Annotation of src/usr.bin/ssh/sshd.8, Revision 1.206.2.2
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: .\"
1.206.2.2! brad 37: .\" $OpenBSD: sshd.8,v 1.215 2006/02/01 09:11:41 jmc 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
1.206.2.2! brad 59: (OpenSSH 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.
1.2 deraadt 64: .Pp
65: .Nm
1.206.2.2! brad 66: 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.
1.2 deraadt 74: .Pp
75: .Nm
1.200 jmc 76: can be configured using command-line options or a configuration file
77: (by default
1.206.2.2! brad 78: .Xr sshd_config 5 ) ;
! 79: command-line options override values specified in the
1.1 deraadt 80: configuration file.
1.25 markus 81: .Nm
82: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 83: .Dv SIGHUP ,
1.201 dtucker 84: by executing itself with the name and options it was started with, e.g.,
1.97 deraadt 85: .Pa /usr/sbin/sshd .
1.18 aaron 86: .Pp
87: The options are as follows:
1.2 deraadt 88: .Bl -tag -width Ds
1.200 jmc 89: .It Fl 4
90: Forces
91: .Nm
92: to use IPv4 addresses only.
93: .It Fl 6
94: Forces
95: .Nm
96: to use IPv6 addresses only.
1.2 deraadt 97: .It Fl b Ar bits
1.120 markus 98: Specifies the number of bits in the ephemeral protocol version 1
99: server key (default 768).
1.200 jmc 100: .It Fl D
101: When this option is specified,
102: .Nm
103: will not detach and does not become a daemon.
104: This allows easy monitoring of
105: .Nm sshd .
1.2 deraadt 106: .It Fl d
1.36 aaron 107: Debug mode.
108: The server sends verbose debug output to the system
109: log, and does not put itself in the background.
110: The server also will not fork and will only process one connection.
111: This option is only intended for debugging for the server.
1.194 jmc 112: Multiple
113: .Fl d
114: options increase the debugging level.
1.67 aaron 115: Maximum is 3.
1.120 markus 116: .It Fl e
117: When this option is specified,
118: .Nm
119: will send the output to the standard error instead of the system log.
1.2 deraadt 120: .It Fl f Ar configuration_file
1.36 aaron 121: Specifies the name of the configuration file.
122: The default is
1.167 deraadt 123: .Pa /etc/ssh/sshd_config .
1.16 markus 124: .Nm
125: refuses to start if there is no configuration file.
1.2 deraadt 126: .It Fl g Ar login_grace_time
1.1 deraadt 127: Gives the grace time for clients to authenticate themselves (default
1.191 stevesk 128: 120 seconds).
1.36 aaron 129: If the client fails to authenticate the user within
130: this many seconds, the server disconnects and exits.
131: A value of zero indicates no limit.
1.2 deraadt 132: .It Fl h Ar host_key_file
1.160 stevesk 133: Specifies a file from which a host key is read.
1.7 markus 134: This option must be given if
135: .Nm
136: is not run as root (as the normal
1.160 stevesk 137: host key files are normally not readable by anyone but root).
138: The default is
1.167 deraadt 139: .Pa /etc/ssh/ssh_host_key
1.160 stevesk 140: for protocol version 1, and
1.167 deraadt 141: .Pa /etc/ssh/ssh_host_rsa_key
1.160 stevesk 142: and
1.167 deraadt 143: .Pa /etc/ssh/ssh_host_dsa_key
1.160 stevesk 144: for protocol version 2.
1.75 markus 145: It is possible to have multiple host key files for
1.120 markus 146: the different protocol versions and host key algorithms.
1.2 deraadt 147: .It Fl i
1.7 markus 148: Specifies that
149: .Nm
1.194 jmc 150: is being run from
151: .Xr inetd 8 .
1.7 markus 152: .Nm
153: is normally not run
1.1 deraadt 154: from inetd because it needs to generate the server key before it can
1.36 aaron 155: respond to the client, and this may take tens of seconds.
156: Clients would have to wait too long if the key was regenerated every time.
1.35 aaron 157: However, with small key sizes (e.g., 512) using
1.7 markus 158: .Nm
159: from inetd may
1.1 deraadt 160: be feasible.
1.2 deraadt 161: .It Fl k Ar key_gen_time
1.120 markus 162: Specifies how often the ephemeral protocol version 1 server key is
163: regenerated (default 3600 seconds, or one hour).
1.36 aaron 164: The motivation for regenerating the key fairly
1.200 jmc 165: often is that the key is not stored anywhere, and after about an hour
1.1 deraadt 166: it becomes impossible to recover the key for decrypting intercepted
167: communications even if the machine is cracked into or physically
1.36 aaron 168: seized.
169: A value of zero indicates that the key will never be regenerated.
1.156 markus 170: .It Fl o Ar option
171: Can be used to give options in the format used in the configuration file.
172: This is useful for specifying options for which there is no separate
173: command-line flag.
1.200 jmc 174: For full details of the options, and their values, see
175: .Xr sshd_config 5 .
1.2 deraadt 176: .It Fl p Ar port
1.1 deraadt 177: Specifies the port on which the server listens for connections
178: (default 22).
1.158 stevesk 179: Multiple port options are permitted.
1.206.2.2! brad 180: Ports specified in the configuration file with the
! 181: .Cm Port
! 182: option are ignored when a command-line port is specified.
! 183: Ports specified using the
! 184: .Cm ListenAddress
! 185: option override command-line ports.
1.2 deraadt 186: .It Fl q
1.36 aaron 187: Quiet mode.
188: Nothing is sent to the system log.
189: Normally the beginning,
1.1 deraadt 190: authentication, and termination of each connection is logged.
1.137 stevesk 191: .It Fl t
192: Test mode.
193: Only check the validity of the configuration file and sanity of the keys.
1.157 deraadt 194: This is useful for updating
1.137 stevesk 195: .Nm
196: reliably as configuration options may change.
1.61 markus 197: .It Fl u Ar len
198: This option is used to specify the size of the field
199: in the
200: .Li utmp
201: structure that holds the remote host name.
202: If the resolved host name is longer than
203: .Ar len ,
204: the dotted decimal value will be used instead.
205: This allows hosts with very long host names that
206: overflow this field to still be uniquely identified.
207: Specifying
208: .Fl u0
209: indicates that only dotted decimal addresses
210: should be put into the
211: .Pa utmp
212: file.
1.144 stevesk 213: .Fl u0
1.194 jmc 214: may also be used to prevent
1.144 stevesk 215: .Nm
216: from making DNS requests unless the authentication
217: mechanism or configuration requires it.
218: Authentication mechanisms that may require DNS include
219: .Cm RhostsRSAAuthentication ,
1.206.2.2! brad 220: .Cm HostbasedAuthentication ,
1.144 stevesk 221: and using a
222: .Cm from="pattern-list"
223: option in a key file.
1.170 stevesk 224: Configuration options that require DNS include using a
225: USER@HOST pattern in
226: .Cm AllowUsers
227: or
228: .Cm DenyUsers .
1.2 deraadt 229: .El
1.206.2.2! brad 230: .Sh AUTHENTICATION
! 231: The OpenSSH SSH daemon supports SSH protocols 1 and 2.
! 232: Both protocols are supported by default,
! 233: though this can be changed via the
! 234: .Cm Protocol
! 235: option in
1.184 stevesk 236: .Xr sshd_config 5 .
1.206.2.2! brad 237: Protocol 2 supports both RSA and DSA keys;
! 238: protocol 1 only supports RSA keys.
! 239: For both protocols,
! 240: each host has a host-specific key,
! 241: normally 2048 bits,
! 242: used to identify the host.
! 243: .Pp
! 244: Forward security for protocol 1 is provided through
! 245: an additional server key,
! 246: normally 768 bits,
! 247: generated when the server starts.
! 248: This key is normally regenerated every hour if it has been used, and
! 249: is never stored on disk.
! 250: Whenever a client connects, the daemon responds with its public
! 251: host and server keys.
! 252: The client compares the
! 253: RSA host key against its own database to verify that it has not changed.
! 254: The client then generates a 256-bit random number.
! 255: It encrypts this
! 256: random number using both the host key and the server key, and sends
! 257: the encrypted number to the server.
! 258: Both sides then use this
! 259: random number as a session key which is used to encrypt all further
! 260: communications in the session.
! 261: The rest of the session is encrypted
! 262: using a conventional cipher, currently Blowfish or 3DES, with 3DES
! 263: being used by default.
! 264: The client selects the encryption algorithm
! 265: to use from those offered by the server.
! 266: .Pp
! 267: For protocol 2,
! 268: forward security is provided through a Diffie-Hellman key agreement.
! 269: This key agreement results in a shared session key.
! 270: The rest of the session is encrypted using a symmetric cipher, currently
! 271: 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
! 272: The client selects the encryption algorithm
! 273: to use from those offered by the server.
! 274: Additionally, session integrity is provided
! 275: through a cryptographic message authentication code
! 276: (hmac-sha1 or hmac-md5).
! 277: .Pp
! 278: Finally, the server and the client enter an authentication dialog.
! 279: The client tries to authenticate itself using
! 280: host-based authentication,
! 281: public key authentication,
! 282: challenge-response authentication,
! 283: or password authentication.
! 284: .Pp
! 285: System security is not improved unless
! 286: .Nm rshd ,
! 287: .Nm rlogind ,
! 288: and
! 289: .Nm rexecd
! 290: are disabled (thus completely disabling
! 291: .Xr rlogin
! 292: and
! 293: .Xr rsh
! 294: into the machine).
! 295: .Sh COMMAND EXECUTION AND DATA FORWARDING
! 296: If the client successfully authenticates itself, a dialog for
! 297: preparing the session is entered.
! 298: At this time the client may request
! 299: things like allocating a pseudo-tty, forwarding X11 connections,
! 300: forwarding TCP connections, or forwarding the authentication agent
! 301: connection over the secure channel.
! 302: .Pp
! 303: Finally, the client either requests a shell or execution of a command.
! 304: The sides then enter session mode.
! 305: In this mode, either side may send
! 306: data at any time, and such data is forwarded to/from the shell or
! 307: command on the server side, and the user terminal in the client side.
! 308: .Pp
! 309: When the user program terminates and all forwarded X11 and other
! 310: connections have been closed, the server sends command exit status to
! 311: the client, and both sides exit.
1.2 deraadt 312: .Sh LOGIN PROCESS
1.1 deraadt 313: When a user successfully logs in,
1.2 deraadt 314: .Nm
1.1 deraadt 315: does the following:
1.2 deraadt 316: .Bl -enum -offset indent
317: .It
1.1 deraadt 318: If the login is on a tty, and no command has been specified,
1.40 aaron 319: prints last login time and
1.2 deraadt 320: .Pa /etc/motd
1.1 deraadt 321: (unless prevented in the configuration file or by
1.206.2.1 brad 322: .Pa ~/.hushlogin ;
1.2 deraadt 323: see the
1.40 aaron 324: .Sx FILES
1.2 deraadt 325: section).
326: .It
1.1 deraadt 327: If the login is on a tty, records login time.
1.2 deraadt 328: .It
329: Checks
330: .Pa /etc/nologin ;
331: if it exists, prints contents and quits
1.1 deraadt 332: (unless root).
1.2 deraadt 333: .It
1.1 deraadt 334: Changes to run with normal user privileges.
1.2 deraadt 335: .It
1.1 deraadt 336: Sets up basic environment.
1.2 deraadt 337: .It
1.200 jmc 338: Reads the file
1.206.2.1 brad 339: .Pa ~/.ssh/environment ,
1.200 jmc 340: if it exists, and users are allowed to change their environment.
1.188 stevesk 341: See the
1.187 marc 342: .Cm PermitUserEnvironment
1.188 stevesk 343: option in
1.187 marc 344: .Xr sshd_config 5 .
1.2 deraadt 345: .It
1.1 deraadt 346: Changes to user's home directory.
1.2 deraadt 347: .It
348: If
1.206.2.1 brad 349: .Pa ~/.ssh/rc
1.2 deraadt 350: exists, runs it; else if
1.168 deraadt 351: .Pa /etc/ssh/sshrc
1.2 deraadt 352: exists, runs
1.36 aaron 353: it; otherwise runs xauth.
354: The
1.2 deraadt 355: .Dq rc
356: files are given the X11
1.1 deraadt 357: authentication protocol and cookie in standard input.
1.2 deraadt 358: .It
1.1 deraadt 359: Runs user's shell or command.
1.2 deraadt 360: .El
361: .Sh AUTHORIZED_KEYS FILE FORMAT
1.206.2.1 brad 362: .Pa ~/.ssh/authorized_keys
1.130 markus 363: is the default file that lists the public keys that are
364: permitted for RSA authentication in protocol version 1
365: and for public key authentication (PubkeyAuthentication)
366: in protocol version 2.
1.125 markus 367: .Cm AuthorizedKeysFile
368: may be used to specify an alternative file.
1.75 markus 369: .Pp
1.36 aaron 370: Each line of the file contains one
1.2 deraadt 371: key (empty lines and lines starting with a
372: .Ql #
373: are ignored as
1.36 aaron 374: comments).
1.75 markus 375: Each RSA public key consists of the following fields, separated by
1.36 aaron 376: spaces: options, bits, exponent, modulus, comment.
1.75 markus 377: Each protocol version 2 public key consists of:
378: options, keytype, base64 encoded key, comment.
1.189 espie 379: The options field
380: is optional; its presence is determined by whether the line starts
381: with a number or not (the options field never starts with a number).
1.75 markus 382: The bits, exponent, modulus and comment fields give the RSA key for
383: protocol version 1; the
1.1 deraadt 384: comment field is not used for anything (but may be convenient for the
385: user to identify the key).
1.75 markus 386: For protocol version 2 the keytype is
387: .Dq ssh-dss
388: or
389: .Dq ssh-rsa .
1.2 deraadt 390: .Pp
1.1 deraadt 391: Note that lines in this file are usually several hundred bytes long
1.203 dtucker 392: (because of the size of the public key encoding) up to a limit of
393: 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
394: keys up to 16 kilobits.
1.36 aaron 395: You don't want to type them in; instead, copy the
1.113 itojun 396: .Pa identity.pub ,
397: .Pa id_dsa.pub
1.75 markus 398: or the
1.113 itojun 399: .Pa id_rsa.pub
1.1 deraadt 400: file and edit it.
1.175 stevesk 401: .Pp
402: .Nm
403: enforces a minimum RSA key modulus size for protocol 1
404: and protocol 2 keys of 768 bits.
1.2 deraadt 405: .Pp
1.58 deraadt 406: The options (if present) consist of comma-separated option
1.36 aaron 407: specifications.
408: No spaces are permitted, except within double quotes.
1.141 stevesk 409: The following option specifications are supported (note
410: that option keywords are case-insensitive):
1.2 deraadt 411: .Bl -tag -width Ds
412: .It Cm from="pattern-list"
1.189 espie 413: Specifies that in addition to public key authentication, the canonical name
1.1 deraadt 414: of the remote host must be present in the comma-separated list of
1.36 aaron 415: patterns
1.196 david 416: .Pf ( Ql \&*
1.36 aaron 417: and
1.195 mouring 418: .Ql \&?
1.36 aaron 419: serve as wildcards).
420: The list may also contain
421: patterns negated by prefixing them with
1.195 mouring 422: .Ql \&! ;
1.36 aaron 423: if the canonical host name matches a negated pattern, the key is not accepted.
424: The purpose
1.189 espie 425: of this option is to optionally increase security: public key authentication
1.1 deraadt 426: by itself does not trust the network or name servers or anything (but
427: the key); however, if somebody somehow steals the key, the key
1.36 aaron 428: permits an intruder to log in from anywhere in the world.
429: This additional option makes using a stolen key more difficult (name
1.1 deraadt 430: servers and/or routers would have to be compromised in addition to
431: just the key).
1.2 deraadt 432: .It Cm command="command"
1.1 deraadt 433: Specifies that the command is executed whenever this key is used for
1.36 aaron 434: authentication.
435: The command supplied by the user (if any) is ignored.
1.148 markus 436: The command is run on a pty if the client requests a pty;
1.36 aaron 437: otherwise it is run without a tty.
1.194 jmc 438: If an 8-bit clean channel is required,
1.147 deraadt 439: one must not request a pty or should specify
1.89 markus 440: .Cm no-pty .
1.36 aaron 441: A quote may be included in the command by quoting it with a backslash.
442: This option might be useful
1.189 espie 443: to restrict certain public keys to perform just a specific operation.
1.36 aaron 444: An example might be a key that permits remote backups but nothing else.
1.206.2.2! brad 445: Note that the client may specify TCP and/or X11
1.51 hugh 446: forwarding unless they are explicitly prohibited.
1.149 markus 447: Note that this option applies to shell, command or subsystem execution.
1.2 deraadt 448: .It Cm environment="NAME=value"
1.1 deraadt 449: Specifies that the string is to be added to the environment when
1.36 aaron 450: logging in using this key.
451: Environment variables set this way
452: override other default environment values.
453: Multiple options of this type are permitted.
1.188 stevesk 454: Environment processing is disabled by default and is
455: controlled via the
456: .Cm PermitUserEnvironment
457: option.
1.155 markus 458: This option is automatically disabled if
459: .Cm UseLogin
460: is enabled.
1.2 deraadt 461: .It Cm no-port-forwarding
1.206.2.2! brad 462: Forbids TCP forwarding when this key is used for authentication.
1.36 aaron 463: Any port forward requests by the client will return an error.
464: This might be used, e.g., in connection with the
1.2 deraadt 465: .Cm command
1.1 deraadt 466: option.
1.2 deraadt 467: .It Cm no-X11-forwarding
1.1 deraadt 468: Forbids X11 forwarding when this key is used for authentication.
469: Any X11 forward requests by the client will return an error.
1.2 deraadt 470: .It Cm no-agent-forwarding
1.1 deraadt 471: Forbids authentication agent forwarding when this key is used for
472: authentication.
1.2 deraadt 473: .It Cm no-pty
1.1 deraadt 474: Prevents tty allocation (a request to allocate a pty will fail).
1.107 djm 475: .It Cm permitopen="host:port"
1.133 itojun 476: Limit local
1.107 djm 477: .Li ``ssh -L''
1.111 stevesk 478: port forwarding such that it may only connect to the specified host and
1.146 stevesk 479: port.
480: IPv6 addresses can be specified with an alternative syntax:
1.200 jmc 481: .Ar host Ns / Ns Ar port .
1.146 stevesk 482: Multiple
1.107 djm 483: .Cm permitopen
1.197 jmc 484: options may be applied separated by commas.
485: No pattern matching is performed on the specified hostnames,
486: they must be literal domains or addresses.
1.206.2.2! brad 487: .It Cm tunnel="n"
! 488: Force a
! 489: .Xr tun 4
! 490: device on the server.
! 491: Without this option, the next available device will be used if
! 492: the client requests a tunnel.
1.2 deraadt 493: .El
494: .Ss Examples
1.200 jmc 495: 1024 33 12121...312314325 ylo@foo.bar
1.2 deraadt 496: .Pp
1.200 jmc 497: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula
1.2 deraadt 498: .Pp
1.200 jmc 499: command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi
1.107 djm 500: .Pp
1.200 jmc 501: permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323
1.206.2.2! brad 502: .Pp
! 503: tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== reyk@openbsd.org
1.2 deraadt 504: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 505: The
1.194 jmc 506: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 507: and
1.206.2.1 brad 508: .Pa ~/.ssh/known_hosts
1.36 aaron 509: files contain host public keys for all known hosts.
510: The global file should
1.37 brad 511: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 512: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 513: its key is added to the per-user file.
1.2 deraadt 514: .Pp
1.1 deraadt 515: Each line in these files contains the following fields: hostnames,
1.36 aaron 516: bits, exponent, modulus, comment.
517: The fields are separated by spaces.
1.2 deraadt 518: .Pp
1.196 david 519: Hostnames is a comma-separated list of patterns
1.197 jmc 520: .Pf ( Ql \&*
521: and
522: .Ql \&?
1.195 mouring 523: act as
1.1 deraadt 524: wildcards); each pattern in turn is matched against the canonical host
525: name (when authenticating a client) or against the user-supplied
1.36 aaron 526: name (when authenticating a server).
527: A pattern may also be preceded by
1.195 mouring 528: .Ql \&!
1.2 deraadt 529: to indicate negation: if the host name matches a negated
1.1 deraadt 530: pattern, it is not accepted (by that line) even if it matched another
531: pattern on the line.
1.2 deraadt 532: .Pp
1.205 djm 533: Alternately, hostnames may be stored in a hashed form which hides host names
1.206 jmc 534: and addresses should the file's contents be disclosed.
535: Hashed hostnames start with a
536: .Ql |
1.205 djm 537: character.
538: Only one hashed hostname may appear on a single line and none of the above
539: negation or wildcard operators may be applied.
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
1.200 jmc 569: closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
1.120 markus 570: cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1.205 djm 571: .Ed
572: .Bd -literal
573: # A hashed hostname
574: |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
575: AAAA1234.....=
1.120 markus 576: .Ed
1.2 deraadt 577: .Sh FILES
578: .Bl -tag -width Ds
1.167 deraadt 579: .It Pa /etc/ssh/sshd_config
1.1 deraadt 580: Contains configuration data for
1.2 deraadt 581: .Nm sshd .
1.184 stevesk 582: The file format and configuration options are described in
583: .Xr sshd_config 5 .
1.167 deraadt 584: .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1.120 markus 585: These three files contain the private parts of the host keys.
1.98 deraadt 586: These files should only be owned by root, readable only by root, and not
1.1 deraadt 587: accessible to others.
1.14 markus 588: Note that
589: .Nm
590: does not start if this file is group/world-accessible.
1.167 deraadt 591: .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 592: These three files contain the public parts of the host keys.
1.98 deraadt 593: These files should be world-readable but writable only by
1.36 aaron 594: root.
1.98 deraadt 595: Their contents should match the respective private parts.
596: These files are not
597: really used for anything; they are provided for the convenience of
598: the user so their contents can be copied to known hosts files.
599: These files are created using
1.7 markus 600: .Xr ssh-keygen 1 .
1.129 provos 601: .It Pa /etc/moduli
1.73 provos 602: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1.192 stevesk 603: The file format is described in
604: .Xr moduli 5 .
1.185 stevesk 605: .It Pa /var/empty
606: .Xr chroot 2
607: directory used by
608: .Nm
609: during privilege separation in the pre-authentication phase.
610: The directory should not contain any files and must be owned by root
611: and not group or world-writable.
1.2 deraadt 612: .It Pa /var/run/sshd.pid
613: Contains the process ID of the
614: .Nm
1.1 deraadt 615: listening for connections (if there are several daemons running
1.186 stevesk 616: concurrently for different ports, this contains the process ID of the one
1.36 aaron 617: started last).
1.58 deraadt 618: The content of this file is not sensitive; it can be world-readable.
1.206.2.1 brad 619: .It Pa ~/.ssh/authorized_keys
1.120 markus 620: Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1.44 deraadt 621: This file must be readable by root (which may on some machines imply
622: it being world-readable if the user's home directory resides on an NFS
623: volume).
624: It is recommended that it not be accessible by others.
625: The format of this file is described above.
626: Users will place the contents of their
1.130 markus 627: .Pa identity.pub ,
1.44 deraadt 628: .Pa id_dsa.pub
1.113 itojun 629: and/or
630: .Pa id_rsa.pub
1.44 deraadt 631: files into this file, as described in
632: .Xr ssh-keygen 1 .
1.206.2.1 brad 633: .It Pa "/etc/ssh/ssh_known_hosts", "~/.ssh/known_hosts"
1.22 markus 634: These files are consulted when using rhosts with RSA host
1.131 markus 635: authentication or protocol version 2 hostbased authentication
636: to check the public key of the host.
1.36 aaron 637: The key must be listed in one of these files to be accepted.
1.22 markus 638: The client uses the same files
1.96 markus 639: to verify that it is connecting to the correct remote host.
1.36 aaron 640: These files should be writable only by root/the owner.
1.167 deraadt 641: .Pa /etc/ssh/ssh_known_hosts
1.2 deraadt 642: should be world-readable, and
1.206.2.1 brad 643: .Pa ~/.ssh/known_hosts
1.194 jmc 644: can, but need not be, world-readable.
1.204 jmc 645: .It Pa /etc/motd
646: See
647: .Xr motd 5 .
1.206.2.1 brad 648: .It Pa ~/.hushlogin
1.204 jmc 649: This file is used to suppress printing the last login time and
650: .Pa /etc/motd ,
651: if
652: .Cm PrintLastLog
653: and
654: .Cm PrintMotd ,
655: respectively,
656: are enabled.
657: It does not suppress printing of the banner specified by
658: .Cm Banner .
1.6 aaron 659: .It Pa /etc/nologin
1.40 aaron 660: If this file exists,
1.2 deraadt 661: .Nm
1.36 aaron 662: refuses to let anyone except root log in.
663: The contents of the file
1.1 deraadt 664: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 665: refused.
666: The file should be world-readable.
1.19 dugsong 667: .It Pa /etc/hosts.allow, /etc/hosts.deny
1.153 camield 668: Access controls that should be enforced by tcp-wrappers are defined here.
669: Further details are described in
1.19 dugsong 670: .Xr hosts_access 5 .
1.206.2.1 brad 671: .It Pa ~/.rhosts
1.202 markus 672: This file is used during
673: .Cm RhostsRSAAuthentication
674: and
675: .Cm HostbasedAuthentication
676: and contains host-username pairs, separated by a space, one per
1.36 aaron 677: line.
678: The given user on the corresponding host is permitted to log in
1.194 jmc 679: without a password.
1.36 aaron 680: The same file is used by rlogind and rshd.
1.6 aaron 681: The file must
1.1 deraadt 682: be writable only by the user; it is recommended that it not be
683: accessible by others.
1.2 deraadt 684: .Pp
1.200 jmc 685: It is also possible to use netgroups in the file.
1.36 aaron 686: Either host or user
1.1 deraadt 687: name may be of the form +@groupname to specify all hosts or all users
688: in the group.
1.206.2.1 brad 689: .It Pa ~/.shosts
1.2 deraadt 690: For ssh,
691: this file is exactly the same as for
692: .Pa .rhosts .
693: However, this file is
694: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 695: .It Pa /etc/hosts.equiv
1.2 deraadt 696: This file is used during
1.202 markus 697: .Cm RhostsRSAAuthentication
698: and
699: .Cm HostbasedAuthentication
1.36 aaron 700: authentication.
701: In the simplest form, this file contains host names, one per line.
702: Users on
1.1 deraadt 703: those hosts are permitted to log in without a password, provided they
1.36 aaron 704: have the same user name on both machines.
705: The host name may also be
1.1 deraadt 706: followed by a user name; such users are permitted to log in as
1.2 deraadt 707: .Em any
1.36 aaron 708: user on this machine (except root).
709: Additionally, the syntax
1.2 deraadt 710: .Dq +@group
1.36 aaron 711: can be used to specify netgroups.
712: Negated entries start with
1.2 deraadt 713: .Ql \&- .
714: .Pp
1.1 deraadt 715: If the client host/user is successfully matched in this file, login is
716: automatically permitted provided the client and server user names are the
1.36 aaron 717: same.
1.202 markus 718: Additionally, successful client host key authentication is required.
1.36 aaron 719: This file must be writable only by root; it is recommended
1.1 deraadt 720: that it be world-readable.
1.2 deraadt 721: .Pp
1.6 aaron 722: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 723: .Pa hosts.equiv .
1.1 deraadt 724: Beware that it really means that the named user(s) can log in as
1.2 deraadt 725: .Em anybody ,
1.1 deraadt 726: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 727: binaries and directories.
728: Using a user name practically grants the user root access.
729: The only valid use for user names that I can think
1.1 deraadt 730: of is in negative entries.
1.2 deraadt 731: .Pp
732: Note that this warning also applies to rsh/rlogin.
733: .It Pa /etc/shosts.equiv
1.1 deraadt 734: This is processed exactly as
1.2 deraadt 735: .Pa /etc/hosts.equiv .
1.1 deraadt 736: However, this file may be useful in environments that want to run both
1.2 deraadt 737: rsh/rlogin and ssh.
1.206.2.1 brad 738: .It Pa ~/.ssh/environment
1.36 aaron 739: This file is read into the environment at login (if it exists).
740: It can only contain empty lines, comment lines (that start with
1.2 deraadt 741: .Ql # ) ,
1.36 aaron 742: and assignment lines of the form name=value.
743: The file should be writable
1.6 aaron 744: only by the user; it need not be readable by anyone else.
1.188 stevesk 745: Environment processing is disabled by default and is
746: controlled via the
747: .Cm PermitUserEnvironment
748: option.
1.206.2.1 brad 749: .It Pa ~/.ssh/rc
1.194 jmc 750: If this file exists, it is run with
751: .Pa /bin/sh
752: after reading the
1.36 aaron 753: environment files but before starting the user's shell or command.
1.176 stevesk 754: It must not produce any output on stdout; stderr must be used
755: instead.
756: If X11 forwarding is in use, it will receive the "proto cookie" pair in
757: its standard input (and
1.2 deraadt 758: .Ev DISPLAY
1.176 stevesk 759: in its environment).
760: The script must call
1.2 deraadt 761: .Xr xauth 1
1.176 stevesk 762: because
763: .Nm
764: will not run xauth automatically to add X11 cookies.
1.2 deraadt 765: .Pp
1.1 deraadt 766: The primary purpose of this file is to run any initialization routines
767: which may be needed before the user's home directory becomes
768: accessible; AFS is a particular example of such an environment.
1.2 deraadt 769: .Pp
1.1 deraadt 770: This file will probably contain some initialization code followed by
1.120 markus 771: something similar to:
772: .Bd -literal
1.176 stevesk 773: if read proto cookie && [ -n "$DISPLAY" ]; then
774: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
775: # X11UseLocalhost=yes
1.193 todd 776: echo add unix:`echo $DISPLAY |
1.176 stevesk 777: cut -c11-` $proto $cookie
778: else
779: # X11UseLocalhost=no
1.193 todd 780: echo add $DISPLAY $proto $cookie
781: fi | xauth -q -
1.176 stevesk 782: fi
1.120 markus 783: .Ed
1.2 deraadt 784: .Pp
785: If this file does not exist,
1.167 deraadt 786: .Pa /etc/ssh/sshrc
1.2 deraadt 787: is run, and if that
1.176 stevesk 788: does not exist either, xauth is used to add the cookie.
1.2 deraadt 789: .Pp
1.1 deraadt 790: This file should be writable only by the user, and need not be
791: readable by anyone else.
1.167 deraadt 792: .It Pa /etc/ssh/sshrc
1.2 deraadt 793: Like
1.206.2.1 brad 794: .Pa ~/.ssh/rc .
1.2 deraadt 795: This can be used to specify
1.36 aaron 796: machine-specific login-time initializations globally.
797: This file should be writable only by root, and should be world-readable.
1.56 aaron 798: .El
1.2 deraadt 799: .Sh SEE ALSO
800: .Xr scp 1 ,
1.90 djm 801: .Xr sftp 1 ,
1.2 deraadt 802: .Xr ssh 1 ,
1.5 deraadt 803: .Xr ssh-add 1 ,
1.2 deraadt 804: .Xr ssh-agent 1 ,
1.5 deraadt 805: .Xr ssh-keygen 1 ,
1.200 jmc 806: .Xr chroot 2 ,
807: .Xr hosts_access 5 ,
1.136 markus 808: .Xr login.conf 5 ,
809: .Xr moduli 5 ,
1.184 stevesk 810: .Xr sshd_config 5 ,
1.200 jmc 811: .Xr inetd 8 ,
1.128 mpech 812: .Xr sftp-server 8
1.119 markus 813: .Rs
814: .%A T. Ylonen
815: .%A T. Kivinen
816: .%A M. Saarinen
817: .%A T. Rinne
818: .%A S. Lehtinen
819: .%T "SSH Protocol Architecture"
1.174 markus 820: .%N draft-ietf-secsh-architecture-12.txt
821: .%D January 2002
1.120 markus 822: .%O work in progress material
823: .Re
824: .Rs
825: .%A M. Friedl
826: .%A N. Provos
827: .%A W. A. Simpson
828: .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1.174 markus 829: .%N draft-ietf-secsh-dh-group-exchange-02.txt
830: .%D January 2002
1.119 markus 831: .%O work in progress material
832: .Re
1.198 jmc 833: .Sh AUTHORS
834: OpenSSH is a derivative of the original and free
835: ssh 1.2.12 release by Tatu Ylonen.
836: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
837: Theo de Raadt and Dug Song
838: removed many bugs, re-added newer features and
839: created OpenSSH.
840: Markus Friedl contributed the support for SSH
841: protocol versions 1.5 and 2.0.
842: Niels Provos and Markus Friedl contributed support
843: for privilege separation.
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