1.\" $NetBSD: ssh_config.5,v 1.35 2022/10/05 22:39:36 christos Exp $ 2.\" -*- nroff -*- 3.\" 4.\" Author: Tatu Ylonen <ylo@cs.hut.fi> 5.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland 6.\" All rights reserved 7.\" 8.\" As far as I am concerned, the code I have written for this software 9.\" can be used freely for any purpose. Any derived versions of this 10.\" software must be clearly marked as such, and if the derived work is 11.\" incompatible with the protocol description in the RFC file, it must be 12.\" called by a name other than "ssh" or "Secure Shell". 13.\" 14.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. 15.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. 16.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. 17.\" 18.\" Redistribution and use in source and binary forms, with or without 19.\" modification, are permitted provided that the following conditions 20.\" are met: 21.\" 1. Redistributions of source code must retain the above copyright 22.\" notice, this list of conditions and the following disclaimer. 23.\" 2. Redistributions in binary form must reproduce the above copyright 24.\" notice, this list of conditions and the following disclaimer in the 25.\" documentation and/or other materials provided with the distribution. 26.\" 27.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 28.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 29.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 30.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 31.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 32.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 33.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 34.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 35.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 36.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 37.\" 38.\" $OpenBSD: ssh_config.5,v 1.374 2022/09/17 10:33:18 djm Exp $ 39.Dd September 17 2022 40.Dt SSH_CONFIG 5 41.Os 42.Sh NAME 43.Nm ssh_config 44.Nd OpenSSH client configuration file 45.Sh DESCRIPTION 46.Xr ssh 1 47obtains configuration data from the following sources in 48the following order: 49.Pp 50.Bl -enum -offset indent -compact 51.It 52command-line options 53.It 54user's configuration file 55.Pq Pa ~/.ssh/config 56.It 57system-wide configuration file 58.Pq Pa /etc/ssh/ssh_config 59.El 60.Pp 61For each parameter, the first obtained value 62will be used. 63The configuration files contain sections separated by 64.Cm Host 65specifications, and that section is only applied for hosts that 66match one of the patterns given in the specification. 67The matched host name is usually the one given on the command line 68(see the 69.Cm CanonicalizeHostname 70option for exceptions). 71.Pp 72Since the first obtained value for each parameter is used, more 73host-specific declarations should be given near the beginning of the 74file, and general defaults at the end. 75.Pp 76The file contains keyword-argument pairs, one per line. 77Lines starting with 78.Ql # 79and empty lines are interpreted as comments. 80Arguments may optionally be enclosed in double quotes 81.Pq \&" 82in order to represent arguments containing spaces. 83Configuration options may be separated by whitespace or 84optional whitespace and exactly one 85.Ql = ; 86the latter format is useful to avoid the need to quote whitespace 87when specifying configuration options using the 88.Nm ssh , 89.Nm scp , 90and 91.Nm sftp 92.Fl o 93option. 94.Pp 95The possible 96keywords and their meanings are as follows (note that 97keywords are case-insensitive and arguments are case-sensitive): 98.Bl -tag -width Ds 99.It Cm Host 100Restricts the following declarations (up to the next 101.Cm Host 102or 103.Cm Match 104keyword) to be only for those hosts that match one of the patterns 105given after the keyword. 106If more than one pattern is provided, they should be separated by whitespace. 107A single 108.Ql * 109as a pattern can be used to provide global 110defaults for all hosts. 111The host is usually the 112.Ar hostname 113argument given on the command line 114(see the 115.Cm CanonicalizeHostname 116keyword for exceptions). 117.Pp 118A pattern entry may be negated by prefixing it with an exclamation mark 119.Pq Sq !\& . 120If a negated entry is matched, then the 121.Cm Host 122entry is ignored, regardless of whether any other patterns on the line 123match. 124Negated matches are therefore useful to provide exceptions for wildcard 125matches. 126.Pp 127See 128.Sx PATTERNS 129for more information on patterns. 130.It Cm Match 131Restricts the following declarations (up to the next 132.Cm Host 133or 134.Cm Match 135keyword) to be used only when the conditions following the 136.Cm Match 137keyword are satisfied. 138Match conditions are specified using one or more criteria 139or the single token 140.Cm all 141which always matches. 142The available criteria keywords are: 143.Cm canonical , 144.Cm final , 145.Cm exec , 146.Cm host , 147.Cm originalhost , 148.Cm user , 149and 150.Cm localuser . 151The 152.Cm all 153criteria must appear alone or immediately after 154.Cm canonical 155or 156.Cm final . 157Other criteria may be combined arbitrarily. 158All criteria but 159.Cm all , 160.Cm canonical , 161and 162.Cm final 163require an argument. 164Criteria may be negated by prepending an exclamation mark 165.Pq Sq !\& . 166.Pp 167The 168.Cm canonical 169keyword matches only when the configuration file is being re-parsed 170after hostname canonicalization (see the 171.Cm CanonicalizeHostname 172option). 173This may be useful to specify conditions that work with canonical host 174names only. 175.Pp 176The 177.Cm final 178keyword requests that the configuration be re-parsed (regardless of whether 179.Cm CanonicalizeHostname 180is enabled), and matches only during this final pass. 181If 182.Cm CanonicalizeHostname 183is enabled, then 184.Cm canonical 185and 186.Cm final 187match during the same pass. 188.Pp 189The 190.Cm exec 191keyword executes the specified command under the user's shell. 192If the command returns a zero exit status then the condition is considered true. 193Commands containing whitespace characters must be quoted. 194Arguments to 195.Cm exec 196accept the tokens described in the 197.Sx TOKENS 198section. 199.Pp 200The other keywords' criteria must be single entries or comma-separated 201lists and may use the wildcard and negation operators described in the 202.Sx PATTERNS 203section. 204The criteria for the 205.Cm host 206keyword are matched against the target hostname, after any substitution 207by the 208.Cm Hostname 209or 210.Cm CanonicalizeHostname 211options. 212The 213.Cm originalhost 214keyword matches against the hostname as it was specified on the command-line. 215The 216.Cm user 217keyword matches against the target username on the remote host. 218The 219.Cm localuser 220keyword matches against the name of the local user running 221.Xr ssh 1 222(this keyword may be useful in system-wide 223.Nm 224files). 225.It Cm AddKeysToAgent 226Specifies whether keys should be automatically added to a running 227.Xr ssh-agent 1 . 228If this option is set to 229.Cm yes 230and a key is loaded from a file, the key and its passphrase are added to 231the agent with the default lifetime, as if by 232.Xr ssh-add 1 . 233If this option is set to 234.Cm ask , 235.Xr ssh 1 236will require confirmation using the 237.Ev SSH_ASKPASS 238program before adding a key (see 239.Xr ssh-add 1 240for details). 241If this option is set to 242.Cm confirm , 243each use of the key must be confirmed, as if the 244.Fl c 245option was specified to 246.Xr ssh-add 1 . 247If this option is set to 248.Cm no , 249no keys are added to the agent. 250Alternately, this option may be specified as a time interval 251using the format described in the 252.Sx TIME FORMATS 253section of 254.Xr sshd_config 5 255to specify the key's lifetime in 256.Xr ssh-agent 1 , 257after which it will automatically be removed. 258The argument must be 259.Cm no 260(the default), 261.Cm yes , 262.Cm confirm 263(optionally followed by a time interval), 264.Cm ask 265or a time interval. 266.It Cm AddressFamily 267Specifies which address family to use when connecting. 268Valid arguments are 269.Cm any 270(the default), 271.Cm inet 272(use IPv4 only), or 273.Cm inet6 274(use IPv6 only). 275.It Cm BatchMode 276If set to 277.Cm yes , 278user interaction such as password prompts and host key confirmation requests 279will be disabled. 280This option is useful in scripts and other batch jobs where no user 281is present to interact with 282.Xr ssh 1 . 283The argument must be 284.Cm yes 285or 286.Cm no 287(the default). 288.It Cm BindAddress 289Use the specified address on the local machine as the source address of 290the connection. 291Only useful on systems with more than one address. 292.It Cm BindInterface 293Use the address of the specified interface on the local machine as the 294source address of the connection. 295.It Cm CanonicalDomains 296When 297.Cm CanonicalizeHostname 298is enabled, this option specifies the list of domain suffixes in which to 299search for the specified destination host. 300.It Cm CanonicalizeFallbackLocal 301Specifies whether to fail with an error when hostname canonicalization fails. 302The default, 303.Cm yes , 304will attempt to look up the unqualified hostname using the system resolver's 305search rules. 306A value of 307.Cm no 308will cause 309.Xr ssh 1 310to fail instantly if 311.Cm CanonicalizeHostname 312is enabled and the target hostname cannot be found in any of the domains 313specified by 314.Cm CanonicalDomains . 315.It Cm CanonicalizeHostname 316Controls whether explicit hostname canonicalization is performed. 317The default, 318.Cm no , 319is not to perform any name rewriting and let the system resolver handle all 320hostname lookups. 321If set to 322.Cm yes 323then, for connections that do not use a 324.Cm ProxyCommand 325or 326.Cm ProxyJump , 327.Xr ssh 1 328will attempt to canonicalize the hostname specified on the command line 329using the 330.Cm CanonicalDomains 331suffixes and 332.Cm CanonicalizePermittedCNAMEs 333rules. 334If 335.Cm CanonicalizeHostname 336is set to 337.Cm always , 338then canonicalization is applied to proxied connections too. 339.Pp 340If this option is enabled, then the configuration files are processed 341again using the new target name to pick up any new configuration in matching 342.Cm Host 343and 344.Cm Match 345stanzas. 346A value of 347.Cm none 348disables the use of a 349.Cm ProxyJump 350host. 351.It Cm CanonicalizeMaxDots 352Specifies the maximum number of dot characters in a hostname before 353canonicalization is disabled. 354The default, 1, 355allows a single dot (i.e. hostname.subdomain). 356.It Cm CanonicalizePermittedCNAMEs 357Specifies rules to determine whether CNAMEs should be followed when 358canonicalizing hostnames. 359The rules consist of one or more arguments of 360.Ar source_domain_list : Ns Ar target_domain_list , 361where 362.Ar source_domain_list 363is a pattern-list of domains that may follow CNAMEs in canonicalization, 364and 365.Ar target_domain_list 366is a pattern-list of domains that they may resolve to. 367.Pp 368For example, 369.Qq *.a.example.com:*.b.example.com,*.c.example.com 370will allow hostnames matching 371.Qq *.a.example.com 372to be canonicalized to names in the 373.Qq *.b.example.com 374or 375.Qq *.c.example.com 376domains. 377.Pp 378A single argument of 379.Qq none 380causes no CNAMEs to be considered for canonicalization. 381This is the default behaviour. 382.It Cm CASignatureAlgorithms 383Specifies which algorithms are allowed for signing of certificates 384by certificate authorities (CAs). 385The default is: 386.Bd -literal -offset indent 387ssh-ed25519,ecdsa-sha2-nistp256, 388ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 389sk-ssh-ed25519@openssh.com, 390sk-ecdsa-sha2-nistp256@openssh.com, 391rsa-sha2-512,rsa-sha2-256 392.Ed 393.Pp 394If the specified list begins with a 395.Sq + 396character, then the specified algorithms will be appended to the default set 397instead of replacing them. 398If the specified list begins with a 399.Sq - 400character, then the specified algorithms (including wildcards) will be removed 401from the default set instead of replacing them. 402.Pp 403.Xr ssh 1 404will not accept host certificates signed using algorithms other than those 405specified. 406.It Cm CertificateFile 407Specifies a file from which the user's certificate is read. 408A corresponding private key must be provided separately in order 409to use this certificate either 410from an 411.Cm IdentityFile 412directive or 413.Fl i 414flag to 415.Xr ssh 1 , 416via 417.Xr ssh-agent 1 , 418or via a 419.Cm PKCS11Provider 420or 421.Cm SecurityKeyProvider . 422.Pp 423Arguments to 424.Cm CertificateFile 425may use the tilde syntax to refer to a user's home directory, 426the tokens described in the 427.Sx TOKENS 428section and environment variables as described in the 429.Sx ENVIRONMENT VARIABLES 430section. 431.Pp 432It is possible to have multiple certificate files specified in 433configuration files; these certificates will be tried in sequence. 434Multiple 435.Cm CertificateFile 436directives will add to the list of certificates used for 437authentication. 438.It Cm CheckHostIP 439If set to 440.Cm yes , 441.Xr ssh 1 442will additionally check the host IP address in the 443.Pa known_hosts 444file. 445This allows it to detect if a host key changed due to DNS spoofing 446and will add addresses of destination hosts to 447.Pa ~/.ssh/known_hosts 448in the process, regardless of the setting of 449.Cm StrictHostKeyChecking . 450If the option is set to 451.Cm no 452(the default), 453the check will not be executed. 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 EnableSSHKeysign 649Setting this option to 650.Cm yes 651in the global client configuration file 652.Pa /etc/ssh/ssh_config 653enables the use of the helper program 654.Xr ssh-keysign 8 655during 656.Cm HostbasedAuthentication . 657The argument must be 658.Cm yes 659or 660.Cm no 661(the default). 662This option should be placed in the non-hostspecific section. 663See 664.Xr ssh-keysign 8 665for more information. 666.It Cm EscapeChar 667Sets the escape character (default: 668.Ql ~ ) . 669The escape character can also 670be set on the command line. 671The argument should be a single character, 672.Ql ^ 673followed by a letter, or 674.Cm none 675to disable the escape 676character entirely (making the connection transparent for binary 677data). 678.It Cm ExitOnForwardFailure 679Specifies whether 680.Xr ssh 1 681should terminate the connection if it cannot set up all requested 682dynamic, tunnel, local, and remote port forwardings, (e.g.\& 683if either end is unable to bind and listen on a specified port). 684Note that 685.Cm ExitOnForwardFailure 686does not apply to connections made over port forwardings and will not, 687for example, cause 688.Xr ssh 1 689to exit if TCP connections to the ultimate forwarding destination fail. 690The argument must be 691.Cm yes 692or 693.Cm no 694(the default). 695.It Cm FingerprintHash 696Specifies the hash algorithm used when displaying key fingerprints. 697Valid options are: 698.Cm md5 699and 700.Cm sha256 701(the default). 702.It Cm ForkAfterAuthentication 703Requests 704.Nm ssh 705to go to background just before command execution. 706This is useful if 707.Nm ssh 708is going to ask for passwords or passphrases, but the user 709wants it in the background. 710This implies the 711.Cm StdinNull 712configuration option being set to 713.Dq yes . 714The recommended way to start X11 programs at a remote site is with 715something like 716.Ic ssh -f host xterm , 717which is the same as 718.Ic ssh host xterm 719if the 720.Cm ForkAfterAuthentication 721configuration option is set to 722.Dq yes . 723.Pp 724If the 725.Cm ExitOnForwardFailure 726configuration option is set to 727.Dq yes , 728then a client started with the 729.Cm ForkAfterAuthentication 730configuration option being set to 731.Dq yes 732will wait for all remote port forwards to be successfully established 733before placing itself in the background. 734The argument to this keyword must be 735.Cm yes 736(same as the 737.Fl f 738option) or 739.Cm no 740(the default). 741.It Cm ForwardAgent 742Specifies whether the connection to the authentication agent (if any) 743will be forwarded to the remote machine. 744The argument may be 745.Cm yes , 746.Cm no 747(the default), 748an explicit path to an agent socket or the name of an environment variable 749(beginning with 750.Sq $ ) 751in which to find the path. 752.Pp 753Agent forwarding should be enabled with caution. 754Users with the ability to bypass file permissions on the remote host 755(for the agent's Unix-domain socket) 756can access the local agent through the forwarded connection. 757An attacker cannot obtain key material from the agent, 758however they can perform operations on the keys that enable them to 759authenticate using the identities loaded into the agent. 760.It Cm ForwardX11 761Specifies whether X11 connections will be automatically redirected 762over the secure channel and 763.Ev DISPLAY 764set. 765The argument must be 766.Cm yes 767or 768.Cm no 769(the default). 770.Pp 771X11 forwarding should be enabled with caution. 772Users with the ability to bypass file permissions on the remote host 773(for the user's X11 authorization database) 774can access the local X11 display through the forwarded connection. 775An attacker may then be able to perform activities such as keystroke monitoring 776if the 777.Cm ForwardX11Trusted 778option is also enabled. 779.It Cm ForwardX11Timeout 780Specify a timeout for untrusted X11 forwarding 781using the format described in the 782.Sx TIME FORMATS 783section of 784.Xr sshd_config 5 . 785X11 connections received by 786.Xr ssh 1 787after this time will be refused. 788Setting 789.Cm ForwardX11Timeout 790to zero will disable the timeout and permit X11 forwarding for the life 791of the connection. 792The default is to disable untrusted X11 forwarding after twenty minutes has 793elapsed. 794.It Cm ForwardX11Trusted 795If this option is set to 796.Cm yes , 797remote X11 clients will have full access to the original X11 display. 798.Pp 799If this option is set to 800.Cm no 801(the default), 802remote X11 clients will be considered untrusted and prevented 803from stealing or tampering with data belonging to trusted X11 804clients. 805Furthermore, the 806.Xr xauth 1 807token used for the session will be set to expire after 20 minutes. 808Remote clients will be refused access after this time. 809.Pp 810See the X11 SECURITY extension specification for full details on 811the restrictions imposed on untrusted clients. 812.It Cm GatewayPorts 813Specifies whether remote hosts are allowed to connect to local 814forwarded ports. 815By default, 816.Xr ssh 1 817binds local port forwardings to the loopback address. 818This prevents other remote hosts from connecting to forwarded ports. 819.Cm GatewayPorts 820can be used to specify that ssh 821should bind local port forwardings to the wildcard address, 822thus allowing remote hosts to connect to forwarded ports. 823The argument must be 824.Cm yes 825or 826.Cm no 827(the default). 828.It Cm GlobalKnownHostsFile 829Specifies one or more files to use for the global 830host key database, separated by whitespace. 831The default is 832.Pa /etc/ssh/ssh_known_hosts , 833.Pa /etc/ssh/ssh_known_hosts2 . 834.It Cm GSSAPIAuthentication 835Specifies whether user authentication based on GSSAPI is allowed. 836The default is 837.Cm no . 838.It Cm GSSAPIDelegateCredentials 839Forward (delegate) credentials to the server. 840The default is 841.Cm no . 842.It Cm HashKnownHosts 843Indicates that 844.Xr ssh 1 845should hash host names and addresses when they are added to 846.Pa ~/.ssh/known_hosts . 847These hashed names may be used normally by 848.Xr ssh 1 849and 850.Xr sshd 8 , 851but they do not visually reveal identifying information if the 852file's contents are disclosed. 853The default is 854.Cm no . 855Note that existing names and addresses in known hosts files 856will not be converted automatically, 857but may be manually hashed using 858.Xr ssh-keygen 1 . 859.It Cm HostbasedAcceptedAlgorithms 860Specifies the signature algorithms that will be used for hostbased 861authentication as a comma-separated list of patterns. 862Alternately if the specified list begins with a 863.Sq + 864character, then the specified signature algorithms will be appended 865to the default set instead of replacing them. 866If the specified list begins with a 867.Sq - 868character, then the specified signature algorithms (including wildcards) 869will be removed from the default set instead of replacing them. 870If the specified list begins with a 871.Sq ^ 872character, then the specified signature algorithms will be placed 873at the head of the default set. 874The default for this option is: 875.Bd -literal -offset 3n 876ssh-ed25519-cert-v01@openssh.com, 877ecdsa-sha2-nistp256-cert-v01@openssh.com, 878ecdsa-sha2-nistp384-cert-v01@openssh.com, 879ecdsa-sha2-nistp521-cert-v01@openssh.com, 880sk-ssh-ed25519-cert-v01@openssh.com, 881sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 882rsa-sha2-512-cert-v01@openssh.com, 883rsa-sha2-256-cert-v01@openssh.com, 884ssh-ed25519, 885ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 886sk-ssh-ed25519@openssh.com, 887sk-ecdsa-sha2-nistp256@openssh.com, 888rsa-sha2-512,rsa-sha2-256 889.Ed 890.Pp 891The 892.Fl Q 893option of 894.Xr ssh 1 895may be used to list supported signature algorithms. 896This was formerly named HostbasedKeyTypes. 897.It Cm HostbasedAuthentication 898Specifies whether to try rhosts based authentication with public key 899authentication. 900The argument must be 901.Cm yes 902or 903.Cm no 904(the default). 905.It Cm HostKeyAlgorithms 906Specifies the host key signature algorithms 907that the client wants to use in order of preference. 908Alternately if the specified list begins with a 909.Sq + 910character, then the specified signature algorithms will be appended to 911the default set instead of replacing them. 912If the specified list begins with a 913.Sq - 914character, then the specified signature algorithms (including wildcards) 915will be removed from the default set instead of replacing them. 916If the specified list begins with a 917.Sq ^ 918character, then the specified signature algorithms will be placed 919at the head of the default set. 920The default for this option is: 921.Bd -literal -offset 3n 922ssh-ed25519-cert-v01@openssh.com, 923ecdsa-sha2-nistp256-cert-v01@openssh.com, 924ecdsa-sha2-nistp384-cert-v01@openssh.com, 925ecdsa-sha2-nistp521-cert-v01@openssh.com, 926sk-ssh-ed25519-cert-v01@openssh.com, 927sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 928rsa-sha2-512-cert-v01@openssh.com, 929rsa-sha2-256-cert-v01@openssh.com, 930ssh-ed25519, 931ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 932sk-ecdsa-sha2-nistp256@openssh.com, 933sk-ssh-ed25519@openssh.com, 934rsa-sha2-512,rsa-sha2-256 935.Ed 936.Pp 937If hostkeys are known for the destination host then this default is modified 938to prefer their algorithms. 939.Pp 940The list of available signature algorithms may also be obtained using 941.Qq ssh -Q HostKeyAlgorithms . 942.It Cm HostKeyAlias 943Specifies an alias that should be used instead of the 944real host name when looking up or saving the host key 945in the host key database files and when validating host certificates. 946This option is useful for tunneling SSH connections 947or for multiple servers running on a single host. 948.It Cm Hostname 949Specifies the real host name to log into. 950This can be used to specify nicknames or abbreviations for hosts. 951Arguments to 952.Cm Hostname 953accept the tokens described in the 954.Sx TOKENS 955section. 956Numeric IP addresses are also permitted (both on the command line and in 957.Cm Hostname 958specifications). 959The default is the name given on the command line. 960.It Cm IdentitiesOnly 961Specifies that 962.Xr ssh 1 963should only use the configured authentication identity and certificate files 964(either the default files, or those explicitly configured in the 965.Nm 966files 967or passed on the 968.Xr ssh 1 969command-line), 970even if 971.Xr ssh-agent 1 972or a 973.Cm PKCS11Provider 974or 975.Cm SecurityKeyProvider 976offers more identities. 977The argument to this keyword must be 978.Cm yes 979or 980.Cm no 981(the default). 982This option is intended for situations where ssh-agent 983offers many different identities. 984.It Cm IdentityAgent 985Specifies the 986.Ux Ns -domain 987socket used to communicate with the authentication agent. 988.Pp 989This option overrides the 990.Ev SSH_AUTH_SOCK 991environment variable and can be used to select a specific agent. 992Setting the socket name to 993.Cm none 994disables the use of an authentication agent. 995If the string 996.Qq SSH_AUTH_SOCK 997is specified, the location of the socket will be read from the 998.Ev SSH_AUTH_SOCK 999environment variable. 1000Otherwise if the specified value begins with a 1001.Sq $ 1002character, then it will be treated as an environment variable containing 1003the location of the socket. 1004.Pp 1005Arguments to 1006.Cm IdentityAgent 1007may use the tilde syntax to refer to a user's home directory, 1008the tokens described in the 1009.Sx TOKENS 1010section and environment variables as described in the 1011.Sx ENVIRONMENT VARIABLES 1012section. 1013.It Cm IdentityFile 1014Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA, 1015Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read. 1016The default is 1017.Pa ~/.ssh/id_rsa , 1018.Pa ~/.ssh/id_ecdsa , 1019.Pa ~/.ssh/id_ecdsa_sk , 1020.Pa ~/.ssh/id_ed25519 , 1021.Pa ~/.ssh/id_ed25519_sk 1022and 1023.Pa ~/.ssh/id_dsa . 1024Additionally, any identities represented by the authentication agent 1025will be used for authentication unless 1026.Cm IdentitiesOnly 1027is set. 1028If no certificates have been explicitly specified by 1029.Cm CertificateFile , 1030.Xr ssh 1 1031will try to load certificate information from the filename obtained by 1032appending 1033.Pa -cert.pub 1034to the path of a specified 1035.Cm IdentityFile . 1036.Pp 1037Arguments to 1038.Cm IdentityFile 1039may use the tilde syntax to refer to a user's home directory 1040or the tokens described in the 1041.Sx TOKENS 1042section. 1043.Pp 1044It is possible to have 1045multiple identity files specified in configuration files; all these 1046identities will be tried in sequence. 1047Multiple 1048.Cm IdentityFile 1049directives will add to the list of identities tried (this behaviour 1050differs from that of other configuration directives). 1051.Pp 1052.Cm IdentityFile 1053may be used in conjunction with 1054.Cm IdentitiesOnly 1055to select which identities in an agent are offered during authentication. 1056.Cm IdentityFile 1057may also be used in conjunction with 1058.Cm CertificateFile 1059in order to provide any certificate also needed for authentication with 1060the identity. 1061.It Cm IgnoreUnknown 1062Specifies a pattern-list of unknown options to be ignored if they are 1063encountered in configuration parsing. 1064This may be used to suppress errors if 1065.Nm 1066contains options that are unrecognised by 1067.Xr ssh 1 . 1068It is recommended that 1069.Cm IgnoreUnknown 1070be listed early in the configuration file as it will not be applied 1071to unknown options that appear before it. 1072.It Cm Include 1073Include the specified configuration file(s). 1074Multiple pathnames may be specified and each pathname may contain 1075.Xr glob 7 1076wildcards and, for user configurations, shell-like 1077.Sq ~ 1078references to user home directories. 1079Wildcards will be expanded and processed in lexical order. 1080Files without absolute paths are assumed to be in 1081.Pa ~/.ssh 1082if included in a user configuration file or 1083.Pa /etc/ssh 1084if included from the system configuration file. 1085.Cm Include 1086directive may appear inside a 1087.Cm Match 1088or 1089.Cm Host 1090block 1091to perform conditional inclusion. 1092.It Cm IPQoS 1093Specifies the IPv4 type-of-service or DSCP class for connections. 1094Accepted values are 1095.Cm af11 , 1096.Cm af12 , 1097.Cm af13 , 1098.Cm af21 , 1099.Cm af22 , 1100.Cm af23 , 1101.Cm af31 , 1102.Cm af32 , 1103.Cm af33 , 1104.Cm af41 , 1105.Cm af42 , 1106.Cm af43 , 1107.Cm cs0 , 1108.Cm cs1 , 1109.Cm cs2 , 1110.Cm cs3 , 1111.Cm cs4 , 1112.Cm cs5 , 1113.Cm cs6 , 1114.Cm cs7 , 1115.Cm ef , 1116.Cm le , 1117.Cm lowdelay , 1118.Cm throughput , 1119.Cm reliability , 1120a numeric value, or 1121.Cm none 1122to use the operating system default. 1123This option may take one or two arguments, separated by whitespace. 1124If one argument is specified, it is used as the packet class unconditionally. 1125If two values are specified, the first is automatically selected for 1126interactive sessions and the second for non-interactive sessions. 1127The default is 1128.Cm af21 1129(Low-Latency Data) 1130for interactive sessions and 1131.Cm cs1 1132(Lower Effort) 1133for non-interactive sessions. 1134.It Cm KbdInteractiveAuthentication 1135Specifies whether to use keyboard-interactive authentication. 1136The argument to this keyword must be 1137.Cm yes 1138(the default) 1139or 1140.Cm no . 1141.Cm ChallengeResponseAuthentication 1142is a deprecated alias for this. 1143.It Cm KbdInteractiveDevices 1144Specifies the list of methods to use in keyboard-interactive authentication. 1145Multiple method names must be comma-separated. 1146The default is to use the server specified list. 1147The methods available vary depending on what the server supports. 1148For an OpenSSH server, 1149it may be zero or more of: 1150.Cm bsdauth , 1151.Cm pam , 1152and 1153.Cm skey . 1154.It Cm KexAlgorithms 1155Specifies the available KEX (Key Exchange) algorithms. 1156Multiple algorithms must be comma-separated. 1157If the specified list begins with a 1158.Sq + 1159character, then the specified algorithms will be appended to the default set 1160instead of replacing them. 1161If the specified list begins with a 1162.Sq - 1163character, then the specified algorithms (including wildcards) will be removed 1164from the default set instead of replacing them. 1165If the specified list begins with a 1166.Sq ^ 1167character, then the specified algorithms will be placed at the head of the 1168default set. 1169The default is: 1170.Bd -literal -offset indent 1171sntrup761x25519-sha512@openssh.com, 1172curve25519-sha256,curve25519-sha256@libssh.org, 1173ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1174diffie-hellman-group-exchange-sha256, 1175diffie-hellman-group16-sha512, 1176diffie-hellman-group18-sha512, 1177diffie-hellman-group14-sha256 1178.Ed 1179.Pp 1180The list of available key exchange algorithms may also be obtained using 1181.Qq ssh -Q kex . 1182.It Cm KnownHostsCommand 1183Specifies a command to use to obtain a list of host keys, in addition to 1184those listed in 1185.Cm UserKnownHostsFile 1186and 1187.Cm GlobalKnownHostsFile . 1188This command is executed after the files have been read. 1189It may write host key lines to standard output in identical format to the 1190usual files (described in the 1191.Sx VERIFYING HOST KEYS 1192section in 1193.Xr ssh 1 ) . 1194Arguments to 1195.Cm KnownHostsCommand 1196accept the tokens described in the 1197.Sx TOKENS 1198section. 1199The command may be invoked multiple times per connection: once when preparing 1200the preference list of host key algorithms to use, again to obtain the 1201host key for the requested host name and, if 1202.Cm CheckHostIP 1203is enabled, one more time to obtain the host key matching the server's 1204address. 1205If the command exits abnormally or returns a non-zero exit status then the 1206connection is terminated. 1207.It Cm LocalCommand 1208Specifies a command to execute on the local machine after successfully 1209connecting to the server. 1210The command string extends to the end of the line, and is executed with 1211the user's shell. 1212Arguments to 1213.Cm LocalCommand 1214accept the tokens described in the 1215.Sx TOKENS 1216section. 1217.Pp 1218The command is run synchronously and does not have access to the 1219session of the 1220.Xr ssh 1 1221that spawned it. 1222It should not be used for interactive commands. 1223.Pp 1224This directive is ignored unless 1225.Cm PermitLocalCommand 1226has been enabled. 1227.It Cm LocalForward 1228Specifies that a TCP port on the local machine be forwarded over 1229the secure channel to the specified host and port from the remote machine. 1230The first argument specifies the listener and may be 1231.Sm off 1232.Oo Ar bind_address : Oc Ar port 1233.Sm on 1234or a Unix domain socket path. 1235The second argument is the destination and may be 1236.Ar host : Ns Ar hostport 1237or a Unix domain socket path if the remote host supports it. 1238.Pp 1239IPv6 addresses can be specified by enclosing addresses in square brackets. 1240Multiple forwardings may be specified, and additional forwardings can be 1241given on the command line. 1242Only the superuser can forward privileged ports. 1243By default, the local port is bound in accordance with the 1244.Cm GatewayPorts 1245setting. 1246However, an explicit 1247.Ar bind_address 1248may be used to bind the connection to a specific address. 1249The 1250.Ar bind_address 1251of 1252.Cm localhost 1253indicates that the listening port be bound for local use only, while an 1254empty address or 1255.Sq * 1256indicates that the port should be available from all interfaces. 1257Unix domain socket paths may use the tokens described in the 1258.Sx TOKENS 1259section and environment variables as described in the 1260.Sx ENVIRONMENT VARIABLES 1261section. 1262.It Cm LogLevel 1263Gives the verbosity level that is used when logging messages from 1264.Xr ssh 1 . 1265The possible values are: 1266QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1267The default is INFO. 1268DEBUG and DEBUG1 are equivalent. 1269DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1270.It Cm LogVerbose 1271Specify one or more overrides to LogLevel. 1272An override consists of a pattern lists that matches the source file, function 1273and line number to force detailed logging for. 1274For example, an override pattern of: 1275.Bd -literal -offset indent 1276kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1277.Ed 1278.Pp 1279would enable detailed logging for line 1000 of 1280.Pa kex.c , 1281everything in the 1282.Fn kex_exchange_identification 1283function, and all code in the 1284.Pa packet.c 1285file. 1286This option is intended for debugging and no overrides are enabled by default. 1287.It Cm MACs 1288Specifies the MAC (message authentication code) algorithms 1289in order of preference. 1290The MAC algorithm is used for data integrity protection. 1291Multiple algorithms must be comma-separated. 1292If the specified list begins with a 1293.Sq + 1294character, then the specified algorithms will be appended to the default set 1295instead of replacing them. 1296If the specified list begins with a 1297.Sq - 1298character, then the specified algorithms (including wildcards) will be removed 1299from the default set instead of replacing them. 1300If the specified list begins with a 1301.Sq ^ 1302character, then the specified algorithms will be placed at the head of the 1303default set. 1304.Pp 1305The algorithms that contain 1306.Qq -etm 1307calculate the MAC after encryption (encrypt-then-mac). 1308These are considered safer and their use recommended. 1309.Pp 1310The default is: 1311.Bd -literal -offset indent 1312umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1313hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1314hmac-sha1-etm@openssh.com, 1315umac-64@openssh.com,umac-128@openssh.com, 1316hmac-sha2-256,hmac-sha2-512,hmac-sha1 1317.Ed 1318.Pp 1319The list of available MAC algorithms may also be obtained using 1320.Qq ssh -Q mac . 1321.It Cm NoHostAuthenticationForLocalhost 1322Disable host authentication for localhost (loopback addresses). 1323The argument to this keyword must be 1324.Cm yes 1325or 1326.Cm no 1327(the default). 1328.It Cm NumberOfPasswordPrompts 1329Specifies the number of password prompts before giving up. 1330The argument to this keyword must be an integer. 1331The default is 3. 1332.It Cm PasswordAuthentication 1333Specifies whether to use password authentication. 1334The argument to this keyword must be 1335.Cm yes 1336(the default) 1337or 1338.Cm no . 1339.It Cm PermitLocalCommand 1340Allow local command execution via the 1341.Ic LocalCommand 1342option or using the 1343.Ic !\& Ns Ar command 1344escape sequence in 1345.Xr ssh 1 . 1346The argument must be 1347.Cm yes 1348or 1349.Cm no 1350(the default). 1351.It Cm PermitRemoteOpen 1352Specifies the destinations to which remote TCP port forwarding is permitted when 1353.Cm RemoteForward 1354is used as a SOCKS proxy. 1355The forwarding specification must be one of the following forms: 1356.Pp 1357.Bl -item -offset indent -compact 1358.It 1359.Cm PermitRemoteOpen 1360.Sm off 1361.Ar host : port 1362.Sm on 1363.It 1364.Cm PermitRemoteOpen 1365.Sm off 1366.Ar IPv4_addr : port 1367.Sm on 1368.It 1369.Cm PermitRemoteOpen 1370.Sm off 1371.Ar \&[ IPv6_addr \&] : port 1372.Sm on 1373.El 1374.Pp 1375Multiple forwards may be specified by separating them with whitespace. 1376An argument of 1377.Cm any 1378can be used to remove all restrictions and permit any forwarding requests. 1379An argument of 1380.Cm none 1381can be used to prohibit all forwarding requests. 1382The wildcard 1383.Sq * 1384can be used for host or port to allow all hosts or ports respectively. 1385Otherwise, no pattern matching or address lookups are performed on supplied 1386names. 1387.It Cm PKCS11Provider 1388Specifies which PKCS#11 provider to use or 1389.Cm none 1390to indicate that no provider should be used (the default). 1391The argument to this keyword is a path to the PKCS#11 shared library 1392.Xr ssh 1 1393should use to communicate with a PKCS#11 token providing keys for user 1394authentication. 1395.It Cm Port 1396Specifies the port number to connect on the remote host. 1397The default is 22. 1398.It Cm PreferredAuthentications 1399Specifies the order in which the client should try authentication methods. 1400This allows a client to prefer one method (e.g.\& 1401.Cm keyboard-interactive ) 1402over another method (e.g.\& 1403.Cm password ) . 1404The default is: 1405.Bd -literal -offset indent 1406gssapi-with-mic,hostbased,publickey, 1407keyboard-interactive,password 1408.Ed 1409.It Cm ProxyCommand 1410Specifies the command to use to connect to the server. 1411The command 1412string extends to the end of the line, and is executed 1413using the user's shell 1414.Ql exec 1415directive to avoid a lingering shell process. 1416.Pp 1417Arguments to 1418.Cm ProxyCommand 1419accept the tokens described in the 1420.Sx TOKENS 1421section. 1422The command can be basically anything, 1423and should read from its standard input and write to its standard output. 1424It should eventually connect an 1425.Xr sshd 8 1426server running on some machine, or execute 1427.Ic sshd -i 1428somewhere. 1429Host key management will be done using the 1430.Cm Hostname 1431of the host being connected (defaulting to the name typed by the user). 1432Setting the command to 1433.Cm none 1434disables this option entirely. 1435Note that 1436.Cm CheckHostIP 1437is not available for connects with a proxy command. 1438.Pp 1439This directive is useful in conjunction with 1440.Xr nc 1 1441and its proxy support. 1442For example, the following directive would connect via an HTTP proxy at 1443192.0.2.0: 1444.Bd -literal -offset 3n 1445ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1446.Ed 1447.It Cm ProxyJump 1448Specifies one or more jump proxies as either 1449.Xo 1450.Sm off 1451.Op Ar user No @ 1452.Ar host 1453.Op : Ns Ar port 1454.Sm on 1455or an ssh URI 1456.Xc . 1457Multiple proxies may be separated by comma characters and will be visited 1458sequentially. 1459Setting this option will cause 1460.Xr ssh 1 1461to connect to the target host by first making a 1462.Xr ssh 1 1463connection to the specified 1464.Cm ProxyJump 1465host and then establishing a 1466TCP forwarding to the ultimate target from there. 1467Setting the host to 1468.Cm none 1469disables this option entirely. 1470.Pp 1471Note that this option will compete with the 1472.Cm ProxyCommand 1473option - whichever is specified first will prevent later instances of the 1474other from taking effect. 1475.Pp 1476Note also that the configuration for the destination host (either supplied 1477via the command-line or the configuration file) is not generally applied 1478to jump hosts. 1479.Pa ~/.ssh/config 1480should be used if specific configuration is required for jump hosts. 1481.It Cm ProxyUseFdpass 1482Specifies that 1483.Cm ProxyCommand 1484will pass a connected file descriptor back to 1485.Xr ssh 1 1486instead of continuing to execute and pass data. 1487The default is 1488.Cm no . 1489.It Cm PubkeyAcceptedAlgorithms 1490Specifies the signature algorithms that will be used for public key 1491authentication as a comma-separated list of patterns. 1492If the specified list begins with a 1493.Sq + 1494character, then the algorithms after it will be appended to the default 1495instead of replacing it. 1496If the specified list begins with a 1497.Sq - 1498character, then the specified algorithms (including wildcards) will be removed 1499from the default set instead of replacing them. 1500If the specified list begins with a 1501.Sq ^ 1502character, then the specified algorithms will be placed at the head of the 1503default set. 1504The default for this option is: 1505.Bd -literal -offset 3n 1506ssh-ed25519-cert-v01@openssh.com, 1507ecdsa-sha2-nistp256-cert-v01@openssh.com, 1508ecdsa-sha2-nistp384-cert-v01@openssh.com, 1509ecdsa-sha2-nistp521-cert-v01@openssh.com, 1510sk-ssh-ed25519-cert-v01@openssh.com, 1511sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1512rsa-sha2-512-cert-v01@openssh.com, 1513rsa-sha2-256-cert-v01@openssh.com, 1514ssh-ed25519, 1515ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1516sk-ssh-ed25519@openssh.com, 1517sk-ecdsa-sha2-nistp256@openssh.com, 1518rsa-sha2-512,rsa-sha2-256 1519.Ed 1520.Pp 1521The list of available signature algorithms may also be obtained using 1522.Qq ssh -Q PubkeyAcceptedAlgorithms . 1523.It Cm PubkeyAuthentication 1524Specifies whether to try public key authentication. 1525The argument to this keyword must be 1526.Cm yes 1527(the default), 1528.Cm no , 1529.Cm unbound 1530or 1531.Cm host-bound . 1532The final two options enable public key authentication while respectively 1533disabling or enabling the OpenSSH host-bound authentication protocol 1534extension required for restricted 1535.Xr ssh-agent 1 1536forwarding. 1537.It Cm RekeyLimit 1538Specifies the maximum amount of data that may be transmitted or received 1539before the session key is renegotiated, optionally followed by a maximum 1540amount of time that may pass before the session key is renegotiated. 1541The first argument is specified in bytes and may have a suffix of 1542.Sq K , 1543.Sq M , 1544or 1545.Sq G 1546to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1547The default is between 1548.Sq 1G 1549and 1550.Sq 4G , 1551depending on the cipher. 1552The optional second value is specified in seconds and may use any of the 1553units documented in the TIME FORMATS section of 1554.Xr sshd_config 5 . 1555The default value for 1556.Cm RekeyLimit 1557is 1558.Cm default none , 1559which means that rekeying is performed after the cipher's default amount 1560of data has been sent or received and no time based rekeying is done. 1561.It Cm RemoteCommand 1562Specifies a command to execute on the remote machine after successfully 1563connecting to the server. 1564The command string extends to the end of the line, and is executed with 1565the user's shell. 1566Arguments to 1567.Cm RemoteCommand 1568accept the tokens described in the 1569.Sx TOKENS 1570section. 1571.It Cm RemoteForward 1572Specifies that a TCP port on the remote machine be forwarded over 1573the secure channel. 1574The remote port may either be forwarded to a specified host and port 1575from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote 1576client to connect to arbitrary destinations from the local machine. 1577The first argument is the listening specification and may be 1578.Sm off 1579.Oo Ar bind_address : Oc Ar port 1580.Sm on 1581or, if the remote host supports it, a Unix domain socket path. 1582If forwarding to a specific destination then the second argument must be 1583.Ar host : Ns Ar hostport 1584or a Unix domain socket path, 1585otherwise if no destination argument is specified then the remote forwarding 1586will be established as a SOCKS proxy. 1587When acting as a SOCKS proxy, the destination of the connection can be 1588restricted by 1589.Cm PermitRemoteOpen . 1590.Pp 1591IPv6 addresses can be specified by enclosing addresses in square brackets. 1592Multiple forwardings may be specified, and additional 1593forwardings can be given on the command line. 1594Privileged ports can be forwarded only when 1595logging in as root on the remote machine. 1596Unix domain socket paths may use the tokens described in the 1597.Sx TOKENS 1598section and environment variables as described in the 1599.Sx ENVIRONMENT VARIABLES 1600section. 1601.Pp 1602If the 1603.Ar port 1604argument is 0, 1605the listen port will be dynamically allocated on the server and reported 1606to the client at run time. 1607.Pp 1608If the 1609.Ar bind_address 1610is not specified, the default is to only bind to loopback addresses. 1611If the 1612.Ar bind_address 1613is 1614.Ql * 1615or an empty string, then the forwarding is requested to listen on all 1616interfaces. 1617Specifying a remote 1618.Ar bind_address 1619will only succeed if the server's 1620.Cm GatewayPorts 1621option is enabled (see 1622.Xr sshd_config 5 ) . 1623.It Cm RequestTTY 1624Specifies whether to request a pseudo-tty for the session. 1625The argument may be one of: 1626.Cm no 1627(never request a TTY), 1628.Cm yes 1629(always request a TTY when standard input is a TTY), 1630.Cm force 1631(always request a TTY) or 1632.Cm auto 1633(request a TTY when opening a login session). 1634This option mirrors the 1635.Fl t 1636and 1637.Fl T 1638flags for 1639.Xr ssh 1 . 1640.It Cm RequiredRSASize 1641Specifies the minimum RSA key size (in bits) that 1642.Xr ssh 1 1643will accept. 1644User authentication keys smaller than this limit will be ignored. 1645Servers that present host keys smaller than this limit will cause the 1646connection to be terminated. 1647The default is 1648.Cm 1024 1649bits. 1650Note that this limit may only be raised from the default. 1651.It Cm RevokedHostKeys 1652Specifies revoked host public keys. 1653Keys listed in this file will be refused for host authentication. 1654Note that if this file does not exist or is not readable, 1655then host authentication will be refused for all hosts. 1656Keys may be specified as a text file, listing one public key per line, or as 1657an OpenSSH Key Revocation List (KRL) as generated by 1658.Xr ssh-keygen 1 . 1659For more information on KRLs, see the KEY REVOCATION LISTS section in 1660.Xr ssh-keygen 1 . 1661.It Cm SecurityKeyProvider 1662Specifies a path to a library that will be used when loading any 1663FIDO authenticator-hosted keys, overriding the default of using 1664the built-in USB HID support. 1665.Pp 1666If the specified value begins with a 1667.Sq $ 1668character, then it will be treated as an environment variable containing 1669the path to the library. 1670.It Cm SendEnv 1671Specifies what variables from the local 1672.Xr environ 7 1673should be sent to the server. 1674The server must also support it, and the server must be configured to 1675accept these environment variables. 1676Note that the 1677.Ev TERM 1678environment variable is always sent whenever a 1679pseudo-terminal is requested as it is required by the protocol. 1680Refer to 1681.Cm AcceptEnv 1682in 1683.Xr sshd_config 5 1684for how to configure the server. 1685Variables are specified by name, which may contain wildcard characters. 1686Multiple environment variables may be separated by whitespace or spread 1687across multiple 1688.Cm SendEnv 1689directives. 1690.Pp 1691See 1692.Sx PATTERNS 1693for more information on patterns. 1694.Pp 1695It is possible to clear previously set 1696.Cm SendEnv 1697variable names by prefixing patterns with 1698.Pa - . 1699The default is not to send any environment variables. 1700.It Cm ServerAliveCountMax 1701Sets the number of server alive messages (see below) which may be 1702sent without 1703.Xr ssh 1 1704receiving any messages back from the server. 1705If this threshold is reached while server alive messages are being sent, 1706ssh will disconnect from the server, terminating the session. 1707It is important to note that the use of server alive messages is very 1708different from 1709.Cm TCPKeepAlive 1710(below). 1711The server alive messages are sent through the encrypted channel 1712and therefore will not be spoofable. 1713The TCP keepalive option enabled by 1714.Cm TCPKeepAlive 1715is spoofable. 1716The server alive mechanism is valuable when the client or 1717server depend on knowing when a connection has become unresponsive. 1718.Pp 1719The default value is 3. 1720If, for example, 1721.Cm ServerAliveInterval 1722(see below) is set to 15 and 1723.Cm ServerAliveCountMax 1724is left at the default, if the server becomes unresponsive, 1725ssh will disconnect after approximately 45 seconds. 1726.It Cm ServerAliveInterval 1727Sets a timeout interval in seconds after which if no data has been received 1728from the server, 1729.Xr ssh 1 1730will send a message through the encrypted 1731channel to request a response from the server. 1732The default 1733is 0, indicating that these messages will not be sent to the server. 1734.It Cm SessionType 1735May be used to either request invocation of a subsystem on the remote system, 1736or to prevent the execution of a remote command at all. 1737The latter is useful for just forwarding ports. 1738The argument to this keyword must be 1739.Cm none 1740(same as the 1741.Fl N 1742option), 1743.Cm subsystem 1744(same as the 1745.Fl s 1746option) or 1747.Cm default 1748(shell or command execution). 1749.It Cm SetEnv 1750Directly specify one or more environment variables and their contents to 1751be sent to the server. 1752Similarly to 1753.Cm SendEnv , 1754with the exception of the 1755.Ev TERM 1756variable, the server must be prepared to accept the environment variable. 1757.It Cm StdinNull 1758Redirects stdin from 1759.Pa /dev/null 1760(actually, prevents reading from stdin). 1761Either this or the equivalent 1762.Fl n 1763option must be used when 1764.Nm ssh 1765is run in the background. 1766The argument to this keyword must be 1767.Cm yes 1768(same as the 1769.Fl n 1770option) or 1771.Cm no 1772(the default). 1773.It Cm StreamLocalBindMask 1774Sets the octal file creation mode mask 1775.Pq umask 1776used when creating a Unix-domain socket file for local or remote 1777port forwarding. 1778This option is only used for port forwarding to a Unix-domain socket file. 1779.Pp 1780The default value is 0177, which creates a Unix-domain socket file that is 1781readable and writable only by the owner. 1782Note that not all operating systems honor the file mode on Unix-domain 1783socket files. 1784.It Cm StreamLocalBindUnlink 1785Specifies whether to remove an existing Unix-domain socket file for local 1786or remote port forwarding before creating a new one. 1787If the socket file already exists and 1788.Cm StreamLocalBindUnlink 1789is not enabled, 1790.Nm ssh 1791will be unable to forward the port to the Unix-domain socket file. 1792This option is only used for port forwarding to a Unix-domain socket file. 1793.Pp 1794The argument must be 1795.Cm yes 1796or 1797.Cm no 1798(the default). 1799.It Cm StrictHostKeyChecking 1800If this flag is set to 1801.Cm yes , 1802.Xr ssh 1 1803will never automatically add host keys to the 1804.Pa ~/.ssh/known_hosts 1805file, and refuses to connect to hosts whose host key has changed. 1806This provides maximum protection against man-in-the-middle (MITM) attacks, 1807though it can be annoying when the 1808.Pa /etc/ssh/ssh_known_hosts 1809file is poorly maintained or when connections to new hosts are 1810frequently made. 1811This option forces the user to manually 1812add all new hosts. 1813.Pp 1814If this flag is set to 1815.Cm accept-new 1816then ssh will automatically add new host keys to the user's 1817.Pa known_hosts 1818file, but will not permit connections to hosts with 1819changed host keys. 1820If this flag is set to 1821.Cm no 1822or 1823.Cm off , 1824ssh will automatically add new host keys to the user known hosts files 1825and allow connections to hosts with changed hostkeys to proceed, 1826subject to some restrictions. 1827If this flag is set to 1828.Cm ask 1829(the default), 1830new host keys 1831will be added to the user known host files only after the user 1832has confirmed that is what they really want to do, and 1833ssh will refuse to connect to hosts whose host key has changed. 1834The host keys of 1835known hosts will be verified automatically in all cases. 1836.It Cm SyslogFacility 1837Gives the facility code that is used when logging messages from 1838.Xr ssh 1 . 1839The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1840LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1841The default is USER. 1842.It Cm TCPKeepAlive 1843Specifies whether the system should send TCP keepalive messages to the 1844other side. 1845If they are sent, death of the connection or crash of one 1846of the machines will be properly noticed. 1847However, this means that 1848connections will die if the route is down temporarily, and some people 1849find it annoying. 1850.Pp 1851The default is 1852.Cm yes 1853(to send TCP keepalive messages), and the client will notice 1854if the network goes down or the remote host dies. 1855This is important in scripts, and many users want it too. 1856.Pp 1857To disable TCP keepalive messages, the value should be set to 1858.Cm no . 1859See also 1860.Cm ServerAliveInterval 1861for protocol-level keepalives. 1862.It Cm Tunnel 1863Request 1864.Xr tun 4 1865device forwarding between the client and the server. 1866The argument must be 1867.Cm yes , 1868.Cm point-to-point 1869(layer 3), 1870.Cm ethernet 1871(layer 2), 1872or 1873.Cm no 1874(the default). 1875Specifying 1876.Cm yes 1877requests the default tunnel mode, which is 1878.Cm point-to-point . 1879.It Cm TunnelDevice 1880Specifies the 1881.Xr tun 4 1882devices to open on the client 1883.Pq Ar local_tun 1884and the server 1885.Pq Ar remote_tun . 1886.Pp 1887The argument must be 1888.Sm off 1889.Ar local_tun Op : Ar remote_tun . 1890.Sm on 1891The devices may be specified by numerical ID or the keyword 1892.Cm any , 1893which uses the next available tunnel device. 1894If 1895.Ar remote_tun 1896is not specified, it defaults to 1897.Cm any . 1898The default is 1899.Cm any:any . 1900.It Cm UpdateHostKeys 1901Specifies whether 1902.Xr ssh 1 1903should accept notifications of additional hostkeys from the server sent 1904after authentication has completed and add them to 1905.Cm UserKnownHostsFile . 1906The argument must be 1907.Cm yes , 1908.Cm no 1909or 1910.Cm ask . 1911This option allows learning alternate hostkeys for a server 1912and supports graceful key rotation by allowing a server to send replacement 1913public keys before old ones are removed. 1914.Pp 1915Additional hostkeys are only accepted if the key used to authenticate the 1916host was already trusted or explicitly accepted by the user, the host was 1917authenticated via 1918.Cm UserKnownHostsFile 1919(i.e. not 1920.Cm GlobalKnownHostsFile ) 1921and the host was authenticated using a plain key and not a certificate. 1922.Pp 1923.Cm UpdateHostKeys 1924is enabled by default if the user has not overridden the default 1925.Cm UserKnownHostsFile 1926setting and has not enabled 1927.Cm VerifyHostKeyDNS , 1928otherwise 1929.Cm UpdateHostKeys 1930will be set to 1931.Cm no . 1932.Pp 1933If 1934.Cm UpdateHostKeys 1935is set to 1936.Cm ask , 1937then the user is asked to confirm the modifications to the known_hosts file. 1938Confirmation is currently incompatible with 1939.Cm ControlPersist , 1940and will be disabled if it is enabled. 1941.Pp 1942Presently, only 1943.Xr sshd 8 1944from OpenSSH 6.8 and greater support the 1945.Qq hostkeys@openssh.com 1946protocol extension used to inform the client of all the server's hostkeys. 1947.It Cm User 1948Specifies the user to log in as. 1949This can be useful when a different user name is used on different machines. 1950This saves the trouble of 1951having to remember to give the user name on the command line. 1952.It Cm UserKnownHostsFile 1953Specifies one or more files to use for the user 1954host key database, separated by whitespace. 1955Each filename may use tilde notation to refer to the user's home directory, 1956the tokens described in the 1957.Sx TOKENS 1958section and environment variables as described in the 1959.Sx ENVIRONMENT VARIABLES 1960section. 1961The default is 1962.Pa ~/.ssh/known_hosts , 1963.Pa ~/.ssh/known_hosts2 . 1964.It Cm VerifyHostKeyDNS 1965Specifies whether to verify the remote key using DNS and SSHFP resource 1966records. 1967If this option is set to 1968.Cm yes , 1969the client will implicitly trust keys that match a secure fingerprint 1970from DNS. 1971Insecure fingerprints will be handled as if this option was set to 1972.Cm ask . 1973If this option is set to 1974.Cm ask , 1975information on fingerprint match will be displayed, but the user will still 1976need to confirm new host keys according to the 1977.Cm StrictHostKeyChecking 1978option. 1979The default is 1980.Cm no . 1981.Pp 1982See also 1983.Sx VERIFYING HOST KEYS 1984in 1985.Xr ssh 1 . 1986.It Cm VisualHostKey 1987If this flag is set to 1988.Cm yes , 1989an ASCII art representation of the remote host key fingerprint is 1990printed in addition to the fingerprint string at login and 1991for unknown host keys. 1992If this flag is set to 1993.Cm no 1994(the default), 1995no fingerprint strings are printed at login and 1996only the fingerprint string will be printed for unknown host keys. 1997.It Cm XAuthLocation 1998Specifies the full pathname of the 1999.Xr xauth 1 2000program. 2001The default is 2002.Pa /usr/X11R6/bin/xauth . 2003.El 2004.Sh PATTERNS 2005A 2006.Em pattern 2007consists of zero or more non-whitespace characters, 2008.Sq * 2009(a wildcard that matches zero or more characters), 2010or 2011.Sq ?\& 2012(a wildcard that matches exactly one character). 2013For example, to specify a set of declarations for any host in the 2014.Qq .co.uk 2015set of domains, 2016the following pattern could be used: 2017.Pp 2018.Dl Host *.co.uk 2019.Pp 2020The following pattern 2021would match any host in the 192.168.0.[0-9] network range: 2022.Pp 2023.Dl Host 192.168.0.? 2024.Pp 2025A 2026.Em pattern-list 2027is a comma-separated list of patterns. 2028Patterns within pattern-lists may be negated 2029by preceding them with an exclamation mark 2030.Pq Sq !\& . 2031For example, 2032to allow a key to be used from anywhere within an organization 2033except from the 2034.Qq dialup 2035pool, 2036the following entry (in authorized_keys) could be used: 2037.Pp 2038.Dl from=\&"!*.dialup.example.com,*.example.com\&" 2039.Pp 2040Note that a negated match will never produce a positive result by itself. 2041For example, attempting to match 2042.Qq host3 2043against the following pattern-list will fail: 2044.Pp 2045.Dl from=\&"!host1,!host2\&" 2046.Pp 2047The solution here is to include a term that will yield a positive match, 2048such as a wildcard: 2049.Pp 2050.Dl from=\&"!host1,!host2,*\&" 2051.Sh TOKENS 2052Arguments to some keywords can make use of tokens, 2053which are expanded at runtime: 2054.Pp 2055.Bl -tag -width XXXX -offset indent -compact 2056.It %% 2057A literal 2058.Sq % . 2059.It \&%C 2060Hash of %l%h%p%r. 2061.It %d 2062Local user's home directory. 2063.It %f 2064The fingerprint of the server's host key. 2065.It %H 2066The 2067.Pa known_hosts 2068hostname or address that is being searched for. 2069.It %h 2070The remote hostname. 2071.It \%%I 2072A string describing the reason for a 2073.Cm KnownHostsCommand 2074execution: either 2075.Cm ADDRESS 2076when looking up a host by address (only when 2077.Cm CheckHostIP 2078is enabled), 2079.Cm HOSTNAME 2080when searching by hostname, or 2081.Cm ORDER 2082when preparing the host key algorithm preference list to use for the 2083destination host. 2084.It %i 2085The local user ID. 2086.It %K 2087The base64 encoded host key. 2088.It %k 2089The host key alias if specified, otherwise the original remote hostname given 2090on the command line. 2091.It %L 2092The local hostname. 2093.It %l 2094The local hostname, including the domain name. 2095.It %n 2096The original remote hostname, as given on the command line. 2097.It %p 2098The remote port. 2099.It %r 2100The remote username. 2101.It \&%T 2102The local 2103.Xr tun 4 2104or 2105.Xr tap 4 2106network interface assigned if 2107tunnel forwarding was requested, or 2108.Qq NONE 2109otherwise. 2110.It %t 2111The type of the server host key, e.g. 2112.Cm ssh-ed25519 . 2113.It %u 2114The local username. 2115.El 2116.Pp 2117.Cm CertificateFile , 2118.Cm ControlPath , 2119.Cm IdentityAgent , 2120.Cm IdentityFile , 2121.Cm KnownHostsCommand , 2122.Cm LocalForward , 2123.Cm Match exec , 2124.Cm RemoteCommand , 2125.Cm RemoteForward , 2126and 2127.Cm UserKnownHostsFile 2128accept the tokens %%, %C, %d, %h, %i, %k, %L, %l, %n, %p, %r, and %u. 2129.Pp 2130.Cm KnownHostsCommand 2131additionally accepts the tokens %f, %H, %I, %K and %t. 2132.Pp 2133.Cm Hostname 2134accepts the tokens %% and %h. 2135.Pp 2136.Cm LocalCommand 2137accepts all tokens. 2138.Pp 2139.Cm ProxyCommand 2140and 2141.Cm ProxyJump 2142accept the tokens %%, %h, %n, %p, and %r. 2143.Sh ENVIRONMENT VARIABLES 2144Arguments to some keywords can be expanded at runtime from environment 2145variables on the client by enclosing them in 2146.Ic ${} , 2147for example 2148.Ic ${HOME}/.ssh 2149would refer to the user's .ssh directory. 2150If a specified environment variable does not exist then an error will be 2151returned and the setting for that keyword will be ignored. 2152.Pp 2153The keywords 2154.Cm CertificateFile , 2155.Cm ControlPath , 2156.Cm IdentityAgent , 2157.Cm IdentityFile , 2158.Cm KnownHostsCommand , 2159and 2160.Cm UserKnownHostsFile 2161support environment variables. 2162The keywords 2163.Cm LocalForward 2164and 2165.Cm RemoteForward 2166support environment variables only for Unix domain socket paths. 2167.Sh FILES 2168.Bl -tag -width Ds 2169.It Pa ~/.ssh/config 2170This is the per-user configuration file. 2171The format of this file is described above. 2172This file is used by the SSH client. 2173Because of the potential for abuse, this file must have strict permissions: 2174read/write for the user, and not writable by others. 2175.It Pa /etc/ssh/ssh_config 2176Systemwide configuration file. 2177This file provides defaults for those 2178values that are not specified in the user's configuration file, and 2179for those users who do not have a configuration file. 2180This file must be world-readable. 2181.El 2182.Sh SEE ALSO 2183.Xr ssh 1 2184.Sh AUTHORS 2185.An -nosplit 2186OpenSSH is a derivative of the original and free 2187ssh 1.2.12 release by 2188.An Tatu Ylonen . 2189.An Aaron Campbell , Bob Beck , Markus Friedl , 2190.An Niels Provos , Theo de Raadt 2191and 2192.An Dug Song 2193removed many bugs, re-added newer features and 2194created OpenSSH. 2195.An Markus Friedl 2196contributed the support for SSH protocol versions 1.5 and 2.0. 2197