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 1.312 2020/05/29 05:37:03 djm Exp $ 37.Dd $Mdocdate: May 29 2020 $ 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 381ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 382ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 383.Ed 384.Pp 385Certificates signed using other algorithms will not be accepted for 386public key or host-based authentication. 387.It Cm ChallengeResponseAuthentication 388Specifies whether challenge-response authentication is allowed. 389All authentication styles from 390.Xr login.conf 5 391are supported. 392The default is 393.Cm yes . 394.It Cm ChrootDirectory 395Specifies the pathname of a directory to 396.Xr chroot 2 397to after authentication. 398At session startup 399.Xr sshd 8 400checks that all components of the pathname are root-owned directories 401which are not writable by any other user or group. 402After the chroot, 403.Xr sshd 8 404changes the working directory to the user's home directory. 405Arguments to 406.Cm ChrootDirectory 407accept the tokens described in the 408.Sx TOKENS 409section. 410.Pp 411The 412.Cm ChrootDirectory 413must contain the necessary files and directories to support the 414user's session. 415For an interactive session this requires at least a shell, typically 416.Xr sh 1 , 417and basic 418.Pa /dev 419nodes such as 420.Xr null 4 , 421.Xr zero 4 , 422.Xr stdin 4 , 423.Xr stdout 4 , 424.Xr stderr 4 , 425and 426.Xr tty 4 427devices. 428For file transfer sessions using SFTP 429no additional configuration of the environment is necessary if the in-process 430sftp-server is used, 431though sessions which use logging may require 432.Pa /dev/log 433inside the chroot directory on some operating systems (see 434.Xr sftp-server 8 435for details). 436.Pp 437For safety, it is very important that the directory hierarchy be 438prevented from modification by other processes on the system (especially 439those outside the jail). 440Misconfiguration can lead to unsafe environments which 441.Xr sshd 8 442cannot detect. 443.Pp 444The default is 445.Cm none , 446indicating not to 447.Xr chroot 2 . 448.It Cm Ciphers 449Specifies the ciphers allowed. 450Multiple ciphers must be comma-separated. 451If the specified list begins with a 452.Sq + 453character, then the specified ciphers will be appended to the default set 454instead of replacing them. 455If the specified list begins with a 456.Sq - 457character, then the specified ciphers (including wildcards) will be removed 458from the default set instead of replacing them. 459If the specified list begins with a 460.Sq ^ 461character, then the specified ciphers will be placed at the head of the 462default set. 463.Pp 464The supported ciphers are: 465.Pp 466.Bl -item -compact -offset indent 467.It 4683des-cbc 469.It 470aes128-cbc 471.It 472aes192-cbc 473.It 474aes256-cbc 475.It 476aes128-ctr 477.It 478aes192-ctr 479.It 480aes256-ctr 481.It 482aes128-gcm@openssh.com 483.It 484aes256-gcm@openssh.com 485.It 486chacha20-poly1305@openssh.com 487.El 488.Pp 489The default is: 490.Bd -literal -offset indent 491chacha20-poly1305@openssh.com, 492aes128-ctr,aes192-ctr,aes256-ctr, 493aes128-gcm@openssh.com,aes256-gcm@openssh.com 494.Ed 495.Pp 496The list of available ciphers may also be obtained using 497.Qq ssh -Q cipher . 498.It Cm ClientAliveCountMax 499Sets the number of client alive messages which may be sent without 500.Xr sshd 8 501receiving any messages back from the client. 502If this threshold is reached while client alive messages are being sent, 503sshd will disconnect the client, terminating the session. 504It is important to note that the use of client alive messages is very 505different from 506.Cm TCPKeepAlive . 507The client alive messages are sent through the encrypted channel 508and therefore will not be spoofable. 509The TCP keepalive option enabled by 510.Cm TCPKeepAlive 511is spoofable. 512The client alive mechanism is valuable when the client or 513server depend on knowing when a connection has become unresponsive. 514.Pp 515The default value is 3. 516If 517.Cm ClientAliveInterval 518is set to 15, and 519.Cm ClientAliveCountMax 520is left at the default, unresponsive SSH clients 521will be disconnected after approximately 45 seconds. 522Setting a zero 523.Cm ClientAliveCountMax 524disables connection termination. 525.It Cm ClientAliveInterval 526Sets a timeout interval in seconds after which if no data has been received 527from the client, 528.Xr sshd 8 529will send a message through the encrypted 530channel to request a response from the client. 531The default 532is 0, indicating that these messages will not be sent to the client. 533.It Cm Compression 534Specifies whether compression is enabled after 535the user has authenticated successfully. 536The argument must be 537.Cm yes , 538.Cm delayed 539(a legacy synonym for 540.Cm yes ) 541or 542.Cm no . 543The default is 544.Cm yes . 545.It Cm DenyGroups 546This keyword can be followed by a list of group name patterns, separated 547by spaces. 548Login is disallowed for users whose primary group or supplementary 549group list matches one of the patterns. 550Only group names are valid; a numerical group ID is not recognized. 551By default, login is allowed for all groups. 552The allow/deny groups directives are processed in the following order: 553.Cm DenyGroups , 554.Cm AllowGroups . 555.Pp 556See PATTERNS in 557.Xr ssh_config 5 558for more information on patterns. 559.It Cm DenyUsers 560This keyword can be followed by a list of user name patterns, separated 561by spaces. 562Login is disallowed for user names that match one of the patterns. 563Only user names are valid; a numerical user ID is not recognized. 564By default, login is allowed for all users. 565If the pattern takes the form USER@HOST then USER and HOST 566are separately checked, restricting logins to particular 567users from particular hosts. 568HOST criteria may additionally contain addresses to match in CIDR 569address/masklen format. 570The allow/deny users directives are processed in the following order: 571.Cm DenyUsers , 572.Cm AllowUsers . 573.Pp 574See PATTERNS in 575.Xr ssh_config 5 576for more information on patterns. 577.It Cm DisableForwarding 578Disables all forwarding features, including X11, 579.Xr ssh-agent 1 , 580TCP and StreamLocal. 581This option overrides all other forwarding-related options and may 582simplify restricted configurations. 583.It Cm ExposeAuthInfo 584Writes a temporary file containing a list of authentication methods and 585public credentials (e.g. keys) used to authenticate the user. 586The location of the file is exposed to the user session through the 587.Ev SSH_USER_AUTH 588environment variable. 589The default is 590.Cm no . 591.It Cm FingerprintHash 592Specifies the hash algorithm used when logging key fingerprints. 593Valid options are: 594.Cm md5 595and 596.Cm sha256 . 597The default is 598.Cm sha256 . 599.It Cm ForceCommand 600Forces the execution of the command specified by 601.Cm ForceCommand , 602ignoring any command supplied by the client and 603.Pa ~/.ssh/rc 604if present. 605The command is invoked by using the user's login shell with the -c option. 606This applies to shell, command, or subsystem execution. 607It is most useful inside a 608.Cm Match 609block. 610The command originally supplied by the client is available in the 611.Ev SSH_ORIGINAL_COMMAND 612environment variable. 613Specifying a command of 614.Cm internal-sftp 615will force the use of an in-process SFTP server that requires no support 616files when used with 617.Cm ChrootDirectory . 618The default is 619.Cm none . 620.It Cm GatewayPorts 621Specifies whether remote hosts are allowed to connect to ports 622forwarded for the client. 623By default, 624.Xr sshd 8 625binds remote port forwardings to the loopback address. 626This prevents other remote hosts from connecting to forwarded ports. 627.Cm GatewayPorts 628can be used to specify that sshd 629should allow remote port forwardings to bind to non-loopback addresses, thus 630allowing other hosts to connect. 631The argument may be 632.Cm no 633to force remote port forwardings to be available to the local host only, 634.Cm yes 635to force remote port forwardings to bind to the wildcard address, or 636.Cm clientspecified 637to allow the client to select the address to which the forwarding is bound. 638The default is 639.Cm no . 640.It Cm GSSAPIAuthentication 641Specifies whether user authentication based on GSSAPI is allowed. 642The default is 643.Cm no . 644.It Cm GSSAPICleanupCredentials 645Specifies whether to automatically destroy the user's credentials cache 646on logout. 647The default is 648.Cm yes . 649.It Cm GSSAPIStrictAcceptorCheck 650Determines whether to be strict about the identity of the GSSAPI acceptor 651a client authenticates against. 652If set to 653.Cm yes 654then the client must authenticate against the host 655service on the current hostname. 656If set to 657.Cm no 658then the client may authenticate against any service key stored in the 659machine's default store. 660This facility is provided to assist with operation on multi homed machines. 661The default is 662.Cm yes . 663.It Cm HostbasedAcceptedKeyTypes 664Specifies the key types that will be accepted for hostbased authentication 665as a list of comma-separated patterns. 666Alternately if the specified list begins with a 667.Sq + 668character, then the specified key types will be appended to the default set 669instead of replacing them. 670If the specified list begins with a 671.Sq - 672character, then the specified key types (including wildcards) will be removed 673from the default set instead of replacing them. 674If the specified list begins with a 675.Sq ^ 676character, then the specified key types will be placed at the head of the 677default set. 678The default for this option is: 679.Bd -literal -offset 3n 680ecdsa-sha2-nistp256-cert-v01@openssh.com, 681ecdsa-sha2-nistp384-cert-v01@openssh.com, 682ecdsa-sha2-nistp521-cert-v01@openssh.com, 683sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 684ssh-ed25519-cert-v01@openssh.com, 685sk-ssh-ed25519-cert-v01@openssh.com, 686rsa-sha2-512-cert-v01@openssh.com, 687rsa-sha2-256-cert-v01@openssh.com, 688ssh-rsa-cert-v01@openssh.com, 689ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 690sk-ecdsa-sha2-nistp256@openssh.com, 691ssh-ed25519,sk-ssh-ed25519@openssh.com, 692rsa-sha2-512,rsa-sha2-256,ssh-rsa 693.Ed 694.Pp 695The list of available key types may also be obtained using 696.Qq ssh -Q HostbasedAcceptedKeyTypes . 697.It Cm HostbasedAuthentication 698Specifies whether rhosts or /etc/hosts.equiv authentication together 699with successful public key client host authentication is allowed 700(host-based authentication). 701The default is 702.Cm no . 703.It Cm HostbasedUsesNameFromPacketOnly 704Specifies whether or not the server will attempt to perform a reverse 705name lookup when matching the name in the 706.Pa ~/.shosts , 707.Pa ~/.rhosts , 708and 709.Pa /etc/hosts.equiv 710files during 711.Cm HostbasedAuthentication . 712A setting of 713.Cm yes 714means that 715.Xr sshd 8 716uses the name supplied by the client rather than 717attempting to resolve the name from the TCP connection itself. 718The default is 719.Cm no . 720.It Cm HostCertificate 721Specifies a file containing a public host certificate. 722The certificate's public key must match a private host key already specified 723by 724.Cm HostKey . 725The default behaviour of 726.Xr sshd 8 727is not to load any certificates. 728.It Cm HostKey 729Specifies a file containing a private host key 730used by SSH. 731The defaults are 732.Pa /etc/ssh/ssh_host_ecdsa_key , 733.Pa /etc/ssh/ssh_host_ed25519_key 734and 735.Pa /etc/ssh/ssh_host_rsa_key . 736.Pp 737Note that 738.Xr sshd 8 739will refuse to use a file if it is group/world-accessible 740and that the 741.Cm HostKeyAlgorithms 742option restricts which of the keys are actually used by 743.Xr sshd 8 . 744.Pp 745It is possible to have multiple host key files. 746It is also possible to specify public host key files instead. 747In this case operations on the private key will be delegated 748to an 749.Xr ssh-agent 1 . 750.It Cm HostKeyAgent 751Identifies the UNIX-domain socket used to communicate 752with an agent that has access to the private host keys. 753If the string 754.Qq SSH_AUTH_SOCK 755is specified, the location of the socket will be read from the 756.Ev SSH_AUTH_SOCK 757environment variable. 758.It Cm HostKeyAlgorithms 759Specifies the host key algorithms 760that the server offers. 761The default for this option is: 762.Bd -literal -offset 3n 763ecdsa-sha2-nistp256-cert-v01@openssh.com, 764ecdsa-sha2-nistp384-cert-v01@openssh.com, 765ecdsa-sha2-nistp521-cert-v01@openssh.com, 766sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 767ssh-ed25519-cert-v01@openssh.com, 768sk-ssh-ed25519-cert-v01@openssh.com, 769rsa-sha2-512-cert-v01@openssh.com, 770rsa-sha2-256-cert-v01@openssh.com, 771ssh-rsa-cert-v01@openssh.com, 772ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 773sk-ecdsa-sha2-nistp256@openssh.com, 774ssh-ed25519,sk-ssh-ed25519@openssh.com, 775rsa-sha2-512,rsa-sha2-256,ssh-rsa 776.Ed 777.Pp 778The list of available key types may also be obtained using 779.Qq ssh -Q HostKeyAlgorithms . 780.It Cm IgnoreRhosts 781Specifies whether to ignore per-user 782.Pa .rhosts 783and 784.Pa .shosts 785files during 786.Cm HostbasedAuthentication . 787The system-wide 788.Pa /etc/hosts.equiv 789and 790.Pa /etc/shosts.equiv 791are still used regardless of this setting. 792.Pp 793Accepted values are 794.Cm yes 795(the default) to ignore all per-user files, 796.Cm shosts-only 797to allow the use of 798.Pa .shosts 799but to ignore 800.Pa .rhosts 801or 802.Cm no 803to allow both 804.Pa .shosts 805and 806.Pa rhosts . 807.It Cm IgnoreUserKnownHosts 808Specifies whether 809.Xr sshd 8 810should ignore the user's 811.Pa ~/.ssh/known_hosts 812during 813.Cm HostbasedAuthentication 814and use only the system-wide known hosts file 815.Pa /etc/ssh/known_hosts . 816The default is 817.Dq no . 818.It Cm Include 819Include the specified configuration file(s). 820Multiple pathnames may be specified and each pathname may contain 821.Xr glob 7 822wildcards that will be expanded and processed in lexical order. 823Files without absolute paths are assumed to be in 824.Pa /etc/ssh . 825An 826.Cm Include 827directive may appear inside a 828.Cm Match 829block 830to perform conditional inclusion. 831.It Cm IPQoS 832Specifies the IPv4 type-of-service or DSCP class for the connection. 833Accepted values are 834.Cm af11 , 835.Cm af12 , 836.Cm af13 , 837.Cm af21 , 838.Cm af22 , 839.Cm af23 , 840.Cm af31 , 841.Cm af32 , 842.Cm af33 , 843.Cm af41 , 844.Cm af42 , 845.Cm af43 , 846.Cm cs0 , 847.Cm cs1 , 848.Cm cs2 , 849.Cm cs3 , 850.Cm cs4 , 851.Cm cs5 , 852.Cm cs6 , 853.Cm cs7 , 854.Cm ef , 855.Cm le , 856.Cm lowdelay , 857.Cm throughput , 858.Cm reliability , 859a numeric value, or 860.Cm none 861to use the operating system default. 862This option may take one or two arguments, separated by whitespace. 863If one argument is specified, it is used as the packet class unconditionally. 864If two values are specified, the first is automatically selected for 865interactive sessions and the second for non-interactive sessions. 866The default is 867.Cm af21 868(Low-Latency Data) 869for interactive sessions and 870.Cm cs1 871(Lower Effort) 872for non-interactive sessions. 873.It Cm KbdInteractiveAuthentication 874Specifies whether to allow keyboard-interactive authentication. 875The argument to this keyword must be 876.Cm yes 877or 878.Cm no . 879The default is to use whatever value 880.Cm ChallengeResponseAuthentication 881is set to 882(by default 883.Cm yes ) . 884.It Cm KerberosAuthentication 885Specifies whether the password provided by the user for 886.Cm PasswordAuthentication 887will be validated through the Kerberos KDC. 888To use this option, the server needs a 889Kerberos servtab which allows the verification of the KDC's identity. 890The default is 891.Cm no . 892.It Cm KerberosGetAFSToken 893If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 894an AFS token before accessing the user's home directory. 895The default is 896.Cm no . 897.It Cm KerberosOrLocalPasswd 898If password authentication through Kerberos fails then 899the password will be validated via any additional local mechanism 900such as 901.Pa /etc/passwd . 902The default is 903.Cm yes . 904.It Cm KerberosTicketCleanup 905Specifies whether to automatically destroy the user's ticket cache 906file on logout. 907The default is 908.Cm yes . 909.It Cm KexAlgorithms 910Specifies the available KEX (Key Exchange) algorithms. 911Multiple algorithms must be comma-separated. 912Alternately if the specified list begins with a 913.Sq + 914character, then the specified methods will be appended to the default set 915instead of replacing them. 916If the specified list begins with a 917.Sq - 918character, then the specified methods (including wildcards) will be removed 919from the default set instead of replacing them. 920If the specified list begins with a 921.Sq ^ 922character, then the specified methods will be placed at the head of the 923default set. 924The supported algorithms are: 925.Pp 926.Bl -item -compact -offset indent 927.It 928curve25519-sha256 929.It 930curve25519-sha256@libssh.org 931.It 932diffie-hellman-group1-sha1 933.It 934diffie-hellman-group14-sha1 935.It 936diffie-hellman-group14-sha256 937.It 938diffie-hellman-group16-sha512 939.It 940diffie-hellman-group18-sha512 941.It 942diffie-hellman-group-exchange-sha1 943.It 944diffie-hellman-group-exchange-sha256 945.It 946ecdh-sha2-nistp256 947.It 948ecdh-sha2-nistp384 949.It 950ecdh-sha2-nistp521 951.It 952sntrup4591761x25519-sha512@tinyssh.org 953.El 954.Pp 955The default is: 956.Bd -literal -offset indent 957curve25519-sha256,curve25519-sha256@libssh.org, 958ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 959diffie-hellman-group-exchange-sha256, 960diffie-hellman-group16-sha512,diffie-hellman-group18-sha512, 961diffie-hellman-group14-sha256 962.Ed 963.Pp 964The list of available key exchange algorithms may also be obtained using 965.Qq ssh -Q KexAlgorithms . 966.It Cm ListenAddress 967Specifies the local addresses 968.Xr sshd 8 969should listen on. 970The following forms may be used: 971.Pp 972.Bl -item -offset indent -compact 973.It 974.Cm ListenAddress 975.Sm off 976.Ar hostname | address 977.Sm on 978.Op Cm rdomain Ar domain 979.It 980.Cm ListenAddress 981.Sm off 982.Ar hostname : port 983.Sm on 984.Op Cm rdomain Ar domain 985.It 986.Cm ListenAddress 987.Sm off 988.Ar IPv4_address : port 989.Sm on 990.Op Cm rdomain Ar domain 991.It 992.Cm ListenAddress 993.Sm off 994.Oo Ar hostname | address Oc : Ar port 995.Sm on 996.Op Cm rdomain Ar domain 997.El 998.Pp 999The optional 1000.Cm rdomain 1001qualifier requests 1002.Xr sshd 8 1003listen in an explicit routing domain. 1004If 1005.Ar port 1006is not specified, 1007sshd will listen on the address and all 1008.Cm Port 1009options specified. 1010The default is to listen on all local addresses on the current default 1011routing domain. 1012Multiple 1013.Cm ListenAddress 1014options are permitted. 1015For more information on routing domains, see 1016.Xr rdomain 4 . 1017.It Cm LoginGraceTime 1018The server disconnects after this time if the user has not 1019successfully logged in. 1020If the value is 0, there is no time limit. 1021The default is 120 seconds. 1022.It Cm LogLevel 1023Gives the verbosity level that is used when logging messages from 1024.Xr sshd 8 . 1025The possible values are: 1026QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1027The default is INFO. 1028DEBUG and DEBUG1 are equivalent. 1029DEBUG2 and DEBUG3 each specify higher levels of debugging output. 1030Logging with a DEBUG level violates the privacy of users and is not recommended. 1031.It Cm MACs 1032Specifies the available MAC (message authentication code) algorithms. 1033The MAC algorithm is used for data integrity protection. 1034Multiple algorithms must be comma-separated. 1035If the specified list begins with a 1036.Sq + 1037character, then the specified algorithms will be appended to the default set 1038instead of replacing them. 1039If the specified list begins with a 1040.Sq - 1041character, then the specified algorithms (including wildcards) will be removed 1042from the default set instead of replacing them. 1043If the specified list begins with a 1044.Sq ^ 1045character, then the specified algorithms will be placed at the head of the 1046default set. 1047.Pp 1048The algorithms that contain 1049.Qq -etm 1050calculate the MAC after encryption (encrypt-then-mac). 1051These are considered safer and their use recommended. 1052The supported MACs are: 1053.Pp 1054.Bl -item -compact -offset indent 1055.It 1056hmac-md5 1057.It 1058hmac-md5-96 1059.It 1060hmac-sha1 1061.It 1062hmac-sha1-96 1063.It 1064hmac-sha2-256 1065.It 1066hmac-sha2-512 1067.It 1068umac-64@openssh.com 1069.It 1070umac-128@openssh.com 1071.It 1072hmac-md5-etm@openssh.com 1073.It 1074hmac-md5-96-etm@openssh.com 1075.It 1076hmac-sha1-etm@openssh.com 1077.It 1078hmac-sha1-96-etm@openssh.com 1079.It 1080hmac-sha2-256-etm@openssh.com 1081.It 1082hmac-sha2-512-etm@openssh.com 1083.It 1084umac-64-etm@openssh.com 1085.It 1086umac-128-etm@openssh.com 1087.El 1088.Pp 1089The default is: 1090.Bd -literal -offset indent 1091umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1092hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1093hmac-sha1-etm@openssh.com, 1094umac-64@openssh.com,umac-128@openssh.com, 1095hmac-sha2-256,hmac-sha2-512,hmac-sha1 1096.Ed 1097.Pp 1098The list of available MAC algorithms may also be obtained using 1099.Qq ssh -Q mac . 1100.It Cm Match 1101Introduces a conditional block. 1102If all of the criteria on the 1103.Cm Match 1104line are satisfied, the keywords on the following lines override those 1105set in the global section of the config file, until either another 1106.Cm Match 1107line or the end of the file. 1108If a keyword appears in multiple 1109.Cm Match 1110blocks that are satisfied, only the first instance of the keyword is 1111applied. 1112.Pp 1113The arguments to 1114.Cm Match 1115are one or more criteria-pattern pairs or the single token 1116.Cm All 1117which matches all criteria. 1118The available criteria are 1119.Cm User , 1120.Cm Group , 1121.Cm Host , 1122.Cm LocalAddress , 1123.Cm LocalPort , 1124.Cm RDomain , 1125and 1126.Cm Address 1127(with 1128.Cm RDomain 1129representing the 1130.Xr rdomain 4 1131on which the connection was received). 1132.Pp 1133The match patterns may consist of single entries or comma-separated 1134lists and may use the wildcard and negation operators described in the 1135.Sx PATTERNS 1136section of 1137.Xr ssh_config 5 . 1138.Pp 1139The patterns in an 1140.Cm Address 1141criteria may additionally contain addresses to match in CIDR 1142address/masklen format, 1143such as 192.0.2.0/24 or 2001:db8::/32. 1144Note that the mask length provided must be consistent with the address - 1145it is an error to specify a mask length that is too long for the address 1146or one with bits set in this host portion of the address. 1147For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. 1148.Pp 1149Only a subset of keywords may be used on the lines following a 1150.Cm Match 1151keyword. 1152Available keywords are 1153.Cm AcceptEnv , 1154.Cm AllowAgentForwarding , 1155.Cm AllowGroups , 1156.Cm AllowStreamLocalForwarding , 1157.Cm AllowTcpForwarding , 1158.Cm AllowUsers , 1159.Cm AuthenticationMethods , 1160.Cm AuthorizedKeysCommand , 1161.Cm AuthorizedKeysCommandUser , 1162.Cm AuthorizedKeysFile , 1163.Cm AuthorizedPrincipalsCommand , 1164.Cm AuthorizedPrincipalsCommandUser , 1165.Cm AuthorizedPrincipalsFile , 1166.Cm Banner , 1167.Cm ChrootDirectory , 1168.Cm ClientAliveCountMax , 1169.Cm ClientAliveInterval , 1170.Cm DenyGroups , 1171.Cm DenyUsers , 1172.Cm ForceCommand , 1173.Cm GatewayPorts , 1174.Cm GSSAPIAuthentication , 1175.Cm HostbasedAcceptedKeyTypes , 1176.Cm HostbasedAuthentication , 1177.Cm HostbasedUsesNameFromPacketOnly , 1178.Cm IgnoreRhosts , 1179.Cm Include , 1180.Cm IPQoS , 1181.Cm KbdInteractiveAuthentication , 1182.Cm KerberosAuthentication , 1183.Cm LogLevel , 1184.Cm MaxAuthTries , 1185.Cm MaxSessions , 1186.Cm PasswordAuthentication , 1187.Cm PermitEmptyPasswords , 1188.Cm PermitListen , 1189.Cm PermitOpen , 1190.Cm PermitRootLogin , 1191.Cm PermitTTY , 1192.Cm PermitTunnel , 1193.Cm PermitUserRC , 1194.Cm PubkeyAcceptedKeyTypes , 1195.Cm PubkeyAuthentication , 1196.Cm RekeyLimit , 1197.Cm RevokedKeys , 1198.Cm RDomain , 1199.Cm SetEnv , 1200.Cm StreamLocalBindMask , 1201.Cm StreamLocalBindUnlink , 1202.Cm TrustedUserCAKeys , 1203.Cm X11DisplayOffset , 1204.Cm X11Forwarding 1205and 1206.Cm X11UseLocalhost . 1207.It Cm MaxAuthTries 1208Specifies the maximum number of authentication attempts permitted per 1209connection. 1210Once the number of failures reaches half this value, 1211additional failures are logged. 1212The default is 6. 1213.It Cm MaxSessions 1214Specifies the maximum number of open shell, login or subsystem (e.g. sftp) 1215sessions permitted per network connection. 1216Multiple sessions may be established by clients that support connection 1217multiplexing. 1218Setting 1219.Cm MaxSessions 1220to 1 will effectively disable session multiplexing, whereas setting it to 0 1221will prevent all shell, login and subsystem sessions while still permitting 1222forwarding. 1223The default is 10. 1224.It Cm MaxStartups 1225Specifies the maximum number of concurrent unauthenticated connections to the 1226SSH daemon. 1227Additional connections will be dropped until authentication succeeds or the 1228.Cm LoginGraceTime 1229expires for a connection. 1230The default is 10:30:100. 1231.Pp 1232Alternatively, random early drop can be enabled by specifying 1233the three colon separated values 1234start:rate:full (e.g. "10:30:60"). 1235.Xr sshd 8 1236will refuse connection attempts with a probability of rate/100 (30%) 1237if there are currently start (10) unauthenticated connections. 1238The probability increases linearly and all connection attempts 1239are refused if the number of unauthenticated connections reaches full (60). 1240.It Cm PasswordAuthentication 1241Specifies whether password authentication is allowed. 1242The default is 1243.Cm yes . 1244.It Cm PermitEmptyPasswords 1245When password authentication is allowed, it specifies whether the 1246server allows login to accounts with empty password strings. 1247The default is 1248.Cm no . 1249.It Cm PermitListen 1250Specifies the addresses/ports on which a remote TCP port forwarding may listen. 1251The listen specification must be one of the following forms: 1252.Pp 1253.Bl -item -offset indent -compact 1254.It 1255.Cm PermitListen 1256.Sm off 1257.Ar port 1258.Sm on 1259.It 1260.Cm PermitListen 1261.Sm off 1262.Ar host : port 1263.Sm on 1264.El 1265.Pp 1266Multiple permissions may be specified by separating them with whitespace. 1267An argument of 1268.Cm any 1269can be used to remove all restrictions and permit any listen requests. 1270An argument of 1271.Cm none 1272can be used to prohibit all listen requests. 1273The host name may contain wildcards as described in the PATTERNS section in 1274.Xr ssh_config 5 . 1275The wildcard 1276.Sq * 1277can also be used in place of a port number to allow all ports. 1278By default all port forwarding listen requests are permitted. 1279Note that the 1280.Cm GatewayPorts 1281option may further restrict which addresses may be listened on. 1282Note also that 1283.Xr ssh 1 1284will request a listen host of 1285.Dq localhost 1286if no listen host was specifically requested, and this name is 1287treated differently to explicit localhost addresses of 1288.Dq 127.0.0.1 1289and 1290.Dq ::1 . 1291.It Cm PermitOpen 1292Specifies the destinations to which TCP port forwarding is permitted. 1293The forwarding specification must be one of the following forms: 1294.Pp 1295.Bl -item -offset indent -compact 1296.It 1297.Cm PermitOpen 1298.Sm off 1299.Ar host : port 1300.Sm on 1301.It 1302.Cm PermitOpen 1303.Sm off 1304.Ar IPv4_addr : port 1305.Sm on 1306.It 1307.Cm PermitOpen 1308.Sm off 1309.Ar \&[ IPv6_addr \&] : port 1310.Sm on 1311.El 1312.Pp 1313Multiple forwards may be specified by separating them with whitespace. 1314An argument of 1315.Cm any 1316can be used to remove all restrictions and permit any forwarding requests. 1317An argument of 1318.Cm none 1319can be used to prohibit all forwarding requests. 1320The wildcard 1321.Sq * 1322can be used for host or port to allow all hosts or ports respectively. 1323Otherwise, no pattern matching or address lookups are performed on supplied 1324names. 1325By default all port forwarding requests are permitted. 1326.It Cm PermitRootLogin 1327Specifies whether root can log in using 1328.Xr ssh 1 . 1329The argument must be 1330.Cm yes , 1331.Cm prohibit-password , 1332.Cm forced-commands-only , 1333or 1334.Cm no . 1335The default is 1336.Cm prohibit-password . 1337.Pp 1338If this option is set to 1339.Cm prohibit-password 1340(or its deprecated alias, 1341.Cm without-password ) , 1342password and keyboard-interactive authentication are disabled for root. 1343.Pp 1344If this option is set to 1345.Cm forced-commands-only , 1346root login with public key authentication will be allowed, 1347but only if the 1348.Ar command 1349option has been specified 1350(which may be useful for taking remote backups even if root login is 1351normally not allowed). 1352All other authentication methods are disabled for root. 1353.Pp 1354If this option is set to 1355.Cm no , 1356root is not allowed to log in. 1357.It Cm PermitTTY 1358Specifies whether 1359.Xr pty 4 1360allocation is permitted. 1361The default is 1362.Cm yes . 1363.It Cm PermitTunnel 1364Specifies whether 1365.Xr tun 4 1366device forwarding is allowed. 1367The argument must be 1368.Cm yes , 1369.Cm point-to-point 1370(layer 3), 1371.Cm ethernet 1372(layer 2), or 1373.Cm no . 1374Specifying 1375.Cm yes 1376permits both 1377.Cm point-to-point 1378and 1379.Cm ethernet . 1380The default is 1381.Cm no . 1382.Pp 1383Independent of this setting, the permissions of the selected 1384.Xr tun 4 1385device must allow access to the user. 1386.It Cm PermitUserEnvironment 1387Specifies whether 1388.Pa ~/.ssh/environment 1389and 1390.Cm environment= 1391options in 1392.Pa ~/.ssh/authorized_keys 1393are processed by 1394.Xr sshd 8 . 1395Valid options are 1396.Cm yes , 1397.Cm no 1398or a pattern-list specifying which environment variable names to accept 1399(for example 1400.Qq LANG,LC_* ) . 1401The default is 1402.Cm no . 1403Enabling environment processing may enable users to bypass access 1404restrictions in some configurations using mechanisms such as 1405.Ev LD_PRELOAD . 1406.It Cm PermitUserRC 1407Specifies whether any 1408.Pa ~/.ssh/rc 1409file is executed. 1410The default is 1411.Cm yes . 1412.It Cm PidFile 1413Specifies the file that contains the process ID of the 1414SSH daemon, or 1415.Cm none 1416to not write one. 1417The default is 1418.Pa /var/run/sshd.pid . 1419.It Cm Port 1420Specifies the port number that 1421.Xr sshd 8 1422listens on. 1423The default is 22. 1424Multiple options of this type are permitted. 1425See also 1426.Cm ListenAddress . 1427.It Cm PrintLastLog 1428Specifies whether 1429.Xr sshd 8 1430should print the date and time of the last user login when a user logs 1431in interactively. 1432The default is 1433.Cm yes . 1434.It Cm PrintMotd 1435Specifies whether 1436.Xr sshd 8 1437should print 1438.Pa /etc/motd 1439when a user logs in interactively. 1440(On some systems it is also printed by the shell, 1441.Pa /etc/profile , 1442or equivalent.) 1443The default is 1444.Cm yes . 1445.It Cm PubkeyAcceptedKeyTypes 1446Specifies the key types that will be accepted for public key authentication 1447as a list of comma-separated patterns. 1448Alternately if the specified list begins with a 1449.Sq + 1450character, then the specified key types will be appended to the default set 1451instead of replacing them. 1452If the specified list begins with a 1453.Sq - 1454character, then the specified key types (including wildcards) will be removed 1455from the default set instead of replacing them. 1456If the specified list begins with a 1457.Sq ^ 1458character, then the specified key types will be placed at the head of the 1459default set. 1460The default for this option is: 1461.Bd -literal -offset 3n 1462ecdsa-sha2-nistp256-cert-v01@openssh.com, 1463ecdsa-sha2-nistp384-cert-v01@openssh.com, 1464ecdsa-sha2-nistp521-cert-v01@openssh.com, 1465sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1466ssh-ed25519-cert-v01@openssh.com, 1467sk-ssh-ed25519-cert-v01@openssh.com, 1468rsa-sha2-512-cert-v01@openssh.com, 1469rsa-sha2-256-cert-v01@openssh.com, 1470ssh-rsa-cert-v01@openssh.com, 1471ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1472sk-ecdsa-sha2-nistp256@openssh.com, 1473ssh-ed25519,sk-ssh-ed25519@openssh.com, 1474rsa-sha2-512,rsa-sha2-256,ssh-rsa 1475.Ed 1476.Pp 1477The list of available key types may also be obtained using 1478.Qq ssh -Q PubkeyAcceptedKeyTypes . 1479.It Cm PubkeyAuthOptions 1480Sets one or more public key authentication options. 1481Two option keywords are currently supported: 1482.Cm none 1483(the default; indicating no additional options are enabled) 1484and 1485.Cm touch-required . 1486.Pp 1487The 1488.Cm touch-required 1489option causes public key authentication using a FIDO authenticator algorithm 1490(i.e.\& 1491.Cm ecdsa-sk 1492or 1493.Cm ed25519-sk ) 1494to always require the signature to attest that a physically present user 1495explicitly confirmed the authentication (usually by touching the authenticator). 1496By default, 1497.Xr sshd 8 1498requires user presence unless overridden with an authorized_keys option. 1499The 1500.Cm touch-required 1501flag disables this override. 1502This option has no effect for other, non-authenticator public key types. 1503.It Cm PubkeyAuthentication 1504Specifies whether public key authentication is allowed. 1505The default is 1506.Cm yes . 1507.It Cm RekeyLimit 1508Specifies the maximum amount of data that may be transmitted before the 1509session key is renegotiated, optionally followed a maximum amount of 1510time that may pass before the session key is renegotiated. 1511The first argument is specified in bytes and may have a suffix of 1512.Sq K , 1513.Sq M , 1514or 1515.Sq G 1516to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1517The default is between 1518.Sq 1G 1519and 1520.Sq 4G , 1521depending on the cipher. 1522The optional second value is specified in seconds and may use any of the 1523units documented in the 1524.Sx TIME FORMATS 1525section. 1526The default value for 1527.Cm RekeyLimit 1528is 1529.Cm default none , 1530which means that rekeying is performed after the cipher's default amount 1531of data has been sent or received and no time based rekeying is done. 1532.It Cm RevokedKeys 1533Specifies revoked public keys file, or 1534.Cm none 1535to not use one. 1536Keys listed in this file will be refused for public key authentication. 1537Note that if this file is not readable, then public key authentication will 1538be refused for all users. 1539Keys may be specified as a text file, listing one public key per line, or as 1540an OpenSSH Key Revocation List (KRL) as generated by 1541.Xr ssh-keygen 1 . 1542For more information on KRLs, see the KEY REVOCATION LISTS section in 1543.Xr ssh-keygen 1 . 1544.It Cm RDomain 1545Specifies an explicit routing domain that is applied after authentication 1546has completed. 1547The user session, as well and any forwarded or listening IP sockets, 1548will be bound to this 1549.Xr rdomain 4 . 1550If the routing domain is set to 1551.Cm \&%D , 1552then the domain in which the incoming connection was received will be applied. 1553.It Cm SecurityKeyProvider 1554Specifies a path to a library that will be used when loading 1555FIDO authenticator-hosted keys, overriding the default of using 1556the built-in USB HID support. 1557.It Cm SetEnv 1558Specifies one or more environment variables to set in child sessions started 1559by 1560.Xr sshd 8 1561as 1562.Dq NAME=VALUE . 1563The environment value may be quoted (e.g. if it contains whitespace 1564characters). 1565Environment variables set by 1566.Cm SetEnv 1567override the default environment and any variables specified by the user 1568via 1569.Cm AcceptEnv 1570or 1571.Cm PermitUserEnvironment . 1572.It Cm StreamLocalBindMask 1573Sets the octal file creation mode mask 1574.Pq umask 1575used when creating a Unix-domain socket file for local or remote 1576port forwarding. 1577This option is only used for port forwarding to a Unix-domain socket file. 1578.Pp 1579The default value is 0177, which creates a Unix-domain socket file that is 1580readable and writable only by the owner. 1581Note that not all operating systems honor the file mode on Unix-domain 1582socket files. 1583.It Cm StreamLocalBindUnlink 1584Specifies whether to remove an existing Unix-domain socket file for local 1585or remote port forwarding before creating a new one. 1586If the socket file already exists and 1587.Cm StreamLocalBindUnlink 1588is not enabled, 1589.Nm sshd 1590will be unable to forward the port to the Unix-domain socket file. 1591This option is only used for port forwarding to a Unix-domain socket file. 1592.Pp 1593The argument must be 1594.Cm yes 1595or 1596.Cm no . 1597The default is 1598.Cm no . 1599.It Cm StrictModes 1600Specifies whether 1601.Xr sshd 8 1602should check file modes and ownership of the 1603user's files and home directory before accepting login. 1604This is normally desirable because novices sometimes accidentally leave their 1605directory or files world-writable. 1606The default is 1607.Cm yes . 1608Note that this does not apply to 1609.Cm ChrootDirectory , 1610whose permissions and ownership are checked unconditionally. 1611.It Cm Subsystem 1612Configures an external subsystem (e.g. file transfer daemon). 1613Arguments should be a subsystem name and a command (with optional arguments) 1614to execute upon subsystem request. 1615.Pp 1616The command 1617.Cm sftp-server 1618implements the SFTP file transfer subsystem. 1619.Pp 1620Alternately the name 1621.Cm internal-sftp 1622implements an in-process SFTP server. 1623This may simplify configurations using 1624.Cm ChrootDirectory 1625to force a different filesystem root on clients. 1626.Pp 1627By default no subsystems are defined. 1628.It Cm SyslogFacility 1629Gives the facility code that is used when logging messages from 1630.Xr sshd 8 . 1631The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1632LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1633The default is AUTH. 1634.It Cm TCPKeepAlive 1635Specifies whether the system should send TCP keepalive messages to the 1636other side. 1637If they are sent, death of the connection or crash of one 1638of the machines will be properly noticed. 1639However, this means that 1640connections will die if the route is down temporarily, and some people 1641find it annoying. 1642On the other hand, if TCP keepalives are not sent, 1643sessions may hang indefinitely on the server, leaving 1644.Qq ghost 1645users and consuming server resources. 1646.Pp 1647The default is 1648.Cm yes 1649(to send TCP keepalive messages), and the server will notice 1650if the network goes down or the client host crashes. 1651This avoids infinitely hanging sessions. 1652.Pp 1653To disable TCP keepalive messages, the value should be set to 1654.Cm no . 1655.It Cm TrustedUserCAKeys 1656Specifies a file containing public keys of certificate authorities that are 1657trusted to sign user certificates for authentication, or 1658.Cm none 1659to not use one. 1660Keys are listed one per line; empty lines and comments starting with 1661.Ql # 1662are allowed. 1663If a certificate is presented for authentication and has its signing CA key 1664listed in this file, then it may be used for authentication for any user 1665listed in the certificate's principals list. 1666Note that certificates that lack a list of principals will not be permitted 1667for authentication using 1668.Cm TrustedUserCAKeys . 1669For more details on certificates, see the CERTIFICATES section in 1670.Xr ssh-keygen 1 . 1671.It Cm UseDNS 1672Specifies whether 1673.Xr sshd 8 1674should look up the remote host name, and to check that 1675the resolved host name for the remote IP address maps back to the 1676very same IP address. 1677.Pp 1678If this option is set to 1679.Cm no 1680(the default) then only addresses and not host names may be used in 1681.Pa ~/.ssh/authorized_keys 1682.Cm from 1683and 1684.Nm 1685.Cm Match 1686.Cm Host 1687directives. 1688.It Cm VersionAddendum 1689Optionally specifies additional text to append to the SSH protocol banner 1690sent by the server upon connection. 1691The default is 1692.Cm none . 1693.It Cm X11DisplayOffset 1694Specifies the first display number available for 1695.Xr sshd 8 Ns 's 1696X11 forwarding. 1697This prevents sshd from interfering with real X11 servers. 1698The default is 10. 1699.It Cm X11Forwarding 1700Specifies whether X11 forwarding is permitted. 1701The argument must be 1702.Cm yes 1703or 1704.Cm no . 1705The default is 1706.Cm no . 1707.Pp 1708When X11 forwarding is enabled, there may be additional exposure to 1709the server and to client displays if the 1710.Xr sshd 8 1711proxy display is configured to listen on the wildcard address (see 1712.Cm X11UseLocalhost ) , 1713though this is not the default. 1714Additionally, the authentication spoofing and authentication data 1715verification and substitution occur on the client side. 1716The security risk of using X11 forwarding is that the client's X11 1717display server may be exposed to attack when the SSH client requests 1718forwarding (see the warnings for 1719.Cm ForwardX11 1720in 1721.Xr ssh_config 5 ) . 1722A system administrator may have a stance in which they want to 1723protect clients that may expose themselves to attack by unwittingly 1724requesting X11 forwarding, which can warrant a 1725.Cm no 1726setting. 1727.Pp 1728Note that disabling X11 forwarding does not prevent users from 1729forwarding X11 traffic, as users can always install their own forwarders. 1730.It Cm X11UseLocalhost 1731Specifies whether 1732.Xr sshd 8 1733should bind the X11 forwarding server to the loopback address or to 1734the wildcard address. 1735By default, 1736sshd binds the forwarding server to the loopback address and sets the 1737hostname part of the 1738.Ev DISPLAY 1739environment variable to 1740.Cm localhost . 1741This prevents remote hosts from connecting to the proxy display. 1742However, some older X11 clients may not function with this 1743configuration. 1744.Cm X11UseLocalhost 1745may be set to 1746.Cm no 1747to specify that the forwarding server should be bound to the wildcard 1748address. 1749The argument must be 1750.Cm yes 1751or 1752.Cm no . 1753The default is 1754.Cm yes . 1755.It Cm XAuthLocation 1756Specifies the full pathname of the 1757.Xr xauth 1 1758program, or 1759.Cm none 1760to not use one. 1761The default is 1762.Pa /usr/X11R6/bin/xauth . 1763.El 1764.Sh TIME FORMATS 1765.Xr sshd 8 1766command-line arguments and configuration file options that specify time 1767may be expressed using a sequence of the form: 1768.Sm off 1769.Ar time Op Ar qualifier , 1770.Sm on 1771where 1772.Ar time 1773is a positive integer value and 1774.Ar qualifier 1775is one of the following: 1776.Pp 1777.Bl -tag -width Ds -compact -offset indent 1778.It Aq Cm none 1779seconds 1780.It Cm s | Cm S 1781seconds 1782.It Cm m | Cm M 1783minutes 1784.It Cm h | Cm H 1785hours 1786.It Cm d | Cm D 1787days 1788.It Cm w | Cm W 1789weeks 1790.El 1791.Pp 1792Each member of the sequence is added together to calculate 1793the total time value. 1794.Pp 1795Time format examples: 1796.Pp 1797.Bl -tag -width Ds -compact -offset indent 1798.It 600 1799600 seconds (10 minutes) 1800.It 10m 180110 minutes 1802.It 1h30m 18031 hour 30 minutes (90 minutes) 1804.El 1805.Sh TOKENS 1806Arguments to some keywords can make use of tokens, 1807which are expanded at runtime: 1808.Pp 1809.Bl -tag -width XXXX -offset indent -compact 1810.It %% 1811A literal 1812.Sq % . 1813.It \&%D 1814The routing domain in which the incoming connection was received. 1815.It %F 1816The fingerprint of the CA key. 1817.It %f 1818The fingerprint of the key or certificate. 1819.It %h 1820The home directory of the user. 1821.It %i 1822The key ID in the certificate. 1823.It %K 1824The base64-encoded CA key. 1825.It %k 1826The base64-encoded key or certificate for authentication. 1827.It %s 1828The serial number of the certificate. 1829.It \&%T 1830The type of the CA key. 1831.It %t 1832The key or certificate type. 1833.It \&%U 1834The numeric user ID of the target user. 1835.It %u 1836The username. 1837.El 1838.Pp 1839.Cm AuthorizedKeysCommand 1840accepts the tokens %%, %f, %h, %k, %t, %U, and %u. 1841.Pp 1842.Cm AuthorizedKeysFile 1843accepts the tokens %%, %h, %U, and %u. 1844.Pp 1845.Cm AuthorizedPrincipalsCommand 1846accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u. 1847.Pp 1848.Cm AuthorizedPrincipalsFile 1849accepts the tokens %%, %h, %U, and %u. 1850.Pp 1851.Cm ChrootDirectory 1852accepts the tokens %%, %h, %U, and %u. 1853.Pp 1854.Cm RoutingDomain 1855accepts the token %D. 1856.Sh FILES 1857.Bl -tag -width Ds 1858.It Pa /etc/ssh/sshd_config 1859Contains configuration data for 1860.Xr sshd 8 . 1861This file should be writable by root only, but it is recommended 1862(though not necessary) that it be world-readable. 1863.El 1864.Sh SEE ALSO 1865.Xr sftp-server 8 , 1866.Xr sshd 8 1867.Sh AUTHORS 1868.An -nosplit 1869OpenSSH is a derivative of the original and free 1870ssh 1.2.12 release by 1871.An Tatu Ylonen . 1872.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , 1873.An Theo de Raadt 1874and 1875.An Dug Song 1876removed many bugs, re-added newer features and 1877created OpenSSH. 1878.An Markus Friedl 1879contributed the support for SSH protocol versions 1.5 and 2.0. 1880.An Niels Provos 1881and 1882.An Markus Friedl 1883contributed support for privilege separation. 1884