Annotation of src/usr.bin/ssh/sshd.8, Revision 1.196
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: .\"
1.196 ! david 37: .\" $OpenBSD: sshd.8,v 1.195 2003/04/30 01:16:20 mouring 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
1.150 stevesk 47: .Op Fl deiqtD46
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: .Pp
82: .Ss SSH protocol version 1
83: .Pp
1.36 aaron 84: Each host has a host-specific RSA key
85: (normally 1024 bits) used to identify the host.
86: Additionally, when
1.1 deraadt 87: the daemon starts, it generates a server RSA key (normally 768 bits).
88: This key is normally regenerated every hour if it has been used, and
89: is never stored on disk.
1.2 deraadt 90: .Pp
1.194 jmc 91: Whenever a client connects, the daemon responds with its public
1.42 hugh 92: host and server keys.
1.36 aaron 93: The client compares the
1.49 markus 94: RSA host key against its own database to verify that it has not changed.
1.36 aaron 95: The client then generates a 256 bit random number.
96: It encrypts this
1.1 deraadt 97: random number using both the host key and the server key, and sends
1.36 aaron 98: the encrypted number to the server.
1.42 hugh 99: Both sides then use this
1.1 deraadt 100: random number as a session key which is used to encrypt all further
1.36 aaron 101: communications in the session.
102: The rest of the session is encrypted
1.42 hugh 103: using a conventional cipher, currently Blowfish or 3DES, with 3DES
1.39 deraadt 104: being used by default.
1.36 aaron 105: The client selects the encryption algorithm
1.5 deraadt 106: to use from those offered by the server.
1.2 deraadt 107: .Pp
1.36 aaron 108: Next, the server and the client enter an authentication dialog.
109: The client tries to authenticate itself using
1.2 deraadt 110: .Pa .rhosts
111: authentication,
112: .Pa .rhosts
113: authentication combined with RSA host
1.1 deraadt 114: authentication, RSA challenge-response authentication, or password
115: based authentication.
1.2 deraadt 116: .Pp
1.1 deraadt 117: Rhosts authentication is normally disabled
118: because it is fundamentally insecure, but can be enabled in the server
1.36 aaron 119: configuration file if desired.
120: System security is not improved unless
1.180 millert 121: .Nm rshd ,
122: .Nm rlogind ,
1.2 deraadt 123: and
1.194 jmc 124: .Nm rexecd
1.1 deraadt 125: are disabled (thus completely disabling
1.180 millert 126: .Xr rlogin
1.1 deraadt 127: and
1.180 millert 128: .Xr rsh
1.42 hugh 129: into the machine).
1.2 deraadt 130: .Pp
1.49 markus 131: .Ss SSH protocol version 2
132: .Pp
1.58 deraadt 133: Version 2 works similarly:
1.138 markus 134: Each host has a host-specific key (RSA or DSA) used to identify the host.
1.49 markus 135: However, when the daemon starts, it does not generate a server key.
136: Forward security is provided through a Diffie-Hellman key agreement.
137: This key agreement results in a shared session key.
1.120 markus 138: .Pp
1.103 deraadt 139: The rest of the session is encrypted using a symmetric cipher, currently
1.120 markus 140: 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
1.49 markus 141: The client selects the encryption algorithm
142: to use from those offered by the server.
143: Additionally, session integrity is provided
1.51 hugh 144: through a cryptographic message authentication code
1.49 markus 145: (hmac-sha1 or hmac-md5).
146: .Pp
147: Protocol version 2 provides a public key based
1.120 markus 148: user (PubkeyAuthentication) or
149: client host (HostbasedAuthentication) authentication method,
150: conventional password authentication and challenge response based methods.
1.49 markus 151: .Pp
152: .Ss Command execution and data forwarding
153: .Pp
1.1 deraadt 154: If the client successfully authenticates itself, a dialog for
1.36 aaron 155: preparing the session is entered.
156: At this time the client may request
1.1 deraadt 157: things like allocating a pseudo-tty, forwarding X11 connections,
158: forwarding TCP/IP connections, or forwarding the authentication agent
159: connection over the secure channel.
1.2 deraadt 160: .Pp
1.1 deraadt 161: Finally, the client either requests a shell or execution of a command.
1.36 aaron 162: The sides then enter session mode.
163: In this mode, either side may send
1.1 deraadt 164: data at any time, and such data is forwarded to/from the shell or
165: command on the server side, and the user terminal in the client side.
1.2 deraadt 166: .Pp
1.1 deraadt 167: When the user program terminates and all forwarded X11 and other
168: connections have been closed, the server sends command exit status to
169: the client, and both sides exit.
1.2 deraadt 170: .Pp
171: .Nm
1.1 deraadt 172: can be configured using command-line options or a configuration
1.36 aaron 173: file.
174: Command-line options override values specified in the
1.1 deraadt 175: configuration file.
1.25 markus 176: .Pp
177: .Nm
178: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 179: .Dv SIGHUP ,
1.128 mpech 180: by executing itself with the name it was started as, i.e.,
1.97 deraadt 181: .Pa /usr/sbin/sshd .
1.18 aaron 182: .Pp
183: The options are as follows:
1.2 deraadt 184: .Bl -tag -width Ds
185: .It Fl b Ar bits
1.120 markus 186: Specifies the number of bits in the ephemeral protocol version 1
187: server key (default 768).
1.2 deraadt 188: .It Fl d
1.36 aaron 189: Debug mode.
190: The server sends verbose debug output to the system
191: log, and does not put itself in the background.
192: The server also will not fork and will only process one connection.
193: This option is only intended for debugging for the server.
1.194 jmc 194: Multiple
195: .Fl d
196: options increase the debugging level.
1.67 aaron 197: Maximum is 3.
1.120 markus 198: .It Fl e
199: When this option is specified,
200: .Nm
201: will send the output to the standard error instead of the system log.
1.2 deraadt 202: .It Fl f Ar configuration_file
1.36 aaron 203: Specifies the name of the configuration file.
204: The default is
1.167 deraadt 205: .Pa /etc/ssh/sshd_config .
1.16 markus 206: .Nm
207: refuses to start if there is no configuration file.
1.2 deraadt 208: .It Fl g Ar login_grace_time
1.1 deraadt 209: Gives the grace time for clients to authenticate themselves (default
1.191 stevesk 210: 120 seconds).
1.36 aaron 211: If the client fails to authenticate the user within
212: this many seconds, the server disconnects and exits.
213: A value of zero indicates no limit.
1.2 deraadt 214: .It Fl h Ar host_key_file
1.160 stevesk 215: Specifies a file from which a host key is read.
1.7 markus 216: This option must be given if
217: .Nm
218: is not run as root (as the normal
1.160 stevesk 219: host key files are normally not readable by anyone but root).
220: The default is
1.167 deraadt 221: .Pa /etc/ssh/ssh_host_key
1.160 stevesk 222: for protocol version 1, and
1.167 deraadt 223: .Pa /etc/ssh/ssh_host_rsa_key
1.160 stevesk 224: and
1.167 deraadt 225: .Pa /etc/ssh/ssh_host_dsa_key
1.160 stevesk 226: for protocol version 2.
1.75 markus 227: It is possible to have multiple host key files for
1.120 markus 228: the different protocol versions and host key algorithms.
1.2 deraadt 229: .It Fl i
1.7 markus 230: Specifies that
231: .Nm
1.194 jmc 232: is being run from
233: .Xr inetd 8 .
1.7 markus 234: .Nm
235: is normally not run
1.1 deraadt 236: from inetd because it needs to generate the server key before it can
1.36 aaron 237: respond to the client, and this may take tens of seconds.
238: Clients would have to wait too long if the key was regenerated every time.
1.35 aaron 239: However, with small key sizes (e.g., 512) using
1.7 markus 240: .Nm
241: from inetd may
1.1 deraadt 242: be feasible.
1.2 deraadt 243: .It Fl k Ar key_gen_time
1.120 markus 244: Specifies how often the ephemeral protocol version 1 server key is
245: regenerated (default 3600 seconds, or one hour).
1.36 aaron 246: The motivation for regenerating the key fairly
1.1 deraadt 247: often is that the key is not stored anywhere, and after about an hour,
248: it becomes impossible to recover the key for decrypting intercepted
249: communications even if the machine is cracked into or physically
1.36 aaron 250: seized.
251: A value of zero indicates that the key will never be regenerated.
1.156 markus 252: .It Fl o Ar option
253: Can be used to give options in the format used in the configuration file.
254: This is useful for specifying options for which there is no separate
255: command-line flag.
1.2 deraadt 256: .It Fl p Ar port
1.1 deraadt 257: Specifies the port on which the server listens for connections
258: (default 22).
1.158 stevesk 259: Multiple port options are permitted.
260: Ports specified in the configuration file are ignored when a
261: command-line port is specified.
1.2 deraadt 262: .It Fl q
1.36 aaron 263: Quiet mode.
264: Nothing is sent to the system log.
265: Normally the beginning,
1.1 deraadt 266: authentication, and termination of each connection is logged.
1.137 stevesk 267: .It Fl t
268: Test mode.
269: Only check the validity of the configuration file and sanity of the keys.
1.157 deraadt 270: This is useful for updating
1.137 stevesk 271: .Nm
272: reliably as configuration options may change.
1.61 markus 273: .It Fl u Ar len
274: This option is used to specify the size of the field
275: in the
276: .Li utmp
277: structure that holds the remote host name.
278: If the resolved host name is longer than
279: .Ar len ,
280: the dotted decimal value will be used instead.
281: This allows hosts with very long host names that
282: overflow this field to still be uniquely identified.
283: Specifying
284: .Fl u0
285: indicates that only dotted decimal addresses
286: should be put into the
287: .Pa utmp
288: file.
1.144 stevesk 289: .Fl u0
1.194 jmc 290: may also be used to prevent
1.144 stevesk 291: .Nm
292: from making DNS requests unless the authentication
293: mechanism or configuration requires it.
294: Authentication mechanisms that may require DNS include
295: .Cm RhostsAuthentication ,
296: .Cm RhostsRSAAuthentication ,
297: .Cm HostbasedAuthentication
298: and using a
299: .Cm from="pattern-list"
300: option in a key file.
1.170 stevesk 301: Configuration options that require DNS include using a
302: USER@HOST pattern in
303: .Cm AllowUsers
304: or
305: .Cm DenyUsers .
1.74 markus 306: .It Fl D
307: When this option is specified
308: .Nm
309: will not detach and does not become a daemon.
310: This allows easy monitoring of
1.76 markus 311: .Nm sshd .
1.29 markus 312: .It Fl 4
313: Forces
314: .Nm
315: to use IPv4 addresses only.
316: .It Fl 6
317: Forces
318: .Nm
319: to use IPv6 addresses only.
1.2 deraadt 320: .El
321: .Sh CONFIGURATION FILE
322: .Nm
1.40 aaron 323: reads configuration data from
1.167 deraadt 324: .Pa /etc/ssh/sshd_config
1.2 deraadt 325: (or the file specified with
326: .Fl f
1.36 aaron 327: on the command line).
1.184 stevesk 328: The file format and configuration options are described in
329: .Xr sshd_config 5 .
1.2 deraadt 330: .Sh LOGIN PROCESS
1.1 deraadt 331: When a user successfully logs in,
1.2 deraadt 332: .Nm
1.1 deraadt 333: does the following:
1.2 deraadt 334: .Bl -enum -offset indent
335: .It
1.1 deraadt 336: If the login is on a tty, and no command has been specified,
1.40 aaron 337: prints last login time and
1.2 deraadt 338: .Pa /etc/motd
1.1 deraadt 339: (unless prevented in the configuration file or by
1.2 deraadt 340: .Pa $HOME/.hushlogin ;
341: see the
1.40 aaron 342: .Sx FILES
1.2 deraadt 343: section).
344: .It
1.1 deraadt 345: If the login is on a tty, records login time.
1.2 deraadt 346: .It
347: Checks
348: .Pa /etc/nologin ;
349: if it exists, prints contents and quits
1.1 deraadt 350: (unless root).
1.2 deraadt 351: .It
1.1 deraadt 352: Changes to run with normal user privileges.
1.2 deraadt 353: .It
1.1 deraadt 354: Sets up basic environment.
1.2 deraadt 355: .It
356: Reads
357: .Pa $HOME/.ssh/environment
1.187 marc 358: if it exists and users are allowed to change their environment.
1.188 stevesk 359: See the
1.187 marc 360: .Cm PermitUserEnvironment
1.188 stevesk 361: option in
1.187 marc 362: .Xr sshd_config 5 .
1.2 deraadt 363: .It
1.1 deraadt 364: Changes to user's home directory.
1.2 deraadt 365: .It
366: If
367: .Pa $HOME/.ssh/rc
368: exists, runs it; else if
1.168 deraadt 369: .Pa /etc/ssh/sshrc
1.2 deraadt 370: exists, runs
1.36 aaron 371: it; otherwise runs xauth.
372: The
1.2 deraadt 373: .Dq rc
374: files are given the X11
1.1 deraadt 375: authentication protocol and cookie in standard input.
1.2 deraadt 376: .It
1.1 deraadt 377: Runs user's shell or command.
1.2 deraadt 378: .El
379: .Sh AUTHORIZED_KEYS FILE FORMAT
380: .Pa $HOME/.ssh/authorized_keys
1.130 markus 381: is the default file that lists the public keys that are
382: permitted for RSA authentication in protocol version 1
383: and for public key authentication (PubkeyAuthentication)
384: in protocol version 2.
1.125 markus 385: .Cm AuthorizedKeysFile
386: may be used to specify an alternative file.
1.75 markus 387: .Pp
1.36 aaron 388: Each line of the file contains one
1.2 deraadt 389: key (empty lines and lines starting with a
390: .Ql #
391: are ignored as
1.36 aaron 392: comments).
1.75 markus 393: Each RSA public key consists of the following fields, separated by
1.36 aaron 394: spaces: options, bits, exponent, modulus, comment.
1.75 markus 395: Each protocol version 2 public key consists of:
396: options, keytype, base64 encoded key, comment.
1.189 espie 397: The options field
398: is optional; its presence is determined by whether the line starts
399: with a number or not (the options field never starts with a number).
1.75 markus 400: The bits, exponent, modulus and comment fields give the RSA key for
401: protocol version 1; the
1.1 deraadt 402: comment field is not used for anything (but may be convenient for the
403: user to identify the key).
1.75 markus 404: For protocol version 2 the keytype is
405: .Dq ssh-dss
406: or
407: .Dq ssh-rsa .
1.2 deraadt 408: .Pp
1.1 deraadt 409: Note that lines in this file are usually several hundred bytes long
1.189 espie 410: (because of the size of the public key encoding).
1.36 aaron 411: You don't want to type them in; instead, copy the
1.113 itojun 412: .Pa identity.pub ,
413: .Pa id_dsa.pub
1.75 markus 414: or the
1.113 itojun 415: .Pa id_rsa.pub
1.1 deraadt 416: file and edit it.
1.175 stevesk 417: .Pp
418: .Nm
419: enforces a minimum RSA key modulus size for protocol 1
420: and protocol 2 keys of 768 bits.
1.2 deraadt 421: .Pp
1.58 deraadt 422: The options (if present) consist of comma-separated option
1.36 aaron 423: specifications.
424: No spaces are permitted, except within double quotes.
1.141 stevesk 425: The following option specifications are supported (note
426: that option keywords are case-insensitive):
1.2 deraadt 427: .Bl -tag -width Ds
428: .It Cm from="pattern-list"
1.189 espie 429: Specifies that in addition to public key authentication, the canonical name
1.1 deraadt 430: of the remote host must be present in the comma-separated list of
1.36 aaron 431: patterns
1.196 ! david 432: .Pf ( Ql \&*
1.36 aaron 433: and
1.195 mouring 434: .Ql \&?
1.36 aaron 435: serve as wildcards).
436: The list may also contain
437: patterns negated by prefixing them with
1.195 mouring 438: .Ql \&! ;
1.36 aaron 439: if the canonical host name matches a negated pattern, the key is not accepted.
440: The purpose
1.189 espie 441: of this option is to optionally increase security: public key authentication
1.1 deraadt 442: by itself does not trust the network or name servers or anything (but
443: the key); however, if somebody somehow steals the key, the key
1.36 aaron 444: permits an intruder to log in from anywhere in the world.
445: This additional option makes using a stolen key more difficult (name
1.1 deraadt 446: servers and/or routers would have to be compromised in addition to
447: just the key).
1.2 deraadt 448: .It Cm command="command"
1.1 deraadt 449: Specifies that the command is executed whenever this key is used for
1.36 aaron 450: authentication.
451: The command supplied by the user (if any) is ignored.
1.148 markus 452: The command is run on a pty if the client requests a pty;
1.36 aaron 453: otherwise it is run without a tty.
1.194 jmc 454: If an 8-bit clean channel is required,
1.147 deraadt 455: one must not request a pty or should specify
1.89 markus 456: .Cm no-pty .
1.36 aaron 457: A quote may be included in the command by quoting it with a backslash.
458: This option might be useful
1.189 espie 459: to restrict certain public keys to perform just a specific operation.
1.36 aaron 460: An example might be a key that permits remote backups but nothing else.
1.51 hugh 461: Note that the client may specify TCP/IP and/or X11
462: forwarding unless they are explicitly prohibited.
1.149 markus 463: Note that this option applies to shell, command or subsystem execution.
1.2 deraadt 464: .It Cm environment="NAME=value"
1.1 deraadt 465: Specifies that the string is to be added to the environment when
1.36 aaron 466: logging in using this key.
467: Environment variables set this way
468: override other default environment values.
469: Multiple options of this type are permitted.
1.188 stevesk 470: Environment processing is disabled by default and is
471: controlled via the
472: .Cm PermitUserEnvironment
473: option.
1.155 markus 474: This option is automatically disabled if
475: .Cm UseLogin
476: is enabled.
1.2 deraadt 477: .It Cm no-port-forwarding
1.1 deraadt 478: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 479: Any port forward requests by the client will return an error.
480: This might be used, e.g., in connection with the
1.2 deraadt 481: .Cm command
1.1 deraadt 482: option.
1.2 deraadt 483: .It Cm no-X11-forwarding
1.1 deraadt 484: Forbids X11 forwarding when this key is used for authentication.
485: Any X11 forward requests by the client will return an error.
1.2 deraadt 486: .It Cm no-agent-forwarding
1.1 deraadt 487: Forbids authentication agent forwarding when this key is used for
488: authentication.
1.2 deraadt 489: .It Cm no-pty
1.1 deraadt 490: Prevents tty allocation (a request to allocate a pty will fail).
1.107 djm 491: .It Cm permitopen="host:port"
1.133 itojun 492: Limit local
1.107 djm 493: .Li ``ssh -L''
1.111 stevesk 494: port forwarding such that it may only connect to the specified host and
1.146 stevesk 495: port.
496: IPv6 addresses can be specified with an alternative syntax:
497: .Ar host/port .
498: Multiple
1.107 djm 499: .Cm permitopen
1.133 itojun 500: options may be applied separated by commas. No pattern matching is
501: performed on the specified hostnames, they must be literal domains or
1.107 djm 502: addresses.
1.2 deraadt 503: .El
504: .Ss Examples
1.1 deraadt 505: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 506: .Pp
1.1 deraadt 507: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 508: .Pp
1.1 deraadt 509: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.107 djm 510: .Pp
511: permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1.2 deraadt 512: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 513: The
1.194 jmc 514: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 515: and
1.131 markus 516: .Pa $HOME/.ssh/known_hosts
1.36 aaron 517: files contain host public keys for all known hosts.
518: The global file should
1.37 brad 519: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 520: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 521: its key is added to the per-user file.
1.2 deraadt 522: .Pp
1.1 deraadt 523: Each line in these files contains the following fields: hostnames,
1.36 aaron 524: bits, exponent, modulus, comment.
525: The fields are separated by spaces.
1.2 deraadt 526: .Pp
1.196 ! david 527: Hostnames is a comma-separated list of patterns
! 528: .Pf ( Ql \&*
1.195 mouring 529: and
530: .Ql \&?
531: act as
1.1 deraadt 532: wildcards); each pattern in turn is matched against the canonical host
533: name (when authenticating a client) or against the user-supplied
1.36 aaron 534: name (when authenticating a server).
535: A pattern may also be preceded by
1.195 mouring 536: .Ql \&!
1.2 deraadt 537: to indicate negation: if the host name matches a negated
1.1 deraadt 538: pattern, it is not accepted (by that line) even if it matched another
539: pattern on the line.
1.2 deraadt 540: .Pp
1.49 markus 541: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 542: can be obtained, e.g., from
1.167 deraadt 543: .Pa /etc/ssh/ssh_host_key.pub .
1.1 deraadt 544: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 545: .Pp
546: Lines starting with
547: .Ql #
548: and empty lines are ignored as comments.
549: .Pp
1.1 deraadt 550: When performing host authentication, authentication is accepted if any
1.36 aaron 551: matching line has the proper key.
552: It is thus permissible (but not
1.1 deraadt 553: recommended) to have several lines or different host keys for the same
1.36 aaron 554: names.
555: This will inevitably happen when short forms of host names
556: from different domains are put in the file.
557: It is possible
1.1 deraadt 558: that the files contain conflicting information; authentication is
559: accepted if valid information can be found from either file.
1.2 deraadt 560: .Pp
1.1 deraadt 561: Note that the lines in these files are typically hundreds of characters
562: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 563: Rather, generate them by a script
1.40 aaron 564: or by taking
1.167 deraadt 565: .Pa /etc/ssh/ssh_host_key.pub
1.1 deraadt 566: and adding the host names at the front.
1.2 deraadt 567: .Ss Examples
1.120 markus 568: .Bd -literal
569: closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
570: cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
571: .Ed
1.2 deraadt 572: .Sh FILES
573: .Bl -tag -width Ds
1.167 deraadt 574: .It Pa /etc/ssh/sshd_config
1.1 deraadt 575: Contains configuration data for
1.2 deraadt 576: .Nm sshd .
1.184 stevesk 577: The file format and configuration options are described in
578: .Xr sshd_config 5 .
1.167 deraadt 579: .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1.120 markus 580: These three files contain the private parts of the host keys.
1.98 deraadt 581: These files should only be owned by root, readable only by root, and not
1.1 deraadt 582: accessible to others.
1.14 markus 583: Note that
584: .Nm
585: does not start if this file is group/world-accessible.
1.167 deraadt 586: .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 587: These three files contain the public parts of the host keys.
1.98 deraadt 588: These files should be world-readable but writable only by
1.36 aaron 589: root.
1.98 deraadt 590: Their contents should match the respective private parts.
591: These files are not
592: really used for anything; they are provided for the convenience of
593: the user so their contents can be copied to known hosts files.
594: These files are created using
1.7 markus 595: .Xr ssh-keygen 1 .
1.129 provos 596: .It Pa /etc/moduli
1.73 provos 597: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1.192 stevesk 598: The file format is described in
599: .Xr moduli 5 .
1.185 stevesk 600: .It Pa /var/empty
601: .Xr chroot 2
602: directory used by
603: .Nm
604: during privilege separation in the pre-authentication phase.
605: The directory should not contain any files and must be owned by root
606: and not group or world-writable.
1.2 deraadt 607: .It Pa /var/run/sshd.pid
608: Contains the process ID of the
609: .Nm
1.1 deraadt 610: listening for connections (if there are several daemons running
1.186 stevesk 611: concurrently for different ports, this contains the process ID of the one
1.36 aaron 612: started last).
1.58 deraadt 613: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 614: .It Pa $HOME/.ssh/authorized_keys
1.120 markus 615: Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1.44 deraadt 616: This file must be readable by root (which may on some machines imply
617: it being world-readable if the user's home directory resides on an NFS
618: volume).
619: It is recommended that it not be accessible by others.
620: The format of this file is described above.
621: Users will place the contents of their
1.130 markus 622: .Pa identity.pub ,
1.44 deraadt 623: .Pa id_dsa.pub
1.113 itojun 624: and/or
625: .Pa id_rsa.pub
1.44 deraadt 626: files into this file, as described in
627: .Xr ssh-keygen 1 .
1.167 deraadt 628: .It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1.22 markus 629: These files are consulted when using rhosts with RSA host
1.131 markus 630: authentication or protocol version 2 hostbased authentication
631: to check the public key of the host.
1.36 aaron 632: The key must be listed in one of these files to be accepted.
1.22 markus 633: The client uses the same files
1.96 markus 634: to verify that it is connecting to the correct remote host.
1.36 aaron 635: These files should be writable only by root/the owner.
1.167 deraadt 636: .Pa /etc/ssh/ssh_known_hosts
1.2 deraadt 637: should be world-readable, and
638: .Pa $HOME/.ssh/known_hosts
1.194 jmc 639: can, but need not be, world-readable.
1.6 aaron 640: .It Pa /etc/nologin
1.40 aaron 641: If this file exists,
1.2 deraadt 642: .Nm
1.36 aaron 643: refuses to let anyone except root log in.
644: The contents of the file
1.1 deraadt 645: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 646: refused.
647: The file should be world-readable.
1.19 dugsong 648: .It Pa /etc/hosts.allow, /etc/hosts.deny
1.153 camield 649: Access controls that should be enforced by tcp-wrappers are defined here.
650: Further details are described in
1.19 dugsong 651: .Xr hosts_access 5 .
1.6 aaron 652: .It Pa $HOME/.rhosts
1.1 deraadt 653: This file contains host-username pairs, separated by a space, one per
1.36 aaron 654: line.
655: The given user on the corresponding host is permitted to log in
1.194 jmc 656: without a password.
1.36 aaron 657: The same file is used by rlogind and rshd.
1.6 aaron 658: The file must
1.1 deraadt 659: be writable only by the user; it is recommended that it not be
660: accessible by others.
1.2 deraadt 661: .Pp
1.36 aaron 662: If is also possible to use netgroups in the file.
663: Either host or user
1.1 deraadt 664: name may be of the form +@groupname to specify all hosts or all users
665: in the group.
1.2 deraadt 666: .It Pa $HOME/.shosts
667: For ssh,
668: this file is exactly the same as for
669: .Pa .rhosts .
670: However, this file is
671: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 672: .It Pa /etc/hosts.equiv
1.2 deraadt 673: This file is used during
674: .Pa .rhosts
1.36 aaron 675: authentication.
676: In the simplest form, this file contains host names, one per line.
677: Users on
1.1 deraadt 678: those hosts are permitted to log in without a password, provided they
1.36 aaron 679: have the same user name on both machines.
680: The host name may also be
1.1 deraadt 681: followed by a user name; such users are permitted to log in as
1.2 deraadt 682: .Em any
1.36 aaron 683: user on this machine (except root).
684: Additionally, the syntax
1.2 deraadt 685: .Dq +@group
1.36 aaron 686: can be used to specify netgroups.
687: Negated entries start with
1.2 deraadt 688: .Ql \&- .
689: .Pp
1.1 deraadt 690: If the client host/user is successfully matched in this file, login is
691: automatically permitted provided the client and server user names are the
1.36 aaron 692: same.
693: Additionally, successful RSA host authentication is normally required.
694: This file must be writable only by root; it is recommended
1.1 deraadt 695: that it be world-readable.
1.2 deraadt 696: .Pp
1.6 aaron 697: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 698: .Pa hosts.equiv .
1.1 deraadt 699: Beware that it really means that the named user(s) can log in as
1.2 deraadt 700: .Em anybody ,
1.1 deraadt 701: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 702: binaries and directories.
703: Using a user name practically grants the user root access.
704: The only valid use for user names that I can think
1.1 deraadt 705: of is in negative entries.
1.2 deraadt 706: .Pp
707: Note that this warning also applies to rsh/rlogin.
708: .It Pa /etc/shosts.equiv
1.1 deraadt 709: This is processed exactly as
1.2 deraadt 710: .Pa /etc/hosts.equiv .
1.1 deraadt 711: However, this file may be useful in environments that want to run both
1.2 deraadt 712: rsh/rlogin and ssh.
1.6 aaron 713: .It Pa $HOME/.ssh/environment
1.36 aaron 714: This file is read into the environment at login (if it exists).
715: It can only contain empty lines, comment lines (that start with
1.2 deraadt 716: .Ql # ) ,
1.36 aaron 717: and assignment lines of the form name=value.
718: The file should be writable
1.6 aaron 719: only by the user; it need not be readable by anyone else.
1.188 stevesk 720: Environment processing is disabled by default and is
721: controlled via the
722: .Cm PermitUserEnvironment
723: option.
1.2 deraadt 724: .It Pa $HOME/.ssh/rc
1.194 jmc 725: If this file exists, it is run with
726: .Pa /bin/sh
727: after reading the
1.36 aaron 728: environment files but before starting the user's shell or command.
1.176 stevesk 729: It must not produce any output on stdout; stderr must be used
730: instead.
731: If X11 forwarding is in use, it will receive the "proto cookie" pair in
732: its standard input (and
1.2 deraadt 733: .Ev DISPLAY
1.176 stevesk 734: in its environment).
735: The script must call
1.2 deraadt 736: .Xr xauth 1
1.176 stevesk 737: because
738: .Nm
739: will not run xauth automatically to add X11 cookies.
1.2 deraadt 740: .Pp
1.1 deraadt 741: The primary purpose of this file is to run any initialization routines
742: which may be needed before the user's home directory becomes
743: accessible; AFS is a particular example of such an environment.
1.2 deraadt 744: .Pp
1.1 deraadt 745: This file will probably contain some initialization code followed by
1.120 markus 746: something similar to:
747: .Bd -literal
1.176 stevesk 748: if read proto cookie && [ -n "$DISPLAY" ]; then
749: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
750: # X11UseLocalhost=yes
1.193 todd 751: echo add unix:`echo $DISPLAY |
1.176 stevesk 752: cut -c11-` $proto $cookie
753: else
754: # X11UseLocalhost=no
1.193 todd 755: echo add $DISPLAY $proto $cookie
756: fi | xauth -q -
1.176 stevesk 757: fi
1.120 markus 758: .Ed
1.2 deraadt 759: .Pp
760: If this file does not exist,
1.167 deraadt 761: .Pa /etc/ssh/sshrc
1.2 deraadt 762: is run, and if that
1.176 stevesk 763: does not exist either, xauth is used to add the cookie.
1.2 deraadt 764: .Pp
1.1 deraadt 765: This file should be writable only by the user, and need not be
766: readable by anyone else.
1.167 deraadt 767: .It Pa /etc/ssh/sshrc
1.2 deraadt 768: Like
769: .Pa $HOME/.ssh/rc .
770: This can be used to specify
1.36 aaron 771: machine-specific login-time initializations globally.
772: This file should be writable only by root, and should be world-readable.
1.56 aaron 773: .El
1.71 aaron 774: .Sh AUTHORS
1.84 markus 775: OpenSSH is a derivative of the original and free
776: ssh 1.2.12 release by Tatu Ylonen.
777: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
778: Theo de Raadt and Dug Song
779: removed many bugs, re-added newer features and
780: created OpenSSH.
781: Markus Friedl contributed the support for SSH
782: protocol versions 1.5 and 2.0.
1.171 provos 783: Niels Provos and Markus Friedl contributed support
784: for privilege separation.
1.2 deraadt 785: .Sh SEE ALSO
786: .Xr scp 1 ,
1.90 djm 787: .Xr sftp 1 ,
1.2 deraadt 788: .Xr ssh 1 ,
1.5 deraadt 789: .Xr ssh-add 1 ,
1.2 deraadt 790: .Xr ssh-agent 1 ,
1.5 deraadt 791: .Xr ssh-keygen 1 ,
1.136 markus 792: .Xr login.conf 5 ,
793: .Xr moduli 5 ,
1.184 stevesk 794: .Xr sshd_config 5 ,
1.128 mpech 795: .Xr sftp-server 8
1.119 markus 796: .Rs
797: .%A T. Ylonen
798: .%A T. Kivinen
799: .%A M. Saarinen
800: .%A T. Rinne
801: .%A S. Lehtinen
802: .%T "SSH Protocol Architecture"
1.174 markus 803: .%N draft-ietf-secsh-architecture-12.txt
804: .%D January 2002
1.120 markus 805: .%O work in progress material
806: .Re
807: .Rs
808: .%A M. Friedl
809: .%A N. Provos
810: .%A W. A. Simpson
811: .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1.174 markus 812: .%N draft-ietf-secsh-dh-group-exchange-02.txt
813: .%D January 2002
1.119 markus 814: .%O work in progress material
815: .Re
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