1.\" $NetBSD: sshd_config.5,v 1.26 2018/04/06 18:59:00 christos Exp $ 2.\" -*- nroff -*- 3.\" 4.\" Author: Tatu Ylonen <ylo@cs.hut.fi> 5.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland 6.\" All rights reserved 7.\" 8.\" As far as I am concerned, the code I have written for this software 9.\" can be used freely for any purpose. Any derived versions of this 10.\" software must be clearly marked as such, and if the derived work is 11.\" incompatible with the protocol description in the RFC file, it must be 12.\" called by a name other than "ssh" or "Secure Shell". 13.\" 14.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. 15.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. 16.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. 17.\" 18.\" Redistribution and use in source and binary forms, with or without 19.\" modification, are permitted provided that the following conditions 20.\" are met: 21.\" 1. Redistributions of source code must retain the above copyright 22.\" notice, this list of conditions and the following disclaimer. 23.\" 2. Redistributions in binary form must reproduce the above copyright 24.\" notice, this list of conditions and the following disclaimer in the 25.\" documentation and/or other materials provided with the distribution. 26.\" 27.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 28.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 29.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 30.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 31.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 32.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 33.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 34.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 35.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 36.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 37.\" 38.\" $OpenBSD: sshd_config.5,v 1.263 2018/02/16 02:40:45 djm Exp $ 39.Dd February 16 2018 40.Dt SSHD_CONFIG 5 41.Os 42.Sh NAME 43.Nm sshd_config 44.Nd OpenSSH SSH daemon configuration file 45.Sh DESCRIPTION 46.Xr sshd 8 47reads configuration data from 48.Pa /etc/ssh/sshd_config 49(or the file specified with 50.Fl f 51on the command line). 52The file contains keyword-argument pairs, one per line. 53For each keyword, the first obtained value will be used. 54Lines starting with 55.Ql # 56and empty lines are interpreted as comments. 57Arguments may optionally be enclosed in double quotes 58.Pq \&" 59in order to represent arguments containing spaces. 60.Pp 61The possible 62keywords and their meanings are as follows (note that 63keywords are case-insensitive and arguments are case-sensitive): 64.Bl -tag -width Ds 65.It Cm AcceptEnv 66Specifies what environment variables sent by the client will be copied into 67the session's 68.Xr environ 7 . 69See 70.Cm SendEnv 71in 72.Xr ssh_config 5 73for how to configure the client. 74The 75.Ev TERM 76environment variable is always sent whenever the client 77requests a pseudo-terminal as it is required by the protocol. 78Variables are specified by name, which may contain the wildcard characters 79.Ql * 80and 81.Ql \&? . 82Multiple environment variables may be separated by whitespace or spread 83across multiple 84.Cm AcceptEnv 85directives. 86Be warned that some environment variables could be used to bypass restricted 87user environments. 88For this reason, care should be taken in the use of this directive. 89The default is not to accept any environment variables. 90.It Cm AddressFamily 91Specifies which address family should be used by 92.Xr sshd 8 . 93Valid arguments are 94.Cm any 95(the default), 96.Cm inet 97(use IPv4 only), or 98.Cm inet6 99(use IPv6 only). 100.It Cm AllowAgentForwarding 101Specifies whether 102.Xr ssh-agent 1 103forwarding is permitted. 104The default is 105.Cm yes . 106Note that disabling agent forwarding does not improve security 107unless users are also denied shell access, as they can always install 108their own forwarders. 109.It Cm AllowGroups 110This keyword can be followed by a list of group name patterns, separated 111by spaces. 112If specified, login is allowed only for users whose primary 113group or supplementary group list matches one of the patterns. 114Only group names are valid; a numerical group ID is not recognized. 115By default, login is allowed for all groups. 116The allow/deny directives are processed in the following order: 117.Cm DenyUsers , 118.Cm AllowUsers , 119.Cm DenyGroups , 120and finally 121.Cm AllowGroups . 122.Pp 123See PATTERNS in 124.Xr ssh_config 5 125for more information on patterns. 126.It Cm AllowStreamLocalForwarding 127Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. 128The available options are 129.Cm yes 130(the default) 131or 132.Cm all 133to allow StreamLocal forwarding, 134.Cm no 135to prevent all StreamLocal forwarding, 136.Cm local 137to allow local (from the perspective of 138.Xr ssh 1 ) 139forwarding only or 140.Cm remote 141to allow remote forwarding only. 142Note that disabling StreamLocal forwarding does not improve security unless 143users are also denied shell access, as they can always install their 144own forwarders. 145.It Cm AllowTcpForwarding 146Specifies whether TCP forwarding is permitted. 147The available options are 148.Cm yes 149(the default) 150or 151.Cm all 152to allow TCP forwarding, 153.Cm no 154to prevent all TCP forwarding, 155.Cm local 156to allow local (from the perspective of 157.Xr ssh 1 ) 158forwarding only or 159.Cm remote 160to allow remote forwarding only. 161Note that disabling TCP forwarding does not improve security unless 162users are also denied shell access, as they can always install their 163own forwarders. 164.It Cm AllowUsers 165This keyword can be followed by a list of user name patterns, separated 166by spaces. 167If specified, login is allowed only for user names that 168match one of the patterns. 169Only user names are valid; a numerical user ID is not recognized. 170By default, login is allowed for all users. 171If the pattern takes the form USER@HOST then USER and HOST 172are separately checked, restricting logins to particular 173users from particular hosts. 174HOST criteria may additionally contain addresses to match in CIDR 175address/masklen format. 176The allow/deny directives are processed in the following order: 177.Cm DenyUsers , 178.Cm AllowUsers , 179.Cm DenyGroups , 180and finally 181.Cm AllowGroups . 182.Pp 183See PATTERNS in 184.Xr ssh_config 5 185for more information on patterns. 186.It Cm AuthenticationMethods 187Specifies the authentication methods that must be successfully completed 188for a user to be granted access. 189This option must be followed by one or more comma-separated lists of 190authentication method names, or by the single string 191.Cm any 192to indicate the default behaviour of accepting any single authentication 193method. 194If the default is overridden, then successful authentication requires 195completion of every method in at least one of these lists. 196.Pp 197For example, 198.Qq publickey,password publickey,keyboard-interactive 199would require the user to complete public key authentication, followed by 200either password or keyboard interactive authentication. 201Only methods that are next in one or more lists are offered at each stage, 202so for this example it would not be possible to attempt password or 203keyboard-interactive authentication before public key. 204.Pp 205For keyboard interactive authentication it is also possible to 206restrict authentication to a specific device by appending a 207colon followed by the device identifier 208.Cm bsdauth , 209.Cm pam , 210or 211.Cm skey , 212depending on the server configuration. 213For example, 214.Qq keyboard-interactive:bsdauth 215would restrict keyboard interactive authentication to the 216.Cm bsdauth 217device. 218.Pp 219If the publickey method is listed more than once, 220.Xr sshd 8 221verifies that keys that have been used successfully are not reused for 222subsequent authentications. 223For example, 224.Qq publickey,publickey 225requires successful authentication using two different public keys. 226.Pp 227Note that each authentication method listed should also be explicitly enabled 228in the configuration. 229.Pp 230The available authentication methods are: 231.Qq gssapi-with-mic , 232.Qq hostbased , 233.Qq keyboard-interactive , 234.Qq none 235(used for access to password-less accounts when 236.Cm PermitEmptyPassword 237is enabled), 238.Qq password 239and 240.Qq publickey . 241.It Cm AuthorizedKeysCommand 242Specifies a program to be used to look up the user's public keys. 243The program must be owned by root, not writable by group or others and 244specified by an absolute path. 245Arguments to 246.Cm AuthorizedKeysCommand 247accept the tokens described in the 248.Sx TOKENS 249section. 250If no arguments are specified then the username of the target user is used. 251.Pp 252The program should produce on standard output zero or 253more lines of authorized_keys output (see 254.Sx AUTHORIZED_KEYS 255in 256.Xr sshd 8 ) . 257If a key supplied by 258.Cm AuthorizedKeysCommand 259does not successfully authenticate 260and authorize the user then public key authentication continues using the usual 261.Cm AuthorizedKeysFile 262files. 263By default, no 264.Cm AuthorizedKeysCommand 265is run. 266.It Cm AuthorizedKeysCommandUser 267Specifies the user under whose account the 268.Cm AuthorizedKeysCommand 269is run. 270It is recommended to use a dedicated user that has no other role on the host 271than running authorized keys commands. 272If 273.Cm AuthorizedKeysCommand 274is specified but 275.Cm AuthorizedKeysCommandUser 276is not, then 277.Xr sshd 8 278will refuse to start. 279.It Cm AuthorizedKeysFile 280Specifies the file that contains the public keys used for user authentication. 281The format is described in the 282.Sx AUTHORIZED_KEYS FILE FORMAT 283section of 284.Xr sshd 8 . 285Arguments to 286.Cm AuthorizedKeysFile 287accept the tokens described in the 288.Sx TOKENS 289section. 290After expansion, 291.Cm AuthorizedKeysFile 292is taken to be an absolute path or one relative to the user's home 293directory. 294Multiple files may be listed, separated by whitespace. 295Alternately this option may be set to 296.Cm none 297to skip checking for user keys in files. 298The default is 299.Qq .ssh/authorized_keys .ssh/authorized_keys2 . 300.It Cm AuthorizedPrincipalsCommand 301Specifies a program to be used to generate the list of allowed 302certificate principals as per 303.Cm AuthorizedPrincipalsFile . 304The program must be owned by root, not writable by group or others and 305specified by an absolute path. 306Arguments to 307.Cm AuthorizedPrincipalsCommand 308accept the tokens described in the 309.Sx TOKENS 310section. 311If no arguments are specified then the username of the target user is used. 312.Pp 313The program should produce on standard output zero or 314more lines of 315.Cm AuthorizedPrincipalsFile 316output. 317If either 318.Cm AuthorizedPrincipalsCommand 319or 320.Cm AuthorizedPrincipalsFile 321is specified, then certificates offered by the client for authentication 322must contain a principal that is listed. 323By default, no 324.Cm AuthorizedPrincipalsCommand 325is run. 326.It Cm AuthorizedPrincipalsCommandUser 327Specifies the user under whose account the 328.Cm AuthorizedPrincipalsCommand 329is run. 330It is recommended to use a dedicated user that has no other role on the host 331than running authorized principals commands. 332If 333.Cm AuthorizedPrincipalsCommand 334is specified but 335.Cm AuthorizedPrincipalsCommandUser 336is not, then 337.Xr sshd 8 338will refuse to start. 339.It Cm AuthorizedPrincipalsFile 340Specifies a file that lists principal names that are accepted for 341certificate authentication. 342When using certificates signed by a key listed in 343.Cm TrustedUserCAKeys , 344this file lists names, one of which must appear in the certificate for it 345to be accepted for authentication. 346Names are listed one per line preceded by key options (as described in 347.Sx AUTHORIZED_KEYS FILE FORMAT 348in 349.Xr sshd 8 ) . 350Empty lines and comments starting with 351.Ql # 352are ignored. 353.Pp 354Arguments to 355.Cm AuthorizedPrincipalsFile 356accept the tokens described in the 357.Sx TOKENS 358section. 359After expansion, 360.Cm AuthorizedPrincipalsFile 361is taken to be an absolute path or one relative to the user's home directory. 362The default is 363.Cm none , 364i.e. not to use a principals file \(en in this case, the username 365of the user must appear in a certificate's principals list for it to be 366accepted. 367.Pp 368Note that 369.Cm AuthorizedPrincipalsFile 370is only used when authentication proceeds using a CA listed in 371.Cm TrustedUserCAKeys 372and is not consulted for certification authorities trusted via 373.Pa ~/.ssh/authorized_keys , 374though the 375.Cm principals= 376key option offers a similar facility (see 377.Xr sshd 8 378for details). 379.It Cm Banner 380The contents of the specified file are sent to the remote user before 381authentication is allowed. 382If the argument is 383.Cm none 384then no banner is displayed. 385By default, no banner is displayed. 386.It Cm ChallengeResponseAuthentication 387Specifies whether challenge-response authentication is allowed. 388All authentication styles from 389.Xr login.conf 5 390are supported. 391The default is 392.Cm yes . 393.It Cm ChrootDirectory 394Specifies the pathname of a directory to 395.Xr chroot 2 396to after authentication. 397At session startup 398.Xr sshd 8 399checks that all components of the pathname are root-owned directories 400which are not writable by any other user or group. 401After the chroot, 402.Xr sshd 8 403changes the working directory to the user's home directory. 404Arguments to 405.Cm ChrootDirectory 406accept the tokens described in the 407.Sx TOKENS 408section. 409.Pp 410The 411.Cm ChrootDirectory 412must contain the necessary files and directories to support the 413user's session. 414For an interactive session this requires at least a shell, typically 415.Xr sh 1 , 416and basic 417.Pa /dev 418nodes such as 419.Xr null 4 , 420.Xr zero 4 , 421.Xr stdin 4 , 422.Xr stdout 4 , 423.Xr stderr 4 , 424and 425.Xr tty 4 426devices. 427For file transfer sessions using SFTP 428no additional configuration of the environment is necessary if the in-process 429sftp-server is used, 430though sessions which use logging may require 431.Pa /dev/log 432inside the chroot directory on some operating systems (see 433.Xr sftp-server 8 434for details). 435.Pp 436For safety, it is very important that the directory hierarchy be 437prevented from modification by other processes on the system (especially 438those outside the jail). 439Misconfiguration can lead to unsafe environments which 440.Xr sshd 8 441cannot detect. 442.Pp 443The default is 444.Cm none , 445indicating not to 446.Xr chroot 2 . 447.It Cm Ciphers 448Specifies the ciphers allowed. 449Multiple ciphers must be comma-separated. 450If the specified value begins with a 451.Sq + 452character, then the specified ciphers will be appended to the default set 453instead of replacing them. 454If the specified value begins with a 455.Sq - 456character, then the specified ciphers (including wildcards) will be removed 457from the default set instead of replacing them. 458.Pp 459The supported ciphers are: 460.Pp 461.Bl -item -compact -offset indent 462.It 4633des-cbc 464.It 465aes128-cbc 466.It 467aes192-cbc 468.It 469aes256-cbc 470.It 471aes128-ctr 472.It 473aes192-ctr 474.It 475aes256-ctr 476.It 477aes128-gcm@openssh.com 478.It 479aes256-gcm@openssh.com 480.It 481chacha20-poly1305@openssh.com 482.El 483.Pp 484The default is: 485.Bd -literal -offset indent 486chacha20-poly1305@openssh.com, 487aes128-ctr,aes192-ctr,aes256-ctr, 488aes128-gcm@openssh.com,aes256-gcm@openssh.com 489.Ed 490.Pp 491The list of available ciphers may also be obtained using 492.Qq ssh -Q cipher . 493.It Cm ClientAliveCountMax 494Sets the number of client alive messages which may be sent without 495.Xr sshd 8 496receiving any messages back from the client. 497If this threshold is reached while client alive messages are being sent, 498sshd will disconnect the client, terminating the session. 499It is important to note that the use of client alive messages is very 500different from 501.Cm TCPKeepAlive . 502The client alive messages are sent through the encrypted channel 503and therefore will not be spoofable. 504The TCP keepalive option enabled by 505.Cm TCPKeepAlive 506is spoofable. 507The client alive mechanism is valuable when the client or 508server depend on knowing when a connection has become inactive. 509.Pp 510The default value is 3. 511If 512.Cm ClientAliveInterval 513is set to 15, and 514.Cm ClientAliveCountMax 515is left at the default, unresponsive SSH clients 516will be disconnected after approximately 45 seconds. 517.It Cm ClientAliveInterval 518Sets a timeout interval in seconds after which if no data has been received 519from the client, 520.Xr sshd 8 521will send a message through the encrypted 522channel to request a response from the client. 523The default 524is 0, indicating that these messages will not be sent to the client. 525.It Cm Compression 526Specifies whether compression is enabled after 527the user has authenticated successfully. 528The argument must be 529.Cm yes , 530.Cm delayed 531(a legacy synonym for 532.Cm yes ) 533or 534.Cm no . 535The default is 536.Cm yes . 537.It Cm DenyGroups 538This keyword can be followed by a list of group name patterns, separated 539by spaces. 540Login is disallowed for users whose primary group or supplementary 541group list matches one of the patterns. 542Only group names are valid; a numerical group ID is not recognized. 543By default, login is allowed for all groups. 544The allow/deny directives are processed in the following order: 545.Cm DenyUsers , 546.Cm AllowUsers , 547.Cm DenyGroups , 548and finally 549.Cm AllowGroups . 550.Pp 551See PATTERNS in 552.Xr ssh_config 5 553for more information on patterns. 554.It Cm DenyUsers 555This keyword can be followed by a list of user name patterns, separated 556by spaces. 557Login is disallowed for user names that match one of the patterns. 558Only user names are valid; a numerical user ID is not recognized. 559By default, login is allowed for all users. 560If the pattern takes the form USER@HOST then USER and HOST 561are separately checked, restricting logins to particular 562users from particular hosts. 563HOST criteria may additionally contain addresses to match in CIDR 564address/masklen format. 565The allow/deny directives are processed in the following order: 566.Cm DenyUsers , 567.Cm AllowUsers , 568.Cm DenyGroups , 569and finally 570.Cm AllowGroups . 571.Pp 572See PATTERNS in 573.Xr ssh_config 5 574for more information on patterns. 575.It Cm DisableForwarding 576Disables all forwarding features, including X11, 577.Xr ssh-agent 1 , 578TCP and StreamLocal. 579This option overrides all other forwarding-related options and may 580simplify restricted configurations. 581.It Cm ExposeAuthInfo 582Writes a temporary file containing a list of authentication methods and 583public credentials (e.g. keys) used to authenticate the user. 584The location of the file is exposed to the user session through the 585.Ev SSH_USER_AUTH 586environment variable. 587The default is 588.Cm no . 589.It Cm FingerprintHash 590Specifies the hash algorithm used when logging key fingerprints. 591Valid options are: 592.Cm md5 593and 594.Cm sha256 . 595The default is 596.Cm sha256 . 597.It Cm ForceCommand 598Forces the execution of the command specified by 599.Cm ForceCommand , 600ignoring any command supplied by the client and 601.Pa ~/.ssh/rc 602if present. 603The command is invoked by using the user's login shell with the -c option. 604This applies to shell, command, or subsystem execution. 605It is most useful inside a 606.Cm Match 607block. 608The command originally supplied by the client is available in the 609.Ev SSH_ORIGINAL_COMMAND 610environment variable. 611Specifying a command of 612.Cm internal-sftp 613will force the use of an in-process SFTP server that requires no support 614files when used with 615.Cm ChrootDirectory . 616The default is 617.Cm none . 618.It Cm GatewayPorts 619Specifies whether remote hosts are allowed to connect to ports 620forwarded for the client. 621By default, 622.Xr sshd 8 623binds remote port forwardings to the loopback address. 624This prevents other remote hosts from connecting to forwarded ports. 625.Cm GatewayPorts 626can be used to specify that sshd 627should allow remote port forwardings to bind to non-loopback addresses, thus 628allowing other hosts to connect. 629The argument may be 630.Cm no 631to force remote port forwardings to be available to the local host only, 632.Cm yes 633to force remote port forwardings to bind to the wildcard address, or 634.Cm clientspecified 635to allow the client to select the address to which the forwarding is bound. 636The default is 637.Cm no . 638.It Cm GSSAPIAuthentication 639Specifies whether user authentication based on GSSAPI is allowed. 640The default is 641.Cm no . 642.It Cm GSSAPICleanupCredentials 643Specifies whether to automatically destroy the user's credentials cache 644on logout. 645The default is 646.Cm yes . 647.It Cm GSSAPIStrictAcceptorCheck 648Determines whether to be strict about the identity of the GSSAPI acceptor 649a client authenticates against. 650If set to 651.Cm yes 652then the client must authenticate against the host 653service on the current hostname. 654If set to 655.Cm no 656then the client may authenticate against any service key stored in the 657machine's default store. 658This facility is provided to assist with operation on multi homed machines. 659The default is 660.Cm yes . 661.It Cm HostbasedAcceptedKeyTypes 662Specifies the key types that will be accepted for hostbased authentication 663as a comma-separated pattern list. 664Alternately if the specified value begins with a 665.Sq + 666character, then the specified key types will be appended to the default set 667instead of replacing them. 668If the specified value begins with a 669.Sq - 670character, then the specified key types (including wildcards) will be removed 671from the default set instead of replacing them. 672The default for this option is: 673.Bd -literal -offset 3n 674ecdsa-sha2-nistp256-cert-v01@openssh.com, 675ecdsa-sha2-nistp384-cert-v01@openssh.com, 676ecdsa-sha2-nistp521-cert-v01@openssh.com, 677ssh-ed25519-cert-v01@openssh.com, 678ssh-rsa-cert-v01@openssh.com, 679ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 680ssh-ed25519,ssh-rsa 681.Ed 682.Pp 683The list of available key types may also be obtained using 684.Qq ssh -Q key . 685.It Cm HostbasedAuthentication 686Specifies whether rhosts or /etc/hosts.equiv authentication together 687with successful public key client host authentication is allowed 688(host-based authentication). 689The default is 690.Cm no . 691.It Cm HostbasedUsesNameFromPacketOnly 692Specifies whether or not the server will attempt to perform a reverse 693name lookup when matching the name in the 694.Pa ~/.shosts , 695.Pa ~/.rhosts , 696and 697.Pa /etc/hosts.equiv 698files during 699.Cm HostbasedAuthentication . 700A setting of 701.Cm yes 702means that 703.Xr sshd 8 704uses the name supplied by the client rather than 705attempting to resolve the name from the TCP connection itself. 706The default is 707.Cm no . 708.It Cm HostCertificate 709Specifies a file containing a public host certificate. 710The certificate's public key must match a private host key already specified 711by 712.Cm HostKey . 713The default behaviour of 714.Xr sshd 8 715is not to load any certificates. 716.It Cm HostKey 717Specifies a file containing a private host key 718used by SSH. 719The defaults are 720.Pa /etc/ssh/ssh_host_ecdsa_key , 721.Pa /etc/ssh/ssh_host_ed25519_key 722and 723.Pa /etc/ssh/ssh_host_rsa_key . 724.Pp 725Note that 726.Xr sshd 8 727will refuse to use a file if it is group/world-accessible 728and that the 729.Cm HostKeyAlgorithms 730option restricts which of the keys are actually used by 731.Xr sshd 8 . 732.Pp 733It is possible to have multiple host key files. 734It is also possible to specify public host key files instead. 735In this case operations on the private key will be delegated 736to an 737.Xr ssh-agent 1 . 738.It Cm HostKeyAgent 739Identifies the UNIX-domain socket used to communicate 740with an agent that has access to the private host keys. 741If the string 742.Qq SSH_AUTH_SOCK 743is specified, the location of the socket will be read from the 744.Ev SSH_AUTH_SOCK 745environment variable. 746.It Cm HostKeyAlgorithms 747Specifies the host key algorithms 748that the server offers. 749The default for this option is: 750.Bd -literal -offset 3n 751ecdsa-sha2-nistp256-cert-v01@openssh.com, 752ecdsa-sha2-nistp384-cert-v01@openssh.com, 753ecdsa-sha2-nistp521-cert-v01@openssh.com, 754ssh-ed25519-cert-v01@openssh.com, 755ssh-rsa-cert-v01@openssh.com, 756ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 757ssh-ed25519,ssh-rsa 758.Ed 759.Pp 760The list of available key types may also be obtained using 761.Qq ssh -Q key . 762.It Cm IgnoreRhosts 763Specifies that 764.Pa .rhosts 765and 766.Pa .shosts 767files will not be used in 768.Cm HostbasedAuthentication . 769.Pp 770.Pa /etc/hosts.equiv 771and 772.Pa /etc/shosts.equiv 773are still used. 774The default is 775.Cm yes . 776.It Cm IgnoreUserKnownHosts 777Specifies whether 778.Xr sshd 8 779should ignore the user's 780.Pa ~/.ssh/known_hosts 781during 782.Cm HostbasedAuthentication 783and use only the system-wide known hosts file 784.Pa /etc/ssh/known_hosts . 785The default is 786.Cm no . 787.It Cm IPQoS 788Specifies the IPv4 type-of-service or DSCP class for the connection. 789Accepted values are 790.Cm af11 , 791.Cm af12 , 792.Cm af13 , 793.Cm af21 , 794.Cm af22 , 795.Cm af23 , 796.Cm af31 , 797.Cm af32 , 798.Cm af33 , 799.Cm af41 , 800.Cm af42 , 801.Cm af43 , 802.Cm cs0 , 803.Cm cs1 , 804.Cm cs2 , 805.Cm cs3 , 806.Cm cs4 , 807.Cm cs5 , 808.Cm cs6 , 809.Cm cs7 , 810.Cm ef , 811.Cm lowdelay , 812.Cm throughput , 813.Cm reliability , 814a numeric value, or 815.Cm none 816to use the operating system default. 817This option may take one or two arguments, separated by whitespace. 818If one argument is specified, it is used as the packet class unconditionally. 819If two values are specified, the first is automatically selected for 820interactive sessions and the second for non-interactive sessions. 821The default is 822.Cm lowdelay 823for interactive sessions and 824.Cm throughput 825for non-interactive sessions. 826.It Cm KbdInteractiveAuthentication 827Specifies whether to allow keyboard-interactive authentication. 828The argument to this keyword must be 829.Cm yes 830or 831.Cm no . 832The default is to use whatever value 833.Cm ChallengeResponseAuthentication 834is set to 835(by default 836.Cm yes ) . 837.It Cm KerberosAuthentication 838Specifies whether the password provided by the user for 839.Cm PasswordAuthentication 840will be validated through the Kerberos KDC. 841To use this option, the server needs a 842Kerberos servtab which allows the verification of the KDC's identity. 843The default is 844.Cm no . 845.It Cm KerberosGetAFSToken 846If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 847an AFS token before accessing the user's home directory. 848The default is 849.Cm no . 850.It Cm KerberosOrLocalPasswd 851If password authentication through Kerberos fails then 852the password will be validated via any additional local mechanism 853such as 854.Pa /etc/passwd . 855The default is 856.Cm yes . 857.It Cm KerberosTicketCleanup 858Specifies whether to automatically destroy the user's ticket cache 859file on logout. 860The default is 861.Cm yes . 862.It Cm KexAlgorithms 863Specifies the available KEX (Key Exchange) algorithms. 864Multiple algorithms must be comma-separated. 865Alternately if the specified value begins with a 866.Sq + 867character, then the specified methods will be appended to the default set 868instead of replacing them. 869If the specified value begins with a 870.Sq - 871character, then the specified methods (including wildcards) will be removed 872from the default set instead of replacing them. 873The supported algorithms are: 874.Pp 875.Bl -item -compact -offset indent 876.It 877curve25519-sha256 878.It 879curve25519-sha256@libssh.org 880.It 881diffie-hellman-group1-sha1 882.It 883diffie-hellman-group14-sha1 884.It 885diffie-hellman-group14-sha256 886.It 887diffie-hellman-group16-sha512 888.It 889diffie-hellman-group18-sha512 890.It 891diffie-hellman-group-exchange-sha1 892.It 893diffie-hellman-group-exchange-sha256 894.It 895ecdh-sha2-nistp256 896.It 897ecdh-sha2-nistp384 898.It 899ecdh-sha2-nistp521 900.El 901.Pp 902The default is: 903.Bd -literal -offset indent 904curve25519-sha256,curve25519-sha256@libssh.org, 905ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 906diffie-hellman-group-exchange-sha256, 907diffie-hellman-group16-sha512,diffie-hellman-group18-sha512, 908diffie-hellman-group14-sha256,diffie-hellman-group14-sha1 909.Ed 910.Pp 911The list of available key exchange algorithms may also be obtained using 912.Qq ssh -Q kex . 913.It Cm ListenAddress 914Specifies the local addresses 915.Xr sshd 8 916should listen on. 917The following forms may be used: 918.Pp 919.Bl -item -offset indent -compact 920.It 921.Cm ListenAddress 922.Sm off 923.Ar hostname | address 924.Sm on 925.Op Cm rdomain Ar domain 926.It 927.Cm ListenAddress 928.Sm off 929.Ar hostname : port 930.Sm on 931.Op Cm rdomain Ar domain 932.It 933.Cm ListenAddress 934.Sm off 935.Ar IPv4_address : port 936.Sm on 937.Op Cm rdomain Ar domain 938.It 939.Cm ListenAddress 940.Sm off 941.Oo Ar hostname | address Oc : Ar port 942.Sm on 943.Op Cm rdomain Ar domain 944.El 945.Pp 946The optional 947.Cm rdomain 948qualifier requests 949.Xr sshd 8 950listen in an explicit routing domain. 951If 952.Ar port 953is not specified, 954sshd will listen on the address and all 955.Cm Port 956options specified. 957The default is to listen on all local addresses on the current default 958routing domain. 959Multiple 960.Cm ListenAddress 961options are permitted. 962For more information on routing domains, see 963.Xr rdomain 4 . 964.It Cm LoginGraceTime 965The server disconnects after this time if the user has not 966successfully logged in. 967If the value is 0, there is no time limit. 968The default is 120 seconds but the default 969.Pa /etc/ssh/sshd_config 970overwrites it to 600 seconds. 971.It Cm LogLevel 972Gives the verbosity level that is used when logging messages from 973.Xr sshd 8 . 974The possible values are: 975QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 976The default is INFO. 977DEBUG and DEBUG1 are equivalent. 978DEBUG2 and DEBUG3 each specify higher levels of debugging output. 979Logging with a DEBUG level violates the privacy of users and is not recommended. 980.It Cm MACs 981Specifies the available MAC (message authentication code) algorithms. 982The MAC algorithm is used for data integrity protection. 983Multiple algorithms must be comma-separated. 984If the specified value begins with a 985.Sq + 986character, then the specified algorithms will be appended to the default set 987instead of replacing them. 988If the specified value begins with a 989.Sq - 990character, then the specified algorithms (including wildcards) will be removed 991from the default set instead of replacing them. 992.Pp 993The algorithms that contain 994.Qq -etm 995calculate the MAC after encryption (encrypt-then-mac). 996These are considered safer and their use recommended. 997The supported MACs are: 998.Pp 999.Bl -item -compact -offset indent 1000.It 1001hmac-md5 1002.It 1003hmac-md5-96 1004.It 1005hmac-sha1 1006.It 1007hmac-sha1-96 1008.It 1009hmac-sha2-256 1010.It 1011hmac-sha2-512 1012.It 1013umac-64@openssh.com 1014.It 1015umac-128@openssh.com 1016.It 1017hmac-md5-etm@openssh.com 1018.It 1019hmac-md5-96-etm@openssh.com 1020.It 1021hmac-sha1-etm@openssh.com 1022.It 1023hmac-sha1-96-etm@openssh.com 1024.It 1025hmac-sha2-256-etm@openssh.com 1026.It 1027hmac-sha2-512-etm@openssh.com 1028.It 1029umac-64-etm@openssh.com 1030.It 1031umac-128-etm@openssh.com 1032.El 1033.Pp 1034The default is: 1035.Bd -literal -offset indent 1036umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1037hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1038hmac-sha1-etm@openssh.com, 1039umac-64@openssh.com,umac-128@openssh.com, 1040hmac-sha2-256,hmac-sha2-512,hmac-sha1 1041.Ed 1042.Pp 1043The list of available MAC algorithms may also be obtained using 1044.Qq ssh -Q mac . 1045.It Cm Match 1046Introduces a conditional block. 1047If all of the criteria on the 1048.Cm Match 1049line are satisfied, the keywords on the following lines override those 1050set in the global section of the config file, until either another 1051.Cm Match 1052line or the end of the file. 1053If a keyword appears in multiple 1054.Cm Match 1055blocks that are satisfied, only the first instance of the keyword is 1056applied. 1057.Pp 1058The arguments to 1059.Cm Match 1060are one or more criteria-pattern pairs or the single token 1061.Cm All 1062which matches all criteria. 1063The available criteria are 1064.Cm User , 1065.Cm Group , 1066.Cm Host , 1067.Cm LocalAddress , 1068.Cm LocalPort , 1069.Cm RDomain , 1070and 1071.Cm Address 1072(with 1073.Cm RDomain 1074representing the 1075.Xr rdomain 4 1076on which the connection was received.) 1077.Pp 1078The match patterns may consist of single entries or comma-separated 1079lists and may use the wildcard and negation operators described in the 1080.Sx PATTERNS 1081section of 1082.Xr ssh_config 5 . 1083.Pp 1084The patterns in an 1085.Cm Address 1086criteria may additionally contain addresses to match in CIDR 1087address/masklen format, 1088such as 192.0.2.0/24 or 2001:db8::/32. 1089Note that the mask length provided must be consistent with the address - 1090it is an error to specify a mask length that is too long for the address 1091or one with bits set in this host portion of the address. 1092For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. 1093.Pp 1094Only a subset of keywords may be used on the lines following a 1095.Cm Match 1096keyword. 1097Available keywords are 1098.Cm AcceptEnv , 1099.Cm AllowAgentForwarding , 1100.Cm AllowGroups , 1101.Cm AllowStreamLocalForwarding , 1102.Cm AllowTcpForwarding , 1103.Cm AllowUsers , 1104.Cm AuthenticationMethods , 1105.Cm AuthorizedKeysCommand , 1106.Cm AuthorizedKeysCommandUser , 1107.Cm AuthorizedKeysFile , 1108.Cm AuthorizedPrincipalsCommand , 1109.Cm AuthorizedPrincipalsCommandUser , 1110.Cm AuthorizedPrincipalsFile , 1111.Cm Banner , 1112.Cm ChrootDirectory , 1113.Cm ClientAliveCountMax , 1114.Cm ClientAliveInterval , 1115.Cm DenyGroups , 1116.Cm DenyUsers , 1117.Cm ForceCommand , 1118.Cm GatewayPorts , 1119.Cm GSSAPIAuthentication , 1120.Cm HostbasedAcceptedKeyTypes , 1121.Cm HostbasedAuthentication , 1122.Cm HostbasedUsesNameFromPacketOnly , 1123.Cm IPQoS , 1124.Cm KbdInteractiveAuthentication , 1125.Cm KerberosAuthentication , 1126.Cm LogLevel , 1127.Cm MaxAuthTries , 1128.Cm MaxSessions , 1129.Cm PasswordAuthentication , 1130.Cm PermitEmptyPasswords , 1131.Cm PermitOpen , 1132.Cm PermitRootLogin , 1133.Cm PermitTTY , 1134.Cm PermitTunnel , 1135.Cm PermitUserRC , 1136.Cm PubkeyAcceptedKeyTypes , 1137.Cm PubkeyAuthentication , 1138.Cm RekeyLimit , 1139.Cm RevokedKeys , 1140.Cm RDomain , 1141.Cm StreamLocalBindMask , 1142.Cm StreamLocalBindUnlink , 1143.Cm TrustedUserCAKeys , 1144.Cm X11DisplayOffset , 1145.Cm X11Forwarding 1146and 1147.Cm X11UseLocalHost . 1148.It Cm MaxAuthTries 1149Specifies the maximum number of authentication attempts permitted per 1150connection. 1151Once the number of failures reaches half this value, 1152additional failures are logged. 1153The default is 6. 1154.It Cm MaxSessions 1155Specifies the maximum number of open shell, login or subsystem (e.g. sftp) 1156sessions permitted per network connection. 1157Multiple sessions may be established by clients that support connection 1158multiplexing. 1159Setting 1160.Cm MaxSessions 1161to 1 will effectively disable session multiplexing, whereas setting it to 0 1162will prevent all shell, login and subsystem sessions while still permitting 1163forwarding. 1164The default is 10. 1165.It Cm MaxStartups 1166Specifies the maximum number of concurrent unauthenticated connections to the 1167SSH daemon. 1168Additional connections will be dropped until authentication succeeds or the 1169.Cm LoginGraceTime 1170expires for a connection. 1171The default is 10:30:100. 1172.Pp 1173Alternatively, random early drop can be enabled by specifying 1174the three colon separated values 1175start:rate:full (e.g. "10:30:60"). 1176.Xr sshd 8 1177will refuse connection attempts with a probability of rate/100 (30%) 1178if there are currently start (10) unauthenticated connections. 1179The probability increases linearly and all connection attempts 1180are refused if the number of unauthenticated connections reaches full (60). 1181.It Cm PasswordAuthentication 1182Specifies whether password authentication is allowed. 1183The default is 1184.Cm yes . 1185.It Cm PermitEmptyPasswords 1186When password authentication is allowed, it specifies whether the 1187server allows login to accounts with empty password strings. 1188The default is 1189.Cm no . 1190.It Cm PermitOpen 1191Specifies the destinations to which TCP port forwarding is permitted. 1192The forwarding specification must be one of the following forms: 1193.Pp 1194.Bl -item -offset indent -compact 1195.It 1196.Cm PermitOpen 1197.Sm off 1198.Ar host : port 1199.Sm on 1200.It 1201.Cm PermitOpen 1202.Sm off 1203.Ar IPv4_addr : port 1204.Sm on 1205.It 1206.Cm PermitOpen 1207.Sm off 1208.Ar \&[ IPv6_addr \&] : port 1209.Sm on 1210.El 1211.Pp 1212Multiple forwards may be specified by separating them with whitespace. 1213An argument of 1214.Cm any 1215can be used to remove all restrictions and permit any forwarding requests. 1216An argument of 1217.Cm none 1218can be used to prohibit all forwarding requests. 1219The wildcard 1220.Sq * 1221can be used for host or port to allow all hosts or ports, respectively. 1222By default all port forwarding requests are permitted. 1223.It Cm PermitRootLogin 1224Specifies whether root can log in using 1225.Xr ssh 1 . 1226The argument must be 1227.Cm yes , 1228.Cm prohibit-password , 1229.Cm forced-commands-only , 1230or 1231.Cm no . 1232The default is 1233.Cm prohibit-password . 1234.Pp 1235If this option is set to 1236.Cm prohibit-password 1237(or its deprecated alias, 1238.Cm without-password ) , 1239password and keyboard-interactive authentication are disabled for root. 1240.Pp 1241If this option is set to 1242.Cm forced-commands-only , 1243root login with public key authentication will be allowed, 1244but only if the 1245.Ar command 1246option has been specified 1247(which may be useful for taking remote backups even if root login is 1248normally not allowed). 1249All other authentication methods are disabled for root. 1250.Pp 1251If this option is set to 1252.Cm no , 1253root is not allowed to log in. 1254.It Cm PermitTTY 1255Specifies whether 1256.Xr pty 4 1257allocation is permitted. 1258The default is 1259.Cm yes . 1260.It Cm PermitTunnel 1261Specifies whether 1262.Xr tun 4 1263device forwarding is allowed. 1264The argument must be 1265.Cm yes , 1266.Cm point-to-point 1267(layer 3), 1268.Cm ethernet 1269(layer 2), or 1270.Cm no . 1271Specifying 1272.Cm yes 1273permits both 1274.Cm point-to-point 1275and 1276.Cm ethernet . 1277The default is 1278.Cm no . 1279.Pp 1280Independent of this setting, the permissions of the selected 1281.Xr tun 4 1282device must allow access to the user. 1283.It Cm PermitUserEnvironment 1284Specifies whether 1285.Pa ~/.ssh/environment 1286and 1287.Cm environment= 1288options in 1289.Pa ~/.ssh/authorized_keys 1290are processed by 1291.Xr sshd 8 . 1292The default is 1293.Cm no . 1294Enabling environment processing may enable users to bypass access 1295restrictions in some configurations using mechanisms such as 1296.Ev LD_PRELOAD . 1297.It Cm PermitUserRC 1298Specifies whether any 1299.Pa ~/.ssh/rc 1300file is executed. 1301The default is 1302.Cm yes . 1303.It Cm PidFile 1304Specifies the file that contains the process ID of the 1305SSH daemon, or 1306.Cm none 1307to not write one. 1308The default is 1309.Pa /var/run/sshd.pid . 1310.It Cm Port 1311Specifies the port number that 1312.Xr sshd 8 1313listens on. 1314The default is 22. 1315Multiple options of this type are permitted. 1316See also 1317.Cm ListenAddress . 1318.It Cm PrintLastLog 1319Specifies whether 1320.Xr sshd 8 1321should print the date and time of the last user login when a user logs 1322in interactively. 1323The default is 1324.Cm yes . 1325.It Cm PrintMotd 1326Specifies whether 1327.Xr sshd 8 1328should print 1329.Pa /etc/motd 1330when a user logs in interactively. 1331(On some systems it is also printed by the shell, 1332.Pa /etc/profile , 1333or equivalent.) 1334The default is 1335.Cm yes . 1336.It Cm PubkeyAcceptedKeyTypes 1337Specifies the key types that will be accepted for public key authentication 1338as a comma-separated pattern list. 1339Alternately if the specified value begins with a 1340.Sq + 1341character, then the specified key types will be appended to the default set 1342instead of replacing them. 1343If the specified value begins with a 1344.Sq - 1345character, then the specified key types (including wildcards) will be removed 1346from the default set instead of replacing them. 1347The default for this option is: 1348.Bd -literal -offset 3n 1349ecdsa-sha2-nistp256-cert-v01@openssh.com, 1350ecdsa-sha2-nistp384-cert-v01@openssh.com, 1351ecdsa-sha2-nistp521-cert-v01@openssh.com, 1352ssh-ed25519-cert-v01@openssh.com, 1353ssh-rsa-cert-v01@openssh.com, 1354ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1355ssh-ed25519,ssh-rsa 1356.Ed 1357.Pp 1358The list of available key types may also be obtained using 1359.Qq ssh -Q key . 1360.It Cm PubkeyAuthentication 1361Specifies whether public key authentication is allowed. 1362The default is 1363.Cm yes . 1364.It Cm RekeyLimit 1365Specifies the maximum amount of data that may be transmitted before the 1366session key is renegotiated, optionally followed a maximum amount of 1367time that may pass before the session key is renegotiated. 1368The first argument is specified in bytes and may have a suffix of 1369.Sq K , 1370.Sq M , 1371or 1372.Sq G 1373to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1374The default is between 1375.Sq 1G 1376and 1377.Sq 4G , 1378depending on the cipher. 1379The optional second value is specified in seconds and may use any of the 1380units documented in the 1381.Sx TIME FORMATS 1382section. 1383The default value for 1384.Cm RekeyLimit 1385is 1386.Cm default none , 1387which means that rekeying is performed after the cipher's default amount 1388of data has been sent or received and no time based rekeying is done. 1389.It Cm RevokedKeys 1390Specifies revoked public keys file, or 1391.Cm none 1392to not use one. 1393Keys listed in this file will be refused for public key authentication. 1394Note that if this file is not readable, then public key authentication will 1395be refused for all users. 1396Keys may be specified as a text file, listing one public key per line, or as 1397an OpenSSH Key Revocation List (KRL) as generated by 1398.Xr ssh-keygen 1 . 1399For more information on KRLs, see the KEY REVOCATION LISTS section in 1400.Xr ssh-keygen 1 . 1401.It Cm RDomain 1402Specifies an explicit routing domain that is applied after authentication 1403has completed. 1404The user session, as well and any forwarded or listening IP sockets, 1405will be bound to this 1406.Xr rdomain 4 . 1407If the routing domain is set to 1408.Cm \&%D , 1409then the domain in which the incoming connection was received will be applied. 1410.It Cm StreamLocalBindMask 1411Sets the octal file creation mode mask 1412.Pq umask 1413used when creating a Unix-domain socket file for local or remote 1414port forwarding. 1415This option is only used for port forwarding to a Unix-domain socket file. 1416.Pp 1417The default value is 0177, which creates a Unix-domain socket file that is 1418readable and writable only by the owner. 1419Note that not all operating systems honor the file mode on Unix-domain 1420socket files. 1421.It Cm StreamLocalBindUnlink 1422Specifies whether to remove an existing Unix-domain socket file for local 1423or remote port forwarding before creating a new one. 1424If the socket file already exists and 1425.Cm StreamLocalBindUnlink 1426is not enabled, 1427.Nm sshd 1428will be unable to forward the port to the Unix-domain socket file. 1429This option is only used for port forwarding to a Unix-domain socket file. 1430.Pp 1431The argument must be 1432.Cm yes 1433or 1434.Cm no . 1435The default is 1436.Cm no . 1437.It Cm StrictModes 1438Specifies whether 1439.Xr sshd 8 1440should check file modes and ownership of the 1441user's files and home directory before accepting login. 1442This is normally desirable because novices sometimes accidentally leave their 1443directory or files world-writable. 1444The default is 1445.Cm yes . 1446Note that this does not apply to 1447.Cm ChrootDirectory , 1448whose permissions and ownership are checked unconditionally. 1449.It Cm Subsystem 1450Configures an external subsystem (e.g. file transfer daemon). 1451Arguments should be a subsystem name and a command (with optional arguments) 1452to execute upon subsystem request. 1453.Pp 1454The command 1455.Cm sftp-server 1456implements the SFTP file transfer subsystem. 1457.Pp 1458Alternately the name 1459.Cm internal-sftp 1460implements an in-process SFTP server. 1461This may simplify configurations using 1462.Cm ChrootDirectory 1463to force a different filesystem root on clients. 1464.Pp 1465By default no subsystems are defined. 1466.It Cm SyslogFacility 1467Gives the facility code that is used when logging messages from 1468.Xr sshd 8 . 1469The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1470LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1471The default is AUTH. 1472.It Cm TCPKeepAlive 1473Specifies whether the system should send TCP keepalive messages to the 1474other side. 1475If they are sent, death of the connection or crash of one 1476of the machines will be properly noticed. 1477However, this means that 1478connections will die if the route is down temporarily, and some people 1479find it annoying. 1480On the other hand, if TCP keepalives are not sent, 1481sessions may hang indefinitely on the server, leaving 1482.Qq ghost 1483users and consuming server resources. 1484.Pp 1485The default is 1486.Cm yes 1487(to send TCP keepalive messages), and the server will notice 1488if the network goes down or the client host crashes. 1489This avoids infinitely hanging sessions. 1490.Pp 1491To disable TCP keepalive messages, the value should be set to 1492.Cm no . 1493.It Cm TrustedUserCAKeys 1494Specifies a file containing public keys of certificate authorities that are 1495trusted to sign user certificates for authentication, or 1496.Cm none 1497to not use one. 1498Keys are listed one per line; empty lines and comments starting with 1499.Ql # 1500are allowed. 1501If a certificate is presented for authentication and has its signing CA key 1502listed in this file, then it may be used for authentication for any user 1503listed in the certificate's principals list. 1504Note that certificates that lack a list of principals will not be permitted 1505for authentication using 1506.Cm TrustedUserCAKeys . 1507For more details on certificates, see the CERTIFICATES section in 1508.Xr ssh-keygen 1 . 1509.It Cm UseDNS 1510Specifies whether 1511.Xr sshd 8 1512should look up the remote host name, and to check that 1513the resolved host name for the remote IP address maps back to the 1514very same IP address. 1515.Pp 1516If this option is set to 1517.Cm no 1518(the default) then only addresses and not host names may be used in 1519.Pa ~/.ssh/authorized_keys 1520.Cm from 1521and 1522.Nm 1523.Cm Match 1524.Cm Host 1525directives. 1526.It Cm VersionAddendum 1527Optionally specifies additional text to append to the SSH protocol banner 1528sent by the server upon connection. 1529The default is 1530.Cm none . 1531.It Cm X11DisplayOffset 1532Specifies the first display number available for 1533.Xr sshd 8 Ns 's 1534X11 forwarding. 1535This prevents sshd from interfering with real X11 servers. 1536The default is 10. 1537.It Cm X11Forwarding 1538Specifies whether X11 forwarding is permitted. 1539The argument must be 1540.Cm yes 1541or 1542.Cm no . 1543The default is 1544.Cm no . 1545.Pp 1546When X11 forwarding is enabled, there may be additional exposure to 1547the server and to client displays if the 1548.Xr sshd 8 1549proxy display is configured to listen on the wildcard address (see 1550.Cm X11UseLocalhost ) , 1551though this is not the default. 1552Additionally, the authentication spoofing and authentication data 1553verification and substitution occur on the client side. 1554The security risk of using X11 forwarding is that the client's X11 1555display server may be exposed to attack when the SSH client requests 1556forwarding (see the warnings for 1557.Cm ForwardX11 1558in 1559.Xr ssh_config 5 ) . 1560A system administrator may have a stance in which they want to 1561protect clients that may expose themselves to attack by unwittingly 1562requesting X11 forwarding, which can warrant a 1563.Cm no 1564setting. 1565.Pp 1566Note that disabling X11 forwarding does not prevent users from 1567forwarding X11 traffic, as users can always install their own forwarders. 1568.It Cm X11UseLocalhost 1569Specifies whether 1570.Xr sshd 8 1571should bind the X11 forwarding server to the loopback address or to 1572the wildcard address. 1573By default, 1574sshd binds the forwarding server to the loopback address and sets the 1575hostname part of the 1576.Ev DISPLAY 1577environment variable to 1578.Cm localhost . 1579This prevents remote hosts from connecting to the proxy display. 1580However, some older X11 clients may not function with this 1581configuration. 1582.Cm X11UseLocalhost 1583may be set to 1584.Cm no 1585to specify that the forwarding server should be bound to the wildcard 1586address. 1587The argument must be 1588.Cm yes 1589or 1590.Cm no . 1591The default is 1592.Cm yes . 1593.It Cm XAuthLocation 1594Specifies the full pathname of the 1595.Xr xauth 1 1596program, or 1597.Cm none 1598to not use one. 1599The default is 1600.Pa /usr/X11R6/bin/xauth . 1601.It Cm UseLPK 1602Specifies whether LDAP public key retrieval must be used or not. It allow 1603an easy centralisation of public keys within an LDAP directory. The argument must be 1604.Dq yes 1605or 1606.Dq no . 1607.It Cm LpkLdapConf 1608Specifies whether LDAP Public keys should parse the specified ldap.conf file 1609instead of sshd_config Tokens. The argument must be a valid path to an ldap.conf 1610file like 1611.Pa /etc/ldap.conf 1612.It Cm LpkServers 1613Specifies LDAP one or more [:space:] separated server's url the following form may be used: 1614.Pp 1615LpkServers ldaps://127.0.0.1 ldap://127.0.0.2 ldap://127.0.0.3 1616.It Cm LpkUserDN 1617Specifies the LDAP user DN. 1618.Pp 1619LpkUserDN ou=users,dc=phear,dc=org 1620.It Cm LpkGroupDN 1621Specifies the LDAP groups DN. 1622.Pp 1623LpkGroupDN ou=groups,dc=phear,dc=org 1624.It Cm LpkBindDN 1625Specifies the LDAP bind DN to use if necessary. 1626.Pp 1627LpkBindDN cn=Manager,dc=phear,dc=org 1628.It Cm LpkBindPw 1629Specifies the LDAP bind credential. 1630.Pp 1631LpkBindPw secret 1632.It Cm LpkServerGroup 1633Specifies one or more [:space:] separated group the server is part of. 1634.Pp 1635LpkServerGroup unix mail prod 1636.It Cm LpkFilter 1637Specifies an additional LDAP filter to use for finding SSH keys 1638.Pp 1639LpkFilter (hostAccess=master.phear.org) 1640.It Cm LpkForceTLS 1641Specifies if the LDAP server connection must be tried, forced or not used. The argument must be 1642.Dq yes 1643or 1644.Dq no 1645or 1646.Dq try . 1647.It Cm LpkSearchTimelimit 1648Specifies the search time limit before the search is considered over. value is 1649in seconds. 1650.Pp 1651LpkSearchTimelimit 3 1652.It Cm LpkBindTimelimit 1653Specifies the bind time limit before the connection is considered dead. value is 1654in seconds. 1655.Pp 1656LpkBindTimelimit 3 1657.El 1658.Sh TIME FORMATS 1659.Xr sshd 8 1660command-line arguments and configuration file options that specify time 1661may be expressed using a sequence of the form: 1662.Sm off 1663.Ar time Op Ar qualifier , 1664.Sm on 1665where 1666.Ar time 1667is a positive integer value and 1668.Ar qualifier 1669is one of the following: 1670.Pp 1671.Bl -tag -width Ds -compact -offset indent 1672.It Aq Cm none 1673seconds 1674.It Cm s | Cm S 1675seconds 1676.It Cm m | Cm M 1677minutes 1678.It Cm h | Cm H 1679hours 1680.It Cm d | Cm D 1681days 1682.It Cm w | Cm W 1683weeks 1684.El 1685.Pp 1686Each member of the sequence is added together to calculate 1687the total time value. 1688.Pp 1689Time format examples: 1690.Pp 1691.Bl -tag -width Ds -compact -offset indent 1692.It 600 1693600 seconds (10 minutes) 1694.It 10m 169510 minutes 1696.It 1h30m 16971 hour 30 minutes (90 minutes) 1698.El 1699.Sh TOKENS 1700Arguments to some keywords can make use of tokens, 1701which are expanded at runtime: 1702.Pp 1703.Bl -tag -width XXXX -offset indent -compact 1704.It %% 1705A literal 1706.Sq % . 1707.It \&%D 1708The routing domain in which the incoming connection was received. 1709.It %F 1710The fingerprint of the CA key. 1711.It %f 1712The fingerprint of the key or certificate. 1713.It %h 1714The home directory of the user. 1715.It %i 1716The key ID in the certificate. 1717.It %K 1718The base64-encoded CA key. 1719.It %k 1720The base64-encoded key or certificate for authentication. 1721.It %s 1722The serial number of the certificate. 1723.It \&%T 1724The type of the CA key. 1725.It %t 1726The key or certificate type. 1727.It %u 1728The username. 1729.El 1730.Pp 1731.Cm AuthorizedKeysCommand 1732accepts the tokens %%, %f, %h, %k, %t, and %u. 1733.Pp 1734.Cm AuthorizedKeysFile 1735accepts the tokens %%, %h, and %u. 1736.Pp 1737.Cm AuthorizedPrincipalsCommand 1738accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, and %u. 1739.Pp 1740.Cm AuthorizedPrincipalsFile 1741accepts the tokens %%, %h, and %u. 1742.Pp 1743.Cm ChrootDirectory 1744accepts the tokens %%, %h, and %u. 1745.Pp 1746.Cm RoutingDomain 1747accepts the token %D. 1748.Sh FILES 1749.Bl -tag -width Ds 1750.It Pa /etc/ssh/sshd_config 1751Contains configuration data for 1752.Xr sshd 8 . 1753This file should be writable by root only, but it is recommended 1754(though not necessary) that it be world-readable. 1755.El 1756.Sh SEE ALSO 1757.Xr sftp-server 8 , 1758.Xr sshd 8 1759.Sh AUTHORS 1760.An -nosplit 1761OpenSSH is a derivative of the original and free 1762ssh 1.2.12 release by 1763.An Tatu Ylonen . 1764.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , 1765.An Theo de Raadt 1766and 1767.An Dug Song 1768removed many bugs, re-added newer features and 1769created OpenSSH. 1770.An Markus Friedl 1771contributed the support for SSH protocol versions 1.5 and 2.0. 1772.An Niels Provos 1773and 1774.An Markus Friedl 1775contributed support for privilege separation. 1776