Annotation of src/usr.bin/ssh/sshd.8, Revision 1.29
1.1 deraadt 1: .\" -*- nroff -*-
2: .\"
3: .\" sshd.8.in
4: .\"
5: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
6: .\"
7: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
8: .\" All rights reserved
9: .\"
10: .\" Created: Sat Apr 22 21:55:14 1995 ylo
11: .\"
1.29 ! markus 12: .\" $Id: sshd.8,v 1.28 2000/01/04 00:08:00 markus Exp $
1.1 deraadt 13: .\"
1.2 deraadt 14: .Dd September 25, 1999
15: .Dt SSHD 8
16: .Os
17: .Sh NAME
18: .Nm sshd
19: .Nd secure shell daemon
20: .Sh SYNOPSIS
21: .Nm sshd
1.29 ! markus 22: .Op Fl diqQ46
1.2 deraadt 23: .Op Fl b Ar bits
24: .Op Fl f Ar config_file
25: .Op Fl g Ar login_grace_time
26: .Op Fl h Ar host_key_file
27: .Op Fl k Ar key_gen_time
28: .Op Fl p Ar port
1.26 markus 29: .Op Fl V Ar client_protocol_id
1.2 deraadt 30: .Sh DESCRIPTION
31: .Nm
1.1 deraadt 32: (Secure Shell Daemon) is the daemon program for
1.2 deraadt 33: .Xr ssh 1 .
1.1 deraadt 34: Together these programs replace rlogin and rsh programs, and
35: provide secure encrypted communications between two untrusted hosts
36: over an insecure network. The programs are intended to be as easy to
37: install and use as possible.
1.2 deraadt 38: .Pp
39: .Nm
1.1 deraadt 40: is the daemon that listens for connections from clients. It is
41: normally started at boot from
1.2 deraadt 42: .Pa /etc/rc .
43: It forks a new
1.1 deraadt 44: daemon for each incoming connection. The forked daemons handle
45: key exchange, encryption, authentication, command execution,
46: and data exchange.
1.2 deraadt 47: .Pp
48: .Nm
49: works as follows. Each host has a host-specific RSA key
1.1 deraadt 50: (normally 1024 bits) used to identify the host. Additionally, when
51: the daemon starts, it generates a server RSA key (normally 768 bits).
52: This key is normally regenerated every hour if it has been used, and
53: is never stored on disk.
1.2 deraadt 54: .Pp
1.1 deraadt 55: Whenever a client connects the daemon, the daemon sends its host
56: and server public keys to the client. The client compares the
57: host key against its own database to verify that it has not changed.
58: The client then generates a 256 bit random number. It encrypts this
59: random number using both the host key and the server key, and sends
60: the encrypted number to the server. Both sides then start to use this
61: random number as a session key which is used to encrypt all further
62: communications in the session. The rest of the session is encrypted
1.5 deraadt 63: using a conventional cipher, currently Blowfish and 3DES, with 3DES
64: being is used by default. The client selects the encryption algorithm
65: to use from those offered by the server.
1.2 deraadt 66: .Pp
1.1 deraadt 67: Next, the server and the client enter an authentication dialog. The
1.2 deraadt 68: client tries to authenticate itself using
69: .Pa .rhosts
70: authentication,
71: .Pa .rhosts
72: authentication combined with RSA host
1.1 deraadt 73: authentication, RSA challenge-response authentication, or password
74: based authentication.
1.2 deraadt 75: .Pp
1.1 deraadt 76: Rhosts authentication is normally disabled
77: because it is fundamentally insecure, but can be enabled in the server
78: configuration file if desired. System security is not improved unless
1.2 deraadt 79: .Xr rshd 8 ,
80: .Xr rlogind 8 ,
81: .Xr rexecd 8 ,
82: and
83: .Xr rexd 8
1.1 deraadt 84: are disabled (thus completely disabling
1.2 deraadt 85: .Xr rlogin 1
1.1 deraadt 86: and
1.2 deraadt 87: .Xr rsh 1
1.1 deraadt 88: into that machine).
1.2 deraadt 89: .Pp
1.1 deraadt 90: If the client successfully authenticates itself, a dialog for
91: preparing the session is entered. At this time the client may request
92: things like allocating a pseudo-tty, forwarding X11 connections,
93: forwarding TCP/IP connections, or forwarding the authentication agent
94: connection over the secure channel.
1.2 deraadt 95: .Pp
1.1 deraadt 96: Finally, the client either requests a shell or execution of a command.
97: The sides then enter session mode. In this mode, either side may send
98: data at any time, and such data is forwarded to/from the shell or
99: command on the server side, and the user terminal in the client side.
1.2 deraadt 100: .Pp
1.1 deraadt 101: When the user program terminates and all forwarded X11 and other
102: connections have been closed, the server sends command exit status to
103: the client, and both sides exit.
1.2 deraadt 104: .Pp
105: .Nm
1.1 deraadt 106: can be configured using command-line options or a configuration
107: file. Command-line options override values specified in the
108: configuration file.
1.25 markus 109: .Pp
110: .Nm
111: rereads its configuration file when it receives a hangup signal,
112: .Dv SIGHUP .
1.18 aaron 113: .Pp
114: The options are as follows:
1.2 deraadt 115: .Bl -tag -width Ds
116: .It Fl b Ar bits
1.1 deraadt 117: Specifies the number of bits in the server key (default 768).
1.2 deraadt 118: .Pp
119: .It Fl d
1.1 deraadt 120: Debug mode. The server sends verbose debug output to the system
121: log, and does not put itself in the background. The server also will
122: not fork and will only process one connection. This option is only
123: intended for debugging for the server.
1.2 deraadt 124: .It Fl f Ar configuration_file
1.1 deraadt 125: Specifies the name of the configuration file. The default is
1.2 deraadt 126: .Pa /etc/sshd_config .
1.16 markus 127: .Nm
128: refuses to start if there is no configuration file.
1.2 deraadt 129: .It Fl g Ar login_grace_time
1.1 deraadt 130: Gives the grace time for clients to authenticate themselves (default
131: 300 seconds). If the client fails to authenticate the user within
132: this many seconds, the server disconnects and exits. A value of zero
133: indicates no limit.
1.2 deraadt 134: .It Fl h Ar host_key_file
1.1 deraadt 135: Specifies the file from which the host key is read (default
1.2 deraadt 136: .Pa /etc/ssh_host_key ) .
1.7 markus 137: This option must be given if
138: .Nm
139: is not run as root (as the normal
1.1 deraadt 140: host file is normally not readable by anyone but root).
1.2 deraadt 141: .It Fl i
1.7 markus 142: Specifies that
143: .Nm
144: is being run from inetd.
145: .Nm
146: is normally not run
1.1 deraadt 147: from inetd because it needs to generate the server key before it can
148: respond to the client, and this may take tens of seconds. Clients
149: would have to wait too long if the key was regenerated every time.
1.7 markus 150: However, with small key sizes (e.g. 512) using
151: .Nm
152: from inetd may
1.1 deraadt 153: be feasible.
1.2 deraadt 154: .It Fl k Ar key_gen_time
1.1 deraadt 155: Specifies how often the server key is regenerated (default 3600
156: seconds, or one hour). The motivation for regenerating the key fairly
157: often is that the key is not stored anywhere, and after about an hour,
158: it becomes impossible to recover the key for decrypting intercepted
159: communications even if the machine is cracked into or physically
160: seized. A value of zero indicates that the key will never be regenerated.
1.2 deraadt 161: .It Fl p Ar port
1.1 deraadt 162: Specifies the port on which the server listens for connections
163: (default 22).
1.2 deraadt 164: .It Fl q
1.1 deraadt 165: Quiet mode. Nothing is sent to the system log. Normally the beginning,
166: authentication, and termination of each connection is logged.
1.18 aaron 167: .It Fl Q
168: Do not print an error message if RSA support is missing.
1.26 markus 169: .It Fl V Ar client_protocol_id
170: SSH2 compatibility mode.
171: When this options is specified
172: .Nm
173: assumes the client has sent the given version string
174: and skips the
175: Protocol Version Identification Exchange.
1.29 ! markus 176: .It Fl 4
! 177: Forces
! 178: .Nm
! 179: to use IPv4 addresses only.
! 180: .It Fl 6
! 181: Forces
! 182: .Nm
! 183: to use IPv6 addresses only.
1.2 deraadt 184: .El
185: .Sh CONFIGURATION FILE
186: .Nm
1.1 deraadt 187: reads configuration data from
1.2 deraadt 188: .Pa /etc/sshd_config
189: (or the file specified with
190: .Fl f
191: on the command line). The file
192: contains keyword-value pairs, one per line. Lines starting with
193: .Ql #
1.1 deraadt 194: and empty lines are interpreted as comments.
1.2 deraadt 195: .Pp
1.1 deraadt 196: The following keywords are possible.
1.2 deraadt 197: .Bl -tag -width Ds
198: .It Cm AFSTokenPassing
1.3 dugsong 199: Specifies whether an AFS token may be forwarded to the server. Default is
1.2 deraadt 200: .Dq yes .
1.11 markus 201: .It Cm AllowGroups
202: This keyword can be followed by a number of group names, separated
203: by spaces. If specified, login is allowed only for users whose primary
204: group matches one of the patterns.
205: .Ql \&*
206: and
207: .Ql ?
208: can be used as
209: wildcards in the patterns. Only group names are valid, a numerical group
210: id isn't recognized. By default login is allowed regardless of
211: the primary group.
1.3 dugsong 212: .Pp
1.11 markus 213: .It Cm AllowUsers
214: This keyword can be followed by a number of user names, separated
215: by spaces. If specified, login is allowed only for users names that
216: match one of the patterns.
217: .Ql \&*
218: and
219: .Ql ?
220: can be used as
221: wildcards in the patterns. Only user names are valid, a numerical user
222: id isn't recognized. By default login is allowed regardless of
223: the user name.
224: .Pp
1.8 markus 225: .It Cm CheckMail
226: Specifies whether
227: .Nm
228: should check for new mail for interactive logins.
229: The default is
230: .Dq no .
1.11 markus 231: .It Cm DenyGroups
232: This keyword can be followed by a number of group names, separated
233: by spaces. Users whose primary group matches one of the patterns
234: aren't allowed to log in.
235: .Ql \&*
236: and
237: .Ql ?
238: can be used as
239: wildcards in the patterns. Only group names are valid, a numerical group
240: id isn't recognized. By default login is allowed regardless of
241: the primary group.
242: .Pp
243: .It Cm DenyUsers
244: This keyword can be followed by a number of user names, separated
245: by spaces. Login is allowed disallowed for user names that match
246: one of the patterns.
247: .Ql \&*
248: and
249: .Ql ?
250: can be used as
251: wildcards in the patterns. Only user names are valid, a numerical user
252: id isn't recognized. By default login is allowed regardless of
253: the user name.
1.2 deraadt 254: .It Cm HostKey
1.1 deraadt 255: Specifies the file containing the private host key (default
1.2 deraadt 256: .Pa /etc/ssh_host_key ) .
1.9 markus 257: Note that
258: .Nm
1.14 markus 259: does not start if this file is group/world-accessible.
1.2 deraadt 260: .It Cm IgnoreRhosts
1.1 deraadt 261: Specifies that rhosts and shosts files will not be used in
262: authentication.
1.2 deraadt 263: .Pa /etc/hosts.equiv
1.1 deraadt 264: and
1.2 deraadt 265: .Pa /etc/shosts.equiv
266: are still used. The default is
1.24 markus 267: .Dq no .
268: .It Cm IgnoreUserKnownHosts
269: Specifies whether
270: .Nm
271: should ignore the user's
272: .Pa $HOME/.ssh/known_hosts
273: during
274: .Cm RhostsRSAAuthentication .
275: The default is
1.2 deraadt 276: .Dq no .
277: .It Cm KeepAlive
1.1 deraadt 278: Specifies whether the system should send keepalive messages to the
279: other side. If they are sent, death of the connection or crash of one
280: of the machines will be properly noticed. However, this means that
281: connections will die if the route is down temporarily, and some people
282: find it annoying. On the other hand, if keepalives are not send,
1.2 deraadt 283: sessions may hang indefinitely on the server, leaving
284: .Dq ghost
285: users and consuming server resources.
286: .Pp
287: The default is
288: .Dq yes
289: (to send keepalives), and the server will notice
1.1 deraadt 290: if the network goes down or the client host reboots. This avoids
291: infinitely hanging sessions.
1.2 deraadt 292: .Pp
293: To disable keepalives, the value should be set to
294: .Dq no
295: in both the server and the client configuration files.
296: .It Cm KerberosAuthentication
1.1 deraadt 297: Specifies whether Kerberos authentication is allowed. This can
1.7 markus 298: be in the form of a Kerberos ticket, or if
299: .Cm PasswordAuthentication
1.1 deraadt 300: is yes, the password provided by the user will be validated through
1.3 dugsong 301: the Kerberos KDC. Default is
302: .Dq yes .
1.2 deraadt 303: .It Cm KerberosOrLocalPasswd
1.1 deraadt 304: If set then if password authentication through Kerberos fails then
305: the password will be validated via any additional local mechanism
1.2 deraadt 306: such as
307: .Pa /etc/passwd
308: or SecurID. Default is
1.20 dugsong 309: .Dq yes .
1.2 deraadt 310: .It Cm KerberosTgtPassing
1.1 deraadt 311: Specifies whether a Kerberos TGT may be forwarded to the server.
1.3 dugsong 312: Default is
313: .Dq no ,
314: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 315: .It Cm KerberosTicketCleanup
1.7 markus 316: Specifies whether to automatically destroy the user's ticket cache
317: file on logout. Default is
1.3 dugsong 318: .Dq yes .
1.2 deraadt 319: .It Cm KeyRegenerationInterval
1.1 deraadt 320: The server key is automatically regenerated after this many seconds
321: (if it has been used). The purpose of regeneration is to prevent
322: decrypting captured sessions by later breaking into the machine and
323: stealing the keys. The key is never stored anywhere. If the value is
324: 0, the key is never regenerated. The default is 3600
325: (seconds).
1.7 markus 326: .It Cm ListenAddress
327: Specifies what local address
328: .Nm
329: should listen on.
330: The default is to listen to all local addresses.
1.28 markus 331: Multiple options of this type are permitted.
332: Additionally, the
333: .Cm Ports
334: options must precede this option.
1.2 deraadt 335: .It Cm LoginGraceTime
1.1 deraadt 336: The server disconnects after this time if the user has not
337: successfully logged in. If the value is 0, there is no time limit.
338: The default is 600 (seconds).
1.23 markus 339: .It Cm LogLevel
340: Gives the verbosity level that is used when logging messages from
341: .Nm sshd .
342: The possible values are:
1.27 markus 343: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
1.23 markus 344: The default is INFO.
345: Logging with level DEBUG violates the privacy of users
346: and is not recommended.
1.2 deraadt 347: .It Cm PasswordAuthentication
1.1 deraadt 348: Specifies whether password authentication is allowed.
1.2 deraadt 349: The default is
350: .Dq yes .
351: .It Cm PermitEmptyPasswords
1.1 deraadt 352: When password authentication is allowed, it specifies whether the
353: server allows login to accounts with empty password strings. The default
1.2 deraadt 354: is
355: .Dq yes .
356: .It Cm PermitRootLogin
1.1 deraadt 357: Specifies whether the root can log in using
1.2 deraadt 358: .Xr ssh 1 .
1.15 markus 359: The argument must be
360: .Dq yes ,
361: .Dq without-password
362: or
363: .Dq no .
1.2 deraadt 364: The default is
365: .Dq yes .
1.15 markus 366: If this options is set to
367: .Dq without-password
368: only password authentication is disabled for root.
1.2 deraadt 369: .Pp
370: Root login with RSA authentication when the
371: .Ar command
372: option has been
1.1 deraadt 373: specified will be allowed regardless of the value of this setting
374: (which may be useful for taking remote backups even if root login is
375: normally not allowed).
1.2 deraadt 376: .It Cm Port
1.1 deraadt 377: Specifies the port number that
1.2 deraadt 378: .Nm
1.1 deraadt 379: listens on. The default is 22.
1.28 markus 380: Multiple options of this type are permitted.
1.2 deraadt 381: .It Cm PrintMotd
1.1 deraadt 382: Specifies whether
1.2 deraadt 383: .Nm
1.1 deraadt 384: should print
1.2 deraadt 385: .Pa /etc/motd
1.1 deraadt 386: when a user logs in interactively. (On some systems it is also
1.2 deraadt 387: printed by the shell,
388: .Pa /etc/profile ,
389: or equivalent.) The default is
390: .Dq yes .
391: .It Cm RandomSeed
1.4 deraadt 392: Obsolete. Random number generation uses other techniques.
1.2 deraadt 393: .It Cm RhostsAuthentication
1.1 deraadt 394: Specifies whether authentication using rhosts or /etc/hosts.equiv
395: files is sufficient. Normally, this method should not be permitted
1.7 markus 396: because it is insecure.
397: .Cm RhostsRSAAuthentication
398: should be used
1.1 deraadt 399: instead, because it performs RSA-based host authentication in addition
400: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 401: The default is
402: .Dq no .
403: .It Cm RhostsRSAAuthentication
1.1 deraadt 404: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.2 deraadt 405: with successful RSA host authentication is allowed. The default is
406: .Dq yes .
407: .It Cm RSAAuthentication
408: Specifies whether pure RSA authentication is allowed. The default is
409: .Dq yes .
410: .It Cm ServerKeyBits
1.1 deraadt 411: Defines the number of bits in the server key. The minimum value is
412: 512, and the default is 768.
1.7 markus 413: .It Cm SkeyAuthentication
414: Specifies whether
415: .Xr skey 1
416: authentication is allowed. The default is
417: .Dq yes .
418: Note that s/key authentication is enabled only if
419: .Cm PasswordAuthentication
420: is allowed, too.
1.2 deraadt 421: .It Cm StrictModes
1.12 markus 422: Specifies whether
423: .Nm
424: should check file modes and ownership of the
425: user's files and home directory before accepting login. This
1.1 deraadt 426: is normally desirable because novices sometimes accidentally leave their
1.7 markus 427: directory or files world-writable. The default is
428: .Dq yes .
1.2 deraadt 429: .It Cm SyslogFacility
1.1 deraadt 430: Gives the facility code that is used when logging messages from
1.2 deraadt 431: .Nm sshd .
1.1 deraadt 432: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.21 markus 433: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is AUTH.
1.10 markus 434: .It Cm UseLogin
435: Specifies whether
436: .Xr login 1
437: is used. The default is
438: .Dq no .
1.2 deraadt 439: .It Cm X11Forwarding
440: Specifies whether X11 forwarding is permitted. The default is
441: .Dq yes .
1.1 deraadt 442: Note that disabling X11 forwarding does not improve security in any
443: way, as users can always install their own forwarders.
1.6 aaron 444: .It Cm X11DisplayOffset
445: Specifies the first display number available for
446: .Nm sshd Ns 's
447: X11 forwarding. This prevents
448: .Nm
449: from interfering with real X11 servers.
1.2 deraadt 450: .El
451: .Sh LOGIN PROCESS
1.1 deraadt 452: When a user successfully logs in,
1.2 deraadt 453: .Nm
1.1 deraadt 454: does the following:
1.2 deraadt 455: .Bl -enum -offset indent
456: .It
1.1 deraadt 457: If the login is on a tty, and no command has been specified,
458: prints last login time and
1.2 deraadt 459: .Pa /etc/motd
1.1 deraadt 460: (unless prevented in the configuration file or by
1.2 deraadt 461: .Pa $HOME/.hushlogin ;
462: see the
463: .Sx FILES
464: section).
465: .It
1.1 deraadt 466: If the login is on a tty, records login time.
1.2 deraadt 467: .It
468: Checks
469: .Pa /etc/nologin ;
470: if it exists, prints contents and quits
1.1 deraadt 471: (unless root).
1.2 deraadt 472: .It
1.1 deraadt 473: Changes to run with normal user privileges.
1.2 deraadt 474: .It
1.1 deraadt 475: Sets up basic environment.
1.2 deraadt 476: .It
477: Reads
478: .Pa $HOME/.ssh/environment
479: if it exists.
480: .It
1.1 deraadt 481: Changes to user's home directory.
1.2 deraadt 482: .It
483: If
484: .Pa $HOME/.ssh/rc
485: exists, runs it; else if
486: .Pa /etc/sshrc
487: exists, runs
488: it; otherwise runs xauth. The
489: .Dq rc
490: files are given the X11
1.1 deraadt 491: authentication protocol and cookie in standard input.
1.2 deraadt 492: .It
1.1 deraadt 493: Runs user's shell or command.
1.2 deraadt 494: .El
495: .Sh AUTHORIZED_KEYS FILE FORMAT
1.1 deraadt 496: The
1.2 deraadt 497: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 498: file lists the RSA keys that are
499: permitted for RSA authentication. Each line of the file contains one
1.2 deraadt 500: key (empty lines and lines starting with a
501: .Ql #
502: are ignored as
1.1 deraadt 503: comments). Each line consists of the following fields, separated by
504: spaces: options, bits, exponent, modulus, comment. The options field
505: is optional; its presence is determined by whether the line starts
506: with a number or not (the option field never starts with a number).
507: The bits, exponent, modulus and comment fields give the RSA key; the
508: comment field is not used for anything (but may be convenient for the
509: user to identify the key).
1.2 deraadt 510: .Pp
1.1 deraadt 511: Note that lines in this file are usually several hundred bytes long
512: (because of the size of the RSA key modulus). You don't want to type
513: them in; instead, copy the
1.2 deraadt 514: .Pa identity.pub
1.1 deraadt 515: file and edit it.
1.2 deraadt 516: .Pp
1.1 deraadt 517: The options (if present) consists of comma-separated option
518: specifications. No spaces are permitted, except within double quotes.
519: The following option specifications are supported:
1.2 deraadt 520: .Bl -tag -width Ds
521: .It Cm from="pattern-list"
1.1 deraadt 522: Specifies that in addition to RSA authentication, the canonical name
523: of the remote host must be present in the comma-separated list of
524: patterns ('*' and '?' serve as wildcards). The list may also contain
525: patterns negated by prefixing them with '!'; if the canonical host
526: name matches a negated pattern, the key is not accepted. The purpose
527: of this option is to optionally increase security: RSA authentication
528: by itself does not trust the network or name servers or anything (but
529: the key); however, if somebody somehow steals the key, the key
530: permits an intruder to log in from anywhere in the world. This
531: additional option makes using a stolen key more difficult (name
532: servers and/or routers would have to be compromised in addition to
533: just the key).
1.2 deraadt 534: .It Cm command="command"
1.1 deraadt 535: Specifies that the command is executed whenever this key is used for
536: authentication. The command supplied by the user (if any) is ignored.
537: The command is run on a pty if the connection requests a pty;
538: otherwise it is run without a tty. A quote may be included in the
539: command by quoting it with a backslash. This option might be useful
540: to restrict certain RSA keys to perform just a specific operation. An
541: example might be a key that permits remote backups but nothing
542: else. Notice that the client may specify TCP/IP and/or X11
543: forwardings unless they are explicitly prohibited.
1.2 deraadt 544: .It Cm environment="NAME=value"
1.1 deraadt 545: Specifies that the string is to be added to the environment when
546: logging in using this key. Environment variables set this way
547: override other default environment values. Multiple options of this
548: type are permitted.
1.2 deraadt 549: .It Cm no-port-forwarding
1.1 deraadt 550: Forbids TCP/IP forwarding when this key is used for authentication.
551: Any port forward requests by the client will return an error. This
1.2 deraadt 552: might be used, e.g., in connection with the
553: .Cm command
1.1 deraadt 554: option.
1.2 deraadt 555: .It Cm no-X11-forwarding
1.1 deraadt 556: Forbids X11 forwarding when this key is used for authentication.
557: Any X11 forward requests by the client will return an error.
1.2 deraadt 558: .It Cm no-agent-forwarding
1.1 deraadt 559: Forbids authentication agent forwarding when this key is used for
560: authentication.
1.2 deraadt 561: .It Cm no-pty
1.1 deraadt 562: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 563: .El
564: .Ss Examples
1.1 deraadt 565: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 566: .Pp
1.1 deraadt 567: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 568: .Pp
1.1 deraadt 569: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.2 deraadt 570: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.1 deraadt 571: The
1.2 deraadt 572: .Pa /etc/ssh_known_hosts
1.1 deraadt 573: and
1.2 deraadt 574: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 575: files contain host public keys for all known hosts. The global file should
576: be prepared by the admistrator (optional), and the per-user file is
577: maintained automatically: whenever the user connects an unknown host
1.6 aaron 578: its key is added to the per-user file.
1.2 deraadt 579: .Pp
1.1 deraadt 580: Each line in these files contains the following fields: hostnames,
581: bits, exponent, modulus, comment. The fields are separated by spaces.
1.2 deraadt 582: .Pp
1.1 deraadt 583: Hostnames is a comma-separated list of patterns ('*' and '?' act as
584: wildcards); each pattern in turn is matched against the canonical host
585: name (when authenticating a client) or against the user-supplied
586: name (when authenticating a server). A pattern may also be preceded
1.2 deraadt 587: by
588: .Ql !
589: to indicate negation: if the host name matches a negated
1.1 deraadt 590: pattern, it is not accepted (by that line) even if it matched another
591: pattern on the line.
1.2 deraadt 592: .Pp
1.1 deraadt 593: Bits, exponent, and modulus are taken directly from the host key; they
1.2 deraadt 594: can be obtained, e.g., from
595: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 596: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 597: .Pp
598: Lines starting with
599: .Ql #
600: and empty lines are ignored as comments.
601: .Pp
1.1 deraadt 602: When performing host authentication, authentication is accepted if any
603: matching line has the proper key. It is thus permissible (but not
604: recommended) to have several lines or different host keys for the same
605: names. This will inevitably happen when short forms of host names
606: from different domains are put in the file. It is possible
607: that the files contain conflicting information; authentication is
608: accepted if valid information can be found from either file.
1.2 deraadt 609: .Pp
1.1 deraadt 610: Note that the lines in these files are typically hundreds of characters
611: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 612: Rather, generate them by a script
1.1 deraadt 613: or by taking
1.2 deraadt 614: .Pa /etc/ssh_host_key.pub
1.1 deraadt 615: and adding the host names at the front.
1.2 deraadt 616: .Ss Examples
1.1 deraadt 617: closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1.2 deraadt 618: .Sh FILES
619: .Bl -tag -width Ds
620: .It Pa /etc/sshd_config
1.1 deraadt 621: Contains configuration data for
1.2 deraadt 622: .Nm sshd .
1.1 deraadt 623: This file should be writable by root only, but it is recommended
624: (though not necessary) that it be world-readable.
1.2 deraadt 625: .It Pa /etc/ssh_host_key
1.7 markus 626: Contains the private part of the host key.
1.1 deraadt 627: This file should only be owned by root, readable only by root, and not
628: accessible to others.
1.14 markus 629: Note that
630: .Nm
631: does not start if this file is group/world-accessible.
1.2 deraadt 632: .It Pa /etc/ssh_host_key.pub
1.7 markus 633: Contains the public part of the host key.
634: This file should be world-readable but writable only by
1.1 deraadt 635: root. Its contents should match the private part. This file is not
636: really used for anything; it is only provided for the convenience of
637: the user so its contents can be copied to known hosts files.
1.7 markus 638: These two files are created using
639: .Xr ssh-keygen 1 .
1.2 deraadt 640: .It Pa /var/run/sshd.pid
641: Contains the process ID of the
642: .Nm
1.1 deraadt 643: listening for connections (if there are several daemons running
644: concurrently for different ports, this contains the pid of the one
645: started last). The contents of this file are not sensitive; it can be
646: world-readable.
1.2 deraadt 647: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 648: Lists the RSA keys that can be used to log into the user's account.
649: This file must be readable by root (which may on some machines imply
650: it being world-readable if the user's home directory resides on an NFS
651: volume). It is recommended that it not be accessible by others. The
652: format of this file is described above.
1.22 markus 653: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
654: These files are consulted when using rhosts with RSA host
1.1 deraadt 655: authentication to check the public key of the host. The key must be
1.22 markus 656: listed in one of these files to be accepted.
657: The client uses the same files
1.6 aaron 658: to verify that the remote host is the one we intended to
659: connect. These files should be writable only by root/the owner.
1.2 deraadt 660: .Pa /etc/ssh_known_hosts
661: should be world-readable, and
662: .Pa $HOME/.ssh/known_hosts
663: can but need not be world-readable.
1.6 aaron 664: .It Pa /etc/nologin
1.1 deraadt 665: If this file exists,
1.2 deraadt 666: .Nm
1.1 deraadt 667: refuses to let anyone except root log in. The contents of the file
668: are displayed to anyone trying to log in, and non-root connections are
669: refused. The file should be world-readable.
1.19 dugsong 670: .It Pa /etc/hosts.allow, /etc/hosts.deny
671: If compiled with
672: .Sy LIBWRAP
673: support, tcp-wrappers access controls may be defined here as described in
674: .Xr hosts_access 5 .
1.6 aaron 675: .It Pa $HOME/.rhosts
1.1 deraadt 676: This file contains host-username pairs, separated by a space, one per
677: line. The given user on the corresponding host is permitted to log in
678: without password. The same file is used by rlogind and rshd.
1.6 aaron 679: The file must
1.1 deraadt 680: be writable only by the user; it is recommended that it not be
681: accessible by others.
1.2 deraadt 682: .Pp
1.1 deraadt 683: If is also possible to use netgroups in the file. Either host or user
684: name may be of the form +@groupname to specify all hosts or all users
685: in the group.
1.2 deraadt 686: .It Pa $HOME/.shosts
687: For ssh,
688: this file is exactly the same as for
689: .Pa .rhosts .
690: However, this file is
691: not used by rlogin and rshd, so using this permits access using SSH only.
692: .Pa /etc/hosts.equiv
693: This file is used during
694: .Pa .rhosts
695: authentication. In the
1.1 deraadt 696: simplest form, this file contains host names, one per line. Users on
697: those hosts are permitted to log in without a password, provided they
698: have the same user name on both machines. The host name may also be
699: followed by a user name; such users are permitted to log in as
1.2 deraadt 700: .Em any
701: user on this machine (except root). Additionally, the syntax
702: .Dq +@group
703: can be used to specify netgroups. Negated entries start with
704: .Ql \&- .
705: .Pp
1.1 deraadt 706: If the client host/user is successfully matched in this file, login is
707: automatically permitted provided the client and server user names are the
708: same. Additionally, successful RSA host authentication is normally
709: required. This file must be writable only by root; it is recommended
710: that it be world-readable.
1.2 deraadt 711: .Pp
1.6 aaron 712: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 713: .Pa hosts.equiv .
1.1 deraadt 714: Beware that it really means that the named user(s) can log in as
1.2 deraadt 715: .Em anybody ,
1.1 deraadt 716: which includes bin, daemon, adm, and other accounts that own critical
717: binaries and directories. Using a user name practically grants the
718: user root access. The only valid use for user names that I can think
719: of is in negative entries.
1.2 deraadt 720: .Pp
721: Note that this warning also applies to rsh/rlogin.
722: .It Pa /etc/shosts.equiv
1.1 deraadt 723: This is processed exactly as
1.2 deraadt 724: .Pa /etc/hosts.equiv .
1.1 deraadt 725: However, this file may be useful in environments that want to run both
1.2 deraadt 726: rsh/rlogin and ssh.
1.6 aaron 727: .It Pa $HOME/.ssh/environment
1.1 deraadt 728: This file is read into the environment at login (if it exists). It
1.2 deraadt 729: can only contain empty lines, comment lines (that start with
730: .Ql # ) ,
1.6 aaron 731: and assignment lines of the form name=value. The file should be writable
732: only by the user; it need not be readable by anyone else.
1.2 deraadt 733: .It Pa $HOME/.ssh/rc
1.1 deraadt 734: If this file exists, it is run with /bin/sh after reading the
735: environment files but before starting the user's shell or command. If
736: X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 737: standard input (and
738: .Ev DISPLAY
739: in environment). This must call
740: .Xr xauth 1
741: in that case.
742: .Pp
1.1 deraadt 743: The primary purpose of this file is to run any initialization routines
744: which may be needed before the user's home directory becomes
745: accessible; AFS is a particular example of such an environment.
1.2 deraadt 746: .Pp
1.1 deraadt 747: This file will probably contain some initialization code followed by
748: something similar to: "if read proto cookie; then echo add $DISPLAY
749: $proto $cookie | xauth -q -; fi".
1.2 deraadt 750: .Pp
751: If this file does not exist,
752: .Pa /etc/sshrc
753: is run, and if that
1.1 deraadt 754: does not exist either, xauth is used to store the cookie.
1.2 deraadt 755: .Pp
1.1 deraadt 756: This file should be writable only by the user, and need not be
757: readable by anyone else.
1.2 deraadt 758: .It Pa /etc/sshrc
759: Like
760: .Pa $HOME/.ssh/rc .
761: This can be used to specify
1.1 deraadt 762: machine-specific login-time initializations globally. This file
763: should be writable only by root, and should be world-readable.
1.2 deraadt 764: .Sh AUTHOR
1.1 deraadt 765: Tatu Ylonen <ylo@cs.hut.fi>
1.2 deraadt 766: .Pp
1.1 deraadt 767: Information about new releases, mailing lists, and other related
1.2 deraadt 768: issues can be found from the SSH WWW home page:
769: .Pp
770: .Dl http://www.cs.hut.fi/ssh.
1.5 deraadt 771: .Pp
1.17 deraadt 772: OpenSSH
773: is a derivative of the original (free) ssh 1.2.12 release, but with bugs
774: removed and newer features re-added. Rapidly after the 1.2.12 release,
775: newer versions bore successively more restrictive licenses. This version
776: of OpenSSH
777: .Bl -bullet
778: .It
779: has all components of a restrictive nature (ie. patents, see
780: .Xr ssl 8 )
781: directly removed from the source code; any licensed or patented components
782: are chosen from
783: external libraries.
784: .It
785: has been updated to support ssh protocol 1.5.
786: .It
787: contains added support for
788: .Xr kerberos 8
789: authentication and ticket passing.
790: .It
791: supports one-time password authentication with
792: .Xr skey 1 .
793: .El
794: .Pp
795: The libraries described in
1.5 deraadt 796: .Xr ssl 8
797: are required for proper operation.
1.2 deraadt 798: .Sh SEE ALSO
799: .Xr rlogin 1 ,
800: .Xr rsh 1 ,
801: .Xr scp 1 ,
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 ,
806: .Xr ssl 8
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