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 1.284 2019/03/22 20:58:34 jmc Exp $ 37.Dd $Mdocdate: March 22 2019 $ 38.Dt SSHD_CONFIG 5 39.Os 40.Sh NAME 41.Nm sshd_config 42.Nd OpenSSH SSH 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 directives are processed in the following order: 117.Cm DenyUsers , 118.Cm AllowUsers , 119.Cm DenyGroups , 120and finally 121.Cm AllowGroups . 122.Pp 123See PATTERNS in 124.Xr ssh_config 5 125for more information on patterns. 126.It Cm AllowStreamLocalForwarding 127Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. 128The available options are 129.Cm yes 130(the default) 131or 132.Cm all 133to allow StreamLocal forwarding, 134.Cm no 135to prevent all StreamLocal forwarding, 136.Cm local 137to allow local (from the perspective of 138.Xr ssh 1 ) 139forwarding only or 140.Cm remote 141to allow remote forwarding only. 142Note that disabling StreamLocal forwarding does not improve security unless 143users are also denied shell access, as they can always install their 144own forwarders. 145.It Cm AllowTcpForwarding 146Specifies whether TCP forwarding is permitted. 147The available options are 148.Cm yes 149(the default) 150or 151.Cm all 152to allow TCP forwarding, 153.Cm no 154to prevent all TCP forwarding, 155.Cm local 156to allow local (from the perspective of 157.Xr ssh 1 ) 158forwarding only or 159.Cm remote 160to allow remote forwarding only. 161Note that disabling TCP forwarding does not improve security unless 162users are also denied shell access, as they can always install their 163own forwarders. 164.It Cm AllowUsers 165This keyword can be followed by a list of user name patterns, separated 166by spaces. 167If specified, login is allowed only for user names that 168match one of the patterns. 169Only user names are valid; a numerical user ID is not recognized. 170By default, login is allowed for all users. 171If the pattern takes the form USER@HOST then USER and HOST 172are separately checked, restricting logins to particular 173users from particular hosts. 174HOST criteria may additionally contain addresses to match in CIDR 175address/masklen format. 176The allow/deny directives are processed in the following order: 177.Cm DenyUsers , 178.Cm AllowUsers , 179.Cm DenyGroups , 180and finally 181.Cm AllowGroups . 182.Pp 183See PATTERNS in 184.Xr ssh_config 5 185for more information on patterns. 186.It Cm AuthenticationMethods 187Specifies the authentication methods that must be successfully completed 188for a user to be granted access. 189This option must be followed by one or more lists of comma-separated 190authentication method names, or by the single string 191.Cm any 192to indicate the default behaviour of accepting any single authentication 193method. 194If the default is overridden, then successful authentication requires 195completion of every method in at least one of these lists. 196.Pp 197For example, 198.Qq publickey,password publickey,keyboard-interactive 199would require the user to complete public key authentication, followed by 200either password or keyboard interactive authentication. 201Only methods that are next in one or more lists are offered at each stage, 202so for this example it would not be possible to attempt password or 203keyboard-interactive authentication before public key. 204.Pp 205For keyboard interactive authentication it is also possible to 206restrict authentication to a specific device by appending a 207colon followed by the device identifier 208.Cm bsdauth , 209.Cm pam , 210or 211.Cm skey , 212depending on the server configuration. 213For example, 214.Qq keyboard-interactive:bsdauth 215would restrict keyboard interactive authentication to the 216.Cm bsdauth 217device. 218.Pp 219If the publickey method is listed more than once, 220.Xr sshd 8 221verifies that keys that have been used successfully are not reused for 222subsequent authentications. 223For example, 224.Qq publickey,publickey 225requires successful authentication using two different public keys. 226.Pp 227Note that each authentication method listed should also be explicitly enabled 228in the configuration. 229.Pp 230The available authentication methods are: 231.Qq gssapi-with-mic , 232.Qq hostbased , 233.Qq keyboard-interactive , 234.Qq none 235(used for access to password-less accounts when 236.Cm PermitEmptyPasswords 237is enabled), 238.Qq password 239and 240.Qq publickey . 241.It Cm AuthorizedKeysCommand 242Specifies a program to be used to look up the user's public keys. 243The program must be owned by root, not writable by group or others and 244specified by an absolute path. 245Arguments to 246.Cm AuthorizedKeysCommand 247accept the tokens described in the 248.Sx TOKENS 249section. 250If no arguments are specified then the username of the target user is used. 251.Pp 252The program should produce on standard output zero or 253more lines of authorized_keys output (see 254.Sx AUTHORIZED_KEYS 255in 256.Xr sshd 8 ) . 257If a key supplied by 258.Cm AuthorizedKeysCommand 259does not successfully authenticate 260and authorize the user then public key authentication continues using the usual 261.Cm AuthorizedKeysFile 262files. 263By default, no 264.Cm AuthorizedKeysCommand 265is run. 266.It Cm AuthorizedKeysCommandUser 267Specifies the user under whose account the 268.Cm AuthorizedKeysCommand 269is run. 270It is recommended to use a dedicated user that has no other role on the host 271than running authorized keys commands. 272If 273.Cm AuthorizedKeysCommand 274is specified but 275.Cm AuthorizedKeysCommandUser 276is not, then 277.Xr sshd 8 278will refuse to start. 279.It Cm AuthorizedKeysFile 280Specifies the file that contains the public keys used for user authentication. 281The format is described in the 282.Sx AUTHORIZED_KEYS FILE FORMAT 283section of 284.Xr sshd 8 . 285Arguments to 286.Cm AuthorizedKeysFile 287accept the tokens described in the 288.Sx TOKENS 289section. 290After expansion, 291.Cm AuthorizedKeysFile 292is taken to be an absolute path or one relative to the user's home 293directory. 294Multiple files may be listed, separated by whitespace. 295Alternately this option may be set to 296.Cm none 297to skip checking for user keys in files. 298The default is 299.Qq .ssh/authorized_keys .ssh/authorized_keys2 . 300.It Cm AuthorizedPrincipalsCommand 301Specifies a program to be used to generate the list of allowed 302certificate principals as per 303.Cm AuthorizedPrincipalsFile . 304The program must be owned by root, not writable by group or others and 305specified by an absolute path. 306Arguments to 307.Cm AuthorizedPrincipalsCommand 308accept the tokens described in the 309.Sx TOKENS 310section. 311If no arguments are specified then the username of the target user is used. 312.Pp 313The program should produce on standard output zero or 314more lines of 315.Cm AuthorizedPrincipalsFile 316output. 317If either 318.Cm AuthorizedPrincipalsCommand 319or 320.Cm AuthorizedPrincipalsFile 321is specified, then certificates offered by the client for authentication 322must contain a principal that is listed. 323By default, no 324.Cm AuthorizedPrincipalsCommand 325is run. 326.It Cm AuthorizedPrincipalsCommandUser 327Specifies the user under whose account the 328.Cm AuthorizedPrincipalsCommand 329is run. 330It is recommended to use a dedicated user that has no other role on the host 331than running authorized principals commands. 332If 333.Cm AuthorizedPrincipalsCommand 334is specified but 335.Cm AuthorizedPrincipalsCommandUser 336is not, then 337.Xr sshd 8 338will refuse to start. 339.It Cm AuthorizedPrincipalsFile 340Specifies a file that lists principal names that are accepted for 341certificate authentication. 342When using certificates signed by a key listed in 343.Cm TrustedUserCAKeys , 344this file lists names, one of which must appear in the certificate for it 345to be accepted for authentication. 346Names are listed one per line preceded by key options (as described in 347.Sx AUTHORIZED_KEYS FILE FORMAT 348in 349.Xr sshd 8 ) . 350Empty lines and comments starting with 351.Ql # 352are ignored. 353.Pp 354Arguments to 355.Cm AuthorizedPrincipalsFile 356accept the tokens described in the 357.Sx TOKENS 358section. 359After expansion, 360.Cm AuthorizedPrincipalsFile 361is taken to be an absolute path or one relative to the user's home directory. 362The default is 363.Cm none , 364i.e. not to use a principals file \(en in this case, the username 365of the user must appear in a certificate's principals list for it to be 366accepted. 367.Pp 368Note that 369.Cm AuthorizedPrincipalsFile 370is only used when authentication proceeds using a CA listed in 371.Cm TrustedUserCAKeys 372and is not consulted for certification authorities trusted via 373.Pa ~/.ssh/authorized_keys , 374though the 375.Cm principals= 376key option offers a similar facility (see 377.Xr sshd 8 378for details). 379.It Cm Banner 380The contents of the specified file are sent to the remote user before 381authentication is allowed. 382If the argument is 383.Cm none 384then no banner is displayed. 385By default, no banner is displayed. 386.It Cm CASignatureAlgorithms 387Specifies which algorithms are allowed for signing of certificates 388by certificate authorities (CAs). 389The default is: 390.Bd -literal -offset indent 391ecdsa-sha2-nistp256.ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 392ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 393.Ed 394.Pp 395Certificates signed using other algorithms will not be accepted for 396public key or host-based authentication. 397.It Cm ChallengeResponseAuthentication 398Specifies whether challenge-response authentication is allowed. 399All authentication styles from 400.Xr login.conf 5 401are supported. 402The default is 403.Cm yes . 404.It Cm ChrootDirectory 405Specifies the pathname of a directory to 406.Xr chroot 2 407to after authentication. 408At session startup 409.Xr sshd 8 410checks that all components of the pathname are root-owned directories 411which are not writable by any other user or group. 412After the chroot, 413.Xr sshd 8 414changes the working directory to the user's home directory. 415Arguments to 416.Cm ChrootDirectory 417accept the tokens described in the 418.Sx TOKENS 419section. 420.Pp 421The 422.Cm ChrootDirectory 423must contain the necessary files and directories to support the 424user's session. 425For an interactive session this requires at least a shell, typically 426.Xr sh 1 , 427and basic 428.Pa /dev 429nodes such as 430.Xr null 4 , 431.Xr zero 4 , 432.Xr stdin 4 , 433.Xr stdout 4 , 434.Xr stderr 4 , 435and 436.Xr tty 4 437devices. 438For file transfer sessions using SFTP 439no additional configuration of the environment is necessary if the in-process 440sftp-server is used, 441though sessions which use logging may require 442.Pa /dev/log 443inside the chroot directory on some operating systems (see 444.Xr sftp-server 8 445for details). 446.Pp 447For safety, it is very important that the directory hierarchy be 448prevented from modification by other processes on the system (especially 449those outside the jail). 450Misconfiguration can lead to unsafe environments which 451.Xr sshd 8 452cannot detect. 453.Pp 454The default is 455.Cm none , 456indicating not to 457.Xr chroot 2 . 458.It Cm Ciphers 459Specifies the ciphers allowed. 460Multiple ciphers must be comma-separated. 461If the specified value begins with a 462.Sq + 463character, then the specified ciphers will be appended to the default set 464instead of replacing them. 465If the specified value begins with a 466.Sq - 467character, then the specified ciphers (including wildcards) will be removed 468from the default set instead of replacing them. 469.Pp 470The supported ciphers are: 471.Pp 472.Bl -item -compact -offset indent 473.It 4743des-cbc 475.It 476aes128-cbc 477.It 478aes192-cbc 479.It 480aes256-cbc 481.It 482aes128-ctr 483.It 484aes192-ctr 485.It 486aes256-ctr 487.It 488aes128-gcm@openssh.com 489.It 490aes256-gcm@openssh.com 491.It 492chacha20-poly1305@openssh.com 493.El 494.Pp 495The default is: 496.Bd -literal -offset indent 497chacha20-poly1305@openssh.com, 498aes128-ctr,aes192-ctr,aes256-ctr, 499aes128-gcm@openssh.com,aes256-gcm@openssh.com 500.Ed 501.Pp 502The list of available ciphers may also be obtained using 503.Qq ssh -Q cipher . 504.It Cm ClientAliveCountMax 505Sets the number of client alive messages which may be sent without 506.Xr sshd 8 507receiving any messages back from the client. 508If this threshold is reached while client alive messages are being sent, 509sshd will disconnect the client, terminating the session. 510It is important to note that the use of client alive messages is very 511different from 512.Cm TCPKeepAlive . 513The client alive messages are sent through the encrypted channel 514and therefore will not be spoofable. 515The TCP keepalive option enabled by 516.Cm TCPKeepAlive 517is spoofable. 518The client alive mechanism is valuable when the client or 519server depend on knowing when a connection has become inactive. 520.Pp 521The default value is 3. 522If 523.Cm ClientAliveInterval 524is set to 15, and 525.Cm ClientAliveCountMax 526is left at the default, unresponsive SSH clients 527will be disconnected after approximately 45 seconds. 528.It Cm ClientAliveInterval 529Sets a timeout interval in seconds after which if no data has been received 530from the client, 531.Xr sshd 8 532will send a message through the encrypted 533channel to request a response from the client. 534The default 535is 0, indicating that these messages will not be sent to the client. 536.It Cm Compression 537Specifies whether compression is enabled after 538the user has authenticated successfully. 539The argument must be 540.Cm yes , 541.Cm delayed 542(a legacy synonym for 543.Cm yes ) 544or 545.Cm no . 546The default is 547.Cm yes . 548.It Cm DenyGroups 549This keyword can be followed by a list of group name patterns, separated 550by spaces. 551Login is disallowed for users whose primary group or supplementary 552group list matches one of the patterns. 553Only group names are valid; a numerical group ID is not recognized. 554By default, login is allowed for all groups. 555The allow/deny directives are processed in the following order: 556.Cm DenyUsers , 557.Cm AllowUsers , 558.Cm DenyGroups , 559and finally 560.Cm AllowGroups . 561.Pp 562See PATTERNS in 563.Xr ssh_config 5 564for more information on patterns. 565.It Cm DenyUsers 566This keyword can be followed by a list of user name patterns, separated 567by spaces. 568Login is disallowed for user names that match one of the patterns. 569Only user names are valid; a numerical user ID is not recognized. 570By default, login is allowed for all users. 571If the pattern takes the form USER@HOST then USER and HOST 572are separately checked, restricting logins to particular 573users from particular hosts. 574HOST criteria may additionally contain addresses to match in CIDR 575address/masklen format. 576The allow/deny directives are processed in the following order: 577.Cm DenyUsers , 578.Cm AllowUsers , 579.Cm DenyGroups , 580and finally 581.Cm AllowGroups . 582.Pp 583See PATTERNS in 584.Xr ssh_config 5 585for more information on patterns. 586.It Cm DisableForwarding 587Disables all forwarding features, including X11, 588.Xr ssh-agent 1 , 589TCP and StreamLocal. 590This option overrides all other forwarding-related options and may 591simplify restricted configurations. 592.It Cm ExposeAuthInfo 593Writes a temporary file containing a list of authentication methods and 594public credentials (e.g. keys) used to authenticate the user. 595The location of the file is exposed to the user session through the 596.Ev SSH_USER_AUTH 597environment variable. 598The default is 599.Cm no . 600.It Cm FingerprintHash 601Specifies the hash algorithm used when logging key fingerprints. 602Valid options are: 603.Cm md5 604and 605.Cm sha256 . 606The default is 607.Cm sha256 . 608.It Cm ForceCommand 609Forces the execution of the command specified by 610.Cm ForceCommand , 611ignoring any command supplied by the client and 612.Pa ~/.ssh/rc 613if present. 614The command is invoked by using the user's login shell with the -c option. 615This applies to shell, command, or subsystem execution. 616It is most useful inside a 617.Cm Match 618block. 619The command originally supplied by the client is available in the 620.Ev SSH_ORIGINAL_COMMAND 621environment variable. 622Specifying a command of 623.Cm internal-sftp 624will force the use of an in-process SFTP server that requires no support 625files when used with 626.Cm ChrootDirectory . 627The default is 628.Cm none . 629.It Cm GatewayPorts 630Specifies whether remote hosts are allowed to connect to ports 631forwarded for the client. 632By default, 633.Xr sshd 8 634binds remote port forwardings to the loopback address. 635This prevents other remote hosts from connecting to forwarded ports. 636.Cm GatewayPorts 637can be used to specify that sshd 638should allow remote port forwardings to bind to non-loopback addresses, thus 639allowing other hosts to connect. 640The argument may be 641.Cm no 642to force remote port forwardings to be available to the local host only, 643.Cm yes 644to force remote port forwardings to bind to the wildcard address, or 645.Cm clientspecified 646to allow the client to select the address to which the forwarding is bound. 647The default is 648.Cm no . 649.It Cm GSSAPIAuthentication 650Specifies whether user authentication based on GSSAPI is allowed. 651The default is 652.Cm no . 653.It Cm GSSAPICleanupCredentials 654Specifies whether to automatically destroy the user's credentials cache 655on logout. 656The default is 657.Cm yes . 658.It Cm GSSAPIStrictAcceptorCheck 659Determines whether to be strict about the identity of the GSSAPI acceptor 660a client authenticates against. 661If set to 662.Cm yes 663then the client must authenticate against the host 664service on the current hostname. 665If set to 666.Cm no 667then the client may authenticate against any service key stored in the 668machine's default store. 669This facility is provided to assist with operation on multi homed machines. 670The default is 671.Cm yes . 672.It Cm HostbasedAcceptedKeyTypes 673Specifies the key types that will be accepted for hostbased authentication 674as a list of comma-separated patterns. 675Alternately if the specified value begins with a 676.Sq + 677character, then the specified key types will be appended to the default set 678instead of replacing them. 679If the specified value begins with a 680.Sq - 681character, then the specified key types (including wildcards) will be removed 682from the default set instead of replacing them. 683The default for this option is: 684.Bd -literal -offset 3n 685ecdsa-sha2-nistp256-cert-v01@openssh.com, 686ecdsa-sha2-nistp384-cert-v01@openssh.com, 687ecdsa-sha2-nistp521-cert-v01@openssh.com, 688ssh-ed25519-cert-v01@openssh.com, 689rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com, 690ssh-rsa-cert-v01@openssh.com, 691ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 692ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 693.Ed 694.Pp 695The list of available key types may also be obtained using 696.Qq ssh -Q key . 697.It Cm HostbasedAuthentication 698Specifies whether rhosts or /etc/hosts.equiv authentication together 699with successful public key client host authentication is allowed 700(host-based authentication). 701The default is 702.Cm no . 703.It Cm HostbasedUsesNameFromPacketOnly 704Specifies whether or not the server will attempt to perform a reverse 705name lookup when matching the name in the 706.Pa ~/.shosts , 707.Pa ~/.rhosts , 708and 709.Pa /etc/hosts.equiv 710files during 711.Cm HostbasedAuthentication . 712A setting of 713.Cm yes 714means that 715.Xr sshd 8 716uses the name supplied by the client rather than 717attempting to resolve the name from the TCP connection itself. 718The default is 719.Cm no . 720.It Cm HostCertificate 721Specifies a file containing a public host certificate. 722The certificate's public key must match a private host key already specified 723by 724.Cm HostKey . 725The default behaviour of 726.Xr sshd 8 727is not to load any certificates. 728.It Cm HostKey 729Specifies a file containing a private host key 730used by SSH. 731The defaults are 732.Pa /etc/ssh/ssh_host_ecdsa_key , 733.Pa /etc/ssh/ssh_host_ed25519_key 734and 735.Pa /etc/ssh/ssh_host_rsa_key . 736.Pp 737Note that 738.Xr sshd 8 739will refuse to use a file if it is group/world-accessible 740and that the 741.Cm HostKeyAlgorithms 742option restricts which of the keys are actually used by 743.Xr sshd 8 . 744.Pp 745It is possible to have multiple host key files. 746It is also possible to specify public host key files instead. 747In this case operations on the private key will be delegated 748to an 749.Xr ssh-agent 1 . 750.It Cm HostKeyAgent 751Identifies the UNIX-domain socket used to communicate 752with an agent that has access to the private host keys. 753If the string 754.Qq SSH_AUTH_SOCK 755is specified, the location of the socket will be read from the 756.Ev SSH_AUTH_SOCK 757environment variable. 758.It Cm HostKeyAlgorithms 759Specifies the host key algorithms 760that the server offers. 761The default for this option is: 762.Bd -literal -offset 3n 763ecdsa-sha2-nistp256-cert-v01@openssh.com, 764ecdsa-sha2-nistp384-cert-v01@openssh.com, 765ecdsa-sha2-nistp521-cert-v01@openssh.com, 766ssh-ed25519-cert-v01@openssh.com, 767rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com, 768ssh-rsa-cert-v01@openssh.com, 769ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 770ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 771.Ed 772.Pp 773The list of available key types may also be obtained using 774.Qq ssh -Q key . 775.It Cm IgnoreRhosts 776Specifies that 777.Pa .rhosts 778and 779.Pa .shosts 780files will not be used in 781.Cm HostbasedAuthentication . 782.Pp 783.Pa /etc/hosts.equiv 784and 785.Pa /etc/shosts.equiv 786are still used. 787The default is 788.Cm yes . 789.It Cm IgnoreUserKnownHosts 790Specifies whether 791.Xr sshd 8 792should ignore the user's 793.Pa ~/.ssh/known_hosts 794during 795.Cm HostbasedAuthentication 796and use only the system-wide known hosts file 797.Pa /etc/ssh/known_hosts . 798The default is 799.Cm no . 800.It Cm IPQoS 801Specifies the IPv4 type-of-service or DSCP class for the connection. 802Accepted values are 803.Cm af11 , 804.Cm af12 , 805.Cm af13 , 806.Cm af21 , 807.Cm af22 , 808.Cm af23 , 809.Cm af31 , 810.Cm af32 , 811.Cm af33 , 812.Cm af41 , 813.Cm af42 , 814.Cm af43 , 815.Cm cs0 , 816.Cm cs1 , 817.Cm cs2 , 818.Cm cs3 , 819.Cm cs4 , 820.Cm cs5 , 821.Cm cs6 , 822.Cm cs7 , 823.Cm ef , 824.Cm lowdelay , 825.Cm throughput , 826.Cm reliability , 827a numeric value, or 828.Cm none 829to use the operating system default. 830This option may take one or two arguments, separated by whitespace. 831If one argument is specified, it is used as the packet class unconditionally. 832If two values are specified, the first is automatically selected for 833interactive sessions and the second for non-interactive sessions. 834The default is 835.Cm af21 836(Low-Latency Data) 837for interactive sessions and 838.Cm cs1 839(Lower Effort) 840for non-interactive sessions. 841.It Cm KbdInteractiveAuthentication 842Specifies whether to allow keyboard-interactive authentication. 843The argument to this keyword must be 844.Cm yes 845or 846.Cm no . 847The default is to use whatever value 848.Cm ChallengeResponseAuthentication 849is set to 850(by default 851.Cm yes ) . 852.It Cm KerberosAuthentication 853Specifies whether the password provided by the user for 854.Cm PasswordAuthentication 855will be validated through the Kerberos KDC. 856To use this option, the server needs a 857Kerberos servtab which allows the verification of the KDC's identity. 858The default is 859.Cm no . 860.It Cm KerberosGetAFSToken 861If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 862an AFS token before accessing the user's home directory. 863The default is 864.Cm no . 865.It Cm KerberosOrLocalPasswd 866If password authentication through Kerberos fails then 867the password will be validated via any additional local mechanism 868such as 869.Pa /etc/passwd . 870The default is 871.Cm yes . 872.It Cm KerberosTicketCleanup 873Specifies whether to automatically destroy the user's ticket cache 874file on logout. 875The default is 876.Cm yes . 877.It Cm KexAlgorithms 878Specifies the available KEX (Key Exchange) algorithms. 879Multiple algorithms must be comma-separated. 880Alternately if the specified value begins with a 881.Sq + 882character, then the specified methods will be appended to the default set 883instead of replacing them. 884If the specified value begins with a 885.Sq - 886character, then the specified methods (including wildcards) will be removed 887from the default set instead of replacing them. 888The supported algorithms are: 889.Pp 890.Bl -item -compact -offset indent 891.It 892curve25519-sha256 893.It 894curve25519-sha256@libssh.org 895.It 896diffie-hellman-group1-sha1 897.It 898diffie-hellman-group14-sha1 899.It 900diffie-hellman-group14-sha256 901.It 902diffie-hellman-group16-sha512 903.It 904diffie-hellman-group18-sha512 905.It 906diffie-hellman-group-exchange-sha1 907.It 908diffie-hellman-group-exchange-sha256 909.It 910ecdh-sha2-nistp256 911.It 912ecdh-sha2-nistp384 913.It 914ecdh-sha2-nistp521 915.El 916.Pp 917The default is: 918.Bd -literal -offset indent 919curve25519-sha256,curve25519-sha256@libssh.org, 920ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 921diffie-hellman-group-exchange-sha256, 922diffie-hellman-group16-sha512,diffie-hellman-group18-sha512, 923diffie-hellman-group14-sha256,diffie-hellman-group14-sha1 924.Ed 925.Pp 926The list of available key exchange algorithms may also be obtained using 927.Qq ssh -Q kex . 928.It Cm ListenAddress 929Specifies the local addresses 930.Xr sshd 8 931should listen on. 932The following forms may be used: 933.Pp 934.Bl -item -offset indent -compact 935.It 936.Cm ListenAddress 937.Sm off 938.Ar hostname | address 939.Sm on 940.Op Cm rdomain Ar domain 941.It 942.Cm ListenAddress 943.Sm off 944.Ar hostname : port 945.Sm on 946.Op Cm rdomain Ar domain 947.It 948.Cm ListenAddress 949.Sm off 950.Ar IPv4_address : port 951.Sm on 952.Op Cm rdomain Ar domain 953.It 954.Cm ListenAddress 955.Sm off 956.Oo Ar hostname | address Oc : Ar port 957.Sm on 958.Op Cm rdomain Ar domain 959.El 960.Pp 961The optional 962.Cm rdomain 963qualifier requests 964.Xr sshd 8 965listen in an explicit routing domain. 966If 967.Ar port 968is not specified, 969sshd will listen on the address and all 970.Cm Port 971options specified. 972The default is to listen on all local addresses on the current default 973routing domain. 974Multiple 975.Cm ListenAddress 976options are permitted. 977For more information on routing domains, see 978.Xr rdomain 4 . 979.It Cm LoginGraceTime 980The server disconnects after this time if the user has not 981successfully logged in. 982If the value is 0, there is no time limit. 983The default is 120 seconds. 984.It Cm LogLevel 985Gives the verbosity level that is used when logging messages from 986.Xr sshd 8 . 987The possible values are: 988QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 989The default is INFO. 990DEBUG and DEBUG1 are equivalent. 991DEBUG2 and DEBUG3 each specify higher levels of debugging output. 992Logging with a DEBUG level violates the privacy of users and is not recommended. 993.It Cm MACs 994Specifies the available MAC (message authentication code) algorithms. 995The MAC algorithm is used for data integrity protection. 996Multiple algorithms must be comma-separated. 997If the specified value begins with a 998.Sq + 999character, then the specified algorithms will be appended to the default set 1000instead of replacing them. 1001If the specified value begins with a 1002.Sq - 1003character, then the specified algorithms (including wildcards) will be removed 1004from the default set instead of replacing them. 1005.Pp 1006The algorithms that contain 1007.Qq -etm 1008calculate the MAC after encryption (encrypt-then-mac). 1009These are considered safer and their use recommended. 1010The supported MACs are: 1011.Pp 1012.Bl -item -compact -offset indent 1013.It 1014hmac-md5 1015.It 1016hmac-md5-96 1017.It 1018hmac-sha1 1019.It 1020hmac-sha1-96 1021.It 1022hmac-sha2-256 1023.It 1024hmac-sha2-512 1025.It 1026umac-64@openssh.com 1027.It 1028umac-128@openssh.com 1029.It 1030hmac-md5-etm@openssh.com 1031.It 1032hmac-md5-96-etm@openssh.com 1033.It 1034hmac-sha1-etm@openssh.com 1035.It 1036hmac-sha1-96-etm@openssh.com 1037.It 1038hmac-sha2-256-etm@openssh.com 1039.It 1040hmac-sha2-512-etm@openssh.com 1041.It 1042umac-64-etm@openssh.com 1043.It 1044umac-128-etm@openssh.com 1045.El 1046.Pp 1047The default is: 1048.Bd -literal -offset indent 1049umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1050hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1051hmac-sha1-etm@openssh.com, 1052umac-64@openssh.com,umac-128@openssh.com, 1053hmac-sha2-256,hmac-sha2-512,hmac-sha1 1054.Ed 1055.Pp 1056The list of available MAC algorithms may also be obtained using 1057.Qq ssh -Q mac . 1058.It Cm Match 1059Introduces a conditional block. 1060If all of the criteria on the 1061.Cm Match 1062line are satisfied, the keywords on the following lines override those 1063set in the global section of the config file, until either another 1064.Cm Match 1065line or the end of the file. 1066If a keyword appears in multiple 1067.Cm Match 1068blocks that are satisfied, only the first instance of the keyword is 1069applied. 1070.Pp 1071The arguments to 1072.Cm Match 1073are one or more criteria-pattern pairs or the single token 1074.Cm All 1075which matches all criteria. 1076The available criteria are 1077.Cm User , 1078.Cm Group , 1079.Cm Host , 1080.Cm LocalAddress , 1081.Cm LocalPort , 1082.Cm RDomain , 1083and 1084.Cm Address 1085(with 1086.Cm RDomain 1087representing the 1088.Xr rdomain 4 1089on which the connection was received). 1090.Pp 1091The match patterns may consist of single entries or comma-separated 1092lists and may use the wildcard and negation operators described in the 1093.Sx PATTERNS 1094section of 1095.Xr ssh_config 5 . 1096.Pp 1097The patterns in an 1098.Cm Address 1099criteria may additionally contain addresses to match in CIDR 1100address/masklen format, 1101such as 192.0.2.0/24 or 2001:db8::/32. 1102Note that the mask length provided must be consistent with the address - 1103it is an error to specify a mask length that is too long for the address 1104or one with bits set in this host portion of the address. 1105For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. 1106.Pp 1107Only a subset of keywords may be used on the lines following a 1108.Cm Match 1109keyword. 1110Available keywords are 1111.Cm AcceptEnv , 1112.Cm AllowAgentForwarding , 1113.Cm AllowGroups , 1114.Cm AllowStreamLocalForwarding , 1115.Cm AllowTcpForwarding , 1116.Cm AllowUsers , 1117.Cm AuthenticationMethods , 1118.Cm AuthorizedKeysCommand , 1119.Cm AuthorizedKeysCommandUser , 1120.Cm AuthorizedKeysFile , 1121.Cm AuthorizedPrincipalsCommand , 1122.Cm AuthorizedPrincipalsCommandUser , 1123.Cm AuthorizedPrincipalsFile , 1124.Cm Banner , 1125.Cm ChrootDirectory , 1126.Cm ClientAliveCountMax , 1127.Cm ClientAliveInterval , 1128.Cm DenyGroups , 1129.Cm DenyUsers , 1130.Cm ForceCommand , 1131.Cm GatewayPorts , 1132.Cm GSSAPIAuthentication , 1133.Cm HostbasedAcceptedKeyTypes , 1134.Cm HostbasedAuthentication , 1135.Cm HostbasedUsesNameFromPacketOnly , 1136.Cm IPQoS , 1137.Cm KbdInteractiveAuthentication , 1138.Cm KerberosAuthentication , 1139.Cm LogLevel , 1140.Cm MaxAuthTries , 1141.Cm MaxSessions , 1142.Cm PasswordAuthentication , 1143.Cm PermitEmptyPasswords , 1144.Cm PermitListen , 1145.Cm PermitOpen , 1146.Cm PermitRootLogin , 1147.Cm PermitTTY , 1148.Cm PermitTunnel , 1149.Cm PermitUserRC , 1150.Cm PubkeyAcceptedKeyTypes , 1151.Cm PubkeyAuthentication , 1152.Cm RekeyLimit , 1153.Cm RevokedKeys , 1154.Cm RDomain , 1155.Cm SetEnv , 1156.Cm StreamLocalBindMask , 1157.Cm StreamLocalBindUnlink , 1158.Cm TrustedUserCAKeys , 1159.Cm X11DisplayOffset , 1160.Cm X11Forwarding 1161and 1162.Cm X11UseLocalHost . 1163.It Cm MaxAuthTries 1164Specifies the maximum number of authentication attempts permitted per 1165connection. 1166Once the number of failures reaches half this value, 1167additional failures are logged. 1168The default is 6. 1169.It Cm MaxSessions 1170Specifies the maximum number of open shell, login or subsystem (e.g. sftp) 1171sessions permitted per network connection. 1172Multiple sessions may be established by clients that support connection 1173multiplexing. 1174Setting 1175.Cm MaxSessions 1176to 1 will effectively disable session multiplexing, whereas setting it to 0 1177will prevent all shell, login and subsystem sessions while still permitting 1178forwarding. 1179The default is 10. 1180.It Cm MaxStartups 1181Specifies the maximum number of concurrent unauthenticated connections to the 1182SSH daemon. 1183Additional connections will be dropped until authentication succeeds or the 1184.Cm LoginGraceTime 1185expires for a connection. 1186The default is 10:30:100. 1187.Pp 1188Alternatively, random early drop can be enabled by specifying 1189the three colon separated values 1190start:rate:full (e.g. "10:30:60"). 1191.Xr sshd 8 1192will refuse connection attempts with a probability of rate/100 (30%) 1193if there are currently start (10) unauthenticated connections. 1194The probability increases linearly and all connection attempts 1195are refused if the number of unauthenticated connections reaches full (60). 1196.It Cm PasswordAuthentication 1197Specifies whether password authentication is allowed. 1198The default is 1199.Cm yes . 1200.It Cm PermitEmptyPasswords 1201When password authentication is allowed, it specifies whether the 1202server allows login to accounts with empty password strings. 1203The default is 1204.Cm no . 1205.It Cm PermitListen 1206Specifies the addresses/ports on which a remote TCP port forwarding may listen. 1207The listen specification must be one of the following forms: 1208.Pp 1209.Bl -item -offset indent -compact 1210.It 1211.Cm PermitListen 1212.Sm off 1213.Ar port 1214.Sm on 1215.It 1216.Cm PermitListen 1217.Sm off 1218.Ar host : port 1219.Sm on 1220.El 1221.Pp 1222Multiple permissions may be specified by separating them with whitespace. 1223An argument of 1224.Cm any 1225can be used to remove all restrictions and permit any listen requests. 1226An argument of 1227.Cm none 1228can be used to prohibit all listen requests. 1229The host name may contain wildcards as described in the PATTERNS section in 1230.Xr ssh_config 5 . 1231The wildcard 1232.Sq * 1233can also be used in place of a port number to allow all ports. 1234By default all port forwarding listen requests are permitted. 1235Note that the 1236.Cm GatewayPorts 1237option may further restrict which addresses may be listened on. 1238Note also that 1239.Xr ssh 1 1240will request a listen host of 1241.Dq localhost 1242if no listen host was specifically requested, and this name is 1243treated differently to explicit localhost addresses of 1244.Dq 127.0.0.1 1245and 1246.Dq ::1 . 1247.It Cm PermitOpen 1248Specifies the destinations to which TCP port forwarding is permitted. 1249The forwarding specification must be one of the following forms: 1250.Pp 1251.Bl -item -offset indent -compact 1252.It 1253.Cm PermitOpen 1254.Sm off 1255.Ar host : port 1256.Sm on 1257.It 1258.Cm PermitOpen 1259.Sm off 1260.Ar IPv4_addr : port 1261.Sm on 1262.It 1263.Cm PermitOpen 1264.Sm off 1265.Ar \&[ IPv6_addr \&] : port 1266.Sm on 1267.El 1268.Pp 1269Multiple forwards may be specified by separating them with whitespace. 1270An argument of 1271.Cm any 1272can be used to remove all restrictions and permit any forwarding requests. 1273An argument of 1274.Cm none 1275can be used to prohibit all forwarding requests. 1276The wildcard 1277.Sq * 1278can be used for host or port to allow all hosts or ports, respectively. 1279By default all port forwarding requests are permitted. 1280.It Cm PermitRootLogin 1281Specifies whether root can log in using 1282.Xr ssh 1 . 1283The argument must be 1284.Cm yes , 1285.Cm prohibit-password , 1286.Cm forced-commands-only , 1287or 1288.Cm no . 1289The default is 1290.Cm prohibit-password . 1291.Pp 1292If this option is set to 1293.Cm prohibit-password 1294(or its deprecated alias, 1295.Cm without-password ) , 1296password and keyboard-interactive authentication are disabled for root. 1297.Pp 1298If this option is set to 1299.Cm forced-commands-only , 1300root login with public key authentication will be allowed, 1301but only if the 1302.Ar command 1303option has been specified 1304(which may be useful for taking remote backups even if root login is 1305normally not allowed). 1306All other authentication methods are disabled for root. 1307.Pp 1308If this option is set to 1309.Cm no , 1310root is not allowed to log in. 1311.It Cm PermitTTY 1312Specifies whether 1313.Xr pty 4 1314allocation is permitted. 1315The default is 1316.Cm yes . 1317.It Cm PermitTunnel 1318Specifies whether 1319.Xr tun 4 1320device forwarding is allowed. 1321The argument must be 1322.Cm yes , 1323.Cm point-to-point 1324(layer 3), 1325.Cm ethernet 1326(layer 2), or 1327.Cm no . 1328Specifying 1329.Cm yes 1330permits both 1331.Cm point-to-point 1332and 1333.Cm ethernet . 1334The default is 1335.Cm no . 1336.Pp 1337Independent of this setting, the permissions of the selected 1338.Xr tun 4 1339device must allow access to the user. 1340.It Cm PermitUserEnvironment 1341Specifies whether 1342.Pa ~/.ssh/environment 1343and 1344.Cm environment= 1345options in 1346.Pa ~/.ssh/authorized_keys 1347are processed by 1348.Xr sshd 8 . 1349Valid options are 1350.Cm yes , 1351.Cm no 1352or a pattern-list specifying which environment variable names to accept 1353(for example 1354.Qq LANG,LC_* ) . 1355The default is 1356.Cm no . 1357Enabling environment processing may enable users to bypass access 1358restrictions in some configurations using mechanisms such as 1359.Ev LD_PRELOAD . 1360.It Cm PermitUserRC 1361Specifies whether any 1362.Pa ~/.ssh/rc 1363file is executed. 1364The default is 1365.Cm yes . 1366.It Cm PidFile 1367Specifies the file that contains the process ID of the 1368SSH daemon, or 1369.Cm none 1370to not write one. 1371The default is 1372.Pa /var/run/sshd.pid . 1373.It Cm Port 1374Specifies the port number that 1375.Xr sshd 8 1376listens on. 1377The default is 22. 1378Multiple options of this type are permitted. 1379See also 1380.Cm ListenAddress . 1381.It Cm PrintLastLog 1382Specifies whether 1383.Xr sshd 8 1384should print the date and time of the last user login when a user logs 1385in interactively. 1386The default is 1387.Cm yes . 1388.It Cm PrintMotd 1389Specifies whether 1390.Xr sshd 8 1391should print 1392.Pa /etc/motd 1393when a user logs in interactively. 1394(On some systems it is also printed by the shell, 1395.Pa /etc/profile , 1396or equivalent.) 1397The default is 1398.Cm yes . 1399.It Cm PubkeyAcceptedKeyTypes 1400Specifies the key types that will be accepted for public key authentication 1401as a list of comma-separated patterns. 1402Alternately if the specified value begins with a 1403.Sq + 1404character, then the specified key types will be appended to the default set 1405instead of replacing them. 1406If the specified value begins with a 1407.Sq - 1408character, then the specified key types (including wildcards) will be removed 1409from the default set instead of replacing them. 1410The default for this option is: 1411.Bd -literal -offset 3n 1412ecdsa-sha2-nistp256-cert-v01@openssh.com, 1413ecdsa-sha2-nistp384-cert-v01@openssh.com, 1414ecdsa-sha2-nistp521-cert-v01@openssh.com, 1415ssh-ed25519-cert-v01@openssh.com, 1416rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com, 1417ssh-rsa-cert-v01@openssh.com, 1418ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1419ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 1420.Ed 1421.Pp 1422The list of available key types may also be obtained using 1423.Qq ssh -Q key . 1424.It Cm PubkeyAuthentication 1425Specifies whether public key authentication is allowed. 1426The default is 1427.Cm yes . 1428.It Cm RekeyLimit 1429Specifies the maximum amount of data that may be transmitted before the 1430session key is renegotiated, optionally followed a maximum amount of 1431time that may pass before the session key is renegotiated. 1432The first argument is specified in bytes and may have a suffix of 1433.Sq K , 1434.Sq M , 1435or 1436.Sq G 1437to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1438The default is between 1439.Sq 1G 1440and 1441.Sq 4G , 1442depending on the cipher. 1443The optional second value is specified in seconds and may use any of the 1444units documented in the 1445.Sx TIME FORMATS 1446section. 1447The default value for 1448.Cm RekeyLimit 1449is 1450.Cm default none , 1451which means that rekeying is performed after the cipher's default amount 1452of data has been sent or received and no time based rekeying is done. 1453.It Cm RevokedKeys 1454Specifies revoked public keys file, or 1455.Cm none 1456to not use one. 1457Keys listed in this file will be refused for public key authentication. 1458Note that if this file is not readable, then public key authentication will 1459be refused for all users. 1460Keys may be specified as a text file, listing one public key per line, or as 1461an OpenSSH Key Revocation List (KRL) as generated by 1462.Xr ssh-keygen 1 . 1463For more information on KRLs, see the KEY REVOCATION LISTS section in 1464.Xr ssh-keygen 1 . 1465.It Cm RDomain 1466Specifies an explicit routing domain that is applied after authentication 1467has completed. 1468The user session, as well and any forwarded or listening IP sockets, 1469will be bound to this 1470.Xr rdomain 4 . 1471If the routing domain is set to 1472.Cm \&%D , 1473then the domain in which the incoming connection was received will be applied. 1474.It Cm SetEnv 1475Specifies one or more environment variables to set in child sessions started 1476by 1477.Xr sshd 8 1478as 1479.Dq NAME=VALUE . 1480The environment value may be quoted (e.g. if it contains whitespace 1481characters). 1482Environment variables set by 1483.Cm SetEnv 1484override the default environment and any variables specified by the user 1485via 1486.Cm AcceptEnv 1487or 1488.Cm PermitUserEnvironment . 1489.It Cm StreamLocalBindMask 1490Sets the octal file creation mode mask 1491.Pq umask 1492used when creating a Unix-domain socket file for local or remote 1493port forwarding. 1494This option is only used for port forwarding to a Unix-domain socket file. 1495.Pp 1496The default value is 0177, which creates a Unix-domain socket file that is 1497readable and writable only by the owner. 1498Note that not all operating systems honor the file mode on Unix-domain 1499socket files. 1500.It Cm StreamLocalBindUnlink 1501Specifies whether to remove an existing Unix-domain socket file for local 1502or remote port forwarding before creating a new one. 1503If the socket file already exists and 1504.Cm StreamLocalBindUnlink 1505is not enabled, 1506.Nm sshd 1507will be unable to forward the port to the Unix-domain socket file. 1508This option is only used for port forwarding to a Unix-domain socket file. 1509.Pp 1510The argument must be 1511.Cm yes 1512or 1513.Cm no . 1514The default is 1515.Cm no . 1516.It Cm StrictModes 1517Specifies whether 1518.Xr sshd 8 1519should check file modes and ownership of the 1520user's files and home directory before accepting login. 1521This is normally desirable because novices sometimes accidentally leave their 1522directory or files world-writable. 1523The default is 1524.Cm yes . 1525Note that this does not apply to 1526.Cm ChrootDirectory , 1527whose permissions and ownership are checked unconditionally. 1528.It Cm Subsystem 1529Configures an external subsystem (e.g. file transfer daemon). 1530Arguments should be a subsystem name and a command (with optional arguments) 1531to execute upon subsystem request. 1532.Pp 1533The command 1534.Cm sftp-server 1535implements the SFTP file transfer subsystem. 1536.Pp 1537Alternately the name 1538.Cm internal-sftp 1539implements an in-process SFTP server. 1540This may simplify configurations using 1541.Cm ChrootDirectory 1542to force a different filesystem root on clients. 1543.Pp 1544By default no subsystems are defined. 1545.It Cm SyslogFacility 1546Gives the facility code that is used when logging messages from 1547.Xr sshd 8 . 1548The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1549LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1550The default is AUTH. 1551.It Cm TCPKeepAlive 1552Specifies whether the system should send TCP keepalive messages to the 1553other side. 1554If they are sent, death of the connection or crash of one 1555of the machines will be properly noticed. 1556However, this means that 1557connections will die if the route is down temporarily, and some people 1558find it annoying. 1559On the other hand, if TCP keepalives are not sent, 1560sessions may hang indefinitely on the server, leaving 1561.Qq ghost 1562users and consuming server resources. 1563.Pp 1564The default is 1565.Cm yes 1566(to send TCP keepalive messages), and the server will notice 1567if the network goes down or the client host crashes. 1568This avoids infinitely hanging sessions. 1569.Pp 1570To disable TCP keepalive messages, the value should be set to 1571.Cm no . 1572.It Cm TrustedUserCAKeys 1573Specifies a file containing public keys of certificate authorities that are 1574trusted to sign user certificates for authentication, or 1575.Cm none 1576to not use one. 1577Keys are listed one per line; empty lines and comments starting with 1578.Ql # 1579are allowed. 1580If a certificate is presented for authentication and has its signing CA key 1581listed in this file, then it may be used for authentication for any user 1582listed in the certificate's principals list. 1583Note that certificates that lack a list of principals will not be permitted 1584for authentication using 1585.Cm TrustedUserCAKeys . 1586For more details on certificates, see the CERTIFICATES section in 1587.Xr ssh-keygen 1 . 1588.It Cm UseDNS 1589Specifies whether 1590.Xr sshd 8 1591should look up the remote host name, and to check that 1592the resolved host name for the remote IP address maps back to the 1593very same IP address. 1594.Pp 1595If this option is set to 1596.Cm no 1597(the default) then only addresses and not host names may be used in 1598.Pa ~/.ssh/authorized_keys 1599.Cm from 1600and 1601.Nm 1602.Cm Match 1603.Cm Host 1604directives. 1605.It Cm VersionAddendum 1606Optionally specifies additional text to append to the SSH protocol banner 1607sent by the server upon connection. 1608The default is 1609.Cm none . 1610.It Cm X11DisplayOffset 1611Specifies the first display number available for 1612.Xr sshd 8 Ns 's 1613X11 forwarding. 1614This prevents sshd from interfering with real X11 servers. 1615The default is 10. 1616.It Cm X11Forwarding 1617Specifies whether X11 forwarding is permitted. 1618The argument must be 1619.Cm yes 1620or 1621.Cm no . 1622The default is 1623.Cm no . 1624.Pp 1625When X11 forwarding is enabled, there may be additional exposure to 1626the server and to client displays if the 1627.Xr sshd 8 1628proxy display is configured to listen on the wildcard address (see 1629.Cm X11UseLocalhost ) , 1630though this is not the default. 1631Additionally, the authentication spoofing and authentication data 1632verification and substitution occur on the client side. 1633The security risk of using X11 forwarding is that the client's X11 1634display server may be exposed to attack when the SSH client requests 1635forwarding (see the warnings for 1636.Cm ForwardX11 1637in 1638.Xr ssh_config 5 ) . 1639A system administrator may have a stance in which they want to 1640protect clients that may expose themselves to attack by unwittingly 1641requesting X11 forwarding, which can warrant a 1642.Cm no 1643setting. 1644.Pp 1645Note that disabling X11 forwarding does not prevent users from 1646forwarding X11 traffic, as users can always install their own forwarders. 1647.It Cm X11UseLocalhost 1648Specifies whether 1649.Xr sshd 8 1650should bind the X11 forwarding server to the loopback address or to 1651the wildcard address. 1652By default, 1653sshd binds the forwarding server to the loopback address and sets the 1654hostname part of the 1655.Ev DISPLAY 1656environment variable to 1657.Cm localhost . 1658This prevents remote hosts from connecting to the proxy display. 1659However, some older X11 clients may not function with this 1660configuration. 1661.Cm X11UseLocalhost 1662may be set to 1663.Cm no 1664to specify that the forwarding server should be bound to the wildcard 1665address. 1666The argument must be 1667.Cm yes 1668or 1669.Cm no . 1670The default is 1671.Cm yes . 1672.It Cm XAuthLocation 1673Specifies the full pathname of the 1674.Xr xauth 1 1675program, or 1676.Cm none 1677to not use one. 1678The default is 1679.Pa /usr/X11R6/bin/xauth . 1680.El 1681.Sh TIME FORMATS 1682.Xr sshd 8 1683command-line arguments and configuration file options that specify time 1684may be expressed using a sequence of the form: 1685.Sm off 1686.Ar time Op Ar qualifier , 1687.Sm on 1688where 1689.Ar time 1690is a positive integer value and 1691.Ar qualifier 1692is one of the following: 1693.Pp 1694.Bl -tag -width Ds -compact -offset indent 1695.It Aq Cm none 1696seconds 1697.It Cm s | Cm S 1698seconds 1699.It Cm m | Cm M 1700minutes 1701.It Cm h | Cm H 1702hours 1703.It Cm d | Cm D 1704days 1705.It Cm w | Cm W 1706weeks 1707.El 1708.Pp 1709Each member of the sequence is added together to calculate 1710the total time value. 1711.Pp 1712Time format examples: 1713.Pp 1714.Bl -tag -width Ds -compact -offset indent 1715.It 600 1716600 seconds (10 minutes) 1717.It 10m 171810 minutes 1719.It 1h30m 17201 hour 30 minutes (90 minutes) 1721.El 1722.Sh TOKENS 1723Arguments to some keywords can make use of tokens, 1724which are expanded at runtime: 1725.Pp 1726.Bl -tag -width XXXX -offset indent -compact 1727.It %% 1728A literal 1729.Sq % . 1730.It \&%D 1731The routing domain in which the incoming connection was received. 1732.It %F 1733The fingerprint of the CA key. 1734.It %f 1735The fingerprint of the key or certificate. 1736.It %h 1737The home directory of the user. 1738.It %i 1739The key ID in the certificate. 1740.It %K 1741The base64-encoded CA key. 1742.It %k 1743The base64-encoded key or certificate for authentication. 1744.It %s 1745The serial number of the certificate. 1746.It \&%T 1747The type of the CA key. 1748.It %t 1749The key or certificate type. 1750.It \&%U 1751The numeric user ID of the target user. 1752.It %u 1753The username. 1754.El 1755.Pp 1756.Cm AuthorizedKeysCommand 1757accepts the tokens %%, %f, %h, %k, %t, %U, and %u. 1758.Pp 1759.Cm AuthorizedKeysFile 1760accepts the tokens %%, %h, %U, and %u. 1761.Pp 1762.Cm AuthorizedPrincipalsCommand 1763accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u. 1764.Pp 1765.Cm AuthorizedPrincipalsFile 1766accepts the tokens %%, %h, %U, and %u. 1767.Pp 1768.Cm ChrootDirectory 1769accepts the tokens %%, %h, %U, and %u. 1770.Pp 1771.Cm RoutingDomain 1772accepts the token %D. 1773.Sh FILES 1774.Bl -tag -width Ds 1775.It Pa /etc/ssh/sshd_config 1776Contains configuration data for 1777.Xr sshd 8 . 1778This file should be writable by root only, but it is recommended 1779(though not necessary) that it be world-readable. 1780.El 1781.Sh SEE ALSO 1782.Xr sftp-server 8 , 1783.Xr sshd 8 1784.Sh AUTHORS 1785.An -nosplit 1786OpenSSH is a derivative of the original and free 1787ssh 1.2.12 release by 1788.An Tatu Ylonen . 1789.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , 1790.An Theo de Raadt 1791and 1792.An Dug Song 1793removed many bugs, re-added newer features and 1794created OpenSSH. 1795.An Markus Friedl 1796contributed the support for SSH protocol versions 1.5 and 2.0. 1797.An Niels Provos 1798and 1799.An Markus Friedl 1800contributed support for privilege separation. 1801