Annotation of src/usr.bin/ssh/sshd.8, Revision 1.27
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: .\"
1.27 ! markus 12: .\" $Id: sshd.8,v 1.26 1999/11/22 20:02:45 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.
1.2 deraadt 323: .It Cm LoginGraceTime
1.1 deraadt 324: The server disconnects after this time if the user has not
325: successfully logged in. If the value is 0, there is no time limit.
326: The default is 600 (seconds).
1.23 markus 327: .It Cm LogLevel
328: Gives the verbosity level that is used when logging messages from
329: .Nm sshd .
330: The possible values are:
1.27 ! markus 331: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
1.23 markus 332: The default is INFO.
333: Logging with level DEBUG violates the privacy of users
334: and is not recommended.
1.2 deraadt 335: .It Cm PasswordAuthentication
1.1 deraadt 336: Specifies whether password authentication is allowed.
1.2 deraadt 337: The default is
338: .Dq yes .
339: .It Cm PermitEmptyPasswords
1.1 deraadt 340: When password authentication is allowed, it specifies whether the
341: server allows login to accounts with empty password strings. The default
1.2 deraadt 342: is
343: .Dq yes .
344: .It Cm PermitRootLogin
1.1 deraadt 345: Specifies whether the root can log in using
1.2 deraadt 346: .Xr ssh 1 .
1.15 markus 347: The argument must be
348: .Dq yes ,
349: .Dq without-password
350: or
351: .Dq no .
1.2 deraadt 352: The default is
353: .Dq yes .
1.15 markus 354: If this options is set to
355: .Dq without-password
356: only password authentication is disabled for root.
1.2 deraadt 357: .Pp
358: Root login with RSA authentication when the
359: .Ar command
360: option has been
1.1 deraadt 361: specified will be allowed regardless of the value of this setting
362: (which may be useful for taking remote backups even if root login is
363: normally not allowed).
1.2 deraadt 364: .It Cm Port
1.1 deraadt 365: Specifies the port number that
1.2 deraadt 366: .Nm
1.1 deraadt 367: listens on. The default is 22.
1.2 deraadt 368: .It Cm PrintMotd
1.1 deraadt 369: Specifies whether
1.2 deraadt 370: .Nm
1.1 deraadt 371: should print
1.2 deraadt 372: .Pa /etc/motd
1.1 deraadt 373: when a user logs in interactively. (On some systems it is also
1.2 deraadt 374: printed by the shell,
375: .Pa /etc/profile ,
376: or equivalent.) The default is
377: .Dq yes .
378: .It Cm RandomSeed
1.4 deraadt 379: Obsolete. Random number generation uses other techniques.
1.2 deraadt 380: .It Cm RhostsAuthentication
1.1 deraadt 381: Specifies whether authentication using rhosts or /etc/hosts.equiv
382: files is sufficient. Normally, this method should not be permitted
1.7 markus 383: because it is insecure.
384: .Cm RhostsRSAAuthentication
385: should be used
1.1 deraadt 386: instead, because it performs RSA-based host authentication in addition
387: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 388: The default is
389: .Dq no .
390: .It Cm RhostsRSAAuthentication
1.1 deraadt 391: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.2 deraadt 392: with successful RSA host authentication is allowed. The default is
393: .Dq yes .
394: .It Cm RSAAuthentication
395: Specifies whether pure RSA authentication is allowed. The default is
396: .Dq yes .
397: .It Cm ServerKeyBits
1.1 deraadt 398: Defines the number of bits in the server key. The minimum value is
399: 512, and the default is 768.
1.7 markus 400: .It Cm SkeyAuthentication
401: Specifies whether
402: .Xr skey 1
403: authentication is allowed. The default is
404: .Dq yes .
405: Note that s/key authentication is enabled only if
406: .Cm PasswordAuthentication
407: is allowed, too.
1.2 deraadt 408: .It Cm StrictModes
1.12 markus 409: Specifies whether
410: .Nm
411: should check file modes and ownership of the
412: user's files and home directory before accepting login. This
1.1 deraadt 413: is normally desirable because novices sometimes accidentally leave their
1.7 markus 414: directory or files world-writable. The default is
415: .Dq yes .
1.2 deraadt 416: .It Cm SyslogFacility
1.1 deraadt 417: Gives the facility code that is used when logging messages from
1.2 deraadt 418: .Nm sshd .
1.1 deraadt 419: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.21 markus 420: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is AUTH.
1.10 markus 421: .It Cm UseLogin
422: Specifies whether
423: .Xr login 1
424: is used. The default is
425: .Dq no .
1.2 deraadt 426: .It Cm X11Forwarding
427: Specifies whether X11 forwarding is permitted. The default is
428: .Dq yes .
1.1 deraadt 429: Note that disabling X11 forwarding does not improve security in any
430: way, as users can always install their own forwarders.
1.6 aaron 431: .It Cm X11DisplayOffset
432: Specifies the first display number available for
433: .Nm sshd Ns 's
434: X11 forwarding. This prevents
435: .Nm
436: from interfering with real X11 servers.
1.2 deraadt 437: .El
438: .Sh LOGIN PROCESS
1.1 deraadt 439: When a user successfully logs in,
1.2 deraadt 440: .Nm
1.1 deraadt 441: does the following:
1.2 deraadt 442: .Bl -enum -offset indent
443: .It
1.1 deraadt 444: If the login is on a tty, and no command has been specified,
445: prints last login time and
1.2 deraadt 446: .Pa /etc/motd
1.1 deraadt 447: (unless prevented in the configuration file or by
1.2 deraadt 448: .Pa $HOME/.hushlogin ;
449: see the
450: .Sx FILES
451: section).
452: .It
1.1 deraadt 453: If the login is on a tty, records login time.
1.2 deraadt 454: .It
455: Checks
456: .Pa /etc/nologin ;
457: if it exists, prints contents and quits
1.1 deraadt 458: (unless root).
1.2 deraadt 459: .It
1.1 deraadt 460: Changes to run with normal user privileges.
1.2 deraadt 461: .It
1.1 deraadt 462: Sets up basic environment.
1.2 deraadt 463: .It
464: Reads
465: .Pa $HOME/.ssh/environment
466: if it exists.
467: .It
1.1 deraadt 468: Changes to user's home directory.
1.2 deraadt 469: .It
470: If
471: .Pa $HOME/.ssh/rc
472: exists, runs it; else if
473: .Pa /etc/sshrc
474: exists, runs
475: it; otherwise runs xauth. The
476: .Dq rc
477: files are given the X11
1.1 deraadt 478: authentication protocol and cookie in standard input.
1.2 deraadt 479: .It
1.1 deraadt 480: Runs user's shell or command.
1.2 deraadt 481: .El
482: .Sh AUTHORIZED_KEYS FILE FORMAT
1.1 deraadt 483: The
1.2 deraadt 484: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 485: file lists the RSA keys that are
486: permitted for RSA authentication. Each line of the file contains one
1.2 deraadt 487: key (empty lines and lines starting with a
488: .Ql #
489: are ignored as
1.1 deraadt 490: comments). Each line consists of the following fields, separated by
491: spaces: options, bits, exponent, modulus, comment. The options field
492: is optional; its presence is determined by whether the line starts
493: with a number or not (the option field never starts with a number).
494: The bits, exponent, modulus and comment fields give the RSA key; the
495: comment field is not used for anything (but may be convenient for the
496: user to identify the key).
1.2 deraadt 497: .Pp
1.1 deraadt 498: Note that lines in this file are usually several hundred bytes long
499: (because of the size of the RSA key modulus). You don't want to type
500: them in; instead, copy the
1.2 deraadt 501: .Pa identity.pub
1.1 deraadt 502: file and edit it.
1.2 deraadt 503: .Pp
1.1 deraadt 504: The options (if present) consists of comma-separated option
505: specifications. No spaces are permitted, except within double quotes.
506: The following option specifications are supported:
1.2 deraadt 507: .Bl -tag -width Ds
508: .It Cm from="pattern-list"
1.1 deraadt 509: Specifies that in addition to RSA authentication, the canonical name
510: of the remote host must be present in the comma-separated list of
511: patterns ('*' and '?' serve as wildcards). The list may also contain
512: patterns negated by prefixing them with '!'; if the canonical host
513: name matches a negated pattern, the key is not accepted. The purpose
514: of this option is to optionally increase security: RSA authentication
515: by itself does not trust the network or name servers or anything (but
516: the key); however, if somebody somehow steals the key, the key
517: permits an intruder to log in from anywhere in the world. This
518: additional option makes using a stolen key more difficult (name
519: servers and/or routers would have to be compromised in addition to
520: just the key).
1.2 deraadt 521: .It Cm command="command"
1.1 deraadt 522: Specifies that the command is executed whenever this key is used for
523: authentication. The command supplied by the user (if any) is ignored.
524: The command is run on a pty if the connection requests a pty;
525: otherwise it is run without a tty. A quote may be included in the
526: command by quoting it with a backslash. This option might be useful
527: to restrict certain RSA keys to perform just a specific operation. An
528: example might be a key that permits remote backups but nothing
529: else. Notice that the client may specify TCP/IP and/or X11
530: forwardings unless they are explicitly prohibited.
1.2 deraadt 531: .It Cm environment="NAME=value"
1.1 deraadt 532: Specifies that the string is to be added to the environment when
533: logging in using this key. Environment variables set this way
534: override other default environment values. Multiple options of this
535: type are permitted.
1.2 deraadt 536: .It Cm no-port-forwarding
1.1 deraadt 537: Forbids TCP/IP forwarding when this key is used for authentication.
538: Any port forward requests by the client will return an error. This
1.2 deraadt 539: might be used, e.g., in connection with the
540: .Cm command
1.1 deraadt 541: option.
1.2 deraadt 542: .It Cm no-X11-forwarding
1.1 deraadt 543: Forbids X11 forwarding when this key is used for authentication.
544: Any X11 forward requests by the client will return an error.
1.2 deraadt 545: .It Cm no-agent-forwarding
1.1 deraadt 546: Forbids authentication agent forwarding when this key is used for
547: authentication.
1.2 deraadt 548: .It Cm no-pty
1.1 deraadt 549: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 550: .El
551: .Ss Examples
1.1 deraadt 552: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 553: .Pp
1.1 deraadt 554: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 555: .Pp
1.1 deraadt 556: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.2 deraadt 557: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.1 deraadt 558: The
1.2 deraadt 559: .Pa /etc/ssh_known_hosts
1.1 deraadt 560: and
1.2 deraadt 561: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 562: files contain host public keys for all known hosts. The global file should
563: be prepared by the admistrator (optional), and the per-user file is
564: maintained automatically: whenever the user connects an unknown host
1.6 aaron 565: its key is added to the per-user file.
1.2 deraadt 566: .Pp
1.1 deraadt 567: Each line in these files contains the following fields: hostnames,
568: bits, exponent, modulus, comment. The fields are separated by spaces.
1.2 deraadt 569: .Pp
1.1 deraadt 570: Hostnames is a comma-separated list of patterns ('*' and '?' act as
571: wildcards); each pattern in turn is matched against the canonical host
572: name (when authenticating a client) or against the user-supplied
573: name (when authenticating a server). A pattern may also be preceded
1.2 deraadt 574: by
575: .Ql !
576: to indicate negation: if the host name matches a negated
1.1 deraadt 577: pattern, it is not accepted (by that line) even if it matched another
578: pattern on the line.
1.2 deraadt 579: .Pp
1.1 deraadt 580: Bits, exponent, and modulus are taken directly from the host key; they
1.2 deraadt 581: can be obtained, e.g., from
582: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 583: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 584: .Pp
585: Lines starting with
586: .Ql #
587: and empty lines are ignored as comments.
588: .Pp
1.1 deraadt 589: When performing host authentication, authentication is accepted if any
590: matching line has the proper key. It is thus permissible (but not
591: recommended) to have several lines or different host keys for the same
592: names. This will inevitably happen when short forms of host names
593: from different domains are put in the file. It is possible
594: that the files contain conflicting information; authentication is
595: accepted if valid information can be found from either file.
1.2 deraadt 596: .Pp
1.1 deraadt 597: Note that the lines in these files are typically hundreds of characters
598: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 599: Rather, generate them by a script
1.1 deraadt 600: or by taking
1.2 deraadt 601: .Pa /etc/ssh_host_key.pub
1.1 deraadt 602: and adding the host names at the front.
1.2 deraadt 603: .Ss Examples
1.1 deraadt 604: closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1.2 deraadt 605: .Sh FILES
606: .Bl -tag -width Ds
607: .It Pa /etc/sshd_config
1.1 deraadt 608: Contains configuration data for
1.2 deraadt 609: .Nm sshd .
1.1 deraadt 610: This file should be writable by root only, but it is recommended
611: (though not necessary) that it be world-readable.
1.2 deraadt 612: .It Pa /etc/ssh_host_key
1.7 markus 613: Contains the private part of the host key.
1.1 deraadt 614: This file should only be owned by root, readable only by root, and not
615: accessible to others.
1.14 markus 616: Note that
617: .Nm
618: does not start if this file is group/world-accessible.
1.2 deraadt 619: .It Pa /etc/ssh_host_key.pub
1.7 markus 620: Contains the public part of the host key.
621: This file should be world-readable but writable only by
1.1 deraadt 622: root. Its contents should match the private part. This file is not
623: really used for anything; it is only provided for the convenience of
624: the user so its contents can be copied to known hosts files.
1.7 markus 625: These two files are created using
626: .Xr ssh-keygen 1 .
1.2 deraadt 627: .It Pa /var/run/sshd.pid
628: Contains the process ID of the
629: .Nm
1.1 deraadt 630: listening for connections (if there are several daemons running
631: concurrently for different ports, this contains the pid of the one
632: started last). The contents of this file are not sensitive; it can be
633: world-readable.
1.2 deraadt 634: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 635: Lists the RSA keys that can be used to log into the user's account.
636: This file must be readable by root (which may on some machines imply
637: it being world-readable if the user's home directory resides on an NFS
638: volume). It is recommended that it not be accessible by others. The
639: format of this file is described above.
1.22 markus 640: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
641: These files are consulted when using rhosts with RSA host
1.1 deraadt 642: authentication to check the public key of the host. The key must be
1.22 markus 643: listed in one of these files to be accepted.
644: The client uses the same files
1.6 aaron 645: to verify that the remote host is the one we intended to
646: connect. These files should be writable only by root/the owner.
1.2 deraadt 647: .Pa /etc/ssh_known_hosts
648: should be world-readable, and
649: .Pa $HOME/.ssh/known_hosts
650: can but need not be world-readable.
1.6 aaron 651: .It Pa /etc/nologin
1.1 deraadt 652: If this file exists,
1.2 deraadt 653: .Nm
1.1 deraadt 654: refuses to let anyone except root log in. The contents of the file
655: are displayed to anyone trying to log in, and non-root connections are
656: refused. The file should be world-readable.
1.19 dugsong 657: .It Pa /etc/hosts.allow, /etc/hosts.deny
658: If compiled with
659: .Sy LIBWRAP
660: support, tcp-wrappers access controls may be defined here as described in
661: .Xr hosts_access 5 .
1.6 aaron 662: .It Pa $HOME/.rhosts
1.1 deraadt 663: This file contains host-username pairs, separated by a space, one per
664: line. The given user on the corresponding host is permitted to log in
665: without password. The same file is used by rlogind and rshd.
1.6 aaron 666: The file must
1.1 deraadt 667: be writable only by the user; it is recommended that it not be
668: accessible by others.
1.2 deraadt 669: .Pp
1.1 deraadt 670: If is also possible to use netgroups in the file. Either host or user
671: name may be of the form +@groupname to specify all hosts or all users
672: in the group.
1.2 deraadt 673: .It Pa $HOME/.shosts
674: For ssh,
675: this file is exactly the same as for
676: .Pa .rhosts .
677: However, this file is
678: not used by rlogin and rshd, so using this permits access using SSH only.
679: .Pa /etc/hosts.equiv
680: This file is used during
681: .Pa .rhosts
682: authentication. In the
1.1 deraadt 683: simplest form, this file contains host names, one per line. Users on
684: those hosts are permitted to log in without a password, provided they
685: have the same user name on both machines. The host name may also be
686: followed by a user name; such users are permitted to log in as
1.2 deraadt 687: .Em any
688: user on this machine (except root). Additionally, the syntax
689: .Dq +@group
690: can be used to specify netgroups. Negated entries start with
691: .Ql \&- .
692: .Pp
1.1 deraadt 693: If the client host/user is successfully matched in this file, login is
694: automatically permitted provided the client and server user names are the
695: same. Additionally, successful RSA host authentication is normally
696: required. This file must be writable only by root; it is recommended
697: that it be world-readable.
1.2 deraadt 698: .Pp
1.6 aaron 699: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 700: .Pa hosts.equiv .
1.1 deraadt 701: Beware that it really means that the named user(s) can log in as
1.2 deraadt 702: .Em anybody ,
1.1 deraadt 703: which includes bin, daemon, adm, and other accounts that own critical
704: binaries and directories. Using a user name practically grants the
705: user root access. The only valid use for user names that I can think
706: of is in negative entries.
1.2 deraadt 707: .Pp
708: Note that this warning also applies to rsh/rlogin.
709: .It Pa /etc/shosts.equiv
1.1 deraadt 710: This is processed exactly as
1.2 deraadt 711: .Pa /etc/hosts.equiv .
1.1 deraadt 712: However, this file may be useful in environments that want to run both
1.2 deraadt 713: rsh/rlogin and ssh.
1.6 aaron 714: .It Pa $HOME/.ssh/environment
1.1 deraadt 715: This file is read into the environment at login (if it exists). It
1.2 deraadt 716: can only contain empty lines, comment lines (that start with
717: .Ql # ) ,
1.6 aaron 718: and assignment lines of the form name=value. The file should be writable
719: only by the user; it need not be readable by anyone else.
1.2 deraadt 720: .It Pa $HOME/.ssh/rc
1.1 deraadt 721: If this file exists, it is run with /bin/sh after reading the
722: environment files but before starting the user's shell or command. If
723: X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 724: standard input (and
725: .Ev DISPLAY
726: in environment). This must call
727: .Xr xauth 1
728: in that case.
729: .Pp
1.1 deraadt 730: The primary purpose of this file is to run any initialization routines
731: which may be needed before the user's home directory becomes
732: accessible; AFS is a particular example of such an environment.
1.2 deraadt 733: .Pp
1.1 deraadt 734: This file will probably contain some initialization code followed by
735: something similar to: "if read proto cookie; then echo add $DISPLAY
736: $proto $cookie | xauth -q -; fi".
1.2 deraadt 737: .Pp
738: If this file does not exist,
739: .Pa /etc/sshrc
740: is run, and if that
1.1 deraadt 741: does not exist either, xauth is used to store the cookie.
1.2 deraadt 742: .Pp
1.1 deraadt 743: This file should be writable only by the user, and need not be
744: readable by anyone else.
1.2 deraadt 745: .It Pa /etc/sshrc
746: Like
747: .Pa $HOME/.ssh/rc .
748: This can be used to specify
1.1 deraadt 749: machine-specific login-time initializations globally. This file
750: should be writable only by root, and should be world-readable.
1.2 deraadt 751: .Sh AUTHOR
1.1 deraadt 752: Tatu Ylonen <ylo@cs.hut.fi>
1.2 deraadt 753: .Pp
1.1 deraadt 754: Information about new releases, mailing lists, and other related
1.2 deraadt 755: issues can be found from the SSH WWW home page:
756: .Pp
757: .Dl http://www.cs.hut.fi/ssh.
1.5 deraadt 758: .Pp
1.17 deraadt 759: OpenSSH
760: is a derivative of the original (free) ssh 1.2.12 release, but with bugs
761: removed and newer features re-added. Rapidly after the 1.2.12 release,
762: newer versions bore successively more restrictive licenses. This version
763: of OpenSSH
764: .Bl -bullet
765: .It
766: has all components of a restrictive nature (ie. patents, see
767: .Xr ssl 8 )
768: directly removed from the source code; any licensed or patented components
769: are chosen from
770: external libraries.
771: .It
772: has been updated to support ssh protocol 1.5.
773: .It
774: contains added support for
775: .Xr kerberos 8
776: authentication and ticket passing.
777: .It
778: supports one-time password authentication with
779: .Xr skey 1 .
780: .El
781: .Pp
782: The libraries described in
1.5 deraadt 783: .Xr ssl 8
784: are required for proper operation.
1.2 deraadt 785: .Sh SEE ALSO
786: .Xr rlogin 1 ,
787: .Xr rsh 1 ,
788: .Xr scp 1 ,
789: .Xr ssh 1 ,
1.5 deraadt 790: .Xr ssh-add 1 ,
1.2 deraadt 791: .Xr ssh-agent 1 ,
1.5 deraadt 792: .Xr ssh-keygen 1 ,
793: .Xr ssl 8
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