Annotation of src/usr.bin/ssh/sshd.8, Revision 1.210
1.1 deraadt 1: .\" -*- nroff -*-
2: .\"
3: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5: .\" All rights reserved
6: .\"
1.64 deraadt 7: .\" As far as I am concerned, the code I have written for this software
8: .\" can be used freely for any purpose. Any derived versions of this
9: .\" software must be clearly marked as such, and if the derived work is
10: .\" incompatible with the protocol description in the RFC file, it must be
11: .\" called by a name other than "ssh" or "Secure Shell".
12: .\"
1.99 deraadt 13: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
1.64 deraadt 16: .\"
17: .\" Redistribution and use in source and binary forms, with or without
18: .\" modification, are permitted provided that the following conditions
19: .\" are met:
20: .\" 1. Redistributions of source code must retain the above copyright
21: .\" notice, this list of conditions and the following disclaimer.
22: .\" 2. Redistributions in binary form must reproduce the above copyright
23: .\" notice, this list of conditions and the following disclaimer in the
24: .\" documentation and/or other materials provided with the distribution.
1.1 deraadt 25: .\"
1.64 deraadt 26: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
32: .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
33: .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
34: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 36: .\"
1.210 ! stevesk 37: .\" $OpenBSD: sshd.8,v 1.209 2005/12/06 22:38:28 reyk Exp $
1.2 deraadt 38: .Dd September 25, 1999
39: .Dt SSHD 8
40: .Os
41: .Sh NAME
42: .Nm sshd
1.120 markus 43: .Nd OpenSSH SSH daemon
1.2 deraadt 44: .Sh SYNOPSIS
45: .Nm sshd
1.194 jmc 46: .Bk -words
1.200 jmc 47: .Op Fl 46Ddeiqt
1.2 deraadt 48: .Op Fl b Ar bits
49: .Op Fl f Ar config_file
50: .Op Fl g Ar login_grace_time
51: .Op Fl h Ar host_key_file
52: .Op Fl k Ar key_gen_time
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
1.106 deraadt 59: (SSH Daemon) is the daemon program for
1.2 deraadt 60: .Xr ssh 1 .
1.42 hugh 61: Together these programs replace rlogin and rsh, and
1.1 deraadt 62: provide secure encrypted communications between two untrusted hosts
1.36 aaron 63: over an insecure network.
64: The programs are intended to be as easy to
1.1 deraadt 65: install and use as possible.
1.2 deraadt 66: .Pp
67: .Nm
1.36 aaron 68: is the daemon that 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.
1.49 markus 76: This implementation of
77: .Nm
78: supports both SSH protocol version 1 and 2 simultaneously.
1.2 deraadt 79: .Nm
1.194 jmc 80: works as follows:
1.49 markus 81: .Ss SSH protocol version 1
1.36 aaron 82: Each host has a host-specific RSA key
1.208 djm 83: (normally 2048 bits) used to identify the host.
1.36 aaron 84: Additionally, when
1.1 deraadt 85: the daemon starts, it generates a server RSA key (normally 768 bits).
86: This key is normally regenerated every hour if it has been used, and
87: is never stored on disk.
1.2 deraadt 88: .Pp
1.194 jmc 89: Whenever a client connects, the daemon responds with its public
1.42 hugh 90: host and server keys.
1.36 aaron 91: The client compares the
1.49 markus 92: RSA host key against its own database to verify that it has not changed.
1.200 jmc 93: The client then generates a 256-bit random number.
1.36 aaron 94: It encrypts this
1.1 deraadt 95: random number using both the host key and the server key, and sends
1.36 aaron 96: the encrypted number to the server.
1.42 hugh 97: Both sides then use this
1.1 deraadt 98: random number as a session key which is used to encrypt all further
1.36 aaron 99: communications in the session.
100: The rest of the session is encrypted
1.42 hugh 101: using a conventional cipher, currently Blowfish or 3DES, with 3DES
1.39 deraadt 102: being used by default.
1.36 aaron 103: The client selects the encryption algorithm
1.5 deraadt 104: to use from those offered by the server.
1.2 deraadt 105: .Pp
1.36 aaron 106: Next, the server and the client enter an authentication dialog.
107: The client tries to authenticate itself using
1.200 jmc 108: .Em rhosts
1.2 deraadt 109: authentication combined with RSA host
1.1 deraadt 110: authentication, RSA challenge-response authentication, or password
111: based authentication.
1.2 deraadt 112: .Pp
1.36 aaron 113: System security is not improved unless
1.180 millert 114: .Nm rshd ,
115: .Nm rlogind ,
1.2 deraadt 116: and
1.194 jmc 117: .Nm rexecd
1.1 deraadt 118: are disabled (thus completely disabling
1.180 millert 119: .Xr rlogin
1.1 deraadt 120: and
1.180 millert 121: .Xr rsh
1.42 hugh 122: into the machine).
1.49 markus 123: .Ss SSH protocol version 2
1.58 deraadt 124: Version 2 works similarly:
1.138 markus 125: Each host has a host-specific key (RSA or DSA) used to identify the host.
1.49 markus 126: However, when the daemon starts, it does not generate a server key.
127: Forward security is provided through a Diffie-Hellman key agreement.
128: This key agreement results in a shared session key.
1.120 markus 129: .Pp
1.103 deraadt 130: The rest of the session is encrypted using a symmetric cipher, currently
1.200 jmc 131: 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
1.49 markus 132: The client selects the encryption algorithm
133: to use from those offered by the server.
134: Additionally, session integrity is provided
1.51 hugh 135: through a cryptographic message authentication code
1.49 markus 136: (hmac-sha1 or hmac-md5).
137: .Pp
138: Protocol version 2 provides a public key based
1.120 markus 139: user (PubkeyAuthentication) or
140: client host (HostbasedAuthentication) authentication method,
141: conventional password authentication and challenge response based methods.
1.49 markus 142: .Ss Command execution and data forwarding
1.1 deraadt 143: If the client successfully authenticates itself, a dialog for
1.36 aaron 144: preparing the session is entered.
145: At this time the client may request
1.1 deraadt 146: things like allocating a pseudo-tty, forwarding X11 connections,
147: forwarding TCP/IP connections, or forwarding the authentication agent
148: connection over the secure channel.
1.2 deraadt 149: .Pp
1.1 deraadt 150: Finally, the client either requests a shell or execution of a command.
1.36 aaron 151: The sides then enter session mode.
152: In this mode, either side may send
1.1 deraadt 153: data at any time, and such data is forwarded to/from the shell or
154: command on the server side, and the user terminal in the client side.
1.2 deraadt 155: .Pp
1.1 deraadt 156: When the user program terminates and all forwarded X11 and other
157: connections have been closed, the server sends command exit status to
158: the client, and both sides exit.
1.2 deraadt 159: .Pp
160: .Nm
1.200 jmc 161: can be configured using command-line options or a configuration file
162: (by default
163: .Xr sshd_config 5 ) .
1.36 aaron 164: Command-line options override values specified in the
1.1 deraadt 165: configuration file.
1.25 markus 166: .Pp
167: .Nm
168: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 169: .Dv SIGHUP ,
1.201 dtucker 170: by executing itself with the name and options it was started with, e.g.,
1.97 deraadt 171: .Pa /usr/sbin/sshd .
1.18 aaron 172: .Pp
173: The options are as follows:
1.2 deraadt 174: .Bl -tag -width Ds
1.200 jmc 175: .It Fl 4
176: Forces
177: .Nm
178: to use IPv4 addresses only.
179: .It Fl 6
180: Forces
181: .Nm
182: to use IPv6 addresses only.
1.2 deraadt 183: .It Fl b Ar bits
1.120 markus 184: Specifies the number of bits in the ephemeral protocol version 1
185: server key (default 768).
1.200 jmc 186: .It Fl D
187: When this option is specified,
188: .Nm
189: will not detach and does not become a daemon.
190: This allows easy monitoring of
191: .Nm sshd .
1.2 deraadt 192: .It Fl d
1.36 aaron 193: Debug mode.
194: The server sends verbose debug output to the system
195: log, and does not put itself in the background.
196: The server also will not fork and will only process one connection.
197: This option is only intended for debugging for the server.
1.194 jmc 198: Multiple
199: .Fl d
200: options increase the debugging level.
1.67 aaron 201: Maximum is 3.
1.120 markus 202: .It Fl e
203: When this option is specified,
204: .Nm
205: will send the output to the standard error instead of the system log.
1.2 deraadt 206: .It Fl f Ar configuration_file
1.36 aaron 207: Specifies the name of the configuration file.
208: The default is
1.167 deraadt 209: .Pa /etc/ssh/sshd_config .
1.16 markus 210: .Nm
211: refuses to start if there is no configuration file.
1.2 deraadt 212: .It Fl g Ar login_grace_time
1.1 deraadt 213: Gives the grace time for clients to authenticate themselves (default
1.191 stevesk 214: 120 seconds).
1.36 aaron 215: If the client fails to authenticate the user within
216: this many seconds, the server disconnects and exits.
217: A value of zero indicates no limit.
1.2 deraadt 218: .It Fl h Ar host_key_file
1.160 stevesk 219: Specifies a file from which a host key is read.
1.7 markus 220: This option must be given if
221: .Nm
222: is not run as root (as the normal
1.160 stevesk 223: host key files are normally not readable by anyone but root).
224: The default is
1.167 deraadt 225: .Pa /etc/ssh/ssh_host_key
1.160 stevesk 226: for protocol version 1, and
1.167 deraadt 227: .Pa /etc/ssh/ssh_host_rsa_key
1.160 stevesk 228: and
1.167 deraadt 229: .Pa /etc/ssh/ssh_host_dsa_key
1.160 stevesk 230: for protocol version 2.
1.75 markus 231: It is possible to have multiple host key files for
1.120 markus 232: the different protocol versions and host key algorithms.
1.2 deraadt 233: .It Fl i
1.7 markus 234: Specifies that
235: .Nm
1.194 jmc 236: is being run from
237: .Xr inetd 8 .
1.7 markus 238: .Nm
239: is normally not run
1.1 deraadt 240: from inetd because it needs to generate the server key before it can
1.36 aaron 241: respond to the client, and this may take tens of seconds.
242: Clients would have to wait too long if the key was regenerated every time.
1.35 aaron 243: However, with small key sizes (e.g., 512) using
1.7 markus 244: .Nm
245: from inetd may
1.1 deraadt 246: be feasible.
1.2 deraadt 247: .It Fl k Ar key_gen_time
1.120 markus 248: Specifies how often the ephemeral protocol version 1 server key is
249: regenerated (default 3600 seconds, or one hour).
1.36 aaron 250: The motivation for regenerating the key fairly
1.200 jmc 251: often is that the key is not stored anywhere, and after about an hour
1.1 deraadt 252: it becomes impossible to recover the key for decrypting intercepted
253: communications even if the machine is cracked into or physically
1.36 aaron 254: seized.
255: A value of zero indicates that the key will never be regenerated.
1.156 markus 256: .It Fl o Ar option
257: Can be used to give options in the format used in the configuration file.
258: This is useful for specifying options for which there is no separate
259: command-line flag.
1.200 jmc 260: For full details of the options, and their values, see
261: .Xr sshd_config 5 .
1.2 deraadt 262: .It Fl p Ar port
1.1 deraadt 263: Specifies the port on which the server listens for connections
264: (default 22).
1.158 stevesk 265: Multiple port options are permitted.
1.210 ! stevesk 266: Ports specified in the configuration file with the
! 267: .Cm Port
! 268: option are ignored when a command-line port is specified.
! 269: Ports specified using the
! 270: .Cm ListenAddress
! 271: option override command-line ports.
1.2 deraadt 272: .It Fl q
1.36 aaron 273: Quiet mode.
274: Nothing is sent to the system log.
275: Normally the beginning,
1.1 deraadt 276: authentication, and termination of each connection is logged.
1.137 stevesk 277: .It Fl t
278: Test mode.
279: Only check the validity of the configuration file and sanity of the keys.
1.157 deraadt 280: This is useful for updating
1.137 stevesk 281: .Nm
282: reliably as configuration options may change.
1.61 markus 283: .It Fl u Ar len
284: This option is used to specify the size of the field
285: in the
286: .Li utmp
287: structure that holds the remote host name.
288: If the resolved host name is longer than
289: .Ar len ,
290: the dotted decimal value will be used instead.
291: This allows hosts with very long host names that
292: overflow this field to still be uniquely identified.
293: Specifying
294: .Fl u0
295: indicates that only dotted decimal addresses
296: should be put into the
297: .Pa utmp
298: file.
1.144 stevesk 299: .Fl u0
1.194 jmc 300: may also be used to prevent
1.144 stevesk 301: .Nm
302: from making DNS requests unless the authentication
303: mechanism or configuration requires it.
304: Authentication mechanisms that may require DNS include
305: .Cm RhostsRSAAuthentication ,
306: .Cm HostbasedAuthentication
307: and using a
308: .Cm from="pattern-list"
309: option in a key file.
1.170 stevesk 310: Configuration options that require DNS include using a
311: USER@HOST pattern in
312: .Cm AllowUsers
313: or
314: .Cm DenyUsers .
1.2 deraadt 315: .El
316: .Sh CONFIGURATION FILE
317: .Nm
1.40 aaron 318: reads configuration data from
1.167 deraadt 319: .Pa /etc/ssh/sshd_config
1.2 deraadt 320: (or the file specified with
321: .Fl f
1.36 aaron 322: on the command line).
1.184 stevesk 323: The file format and configuration options are described in
324: .Xr sshd_config 5 .
1.2 deraadt 325: .Sh LOGIN PROCESS
1.1 deraadt 326: When a user successfully logs in,
1.2 deraadt 327: .Nm
1.1 deraadt 328: does the following:
1.2 deraadt 329: .Bl -enum -offset indent
330: .It
1.1 deraadt 331: If the login is on a tty, and no command has been specified,
1.40 aaron 332: prints last login time and
1.2 deraadt 333: .Pa /etc/motd
1.1 deraadt 334: (unless prevented in the configuration file or by
1.207 djm 335: .Pa ~/.hushlogin ;
1.2 deraadt 336: see the
1.40 aaron 337: .Sx FILES
1.2 deraadt 338: section).
339: .It
1.1 deraadt 340: If the login is on a tty, records login time.
1.2 deraadt 341: .It
342: Checks
343: .Pa /etc/nologin ;
344: if it exists, prints contents and quits
1.1 deraadt 345: (unless root).
1.2 deraadt 346: .It
1.1 deraadt 347: Changes to run with normal user privileges.
1.2 deraadt 348: .It
1.1 deraadt 349: Sets up basic environment.
1.2 deraadt 350: .It
1.200 jmc 351: Reads the file
1.207 djm 352: .Pa ~/.ssh/environment ,
1.200 jmc 353: if it exists, and users are allowed to change their environment.
1.188 stevesk 354: See the
1.187 marc 355: .Cm PermitUserEnvironment
1.188 stevesk 356: option in
1.187 marc 357: .Xr sshd_config 5 .
1.2 deraadt 358: .It
1.1 deraadt 359: Changes to user's home directory.
1.2 deraadt 360: .It
361: If
1.207 djm 362: .Pa ~/.ssh/rc
1.2 deraadt 363: exists, runs it; else if
1.168 deraadt 364: .Pa /etc/ssh/sshrc
1.2 deraadt 365: exists, runs
1.36 aaron 366: it; otherwise runs xauth.
367: The
1.2 deraadt 368: .Dq rc
369: files are given the X11
1.1 deraadt 370: authentication protocol and cookie in standard input.
1.2 deraadt 371: .It
1.1 deraadt 372: Runs user's shell or command.
1.2 deraadt 373: .El
374: .Sh AUTHORIZED_KEYS FILE FORMAT
1.207 djm 375: .Pa ~/.ssh/authorized_keys
1.130 markus 376: is the default file that lists the public keys that are
377: permitted for RSA authentication in protocol version 1
378: and for public key authentication (PubkeyAuthentication)
379: in protocol version 2.
1.125 markus 380: .Cm AuthorizedKeysFile
381: may be used to specify an alternative file.
1.75 markus 382: .Pp
1.36 aaron 383: Each line of the file contains one
1.2 deraadt 384: key (empty lines and lines starting with a
385: .Ql #
386: are ignored as
1.36 aaron 387: comments).
1.75 markus 388: Each RSA public key consists of the following fields, separated by
1.36 aaron 389: spaces: options, bits, exponent, modulus, comment.
1.75 markus 390: Each protocol version 2 public key consists of:
391: options, keytype, base64 encoded key, comment.
1.189 espie 392: The options field
393: is optional; its presence is determined by whether the line starts
394: with a number or not (the options field never starts with a number).
1.75 markus 395: The bits, exponent, modulus and comment fields give the RSA key for
396: protocol version 1; the
1.1 deraadt 397: comment field is not used for anything (but may be convenient for the
398: user to identify the key).
1.75 markus 399: For protocol version 2 the keytype is
400: .Dq ssh-dss
401: or
402: .Dq ssh-rsa .
1.2 deraadt 403: .Pp
1.1 deraadt 404: Note that lines in this file are usually several hundred bytes long
1.203 dtucker 405: (because of the size of the public key encoding) up to a limit of
406: 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
407: keys up to 16 kilobits.
1.36 aaron 408: You don't want to type them in; instead, copy the
1.113 itojun 409: .Pa identity.pub ,
410: .Pa id_dsa.pub
1.75 markus 411: or the
1.113 itojun 412: .Pa id_rsa.pub
1.1 deraadt 413: file and edit it.
1.175 stevesk 414: .Pp
415: .Nm
416: enforces a minimum RSA key modulus size for protocol 1
417: and protocol 2 keys of 768 bits.
1.2 deraadt 418: .Pp
1.58 deraadt 419: The options (if present) consist of comma-separated option
1.36 aaron 420: specifications.
421: No spaces are permitted, except within double quotes.
1.141 stevesk 422: The following option specifications are supported (note
423: that option keywords are case-insensitive):
1.2 deraadt 424: .Bl -tag -width Ds
425: .It Cm from="pattern-list"
1.189 espie 426: Specifies that in addition to public key authentication, the canonical name
1.1 deraadt 427: of the remote host must be present in the comma-separated list of
1.36 aaron 428: patterns
1.196 david 429: .Pf ( Ql \&*
1.36 aaron 430: and
1.195 mouring 431: .Ql \&?
1.36 aaron 432: serve as wildcards).
433: The list may also contain
434: patterns negated by prefixing them with
1.195 mouring 435: .Ql \&! ;
1.36 aaron 436: if the canonical host name matches a negated pattern, the key is not accepted.
437: The purpose
1.189 espie 438: of this option is to optionally increase security: public key authentication
1.1 deraadt 439: by itself does not trust the network or name servers or anything (but
440: the key); however, if somebody somehow steals the key, the key
1.36 aaron 441: permits an intruder to log in from anywhere in the world.
442: This additional option makes using a stolen key more difficult (name
1.1 deraadt 443: servers and/or routers would have to be compromised in addition to
444: just the key).
1.2 deraadt 445: .It Cm command="command"
1.1 deraadt 446: Specifies that the command is executed whenever this key is used for
1.36 aaron 447: authentication.
448: The command supplied by the user (if any) is ignored.
1.148 markus 449: The command is run on a pty if the client requests a pty;
1.36 aaron 450: otherwise it is run without a tty.
1.194 jmc 451: If an 8-bit clean channel is required,
1.147 deraadt 452: one must not request a pty or should specify
1.89 markus 453: .Cm no-pty .
1.36 aaron 454: A quote may be included in the command by quoting it with a backslash.
455: This option might be useful
1.189 espie 456: to restrict certain public keys to perform just a specific operation.
1.36 aaron 457: An example might be a key that permits remote backups but nothing else.
1.51 hugh 458: Note that the client may specify TCP/IP and/or X11
459: forwarding unless they are explicitly prohibited.
1.149 markus 460: Note that this option applies to shell, command or subsystem execution.
1.2 deraadt 461: .It Cm environment="NAME=value"
1.1 deraadt 462: Specifies that the string is to be added to the environment when
1.36 aaron 463: logging in using this key.
464: Environment variables set this way
465: override other default environment values.
466: Multiple options of this type are permitted.
1.188 stevesk 467: Environment processing is disabled by default and is
468: controlled via the
469: .Cm PermitUserEnvironment
470: option.
1.155 markus 471: This option is automatically disabled if
472: .Cm UseLogin
473: is enabled.
1.2 deraadt 474: .It Cm no-port-forwarding
1.1 deraadt 475: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 476: Any port forward requests by the client will return an error.
477: This might be used, e.g., in connection with the
1.2 deraadt 478: .Cm command
1.1 deraadt 479: option.
1.2 deraadt 480: .It Cm no-X11-forwarding
1.1 deraadt 481: Forbids X11 forwarding when this key is used for authentication.
482: Any X11 forward requests by the client will return an error.
1.2 deraadt 483: .It Cm no-agent-forwarding
1.1 deraadt 484: Forbids authentication agent forwarding when this key is used for
485: authentication.
1.2 deraadt 486: .It Cm no-pty
1.1 deraadt 487: Prevents tty allocation (a request to allocate a pty will fail).
1.107 djm 488: .It Cm permitopen="host:port"
1.133 itojun 489: Limit local
1.107 djm 490: .Li ``ssh -L''
1.111 stevesk 491: port forwarding such that it may only connect to the specified host and
1.146 stevesk 492: port.
493: IPv6 addresses can be specified with an alternative syntax:
1.200 jmc 494: .Ar host Ns / Ns Ar port .
1.146 stevesk 495: Multiple
1.107 djm 496: .Cm permitopen
1.197 jmc 497: options may be applied separated by commas.
498: No pattern matching is performed on the specified hostnames,
499: they must be literal domains or addresses.
1.209 reyk 500: .It Cm tunnel="n"
501: Force a
502: .Xr tun 4
503: device on the server.
504: Without this option, the next available device will be used if
505: the client requests a tunnel.
1.2 deraadt 506: .El
507: .Ss Examples
1.200 jmc 508: 1024 33 12121...312314325 ylo@foo.bar
1.2 deraadt 509: .Pp
1.200 jmc 510: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula
1.2 deraadt 511: .Pp
1.200 jmc 512: command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi
1.107 djm 513: .Pp
1.200 jmc 514: permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323
1.209 reyk 515: .Pp
516: tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== reyk@openbsd.org
1.2 deraadt 517: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 518: The
1.194 jmc 519: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 520: and
1.207 djm 521: .Pa ~/.ssh/known_hosts
1.36 aaron 522: files contain host public keys for all known hosts.
523: The global file should
1.37 brad 524: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 525: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 526: its key is added to the per-user file.
1.2 deraadt 527: .Pp
1.1 deraadt 528: Each line in these files contains the following fields: hostnames,
1.36 aaron 529: bits, exponent, modulus, comment.
530: The fields are separated by spaces.
1.2 deraadt 531: .Pp
1.196 david 532: Hostnames is a comma-separated list of patterns
1.197 jmc 533: .Pf ( Ql \&*
534: and
535: .Ql \&?
1.195 mouring 536: act as
1.1 deraadt 537: wildcards); each pattern in turn is matched against the canonical host
538: name (when authenticating a client) or against the user-supplied
1.36 aaron 539: name (when authenticating a server).
540: A pattern may also be preceded by
1.195 mouring 541: .Ql \&!
1.2 deraadt 542: to indicate negation: if the host name matches a negated
1.1 deraadt 543: pattern, it is not accepted (by that line) even if it matched another
544: pattern on the line.
1.2 deraadt 545: .Pp
1.205 djm 546: Alternately, hostnames may be stored in a hashed form which hides host names
1.206 jmc 547: and addresses should the file's contents be disclosed.
548: Hashed hostnames start with a
549: .Ql |
1.205 djm 550: character.
551: Only one hashed hostname may appear on a single line and none of the above
552: negation or wildcard operators may be applied.
553: .Pp
1.49 markus 554: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 555: can be obtained, e.g., from
1.167 deraadt 556: .Pa /etc/ssh/ssh_host_key.pub .
1.1 deraadt 557: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 558: .Pp
559: Lines starting with
560: .Ql #
561: and empty lines are ignored as comments.
562: .Pp
1.1 deraadt 563: When performing host authentication, authentication is accepted if any
1.36 aaron 564: matching line has the proper key.
565: It is thus permissible (but not
1.1 deraadt 566: recommended) to have several lines or different host keys for the same
1.36 aaron 567: names.
568: This will inevitably happen when short forms of host names
569: from different domains are put in the file.
570: It is possible
1.1 deraadt 571: that the files contain conflicting information; authentication is
572: accepted if valid information can be found from either file.
1.2 deraadt 573: .Pp
1.1 deraadt 574: Note that the lines in these files are typically hundreds of characters
575: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 576: Rather, generate them by a script
1.40 aaron 577: or by taking
1.167 deraadt 578: .Pa /etc/ssh/ssh_host_key.pub
1.1 deraadt 579: and adding the host names at the front.
1.2 deraadt 580: .Ss Examples
1.120 markus 581: .Bd -literal
1.200 jmc 582: closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
1.120 markus 583: cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1.205 djm 584: .Ed
585: .Bd -literal
586: # A hashed hostname
587: |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
588: AAAA1234.....=
1.120 markus 589: .Ed
1.2 deraadt 590: .Sh FILES
591: .Bl -tag -width Ds
1.167 deraadt 592: .It Pa /etc/ssh/sshd_config
1.1 deraadt 593: Contains configuration data for
1.2 deraadt 594: .Nm sshd .
1.184 stevesk 595: The file format and configuration options are described in
596: .Xr sshd_config 5 .
1.167 deraadt 597: .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1.120 markus 598: These three files contain the private parts of the host keys.
1.98 deraadt 599: These files should only be owned by root, readable only by root, and not
1.1 deraadt 600: accessible to others.
1.14 markus 601: Note that
602: .Nm
603: does not start if this file is group/world-accessible.
1.167 deraadt 604: .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub
1.120 markus 605: These three files contain the public parts of the host keys.
1.98 deraadt 606: These files should be world-readable but writable only by
1.36 aaron 607: root.
1.98 deraadt 608: Their contents should match the respective private parts.
609: These files are not
610: really used for anything; they are provided for the convenience of
611: the user so their contents can be copied to known hosts files.
612: These files are created using
1.7 markus 613: .Xr ssh-keygen 1 .
1.129 provos 614: .It Pa /etc/moduli
1.73 provos 615: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1.192 stevesk 616: The file format is described in
617: .Xr moduli 5 .
1.185 stevesk 618: .It Pa /var/empty
619: .Xr chroot 2
620: directory used by
621: .Nm
622: during privilege separation in the pre-authentication phase.
623: The directory should not contain any files and must be owned by root
624: and not group or world-writable.
1.2 deraadt 625: .It Pa /var/run/sshd.pid
626: Contains the process ID of the
627: .Nm
1.1 deraadt 628: listening for connections (if there are several daemons running
1.186 stevesk 629: concurrently for different ports, this contains the process ID of the one
1.36 aaron 630: started last).
1.58 deraadt 631: The content of this file is not sensitive; it can be world-readable.
1.207 djm 632: .It Pa ~/.ssh/authorized_keys
1.120 markus 633: Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1.44 deraadt 634: This file must be readable by root (which may on some machines imply
635: it being world-readable if the user's home directory resides on an NFS
636: volume).
637: It is recommended that it not be accessible by others.
638: The format of this file is described above.
639: Users will place the contents of their
1.130 markus 640: .Pa identity.pub ,
1.44 deraadt 641: .Pa id_dsa.pub
1.113 itojun 642: and/or
643: .Pa id_rsa.pub
1.44 deraadt 644: files into this file, as described in
645: .Xr ssh-keygen 1 .
1.207 djm 646: .It Pa "/etc/ssh/ssh_known_hosts", "~/.ssh/known_hosts"
1.22 markus 647: These files are consulted when using rhosts with RSA host
1.131 markus 648: authentication or protocol version 2 hostbased authentication
649: to check the public key of the host.
1.36 aaron 650: The key must be listed in one of these files to be accepted.
1.22 markus 651: The client uses the same files
1.96 markus 652: to verify that it is connecting to the correct remote host.
1.36 aaron 653: These files should be writable only by root/the owner.
1.167 deraadt 654: .Pa /etc/ssh/ssh_known_hosts
1.2 deraadt 655: should be world-readable, and
1.207 djm 656: .Pa ~/.ssh/known_hosts
1.194 jmc 657: can, but need not be, world-readable.
1.204 jmc 658: .It Pa /etc/motd
659: See
660: .Xr motd 5 .
1.207 djm 661: .It Pa ~/.hushlogin
1.204 jmc 662: This file is used to suppress printing the last login time and
663: .Pa /etc/motd ,
664: if
665: .Cm PrintLastLog
666: and
667: .Cm PrintMotd ,
668: respectively,
669: are enabled.
670: It does not suppress printing of the banner specified by
671: .Cm Banner .
1.6 aaron 672: .It Pa /etc/nologin
1.40 aaron 673: If this file exists,
1.2 deraadt 674: .Nm
1.36 aaron 675: refuses to let anyone except root log in.
676: The contents of the file
1.1 deraadt 677: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 678: refused.
679: The file should be world-readable.
1.19 dugsong 680: .It Pa /etc/hosts.allow, /etc/hosts.deny
1.153 camield 681: Access controls that should be enforced by tcp-wrappers are defined here.
682: Further details are described in
1.19 dugsong 683: .Xr hosts_access 5 .
1.207 djm 684: .It Pa ~/.rhosts
1.202 markus 685: This file is used during
686: .Cm RhostsRSAAuthentication
687: and
688: .Cm HostbasedAuthentication
689: and contains host-username pairs, separated by a space, one per
1.36 aaron 690: line.
691: The given user on the corresponding host is permitted to log in
1.194 jmc 692: without a password.
1.36 aaron 693: The same file is used by rlogind and rshd.
1.6 aaron 694: The file must
1.1 deraadt 695: be writable only by the user; it is recommended that it not be
696: accessible by others.
1.2 deraadt 697: .Pp
1.200 jmc 698: It is also possible to use netgroups in the file.
1.36 aaron 699: Either host or user
1.1 deraadt 700: name may be of the form +@groupname to specify all hosts or all users
701: in the group.
1.207 djm 702: .It Pa ~/.shosts
1.2 deraadt 703: For ssh,
704: this file is exactly the same as for
705: .Pa .rhosts .
706: However, this file is
707: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 708: .It Pa /etc/hosts.equiv
1.2 deraadt 709: This file is used during
1.202 markus 710: .Cm RhostsRSAAuthentication
711: and
712: .Cm HostbasedAuthentication
1.36 aaron 713: authentication.
714: In the simplest form, this file contains host names, one per line.
715: Users on
1.1 deraadt 716: those hosts are permitted to log in without a password, provided they
1.36 aaron 717: have the same user name on both machines.
718: The host name may also be
1.1 deraadt 719: followed by a user name; such users are permitted to log in as
1.2 deraadt 720: .Em any
1.36 aaron 721: user on this machine (except root).
722: Additionally, the syntax
1.2 deraadt 723: .Dq +@group
1.36 aaron 724: can be used to specify netgroups.
725: Negated entries start with
1.2 deraadt 726: .Ql \&- .
727: .Pp
1.1 deraadt 728: If the client host/user is successfully matched in this file, login is
729: automatically permitted provided the client and server user names are the
1.36 aaron 730: same.
1.202 markus 731: Additionally, successful client host key authentication is required.
1.36 aaron 732: This file must be writable only by root; it is recommended
1.1 deraadt 733: that it be world-readable.
1.2 deraadt 734: .Pp
1.6 aaron 735: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 736: .Pa hosts.equiv .
1.1 deraadt 737: Beware that it really means that the named user(s) can log in as
1.2 deraadt 738: .Em anybody ,
1.1 deraadt 739: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 740: binaries and directories.
741: Using a user name practically grants the user root access.
742: The only valid use for user names that I can think
1.1 deraadt 743: of is in negative entries.
1.2 deraadt 744: .Pp
745: Note that this warning also applies to rsh/rlogin.
746: .It Pa /etc/shosts.equiv
1.1 deraadt 747: This is processed exactly as
1.2 deraadt 748: .Pa /etc/hosts.equiv .
1.1 deraadt 749: However, this file may be useful in environments that want to run both
1.2 deraadt 750: rsh/rlogin and ssh.
1.207 djm 751: .It Pa ~/.ssh/environment
1.36 aaron 752: This file is read into the environment at login (if it exists).
753: It can only contain empty lines, comment lines (that start with
1.2 deraadt 754: .Ql # ) ,
1.36 aaron 755: and assignment lines of the form name=value.
756: The file should be writable
1.6 aaron 757: only by the user; it need not be readable by anyone else.
1.188 stevesk 758: Environment processing is disabled by default and is
759: controlled via the
760: .Cm PermitUserEnvironment
761: option.
1.207 djm 762: .It Pa ~/.ssh/rc
1.194 jmc 763: If this file exists, it is run with
764: .Pa /bin/sh
765: after reading the
1.36 aaron 766: environment files but before starting the user's shell or command.
1.176 stevesk 767: It must not produce any output on stdout; stderr must be used
768: instead.
769: If X11 forwarding is in use, it will receive the "proto cookie" pair in
770: its standard input (and
1.2 deraadt 771: .Ev DISPLAY
1.176 stevesk 772: in its environment).
773: The script must call
1.2 deraadt 774: .Xr xauth 1
1.176 stevesk 775: because
776: .Nm
777: will not run xauth automatically to add X11 cookies.
1.2 deraadt 778: .Pp
1.1 deraadt 779: The primary purpose of this file is to run any initialization routines
780: which may be needed before the user's home directory becomes
781: accessible; AFS is a particular example of such an environment.
1.2 deraadt 782: .Pp
1.1 deraadt 783: This file will probably contain some initialization code followed by
1.120 markus 784: something similar to:
785: .Bd -literal
1.176 stevesk 786: if read proto cookie && [ -n "$DISPLAY" ]; then
787: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
788: # X11UseLocalhost=yes
1.193 todd 789: echo add unix:`echo $DISPLAY |
1.176 stevesk 790: cut -c11-` $proto $cookie
791: else
792: # X11UseLocalhost=no
1.193 todd 793: echo add $DISPLAY $proto $cookie
794: fi | xauth -q -
1.176 stevesk 795: fi
1.120 markus 796: .Ed
1.2 deraadt 797: .Pp
798: If this file does not exist,
1.167 deraadt 799: .Pa /etc/ssh/sshrc
1.2 deraadt 800: is run, and if that
1.176 stevesk 801: does not exist either, xauth is used to add the cookie.
1.2 deraadt 802: .Pp
1.1 deraadt 803: This file should be writable only by the user, and need not be
804: readable by anyone else.
1.167 deraadt 805: .It Pa /etc/ssh/sshrc
1.2 deraadt 806: Like
1.207 djm 807: .Pa ~/.ssh/rc .
1.2 deraadt 808: This can be used to specify
1.36 aaron 809: machine-specific login-time initializations globally.
810: This file should be writable only by root, and should be world-readable.
1.56 aaron 811: .El
1.2 deraadt 812: .Sh SEE ALSO
813: .Xr scp 1 ,
1.90 djm 814: .Xr sftp 1 ,
1.2 deraadt 815: .Xr ssh 1 ,
1.5 deraadt 816: .Xr ssh-add 1 ,
1.2 deraadt 817: .Xr ssh-agent 1 ,
1.5 deraadt 818: .Xr ssh-keygen 1 ,
1.200 jmc 819: .Xr chroot 2 ,
820: .Xr hosts_access 5 ,
1.136 markus 821: .Xr login.conf 5 ,
822: .Xr moduli 5 ,
1.184 stevesk 823: .Xr sshd_config 5 ,
1.200 jmc 824: .Xr inetd 8 ,
1.128 mpech 825: .Xr sftp-server 8
1.119 markus 826: .Rs
827: .%A T. Ylonen
828: .%A T. Kivinen
829: .%A M. Saarinen
830: .%A T. Rinne
831: .%A S. Lehtinen
832: .%T "SSH Protocol Architecture"
1.174 markus 833: .%N draft-ietf-secsh-architecture-12.txt
834: .%D January 2002
1.120 markus 835: .%O work in progress material
836: .Re
837: .Rs
838: .%A M. Friedl
839: .%A N. Provos
840: .%A W. A. Simpson
841: .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1.174 markus 842: .%N draft-ietf-secsh-dh-group-exchange-02.txt
843: .%D January 2002
1.119 markus 844: .%O work in progress material
845: .Re
1.198 jmc 846: .Sh AUTHORS
847: OpenSSH is a derivative of the original and free
848: ssh 1.2.12 release by Tatu Ylonen.
849: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
850: Theo de Raadt and Dug Song
851: removed many bugs, re-added newer features and
852: created OpenSSH.
853: Markus Friedl contributed the support for SSH
854: protocol versions 1.5 and 2.0.
855: Niels Provos and Markus Friedl contributed support
856: for privilege separation.
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