Annotation of src/usr.bin/ssh/sshd.8, Revision 1.16
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: .\"
1.16 ! markus 12: .\" $Id: sshd.8,v 1.15 1999/10/14 19:56:02 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
22: .Op Fl diq
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
29: .Sh DESCRIPTION
30: .Nm
1.1 deraadt 31: (Secure Shell Daemon) is the daemon program for
1.2 deraadt 32: .Xr ssh 1 .
1.1 deraadt 33: Together these programs replace rlogin and rsh programs, and
34: provide secure encrypted communications between two untrusted hosts
35: over an insecure network. The programs are intended to be as easy to
36: install and use as possible.
1.2 deraadt 37: .Pp
38: .Nm
1.1 deraadt 39: is the daemon that listens for connections from clients. It is
40: normally started at boot from
1.2 deraadt 41: .Pa /etc/rc .
42: It forks a new
1.1 deraadt 43: daemon for each incoming connection. The forked daemons handle
44: key exchange, encryption, authentication, command execution,
45: and data exchange.
1.2 deraadt 46: .Pp
47: .Nm
48: works as follows. Each host has a host-specific RSA key
1.1 deraadt 49: (normally 1024 bits) used to identify the host. Additionally, when
50: the daemon starts, it generates a server RSA key (normally 768 bits).
51: This key is normally regenerated every hour if it has been used, and
52: is never stored on disk.
1.2 deraadt 53: .Pp
1.1 deraadt 54: Whenever a client connects the daemon, the daemon sends its host
55: and server public keys to the client. The client compares the
56: host key against its own database to verify that it has not changed.
57: The client then generates a 256 bit random number. It encrypts this
58: random number using both the host key and the server key, and sends
59: the encrypted number to the server. Both sides then start to use this
60: random number as a session key which is used to encrypt all further
61: communications in the session. The rest of the session is encrypted
1.5 deraadt 62: using a conventional cipher, currently Blowfish and 3DES, with 3DES
63: being is used by default. The client selects the encryption algorithm
64: to use from those offered by the server.
1.2 deraadt 65: .Pp
1.1 deraadt 66: Next, the server and the client enter an authentication dialog. The
1.2 deraadt 67: client tries to authenticate itself using
68: .Pa .rhosts
69: authentication,
70: .Pa .rhosts
71: authentication combined with RSA host
1.1 deraadt 72: authentication, RSA challenge-response authentication, or password
73: based authentication.
1.2 deraadt 74: .Pp
1.1 deraadt 75: Rhosts authentication is normally disabled
76: because it is fundamentally insecure, but can be enabled in the server
77: configuration file if desired. System security is not improved unless
1.2 deraadt 78: .Xr rshd 8 ,
79: .Xr rlogind 8 ,
80: .Xr rexecd 8 ,
81: and
82: .Xr rexd 8
1.1 deraadt 83: are disabled (thus completely disabling
1.2 deraadt 84: .Xr rlogin 1
1.1 deraadt 85: and
1.2 deraadt 86: .Xr rsh 1
1.1 deraadt 87: into that machine).
1.2 deraadt 88: .Pp
1.1 deraadt 89: If the client successfully authenticates itself, a dialog for
90: preparing the session is entered. At this time the client may request
91: things like allocating a pseudo-tty, forwarding X11 connections,
92: forwarding TCP/IP connections, or forwarding the authentication agent
93: connection over the secure channel.
1.2 deraadt 94: .Pp
1.1 deraadt 95: Finally, the client either requests a shell or execution of a command.
96: The sides then enter session mode. In this mode, either side may send
97: data at any time, and such data is forwarded to/from the shell or
98: command on the server side, and the user terminal in the client side.
1.2 deraadt 99: .Pp
1.1 deraadt 100: When the user program terminates and all forwarded X11 and other
101: connections have been closed, the server sends command exit status to
102: the client, and both sides exit.
1.2 deraadt 103: .Pp
104: .Nm
1.1 deraadt 105: can be configured using command-line options or a configuration
106: file. Command-line options override values specified in the
107: configuration file.
1.2 deraadt 108: .Sh OPTIONS
109: .Bl -tag -width Ds
110: .It Fl b Ar bits
1.1 deraadt 111: Specifies the number of bits in the server key (default 768).
1.2 deraadt 112: .Pp
113: .It Fl d
1.1 deraadt 114: Debug mode. The server sends verbose debug output to the system
115: log, and does not put itself in the background. The server also will
116: not fork and will only process one connection. This option is only
117: intended for debugging for the server.
1.2 deraadt 118: .It Fl f Ar configuration_file
1.1 deraadt 119: Specifies the name of the configuration file. The default is
1.2 deraadt 120: .Pa /etc/sshd_config .
1.16 ! markus 121: .Nm
! 122: refuses to start if there is no configuration file.
1.2 deraadt 123: .It Fl g Ar login_grace_time
1.1 deraadt 124: Gives the grace time for clients to authenticate themselves (default
125: 300 seconds). If the client fails to authenticate the user within
126: this many seconds, the server disconnects and exits. A value of zero
127: indicates no limit.
1.2 deraadt 128: .It Fl h Ar host_key_file
1.1 deraadt 129: Specifies the file from which the host key is read (default
1.2 deraadt 130: .Pa /etc/ssh_host_key ) .
1.7 markus 131: This option must be given if
132: .Nm
133: is not run as root (as the normal
1.1 deraadt 134: host file is normally not readable by anyone but root).
1.2 deraadt 135: .It Fl i
1.7 markus 136: Specifies that
137: .Nm
138: is being run from inetd.
139: .Nm
140: is normally not run
1.1 deraadt 141: from inetd because it needs to generate the server key before it can
142: respond to the client, and this may take tens of seconds. Clients
143: would have to wait too long if the key was regenerated every time.
1.7 markus 144: However, with small key sizes (e.g. 512) using
145: .Nm
146: from inetd may
1.1 deraadt 147: be feasible.
1.2 deraadt 148: .It Fl k Ar key_gen_time
1.1 deraadt 149: Specifies how often the server key is regenerated (default 3600
150: seconds, or one hour). The motivation for regenerating the key fairly
151: often is that the key is not stored anywhere, and after about an hour,
152: it becomes impossible to recover the key for decrypting intercepted
153: communications even if the machine is cracked into or physically
154: seized. A value of zero indicates that the key will never be regenerated.
1.2 deraadt 155: .It Fl p Ar port
1.1 deraadt 156: Specifies the port on which the server listens for connections
157: (default 22).
1.2 deraadt 158: .It Fl q
1.1 deraadt 159: Quiet mode. Nothing is sent to the system log. Normally the beginning,
160: authentication, and termination of each connection is logged.
1.2 deraadt 161: .El
162: .Sh CONFIGURATION FILE
163: .Nm
1.1 deraadt 164: reads configuration data from
1.2 deraadt 165: .Pa /etc/sshd_config
166: (or the file specified with
167: .Fl f
168: on the command line). The file
169: contains keyword-value pairs, one per line. Lines starting with
170: .Ql #
1.1 deraadt 171: and empty lines are interpreted as comments.
1.2 deraadt 172: .Pp
1.1 deraadt 173: The following keywords are possible.
1.2 deraadt 174: .Bl -tag -width Ds
175: .It Cm AFSTokenPassing
1.3 dugsong 176: Specifies whether an AFS token may be forwarded to the server. Default is
1.2 deraadt 177: .Dq yes .
1.11 markus 178: .It Cm AllowGroups
179: This keyword can be followed by a number of group names, separated
180: by spaces. If specified, login is allowed only for users whose primary
181: group matches one of the patterns.
182: .Ql \&*
183: and
184: .Ql ?
185: can be used as
186: wildcards in the patterns. Only group names are valid, a numerical group
187: id isn't recognized. By default login is allowed regardless of
188: the primary group.
1.3 dugsong 189: .Pp
1.2 deraadt 190: .It Cm AllowHosts
1.1 deraadt 191: This keyword can be followed by any number of host name patterns,
192: separated by spaces. If specified, login is allowed only from hosts
1.2 deraadt 193: whose name matches one of the patterns.
194: .Ql \&*
195: and
196: .Ql ?
197: can be used as
1.1 deraadt 198: wildcards in the patterns. Normal name servers are used to map the
199: client's host into a canonical host name. If the name cannot be
200: mapped, its IP-address is used as the host name. By default all hosts
201: are allowed to connect.
1.2 deraadt 202: .Pp
1.1 deraadt 203: Note that
1.2 deraadt 204: .Nm
205: can also be configured to use tcp_wrappers using the
1.7 markus 206: .Sy LIBWARP
1.6 aaron 207: compile-time option.
1.11 markus 208: .It Cm AllowUsers
209: This keyword can be followed by a number of user names, separated
210: by spaces. If specified, login is allowed only for users names that
211: match one of the patterns.
212: .Ql \&*
213: and
214: .Ql ?
215: can be used as
216: wildcards in the patterns. Only user names are valid, a numerical user
217: id isn't recognized. By default login is allowed regardless of
218: the user name.
219: .Pp
1.8 markus 220: .It Cm CheckMail
221: Specifies whether
222: .Nm
223: should check for new mail for interactive logins.
224: The default is
225: .Dq no .
1.11 markus 226: .It Cm DenyGroups
227: This keyword can be followed by a number of group names, separated
228: by spaces. Users whose primary group matches one of the patterns
229: aren't allowed to log in.
230: .Ql \&*
231: and
232: .Ql ?
233: can be used as
234: wildcards in the patterns. Only group names are valid, a numerical group
235: id isn't recognized. By default login is allowed regardless of
236: the primary group.
237: .Pp
1.2 deraadt 238: .It Cm DenyHosts
1.1 deraadt 239: This keyword can be followed by any number of host name patterns,
240: separated by spaces. If specified, login is disallowed from the hosts
241: whose name matches any of the patterns.
1.11 markus 242: .It Cm DenyUsers
243: This keyword can be followed by a number of user names, separated
244: by spaces. Login is allowed disallowed for user names that match
245: one of the patterns.
246: .Ql \&*
247: and
248: .Ql ?
249: can be used as
250: wildcards in the patterns. Only user names are valid, a numerical user
251: id isn't recognized. By default login is allowed regardless of
252: the user name.
253: .Pp
1.2 deraadt 254: .It Cm FascistLogging
1.1 deraadt 255: Specifies whether to use verbose logging. Verbose logging violates
256: the privacy of users and is not recommended. The argument must be
1.2 deraadt 257: .Dq yes
258: or
259: .Dq no .
260: The default is
261: .Dq no .
262: .It Cm HostKey
1.1 deraadt 263: Specifies the file containing the private host key (default
1.2 deraadt 264: .Pa /etc/ssh_host_key ) .
1.9 markus 265: Note that
266: .Nm
1.14 markus 267: does not start if this file is group/world-accessible.
1.2 deraadt 268: .It Cm IgnoreRhosts
1.1 deraadt 269: Specifies that rhosts and shosts files will not be used in
270: authentication.
1.2 deraadt 271: .Pa /etc/hosts.equiv
1.1 deraadt 272: and
1.2 deraadt 273: .Pa /etc/shosts.equiv
274: are still used. The default is
275: .Dq no .
276: .It Cm KeepAlive
1.1 deraadt 277: Specifies whether the system should send keepalive messages to the
278: other side. If they are sent, death of the connection or crash of one
279: of the machines will be properly noticed. However, this means that
280: connections will die if the route is down temporarily, and some people
281: find it annoying. On the other hand, if keepalives are not send,
1.2 deraadt 282: sessions may hang indefinitely on the server, leaving
283: .Dq ghost
284: users and consuming server resources.
285: .Pp
286: The default is
287: .Dq yes
288: (to send keepalives), and the server will notice
1.1 deraadt 289: if the network goes down or the client host reboots. This avoids
290: infinitely hanging sessions.
1.2 deraadt 291: .Pp
292: To disable keepalives, the value should be set to
293: .Dq no
294: in both the server and the client configuration files.
295: .It Cm KerberosAuthentication
1.1 deraadt 296: Specifies whether Kerberos authentication is allowed. This can
1.7 markus 297: be in the form of a Kerberos ticket, or if
298: .Cm PasswordAuthentication
1.1 deraadt 299: is yes, the password provided by the user will be validated through
1.3 dugsong 300: the Kerberos KDC. Default is
301: .Dq yes .
1.2 deraadt 302: .It Cm KerberosOrLocalPasswd
1.1 deraadt 303: If set then if password authentication through Kerberos fails then
304: the password will be validated via any additional local mechanism
1.2 deraadt 305: such as
306: .Pa /etc/passwd
307: or SecurID. Default is
308: .Dq no .
309: .It Cm KerberosTgtPassing
1.1 deraadt 310: Specifies whether a Kerberos TGT may be forwarded to the server.
1.3 dugsong 311: Default is
312: .Dq no ,
313: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 314: .It Cm KerberosTicketCleanup
1.7 markus 315: Specifies whether to automatically destroy the user's ticket cache
316: file on logout. Default is
1.3 dugsong 317: .Dq yes .
1.2 deraadt 318: .It Cm KeyRegenerationInterval
1.1 deraadt 319: The server key is automatically regenerated after this many seconds
320: (if it has been used). The purpose of regeneration is to prevent
321: decrypting captured sessions by later breaking into the machine and
322: stealing the keys. The key is never stored anywhere. If the value is
323: 0, the key is never regenerated. The default is 3600
324: (seconds).
1.7 markus 325: .It Cm ListenAddress
326: Specifies what local address
327: .Nm
328: should listen on.
329: The default is to listen to all local addresses.
1.2 deraadt 330: .It Cm LoginGraceTime
1.1 deraadt 331: The server disconnects after this time if the user has not
332: successfully logged in. If the value is 0, there is no time limit.
333: The default is 600 (seconds).
1.2 deraadt 334: .It Cm PasswordAuthentication
1.1 deraadt 335: Specifies whether password authentication is allowed.
1.2 deraadt 336: The default is
337: .Dq yes .
338: .It Cm PermitEmptyPasswords
1.1 deraadt 339: When password authentication is allowed, it specifies whether the
340: server allows login to accounts with empty password strings. The default
1.2 deraadt 341: is
342: .Dq yes .
343: .It Cm PermitRootLogin
1.1 deraadt 344: Specifies whether the root can log in using
1.2 deraadt 345: .Xr ssh 1 .
1.15 markus 346: The argument must be
347: .Dq yes ,
348: .Dq without-password
349: or
350: .Dq no .
1.2 deraadt 351: The default is
352: .Dq yes .
1.15 markus 353: If this options is set to
354: .Dq without-password
355: only password authentication is disabled for root.
1.2 deraadt 356: .Pp
357: Root login with RSA authentication when the
358: .Ar command
359: option has been
1.1 deraadt 360: specified will be allowed regardless of the value of this setting
361: (which may be useful for taking remote backups even if root login is
362: normally not allowed).
1.2 deraadt 363: .It Cm Port
1.1 deraadt 364: Specifies the port number that
1.2 deraadt 365: .Nm
1.1 deraadt 366: listens on. The default is 22.
1.2 deraadt 367: .It Cm PrintMotd
1.1 deraadt 368: Specifies whether
1.2 deraadt 369: .Nm
1.1 deraadt 370: should print
1.2 deraadt 371: .Pa /etc/motd
1.1 deraadt 372: when a user logs in interactively. (On some systems it is also
1.2 deraadt 373: printed by the shell,
374: .Pa /etc/profile ,
375: or equivalent.) The default is
376: .Dq yes .
377: .It Cm QuietMode
1.1 deraadt 378: Specifies whether the system runs in quiet mode. In quiet mode,
379: nothing is logged in the system log, except fatal errors. The default
1.2 deraadt 380: is
381: .Dq no .
382: .It Cm RandomSeed
1.4 deraadt 383: Obsolete. Random number generation uses other techniques.
1.2 deraadt 384: .It Cm RhostsAuthentication
1.1 deraadt 385: Specifies whether authentication using rhosts or /etc/hosts.equiv
386: files is sufficient. Normally, this method should not be permitted
1.7 markus 387: because it is insecure.
388: .Cm RhostsRSAAuthentication
389: should be used
1.1 deraadt 390: instead, because it performs RSA-based host authentication in addition
391: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 392: The default is
393: .Dq no .
394: .It Cm RhostsRSAAuthentication
1.1 deraadt 395: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.2 deraadt 396: with successful RSA host authentication is allowed. The default is
397: .Dq yes .
398: .It Cm RSAAuthentication
399: Specifies whether pure RSA authentication is allowed. The default is
400: .Dq yes .
401: .It Cm ServerKeyBits
1.1 deraadt 402: Defines the number of bits in the server key. The minimum value is
403: 512, and the default is 768.
1.13 markus 404: .It Cm SilentDeny
405: Specifies whether
406: .Nm
407: should log and complain to denied hosts or close the
408: connection silently.
409: The default is
410: .Dq no .
1.7 markus 411: .It Cm SkeyAuthentication
412: Specifies whether
413: .Xr skey 1
414: authentication is allowed. The default is
415: .Dq yes .
416: Note that s/key authentication is enabled only if
417: .Cm PasswordAuthentication
418: is allowed, too.
1.2 deraadt 419: .It Cm StrictModes
1.12 markus 420: Specifies whether
421: .Nm
422: should check file modes and ownership of the
423: user's files and home directory before accepting login. This
1.1 deraadt 424: is normally desirable because novices sometimes accidentally leave their
1.7 markus 425: directory or files world-writable. The default is
426: .Dq yes .
1.2 deraadt 427: .It Cm SyslogFacility
1.1 deraadt 428: Gives the facility code that is used when logging messages from
1.2 deraadt 429: .Nm sshd .
1.1 deraadt 430: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
431: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is DAEMON.
1.10 markus 432: .It Cm UseLogin
433: Specifies whether
434: .Xr login 1
435: is used. The default is
436: .Dq no .
1.2 deraadt 437: .It Cm X11Forwarding
438: Specifies whether X11 forwarding is permitted. The default is
439: .Dq yes .
1.1 deraadt 440: Note that disabling X11 forwarding does not improve security in any
441: way, as users can always install their own forwarders.
1.6 aaron 442: .It Cm X11DisplayOffset
443: Specifies the first display number available for
444: .Nm sshd Ns 's
445: X11 forwarding. This prevents
446: .Nm
447: from interfering with real X11 servers.
1.2 deraadt 448: .El
449: .Sh LOGIN PROCESS
1.1 deraadt 450: When a user successfully logs in,
1.2 deraadt 451: .Nm
1.1 deraadt 452: does the following:
1.2 deraadt 453: .Bl -enum -offset indent
454: .It
1.1 deraadt 455: If the login is on a tty, and no command has been specified,
456: prints last login time and
1.2 deraadt 457: .Pa /etc/motd
1.1 deraadt 458: (unless prevented in the configuration file or by
1.2 deraadt 459: .Pa $HOME/.hushlogin ;
460: see the
461: .Sx FILES
462: section).
463: .It
1.1 deraadt 464: If the login is on a tty, records login time.
1.2 deraadt 465: .It
466: Checks
467: .Pa /etc/nologin ;
468: if it exists, prints contents and quits
1.1 deraadt 469: (unless root).
1.2 deraadt 470: .It
1.1 deraadt 471: Changes to run with normal user privileges.
1.2 deraadt 472: .It
1.1 deraadt 473: Sets up basic environment.
1.2 deraadt 474: .It
475: Reads
476: .Pa $HOME/.ssh/environment
477: if it exists.
478: .It
1.1 deraadt 479: Changes to user's home directory.
1.2 deraadt 480: .It
481: If
482: .Pa $HOME/.ssh/rc
483: exists, runs it; else if
484: .Pa /etc/sshrc
485: exists, runs
486: it; otherwise runs xauth. The
487: .Dq rc
488: files are given the X11
1.1 deraadt 489: authentication protocol and cookie in standard input.
1.2 deraadt 490: .It
1.1 deraadt 491: Runs user's shell or command.
1.2 deraadt 492: .El
493: .Sh AUTHORIZED_KEYS FILE FORMAT
1.1 deraadt 494: The
1.2 deraadt 495: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 496: file lists the RSA keys that are
497: permitted for RSA authentication. Each line of the file contains one
1.2 deraadt 498: key (empty lines and lines starting with a
499: .Ql #
500: are ignored as
1.1 deraadt 501: comments). Each line consists of the following fields, separated by
502: spaces: options, bits, exponent, modulus, comment. The options field
503: is optional; its presence is determined by whether the line starts
504: with a number or not (the option field never starts with a number).
505: The bits, exponent, modulus and comment fields give the RSA key; the
506: comment field is not used for anything (but may be convenient for the
507: user to identify the key).
1.2 deraadt 508: .Pp
1.1 deraadt 509: Note that lines in this file are usually several hundred bytes long
510: (because of the size of the RSA key modulus). You don't want to type
511: them in; instead, copy the
1.2 deraadt 512: .Pa identity.pub
1.1 deraadt 513: file and edit it.
1.2 deraadt 514: .Pp
1.1 deraadt 515: The options (if present) consists of comma-separated option
516: specifications. No spaces are permitted, except within double quotes.
517: The following option specifications are supported:
1.2 deraadt 518: .Bl -tag -width Ds
519: .It Cm from="pattern-list"
1.1 deraadt 520: Specifies that in addition to RSA authentication, the canonical name
521: of the remote host must be present in the comma-separated list of
522: patterns ('*' and '?' serve as wildcards). The list may also contain
523: patterns negated by prefixing them with '!'; if the canonical host
524: name matches a negated pattern, the key is not accepted. The purpose
525: of this option is to optionally increase security: RSA authentication
526: by itself does not trust the network or name servers or anything (but
527: the key); however, if somebody somehow steals the key, the key
528: permits an intruder to log in from anywhere in the world. This
529: additional option makes using a stolen key more difficult (name
530: servers and/or routers would have to be compromised in addition to
531: just the key).
1.2 deraadt 532: .It Cm command="command"
1.1 deraadt 533: Specifies that the command is executed whenever this key is used for
534: authentication. The command supplied by the user (if any) is ignored.
535: The command is run on a pty if the connection requests a pty;
536: otherwise it is run without a tty. A quote may be included in the
537: command by quoting it with a backslash. This option might be useful
538: to restrict certain RSA keys to perform just a specific operation. An
539: example might be a key that permits remote backups but nothing
540: else. Notice that the client may specify TCP/IP and/or X11
541: forwardings unless they are explicitly prohibited.
1.2 deraadt 542: .It Cm environment="NAME=value"
1.1 deraadt 543: Specifies that the string is to be added to the environment when
544: logging in using this key. Environment variables set this way
545: override other default environment values. Multiple options of this
546: type are permitted.
1.2 deraadt 547: .It Cm no-port-forwarding
1.1 deraadt 548: Forbids TCP/IP forwarding when this key is used for authentication.
549: Any port forward requests by the client will return an error. This
1.2 deraadt 550: might be used, e.g., in connection with the
551: .Cm command
1.1 deraadt 552: option.
1.2 deraadt 553: .It Cm no-X11-forwarding
1.1 deraadt 554: Forbids X11 forwarding when this key is used for authentication.
555: Any X11 forward requests by the client will return an error.
1.2 deraadt 556: .It Cm no-agent-forwarding
1.1 deraadt 557: Forbids authentication agent forwarding when this key is used for
558: authentication.
1.2 deraadt 559: .It Cm no-pty
1.1 deraadt 560: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 561: .El
562: .Ss Examples
1.1 deraadt 563: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 564: .Pp
1.1 deraadt 565: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 566: .Pp
1.1 deraadt 567: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.2 deraadt 568: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.1 deraadt 569: The
1.2 deraadt 570: .Pa /etc/ssh_known_hosts
1.1 deraadt 571: and
1.2 deraadt 572: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 573: files contain host public keys for all known hosts. The global file should
574: be prepared by the admistrator (optional), and the per-user file is
575: maintained automatically: whenever the user connects an unknown host
1.6 aaron 576: its key is added to the per-user file.
1.2 deraadt 577: .Pp
1.1 deraadt 578: Each line in these files contains the following fields: hostnames,
579: bits, exponent, modulus, comment. The fields are separated by spaces.
1.2 deraadt 580: .Pp
1.1 deraadt 581: Hostnames is a comma-separated list of patterns ('*' and '?' act as
582: wildcards); each pattern in turn is matched against the canonical host
583: name (when authenticating a client) or against the user-supplied
584: name (when authenticating a server). A pattern may also be preceded
1.2 deraadt 585: by
586: .Ql !
587: to indicate negation: if the host name matches a negated
1.1 deraadt 588: pattern, it is not accepted (by that line) even if it matched another
589: pattern on the line.
1.2 deraadt 590: .Pp
1.1 deraadt 591: Bits, exponent, and modulus are taken directly from the host key; they
1.2 deraadt 592: can be obtained, e.g., from
593: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 594: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 595: .Pp
596: Lines starting with
597: .Ql #
598: and empty lines are ignored as comments.
599: .Pp
1.1 deraadt 600: When performing host authentication, authentication is accepted if any
601: matching line has the proper key. It is thus permissible (but not
602: recommended) to have several lines or different host keys for the same
603: names. This will inevitably happen when short forms of host names
604: from different domains are put in the file. It is possible
605: that the files contain conflicting information; authentication is
606: accepted if valid information can be found from either file.
1.2 deraadt 607: .Pp
1.1 deraadt 608: Note that the lines in these files are typically hundreds of characters
609: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 610: Rather, generate them by a script
1.1 deraadt 611: or by taking
1.2 deraadt 612: .Pa /etc/ssh_host_key.pub
1.1 deraadt 613: and adding the host names at the front.
1.2 deraadt 614: .Ss Examples
1.1 deraadt 615: closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1.2 deraadt 616: .Sh FILES
617: .Bl -tag -width Ds
618: .It Pa /etc/sshd_config
1.1 deraadt 619: Contains configuration data for
1.2 deraadt 620: .Nm sshd .
1.1 deraadt 621: This file should be writable by root only, but it is recommended
622: (though not necessary) that it be world-readable.
1.2 deraadt 623: .It Pa /etc/ssh_host_key
1.7 markus 624: Contains the private part of the host key.
1.1 deraadt 625: This file should only be owned by root, readable only by root, and not
626: accessible to others.
1.14 markus 627: Note that
628: .Nm
629: does not start if this file is group/world-accessible.
1.2 deraadt 630: .It Pa /etc/ssh_host_key.pub
1.7 markus 631: Contains the public part of the host key.
632: This file should be world-readable but writable only by
1.1 deraadt 633: root. Its contents should match the private part. This file is not
634: really used for anything; it is only provided for the convenience of
635: the user so its contents can be copied to known hosts files.
1.7 markus 636: These two files are created using
637: .Xr ssh-keygen 1 .
1.2 deraadt 638: .It Pa /var/run/sshd.pid
639: Contains the process ID of the
640: .Nm
1.1 deraadt 641: listening for connections (if there are several daemons running
642: concurrently for different ports, this contains the pid of the one
643: started last). The contents of this file are not sensitive; it can be
644: world-readable.
1.2 deraadt 645: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 646: Lists the RSA keys that can be used to log into the user's account.
647: This file must be readable by root (which may on some machines imply
648: it being world-readable if the user's home directory resides on an NFS
649: volume). It is recommended that it not be accessible by others. The
650: format of this file is described above.
1.6 aaron 651: .It Pa /etc/ssh_known_hosts
652: This file is consulted when using rhosts with RSA host
1.1 deraadt 653: authentication to check the public key of the host. The key must be
1.6 aaron 654: listed in this file to be accepted.
655: .It Pa $HOME/.ssh/known_hosts
656: The client uses this file
657: and
658: .Pa /etc/ssh_known_hosts
659: to verify that the remote host is the one we intended to
660: connect. These files should be writable only by root/the owner.
1.2 deraadt 661: .Pa /etc/ssh_known_hosts
662: should be world-readable, and
663: .Pa $HOME/.ssh/known_hosts
664: can but need not be world-readable.
1.6 aaron 665: .It Pa /etc/nologin
1.1 deraadt 666: If this file exists,
1.2 deraadt 667: .Nm
1.1 deraadt 668: refuses to let anyone except root log in. The contents of the file
669: are displayed to anyone trying to log in, and non-root connections are
670: refused. The file should be world-readable.
1.6 aaron 671: .It Pa $HOME/.rhosts
1.1 deraadt 672: This file contains host-username pairs, separated by a space, one per
673: line. The given user on the corresponding host is permitted to log in
674: without password. The same file is used by rlogind and rshd.
1.6 aaron 675: The file must
1.1 deraadt 676: be writable only by the user; it is recommended that it not be
677: accessible by others.
1.2 deraadt 678: .Pp
1.1 deraadt 679: If is also possible to use netgroups in the file. Either host or user
680: name may be of the form +@groupname to specify all hosts or all users
681: in the group.
1.2 deraadt 682: .It Pa $HOME/.shosts
683: For ssh,
684: this file is exactly the same as for
685: .Pa .rhosts .
686: However, this file is
687: not used by rlogin and rshd, so using this permits access using SSH only.
688: .Pa /etc/hosts.equiv
689: This file is used during
690: .Pa .rhosts
691: authentication. In the
1.1 deraadt 692: simplest form, this file contains host names, one per line. Users on
693: those hosts are permitted to log in without a password, provided they
694: have the same user name on both machines. The host name may also be
695: followed by a user name; such users are permitted to log in as
1.2 deraadt 696: .Em any
697: user on this machine (except root). Additionally, the syntax
698: .Dq +@group
699: can be used to specify netgroups. Negated entries start with
700: .Ql \&- .
701: .Pp
1.1 deraadt 702: If the client host/user is successfully matched in this file, login is
703: automatically permitted provided the client and server user names are the
704: same. Additionally, successful RSA host authentication is normally
705: required. This file must be writable only by root; it is recommended
706: that it be world-readable.
1.2 deraadt 707: .Pp
1.6 aaron 708: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 709: .Pa hosts.equiv .
1.1 deraadt 710: Beware that it really means that the named user(s) can log in as
1.2 deraadt 711: .Em anybody ,
1.1 deraadt 712: which includes bin, daemon, adm, and other accounts that own critical
713: binaries and directories. Using a user name practically grants the
714: user root access. The only valid use for user names that I can think
715: of is in negative entries.
1.2 deraadt 716: .Pp
717: Note that this warning also applies to rsh/rlogin.
718: .It Pa /etc/shosts.equiv
1.1 deraadt 719: This is processed exactly as
1.2 deraadt 720: .Pa /etc/hosts.equiv .
1.1 deraadt 721: However, this file may be useful in environments that want to run both
1.2 deraadt 722: rsh/rlogin and ssh.
1.6 aaron 723: .It Pa $HOME/.ssh/environment
1.1 deraadt 724: This file is read into the environment at login (if it exists). It
1.2 deraadt 725: can only contain empty lines, comment lines (that start with
726: .Ql # ) ,
1.6 aaron 727: and assignment lines of the form name=value. The file should be writable
728: only by the user; it need not be readable by anyone else.
1.2 deraadt 729: .It Pa $HOME/.ssh/rc
1.1 deraadt 730: If this file exists, it is run with /bin/sh after reading the
731: environment files but before starting the user's shell or command. If
732: X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 733: standard input (and
734: .Ev DISPLAY
735: in environment). This must call
736: .Xr xauth 1
737: in that case.
738: .Pp
1.1 deraadt 739: The primary purpose of this file is to run any initialization routines
740: which may be needed before the user's home directory becomes
741: accessible; AFS is a particular example of such an environment.
1.2 deraadt 742: .Pp
1.1 deraadt 743: This file will probably contain some initialization code followed by
744: something similar to: "if read proto cookie; then echo add $DISPLAY
745: $proto $cookie | xauth -q -; fi".
1.2 deraadt 746: .Pp
747: If this file does not exist,
748: .Pa /etc/sshrc
749: is run, and if that
1.1 deraadt 750: does not exist either, xauth is used to store the cookie.
1.2 deraadt 751: .Pp
1.1 deraadt 752: This file should be writable only by the user, and need not be
753: readable by anyone else.
1.2 deraadt 754: .It Pa /etc/sshrc
755: Like
756: .Pa $HOME/.ssh/rc .
757: This can be used to specify
1.1 deraadt 758: machine-specific login-time initializations globally. This file
759: should be writable only by root, and should be world-readable.
1.2 deraadt 760: .Sh AUTHOR
1.1 deraadt 761: Tatu Ylonen <ylo@cs.hut.fi>
1.2 deraadt 762: .Pp
1.1 deraadt 763: Information about new releases, mailing lists, and other related
1.2 deraadt 764: issues can be found from the SSH WWW home page:
765: .Pp
766: .Dl http://www.cs.hut.fi/ssh.
1.5 deraadt 767: .Pp
768: This version of
769: .Nm
770: is a derivative of the original 1.2.12 release, but with bugs removed and
771: newer features re-added. Rapidly after the 1.2.12 release, newer versions
772: bore successively more restrictive licenses. In this version, all components
773: of a restrictive nature (ie. patents) have been directly removed from the
774: source code; any licensed or patented components are chosen from external
775: libraries. The libraries described in
776: .Xr ssl 8
777: are required for proper operation.
1.2 deraadt 778: .Sh SEE ALSO
779: .Xr rlogin 1 ,
780: .Xr rsh 1 ,
781: .Xr scp 1 ,
782: .Xr ssh 1 ,
1.5 deraadt 783: .Xr ssh-add 1 ,
1.2 deraadt 784: .Xr ssh-agent 1 ,
1.5 deraadt 785: .Xr ssh-keygen 1 ,
786: .Xr ssl 8
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