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 1.331 2021/04/04 11:36:56 jmc Exp $ 37.Dd $Mdocdate: April 4 2021 $ 38.Dt SSHD_CONFIG 5 39.Os 40.Sh NAME 41.Nm sshd_config 42.Nd OpenSSH daemon configuration file 43.Sh DESCRIPTION 44.Xr sshd 8 45reads configuration data from 46.Pa /etc/ssh/sshd_config 47(or the file specified with 48.Fl f 49on the command line). 50The file contains keyword-argument pairs, one per line. 51For each keyword, the first obtained value will be used. 52Lines starting with 53.Ql # 54and empty lines are interpreted as comments. 55Arguments may optionally be enclosed in double quotes 56.Pq \&" 57in order to represent arguments containing spaces. 58.Pp 59The possible 60keywords and their meanings are as follows (note that 61keywords are case-insensitive and arguments are case-sensitive): 62.Bl -tag -width Ds 63.It Cm AcceptEnv 64Specifies what environment variables sent by the client will be copied into 65the session's 66.Xr environ 7 . 67See 68.Cm SendEnv 69and 70.Cm SetEnv 71in 72.Xr ssh_config 5 73for how to configure the client. 74The 75.Ev TERM 76environment variable is always accepted whenever the client 77requests a pseudo-terminal as it is required by the protocol. 78Variables are specified by name, which may contain the wildcard characters 79.Ql * 80and 81.Ql \&? . 82Multiple environment variables may be separated by whitespace or spread 83across multiple 84.Cm AcceptEnv 85directives. 86Be warned that some environment variables could be used to bypass restricted 87user environments. 88For this reason, care should be taken in the use of this directive. 89The default is not to accept any environment variables. 90.It Cm AddressFamily 91Specifies which address family should be used by 92.Xr sshd 8 . 93Valid arguments are 94.Cm any 95(the default), 96.Cm inet 97(use IPv4 only), or 98.Cm inet6 99(use IPv6 only). 100.It Cm AllowAgentForwarding 101Specifies whether 102.Xr ssh-agent 1 103forwarding is permitted. 104The default is 105.Cm yes . 106Note that disabling agent forwarding does not improve security 107unless users are also denied shell access, as they can always install 108their own forwarders. 109.It Cm AllowGroups 110This keyword can be followed by a list of group name patterns, separated 111by spaces. 112If specified, login is allowed only for users whose primary 113group or supplementary group list matches one of the patterns. 114Only group names are valid; a numerical group ID is not recognized. 115By default, login is allowed for all groups. 116The allow/deny groups directives are processed in the following order: 117.Cm DenyGroups , 118.Cm AllowGroups . 119.Pp 120See PATTERNS in 121.Xr ssh_config 5 122for more information on patterns. 123.It Cm AllowStreamLocalForwarding 124Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. 125The available options are 126.Cm yes 127(the default) 128or 129.Cm all 130to allow StreamLocal forwarding, 131.Cm no 132to prevent all StreamLocal forwarding, 133.Cm local 134to allow local (from the perspective of 135.Xr ssh 1 ) 136forwarding only or 137.Cm remote 138to allow remote forwarding only. 139Note that disabling StreamLocal forwarding does not improve security unless 140users are also denied shell access, as they can always install their 141own forwarders. 142.It Cm AllowTcpForwarding 143Specifies whether TCP forwarding is permitted. 144The available options are 145.Cm yes 146(the default) 147or 148.Cm all 149to allow TCP forwarding, 150.Cm no 151to prevent all TCP forwarding, 152.Cm local 153to allow local (from the perspective of 154.Xr ssh 1 ) 155forwarding only or 156.Cm remote 157to allow remote forwarding only. 158Note that disabling TCP forwarding does not improve security unless 159users are also denied shell access, as they can always install their 160own forwarders. 161.It Cm AllowUsers 162This keyword can be followed by a list of user name patterns, separated 163by spaces. 164If specified, login is allowed only for user names that 165match one of the patterns. 166Only user names are valid; a numerical user ID is not recognized. 167By default, login is allowed for all users. 168If the pattern takes the form USER@HOST then USER and HOST 169are separately checked, restricting logins to particular 170users from particular hosts. 171HOST criteria may additionally contain addresses to match in CIDR 172address/masklen format. 173The allow/deny users directives are processed in the following order: 174.Cm DenyUsers , 175.Cm AllowUsers . 176.Pp 177See PATTERNS in 178.Xr ssh_config 5 179for more information on patterns. 180.It Cm AuthenticationMethods 181Specifies the authentication methods that must be successfully completed 182for a user to be granted access. 183This option must be followed by one or more lists of comma-separated 184authentication method names, or by the single string 185.Cm any 186to indicate the default behaviour of accepting any single authentication 187method. 188If the default is overridden, then successful authentication requires 189completion of every method in at least one of these lists. 190.Pp 191For example, 192.Qq publickey,password publickey,keyboard-interactive 193would require the user to complete public key authentication, followed by 194either password or keyboard interactive authentication. 195Only methods that are next in one or more lists are offered at each stage, 196so for this example it would not be possible to attempt password or 197keyboard-interactive authentication before public key. 198.Pp 199For keyboard interactive authentication it is also possible to 200restrict authentication to a specific device by appending a 201colon followed by the device identifier 202.Cm bsdauth , 203.Cm pam , 204or 205.Cm skey , 206depending on the server configuration. 207For example, 208.Qq keyboard-interactive:bsdauth 209would restrict keyboard interactive authentication to the 210.Cm bsdauth 211device. 212.Pp 213If the publickey method is listed more than once, 214.Xr sshd 8 215verifies that keys that have been used successfully are not reused for 216subsequent authentications. 217For example, 218.Qq publickey,publickey 219requires successful authentication using two different public keys. 220.Pp 221Note that each authentication method listed should also be explicitly enabled 222in the configuration. 223.Pp 224The available authentication methods are: 225.Qq gssapi-with-mic , 226.Qq hostbased , 227.Qq keyboard-interactive , 228.Qq none 229(used for access to password-less accounts when 230.Cm PermitEmptyPasswords 231is enabled), 232.Qq password 233and 234.Qq publickey . 235.It Cm AuthorizedKeysCommand 236Specifies a program to be used to look up the user's public keys. 237The program must be owned by root, not writable by group or others and 238specified by an absolute path. 239Arguments to 240.Cm AuthorizedKeysCommand 241accept the tokens described in the 242.Sx TOKENS 243section. 244If no arguments are specified then the username of the target user is used. 245.Pp 246The program should produce on standard output zero or 247more lines of authorized_keys output (see 248.Sx AUTHORIZED_KEYS 249in 250.Xr sshd 8 ) . 251.Cm AuthorizedKeysCommand 252is tried after the usual 253.Cm AuthorizedKeysFile 254files and will not be executed if a matching key is found there. 255By default, no 256.Cm AuthorizedKeysCommand 257is run. 258.It Cm AuthorizedKeysCommandUser 259Specifies the user under whose account the 260.Cm AuthorizedKeysCommand 261is run. 262It is recommended to use a dedicated user that has no other role on the host 263than running authorized keys commands. 264If 265.Cm AuthorizedKeysCommand 266is specified but 267.Cm AuthorizedKeysCommandUser 268is not, then 269.Xr sshd 8 270will refuse to start. 271.It Cm AuthorizedKeysFile 272Specifies the file that contains the public keys used for user authentication. 273The format is described in the AUTHORIZED_KEYS FILE FORMAT section of 274.Xr sshd 8 . 275Arguments to 276.Cm AuthorizedKeysFile 277accept the tokens described in the 278.Sx TOKENS 279section. 280After expansion, 281.Cm AuthorizedKeysFile 282is taken to be an absolute path or one relative to the user's home 283directory. 284Multiple files may be listed, separated by whitespace. 285Alternately this option may be set to 286.Cm none 287to skip checking for user keys in files. 288The default is 289.Qq .ssh/authorized_keys .ssh/authorized_keys2 . 290.It Cm AuthorizedPrincipalsCommand 291Specifies a program to be used to generate the list of allowed 292certificate principals as per 293.Cm AuthorizedPrincipalsFile . 294The program must be owned by root, not writable by group or others and 295specified by an absolute path. 296Arguments to 297.Cm AuthorizedPrincipalsCommand 298accept the tokens described in the 299.Sx TOKENS 300section. 301If no arguments are specified then the username of the target user is used. 302.Pp 303The program should produce on standard output zero or 304more lines of 305.Cm AuthorizedPrincipalsFile 306output. 307If either 308.Cm AuthorizedPrincipalsCommand 309or 310.Cm AuthorizedPrincipalsFile 311is specified, then certificates offered by the client for authentication 312must contain a principal that is listed. 313By default, no 314.Cm AuthorizedPrincipalsCommand 315is run. 316.It Cm AuthorizedPrincipalsCommandUser 317Specifies the user under whose account the 318.Cm AuthorizedPrincipalsCommand 319is run. 320It is recommended to use a dedicated user that has no other role on the host 321than running authorized principals commands. 322If 323.Cm AuthorizedPrincipalsCommand 324is specified but 325.Cm AuthorizedPrincipalsCommandUser 326is not, then 327.Xr sshd 8 328will refuse to start. 329.It Cm AuthorizedPrincipalsFile 330Specifies a file that lists principal names that are accepted for 331certificate authentication. 332When using certificates signed by a key listed in 333.Cm TrustedUserCAKeys , 334this file lists names, one of which must appear in the certificate for it 335to be accepted for authentication. 336Names are listed one per line preceded by key options (as described in 337.Sx AUTHORIZED_KEYS FILE FORMAT 338in 339.Xr sshd 8 ) . 340Empty lines and comments starting with 341.Ql # 342are ignored. 343.Pp 344Arguments to 345.Cm AuthorizedPrincipalsFile 346accept the tokens described in the 347.Sx TOKENS 348section. 349After expansion, 350.Cm AuthorizedPrincipalsFile 351is taken to be an absolute path or one relative to the user's home directory. 352The default is 353.Cm none , 354i.e. not to use a principals file \(en in this case, the username 355of the user must appear in a certificate's principals list for it to be 356accepted. 357.Pp 358Note that 359.Cm AuthorizedPrincipalsFile 360is only used when authentication proceeds using a CA listed in 361.Cm TrustedUserCAKeys 362and is not consulted for certification authorities trusted via 363.Pa ~/.ssh/authorized_keys , 364though the 365.Cm principals= 366key option offers a similar facility (see 367.Xr sshd 8 368for details). 369.It Cm Banner 370The contents of the specified file are sent to the remote user before 371authentication is allowed. 372If the argument is 373.Cm none 374then no banner is displayed. 375By default, no banner is displayed. 376.It Cm CASignatureAlgorithms 377Specifies which algorithms are allowed for signing of certificates 378by certificate authorities (CAs). 379The default is: 380.Bd -literal -offset indent 381ssh-ed25519,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 382sk-ssh-ed25519@openssh.com,sk-ecdsa-sha2-nistp256@openssh.com, 383rsa-sha2-512,rsa-sha2-256 384.Ed 385.Pp 386Certificates signed using other algorithms will not be accepted for 387public key or host-based authentication. 388.It Cm ChallengeResponseAuthentication 389Specifies whether challenge-response authentication is allowed. 390All authentication styles from 391.Xr login.conf 5 392are supported. 393The default is 394.Cm yes . 395.It Cm ChrootDirectory 396Specifies the pathname of a directory to 397.Xr chroot 2 398to after authentication. 399At session startup 400.Xr sshd 8 401checks that all components of the pathname are root-owned directories 402which are not writable by any other user or group. 403After the chroot, 404.Xr sshd 8 405changes the working directory to the user's home directory. 406Arguments to 407.Cm ChrootDirectory 408accept the tokens described in the 409.Sx TOKENS 410section. 411.Pp 412The 413.Cm ChrootDirectory 414must contain the necessary files and directories to support the 415user's session. 416For an interactive session this requires at least a shell, typically 417.Xr sh 1 , 418and basic 419.Pa /dev 420nodes such as 421.Xr null 4 , 422.Xr zero 4 , 423.Xr stdin 4 , 424.Xr stdout 4 , 425.Xr stderr 4 , 426and 427.Xr tty 4 428devices. 429For file transfer sessions using SFTP 430no additional configuration of the environment is necessary if the in-process 431sftp-server is used, 432though sessions which use logging may require 433.Pa /dev/log 434inside the chroot directory on some operating systems (see 435.Xr sftp-server 8 436for details). 437.Pp 438For safety, it is very important that the directory hierarchy be 439prevented from modification by other processes on the system (especially 440those outside the jail). 441Misconfiguration can lead to unsafe environments which 442.Xr sshd 8 443cannot detect. 444.Pp 445The default is 446.Cm none , 447indicating not to 448.Xr chroot 2 . 449.It Cm Ciphers 450Specifies the ciphers allowed. 451Multiple ciphers must be comma-separated. 452If the specified list begins with a 453.Sq + 454character, then the specified ciphers will be appended to the default set 455instead of replacing them. 456If the specified list begins with a 457.Sq - 458character, then the specified ciphers (including wildcards) will be removed 459from the default set instead of replacing them. 460If the specified list begins with a 461.Sq ^ 462character, then the specified ciphers will be placed at the head of the 463default set. 464.Pp 465The supported ciphers are: 466.Pp 467.Bl -item -compact -offset indent 468.It 4693des-cbc 470.It 471aes128-cbc 472.It 473aes192-cbc 474.It 475aes256-cbc 476.It 477aes128-ctr 478.It 479aes192-ctr 480.It 481aes256-ctr 482.It 483aes128-gcm@openssh.com 484.It 485aes256-gcm@openssh.com 486.It 487chacha20-poly1305@openssh.com 488.El 489.Pp 490The default is: 491.Bd -literal -offset indent 492chacha20-poly1305@openssh.com, 493aes128-ctr,aes192-ctr,aes256-ctr, 494aes128-gcm@openssh.com,aes256-gcm@openssh.com 495.Ed 496.Pp 497The list of available ciphers may also be obtained using 498.Qq ssh -Q cipher . 499.It Cm ClientAliveCountMax 500Sets the number of client alive messages which may be sent without 501.Xr sshd 8 502receiving any messages back from the client. 503If this threshold is reached while client alive messages are being sent, 504sshd will disconnect the client, terminating the session. 505It is important to note that the use of client alive messages is very 506different from 507.Cm TCPKeepAlive . 508The client alive messages are sent through the encrypted channel 509and therefore will not be spoofable. 510The TCP keepalive option enabled by 511.Cm TCPKeepAlive 512is spoofable. 513The client alive mechanism is valuable when the client or 514server depend on knowing when a connection has become unresponsive. 515.Pp 516The default value is 3. 517If 518.Cm ClientAliveInterval 519is set to 15, and 520.Cm ClientAliveCountMax 521is left at the default, unresponsive SSH clients 522will be disconnected after approximately 45 seconds. 523Setting a zero 524.Cm ClientAliveCountMax 525disables connection termination. 526.It Cm ClientAliveInterval 527Sets a timeout interval in seconds after which if no data has been received 528from the client, 529.Xr sshd 8 530will send a message through the encrypted 531channel to request a response from the client. 532The default 533is 0, indicating that these messages will not be sent to the client. 534.It Cm Compression 535Specifies whether compression is enabled after 536the user has authenticated successfully. 537The argument must be 538.Cm yes , 539.Cm delayed 540(a legacy synonym for 541.Cm yes ) 542or 543.Cm no . 544The default is 545.Cm yes . 546.It Cm DenyGroups 547This keyword can be followed by a list of group name patterns, separated 548by spaces. 549Login is disallowed for users whose primary group or supplementary 550group list matches one of the patterns. 551Only group names are valid; a numerical group ID is not recognized. 552By default, login is allowed for all groups. 553The allow/deny groups directives are processed in the following order: 554.Cm DenyGroups , 555.Cm AllowGroups . 556.Pp 557See PATTERNS in 558.Xr ssh_config 5 559for more information on patterns. 560.It Cm DenyUsers 561This keyword can be followed by a list of user name patterns, separated 562by spaces. 563Login is disallowed for user names that match one of the patterns. 564Only user names are valid; a numerical user ID is not recognized. 565By default, login is allowed for all users. 566If the pattern takes the form USER@HOST then USER and HOST 567are separately checked, restricting logins to particular 568users from particular hosts. 569HOST criteria may additionally contain addresses to match in CIDR 570address/masklen format. 571The allow/deny users directives are processed in the following order: 572.Cm DenyUsers , 573.Cm AllowUsers . 574.Pp 575See PATTERNS in 576.Xr ssh_config 5 577for more information on patterns. 578.It Cm DisableForwarding 579Disables all forwarding features, including X11, 580.Xr ssh-agent 1 , 581TCP and StreamLocal. 582This option overrides all other forwarding-related options and may 583simplify restricted configurations. 584.It Cm ExposeAuthInfo 585Writes a temporary file containing a list of authentication methods and 586public credentials (e.g. keys) used to authenticate the user. 587The location of the file is exposed to the user session through the 588.Ev SSH_USER_AUTH 589environment variable. 590The default is 591.Cm no . 592.It Cm FingerprintHash 593Specifies the hash algorithm used when logging key fingerprints. 594Valid options are: 595.Cm md5 596and 597.Cm sha256 . 598The default is 599.Cm sha256 . 600.It Cm ForceCommand 601Forces the execution of the command specified by 602.Cm ForceCommand , 603ignoring any command supplied by the client and 604.Pa ~/.ssh/rc 605if present. 606The command is invoked by using the user's login shell with the -c option. 607This applies to shell, command, or subsystem execution. 608It is most useful inside a 609.Cm Match 610block. 611The command originally supplied by the client is available in the 612.Ev SSH_ORIGINAL_COMMAND 613environment variable. 614Specifying a command of 615.Cm internal-sftp 616will force the use of an in-process SFTP server that requires no support 617files when used with 618.Cm ChrootDirectory . 619The default is 620.Cm none . 621.It Cm GatewayPorts 622Specifies whether remote hosts are allowed to connect to ports 623forwarded for the client. 624By default, 625.Xr sshd 8 626binds remote port forwardings to the loopback address. 627This prevents other remote hosts from connecting to forwarded ports. 628.Cm GatewayPorts 629can be used to specify that sshd 630should allow remote port forwardings to bind to non-loopback addresses, thus 631allowing other hosts to connect. 632The argument may be 633.Cm no 634to force remote port forwardings to be available to the local host only, 635.Cm yes 636to force remote port forwardings to bind to the wildcard address, or 637.Cm clientspecified 638to allow the client to select the address to which the forwarding is bound. 639The default is 640.Cm no . 641.It Cm GSSAPIAuthentication 642Specifies whether user authentication based on GSSAPI is allowed. 643The default is 644.Cm no . 645.It Cm GSSAPICleanupCredentials 646Specifies whether to automatically destroy the user's credentials cache 647on logout. 648The default is 649.Cm yes . 650.It Cm GSSAPIStrictAcceptorCheck 651Determines whether to be strict about the identity of the GSSAPI acceptor 652a client authenticates against. 653If set to 654.Cm yes 655then the client must authenticate against the host 656service on the current hostname. 657If set to 658.Cm no 659then the client may authenticate against any service key stored in the 660machine's default store. 661This facility is provided to assist with operation on multi homed machines. 662The default is 663.Cm yes . 664.It Cm HostbasedAcceptedAlgorithms 665Specifies the signature algorithms that will be accepted for hostbased 666authentication as a list of comma-separated patterns. 667Alternately if the specified list begins with a 668.Sq + 669character, then the specified signature algorithms will be appended to 670the default set instead of replacing them. 671If the specified list begins with a 672.Sq - 673character, then the specified signature algorithms (including wildcards) 674will be removed from the default set instead of replacing them. 675If the specified list begins with a 676.Sq ^ 677character, then the specified signature algorithms will be placed at 678the head of the default set. 679The default for this option is: 680.Bd -literal -offset 3n 681ssh-ed25519-cert-v01@openssh.com, 682ecdsa-sha2-nistp256-cert-v01@openssh.com, 683ecdsa-sha2-nistp384-cert-v01@openssh.com, 684ecdsa-sha2-nistp521-cert-v01@openssh.com, 685sk-ssh-ed25519-cert-v01@openssh.com, 686sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 687rsa-sha2-512-cert-v01@openssh.com, 688rsa-sha2-256-cert-v01@openssh.com, 689ssh-rsa-cert-v01@openssh.com, 690ssh-ed25519, 691ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 692sk-ssh-ed25519@openssh.com, 693sk-ecdsa-sha2-nistp256@openssh.com, 694rsa-sha2-512,rsa-sha2-256,ssh-rsa 695.Ed 696.Pp 697The list of available signature algorithms may also be obtained using 698.Qq ssh -Q HostbasedAcceptedAlgorithms . 699This was formerly named HostbasedAcceptedKeyTypes. 700.It Cm HostbasedAuthentication 701Specifies whether rhosts or /etc/hosts.equiv authentication together 702with successful public key client host authentication is allowed 703(host-based authentication). 704The default is 705.Cm no . 706.It Cm HostbasedUsesNameFromPacketOnly 707Specifies whether or not the server will attempt to perform a reverse 708name lookup when matching the name in the 709.Pa ~/.shosts , 710.Pa ~/.rhosts , 711and 712.Pa /etc/hosts.equiv 713files during 714.Cm HostbasedAuthentication . 715A setting of 716.Cm yes 717means that 718.Xr sshd 8 719uses the name supplied by the client rather than 720attempting to resolve the name from the TCP connection itself. 721The default is 722.Cm no . 723.It Cm HostCertificate 724Specifies a file containing a public host certificate. 725The certificate's public key must match a private host key already specified 726by 727.Cm HostKey . 728The default behaviour of 729.Xr sshd 8 730is not to load any certificates. 731.It Cm HostKey 732Specifies a file containing a private host key 733used by SSH. 734The defaults are 735.Pa /etc/ssh/ssh_host_ecdsa_key , 736.Pa /etc/ssh/ssh_host_ed25519_key 737and 738.Pa /etc/ssh/ssh_host_rsa_key . 739.Pp 740Note that 741.Xr sshd 8 742will refuse to use a file if it is group/world-accessible 743and that the 744.Cm HostKeyAlgorithms 745option restricts which of the keys are actually used by 746.Xr sshd 8 . 747.Pp 748It is possible to have multiple host key files. 749It is also possible to specify public host key files instead. 750In this case operations on the private key will be delegated 751to an 752.Xr ssh-agent 1 . 753.It Cm HostKeyAgent 754Identifies the UNIX-domain socket used to communicate 755with an agent that has access to the private host keys. 756If the string 757.Qq SSH_AUTH_SOCK 758is specified, the location of the socket will be read from the 759.Ev SSH_AUTH_SOCK 760environment variable. 761.It Cm HostKeyAlgorithms 762Specifies the host key signature algorithms 763that the server offers. 764The default for this option is: 765.Bd -literal -offset 3n 766ssh-ed25519-cert-v01@openssh.com, 767ecdsa-sha2-nistp256-cert-v01@openssh.com, 768ecdsa-sha2-nistp384-cert-v01@openssh.com, 769ecdsa-sha2-nistp521-cert-v01@openssh.com, 770sk-ssh-ed25519-cert-v01@openssh.com, 771sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 772rsa-sha2-512-cert-v01@openssh.com, 773rsa-sha2-256-cert-v01@openssh.com, 774ssh-rsa-cert-v01@openssh.com, 775ssh-ed25519, 776ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 777sk-ssh-ed25519@openssh.com, 778sk-ecdsa-sha2-nistp256@openssh.com, 779rsa-sha2-512,rsa-sha2-256,ssh-rsa 780.Ed 781.Pp 782The list of available signature algorithms may also be obtained using 783.Qq ssh -Q HostKeyAlgorithms . 784.It Cm IgnoreRhosts 785Specifies whether to ignore per-user 786.Pa .rhosts 787and 788.Pa .shosts 789files during 790.Cm HostbasedAuthentication . 791The system-wide 792.Pa /etc/hosts.equiv 793and 794.Pa /etc/shosts.equiv 795are still used regardless of this setting. 796.Pp 797Accepted values are 798.Cm yes 799(the default) to ignore all per-user files, 800.Cm shosts-only 801to allow the use of 802.Pa .shosts 803but to ignore 804.Pa .rhosts 805or 806.Cm no 807to allow both 808.Pa .shosts 809and 810.Pa rhosts . 811.It Cm IgnoreUserKnownHosts 812Specifies whether 813.Xr sshd 8 814should ignore the user's 815.Pa ~/.ssh/known_hosts 816during 817.Cm HostbasedAuthentication 818and use only the system-wide known hosts file 819.Pa /etc/ssh/known_hosts . 820The default is 821.Dq no . 822.It Cm Include 823Include the specified configuration file(s). 824Multiple pathnames may be specified and each pathname may contain 825.Xr glob 7 826wildcards that will be expanded and processed in lexical order. 827Files without absolute paths are assumed to be in 828.Pa /etc/ssh . 829An 830.Cm Include 831directive may appear inside a 832.Cm Match 833block 834to perform conditional inclusion. 835.It Cm IPQoS 836Specifies the IPv4 type-of-service or DSCP class for the connection. 837Accepted values are 838.Cm af11 , 839.Cm af12 , 840.Cm af13 , 841.Cm af21 , 842.Cm af22 , 843.Cm af23 , 844.Cm af31 , 845.Cm af32 , 846.Cm af33 , 847.Cm af41 , 848.Cm af42 , 849.Cm af43 , 850.Cm cs0 , 851.Cm cs1 , 852.Cm cs2 , 853.Cm cs3 , 854.Cm cs4 , 855.Cm cs5 , 856.Cm cs6 , 857.Cm cs7 , 858.Cm ef , 859.Cm le , 860.Cm lowdelay , 861.Cm throughput , 862.Cm reliability , 863a numeric value, or 864.Cm none 865to use the operating system default. 866This option may take one or two arguments, separated by whitespace. 867If one argument is specified, it is used as the packet class unconditionally. 868If two values are specified, the first is automatically selected for 869interactive sessions and the second for non-interactive sessions. 870The default is 871.Cm af21 872(Low-Latency Data) 873for interactive sessions and 874.Cm cs1 875(Lower Effort) 876for non-interactive sessions. 877.It Cm KbdInteractiveAuthentication 878Specifies whether to allow keyboard-interactive authentication. 879The argument to this keyword must be 880.Cm yes 881or 882.Cm no . 883The default is to use whatever value 884.Cm ChallengeResponseAuthentication 885is set to 886(by default 887.Cm yes ) . 888.It Cm KerberosAuthentication 889Specifies whether the password provided by the user for 890.Cm PasswordAuthentication 891will be validated through the Kerberos KDC. 892To use this option, the server needs a 893Kerberos servtab which allows the verification of the KDC's identity. 894The default is 895.Cm no . 896.It Cm KerberosGetAFSToken 897If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 898an AFS token before accessing the user's home directory. 899The default is 900.Cm no . 901.It Cm KerberosOrLocalPasswd 902If password authentication through Kerberos fails then 903the password will be validated via any additional local mechanism 904such as 905.Pa /etc/passwd . 906The default is 907.Cm yes . 908.It Cm KerberosTicketCleanup 909Specifies whether to automatically destroy the user's ticket cache 910file on logout. 911The default is 912.Cm yes . 913.It Cm KexAlgorithms 914Specifies the available KEX (Key Exchange) algorithms. 915Multiple algorithms must be comma-separated. 916Alternately if the specified list begins with a 917.Sq + 918character, then the specified methods will be appended to the default set 919instead of replacing them. 920If the specified list begins with a 921.Sq - 922character, then the specified methods (including wildcards) will be removed 923from the default set instead of replacing them. 924If the specified list begins with a 925.Sq ^ 926character, then the specified methods will be placed at the head of the 927default set. 928The supported algorithms are: 929.Pp 930.Bl -item -compact -offset indent 931.It 932curve25519-sha256 933.It 934curve25519-sha256@libssh.org 935.It 936diffie-hellman-group1-sha1 937.It 938diffie-hellman-group14-sha1 939.It 940diffie-hellman-group14-sha256 941.It 942diffie-hellman-group16-sha512 943.It 944diffie-hellman-group18-sha512 945.It 946diffie-hellman-group-exchange-sha1 947.It 948diffie-hellman-group-exchange-sha256 949.It 950ecdh-sha2-nistp256 951.It 952ecdh-sha2-nistp384 953.It 954ecdh-sha2-nistp521 955.It 956sntrup761x25519-sha512@openssh.com 957.El 958.Pp 959The default is: 960.Bd -literal -offset indent 961curve25519-sha256,curve25519-sha256@libssh.org, 962ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 963diffie-hellman-group-exchange-sha256, 964diffie-hellman-group16-sha512,diffie-hellman-group18-sha512, 965diffie-hellman-group14-sha256 966.Ed 967.Pp 968The list of available key exchange algorithms may also be obtained using 969.Qq ssh -Q KexAlgorithms . 970.It Cm ListenAddress 971Specifies the local addresses 972.Xr sshd 8 973should listen on. 974The following forms may be used: 975.Pp 976.Bl -item -offset indent -compact 977.It 978.Cm ListenAddress 979.Sm off 980.Ar hostname | address 981.Sm on 982.Op Cm rdomain Ar domain 983.It 984.Cm ListenAddress 985.Sm off 986.Ar hostname : port 987.Sm on 988.Op Cm rdomain Ar domain 989.It 990.Cm ListenAddress 991.Sm off 992.Ar IPv4_address : port 993.Sm on 994.Op Cm rdomain Ar domain 995.It 996.Cm ListenAddress 997.Sm off 998.Oo Ar hostname | address Oc : Ar port 999.Sm on 1000.Op Cm rdomain Ar domain 1001.El 1002.Pp 1003The optional 1004.Cm rdomain 1005qualifier requests 1006.Xr sshd 8 1007listen in an explicit routing domain. 1008If 1009.Ar port 1010is not specified, 1011sshd will listen on the address and all 1012.Cm Port 1013options specified. 1014The default is to listen on all local addresses on the current default 1015routing domain. 1016Multiple 1017.Cm ListenAddress 1018options are permitted. 1019For more information on routing domains, see 1020.Xr rdomain 4 . 1021.It Cm LoginGraceTime 1022The server disconnects after this time if the user has not 1023successfully logged in. 1024If the value is 0, there is no time limit. 1025The default is 120 seconds. 1026.It Cm LogLevel 1027Gives the verbosity level that is used when logging messages from 1028.Xr sshd 8 . 1029The possible values are: 1030QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1031The default is INFO. 1032DEBUG and DEBUG1 are equivalent. 1033DEBUG2 and DEBUG3 each specify higher levels of debugging output. 1034Logging with a DEBUG level violates the privacy of users and is not recommended. 1035.It Cm LogVerbose 1036Specify one or more overrides to LogLevel. 1037An override consists of a pattern lists that matches the source file, function 1038and line number to force detailed logging for. 1039For example, an override pattern of: 1040.Bd -literal -offset indent 1041kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1042.Ed 1043.Pp 1044would enable detailed logging for line 1000 of 1045.Pa kex.c , 1046everything in the 1047.Fn kex_exchange_identification 1048function, and all code in the 1049.Pa packet.c 1050file. 1051This option is intended for debugging and no overrides are enabled by default. 1052.It Cm MACs 1053Specifies the available MAC (message authentication code) algorithms. 1054The MAC algorithm is used for data integrity protection. 1055Multiple algorithms must be comma-separated. 1056If the specified list begins with a 1057.Sq + 1058character, then the specified algorithms will be appended to the default set 1059instead of replacing them. 1060If the specified list begins with a 1061.Sq - 1062character, then the specified algorithms (including wildcards) will be removed 1063from the default set instead of replacing them. 1064If the specified list begins with a 1065.Sq ^ 1066character, then the specified algorithms will be placed at the head of the 1067default set. 1068.Pp 1069The algorithms that contain 1070.Qq -etm 1071calculate the MAC after encryption (encrypt-then-mac). 1072These are considered safer and their use recommended. 1073The supported MACs are: 1074.Pp 1075.Bl -item -compact -offset indent 1076.It 1077hmac-md5 1078.It 1079hmac-md5-96 1080.It 1081hmac-sha1 1082.It 1083hmac-sha1-96 1084.It 1085hmac-sha2-256 1086.It 1087hmac-sha2-512 1088.It 1089umac-64@openssh.com 1090.It 1091umac-128@openssh.com 1092.It 1093hmac-md5-etm@openssh.com 1094.It 1095hmac-md5-96-etm@openssh.com 1096.It 1097hmac-sha1-etm@openssh.com 1098.It 1099hmac-sha1-96-etm@openssh.com 1100.It 1101hmac-sha2-256-etm@openssh.com 1102.It 1103hmac-sha2-512-etm@openssh.com 1104.It 1105umac-64-etm@openssh.com 1106.It 1107umac-128-etm@openssh.com 1108.El 1109.Pp 1110The default is: 1111.Bd -literal -offset indent 1112umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1113hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1114hmac-sha1-etm@openssh.com, 1115umac-64@openssh.com,umac-128@openssh.com, 1116hmac-sha2-256,hmac-sha2-512,hmac-sha1 1117.Ed 1118.Pp 1119The list of available MAC algorithms may also be obtained using 1120.Qq ssh -Q mac . 1121.It Cm Match 1122Introduces a conditional block. 1123If all of the criteria on the 1124.Cm Match 1125line are satisfied, the keywords on the following lines override those 1126set in the global section of the config file, until either another 1127.Cm Match 1128line or the end of the file. 1129If a keyword appears in multiple 1130.Cm Match 1131blocks that are satisfied, only the first instance of the keyword is 1132applied. 1133.Pp 1134The arguments to 1135.Cm Match 1136are one or more criteria-pattern pairs or the single token 1137.Cm All 1138which matches all criteria. 1139The available criteria are 1140.Cm User , 1141.Cm Group , 1142.Cm Host , 1143.Cm LocalAddress , 1144.Cm LocalPort , 1145.Cm RDomain , 1146and 1147.Cm Address 1148(with 1149.Cm RDomain 1150representing the 1151.Xr rdomain 4 1152on which the connection was received). 1153.Pp 1154The match patterns may consist of single entries or comma-separated 1155lists and may use the wildcard and negation operators described in the 1156.Sx PATTERNS 1157section of 1158.Xr ssh_config 5 . 1159.Pp 1160The patterns in an 1161.Cm Address 1162criteria may additionally contain addresses to match in CIDR 1163address/masklen format, 1164such as 192.0.2.0/24 or 2001:db8::/32. 1165Note that the mask length provided must be consistent with the address - 1166it is an error to specify a mask length that is too long for the address 1167or one with bits set in this host portion of the address. 1168For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. 1169.Pp 1170Only a subset of keywords may be used on the lines following a 1171.Cm Match 1172keyword. 1173Available keywords are 1174.Cm AcceptEnv , 1175.Cm AllowAgentForwarding , 1176.Cm AllowGroups , 1177.Cm AllowStreamLocalForwarding , 1178.Cm AllowTcpForwarding , 1179.Cm AllowUsers , 1180.Cm AuthenticationMethods , 1181.Cm AuthorizedKeysCommand , 1182.Cm AuthorizedKeysCommandUser , 1183.Cm AuthorizedKeysFile , 1184.Cm AuthorizedPrincipalsCommand , 1185.Cm AuthorizedPrincipalsCommandUser , 1186.Cm AuthorizedPrincipalsFile , 1187.Cm Banner , 1188.Cm ChrootDirectory , 1189.Cm ClientAliveCountMax , 1190.Cm ClientAliveInterval , 1191.Cm DenyGroups , 1192.Cm DenyUsers , 1193.Cm DisableForwarding , 1194.Cm ForceCommand , 1195.Cm GatewayPorts , 1196.Cm GSSAPIAuthentication , 1197.Cm HostbasedAcceptedAlgorithms , 1198.Cm HostbasedAuthentication , 1199.Cm HostbasedUsesNameFromPacketOnly , 1200.Cm IgnoreRhosts , 1201.Cm Include , 1202.Cm IPQoS , 1203.Cm KbdInteractiveAuthentication , 1204.Cm KerberosAuthentication , 1205.Cm LogLevel , 1206.Cm MaxAuthTries , 1207.Cm MaxSessions , 1208.Cm PasswordAuthentication , 1209.Cm PermitEmptyPasswords , 1210.Cm PermitListen , 1211.Cm PermitOpen , 1212.Cm PermitRootLogin , 1213.Cm PermitTTY , 1214.Cm PermitTunnel , 1215.Cm PermitUserRC , 1216.Cm PubkeyAcceptedAlgorithms , 1217.Cm PubkeyAuthentication , 1218.Cm RekeyLimit , 1219.Cm RevokedKeys , 1220.Cm RDomain , 1221.Cm SetEnv , 1222.Cm StreamLocalBindMask , 1223.Cm StreamLocalBindUnlink , 1224.Cm TrustedUserCAKeys , 1225.Cm X11DisplayOffset , 1226.Cm X11Forwarding 1227and 1228.Cm X11UseLocalhost . 1229.It Cm MaxAuthTries 1230Specifies the maximum number of authentication attempts permitted per 1231connection. 1232Once the number of failures reaches half this value, 1233additional failures are logged. 1234The default is 6. 1235.It Cm MaxSessions 1236Specifies the maximum number of open shell, login or subsystem (e.g. sftp) 1237sessions permitted per network connection. 1238Multiple sessions may be established by clients that support connection 1239multiplexing. 1240Setting 1241.Cm MaxSessions 1242to 1 will effectively disable session multiplexing, whereas setting it to 0 1243will prevent all shell, login and subsystem sessions while still permitting 1244forwarding. 1245The default is 10. 1246.It Cm MaxStartups 1247Specifies the maximum number of concurrent unauthenticated connections to the 1248SSH daemon. 1249Additional connections will be dropped until authentication succeeds or the 1250.Cm LoginGraceTime 1251expires for a connection. 1252The default is 10:30:100. 1253.Pp 1254Alternatively, random early drop can be enabled by specifying 1255the three colon separated values 1256start:rate:full (e.g. "10:30:60"). 1257.Xr sshd 8 1258will refuse connection attempts with a probability of rate/100 (30%) 1259if there are currently start (10) unauthenticated connections. 1260The probability increases linearly and all connection attempts 1261are refused if the number of unauthenticated connections reaches full (60). 1262.It Cm ModuliFile 1263Specifies the 1264.Xr moduli 5 1265file that contains the Diffie-Hellman groups used for the 1266.Dq diffie-hellman-group-exchange-sha1 1267and 1268.Dq diffie-hellman-group-exchange-sha256 1269key exchange methods. 1270The default is 1271.Pa /etc/moduli . 1272.It Cm PasswordAuthentication 1273Specifies whether password authentication is allowed. 1274The default is 1275.Cm yes . 1276.It Cm PermitEmptyPasswords 1277When password authentication is allowed, it specifies whether the 1278server allows login to accounts with empty password strings. 1279The default is 1280.Cm no . 1281.It Cm PermitListen 1282Specifies the addresses/ports on which a remote TCP port forwarding may listen. 1283The listen specification must be one of the following forms: 1284.Pp 1285.Bl -item -offset indent -compact 1286.It 1287.Cm PermitListen 1288.Sm off 1289.Ar port 1290.Sm on 1291.It 1292.Cm PermitListen 1293.Sm off 1294.Ar host : port 1295.Sm on 1296.El 1297.Pp 1298Multiple permissions may be specified by separating them with whitespace. 1299An argument of 1300.Cm any 1301can be used to remove all restrictions and permit any listen requests. 1302An argument of 1303.Cm none 1304can be used to prohibit all listen requests. 1305The host name may contain wildcards as described in the PATTERNS section in 1306.Xr ssh_config 5 . 1307The wildcard 1308.Sq * 1309can also be used in place of a port number to allow all ports. 1310By default all port forwarding listen requests are permitted. 1311Note that the 1312.Cm GatewayPorts 1313option may further restrict which addresses may be listened on. 1314Note also that 1315.Xr ssh 1 1316will request a listen host of 1317.Dq localhost 1318if no listen host was specifically requested, and this name is 1319treated differently to explicit localhost addresses of 1320.Dq 127.0.0.1 1321and 1322.Dq ::1 . 1323.It Cm PermitOpen 1324Specifies the destinations to which TCP port forwarding is permitted. 1325The forwarding specification must be one of the following forms: 1326.Pp 1327.Bl -item -offset indent -compact 1328.It 1329.Cm PermitOpen 1330.Sm off 1331.Ar host : port 1332.Sm on 1333.It 1334.Cm PermitOpen 1335.Sm off 1336.Ar IPv4_addr : port 1337.Sm on 1338.It 1339.Cm PermitOpen 1340.Sm off 1341.Ar \&[ IPv6_addr \&] : port 1342.Sm on 1343.El 1344.Pp 1345Multiple forwards may be specified by separating them with whitespace. 1346An argument of 1347.Cm any 1348can be used to remove all restrictions and permit any forwarding requests. 1349An argument of 1350.Cm none 1351can be used to prohibit all forwarding requests. 1352The wildcard 1353.Sq * 1354can be used for host or port to allow all hosts or ports respectively. 1355Otherwise, no pattern matching or address lookups are performed on supplied 1356names. 1357By default all port forwarding requests are permitted. 1358.It Cm PermitRootLogin 1359Specifies whether root can log in using 1360.Xr ssh 1 . 1361The argument must be 1362.Cm yes , 1363.Cm prohibit-password , 1364.Cm forced-commands-only , 1365or 1366.Cm no . 1367The default is 1368.Cm prohibit-password . 1369.Pp 1370If this option is set to 1371.Cm prohibit-password 1372(or its deprecated alias, 1373.Cm without-password ) , 1374password and keyboard-interactive authentication are disabled for root. 1375.Pp 1376If this option is set to 1377.Cm forced-commands-only , 1378root login with public key authentication will be allowed, 1379but only if the 1380.Ar command 1381option has been specified 1382(which may be useful for taking remote backups even if root login is 1383normally not allowed). 1384All other authentication methods are disabled for root. 1385.Pp 1386If this option is set to 1387.Cm no , 1388root is not allowed to log in. 1389.It Cm PermitTTY 1390Specifies whether 1391.Xr pty 4 1392allocation is permitted. 1393The default is 1394.Cm yes . 1395.It Cm PermitTunnel 1396Specifies whether 1397.Xr tun 4 1398device forwarding is allowed. 1399The argument must be 1400.Cm yes , 1401.Cm point-to-point 1402(layer 3), 1403.Cm ethernet 1404(layer 2), or 1405.Cm no . 1406Specifying 1407.Cm yes 1408permits both 1409.Cm point-to-point 1410and 1411.Cm ethernet . 1412The default is 1413.Cm no . 1414.Pp 1415Independent of this setting, the permissions of the selected 1416.Xr tun 4 1417device must allow access to the user. 1418.It Cm PermitUserEnvironment 1419Specifies whether 1420.Pa ~/.ssh/environment 1421and 1422.Cm environment= 1423options in 1424.Pa ~/.ssh/authorized_keys 1425are processed by 1426.Xr sshd 8 . 1427Valid options are 1428.Cm yes , 1429.Cm no 1430or a pattern-list specifying which environment variable names to accept 1431(for example 1432.Qq LANG,LC_* ) . 1433The default is 1434.Cm no . 1435Enabling environment processing may enable users to bypass access 1436restrictions in some configurations using mechanisms such as 1437.Ev LD_PRELOAD . 1438.It Cm PermitUserRC 1439Specifies whether any 1440.Pa ~/.ssh/rc 1441file is executed. 1442The default is 1443.Cm yes . 1444.It Cm PerSourceMaxStartups 1445Specifies the number of unauthenticated connections allowed from a 1446given source address, or 1447.Dq none 1448if there is no limit. 1449This limit is applied in addition to 1450.Cm MaxStartups , 1451whichever is lower. 1452The default is 1453.Cm none . 1454.It Cm PerSourceNetBlockSize 1455Specifies the number of bits of source address that are grouped together 1456for the purposes of applying PerSourceMaxStartups limits. 1457Values for IPv4 and optionally IPv6 may be specified, separated by a colon. 1458The default is 1459.Cm 32:128 , 1460which means each address is considered individually. 1461.It Cm PidFile 1462Specifies the file that contains the process ID of the 1463SSH daemon, or 1464.Cm none 1465to not write one. 1466The default is 1467.Pa /var/run/sshd.pid . 1468.It Cm Port 1469Specifies the port number that 1470.Xr sshd 8 1471listens on. 1472The default is 22. 1473Multiple options of this type are permitted. 1474See also 1475.Cm ListenAddress . 1476.It Cm PrintLastLog 1477Specifies whether 1478.Xr sshd 8 1479should print the date and time of the last user login when a user logs 1480in interactively. 1481The default is 1482.Cm yes . 1483.It Cm PrintMotd 1484Specifies whether 1485.Xr sshd 8 1486should print 1487.Pa /etc/motd 1488when a user logs in interactively. 1489(On some systems it is also printed by the shell, 1490.Pa /etc/profile , 1491or equivalent.) 1492The default is 1493.Cm yes . 1494.It Cm PubkeyAcceptedAlgorithms 1495Specifies the signature algorithms that will be accepted for public key 1496authentication as a list of comma-separated patterns. 1497Alternately if the specified list begins with a 1498.Sq + 1499character, then the specified algorithms will be appended to the default set 1500instead of replacing them. 1501If the specified list begins with a 1502.Sq - 1503character, then the specified algorithms (including wildcards) will be removed 1504from the default set instead of replacing them. 1505If the specified list begins with a 1506.Sq ^ 1507character, then the specified algorithms will be placed at the head of the 1508default set. 1509The default for this option is: 1510.Bd -literal -offset 3n 1511ssh-ed25519-cert-v01@openssh.com, 1512ecdsa-sha2-nistp256-cert-v01@openssh.com, 1513ecdsa-sha2-nistp384-cert-v01@openssh.com, 1514ecdsa-sha2-nistp521-cert-v01@openssh.com, 1515sk-ssh-ed25519-cert-v01@openssh.com, 1516sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1517rsa-sha2-512-cert-v01@openssh.com, 1518rsa-sha2-256-cert-v01@openssh.com, 1519ssh-rsa-cert-v01@openssh.com, 1520ssh-ed25519, 1521ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1522sk-ssh-ed25519@openssh.com, 1523sk-ecdsa-sha2-nistp256@openssh.com, 1524rsa-sha2-512,rsa-sha2-256,ssh-rsa 1525.Ed 1526.Pp 1527The list of available signature algorithms may also be obtained using 1528.Qq ssh -Q PubkeyAcceptedAlgorithms . 1529.It Cm PubkeyAuthOptions 1530Sets one or more public key authentication options. 1531The supported keywords are: 1532.Cm none 1533(the default; indicating no additional options are enabled), 1534.Cm touch-required 1535and 1536.Cm verify-required . 1537.Pp 1538The 1539.Cm touch-required 1540option causes public key authentication using a FIDO authenticator algorithm 1541(i.e.\& 1542.Cm ecdsa-sk 1543or 1544.Cm ed25519-sk ) 1545to always require the signature to attest that a physically present user 1546explicitly confirmed the authentication (usually by touching the authenticator). 1547By default, 1548.Xr sshd 8 1549requires user presence unless overridden with an authorized_keys option. 1550The 1551.Cm touch-required 1552flag disables this override. 1553.Pp 1554The 1555.Cm verify-required 1556option requires a FIDO key signature attest that the user was verified, 1557e.g. via a PIN. 1558.Pp 1559Neither the 1560.Cm touch-required 1561or 1562.Cm verify-required 1563options have any effect for other, non-FIDO, public key types. 1564.It Cm PubkeyAuthentication 1565Specifies whether public key authentication is allowed. 1566The default is 1567.Cm yes . 1568.It Cm RekeyLimit 1569Specifies the maximum amount of data that may be transmitted before the 1570session key is renegotiated, optionally followed by a maximum amount of 1571time that may pass before the session key is renegotiated. 1572The first argument is specified in bytes and may have a suffix of 1573.Sq K , 1574.Sq M , 1575or 1576.Sq G 1577to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1578The default is between 1579.Sq 1G 1580and 1581.Sq 4G , 1582depending on the cipher. 1583The optional second value is specified in seconds and may use any of the 1584units documented in the 1585.Sx TIME FORMATS 1586section. 1587The default value for 1588.Cm RekeyLimit 1589is 1590.Cm default none , 1591which means that rekeying is performed after the cipher's default amount 1592of data has been sent or received and no time based rekeying is done. 1593.It Cm RevokedKeys 1594Specifies revoked public keys file, or 1595.Cm none 1596to not use one. 1597Keys listed in this file will be refused for public key authentication. 1598Note that if this file is not readable, then public key authentication will 1599be refused for all users. 1600Keys may be specified as a text file, listing one public key per line, or as 1601an OpenSSH Key Revocation List (KRL) as generated by 1602.Xr ssh-keygen 1 . 1603For more information on KRLs, see the KEY REVOCATION LISTS section in 1604.Xr ssh-keygen 1 . 1605.It Cm RDomain 1606Specifies an explicit routing domain that is applied after authentication 1607has completed. 1608The user session, as well and any forwarded or listening IP sockets, 1609will be bound to this 1610.Xr rdomain 4 . 1611If the routing domain is set to 1612.Cm \&%D , 1613then the domain in which the incoming connection was received will be applied. 1614.It Cm SecurityKeyProvider 1615Specifies a path to a library that will be used when loading 1616FIDO authenticator-hosted keys, overriding the default of using 1617the built-in USB HID support. 1618.It Cm SetEnv 1619Specifies one or more environment variables to set in child sessions started 1620by 1621.Xr sshd 8 1622as 1623.Dq NAME=VALUE . 1624The environment value may be quoted (e.g. if it contains whitespace 1625characters). 1626Environment variables set by 1627.Cm SetEnv 1628override the default environment and any variables specified by the user 1629via 1630.Cm AcceptEnv 1631or 1632.Cm PermitUserEnvironment . 1633.It Cm StreamLocalBindMask 1634Sets the octal file creation mode mask 1635.Pq umask 1636used when creating a Unix-domain socket file for local or remote 1637port forwarding. 1638This option is only used for port forwarding to a Unix-domain socket file. 1639.Pp 1640The default value is 0177, which creates a Unix-domain socket file that is 1641readable and writable only by the owner. 1642Note that not all operating systems honor the file mode on Unix-domain 1643socket files. 1644.It Cm StreamLocalBindUnlink 1645Specifies whether to remove an existing Unix-domain socket file for local 1646or remote port forwarding before creating a new one. 1647If the socket file already exists and 1648.Cm StreamLocalBindUnlink 1649is not enabled, 1650.Nm sshd 1651will be unable to forward the port to the Unix-domain socket file. 1652This option is only used for port forwarding to a Unix-domain socket file. 1653.Pp 1654The argument must be 1655.Cm yes 1656or 1657.Cm no . 1658The default is 1659.Cm no . 1660.It Cm StrictModes 1661Specifies whether 1662.Xr sshd 8 1663should check file modes and ownership of the 1664user's files and home directory before accepting login. 1665This is normally desirable because novices sometimes accidentally leave their 1666directory or files world-writable. 1667The default is 1668.Cm yes . 1669Note that this does not apply to 1670.Cm ChrootDirectory , 1671whose permissions and ownership are checked unconditionally. 1672.It Cm Subsystem 1673Configures an external subsystem (e.g. file transfer daemon). 1674Arguments should be a subsystem name and a command (with optional arguments) 1675to execute upon subsystem request. 1676.Pp 1677The command 1678.Cm sftp-server 1679implements the SFTP file transfer subsystem. 1680.Pp 1681Alternately the name 1682.Cm internal-sftp 1683implements an in-process SFTP server. 1684This may simplify configurations using 1685.Cm ChrootDirectory 1686to force a different filesystem root on clients. 1687.Pp 1688By default no subsystems are defined. 1689.It Cm SyslogFacility 1690Gives the facility code that is used when logging messages from 1691.Xr sshd 8 . 1692The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1693LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1694The default is AUTH. 1695.It Cm TCPKeepAlive 1696Specifies whether the system should send TCP keepalive messages to the 1697other side. 1698If they are sent, death of the connection or crash of one 1699of the machines will be properly noticed. 1700However, this means that 1701connections will die if the route is down temporarily, and some people 1702find it annoying. 1703On the other hand, if TCP keepalives are not sent, 1704sessions may hang indefinitely on the server, leaving 1705.Qq ghost 1706users and consuming server resources. 1707.Pp 1708The default is 1709.Cm yes 1710(to send TCP keepalive messages), and the server will notice 1711if the network goes down or the client host crashes. 1712This avoids infinitely hanging sessions. 1713.Pp 1714To disable TCP keepalive messages, the value should be set to 1715.Cm no . 1716.It Cm TrustedUserCAKeys 1717Specifies a file containing public keys of certificate authorities that are 1718trusted to sign user certificates for authentication, or 1719.Cm none 1720to not use one. 1721Keys are listed one per line; empty lines and comments starting with 1722.Ql # 1723are allowed. 1724If a certificate is presented for authentication and has its signing CA key 1725listed in this file, then it may be used for authentication for any user 1726listed in the certificate's principals list. 1727Note that certificates that lack a list of principals will not be permitted 1728for authentication using 1729.Cm TrustedUserCAKeys . 1730For more details on certificates, see the CERTIFICATES section in 1731.Xr ssh-keygen 1 . 1732.It Cm UseDNS 1733Specifies whether 1734.Xr sshd 8 1735should look up the remote host name, and to check that 1736the resolved host name for the remote IP address maps back to the 1737very same IP address. 1738.Pp 1739If this option is set to 1740.Cm no 1741(the default) then only addresses and not host names may be used in 1742.Pa ~/.ssh/authorized_keys 1743.Cm from 1744and 1745.Nm 1746.Cm Match 1747.Cm Host 1748directives. 1749.It Cm VersionAddendum 1750Optionally specifies additional text to append to the SSH protocol banner 1751sent by the server upon connection. 1752The default is 1753.Cm none . 1754.It Cm X11DisplayOffset 1755Specifies the first display number available for 1756.Xr sshd 8 Ns 's 1757X11 forwarding. 1758This prevents sshd from interfering with real X11 servers. 1759The default is 10. 1760.It Cm X11Forwarding 1761Specifies whether X11 forwarding is permitted. 1762The argument must be 1763.Cm yes 1764or 1765.Cm no . 1766The default is 1767.Cm no . 1768.Pp 1769When X11 forwarding is enabled, there may be additional exposure to 1770the server and to client displays if the 1771.Xr sshd 8 1772proxy display is configured to listen on the wildcard address (see 1773.Cm X11UseLocalhost ) , 1774though this is not the default. 1775Additionally, the authentication spoofing and authentication data 1776verification and substitution occur on the client side. 1777The security risk of using X11 forwarding is that the client's X11 1778display server may be exposed to attack when the SSH client requests 1779forwarding (see the warnings for 1780.Cm ForwardX11 1781in 1782.Xr ssh_config 5 ) . 1783A system administrator may have a stance in which they want to 1784protect clients that may expose themselves to attack by unwittingly 1785requesting X11 forwarding, which can warrant a 1786.Cm no 1787setting. 1788.Pp 1789Note that disabling X11 forwarding does not prevent users from 1790forwarding X11 traffic, as users can always install their own forwarders. 1791.It Cm X11UseLocalhost 1792Specifies whether 1793.Xr sshd 8 1794should bind the X11 forwarding server to the loopback address or to 1795the wildcard address. 1796By default, 1797sshd binds the forwarding server to the loopback address and sets the 1798hostname part of the 1799.Ev DISPLAY 1800environment variable to 1801.Cm localhost . 1802This prevents remote hosts from connecting to the proxy display. 1803However, some older X11 clients may not function with this 1804configuration. 1805.Cm X11UseLocalhost 1806may be set to 1807.Cm no 1808to specify that the forwarding server should be bound to the wildcard 1809address. 1810The argument must be 1811.Cm yes 1812or 1813.Cm no . 1814The default is 1815.Cm yes . 1816.It Cm XAuthLocation 1817Specifies the full pathname of the 1818.Xr xauth 1 1819program, or 1820.Cm none 1821to not use one. 1822The default is 1823.Pa /usr/X11R6/bin/xauth . 1824.El 1825.Sh TIME FORMATS 1826.Xr sshd 8 1827command-line arguments and configuration file options that specify time 1828may be expressed using a sequence of the form: 1829.Sm off 1830.Ar time Op Ar qualifier , 1831.Sm on 1832where 1833.Ar time 1834is a positive integer value and 1835.Ar qualifier 1836is one of the following: 1837.Pp 1838.Bl -tag -width Ds -compact -offset indent 1839.It Aq Cm none 1840seconds 1841.It Cm s | Cm S 1842seconds 1843.It Cm m | Cm M 1844minutes 1845.It Cm h | Cm H 1846hours 1847.It Cm d | Cm D 1848days 1849.It Cm w | Cm W 1850weeks 1851.El 1852.Pp 1853Each member of the sequence is added together to calculate 1854the total time value. 1855.Pp 1856Time format examples: 1857.Pp 1858.Bl -tag -width Ds -compact -offset indent 1859.It 600 1860600 seconds (10 minutes) 1861.It 10m 186210 minutes 1863.It 1h30m 18641 hour 30 minutes (90 minutes) 1865.El 1866.Sh TOKENS 1867Arguments to some keywords can make use of tokens, 1868which are expanded at runtime: 1869.Pp 1870.Bl -tag -width XXXX -offset indent -compact 1871.It %% 1872A literal 1873.Sq % . 1874.It \&%D 1875The routing domain in which the incoming connection was received. 1876.It %F 1877The fingerprint of the CA key. 1878.It %f 1879The fingerprint of the key or certificate. 1880.It %h 1881The home directory of the user. 1882.It %i 1883The key ID in the certificate. 1884.It %K 1885The base64-encoded CA key. 1886.It %k 1887The base64-encoded key or certificate for authentication. 1888.It %s 1889The serial number of the certificate. 1890.It \&%T 1891The type of the CA key. 1892.It %t 1893The key or certificate type. 1894.It \&%U 1895The numeric user ID of the target user. 1896.It %u 1897The username. 1898.El 1899.Pp 1900.Cm AuthorizedKeysCommand 1901accepts the tokens %%, %f, %h, %k, %t, %U, and %u. 1902.Pp 1903.Cm AuthorizedKeysFile 1904accepts the tokens %%, %h, %U, and %u. 1905.Pp 1906.Cm AuthorizedPrincipalsCommand 1907accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u. 1908.Pp 1909.Cm AuthorizedPrincipalsFile 1910accepts the tokens %%, %h, %U, and %u. 1911.Pp 1912.Cm ChrootDirectory 1913accepts the tokens %%, %h, %U, and %u. 1914.Pp 1915.Cm RoutingDomain 1916accepts the token %D. 1917.Sh FILES 1918.Bl -tag -width Ds 1919.It Pa /etc/ssh/sshd_config 1920Contains configuration data for 1921.Xr sshd 8 . 1922This file should be writable by root only, but it is recommended 1923(though not necessary) that it be world-readable. 1924.El 1925.Sh SEE ALSO 1926.Xr sftp-server 8 , 1927.Xr sshd 8 1928.Sh AUTHORS 1929.An -nosplit 1930OpenSSH is a derivative of the original and free 1931ssh 1.2.12 release by 1932.An Tatu Ylonen . 1933.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , 1934.An Theo de Raadt 1935and 1936.An Dug Song 1937removed many bugs, re-added newer features and 1938created OpenSSH. 1939.An Markus Friedl 1940contributed the support for SSH protocol versions 1.5 and 2.0. 1941.An Niels Provos 1942and 1943.An Markus Friedl 1944contributed support for privilege separation. 1945