Annotation of src/usr.bin/ssh/sshd.8, Revision 1.187
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,
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35: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 36: .\"
1.187 ! marc 37: .\" $OpenBSD: sshd.8,v 1.186 2002/06/22 16:45:29 stevesk 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
1.150 stevesk 46: .Op Fl deiqtD46
1.2 deraadt 47: .Op Fl b Ar bits
48: .Op Fl f Ar config_file
49: .Op Fl g Ar login_grace_time
50: .Op Fl h Ar host_key_file
51: .Op Fl k Ar key_gen_time
1.156 markus 52: .Op Fl o Ar option
1.2 deraadt 53: .Op Fl p Ar port
1.61 markus 54: .Op Fl u Ar len
1.40 aaron 55: .Sh DESCRIPTION
1.2 deraadt 56: .Nm
1.106 deraadt 57: (SSH Daemon) is the daemon program for
1.2 deraadt 58: .Xr ssh 1 .
1.42 hugh 59: Together these programs replace rlogin and rsh, and
1.1 deraadt 60: provide secure encrypted communications between two untrusted hosts
1.36 aaron 61: over an insecure network.
62: The programs are intended to be as easy to
1.1 deraadt 63: install and use as possible.
1.2 deraadt 64: .Pp
65: .Nm
1.36 aaron 66: is the daemon that listens for connections from clients.
1.40 aaron 67: It is normally started at boot from
1.2 deraadt 68: .Pa /etc/rc .
69: It forks a new
1.36 aaron 70: daemon for each incoming connection.
71: The forked daemons handle
1.1 deraadt 72: key exchange, encryption, authentication, command execution,
73: and data exchange.
1.49 markus 74: This implementation of
75: .Nm
76: supports both SSH protocol version 1 and 2 simultaneously.
1.2 deraadt 77: .Nm
1.36 aaron 78: works as follows.
1.49 markus 79: .Pp
80: .Ss SSH protocol version 1
81: .Pp
1.36 aaron 82: Each host has a host-specific RSA key
83: (normally 1024 bits) used to identify the host.
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
1.42 hugh 89: Whenever a client connects the daemon responds with its public
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.
1.36 aaron 93: The client then generates a 256 bit random number.
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
1.2 deraadt 108: .Pa .rhosts
109: authentication,
110: .Pa .rhosts
111: authentication combined with RSA host
1.1 deraadt 112: authentication, RSA challenge-response authentication, or password
113: based authentication.
1.2 deraadt 114: .Pp
1.1 deraadt 115: Rhosts authentication is normally disabled
116: because it is fundamentally insecure, but can be enabled in the server
1.36 aaron 117: configuration file if desired.
118: System security is not improved unless
1.180 millert 119: .Nm rshd ,
120: .Nm rlogind ,
1.2 deraadt 121: and
1.180 millert 122: .Xr rexecd
1.1 deraadt 123: are disabled (thus completely disabling
1.180 millert 124: .Xr rlogin
1.1 deraadt 125: and
1.180 millert 126: .Xr rsh
1.42 hugh 127: into the machine).
1.2 deraadt 128: .Pp
1.49 markus 129: .Ss SSH protocol version 2
130: .Pp
1.58 deraadt 131: Version 2 works similarly:
1.138 markus 132: Each host has a host-specific key (RSA or DSA) used to identify the host.
1.49 markus 133: However, when the daemon starts, it does not generate a server key.
134: Forward security is provided through a Diffie-Hellman key agreement.
135: This key agreement results in a shared session key.
1.120 markus 136: .Pp
1.103 deraadt 137: The rest of the session is encrypted using a symmetric cipher, currently
1.120 markus 138: 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
1.49 markus 139: The client selects the encryption algorithm
140: to use from those offered by the server.
141: Additionally, session integrity is provided
1.51 hugh 142: through a cryptographic message authentication code
1.49 markus 143: (hmac-sha1 or hmac-md5).
144: .Pp
145: Protocol version 2 provides a public key based
1.120 markus 146: user (PubkeyAuthentication) or
147: client host (HostbasedAuthentication) authentication method,
148: conventional password authentication and challenge response based methods.
1.49 markus 149: .Pp
150: .Ss Command execution and data forwarding
151: .Pp
1.1 deraadt 152: If the client successfully authenticates itself, a dialog for
1.36 aaron 153: preparing the session is entered.
154: At this time the client may request
1.1 deraadt 155: things like allocating a pseudo-tty, forwarding X11 connections,
156: forwarding TCP/IP connections, or forwarding the authentication agent
157: connection over the secure channel.
1.2 deraadt 158: .Pp
1.1 deraadt 159: Finally, the client either requests a shell or execution of a command.
1.36 aaron 160: The sides then enter session mode.
161: In this mode, either side may send
1.1 deraadt 162: data at any time, and such data is forwarded to/from the shell or
163: command on the server side, and the user terminal in the client side.
1.2 deraadt 164: .Pp
1.1 deraadt 165: When the user program terminates and all forwarded X11 and other
166: connections have been closed, the server sends command exit status to
167: the client, and both sides exit.
1.2 deraadt 168: .Pp
169: .Nm
1.1 deraadt 170: can be configured using command-line options or a configuration
1.36 aaron 171: file.
172: Command-line options override values specified in the
1.1 deraadt 173: configuration file.
1.25 markus 174: .Pp
175: .Nm
176: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 177: .Dv SIGHUP ,
1.128 mpech 178: by executing itself with the name it was started as, i.e.,
1.97 deraadt 179: .Pa /usr/sbin/sshd .
1.18 aaron 180: .Pp
181: The options are as follows:
1.2 deraadt 182: .Bl -tag -width Ds
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).
1.2 deraadt 186: .It Fl d
1.36 aaron 187: Debug mode.
188: The server sends verbose debug output to the system
189: log, and does not put itself in the background.
190: The server also will not fork and will only process one connection.
191: This option is only intended for debugging for the server.
1.120 markus 192: Multiple -d options increase the debugging level.
1.67 aaron 193: Maximum is 3.
1.120 markus 194: .It Fl e
195: When this option is specified,
196: .Nm
197: will send the output to the standard error instead of the system log.
1.2 deraadt 198: .It Fl f Ar configuration_file
1.36 aaron 199: Specifies the name of the configuration file.
200: The default is
1.167 deraadt 201: .Pa /etc/ssh/sshd_config .
1.16 markus 202: .Nm
203: refuses to start if there is no configuration file.
1.2 deraadt 204: .It Fl g Ar login_grace_time
1.1 deraadt 205: Gives the grace time for clients to authenticate themselves (default
1.77 markus 206: 600 seconds).
1.36 aaron 207: If the client fails to authenticate the user within
208: this many seconds, the server disconnects and exits.
209: A value of zero indicates no limit.
1.2 deraadt 210: .It Fl h Ar host_key_file
1.160 stevesk 211: Specifies a file from which a host key is read.
1.7 markus 212: This option must be given if
213: .Nm
214: is not run as root (as the normal
1.160 stevesk 215: host key files are normally not readable by anyone but root).
216: The default is
1.167 deraadt 217: .Pa /etc/ssh/ssh_host_key
1.160 stevesk 218: for protocol version 1, and
1.167 deraadt 219: .Pa /etc/ssh/ssh_host_rsa_key
1.160 stevesk 220: and
1.167 deraadt 221: .Pa /etc/ssh/ssh_host_dsa_key
1.160 stevesk 222: for protocol version 2.
1.75 markus 223: It is possible to have multiple host key files for
1.120 markus 224: the different protocol versions and host key algorithms.
1.2 deraadt 225: .It Fl i
1.7 markus 226: Specifies that
227: .Nm
1.40 aaron 228: is being run from inetd.
1.7 markus 229: .Nm
230: is normally not run
1.1 deraadt 231: from inetd because it needs to generate the server key before it can
1.36 aaron 232: respond to the client, and this may take tens of seconds.
233: Clients would have to wait too long if the key was regenerated every time.
1.35 aaron 234: However, with small key sizes (e.g., 512) using
1.7 markus 235: .Nm
236: from inetd may
1.1 deraadt 237: be feasible.
1.2 deraadt 238: .It Fl k Ar key_gen_time
1.120 markus 239: Specifies how often the ephemeral protocol version 1 server key is
240: regenerated (default 3600 seconds, or one hour).
1.36 aaron 241: The motivation for regenerating the key fairly
1.1 deraadt 242: often is that the key is not stored anywhere, and after about an hour,
243: it becomes impossible to recover the key for decrypting intercepted
244: communications even if the machine is cracked into or physically
1.36 aaron 245: seized.
246: A value of zero indicates that the key will never be regenerated.
1.156 markus 247: .It Fl o Ar option
248: Can be used to give options in the format used in the configuration file.
249: This is useful for specifying options for which there is no separate
250: command-line flag.
1.2 deraadt 251: .It Fl p Ar port
1.1 deraadt 252: Specifies the port on which the server listens for connections
253: (default 22).
1.158 stevesk 254: Multiple port options are permitted.
255: Ports specified in the configuration file are ignored when a
256: command-line port is specified.
1.2 deraadt 257: .It Fl q
1.36 aaron 258: Quiet mode.
259: Nothing is sent to the system log.
260: Normally the beginning,
1.1 deraadt 261: authentication, and termination of each connection is logged.
1.137 stevesk 262: .It Fl t
263: Test mode.
264: Only check the validity of the configuration file and sanity of the keys.
1.157 deraadt 265: This is useful for updating
1.137 stevesk 266: .Nm
267: reliably as configuration options may change.
1.61 markus 268: .It Fl u Ar len
269: This option is used to specify the size of the field
270: in the
271: .Li utmp
272: structure that holds the remote host name.
273: If the resolved host name is longer than
274: .Ar len ,
275: the dotted decimal value will be used instead.
276: This allows hosts with very long host names that
277: overflow this field to still be uniquely identified.
278: Specifying
279: .Fl u0
280: indicates that only dotted decimal addresses
281: should be put into the
282: .Pa utmp
283: file.
1.144 stevesk 284: .Fl u0
285: is also be used to prevent
286: .Nm
287: from making DNS requests unless the authentication
288: mechanism or configuration requires it.
289: Authentication mechanisms that may require DNS include
290: .Cm RhostsAuthentication ,
291: .Cm RhostsRSAAuthentication ,
292: .Cm HostbasedAuthentication
293: and using a
294: .Cm from="pattern-list"
295: option in a key file.
1.170 stevesk 296: Configuration options that require DNS include using a
297: USER@HOST pattern in
298: .Cm AllowUsers
299: or
300: .Cm DenyUsers .
1.74 markus 301: .It Fl D
302: When this option is specified
303: .Nm
304: will not detach and does not become a daemon.
305: This allows easy monitoring of
1.76 markus 306: .Nm sshd .
1.29 markus 307: .It Fl 4
308: Forces
309: .Nm
310: to use IPv4 addresses only.
311: .It Fl 6
312: Forces
313: .Nm
314: to use IPv6 addresses only.
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
1.2 deraadt 335: .Pa $HOME/.hushlogin ;
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
351: Reads
352: .Pa $HOME/.ssh/environment
1.187 ! marc 353: if it exists and users are allowed to change their environment.
! 354: See
! 355: .Cm PermitUserEnvironment
! 356: in
! 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
362: .Pa $HOME/.ssh/rc
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
375: .Pa $HOME/.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.
392: The options fields
393: are optional; its presence is determined by whether the line starts
1.1 deraadt 394: with a number or not (the option 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
1.36 aaron 405: (because of the size of the RSA key modulus).
406: You don't want to type them in; instead, copy the
1.113 itojun 407: .Pa identity.pub ,
408: .Pa id_dsa.pub
1.75 markus 409: or the
1.113 itojun 410: .Pa id_rsa.pub
1.1 deraadt 411: file and edit it.
1.175 stevesk 412: .Pp
413: .Nm
414: enforces a minimum RSA key modulus size for protocol 1
415: and protocol 2 keys of 768 bits.
1.2 deraadt 416: .Pp
1.58 deraadt 417: The options (if present) consist of comma-separated option
1.36 aaron 418: specifications.
419: No spaces are permitted, except within double quotes.
1.141 stevesk 420: The following option specifications are supported (note
421: that option keywords are case-insensitive):
1.2 deraadt 422: .Bl -tag -width Ds
423: .It Cm from="pattern-list"
1.1 deraadt 424: Specifies that in addition to RSA authentication, the canonical name
425: of the remote host must be present in the comma-separated list of
1.36 aaron 426: patterns
427: .Pf ( Ql *
428: and
429: .Ql ?
430: serve as wildcards).
431: The list may also contain
432: patterns negated by prefixing them with
433: .Ql ! ;
434: if the canonical host name matches a negated pattern, the key is not accepted.
435: The purpose
1.1 deraadt 436: of this option is to optionally increase security: RSA authentication
437: by itself does not trust the network or name servers or anything (but
438: the key); however, if somebody somehow steals the key, the key
1.36 aaron 439: permits an intruder to log in from anywhere in the world.
440: This additional option makes using a stolen key more difficult (name
1.1 deraadt 441: servers and/or routers would have to be compromised in addition to
442: just the key).
1.2 deraadt 443: .It Cm command="command"
1.1 deraadt 444: Specifies that the command is executed whenever this key is used for
1.36 aaron 445: authentication.
446: The command supplied by the user (if any) is ignored.
1.148 markus 447: The command is run on a pty if the client requests a pty;
1.36 aaron 448: otherwise it is run without a tty.
1.147 deraadt 449: If a 8-bit clean channel is required,
450: one must not request a pty or should specify
1.89 markus 451: .Cm no-pty .
1.36 aaron 452: A quote may be included in the command by quoting it with a backslash.
453: This option might be useful
454: to restrict certain RSA keys to perform just a specific operation.
455: An example might be a key that permits remote backups but nothing else.
1.51 hugh 456: Note that the client may specify TCP/IP and/or X11
457: forwarding unless they are explicitly prohibited.
1.149 markus 458: Note that this option applies to shell, command or subsystem execution.
1.2 deraadt 459: .It Cm environment="NAME=value"
1.1 deraadt 460: Specifies that the string is to be added to the environment when
1.36 aaron 461: logging in using this key.
462: Environment variables set this way
463: override other default environment values.
464: Multiple options of this type are permitted.
1.155 markus 465: This option is automatically disabled if
466: .Cm UseLogin
467: is enabled.
1.2 deraadt 468: .It Cm no-port-forwarding
1.1 deraadt 469: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 470: Any port forward requests by the client will return an error.
471: This might be used, e.g., in connection with the
1.2 deraadt 472: .Cm command
1.1 deraadt 473: option.
1.2 deraadt 474: .It Cm no-X11-forwarding
1.1 deraadt 475: Forbids X11 forwarding when this key is used for authentication.
476: Any X11 forward requests by the client will return an error.
1.2 deraadt 477: .It Cm no-agent-forwarding
1.1 deraadt 478: Forbids authentication agent forwarding when this key is used for
479: authentication.
1.2 deraadt 480: .It Cm no-pty
1.1 deraadt 481: Prevents tty allocation (a request to allocate a pty will fail).
1.107 djm 482: .It Cm permitopen="host:port"
1.133 itojun 483: Limit local
1.107 djm 484: .Li ``ssh -L''
1.111 stevesk 485: port forwarding such that it may only connect to the specified host and
1.146 stevesk 486: port.
487: IPv6 addresses can be specified with an alternative syntax:
488: .Ar host/port .
489: Multiple
1.107 djm 490: .Cm permitopen
1.133 itojun 491: options may be applied separated by commas. No pattern matching is
492: performed on the specified hostnames, they must be literal domains or
1.107 djm 493: addresses.
1.2 deraadt 494: .El
495: .Ss Examples
1.1 deraadt 496: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 497: .Pp
1.1 deraadt 498: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 499: .Pp
1.1 deraadt 500: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.107 djm 501: .Pp
502: permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1.2 deraadt 503: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 504: The
1.167 deraadt 505: .Pa /etc/ssh/ssh_known_hosts ,
1.40 aaron 506: and
1.131 markus 507: .Pa $HOME/.ssh/known_hosts
1.36 aaron 508: files contain host public keys for all known hosts.
509: The global file should
1.37 brad 510: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 511: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 512: its key is added to the per-user file.
1.2 deraadt 513: .Pp
1.1 deraadt 514: Each line in these files contains the following fields: hostnames,
1.36 aaron 515: bits, exponent, modulus, comment.
516: The fields are separated by spaces.
1.2 deraadt 517: .Pp
1.1 deraadt 518: Hostnames is a comma-separated list of patterns ('*' and '?' act as
519: wildcards); each pattern in turn is matched against the canonical host
520: name (when authenticating a client) or against the user-supplied
1.36 aaron 521: name (when authenticating a server).
522: A pattern may also be preceded by
1.2 deraadt 523: .Ql !
524: to indicate negation: if the host name matches a negated
1.1 deraadt 525: pattern, it is not accepted (by that line) even if it matched another
526: pattern on the line.
1.2 deraadt 527: .Pp
1.49 markus 528: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 529: can be obtained, e.g., from
1.167 deraadt 530: .Pa /etc/ssh/ssh_host_key.pub .
1.1 deraadt 531: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 532: .Pp
533: Lines starting with
534: .Ql #
535: and empty lines are ignored as comments.
536: .Pp
1.1 deraadt 537: When performing host authentication, authentication is accepted if any
1.36 aaron 538: matching line has the proper key.
539: It is thus permissible (but not
1.1 deraadt 540: recommended) to have several lines or different host keys for the same
1.36 aaron 541: names.
542: This will inevitably happen when short forms of host names
543: from different domains are put in the file.
544: It is possible
1.1 deraadt 545: that the files contain conflicting information; authentication is
546: accepted if valid information can be found from either file.
1.2 deraadt 547: .Pp
1.1 deraadt 548: Note that the lines in these files are typically hundreds of characters
549: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 550: Rather, generate them by a script
1.40 aaron 551: or by taking
1.167 deraadt 552: .Pa /etc/ssh/ssh_host_key.pub
1.1 deraadt 553: and adding the host names at the front.
1.2 deraadt 554: .Ss Examples
1.120 markus 555: .Bd -literal
556: closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
557: cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
558: .Ed
1.2 deraadt 559: .Sh FILES
560: .Bl -tag -width Ds
1.167 deraadt 561: .It Pa /etc/ssh/sshd_config
1.1 deraadt 562: Contains configuration data for
1.2 deraadt 563: .Nm sshd .
1.184 stevesk 564: The file format and configuration options are described in
565: .Xr sshd_config 5 .
1.167 deraadt 566: .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1.120 markus 567: These three files contain the private parts of the host keys.
1.98 deraadt 568: These files should only be owned by root, readable only by root, and not
1.1 deraadt 569: accessible to others.
1.14 markus 570: Note that
571: .Nm
572: does not start if this file is group/world-accessible.
1.167 deraadt 573: .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 574: These three files contain the public parts of the host keys.
1.98 deraadt 575: These files should be world-readable but writable only by
1.36 aaron 576: root.
1.98 deraadt 577: Their contents should match the respective private parts.
578: These files are not
579: really used for anything; they are provided for the convenience of
580: the user so their contents can be copied to known hosts files.
581: These files are created using
1.7 markus 582: .Xr ssh-keygen 1 .
1.129 provos 583: .It Pa /etc/moduli
1.73 provos 584: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1.185 stevesk 585: .It Pa /var/empty
586: .Xr chroot 2
587: directory used by
588: .Nm
589: during privilege separation in the pre-authentication phase.
590: The directory should not contain any files and must be owned by root
591: and not group or world-writable.
1.2 deraadt 592: .It Pa /var/run/sshd.pid
593: Contains the process ID of the
594: .Nm
1.1 deraadt 595: listening for connections (if there are several daemons running
1.186 stevesk 596: concurrently for different ports, this contains the process ID of the one
1.36 aaron 597: started last).
1.58 deraadt 598: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 599: .It Pa $HOME/.ssh/authorized_keys
1.120 markus 600: Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1.44 deraadt 601: This file must be readable by root (which may on some machines imply
602: it being world-readable if the user's home directory resides on an NFS
603: volume).
604: It is recommended that it not be accessible by others.
605: The format of this file is described above.
606: Users will place the contents of their
1.130 markus 607: .Pa identity.pub ,
1.44 deraadt 608: .Pa id_dsa.pub
1.113 itojun 609: and/or
610: .Pa id_rsa.pub
1.44 deraadt 611: files into this file, as described in
612: .Xr ssh-keygen 1 .
1.167 deraadt 613: .It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1.22 markus 614: These files are consulted when using rhosts with RSA host
1.131 markus 615: authentication or protocol version 2 hostbased authentication
616: to check the public key of the host.
1.36 aaron 617: The key must be listed in one of these files to be accepted.
1.22 markus 618: The client uses the same files
1.96 markus 619: to verify that it is connecting to the correct remote host.
1.36 aaron 620: These files should be writable only by root/the owner.
1.167 deraadt 621: .Pa /etc/ssh/ssh_known_hosts
1.2 deraadt 622: should be world-readable, and
623: .Pa $HOME/.ssh/known_hosts
1.120 markus 624: can but need not be world-readable.
1.6 aaron 625: .It Pa /etc/nologin
1.40 aaron 626: If this file exists,
1.2 deraadt 627: .Nm
1.36 aaron 628: refuses to let anyone except root log in.
629: The contents of the file
1.1 deraadt 630: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 631: refused.
632: The file should be world-readable.
1.19 dugsong 633: .It Pa /etc/hosts.allow, /etc/hosts.deny
1.153 camield 634: Access controls that should be enforced by tcp-wrappers are defined here.
635: Further details are described in
1.19 dugsong 636: .Xr hosts_access 5 .
1.6 aaron 637: .It Pa $HOME/.rhosts
1.1 deraadt 638: This file contains host-username pairs, separated by a space, one per
1.36 aaron 639: line.
640: The given user on the corresponding host is permitted to log in
641: without password.
642: The same file is used by rlogind and rshd.
1.6 aaron 643: The file must
1.1 deraadt 644: be writable only by the user; it is recommended that it not be
645: accessible by others.
1.2 deraadt 646: .Pp
1.36 aaron 647: If is also possible to use netgroups in the file.
648: Either host or user
1.1 deraadt 649: name may be of the form +@groupname to specify all hosts or all users
650: in the group.
1.2 deraadt 651: .It Pa $HOME/.shosts
652: For ssh,
653: this file is exactly the same as for
654: .Pa .rhosts .
655: However, this file is
656: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 657: .It Pa /etc/hosts.equiv
1.2 deraadt 658: This file is used during
659: .Pa .rhosts
1.36 aaron 660: authentication.
661: In the simplest form, this file contains host names, one per line.
662: Users on
1.1 deraadt 663: those hosts are permitted to log in without a password, provided they
1.36 aaron 664: have the same user name on both machines.
665: The host name may also be
1.1 deraadt 666: followed by a user name; such users are permitted to log in as
1.2 deraadt 667: .Em any
1.36 aaron 668: user on this machine (except root).
669: Additionally, the syntax
1.2 deraadt 670: .Dq +@group
1.36 aaron 671: can be used to specify netgroups.
672: Negated entries start with
1.2 deraadt 673: .Ql \&- .
674: .Pp
1.1 deraadt 675: If the client host/user is successfully matched in this file, login is
676: automatically permitted provided the client and server user names are the
1.36 aaron 677: same.
678: Additionally, successful RSA host authentication is normally required.
679: This file must be writable only by root; it is recommended
1.1 deraadt 680: that it be world-readable.
1.2 deraadt 681: .Pp
1.6 aaron 682: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 683: .Pa hosts.equiv .
1.1 deraadt 684: Beware that it really means that the named user(s) can log in as
1.2 deraadt 685: .Em anybody ,
1.1 deraadt 686: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 687: binaries and directories.
688: Using a user name practically grants the user root access.
689: The only valid use for user names that I can think
1.1 deraadt 690: of is in negative entries.
1.2 deraadt 691: .Pp
692: Note that this warning also applies to rsh/rlogin.
693: .It Pa /etc/shosts.equiv
1.1 deraadt 694: This is processed exactly as
1.2 deraadt 695: .Pa /etc/hosts.equiv .
1.1 deraadt 696: However, this file may be useful in environments that want to run both
1.2 deraadt 697: rsh/rlogin and ssh.
1.6 aaron 698: .It Pa $HOME/.ssh/environment
1.36 aaron 699: This file is read into the environment at login (if it exists).
700: It can only contain empty lines, comment lines (that start with
1.2 deraadt 701: .Ql # ) ,
1.36 aaron 702: and assignment lines of the form name=value.
703: The file should be writable
1.6 aaron 704: only by the user; it need not be readable by anyone else.
1.2 deraadt 705: .It Pa $HOME/.ssh/rc
1.1 deraadt 706: If this file exists, it is run with /bin/sh after reading the
1.36 aaron 707: environment files but before starting the user's shell or command.
1.176 stevesk 708: It must not produce any output on stdout; stderr must be used
709: instead.
710: If X11 forwarding is in use, it will receive the "proto cookie" pair in
711: its standard input (and
1.2 deraadt 712: .Ev DISPLAY
1.176 stevesk 713: in its environment).
714: The script must call
1.2 deraadt 715: .Xr xauth 1
1.176 stevesk 716: because
717: .Nm
718: will not run xauth automatically to add X11 cookies.
1.2 deraadt 719: .Pp
1.1 deraadt 720: The primary purpose of this file is to run any initialization routines
721: which may be needed before the user's home directory becomes
722: accessible; AFS is a particular example of such an environment.
1.2 deraadt 723: .Pp
1.1 deraadt 724: This file will probably contain some initialization code followed by
1.120 markus 725: something similar to:
726: .Bd -literal
1.176 stevesk 727: if read proto cookie && [ -n "$DISPLAY" ]; then
728: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
729: # X11UseLocalhost=yes
730: xauth add unix:`echo $DISPLAY |
731: cut -c11-` $proto $cookie
732: else
733: # X11UseLocalhost=no
734: xauth add $DISPLAY $proto $cookie
1.120 markus 735: fi
1.176 stevesk 736: fi
1.120 markus 737: .Ed
1.2 deraadt 738: .Pp
739: If this file does not exist,
1.167 deraadt 740: .Pa /etc/ssh/sshrc
1.2 deraadt 741: is run, and if that
1.176 stevesk 742: does not exist either, xauth is used to add the cookie.
1.2 deraadt 743: .Pp
1.1 deraadt 744: This file should be writable only by the user, and need not be
745: readable by anyone else.
1.167 deraadt 746: .It Pa /etc/ssh/sshrc
1.2 deraadt 747: Like
748: .Pa $HOME/.ssh/rc .
749: This can be used to specify
1.36 aaron 750: machine-specific login-time initializations globally.
751: This file should be writable only by root, and should be world-readable.
1.56 aaron 752: .El
1.71 aaron 753: .Sh AUTHORS
1.84 markus 754: OpenSSH is a derivative of the original and free
755: ssh 1.2.12 release by Tatu Ylonen.
756: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
757: Theo de Raadt and Dug Song
758: removed many bugs, re-added newer features and
759: created OpenSSH.
760: Markus Friedl contributed the support for SSH
761: protocol versions 1.5 and 2.0.
1.171 provos 762: Niels Provos and Markus Friedl contributed support
763: for privilege separation.
1.2 deraadt 764: .Sh SEE ALSO
765: .Xr scp 1 ,
1.90 djm 766: .Xr sftp 1 ,
1.2 deraadt 767: .Xr ssh 1 ,
1.5 deraadt 768: .Xr ssh-add 1 ,
1.2 deraadt 769: .Xr ssh-agent 1 ,
1.5 deraadt 770: .Xr ssh-keygen 1 ,
1.136 markus 771: .Xr login.conf 5 ,
772: .Xr moduli 5 ,
1.184 stevesk 773: .Xr sshd_config 5 ,
1.128 mpech 774: .Xr sftp-server 8
1.119 markus 775: .Rs
776: .%A T. Ylonen
777: .%A T. Kivinen
778: .%A M. Saarinen
779: .%A T. Rinne
780: .%A S. Lehtinen
781: .%T "SSH Protocol Architecture"
1.174 markus 782: .%N draft-ietf-secsh-architecture-12.txt
783: .%D January 2002
1.120 markus 784: .%O work in progress material
785: .Re
786: .Rs
787: .%A M. Friedl
788: .%A N. Provos
789: .%A W. A. Simpson
790: .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1.174 markus 791: .%N draft-ietf-secsh-dh-group-exchange-02.txt
792: .%D January 2002
1.119 markus 793: .%O work in progress material
794: .Re
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