Annotation of src/usr.bin/ssh/sshd.8, Revision 1.65
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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35: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.
1.65 ! markus 189: Multiple -d options increases the debugging level. Maximum is 3.
1.2 deraadt 190: .It Fl f Ar configuration_file
1.36 aaron 191: Specifies the name of the configuration file.
192: The default is
1.2 deraadt 193: .Pa /etc/sshd_config .
1.16 markus 194: .Nm
195: refuses to start if there is no configuration file.
1.2 deraadt 196: .It Fl g Ar login_grace_time
1.1 deraadt 197: Gives the grace time for clients to authenticate themselves (default
1.36 aaron 198: 300 seconds).
199: If the client fails to authenticate the user within
200: this many seconds, the server disconnects and exits.
201: A value of zero indicates no limit.
1.2 deraadt 202: .It Fl h Ar host_key_file
1.49 markus 203: Specifies the file from which the RSA host key is read (default
1.2 deraadt 204: .Pa /etc/ssh_host_key ) .
1.7 markus 205: This option must be given if
206: .Nm
207: is not run as root (as the normal
1.1 deraadt 208: host file is normally not readable by anyone but root).
1.2 deraadt 209: .It Fl i
1.7 markus 210: Specifies that
211: .Nm
1.40 aaron 212: is being run from inetd.
1.7 markus 213: .Nm
214: is normally not run
1.1 deraadt 215: from inetd because it needs to generate the server key before it can
1.36 aaron 216: respond to the client, and this may take tens of seconds.
217: Clients would have to wait too long if the key was regenerated every time.
1.35 aaron 218: However, with small key sizes (e.g., 512) using
1.7 markus 219: .Nm
220: from inetd may
1.1 deraadt 221: be feasible.
1.2 deraadt 222: .It Fl k Ar key_gen_time
1.1 deraadt 223: Specifies how often the server key is regenerated (default 3600
1.36 aaron 224: seconds, or one hour).
225: The motivation for regenerating the key fairly
1.1 deraadt 226: often is that the key is not stored anywhere, and after about an hour,
227: it becomes impossible to recover the key for decrypting intercepted
228: communications even if the machine is cracked into or physically
1.36 aaron 229: seized.
230: A value of zero indicates that the key will never be regenerated.
1.2 deraadt 231: .It Fl p Ar port
1.1 deraadt 232: Specifies the port on which the server listens for connections
233: (default 22).
1.2 deraadt 234: .It Fl q
1.36 aaron 235: Quiet mode.
236: Nothing is sent to the system log.
237: Normally the beginning,
1.1 deraadt 238: authentication, and termination of each connection is logged.
1.61 markus 239: .It Fl u Ar len
240: This option is used to specify the size of the field
241: in the
242: .Li utmp
243: structure that holds the remote host name.
244: If the resolved host name is longer than
245: .Ar len ,
246: the dotted decimal value will be used instead.
247: This allows hosts with very long host names that
248: overflow this field to still be uniquely identified.
249: Specifying
250: .Fl u0
251: indicates that only dotted decimal addresses
252: should be put into the
253: .Pa utmp
254: file.
1.18 aaron 255: .It Fl Q
256: Do not print an error message if RSA support is missing.
1.26 markus 257: .It Fl V Ar client_protocol_id
258: SSH2 compatibility mode.
1.42 hugh 259: When this option is specified
1.26 markus 260: .Nm
1.42 hugh 261: assumes the client has sent the supplied version string
1.26 markus 262: and skips the
263: Protocol Version Identification Exchange.
1.29 markus 264: .It Fl 4
265: Forces
266: .Nm
267: to use IPv4 addresses only.
268: .It Fl 6
269: Forces
270: .Nm
271: to use IPv6 addresses only.
1.2 deraadt 272: .El
273: .Sh CONFIGURATION FILE
274: .Nm
1.40 aaron 275: reads configuration data from
1.2 deraadt 276: .Pa /etc/sshd_config
277: (or the file specified with
278: .Fl f
1.36 aaron 279: on the command line).
280: The file contains keyword-value pairs, one per line.
281: Lines starting with
1.2 deraadt 282: .Ql #
1.1 deraadt 283: and empty lines are interpreted as comments.
1.2 deraadt 284: .Pp
1.1 deraadt 285: The following keywords are possible.
1.2 deraadt 286: .Bl -tag -width Ds
287: .It Cm AFSTokenPassing
1.36 aaron 288: Specifies whether an AFS token may be forwarded to the server.
289: Default is
1.2 deraadt 290: .Dq yes .
1.11 markus 291: .It Cm AllowGroups
292: This keyword can be followed by a number of group names, separated
1.36 aaron 293: by spaces.
294: If specified, login is allowed only for users whose primary
1.11 markus 295: group matches one of the patterns.
296: .Ql \&*
297: and
298: .Ql ?
299: can be used as
1.36 aaron 300: wildcards in the patterns.
1.58 deraadt 301: Only group names are valid; a numerical group ID isn't recognized.
1.36 aaron 302: By default login is allowed regardless of the primary group.
1.3 dugsong 303: .Pp
1.11 markus 304: .It Cm AllowUsers
305: This keyword can be followed by a number of user names, separated
1.36 aaron 306: by spaces.
307: If specified, login is allowed only for users names that
1.11 markus 308: match one of the patterns.
309: .Ql \&*
310: and
311: .Ql ?
312: can be used as
1.36 aaron 313: wildcards in the patterns.
1.58 deraadt 314: Only user names are valid; a numerical user ID isn't recognized.
1.36 aaron 315: By default login is allowed regardless of the user name.
1.11 markus 316: .Pp
1.41 markus 317: .It Cm Ciphers
318: Specifies the ciphers allowed for protocol version 2.
319: Multiple ciphers must be comma-separated.
320: The default is
1.50 markus 321: .Dq 3des-cbc,blowfish-cbc,arcfour,cast128-cbc .
1.8 markus 322: .It Cm CheckMail
323: Specifies whether
324: .Nm
325: should check for new mail for interactive logins.
326: The default is
327: .Dq no .
1.11 markus 328: .It Cm DenyGroups
329: This keyword can be followed by a number of group names, separated
1.36 aaron 330: by spaces.
331: Users whose primary group matches one of the patterns
1.11 markus 332: aren't allowed to log in.
333: .Ql \&*
334: and
335: .Ql ?
336: can be used as
1.36 aaron 337: wildcards in the patterns.
1.58 deraadt 338: Only group names are valid; a numerical group ID isn't recognized.
1.36 aaron 339: By default login is allowed regardless of the primary group.
1.11 markus 340: .Pp
341: .It Cm DenyUsers
342: This keyword can be followed by a number of user names, separated
1.36 aaron 343: by spaces.
344: Login is disallowed for user names that match one of the patterns.
1.11 markus 345: .Ql \&*
346: and
347: .Ql ?
1.36 aaron 348: can be used as wildcards in the patterns.
1.58 deraadt 349: Only user names are valid; a numerical user ID isn't recognized.
1.36 aaron 350: By default login is allowed regardless of the user name.
1.49 markus 351: .It Cm DSAAuthentication
352: Specifies whether DSA authentication is allowed.
353: The default is
354: .Dq yes .
355: Note that this option applies to protocol version 2 only.
1.47 markus 356: .It Cm GatewayPorts
357: Specifies whether remote hosts are allowed to connect to ports
358: forwarded for the client.
359: The argument must be
360: .Dq yes
361: or
362: .Dq no .
363: The default is
364: .Dq no .
1.58 deraadt 365: .It Cm HostDSAKey
1.46 markus 366: Specifies the file containing the private DSA host key (default
367: .Pa /etc/ssh_host_dsa_key )
368: used by SSH protocol 2.0.
369: Note that
370: .Nm
1.51 hugh 371: disables protocol 2.0 if this file is group/world-accessible.
1.2 deraadt 372: .It Cm HostKey
1.46 markus 373: Specifies the file containing the private RSA host key (default
374: .Pa /etc/ssh_host_key )
375: used by SSH protocols 1.3 and 1.5.
1.9 markus 376: Note that
377: .Nm
1.51 hugh 378: disables protocols 1.3 and 1.5 if this file is group/world-accessible.
1.2 deraadt 379: .It Cm IgnoreRhosts
1.34 markus 380: Specifies that
381: .Pa .rhosts
1.40 aaron 382: and
1.34 markus 383: .Pa .shosts
384: files will not be used in authentication.
1.2 deraadt 385: .Pa /etc/hosts.equiv
1.1 deraadt 386: and
1.40 aaron 387: .Pa /etc/shosts.equiv
1.36 aaron 388: are still used.
1.40 aaron 389: The default is
1.34 markus 390: .Dq yes .
1.24 markus 391: .It Cm IgnoreUserKnownHosts
392: Specifies whether
393: .Nm
394: should ignore the user's
395: .Pa $HOME/.ssh/known_hosts
1.45 markus 396: during
1.24 markus 397: .Cm RhostsRSAAuthentication .
398: The default is
1.2 deraadt 399: .Dq no .
400: .It Cm KeepAlive
1.1 deraadt 401: Specifies whether the system should send keepalive messages to the
1.36 aaron 402: other side.
403: If they are sent, death of the connection or crash of one
404: of the machines will be properly noticed.
405: However, this means that
1.1 deraadt 406: connections will die if the route is down temporarily, and some people
1.36 aaron 407: find it annoying.
1.51 hugh 408: On the other hand, if keepalives are not sent,
1.2 deraadt 409: sessions may hang indefinitely on the server, leaving
410: .Dq ghost
411: users and consuming server resources.
412: .Pp
413: The default is
414: .Dq yes
415: (to send keepalives), and the server will notice
1.36 aaron 416: if the network goes down or the client host reboots.
417: This avoids infinitely hanging sessions.
1.2 deraadt 418: .Pp
419: To disable keepalives, the value should be set to
420: .Dq no
421: in both the server and the client configuration files.
422: .It Cm KerberosAuthentication
1.36 aaron 423: Specifies whether Kerberos authentication is allowed.
424: This can be in the form of a Kerberos ticket, or if
1.7 markus 425: .Cm PasswordAuthentication
1.1 deraadt 426: is yes, the password provided by the user will be validated through
1.59 provos 427: the Kerberos KDC. To use this option, the server needs a
428: Kerberos servtab which allows the verification of the KDC's identity.
1.36 aaron 429: Default is
1.60 provos 430: .Dq yes .
1.2 deraadt 431: .It Cm KerberosOrLocalPasswd
1.1 deraadt 432: If set then if password authentication through Kerberos fails then
433: the password will be validated via any additional local mechanism
1.2 deraadt 434: such as
435: .Pa /etc/passwd
1.36 aaron 436: or SecurID.
437: Default is
1.20 dugsong 438: .Dq yes .
1.2 deraadt 439: .It Cm KerberosTgtPassing
1.1 deraadt 440: Specifies whether a Kerberos TGT may be forwarded to the server.
1.40 aaron 441: Default is
1.3 dugsong 442: .Dq no ,
443: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 444: .It Cm KerberosTicketCleanup
1.7 markus 445: Specifies whether to automatically destroy the user's ticket cache
1.36 aaron 446: file on logout.
447: Default is
1.3 dugsong 448: .Dq yes .
1.2 deraadt 449: .It Cm KeyRegenerationInterval
1.1 deraadt 450: The server key is automatically regenerated after this many seconds
1.36 aaron 451: (if it has been used).
452: The purpose of regeneration is to prevent
1.1 deraadt 453: decrypting captured sessions by later breaking into the machine and
1.36 aaron 454: stealing the keys.
455: The key is never stored anywhere.
456: If the value is 0, the key is never regenerated.
457: The default is 3600 (seconds).
1.7 markus 458: .It Cm ListenAddress
459: Specifies what local address
460: .Nm
461: should listen on.
462: The default is to listen to all local addresses.
1.28 markus 463: Multiple options of this type are permitted.
464: Additionally, the
465: .Cm Ports
466: options must precede this option.
1.2 deraadt 467: .It Cm LoginGraceTime
1.1 deraadt 468: The server disconnects after this time if the user has not
1.36 aaron 469: successfully logged in.
470: If the value is 0, there is no time limit.
1.1 deraadt 471: The default is 600 (seconds).
1.23 markus 472: .It Cm LogLevel
473: Gives the verbosity level that is used when logging messages from
474: .Nm sshd .
475: The possible values are:
1.27 markus 476: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
1.23 markus 477: The default is INFO.
478: Logging with level DEBUG violates the privacy of users
479: and is not recommended.
1.55 markus 480: .It Cm MaxStartups
481: Specifies the maximum number of concurrent unauthenticated connections to the
482: .Nm
483: daemon.
484: Additional connections will be dropped until authentication succeeds or the
485: .Cm LoginGraceTime
486: expires for a connection.
487: The default is 10.
1.57 markus 488: .Pp
489: Alternatively, random early drop can be enabled by specifying
490: the three colon separated values
491: .Dq start:rate:full
492: (e.g. "10:30:60").
493: .Nm
494: will refuse connection attempts with a probabillity of
495: .Dq rate/100
496: (30%)
497: if there are currently
498: .Dq start
499: (10)
500: unauthenticated connections.
501: The probabillity increases linearly and all connection attempts
502: are refused if the number of unauthenticated connections reaches
503: .Dq full
504: (60).
1.2 deraadt 505: .It Cm PasswordAuthentication
1.1 deraadt 506: Specifies whether password authentication is allowed.
1.2 deraadt 507: The default is
508: .Dq yes .
1.58 deraadt 509: Note that this option applies to both protocol versions 1 and 2.
1.2 deraadt 510: .It Cm PermitEmptyPasswords
1.1 deraadt 511: When password authentication is allowed, it specifies whether the
1.36 aaron 512: server allows login to accounts with empty password strings.
513: The default is
1.34 markus 514: .Dq no .
1.2 deraadt 515: .It Cm PermitRootLogin
1.1 deraadt 516: Specifies whether the root can log in using
1.2 deraadt 517: .Xr ssh 1 .
1.15 markus 518: The argument must be
519: .Dq yes ,
520: .Dq without-password
521: or
522: .Dq no .
1.2 deraadt 523: The default is
524: .Dq yes .
1.15 markus 525: If this options is set to
526: .Dq without-password
527: only password authentication is disabled for root.
1.2 deraadt 528: .Pp
529: Root login with RSA authentication when the
530: .Ar command
531: option has been
1.1 deraadt 532: specified will be allowed regardless of the value of this setting
533: (which may be useful for taking remote backups even if root login is
534: normally not allowed).
1.43 markus 535: .It Cm PidFile
536: Specifies the file that contains the process identifier of the
537: .Nm
538: daemon.
539: The default is
540: .Pa /var/run/sshd.pid .
1.2 deraadt 541: .It Cm Port
1.1 deraadt 542: Specifies the port number that
1.2 deraadt 543: .Nm
1.36 aaron 544: listens on.
545: The default is 22.
1.28 markus 546: Multiple options of this type are permitted.
1.2 deraadt 547: .It Cm PrintMotd
1.1 deraadt 548: Specifies whether
1.2 deraadt 549: .Nm
1.40 aaron 550: should print
1.2 deraadt 551: .Pa /etc/motd
1.36 aaron 552: when a user logs in interactively.
553: (On some systems it is also printed by the shell,
1.2 deraadt 554: .Pa /etc/profile ,
1.36 aaron 555: or equivalent.)
556: The default is
1.2 deraadt 557: .Dq yes .
1.41 markus 558: .It Cm Protocol
559: Specifies the protocol versions
560: .Nm
561: should support.
562: The possible values are
563: .Dq 1
564: and
565: .Dq 2 .
566: Multiple versions must be comma-separated.
567: The default is
568: .Dq 1 .
1.2 deraadt 569: .It Cm RandomSeed
1.36 aaron 570: Obsolete.
571: Random number generation uses other techniques.
1.2 deraadt 572: .It Cm RhostsAuthentication
1.1 deraadt 573: Specifies whether authentication using rhosts or /etc/hosts.equiv
1.36 aaron 574: files is sufficient.
575: Normally, this method should not be permitted because it is insecure.
1.7 markus 576: .Cm RhostsRSAAuthentication
577: should be used
1.1 deraadt 578: instead, because it performs RSA-based host authentication in addition
579: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 580: The default is
581: .Dq no .
582: .It Cm RhostsRSAAuthentication
1.1 deraadt 583: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.36 aaron 584: with successful RSA host authentication is allowed.
585: The default is
1.34 markus 586: .Dq no .
1.2 deraadt 587: .It Cm RSAAuthentication
1.36 aaron 588: Specifies whether pure RSA authentication is allowed.
589: The default is
1.2 deraadt 590: .Dq yes .
1.49 markus 591: Note that this option applies to protocol version 1 only.
1.2 deraadt 592: .It Cm ServerKeyBits
1.36 aaron 593: Defines the number of bits in the server key.
594: The minimum value is 512, and the default is 768.
1.7 markus 595: .It Cm SkeyAuthentication
596: Specifies whether
1.40 aaron 597: .Xr skey 1
1.36 aaron 598: authentication is allowed.
599: The default is
1.7 markus 600: .Dq yes .
601: Note that s/key authentication is enabled only if
602: .Cm PasswordAuthentication
603: is allowed, too.
1.2 deraadt 604: .It Cm StrictModes
1.12 markus 605: Specifies whether
606: .Nm
607: should check file modes and ownership of the
1.36 aaron 608: user's files and home directory before accepting login.
609: This is normally desirable because novices sometimes accidentally leave their
610: directory or files world-writable.
611: The default is
1.7 markus 612: .Dq yes .
1.54 jakob 613: .It Cm Subsystem
614: Configures an external subsystem (e.g. file transfer daemon).
615: Arguments should be a subsystem name and a command to execute upon subsystem request.
1.63 markus 616: The command
617: .Xr sftp-server 8
618: implements the
619: .Dq sftp
620: file transfer subsystem.
1.54 jakob 621: By default no subsystems are defined.
622: Note that this option applies to protocol version 2 only.
1.2 deraadt 623: .It Cm SyslogFacility
1.1 deraadt 624: Gives the facility code that is used when logging messages from
1.2 deraadt 625: .Nm sshd .
1.1 deraadt 626: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.36 aaron 627: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
628: The default is AUTH.
1.10 markus 629: .It Cm UseLogin
630: Specifies whether
631: .Xr login 1
1.53 markus 632: is used for interactive login sessions.
633: Note that
634: .Xr login 1
1.58 deraadt 635: is never used for remote command execution.
1.36 aaron 636: The default is
1.10 markus 637: .Dq no .
1.6 aaron 638: .It Cm X11DisplayOffset
639: Specifies the first display number available for
640: .Nm sshd Ns 's
1.36 aaron 641: X11 forwarding.
642: This prevents
1.6 aaron 643: .Nm
644: from interfering with real X11 servers.
1.34 markus 645: The default is 10.
1.30 markus 646: .It Cm X11Forwarding
1.36 aaron 647: Specifies whether X11 forwarding is permitted.
648: The default is
1.34 markus 649: .Dq no .
1.30 markus 650: Note that disabling X11 forwarding does not improve security in any
651: way, as users can always install their own forwarders.
1.52 markus 652: .It Cm XAuthLocation
653: Specifies the location of the
654: .Xr xauth 1
655: program.
656: The default is
657: .Pa /usr/X11R6/bin/xauth .
1.2 deraadt 658: .El
659: .Sh LOGIN PROCESS
1.1 deraadt 660: When a user successfully logs in,
1.2 deraadt 661: .Nm
1.1 deraadt 662: does the following:
1.2 deraadt 663: .Bl -enum -offset indent
664: .It
1.1 deraadt 665: If the login is on a tty, and no command has been specified,
1.40 aaron 666: prints last login time and
1.2 deraadt 667: .Pa /etc/motd
1.1 deraadt 668: (unless prevented in the configuration file or by
1.2 deraadt 669: .Pa $HOME/.hushlogin ;
670: see the
1.40 aaron 671: .Sx FILES
1.2 deraadt 672: section).
673: .It
1.1 deraadt 674: If the login is on a tty, records login time.
1.2 deraadt 675: .It
676: Checks
677: .Pa /etc/nologin ;
678: if it exists, prints contents and quits
1.1 deraadt 679: (unless root).
1.2 deraadt 680: .It
1.1 deraadt 681: Changes to run with normal user privileges.
1.2 deraadt 682: .It
1.1 deraadt 683: Sets up basic environment.
1.2 deraadt 684: .It
685: Reads
686: .Pa $HOME/.ssh/environment
687: if it exists.
688: .It
1.1 deraadt 689: Changes to user's home directory.
1.2 deraadt 690: .It
691: If
692: .Pa $HOME/.ssh/rc
693: exists, runs it; else if
694: .Pa /etc/sshrc
695: exists, runs
1.36 aaron 696: it; otherwise runs xauth.
697: The
1.2 deraadt 698: .Dq rc
699: files are given the X11
1.1 deraadt 700: authentication protocol and cookie in standard input.
1.2 deraadt 701: .It
1.1 deraadt 702: Runs user's shell or command.
1.2 deraadt 703: .El
704: .Sh AUTHORIZED_KEYS FILE FORMAT
1.40 aaron 705: The
1.2 deraadt 706: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 707: file lists the RSA keys that are
1.44 deraadt 708: permitted for RSA authentication in SSH protocols 1.3 and 1.5
1.51 hugh 709: Similarly, the
1.44 deraadt 710: .Pa $HOME/.ssh/authorized_keys2
711: file lists the DSA keys that are
712: permitted for DSA authentication in SSH protocol 2.0.
1.36 aaron 713: Each line of the file contains one
1.2 deraadt 714: key (empty lines and lines starting with a
715: .Ql #
716: are ignored as
1.36 aaron 717: comments).
718: Each line consists of the following fields, separated by
719: spaces: options, bits, exponent, modulus, comment.
720: The options field
1.1 deraadt 721: is optional; its presence is determined by whether the line starts
722: with a number or not (the option field never starts with a number).
723: The bits, exponent, modulus and comment fields give the RSA key; the
724: comment field is not used for anything (but may be convenient for the
725: user to identify the key).
1.2 deraadt 726: .Pp
1.1 deraadt 727: Note that lines in this file are usually several hundred bytes long
1.36 aaron 728: (because of the size of the RSA key modulus).
729: You don't want to type them in; instead, copy the
1.2 deraadt 730: .Pa identity.pub
1.1 deraadt 731: file and edit it.
1.2 deraadt 732: .Pp
1.58 deraadt 733: The options (if present) consist of comma-separated option
1.36 aaron 734: specifications.
735: No spaces are permitted, except within double quotes.
1.1 deraadt 736: The following option specifications are supported:
1.2 deraadt 737: .Bl -tag -width Ds
738: .It Cm from="pattern-list"
1.1 deraadt 739: Specifies that in addition to RSA authentication, the canonical name
740: of the remote host must be present in the comma-separated list of
1.36 aaron 741: patterns
742: .Pf ( Ql *
743: and
744: .Ql ?
745: serve as wildcards).
746: The list may also contain
747: patterns negated by prefixing them with
748: .Ql ! ;
749: if the canonical host name matches a negated pattern, the key is not accepted.
750: The purpose
1.1 deraadt 751: of this option is to optionally increase security: RSA authentication
752: by itself does not trust the network or name servers or anything (but
753: the key); however, if somebody somehow steals the key, the key
1.36 aaron 754: permits an intruder to log in from anywhere in the world.
755: This additional option makes using a stolen key more difficult (name
1.1 deraadt 756: servers and/or routers would have to be compromised in addition to
757: just the key).
1.2 deraadt 758: .It Cm command="command"
1.1 deraadt 759: Specifies that the command is executed whenever this key is used for
1.36 aaron 760: authentication.
761: The command supplied by the user (if any) is ignored.
1.1 deraadt 762: The command is run on a pty if the connection requests a pty;
1.36 aaron 763: otherwise it is run without a tty.
764: A quote may be included in the command by quoting it with a backslash.
765: This option might be useful
766: to restrict certain RSA keys to perform just a specific operation.
767: An example might be a key that permits remote backups but nothing else.
1.51 hugh 768: Note that the client may specify TCP/IP and/or X11
769: forwarding unless they are explicitly prohibited.
1.2 deraadt 770: .It Cm environment="NAME=value"
1.1 deraadt 771: Specifies that the string is to be added to the environment when
1.36 aaron 772: logging in using this key.
773: Environment variables set this way
774: override other default environment values.
775: Multiple options of this type are permitted.
1.2 deraadt 776: .It Cm no-port-forwarding
1.1 deraadt 777: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 778: Any port forward requests by the client will return an error.
779: This might be used, e.g., in connection with the
1.2 deraadt 780: .Cm command
1.1 deraadt 781: option.
1.2 deraadt 782: .It Cm no-X11-forwarding
1.1 deraadt 783: Forbids X11 forwarding when this key is used for authentication.
784: Any X11 forward requests by the client will return an error.
1.2 deraadt 785: .It Cm no-agent-forwarding
1.1 deraadt 786: Forbids authentication agent forwarding when this key is used for
787: authentication.
1.2 deraadt 788: .It Cm no-pty
1.1 deraadt 789: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 790: .El
791: .Ss Examples
1.1 deraadt 792: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 793: .Pp
1.1 deraadt 794: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 795: .Pp
1.1 deraadt 796: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.2 deraadt 797: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 798: The
1.44 deraadt 799: .Pa /etc/ssh_known_hosts ,
800: .Pa /etc/ssh_known_hosts2 ,
801: .Pa $HOME/.ssh/known_hosts ,
1.40 aaron 802: and
1.44 deraadt 803: .Pa $HOME/.ssh/known_hosts2
1.36 aaron 804: files contain host public keys for all known hosts.
805: The global file should
1.37 brad 806: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 807: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 808: its key is added to the per-user file.
1.2 deraadt 809: .Pp
1.1 deraadt 810: Each line in these files contains the following fields: hostnames,
1.36 aaron 811: bits, exponent, modulus, comment.
812: The fields are separated by spaces.
1.2 deraadt 813: .Pp
1.1 deraadt 814: Hostnames is a comma-separated list of patterns ('*' and '?' act as
815: wildcards); each pattern in turn is matched against the canonical host
816: name (when authenticating a client) or against the user-supplied
1.36 aaron 817: name (when authenticating a server).
818: A pattern may also be preceded by
1.2 deraadt 819: .Ql !
820: to indicate negation: if the host name matches a negated
1.1 deraadt 821: pattern, it is not accepted (by that line) even if it matched another
822: pattern on the line.
1.2 deraadt 823: .Pp
1.49 markus 824: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 825: can be obtained, e.g., from
826: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 827: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 828: .Pp
829: Lines starting with
830: .Ql #
831: and empty lines are ignored as comments.
832: .Pp
1.1 deraadt 833: When performing host authentication, authentication is accepted if any
1.36 aaron 834: matching line has the proper key.
835: It is thus permissible (but not
1.1 deraadt 836: recommended) to have several lines or different host keys for the same
1.36 aaron 837: names.
838: This will inevitably happen when short forms of host names
839: from different domains are put in the file.
840: It is possible
1.1 deraadt 841: that the files contain conflicting information; authentication is
842: accepted if valid information can be found from either file.
1.2 deraadt 843: .Pp
1.1 deraadt 844: Note that the lines in these files are typically hundreds of characters
845: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 846: Rather, generate them by a script
1.40 aaron 847: or by taking
1.2 deraadt 848: .Pa /etc/ssh_host_key.pub
1.1 deraadt 849: and adding the host names at the front.
1.2 deraadt 850: .Ss Examples
1.1 deraadt 851: closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1.2 deraadt 852: .Sh FILES
853: .Bl -tag -width Ds
854: .It Pa /etc/sshd_config
1.1 deraadt 855: Contains configuration data for
1.2 deraadt 856: .Nm sshd .
1.1 deraadt 857: This file should be writable by root only, but it is recommended
858: (though not necessary) that it be world-readable.
1.2 deraadt 859: .It Pa /etc/ssh_host_key
1.7 markus 860: Contains the private part of the host key.
1.1 deraadt 861: This file should only be owned by root, readable only by root, and not
862: accessible to others.
1.14 markus 863: Note that
864: .Nm
865: does not start if this file is group/world-accessible.
1.2 deraadt 866: .It Pa /etc/ssh_host_key.pub
1.7 markus 867: Contains the public part of the host key.
868: This file should be world-readable but writable only by
1.36 aaron 869: root.
870: Its contents should match the private part.
871: This file is not
1.1 deraadt 872: really used for anything; it is only provided for the convenience of
873: the user so its contents can be copied to known hosts files.
1.7 markus 874: These two files are created using
875: .Xr ssh-keygen 1 .
1.2 deraadt 876: .It Pa /var/run/sshd.pid
877: Contains the process ID of the
878: .Nm
1.1 deraadt 879: listening for connections (if there are several daemons running
880: concurrently for different ports, this contains the pid of the one
1.36 aaron 881: started last).
1.58 deraadt 882: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 883: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 884: Lists the RSA keys that can be used to log into the user's account.
885: This file must be readable by root (which may on some machines imply
886: it being world-readable if the user's home directory resides on an NFS
1.36 aaron 887: volume).
888: It is recommended that it not be accessible by others.
889: The format of this file is described above.
1.44 deraadt 890: Users will place the contents of their
891: .Pa identity.pub
892: files into this file, as described in
893: .Xr ssh-keygen 1 .
894: .It Pa $HOME/.ssh/authorized_keys2
895: Lists the DSA keys that can be used to log into the user's account.
896: This file must be readable by root (which may on some machines imply
897: it being world-readable if the user's home directory resides on an NFS
898: volume).
899: It is recommended that it not be accessible by others.
900: The format of this file is described above.
901: Users will place the contents of their
902: .Pa id_dsa.pub
903: files into this file, as described in
904: .Xr ssh-keygen 1 .
1.22 markus 905: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
906: These files are consulted when using rhosts with RSA host
1.36 aaron 907: authentication to check the public key of the host.
908: The key must be listed in one of these files to be accepted.
1.22 markus 909: The client uses the same files
1.58 deraadt 910: to verify that the remote host is the one it intended to connect.
1.36 aaron 911: These files should be writable only by root/the owner.
1.2 deraadt 912: .Pa /etc/ssh_known_hosts
913: should be world-readable, and
914: .Pa $HOME/.ssh/known_hosts
915: can but need not be world-readable.
1.6 aaron 916: .It Pa /etc/nologin
1.40 aaron 917: If this file exists,
1.2 deraadt 918: .Nm
1.36 aaron 919: refuses to let anyone except root log in.
920: The contents of the file
1.1 deraadt 921: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 922: refused.
923: The file should be world-readable.
1.19 dugsong 924: .It Pa /etc/hosts.allow, /etc/hosts.deny
925: If compiled with
926: .Sy LIBWRAP
927: support, tcp-wrappers access controls may be defined here as described in
928: .Xr hosts_access 5 .
1.6 aaron 929: .It Pa $HOME/.rhosts
1.1 deraadt 930: This file contains host-username pairs, separated by a space, one per
1.36 aaron 931: line.
932: The given user on the corresponding host is permitted to log in
933: without password.
934: The same file is used by rlogind and rshd.
1.6 aaron 935: The file must
1.1 deraadt 936: be writable only by the user; it is recommended that it not be
937: accessible by others.
1.2 deraadt 938: .Pp
1.36 aaron 939: If is also possible to use netgroups in the file.
940: Either host or user
1.1 deraadt 941: name may be of the form +@groupname to specify all hosts or all users
942: in the group.
1.2 deraadt 943: .It Pa $HOME/.shosts
944: For ssh,
945: this file is exactly the same as for
946: .Pa .rhosts .
947: However, this file is
948: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 949: .It Pa /etc/hosts.equiv
1.2 deraadt 950: This file is used during
951: .Pa .rhosts
1.36 aaron 952: authentication.
953: In the simplest form, this file contains host names, one per line.
954: Users on
1.1 deraadt 955: those hosts are permitted to log in without a password, provided they
1.36 aaron 956: have the same user name on both machines.
957: The host name may also be
1.1 deraadt 958: followed by a user name; such users are permitted to log in as
1.2 deraadt 959: .Em any
1.36 aaron 960: user on this machine (except root).
961: Additionally, the syntax
1.2 deraadt 962: .Dq +@group
1.36 aaron 963: can be used to specify netgroups.
964: Negated entries start with
1.2 deraadt 965: .Ql \&- .
966: .Pp
1.1 deraadt 967: If the client host/user is successfully matched in this file, login is
968: automatically permitted provided the client and server user names are the
1.36 aaron 969: same.
970: Additionally, successful RSA host authentication is normally required.
971: This file must be writable only by root; it is recommended
1.1 deraadt 972: that it be world-readable.
1.2 deraadt 973: .Pp
1.6 aaron 974: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 975: .Pa hosts.equiv .
1.1 deraadt 976: Beware that it really means that the named user(s) can log in as
1.2 deraadt 977: .Em anybody ,
1.1 deraadt 978: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 979: binaries and directories.
980: Using a user name practically grants the user root access.
981: The only valid use for user names that I can think
1.1 deraadt 982: of is in negative entries.
1.2 deraadt 983: .Pp
984: Note that this warning also applies to rsh/rlogin.
985: .It Pa /etc/shosts.equiv
1.1 deraadt 986: This is processed exactly as
1.2 deraadt 987: .Pa /etc/hosts.equiv .
1.1 deraadt 988: However, this file may be useful in environments that want to run both
1.2 deraadt 989: rsh/rlogin and ssh.
1.6 aaron 990: .It Pa $HOME/.ssh/environment
1.36 aaron 991: This file is read into the environment at login (if it exists).
992: It can only contain empty lines, comment lines (that start with
1.2 deraadt 993: .Ql # ) ,
1.36 aaron 994: and assignment lines of the form name=value.
995: The file should be writable
1.6 aaron 996: only by the user; it need not be readable by anyone else.
1.2 deraadt 997: .It Pa $HOME/.ssh/rc
1.1 deraadt 998: If this file exists, it is run with /bin/sh after reading the
1.36 aaron 999: environment files but before starting the user's shell or command.
1000: If X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 1001: standard input (and
1002: .Ev DISPLAY
1.36 aaron 1003: in environment).
1004: This must call
1.2 deraadt 1005: .Xr xauth 1
1006: in that case.
1007: .Pp
1.1 deraadt 1008: The primary purpose of this file is to run any initialization routines
1009: which may be needed before the user's home directory becomes
1010: accessible; AFS is a particular example of such an environment.
1.2 deraadt 1011: .Pp
1.1 deraadt 1012: This file will probably contain some initialization code followed by
1013: something similar to: "if read proto cookie; then echo add $DISPLAY
1014: $proto $cookie | xauth -q -; fi".
1.2 deraadt 1015: .Pp
1016: If this file does not exist,
1017: .Pa /etc/sshrc
1018: is run, and if that
1.1 deraadt 1019: does not exist either, xauth is used to store the cookie.
1.2 deraadt 1020: .Pp
1.1 deraadt 1021: This file should be writable only by the user, and need not be
1022: readable by anyone else.
1.2 deraadt 1023: .It Pa /etc/sshrc
1024: Like
1025: .Pa $HOME/.ssh/rc .
1026: This can be used to specify
1.36 aaron 1027: machine-specific login-time initializations globally.
1028: This file should be writable only by root, and should be world-readable.
1.56 aaron 1029: .El
1.2 deraadt 1030: .Sh AUTHOR
1.17 deraadt 1031: OpenSSH
1.33 deraadt 1032: is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
1.36 aaron 1033: but with bugs removed and newer features re-added.
1034: Rapidly after the
1.33 deraadt 1035: 1.2.12 release, newer versions of the original ssh bore successively
1036: more restrictive licenses, and thus demand for a free version was born.
1.48 markus 1037: .Pp
1.33 deraadt 1038: This version of OpenSSH
1.17 deraadt 1039: .Bl -bullet
1040: .It
1.32 aaron 1041: has all components of a restrictive nature (i.e., patents, see
1.17 deraadt 1042: .Xr ssl 8 )
1043: directly removed from the source code; any licensed or patented components
1044: are chosen from
1045: external libraries.
1046: .It
1.48 markus 1047: has been updated to support SSH protocol 1.5 and 2, making it compatible with
1048: all other SSH clients and servers.
1.17 deraadt 1049: .It
1.40 aaron 1050: contains added support for
1.17 deraadt 1051: .Xr kerberos 8
1052: authentication and ticket passing.
1053: .It
1054: supports one-time password authentication with
1055: .Xr skey 1 .
1056: .El
1.48 markus 1057: .Pp
1058: OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
1059: Niels Provos, Theo de Raadt, and Dug Song.
1060: .Pp
1061: The support for SSH protocol 2 was written by Markus Friedl.
1.2 deraadt 1062: .Sh SEE ALSO
1063: .Xr scp 1 ,
1.63 markus 1064: .Xr sftp-server 8 ,
1.2 deraadt 1065: .Xr ssh 1 ,
1.5 deraadt 1066: .Xr ssh-add 1 ,
1.2 deraadt 1067: .Xr ssh-agent 1 ,
1.5 deraadt 1068: .Xr ssh-keygen 1 ,
1.38 deraadt 1069: .Xr ssl 8 ,
1070: .Xr rlogin 1 ,
1071: .Xr rsh 1
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