Annotation of src/usr.bin/ssh/sshd.8, Revision 1.7
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: .\"
1.7 ! markus 12: .\" $Id: sshd.8,v 1.6 1999/10/03 18:46:12 aaron 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 .
1.3 dugsong 176: .Pp
1.2 deraadt 177: .It Cm AllowHosts
1.1 deraadt 178: This keyword can be followed by any number of host name patterns,
179: separated by spaces. If specified, login is allowed only from hosts
1.2 deraadt 180: whose name matches one of the patterns.
181: .Ql \&*
182: and
183: .Ql ?
184: can be used as
1.1 deraadt 185: wildcards in the patterns. Normal name servers are used to map the
186: client's host into a canonical host name. If the name cannot be
187: mapped, its IP-address is used as the host name. By default all hosts
188: are allowed to connect.
1.2 deraadt 189: .Pp
1.1 deraadt 190: Note that
1.2 deraadt 191: .Nm
192: can also be configured to use tcp_wrappers using the
1.7 ! markus 193: .Sy LIBWARP
1.6 aaron 194: compile-time option.
1.2 deraadt 195: .It Cm DenyHosts
1.1 deraadt 196: This keyword can be followed by any number of host name patterns,
197: separated by spaces. If specified, login is disallowed from the hosts
198: whose name matches any of the patterns.
1.2 deraadt 199: .It Cm FascistLogging
1.1 deraadt 200: Specifies whether to use verbose logging. Verbose logging violates
201: the privacy of users and is not recommended. The argument must be
1.2 deraadt 202: .Dq yes
203: or
204: .Dq no .
205: The default is
206: .Dq no .
207: .It Cm HostKey
1.1 deraadt 208: Specifies the file containing the private host key (default
1.2 deraadt 209: .Pa /etc/ssh_host_key ) .
210: .It Cm IgnoreRhosts
1.1 deraadt 211: Specifies that rhosts and shosts files will not be used in
212: authentication.
1.2 deraadt 213: .Pa /etc/hosts.equiv
1.1 deraadt 214: and
1.2 deraadt 215: .Pa /etc/shosts.equiv
216: are still used. The default is
217: .Dq no .
218: .It Cm KeepAlive
1.1 deraadt 219: Specifies whether the system should send keepalive messages to the
220: other side. If they are sent, death of the connection or crash of one
221: of the machines will be properly noticed. However, this means that
222: connections will die if the route is down temporarily, and some people
223: find it annoying. On the other hand, if keepalives are not send,
1.2 deraadt 224: sessions may hang indefinitely on the server, leaving
225: .Dq ghost
226: users and consuming server resources.
227: .Pp
228: The default is
229: .Dq yes
230: (to send keepalives), and the server will notice
1.1 deraadt 231: if the network goes down or the client host reboots. This avoids
232: infinitely hanging sessions.
1.2 deraadt 233: .Pp
234: To disable keepalives, the value should be set to
235: .Dq no
236: in both the server and the client configuration files.
237: .It Cm KerberosAuthentication
1.1 deraadt 238: Specifies whether Kerberos authentication is allowed. This can
1.7 ! markus 239: be in the form of a Kerberos ticket, or if
! 240: .Cm PasswordAuthentication
1.1 deraadt 241: is yes, the password provided by the user will be validated through
1.3 dugsong 242: the Kerberos KDC. Default is
243: .Dq yes .
1.2 deraadt 244: .It Cm KerberosOrLocalPasswd
1.1 deraadt 245: If set then if password authentication through Kerberos fails then
246: the password will be validated via any additional local mechanism
1.2 deraadt 247: such as
248: .Pa /etc/passwd
249: or SecurID. Default is
250: .Dq no .
251: .It Cm KerberosTgtPassing
1.1 deraadt 252: Specifies whether a Kerberos TGT may be forwarded to the server.
1.3 dugsong 253: Default is
254: .Dq no ,
255: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 256: .It Cm KerberosTicketCleanup
1.7 ! markus 257: Specifies whether to automatically destroy the user's ticket cache
! 258: file on logout. Default is
1.3 dugsong 259: .Dq yes .
1.2 deraadt 260: .It Cm KeyRegenerationInterval
1.1 deraadt 261: The server key is automatically regenerated after this many seconds
262: (if it has been used). The purpose of regeneration is to prevent
263: decrypting captured sessions by later breaking into the machine and
264: stealing the keys. The key is never stored anywhere. If the value is
265: 0, the key is never regenerated. The default is 3600
266: (seconds).
1.7 ! markus 267: .It Cm ListenAddress
! 268: Specifies what local address
! 269: .Nm
! 270: should listen on.
! 271: The default is to listen to all local addresses.
1.2 deraadt 272: .It Cm LoginGraceTime
1.1 deraadt 273: The server disconnects after this time if the user has not
274: successfully logged in. If the value is 0, there is no time limit.
275: The default is 600 (seconds).
1.2 deraadt 276: .It Cm PasswordAuthentication
1.1 deraadt 277: Specifies whether password authentication is allowed.
1.2 deraadt 278: The default is
279: .Dq yes .
280: .It Cm PermitEmptyPasswords
1.1 deraadt 281: When password authentication is allowed, it specifies whether the
282: server allows login to accounts with empty password strings. The default
1.2 deraadt 283: is
284: .Dq yes .
285: .It Cm PermitRootLogin
1.1 deraadt 286: Specifies whether the root can log in using
1.2 deraadt 287: .Xr ssh 1 .
288: The default is
289: .Dq yes .
290: .Pp
291: Root login with RSA authentication when the
292: .Ar command
293: option has been
1.1 deraadt 294: specified will be allowed regardless of the value of this setting
295: (which may be useful for taking remote backups even if root login is
296: normally not allowed).
1.2 deraadt 297: .It Cm Port
1.1 deraadt 298: Specifies the port number that
1.2 deraadt 299: .Nm
1.1 deraadt 300: listens on. The default is 22.
1.2 deraadt 301: .It Cm PrintMotd
1.1 deraadt 302: Specifies whether
1.2 deraadt 303: .Nm
1.1 deraadt 304: should print
1.2 deraadt 305: .Pa /etc/motd
1.1 deraadt 306: when a user logs in interactively. (On some systems it is also
1.2 deraadt 307: printed by the shell,
308: .Pa /etc/profile ,
309: or equivalent.) The default is
310: .Dq yes .
311: .It Cm QuietMode
1.1 deraadt 312: Specifies whether the system runs in quiet mode. In quiet mode,
313: nothing is logged in the system log, except fatal errors. The default
1.2 deraadt 314: is
315: .Dq no .
316: .It Cm RandomSeed
1.4 deraadt 317: Obsolete. Random number generation uses other techniques.
1.2 deraadt 318: .It Cm RhostsAuthentication
1.1 deraadt 319: Specifies whether authentication using rhosts or /etc/hosts.equiv
320: files is sufficient. Normally, this method should not be permitted
1.7 ! markus 321: because it is insecure.
! 322: .Cm RhostsRSAAuthentication
! 323: should be used
1.1 deraadt 324: instead, because it performs RSA-based host authentication in addition
325: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 326: The default is
327: .Dq no .
328: .It Cm RhostsRSAAuthentication
1.1 deraadt 329: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.2 deraadt 330: with successful RSA host authentication is allowed. The default is
331: .Dq yes .
332: .It Cm RSAAuthentication
333: Specifies whether pure RSA authentication is allowed. The default is
334: .Dq yes .
335: .It Cm ServerKeyBits
1.1 deraadt 336: Defines the number of bits in the server key. The minimum value is
337: 512, and the default is 768.
1.7 ! markus 338: .It Cm SkeyAuthentication
! 339: Specifies whether
! 340: .Xr skey 1
! 341: authentication is allowed. The default is
! 342: .Dq yes .
! 343: Note that s/key authentication is enabled only if
! 344: .Cm PasswordAuthentication
! 345: is allowed, too.
1.2 deraadt 346: .It Cm StrictModes
347: Specifies whether SSH should check file modes and ownership of the
1.1 deraadt 348: user's home directory and rhosts files before accepting login. This
349: is normally desirable because novices sometimes accidentally leave their
1.7 ! markus 350: directory or files world-writable. The default is
! 351: .Dq yes .
1.2 deraadt 352: .It Cm SyslogFacility
1.1 deraadt 353: Gives the facility code that is used when logging messages from
1.2 deraadt 354: .Nm sshd .
1.1 deraadt 355: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
356: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is DAEMON.
1.2 deraadt 357: .It Cm X11Forwarding
358: Specifies whether X11 forwarding is permitted. The default is
359: .Dq yes .
1.1 deraadt 360: Note that disabling X11 forwarding does not improve security in any
361: way, as users can always install their own forwarders.
1.6 aaron 362: .It Cm X11DisplayOffset
363: Specifies the first display number available for
364: .Nm sshd Ns 's
365: X11 forwarding. This prevents
366: .Nm
367: from interfering with real X11 servers.
1.2 deraadt 368: .El
369: .Sh LOGIN PROCESS
1.1 deraadt 370: When a user successfully logs in,
1.2 deraadt 371: .Nm
1.1 deraadt 372: does the following:
1.2 deraadt 373: .Bl -enum -offset indent
374: .It
1.1 deraadt 375: If the login is on a tty, and no command has been specified,
376: prints last login time and
1.2 deraadt 377: .Pa /etc/motd
1.1 deraadt 378: (unless prevented in the configuration file or by
1.2 deraadt 379: .Pa $HOME/.hushlogin ;
380: see the
381: .Sx FILES
382: section).
383: .It
1.1 deraadt 384: If the login is on a tty, records login time.
1.2 deraadt 385: .It
386: Checks
387: .Pa /etc/nologin ;
388: if it exists, prints contents and quits
1.1 deraadt 389: (unless root).
1.2 deraadt 390: .It
1.1 deraadt 391: Changes to run with normal user privileges.
1.2 deraadt 392: .It
1.1 deraadt 393: Sets up basic environment.
1.2 deraadt 394: .It
395: Reads
396: .Pa $HOME/.ssh/environment
397: if it exists.
398: .It
1.1 deraadt 399: Changes to user's home directory.
1.2 deraadt 400: .It
401: If
402: .Pa $HOME/.ssh/rc
403: exists, runs it; else if
404: .Pa /etc/sshrc
405: exists, runs
406: it; otherwise runs xauth. The
407: .Dq rc
408: files are given the X11
1.1 deraadt 409: authentication protocol and cookie in standard input.
1.2 deraadt 410: .It
1.1 deraadt 411: Runs user's shell or command.
1.2 deraadt 412: .El
413: .Sh AUTHORIZED_KEYS FILE FORMAT
1.1 deraadt 414: The
1.2 deraadt 415: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 416: file lists the RSA keys that are
417: permitted for RSA authentication. Each line of the file contains one
1.2 deraadt 418: key (empty lines and lines starting with a
419: .Ql #
420: are ignored as
1.1 deraadt 421: comments). Each line consists of the following fields, separated by
422: spaces: options, bits, exponent, modulus, comment. The options field
423: is optional; its presence is determined by whether the line starts
424: with a number or not (the option field never starts with a number).
425: The bits, exponent, modulus and comment fields give the RSA key; the
426: comment field is not used for anything (but may be convenient for the
427: user to identify the key).
1.2 deraadt 428: .Pp
1.1 deraadt 429: Note that lines in this file are usually several hundred bytes long
430: (because of the size of the RSA key modulus). You don't want to type
431: them in; instead, copy the
1.2 deraadt 432: .Pa identity.pub
1.1 deraadt 433: file and edit it.
1.2 deraadt 434: .Pp
1.1 deraadt 435: The options (if present) consists of comma-separated option
436: specifications. No spaces are permitted, except within double quotes.
437: The following option specifications are supported:
1.2 deraadt 438: .Bl -tag -width Ds
439: .It Cm from="pattern-list"
1.1 deraadt 440: Specifies that in addition to RSA authentication, the canonical name
441: of the remote host must be present in the comma-separated list of
442: patterns ('*' and '?' serve as wildcards). The list may also contain
443: patterns negated by prefixing them with '!'; if the canonical host
444: name matches a negated pattern, the key is not accepted. The purpose
445: of this option is to optionally increase security: RSA authentication
446: by itself does not trust the network or name servers or anything (but
447: the key); however, if somebody somehow steals the key, the key
448: permits an intruder to log in from anywhere in the world. This
449: additional option makes using a stolen key more difficult (name
450: servers and/or routers would have to be compromised in addition to
451: just the key).
1.2 deraadt 452: .It Cm command="command"
1.1 deraadt 453: Specifies that the command is executed whenever this key is used for
454: authentication. The command supplied by the user (if any) is ignored.
455: The command is run on a pty if the connection requests a pty;
456: otherwise it is run without a tty. A quote may be included in the
457: command by quoting it with a backslash. This option might be useful
458: to restrict certain RSA keys to perform just a specific operation. An
459: example might be a key that permits remote backups but nothing
460: else. Notice that the client may specify TCP/IP and/or X11
461: forwardings unless they are explicitly prohibited.
1.2 deraadt 462: .It Cm environment="NAME=value"
1.1 deraadt 463: Specifies that the string is to be added to the environment when
464: logging in using this key. Environment variables set this way
465: override other default environment values. Multiple options of this
466: type are permitted.
1.2 deraadt 467: .It Cm no-port-forwarding
1.1 deraadt 468: Forbids TCP/IP forwarding when this key is used for authentication.
469: Any port forward requests by the client will return an error. This
1.2 deraadt 470: might be used, e.g., in connection with the
471: .Cm command
1.1 deraadt 472: option.
1.2 deraadt 473: .It Cm no-X11-forwarding
1.1 deraadt 474: Forbids X11 forwarding when this key is used for authentication.
475: Any X11 forward requests by the client will return an error.
1.2 deraadt 476: .It Cm no-agent-forwarding
1.1 deraadt 477: Forbids authentication agent forwarding when this key is used for
478: authentication.
1.2 deraadt 479: .It Cm no-pty
1.1 deraadt 480: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 481: .El
482: .Ss Examples
1.1 deraadt 483: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 484: .Pp
1.1 deraadt 485: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 486: .Pp
1.1 deraadt 487: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.2 deraadt 488: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.1 deraadt 489: The
1.2 deraadt 490: .Pa /etc/ssh_known_hosts
1.1 deraadt 491: and
1.2 deraadt 492: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 493: files contain host public keys for all known hosts. The global file should
494: be prepared by the admistrator (optional), and the per-user file is
495: maintained automatically: whenever the user connects an unknown host
1.6 aaron 496: its key is added to the per-user file.
1.2 deraadt 497: .Pp
1.1 deraadt 498: Each line in these files contains the following fields: hostnames,
499: bits, exponent, modulus, comment. The fields are separated by spaces.
1.2 deraadt 500: .Pp
1.1 deraadt 501: Hostnames is a comma-separated list of patterns ('*' and '?' act as
502: wildcards); each pattern in turn is matched against the canonical host
503: name (when authenticating a client) or against the user-supplied
504: name (when authenticating a server). A pattern may also be preceded
1.2 deraadt 505: by
506: .Ql !
507: to indicate negation: if the host name matches a negated
1.1 deraadt 508: pattern, it is not accepted (by that line) even if it matched another
509: pattern on the line.
1.2 deraadt 510: .Pp
1.1 deraadt 511: Bits, exponent, and modulus are taken directly from the host key; they
1.2 deraadt 512: can be obtained, e.g., from
513: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 514: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 515: .Pp
516: Lines starting with
517: .Ql #
518: and empty lines are ignored as comments.
519: .Pp
1.1 deraadt 520: When performing host authentication, authentication is accepted if any
521: matching line has the proper key. It is thus permissible (but not
522: recommended) to have several lines or different host keys for the same
523: names. This will inevitably happen when short forms of host names
524: from different domains are put in the file. It is possible
525: that the files contain conflicting information; authentication is
526: accepted if valid information can be found from either file.
1.2 deraadt 527: .Pp
1.1 deraadt 528: Note that the lines in these files are typically hundreds of characters
529: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 530: Rather, generate them by a script
1.1 deraadt 531: or by taking
1.2 deraadt 532: .Pa /etc/ssh_host_key.pub
1.1 deraadt 533: and adding the host names at the front.
1.2 deraadt 534: .Ss Examples
1.1 deraadt 535: closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1.2 deraadt 536: .Sh FILES
537: .Bl -tag -width Ds
538: .It Pa /etc/sshd_config
1.1 deraadt 539: Contains configuration data for
1.2 deraadt 540: .Nm sshd .
1.1 deraadt 541: This file should be writable by root only, but it is recommended
542: (though not necessary) that it be world-readable.
1.2 deraadt 543: .It Pa /etc/ssh_host_key
1.7 ! markus 544: Contains the private part of the host key.
1.1 deraadt 545: This file should only be owned by root, readable only by root, and not
546: accessible to others.
1.2 deraadt 547: .It Pa /etc/ssh_host_key.pub
1.7 ! markus 548: Contains the public part of the host key.
! 549: This file should be world-readable but writable only by
1.1 deraadt 550: root. Its contents should match the private part. This file is not
551: really used for anything; it is only provided for the convenience of
552: the user so its contents can be copied to known hosts files.
1.7 ! markus 553: These two files are created using
! 554: .Xr ssh-keygen 1 .
1.2 deraadt 555: .It Pa /var/run/sshd.pid
556: Contains the process ID of the
557: .Nm
1.1 deraadt 558: listening for connections (if there are several daemons running
559: concurrently for different ports, this contains the pid of the one
560: started last). The contents of this file are not sensitive; it can be
561: world-readable.
1.2 deraadt 562: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 563: Lists the RSA keys that can be used to log into the user's account.
564: This file must be readable by root (which may on some machines imply
565: it being world-readable if the user's home directory resides on an NFS
566: volume). It is recommended that it not be accessible by others. The
567: format of this file is described above.
1.6 aaron 568: .It Pa /etc/ssh_known_hosts
569: This file is consulted when using rhosts with RSA host
1.1 deraadt 570: authentication to check the public key of the host. The key must be
1.6 aaron 571: listed in this file to be accepted.
572: .It Pa $HOME/.ssh/known_hosts
573: The client uses this file
574: and
575: .Pa /etc/ssh_known_hosts
576: to verify that the remote host is the one we intended to
577: connect. These files should be writable only by root/the owner.
1.2 deraadt 578: .Pa /etc/ssh_known_hosts
579: should be world-readable, and
580: .Pa $HOME/.ssh/known_hosts
581: can but need not be world-readable.
1.6 aaron 582: .It Pa /etc/nologin
1.1 deraadt 583: If this file exists,
1.2 deraadt 584: .Nm
1.1 deraadt 585: refuses to let anyone except root log in. The contents of the file
586: are displayed to anyone trying to log in, and non-root connections are
587: refused. The file should be world-readable.
1.6 aaron 588: .It Pa $HOME/.rhosts
1.1 deraadt 589: This file contains host-username pairs, separated by a space, one per
590: line. The given user on the corresponding host is permitted to log in
591: without password. The same file is used by rlogind and rshd.
1.6 aaron 592: The file must
1.1 deraadt 593: be writable only by the user; it is recommended that it not be
594: accessible by others.
1.2 deraadt 595: .Pp
1.1 deraadt 596: If is also possible to use netgroups in the file. Either host or user
597: name may be of the form +@groupname to specify all hosts or all users
598: in the group.
1.2 deraadt 599: .It Pa $HOME/.shosts
600: For ssh,
601: this file is exactly the same as for
602: .Pa .rhosts .
603: However, this file is
604: not used by rlogin and rshd, so using this permits access using SSH only.
605: .Pa /etc/hosts.equiv
606: This file is used during
607: .Pa .rhosts
608: authentication. In the
1.1 deraadt 609: simplest form, this file contains host names, one per line. Users on
610: those hosts are permitted to log in without a password, provided they
611: have the same user name on both machines. The host name may also be
612: followed by a user name; such users are permitted to log in as
1.2 deraadt 613: .Em any
614: user on this machine (except root). Additionally, the syntax
615: .Dq +@group
616: can be used to specify netgroups. Negated entries start with
617: .Ql \&- .
618: .Pp
1.1 deraadt 619: If the client host/user is successfully matched in this file, login is
620: automatically permitted provided the client and server user names are the
621: same. Additionally, successful RSA host authentication is normally
622: required. This file must be writable only by root; it is recommended
623: that it be world-readable.
1.2 deraadt 624: .Pp
1.6 aaron 625: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 626: .Pa hosts.equiv .
1.1 deraadt 627: Beware that it really means that the named user(s) can log in as
1.2 deraadt 628: .Em anybody ,
1.1 deraadt 629: which includes bin, daemon, adm, and other accounts that own critical
630: binaries and directories. Using a user name practically grants the
631: user root access. The only valid use for user names that I can think
632: of is in negative entries.
1.2 deraadt 633: .Pp
634: Note that this warning also applies to rsh/rlogin.
635: .It Pa /etc/shosts.equiv
1.1 deraadt 636: This is processed exactly as
1.2 deraadt 637: .Pa /etc/hosts.equiv .
1.1 deraadt 638: However, this file may be useful in environments that want to run both
1.2 deraadt 639: rsh/rlogin and ssh.
1.6 aaron 640: .It Pa $HOME/.ssh/environment
1.1 deraadt 641: This file is read into the environment at login (if it exists). It
1.2 deraadt 642: can only contain empty lines, comment lines (that start with
643: .Ql # ) ,
1.6 aaron 644: and assignment lines of the form name=value. The file should be writable
645: only by the user; it need not be readable by anyone else.
1.2 deraadt 646: .It Pa $HOME/.ssh/rc
1.1 deraadt 647: If this file exists, it is run with /bin/sh after reading the
648: environment files but before starting the user's shell or command. If
649: X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 650: standard input (and
651: .Ev DISPLAY
652: in environment). This must call
653: .Xr xauth 1
654: in that case.
655: .Pp
1.1 deraadt 656: The primary purpose of this file is to run any initialization routines
657: which may be needed before the user's home directory becomes
658: accessible; AFS is a particular example of such an environment.
1.2 deraadt 659: .Pp
1.1 deraadt 660: This file will probably contain some initialization code followed by
661: something similar to: "if read proto cookie; then echo add $DISPLAY
662: $proto $cookie | xauth -q -; fi".
1.2 deraadt 663: .Pp
664: If this file does not exist,
665: .Pa /etc/sshrc
666: is run, and if that
1.1 deraadt 667: does not exist either, xauth is used to store the cookie.
1.2 deraadt 668: .Pp
1.1 deraadt 669: This file should be writable only by the user, and need not be
670: readable by anyone else.
1.2 deraadt 671: .It Pa /etc/sshrc
672: Like
673: .Pa $HOME/.ssh/rc .
674: This can be used to specify
1.1 deraadt 675: machine-specific login-time initializations globally. This file
676: should be writable only by root, and should be world-readable.
1.2 deraadt 677: .Sh AUTHOR
1.1 deraadt 678: Tatu Ylonen <ylo@cs.hut.fi>
1.2 deraadt 679: .Pp
1.1 deraadt 680: Information about new releases, mailing lists, and other related
1.2 deraadt 681: issues can be found from the SSH WWW home page:
682: .Pp
683: .Dl http://www.cs.hut.fi/ssh.
1.5 deraadt 684: .Pp
685: This version of
686: .Nm
687: is a derivative of the original 1.2.12 release, but with bugs removed and
688: newer features re-added. Rapidly after the 1.2.12 release, newer versions
689: bore successively more restrictive licenses. In this version, all components
690: of a restrictive nature (ie. patents) have been directly removed from the
691: source code; any licensed or patented components are chosen from external
692: libraries. The libraries described in
693: .Xr ssl 8
694: are required for proper operation.
1.2 deraadt 695: .Sh SEE ALSO
696: .Xr rlogin 1 ,
697: .Xr rsh 1 ,
698: .Xr scp 1 ,
699: .Xr ssh 1 ,
1.5 deraadt 700: .Xr ssh-add 1 ,
1.2 deraadt 701: .Xr ssh-agent 1 ,
1.5 deraadt 702: .Xr ssh-keygen 1 ,
703: .Xr ssl 8
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