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 1.335 2021/09/03 05:25:50 dtucker Exp $ 37.Dd $Mdocdate: September 3 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 203or 204.Cm pam . 205depending on the server configuration. 206For example, 207.Qq keyboard-interactive:bsdauth 208would restrict keyboard interactive authentication to the 209.Cm bsdauth 210device. 211.Pp 212If the publickey method is listed more than once, 213.Xr sshd 8 214verifies that keys that have been used successfully are not reused for 215subsequent authentications. 216For example, 217.Qq publickey,publickey 218requires successful authentication using two different public keys. 219.Pp 220Note that each authentication method listed should also be explicitly enabled 221in the configuration. 222.Pp 223The available authentication methods are: 224.Qq gssapi-with-mic , 225.Qq hostbased , 226.Qq keyboard-interactive , 227.Qq none 228(used for access to password-less accounts when 229.Cm PermitEmptyPasswords 230is enabled), 231.Qq password 232and 233.Qq publickey . 234.It Cm AuthorizedKeysCommand 235Specifies a program to be used to look up the user's public keys. 236The program must be owned by root, not writable by group or others and 237specified by an absolute path. 238Arguments to 239.Cm AuthorizedKeysCommand 240accept the tokens described in the 241.Sx TOKENS 242section. 243If no arguments are specified then the username of the target user is used. 244.Pp 245The program should produce on standard output zero or 246more lines of authorized_keys output (see 247.Sx AUTHORIZED_KEYS 248in 249.Xr sshd 8 ) . 250.Cm AuthorizedKeysCommand 251is tried after the usual 252.Cm AuthorizedKeysFile 253files and will not be executed if a matching key is found there. 254By default, no 255.Cm AuthorizedKeysCommand 256is run. 257.It Cm AuthorizedKeysCommandUser 258Specifies the user under whose account the 259.Cm AuthorizedKeysCommand 260is run. 261It is recommended to use a dedicated user that has no other role on the host 262than running authorized keys commands. 263If 264.Cm AuthorizedKeysCommand 265is specified but 266.Cm AuthorizedKeysCommandUser 267is not, then 268.Xr sshd 8 269will refuse to start. 270.It Cm AuthorizedKeysFile 271Specifies the file that contains the public keys used for user authentication. 272The format is described in the AUTHORIZED_KEYS FILE FORMAT section of 273.Xr sshd 8 . 274Arguments to 275.Cm AuthorizedKeysFile 276accept the tokens described in the 277.Sx TOKENS 278section. 279After expansion, 280.Cm AuthorizedKeysFile 281is taken to be an absolute path or one relative to the user's home 282directory. 283Multiple files may be listed, separated by whitespace. 284Alternately this option may be set to 285.Cm none 286to skip checking for user keys in files. 287The default is 288.Qq .ssh/authorized_keys .ssh/authorized_keys2 . 289.It Cm AuthorizedPrincipalsCommand 290Specifies a program to be used to generate the list of allowed 291certificate principals as per 292.Cm AuthorizedPrincipalsFile . 293The program must be owned by root, not writable by group or others and 294specified by an absolute path. 295Arguments to 296.Cm AuthorizedPrincipalsCommand 297accept the tokens described in the 298.Sx TOKENS 299section. 300If no arguments are specified then the username of the target user is used. 301.Pp 302The program should produce on standard output zero or 303more lines of 304.Cm AuthorizedPrincipalsFile 305output. 306If either 307.Cm AuthorizedPrincipalsCommand 308or 309.Cm AuthorizedPrincipalsFile 310is specified, then certificates offered by the client for authentication 311must contain a principal that is listed. 312By default, no 313.Cm AuthorizedPrincipalsCommand 314is run. 315.It Cm AuthorizedPrincipalsCommandUser 316Specifies the user under whose account the 317.Cm AuthorizedPrincipalsCommand 318is run. 319It is recommended to use a dedicated user that has no other role on the host 320than running authorized principals commands. 321If 322.Cm AuthorizedPrincipalsCommand 323is specified but 324.Cm AuthorizedPrincipalsCommandUser 325is not, then 326.Xr sshd 8 327will refuse to start. 328.It Cm AuthorizedPrincipalsFile 329Specifies a file that lists principal names that are accepted for 330certificate authentication. 331When using certificates signed by a key listed in 332.Cm TrustedUserCAKeys , 333this file lists names, one of which must appear in the certificate for it 334to be accepted for authentication. 335Names are listed one per line preceded by key options (as described in 336.Sx AUTHORIZED_KEYS FILE FORMAT 337in 338.Xr sshd 8 ) . 339Empty lines and comments starting with 340.Ql # 341are ignored. 342.Pp 343Arguments to 344.Cm AuthorizedPrincipalsFile 345accept the tokens described in the 346.Sx TOKENS 347section. 348After expansion, 349.Cm AuthorizedPrincipalsFile 350is taken to be an absolute path or one relative to the user's home directory. 351The default is 352.Cm none , 353i.e. not to use a principals file \(en in this case, the username 354of the user must appear in a certificate's principals list for it to be 355accepted. 356.Pp 357Note that 358.Cm AuthorizedPrincipalsFile 359is only used when authentication proceeds using a CA listed in 360.Cm TrustedUserCAKeys 361and is not consulted for certification authorities trusted via 362.Pa ~/.ssh/authorized_keys , 363though the 364.Cm principals= 365key option offers a similar facility (see 366.Xr sshd 8 367for details). 368.It Cm Banner 369The contents of the specified file are sent to the remote user before 370authentication is allowed. 371If the argument is 372.Cm none 373then no banner is displayed. 374By default, no banner is displayed. 375.It Cm CASignatureAlgorithms 376Specifies which algorithms are allowed for signing of certificates 377by certificate authorities (CAs). 378The default is: 379.Bd -literal -offset indent 380ssh-ed25519,ecdsa-sha2-nistp256, 381ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 382sk-ssh-ed25519@openssh.com, 383sk-ecdsa-sha2-nistp256@openssh.com, 384rsa-sha2-512,rsa-sha2-256 385.Ed 386.Pp 387If the specified list begins with a 388.Sq + 389character, then the specified algorithms will be appended to the default set 390instead of replacing them. 391If the specified list begins with a 392.Sq - 393character, then the specified algorithms (including wildcards) will be removed 394from the default set instead of replacing them. 395.Pp 396Certificates signed using other algorithms will not be accepted for 397public key or host-based authentication. 398.It Cm ChrootDirectory 399Specifies the pathname of a directory to 400.Xr chroot 2 401to after authentication. 402At session startup 403.Xr sshd 8 404checks that all components of the pathname are root-owned directories 405which are not writable by any other user or group. 406After the chroot, 407.Xr sshd 8 408changes the working directory to the user's home directory. 409Arguments to 410.Cm ChrootDirectory 411accept the tokens described in the 412.Sx TOKENS 413section. 414.Pp 415The 416.Cm ChrootDirectory 417must contain the necessary files and directories to support the 418user's session. 419For an interactive session this requires at least a shell, typically 420.Xr sh 1 , 421and basic 422.Pa /dev 423nodes such as 424.Xr null 4 , 425.Xr zero 4 , 426.Xr stdin 4 , 427.Xr stdout 4 , 428.Xr stderr 4 , 429and 430.Xr tty 4 431devices. 432For file transfer sessions using SFTP 433no additional configuration of the environment is necessary if the in-process 434sftp-server is used, 435though sessions which use logging may require 436.Pa /dev/log 437inside the chroot directory on some operating systems (see 438.Xr sftp-server 8 439for details). 440.Pp 441For safety, it is very important that the directory hierarchy be 442prevented from modification by other processes on the system (especially 443those outside the jail). 444Misconfiguration can lead to unsafe environments which 445.Xr sshd 8 446cannot detect. 447.Pp 448The default is 449.Cm none , 450indicating not to 451.Xr chroot 2 . 452.It Cm Ciphers 453Specifies the ciphers allowed. 454Multiple ciphers must be comma-separated. 455If the specified list begins with a 456.Sq + 457character, then the specified ciphers will be appended to the default set 458instead of replacing them. 459If the specified list begins with a 460.Sq - 461character, then the specified ciphers (including wildcards) will be removed 462from the default set instead of replacing them. 463If the specified list begins with a 464.Sq ^ 465character, then the specified ciphers will be placed at the head of the 466default set. 467.Pp 468The supported ciphers are: 469.Pp 470.Bl -item -compact -offset indent 471.It 4723des-cbc 473.It 474aes128-cbc 475.It 476aes192-cbc 477.It 478aes256-cbc 479.It 480aes128-ctr 481.It 482aes192-ctr 483.It 484aes256-ctr 485.It 486aes128-gcm@openssh.com 487.It 488aes256-gcm@openssh.com 489.It 490chacha20-poly1305@openssh.com 491.El 492.Pp 493The default is: 494.Bd -literal -offset indent 495chacha20-poly1305@openssh.com, 496aes128-ctr,aes192-ctr,aes256-ctr, 497aes128-gcm@openssh.com,aes256-gcm@openssh.com 498.Ed 499.Pp 500The list of available ciphers may also be obtained using 501.Qq ssh -Q cipher . 502.It Cm ClientAliveCountMax 503Sets the number of client alive messages which may be sent without 504.Xr sshd 8 505receiving any messages back from the client. 506If this threshold is reached while client alive messages are being sent, 507sshd will disconnect the client, terminating the session. 508It is important to note that the use of client alive messages is very 509different from 510.Cm TCPKeepAlive . 511The client alive messages are sent through the encrypted channel 512and therefore will not be spoofable. 513The TCP keepalive option enabled by 514.Cm TCPKeepAlive 515is spoofable. 516The client alive mechanism is valuable when the client or 517server depend on knowing when a connection has become unresponsive. 518.Pp 519The default value is 3. 520If 521.Cm ClientAliveInterval 522is set to 15, and 523.Cm ClientAliveCountMax 524is left at the default, unresponsive SSH clients 525will be disconnected after approximately 45 seconds. 526Setting a zero 527.Cm ClientAliveCountMax 528disables connection termination. 529.It Cm ClientAliveInterval 530Sets a timeout interval in seconds after which if no data has been received 531from the client, 532.Xr sshd 8 533will send a message through the encrypted 534channel to request a response from the client. 535The default 536is 0, indicating that these messages will not be sent to the client. 537.It Cm Compression 538Specifies whether compression is enabled after 539the user has authenticated successfully. 540The argument must be 541.Cm yes , 542.Cm delayed 543(a legacy synonym for 544.Cm yes ) 545or 546.Cm no . 547The default is 548.Cm yes . 549.It Cm DenyGroups 550This keyword can be followed by a list of group name patterns, separated 551by spaces. 552Login is disallowed for users whose primary group or supplementary 553group list matches one of the patterns. 554Only group names are valid; a numerical group ID is not recognized. 555By default, login is allowed for all groups. 556The allow/deny groups directives are processed in the following order: 557.Cm DenyGroups , 558.Cm AllowGroups . 559.Pp 560See PATTERNS in 561.Xr ssh_config 5 562for more information on patterns. 563.It Cm DenyUsers 564This keyword can be followed by a list of user name patterns, separated 565by spaces. 566Login is disallowed for user names that match one of the patterns. 567Only user names are valid; a numerical user ID is not recognized. 568By default, login is allowed for all users. 569If the pattern takes the form USER@HOST then USER and HOST 570are separately checked, restricting logins to particular 571users from particular hosts. 572HOST criteria may additionally contain addresses to match in CIDR 573address/masklen format. 574The allow/deny users directives are processed in the following order: 575.Cm DenyUsers , 576.Cm AllowUsers . 577.Pp 578See PATTERNS in 579.Xr ssh_config 5 580for more information on patterns. 581.It Cm DisableForwarding 582Disables all forwarding features, including X11, 583.Xr ssh-agent 1 , 584TCP and StreamLocal. 585This option overrides all other forwarding-related options and may 586simplify restricted configurations. 587.It Cm ExposeAuthInfo 588Writes a temporary file containing a list of authentication methods and 589public credentials (e.g. keys) used to authenticate the user. 590The location of the file is exposed to the user session through the 591.Ev SSH_USER_AUTH 592environment variable. 593The default is 594.Cm no . 595.It Cm FingerprintHash 596Specifies the hash algorithm used when logging key fingerprints. 597Valid options are: 598.Cm md5 599and 600.Cm sha256 . 601The default is 602.Cm sha256 . 603.It Cm ForceCommand 604Forces the execution of the command specified by 605.Cm ForceCommand , 606ignoring any command supplied by the client and 607.Pa ~/.ssh/rc 608if present. 609The command is invoked by using the user's login shell with the -c option. 610This applies to shell, command, or subsystem execution. 611It is most useful inside a 612.Cm Match 613block. 614The command originally supplied by the client is available in the 615.Ev SSH_ORIGINAL_COMMAND 616environment variable. 617Specifying a command of 618.Cm internal-sftp 619will force the use of an in-process SFTP server that requires no support 620files when used with 621.Cm ChrootDirectory . 622The default is 623.Cm none . 624.It Cm GatewayPorts 625Specifies whether remote hosts are allowed to connect to ports 626forwarded for the client. 627By default, 628.Xr sshd 8 629binds remote port forwardings to the loopback address. 630This prevents other remote hosts from connecting to forwarded ports. 631.Cm GatewayPorts 632can be used to specify that sshd 633should allow remote port forwardings to bind to non-loopback addresses, thus 634allowing other hosts to connect. 635The argument may be 636.Cm no 637to force remote port forwardings to be available to the local host only, 638.Cm yes 639to force remote port forwardings to bind to the wildcard address, or 640.Cm clientspecified 641to allow the client to select the address to which the forwarding is bound. 642The default is 643.Cm no . 644.It Cm GSSAPIAuthentication 645Specifies whether user authentication based on GSSAPI is allowed. 646The default is 647.Cm no . 648.It Cm GSSAPICleanupCredentials 649Specifies whether to automatically destroy the user's credentials cache 650on logout. 651The default is 652.Cm yes . 653.It Cm GSSAPIStrictAcceptorCheck 654Determines whether to be strict about the identity of the GSSAPI acceptor 655a client authenticates against. 656If set to 657.Cm yes 658then the client must authenticate against the host 659service on the current hostname. 660If set to 661.Cm no 662then the client may authenticate against any service key stored in the 663machine's default store. 664This facility is provided to assist with operation on multi homed machines. 665The default is 666.Cm yes . 667.It Cm HostbasedAcceptedAlgorithms 668Specifies the signature algorithms that will be accepted for hostbased 669authentication as a list of comma-separated patterns. 670Alternately if the specified list begins with a 671.Sq + 672character, then the specified signature algorithms will be appended to 673the default set instead of replacing them. 674If the specified list begins with a 675.Sq - 676character, then the specified signature algorithms (including wildcards) 677will be removed from the default set instead of replacing them. 678If the specified list begins with a 679.Sq ^ 680character, then the specified signature algorithms will be placed at 681the head of the default set. 682The default for this option is: 683.Bd -literal -offset 3n 684ssh-ed25519-cert-v01@openssh.com, 685ecdsa-sha2-nistp256-cert-v01@openssh.com, 686ecdsa-sha2-nistp384-cert-v01@openssh.com, 687ecdsa-sha2-nistp521-cert-v01@openssh.com, 688sk-ssh-ed25519-cert-v01@openssh.com, 689sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 690rsa-sha2-512-cert-v01@openssh.com, 691rsa-sha2-256-cert-v01@openssh.com, 692ssh-rsa-cert-v01@openssh.com, 693ssh-ed25519, 694ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 695sk-ssh-ed25519@openssh.com, 696sk-ecdsa-sha2-nistp256@openssh.com, 697rsa-sha2-512,rsa-sha2-256,ssh-rsa 698.Ed 699.Pp 700The list of available signature algorithms may also be obtained using 701.Qq ssh -Q HostbasedAcceptedAlgorithms . 702This was formerly named HostbasedAcceptedKeyTypes. 703.It Cm HostbasedAuthentication 704Specifies whether rhosts or /etc/hosts.equiv authentication together 705with successful public key client host authentication is allowed 706(host-based authentication). 707The default is 708.Cm no . 709.It Cm HostbasedUsesNameFromPacketOnly 710Specifies whether or not the server will attempt to perform a reverse 711name lookup when matching the name in the 712.Pa ~/.shosts , 713.Pa ~/.rhosts , 714and 715.Pa /etc/hosts.equiv 716files during 717.Cm HostbasedAuthentication . 718A setting of 719.Cm yes 720means that 721.Xr sshd 8 722uses the name supplied by the client rather than 723attempting to resolve the name from the TCP connection itself. 724The default is 725.Cm no . 726.It Cm HostCertificate 727Specifies a file containing a public host certificate. 728The certificate's public key must match a private host key already specified 729by 730.Cm HostKey . 731The default behaviour of 732.Xr sshd 8 733is not to load any certificates. 734.It Cm HostKey 735Specifies a file containing a private host key 736used by SSH. 737The defaults are 738.Pa /etc/ssh/ssh_host_ecdsa_key , 739.Pa /etc/ssh/ssh_host_ed25519_key 740and 741.Pa /etc/ssh/ssh_host_rsa_key . 742.Pp 743Note that 744.Xr sshd 8 745will refuse to use a file if it is group/world-accessible 746and that the 747.Cm HostKeyAlgorithms 748option restricts which of the keys are actually used by 749.Xr sshd 8 . 750.Pp 751It is possible to have multiple host key files. 752It is also possible to specify public host key files instead. 753In this case operations on the private key will be delegated 754to an 755.Xr ssh-agent 1 . 756.It Cm HostKeyAgent 757Identifies the UNIX-domain socket used to communicate 758with an agent that has access to the private host keys. 759If the string 760.Qq SSH_AUTH_SOCK 761is specified, the location of the socket will be read from the 762.Ev SSH_AUTH_SOCK 763environment variable. 764.It Cm HostKeyAlgorithms 765Specifies the host key signature algorithms 766that the server offers. 767The default for this option is: 768.Bd -literal -offset 3n 769ssh-ed25519-cert-v01@openssh.com, 770ecdsa-sha2-nistp256-cert-v01@openssh.com, 771ecdsa-sha2-nistp384-cert-v01@openssh.com, 772ecdsa-sha2-nistp521-cert-v01@openssh.com, 773sk-ssh-ed25519-cert-v01@openssh.com, 774sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 775rsa-sha2-512-cert-v01@openssh.com, 776rsa-sha2-256-cert-v01@openssh.com, 777ssh-rsa-cert-v01@openssh.com, 778ssh-ed25519, 779ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 780sk-ssh-ed25519@openssh.com, 781sk-ecdsa-sha2-nistp256@openssh.com, 782rsa-sha2-512,rsa-sha2-256,ssh-rsa 783.Ed 784.Pp 785The list of available signature algorithms may also be obtained using 786.Qq ssh -Q HostKeyAlgorithms . 787.It Cm IgnoreRhosts 788Specifies whether to ignore per-user 789.Pa .rhosts 790and 791.Pa .shosts 792files during 793.Cm HostbasedAuthentication . 794The system-wide 795.Pa /etc/hosts.equiv 796and 797.Pa /etc/shosts.equiv 798are still used regardless of this setting. 799.Pp 800Accepted values are 801.Cm yes 802(the default) to ignore all per-user files, 803.Cm shosts-only 804to allow the use of 805.Pa .shosts 806but to ignore 807.Pa .rhosts 808or 809.Cm no 810to allow both 811.Pa .shosts 812and 813.Pa rhosts . 814.It Cm IgnoreUserKnownHosts 815Specifies whether 816.Xr sshd 8 817should ignore the user's 818.Pa ~/.ssh/known_hosts 819during 820.Cm HostbasedAuthentication 821and use only the system-wide known hosts file 822.Pa /etc/ssh/known_hosts . 823The default is 824.Dq no . 825.It Cm Include 826Include the specified configuration file(s). 827Multiple pathnames may be specified and each pathname may contain 828.Xr glob 7 829wildcards that will be expanded and processed in lexical order. 830Files without absolute paths are assumed to be in 831.Pa /etc/ssh . 832An 833.Cm Include 834directive may appear inside a 835.Cm Match 836block 837to perform conditional inclusion. 838.It Cm IPQoS 839Specifies the IPv4 type-of-service or DSCP class for the connection. 840Accepted values are 841.Cm af11 , 842.Cm af12 , 843.Cm af13 , 844.Cm af21 , 845.Cm af22 , 846.Cm af23 , 847.Cm af31 , 848.Cm af32 , 849.Cm af33 , 850.Cm af41 , 851.Cm af42 , 852.Cm af43 , 853.Cm cs0 , 854.Cm cs1 , 855.Cm cs2 , 856.Cm cs3 , 857.Cm cs4 , 858.Cm cs5 , 859.Cm cs6 , 860.Cm cs7 , 861.Cm ef , 862.Cm le , 863.Cm lowdelay , 864.Cm throughput , 865.Cm reliability , 866a numeric value, or 867.Cm none 868to use the operating system default. 869This option may take one or two arguments, separated by whitespace. 870If one argument is specified, it is used as the packet class unconditionally. 871If two values are specified, the first is automatically selected for 872interactive sessions and the second for non-interactive sessions. 873The default is 874.Cm af21 875(Low-Latency Data) 876for interactive sessions and 877.Cm cs1 878(Lower Effort) 879for non-interactive sessions. 880.It Cm KbdInteractiveAuthentication 881Specifies whether to allow keyboard-interactive authentication. 882All authentication styles from 883.Xr login.conf 5 884are supported. 885The default is 886.Cm yes . 887The argument to this keyword must be 888.Cm yes 889or 890.Cm no . 891.Cm ChallengeResponseAuthentication 892is a deprecated alias for this. 893.It Cm KerberosAuthentication 894Specifies whether the password provided by the user for 895.Cm PasswordAuthentication 896will be validated through the Kerberos KDC. 897To use this option, the server needs a 898Kerberos servtab which allows the verification of the KDC's identity. 899The default is 900.Cm no . 901.It Cm KerberosGetAFSToken 902If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 903an AFS token before accessing the user's home directory. 904The default is 905.Cm no . 906.It Cm KerberosOrLocalPasswd 907If password authentication through Kerberos fails then 908the password will be validated via any additional local mechanism 909such as 910.Pa /etc/passwd . 911The default is 912.Cm yes . 913.It Cm KerberosTicketCleanup 914Specifies whether to automatically destroy the user's ticket cache 915file on logout. 916The default is 917.Cm yes . 918.It Cm KexAlgorithms 919Specifies the available KEX (Key Exchange) algorithms. 920Multiple algorithms must be comma-separated. 921Alternately if the specified list begins with a 922.Sq + 923character, then the specified algorithms will be appended to the default set 924instead of replacing them. 925If the specified list begins with a 926.Sq - 927character, then the specified algorithms (including wildcards) will be removed 928from the default set instead of replacing them. 929If the specified list begins with a 930.Sq ^ 931character, then the specified algorithms will be placed at the head of the 932default set. 933The supported algorithms are: 934.Pp 935.Bl -item -compact -offset indent 936.It 937curve25519-sha256 938.It 939curve25519-sha256@libssh.org 940.It 941diffie-hellman-group1-sha1 942.It 943diffie-hellman-group14-sha1 944.It 945diffie-hellman-group14-sha256 946.It 947diffie-hellman-group16-sha512 948.It 949diffie-hellman-group18-sha512 950.It 951diffie-hellman-group-exchange-sha1 952.It 953diffie-hellman-group-exchange-sha256 954.It 955ecdh-sha2-nistp256 956.It 957ecdh-sha2-nistp384 958.It 959ecdh-sha2-nistp521 960.It 961sntrup761x25519-sha512@openssh.com 962.El 963.Pp 964The default is: 965.Bd -literal -offset indent 966curve25519-sha256,curve25519-sha256@libssh.org, 967ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 968diffie-hellman-group-exchange-sha256, 969diffie-hellman-group16-sha512,diffie-hellman-group18-sha512, 970diffie-hellman-group14-sha256 971.Ed 972.Pp 973The list of available key exchange algorithms may also be obtained using 974.Qq ssh -Q KexAlgorithms . 975.It Cm ListenAddress 976Specifies the local addresses 977.Xr sshd 8 978should listen on. 979The following forms may be used: 980.Pp 981.Bl -item -offset indent -compact 982.It 983.Cm ListenAddress 984.Sm off 985.Ar hostname | address 986.Sm on 987.Op Cm rdomain Ar domain 988.It 989.Cm ListenAddress 990.Sm off 991.Ar hostname : port 992.Sm on 993.Op Cm rdomain Ar domain 994.It 995.Cm ListenAddress 996.Sm off 997.Ar IPv4_address : port 998.Sm on 999.Op Cm rdomain Ar domain 1000.It 1001.Cm ListenAddress 1002.Sm off 1003.Oo Ar hostname | address Oc : Ar port 1004.Sm on 1005.Op Cm rdomain Ar domain 1006.El 1007.Pp 1008The optional 1009.Cm rdomain 1010qualifier requests 1011.Xr sshd 8 1012listen in an explicit routing domain. 1013If 1014.Ar port 1015is not specified, 1016sshd will listen on the address and all 1017.Cm Port 1018options specified. 1019The default is to listen on all local addresses on the current default 1020routing domain. 1021Multiple 1022.Cm ListenAddress 1023options are permitted. 1024For more information on routing domains, see 1025.Xr rdomain 4 . 1026.It Cm LoginGraceTime 1027The server disconnects after this time if the user has not 1028successfully logged in. 1029If the value is 0, there is no time limit. 1030The default is 120 seconds. 1031.It Cm LogLevel 1032Gives the verbosity level that is used when logging messages from 1033.Xr sshd 8 . 1034The possible values are: 1035QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1036The default is INFO. 1037DEBUG and DEBUG1 are equivalent. 1038DEBUG2 and DEBUG3 each specify higher levels of debugging output. 1039Logging with a DEBUG level violates the privacy of users and is not recommended. 1040.It Cm LogVerbose 1041Specify one or more overrides to LogLevel. 1042An override consists of a pattern lists that matches the source file, function 1043and line number to force detailed logging for. 1044For example, an override pattern of: 1045.Bd -literal -offset indent 1046kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1047.Ed 1048.Pp 1049would enable detailed logging for line 1000 of 1050.Pa kex.c , 1051everything in the 1052.Fn kex_exchange_identification 1053function, and all code in the 1054.Pa packet.c 1055file. 1056This option is intended for debugging and no overrides are enabled by default. 1057.It Cm MACs 1058Specifies the available MAC (message authentication code) algorithms. 1059The MAC algorithm is used for data integrity protection. 1060Multiple algorithms must be comma-separated. 1061If the specified list begins with a 1062.Sq + 1063character, then the specified algorithms will be appended to the default set 1064instead of replacing them. 1065If the specified list begins with a 1066.Sq - 1067character, then the specified algorithms (including wildcards) will be removed 1068from the default set instead of replacing them. 1069If the specified list begins with a 1070.Sq ^ 1071character, then the specified algorithms will be placed at the head of the 1072default set. 1073.Pp 1074The algorithms that contain 1075.Qq -etm 1076calculate the MAC after encryption (encrypt-then-mac). 1077These are considered safer and their use recommended. 1078The supported MACs are: 1079.Pp 1080.Bl -item -compact -offset indent 1081.It 1082hmac-md5 1083.It 1084hmac-md5-96 1085.It 1086hmac-sha1 1087.It 1088hmac-sha1-96 1089.It 1090hmac-sha2-256 1091.It 1092hmac-sha2-512 1093.It 1094umac-64@openssh.com 1095.It 1096umac-128@openssh.com 1097.It 1098hmac-md5-etm@openssh.com 1099.It 1100hmac-md5-96-etm@openssh.com 1101.It 1102hmac-sha1-etm@openssh.com 1103.It 1104hmac-sha1-96-etm@openssh.com 1105.It 1106hmac-sha2-256-etm@openssh.com 1107.It 1108hmac-sha2-512-etm@openssh.com 1109.It 1110umac-64-etm@openssh.com 1111.It 1112umac-128-etm@openssh.com 1113.El 1114.Pp 1115The default is: 1116.Bd -literal -offset indent 1117umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1118hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1119hmac-sha1-etm@openssh.com, 1120umac-64@openssh.com,umac-128@openssh.com, 1121hmac-sha2-256,hmac-sha2-512,hmac-sha1 1122.Ed 1123.Pp 1124The list of available MAC algorithms may also be obtained using 1125.Qq ssh -Q mac . 1126.It Cm Match 1127Introduces a conditional block. 1128If all of the criteria on the 1129.Cm Match 1130line are satisfied, the keywords on the following lines override those 1131set in the global section of the config file, until either another 1132.Cm Match 1133line or the end of the file. 1134If a keyword appears in multiple 1135.Cm Match 1136blocks that are satisfied, only the first instance of the keyword is 1137applied. 1138.Pp 1139The arguments to 1140.Cm Match 1141are one or more criteria-pattern pairs or the single token 1142.Cm All 1143which matches all criteria. 1144The available criteria are 1145.Cm User , 1146.Cm Group , 1147.Cm Host , 1148.Cm LocalAddress , 1149.Cm LocalPort , 1150.Cm RDomain , 1151and 1152.Cm Address 1153(with 1154.Cm RDomain 1155representing the 1156.Xr rdomain 4 1157on which the connection was received). 1158.Pp 1159The match patterns may consist of single entries or comma-separated 1160lists and may use the wildcard and negation operators described in the 1161.Sx PATTERNS 1162section of 1163.Xr ssh_config 5 . 1164.Pp 1165The patterns in an 1166.Cm Address 1167criteria may additionally contain addresses to match in CIDR 1168address/masklen format, 1169such as 192.0.2.0/24 or 2001:db8::/32. 1170Note that the mask length provided must be consistent with the address - 1171it is an error to specify a mask length that is too long for the address 1172or one with bits set in this host portion of the address. 1173For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. 1174.Pp 1175Only a subset of keywords may be used on the lines following a 1176.Cm Match 1177keyword. 1178Available keywords are 1179.Cm AcceptEnv , 1180.Cm AllowAgentForwarding , 1181.Cm AllowGroups , 1182.Cm AllowStreamLocalForwarding , 1183.Cm AllowTcpForwarding , 1184.Cm AllowUsers , 1185.Cm AuthenticationMethods , 1186.Cm AuthorizedKeysCommand , 1187.Cm AuthorizedKeysCommandUser , 1188.Cm AuthorizedKeysFile , 1189.Cm AuthorizedPrincipalsCommand , 1190.Cm AuthorizedPrincipalsCommandUser , 1191.Cm AuthorizedPrincipalsFile , 1192.Cm Banner , 1193.Cm ChrootDirectory , 1194.Cm ClientAliveCountMax , 1195.Cm ClientAliveInterval , 1196.Cm DenyGroups , 1197.Cm DenyUsers , 1198.Cm DisableForwarding , 1199.Cm ForceCommand , 1200.Cm GatewayPorts , 1201.Cm GSSAPIAuthentication , 1202.Cm HostbasedAcceptedAlgorithms , 1203.Cm HostbasedAuthentication , 1204.Cm HostbasedUsesNameFromPacketOnly , 1205.Cm IgnoreRhosts , 1206.Cm Include , 1207.Cm IPQoS , 1208.Cm KbdInteractiveAuthentication , 1209.Cm KerberosAuthentication , 1210.Cm LogLevel , 1211.Cm MaxAuthTries , 1212.Cm MaxSessions , 1213.Cm PasswordAuthentication , 1214.Cm PermitEmptyPasswords , 1215.Cm PermitListen , 1216.Cm PermitOpen , 1217.Cm PermitRootLogin , 1218.Cm PermitTTY , 1219.Cm PermitTunnel , 1220.Cm PermitUserRC , 1221.Cm PubkeyAcceptedAlgorithms , 1222.Cm PubkeyAuthentication , 1223.Cm RekeyLimit , 1224.Cm RevokedKeys , 1225.Cm RDomain , 1226.Cm SetEnv , 1227.Cm StreamLocalBindMask , 1228.Cm StreamLocalBindUnlink , 1229.Cm TrustedUserCAKeys , 1230.Cm X11DisplayOffset , 1231.Cm X11Forwarding 1232and 1233.Cm X11UseLocalhost . 1234.It Cm MaxAuthTries 1235Specifies the maximum number of authentication attempts permitted per 1236connection. 1237Once the number of failures reaches half this value, 1238additional failures are logged. 1239The default is 6. 1240.It Cm MaxSessions 1241Specifies the maximum number of open shell, login or subsystem (e.g. sftp) 1242sessions permitted per network connection. 1243Multiple sessions may be established by clients that support connection 1244multiplexing. 1245Setting 1246.Cm MaxSessions 1247to 1 will effectively disable session multiplexing, whereas setting it to 0 1248will prevent all shell, login and subsystem sessions while still permitting 1249forwarding. 1250The default is 10. 1251.It Cm MaxStartups 1252Specifies the maximum number of concurrent unauthenticated connections to the 1253SSH daemon. 1254Additional connections will be dropped until authentication succeeds or the 1255.Cm LoginGraceTime 1256expires for a connection. 1257The default is 10:30:100. 1258.Pp 1259Alternatively, random early drop can be enabled by specifying 1260the three colon separated values 1261start:rate:full (e.g. "10:30:60"). 1262.Xr sshd 8 1263will refuse connection attempts with a probability of rate/100 (30%) 1264if there are currently start (10) unauthenticated connections. 1265The probability increases linearly and all connection attempts 1266are refused if the number of unauthenticated connections reaches full (60). 1267.It Cm ModuliFile 1268Specifies the 1269.Xr moduli 5 1270file that contains the Diffie-Hellman groups used for the 1271.Dq diffie-hellman-group-exchange-sha1 1272and 1273.Dq diffie-hellman-group-exchange-sha256 1274key exchange methods. 1275The default is 1276.Pa /etc/moduli . 1277.It Cm PasswordAuthentication 1278Specifies whether password authentication is allowed. 1279The default is 1280.Cm yes . 1281.It Cm PermitEmptyPasswords 1282When password authentication is allowed, it specifies whether the 1283server allows login to accounts with empty password strings. 1284The default is 1285.Cm no . 1286.It Cm PermitListen 1287Specifies the addresses/ports on which a remote TCP port forwarding may listen. 1288The listen specification must be one of the following forms: 1289.Pp 1290.Bl -item -offset indent -compact 1291.It 1292.Cm PermitListen 1293.Sm off 1294.Ar port 1295.Sm on 1296.It 1297.Cm PermitListen 1298.Sm off 1299.Ar host : port 1300.Sm on 1301.El 1302.Pp 1303Multiple permissions may be specified by separating them with whitespace. 1304An argument of 1305.Cm any 1306can be used to remove all restrictions and permit any listen requests. 1307An argument of 1308.Cm none 1309can be used to prohibit all listen requests. 1310The host name may contain wildcards as described in the PATTERNS section in 1311.Xr ssh_config 5 . 1312The wildcard 1313.Sq * 1314can also be used in place of a port number to allow all ports. 1315By default all port forwarding listen requests are permitted. 1316Note that the 1317.Cm GatewayPorts 1318option may further restrict which addresses may be listened on. 1319Note also that 1320.Xr ssh 1 1321will request a listen host of 1322.Dq localhost 1323if no listen host was specifically requested, and this name is 1324treated differently to explicit localhost addresses of 1325.Dq 127.0.0.1 1326and 1327.Dq ::1 . 1328.It Cm PermitOpen 1329Specifies the destinations to which TCP port forwarding is permitted. 1330The forwarding specification must be one of the following forms: 1331.Pp 1332.Bl -item -offset indent -compact 1333.It 1334.Cm PermitOpen 1335.Sm off 1336.Ar host : port 1337.Sm on 1338.It 1339.Cm PermitOpen 1340.Sm off 1341.Ar IPv4_addr : port 1342.Sm on 1343.It 1344.Cm PermitOpen 1345.Sm off 1346.Ar \&[ IPv6_addr \&] : port 1347.Sm on 1348.El 1349.Pp 1350Multiple forwards may be specified by separating them with whitespace. 1351An argument of 1352.Cm any 1353can be used to remove all restrictions and permit any forwarding requests. 1354An argument of 1355.Cm none 1356can be used to prohibit all forwarding requests. 1357The wildcard 1358.Sq * 1359can be used for host or port to allow all hosts or ports respectively. 1360Otherwise, no pattern matching or address lookups are performed on supplied 1361names. 1362By default all port forwarding requests are permitted. 1363.It Cm PermitRootLogin 1364Specifies whether root can log in using 1365.Xr ssh 1 . 1366The argument must be 1367.Cm yes , 1368.Cm prohibit-password , 1369.Cm forced-commands-only , 1370or 1371.Cm no . 1372The default is 1373.Cm prohibit-password . 1374.Pp 1375If this option is set to 1376.Cm prohibit-password 1377(or its deprecated alias, 1378.Cm without-password ) , 1379password and keyboard-interactive authentication are disabled for root. 1380.Pp 1381If this option is set to 1382.Cm forced-commands-only , 1383root login with public key authentication will be allowed, 1384but only if the 1385.Ar command 1386option has been specified 1387(which may be useful for taking remote backups even if root login is 1388normally not allowed). 1389All other authentication methods are disabled for root. 1390.Pp 1391If this option is set to 1392.Cm no , 1393root is not allowed to log in. 1394.It Cm PermitTTY 1395Specifies whether 1396.Xr pty 4 1397allocation is permitted. 1398The default is 1399.Cm yes . 1400.It Cm PermitTunnel 1401Specifies whether 1402.Xr tun 4 1403device forwarding is allowed. 1404The argument must be 1405.Cm yes , 1406.Cm point-to-point 1407(layer 3), 1408.Cm ethernet 1409(layer 2), or 1410.Cm no . 1411Specifying 1412.Cm yes 1413permits both 1414.Cm point-to-point 1415and 1416.Cm ethernet . 1417The default is 1418.Cm no . 1419.Pp 1420Independent of this setting, the permissions of the selected 1421.Xr tun 4 1422device must allow access to the user. 1423.It Cm PermitUserEnvironment 1424Specifies whether 1425.Pa ~/.ssh/environment 1426and 1427.Cm environment= 1428options in 1429.Pa ~/.ssh/authorized_keys 1430are processed by 1431.Xr sshd 8 . 1432Valid options are 1433.Cm yes , 1434.Cm no 1435or a pattern-list specifying which environment variable names to accept 1436(for example 1437.Qq LANG,LC_* ) . 1438The default is 1439.Cm no . 1440Enabling environment processing may enable users to bypass access 1441restrictions in some configurations using mechanisms such as 1442.Ev LD_PRELOAD . 1443.It Cm PermitUserRC 1444Specifies whether any 1445.Pa ~/.ssh/rc 1446file is executed. 1447The default is 1448.Cm yes . 1449.It Cm PerSourceMaxStartups 1450Specifies the number of unauthenticated connections allowed from a 1451given source address, or 1452.Dq none 1453if there is no limit. 1454This limit is applied in addition to 1455.Cm MaxStartups , 1456whichever is lower. 1457The default is 1458.Cm none . 1459.It Cm PerSourceNetBlockSize 1460Specifies the number of bits of source address that are grouped together 1461for the purposes of applying PerSourceMaxStartups limits. 1462Values for IPv4 and optionally IPv6 may be specified, separated by a colon. 1463The default is 1464.Cm 32:128 , 1465which means each address is considered individually. 1466.It Cm PidFile 1467Specifies the file that contains the process ID of the 1468SSH daemon, or 1469.Cm none 1470to not write one. 1471The default is 1472.Pa /var/run/sshd.pid . 1473.It Cm Port 1474Specifies the port number that 1475.Xr sshd 8 1476listens on. 1477The default is 22. 1478Multiple options of this type are permitted. 1479See also 1480.Cm ListenAddress . 1481.It Cm PrintLastLog 1482Specifies whether 1483.Xr sshd 8 1484should print the date and time of the last user login when a user logs 1485in interactively. 1486The default is 1487.Cm yes . 1488.It Cm PrintMotd 1489Specifies whether 1490.Xr sshd 8 1491should print 1492.Pa /etc/motd 1493when a user logs in interactively. 1494(On some systems it is also printed by the shell, 1495.Pa /etc/profile , 1496or equivalent.) 1497The default is 1498.Cm yes . 1499.It Cm PubkeyAcceptedAlgorithms 1500Specifies the signature algorithms that will be accepted for public key 1501authentication as a list of comma-separated patterns. 1502Alternately if the specified list begins with a 1503.Sq + 1504character, then the specified algorithms will be appended to the default set 1505instead of replacing them. 1506If the specified list begins with a 1507.Sq - 1508character, then the specified algorithms (including wildcards) will be removed 1509from the default set instead of replacing them. 1510If the specified list begins with a 1511.Sq ^ 1512character, then the specified algorithms will be placed at the head of the 1513default set. 1514The default for this option is: 1515.Bd -literal -offset 3n 1516ssh-ed25519-cert-v01@openssh.com, 1517ecdsa-sha2-nistp256-cert-v01@openssh.com, 1518ecdsa-sha2-nistp384-cert-v01@openssh.com, 1519ecdsa-sha2-nistp521-cert-v01@openssh.com, 1520sk-ssh-ed25519-cert-v01@openssh.com, 1521sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1522rsa-sha2-512-cert-v01@openssh.com, 1523rsa-sha2-256-cert-v01@openssh.com, 1524ssh-rsa-cert-v01@openssh.com, 1525ssh-ed25519, 1526ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1527sk-ssh-ed25519@openssh.com, 1528sk-ecdsa-sha2-nistp256@openssh.com, 1529rsa-sha2-512,rsa-sha2-256,ssh-rsa 1530.Ed 1531.Pp 1532The list of available signature algorithms may also be obtained using 1533.Qq ssh -Q PubkeyAcceptedAlgorithms . 1534.It Cm PubkeyAuthOptions 1535Sets one or more public key authentication options. 1536The supported keywords are: 1537.Cm none 1538(the default; indicating no additional options are enabled), 1539.Cm touch-required 1540and 1541.Cm verify-required . 1542.Pp 1543The 1544.Cm touch-required 1545option causes public key authentication using a FIDO authenticator algorithm 1546(i.e.\& 1547.Cm ecdsa-sk 1548or 1549.Cm ed25519-sk ) 1550to always require the signature to attest that a physically present user 1551explicitly confirmed the authentication (usually by touching the authenticator). 1552By default, 1553.Xr sshd 8 1554requires user presence unless overridden with an authorized_keys option. 1555The 1556.Cm touch-required 1557flag disables this override. 1558.Pp 1559The 1560.Cm verify-required 1561option requires a FIDO key signature attest that the user was verified, 1562e.g. via a PIN. 1563.Pp 1564Neither the 1565.Cm touch-required 1566or 1567.Cm verify-required 1568options have any effect for other, non-FIDO, public key types. 1569.It Cm PubkeyAuthentication 1570Specifies whether public key authentication is allowed. 1571The default is 1572.Cm yes . 1573.It Cm RekeyLimit 1574Specifies the maximum amount of data that may be transmitted before the 1575session key is renegotiated, optionally followed by a maximum amount of 1576time that may pass before the session key is renegotiated. 1577The first argument is specified in bytes and may have a suffix of 1578.Sq K , 1579.Sq M , 1580or 1581.Sq G 1582to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1583The default is between 1584.Sq 1G 1585and 1586.Sq 4G , 1587depending on the cipher. 1588The optional second value is specified in seconds and may use any of the 1589units documented in the 1590.Sx TIME FORMATS 1591section. 1592The default value for 1593.Cm RekeyLimit 1594is 1595.Cm default none , 1596which means that rekeying is performed after the cipher's default amount 1597of data has been sent or received and no time based rekeying is done. 1598.It Cm RevokedKeys 1599Specifies revoked public keys file, or 1600.Cm none 1601to not use one. 1602Keys listed in this file will be refused for public key authentication. 1603Note that if this file is not readable, then public key authentication will 1604be refused for all users. 1605Keys may be specified as a text file, listing one public key per line, or as 1606an OpenSSH Key Revocation List (KRL) as generated by 1607.Xr ssh-keygen 1 . 1608For more information on KRLs, see the KEY REVOCATION LISTS section in 1609.Xr ssh-keygen 1 . 1610.It Cm RDomain 1611Specifies an explicit routing domain that is applied after authentication 1612has completed. 1613The user session, as well as any forwarded or listening IP sockets, 1614will be bound to this 1615.Xr rdomain 4 . 1616If the routing domain is set to 1617.Cm \&%D , 1618then the domain in which the incoming connection was received will be applied. 1619.It Cm SecurityKeyProvider 1620Specifies a path to a library that will be used when loading 1621FIDO authenticator-hosted keys, overriding the default of using 1622the built-in USB HID support. 1623.It Cm SetEnv 1624Specifies one or more environment variables to set in child sessions started 1625by 1626.Xr sshd 8 1627as 1628.Dq NAME=VALUE . 1629The environment value may be quoted (e.g. if it contains whitespace 1630characters). 1631Environment variables set by 1632.Cm SetEnv 1633override the default environment and any variables specified by the user 1634via 1635.Cm AcceptEnv 1636or 1637.Cm PermitUserEnvironment . 1638.It Cm StreamLocalBindMask 1639Sets the octal file creation mode mask 1640.Pq umask 1641used when creating a Unix-domain socket file for local or remote 1642port forwarding. 1643This option is only used for port forwarding to a Unix-domain socket file. 1644.Pp 1645The default value is 0177, which creates a Unix-domain socket file that is 1646readable and writable only by the owner. 1647Note that not all operating systems honor the file mode on Unix-domain 1648socket files. 1649.It Cm StreamLocalBindUnlink 1650Specifies whether to remove an existing Unix-domain socket file for local 1651or remote port forwarding before creating a new one. 1652If the socket file already exists and 1653.Cm StreamLocalBindUnlink 1654is not enabled, 1655.Nm sshd 1656will be unable to forward the port to the Unix-domain socket file. 1657This option is only used for port forwarding to a Unix-domain socket file. 1658.Pp 1659The argument must be 1660.Cm yes 1661or 1662.Cm no . 1663The default is 1664.Cm no . 1665.It Cm StrictModes 1666Specifies whether 1667.Xr sshd 8 1668should check file modes and ownership of the 1669user's files and home directory before accepting login. 1670This is normally desirable because novices sometimes accidentally leave their 1671directory or files world-writable. 1672The default is 1673.Cm yes . 1674Note that this does not apply to 1675.Cm ChrootDirectory , 1676whose permissions and ownership are checked unconditionally. 1677.It Cm Subsystem 1678Configures an external subsystem (e.g. file transfer daemon). 1679Arguments should be a subsystem name and a command (with optional arguments) 1680to execute upon subsystem request. 1681.Pp 1682The command 1683.Cm sftp-server 1684implements the SFTP file transfer subsystem. 1685.Pp 1686Alternately the name 1687.Cm internal-sftp 1688implements an in-process SFTP server. 1689This may simplify configurations using 1690.Cm ChrootDirectory 1691to force a different filesystem root on clients. 1692.Pp 1693By default no subsystems are defined. 1694.It Cm SyslogFacility 1695Gives the facility code that is used when logging messages from 1696.Xr sshd 8 . 1697The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1698LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1699The default is AUTH. 1700.It Cm TCPKeepAlive 1701Specifies whether the system should send TCP keepalive messages to the 1702other side. 1703If they are sent, death of the connection or crash of one 1704of the machines will be properly noticed. 1705However, this means that 1706connections will die if the route is down temporarily, and some people 1707find it annoying. 1708On the other hand, if TCP keepalives are not sent, 1709sessions may hang indefinitely on the server, leaving 1710.Qq ghost 1711users and consuming server resources. 1712.Pp 1713The default is 1714.Cm yes 1715(to send TCP keepalive messages), and the server will notice 1716if the network goes down or the client host crashes. 1717This avoids infinitely hanging sessions. 1718.Pp 1719To disable TCP keepalive messages, the value should be set to 1720.Cm no . 1721.It Cm TrustedUserCAKeys 1722Specifies a file containing public keys of certificate authorities that are 1723trusted to sign user certificates for authentication, or 1724.Cm none 1725to not use one. 1726Keys are listed one per line; empty lines and comments starting with 1727.Ql # 1728are allowed. 1729If a certificate is presented for authentication and has its signing CA key 1730listed in this file, then it may be used for authentication for any user 1731listed in the certificate's principals list. 1732Note that certificates that lack a list of principals will not be permitted 1733for authentication using 1734.Cm TrustedUserCAKeys . 1735For more details on certificates, see the CERTIFICATES section in 1736.Xr ssh-keygen 1 . 1737.It Cm UseDNS 1738Specifies whether 1739.Xr sshd 8 1740should look up the remote host name, and to check that 1741the resolved host name for the remote IP address maps back to the 1742very same IP address. 1743.Pp 1744If this option is set to 1745.Cm no 1746(the default) then only addresses and not host names may be used in 1747.Pa ~/.ssh/authorized_keys 1748.Cm from 1749and 1750.Nm 1751.Cm Match 1752.Cm Host 1753directives. 1754.It Cm UsePAM 1755Enables the Pluggable Authentication Module interface. 1756If set to 1757.Cm yes 1758this will enable PAM authentication using 1759.Cm KbdInteractiveAuthentication 1760and 1761.Cm PasswordAuthentication 1762in addition to PAM account and session module processing for all 1763authentication types. 1764.Pp 1765Because PAM keyboard-interactive authentication usually serves an equivalent 1766role to password authentication, you should disable either 1767.Cm PasswordAuthentication 1768or 1769.Cm KbdInteractiveAuthentication . 1770.Pp 1771If 1772.Cm UsePAM 1773is enabled, you will not be able to run 1774.Xr sshd 8 1775as a non-root user. 1776The default is 1777.Cm no . 1778.It Cm VersionAddendum 1779Optionally specifies additional text to append to the SSH protocol banner 1780sent by the server upon connection. 1781The default is 1782.Cm none . 1783.It Cm X11DisplayOffset 1784Specifies the first display number available for 1785.Xr sshd 8 Ns 's 1786X11 forwarding. 1787This prevents sshd from interfering with real X11 servers. 1788The default is 10. 1789.It Cm X11Forwarding 1790Specifies whether X11 forwarding is permitted. 1791The argument must be 1792.Cm yes 1793or 1794.Cm no . 1795The default is 1796.Cm no . 1797.Pp 1798When X11 forwarding is enabled, there may be additional exposure to 1799the server and to client displays if the 1800.Xr sshd 8 1801proxy display is configured to listen on the wildcard address (see 1802.Cm X11UseLocalhost ) , 1803though this is not the default. 1804Additionally, the authentication spoofing and authentication data 1805verification and substitution occur on the client side. 1806The security risk of using X11 forwarding is that the client's X11 1807display server may be exposed to attack when the SSH client requests 1808forwarding (see the warnings for 1809.Cm ForwardX11 1810in 1811.Xr ssh_config 5 ) . 1812A system administrator may have a stance in which they want to 1813protect clients that may expose themselves to attack by unwittingly 1814requesting X11 forwarding, which can warrant a 1815.Cm no 1816setting. 1817.Pp 1818Note that disabling X11 forwarding does not prevent users from 1819forwarding X11 traffic, as users can always install their own forwarders. 1820.It Cm X11UseLocalhost 1821Specifies whether 1822.Xr sshd 8 1823should bind the X11 forwarding server to the loopback address or to 1824the wildcard address. 1825By default, 1826sshd binds the forwarding server to the loopback address and sets the 1827hostname part of the 1828.Ev DISPLAY 1829environment variable to 1830.Cm localhost . 1831This prevents remote hosts from connecting to the proxy display. 1832However, some older X11 clients may not function with this 1833configuration. 1834.Cm X11UseLocalhost 1835may be set to 1836.Cm no 1837to specify that the forwarding server should be bound to the wildcard 1838address. 1839The argument must be 1840.Cm yes 1841or 1842.Cm no . 1843The default is 1844.Cm yes . 1845.It Cm XAuthLocation 1846Specifies the full pathname of the 1847.Xr xauth 1 1848program, or 1849.Cm none 1850to not use one. 1851The default is 1852.Pa /usr/X11R6/bin/xauth . 1853.El 1854.Sh TIME FORMATS 1855.Xr sshd 8 1856command-line arguments and configuration file options that specify time 1857may be expressed using a sequence of the form: 1858.Sm off 1859.Ar time Op Ar qualifier , 1860.Sm on 1861where 1862.Ar time 1863is a positive integer value and 1864.Ar qualifier 1865is one of the following: 1866.Pp 1867.Bl -tag -width Ds -compact -offset indent 1868.It Aq Cm none 1869seconds 1870.It Cm s | Cm S 1871seconds 1872.It Cm m | Cm M 1873minutes 1874.It Cm h | Cm H 1875hours 1876.It Cm d | Cm D 1877days 1878.It Cm w | Cm W 1879weeks 1880.El 1881.Pp 1882Each member of the sequence is added together to calculate 1883the total time value. 1884.Pp 1885Time format examples: 1886.Pp 1887.Bl -tag -width Ds -compact -offset indent 1888.It 600 1889600 seconds (10 minutes) 1890.It 10m 189110 minutes 1892.It 1h30m 18931 hour 30 minutes (90 minutes) 1894.El 1895.Sh TOKENS 1896Arguments to some keywords can make use of tokens, 1897which are expanded at runtime: 1898.Pp 1899.Bl -tag -width XXXX -offset indent -compact 1900.It %% 1901A literal 1902.Sq % . 1903.It \&%D 1904The routing domain in which the incoming connection was received. 1905.It %F 1906The fingerprint of the CA key. 1907.It %f 1908The fingerprint of the key or certificate. 1909.It %h 1910The home directory of the user. 1911.It %i 1912The key ID in the certificate. 1913.It %K 1914The base64-encoded CA key. 1915.It %k 1916The base64-encoded key or certificate for authentication. 1917.It %s 1918The serial number of the certificate. 1919.It \&%T 1920The type of the CA key. 1921.It %t 1922The key or certificate type. 1923.It \&%U 1924The numeric user ID of the target user. 1925.It %u 1926The username. 1927.El 1928.Pp 1929.Cm AuthorizedKeysCommand 1930accepts the tokens %%, %f, %h, %k, %t, %U, and %u. 1931.Pp 1932.Cm AuthorizedKeysFile 1933accepts the tokens %%, %h, %U, and %u. 1934.Pp 1935.Cm AuthorizedPrincipalsCommand 1936accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u. 1937.Pp 1938.Cm AuthorizedPrincipalsFile 1939accepts the tokens %%, %h, %U, and %u. 1940.Pp 1941.Cm ChrootDirectory 1942accepts the tokens %%, %h, %U, and %u. 1943.Pp 1944.Cm RoutingDomain 1945accepts the token %D. 1946.Sh FILES 1947.Bl -tag -width Ds 1948.It Pa /etc/ssh/sshd_config 1949Contains configuration data for 1950.Xr sshd 8 . 1951This file should be writable by root only, but it is recommended 1952(though not necessary) that it be world-readable. 1953.El 1954.Sh SEE ALSO 1955.Xr sftp-server 8 , 1956.Xr sshd 8 1957.Sh AUTHORS 1958.An -nosplit 1959OpenSSH is a derivative of the original and free 1960ssh 1.2.12 release by 1961.An Tatu Ylonen . 1962.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , 1963.An Theo de Raadt 1964and 1965.An Dug Song 1966removed many bugs, re-added newer features and 1967created OpenSSH. 1968.An Markus Friedl 1969contributed the support for SSH protocol versions 1.5 and 2.0. 1970.An Niels Provos 1971and 1972.An Markus Friedl 1973contributed support for privilege separation. 1974