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 1.144 2012/06/29 13:57:25 naddy Exp $ 37.Dd $Mdocdate: June 29 2012 $ 38.Dt SSHD_CONFIG 5 39.Os 40.Sh NAME 41.Nm sshd_config 42.Nd OpenSSH SSH daemon configuration file 43.Sh SYNOPSIS 44.Nm /etc/ssh/sshd_config 45.Sh DESCRIPTION 46.Xr sshd 8 47reads configuration data from 48.Pa /etc/ssh/sshd_config 49(or the file specified with 50.Fl f 51on the command line). 52The file contains keyword-argument pairs, one per line. 53Lines starting with 54.Ql # 55and empty lines are interpreted as comments. 56Arguments may optionally be enclosed in double quotes 57.Pq \&" 58in order to represent arguments containing spaces. 59.Pp 60The possible 61keywords and their meanings are as follows (note that 62keywords are case-insensitive and arguments are case-sensitive): 63.Bl -tag -width Ds 64.It Cm AcceptEnv 65Specifies what environment variables sent by the client will be copied into 66the session's 67.Xr environ 7 . 68See 69.Cm SendEnv 70in 71.Xr ssh_config 5 72for how to configure the client. 73Note that environment passing is only supported for protocol 2. 74Variables are specified by name, which may contain the wildcard characters 75.Ql * 76and 77.Ql \&? . 78Multiple environment variables may be separated by whitespace or spread 79across multiple 80.Cm AcceptEnv 81directives. 82Be warned that some environment variables could be used to bypass restricted 83user environments. 84For this reason, care should be taken in the use of this directive. 85The default is not to accept any environment variables. 86.It Cm AddressFamily 87Specifies which address family should be used by 88.Xr sshd 8 . 89Valid arguments are 90.Dq any , 91.Dq inet 92(use IPv4 only), or 93.Dq inet6 94(use IPv6 only). 95The default is 96.Dq any . 97.It Cm AllowAgentForwarding 98Specifies whether 99.Xr ssh-agent 1 100forwarding is permitted. 101The default is 102.Dq yes . 103Note that disabling agent forwarding does not improve security 104unless users are also denied shell access, as they can always install 105their own forwarders. 106.It Cm AllowGroups 107This keyword can be followed by a list of group name patterns, separated 108by spaces. 109If specified, login is allowed only for users whose primary 110group or supplementary group list matches one of the patterns. 111Only group names are valid; a numerical group ID is not recognized. 112By default, login is allowed for all groups. 113The allow/deny directives are processed in the following order: 114.Cm DenyUsers , 115.Cm AllowUsers , 116.Cm DenyGroups , 117and finally 118.Cm AllowGroups . 119.Pp 120See 121.Sx PATTERNS 122in 123.Xr ssh_config 5 124for more information on patterns. 125.It Cm AllowTcpForwarding 126Specifies whether TCP forwarding is permitted. 127The default is 128.Dq yes . 129Note that disabling TCP forwarding does not improve security unless 130users are also denied shell access, as they can always install their 131own forwarders. 132.It Cm AllowUsers 133This keyword can be followed by a list of user name patterns, separated 134by spaces. 135If specified, login is allowed only for user names that 136match one of the patterns. 137Only user names are valid; a numerical user ID is not recognized. 138By default, login is allowed for all users. 139If the pattern takes the form USER@HOST then USER and HOST 140are separately checked, restricting logins to particular 141users from particular hosts. 142The allow/deny directives are processed in the following order: 143.Cm DenyUsers , 144.Cm AllowUsers , 145.Cm DenyGroups , 146and finally 147.Cm AllowGroups . 148.Pp 149See 150.Sx PATTERNS 151in 152.Xr ssh_config 5 153for more information on patterns. 154.It Cm AuthorizedKeysFile 155Specifies the file that contains the public keys that can be used 156for user authentication. 157The format is described in the 158.Sx AUTHORIZED_KEYS FILE FORMAT 159section of 160.Xr sshd 8 . 161.Cm AuthorizedKeysFile 162may contain tokens of the form %T which are substituted during connection 163setup. 164The following tokens are defined: %% is replaced by a literal '%', 165%h is replaced by the home directory of the user being authenticated, and 166%u is replaced by the username of that user. 167After expansion, 168.Cm AuthorizedKeysFile 169is taken to be an absolute path or one relative to the user's home 170directory. 171Multiple files may be listed, separated by whitespace. 172The default is 173.Dq .ssh/authorized_keys .ssh/authorized_keys2 . 174.It Cm AuthorizedPrincipalsFile 175Specifies a file that lists principal names that are accepted for 176certificate authentication. 177When using certificates signed by a key listed in 178.Cm TrustedUserCAKeys , 179this file lists names, one of which must appear in the certificate for it 180to be accepted for authentication. 181Names are listed one per line preceded by key options (as described 182in 183.Sx AUTHORIZED_KEYS FILE FORMAT 184in 185.Xr sshd 8 ) . 186Empty lines and comments starting with 187.Ql # 188are ignored. 189.Pp 190.Cm AuthorizedPrincipalsFile 191may contain tokens of the form %T which are substituted during connection 192setup. 193The following tokens are defined: %% is replaced by a literal '%', 194%h is replaced by the home directory of the user being authenticated, and 195%u is replaced by the username of that user. 196After expansion, 197.Cm AuthorizedPrincipalsFile 198is taken to be an absolute path or one relative to the user's home 199directory. 200.Pp 201The default is 202.Dq none , 203i.e. not to use a principals file \(en in this case, the username 204of the user must appear in a certificate's principals list for it to be 205accepted. 206Note that 207.Cm AuthorizedPrincipalsFile 208is only used when authentication proceeds using a CA listed in 209.Cm TrustedUserCAKeys 210and is not consulted for certification authorities trusted via 211.Pa ~/.ssh/authorized_keys , 212though the 213.Cm principals= 214key option offers a similar facility (see 215.Xr sshd 8 216for details). 217.It Cm Banner 218The contents of the specified file are sent to the remote user before 219authentication is allowed. 220If the argument is 221.Dq none 222then no banner is displayed. 223This option is only available for protocol version 2. 224By default, no banner is displayed. 225.It Cm ChallengeResponseAuthentication 226Specifies whether challenge-response authentication is allowed. 227All authentication styles from 228.Xr login.conf 5 229are supported. 230The default is 231.Dq yes . 232.It Cm ChrootDirectory 233Specifies the pathname of a directory to 234.Xr chroot 2 235to after authentication. 236All components of the pathname must be root-owned directories that are 237not writable by any other user or group. 238After the chroot, 239.Xr sshd 8 240changes the working directory to the user's home directory. 241.Pp 242The pathname may contain the following tokens that are expanded at runtime once 243the connecting user has been authenticated: %% is replaced by a literal '%', 244%h is replaced by the home directory of the user being authenticated, and 245%u is replaced by the username of that user. 246.Pp 247The 248.Cm ChrootDirectory 249must contain the necessary files and directories to support the 250user's session. 251For an interactive session this requires at least a shell, typically 252.Xr sh 1 , 253and basic 254.Pa /dev 255nodes such as 256.Xr null 4 , 257.Xr zero 4 , 258.Xr stdin 4 , 259.Xr stdout 4 , 260.Xr stderr 4 , 261.Xr arandom 4 262and 263.Xr tty 4 264devices. 265For file transfer sessions using 266.Dq sftp , 267no additional configuration of the environment is necessary if the 268in-process sftp server is used, 269though sessions which use logging do require 270.Pa /dev/log 271inside the chroot directory (see 272.Xr sftp-server 8 273for details). 274.Pp 275The default is not to 276.Xr chroot 2 . 277.It Cm Ciphers 278Specifies the ciphers allowed for protocol version 2. 279Multiple ciphers must be comma-separated. 280The supported ciphers are 281.Dq 3des-cbc , 282.Dq aes128-cbc , 283.Dq aes192-cbc , 284.Dq aes256-cbc , 285.Dq aes128-ctr , 286.Dq aes192-ctr , 287.Dq aes256-ctr , 288.Dq arcfour128 , 289.Dq arcfour256 , 290.Dq arcfour , 291.Dq blowfish-cbc , 292and 293.Dq cast128-cbc . 294The default is: 295.Bd -literal -offset 3n 296aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, 297aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, 298aes256-cbc,arcfour 299.Ed 300.It Cm ClientAliveCountMax 301Sets the number of client alive messages (see below) which may be 302sent without 303.Xr sshd 8 304receiving any messages back from the client. 305If this threshold is reached while client alive messages are being sent, 306sshd will disconnect the client, terminating the session. 307It is important to note that the use of client alive messages is very 308different from 309.Cm TCPKeepAlive 310(below). 311The client alive messages are sent through the encrypted channel 312and therefore will not be spoofable. 313The TCP keepalive option enabled by 314.Cm TCPKeepAlive 315is spoofable. 316The client alive mechanism is valuable when the client or 317server depend on knowing when a connection has become inactive. 318.Pp 319The default value is 3. 320If 321.Cm ClientAliveInterval 322(see below) is set to 15, and 323.Cm ClientAliveCountMax 324is left at the default, unresponsive SSH clients 325will be disconnected after approximately 45 seconds. 326This option applies to protocol version 2 only. 327.It Cm ClientAliveInterval 328Sets a timeout interval in seconds after which if no data has been received 329from the client, 330.Xr sshd 8 331will send a message through the encrypted 332channel to request a response from the client. 333The default 334is 0, indicating that these messages will not be sent to the client. 335This option applies to protocol version 2 only. 336.It Cm Compression 337Specifies whether compression is allowed, or delayed until 338the user has authenticated successfully. 339The argument must be 340.Dq yes , 341.Dq delayed , 342or 343.Dq no . 344The default is 345.Dq delayed . 346.It Cm DenyGroups 347This keyword can be followed by a list of group name patterns, separated 348by spaces. 349Login is disallowed for users whose primary group or supplementary 350group list matches one of the patterns. 351Only group names are valid; a numerical group ID is not recognized. 352By default, login is allowed for all groups. 353The allow/deny directives are processed in the following order: 354.Cm DenyUsers , 355.Cm AllowUsers , 356.Cm DenyGroups , 357and finally 358.Cm AllowGroups . 359.Pp 360See 361.Sx PATTERNS 362in 363.Xr ssh_config 5 364for more information on patterns. 365.It Cm DenyUsers 366This keyword can be followed by a list of user name patterns, separated 367by spaces. 368Login is disallowed for user names that match one of the patterns. 369Only user names are valid; a numerical user ID is not recognized. 370By default, login is allowed for all users. 371If the pattern takes the form USER@HOST then USER and HOST 372are separately checked, restricting logins to particular 373users from particular hosts. 374The allow/deny directives are processed in the following order: 375.Cm DenyUsers , 376.Cm AllowUsers , 377.Cm DenyGroups , 378and finally 379.Cm AllowGroups . 380.Pp 381See 382.Sx PATTERNS 383in 384.Xr ssh_config 5 385for more information on patterns. 386.It Cm ForceCommand 387Forces the execution of the command specified by 388.Cm ForceCommand , 389ignoring any command supplied by the client and 390.Pa ~/.ssh/rc 391if present. 392The command is invoked by using the user's login shell with the -c option. 393This applies to shell, command, or subsystem execution. 394It is most useful inside a 395.Cm Match 396block. 397The command originally supplied by the client is available in the 398.Ev SSH_ORIGINAL_COMMAND 399environment variable. 400Specifying a command of 401.Dq internal-sftp 402will force the use of an in-process sftp server that requires no support 403files when used with 404.Cm ChrootDirectory . 405.It Cm GatewayPorts 406Specifies whether remote hosts are allowed to connect to ports 407forwarded for the client. 408By default, 409.Xr sshd 8 410binds remote port forwardings to the loopback address. 411This prevents other remote hosts from connecting to forwarded ports. 412.Cm GatewayPorts 413can be used to specify that sshd 414should allow remote port forwardings to bind to non-loopback addresses, thus 415allowing other hosts to connect. 416The argument may be 417.Dq no 418to force remote port forwardings to be available to the local host only, 419.Dq yes 420to force remote port forwardings to bind to the wildcard address, or 421.Dq clientspecified 422to allow the client to select the address to which the forwarding is bound. 423The default is 424.Dq no . 425.It Cm GSSAPIAuthentication 426Specifies whether user authentication based on GSSAPI is allowed. 427The default is 428.Dq no . 429Note that this option applies to protocol version 2 only. 430.It Cm GSSAPICleanupCredentials 431Specifies whether to automatically destroy the user's credentials cache 432on logout. 433The default is 434.Dq yes . 435Note that this option applies to protocol version 2 only. 436.It Cm HostbasedAuthentication 437Specifies whether rhosts or /etc/hosts.equiv authentication together 438with successful public key client host authentication is allowed 439(host-based authentication). 440This option is similar to 441.Cm RhostsRSAAuthentication 442and applies to protocol version 2 only. 443The default is 444.Dq no . 445.It Cm HostbasedUsesNameFromPacketOnly 446Specifies whether or not the server will attempt to perform a reverse 447name lookup when matching the name in the 448.Pa ~/.shosts , 449.Pa ~/.rhosts , 450and 451.Pa /etc/hosts.equiv 452files during 453.Cm HostbasedAuthentication . 454A setting of 455.Dq yes 456means that 457.Xr sshd 8 458uses the name supplied by the client rather than 459attempting to resolve the name from the TCP connection itself. 460The default is 461.Dq no . 462.It Cm HostCertificate 463Specifies a file containing a public host certificate. 464The certificate's public key must match a private host key already specified 465by 466.Cm HostKey . 467The default behaviour of 468.Xr sshd 8 469is not to load any certificates. 470.It Cm HostKey 471Specifies a file containing a private host key 472used by SSH. 473The default is 474.Pa /etc/ssh/ssh_host_key 475for protocol version 1, and 476.Pa /etc/ssh/ssh_host_dsa_key , 477.Pa /etc/ssh/ssh_host_ecdsa_key 478and 479.Pa /etc/ssh/ssh_host_rsa_key 480for protocol version 2. 481Note that 482.Xr sshd 8 483will refuse to use a file if it is group/world-accessible. 484It is possible to have multiple host key files. 485.Dq rsa1 486keys are used for version 1 and 487.Dq dsa , 488.Dq ecdsa 489or 490.Dq rsa 491are used for version 2 of the SSH protocol. 492.It Cm IgnoreRhosts 493Specifies that 494.Pa .rhosts 495and 496.Pa .shosts 497files will not be used in 498.Cm RhostsRSAAuthentication 499or 500.Cm HostbasedAuthentication . 501.Pp 502.Pa /etc/hosts.equiv 503and 504.Pa /etc/shosts.equiv 505are still used. 506The default is 507.Dq yes . 508.It Cm IgnoreUserKnownHosts 509Specifies whether 510.Xr sshd 8 511should ignore the user's 512.Pa ~/.ssh/known_hosts 513during 514.Cm RhostsRSAAuthentication 515or 516.Cm HostbasedAuthentication . 517The default is 518.Dq no . 519.It Cm IPQoS 520Specifies the IPv4 type-of-service or DSCP class for the connection. 521Accepted values are 522.Dq af11 , 523.Dq af12 , 524.Dq af13 , 525.Dq af21 , 526.Dq af22 , 527.Dq af23 , 528.Dq af31 , 529.Dq af32 , 530.Dq af33 , 531.Dq af41 , 532.Dq af42 , 533.Dq af43 , 534.Dq cs0 , 535.Dq cs1 , 536.Dq cs2 , 537.Dq cs3 , 538.Dq cs4 , 539.Dq cs5 , 540.Dq cs6 , 541.Dq cs7 , 542.Dq ef , 543.Dq lowdelay , 544.Dq throughput , 545.Dq reliability , 546or a numeric value. 547This option may take one or two arguments, separated by whitespace. 548If one argument is specified, it is used as the packet class unconditionally. 549If two values are specified, the first is automatically selected for 550interactive sessions and the second for non-interactive sessions. 551The default is 552.Dq lowdelay 553for interactive sessions and 554.Dq throughput 555for non-interactive sessions. 556.It Cm KerberosAuthentication 557Specifies whether the password provided by the user for 558.Cm PasswordAuthentication 559will be validated through the Kerberos KDC. 560To use this option, the server needs a 561Kerberos servtab which allows the verification of the KDC's identity. 562The default is 563.Dq no . 564.It Cm KerberosGetAFSToken 565If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 566an AFS token before accessing the user's home directory. 567The default is 568.Dq no . 569.It Cm KerberosOrLocalPasswd 570If password authentication through Kerberos fails then 571the password will be validated via any additional local mechanism 572such as 573.Pa /etc/passwd . 574The default is 575.Dq yes . 576.It Cm KerberosTicketCleanup 577Specifies whether to automatically destroy the user's ticket cache 578file on logout. 579The default is 580.Dq yes . 581.It Cm KexAlgorithms 582Specifies the available KEX (Key Exchange) algorithms. 583Multiple algorithms must be comma-separated. 584The default is 585.Dq ecdh-sha2-nistp256 , 586.Dq ecdh-sha2-nistp384 , 587.Dq ecdh-sha2-nistp521 , 588.Dq diffie-hellman-group-exchange-sha256 , 589.Dq diffie-hellman-group-exchange-sha1 , 590.Dq diffie-hellman-group14-sha1 , 591.Dq diffie-hellman-group1-sha1 . 592.It Cm KeyRegenerationInterval 593In protocol version 1, the ephemeral server key is automatically regenerated 594after this many seconds (if it has been used). 595The purpose of regeneration is to prevent 596decrypting captured sessions by later breaking into the machine and 597stealing the keys. 598The key is never stored anywhere. 599If the value is 0, the key is never regenerated. 600The default is 3600 (seconds). 601.It Cm ListenAddress 602Specifies the local addresses 603.Xr sshd 8 604should listen on. 605The following forms may be used: 606.Pp 607.Bl -item -offset indent -compact 608.It 609.Cm ListenAddress 610.Sm off 611.Ar host No | Ar IPv4_addr No | Ar IPv6_addr 612.Sm on 613.It 614.Cm ListenAddress 615.Sm off 616.Ar host No | Ar IPv4_addr No : Ar port 617.Sm on 618.It 619.Cm ListenAddress 620.Sm off 621.Oo 622.Ar host No | Ar IPv6_addr Oc : Ar port 623.Sm on 624.El 625.Pp 626If 627.Ar port 628is not specified, 629sshd will listen on the address and all prior 630.Cm Port 631options specified. 632The default is to listen on all local addresses. 633Multiple 634.Cm ListenAddress 635options are permitted. 636Additionally, any 637.Cm Port 638options must precede this option for non-port qualified addresses. 639.It Cm LoginGraceTime 640The server disconnects after this time if the user has not 641successfully logged in. 642If the value is 0, there is no time limit. 643The default is 120 seconds. 644.It Cm LogLevel 645Gives the verbosity level that is used when logging messages from 646.Xr sshd 8 . 647The possible values are: 648QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 649The default is INFO. 650DEBUG and DEBUG1 are equivalent. 651DEBUG2 and DEBUG3 each specify higher levels of debugging output. 652Logging with a DEBUG level violates the privacy of users and is not recommended. 653.It Cm MACs 654Specifies the available MAC (message authentication code) algorithms. 655The MAC algorithm is used in protocol version 2 656for data integrity protection. 657Multiple algorithms must be comma-separated. 658The default is: 659.Bd -literal -offset indent 660hmac-md5,hmac-sha1,umac-64@openssh.com, 661hmac-sha2-256,hmac-sha2-512,hmac-ripemd160, 662hmac-sha1-96,hmac-md5-96 663.Ed 664.It Cm Match 665Introduces a conditional block. 666If all of the criteria on the 667.Cm Match 668line are satisfied, the keywords on the following lines override those 669set in the global section of the config file, until either another 670.Cm Match 671line or the end of the file. 672.Pp 673The arguments to 674.Cm Match 675are one or more criteria-pattern pairs. 676The available criteria are 677.Cm User , 678.Cm Group , 679.Cm Host , 680.Cm LocalAddress , 681.Cm LocalPort , 682and 683.Cm Address . 684The match patterns may consist of single entries or comma-separated 685lists and may use the wildcard and negation operators described in the 686.Sx PATTERNS 687section of 688.Xr ssh_config 5 . 689.Pp 690The patterns in an 691.Cm Address 692criteria may additionally contain addresses to match in CIDR 693address/masklen format, e.g.\& 694.Dq 192.0.2.0/24 695or 696.Dq 3ffe:ffff::/32 . 697Note that the mask length provided must be consistent with the address - 698it is an error to specify a mask length that is too long for the address 699or one with bits set in this host portion of the address. 700For example, 701.Dq 192.0.2.0/33 702and 703.Dq 192.0.2.0/8 704respectively. 705.Pp 706Only a subset of keywords may be used on the lines following a 707.Cm Match 708keyword. 709Available keywords are 710.Cm AcceptEnv , 711.Cm AllowAgentForwarding , 712.Cm AllowGroups , 713.Cm AllowTcpForwarding , 714.Cm AllowUsers , 715.Cm AuthorizedKeysFile , 716.Cm AuthorizedPrincipalsFile , 717.Cm Banner , 718.Cm ChrootDirectory , 719.Cm DenyGroups , 720.Cm DenyUsers , 721.Cm ForceCommand , 722.Cm GatewayPorts , 723.Cm GSSAPIAuthentication , 724.Cm HostbasedAuthentication , 725.Cm HostbasedUsesNameFromPacketOnly , 726.Cm KbdInteractiveAuthentication , 727.Cm KerberosAuthentication , 728.Cm MaxAuthTries , 729.Cm MaxSessions , 730.Cm PasswordAuthentication , 731.Cm PermitEmptyPasswords , 732.Cm PermitOpen , 733.Cm PermitRootLogin , 734.Cm PermitTunnel , 735.Cm PubkeyAuthentication , 736.Cm RhostsRSAAuthentication , 737.Cm RSAAuthentication , 738.Cm X11DisplayOffset , 739.Cm X11Forwarding 740and 741.Cm X11UseLocalHost . 742.It Cm MaxAuthTries 743Specifies the maximum number of authentication attempts permitted per 744connection. 745Once the number of failures reaches half this value, 746additional failures are logged. 747The default is 6. 748.It Cm MaxSessions 749Specifies the maximum number of open sessions permitted per network connection. 750The default is 10. 751.It Cm MaxStartups 752Specifies the maximum number of concurrent unauthenticated connections to the 753SSH daemon. 754Additional connections will be dropped until authentication succeeds or the 755.Cm LoginGraceTime 756expires for a connection. 757The default is 10. 758.Pp 759Alternatively, random early drop can be enabled by specifying 760the three colon separated values 761.Dq start:rate:full 762(e.g. "10:30:60"). 763.Xr sshd 8 764will refuse connection attempts with a probability of 765.Dq rate/100 766(30%) 767if there are currently 768.Dq start 769(10) 770unauthenticated connections. 771The probability increases linearly and all connection attempts 772are refused if the number of unauthenticated connections reaches 773.Dq full 774(60). 775.It Cm PasswordAuthentication 776Specifies whether password authentication is allowed. 777The default is 778.Dq yes . 779.It Cm PermitEmptyPasswords 780When password authentication is allowed, it specifies whether the 781server allows login to accounts with empty password strings. 782The default is 783.Dq no . 784.It Cm PermitOpen 785Specifies the destinations to which TCP port forwarding is permitted. 786The forwarding specification must be one of the following forms: 787.Pp 788.Bl -item -offset indent -compact 789.It 790.Cm PermitOpen 791.Sm off 792.Ar host : port 793.Sm on 794.It 795.Cm PermitOpen 796.Sm off 797.Ar IPv4_addr : port 798.Sm on 799.It 800.Cm PermitOpen 801.Sm off 802.Ar \&[ IPv6_addr \&] : port 803.Sm on 804.El 805.Pp 806Multiple forwards may be specified by separating them with whitespace. 807An argument of 808.Dq any 809can be used to remove all restrictions and permit any forwarding requests. 810An argument of 811.Dq none 812can be used to prohibit all forwarding requests. 813By default all port forwarding requests are permitted. 814.It Cm PermitRootLogin 815Specifies whether root can log in using 816.Xr ssh 1 . 817The argument must be 818.Dq yes , 819.Dq without-password , 820.Dq forced-commands-only , 821or 822.Dq no . 823The default is 824.Dq yes . 825.Pp 826If this option is set to 827.Dq without-password , 828password authentication is disabled for root. 829.Pp 830If this option is set to 831.Dq forced-commands-only , 832root login with public key authentication will be allowed, 833but only if the 834.Ar command 835option has been specified 836(which may be useful for taking remote backups even if root login is 837normally not allowed). 838All other authentication methods are disabled for root. 839.Pp 840If this option is set to 841.Dq no , 842root is not allowed to log in. 843.It Cm PermitTunnel 844Specifies whether 845.Xr tun 4 846device forwarding is allowed. 847The argument must be 848.Dq yes , 849.Dq point-to-point 850(layer 3), 851.Dq ethernet 852(layer 2), or 853.Dq no . 854Specifying 855.Dq yes 856permits both 857.Dq point-to-point 858and 859.Dq ethernet . 860The default is 861.Dq no . 862.It Cm PermitUserEnvironment 863Specifies whether 864.Pa ~/.ssh/environment 865and 866.Cm environment= 867options in 868.Pa ~/.ssh/authorized_keys 869are processed by 870.Xr sshd 8 . 871The default is 872.Dq no . 873Enabling environment processing may enable users to bypass access 874restrictions in some configurations using mechanisms such as 875.Ev LD_PRELOAD . 876.It Cm PidFile 877Specifies the file that contains the process ID of the 878SSH daemon. 879The default is 880.Pa /var/run/sshd.pid . 881.It Cm Port 882Specifies the port number that 883.Xr sshd 8 884listens on. 885The default is 22. 886Multiple options of this type are permitted. 887See also 888.Cm ListenAddress . 889.It Cm PrintLastLog 890Specifies whether 891.Xr sshd 8 892should print the date and time of the last user login when a user logs 893in interactively. 894The default is 895.Dq yes . 896.It Cm PrintMotd 897Specifies whether 898.Xr sshd 8 899should print 900.Pa /etc/motd 901when a user logs in interactively. 902(On some systems it is also printed by the shell, 903.Pa /etc/profile , 904or equivalent.) 905The default is 906.Dq yes . 907.It Cm Protocol 908Specifies the protocol versions 909.Xr sshd 8 910supports. 911The possible values are 912.Sq 1 913and 914.Sq 2 . 915Multiple versions must be comma-separated. 916The default is 917.Sq 2 . 918Note that the order of the protocol list does not indicate preference, 919because the client selects among multiple protocol versions offered 920by the server. 921Specifying 922.Dq 2,1 923is identical to 924.Dq 1,2 . 925.It Cm PubkeyAuthentication 926Specifies whether public key authentication is allowed. 927The default is 928.Dq yes . 929Note that this option applies to protocol version 2 only. 930.It Cm RevokedKeys 931Specifies a list of revoked public keys. 932Keys listed in this file will be refused for public key authentication. 933Note that if this file is not readable, then public key authentication will 934be refused for all users. 935.It Cm RhostsRSAAuthentication 936Specifies whether rhosts or /etc/hosts.equiv authentication together 937with successful RSA host authentication is allowed. 938The default is 939.Dq no . 940This option applies to protocol version 1 only. 941.It Cm RSAAuthentication 942Specifies whether pure RSA authentication is allowed. 943The default is 944.Dq yes . 945This option applies to protocol version 1 only. 946.It Cm ServerKeyBits 947Defines the number of bits in the ephemeral protocol version 1 server key. 948The minimum value is 512, and the default is 1024. 949.It Cm StrictModes 950Specifies whether 951.Xr sshd 8 952should check file modes and ownership of the 953user's files and home directory before accepting login. 954This is normally desirable because novices sometimes accidentally leave their 955directory or files world-writable. 956The default is 957.Dq yes . 958Note that this does not apply to 959.Cm ChrootDirectory , 960whose permissions and ownership are checked unconditionally. 961.It Cm Subsystem 962Configures an external subsystem (e.g. file transfer daemon). 963Arguments should be a subsystem name and a command (with optional arguments) 964to execute upon subsystem request. 965.Pp 966The command 967.Xr sftp-server 8 968implements the 969.Dq sftp 970file transfer subsystem. 971.Pp 972Alternately the name 973.Dq internal-sftp 974implements an in-process 975.Dq sftp 976server. 977This may simplify configurations using 978.Cm ChrootDirectory 979to force a different filesystem root on clients. 980.Pp 981By default no subsystems are defined. 982Note that this option applies to protocol version 2 only. 983.It Cm SyslogFacility 984Gives the facility code that is used when logging messages from 985.Xr sshd 8 . 986The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 987LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 988The default is AUTH. 989.It Cm TCPKeepAlive 990Specifies whether the system should send TCP keepalive messages to the 991other side. 992If they are sent, death of the connection or crash of one 993of the machines will be properly noticed. 994However, this means that 995connections will die if the route is down temporarily, and some people 996find it annoying. 997On the other hand, if TCP keepalives are not sent, 998sessions may hang indefinitely on the server, leaving 999.Dq ghost 1000users and consuming server resources. 1001.Pp 1002The default is 1003.Dq yes 1004(to send TCP keepalive messages), and the server will notice 1005if the network goes down or the client host crashes. 1006This avoids infinitely hanging sessions. 1007.Pp 1008To disable TCP keepalive messages, the value should be set to 1009.Dq no . 1010.It Cm TrustedUserCAKeys 1011Specifies a file containing public keys of certificate authorities that are 1012trusted to sign user certificates for authentication. 1013Keys are listed one per line; empty lines and comments starting with 1014.Ql # 1015are allowed. 1016If a certificate is presented for authentication and has its signing CA key 1017listed in this file, then it may be used for authentication for any user 1018listed in the certificate's principals list. 1019Note that certificates that lack a list of principals will not be permitted 1020for authentication using 1021.Cm TrustedUserCAKeys . 1022For more details on certificates, see the 1023.Sx CERTIFICATES 1024section in 1025.Xr ssh-keygen 1 . 1026.It Cm UseDNS 1027Specifies whether 1028.Xr sshd 8 1029should look up the remote host name and check that 1030the resolved host name for the remote IP address maps back to the 1031very same IP address. 1032The default is 1033.Dq yes . 1034.It Cm UseLogin 1035Specifies whether 1036.Xr login 1 1037is used for interactive login sessions. 1038The default is 1039.Dq no . 1040Note that 1041.Xr login 1 1042is never used for remote command execution. 1043Note also, that if this is enabled, 1044.Cm X11Forwarding 1045will be disabled because 1046.Xr login 1 1047does not know how to handle 1048.Xr xauth 1 1049cookies. 1050If 1051.Cm UsePrivilegeSeparation 1052is specified, it will be disabled after authentication. 1053.It Cm UsePrivilegeSeparation 1054Specifies whether 1055.Xr sshd 8 1056separates privileges by creating an unprivileged child process 1057to deal with incoming network traffic. 1058After successful authentication, another process will be created that has 1059the privilege of the authenticated user. 1060The goal of privilege separation is to prevent privilege 1061escalation by containing any corruption within the unprivileged processes. 1062The default is 1063.Dq yes . 1064If 1065.Cm UsePrivilegeSeparation 1066is set to 1067.Dq sandbox 1068then the pre-authentication unprivileged process is subject to additional 1069restrictions. 1070.It Cm VersionAddendum 1071Optionally specifies additional text to append to the SSH protocol banner 1072sent by the server upon connection. 1073The default is 1074.Dq none . 1075.It Cm X11DisplayOffset 1076Specifies the first display number available for 1077.Xr sshd 8 Ns 's 1078X11 forwarding. 1079This prevents sshd from interfering with real X11 servers. 1080The default is 10. 1081.It Cm X11Forwarding 1082Specifies whether X11 forwarding is permitted. 1083The argument must be 1084.Dq yes 1085or 1086.Dq no . 1087The default is 1088.Dq no . 1089.Pp 1090When X11 forwarding is enabled, there may be additional exposure to 1091the server and to client displays if the 1092.Xr sshd 8 1093proxy display is configured to listen on the wildcard address (see 1094.Cm X11UseLocalhost 1095below), though this is not the default. 1096Additionally, the authentication spoofing and authentication data 1097verification and substitution occur on the client side. 1098The security risk of using X11 forwarding is that the client's X11 1099display server may be exposed to attack when the SSH client requests 1100forwarding (see the warnings for 1101.Cm ForwardX11 1102in 1103.Xr ssh_config 5 ) . 1104A system administrator may have a stance in which they want to 1105protect clients that may expose themselves to attack by unwittingly 1106requesting X11 forwarding, which can warrant a 1107.Dq no 1108setting. 1109.Pp 1110Note that disabling X11 forwarding does not prevent users from 1111forwarding X11 traffic, as users can always install their own forwarders. 1112X11 forwarding is automatically disabled if 1113.Cm UseLogin 1114is enabled. 1115.It Cm X11UseLocalhost 1116Specifies whether 1117.Xr sshd 8 1118should bind the X11 forwarding server to the loopback address or to 1119the wildcard address. 1120By default, 1121sshd binds the forwarding server to the loopback address and sets the 1122hostname part of the 1123.Ev DISPLAY 1124environment variable to 1125.Dq localhost . 1126This prevents remote hosts from connecting to the proxy display. 1127However, some older X11 clients may not function with this 1128configuration. 1129.Cm X11UseLocalhost 1130may be set to 1131.Dq no 1132to specify that the forwarding server should be bound to the wildcard 1133address. 1134The argument must be 1135.Dq yes 1136or 1137.Dq no . 1138The default is 1139.Dq yes . 1140.It Cm XAuthLocation 1141Specifies the full pathname of the 1142.Xr xauth 1 1143program. 1144The default is 1145.Pa /usr/X11R6/bin/xauth . 1146.El 1147.Sh TIME FORMATS 1148.Xr sshd 8 1149command-line arguments and configuration file options that specify time 1150may be expressed using a sequence of the form: 1151.Sm off 1152.Ar time Op Ar qualifier , 1153.Sm on 1154where 1155.Ar time 1156is a positive integer value and 1157.Ar qualifier 1158is one of the following: 1159.Pp 1160.Bl -tag -width Ds -compact -offset indent 1161.It Aq Cm none 1162seconds 1163.It Cm s | Cm S 1164seconds 1165.It Cm m | Cm M 1166minutes 1167.It Cm h | Cm H 1168hours 1169.It Cm d | Cm D 1170days 1171.It Cm w | Cm W 1172weeks 1173.El 1174.Pp 1175Each member of the sequence is added together to calculate 1176the total time value. 1177.Pp 1178Time format examples: 1179.Pp 1180.Bl -tag -width Ds -compact -offset indent 1181.It 600 1182600 seconds (10 minutes) 1183.It 10m 118410 minutes 1185.It 1h30m 11861 hour 30 minutes (90 minutes) 1187.El 1188.Sh FILES 1189.Bl -tag -width Ds 1190.It Pa /etc/ssh/sshd_config 1191Contains configuration data for 1192.Xr sshd 8 . 1193This file should be writable by root only, but it is recommended 1194(though not necessary) that it be world-readable. 1195.El 1196.Sh SEE ALSO 1197.Xr sshd 8 1198.Sh AUTHORS 1199OpenSSH is a derivative of the original and free 1200ssh 1.2.12 release by Tatu Ylonen. 1201Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1202Theo de Raadt and Dug Song 1203removed many bugs, re-added newer features and 1204created OpenSSH. 1205Markus Friedl contributed the support for SSH 1206protocol versions 1.5 and 2.0. 1207Niels Provos and Markus Friedl contributed support 1208for privilege separation. 1209