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 1.282 2018/09/20 03:28:06 djm Exp $ 37.\" $FreeBSD$ 38.Dd $Mdocdate: July 28 2020 $ 39.Dt SSHD_CONFIG 5 40.Os 41.Sh NAME 42.Nm sshd_config 43.Nd OpenSSH SSH daemon configuration file 44.Sh DESCRIPTION 45.Xr sshd 8 46reads configuration data from 47.Pa /etc/ssh/sshd_config 48(or the file specified with 49.Fl f 50on the command line). 51The file contains keyword-argument pairs, one per line. 52For each keyword, the first obtained value will be used. 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 70and 71.Cm SetEnv 72in 73.Xr ssh_config 5 74for how to configure the client. 75The 76.Ev TERM 77environment variable is always accepted whenever the client 78requests a pseudo-terminal as it is required by the protocol. 79Variables are specified by name, which may contain the wildcard characters 80.Ql * 81and 82.Ql \&? . 83Multiple environment variables may be separated by whitespace or spread 84across multiple 85.Cm AcceptEnv 86directives. 87Be warned that some environment variables could be used to bypass restricted 88user environments. 89For this reason, care should be taken in the use of this directive. 90The default is not to accept any environment variables. 91.It Cm AddressFamily 92Specifies which address family should be used by 93.Xr sshd 8 . 94Valid arguments are 95.Cm any 96(the default), 97.Cm inet 98(use IPv4 only), or 99.Cm inet6 100(use IPv6 only). 101.It Cm AllowAgentForwarding 102Specifies whether 103.Xr ssh-agent 1 104forwarding is permitted. 105The default is 106.Cm yes . 107Note that disabling agent forwarding does not improve security 108unless users are also denied shell access, as they can always install 109their own forwarders. 110.It Cm AllowGroups 111This keyword can be followed by a list of group name patterns, separated 112by spaces. 113If specified, login is allowed only for users whose primary 114group or supplementary group list matches one of the patterns. 115Only group names are valid; a numerical group ID is not recognized. 116By default, login is allowed for all groups. 117The allow/deny directives are processed in the following order: 118.Cm DenyUsers , 119.Cm AllowUsers , 120.Cm DenyGroups , 121and finally 122.Cm AllowGroups . 123.Pp 124See PATTERNS in 125.Xr ssh_config 5 126for more information on patterns. 127.It Cm AllowStreamLocalForwarding 128Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. 129The available options are 130.Cm yes 131(the default) 132or 133.Cm all 134to allow StreamLocal forwarding, 135.Cm no 136to prevent all StreamLocal forwarding, 137.Cm local 138to allow local (from the perspective of 139.Xr ssh 1 ) 140forwarding only or 141.Cm remote 142to allow remote forwarding only. 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.Cm yes 150(the default) 151or 152.Cm all 153to allow TCP forwarding, 154.Cm no 155to prevent all TCP forwarding, 156.Cm local 157to allow local (from the perspective of 158.Xr ssh 1 ) 159forwarding only or 160.Cm remote 161to allow remote forwarding only. 162Note that disabling TCP forwarding does not improve security unless 163users are also denied shell access, as they can always install their 164own forwarders. 165.It Cm AllowUsers 166This keyword can be followed by a list of user name patterns, separated 167by spaces. 168If specified, login is allowed only for user names that 169match one of the patterns. 170Only user names are valid; a numerical user ID is not recognized. 171By default, login is allowed for all users. 172If the pattern takes the form USER@HOST then USER and HOST 173are separately checked, restricting logins to particular 174users from particular hosts. 175HOST criteria may additionally contain addresses to match in CIDR 176address/masklen format. 177The allow/deny directives are processed in the following order: 178.Cm DenyUsers , 179.Cm AllowUsers , 180.Cm DenyGroups , 181and finally 182.Cm AllowGroups . 183.Pp 184See PATTERNS in 185.Xr ssh_config 5 186for more information on patterns. 187.It Cm AuthenticationMethods 188Specifies the authentication methods that must be successfully completed 189for a user to be granted access. 190This option must be followed by one or more lists of comma-separated 191authentication method names, or by the single string 192.Cm any 193to indicate the default behaviour of accepting any single authentication 194method. 195If the default is overridden, then successful authentication requires 196completion of every method in at least one of these lists. 197.Pp 198For example, 199.Qq publickey,password publickey,keyboard-interactive 200would require the user to complete public key authentication, followed by 201either password or keyboard interactive authentication. 202Only methods that are next in one or more lists are offered at each stage, 203so for this example it would not be possible to attempt password or 204keyboard-interactive authentication before public key. 205.Pp 206For keyboard interactive authentication it is also possible to 207restrict authentication to a specific device by appending a 208colon followed by the device identifier 209.Cm bsdauth 210or 211.Cm pam . 212depending on the server configuration. 213For example, 214.Qq keyboard-interactive:bsdauth 215would restrict keyboard interactive authentication to the 216.Cm bsdauth 217device. 218.Pp 219If the publickey method is listed more than once, 220.Xr sshd 8 221verifies that keys that have been used successfully are not reused for 222subsequent authentications. 223For example, 224.Qq publickey,publickey 225requires successful authentication using two different public keys. 226.Pp 227Note that each authentication method listed should also be explicitly enabled 228in the configuration. 229.Pp 230The available authentication methods are: 231.Qq gssapi-with-mic , 232.Qq hostbased , 233.Qq keyboard-interactive , 234.Qq none 235(used for access to password-less accounts when 236.Cm PermitEmptyPasswords 237is enabled), 238.Qq password 239and 240.Qq publickey . 241.It Cm AuthorizedKeysCommand 242Specifies a program to be used to look up the user's public keys. 243The program must be owned by root, not writable by group or others and 244specified by an absolute path. 245Arguments to 246.Cm AuthorizedKeysCommand 247accept the tokens described in the 248.Sx TOKENS 249section. 250If no arguments are specified then the username of the target user is used. 251.Pp 252The program should produce on standard output zero or 253more lines of authorized_keys output (see 254.Sx AUTHORIZED_KEYS 255in 256.Xr sshd 8 ) . 257If a key supplied by 258.Cm AuthorizedKeysCommand 259does not successfully authenticate 260and authorize the user then public key authentication continues using the usual 261.Cm AuthorizedKeysFile 262files. 263By default, no 264.Cm AuthorizedKeysCommand 265is run. 266.It Cm AuthorizedKeysCommandUser 267Specifies the user under whose account the 268.Cm AuthorizedKeysCommand 269is run. 270It is recommended to use a dedicated user that has no other role on the host 271than running authorized keys commands. 272If 273.Cm AuthorizedKeysCommand 274is specified but 275.Cm AuthorizedKeysCommandUser 276is not, then 277.Xr sshd 8 278will refuse to start. 279.It Cm AuthorizedKeysFile 280Specifies the file that contains the public keys used for user authentication. 281The format is described in the 282.Sx AUTHORIZED_KEYS FILE FORMAT 283section of 284.Xr sshd 8 . 285Arguments to 286.Cm AuthorizedKeysFile 287accept the tokens described in the 288.Sx TOKENS 289section. 290After expansion, 291.Cm AuthorizedKeysFile 292is taken to be an absolute path or one relative to the user's home 293directory. 294Multiple files may be listed, separated by whitespace. 295Alternately this option may be set to 296.Cm none 297to skip checking for user keys in files. 298The default is 299.Qq .ssh/authorized_keys .ssh/authorized_keys2 . 300.It Cm AuthorizedPrincipalsCommand 301Specifies a program to be used to generate the list of allowed 302certificate principals as per 303.Cm AuthorizedPrincipalsFile . 304The program must be owned by root, not writable by group or others and 305specified by an absolute path. 306Arguments to 307.Cm AuthorizedPrincipalsCommand 308accept the tokens described in the 309.Sx TOKENS 310section. 311If no arguments are specified then the username of the target user is used. 312.Pp 313The program should produce on standard output zero or 314more lines of 315.Cm AuthorizedPrincipalsFile 316output. 317If either 318.Cm AuthorizedPrincipalsCommand 319or 320.Cm AuthorizedPrincipalsFile 321is specified, then certificates offered by the client for authentication 322must contain a principal that is listed. 323By default, no 324.Cm AuthorizedPrincipalsCommand 325is run. 326.It Cm AuthorizedPrincipalsCommandUser 327Specifies the user under whose account the 328.Cm AuthorizedPrincipalsCommand 329is run. 330It is recommended to use a dedicated user that has no other role on the host 331than running authorized principals commands. 332If 333.Cm AuthorizedPrincipalsCommand 334is specified but 335.Cm AuthorizedPrincipalsCommandUser 336is not, then 337.Xr sshd 8 338will refuse to start. 339.It Cm AuthorizedPrincipalsFile 340Specifies a file that lists principal names that are accepted for 341certificate authentication. 342When using certificates signed by a key listed in 343.Cm TrustedUserCAKeys , 344this file lists names, one of which must appear in the certificate for it 345to be accepted for authentication. 346Names are listed one per line preceded by key options (as described in 347.Sx AUTHORIZED_KEYS FILE FORMAT 348in 349.Xr sshd 8 ) . 350Empty lines and comments starting with 351.Ql # 352are ignored. 353.Pp 354Arguments to 355.Cm AuthorizedPrincipalsFile 356accept the tokens described in the 357.Sx TOKENS 358section. 359After expansion, 360.Cm AuthorizedPrincipalsFile 361is taken to be an absolute path or one relative to the user's home directory. 362The default is 363.Cm none , 364i.e. not to use a principals file \(en in this case, the username 365of the user must appear in a certificate's principals list for it to be 366accepted. 367.Pp 368Note that 369.Cm AuthorizedPrincipalsFile 370is only used when authentication proceeds using a CA listed in 371.Cm TrustedUserCAKeys 372and is not consulted for certification authorities trusted via 373.Pa ~/.ssh/authorized_keys , 374though the 375.Cm principals= 376key option offers a similar facility (see 377.Xr sshd 8 378for details). 379.It Cm Banner 380The contents of the specified file are sent to the remote user before 381authentication is allowed. 382If the argument is 383.Cm none 384then no banner is displayed. 385By default, no banner is displayed. 386.It Cm CASignatureAlgorithms 387Specifies which algorithms are allowed for signing of certificates 388by certificate authorities (CAs). 389The default is: 390.Bd -literal -offset indent 391ecdsa-sha2-nistp256.ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 392ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 393.Ed 394.Pp 395Certificates signed using other algorithms will not be accepted for 396public key or host-based authentication. 397.It Cm ChallengeResponseAuthentication 398Specifies whether challenge-response authentication is allowed (e.g. via 399PAM or through authentication styles supported in 400.Xr login.conf 5 ) 401The default is 402.Cm yes . 403.It Cm ChrootDirectory 404Specifies the pathname of a directory to 405.Xr chroot 2 406to after authentication. 407At session startup 408.Xr sshd 8 409checks that all components of the pathname are root-owned directories 410which are not writable by any other user or group. 411After the chroot, 412.Xr sshd 8 413changes the working directory to the user's home directory. 414Arguments to 415.Cm ChrootDirectory 416accept the tokens described in the 417.Sx TOKENS 418section. 419.Pp 420The 421.Cm ChrootDirectory 422must contain the necessary files and directories to support the 423user's session. 424For an interactive session this requires at least a shell, typically 425.Xr sh 1 , 426and basic 427.Pa /dev 428nodes such as 429.Xr null 4 , 430.Xr zero 4 , 431.Xr stdin 4 , 432.Xr stdout 4 , 433.Xr stderr 4 , 434and 435.Xr tty 4 436devices. 437For file transfer sessions using SFTP 438no additional configuration of the environment is necessary if the in-process 439sftp-server is used, 440though sessions which use logging may require 441.Pa /dev/log 442inside the chroot directory on some operating systems (see 443.Xr sftp-server 8 444for details). 445.Pp 446For safety, it is very important that the directory hierarchy be 447prevented from modification by other processes on the system (especially 448those outside the jail). 449Misconfiguration can lead to unsafe environments which 450.Xr sshd 8 451cannot detect. 452.Pp 453The default is 454.Cm none , 455indicating not to 456.Xr chroot 2 . 457.It Cm Ciphers 458Specifies the ciphers allowed. 459Multiple ciphers must be comma-separated. 460If the specified value begins with a 461.Sq + 462character, then the specified ciphers will be appended to the default set 463instead of replacing them. 464If the specified value begins with a 465.Sq - 466character, then the specified ciphers (including wildcards) will be removed 467from the default set instead of replacing them. 468.Pp 469The supported ciphers are: 470.Pp 471.Bl -item -compact -offset indent 472.It 4733des-cbc 474.It 475aes128-cbc 476.It 477aes192-cbc 478.It 479aes256-cbc 480.It 481aes128-ctr 482.It 483aes192-ctr 484.It 485aes256-ctr 486.It 487aes128-gcm@openssh.com 488.It 489aes256-gcm@openssh.com 490.It 491chacha20-poly1305@openssh.com 492.El 493.Pp 494The default is: 495.Bd -literal -offset indent 496chacha20-poly1305@openssh.com, 497aes128-ctr,aes192-ctr,aes256-ctr, 498aes128-gcm@openssh.com,aes256-gcm@openssh.com 499.Ed 500.Pp 501The list of available ciphers may also be obtained using 502.Qq ssh -Q cipher . 503.It Cm ClientAliveCountMax 504Sets the number of client alive messages which may be sent without 505.Xr sshd 8 506receiving any messages back from the client. 507If this threshold is reached while client alive messages are being sent, 508sshd will disconnect the client, terminating the session. 509It is important to note that the use of client alive messages is very 510different from 511.Cm TCPKeepAlive . 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 523is 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 enabled after 537the user has authenticated successfully. 538The argument must be 539.Cm yes , 540.Cm delayed 541(a legacy synonym for 542.Cm yes ) 543or 544.Cm no . 545The default is 546.Cm yes . 547.It Cm DenyGroups 548This keyword can be followed by a list of group name patterns, separated 549by spaces. 550Login is disallowed for users whose primary group or supplementary 551group list matches one of the patterns. 552Only group names are valid; a numerical group ID is not recognized. 553By default, login is allowed for all groups. 554The allow/deny directives are processed in the following order: 555.Cm DenyUsers , 556.Cm AllowUsers , 557.Cm DenyGroups , 558and finally 559.Cm AllowGroups . 560.Pp 561See PATTERNS in 562.Xr ssh_config 5 563for more information on patterns. 564.It Cm DenyUsers 565This keyword can be followed by a list of user name patterns, separated 566by spaces. 567Login is disallowed for user names that match one of the patterns. 568Only user names are valid; a numerical user ID is not recognized. 569By default, login is allowed for all users. 570If the pattern takes the form USER@HOST then USER and HOST 571are separately checked, restricting logins to particular 572users from particular hosts. 573HOST criteria may additionally contain addresses to match in CIDR 574address/masklen format. 575The allow/deny directives are processed in the following order: 576.Cm DenyUsers , 577.Cm AllowUsers , 578.Cm DenyGroups , 579and finally 580.Cm AllowGroups . 581.Pp 582See PATTERNS in 583.Xr ssh_config 5 584for more information on patterns. 585.It Cm DisableForwarding 586Disables all forwarding features, including X11, 587.Xr ssh-agent 1 , 588TCP and StreamLocal. 589This option overrides all other forwarding-related options and may 590simplify restricted configurations. 591.It Cm ExposeAuthInfo 592Writes a temporary file containing a list of authentication methods and 593public credentials (e.g. keys) used to authenticate the user. 594The location of the file is exposed to the user session through the 595.Ev SSH_USER_AUTH 596environment variable. 597The default is 598.Cm no . 599.It Cm FingerprintHash 600Specifies the hash algorithm used when logging key fingerprints. 601Valid options are: 602.Cm md5 603and 604.Cm sha256 . 605The default is 606.Cm sha256 . 607.It Cm ForceCommand 608Forces the execution of the command specified by 609.Cm ForceCommand , 610ignoring any command supplied by the client and 611.Pa ~/.ssh/rc 612if present. 613The command is invoked by using the user's login shell with the -c option. 614This applies to shell, command, or subsystem execution. 615It is most useful inside a 616.Cm Match 617block. 618The command originally supplied by the client is available in the 619.Ev SSH_ORIGINAL_COMMAND 620environment variable. 621Specifying a command of 622.Cm internal-sftp 623will force the use of an in-process SFTP server that requires no support 624files when used with 625.Cm ChrootDirectory . 626The default is 627.Cm none . 628.It Cm GatewayPorts 629Specifies whether remote hosts are allowed to connect to ports 630forwarded for the client. 631By default, 632.Xr sshd 8 633binds remote port forwardings to the loopback address. 634This prevents other remote hosts from connecting to forwarded ports. 635.Cm GatewayPorts 636can be used to specify that sshd 637should allow remote port forwardings to bind to non-loopback addresses, thus 638allowing other hosts to connect. 639The argument may be 640.Cm no 641to force remote port forwardings to be available to the local host only, 642.Cm yes 643to force remote port forwardings to bind to the wildcard address, or 644.Cm clientspecified 645to allow the client to select the address to which the forwarding is bound. 646The default is 647.Cm no . 648.It Cm GSSAPIAuthentication 649Specifies whether user authentication based on GSSAPI is allowed. 650The default is 651.Cm no . 652.It Cm GSSAPICleanupCredentials 653Specifies whether to automatically destroy the user's credentials cache 654on logout. 655The default is 656.Cm yes . 657.It Cm GSSAPIStrictAcceptorCheck 658Determines whether to be strict about the identity of the GSSAPI acceptor 659a client authenticates against. 660If set to 661.Cm yes 662then the client must authenticate against the host 663service on the current hostname. 664If set to 665.Cm no 666then the client may authenticate against any service key stored in the 667machine's default store. 668This facility is provided to assist with operation on multi homed machines. 669The default is 670.Cm yes . 671.It Cm HostbasedAcceptedKeyTypes 672Specifies the key types that will be accepted for hostbased authentication 673as a list of comma-separated patterns. 674Alternately if the specified value begins with a 675.Sq + 676character, then the specified key types will be appended to the default set 677instead of replacing them. 678If the specified value begins with a 679.Sq - 680character, then the specified key types (including wildcards) will be removed 681from the default set instead of replacing them. 682The default for this option is: 683.Bd -literal -offset 3n 684ecdsa-sha2-nistp256-cert-v01@openssh.com, 685ecdsa-sha2-nistp384-cert-v01@openssh.com, 686ecdsa-sha2-nistp521-cert-v01@openssh.com, 687ssh-ed25519-cert-v01@openssh.com, 688rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com, 689ssh-rsa-cert-v01@openssh.com, 690ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 691ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 692.Ed 693.Pp 694The list of available key types may also be obtained using 695.Qq ssh -Q key . 696.It Cm HostbasedAuthentication 697Specifies whether rhosts or /etc/hosts.equiv authentication together 698with successful public key client host authentication is allowed 699(host-based authentication). 700The default is 701.Cm no . 702.It Cm HostbasedUsesNameFromPacketOnly 703Specifies whether or not the server will attempt to perform a reverse 704name lookup when matching the name in the 705.Pa ~/.shosts , 706.Pa ~/.rhosts , 707and 708.Pa /etc/hosts.equiv 709files during 710.Cm HostbasedAuthentication . 711A setting of 712.Cm yes 713means that 714.Xr sshd 8 715uses the name supplied by the client rather than 716attempting to resolve the name from the TCP connection itself. 717The default is 718.Cm no . 719.It Cm HostCertificate 720Specifies a file containing a public host certificate. 721The certificate's public key must match a private host key already specified 722by 723.Cm HostKey . 724The default behaviour of 725.Xr sshd 8 726is not to load any certificates. 727.It Cm HostKey 728Specifies a file containing a private host key 729used by SSH. 730The defaults are 731.Pa /etc/ssh/ssh_host_ecdsa_key , 732.Pa /etc/ssh/ssh_host_ed25519_key 733and 734.Pa /etc/ssh/ssh_host_rsa_key . 735.Pp 736Note that 737.Xr sshd 8 738will refuse to use a file if it is group/world-accessible 739and that the 740.Cm HostKeyAlgorithms 741option restricts which of the keys are actually used by 742.Xr sshd 8 . 743.Pp 744It is possible to have multiple host key files. 745It is also possible to specify public host key files instead. 746In this case operations on the private key will be delegated 747to an 748.Xr ssh-agent 1 . 749.It Cm HostKeyAgent 750Identifies the UNIX-domain socket used to communicate 751with an agent that has access to the private host keys. 752If the string 753.Qq SSH_AUTH_SOCK 754is specified, the location of the socket will be read from the 755.Ev SSH_AUTH_SOCK 756environment variable. 757.It Cm HostKeyAlgorithms 758Specifies the host key algorithms 759that the server offers. 760The default for this option is: 761.Bd -literal -offset 3n 762ecdsa-sha2-nistp256-cert-v01@openssh.com, 763ecdsa-sha2-nistp384-cert-v01@openssh.com, 764ecdsa-sha2-nistp521-cert-v01@openssh.com, 765ssh-ed25519-cert-v01@openssh.com, 766rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com, 767ssh-rsa-cert-v01@openssh.com, 768ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 769ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 770.Ed 771.Pp 772The list of available key types may also be obtained using 773.Qq ssh -Q key . 774.It Cm IgnoreRhosts 775Specifies that 776.Pa .rhosts 777and 778.Pa .shosts 779files will not be used in 780.Cm HostbasedAuthentication . 781.Pp 782.Pa /etc/hosts.equiv 783and 784.Pa /etc/ssh/shosts.equiv 785are still used. 786The default is 787.Cm yes . 788.It Cm IgnoreUserKnownHosts 789Specifies whether 790.Xr sshd 8 791should ignore the user's 792.Pa ~/.ssh/known_hosts 793during 794.Cm HostbasedAuthentication 795and use only the system-wide known hosts file 796.Pa /etc/ssh/known_hosts . 797The default is 798.Cm no . 799.It Cm IPQoS 800Specifies the IPv4 type-of-service or DSCP class for the connection. 801Accepted values are 802.Cm af11 , 803.Cm af12 , 804.Cm af13 , 805.Cm af21 , 806.Cm af22 , 807.Cm af23 , 808.Cm af31 , 809.Cm af32 , 810.Cm af33 , 811.Cm af41 , 812.Cm af42 , 813.Cm af43 , 814.Cm cs0 , 815.Cm cs1 , 816.Cm cs2 , 817.Cm cs3 , 818.Cm cs4 , 819.Cm cs5 , 820.Cm cs6 , 821.Cm cs7 , 822.Cm ef , 823.Cm lowdelay , 824.Cm throughput , 825.Cm reliability , 826a numeric value, or 827.Cm none 828to use the operating system default. 829This option may take one or two arguments, separated by whitespace. 830If one argument is specified, it is used as the packet class unconditionally. 831If two values are specified, the first is automatically selected for 832interactive sessions and the second for non-interactive sessions. 833The default is 834.Cm af21 835(Low-Latency Data) 836for interactive sessions and 837.Cm cs1 838(Lower Effort) 839for non-interactive sessions. 840.It Cm KbdInteractiveAuthentication 841Specifies whether to allow keyboard-interactive authentication. 842The argument to this keyword must be 843.Cm yes 844or 845.Cm no . 846The default is to use whatever value 847.Cm ChallengeResponseAuthentication 848is set to 849(by default 850.Cm yes ) . 851.It Cm KerberosAuthentication 852Specifies whether the password provided by the user for 853.Cm PasswordAuthentication 854will be validated through the Kerberos KDC. 855To use this option, the server needs a 856Kerberos servtab which allows the verification of the KDC's identity. 857The default is 858.Cm no . 859.It Cm KerberosGetAFSToken 860If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 861an AFS token before accessing the user's home directory. 862The default is 863.Cm no . 864.It Cm KerberosOrLocalPasswd 865If password authentication through Kerberos fails then 866the password will be validated via any additional local mechanism 867such as 868.Pa /etc/passwd . 869The default is 870.Cm yes . 871.It Cm KerberosTicketCleanup 872Specifies whether to automatically destroy the user's ticket cache 873file on logout. 874The default is 875.Cm yes . 876.It Cm KexAlgorithms 877Specifies the available KEX (Key Exchange) algorithms. 878Multiple algorithms must be comma-separated. 879Alternately if the specified value begins with a 880.Sq + 881character, then the specified methods will be appended to the default set 882instead of replacing them. 883If the specified value begins with a 884.Sq - 885character, then the specified methods (including wildcards) will be removed 886from the default set instead of replacing them. 887The supported algorithms are: 888.Pp 889.Bl -item -compact -offset indent 890.It 891curve25519-sha256 892.It 893curve25519-sha256@libssh.org 894.It 895diffie-hellman-group1-sha1 896.It 897diffie-hellman-group14-sha1 898.It 899diffie-hellman-group14-sha256 900.It 901diffie-hellman-group16-sha512 902.It 903diffie-hellman-group18-sha512 904.It 905diffie-hellman-group-exchange-sha1 906.It 907diffie-hellman-group-exchange-sha256 908.It 909ecdh-sha2-nistp256 910.It 911ecdh-sha2-nistp384 912.It 913ecdh-sha2-nistp521 914.El 915.Pp 916The default is: 917.Bd -literal -offset indent 918curve25519-sha256,curve25519-sha256@libssh.org, 919ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 920diffie-hellman-group-exchange-sha256, 921diffie-hellman-group16-sha512,diffie-hellman-group18-sha512, 922diffie-hellman-group14-sha256,diffie-hellman-group14-sha1 923.Ed 924.Pp 925The list of available key exchange algorithms may also be obtained using 926.Qq ssh -Q kex . 927.It Cm ListenAddress 928Specifies the local addresses 929.Xr sshd 8 930should listen on. 931The following forms may be used: 932.Pp 933.Bl -item -offset indent -compact 934.It 935.Cm ListenAddress 936.Sm off 937.Ar hostname | address 938.Sm on 939.Op Cm rdomain Ar domain 940.It 941.Cm ListenAddress 942.Sm off 943.Ar hostname : port 944.Sm on 945.Op Cm rdomain Ar domain 946.It 947.Cm ListenAddress 948.Sm off 949.Ar IPv4_address : port 950.Sm on 951.Op Cm rdomain Ar domain 952.It 953.Cm ListenAddress 954.Sm off 955.Oo Ar hostname | address Oc : Ar port 956.Sm on 957.Op Cm rdomain Ar domain 958.El 959.Pp 960The optional 961.Cm rdomain 962qualifier requests 963.Xr sshd 8 964listen in an explicit routing domain. 965If 966.Ar port 967is not specified, 968sshd will listen on the address and all 969.Cm Port 970options specified. 971The default is to listen on all local addresses on the current default 972routing domain. 973Multiple 974.Cm ListenAddress 975options are permitted. 976For more information on routing domains, see 977.Xr rdomain 4 . 978.It Cm LoginGraceTime 979The server disconnects after this time if the user has not 980successfully logged in. 981If the value is 0, there is no time limit. 982The default is 120 seconds. 983.It Cm LogLevel 984Gives the verbosity level that is used when logging messages from 985.Xr sshd 8 . 986The possible values are: 987QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 988The default is INFO. 989DEBUG and DEBUG1 are equivalent. 990DEBUG2 and DEBUG3 each specify higher levels of debugging output. 991Logging with a DEBUG level violates the privacy of users and is not recommended. 992.It Cm MACs 993Specifies the available MAC (message authentication code) algorithms. 994The MAC algorithm is used for data integrity protection. 995Multiple algorithms must be comma-separated. 996If the specified value begins with a 997.Sq + 998character, then the specified algorithms will be appended to the default set 999instead of replacing them. 1000If the specified value begins with a 1001.Sq - 1002character, then the specified algorithms (including wildcards) will be removed 1003from the default set instead of replacing them. 1004.Pp 1005The algorithms that contain 1006.Qq -etm 1007calculate the MAC after encryption (encrypt-then-mac). 1008These are considered safer and their use recommended. 1009The supported MACs are: 1010.Pp 1011.Bl -item -compact -offset indent 1012.It 1013hmac-md5 1014.It 1015hmac-md5-96 1016.It 1017hmac-sha1 1018.It 1019hmac-sha1-96 1020.It 1021hmac-sha2-256 1022.It 1023hmac-sha2-512 1024.It 1025umac-64@openssh.com 1026.It 1027umac-128@openssh.com 1028.It 1029hmac-md5-etm@openssh.com 1030.It 1031hmac-md5-96-etm@openssh.com 1032.It 1033hmac-sha1-etm@openssh.com 1034.It 1035hmac-sha1-96-etm@openssh.com 1036.It 1037hmac-sha2-256-etm@openssh.com 1038.It 1039hmac-sha2-512-etm@openssh.com 1040.It 1041umac-64-etm@openssh.com 1042.It 1043umac-128-etm@openssh.com 1044.El 1045.Pp 1046The default is: 1047.Bd -literal -offset indent 1048umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1049hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1050hmac-sha1-etm@openssh.com, 1051umac-64@openssh.com,umac-128@openssh.com, 1052hmac-sha2-256,hmac-sha2-512,hmac-sha1 1053.Ed 1054.Pp 1055The list of available MAC algorithms may also be obtained using 1056.Qq ssh -Q mac . 1057.It Cm Match 1058Introduces a conditional block. 1059If all of the criteria on the 1060.Cm Match 1061line are satisfied, the keywords on the following lines override those 1062set in the global section of the config file, until either another 1063.Cm Match 1064line or the end of the file. 1065If a keyword appears in multiple 1066.Cm Match 1067blocks that are satisfied, only the first instance of the keyword is 1068applied. 1069.Pp 1070The arguments to 1071.Cm Match 1072are one or more criteria-pattern pairs or the single token 1073.Cm All 1074which matches all criteria. 1075The available criteria are 1076.Cm User , 1077.Cm Group , 1078.Cm Host , 1079.Cm LocalAddress , 1080.Cm LocalPort , 1081.Cm RDomain , 1082and 1083.Cm Address 1084(with 1085.Cm RDomain 1086representing the 1087.Xr rdomain 4 1088on which the connection was received.) 1089.Pp 1090The match patterns may consist of single entries or comma-separated 1091lists and may use the wildcard and negation operators described in the 1092.Sx PATTERNS 1093section of 1094.Xr ssh_config 5 . 1095.Pp 1096The patterns in an 1097.Cm Address 1098criteria may additionally contain addresses to match in CIDR 1099address/masklen format, 1100such as 192.0.2.0/24 or 2001:db8::/32. 1101Note that the mask length provided must be consistent with the address - 1102it is an error to specify a mask length that is too long for the address 1103or one with bits set in this host portion of the address. 1104For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. 1105.Pp 1106Only a subset of keywords may be used on the lines following a 1107.Cm Match 1108keyword. 1109Available keywords are 1110.Cm AcceptEnv , 1111.Cm AllowAgentForwarding , 1112.Cm AllowGroups , 1113.Cm AllowStreamLocalForwarding , 1114.Cm AllowTcpForwarding , 1115.Cm AllowUsers , 1116.Cm AuthenticationMethods , 1117.Cm AuthorizedKeysCommand , 1118.Cm AuthorizedKeysCommandUser , 1119.Cm AuthorizedKeysFile , 1120.Cm AuthorizedPrincipalsCommand , 1121.Cm AuthorizedPrincipalsCommandUser , 1122.Cm AuthorizedPrincipalsFile , 1123.Cm Banner , 1124.Cm ChrootDirectory , 1125.Cm ClientAliveCountMax , 1126.Cm ClientAliveInterval , 1127.Cm DenyGroups , 1128.Cm DenyUsers , 1129.Cm ForceCommand , 1130.Cm GatewayPorts , 1131.Cm GSSAPIAuthentication , 1132.Cm HostbasedAcceptedKeyTypes , 1133.Cm HostbasedAuthentication , 1134.Cm HostbasedUsesNameFromPacketOnly , 1135.Cm IPQoS , 1136.Cm KbdInteractiveAuthentication , 1137.Cm KerberosAuthentication , 1138.Cm LogLevel , 1139.Cm MaxAuthTries , 1140.Cm MaxSessions , 1141.Cm PasswordAuthentication , 1142.Cm PermitEmptyPasswords , 1143.Cm PermitListen , 1144.Cm PermitOpen , 1145.Cm PermitRootLogin , 1146.Cm PermitTTY , 1147.Cm PermitTunnel , 1148.Cm PermitUserRC , 1149.Cm PubkeyAcceptedKeyTypes , 1150.Cm PubkeyAuthentication , 1151.Cm RekeyLimit , 1152.Cm RevokedKeys , 1153.Cm RDomain , 1154.Cm SetEnv , 1155.Cm StreamLocalBindMask , 1156.Cm StreamLocalBindUnlink , 1157.Cm TrustedUserCAKeys , 1158.Cm X11DisplayOffset , 1159.Cm X11Forwarding 1160and 1161.Cm X11UseLocalHost . 1162.It Cm MaxAuthTries 1163Specifies the maximum number of authentication attempts permitted per 1164connection. 1165Once the number of failures reaches half this value, 1166additional failures are logged. 1167The default is 6. 1168.It Cm MaxSessions 1169Specifies the maximum number of open shell, login or subsystem (e.g. sftp) 1170sessions permitted per network connection. 1171Multiple sessions may be established by clients that support connection 1172multiplexing. 1173Setting 1174.Cm MaxSessions 1175to 1 will effectively disable session multiplexing, whereas setting it to 0 1176will prevent all shell, login and subsystem sessions while still permitting 1177forwarding. 1178The default is 10. 1179.It Cm MaxStartups 1180Specifies the maximum number of concurrent unauthenticated connections to the 1181SSH daemon. 1182Additional connections will be dropped until authentication succeeds or the 1183.Cm LoginGraceTime 1184expires for a connection. 1185The default is 10:30:100. 1186.Pp 1187Alternatively, random early drop can be enabled by specifying 1188the three colon separated values 1189start:rate:full (e.g. "10:30:60"). 1190.Xr sshd 8 1191will refuse connection attempts with a probability of rate/100 (30%) 1192if there are currently start (10) unauthenticated connections. 1193The probability increases linearly and all connection attempts 1194are refused if the number of unauthenticated connections reaches full (60). 1195.It Cm PasswordAuthentication 1196Specifies whether password authentication is allowed. 1197See also 1198.Cm UsePAM . 1199The default is 1200.Cm no . 1201.It Cm PermitEmptyPasswords 1202When password authentication is allowed, it specifies whether the 1203server allows login to accounts with empty password strings. 1204The default is 1205.Cm no . 1206.It Cm PermitListen 1207Specifies the addresses/ports on which a remote TCP port forwarding may listen. 1208The listen specification must be one of the following forms: 1209.Pp 1210.Bl -item -offset indent -compact 1211.It 1212.Cm PermitListen 1213.Sm off 1214.Ar port 1215.Sm on 1216.It 1217.Cm PermitListen 1218.Sm off 1219.Ar host : port 1220.Sm on 1221.El 1222.Pp 1223Multiple permissions may be specified by separating them with whitespace. 1224An argument of 1225.Cm any 1226can be used to remove all restrictions and permit any listen requests. 1227An argument of 1228.Cm none 1229can be used to prohibit all listen requests. 1230The host name may contain wildcards as described in the PATTERNS section in 1231.Xr ssh_config 5 . 1232The wildcard 1233.Sq * 1234can also be used in place of a port number to allow all ports. 1235By default all port forwarding listen requests are permitted. 1236Note that the 1237.Cm GatewayPorts 1238option may further restrict which addresses may be listened on. 1239Note also that 1240.Xr ssh 1 1241will request a listen host of 1242.Dq localhost 1243if no listen host was specifically requested, and this this name is 1244treated differently to explicit localhost addresses of 1245.Dq 127.0.0.1 1246and 1247.Dq ::1 . 1248.It Cm PermitOpen 1249Specifies the destinations to which TCP port forwarding is permitted. 1250The forwarding specification must be one of the following forms: 1251.Pp 1252.Bl -item -offset indent -compact 1253.It 1254.Cm PermitOpen 1255.Sm off 1256.Ar host : port 1257.Sm on 1258.It 1259.Cm PermitOpen 1260.Sm off 1261.Ar IPv4_addr : port 1262.Sm on 1263.It 1264.Cm PermitOpen 1265.Sm off 1266.Ar \&[ IPv6_addr \&] : port 1267.Sm on 1268.El 1269.Pp 1270Multiple forwards may be specified by separating them with whitespace. 1271An argument of 1272.Cm any 1273can be used to remove all restrictions and permit any forwarding requests. 1274An argument of 1275.Cm none 1276can be used to prohibit all forwarding requests. 1277The wildcard 1278.Sq * 1279can be used for host or port to allow all hosts or ports, respectively. 1280By default all port forwarding requests are permitted. 1281.It Cm PermitRootLogin 1282Specifies whether root can log in using 1283.Xr ssh 1 . 1284The argument must be 1285.Cm yes , 1286.Cm prohibit-password , 1287.Cm forced-commands-only , 1288or 1289.Cm no . 1290The default is 1291.Cm no . 1292Note that if 1293.Cm ChallengeResponseAuthentication 1294and 1295.Cm UsePAM 1296are both 1297.Cm yes , 1298this setting may be overridden by the PAM policy. 1299.Pp 1300If this option is set to 1301.Cm prohibit-password 1302(or its deprecated alias, 1303.Cm without-password ) , 1304password and keyboard-interactive authentication are disabled for root. 1305.Pp 1306If this option is set to 1307.Cm forced-commands-only , 1308root login with public key authentication will be allowed, 1309but only if the 1310.Ar command 1311option has been specified 1312(which may be useful for taking remote backups even if root login is 1313normally not allowed). 1314All other authentication methods are disabled for root. 1315.Pp 1316If this option is set to 1317.Cm no , 1318root is not allowed to log in. 1319.It Cm PermitTTY 1320Specifies whether 1321.Xr pty 4 1322allocation is permitted. 1323The default is 1324.Cm yes . 1325.It Cm PermitTunnel 1326Specifies whether 1327.Xr tun 4 1328device forwarding is allowed. 1329The argument must be 1330.Cm yes , 1331.Cm point-to-point 1332(layer 3), 1333.Cm ethernet 1334(layer 2), or 1335.Cm no . 1336Specifying 1337.Cm yes 1338permits both 1339.Cm point-to-point 1340and 1341.Cm ethernet . 1342The default is 1343.Cm no . 1344.Pp 1345Independent of this setting, the permissions of the selected 1346.Xr tun 4 1347device must allow access to the user. 1348.It Cm PermitUserEnvironment 1349Specifies whether 1350.Pa ~/.ssh/environment 1351and 1352.Cm environment= 1353options in 1354.Pa ~/.ssh/authorized_keys 1355are processed by 1356.Xr sshd 8 . 1357Valid options are 1358.Cm yes , 1359.Cm no 1360or a pattern-list specifying which environment variable names to accept 1361(for example 1362.Qq LANG,LC_* ) . 1363The default is 1364.Cm no . 1365Enabling environment processing may enable users to bypass access 1366restrictions in some configurations using mechanisms such as 1367.Ev LD_PRELOAD . 1368.It Cm PermitUserRC 1369Specifies whether any 1370.Pa ~/.ssh/rc 1371file is executed. 1372The default is 1373.Cm yes . 1374.It Cm PidFile 1375Specifies the file that contains the process ID of the 1376SSH daemon, or 1377.Cm none 1378to not write one. 1379The default is 1380.Pa /var/run/sshd.pid . 1381.It Cm Port 1382Specifies the port number that 1383.Xr sshd 8 1384listens on. 1385The default is 22. 1386Multiple options of this type are permitted. 1387See also 1388.Cm ListenAddress . 1389.It Cm PrintLastLog 1390Specifies whether 1391.Xr sshd 8 1392should print the date and time of the last user login when a user logs 1393in interactively. 1394The default is 1395.Cm yes . 1396.It Cm PrintMotd 1397Specifies whether 1398.Xr sshd 8 1399should print 1400.Pa /etc/motd 1401when a user logs in interactively. 1402(On some systems it is also printed by the shell, 1403.Pa /etc/profile , 1404or equivalent.) 1405The default is 1406.Cm yes . 1407.It Cm PubkeyAcceptedKeyTypes 1408Specifies the key types that will be accepted for public key authentication 1409as a list of comma-separated patterns. 1410Alternately if the specified value begins with a 1411.Sq + 1412character, then the specified key types will be appended to the default set 1413instead of replacing them. 1414If the specified value begins with a 1415.Sq - 1416character, then the specified key types (including wildcards) will be removed 1417from the default set instead of replacing them. 1418The default for this option is: 1419.Bd -literal -offset 3n 1420ecdsa-sha2-nistp256-cert-v01@openssh.com, 1421ecdsa-sha2-nistp384-cert-v01@openssh.com, 1422ecdsa-sha2-nistp521-cert-v01@openssh.com, 1423ssh-ed25519-cert-v01@openssh.com, 1424rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com, 1425ssh-rsa-cert-v01@openssh.com, 1426ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1427ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 1428.Ed 1429.Pp 1430The list of available key types may also be obtained using 1431.Qq ssh -Q key . 1432.It Cm PubkeyAuthentication 1433Specifies whether public key authentication is allowed. 1434The default is 1435.Cm yes . 1436.It Cm RekeyLimit 1437Specifies the maximum amount of data that may be transmitted before the 1438session key is renegotiated, optionally followed a maximum amount of 1439time that may pass before the session key is renegotiated. 1440The first argument is specified in bytes and may have a suffix of 1441.Sq K , 1442.Sq M , 1443or 1444.Sq G 1445to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1446The default is between 1447.Sq 1G 1448and 1449.Sq 4G , 1450depending on the cipher. 1451The optional second value is specified in seconds and may use any of the 1452units documented in the 1453.Sx TIME FORMATS 1454section. 1455The default value for 1456.Cm RekeyLimit 1457is 1458.Cm default none , 1459which means that rekeying is performed after the cipher's default amount 1460of data has been sent or received and no time based rekeying is done. 1461.It Cm RevokedKeys 1462Specifies revoked public keys file, or 1463.Cm none 1464to not use one. 1465Keys listed in this file will be refused for public key authentication. 1466Note that if this file is not readable, then public key authentication will 1467be refused for all users. 1468Keys may be specified as a text file, listing one public key per line, or as 1469an OpenSSH Key Revocation List (KRL) as generated by 1470.Xr ssh-keygen 1 . 1471For more information on KRLs, see the KEY REVOCATION LISTS section in 1472.Xr ssh-keygen 1 . 1473.It Cm RDomain 1474Specifies an explicit routing domain that is applied after authentication 1475has completed. 1476The user session, as well and any forwarded or listening IP sockets, 1477will be bound to this 1478.Xr rdomain 4 . 1479If the routing domain is set to 1480.Cm \&%D , 1481then the domain in which the incoming connection was received will be applied. 1482.It Cm SetEnv 1483Specifies one or more environment variables to set in child sessions started 1484by 1485.Xr sshd 8 1486as 1487.Dq NAME=VALUE . 1488The environment value may be quoted (e.g. if it contains whitespace 1489characters). 1490Environment variables set by 1491.Cm SetEnv 1492override the default environment and any variables specified by the user 1493via 1494.Cm AcceptEnv 1495or 1496.Cm PermitUserEnvironment . 1497.It Cm StreamLocalBindMask 1498Sets the octal file creation mode mask 1499.Pq umask 1500used when creating a Unix-domain socket file for local or remote 1501port forwarding. 1502This option is only used for port forwarding to a Unix-domain socket file. 1503.Pp 1504The default value is 0177, which creates a Unix-domain socket file that is 1505readable and writable only by the owner. 1506Note that not all operating systems honor the file mode on Unix-domain 1507socket files. 1508.It Cm StreamLocalBindUnlink 1509Specifies whether to remove an existing Unix-domain socket file for local 1510or remote port forwarding before creating a new one. 1511If the socket file already exists and 1512.Cm StreamLocalBindUnlink 1513is not enabled, 1514.Nm sshd 1515will be unable to forward the port to the Unix-domain socket file. 1516This option is only used for port forwarding to a Unix-domain socket file. 1517.Pp 1518The argument must be 1519.Cm yes 1520or 1521.Cm no . 1522The default is 1523.Cm no . 1524.It Cm StrictModes 1525Specifies whether 1526.Xr sshd 8 1527should check file modes and ownership of the 1528user's files and home directory before accepting login. 1529This is normally desirable because novices sometimes accidentally leave their 1530directory or files world-writable. 1531The default is 1532.Cm yes . 1533Note that this does not apply to 1534.Cm ChrootDirectory , 1535whose permissions and ownership are checked unconditionally. 1536.It Cm Subsystem 1537Configures an external subsystem (e.g. file transfer daemon). 1538Arguments should be a subsystem name and a command (with optional arguments) 1539to execute upon subsystem request. 1540.Pp 1541The command 1542.Cm sftp-server 1543implements the SFTP file transfer subsystem. 1544.Pp 1545Alternately the name 1546.Cm internal-sftp 1547implements an in-process SFTP server. 1548This may simplify configurations using 1549.Cm ChrootDirectory 1550to force a different filesystem root on clients. 1551.Pp 1552By default no subsystems are defined. 1553.It Cm SyslogFacility 1554Gives the facility code that is used when logging messages from 1555.Xr sshd 8 . 1556The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1557LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1558The default is AUTH. 1559.It Cm TCPKeepAlive 1560Specifies whether the system should send TCP keepalive messages to the 1561other side. 1562If they are sent, death of the connection or crash of one 1563of the machines will be properly noticed. 1564However, this means that 1565connections will die if the route is down temporarily, and some people 1566find it annoying. 1567On the other hand, if TCP keepalives are not sent, 1568sessions may hang indefinitely on the server, leaving 1569.Qq ghost 1570users and consuming server resources. 1571.Pp 1572The default is 1573.Cm yes 1574(to send TCP keepalive messages), and the server will notice 1575if the network goes down or the client host crashes. 1576This avoids infinitely hanging sessions. 1577.Pp 1578To disable TCP keepalive messages, the value should be set to 1579.Cm no . 1580.It Cm TrustedUserCAKeys 1581Specifies a file containing public keys of certificate authorities that are 1582trusted to sign user certificates for authentication, or 1583.Cm none 1584to not use one. 1585Keys are listed one per line; empty lines and comments starting with 1586.Ql # 1587are allowed. 1588If a certificate is presented for authentication and has its signing CA key 1589listed in this file, then it may be used for authentication for any user 1590listed in the certificate's principals list. 1591Note that certificates that lack a list of principals will not be permitted 1592for authentication using 1593.Cm TrustedUserCAKeys . 1594For more details on certificates, see the CERTIFICATES section in 1595.Xr ssh-keygen 1 . 1596.It Cm UseBlacklist 1597Specifies whether 1598.Xr sshd 8 1599attempts to send authentication success and failure messages 1600to the 1601.Xr blacklistd 8 1602daemon. 1603The default is 1604.Cm no . 1605For forward compatibility with an upcoming 1606.Xr blacklistd 1607rename, the 1608.Cm UseBlocklist 1609alias can be used instead. 1610.It Cm UseDNS 1611Specifies whether 1612.Xr sshd 8 1613should look up the remote host name, and to check that 1614the resolved host name for the remote IP address maps back to the 1615very same IP address. 1616.Pp 1617If this option is set to 1618.Cm no , 1619then only addresses and not host names may be used in 1620.Pa ~/.ssh/authorized_keys 1621.Cm from 1622and 1623.Nm 1624.Cm Match 1625.Cm Host 1626directives. 1627The default is 1628.Dq yes . 1629.It Cm UsePAM 1630Enables the Pluggable Authentication Module interface. 1631If set to 1632.Cm yes 1633this will enable PAM authentication using 1634.Cm ChallengeResponseAuthentication 1635and 1636.Cm PasswordAuthentication 1637in addition to PAM account and session module processing for all 1638authentication types. 1639.Pp 1640Because PAM challenge-response authentication usually serves an equivalent 1641role to password authentication, you should disable either 1642.Cm PasswordAuthentication 1643or 1644.Cm ChallengeResponseAuthentication. 1645.Pp 1646If 1647.Cm UsePAM 1648is enabled, you will not be able to run 1649.Xr sshd 8 1650as a non-root user. 1651The default is 1652.Cm yes . 1653.It Cm VersionAddendum 1654Optionally specifies additional text to append to the SSH protocol banner 1655sent by the server upon connection. 1656The default is 1657.Qq FreeBSD-20180909 . 1658The value 1659.Cm none 1660may be used to disable this. 1661.It Cm X11DisplayOffset 1662Specifies the first display number available for 1663.Xr sshd 8 Ns 's 1664X11 forwarding. 1665This prevents sshd from interfering with real X11 servers. 1666The default is 10. 1667.It Cm X11Forwarding 1668Specifies whether X11 forwarding is permitted. 1669The argument must be 1670.Cm yes 1671or 1672.Cm no . 1673The default is 1674.Cm yes . 1675.Pp 1676When X11 forwarding is enabled, there may be additional exposure to 1677the server and to client displays if the 1678.Xr sshd 8 1679proxy display is configured to listen on the wildcard address (see 1680.Cm X11UseLocalhost ) , 1681though this is not the default. 1682Additionally, the authentication spoofing and authentication data 1683verification and substitution occur on the client side. 1684The security risk of using X11 forwarding is that the client's X11 1685display server may be exposed to attack when the SSH client requests 1686forwarding (see the warnings for 1687.Cm ForwardX11 1688in 1689.Xr ssh_config 5 ) . 1690A system administrator may have a stance in which they want to 1691protect clients that may expose themselves to attack by unwittingly 1692requesting X11 forwarding, which can warrant a 1693.Cm no 1694setting. 1695.Pp 1696Note that disabling X11 forwarding does not prevent users from 1697forwarding X11 traffic, as users can always install their own forwarders. 1698.It Cm X11UseLocalhost 1699Specifies whether 1700.Xr sshd 8 1701should bind the X11 forwarding server to the loopback address or to 1702the wildcard address. 1703By default, 1704sshd binds the forwarding server to the loopback address and sets the 1705hostname part of the 1706.Ev DISPLAY 1707environment variable to 1708.Cm localhost . 1709This prevents remote hosts from connecting to the proxy display. 1710However, some older X11 clients may not function with this 1711configuration. 1712.Cm X11UseLocalhost 1713may be set to 1714.Cm no 1715to specify that the forwarding server should be bound to the wildcard 1716address. 1717The argument must be 1718.Cm yes 1719or 1720.Cm no . 1721The default is 1722.Cm yes . 1723.It Cm XAuthLocation 1724Specifies the full pathname of the 1725.Xr xauth 1 1726program, or 1727.Cm none 1728to not use one. 1729The default is 1730.Pa /usr/local/bin/xauth . 1731.El 1732.Sh TIME FORMATS 1733.Xr sshd 8 1734command-line arguments and configuration file options that specify time 1735may be expressed using a sequence of the form: 1736.Sm off 1737.Ar time Op Ar qualifier , 1738.Sm on 1739where 1740.Ar time 1741is a positive integer value and 1742.Ar qualifier 1743is one of the following: 1744.Pp 1745.Bl -tag -width Ds -compact -offset indent 1746.It Aq Cm none 1747seconds 1748.It Cm s | Cm S 1749seconds 1750.It Cm m | Cm M 1751minutes 1752.It Cm h | Cm H 1753hours 1754.It Cm d | Cm D 1755days 1756.It Cm w | Cm W 1757weeks 1758.El 1759.Pp 1760Each member of the sequence is added together to calculate 1761the total time value. 1762.Pp 1763Time format examples: 1764.Pp 1765.Bl -tag -width Ds -compact -offset indent 1766.It 600 1767600 seconds (10 minutes) 1768.It 10m 176910 minutes 1770.It 1h30m 17711 hour 30 minutes (90 minutes) 1772.El 1773.Sh TOKENS 1774Arguments to some keywords can make use of tokens, 1775which are expanded at runtime: 1776.Pp 1777.Bl -tag -width XXXX -offset indent -compact 1778.It %% 1779A literal 1780.Sq % . 1781.It \&%D 1782The routing domain in which the incoming connection was received. 1783.It %F 1784The fingerprint of the CA key. 1785.It %f 1786The fingerprint of the key or certificate. 1787.It %h 1788The home directory of the user. 1789.It %i 1790The key ID in the certificate. 1791.It %K 1792The base64-encoded CA key. 1793.It %k 1794The base64-encoded key or certificate for authentication. 1795.It %s 1796The serial number of the certificate. 1797.It \&%T 1798The type of the CA key. 1799.It %t 1800The key or certificate type. 1801.It \&%U 1802The numeric user ID of the target user. 1803.It %u 1804The username. 1805.El 1806.Pp 1807.Cm AuthorizedKeysCommand 1808accepts the tokens %%, %f, %h, %k, %t, %U, and %u. 1809.Pp 1810.Cm AuthorizedKeysFile 1811accepts the tokens %%, %h, %U, and %u. 1812.Pp 1813.Cm AuthorizedPrincipalsCommand 1814accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u. 1815.Pp 1816.Cm AuthorizedPrincipalsFile 1817accepts the tokens %%, %h, %U, and %u. 1818.Pp 1819.Cm ChrootDirectory 1820accepts the tokens %%, %h, %U, and %u. 1821.Pp 1822.Cm RoutingDomain 1823accepts the token %D. 1824.Sh FILES 1825.Bl -tag -width Ds 1826.It Pa /etc/ssh/sshd_config 1827Contains configuration data for 1828.Xr sshd 8 . 1829This file should be writable by root only, but it is recommended 1830(though not necessary) that it be world-readable. 1831.El 1832.Sh SEE ALSO 1833.Xr sftp-server 8 , 1834.Xr sshd 8 1835.Sh AUTHORS 1836.An -nosplit 1837OpenSSH is a derivative of the original and free 1838ssh 1.2.12 release by 1839.An Tatu Ylonen . 1840.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , 1841.An Theo de Raadt 1842and 1843.An Dug Song 1844removed many bugs, re-added newer features and 1845created OpenSSH. 1846.An Markus Friedl 1847contributed the support for SSH protocol versions 1.5 and 2.0. 1848.An Niels Provos 1849and 1850.An Markus Friedl 1851contributed support for privilege separation. 1852