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 1.379 2023/03/10 02:32:04 djm Exp $ 37.Dd $Mdocdate: March 10 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. 452The default is 453.Cm no . 454.It Cm Ciphers 455Specifies the ciphers allowed and their order of preference. 456Multiple ciphers must be comma-separated. 457If the specified list begins with a 458.Sq + 459character, then the specified ciphers will be appended to the default set 460instead of replacing them. 461If the specified list begins with a 462.Sq - 463character, then the specified ciphers (including wildcards) will be removed 464from the default set instead of replacing them. 465If the specified list begins with a 466.Sq ^ 467character, then the specified ciphers will be placed at the head of the 468default set. 469.Pp 470The supported ciphers are: 471.Bd -literal -offset indent 4723des-cbc 473aes128-cbc 474aes192-cbc 475aes256-cbc 476aes128-ctr 477aes192-ctr 478aes256-ctr 479aes128-gcm@openssh.com 480aes256-gcm@openssh.com 481chacha20-poly1305@openssh.com 482.Ed 483.Pp 484The default is: 485.Bd -literal -offset indent 486chacha20-poly1305@openssh.com, 487aes128-ctr,aes192-ctr,aes256-ctr, 488aes128-gcm@openssh.com,aes256-gcm@openssh.com 489.Ed 490.Pp 491The list of available ciphers may also be obtained using 492.Qq ssh -Q cipher . 493.It Cm ClearAllForwardings 494Specifies that all local, remote, and dynamic port forwardings 495specified in the configuration files or on the command line be 496cleared. 497This option is primarily useful when used from the 498.Xr ssh 1 499command line to clear port forwardings set in 500configuration files, and is automatically set by 501.Xr scp 1 502and 503.Xr sftp 1 . 504The argument must be 505.Cm yes 506or 507.Cm no 508(the default). 509.It Cm Compression 510Specifies whether to use compression. 511The argument must be 512.Cm yes 513or 514.Cm no 515(the default). 516.It Cm ConnectionAttempts 517Specifies the number of tries (one per second) to make before exiting. 518The argument must be an integer. 519This may be useful in scripts if the connection sometimes fails. 520The default is 1. 521.It Cm ConnectTimeout 522Specifies the timeout (in seconds) used when connecting to the 523SSH server, instead of using the default system TCP timeout. 524This timeout is applied both to establishing the connection and to performing 525the initial SSH protocol handshake and key exchange. 526.It Cm ControlMaster 527Enables the sharing of multiple sessions over a single network connection. 528When set to 529.Cm yes , 530.Xr ssh 1 531will listen for connections on a control socket specified using the 532.Cm ControlPath 533argument. 534Additional sessions can connect to this socket using the same 535.Cm ControlPath 536with 537.Cm ControlMaster 538set to 539.Cm no 540(the default). 541These sessions will try to reuse the master instance's network connection 542rather than initiating new ones, but will fall back to connecting normally 543if the control socket does not exist, or is not listening. 544.Pp 545Setting this to 546.Cm ask 547will cause 548.Xr ssh 1 549to listen for control connections, but require confirmation using 550.Xr ssh-askpass 1 . 551If the 552.Cm ControlPath 553cannot be opened, 554.Xr ssh 1 555will continue without connecting to a master instance. 556.Pp 557X11 and 558.Xr ssh-agent 1 559forwarding is supported over these multiplexed connections, however the 560display and agent forwarded will be the one belonging to the master 561connection i.e. it is not possible to forward multiple displays or agents. 562.Pp 563Two additional options allow for opportunistic multiplexing: try to use a 564master connection but fall back to creating a new one if one does not already 565exist. 566These options are: 567.Cm auto 568and 569.Cm autoask . 570The latter requires confirmation like the 571.Cm ask 572option. 573.It Cm ControlPath 574Specify the path to the control socket used for connection sharing as described 575in the 576.Cm ControlMaster 577section above or the string 578.Cm none 579to disable connection sharing. 580Arguments to 581.Cm ControlPath 582may use the tilde syntax to refer to a user's home directory, 583the tokens described in the 584.Sx TOKENS 585section and environment variables as described in the 586.Sx ENVIRONMENT VARIABLES 587section. 588It is recommended that any 589.Cm ControlPath 590used for opportunistic connection sharing include 591at least %h, %p, and %r (or alternatively %C) and be placed in a directory 592that is not writable by other users. 593This ensures that shared connections are uniquely identified. 594.It Cm ControlPersist 595When used in conjunction with 596.Cm ControlMaster , 597specifies that the master connection should remain open 598in the background (waiting for future client connections) 599after the initial client connection has been closed. 600If set to 601.Cm no 602(the default), 603then the master connection will not be placed into the background, 604and will close as soon as the initial client connection is closed. 605If set to 606.Cm yes 607or 0, 608then the master connection will remain in the background indefinitely 609(until killed or closed via a mechanism such as the 610.Qq ssh -O exit ) . 611If set to a time in seconds, or a time in any of the formats documented in 612.Xr sshd_config 5 , 613then the backgrounded master connection will automatically terminate 614after it has remained idle (with no client connections) for the 615specified time. 616.It Cm DynamicForward 617Specifies that a TCP port on the local machine be forwarded 618over the secure channel, and the application 619protocol is then used to determine where to connect to from the 620remote machine. 621.Pp 622The argument must be 623.Sm off 624.Oo Ar bind_address : Oc Ar port . 625.Sm on 626IPv6 addresses can be specified by enclosing addresses in square brackets. 627By default, the local port is bound in accordance with the 628.Cm GatewayPorts 629setting. 630However, an explicit 631.Ar bind_address 632may be used to bind the connection to a specific address. 633The 634.Ar bind_address 635of 636.Cm localhost 637indicates that the listening port be bound for local use only, while an 638empty address or 639.Sq * 640indicates that the port should be available from all interfaces. 641.Pp 642Currently the SOCKS4 and SOCKS5 protocols are supported, and 643.Xr ssh 1 644will act as a SOCKS server. 645Multiple forwardings may be specified, and 646additional forwardings can be given on the command line. 647Only the superuser can forward privileged ports. 648.It Cm EnableEscapeCommandline 649Enables the command line option in the 650.Cm EscapeChar 651menu for interactive sessions (default 652.Ql ~C ) . 653By default, the command line is disabled. 654.It Cm EnableSSHKeysign 655Setting this option to 656.Cm yes 657in the global client configuration file 658.Pa /etc/ssh/ssh_config 659enables the use of the helper program 660.Xr ssh-keysign 8 661during 662.Cm HostbasedAuthentication . 663The argument must be 664.Cm yes 665or 666.Cm no 667(the default). 668This option should be placed in the non-hostspecific section. 669See 670.Xr ssh-keysign 8 671for more information. 672.It Cm EscapeChar 673Sets the escape character (default: 674.Ql ~ ) . 675The escape character can also 676be set on the command line. 677The argument should be a single character, 678.Ql ^ 679followed by a letter, or 680.Cm none 681to disable the escape 682character entirely (making the connection transparent for binary 683data). 684.It Cm ExitOnForwardFailure 685Specifies whether 686.Xr ssh 1 687should terminate the connection if it cannot set up all requested 688dynamic, tunnel, local, and remote port forwardings, (e.g.\& 689if either end is unable to bind and listen on a specified port). 690Note that 691.Cm ExitOnForwardFailure 692does not apply to connections made over port forwardings and will not, 693for example, cause 694.Xr ssh 1 695to exit if TCP connections to the ultimate forwarding destination fail. 696The argument must be 697.Cm yes 698or 699.Cm no 700(the default). 701.It Cm FingerprintHash 702Specifies the hash algorithm used when displaying key fingerprints. 703Valid options are: 704.Cm md5 705and 706.Cm sha256 707(the default). 708.It Cm ForkAfterAuthentication 709Requests 710.Nm ssh 711to go to background just before command execution. 712This is useful if 713.Nm ssh 714is going to ask for passwords or passphrases, but the user 715wants it in the background. 716This implies the 717.Cm StdinNull 718configuration option being set to 719.Dq yes . 720The recommended way to start X11 programs at a remote site is with 721something like 722.Ic ssh -f host xterm , 723which is the same as 724.Ic ssh host xterm 725if the 726.Cm ForkAfterAuthentication 727configuration option is set to 728.Dq yes . 729.Pp 730If the 731.Cm ExitOnForwardFailure 732configuration option is set to 733.Dq yes , 734then a client started with the 735.Cm ForkAfterAuthentication 736configuration option being set to 737.Dq yes 738will wait for all remote port forwards to be successfully established 739before placing itself in the background. 740The argument to this keyword must be 741.Cm yes 742(same as the 743.Fl f 744option) or 745.Cm no 746(the default). 747.It Cm ForwardAgent 748Specifies whether the connection to the authentication agent (if any) 749will be forwarded to the remote machine. 750The argument may be 751.Cm yes , 752.Cm no 753(the default), 754an explicit path to an agent socket or the name of an environment variable 755(beginning with 756.Sq $ ) 757in which to find the path. 758.Pp 759Agent forwarding should be enabled with caution. 760Users with the ability to bypass file permissions on the remote host 761(for the agent's Unix-domain socket) 762can access the local agent through the forwarded connection. 763An attacker cannot obtain key material from the agent, 764however they can perform operations on the keys that enable them to 765authenticate using the identities loaded into the agent. 766.It Cm ForwardX11 767Specifies whether X11 connections will be automatically redirected 768over the secure channel and 769.Ev DISPLAY 770set. 771The argument must be 772.Cm yes 773or 774.Cm no 775(the default). 776.Pp 777X11 forwarding should be enabled with caution. 778Users with the ability to bypass file permissions on the remote host 779(for the user's X11 authorization database) 780can access the local X11 display through the forwarded connection. 781An attacker may then be able to perform activities such as keystroke monitoring 782if the 783.Cm ForwardX11Trusted 784option is also enabled. 785.It Cm ForwardX11Timeout 786Specify a timeout for untrusted X11 forwarding 787using the format described in the 788.Sx TIME FORMATS 789section of 790.Xr sshd_config 5 . 791X11 connections received by 792.Xr ssh 1 793after this time will be refused. 794Setting 795.Cm ForwardX11Timeout 796to zero will disable the timeout and permit X11 forwarding for the life 797of the connection. 798The default is to disable untrusted X11 forwarding after twenty minutes has 799elapsed. 800.It Cm ForwardX11Trusted 801If this option is set to 802.Cm yes , 803remote X11 clients will have full access to the original X11 display. 804.Pp 805If this option is set to 806.Cm no 807(the default), 808remote X11 clients will be considered untrusted and prevented 809from stealing or tampering with data belonging to trusted X11 810clients. 811Furthermore, the 812.Xr xauth 1 813token used for the session will be set to expire after 20 minutes. 814Remote clients will be refused access after this time. 815.Pp 816See the X11 SECURITY extension specification for full details on 817the restrictions imposed on untrusted clients. 818.It Cm GatewayPorts 819Specifies whether remote hosts are allowed to connect to local 820forwarded ports. 821By default, 822.Xr ssh 1 823binds local port forwardings to the loopback address. 824This prevents other remote hosts from connecting to forwarded ports. 825.Cm GatewayPorts 826can be used to specify that ssh 827should bind local port forwardings to the wildcard address, 828thus allowing remote hosts to connect to forwarded ports. 829The argument must be 830.Cm yes 831or 832.Cm no 833(the default). 834.It Cm GlobalKnownHostsFile 835Specifies one or more files to use for the global 836host key database, separated by whitespace. 837The default is 838.Pa /etc/ssh/ssh_known_hosts , 839.Pa /etc/ssh/ssh_known_hosts2 . 840.It Cm GSSAPIAuthentication 841Specifies whether user authentication based on GSSAPI is allowed. 842The default is 843.Cm no . 844.It Cm GSSAPIDelegateCredentials 845Forward (delegate) credentials to the server. 846The default is 847.Cm no . 848.It Cm HashKnownHosts 849Indicates that 850.Xr ssh 1 851should hash host names and addresses when they are added to 852.Pa ~/.ssh/known_hosts . 853These hashed names may be used normally by 854.Xr ssh 1 855and 856.Xr sshd 8 , 857but they do not visually reveal identifying information if the 858file's contents are disclosed. 859The default is 860.Cm no . 861Note that existing names and addresses in known hosts files 862will not be converted automatically, 863but may be manually hashed using 864.Xr ssh-keygen 1 . 865.It Cm HostbasedAcceptedAlgorithms 866Specifies the signature algorithms that will be used for hostbased 867authentication as a comma-separated list of patterns. 868Alternately if the specified list begins with a 869.Sq + 870character, then the specified signature algorithms will be appended 871to the default set instead of replacing them. 872If the specified list begins with a 873.Sq - 874character, then the specified signature algorithms (including wildcards) 875will be removed from the default set instead of replacing them. 876If the specified list begins with a 877.Sq ^ 878character, then the specified signature algorithms will be placed 879at the head of the default set. 880The default for this option is: 881.Bd -literal -offset 3n 882ssh-ed25519-cert-v01@openssh.com, 883ecdsa-sha2-nistp256-cert-v01@openssh.com, 884ecdsa-sha2-nistp384-cert-v01@openssh.com, 885ecdsa-sha2-nistp521-cert-v01@openssh.com, 886sk-ssh-ed25519-cert-v01@openssh.com, 887sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 888rsa-sha2-512-cert-v01@openssh.com, 889rsa-sha2-256-cert-v01@openssh.com, 890ssh-ed25519, 891ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 892sk-ssh-ed25519@openssh.com, 893sk-ecdsa-sha2-nistp256@openssh.com, 894rsa-sha2-512,rsa-sha2-256 895.Ed 896.Pp 897The 898.Fl Q 899option of 900.Xr ssh 1 901may be used to list supported signature algorithms. 902This was formerly named HostbasedKeyTypes. 903.It Cm HostbasedAuthentication 904Specifies whether to try rhosts based authentication with public key 905authentication. 906The argument must be 907.Cm yes 908or 909.Cm no 910(the default). 911.It Cm HostKeyAlgorithms 912Specifies the host key signature algorithms 913that the client wants to use in order of preference. 914Alternately if the specified list begins with a 915.Sq + 916character, then the specified signature algorithms will be appended to 917the default set instead of replacing them. 918If the specified list begins with a 919.Sq - 920character, then the specified signature algorithms (including wildcards) 921will be removed from the default set instead of replacing them. 922If the specified list begins with a 923.Sq ^ 924character, then the specified signature algorithms will be placed 925at the head of the default set. 926The default for this option is: 927.Bd -literal -offset 3n 928ssh-ed25519-cert-v01@openssh.com, 929ecdsa-sha2-nistp256-cert-v01@openssh.com, 930ecdsa-sha2-nistp384-cert-v01@openssh.com, 931ecdsa-sha2-nistp521-cert-v01@openssh.com, 932sk-ssh-ed25519-cert-v01@openssh.com, 933sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 934rsa-sha2-512-cert-v01@openssh.com, 935rsa-sha2-256-cert-v01@openssh.com, 936ssh-ed25519, 937ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 938sk-ecdsa-sha2-nistp256@openssh.com, 939sk-ssh-ed25519@openssh.com, 940rsa-sha2-512,rsa-sha2-256 941.Ed 942.Pp 943If hostkeys are known for the destination host then this default is modified 944to prefer their algorithms. 945.Pp 946The list of available signature algorithms may also be obtained using 947.Qq ssh -Q HostKeyAlgorithms . 948.It Cm HostKeyAlias 949Specifies an alias that should be used instead of the 950real host name when looking up or saving the host key 951in the host key database files and when validating host certificates. 952This option is useful for tunneling SSH connections 953or for multiple servers running on a single host. 954.It Cm Hostname 955Specifies the real host name to log into. 956This can be used to specify nicknames or abbreviations for hosts. 957Arguments to 958.Cm Hostname 959accept the tokens described in the 960.Sx TOKENS 961section. 962Numeric IP addresses are also permitted (both on the command line and in 963.Cm Hostname 964specifications). 965The default is the name given on the command line. 966.It Cm IdentitiesOnly 967Specifies that 968.Xr ssh 1 969should only use the configured authentication identity and certificate files 970(either the default files, or those explicitly configured in the 971.Nm 972files 973or passed on the 974.Xr ssh 1 975command-line), 976even if 977.Xr ssh-agent 1 978or a 979.Cm PKCS11Provider 980or 981.Cm SecurityKeyProvider 982offers more identities. 983The argument to this keyword must be 984.Cm yes 985or 986.Cm no 987(the default). 988This option is intended for situations where ssh-agent 989offers many different identities. 990.It Cm IdentityAgent 991Specifies the 992.Ux Ns -domain 993socket used to communicate with the authentication agent. 994.Pp 995This option overrides the 996.Ev SSH_AUTH_SOCK 997environment variable and can be used to select a specific agent. 998Setting the socket name to 999.Cm none 1000disables the use of an authentication agent. 1001If the string 1002.Qq SSH_AUTH_SOCK 1003is specified, the location of the socket will be read from the 1004.Ev SSH_AUTH_SOCK 1005environment variable. 1006Otherwise if the specified value begins with a 1007.Sq $ 1008character, then it will be treated as an environment variable containing 1009the location of the socket. 1010.Pp 1011Arguments to 1012.Cm IdentityAgent 1013may use the tilde syntax to refer to a user's home directory, 1014the tokens described in the 1015.Sx TOKENS 1016section and environment variables as described in the 1017.Sx ENVIRONMENT VARIABLES 1018section. 1019.It Cm IdentityFile 1020Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA, 1021Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read. 1022You can also specify a public key file to use the corresponding 1023private key that is loaded in 1024.Xr ssh-agent 1 1025when the private key file is not present locally. 1026The default is 1027.Pa ~/.ssh/id_rsa , 1028.Pa ~/.ssh/id_ecdsa , 1029.Pa ~/.ssh/id_ecdsa_sk , 1030.Pa ~/.ssh/id_ed25519 , 1031.Pa ~/.ssh/id_ed25519_sk 1032and 1033.Pa ~/.ssh/id_dsa . 1034Additionally, any identities represented by the authentication agent 1035will be used for authentication unless 1036.Cm IdentitiesOnly 1037is set. 1038If no certificates have been explicitly specified by 1039.Cm CertificateFile , 1040.Xr ssh 1 1041will try to load certificate information from the filename obtained by 1042appending 1043.Pa -cert.pub 1044to the path of a specified 1045.Cm IdentityFile . 1046.Pp 1047Arguments to 1048.Cm IdentityFile 1049may use the tilde syntax to refer to a user's home directory 1050or the tokens described in the 1051.Sx TOKENS 1052section. 1053.Pp 1054It is possible to have 1055multiple identity files specified in configuration files; all these 1056identities will be tried in sequence. 1057Multiple 1058.Cm IdentityFile 1059directives will add to the list of identities tried (this behaviour 1060differs from that of other configuration directives). 1061.Pp 1062.Cm IdentityFile 1063may be used in conjunction with 1064.Cm IdentitiesOnly 1065to select which identities in an agent are offered during authentication. 1066.Cm IdentityFile 1067may also be used in conjunction with 1068.Cm CertificateFile 1069in order to provide any certificate also needed for authentication with 1070the identity. 1071.It Cm IgnoreUnknown 1072Specifies a pattern-list of unknown options to be ignored if they are 1073encountered in configuration parsing. 1074This may be used to suppress errors if 1075.Nm 1076contains options that are unrecognised by 1077.Xr ssh 1 . 1078It is recommended that 1079.Cm IgnoreUnknown 1080be listed early in the configuration file as it will not be applied 1081to unknown options that appear before it. 1082.It Cm Include 1083Include the specified configuration file(s). 1084Multiple pathnames may be specified and each pathname may contain 1085.Xr glob 7 1086wildcards and, for user configurations, shell-like 1087.Sq ~ 1088references to user home directories. 1089Wildcards will be expanded and processed in lexical order. 1090Files without absolute paths are assumed to be in 1091.Pa ~/.ssh 1092if included in a user configuration file or 1093.Pa /etc/ssh 1094if included from the system configuration file. 1095.Cm Include 1096directive may appear inside a 1097.Cm Match 1098or 1099.Cm Host 1100block 1101to perform conditional inclusion. 1102.It Cm IPQoS 1103Specifies the IPv4 type-of-service or DSCP class for connections. 1104Accepted values are 1105.Cm af11 , 1106.Cm af12 , 1107.Cm af13 , 1108.Cm af21 , 1109.Cm af22 , 1110.Cm af23 , 1111.Cm af31 , 1112.Cm af32 , 1113.Cm af33 , 1114.Cm af41 , 1115.Cm af42 , 1116.Cm af43 , 1117.Cm cs0 , 1118.Cm cs1 , 1119.Cm cs2 , 1120.Cm cs3 , 1121.Cm cs4 , 1122.Cm cs5 , 1123.Cm cs6 , 1124.Cm cs7 , 1125.Cm ef , 1126.Cm le , 1127.Cm lowdelay , 1128.Cm throughput , 1129.Cm reliability , 1130a numeric value, or 1131.Cm none 1132to use the operating system default. 1133This option may take one or two arguments, separated by whitespace. 1134If one argument is specified, it is used as the packet class unconditionally. 1135If two values are specified, the first is automatically selected for 1136interactive sessions and the second for non-interactive sessions. 1137The default is 1138.Cm af21 1139(Low-Latency Data) 1140for interactive sessions and 1141.Cm cs1 1142(Lower Effort) 1143for non-interactive sessions. 1144.It Cm KbdInteractiveAuthentication 1145Specifies whether to use keyboard-interactive authentication. 1146The argument to this keyword must be 1147.Cm yes 1148(the default) 1149or 1150.Cm no . 1151.Cm ChallengeResponseAuthentication 1152is a deprecated alias for this. 1153.It Cm KbdInteractiveDevices 1154Specifies the list of methods to use in keyboard-interactive authentication. 1155Multiple method names must be comma-separated. 1156The default is to use the server specified list. 1157The methods available vary depending on what the server supports. 1158For an OpenSSH server, 1159it may be zero or more of: 1160.Cm bsdauth 1161and 1162.Cm pam . 1163.It Cm KexAlgorithms 1164Specifies the available KEX (Key Exchange) algorithms. 1165Multiple algorithms must be comma-separated. 1166If the specified list begins with a 1167.Sq + 1168character, then the specified algorithms will be appended to the default set 1169instead of replacing them. 1170If the specified list begins with a 1171.Sq - 1172character, then the specified algorithms (including wildcards) will be removed 1173from the default set instead of replacing them. 1174If the specified list begins with a 1175.Sq ^ 1176character, then the specified algorithms will be placed at the head of the 1177default set. 1178The default is: 1179.Bd -literal -offset indent 1180sntrup761x25519-sha512@openssh.com, 1181curve25519-sha256,curve25519-sha256@libssh.org, 1182ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1183diffie-hellman-group-exchange-sha256, 1184diffie-hellman-group16-sha512, 1185diffie-hellman-group18-sha512, 1186diffie-hellman-group14-sha256 1187.Ed 1188.Pp 1189The list of available key exchange algorithms may also be obtained using 1190.Qq ssh -Q kex . 1191.It Cm KnownHostsCommand 1192Specifies a command to use to obtain a list of host keys, in addition to 1193those listed in 1194.Cm UserKnownHostsFile 1195and 1196.Cm GlobalKnownHostsFile . 1197This command is executed after the files have been read. 1198It may write host key lines to standard output in identical format to the 1199usual files (described in the 1200.Sx VERIFYING HOST KEYS 1201section in 1202.Xr ssh 1 ) . 1203Arguments to 1204.Cm KnownHostsCommand 1205accept the tokens described in the 1206.Sx TOKENS 1207section. 1208The command may be invoked multiple times per connection: once when preparing 1209the preference list of host key algorithms to use, again to obtain the 1210host key for the requested host name and, if 1211.Cm CheckHostIP 1212is enabled, one more time to obtain the host key matching the server's 1213address. 1214If the command exits abnormally or returns a non-zero exit status then the 1215connection is terminated. 1216.It Cm LocalCommand 1217Specifies a command to execute on the local machine after successfully 1218connecting to the server. 1219The command string extends to the end of the line, and is executed with 1220the user's shell. 1221Arguments to 1222.Cm LocalCommand 1223accept the tokens described in the 1224.Sx TOKENS 1225section. 1226.Pp 1227The command is run synchronously and does not have access to the 1228session of the 1229.Xr ssh 1 1230that spawned it. 1231It should not be used for interactive commands. 1232.Pp 1233This directive is ignored unless 1234.Cm PermitLocalCommand 1235has been enabled. 1236.It Cm LocalForward 1237Specifies that a TCP port on the local machine be forwarded over 1238the secure channel to the specified host and port from the remote machine. 1239The first argument specifies the listener and may be 1240.Sm off 1241.Oo Ar bind_address : Oc Ar port 1242.Sm on 1243or a Unix domain socket path. 1244The second argument is the destination and may be 1245.Ar host : Ns Ar hostport 1246or a Unix domain socket path if the remote host supports it. 1247.Pp 1248IPv6 addresses can be specified by enclosing addresses in square brackets. 1249Multiple forwardings may be specified, and additional forwardings can be 1250given on the command line. 1251Only the superuser can forward privileged ports. 1252By default, the local port is bound in accordance with the 1253.Cm GatewayPorts 1254setting. 1255However, an explicit 1256.Ar bind_address 1257may be used to bind the connection to a specific address. 1258The 1259.Ar bind_address 1260of 1261.Cm localhost 1262indicates that the listening port be bound for local use only, while an 1263empty address or 1264.Sq * 1265indicates that the port should be available from all interfaces. 1266Unix domain socket paths may use the tokens described in the 1267.Sx TOKENS 1268section and environment variables as described in the 1269.Sx ENVIRONMENT VARIABLES 1270section. 1271.It Cm LogLevel 1272Gives the verbosity level that is used when logging messages from 1273.Xr ssh 1 . 1274The possible values are: 1275QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1276The default is INFO. 1277DEBUG and DEBUG1 are equivalent. 1278DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1279.It Cm LogVerbose 1280Specify one or more overrides to LogLevel. 1281An override consists of a pattern lists that matches the source file, function 1282and line number to force detailed logging for. 1283For example, an override pattern of: 1284.Bd -literal -offset indent 1285kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1286.Ed 1287.Pp 1288would enable detailed logging for line 1000 of 1289.Pa kex.c , 1290everything in the 1291.Fn kex_exchange_identification 1292function, and all code in the 1293.Pa packet.c 1294file. 1295This option is intended for debugging and no overrides are enabled by default. 1296.It Cm MACs 1297Specifies the MAC (message authentication code) algorithms 1298in order of preference. 1299The MAC algorithm is used for data integrity protection. 1300Multiple algorithms must be comma-separated. 1301If the specified list begins with a 1302.Sq + 1303character, then the specified algorithms will be appended to the default set 1304instead of replacing them. 1305If the specified list begins with a 1306.Sq - 1307character, then the specified algorithms (including wildcards) will be removed 1308from the default set instead of replacing them. 1309If the specified list begins with a 1310.Sq ^ 1311character, then the specified algorithms will be placed at the head of the 1312default set. 1313.Pp 1314The algorithms that contain 1315.Qq -etm 1316calculate the MAC after encryption (encrypt-then-mac). 1317These are considered safer and their use recommended. 1318.Pp 1319The default is: 1320.Bd -literal -offset indent 1321umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1322hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1323hmac-sha1-etm@openssh.com, 1324umac-64@openssh.com,umac-128@openssh.com, 1325hmac-sha2-256,hmac-sha2-512,hmac-sha1 1326.Ed 1327.Pp 1328The list of available MAC algorithms may also be obtained using 1329.Qq ssh -Q mac . 1330.It Cm NoHostAuthenticationForLocalhost 1331Disable host authentication for localhost (loopback addresses). 1332The argument to this keyword must be 1333.Cm yes 1334or 1335.Cm no 1336(the default). 1337.It Cm NumberOfPasswordPrompts 1338Specifies the number of password prompts before giving up. 1339The argument to this keyword must be an integer. 1340The default is 3. 1341.It Cm PasswordAuthentication 1342Specifies whether to use password authentication. 1343The argument to this keyword must be 1344.Cm yes 1345(the default) 1346or 1347.Cm no . 1348.It Cm PermitLocalCommand 1349Allow local command execution via the 1350.Ic LocalCommand 1351option or using the 1352.Ic !\& Ns Ar command 1353escape sequence in 1354.Xr ssh 1 . 1355The argument must be 1356.Cm yes 1357or 1358.Cm no 1359(the default). 1360.It Cm PermitRemoteOpen 1361Specifies the destinations to which remote TCP port forwarding is permitted when 1362.Cm RemoteForward 1363is used as a SOCKS proxy. 1364The forwarding specification must be one of the following forms: 1365.Pp 1366.Bl -item -offset indent -compact 1367.It 1368.Cm PermitRemoteOpen 1369.Sm off 1370.Ar host : port 1371.Sm on 1372.It 1373.Cm PermitRemoteOpen 1374.Sm off 1375.Ar IPv4_addr : port 1376.Sm on 1377.It 1378.Cm PermitRemoteOpen 1379.Sm off 1380.Ar \&[ IPv6_addr \&] : port 1381.Sm on 1382.El 1383.Pp 1384Multiple forwards may be specified by separating them with whitespace. 1385An argument of 1386.Cm any 1387can be used to remove all restrictions and permit any forwarding requests. 1388An argument of 1389.Cm none 1390can be used to prohibit all forwarding requests. 1391The wildcard 1392.Sq * 1393can be used for host or port to allow all hosts or ports respectively. 1394Otherwise, no pattern matching or address lookups are performed on supplied 1395names. 1396.It Cm PKCS11Provider 1397Specifies which PKCS#11 provider to use or 1398.Cm none 1399to indicate that no provider should be used (the default). 1400The argument to this keyword is a path to the PKCS#11 shared library 1401.Xr ssh 1 1402should use to communicate with a PKCS#11 token providing keys for user 1403authentication. 1404.It Cm Port 1405Specifies the port number to connect on the remote host. 1406The default is 22. 1407.It Cm PreferredAuthentications 1408Specifies the order in which the client should try authentication methods. 1409This allows a client to prefer one method (e.g.\& 1410.Cm keyboard-interactive ) 1411over another method (e.g.\& 1412.Cm password ) . 1413The default is: 1414.Bd -literal -offset indent 1415gssapi-with-mic,hostbased,publickey, 1416keyboard-interactive,password 1417.Ed 1418.It Cm ProxyCommand 1419Specifies the command to use to connect to the server. 1420The command 1421string extends to the end of the line, and is executed 1422using the user's shell 1423.Ql exec 1424directive to avoid a lingering shell process. 1425.Pp 1426Arguments to 1427.Cm ProxyCommand 1428accept the tokens described in the 1429.Sx TOKENS 1430section. 1431The command can be basically anything, 1432and should read from its standard input and write to its standard output. 1433It should eventually connect an 1434.Xr sshd 8 1435server running on some machine, or execute 1436.Ic sshd -i 1437somewhere. 1438Host key management will be done using the 1439.Cm Hostname 1440of the host being connected (defaulting to the name typed by the user). 1441Setting the command to 1442.Cm none 1443disables this option entirely. 1444Note that 1445.Cm CheckHostIP 1446is not available for connects with a proxy command. 1447.Pp 1448This directive is useful in conjunction with 1449.Xr nc 1 1450and its proxy support. 1451For example, the following directive would connect via an HTTP proxy at 1452192.0.2.0: 1453.Bd -literal -offset 3n 1454ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1455.Ed 1456.It Cm ProxyJump 1457Specifies one or more jump proxies as either 1458.Xo 1459.Sm off 1460.Op Ar user No @ 1461.Ar host 1462.Op : Ns Ar port 1463.Sm on 1464or an ssh URI 1465.Xc . 1466Multiple proxies may be separated by comma characters and will be visited 1467sequentially. 1468Setting this option will cause 1469.Xr ssh 1 1470to connect to the target host by first making a 1471.Xr ssh 1 1472connection to the specified 1473.Cm ProxyJump 1474host and then establishing a 1475TCP forwarding to the ultimate target from there. 1476Setting the host to 1477.Cm none 1478disables this option entirely. 1479.Pp 1480Note that this option will compete with the 1481.Cm ProxyCommand 1482option - whichever is specified first will prevent later instances of the 1483other from taking effect. 1484.Pp 1485Note also that the configuration for the destination host (either supplied 1486via the command-line or the configuration file) is not generally applied 1487to jump hosts. 1488.Pa ~/.ssh/config 1489should be used if specific configuration is required for jump hosts. 1490.It Cm ProxyUseFdpass 1491Specifies that 1492.Cm ProxyCommand 1493will pass a connected file descriptor back to 1494.Xr ssh 1 1495instead of continuing to execute and pass data. 1496The default is 1497.Cm no . 1498.It Cm PubkeyAcceptedAlgorithms 1499Specifies the signature algorithms that will be used for public key 1500authentication as a comma-separated list of patterns. 1501If the specified list begins with a 1502.Sq + 1503character, then the algorithms after it will be appended to the default 1504instead of replacing it. 1505If the specified list begins with a 1506.Sq - 1507character, then the specified algorithms (including wildcards) will be removed 1508from the default set instead of replacing them. 1509If the specified list begins with a 1510.Sq ^ 1511character, then the specified algorithms will be placed at the head of the 1512default set. 1513The default for this option is: 1514.Bd -literal -offset 3n 1515ssh-ed25519-cert-v01@openssh.com, 1516ecdsa-sha2-nistp256-cert-v01@openssh.com, 1517ecdsa-sha2-nistp384-cert-v01@openssh.com, 1518ecdsa-sha2-nistp521-cert-v01@openssh.com, 1519sk-ssh-ed25519-cert-v01@openssh.com, 1520sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1521rsa-sha2-512-cert-v01@openssh.com, 1522rsa-sha2-256-cert-v01@openssh.com, 1523ssh-ed25519, 1524ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1525sk-ssh-ed25519@openssh.com, 1526sk-ecdsa-sha2-nistp256@openssh.com, 1527rsa-sha2-512,rsa-sha2-256 1528.Ed 1529.Pp 1530The list of available signature algorithms may also be obtained using 1531.Qq ssh -Q PubkeyAcceptedAlgorithms . 1532.It Cm PubkeyAuthentication 1533Specifies whether to try public key authentication. 1534The argument to this keyword must be 1535.Cm yes 1536(the default), 1537.Cm no , 1538.Cm unbound 1539or 1540.Cm host-bound . 1541The final two options enable public key authentication while respectively 1542disabling or enabling the OpenSSH host-bound authentication protocol 1543extension required for restricted 1544.Xr ssh-agent 1 1545forwarding. 1546.It Cm RekeyLimit 1547Specifies the maximum amount of data that may be transmitted or received 1548before the session key is renegotiated, optionally followed by a maximum 1549amount of time that may pass before the session key is renegotiated. 1550The first argument is specified in bytes and may have a suffix of 1551.Sq K , 1552.Sq M , 1553or 1554.Sq G 1555to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1556The default is between 1557.Sq 1G 1558and 1559.Sq 4G , 1560depending on the cipher. 1561The optional second value is specified in seconds and may use any of the 1562units documented in the TIME FORMATS section of 1563.Xr sshd_config 5 . 1564The default value for 1565.Cm RekeyLimit 1566is 1567.Cm default none , 1568which means that rekeying is performed after the cipher's default amount 1569of data has been sent or received and no time based rekeying is done. 1570.It Cm RemoteCommand 1571Specifies a command to execute on the remote machine after successfully 1572connecting to the server. 1573The command string extends to the end of the line, and is executed with 1574the user's shell. 1575Arguments to 1576.Cm RemoteCommand 1577accept the tokens described in the 1578.Sx TOKENS 1579section. 1580.It Cm RemoteForward 1581Specifies that a TCP port on the remote machine be forwarded over 1582the secure channel. 1583The remote port may either be forwarded to a specified host and port 1584from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote 1585client to connect to arbitrary destinations from the local machine. 1586The first argument is the listening specification and may be 1587.Sm off 1588.Oo Ar bind_address : Oc Ar port 1589.Sm on 1590or, if the remote host supports it, a Unix domain socket path. 1591If forwarding to a specific destination then the second argument must be 1592.Ar host : Ns Ar hostport 1593or a Unix domain socket path, 1594otherwise if no destination argument is specified then the remote forwarding 1595will be established as a SOCKS proxy. 1596When acting as a SOCKS proxy, the destination of the connection can be 1597restricted by 1598.Cm PermitRemoteOpen . 1599.Pp 1600IPv6 addresses can be specified by enclosing addresses in square brackets. 1601Multiple forwardings may be specified, and additional 1602forwardings can be given on the command line. 1603Privileged ports can be forwarded only when 1604logging in as root on the remote machine. 1605Unix domain socket paths may use the tokens described in the 1606.Sx TOKENS 1607section and environment variables as described in the 1608.Sx ENVIRONMENT VARIABLES 1609section. 1610.Pp 1611If the 1612.Ar port 1613argument is 0, 1614the listen port will be dynamically allocated on the server and reported 1615to the client at run time. 1616.Pp 1617If the 1618.Ar bind_address 1619is not specified, the default is to only bind to loopback addresses. 1620If the 1621.Ar bind_address 1622is 1623.Ql * 1624or an empty string, then the forwarding is requested to listen on all 1625interfaces. 1626Specifying a remote 1627.Ar bind_address 1628will only succeed if the server's 1629.Cm GatewayPorts 1630option is enabled (see 1631.Xr sshd_config 5 ) . 1632.It Cm RequestTTY 1633Specifies whether to request a pseudo-tty for the session. 1634The argument may be one of: 1635.Cm no 1636(never request a TTY), 1637.Cm yes 1638(always request a TTY when standard input is a TTY), 1639.Cm force 1640(always request a TTY) or 1641.Cm auto 1642(request a TTY when opening a login session). 1643This option mirrors the 1644.Fl t 1645and 1646.Fl T 1647flags for 1648.Xr ssh 1 . 1649.It Cm RequiredRSASize 1650Specifies the minimum RSA key size (in bits) that 1651.Xr ssh 1 1652will accept. 1653User authentication keys smaller than this limit will be ignored. 1654Servers that present host keys smaller than this limit will cause the 1655connection to be terminated. 1656The default is 1657.Cm 1024 1658bits. 1659Note that this limit may only be raised from the default. 1660.It Cm RevokedHostKeys 1661Specifies revoked host public keys. 1662Keys listed in this file will be refused for host authentication. 1663Note that if this file does not exist or is not readable, 1664then host authentication will be refused for all hosts. 1665Keys may be specified as a text file, listing one public key per line, or as 1666an OpenSSH Key Revocation List (KRL) as generated by 1667.Xr ssh-keygen 1 . 1668For more information on KRLs, see the KEY REVOCATION LISTS section in 1669.Xr ssh-keygen 1 . 1670.It Cm SecurityKeyProvider 1671Specifies a path to a library that will be used when loading any 1672FIDO authenticator-hosted keys, overriding the default of using 1673the built-in USB HID support. 1674.Pp 1675If the specified value begins with a 1676.Sq $ 1677character, then it will be treated as an environment variable containing 1678the path to the library. 1679.It Cm SendEnv 1680Specifies what variables from the local 1681.Xr environ 7 1682should be sent to the server. 1683The server must also support it, and the server must be configured to 1684accept these environment variables. 1685Note that the 1686.Ev TERM 1687environment variable is always sent whenever a 1688pseudo-terminal is requested as it is required by the protocol. 1689Refer to 1690.Cm AcceptEnv 1691in 1692.Xr sshd_config 5 1693for how to configure the server. 1694Variables are specified by name, which may contain wildcard characters. 1695Multiple environment variables may be separated by whitespace or spread 1696across multiple 1697.Cm SendEnv 1698directives. 1699.Pp 1700See 1701.Sx PATTERNS 1702for more information on patterns. 1703.Pp 1704It is possible to clear previously set 1705.Cm SendEnv 1706variable names by prefixing patterns with 1707.Pa - . 1708The default is not to send any environment variables. 1709.It Cm ServerAliveCountMax 1710Sets the number of server alive messages (see below) which may be 1711sent without 1712.Xr ssh 1 1713receiving any messages back from the server. 1714If this threshold is reached while server alive messages are being sent, 1715ssh will disconnect from the server, terminating the session. 1716It is important to note that the use of server alive messages is very 1717different from 1718.Cm TCPKeepAlive 1719(below). 1720The server alive messages are sent through the encrypted channel 1721and therefore will not be spoofable. 1722The TCP keepalive option enabled by 1723.Cm TCPKeepAlive 1724is spoofable. 1725The server alive mechanism is valuable when the client or 1726server depend on knowing when a connection has become unresponsive. 1727.Pp 1728The default value is 3. 1729If, for example, 1730.Cm ServerAliveInterval 1731(see below) is set to 15 and 1732.Cm ServerAliveCountMax 1733is left at the default, if the server becomes unresponsive, 1734ssh will disconnect after approximately 45 seconds. 1735.It Cm ServerAliveInterval 1736Sets a timeout interval in seconds after which if no data has been received 1737from the server, 1738.Xr ssh 1 1739will send a message through the encrypted 1740channel to request a response from the server. 1741The default 1742is 0, indicating that these messages will not be sent to the server. 1743.It Cm SessionType 1744May be used to either request invocation of a subsystem on the remote system, 1745or to prevent the execution of a remote command at all. 1746The latter is useful for just forwarding ports. 1747The argument to this keyword must be 1748.Cm none 1749(same as the 1750.Fl N 1751option), 1752.Cm subsystem 1753(same as the 1754.Fl s 1755option) or 1756.Cm default 1757(shell or command execution). 1758.It Cm SetEnv 1759Directly specify one or more environment variables and their contents to 1760be sent to the server. 1761Similarly to 1762.Cm SendEnv , 1763with the exception of the 1764.Ev TERM 1765variable, the server must be prepared to accept the environment variable. 1766.It Cm StdinNull 1767Redirects stdin from 1768.Pa /dev/null 1769(actually, prevents reading from stdin). 1770Either this or the equivalent 1771.Fl n 1772option must be used when 1773.Nm ssh 1774is run in the background. 1775The argument to this keyword must be 1776.Cm yes 1777(same as the 1778.Fl n 1779option) or 1780.Cm no 1781(the default). 1782.It Cm StreamLocalBindMask 1783Sets the octal file creation mode mask 1784.Pq umask 1785used when creating a Unix-domain socket file for local or remote 1786port forwarding. 1787This option is only used for port forwarding to a Unix-domain socket file. 1788.Pp 1789The default value is 0177, which creates a Unix-domain socket file that is 1790readable and writable only by the owner. 1791Note that not all operating systems honor the file mode on Unix-domain 1792socket files. 1793.It Cm StreamLocalBindUnlink 1794Specifies whether to remove an existing Unix-domain socket file for local 1795or remote port forwarding before creating a new one. 1796If the socket file already exists and 1797.Cm StreamLocalBindUnlink 1798is not enabled, 1799.Nm ssh 1800will be unable to forward the port to the Unix-domain socket file. 1801This option is only used for port forwarding to a Unix-domain socket file. 1802.Pp 1803The argument must be 1804.Cm yes 1805or 1806.Cm no 1807(the default). 1808.It Cm StrictHostKeyChecking 1809If this flag is set to 1810.Cm yes , 1811.Xr ssh 1 1812will never automatically add host keys to the 1813.Pa ~/.ssh/known_hosts 1814file, and refuses to connect to hosts whose host key has changed. 1815This provides maximum protection against man-in-the-middle (MITM) attacks, 1816though it can be annoying when the 1817.Pa /etc/ssh/ssh_known_hosts 1818file is poorly maintained or when connections to new hosts are 1819frequently made. 1820This option forces the user to manually 1821add all new hosts. 1822.Pp 1823If this flag is set to 1824.Cm accept-new 1825then ssh will automatically add new host keys to the user's 1826.Pa known_hosts 1827file, but will not permit connections to hosts with 1828changed host keys. 1829If this flag is set to 1830.Cm no 1831or 1832.Cm off , 1833ssh will automatically add new host keys to the user known hosts files 1834and allow connections to hosts with changed hostkeys to proceed, 1835subject to some restrictions. 1836If this flag is set to 1837.Cm ask 1838(the default), 1839new host keys 1840will be added to the user known host files only after the user 1841has confirmed that is what they really want to do, and 1842ssh will refuse to connect to hosts whose host key has changed. 1843The host keys of 1844known hosts will be verified automatically in all cases. 1845.It Cm SyslogFacility 1846Gives the facility code that is used when logging messages from 1847.Xr ssh 1 . 1848The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1849LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1850The default is USER. 1851.It Cm TCPKeepAlive 1852Specifies whether the system should send TCP keepalive messages to the 1853other side. 1854If they are sent, death of the connection or crash of one 1855of the machines will be properly noticed. 1856However, this means that 1857connections will die if the route is down temporarily, and some people 1858find it annoying. 1859.Pp 1860The default is 1861.Cm yes 1862(to send TCP keepalive messages), and the client will notice 1863if the network goes down or the remote host dies. 1864This is important in scripts, and many users want it too. 1865.Pp 1866To disable TCP keepalive messages, the value should be set to 1867.Cm no . 1868See also 1869.Cm ServerAliveInterval 1870for protocol-level keepalives. 1871.It Cm Tunnel 1872Request 1873.Xr tun 4 1874device forwarding between the client and the server. 1875The argument must be 1876.Cm yes , 1877.Cm point-to-point 1878(layer 3), 1879.Cm ethernet 1880(layer 2), 1881or 1882.Cm no 1883(the default). 1884Specifying 1885.Cm yes 1886requests the default tunnel mode, which is 1887.Cm point-to-point . 1888.It Cm TunnelDevice 1889Specifies the 1890.Xr tun 4 1891devices to open on the client 1892.Pq Ar local_tun 1893and the server 1894.Pq Ar remote_tun . 1895.Pp 1896The argument must be 1897.Sm off 1898.Ar local_tun Op : Ar remote_tun . 1899.Sm on 1900The devices may be specified by numerical ID or the keyword 1901.Cm any , 1902which uses the next available tunnel device. 1903If 1904.Ar remote_tun 1905is not specified, it defaults to 1906.Cm any . 1907The default is 1908.Cm any:any . 1909.It Cm UpdateHostKeys 1910Specifies whether 1911.Xr ssh 1 1912should accept notifications of additional hostkeys from the server sent 1913after authentication has completed and add them to 1914.Cm UserKnownHostsFile . 1915The argument must be 1916.Cm yes , 1917.Cm no 1918or 1919.Cm ask . 1920This option allows learning alternate hostkeys for a server 1921and supports graceful key rotation by allowing a server to send replacement 1922public keys before old ones are removed. 1923.Pp 1924Additional hostkeys are only accepted if the key used to authenticate the 1925host was already trusted or explicitly accepted by the user, the host was 1926authenticated via 1927.Cm UserKnownHostsFile 1928(i.e. not 1929.Cm GlobalKnownHostsFile ) 1930and the host was authenticated using a plain key and not a certificate. 1931.Pp 1932.Cm UpdateHostKeys 1933is enabled by default if the user has not overridden the default 1934.Cm UserKnownHostsFile 1935setting and has not enabled 1936.Cm VerifyHostKeyDNS , 1937otherwise 1938.Cm UpdateHostKeys 1939will be set to 1940.Cm no . 1941.Pp 1942If 1943.Cm UpdateHostKeys 1944is set to 1945.Cm ask , 1946then the user is asked to confirm the modifications to the known_hosts file. 1947Confirmation is currently incompatible with 1948.Cm ControlPersist , 1949and will be disabled if it is enabled. 1950.Pp 1951Presently, only 1952.Xr sshd 8 1953from OpenSSH 6.8 and greater support the 1954.Qq hostkeys@openssh.com 1955protocol extension used to inform the client of all the server's hostkeys. 1956.It Cm User 1957Specifies the user to log in as. 1958This can be useful when a different user name is used on different machines. 1959This saves the trouble of 1960having to remember to give the user name on the command line. 1961.It Cm UserKnownHostsFile 1962Specifies one or more files to use for the user 1963host key database, separated by whitespace. 1964Each filename may use tilde notation to refer to the user's home directory, 1965the tokens described in the 1966.Sx TOKENS 1967section and environment variables as described in the 1968.Sx ENVIRONMENT VARIABLES 1969section. 1970A value of 1971.Cm none 1972causes 1973.Xr ssh 1 1974to ignore any user-specific known hosts files. 1975The default is 1976.Pa ~/.ssh/known_hosts , 1977.Pa ~/.ssh/known_hosts2 . 1978.It Cm VerifyHostKeyDNS 1979Specifies whether to verify the remote key using DNS and SSHFP resource 1980records. 1981If this option is set to 1982.Cm yes , 1983the client will implicitly trust keys that match a secure fingerprint 1984from DNS. 1985Insecure fingerprints will be handled as if this option was set to 1986.Cm ask . 1987If this option is set to 1988.Cm ask , 1989information on fingerprint match will be displayed, but the user will still 1990need to confirm new host keys according to the 1991.Cm StrictHostKeyChecking 1992option. 1993The default is 1994.Cm no . 1995.Pp 1996See also 1997.Sx VERIFYING HOST KEYS 1998in 1999.Xr ssh 1 . 2000.It Cm VisualHostKey 2001If this flag is set to 2002.Cm yes , 2003an ASCII art representation of the remote host key fingerprint is 2004printed in addition to the fingerprint string at login and 2005for unknown host keys. 2006If this flag is set to 2007.Cm no 2008(the default), 2009no fingerprint strings are printed at login and 2010only the fingerprint string will be printed for unknown host keys. 2011.It Cm XAuthLocation 2012Specifies the full pathname of the 2013.Xr xauth 1 2014program. 2015The default is 2016.Pa /usr/local/bin/xauth . 2017.El 2018.Sh PATTERNS 2019A 2020.Em pattern 2021consists of zero or more non-whitespace characters, 2022.Sq * 2023(a wildcard that matches zero or more characters), 2024or 2025.Sq ?\& 2026(a wildcard that matches exactly one character). 2027For example, to specify a set of declarations for any host in the 2028.Qq .co.uk 2029set of domains, 2030the following pattern could be used: 2031.Pp 2032.Dl Host *.co.uk 2033.Pp 2034The following pattern 2035would match any host in the 192.168.0.[0-9] network range: 2036.Pp 2037.Dl Host 192.168.0.? 2038.Pp 2039A 2040.Em pattern-list 2041is a comma-separated list of patterns. 2042Patterns within pattern-lists may be negated 2043by preceding them with an exclamation mark 2044.Pq Sq !\& . 2045For example, 2046to allow a key to be used from anywhere within an organization 2047except from the 2048.Qq dialup 2049pool, 2050the following entry (in authorized_keys) could be used: 2051.Pp 2052.Dl from=\&"!*.dialup.example.com,*.example.com\&" 2053.Pp 2054Note that a negated match will never produce a positive result by itself. 2055For example, attempting to match 2056.Qq host3 2057against the following pattern-list will fail: 2058.Pp 2059.Dl from=\&"!host1,!host2\&" 2060.Pp 2061The solution here is to include a term that will yield a positive match, 2062such as a wildcard: 2063.Pp 2064.Dl from=\&"!host1,!host2,*\&" 2065.Sh TOKENS 2066Arguments to some keywords can make use of tokens, 2067which are expanded at runtime: 2068.Pp 2069.Bl -tag -width XXXX -offset indent -compact 2070.It %% 2071A literal 2072.Sq % . 2073.It \&%C 2074Hash of %l%h%p%r. 2075.It %d 2076Local user's home directory. 2077.It %f 2078The fingerprint of the server's host key. 2079.It %H 2080The 2081.Pa known_hosts 2082hostname or address that is being searched for. 2083.It %h 2084The remote hostname. 2085.It \%%I 2086A string describing the reason for a 2087.Cm KnownHostsCommand 2088execution: either 2089.Cm ADDRESS 2090when looking up a host by address (only when 2091.Cm CheckHostIP 2092is enabled), 2093.Cm HOSTNAME 2094when searching by hostname, or 2095.Cm ORDER 2096when preparing the host key algorithm preference list to use for the 2097destination host. 2098.It %i 2099The local user ID. 2100.It %K 2101The base64 encoded host key. 2102.It %k 2103The host key alias if specified, otherwise the original remote hostname given 2104on the command line. 2105.It %L 2106The local hostname. 2107.It %l 2108The local hostname, including the domain name. 2109.It %n 2110The original remote hostname, as given on the command line. 2111.It %p 2112The remote port. 2113.It %r 2114The remote username. 2115.It \&%T 2116The local 2117.Xr tun 4 2118or 2119.Xr tap 4 2120network interface assigned if 2121tunnel forwarding was requested, or 2122.Qq NONE 2123otherwise. 2124.It %t 2125The type of the server host key, e.g. 2126.Cm ssh-ed25519 . 2127.It %u 2128The local username. 2129.El 2130.Pp 2131.Cm CertificateFile , 2132.Cm ControlPath , 2133.Cm IdentityAgent , 2134.Cm IdentityFile , 2135.Cm KnownHostsCommand , 2136.Cm LocalForward , 2137.Cm Match exec , 2138.Cm RemoteCommand , 2139.Cm RemoteForward , 2140and 2141.Cm UserKnownHostsFile 2142accept the tokens %%, %C, %d, %h, %i, %k, %L, %l, %n, %p, %r, and %u. 2143.Pp 2144.Cm KnownHostsCommand 2145additionally accepts the tokens %f, %H, %I, %K and %t. 2146.Pp 2147.Cm Hostname 2148accepts the tokens %% and %h. 2149.Pp 2150.Cm LocalCommand 2151accepts all tokens. 2152.Pp 2153.Cm ProxyCommand 2154and 2155.Cm ProxyJump 2156accept the tokens %%, %h, %n, %p, and %r. 2157.Sh ENVIRONMENT VARIABLES 2158Arguments to some keywords can be expanded at runtime from environment 2159variables on the client by enclosing them in 2160.Ic ${} , 2161for example 2162.Ic ${HOME}/.ssh 2163would refer to the user's .ssh directory. 2164If a specified environment variable does not exist then an error will be 2165returned and the setting for that keyword will be ignored. 2166.Pp 2167The keywords 2168.Cm CertificateFile , 2169.Cm ControlPath , 2170.Cm IdentityAgent , 2171.Cm IdentityFile , 2172.Cm KnownHostsCommand , 2173and 2174.Cm UserKnownHostsFile 2175support environment variables. 2176The keywords 2177.Cm LocalForward 2178and 2179.Cm RemoteForward 2180support environment variables only for Unix domain socket paths. 2181.Sh FILES 2182.Bl -tag -width Ds 2183.It Pa ~/.ssh/config 2184This is the per-user configuration file. 2185The format of this file is described above. 2186This file is used by the SSH client. 2187Because of the potential for abuse, this file must have strict permissions: 2188read/write for the user, and not writable by others. 2189.It Pa /etc/ssh/ssh_config 2190Systemwide configuration file. 2191This file provides defaults for those 2192values that are not specified in the user's configuration file, and 2193for those users who do not have a configuration file. 2194This file must be world-readable. 2195.El 2196.Sh SEE ALSO 2197.Xr ssh 1 2198.Sh AUTHORS 2199.An -nosplit 2200OpenSSH is a derivative of the original and free 2201ssh 1.2.12 release by 2202.An Tatu Ylonen . 2203.An Aaron Campbell , Bob Beck , Markus Friedl , 2204.An Niels Provos , Theo de Raadt 2205and 2206.An Dug Song 2207removed many bugs, re-added newer features and 2208created OpenSSH. 2209.An Markus Friedl 2210contributed the support for SSH protocol versions 1.5 and 2.0. 2211