Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.242
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: .\"
1.242 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.241 2017/01/06 16:28:12 jmc Exp $
1.241 jmc 37: .Dd $Mdocdate: January 6 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: arcfour
469: .It
470: arcfour128
471: .It
472: arcfour256
473: .It
474: blowfish-cbc
475: .It
476: cast128-cbc
477: .It
478: chacha20-poly1305@openssh.com
479: .El
1.167 djm 480: .Pp
1.52 jmc 481: The default is:
1.173 naddy 482: .Bd -literal -offset indent
1.211 jmc 483: chacha20-poly1305@openssh.com,
1.173 naddy 484: aes128-ctr,aes192-ctr,aes256-ctr,
1.211 jmc 485: aes128-gcm@openssh.com,aes256-gcm@openssh.com
1.1 stevesk 486: .Ed
1.167 djm 487: .Pp
1.237 jmc 488: The list of available ciphers may also be obtained using
489: .Qq ssh -Q cipher .
1.1 stevesk 490: .It Cm ClientAliveCountMax
1.237 jmc 491: Sets the number of client alive messages which may be sent without
1.52 jmc 492: .Xr sshd 8
1.17 jmc 493: receiving any messages back from the client.
494: If this threshold is reached while client alive messages are being sent,
1.52 jmc 495: sshd will disconnect the client, terminating the session.
1.17 jmc 496: It is important to note that the use of client alive messages is very
497: different from
1.237 jmc 498: .Cm TCPKeepAlive .
1.17 jmc 499: The client alive messages are sent through the encrypted channel
500: and therefore will not be spoofable.
501: The TCP keepalive option enabled by
1.27 markus 502: .Cm TCPKeepAlive
1.17 jmc 503: is spoofable.
504: The client alive mechanism is valuable when the client or
1.1 stevesk 505: server depend on knowing when a connection has become inactive.
506: .Pp
1.17 jmc 507: The default value is 3.
508: If
1.1 stevesk 509: .Cm ClientAliveInterval
1.237 jmc 510: is set to 15, and
1.1 stevesk 511: .Cm ClientAliveCountMax
1.52 jmc 512: is left at the default, unresponsive SSH clients
1.1 stevesk 513: will be disconnected after approximately 45 seconds.
1.42 djm 514: .It Cm ClientAliveInterval
515: Sets a timeout interval in seconds after which if no data has been received
516: from the client,
1.52 jmc 517: .Xr sshd 8
1.42 djm 518: will send a message through the encrypted
519: channel to request a response from the client.
520: The default
521: is 0, indicating that these messages will not be sent to the client.
1.3 markus 522: .It Cm Compression
1.236 djm 523: Specifies whether compression is enabled after
1.44 markus 524: the user has authenticated successfully.
1.3 markus 525: The argument must be
1.237 jmc 526: .Cm yes ,
527: .Cm delayed
1.236 djm 528: (a legacy synonym for
1.237 jmc 529: .Cm yes )
1.3 markus 530: or
1.237 jmc 531: .Cm no .
1.3 markus 532: The default is
1.237 jmc 533: .Cm yes .
1.1 stevesk 534: .It Cm DenyGroups
535: This keyword can be followed by a list of group name patterns, separated
536: by spaces.
537: Login is disallowed for users whose primary group or supplementary
538: group list matches one of the patterns.
539: Only group names are valid; a numerical group ID is not recognized.
540: By default, login is allowed for all groups.
1.54 jmc 541: The allow/deny directives are processed in the following order:
542: .Cm DenyUsers ,
543: .Cm AllowUsers ,
544: .Cm DenyGroups ,
545: and finally
546: .Cm AllowGroups .
1.49 jmc 547: .Pp
1.161 jmc 548: See PATTERNS in
1.49 jmc 549: .Xr ssh_config 5
550: for more information on patterns.
1.1 stevesk 551: .It Cm DenyUsers
552: This keyword can be followed by a list of user name patterns, separated
553: by spaces.
554: Login is disallowed for user names that match one of the patterns.
555: Only user names are valid; a numerical user ID is not recognized.
556: By default, login is allowed for all users.
557: If the pattern takes the form USER@HOST then USER and HOST
558: are separately checked, restricting logins to particular
559: users from particular hosts.
1.222 jmc 560: HOST criteria may additionally contain addresses to match in CIDR
561: address/masklen format.
1.54 jmc 562: The allow/deny directives are processed in the following order:
563: .Cm DenyUsers ,
564: .Cm AllowUsers ,
565: .Cm DenyGroups ,
566: and finally
567: .Cm AllowGroups .
1.49 jmc 568: .Pp
1.161 jmc 569: See PATTERNS in
1.49 jmc 570: .Xr ssh_config 5
571: for more information on patterns.
1.239 djm 572: .It Cm DisableForwarding
573: Disables all forwarding features, including X11,
574: .Xr ssh-agent 1 ,
575: TCP and StreamLocal.
576: This option overrides all other forwarding-related options and may
577: simplify restricted configurations.
1.183 djm 578: .It Cm FingerprintHash
579: Specifies the hash algorithm used when logging key fingerprints.
580: Valid options are:
1.237 jmc 581: .Cm md5
1.183 djm 582: and
1.237 jmc 583: .Cm sha256 .
1.183 djm 584: The default is
1.237 jmc 585: .Cm sha256 .
1.67 dtucker 586: .It Cm ForceCommand
587: Forces the execution of the command specified by
588: .Cm ForceCommand ,
1.84 djm 589: ignoring any command supplied by the client and
590: .Pa ~/.ssh/rc
591: if present.
1.67 dtucker 592: The command is invoked by using the user's login shell with the -c option.
593: This applies to shell, command, or subsystem execution.
594: It is most useful inside a
595: .Cm Match
596: block.
597: The command originally supplied by the client is available in the
598: .Ev SSH_ORIGINAL_COMMAND
599: environment variable.
1.82 djm 600: Specifying a command of
1.237 jmc 601: .Cm internal-sftp
602: will force the use of an in-process SFTP server that requires no support
1.82 djm 603: files when used with
604: .Cm ChrootDirectory .
1.215 djm 605: The default is
1.237 jmc 606: .Cm none .
1.1 stevesk 607: .It Cm GatewayPorts
608: Specifies whether remote hosts are allowed to connect to ports
609: forwarded for the client.
610: By default,
1.52 jmc 611: .Xr sshd 8
1.15 jmc 612: binds remote port forwardings to the loopback address.
613: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 614: .Cm GatewayPorts
1.52 jmc 615: can be used to specify that sshd
1.39 djm 616: should allow remote port forwardings to bind to non-loopback addresses, thus
617: allowing other hosts to connect.
618: The argument may be
1.237 jmc 619: .Cm no
1.39 djm 620: to force remote port forwardings to be available to the local host only,
1.237 jmc 621: .Cm yes
1.39 djm 622: to force remote port forwardings to bind to the wildcard address, or
1.237 jmc 623: .Cm clientspecified
1.39 djm 624: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 625: The default is
1.237 jmc 626: .Cm no .
1.23 markus 627: .It Cm GSSAPIAuthentication
1.25 markus 628: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 629: The default is
1.237 jmc 630: .Cm no .
1.23 markus 631: .It Cm GSSAPICleanupCredentials
632: Specifies whether to automatically destroy the user's credentials cache
633: on logout.
634: The default is
1.237 jmc 635: .Cm yes .
1.203 djm 636: .It Cm GSSAPIStrictAcceptorCheck
637: Determines whether to be strict about the identity of the GSSAPI acceptor
638: a client authenticates against.
639: If set to
1.237 jmc 640: .Cm yes
641: then the client must authenticate against the host
1.203 djm 642: service on the current hostname.
643: If set to
1.237 jmc 644: .Cm no
1.203 djm 645: then the client may authenticate against any service key stored in the
646: machine's default store.
647: This facility is provided to assist with operation on multi homed machines.
648: The default is
1.237 jmc 649: .Cm yes .
1.189 djm 650: .It Cm HostbasedAcceptedKeyTypes
651: Specifies the key types that will be accepted for hostbased authentication
652: as a comma-separated pattern list.
1.208 djm 653: Alternately if the specified value begins with a
654: .Sq +
655: character, then the specified key types will be appended to the default set
656: instead of replacing them.
1.242 ! djm 657: If the specified value begins with a
! 658: .Sq -
! 659: character, then the specified key types (including wildcards) will be removed
! 660: from the default set instead of replacing them.
1.206 markus 661: The default for this option is:
662: .Bd -literal -offset 3n
663: ecdsa-sha2-nistp256-cert-v01@openssh.com,
664: ecdsa-sha2-nistp384-cert-v01@openssh.com,
665: ecdsa-sha2-nistp521-cert-v01@openssh.com,
666: ssh-ed25519-cert-v01@openssh.com,
667: ssh-rsa-cert-v01@openssh.com,
668: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.220 djm 669: ssh-ed25519,ssh-rsa
1.206 markus 670: .Ed
671: .Pp
1.237 jmc 672: The list of available key types may also be obtained using
673: .Qq ssh -Q key .
1.1 stevesk 674: .It Cm HostbasedAuthentication
675: Specifies whether rhosts or /etc/hosts.equiv authentication together
676: with successful public key client host authentication is allowed
1.50 jmc 677: (host-based authentication).
1.70 dtucker 678: The default is
1.237 jmc 679: .Cm no .
1.70 dtucker 680: .It Cm HostbasedUsesNameFromPacketOnly
681: Specifies whether or not the server will attempt to perform a reverse
682: name lookup when matching the name in the
683: .Pa ~/.shosts ,
684: .Pa ~/.rhosts ,
685: and
686: .Pa /etc/hosts.equiv
687: files during
688: .Cm HostbasedAuthentication .
689: A setting of
1.237 jmc 690: .Cm yes
1.70 dtucker 691: means that
692: .Xr sshd 8
693: uses the name supplied by the client rather than
694: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 695: The default is
1.237 jmc 696: .Cm no .
1.117 djm 697: .It Cm HostCertificate
698: Specifies a file containing a public host certificate.
699: The certificate's public key must match a private host key already specified
700: by
701: .Cm HostKey .
702: The default behaviour of
703: .Xr sshd 8
704: is not to load any certificates.
1.1 stevesk 705: .It Cm HostKey
706: Specifies a file containing a private host key
707: used by SSH.
1.229 naddy 708: The defaults are
1.126 djm 709: .Pa /etc/ssh/ssh_host_dsa_key ,
1.169 naddy 710: .Pa /etc/ssh/ssh_host_ecdsa_key ,
711: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 712: and
1.229 naddy 713: .Pa /etc/ssh/ssh_host_rsa_key .
1.206 markus 714: .Pp
1.1 stevesk 715: Note that
1.52 jmc 716: .Xr sshd 8
1.206 markus 717: will refuse to use a file if it is group/world-accessible
718: and that the
719: .Cm HostKeyAlgorithms
720: option restricts which of the keys are actually used by
721: .Xr sshd 8 .
722: .Pp
1.1 stevesk 723: It is possible to have multiple host key files.
1.162 markus 724: It is also possible to specify public host key files instead.
725: In this case operations on the private key will be delegated
726: to an
727: .Xr ssh-agent 1 .
728: .It Cm HostKeyAgent
729: Identifies the UNIX-domain socket used to communicate
730: with an agent that has access to the private host keys.
1.223 markus 731: If the string
1.237 jmc 732: .Qq SSH_AUTH_SOCK
1.162 markus 733: is specified, the location of the socket will be read from the
734: .Ev SSH_AUTH_SOCK
735: environment variable.
1.206 markus 736: .It Cm HostKeyAlgorithms
1.219 jmc 737: Specifies the host key algorithms
1.206 markus 738: that the server offers.
739: The default for this option is:
740: .Bd -literal -offset 3n
741: ecdsa-sha2-nistp256-cert-v01@openssh.com,
742: ecdsa-sha2-nistp384-cert-v01@openssh.com,
743: ecdsa-sha2-nistp521-cert-v01@openssh.com,
744: ssh-ed25519-cert-v01@openssh.com,
745: ssh-rsa-cert-v01@openssh.com,
746: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.220 djm 747: ssh-ed25519,ssh-rsa
1.206 markus 748: .Ed
749: .Pp
1.237 jmc 750: The list of available key types may also be obtained using
751: .Qq ssh -Q key .
1.1 stevesk 752: .It Cm IgnoreRhosts
753: Specifies that
754: .Pa .rhosts
755: and
756: .Pa .shosts
757: files will not be used in
758: .Cm HostbasedAuthentication .
759: .Pp
760: .Pa /etc/hosts.equiv
761: and
762: .Pa /etc/shosts.equiv
763: are still used.
764: The default is
1.237 jmc 765: .Cm yes .
1.1 stevesk 766: .It Cm IgnoreUserKnownHosts
767: Specifies whether
1.52 jmc 768: .Xr sshd 8
1.1 stevesk 769: should ignore the user's
1.41 djm 770: .Pa ~/.ssh/known_hosts
1.1 stevesk 771: during
772: .Cm HostbasedAuthentication .
773: The default is
1.237 jmc 774: .Cm no .
1.129 djm 775: .It Cm IPQoS
776: Specifies the IPv4 type-of-service or DSCP class for the connection.
777: Accepted values are
1.237 jmc 778: .Cm af11 ,
779: .Cm af12 ,
780: .Cm af13 ,
781: .Cm af21 ,
782: .Cm af22 ,
783: .Cm af23 ,
784: .Cm af31 ,
785: .Cm af32 ,
786: .Cm af33 ,
787: .Cm af41 ,
788: .Cm af42 ,
789: .Cm af43 ,
790: .Cm cs0 ,
791: .Cm cs1 ,
792: .Cm cs2 ,
793: .Cm cs3 ,
794: .Cm cs4 ,
795: .Cm cs5 ,
796: .Cm cs6 ,
797: .Cm cs7 ,
798: .Cm ef ,
799: .Cm lowdelay ,
800: .Cm throughput ,
801: .Cm reliability ,
1.129 djm 802: or a numeric value.
1.131 djm 803: This option may take one or two arguments, separated by whitespace.
1.129 djm 804: If one argument is specified, it is used as the packet class unconditionally.
805: If two values are specified, the first is automatically selected for
806: interactive sessions and the second for non-interactive sessions.
807: The default is
1.237 jmc 808: .Cm lowdelay
1.129 djm 809: for interactive sessions and
1.237 jmc 810: .Cm throughput
1.129 djm 811: for non-interactive sessions.
1.171 jmc 812: .It Cm KbdInteractiveAuthentication
813: Specifies whether to allow keyboard-interactive authentication.
814: The argument to this keyword must be
1.237 jmc 815: .Cm yes
1.171 jmc 816: or
1.237 jmc 817: .Cm no .
1.171 jmc 818: The default is to use whatever value
819: .Cm ChallengeResponseAuthentication
820: is set to
821: (by default
1.237 jmc 822: .Cm yes ) .
1.1 stevesk 823: .It Cm KerberosAuthentication
1.24 markus 824: Specifies whether the password provided by the user for
1.1 stevesk 825: .Cm PasswordAuthentication
1.24 markus 826: will be validated through the Kerberos KDC.
1.1 stevesk 827: To use this option, the server needs a
828: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 829: The default is
1.237 jmc 830: .Cm no .
1.29 dtucker 831: .It Cm KerberosGetAFSToken
1.45 djm 832: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 833: an AFS token before accessing the user's home directory.
1.52 jmc 834: The default is
1.237 jmc 835: .Cm no .
1.1 stevesk 836: .It Cm KerberosOrLocalPasswd
1.52 jmc 837: If password authentication through Kerberos fails then
1.1 stevesk 838: the password will be validated via any additional local mechanism
839: such as
840: .Pa /etc/passwd .
1.52 jmc 841: The default is
1.237 jmc 842: .Cm yes .
1.1 stevesk 843: .It Cm KerberosTicketCleanup
844: Specifies whether to automatically destroy the user's ticket cache
845: file on logout.
1.52 jmc 846: The default is
1.237 jmc 847: .Cm yes .
1.127 djm 848: .It Cm KexAlgorithms
849: Specifies the available KEX (Key Exchange) algorithms.
850: Multiple algorithms must be comma-separated.
1.208 djm 851: Alternately if the specified value begins with a
852: .Sq +
853: character, then the specified methods will be appended to the default set
854: instead of replacing them.
1.242 ! djm 855: If the specified value begins with a
! 856: .Sq -
! 857: character, then the specified methods (including wildcards) will be removed
! 858: from the default set instead of replacing them.
1.173 naddy 859: The supported algorithms are:
860: .Pp
861: .Bl -item -compact -offset indent
862: .It
1.234 djm 863: curve25519-sha256
864: .It
1.173 naddy 865: curve25519-sha256@libssh.org
866: .It
867: diffie-hellman-group1-sha1
868: .It
869: diffie-hellman-group14-sha1
870: .It
871: diffie-hellman-group-exchange-sha1
872: .It
873: diffie-hellman-group-exchange-sha256
874: .It
875: ecdh-sha2-nistp256
876: .It
877: ecdh-sha2-nistp384
878: .It
879: ecdh-sha2-nistp521
880: .El
881: .Pp
882: The default is:
1.170 dtucker 883: .Bd -literal -offset indent
1.234 djm 884: curve25519-sha256,curve25519-sha256@libssh.org,
1.170 dtucker 885: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
886: diffie-hellman-group-exchange-sha256,
1.173 naddy 887: diffie-hellman-group14-sha1
1.170 dtucker 888: .Ed
1.188 djm 889: .Pp
1.237 jmc 890: The list of available key exchange algorithms may also be obtained using
891: .Qq ssh -Q kex .
1.1 stevesk 892: .It Cm ListenAddress
893: Specifies the local addresses
1.52 jmc 894: .Xr sshd 8
1.1 stevesk 895: should listen on.
896: The following forms may be used:
897: .Pp
898: .Bl -item -offset indent -compact
899: .It
900: .Cm ListenAddress
901: .Sm off
1.180 jmc 902: .Ar host | Ar IPv4_addr | Ar IPv6_addr
1.1 stevesk 903: .Sm on
904: .It
905: .Cm ListenAddress
906: .Sm off
1.180 jmc 907: .Ar host | Ar IPv4_addr : Ar port
1.1 stevesk 908: .Sm on
909: .It
910: .Cm ListenAddress
911: .Sm off
912: .Oo
1.180 jmc 913: .Ar host | Ar IPv6_addr Oc : Ar port
1.1 stevesk 914: .Sm on
915: .El
916: .Pp
917: If
918: .Ar port
919: is not specified,
1.200 dtucker 920: sshd will listen on the address and all
1.1 stevesk 921: .Cm Port
1.17 jmc 922: options specified.
923: The default is to listen on all local addresses.
1.15 jmc 924: Multiple
1.1 stevesk 925: .Cm ListenAddress
1.17 jmc 926: options are permitted.
1.1 stevesk 927: .It Cm LoginGraceTime
928: The server disconnects after this time if the user has not
929: successfully logged in.
930: If the value is 0, there is no time limit.
1.12 stevesk 931: The default is 120 seconds.
1.1 stevesk 932: .It Cm LogLevel
933: Gives the verbosity level that is used when logging messages from
1.53 jmc 934: .Xr sshd 8 .
1.1 stevesk 935: The possible values are:
1.52 jmc 936: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 937: The default is INFO.
938: DEBUG and DEBUG1 are equivalent.
939: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
940: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.1 stevesk 941: .It Cm MACs
942: Specifies the available MAC (message authentication code) algorithms.
1.219 jmc 943: The MAC algorithm is used for data integrity protection.
1.1 stevesk 944: Multiple algorithms must be comma-separated.
1.208 djm 945: If the specified value begins with a
946: .Sq +
947: character, then the specified algorithms will be appended to the default set
948: instead of replacing them.
1.242 ! djm 949: If the specified value begins with a
! 950: .Sq -
! 951: character, then the specified algorithms (including wildcards) will be removed
! 952: from the default set instead of replacing them.
1.208 djm 953: .Pp
1.152 markus 954: The algorithms that contain
1.237 jmc 955: .Qq -etm
1.152 markus 956: calculate the MAC after encryption (encrypt-then-mac).
957: These are considered safer and their use recommended.
1.173 naddy 958: The supported MACs are:
959: .Pp
960: .Bl -item -compact -offset indent
961: .It
962: hmac-md5
963: .It
964: hmac-md5-96
965: .It
966: hmac-ripemd160
967: .It
968: hmac-sha1
969: .It
970: hmac-sha1-96
971: .It
972: hmac-sha2-256
973: .It
974: hmac-sha2-512
975: .It
976: umac-64@openssh.com
977: .It
978: umac-128@openssh.com
979: .It
980: hmac-md5-etm@openssh.com
981: .It
982: hmac-md5-96-etm@openssh.com
983: .It
984: hmac-ripemd160-etm@openssh.com
985: .It
986: hmac-sha1-etm@openssh.com
987: .It
988: hmac-sha1-96-etm@openssh.com
989: .It
990: hmac-sha2-256-etm@openssh.com
991: .It
992: hmac-sha2-512-etm@openssh.com
993: .It
994: umac-64-etm@openssh.com
995: .It
996: umac-128-etm@openssh.com
997: .El
998: .Pp
1.52 jmc 999: The default is:
1.77 jmc 1000: .Bd -literal -offset indent
1.152 markus 1001: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1002: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.217 djm 1003: hmac-sha1-etm@openssh.com,
1.173 naddy 1004: umac-64@openssh.com,umac-128@openssh.com,
1.217 djm 1005: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.77 jmc 1006: .Ed
1.188 djm 1007: .Pp
1.237 jmc 1008: The list of available MAC algorithms may also be obtained using
1009: .Qq ssh -Q mac .
1.60 dtucker 1010: .It Cm Match
1.61 jmc 1011: Introduces a conditional block.
1.65 dtucker 1012: If all of the criteria on the
1.60 dtucker 1013: .Cm Match
1.65 dtucker 1014: line are satisfied, the keywords on the following lines override those
1015: set in the global section of the config file, until either another
1.60 dtucker 1016: .Cm Match
1.65 dtucker 1017: line or the end of the file.
1.172 djm 1018: If a keyword appears in multiple
1019: .Cm Match
1.177 sobrado 1020: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1021: applied.
1.91 djm 1022: .Pp
1.61 jmc 1023: The arguments to
1.60 dtucker 1024: .Cm Match
1.163 dtucker 1025: are one or more criteria-pattern pairs or the single token
1026: .Cm All
1027: which matches all criteria.
1.60 dtucker 1028: The available criteria are
1029: .Cm User ,
1.69 dtucker 1030: .Cm Group ,
1.60 dtucker 1031: .Cm Host ,
1.139 dtucker 1032: .Cm LocalAddress ,
1033: .Cm LocalPort ,
1.60 dtucker 1034: and
1035: .Cm Address .
1.91 djm 1036: The match patterns may consist of single entries or comma-separated
1037: lists and may use the wildcard and negation operators described in the
1.237 jmc 1038: .Sx PATTERNS
1039: section of
1.92 djm 1040: .Xr ssh_config 5 .
1.91 djm 1041: .Pp
1042: The patterns in an
1043: .Cm Address
1044: criteria may additionally contain addresses to match in CIDR
1.237 jmc 1045: address/masklen format,
1046: such as 192.0.2.0/24 or 2001:db8::/32.
1.91 djm 1047: Note that the mask length provided must be consistent with the address -
1048: it is an error to specify a mask length that is too long for the address
1.93 jmc 1049: or one with bits set in this host portion of the address.
1.237 jmc 1050: For example, 192.0.2.0/33 and 192.0.2.0/8, respectively.
1.91 djm 1051: .Pp
1.60 dtucker 1052: Only a subset of keywords may be used on the lines following a
1053: .Cm Match
1054: keyword.
1055: Available keywords are
1.142 jmc 1056: .Cm AcceptEnv ,
1.99 okan 1057: .Cm AllowAgentForwarding ,
1.142 jmc 1058: .Cm AllowGroups ,
1.193 djm 1059: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1060: .Cm AllowTcpForwarding ,
1.141 markus 1061: .Cm AllowUsers ,
1.149 djm 1062: .Cm AuthenticationMethods ,
1.146 djm 1063: .Cm AuthorizedKeysCommand ,
1064: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1065: .Cm AuthorizedKeysFile ,
1.214 djm 1066: .Cm AuthorizedPrincipalsCommand ,
1067: .Cm AuthorizedPrincipalsCommandUser ,
1.123 djm 1068: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1069: .Cm Banner ,
1.85 djm 1070: .Cm ChrootDirectory ,
1.238 markus 1071: .Cm ClientAliveCountMax ,
1072: .Cm ClientAliveInterval ,
1.141 markus 1073: .Cm DenyGroups ,
1074: .Cm DenyUsers ,
1.67 dtucker 1075: .Cm ForceCommand ,
1.194 djm 1076: .Cm GatewayPorts ,
1.193 djm 1077: .Cm GSSAPIAuthentication ,
1.189 djm 1078: .Cm HostbasedAcceptedKeyTypes ,
1.87 djm 1079: .Cm HostbasedAuthentication ,
1.123 djm 1080: .Cm HostbasedUsesNameFromPacketOnly ,
1.193 djm 1081: .Cm IPQoS ,
1.74 jmc 1082: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1083: .Cm KerberosAuthentication ,
1.95 dtucker 1084: .Cm MaxAuthTries ,
1.94 dtucker 1085: .Cm MaxSessions ,
1.72 dtucker 1086: .Cm PasswordAuthentication ,
1.97 djm 1087: .Cm PermitEmptyPasswords ,
1.66 dtucker 1088: .Cm PermitOpen ,
1.79 dtucker 1089: .Cm PermitRootLogin ,
1.164 djm 1090: .Cm PermitTTY ,
1.123 djm 1091: .Cm PermitTunnel ,
1.174 djm 1092: .Cm PermitUserRC ,
1.189 djm 1093: .Cm PubkeyAcceptedKeyTypes ,
1.107 dtucker 1094: .Cm PubkeyAuthentication ,
1.159 dtucker 1095: .Cm RekeyLimit ,
1.193 djm 1096: .Cm RevokedKeys ,
1097: .Cm StreamLocalBindMask ,
1098: .Cm StreamLocalBindUnlink ,
1099: .Cm TrustedUserCAKeys ,
1.66 dtucker 1100: .Cm X11DisplayOffset ,
1.101 djm 1101: .Cm X11Forwarding
1.60 dtucker 1102: and
1.102 djm 1103: .Cm X11UseLocalHost .
1.33 dtucker 1104: .It Cm MaxAuthTries
1105: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1106: connection.
1107: Once the number of failures reaches half this value,
1108: additional failures are logged.
1109: The default is 6.
1.90 djm 1110: .It Cm MaxSessions
1.216 djm 1111: Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
1112: sessions permitted per network connection.
1113: Multiple sessions may be established by clients that support connection
1114: multiplexing.
1115: Setting
1116: .Cm MaxSessions
1117: to 1 will effectively disable session multiplexing, whereas setting it to 0
1118: will prevent all shell, login and subsystem sessions while still permitting
1119: forwarding.
1.90 djm 1120: The default is 10.
1.1 stevesk 1121: .It Cm MaxStartups
1122: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1123: SSH daemon.
1.1 stevesk 1124: Additional connections will be dropped until authentication succeeds or the
1125: .Cm LoginGraceTime
1126: expires for a connection.
1.156 dtucker 1127: The default is 10:30:100.
1.1 stevesk 1128: .Pp
1129: Alternatively, random early drop can be enabled by specifying
1130: the three colon separated values
1.237 jmc 1131: start:rate:full (e.g. "10:30:60").
1.53 jmc 1132: .Xr sshd 8
1.237 jmc 1133: will refuse connection attempts with a probability of rate/100 (30%)
1134: if there are currently start (10) unauthenticated connections.
1.1 stevesk 1135: The probability increases linearly and all connection attempts
1.237 jmc 1136: are refused if the number of unauthenticated connections reaches full (60).
1.1 stevesk 1137: .It Cm PasswordAuthentication
1138: Specifies whether password authentication is allowed.
1139: The default is
1.237 jmc 1140: .Cm yes .
1.1 stevesk 1141: .It Cm PermitEmptyPasswords
1142: When password authentication is allowed, it specifies whether the
1143: server allows login to accounts with empty password strings.
1144: The default is
1.237 jmc 1145: .Cm no .
1.62 dtucker 1146: .It Cm PermitOpen
1147: Specifies the destinations to which TCP port forwarding is permitted.
1148: The forwarding specification must be one of the following forms:
1149: .Pp
1150: .Bl -item -offset indent -compact
1151: .It
1152: .Cm PermitOpen
1153: .Sm off
1154: .Ar host : port
1155: .Sm on
1156: .It
1157: .Cm PermitOpen
1158: .Sm off
1159: .Ar IPv4_addr : port
1160: .Sm on
1161: .It
1162: .Cm PermitOpen
1163: .Sm off
1164: .Ar \&[ IPv6_addr \&] : port
1165: .Sm on
1166: .El
1167: .Pp
1.68 dtucker 1168: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1169: An argument of
1.237 jmc 1170: .Cm any
1.62 dtucker 1171: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1172: An argument of
1.237 jmc 1173: .Cm none
1.140 dtucker 1174: can be used to prohibit all forwarding requests.
1.227 jmc 1175: The wildcard
1.237 jmc 1176: .Sq *
1.227 jmc 1177: can be used for host or port to allow all hosts or ports, respectively.
1.63 jmc 1178: By default all port forwarding requests are permitted.
1.1 stevesk 1179: .It Cm PermitRootLogin
1.38 jmc 1180: Specifies whether root can log in using
1.1 stevesk 1181: .Xr ssh 1 .
1182: The argument must be
1.237 jmc 1183: .Cm yes ,
1184: .Cm prohibit-password ,
1185: .Cm without-password ,
1186: .Cm forced-commands-only ,
1.1 stevesk 1187: or
1.237 jmc 1188: .Cm no .
1.1 stevesk 1189: The default is
1.237 jmc 1190: .Cm prohibit-password .
1.1 stevesk 1191: .Pp
1192: If this option is set to
1.237 jmc 1193: .Cm prohibit-password
1.210 deraadt 1194: or
1.237 jmc 1195: .Cm without-password ,
1.210 deraadt 1196: password and keyboard-interactive authentication are disabled for root.
1.1 stevesk 1197: .Pp
1198: If this option is set to
1.237 jmc 1199: .Cm forced-commands-only ,
1.1 stevesk 1200: root login with public key authentication will be allowed,
1201: but only if the
1202: .Ar command
1203: option has been specified
1204: (which may be useful for taking remote backups even if root login is
1.17 jmc 1205: normally not allowed).
1206: All other authentication methods are disabled for root.
1.1 stevesk 1207: .Pp
1208: If this option is set to
1.237 jmc 1209: .Cm no ,
1.38 jmc 1210: root is not allowed to log in.
1.231 jmc 1211: .It Cm PermitTTY
1212: Specifies whether
1213: .Xr pty 4
1214: allocation is permitted.
1215: The default is
1.237 jmc 1216: .Cm yes .
1.46 reyk 1217: .It Cm PermitTunnel
1218: Specifies whether
1219: .Xr tun 4
1220: device forwarding is allowed.
1.47 reyk 1221: The argument must be
1.237 jmc 1222: .Cm yes ,
1223: .Cm point-to-point
1.58 stevesk 1224: (layer 3),
1.237 jmc 1225: .Cm ethernet
1.58 stevesk 1226: (layer 2), or
1.237 jmc 1227: .Cm no .
1.58 stevesk 1228: Specifying
1.237 jmc 1229: .Cm yes
1.58 stevesk 1230: permits both
1.237 jmc 1231: .Cm point-to-point
1.58 stevesk 1232: and
1.237 jmc 1233: .Cm ethernet .
1.46 reyk 1234: The default is
1.237 jmc 1235: .Cm no .
1.178 djm 1236: .Pp
1237: Independent of this setting, the permissions of the selected
1238: .Xr tun 4
1239: device must allow access to the user.
1.6 markus 1240: .It Cm PermitUserEnvironment
1241: Specifies whether
1242: .Pa ~/.ssh/environment
1.9 stevesk 1243: and
1.6 markus 1244: .Cm environment=
1245: options in
1246: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1247: are processed by
1.52 jmc 1248: .Xr sshd 8 .
1.6 markus 1249: The default is
1.237 jmc 1250: .Cm no .
1.9 stevesk 1251: Enabling environment processing may enable users to bypass access
1252: restrictions in some configurations using mechanisms such as
1253: .Ev LD_PRELOAD .
1.174 djm 1254: .It Cm PermitUserRC
1255: Specifies whether any
1256: .Pa ~/.ssh/rc
1257: file is executed.
1258: The default is
1.237 jmc 1259: .Cm yes .
1.1 stevesk 1260: .It Cm PidFile
1.4 stevesk 1261: Specifies the file that contains the process ID of the
1.195 dtucker 1262: SSH daemon, or
1.237 jmc 1263: .Cm none
1.195 dtucker 1264: to not write one.
1.1 stevesk 1265: The default is
1266: .Pa /var/run/sshd.pid .
1267: .It Cm Port
1268: Specifies the port number that
1.52 jmc 1269: .Xr sshd 8
1.1 stevesk 1270: listens on.
1271: The default is 22.
1272: Multiple options of this type are permitted.
1273: See also
1274: .Cm ListenAddress .
1275: .It Cm PrintLastLog
1276: Specifies whether
1.52 jmc 1277: .Xr sshd 8
1.36 jaredy 1278: should print the date and time of the last user login when a user logs
1279: in interactively.
1.1 stevesk 1280: The default is
1.237 jmc 1281: .Cm yes .
1.1 stevesk 1282: .It Cm PrintMotd
1283: Specifies whether
1.52 jmc 1284: .Xr sshd 8
1.1 stevesk 1285: should print
1286: .Pa /etc/motd
1287: when a user logs in interactively.
1288: (On some systems it is also printed by the shell,
1289: .Pa /etc/profile ,
1290: or equivalent.)
1291: The default is
1.237 jmc 1292: .Cm yes .
1.189 djm 1293: .It Cm PubkeyAcceptedKeyTypes
1294: Specifies the key types that will be accepted for public key authentication
1295: as a comma-separated pattern list.
1.208 djm 1296: Alternately if the specified value begins with a
1297: .Sq +
1298: character, then the specified key types will be appended to the default set
1299: instead of replacing them.
1.242 ! djm 1300: If the specified value begins with a
! 1301: .Sq -
! 1302: character, then the specified key types (including wildcards) will be removed
! 1303: from the default set instead of replacing them.
1.206 markus 1304: The default for this option is:
1305: .Bd -literal -offset 3n
1306: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1307: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1308: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1309: ssh-ed25519-cert-v01@openssh.com,
1310: ssh-rsa-cert-v01@openssh.com,
1311: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.220 djm 1312: ssh-ed25519,ssh-rsa
1.206 markus 1313: .Ed
1314: .Pp
1.237 jmc 1315: The list of available key types may also be obtained using
1316: .Qq ssh -Q key .
1.1 stevesk 1317: .It Cm PubkeyAuthentication
1318: Specifies whether public key authentication is allowed.
1319: The default is
1.237 jmc 1320: .Cm yes .
1.159 dtucker 1321: .It Cm RekeyLimit
1322: Specifies the maximum amount of data that may be transmitted before the
1323: session key is renegotiated, optionally followed a maximum amount of
1324: time that may pass before the session key is renegotiated.
1325: The first argument is specified in bytes and may have a suffix of
1326: .Sq K ,
1327: .Sq M ,
1328: or
1329: .Sq G
1330: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1331: The default is between
1332: .Sq 1G
1333: and
1334: .Sq 4G ,
1335: depending on the cipher.
1336: The optional second value is specified in seconds and may use any of the
1337: units documented in the
1338: .Sx TIME FORMATS
1.160 jmc 1339: section.
1.159 dtucker 1340: The default value for
1341: .Cm RekeyLimit
1342: is
1.237 jmc 1343: .Cm default none ,
1.159 dtucker 1344: which means that rekeying is performed after the cipher's default amount
1345: of data has been sent or received and no time based rekeying is done.
1.118 djm 1346: .It Cm RevokedKeys
1.195 dtucker 1347: Specifies revoked public keys file, or
1.237 jmc 1348: .Cm none
1.195 dtucker 1349: to not use one.
1.118 djm 1350: Keys listed in this file will be refused for public key authentication.
1351: Note that if this file is not readable, then public key authentication will
1352: be refused for all users.
1.154 djm 1353: Keys may be specified as a text file, listing one public key per line, or as
1354: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1355: .Xr ssh-keygen 1 .
1.161 jmc 1356: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1357: .Xr ssh-keygen 1 .
1.175 millert 1358: .It Cm StreamLocalBindMask
1359: Sets the octal file creation mode mask
1360: .Pq umask
1361: used when creating a Unix-domain socket file for local or remote
1362: port forwarding.
1363: This option is only used for port forwarding to a Unix-domain socket file.
1364: .Pp
1365: The default value is 0177, which creates a Unix-domain socket file that is
1366: readable and writable only by the owner.
1367: Note that not all operating systems honor the file mode on Unix-domain
1368: socket files.
1369: .It Cm StreamLocalBindUnlink
1370: Specifies whether to remove an existing Unix-domain socket file for local
1371: or remote port forwarding before creating a new one.
1372: If the socket file already exists and
1373: .Cm StreamLocalBindUnlink
1374: is not enabled,
1375: .Nm sshd
1376: will be unable to forward the port to the Unix-domain socket file.
1377: This option is only used for port forwarding to a Unix-domain socket file.
1378: .Pp
1379: The argument must be
1.237 jmc 1380: .Cm yes
1.175 millert 1381: or
1.237 jmc 1382: .Cm no .
1.175 millert 1383: The default is
1.237 jmc 1384: .Cm no .
1.1 stevesk 1385: .It Cm StrictModes
1386: Specifies whether
1.52 jmc 1387: .Xr sshd 8
1.1 stevesk 1388: should check file modes and ownership of the
1389: user's files and home directory before accepting login.
1390: This is normally desirable because novices sometimes accidentally leave their
1391: directory or files world-writable.
1392: The default is
1.237 jmc 1393: .Cm yes .
1.112 djm 1394: Note that this does not apply to
1395: .Cm ChrootDirectory ,
1396: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1397: .It Cm Subsystem
1.51 jmc 1398: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1399: Arguments should be a subsystem name and a command (with optional arguments)
1400: to execute upon subsystem request.
1.80 djm 1401: .Pp
1.1 stevesk 1402: The command
1.237 jmc 1403: .Cm sftp-server
1404: implements the SFTP file transfer subsystem.
1.80 djm 1405: .Pp
1406: Alternately the name
1.237 jmc 1407: .Cm internal-sftp
1408: implements an in-process SFTP server.
1.80 djm 1409: This may simplify configurations using
1410: .Cm ChrootDirectory
1411: to force a different filesystem root on clients.
1412: .Pp
1.1 stevesk 1413: By default no subsystems are defined.
1414: .It Cm SyslogFacility
1415: Gives the facility code that is used when logging messages from
1.53 jmc 1416: .Xr sshd 8 .
1.1 stevesk 1417: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1418: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1419: The default is AUTH.
1.27 markus 1420: .It Cm TCPKeepAlive
1421: Specifies whether the system should send TCP keepalive messages to the
1422: other side.
1423: If they are sent, death of the connection or crash of one
1424: of the machines will be properly noticed.
1425: However, this means that
1426: connections will die if the route is down temporarily, and some people
1427: find it annoying.
1428: On the other hand, if TCP keepalives are not sent,
1429: sessions may hang indefinitely on the server, leaving
1.237 jmc 1430: .Qq ghost
1.27 markus 1431: users and consuming server resources.
1432: .Pp
1433: The default is
1.237 jmc 1434: .Cm yes
1.27 markus 1435: (to send TCP keepalive messages), and the server will notice
1436: if the network goes down or the client host crashes.
1437: This avoids infinitely hanging sessions.
1438: .Pp
1439: To disable TCP keepalive messages, the value should be set to
1.237 jmc 1440: .Cm no .
1.118 djm 1441: .It Cm TrustedUserCAKeys
1442: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1443: trusted to sign user certificates for authentication, or
1.237 jmc 1444: .Cm none
1.195 dtucker 1445: to not use one.
1.119 jmc 1446: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1447: .Ql #
1448: are allowed.
1449: If a certificate is presented for authentication and has its signing CA key
1450: listed in this file, then it may be used for authentication for any user
1451: listed in the certificate's principals list.
1452: Note that certificates that lack a list of principals will not be permitted
1453: for authentication using
1454: .Cm TrustedUserCAKeys .
1.161 jmc 1455: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1456: .Xr ssh-keygen 1 .
1.18 markus 1457: .It Cm UseDNS
1458: Specifies whether
1.52 jmc 1459: .Xr sshd 8
1.207 djm 1460: should look up the remote host name, and to check that
1.18 markus 1461: the resolved host name for the remote IP address maps back to the
1462: very same IP address.
1.207 djm 1463: .Pp
1464: If this option is set to
1.237 jmc 1465: .Cm no
1.207 djm 1466: (the default) then only addresses and not host names may be used in
1.221 djm 1467: .Pa ~/.ssh/authorized_keys
1.207 djm 1468: .Cm from
1469: and
1.211 jmc 1470: .Nm
1.207 djm 1471: .Cm Match
1472: .Cm Host
1473: directives.
1.1 stevesk 1474: .It Cm UsePrivilegeSeparation
1475: Specifies whether
1.52 jmc 1476: .Xr sshd 8
1.2 stevesk 1477: separates privileges by creating an unprivileged child process
1.15 jmc 1478: to deal with incoming network traffic.
1479: After successful authentication, another process will be created that has
1480: the privilege of the authenticated user.
1481: The goal of privilege separation is to prevent privilege
1.1 stevesk 1482: escalation by containing any corruption within the unprivileged processes.
1.213 sobrado 1483: The argument must be
1.237 jmc 1484: .Cm yes ,
1485: .Cm no ,
1.213 sobrado 1486: or
1.237 jmc 1487: .Cm sandbox .
1.134 djm 1488: If
1489: .Cm UsePrivilegeSeparation
1490: is set to
1.237 jmc 1491: .Cm sandbox
1.134 djm 1492: then the pre-authentication unprivileged process is subject to additional
1493: restrictions.
1.213 sobrado 1494: The default is
1.237 jmc 1495: .Cm sandbox .
1.137 djm 1496: .It Cm VersionAddendum
1497: Optionally specifies additional text to append to the SSH protocol banner
1498: sent by the server upon connection.
1499: The default is
1.237 jmc 1500: .Cm none .
1.1 stevesk 1501: .It Cm X11DisplayOffset
1502: Specifies the first display number available for
1.52 jmc 1503: .Xr sshd 8 Ns 's
1.1 stevesk 1504: X11 forwarding.
1.52 jmc 1505: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1506: The default is 10.
1507: .It Cm X11Forwarding
1508: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1509: The argument must be
1.237 jmc 1510: .Cm yes
1.13 stevesk 1511: or
1.237 jmc 1512: .Cm no .
1.1 stevesk 1513: The default is
1.237 jmc 1514: .Cm no .
1.13 stevesk 1515: .Pp
1516: When X11 forwarding is enabled, there may be additional exposure to
1517: the server and to client displays if the
1.52 jmc 1518: .Xr sshd 8
1.13 stevesk 1519: proxy display is configured to listen on the wildcard address (see
1.237 jmc 1520: .Cm X11UseLocalhost ) ,
1521: though this is not the default.
1.13 stevesk 1522: Additionally, the authentication spoofing and authentication data
1523: verification and substitution occur on the client side.
1524: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1525: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1526: forwarding (see the warnings for
1527: .Cm ForwardX11
1528: in
1.19 jmc 1529: .Xr ssh_config 5 ) .
1.13 stevesk 1530: A system administrator may have a stance in which they want to
1531: protect clients that may expose themselves to attack by unwittingly
1532: requesting X11 forwarding, which can warrant a
1.237 jmc 1533: .Cm no
1.13 stevesk 1534: setting.
1535: .Pp
1536: Note that disabling X11 forwarding does not prevent users from
1537: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1538: .It Cm X11UseLocalhost
1539: Specifies whether
1.52 jmc 1540: .Xr sshd 8
1.1 stevesk 1541: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1542: the wildcard address.
1543: By default,
1.52 jmc 1544: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1545: hostname part of the
1546: .Ev DISPLAY
1547: environment variable to
1.237 jmc 1548: .Cm localhost .
1.8 stevesk 1549: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1550: However, some older X11 clients may not function with this
1551: configuration.
1552: .Cm X11UseLocalhost
1553: may be set to
1.237 jmc 1554: .Cm no
1.1 stevesk 1555: to specify that the forwarding server should be bound to the wildcard
1556: address.
1557: The argument must be
1.237 jmc 1558: .Cm yes
1.1 stevesk 1559: or
1.237 jmc 1560: .Cm no .
1.1 stevesk 1561: The default is
1.237 jmc 1562: .Cm yes .
1.1 stevesk 1563: .It Cm XAuthLocation
1.11 stevesk 1564: Specifies the full pathname of the
1.1 stevesk 1565: .Xr xauth 1
1.195 dtucker 1566: program, or
1.237 jmc 1567: .Cm none
1.195 dtucker 1568: to not use one.
1.1 stevesk 1569: The default is
1570: .Pa /usr/X11R6/bin/xauth .
1571: .El
1.55 jmc 1572: .Sh TIME FORMATS
1.53 jmc 1573: .Xr sshd 8
1.1 stevesk 1574: command-line arguments and configuration file options that specify time
1575: may be expressed using a sequence of the form:
1576: .Sm off
1.7 stevesk 1577: .Ar time Op Ar qualifier ,
1.1 stevesk 1578: .Sm on
1579: where
1580: .Ar time
1581: is a positive integer value and
1582: .Ar qualifier
1583: is one of the following:
1584: .Pp
1585: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1586: .It Aq Cm none
1.1 stevesk 1587: seconds
1588: .It Cm s | Cm S
1589: seconds
1590: .It Cm m | Cm M
1591: minutes
1592: .It Cm h | Cm H
1593: hours
1594: .It Cm d | Cm D
1595: days
1596: .It Cm w | Cm W
1597: weeks
1598: .El
1599: .Pp
1600: Each member of the sequence is added together to calculate
1601: the total time value.
1602: .Pp
1603: Time format examples:
1604: .Pp
1605: .Bl -tag -width Ds -compact -offset indent
1606: .It 600
1607: 600 seconds (10 minutes)
1608: .It 10m
1609: 10 minutes
1610: .It 1h30m
1611: 1 hour 30 minutes (90 minutes)
1612: .El
1.235 jmc 1613: .Sh TOKENS
1614: Arguments to some keywords can make use of tokens,
1615: which are expanded at runtime:
1616: .Pp
1617: .Bl -tag -width XXXX -offset indent -compact
1618: .It %%
1619: A literal
1620: .Sq % .
1621: .It %F
1622: The fingerprint of the CA key.
1623: .It %f
1624: The fingerprint of the key or certificate.
1625: .It %h
1626: The home directory of the user.
1627: .It %i
1628: The key ID in the certificate.
1629: .It %K
1630: The base64-encoded CA key.
1631: .It %k
1632: The base64-encoded key or certificate for authentication.
1633: .It %s
1634: The serial number of the certificate.
1635: .It \&%T
1636: The type of the CA key.
1637: .It %t
1638: The key or certificate type.
1639: .It %u
1640: The username.
1641: .El
1642: .Pp
1643: .Cm AuthorizedKeysCommand
1.241 jmc 1644: accepts the tokens %%, %f, %h, %k, %t, and %u.
1.235 jmc 1645: .Pp
1646: .Cm AuthorizedKeysFile
1647: accepts the tokens %%, %h, and %u.
1648: .Pp
1649: .Cm AuthorizedPrincipalsCommand
1.241 jmc 1650: accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, and %u.
1.235 jmc 1651: .Pp
1652: .Cm AuthorizedPrincipalsFile
1653: accepts the tokens %%, %h, and %u.
1654: .Pp
1655: .Cm ChrootDirectory
1656: accepts the tokens %%, %h, and %u.
1.1 stevesk 1657: .Sh FILES
1658: .Bl -tag -width Ds
1659: .It Pa /etc/ssh/sshd_config
1660: Contains configuration data for
1.53 jmc 1661: .Xr sshd 8 .
1.1 stevesk 1662: This file should be writable by root only, but it is recommended
1663: (though not necessary) that it be world-readable.
1664: .El
1.19 jmc 1665: .Sh SEE ALSO
1.237 jmc 1666: .Xr sftp-server 8 ,
1.19 jmc 1667: .Xr sshd 8
1.1 stevesk 1668: .Sh AUTHORS
1.237 jmc 1669: .An -nosplit
1.1 stevesk 1670: OpenSSH is a derivative of the original and free
1.237 jmc 1671: ssh 1.2.12 release by
1672: .An Tatu Ylonen .
1673: .An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos ,
1674: .An Theo de Raadt
1675: and
1676: .An Dug Song
1.1 stevesk 1677: removed many bugs, re-added newer features and
1678: created OpenSSH.
1.237 jmc 1679: .An Markus Friedl
1680: contributed the support for SSH protocol versions 1.5 and 2.0.
1681: .An Niels Provos
1682: and
1683: .An Markus Friedl
1684: contributed support for privilege separation.
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