Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.248
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
18: .\" are met:
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
23: .\" documentation and/or other materials provided with the distribution.
24: .\"
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
27: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31: .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32: .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
35: .\"
1.248 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.247 2017/06/24 06:57:04 jmc Exp $
1.247 jmc 37: .Dd $Mdocdate: June 24 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 SYNOPSIS
1.71 jmc 44: .Nm /etc/ssh/sshd_config
1.1 stevesk 45: .Sh DESCRIPTION
1.53 jmc 46: .Xr sshd 8
1.1 stevesk 47: reads configuration data from
48: .Pa /etc/ssh/sshd_config
49: (or the file specified with
50: .Fl f
51: on the command line).
52: The file contains keyword-argument pairs, one per line.
53: Lines starting with
54: .Ql #
55: and empty lines are interpreted as comments.
1.56 dtucker 56: Arguments may optionally be enclosed in double quotes
57: .Pq \&"
58: in order to represent arguments containing spaces.
1.1 stevesk 59: .Pp
60: The possible
61: keywords and their meanings are as follows (note that
62: keywords are case-insensitive and arguments are case-sensitive):
63: .Bl -tag -width Ds
1.30 djm 64: .It Cm AcceptEnv
65: Specifies what environment variables sent by the client will be copied into
66: the session's
67: .Xr environ 7 .
68: See
69: .Cm SendEnv
70: in
71: .Xr ssh_config 5
72: for how to configure the client.
1.219 jmc 73: The
1.197 dtucker 74: .Ev TERM
1.199 jmc 75: environment variable is always sent whenever the client
1.204 djm 76: requests a pseudo-terminal as it is required by the protocol.
1.30 djm 77: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 78: .Ql *
1.30 djm 79: and
80: .Ql \&? .
1.31 djm 81: Multiple environment variables may be separated by whitespace or spread
1.30 djm 82: across multiple
83: .Cm AcceptEnv
84: directives.
1.31 djm 85: Be warned that some environment variables could be used to bypass restricted
1.30 djm 86: user environments.
87: For this reason, care should be taken in the use of this directive.
88: The default is not to accept any environment variables.
1.37 djm 89: .It Cm AddressFamily
90: Specifies which address family should be used by
1.53 jmc 91: .Xr sshd 8 .
1.37 djm 92: Valid arguments are
1.237 jmc 93: .Cm any
94: (the default),
95: .Cm inet
1.52 jmc 96: (use IPv4 only), or
1.237 jmc 97: .Cm inet6
1.37 djm 98: (use IPv6 only).
1.89 jmc 99: .It Cm AllowAgentForwarding
100: Specifies whether
101: .Xr ssh-agent 1
102: forwarding is permitted.
103: The default is
1.237 jmc 104: .Cm yes .
1.89 jmc 105: Note that disabling agent forwarding does not improve security
106: unless users are also denied shell access, as they can always install
107: their own forwarders.
1.1 stevesk 108: .It Cm AllowGroups
109: This keyword can be followed by a list of group name patterns, separated
110: by spaces.
111: If specified, login is allowed only for users whose primary
112: group or supplementary group list matches one of the patterns.
113: Only group names are valid; a numerical group ID is not recognized.
114: By default, login is allowed for all groups.
1.54 jmc 115: The allow/deny directives are processed in the following order:
116: .Cm DenyUsers ,
117: .Cm AllowUsers ,
118: .Cm DenyGroups ,
119: and finally
120: .Cm AllowGroups .
1.49 jmc 121: .Pp
1.161 jmc 122: See PATTERNS in
1.49 jmc 123: .Xr ssh_config 5
124: for more information on patterns.
1.231 jmc 125: .It Cm AllowStreamLocalForwarding
126: Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
1.150 djm 127: The available options are
1.237 jmc 128: .Cm yes
129: (the default)
1.150 djm 130: or
1.237 jmc 131: .Cm all
1.231 jmc 132: to allow StreamLocal forwarding,
1.237 jmc 133: .Cm no
1.231 jmc 134: to prevent all StreamLocal forwarding,
1.237 jmc 135: .Cm local
1.150 djm 136: to allow local (from the perspective of
1.151 jmc 137: .Xr ssh 1 )
138: forwarding only or
1.237 jmc 139: .Cm remote
1.150 djm 140: to allow remote forwarding only.
1.231 jmc 141: Note that disabling StreamLocal forwarding does not improve security unless
1.1 stevesk 142: users are also denied shell access, as they can always install their
143: own forwarders.
1.231 jmc 144: .It Cm AllowTcpForwarding
145: Specifies whether TCP forwarding is permitted.
1.175 millert 146: The available options are
1.237 jmc 147: .Cm yes
148: (the default)
1.175 millert 149: or
1.237 jmc 150: .Cm all
1.231 jmc 151: to allow TCP forwarding,
1.237 jmc 152: .Cm no
1.231 jmc 153: to prevent all TCP forwarding,
1.237 jmc 154: .Cm local
1.175 millert 155: to allow local (from the perspective of
156: .Xr ssh 1 )
157: forwarding only or
1.237 jmc 158: .Cm remote
1.175 millert 159: to allow remote forwarding only.
1.231 jmc 160: Note that disabling TCP forwarding does not improve security unless
1.175 millert 161: users are also denied shell access, as they can always install their
162: own forwarders.
1.1 stevesk 163: .It Cm AllowUsers
164: This keyword can be followed by a list of user name patterns, separated
165: by spaces.
1.14 jmc 166: If specified, login is allowed only for user names that
1.1 stevesk 167: match one of the patterns.
168: Only user names are valid; a numerical user ID is not recognized.
169: By default, login is allowed for all users.
170: If the pattern takes the form USER@HOST then USER and HOST
171: are separately checked, restricting logins to particular
172: users from particular hosts.
1.222 jmc 173: HOST criteria may additionally contain addresses to match in CIDR
174: address/masklen format.
1.54 jmc 175: The allow/deny directives are processed in the following order:
176: .Cm DenyUsers ,
177: .Cm AllowUsers ,
178: .Cm DenyGroups ,
179: and finally
180: .Cm AllowGroups .
1.49 jmc 181: .Pp
1.161 jmc 182: See PATTERNS in
1.49 jmc 183: .Xr ssh_config 5
184: for more information on patterns.
1.149 djm 185: .It Cm AuthenticationMethods
186: Specifies the authentication methods that must be successfully completed
187: for a user to be granted access.
188: This option must be followed by one or more comma-separated lists of
1.224 djm 189: authentication method names, or by the single string
1.237 jmc 190: .Cm any
1.224 djm 191: to indicate the default behaviour of accepting any single authentication
1.225 jmc 192: method.
1.237 jmc 193: If the default is overridden, then successful authentication requires
1.224 djm 194: completion of every method in at least one of these lists.
1.149 djm 195: .Pp
1.237 jmc 196: For example,
197: .Qq publickey,password publickey,keyboard-interactive
1.149 djm 198: would require the user to complete public key authentication, followed by
199: either password or keyboard interactive authentication.
200: Only methods that are next in one or more lists are offered at each stage,
1.237 jmc 201: so for this example it would not be possible to attempt password or
1.149 djm 202: keyboard-interactive authentication before public key.
1.157 markus 203: .Pp
204: For keyboard interactive authentication it is also possible to
205: restrict authentication to a specific device by appending a
206: colon followed by the device identifier
1.237 jmc 207: .Cm bsdauth ,
208: .Cm pam ,
1.157 markus 209: or
1.237 jmc 210: .Cm skey ,
1.157 markus 211: depending on the server configuration.
212: For example,
1.237 jmc 213: .Qq keyboard-interactive:bsdauth
1.157 markus 214: would restrict keyboard interactive authentication to the
1.237 jmc 215: .Cm bsdauth
1.157 markus 216: device.
1.185 djm 217: .Pp
1.237 jmc 218: If the publickey method is listed more than once,
1.185 djm 219: .Xr sshd 8
220: verifies that keys that have been used successfully are not reused for
221: subsequent authentications.
1.237 jmc 222: For example,
223: .Qq publickey,publickey
224: requires successful authentication using two different public keys.
1.181 djm 225: .Pp
1.149 djm 226: Note that each authentication method listed should also be explicitly enabled
227: in the configuration.
1.146 djm 228: .It Cm AuthorizedKeysCommand
1.147 jmc 229: Specifies a program to be used to look up the user's public keys.
1.201 djm 230: The program must be owned by root, not writable by group or others and
231: specified by an absolute path.
232: Arguments to
233: .Cm AuthorizedKeysCommand
1.235 jmc 234: accept the tokens described in the
235: .Sx TOKENS
236: section.
237: If no arguments are specified then the username of the target user is used.
1.201 djm 238: .Pp
239: The program should produce on standard output zero or
1.235 jmc 240: more lines of authorized_keys output (see
241: .Sx AUTHORIZED_KEYS
242: in
1.147 jmc 243: .Xr sshd 8 ) .
1.235 jmc 244: If a key supplied by
245: .Cm AuthorizedKeysCommand
246: does not successfully authenticate
1.146 djm 247: and authorize the user then public key authentication continues using the usual
248: .Cm AuthorizedKeysFile
249: files.
1.237 jmc 250: By default, no
251: .Cm AuthorizedKeysCommand
252: is run.
1.146 djm 253: .It Cm AuthorizedKeysCommandUser
1.237 jmc 254: Specifies the user under whose account the
255: .Cm AuthorizedKeysCommand
256: is run.
1.146 djm 257: It is recommended to use a dedicated user that has no other role on the host
258: than running authorized keys commands.
1.186 djm 259: If
1.181 djm 260: .Cm AuthorizedKeysCommand
1.186 djm 261: is specified but
262: .Cm AuthorizedKeysCommandUser
263: is not, then
264: .Xr sshd 8
265: will refuse to start.
1.1 stevesk 266: .It Cm AuthorizedKeysFile
1.235 jmc 267: Specifies the file that contains the public keys used for user authentication.
1.124 djm 268: The format is described in the
1.235 jmc 269: .Sx AUTHORIZED_KEYS FILE FORMAT
1.124 djm 270: section of
271: .Xr sshd 8 .
1.235 jmc 272: Arguments to
1.1 stevesk 273: .Cm AuthorizedKeysFile
1.235 jmc 274: accept the tokens described in the
275: .Sx TOKENS
276: section.
1.1 stevesk 277: After expansion,
278: .Cm AuthorizedKeysFile
279: is taken to be an absolute path or one relative to the user's home
280: directory.
1.133 jmc 281: Multiple files may be listed, separated by whitespace.
1.212 djm 282: Alternately this option may be set to
1.237 jmc 283: .Cm none
1.212 djm 284: to skip checking for user keys in files.
1.133 jmc 285: The default is
1.237 jmc 286: .Qq .ssh/authorized_keys .ssh/authorized_keys2 .
1.202 djm 287: .It Cm AuthorizedPrincipalsCommand
288: Specifies a program to be used to generate the list of allowed
289: certificate principals as per
290: .Cm AuthorizedPrincipalsFile .
291: The program must be owned by root, not writable by group or others and
292: specified by an absolute path.
293: Arguments to
294: .Cm AuthorizedPrincipalsCommand
1.235 jmc 295: accept the tokens described in the
296: .Sx TOKENS
297: section.
298: If no arguments are specified then the username of the target user is used.
1.202 djm 299: .Pp
300: The program should produce on standard output zero or
301: more lines of
302: .Cm AuthorizedPrincipalsFile
303: output.
304: If either
305: .Cm AuthorizedPrincipalsCommand
306: or
307: .Cm AuthorizedPrincipalsFile
308: is specified, then certificates offered by the client for authentication
309: must contain a principal that is listed.
1.235 jmc 310: By default, no
311: .Cm AuthorizedPrincipalsCommand
312: is run.
1.202 djm 313: .It Cm AuthorizedPrincipalsCommandUser
1.235 jmc 314: Specifies the user under whose account the
315: .Cm AuthorizedPrincipalsCommand
316: is run.
1.202 djm 317: It is recommended to use a dedicated user that has no other role on the host
318: than running authorized principals commands.
319: If
320: .Cm AuthorizedPrincipalsCommand
321: is specified but
322: .Cm AuthorizedPrincipalsCommandUser
323: is not, then
324: .Xr sshd 8
325: will refuse to start.
1.121 djm 326: .It Cm AuthorizedPrincipalsFile
327: Specifies a file that lists principal names that are accepted for
328: certificate authentication.
329: When using certificates signed by a key listed in
330: .Cm TrustedUserCAKeys ,
331: this file lists names, one of which must appear in the certificate for it
332: to be accepted for authentication.
1.235 jmc 333: Names are listed one per line preceded by key options (as described in
334: .Sx AUTHORIZED_KEYS FILE FORMAT
335: in
1.125 jmc 336: .Xr sshd 8 ) .
1.124 djm 337: Empty lines and comments starting with
1.121 djm 338: .Ql #
339: are ignored.
340: .Pp
1.235 jmc 341: Arguments to
1.121 djm 342: .Cm AuthorizedPrincipalsFile
1.235 jmc 343: accept the tokens described in the
344: .Sx TOKENS
345: section.
1.121 djm 346: After expansion,
347: .Cm AuthorizedPrincipalsFile
1.235 jmc 348: is taken to be an absolute path or one relative to the user's home directory.
1.138 djm 349: The default is
1.237 jmc 350: .Cm none ,
1.138 djm 351: i.e. not to use a principals file \(en in this case, the username
1.121 djm 352: of the user must appear in a certificate's principals list for it to be
353: accepted.
1.235 jmc 354: .Pp
1.121 djm 355: Note that
356: .Cm AuthorizedPrincipalsFile
357: is only used when authentication proceeds using a CA listed in
358: .Cm TrustedUserCAKeys
359: and is not consulted for certification authorities trusted via
360: .Pa ~/.ssh/authorized_keys ,
361: though the
362: .Cm principals=
363: key option offers a similar facility (see
364: .Xr sshd 8
365: for details).
1.1 stevesk 366: .It Cm Banner
367: The contents of the specified file are sent to the remote user before
368: authentication is allowed.
1.78 djm 369: If the argument is
1.237 jmc 370: .Cm none
1.78 djm 371: then no banner is displayed.
1.1 stevesk 372: By default, no banner is displayed.
373: .It Cm ChallengeResponseAuthentication
1.50 jmc 374: Specifies whether challenge-response authentication is allowed.
1.1 stevesk 375: All authentication styles from
376: .Xr login.conf 5
377: are supported.
378: The default is
1.237 jmc 379: .Cm yes .
1.80 djm 380: .It Cm ChrootDirectory
1.113 stevesk 381: Specifies the pathname of a directory to
1.80 djm 382: .Xr chroot 2
383: to after authentication.
1.190 deraadt 384: At session startup
385: .Xr sshd 8
386: checks that all components of the pathname are root-owned directories
387: which are not writable by any other user or group.
1.106 stevesk 388: After the chroot,
389: .Xr sshd 8
390: changes the working directory to the user's home directory.
1.235 jmc 391: Arguments to
392: .Cm ChrootDirectory
393: accept the tokens described in the
394: .Sx TOKENS
395: section.
1.80 djm 396: .Pp
397: The
398: .Cm ChrootDirectory
399: must contain the necessary files and directories to support the
1.103 stevesk 400: user's session.
1.80 djm 401: For an interactive session this requires at least a shell, typically
402: .Xr sh 1 ,
403: and basic
404: .Pa /dev
405: nodes such as
406: .Xr null 4 ,
407: .Xr zero 4 ,
408: .Xr stdin 4 ,
409: .Xr stdout 4 ,
410: .Xr stderr 4 ,
1.180 jmc 411: and
1.80 djm 412: .Xr tty 4
413: devices.
1.237 jmc 414: For file transfer sessions using SFTP
415: no additional configuration of the environment is necessary if the in-process
416: sftp-server is used,
1.176 schwarze 417: though sessions which use logging may require
1.104 stevesk 418: .Pa /dev/log
1.176 schwarze 419: inside the chroot directory on some operating systems (see
1.104 stevesk 420: .Xr sftp-server 8
1.81 jmc 421: for details).
1.190 deraadt 422: .Pp
1.191 jmc 423: For safety, it is very important that the directory hierarchy be
1.190 deraadt 424: prevented from modification by other processes on the system (especially
425: those outside the jail).
426: Misconfiguration can lead to unsafe environments which
427: .Xr sshd 8
428: cannot detect.
1.80 djm 429: .Pp
1.215 djm 430: The default is
1.237 jmc 431: .Cm none ,
1.215 djm 432: indicating not to
1.80 djm 433: .Xr chroot 2 .
1.1 stevesk 434: .It Cm Ciphers
1.219 jmc 435: Specifies the ciphers allowed.
1.1 stevesk 436: Multiple ciphers must be comma-separated.
1.208 djm 437: If the specified value begins with a
438: .Sq +
439: character, then the specified ciphers will be appended to the default set
440: instead of replacing them.
1.242 djm 441: If the specified value begins with a
442: .Sq -
443: character, then the specified ciphers (including wildcards) will be removed
444: from the default set instead of replacing them.
1.208 djm 445: .Pp
1.167 djm 446: The supported ciphers are:
447: .Pp
1.173 naddy 448: .Bl -item -compact -offset indent
449: .It
450: 3des-cbc
451: .It
452: aes128-cbc
453: .It
454: aes192-cbc
455: .It
456: aes256-cbc
457: .It
458: aes128-ctr
459: .It
460: aes192-ctr
461: .It
462: aes256-ctr
463: .It
464: aes128-gcm@openssh.com
465: .It
466: aes256-gcm@openssh.com
467: .It
468: chacha20-poly1305@openssh.com
469: .El
1.167 djm 470: .Pp
1.52 jmc 471: The default is:
1.173 naddy 472: .Bd -literal -offset indent
1.211 jmc 473: chacha20-poly1305@openssh.com,
1.173 naddy 474: aes128-ctr,aes192-ctr,aes256-ctr,
1.211 jmc 475: aes128-gcm@openssh.com,aes256-gcm@openssh.com
1.1 stevesk 476: .Ed
1.167 djm 477: .Pp
1.237 jmc 478: The list of available ciphers may also be obtained using
479: .Qq ssh -Q cipher .
1.1 stevesk 480: .It Cm ClientAliveCountMax
1.237 jmc 481: Sets the number of client alive messages which may be sent without
1.52 jmc 482: .Xr sshd 8
1.17 jmc 483: receiving any messages back from the client.
484: If this threshold is reached while client alive messages are being sent,
1.52 jmc 485: sshd will disconnect the client, terminating the session.
1.17 jmc 486: It is important to note that the use of client alive messages is very
487: different from
1.237 jmc 488: .Cm TCPKeepAlive .
1.17 jmc 489: The client alive messages are sent through the encrypted channel
490: and therefore will not be spoofable.
491: The TCP keepalive option enabled by
1.27 markus 492: .Cm TCPKeepAlive
1.17 jmc 493: is spoofable.
494: The client alive mechanism is valuable when the client or
1.1 stevesk 495: server depend on knowing when a connection has become inactive.
496: .Pp
1.17 jmc 497: The default value is 3.
498: If
1.1 stevesk 499: .Cm ClientAliveInterval
1.237 jmc 500: is set to 15, and
1.1 stevesk 501: .Cm ClientAliveCountMax
1.52 jmc 502: is left at the default, unresponsive SSH clients
1.1 stevesk 503: will be disconnected after approximately 45 seconds.
1.42 djm 504: .It Cm ClientAliveInterval
505: Sets a timeout interval in seconds after which if no data has been received
506: from the client,
1.52 jmc 507: .Xr sshd 8
1.42 djm 508: will send a message through the encrypted
509: channel to request a response from the client.
510: The default
511: is 0, indicating that these messages will not be sent to the client.
1.3 markus 512: .It Cm Compression
1.236 djm 513: Specifies whether compression is enabled after
1.44 markus 514: the user has authenticated successfully.
1.3 markus 515: The argument must be
1.237 jmc 516: .Cm yes ,
517: .Cm delayed
1.236 djm 518: (a legacy synonym for
1.237 jmc 519: .Cm yes )
1.3 markus 520: or
1.237 jmc 521: .Cm no .
1.3 markus 522: The default is
1.237 jmc 523: .Cm yes .
1.1 stevesk 524: .It Cm DenyGroups
525: This keyword can be followed by a list of group name patterns, separated
526: by spaces.
527: Login is disallowed for users whose primary group or supplementary
528: group list matches one of the patterns.
529: Only group names are valid; a numerical group ID is not recognized.
530: By default, login is allowed for all groups.
1.54 jmc 531: The allow/deny directives are processed in the following order:
532: .Cm DenyUsers ,
533: .Cm AllowUsers ,
534: .Cm DenyGroups ,
535: and finally
536: .Cm AllowGroups .
1.49 jmc 537: .Pp
1.161 jmc 538: See PATTERNS in
1.49 jmc 539: .Xr ssh_config 5
540: for more information on patterns.
1.1 stevesk 541: .It Cm DenyUsers
542: This keyword can be followed by a list of user name patterns, separated
543: by spaces.
544: Login is disallowed for user names that match one of the patterns.
545: Only user names are valid; a numerical user ID is not recognized.
546: By default, login is allowed for all users.
547: If the pattern takes the form USER@HOST then USER and HOST
548: are separately checked, restricting logins to particular
549: users from particular hosts.
1.222 jmc 550: HOST criteria may additionally contain addresses to match in CIDR
551: address/masklen format.
1.54 jmc 552: The allow/deny directives are processed in the following order:
553: .Cm DenyUsers ,
554: .Cm AllowUsers ,
555: .Cm DenyGroups ,
556: and finally
557: .Cm AllowGroups .
1.49 jmc 558: .Pp
1.161 jmc 559: See PATTERNS in
1.49 jmc 560: .Xr ssh_config 5
561: for more information on patterns.
1.239 djm 562: .It Cm DisableForwarding
563: Disables all forwarding features, including X11,
564: .Xr ssh-agent 1 ,
565: TCP and StreamLocal.
566: This option overrides all other forwarding-related options and may
567: simplify restricted configurations.
1.246 djm 568: .It Cm ExposeAuthInfo
569: Enables writing a file containing a list of authentication methods and
570: public credentials (e.g. keys) used to authenticate the user.
1.247 jmc 571: The location of the file is exposed to the user session through the
1.248 ! djm 572: .Ev SSH_USER_AUTH
1.247 jmc 573: environment variable.
1.183 djm 574: .It Cm FingerprintHash
575: Specifies the hash algorithm used when logging key fingerprints.
576: Valid options are:
1.237 jmc 577: .Cm md5
1.183 djm 578: and
1.237 jmc 579: .Cm sha256 .
1.183 djm 580: The default is
1.237 jmc 581: .Cm sha256 .
1.67 dtucker 582: .It Cm ForceCommand
583: Forces the execution of the command specified by
584: .Cm ForceCommand ,
1.84 djm 585: ignoring any command supplied by the client and
586: .Pa ~/.ssh/rc
587: if present.
1.67 dtucker 588: The command is invoked by using the user's login shell with the -c option.
589: This applies to shell, command, or subsystem execution.
590: It is most useful inside a
591: .Cm Match
592: block.
593: The command originally supplied by the client is available in the
594: .Ev SSH_ORIGINAL_COMMAND
595: environment variable.
1.82 djm 596: Specifying a command of
1.237 jmc 597: .Cm internal-sftp
598: will force the use of an in-process SFTP server that requires no support
1.82 djm 599: files when used with
600: .Cm ChrootDirectory .
1.215 djm 601: The default is
1.237 jmc 602: .Cm none .
1.1 stevesk 603: .It Cm GatewayPorts
604: Specifies whether remote hosts are allowed to connect to ports
605: forwarded for the client.
606: By default,
1.52 jmc 607: .Xr sshd 8
1.15 jmc 608: binds remote port forwardings to the loopback address.
609: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 610: .Cm GatewayPorts
1.52 jmc 611: can be used to specify that sshd
1.39 djm 612: should allow remote port forwardings to bind to non-loopback addresses, thus
613: allowing other hosts to connect.
614: The argument may be
1.237 jmc 615: .Cm no
1.39 djm 616: to force remote port forwardings to be available to the local host only,
1.237 jmc 617: .Cm yes
1.39 djm 618: to force remote port forwardings to bind to the wildcard address, or
1.237 jmc 619: .Cm clientspecified
1.39 djm 620: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 621: The default is
1.237 jmc 622: .Cm no .
1.23 markus 623: .It Cm GSSAPIAuthentication
1.25 markus 624: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 625: The default is
1.237 jmc 626: .Cm no .
1.23 markus 627: .It Cm GSSAPICleanupCredentials
628: Specifies whether to automatically destroy the user's credentials cache
629: on logout.
630: The default is
1.237 jmc 631: .Cm yes .
1.203 djm 632: .It Cm GSSAPIStrictAcceptorCheck
633: Determines whether to be strict about the identity of the GSSAPI acceptor
634: a client authenticates against.
635: If set to
1.237 jmc 636: .Cm yes
637: then the client must authenticate against the host
1.203 djm 638: service on the current hostname.
639: If set to
1.237 jmc 640: .Cm no
1.203 djm 641: then the client may authenticate against any service key stored in the
642: machine's default store.
643: This facility is provided to assist with operation on multi homed machines.
644: The default is
1.237 jmc 645: .Cm yes .
1.189 djm 646: .It Cm HostbasedAcceptedKeyTypes
647: Specifies the key types that will be accepted for hostbased authentication
648: as a comma-separated pattern list.
1.208 djm 649: Alternately if the specified value begins with a
650: .Sq +
651: character, then the specified key types will be appended to the default set
652: instead of replacing them.
1.242 djm 653: If the specified value begins with a
654: .Sq -
655: character, then the specified key types (including wildcards) will be removed
656: from the default set instead of replacing them.
1.206 markus 657: The default for this option is:
658: .Bd -literal -offset 3n
659: ecdsa-sha2-nistp256-cert-v01@openssh.com,
660: ecdsa-sha2-nistp384-cert-v01@openssh.com,
661: ecdsa-sha2-nistp521-cert-v01@openssh.com,
662: ssh-ed25519-cert-v01@openssh.com,
663: ssh-rsa-cert-v01@openssh.com,
664: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.220 djm 665: ssh-ed25519,ssh-rsa
1.206 markus 666: .Ed
667: .Pp
1.237 jmc 668: The list of available key types may also be obtained using
669: .Qq ssh -Q key .
1.1 stevesk 670: .It Cm HostbasedAuthentication
671: Specifies whether rhosts or /etc/hosts.equiv authentication together
672: with successful public key client host authentication is allowed
1.50 jmc 673: (host-based authentication).
1.70 dtucker 674: The default is
1.237 jmc 675: .Cm no .
1.70 dtucker 676: .It Cm HostbasedUsesNameFromPacketOnly
677: Specifies whether or not the server will attempt to perform a reverse
678: name lookup when matching the name in the
679: .Pa ~/.shosts ,
680: .Pa ~/.rhosts ,
681: and
682: .Pa /etc/hosts.equiv
683: files during
684: .Cm HostbasedAuthentication .
685: A setting of
1.237 jmc 686: .Cm yes
1.70 dtucker 687: means that
688: .Xr sshd 8
689: uses the name supplied by the client rather than
690: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 691: The default is
1.237 jmc 692: .Cm no .
1.117 djm 693: .It Cm HostCertificate
694: Specifies a file containing a public host certificate.
695: The certificate's public key must match a private host key already specified
696: by
697: .Cm HostKey .
698: The default behaviour of
699: .Xr sshd 8
700: is not to load any certificates.
1.1 stevesk 701: .It Cm HostKey
702: Specifies a file containing a private host key
703: used by SSH.
1.229 naddy 704: The defaults are
1.126 djm 705: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 706: .Pa /etc/ssh/ssh_host_ecdsa_key ,
707: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 708: and
1.229 naddy 709: .Pa /etc/ssh/ssh_host_rsa_key .
1.206 markus 710: .Pp
1.1 stevesk 711: Note that
1.52 jmc 712: .Xr sshd 8
1.206 markus 713: will refuse to use a file if it is group/world-accessible
714: and that the
715: .Cm HostKeyAlgorithms
716: option restricts which of the keys are actually used by
717: .Xr sshd 8 .
718: .Pp
1.1 stevesk 719: It is possible to have multiple host key files.
1.162 markus 720: It is also possible to specify public host key files instead.
721: In this case operations on the private key will be delegated
722: to an
723: .Xr ssh-agent 1 .
724: .It Cm HostKeyAgent
725: Identifies the UNIX-domain socket used to communicate
726: with an agent that has access to the private host keys.
1.223 markus 727: If the string
1.237 jmc 728: .Qq SSH_AUTH_SOCK
1.162 markus 729: is specified, the location of the socket will be read from the
730: .Ev SSH_AUTH_SOCK
731: environment variable.
1.206 markus 732: .It Cm HostKeyAlgorithms
1.219 jmc 733: Specifies the host key algorithms
1.206 markus 734: that the server offers.
735: The default for this option is:
736: .Bd -literal -offset 3n
737: ecdsa-sha2-nistp256-cert-v01@openssh.com,
738: ecdsa-sha2-nistp384-cert-v01@openssh.com,
739: ecdsa-sha2-nistp521-cert-v01@openssh.com,
740: ssh-ed25519-cert-v01@openssh.com,
741: ssh-rsa-cert-v01@openssh.com,
742: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.220 djm 743: ssh-ed25519,ssh-rsa
1.206 markus 744: .Ed
745: .Pp
1.237 jmc 746: The list of available key types may also be obtained using
747: .Qq ssh -Q key .
1.1 stevesk 748: .It Cm IgnoreRhosts
749: Specifies that
750: .Pa .rhosts
751: and
752: .Pa .shosts
753: files will not be used in
754: .Cm HostbasedAuthentication .
755: .Pp
756: .Pa /etc/hosts.equiv
757: and
758: .Pa /etc/shosts.equiv
759: are still used.
760: The default is
1.237 jmc 761: .Cm yes .
1.1 stevesk 762: .It Cm IgnoreUserKnownHosts
763: Specifies whether
1.52 jmc 764: .Xr sshd 8
1.1 stevesk 765: should ignore the user's
1.41 djm 766: .Pa ~/.ssh/known_hosts
1.1 stevesk 767: during
768: .Cm HostbasedAuthentication .
769: The default is
1.237 jmc 770: .Cm no .
1.129 djm 771: .It Cm IPQoS
772: Specifies the IPv4 type-of-service or DSCP class for the connection.
773: Accepted values are
1.237 jmc 774: .Cm af11 ,
775: .Cm af12 ,
776: .Cm af13 ,
777: .Cm af21 ,
778: .Cm af22 ,
779: .Cm af23 ,
780: .Cm af31 ,
781: .Cm af32 ,
782: .Cm af33 ,
783: .Cm af41 ,
784: .Cm af42 ,
785: .Cm af43 ,
786: .Cm cs0 ,
787: .Cm cs1 ,
788: .Cm cs2 ,
789: .Cm cs3 ,
790: .Cm cs4 ,
791: .Cm cs5 ,
792: .Cm cs6 ,
793: .Cm cs7 ,
794: .Cm ef ,
795: .Cm lowdelay ,
796: .Cm throughput ,
797: .Cm reliability ,
1.129 djm 798: or a numeric value.
1.131 djm 799: This option may take one or two arguments, separated by whitespace.
1.129 djm 800: If one argument is specified, it is used as the packet class unconditionally.
801: If two values are specified, the first is automatically selected for
802: interactive sessions and the second for non-interactive sessions.
803: The default is
1.237 jmc 804: .Cm lowdelay
1.129 djm 805: for interactive sessions and
1.237 jmc 806: .Cm throughput
1.129 djm 807: for non-interactive sessions.
1.171 jmc 808: .It Cm KbdInteractiveAuthentication
809: Specifies whether to allow keyboard-interactive authentication.
810: The argument to this keyword must be
1.237 jmc 811: .Cm yes
1.171 jmc 812: or
1.237 jmc 813: .Cm no .
1.171 jmc 814: The default is to use whatever value
815: .Cm ChallengeResponseAuthentication
816: is set to
817: (by default
1.237 jmc 818: .Cm yes ) .
1.1 stevesk 819: .It Cm KerberosAuthentication
1.24 markus 820: Specifies whether the password provided by the user for
1.1 stevesk 821: .Cm PasswordAuthentication
1.24 markus 822: will be validated through the Kerberos KDC.
1.1 stevesk 823: To use this option, the server needs a
824: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 825: The default is
1.237 jmc 826: .Cm no .
1.29 dtucker 827: .It Cm KerberosGetAFSToken
1.45 djm 828: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 829: an AFS token before accessing the user's home directory.
1.52 jmc 830: The default is
1.237 jmc 831: .Cm no .
1.1 stevesk 832: .It Cm KerberosOrLocalPasswd
1.52 jmc 833: If password authentication through Kerberos fails then
1.1 stevesk 834: the password will be validated via any additional local mechanism
835: such as
836: .Pa /etc/passwd .
1.52 jmc 837: The default is
1.237 jmc 838: .Cm yes .
1.1 stevesk 839: .It Cm KerberosTicketCleanup
840: Specifies whether to automatically destroy the user's ticket cache
841: file on logout.
1.52 jmc 842: The default is
1.237 jmc 843: .Cm yes .
1.127 djm 844: .It Cm KexAlgorithms
845: Specifies the available KEX (Key Exchange) algorithms.
846: Multiple algorithms must be comma-separated.
1.208 djm 847: Alternately if the specified value begins with a
848: .Sq +
849: character, then the specified methods will be appended to the default set
850: instead of replacing them.
1.242 djm 851: If the specified value begins with a
852: .Sq -
853: character, then the specified methods (including wildcards) will be removed
854: from the default set instead of replacing them.
1.173 naddy 855: The supported algorithms are:
856: .Pp
857: .Bl -item -compact -offset indent
858: .It
1.234 djm 859: curve25519-sha256
860: .It
1.173 naddy 861: curve25519-sha256@libssh.org
862: .It
863: diffie-hellman-group1-sha1
864: .It
865: diffie-hellman-group14-sha1
866: .It
867: diffie-hellman-group-exchange-sha1
868: .It
869: diffie-hellman-group-exchange-sha256
870: .It
871: ecdh-sha2-nistp256
872: .It
873: ecdh-sha2-nistp384
874: .It
875: ecdh-sha2-nistp521
876: .El
877: .Pp
878: The default is:
1.170 dtucker 879: .Bd -literal -offset indent
1.234 djm 880: curve25519-sha256,curve25519-sha256@libssh.org,
1.170 dtucker 881: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
882: diffie-hellman-group-exchange-sha256,
1.173 naddy 883: diffie-hellman-group14-sha1
1.170 dtucker 884: .Ed
1.188 djm 885: .Pp
1.237 jmc 886: The list of available key exchange algorithms may also be obtained using
887: .Qq ssh -Q kex .
1.1 stevesk 888: .It Cm ListenAddress
889: Specifies the local addresses
1.52 jmc 890: .Xr sshd 8
1.1 stevesk 891: should listen on.
892: The following forms may be used:
893: .Pp
894: .Bl -item -offset indent -compact
895: .It
896: .Cm ListenAddress
897: .Sm off
1.180 jmc 898: .Ar host | Ar IPv4_addr | Ar IPv6_addr
1.1 stevesk 899: .Sm on
900: .It
901: .Cm ListenAddress
902: .Sm off
1.180 jmc 903: .Ar host | Ar IPv4_addr : Ar port
1.1 stevesk 904: .Sm on
905: .It
906: .Cm ListenAddress
907: .Sm off
908: .Oo
1.180 jmc 909: .Ar host | Ar IPv6_addr Oc : Ar port
1.1 stevesk 910: .Sm on
911: .El
912: .Pp
913: If
914: .Ar port
915: is not specified,
1.200 dtucker 916: sshd will listen on the address and all
1.1 stevesk 917: .Cm Port
1.17 jmc 918: options specified.
919: The default is to listen on all local addresses.
1.15 jmc 920: Multiple
1.1 stevesk 921: .Cm ListenAddress
1.17 jmc 922: options are permitted.
1.1 stevesk 923: .It Cm LoginGraceTime
924: The server disconnects after this time if the user has not
925: successfully logged in.
926: If the value is 0, there is no time limit.
1.12 stevesk 927: The default is 120 seconds.
1.1 stevesk 928: .It Cm LogLevel
929: Gives the verbosity level that is used when logging messages from
1.53 jmc 930: .Xr sshd 8 .
1.1 stevesk 931: The possible values are:
1.52 jmc 932: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 933: The default is INFO.
934: DEBUG and DEBUG1 are equivalent.
935: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
936: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 937: .It Cm MACs
938: Specifies the available MAC (message authentication code) algorithms.
1.219 jmc 939: The MAC algorithm is used for data integrity protection.
1.1 stevesk 940: Multiple algorithms must be comma-separated.
1.208 djm 941: If the specified value begins with a
942: .Sq +
943: character, then the specified algorithms will be appended to the default set
944: instead of replacing them.
1.242 djm 945: If the specified value begins with a
946: .Sq -
947: character, then the specified algorithms (including wildcards) will be removed
948: from the default set instead of replacing them.
1.208 djm 949: .Pp
1.152 markus 950: The algorithms that contain
1.237 jmc 951: .Qq -etm
1.152 markus 952: calculate the MAC after encryption (encrypt-then-mac).
953: These are considered safer and their use recommended.
1.173 naddy 954: The supported MACs are:
955: .Pp
956: .Bl -item -compact -offset indent
957: .It
958: hmac-md5
959: .It
960: hmac-md5-96
961: .It
962: hmac-sha1
963: .It
964: hmac-sha1-96
965: .It
966: hmac-sha2-256
967: .It
968: hmac-sha2-512
969: .It
970: umac-64@openssh.com
971: .It
972: umac-128@openssh.com
973: .It
974: hmac-md5-etm@openssh.com
975: .It
976: hmac-md5-96-etm@openssh.com
977: .It
978: hmac-sha1-etm@openssh.com
979: .It
980: hmac-sha1-96-etm@openssh.com
981: .It
982: hmac-sha2-256-etm@openssh.com
983: .It
984: hmac-sha2-512-etm@openssh.com
985: .It
986: umac-64-etm@openssh.com
987: .It
988: umac-128-etm@openssh.com
989: .El
990: .Pp
1.52 jmc 991: The default is:
1.77 jmc 992: .Bd -literal -offset indent
1.152 markus 993: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
994: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.217 djm 995: hmac-sha1-etm@openssh.com,
1.173 naddy 996: umac-64@openssh.com,umac-128@openssh.com,
1.217 djm 997: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.77 jmc 998: .Ed
1.188 djm 999: .Pp
1.237 jmc 1000: The list of available MAC algorithms may also be obtained using
1001: .Qq ssh -Q mac .
1.60 dtucker 1002: .It Cm Match
1.61 jmc 1003: Introduces a conditional block.
1.65 dtucker 1004: If all of the criteria on the
1.60 dtucker 1005: .Cm Match
1.65 dtucker 1006: line are satisfied, the keywords on the following lines override those
1007: set in the global section of the config file, until either another
1.60 dtucker 1008: .Cm Match
1.65 dtucker 1009: line or the end of the file.
1.172 djm 1010: If a keyword appears in multiple
1011: .Cm Match
1.177 sobrado 1012: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1013: applied.
1.91 djm 1014: .Pp
1.61 jmc 1015: The arguments to
1.60 dtucker 1016: .Cm Match
1.163 dtucker 1017: are one or more criteria-pattern pairs or the single token
1018: .Cm All
1019: which matches all criteria.
1.60 dtucker 1020: The available criteria are
1021: .Cm User ,
1.69 dtucker 1022: .Cm Group ,
1.60 dtucker 1023: .Cm Host ,
1.139 dtucker 1024: .Cm LocalAddress ,
1025: .Cm LocalPort ,
1.60 dtucker 1026: and
1027: .Cm Address .
1.91 djm 1028: The match patterns may consist of single entries or comma-separated
1029: lists and may use the wildcard and negation operators described in the
1.237 jmc 1030: .Sx PATTERNS
1031: section of
1.92 djm 1032: .Xr ssh_config 5 .
1.91 djm 1033: .Pp
1034: The patterns in an
1035: .Cm Address
1036: criteria may additionally contain addresses to match in CIDR
1.237 jmc 1037: address/masklen format,
1038: such as 192.0.2.0/24 or 2001:db8::/32.
1.91 djm 1039: Note that the mask length provided must be consistent with the address -
1040: it is an error to specify a mask length that is too long for the address
1.93 jmc 1041: or one with bits set in this host portion of the address.
1.237 jmc 1042: For example, 192.0.2.0/33 and 192.0.2.0/8, respectively.
1.91 djm 1043: .Pp
1.60 dtucker 1044: Only a subset of keywords may be used on the lines following a
1045: .Cm Match
1046: keyword.
1047: Available keywords are
1.142 jmc 1048: .Cm AcceptEnv ,
1.99 okan 1049: .Cm AllowAgentForwarding ,
1.142 jmc 1050: .Cm AllowGroups ,
1.193 djm 1051: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1052: .Cm AllowTcpForwarding ,
1.141 markus 1053: .Cm AllowUsers ,
1.149 djm 1054: .Cm AuthenticationMethods ,
1.146 djm 1055: .Cm AuthorizedKeysCommand ,
1056: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1057: .Cm AuthorizedKeysFile ,
1.214 djm 1058: .Cm AuthorizedPrincipalsCommand ,
1059: .Cm AuthorizedPrincipalsCommandUser ,
1.123 djm 1060: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1061: .Cm Banner ,
1.85 djm 1062: .Cm ChrootDirectory ,
1.238 markus 1063: .Cm ClientAliveCountMax ,
1064: .Cm ClientAliveInterval ,
1.141 markus 1065: .Cm DenyGroups ,
1066: .Cm DenyUsers ,
1.67 dtucker 1067: .Cm ForceCommand ,
1.194 djm 1068: .Cm GatewayPorts ,
1.193 djm 1069: .Cm GSSAPIAuthentication ,
1.189 djm 1070: .Cm HostbasedAcceptedKeyTypes ,
1.87 djm 1071: .Cm HostbasedAuthentication ,
1.123 djm 1072: .Cm HostbasedUsesNameFromPacketOnly ,
1.193 djm 1073: .Cm IPQoS ,
1.74 jmc 1074: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1075: .Cm KerberosAuthentication ,
1.245 djm 1076: .Cm LogLevel ,
1.95 dtucker 1077: .Cm MaxAuthTries ,
1.94 dtucker 1078: .Cm MaxSessions ,
1.72 dtucker 1079: .Cm PasswordAuthentication ,
1.97 djm 1080: .Cm PermitEmptyPasswords ,
1.66 dtucker 1081: .Cm PermitOpen ,
1.79 dtucker 1082: .Cm PermitRootLogin ,
1.164 djm 1083: .Cm PermitTTY ,
1.123 djm 1084: .Cm PermitTunnel ,
1.174 djm 1085: .Cm PermitUserRC ,
1.189 djm 1086: .Cm PubkeyAcceptedKeyTypes ,
1.107 dtucker 1087: .Cm PubkeyAuthentication ,
1.159 dtucker 1088: .Cm RekeyLimit ,
1.193 djm 1089: .Cm RevokedKeys ,
1090: .Cm StreamLocalBindMask ,
1091: .Cm StreamLocalBindUnlink ,
1092: .Cm TrustedUserCAKeys ,
1.66 dtucker 1093: .Cm X11DisplayOffset ,
1.101 djm 1094: .Cm X11Forwarding
1.60 dtucker 1095: and
1.102 djm 1096: .Cm X11UseLocalHost .
1.33 dtucker 1097: .It Cm MaxAuthTries
1098: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1099: connection.
1100: Once the number of failures reaches half this value,
1101: additional failures are logged.
1102: The default is 6.
1.90 djm 1103: .It Cm MaxSessions
1.216 djm 1104: Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
1105: sessions permitted per network connection.
1106: Multiple sessions may be established by clients that support connection
1107: multiplexing.
1108: Setting
1109: .Cm MaxSessions
1110: to 1 will effectively disable session multiplexing, whereas setting it to 0
1111: will prevent all shell, login and subsystem sessions while still permitting
1112: forwarding.
1.90 djm 1113: The default is 10.
1.1 stevesk 1114: .It Cm MaxStartups
1115: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1116: SSH daemon.
1.1 stevesk 1117: Additional connections will be dropped until authentication succeeds or the
1118: .Cm LoginGraceTime
1119: expires for a connection.
1.156 dtucker 1120: The default is 10:30:100.
1.1 stevesk 1121: .Pp
1122: Alternatively, random early drop can be enabled by specifying
1123: the three colon separated values
1.237 jmc 1124: start:rate:full (e.g. "10:30:60").
1.53 jmc 1125: .Xr sshd 8
1.237 jmc 1126: will refuse connection attempts with a probability of rate/100 (30%)
1127: if there are currently start (10) unauthenticated connections.
1.1 stevesk 1128: The probability increases linearly and all connection attempts
1.237 jmc 1129: are refused if the number of unauthenticated connections reaches full (60).
1.1 stevesk 1130: .It Cm PasswordAuthentication
1131: Specifies whether password authentication is allowed.
1132: The default is
1.237 jmc 1133: .Cm yes .
1.1 stevesk 1134: .It Cm PermitEmptyPasswords
1135: When password authentication is allowed, it specifies whether the
1136: server allows login to accounts with empty password strings.
1137: The default is
1.237 jmc 1138: .Cm no .
1.62 dtucker 1139: .It Cm PermitOpen
1140: Specifies the destinations to which TCP port forwarding is permitted.
1141: The forwarding specification must be one of the following forms:
1142: .Pp
1143: .Bl -item -offset indent -compact
1144: .It
1145: .Cm PermitOpen
1146: .Sm off
1147: .Ar host : port
1148: .Sm on
1149: .It
1150: .Cm PermitOpen
1151: .Sm off
1152: .Ar IPv4_addr : port
1153: .Sm on
1154: .It
1155: .Cm PermitOpen
1156: .Sm off
1157: .Ar \&[ IPv6_addr \&] : port
1158: .Sm on
1159: .El
1160: .Pp
1.68 dtucker 1161: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1162: An argument of
1.237 jmc 1163: .Cm any
1.62 dtucker 1164: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1165: An argument of
1.237 jmc 1166: .Cm none
1.140 dtucker 1167: can be used to prohibit all forwarding requests.
1.227 jmc 1168: The wildcard
1.237 jmc 1169: .Sq *
1.227 jmc 1170: can be used for host or port to allow all hosts or ports, respectively.
1.63 jmc 1171: By default all port forwarding requests are permitted.
1.1 stevesk 1172: .It Cm PermitRootLogin
1.38 jmc 1173: Specifies whether root can log in using
1.1 stevesk 1174: .Xr ssh 1 .
1175: The argument must be
1.237 jmc 1176: .Cm yes ,
1177: .Cm prohibit-password ,
1178: .Cm without-password ,
1179: .Cm forced-commands-only ,
1.1 stevesk 1180: or
1.237 jmc 1181: .Cm no .
1.1 stevesk 1182: The default is
1.237 jmc 1183: .Cm prohibit-password .
1.1 stevesk 1184: .Pp
1185: If this option is set to
1.237 jmc 1186: .Cm prohibit-password
1.210 deraadt 1187: or
1.237 jmc 1188: .Cm without-password ,
1.210 deraadt 1189: password and keyboard-interactive authentication are disabled for root.
1.1 stevesk 1190: .Pp
1191: If this option is set to
1.237 jmc 1192: .Cm forced-commands-only ,
1.1 stevesk 1193: root login with public key authentication will be allowed,
1194: but only if the
1195: .Ar command
1196: option has been specified
1197: (which may be useful for taking remote backups even if root login is
1.17 jmc 1198: normally not allowed).
1199: All other authentication methods are disabled for root.
1.1 stevesk 1200: .Pp
1201: If this option is set to
1.237 jmc 1202: .Cm no ,
1.38 jmc 1203: root is not allowed to log in.
1.231 jmc 1204: .It Cm PermitTTY
1205: Specifies whether
1206: .Xr pty 4
1207: allocation is permitted.
1208: The default is
1.237 jmc 1209: .Cm yes .
1.46 reyk 1210: .It Cm PermitTunnel
1211: Specifies whether
1212: .Xr tun 4
1213: device forwarding is allowed.
1.47 reyk 1214: The argument must be
1.237 jmc 1215: .Cm yes ,
1216: .Cm point-to-point
1.58 stevesk 1217: (layer 3),
1.237 jmc 1218: .Cm ethernet
1.58 stevesk 1219: (layer 2), or
1.237 jmc 1220: .Cm no .
1.58 stevesk 1221: Specifying
1.237 jmc 1222: .Cm yes
1.58 stevesk 1223: permits both
1.237 jmc 1224: .Cm point-to-point
1.58 stevesk 1225: and
1.237 jmc 1226: .Cm ethernet .
1.46 reyk 1227: The default is
1.237 jmc 1228: .Cm no .
1.178 djm 1229: .Pp
1230: Independent of this setting, the permissions of the selected
1231: .Xr tun 4
1232: device must allow access to the user.
1.6 markus 1233: .It Cm PermitUserEnvironment
1234: Specifies whether
1235: .Pa ~/.ssh/environment
1.9 stevesk 1236: and
1.6 markus 1237: .Cm environment=
1238: options in
1239: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1240: are processed by
1.52 jmc 1241: .Xr sshd 8 .
1.6 markus 1242: The default is
1.237 jmc 1243: .Cm no .
1.9 stevesk 1244: Enabling environment processing may enable users to bypass access
1245: restrictions in some configurations using mechanisms such as
1246: .Ev LD_PRELOAD .
1.174 djm 1247: .It Cm PermitUserRC
1248: Specifies whether any
1249: .Pa ~/.ssh/rc
1250: file is executed.
1251: The default is
1.237 jmc 1252: .Cm yes .
1.1 stevesk 1253: .It Cm PidFile
1.4 stevesk 1254: Specifies the file that contains the process ID of the
1.195 dtucker 1255: SSH daemon, or
1.237 jmc 1256: .Cm none
1.195 dtucker 1257: to not write one.
1.1 stevesk 1258: The default is
1259: .Pa /var/run/sshd.pid .
1260: .It Cm Port
1261: Specifies the port number that
1.52 jmc 1262: .Xr sshd 8
1.1 stevesk 1263: listens on.
1264: The default is 22.
1265: Multiple options of this type are permitted.
1266: See also
1267: .Cm ListenAddress .
1268: .It Cm PrintLastLog
1269: Specifies whether
1.52 jmc 1270: .Xr sshd 8
1.36 jaredy 1271: should print the date and time of the last user login when a user logs
1272: in interactively.
1.1 stevesk 1273: The default is
1.237 jmc 1274: .Cm yes .
1.1 stevesk 1275: .It Cm PrintMotd
1276: Specifies whether
1.52 jmc 1277: .Xr sshd 8
1.1 stevesk 1278: should print
1279: .Pa /etc/motd
1280: when a user logs in interactively.
1281: (On some systems it is also printed by the shell,
1282: .Pa /etc/profile ,
1283: or equivalent.)
1284: The default is
1.237 jmc 1285: .Cm yes .
1.189 djm 1286: .It Cm PubkeyAcceptedKeyTypes
1287: Specifies the key types that will be accepted for public key authentication
1288: as a comma-separated pattern list.
1.208 djm 1289: Alternately if the specified value begins with a
1290: .Sq +
1291: character, then the specified key types will be appended to the default set
1292: instead of replacing them.
1.242 djm 1293: If the specified value begins with a
1294: .Sq -
1295: character, then the specified key types (including wildcards) will be removed
1296: from the default set instead of replacing them.
1.206 markus 1297: The default for this option is:
1298: .Bd -literal -offset 3n
1299: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1300: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1301: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1302: ssh-ed25519-cert-v01@openssh.com,
1303: ssh-rsa-cert-v01@openssh.com,
1304: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.220 djm 1305: ssh-ed25519,ssh-rsa
1.206 markus 1306: .Ed
1307: .Pp
1.237 jmc 1308: The list of available key types may also be obtained using
1309: .Qq ssh -Q key .
1.1 stevesk 1310: .It Cm PubkeyAuthentication
1311: Specifies whether public key authentication is allowed.
1312: The default is
1.237 jmc 1313: .Cm yes .
1.159 dtucker 1314: .It Cm RekeyLimit
1315: Specifies the maximum amount of data that may be transmitted before the
1316: session key is renegotiated, optionally followed a maximum amount of
1317: time that may pass before the session key is renegotiated.
1318: The first argument is specified in bytes and may have a suffix of
1319: .Sq K ,
1320: .Sq M ,
1321: or
1322: .Sq G
1323: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1324: The default is between
1325: .Sq 1G
1326: and
1327: .Sq 4G ,
1328: depending on the cipher.
1329: The optional second value is specified in seconds and may use any of the
1330: units documented in the
1331: .Sx TIME FORMATS
1.160 jmc 1332: section.
1.159 dtucker 1333: The default value for
1334: .Cm RekeyLimit
1335: is
1.237 jmc 1336: .Cm default none ,
1.159 dtucker 1337: which means that rekeying is performed after the cipher's default amount
1338: of data has been sent or received and no time based rekeying is done.
1.118 djm 1339: .It Cm RevokedKeys
1.195 dtucker 1340: Specifies revoked public keys file, or
1.237 jmc 1341: .Cm none
1.195 dtucker 1342: to not use one.
1.118 djm 1343: Keys listed in this file will be refused for public key authentication.
1344: Note that if this file is not readable, then public key authentication will
1345: be refused for all users.
1.154 djm 1346: Keys may be specified as a text file, listing one public key per line, or as
1347: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1348: .Xr ssh-keygen 1 .
1.161 jmc 1349: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1350: .Xr ssh-keygen 1 .
1.175 millert 1351: .It Cm StreamLocalBindMask
1352: Sets the octal file creation mode mask
1353: .Pq umask
1354: used when creating a Unix-domain socket file for local or remote
1355: port forwarding.
1356: This option is only used for port forwarding to a Unix-domain socket file.
1357: .Pp
1358: The default value is 0177, which creates a Unix-domain socket file that is
1359: readable and writable only by the owner.
1360: Note that not all operating systems honor the file mode on Unix-domain
1361: socket files.
1362: .It Cm StreamLocalBindUnlink
1363: Specifies whether to remove an existing Unix-domain socket file for local
1364: or remote port forwarding before creating a new one.
1365: If the socket file already exists and
1366: .Cm StreamLocalBindUnlink
1367: is not enabled,
1368: .Nm sshd
1369: will be unable to forward the port to the Unix-domain socket file.
1370: This option is only used for port forwarding to a Unix-domain socket file.
1371: .Pp
1372: The argument must be
1.237 jmc 1373: .Cm yes
1.175 millert 1374: or
1.237 jmc 1375: .Cm no .
1.175 millert 1376: The default is
1.237 jmc 1377: .Cm no .
1.1 stevesk 1378: .It Cm StrictModes
1379: Specifies whether
1.52 jmc 1380: .Xr sshd 8
1.1 stevesk 1381: should check file modes and ownership of the
1382: user's files and home directory before accepting login.
1383: This is normally desirable because novices sometimes accidentally leave their
1384: directory or files world-writable.
1385: The default is
1.237 jmc 1386: .Cm yes .
1.112 djm 1387: Note that this does not apply to
1388: .Cm ChrootDirectory ,
1389: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1390: .It Cm Subsystem
1.51 jmc 1391: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1392: Arguments should be a subsystem name and a command (with optional arguments)
1393: to execute upon subsystem request.
1.80 djm 1394: .Pp
1.1 stevesk 1395: The command
1.237 jmc 1396: .Cm sftp-server
1397: implements the SFTP file transfer subsystem.
1.80 djm 1398: .Pp
1399: Alternately the name
1.237 jmc 1400: .Cm internal-sftp
1401: implements an in-process SFTP server.
1.80 djm 1402: This may simplify configurations using
1403: .Cm ChrootDirectory
1404: to force a different filesystem root on clients.
1405: .Pp
1.1 stevesk 1406: By default no subsystems are defined.
1407: .It Cm SyslogFacility
1408: Gives the facility code that is used when logging messages from
1.53 jmc 1409: .Xr sshd 8 .
1.1 stevesk 1410: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1411: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1412: The default is AUTH.
1.27 markus 1413: .It Cm TCPKeepAlive
1414: Specifies whether the system should send TCP keepalive messages to the
1415: other side.
1416: If they are sent, death of the connection or crash of one
1417: of the machines will be properly noticed.
1418: However, this means that
1419: connections will die if the route is down temporarily, and some people
1420: find it annoying.
1421: On the other hand, if TCP keepalives are not sent,
1422: sessions may hang indefinitely on the server, leaving
1.237 jmc 1423: .Qq ghost
1.27 markus 1424: users and consuming server resources.
1425: .Pp
1426: The default is
1.237 jmc 1427: .Cm yes
1.27 markus 1428: (to send TCP keepalive messages), and the server will notice
1429: if the network goes down or the client host crashes.
1430: This avoids infinitely hanging sessions.
1431: .Pp
1432: To disable TCP keepalive messages, the value should be set to
1.237 jmc 1433: .Cm no .
1.118 djm 1434: .It Cm TrustedUserCAKeys
1435: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1436: trusted to sign user certificates for authentication, or
1.237 jmc 1437: .Cm none
1.195 dtucker 1438: to not use one.
1.119 jmc 1439: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1440: .Ql #
1441: are allowed.
1442: If a certificate is presented for authentication and has its signing CA key
1443: listed in this file, then it may be used for authentication for any user
1444: listed in the certificate's principals list.
1445: Note that certificates that lack a list of principals will not be permitted
1446: for authentication using
1447: .Cm TrustedUserCAKeys .
1.161 jmc 1448: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1449: .Xr ssh-keygen 1 .
1.18 markus 1450: .It Cm UseDNS
1451: Specifies whether
1.52 jmc 1452: .Xr sshd 8
1.207 djm 1453: should look up the remote host name, and to check that
1.18 markus 1454: the resolved host name for the remote IP address maps back to the
1455: very same IP address.
1.207 djm 1456: .Pp
1457: If this option is set to
1.237 jmc 1458: .Cm no
1.207 djm 1459: (the default) then only addresses and not host names may be used in
1.221 djm 1460: .Pa ~/.ssh/authorized_keys
1.207 djm 1461: .Cm from
1462: and
1.211 jmc 1463: .Nm
1.207 djm 1464: .Cm Match
1465: .Cm Host
1466: directives.
1.137 djm 1467: .It Cm VersionAddendum
1468: Optionally specifies additional text to append to the SSH protocol banner
1469: sent by the server upon connection.
1470: The default is
1.237 jmc 1471: .Cm none .
1.1 stevesk 1472: .It Cm X11DisplayOffset
1473: Specifies the first display number available for
1.52 jmc 1474: .Xr sshd 8 Ns 's
1.1 stevesk 1475: X11 forwarding.
1.52 jmc 1476: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1477: The default is 10.
1478: .It Cm X11Forwarding
1479: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1480: The argument must be
1.237 jmc 1481: .Cm yes
1.13 stevesk 1482: or
1.237 jmc 1483: .Cm no .
1.1 stevesk 1484: The default is
1.237 jmc 1485: .Cm no .
1.13 stevesk 1486: .Pp
1487: When X11 forwarding is enabled, there may be additional exposure to
1488: the server and to client displays if the
1.52 jmc 1489: .Xr sshd 8
1.13 stevesk 1490: proxy display is configured to listen on the wildcard address (see
1.237 jmc 1491: .Cm X11UseLocalhost ) ,
1492: though this is not the default.
1.13 stevesk 1493: Additionally, the authentication spoofing and authentication data
1494: verification and substitution occur on the client side.
1495: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1496: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1497: forwarding (see the warnings for
1498: .Cm ForwardX11
1499: in
1.19 jmc 1500: .Xr ssh_config 5 ) .
1.13 stevesk 1501: A system administrator may have a stance in which they want to
1502: protect clients that may expose themselves to attack by unwittingly
1503: requesting X11 forwarding, which can warrant a
1.237 jmc 1504: .Cm no
1.13 stevesk 1505: setting.
1506: .Pp
1507: Note that disabling X11 forwarding does not prevent users from
1508: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1509: .It Cm X11UseLocalhost
1510: Specifies whether
1.52 jmc 1511: .Xr sshd 8
1.1 stevesk 1512: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1513: the wildcard address.
1514: By default,
1.52 jmc 1515: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1516: hostname part of the
1517: .Ev DISPLAY
1518: environment variable to
1.237 jmc 1519: .Cm localhost .
1.8 stevesk 1520: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1521: However, some older X11 clients may not function with this
1522: configuration.
1523: .Cm X11UseLocalhost
1524: may be set to
1.237 jmc 1525: .Cm no
1.1 stevesk 1526: to specify that the forwarding server should be bound to the wildcard
1527: address.
1528: The argument must be
1.237 jmc 1529: .Cm yes
1.1 stevesk 1530: or
1.237 jmc 1531: .Cm no .
1.1 stevesk 1532: The default is
1.237 jmc 1533: .Cm yes .
1.1 stevesk 1534: .It Cm XAuthLocation
1.11 stevesk 1535: Specifies the full pathname of the
1.1 stevesk 1536: .Xr xauth 1
1.195 dtucker 1537: program, or
1.237 jmc 1538: .Cm none
1.195 dtucker 1539: to not use one.
1.1 stevesk 1540: The default is
1541: .Pa /usr/X11R6/bin/xauth .
1542: .El
1.55 jmc 1543: .Sh TIME FORMATS
1.53 jmc 1544: .Xr sshd 8
1.1 stevesk 1545: command-line arguments and configuration file options that specify time
1546: may be expressed using a sequence of the form:
1547: .Sm off
1.7 stevesk 1548: .Ar time Op Ar qualifier ,
1.1 stevesk 1549: .Sm on
1550: where
1551: .Ar time
1552: is a positive integer value and
1553: .Ar qualifier
1554: is one of the following:
1555: .Pp
1556: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1557: .It Aq Cm none
1.1 stevesk 1558: seconds
1559: .It Cm s | Cm S
1560: seconds
1561: .It Cm m | Cm M
1562: minutes
1563: .It Cm h | Cm H
1564: hours
1565: .It Cm d | Cm D
1566: days
1567: .It Cm w | Cm W
1568: weeks
1569: .El
1570: .Pp
1571: Each member of the sequence is added together to calculate
1572: the total time value.
1573: .Pp
1574: Time format examples:
1575: .Pp
1576: .Bl -tag -width Ds -compact -offset indent
1577: .It 600
1578: 600 seconds (10 minutes)
1579: .It 10m
1580: 10 minutes
1581: .It 1h30m
1582: 1 hour 30 minutes (90 minutes)
1583: .El
1.235 jmc 1584: .Sh TOKENS
1585: Arguments to some keywords can make use of tokens,
1586: which are expanded at runtime:
1587: .Pp
1588: .Bl -tag -width XXXX -offset indent -compact
1589: .It %%
1590: A literal
1591: .Sq % .
1592: .It %F
1593: The fingerprint of the CA key.
1594: .It %f
1595: The fingerprint of the key or certificate.
1596: .It %h
1597: The home directory of the user.
1598: .It %i
1599: The key ID in the certificate.
1600: .It %K
1601: The base64-encoded CA key.
1602: .It %k
1603: The base64-encoded key or certificate for authentication.
1604: .It %s
1605: The serial number of the certificate.
1606: .It \&%T
1607: The type of the CA key.
1608: .It %t
1609: The key or certificate type.
1610: .It %u
1611: The username.
1612: .El
1613: .Pp
1614: .Cm AuthorizedKeysCommand
1.241 jmc 1615: accepts the tokens %%, %f, %h, %k, %t, and %u.
1.235 jmc 1616: .Pp
1617: .Cm AuthorizedKeysFile
1618: accepts the tokens %%, %h, and %u.
1619: .Pp
1620: .Cm AuthorizedPrincipalsCommand
1.241 jmc 1621: accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, and %u.
1.235 jmc 1622: .Pp
1623: .Cm AuthorizedPrincipalsFile
1624: accepts the tokens %%, %h, and %u.
1625: .Pp
1626: .Cm ChrootDirectory
1627: accepts the tokens %%, %h, and %u.
1.1 stevesk 1628: .Sh FILES
1629: .Bl -tag -width Ds
1630: .It Pa /etc/ssh/sshd_config
1631: Contains configuration data for
1.53 jmc 1632: .Xr sshd 8 .
1.1 stevesk 1633: This file should be writable by root only, but it is recommended
1634: (though not necessary) that it be world-readable.
1635: .El
1.19 jmc 1636: .Sh SEE ALSO
1.237 jmc 1637: .Xr sftp-server 8 ,
1.19 jmc 1638: .Xr sshd 8
1.1 stevesk 1639: .Sh AUTHORS
1.237 jmc 1640: .An -nosplit
1.1 stevesk 1641: OpenSSH is a derivative of the original and free
1.237 jmc 1642: ssh 1.2.12 release by
1643: .An Tatu Ylonen .
1644: .An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos ,
1645: .An Theo de Raadt
1646: and
1647: .An Dug Song
1.1 stevesk 1648: removed many bugs, re-added newer features and
1649: created OpenSSH.
1.237 jmc 1650: .An Markus Friedl
1651: contributed the support for SSH protocol versions 1.5 and 2.0.
1652: .An Niels Provos
1653: and
1654: .An Markus Friedl
1655: contributed support for privilege separation.
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