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.\" 36.\" $OpenBSD: sshd_config.5,v 1.232 2016/09/14 05:42:25 djm Exp $ 37.Dd $Mdocdate: September 14 2016 $ 38.Dt SSHD_CONFIG 5 39.Os 40.Sh NAME 41.Nm sshd_config 42.Nd OpenSSH SSH daemon configuration file 43.Sh SYNOPSIS 44.Nm /etc/ssh/sshd_config 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. 53Lines starting with 54.Ql # 55and empty lines are interpreted as comments. 56Arguments may optionally be enclosed in double quotes 57.Pq \&" 58in order to represent arguments containing spaces. 59.Pp 60The possible 61keywords and their meanings are as follows (note that 62keywords are case-insensitive and arguments are case-sensitive): 63.Bl -tag -width Ds 64.It Cm AcceptEnv 65Specifies what environment variables sent by the client will be copied into 66the session's 67.Xr environ 7 . 68See 69.Cm SendEnv 70in 71.Xr ssh_config 5 72for how to configure the client. 73The 74.Ev TERM 75environment variable is always sent whenever the client 76requests a pseudo-terminal as it is required by the protocol. 77Variables are specified by name, which may contain the wildcard characters 78.Ql * 79and 80.Ql \&? . 81Multiple environment variables may be separated by whitespace or spread 82across multiple 83.Cm AcceptEnv 84directives. 85Be warned that some environment variables could be used to bypass restricted 86user environments. 87For this reason, care should be taken in the use of this directive. 88The default is not to accept any environment variables. 89.It Cm AddressFamily 90Specifies which address family should be used by 91.Xr sshd 8 . 92Valid arguments are 93.Dq any , 94.Dq inet 95(use IPv4 only), or 96.Dq inet6 97(use IPv6 only). 98The default is 99.Dq any . 100.It Cm AllowAgentForwarding 101Specifies whether 102.Xr ssh-agent 1 103forwarding is permitted. 104The default is 105.Dq 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.Dq yes 130or 131.Dq all 132to allow StreamLocal forwarding, 133.Dq no 134to prevent all StreamLocal forwarding, 135.Dq local 136to allow local (from the perspective of 137.Xr ssh 1 ) 138forwarding only or 139.Dq remote 140to allow remote forwarding only. 141The default is 142.Dq yes . 143Note that disabling StreamLocal forwarding does not improve security unless 144users are also denied shell access, as they can always install their 145own forwarders. 146.It Cm AllowTcpForwarding 147Specifies whether TCP forwarding is permitted. 148The available options are 149.Dq yes 150or 151.Dq all 152to allow TCP forwarding, 153.Dq no 154to prevent all TCP forwarding, 155.Dq local 156to allow local (from the perspective of 157.Xr ssh 1 ) 158forwarding only or 159.Dq remote 160to allow remote forwarding only. 161The default is 162.Dq yes . 163Note that disabling TCP forwarding does not improve security unless 164users are also denied shell access, as they can always install their 165own forwarders. 166.It Cm AllowUsers 167This keyword can be followed by a list of user name patterns, separated 168by spaces. 169If specified, login is allowed only for user names that 170match one of the patterns. 171Only user names are valid; a numerical user ID is not recognized. 172By default, login is allowed for all users. 173If the pattern takes the form USER@HOST then USER and HOST 174are separately checked, restricting logins to particular 175users from particular hosts. 176HOST criteria may additionally contain addresses to match in CIDR 177address/masklen format. 178The allow/deny directives are processed in the following order: 179.Cm DenyUsers , 180.Cm AllowUsers , 181.Cm DenyGroups , 182and finally 183.Cm AllowGroups . 184.Pp 185See PATTERNS in 186.Xr ssh_config 5 187for more information on patterns. 188.It Cm AuthenticationMethods 189Specifies the authentication methods that must be successfully completed 190for a user to be granted access. 191This option must be followed by one or more comma-separated lists of 192authentication method names, or by the single string 193.Dq any 194to indicate the default behaviour of accepting any single authentication 195method. 196if the default is overridden, then successful authentication requires 197completion of every method in at least one of these lists. 198.Pp 199For example, an argument of 200.Dq publickey,password publickey,keyboard-interactive 201would require the user to complete public key authentication, followed by 202either password or keyboard interactive authentication. 203Only methods that are next in one or more lists are offered at each stage, 204so for this example, it would not be possible to attempt password or 205keyboard-interactive authentication before public key. 206.Pp 207For keyboard interactive authentication it is also possible to 208restrict authentication to a specific device by appending a 209colon followed by the device identifier 210.Dq bsdauth , 211.Dq pam , 212or 213.Dq skey , 214depending on the server configuration. 215For example, 216.Dq keyboard-interactive:bsdauth 217would restrict keyboard interactive authentication to the 218.Dq bsdauth 219device. 220.Pp 221If the 222.Dq publickey 223method is listed more than once, 224.Xr sshd 8 225verifies that keys that have been used successfully are not reused for 226subsequent authentications. 227For example, an 228.Cm AuthenticationMethods 229of 230.Dq publickey,publickey 231will require successful authentication using two different public keys. 232.Pp 233Note that each authentication method listed should also be explicitly enabled 234in the configuration. 235The default 236.Dq any 237is not to require multiple authentication; successful completion 238of a single authentication method is sufficient. 239.It Cm AuthorizedKeysCommand 240Specifies a program to be used to look up the user's public keys. 241The program must be owned by root, not writable by group or others and 242specified by an absolute path. 243.Pp 244Arguments to 245.Cm AuthorizedKeysCommand 246may be provided using the following tokens, which will be expanded 247at runtime: %% is replaced by a literal '%', %u is replaced by the 248username being authenticated, %h is replaced by the home directory 249of the user being authenticated, %t is replaced with the key type 250offered for authentication, %f is replaced with the fingerprint of 251the key, and %k is replaced with the key being offered for authentication. 252If no arguments are specified then the username of the target user 253will be supplied. 254.Pp 255The program should produce on standard output zero or 256more lines of authorized_keys output (see AUTHORIZED_KEYS in 257.Xr sshd 8 ) . 258If a key supplied by AuthorizedKeysCommand does not successfully authenticate 259and authorize the user then public key authentication continues using the usual 260.Cm AuthorizedKeysFile 261files. 262By default, no AuthorizedKeysCommand is run. 263.It Cm AuthorizedKeysCommandUser 264Specifies the user under whose account the AuthorizedKeysCommand is run. 265It is recommended to use a dedicated user that has no other role on the host 266than running authorized keys commands. 267If 268.Cm AuthorizedKeysCommand 269is specified but 270.Cm AuthorizedKeysCommandUser 271is not, then 272.Xr sshd 8 273will refuse to start. 274.It Cm AuthorizedKeysFile 275Specifies the file that contains the public keys that can be used 276for user authentication. 277The format is described in the 278AUTHORIZED_KEYS FILE FORMAT 279section of 280.Xr sshd 8 . 281.Cm AuthorizedKeysFile 282may contain tokens of the form %T which are substituted during connection 283setup. 284The following tokens are defined: %% is replaced by a literal '%', 285%h is replaced by the home directory of the user being authenticated, and 286%u is replaced by the username of that user. 287After expansion, 288.Cm AuthorizedKeysFile 289is taken to be an absolute path or one relative to the user's home 290directory. 291Multiple files may be listed, separated by whitespace. 292Alternately this option may be set to 293.Dq none 294to skip checking for user keys in files. 295The default is 296.Dq .ssh/authorized_keys .ssh/authorized_keys2 . 297.It Cm AuthorizedPrincipalsCommand 298Specifies a program to be used to generate the list of allowed 299certificate principals as per 300.Cm AuthorizedPrincipalsFile . 301The program must be owned by root, not writable by group or others and 302specified by an absolute path. 303.Pp 304Arguments to 305.Cm AuthorizedPrincipalsCommand 306may be provided using the following tokens, which will be expanded 307at runtime: 308%% is replaced by a literal '%', 309%u is replaced by the username being authenticated, 310%h is replaced by the home directory of the user being authenticated, 311%t is replaced with type of the certificate being offered, 312%T with the type of the CA key, 313%f is replaced with certificate fingerprint, 314%F with the fingerprint of the CA key, 315%k is replaced with the full base-64 encoded certificate and 316%K is replaced with the base-64 encoded CA key. 317If no arguments are specified then the username of the target user 318will be supplied. 319.Pp 320The program should produce on standard output zero or 321more lines of 322.Cm AuthorizedPrincipalsFile 323output. 324If either 325.Cm AuthorizedPrincipalsCommand 326or 327.Cm AuthorizedPrincipalsFile 328is specified, then certificates offered by the client for authentication 329must contain a principal that is listed. 330By default, no AuthorizedPrincipalsCommand is run. 331.It Cm AuthorizedPrincipalsCommandUser 332Specifies the user under whose account the AuthorizedPrincipalsCommand is run. 333It is recommended to use a dedicated user that has no other role on the host 334than running authorized principals commands. 335If 336.Cm AuthorizedPrincipalsCommand 337is specified but 338.Cm AuthorizedPrincipalsCommandUser 339is not, then 340.Xr sshd 8 341will refuse to start. 342.It Cm AuthorizedPrincipalsFile 343Specifies a file that lists principal names that are accepted for 344certificate authentication. 345When using certificates signed by a key listed in 346.Cm TrustedUserCAKeys , 347this file lists names, one of which must appear in the certificate for it 348to be accepted for authentication. 349Names are listed one per line preceded by key options (as described 350in AUTHORIZED_KEYS FILE FORMAT in 351.Xr sshd 8 ) . 352Empty lines and comments starting with 353.Ql # 354are ignored. 355.Pp 356.Cm AuthorizedPrincipalsFile 357may contain tokens of the form %T which are substituted during connection 358setup. 359The following tokens are defined: %% is replaced by a literal '%', 360%h is replaced by the home directory of the user being authenticated, and 361%u is replaced by the username of that user. 362After expansion, 363.Cm AuthorizedPrincipalsFile 364is taken to be an absolute path or one relative to the user's home 365directory. 366.Pp 367The default is 368.Dq none , 369i.e. not to use a principals file \(en in this case, the username 370of the user must appear in a certificate's principals list for it to be 371accepted. 372Note that 373.Cm AuthorizedPrincipalsFile 374is only used when authentication proceeds using a CA listed in 375.Cm TrustedUserCAKeys 376and is not consulted for certification authorities trusted via 377.Pa ~/.ssh/authorized_keys , 378though the 379.Cm principals= 380key option offers a similar facility (see 381.Xr sshd 8 382for details). 383.It Cm Banner 384The contents of the specified file are sent to the remote user before 385authentication is allowed. 386If the argument is 387.Dq none 388then no banner is displayed. 389By default, no banner is displayed. 390.It Cm ChallengeResponseAuthentication 391Specifies whether challenge-response authentication is allowed. 392All authentication styles from 393.Xr login.conf 5 394are supported. 395The default is 396.Dq yes . 397.It Cm ChrootDirectory 398Specifies the pathname of a directory to 399.Xr chroot 2 400to after authentication. 401At session startup 402.Xr sshd 8 403checks that all components of the pathname are root-owned directories 404which are not writable by any other user or group. 405After the chroot, 406.Xr sshd 8 407changes the working directory to the user's home directory. 408.Pp 409The pathname may contain the following tokens that are expanded at runtime once 410the connecting user has been authenticated: %% is replaced by a literal '%', 411%h is replaced by the home directory of the user being authenticated, and 412%u is replaced by the username of that user. 413.Pp 414The 415.Cm ChrootDirectory 416must contain the necessary files and directories to support the 417user's session. 418For an interactive session this requires at least a shell, typically 419.Xr sh 1 , 420and basic 421.Pa /dev 422nodes such as 423.Xr null 4 , 424.Xr zero 4 , 425.Xr stdin 4 , 426.Xr stdout 4 , 427.Xr stderr 4 , 428and 429.Xr tty 4 430devices. 431For file transfer sessions using 432.Dq sftp , 433no additional configuration of the environment is necessary if the 434in-process sftp server is used, 435though sessions which use logging may require 436.Pa /dev/log 437inside the chroot directory on some operating systems (see 438.Xr sftp-server 8 439for details). 440.Pp 441For safety, it is very important that the directory hierarchy be 442prevented from modification by other processes on the system (especially 443those outside the jail). 444Misconfiguration can lead to unsafe environments which 445.Xr sshd 8 446cannot detect. 447.Pp 448The default is 449.Dq none , 450indicating not to 451.Xr chroot 2 . 452.It Cm Ciphers 453Specifies the ciphers allowed. 454Multiple ciphers must be comma-separated. 455If the specified value begins with a 456.Sq + 457character, then the specified ciphers will be appended to the default set 458instead of replacing them. 459.Pp 460The supported ciphers are: 461.Pp 462.Bl -item -compact -offset indent 463.It 4643des-cbc 465.It 466aes128-cbc 467.It 468aes192-cbc 469.It 470aes256-cbc 471.It 472aes128-ctr 473.It 474aes192-ctr 475.It 476aes256-ctr 477.It 478aes128-gcm@openssh.com 479.It 480aes256-gcm@openssh.com 481.It 482arcfour 483.It 484arcfour128 485.It 486arcfour256 487.It 488blowfish-cbc 489.It 490cast128-cbc 491.It 492chacha20-poly1305@openssh.com 493.El 494.Pp 495The default is: 496.Bd -literal -offset indent 497chacha20-poly1305@openssh.com, 498aes128-ctr,aes192-ctr,aes256-ctr, 499aes128-gcm@openssh.com,aes256-gcm@openssh.com 500.Ed 501.Pp 502The list of available ciphers may also be obtained using the 503.Fl Q 504option of 505.Xr ssh 1 506with an argument of 507.Dq cipher . 508.It Cm ClientAliveCountMax 509Sets the number of client alive messages (see below) which may be 510sent without 511.Xr sshd 8 512receiving any messages back from the client. 513If this threshold is reached while client alive messages are being sent, 514sshd will disconnect the client, terminating the session. 515It is important to note that the use of client alive messages is very 516different from 517.Cm TCPKeepAlive 518(below). 519The client alive messages are sent through the encrypted channel 520and therefore will not be spoofable. 521The TCP keepalive option enabled by 522.Cm TCPKeepAlive 523is spoofable. 524The client alive mechanism is valuable when the client or 525server depend on knowing when a connection has become inactive. 526.Pp 527The default value is 3. 528If 529.Cm ClientAliveInterval 530(see below) is set to 15, and 531.Cm ClientAliveCountMax 532is left at the default, unresponsive SSH clients 533will be disconnected after approximately 45 seconds. 534.It Cm ClientAliveInterval 535Sets a timeout interval in seconds after which if no data has been received 536from the client, 537.Xr sshd 8 538will send a message through the encrypted 539channel to request a response from the client. 540The default 541is 0, indicating that these messages will not be sent to the client. 542.It Cm Compression 543Specifies whether compression is allowed, or delayed until 544the user has authenticated successfully. 545The argument must be 546.Dq yes , 547.Dq delayed , 548or 549.Dq no . 550The default is 551.Dq delayed . 552.It Cm DenyGroups 553This keyword can be followed by a list of group name patterns, separated 554by spaces. 555Login is disallowed for users whose primary group or supplementary 556group list matches one of the patterns. 557Only group names are valid; a numerical group ID is not recognized. 558By default, login is allowed for all groups. 559The allow/deny directives are processed in the following order: 560.Cm DenyUsers , 561.Cm AllowUsers , 562.Cm DenyGroups , 563and finally 564.Cm AllowGroups . 565.Pp 566See PATTERNS in 567.Xr ssh_config 5 568for more information on patterns. 569.It Cm DenyUsers 570This keyword can be followed by a list of user name patterns, separated 571by spaces. 572Login is disallowed for user names that match one of the patterns. 573Only user names are valid; a numerical user ID is not recognized. 574By default, login is allowed for all users. 575If the pattern takes the form USER@HOST then USER and HOST 576are separately checked, restricting logins to particular 577users from particular hosts. 578HOST criteria may additionally contain addresses to match in CIDR 579address/masklen format. 580The allow/deny directives are processed in the following order: 581.Cm DenyUsers , 582.Cm AllowUsers , 583.Cm DenyGroups , 584and finally 585.Cm AllowGroups . 586.Pp 587See PATTERNS in 588.Xr ssh_config 5 589for more information on patterns. 590.It Cm FingerprintHash 591Specifies the hash algorithm used when logging key fingerprints. 592Valid options are: 593.Dq md5 594and 595.Dq sha256 . 596The default is 597.Dq sha256 . 598.It Cm ForceCommand 599Forces the execution of the command specified by 600.Cm ForceCommand , 601ignoring any command supplied by the client and 602.Pa ~/.ssh/rc 603if present. 604The command is invoked by using the user's login shell with the -c option. 605This applies to shell, command, or subsystem execution. 606It is most useful inside a 607.Cm Match 608block. 609The command originally supplied by the client is available in the 610.Ev SSH_ORIGINAL_COMMAND 611environment variable. 612Specifying a command of 613.Dq internal-sftp 614will force the use of an in-process sftp server that requires no support 615files when used with 616.Cm ChrootDirectory . 617The default is 618.Dq none . 619.It Cm GatewayPorts 620Specifies whether remote hosts are allowed to connect to ports 621forwarded for the client. 622By default, 623.Xr sshd 8 624binds remote port forwardings to the loopback address. 625This prevents other remote hosts from connecting to forwarded ports. 626.Cm GatewayPorts 627can be used to specify that sshd 628should allow remote port forwardings to bind to non-loopback addresses, thus 629allowing other hosts to connect. 630The argument may be 631.Dq no 632to force remote port forwardings to be available to the local host only, 633.Dq yes 634to force remote port forwardings to bind to the wildcard address, or 635.Dq clientspecified 636to allow the client to select the address to which the forwarding is bound. 637The default is 638.Dq no . 639.It Cm GSSAPIAuthentication 640Specifies whether user authentication based on GSSAPI is allowed. 641The default is 642.Dq no . 643.It Cm GSSAPICleanupCredentials 644Specifies whether to automatically destroy the user's credentials cache 645on logout. 646The default is 647.Dq yes . 648.It Cm GSSAPIStrictAcceptorCheck 649Determines whether to be strict about the identity of the GSSAPI acceptor 650a client authenticates against. 651If set to 652.Dq yes 653then the client must authenticate against the 654.Pa host 655service on the current hostname. 656If set to 657.Dq no 658then the client may authenticate against any service key stored in the 659machine's default store. 660This facility is provided to assist with operation on multi homed machines. 661The default is 662.Dq yes . 663.It Cm HostbasedAcceptedKeyTypes 664Specifies the key types that will be accepted for hostbased authentication 665as a comma-separated pattern list. 666Alternately if the specified value begins with a 667.Sq + 668character, then the specified key types will be appended to the default set 669instead of replacing them. 670The default for this option is: 671.Bd -literal -offset 3n 672ecdsa-sha2-nistp256-cert-v01@openssh.com, 673ecdsa-sha2-nistp384-cert-v01@openssh.com, 674ecdsa-sha2-nistp521-cert-v01@openssh.com, 675ssh-ed25519-cert-v01@openssh.com, 676ssh-rsa-cert-v01@openssh.com, 677ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 678ssh-ed25519,ssh-rsa 679.Ed 680.Pp 681The 682.Fl Q 683option of 684.Xr ssh 1 685may be used to list supported key types. 686.It Cm HostbasedAuthentication 687Specifies whether rhosts or /etc/hosts.equiv authentication together 688with successful public key client host authentication is allowed 689(host-based authentication). 690The default is 691.Dq no . 692.It Cm HostbasedUsesNameFromPacketOnly 693Specifies whether or not the server will attempt to perform a reverse 694name lookup when matching the name in the 695.Pa ~/.shosts , 696.Pa ~/.rhosts , 697and 698.Pa /etc/hosts.equiv 699files during 700.Cm HostbasedAuthentication . 701A setting of 702.Dq yes 703means that 704.Xr sshd 8 705uses the name supplied by the client rather than 706attempting to resolve the name from the TCP connection itself. 707The default is 708.Dq no . 709.It Cm HostCertificate 710Specifies a file containing a public host certificate. 711The certificate's public key must match a private host key already specified 712by 713.Cm HostKey . 714The default behaviour of 715.Xr sshd 8 716is not to load any certificates. 717.It Cm HostKey 718Specifies a file containing a private host key 719used by SSH. 720The defaults are 721.Pa /etc/ssh/ssh_host_dsa_key , 722.Pa /etc/ssh/ssh_host_ecdsa_key , 723.Pa /etc/ssh/ssh_host_ed25519_key 724and 725.Pa /etc/ssh/ssh_host_rsa_key . 726.Pp 727Note that 728.Xr sshd 8 729will refuse to use a file if it is group/world-accessible 730and that the 731.Cm HostKeyAlgorithms 732option restricts which of the keys are actually used by 733.Xr sshd 8 . 734.Pp 735It is possible to have multiple host key files. 736It is also possible to specify public host key files instead. 737In this case operations on the private key will be delegated 738to an 739.Xr ssh-agent 1 . 740.It Cm HostKeyAgent 741Identifies the UNIX-domain socket used to communicate 742with an agent that has access to the private host keys. 743If the string 744.Dq SSH_AUTH_SOCK 745is specified, the location of the socket will be read from the 746.Ev SSH_AUTH_SOCK 747environment variable. 748.It Cm HostKeyAlgorithms 749Specifies the host key algorithms 750that the server offers. 751The default for this option is: 752.Bd -literal -offset 3n 753ecdsa-sha2-nistp256-cert-v01@openssh.com, 754ecdsa-sha2-nistp384-cert-v01@openssh.com, 755ecdsa-sha2-nistp521-cert-v01@openssh.com, 756ssh-ed25519-cert-v01@openssh.com, 757ssh-rsa-cert-v01@openssh.com, 758ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 759ssh-ed25519,ssh-rsa 760.Ed 761.Pp 762The list of available key types may also be obtained using the 763.Fl Q 764option of 765.Xr ssh 1 766with an argument of 767.Dq key . 768.It Cm IgnoreRhosts 769Specifies that 770.Pa .rhosts 771and 772.Pa .shosts 773files will not be used in 774.Cm HostbasedAuthentication . 775.Pp 776.Pa /etc/hosts.equiv 777and 778.Pa /etc/shosts.equiv 779are still used. 780The default is 781.Dq yes . 782.It Cm IgnoreUserKnownHosts 783Specifies whether 784.Xr sshd 8 785should ignore the user's 786.Pa ~/.ssh/known_hosts 787during 788.Cm HostbasedAuthentication . 789The default is 790.Dq no . 791.It Cm IPQoS 792Specifies the IPv4 type-of-service or DSCP class for the connection. 793Accepted values are 794.Dq af11 , 795.Dq af12 , 796.Dq af13 , 797.Dq af21 , 798.Dq af22 , 799.Dq af23 , 800.Dq af31 , 801.Dq af32 , 802.Dq af33 , 803.Dq af41 , 804.Dq af42 , 805.Dq af43 , 806.Dq cs0 , 807.Dq cs1 , 808.Dq cs2 , 809.Dq cs3 , 810.Dq cs4 , 811.Dq cs5 , 812.Dq cs6 , 813.Dq cs7 , 814.Dq ef , 815.Dq lowdelay , 816.Dq throughput , 817.Dq reliability , 818or a numeric value. 819This option may take one or two arguments, separated by whitespace. 820If one argument is specified, it is used as the packet class unconditionally. 821If two values are specified, the first is automatically selected for 822interactive sessions and the second for non-interactive sessions. 823The default is 824.Dq lowdelay 825for interactive sessions and 826.Dq throughput 827for non-interactive sessions. 828.It Cm KbdInteractiveAuthentication 829Specifies whether to allow keyboard-interactive authentication. 830The argument to this keyword must be 831.Dq yes 832or 833.Dq no . 834The default is to use whatever value 835.Cm ChallengeResponseAuthentication 836is set to 837(by default 838.Dq yes ) . 839.It Cm KerberosAuthentication 840Specifies whether the password provided by the user for 841.Cm PasswordAuthentication 842will be validated through the Kerberos KDC. 843To use this option, the server needs a 844Kerberos servtab which allows the verification of the KDC's identity. 845The default is 846.Dq no . 847.It Cm KerberosGetAFSToken 848If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 849an AFS token before accessing the user's home directory. 850The default is 851.Dq no . 852.It Cm KerberosOrLocalPasswd 853If password authentication through Kerberos fails then 854the password will be validated via any additional local mechanism 855such as 856.Pa /etc/passwd . 857The default is 858.Dq yes . 859.It Cm KerberosTicketCleanup 860Specifies whether to automatically destroy the user's ticket cache 861file on logout. 862The default is 863.Dq yes . 864.It Cm KexAlgorithms 865Specifies the available KEX (Key Exchange) algorithms. 866Multiple algorithms must be comma-separated. 867Alternately if the specified value begins with a 868.Sq + 869character, then the specified methods will be appended to the default set 870instead of replacing them. 871The supported algorithms are: 872.Pp 873.Bl -item -compact -offset indent 874.It 875curve25519-sha256@libssh.org 876.It 877diffie-hellman-group1-sha1 878.It 879diffie-hellman-group14-sha1 880.It 881diffie-hellman-group-exchange-sha1 882.It 883diffie-hellman-group-exchange-sha256 884.It 885ecdh-sha2-nistp256 886.It 887ecdh-sha2-nistp384 888.It 889ecdh-sha2-nistp521 890.El 891.Pp 892The default is: 893.Bd -literal -offset indent 894curve25519-sha256@libssh.org, 895ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 896diffie-hellman-group-exchange-sha256, 897diffie-hellman-group14-sha1 898.Ed 899.Pp 900The list of available key exchange algorithms may also be obtained using the 901.Fl Q 902option of 903.Xr ssh 1 904with an argument of 905.Dq kex . 906.It Cm ListenAddress 907Specifies the local addresses 908.Xr sshd 8 909should listen on. 910The following forms may be used: 911.Pp 912.Bl -item -offset indent -compact 913.It 914.Cm ListenAddress 915.Sm off 916.Ar host | Ar IPv4_addr | Ar IPv6_addr 917.Sm on 918.It 919.Cm ListenAddress 920.Sm off 921.Ar host | Ar IPv4_addr : Ar port 922.Sm on 923.It 924.Cm ListenAddress 925.Sm off 926.Oo 927.Ar host | Ar IPv6_addr Oc : Ar port 928.Sm on 929.El 930.Pp 931If 932.Ar port 933is not specified, 934sshd will listen on the address and all 935.Cm Port 936options specified. 937The default is to listen on all local addresses. 938Multiple 939.Cm ListenAddress 940options are permitted. 941.It Cm LoginGraceTime 942The server disconnects after this time if the user has not 943successfully logged in. 944If the value is 0, there is no time limit. 945The default is 120 seconds. 946.It Cm LogLevel 947Gives the verbosity level that is used when logging messages from 948.Xr sshd 8 . 949The possible values are: 950QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 951The default is INFO. 952DEBUG and DEBUG1 are equivalent. 953DEBUG2 and DEBUG3 each specify higher levels of debugging output. 954Logging with a DEBUG level violates the privacy of users and is not recommended. 955.It Cm MACs 956Specifies the available MAC (message authentication code) algorithms. 957The MAC algorithm is used for data integrity protection. 958Multiple algorithms must be comma-separated. 959If the specified value begins with a 960.Sq + 961character, then the specified algorithms will be appended to the default set 962instead of replacing them. 963.Pp 964The algorithms that contain 965.Dq -etm 966calculate the MAC after encryption (encrypt-then-mac). 967These are considered safer and their use recommended. 968The supported MACs are: 969.Pp 970.Bl -item -compact -offset indent 971.It 972hmac-md5 973.It 974hmac-md5-96 975.It 976hmac-ripemd160 977.It 978hmac-sha1 979.It 980hmac-sha1-96 981.It 982hmac-sha2-256 983.It 984hmac-sha2-512 985.It 986umac-64@openssh.com 987.It 988umac-128@openssh.com 989.It 990hmac-md5-etm@openssh.com 991.It 992hmac-md5-96-etm@openssh.com 993.It 994hmac-ripemd160-etm@openssh.com 995.It 996hmac-sha1-etm@openssh.com 997.It 998hmac-sha1-96-etm@openssh.com 999.It 1000hmac-sha2-256-etm@openssh.com 1001.It 1002hmac-sha2-512-etm@openssh.com 1003.It 1004umac-64-etm@openssh.com 1005.It 1006umac-128-etm@openssh.com 1007.El 1008.Pp 1009The default is: 1010.Bd -literal -offset indent 1011umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1012hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1013hmac-sha1-etm@openssh.com, 1014umac-64@openssh.com,umac-128@openssh.com, 1015hmac-sha2-256,hmac-sha2-512,hmac-sha1 1016.Ed 1017.Pp 1018The list of available MAC algorithms may also be obtained using the 1019.Fl Q 1020option of 1021.Xr ssh 1 1022with an argument of 1023.Dq mac . 1024.It Cm Match 1025Introduces a conditional block. 1026If all of the criteria on the 1027.Cm Match 1028line are satisfied, the keywords on the following lines override those 1029set in the global section of the config file, until either another 1030.Cm Match 1031line or the end of the file. 1032If a keyword appears in multiple 1033.Cm Match 1034blocks that are satisfied, only the first instance of the keyword is 1035applied. 1036.Pp 1037The arguments to 1038.Cm Match 1039are one or more criteria-pattern pairs or the single token 1040.Cm All 1041which matches all criteria. 1042The available criteria are 1043.Cm User , 1044.Cm Group , 1045.Cm Host , 1046.Cm LocalAddress , 1047.Cm LocalPort , 1048and 1049.Cm Address . 1050The match patterns may consist of single entries or comma-separated 1051lists and may use the wildcard and negation operators described in the 1052PATTERNS section of 1053.Xr ssh_config 5 . 1054.Pp 1055The patterns in an 1056.Cm Address 1057criteria may additionally contain addresses to match in CIDR 1058address/masklen format, e.g.\& 1059.Dq 192.0.2.0/24 1060or 1061.Dq 2001:db8::/32 . 1062Note that the mask length provided must be consistent with the address - 1063it is an error to specify a mask length that is too long for the address 1064or one with bits set in this host portion of the address. 1065For example, 1066.Dq 192.0.2.0/33 1067and 1068.Dq 192.0.2.0/8 1069respectively. 1070.Pp 1071Only a subset of keywords may be used on the lines following a 1072.Cm Match 1073keyword. 1074Available keywords are 1075.Cm AcceptEnv , 1076.Cm AllowAgentForwarding , 1077.Cm AllowGroups , 1078.Cm AllowStreamLocalForwarding , 1079.Cm AllowTcpForwarding , 1080.Cm AllowUsers , 1081.Cm AuthenticationMethods , 1082.Cm AuthorizedKeysCommand , 1083.Cm AuthorizedKeysCommandUser , 1084.Cm AuthorizedKeysFile , 1085.Cm AuthorizedPrincipalsCommand , 1086.Cm AuthorizedPrincipalsCommandUser , 1087.Cm AuthorizedPrincipalsFile , 1088.Cm Banner , 1089.Cm ChrootDirectory , 1090.Cm DenyGroups , 1091.Cm DenyUsers , 1092.Cm ForceCommand , 1093.Cm GatewayPorts , 1094.Cm GSSAPIAuthentication , 1095.Cm HostbasedAcceptedKeyTypes , 1096.Cm HostbasedAuthentication , 1097.Cm HostbasedUsesNameFromPacketOnly , 1098.Cm IPQoS , 1099.Cm KbdInteractiveAuthentication , 1100.Cm KerberosAuthentication , 1101.Cm MaxAuthTries , 1102.Cm MaxSessions , 1103.Cm PasswordAuthentication , 1104.Cm PermitEmptyPasswords , 1105.Cm PermitOpen , 1106.Cm PermitRootLogin , 1107.Cm PermitTTY , 1108.Cm PermitTunnel , 1109.Cm PermitUserRC , 1110.Cm PubkeyAcceptedKeyTypes , 1111.Cm PubkeyAuthentication , 1112.Cm RekeyLimit , 1113.Cm RevokedKeys , 1114.Cm StreamLocalBindMask , 1115.Cm StreamLocalBindUnlink , 1116.Cm TrustedUserCAKeys , 1117.Cm X11DisplayOffset , 1118.Cm X11Forwarding 1119and 1120.Cm X11UseLocalHost . 1121.It Cm MaxAuthTries 1122Specifies the maximum number of authentication attempts permitted per 1123connection. 1124Once the number of failures reaches half this value, 1125additional failures are logged. 1126The default is 6. 1127.It Cm MaxSessions 1128Specifies the maximum number of open shell, login or subsystem (e.g. sftp) 1129sessions permitted per network connection. 1130Multiple sessions may be established by clients that support connection 1131multiplexing. 1132Setting 1133.Cm MaxSessions 1134to 1 will effectively disable session multiplexing, whereas setting it to 0 1135will prevent all shell, login and subsystem sessions while still permitting 1136forwarding. 1137The default is 10. 1138.It Cm MaxStartups 1139Specifies the maximum number of concurrent unauthenticated connections to the 1140SSH daemon. 1141Additional connections will be dropped until authentication succeeds or the 1142.Cm LoginGraceTime 1143expires for a connection. 1144The default is 10:30:100. 1145.Pp 1146Alternatively, random early drop can be enabled by specifying 1147the three colon separated values 1148.Dq start:rate:full 1149(e.g. "10:30:60"). 1150.Xr sshd 8 1151will refuse connection attempts with a probability of 1152.Dq rate/100 1153(30%) 1154if there are currently 1155.Dq start 1156(10) 1157unauthenticated connections. 1158The probability increases linearly and all connection attempts 1159are refused if the number of unauthenticated connections reaches 1160.Dq full 1161(60). 1162.It Cm PasswordAuthentication 1163Specifies whether password authentication is allowed. 1164The default is 1165.Dq yes . 1166.It Cm PermitEmptyPasswords 1167When password authentication is allowed, it specifies whether the 1168server allows login to accounts with empty password strings. 1169The default is 1170.Dq no . 1171.It Cm PermitOpen 1172Specifies the destinations to which TCP port forwarding is permitted. 1173The forwarding specification must be one of the following forms: 1174.Pp 1175.Bl -item -offset indent -compact 1176.It 1177.Cm PermitOpen 1178.Sm off 1179.Ar host : port 1180.Sm on 1181.It 1182.Cm PermitOpen 1183.Sm off 1184.Ar IPv4_addr : port 1185.Sm on 1186.It 1187.Cm PermitOpen 1188.Sm off 1189.Ar \&[ IPv6_addr \&] : port 1190.Sm on 1191.El 1192.Pp 1193Multiple forwards may be specified by separating them with whitespace. 1194An argument of 1195.Dq any 1196can be used to remove all restrictions and permit any forwarding requests. 1197An argument of 1198.Dq none 1199can be used to prohibit all forwarding requests. 1200The wildcard 1201.Dq * 1202can be used for host or port to allow all hosts or ports, respectively. 1203By default all port forwarding requests are permitted. 1204.It Cm PermitRootLogin 1205Specifies whether root can log in using 1206.Xr ssh 1 . 1207The argument must be 1208.Dq yes , 1209.Dq prohibit-password , 1210.Dq without-password , 1211.Dq forced-commands-only , 1212or 1213.Dq no . 1214The default is 1215.Dq prohibit-password . 1216.Pp 1217If this option is set to 1218.Dq prohibit-password 1219or 1220.Dq without-password , 1221password and keyboard-interactive authentication are disabled for root. 1222.Pp 1223If this option is set to 1224.Dq forced-commands-only , 1225root login with public key authentication will be allowed, 1226but only if the 1227.Ar command 1228option has been specified 1229(which may be useful for taking remote backups even if root login is 1230normally not allowed). 1231All other authentication methods are disabled for root. 1232.Pp 1233If this option is set to 1234.Dq no , 1235root is not allowed to log in. 1236.It Cm PermitTTY 1237Specifies whether 1238.Xr pty 4 1239allocation is permitted. 1240The default is 1241.Dq yes . 1242.It Cm PermitTunnel 1243Specifies whether 1244.Xr tun 4 1245device forwarding is allowed. 1246The argument must be 1247.Dq yes , 1248.Dq point-to-point 1249(layer 3), 1250.Dq ethernet 1251(layer 2), or 1252.Dq no . 1253Specifying 1254.Dq yes 1255permits both 1256.Dq point-to-point 1257and 1258.Dq ethernet . 1259The default is 1260.Dq no . 1261.Pp 1262Independent of this setting, the permissions of the selected 1263.Xr tun 4 1264device must allow access to the user. 1265.It Cm PermitUserEnvironment 1266Specifies whether 1267.Pa ~/.ssh/environment 1268and 1269.Cm environment= 1270options in 1271.Pa ~/.ssh/authorized_keys 1272are processed by 1273.Xr sshd 8 . 1274The default is 1275.Dq no . 1276Enabling environment processing may enable users to bypass access 1277restrictions in some configurations using mechanisms such as 1278.Ev LD_PRELOAD . 1279.It Cm PermitUserRC 1280Specifies whether any 1281.Pa ~/.ssh/rc 1282file is executed. 1283The default is 1284.Dq yes . 1285.It Cm PidFile 1286Specifies the file that contains the process ID of the 1287SSH daemon, or 1288.Dq none 1289to not write one. 1290The default is 1291.Pa /var/run/sshd.pid . 1292.It Cm Port 1293Specifies the port number that 1294.Xr sshd 8 1295listens on. 1296The default is 22. 1297Multiple options of this type are permitted. 1298See also 1299.Cm ListenAddress . 1300.It Cm PrintLastLog 1301Specifies whether 1302.Xr sshd 8 1303should print the date and time of the last user login when a user logs 1304in interactively. 1305The default is 1306.Dq yes . 1307.It Cm PrintMotd 1308Specifies whether 1309.Xr sshd 8 1310should print 1311.Pa /etc/motd 1312when a user logs in interactively. 1313(On some systems it is also printed by the shell, 1314.Pa /etc/profile , 1315or equivalent.) 1316The default is 1317.Dq yes . 1318.It Cm PubkeyAcceptedKeyTypes 1319Specifies the key types that will be accepted for public key authentication 1320as a comma-separated pattern list. 1321Alternately if the specified value begins with a 1322.Sq + 1323character, then the specified key types will be appended to the default set 1324instead of replacing them. 1325The default for this option is: 1326.Bd -literal -offset 3n 1327ecdsa-sha2-nistp256-cert-v01@openssh.com, 1328ecdsa-sha2-nistp384-cert-v01@openssh.com, 1329ecdsa-sha2-nistp521-cert-v01@openssh.com, 1330ssh-ed25519-cert-v01@openssh.com, 1331ssh-rsa-cert-v01@openssh.com, 1332ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1333ssh-ed25519,ssh-rsa 1334.Ed 1335.Pp 1336The 1337.Fl Q 1338option of 1339.Xr ssh 1 1340may be used to list supported key types. 1341.It Cm PubkeyAuthentication 1342Specifies whether public key authentication is allowed. 1343The default is 1344.Dq yes . 1345.It Cm RekeyLimit 1346Specifies the maximum amount of data that may be transmitted before the 1347session key is renegotiated, optionally followed a maximum amount of 1348time that may pass before the session key is renegotiated. 1349The first argument is specified in bytes and may have a suffix of 1350.Sq K , 1351.Sq M , 1352or 1353.Sq G 1354to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1355The default is between 1356.Sq 1G 1357and 1358.Sq 4G , 1359depending on the cipher. 1360The optional second value is specified in seconds and may use any of the 1361units documented in the 1362.Sx TIME FORMATS 1363section. 1364The default value for 1365.Cm RekeyLimit 1366is 1367.Dq default none , 1368which means that rekeying is performed after the cipher's default amount 1369of data has been sent or received and no time based rekeying is done. 1370.It Cm RevokedKeys 1371Specifies revoked public keys file, or 1372.Dq none 1373to not use one. 1374Keys listed in this file will be refused for public key authentication. 1375Note that if this file is not readable, then public key authentication will 1376be refused for all users. 1377Keys may be specified as a text file, listing one public key per line, or as 1378an OpenSSH Key Revocation List (KRL) as generated by 1379.Xr ssh-keygen 1 . 1380For more information on KRLs, see the KEY REVOCATION LISTS section in 1381.Xr ssh-keygen 1 . 1382.It Cm StreamLocalBindMask 1383Sets the octal file creation mode mask 1384.Pq umask 1385used when creating a Unix-domain socket file for local or remote 1386port forwarding. 1387This option is only used for port forwarding to a Unix-domain socket file. 1388.Pp 1389The default value is 0177, which creates a Unix-domain socket file that is 1390readable and writable only by the owner. 1391Note that not all operating systems honor the file mode on Unix-domain 1392socket files. 1393.It Cm StreamLocalBindUnlink 1394Specifies whether to remove an existing Unix-domain socket file for local 1395or remote port forwarding before creating a new one. 1396If the socket file already exists and 1397.Cm StreamLocalBindUnlink 1398is not enabled, 1399.Nm sshd 1400will be unable to forward the port to the Unix-domain socket file. 1401This option is only used for port forwarding to a Unix-domain socket file. 1402.Pp 1403The argument must be 1404.Dq yes 1405or 1406.Dq no . 1407The default is 1408.Dq no . 1409.It Cm StrictModes 1410Specifies whether 1411.Xr sshd 8 1412should check file modes and ownership of the 1413user's files and home directory before accepting login. 1414This is normally desirable because novices sometimes accidentally leave their 1415directory or files world-writable. 1416The default is 1417.Dq yes . 1418Note that this does not apply to 1419.Cm ChrootDirectory , 1420whose permissions and ownership are checked unconditionally. 1421.It Cm Subsystem 1422Configures an external subsystem (e.g. file transfer daemon). 1423Arguments should be a subsystem name and a command (with optional arguments) 1424to execute upon subsystem request. 1425.Pp 1426The command 1427.Xr sftp-server 8 1428implements the 1429.Dq sftp 1430file transfer subsystem. 1431.Pp 1432Alternately the name 1433.Dq internal-sftp 1434implements an in-process 1435.Dq sftp 1436server. 1437This may simplify configurations using 1438.Cm ChrootDirectory 1439to force a different filesystem root on clients. 1440.Pp 1441By default no subsystems are defined. 1442.It Cm SyslogFacility 1443Gives the facility code that is used when logging messages from 1444.Xr sshd 8 . 1445The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1446LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1447The default is AUTH. 1448.It Cm TCPKeepAlive 1449Specifies whether the system should send TCP keepalive messages to the 1450other side. 1451If they are sent, death of the connection or crash of one 1452of the machines will be properly noticed. 1453However, this means that 1454connections will die if the route is down temporarily, and some people 1455find it annoying. 1456On the other hand, if TCP keepalives are not sent, 1457sessions may hang indefinitely on the server, leaving 1458.Dq ghost 1459users and consuming server resources. 1460.Pp 1461The default is 1462.Dq yes 1463(to send TCP keepalive messages), and the server will notice 1464if the network goes down or the client host crashes. 1465This avoids infinitely hanging sessions. 1466.Pp 1467To disable TCP keepalive messages, the value should be set to 1468.Dq no . 1469.It Cm TrustedUserCAKeys 1470Specifies a file containing public keys of certificate authorities that are 1471trusted to sign user certificates for authentication, or 1472.Dq none 1473to not use one. 1474Keys are listed one per line; empty lines and comments starting with 1475.Ql # 1476are allowed. 1477If a certificate is presented for authentication and has its signing CA key 1478listed in this file, then it may be used for authentication for any user 1479listed in the certificate's principals list. 1480Note that certificates that lack a list of principals will not be permitted 1481for authentication using 1482.Cm TrustedUserCAKeys . 1483For more details on certificates, see the CERTIFICATES section in 1484.Xr ssh-keygen 1 . 1485.It Cm UseDNS 1486Specifies whether 1487.Xr sshd 8 1488should look up the remote host name, and to check that 1489the resolved host name for the remote IP address maps back to the 1490very same IP address. 1491.Pp 1492If this option is set to 1493.Dq no 1494(the default) then only addresses and not host names may be used in 1495.Pa ~/.ssh/authorized_keys 1496.Cm from 1497and 1498.Nm 1499.Cm Match 1500.Cm Host 1501directives. 1502.It Cm UsePrivilegeSeparation 1503Specifies whether 1504.Xr sshd 8 1505separates privileges by creating an unprivileged child process 1506to deal with incoming network traffic. 1507After successful authentication, another process will be created that has 1508the privilege of the authenticated user. 1509The goal of privilege separation is to prevent privilege 1510escalation by containing any corruption within the unprivileged processes. 1511The argument must be 1512.Dq yes , 1513.Dq no , 1514or 1515.Dq sandbox . 1516If 1517.Cm UsePrivilegeSeparation 1518is set to 1519.Dq sandbox 1520then the pre-authentication unprivileged process is subject to additional 1521restrictions. 1522The default is 1523.Dq sandbox . 1524.It Cm VersionAddendum 1525Optionally specifies additional text to append to the SSH protocol banner 1526sent by the server upon connection. 1527The default is 1528.Dq none . 1529.It Cm X11DisplayOffset 1530Specifies the first display number available for 1531.Xr sshd 8 Ns 's 1532X11 forwarding. 1533This prevents sshd from interfering with real X11 servers. 1534The default is 10. 1535.It Cm X11Forwarding 1536Specifies whether X11 forwarding is permitted. 1537The argument must be 1538.Dq yes 1539or 1540.Dq no . 1541The default is 1542.Dq no . 1543.Pp 1544When X11 forwarding is enabled, there may be additional exposure to 1545the server and to client displays if the 1546.Xr sshd 8 1547proxy display is configured to listen on the wildcard address (see 1548.Cm X11UseLocalhost 1549below), though this is not the default. 1550Additionally, the authentication spoofing and authentication data 1551verification and substitution occur on the client side. 1552The security risk of using X11 forwarding is that the client's X11 1553display server may be exposed to attack when the SSH client requests 1554forwarding (see the warnings for 1555.Cm ForwardX11 1556in 1557.Xr ssh_config 5 ) . 1558A system administrator may have a stance in which they want to 1559protect clients that may expose themselves to attack by unwittingly 1560requesting X11 forwarding, which can warrant a 1561.Dq no 1562setting. 1563.Pp 1564Note that disabling X11 forwarding does not prevent users from 1565forwarding X11 traffic, as users can always install their own forwarders. 1566.It Cm X11UseLocalhost 1567Specifies whether 1568.Xr sshd 8 1569should bind the X11 forwarding server to the loopback address or to 1570the wildcard address. 1571By default, 1572sshd binds the forwarding server to the loopback address and sets the 1573hostname part of the 1574.Ev DISPLAY 1575environment variable to 1576.Dq localhost . 1577This prevents remote hosts from connecting to the proxy display. 1578However, some older X11 clients may not function with this 1579configuration. 1580.Cm X11UseLocalhost 1581may be set to 1582.Dq no 1583to specify that the forwarding server should be bound to the wildcard 1584address. 1585The argument must be 1586.Dq yes 1587or 1588.Dq no . 1589The default is 1590.Dq yes . 1591.It Cm XAuthLocation 1592Specifies the full pathname of the 1593.Xr xauth 1 1594program, or 1595.Dq none 1596to not use one. 1597The default is 1598.Pa /usr/X11R6/bin/xauth . 1599.El 1600.Sh TIME FORMATS 1601.Xr sshd 8 1602command-line arguments and configuration file options that specify time 1603may be expressed using a sequence of the form: 1604.Sm off 1605.Ar time Op Ar qualifier , 1606.Sm on 1607where 1608.Ar time 1609is a positive integer value and 1610.Ar qualifier 1611is one of the following: 1612.Pp 1613.Bl -tag -width Ds -compact -offset indent 1614.It Aq Cm none 1615seconds 1616.It Cm s | Cm S 1617seconds 1618.It Cm m | Cm M 1619minutes 1620.It Cm h | Cm H 1621hours 1622.It Cm d | Cm D 1623days 1624.It Cm w | Cm W 1625weeks 1626.El 1627.Pp 1628Each member of the sequence is added together to calculate 1629the total time value. 1630.Pp 1631Time format examples: 1632.Pp 1633.Bl -tag -width Ds -compact -offset indent 1634.It 600 1635600 seconds (10 minutes) 1636.It 10m 163710 minutes 1638.It 1h30m 16391 hour 30 minutes (90 minutes) 1640.El 1641.Sh FILES 1642.Bl -tag -width Ds 1643.It Pa /etc/ssh/sshd_config 1644Contains configuration data for 1645.Xr sshd 8 . 1646This file should be writable by root only, but it is recommended 1647(though not necessary) that it be world-readable. 1648.El 1649.Sh SEE ALSO 1650.Xr sshd 8 1651.Sh AUTHORS 1652OpenSSH is a derivative of the original and free 1653ssh 1.2.12 release by Tatu Ylonen. 1654Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1655Theo de Raadt and Dug Song 1656removed many bugs, re-added newer features and 1657created OpenSSH. 1658Markus Friedl contributed the support for SSH 1659protocol versions 1.5 and 2.0. 1660Niels Provos and Markus Friedl contributed support 1661for privilege separation. 1662