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 1.380 2023/03/27 03:56:11 dtucker Exp $ 37.Dd $Mdocdate: March 27 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 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.Pp 376A single argument of 377.Qq none 378causes no CNAMEs to be considered for canonicalization. 379This is the default behaviour. 380.It Cm CASignatureAlgorithms 381Specifies which algorithms are allowed for signing of certificates 382by certificate authorities (CAs). 383The default is: 384.Bd -literal -offset indent 385ssh-ed25519,ecdsa-sha2-nistp256, 386ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 387sk-ssh-ed25519@openssh.com, 388sk-ecdsa-sha2-nistp256@openssh.com, 389rsa-sha2-512,rsa-sha2-256 390.Ed 391.Pp 392If the specified list begins with a 393.Sq + 394character, then the specified algorithms will be appended to the default set 395instead of replacing them. 396If the specified list begins with a 397.Sq - 398character, then the specified algorithms (including wildcards) will be removed 399from the default set instead of replacing them. 400.Pp 401.Xr ssh 1 402will not accept host certificates signed using algorithms other than those 403specified. 404.It Cm CertificateFile 405Specifies a file from which the user's certificate is read. 406A corresponding private key must be provided separately in order 407to use this certificate either 408from an 409.Cm IdentityFile 410directive or 411.Fl i 412flag to 413.Xr ssh 1 , 414via 415.Xr ssh-agent 1 , 416or via a 417.Cm PKCS11Provider 418or 419.Cm SecurityKeyProvider . 420.Pp 421Arguments to 422.Cm CertificateFile 423may use the tilde syntax to refer to a user's home directory, 424the tokens described in the 425.Sx TOKENS 426section and environment variables as described in the 427.Sx ENVIRONMENT VARIABLES 428section. 429.Pp 430It is possible to have multiple certificate files specified in 431configuration files; these certificates will be tried in sequence. 432Multiple 433.Cm CertificateFile 434directives will add to the list of certificates used for 435authentication. 436.It Cm CheckHostIP 437If set to 438.Cm yes , 439.Xr ssh 1 440will additionally check the host IP address in the 441.Pa known_hosts 442file. 443This allows it to detect if a host key changed due to DNS spoofing 444and will add addresses of destination hosts to 445.Pa ~/.ssh/known_hosts 446in the process, regardless of the setting of 447.Cm StrictHostKeyChecking . 448If the option is set to 449.Cm no 450(the default), 451the check will not be executed. 452.It Cm Ciphers 453Specifies the ciphers allowed and their order of preference. 454Multiple ciphers must be comma-separated. 455If the specified list begins with a 456.Sq + 457character, then the specified ciphers will be appended to the default set 458instead of replacing them. 459If the specified list begins with a 460.Sq - 461character, then the specified ciphers (including wildcards) will be removed 462from the default set instead of replacing them. 463If the specified list begins with a 464.Sq ^ 465character, then the specified ciphers will be placed at the head of the 466default set. 467.Pp 468The supported ciphers are: 469.Bd -literal -offset indent 4703des-cbc 471aes128-cbc 472aes192-cbc 473aes256-cbc 474aes128-ctr 475aes192-ctr 476aes256-ctr 477aes128-gcm@openssh.com 478aes256-gcm@openssh.com 479chacha20-poly1305@openssh.com 480.Ed 481.Pp 482The default is: 483.Bd -literal -offset indent 484chacha20-poly1305@openssh.com, 485aes128-ctr,aes192-ctr,aes256-ctr, 486aes128-gcm@openssh.com,aes256-gcm@openssh.com 487.Ed 488.Pp 489The list of available ciphers may also be obtained using 490.Qq ssh -Q cipher . 491.It Cm ClearAllForwardings 492Specifies that all local, remote, and dynamic port forwardings 493specified in the configuration files or on the command line be 494cleared. 495This option is primarily useful when used from the 496.Xr ssh 1 497command line to clear port forwardings set in 498configuration files, and is automatically set by 499.Xr scp 1 500and 501.Xr sftp 1 . 502The argument must be 503.Cm yes 504or 505.Cm no 506(the default). 507.It Cm Compression 508Specifies whether to use compression. 509The argument must be 510.Cm yes 511or 512.Cm no 513(the default). 514.It Cm ConnectionAttempts 515Specifies the number of tries (one per second) to make before exiting. 516The argument must be an integer. 517This may be useful in scripts if the connection sometimes fails. 518The default is 1. 519.It Cm ConnectTimeout 520Specifies the timeout (in seconds) used when connecting to the 521SSH server, instead of using the default system TCP timeout. 522This timeout is applied both to establishing the connection and to performing 523the initial SSH protocol handshake and key exchange. 524.It Cm ControlMaster 525Enables the sharing of multiple sessions over a single network connection. 526When set to 527.Cm yes , 528.Xr ssh 1 529will listen for connections on a control socket specified using the 530.Cm ControlPath 531argument. 532Additional sessions can connect to this socket using the same 533.Cm ControlPath 534with 535.Cm ControlMaster 536set to 537.Cm no 538(the default). 539These sessions will try to reuse the master instance's network connection 540rather than initiating new ones, but will fall back to connecting normally 541if the control socket does not exist, or is not listening. 542.Pp 543Setting this to 544.Cm ask 545will cause 546.Xr ssh 1 547to listen for control connections, but require confirmation using 548.Xr ssh-askpass 1 . 549If the 550.Cm ControlPath 551cannot be opened, 552.Xr ssh 1 553will continue without connecting to a master instance. 554.Pp 555X11 and 556.Xr ssh-agent 1 557forwarding is supported over these multiplexed connections, however the 558display and agent forwarded will be the one belonging to the master 559connection i.e. it is not possible to forward multiple displays or agents. 560.Pp 561Two additional options allow for opportunistic multiplexing: try to use a 562master connection but fall back to creating a new one if one does not already 563exist. 564These options are: 565.Cm auto 566and 567.Cm autoask . 568The latter requires confirmation like the 569.Cm ask 570option. 571.It Cm ControlPath 572Specify the path to the control socket used for connection sharing as described 573in the 574.Cm ControlMaster 575section above or the string 576.Cm none 577to disable connection sharing. 578Arguments to 579.Cm ControlPath 580may use the tilde syntax to refer to a user's home directory, 581the tokens described in the 582.Sx TOKENS 583section and environment variables as described in the 584.Sx ENVIRONMENT VARIABLES 585section. 586It is recommended that any 587.Cm ControlPath 588used for opportunistic connection sharing include 589at least %h, %p, and %r (or alternatively %C) and be placed in a directory 590that is not writable by other users. 591This ensures that shared connections are uniquely identified. 592.It Cm ControlPersist 593When used in conjunction with 594.Cm ControlMaster , 595specifies that the master connection should remain open 596in the background (waiting for future client connections) 597after the initial client connection has been closed. 598If set to 599.Cm no 600(the default), 601then the master connection will not be placed into the background, 602and will close as soon as the initial client connection is closed. 603If set to 604.Cm yes 605or 0, 606then the master connection will remain in the background indefinitely 607(until killed or closed via a mechanism such as the 608.Qq ssh -O exit ) . 609If set to a time in seconds, or a time in any of the formats documented in 610.Xr sshd_config 5 , 611then the backgrounded master connection will automatically terminate 612after it has remained idle (with no client connections) for the 613specified time. 614.It Cm DynamicForward 615Specifies that a TCP port on the local machine be forwarded 616over the secure channel, and the application 617protocol is then used to determine where to connect to from the 618remote machine. 619.Pp 620The argument must be 621.Sm off 622.Oo Ar bind_address : Oc Ar port . 623.Sm on 624IPv6 addresses can be specified by enclosing addresses in square brackets. 625By default, the local port is bound in accordance with the 626.Cm GatewayPorts 627setting. 628However, an explicit 629.Ar bind_address 630may be used to bind the connection to a specific address. 631The 632.Ar bind_address 633of 634.Cm localhost 635indicates that the listening port be bound for local use only, while an 636empty address or 637.Sq * 638indicates that the port should be available from all interfaces. 639.Pp 640Currently the SOCKS4 and SOCKS5 protocols are supported, and 641.Xr ssh 1 642will act as a SOCKS server. 643Multiple forwardings may be specified, and 644additional forwardings can be given on the command line. 645Only the superuser can forward privileged ports. 646.It Cm EnableEscapeCommandline 647Enables the command line option in the 648.Cm EscapeChar 649menu for interactive sessions (default 650.Ql ~C ) . 651By default, the command line is disabled. 652.It Cm EnableSSHKeysign 653Setting this option to 654.Cm yes 655in the global client configuration file 656.Pa /etc/ssh/ssh_config 657enables the use of the helper program 658.Xr ssh-keysign 8 659during 660.Cm HostbasedAuthentication . 661The argument must be 662.Cm yes 663or 664.Cm no 665(the default). 666This option should be placed in the non-hostspecific section. 667See 668.Xr ssh-keysign 8 669for more information. 670.It Cm EscapeChar 671Sets the escape character (default: 672.Ql ~ ) . 673The escape character can also 674be set on the command line. 675The argument should be a single character, 676.Ql ^ 677followed by a letter, or 678.Cm none 679to disable the escape 680character entirely (making the connection transparent for binary 681data). 682.It Cm ExitOnForwardFailure 683Specifies whether 684.Xr ssh 1 685should terminate the connection if it cannot set up all requested 686dynamic, tunnel, local, and remote port forwardings, (e.g.\& 687if either end is unable to bind and listen on a specified port). 688Note that 689.Cm ExitOnForwardFailure 690does not apply to connections made over port forwardings and will not, 691for example, cause 692.Xr ssh 1 693to exit if TCP connections to the ultimate forwarding destination fail. 694The argument must be 695.Cm yes 696or 697.Cm no 698(the default). 699.It Cm FingerprintHash 700Specifies the hash algorithm used when displaying key fingerprints. 701Valid options are: 702.Cm md5 703and 704.Cm sha256 705(the default). 706.It Cm ForkAfterAuthentication 707Requests 708.Nm ssh 709to go to background just before command execution. 710This is useful if 711.Nm ssh 712is going to ask for passwords or passphrases, but the user 713wants it in the background. 714This implies the 715.Cm StdinNull 716configuration option being set to 717.Dq yes . 718The recommended way to start X11 programs at a remote site is with 719something like 720.Ic ssh -f host xterm , 721which is the same as 722.Ic ssh host xterm 723if the 724.Cm ForkAfterAuthentication 725configuration option is set to 726.Dq yes . 727.Pp 728If the 729.Cm ExitOnForwardFailure 730configuration option is set to 731.Dq yes , 732then a client started with the 733.Cm ForkAfterAuthentication 734configuration option being set to 735.Dq yes 736will wait for all remote port forwards to be successfully established 737before placing itself in the background. 738The argument to this keyword must be 739.Cm yes 740(same as the 741.Fl f 742option) or 743.Cm no 744(the default). 745.It Cm ForwardAgent 746Specifies whether the connection to the authentication agent (if any) 747will be forwarded to the remote machine. 748The argument may be 749.Cm yes , 750.Cm no 751(the default), 752an explicit path to an agent socket or the name of an environment variable 753(beginning with 754.Sq $ ) 755in which to find the path. 756.Pp 757Agent forwarding should be enabled with caution. 758Users with the ability to bypass file permissions on the remote host 759(for the agent's Unix-domain socket) 760can access the local agent through the forwarded connection. 761An attacker cannot obtain key material from the agent, 762however they can perform operations on the keys that enable them to 763authenticate using the identities loaded into the agent. 764.It Cm ForwardX11 765Specifies whether X11 connections will be automatically redirected 766over the secure channel and 767.Ev DISPLAY 768set. 769The argument must be 770.Cm yes 771or 772.Cm no 773(the default). 774.Pp 775X11 forwarding should be enabled with caution. 776Users with the ability to bypass file permissions on the remote host 777(for the user's X11 authorization database) 778can access the local X11 display through the forwarded connection. 779An attacker may then be able to perform activities such as keystroke monitoring 780if the 781.Cm ForwardX11Trusted 782option is also enabled. 783.It Cm ForwardX11Timeout 784Specify a timeout for untrusted X11 forwarding 785using the format described in the 786.Sx TIME FORMATS 787section of 788.Xr sshd_config 5 . 789X11 connections received by 790.Xr ssh 1 791after this time will be refused. 792Setting 793.Cm ForwardX11Timeout 794to zero will disable the timeout and permit X11 forwarding for the life 795of the connection. 796The default is to disable untrusted X11 forwarding after twenty minutes has 797elapsed. 798.It Cm ForwardX11Trusted 799If this option is set to 800.Cm yes , 801remote X11 clients will have full access to the original X11 display. 802.Pp 803If this option is set to 804.Cm no 805(the default), 806remote X11 clients will be considered untrusted and prevented 807from stealing or tampering with data belonging to trusted X11 808clients. 809Furthermore, the 810.Xr xauth 1 811token used for the session will be set to expire after 20 minutes. 812Remote clients will be refused access after this time. 813.Pp 814See the X11 SECURITY extension specification for full details on 815the restrictions imposed on untrusted clients. 816.It Cm GatewayPorts 817Specifies whether remote hosts are allowed to connect to local 818forwarded ports. 819By default, 820.Xr ssh 1 821binds local port forwardings to the loopback address. 822This prevents other remote hosts from connecting to forwarded ports. 823.Cm GatewayPorts 824can be used to specify that ssh 825should bind local port forwardings to the wildcard address, 826thus allowing remote hosts to connect to forwarded ports. 827The argument must be 828.Cm yes 829or 830.Cm no 831(the default). 832.It Cm GlobalKnownHostsFile 833Specifies one or more files to use for the global 834host key database, separated by whitespace. 835The default is 836.Pa /etc/ssh/ssh_known_hosts , 837.Pa /etc/ssh/ssh_known_hosts2 . 838.It Cm GSSAPIAuthentication 839Specifies whether user authentication based on GSSAPI is allowed. 840The default is 841.Cm no . 842.It Cm GSSAPIDelegateCredentials 843Forward (delegate) credentials to the server. 844The default is 845.Cm no . 846.It Cm HashKnownHosts 847Indicates that 848.Xr ssh 1 849should hash host names and addresses when they are added to 850.Pa ~/.ssh/known_hosts . 851These hashed names may be used normally by 852.Xr ssh 1 853and 854.Xr sshd 8 , 855but they do not visually reveal identifying information if the 856file's contents are disclosed. 857The default is 858.Cm no . 859Note that existing names and addresses in known hosts files 860will not be converted automatically, 861but may be manually hashed using 862.Xr ssh-keygen 1 . 863.It Cm HostbasedAcceptedAlgorithms 864Specifies the signature algorithms that will be used for hostbased 865authentication as a comma-separated list of patterns. 866Alternately if the specified list begins with a 867.Sq + 868character, then the specified signature algorithms will be appended 869to the default set instead of replacing them. 870If the specified list begins with a 871.Sq - 872character, then the specified signature algorithms (including wildcards) 873will be removed from the default set instead of replacing them. 874If the specified list begins with a 875.Sq ^ 876character, then the specified signature algorithms will be placed 877at the head of the default set. 878The default for this option is: 879.Bd -literal -offset 3n 880ssh-ed25519-cert-v01@openssh.com, 881ecdsa-sha2-nistp256-cert-v01@openssh.com, 882ecdsa-sha2-nistp384-cert-v01@openssh.com, 883ecdsa-sha2-nistp521-cert-v01@openssh.com, 884sk-ssh-ed25519-cert-v01@openssh.com, 885sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 886rsa-sha2-512-cert-v01@openssh.com, 887rsa-sha2-256-cert-v01@openssh.com, 888ssh-ed25519, 889ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 890sk-ssh-ed25519@openssh.com, 891sk-ecdsa-sha2-nistp256@openssh.com, 892rsa-sha2-512,rsa-sha2-256 893.Ed 894.Pp 895The 896.Fl Q 897option of 898.Xr ssh 1 899may be used to list supported signature algorithms. 900This was formerly named HostbasedKeyTypes. 901.It Cm HostbasedAuthentication 902Specifies whether to try rhosts based authentication with public key 903authentication. 904The argument must be 905.Cm yes 906or 907.Cm no 908(the default). 909.It Cm HostKeyAlgorithms 910Specifies the host key signature algorithms 911that the client wants to use in order of preference. 912Alternately if the specified list begins with a 913.Sq + 914character, then the specified signature algorithms will be appended to 915the default set instead of replacing them. 916If the specified list begins with a 917.Sq - 918character, then the specified signature algorithms (including wildcards) 919will be removed from the default set instead of replacing them. 920If the specified list begins with a 921.Sq ^ 922character, then the specified signature algorithms will be placed 923at the head of the default set. 924The default for this option is: 925.Bd -literal -offset 3n 926ssh-ed25519-cert-v01@openssh.com, 927ecdsa-sha2-nistp256-cert-v01@openssh.com, 928ecdsa-sha2-nistp384-cert-v01@openssh.com, 929ecdsa-sha2-nistp521-cert-v01@openssh.com, 930sk-ssh-ed25519-cert-v01@openssh.com, 931sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 932rsa-sha2-512-cert-v01@openssh.com, 933rsa-sha2-256-cert-v01@openssh.com, 934ssh-ed25519, 935ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 936sk-ecdsa-sha2-nistp256@openssh.com, 937sk-ssh-ed25519@openssh.com, 938rsa-sha2-512,rsa-sha2-256 939.Ed 940.Pp 941If hostkeys are known for the destination host then this default is modified 942to prefer their algorithms. 943.Pp 944The list of available signature algorithms may also be obtained using 945.Qq ssh -Q HostKeyAlgorithms . 946.It Cm HostKeyAlias 947Specifies an alias that should be used instead of the 948real host name when looking up or saving the host key 949in the host key database files and when validating host certificates. 950This option is useful for tunneling SSH connections 951or for multiple servers running on a single host. 952.It Cm Hostname 953Specifies the real host name to log into. 954This can be used to specify nicknames or abbreviations for hosts. 955Arguments to 956.Cm Hostname 957accept the tokens described in the 958.Sx TOKENS 959section. 960Numeric IP addresses are also permitted (both on the command line and in 961.Cm Hostname 962specifications). 963The default is the name given on the command line. 964.It Cm IdentitiesOnly 965Specifies that 966.Xr ssh 1 967should only use the configured authentication identity and certificate files 968(either the default files, or those explicitly configured in the 969.Nm 970files 971or passed on the 972.Xr ssh 1 973command-line), 974even if 975.Xr ssh-agent 1 976or a 977.Cm PKCS11Provider 978or 979.Cm SecurityKeyProvider 980offers more identities. 981The argument to this keyword must be 982.Cm yes 983or 984.Cm no 985(the default). 986This option is intended for situations where ssh-agent 987offers many different identities. 988.It Cm IdentityAgent 989Specifies the 990.Ux Ns -domain 991socket used to communicate with the authentication agent. 992.Pp 993This option overrides the 994.Ev SSH_AUTH_SOCK 995environment variable and can be used to select a specific agent. 996Setting the socket name to 997.Cm none 998disables the use of an authentication agent. 999If the string 1000.Qq SSH_AUTH_SOCK 1001is specified, the location of the socket will be read from the 1002.Ev SSH_AUTH_SOCK 1003environment variable. 1004Otherwise if the specified value begins with a 1005.Sq $ 1006character, then it will be treated as an environment variable containing 1007the location of the socket. 1008.Pp 1009Arguments to 1010.Cm IdentityAgent 1011may use the tilde syntax to refer to a user's home directory, 1012the tokens described in the 1013.Sx TOKENS 1014section and environment variables as described in the 1015.Sx ENVIRONMENT VARIABLES 1016section. 1017.It Cm IdentityFile 1018Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA, 1019Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read. 1020You can also specify a public key file to use the corresponding 1021private key that is loaded in 1022.Xr ssh-agent 1 1023when the private key file is not present locally. 1024The default is 1025.Pa ~/.ssh/id_rsa , 1026.Pa ~/.ssh/id_ecdsa , 1027.Pa ~/.ssh/id_ecdsa_sk , 1028.Pa ~/.ssh/id_ed25519 , 1029.Pa ~/.ssh/id_ed25519_sk 1030and 1031.Pa ~/.ssh/id_dsa . 1032Additionally, any identities represented by the authentication agent 1033will be used for authentication unless 1034.Cm IdentitiesOnly 1035is set. 1036If no certificates have been explicitly specified by 1037.Cm CertificateFile , 1038.Xr ssh 1 1039will try to load certificate information from the filename obtained by 1040appending 1041.Pa -cert.pub 1042to the path of a specified 1043.Cm IdentityFile . 1044.Pp 1045Arguments to 1046.Cm IdentityFile 1047may use the tilde syntax to refer to a user's home directory 1048or the tokens described in the 1049.Sx TOKENS 1050section. 1051.Pp 1052It is possible to have 1053multiple identity files specified in configuration files; all these 1054identities will be tried in sequence. 1055Multiple 1056.Cm IdentityFile 1057directives will add to the list of identities tried (this behaviour 1058differs from that of other configuration directives). 1059.Pp 1060.Cm IdentityFile 1061may be used in conjunction with 1062.Cm IdentitiesOnly 1063to select which identities in an agent are offered during authentication. 1064.Cm IdentityFile 1065may also be used in conjunction with 1066.Cm CertificateFile 1067in order to provide any certificate also needed for authentication with 1068the identity. 1069.It Cm IgnoreUnknown 1070Specifies a pattern-list of unknown options to be ignored if they are 1071encountered in configuration parsing. 1072This may be used to suppress errors if 1073.Nm 1074contains options that are unrecognised by 1075.Xr ssh 1 . 1076It is recommended that 1077.Cm IgnoreUnknown 1078be listed early in the configuration file as it will not be applied 1079to unknown options that appear before it. 1080.It Cm Include 1081Include the specified configuration file(s). 1082Multiple pathnames may be specified and each pathname may contain 1083.Xr glob 7 1084wildcards and, for user configurations, shell-like 1085.Sq ~ 1086references to user home directories. 1087Wildcards will be expanded and processed in lexical order. 1088Files without absolute paths are assumed to be in 1089.Pa ~/.ssh 1090if included in a user configuration file or 1091.Pa /etc/ssh 1092if included from the system configuration file. 1093.Cm Include 1094directive may appear inside a 1095.Cm Match 1096or 1097.Cm Host 1098block 1099to perform conditional inclusion. 1100.It Cm IPQoS 1101Specifies the IPv4 type-of-service or DSCP class for connections. 1102Accepted values are 1103.Cm af11 , 1104.Cm af12 , 1105.Cm af13 , 1106.Cm af21 , 1107.Cm af22 , 1108.Cm af23 , 1109.Cm af31 , 1110.Cm af32 , 1111.Cm af33 , 1112.Cm af41 , 1113.Cm af42 , 1114.Cm af43 , 1115.Cm cs0 , 1116.Cm cs1 , 1117.Cm cs2 , 1118.Cm cs3 , 1119.Cm cs4 , 1120.Cm cs5 , 1121.Cm cs6 , 1122.Cm cs7 , 1123.Cm ef , 1124.Cm le , 1125.Cm lowdelay , 1126.Cm throughput , 1127.Cm reliability , 1128a numeric value, or 1129.Cm none 1130to use the operating system default. 1131This option may take one or two arguments, separated by whitespace. 1132If one argument is specified, it is used as the packet class unconditionally. 1133If two values are specified, the first is automatically selected for 1134interactive sessions and the second for non-interactive sessions. 1135The default is 1136.Cm af21 1137(Low-Latency Data) 1138for interactive sessions and 1139.Cm cs1 1140(Lower Effort) 1141for non-interactive sessions. 1142.It Cm KbdInteractiveAuthentication 1143Specifies whether to use keyboard-interactive authentication. 1144The argument to this keyword must be 1145.Cm yes 1146(the default) 1147or 1148.Cm no . 1149.Cm ChallengeResponseAuthentication 1150is a deprecated alias for this. 1151.It Cm KbdInteractiveDevices 1152Specifies the list of methods to use in keyboard-interactive authentication. 1153Multiple method names must be comma-separated. 1154The default is to use the server specified list. 1155The methods available vary depending on what the server supports. 1156For an OpenSSH server, 1157it may be zero or more of: 1158.Cm bsdauth , 1159.Cm pam , 1160and 1161.Cm skey . 1162.It Cm KexAlgorithms 1163Specifies the available KEX (Key Exchange) algorithms. 1164Multiple algorithms must be comma-separated. 1165If the specified list begins with a 1166.Sq + 1167character, then the specified algorithms will be appended to the default set 1168instead of replacing them. 1169If the specified list begins with a 1170.Sq - 1171character, then the specified algorithms (including wildcards) will be removed 1172from the default set instead of replacing them. 1173If the specified list begins with a 1174.Sq ^ 1175character, then the specified algorithms will be placed at the head of the 1176default set. 1177The default is: 1178.Bd -literal -offset indent 1179sntrup761x25519-sha512@openssh.com, 1180curve25519-sha256,curve25519-sha256@libssh.org, 1181ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1182diffie-hellman-group-exchange-sha256, 1183diffie-hellman-group16-sha512, 1184diffie-hellman-group18-sha512, 1185diffie-hellman-group14-sha256 1186.Ed 1187.Pp 1188The list of available key exchange algorithms may also be obtained using 1189.Qq ssh -Q kex . 1190.It Cm KnownHostsCommand 1191Specifies a command to use to obtain a list of host keys, in addition to 1192those listed in 1193.Cm UserKnownHostsFile 1194and 1195.Cm GlobalKnownHostsFile . 1196This command is executed after the files have been read. 1197It may write host key lines to standard output in identical format to the 1198usual files (described in the 1199.Sx VERIFYING HOST KEYS 1200section in 1201.Xr ssh 1 ) . 1202Arguments to 1203.Cm KnownHostsCommand 1204accept the tokens described in the 1205.Sx TOKENS 1206section. 1207The command may be invoked multiple times per connection: once when preparing 1208the preference list of host key algorithms to use, again to obtain the 1209host key for the requested host name and, if 1210.Cm CheckHostIP 1211is enabled, one more time to obtain the host key matching the server's 1212address. 1213If the command exits abnormally or returns a non-zero exit status then the 1214connection is terminated. 1215.It Cm LocalCommand 1216Specifies a command to execute on the local machine after successfully 1217connecting to the server. 1218The command string extends to the end of the line, and is executed with 1219the user's shell. 1220Arguments to 1221.Cm LocalCommand 1222accept the tokens described in the 1223.Sx TOKENS 1224section. 1225.Pp 1226The command is run synchronously and does not have access to the 1227session of the 1228.Xr ssh 1 1229that spawned it. 1230It should not be used for interactive commands. 1231.Pp 1232This directive is ignored unless 1233.Cm PermitLocalCommand 1234has been enabled. 1235.It Cm LocalForward 1236Specifies that a TCP port on the local machine be forwarded over 1237the secure channel to the specified host and port from the remote machine. 1238The first argument specifies the listener and may be 1239.Sm off 1240.Oo Ar bind_address : Oc Ar port 1241.Sm on 1242or a Unix domain socket path. 1243The second argument is the destination and may be 1244.Ar host : Ns Ar hostport 1245or a Unix domain socket path if the remote host supports it. 1246.Pp 1247IPv6 addresses can be specified by enclosing addresses in square brackets. 1248Multiple forwardings may be specified, and additional forwardings can be 1249given on the command line. 1250Only the superuser can forward privileged ports. 1251By default, the local port is bound in accordance with the 1252.Cm GatewayPorts 1253setting. 1254However, an explicit 1255.Ar bind_address 1256may be used to bind the connection to a specific address. 1257The 1258.Ar bind_address 1259of 1260.Cm localhost 1261indicates that the listening port be bound for local use only, while an 1262empty address or 1263.Sq * 1264indicates that the port should be available from all interfaces. 1265Unix domain socket paths may use the tokens described in the 1266.Sx TOKENS 1267section and environment variables as described in the 1268.Sx ENVIRONMENT VARIABLES 1269section. 1270.It Cm LogLevel 1271Gives the verbosity level that is used when logging messages from 1272.Xr ssh 1 . 1273The possible values are: 1274QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1275The default is INFO. 1276DEBUG and DEBUG1 are equivalent. 1277DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1278.It Cm LogVerbose 1279Specify one or more overrides to LogLevel. 1280An override consists of a pattern lists that matches the source file, function 1281and line number to force detailed logging for. 1282For example, an override pattern of: 1283.Bd -literal -offset indent 1284kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1285.Ed 1286.Pp 1287would enable detailed logging for line 1000 of 1288.Pa kex.c , 1289everything in the 1290.Fn kex_exchange_identification 1291function, and all code in the 1292.Pa packet.c 1293file. 1294This option is intended for debugging and no overrides are enabled by default. 1295.It Cm MACs 1296Specifies the MAC (message authentication code) algorithms 1297in order of preference. 1298The MAC algorithm is used for data integrity protection. 1299Multiple algorithms must be comma-separated. 1300If the specified list begins with a 1301.Sq + 1302character, then the specified algorithms will be appended to the default set 1303instead of replacing them. 1304If the specified list begins with a 1305.Sq - 1306character, then the specified algorithms (including wildcards) will be removed 1307from the default set instead of replacing them. 1308If the specified list begins with a 1309.Sq ^ 1310character, then the specified algorithms will be placed at the head of the 1311default set. 1312.Pp 1313The algorithms that contain 1314.Qq -etm 1315calculate the MAC after encryption (encrypt-then-mac). 1316These are considered safer and their use recommended. 1317.Pp 1318The default is: 1319.Bd -literal -offset indent 1320umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1321hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1322hmac-sha1-etm@openssh.com, 1323umac-64@openssh.com,umac-128@openssh.com, 1324hmac-sha2-256,hmac-sha2-512,hmac-sha1 1325.Ed 1326.Pp 1327The list of available MAC algorithms may also be obtained using 1328.Qq ssh -Q mac . 1329.It Cm NoHostAuthenticationForLocalhost 1330Disable host authentication for localhost (loopback addresses). 1331The argument to this keyword must be 1332.Cm yes 1333or 1334.Cm no 1335(the default). 1336.It Cm NumberOfPasswordPrompts 1337Specifies the number of password prompts before giving up. 1338The argument to this keyword must be an integer. 1339The default is 3. 1340.It Cm PasswordAuthentication 1341Specifies whether to use password authentication. 1342The argument to this keyword must be 1343.Cm yes 1344(the default) 1345or 1346.Cm no . 1347.It Cm PermitLocalCommand 1348Allow local command execution via the 1349.Ic LocalCommand 1350option or using the 1351.Ic !\& Ns Ar command 1352escape sequence in 1353.Xr ssh 1 . 1354The argument must be 1355.Cm yes 1356or 1357.Cm no 1358(the default). 1359.It Cm PermitRemoteOpen 1360Specifies the destinations to which remote TCP port forwarding is permitted when 1361.Cm RemoteForward 1362is used as a SOCKS proxy. 1363The forwarding specification must be one of the following forms: 1364.Pp 1365.Bl -item -offset indent -compact 1366.It 1367.Cm PermitRemoteOpen 1368.Sm off 1369.Ar host : port 1370.Sm on 1371.It 1372.Cm PermitRemoteOpen 1373.Sm off 1374.Ar IPv4_addr : port 1375.Sm on 1376.It 1377.Cm PermitRemoteOpen 1378.Sm off 1379.Ar \&[ IPv6_addr \&] : port 1380.Sm on 1381.El 1382.Pp 1383Multiple forwards may be specified by separating them with whitespace. 1384An argument of 1385.Cm any 1386can be used to remove all restrictions and permit any forwarding requests. 1387An argument of 1388.Cm none 1389can be used to prohibit all forwarding requests. 1390The wildcard 1391.Sq * 1392can be used for host or port to allow all hosts or ports respectively. 1393Otherwise, no pattern matching or address lookups are performed on supplied 1394names. 1395.It Cm PKCS11Provider 1396Specifies which PKCS#11 provider to use or 1397.Cm none 1398to indicate that no provider should be used (the default). 1399The argument to this keyword is a path to the PKCS#11 shared library 1400.Xr ssh 1 1401should use to communicate with a PKCS#11 token providing keys for user 1402authentication. 1403.It Cm Port 1404Specifies the port number to connect on the remote host. 1405The default is 22. 1406.It Cm PreferredAuthentications 1407Specifies the order in which the client should try authentication methods. 1408This allows a client to prefer one method (e.g.\& 1409.Cm keyboard-interactive ) 1410over another method (e.g.\& 1411.Cm password ) . 1412The default is: 1413.Bd -literal -offset indent 1414gssapi-with-mic,hostbased,publickey, 1415keyboard-interactive,password 1416.Ed 1417.It Cm ProxyCommand 1418Specifies the command to use to connect to the server. 1419The command 1420string extends to the end of the line, and is executed 1421using the user's shell 1422.Ql exec 1423directive to avoid a lingering shell process. 1424.Pp 1425Arguments to 1426.Cm ProxyCommand 1427accept the tokens described in the 1428.Sx TOKENS 1429section. 1430The command can be basically anything, 1431and should read from its standard input and write to its standard output. 1432It should eventually connect an 1433.Xr sshd 8 1434server running on some machine, or execute 1435.Ic sshd -i 1436somewhere. 1437Host key management will be done using the 1438.Cm Hostname 1439of the host being connected (defaulting to the name typed by the user). 1440Setting the command to 1441.Cm none 1442disables this option entirely. 1443Note that 1444.Cm CheckHostIP 1445is not available for connects with a proxy command. 1446.Pp 1447This directive is useful in conjunction with 1448.Xr nc 1 1449and its proxy support. 1450For example, the following directive would connect via an HTTP proxy at 1451192.0.2.0: 1452.Bd -literal -offset 3n 1453ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1454.Ed 1455.It Cm ProxyJump 1456Specifies one or more jump proxies as either 1457.Xo 1458.Sm off 1459.Op Ar user No @ 1460.Ar host 1461.Op : Ns Ar port 1462.Sm on 1463or an ssh URI 1464.Xc . 1465Multiple proxies may be separated by comma characters and will be visited 1466sequentially. 1467Setting this option will cause 1468.Xr ssh 1 1469to connect to the target host by first making a 1470.Xr ssh 1 1471connection to the specified 1472.Cm ProxyJump 1473host and then establishing a 1474TCP forwarding to the ultimate target from there. 1475Setting the host to 1476.Cm none 1477disables this option entirely. 1478.Pp 1479Note that this option will compete with the 1480.Cm ProxyCommand 1481option - whichever is specified first will prevent later instances of the 1482other from taking effect. 1483.Pp 1484Note also that the configuration for the destination host (either supplied 1485via the command-line or the configuration file) is not generally applied 1486to jump hosts. 1487.Pa ~/.ssh/config 1488should be used if specific configuration is required for jump hosts. 1489.It Cm ProxyUseFdpass 1490Specifies that 1491.Cm ProxyCommand 1492will pass a connected file descriptor back to 1493.Xr ssh 1 1494instead of continuing to execute and pass data. 1495The default is 1496.Cm no . 1497.It Cm PubkeyAcceptedAlgorithms 1498Specifies the signature algorithms that will be used for public key 1499authentication as a comma-separated list of patterns. 1500If the specified list begins with a 1501.Sq + 1502character, then the algorithms after it will be appended to the default 1503instead of replacing it. 1504If the specified list begins with a 1505.Sq - 1506character, then the specified algorithms (including wildcards) will be removed 1507from the default set instead of replacing them. 1508If the specified list begins with a 1509.Sq ^ 1510character, then the specified algorithms will be placed at the head of the 1511default set. 1512The default for this option is: 1513.Bd -literal -offset 3n 1514ssh-ed25519-cert-v01@openssh.com, 1515ecdsa-sha2-nistp256-cert-v01@openssh.com, 1516ecdsa-sha2-nistp384-cert-v01@openssh.com, 1517ecdsa-sha2-nistp521-cert-v01@openssh.com, 1518sk-ssh-ed25519-cert-v01@openssh.com, 1519sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1520rsa-sha2-512-cert-v01@openssh.com, 1521rsa-sha2-256-cert-v01@openssh.com, 1522ssh-ed25519, 1523ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1524sk-ssh-ed25519@openssh.com, 1525sk-ecdsa-sha2-nistp256@openssh.com, 1526rsa-sha2-512,rsa-sha2-256 1527.Ed 1528.Pp 1529The list of available signature algorithms may also be obtained using 1530.Qq ssh -Q PubkeyAcceptedAlgorithms . 1531.It Cm PubkeyAuthentication 1532Specifies whether to try public key authentication. 1533The argument to this keyword must be 1534.Cm yes 1535(the default), 1536.Cm no , 1537.Cm unbound 1538or 1539.Cm host-bound . 1540The final two options enable public key authentication while respectively 1541disabling or enabling the OpenSSH host-bound authentication protocol 1542extension required for restricted 1543.Xr ssh-agent 1 1544forwarding. 1545.It Cm RekeyLimit 1546Specifies the maximum amount of data that may be transmitted or received 1547before the session key is renegotiated, optionally followed by a maximum 1548amount of time that may pass before the session key is renegotiated. 1549The first argument is specified in bytes and may have a suffix of 1550.Sq K , 1551.Sq M , 1552or 1553.Sq G 1554to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1555The default is between 1556.Sq 1G 1557and 1558.Sq 4G , 1559depending on the cipher. 1560The optional second value is specified in seconds and may use any of the 1561units documented in the TIME FORMATS section of 1562.Xr sshd_config 5 . 1563The default value for 1564.Cm RekeyLimit 1565is 1566.Cm default none , 1567which means that rekeying is performed after the cipher's default amount 1568of data has been sent or received and no time based rekeying is done. 1569.It Cm RemoteCommand 1570Specifies a command to execute on the remote machine after successfully 1571connecting to the server. 1572The command string extends to the end of the line, and is executed with 1573the user's shell. 1574Arguments to 1575.Cm RemoteCommand 1576accept the tokens described in the 1577.Sx TOKENS 1578section. 1579.It Cm RemoteForward 1580Specifies that a TCP port on the remote machine be forwarded over 1581the secure channel. 1582The remote port may either be forwarded to a specified host and port 1583from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote 1584client to connect to arbitrary destinations from the local machine. 1585The first argument is the listening specification and may be 1586.Sm off 1587.Oo Ar bind_address : Oc Ar port 1588.Sm on 1589or, if the remote host supports it, a Unix domain socket path. 1590If forwarding to a specific destination then the second argument must be 1591.Ar host : Ns Ar hostport 1592or a Unix domain socket path, 1593otherwise if no destination argument is specified then the remote forwarding 1594will be established as a SOCKS proxy. 1595When acting as a SOCKS proxy, the destination of the connection can be 1596restricted by 1597.Cm PermitRemoteOpen . 1598.Pp 1599IPv6 addresses can be specified by enclosing addresses in square brackets. 1600Multiple forwardings may be specified, and additional 1601forwardings can be given on the command line. 1602Privileged ports can be forwarded only when 1603logging in as root on the remote machine. 1604Unix domain socket paths may use the tokens described in the 1605.Sx TOKENS 1606section and environment variables as described in the 1607.Sx ENVIRONMENT VARIABLES 1608section. 1609.Pp 1610If the 1611.Ar port 1612argument is 0, 1613the listen port will be dynamically allocated on the server and reported 1614to the client at run time. 1615.Pp 1616If the 1617.Ar bind_address 1618is not specified, the default is to only bind to loopback addresses. 1619If the 1620.Ar bind_address 1621is 1622.Ql * 1623or an empty string, then the forwarding is requested to listen on all 1624interfaces. 1625Specifying a remote 1626.Ar bind_address 1627will only succeed if the server's 1628.Cm GatewayPorts 1629option is enabled (see 1630.Xr sshd_config 5 ) . 1631.It Cm RequestTTY 1632Specifies whether to request a pseudo-tty for the session. 1633The argument may be one of: 1634.Cm no 1635(never request a TTY), 1636.Cm yes 1637(always request a TTY when standard input is a TTY), 1638.Cm force 1639(always request a TTY) or 1640.Cm auto 1641(request a TTY when opening a login session). 1642This option mirrors the 1643.Fl t 1644and 1645.Fl T 1646flags for 1647.Xr ssh 1 . 1648.It Cm RequiredRSASize 1649Specifies the minimum RSA key size (in bits) that 1650.Xr ssh 1 1651will accept. 1652User authentication keys smaller than this limit will be ignored. 1653Servers that present host keys smaller than this limit will cause the 1654connection to be terminated. 1655The default is 1656.Cm 1024 1657bits. 1658Note that this limit may only be raised from the default. 1659.It Cm RevokedHostKeys 1660Specifies revoked host public keys. 1661Keys listed in this file will be refused for host authentication. 1662Note that if this file does not exist or is not readable, 1663then host authentication will be refused for all hosts. 1664Keys may be specified as a text file, listing one public key per line, or as 1665an OpenSSH Key Revocation List (KRL) as generated by 1666.Xr ssh-keygen 1 . 1667For more information on KRLs, see the KEY REVOCATION LISTS section in 1668.Xr ssh-keygen 1 . 1669Arguments to 1670.Cm RevokedHostKeys 1671may use the tilde syntax to refer to a user's home directory, 1672the tokens described in the 1673.Sx TOKENS 1674section and environment variables as described in the 1675.Sx ENVIRONMENT VARIABLES 1676section. 1677.It Cm SecurityKeyProvider 1678Specifies a path to a library that will be used when loading any 1679FIDO authenticator-hosted keys, overriding the default of using 1680the built-in USB HID support. 1681.Pp 1682If the specified value begins with a 1683.Sq $ 1684character, then it will be treated as an environment variable containing 1685the path to the library. 1686.It Cm SendEnv 1687Specifies what variables from the local 1688.Xr environ 7 1689should be sent to the server. 1690The server must also support it, and the server must be configured to 1691accept these environment variables. 1692Note that the 1693.Ev TERM 1694environment variable is always sent whenever a 1695pseudo-terminal is requested as it is required by the protocol. 1696Refer to 1697.Cm AcceptEnv 1698in 1699.Xr sshd_config 5 1700for how to configure the server. 1701Variables are specified by name, which may contain wildcard characters. 1702Multiple environment variables may be separated by whitespace or spread 1703across multiple 1704.Cm SendEnv 1705directives. 1706.Pp 1707See 1708.Sx PATTERNS 1709for more information on patterns. 1710.Pp 1711It is possible to clear previously set 1712.Cm SendEnv 1713variable names by prefixing patterns with 1714.Pa - . 1715The default is not to send any environment variables. 1716.It Cm ServerAliveCountMax 1717Sets the number of server alive messages (see below) which may be 1718sent without 1719.Xr ssh 1 1720receiving any messages back from the server. 1721If this threshold is reached while server alive messages are being sent, 1722ssh will disconnect from the server, terminating the session. 1723It is important to note that the use of server alive messages is very 1724different from 1725.Cm TCPKeepAlive 1726(below). 1727The server alive messages are sent through the encrypted channel 1728and therefore will not be spoofable. 1729The TCP keepalive option enabled by 1730.Cm TCPKeepAlive 1731is spoofable. 1732The server alive mechanism is valuable when the client or 1733server depend on knowing when a connection has become unresponsive. 1734.Pp 1735The default value is 3. 1736If, for example, 1737.Cm ServerAliveInterval 1738(see below) is set to 15 and 1739.Cm ServerAliveCountMax 1740is left at the default, if the server becomes unresponsive, 1741ssh will disconnect after approximately 45 seconds. 1742.It Cm ServerAliveInterval 1743Sets a timeout interval in seconds after which if no data has been received 1744from the server, 1745.Xr ssh 1 1746will send a message through the encrypted 1747channel to request a response from the server. 1748The default 1749is 0, indicating that these messages will not be sent to the server. 1750.It Cm SessionType 1751May be used to either request invocation of a subsystem on the remote system, 1752or to prevent the execution of a remote command at all. 1753The latter is useful for just forwarding ports. 1754The argument to this keyword must be 1755.Cm none 1756(same as the 1757.Fl N 1758option), 1759.Cm subsystem 1760(same as the 1761.Fl s 1762option) or 1763.Cm default 1764(shell or command execution). 1765.It Cm SetEnv 1766Directly specify one or more environment variables and their contents to 1767be sent to the server. 1768Similarly to 1769.Cm SendEnv , 1770with the exception of the 1771.Ev TERM 1772variable, the server must be prepared to accept the environment variable. 1773.It Cm StdinNull 1774Redirects stdin from 1775.Pa /dev/null 1776(actually, prevents reading from stdin). 1777Either this or the equivalent 1778.Fl n 1779option must be used when 1780.Nm ssh 1781is run in the background. 1782The argument to this keyword must be 1783.Cm yes 1784(same as the 1785.Fl n 1786option) or 1787.Cm no 1788(the default). 1789.It Cm StreamLocalBindMask 1790Sets the octal file creation mode mask 1791.Pq umask 1792used when creating a Unix-domain socket file for local or remote 1793port forwarding. 1794This option is only used for port forwarding to a Unix-domain socket file. 1795.Pp 1796The default value is 0177, which creates a Unix-domain socket file that is 1797readable and writable only by the owner. 1798Note that not all operating systems honor the file mode on Unix-domain 1799socket files. 1800.It Cm StreamLocalBindUnlink 1801Specifies whether to remove an existing Unix-domain socket file for local 1802or remote port forwarding before creating a new one. 1803If the socket file already exists and 1804.Cm StreamLocalBindUnlink 1805is not enabled, 1806.Nm ssh 1807will be unable to forward the port to the Unix-domain socket file. 1808This option is only used for port forwarding to a Unix-domain socket file. 1809.Pp 1810The argument must be 1811.Cm yes 1812or 1813.Cm no 1814(the default). 1815.It Cm StrictHostKeyChecking 1816If this flag is set to 1817.Cm yes , 1818.Xr ssh 1 1819will never automatically add host keys to the 1820.Pa ~/.ssh/known_hosts 1821file, and refuses to connect to hosts whose host key has changed. 1822This provides maximum protection against man-in-the-middle (MITM) attacks, 1823though it can be annoying when the 1824.Pa /etc/ssh/ssh_known_hosts 1825file is poorly maintained or when connections to new hosts are 1826frequently made. 1827This option forces the user to manually 1828add all new hosts. 1829.Pp 1830If this flag is set to 1831.Cm accept-new 1832then ssh will automatically add new host keys to the user's 1833.Pa known_hosts 1834file, but will not permit connections to hosts with 1835changed host keys. 1836If this flag is set to 1837.Cm no 1838or 1839.Cm off , 1840ssh will automatically add new host keys to the user known hosts files 1841and allow connections to hosts with changed hostkeys to proceed, 1842subject to some restrictions. 1843If this flag is set to 1844.Cm ask 1845(the default), 1846new host keys 1847will be added to the user known host files only after the user 1848has confirmed that is what they really want to do, and 1849ssh will refuse to connect to hosts whose host key has changed. 1850The host keys of 1851known hosts will be verified automatically in all cases. 1852.It Cm SyslogFacility 1853Gives the facility code that is used when logging messages from 1854.Xr ssh 1 . 1855The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1856LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1857The default is USER. 1858.It Cm TCPKeepAlive 1859Specifies whether the system should send TCP keepalive messages to the 1860other side. 1861If they are sent, death of the connection or crash of one 1862of the machines will be properly noticed. 1863However, this means that 1864connections will die if the route is down temporarily, and some people 1865find it annoying. 1866.Pp 1867The default is 1868.Cm yes 1869(to send TCP keepalive messages), and the client will notice 1870if the network goes down or the remote host dies. 1871This is important in scripts, and many users want it too. 1872.Pp 1873To disable TCP keepalive messages, the value should be set to 1874.Cm no . 1875See also 1876.Cm ServerAliveInterval 1877for protocol-level keepalives. 1878.It Cm Tunnel 1879Request 1880.Xr tun 4 1881device forwarding between the client and the server. 1882The argument must be 1883.Cm yes , 1884.Cm point-to-point 1885(layer 3), 1886.Cm ethernet 1887(layer 2), 1888or 1889.Cm no 1890(the default). 1891Specifying 1892.Cm yes 1893requests the default tunnel mode, which is 1894.Cm point-to-point . 1895.It Cm TunnelDevice 1896Specifies the 1897.Xr tun 4 1898devices to open on the client 1899.Pq Ar local_tun 1900and the server 1901.Pq Ar remote_tun . 1902.Pp 1903The argument must be 1904.Sm off 1905.Ar local_tun Op : Ar remote_tun . 1906.Sm on 1907The devices may be specified by numerical ID or the keyword 1908.Cm any , 1909which uses the next available tunnel device. 1910If 1911.Ar remote_tun 1912is not specified, it defaults to 1913.Cm any . 1914The default is 1915.Cm any:any . 1916.It Cm UpdateHostKeys 1917Specifies whether 1918.Xr ssh 1 1919should accept notifications of additional hostkeys from the server sent 1920after authentication has completed and add them to 1921.Cm UserKnownHostsFile . 1922The argument must be 1923.Cm yes , 1924.Cm no 1925or 1926.Cm ask . 1927This option allows learning alternate hostkeys for a server 1928and supports graceful key rotation by allowing a server to send replacement 1929public keys before old ones are removed. 1930.Pp 1931Additional hostkeys are only accepted if the key used to authenticate the 1932host was already trusted or explicitly accepted by the user, the host was 1933authenticated via 1934.Cm UserKnownHostsFile 1935(i.e. not 1936.Cm GlobalKnownHostsFile ) 1937and the host was authenticated using a plain key and not a certificate. 1938.Pp 1939.Cm UpdateHostKeys 1940is enabled by default if the user has not overridden the default 1941.Cm UserKnownHostsFile 1942setting and has not enabled 1943.Cm VerifyHostKeyDNS , 1944otherwise 1945.Cm UpdateHostKeys 1946will be set to 1947.Cm no . 1948.Pp 1949If 1950.Cm UpdateHostKeys 1951is set to 1952.Cm ask , 1953then the user is asked to confirm the modifications to the known_hosts file. 1954Confirmation is currently incompatible with 1955.Cm ControlPersist , 1956and will be disabled if it is enabled. 1957.Pp 1958Presently, only 1959.Xr sshd 8 1960from OpenSSH 6.8 and greater support the 1961.Qq hostkeys@openssh.com 1962protocol extension used to inform the client of all the server's hostkeys. 1963.It Cm User 1964Specifies the user to log in as. 1965This can be useful when a different user name is used on different machines. 1966This saves the trouble of 1967having to remember to give the user name on the command line. 1968.It Cm UserKnownHostsFile 1969Specifies one or more files to use for the user 1970host key database, separated by whitespace. 1971Each filename may use tilde notation to refer to the user's home directory, 1972the tokens described in the 1973.Sx TOKENS 1974section and environment variables as described in the 1975.Sx ENVIRONMENT VARIABLES 1976section. 1977A value of 1978.Cm none 1979causes 1980.Xr ssh 1 1981to ignore any user-specific known hosts files. 1982The default is 1983.Pa ~/.ssh/known_hosts , 1984.Pa ~/.ssh/known_hosts2 . 1985.It Cm VerifyHostKeyDNS 1986Specifies whether to verify the remote key using DNS and SSHFP resource 1987records. 1988If this option is set to 1989.Cm yes , 1990the client will implicitly trust keys that match a secure fingerprint 1991from DNS. 1992Insecure fingerprints will be handled as if this option was set to 1993.Cm ask . 1994If this option is set to 1995.Cm ask , 1996information on fingerprint match will be displayed, but the user will still 1997need to confirm new host keys according to the 1998.Cm StrictHostKeyChecking 1999option. 2000The default is 2001.Cm no . 2002.Pp 2003See also 2004.Sx VERIFYING HOST KEYS 2005in 2006.Xr ssh 1 . 2007.It Cm VisualHostKey 2008If this flag is set to 2009.Cm yes , 2010an ASCII art representation of the remote host key fingerprint is 2011printed in addition to the fingerprint string at login and 2012for unknown host keys. 2013If this flag is set to 2014.Cm no 2015(the default), 2016no fingerprint strings are printed at login and 2017only the fingerprint string will be printed for unknown host keys. 2018.It Cm XAuthLocation 2019Specifies the full pathname of the 2020.Xr xauth 1 2021program. 2022The default is 2023.Pa /usr/X11R6/bin/xauth . 2024.El 2025.Sh PATTERNS 2026A 2027.Em pattern 2028consists of zero or more non-whitespace characters, 2029.Sq * 2030(a wildcard that matches zero or more characters), 2031or 2032.Sq ?\& 2033(a wildcard that matches exactly one character). 2034For example, to specify a set of declarations for any host in the 2035.Qq .co.uk 2036set of domains, 2037the following pattern could be used: 2038.Pp 2039.Dl Host *.co.uk 2040.Pp 2041The following pattern 2042would match any host in the 192.168.0.[0-9] network range: 2043.Pp 2044.Dl Host 192.168.0.? 2045.Pp 2046A 2047.Em pattern-list 2048is a comma-separated list of patterns. 2049Patterns within pattern-lists may be negated 2050by preceding them with an exclamation mark 2051.Pq Sq !\& . 2052For example, 2053to allow a key to be used from anywhere within an organization 2054except from the 2055.Qq dialup 2056pool, 2057the following entry (in authorized_keys) could be used: 2058.Pp 2059.Dl from=\&"!*.dialup.example.com,*.example.com\&" 2060.Pp 2061Note that a negated match will never produce a positive result by itself. 2062For example, attempting to match 2063.Qq host3 2064against the following pattern-list will fail: 2065.Pp 2066.Dl from=\&"!host1,!host2\&" 2067.Pp 2068The solution here is to include a term that will yield a positive match, 2069such as a wildcard: 2070.Pp 2071.Dl from=\&"!host1,!host2,*\&" 2072.Sh TOKENS 2073Arguments to some keywords can make use of tokens, 2074which are expanded at runtime: 2075.Pp 2076.Bl -tag -width XXXX -offset indent -compact 2077.It %% 2078A literal 2079.Sq % . 2080.It \&%C 2081Hash of %l%h%p%r. 2082.It %d 2083Local user's home directory. 2084.It %f 2085The fingerprint of the server's host key. 2086.It %H 2087The 2088.Pa known_hosts 2089hostname or address that is being searched for. 2090.It %h 2091The remote hostname. 2092.It \%%I 2093A string describing the reason for a 2094.Cm KnownHostsCommand 2095execution: either 2096.Cm ADDRESS 2097when looking up a host by address (only when 2098.Cm CheckHostIP 2099is enabled), 2100.Cm HOSTNAME 2101when searching by hostname, or 2102.Cm ORDER 2103when preparing the host key algorithm preference list to use for the 2104destination host. 2105.It %i 2106The local user ID. 2107.It %K 2108The base64 encoded host key. 2109.It %k 2110The host key alias if specified, otherwise the original remote hostname given 2111on the command line. 2112.It %L 2113The local hostname. 2114.It %l 2115The local hostname, including the domain name. 2116.It %n 2117The original remote hostname, as given on the command line. 2118.It %p 2119The remote port. 2120.It %r 2121The remote username. 2122.It \&%T 2123The local 2124.Xr tun 4 2125or 2126.Xr tap 4 2127network interface assigned if 2128tunnel forwarding was requested, or 2129.Qq NONE 2130otherwise. 2131.It %t 2132The type of the server host key, e.g. 2133.Cm ssh-ed25519 . 2134.It %u 2135The local username. 2136.El 2137.Pp 2138.Cm CertificateFile , 2139.Cm ControlPath , 2140.Cm IdentityAgent , 2141.Cm IdentityFile , 2142.Cm KnownHostsCommand , 2143.Cm LocalForward , 2144.Cm Match exec , 2145.Cm RemoteCommand , 2146.Cm RemoteForward , 2147.Cm RevokedHostKeys , 2148and 2149.Cm UserKnownHostsFile 2150accept the tokens %%, %C, %d, %h, %i, %k, %L, %l, %n, %p, %r, and %u. 2151.Pp 2152.Cm KnownHostsCommand 2153additionally accepts the tokens %f, %H, %I, %K and %t. 2154.Pp 2155.Cm Hostname 2156accepts the tokens %% and %h. 2157.Pp 2158.Cm LocalCommand 2159accepts all tokens. 2160.Pp 2161.Cm ProxyCommand 2162and 2163.Cm ProxyJump 2164accept the tokens %%, %h, %n, %p, and %r. 2165.Sh ENVIRONMENT VARIABLES 2166Arguments to some keywords can be expanded at runtime from environment 2167variables on the client by enclosing them in 2168.Ic ${} , 2169for example 2170.Ic ${HOME}/.ssh 2171would refer to the user's .ssh directory. 2172If a specified environment variable does not exist then an error will be 2173returned and the setting for that keyword will be ignored. 2174.Pp 2175The keywords 2176.Cm CertificateFile , 2177.Cm ControlPath , 2178.Cm IdentityAgent , 2179.Cm IdentityFile , 2180.Cm KnownHostsCommand , 2181and 2182.Cm UserKnownHostsFile 2183support environment variables. 2184The keywords 2185.Cm LocalForward 2186and 2187.Cm RemoteForward 2188support environment variables only for Unix domain socket paths. 2189.Sh FILES 2190.Bl -tag -width Ds 2191.It Pa ~/.ssh/config 2192This is the per-user configuration file. 2193The format of this file is described above. 2194This file is used by the SSH client. 2195Because of the potential for abuse, this file must have strict permissions: 2196read/write for the user, and not writable by others. 2197.It Pa /etc/ssh/ssh_config 2198Systemwide configuration file. 2199This file provides defaults for those 2200values that are not specified in the user's configuration file, and 2201for those users who do not have a configuration file. 2202This file must be world-readable. 2203.El 2204.Sh SEE ALSO 2205.Xr ssh 1 2206.Sh AUTHORS 2207.An -nosplit 2208OpenSSH is a derivative of the original and free 2209ssh 1.2.12 release by 2210.An Tatu Ylonen . 2211.An Aaron Campbell , Bob Beck , Markus Friedl , 2212.An Niels Provos , Theo de Raadt 2213and 2214.An Dug Song 2215removed many bugs, re-added newer features and 2216created OpenSSH. 2217.An Markus Friedl 2218contributed the support for SSH protocol versions 1.5 and 2.0. 2219