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 1.136 2011/09/09 00:43:00 djm Exp $ 37.Dd $Mdocdate: September 9 2011 $ 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 not to use a principals file \(en in this case, the username 202of the user must appear in a certificate's principals list for it to be 203accepted. 204Note that 205.Cm AuthorizedPrincipalsFile 206is only used when authentication proceeds using a CA listed in 207.Cm TrustedUserCAKeys 208and is not consulted for certification authorities trusted via 209.Pa ~/.ssh/authorized_keys , 210though the 211.Cm principals= 212key option offers a similar facility (see 213.Xr sshd 8 214for details). 215.It Cm Banner 216The contents of the specified file are sent to the remote user before 217authentication is allowed. 218If the argument is 219.Dq none 220then no banner is displayed. 221This option is only available for protocol version 2. 222By default, no banner is displayed. 223.It Cm ChallengeResponseAuthentication 224Specifies whether challenge-response authentication is allowed. 225All authentication styles from 226.Xr login.conf 5 227are supported. 228The default is 229.Dq yes . 230.It Cm ChrootDirectory 231Specifies the pathname of a directory to 232.Xr chroot 2 233to after authentication. 234All components of the pathname must be root-owned directories that are 235not writable by any other user or group. 236After the chroot, 237.Xr sshd 8 238changes the working directory to the user's home directory. 239.Pp 240The pathname may contain the following tokens that are expanded at runtime once 241the connecting user has been authenticated: %% is replaced by a literal '%', 242%h is replaced by the home directory of the user being authenticated, and 243%u is replaced by the username of that user. 244.Pp 245The 246.Cm ChrootDirectory 247must contain the necessary files and directories to support the 248user's session. 249For an interactive session this requires at least a shell, typically 250.Xr sh 1 , 251and basic 252.Pa /dev 253nodes such as 254.Xr null 4 , 255.Xr zero 4 , 256.Xr stdin 4 , 257.Xr stdout 4 , 258.Xr stderr 4 , 259.Xr arandom 4 260and 261.Xr tty 4 262devices. 263For file transfer sessions using 264.Dq sftp , 265no additional configuration of the environment is necessary if the 266in-process sftp server is used, 267though sessions which use logging do require 268.Pa /dev/log 269inside the chroot directory (see 270.Xr sftp-server 8 271for details). 272.Pp 273The default is not to 274.Xr chroot 2 . 275.It Cm Ciphers 276Specifies the ciphers allowed for protocol version 2. 277Multiple ciphers must be comma-separated. 278The supported ciphers are 279.Dq 3des-cbc , 280.Dq aes128-cbc , 281.Dq aes192-cbc , 282.Dq aes256-cbc , 283.Dq aes128-ctr , 284.Dq aes192-ctr , 285.Dq aes256-ctr , 286.Dq arcfour128 , 287.Dq arcfour256 , 288.Dq arcfour , 289.Dq blowfish-cbc , 290and 291.Dq cast128-cbc . 292The default is: 293.Bd -literal -offset 3n 294aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, 295aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, 296aes256-cbc,arcfour 297.Ed 298.It Cm ClientAliveCountMax 299Sets the number of client alive messages (see below) which may be 300sent without 301.Xr sshd 8 302receiving any messages back from the client. 303If this threshold is reached while client alive messages are being sent, 304sshd will disconnect the client, terminating the session. 305It is important to note that the use of client alive messages is very 306different from 307.Cm TCPKeepAlive 308(below). 309The client alive messages are sent through the encrypted channel 310and therefore will not be spoofable. 311The TCP keepalive option enabled by 312.Cm TCPKeepAlive 313is spoofable. 314The client alive mechanism is valuable when the client or 315server depend on knowing when a connection has become inactive. 316.Pp 317The default value is 3. 318If 319.Cm ClientAliveInterval 320(see below) is set to 15, and 321.Cm ClientAliveCountMax 322is left at the default, unresponsive SSH clients 323will be disconnected after approximately 45 seconds. 324This option applies to protocol version 2 only. 325.It Cm ClientAliveInterval 326Sets a timeout interval in seconds after which if no data has been received 327from the client, 328.Xr sshd 8 329will send a message through the encrypted 330channel to request a response from the client. 331The default 332is 0, indicating that these messages will not be sent to the client. 333This option applies to protocol version 2 only. 334.It Cm Compression 335Specifies whether compression is allowed, or delayed until 336the user has authenticated successfully. 337The argument must be 338.Dq yes , 339.Dq delayed , 340or 341.Dq no . 342The default is 343.Dq delayed . 344.It Cm DenyGroups 345This keyword can be followed by a list of group name patterns, separated 346by spaces. 347Login is disallowed for users whose primary group or supplementary 348group list matches one of the patterns. 349Only group names are valid; a numerical group ID is not recognized. 350By default, login is allowed for all groups. 351The allow/deny directives are processed in the following order: 352.Cm DenyUsers , 353.Cm AllowUsers , 354.Cm DenyGroups , 355and finally 356.Cm AllowGroups . 357.Pp 358See 359.Sx PATTERNS 360in 361.Xr ssh_config 5 362for more information on patterns. 363.It Cm DenyUsers 364This keyword can be followed by a list of user name patterns, separated 365by spaces. 366Login is disallowed for user names that match one of the patterns. 367Only user names are valid; a numerical user ID is not recognized. 368By default, login is allowed for all users. 369If the pattern takes the form USER@HOST then USER and HOST 370are separately checked, restricting logins to particular 371users from particular hosts. 372The allow/deny directives are processed in the following order: 373.Cm DenyUsers , 374.Cm AllowUsers , 375.Cm DenyGroups , 376and finally 377.Cm AllowGroups . 378.Pp 379See 380.Sx PATTERNS 381in 382.Xr ssh_config 5 383for more information on patterns. 384.It Cm ForceCommand 385Forces the execution of the command specified by 386.Cm ForceCommand , 387ignoring any command supplied by the client and 388.Pa ~/.ssh/rc 389if present. 390The command is invoked by using the user's login shell with the -c option. 391This applies to shell, command, or subsystem execution. 392It is most useful inside a 393.Cm Match 394block. 395The command originally supplied by the client is available in the 396.Ev SSH_ORIGINAL_COMMAND 397environment variable. 398Specifying a command of 399.Dq internal-sftp 400will force the use of an in-process sftp server that requires no support 401files when used with 402.Cm ChrootDirectory . 403.It Cm GatewayPorts 404Specifies whether remote hosts are allowed to connect to ports 405forwarded for the client. 406By default, 407.Xr sshd 8 408binds remote port forwardings to the loopback address. 409This prevents other remote hosts from connecting to forwarded ports. 410.Cm GatewayPorts 411can be used to specify that sshd 412should allow remote port forwardings to bind to non-loopback addresses, thus 413allowing other hosts to connect. 414The argument may be 415.Dq no 416to force remote port forwardings to be available to the local host only, 417.Dq yes 418to force remote port forwardings to bind to the wildcard address, or 419.Dq clientspecified 420to allow the client to select the address to which the forwarding is bound. 421The default is 422.Dq no . 423.It Cm GSSAPIAuthentication 424Specifies whether user authentication based on GSSAPI is allowed. 425The default is 426.Dq no . 427Note that this option applies to protocol version 2 only. 428.It Cm GSSAPICleanupCredentials 429Specifies whether to automatically destroy the user's credentials cache 430on logout. 431The default is 432.Dq yes . 433Note that this option applies to protocol version 2 only. 434.It Cm HostbasedAuthentication 435Specifies whether rhosts or /etc/hosts.equiv authentication together 436with successful public key client host authentication is allowed 437(host-based authentication). 438This option is similar to 439.Cm RhostsRSAAuthentication 440and applies to protocol version 2 only. 441The default is 442.Dq no . 443.It Cm HostbasedUsesNameFromPacketOnly 444Specifies whether or not the server will attempt to perform a reverse 445name lookup when matching the name in the 446.Pa ~/.shosts , 447.Pa ~/.rhosts , 448and 449.Pa /etc/hosts.equiv 450files during 451.Cm HostbasedAuthentication . 452A setting of 453.Dq yes 454means that 455.Xr sshd 8 456uses the name supplied by the client rather than 457attempting to resolve the name from the TCP connection itself. 458The default is 459.Dq no . 460.It Cm HostCertificate 461Specifies a file containing a public host certificate. 462The certificate's public key must match a private host key already specified 463by 464.Cm HostKey . 465The default behaviour of 466.Xr sshd 8 467is not to load any certificates. 468.It Cm HostKey 469Specifies a file containing a private host key 470used by SSH. 471The default is 472.Pa /etc/ssh/ssh_host_key 473for protocol version 1, and 474.Pa /etc/ssh/ssh_host_dsa_key , 475.Pa /etc/ssh/ssh_host_ecdsa_key 476and 477.Pa /etc/ssh/ssh_host_rsa_key 478for protocol version 2. 479Note that 480.Xr sshd 8 481will refuse to use a file if it is group/world-accessible. 482It is possible to have multiple host key files. 483.Dq rsa1 484keys are used for version 1 and 485.Dq dsa , 486.Dq ecdsa 487or 488.Dq rsa 489are used for version 2 of the SSH protocol. 490.It Cm IgnoreRhosts 491Specifies that 492.Pa .rhosts 493and 494.Pa .shosts 495files will not be used in 496.Cm RhostsRSAAuthentication 497or 498.Cm HostbasedAuthentication . 499.Pp 500.Pa /etc/hosts.equiv 501and 502.Pa /etc/shosts.equiv 503are still used. 504The default is 505.Dq yes . 506.It Cm IgnoreUserKnownHosts 507Specifies whether 508.Xr sshd 8 509should ignore the user's 510.Pa ~/.ssh/known_hosts 511during 512.Cm RhostsRSAAuthentication 513or 514.Cm HostbasedAuthentication . 515The default is 516.Dq no . 517.It Cm IPQoS 518Specifies the IPv4 type-of-service or DSCP class for the connection. 519Accepted values are 520.Dq af11 , 521.Dq af12 , 522.Dq af13 , 523.Dq af21 , 524.Dq af22 , 525.Dq af23 , 526.Dq af31 , 527.Dq af32 , 528.Dq af33 , 529.Dq af41 , 530.Dq af42 , 531.Dq af43 , 532.Dq cs0 , 533.Dq cs1 , 534.Dq cs2 , 535.Dq cs3 , 536.Dq cs4 , 537.Dq cs5 , 538.Dq cs6 , 539.Dq cs7 , 540.Dq ef , 541.Dq lowdelay , 542.Dq throughput , 543.Dq reliability , 544or a numeric value. 545This option may take one or two arguments, separated by whitespace. 546If one argument is specified, it is used as the packet class unconditionally. 547If two values are specified, the first is automatically selected for 548interactive sessions and the second for non-interactive sessions. 549The default is 550.Dq lowdelay 551for interactive sessions and 552.Dq throughput 553for non-interactive sessions. 554.It Cm KerberosAuthentication 555Specifies whether the password provided by the user for 556.Cm PasswordAuthentication 557will be validated through the Kerberos KDC. 558To use this option, the server needs a 559Kerberos servtab which allows the verification of the KDC's identity. 560The default is 561.Dq no . 562.It Cm KerberosGetAFSToken 563If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 564an AFS token before accessing the user's home directory. 565The default is 566.Dq no . 567.It Cm KerberosOrLocalPasswd 568If password authentication through Kerberos fails then 569the password will be validated via any additional local mechanism 570such as 571.Pa /etc/passwd . 572The default is 573.Dq yes . 574.It Cm KerberosTicketCleanup 575Specifies whether to automatically destroy the user's ticket cache 576file on logout. 577The default is 578.Dq yes . 579.It Cm KexAlgorithms 580Specifies the available KEX (Key Exchange) algorithms. 581Multiple algorithms must be comma-separated. 582The default is 583.Dq ecdh-sha2-nistp256 , 584.Dq ecdh-sha2-nistp384 , 585.Dq ecdh-sha2-nistp521 , 586.Dq diffie-hellman-group-exchange-sha256 , 587.Dq diffie-hellman-group-exchange-sha1 , 588.Dq diffie-hellman-group14-sha1 , 589.Dq diffie-hellman-group1-sha1 . 590.It Cm KeyRegenerationInterval 591In protocol version 1, the ephemeral server key is automatically regenerated 592after this many seconds (if it has been used). 593The purpose of regeneration is to prevent 594decrypting captured sessions by later breaking into the machine and 595stealing the keys. 596The key is never stored anywhere. 597If the value is 0, the key is never regenerated. 598The default is 3600 (seconds). 599.It Cm ListenAddress 600Specifies the local addresses 601.Xr sshd 8 602should listen on. 603The following forms may be used: 604.Pp 605.Bl -item -offset indent -compact 606.It 607.Cm ListenAddress 608.Sm off 609.Ar host No | Ar IPv4_addr No | Ar IPv6_addr 610.Sm on 611.It 612.Cm ListenAddress 613.Sm off 614.Ar host No | Ar IPv4_addr No : Ar port 615.Sm on 616.It 617.Cm ListenAddress 618.Sm off 619.Oo 620.Ar host No | Ar IPv6_addr Oc : Ar port 621.Sm on 622.El 623.Pp 624If 625.Ar port 626is not specified, 627sshd will listen on the address and all prior 628.Cm Port 629options specified. 630The default is to listen on all local addresses. 631Multiple 632.Cm ListenAddress 633options are permitted. 634Additionally, any 635.Cm Port 636options must precede this option for non-port qualified addresses. 637.It Cm LoginGraceTime 638The server disconnects after this time if the user has not 639successfully logged in. 640If the value is 0, there is no time limit. 641The default is 120 seconds. 642.It Cm LogLevel 643Gives the verbosity level that is used when logging messages from 644.Xr sshd 8 . 645The possible values are: 646QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 647The default is INFO. 648DEBUG and DEBUG1 are equivalent. 649DEBUG2 and DEBUG3 each specify higher levels of debugging output. 650Logging with a DEBUG level violates the privacy of users and is not recommended. 651.It Cm MACs 652Specifies the available MAC (message authentication code) algorithms. 653The MAC algorithm is used in protocol version 2 654for data integrity protection. 655Multiple algorithms must be comma-separated. 656The default is: 657.Bd -literal -offset indent 658hmac-md5,hmac-sha1,umac-64@openssh.com, 659hmac-ripemd160,hmac-sha1-96,hmac-md5-96, 660hmac-sha2-256,hmac-sha256-96,hmac-sha2-512, 661hmac-sha2-512-96 662.Ed 663.It Cm Match 664Introduces a conditional block. 665If all of the criteria on the 666.Cm Match 667line are satisfied, the keywords on the following lines override those 668set in the global section of the config file, until either another 669.Cm Match 670line or the end of the file. 671.Pp 672The arguments to 673.Cm Match 674are one or more criteria-pattern pairs. 675The available criteria are 676.Cm User , 677.Cm Group , 678.Cm Host , 679and 680.Cm Address . 681The match patterns may consist of single entries or comma-separated 682lists and may use the wildcard and negation operators described in the 683.Sx PATTERNS 684section of 685.Xr ssh_config 5 . 686.Pp 687The patterns in an 688.Cm Address 689criteria may additionally contain addresses to match in CIDR 690address/masklen format, e.g.\& 691.Dq 192.0.2.0/24 692or 693.Dq 3ffe:ffff::/32 . 694Note that the mask length provided must be consistent with the address - 695it is an error to specify a mask length that is too long for the address 696or one with bits set in this host portion of the address. 697For example, 698.Dq 192.0.2.0/33 699and 700.Dq 192.0.2.0/8 701respectively. 702.Pp 703Only a subset of keywords may be used on the lines following a 704.Cm Match 705keyword. 706Available keywords are 707.Cm AllowAgentForwarding , 708.Cm AllowTcpForwarding , 709.Cm AuthorizedKeysFile , 710.Cm AuthorizedPrincipalsFile , 711.Cm Banner , 712.Cm ChrootDirectory , 713.Cm ForceCommand , 714.Cm GatewayPorts , 715.Cm GSSAPIAuthentication , 716.Cm HostbasedAuthentication , 717.Cm HostbasedUsesNameFromPacketOnly , 718.Cm KbdInteractiveAuthentication , 719.Cm KerberosAuthentication , 720.Cm MaxAuthTries , 721.Cm MaxSessions , 722.Cm PasswordAuthentication , 723.Cm PermitEmptyPasswords , 724.Cm PermitOpen , 725.Cm PermitRootLogin , 726.Cm PermitTunnel , 727.Cm PubkeyAuthentication , 728.Cm RhostsRSAAuthentication , 729.Cm RSAAuthentication , 730.Cm X11DisplayOffset , 731.Cm X11Forwarding 732and 733.Cm X11UseLocalHost . 734.It Cm MaxAuthTries 735Specifies the maximum number of authentication attempts permitted per 736connection. 737Once the number of failures reaches half this value, 738additional failures are logged. 739The default is 6. 740.It Cm MaxSessions 741Specifies the maximum number of open sessions permitted per network connection. 742The default is 10. 743.It Cm MaxStartups 744Specifies the maximum number of concurrent unauthenticated connections to the 745SSH daemon. 746Additional connections will be dropped until authentication succeeds or the 747.Cm LoginGraceTime 748expires for a connection. 749The default is 10. 750.Pp 751Alternatively, random early drop can be enabled by specifying 752the three colon separated values 753.Dq start:rate:full 754(e.g. "10:30:60"). 755.Xr sshd 8 756will refuse connection attempts with a probability of 757.Dq rate/100 758(30%) 759if there are currently 760.Dq start 761(10) 762unauthenticated connections. 763The probability increases linearly and all connection attempts 764are refused if the number of unauthenticated connections reaches 765.Dq full 766(60). 767.It Cm PasswordAuthentication 768Specifies whether password authentication is allowed. 769The default is 770.Dq yes . 771.It Cm PermitEmptyPasswords 772When password authentication is allowed, it specifies whether the 773server allows login to accounts with empty password strings. 774The default is 775.Dq no . 776.It Cm PermitOpen 777Specifies the destinations to which TCP port forwarding is permitted. 778The forwarding specification must be one of the following forms: 779.Pp 780.Bl -item -offset indent -compact 781.It 782.Cm PermitOpen 783.Sm off 784.Ar host : port 785.Sm on 786.It 787.Cm PermitOpen 788.Sm off 789.Ar IPv4_addr : port 790.Sm on 791.It 792.Cm PermitOpen 793.Sm off 794.Ar \&[ IPv6_addr \&] : port 795.Sm on 796.El 797.Pp 798Multiple forwards may be specified by separating them with whitespace. 799An argument of 800.Dq any 801can be used to remove all restrictions and permit any forwarding requests. 802By default all port forwarding requests are permitted. 803.It Cm PermitRootLogin 804Specifies whether root can log in using 805.Xr ssh 1 . 806The argument must be 807.Dq yes , 808.Dq without-password , 809.Dq forced-commands-only , 810or 811.Dq no . 812The default is 813.Dq yes . 814.Pp 815If this option is set to 816.Dq without-password , 817password authentication is disabled for root. 818.Pp 819If this option is set to 820.Dq forced-commands-only , 821root login with public key authentication will be allowed, 822but only if the 823.Ar command 824option has been specified 825(which may be useful for taking remote backups even if root login is 826normally not allowed). 827All other authentication methods are disabled for root. 828.Pp 829If this option is set to 830.Dq no , 831root is not allowed to log in. 832.It Cm PermitTunnel 833Specifies whether 834.Xr tun 4 835device forwarding is allowed. 836The argument must be 837.Dq yes , 838.Dq point-to-point 839(layer 3), 840.Dq ethernet 841(layer 2), or 842.Dq no . 843Specifying 844.Dq yes 845permits both 846.Dq point-to-point 847and 848.Dq ethernet . 849The default is 850.Dq no . 851.It Cm PermitUserEnvironment 852Specifies whether 853.Pa ~/.ssh/environment 854and 855.Cm environment= 856options in 857.Pa ~/.ssh/authorized_keys 858are processed by 859.Xr sshd 8 . 860The default is 861.Dq no . 862Enabling environment processing may enable users to bypass access 863restrictions in some configurations using mechanisms such as 864.Ev LD_PRELOAD . 865.It Cm PidFile 866Specifies the file that contains the process ID of the 867SSH daemon. 868The default is 869.Pa /var/run/sshd.pid . 870.It Cm Port 871Specifies the port number that 872.Xr sshd 8 873listens on. 874The default is 22. 875Multiple options of this type are permitted. 876See also 877.Cm ListenAddress . 878.It Cm PrintLastLog 879Specifies whether 880.Xr sshd 8 881should print the date and time of the last user login when a user logs 882in interactively. 883The default is 884.Dq yes . 885.It Cm PrintMotd 886Specifies whether 887.Xr sshd 8 888should print 889.Pa /etc/motd 890when a user logs in interactively. 891(On some systems it is also printed by the shell, 892.Pa /etc/profile , 893or equivalent.) 894The default is 895.Dq yes . 896.It Cm Protocol 897Specifies the protocol versions 898.Xr sshd 8 899supports. 900The possible values are 901.Sq 1 902and 903.Sq 2 . 904Multiple versions must be comma-separated. 905The default is 906.Sq 2 . 907Note that the order of the protocol list does not indicate preference, 908because the client selects among multiple protocol versions offered 909by the server. 910Specifying 911.Dq 2,1 912is identical to 913.Dq 1,2 . 914.It Cm PubkeyAuthentication 915Specifies whether public key authentication is allowed. 916The default is 917.Dq yes . 918Note that this option applies to protocol version 2 only. 919.It Cm RevokedKeys 920Specifies a list of revoked public keys. 921Keys listed in this file will be refused for public key authentication. 922Note that if this file is not readable, then public key authentication will 923be refused for all users. 924.It Cm RhostsRSAAuthentication 925Specifies whether rhosts or /etc/hosts.equiv authentication together 926with successful RSA host authentication is allowed. 927The default is 928.Dq no . 929This option applies to protocol version 1 only. 930.It Cm RSAAuthentication 931Specifies whether pure RSA authentication is allowed. 932The default is 933.Dq yes . 934This option applies to protocol version 1 only. 935.It Cm ServerKeyBits 936Defines the number of bits in the ephemeral protocol version 1 server key. 937The minimum value is 512, and the default is 1024. 938.It Cm StrictModes 939Specifies whether 940.Xr sshd 8 941should check file modes and ownership of the 942user's files and home directory before accepting login. 943This is normally desirable because novices sometimes accidentally leave their 944directory or files world-writable. 945The default is 946.Dq yes . 947Note that this does not apply to 948.Cm ChrootDirectory , 949whose permissions and ownership are checked unconditionally. 950.It Cm Subsystem 951Configures an external subsystem (e.g. file transfer daemon). 952Arguments should be a subsystem name and a command (with optional arguments) 953to execute upon subsystem request. 954.Pp 955The command 956.Xr sftp-server 8 957implements the 958.Dq sftp 959file transfer subsystem. 960.Pp 961Alternately the name 962.Dq internal-sftp 963implements an in-process 964.Dq sftp 965server. 966This may simplify configurations using 967.Cm ChrootDirectory 968to force a different filesystem root on clients. 969.Pp 970By default no subsystems are defined. 971Note that this option applies to protocol version 2 only. 972.It Cm SyslogFacility 973Gives the facility code that is used when logging messages from 974.Xr sshd 8 . 975The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 976LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 977The default is AUTH. 978.It Cm TCPKeepAlive 979Specifies whether the system should send TCP keepalive messages to the 980other side. 981If they are sent, death of the connection or crash of one 982of the machines will be properly noticed. 983However, this means that 984connections will die if the route is down temporarily, and some people 985find it annoying. 986On the other hand, if TCP keepalives are not sent, 987sessions may hang indefinitely on the server, leaving 988.Dq ghost 989users and consuming server resources. 990.Pp 991The default is 992.Dq yes 993(to send TCP keepalive messages), and the server will notice 994if the network goes down or the client host crashes. 995This avoids infinitely hanging sessions. 996.Pp 997To disable TCP keepalive messages, the value should be set to 998.Dq no . 999.It Cm TrustedUserCAKeys 1000Specifies a file containing public keys of certificate authorities that are 1001trusted to sign user certificates for authentication. 1002Keys are listed one per line; empty lines and comments starting with 1003.Ql # 1004are allowed. 1005If a certificate is presented for authentication and has its signing CA key 1006listed in this file, then it may be used for authentication for any user 1007listed in the certificate's principals list. 1008Note that certificates that lack a list of principals will not be permitted 1009for authentication using 1010.Cm TrustedUserCAKeys . 1011For more details on certificates, see the 1012.Sx CERTIFICATES 1013section in 1014.Xr ssh-keygen 1 . 1015.It Cm UseDNS 1016Specifies whether 1017.Xr sshd 8 1018should look up the remote host name and check that 1019the resolved host name for the remote IP address maps back to the 1020very same IP address. 1021The default is 1022.Dq yes . 1023.It Cm UseLogin 1024Specifies whether 1025.Xr login 1 1026is used for interactive login sessions. 1027The default is 1028.Dq no . 1029Note that 1030.Xr login 1 1031is never used for remote command execution. 1032Note also, that if this is enabled, 1033.Cm X11Forwarding 1034will be disabled because 1035.Xr login 1 1036does not know how to handle 1037.Xr xauth 1 1038cookies. 1039If 1040.Cm UsePrivilegeSeparation 1041is specified, it will be disabled after authentication. 1042.It Cm UsePrivilegeSeparation 1043Specifies whether 1044.Xr sshd 8 1045separates privileges by creating an unprivileged child process 1046to deal with incoming network traffic. 1047After successful authentication, another process will be created that has 1048the privilege of the authenticated user. 1049The goal of privilege separation is to prevent privilege 1050escalation by containing any corruption within the unprivileged processes. 1051The default is 1052.Dq yes . 1053If 1054.Cm UsePrivilegeSeparation 1055is set to 1056.Dq sandbox 1057then the pre-authentication unprivileged process is subject to additional 1058restrictions. 1059.It Cm X11DisplayOffset 1060Specifies the first display number available for 1061.Xr sshd 8 Ns 's 1062X11 forwarding. 1063This prevents sshd from interfering with real X11 servers. 1064The default is 10. 1065.It Cm X11Forwarding 1066Specifies whether X11 forwarding is permitted. 1067The argument must be 1068.Dq yes 1069or 1070.Dq no . 1071The default is 1072.Dq no . 1073.Pp 1074When X11 forwarding is enabled, there may be additional exposure to 1075the server and to client displays if the 1076.Xr sshd 8 1077proxy display is configured to listen on the wildcard address (see 1078.Cm X11UseLocalhost 1079below), though this is not the default. 1080Additionally, the authentication spoofing and authentication data 1081verification and substitution occur on the client side. 1082The security risk of using X11 forwarding is that the client's X11 1083display server may be exposed to attack when the SSH client requests 1084forwarding (see the warnings for 1085.Cm ForwardX11 1086in 1087.Xr ssh_config 5 ) . 1088A system administrator may have a stance in which they want to 1089protect clients that may expose themselves to attack by unwittingly 1090requesting X11 forwarding, which can warrant a 1091.Dq no 1092setting. 1093.Pp 1094Note that disabling X11 forwarding does not prevent users from 1095forwarding X11 traffic, as users can always install their own forwarders. 1096X11 forwarding is automatically disabled if 1097.Cm UseLogin 1098is enabled. 1099.It Cm X11UseLocalhost 1100Specifies whether 1101.Xr sshd 8 1102should bind the X11 forwarding server to the loopback address or to 1103the wildcard address. 1104By default, 1105sshd binds the forwarding server to the loopback address and sets the 1106hostname part of the 1107.Ev DISPLAY 1108environment variable to 1109.Dq localhost . 1110This prevents remote hosts from connecting to the proxy display. 1111However, some older X11 clients may not function with this 1112configuration. 1113.Cm X11UseLocalhost 1114may be set to 1115.Dq no 1116to specify that the forwarding server should be bound to the wildcard 1117address. 1118The argument must be 1119.Dq yes 1120or 1121.Dq no . 1122The default is 1123.Dq yes . 1124.It Cm XAuthLocation 1125Specifies the full pathname of the 1126.Xr xauth 1 1127program. 1128The default is 1129.Pa /usr/X11R6/bin/xauth . 1130.El 1131.Sh TIME FORMATS 1132.Xr sshd 8 1133command-line arguments and configuration file options that specify time 1134may be expressed using a sequence of the form: 1135.Sm off 1136.Ar time Op Ar qualifier , 1137.Sm on 1138where 1139.Ar time 1140is a positive integer value and 1141.Ar qualifier 1142is one of the following: 1143.Pp 1144.Bl -tag -width Ds -compact -offset indent 1145.It Aq Cm none 1146seconds 1147.It Cm s | Cm S 1148seconds 1149.It Cm m | Cm M 1150minutes 1151.It Cm h | Cm H 1152hours 1153.It Cm d | Cm D 1154days 1155.It Cm w | Cm W 1156weeks 1157.El 1158.Pp 1159Each member of the sequence is added together to calculate 1160the total time value. 1161.Pp 1162Time format examples: 1163.Pp 1164.Bl -tag -width Ds -compact -offset indent 1165.It 600 1166600 seconds (10 minutes) 1167.It 10m 116810 minutes 1169.It 1h30m 11701 hour 30 minutes (90 minutes) 1171.El 1172.Sh FILES 1173.Bl -tag -width Ds 1174.It Pa /etc/ssh/sshd_config 1175Contains configuration data for 1176.Xr sshd 8 . 1177This file should be writable by root only, but it is recommended 1178(though not necessary) that it be world-readable. 1179.El 1180.Sh SEE ALSO 1181.Xr sshd 8 1182.Sh AUTHORS 1183OpenSSH is a derivative of the original and free 1184ssh 1.2.12 release by Tatu Ylonen. 1185Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1186Theo de Raadt and Dug Song 1187removed many bugs, re-added newer features and 1188created OpenSSH. 1189Markus Friedl contributed the support for SSH 1190protocol versions 1.5 and 2.0. 1191Niels Provos and Markus Friedl contributed support 1192for privilege separation. 1193