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 1.227 2016/07/19 12:59:16 jmc Exp $ 37.Dd $Mdocdate: July 19 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 AllowTcpForwarding 127Specifies whether TCP forwarding is permitted. 128The available options are 129.Dq yes 130or 131.Dq all 132to allow TCP forwarding, 133.Dq no 134to prevent all TCP 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 TCP forwarding does not improve security unless 144users are also denied shell access, as they can always install their 145own forwarders. 146.It Cm AllowStreamLocalForwarding 147Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. 148The available options are 149.Dq yes 150or 151.Dq all 152to allow StreamLocal forwarding, 153.Dq no 154to prevent all StreamLocal 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 StreamLocal 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 233This option will yield a fatal 234error if enabled if protocol 1 is also enabled. 235Note that each authentication method listed should also be explicitly enabled 236in the configuration. 237The default 238.Dq any 239is not to require multiple authentication; successful completion 240of a single authentication method is sufficient. 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. 245.Pp 246Arguments to 247.Cm AuthorizedKeysCommand 248may be provided using the following tokens, which will be expanded 249at runtime: %% is replaced by a literal '%', %u is replaced by the 250username being authenticated, %h is replaced by the home directory 251of the user being authenticated, %t is replaced with the key type 252offered for authentication, %f is replaced with the fingerprint of 253the key, and %k is replaced with the key being offered for authentication. 254If no arguments are specified then the username of the target user 255will be supplied. 256.Pp 257The program should produce on standard output zero or 258more lines of authorized_keys output (see AUTHORIZED_KEYS in 259.Xr sshd 8 ) . 260If a key supplied by AuthorizedKeysCommand does not successfully authenticate 261and authorize the user then public key authentication continues using the usual 262.Cm AuthorizedKeysFile 263files. 264By default, no AuthorizedKeysCommand is run. 265.It Cm AuthorizedKeysCommandUser 266Specifies the user under whose account the AuthorizedKeysCommand is run. 267It is recommended to use a dedicated user that has no other role on the host 268than running authorized keys commands. 269If 270.Cm AuthorizedKeysCommand 271is specified but 272.Cm AuthorizedKeysCommandUser 273is not, then 274.Xr sshd 8 275will refuse to start. 276.It Cm AuthorizedKeysFile 277Specifies the file that contains the public keys that can be used 278for user authentication. 279The format is described in the 280AUTHORIZED_KEYS FILE FORMAT 281section of 282.Xr sshd 8 . 283.Cm AuthorizedKeysFile 284may contain tokens of the form %T which are substituted during connection 285setup. 286The following tokens are defined: %% is replaced by a literal '%', 287%h is replaced by the home directory of the user being authenticated, and 288%u is replaced by the username of that user. 289After expansion, 290.Cm AuthorizedKeysFile 291is taken to be an absolute path or one relative to the user's home 292directory. 293Multiple files may be listed, separated by whitespace. 294Alternately this option may be set to 295.Dq none 296to skip checking for user keys in files. 297The default is 298.Dq .ssh/authorized_keys .ssh/authorized_keys2 . 299.It Cm AuthorizedPrincipalsCommand 300Specifies a program to be used to generate the list of allowed 301certificate principals as per 302.Cm AuthorizedPrincipalsFile . 303The program must be owned by root, not writable by group or others and 304specified by an absolute path. 305.Pp 306Arguments to 307.Cm AuthorizedPrincipalsCommand 308may be provided using the following tokens, which will be expanded 309at runtime: %% is replaced by a literal '%', %u is replaced by the 310username being authenticated and %h is replaced by the home directory 311of the user being authenticated. 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 AuthorizedPrincipalsCommand is run. 324.It Cm AuthorizedPrincipalsCommandUser 325Specifies the user under whose account the AuthorizedPrincipalsCommand is run. 326It is recommended to use a dedicated user that has no other role on the host 327than running authorized principals commands. 328If 329.Cm AuthorizedPrincipalsCommand 330is specified but 331.Cm AuthorizedPrincipalsCommandUser 332is not, then 333.Xr sshd 8 334will refuse to start. 335.It Cm AuthorizedPrincipalsFile 336Specifies a file that lists principal names that are accepted for 337certificate authentication. 338When using certificates signed by a key listed in 339.Cm TrustedUserCAKeys , 340this file lists names, one of which must appear in the certificate for it 341to be accepted for authentication. 342Names are listed one per line preceded by key options (as described 343in AUTHORIZED_KEYS FILE FORMAT in 344.Xr sshd 8 ) . 345Empty lines and comments starting with 346.Ql # 347are ignored. 348.Pp 349.Cm AuthorizedPrincipalsFile 350may contain tokens of the form %T which are substituted during connection 351setup. 352The following tokens are defined: %% is replaced by a literal '%', 353%h is replaced by the home directory of the user being authenticated, and 354%u is replaced by the username of that user. 355After expansion, 356.Cm AuthorizedPrincipalsFile 357is taken to be an absolute path or one relative to the user's home 358directory. 359.Pp 360The default is 361.Dq none , 362i.e. not to use a principals file \(en in this case, the username 363of the user must appear in a certificate's principals list for it to be 364accepted. 365Note that 366.Cm AuthorizedPrincipalsFile 367is only used when authentication proceeds using a CA listed in 368.Cm TrustedUserCAKeys 369and is not consulted for certification authorities trusted via 370.Pa ~/.ssh/authorized_keys , 371though the 372.Cm principals= 373key option offers a similar facility (see 374.Xr sshd 8 375for details). 376.It Cm Banner 377The contents of the specified file are sent to the remote user before 378authentication is allowed. 379If the argument is 380.Dq none 381then no banner is displayed. 382By default, no banner is displayed. 383.It Cm ChallengeResponseAuthentication 384Specifies whether challenge-response authentication is allowed. 385All authentication styles from 386.Xr login.conf 5 387are supported. 388The default is 389.Dq yes . 390.It Cm ChrootDirectory 391Specifies the pathname of a directory to 392.Xr chroot 2 393to after authentication. 394At session startup 395.Xr sshd 8 396checks that all components of the pathname are root-owned directories 397which are not writable by any other user or group. 398After the chroot, 399.Xr sshd 8 400changes the working directory to the user's home directory. 401.Pp 402The pathname may contain the following tokens that are expanded at runtime once 403the connecting user has been authenticated: %% is replaced by a literal '%', 404%h is replaced by the home directory of the user being authenticated, and 405%u is replaced by the username of that user. 406.Pp 407The 408.Cm ChrootDirectory 409must contain the necessary files and directories to support the 410user's session. 411For an interactive session this requires at least a shell, typically 412.Xr sh 1 , 413and basic 414.Pa /dev 415nodes such as 416.Xr null 4 , 417.Xr zero 4 , 418.Xr stdin 4 , 419.Xr stdout 4 , 420.Xr stderr 4 , 421and 422.Xr tty 4 423devices. 424For file transfer sessions using 425.Dq sftp , 426no additional configuration of the environment is necessary if the 427in-process sftp server is used, 428though sessions which use logging may require 429.Pa /dev/log 430inside the chroot directory on some operating systems (see 431.Xr sftp-server 8 432for details). 433.Pp 434For safety, it is very important that the directory hierarchy be 435prevented from modification by other processes on the system (especially 436those outside the jail). 437Misconfiguration can lead to unsafe environments which 438.Xr sshd 8 439cannot detect. 440.Pp 441The default is 442.Dq none , 443indicating not to 444.Xr chroot 2 . 445.It Cm Ciphers 446Specifies the ciphers allowed. 447Multiple ciphers must be comma-separated. 448If the specified value begins with a 449.Sq + 450character, then the specified ciphers will be appended to the default set 451instead of replacing them. 452.Pp 453The supported ciphers are: 454.Pp 455.Bl -item -compact -offset indent 456.It 4573des-cbc 458.It 459aes128-cbc 460.It 461aes192-cbc 462.It 463aes256-cbc 464.It 465aes128-ctr 466.It 467aes192-ctr 468.It 469aes256-ctr 470.It 471aes128-gcm@openssh.com 472.It 473aes256-gcm@openssh.com 474.It 475arcfour 476.It 477arcfour128 478.It 479arcfour256 480.It 481blowfish-cbc 482.It 483cast128-cbc 484.It 485chacha20-poly1305@openssh.com 486.El 487.Pp 488The default is: 489.Bd -literal -offset indent 490chacha20-poly1305@openssh.com, 491aes128-ctr,aes192-ctr,aes256-ctr, 492aes128-gcm@openssh.com,aes256-gcm@openssh.com 493.Ed 494.Pp 495The list of available ciphers may also be obtained using the 496.Fl Q 497option of 498.Xr ssh 1 499with an argument of 500.Dq cipher . 501.It Cm ClientAliveCountMax 502Sets the number of client alive messages (see below) which may be 503sent without 504.Xr sshd 8 505receiving any messages back from the client. 506If this threshold is reached while client alive messages are being sent, 507sshd will disconnect the client, terminating the session. 508It is important to note that the use of client alive messages is very 509different from 510.Cm TCPKeepAlive 511(below). 512The client alive messages are sent through the encrypted channel 513and therefore will not be spoofable. 514The TCP keepalive option enabled by 515.Cm TCPKeepAlive 516is spoofable. 517The client alive mechanism is valuable when the client or 518server depend on knowing when a connection has become inactive. 519.Pp 520The default value is 3. 521If 522.Cm ClientAliveInterval 523(see below) is set to 15, and 524.Cm ClientAliveCountMax 525is left at the default, unresponsive SSH clients 526will be disconnected after approximately 45 seconds. 527.It Cm ClientAliveInterval 528Sets a timeout interval in seconds after which if no data has been received 529from the client, 530.Xr sshd 8 531will send a message through the encrypted 532channel to request a response from the client. 533The default 534is 0, indicating that these messages will not be sent to the client. 535.It Cm Compression 536Specifies whether compression is allowed, or delayed until 537the user has authenticated successfully. 538The argument must be 539.Dq yes , 540.Dq delayed , 541or 542.Dq no . 543The default is 544.Dq delayed . 545.It Cm DenyGroups 546This keyword can be followed by a list of group name patterns, separated 547by spaces. 548Login is disallowed for users whose primary group or supplementary 549group list matches one of the patterns. 550Only group names are valid; a numerical group ID is not recognized. 551By default, login is allowed for all groups. 552The allow/deny directives are processed in the following order: 553.Cm DenyUsers , 554.Cm AllowUsers , 555.Cm DenyGroups , 556and finally 557.Cm AllowGroups . 558.Pp 559See PATTERNS in 560.Xr ssh_config 5 561for more information on patterns. 562.It Cm DenyUsers 563This keyword can be followed by a list of user name patterns, separated 564by spaces. 565Login is disallowed for user names that match one of the patterns. 566Only user names are valid; a numerical user ID is not recognized. 567By default, login is allowed for all users. 568If the pattern takes the form USER@HOST then USER and HOST 569are separately checked, restricting logins to particular 570users from particular hosts. 571HOST criteria may additionally contain addresses to match in CIDR 572address/masklen format. 573The allow/deny directives are processed in the following order: 574.Cm DenyUsers , 575.Cm AllowUsers , 576.Cm DenyGroups , 577and finally 578.Cm AllowGroups . 579.Pp 580See PATTERNS in 581.Xr ssh_config 5 582for more information on patterns. 583.It Cm FingerprintHash 584Specifies the hash algorithm used when logging key fingerprints. 585Valid options are: 586.Dq md5 587and 588.Dq sha256 . 589The default is 590.Dq sha256 . 591.It Cm ForceCommand 592Forces the execution of the command specified by 593.Cm ForceCommand , 594ignoring any command supplied by the client and 595.Pa ~/.ssh/rc 596if present. 597The command is invoked by using the user's login shell with the -c option. 598This applies to shell, command, or subsystem execution. 599It is most useful inside a 600.Cm Match 601block. 602The command originally supplied by the client is available in the 603.Ev SSH_ORIGINAL_COMMAND 604environment variable. 605Specifying a command of 606.Dq internal-sftp 607will force the use of an in-process sftp server that requires no support 608files when used with 609.Cm ChrootDirectory . 610The default is 611.Dq none . 612.It Cm GatewayPorts 613Specifies whether remote hosts are allowed to connect to ports 614forwarded for the client. 615By default, 616.Xr sshd 8 617binds remote port forwardings to the loopback address. 618This prevents other remote hosts from connecting to forwarded ports. 619.Cm GatewayPorts 620can be used to specify that sshd 621should allow remote port forwardings to bind to non-loopback addresses, thus 622allowing other hosts to connect. 623The argument may be 624.Dq no 625to force remote port forwardings to be available to the local host only, 626.Dq yes 627to force remote port forwardings to bind to the wildcard address, or 628.Dq clientspecified 629to allow the client to select the address to which the forwarding is bound. 630The default is 631.Dq no . 632.It Cm GSSAPIAuthentication 633Specifies whether user authentication based on GSSAPI is allowed. 634The default is 635.Dq no . 636.It Cm GSSAPICleanupCredentials 637Specifies whether to automatically destroy the user's credentials cache 638on logout. 639The default is 640.Dq yes . 641.It Cm GSSAPIStrictAcceptorCheck 642Determines whether to be strict about the identity of the GSSAPI acceptor 643a client authenticates against. 644If set to 645.Dq yes 646then the client must authenticate against the 647.Pa host 648service on the current hostname. 649If set to 650.Dq no 651then the client may authenticate against any service key stored in the 652machine's default store. 653This facility is provided to assist with operation on multi homed machines. 654The default is 655.Dq yes . 656.It Cm HostbasedAcceptedKeyTypes 657Specifies the key types that will be accepted for hostbased authentication 658as a comma-separated pattern list. 659Alternately if the specified value begins with a 660.Sq + 661character, then the specified key types will be appended to the default set 662instead of replacing them. 663The default for this option is: 664.Bd -literal -offset 3n 665ecdsa-sha2-nistp256-cert-v01@openssh.com, 666ecdsa-sha2-nistp384-cert-v01@openssh.com, 667ecdsa-sha2-nistp521-cert-v01@openssh.com, 668ssh-ed25519-cert-v01@openssh.com, 669ssh-rsa-cert-v01@openssh.com, 670ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 671ssh-ed25519,ssh-rsa 672.Ed 673.Pp 674The 675.Fl Q 676option of 677.Xr ssh 1 678may be used to list supported key types. 679.It Cm HostbasedAuthentication 680Specifies whether rhosts or /etc/hosts.equiv authentication together 681with successful public key client host authentication is allowed 682(host-based authentication). 683The default is 684.Dq no . 685.It Cm HostbasedUsesNameFromPacketOnly 686Specifies whether or not the server will attempt to perform a reverse 687name lookup when matching the name in the 688.Pa ~/.shosts , 689.Pa ~/.rhosts , 690and 691.Pa /etc/hosts.equiv 692files during 693.Cm HostbasedAuthentication . 694A setting of 695.Dq yes 696means that 697.Xr sshd 8 698uses the name supplied by the client rather than 699attempting to resolve the name from the TCP connection itself. 700The default is 701.Dq no . 702.It Cm HostCertificate 703Specifies a file containing a public host certificate. 704The certificate's public key must match a private host key already specified 705by 706.Cm HostKey . 707The default behaviour of 708.Xr sshd 8 709is not to load any certificates. 710.It Cm HostKey 711Specifies a file containing a private host key 712used by SSH. 713The default is 714.Pa /etc/ssh/ssh_host_key 715for protocol version 1, and 716.Pa /etc/ssh/ssh_host_dsa_key , 717.Pa /etc/ssh/ssh_host_ecdsa_key , 718.Pa /etc/ssh/ssh_host_ed25519_key 719and 720.Pa /etc/ssh/ssh_host_rsa_key 721for protocol version 2. 722.Pp 723Note that 724.Xr sshd 8 725will refuse to use a file if it is group/world-accessible 726and that the 727.Cm HostKeyAlgorithms 728option restricts which of the keys are actually used by 729.Xr sshd 8 . 730.Pp 731It is possible to have multiple host key files. 732.Dq rsa1 733keys are used for version 1 and 734.Dq dsa , 735.Dq ecdsa , 736.Dq ed25519 737or 738.Dq rsa 739are used for version 2 of the SSH protocol. 740It is also possible to specify public host key files instead. 741In this case operations on the private key will be delegated 742to an 743.Xr ssh-agent 1 . 744.It Cm HostKeyAgent 745Identifies the UNIX-domain socket used to communicate 746with an agent that has access to the private host keys. 747If the string 748.Dq SSH_AUTH_SOCK 749is specified, the location of the socket will be read from the 750.Ev SSH_AUTH_SOCK 751environment variable. 752.It Cm HostKeyAlgorithms 753Specifies the host key algorithms 754that the server offers. 755The default for this option is: 756.Bd -literal -offset 3n 757ecdsa-sha2-nistp256-cert-v01@openssh.com, 758ecdsa-sha2-nistp384-cert-v01@openssh.com, 759ecdsa-sha2-nistp521-cert-v01@openssh.com, 760ssh-ed25519-cert-v01@openssh.com, 761ssh-rsa-cert-v01@openssh.com, 762ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 763ssh-ed25519,ssh-rsa 764.Ed 765.Pp 766The list of available key types may also be obtained using the 767.Fl Q 768option of 769.Xr ssh 1 770with an argument of 771.Dq key . 772.It Cm IgnoreRhosts 773Specifies that 774.Pa .rhosts 775and 776.Pa .shosts 777files will not be used in 778.Cm RhostsRSAAuthentication 779or 780.Cm HostbasedAuthentication . 781.Pp 782.Pa /etc/hosts.equiv 783and 784.Pa /etc/shosts.equiv 785are still used. 786The default is 787.Dq yes . 788.It Cm IgnoreUserKnownHosts 789Specifies whether 790.Xr sshd 8 791should ignore the user's 792.Pa ~/.ssh/known_hosts 793during 794.Cm RhostsRSAAuthentication 795or 796.Cm HostbasedAuthentication . 797The default is 798.Dq no . 799.It Cm IPQoS 800Specifies the IPv4 type-of-service or DSCP class for the connection. 801Accepted values are 802.Dq af11 , 803.Dq af12 , 804.Dq af13 , 805.Dq af21 , 806.Dq af22 , 807.Dq af23 , 808.Dq af31 , 809.Dq af32 , 810.Dq af33 , 811.Dq af41 , 812.Dq af42 , 813.Dq af43 , 814.Dq cs0 , 815.Dq cs1 , 816.Dq cs2 , 817.Dq cs3 , 818.Dq cs4 , 819.Dq cs5 , 820.Dq cs6 , 821.Dq cs7 , 822.Dq ef , 823.Dq lowdelay , 824.Dq throughput , 825.Dq reliability , 826or a numeric value. 827This option may take one or two arguments, separated by whitespace. 828If one argument is specified, it is used as the packet class unconditionally. 829If two values are specified, the first is automatically selected for 830interactive sessions and the second for non-interactive sessions. 831The default is 832.Dq lowdelay 833for interactive sessions and 834.Dq throughput 835for non-interactive sessions. 836.It Cm KbdInteractiveAuthentication 837Specifies whether to allow keyboard-interactive authentication. 838The argument to this keyword must be 839.Dq yes 840or 841.Dq no . 842The default is to use whatever value 843.Cm ChallengeResponseAuthentication 844is set to 845(by default 846.Dq yes ) . 847.It Cm KerberosAuthentication 848Specifies whether the password provided by the user for 849.Cm PasswordAuthentication 850will be validated through the Kerberos KDC. 851To use this option, the server needs a 852Kerberos servtab which allows the verification of the KDC's identity. 853The default is 854.Dq no . 855.It Cm KerberosGetAFSToken 856If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 857an AFS token before accessing the user's home directory. 858The default is 859.Dq no . 860.It Cm KerberosOrLocalPasswd 861If password authentication through Kerberos fails then 862the password will be validated via any additional local mechanism 863such as 864.Pa /etc/passwd . 865The default is 866.Dq yes . 867.It Cm KerberosTicketCleanup 868Specifies whether to automatically destroy the user's ticket cache 869file on logout. 870The default is 871.Dq yes . 872.It Cm KexAlgorithms 873Specifies the available KEX (Key Exchange) algorithms. 874Multiple algorithms must be comma-separated. 875Alternately if the specified value begins with a 876.Sq + 877character, then the specified methods will be appended to the default set 878instead of replacing them. 879The supported algorithms are: 880.Pp 881.Bl -item -compact -offset indent 882.It 883curve25519-sha256@libssh.org 884.It 885diffie-hellman-group1-sha1 886.It 887diffie-hellman-group14-sha1 888.It 889diffie-hellman-group-exchange-sha1 890.It 891diffie-hellman-group-exchange-sha256 892.It 893ecdh-sha2-nistp256 894.It 895ecdh-sha2-nistp384 896.It 897ecdh-sha2-nistp521 898.El 899.Pp 900The default is: 901.Bd -literal -offset indent 902curve25519-sha256@libssh.org, 903ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 904diffie-hellman-group-exchange-sha256, 905diffie-hellman-group14-sha1 906.Ed 907.Pp 908The list of available key exchange algorithms may also be obtained using the 909.Fl Q 910option of 911.Xr ssh 1 912with an argument of 913.Dq kex . 914.It Cm KeyRegenerationInterval 915In protocol version 1, the ephemeral server key is automatically regenerated 916after this many seconds (if it has been used). 917The purpose of regeneration is to prevent 918decrypting captured sessions by later breaking into the machine and 919stealing the keys. 920The key is never stored anywhere. 921If the value is 0, the key is never regenerated. 922The default is 3600 (seconds). 923.It Cm ListenAddress 924Specifies the local addresses 925.Xr sshd 8 926should listen on. 927The following forms may be used: 928.Pp 929.Bl -item -offset indent -compact 930.It 931.Cm ListenAddress 932.Sm off 933.Ar host | Ar IPv4_addr | Ar IPv6_addr 934.Sm on 935.It 936.Cm ListenAddress 937.Sm off 938.Ar host | Ar IPv4_addr : Ar port 939.Sm on 940.It 941.Cm ListenAddress 942.Sm off 943.Oo 944.Ar host | Ar IPv6_addr Oc : Ar port 945.Sm on 946.El 947.Pp 948If 949.Ar port 950is not specified, 951sshd will listen on the address and all 952.Cm Port 953options specified. 954The default is to listen on all local addresses. 955Multiple 956.Cm ListenAddress 957options are permitted. 958.It Cm LoginGraceTime 959The server disconnects after this time if the user has not 960successfully logged in. 961If the value is 0, there is no time limit. 962The default is 120 seconds. 963.It Cm LogLevel 964Gives the verbosity level that is used when logging messages from 965.Xr sshd 8 . 966The possible values are: 967QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 968The default is INFO. 969DEBUG and DEBUG1 are equivalent. 970DEBUG2 and DEBUG3 each specify higher levels of debugging output. 971Logging with a DEBUG level violates the privacy of users and is not recommended. 972.It Cm MACs 973Specifies the available MAC (message authentication code) algorithms. 974The MAC algorithm is used for data integrity protection. 975Multiple algorithms must be comma-separated. 976If the specified value begins with a 977.Sq + 978character, then the specified algorithms will be appended to the default set 979instead of replacing them. 980.Pp 981The algorithms that contain 982.Dq -etm 983calculate the MAC after encryption (encrypt-then-mac). 984These are considered safer and their use recommended. 985The supported MACs are: 986.Pp 987.Bl -item -compact -offset indent 988.It 989hmac-md5 990.It 991hmac-md5-96 992.It 993hmac-ripemd160 994.It 995hmac-sha1 996.It 997hmac-sha1-96 998.It 999hmac-sha2-256 1000.It 1001hmac-sha2-512 1002.It 1003umac-64@openssh.com 1004.It 1005umac-128@openssh.com 1006.It 1007hmac-md5-etm@openssh.com 1008.It 1009hmac-md5-96-etm@openssh.com 1010.It 1011hmac-ripemd160-etm@openssh.com 1012.It 1013hmac-sha1-etm@openssh.com 1014.It 1015hmac-sha1-96-etm@openssh.com 1016.It 1017hmac-sha2-256-etm@openssh.com 1018.It 1019hmac-sha2-512-etm@openssh.com 1020.It 1021umac-64-etm@openssh.com 1022.It 1023umac-128-etm@openssh.com 1024.El 1025.Pp 1026The default is: 1027.Bd -literal -offset indent 1028umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1029hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1030hmac-sha1-etm@openssh.com, 1031umac-64@openssh.com,umac-128@openssh.com, 1032hmac-sha2-256,hmac-sha2-512,hmac-sha1 1033.Ed 1034.Pp 1035The list of available MAC algorithms may also be obtained using the 1036.Fl Q 1037option of 1038.Xr ssh 1 1039with an argument of 1040.Dq mac . 1041.It Cm Match 1042Introduces a conditional block. 1043If all of the criteria on the 1044.Cm Match 1045line are satisfied, the keywords on the following lines override those 1046set in the global section of the config file, until either another 1047.Cm Match 1048line or the end of the file. 1049If a keyword appears in multiple 1050.Cm Match 1051blocks that are satisfied, only the first instance of the keyword is 1052applied. 1053.Pp 1054The arguments to 1055.Cm Match 1056are one or more criteria-pattern pairs or the single token 1057.Cm All 1058which matches all criteria. 1059The available criteria are 1060.Cm User , 1061.Cm Group , 1062.Cm Host , 1063.Cm LocalAddress , 1064.Cm LocalPort , 1065and 1066.Cm Address . 1067The match patterns may consist of single entries or comma-separated 1068lists and may use the wildcard and negation operators described in the 1069PATTERNS section of 1070.Xr ssh_config 5 . 1071.Pp 1072The patterns in an 1073.Cm Address 1074criteria may additionally contain addresses to match in CIDR 1075address/masklen format, e.g.\& 1076.Dq 192.0.2.0/24 1077or 1078.Dq 3ffe:ffff::/32 . 1079Note that the mask length provided must be consistent with the address - 1080it is an error to specify a mask length that is too long for the address 1081or one with bits set in this host portion of the address. 1082For example, 1083.Dq 192.0.2.0/33 1084and 1085.Dq 192.0.2.0/8 1086respectively. 1087.Pp 1088Only a subset of keywords may be used on the lines following a 1089.Cm Match 1090keyword. 1091Available keywords are 1092.Cm AcceptEnv , 1093.Cm AllowAgentForwarding , 1094.Cm AllowGroups , 1095.Cm AllowStreamLocalForwarding , 1096.Cm AllowTcpForwarding , 1097.Cm AllowUsers , 1098.Cm AuthenticationMethods , 1099.Cm AuthorizedKeysCommand , 1100.Cm AuthorizedKeysCommandUser , 1101.Cm AuthorizedKeysFile , 1102.Cm AuthorizedPrincipalsCommand , 1103.Cm AuthorizedPrincipalsCommandUser , 1104.Cm AuthorizedPrincipalsFile , 1105.Cm Banner , 1106.Cm ChrootDirectory , 1107.Cm DenyGroups , 1108.Cm DenyUsers , 1109.Cm ForceCommand , 1110.Cm GatewayPorts , 1111.Cm GSSAPIAuthentication , 1112.Cm HostbasedAcceptedKeyTypes , 1113.Cm HostbasedAuthentication , 1114.Cm HostbasedUsesNameFromPacketOnly , 1115.Cm IPQoS , 1116.Cm KbdInteractiveAuthentication , 1117.Cm KerberosAuthentication , 1118.Cm MaxAuthTries , 1119.Cm MaxSessions , 1120.Cm PasswordAuthentication , 1121.Cm PermitEmptyPasswords , 1122.Cm PermitOpen , 1123.Cm PermitRootLogin , 1124.Cm PermitTTY , 1125.Cm PermitTunnel , 1126.Cm PermitUserRC , 1127.Cm PubkeyAcceptedKeyTypes , 1128.Cm PubkeyAuthentication , 1129.Cm RekeyLimit , 1130.Cm RevokedKeys , 1131.Cm RhostsRSAAuthentication , 1132.Cm RSAAuthentication , 1133.Cm StreamLocalBindMask , 1134.Cm StreamLocalBindUnlink , 1135.Cm TrustedUserCAKeys , 1136.Cm X11DisplayOffset , 1137.Cm X11Forwarding 1138and 1139.Cm X11UseLocalHost . 1140.It Cm MaxAuthTries 1141Specifies the maximum number of authentication attempts permitted per 1142connection. 1143Once the number of failures reaches half this value, 1144additional failures are logged. 1145The default is 6. 1146.It Cm MaxSessions 1147Specifies the maximum number of open shell, login or subsystem (e.g. sftp) 1148sessions permitted per network connection. 1149Multiple sessions may be established by clients that support connection 1150multiplexing. 1151Setting 1152.Cm MaxSessions 1153to 1 will effectively disable session multiplexing, whereas setting it to 0 1154will prevent all shell, login and subsystem sessions while still permitting 1155forwarding. 1156The default is 10. 1157.It Cm MaxStartups 1158Specifies the maximum number of concurrent unauthenticated connections to the 1159SSH daemon. 1160Additional connections will be dropped until authentication succeeds or the 1161.Cm LoginGraceTime 1162expires for a connection. 1163The default is 10:30:100. 1164.Pp 1165Alternatively, random early drop can be enabled by specifying 1166the three colon separated values 1167.Dq start:rate:full 1168(e.g. "10:30:60"). 1169.Xr sshd 8 1170will refuse connection attempts with a probability of 1171.Dq rate/100 1172(30%) 1173if there are currently 1174.Dq start 1175(10) 1176unauthenticated connections. 1177The probability increases linearly and all connection attempts 1178are refused if the number of unauthenticated connections reaches 1179.Dq full 1180(60). 1181.It Cm PasswordAuthentication 1182Specifies whether password authentication is allowed. 1183The default is 1184.Dq 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.Dq 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.Dq any 1215can be used to remove all restrictions and permit any forwarding requests. 1216An argument of 1217.Dq none 1218can be used to prohibit all forwarding requests. 1219The wildcard 1220.Dq * 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.Dq yes , 1228.Dq prohibit-password , 1229.Dq without-password , 1230.Dq forced-commands-only , 1231or 1232.Dq no . 1233The default is 1234.Dq prohibit-password . 1235.Pp 1236If this option is set to 1237.Dq prohibit-password 1238or 1239.Dq without-password , 1240password and keyboard-interactive authentication are disabled for root. 1241.Pp 1242If this option is set to 1243.Dq forced-commands-only , 1244root login with public key authentication will be allowed, 1245but only if the 1246.Ar command 1247option has been specified 1248(which may be useful for taking remote backups even if root login is 1249normally not allowed). 1250All other authentication methods are disabled for root. 1251.Pp 1252If this option is set to 1253.Dq no , 1254root is not allowed to log in. 1255.It Cm PermitTunnel 1256Specifies whether 1257.Xr tun 4 1258device forwarding is allowed. 1259The argument must be 1260.Dq yes , 1261.Dq point-to-point 1262(layer 3), 1263.Dq ethernet 1264(layer 2), or 1265.Dq no . 1266Specifying 1267.Dq yes 1268permits both 1269.Dq point-to-point 1270and 1271.Dq ethernet . 1272The default is 1273.Dq no . 1274.Pp 1275Independent of this setting, the permissions of the selected 1276.Xr tun 4 1277device must allow access to the user. 1278.It Cm PermitTTY 1279Specifies whether 1280.Xr pty 4 1281allocation is permitted. 1282The default is 1283.Dq yes . 1284.It Cm PermitUserEnvironment 1285Specifies whether 1286.Pa ~/.ssh/environment 1287and 1288.Cm environment= 1289options in 1290.Pa ~/.ssh/authorized_keys 1291are processed by 1292.Xr sshd 8 . 1293The default is 1294.Dq no . 1295Enabling environment processing may enable users to bypass access 1296restrictions in some configurations using mechanisms such as 1297.Ev LD_PRELOAD . 1298.It Cm PermitUserRC 1299Specifies whether any 1300.Pa ~/.ssh/rc 1301file is executed. 1302The default is 1303.Dq yes . 1304.It Cm PidFile 1305Specifies the file that contains the process ID of the 1306SSH daemon, or 1307.Dq none 1308to not write one. 1309The default is 1310.Pa /var/run/sshd.pid . 1311.It Cm Port 1312Specifies the port number that 1313.Xr sshd 8 1314listens on. 1315The default is 22. 1316Multiple options of this type are permitted. 1317See also 1318.Cm ListenAddress . 1319.It Cm PrintLastLog 1320Specifies whether 1321.Xr sshd 8 1322should print the date and time of the last user login when a user logs 1323in interactively. 1324The default is 1325.Dq yes . 1326.It Cm PrintMotd 1327Specifies whether 1328.Xr sshd 8 1329should print 1330.Pa /etc/motd 1331when a user logs in interactively. 1332(On some systems it is also printed by the shell, 1333.Pa /etc/profile , 1334or equivalent.) 1335The default is 1336.Dq yes . 1337.It Cm Protocol 1338Specifies the protocol versions 1339.Xr sshd 8 1340supports. 1341The possible values are 1342.Sq 1 1343and 1344.Sq 2 . 1345Multiple versions must be comma-separated. 1346The default is 1347.Sq 2 . 1348Protocol 1 suffers from a number of cryptographic weaknesses and should 1349not be used. 1350It is only offered to support legacy devices. 1351.Pp 1352Note that the order of the protocol list does not indicate preference, 1353because the client selects among multiple protocol versions offered 1354by the server. 1355Specifying 1356.Dq 2,1 1357is identical to 1358.Dq 1,2 . 1359.It Cm PubkeyAcceptedKeyTypes 1360Specifies the key types that will be accepted for public key authentication 1361as a comma-separated pattern list. 1362Alternately if the specified value begins with a 1363.Sq + 1364character, then the specified key types will be appended to the default set 1365instead of replacing them. 1366The default for this option is: 1367.Bd -literal -offset 3n 1368ecdsa-sha2-nistp256-cert-v01@openssh.com, 1369ecdsa-sha2-nistp384-cert-v01@openssh.com, 1370ecdsa-sha2-nistp521-cert-v01@openssh.com, 1371ssh-ed25519-cert-v01@openssh.com, 1372ssh-rsa-cert-v01@openssh.com, 1373ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1374ssh-ed25519,ssh-rsa 1375.Ed 1376.Pp 1377The 1378.Fl Q 1379option of 1380.Xr ssh 1 1381may be used to list supported key types. 1382.It Cm PubkeyAuthentication 1383Specifies whether public key authentication is allowed. 1384The default is 1385.Dq yes . 1386.It Cm RekeyLimit 1387Specifies the maximum amount of data that may be transmitted before the 1388session key is renegotiated, optionally followed a maximum amount of 1389time that may pass before the session key is renegotiated. 1390The first argument is specified in bytes and may have a suffix of 1391.Sq K , 1392.Sq M , 1393or 1394.Sq G 1395to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1396The default is between 1397.Sq 1G 1398and 1399.Sq 4G , 1400depending on the cipher. 1401The optional second value is specified in seconds and may use any of the 1402units documented in the 1403.Sx TIME FORMATS 1404section. 1405The default value for 1406.Cm RekeyLimit 1407is 1408.Dq default none , 1409which means that rekeying is performed after the cipher's default amount 1410of data has been sent or received and no time based rekeying is done. 1411.It Cm RevokedKeys 1412Specifies revoked public keys file, or 1413.Dq none 1414to not use one. 1415Keys listed in this file will be refused for public key authentication. 1416Note that if this file is not readable, then public key authentication will 1417be refused for all users. 1418Keys may be specified as a text file, listing one public key per line, or as 1419an OpenSSH Key Revocation List (KRL) as generated by 1420.Xr ssh-keygen 1 . 1421For more information on KRLs, see the KEY REVOCATION LISTS section in 1422.Xr ssh-keygen 1 . 1423.It Cm RhostsRSAAuthentication 1424Specifies whether rhosts or /etc/hosts.equiv authentication together 1425with successful RSA host authentication is allowed. 1426The default is 1427.Dq no . 1428This option applies to protocol version 1 only. 1429.It Cm RSAAuthentication 1430Specifies whether pure RSA authentication is allowed. 1431The default is 1432.Dq yes . 1433This option applies to protocol version 1 only. 1434.It Cm ServerKeyBits 1435Defines the number of bits in the ephemeral protocol version 1 server key. 1436The default and minimum value is 1024. 1437.It Cm StreamLocalBindMask 1438Sets the octal file creation mode mask 1439.Pq umask 1440used when creating a Unix-domain socket file for local or remote 1441port forwarding. 1442This option is only used for port forwarding to a Unix-domain socket file. 1443.Pp 1444The default value is 0177, which creates a Unix-domain socket file that is 1445readable and writable only by the owner. 1446Note that not all operating systems honor the file mode on Unix-domain 1447socket files. 1448.It Cm StreamLocalBindUnlink 1449Specifies whether to remove an existing Unix-domain socket file for local 1450or remote port forwarding before creating a new one. 1451If the socket file already exists and 1452.Cm StreamLocalBindUnlink 1453is not enabled, 1454.Nm sshd 1455will be unable to forward the port to the Unix-domain socket file. 1456This option is only used for port forwarding to a Unix-domain socket file. 1457.Pp 1458The argument must be 1459.Dq yes 1460or 1461.Dq no . 1462The default is 1463.Dq no . 1464.It Cm StrictModes 1465Specifies whether 1466.Xr sshd 8 1467should check file modes and ownership of the 1468user's files and home directory before accepting login. 1469This is normally desirable because novices sometimes accidentally leave their 1470directory or files world-writable. 1471The default is 1472.Dq yes . 1473Note that this does not apply to 1474.Cm ChrootDirectory , 1475whose permissions and ownership are checked unconditionally. 1476.It Cm Subsystem 1477Configures an external subsystem (e.g. file transfer daemon). 1478Arguments should be a subsystem name and a command (with optional arguments) 1479to execute upon subsystem request. 1480.Pp 1481The command 1482.Xr sftp-server 8 1483implements the 1484.Dq sftp 1485file transfer subsystem. 1486.Pp 1487Alternately the name 1488.Dq internal-sftp 1489implements an in-process 1490.Dq sftp 1491server. 1492This may simplify configurations using 1493.Cm ChrootDirectory 1494to force a different filesystem root on clients. 1495.Pp 1496By default no subsystems are defined. 1497.It Cm SyslogFacility 1498Gives the facility code that is used when logging messages from 1499.Xr sshd 8 . 1500The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1501LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1502The default is AUTH. 1503.It Cm TCPKeepAlive 1504Specifies whether the system should send TCP keepalive messages to the 1505other side. 1506If they are sent, death of the connection or crash of one 1507of the machines will be properly noticed. 1508However, this means that 1509connections will die if the route is down temporarily, and some people 1510find it annoying. 1511On the other hand, if TCP keepalives are not sent, 1512sessions may hang indefinitely on the server, leaving 1513.Dq ghost 1514users and consuming server resources. 1515.Pp 1516The default is 1517.Dq yes 1518(to send TCP keepalive messages), and the server will notice 1519if the network goes down or the client host crashes. 1520This avoids infinitely hanging sessions. 1521.Pp 1522To disable TCP keepalive messages, the value should be set to 1523.Dq no . 1524.It Cm TrustedUserCAKeys 1525Specifies a file containing public keys of certificate authorities that are 1526trusted to sign user certificates for authentication, or 1527.Dq none 1528to not use one. 1529Keys are listed one per line; empty lines and comments starting with 1530.Ql # 1531are allowed. 1532If a certificate is presented for authentication and has its signing CA key 1533listed in this file, then it may be used for authentication for any user 1534listed in the certificate's principals list. 1535Note that certificates that lack a list of principals will not be permitted 1536for authentication using 1537.Cm TrustedUserCAKeys . 1538For more details on certificates, see the CERTIFICATES section in 1539.Xr ssh-keygen 1 . 1540.It Cm UseDNS 1541Specifies whether 1542.Xr sshd 8 1543should look up the remote host name, and to check that 1544the resolved host name for the remote IP address maps back to the 1545very same IP address. 1546.Pp 1547If this option is set to 1548.Dq no 1549(the default) then only addresses and not host names may be used in 1550.Pa ~/.ssh/authorized_keys 1551.Cm from 1552and 1553.Nm 1554.Cm Match 1555.Cm Host 1556directives. 1557.It Cm UseLogin 1558Specifies whether 1559.Xr login 1 1560is used for interactive login sessions. 1561The default is 1562.Dq no . 1563Note that 1564.Xr login 1 1565is never used for remote command execution. 1566Note also, that if this is enabled, 1567.Cm X11Forwarding 1568will be disabled because 1569.Xr login 1 1570does not know how to handle 1571.Xr xauth 1 1572cookies. 1573If 1574.Cm UsePrivilegeSeparation 1575is specified, it will be disabled after authentication. 1576.It Cm UsePrivilegeSeparation 1577Specifies whether 1578.Xr sshd 8 1579separates privileges by creating an unprivileged child process 1580to deal with incoming network traffic. 1581After successful authentication, another process will be created that has 1582the privilege of the authenticated user. 1583The goal of privilege separation is to prevent privilege 1584escalation by containing any corruption within the unprivileged processes. 1585The argument must be 1586.Dq yes , 1587.Dq no , 1588or 1589.Dq sandbox . 1590If 1591.Cm UsePrivilegeSeparation 1592is set to 1593.Dq sandbox 1594then the pre-authentication unprivileged process is subject to additional 1595restrictions. 1596The default is 1597.Dq sandbox . 1598.It Cm VersionAddendum 1599Optionally specifies additional text to append to the SSH protocol banner 1600sent by the server upon connection. 1601The default is 1602.Dq none . 1603.It Cm X11DisplayOffset 1604Specifies the first display number available for 1605.Xr sshd 8 Ns 's 1606X11 forwarding. 1607This prevents sshd from interfering with real X11 servers. 1608The default is 10. 1609.It Cm X11Forwarding 1610Specifies whether X11 forwarding is permitted. 1611The argument must be 1612.Dq yes 1613or 1614.Dq no . 1615The default is 1616.Dq no . 1617.Pp 1618When X11 forwarding is enabled, there may be additional exposure to 1619the server and to client displays if the 1620.Xr sshd 8 1621proxy display is configured to listen on the wildcard address (see 1622.Cm X11UseLocalhost 1623below), though this is not the default. 1624Additionally, the authentication spoofing and authentication data 1625verification and substitution occur on the client side. 1626The security risk of using X11 forwarding is that the client's X11 1627display server may be exposed to attack when the SSH client requests 1628forwarding (see the warnings for 1629.Cm ForwardX11 1630in 1631.Xr ssh_config 5 ) . 1632A system administrator may have a stance in which they want to 1633protect clients that may expose themselves to attack by unwittingly 1634requesting X11 forwarding, which can warrant a 1635.Dq no 1636setting. 1637.Pp 1638Note that disabling X11 forwarding does not prevent users from 1639forwarding X11 traffic, as users can always install their own forwarders. 1640X11 forwarding is automatically disabled if 1641.Cm UseLogin 1642is enabled. 1643.It Cm X11UseLocalhost 1644Specifies whether 1645.Xr sshd 8 1646should bind the X11 forwarding server to the loopback address or to 1647the wildcard address. 1648By default, 1649sshd binds the forwarding server to the loopback address and sets the 1650hostname part of the 1651.Ev DISPLAY 1652environment variable to 1653.Dq localhost . 1654This prevents remote hosts from connecting to the proxy display. 1655However, some older X11 clients may not function with this 1656configuration. 1657.Cm X11UseLocalhost 1658may be set to 1659.Dq no 1660to specify that the forwarding server should be bound to the wildcard 1661address. 1662The argument must be 1663.Dq yes 1664or 1665.Dq no . 1666The default is 1667.Dq yes . 1668.It Cm XAuthLocation 1669Specifies the full pathname of the 1670.Xr xauth 1 1671program, or 1672.Dq none 1673to not use one. 1674The default is 1675.Pa /usr/X11R6/bin/xauth . 1676.El 1677.Sh TIME FORMATS 1678.Xr sshd 8 1679command-line arguments and configuration file options that specify time 1680may be expressed using a sequence of the form: 1681.Sm off 1682.Ar time Op Ar qualifier , 1683.Sm on 1684where 1685.Ar time 1686is a positive integer value and 1687.Ar qualifier 1688is one of the following: 1689.Pp 1690.Bl -tag -width Ds -compact -offset indent 1691.It Aq Cm none 1692seconds 1693.It Cm s | Cm S 1694seconds 1695.It Cm m | Cm M 1696minutes 1697.It Cm h | Cm H 1698hours 1699.It Cm d | Cm D 1700days 1701.It Cm w | Cm W 1702weeks 1703.El 1704.Pp 1705Each member of the sequence is added together to calculate 1706the total time value. 1707.Pp 1708Time format examples: 1709.Pp 1710.Bl -tag -width Ds -compact -offset indent 1711.It 600 1712600 seconds (10 minutes) 1713.It 10m 171410 minutes 1715.It 1h30m 17161 hour 30 minutes (90 minutes) 1717.El 1718.Sh FILES 1719.Bl -tag -width Ds 1720.It Pa /etc/ssh/sshd_config 1721Contains configuration data for 1722.Xr sshd 8 . 1723This file should be writable by root only, but it is recommended 1724(though not necessary) that it be world-readable. 1725.El 1726.Sh SEE ALSO 1727.Xr sshd 8 1728.Sh AUTHORS 1729OpenSSH is a derivative of the original and free 1730ssh 1.2.12 release by Tatu Ylonen. 1731Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1732Theo de Raadt and Dug Song 1733removed many bugs, re-added newer features and 1734created OpenSSH. 1735Markus Friedl contributed the support for SSH 1736protocol versions 1.5 and 2.0. 1737Niels Provos and Markus Friedl contributed support 1738for privilege separation. 1739