Annotation of src/usr.bin/ssh/sshd.8, Revision 1.28
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: .\"
1.28 ! markus 12: .\" $Id: sshd.8,v 1.27 1999/11/22 21:02:39 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
1.25 markus 22: .Op Fl diqQ
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.
1.2 deraadt 176: .El
177: .Sh CONFIGURATION FILE
178: .Nm
1.1 deraadt 179: reads configuration data from
1.2 deraadt 180: .Pa /etc/sshd_config
181: (or the file specified with
182: .Fl f
183: on the command line). The file
184: contains keyword-value pairs, one per line. Lines starting with
185: .Ql #
1.1 deraadt 186: and empty lines are interpreted as comments.
1.2 deraadt 187: .Pp
1.1 deraadt 188: The following keywords are possible.
1.2 deraadt 189: .Bl -tag -width Ds
190: .It Cm AFSTokenPassing
1.3 dugsong 191: Specifies whether an AFS token may be forwarded to the server. Default is
1.2 deraadt 192: .Dq yes .
1.11 markus 193: .It Cm AllowGroups
194: This keyword can be followed by a number of group names, separated
195: by spaces. If specified, login is allowed only for users whose primary
196: group matches one of the patterns.
197: .Ql \&*
198: and
199: .Ql ?
200: can be used as
201: wildcards in the patterns. Only group names are valid, a numerical group
202: id isn't recognized. By default login is allowed regardless of
203: the primary group.
1.3 dugsong 204: .Pp
1.11 markus 205: .It Cm AllowUsers
206: This keyword can be followed by a number of user names, separated
207: by spaces. If specified, login is allowed only for users names that
208: match one of the patterns.
209: .Ql \&*
210: and
211: .Ql ?
212: can be used as
213: wildcards in the patterns. Only user names are valid, a numerical user
214: id isn't recognized. By default login is allowed regardless of
215: the user name.
216: .Pp
1.8 markus 217: .It Cm CheckMail
218: Specifies whether
219: .Nm
220: should check for new mail for interactive logins.
221: The default is
222: .Dq no .
1.11 markus 223: .It Cm DenyGroups
224: This keyword can be followed by a number of group names, separated
225: by spaces. Users whose primary group matches one of the patterns
226: aren't allowed to log in.
227: .Ql \&*
228: and
229: .Ql ?
230: can be used as
231: wildcards in the patterns. Only group names are valid, a numerical group
232: id isn't recognized. By default login is allowed regardless of
233: the primary group.
234: .Pp
235: .It Cm DenyUsers
236: This keyword can be followed by a number of user names, separated
237: by spaces. Login is allowed disallowed for user names that match
238: one of the patterns.
239: .Ql \&*
240: and
241: .Ql ?
242: can be used as
243: wildcards in the patterns. Only user names are valid, a numerical user
244: id isn't recognized. By default login is allowed regardless of
245: the user name.
1.2 deraadt 246: .It Cm HostKey
1.1 deraadt 247: Specifies the file containing the private host key (default
1.2 deraadt 248: .Pa /etc/ssh_host_key ) .
1.9 markus 249: Note that
250: .Nm
1.14 markus 251: does not start if this file is group/world-accessible.
1.2 deraadt 252: .It Cm IgnoreRhosts
1.1 deraadt 253: Specifies that rhosts and shosts files will not be used in
254: authentication.
1.2 deraadt 255: .Pa /etc/hosts.equiv
1.1 deraadt 256: and
1.2 deraadt 257: .Pa /etc/shosts.equiv
258: are still used. The default is
1.24 markus 259: .Dq no .
260: .It Cm IgnoreUserKnownHosts
261: Specifies whether
262: .Nm
263: should ignore the user's
264: .Pa $HOME/.ssh/known_hosts
265: during
266: .Cm RhostsRSAAuthentication .
267: The default is
1.2 deraadt 268: .Dq no .
269: .It Cm KeepAlive
1.1 deraadt 270: Specifies whether the system should send keepalive messages to the
271: other side. If they are sent, death of the connection or crash of one
272: of the machines will be properly noticed. However, this means that
273: connections will die if the route is down temporarily, and some people
274: find it annoying. On the other hand, if keepalives are not send,
1.2 deraadt 275: sessions may hang indefinitely on the server, leaving
276: .Dq ghost
277: users and consuming server resources.
278: .Pp
279: The default is
280: .Dq yes
281: (to send keepalives), and the server will notice
1.1 deraadt 282: if the network goes down or the client host reboots. This avoids
283: infinitely hanging sessions.
1.2 deraadt 284: .Pp
285: To disable keepalives, the value should be set to
286: .Dq no
287: in both the server and the client configuration files.
288: .It Cm KerberosAuthentication
1.1 deraadt 289: Specifies whether Kerberos authentication is allowed. This can
1.7 markus 290: be in the form of a Kerberos ticket, or if
291: .Cm PasswordAuthentication
1.1 deraadt 292: is yes, the password provided by the user will be validated through
1.3 dugsong 293: the Kerberos KDC. Default is
294: .Dq yes .
1.2 deraadt 295: .It Cm KerberosOrLocalPasswd
1.1 deraadt 296: If set then if password authentication through Kerberos fails then
297: the password will be validated via any additional local mechanism
1.2 deraadt 298: such as
299: .Pa /etc/passwd
300: or SecurID. Default is
1.20 dugsong 301: .Dq yes .
1.2 deraadt 302: .It Cm KerberosTgtPassing
1.1 deraadt 303: Specifies whether a Kerberos TGT may be forwarded to the server.
1.3 dugsong 304: Default is
305: .Dq no ,
306: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 307: .It Cm KerberosTicketCleanup
1.7 markus 308: Specifies whether to automatically destroy the user's ticket cache
309: file on logout. Default is
1.3 dugsong 310: .Dq yes .
1.2 deraadt 311: .It Cm KeyRegenerationInterval
1.1 deraadt 312: The server key is automatically regenerated after this many seconds
313: (if it has been used). The purpose of regeneration is to prevent
314: decrypting captured sessions by later breaking into the machine and
315: stealing the keys. The key is never stored anywhere. If the value is
316: 0, the key is never regenerated. The default is 3600
317: (seconds).
1.7 markus 318: .It Cm ListenAddress
319: Specifies what local address
320: .Nm
321: should listen on.
322: The default is to listen to all local addresses.
1.28 ! markus 323: Multiple options of this type are permitted.
! 324: Additionally, the
! 325: .Cm Ports
! 326: options must precede this option.
1.2 deraadt 327: .It Cm LoginGraceTime
1.1 deraadt 328: The server disconnects after this time if the user has not
329: successfully logged in. If the value is 0, there is no time limit.
330: The default is 600 (seconds).
1.23 markus 331: .It Cm LogLevel
332: Gives the verbosity level that is used when logging messages from
333: .Nm sshd .
334: The possible values are:
1.27 markus 335: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
1.23 markus 336: The default is INFO.
337: Logging with level DEBUG violates the privacy of users
338: and is not recommended.
1.2 deraadt 339: .It Cm PasswordAuthentication
1.1 deraadt 340: Specifies whether password authentication is allowed.
1.2 deraadt 341: The default is
342: .Dq yes .
343: .It Cm PermitEmptyPasswords
1.1 deraadt 344: When password authentication is allowed, it specifies whether the
345: server allows login to accounts with empty password strings. The default
1.2 deraadt 346: is
347: .Dq yes .
348: .It Cm PermitRootLogin
1.1 deraadt 349: Specifies whether the root can log in using
1.2 deraadt 350: .Xr ssh 1 .
1.15 markus 351: The argument must be
352: .Dq yes ,
353: .Dq without-password
354: or
355: .Dq no .
1.2 deraadt 356: The default is
357: .Dq yes .
1.15 markus 358: If this options is set to
359: .Dq without-password
360: only password authentication is disabled for root.
1.2 deraadt 361: .Pp
362: Root login with RSA authentication when the
363: .Ar command
364: option has been
1.1 deraadt 365: specified will be allowed regardless of the value of this setting
366: (which may be useful for taking remote backups even if root login is
367: normally not allowed).
1.2 deraadt 368: .It Cm Port
1.1 deraadt 369: Specifies the port number that
1.2 deraadt 370: .Nm
1.1 deraadt 371: listens on. The default is 22.
1.28 ! markus 372: Multiple options of this type are permitted.
1.2 deraadt 373: .It Cm PrintMotd
1.1 deraadt 374: Specifies whether
1.2 deraadt 375: .Nm
1.1 deraadt 376: should print
1.2 deraadt 377: .Pa /etc/motd
1.1 deraadt 378: when a user logs in interactively. (On some systems it is also
1.2 deraadt 379: printed by the shell,
380: .Pa /etc/profile ,
381: or equivalent.) The default is
382: .Dq yes .
383: .It Cm RandomSeed
1.4 deraadt 384: Obsolete. Random number generation uses other techniques.
1.2 deraadt 385: .It Cm RhostsAuthentication
1.1 deraadt 386: Specifies whether authentication using rhosts or /etc/hosts.equiv
387: files is sufficient. Normally, this method should not be permitted
1.7 markus 388: because it is insecure.
389: .Cm RhostsRSAAuthentication
390: should be used
1.1 deraadt 391: instead, because it performs RSA-based host authentication in addition
392: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 393: The default is
394: .Dq no .
395: .It Cm RhostsRSAAuthentication
1.1 deraadt 396: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.2 deraadt 397: with successful RSA host authentication is allowed. The default is
398: .Dq yes .
399: .It Cm RSAAuthentication
400: Specifies whether pure RSA authentication is allowed. The default is
401: .Dq yes .
402: .It Cm ServerKeyBits
1.1 deraadt 403: Defines the number of bits in the server key. The minimum value is
404: 512, and the default is 768.
1.7 markus 405: .It Cm SkeyAuthentication
406: Specifies whether
407: .Xr skey 1
408: authentication is allowed. The default is
409: .Dq yes .
410: Note that s/key authentication is enabled only if
411: .Cm PasswordAuthentication
412: is allowed, too.
1.2 deraadt 413: .It Cm StrictModes
1.12 markus 414: Specifies whether
415: .Nm
416: should check file modes and ownership of the
417: user's files and home directory before accepting login. This
1.1 deraadt 418: is normally desirable because novices sometimes accidentally leave their
1.7 markus 419: directory or files world-writable. The default is
420: .Dq yes .
1.2 deraadt 421: .It Cm SyslogFacility
1.1 deraadt 422: Gives the facility code that is used when logging messages from
1.2 deraadt 423: .Nm sshd .
1.1 deraadt 424: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.21 markus 425: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is AUTH.
1.10 markus 426: .It Cm UseLogin
427: Specifies whether
428: .Xr login 1
429: is used. The default is
430: .Dq no .
1.2 deraadt 431: .It Cm X11Forwarding
432: Specifies whether X11 forwarding is permitted. The default is
433: .Dq yes .
1.1 deraadt 434: Note that disabling X11 forwarding does not improve security in any
435: way, as users can always install their own forwarders.
1.6 aaron 436: .It Cm X11DisplayOffset
437: Specifies the first display number available for
438: .Nm sshd Ns 's
439: X11 forwarding. This prevents
440: .Nm
441: from interfering with real X11 servers.
1.2 deraadt 442: .El
443: .Sh LOGIN PROCESS
1.1 deraadt 444: When a user successfully logs in,
1.2 deraadt 445: .Nm
1.1 deraadt 446: does the following:
1.2 deraadt 447: .Bl -enum -offset indent
448: .It
1.1 deraadt 449: If the login is on a tty, and no command has been specified,
450: prints last login time and
1.2 deraadt 451: .Pa /etc/motd
1.1 deraadt 452: (unless prevented in the configuration file or by
1.2 deraadt 453: .Pa $HOME/.hushlogin ;
454: see the
455: .Sx FILES
456: section).
457: .It
1.1 deraadt 458: If the login is on a tty, records login time.
1.2 deraadt 459: .It
460: Checks
461: .Pa /etc/nologin ;
462: if it exists, prints contents and quits
1.1 deraadt 463: (unless root).
1.2 deraadt 464: .It
1.1 deraadt 465: Changes to run with normal user privileges.
1.2 deraadt 466: .It
1.1 deraadt 467: Sets up basic environment.
1.2 deraadt 468: .It
469: Reads
470: .Pa $HOME/.ssh/environment
471: if it exists.
472: .It
1.1 deraadt 473: Changes to user's home directory.
1.2 deraadt 474: .It
475: If
476: .Pa $HOME/.ssh/rc
477: exists, runs it; else if
478: .Pa /etc/sshrc
479: exists, runs
480: it; otherwise runs xauth. The
481: .Dq rc
482: files are given the X11
1.1 deraadt 483: authentication protocol and cookie in standard input.
1.2 deraadt 484: .It
1.1 deraadt 485: Runs user's shell or command.
1.2 deraadt 486: .El
487: .Sh AUTHORIZED_KEYS FILE FORMAT
1.1 deraadt 488: The
1.2 deraadt 489: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 490: file lists the RSA keys that are
491: permitted for RSA authentication. Each line of the file contains one
1.2 deraadt 492: key (empty lines and lines starting with a
493: .Ql #
494: are ignored as
1.1 deraadt 495: comments). Each line consists of the following fields, separated by
496: spaces: options, bits, exponent, modulus, comment. The options field
497: is optional; its presence is determined by whether the line starts
498: with a number or not (the option field never starts with a number).
499: The bits, exponent, modulus and comment fields give the RSA key; the
500: comment field is not used for anything (but may be convenient for the
501: user to identify the key).
1.2 deraadt 502: .Pp
1.1 deraadt 503: Note that lines in this file are usually several hundred bytes long
504: (because of the size of the RSA key modulus). You don't want to type
505: them in; instead, copy the
1.2 deraadt 506: .Pa identity.pub
1.1 deraadt 507: file and edit it.
1.2 deraadt 508: .Pp
1.1 deraadt 509: The options (if present) consists of comma-separated option
510: specifications. No spaces are permitted, except within double quotes.
511: The following option specifications are supported:
1.2 deraadt 512: .Bl -tag -width Ds
513: .It Cm from="pattern-list"
1.1 deraadt 514: Specifies that in addition to RSA authentication, the canonical name
515: of the remote host must be present in the comma-separated list of
516: patterns ('*' and '?' serve as wildcards). The list may also contain
517: patterns negated by prefixing them with '!'; if the canonical host
518: name matches a negated pattern, the key is not accepted. The purpose
519: of this option is to optionally increase security: RSA authentication
520: by itself does not trust the network or name servers or anything (but
521: the key); however, if somebody somehow steals the key, the key
522: permits an intruder to log in from anywhere in the world. This
523: additional option makes using a stolen key more difficult (name
524: servers and/or routers would have to be compromised in addition to
525: just the key).
1.2 deraadt 526: .It Cm command="command"
1.1 deraadt 527: Specifies that the command is executed whenever this key is used for
528: authentication. The command supplied by the user (if any) is ignored.
529: The command is run on a pty if the connection requests a pty;
530: otherwise it is run without a tty. A quote may be included in the
531: command by quoting it with a backslash. This option might be useful
532: to restrict certain RSA keys to perform just a specific operation. An
533: example might be a key that permits remote backups but nothing
534: else. Notice that the client may specify TCP/IP and/or X11
535: forwardings unless they are explicitly prohibited.
1.2 deraadt 536: .It Cm environment="NAME=value"
1.1 deraadt 537: Specifies that the string is to be added to the environment when
538: logging in using this key. Environment variables set this way
539: override other default environment values. Multiple options of this
540: type are permitted.
1.2 deraadt 541: .It Cm no-port-forwarding
1.1 deraadt 542: Forbids TCP/IP forwarding when this key is used for authentication.
543: Any port forward requests by the client will return an error. This
1.2 deraadt 544: might be used, e.g., in connection with the
545: .Cm command
1.1 deraadt 546: option.
1.2 deraadt 547: .It Cm no-X11-forwarding
1.1 deraadt 548: Forbids X11 forwarding when this key is used for authentication.
549: Any X11 forward requests by the client will return an error.
1.2 deraadt 550: .It Cm no-agent-forwarding
1.1 deraadt 551: Forbids authentication agent forwarding when this key is used for
552: authentication.
1.2 deraadt 553: .It Cm no-pty
1.1 deraadt 554: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 555: .El
556: .Ss Examples
1.1 deraadt 557: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 558: .Pp
1.1 deraadt 559: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 560: .Pp
1.1 deraadt 561: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.2 deraadt 562: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.1 deraadt 563: The
1.2 deraadt 564: .Pa /etc/ssh_known_hosts
1.1 deraadt 565: and
1.2 deraadt 566: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 567: files contain host public keys for all known hosts. The global file should
568: be prepared by the admistrator (optional), and the per-user file is
569: maintained automatically: whenever the user connects an unknown host
1.6 aaron 570: its key is added to the per-user file.
1.2 deraadt 571: .Pp
1.1 deraadt 572: Each line in these files contains the following fields: hostnames,
573: bits, exponent, modulus, comment. The fields are separated by spaces.
1.2 deraadt 574: .Pp
1.1 deraadt 575: Hostnames is a comma-separated list of patterns ('*' and '?' act as
576: wildcards); each pattern in turn is matched against the canonical host
577: name (when authenticating a client) or against the user-supplied
578: name (when authenticating a server). A pattern may also be preceded
1.2 deraadt 579: by
580: .Ql !
581: to indicate negation: if the host name matches a negated
1.1 deraadt 582: pattern, it is not accepted (by that line) even if it matched another
583: pattern on the line.
1.2 deraadt 584: .Pp
1.1 deraadt 585: Bits, exponent, and modulus are taken directly from the host key; they
1.2 deraadt 586: can be obtained, e.g., from
587: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 588: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 589: .Pp
590: Lines starting with
591: .Ql #
592: and empty lines are ignored as comments.
593: .Pp
1.1 deraadt 594: When performing host authentication, authentication is accepted if any
595: matching line has the proper key. It is thus permissible (but not
596: recommended) to have several lines or different host keys for the same
597: names. This will inevitably happen when short forms of host names
598: from different domains are put in the file. It is possible
599: that the files contain conflicting information; authentication is
600: accepted if valid information can be found from either file.
1.2 deraadt 601: .Pp
1.1 deraadt 602: Note that the lines in these files are typically hundreds of characters
603: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 604: Rather, generate them by a script
1.1 deraadt 605: or by taking
1.2 deraadt 606: .Pa /etc/ssh_host_key.pub
1.1 deraadt 607: and adding the host names at the front.
1.2 deraadt 608: .Ss Examples
1.1 deraadt 609: closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1.2 deraadt 610: .Sh FILES
611: .Bl -tag -width Ds
612: .It Pa /etc/sshd_config
1.1 deraadt 613: Contains configuration data for
1.2 deraadt 614: .Nm sshd .
1.1 deraadt 615: This file should be writable by root only, but it is recommended
616: (though not necessary) that it be world-readable.
1.2 deraadt 617: .It Pa /etc/ssh_host_key
1.7 markus 618: Contains the private part of the host key.
1.1 deraadt 619: This file should only be owned by root, readable only by root, and not
620: accessible to others.
1.14 markus 621: Note that
622: .Nm
623: does not start if this file is group/world-accessible.
1.2 deraadt 624: .It Pa /etc/ssh_host_key.pub
1.7 markus 625: Contains the public part of the host key.
626: This file should be world-readable but writable only by
1.1 deraadt 627: root. Its contents should match the private part. This file is not
628: really used for anything; it is only provided for the convenience of
629: the user so its contents can be copied to known hosts files.
1.7 markus 630: These two files are created using
631: .Xr ssh-keygen 1 .
1.2 deraadt 632: .It Pa /var/run/sshd.pid
633: Contains the process ID of the
634: .Nm
1.1 deraadt 635: listening for connections (if there are several daemons running
636: concurrently for different ports, this contains the pid of the one
637: started last). The contents of this file are not sensitive; it can be
638: world-readable.
1.2 deraadt 639: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 640: Lists the RSA keys that can be used to log into the user's account.
641: This file must be readable by root (which may on some machines imply
642: it being world-readable if the user's home directory resides on an NFS
643: volume). It is recommended that it not be accessible by others. The
644: format of this file is described above.
1.22 markus 645: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
646: These files are consulted when using rhosts with RSA host
1.1 deraadt 647: authentication to check the public key of the host. The key must be
1.22 markus 648: listed in one of these files to be accepted.
649: The client uses the same files
1.6 aaron 650: to verify that the remote host is the one we intended to
651: connect. These files should be writable only by root/the owner.
1.2 deraadt 652: .Pa /etc/ssh_known_hosts
653: should be world-readable, and
654: .Pa $HOME/.ssh/known_hosts
655: can but need not be world-readable.
1.6 aaron 656: .It Pa /etc/nologin
1.1 deraadt 657: If this file exists,
1.2 deraadt 658: .Nm
1.1 deraadt 659: refuses to let anyone except root log in. The contents of the file
660: are displayed to anyone trying to log in, and non-root connections are
661: refused. The file should be world-readable.
1.19 dugsong 662: .It Pa /etc/hosts.allow, /etc/hosts.deny
663: If compiled with
664: .Sy LIBWRAP
665: support, tcp-wrappers access controls may be defined here as described in
666: .Xr hosts_access 5 .
1.6 aaron 667: .It Pa $HOME/.rhosts
1.1 deraadt 668: This file contains host-username pairs, separated by a space, one per
669: line. The given user on the corresponding host is permitted to log in
670: without password. The same file is used by rlogind and rshd.
1.6 aaron 671: The file must
1.1 deraadt 672: be writable only by the user; it is recommended that it not be
673: accessible by others.
1.2 deraadt 674: .Pp
1.1 deraadt 675: If is also possible to use netgroups in the file. Either host or user
676: name may be of the form +@groupname to specify all hosts or all users
677: in the group.
1.2 deraadt 678: .It Pa $HOME/.shosts
679: For ssh,
680: this file is exactly the same as for
681: .Pa .rhosts .
682: However, this file is
683: not used by rlogin and rshd, so using this permits access using SSH only.
684: .Pa /etc/hosts.equiv
685: This file is used during
686: .Pa .rhosts
687: authentication. In the
1.1 deraadt 688: simplest form, this file contains host names, one per line. Users on
689: those hosts are permitted to log in without a password, provided they
690: have the same user name on both machines. The host name may also be
691: followed by a user name; such users are permitted to log in as
1.2 deraadt 692: .Em any
693: user on this machine (except root). Additionally, the syntax
694: .Dq +@group
695: can be used to specify netgroups. Negated entries start with
696: .Ql \&- .
697: .Pp
1.1 deraadt 698: If the client host/user is successfully matched in this file, login is
699: automatically permitted provided the client and server user names are the
700: same. Additionally, successful RSA host authentication is normally
701: required. This file must be writable only by root; it is recommended
702: that it be world-readable.
1.2 deraadt 703: .Pp
1.6 aaron 704: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 705: .Pa hosts.equiv .
1.1 deraadt 706: Beware that it really means that the named user(s) can log in as
1.2 deraadt 707: .Em anybody ,
1.1 deraadt 708: which includes bin, daemon, adm, and other accounts that own critical
709: binaries and directories. Using a user name practically grants the
710: user root access. The only valid use for user names that I can think
711: of is in negative entries.
1.2 deraadt 712: .Pp
713: Note that this warning also applies to rsh/rlogin.
714: .It Pa /etc/shosts.equiv
1.1 deraadt 715: This is processed exactly as
1.2 deraadt 716: .Pa /etc/hosts.equiv .
1.1 deraadt 717: However, this file may be useful in environments that want to run both
1.2 deraadt 718: rsh/rlogin and ssh.
1.6 aaron 719: .It Pa $HOME/.ssh/environment
1.1 deraadt 720: This file is read into the environment at login (if it exists). It
1.2 deraadt 721: can only contain empty lines, comment lines (that start with
722: .Ql # ) ,
1.6 aaron 723: and assignment lines of the form name=value. The file should be writable
724: only by the user; it need not be readable by anyone else.
1.2 deraadt 725: .It Pa $HOME/.ssh/rc
1.1 deraadt 726: If this file exists, it is run with /bin/sh after reading the
727: environment files but before starting the user's shell or command. If
728: X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 729: standard input (and
730: .Ev DISPLAY
731: in environment). This must call
732: .Xr xauth 1
733: in that case.
734: .Pp
1.1 deraadt 735: The primary purpose of this file is to run any initialization routines
736: which may be needed before the user's home directory becomes
737: accessible; AFS is a particular example of such an environment.
1.2 deraadt 738: .Pp
1.1 deraadt 739: This file will probably contain some initialization code followed by
740: something similar to: "if read proto cookie; then echo add $DISPLAY
741: $proto $cookie | xauth -q -; fi".
1.2 deraadt 742: .Pp
743: If this file does not exist,
744: .Pa /etc/sshrc
745: is run, and if that
1.1 deraadt 746: does not exist either, xauth is used to store the cookie.
1.2 deraadt 747: .Pp
1.1 deraadt 748: This file should be writable only by the user, and need not be
749: readable by anyone else.
1.2 deraadt 750: .It Pa /etc/sshrc
751: Like
752: .Pa $HOME/.ssh/rc .
753: This can be used to specify
1.1 deraadt 754: machine-specific login-time initializations globally. This file
755: should be writable only by root, and should be world-readable.
1.2 deraadt 756: .Sh AUTHOR
1.1 deraadt 757: Tatu Ylonen <ylo@cs.hut.fi>
1.2 deraadt 758: .Pp
1.1 deraadt 759: Information about new releases, mailing lists, and other related
1.2 deraadt 760: issues can be found from the SSH WWW home page:
761: .Pp
762: .Dl http://www.cs.hut.fi/ssh.
1.5 deraadt 763: .Pp
1.17 deraadt 764: OpenSSH
765: is a derivative of the original (free) ssh 1.2.12 release, but with bugs
766: removed and newer features re-added. Rapidly after the 1.2.12 release,
767: newer versions bore successively more restrictive licenses. This version
768: of OpenSSH
769: .Bl -bullet
770: .It
771: has all components of a restrictive nature (ie. patents, see
772: .Xr ssl 8 )
773: directly removed from the source code; any licensed or patented components
774: are chosen from
775: external libraries.
776: .It
777: has been updated to support ssh protocol 1.5.
778: .It
779: contains added support for
780: .Xr kerberos 8
781: authentication and ticket passing.
782: .It
783: supports one-time password authentication with
784: .Xr skey 1 .
785: .El
786: .Pp
787: The libraries described in
1.5 deraadt 788: .Xr ssl 8
789: are required for proper operation.
1.2 deraadt 790: .Sh SEE ALSO
791: .Xr rlogin 1 ,
792: .Xr rsh 1 ,
793: .Xr scp 1 ,
794: .Xr ssh 1 ,
1.5 deraadt 795: .Xr ssh-add 1 ,
1.2 deraadt 796: .Xr ssh-agent 1 ,
1.5 deraadt 797: .Xr ssh-keygen 1 ,
798: .Xr ssl 8
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