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 1.383 2023/07/17 05:36:14 jsg Exp $ 37.Dd $Mdocdate: July 17 2023 $ 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 59Unless noted otherwise, for 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 localnetwork , 145.Cm host , 146.Cm originalhost , 147.Cm Tag , 148.Cm user , 149and 150.Cm localuser . 151The 152.Cm all 153criteria must appear alone or immediately after 154.Cm canonical 155or 156.Cm final . 157Other criteria may be combined arbitrarily. 158All criteria but 159.Cm all , 160.Cm canonical , 161and 162.Cm final 163require an argument. 164Criteria may be negated by prepending an exclamation mark 165.Pq Sq !\& . 166.Pp 167The 168.Cm canonical 169keyword matches only when the configuration file is being re-parsed 170after hostname canonicalization (see the 171.Cm CanonicalizeHostname 172option). 173This may be useful to specify conditions that work with canonical host 174names only. 175.Pp 176The 177.Cm final 178keyword requests that the configuration be re-parsed (regardless of whether 179.Cm CanonicalizeHostname 180is enabled), and matches only during this final pass. 181If 182.Cm CanonicalizeHostname 183is enabled, then 184.Cm canonical 185and 186.Cm final 187match during the same pass. 188.Pp 189The 190.Cm exec 191keyword executes the specified command under the user's shell. 192If the command returns a zero exit status then the condition is considered true. 193Commands containing whitespace characters must be quoted. 194Arguments to 195.Cm exec 196accept the tokens described in the 197.Sx TOKENS 198section. 199.Pp 200The 201.Cm localnetwork 202keyword matches the addresses of active local network interfaces against the 203supplied list of networks in CIDR format. 204This may be convenient for varying the effective configuration on devices that 205roam between networks. 206Note that network address is not a trustworthy criteria in many 207situations (e.g. when the network is automatically configured using DHCP) 208and so caution should be applied if using it to control security-sensitive 209configuration. 210.Pp 211The other keywords' criteria must be single entries or comma-separated 212lists and may use the wildcard and negation operators described in the 213.Sx PATTERNS 214section. 215The criteria for the 216.Cm host 217keyword are matched against the target hostname, after any substitution 218by the 219.Cm Hostname 220or 221.Cm CanonicalizeHostname 222options. 223The 224.Cm originalhost 225keyword matches against the hostname as it was specified on the command-line. 226The 227.Cm tagged 228keyword matches a tag name specified by a prior 229.Cm Tag 230directive or on the 231.Xr ssh 1 232command-line using the 233.Fl P 234flag. 235The 236.Cm user 237keyword matches against the target username on the remote host. 238The 239.Cm localuser 240keyword matches against the name of the local user running 241.Xr ssh 1 242(this keyword may be useful in system-wide 243.Nm 244files). 245.It Cm AddKeysToAgent 246Specifies whether keys should be automatically added to a running 247.Xr ssh-agent 1 . 248If this option is set to 249.Cm yes 250and a key is loaded from a file, the key and its passphrase are added to 251the agent with the default lifetime, as if by 252.Xr ssh-add 1 . 253If this option is set to 254.Cm ask , 255.Xr ssh 1 256will require confirmation using the 257.Ev SSH_ASKPASS 258program before adding a key (see 259.Xr ssh-add 1 260for details). 261If this option is set to 262.Cm confirm , 263each use of the key must be confirmed, as if the 264.Fl c 265option was specified to 266.Xr ssh-add 1 . 267If this option is set to 268.Cm no , 269no keys are added to the agent. 270Alternately, this option may be specified as a time interval 271using the format described in the 272.Sx TIME FORMATS 273section of 274.Xr sshd_config 5 275to specify the key's lifetime in 276.Xr ssh-agent 1 , 277after which it will automatically be removed. 278The argument must be 279.Cm no 280(the default), 281.Cm yes , 282.Cm confirm 283(optionally followed by a time interval), 284.Cm ask 285or a time interval. 286.It Cm AddressFamily 287Specifies which address family to use when connecting. 288Valid arguments are 289.Cm any 290(the default), 291.Cm inet 292(use IPv4 only), or 293.Cm inet6 294(use IPv6 only). 295.It Cm BatchMode 296If set to 297.Cm yes , 298user interaction such as password prompts and host key confirmation requests 299will be disabled. 300This option is useful in scripts and other batch jobs where no user 301is present to interact with 302.Xr ssh 1 . 303The argument must be 304.Cm yes 305or 306.Cm no 307(the default). 308.It Cm BindAddress 309Use the specified address on the local machine as the source address of 310the connection. 311Only useful on systems with more than one address. 312.It Cm BindInterface 313Use the address of the specified interface on the local machine as the 314source address of the connection. 315.It Cm CanonicalDomains 316When 317.Cm CanonicalizeHostname 318is enabled, this option specifies the list of domain suffixes in which to 319search for the specified destination host. 320.It Cm CanonicalizeFallbackLocal 321Specifies whether to fail with an error when hostname canonicalization fails. 322The default, 323.Cm yes , 324will attempt to look up the unqualified hostname using the system resolver's 325search rules. 326A value of 327.Cm no 328will cause 329.Xr ssh 1 330to fail instantly if 331.Cm CanonicalizeHostname 332is enabled and the target hostname cannot be found in any of the domains 333specified by 334.Cm CanonicalDomains . 335.It Cm CanonicalizeHostname 336Controls whether explicit hostname canonicalization is performed. 337The default, 338.Cm no , 339is not to perform any name rewriting and let the system resolver handle all 340hostname lookups. 341If set to 342.Cm yes 343then, for connections that do not use a 344.Cm ProxyCommand 345or 346.Cm ProxyJump , 347.Xr ssh 1 348will attempt to canonicalize the hostname specified on the command line 349using the 350.Cm CanonicalDomains 351suffixes and 352.Cm CanonicalizePermittedCNAMEs 353rules. 354If 355.Cm CanonicalizeHostname 356is set to 357.Cm always , 358then canonicalization is applied to proxied connections too. 359.Pp 360If this option is enabled, then the configuration files are processed 361again using the new target name to pick up any new configuration in matching 362.Cm Host 363and 364.Cm Match 365stanzas. 366A value of 367.Cm none 368disables the use of a 369.Cm ProxyJump 370host. 371.It Cm CanonicalizeMaxDots 372Specifies the maximum number of dot characters in a hostname before 373canonicalization is disabled. 374The default, 1, 375allows a single dot (i.e. hostname.subdomain). 376.It Cm CanonicalizePermittedCNAMEs 377Specifies rules to determine whether CNAMEs should be followed when 378canonicalizing hostnames. 379The rules consist of one or more arguments of 380.Ar source_domain_list : Ns Ar target_domain_list , 381where 382.Ar source_domain_list 383is a pattern-list of domains that may follow CNAMEs in canonicalization, 384and 385.Ar target_domain_list 386is a pattern-list of domains that they may resolve to. 387.Pp 388For example, 389.Qq *.a.example.com:*.b.example.com,*.c.example.com 390will allow hostnames matching 391.Qq *.a.example.com 392to be canonicalized to names in the 393.Qq *.b.example.com 394or 395.Qq *.c.example.com 396domains. 397.Pp 398A single argument of 399.Qq none 400causes no CNAMEs to be considered for canonicalization. 401This is the default behaviour. 402.It Cm CASignatureAlgorithms 403Specifies which algorithms are allowed for signing of certificates 404by certificate authorities (CAs). 405The default is: 406.Bd -literal -offset indent 407ssh-ed25519,ecdsa-sha2-nistp256, 408ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 409sk-ssh-ed25519@openssh.com, 410sk-ecdsa-sha2-nistp256@openssh.com, 411rsa-sha2-512,rsa-sha2-256 412.Ed 413.Pp 414If the specified list begins with a 415.Sq + 416character, then the specified algorithms will be appended to the default set 417instead of replacing them. 418If the specified list begins with a 419.Sq - 420character, then the specified algorithms (including wildcards) will be removed 421from the default set instead of replacing them. 422.Pp 423.Xr ssh 1 424will not accept host certificates signed using algorithms other than those 425specified. 426.It Cm CertificateFile 427Specifies a file from which the user's certificate is read. 428A corresponding private key must be provided separately in order 429to use this certificate either 430from an 431.Cm IdentityFile 432directive or 433.Fl i 434flag to 435.Xr ssh 1 , 436via 437.Xr ssh-agent 1 , 438or via a 439.Cm PKCS11Provider 440or 441.Cm SecurityKeyProvider . 442.Pp 443Arguments to 444.Cm CertificateFile 445may use the tilde syntax to refer to a user's home directory, 446the tokens described in the 447.Sx TOKENS 448section and environment variables as described in the 449.Sx ENVIRONMENT VARIABLES 450section. 451.Pp 452It is possible to have multiple certificate files specified in 453configuration files; these certificates will be tried in sequence. 454Multiple 455.Cm CertificateFile 456directives will add to the list of certificates used for 457authentication. 458.It Cm CheckHostIP 459If set to 460.Cm yes , 461.Xr ssh 1 462will additionally check the host IP address in the 463.Pa known_hosts 464file. 465This allows it to detect if a host key changed due to DNS spoofing 466and will add addresses of destination hosts to 467.Pa ~/.ssh/known_hosts 468in the process, regardless of the setting of 469.Cm StrictHostKeyChecking . 470If the option is set to 471.Cm no 472(the default), 473the check will not be executed. 474The default is 475.Cm no . 476.It Cm Ciphers 477Specifies the ciphers allowed and their order of preference. 478Multiple ciphers must be comma-separated. 479If the specified list begins with a 480.Sq + 481character, then the specified ciphers will be appended to the default set 482instead of replacing them. 483If the specified list begins with a 484.Sq - 485character, then the specified ciphers (including wildcards) will be removed 486from the default set instead of replacing them. 487If the specified list begins with a 488.Sq ^ 489character, then the specified ciphers will be placed at the head of the 490default set. 491.Pp 492The supported ciphers are: 493.Bd -literal -offset indent 4943des-cbc 495aes128-cbc 496aes192-cbc 497aes256-cbc 498aes128-ctr 499aes192-ctr 500aes256-ctr 501aes128-gcm@openssh.com 502aes256-gcm@openssh.com 503chacha20-poly1305@openssh.com 504.Ed 505.Pp 506The default is: 507.Bd -literal -offset indent 508chacha20-poly1305@openssh.com, 509aes128-ctr,aes192-ctr,aes256-ctr, 510aes128-gcm@openssh.com,aes256-gcm@openssh.com 511.Ed 512.Pp 513The list of available ciphers may also be obtained using 514.Qq ssh -Q cipher . 515.It Cm ClearAllForwardings 516Specifies that all local, remote, and dynamic port forwardings 517specified in the configuration files or on the command line be 518cleared. 519This option is primarily useful when used from the 520.Xr ssh 1 521command line to clear port forwardings set in 522configuration files, and is automatically set by 523.Xr scp 1 524and 525.Xr sftp 1 . 526The argument must be 527.Cm yes 528or 529.Cm no 530(the default). 531.It Cm Compression 532Specifies whether to use compression. 533The argument must be 534.Cm yes 535or 536.Cm no 537(the default). 538.It Cm ConnectionAttempts 539Specifies the number of tries (one per second) to make before exiting. 540The argument must be an integer. 541This may be useful in scripts if the connection sometimes fails. 542The default is 1. 543.It Cm ConnectTimeout 544Specifies the timeout (in seconds) used when connecting to the 545SSH server, instead of using the default system TCP timeout. 546This timeout is applied both to establishing the connection and to performing 547the initial SSH protocol handshake and key exchange. 548.It Cm ControlMaster 549Enables the sharing of multiple sessions over a single network connection. 550When set to 551.Cm yes , 552.Xr ssh 1 553will listen for connections on a control socket specified using the 554.Cm ControlPath 555argument. 556Additional sessions can connect to this socket using the same 557.Cm ControlPath 558with 559.Cm ControlMaster 560set to 561.Cm no 562(the default). 563These sessions will try to reuse the master instance's network connection 564rather than initiating new ones, but will fall back to connecting normally 565if the control socket does not exist, or is not listening. 566.Pp 567Setting this to 568.Cm ask 569will cause 570.Xr ssh 1 571to listen for control connections, but require confirmation using 572.Xr ssh-askpass 1 . 573If the 574.Cm ControlPath 575cannot be opened, 576.Xr ssh 1 577will continue without connecting to a master instance. 578.Pp 579X11 and 580.Xr ssh-agent 1 581forwarding is supported over these multiplexed connections, however the 582display and agent forwarded will be the one belonging to the master 583connection i.e. it is not possible to forward multiple displays or agents. 584.Pp 585Two additional options allow for opportunistic multiplexing: try to use a 586master connection but fall back to creating a new one if one does not already 587exist. 588These options are: 589.Cm auto 590and 591.Cm autoask . 592The latter requires confirmation like the 593.Cm ask 594option. 595.It Cm ControlPath 596Specify the path to the control socket used for connection sharing as described 597in the 598.Cm ControlMaster 599section above or the string 600.Cm none 601to disable connection sharing. 602Arguments to 603.Cm ControlPath 604may use the tilde syntax to refer to a user's home directory, 605the tokens described in the 606.Sx TOKENS 607section and environment variables as described in the 608.Sx ENVIRONMENT VARIABLES 609section. 610It is recommended that any 611.Cm ControlPath 612used for opportunistic connection sharing include 613at least %h, %p, and %r (or alternatively %C) and be placed in a directory 614that is not writable by other users. 615This ensures that shared connections are uniquely identified. 616.It Cm ControlPersist 617When used in conjunction with 618.Cm ControlMaster , 619specifies that the master connection should remain open 620in the background (waiting for future client connections) 621after the initial client connection has been closed. 622If set to 623.Cm no 624(the default), 625then the master connection will not be placed into the background, 626and will close as soon as the initial client connection is closed. 627If set to 628.Cm yes 629or 0, 630then the master connection will remain in the background indefinitely 631(until killed or closed via a mechanism such as the 632.Qq ssh -O exit ) . 633If set to a time in seconds, or a time in any of the formats documented in 634.Xr sshd_config 5 , 635then the backgrounded master connection will automatically terminate 636after it has remained idle (with no client connections) for the 637specified time. 638.It Cm DynamicForward 639Specifies that a TCP port on the local machine be forwarded 640over the secure channel, and the application 641protocol is then used to determine where to connect to from the 642remote machine. 643.Pp 644The argument must be 645.Sm off 646.Oo Ar bind_address : Oc Ar port . 647.Sm on 648IPv6 addresses can be specified by enclosing addresses in square brackets. 649By default, the local port is bound in accordance with the 650.Cm GatewayPorts 651setting. 652However, an explicit 653.Ar bind_address 654may be used to bind the connection to a specific address. 655The 656.Ar bind_address 657of 658.Cm localhost 659indicates that the listening port be bound for local use only, while an 660empty address or 661.Sq * 662indicates that the port should be available from all interfaces. 663.Pp 664Currently the SOCKS4 and SOCKS5 protocols are supported, and 665.Xr ssh 1 666will act as a SOCKS server. 667Multiple forwardings may be specified, and 668additional forwardings can be given on the command line. 669Only the superuser can forward privileged ports. 670.It Cm EnableEscapeCommandline 671Enables the command line option in the 672.Cm EscapeChar 673menu for interactive sessions (default 674.Ql ~C ) . 675By default, the command line is disabled. 676.It Cm EnableSSHKeysign 677Setting this option to 678.Cm yes 679in the global client configuration file 680.Pa /etc/ssh/ssh_config 681enables the use of the helper program 682.Xr ssh-keysign 8 683during 684.Cm HostbasedAuthentication . 685The argument must be 686.Cm yes 687or 688.Cm no 689(the default). 690This option should be placed in the non-hostspecific section. 691See 692.Xr ssh-keysign 8 693for more information. 694.It Cm EscapeChar 695Sets the escape character (default: 696.Ql ~ ) . 697The escape character can also 698be set on the command line. 699The argument should be a single character, 700.Ql ^ 701followed by a letter, or 702.Cm none 703to disable the escape 704character entirely (making the connection transparent for binary 705data). 706.It Cm ExitOnForwardFailure 707Specifies whether 708.Xr ssh 1 709should terminate the connection if it cannot set up all requested 710dynamic, tunnel, local, and remote port forwardings, (e.g.\& 711if either end is unable to bind and listen on a specified port). 712Note that 713.Cm ExitOnForwardFailure 714does not apply to connections made over port forwardings and will not, 715for example, cause 716.Xr ssh 1 717to exit if TCP connections to the ultimate forwarding destination fail. 718The argument must be 719.Cm yes 720or 721.Cm no 722(the default). 723.It Cm FingerprintHash 724Specifies the hash algorithm used when displaying key fingerprints. 725Valid options are: 726.Cm md5 727and 728.Cm sha256 729(the default). 730.It Cm ForkAfterAuthentication 731Requests 732.Nm ssh 733to go to background just before command execution. 734This is useful if 735.Nm ssh 736is going to ask for passwords or passphrases, but the user 737wants it in the background. 738This implies the 739.Cm StdinNull 740configuration option being set to 741.Dq yes . 742The recommended way to start X11 programs at a remote site is with 743something like 744.Ic ssh -f host xterm , 745which is the same as 746.Ic ssh host xterm 747if the 748.Cm ForkAfterAuthentication 749configuration option is set to 750.Dq yes . 751.Pp 752If the 753.Cm ExitOnForwardFailure 754configuration option is set to 755.Dq yes , 756then a client started with the 757.Cm ForkAfterAuthentication 758configuration option being set to 759.Dq yes 760will wait for all remote port forwards to be successfully established 761before placing itself in the background. 762The argument to this keyword must be 763.Cm yes 764(same as the 765.Fl f 766option) or 767.Cm no 768(the default). 769.It Cm ForwardAgent 770Specifies whether the connection to the authentication agent (if any) 771will be forwarded to the remote machine. 772The argument may be 773.Cm yes , 774.Cm no 775(the default), 776an explicit path to an agent socket or the name of an environment variable 777(beginning with 778.Sq $ ) 779in which to find the path. 780.Pp 781Agent forwarding should be enabled with caution. 782Users with the ability to bypass file permissions on the remote host 783(for the agent's Unix-domain socket) 784can access the local agent through the forwarded connection. 785An attacker cannot obtain key material from the agent, 786however they can perform operations on the keys that enable them to 787authenticate using the identities loaded into the agent. 788.It Cm ForwardX11 789Specifies whether X11 connections will be automatically redirected 790over the secure channel and 791.Ev DISPLAY 792set. 793The argument must be 794.Cm yes 795or 796.Cm no 797(the default). 798.Pp 799X11 forwarding should be enabled with caution. 800Users with the ability to bypass file permissions on the remote host 801(for the user's X11 authorization database) 802can access the local X11 display through the forwarded connection. 803An attacker may then be able to perform activities such as keystroke monitoring 804if the 805.Cm ForwardX11Trusted 806option is also enabled. 807.It Cm ForwardX11Timeout 808Specify a timeout for untrusted X11 forwarding 809using the format described in the 810.Sx TIME FORMATS 811section of 812.Xr sshd_config 5 . 813X11 connections received by 814.Xr ssh 1 815after this time will be refused. 816Setting 817.Cm ForwardX11Timeout 818to zero will disable the timeout and permit X11 forwarding for the life 819of the connection. 820The default is to disable untrusted X11 forwarding after twenty minutes has 821elapsed. 822.It Cm ForwardX11Trusted 823If this option is set to 824.Cm yes , 825remote X11 clients will have full access to the original X11 display. 826.Pp 827If this option is set to 828.Cm no 829(the default), 830remote X11 clients will be considered untrusted and prevented 831from stealing or tampering with data belonging to trusted X11 832clients. 833Furthermore, the 834.Xr xauth 1 835token used for the session will be set to expire after 20 minutes. 836Remote clients will be refused access after this time. 837.Pp 838See the X11 SECURITY extension specification for full details on 839the restrictions imposed on untrusted clients. 840.It Cm GatewayPorts 841Specifies whether remote hosts are allowed to connect to local 842forwarded ports. 843By default, 844.Xr ssh 1 845binds local port forwardings to the loopback address. 846This prevents other remote hosts from connecting to forwarded ports. 847.Cm GatewayPorts 848can be used to specify that ssh 849should bind local port forwardings to the wildcard address, 850thus allowing remote hosts to connect to forwarded ports. 851The argument must be 852.Cm yes 853or 854.Cm no 855(the default). 856.It Cm GlobalKnownHostsFile 857Specifies one or more files to use for the global 858host key database, separated by whitespace. 859The default is 860.Pa /etc/ssh/ssh_known_hosts , 861.Pa /etc/ssh/ssh_known_hosts2 . 862.It Cm GSSAPIAuthentication 863Specifies whether user authentication based on GSSAPI is allowed. 864The default is 865.Cm no . 866.It Cm GSSAPIDelegateCredentials 867Forward (delegate) credentials to the server. 868The default is 869.Cm no . 870.It Cm HashKnownHosts 871Indicates that 872.Xr ssh 1 873should hash host names and addresses when they are added to 874.Pa ~/.ssh/known_hosts . 875These hashed names may be used normally by 876.Xr ssh 1 877and 878.Xr sshd 8 , 879but they do not visually reveal identifying information if the 880file's contents are disclosed. 881The default is 882.Cm no . 883Note that existing names and addresses in known hosts files 884will not be converted automatically, 885but may be manually hashed using 886.Xr ssh-keygen 1 . 887.It Cm HostbasedAcceptedAlgorithms 888Specifies the signature algorithms that will be used for hostbased 889authentication as a comma-separated list of patterns. 890Alternately if the specified list begins with a 891.Sq + 892character, then the specified signature algorithms will be appended 893to the default set instead of replacing them. 894If the specified list begins with a 895.Sq - 896character, then the specified signature algorithms (including wildcards) 897will be removed from the default set instead of replacing them. 898If the specified list begins with a 899.Sq ^ 900character, then the specified signature algorithms will be placed 901at the head of the default set. 902The default for this option is: 903.Bd -literal -offset 3n 904ssh-ed25519-cert-v01@openssh.com, 905ecdsa-sha2-nistp256-cert-v01@openssh.com, 906ecdsa-sha2-nistp384-cert-v01@openssh.com, 907ecdsa-sha2-nistp521-cert-v01@openssh.com, 908sk-ssh-ed25519-cert-v01@openssh.com, 909sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 910rsa-sha2-512-cert-v01@openssh.com, 911rsa-sha2-256-cert-v01@openssh.com, 912ssh-ed25519, 913ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 914sk-ssh-ed25519@openssh.com, 915sk-ecdsa-sha2-nistp256@openssh.com, 916rsa-sha2-512,rsa-sha2-256 917.Ed 918.Pp 919The 920.Fl Q 921option of 922.Xr ssh 1 923may be used to list supported signature algorithms. 924This was formerly named HostbasedKeyTypes. 925.It Cm HostbasedAuthentication 926Specifies whether to try rhosts based authentication with public key 927authentication. 928The argument must be 929.Cm yes 930or 931.Cm no 932(the default). 933.It Cm HostKeyAlgorithms 934Specifies the host key signature algorithms 935that the client wants to use in order of preference. 936Alternately if the specified list begins with a 937.Sq + 938character, then the specified signature algorithms will be appended to 939the default set instead of replacing them. 940If the specified list begins with a 941.Sq - 942character, then the specified signature algorithms (including wildcards) 943will be removed from the default set instead of replacing them. 944If the specified list begins with a 945.Sq ^ 946character, then the specified signature algorithms will be placed 947at the head of the default set. 948The default for this option is: 949.Bd -literal -offset 3n 950ssh-ed25519-cert-v01@openssh.com, 951ecdsa-sha2-nistp256-cert-v01@openssh.com, 952ecdsa-sha2-nistp384-cert-v01@openssh.com, 953ecdsa-sha2-nistp521-cert-v01@openssh.com, 954sk-ssh-ed25519-cert-v01@openssh.com, 955sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 956rsa-sha2-512-cert-v01@openssh.com, 957rsa-sha2-256-cert-v01@openssh.com, 958ssh-ed25519, 959ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 960sk-ecdsa-sha2-nistp256@openssh.com, 961sk-ssh-ed25519@openssh.com, 962rsa-sha2-512,rsa-sha2-256 963.Ed 964.Pp 965If hostkeys are known for the destination host then this default is modified 966to prefer their algorithms. 967.Pp 968The list of available signature algorithms may also be obtained using 969.Qq ssh -Q HostKeyAlgorithms . 970.It Cm HostKeyAlias 971Specifies an alias that should be used instead of the 972real host name when looking up or saving the host key 973in the host key database files and when validating host certificates. 974This option is useful for tunneling SSH connections 975or for multiple servers running on a single host. 976.It Cm Hostname 977Specifies the real host name to log into. 978This can be used to specify nicknames or abbreviations for hosts. 979Arguments to 980.Cm Hostname 981accept the tokens described in the 982.Sx TOKENS 983section. 984Numeric IP addresses are also permitted (both on the command line and in 985.Cm Hostname 986specifications). 987The default is the name given on the command line. 988.It Cm IdentitiesOnly 989Specifies that 990.Xr ssh 1 991should only use the configured authentication identity and certificate files 992(either the default files, or those explicitly configured in the 993.Nm 994files 995or passed on the 996.Xr ssh 1 997command-line), 998even if 999.Xr ssh-agent 1 1000or a 1001.Cm PKCS11Provider 1002or 1003.Cm SecurityKeyProvider 1004offers more identities. 1005The argument to this keyword must be 1006.Cm yes 1007or 1008.Cm no 1009(the default). 1010This option is intended for situations where ssh-agent 1011offers many different identities. 1012.It Cm IdentityAgent 1013Specifies the 1014.Ux Ns -domain 1015socket used to communicate with the authentication agent. 1016.Pp 1017This option overrides the 1018.Ev SSH_AUTH_SOCK 1019environment variable and can be used to select a specific agent. 1020Setting the socket name to 1021.Cm none 1022disables the use of an authentication agent. 1023If the string 1024.Qq SSH_AUTH_SOCK 1025is specified, the location of the socket will be read from the 1026.Ev SSH_AUTH_SOCK 1027environment variable. 1028Otherwise if the specified value begins with a 1029.Sq $ 1030character, then it will be treated as an environment variable containing 1031the location of the socket. 1032.Pp 1033Arguments to 1034.Cm IdentityAgent 1035may use the tilde syntax to refer to a user's home directory, 1036the tokens described in the 1037.Sx TOKENS 1038section and environment variables as described in the 1039.Sx ENVIRONMENT VARIABLES 1040section. 1041.It Cm IdentityFile 1042Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA, 1043Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read. 1044You can also specify a public key file to use the corresponding 1045private key that is loaded in 1046.Xr ssh-agent 1 1047when the private key file is not present locally. 1048The default is 1049.Pa ~/.ssh/id_rsa , 1050.Pa ~/.ssh/id_ecdsa , 1051.Pa ~/.ssh/id_ecdsa_sk , 1052.Pa ~/.ssh/id_ed25519 , 1053.Pa ~/.ssh/id_ed25519_sk 1054and 1055.Pa ~/.ssh/id_dsa . 1056Additionally, any identities represented by the authentication agent 1057will be used for authentication unless 1058.Cm IdentitiesOnly 1059is set. 1060If no certificates have been explicitly specified by 1061.Cm CertificateFile , 1062.Xr ssh 1 1063will try to load certificate information from the filename obtained by 1064appending 1065.Pa -cert.pub 1066to the path of a specified 1067.Cm IdentityFile . 1068.Pp 1069Arguments to 1070.Cm IdentityFile 1071may use the tilde syntax to refer to a user's home directory 1072or the tokens described in the 1073.Sx TOKENS 1074section. 1075.Pp 1076It is possible to have 1077multiple identity files specified in configuration files; all these 1078identities will be tried in sequence. 1079Multiple 1080.Cm IdentityFile 1081directives will add to the list of identities tried (this behaviour 1082differs from that of other configuration directives). 1083.Pp 1084.Cm IdentityFile 1085may be used in conjunction with 1086.Cm IdentitiesOnly 1087to select which identities in an agent are offered during authentication. 1088.Cm IdentityFile 1089may also be used in conjunction with 1090.Cm CertificateFile 1091in order to provide any certificate also needed for authentication with 1092the identity. 1093.It Cm IgnoreUnknown 1094Specifies a pattern-list of unknown options to be ignored if they are 1095encountered in configuration parsing. 1096This may be used to suppress errors if 1097.Nm 1098contains options that are unrecognised by 1099.Xr ssh 1 . 1100It is recommended that 1101.Cm IgnoreUnknown 1102be listed early in the configuration file as it will not be applied 1103to unknown options that appear before it. 1104.It Cm Include 1105Include the specified configuration file(s). 1106Multiple pathnames may be specified and each pathname may contain 1107.Xr glob 7 1108wildcards and, for user configurations, shell-like 1109.Sq ~ 1110references to user home directories. 1111Wildcards will be expanded and processed in lexical order. 1112Files without absolute paths are assumed to be in 1113.Pa ~/.ssh 1114if included in a user configuration file or 1115.Pa /etc/ssh 1116if included from the system configuration file. 1117.Cm Include 1118directive may appear inside a 1119.Cm Match 1120or 1121.Cm Host 1122block 1123to perform conditional inclusion. 1124.It Cm IPQoS 1125Specifies the IPv4 type-of-service or DSCP class for connections. 1126Accepted values are 1127.Cm af11 , 1128.Cm af12 , 1129.Cm af13 , 1130.Cm af21 , 1131.Cm af22 , 1132.Cm af23 , 1133.Cm af31 , 1134.Cm af32 , 1135.Cm af33 , 1136.Cm af41 , 1137.Cm af42 , 1138.Cm af43 , 1139.Cm cs0 , 1140.Cm cs1 , 1141.Cm cs2 , 1142.Cm cs3 , 1143.Cm cs4 , 1144.Cm cs5 , 1145.Cm cs6 , 1146.Cm cs7 , 1147.Cm ef , 1148.Cm le , 1149.Cm lowdelay , 1150.Cm throughput , 1151.Cm reliability , 1152a numeric value, or 1153.Cm none 1154to use the operating system default. 1155This option may take one or two arguments, separated by whitespace. 1156If one argument is specified, it is used as the packet class unconditionally. 1157If two values are specified, the first is automatically selected for 1158interactive sessions and the second for non-interactive sessions. 1159The default is 1160.Cm af21 1161(Low-Latency Data) 1162for interactive sessions and 1163.Cm cs1 1164(Lower Effort) 1165for non-interactive sessions. 1166.It Cm KbdInteractiveAuthentication 1167Specifies whether to use keyboard-interactive authentication. 1168The argument to this keyword must be 1169.Cm yes 1170(the default) 1171or 1172.Cm no . 1173.Cm ChallengeResponseAuthentication 1174is a deprecated alias for this. 1175.It Cm KbdInteractiveDevices 1176Specifies the list of methods to use in keyboard-interactive authentication. 1177Multiple method names must be comma-separated. 1178The default is to use the server specified list. 1179The methods available vary depending on what the server supports. 1180For an OpenSSH server, 1181it may be zero or more of: 1182.Cm bsdauth 1183and 1184.Cm pam . 1185.It Cm KexAlgorithms 1186Specifies the available KEX (Key Exchange) algorithms. 1187Multiple algorithms must be comma-separated. 1188If the specified list begins with a 1189.Sq + 1190character, then the specified algorithms will be appended to the default set 1191instead of replacing them. 1192If the specified list begins with a 1193.Sq - 1194character, then the specified algorithms (including wildcards) will be removed 1195from the default set instead of replacing them. 1196If the specified list begins with a 1197.Sq ^ 1198character, then the specified algorithms will be placed at the head of the 1199default set. 1200The default is: 1201.Bd -literal -offset indent 1202sntrup761x25519-sha512@openssh.com, 1203curve25519-sha256,curve25519-sha256@libssh.org, 1204ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1205diffie-hellman-group-exchange-sha256, 1206diffie-hellman-group16-sha512, 1207diffie-hellman-group18-sha512, 1208diffie-hellman-group14-sha256 1209.Ed 1210.Pp 1211The list of available key exchange algorithms may also be obtained using 1212.Qq ssh -Q kex . 1213.It Cm KnownHostsCommand 1214Specifies a command to use to obtain a list of host keys, in addition to 1215those listed in 1216.Cm UserKnownHostsFile 1217and 1218.Cm GlobalKnownHostsFile . 1219This command is executed after the files have been read. 1220It may write host key lines to standard output in identical format to the 1221usual files (described in the 1222.Sx VERIFYING HOST KEYS 1223section in 1224.Xr ssh 1 ) . 1225Arguments to 1226.Cm KnownHostsCommand 1227accept the tokens described in the 1228.Sx TOKENS 1229section. 1230The command may be invoked multiple times per connection: once when preparing 1231the preference list of host key algorithms to use, again to obtain the 1232host key for the requested host name and, if 1233.Cm CheckHostIP 1234is enabled, one more time to obtain the host key matching the server's 1235address. 1236If the command exits abnormally or returns a non-zero exit status then the 1237connection is terminated. 1238.It Cm LocalCommand 1239Specifies a command to execute on the local machine after successfully 1240connecting to the server. 1241The command string extends to the end of the line, and is executed with 1242the user's shell. 1243Arguments to 1244.Cm LocalCommand 1245accept the tokens described in the 1246.Sx TOKENS 1247section. 1248.Pp 1249The command is run synchronously and does not have access to the 1250session of the 1251.Xr ssh 1 1252that spawned it. 1253It should not be used for interactive commands. 1254.Pp 1255This directive is ignored unless 1256.Cm PermitLocalCommand 1257has been enabled. 1258.It Cm LocalForward 1259Specifies that a TCP port on the local machine be forwarded over 1260the secure channel to the specified host and port from the remote machine. 1261The first argument specifies the listener and may be 1262.Sm off 1263.Oo Ar bind_address : Oc Ar port 1264.Sm on 1265or a Unix domain socket path. 1266The second argument is the destination and may be 1267.Ar host : Ns Ar hostport 1268or a Unix domain socket path if the remote host supports it. 1269.Pp 1270IPv6 addresses can be specified by enclosing addresses in square brackets. 1271Multiple forwardings may be specified, and additional forwardings can be 1272given on the command line. 1273Only the superuser can forward privileged ports. 1274By default, the local port is bound in accordance with the 1275.Cm GatewayPorts 1276setting. 1277However, an explicit 1278.Ar bind_address 1279may be used to bind the connection to a specific address. 1280The 1281.Ar bind_address 1282of 1283.Cm localhost 1284indicates that the listening port be bound for local use only, while an 1285empty address or 1286.Sq * 1287indicates that the port should be available from all interfaces. 1288Unix domain socket paths may use the tokens described in the 1289.Sx TOKENS 1290section and environment variables as described in the 1291.Sx ENVIRONMENT VARIABLES 1292section. 1293.It Cm LogLevel 1294Gives the verbosity level that is used when logging messages from 1295.Xr ssh 1 . 1296The possible values are: 1297QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1298The default is INFO. 1299DEBUG and DEBUG1 are equivalent. 1300DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1301.It Cm LogVerbose 1302Specify one or more overrides to LogLevel. 1303An override consists of a pattern lists that matches the source file, function 1304and line number to force detailed logging for. 1305For example, an override pattern of: 1306.Bd -literal -offset indent 1307kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1308.Ed 1309.Pp 1310would enable detailed logging for line 1000 of 1311.Pa kex.c , 1312everything in the 1313.Fn kex_exchange_identification 1314function, and all code in the 1315.Pa packet.c 1316file. 1317This option is intended for debugging and no overrides are enabled by default. 1318.It Cm MACs 1319Specifies the MAC (message authentication code) algorithms 1320in order of preference. 1321The MAC algorithm is used for data integrity protection. 1322Multiple algorithms must be comma-separated. 1323If the specified list begins with a 1324.Sq + 1325character, then the specified algorithms will be appended to the default set 1326instead of replacing them. 1327If the specified list begins with a 1328.Sq - 1329character, then the specified algorithms (including wildcards) will be removed 1330from the default set instead of replacing them. 1331If the specified list begins with a 1332.Sq ^ 1333character, then the specified algorithms will be placed at the head of the 1334default set. 1335.Pp 1336The algorithms that contain 1337.Qq -etm 1338calculate the MAC after encryption (encrypt-then-mac). 1339These are considered safer and their use recommended. 1340.Pp 1341The default is: 1342.Bd -literal -offset indent 1343umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1344hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1345hmac-sha1-etm@openssh.com, 1346umac-64@openssh.com,umac-128@openssh.com, 1347hmac-sha2-256,hmac-sha2-512,hmac-sha1 1348.Ed 1349.Pp 1350The list of available MAC algorithms may also be obtained using 1351.Qq ssh -Q mac . 1352.It Cm NoHostAuthenticationForLocalhost 1353Disable host authentication for localhost (loopback addresses). 1354The argument to this keyword must be 1355.Cm yes 1356or 1357.Cm no 1358(the default). 1359.It Cm NumberOfPasswordPrompts 1360Specifies the number of password prompts before giving up. 1361The argument to this keyword must be an integer. 1362The default is 3. 1363.It Cm PasswordAuthentication 1364Specifies whether to use password authentication. 1365The argument to this keyword must be 1366.Cm yes 1367(the default) 1368or 1369.Cm no . 1370.It Cm PermitLocalCommand 1371Allow local command execution via the 1372.Ic LocalCommand 1373option or using the 1374.Ic !\& Ns Ar command 1375escape sequence in 1376.Xr ssh 1 . 1377The argument must be 1378.Cm yes 1379or 1380.Cm no 1381(the default). 1382.It Cm PermitRemoteOpen 1383Specifies the destinations to which remote TCP port forwarding is permitted when 1384.Cm RemoteForward 1385is used as a SOCKS proxy. 1386The forwarding specification must be one of the following forms: 1387.Pp 1388.Bl -item -offset indent -compact 1389.It 1390.Cm PermitRemoteOpen 1391.Sm off 1392.Ar host : port 1393.Sm on 1394.It 1395.Cm PermitRemoteOpen 1396.Sm off 1397.Ar IPv4_addr : port 1398.Sm on 1399.It 1400.Cm PermitRemoteOpen 1401.Sm off 1402.Ar \&[ IPv6_addr \&] : port 1403.Sm on 1404.El 1405.Pp 1406Multiple forwards may be specified by separating them with whitespace. 1407An argument of 1408.Cm any 1409can be used to remove all restrictions and permit any forwarding requests. 1410An argument of 1411.Cm none 1412can be used to prohibit all forwarding requests. 1413The wildcard 1414.Sq * 1415can be used for host or port to allow all hosts or ports respectively. 1416Otherwise, no pattern matching or address lookups are performed on supplied 1417names. 1418.It Cm PKCS11Provider 1419Specifies which PKCS#11 provider to use or 1420.Cm none 1421to indicate that no provider should be used (the default). 1422The argument to this keyword is a path to the PKCS#11 shared library 1423.Xr ssh 1 1424should use to communicate with a PKCS#11 token providing keys for user 1425authentication. 1426.It Cm Port 1427Specifies the port number to connect on the remote host. 1428The default is 22. 1429.It Cm PreferredAuthentications 1430Specifies the order in which the client should try authentication methods. 1431This allows a client to prefer one method (e.g.\& 1432.Cm keyboard-interactive ) 1433over another method (e.g.\& 1434.Cm password ) . 1435The default is: 1436.Bd -literal -offset indent 1437gssapi-with-mic,hostbased,publickey, 1438keyboard-interactive,password 1439.Ed 1440.It Cm ProxyCommand 1441Specifies the command to use to connect to the server. 1442The command 1443string extends to the end of the line, and is executed 1444using the user's shell 1445.Ql exec 1446directive to avoid a lingering shell process. 1447.Pp 1448Arguments to 1449.Cm ProxyCommand 1450accept the tokens described in the 1451.Sx TOKENS 1452section. 1453The command can be basically anything, 1454and should read from its standard input and write to its standard output. 1455It should eventually connect an 1456.Xr sshd 8 1457server running on some machine, or execute 1458.Ic sshd -i 1459somewhere. 1460Host key management will be done using the 1461.Cm Hostname 1462of the host being connected (defaulting to the name typed by the user). 1463Setting the command to 1464.Cm none 1465disables this option entirely. 1466Note that 1467.Cm CheckHostIP 1468is not available for connects with a proxy command. 1469.Pp 1470This directive is useful in conjunction with 1471.Xr nc 1 1472and its proxy support. 1473For example, the following directive would connect via an HTTP proxy at 1474192.0.2.0: 1475.Bd -literal -offset 3n 1476ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1477.Ed 1478.It Cm ProxyJump 1479Specifies one or more jump proxies as either 1480.Xo 1481.Sm off 1482.Op Ar user No @ 1483.Ar host 1484.Op : Ns Ar port 1485.Sm on 1486or an ssh URI 1487.Xc . 1488Multiple proxies may be separated by comma characters and will be visited 1489sequentially. 1490Setting this option will cause 1491.Xr ssh 1 1492to connect to the target host by first making a 1493.Xr ssh 1 1494connection to the specified 1495.Cm ProxyJump 1496host and then establishing a 1497TCP forwarding to the ultimate target from there. 1498Setting the host to 1499.Cm none 1500disables this option entirely. 1501.Pp 1502Note that this option will compete with the 1503.Cm ProxyCommand 1504option - whichever is specified first will prevent later instances of the 1505other from taking effect. 1506.Pp 1507Note also that the configuration for the destination host (either supplied 1508via the command-line or the configuration file) is not generally applied 1509to jump hosts. 1510.Pa ~/.ssh/config 1511should be used if specific configuration is required for jump hosts. 1512.It Cm ProxyUseFdpass 1513Specifies that 1514.Cm ProxyCommand 1515will pass a connected file descriptor back to 1516.Xr ssh 1 1517instead of continuing to execute and pass data. 1518The default is 1519.Cm no . 1520.It Cm PubkeyAcceptedAlgorithms 1521Specifies the signature algorithms that will be used for public key 1522authentication as a comma-separated list of patterns. 1523If the specified list begins with a 1524.Sq + 1525character, then the algorithms after it will be appended to the default 1526instead of replacing it. 1527If the specified list begins with a 1528.Sq - 1529character, then the specified algorithms (including wildcards) will be removed 1530from the default set instead of replacing them. 1531If the specified list begins with a 1532.Sq ^ 1533character, then the specified algorithms will be placed at the head of the 1534default set. 1535The default for this option is: 1536.Bd -literal -offset 3n 1537ssh-ed25519-cert-v01@openssh.com, 1538ecdsa-sha2-nistp256-cert-v01@openssh.com, 1539ecdsa-sha2-nistp384-cert-v01@openssh.com, 1540ecdsa-sha2-nistp521-cert-v01@openssh.com, 1541sk-ssh-ed25519-cert-v01@openssh.com, 1542sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1543rsa-sha2-512-cert-v01@openssh.com, 1544rsa-sha2-256-cert-v01@openssh.com, 1545ssh-ed25519, 1546ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1547sk-ssh-ed25519@openssh.com, 1548sk-ecdsa-sha2-nistp256@openssh.com, 1549rsa-sha2-512,rsa-sha2-256 1550.Ed 1551.Pp 1552The list of available signature algorithms may also be obtained using 1553.Qq ssh -Q PubkeyAcceptedAlgorithms . 1554.It Cm PubkeyAuthentication 1555Specifies whether to try public key authentication. 1556The argument to this keyword must be 1557.Cm yes 1558(the default), 1559.Cm no , 1560.Cm unbound 1561or 1562.Cm host-bound . 1563The final two options enable public key authentication while respectively 1564disabling or enabling the OpenSSH host-bound authentication protocol 1565extension required for restricted 1566.Xr ssh-agent 1 1567forwarding. 1568.It Cm RekeyLimit 1569Specifies the maximum amount of data that may be transmitted or received 1570before the session key is renegotiated, optionally followed by a maximum 1571amount of time that may pass before the session key is renegotiated. 1572The first argument is specified in bytes and may have a suffix of 1573.Sq K , 1574.Sq M , 1575or 1576.Sq G 1577to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1578The default is between 1579.Sq 1G 1580and 1581.Sq 4G , 1582depending on the cipher. 1583The optional second value is specified in seconds and may use any of the 1584units documented in the TIME FORMATS section of 1585.Xr sshd_config 5 . 1586The default value for 1587.Cm RekeyLimit 1588is 1589.Cm default none , 1590which means that rekeying is performed after the cipher's default amount 1591of data has been sent or received and no time based rekeying is done. 1592.It Cm RemoteCommand 1593Specifies a command to execute on the remote machine after successfully 1594connecting to the server. 1595The command string extends to the end of the line, and is executed with 1596the user's shell. 1597Arguments to 1598.Cm RemoteCommand 1599accept the tokens described in the 1600.Sx TOKENS 1601section. 1602.It Cm RemoteForward 1603Specifies that a TCP port on the remote machine be forwarded over 1604the secure channel. 1605The remote port may either be forwarded to a specified host and port 1606from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote 1607client to connect to arbitrary destinations from the local machine. 1608The first argument is the listening specification and may be 1609.Sm off 1610.Oo Ar bind_address : Oc Ar port 1611.Sm on 1612or, if the remote host supports it, a Unix domain socket path. 1613If forwarding to a specific destination then the second argument must be 1614.Ar host : Ns Ar hostport 1615or a Unix domain socket path, 1616otherwise if no destination argument is specified then the remote forwarding 1617will be established as a SOCKS proxy. 1618When acting as a SOCKS proxy, the destination of the connection can be 1619restricted by 1620.Cm PermitRemoteOpen . 1621.Pp 1622IPv6 addresses can be specified by enclosing addresses in square brackets. 1623Multiple forwardings may be specified, and additional 1624forwardings can be given on the command line. 1625Privileged ports can be forwarded only when 1626logging in as root on the remote machine. 1627Unix domain socket paths may use the tokens described in the 1628.Sx TOKENS 1629section and environment variables as described in the 1630.Sx ENVIRONMENT VARIABLES 1631section. 1632.Pp 1633If the 1634.Ar port 1635argument is 0, 1636the listen port will be dynamically allocated on the server and reported 1637to the client at run time. 1638.Pp 1639If the 1640.Ar bind_address 1641is not specified, the default is to only bind to loopback addresses. 1642If the 1643.Ar bind_address 1644is 1645.Ql * 1646or an empty string, then the forwarding is requested to listen on all 1647interfaces. 1648Specifying a remote 1649.Ar bind_address 1650will only succeed if the server's 1651.Cm GatewayPorts 1652option is enabled (see 1653.Xr sshd_config 5 ) . 1654.It Cm RequestTTY 1655Specifies whether to request a pseudo-tty for the session. 1656The argument may be one of: 1657.Cm no 1658(never request a TTY), 1659.Cm yes 1660(always request a TTY when standard input is a TTY), 1661.Cm force 1662(always request a TTY) or 1663.Cm auto 1664(request a TTY when opening a login session). 1665This option mirrors the 1666.Fl t 1667and 1668.Fl T 1669flags for 1670.Xr ssh 1 . 1671.It Cm RequiredRSASize 1672Specifies the minimum RSA key size (in bits) that 1673.Xr ssh 1 1674will accept. 1675User authentication keys smaller than this limit will be ignored. 1676Servers that present host keys smaller than this limit will cause the 1677connection to be terminated. 1678The default is 1679.Cm 1024 1680bits. 1681Note that this limit may only be raised from the default. 1682.It Cm RevokedHostKeys 1683Specifies revoked host public keys. 1684Keys listed in this file will be refused for host authentication. 1685Note that if this file does not exist or is not readable, 1686then host authentication will be refused for all hosts. 1687Keys may be specified as a text file, listing one public key per line, or as 1688an OpenSSH Key Revocation List (KRL) as generated by 1689.Xr ssh-keygen 1 . 1690For more information on KRLs, see the KEY REVOCATION LISTS section in 1691.Xr ssh-keygen 1 . 1692Arguments to 1693.Cm RevokedHostKeys 1694may use the tilde syntax to refer to a user's home directory, 1695the tokens described in the 1696.Sx TOKENS 1697section and environment variables as described in the 1698.Sx ENVIRONMENT VARIABLES 1699section. 1700.It Cm SecurityKeyProvider 1701Specifies a path to a library that will be used when loading any 1702FIDO authenticator-hosted keys, overriding the default of using 1703the built-in USB HID support. 1704.Pp 1705If the specified value begins with a 1706.Sq $ 1707character, then it will be treated as an environment variable containing 1708the path to the library. 1709.It Cm SendEnv 1710Specifies what variables from the local 1711.Xr environ 7 1712should be sent to the server. 1713The server must also support it, and the server must be configured to 1714accept these environment variables. 1715Note that the 1716.Ev TERM 1717environment variable is always sent whenever a 1718pseudo-terminal is requested as it is required by the protocol. 1719Refer to 1720.Cm AcceptEnv 1721in 1722.Xr sshd_config 5 1723for how to configure the server. 1724Variables are specified by name, which may contain wildcard characters. 1725Multiple environment variables may be separated by whitespace or spread 1726across multiple 1727.Cm SendEnv 1728directives. 1729.Pp 1730See 1731.Sx PATTERNS 1732for more information on patterns. 1733.Pp 1734It is possible to clear previously set 1735.Cm SendEnv 1736variable names by prefixing patterns with 1737.Pa - . 1738The default is not to send any environment variables. 1739.It Cm ServerAliveCountMax 1740Sets the number of server alive messages (see below) which may be 1741sent without 1742.Xr ssh 1 1743receiving any messages back from the server. 1744If this threshold is reached while server alive messages are being sent, 1745ssh will disconnect from the server, terminating the session. 1746It is important to note that the use of server alive messages is very 1747different from 1748.Cm TCPKeepAlive 1749(below). 1750The server alive messages are sent through the encrypted channel 1751and therefore will not be spoofable. 1752The TCP keepalive option enabled by 1753.Cm TCPKeepAlive 1754is spoofable. 1755The server alive mechanism is valuable when the client or 1756server depend on knowing when a connection has become unresponsive. 1757.Pp 1758The default value is 3. 1759If, for example, 1760.Cm ServerAliveInterval 1761(see below) is set to 15 and 1762.Cm ServerAliveCountMax 1763is left at the default, if the server becomes unresponsive, 1764ssh will disconnect after approximately 45 seconds. 1765.It Cm ServerAliveInterval 1766Sets a timeout interval in seconds after which if no data has been received 1767from the server, 1768.Xr ssh 1 1769will send a message through the encrypted 1770channel to request a response from the server. 1771The default 1772is 0, indicating that these messages will not be sent to the server. 1773.It Cm SessionType 1774May be used to either request invocation of a subsystem on the remote system, 1775or to prevent the execution of a remote command at all. 1776The latter is useful for just forwarding ports. 1777The argument to this keyword must be 1778.Cm none 1779(same as the 1780.Fl N 1781option), 1782.Cm subsystem 1783(same as the 1784.Fl s 1785option) or 1786.Cm default 1787(shell or command execution). 1788.It Cm SetEnv 1789Directly specify one or more environment variables and their contents to 1790be sent to the server. 1791Similarly to 1792.Cm SendEnv , 1793with the exception of the 1794.Ev TERM 1795variable, the server must be prepared to accept the environment variable. 1796.It Cm StdinNull 1797Redirects stdin from 1798.Pa /dev/null 1799(actually, prevents reading from stdin). 1800Either this or the equivalent 1801.Fl n 1802option must be used when 1803.Nm ssh 1804is run in the background. 1805The argument to this keyword must be 1806.Cm yes 1807(same as the 1808.Fl n 1809option) or 1810.Cm no 1811(the default). 1812.It Cm StreamLocalBindMask 1813Sets the octal file creation mode mask 1814.Pq umask 1815used when creating a Unix-domain socket file for local or remote 1816port forwarding. 1817This option is only used for port forwarding to a Unix-domain socket file. 1818.Pp 1819The default value is 0177, which creates a Unix-domain socket file that is 1820readable and writable only by the owner. 1821Note that not all operating systems honor the file mode on Unix-domain 1822socket files. 1823.It Cm StreamLocalBindUnlink 1824Specifies whether to remove an existing Unix-domain socket file for local 1825or remote port forwarding before creating a new one. 1826If the socket file already exists and 1827.Cm StreamLocalBindUnlink 1828is not enabled, 1829.Nm ssh 1830will be unable to forward the port to the Unix-domain socket file. 1831This option is only used for port forwarding to a Unix-domain socket file. 1832.Pp 1833The argument must be 1834.Cm yes 1835or 1836.Cm no 1837(the default). 1838.It Cm StrictHostKeyChecking 1839If this flag is set to 1840.Cm yes , 1841.Xr ssh 1 1842will never automatically add host keys to the 1843.Pa ~/.ssh/known_hosts 1844file, and refuses to connect to hosts whose host key has changed. 1845This provides maximum protection against man-in-the-middle (MITM) attacks, 1846though it can be annoying when the 1847.Pa /etc/ssh/ssh_known_hosts 1848file is poorly maintained or when connections to new hosts are 1849frequently made. 1850This option forces the user to manually 1851add all new hosts. 1852.Pp 1853If this flag is set to 1854.Cm accept-new 1855then ssh will automatically add new host keys to the user's 1856.Pa known_hosts 1857file, but will not permit connections to hosts with 1858changed host keys. 1859If this flag is set to 1860.Cm no 1861or 1862.Cm off , 1863ssh will automatically add new host keys to the user known hosts files 1864and allow connections to hosts with changed hostkeys to proceed, 1865subject to some restrictions. 1866If this flag is set to 1867.Cm ask 1868(the default), 1869new host keys 1870will be added to the user known host files only after the user 1871has confirmed that is what they really want to do, and 1872ssh will refuse to connect to hosts whose host key has changed. 1873The host keys of 1874known hosts will be verified automatically in all cases. 1875.It Cm SyslogFacility 1876Gives the facility code that is used when logging messages from 1877.Xr ssh 1 . 1878The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1879LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1880The default is USER. 1881.It Cm TCPKeepAlive 1882Specifies whether the system should send TCP keepalive messages to the 1883other side. 1884If they are sent, death of the connection or crash of one 1885of the machines will be properly noticed. 1886However, this means that 1887connections will die if the route is down temporarily, and some people 1888find it annoying. 1889.Pp 1890The default is 1891.Cm yes 1892(to send TCP keepalive messages), and the client will notice 1893if the network goes down or the remote host dies. 1894This is important in scripts, and many users want it too. 1895.Pp 1896To disable TCP keepalive messages, the value should be set to 1897.Cm no . 1898See also 1899.Cm ServerAliveInterval 1900for protocol-level keepalives. 1901.It Cm Tag 1902Specify a configuration tag name that may be later used by a 1903.Cm Match 1904directive to select a block of configuration. 1905.It Cm Tunnel 1906Request 1907.Xr tun 4 1908device forwarding between the client and the server. 1909The argument must be 1910.Cm yes , 1911.Cm point-to-point 1912(layer 3), 1913.Cm ethernet 1914(layer 2), 1915or 1916.Cm no 1917(the default). 1918Specifying 1919.Cm yes 1920requests the default tunnel mode, which is 1921.Cm point-to-point . 1922.It Cm TunnelDevice 1923Specifies the 1924.Xr tun 4 1925devices to open on the client 1926.Pq Ar local_tun 1927and the server 1928.Pq Ar remote_tun . 1929.Pp 1930The argument must be 1931.Sm off 1932.Ar local_tun Op : Ar remote_tun . 1933.Sm on 1934The devices may be specified by numerical ID or the keyword 1935.Cm any , 1936which uses the next available tunnel device. 1937If 1938.Ar remote_tun 1939is not specified, it defaults to 1940.Cm any . 1941The default is 1942.Cm any:any . 1943.It Cm UpdateHostKeys 1944Specifies whether 1945.Xr ssh 1 1946should accept notifications of additional hostkeys from the server sent 1947after authentication has completed and add them to 1948.Cm UserKnownHostsFile . 1949The argument must be 1950.Cm yes , 1951.Cm no 1952or 1953.Cm ask . 1954This option allows learning alternate hostkeys for a server 1955and supports graceful key rotation by allowing a server to send replacement 1956public keys before old ones are removed. 1957.Pp 1958Additional hostkeys are only accepted if the key used to authenticate the 1959host was already trusted or explicitly accepted by the user, the host was 1960authenticated via 1961.Cm UserKnownHostsFile 1962(i.e. not 1963.Cm GlobalKnownHostsFile ) 1964and the host was authenticated using a plain key and not a certificate. 1965.Pp 1966.Cm UpdateHostKeys 1967is enabled by default if the user has not overridden the default 1968.Cm UserKnownHostsFile 1969setting and has not enabled 1970.Cm VerifyHostKeyDNS , 1971otherwise 1972.Cm UpdateHostKeys 1973will be set to 1974.Cm no . 1975.Pp 1976If 1977.Cm UpdateHostKeys 1978is set to 1979.Cm ask , 1980then the user is asked to confirm the modifications to the known_hosts file. 1981Confirmation is currently incompatible with 1982.Cm ControlPersist , 1983and will be disabled if it is enabled. 1984.Pp 1985Presently, only 1986.Xr sshd 8 1987from OpenSSH 6.8 and greater support the 1988.Qq hostkeys@openssh.com 1989protocol extension used to inform the client of all the server's hostkeys. 1990.It Cm User 1991Specifies the user to log in as. 1992This can be useful when a different user name is used on different machines. 1993This saves the trouble of 1994having to remember to give the user name on the command line. 1995.It Cm UserKnownHostsFile 1996Specifies one or more files to use for the user 1997host key database, separated by whitespace. 1998Each filename may use tilde notation to refer to the user's home directory, 1999the tokens described in the 2000.Sx TOKENS 2001section and environment variables as described in the 2002.Sx ENVIRONMENT VARIABLES 2003section. 2004A value of 2005.Cm none 2006causes 2007.Xr ssh 1 2008to ignore any user-specific known hosts files. 2009The default is 2010.Pa ~/.ssh/known_hosts , 2011.Pa ~/.ssh/known_hosts2 . 2012.It Cm VerifyHostKeyDNS 2013Specifies whether to verify the remote key using DNS and SSHFP resource 2014records. 2015If this option is set to 2016.Cm yes , 2017the client will implicitly trust keys that match a secure fingerprint 2018from DNS. 2019Insecure fingerprints will be handled as if this option was set to 2020.Cm ask . 2021If this option is set to 2022.Cm ask , 2023information on fingerprint match will be displayed, but the user will still 2024need to confirm new host keys according to the 2025.Cm StrictHostKeyChecking 2026option. 2027The default is 2028.Cm no . 2029.Pp 2030See also 2031.Sx VERIFYING HOST KEYS 2032in 2033.Xr ssh 1 . 2034.It Cm VisualHostKey 2035If this flag is set to 2036.Cm yes , 2037an ASCII art representation of the remote host key fingerprint is 2038printed in addition to the fingerprint string at login and 2039for unknown host keys. 2040If this flag is set to 2041.Cm no 2042(the default), 2043no fingerprint strings are printed at login and 2044only the fingerprint string will be printed for unknown host keys. 2045.It Cm XAuthLocation 2046Specifies the full pathname of the 2047.Xr xauth 1 2048program. 2049The default is 2050.Pa /usr/local/bin/xauth . 2051.El 2052.Sh PATTERNS 2053A 2054.Em pattern 2055consists of zero or more non-whitespace characters, 2056.Sq * 2057(a wildcard that matches zero or more characters), 2058or 2059.Sq ?\& 2060(a wildcard that matches exactly one character). 2061For example, to specify a set of declarations for any host in the 2062.Qq .co.uk 2063set of domains, 2064the following pattern could be used: 2065.Pp 2066.Dl Host *.co.uk 2067.Pp 2068The following pattern 2069would match any host in the 192.168.0.[0-9] network range: 2070.Pp 2071.Dl Host 192.168.0.? 2072.Pp 2073A 2074.Em pattern-list 2075is a comma-separated list of patterns. 2076Patterns within pattern-lists may be negated 2077by preceding them with an exclamation mark 2078.Pq Sq !\& . 2079For example, 2080to allow a key to be used from anywhere within an organization 2081except from the 2082.Qq dialup 2083pool, 2084the following entry (in authorized_keys) could be used: 2085.Pp 2086.Dl from=\&"!*.dialup.example.com,*.example.com\&" 2087.Pp 2088Note that a negated match will never produce a positive result by itself. 2089For example, attempting to match 2090.Qq host3 2091against the following pattern-list will fail: 2092.Pp 2093.Dl from=\&"!host1,!host2\&" 2094.Pp 2095The solution here is to include a term that will yield a positive match, 2096such as a wildcard: 2097.Pp 2098.Dl from=\&"!host1,!host2,*\&" 2099.Sh TOKENS 2100Arguments to some keywords can make use of tokens, 2101which are expanded at runtime: 2102.Pp 2103.Bl -tag -width XXXX -offset indent -compact 2104.It %% 2105A literal 2106.Sq % . 2107.It \&%C 2108Hash of %l%h%p%r. 2109.It %d 2110Local user's home directory. 2111.It %f 2112The fingerprint of the server's host key. 2113.It %H 2114The 2115.Pa known_hosts 2116hostname or address that is being searched for. 2117.It %h 2118The remote hostname. 2119.It \%%I 2120A string describing the reason for a 2121.Cm KnownHostsCommand 2122execution: either 2123.Cm ADDRESS 2124when looking up a host by address (only when 2125.Cm CheckHostIP 2126is enabled), 2127.Cm HOSTNAME 2128when searching by hostname, or 2129.Cm ORDER 2130when preparing the host key algorithm preference list to use for the 2131destination host. 2132.It %i 2133The local user ID. 2134.It %K 2135The base64 encoded host key. 2136.It %k 2137The host key alias if specified, otherwise the original remote hostname given 2138on the command line. 2139.It %L 2140The local hostname. 2141.It %l 2142The local hostname, including the domain name. 2143.It %n 2144The original remote hostname, as given on the command line. 2145.It %p 2146The remote port. 2147.It %r 2148The remote username. 2149.It \&%T 2150The local 2151.Xr tun 4 2152or 2153.Xr tap 4 2154network interface assigned if 2155tunnel forwarding was requested, or 2156.Qq NONE 2157otherwise. 2158.It %t 2159The type of the server host key, e.g. 2160.Cm ssh-ed25519 . 2161.It %u 2162The local username. 2163.El 2164.Pp 2165.Cm CertificateFile , 2166.Cm ControlPath , 2167.Cm IdentityAgent , 2168.Cm IdentityFile , 2169.Cm KnownHostsCommand , 2170.Cm LocalForward , 2171.Cm Match exec , 2172.Cm RemoteCommand , 2173.Cm RemoteForward , 2174.Cm RevokedHostKeys , 2175and 2176.Cm UserKnownHostsFile 2177accept the tokens %%, %C, %d, %h, %i, %k, %L, %l, %n, %p, %r, and %u. 2178.Pp 2179.Cm KnownHostsCommand 2180additionally accepts the tokens %f, %H, %I, %K and %t. 2181.Pp 2182.Cm Hostname 2183accepts the tokens %% and %h. 2184.Pp 2185.Cm LocalCommand 2186accepts all tokens. 2187.Pp 2188.Cm ProxyCommand 2189and 2190.Cm ProxyJump 2191accept the tokens %%, %h, %n, %p, and %r. 2192.Sh ENVIRONMENT VARIABLES 2193Arguments to some keywords can be expanded at runtime from environment 2194variables on the client by enclosing them in 2195.Ic ${} , 2196for example 2197.Ic ${HOME}/.ssh 2198would refer to the user's .ssh directory. 2199If a specified environment variable does not exist then an error will be 2200returned and the setting for that keyword will be ignored. 2201.Pp 2202The keywords 2203.Cm CertificateFile , 2204.Cm ControlPath , 2205.Cm IdentityAgent , 2206.Cm IdentityFile , 2207.Cm KnownHostsCommand , 2208and 2209.Cm UserKnownHostsFile 2210support environment variables. 2211The keywords 2212.Cm LocalForward 2213and 2214.Cm RemoteForward 2215support environment variables only for Unix domain socket paths. 2216.Sh FILES 2217.Bl -tag -width Ds 2218.It Pa ~/.ssh/config 2219This is the per-user configuration file. 2220The format of this file is described above. 2221This file is used by the SSH client. 2222Because of the potential for abuse, this file must have strict permissions: 2223read/write for the user, and not writable by others. 2224.It Pa /etc/ssh/ssh_config 2225Systemwide configuration file. 2226This file provides defaults for those 2227values that are not specified in the user's configuration file, and 2228for those users who do not have a configuration file. 2229This file must be world-readable. 2230.El 2231.Sh SEE ALSO 2232.Xr ssh 1 2233.Sh AUTHORS 2234.An -nosplit 2235OpenSSH is a derivative of the original and free 2236ssh 1.2.12 release by 2237.An Tatu Ylonen . 2238.An Aaron Campbell , Bob Beck , Markus Friedl , 2239.An Niels Provos , Theo de Raadt 2240and 2241.An Dug Song 2242removed many bugs, re-added newer features and 2243created OpenSSH. 2244.An Markus Friedl 2245contributed the support for SSH protocol versions 1.5 and 2.0. 2246