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