Annotation of src/usr.bin/ssh/sshd.8, Revision 1.2
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: .\"
12: .\" $Id: sshd.8.in,v 1.7 1999/06/06 08:38:58 bg Exp $
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
62: using a conventional cipher. Currently,
1.2 ! deraadt 63: Blowfish, IDEA, DES, and 3DES.
! 64: 3DES
1.1 deraadt 65: is used by default. The client selects the encryption algorithm to use
66: from those offered by the server.
1.2 ! deraadt 67: .Pp
1.1 deraadt 68: Next, the server and the client enter an authentication dialog. The
1.2 ! deraadt 69: client tries to authenticate itself using
! 70: .Pa .rhosts
! 71: authentication,
! 72: .Pa .rhosts
! 73: authentication combined with RSA host
1.1 deraadt 74: authentication, RSA challenge-response authentication, or password
75: based authentication.
1.2 ! deraadt 76: .Pp
1.1 deraadt 77: Rhosts authentication is normally disabled
78: because it is fundamentally insecure, but can be enabled in the server
79: configuration file if desired. System security is not improved unless
1.2 ! deraadt 80: .Xr rshd 8 ,
! 81: .Xr rlogind 8 ,
! 82: .Xr rexecd 8 ,
! 83: and
! 84: .Xr rexd 8
1.1 deraadt 85: are disabled (thus completely disabling
1.2 ! deraadt 86: .Xr rlogin 1
1.1 deraadt 87: and
1.2 ! deraadt 88: .Xr rsh 1
1.1 deraadt 89: into that machine).
1.2 ! deraadt 90: .Pp
1.1 deraadt 91: If the client successfully authenticates itself, a dialog for
92: preparing the session is entered. At this time the client may request
93: things like allocating a pseudo-tty, forwarding X11 connections,
94: forwarding TCP/IP connections, or forwarding the authentication agent
95: connection over the secure channel.
1.2 ! deraadt 96: .Pp
1.1 deraadt 97: Finally, the client either requests a shell or execution of a command.
98: The sides then enter session mode. In this mode, either side may send
99: data at any time, and such data is forwarded to/from the shell or
100: command on the server side, and the user terminal in the client side.
1.2 ! deraadt 101: .Pp
1.1 deraadt 102: When the user program terminates and all forwarded X11 and other
103: connections have been closed, the server sends command exit status to
104: the client, and both sides exit.
1.2 ! deraadt 105: .Pp
! 106: .Nm
1.1 deraadt 107: can be configured using command-line options or a configuration
108: file. Command-line options override values specified in the
109: configuration file.
1.2 ! deraadt 110: .Sh OPTIONS
! 111: .Bl -tag -width Ds
! 112: .It Fl b Ar bits
1.1 deraadt 113: Specifies the number of bits in the server key (default 768).
1.2 ! deraadt 114: .Pp
! 115: .It Fl d
1.1 deraadt 116: Debug mode. The server sends verbose debug output to the system
117: log, and does not put itself in the background. The server also will
118: not fork and will only process one connection. This option is only
119: intended for debugging for the server.
1.2 ! deraadt 120: .It Fl f Ar configuration_file
1.1 deraadt 121: Specifies the name of the configuration file. The default is
1.2 ! deraadt 122: .Pa /etc/sshd_config .
! 123: .It Fl g Ar login_grace_time
1.1 deraadt 124: Gives the grace time for clients to authenticate themselves (default
125: 300 seconds). If the client fails to authenticate the user within
126: this many seconds, the server disconnects and exits. A value of zero
127: indicates no limit.
1.2 ! deraadt 128: .It Fl h Ar host_key_file
1.1 deraadt 129: Specifies the file from which the host key is read (default
1.2 ! deraadt 130: .Pa /etc/ssh_host_key ) .
1.1 deraadt 131: This option must be given if sshd is not run as root (as the normal
132: host file is normally not readable by anyone but root).
1.2 ! deraadt 133: .It Fl i
1.1 deraadt 134: Specifies that sshd is being run from inetd. Sshd is normally not run
135: from inetd because it needs to generate the server key before it can
136: respond to the client, and this may take tens of seconds. Clients
137: would have to wait too long if the key was regenerated every time.
138: However, with small key sizes (e.g. 512) using sshd from inetd may
139: be feasible.
1.2 ! deraadt 140: .It Fl k Ar key_gen_time
1.1 deraadt 141: Specifies how often the server key is regenerated (default 3600
142: seconds, or one hour). The motivation for regenerating the key fairly
143: often is that the key is not stored anywhere, and after about an hour,
144: it becomes impossible to recover the key for decrypting intercepted
145: communications even if the machine is cracked into or physically
146: seized. A value of zero indicates that the key will never be regenerated.
1.2 ! deraadt 147: .It Fl p Ar port
1.1 deraadt 148: Specifies the port on which the server listens for connections
149: (default 22).
1.2 ! deraadt 150: .It Fl q
1.1 deraadt 151: Quiet mode. Nothing is sent to the system log. Normally the beginning,
152: authentication, and termination of each connection is logged.
1.2 ! deraadt 153: .El
! 154: .Sh CONFIGURATION FILE
! 155: .Nm
1.1 deraadt 156: reads configuration data from
1.2 ! deraadt 157: .Pa /etc/sshd_config
! 158: (or the file specified with
! 159: .Fl f
! 160: on the command line). The file
! 161: contains keyword-value pairs, one per line. Lines starting with
! 162: .Ql #
1.1 deraadt 163: and empty lines are interpreted as comments.
1.2 ! deraadt 164: .Pp
1.1 deraadt 165: The following keywords are possible.
1.2 ! deraadt 166: .Bl -tag -width Ds
! 167: .It Cm AFSTokenPassing
1.1 deraadt 168: Specifies whether to accept AFS tokens passed from the client. Default
1.2 ! deraadt 169: is
! 170: .Dq yes .
! 171: .It Cm AllowHosts
1.1 deraadt 172: This keyword can be followed by any number of host name patterns,
173: separated by spaces. If specified, login is allowed only from hosts
1.2 ! deraadt 174: whose name matches one of the patterns.
! 175: .Ql \&*
! 176: and
! 177: .Ql ?
! 178: can be used as
1.1 deraadt 179: wildcards in the patterns. Normal name servers are used to map the
180: client's host into a canonical host name. If the name cannot be
181: mapped, its IP-address is used as the host name. By default all hosts
182: are allowed to connect.
1.2 ! deraadt 183: .Pp
1.1 deraadt 184: Note that
1.2 ! deraadt 185: .Nm
! 186: can also be configured to use tcp_wrappers using the
! 187: .Fl -with-libwrap
1.1 deraadt 188: compile-time configuration option.
1.2 ! deraadt 189: .It Cm DenyHosts
1.1 deraadt 190: This keyword can be followed by any number of host name patterns,
191: separated by spaces. If specified, login is disallowed from the hosts
192: whose name matches any of the patterns.
1.2 ! deraadt 193: .It Cm FascistLogging
1.1 deraadt 194: Specifies whether to use verbose logging. Verbose logging violates
195: the privacy of users and is not recommended. The argument must be
1.2 ! deraadt 196: .Dq yes
! 197: or
! 198: .Dq no .
! 199: The default is
! 200: .Dq no .
! 201: .It Cm HostKey
1.1 deraadt 202: Specifies the file containing the private host key (default
1.2 ! deraadt 203: .Pa /etc/ssh_host_key ) .
! 204: .It Cm IgnoreRhosts
1.1 deraadt 205: Specifies that rhosts and shosts files will not be used in
206: authentication.
1.2 ! deraadt 207: .Pa /etc/hosts.equiv
1.1 deraadt 208: and
1.2 ! deraadt 209: .Pa /etc/shosts.equiv
! 210: are still used. The default is
! 211: .Dq no .
! 212: .It Cm KeepAlive
1.1 deraadt 213: Specifies whether the system should send keepalive messages to the
214: other side. If they are sent, death of the connection or crash of one
215: of the machines will be properly noticed. However, this means that
216: connections will die if the route is down temporarily, and some people
217: find it annoying. On the other hand, if keepalives are not send,
1.2 ! deraadt 218: sessions may hang indefinitely on the server, leaving
! 219: .Dq ghost
! 220: users and consuming server resources.
! 221: .Pp
! 222: The default is
! 223: .Dq yes
! 224: (to send keepalives), and the server will notice
1.1 deraadt 225: if the network goes down or the client host reboots. This avoids
226: infinitely hanging sessions.
1.2 ! deraadt 227: .Pp
! 228: To disable keepalives, the value should be set to
! 229: .Dq no
! 230: in both the server and the client configuration files.
! 231: .It Cm KerberosAuthentication
1.1 deraadt 232: Specifies whether Kerberos authentication is allowed. This can
233: be in the form of a Kerberos ticket, or if PasswordAuthentication
234: is yes, the password provided by the user will be validated through
235: the Kerberos KDC / AFS kaserver / DCE Security Server. Default is yes.
1.2 ! deraadt 236: .It Cm KerberosOrLocalPasswd
1.1 deraadt 237: If set then if password authentication through Kerberos fails then
238: the password will be validated via any additional local mechanism
1.2 ! deraadt 239: such as
! 240: .Pa /etc/passwd
! 241: or SecurID. Default is
! 242: .Dq no .
! 243: .It Cm KerberosTgtPassing
1.1 deraadt 244: Specifies whether a Kerberos TGT may be forwarded to the server.
245: Default is no, TGT forwarding does only work with the AFS kaserver.
1.2 ! deraadt 246: .It Cm KerberosTicketCleanup
1.1 deraadt 247: Specifies whether to automatically destroy the user's
248: ticket cache file on logout. Default is yes.
1.2 ! deraadt 249: .It Cm KeyRegenerationInterval
1.1 deraadt 250: The server key is automatically regenerated after this many seconds
251: (if it has been used). The purpose of regeneration is to prevent
252: decrypting captured sessions by later breaking into the machine and
253: stealing the keys. The key is never stored anywhere. If the value is
254: 0, the key is never regenerated. The default is 3600
255: (seconds).
1.2 ! deraadt 256: .It Cm LoginGraceTime
1.1 deraadt 257: The server disconnects after this time if the user has not
258: successfully logged in. If the value is 0, there is no time limit.
259: The default is 600 (seconds).
1.2 ! deraadt 260: .It Cm PasswordAuthentication
1.1 deraadt 261: Specifies whether password authentication is allowed.
1.2 ! deraadt 262: The default is
! 263: .Dq yes .
! 264: .It Cm PermitEmptyPasswords
1.1 deraadt 265: When password authentication is allowed, it specifies whether the
266: server allows login to accounts with empty password strings. The default
1.2 ! deraadt 267: is
! 268: .Dq yes .
! 269: .It Cm PermitRootLogin
1.1 deraadt 270: Specifies whether the root can log in using
1.2 ! deraadt 271: .Xr ssh 1 .
! 272: The default is
! 273: .Dq yes .
! 274: .Pp
! 275: Root login with RSA authentication when the
! 276: .Ar command
! 277: option has been
1.1 deraadt 278: specified will be allowed regardless of the value of this setting
279: (which may be useful for taking remote backups even if root login is
280: normally not allowed).
1.2 ! deraadt 281: .It Cm Port
1.1 deraadt 282: Specifies the port number that
1.2 ! deraadt 283: .Nm
1.1 deraadt 284: listens on. The default is 22.
1.2 ! deraadt 285: .It Cm PrintMotd
1.1 deraadt 286: Specifies whether
1.2 ! deraadt 287: .Nm
1.1 deraadt 288: should print
1.2 ! deraadt 289: .Pa /etc/motd
1.1 deraadt 290: when a user logs in interactively. (On some systems it is also
1.2 ! deraadt 291: printed by the shell,
! 292: .Pa /etc/profile ,
! 293: or equivalent.) The default is
! 294: .Dq yes .
! 295: .It Cm QuietMode
1.1 deraadt 296: Specifies whether the system runs in quiet mode. In quiet mode,
297: nothing is logged in the system log, except fatal errors. The default
1.2 ! deraadt 298: is
! 299: .Dq no .
! 300: .It Cm RandomSeed
1.1 deraadt 301: Specifies the file containing the random seed for the server; this
302: file is created automatically and updated regularly. The default is
1.2 ! deraadt 303: .Pa /etc/ssh_random_seed .
! 304: .It Cm RhostsAuthentication
1.1 deraadt 305: Specifies whether authentication using rhosts or /etc/hosts.equiv
306: files is sufficient. Normally, this method should not be permitted
307: because it is insecure. RhostsRSAAuthentication should be used
308: instead, because it performs RSA-based host authentication in addition
309: to normal rhosts or /etc/hosts.equiv authentication.
1.2 ! deraadt 310: The default is
! 311: .Dq no .
! 312: .It Cm RhostsRSAAuthentication
1.1 deraadt 313: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.2 ! deraadt 314: with successful RSA host authentication is allowed. The default is
! 315: .Dq yes .
! 316: .It Cm RSAAuthentication
! 317: Specifies whether pure RSA authentication is allowed. The default is
! 318: .Dq yes .
! 319: .It Cm ServerKeyBits
1.1 deraadt 320: Defines the number of bits in the server key. The minimum value is
321: 512, and the default is 768.
1.2 ! deraadt 322: .It Cm StrictModes
! 323: Specifies whether SSH should check file modes and ownership of the
1.1 deraadt 324: user's home directory and rhosts files before accepting login. This
325: is normally desirable because novices sometimes accidentally leave their
326: directory or files world-writable. The default is "yes".
1.2 ! deraadt 327: .It Cm SyslogFacility
1.1 deraadt 328: Gives the facility code that is used when logging messages from
1.2 ! deraadt 329: .Nm sshd .
1.1 deraadt 330: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
331: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is DAEMON.
1.2 ! deraadt 332: .It Cm X11Forwarding
! 333: Specifies whether X11 forwarding is permitted. The default is
! 334: .Dq yes .
1.1 deraadt 335: Note that disabling X11 forwarding does not improve security in any
336: way, as users can always install their own forwarders.
1.2 ! deraadt 337: .El
! 338: .Sh LOGIN PROCESS
1.1 deraadt 339: When a user successfully logs in,
1.2 ! deraadt 340: .Nm
1.1 deraadt 341: does the following:
1.2 ! deraadt 342: .Bl -enum -offset indent
! 343: .It
1.1 deraadt 344: If the login is on a tty, and no command has been specified,
345: prints last login time and
1.2 ! deraadt 346: .Pa /etc/motd
1.1 deraadt 347: (unless prevented in the configuration file or by
1.2 ! deraadt 348: .Pa $HOME/.hushlogin ;
! 349: see the
! 350: .Sx FILES
! 351: section).
! 352: .It
1.1 deraadt 353: If the login is on a tty, records login time.
1.2 ! deraadt 354: .It
! 355: Checks
! 356: .Pa /etc/nologin ;
! 357: if it exists, prints contents and quits
1.1 deraadt 358: (unless root).
1.2 ! deraadt 359: .It
1.1 deraadt 360: Changes to run with normal user privileges.
1.2 ! deraadt 361: .It
1.1 deraadt 362: Sets up basic environment.
1.2 ! deraadt 363: .It
! 364: Reads
! 365: .Pa /etc/environment
! 366: if it exists.
! 367: .It
! 368: Reads
! 369: .Pa $HOME/.ssh/environment
! 370: if it exists.
! 371: .It
1.1 deraadt 372: Changes to user's home directory.
1.2 ! deraadt 373: .It
! 374: If
! 375: .Pa $HOME/.ssh/rc
! 376: exists, runs it; else if
! 377: .Pa /etc/sshrc
! 378: exists, runs
! 379: it; otherwise runs xauth. The
! 380: .Dq rc
! 381: files are given the X11
1.1 deraadt 382: authentication protocol and cookie in standard input.
1.2 ! deraadt 383: .It
1.1 deraadt 384: Runs user's shell or command.
1.2 ! deraadt 385: .El
! 386: .Sh AUTHORIZED_KEYS FILE FORMAT
1.1 deraadt 387: The
1.2 ! deraadt 388: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 389: file lists the RSA keys that are
390: permitted for RSA authentication. Each line of the file contains one
1.2 ! deraadt 391: key (empty lines and lines starting with a
! 392: .Ql #
! 393: are ignored as
1.1 deraadt 394: comments). Each line consists of the following fields, separated by
395: spaces: options, bits, exponent, modulus, comment. The options field
396: is optional; its presence is determined by whether the line starts
397: with a number or not (the option field never starts with a number).
398: The bits, exponent, modulus and comment fields give the RSA key; the
399: comment field is not used for anything (but may be convenient for the
400: user to identify the key).
1.2 ! deraadt 401: .Pp
1.1 deraadt 402: Note that lines in this file are usually several hundred bytes long
403: (because of the size of the RSA key modulus). You don't want to type
404: them in; instead, copy the
1.2 ! deraadt 405: .Pa identity.pub
1.1 deraadt 406: file and edit it.
1.2 ! deraadt 407: .Pp
1.1 deraadt 408: The options (if present) consists of comma-separated option
409: specifications. No spaces are permitted, except within double quotes.
410: The following option specifications are supported:
1.2 ! deraadt 411: .Bl -tag -width Ds
! 412: .It Cm from="pattern-list"
1.1 deraadt 413: Specifies that in addition to RSA authentication, the canonical name
414: of the remote host must be present in the comma-separated list of
415: patterns ('*' and '?' serve as wildcards). The list may also contain
416: patterns negated by prefixing them with '!'; if the canonical host
417: name matches a negated pattern, the key is not accepted. The purpose
418: of this option is to optionally increase security: RSA authentication
419: by itself does not trust the network or name servers or anything (but
420: the key); however, if somebody somehow steals the key, the key
421: permits an intruder to log in from anywhere in the world. This
422: additional option makes using a stolen key more difficult (name
423: servers and/or routers would have to be compromised in addition to
424: just the key).
1.2 ! deraadt 425: .It Cm command="command"
1.1 deraadt 426: Specifies that the command is executed whenever this key is used for
427: authentication. The command supplied by the user (if any) is ignored.
428: The command is run on a pty if the connection requests a pty;
429: otherwise it is run without a tty. A quote may be included in the
430: command by quoting it with a backslash. This option might be useful
431: to restrict certain RSA keys to perform just a specific operation. An
432: example might be a key that permits remote backups but nothing
433: else. Notice that the client may specify TCP/IP and/or X11
434: forwardings unless they are explicitly prohibited.
1.2 ! deraadt 435: .It Cm environment="NAME=value"
1.1 deraadt 436: Specifies that the string is to be added to the environment when
437: logging in using this key. Environment variables set this way
438: override other default environment values. Multiple options of this
439: type are permitted.
1.2 ! deraadt 440: .It Cm no-port-forwarding
1.1 deraadt 441: Forbids TCP/IP forwarding when this key is used for authentication.
442: Any port forward requests by the client will return an error. This
1.2 ! deraadt 443: might be used, e.g., in connection with the
! 444: .Cm command
1.1 deraadt 445: option.
1.2 ! deraadt 446: .It Cm no-X11-forwarding
1.1 deraadt 447: Forbids X11 forwarding when this key is used for authentication.
448: Any X11 forward requests by the client will return an error.
1.2 ! deraadt 449: .It Cm no-agent-forwarding
1.1 deraadt 450: Forbids authentication agent forwarding when this key is used for
451: authentication.
1.2 ! deraadt 452: .It Cm no-pty
1.1 deraadt 453: Prevents tty allocation (a request to allocate a pty will fail).
1.2 ! deraadt 454: .El
! 455: .Ss Examples
1.1 deraadt 456: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 ! deraadt 457: .Pp
1.1 deraadt 458: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 ! deraadt 459: .Pp
1.1 deraadt 460: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.2 ! deraadt 461: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.1 deraadt 462: The
1.2 ! deraadt 463: .Pa /etc/ssh_known_hosts
1.1 deraadt 464: and
1.2 ! deraadt 465: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 466: files contain host public keys for all known hosts. The global file should
467: be prepared by the admistrator (optional), and the per-user file is
468: maintained automatically: whenever the user connects an unknown host
469: its key is added to the per-user file. The recommended way to create
1.2 ! deraadt 470: .Pa /etc/ssh_known_hosts
1.1 deraadt 471: is to use the
1.2 ! deraadt 472: .Xr make-ssh-known-hosts 1
1.1 deraadt 473: command.
1.2 ! deraadt 474: .Pp
1.1 deraadt 475: Each line in these files contains the following fields: hostnames,
476: bits, exponent, modulus, comment. The fields are separated by spaces.
1.2 ! deraadt 477: .Pp
1.1 deraadt 478: Hostnames is a comma-separated list of patterns ('*' and '?' act as
479: wildcards); each pattern in turn is matched against the canonical host
480: name (when authenticating a client) or against the user-supplied
481: name (when authenticating a server). A pattern may also be preceded
1.2 ! deraadt 482: by
! 483: .Ql !
! 484: to indicate negation: if the host name matches a negated
1.1 deraadt 485: pattern, it is not accepted (by that line) even if it matched another
486: pattern on the line.
1.2 ! deraadt 487: .Pp
1.1 deraadt 488: Bits, exponent, and modulus are taken directly from the host key; they
1.2 ! deraadt 489: can be obtained, e.g., from
! 490: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 491: The optional comment field continues to the end of the line, and is not used.
1.2 ! deraadt 492: .Pp
! 493: Lines starting with
! 494: .Ql #
! 495: and empty lines are ignored as comments.
! 496: .Pp
1.1 deraadt 497: When performing host authentication, authentication is accepted if any
498: matching line has the proper key. It is thus permissible (but not
499: recommended) to have several lines or different host keys for the same
500: names. This will inevitably happen when short forms of host names
501: from different domains are put in the file. It is possible
502: that the files contain conflicting information; authentication is
503: accepted if valid information can be found from either file.
1.2 ! deraadt 504: .Pp
1.1 deraadt 505: Note that the lines in these files are typically hundreds of characters
506: long, and you definitely don't want to type in the host keys by hand.
507: Rather, generate them by a script (see
1.2 ! deraadt 508: .Xr make-ssh-known-hosts 1 )
1.1 deraadt 509: or by taking
1.2 ! deraadt 510: .Pa /etc/ssh_host_key.pub
1.1 deraadt 511: and adding the host names at the front.
1.2 ! deraadt 512: .Ss Examples
1.1 deraadt 513: closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1.2 ! deraadt 514: .Sh FILES
! 515: .Bl -tag -width Ds
! 516: .It Pa /etc/sshd_config
1.1 deraadt 517: Contains configuration data for
1.2 ! deraadt 518: .Nm sshd .
1.1 deraadt 519: This file should be writable by root only, but it is recommended
520: (though not necessary) that it be world-readable.
1.2 ! deraadt 521: .It Pa /etc/ssh_host_key
1.1 deraadt 522: Contains the private part of the host key. This file is normally
523: created automatically by "make install", but can also be created
524: manually using
1.2 ! deraadt 525: .Xr ssh-keygen 1 .
1.1 deraadt 526: This file should only be owned by root, readable only by root, and not
527: accessible to others.
1.2 ! deraadt 528: .It Pa /etc/ssh_host_key.pub
1.1 deraadt 529: Contains the public part of the host key. This file is normally
530: created automatically by "make install", but can also be created
531: manually. This file should be world-readable but writable only by
532: root. Its contents should match the private part. This file is not
533: really used for anything; it is only provided for the convenience of
534: the user so its contents can be copied to known hosts files.
1.2 ! deraadt 535: .It Pa /etc/ssh_random_seed
1.1 deraadt 536: This file contains a seed for the random number generator. This file
537: should only be accessible by root.
1.2 ! deraadt 538: .It Pa /var/run/sshd.pid
! 539: Contains the process ID of the
! 540: .Nm
1.1 deraadt 541: listening for connections (if there are several daemons running
542: concurrently for different ports, this contains the pid of the one
543: started last). The contents of this file are not sensitive; it can be
544: world-readable.
1.2 ! deraadt 545: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 546: Lists the RSA keys that can be used to log into the user's account.
547: This file must be readable by root (which may on some machines imply
548: it being world-readable if the user's home directory resides on an NFS
549: volume). It is recommended that it not be accessible by others. The
550: format of this file is described above.
1.2 ! deraadt 551: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1.1 deraadt 552: These files are consulted when using rhosts with RSA host
553: authentication to check the public key of the host. The key must be
554: listed in one of these files to be accepted. (The client uses the
555: same files to verify that the remote host is the one we intended to
556: connect.) These files should be writable only by root/the owner.
1.2 ! deraadt 557: .Pa /etc/ssh_known_hosts
! 558: should be world-readable, and
! 559: .Pa $HOME/.ssh/known_hosts
! 560: can but need not be world-readable.
! 561: .Pa /etc/nologin
1.1 deraadt 562: If this file exists,
1.2 ! deraadt 563: .Nm
1.1 deraadt 564: refuses to let anyone except root log in. The contents of the file
565: are displayed to anyone trying to log in, and non-root connections are
566: refused. The file should be world-readable.
1.2 ! deraadt 567: .Pa $HOME/.rhosts
1.1 deraadt 568: This file contains host-username pairs, separated by a space, one per
569: line. The given user on the corresponding host is permitted to log in
570: without password. The same file is used by rlogind and rshd.
1.2 ! deraadt 571: Ssh differs from rlogind
1.1 deraadt 572: and rshd in that it requires RSA host authentication in addition to
573: validating the host name retrieved from domain name servers (unless
1.2 ! deraadt 574: compiled with the
! 575: .Fl -with-rhosts
! 576: configuration option). The file must
1.1 deraadt 577: be writable only by the user; it is recommended that it not be
578: accessible by others.
1.2 ! deraadt 579: .Pp
1.1 deraadt 580: If is also possible to use netgroups in the file. Either host or user
581: name may be of the form +@groupname to specify all hosts or all users
582: in the group.
1.2 ! deraadt 583: .It Pa $HOME/.shosts
! 584: For ssh,
! 585: this file is exactly the same as for
! 586: .Pa .rhosts .
! 587: However, this file is
! 588: not used by rlogin and rshd, so using this permits access using SSH only.
! 589: .Pa /etc/hosts.equiv
! 590: This file is used during
! 591: .Pa .rhosts
! 592: authentication. In the
1.1 deraadt 593: simplest form, this file contains host names, one per line. Users on
594: those hosts are permitted to log in without a password, provided they
595: have the same user name on both machines. The host name may also be
596: followed by a user name; such users are permitted to log in as
1.2 ! deraadt 597: .Em any
! 598: user on this machine (except root). Additionally, the syntax
! 599: .Dq +@group
! 600: can be used to specify netgroups. Negated entries start with
! 601: .Ql \&- .
! 602: .Pp
1.1 deraadt 603: If the client host/user is successfully matched in this file, login is
604: automatically permitted provided the client and server user names are the
605: same. Additionally, successful RSA host authentication is normally
606: required. This file must be writable only by root; it is recommended
607: that it be world-readable.
1.2 ! deraadt 608: .Pp
! 609: .Sy Warning: It is almost never a good idea to use user names in
! 610: .Pa hosts.equiv .
1.1 deraadt 611: Beware that it really means that the named user(s) can log in as
1.2 ! deraadt 612: .Em anybody ,
1.1 deraadt 613: which includes bin, daemon, adm, and other accounts that own critical
614: binaries and directories. Using a user name practically grants the
615: user root access. The only valid use for user names that I can think
616: of is in negative entries.
1.2 ! deraadt 617: .Pp
! 618: Note that this warning also applies to rsh/rlogin.
! 619: .It Pa /etc/shosts.equiv
1.1 deraadt 620: This is processed exactly as
1.2 ! deraadt 621: .Pa /etc/hosts.equiv .
1.1 deraadt 622: However, this file may be useful in environments that want to run both
1.2 ! deraadt 623: rsh/rlogin and ssh.
! 624: .It Pa /etc/environment
1.1 deraadt 625: This file is read into the environment at login (if it exists). It
1.2 ! deraadt 626: can only contain empty lines, comment lines (that start with
! 627: .Ql # ) ,
! 628: and assignment lines of the form name=value. This file is processed in
1.1 deraadt 629: all environments (normal rsh/rlogin only process it on AIX and
630: potentially some other systems). The file should be writable only by
631: root, and should be world-readable.
1.2 ! deraadt 632: .It Pa $HOME/.ssh/environment
1.1 deraadt 633: This file is read into the environment after /etc/environment. It has
634: the same format. The file should be writable only by the user; it
635: need not be readable by anyone else.
1.2 ! deraadt 636: .It Pa $HOME/.ssh/rc
1.1 deraadt 637: If this file exists, it is run with /bin/sh after reading the
638: environment files but before starting the user's shell or command. If
639: X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 ! deraadt 640: standard input (and
! 641: .Ev DISPLAY
! 642: in environment). This must call
! 643: .Xr xauth 1
! 644: in that case.
! 645: .Pp
1.1 deraadt 646: The primary purpose of this file is to run any initialization routines
647: which may be needed before the user's home directory becomes
648: accessible; AFS is a particular example of such an environment.
1.2 ! deraadt 649: .Pp
1.1 deraadt 650: This file will probably contain some initialization code followed by
651: something similar to: "if read proto cookie; then echo add $DISPLAY
652: $proto $cookie | xauth -q -; fi".
1.2 ! deraadt 653: .Pp
! 654: If this file does not exist,
! 655: .Pa /etc/sshrc
! 656: is run, and if that
1.1 deraadt 657: does not exist either, xauth is used to store the cookie.
1.2 ! deraadt 658: .Pp
1.1 deraadt 659: This file should be writable only by the user, and need not be
660: readable by anyone else.
1.2 ! deraadt 661: .It Pa /etc/sshrc
! 662: Like
! 663: .Pa $HOME/.ssh/rc .
! 664: This can be used to specify
1.1 deraadt 665: machine-specific login-time initializations globally. This file
666: should be writable only by root, and should be world-readable.
1.2 ! deraadt 667: .Sh INSTALLATION
! 668: .Nm
1.1 deraadt 669: is normally run as root. If it is not run as root, it can
670: only log in as the user it is running as, and password authentication
671: may not work if the system uses shadow passwords. An alternative
672: host key file must also be used.
1.2 ! deraadt 673: .Pp
! 674: .Nm
1.1 deraadt 675: is normally started from
1.2 ! deraadt 676: .Pa /etc/rc
1.1 deraadt 677: or equivalent at system boot.
1.2 ! deraadt 678: .Pp
1.1 deraadt 679: Considerable work has been put to making
1.2 ! deraadt 680: .Nm
1.1 deraadt 681: secure. However, if you find a security problem, please report it
682: immediately to <ssh-bugs@cs.hut.fi>.
1.2 ! deraadt 683: .Sh AUTHOR
1.1 deraadt 684: Tatu Ylonen <ylo@cs.hut.fi>
1.2 ! deraadt 685: .Pp
1.1 deraadt 686: Information about new releases, mailing lists, and other related
1.2 ! deraadt 687: issues can be found from the SSH WWW home page:
! 688: .Pp
! 689: .Dl http://www.cs.hut.fi/ssh.
! 690: .Sh SEE ALSO
! 691: .Xr make-ssh-known-hosts 1 ,
! 692: .Xr rlogin 1 ,
! 693: .Xr rsh 1 ,
! 694: .Xr scp 1 ,
! 695: .Xr ssh 1 ,
! 696: .Xr ssh-add x 1 ,
! 697: .Xr ssh-agent 1 ,
! 698: .Xr ssh-keygen 1
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