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: ssh_config.5,v 1.364 2021/09/03 07:43:23 dtucker Exp $ 37.Dd $Mdocdate: September 3 2021 $ 38.Dt SSH_CONFIG 5 39.Os 40.Sh NAME 41.Nm ssh_config 42.Nd OpenSSH client configuration file 43.Sh DESCRIPTION 44.Xr ssh 1 45obtains configuration data from the following sources in 46the following order: 47.Pp 48.Bl -enum -offset indent -compact 49.It 50command-line options 51.It 52user's configuration file 53.Pq Pa ~/.ssh/config 54.It 55system-wide configuration file 56.Pq Pa /etc/ssh/ssh_config 57.El 58.Pp 59For each parameter, the first obtained value 60will be used. 61The configuration files contain sections separated by 62.Cm Host 63specifications, and that section is only applied for hosts that 64match one of the patterns given in the specification. 65The matched host name is usually the one given on the command line 66(see the 67.Cm CanonicalizeHostname 68option for exceptions). 69.Pp 70Since the first obtained value for each parameter is used, more 71host-specific declarations should be given near the beginning of the 72file, and general defaults at the end. 73.Pp 74The file contains keyword-argument pairs, one per line. 75Lines starting with 76.Ql # 77and empty lines are interpreted as comments. 78Arguments may optionally be enclosed in double quotes 79.Pq \&" 80in order to represent arguments containing spaces. 81Configuration options may be separated by whitespace or 82optional whitespace and exactly one 83.Ql = ; 84the latter format is useful to avoid the need to quote whitespace 85when specifying configuration options using the 86.Nm ssh , 87.Nm scp , 88and 89.Nm sftp 90.Fl o 91option. 92.Pp 93The possible 94keywords and their meanings are as follows (note that 95keywords are case-insensitive and arguments are case-sensitive): 96.Bl -tag -width Ds 97.It Cm Host 98Restricts the following declarations (up to the next 99.Cm Host 100or 101.Cm Match 102keyword) to be only for those hosts that match one of the patterns 103given after the keyword. 104If more than one pattern is provided, they should be separated by whitespace. 105A single 106.Ql * 107as a pattern can be used to provide global 108defaults for all hosts. 109The host is usually the 110.Ar hostname 111argument given on the command line 112(see the 113.Cm CanonicalizeHostname 114keyword for exceptions). 115.Pp 116A pattern entry may be negated by prefixing it with an exclamation mark 117.Pq Sq !\& . 118If a negated entry is matched, then the 119.Cm Host 120entry is ignored, regardless of whether any other patterns on the line 121match. 122Negated matches are therefore useful to provide exceptions for wildcard 123matches. 124.Pp 125See 126.Sx PATTERNS 127for more information on patterns. 128.It Cm Match 129Restricts the following declarations (up to the next 130.Cm Host 131or 132.Cm Match 133keyword) to be used only when the conditions following the 134.Cm Match 135keyword are satisfied. 136Match conditions are specified using one or more criteria 137or the single token 138.Cm all 139which always matches. 140The available criteria keywords are: 141.Cm canonical , 142.Cm final , 143.Cm exec , 144.Cm host , 145.Cm originalhost , 146.Cm user , 147and 148.Cm localuser . 149The 150.Cm all 151criteria must appear alone or immediately after 152.Cm canonical 153or 154.Cm final . 155Other criteria may be combined arbitrarily. 156All criteria but 157.Cm all , 158.Cm canonical , 159and 160.Cm final 161require an argument. 162Criteria may be negated by prepending an exclamation mark 163.Pq Sq !\& . 164.Pp 165The 166.Cm canonical 167keyword matches only when the configuration file is being re-parsed 168after hostname canonicalization (see the 169.Cm CanonicalizeHostname 170option). 171This may be useful to specify conditions that work with canonical host 172names only. 173.Pp 174The 175.Cm final 176keyword requests that the configuration be re-parsed (regardless of whether 177.Cm CanonicalizeHostname 178is enabled), and matches only during this final pass. 179If 180.Cm CanonicalizeHostname 181is enabled, then 182.Cm canonical 183and 184.Cm final 185match during the same pass. 186.Pp 187The 188.Cm exec 189keyword executes the specified command under the user's shell. 190If the command returns a zero exit status then the condition is considered true. 191Commands containing whitespace characters must be quoted. 192Arguments to 193.Cm exec 194accept the tokens described in the 195.Sx TOKENS 196section. 197.Pp 198The other keywords' criteria must be single entries or comma-separated 199lists and may use the wildcard and negation operators described in the 200.Sx PATTERNS 201section. 202The criteria for the 203.Cm host 204keyword are matched against the target hostname, after any substitution 205by the 206.Cm Hostname 207or 208.Cm CanonicalizeHostname 209options. 210The 211.Cm originalhost 212keyword matches against the hostname as it was specified on the command-line. 213The 214.Cm user 215keyword matches against the target username on the remote host. 216The 217.Cm localuser 218keyword matches against the name of the local user running 219.Xr ssh 1 220(this keyword may be useful in system-wide 221.Nm 222files). 223.It Cm AddKeysToAgent 224Specifies whether keys should be automatically added to a running 225.Xr ssh-agent 1 . 226If this option is set to 227.Cm yes 228and a key is loaded from a file, the key and its passphrase are added to 229the agent with the default lifetime, as if by 230.Xr ssh-add 1 . 231If this option is set to 232.Cm ask , 233.Xr ssh 1 234will require confirmation using the 235.Ev SSH_ASKPASS 236program before adding a key (see 237.Xr ssh-add 1 238for details). 239If this option is set to 240.Cm confirm , 241each use of the key must be confirmed, as if the 242.Fl c 243option was specified to 244.Xr ssh-add 1 . 245If this option is set to 246.Cm no , 247no keys are added to the agent. 248Alternately, this option may be specified as a time interval 249using the format described in the 250.Sx TIME FORMATS 251section of 252.Xr sshd_config 5 253to specify the key's lifetime in 254.Xr ssh-agent 1 , 255after which it will automatically be removed. 256The argument must be 257.Cm no 258(the default), 259.Cm yes , 260.Cm confirm 261(optionally followed by a time interval), 262.Cm ask 263or a time interval. 264.It Cm AddressFamily 265Specifies which address family to use when connecting. 266Valid arguments are 267.Cm any 268(the default), 269.Cm inet 270(use IPv4 only), or 271.Cm inet6 272(use IPv6 only). 273.It Cm BatchMode 274If set to 275.Cm yes , 276user interaction such as password prompts and host key confirmation requests 277will be disabled. 278This option is useful in scripts and other batch jobs where no user 279is present to interact with 280.Xr ssh 1 . 281The argument must be 282.Cm yes 283or 284.Cm no 285(the default). 286.It Cm BindAddress 287Use the specified address on the local machine as the source address of 288the connection. 289Only useful on systems with more than one address. 290.It Cm BindInterface 291Use the address of the specified interface on the local machine as the 292source address of the connection. 293.It Cm CanonicalDomains 294When 295.Cm CanonicalizeHostname 296is enabled, this option specifies the list of domain suffixes in which to 297search for the specified destination host. 298.It Cm CanonicalizeFallbackLocal 299Specifies whether to fail with an error when hostname canonicalization fails. 300The default, 301.Cm yes , 302will attempt to look up the unqualified hostname using the system resolver's 303search rules. 304A value of 305.Cm no 306will cause 307.Xr ssh 1 308to fail instantly if 309.Cm CanonicalizeHostname 310is enabled and the target hostname cannot be found in any of the domains 311specified by 312.Cm CanonicalDomains . 313.It Cm CanonicalizeHostname 314Controls whether explicit hostname canonicalization is performed. 315The default, 316.Cm no , 317is not to perform any name rewriting and let the system resolver handle all 318hostname lookups. 319If set to 320.Cm yes 321then, for connections that do not use a 322.Cm ProxyCommand 323or 324.Cm ProxyJump , 325.Xr ssh 1 326will attempt to canonicalize the hostname specified on the command line 327using the 328.Cm CanonicalDomains 329suffixes and 330.Cm CanonicalizePermittedCNAMEs 331rules. 332If 333.Cm CanonicalizeHostname 334is set to 335.Cm always , 336then canonicalization is applied to proxied connections too. 337.Pp 338If this option is enabled, then the configuration files are processed 339again using the new target name to pick up any new configuration in matching 340.Cm Host 341and 342.Cm Match 343stanzas. 344A value of 345.Cm none 346disables the use of a 347.Cm ProxyJump 348host. 349.It Cm CanonicalizeMaxDots 350Specifies the maximum number of dot characters in a hostname before 351canonicalization is disabled. 352The default, 1, 353allows a single dot (i.e. hostname.subdomain). 354.It Cm CanonicalizePermittedCNAMEs 355Specifies rules to determine whether CNAMEs should be followed when 356canonicalizing hostnames. 357The rules consist of one or more arguments of 358.Ar source_domain_list : Ns Ar target_domain_list , 359where 360.Ar source_domain_list 361is a pattern-list of domains that may follow CNAMEs in canonicalization, 362and 363.Ar target_domain_list 364is a pattern-list of domains that they may resolve to. 365.Pp 366For example, 367.Qq *.a.example.com:*.b.example.com,*.c.example.com 368will allow hostnames matching 369.Qq *.a.example.com 370to be canonicalized to names in the 371.Qq *.b.example.com 372or 373.Qq *.c.example.com 374domains. 375.It Cm CASignatureAlgorithms 376Specifies which algorithms are allowed for signing of certificates 377by certificate authorities (CAs). 378The default is: 379.Bd -literal -offset indent 380ssh-ed25519,ecdsa-sha2-nistp256, 381ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 382sk-ssh-ed25519@openssh.com, 383sk-ecdsa-sha2-nistp256@openssh.com, 384rsa-sha2-512,rsa-sha2-256 385.Ed 386.Pp 387If the specified list begins with a 388.Sq + 389character, then the specified algorithms will be appended to the default set 390instead of replacing them. 391If the specified list begins with a 392.Sq - 393character, then the specified algorithms (including wildcards) will be removed 394from the default set instead of replacing them. 395.Pp 396.Xr ssh 1 397will not accept host certificates signed using algorithms other than those 398specified. 399.It Cm CertificateFile 400Specifies a file from which the user's certificate is read. 401A corresponding private key must be provided separately in order 402to use this certificate either 403from an 404.Cm IdentityFile 405directive or 406.Fl i 407flag to 408.Xr ssh 1 , 409via 410.Xr ssh-agent 1 , 411or via a 412.Cm PKCS11Provider 413or 414.Cm SecurityKeyProvider . 415.Pp 416Arguments to 417.Cm CertificateFile 418may use the tilde syntax to refer to a user's home directory, 419the tokens described in the 420.Sx TOKENS 421section and environment variables as described in the 422.Sx ENVIRONMENT VARIABLES 423section. 424.Pp 425It is possible to have multiple certificate files specified in 426configuration files; these certificates will be tried in sequence. 427Multiple 428.Cm CertificateFile 429directives will add to the list of certificates used for 430authentication. 431.It Cm CheckHostIP 432If set to 433.Cm yes 434.Xr ssh 1 435will additionally check the host IP address in the 436.Pa known_hosts 437file. 438This allows it to detect if a host key changed due to DNS spoofing 439and will add addresses of destination hosts to 440.Pa ~/.ssh/known_hosts 441in the process, regardless of the setting of 442.Cm StrictHostKeyChecking . 443If the option is set to 444.Cm no 445(the default), 446the check will not be executed. 447.It Cm Ciphers 448Specifies the ciphers allowed and their order of preference. 449Multiple ciphers must be comma-separated. 450If the specified list begins with a 451.Sq + 452character, then the specified ciphers will be appended to the default set 453instead of replacing them. 454If the specified list begins with a 455.Sq - 456character, then the specified ciphers (including wildcards) will be removed 457from the default set instead of replacing them. 458If the specified list begins with a 459.Sq ^ 460character, then the specified ciphers will be placed at the head of the 461default set. 462.Pp 463The supported ciphers are: 464.Bd -literal -offset indent 4653des-cbc 466aes128-cbc 467aes192-cbc 468aes256-cbc 469aes128-ctr 470aes192-ctr 471aes256-ctr 472aes128-gcm@openssh.com 473aes256-gcm@openssh.com 474chacha20-poly1305@openssh.com 475.Ed 476.Pp 477The default is: 478.Bd -literal -offset indent 479chacha20-poly1305@openssh.com, 480aes128-ctr,aes192-ctr,aes256-ctr, 481aes128-gcm@openssh.com,aes256-gcm@openssh.com 482.Ed 483.Pp 484The list of available ciphers may also be obtained using 485.Qq ssh -Q cipher . 486.It Cm ClearAllForwardings 487Specifies that all local, remote, and dynamic port forwardings 488specified in the configuration files or on the command line be 489cleared. 490This option is primarily useful when used from the 491.Xr ssh 1 492command line to clear port forwardings set in 493configuration files, and is automatically set by 494.Xr scp 1 495and 496.Xr sftp 1 . 497The argument must be 498.Cm yes 499or 500.Cm no 501(the default). 502.It Cm Compression 503Specifies whether to use compression. 504The argument must be 505.Cm yes 506or 507.Cm no 508(the default). 509.It Cm ConnectionAttempts 510Specifies the number of tries (one per second) to make before exiting. 511The argument must be an integer. 512This may be useful in scripts if the connection sometimes fails. 513The default is 1. 514.It Cm ConnectTimeout 515Specifies the timeout (in seconds) used when connecting to the 516SSH server, instead of using the default system TCP timeout. 517This timeout is applied both to establishing the connection and to performing 518the initial SSH protocol handshake and key exchange. 519.It Cm ControlMaster 520Enables the sharing of multiple sessions over a single network connection. 521When set to 522.Cm yes , 523.Xr ssh 1 524will listen for connections on a control socket specified using the 525.Cm ControlPath 526argument. 527Additional sessions can connect to this socket using the same 528.Cm ControlPath 529with 530.Cm ControlMaster 531set to 532.Cm no 533(the default). 534These sessions will try to reuse the master instance's network connection 535rather than initiating new ones, but will fall back to connecting normally 536if the control socket does not exist, or is not listening. 537.Pp 538Setting this to 539.Cm ask 540will cause 541.Xr ssh 1 542to listen for control connections, but require confirmation using 543.Xr ssh-askpass 1 . 544If the 545.Cm ControlPath 546cannot be opened, 547.Xr ssh 1 548will continue without connecting to a master instance. 549.Pp 550X11 and 551.Xr ssh-agent 1 552forwarding is supported over these multiplexed connections, however the 553display and agent forwarded will be the one belonging to the master 554connection i.e. it is not possible to forward multiple displays or agents. 555.Pp 556Two additional options allow for opportunistic multiplexing: try to use a 557master connection but fall back to creating a new one if one does not already 558exist. 559These options are: 560.Cm auto 561and 562.Cm autoask . 563The latter requires confirmation like the 564.Cm ask 565option. 566.It Cm ControlPath 567Specify the path to the control socket used for connection sharing as described 568in the 569.Cm ControlMaster 570section above or the string 571.Cm none 572to disable connection sharing. 573Arguments to 574.Cm ControlPath 575may use the tilde syntax to refer to a user's home directory, 576the tokens described in the 577.Sx TOKENS 578section and environment variables as described in the 579.Sx ENVIRONMENT VARIABLES 580section. 581It is recommended that any 582.Cm ControlPath 583used for opportunistic connection sharing include 584at least %h, %p, and %r (or alternatively %C) and be placed in a directory 585that is not writable by other users. 586This ensures that shared connections are uniquely identified. 587.It Cm ControlPersist 588When used in conjunction with 589.Cm ControlMaster , 590specifies that the master connection should remain open 591in the background (waiting for future client connections) 592after the initial client connection has been closed. 593If set to 594.Cm no 595(the default), 596then the master connection will not be placed into the background, 597and will close as soon as the initial client connection is closed. 598If set to 599.Cm yes 600or 0, 601then the master connection will remain in the background indefinitely 602(until killed or closed via a mechanism such as the 603.Qq ssh -O exit ) . 604If set to a time in seconds, or a time in any of the formats documented in 605.Xr sshd_config 5 , 606then the backgrounded master connection will automatically terminate 607after it has remained idle (with no client connections) for the 608specified time. 609.It Cm DynamicForward 610Specifies that a TCP port on the local machine be forwarded 611over the secure channel, and the application 612protocol is then used to determine where to connect to from the 613remote machine. 614.Pp 615The argument must be 616.Sm off 617.Oo Ar bind_address : Oc Ar port . 618.Sm on 619IPv6 addresses can be specified by enclosing addresses in square brackets. 620By default, the local port is bound in accordance with the 621.Cm GatewayPorts 622setting. 623However, an explicit 624.Ar bind_address 625may be used to bind the connection to a specific address. 626The 627.Ar bind_address 628of 629.Cm localhost 630indicates that the listening port be bound for local use only, while an 631empty address or 632.Sq * 633indicates that the port should be available from all interfaces. 634.Pp 635Currently the SOCKS4 and SOCKS5 protocols are supported, and 636.Xr ssh 1 637will act as a SOCKS server. 638Multiple forwardings may be specified, and 639additional forwardings can be given on the command line. 640Only the superuser can forward privileged ports. 641.It Cm EnableSSHKeysign 642Setting this option to 643.Cm yes 644in the global client configuration file 645.Pa /etc/ssh/ssh_config 646enables the use of the helper program 647.Xr ssh-keysign 8 648during 649.Cm HostbasedAuthentication . 650The argument must be 651.Cm yes 652or 653.Cm no 654(the default). 655This option should be placed in the non-hostspecific section. 656See 657.Xr ssh-keysign 8 658for more information. 659.It Cm EscapeChar 660Sets the escape character (default: 661.Ql ~ ) . 662The escape character can also 663be set on the command line. 664The argument should be a single character, 665.Ql ^ 666followed by a letter, or 667.Cm none 668to disable the escape 669character entirely (making the connection transparent for binary 670data). 671.It Cm ExitOnForwardFailure 672Specifies whether 673.Xr ssh 1 674should terminate the connection if it cannot set up all requested 675dynamic, tunnel, local, and remote port forwardings, (e.g.\& 676if either end is unable to bind and listen on a specified port). 677Note that 678.Cm ExitOnForwardFailure 679does not apply to connections made over port forwardings and will not, 680for example, cause 681.Xr ssh 1 682to exit if TCP connections to the ultimate forwarding destination fail. 683The argument must be 684.Cm yes 685or 686.Cm no 687(the default). 688.It Cm FingerprintHash 689Specifies the hash algorithm used when displaying key fingerprints. 690Valid options are: 691.Cm md5 692and 693.Cm sha256 694(the default). 695.It Cm ForkAfterAuthentication 696Requests 697.Nm ssh 698to go to background just before command execution. 699This is useful if 700.Nm ssh 701is going to ask for passwords or passphrases, but the user 702wants it in the background. 703This implies the 704.Cm StdinNull 705configuration option being set to 706.Dq yes . 707The recommended way to start X11 programs at a remote site is with 708something like 709.Ic ssh -f host xterm , 710which is the same as 711.Ic ssh host xterm 712if the 713.Cm ForkAfterAuthentication 714configuration option is set to 715.Dq yes . 716.Pp 717If the 718.Cm ExitOnForwardFailure 719configuration option is set to 720.Dq yes , 721then a client started with the 722.Cm ForkAfterAuthentication 723configuration option being set to 724.Dq yes 725will wait for all remote port forwards to be successfully established 726before placing itself in the background. 727The argument to this keyword must be 728.Cm yes 729(same as the 730.Fl f 731option) or 732.Cm no 733(the default). 734.It Cm ForwardAgent 735Specifies whether the connection to the authentication agent (if any) 736will be forwarded to the remote machine. 737The argument may be 738.Cm yes , 739.Cm no 740(the default), 741an explicit path to an agent socket or the name of an environment variable 742(beginning with 743.Sq $ ) 744in which to find the path. 745.Pp 746Agent forwarding should be enabled with caution. 747Users with the ability to bypass file permissions on the remote host 748(for the agent's Unix-domain socket) 749can access the local agent through the forwarded connection. 750An attacker cannot obtain key material from the agent, 751however they can perform operations on the keys that enable them to 752authenticate using the identities loaded into the agent. 753.It Cm ForwardX11 754Specifies whether X11 connections will be automatically redirected 755over the secure channel and 756.Ev DISPLAY 757set. 758The argument must be 759.Cm yes 760or 761.Cm no 762(the default). 763.Pp 764X11 forwarding should be enabled with caution. 765Users with the ability to bypass file permissions on the remote host 766(for the user's X11 authorization database) 767can access the local X11 display through the forwarded connection. 768An attacker may then be able to perform activities such as keystroke monitoring 769if the 770.Cm ForwardX11Trusted 771option is also enabled. 772.It Cm ForwardX11Timeout 773Specify a timeout for untrusted X11 forwarding 774using the format described in the 775.Sx TIME FORMATS 776section of 777.Xr sshd_config 5 . 778X11 connections received by 779.Xr ssh 1 780after this time will be refused. 781Setting 782.Cm ForwardX11Timeout 783to zero will disable the timeout and permit X11 forwarding for the life 784of the connection. 785The default is to disable untrusted X11 forwarding after twenty minutes has 786elapsed. 787.It Cm ForwardX11Trusted 788If this option is set to 789.Cm yes , 790remote X11 clients will have full access to the original X11 display. 791.Pp 792If this option is set to 793.Cm no 794(the default), 795remote X11 clients will be considered untrusted and prevented 796from stealing or tampering with data belonging to trusted X11 797clients. 798Furthermore, the 799.Xr xauth 1 800token used for the session will be set to expire after 20 minutes. 801Remote clients will be refused access after this time. 802.Pp 803See the X11 SECURITY extension specification for full details on 804the restrictions imposed on untrusted clients. 805.It Cm GatewayPorts 806Specifies whether remote hosts are allowed to connect to local 807forwarded ports. 808By default, 809.Xr ssh 1 810binds local port forwardings to the loopback address. 811This prevents other remote hosts from connecting to forwarded ports. 812.Cm GatewayPorts 813can be used to specify that ssh 814should bind local port forwardings to the wildcard address, 815thus allowing remote hosts to connect to forwarded ports. 816The argument must be 817.Cm yes 818or 819.Cm no 820(the default). 821.It Cm GlobalKnownHostsFile 822Specifies one or more files to use for the global 823host key database, separated by whitespace. 824The default is 825.Pa /etc/ssh/ssh_known_hosts , 826.Pa /etc/ssh/ssh_known_hosts2 . 827.It Cm GSSAPIAuthentication 828Specifies whether user authentication based on GSSAPI is allowed. 829The default is 830.Cm no . 831.It Cm GSSAPIDelegateCredentials 832Forward (delegate) credentials to the server. 833The default is 834.Cm no . 835.It Cm HashKnownHosts 836Indicates that 837.Xr ssh 1 838should hash host names and addresses when they are added to 839.Pa ~/.ssh/known_hosts . 840These hashed names may be used normally by 841.Xr ssh 1 842and 843.Xr sshd 8 , 844but they do not visually reveal identifying information if the 845file's contents are disclosed. 846The default is 847.Cm no . 848Note that existing names and addresses in known hosts files 849will not be converted automatically, 850but may be manually hashed using 851.Xr ssh-keygen 1 . 852.It Cm HostbasedAcceptedAlgorithms 853Specifies the signature algorithms that will be used for hostbased 854authentication as a comma-separated list of patterns. 855Alternately if the specified list begins with a 856.Sq + 857character, then the specified signature algorithms will be appended 858to the default set instead of replacing them. 859If the specified list begins with a 860.Sq - 861character, then the specified signature algorithms (including wildcards) 862will be removed from the default set instead of replacing them. 863If the specified list begins with a 864.Sq ^ 865character, then the specified signature algorithms will be placed 866at the head of the default set. 867The default for this option is: 868.Bd -literal -offset 3n 869ssh-ed25519-cert-v01@openssh.com, 870ecdsa-sha2-nistp256-cert-v01@openssh.com, 871ecdsa-sha2-nistp384-cert-v01@openssh.com, 872ecdsa-sha2-nistp521-cert-v01@openssh.com, 873sk-ssh-ed25519-cert-v01@openssh.com, 874sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 875rsa-sha2-512-cert-v01@openssh.com, 876rsa-sha2-256-cert-v01@openssh.com, 877ssh-rsa-cert-v01@openssh.com, 878ssh-ed25519, 879ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 880sk-ssh-ed25519@openssh.com, 881sk-ecdsa-sha2-nistp256@openssh.com, 882rsa-sha2-512,rsa-sha2-256,ssh-rsa 883.Ed 884.Pp 885The 886.Fl Q 887option of 888.Xr ssh 1 889may be used to list supported signature algorithms. 890This was formerly named HostbasedKeyTypes. 891.It Cm HostbasedAuthentication 892Specifies whether to try rhosts based authentication with public key 893authentication. 894The argument must be 895.Cm yes 896or 897.Cm no 898(the default). 899.It Cm HostKeyAlgorithms 900Specifies the host key signature algorithms 901that the client wants to use in order of preference. 902Alternately if the specified list begins with a 903.Sq + 904character, then the specified signature algorithms will be appended to 905the default set instead of replacing them. 906If the specified list begins with a 907.Sq - 908character, then the specified signature algorithms (including wildcards) 909will be removed from the default set instead of replacing them. 910If the specified list begins with a 911.Sq ^ 912character, then the specified signature algorithms will be placed 913at the head of the default set. 914The default for this option is: 915.Bd -literal -offset 3n 916ssh-ed25519-cert-v01@openssh.com, 917ecdsa-sha2-nistp256-cert-v01@openssh.com, 918ecdsa-sha2-nistp384-cert-v01@openssh.com, 919ecdsa-sha2-nistp521-cert-v01@openssh.com, 920sk-ssh-ed25519-cert-v01@openssh.com, 921sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 922rsa-sha2-512-cert-v01@openssh.com, 923rsa-sha2-256-cert-v01@openssh.com, 924ssh-rsa-cert-v01@openssh.com, 925ssh-ed25519, 926ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 927sk-ecdsa-sha2-nistp256@openssh.com, 928sk-ssh-ed25519@openssh.com, 929rsa-sha2-512,rsa-sha2-256,ssh-rsa 930.Ed 931.Pp 932If hostkeys are known for the destination host then this default is modified 933to prefer their algorithms. 934.Pp 935The list of available signature algorithms may also be obtained using 936.Qq ssh -Q HostKeyAlgorithms . 937.It Cm HostKeyAlias 938Specifies an alias that should be used instead of the 939real host name when looking up or saving the host key 940in the host key database files and when validating host certificates. 941This option is useful for tunneling SSH connections 942or for multiple servers running on a single host. 943.It Cm Hostname 944Specifies the real host name to log into. 945This can be used to specify nicknames or abbreviations for hosts. 946Arguments to 947.Cm Hostname 948accept the tokens described in the 949.Sx TOKENS 950section. 951Numeric IP addresses are also permitted (both on the command line and in 952.Cm Hostname 953specifications). 954The default is the name given on the command line. 955.It Cm IdentitiesOnly 956Specifies that 957.Xr ssh 1 958should only use the configured authentication identity and certificate files 959(either the default files, or those explicitly configured in the 960.Nm 961files 962or passed on the 963.Xr ssh 1 964command-line), 965even if 966.Xr ssh-agent 1 967or a 968.Cm PKCS11Provider 969or 970.Cm SecurityKeyProvider 971offers more identities. 972The argument to this keyword must be 973.Cm yes 974or 975.Cm no 976(the default). 977This option is intended for situations where ssh-agent 978offers many different identities. 979.It Cm IdentityAgent 980Specifies the 981.Ux Ns -domain 982socket used to communicate with the authentication agent. 983.Pp 984This option overrides the 985.Ev SSH_AUTH_SOCK 986environment variable and can be used to select a specific agent. 987Setting the socket name to 988.Cm none 989disables the use of an authentication agent. 990If the string 991.Qq SSH_AUTH_SOCK 992is specified, the location of the socket will be read from the 993.Ev SSH_AUTH_SOCK 994environment variable. 995Otherwise if the specified value begins with a 996.Sq $ 997character, then it will be treated as an environment variable containing 998the location of the socket. 999.Pp 1000Arguments to 1001.Cm IdentityAgent 1002may use the tilde syntax to refer to a user's home directory, 1003the tokens described in the 1004.Sx TOKENS 1005section and environment variables as described in the 1006.Sx ENVIRONMENT VARIABLES 1007section. 1008.It Cm IdentityFile 1009Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA, 1010Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read. 1011The default is 1012.Pa ~/.ssh/id_dsa , 1013.Pa ~/.ssh/id_ecdsa , 1014.Pa ~/.ssh/id_ecdsa_sk , 1015.Pa ~/.ssh/id_ed25519 , 1016.Pa ~/.ssh/id_ed25519_sk 1017and 1018.Pa ~/.ssh/id_rsa . 1019Additionally, any identities represented by the authentication agent 1020will be used for authentication unless 1021.Cm IdentitiesOnly 1022is set. 1023If no certificates have been explicitly specified by 1024.Cm CertificateFile , 1025.Xr ssh 1 1026will try to load certificate information from the filename obtained by 1027appending 1028.Pa -cert.pub 1029to the path of a specified 1030.Cm IdentityFile . 1031.Pp 1032Arguments to 1033.Cm IdentityFile 1034may use the tilde syntax to refer to a user's home directory 1035or the tokens described in the 1036.Sx TOKENS 1037section. 1038.Pp 1039It is possible to have 1040multiple identity files specified in configuration files; all these 1041identities will be tried in sequence. 1042Multiple 1043.Cm IdentityFile 1044directives will add to the list of identities tried (this behaviour 1045differs from that of other configuration directives). 1046.Pp 1047.Cm IdentityFile 1048may be used in conjunction with 1049.Cm IdentitiesOnly 1050to select which identities in an agent are offered during authentication. 1051.Cm IdentityFile 1052may also be used in conjunction with 1053.Cm CertificateFile 1054in order to provide any certificate also needed for authentication with 1055the identity. 1056.It Cm IgnoreUnknown 1057Specifies a pattern-list of unknown options to be ignored if they are 1058encountered in configuration parsing. 1059This may be used to suppress errors if 1060.Nm 1061contains options that are unrecognised by 1062.Xr ssh 1 . 1063It is recommended that 1064.Cm IgnoreUnknown 1065be listed early in the configuration file as it will not be applied 1066to unknown options that appear before it. 1067.It Cm Include 1068Include the specified configuration file(s). 1069Multiple pathnames may be specified and each pathname may contain 1070.Xr glob 7 1071wildcards and, for user configurations, shell-like 1072.Sq ~ 1073references to user home directories. 1074Wildcards will be expanded and processed in lexical order. 1075Files without absolute paths are assumed to be in 1076.Pa ~/.ssh 1077if included in a user configuration file or 1078.Pa /etc/ssh 1079if included from the system configuration file. 1080.Cm Include 1081directive may appear inside a 1082.Cm Match 1083or 1084.Cm Host 1085block 1086to perform conditional inclusion. 1087.It Cm IPQoS 1088Specifies the IPv4 type-of-service or DSCP class for connections. 1089Accepted values are 1090.Cm af11 , 1091.Cm af12 , 1092.Cm af13 , 1093.Cm af21 , 1094.Cm af22 , 1095.Cm af23 , 1096.Cm af31 , 1097.Cm af32 , 1098.Cm af33 , 1099.Cm af41 , 1100.Cm af42 , 1101.Cm af43 , 1102.Cm cs0 , 1103.Cm cs1 , 1104.Cm cs2 , 1105.Cm cs3 , 1106.Cm cs4 , 1107.Cm cs5 , 1108.Cm cs6 , 1109.Cm cs7 , 1110.Cm ef , 1111.Cm le , 1112.Cm lowdelay , 1113.Cm throughput , 1114.Cm reliability , 1115a numeric value, or 1116.Cm none 1117to use the operating system default. 1118This option may take one or two arguments, separated by whitespace. 1119If one argument is specified, it is used as the packet class unconditionally. 1120If two values are specified, the first is automatically selected for 1121interactive sessions and the second for non-interactive sessions. 1122The default is 1123.Cm af21 1124(Low-Latency Data) 1125for interactive sessions and 1126.Cm cs1 1127(Lower Effort) 1128for non-interactive sessions. 1129.It Cm KbdInteractiveAuthentication 1130Specifies whether to use keyboard-interactive authentication. 1131The argument to this keyword must be 1132.Cm yes 1133(the default) 1134or 1135.Cm no . 1136.Cm ChallengeResponseAuthentication 1137is a deprecated alias for this. 1138.It Cm KbdInteractiveDevices 1139Specifies the list of methods to use in keyboard-interactive authentication. 1140Multiple method names must be comma-separated. 1141The default is to use the server specified list. 1142The methods available vary depending on what the server supports. 1143For an OpenSSH server, 1144it may be zero or more of: 1145.Cm bsdauth , 1146.Cm pam , 1147and 1148.Cm skey . 1149.It Cm KexAlgorithms 1150Specifies the available KEX (Key Exchange) algorithms. 1151Multiple algorithms must be comma-separated. 1152If the specified list begins with a 1153.Sq + 1154character, then the specified algorithms will be appended to the default set 1155instead of replacing them. 1156If the specified list begins with a 1157.Sq - 1158character, then the specified algorithms (including wildcards) will be removed 1159from the default set instead of replacing them. 1160If the specified list begins with a 1161.Sq ^ 1162character, then the specified algorithms will be placed at the head of the 1163default set. 1164The default is: 1165.Bd -literal -offset indent 1166curve25519-sha256,curve25519-sha256@libssh.org, 1167ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1168diffie-hellman-group-exchange-sha256, 1169diffie-hellman-group16-sha512, 1170diffie-hellman-group18-sha512, 1171diffie-hellman-group14-sha256 1172.Ed 1173.Pp 1174The list of available key exchange algorithms may also be obtained using 1175.Qq ssh -Q kex . 1176.It Cm KnownHostsCommand 1177Specifies a command to use to obtain a list of host keys, in addition to 1178those listed in 1179.Cm UserKnownHostsFile 1180and 1181.Cm GlobalKnownHostsFile . 1182This command is executed after the files have been read. 1183It may write host key lines to standard output in identical format to the 1184usual files (described in the 1185.Sx VERIFYING HOST KEYS 1186section in 1187.Xr ssh 1 ) . 1188Arguments to 1189.Cm KnownHostsCommand 1190accept the tokens described in the 1191.Sx TOKENS 1192section. 1193The command may be invoked multiple times per connection: once when preparing 1194the preference list of host key algorithms to use, again to obtain the 1195host key for the requested host name and, if 1196.Cm CheckHostIP 1197is enabled, one more time to obtain the host key matching the server's 1198address. 1199If the command exits abnormally or returns a non-zero exit status then the 1200connection is terminated. 1201.It Cm LocalCommand 1202Specifies a command to execute on the local machine after successfully 1203connecting to the server. 1204The command string extends to the end of the line, and is executed with 1205the user's shell. 1206Arguments to 1207.Cm LocalCommand 1208accept the tokens described in the 1209.Sx TOKENS 1210section. 1211.Pp 1212The command is run synchronously and does not have access to the 1213session of the 1214.Xr ssh 1 1215that spawned it. 1216It should not be used for interactive commands. 1217.Pp 1218This directive is ignored unless 1219.Cm PermitLocalCommand 1220has been enabled. 1221.It Cm LocalForward 1222Specifies that a TCP port on the local machine be forwarded over 1223the secure channel to the specified host and port from the remote machine. 1224The first argument specifies the listener and may be 1225.Sm off 1226.Oo Ar bind_address : Oc Ar port 1227.Sm on 1228or a Unix domain socket path. 1229The second argument is the destination and may be 1230.Ar host : Ns Ar hostport 1231or a Unix domain socket path if the remote host supports it. 1232.Pp 1233IPv6 addresses can be specified by enclosing addresses in square brackets. 1234Multiple forwardings may be specified, and additional forwardings can be 1235given on the command line. 1236Only the superuser can forward privileged ports. 1237By default, the local port is bound in accordance with the 1238.Cm GatewayPorts 1239setting. 1240However, an explicit 1241.Ar bind_address 1242may be used to bind the connection to a specific address. 1243The 1244.Ar bind_address 1245of 1246.Cm localhost 1247indicates that the listening port be bound for local use only, while an 1248empty address or 1249.Sq * 1250indicates that the port should be available from all interfaces. 1251Unix domain socket paths may use the tokens described in the 1252.Sx TOKENS 1253section and environment variables as described in the 1254.Sx ENVIRONMENT VARIABLES 1255section. 1256.It Cm LogLevel 1257Gives the verbosity level that is used when logging messages from 1258.Xr ssh 1 . 1259The possible values are: 1260QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1261The default is INFO. 1262DEBUG and DEBUG1 are equivalent. 1263DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1264.It Cm LogVerbose 1265Specify one or more overrides to LogLevel. 1266An override consists of a pattern lists that matches the source file, function 1267and line number to force detailed logging for. 1268For example, an override pattern of: 1269.Bd -literal -offset indent 1270kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1271.Ed 1272.Pp 1273would enable detailed logging for line 1000 of 1274.Pa kex.c , 1275everything in the 1276.Fn kex_exchange_identification 1277function, and all code in the 1278.Pa packet.c 1279file. 1280This option is intended for debugging and no overrides are enabled by default. 1281.It Cm MACs 1282Specifies the MAC (message authentication code) algorithms 1283in order of preference. 1284The MAC algorithm is used for data integrity protection. 1285Multiple algorithms must be comma-separated. 1286If the specified list begins with a 1287.Sq + 1288character, then the specified algorithms will be appended to the default set 1289instead of replacing them. 1290If the specified list begins with a 1291.Sq - 1292character, then the specified algorithms (including wildcards) will be removed 1293from the default set instead of replacing them. 1294If the specified list begins with a 1295.Sq ^ 1296character, then the specified algorithms will be placed at the head of the 1297default set. 1298.Pp 1299The algorithms that contain 1300.Qq -etm 1301calculate the MAC after encryption (encrypt-then-mac). 1302These are considered safer and their use recommended. 1303.Pp 1304The default is: 1305.Bd -literal -offset indent 1306umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1307hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1308hmac-sha1-etm@openssh.com, 1309umac-64@openssh.com,umac-128@openssh.com, 1310hmac-sha2-256,hmac-sha2-512,hmac-sha1 1311.Ed 1312.Pp 1313The list of available MAC algorithms may also be obtained using 1314.Qq ssh -Q mac . 1315.It Cm NoHostAuthenticationForLocalhost 1316Disable host authentication for localhost (loopback addresses). 1317The argument to this keyword must be 1318.Cm yes 1319or 1320.Cm no 1321(the default). 1322.It Cm NumberOfPasswordPrompts 1323Specifies the number of password prompts before giving up. 1324The argument to this keyword must be an integer. 1325The default is 3. 1326.It Cm PasswordAuthentication 1327Specifies whether to use password authentication. 1328The argument to this keyword must be 1329.Cm yes 1330(the default) 1331or 1332.Cm no . 1333.It Cm PermitLocalCommand 1334Allow local command execution via the 1335.Ic LocalCommand 1336option or using the 1337.Ic !\& Ns Ar command 1338escape sequence in 1339.Xr ssh 1 . 1340The argument must be 1341.Cm yes 1342or 1343.Cm no 1344(the default). 1345.It Cm PermitRemoteOpen 1346Specifies the destinations to which remote TCP port forwarding is permitted when 1347.Cm RemoteForward 1348is used as a SOCKS proxy. 1349The forwarding specification must be one of the following forms: 1350.Pp 1351.Bl -item -offset indent -compact 1352.It 1353.Cm PermitRemoteOpen 1354.Sm off 1355.Ar host : port 1356.Sm on 1357.It 1358.Cm PermitRemoteOpen 1359.Sm off 1360.Ar IPv4_addr : port 1361.Sm on 1362.It 1363.Cm PermitRemoteOpen 1364.Sm off 1365.Ar \&[ IPv6_addr \&] : port 1366.Sm on 1367.El 1368.Pp 1369Multiple forwards may be specified by separating them with whitespace. 1370An argument of 1371.Cm any 1372can be used to remove all restrictions and permit any forwarding requests. 1373An argument of 1374.Cm none 1375can be used to prohibit all forwarding requests. 1376The wildcard 1377.Sq * 1378can be used for host or port to allow all hosts or ports respectively. 1379Otherwise, no pattern matching or address lookups are performed on supplied 1380names. 1381.It Cm PKCS11Provider 1382Specifies which PKCS#11 provider to use or 1383.Cm none 1384to indicate that no provider should be used (the default). 1385The argument to this keyword is a path to the PKCS#11 shared library 1386.Xr ssh 1 1387should use to communicate with a PKCS#11 token providing keys for user 1388authentication. 1389.It Cm Port 1390Specifies the port number to connect on the remote host. 1391The default is 22. 1392.It Cm PreferredAuthentications 1393Specifies the order in which the client should try authentication methods. 1394This allows a client to prefer one method (e.g.\& 1395.Cm keyboard-interactive ) 1396over another method (e.g.\& 1397.Cm password ) . 1398The default is: 1399.Bd -literal -offset indent 1400gssapi-with-mic,hostbased,publickey, 1401keyboard-interactive,password 1402.Ed 1403.It Cm ProxyCommand 1404Specifies the command to use to connect to the server. 1405The command 1406string extends to the end of the line, and is executed 1407using the user's shell 1408.Ql exec 1409directive to avoid a lingering shell process. 1410.Pp 1411Arguments to 1412.Cm ProxyCommand 1413accept the tokens described in the 1414.Sx TOKENS 1415section. 1416The command can be basically anything, 1417and should read from its standard input and write to its standard output. 1418It should eventually connect an 1419.Xr sshd 8 1420server running on some machine, or execute 1421.Ic sshd -i 1422somewhere. 1423Host key management will be done using the 1424.Cm Hostname 1425of the host being connected (defaulting to the name typed by the user). 1426Setting the command to 1427.Cm none 1428disables this option entirely. 1429Note that 1430.Cm CheckHostIP 1431is not available for connects with a proxy command. 1432.Pp 1433This directive is useful in conjunction with 1434.Xr nc 1 1435and its proxy support. 1436For example, the following directive would connect via an HTTP proxy at 1437192.0.2.0: 1438.Bd -literal -offset 3n 1439ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1440.Ed 1441.It Cm ProxyJump 1442Specifies one or more jump proxies as either 1443.Xo 1444.Sm off 1445.Op Ar user No @ 1446.Ar host 1447.Op : Ns Ar port 1448.Sm on 1449or an ssh URI 1450.Xc . 1451Multiple proxies may be separated by comma characters and will be visited 1452sequentially. 1453Setting this option will cause 1454.Xr ssh 1 1455to connect to the target host by first making a 1456.Xr ssh 1 1457connection to the specified 1458.Cm ProxyJump 1459host and then establishing a 1460TCP forwarding to the ultimate target from there. 1461Setting the host to 1462.Cm none 1463disables this option entirely. 1464.Pp 1465Note that this option will compete with the 1466.Cm ProxyCommand 1467option - whichever is specified first will prevent later instances of the 1468other from taking effect. 1469.Pp 1470Note also that the configuration for the destination host (either supplied 1471via the command-line or the configuration file) is not generally applied 1472to jump hosts. 1473.Pa ~/.ssh/config 1474should be used if specific configuration is required for jump hosts. 1475.It Cm ProxyUseFdpass 1476Specifies that 1477.Cm ProxyCommand 1478will pass a connected file descriptor back to 1479.Xr ssh 1 1480instead of continuing to execute and pass data. 1481The default is 1482.Cm no . 1483.It Cm PubkeyAcceptedAlgorithms 1484Specifies the signature algorithms that will be used for public key 1485authentication as a comma-separated list of patterns. 1486If the specified list begins with a 1487.Sq + 1488character, then the algorithms after it will be appended to the default 1489instead of replacing it. 1490If the specified list begins with a 1491.Sq - 1492character, then the specified algorithms (including wildcards) will be removed 1493from the default set instead of replacing them. 1494If the specified list begins with a 1495.Sq ^ 1496character, then the specified algorithms will be placed at the head of the 1497default set. 1498The default for this option is: 1499.Bd -literal -offset 3n 1500ssh-ed25519-cert-v01@openssh.com, 1501ecdsa-sha2-nistp256-cert-v01@openssh.com, 1502ecdsa-sha2-nistp384-cert-v01@openssh.com, 1503ecdsa-sha2-nistp521-cert-v01@openssh.com, 1504sk-ssh-ed25519-cert-v01@openssh.com, 1505sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1506rsa-sha2-512-cert-v01@openssh.com, 1507rsa-sha2-256-cert-v01@openssh.com, 1508ssh-rsa-cert-v01@openssh.com, 1509ssh-ed25519, 1510ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1511sk-ssh-ed25519@openssh.com, 1512sk-ecdsa-sha2-nistp256@openssh.com, 1513rsa-sha2-512,rsa-sha2-256,ssh-rsa 1514.Ed 1515.Pp 1516The list of available signature algorithms may also be obtained using 1517.Qq ssh -Q PubkeyAcceptedAlgorithms . 1518.It Cm PubkeyAuthentication 1519Specifies whether to try public key authentication. 1520The argument to this keyword must be 1521.Cm yes 1522(the default) 1523or 1524.Cm no . 1525.It Cm RekeyLimit 1526Specifies the maximum amount of data that may be transmitted before the 1527session key is renegotiated, optionally followed by a maximum amount of 1528time that may pass before the session key is renegotiated. 1529The first argument is specified in bytes and may have a suffix of 1530.Sq K , 1531.Sq M , 1532or 1533.Sq G 1534to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1535The default is between 1536.Sq 1G 1537and 1538.Sq 4G , 1539depending on the cipher. 1540The optional second value is specified in seconds and may use any of the 1541units documented in the TIME FORMATS section of 1542.Xr sshd_config 5 . 1543The default value for 1544.Cm RekeyLimit 1545is 1546.Cm default none , 1547which means that rekeying is performed after the cipher's default amount 1548of data has been sent or received and no time based rekeying is done. 1549.It Cm RemoteCommand 1550Specifies a command to execute on the remote machine after successfully 1551connecting to the server. 1552The command string extends to the end of the line, and is executed with 1553the user's shell. 1554Arguments to 1555.Cm RemoteCommand 1556accept the tokens described in the 1557.Sx TOKENS 1558section. 1559.It Cm RemoteForward 1560Specifies that a TCP port on the remote machine be forwarded over 1561the secure channel. 1562The remote port may either be forwarded to a specified host and port 1563from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote 1564client to connect to arbitrary destinations from the local machine. 1565The first argument is the listening specification and may be 1566.Sm off 1567.Oo Ar bind_address : Oc Ar port 1568.Sm on 1569or, if the remote host supports it, a Unix domain socket path. 1570If forwarding to a specific destination then the second argument must be 1571.Ar host : Ns Ar hostport 1572or a Unix domain socket path, 1573otherwise if no destination argument is specified then the remote forwarding 1574will be established as a SOCKS proxy. 1575When acting as a SOCKS proxy the destination of the connection can be 1576restricted by 1577.Cm PermitRemoteOpen . 1578.Pp 1579IPv6 addresses can be specified by enclosing addresses in square brackets. 1580Multiple forwardings may be specified, and additional 1581forwardings can be given on the command line. 1582Privileged ports can be forwarded only when 1583logging in as root on the remote machine. 1584Unix domain socket paths may use the tokens described in the 1585.Sx TOKENS 1586section and environment variables as described in the 1587.Sx ENVIRONMENT VARIABLES 1588section. 1589.Pp 1590If the 1591.Ar port 1592argument is 0, 1593the listen port will be dynamically allocated on the server and reported 1594to the client at run time. 1595.Pp 1596If the 1597.Ar bind_address 1598is not specified, the default is to only bind to loopback addresses. 1599If the 1600.Ar bind_address 1601is 1602.Ql * 1603or an empty string, then the forwarding is requested to listen on all 1604interfaces. 1605Specifying a remote 1606.Ar bind_address 1607will only succeed if the server's 1608.Cm GatewayPorts 1609option is enabled (see 1610.Xr sshd_config 5 ) . 1611.It Cm RequestTTY 1612Specifies whether to request a pseudo-tty for the session. 1613The argument may be one of: 1614.Cm no 1615(never request a TTY), 1616.Cm yes 1617(always request a TTY when standard input is a TTY), 1618.Cm force 1619(always request a TTY) or 1620.Cm auto 1621(request a TTY when opening a login session). 1622This option mirrors the 1623.Fl t 1624and 1625.Fl T 1626flags for 1627.Xr ssh 1 . 1628.It Cm RevokedHostKeys 1629Specifies revoked host public keys. 1630Keys listed in this file will be refused for host authentication. 1631Note that if this file does not exist or is not readable, 1632then host authentication will be refused for all hosts. 1633Keys may be specified as a text file, listing one public key per line, or as 1634an OpenSSH Key Revocation List (KRL) as generated by 1635.Xr ssh-keygen 1 . 1636For more information on KRLs, see the KEY REVOCATION LISTS section in 1637.Xr ssh-keygen 1 . 1638.It Cm SecurityKeyProvider 1639Specifies a path to a library that will be used when loading any 1640FIDO authenticator-hosted keys, overriding the default of using 1641the built-in USB HID support. 1642.Pp 1643If the specified value begins with a 1644.Sq $ 1645character, then it will be treated as an environment variable containing 1646the path to the library. 1647.It Cm SendEnv 1648Specifies what variables from the local 1649.Xr environ 7 1650should be sent to the server. 1651The server must also support it, and the server must be configured to 1652accept these environment variables. 1653Note that the 1654.Ev TERM 1655environment variable is always sent whenever a 1656pseudo-terminal is requested as it is required by the protocol. 1657Refer to 1658.Cm AcceptEnv 1659in 1660.Xr sshd_config 5 1661for how to configure the server. 1662Variables are specified by name, which may contain wildcard characters. 1663Multiple environment variables may be separated by whitespace or spread 1664across multiple 1665.Cm SendEnv 1666directives. 1667.Pp 1668See 1669.Sx PATTERNS 1670for more information on patterns. 1671.Pp 1672It is possible to clear previously set 1673.Cm SendEnv 1674variable names by prefixing patterns with 1675.Pa - . 1676The default is not to send any environment variables. 1677.It Cm ServerAliveCountMax 1678Sets the number of server alive messages (see below) which may be 1679sent without 1680.Xr ssh 1 1681receiving any messages back from the server. 1682If this threshold is reached while server alive messages are being sent, 1683ssh will disconnect from the server, terminating the session. 1684It is important to note that the use of server alive messages is very 1685different from 1686.Cm TCPKeepAlive 1687(below). 1688The server alive messages are sent through the encrypted channel 1689and therefore will not be spoofable. 1690The TCP keepalive option enabled by 1691.Cm TCPKeepAlive 1692is spoofable. 1693The server alive mechanism is valuable when the client or 1694server depend on knowing when a connection has become unresponsive. 1695.Pp 1696The default value is 3. 1697If, for example, 1698.Cm ServerAliveInterval 1699(see below) is set to 15 and 1700.Cm ServerAliveCountMax 1701is left at the default, if the server becomes unresponsive, 1702ssh will disconnect after approximately 45 seconds. 1703.It Cm ServerAliveInterval 1704Sets a timeout interval in seconds after which if no data has been received 1705from the server, 1706.Xr ssh 1 1707will send a message through the encrypted 1708channel to request a response from the server. 1709The default 1710is 0, indicating that these messages will not be sent to the server. 1711.It Cm SessionType 1712May be used to either request invocation of a subsystem on the remote system, 1713or to prevent the execution of a remote command at all. 1714The latter is useful for just forwarding ports. 1715The argument to this keyword must be 1716.Cm none 1717(same as the 1718.Fl N 1719option), 1720.Cm subsystem 1721(same as the 1722.Fl s 1723option) or 1724.Cm default 1725(shell or command execution). 1726.It Cm SetEnv 1727Directly specify one or more environment variables and their contents to 1728be sent to the server. 1729Similarly to 1730.Cm SendEnv , 1731with the exception of the 1732.Ev TERM 1733variable, the server must be prepared to accept the environment variable. 1734.It Cm StdinNull 1735Redirects stdin from 1736.Pa /dev/null 1737(actually, prevents reading from stdin). 1738Either this or the equivalent 1739.Fl n 1740option must be used when 1741.Nm ssh 1742is run in the background. 1743The argument to this keyword must be 1744.Cm yes 1745(same as the 1746.Fl n 1747option) or 1748.Cm no 1749(the default). 1750.It Cm StreamLocalBindMask 1751Sets the octal file creation mode mask 1752.Pq umask 1753used when creating a Unix-domain socket file for local or remote 1754port forwarding. 1755This option is only used for port forwarding to a Unix-domain socket file. 1756.Pp 1757The default value is 0177, which creates a Unix-domain socket file that is 1758readable and writable only by the owner. 1759Note that not all operating systems honor the file mode on Unix-domain 1760socket files. 1761.It Cm StreamLocalBindUnlink 1762Specifies whether to remove an existing Unix-domain socket file for local 1763or remote port forwarding before creating a new one. 1764If the socket file already exists and 1765.Cm StreamLocalBindUnlink 1766is not enabled, 1767.Nm ssh 1768will be unable to forward the port to the Unix-domain socket file. 1769This option is only used for port forwarding to a Unix-domain socket file. 1770.Pp 1771The argument must be 1772.Cm yes 1773or 1774.Cm no 1775(the default). 1776.It Cm StrictHostKeyChecking 1777If this flag is set to 1778.Cm yes , 1779.Xr ssh 1 1780will never automatically add host keys to the 1781.Pa ~/.ssh/known_hosts 1782file, and refuses to connect to hosts whose host key has changed. 1783This provides maximum protection against man-in-the-middle (MITM) attacks, 1784though it can be annoying when the 1785.Pa /etc/ssh/ssh_known_hosts 1786file is poorly maintained or when connections to new hosts are 1787frequently made. 1788This option forces the user to manually 1789add all new hosts. 1790.Pp 1791If this flag is set to 1792.Cm accept-new 1793then ssh will automatically add new host keys to the user's 1794.Pa known_hosts 1795file, but will not permit connections to hosts with 1796changed host keys. 1797If this flag is set to 1798.Cm no 1799or 1800.Cm off , 1801ssh will automatically add new host keys to the user known hosts files 1802and allow connections to hosts with changed hostkeys to proceed, 1803subject to some restrictions. 1804If this flag is set to 1805.Cm ask 1806(the default), 1807new host keys 1808will be added to the user known host files only after the user 1809has confirmed that is what they really want to do, and 1810ssh will refuse to connect to hosts whose host key has changed. 1811The host keys of 1812known hosts will be verified automatically in all cases. 1813.It Cm SyslogFacility 1814Gives the facility code that is used when logging messages from 1815.Xr ssh 1 . 1816The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1817LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1818The default is USER. 1819.It Cm TCPKeepAlive 1820Specifies whether the system should send TCP keepalive messages to the 1821other side. 1822If they are sent, death of the connection or crash of one 1823of the machines will be properly noticed. 1824However, this means that 1825connections will die if the route is down temporarily, and some people 1826find it annoying. 1827.Pp 1828The default is 1829.Cm yes 1830(to send TCP keepalive messages), and the client will notice 1831if the network goes down or the remote host dies. 1832This is important in scripts, and many users want it too. 1833.Pp 1834To disable TCP keepalive messages, the value should be set to 1835.Cm no . 1836See also 1837.Cm ServerAliveInterval 1838for protocol-level keepalives. 1839.It Cm Tunnel 1840Request 1841.Xr tun 4 1842device forwarding between the client and the server. 1843The argument must be 1844.Cm yes , 1845.Cm point-to-point 1846(layer 3), 1847.Cm ethernet 1848(layer 2), 1849or 1850.Cm no 1851(the default). 1852Specifying 1853.Cm yes 1854requests the default tunnel mode, which is 1855.Cm point-to-point . 1856.It Cm TunnelDevice 1857Specifies the 1858.Xr tun 4 1859devices to open on the client 1860.Pq Ar local_tun 1861and the server 1862.Pq Ar remote_tun . 1863.Pp 1864The argument must be 1865.Sm off 1866.Ar local_tun Op : Ar remote_tun . 1867.Sm on 1868The devices may be specified by numerical ID or the keyword 1869.Cm any , 1870which uses the next available tunnel device. 1871If 1872.Ar remote_tun 1873is not specified, it defaults to 1874.Cm any . 1875The default is 1876.Cm any:any . 1877.It Cm UpdateHostKeys 1878Specifies whether 1879.Xr ssh 1 1880should accept notifications of additional hostkeys from the server sent 1881after authentication has completed and add them to 1882.Cm UserKnownHostsFile . 1883The argument must be 1884.Cm yes , 1885.Cm no 1886or 1887.Cm ask . 1888This option allows learning alternate hostkeys for a server 1889and supports graceful key rotation by allowing a server to send replacement 1890public keys before old ones are removed. 1891.Pp 1892Additional hostkeys are only accepted if the key used to authenticate the 1893host was already trusted or explicitly accepted by the user, the host was 1894authenticated via 1895.Cm UserKnownHostsFile 1896(i.e. not 1897.Cm GlobalKnownHostsFile ) 1898and the host was authenticated using a plain key and not a certificate. 1899.Pp 1900.Cm UpdateHostKeys 1901is enabled by default if the user has not overridden the default 1902.Cm UserKnownHostsFile 1903setting and has not enabled 1904.Cm VerifyHostKeyDNS , 1905otherwise 1906.Cm UpdateHostKeys 1907will be set to 1908.Cm no . 1909.Pp 1910If 1911.Cm UpdateHostKeys 1912is set to 1913.Cm ask , 1914then the user is asked to confirm the modifications to the known_hosts file. 1915Confirmation is currently incompatible with 1916.Cm ControlPersist , 1917and will be disabled if it is enabled. 1918.Pp 1919Presently, only 1920.Xr sshd 8 1921from OpenSSH 6.8 and greater support the 1922.Qq hostkeys@openssh.com 1923protocol extension used to inform the client of all the server's hostkeys. 1924.It Cm User 1925Specifies the user to log in as. 1926This can be useful when a different user name is used on different machines. 1927This saves the trouble of 1928having to remember to give the user name on the command line. 1929.It Cm UserKnownHostsFile 1930Specifies one or more files to use for the user 1931host key database, separated by whitespace. 1932Each filename may use tilde notation to refer to the user's home directory, 1933the tokens described in the 1934.Sx TOKENS 1935section and environment variables as described in the 1936.Sx ENVIRONMENT VARIABLES 1937section. 1938The default is 1939.Pa ~/.ssh/known_hosts , 1940.Pa ~/.ssh/known_hosts2 . 1941.It Cm VerifyHostKeyDNS 1942Specifies whether to verify the remote key using DNS and SSHFP resource 1943records. 1944If this option is set to 1945.Cm yes , 1946the client will implicitly trust keys that match a secure fingerprint 1947from DNS. 1948Insecure fingerprints will be handled as if this option was set to 1949.Cm ask . 1950If this option is set to 1951.Cm ask , 1952information on fingerprint match will be displayed, but the user will still 1953need to confirm new host keys according to the 1954.Cm StrictHostKeyChecking 1955option. 1956The default is 1957.Cm no . 1958.Pp 1959See also 1960.Sx VERIFYING HOST KEYS 1961in 1962.Xr ssh 1 . 1963.It Cm VisualHostKey 1964If this flag is set to 1965.Cm yes , 1966an ASCII art representation of the remote host key fingerprint is 1967printed in addition to the fingerprint string at login and 1968for unknown host keys. 1969If this flag is set to 1970.Cm no 1971(the default), 1972no fingerprint strings are printed at login and 1973only the fingerprint string will be printed for unknown host keys. 1974.It Cm XAuthLocation 1975Specifies the full pathname of the 1976.Xr xauth 1 1977program. 1978The default is 1979.Pa /usr/X11R6/bin/xauth . 1980.El 1981.Sh PATTERNS 1982A 1983.Em pattern 1984consists of zero or more non-whitespace characters, 1985.Sq * 1986(a wildcard that matches zero or more characters), 1987or 1988.Sq ?\& 1989(a wildcard that matches exactly one character). 1990For example, to specify a set of declarations for any host in the 1991.Qq .co.uk 1992set of domains, 1993the following pattern could be used: 1994.Pp 1995.Dl Host *.co.uk 1996.Pp 1997The following pattern 1998would match any host in the 192.168.0.[0-9] network range: 1999.Pp 2000.Dl Host 192.168.0.? 2001.Pp 2002A 2003.Em pattern-list 2004is a comma-separated list of patterns. 2005Patterns within pattern-lists may be negated 2006by preceding them with an exclamation mark 2007.Pq Sq !\& . 2008For example, 2009to allow a key to be used from anywhere within an organization 2010except from the 2011.Qq dialup 2012pool, 2013the following entry (in authorized_keys) could be used: 2014.Pp 2015.Dl from=\&"!*.dialup.example.com,*.example.com\&" 2016.Pp 2017Note that a negated match will never produce a positive result by itself. 2018For example, attempting to match 2019.Qq host3 2020against the following pattern-list will fail: 2021.Pp 2022.Dl from=\&"!host1,!host2\&" 2023.Pp 2024The solution here is to include a term that will yield a positive match, 2025such as a wildcard: 2026.Pp 2027.Dl from=\&"!host1,!host2,*\&" 2028.Sh TOKENS 2029Arguments to some keywords can make use of tokens, 2030which are expanded at runtime: 2031.Pp 2032.Bl -tag -width XXXX -offset indent -compact 2033.It %% 2034A literal 2035.Sq % . 2036.It \&%C 2037Hash of %l%h%p%r. 2038.It %d 2039Local user's home directory. 2040.It %f 2041The fingerprint of the server's host key. 2042.It %H 2043The 2044.Pa known_hosts 2045hostname or address that is being searched for. 2046.It %h 2047The remote hostname. 2048.It \%%I 2049A string describing the reason for a 2050.Cm KnownHostsCommand 2051execution: either 2052.Cm ADDRESS 2053when looking up a host by address (only when 2054.Cm CheckHostIP 2055is enabled), 2056.Cm HOSTNAME 2057when searching by hostname, or 2058.Cm ORDER 2059when preparing the host key algorithm preference list to use for the 2060destination host. 2061.It %i 2062The local user ID. 2063.It %K 2064The base64 encoded host key. 2065.It %k 2066The host key alias if specified, otherwise the original remote hostname given 2067on the command line. 2068.It %L 2069The local hostname. 2070.It %l 2071The local hostname, including the domain name. 2072.It %n 2073The original remote hostname, as given on the command line. 2074.It %p 2075The remote port. 2076.It %r 2077The remote username. 2078.It \&%T 2079The local 2080.Xr tun 4 2081or 2082.Xr tap 4 2083network interface assigned if 2084tunnel forwarding was requested, or 2085.Qq NONE 2086otherwise. 2087.It %t 2088The type of the server host key, e.g. 2089.Cm ssh-ed25519 . 2090.It %u 2091The local username. 2092.El 2093.Pp 2094.Cm CertificateFile , 2095.Cm ControlPath , 2096.Cm IdentityAgent , 2097.Cm IdentityFile , 2098.Cm KnownHostsCommand , 2099.Cm LocalForward , 2100.Cm Match exec , 2101.Cm RemoteCommand , 2102.Cm RemoteForward , 2103and 2104.Cm UserKnownHostsFile 2105accept the tokens %%, %C, %d, %h, %i, %k, %L, %l, %n, %p, %r, and %u. 2106.Pp 2107.Cm KnownHostsCommand 2108additionally accepts the tokens %f, %H, %I, %K and %t. 2109.Pp 2110.Cm Hostname 2111accepts the tokens %% and %h. 2112.Pp 2113.Cm LocalCommand 2114accepts all tokens. 2115.Pp 2116.Cm ProxyCommand 2117accepts the tokens %%, %h, %n, %p, and %r. 2118.Sh ENVIRONMENT VARIABLES 2119Arguments to some keywords can be expanded at runtime from environment 2120variables on the client by enclosing them in 2121.Ic ${} , 2122for example 2123.Ic ${HOME}/.ssh 2124would refer to the user's .ssh directory. 2125If a specified environment variable does not exist then an error will be 2126returned and the setting for that keyword will be ignored. 2127.Pp 2128The keywords 2129.Cm CertificateFile , 2130.Cm ControlPath , 2131.Cm IdentityAgent , 2132.Cm IdentityFile , 2133.Cm KnownHostsCommand , 2134and 2135.Cm UserKnownHostsFile 2136support environment variables. 2137The keywords 2138.Cm LocalForward 2139and 2140.Cm RemoteForward 2141support environment variables only for Unix domain socket paths. 2142.Sh FILES 2143.Bl -tag -width Ds 2144.It Pa ~/.ssh/config 2145This is the per-user configuration file. 2146The format of this file is described above. 2147This file is used by the SSH client. 2148Because of the potential for abuse, this file must have strict permissions: 2149read/write for the user, and not writable by others. 2150.It Pa /etc/ssh/ssh_config 2151Systemwide configuration file. 2152This file provides defaults for those 2153values that are not specified in the user's configuration file, and 2154for those users who do not have a configuration file. 2155This file must be world-readable. 2156.El 2157.Sh SEE ALSO 2158.Xr ssh 1 2159.Sh AUTHORS 2160.An -nosplit 2161OpenSSH is a derivative of the original and free 2162ssh 1.2.12 release by 2163.An Tatu Ylonen . 2164.An Aaron Campbell , Bob Beck , Markus Friedl , 2165.An Niels Provos , Theo de Raadt 2166and 2167.An Dug Song 2168removed many bugs, re-added newer features and 2169created OpenSSH. 2170.An Markus Friedl 2171contributed the support for SSH protocol versions 1.5 and 2.0. 2172