Annotation of src/usr.bin/ssh/sshd.8, Revision 1.69
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: .\"
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.
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,
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31: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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1.1 deraadt 36: .\"
1.2 deraadt 37: .Dd September 25, 1999
38: .Dt SSHD 8
39: .Os
40: .Sh NAME
41: .Nm sshd
42: .Nd secure shell daemon
43: .Sh SYNOPSIS
44: .Nm sshd
1.29 markus 45: .Op Fl diqQ46
1.2 deraadt 46: .Op Fl b Ar bits
47: .Op Fl f Ar config_file
48: .Op Fl g Ar login_grace_time
49: .Op Fl h Ar host_key_file
50: .Op Fl k Ar key_gen_time
51: .Op Fl p Ar port
1.61 markus 52: .Op Fl u Ar len
1.26 markus 53: .Op Fl V Ar client_protocol_id
1.40 aaron 54: .Sh DESCRIPTION
1.2 deraadt 55: .Nm
1.40 aaron 56: (Secure Shell Daemon) is the daemon program for
1.2 deraadt 57: .Xr ssh 1 .
1.42 hugh 58: Together these programs replace rlogin and rsh, and
1.1 deraadt 59: provide secure encrypted communications between two untrusted hosts
1.36 aaron 60: over an insecure network.
61: The programs are intended to be as easy to
1.1 deraadt 62: install and use as possible.
1.2 deraadt 63: .Pp
64: .Nm
1.36 aaron 65: is the daemon that listens for connections from clients.
1.40 aaron 66: It is normally started at boot from
1.2 deraadt 67: .Pa /etc/rc .
68: It forks a new
1.36 aaron 69: daemon for each incoming connection.
70: The forked daemons handle
1.1 deraadt 71: key exchange, encryption, authentication, command execution,
72: and data exchange.
1.49 markus 73: This implementation of
74: .Nm
75: supports both SSH protocol version 1 and 2 simultaneously.
1.2 deraadt 76: .Nm
1.36 aaron 77: works as follows.
1.49 markus 78: .Pp
79: .Ss SSH protocol version 1
80: .Pp
1.36 aaron 81: Each host has a host-specific RSA key
82: (normally 1024 bits) used to identify the host.
83: Additionally, when
1.1 deraadt 84: the daemon starts, it generates a server RSA key (normally 768 bits).
85: This key is normally regenerated every hour if it has been used, and
86: is never stored on disk.
1.2 deraadt 87: .Pp
1.42 hugh 88: Whenever a client connects the daemon responds with its public
89: host and server keys.
1.36 aaron 90: The client compares the
1.49 markus 91: RSA host key against its own database to verify that it has not changed.
1.36 aaron 92: The client then generates a 256 bit random number.
93: It encrypts this
1.1 deraadt 94: random number using both the host key and the server key, and sends
1.36 aaron 95: the encrypted number to the server.
1.42 hugh 96: Both sides then use this
1.1 deraadt 97: random number as a session key which is used to encrypt all further
1.36 aaron 98: communications in the session.
99: The rest of the session is encrypted
1.42 hugh 100: using a conventional cipher, currently Blowfish or 3DES, with 3DES
1.39 deraadt 101: being used by default.
1.36 aaron 102: The client selects the encryption algorithm
1.5 deraadt 103: to use from those offered by the server.
1.2 deraadt 104: .Pp
1.36 aaron 105: Next, the server and the client enter an authentication dialog.
106: The client tries to authenticate itself using
1.2 deraadt 107: .Pa .rhosts
108: authentication,
109: .Pa .rhosts
110: authentication combined with RSA host
1.1 deraadt 111: authentication, RSA challenge-response authentication, or password
112: based authentication.
1.2 deraadt 113: .Pp
1.1 deraadt 114: Rhosts authentication is normally disabled
115: because it is fundamentally insecure, but can be enabled in the server
1.36 aaron 116: configuration file if desired.
117: System security is not improved unless
1.2 deraadt 118: .Xr rshd 8 ,
119: .Xr rlogind 8 ,
120: .Xr rexecd 8 ,
121: and
122: .Xr rexd 8
1.1 deraadt 123: are disabled (thus completely disabling
1.2 deraadt 124: .Xr rlogin 1
1.1 deraadt 125: and
1.2 deraadt 126: .Xr rsh 1
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:
1.49 markus 132: Each host has a host-specific DSA key used to identify the host.
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.
136: The rest of the session is encrypted
137: using a symmetric cipher, currently
138: Blowfish, 3DES or CAST128 in CBC mode or Arcfour.
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
146: user authentication method (DSAAuthentication)
147: and conventional password authentication.
148: .Pp
149: .Ss Command execution and data forwarding
150: .Pp
1.1 deraadt 151: If the client successfully authenticates itself, a dialog for
1.36 aaron 152: preparing the session is entered.
153: At this time the client may request
1.1 deraadt 154: things like allocating a pseudo-tty, forwarding X11 connections,
155: forwarding TCP/IP connections, or forwarding the authentication agent
156: connection over the secure channel.
1.2 deraadt 157: .Pp
1.1 deraadt 158: Finally, the client either requests a shell or execution of a command.
1.36 aaron 159: The sides then enter session mode.
160: In this mode, either side may send
1.1 deraadt 161: data at any time, and such data is forwarded to/from the shell or
162: command on the server side, and the user terminal in the client side.
1.2 deraadt 163: .Pp
1.1 deraadt 164: When the user program terminates and all forwarded X11 and other
165: connections have been closed, the server sends command exit status to
166: the client, and both sides exit.
1.2 deraadt 167: .Pp
168: .Nm
1.1 deraadt 169: can be configured using command-line options or a configuration
1.36 aaron 170: file.
171: Command-line options override values specified in the
1.1 deraadt 172: configuration file.
1.25 markus 173: .Pp
174: .Nm
175: rereads its configuration file when it receives a hangup signal,
176: .Dv SIGHUP .
1.18 aaron 177: .Pp
178: The options are as follows:
1.2 deraadt 179: .Bl -tag -width Ds
180: .It Fl b Ar bits
1.1 deraadt 181: Specifies the number of bits in the server key (default 768).
1.2 deraadt 182: .Pp
183: .It Fl d
1.36 aaron 184: Debug mode.
185: The server sends verbose debug output to the system
186: log, and does not put itself in the background.
187: The server also will not fork and will only process one connection.
188: This option is only intended for debugging for the server.
1.67 aaron 189: Multiple -d options increases the debugging level.
190: Maximum is 3.
1.2 deraadt 191: .It Fl f Ar configuration_file
1.36 aaron 192: Specifies the name of the configuration file.
193: The default is
1.2 deraadt 194: .Pa /etc/sshd_config .
1.16 markus 195: .Nm
196: refuses to start if there is no configuration file.
1.2 deraadt 197: .It Fl g Ar login_grace_time
1.1 deraadt 198: Gives the grace time for clients to authenticate themselves (default
1.36 aaron 199: 300 seconds).
200: If the client fails to authenticate the user within
201: this many seconds, the server disconnects and exits.
202: A value of zero indicates no limit.
1.2 deraadt 203: .It Fl h Ar host_key_file
1.49 markus 204: Specifies the file from which the RSA host key is read (default
1.2 deraadt 205: .Pa /etc/ssh_host_key ) .
1.7 markus 206: This option must be given if
207: .Nm
208: is not run as root (as the normal
1.1 deraadt 209: host file is normally not readable by anyone but root).
1.2 deraadt 210: .It Fl i
1.7 markus 211: Specifies that
212: .Nm
1.40 aaron 213: is being run from inetd.
1.7 markus 214: .Nm
215: is normally not run
1.1 deraadt 216: from inetd because it needs to generate the server key before it can
1.36 aaron 217: respond to the client, and this may take tens of seconds.
218: Clients would have to wait too long if the key was regenerated every time.
1.35 aaron 219: However, with small key sizes (e.g., 512) using
1.7 markus 220: .Nm
221: from inetd may
1.1 deraadt 222: be feasible.
1.2 deraadt 223: .It Fl k Ar key_gen_time
1.1 deraadt 224: Specifies how often the server key is regenerated (default 3600
1.36 aaron 225: seconds, or one hour).
226: The motivation for regenerating the key fairly
1.1 deraadt 227: often is that the key is not stored anywhere, and after about an hour,
228: it becomes impossible to recover the key for decrypting intercepted
229: communications even if the machine is cracked into or physically
1.36 aaron 230: seized.
231: A value of zero indicates that the key will never be regenerated.
1.2 deraadt 232: .It Fl p Ar port
1.1 deraadt 233: Specifies the port on which the server listens for connections
234: (default 22).
1.2 deraadt 235: .It Fl q
1.36 aaron 236: Quiet mode.
237: Nothing is sent to the system log.
238: Normally the beginning,
1.1 deraadt 239: authentication, and termination of each connection is logged.
1.61 markus 240: .It Fl u Ar len
241: This option is used to specify the size of the field
242: in the
243: .Li utmp
244: structure that holds the remote host name.
245: If the resolved host name is longer than
246: .Ar len ,
247: the dotted decimal value will be used instead.
248: This allows hosts with very long host names that
249: overflow this field to still be uniquely identified.
250: Specifying
251: .Fl u0
252: indicates that only dotted decimal addresses
253: should be put into the
254: .Pa utmp
255: file.
1.18 aaron 256: .It Fl Q
257: Do not print an error message if RSA support is missing.
1.26 markus 258: .It Fl V Ar client_protocol_id
1.68 markus 259: SSH-2 compatibility mode.
1.42 hugh 260: When this option is specified
1.26 markus 261: .Nm
1.42 hugh 262: assumes the client has sent the supplied version string
1.26 markus 263: and skips the
264: Protocol Version Identification Exchange.
1.68 markus 265: This option is not intended to be called directly.
1.29 markus 266: .It Fl 4
267: Forces
268: .Nm
269: to use IPv4 addresses only.
270: .It Fl 6
271: Forces
272: .Nm
273: to use IPv6 addresses only.
1.2 deraadt 274: .El
275: .Sh CONFIGURATION FILE
276: .Nm
1.40 aaron 277: reads configuration data from
1.2 deraadt 278: .Pa /etc/sshd_config
279: (or the file specified with
280: .Fl f
1.36 aaron 281: on the command line).
282: The file contains keyword-value pairs, one per line.
283: Lines starting with
1.2 deraadt 284: .Ql #
1.1 deraadt 285: and empty lines are interpreted as comments.
1.2 deraadt 286: .Pp
1.1 deraadt 287: The following keywords are possible.
1.2 deraadt 288: .Bl -tag -width Ds
289: .It Cm AFSTokenPassing
1.36 aaron 290: Specifies whether an AFS token may be forwarded to the server.
291: Default is
1.2 deraadt 292: .Dq yes .
1.11 markus 293: .It Cm AllowGroups
294: This keyword can be followed by a number of group names, separated
1.36 aaron 295: by spaces.
296: If specified, login is allowed only for users whose primary
1.11 markus 297: group matches one of the patterns.
298: .Ql \&*
299: and
300: .Ql ?
301: can be used as
1.36 aaron 302: wildcards in the patterns.
1.58 deraadt 303: Only group names are valid; a numerical group ID isn't recognized.
1.36 aaron 304: By default login is allowed regardless of the primary group.
1.3 dugsong 305: .Pp
1.69 ! markus 306: .It Cm AllowTcpForwarding
! 307: Specifies whether TCP forwarding is permitted.
! 308: The default is
! 309: .Dq yes .
! 310: Note that disabling TCP forwarding does not improve security unless
! 311: users are also denied shell access, as they can always install their
! 312: own forwarders.
! 313: .Pp
1.11 markus 314: .It Cm AllowUsers
315: This keyword can be followed by a number of user names, separated
1.36 aaron 316: by spaces.
317: If specified, login is allowed only for users names that
1.11 markus 318: match one of the patterns.
319: .Ql \&*
320: and
321: .Ql ?
322: can be used as
1.36 aaron 323: wildcards in the patterns.
1.58 deraadt 324: Only user names are valid; a numerical user ID isn't recognized.
1.36 aaron 325: By default login is allowed regardless of the user name.
1.11 markus 326: .Pp
1.41 markus 327: .It Cm Ciphers
328: Specifies the ciphers allowed for protocol version 2.
329: Multiple ciphers must be comma-separated.
330: The default is
1.50 markus 331: .Dq 3des-cbc,blowfish-cbc,arcfour,cast128-cbc .
1.8 markus 332: .It Cm CheckMail
333: Specifies whether
334: .Nm
335: should check for new mail for interactive logins.
336: The default is
337: .Dq no .
1.11 markus 338: .It Cm DenyGroups
339: This keyword can be followed by a number of group names, separated
1.36 aaron 340: by spaces.
341: Users whose primary group matches one of the patterns
1.11 markus 342: aren't allowed to log in.
343: .Ql \&*
344: and
345: .Ql ?
346: can be used as
1.36 aaron 347: wildcards in the patterns.
1.58 deraadt 348: Only group names are valid; a numerical group ID isn't recognized.
1.36 aaron 349: By default login is allowed regardless of the primary group.
1.11 markus 350: .Pp
351: .It Cm DenyUsers
352: This keyword can be followed by a number of user names, separated
1.36 aaron 353: by spaces.
354: Login is disallowed for user names that match one of the patterns.
1.11 markus 355: .Ql \&*
356: and
357: .Ql ?
1.36 aaron 358: can be used as wildcards in the patterns.
1.58 deraadt 359: Only user names are valid; a numerical user ID isn't recognized.
1.36 aaron 360: By default login is allowed regardless of the user name.
1.49 markus 361: .It Cm DSAAuthentication
362: Specifies whether DSA authentication is allowed.
363: The default is
364: .Dq yes .
365: Note that this option applies to protocol version 2 only.
1.47 markus 366: .It Cm GatewayPorts
367: Specifies whether remote hosts are allowed to connect to ports
368: forwarded for the client.
369: The argument must be
370: .Dq yes
371: or
372: .Dq no .
373: The default is
374: .Dq no .
1.58 deraadt 375: .It Cm HostDSAKey
1.46 markus 376: Specifies the file containing the private DSA host key (default
377: .Pa /etc/ssh_host_dsa_key )
378: used by SSH protocol 2.0.
379: Note that
380: .Nm
1.51 hugh 381: disables protocol 2.0 if this file is group/world-accessible.
1.2 deraadt 382: .It Cm HostKey
1.46 markus 383: Specifies the file containing the private RSA host key (default
384: .Pa /etc/ssh_host_key )
385: used by SSH protocols 1.3 and 1.5.
1.9 markus 386: Note that
387: .Nm
1.51 hugh 388: disables protocols 1.3 and 1.5 if this file is group/world-accessible.
1.2 deraadt 389: .It Cm IgnoreRhosts
1.34 markus 390: Specifies that
391: .Pa .rhosts
1.40 aaron 392: and
1.34 markus 393: .Pa .shosts
394: files will not be used in authentication.
1.2 deraadt 395: .Pa /etc/hosts.equiv
1.1 deraadt 396: and
1.40 aaron 397: .Pa /etc/shosts.equiv
1.36 aaron 398: are still used.
1.40 aaron 399: The default is
1.34 markus 400: .Dq yes .
1.24 markus 401: .It Cm IgnoreUserKnownHosts
402: Specifies whether
403: .Nm
404: should ignore the user's
405: .Pa $HOME/.ssh/known_hosts
1.45 markus 406: during
1.24 markus 407: .Cm RhostsRSAAuthentication .
408: The default is
1.2 deraadt 409: .Dq no .
410: .It Cm KeepAlive
1.1 deraadt 411: Specifies whether the system should send keepalive messages to the
1.36 aaron 412: other side.
413: If they are sent, death of the connection or crash of one
414: of the machines will be properly noticed.
415: However, this means that
1.1 deraadt 416: connections will die if the route is down temporarily, and some people
1.36 aaron 417: find it annoying.
1.51 hugh 418: On the other hand, if keepalives are not sent,
1.2 deraadt 419: sessions may hang indefinitely on the server, leaving
420: .Dq ghost
421: users and consuming server resources.
422: .Pp
423: The default is
424: .Dq yes
425: (to send keepalives), and the server will notice
1.36 aaron 426: if the network goes down or the client host reboots.
427: This avoids infinitely hanging sessions.
1.2 deraadt 428: .Pp
429: To disable keepalives, the value should be set to
430: .Dq no
431: in both the server and the client configuration files.
432: .It Cm KerberosAuthentication
1.36 aaron 433: Specifies whether Kerberos authentication is allowed.
434: This can be in the form of a Kerberos ticket, or if
1.7 markus 435: .Cm PasswordAuthentication
1.1 deraadt 436: is yes, the password provided by the user will be validated through
1.67 aaron 437: the Kerberos KDC.
438: To use this option, the server needs a
1.59 provos 439: Kerberos servtab which allows the verification of the KDC's identity.
1.36 aaron 440: Default is
1.60 provos 441: .Dq yes .
1.2 deraadt 442: .It Cm KerberosOrLocalPasswd
1.1 deraadt 443: If set then if password authentication through Kerberos fails then
444: the password will be validated via any additional local mechanism
1.2 deraadt 445: such as
1.66 markus 446: .Pa /etc/passwd .
1.36 aaron 447: Default is
1.20 dugsong 448: .Dq yes .
1.2 deraadt 449: .It Cm KerberosTgtPassing
1.1 deraadt 450: Specifies whether a Kerberos TGT may be forwarded to the server.
1.40 aaron 451: Default is
1.3 dugsong 452: .Dq no ,
453: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 454: .It Cm KerberosTicketCleanup
1.7 markus 455: Specifies whether to automatically destroy the user's ticket cache
1.36 aaron 456: file on logout.
457: Default is
1.3 dugsong 458: .Dq yes .
1.2 deraadt 459: .It Cm KeyRegenerationInterval
1.1 deraadt 460: The server key is automatically regenerated after this many seconds
1.36 aaron 461: (if it has been used).
462: The purpose of regeneration is to prevent
1.1 deraadt 463: decrypting captured sessions by later breaking into the machine and
1.36 aaron 464: stealing the keys.
465: The key is never stored anywhere.
466: If the value is 0, the key is never regenerated.
467: The default is 3600 (seconds).
1.7 markus 468: .It Cm ListenAddress
469: Specifies what local address
470: .Nm
471: should listen on.
472: The default is to listen to all local addresses.
1.28 markus 473: Multiple options of this type are permitted.
474: Additionally, the
475: .Cm Ports
476: options must precede this option.
1.2 deraadt 477: .It Cm LoginGraceTime
1.1 deraadt 478: The server disconnects after this time if the user has not
1.36 aaron 479: successfully logged in.
480: If the value is 0, there is no time limit.
1.1 deraadt 481: The default is 600 (seconds).
1.23 markus 482: .It Cm LogLevel
483: Gives the verbosity level that is used when logging messages from
484: .Nm sshd .
485: The possible values are:
1.27 markus 486: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
1.23 markus 487: The default is INFO.
488: Logging with level DEBUG violates the privacy of users
489: and is not recommended.
1.55 markus 490: .It Cm MaxStartups
491: Specifies the maximum number of concurrent unauthenticated connections to the
492: .Nm
493: daemon.
494: Additional connections will be dropped until authentication succeeds or the
495: .Cm LoginGraceTime
496: expires for a connection.
497: The default is 10.
1.57 markus 498: .Pp
499: Alternatively, random early drop can be enabled by specifying
500: the three colon separated values
501: .Dq start:rate:full
1.67 aaron 502: (e.g., "10:30:60").
1.57 markus 503: .Nm
504: will refuse connection attempts with a probabillity of
505: .Dq rate/100
506: (30%)
507: if there are currently
508: .Dq start
509: (10)
510: unauthenticated connections.
511: The probabillity increases linearly and all connection attempts
512: are refused if the number of unauthenticated connections reaches
513: .Dq full
514: (60).
1.2 deraadt 515: .It Cm PasswordAuthentication
1.1 deraadt 516: Specifies whether password authentication is allowed.
1.2 deraadt 517: The default is
518: .Dq yes .
1.58 deraadt 519: Note that this option applies to both protocol versions 1 and 2.
1.2 deraadt 520: .It Cm PermitEmptyPasswords
1.1 deraadt 521: When password authentication is allowed, it specifies whether the
1.36 aaron 522: server allows login to accounts with empty password strings.
523: The default is
1.34 markus 524: .Dq no .
1.2 deraadt 525: .It Cm PermitRootLogin
1.1 deraadt 526: Specifies whether the root can log in using
1.2 deraadt 527: .Xr ssh 1 .
1.15 markus 528: The argument must be
529: .Dq yes ,
530: .Dq without-password
531: or
532: .Dq no .
1.2 deraadt 533: The default is
534: .Dq yes .
1.15 markus 535: If this options is set to
536: .Dq without-password
537: only password authentication is disabled for root.
1.2 deraadt 538: .Pp
539: Root login with RSA authentication when the
540: .Ar command
541: option has been
1.1 deraadt 542: specified will be allowed regardless of the value of this setting
543: (which may be useful for taking remote backups even if root login is
544: normally not allowed).
1.43 markus 545: .It Cm PidFile
546: Specifies the file that contains the process identifier of the
547: .Nm
548: daemon.
549: The default is
550: .Pa /var/run/sshd.pid .
1.2 deraadt 551: .It Cm Port
1.1 deraadt 552: Specifies the port number that
1.2 deraadt 553: .Nm
1.36 aaron 554: listens on.
555: The default is 22.
1.28 markus 556: Multiple options of this type are permitted.
1.2 deraadt 557: .It Cm PrintMotd
1.1 deraadt 558: Specifies whether
1.2 deraadt 559: .Nm
1.40 aaron 560: should print
1.2 deraadt 561: .Pa /etc/motd
1.36 aaron 562: when a user logs in interactively.
563: (On some systems it is also printed by the shell,
1.2 deraadt 564: .Pa /etc/profile ,
1.36 aaron 565: or equivalent.)
566: The default is
1.2 deraadt 567: .Dq yes .
1.41 markus 568: .It Cm Protocol
569: Specifies the protocol versions
570: .Nm
571: should support.
572: The possible values are
573: .Dq 1
574: and
575: .Dq 2 .
576: Multiple versions must be comma-separated.
577: The default is
578: .Dq 1 .
1.2 deraadt 579: .It Cm RandomSeed
1.36 aaron 580: Obsolete.
581: Random number generation uses other techniques.
1.2 deraadt 582: .It Cm RhostsAuthentication
1.1 deraadt 583: Specifies whether authentication using rhosts or /etc/hosts.equiv
1.36 aaron 584: files is sufficient.
585: Normally, this method should not be permitted because it is insecure.
1.7 markus 586: .Cm RhostsRSAAuthentication
587: should be used
1.1 deraadt 588: instead, because it performs RSA-based host authentication in addition
589: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 590: The default is
591: .Dq no .
592: .It Cm RhostsRSAAuthentication
1.1 deraadt 593: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.36 aaron 594: with successful RSA host authentication is allowed.
595: The default is
1.34 markus 596: .Dq no .
1.2 deraadt 597: .It Cm RSAAuthentication
1.36 aaron 598: Specifies whether pure RSA authentication is allowed.
599: The default is
1.2 deraadt 600: .Dq yes .
1.49 markus 601: Note that this option applies to protocol version 1 only.
1.2 deraadt 602: .It Cm ServerKeyBits
1.36 aaron 603: Defines the number of bits in the server key.
604: The minimum value is 512, and the default is 768.
1.7 markus 605: .It Cm SkeyAuthentication
606: Specifies whether
1.40 aaron 607: .Xr skey 1
1.36 aaron 608: authentication is allowed.
609: The default is
1.7 markus 610: .Dq yes .
611: Note that s/key authentication is enabled only if
612: .Cm PasswordAuthentication
613: is allowed, too.
1.2 deraadt 614: .It Cm StrictModes
1.12 markus 615: Specifies whether
616: .Nm
617: should check file modes and ownership of the
1.36 aaron 618: user's files and home directory before accepting login.
619: This is normally desirable because novices sometimes accidentally leave their
620: directory or files world-writable.
621: The default is
1.7 markus 622: .Dq yes .
1.54 jakob 623: .It Cm Subsystem
1.67 aaron 624: Configures an external subsystem (e.g., file transfer daemon).
625: Arguments should be a subsystem name and a command to execute upon subsystem
626: request.
1.63 markus 627: The command
628: .Xr sftp-server 8
629: implements the
630: .Dq sftp
631: file transfer subsystem.
1.54 jakob 632: By default no subsystems are defined.
633: Note that this option applies to protocol version 2 only.
1.2 deraadt 634: .It Cm SyslogFacility
1.1 deraadt 635: Gives the facility code that is used when logging messages from
1.2 deraadt 636: .Nm sshd .
1.1 deraadt 637: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.36 aaron 638: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
639: The default is AUTH.
1.10 markus 640: .It Cm UseLogin
641: Specifies whether
642: .Xr login 1
1.53 markus 643: is used for interactive login sessions.
644: Note that
645: .Xr login 1
1.58 deraadt 646: is never used for remote command execution.
1.36 aaron 647: The default is
1.10 markus 648: .Dq no .
1.6 aaron 649: .It Cm X11DisplayOffset
650: Specifies the first display number available for
651: .Nm sshd Ns 's
1.36 aaron 652: X11 forwarding.
653: This prevents
1.6 aaron 654: .Nm
655: from interfering with real X11 servers.
1.34 markus 656: The default is 10.
1.30 markus 657: .It Cm X11Forwarding
1.36 aaron 658: Specifies whether X11 forwarding is permitted.
659: The default is
1.34 markus 660: .Dq no .
1.30 markus 661: Note that disabling X11 forwarding does not improve security in any
662: way, as users can always install their own forwarders.
1.52 markus 663: .It Cm XAuthLocation
664: Specifies the location of the
665: .Xr xauth 1
666: program.
667: The default is
668: .Pa /usr/X11R6/bin/xauth .
1.2 deraadt 669: .El
670: .Sh LOGIN PROCESS
1.1 deraadt 671: When a user successfully logs in,
1.2 deraadt 672: .Nm
1.1 deraadt 673: does the following:
1.2 deraadt 674: .Bl -enum -offset indent
675: .It
1.1 deraadt 676: If the login is on a tty, and no command has been specified,
1.40 aaron 677: prints last login time and
1.2 deraadt 678: .Pa /etc/motd
1.1 deraadt 679: (unless prevented in the configuration file or by
1.2 deraadt 680: .Pa $HOME/.hushlogin ;
681: see the
1.40 aaron 682: .Sx FILES
1.2 deraadt 683: section).
684: .It
1.1 deraadt 685: If the login is on a tty, records login time.
1.2 deraadt 686: .It
687: Checks
688: .Pa /etc/nologin ;
689: if it exists, prints contents and quits
1.1 deraadt 690: (unless root).
1.2 deraadt 691: .It
1.1 deraadt 692: Changes to run with normal user privileges.
1.2 deraadt 693: .It
1.1 deraadt 694: Sets up basic environment.
1.2 deraadt 695: .It
696: Reads
697: .Pa $HOME/.ssh/environment
698: if it exists.
699: .It
1.1 deraadt 700: Changes to user's home directory.
1.2 deraadt 701: .It
702: If
703: .Pa $HOME/.ssh/rc
704: exists, runs it; else if
705: .Pa /etc/sshrc
706: exists, runs
1.36 aaron 707: it; otherwise runs xauth.
708: The
1.2 deraadt 709: .Dq rc
710: files are given the X11
1.1 deraadt 711: authentication protocol and cookie in standard input.
1.2 deraadt 712: .It
1.1 deraadt 713: Runs user's shell or command.
1.2 deraadt 714: .El
715: .Sh AUTHORIZED_KEYS FILE FORMAT
1.40 aaron 716: The
1.2 deraadt 717: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 718: file lists the RSA keys that are
1.44 deraadt 719: permitted for RSA authentication in SSH protocols 1.3 and 1.5
1.51 hugh 720: Similarly, the
1.44 deraadt 721: .Pa $HOME/.ssh/authorized_keys2
722: file lists the DSA keys that are
723: permitted for DSA authentication in SSH protocol 2.0.
1.36 aaron 724: Each line of the file contains one
1.2 deraadt 725: key (empty lines and lines starting with a
726: .Ql #
727: are ignored as
1.36 aaron 728: comments).
729: Each line consists of the following fields, separated by
730: spaces: options, bits, exponent, modulus, comment.
731: The options field
1.1 deraadt 732: is optional; its presence is determined by whether the line starts
733: with a number or not (the option field never starts with a number).
734: The bits, exponent, modulus and comment fields give the RSA key; the
735: comment field is not used for anything (but may be convenient for the
736: user to identify the key).
1.2 deraadt 737: .Pp
1.1 deraadt 738: Note that lines in this file are usually several hundred bytes long
1.36 aaron 739: (because of the size of the RSA key modulus).
740: You don't want to type them in; instead, copy the
1.2 deraadt 741: .Pa identity.pub
1.1 deraadt 742: file and edit it.
1.2 deraadt 743: .Pp
1.58 deraadt 744: The options (if present) consist of comma-separated option
1.36 aaron 745: specifications.
746: No spaces are permitted, except within double quotes.
1.1 deraadt 747: The following option specifications are supported:
1.2 deraadt 748: .Bl -tag -width Ds
749: .It Cm from="pattern-list"
1.1 deraadt 750: Specifies that in addition to RSA authentication, the canonical name
751: of the remote host must be present in the comma-separated list of
1.36 aaron 752: patterns
753: .Pf ( Ql *
754: and
755: .Ql ?
756: serve as wildcards).
757: The list may also contain
758: patterns negated by prefixing them with
759: .Ql ! ;
760: if the canonical host name matches a negated pattern, the key is not accepted.
761: The purpose
1.1 deraadt 762: of this option is to optionally increase security: RSA authentication
763: by itself does not trust the network or name servers or anything (but
764: the key); however, if somebody somehow steals the key, the key
1.36 aaron 765: permits an intruder to log in from anywhere in the world.
766: This additional option makes using a stolen key more difficult (name
1.1 deraadt 767: servers and/or routers would have to be compromised in addition to
768: just the key).
1.2 deraadt 769: .It Cm command="command"
1.1 deraadt 770: Specifies that the command is executed whenever this key is used for
1.36 aaron 771: authentication.
772: The command supplied by the user (if any) is ignored.
1.1 deraadt 773: The command is run on a pty if the connection requests a pty;
1.36 aaron 774: otherwise it is run without a tty.
775: A quote may be included in the command by quoting it with a backslash.
776: This option might be useful
777: to restrict certain RSA keys to perform just a specific operation.
778: An example might be a key that permits remote backups but nothing else.
1.51 hugh 779: Note that the client may specify TCP/IP and/or X11
780: forwarding unless they are explicitly prohibited.
1.2 deraadt 781: .It Cm environment="NAME=value"
1.1 deraadt 782: Specifies that the string is to be added to the environment when
1.36 aaron 783: logging in using this key.
784: Environment variables set this way
785: override other default environment values.
786: Multiple options of this type are permitted.
1.2 deraadt 787: .It Cm no-port-forwarding
1.1 deraadt 788: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 789: Any port forward requests by the client will return an error.
790: This might be used, e.g., in connection with the
1.2 deraadt 791: .Cm command
1.1 deraadt 792: option.
1.2 deraadt 793: .It Cm no-X11-forwarding
1.1 deraadt 794: Forbids X11 forwarding when this key is used for authentication.
795: Any X11 forward requests by the client will return an error.
1.2 deraadt 796: .It Cm no-agent-forwarding
1.1 deraadt 797: Forbids authentication agent forwarding when this key is used for
798: authentication.
1.2 deraadt 799: .It Cm no-pty
1.1 deraadt 800: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 801: .El
802: .Ss Examples
1.1 deraadt 803: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 804: .Pp
1.1 deraadt 805: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 806: .Pp
1.1 deraadt 807: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.2 deraadt 808: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 809: The
1.44 deraadt 810: .Pa /etc/ssh_known_hosts ,
811: .Pa /etc/ssh_known_hosts2 ,
812: .Pa $HOME/.ssh/known_hosts ,
1.40 aaron 813: and
1.44 deraadt 814: .Pa $HOME/.ssh/known_hosts2
1.36 aaron 815: files contain host public keys for all known hosts.
816: The global file should
1.37 brad 817: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 818: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 819: its key is added to the per-user file.
1.2 deraadt 820: .Pp
1.1 deraadt 821: Each line in these files contains the following fields: hostnames,
1.36 aaron 822: bits, exponent, modulus, comment.
823: The fields are separated by spaces.
1.2 deraadt 824: .Pp
1.1 deraadt 825: Hostnames is a comma-separated list of patterns ('*' and '?' act as
826: wildcards); each pattern in turn is matched against the canonical host
827: name (when authenticating a client) or against the user-supplied
1.36 aaron 828: name (when authenticating a server).
829: A pattern may also be preceded by
1.2 deraadt 830: .Ql !
831: to indicate negation: if the host name matches a negated
1.1 deraadt 832: pattern, it is not accepted (by that line) even if it matched another
833: pattern on the line.
1.2 deraadt 834: .Pp
1.49 markus 835: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 836: can be obtained, e.g., from
837: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 838: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 839: .Pp
840: Lines starting with
841: .Ql #
842: and empty lines are ignored as comments.
843: .Pp
1.1 deraadt 844: When performing host authentication, authentication is accepted if any
1.36 aaron 845: matching line has the proper key.
846: It is thus permissible (but not
1.1 deraadt 847: recommended) to have several lines or different host keys for the same
1.36 aaron 848: names.
849: This will inevitably happen when short forms of host names
850: from different domains are put in the file.
851: It is possible
1.1 deraadt 852: that the files contain conflicting information; authentication is
853: accepted if valid information can be found from either file.
1.2 deraadt 854: .Pp
1.1 deraadt 855: Note that the lines in these files are typically hundreds of characters
856: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 857: Rather, generate them by a script
1.40 aaron 858: or by taking
1.2 deraadt 859: .Pa /etc/ssh_host_key.pub
1.1 deraadt 860: and adding the host names at the front.
1.2 deraadt 861: .Ss Examples
1.1 deraadt 862: closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1.2 deraadt 863: .Sh FILES
864: .Bl -tag -width Ds
865: .It Pa /etc/sshd_config
1.1 deraadt 866: Contains configuration data for
1.2 deraadt 867: .Nm sshd .
1.1 deraadt 868: This file should be writable by root only, but it is recommended
869: (though not necessary) that it be world-readable.
1.2 deraadt 870: .It Pa /etc/ssh_host_key
1.7 markus 871: Contains the private part of the host key.
1.1 deraadt 872: This file should only be owned by root, readable only by root, and not
873: accessible to others.
1.14 markus 874: Note that
875: .Nm
876: does not start if this file is group/world-accessible.
1.2 deraadt 877: .It Pa /etc/ssh_host_key.pub
1.7 markus 878: Contains the public part of the host key.
879: This file should be world-readable but writable only by
1.36 aaron 880: root.
881: Its contents should match the private part.
882: This file is not
1.1 deraadt 883: really used for anything; it is only provided for the convenience of
884: the user so its contents can be copied to known hosts files.
1.7 markus 885: These two files are created using
886: .Xr ssh-keygen 1 .
1.2 deraadt 887: .It Pa /var/run/sshd.pid
888: Contains the process ID of the
889: .Nm
1.1 deraadt 890: listening for connections (if there are several daemons running
891: concurrently for different ports, this contains the pid of the one
1.36 aaron 892: started last).
1.58 deraadt 893: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 894: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 895: Lists the RSA keys that can be used to log into the user's account.
896: This file must be readable by root (which may on some machines imply
897: it being world-readable if the user's home directory resides on an NFS
1.36 aaron 898: volume).
899: It is recommended that it not be accessible by others.
900: The format of this file is described above.
1.44 deraadt 901: Users will place the contents of their
902: .Pa identity.pub
903: files into this file, as described in
904: .Xr ssh-keygen 1 .
905: .It Pa $HOME/.ssh/authorized_keys2
906: Lists the DSA keys that can be used to log into the user's account.
907: This file must be readable by root (which may on some machines imply
908: it being world-readable if the user's home directory resides on an NFS
909: volume).
910: It is recommended that it not be accessible by others.
911: The format of this file is described above.
912: Users will place the contents of their
913: .Pa id_dsa.pub
914: files into this file, as described in
915: .Xr ssh-keygen 1 .
1.22 markus 916: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
917: These files are consulted when using rhosts with RSA host
1.36 aaron 918: authentication to check the public key of the host.
919: The key must be listed in one of these files to be accepted.
1.22 markus 920: The client uses the same files
1.58 deraadt 921: to verify that the remote host is the one it intended to connect.
1.36 aaron 922: These files should be writable only by root/the owner.
1.2 deraadt 923: .Pa /etc/ssh_known_hosts
924: should be world-readable, and
925: .Pa $HOME/.ssh/known_hosts
926: can but need not be world-readable.
1.6 aaron 927: .It Pa /etc/nologin
1.40 aaron 928: If this file exists,
1.2 deraadt 929: .Nm
1.36 aaron 930: refuses to let anyone except root log in.
931: The contents of the file
1.1 deraadt 932: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 933: refused.
934: The file should be world-readable.
1.19 dugsong 935: .It Pa /etc/hosts.allow, /etc/hosts.deny
936: If compiled with
937: .Sy LIBWRAP
938: support, tcp-wrappers access controls may be defined here as described in
939: .Xr hosts_access 5 .
1.6 aaron 940: .It Pa $HOME/.rhosts
1.1 deraadt 941: This file contains host-username pairs, separated by a space, one per
1.36 aaron 942: line.
943: The given user on the corresponding host is permitted to log in
944: without password.
945: The same file is used by rlogind and rshd.
1.6 aaron 946: The file must
1.1 deraadt 947: be writable only by the user; it is recommended that it not be
948: accessible by others.
1.2 deraadt 949: .Pp
1.36 aaron 950: If is also possible to use netgroups in the file.
951: Either host or user
1.1 deraadt 952: name may be of the form +@groupname to specify all hosts or all users
953: in the group.
1.2 deraadt 954: .It Pa $HOME/.shosts
955: For ssh,
956: this file is exactly the same as for
957: .Pa .rhosts .
958: However, this file is
959: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 960: .It Pa /etc/hosts.equiv
1.2 deraadt 961: This file is used during
962: .Pa .rhosts
1.36 aaron 963: authentication.
964: In the simplest form, this file contains host names, one per line.
965: Users on
1.1 deraadt 966: those hosts are permitted to log in without a password, provided they
1.36 aaron 967: have the same user name on both machines.
968: The host name may also be
1.1 deraadt 969: followed by a user name; such users are permitted to log in as
1.2 deraadt 970: .Em any
1.36 aaron 971: user on this machine (except root).
972: Additionally, the syntax
1.2 deraadt 973: .Dq +@group
1.36 aaron 974: can be used to specify netgroups.
975: Negated entries start with
1.2 deraadt 976: .Ql \&- .
977: .Pp
1.1 deraadt 978: If the client host/user is successfully matched in this file, login is
979: automatically permitted provided the client and server user names are the
1.36 aaron 980: same.
981: Additionally, successful RSA host authentication is normally required.
982: This file must be writable only by root; it is recommended
1.1 deraadt 983: that it be world-readable.
1.2 deraadt 984: .Pp
1.6 aaron 985: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 986: .Pa hosts.equiv .
1.1 deraadt 987: Beware that it really means that the named user(s) can log in as
1.2 deraadt 988: .Em anybody ,
1.1 deraadt 989: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 990: binaries and directories.
991: Using a user name practically grants the user root access.
992: The only valid use for user names that I can think
1.1 deraadt 993: of is in negative entries.
1.2 deraadt 994: .Pp
995: Note that this warning also applies to rsh/rlogin.
996: .It Pa /etc/shosts.equiv
1.1 deraadt 997: This is processed exactly as
1.2 deraadt 998: .Pa /etc/hosts.equiv .
1.1 deraadt 999: However, this file may be useful in environments that want to run both
1.2 deraadt 1000: rsh/rlogin and ssh.
1.6 aaron 1001: .It Pa $HOME/.ssh/environment
1.36 aaron 1002: This file is read into the environment at login (if it exists).
1003: It can only contain empty lines, comment lines (that start with
1.2 deraadt 1004: .Ql # ) ,
1.36 aaron 1005: and assignment lines of the form name=value.
1006: The file should be writable
1.6 aaron 1007: only by the user; it need not be readable by anyone else.
1.2 deraadt 1008: .It Pa $HOME/.ssh/rc
1.1 deraadt 1009: If this file exists, it is run with /bin/sh after reading the
1.36 aaron 1010: environment files but before starting the user's shell or command.
1011: If X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 1012: standard input (and
1013: .Ev DISPLAY
1.36 aaron 1014: in environment).
1015: This must call
1.2 deraadt 1016: .Xr xauth 1
1017: in that case.
1018: .Pp
1.1 deraadt 1019: The primary purpose of this file is to run any initialization routines
1020: which may be needed before the user's home directory becomes
1021: accessible; AFS is a particular example of such an environment.
1.2 deraadt 1022: .Pp
1.1 deraadt 1023: This file will probably contain some initialization code followed by
1024: something similar to: "if read proto cookie; then echo add $DISPLAY
1025: $proto $cookie | xauth -q -; fi".
1.2 deraadt 1026: .Pp
1027: If this file does not exist,
1028: .Pa /etc/sshrc
1029: is run, and if that
1.1 deraadt 1030: does not exist either, xauth is used to store the cookie.
1.2 deraadt 1031: .Pp
1.1 deraadt 1032: This file should be writable only by the user, and need not be
1033: readable by anyone else.
1.2 deraadt 1034: .It Pa /etc/sshrc
1035: Like
1036: .Pa $HOME/.ssh/rc .
1037: This can be used to specify
1.36 aaron 1038: machine-specific login-time initializations globally.
1039: This file should be writable only by root, and should be world-readable.
1.56 aaron 1040: .El
1.2 deraadt 1041: .Sh AUTHOR
1.17 deraadt 1042: OpenSSH
1.33 deraadt 1043: is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
1.36 aaron 1044: but with bugs removed and newer features re-added.
1045: Rapidly after the
1.33 deraadt 1046: 1.2.12 release, newer versions of the original ssh bore successively
1047: more restrictive licenses, and thus demand for a free version was born.
1.48 markus 1048: .Pp
1.33 deraadt 1049: This version of OpenSSH
1.17 deraadt 1050: .Bl -bullet
1051: .It
1.32 aaron 1052: has all components of a restrictive nature (i.e., patents, see
1.17 deraadt 1053: .Xr ssl 8 )
1054: directly removed from the source code; any licensed or patented components
1055: are chosen from
1056: external libraries.
1057: .It
1.48 markus 1058: has been updated to support SSH protocol 1.5 and 2, making it compatible with
1059: all other SSH clients and servers.
1.17 deraadt 1060: .It
1.40 aaron 1061: contains added support for
1.17 deraadt 1062: .Xr kerberos 8
1063: authentication and ticket passing.
1064: .It
1065: supports one-time password authentication with
1066: .Xr skey 1 .
1067: .El
1.48 markus 1068: .Pp
1069: OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
1070: Niels Provos, Theo de Raadt, and Dug Song.
1071: .Pp
1072: The support for SSH protocol 2 was written by Markus Friedl.
1.2 deraadt 1073: .Sh SEE ALSO
1074: .Xr scp 1 ,
1.63 markus 1075: .Xr sftp-server 8 ,
1.2 deraadt 1076: .Xr ssh 1 ,
1.5 deraadt 1077: .Xr ssh-add 1 ,
1.2 deraadt 1078: .Xr ssh-agent 1 ,
1.5 deraadt 1079: .Xr ssh-keygen 1 ,
1.38 deraadt 1080: .Xr ssl 8 ,
1081: .Xr rlogin 1 ,
1082: .Xr rsh 1
![[BACK]](/icons/back.gif){kind=icon}
Return to sshd.8 CVS log ![[TXT]](/icons/text.gif){kind=icon}
![[DIR]](/icons/dir.gif){kind=icon}
Up to [local] / src / usr.bin / ssh