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 1.332 2021/07/02 05:11:21 dtucker Exp $ 37.Dd $Mdocdate: July 2 2021 $ 38.Dt SSHD_CONFIG 5 39.Os 40.Sh NAME 41.Nm sshd_config 42.Nd OpenSSH daemon configuration file 43.Sh DESCRIPTION 44.Xr sshd 8 45reads configuration data from 46.Pa /etc/ssh/sshd_config 47(or the file specified with 48.Fl f 49on the command line). 50The file contains keyword-argument pairs, one per line. 51For each keyword, the first obtained value will be used. 52Lines starting with 53.Ql # 54and empty lines are interpreted as comments. 55Arguments may optionally be enclosed in double quotes 56.Pq \&" 57in order to represent arguments containing spaces. 58.Pp 59The possible 60keywords and their meanings are as follows (note that 61keywords are case-insensitive and arguments are case-sensitive): 62.Bl -tag -width Ds 63.It Cm AcceptEnv 64Specifies what environment variables sent by the client will be copied into 65the session's 66.Xr environ 7 . 67See 68.Cm SendEnv 69and 70.Cm SetEnv 71in 72.Xr ssh_config 5 73for how to configure the client. 74The 75.Ev TERM 76environment variable is always accepted whenever the client 77requests a pseudo-terminal as it is required by the protocol. 78Variables are specified by name, which may contain the wildcard characters 79.Ql * 80and 81.Ql \&? . 82Multiple environment variables may be separated by whitespace or spread 83across multiple 84.Cm AcceptEnv 85directives. 86Be warned that some environment variables could be used to bypass restricted 87user environments. 88For this reason, care should be taken in the use of this directive. 89The default is not to accept any environment variables. 90.It Cm AddressFamily 91Specifies which address family should be used by 92.Xr sshd 8 . 93Valid arguments are 94.Cm any 95(the default), 96.Cm inet 97(use IPv4 only), or 98.Cm inet6 99(use IPv6 only). 100.It Cm AllowAgentForwarding 101Specifies whether 102.Xr ssh-agent 1 103forwarding is permitted. 104The default is 105.Cm yes . 106Note that disabling agent forwarding does not improve security 107unless users are also denied shell access, as they can always install 108their own forwarders. 109.It Cm AllowGroups 110This keyword can be followed by a list of group name patterns, separated 111by spaces. 112If specified, login is allowed only for users whose primary 113group or supplementary group list matches one of the patterns. 114Only group names are valid; a numerical group ID is not recognized. 115By default, login is allowed for all groups. 116The allow/deny groups directives are processed in the following order: 117.Cm DenyGroups , 118.Cm AllowGroups . 119.Pp 120See PATTERNS in 121.Xr ssh_config 5 122for more information on patterns. 123.It Cm AllowStreamLocalForwarding 124Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. 125The available options are 126.Cm yes 127(the default) 128or 129.Cm all 130to allow StreamLocal forwarding, 131.Cm no 132to prevent all StreamLocal forwarding, 133.Cm local 134to allow local (from the perspective of 135.Xr ssh 1 ) 136forwarding only or 137.Cm remote 138to allow remote forwarding only. 139Note that disabling StreamLocal forwarding does not improve security unless 140users are also denied shell access, as they can always install their 141own forwarders. 142.It Cm AllowTcpForwarding 143Specifies whether TCP forwarding is permitted. 144The available options are 145.Cm yes 146(the default) 147or 148.Cm all 149to allow TCP forwarding, 150.Cm no 151to prevent all TCP forwarding, 152.Cm local 153to allow local (from the perspective of 154.Xr ssh 1 ) 155forwarding only or 156.Cm remote 157to allow remote forwarding only. 158Note that disabling TCP forwarding does not improve security unless 159users are also denied shell access, as they can always install their 160own forwarders. 161.It Cm AllowUsers 162This keyword can be followed by a list of user name patterns, separated 163by spaces. 164If specified, login is allowed only for user names that 165match one of the patterns. 166Only user names are valid; a numerical user ID is not recognized. 167By default, login is allowed for all users. 168If the pattern takes the form USER@HOST then USER and HOST 169are separately checked, restricting logins to particular 170users from particular hosts. 171HOST criteria may additionally contain addresses to match in CIDR 172address/masklen format. 173The allow/deny users directives are processed in the following order: 174.Cm DenyUsers , 175.Cm AllowUsers . 176.Pp 177See PATTERNS in 178.Xr ssh_config 5 179for more information on patterns. 180.It Cm AuthenticationMethods 181Specifies the authentication methods that must be successfully completed 182for a user to be granted access. 183This option must be followed by one or more lists of comma-separated 184authentication method names, or by the single string 185.Cm any 186to indicate the default behaviour of accepting any single authentication 187method. 188If the default is overridden, then successful authentication requires 189completion of every method in at least one of these lists. 190.Pp 191For example, 192.Qq publickey,password publickey,keyboard-interactive 193would require the user to complete public key authentication, followed by 194either password or keyboard interactive authentication. 195Only methods that are next in one or more lists are offered at each stage, 196so for this example it would not be possible to attempt password or 197keyboard-interactive authentication before public key. 198.Pp 199For keyboard interactive authentication it is also possible to 200restrict authentication to a specific device by appending a 201colon followed by the device identifier 202.Cm bsdauth , 203.Cm pam , 204or 205.Cm skey , 206depending on the server configuration. 207For example, 208.Qq keyboard-interactive:bsdauth 209would restrict keyboard interactive authentication to the 210.Cm bsdauth 211device. 212.Pp 213If the publickey method is listed more than once, 214.Xr sshd 8 215verifies that keys that have been used successfully are not reused for 216subsequent authentications. 217For example, 218.Qq publickey,publickey 219requires successful authentication using two different public keys. 220.Pp 221Note that each authentication method listed should also be explicitly enabled 222in the configuration. 223.Pp 224The available authentication methods are: 225.Qq gssapi-with-mic , 226.Qq hostbased , 227.Qq keyboard-interactive , 228.Qq none 229(used for access to password-less accounts when 230.Cm PermitEmptyPasswords 231is enabled), 232.Qq password 233and 234.Qq publickey . 235.It Cm AuthorizedKeysCommand 236Specifies a program to be used to look up the user's public keys. 237The program must be owned by root, not writable by group or others and 238specified by an absolute path. 239Arguments to 240.Cm AuthorizedKeysCommand 241accept the tokens described in the 242.Sx TOKENS 243section. 244If no arguments are specified then the username of the target user is used. 245.Pp 246The program should produce on standard output zero or 247more lines of authorized_keys output (see 248.Sx AUTHORIZED_KEYS 249in 250.Xr sshd 8 ) . 251.Cm AuthorizedKeysCommand 252is tried after the usual 253.Cm AuthorizedKeysFile 254files and will not be executed if a matching key is found there. 255By default, no 256.Cm AuthorizedKeysCommand 257is run. 258.It Cm AuthorizedKeysCommandUser 259Specifies the user under whose account the 260.Cm AuthorizedKeysCommand 261is run. 262It is recommended to use a dedicated user that has no other role on the host 263than running authorized keys commands. 264If 265.Cm AuthorizedKeysCommand 266is specified but 267.Cm AuthorizedKeysCommandUser 268is not, then 269.Xr sshd 8 270will refuse to start. 271.It Cm AuthorizedKeysFile 272Specifies the file that contains the public keys used for user authentication. 273The format is described in the AUTHORIZED_KEYS FILE FORMAT section of 274.Xr sshd 8 . 275Arguments to 276.Cm AuthorizedKeysFile 277accept the tokens described in the 278.Sx TOKENS 279section. 280After expansion, 281.Cm AuthorizedKeysFile 282is taken to be an absolute path or one relative to the user's home 283directory. 284Multiple files may be listed, separated by whitespace. 285Alternately this option may be set to 286.Cm none 287to skip checking for user keys in files. 288The default is 289.Qq .ssh/authorized_keys .ssh/authorized_keys2 . 290.It Cm AuthorizedPrincipalsCommand 291Specifies a program to be used to generate the list of allowed 292certificate principals as per 293.Cm AuthorizedPrincipalsFile . 294The program must be owned by root, not writable by group or others and 295specified by an absolute path. 296Arguments to 297.Cm AuthorizedPrincipalsCommand 298accept the tokens described in the 299.Sx TOKENS 300section. 301If no arguments are specified then the username of the target user is used. 302.Pp 303The program should produce on standard output zero or 304more lines of 305.Cm AuthorizedPrincipalsFile 306output. 307If either 308.Cm AuthorizedPrincipalsCommand 309or 310.Cm AuthorizedPrincipalsFile 311is specified, then certificates offered by the client for authentication 312must contain a principal that is listed. 313By default, no 314.Cm AuthorizedPrincipalsCommand 315is run. 316.It Cm AuthorizedPrincipalsCommandUser 317Specifies the user under whose account the 318.Cm AuthorizedPrincipalsCommand 319is run. 320It is recommended to use a dedicated user that has no other role on the host 321than running authorized principals commands. 322If 323.Cm AuthorizedPrincipalsCommand 324is specified but 325.Cm AuthorizedPrincipalsCommandUser 326is not, then 327.Xr sshd 8 328will refuse to start. 329.It Cm AuthorizedPrincipalsFile 330Specifies a file that lists principal names that are accepted for 331certificate authentication. 332When using certificates signed by a key listed in 333.Cm TrustedUserCAKeys , 334this file lists names, one of which must appear in the certificate for it 335to be accepted for authentication. 336Names are listed one per line preceded by key options (as described in 337.Sx AUTHORIZED_KEYS FILE FORMAT 338in 339.Xr sshd 8 ) . 340Empty lines and comments starting with 341.Ql # 342are ignored. 343.Pp 344Arguments to 345.Cm AuthorizedPrincipalsFile 346accept the tokens described in the 347.Sx TOKENS 348section. 349After expansion, 350.Cm AuthorizedPrincipalsFile 351is taken to be an absolute path or one relative to the user's home directory. 352The default is 353.Cm none , 354i.e. not to use a principals file \(en in this case, the username 355of the user must appear in a certificate's principals list for it to be 356accepted. 357.Pp 358Note that 359.Cm AuthorizedPrincipalsFile 360is only used when authentication proceeds using a CA listed in 361.Cm TrustedUserCAKeys 362and is not consulted for certification authorities trusted via 363.Pa ~/.ssh/authorized_keys , 364though the 365.Cm principals= 366key option offers a similar facility (see 367.Xr sshd 8 368for details). 369.It Cm Banner 370The contents of the specified file are sent to the remote user before 371authentication is allowed. 372If the argument is 373.Cm none 374then no banner is displayed. 375By default, no banner is displayed. 376.It Cm CASignatureAlgorithms 377Specifies which algorithms are allowed for signing of certificates 378by certificate authorities (CAs). 379The default is: 380.Bd -literal -offset indent 381ssh-ed25519,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 382sk-ssh-ed25519@openssh.com,sk-ecdsa-sha2-nistp256@openssh.com, 383rsa-sha2-512,rsa-sha2-256 384.Ed 385.Pp 386Certificates signed using other algorithms will not be accepted for 387public key or host-based authentication. 388.It Cm ChrootDirectory 389Specifies the pathname of a directory to 390.Xr chroot 2 391to after authentication. 392At session startup 393.Xr sshd 8 394checks that all components of the pathname are root-owned directories 395which are not writable by any other user or group. 396After the chroot, 397.Xr sshd 8 398changes the working directory to the user's home directory. 399Arguments to 400.Cm ChrootDirectory 401accept the tokens described in the 402.Sx TOKENS 403section. 404.Pp 405The 406.Cm ChrootDirectory 407must contain the necessary files and directories to support the 408user's session. 409For an interactive session this requires at least a shell, typically 410.Xr sh 1 , 411and basic 412.Pa /dev 413nodes such as 414.Xr null 4 , 415.Xr zero 4 , 416.Xr stdin 4 , 417.Xr stdout 4 , 418.Xr stderr 4 , 419and 420.Xr tty 4 421devices. 422For file transfer sessions using SFTP 423no additional configuration of the environment is necessary if the in-process 424sftp-server is used, 425though sessions which use logging may require 426.Pa /dev/log 427inside the chroot directory on some operating systems (see 428.Xr sftp-server 8 429for details). 430.Pp 431For safety, it is very important that the directory hierarchy be 432prevented from modification by other processes on the system (especially 433those outside the jail). 434Misconfiguration can lead to unsafe environments which 435.Xr sshd 8 436cannot detect. 437.Pp 438The default is 439.Cm none , 440indicating not to 441.Xr chroot 2 . 442.It Cm Ciphers 443Specifies the ciphers allowed. 444Multiple ciphers must be comma-separated. 445If the specified list begins with a 446.Sq + 447character, then the specified ciphers will be appended to the default set 448instead of replacing them. 449If the specified list begins with a 450.Sq - 451character, then the specified ciphers (including wildcards) will be removed 452from the default set instead of replacing them. 453If the specified list begins with a 454.Sq ^ 455character, then the specified ciphers will be placed at the head of the 456default set. 457.Pp 458The supported ciphers are: 459.Pp 460.Bl -item -compact -offset indent 461.It 4623des-cbc 463.It 464aes128-cbc 465.It 466aes192-cbc 467.It 468aes256-cbc 469.It 470aes128-ctr 471.It 472aes192-ctr 473.It 474aes256-ctr 475.It 476aes128-gcm@openssh.com 477.It 478aes256-gcm@openssh.com 479.It 480chacha20-poly1305@openssh.com 481.El 482.Pp 483The default is: 484.Bd -literal -offset indent 485chacha20-poly1305@openssh.com, 486aes128-ctr,aes192-ctr,aes256-ctr, 487aes128-gcm@openssh.com,aes256-gcm@openssh.com 488.Ed 489.Pp 490The list of available ciphers may also be obtained using 491.Qq ssh -Q cipher . 492.It Cm ClientAliveCountMax 493Sets the number of client alive messages which may be sent without 494.Xr sshd 8 495receiving any messages back from the client. 496If this threshold is reached while client alive messages are being sent, 497sshd will disconnect the client, terminating the session. 498It is important to note that the use of client alive messages is very 499different from 500.Cm TCPKeepAlive . 501The client alive messages are sent through the encrypted channel 502and therefore will not be spoofable. 503The TCP keepalive option enabled by 504.Cm TCPKeepAlive 505is spoofable. 506The client alive mechanism is valuable when the client or 507server depend on knowing when a connection has become unresponsive. 508.Pp 509The default value is 3. 510If 511.Cm ClientAliveInterval 512is set to 15, and 513.Cm ClientAliveCountMax 514is left at the default, unresponsive SSH clients 515will be disconnected after approximately 45 seconds. 516Setting a zero 517.Cm ClientAliveCountMax 518disables connection termination. 519.It Cm ClientAliveInterval 520Sets a timeout interval in seconds after which if no data has been received 521from the client, 522.Xr sshd 8 523will send a message through the encrypted 524channel to request a response from the client. 525The default 526is 0, indicating that these messages will not be sent to the client. 527.It Cm Compression 528Specifies whether compression is enabled after 529the user has authenticated successfully. 530The argument must be 531.Cm yes , 532.Cm delayed 533(a legacy synonym for 534.Cm yes ) 535or 536.Cm no . 537The default is 538.Cm yes . 539.It Cm DenyGroups 540This keyword can be followed by a list of group name patterns, separated 541by spaces. 542Login is disallowed for users whose primary group or supplementary 543group list matches one of the patterns. 544Only group names are valid; a numerical group ID is not recognized. 545By default, login is allowed for all groups. 546The allow/deny groups directives are processed in the following order: 547.Cm DenyGroups , 548.Cm AllowGroups . 549.Pp 550See PATTERNS in 551.Xr ssh_config 5 552for more information on patterns. 553.It Cm DenyUsers 554This keyword can be followed by a list of user name patterns, separated 555by spaces. 556Login is disallowed for user names that match one of the patterns. 557Only user names are valid; a numerical user ID is not recognized. 558By default, login is allowed for all users. 559If the pattern takes the form USER@HOST then USER and HOST 560are separately checked, restricting logins to particular 561users from particular hosts. 562HOST criteria may additionally contain addresses to match in CIDR 563address/masklen format. 564The allow/deny users directives are processed in the following order: 565.Cm DenyUsers , 566.Cm AllowUsers . 567.Pp 568See PATTERNS in 569.Xr ssh_config 5 570for more information on patterns. 571.It Cm DisableForwarding 572Disables all forwarding features, including X11, 573.Xr ssh-agent 1 , 574TCP and StreamLocal. 575This option overrides all other forwarding-related options and may 576simplify restricted configurations. 577.It Cm ExposeAuthInfo 578Writes a temporary file containing a list of authentication methods and 579public credentials (e.g. keys) used to authenticate the user. 580The location of the file is exposed to the user session through the 581.Ev SSH_USER_AUTH 582environment variable. 583The default is 584.Cm no . 585.It Cm FingerprintHash 586Specifies the hash algorithm used when logging key fingerprints. 587Valid options are: 588.Cm md5 589and 590.Cm sha256 . 591The default is 592.Cm sha256 . 593.It Cm ForceCommand 594Forces the execution of the command specified by 595.Cm ForceCommand , 596ignoring any command supplied by the client and 597.Pa ~/.ssh/rc 598if present. 599The command is invoked by using the user's login shell with the -c option. 600This applies to shell, command, or subsystem execution. 601It is most useful inside a 602.Cm Match 603block. 604The command originally supplied by the client is available in the 605.Ev SSH_ORIGINAL_COMMAND 606environment variable. 607Specifying a command of 608.Cm internal-sftp 609will force the use of an in-process SFTP server that requires no support 610files when used with 611.Cm ChrootDirectory . 612The default is 613.Cm none . 614.It Cm GatewayPorts 615Specifies whether remote hosts are allowed to connect to ports 616forwarded for the client. 617By default, 618.Xr sshd 8 619binds remote port forwardings to the loopback address. 620This prevents other remote hosts from connecting to forwarded ports. 621.Cm GatewayPorts 622can be used to specify that sshd 623should allow remote port forwardings to bind to non-loopback addresses, thus 624allowing other hosts to connect. 625The argument may be 626.Cm no 627to force remote port forwardings to be available to the local host only, 628.Cm yes 629to force remote port forwardings to bind to the wildcard address, or 630.Cm clientspecified 631to allow the client to select the address to which the forwarding is bound. 632The default is 633.Cm no . 634.It Cm GSSAPIAuthentication 635Specifies whether user authentication based on GSSAPI is allowed. 636The default is 637.Cm no . 638.It Cm GSSAPICleanupCredentials 639Specifies whether to automatically destroy the user's credentials cache 640on logout. 641The default is 642.Cm yes . 643.It Cm GSSAPIStrictAcceptorCheck 644Determines whether to be strict about the identity of the GSSAPI acceptor 645a client authenticates against. 646If set to 647.Cm yes 648then the client must authenticate against the host 649service on the current hostname. 650If set to 651.Cm no 652then the client may authenticate against any service key stored in the 653machine's default store. 654This facility is provided to assist with operation on multi homed machines. 655The default is 656.Cm yes . 657.It Cm HostbasedAcceptedAlgorithms 658Specifies the signature algorithms that will be accepted for hostbased 659authentication as a list of comma-separated patterns. 660Alternately if the specified list begins with a 661.Sq + 662character, then the specified signature algorithms will be appended to 663the default set instead of replacing them. 664If the specified list begins with a 665.Sq - 666character, then the specified signature algorithms (including wildcards) 667will be removed from the default set instead of replacing them. 668If the specified list begins with a 669.Sq ^ 670character, then the specified signature algorithms will be placed at 671the head of the default set. 672The default for this option is: 673.Bd -literal -offset 3n 674ssh-ed25519-cert-v01@openssh.com, 675ecdsa-sha2-nistp256-cert-v01@openssh.com, 676ecdsa-sha2-nistp384-cert-v01@openssh.com, 677ecdsa-sha2-nistp521-cert-v01@openssh.com, 678sk-ssh-ed25519-cert-v01@openssh.com, 679sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 680rsa-sha2-512-cert-v01@openssh.com, 681rsa-sha2-256-cert-v01@openssh.com, 682ssh-rsa-cert-v01@openssh.com, 683ssh-ed25519, 684ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 685sk-ssh-ed25519@openssh.com, 686sk-ecdsa-sha2-nistp256@openssh.com, 687rsa-sha2-512,rsa-sha2-256,ssh-rsa 688.Ed 689.Pp 690The list of available signature algorithms may also be obtained using 691.Qq ssh -Q HostbasedAcceptedAlgorithms . 692This was formerly named HostbasedAcceptedKeyTypes. 693.It Cm HostbasedAuthentication 694Specifies whether rhosts or /etc/hosts.equiv authentication together 695with successful public key client host authentication is allowed 696(host-based authentication). 697The default is 698.Cm no . 699.It Cm HostbasedUsesNameFromPacketOnly 700Specifies whether or not the server will attempt to perform a reverse 701name lookup when matching the name in the 702.Pa ~/.shosts , 703.Pa ~/.rhosts , 704and 705.Pa /etc/hosts.equiv 706files during 707.Cm HostbasedAuthentication . 708A setting of 709.Cm yes 710means that 711.Xr sshd 8 712uses the name supplied by the client rather than 713attempting to resolve the name from the TCP connection itself. 714The default is 715.Cm no . 716.It Cm HostCertificate 717Specifies a file containing a public host certificate. 718The certificate's public key must match a private host key already specified 719by 720.Cm HostKey . 721The default behaviour of 722.Xr sshd 8 723is not to load any certificates. 724.It Cm HostKey 725Specifies a file containing a private host key 726used by SSH. 727The defaults are 728.Pa /etc/ssh/ssh_host_ecdsa_key , 729.Pa /etc/ssh/ssh_host_ed25519_key 730and 731.Pa /etc/ssh/ssh_host_rsa_key . 732.Pp 733Note that 734.Xr sshd 8 735will refuse to use a file if it is group/world-accessible 736and that the 737.Cm HostKeyAlgorithms 738option restricts which of the keys are actually used by 739.Xr sshd 8 . 740.Pp 741It is possible to have multiple host key files. 742It is also possible to specify public host key files instead. 743In this case operations on the private key will be delegated 744to an 745.Xr ssh-agent 1 . 746.It Cm HostKeyAgent 747Identifies the UNIX-domain socket used to communicate 748with an agent that has access to the private host keys. 749If the string 750.Qq SSH_AUTH_SOCK 751is specified, the location of the socket will be read from the 752.Ev SSH_AUTH_SOCK 753environment variable. 754.It Cm HostKeyAlgorithms 755Specifies the host key signature algorithms 756that the server offers. 757The default for this option is: 758.Bd -literal -offset 3n 759ssh-ed25519-cert-v01@openssh.com, 760ecdsa-sha2-nistp256-cert-v01@openssh.com, 761ecdsa-sha2-nistp384-cert-v01@openssh.com, 762ecdsa-sha2-nistp521-cert-v01@openssh.com, 763sk-ssh-ed25519-cert-v01@openssh.com, 764sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 765rsa-sha2-512-cert-v01@openssh.com, 766rsa-sha2-256-cert-v01@openssh.com, 767ssh-rsa-cert-v01@openssh.com, 768ssh-ed25519, 769ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 770sk-ssh-ed25519@openssh.com, 771sk-ecdsa-sha2-nistp256@openssh.com, 772rsa-sha2-512,rsa-sha2-256,ssh-rsa 773.Ed 774.Pp 775The list of available signature algorithms may also be obtained using 776.Qq ssh -Q HostKeyAlgorithms . 777.It Cm IgnoreRhosts 778Specifies whether to ignore per-user 779.Pa .rhosts 780and 781.Pa .shosts 782files during 783.Cm HostbasedAuthentication . 784The system-wide 785.Pa /etc/hosts.equiv 786and 787.Pa /etc/shosts.equiv 788are still used regardless of this setting. 789.Pp 790Accepted values are 791.Cm yes 792(the default) to ignore all per-user files, 793.Cm shosts-only 794to allow the use of 795.Pa .shosts 796but to ignore 797.Pa .rhosts 798or 799.Cm no 800to allow both 801.Pa .shosts 802and 803.Pa rhosts . 804.It Cm IgnoreUserKnownHosts 805Specifies whether 806.Xr sshd 8 807should ignore the user's 808.Pa ~/.ssh/known_hosts 809during 810.Cm HostbasedAuthentication 811and use only the system-wide known hosts file 812.Pa /etc/ssh/known_hosts . 813The default is 814.Dq no . 815.It Cm Include 816Include the specified configuration file(s). 817Multiple pathnames may be specified and each pathname may contain 818.Xr glob 7 819wildcards that will be expanded and processed in lexical order. 820Files without absolute paths are assumed to be in 821.Pa /etc/ssh . 822An 823.Cm Include 824directive may appear inside a 825.Cm Match 826block 827to perform conditional inclusion. 828.It Cm IPQoS 829Specifies the IPv4 type-of-service or DSCP class for the connection. 830Accepted values are 831.Cm af11 , 832.Cm af12 , 833.Cm af13 , 834.Cm af21 , 835.Cm af22 , 836.Cm af23 , 837.Cm af31 , 838.Cm af32 , 839.Cm af33 , 840.Cm af41 , 841.Cm af42 , 842.Cm af43 , 843.Cm cs0 , 844.Cm cs1 , 845.Cm cs2 , 846.Cm cs3 , 847.Cm cs4 , 848.Cm cs5 , 849.Cm cs6 , 850.Cm cs7 , 851.Cm ef , 852.Cm le , 853.Cm lowdelay , 854.Cm throughput , 855.Cm reliability , 856a numeric value, or 857.Cm none 858to use the operating system default. 859This option may take one or two arguments, separated by whitespace. 860If one argument is specified, it is used as the packet class unconditionally. 861If two values are specified, the first is automatically selected for 862interactive sessions and the second for non-interactive sessions. 863The default is 864.Cm af21 865(Low-Latency Data) 866for interactive sessions and 867.Cm cs1 868(Lower Effort) 869for non-interactive sessions. 870.It Cm KbdInteractiveAuthentication 871Specifies whether to allow keyboard-interactive authentication. 872All authentication styles from 873.Xr login.conf 5 874are supported. 875The default is 876.Cm yes . 877The argument to this keyword must be 878.Cm yes 879or 880.Cm no . 881.Cm ChallengeResponseAuthentication 882is a deprecated alias for this. 883.It Cm KerberosAuthentication 884Specifies whether the password provided by the user for 885.Cm PasswordAuthentication 886will be validated through the Kerberos KDC. 887To use this option, the server needs a 888Kerberos servtab which allows the verification of the KDC's identity. 889The default is 890.Cm no . 891.It Cm KerberosGetAFSToken 892If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 893an AFS token before accessing the user's home directory. 894The default is 895.Cm no . 896.It Cm KerberosOrLocalPasswd 897If password authentication through Kerberos fails then 898the password will be validated via any additional local mechanism 899such as 900.Pa /etc/passwd . 901The default is 902.Cm yes . 903.It Cm KerberosTicketCleanup 904Specifies whether to automatically destroy the user's ticket cache 905file on logout. 906The default is 907.Cm yes . 908.It Cm KexAlgorithms 909Specifies the available KEX (Key Exchange) algorithms. 910Multiple algorithms must be comma-separated. 911Alternately if the specified list begins with a 912.Sq + 913character, then the specified methods will be appended to the default set 914instead of replacing them. 915If the specified list begins with a 916.Sq - 917character, then the specified methods (including wildcards) will be removed 918from the default set instead of replacing them. 919If the specified list begins with a 920.Sq ^ 921character, then the specified methods will be placed at the head of the 922default set. 923The supported algorithms are: 924.Pp 925.Bl -item -compact -offset indent 926.It 927curve25519-sha256 928.It 929curve25519-sha256@libssh.org 930.It 931diffie-hellman-group1-sha1 932.It 933diffie-hellman-group14-sha1 934.It 935diffie-hellman-group14-sha256 936.It 937diffie-hellman-group16-sha512 938.It 939diffie-hellman-group18-sha512 940.It 941diffie-hellman-group-exchange-sha1 942.It 943diffie-hellman-group-exchange-sha256 944.It 945ecdh-sha2-nistp256 946.It 947ecdh-sha2-nistp384 948.It 949ecdh-sha2-nistp521 950.It 951sntrup761x25519-sha512@openssh.com 952.El 953.Pp 954The default is: 955.Bd -literal -offset indent 956curve25519-sha256,curve25519-sha256@libssh.org, 957ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 958diffie-hellman-group-exchange-sha256, 959diffie-hellman-group16-sha512,diffie-hellman-group18-sha512, 960diffie-hellman-group14-sha256 961.Ed 962.Pp 963The list of available key exchange algorithms may also be obtained using 964.Qq ssh -Q KexAlgorithms . 965.It Cm ListenAddress 966Specifies the local addresses 967.Xr sshd 8 968should listen on. 969The following forms may be used: 970.Pp 971.Bl -item -offset indent -compact 972.It 973.Cm ListenAddress 974.Sm off 975.Ar hostname | address 976.Sm on 977.Op Cm rdomain Ar domain 978.It 979.Cm ListenAddress 980.Sm off 981.Ar hostname : port 982.Sm on 983.Op Cm rdomain Ar domain 984.It 985.Cm ListenAddress 986.Sm off 987.Ar IPv4_address : port 988.Sm on 989.Op Cm rdomain Ar domain 990.It 991.Cm ListenAddress 992.Sm off 993.Oo Ar hostname | address Oc : Ar port 994.Sm on 995.Op Cm rdomain Ar domain 996.El 997.Pp 998The optional 999.Cm rdomain 1000qualifier requests 1001.Xr sshd 8 1002listen in an explicit routing domain. 1003If 1004.Ar port 1005is not specified, 1006sshd will listen on the address and all 1007.Cm Port 1008options specified. 1009The default is to listen on all local addresses on the current default 1010routing domain. 1011Multiple 1012.Cm ListenAddress 1013options are permitted. 1014For more information on routing domains, see 1015.Xr rdomain 4 . 1016.It Cm LoginGraceTime 1017The server disconnects after this time if the user has not 1018successfully logged in. 1019If the value is 0, there is no time limit. 1020The default is 120 seconds. 1021.It Cm LogLevel 1022Gives the verbosity level that is used when logging messages from 1023.Xr sshd 8 . 1024The possible values are: 1025QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1026The default is INFO. 1027DEBUG and DEBUG1 are equivalent. 1028DEBUG2 and DEBUG3 each specify higher levels of debugging output. 1029Logging with a DEBUG level violates the privacy of users and is not recommended. 1030.It Cm LogVerbose 1031Specify one or more overrides to LogLevel. 1032An override consists of a pattern lists that matches the source file, function 1033and line number to force detailed logging for. 1034For example, an override pattern of: 1035.Bd -literal -offset indent 1036kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1037.Ed 1038.Pp 1039would enable detailed logging for line 1000 of 1040.Pa kex.c , 1041everything in the 1042.Fn kex_exchange_identification 1043function, and all code in the 1044.Pa packet.c 1045file. 1046This option is intended for debugging and no overrides are enabled by default. 1047.It Cm MACs 1048Specifies the available MAC (message authentication code) algorithms. 1049The MAC algorithm is used for data integrity protection. 1050Multiple algorithms must be comma-separated. 1051If the specified list begins with a 1052.Sq + 1053character, then the specified algorithms will be appended to the default set 1054instead of replacing them. 1055If the specified list begins with a 1056.Sq - 1057character, then the specified algorithms (including wildcards) will be removed 1058from the default set instead of replacing them. 1059If the specified list begins with a 1060.Sq ^ 1061character, then the specified algorithms will be placed at the head of the 1062default set. 1063.Pp 1064The algorithms that contain 1065.Qq -etm 1066calculate the MAC after encryption (encrypt-then-mac). 1067These are considered safer and their use recommended. 1068The supported MACs are: 1069.Pp 1070.Bl -item -compact -offset indent 1071.It 1072hmac-md5 1073.It 1074hmac-md5-96 1075.It 1076hmac-sha1 1077.It 1078hmac-sha1-96 1079.It 1080hmac-sha2-256 1081.It 1082hmac-sha2-512 1083.It 1084umac-64@openssh.com 1085.It 1086umac-128@openssh.com 1087.It 1088hmac-md5-etm@openssh.com 1089.It 1090hmac-md5-96-etm@openssh.com 1091.It 1092hmac-sha1-etm@openssh.com 1093.It 1094hmac-sha1-96-etm@openssh.com 1095.It 1096hmac-sha2-256-etm@openssh.com 1097.It 1098hmac-sha2-512-etm@openssh.com 1099.It 1100umac-64-etm@openssh.com 1101.It 1102umac-128-etm@openssh.com 1103.El 1104.Pp 1105The default is: 1106.Bd -literal -offset indent 1107umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1108hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1109hmac-sha1-etm@openssh.com, 1110umac-64@openssh.com,umac-128@openssh.com, 1111hmac-sha2-256,hmac-sha2-512,hmac-sha1 1112.Ed 1113.Pp 1114The list of available MAC algorithms may also be obtained using 1115.Qq ssh -Q mac . 1116.It Cm Match 1117Introduces a conditional block. 1118If all of the criteria on the 1119.Cm Match 1120line are satisfied, the keywords on the following lines override those 1121set in the global section of the config file, until either another 1122.Cm Match 1123line or the end of the file. 1124If a keyword appears in multiple 1125.Cm Match 1126blocks that are satisfied, only the first instance of the keyword is 1127applied. 1128.Pp 1129The arguments to 1130.Cm Match 1131are one or more criteria-pattern pairs or the single token 1132.Cm All 1133which matches all criteria. 1134The available criteria are 1135.Cm User , 1136.Cm Group , 1137.Cm Host , 1138.Cm LocalAddress , 1139.Cm LocalPort , 1140.Cm RDomain , 1141and 1142.Cm Address 1143(with 1144.Cm RDomain 1145representing the 1146.Xr rdomain 4 1147on which the connection was received). 1148.Pp 1149The match patterns may consist of single entries or comma-separated 1150lists and may use the wildcard and negation operators described in the 1151.Sx PATTERNS 1152section of 1153.Xr ssh_config 5 . 1154.Pp 1155The patterns in an 1156.Cm Address 1157criteria may additionally contain addresses to match in CIDR 1158address/masklen format, 1159such as 192.0.2.0/24 or 2001:db8::/32. 1160Note that the mask length provided must be consistent with the address - 1161it is an error to specify a mask length that is too long for the address 1162or one with bits set in this host portion of the address. 1163For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. 1164.Pp 1165Only a subset of keywords may be used on the lines following a 1166.Cm Match 1167keyword. 1168Available keywords are 1169.Cm AcceptEnv , 1170.Cm AllowAgentForwarding , 1171.Cm AllowGroups , 1172.Cm AllowStreamLocalForwarding , 1173.Cm AllowTcpForwarding , 1174.Cm AllowUsers , 1175.Cm AuthenticationMethods , 1176.Cm AuthorizedKeysCommand , 1177.Cm AuthorizedKeysCommandUser , 1178.Cm AuthorizedKeysFile , 1179.Cm AuthorizedPrincipalsCommand , 1180.Cm AuthorizedPrincipalsCommandUser , 1181.Cm AuthorizedPrincipalsFile , 1182.Cm Banner , 1183.Cm ChrootDirectory , 1184.Cm ClientAliveCountMax , 1185.Cm ClientAliveInterval , 1186.Cm DenyGroups , 1187.Cm DenyUsers , 1188.Cm DisableForwarding , 1189.Cm ForceCommand , 1190.Cm GatewayPorts , 1191.Cm GSSAPIAuthentication , 1192.Cm HostbasedAcceptedAlgorithms , 1193.Cm HostbasedAuthentication , 1194.Cm HostbasedUsesNameFromPacketOnly , 1195.Cm IgnoreRhosts , 1196.Cm Include , 1197.Cm IPQoS , 1198.Cm KbdInteractiveAuthentication , 1199.Cm KerberosAuthentication , 1200.Cm LogLevel , 1201.Cm MaxAuthTries , 1202.Cm MaxSessions , 1203.Cm PasswordAuthentication , 1204.Cm PermitEmptyPasswords , 1205.Cm PermitListen , 1206.Cm PermitOpen , 1207.Cm PermitRootLogin , 1208.Cm PermitTTY , 1209.Cm PermitTunnel , 1210.Cm PermitUserRC , 1211.Cm PubkeyAcceptedAlgorithms , 1212.Cm PubkeyAuthentication , 1213.Cm RekeyLimit , 1214.Cm RevokedKeys , 1215.Cm RDomain , 1216.Cm SetEnv , 1217.Cm StreamLocalBindMask , 1218.Cm StreamLocalBindUnlink , 1219.Cm TrustedUserCAKeys , 1220.Cm X11DisplayOffset , 1221.Cm X11Forwarding 1222and 1223.Cm X11UseLocalhost . 1224.It Cm MaxAuthTries 1225Specifies the maximum number of authentication attempts permitted per 1226connection. 1227Once the number of failures reaches half this value, 1228additional failures are logged. 1229The default is 6. 1230.It Cm MaxSessions 1231Specifies the maximum number of open shell, login or subsystem (e.g. sftp) 1232sessions permitted per network connection. 1233Multiple sessions may be established by clients that support connection 1234multiplexing. 1235Setting 1236.Cm MaxSessions 1237to 1 will effectively disable session multiplexing, whereas setting it to 0 1238will prevent all shell, login and subsystem sessions while still permitting 1239forwarding. 1240The default is 10. 1241.It Cm MaxStartups 1242Specifies the maximum number of concurrent unauthenticated connections to the 1243SSH daemon. 1244Additional connections will be dropped until authentication succeeds or the 1245.Cm LoginGraceTime 1246expires for a connection. 1247The default is 10:30:100. 1248.Pp 1249Alternatively, random early drop can be enabled by specifying 1250the three colon separated values 1251start:rate:full (e.g. "10:30:60"). 1252.Xr sshd 8 1253will refuse connection attempts with a probability of rate/100 (30%) 1254if there are currently start (10) unauthenticated connections. 1255The probability increases linearly and all connection attempts 1256are refused if the number of unauthenticated connections reaches full (60). 1257.It Cm ModuliFile 1258Specifies the 1259.Xr moduli 5 1260file that contains the Diffie-Hellman groups used for the 1261.Dq diffie-hellman-group-exchange-sha1 1262and 1263.Dq diffie-hellman-group-exchange-sha256 1264key exchange methods. 1265The default is 1266.Pa /etc/moduli . 1267.It Cm PasswordAuthentication 1268Specifies whether password authentication is allowed. 1269The default is 1270.Cm yes . 1271.It Cm PermitEmptyPasswords 1272When password authentication is allowed, it specifies whether the 1273server allows login to accounts with empty password strings. 1274The default is 1275.Cm no . 1276.It Cm PermitListen 1277Specifies the addresses/ports on which a remote TCP port forwarding may listen. 1278The listen specification must be one of the following forms: 1279.Pp 1280.Bl -item -offset indent -compact 1281.It 1282.Cm PermitListen 1283.Sm off 1284.Ar port 1285.Sm on 1286.It 1287.Cm PermitListen 1288.Sm off 1289.Ar host : port 1290.Sm on 1291.El 1292.Pp 1293Multiple permissions may be specified by separating them with whitespace. 1294An argument of 1295.Cm any 1296can be used to remove all restrictions and permit any listen requests. 1297An argument of 1298.Cm none 1299can be used to prohibit all listen requests. 1300The host name may contain wildcards as described in the PATTERNS section in 1301.Xr ssh_config 5 . 1302The wildcard 1303.Sq * 1304can also be used in place of a port number to allow all ports. 1305By default all port forwarding listen requests are permitted. 1306Note that the 1307.Cm GatewayPorts 1308option may further restrict which addresses may be listened on. 1309Note also that 1310.Xr ssh 1 1311will request a listen host of 1312.Dq localhost 1313if no listen host was specifically requested, and this name is 1314treated differently to explicit localhost addresses of 1315.Dq 127.0.0.1 1316and 1317.Dq ::1 . 1318.It Cm PermitOpen 1319Specifies the destinations to which TCP port forwarding is permitted. 1320The forwarding specification must be one of the following forms: 1321.Pp 1322.Bl -item -offset indent -compact 1323.It 1324.Cm PermitOpen 1325.Sm off 1326.Ar host : port 1327.Sm on 1328.It 1329.Cm PermitOpen 1330.Sm off 1331.Ar IPv4_addr : port 1332.Sm on 1333.It 1334.Cm PermitOpen 1335.Sm off 1336.Ar \&[ IPv6_addr \&] : port 1337.Sm on 1338.El 1339.Pp 1340Multiple forwards may be specified by separating them with whitespace. 1341An argument of 1342.Cm any 1343can be used to remove all restrictions and permit any forwarding requests. 1344An argument of 1345.Cm none 1346can be used to prohibit all forwarding requests. 1347The wildcard 1348.Sq * 1349can be used for host or port to allow all hosts or ports respectively. 1350Otherwise, no pattern matching or address lookups are performed on supplied 1351names. 1352By default all port forwarding requests are permitted. 1353.It Cm PermitRootLogin 1354Specifies whether root can log in using 1355.Xr ssh 1 . 1356The argument must be 1357.Cm yes , 1358.Cm prohibit-password , 1359.Cm forced-commands-only , 1360or 1361.Cm no . 1362The default is 1363.Cm prohibit-password . 1364.Pp 1365If this option is set to 1366.Cm prohibit-password 1367(or its deprecated alias, 1368.Cm without-password ) , 1369password and keyboard-interactive authentication are disabled for root. 1370.Pp 1371If this option is set to 1372.Cm forced-commands-only , 1373root login with public key authentication will be allowed, 1374but only if the 1375.Ar command 1376option has been specified 1377(which may be useful for taking remote backups even if root login is 1378normally not allowed). 1379All other authentication methods are disabled for root. 1380.Pp 1381If this option is set to 1382.Cm no , 1383root is not allowed to log in. 1384.It Cm PermitTTY 1385Specifies whether 1386.Xr pty 4 1387allocation is permitted. 1388The default is 1389.Cm yes . 1390.It Cm PermitTunnel 1391Specifies whether 1392.Xr tun 4 1393device forwarding is allowed. 1394The argument must be 1395.Cm yes , 1396.Cm point-to-point 1397(layer 3), 1398.Cm ethernet 1399(layer 2), or 1400.Cm no . 1401Specifying 1402.Cm yes 1403permits both 1404.Cm point-to-point 1405and 1406.Cm ethernet . 1407The default is 1408.Cm no . 1409.Pp 1410Independent of this setting, the permissions of the selected 1411.Xr tun 4 1412device must allow access to the user. 1413.It Cm PermitUserEnvironment 1414Specifies whether 1415.Pa ~/.ssh/environment 1416and 1417.Cm environment= 1418options in 1419.Pa ~/.ssh/authorized_keys 1420are processed by 1421.Xr sshd 8 . 1422Valid options are 1423.Cm yes , 1424.Cm no 1425or a pattern-list specifying which environment variable names to accept 1426(for example 1427.Qq LANG,LC_* ) . 1428The default is 1429.Cm no . 1430Enabling environment processing may enable users to bypass access 1431restrictions in some configurations using mechanisms such as 1432.Ev LD_PRELOAD . 1433.It Cm PermitUserRC 1434Specifies whether any 1435.Pa ~/.ssh/rc 1436file is executed. 1437The default is 1438.Cm yes . 1439.It Cm PerSourceMaxStartups 1440Specifies the number of unauthenticated connections allowed from a 1441given source address, or 1442.Dq none 1443if there is no limit. 1444This limit is applied in addition to 1445.Cm MaxStartups , 1446whichever is lower. 1447The default is 1448.Cm none . 1449.It Cm PerSourceNetBlockSize 1450Specifies the number of bits of source address that are grouped together 1451for the purposes of applying PerSourceMaxStartups limits. 1452Values for IPv4 and optionally IPv6 may be specified, separated by a colon. 1453The default is 1454.Cm 32:128 , 1455which means each address is considered individually. 1456.It Cm PidFile 1457Specifies the file that contains the process ID of the 1458SSH daemon, or 1459.Cm none 1460to not write one. 1461The default is 1462.Pa /var/run/sshd.pid . 1463.It Cm Port 1464Specifies the port number that 1465.Xr sshd 8 1466listens on. 1467The default is 22. 1468Multiple options of this type are permitted. 1469See also 1470.Cm ListenAddress . 1471.It Cm PrintLastLog 1472Specifies whether 1473.Xr sshd 8 1474should print the date and time of the last user login when a user logs 1475in interactively. 1476The default is 1477.Cm yes . 1478.It Cm PrintMotd 1479Specifies whether 1480.Xr sshd 8 1481should print 1482.Pa /etc/motd 1483when a user logs in interactively. 1484(On some systems it is also printed by the shell, 1485.Pa /etc/profile , 1486or equivalent.) 1487The default is 1488.Cm yes . 1489.It Cm PubkeyAcceptedAlgorithms 1490Specifies the signature algorithms that will be accepted for public key 1491authentication as a list of comma-separated patterns. 1492Alternately if the specified list begins with a 1493.Sq + 1494character, then the specified algorithms will be appended to the default set 1495instead of replacing them. 1496If the specified list begins with a 1497.Sq - 1498character, then the specified algorithms (including wildcards) will be removed 1499from the default set instead of replacing them. 1500If the specified list begins with a 1501.Sq ^ 1502character, then the specified algorithms will be placed at the head of the 1503default set. 1504The default for this option is: 1505.Bd -literal -offset 3n 1506ssh-ed25519-cert-v01@openssh.com, 1507ecdsa-sha2-nistp256-cert-v01@openssh.com, 1508ecdsa-sha2-nistp384-cert-v01@openssh.com, 1509ecdsa-sha2-nistp521-cert-v01@openssh.com, 1510sk-ssh-ed25519-cert-v01@openssh.com, 1511sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1512rsa-sha2-512-cert-v01@openssh.com, 1513rsa-sha2-256-cert-v01@openssh.com, 1514ssh-rsa-cert-v01@openssh.com, 1515ssh-ed25519, 1516ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1517sk-ssh-ed25519@openssh.com, 1518sk-ecdsa-sha2-nistp256@openssh.com, 1519rsa-sha2-512,rsa-sha2-256,ssh-rsa 1520.Ed 1521.Pp 1522The list of available signature algorithms may also be obtained using 1523.Qq ssh -Q PubkeyAcceptedAlgorithms . 1524.It Cm PubkeyAuthOptions 1525Sets one or more public key authentication options. 1526The supported keywords are: 1527.Cm none 1528(the default; indicating no additional options are enabled), 1529.Cm touch-required 1530and 1531.Cm verify-required . 1532.Pp 1533The 1534.Cm touch-required 1535option causes public key authentication using a FIDO authenticator algorithm 1536(i.e.\& 1537.Cm ecdsa-sk 1538or 1539.Cm ed25519-sk ) 1540to always require the signature to attest that a physically present user 1541explicitly confirmed the authentication (usually by touching the authenticator). 1542By default, 1543.Xr sshd 8 1544requires user presence unless overridden with an authorized_keys option. 1545The 1546.Cm touch-required 1547flag disables this override. 1548.Pp 1549The 1550.Cm verify-required 1551option requires a FIDO key signature attest that the user was verified, 1552e.g. via a PIN. 1553.Pp 1554Neither the 1555.Cm touch-required 1556or 1557.Cm verify-required 1558options have any effect for other, non-FIDO, public key types. 1559.It Cm PubkeyAuthentication 1560Specifies whether public key authentication is allowed. 1561The default is 1562.Cm yes . 1563.It Cm RekeyLimit 1564Specifies the maximum amount of data that may be transmitted before the 1565session key is renegotiated, optionally followed by a maximum amount of 1566time that may pass before the session key is renegotiated. 1567The first argument is specified in bytes and may have a suffix of 1568.Sq K , 1569.Sq M , 1570or 1571.Sq G 1572to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1573The default is between 1574.Sq 1G 1575and 1576.Sq 4G , 1577depending on the cipher. 1578The optional second value is specified in seconds and may use any of the 1579units documented in the 1580.Sx TIME FORMATS 1581section. 1582The default value for 1583.Cm RekeyLimit 1584is 1585.Cm default none , 1586which means that rekeying is performed after the cipher's default amount 1587of data has been sent or received and no time based rekeying is done. 1588.It Cm RevokedKeys 1589Specifies revoked public keys file, or 1590.Cm none 1591to not use one. 1592Keys listed in this file will be refused for public key authentication. 1593Note that if this file is not readable, then public key authentication will 1594be refused for all users. 1595Keys may be specified as a text file, listing one public key per line, or as 1596an OpenSSH Key Revocation List (KRL) as generated by 1597.Xr ssh-keygen 1 . 1598For more information on KRLs, see the KEY REVOCATION LISTS section in 1599.Xr ssh-keygen 1 . 1600.It Cm RDomain 1601Specifies an explicit routing domain that is applied after authentication 1602has completed. 1603The user session, as well and any forwarded or listening IP sockets, 1604will be bound to this 1605.Xr rdomain 4 . 1606If the routing domain is set to 1607.Cm \&%D , 1608then the domain in which the incoming connection was received will be applied. 1609.It Cm SecurityKeyProvider 1610Specifies a path to a library that will be used when loading 1611FIDO authenticator-hosted keys, overriding the default of using 1612the built-in USB HID support. 1613.It Cm SetEnv 1614Specifies one or more environment variables to set in child sessions started 1615by 1616.Xr sshd 8 1617as 1618.Dq NAME=VALUE . 1619The environment value may be quoted (e.g. if it contains whitespace 1620characters). 1621Environment variables set by 1622.Cm SetEnv 1623override the default environment and any variables specified by the user 1624via 1625.Cm AcceptEnv 1626or 1627.Cm PermitUserEnvironment . 1628.It Cm StreamLocalBindMask 1629Sets the octal file creation mode mask 1630.Pq umask 1631used when creating a Unix-domain socket file for local or remote 1632port forwarding. 1633This option is only used for port forwarding to a Unix-domain socket file. 1634.Pp 1635The default value is 0177, which creates a Unix-domain socket file that is 1636readable and writable only by the owner. 1637Note that not all operating systems honor the file mode on Unix-domain 1638socket files. 1639.It Cm StreamLocalBindUnlink 1640Specifies whether to remove an existing Unix-domain socket file for local 1641or remote port forwarding before creating a new one. 1642If the socket file already exists and 1643.Cm StreamLocalBindUnlink 1644is not enabled, 1645.Nm sshd 1646will be unable to forward the port to the Unix-domain socket file. 1647This option is only used for port forwarding to a Unix-domain socket file. 1648.Pp 1649The argument must be 1650.Cm yes 1651or 1652.Cm no . 1653The default is 1654.Cm no . 1655.It Cm StrictModes 1656Specifies whether 1657.Xr sshd 8 1658should check file modes and ownership of the 1659user's files and home directory before accepting login. 1660This is normally desirable because novices sometimes accidentally leave their 1661directory or files world-writable. 1662The default is 1663.Cm yes . 1664Note that this does not apply to 1665.Cm ChrootDirectory , 1666whose permissions and ownership are checked unconditionally. 1667.It Cm Subsystem 1668Configures an external subsystem (e.g. file transfer daemon). 1669Arguments should be a subsystem name and a command (with optional arguments) 1670to execute upon subsystem request. 1671.Pp 1672The command 1673.Cm sftp-server 1674implements the SFTP file transfer subsystem. 1675.Pp 1676Alternately the name 1677.Cm internal-sftp 1678implements an in-process SFTP server. 1679This may simplify configurations using 1680.Cm ChrootDirectory 1681to force a different filesystem root on clients. 1682.Pp 1683By default no subsystems are defined. 1684.It Cm SyslogFacility 1685Gives the facility code that is used when logging messages from 1686.Xr sshd 8 . 1687The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1688LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1689The default is AUTH. 1690.It Cm TCPKeepAlive 1691Specifies whether the system should send TCP keepalive messages to the 1692other side. 1693If they are sent, death of the connection or crash of one 1694of the machines will be properly noticed. 1695However, this means that 1696connections will die if the route is down temporarily, and some people 1697find it annoying. 1698On the other hand, if TCP keepalives are not sent, 1699sessions may hang indefinitely on the server, leaving 1700.Qq ghost 1701users and consuming server resources. 1702.Pp 1703The default is 1704.Cm yes 1705(to send TCP keepalive messages), and the server will notice 1706if the network goes down or the client host crashes. 1707This avoids infinitely hanging sessions. 1708.Pp 1709To disable TCP keepalive messages, the value should be set to 1710.Cm no . 1711.It Cm TrustedUserCAKeys 1712Specifies a file containing public keys of certificate authorities that are 1713trusted to sign user certificates for authentication, or 1714.Cm none 1715to not use one. 1716Keys are listed one per line; empty lines and comments starting with 1717.Ql # 1718are allowed. 1719If a certificate is presented for authentication and has its signing CA key 1720listed in this file, then it may be used for authentication for any user 1721listed in the certificate's principals list. 1722Note that certificates that lack a list of principals will not be permitted 1723for authentication using 1724.Cm TrustedUserCAKeys . 1725For more details on certificates, see the CERTIFICATES section in 1726.Xr ssh-keygen 1 . 1727.It Cm UseDNS 1728Specifies whether 1729.Xr sshd 8 1730should look up the remote host name, and to check that 1731the resolved host name for the remote IP address maps back to the 1732very same IP address. 1733.Pp 1734If this option is set to 1735.Cm no 1736(the default) then only addresses and not host names may be used in 1737.Pa ~/.ssh/authorized_keys 1738.Cm from 1739and 1740.Nm 1741.Cm Match 1742.Cm Host 1743directives. 1744.It Cm VersionAddendum 1745Optionally specifies additional text to append to the SSH protocol banner 1746sent by the server upon connection. 1747The default is 1748.Cm none . 1749.It Cm X11DisplayOffset 1750Specifies the first display number available for 1751.Xr sshd 8 Ns 's 1752X11 forwarding. 1753This prevents sshd from interfering with real X11 servers. 1754The default is 10. 1755.It Cm X11Forwarding 1756Specifies whether X11 forwarding is permitted. 1757The argument must be 1758.Cm yes 1759or 1760.Cm no . 1761The default is 1762.Cm no . 1763.Pp 1764When X11 forwarding is enabled, there may be additional exposure to 1765the server and to client displays if the 1766.Xr sshd 8 1767proxy display is configured to listen on the wildcard address (see 1768.Cm X11UseLocalhost ) , 1769though this is not the default. 1770Additionally, the authentication spoofing and authentication data 1771verification and substitution occur on the client side. 1772The security risk of using X11 forwarding is that the client's X11 1773display server may be exposed to attack when the SSH client requests 1774forwarding (see the warnings for 1775.Cm ForwardX11 1776in 1777.Xr ssh_config 5 ) . 1778A system administrator may have a stance in which they want to 1779protect clients that may expose themselves to attack by unwittingly 1780requesting X11 forwarding, which can warrant a 1781.Cm no 1782setting. 1783.Pp 1784Note that disabling X11 forwarding does not prevent users from 1785forwarding X11 traffic, as users can always install their own forwarders. 1786.It Cm X11UseLocalhost 1787Specifies whether 1788.Xr sshd 8 1789should bind the X11 forwarding server to the loopback address or to 1790the wildcard address. 1791By default, 1792sshd binds the forwarding server to the loopback address and sets the 1793hostname part of the 1794.Ev DISPLAY 1795environment variable to 1796.Cm localhost . 1797This prevents remote hosts from connecting to the proxy display. 1798However, some older X11 clients may not function with this 1799configuration. 1800.Cm X11UseLocalhost 1801may be set to 1802.Cm no 1803to specify that the forwarding server should be bound to the wildcard 1804address. 1805The argument must be 1806.Cm yes 1807or 1808.Cm no . 1809The default is 1810.Cm yes . 1811.It Cm XAuthLocation 1812Specifies the full pathname of the 1813.Xr xauth 1 1814program, or 1815.Cm none 1816to not use one. 1817The default is 1818.Pa /usr/X11R6/bin/xauth . 1819.El 1820.Sh TIME FORMATS 1821.Xr sshd 8 1822command-line arguments and configuration file options that specify time 1823may be expressed using a sequence of the form: 1824.Sm off 1825.Ar time Op Ar qualifier , 1826.Sm on 1827where 1828.Ar time 1829is a positive integer value and 1830.Ar qualifier 1831is one of the following: 1832.Pp 1833.Bl -tag -width Ds -compact -offset indent 1834.It Aq Cm none 1835seconds 1836.It Cm s | Cm S 1837seconds 1838.It Cm m | Cm M 1839minutes 1840.It Cm h | Cm H 1841hours 1842.It Cm d | Cm D 1843days 1844.It Cm w | Cm W 1845weeks 1846.El 1847.Pp 1848Each member of the sequence is added together to calculate 1849the total time value. 1850.Pp 1851Time format examples: 1852.Pp 1853.Bl -tag -width Ds -compact -offset indent 1854.It 600 1855600 seconds (10 minutes) 1856.It 10m 185710 minutes 1858.It 1h30m 18591 hour 30 minutes (90 minutes) 1860.El 1861.Sh TOKENS 1862Arguments to some keywords can make use of tokens, 1863which are expanded at runtime: 1864.Pp 1865.Bl -tag -width XXXX -offset indent -compact 1866.It %% 1867A literal 1868.Sq % . 1869.It \&%D 1870The routing domain in which the incoming connection was received. 1871.It %F 1872The fingerprint of the CA key. 1873.It %f 1874The fingerprint of the key or certificate. 1875.It %h 1876The home directory of the user. 1877.It %i 1878The key ID in the certificate. 1879.It %K 1880The base64-encoded CA key. 1881.It %k 1882The base64-encoded key or certificate for authentication. 1883.It %s 1884The serial number of the certificate. 1885.It \&%T 1886The type of the CA key. 1887.It %t 1888The key or certificate type. 1889.It \&%U 1890The numeric user ID of the target user. 1891.It %u 1892The username. 1893.El 1894.Pp 1895.Cm AuthorizedKeysCommand 1896accepts the tokens %%, %f, %h, %k, %t, %U, and %u. 1897.Pp 1898.Cm AuthorizedKeysFile 1899accepts the tokens %%, %h, %U, and %u. 1900.Pp 1901.Cm AuthorizedPrincipalsCommand 1902accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u. 1903.Pp 1904.Cm AuthorizedPrincipalsFile 1905accepts the tokens %%, %h, %U, and %u. 1906.Pp 1907.Cm ChrootDirectory 1908accepts the tokens %%, %h, %U, and %u. 1909.Pp 1910.Cm RoutingDomain 1911accepts the token %D. 1912.Sh FILES 1913.Bl -tag -width Ds 1914.It Pa /etc/ssh/sshd_config 1915Contains configuration data for 1916.Xr sshd 8 . 1917This file should be writable by root only, but it is recommended 1918(though not necessary) that it be world-readable. 1919.El 1920.Sh SEE ALSO 1921.Xr sftp-server 8 , 1922.Xr sshd 8 1923.Sh AUTHORS 1924.An -nosplit 1925OpenSSH is a derivative of the original and free 1926ssh 1.2.12 release by 1927.An Tatu Ylonen . 1928.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , 1929.An Theo de Raadt 1930and 1931.An Dug Song 1932removed many bugs, re-added newer features and 1933created OpenSSH. 1934.An Markus Friedl 1935contributed the support for SSH protocol versions 1.5 and 2.0. 1936.An Niels Provos 1937and 1938.An Markus Friedl 1939contributed support for privilege separation. 1940