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 1.347 2023/01/18 06:55:32 jmc Exp $ 37.Dd $Mdocdate: January 18 2023 $ 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, 382ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 383sk-ssh-ed25519@openssh.com, 384sk-ecdsa-sha2-nistp256@openssh.com, 385rsa-sha2-512,rsa-sha2-256 386.Ed 387.Pp 388If the specified list begins with a 389.Sq + 390character, then the specified algorithms will be appended to the default set 391instead of replacing them. 392If the specified list begins with a 393.Sq - 394character, then the specified algorithms (including wildcards) will be removed 395from the default set instead of replacing them. 396.Pp 397Certificates signed using other algorithms will not be accepted for 398public key or host-based authentication. 399.It Cm ChannelTimeout 400Specifies whether and how quickly 401.Xr sshd 8 402should close inactive channels. 403Timeouts are specified as one or more 404.Dq type=interval 405pairs separated by whitespace, where the 406.Dq type 407must be a channel type name (as described in the table below), optionally 408containing wildcard characters. 409.Pp 410The timeout value 411.Dq interval 412is specified in seconds or may use any of the units documented in the 413.Sx TIME FORMATS 414section. 415For example, 416.Dq session:*=5m 417would cause all sessions to terminate after five minutes of inactivity. 418Specifying a zero value disables the inactivity timeout. 419.Pp 420The available channel types include: 421.Bl -tag -width Ds 422.It Cm agent-connection 423Open connections to 424.Xr ssh-agent 1 . 425.It Cm direct-tcpip , Cm direct-streamlocal@openssh.com 426Open TCP or Unix socket (respectively) connections that have 427been established from a 428.Xr ssh 1 429local forwarding, i.e.\& 430.Cm LocalForward 431or 432.Cm DynamicForward . 433.It Cm forwarded-tcpip , Cm forwarded-streamlocal@openssh.com 434Open TCP or Unix socket (respectively) connections that have been 435established to a 436.Xr sshd 8 437listening on behalf of a 438.Xr ssh 1 439remote forwarding, i.e.\& 440.Cm RemoteForward . 441.It Cm session:command 442Command execution sessions. 443.It Cm session:shell 444Interactive shell sessions. 445.It Cm session:subsystem:... 446Subsystem sessions, e.g. for 447.Xr sftp 1 , 448which could be identified as 449.Cm session:subsystem:sftp . 450.It Cm x11-connection 451Open X11 forwarding sessions. 452.El 453.Pp 454Note that in all the above cases, terminating an inactive session does not 455guarantee to remove all resources associated with the session, e.g. shell 456processes or X11 clients relating to the session may continue to execute. 457.Pp 458Moreover, terminating an inactive channel or session does not necessarily 459close the SSH connection, nor does it prevent a client from 460requesting another channel of the same type. 461In particular, expiring an inactive forwarding session does not prevent 462another identical forwarding from being subsequently created. 463See also 464.Cm UnusedConnectionTimeout , 465which may be used in conjunction with this option. 466.Pp 467The default is not to expire channels of any type for inactivity. 468.It Cm ChrootDirectory 469Specifies the pathname of a directory to 470.Xr chroot 2 471to after authentication. 472At session startup 473.Xr sshd 8 474checks that all components of the pathname are root-owned directories 475which are not writable by any other user or group. 476After the chroot, 477.Xr sshd 8 478changes the working directory to the user's home directory. 479Arguments to 480.Cm ChrootDirectory 481accept the tokens described in the 482.Sx TOKENS 483section. 484.Pp 485The 486.Cm ChrootDirectory 487must contain the necessary files and directories to support the 488user's session. 489For an interactive session this requires at least a shell, typically 490.Xr sh 1 , 491and basic 492.Pa /dev 493nodes such as 494.Xr null 4 , 495.Xr zero 4 , 496.Xr stdin 4 , 497.Xr stdout 4 , 498.Xr stderr 4 , 499and 500.Xr tty 4 501devices. 502For file transfer sessions using SFTP 503no additional configuration of the environment is necessary if the in-process 504sftp-server is used, 505though sessions which use logging may require 506.Pa /dev/log 507inside the chroot directory on some operating systems (see 508.Xr sftp-server 8 509for details). 510.Pp 511For safety, it is very important that the directory hierarchy be 512prevented from modification by other processes on the system (especially 513those outside the jail). 514Misconfiguration can lead to unsafe environments which 515.Xr sshd 8 516cannot detect. 517.Pp 518The default is 519.Cm none , 520indicating not to 521.Xr chroot 2 . 522.It Cm Ciphers 523Specifies the ciphers allowed. 524Multiple ciphers must be comma-separated. 525If the specified list begins with a 526.Sq + 527character, then the specified ciphers will be appended to the default set 528instead of replacing them. 529If the specified list begins with a 530.Sq - 531character, then the specified ciphers (including wildcards) will be removed 532from the default set instead of replacing them. 533If the specified list begins with a 534.Sq ^ 535character, then the specified ciphers will be placed at the head of the 536default set. 537.Pp 538The supported ciphers are: 539.Pp 540.Bl -item -compact -offset indent 541.It 5423des-cbc 543.It 544aes128-cbc 545.It 546aes192-cbc 547.It 548aes256-cbc 549.It 550aes128-ctr 551.It 552aes192-ctr 553.It 554aes256-ctr 555.It 556aes128-gcm@openssh.com 557.It 558aes256-gcm@openssh.com 559.It 560chacha20-poly1305@openssh.com 561.El 562.Pp 563The default is: 564.Bd -literal -offset indent 565chacha20-poly1305@openssh.com, 566aes128-ctr,aes192-ctr,aes256-ctr, 567aes128-gcm@openssh.com,aes256-gcm@openssh.com 568.Ed 569.Pp 570The list of available ciphers may also be obtained using 571.Qq ssh -Q cipher . 572.It Cm ClientAliveCountMax 573Sets the number of client alive messages which may be sent without 574.Xr sshd 8 575receiving any messages back from the client. 576If this threshold is reached while client alive messages are being sent, 577sshd will disconnect the client, terminating the session. 578It is important to note that the use of client alive messages is very 579different from 580.Cm TCPKeepAlive . 581The client alive messages are sent through the encrypted channel 582and therefore will not be spoofable. 583The TCP keepalive option enabled by 584.Cm TCPKeepAlive 585is spoofable. 586The client alive mechanism is valuable when the client or 587server depend on knowing when a connection has become unresponsive. 588.Pp 589The default value is 3. 590If 591.Cm ClientAliveInterval 592is set to 15, and 593.Cm ClientAliveCountMax 594is left at the default, unresponsive SSH clients 595will be disconnected after approximately 45 seconds. 596Setting a zero 597.Cm ClientAliveCountMax 598disables connection termination. 599.It Cm ClientAliveInterval 600Sets a timeout interval in seconds after which if no data has been received 601from the client, 602.Xr sshd 8 603will send a message through the encrypted 604channel to request a response from the client. 605The default 606is 0, indicating that these messages will not be sent to the client. 607.It Cm Compression 608Specifies whether compression is enabled after 609the user has authenticated successfully. 610The argument must be 611.Cm yes , 612.Cm delayed 613(a legacy synonym for 614.Cm yes ) 615or 616.Cm no . 617The default is 618.Cm yes . 619.It Cm DenyGroups 620This keyword can be followed by a list of group name patterns, separated 621by spaces. 622Login is disallowed for users whose primary group or supplementary 623group list matches one of the patterns. 624Only group names are valid; a numerical group ID is not recognized. 625By default, login is allowed for all groups. 626The allow/deny groups directives are processed in the following order: 627.Cm DenyGroups , 628.Cm AllowGroups . 629.Pp 630See PATTERNS in 631.Xr ssh_config 5 632for more information on patterns. 633.It Cm DenyUsers 634This keyword can be followed by a list of user name patterns, separated 635by spaces. 636Login is disallowed for user names that match one of the patterns. 637Only user names are valid; a numerical user ID is not recognized. 638By default, login is allowed for all users. 639If the pattern takes the form USER@HOST then USER and HOST 640are separately checked, restricting logins to particular 641users from particular hosts. 642HOST criteria may additionally contain addresses to match in CIDR 643address/masklen format. 644The allow/deny users directives are processed in the following order: 645.Cm DenyUsers , 646.Cm AllowUsers . 647.Pp 648See PATTERNS in 649.Xr ssh_config 5 650for more information on patterns. 651.It Cm DisableForwarding 652Disables all forwarding features, including X11, 653.Xr ssh-agent 1 , 654TCP and StreamLocal. 655This option overrides all other forwarding-related options and may 656simplify restricted configurations. 657.It Cm ExposeAuthInfo 658Writes a temporary file containing a list of authentication methods and 659public credentials (e.g. keys) used to authenticate the user. 660The location of the file is exposed to the user session through the 661.Ev SSH_USER_AUTH 662environment variable. 663The default is 664.Cm no . 665.It Cm FingerprintHash 666Specifies the hash algorithm used when logging key fingerprints. 667Valid options are: 668.Cm md5 669and 670.Cm sha256 . 671The default is 672.Cm sha256 . 673.It Cm ForceCommand 674Forces the execution of the command specified by 675.Cm ForceCommand , 676ignoring any command supplied by the client and 677.Pa ~/.ssh/rc 678if present. 679The command is invoked by using the user's login shell with the -c option. 680This applies to shell, command, or subsystem execution. 681It is most useful inside a 682.Cm Match 683block. 684The command originally supplied by the client is available in the 685.Ev SSH_ORIGINAL_COMMAND 686environment variable. 687Specifying a command of 688.Cm internal-sftp 689will force the use of an in-process SFTP server that requires no support 690files when used with 691.Cm ChrootDirectory . 692The default is 693.Cm none . 694.It Cm GatewayPorts 695Specifies whether remote hosts are allowed to connect to ports 696forwarded for the client. 697By default, 698.Xr sshd 8 699binds remote port forwardings to the loopback address. 700This prevents other remote hosts from connecting to forwarded ports. 701.Cm GatewayPorts 702can be used to specify that sshd 703should allow remote port forwardings to bind to non-loopback addresses, thus 704allowing other hosts to connect. 705The argument may be 706.Cm no 707to force remote port forwardings to be available to the local host only, 708.Cm yes 709to force remote port forwardings to bind to the wildcard address, or 710.Cm clientspecified 711to allow the client to select the address to which the forwarding is bound. 712The default is 713.Cm no . 714.It Cm GSSAPIAuthentication 715Specifies whether user authentication based on GSSAPI is allowed. 716The default is 717.Cm no . 718.It Cm GSSAPICleanupCredentials 719Specifies whether to automatically destroy the user's credentials cache 720on logout. 721The default is 722.Cm yes . 723.It Cm GSSAPIStrictAcceptorCheck 724Determines whether to be strict about the identity of the GSSAPI acceptor 725a client authenticates against. 726If set to 727.Cm yes 728then the client must authenticate against the host 729service on the current hostname. 730If set to 731.Cm no 732then the client may authenticate against any service key stored in the 733machine's default store. 734This facility is provided to assist with operation on multi homed machines. 735The default is 736.Cm yes . 737.It Cm HostbasedAcceptedAlgorithms 738Specifies the signature algorithms that will be accepted for hostbased 739authentication as a list of comma-separated patterns. 740Alternately if the specified list begins with a 741.Sq + 742character, then the specified signature algorithms will be appended to 743the default set instead of replacing them. 744If the specified list begins with a 745.Sq - 746character, then the specified signature algorithms (including wildcards) 747will be removed from the default set instead of replacing them. 748If the specified list begins with a 749.Sq ^ 750character, then the specified signature algorithms will be placed at 751the head of the default set. 752The default for this option is: 753.Bd -literal -offset 3n 754ssh-ed25519-cert-v01@openssh.com, 755ecdsa-sha2-nistp256-cert-v01@openssh.com, 756ecdsa-sha2-nistp384-cert-v01@openssh.com, 757ecdsa-sha2-nistp521-cert-v01@openssh.com, 758sk-ssh-ed25519-cert-v01@openssh.com, 759sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 760rsa-sha2-512-cert-v01@openssh.com, 761rsa-sha2-256-cert-v01@openssh.com, 762ssh-ed25519, 763ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 764sk-ssh-ed25519@openssh.com, 765sk-ecdsa-sha2-nistp256@openssh.com, 766rsa-sha2-512,rsa-sha2-256 767.Ed 768.Pp 769The list of available signature algorithms may also be obtained using 770.Qq ssh -Q HostbasedAcceptedAlgorithms . 771This was formerly named HostbasedAcceptedKeyTypes. 772.It Cm HostbasedAuthentication 773Specifies whether rhosts or /etc/hosts.equiv authentication together 774with successful public key client host authentication is allowed 775(host-based authentication). 776The default is 777.Cm no . 778.It Cm HostbasedUsesNameFromPacketOnly 779Specifies whether or not the server will attempt to perform a reverse 780name lookup when matching the name in the 781.Pa ~/.shosts , 782.Pa ~/.rhosts , 783and 784.Pa /etc/hosts.equiv 785files during 786.Cm HostbasedAuthentication . 787A setting of 788.Cm yes 789means that 790.Xr sshd 8 791uses the name supplied by the client rather than 792attempting to resolve the name from the TCP connection itself. 793The default is 794.Cm no . 795.It Cm HostCertificate 796Specifies a file containing a public host certificate. 797The certificate's public key must match a private host key already specified 798by 799.Cm HostKey . 800The default behaviour of 801.Xr sshd 8 802is not to load any certificates. 803.It Cm HostKey 804Specifies a file containing a private host key 805used by SSH. 806The defaults are 807.Pa /etc/ssh/ssh_host_ecdsa_key , 808.Pa /etc/ssh/ssh_host_ed25519_key 809and 810.Pa /etc/ssh/ssh_host_rsa_key . 811.Pp 812Note that 813.Xr sshd 8 814will refuse to use a file if it is group/world-accessible 815and that the 816.Cm HostKeyAlgorithms 817option restricts which of the keys are actually used by 818.Xr sshd 8 . 819.Pp 820It is possible to have multiple host key files. 821It is also possible to specify public host key files instead. 822In this case operations on the private key will be delegated 823to an 824.Xr ssh-agent 1 . 825.It Cm HostKeyAgent 826Identifies the UNIX-domain socket used to communicate 827with an agent that has access to the private host keys. 828If the string 829.Qq SSH_AUTH_SOCK 830is specified, the location of the socket will be read from the 831.Ev SSH_AUTH_SOCK 832environment variable. 833.It Cm HostKeyAlgorithms 834Specifies the host key signature algorithms 835that the server offers. 836The default for this option is: 837.Bd -literal -offset 3n 838ssh-ed25519-cert-v01@openssh.com, 839ecdsa-sha2-nistp256-cert-v01@openssh.com, 840ecdsa-sha2-nistp384-cert-v01@openssh.com, 841ecdsa-sha2-nistp521-cert-v01@openssh.com, 842sk-ssh-ed25519-cert-v01@openssh.com, 843sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 844rsa-sha2-512-cert-v01@openssh.com, 845rsa-sha2-256-cert-v01@openssh.com, 846ssh-ed25519, 847ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 848sk-ssh-ed25519@openssh.com, 849sk-ecdsa-sha2-nistp256@openssh.com, 850rsa-sha2-512,rsa-sha2-256 851.Ed 852.Pp 853The list of available signature algorithms may also be obtained using 854.Qq ssh -Q HostKeyAlgorithms . 855.It Cm IgnoreRhosts 856Specifies whether to ignore per-user 857.Pa .rhosts 858and 859.Pa .shosts 860files during 861.Cm HostbasedAuthentication . 862The system-wide 863.Pa /etc/hosts.equiv 864and 865.Pa /etc/shosts.equiv 866are still used regardless of this setting. 867.Pp 868Accepted values are 869.Cm yes 870(the default) to ignore all per-user files, 871.Cm shosts-only 872to allow the use of 873.Pa .shosts 874but to ignore 875.Pa .rhosts 876or 877.Cm no 878to allow both 879.Pa .shosts 880and 881.Pa rhosts . 882.It Cm IgnoreUserKnownHosts 883Specifies whether 884.Xr sshd 8 885should ignore the user's 886.Pa ~/.ssh/known_hosts 887during 888.Cm HostbasedAuthentication 889and use only the system-wide known hosts file 890.Pa /etc/ssh/ssh_known_hosts . 891The default is 892.Dq no . 893.It Cm Include 894Include the specified configuration file(s). 895Multiple pathnames may be specified and each pathname may contain 896.Xr glob 7 897wildcards that will be expanded and processed in lexical order. 898Files without absolute paths are assumed to be in 899.Pa /etc/ssh . 900An 901.Cm Include 902directive may appear inside a 903.Cm Match 904block 905to perform conditional inclusion. 906.It Cm IPQoS 907Specifies the IPv4 type-of-service or DSCP class for the connection. 908Accepted values are 909.Cm af11 , 910.Cm af12 , 911.Cm af13 , 912.Cm af21 , 913.Cm af22 , 914.Cm af23 , 915.Cm af31 , 916.Cm af32 , 917.Cm af33 , 918.Cm af41 , 919.Cm af42 , 920.Cm af43 , 921.Cm cs0 , 922.Cm cs1 , 923.Cm cs2 , 924.Cm cs3 , 925.Cm cs4 , 926.Cm cs5 , 927.Cm cs6 , 928.Cm cs7 , 929.Cm ef , 930.Cm le , 931.Cm lowdelay , 932.Cm throughput , 933.Cm reliability , 934a numeric value, or 935.Cm none 936to use the operating system default. 937This option may take one or two arguments, separated by whitespace. 938If one argument is specified, it is used as the packet class unconditionally. 939If two values are specified, the first is automatically selected for 940interactive sessions and the second for non-interactive sessions. 941The default is 942.Cm af21 943(Low-Latency Data) 944for interactive sessions and 945.Cm cs1 946(Lower Effort) 947for non-interactive sessions. 948.It Cm KbdInteractiveAuthentication 949Specifies whether to allow keyboard-interactive authentication. 950All authentication styles from 951.Xr login.conf 5 952are supported. 953The default is 954.Cm yes . 955The argument to this keyword must be 956.Cm yes 957or 958.Cm no . 959.Cm ChallengeResponseAuthentication 960is a deprecated alias for this. 961.It Cm KerberosAuthentication 962Specifies whether the password provided by the user for 963.Cm PasswordAuthentication 964will be validated through the Kerberos KDC. 965To use this option, the server needs a 966Kerberos servtab which allows the verification of the KDC's identity. 967The default is 968.Cm no . 969.It Cm KerberosGetAFSToken 970If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 971an AFS token before accessing the user's home directory. 972The default is 973.Cm no . 974.It Cm KerberosOrLocalPasswd 975If password authentication through Kerberos fails then 976the password will be validated via any additional local mechanism 977such as 978.Pa /etc/passwd . 979The default is 980.Cm yes . 981.It Cm KerberosTicketCleanup 982Specifies whether to automatically destroy the user's ticket cache 983file on logout. 984The default is 985.Cm yes . 986.It Cm KexAlgorithms 987Specifies the available KEX (Key Exchange) algorithms. 988Multiple algorithms must be comma-separated. 989Alternately if the specified list begins with a 990.Sq + 991character, then the specified algorithms will be appended to the default set 992instead of replacing them. 993If the specified list begins with a 994.Sq - 995character, then the specified algorithms (including wildcards) will be removed 996from the default set instead of replacing them. 997If the specified list begins with a 998.Sq ^ 999character, then the specified algorithms will be placed at the head of the 1000default set. 1001The supported algorithms are: 1002.Pp 1003.Bl -item -compact -offset indent 1004.It 1005curve25519-sha256 1006.It 1007curve25519-sha256@libssh.org 1008.It 1009diffie-hellman-group1-sha1 1010.It 1011diffie-hellman-group14-sha1 1012.It 1013diffie-hellman-group14-sha256 1014.It 1015diffie-hellman-group16-sha512 1016.It 1017diffie-hellman-group18-sha512 1018.It 1019diffie-hellman-group-exchange-sha1 1020.It 1021diffie-hellman-group-exchange-sha256 1022.It 1023ecdh-sha2-nistp256 1024.It 1025ecdh-sha2-nistp384 1026.It 1027ecdh-sha2-nistp521 1028.It 1029sntrup761x25519-sha512@openssh.com 1030.El 1031.Pp 1032The default is: 1033.Bd -literal -offset indent 1034sntrup761x25519-sha512@openssh.com, 1035curve25519-sha256,curve25519-sha256@libssh.org, 1036ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1037diffie-hellman-group-exchange-sha256, 1038diffie-hellman-group16-sha512,diffie-hellman-group18-sha512, 1039diffie-hellman-group14-sha256 1040.Ed 1041.Pp 1042The list of available key exchange algorithms may also be obtained using 1043.Qq ssh -Q KexAlgorithms . 1044.It Cm ListenAddress 1045Specifies the local addresses 1046.Xr sshd 8 1047should listen on. 1048The following forms may be used: 1049.Pp 1050.Bl -item -offset indent -compact 1051.It 1052.Cm ListenAddress 1053.Sm off 1054.Ar hostname | address 1055.Sm on 1056.Op Cm rdomain Ar domain 1057.It 1058.Cm ListenAddress 1059.Sm off 1060.Ar hostname : port 1061.Sm on 1062.Op Cm rdomain Ar domain 1063.It 1064.Cm ListenAddress 1065.Sm off 1066.Ar IPv4_address : port 1067.Sm on 1068.Op Cm rdomain Ar domain 1069.It 1070.Cm ListenAddress 1071.Sm off 1072.Oo Ar hostname | address Oc : Ar port 1073.Sm on 1074.Op Cm rdomain Ar domain 1075.El 1076.Pp 1077The optional 1078.Cm rdomain 1079qualifier requests 1080.Xr sshd 8 1081listen in an explicit routing domain. 1082If 1083.Ar port 1084is not specified, 1085sshd will listen on the address and all 1086.Cm Port 1087options specified. 1088The default is to listen on all local addresses on the current default 1089routing domain. 1090Multiple 1091.Cm ListenAddress 1092options are permitted. 1093For more information on routing domains, see 1094.Xr rdomain 4 . 1095.It Cm LoginGraceTime 1096The server disconnects after this time if the user has not 1097successfully logged in. 1098If the value is 0, there is no time limit. 1099The default is 120 seconds. 1100.It Cm LogLevel 1101Gives the verbosity level that is used when logging messages from 1102.Xr sshd 8 . 1103The possible values are: 1104QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1105The default is INFO. 1106DEBUG and DEBUG1 are equivalent. 1107DEBUG2 and DEBUG3 each specify higher levels of debugging output. 1108Logging with a DEBUG level violates the privacy of users and is not recommended. 1109.It Cm LogVerbose 1110Specify one or more overrides to LogLevel. 1111An override consists of a pattern lists that matches the source file, function 1112and line number to force detailed logging for. 1113For example, an override pattern of: 1114.Bd -literal -offset indent 1115kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1116.Ed 1117.Pp 1118would enable detailed logging for line 1000 of 1119.Pa kex.c , 1120everything in the 1121.Fn kex_exchange_identification 1122function, and all code in the 1123.Pa packet.c 1124file. 1125This option is intended for debugging and no overrides are enabled by default. 1126.It Cm MACs 1127Specifies the available MAC (message authentication code) algorithms. 1128The MAC algorithm is used for data integrity protection. 1129Multiple algorithms must be comma-separated. 1130If the specified list begins with a 1131.Sq + 1132character, then the specified algorithms will be appended to the default set 1133instead of replacing them. 1134If the specified list begins with a 1135.Sq - 1136character, then the specified algorithms (including wildcards) will be removed 1137from the default set instead of replacing them. 1138If the specified list begins with a 1139.Sq ^ 1140character, then the specified algorithms will be placed at the head of the 1141default set. 1142.Pp 1143The algorithms that contain 1144.Qq -etm 1145calculate the MAC after encryption (encrypt-then-mac). 1146These are considered safer and their use recommended. 1147The supported MACs are: 1148.Pp 1149.Bl -item -compact -offset indent 1150.It 1151hmac-md5 1152.It 1153hmac-md5-96 1154.It 1155hmac-sha1 1156.It 1157hmac-sha1-96 1158.It 1159hmac-sha2-256 1160.It 1161hmac-sha2-512 1162.It 1163umac-64@openssh.com 1164.It 1165umac-128@openssh.com 1166.It 1167hmac-md5-etm@openssh.com 1168.It 1169hmac-md5-96-etm@openssh.com 1170.It 1171hmac-sha1-etm@openssh.com 1172.It 1173hmac-sha1-96-etm@openssh.com 1174.It 1175hmac-sha2-256-etm@openssh.com 1176.It 1177hmac-sha2-512-etm@openssh.com 1178.It 1179umac-64-etm@openssh.com 1180.It 1181umac-128-etm@openssh.com 1182.El 1183.Pp 1184The default is: 1185.Bd -literal -offset indent 1186umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1187hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1188hmac-sha1-etm@openssh.com, 1189umac-64@openssh.com,umac-128@openssh.com, 1190hmac-sha2-256,hmac-sha2-512,hmac-sha1 1191.Ed 1192.Pp 1193The list of available MAC algorithms may also be obtained using 1194.Qq ssh -Q mac . 1195.It Cm Match 1196Introduces a conditional block. 1197If all of the criteria on the 1198.Cm Match 1199line are satisfied, the keywords on the following lines override those 1200set in the global section of the config file, until either another 1201.Cm Match 1202line or the end of the file. 1203If a keyword appears in multiple 1204.Cm Match 1205blocks that are satisfied, only the first instance of the keyword is 1206applied. 1207.Pp 1208The arguments to 1209.Cm Match 1210are one or more criteria-pattern pairs or the single token 1211.Cm All 1212which matches all criteria. 1213The available criteria are 1214.Cm User , 1215.Cm Group , 1216.Cm Host , 1217.Cm LocalAddress , 1218.Cm LocalPort , 1219.Cm RDomain , 1220and 1221.Cm Address 1222(with 1223.Cm RDomain 1224representing the 1225.Xr rdomain 4 1226on which the connection was received). 1227.Pp 1228The match patterns may consist of single entries or comma-separated 1229lists and may use the wildcard and negation operators described in the 1230.Sx PATTERNS 1231section of 1232.Xr ssh_config 5 . 1233.Pp 1234The patterns in an 1235.Cm Address 1236criteria may additionally contain addresses to match in CIDR 1237address/masklen format, 1238such as 192.0.2.0/24 or 2001:db8::/32. 1239Note that the mask length provided must be consistent with the address - 1240it is an error to specify a mask length that is too long for the address 1241or one with bits set in this host portion of the address. 1242For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. 1243.Pp 1244Only a subset of keywords may be used on the lines following a 1245.Cm Match 1246keyword. 1247Available keywords are 1248.Cm AcceptEnv , 1249.Cm AllowAgentForwarding , 1250.Cm AllowGroups , 1251.Cm AllowStreamLocalForwarding , 1252.Cm AllowTcpForwarding , 1253.Cm AllowUsers , 1254.Cm AuthenticationMethods , 1255.Cm AuthorizedKeysCommand , 1256.Cm AuthorizedKeysCommandUser , 1257.Cm AuthorizedKeysFile , 1258.Cm AuthorizedPrincipalsCommand , 1259.Cm AuthorizedPrincipalsCommandUser , 1260.Cm AuthorizedPrincipalsFile , 1261.Cm Banner , 1262.Cm CASignatureAlgorithms , 1263.Cm ChannelTimeout , 1264.Cm ChrootDirectory , 1265.Cm ClientAliveCountMax , 1266.Cm ClientAliveInterval , 1267.Cm DenyGroups , 1268.Cm DenyUsers , 1269.Cm DisableForwarding , 1270.Cm ExposeAuthInfo , 1271.Cm ForceCommand , 1272.Cm GatewayPorts , 1273.Cm GSSAPIAuthentication , 1274.Cm HostbasedAcceptedAlgorithms , 1275.Cm HostbasedAuthentication , 1276.Cm HostbasedUsesNameFromPacketOnly , 1277.Cm IgnoreRhosts , 1278.Cm Include , 1279.Cm IPQoS , 1280.Cm KbdInteractiveAuthentication , 1281.Cm KerberosAuthentication , 1282.Cm LogLevel , 1283.Cm MaxAuthTries , 1284.Cm MaxSessions , 1285.Cm PasswordAuthentication , 1286.Cm PermitEmptyPasswords , 1287.Cm PermitListen , 1288.Cm PermitOpen , 1289.Cm PermitRootLogin , 1290.Cm PermitTTY , 1291.Cm PermitTunnel , 1292.Cm PermitUserRC , 1293.Cm PubkeyAcceptedAlgorithms , 1294.Cm PubkeyAuthentication , 1295.Cm PubkeyAuthOptions , 1296.Cm RekeyLimit , 1297.Cm RevokedKeys , 1298.Cm RDomain , 1299.Cm SetEnv , 1300.Cm StreamLocalBindMask , 1301.Cm StreamLocalBindUnlink , 1302.Cm TrustedUserCAKeys , 1303.Cm UnusedConnectionTimeout , 1304.Cm X11DisplayOffset , 1305.Cm X11Forwarding 1306and 1307.Cm X11UseLocalhost . 1308.It Cm MaxAuthTries 1309Specifies the maximum number of authentication attempts permitted per 1310connection. 1311Once the number of failures reaches half this value, 1312additional failures are logged. 1313The default is 6. 1314.It Cm MaxSessions 1315Specifies the maximum number of open shell, login or subsystem (e.g. sftp) 1316sessions permitted per network connection. 1317Multiple sessions may be established by clients that support connection 1318multiplexing. 1319Setting 1320.Cm MaxSessions 1321to 1 will effectively disable session multiplexing, whereas setting it to 0 1322will prevent all shell, login and subsystem sessions while still permitting 1323forwarding. 1324The default is 10. 1325.It Cm MaxStartups 1326Specifies the maximum number of concurrent unauthenticated connections to the 1327SSH daemon. 1328Additional connections will be dropped until authentication succeeds or the 1329.Cm LoginGraceTime 1330expires for a connection. 1331The default is 10:30:100. 1332.Pp 1333Alternatively, random early drop can be enabled by specifying 1334the three colon separated values 1335start:rate:full (e.g. "10:30:60"). 1336.Xr sshd 8 1337will refuse connection attempts with a probability of rate/100 (30%) 1338if there are currently start (10) unauthenticated connections. 1339The probability increases linearly and all connection attempts 1340are refused if the number of unauthenticated connections reaches full (60). 1341.It Cm ModuliFile 1342Specifies the 1343.Xr moduli 5 1344file that contains the Diffie-Hellman groups used for the 1345.Dq diffie-hellman-group-exchange-sha1 1346and 1347.Dq diffie-hellman-group-exchange-sha256 1348key exchange methods. 1349The default is 1350.Pa /etc/moduli . 1351.It Cm PasswordAuthentication 1352Specifies whether password authentication is allowed. 1353The default is 1354.Cm yes . 1355.It Cm PermitEmptyPasswords 1356When password authentication is allowed, it specifies whether the 1357server allows login to accounts with empty password strings. 1358The default is 1359.Cm no . 1360.It Cm PermitListen 1361Specifies the addresses/ports on which a remote TCP port forwarding may listen. 1362The listen specification must be one of the following forms: 1363.Pp 1364.Bl -item -offset indent -compact 1365.It 1366.Cm PermitListen 1367.Sm off 1368.Ar port 1369.Sm on 1370.It 1371.Cm PermitListen 1372.Sm off 1373.Ar host : port 1374.Sm on 1375.El 1376.Pp 1377Multiple permissions may be specified by separating them with whitespace. 1378An argument of 1379.Cm any 1380can be used to remove all restrictions and permit any listen requests. 1381An argument of 1382.Cm none 1383can be used to prohibit all listen requests. 1384The host name may contain wildcards as described in the PATTERNS section in 1385.Xr ssh_config 5 . 1386The wildcard 1387.Sq * 1388can also be used in place of a port number to allow all ports. 1389By default all port forwarding listen requests are permitted. 1390Note that the 1391.Cm GatewayPorts 1392option may further restrict which addresses may be listened on. 1393Note also that 1394.Xr ssh 1 1395will request a listen host of 1396.Dq localhost 1397if no listen host was specifically requested, and this name is 1398treated differently to explicit localhost addresses of 1399.Dq 127.0.0.1 1400and 1401.Dq ::1 . 1402.It Cm PermitOpen 1403Specifies the destinations to which TCP port forwarding is permitted. 1404The forwarding specification must be one of the following forms: 1405.Pp 1406.Bl -item -offset indent -compact 1407.It 1408.Cm PermitOpen 1409.Sm off 1410.Ar host : port 1411.Sm on 1412.It 1413.Cm PermitOpen 1414.Sm off 1415.Ar IPv4_addr : port 1416.Sm on 1417.It 1418.Cm PermitOpen 1419.Sm off 1420.Ar \&[ IPv6_addr \&] : port 1421.Sm on 1422.El 1423.Pp 1424Multiple forwards may be specified by separating them with whitespace. 1425An argument of 1426.Cm any 1427can be used to remove all restrictions and permit any forwarding requests. 1428An argument of 1429.Cm none 1430can be used to prohibit all forwarding requests. 1431The wildcard 1432.Sq * 1433can be used for host or port to allow all hosts or ports respectively. 1434Otherwise, no pattern matching or address lookups are performed on supplied 1435names. 1436By default all port forwarding requests are permitted. 1437.It Cm PermitRootLogin 1438Specifies whether root can log in using 1439.Xr ssh 1 . 1440The argument must be 1441.Cm yes , 1442.Cm prohibit-password , 1443.Cm forced-commands-only , 1444or 1445.Cm no . 1446The default is 1447.Cm prohibit-password . 1448.Pp 1449If this option is set to 1450.Cm prohibit-password 1451(or its deprecated alias, 1452.Cm without-password ) , 1453password and keyboard-interactive authentication are disabled for root. 1454.Pp 1455If this option is set to 1456.Cm forced-commands-only , 1457root login with public key authentication will be allowed, 1458but only if the 1459.Ar command 1460option has been specified 1461(which may be useful for taking remote backups even if root login is 1462normally not allowed). 1463All other authentication methods are disabled for root. 1464.Pp 1465If this option is set to 1466.Cm no , 1467root is not allowed to log in. 1468.It Cm PermitTTY 1469Specifies whether 1470.Xr pty 4 1471allocation is permitted. 1472The default is 1473.Cm yes . 1474.It Cm PermitTunnel 1475Specifies whether 1476.Xr tun 4 1477device forwarding is allowed. 1478The argument must be 1479.Cm yes , 1480.Cm point-to-point 1481(layer 3), 1482.Cm ethernet 1483(layer 2), or 1484.Cm no . 1485Specifying 1486.Cm yes 1487permits both 1488.Cm point-to-point 1489and 1490.Cm ethernet . 1491The default is 1492.Cm no . 1493.Pp 1494Independent of this setting, the permissions of the selected 1495.Xr tun 4 1496device must allow access to the user. 1497.It Cm PermitUserEnvironment 1498Specifies whether 1499.Pa ~/.ssh/environment 1500and 1501.Cm environment= 1502options in 1503.Pa ~/.ssh/authorized_keys 1504are processed by 1505.Xr sshd 8 . 1506Valid options are 1507.Cm yes , 1508.Cm no 1509or a pattern-list specifying which environment variable names to accept 1510(for example 1511.Qq LANG,LC_* ) . 1512The default is 1513.Cm no . 1514Enabling environment processing may enable users to bypass access 1515restrictions in some configurations using mechanisms such as 1516.Ev LD_PRELOAD . 1517.It Cm PermitUserRC 1518Specifies whether any 1519.Pa ~/.ssh/rc 1520file is executed. 1521The default is 1522.Cm yes . 1523.It Cm PerSourceMaxStartups 1524Specifies the number of unauthenticated connections allowed from a 1525given source address, or 1526.Dq none 1527if there is no limit. 1528This limit is applied in addition to 1529.Cm MaxStartups , 1530whichever is lower. 1531The default is 1532.Cm none . 1533.It Cm PerSourceNetBlockSize 1534Specifies the number of bits of source address that are grouped together 1535for the purposes of applying PerSourceMaxStartups limits. 1536Values for IPv4 and optionally IPv6 may be specified, separated by a colon. 1537The default is 1538.Cm 32:128 , 1539which means each address is considered individually. 1540.It Cm PidFile 1541Specifies the file that contains the process ID of the 1542SSH daemon, or 1543.Cm none 1544to not write one. 1545The default is 1546.Pa /var/run/sshd.pid . 1547.It Cm Port 1548Specifies the port number that 1549.Xr sshd 8 1550listens on. 1551The default is 22. 1552Multiple options of this type are permitted. 1553See also 1554.Cm ListenAddress . 1555.It Cm PrintLastLog 1556Specifies whether 1557.Xr sshd 8 1558should print the date and time of the last user login when a user logs 1559in interactively. 1560The default is 1561.Cm yes . 1562.It Cm PrintMotd 1563Specifies whether 1564.Xr sshd 8 1565should print 1566.Pa /etc/motd 1567when a user logs in interactively. 1568(On some systems it is also printed by the shell, 1569.Pa /etc/profile , 1570or equivalent.) 1571The default is 1572.Cm yes . 1573.It Cm PubkeyAcceptedAlgorithms 1574Specifies the signature algorithms that will be accepted for public key 1575authentication as a list of comma-separated patterns. 1576Alternately if the specified list begins with a 1577.Sq + 1578character, then the specified algorithms will be appended to the default set 1579instead of replacing them. 1580If the specified list begins with a 1581.Sq - 1582character, then the specified algorithms (including wildcards) will be removed 1583from the default set instead of replacing them. 1584If the specified list begins with a 1585.Sq ^ 1586character, then the specified algorithms will be placed at the head of the 1587default set. 1588The default for this option is: 1589.Bd -literal -offset 3n 1590ssh-ed25519-cert-v01@openssh.com, 1591ecdsa-sha2-nistp256-cert-v01@openssh.com, 1592ecdsa-sha2-nistp384-cert-v01@openssh.com, 1593ecdsa-sha2-nistp521-cert-v01@openssh.com, 1594sk-ssh-ed25519-cert-v01@openssh.com, 1595sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1596rsa-sha2-512-cert-v01@openssh.com, 1597rsa-sha2-256-cert-v01@openssh.com, 1598ssh-ed25519, 1599ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1600sk-ssh-ed25519@openssh.com, 1601sk-ecdsa-sha2-nistp256@openssh.com, 1602rsa-sha2-512,rsa-sha2-256 1603.Ed 1604.Pp 1605The list of available signature algorithms may also be obtained using 1606.Qq ssh -Q PubkeyAcceptedAlgorithms . 1607.It Cm PubkeyAuthOptions 1608Sets one or more public key authentication options. 1609The supported keywords are: 1610.Cm none 1611(the default; indicating no additional options are enabled), 1612.Cm touch-required 1613and 1614.Cm verify-required . 1615.Pp 1616The 1617.Cm touch-required 1618option causes public key authentication using a FIDO authenticator algorithm 1619(i.e.\& 1620.Cm ecdsa-sk 1621or 1622.Cm ed25519-sk ) 1623to always require the signature to attest that a physically present user 1624explicitly confirmed the authentication (usually by touching the authenticator). 1625By default, 1626.Xr sshd 8 1627requires user presence unless overridden with an authorized_keys option. 1628The 1629.Cm touch-required 1630flag disables this override. 1631.Pp 1632The 1633.Cm verify-required 1634option requires a FIDO key signature attest that the user was verified, 1635e.g. via a PIN. 1636.Pp 1637Neither the 1638.Cm touch-required 1639or 1640.Cm verify-required 1641options have any effect for other, non-FIDO, public key types. 1642.It Cm PubkeyAuthentication 1643Specifies whether public key authentication is allowed. 1644The default is 1645.Cm yes . 1646.It Cm RekeyLimit 1647Specifies the maximum amount of data that may be transmitted or received 1648before the session key is renegotiated, optionally followed by a maximum 1649amount of time that may pass before the session key is renegotiated. 1650The first argument is specified in bytes and may have a suffix of 1651.Sq K , 1652.Sq M , 1653or 1654.Sq G 1655to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1656The default is between 1657.Sq 1G 1658and 1659.Sq 4G , 1660depending on the cipher. 1661The optional second value is specified in seconds and may use any of the 1662units documented in the 1663.Sx TIME FORMATS 1664section. 1665The default value for 1666.Cm RekeyLimit 1667is 1668.Cm default none , 1669which means that rekeying is performed after the cipher's default amount 1670of data has been sent or received and no time based rekeying is done. 1671.It Cm RequiredRSASize 1672Specifies the minimum RSA key size (in bits) that 1673.Xr sshd 8 1674will accept. 1675User and host-based authentication keys smaller than this limit will be 1676refused. 1677The default is 1678.Cm 1024 1679bits. 1680Note that this limit may only be raised from the default. 1681.It Cm RevokedKeys 1682Specifies revoked public keys file, or 1683.Cm none 1684to not use one. 1685Keys listed in this file will be refused for public key authentication. 1686Note that if this file is not readable, then public key authentication will 1687be refused for all users. 1688Keys may be specified as a text file, listing one public key per line, or as 1689an OpenSSH Key Revocation List (KRL) as generated by 1690.Xr ssh-keygen 1 . 1691For more information on KRLs, see the KEY REVOCATION LISTS section in 1692.Xr ssh-keygen 1 . 1693.It Cm RDomain 1694Specifies an explicit routing domain that is applied after authentication 1695has completed. 1696The user session, as well as any forwarded or listening IP sockets, 1697will be bound to this 1698.Xr rdomain 4 . 1699If the routing domain is set to 1700.Cm \&%D , 1701then the domain in which the incoming connection was received will be applied. 1702.It Cm SecurityKeyProvider 1703Specifies a path to a library that will be used when loading 1704FIDO authenticator-hosted keys, overriding the default of using 1705the built-in USB HID support. 1706.It Cm SetEnv 1707Specifies one or more environment variables to set in child sessions started 1708by 1709.Xr sshd 8 1710as 1711.Dq NAME=VALUE . 1712The environment value may be quoted (e.g. if it contains whitespace 1713characters). 1714Environment variables set by 1715.Cm SetEnv 1716override the default environment and any variables specified by the user 1717via 1718.Cm AcceptEnv 1719or 1720.Cm PermitUserEnvironment . 1721.It Cm StreamLocalBindMask 1722Sets the octal file creation mode mask 1723.Pq umask 1724used when creating a Unix-domain socket file for local or remote 1725port forwarding. 1726This option is only used for port forwarding to a Unix-domain socket file. 1727.Pp 1728The default value is 0177, which creates a Unix-domain socket file that is 1729readable and writable only by the owner. 1730Note that not all operating systems honor the file mode on Unix-domain 1731socket files. 1732.It Cm StreamLocalBindUnlink 1733Specifies whether to remove an existing Unix-domain socket file for local 1734or remote port forwarding before creating a new one. 1735If the socket file already exists and 1736.Cm StreamLocalBindUnlink 1737is not enabled, 1738.Nm sshd 1739will be unable to forward the port to the Unix-domain socket file. 1740This option is only used for port forwarding to a Unix-domain socket file. 1741.Pp 1742The argument must be 1743.Cm yes 1744or 1745.Cm no . 1746The default is 1747.Cm no . 1748.It Cm StrictModes 1749Specifies whether 1750.Xr sshd 8 1751should check file modes and ownership of the 1752user's files and home directory before accepting login. 1753This is normally desirable because novices sometimes accidentally leave their 1754directory or files world-writable. 1755The default is 1756.Cm yes . 1757Note that this does not apply to 1758.Cm ChrootDirectory , 1759whose permissions and ownership are checked unconditionally. 1760.It Cm Subsystem 1761Configures an external subsystem (e.g. file transfer daemon). 1762Arguments should be a subsystem name and a command (with optional arguments) 1763to execute upon subsystem request. 1764.Pp 1765The command 1766.Cm sftp-server 1767implements the SFTP file transfer subsystem. 1768.Pp 1769Alternately the name 1770.Cm internal-sftp 1771implements an in-process SFTP server. 1772This may simplify configurations using 1773.Cm ChrootDirectory 1774to force a different filesystem root on clients. 1775.Pp 1776By default no subsystems are defined. 1777.It Cm SyslogFacility 1778Gives the facility code that is used when logging messages from 1779.Xr sshd 8 . 1780The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1781LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1782The default is AUTH. 1783.It Cm TCPKeepAlive 1784Specifies whether the system should send TCP keepalive messages to the 1785other side. 1786If they are sent, death of the connection or crash of one 1787of the machines will be properly noticed. 1788However, this means that 1789connections will die if the route is down temporarily, and some people 1790find it annoying. 1791On the other hand, if TCP keepalives are not sent, 1792sessions may hang indefinitely on the server, leaving 1793.Qq ghost 1794users and consuming server resources. 1795.Pp 1796The default is 1797.Cm yes 1798(to send TCP keepalive messages), and the server will notice 1799if the network goes down or the client host crashes. 1800This avoids infinitely hanging sessions. 1801.Pp 1802To disable TCP keepalive messages, the value should be set to 1803.Cm no . 1804.It Cm TrustedUserCAKeys 1805Specifies a file containing public keys of certificate authorities that are 1806trusted to sign user certificates for authentication, or 1807.Cm none 1808to not use one. 1809Keys are listed one per line; empty lines and comments starting with 1810.Ql # 1811are allowed. 1812If a certificate is presented for authentication and has its signing CA key 1813listed in this file, then it may be used for authentication for any user 1814listed in the certificate's principals list. 1815Note that certificates that lack a list of principals will not be permitted 1816for authentication using 1817.Cm TrustedUserCAKeys . 1818For more details on certificates, see the CERTIFICATES section in 1819.Xr ssh-keygen 1 . 1820.It Cm UnusedConnectionTimeout 1821Specifies whether and how quickly 1822.Xr sshd 8 1823should close client connections with no open channels. 1824Open channels include active shell, command execution or subsystem 1825sessions, connected network, socket, agent or X11 forwardings. 1826Forwarding listeners, such as those from the 1827.Xr ssh 1 1828.Fl R 1829flag, are not considered as open channels and do not prevent the timeout. 1830The timeout value 1831is specified in seconds or may use any of the units documented in the 1832.Sx TIME FORMATS 1833section. 1834.Pp 1835Note that this timeout starts when the client connection completes 1836user authentication but before the client has an opportunity to open any 1837channels. 1838Caution should be used when using short timeout values, as they may not 1839provide sufficient time for the client to request and open its channels 1840before terminating the connection. 1841.Pp 1842The default 1843.Cm none 1844is to never expire connections for having no open channels. 1845This option may be useful in conjunction with 1846.Cm ChannelTimeout . 1847.It Cm UseDNS 1848Specifies whether 1849.Xr sshd 8 1850should look up the remote host name, and to check that 1851the resolved host name for the remote IP address maps back to the 1852very same IP address. 1853.Pp 1854If this option is set to 1855.Cm no 1856(the default) then only addresses and not host names may be used in 1857.Pa ~/.ssh/authorized_keys 1858.Cm from 1859and 1860.Nm 1861.Cm Match 1862.Cm Host 1863directives. 1864.It Cm VersionAddendum 1865Optionally specifies additional text to append to the SSH protocol banner 1866sent by the server upon connection. 1867The default is 1868.Cm none . 1869.It Cm X11DisplayOffset 1870Specifies the first display number available for 1871.Xr sshd 8 Ns 's 1872X11 forwarding. 1873This prevents sshd from interfering with real X11 servers. 1874The default is 10. 1875.It Cm X11Forwarding 1876Specifies whether X11 forwarding is permitted. 1877The argument must be 1878.Cm yes 1879or 1880.Cm no . 1881The default is 1882.Cm no . 1883.Pp 1884When X11 forwarding is enabled, there may be additional exposure to 1885the server and to client displays if the 1886.Xr sshd 8 1887proxy display is configured to listen on the wildcard address (see 1888.Cm X11UseLocalhost ) , 1889though this is not the default. 1890Additionally, the authentication spoofing and authentication data 1891verification and substitution occur on the client side. 1892The security risk of using X11 forwarding is that the client's X11 1893display server may be exposed to attack when the SSH client requests 1894forwarding (see the warnings for 1895.Cm ForwardX11 1896in 1897.Xr ssh_config 5 ) . 1898A system administrator may have a stance in which they want to 1899protect clients that may expose themselves to attack by unwittingly 1900requesting X11 forwarding, which can warrant a 1901.Cm no 1902setting. 1903.Pp 1904Note that disabling X11 forwarding does not prevent users from 1905forwarding X11 traffic, as users can always install their own forwarders. 1906.It Cm X11UseLocalhost 1907Specifies whether 1908.Xr sshd 8 1909should bind the X11 forwarding server to the loopback address or to 1910the wildcard address. 1911By default, 1912sshd binds the forwarding server to the loopback address and sets the 1913hostname part of the 1914.Ev DISPLAY 1915environment variable to 1916.Cm localhost . 1917This prevents remote hosts from connecting to the proxy display. 1918However, some older X11 clients may not function with this 1919configuration. 1920.Cm X11UseLocalhost 1921may be set to 1922.Cm no 1923to specify that the forwarding server should be bound to the wildcard 1924address. 1925The argument must be 1926.Cm yes 1927or 1928.Cm no . 1929The default is 1930.Cm yes . 1931.It Cm XAuthLocation 1932Specifies the full pathname of the 1933.Xr xauth 1 1934program, or 1935.Cm none 1936to not use one. 1937The default is 1938.Pa /usr/X11R6/bin/xauth . 1939.El 1940.Sh TIME FORMATS 1941.Xr sshd 8 1942command-line arguments and configuration file options that specify time 1943may be expressed using a sequence of the form: 1944.Sm off 1945.Ar time Op Ar qualifier , 1946.Sm on 1947where 1948.Ar time 1949is a positive integer value and 1950.Ar qualifier 1951is one of the following: 1952.Pp 1953.Bl -tag -width Ds -compact -offset indent 1954.It Aq Cm none 1955seconds 1956.It Cm s | Cm S 1957seconds 1958.It Cm m | Cm M 1959minutes 1960.It Cm h | Cm H 1961hours 1962.It Cm d | Cm D 1963days 1964.It Cm w | Cm W 1965weeks 1966.El 1967.Pp 1968Each member of the sequence is added together to calculate 1969the total time value. 1970.Pp 1971Time format examples: 1972.Pp 1973.Bl -tag -width Ds -compact -offset indent 1974.It 600 1975600 seconds (10 minutes) 1976.It 10m 197710 minutes 1978.It 1h30m 19791 hour 30 minutes (90 minutes) 1980.El 1981.Sh TOKENS 1982Arguments to some keywords can make use of tokens, 1983which are expanded at runtime: 1984.Pp 1985.Bl -tag -width XXXX -offset indent -compact 1986.It %% 1987A literal 1988.Sq % . 1989.It \&%D 1990The routing domain in which the incoming connection was received. 1991.It %F 1992The fingerprint of the CA key. 1993.It %f 1994The fingerprint of the key or certificate. 1995.It %h 1996The home directory of the user. 1997.It %i 1998The key ID in the certificate. 1999.It %K 2000The base64-encoded CA key. 2001.It %k 2002The base64-encoded key or certificate for authentication. 2003.It %s 2004The serial number of the certificate. 2005.It \&%T 2006The type of the CA key. 2007.It %t 2008The key or certificate type. 2009.It \&%U 2010The numeric user ID of the target user. 2011.It %u 2012The username. 2013.El 2014.Pp 2015.Cm AuthorizedKeysCommand 2016accepts the tokens %%, %f, %h, %k, %t, %U, and %u. 2017.Pp 2018.Cm AuthorizedKeysFile 2019accepts the tokens %%, %h, %U, and %u. 2020.Pp 2021.Cm AuthorizedPrincipalsCommand 2022accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u. 2023.Pp 2024.Cm AuthorizedPrincipalsFile 2025accepts the tokens %%, %h, %U, and %u. 2026.Pp 2027.Cm ChrootDirectory 2028accepts the tokens %%, %h, %U, and %u. 2029.Pp 2030.Cm RoutingDomain 2031accepts the token %D. 2032.Sh FILES 2033.Bl -tag -width Ds 2034.It Pa /etc/ssh/sshd_config 2035Contains configuration data for 2036.Xr sshd 8 . 2037This file should be writable by root only, but it is recommended 2038(though not necessary) that it be world-readable. 2039.El 2040.Sh SEE ALSO 2041.Xr sftp-server 8 , 2042.Xr sshd 8 2043.Sh AUTHORS 2044.An -nosplit 2045OpenSSH is a derivative of the original and free 2046ssh 1.2.12 release by 2047.An Tatu Ylonen . 2048.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , 2049.An Theo de Raadt 2050and 2051.An Dug Song 2052removed many bugs, re-added newer features and 2053created OpenSSH. 2054.An Markus Friedl 2055contributed the support for SSH protocol versions 1.5 and 2.0. 2056.An Niels Provos 2057and 2058.An Markus Friedl 2059contributed support for privilege separation. 2060