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 1.253 2017/09/27 06:45:53 jmc Exp $ 37.\" $FreeBSD$ 38.Dd $Mdocdate: September 27 2017 $ 39.Dt SSHD_CONFIG 5 40.Os 41.Sh NAME 42.Nm sshd_config 43.Nd OpenSSH SSH daemon configuration file 44.Sh DESCRIPTION 45.Xr sshd 8 46reads configuration data from 47.Pa /etc/ssh/sshd_config 48(or the file specified with 49.Fl f 50on the command line). 51The file contains keyword-argument pairs, one per line. 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 69in 70.Xr ssh_config 5 71for how to configure the client. 72The 73.Ev TERM 74environment variable is always sent whenever the client 75requests a pseudo-terminal as it is required by the protocol. 76Variables are specified by name, which may contain the wildcard characters 77.Ql * 78and 79.Ql \&? . 80Multiple environment variables may be separated by whitespace or spread 81across multiple 82.Cm AcceptEnv 83directives. 84Be warned that some environment variables could be used to bypass restricted 85user environments. 86For this reason, care should be taken in the use of this directive. 87The default is not to accept any environment variables. 88.It Cm AddressFamily 89Specifies which address family should be used by 90.Xr sshd 8 . 91Valid arguments are 92.Cm any 93(the default), 94.Cm inet 95(use IPv4 only), or 96.Cm inet6 97(use IPv6 only). 98.It Cm AllowAgentForwarding 99Specifies whether 100.Xr ssh-agent 1 101forwarding is permitted. 102The default is 103.Cm yes . 104Note that disabling agent forwarding does not improve security 105unless users are also denied shell access, as they can always install 106their own forwarders. 107.It Cm AllowGroups 108This keyword can be followed by a list of group name patterns, separated 109by spaces. 110If specified, login is allowed only for users whose primary 111group or supplementary group list matches one of the patterns. 112Only group names are valid; a numerical group ID is not recognized. 113By default, login is allowed for all groups. 114The allow/deny directives are processed in the following order: 115.Cm DenyUsers , 116.Cm AllowUsers , 117.Cm DenyGroups , 118and finally 119.Cm AllowGroups . 120.Pp 121See PATTERNS in 122.Xr ssh_config 5 123for more information on patterns. 124.It Cm AllowStreamLocalForwarding 125Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. 126The available options are 127.Cm yes 128(the default) 129or 130.Cm all 131to allow StreamLocal forwarding, 132.Cm no 133to prevent all StreamLocal forwarding, 134.Cm local 135to allow local (from the perspective of 136.Xr ssh 1 ) 137forwarding only or 138.Cm remote 139to allow remote forwarding only. 140Note that disabling StreamLocal forwarding does not improve security unless 141users are also denied shell access, as they can always install their 142own forwarders. 143.It Cm AllowTcpForwarding 144Specifies whether TCP forwarding is permitted. 145The available options are 146.Cm yes 147(the default) 148or 149.Cm all 150to allow TCP forwarding, 151.Cm no 152to prevent all TCP forwarding, 153.Cm local 154to allow local (from the perspective of 155.Xr ssh 1 ) 156forwarding only or 157.Cm remote 158to allow remote forwarding only. 159Note that disabling TCP forwarding does not improve security unless 160users are also denied shell access, as they can always install their 161own forwarders. 162.It Cm AllowUsers 163This keyword can be followed by a list of user name patterns, separated 164by spaces. 165If specified, login is allowed only for user names that 166match one of the patterns. 167Only user names are valid; a numerical user ID is not recognized. 168By default, login is allowed for all users. 169If the pattern takes the form USER@HOST then USER and HOST 170are separately checked, restricting logins to particular 171users from particular hosts. 172HOST criteria may additionally contain addresses to match in CIDR 173address/masklen format. 174The allow/deny directives are processed in the following order: 175.Cm DenyUsers , 176.Cm AllowUsers , 177.Cm DenyGroups , 178and finally 179.Cm AllowGroups . 180.Pp 181See PATTERNS in 182.Xr ssh_config 5 183for more information on patterns. 184.It Cm AuthenticationMethods 185Specifies the authentication methods that must be successfully completed 186for a user to be granted access. 187This option must be followed by one or more comma-separated lists of 188authentication method names, or by the single string 189.Cm any 190to indicate the default behaviour of accepting any single authentication 191method. 192If the default is overridden, then successful authentication requires 193completion of every method in at least one of these lists. 194.Pp 195For example, 196.Qq publickey,password publickey,keyboard-interactive 197would require the user to complete public key authentication, followed by 198either password or keyboard interactive authentication. 199Only methods that are next in one or more lists are offered at each stage, 200so for this example it would not be possible to attempt password or 201keyboard-interactive authentication before public key. 202.Pp 203For keyboard interactive authentication it is also possible to 204restrict authentication to a specific device by appending a 205colon followed by the device identifier 206.Cm bsdauth , 207.Cm pam , 208or 209.Cm skey , 210depending on the server configuration. 211For example, 212.Qq keyboard-interactive:bsdauth 213would restrict keyboard interactive authentication to the 214.Cm bsdauth 215device. 216.Pp 217If the publickey method is listed more than once, 218.Xr sshd 8 219verifies that keys that have been used successfully are not reused for 220subsequent authentications. 221For example, 222.Qq publickey,publickey 223requires successful authentication using two different public keys. 224.Pp 225Note that each authentication method listed should also be explicitly enabled 226in the configuration. 227.Pp 228The available authentication methods are: 229.Qq gssapi-with-mic , 230.Qq hostbased , 231.Qq keyboard-interactive , 232.Qq none 233(used for access to password-less accounts when 234.Cm PermitEmptyPassword 235is enabled), 236.Qq password 237and 238.Qq publickey . 239.It Cm AuthorizedKeysCommand 240Specifies a program to be used to look up the user's public keys. 241The program must be owned by root, not writable by group or others and 242specified by an absolute path. 243Arguments to 244.Cm AuthorizedKeysCommand 245accept the tokens described in the 246.Sx TOKENS 247section. 248If no arguments are specified then the username of the target user is used. 249.Pp 250The program should produce on standard output zero or 251more lines of authorized_keys output (see 252.Sx AUTHORIZED_KEYS 253in 254.Xr sshd 8 ) . 255If a key supplied by 256.Cm AuthorizedKeysCommand 257does not successfully authenticate 258and authorize the user then public key authentication continues using the usual 259.Cm AuthorizedKeysFile 260files. 261By default, no 262.Cm AuthorizedKeysCommand 263is run. 264.It Cm AuthorizedKeysCommandUser 265Specifies the user under whose account the 266.Cm AuthorizedKeysCommand 267is run. 268It is recommended to use a dedicated user that has no other role on the host 269than running authorized keys commands. 270If 271.Cm AuthorizedKeysCommand 272is specified but 273.Cm AuthorizedKeysCommandUser 274is not, then 275.Xr sshd 8 276will refuse to start. 277.It Cm AuthorizedKeysFile 278Specifies the file that contains the public keys used for user authentication. 279The format is described in the 280.Sx AUTHORIZED_KEYS FILE FORMAT 281section of 282.Xr sshd 8 . 283Arguments to 284.Cm AuthorizedKeysFile 285accept the tokens described in the 286.Sx TOKENS 287section. 288After expansion, 289.Cm AuthorizedKeysFile 290is taken to be an absolute path or one relative to the user's home 291directory. 292Multiple files may be listed, separated by whitespace. 293Alternately this option may be set to 294.Cm none 295to skip checking for user keys in files. 296The default is 297.Qq .ssh/authorized_keys .ssh/authorized_keys2 . 298.It Cm AuthorizedPrincipalsCommand 299Specifies a program to be used to generate the list of allowed 300certificate principals as per 301.Cm AuthorizedPrincipalsFile . 302The program must be owned by root, not writable by group or others and 303specified by an absolute path. 304Arguments to 305.Cm AuthorizedPrincipalsCommand 306accept the tokens described in the 307.Sx TOKENS 308section. 309If no arguments are specified then the username of the target user is used. 310.Pp 311The program should produce on standard output zero or 312more lines of 313.Cm AuthorizedPrincipalsFile 314output. 315If either 316.Cm AuthorizedPrincipalsCommand 317or 318.Cm AuthorizedPrincipalsFile 319is specified, then certificates offered by the client for authentication 320must contain a principal that is listed. 321By default, no 322.Cm AuthorizedPrincipalsCommand 323is run. 324.It Cm AuthorizedPrincipalsCommandUser 325Specifies the user under whose account the 326.Cm AuthorizedPrincipalsCommand 327is run. 328It is recommended to use a dedicated user that has no other role on the host 329than running authorized principals commands. 330If 331.Cm AuthorizedPrincipalsCommand 332is specified but 333.Cm AuthorizedPrincipalsCommandUser 334is not, then 335.Xr sshd 8 336will refuse to start. 337.It Cm AuthorizedPrincipalsFile 338Specifies a file that lists principal names that are accepted for 339certificate authentication. 340When using certificates signed by a key listed in 341.Cm TrustedUserCAKeys , 342this file lists names, one of which must appear in the certificate for it 343to be accepted for authentication. 344Names are listed one per line preceded by key options (as described in 345.Sx AUTHORIZED_KEYS FILE FORMAT 346in 347.Xr sshd 8 ) . 348Empty lines and comments starting with 349.Ql # 350are ignored. 351.Pp 352Arguments to 353.Cm AuthorizedPrincipalsFile 354accept the tokens described in the 355.Sx TOKENS 356section. 357After expansion, 358.Cm AuthorizedPrincipalsFile 359is taken to be an absolute path or one relative to the user's home directory. 360The default is 361.Cm none , 362i.e. not to use a principals file \(en in this case, the username 363of the user must appear in a certificate's principals list for it to be 364accepted. 365.Pp 366Note that 367.Cm AuthorizedPrincipalsFile 368is only used when authentication proceeds using a CA listed in 369.Cm TrustedUserCAKeys 370and is not consulted for certification authorities trusted via 371.Pa ~/.ssh/authorized_keys , 372though the 373.Cm principals= 374key option offers a similar facility (see 375.Xr sshd 8 376for details). 377.It Cm Banner 378The contents of the specified file are sent to the remote user before 379authentication is allowed. 380If the argument is 381.Cm none 382then no banner is displayed. 383By default, no banner is displayed. 384.It Cm ChallengeResponseAuthentication 385Specifies whether challenge-response authentication is allowed (e.g. via 386PAM or through authentication styles supported in 387.Xr login.conf 5 ) 388The default is 389.Cm yes . 390.It Cm ChrootDirectory 391Specifies the pathname of a directory to 392.Xr chroot 2 393to after authentication. 394At session startup 395.Xr sshd 8 396checks that all components of the pathname are root-owned directories 397which are not writable by any other user or group. 398After the chroot, 399.Xr sshd 8 400changes the working directory to the user's home directory. 401Arguments to 402.Cm ChrootDirectory 403accept the tokens described in the 404.Sx TOKENS 405section. 406.Pp 407The 408.Cm ChrootDirectory 409must contain the necessary files and directories to support the 410user's session. 411For an interactive session this requires at least a shell, typically 412.Xr sh 1 , 413and basic 414.Pa /dev 415nodes such as 416.Xr null 4 , 417.Xr zero 4 , 418.Xr stdin 4 , 419.Xr stdout 4 , 420.Xr stderr 4 , 421and 422.Xr tty 4 423devices. 424For file transfer sessions using SFTP 425no additional configuration of the environment is necessary if the in-process 426sftp-server is used, 427though sessions which use logging may require 428.Pa /dev/log 429inside the chroot directory on some operating systems (see 430.Xr sftp-server 8 431for details). 432.Pp 433For safety, it is very important that the directory hierarchy be 434prevented from modification by other processes on the system (especially 435those outside the jail). 436Misconfiguration can lead to unsafe environments which 437.Xr sshd 8 438cannot detect. 439.Pp 440The default is 441.Cm none , 442indicating not to 443.Xr chroot 2 . 444.It Cm Ciphers 445Specifies the ciphers allowed. 446Multiple ciphers must be comma-separated. 447If the specified value begins with a 448.Sq + 449character, then the specified ciphers will be appended to the default set 450instead of replacing them. 451If the specified value begins with a 452.Sq - 453character, then the specified ciphers (including wildcards) will be removed 454from the default set instead of replacing them. 455.Pp 456The supported ciphers are: 457.Pp 458.Bl -item -compact -offset indent 459.It 4603des-cbc 461.It 462aes128-cbc 463.It 464aes192-cbc 465.It 466aes256-cbc 467.It 468aes128-ctr 469.It 470aes192-ctr 471.It 472aes256-ctr 473.It 474aes128-gcm@openssh.com 475.It 476aes256-gcm@openssh.com 477.It 478chacha20-poly1305@openssh.com 479.El 480.Pp 481The default is: 482.Bd -literal -offset indent 483chacha20-poly1305@openssh.com, 484aes128-ctr,aes192-ctr,aes256-ctr, 485aes128-gcm@openssh.com,aes256-gcm@openssh.com, 486aes128-cbc,aes192-cbc,aes256-cbc 487.Ed 488.Pp 489The list of available ciphers may also be obtained using 490.Qq ssh -Q cipher . 491.It Cm ClientAliveCountMax 492Sets the number of client alive messages which may be sent without 493.Xr sshd 8 494receiving any messages back from the client. 495If this threshold is reached while client alive messages are being sent, 496sshd will disconnect the client, terminating the session. 497It is important to note that the use of client alive messages is very 498different from 499.Cm TCPKeepAlive . 500The client alive messages are sent through the encrypted channel 501and therefore will not be spoofable. 502The TCP keepalive option enabled by 503.Cm TCPKeepAlive 504is spoofable. 505The client alive mechanism is valuable when the client or 506server depend on knowing when a connection has become inactive. 507.Pp 508The default value is 3. 509If 510.Cm ClientAliveInterval 511is set to 15, and 512.Cm ClientAliveCountMax 513is left at the default, unresponsive SSH clients 514will be disconnected after approximately 45 seconds. 515.It Cm ClientAliveInterval 516Sets a timeout interval in seconds after which if no data has been received 517from the client, 518.Xr sshd 8 519will send a message through the encrypted 520channel to request a response from the client. 521The default 522is 0, indicating that these messages will not be sent to the client. 523.It Cm Compression 524Specifies whether compression is enabled after 525the user has authenticated successfully. 526The argument must be 527.Cm yes , 528.Cm delayed 529(a legacy synonym for 530.Cm yes ) 531or 532.Cm no . 533The default is 534.Cm yes . 535.It Cm DenyGroups 536This keyword can be followed by a list of group name patterns, separated 537by spaces. 538Login is disallowed for users whose primary group or supplementary 539group list matches one of the patterns. 540Only group names are valid; a numerical group ID is not recognized. 541By default, login is allowed for all groups. 542The allow/deny directives are processed in the following order: 543.Cm DenyUsers , 544.Cm AllowUsers , 545.Cm DenyGroups , 546and finally 547.Cm AllowGroups . 548.Pp 549See PATTERNS in 550.Xr ssh_config 5 551for more information on patterns. 552.It Cm DenyUsers 553This keyword can be followed by a list of user name patterns, separated 554by spaces. 555Login is disallowed for user names that match one of the patterns. 556Only user names are valid; a numerical user ID is not recognized. 557By default, login is allowed for all users. 558If the pattern takes the form USER@HOST then USER and HOST 559are separately checked, restricting logins to particular 560users from particular hosts. 561HOST criteria may additionally contain addresses to match in CIDR 562address/masklen format. 563The allow/deny directives are processed in the following order: 564.Cm DenyUsers , 565.Cm AllowUsers , 566.Cm DenyGroups , 567and finally 568.Cm AllowGroups . 569.Pp 570See PATTERNS in 571.Xr ssh_config 5 572for more information on patterns. 573.It Cm DisableForwarding 574Disables all forwarding features, including X11, 575.Xr ssh-agent 1 , 576TCP and StreamLocal. 577This option overrides all other forwarding-related options and may 578simplify restricted configurations. 579.It Cm ExposeAuthInfo 580Writes a temporary file containing a list of authentication methods and 581public credentials (e.g. keys) used to authenticate the user. 582The location of the file is exposed to the user session through the 583.Ev SSH_USER_AUTH 584environment variable. 585The default is 586.Cm no . 587.It Cm FingerprintHash 588Specifies the hash algorithm used when logging key fingerprints. 589Valid options are: 590.Cm md5 591and 592.Cm sha256 . 593The default is 594.Cm sha256 . 595.It Cm ForceCommand 596Forces the execution of the command specified by 597.Cm ForceCommand , 598ignoring any command supplied by the client and 599.Pa ~/.ssh/rc 600if present. 601The command is invoked by using the user's login shell with the -c option. 602This applies to shell, command, or subsystem execution. 603It is most useful inside a 604.Cm Match 605block. 606The command originally supplied by the client is available in the 607.Ev SSH_ORIGINAL_COMMAND 608environment variable. 609Specifying a command of 610.Cm internal-sftp 611will force the use of an in-process SFTP server that requires no support 612files when used with 613.Cm ChrootDirectory . 614The default is 615.Cm none . 616.It Cm GatewayPorts 617Specifies whether remote hosts are allowed to connect to ports 618forwarded for the client. 619By default, 620.Xr sshd 8 621binds remote port forwardings to the loopback address. 622This prevents other remote hosts from connecting to forwarded ports. 623.Cm GatewayPorts 624can be used to specify that sshd 625should allow remote port forwardings to bind to non-loopback addresses, thus 626allowing other hosts to connect. 627The argument may be 628.Cm no 629to force remote port forwardings to be available to the local host only, 630.Cm yes 631to force remote port forwardings to bind to the wildcard address, or 632.Cm clientspecified 633to allow the client to select the address to which the forwarding is bound. 634The default is 635.Cm no . 636.It Cm GSSAPIAuthentication 637Specifies whether user authentication based on GSSAPI is allowed. 638The default is 639.Cm no . 640.It Cm GSSAPICleanupCredentials 641Specifies whether to automatically destroy the user's credentials cache 642on logout. 643The default is 644.Cm yes . 645.It Cm GSSAPIStrictAcceptorCheck 646Determines whether to be strict about the identity of the GSSAPI acceptor 647a client authenticates against. 648If set to 649.Cm yes 650then the client must authenticate against the host 651service on the current hostname. 652If set to 653.Cm no 654then the client may authenticate against any service key stored in the 655machine's default store. 656This facility is provided to assist with operation on multi homed machines. 657The default is 658.Cm yes . 659.It Cm HostbasedAcceptedKeyTypes 660Specifies the key types that will be accepted for hostbased authentication 661as a comma-separated pattern list. 662Alternately if the specified value begins with a 663.Sq + 664character, then the specified key types will be appended to the default set 665instead of replacing them. 666If the specified value begins with a 667.Sq - 668character, then the specified key types (including wildcards) will be removed 669from the default set instead of replacing them. 670The default for this option is: 671.Bd -literal -offset 3n 672ecdsa-sha2-nistp256-cert-v01@openssh.com, 673ecdsa-sha2-nistp384-cert-v01@openssh.com, 674ecdsa-sha2-nistp521-cert-v01@openssh.com, 675ssh-ed25519-cert-v01@openssh.com, 676ssh-rsa-cert-v01@openssh.com, 677ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 678ssh-ed25519,ssh-rsa 679.Ed 680.Pp 681The list of available key types may also be obtained using 682.Qq ssh -Q key . 683.It Cm HostbasedAuthentication 684Specifies whether rhosts or /etc/hosts.equiv authentication together 685with successful public key client host authentication is allowed 686(host-based authentication). 687The default is 688.Cm no . 689.It Cm HostbasedUsesNameFromPacketOnly 690Specifies whether or not the server will attempt to perform a reverse 691name lookup when matching the name in the 692.Pa ~/.shosts , 693.Pa ~/.rhosts , 694and 695.Pa /etc/hosts.equiv 696files during 697.Cm HostbasedAuthentication . 698A setting of 699.Cm yes 700means that 701.Xr sshd 8 702uses the name supplied by the client rather than 703attempting to resolve the name from the TCP connection itself. 704The default is 705.Cm no . 706.It Cm HostCertificate 707Specifies a file containing a public host certificate. 708The certificate's public key must match a private host key already specified 709by 710.Cm HostKey . 711The default behaviour of 712.Xr sshd 8 713is not to load any certificates. 714.It Cm HostKey 715Specifies a file containing a private host key 716used by SSH. 717The defaults are 718.Pa /etc/ssh/ssh_host_dsa_key , 719.Pa /etc/ssh/ssh_host_ecdsa_key , 720.Pa /etc/ssh/ssh_host_ed25519_key 721and 722.Pa /etc/ssh/ssh_host_rsa_key . 723.Pp 724Note that 725.Xr sshd 8 726will refuse to use a file if it is group/world-accessible 727and that the 728.Cm HostKeyAlgorithms 729option restricts which of the keys are actually used by 730.Xr sshd 8 . 731.Pp 732It is possible to have multiple host key files. 733It is also possible to specify public host key files instead. 734In this case operations on the private key will be delegated 735to an 736.Xr ssh-agent 1 . 737.It Cm HostKeyAgent 738Identifies the UNIX-domain socket used to communicate 739with an agent that has access to the private host keys. 740If the string 741.Qq SSH_AUTH_SOCK 742is specified, the location of the socket will be read from the 743.Ev SSH_AUTH_SOCK 744environment variable. 745.It Cm HostKeyAlgorithms 746Specifies the host key algorithms 747that the server offers. 748The default for this option is: 749.Bd -literal -offset 3n 750ecdsa-sha2-nistp256-cert-v01@openssh.com, 751ecdsa-sha2-nistp384-cert-v01@openssh.com, 752ecdsa-sha2-nistp521-cert-v01@openssh.com, 753ssh-ed25519-cert-v01@openssh.com, 754ssh-rsa-cert-v01@openssh.com, 755ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 756ssh-ed25519,ssh-rsa 757.Ed 758.Pp 759The list of available key types may also be obtained using 760.Qq ssh -Q key . 761.It Cm IgnoreRhosts 762Specifies that 763.Pa .rhosts 764and 765.Pa .shosts 766files will not be used in 767.Cm HostbasedAuthentication . 768.Pp 769.Pa /etc/hosts.equiv 770and 771.Pa /etc/ssh/shosts.equiv 772are still used. 773The default is 774.Cm yes . 775.It Cm IgnoreUserKnownHosts 776Specifies whether 777.Xr sshd 8 778should ignore the user's 779.Pa ~/.ssh/known_hosts 780during 781.Cm HostbasedAuthentication . 782The default is 783.Cm no . 784.It Cm IPQoS 785Specifies the IPv4 type-of-service or DSCP class for the connection. 786Accepted values are 787.Cm af11 , 788.Cm af12 , 789.Cm af13 , 790.Cm af21 , 791.Cm af22 , 792.Cm af23 , 793.Cm af31 , 794.Cm af32 , 795.Cm af33 , 796.Cm af41 , 797.Cm af42 , 798.Cm af43 , 799.Cm cs0 , 800.Cm cs1 , 801.Cm cs2 , 802.Cm cs3 , 803.Cm cs4 , 804.Cm cs5 , 805.Cm cs6 , 806.Cm cs7 , 807.Cm ef , 808.Cm lowdelay , 809.Cm throughput , 810.Cm reliability , 811a numeric value, or 812.Cm none 813to use the operating system default. 814This option may take one or two arguments, separated by whitespace. 815If one argument is specified, it is used as the packet class unconditionally. 816If two values are specified, the first is automatically selected for 817interactive sessions and the second for non-interactive sessions. 818The default is 819.Cm lowdelay 820for interactive sessions and 821.Cm throughput 822for non-interactive sessions. 823.It Cm KbdInteractiveAuthentication 824Specifies whether to allow keyboard-interactive authentication. 825The argument to this keyword must be 826.Cm yes 827or 828.Cm no . 829The default is to use whatever value 830.Cm ChallengeResponseAuthentication 831is set to 832(by default 833.Cm yes ) . 834.It Cm KerberosAuthentication 835Specifies whether the password provided by the user for 836.Cm PasswordAuthentication 837will be validated through the Kerberos KDC. 838To use this option, the server needs a 839Kerberos servtab which allows the verification of the KDC's identity. 840The default is 841.Cm no . 842.It Cm KerberosGetAFSToken 843If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 844an AFS token before accessing the user's home directory. 845The default is 846.Cm no . 847.It Cm KerberosOrLocalPasswd 848If password authentication through Kerberos fails then 849the password will be validated via any additional local mechanism 850such as 851.Pa /etc/passwd . 852The default is 853.Cm yes . 854.It Cm KerberosTicketCleanup 855Specifies whether to automatically destroy the user's ticket cache 856file on logout. 857The default is 858.Cm yes . 859.It Cm KexAlgorithms 860Specifies the available KEX (Key Exchange) algorithms. 861Multiple algorithms must be comma-separated. 862Alternately if the specified value begins with a 863.Sq + 864character, then the specified methods will be appended to the default set 865instead of replacing them. 866If the specified value begins with a 867.Sq - 868character, then the specified methods (including wildcards) will be removed 869from the default set instead of replacing them. 870The supported algorithms are: 871.Pp 872.Bl -item -compact -offset indent 873.It 874curve25519-sha256 875.It 876curve25519-sha256@libssh.org 877.It 878diffie-hellman-group1-sha1 879.It 880diffie-hellman-group14-sha1 881.It 882diffie-hellman-group-exchange-sha1 883.It 884diffie-hellman-group-exchange-sha256 885.It 886ecdh-sha2-nistp256 887.It 888ecdh-sha2-nistp384 889.It 890ecdh-sha2-nistp521 891.El 892.Pp 893The default is: 894.Bd -literal -offset indent 895curve25519-sha256,curve25519-sha256@libssh.org, 896ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 897diffie-hellman-group-exchange-sha256, 898diffie-hellman-group14-sha1 899.Ed 900.Pp 901The list of available key exchange algorithms may also be obtained using 902.Qq ssh -Q kex . 903.It Cm ListenAddress 904Specifies the local addresses 905.Xr sshd 8 906should listen on. 907The following forms may be used: 908.Pp 909.Bl -item -offset indent -compact 910.It 911.Cm ListenAddress 912.Sm off 913.Ar host | Ar IPv4_addr | Ar IPv6_addr 914.Sm on 915.It 916.Cm ListenAddress 917.Sm off 918.Ar host | Ar IPv4_addr : Ar port 919.Sm on 920.It 921.Cm ListenAddress 922.Sm off 923.Oo 924.Ar host | Ar IPv6_addr Oc : Ar port 925.Sm on 926.El 927.Pp 928If 929.Ar port 930is not specified, 931sshd will listen on the address and all 932.Cm Port 933options specified. 934The default is to listen on all local addresses. 935Multiple 936.Cm ListenAddress 937options are permitted. 938.It Cm LoginGraceTime 939The server disconnects after this time if the user has not 940successfully logged in. 941If the value is 0, there is no time limit. 942The default is 120 seconds. 943.It Cm LogLevel 944Gives the verbosity level that is used when logging messages from 945.Xr sshd 8 . 946The possible values are: 947QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 948The default is INFO. 949DEBUG and DEBUG1 are equivalent. 950DEBUG2 and DEBUG3 each specify higher levels of debugging output. 951Logging with a DEBUG level violates the privacy of users and is not recommended. 952.It Cm MACs 953Specifies the available MAC (message authentication code) algorithms. 954The MAC algorithm is used for data integrity protection. 955Multiple algorithms must be comma-separated. 956If the specified value begins with a 957.Sq + 958character, then the specified algorithms will be appended to the default set 959instead of replacing them. 960If the specified value begins with a 961.Sq - 962character, then the specified algorithms (including wildcards) will be removed 963from the default set instead of replacing them. 964.Pp 965The algorithms that contain 966.Qq -etm 967calculate the MAC after encryption (encrypt-then-mac). 968These are considered safer and their use recommended. 969The supported MACs are: 970.Pp 971.Bl -item -compact -offset indent 972.It 973hmac-md5 974.It 975hmac-md5-96 976.It 977hmac-sha1 978.It 979hmac-sha1-96 980.It 981hmac-sha2-256 982.It 983hmac-sha2-512 984.It 985umac-64@openssh.com 986.It 987umac-128@openssh.com 988.It 989hmac-md5-etm@openssh.com 990.It 991hmac-md5-96-etm@openssh.com 992.It 993hmac-sha1-etm@openssh.com 994.It 995hmac-sha1-96-etm@openssh.com 996.It 997hmac-sha2-256-etm@openssh.com 998.It 999hmac-sha2-512-etm@openssh.com 1000.It 1001umac-64-etm@openssh.com 1002.It 1003umac-128-etm@openssh.com 1004.El 1005.Pp 1006The default is: 1007.Bd -literal -offset indent 1008umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1009hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1010hmac-sha1-etm@openssh.com, 1011umac-64@openssh.com,umac-128@openssh.com, 1012hmac-sha2-256,hmac-sha2-512,hmac-sha1 1013.Ed 1014.Pp 1015The list of available MAC algorithms may also be obtained using 1016.Qq ssh -Q mac . 1017.It Cm Match 1018Introduces a conditional block. 1019If all of the criteria on the 1020.Cm Match 1021line are satisfied, the keywords on the following lines override those 1022set in the global section of the config file, until either another 1023.Cm Match 1024line or the end of the file. 1025If a keyword appears in multiple 1026.Cm Match 1027blocks that are satisfied, only the first instance of the keyword is 1028applied. 1029.Pp 1030The arguments to 1031.Cm Match 1032are one or more criteria-pattern pairs or the single token 1033.Cm All 1034which matches all criteria. 1035The available criteria are 1036.Cm User , 1037.Cm Group , 1038.Cm Host , 1039.Cm LocalAddress , 1040.Cm LocalPort , 1041and 1042.Cm Address . 1043The match patterns may consist of single entries or comma-separated 1044lists and may use the wildcard and negation operators described in the 1045.Sx PATTERNS 1046section of 1047.Xr ssh_config 5 . 1048.Pp 1049The patterns in an 1050.Cm Address 1051criteria may additionally contain addresses to match in CIDR 1052address/masklen format, 1053such as 192.0.2.0/24 or 2001:db8::/32. 1054Note that the mask length provided must be consistent with the address - 1055it is an error to specify a mask length that is too long for the address 1056or one with bits set in this host portion of the address. 1057For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. 1058.Pp 1059Only a subset of keywords may be used on the lines following a 1060.Cm Match 1061keyword. 1062Available keywords are 1063.Cm AcceptEnv , 1064.Cm AllowAgentForwarding , 1065.Cm AllowGroups , 1066.Cm AllowStreamLocalForwarding , 1067.Cm AllowTcpForwarding , 1068.Cm AllowUsers , 1069.Cm AuthenticationMethods , 1070.Cm AuthorizedKeysCommand , 1071.Cm AuthorizedKeysCommandUser , 1072.Cm AuthorizedKeysFile , 1073.Cm AuthorizedPrincipalsCommand , 1074.Cm AuthorizedPrincipalsCommandUser , 1075.Cm AuthorizedPrincipalsFile , 1076.Cm Banner , 1077.Cm ChrootDirectory , 1078.Cm ClientAliveCountMax , 1079.Cm ClientAliveInterval , 1080.Cm DenyGroups , 1081.Cm DenyUsers , 1082.Cm ForceCommand , 1083.Cm GatewayPorts , 1084.Cm GSSAPIAuthentication , 1085.Cm HostbasedAcceptedKeyTypes , 1086.Cm HostbasedAuthentication , 1087.Cm HostbasedUsesNameFromPacketOnly , 1088.Cm IPQoS , 1089.Cm KbdInteractiveAuthentication , 1090.Cm KerberosAuthentication , 1091.Cm LogLevel , 1092.Cm MaxAuthTries , 1093.Cm MaxSessions , 1094.Cm PasswordAuthentication , 1095.Cm PermitEmptyPasswords , 1096.Cm PermitOpen , 1097.Cm PermitRootLogin , 1098.Cm PermitTTY , 1099.Cm PermitTunnel , 1100.Cm PermitUserRC , 1101.Cm PubkeyAcceptedKeyTypes , 1102.Cm PubkeyAuthentication , 1103.Cm RekeyLimit , 1104.Cm RevokedKeys , 1105.Cm StreamLocalBindMask , 1106.Cm StreamLocalBindUnlink , 1107.Cm TrustedUserCAKeys , 1108.Cm X11DisplayOffset , 1109.Cm X11Forwarding 1110and 1111.Cm X11UseLocalHost . 1112.It Cm MaxAuthTries 1113Specifies the maximum number of authentication attempts permitted per 1114connection. 1115Once the number of failures reaches half this value, 1116additional failures are logged. 1117The default is 6. 1118.It Cm MaxSessions 1119Specifies the maximum number of open shell, login or subsystem (e.g. sftp) 1120sessions permitted per network connection. 1121Multiple sessions may be established by clients that support connection 1122multiplexing. 1123Setting 1124.Cm MaxSessions 1125to 1 will effectively disable session multiplexing, whereas setting it to 0 1126will prevent all shell, login and subsystem sessions while still permitting 1127forwarding. 1128The default is 10. 1129.It Cm MaxStartups 1130Specifies the maximum number of concurrent unauthenticated connections to the 1131SSH daemon. 1132Additional connections will be dropped until authentication succeeds or the 1133.Cm LoginGraceTime 1134expires for a connection. 1135The default is 10:30:100. 1136.Pp 1137Alternatively, random early drop can be enabled by specifying 1138the three colon separated values 1139start:rate:full (e.g. "10:30:60"). 1140.Xr sshd 8 1141will refuse connection attempts with a probability of rate/100 (30%) 1142if there are currently start (10) unauthenticated connections. 1143The probability increases linearly and all connection attempts 1144are refused if the number of unauthenticated connections reaches full (60). 1145.It Cm PasswordAuthentication 1146Specifies whether password authentication is allowed. 1147See also 1148.Cm UsePAM . 1149The default is 1150.Cm no . 1151.It Cm PermitEmptyPasswords 1152When password authentication is allowed, it specifies whether the 1153server allows login to accounts with empty password strings. 1154The default is 1155.Cm no . 1156.It Cm PermitOpen 1157Specifies the destinations to which TCP port forwarding is permitted. 1158The forwarding specification must be one of the following forms: 1159.Pp 1160.Bl -item -offset indent -compact 1161.It 1162.Cm PermitOpen 1163.Sm off 1164.Ar host : port 1165.Sm on 1166.It 1167.Cm PermitOpen 1168.Sm off 1169.Ar IPv4_addr : port 1170.Sm on 1171.It 1172.Cm PermitOpen 1173.Sm off 1174.Ar \&[ IPv6_addr \&] : port 1175.Sm on 1176.El 1177.Pp 1178Multiple forwards may be specified by separating them with whitespace. 1179An argument of 1180.Cm any 1181can be used to remove all restrictions and permit any forwarding requests. 1182An argument of 1183.Cm none 1184can be used to prohibit all forwarding requests. 1185The wildcard 1186.Sq * 1187can be used for host or port to allow all hosts or ports, respectively. 1188By default all port forwarding requests are permitted. 1189.It Cm PermitRootLogin 1190Specifies whether root can log in using 1191.Xr ssh 1 . 1192The argument must be 1193.Cm yes , 1194.Cm prohibit-password , 1195.Cm without-password , 1196.Cm forced-commands-only , 1197or 1198.Cm no . 1199The default is 1200.Cm no . 1201Note that if 1202.Cm ChallengeResponseAuthentication 1203and 1204.Cm UsePAM 1205are both 1206.Cm yes , 1207this setting may be overridden by the PAM policy. 1208.Pp 1209If this option is set to 1210.Cm prohibit-password 1211or 1212.Cm without-password , 1213password and keyboard-interactive authentication are disabled for root. 1214.Pp 1215If this option is set to 1216.Cm forced-commands-only , 1217root login with public key authentication will be allowed, 1218but only if the 1219.Ar command 1220option has been specified 1221(which may be useful for taking remote backups even if root login is 1222normally not allowed). 1223All other authentication methods are disabled for root. 1224.Pp 1225If this option is set to 1226.Cm no , 1227root is not allowed to log in. 1228.It Cm PermitTTY 1229Specifies whether 1230.Xr pty 4 1231allocation is permitted. 1232The default is 1233.Cm yes . 1234.It Cm PermitTunnel 1235Specifies whether 1236.Xr tun 4 1237device forwarding is allowed. 1238The argument must be 1239.Cm yes , 1240.Cm point-to-point 1241(layer 3), 1242.Cm ethernet 1243(layer 2), or 1244.Cm no . 1245Specifying 1246.Cm yes 1247permits both 1248.Cm point-to-point 1249and 1250.Cm ethernet . 1251The default is 1252.Cm no . 1253.Pp 1254Independent of this setting, the permissions of the selected 1255.Xr tun 4 1256device must allow access to the user. 1257.It Cm PermitUserEnvironment 1258Specifies whether 1259.Pa ~/.ssh/environment 1260and 1261.Cm environment= 1262options in 1263.Pa ~/.ssh/authorized_keys 1264are processed by 1265.Xr sshd 8 . 1266The default is 1267.Cm no . 1268Enabling environment processing may enable users to bypass access 1269restrictions in some configurations using mechanisms such as 1270.Ev LD_PRELOAD . 1271.It Cm PermitUserRC 1272Specifies whether any 1273.Pa ~/.ssh/rc 1274file is executed. 1275The default is 1276.Cm yes . 1277.It Cm PidFile 1278Specifies the file that contains the process ID of the 1279SSH daemon, or 1280.Cm none 1281to not write one. 1282The default is 1283.Pa /var/run/sshd.pid . 1284.It Cm Port 1285Specifies the port number that 1286.Xr sshd 8 1287listens on. 1288The default is 22. 1289Multiple options of this type are permitted. 1290See also 1291.Cm ListenAddress . 1292.It Cm PrintLastLog 1293Specifies whether 1294.Xr sshd 8 1295should print the date and time of the last user login when a user logs 1296in interactively. 1297The default is 1298.Cm yes . 1299.It Cm PrintMotd 1300Specifies whether 1301.Xr sshd 8 1302should print 1303.Pa /etc/motd 1304when a user logs in interactively. 1305(On some systems it is also printed by the shell, 1306.Pa /etc/profile , 1307or equivalent.) 1308The default is 1309.Cm yes . 1310.It Cm PubkeyAcceptedKeyTypes 1311Specifies the key types that will be accepted for public key authentication 1312as a comma-separated pattern list. 1313Alternately if the specified value begins with a 1314.Sq + 1315character, then the specified key types will be appended to the default set 1316instead of replacing them. 1317If the specified value begins with a 1318.Sq - 1319character, then the specified key types (including wildcards) will be removed 1320from the default set instead of replacing them. 1321The default for this option is: 1322.Bd -literal -offset 3n 1323ecdsa-sha2-nistp256-cert-v01@openssh.com, 1324ecdsa-sha2-nistp384-cert-v01@openssh.com, 1325ecdsa-sha2-nistp521-cert-v01@openssh.com, 1326ssh-ed25519-cert-v01@openssh.com, 1327ssh-rsa-cert-v01@openssh.com, 1328ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1329ssh-ed25519,ssh-rsa 1330.Ed 1331.Pp 1332The list of available key types may also be obtained using 1333.Qq ssh -Q key . 1334.It Cm PubkeyAuthentication 1335Specifies whether public key authentication is allowed. 1336The default is 1337.Cm yes . 1338.It Cm RekeyLimit 1339Specifies the maximum amount of data that may be transmitted before the 1340session key is renegotiated, optionally followed a maximum amount of 1341time that may pass before the session key is renegotiated. 1342The first argument is specified in bytes and may have a suffix of 1343.Sq K , 1344.Sq M , 1345or 1346.Sq G 1347to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1348The default is between 1349.Sq 1G 1350and 1351.Sq 4G , 1352depending on the cipher. 1353The optional second value is specified in seconds and may use any of the 1354units documented in the 1355.Sx TIME FORMATS 1356section. 1357The default value for 1358.Cm RekeyLimit 1359is 1360.Cm default none , 1361which means that rekeying is performed after the cipher's default amount 1362of data has been sent or received and no time based rekeying is done. 1363.It Cm RevokedKeys 1364Specifies revoked public keys file, or 1365.Cm none 1366to not use one. 1367Keys listed in this file will be refused for public key authentication. 1368Note that if this file is not readable, then public key authentication will 1369be refused for all users. 1370Keys may be specified as a text file, listing one public key per line, or as 1371an OpenSSH Key Revocation List (KRL) as generated by 1372.Xr ssh-keygen 1 . 1373For more information on KRLs, see the KEY REVOCATION LISTS section in 1374.Xr ssh-keygen 1 . 1375.It Cm StreamLocalBindMask 1376Sets the octal file creation mode mask 1377.Pq umask 1378used when creating a Unix-domain socket file for local or remote 1379port forwarding. 1380This option is only used for port forwarding to a Unix-domain socket file. 1381.Pp 1382The default value is 0177, which creates a Unix-domain socket file that is 1383readable and writable only by the owner. 1384Note that not all operating systems honor the file mode on Unix-domain 1385socket files. 1386.It Cm StreamLocalBindUnlink 1387Specifies whether to remove an existing Unix-domain socket file for local 1388or remote port forwarding before creating a new one. 1389If the socket file already exists and 1390.Cm StreamLocalBindUnlink 1391is not enabled, 1392.Nm sshd 1393will be unable to forward the port to the Unix-domain socket file. 1394This option is only used for port forwarding to a Unix-domain socket file. 1395.Pp 1396The argument must be 1397.Cm yes 1398or 1399.Cm no . 1400The default is 1401.Cm no . 1402.It Cm StrictModes 1403Specifies whether 1404.Xr sshd 8 1405should check file modes and ownership of the 1406user's files and home directory before accepting login. 1407This is normally desirable because novices sometimes accidentally leave their 1408directory or files world-writable. 1409The default is 1410.Cm yes . 1411Note that this does not apply to 1412.Cm ChrootDirectory , 1413whose permissions and ownership are checked unconditionally. 1414.It Cm Subsystem 1415Configures an external subsystem (e.g. file transfer daemon). 1416Arguments should be a subsystem name and a command (with optional arguments) 1417to execute upon subsystem request. 1418.Pp 1419The command 1420.Cm sftp-server 1421implements the SFTP file transfer subsystem. 1422.Pp 1423Alternately the name 1424.Cm internal-sftp 1425implements an in-process SFTP server. 1426This may simplify configurations using 1427.Cm ChrootDirectory 1428to force a different filesystem root on clients. 1429.Pp 1430By default no subsystems are defined. 1431.It Cm SyslogFacility 1432Gives the facility code that is used when logging messages from 1433.Xr sshd 8 . 1434The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1435LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1436The default is AUTH. 1437.It Cm TCPKeepAlive 1438Specifies whether the system should send TCP keepalive messages to the 1439other side. 1440If they are sent, death of the connection or crash of one 1441of the machines will be properly noticed. 1442However, this means that 1443connections will die if the route is down temporarily, and some people 1444find it annoying. 1445On the other hand, if TCP keepalives are not sent, 1446sessions may hang indefinitely on the server, leaving 1447.Qq ghost 1448users and consuming server resources. 1449.Pp 1450The default is 1451.Cm yes 1452(to send TCP keepalive messages), and the server will notice 1453if the network goes down or the client host crashes. 1454This avoids infinitely hanging sessions. 1455.Pp 1456To disable TCP keepalive messages, the value should be set to 1457.Cm no . 1458.It Cm TrustedUserCAKeys 1459Specifies a file containing public keys of certificate authorities that are 1460trusted to sign user certificates for authentication, or 1461.Cm none 1462to not use one. 1463Keys are listed one per line; empty lines and comments starting with 1464.Ql # 1465are allowed. 1466If a certificate is presented for authentication and has its signing CA key 1467listed in this file, then it may be used for authentication for any user 1468listed in the certificate's principals list. 1469Note that certificates that lack a list of principals will not be permitted 1470for authentication using 1471.Cm TrustedUserCAKeys . 1472For more details on certificates, see the CERTIFICATES section in 1473.Xr ssh-keygen 1 . 1474.It Cm UseBlacklist 1475Specifies whether 1476.Xr sshd 8 1477attempts to send authentication success and failure messages 1478to the 1479.Xr blacklistd 8 1480daemon. 1481The default is 1482.Cm no . 1483.It Cm UseDNS 1484Specifies whether 1485.Xr sshd 8 1486should look up the remote host name, and to check that 1487the resolved host name for the remote IP address maps back to the 1488very same IP address. 1489.Pp 1490If this option is set to 1491.Cm no , 1492then only addresses and not host names may be used in 1493.Pa ~/.ssh/authorized_keys 1494.Cm from 1495and 1496.Nm 1497.Cm Match 1498.Cm Host 1499directives. 1500The default is 1501.Dq yes . 1502.It Cm UsePAM 1503Enables the Pluggable Authentication Module interface. 1504If set to 1505.Cm yes 1506this will enable PAM authentication using 1507.Cm ChallengeResponseAuthentication 1508and 1509.Cm PasswordAuthentication 1510in addition to PAM account and session module processing for all 1511authentication types. 1512.Pp 1513Because PAM challenge-response authentication usually serves an equivalent 1514role to password authentication, you should disable either 1515.Cm PasswordAuthentication 1516or 1517.Cm ChallengeResponseAuthentication. 1518.Pp 1519If 1520.Cm UsePAM 1521is enabled, you will not be able to run 1522.Xr sshd 8 1523as a non-root user. 1524The default is 1525.Cm yes . 1526.It Cm VersionAddendum 1527Optionally specifies additional text to append to the SSH protocol banner 1528sent by the server upon connection. 1529The default is 1530.Qq FreeBSD-20180507 . 1531The value 1532.Cm none 1533may be used to disable this. 1534.It Cm X11DisplayOffset 1535Specifies the first display number available for 1536.Xr sshd 8 Ns 's 1537X11 forwarding. 1538This prevents sshd from interfering with real X11 servers. 1539The default is 10. 1540.It Cm X11Forwarding 1541Specifies whether X11 forwarding is permitted. 1542The argument must be 1543.Cm yes 1544or 1545.Cm no . 1546The default is 1547.Cm yes . 1548.Pp 1549When X11 forwarding is enabled, there may be additional exposure to 1550the server and to client displays if the 1551.Xr sshd 8 1552proxy display is configured to listen on the wildcard address (see 1553.Cm X11UseLocalhost ) , 1554though this is not the default. 1555Additionally, the authentication spoofing and authentication data 1556verification and substitution occur on the client side. 1557The security risk of using X11 forwarding is that the client's X11 1558display server may be exposed to attack when the SSH client requests 1559forwarding (see the warnings for 1560.Cm ForwardX11 1561in 1562.Xr ssh_config 5 ) . 1563A system administrator may have a stance in which they want to 1564protect clients that may expose themselves to attack by unwittingly 1565requesting X11 forwarding, which can warrant a 1566.Cm no 1567setting. 1568.Pp 1569Note that disabling X11 forwarding does not prevent users from 1570forwarding X11 traffic, as users can always install their own forwarders. 1571.It Cm X11UseLocalhost 1572Specifies whether 1573.Xr sshd 8 1574should bind the X11 forwarding server to the loopback address or to 1575the wildcard address. 1576By default, 1577sshd binds the forwarding server to the loopback address and sets the 1578hostname part of the 1579.Ev DISPLAY 1580environment variable to 1581.Cm localhost . 1582This prevents remote hosts from connecting to the proxy display. 1583However, some older X11 clients may not function with this 1584configuration. 1585.Cm X11UseLocalhost 1586may be set to 1587.Cm no 1588to specify that the forwarding server should be bound to the wildcard 1589address. 1590The argument must be 1591.Cm yes 1592or 1593.Cm no . 1594The default is 1595.Cm yes . 1596.It Cm XAuthLocation 1597Specifies the full pathname of the 1598.Xr xauth 1 1599program, or 1600.Cm none 1601to not use one. 1602The default is 1603.Pa /usr/local/bin/xauth . 1604.El 1605.Sh TIME FORMATS 1606.Xr sshd 8 1607command-line arguments and configuration file options that specify time 1608may be expressed using a sequence of the form: 1609.Sm off 1610.Ar time Op Ar qualifier , 1611.Sm on 1612where 1613.Ar time 1614is a positive integer value and 1615.Ar qualifier 1616is one of the following: 1617.Pp 1618.Bl -tag -width Ds -compact -offset indent 1619.It Aq Cm none 1620seconds 1621.It Cm s | Cm S 1622seconds 1623.It Cm m | Cm M 1624minutes 1625.It Cm h | Cm H 1626hours 1627.It Cm d | Cm D 1628days 1629.It Cm w | Cm W 1630weeks 1631.El 1632.Pp 1633Each member of the sequence is added together to calculate 1634the total time value. 1635.Pp 1636Time format examples: 1637.Pp 1638.Bl -tag -width Ds -compact -offset indent 1639.It 600 1640600 seconds (10 minutes) 1641.It 10m 164210 minutes 1643.It 1h30m 16441 hour 30 minutes (90 minutes) 1645.El 1646.Sh TOKENS 1647Arguments to some keywords can make use of tokens, 1648which are expanded at runtime: 1649.Pp 1650.Bl -tag -width XXXX -offset indent -compact 1651.It %% 1652A literal 1653.Sq % . 1654.It %F 1655The fingerprint of the CA key. 1656.It %f 1657The fingerprint of the key or certificate. 1658.It %h 1659The home directory of the user. 1660.It %i 1661The key ID in the certificate. 1662.It %K 1663The base64-encoded CA key. 1664.It %k 1665The base64-encoded key or certificate for authentication. 1666.It %s 1667The serial number of the certificate. 1668.It \&%T 1669The type of the CA key. 1670.It %t 1671The key or certificate type. 1672.It %u 1673The username. 1674.El 1675.Pp 1676.Cm AuthorizedKeysCommand 1677accepts the tokens %%, %f, %h, %k, %t, and %u. 1678.Pp 1679.Cm AuthorizedKeysFile 1680accepts the tokens %%, %h, and %u. 1681.Pp 1682.Cm AuthorizedPrincipalsCommand 1683accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, and %u. 1684.Pp 1685.Cm AuthorizedPrincipalsFile 1686accepts the tokens %%, %h, and %u. 1687.Pp 1688.Cm ChrootDirectory 1689accepts the tokens %%, %h, and %u. 1690.Sh FILES 1691.Bl -tag -width Ds 1692.It Pa /etc/ssh/sshd_config 1693Contains configuration data for 1694.Xr sshd 8 . 1695This file should be writable by root only, but it is recommended 1696(though not necessary) that it be world-readable. 1697.El 1698.Sh SEE ALSO 1699.Xr sftp-server 8 , 1700.Xr sshd 8 1701.Sh AUTHORS 1702.An -nosplit 1703OpenSSH is a derivative of the original and free 1704ssh 1.2.12 release by 1705.An Tatu Ylonen . 1706.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , 1707.An Theo de Raadt 1708and 1709.An Dug Song 1710removed many bugs, re-added newer features and 1711created OpenSSH. 1712.An Markus Friedl 1713contributed the support for SSH protocol versions 1.5 and 2.0. 1714.An Niels Provos 1715and 1716.An Markus Friedl 1717contributed support for privilege separation. 1718