt 710: .Pp
1.1 deraadt 711: Note that the lines in these files are typically hundreds of characters
712: long, and you definitely don't want to type in the host keys by hand.
1.254 djm 713: Rather, generate them by a script,
714: .Xr ssh-keyscan 1
1.40 aaron 715: or by taking
1.167 deraadt 716: .Pa /etc/ssh/ssh_host_key.pub
1.1 deraadt 717: and adding the host names at the front.
1.254 djm 718: .Xr ssh-keygen 1
1.255 jmc 719: also offers some basic automated editing for
1.254 djm 720: .Pa ~/.ssh/known_hosts
721: including removing hosts matching a host name and converting all host
722: names to their hashed representations.
1.221 jmc 723: .Pp
724: An example ssh_known_hosts file:
725: .Bd -literal -offset 3n
726: # Comments allowed at start of line
727: closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
728: cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
1.205 djm 729: # A hashed hostname
730: |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
731: AAAA1234.....=
1.254 djm 732: # A revoked key
733: @revoked * ssh-rsa AAAAB5W...
734: # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
735: @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
1.120 markus 736: .Ed
1.2 deraadt 737: .Sh FILES
1.223 jmc 738: .Bl -tag -width Ds -compact
1.258 jmc 739: .It Pa ~/.hushlogin
1.204 jmc 740: This file is used to suppress printing the last login time and
741: .Pa /etc/motd ,
742: if
743: .Cm PrintLastLog
744: and
745: .Cm PrintMotd ,
746: respectively,
747: are enabled.
748: It does not suppress printing of the banner specified by
749: .Cm Banner .
1.223 jmc 750: .Pp
1.258 jmc 751: .It Pa ~/.rhosts
1.225 jmc 752: This file is used for host-based authentication (see
753: .Xr ssh 1
754: for more information).
755: On some machines this file may need to be
756: world-readable if the user's home directory is on an NFS partition,
757: because
758: .Nm
759: reads it as root.
760: Additionally, this file must be owned by the user,
761: and must not have write permissions for anyone else.
762: The recommended
763: permission for most machines is read/write for the user, and not
1.1 deraadt 764: accessible by others.
1.2 deraadt 765: .Pp
1.258 jmc 766: .It Pa ~/.shosts
1.225 jmc 767: This file is used in exactly the same way as
768: .Pa .rhosts ,
769: but allows host-based authentication without permitting login with
770: rlogin/rsh.
1.238 mcbride 771: .Pp
1.258 jmc 772: .It Pa ~/.ssh/
1.238 mcbride 773: This directory is the default location for all user-specific configuration
774: and authentication information.
775: There is no general requirement to keep the entire contents of this directory
776: secret, but the recommended permissions are read/write/execute for the user,
777: and not accessible by others.
1.2 deraadt 778: .Pp
1.258 jmc 779: .It Pa ~/.ssh/authorized_keys
1.259 djm 780: Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in
781: as this user.
1.223 jmc 782: The format of this file is described above.
1.234 dtucker 783: The content of the file is not highly sensitive, but the recommended
1.225 jmc 784: permissions are read/write for the user, and not accessible by others.
1.234 dtucker 785: .Pp
786: If this file, the
787: .Pa ~/.ssh
788: directory, or the user's home directory are writable
789: by other users, then the file could be modified or replaced by unauthorized
790: users.
791: In this case,
792: .Nm
793: will not allow it to be used unless the
794: .Cm StrictModes
795: option has been set to
796: .Dq no .
1.2 deraadt 797: .Pp
1.258 jmc 798: .It Pa ~/.ssh/environment
1.36 aaron 799: This file is read into the environment at login (if it exists).
800: It can only contain empty lines, comment lines (that start with
1.2 deraadt 801: .Ql # ) ,
1.36 aaron 802: and assignment lines of the form name=value.
803: The file should be writable
1.6 aaron 804: only by the user; it need not be readable by anyone else.
1.188 stevesk 805: Environment processing is disabled by default and is
806: controlled via the
807: .Cm PermitUserEnvironment
808: option.
1.223 jmc 809: .Pp
1.258 jmc 810: .It Pa ~/.ssh/known_hosts
1.225 jmc 811: Contains a list of host keys for all hosts the user has logged into
812: that are not already in the systemwide list of known host keys.
813: The format of this file is described above.
814: This file should be writable only by root/the owner and
1.223 jmc 815: can, but need not be, world-readable.
816: .Pp
1.258 jmc 817: .It Pa ~/.ssh/rc
1.226 jmc 818: Contains initialization routines to be run before
819: the user's home directory becomes accessible.
1.1 deraadt 820: This file should be writable only by the user, and need not be
821: readable by anyone else.
1.223 jmc 822: .Pp
1.258 jmc 823: .It Pa /etc/hosts.allow
824: .It Pa /etc/hosts.deny
1.223 jmc 825: Access controls that should be enforced by tcp-wrappers are defined here.
826: Further details are described in
827: .Xr hosts_access 5 .
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
1.258 jmc 861: .It Pa /etc/ssh/ssh_host_rsa_key
1.266 ! dtucker 862: These files contain the private parts of the host keys.
1.223 jmc 863: These files should only be owned by root, readable only by root, and not
864: accessible to others.
865: Note that
866: .Nm
1.228 jmc 867: does not start if these files are group/world-accessible.
1.223 jmc 868: .Pp
1.258 jmc 869: .It Pa /etc/ssh/ssh_host_key.pub
870: .It Pa /etc/ssh/ssh_host_dsa_key.pub
1.259 djm 871: .It Pa /etc/ssh/ssh_host_ecdsa_key.pub
1.258 jmc 872: .It Pa /etc/ssh/ssh_host_rsa_key.pub
1.266 ! dtucker 873: These files contain the public parts of the host keys.
1.223 jmc 874: These files should be world-readable but writable only by
875: root.
876: Their contents should match the respective private parts.
877: These files are not
878: really used for anything; they are provided for the convenience of
879: the user so their contents can be copied to known hosts files.
880: These files are created using
881: .Xr ssh-keygen 1 .
882: .Pp
1.258 jmc 883: .It Pa /etc/ssh/ssh_known_hosts
1.235 jmc 884: Systemwide list of known host keys.
885: This file should be prepared by the
886: system administrator to contain the public host keys of all machines in the
887: organization.
888: The format of this file is described above.
889: This file should be writable only by root/the owner and
890: should be world-readable.
891: .Pp
1.258 jmc 892: .It Pa /etc/ssh/sshd_config
1.223 jmc 893: Contains configuration data for
894: .Nm sshd .
895: The file format and configuration options are described in
896: .Xr sshd_config 5 .
897: .Pp
1.258 jmc 898: .It Pa /etc/ssh/sshrc
1.226 jmc 899: Similar to
900: .Pa ~/.ssh/rc ,
901: it can be used to specify
1.36 aaron 902: machine-specific login-time initializations globally.
903: This file should be writable only by root, and should be world-readable.
1.223 jmc 904: .Pp
1.258 jmc 905: .It Pa /var/empty
1.223 jmc 906: .Xr chroot 2
907: directory used by
908: .Nm
909: during privilege separation in the pre-authentication phase.
910: The directory should not contain any files and must be owned by root
911: and not group or world-writable.
912: .Pp
1.258 jmc 913: .It Pa /var/run/sshd.pid
1.223 jmc 914: Contains the process ID of the
915: .Nm
916: listening for connections (if there are several daemons running
917: concurrently for different ports, this contains the process ID of the one
918: started last).
919: The content of this file is not sensitive; it can be world-readable.
1.56 aaron 920: .El
1.2 deraadt 921: .Sh SEE ALSO
922: .Xr scp 1 ,
1.90 djm 923: .Xr sftp 1 ,
1.2 deraadt 924: .Xr ssh 1 ,
1.5 deraadt 925: .Xr ssh-add 1 ,
1.2 deraadt 926: .Xr ssh-agent 1 ,
1.5 deraadt 927: .Xr ssh-keygen 1 ,
1.235 jmc 928: .Xr ssh-keyscan 1 ,
1.200 jmc 929: .Xr chroot 2 ,
930: .Xr hosts_access 5 ,
1.136 markus 931: .Xr login.conf 5 ,
932: .Xr moduli 5 ,
1.184 stevesk 933: .Xr sshd_config 5 ,
1.200 jmc 934: .Xr inetd 8 ,
1.128 mpech 935: .Xr sftp-server 8
1.198 jmc 936: .Sh AUTHORS
937: OpenSSH is a derivative of the original and free
938: ssh 1.2.12 release by Tatu Ylonen.
939: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
940: Theo de Raadt and Dug Song
941: removed many bugs, re-added newer features and
942: created OpenSSH.
943: Markus Friedl contributed the support for SSH
944: protocol versions 1.5 and 2.0.
945: Niels Provos and Markus Friedl contributed support
946: for privilege separation.
1.216 jmc 947: .Sh CAVEATS
948: System security is not improved unless
949: .Nm rshd ,
950: .Nm rlogind ,
951: and
952: .Nm rexecd
953: are disabled (thus completely disabling
954: .Xr rlogin
955: and
956: .Xr rsh
957: into the machine).
![[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