Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.260
1.1 stevesk 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
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19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
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25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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1.260 ! jmc 36: .\" $OpenBSD: sshd_config.5,v 1.259 2017/10/25 06:18:06 jmc Exp $
1.257 djm 37: .Dd $Mdocdate: October 25 2017 $
1.1 stevesk 38: .Dt SSHD_CONFIG 5
39: .Os
40: .Sh NAME
41: .Nm sshd_config
42: .Nd OpenSSH SSH daemon configuration file
43: .Sh DESCRIPTION
1.53 jmc 44: .Xr sshd 8
1.1 stevesk 45: reads configuration data from
46: .Pa /etc/ssh/sshd_config
47: (or the file specified with
48: .Fl f
49: on the command line).
50: The file contains keyword-argument pairs, one per line.
1.254 benno 51: For each keyword, the first obtained value will be used.
1.1 stevesk 52: Lines starting with
53: .Ql #
54: and empty lines are interpreted as comments.
1.56 dtucker 55: Arguments may optionally be enclosed in double quotes
56: .Pq \&"
57: in order to represent arguments containing spaces.
1.1 stevesk 58: .Pp
59: The possible
60: keywords and their meanings are as follows (note that
61: keywords are case-insensitive and arguments are case-sensitive):
62: .Bl -tag -width Ds
1.30 djm 63: .It Cm AcceptEnv
64: Specifies what environment variables sent by the client will be copied into
65: the session's
66: .Xr environ 7 .
67: See
68: .Cm SendEnv
69: in
70: .Xr ssh_config 5
71: for how to configure the client.
1.219 jmc 72: The
1.197 dtucker 73: .Ev TERM
1.199 jmc 74: environment variable is always sent whenever the client
1.204 djm 75: requests a pseudo-terminal as it is required by the protocol.
1.30 djm 76: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 77: .Ql *
1.30 djm 78: and
79: .Ql \&? .
1.31 djm 80: Multiple environment variables may be separated by whitespace or spread
1.30 djm 81: across multiple
82: .Cm AcceptEnv
83: directives.
1.31 djm 84: Be warned that some environment variables could be used to bypass restricted
1.30 djm 85: user environments.
86: For this reason, care should be taken in the use of this directive.
87: The default is not to accept any environment variables.
1.37 djm 88: .It Cm AddressFamily
89: Specifies which address family should be used by
1.53 jmc 90: .Xr sshd 8 .
1.37 djm 91: Valid arguments are
1.237 jmc 92: .Cm any
93: (the default),
94: .Cm inet
1.52 jmc 95: (use IPv4 only), or
1.237 jmc 96: .Cm inet6
1.37 djm 97: (use IPv6 only).
1.89 jmc 98: .It Cm AllowAgentForwarding
99: Specifies whether
100: .Xr ssh-agent 1
101: forwarding is permitted.
102: The default is
1.237 jmc 103: .Cm yes .
1.89 jmc 104: Note that disabling agent forwarding does not improve security
105: unless users are also denied shell access, as they can always install
106: their own forwarders.
1.1 stevesk 107: .It Cm AllowGroups
108: This keyword can be followed by a list of group name patterns, separated
109: by spaces.
110: If specified, login is allowed only for users whose primary
111: group or supplementary group list matches one of the patterns.
112: Only group names are valid; a numerical group ID is not recognized.
113: By default, login is allowed for all groups.
1.54 jmc 114: The allow/deny directives are processed in the following order:
115: .Cm DenyUsers ,
116: .Cm AllowUsers ,
117: .Cm DenyGroups ,
118: and finally
119: .Cm AllowGroups .
1.49 jmc 120: .Pp
1.161 jmc 121: See PATTERNS in
1.49 jmc 122: .Xr ssh_config 5
123: for more information on patterns.
1.231 jmc 124: .It Cm AllowStreamLocalForwarding
125: Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
1.150 djm 126: The available options are
1.237 jmc 127: .Cm yes
128: (the default)
1.150 djm 129: or
1.237 jmc 130: .Cm all
1.231 jmc 131: to allow StreamLocal forwarding,
1.237 jmc 132: .Cm no
1.231 jmc 133: to prevent all StreamLocal forwarding,
1.237 jmc 134: .Cm local
1.150 djm 135: to allow local (from the perspective of
1.151 jmc 136: .Xr ssh 1 )
137: forwarding only or
1.237 jmc 138: .Cm remote
1.150 djm 139: to allow remote forwarding only.
1.231 jmc 140: Note that disabling StreamLocal forwarding does not improve security unless
1.1 stevesk 141: users are also denied shell access, as they can always install their
142: own forwarders.
1.231 jmc 143: .It Cm AllowTcpForwarding
144: Specifies whether TCP forwarding is permitted.
1.175 millert 145: The available options are
1.237 jmc 146: .Cm yes
147: (the default)
1.175 millert 148: or
1.237 jmc 149: .Cm all
1.231 jmc 150: to allow TCP forwarding,
1.237 jmc 151: .Cm no
1.231 jmc 152: to prevent all TCP forwarding,
1.237 jmc 153: .Cm local
1.175 millert 154: to allow local (from the perspective of
155: .Xr ssh 1 )
156: forwarding only or
1.237 jmc 157: .Cm remote
1.175 millert 158: to allow remote forwarding only.
1.231 jmc 159: Note that disabling TCP forwarding does not improve security unless
1.175 millert 160: users are also denied shell access, as they can always install their
161: own forwarders.
1.1 stevesk 162: .It Cm AllowUsers
163: This keyword can be followed by a list of user name patterns, separated
164: by spaces.
1.14 jmc 165: If specified, login is allowed only for user names that
1.1 stevesk 166: match one of the patterns.
167: Only user names are valid; a numerical user ID is not recognized.
168: By default, login is allowed for all users.
169: If the pattern takes the form USER@HOST then USER and HOST
170: are separately checked, restricting logins to particular
171: users from particular hosts.
1.222 jmc 172: HOST criteria may additionally contain addresses to match in CIDR
173: address/masklen format.
1.54 jmc 174: The allow/deny directives are processed in the following order:
175: .Cm DenyUsers ,
176: .Cm AllowUsers ,
177: .Cm DenyGroups ,
178: and finally
179: .Cm AllowGroups .
1.49 jmc 180: .Pp
1.161 jmc 181: See PATTERNS in
1.49 jmc 182: .Xr ssh_config 5
183: for more information on patterns.
1.149 djm 184: .It Cm AuthenticationMethods
185: Specifies the authentication methods that must be successfully completed
186: for a user to be granted access.
187: This option must be followed by one or more comma-separated lists of
1.224 djm 188: authentication method names, or by the single string
1.237 jmc 189: .Cm any
1.224 djm 190: to indicate the default behaviour of accepting any single authentication
1.225 jmc 191: method.
1.237 jmc 192: If the default is overridden, then successful authentication requires
1.224 djm 193: completion of every method in at least one of these lists.
1.149 djm 194: .Pp
1.237 jmc 195: For example,
196: .Qq publickey,password publickey,keyboard-interactive
1.149 djm 197: would require the user to complete public key authentication, followed by
198: either password or keyboard interactive authentication.
199: Only methods that are next in one or more lists are offered at each stage,
1.237 jmc 200: so for this example it would not be possible to attempt password or
1.149 djm 201: keyboard-interactive authentication before public key.
1.157 markus 202: .Pp
203: For keyboard interactive authentication it is also possible to
204: restrict authentication to a specific device by appending a
205: colon followed by the device identifier
1.237 jmc 206: .Cm bsdauth ,
207: .Cm pam ,
1.157 markus 208: or
1.237 jmc 209: .Cm skey ,
1.157 markus 210: depending on the server configuration.
211: For example,
1.237 jmc 212: .Qq keyboard-interactive:bsdauth
1.157 markus 213: would restrict keyboard interactive authentication to the
1.237 jmc 214: .Cm bsdauth
1.157 markus 215: device.
1.185 djm 216: .Pp
1.237 jmc 217: If the publickey method is listed more than once,
1.185 djm 218: .Xr sshd 8
219: verifies that keys that have been used successfully are not reused for
220: subsequent authentications.
1.237 jmc 221: For example,
222: .Qq publickey,publickey
223: requires successful authentication using two different public keys.
1.181 djm 224: .Pp
1.149 djm 225: Note that each authentication method listed should also be explicitly enabled
226: in the configuration.
1.251 djm 227: .Pp
228: The available authentication methods are:
229: .Qq gssapi-with-mic ,
230: .Qq hostbased ,
231: .Qq keyboard-interactive ,
232: .Qq none
233: (used for access to password-less accounts when
234: .Cm PermitEmptyPassword
235: is enabled),
236: .Qq password
237: and
238: .Qq publickey .
1.146 djm 239: .It Cm AuthorizedKeysCommand
1.147 jmc 240: Specifies a program to be used to look up the user's public keys.
1.201 djm 241: The program must be owned by root, not writable by group or others and
242: specified by an absolute path.
243: Arguments to
244: .Cm AuthorizedKeysCommand
1.235 jmc 245: accept the tokens described in the
246: .Sx TOKENS
247: section.
248: If no arguments are specified then the username of the target user is used.
1.201 djm 249: .Pp
250: The program should produce on standard output zero or
1.235 jmc 251: more lines of authorized_keys output (see
252: .Sx AUTHORIZED_KEYS
253: in
1.147 jmc 254: .Xr sshd 8 ) .
1.235 jmc 255: If a key supplied by
256: .Cm AuthorizedKeysCommand
257: does not successfully authenticate
1.146 djm 258: and authorize the user then public key authentication continues using the usual
259: .Cm AuthorizedKeysFile
260: files.
1.237 jmc 261: By default, no
262: .Cm AuthorizedKeysCommand
263: is run.
1.146 djm 264: .It Cm AuthorizedKeysCommandUser
1.237 jmc 265: Specifies the user under whose account the
266: .Cm AuthorizedKeysCommand
267: is run.
1.146 djm 268: It is recommended to use a dedicated user that has no other role on the host
269: than running authorized keys commands.
1.186 djm 270: If
1.181 djm 271: .Cm AuthorizedKeysCommand
1.186 djm 272: is specified but
273: .Cm AuthorizedKeysCommandUser
274: is not, then
275: .Xr sshd 8
276: will refuse to start.
1.1 stevesk 277: .It Cm AuthorizedKeysFile
1.235 jmc 278: Specifies the file that contains the public keys used for user authentication.
1.124 djm 279: The format is described in the
1.235 jmc 280: .Sx AUTHORIZED_KEYS FILE FORMAT
1.124 djm 281: section of
282: .Xr sshd 8 .
1.235 jmc 283: Arguments to
1.1 stevesk 284: .Cm AuthorizedKeysFile
1.235 jmc 285: accept the tokens described in the
286: .Sx TOKENS
287: section.
1.1 stevesk 288: After expansion,
289: .Cm AuthorizedKeysFile
290: is taken to be an absolute path or one relative to the user's home
291: directory.
1.133 jmc 292: Multiple files may be listed, separated by whitespace.
1.212 djm 293: Alternately this option may be set to
1.237 jmc 294: .Cm none
1.212 djm 295: to skip checking for user keys in files.
1.133 jmc 296: The default is
1.237 jmc 297: .Qq .ssh/authorized_keys .ssh/authorized_keys2 .
1.202 djm 298: .It Cm AuthorizedPrincipalsCommand
299: Specifies a program to be used to generate the list of allowed
300: certificate principals as per
301: .Cm AuthorizedPrincipalsFile .
302: The program must be owned by root, not writable by group or others and
303: specified by an absolute path.
304: Arguments to
305: .Cm AuthorizedPrincipalsCommand
1.235 jmc 306: accept the tokens described in the
307: .Sx TOKENS
308: section.
309: If no arguments are specified then the username of the target user is used.
1.202 djm 310: .Pp
311: The program should produce on standard output zero or
312: more lines of
313: .Cm AuthorizedPrincipalsFile
314: output.
315: If either
316: .Cm AuthorizedPrincipalsCommand
317: or
318: .Cm AuthorizedPrincipalsFile
319: is specified, then certificates offered by the client for authentication
320: must contain a principal that is listed.
1.235 jmc 321: By default, no
322: .Cm AuthorizedPrincipalsCommand
323: is run.
1.202 djm 324: .It Cm AuthorizedPrincipalsCommandUser
1.235 jmc 325: Specifies the user under whose account the
326: .Cm AuthorizedPrincipalsCommand
327: is run.
1.202 djm 328: It is recommended to use a dedicated user that has no other role on the host
329: than running authorized principals commands.
330: If
331: .Cm AuthorizedPrincipalsCommand
332: is specified but
333: .Cm AuthorizedPrincipalsCommandUser
334: is not, then
335: .Xr sshd 8
336: will refuse to start.
1.121 djm 337: .It Cm AuthorizedPrincipalsFile
338: Specifies a file that lists principal names that are accepted for
339: certificate authentication.
340: When using certificates signed by a key listed in
341: .Cm TrustedUserCAKeys ,
342: this file lists names, one of which must appear in the certificate for it
343: to be accepted for authentication.
1.235 jmc 344: Names are listed one per line preceded by key options (as described in
345: .Sx AUTHORIZED_KEYS FILE FORMAT
346: in
1.125 jmc 347: .Xr sshd 8 ) .
1.124 djm 348: Empty lines and comments starting with
1.121 djm 349: .Ql #
350: are ignored.
351: .Pp
1.235 jmc 352: Arguments to
1.121 djm 353: .Cm AuthorizedPrincipalsFile
1.235 jmc 354: accept the tokens described in the
355: .Sx TOKENS
356: section.
1.121 djm 357: After expansion,
358: .Cm AuthorizedPrincipalsFile
1.235 jmc 359: is taken to be an absolute path or one relative to the user's home directory.
1.138 djm 360: The default is
1.237 jmc 361: .Cm none ,
1.138 djm 362: i.e. not to use a principals file \(en in this case, the username
1.121 djm 363: of the user must appear in a certificate's principals list for it to be
364: accepted.
1.235 jmc 365: .Pp
1.121 djm 366: Note that
367: .Cm AuthorizedPrincipalsFile
368: is only used when authentication proceeds using a CA listed in
369: .Cm TrustedUserCAKeys
370: and is not consulted for certification authorities trusted via
371: .Pa ~/.ssh/authorized_keys ,
372: though the
373: .Cm principals=
374: key option offers a similar facility (see
375: .Xr sshd 8
376: for details).
1.1 stevesk 377: .It Cm Banner
378: The contents of the specified file are sent to the remote user before
379: authentication is allowed.
1.78 djm 380: If the argument is
1.237 jmc 381: .Cm none
1.78 djm 382: then no banner is displayed.
1.1 stevesk 383: By default, no banner is displayed.
384: .It Cm ChallengeResponseAuthentication
1.50 jmc 385: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 386: All authentication styles from
387: .Xr login.conf 5
388: are supported.
389: The default is
1.237 jmc 390: .Cm yes .
1.80 djm 391: .It Cm ChrootDirectory
1.113 stevesk 392: Specifies the pathname of a directory to
1.80 djm 393: .Xr chroot 2
394: to after authentication.
1.190 deraadt 395: At session startup
396: .Xr sshd 8
397: checks that all components of the pathname are root-owned directories
398: which are not writable by any other user or group.
1.106 stevesk 399: After the chroot,
400: .Xr sshd 8
401: changes the working directory to the user's home directory.
1.235 jmc 402: Arguments to
403: .Cm ChrootDirectory
404: accept the tokens described in the
405: .Sx TOKENS
406: section.
1.80 djm 407: .Pp
408: The
409: .Cm ChrootDirectory
410: must contain the necessary files and directories to support the
1.103 stevesk 411: user's session.
1.80 djm 412: For an interactive session this requires at least a shell, typically
413: .Xr sh 1 ,
414: and basic
415: .Pa /dev
416: nodes such as
417: .Xr null 4 ,
418: .Xr zero 4 ,
419: .Xr stdin 4 ,
420: .Xr stdout 4 ,
421: .Xr stderr 4 ,
1.180 jmc 422: and
1.80 djm 423: .Xr tty 4
424: devices.
1.237 jmc 425: For file transfer sessions using SFTP
426: no additional configuration of the environment is necessary if the in-process
427: sftp-server is used,
1.176 schwarze 428: though sessions which use logging may require
1.104 stevesk 429: .Pa /dev/log
1.176 schwarze 430: inside the chroot directory on some operating systems (see
1.104 stevesk 431: .Xr sftp-server 8
1.81 jmc 432: for details).
1.190 deraadt 433: .Pp
1.191 jmc 434: For safety, it is very important that the directory hierarchy be
1.190 deraadt 435: prevented from modification by other processes on the system (especially
436: those outside the jail).
437: Misconfiguration can lead to unsafe environments which
438: .Xr sshd 8
439: cannot detect.
1.80 djm 440: .Pp
1.215 djm 441: The default is
1.237 jmc 442: .Cm none ,
1.215 djm 443: indicating not to
1.80 djm 444: .Xr chroot 2 .
1.1 stevesk 445: .It Cm Ciphers
1.219 jmc 446: Specifies the ciphers allowed.
1.1 stevesk 447: Multiple ciphers must be comma-separated.
1.208 djm 448: If the specified value begins with a
449: .Sq +
450: character, then the specified ciphers will be appended to the default set
451: instead of replacing them.
1.242 djm 452: If the specified value begins with a
453: .Sq -
454: character, then the specified ciphers (including wildcards) will be removed
455: from the default set instead of replacing them.
1.208 djm 456: .Pp
1.167 djm 457: The supported ciphers are:
458: .Pp
1.173 naddy 459: .Bl -item -compact -offset indent
460: .It
461: 3des-cbc
462: .It
463: aes128-cbc
464: .It
465: aes192-cbc
466: .It
467: aes256-cbc
468: .It
469: aes128-ctr
470: .It
471: aes192-ctr
472: .It
473: aes256-ctr
474: .It
475: aes128-gcm@openssh.com
476: .It
477: aes256-gcm@openssh.com
478: .It
479: chacha20-poly1305@openssh.com
480: .El
1.167 djm 481: .Pp
1.52 jmc 482: The default is:
1.173 naddy 483: .Bd -literal -offset indent
1.211 jmc 484: chacha20-poly1305@openssh.com,
1.173 naddy 485: aes128-ctr,aes192-ctr,aes256-ctr,
1.211 jmc 486: aes128-gcm@openssh.com,aes256-gcm@openssh.com
1.1 stevesk 487: .Ed
1.167 djm 488: .Pp
1.237 jmc 489: The list of available ciphers may also be obtained using
490: .Qq ssh -Q cipher .
1.1 stevesk 491: .It Cm ClientAliveCountMax
1.237 jmc 492: Sets the number of client alive messages which may be sent without
1.52 jmc 493: .Xr sshd 8
1.17 jmc 494: receiving any messages back from the client.
495: If this threshold is reached while client alive messages are being sent,
1.52 jmc 496: sshd will disconnect the client, terminating the session.
1.17 jmc 497: It is important to note that the use of client alive messages is very
498: different from
1.237 jmc 499: .Cm TCPKeepAlive .
1.17 jmc 500: The client alive messages are sent through the encrypted channel
501: and therefore will not be spoofable.
502: The TCP keepalive option enabled by
1.27 markus 503: .Cm TCPKeepAlive
1.17 jmc 504: is spoofable.
505: The client alive mechanism is valuable when the client or
1.1 stevesk 506: server depend on knowing when a connection has become inactive.
507: .Pp
1.17 jmc 508: The default value is 3.
509: If
1.1 stevesk 510: .Cm ClientAliveInterval
1.237 jmc 511: is set to 15, and
1.1 stevesk 512: .Cm ClientAliveCountMax
1.52 jmc 513: is left at the default, unresponsive SSH clients
1.1 stevesk 514: will be disconnected after approximately 45 seconds.
1.42 djm 515: .It Cm ClientAliveInterval
516: Sets a timeout interval in seconds after which if no data has been received
517: from the client,
1.52 jmc 518: .Xr sshd 8
1.42 djm 519: will send a message through the encrypted
520: channel to request a response from the client.
521: The default
522: is 0, indicating that these messages will not be sent to the client.
1.3 markus 523: .It Cm Compression
1.236 djm 524: Specifies whether compression is enabled after
1.44 markus 525: the user has authenticated successfully.
1.3 markus 526: The argument must be
1.237 jmc 527: .Cm yes ,
528: .Cm delayed
1.236 djm 529: (a legacy synonym for
1.237 jmc 530: .Cm yes )
1.3 markus 531: or
1.237 jmc 532: .Cm no .
1.3 markus 533: The default is
1.237 jmc 534: .Cm yes .
1.1 stevesk 535: .It Cm DenyGroups
536: This keyword can be followed by a list of group name patterns, separated
537: by spaces.
538: Login is disallowed for users whose primary group or supplementary
539: group list matches one of the patterns.
540: Only group names are valid; a numerical group ID is not recognized.
541: By default, login is allowed for all groups.
1.54 jmc 542: The allow/deny directives are processed in the following order:
543: .Cm DenyUsers ,
544: .Cm AllowUsers ,
545: .Cm DenyGroups ,
546: and finally
547: .Cm AllowGroups .
1.49 jmc 548: .Pp
1.161 jmc 549: See PATTERNS in
1.49 jmc 550: .Xr ssh_config 5
551: for more information on patterns.
1.1 stevesk 552: .It Cm DenyUsers
553: This keyword can be followed by a list of user name patterns, separated
554: by spaces.
555: Login is disallowed for user names that match one of the patterns.
556: Only user names are valid; a numerical user ID is not recognized.
557: By default, login is allowed for all users.
558: If the pattern takes the form USER@HOST then USER and HOST
559: are separately checked, restricting logins to particular
560: users from particular hosts.
1.222 jmc 561: HOST criteria may additionally contain addresses to match in CIDR
562: address/masklen format.
1.54 jmc 563: The allow/deny directives are processed in the following order:
564: .Cm DenyUsers ,
565: .Cm AllowUsers ,
566: .Cm DenyGroups ,
567: and finally
568: .Cm AllowGroups .
1.49 jmc 569: .Pp
1.161 jmc 570: See PATTERNS in
1.49 jmc 571: .Xr ssh_config 5
572: for more information on patterns.
1.239 djm 573: .It Cm DisableForwarding
574: Disables all forwarding features, including X11,
575: .Xr ssh-agent 1 ,
576: TCP and StreamLocal.
577: This option overrides all other forwarding-related options and may
578: simplify restricted configurations.
1.246 djm 579: .It Cm ExposeAuthInfo
1.253 jmc 580: Writes a temporary file containing a list of authentication methods and
1.246 djm 581: public credentials (e.g. keys) used to authenticate the user.
1.247 jmc 582: The location of the file is exposed to the user session through the
1.248 djm 583: .Ev SSH_USER_AUTH
1.247 jmc 584: environment variable.
1.253 jmc 585: The default is
586: .Cm no .
1.183 djm 587: .It Cm FingerprintHash
588: Specifies the hash algorithm used when logging key fingerprints.
589: Valid options are:
1.237 jmc 590: .Cm md5
1.183 djm 591: and
1.237 jmc 592: .Cm sha256 .
1.183 djm 593: The default is
1.237 jmc 594: .Cm sha256 .
1.67 dtucker 595: .It Cm ForceCommand
596: Forces the execution of the command specified by
597: .Cm ForceCommand ,
1.84 djm 598: ignoring any command supplied by the client and
599: .Pa ~/.ssh/rc
600: if present.
1.67 dtucker 601: The command is invoked by using the user's login shell with the -c option.
602: This applies to shell, command, or subsystem execution.
603: It is most useful inside a
604: .Cm Match
605: block.
606: The command originally supplied by the client is available in the
607: .Ev SSH_ORIGINAL_COMMAND
608: environment variable.
1.82 djm 609: Specifying a command of
1.237 jmc 610: .Cm internal-sftp
611: will force the use of an in-process SFTP server that requires no support
1.82 djm 612: files when used with
613: .Cm ChrootDirectory .
1.215 djm 614: The default is
1.237 jmc 615: .Cm none .
1.1 stevesk 616: .It Cm GatewayPorts
617: Specifies whether remote hosts are allowed to connect to ports
618: forwarded for the client.
619: By default,
1.52 jmc 620: .Xr sshd 8
1.15 jmc 621: binds remote port forwardings to the loopback address.
622: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 623: .Cm GatewayPorts
1.52 jmc 624: can be used to specify that sshd
1.39 djm 625: should allow remote port forwardings to bind to non-loopback addresses, thus
626: allowing other hosts to connect.
627: The argument may be
1.237 jmc 628: .Cm no
1.39 djm 629: to force remote port forwardings to be available to the local host only,
1.237 jmc 630: .Cm yes
1.39 djm 631: to force remote port forwardings to bind to the wildcard address, or
1.237 jmc 632: .Cm clientspecified
1.39 djm 633: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 634: The default is
1.237 jmc 635: .Cm no .
1.23 markus 636: .It Cm GSSAPIAuthentication
1.25 markus 637: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 638: The default is
1.237 jmc 639: .Cm no .
1.23 markus 640: .It Cm GSSAPICleanupCredentials
641: Specifies whether to automatically destroy the user's credentials cache
642: on logout.
643: The default is
1.237 jmc 644: .Cm yes .
1.203 djm 645: .It Cm GSSAPIStrictAcceptorCheck
646: Determines whether to be strict about the identity of the GSSAPI acceptor
647: a client authenticates against.
648: If set to
1.237 jmc 649: .Cm yes
650: then the client must authenticate against the host
1.203 djm 651: service on the current hostname.
652: If set to
1.237 jmc 653: .Cm no
1.203 djm 654: then the client may authenticate against any service key stored in the
655: machine's default store.
656: This facility is provided to assist with operation on multi homed machines.
657: The default is
1.237 jmc 658: .Cm yes .
1.189 djm 659: .It Cm HostbasedAcceptedKeyTypes
660: Specifies the key types that will be accepted for hostbased authentication
661: as a comma-separated pattern list.
1.208 djm 662: Alternately if the specified value begins with a
663: .Sq +
664: character, then the specified key types will be appended to the default set
665: instead of replacing them.
1.242 djm 666: If the specified value begins with a
667: .Sq -
668: character, then the specified key types (including wildcards) will be removed
669: from the default set instead of replacing them.
1.206 markus 670: The default for this option is:
671: .Bd -literal -offset 3n
672: ecdsa-sha2-nistp256-cert-v01@openssh.com,
673: ecdsa-sha2-nistp384-cert-v01@openssh.com,
674: ecdsa-sha2-nistp521-cert-v01@openssh.com,
675: ssh-ed25519-cert-v01@openssh.com,
676: ssh-rsa-cert-v01@openssh.com,
677: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.220 djm 678: ssh-ed25519,ssh-rsa
1.206 markus 679: .Ed
680: .Pp
1.237 jmc 681: The list of available key types may also be obtained using
682: .Qq ssh -Q key .
1.1 stevesk 683: .It Cm HostbasedAuthentication
684: Specifies whether rhosts or /etc/hosts.equiv authentication together
685: with successful public key client host authentication is allowed
1.50 jmc 686: (host-based authentication).
1.70 dtucker 687: The default is
1.237 jmc 688: .Cm no .
1.70 dtucker 689: .It Cm HostbasedUsesNameFromPacketOnly
690: Specifies whether or not the server will attempt to perform a reverse
691: name lookup when matching the name in the
692: .Pa ~/.shosts ,
693: .Pa ~/.rhosts ,
694: and
695: .Pa /etc/hosts.equiv
696: files during
697: .Cm HostbasedAuthentication .
698: A setting of
1.237 jmc 699: .Cm yes
1.70 dtucker 700: means that
701: .Xr sshd 8
702: uses the name supplied by the client rather than
703: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 704: The default is
1.237 jmc 705: .Cm no .
1.117 djm 706: .It Cm HostCertificate
707: Specifies a file containing a public host certificate.
708: The certificate's public key must match a private host key already specified
709: by
710: .Cm HostKey .
711: The default behaviour of
712: .Xr sshd 8
713: is not to load any certificates.
1.1 stevesk 714: .It Cm HostKey
715: Specifies a file containing a private host key
716: used by SSH.
1.229 naddy 717: The defaults are
1.126 djm 718: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 719: .Pa /etc/ssh/ssh_host_ecdsa_key ,
720: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 721: and
1.229 naddy 722: .Pa /etc/ssh/ssh_host_rsa_key .
1.206 markus 723: .Pp
1.1 stevesk 724: Note that
1.52 jmc 725: .Xr sshd 8
1.206 markus 726: will refuse to use a file if it is group/world-accessible
727: and that the
728: .Cm HostKeyAlgorithms
729: option restricts which of the keys are actually used by
730: .Xr sshd 8 .
731: .Pp
1.1 stevesk 732: It is possible to have multiple host key files.
1.162 markus 733: It is also possible to specify public host key files instead.
734: In this case operations on the private key will be delegated
735: to an
736: .Xr ssh-agent 1 .
737: .It Cm HostKeyAgent
738: Identifies the UNIX-domain socket used to communicate
739: with an agent that has access to the private host keys.
1.223 markus 740: If the string
1.237 jmc 741: .Qq SSH_AUTH_SOCK
1.162 markus 742: is specified, the location of the socket will be read from the
743: .Ev SSH_AUTH_SOCK
744: environment variable.
1.206 markus 745: .It Cm HostKeyAlgorithms
1.219 jmc 746: Specifies the host key algorithms
1.206 markus 747: that the server offers.
748: The default for this option is:
749: .Bd -literal -offset 3n
750: ecdsa-sha2-nistp256-cert-v01@openssh.com,
751: ecdsa-sha2-nistp384-cert-v01@openssh.com,
752: ecdsa-sha2-nistp521-cert-v01@openssh.com,
753: ssh-ed25519-cert-v01@openssh.com,
754: ssh-rsa-cert-v01@openssh.com,
755: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.220 djm 756: ssh-ed25519,ssh-rsa
1.206 markus 757: .Ed
758: .Pp
1.237 jmc 759: The list of available key types may also be obtained using
760: .Qq ssh -Q key .
1.1 stevesk 761: .It Cm IgnoreRhosts
762: Specifies that
763: .Pa .rhosts
764: and
765: .Pa .shosts
766: files will not be used in
767: .Cm HostbasedAuthentication .
768: .Pp
769: .Pa /etc/hosts.equiv
770: and
771: .Pa /etc/shosts.equiv
772: are still used.
773: The default is
1.237 jmc 774: .Cm yes .
1.1 stevesk 775: .It Cm IgnoreUserKnownHosts
776: Specifies whether
1.52 jmc 777: .Xr sshd 8
1.1 stevesk 778: should ignore the user's
1.41 djm 779: .Pa ~/.ssh/known_hosts
1.1 stevesk 780: during
781: .Cm HostbasedAuthentication .
782: The default is
1.237 jmc 783: .Cm no .
1.129 djm 784: .It Cm IPQoS
785: Specifies the IPv4 type-of-service or DSCP class for the connection.
786: Accepted values are
1.237 jmc 787: .Cm af11 ,
788: .Cm af12 ,
789: .Cm af13 ,
790: .Cm af21 ,
791: .Cm af22 ,
792: .Cm af23 ,
793: .Cm af31 ,
794: .Cm af32 ,
795: .Cm af33 ,
796: .Cm af41 ,
797: .Cm af42 ,
798: .Cm af43 ,
799: .Cm cs0 ,
800: .Cm cs1 ,
801: .Cm cs2 ,
802: .Cm cs3 ,
803: .Cm cs4 ,
804: .Cm cs5 ,
805: .Cm cs6 ,
806: .Cm cs7 ,
807: .Cm ef ,
808: .Cm lowdelay ,
809: .Cm throughput ,
810: .Cm reliability ,
1.250 djm 811: a numeric value, or
812: .Cm none
813: to use the operating system default.
1.131 djm 814: This option may take one or two arguments, separated by whitespace.
1.129 djm 815: If one argument is specified, it is used as the packet class unconditionally.
816: If two values are specified, the first is automatically selected for
817: interactive sessions and the second for non-interactive sessions.
818: The default is
1.237 jmc 819: .Cm lowdelay
1.129 djm 820: for interactive sessions and
1.237 jmc 821: .Cm throughput
1.129 djm 822: for non-interactive sessions.
1.171 jmc 823: .It Cm KbdInteractiveAuthentication
824: Specifies whether to allow keyboard-interactive authentication.
825: The argument to this keyword must be
1.237 jmc 826: .Cm yes
1.171 jmc 827: or
1.237 jmc 828: .Cm no .
1.171 jmc 829: The default is to use whatever value
830: .Cm ChallengeResponseAuthentication
831: is set to
832: (by default
1.237 jmc 833: .Cm yes ) .
1.1 stevesk 834: .It Cm KerberosAuthentication
1.24 markus 835: Specifies whether the password provided by the user for
1.1 stevesk 836: .Cm PasswordAuthentication
1.24 markus 837: will be validated through the Kerberos KDC.
1.1 stevesk 838: To use this option, the server needs a
839: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 840: The default is
1.237 jmc 841: .Cm no .
1.29 dtucker 842: .It Cm KerberosGetAFSToken
1.45 djm 843: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 844: an AFS token before accessing the user's home directory.
1.52 jmc 845: The default is
1.237 jmc 846: .Cm no .
1.1 stevesk 847: .It Cm KerberosOrLocalPasswd
1.52 jmc 848: If password authentication through Kerberos fails then
1.1 stevesk 849: the password will be validated via any additional local mechanism
850: such as
851: .Pa /etc/passwd .
1.52 jmc 852: The default is
1.237 jmc 853: .Cm yes .
1.1 stevesk 854: .It Cm KerberosTicketCleanup
855: Specifies whether to automatically destroy the user's ticket cache
856: file on logout.
1.52 jmc 857: The default is
1.237 jmc 858: .Cm yes .
1.127 djm 859: .It Cm KexAlgorithms
860: Specifies the available KEX (Key Exchange) algorithms.
861: Multiple algorithms must be comma-separated.
1.208 djm 862: Alternately if the specified value begins with a
863: .Sq +
864: character, then the specified methods will be appended to the default set
865: instead of replacing them.
1.242 djm 866: If the specified value begins with a
867: .Sq -
868: character, then the specified methods (including wildcards) will be removed
869: from the default set instead of replacing them.
1.173 naddy 870: The supported algorithms are:
871: .Pp
872: .Bl -item -compact -offset indent
873: .It
1.234 djm 874: curve25519-sha256
875: .It
1.173 naddy 876: curve25519-sha256@libssh.org
877: .It
878: diffie-hellman-group1-sha1
879: .It
880: diffie-hellman-group14-sha1
881: .It
882: diffie-hellman-group-exchange-sha1
883: .It
884: diffie-hellman-group-exchange-sha256
885: .It
886: ecdh-sha2-nistp256
887: .It
888: ecdh-sha2-nistp384
889: .It
890: ecdh-sha2-nistp521
891: .El
892: .Pp
893: The default is:
1.170 dtucker 894: .Bd -literal -offset indent
1.234 djm 895: curve25519-sha256,curve25519-sha256@libssh.org,
1.170 dtucker 896: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
897: diffie-hellman-group-exchange-sha256,
1.173 naddy 898: diffie-hellman-group14-sha1
1.170 dtucker 899: .Ed
1.188 djm 900: .Pp
1.237 jmc 901: The list of available key exchange algorithms may also be obtained using
902: .Qq ssh -Q kex .
1.1 stevesk 903: .It Cm ListenAddress
904: Specifies the local addresses
1.52 jmc 905: .Xr sshd 8
1.1 stevesk 906: should listen on.
907: The following forms may be used:
908: .Pp
909: .Bl -item -offset indent -compact
910: .It
911: .Cm ListenAddress
912: .Sm off
1.259 jmc 913: .Ar hostname | address
1.1 stevesk 914: .Sm on
1.260 ! jmc 915: .Op Cm rdomain Ar domain
1.1 stevesk 916: .It
917: .Cm ListenAddress
918: .Sm off
1.259 jmc 919: .Ar hostname : port
1.1 stevesk 920: .Sm on
1.260 ! jmc 921: .Op Cm rdomain Ar domain
1.256 djm 922: .It
923: .Cm ListenAddress
924: .Sm off
1.259 jmc 925: .Ar IPv4_address : port
1.256 djm 926: .Sm on
1.260 ! jmc 927: .Op Cm rdomain Ar domain
1.1 stevesk 928: .It
929: .Cm ListenAddress
930: .Sm off
1.259 jmc 931: .Oo Ar hostname | address Oc : Ar port
1.1 stevesk 932: .Sm on
1.260 ! jmc 933: .Op Cm rdomain Ar domain
1.1 stevesk 934: .El
935: .Pp
1.256 djm 936: The optional
937: .Cm rdomain
938: qualifier requests
939: .Xr sshd 8
940: listen in an explicit routing domain.
1.1 stevesk 941: If
942: .Ar port
943: is not specified,
1.200 dtucker 944: sshd will listen on the address and all
1.1 stevesk 945: .Cm Port
1.17 jmc 946: options specified.
1.256 djm 947: The default is to listen on all local addresses on the current default
948: routing domain.
1.15 jmc 949: Multiple
1.1 stevesk 950: .Cm ListenAddress
1.17 jmc 951: options are permitted.
1.256 djm 952: For more information on routing domains, see
1.259 jmc 953: .Xr rdomain 4 .
1.1 stevesk 954: .It Cm LoginGraceTime
955: The server disconnects after this time if the user has not
956: successfully logged in.
957: If the value is 0, there is no time limit.
1.12 stevesk 958: The default is 120 seconds.
1.1 stevesk 959: .It Cm LogLevel
960: Gives the verbosity level that is used when logging messages from
1.53 jmc 961: .Xr sshd 8 .
1.1 stevesk 962: The possible values are:
1.52 jmc 963: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 964: The default is INFO.
965: DEBUG and DEBUG1 are equivalent.
966: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
967: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 968: .It Cm MACs
969: Specifies the available MAC (message authentication code) algorithms.
1.219 jmc 970: The MAC algorithm is used for data integrity protection.
1.1 stevesk 971: Multiple algorithms must be comma-separated.
1.208 djm 972: If the specified value begins with a
973: .Sq +
974: character, then the specified algorithms will be appended to the default set
975: instead of replacing them.
1.242 djm 976: If the specified value begins with a
977: .Sq -
978: character, then the specified algorithms (including wildcards) will be removed
979: from the default set instead of replacing them.
1.208 djm 980: .Pp
1.152 markus 981: The algorithms that contain
1.237 jmc 982: .Qq -etm
1.152 markus 983: calculate the MAC after encryption (encrypt-then-mac).
984: These are considered safer and their use recommended.
1.173 naddy 985: The supported MACs are:
986: .Pp
987: .Bl -item -compact -offset indent
988: .It
989: hmac-md5
990: .It
991: hmac-md5-96
992: .It
993: hmac-sha1
994: .It
995: hmac-sha1-96
996: .It
997: hmac-sha2-256
998: .It
999: hmac-sha2-512
1000: .It
1001: umac-64@openssh.com
1002: .It
1003: umac-128@openssh.com
1004: .It
1005: hmac-md5-etm@openssh.com
1006: .It
1007: hmac-md5-96-etm@openssh.com
1008: .It
1009: hmac-sha1-etm@openssh.com
1010: .It
1011: hmac-sha1-96-etm@openssh.com
1012: .It
1013: hmac-sha2-256-etm@openssh.com
1014: .It
1015: hmac-sha2-512-etm@openssh.com
1016: .It
1017: umac-64-etm@openssh.com
1018: .It
1019: umac-128-etm@openssh.com
1020: .El
1021: .Pp
1.52 jmc 1022: The default is:
1.77 jmc 1023: .Bd -literal -offset indent
1.152 markus 1024: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1025: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.217 djm 1026: hmac-sha1-etm@openssh.com,
1.173 naddy 1027: umac-64@openssh.com,umac-128@openssh.com,
1.217 djm 1028: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.77 jmc 1029: .Ed
1.188 djm 1030: .Pp
1.237 jmc 1031: The list of available MAC algorithms may also be obtained using
1032: .Qq ssh -Q mac .
1.60 dtucker 1033: .It Cm Match
1.61 jmc 1034: Introduces a conditional block.
1.65 dtucker 1035: If all of the criteria on the
1.60 dtucker 1036: .Cm Match
1.65 dtucker 1037: line are satisfied, the keywords on the following lines override those
1038: set in the global section of the config file, until either another
1.60 dtucker 1039: .Cm Match
1.65 dtucker 1040: line or the end of the file.
1.172 djm 1041: If a keyword appears in multiple
1042: .Cm Match
1.177 sobrado 1043: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1044: applied.
1.91 djm 1045: .Pp
1.61 jmc 1046: The arguments to
1.60 dtucker 1047: .Cm Match
1.163 dtucker 1048: are one or more criteria-pattern pairs or the single token
1049: .Cm All
1050: which matches all criteria.
1.60 dtucker 1051: The available criteria are
1052: .Cm User ,
1.69 dtucker 1053: .Cm Group ,
1.60 dtucker 1054: .Cm Host ,
1.139 dtucker 1055: .Cm LocalAddress ,
1056: .Cm LocalPort ,
1.258 djm 1057: .Cm RDomain ,
1.60 dtucker 1058: and
1.258 djm 1059: .Cm Address
1060: (with
1061: .Cm RDomain
1062: representing the
1063: .Xr rdomain 4
1064: on which the connection was received.)
1065: .Pp
1.91 djm 1066: The match patterns may consist of single entries or comma-separated
1067: lists and may use the wildcard and negation operators described in the
1.237 jmc 1068: .Sx PATTERNS
1069: section of
1.92 djm 1070: .Xr ssh_config 5 .
1.91 djm 1071: .Pp
1072: The patterns in an
1073: .Cm Address
1074: criteria may additionally contain addresses to match in CIDR
1.237 jmc 1075: address/masklen format,
1076: such as 192.0.2.0/24 or 2001:db8::/32.
1.91 djm 1077: Note that the mask length provided must be consistent with the address -
1078: it is an error to specify a mask length that is too long for the address
1.93 jmc 1079: or one with bits set in this host portion of the address.
1.237 jmc 1080: For example, 192.0.2.0/33 and 192.0.2.0/8, respectively.
1.91 djm 1081: .Pp
1.60 dtucker 1082: Only a subset of keywords may be used on the lines following a
1083: .Cm Match
1084: keyword.
1085: Available keywords are
1.142 jmc 1086: .Cm AcceptEnv ,
1.99 okan 1087: .Cm AllowAgentForwarding ,
1.142 jmc 1088: .Cm AllowGroups ,
1.193 djm 1089: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1090: .Cm AllowTcpForwarding ,
1.141 markus 1091: .Cm AllowUsers ,
1.149 djm 1092: .Cm AuthenticationMethods ,
1.146 djm 1093: .Cm AuthorizedKeysCommand ,
1094: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1095: .Cm AuthorizedKeysFile ,
1.214 djm 1096: .Cm AuthorizedPrincipalsCommand ,
1097: .Cm AuthorizedPrincipalsCommandUser ,
1.123 djm 1098: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1099: .Cm Banner ,
1.85 djm 1100: .Cm ChrootDirectory ,
1.238 markus 1101: .Cm ClientAliveCountMax ,
1102: .Cm ClientAliveInterval ,
1.141 markus 1103: .Cm DenyGroups ,
1104: .Cm DenyUsers ,
1.67 dtucker 1105: .Cm ForceCommand ,
1.194 djm 1106: .Cm GatewayPorts ,
1.193 djm 1107: .Cm GSSAPIAuthentication ,
1.189 djm 1108: .Cm HostbasedAcceptedKeyTypes ,
1.87 djm 1109: .Cm HostbasedAuthentication ,
1.123 djm 1110: .Cm HostbasedUsesNameFromPacketOnly ,
1.193 djm 1111: .Cm IPQoS ,
1.74 jmc 1112: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1113: .Cm KerberosAuthentication ,
1.245 djm 1114: .Cm LogLevel ,
1.95 dtucker 1115: .Cm MaxAuthTries ,
1.94 dtucker 1116: .Cm MaxSessions ,
1.72 dtucker 1117: .Cm PasswordAuthentication ,
1.97 djm 1118: .Cm PermitEmptyPasswords ,
1.66 dtucker 1119: .Cm PermitOpen ,
1.79 dtucker 1120: .Cm PermitRootLogin ,
1.164 djm 1121: .Cm PermitTTY ,
1.123 djm 1122: .Cm PermitTunnel ,
1.174 djm 1123: .Cm PermitUserRC ,
1.189 djm 1124: .Cm PubkeyAcceptedKeyTypes ,
1.107 dtucker 1125: .Cm PubkeyAuthentication ,
1.159 dtucker 1126: .Cm RekeyLimit ,
1.193 djm 1127: .Cm RevokedKeys ,
1.257 djm 1128: .Cm RDomain ,
1.193 djm 1129: .Cm StreamLocalBindMask ,
1130: .Cm StreamLocalBindUnlink ,
1131: .Cm TrustedUserCAKeys ,
1.66 dtucker 1132: .Cm X11DisplayOffset ,
1.101 djm 1133: .Cm X11Forwarding
1.60 dtucker 1134: and
1.102 djm 1135: .Cm X11UseLocalHost .
1.33 dtucker 1136: .It Cm MaxAuthTries
1137: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1138: connection.
1139: Once the number of failures reaches half this value,
1140: additional failures are logged.
1141: The default is 6.
1.90 djm 1142: .It Cm MaxSessions
1.216 djm 1143: Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
1144: sessions permitted per network connection.
1145: Multiple sessions may be established by clients that support connection
1146: multiplexing.
1147: Setting
1148: .Cm MaxSessions
1149: to 1 will effectively disable session multiplexing, whereas setting it to 0
1150: will prevent all shell, login and subsystem sessions while still permitting
1151: forwarding.
1.90 djm 1152: The default is 10.
1.1 stevesk 1153: .It Cm MaxStartups
1154: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1155: SSH daemon.
1.1 stevesk 1156: Additional connections will be dropped until authentication succeeds or the
1157: .Cm LoginGraceTime
1158: expires for a connection.
1.156 dtucker 1159: The default is 10:30:100.
1.1 stevesk 1160: .Pp
1161: Alternatively, random early drop can be enabled by specifying
1162: the three colon separated values
1.237 jmc 1163: start:rate:full (e.g. "10:30:60").
1.53 jmc 1164: .Xr sshd 8
1.237 jmc 1165: will refuse connection attempts with a probability of rate/100 (30%)
1166: if there are currently start (10) unauthenticated connections.
1.1 stevesk 1167: The probability increases linearly and all connection attempts
1.237 jmc 1168: are refused if the number of unauthenticated connections reaches full (60).
1.1 stevesk 1169: .It Cm PasswordAuthentication
1170: Specifies whether password authentication is allowed.
1171: The default is
1.237 jmc 1172: .Cm yes .
1.1 stevesk 1173: .It Cm PermitEmptyPasswords
1174: When password authentication is allowed, it specifies whether the
1175: server allows login to accounts with empty password strings.
1176: The default is
1.237 jmc 1177: .Cm no .
1.62 dtucker 1178: .It Cm PermitOpen
1179: Specifies the destinations to which TCP port forwarding is permitted.
1180: The forwarding specification must be one of the following forms:
1181: .Pp
1182: .Bl -item -offset indent -compact
1183: .It
1184: .Cm PermitOpen
1185: .Sm off
1186: .Ar host : port
1187: .Sm on
1188: .It
1189: .Cm PermitOpen
1190: .Sm off
1191: .Ar IPv4_addr : port
1192: .Sm on
1193: .It
1194: .Cm PermitOpen
1195: .Sm off
1196: .Ar \&[ IPv6_addr \&] : port
1197: .Sm on
1198: .El
1199: .Pp
1.68 dtucker 1200: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1201: An argument of
1.237 jmc 1202: .Cm any
1.62 dtucker 1203: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1204: An argument of
1.237 jmc 1205: .Cm none
1.140 dtucker 1206: can be used to prohibit all forwarding requests.
1.227 jmc 1207: The wildcard
1.237 jmc 1208: .Sq *
1.227 jmc 1209: can be used for host or port to allow all hosts or ports, respectively.
1.63 jmc 1210: By default all port forwarding requests are permitted.
1.1 stevesk 1211: .It Cm PermitRootLogin
1.38 jmc 1212: Specifies whether root can log in using
1.1 stevesk 1213: .Xr ssh 1 .
1214: The argument must be
1.237 jmc 1215: .Cm yes ,
1216: .Cm prohibit-password ,
1217: .Cm forced-commands-only ,
1.1 stevesk 1218: or
1.237 jmc 1219: .Cm no .
1.1 stevesk 1220: The default is
1.237 jmc 1221: .Cm prohibit-password .
1.1 stevesk 1222: .Pp
1223: If this option is set to
1.237 jmc 1224: .Cm prohibit-password
1.255 jmc 1225: (or its deprecated alias,
1226: .Cm without-password ) ,
1.210 deraadt 1227: password and keyboard-interactive authentication are disabled for root.
1.1 stevesk 1228: .Pp
1229: If this option is set to
1.237 jmc 1230: .Cm forced-commands-only ,
1.1 stevesk 1231: root login with public key authentication will be allowed,
1232: but only if the
1233: .Ar command
1234: option has been specified
1235: (which may be useful for taking remote backups even if root login is
1.17 jmc 1236: normally not allowed).
1237: All other authentication methods are disabled for root.
1.1 stevesk 1238: .Pp
1239: If this option is set to
1.237 jmc 1240: .Cm no ,
1.38 jmc 1241: root is not allowed to log in.
1.231 jmc 1242: .It Cm PermitTTY
1243: Specifies whether
1244: .Xr pty 4
1245: allocation is permitted.
1246: The default is
1.237 jmc 1247: .Cm yes .
1.46 reyk 1248: .It Cm PermitTunnel
1249: Specifies whether
1250: .Xr tun 4
1251: device forwarding is allowed.
1.47 reyk 1252: The argument must be
1.237 jmc 1253: .Cm yes ,
1254: .Cm point-to-point
1.58 stevesk 1255: (layer 3),
1.237 jmc 1256: .Cm ethernet
1.58 stevesk 1257: (layer 2), or
1.237 jmc 1258: .Cm no .
1.58 stevesk 1259: Specifying
1.237 jmc 1260: .Cm yes
1.58 stevesk 1261: permits both
1.237 jmc 1262: .Cm point-to-point
1.58 stevesk 1263: and
1.237 jmc 1264: .Cm ethernet .
1.46 reyk 1265: The default is
1.237 jmc 1266: .Cm no .
1.178 djm 1267: .Pp
1268: Independent of this setting, the permissions of the selected
1269: .Xr tun 4
1270: device must allow access to the user.
1.6 markus 1271: .It Cm PermitUserEnvironment
1272: Specifies whether
1273: .Pa ~/.ssh/environment
1.9 stevesk 1274: and
1.6 markus 1275: .Cm environment=
1276: options in
1277: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1278: are processed by
1.52 jmc 1279: .Xr sshd 8 .
1.6 markus 1280: The default is
1.237 jmc 1281: .Cm no .
1.9 stevesk 1282: Enabling environment processing may enable users to bypass access
1283: restrictions in some configurations using mechanisms such as
1284: .Ev LD_PRELOAD .
1.174 djm 1285: .It Cm PermitUserRC
1286: Specifies whether any
1287: .Pa ~/.ssh/rc
1288: file is executed.
1289: The default is
1.237 jmc 1290: .Cm yes .
1.1 stevesk 1291: .It Cm PidFile
1.4 stevesk 1292: Specifies the file that contains the process ID of the
1.195 dtucker 1293: SSH daemon, or
1.237 jmc 1294: .Cm none
1.195 dtucker 1295: to not write one.
1.1 stevesk 1296: The default is
1297: .Pa /var/run/sshd.pid .
1298: .It Cm Port
1299: Specifies the port number that
1.52 jmc 1300: .Xr sshd 8
1.1 stevesk 1301: listens on.
1302: The default is 22.
1303: Multiple options of this type are permitted.
1304: See also
1305: .Cm ListenAddress .
1306: .It Cm PrintLastLog
1307: Specifies whether
1.52 jmc 1308: .Xr sshd 8
1.36 jaredy 1309: should print the date and time of the last user login when a user logs
1310: in interactively.
1.1 stevesk 1311: The default is
1.237 jmc 1312: .Cm yes .
1.1 stevesk 1313: .It Cm PrintMotd
1314: Specifies whether
1.52 jmc 1315: .Xr sshd 8
1.1 stevesk 1316: should print
1317: .Pa /etc/motd
1318: when a user logs in interactively.
1319: (On some systems it is also printed by the shell,
1320: .Pa /etc/profile ,
1321: or equivalent.)
1322: The default is
1.237 jmc 1323: .Cm yes .
1.189 djm 1324: .It Cm PubkeyAcceptedKeyTypes
1325: Specifies the key types that will be accepted for public key authentication
1326: as a comma-separated pattern list.
1.208 djm 1327: Alternately if the specified value begins with a
1328: .Sq +
1329: character, then the specified key types will be appended to the default set
1330: instead of replacing them.
1.242 djm 1331: If the specified value begins with a
1332: .Sq -
1333: character, then the specified key types (including wildcards) will be removed
1334: from the default set instead of replacing them.
1.206 markus 1335: The default for this option is:
1336: .Bd -literal -offset 3n
1337: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1338: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1339: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1340: ssh-ed25519-cert-v01@openssh.com,
1341: ssh-rsa-cert-v01@openssh.com,
1342: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.220 djm 1343: ssh-ed25519,ssh-rsa
1.206 markus 1344: .Ed
1345: .Pp
1.237 jmc 1346: The list of available key types may also be obtained using
1347: .Qq ssh -Q key .
1.1 stevesk 1348: .It Cm PubkeyAuthentication
1349: Specifies whether public key authentication is allowed.
1350: The default is
1.237 jmc 1351: .Cm yes .
1.159 dtucker 1352: .It Cm RekeyLimit
1353: Specifies the maximum amount of data that may be transmitted before the
1354: session key is renegotiated, optionally followed a maximum amount of
1355: time that may pass before the session key is renegotiated.
1356: The first argument is specified in bytes and may have a suffix of
1357: .Sq K ,
1358: .Sq M ,
1359: or
1360: .Sq G
1361: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1362: The default is between
1363: .Sq 1G
1364: and
1365: .Sq 4G ,
1366: depending on the cipher.
1367: The optional second value is specified in seconds and may use any of the
1368: units documented in the
1369: .Sx TIME FORMATS
1.160 jmc 1370: section.
1.159 dtucker 1371: The default value for
1372: .Cm RekeyLimit
1373: is
1.237 jmc 1374: .Cm default none ,
1.159 dtucker 1375: which means that rekeying is performed after the cipher's default amount
1376: of data has been sent or received and no time based rekeying is done.
1.118 djm 1377: .It Cm RevokedKeys
1.195 dtucker 1378: Specifies revoked public keys file, or
1.237 jmc 1379: .Cm none
1.195 dtucker 1380: to not use one.
1.118 djm 1381: Keys listed in this file will be refused for public key authentication.
1382: Note that if this file is not readable, then public key authentication will
1383: be refused for all users.
1.154 djm 1384: Keys may be specified as a text file, listing one public key per line, or as
1385: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1386: .Xr ssh-keygen 1 .
1.161 jmc 1387: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1388: .Xr ssh-keygen 1 .
1.257 djm 1389: .It Cm RDomain
1390: Specifies an explicit routing domain that is applied after authentication
1391: has completed.
1.259 jmc 1392: The user session, as well and any forwarded or listening IP sockets,
1393: will be bound to this
1.257 djm 1394: .Xr rdomain 4 .
1395: If the routing domain is set to
1396: .Cm \&%D ,
1.259 jmc 1397: then the domain in which the incoming connection was received will be applied.
1.175 millert 1398: .It Cm StreamLocalBindMask
1399: Sets the octal file creation mode mask
1400: .Pq umask
1401: used when creating a Unix-domain socket file for local or remote
1402: port forwarding.
1403: This option is only used for port forwarding to a Unix-domain socket file.
1404: .Pp
1405: The default value is 0177, which creates a Unix-domain socket file that is
1406: readable and writable only by the owner.
1407: Note that not all operating systems honor the file mode on Unix-domain
1408: socket files.
1409: .It Cm StreamLocalBindUnlink
1410: Specifies whether to remove an existing Unix-domain socket file for local
1411: or remote port forwarding before creating a new one.
1412: If the socket file already exists and
1413: .Cm StreamLocalBindUnlink
1414: is not enabled,
1415: .Nm sshd
1416: will be unable to forward the port to the Unix-domain socket file.
1417: This option is only used for port forwarding to a Unix-domain socket file.
1418: .Pp
1419: The argument must be
1.237 jmc 1420: .Cm yes
1.175 millert 1421: or
1.237 jmc 1422: .Cm no .
1.175 millert 1423: The default is
1.237 jmc 1424: .Cm no .
1.1 stevesk 1425: .It Cm StrictModes
1426: Specifies whether
1.52 jmc 1427: .Xr sshd 8
1.1 stevesk 1428: should check file modes and ownership of the
1429: user's files and home directory before accepting login.
1430: This is normally desirable because novices sometimes accidentally leave their
1431: directory or files world-writable.
1432: The default is
1.237 jmc 1433: .Cm yes .
1.112 djm 1434: Note that this does not apply to
1435: .Cm ChrootDirectory ,
1436: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1437: .It Cm Subsystem
1.51 jmc 1438: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1439: Arguments should be a subsystem name and a command (with optional arguments)
1440: to execute upon subsystem request.
1.80 djm 1441: .Pp
1.1 stevesk 1442: The command
1.237 jmc 1443: .Cm sftp-server
1444: implements the SFTP file transfer subsystem.
1.80 djm 1445: .Pp
1446: Alternately the name
1.237 jmc 1447: .Cm internal-sftp
1448: implements an in-process SFTP server.
1.80 djm 1449: This may simplify configurations using
1450: .Cm ChrootDirectory
1451: to force a different filesystem root on clients.
1452: .Pp
1.1 stevesk 1453: By default no subsystems are defined.
1454: .It Cm SyslogFacility
1455: Gives the facility code that is used when logging messages from
1.53 jmc 1456: .Xr sshd 8 .
1.1 stevesk 1457: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1458: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1459: The default is AUTH.
1.27 markus 1460: .It Cm TCPKeepAlive
1461: Specifies whether the system should send TCP keepalive messages to the
1462: other side.
1463: If they are sent, death of the connection or crash of one
1464: of the machines will be properly noticed.
1465: However, this means that
1466: connections will die if the route is down temporarily, and some people
1467: find it annoying.
1468: On the other hand, if TCP keepalives are not sent,
1469: sessions may hang indefinitely on the server, leaving
1.237 jmc 1470: .Qq ghost
1.27 markus 1471: users and consuming server resources.
1472: .Pp
1473: The default is
1.237 jmc 1474: .Cm yes
1.27 markus 1475: (to send TCP keepalive messages), and the server will notice
1476: if the network goes down or the client host crashes.
1477: This avoids infinitely hanging sessions.
1478: .Pp
1479: To disable TCP keepalive messages, the value should be set to
1.237 jmc 1480: .Cm no .
1.118 djm 1481: .It Cm TrustedUserCAKeys
1482: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1483: trusted to sign user certificates for authentication, or
1.237 jmc 1484: .Cm none
1.195 dtucker 1485: to not use one.
1.119 jmc 1486: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1487: .Ql #
1488: are allowed.
1489: If a certificate is presented for authentication and has its signing CA key
1490: listed in this file, then it may be used for authentication for any user
1491: listed in the certificate's principals list.
1492: Note that certificates that lack a list of principals will not be permitted
1493: for authentication using
1494: .Cm TrustedUserCAKeys .
1.161 jmc 1495: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1496: .Xr ssh-keygen 1 .
1.18 markus 1497: .It Cm UseDNS
1498: Specifies whether
1.52 jmc 1499: .Xr sshd 8
1.207 djm 1500: should look up the remote host name, and to check that
1.18 markus 1501: the resolved host name for the remote IP address maps back to the
1502: very same IP address.
1.207 djm 1503: .Pp
1504: If this option is set to
1.237 jmc 1505: .Cm no
1.207 djm 1506: (the default) then only addresses and not host names may be used in
1.221 djm 1507: .Pa ~/.ssh/authorized_keys
1.207 djm 1508: .Cm from
1509: and
1.211 jmc 1510: .Nm
1.207 djm 1511: .Cm Match
1512: .Cm Host
1513: directives.
1.137 djm 1514: .It Cm VersionAddendum
1515: Optionally specifies additional text to append to the SSH protocol banner
1516: sent by the server upon connection.
1517: The default is
1.237 jmc 1518: .Cm none .
1.1 stevesk 1519: .It Cm X11DisplayOffset
1520: Specifies the first display number available for
1.52 jmc 1521: .Xr sshd 8 Ns 's
1.1 stevesk 1522: X11 forwarding.
1.52 jmc 1523: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1524: The default is 10.
1525: .It Cm X11Forwarding
1526: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1527: The argument must be
1.237 jmc 1528: .Cm yes
1.13 stevesk 1529: or
1.237 jmc 1530: .Cm no .
1.1 stevesk 1531: The default is
1.237 jmc 1532: .Cm no .
1.13 stevesk 1533: .Pp
1534: When X11 forwarding is enabled, there may be additional exposure to
1535: the server and to client displays if the
1.52 jmc 1536: .Xr sshd 8
1.13 stevesk 1537: proxy display is configured to listen on the wildcard address (see
1.237 jmc 1538: .Cm X11UseLocalhost ) ,
1539: though this is not the default.
1.13 stevesk 1540: Additionally, the authentication spoofing and authentication data
1541: verification and substitution occur on the client side.
1542: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1543: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1544: forwarding (see the warnings for
1545: .Cm ForwardX11
1546: in
1.19 jmc 1547: .Xr ssh_config 5 ) .
1.13 stevesk 1548: A system administrator may have a stance in which they want to
1549: protect clients that may expose themselves to attack by unwittingly
1550: requesting X11 forwarding, which can warrant a
1.237 jmc 1551: .Cm no
1.13 stevesk 1552: setting.
1553: .Pp
1554: Note that disabling X11 forwarding does not prevent users from
1555: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1556: .It Cm X11UseLocalhost
1557: Specifies whether
1.52 jmc 1558: .Xr sshd 8
1.1 stevesk 1559: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1560: the wildcard address.
1561: By default,
1.52 jmc 1562: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1563: hostname part of the
1564: .Ev DISPLAY
1565: environment variable to
1.237 jmc 1566: .Cm localhost .
1.8 stevesk 1567: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1568: However, some older X11 clients may not function with this
1569: configuration.
1570: .Cm X11UseLocalhost
1571: may be set to
1.237 jmc 1572: .Cm no
1.1 stevesk 1573: to specify that the forwarding server should be bound to the wildcard
1574: address.
1575: The argument must be
1.237 jmc 1576: .Cm yes
1.1 stevesk 1577: or
1.237 jmc 1578: .Cm no .
1.1 stevesk 1579: The default is
1.237 jmc 1580: .Cm yes .
1.1 stevesk 1581: .It Cm XAuthLocation
1.11 stevesk 1582: Specifies the full pathname of the
1.1 stevesk 1583: .Xr xauth 1
1.195 dtucker 1584: program, or
1.237 jmc 1585: .Cm none
1.195 dtucker 1586: to not use one.
1.1 stevesk 1587: The default is
1588: .Pa /usr/X11R6/bin/xauth .
1589: .El
1.55 jmc 1590: .Sh TIME FORMATS
1.53 jmc 1591: .Xr sshd 8
1.1 stevesk 1592: command-line arguments and configuration file options that specify time
1593: may be expressed using a sequence of the form:
1594: .Sm off
1.7 stevesk 1595: .Ar time Op Ar qualifier ,
1.1 stevesk 1596: .Sm on
1597: where
1598: .Ar time
1599: is a positive integer value and
1600: .Ar qualifier
1601: is one of the following:
1602: .Pp
1603: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1604: .It Aq Cm none
1.1 stevesk 1605: seconds
1606: .It Cm s | Cm S
1607: seconds
1608: .It Cm m | Cm M
1609: minutes
1610: .It Cm h | Cm H
1611: hours
1612: .It Cm d | Cm D
1613: days
1614: .It Cm w | Cm W
1615: weeks
1616: .El
1617: .Pp
1618: Each member of the sequence is added together to calculate
1619: the total time value.
1620: .Pp
1621: Time format examples:
1622: .Pp
1623: .Bl -tag -width Ds -compact -offset indent
1624: .It 600
1625: 600 seconds (10 minutes)
1626: .It 10m
1627: 10 minutes
1628: .It 1h30m
1629: 1 hour 30 minutes (90 minutes)
1630: .El
1.235 jmc 1631: .Sh TOKENS
1632: Arguments to some keywords can make use of tokens,
1633: which are expanded at runtime:
1634: .Pp
1635: .Bl -tag -width XXXX -offset indent -compact
1636: .It %%
1637: A literal
1638: .Sq % .
1.257 djm 1639: .It \&%D
1640: The routing domain in which the incoming connection was received.
1.235 jmc 1641: .It %F
1642: The fingerprint of the CA key.
1643: .It %f
1644: The fingerprint of the key or certificate.
1645: .It %h
1646: The home directory of the user.
1647: .It %i
1648: The key ID in the certificate.
1649: .It %K
1650: The base64-encoded CA key.
1651: .It %k
1652: The base64-encoded key or certificate for authentication.
1653: .It %s
1654: The serial number of the certificate.
1655: .It \&%T
1656: The type of the CA key.
1657: .It %t
1658: The key or certificate type.
1659: .It %u
1660: The username.
1661: .El
1662: .Pp
1663: .Cm AuthorizedKeysCommand
1.241 jmc 1664: accepts the tokens %%, %f, %h, %k, %t, and %u.
1.235 jmc 1665: .Pp
1666: .Cm AuthorizedKeysFile
1667: accepts the tokens %%, %h, and %u.
1668: .Pp
1669: .Cm AuthorizedPrincipalsCommand
1.241 jmc 1670: accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, and %u.
1.235 jmc 1671: .Pp
1672: .Cm AuthorizedPrincipalsFile
1673: accepts the tokens %%, %h, and %u.
1674: .Pp
1675: .Cm ChrootDirectory
1676: accepts the tokens %%, %h, and %u.
1.257 djm 1677: .Pp
1678: .Cm RoutingDomain
1679: accepts the token %D.
1.1 stevesk 1680: .Sh FILES
1681: .Bl -tag -width Ds
1682: .It Pa /etc/ssh/sshd_config
1683: Contains configuration data for
1.53 jmc 1684: .Xr sshd 8 .
1.1 stevesk 1685: This file should be writable by root only, but it is recommended
1686: (though not necessary) that it be world-readable.
1687: .El
1.19 jmc 1688: .Sh SEE ALSO
1.237 jmc 1689: .Xr sftp-server 8 ,
1.19 jmc 1690: .Xr sshd 8
1.1 stevesk 1691: .Sh AUTHORS
1.237 jmc 1692: .An -nosplit
1.1 stevesk 1693: OpenSSH is a derivative of the original and free
1.237 jmc 1694: ssh 1.2.12 release by
1695: .An Tatu Ylonen .
1696: .An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos ,
1697: .An Theo de Raadt
1698: and
1699: .An Dug Song
1.1 stevesk 1700: removed many bugs, re-added newer features and
1701: created OpenSSH.
1.237 jmc 1702: .An Markus Friedl
1703: contributed the support for SSH protocol versions 1.5 and 2.0.
1704: .An Niels Provos
1705: and
1706: .An Markus Friedl
1707: contributed support for privilege separation.
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