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 1.394 2024/02/21 06:01:13 djm Exp $ 37.Dd $Mdocdate: February 21 2024 $ 38.Dt SSH_CONFIG 5 39.Os 40.Sh NAME 41.Nm ssh_config 42.Nd OpenSSH client configuration file 43.Sh DESCRIPTION 44.Xr ssh 1 45obtains configuration data from the following sources in 46the following order: 47.Pp 48.Bl -enum -offset indent -compact 49.It 50command-line options 51.It 52user's configuration file 53.Pq Pa ~/.ssh/config 54.It 55system-wide configuration file 56.Pq Pa /etc/ssh/ssh_config 57.El 58.Pp 59Unless noted otherwise, for each parameter, the first obtained value 60will be used. 61The configuration files contain sections separated by 62.Cm Host 63specifications, and that section is only applied for hosts that 64match one of the patterns given in the specification. 65The matched host name is usually the one given on the command line 66(see the 67.Cm CanonicalizeHostname 68option for exceptions). 69.Pp 70Since the first obtained value for each parameter is used, more 71host-specific declarations should be given near the beginning of the 72file, and general defaults at the end. 73.Pp 74The file contains keyword-argument pairs, one per line. 75Lines starting with 76.Ql # 77and empty lines are interpreted as comments. 78Arguments may optionally be enclosed in double quotes 79.Pq \&" 80in order to represent arguments containing spaces. 81Configuration options may be separated by whitespace or 82optional whitespace and exactly one 83.Ql = ; 84the latter format is useful to avoid the need to quote whitespace 85when specifying configuration options using the 86.Nm ssh , 87.Nm scp , 88and 89.Nm sftp 90.Fl o 91option. 92.Pp 93The possible 94keywords and their meanings are as follows (note that 95keywords are case-insensitive and arguments are case-sensitive): 96.Bl -tag -width Ds 97.It Cm Host 98Restricts the following declarations (up to the next 99.Cm Host 100or 101.Cm Match 102keyword) to be only for those hosts that match one of the patterns 103given after the keyword. 104If more than one pattern is provided, they should be separated by whitespace. 105A single 106.Ql * 107as a pattern can be used to provide global 108defaults for all hosts. 109The host is usually the 110.Ar hostname 111argument given on the command line 112(see the 113.Cm CanonicalizeHostname 114keyword for exceptions). 115.Pp 116A pattern entry may be negated by prefixing it with an exclamation mark 117.Pq Sq !\& . 118If a negated entry is matched, then the 119.Cm Host 120entry is ignored, regardless of whether any other patterns on the line 121match. 122Negated matches are therefore useful to provide exceptions for wildcard 123matches. 124.Pp 125See 126.Sx PATTERNS 127for more information on patterns. 128.It Cm Match 129Restricts the following declarations (up to the next 130.Cm Host 131or 132.Cm Match 133keyword) to be used only when the conditions following the 134.Cm Match 135keyword are satisfied. 136Match conditions are specified using one or more criteria 137or the single token 138.Cm all 139which always matches. 140The available criteria keywords are: 141.Cm canonical , 142.Cm final , 143.Cm exec , 144.Cm localnetwork , 145.Cm host , 146.Cm originalhost , 147.Cm tagged , 148.Cm user , 149and 150.Cm localuser . 151The 152.Cm all 153criteria must appear alone or immediately after 154.Cm canonical 155or 156.Cm final . 157Other criteria may be combined arbitrarily. 158All criteria but 159.Cm all , 160.Cm canonical , 161and 162.Cm final 163require an argument. 164Criteria may be negated by prepending an exclamation mark 165.Pq Sq !\& . 166.Pp 167The 168.Cm canonical 169keyword matches only when the configuration file is being re-parsed 170after hostname canonicalization (see the 171.Cm CanonicalizeHostname 172option). 173This may be useful to specify conditions that work with canonical host 174names only. 175.Pp 176The 177.Cm final 178keyword requests that the configuration be re-parsed (regardless of whether 179.Cm CanonicalizeHostname 180is enabled), and matches only during this final pass. 181If 182.Cm CanonicalizeHostname 183is enabled, then 184.Cm canonical 185and 186.Cm final 187match during the same pass. 188.Pp 189The 190.Cm exec 191keyword executes the specified command under the user's shell. 192If the command returns a zero exit status then the condition is considered true. 193Commands containing whitespace characters must be quoted. 194Arguments to 195.Cm exec 196accept the tokens described in the 197.Sx TOKENS 198section. 199.Pp 200The 201.Cm localnetwork 202keyword matches the addresses of active local network interfaces against the 203supplied list of networks in CIDR format. 204This may be convenient for varying the effective configuration on devices that 205roam between networks. 206Note that network address is not a trustworthy criteria in many 207situations (e.g. when the network is automatically configured using DHCP) 208and so caution should be applied if using it to control security-sensitive 209configuration. 210.Pp 211The other keywords' criteria must be single entries or comma-separated 212lists and may use the wildcard and negation operators described in the 213.Sx PATTERNS 214section. 215The criteria for the 216.Cm host 217keyword are matched against the target hostname, after any substitution 218by the 219.Cm Hostname 220or 221.Cm CanonicalizeHostname 222options. 223The 224.Cm originalhost 225keyword matches against the hostname as it was specified on the command-line. 226The 227.Cm tagged 228keyword matches a tag name specified by a prior 229.Cm Tag 230directive or on the 231.Xr ssh 1 232command-line using the 233.Fl P 234flag. 235The 236.Cm user 237keyword matches against the target username on the remote host. 238The 239.Cm localuser 240keyword matches against the name of the local user running 241.Xr ssh 1 242(this keyword may be useful in system-wide 243.Nm 244files). 245.It Cm AddKeysToAgent 246Specifies whether keys should be automatically added to a running 247.Xr ssh-agent 1 . 248If this option is set to 249.Cm yes 250and a key is loaded from a file, the key and its passphrase are added to 251the agent with the default lifetime, as if by 252.Xr ssh-add 1 . 253If this option is set to 254.Cm ask , 255.Xr ssh 1 256will require confirmation using the 257.Ev SSH_ASKPASS 258program before adding a key (see 259.Xr ssh-add 1 260for details). 261If this option is set to 262.Cm confirm , 263each use of the key must be confirmed, as if the 264.Fl c 265option was specified to 266.Xr ssh-add 1 . 267If this option is set to 268.Cm no , 269no keys are added to the agent. 270Alternately, this option may be specified as a time interval 271using the format described in the 272.Sx TIME FORMATS 273section of 274.Xr sshd_config 5 275to specify the key's lifetime in 276.Xr ssh-agent 1 , 277after which it will automatically be removed. 278The argument must be 279.Cm no 280(the default), 281.Cm yes , 282.Cm confirm 283(optionally followed by a time interval), 284.Cm ask 285or a time interval. 286.It Cm AddressFamily 287Specifies which address family to use when connecting. 288Valid arguments are 289.Cm any 290(the default), 291.Cm inet 292(use IPv4 only), or 293.Cm inet6 294(use IPv6 only). 295.It Cm BatchMode 296If set to 297.Cm yes , 298user interaction such as password prompts and host key confirmation requests 299will be disabled. 300This option is useful in scripts and other batch jobs where no user 301is present to interact with 302.Xr ssh 1 . 303The argument must be 304.Cm yes 305or 306.Cm no 307(the default). 308.It Cm BindAddress 309Use the specified address on the local machine as the source address of 310the connection. 311Only useful on systems with more than one address. 312.It Cm BindInterface 313Use the address of the specified interface on the local machine as the 314source address of the connection. 315.It Cm CanonicalDomains 316When 317.Cm CanonicalizeHostname 318is enabled, this option specifies the list of domain suffixes in which to 319search for the specified destination host. 320.It Cm CanonicalizeFallbackLocal 321Specifies whether to fail with an error when hostname canonicalization fails. 322The default, 323.Cm yes , 324will attempt to look up the unqualified hostname using the system resolver's 325search rules. 326A value of 327.Cm no 328will cause 329.Xr ssh 1 330to fail instantly if 331.Cm CanonicalizeHostname 332is enabled and the target hostname cannot be found in any of the domains 333specified by 334.Cm CanonicalDomains . 335.It Cm CanonicalizeHostname 336Controls whether explicit hostname canonicalization is performed. 337The default, 338.Cm no , 339is not to perform any name rewriting and let the system resolver handle all 340hostname lookups. 341If set to 342.Cm yes 343then, for connections that do not use a 344.Cm ProxyCommand 345or 346.Cm ProxyJump , 347.Xr ssh 1 348will attempt to canonicalize the hostname specified on the command line 349using the 350.Cm CanonicalDomains 351suffixes and 352.Cm CanonicalizePermittedCNAMEs 353rules. 354If 355.Cm CanonicalizeHostname 356is set to 357.Cm always , 358then canonicalization is applied to proxied connections too. 359.Pp 360If this option is enabled, then the configuration files are processed 361again using the new target name to pick up any new configuration in matching 362.Cm Host 363and 364.Cm Match 365stanzas. 366A value of 367.Cm none 368disables the use of a 369.Cm ProxyJump 370host. 371.It Cm CanonicalizeMaxDots 372Specifies the maximum number of dot characters in a hostname before 373canonicalization is disabled. 374The default, 1, 375allows a single dot (i.e. hostname.subdomain). 376.It Cm CanonicalizePermittedCNAMEs 377Specifies rules to determine whether CNAMEs should be followed when 378canonicalizing hostnames. 379The rules consist of one or more arguments of 380.Ar source_domain_list : Ns Ar target_domain_list , 381where 382.Ar source_domain_list 383is a pattern-list of domains that may follow CNAMEs in canonicalization, 384and 385.Ar target_domain_list 386is a pattern-list of domains that they may resolve to. 387.Pp 388For example, 389.Qq *.a.example.com:*.b.example.com,*.c.example.com 390will allow hostnames matching 391.Qq *.a.example.com 392to be canonicalized to names in the 393.Qq *.b.example.com 394or 395.Qq *.c.example.com 396domains. 397.Pp 398A single argument of 399.Qq none 400causes no CNAMEs to be considered for canonicalization. 401This is the default behaviour. 402.It Cm CASignatureAlgorithms 403Specifies which algorithms are allowed for signing of certificates 404by certificate authorities (CAs). 405The default is: 406.Bd -literal -offset indent 407ssh-ed25519,ecdsa-sha2-nistp256, 408ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 409sk-ssh-ed25519@openssh.com, 410sk-ecdsa-sha2-nistp256@openssh.com, 411rsa-sha2-512,rsa-sha2-256 412.Ed 413.Pp 414If the specified list begins with a 415.Sq + 416character, then the specified algorithms will be appended to the default set 417instead of replacing them. 418If the specified list begins with a 419.Sq - 420character, then the specified algorithms (including wildcards) will be removed 421from the default set instead of replacing them. 422.Pp 423.Xr ssh 1 424will not accept host certificates signed using algorithms other than those 425specified. 426.It Cm CertificateFile 427Specifies a file from which the user's certificate is read. 428A corresponding private key must be provided separately in order 429to use this certificate either 430from an 431.Cm IdentityFile 432directive or 433.Fl i 434flag to 435.Xr ssh 1 , 436via 437.Xr ssh-agent 1 , 438or via a 439.Cm PKCS11Provider 440or 441.Cm SecurityKeyProvider . 442.Pp 443Arguments to 444.Cm CertificateFile 445may use the tilde syntax to refer to a user's home directory, 446the tokens described in the 447.Sx TOKENS 448section and environment variables as described in the 449.Sx ENVIRONMENT VARIABLES 450section. 451.Pp 452It is possible to have multiple certificate files specified in 453configuration files; these certificates will be tried in sequence. 454Multiple 455.Cm CertificateFile 456directives will add to the list of certificates used for 457authentication. 458.It Cm ChannelTimeout 459Specifies whether and how quickly 460.Xr ssh 1 461should close inactive channels. 462Timeouts are specified as one or more 463.Dq type=interval 464pairs separated by whitespace, where the 465.Dq type 466must be the special keyword 467.Dq global 468or a channel type name from the list below, optionally containing 469wildcard characters. 470.Pp 471The timeout value 472.Dq interval 473is specified in seconds or may use any of the units documented in the 474.Sx TIME FORMATS 475section. 476For example, 477.Dq session=5m 478would cause interactive sessions to terminate after five minutes of 479inactivity. 480Specifying a zero value disables the inactivity timeout. 481.Pp 482The special timeout 483.Dq global 484applies to all active channels, taken together. 485Traffic on any active channel will reset the timeout, but when the timeout 486expires then all open channels will be closed. 487Note that this global timeout is not matched by wildcards and must be 488specified explicitly. 489.Pp 490The available channel type names include: 491.Bl -tag -width Ds 492.It Cm agent-connection 493Open connections to 494.Xr ssh-agent 1 . 495.It Cm direct-tcpip , Cm direct-streamlocal@openssh.com 496Open TCP or Unix socket (respectively) connections that have 497been established from a 498.Xr ssh 1 499local forwarding, i.e.\& 500.Cm LocalForward 501or 502.Cm DynamicForward . 503.It Cm forwarded-tcpip , Cm forwarded-streamlocal@openssh.com 504Open TCP or Unix socket (respectively) connections that have been 505established to a 506.Xr sshd 8 507listening on behalf of a 508.Xr ssh 1 509remote forwarding, i.e.\& 510.Cm RemoteForward . 511.It Cm session 512The interactive main session, including shell session, command execution, 513.Xr scp 1 , 514.Xr sftp 1 , 515etc. 516.It Cm tun-connection 517Open 518.Cm TunnelForward 519connections. 520.It Cm x11-connection 521Open X11 forwarding sessions. 522.El 523.Pp 524Note that in all the above cases, terminating an inactive session does not 525guarantee to remove all resources associated with the session, e.g. shell 526processes or X11 clients relating to the session may continue to execute. 527.Pp 528Moreover, terminating an inactive channel or session does not necessarily 529close the SSH connection, nor does it prevent a client from 530requesting another channel of the same type. 531In particular, expiring an inactive forwarding session does not prevent 532another identical forwarding from being subsequently created. 533.Pp 534The default is not to expire channels of any type for inactivity. 535.It Cm CheckHostIP 536If set to 537.Cm yes , 538.Xr ssh 1 539will additionally check the host IP address in the 540.Pa known_hosts 541file. 542This allows it to detect if a host key changed due to DNS spoofing 543and will add addresses of destination hosts to 544.Pa ~/.ssh/known_hosts 545in the process, regardless of the setting of 546.Cm StrictHostKeyChecking . 547If the option is set to 548.Cm no 549(the default), 550the check will not be executed. 551The default is 552.Cm no . 553.It Cm Ciphers 554Specifies the ciphers allowed and their order of preference. 555Multiple ciphers must be comma-separated. 556If the specified list begins with a 557.Sq + 558character, then the specified ciphers will be appended to the default set 559instead of replacing them. 560If the specified list begins with a 561.Sq - 562character, then the specified ciphers (including wildcards) will be removed 563from the default set instead of replacing them. 564If the specified list begins with a 565.Sq ^ 566character, then the specified ciphers will be placed at the head of the 567default set. 568.Pp 569The supported ciphers are: 570.Bd -literal -offset indent 5713des-cbc 572aes128-cbc 573aes192-cbc 574aes256-cbc 575aes128-ctr 576aes192-ctr 577aes256-ctr 578aes128-gcm@openssh.com 579aes256-gcm@openssh.com 580chacha20-poly1305@openssh.com 581.Ed 582.Pp 583The default is: 584.Bd -literal -offset indent 585chacha20-poly1305@openssh.com, 586aes128-ctr,aes192-ctr,aes256-ctr, 587aes128-gcm@openssh.com,aes256-gcm@openssh.com 588.Ed 589.Pp 590The list of available ciphers may also be obtained using 591.Qq ssh -Q cipher . 592.It Cm ClearAllForwardings 593Specifies that all local, remote, and dynamic port forwardings 594specified in the configuration files or on the command line be 595cleared. 596This option is primarily useful when used from the 597.Xr ssh 1 598command line to clear port forwardings set in 599configuration files, and is automatically set by 600.Xr scp 1 601and 602.Xr sftp 1 . 603The argument must be 604.Cm yes 605or 606.Cm no 607(the default). 608.It Cm Compression 609Specifies whether to use compression. 610The argument must be 611.Cm yes 612or 613.Cm no 614(the default). 615.It Cm ConnectionAttempts 616Specifies the number of tries (one per second) to make before exiting. 617The argument must be an integer. 618This may be useful in scripts if the connection sometimes fails. 619The default is 1. 620.It Cm ConnectTimeout 621Specifies the timeout (in seconds) used when connecting to the 622SSH server, instead of using the default system TCP timeout. 623This timeout is applied both to establishing the connection and to performing 624the initial SSH protocol handshake and key exchange. 625.It Cm ControlMaster 626Enables the sharing of multiple sessions over a single network connection. 627When set to 628.Cm yes , 629.Xr ssh 1 630will listen for connections on a control socket specified using the 631.Cm ControlPath 632argument. 633Additional sessions can connect to this socket using the same 634.Cm ControlPath 635with 636.Cm ControlMaster 637set to 638.Cm no 639(the default). 640These sessions will try to reuse the master instance's network connection 641rather than initiating new ones, but will fall back to connecting normally 642if the control socket does not exist, or is not listening. 643.Pp 644Setting this to 645.Cm ask 646will cause 647.Xr ssh 1 648to listen for control connections, but require confirmation using 649.Xr ssh-askpass 1 . 650If the 651.Cm ControlPath 652cannot be opened, 653.Xr ssh 1 654will continue without connecting to a master instance. 655.Pp 656X11 and 657.Xr ssh-agent 1 658forwarding is supported over these multiplexed connections, however the 659display and agent forwarded will be the one belonging to the master 660connection i.e. it is not possible to forward multiple displays or agents. 661.Pp 662Two additional options allow for opportunistic multiplexing: try to use a 663master connection but fall back to creating a new one if one does not already 664exist. 665These options are: 666.Cm auto 667and 668.Cm autoask . 669The latter requires confirmation like the 670.Cm ask 671option. 672.It Cm ControlPath 673Specify the path to the control socket used for connection sharing as described 674in the 675.Cm ControlMaster 676section above or the string 677.Cm none 678to disable connection sharing. 679Arguments to 680.Cm ControlPath 681may use the tilde syntax to refer to a user's home directory, 682the tokens described in the 683.Sx TOKENS 684section and environment variables as described in the 685.Sx ENVIRONMENT VARIABLES 686section. 687It is recommended that any 688.Cm ControlPath 689used for opportunistic connection sharing include 690at least %h, %p, and %r (or alternatively %C) and be placed in a directory 691that is not writable by other users. 692This ensures that shared connections are uniquely identified. 693.It Cm ControlPersist 694When used in conjunction with 695.Cm ControlMaster , 696specifies that the master connection should remain open 697in the background (waiting for future client connections) 698after the initial client connection has been closed. 699If set to 700.Cm no 701(the default), 702then the master connection will not be placed into the background, 703and will close as soon as the initial client connection is closed. 704If set to 705.Cm yes 706or 0, 707then the master connection will remain in the background indefinitely 708(until killed or closed via a mechanism such as the 709.Qq ssh -O exit ) . 710If set to a time in seconds, or a time in any of the formats documented in 711.Xr sshd_config 5 , 712then the backgrounded master connection will automatically terminate 713after it has remained idle (with no client connections) for the 714specified time. 715.It Cm DynamicForward 716Specifies that a TCP port on the local machine be forwarded 717over the secure channel, and the application 718protocol is then used to determine where to connect to from the 719remote machine. 720.Pp 721The argument must be 722.Sm off 723.Oo Ar bind_address : Oc Ar port . 724.Sm on 725IPv6 addresses can be specified by enclosing addresses in square brackets. 726By default, the local port is bound in accordance with the 727.Cm GatewayPorts 728setting. 729However, an explicit 730.Ar bind_address 731may be used to bind the connection to a specific address. 732The 733.Ar bind_address 734of 735.Cm localhost 736indicates that the listening port be bound for local use only, while an 737empty address or 738.Sq * 739indicates that the port should be available from all interfaces. 740.Pp 741Currently the SOCKS4 and SOCKS5 protocols are supported, and 742.Xr ssh 1 743will act as a SOCKS server. 744Multiple forwardings may be specified, and 745additional forwardings can be given on the command line. 746Only the superuser can forward privileged ports. 747.It Cm EnableEscapeCommandline 748Enables the command line option in the 749.Cm EscapeChar 750menu for interactive sessions (default 751.Ql ~C ) . 752By default, the command line is disabled. 753.It Cm EnableSSHKeysign 754Setting this option to 755.Cm yes 756in the global client configuration file 757.Pa /etc/ssh/ssh_config 758enables the use of the helper program 759.Xr ssh-keysign 8 760during 761.Cm HostbasedAuthentication . 762The argument must be 763.Cm yes 764or 765.Cm no 766(the default). 767This option should be placed in the non-hostspecific section. 768See 769.Xr ssh-keysign 8 770for more information. 771.It Cm EscapeChar 772Sets the escape character (default: 773.Ql ~ ) . 774The escape character can also 775be set on the command line. 776The argument should be a single character, 777.Ql ^ 778followed by a letter, or 779.Cm none 780to disable the escape 781character entirely (making the connection transparent for binary 782data). 783.It Cm ExitOnForwardFailure 784Specifies whether 785.Xr ssh 1 786should terminate the connection if it cannot set up all requested 787dynamic, tunnel, local, and remote port forwardings, (e.g.\& 788if either end is unable to bind and listen on a specified port). 789Note that 790.Cm ExitOnForwardFailure 791does not apply to connections made over port forwardings and will not, 792for example, cause 793.Xr ssh 1 794to exit if TCP connections to the ultimate forwarding destination fail. 795The argument must be 796.Cm yes 797or 798.Cm no 799(the default). 800.It Cm FingerprintHash 801Specifies the hash algorithm used when displaying key fingerprints. 802Valid options are: 803.Cm md5 804and 805.Cm sha256 806(the default). 807.It Cm ForkAfterAuthentication 808Requests 809.Nm ssh 810to go to background just before command execution. 811This is useful if 812.Nm ssh 813is going to ask for passwords or passphrases, but the user 814wants it in the background. 815This implies the 816.Cm StdinNull 817configuration option being set to 818.Dq yes . 819The recommended way to start X11 programs at a remote site is with 820something like 821.Ic ssh -f host xterm , 822which is the same as 823.Ic ssh host xterm 824if the 825.Cm ForkAfterAuthentication 826configuration option is set to 827.Dq yes . 828.Pp 829If the 830.Cm ExitOnForwardFailure 831configuration option is set to 832.Dq yes , 833then a client started with the 834.Cm ForkAfterAuthentication 835configuration option being set to 836.Dq yes 837will wait for all remote port forwards to be successfully established 838before placing itself in the background. 839The argument to this keyword must be 840.Cm yes 841(same as the 842.Fl f 843option) or 844.Cm no 845(the default). 846.It Cm ForwardAgent 847Specifies whether the connection to the authentication agent (if any) 848will be forwarded to the remote machine. 849The argument may be 850.Cm yes , 851.Cm no 852(the default), 853an explicit path to an agent socket or the name of an environment variable 854(beginning with 855.Sq $ ) 856in which to find the path. 857.Pp 858Agent forwarding should be enabled with caution. 859Users with the ability to bypass file permissions on the remote host 860(for the agent's Unix-domain socket) 861can access the local agent through the forwarded connection. 862An attacker cannot obtain key material from the agent, 863however they can perform operations on the keys that enable them to 864authenticate using the identities loaded into the agent. 865.It Cm ForwardX11 866Specifies whether X11 connections will be automatically redirected 867over the secure channel and 868.Ev DISPLAY 869set. 870The argument must be 871.Cm yes 872or 873.Cm no 874(the default). 875.Pp 876X11 forwarding should be enabled with caution. 877Users with the ability to bypass file permissions on the remote host 878(for the user's X11 authorization database) 879can access the local X11 display through the forwarded connection. 880An attacker may then be able to perform activities such as keystroke monitoring 881if the 882.Cm ForwardX11Trusted 883option is also enabled. 884.It Cm ForwardX11Timeout 885Specify a timeout for untrusted X11 forwarding 886using the format described in the 887.Sx TIME FORMATS 888section of 889.Xr sshd_config 5 . 890X11 connections received by 891.Xr ssh 1 892after this time will be refused. 893Setting 894.Cm ForwardX11Timeout 895to zero will disable the timeout and permit X11 forwarding for the life 896of the connection. 897The default is to disable untrusted X11 forwarding after twenty minutes has 898elapsed. 899.It Cm ForwardX11Trusted 900If this option is set to 901.Cm yes , 902remote X11 clients will have full access to the original X11 display. 903.Pp 904If this option is set to 905.Cm no 906(the default), 907remote X11 clients will be considered untrusted and prevented 908from stealing or tampering with data belonging to trusted X11 909clients. 910Furthermore, the 911.Xr xauth 1 912token used for the session will be set to expire after 20 minutes. 913Remote clients will be refused access after this time. 914.Pp 915See the X11 SECURITY extension specification for full details on 916the restrictions imposed on untrusted clients. 917.It Cm GatewayPorts 918Specifies whether remote hosts are allowed to connect to local 919forwarded ports. 920By default, 921.Xr ssh 1 922binds local port forwardings to the loopback address. 923This prevents other remote hosts from connecting to forwarded ports. 924.Cm GatewayPorts 925can be used to specify that ssh 926should bind local port forwardings to the wildcard address, 927thus allowing remote hosts to connect to forwarded ports. 928The argument must be 929.Cm yes 930or 931.Cm no 932(the default). 933.It Cm GlobalKnownHostsFile 934Specifies one or more files to use for the global 935host key database, separated by whitespace. 936The default is 937.Pa /etc/ssh/ssh_known_hosts , 938.Pa /etc/ssh/ssh_known_hosts2 . 939.It Cm GSSAPIAuthentication 940Specifies whether user authentication based on GSSAPI is allowed. 941The default is 942.Cm no . 943.It Cm GSSAPIDelegateCredentials 944Forward (delegate) credentials to the server. 945The default is 946.Cm no . 947.It Cm HashKnownHosts 948Indicates that 949.Xr ssh 1 950should hash host names and addresses when they are added to 951.Pa ~/.ssh/known_hosts . 952These hashed names may be used normally by 953.Xr ssh 1 954and 955.Xr sshd 8 , 956but they do not visually reveal identifying information if the 957file's contents are disclosed. 958The default is 959.Cm no . 960Note that existing names and addresses in known hosts files 961will not be converted automatically, 962but may be manually hashed using 963.Xr ssh-keygen 1 . 964.It Cm HostbasedAcceptedAlgorithms 965Specifies the signature algorithms that will be used for hostbased 966authentication as a comma-separated list of patterns. 967Alternately if the specified list begins with a 968.Sq + 969character, then the specified signature algorithms will be appended 970to the default set instead of replacing them. 971If the specified list begins with a 972.Sq - 973character, then the specified signature algorithms (including wildcards) 974will be removed from the default set instead of replacing them. 975If the specified list begins with a 976.Sq ^ 977character, then the specified signature algorithms will be placed 978at the head of the default set. 979The default for this option is: 980.Bd -literal -offset 3n 981ssh-ed25519-cert-v01@openssh.com, 982ecdsa-sha2-nistp256-cert-v01@openssh.com, 983ecdsa-sha2-nistp384-cert-v01@openssh.com, 984ecdsa-sha2-nistp521-cert-v01@openssh.com, 985sk-ssh-ed25519-cert-v01@openssh.com, 986sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 987rsa-sha2-512-cert-v01@openssh.com, 988rsa-sha2-256-cert-v01@openssh.com, 989ssh-ed25519, 990ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 991sk-ssh-ed25519@openssh.com, 992sk-ecdsa-sha2-nistp256@openssh.com, 993rsa-sha2-512,rsa-sha2-256 994.Ed 995.Pp 996The 997.Fl Q 998option of 999.Xr ssh 1 1000may be used to list supported signature algorithms. 1001This was formerly named HostbasedKeyTypes. 1002.It Cm HostbasedAuthentication 1003Specifies whether to try rhosts based authentication with public key 1004authentication. 1005The argument must be 1006.Cm yes 1007or 1008.Cm no 1009(the default). 1010.It Cm HostKeyAlgorithms 1011Specifies the host key signature algorithms 1012that the client wants to use in order of preference. 1013Alternately if the specified list begins with a 1014.Sq + 1015character, then the specified signature algorithms will be appended to 1016the default set instead of replacing them. 1017If the specified list begins with a 1018.Sq - 1019character, then the specified signature algorithms (including wildcards) 1020will be removed from the default set instead of replacing them. 1021If the specified list begins with a 1022.Sq ^ 1023character, then the specified signature algorithms will be placed 1024at the head of the default set. 1025The default for this option is: 1026.Bd -literal -offset 3n 1027ssh-ed25519-cert-v01@openssh.com, 1028ecdsa-sha2-nistp256-cert-v01@openssh.com, 1029ecdsa-sha2-nistp384-cert-v01@openssh.com, 1030ecdsa-sha2-nistp521-cert-v01@openssh.com, 1031sk-ssh-ed25519-cert-v01@openssh.com, 1032sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1033rsa-sha2-512-cert-v01@openssh.com, 1034rsa-sha2-256-cert-v01@openssh.com, 1035ssh-ed25519, 1036ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1037sk-ecdsa-sha2-nistp256@openssh.com, 1038sk-ssh-ed25519@openssh.com, 1039rsa-sha2-512,rsa-sha2-256 1040.Ed 1041.Pp 1042If hostkeys are known for the destination host then this default is modified 1043to prefer their algorithms. 1044.Pp 1045The list of available signature algorithms may also be obtained using 1046.Qq ssh -Q HostKeyAlgorithms . 1047.It Cm HostKeyAlias 1048Specifies an alias that should be used instead of the 1049real host name when looking up or saving the host key 1050in the host key database files and when validating host certificates. 1051This option is useful for tunneling SSH connections 1052or for multiple servers running on a single host. 1053.It Cm Hostname 1054Specifies the real host name to log into. 1055This can be used to specify nicknames or abbreviations for hosts. 1056Arguments to 1057.Cm Hostname 1058accept the tokens described in the 1059.Sx TOKENS 1060section. 1061Numeric IP addresses are also permitted (both on the command line and in 1062.Cm Hostname 1063specifications). 1064The default is the name given on the command line. 1065.It Cm IdentitiesOnly 1066Specifies that 1067.Xr ssh 1 1068should only use the configured authentication identity and certificate files 1069(either the default files, or those explicitly configured in the 1070.Nm 1071files 1072or passed on the 1073.Xr ssh 1 1074command-line), 1075even if 1076.Xr ssh-agent 1 1077or a 1078.Cm PKCS11Provider 1079or 1080.Cm SecurityKeyProvider 1081offers more identities. 1082The argument to this keyword must be 1083.Cm yes 1084or 1085.Cm no 1086(the default). 1087This option is intended for situations where ssh-agent 1088offers many different identities. 1089.It Cm IdentityAgent 1090Specifies the 1091.Ux Ns -domain 1092socket used to communicate with the authentication agent. 1093.Pp 1094This option overrides the 1095.Ev SSH_AUTH_SOCK 1096environment variable and can be used to select a specific agent. 1097Setting the socket name to 1098.Cm none 1099disables the use of an authentication agent. 1100If the string 1101.Qq SSH_AUTH_SOCK 1102is specified, the location of the socket will be read from the 1103.Ev SSH_AUTH_SOCK 1104environment variable. 1105Otherwise if the specified value begins with a 1106.Sq $ 1107character, then it will be treated as an environment variable containing 1108the location of the socket. 1109.Pp 1110Arguments to 1111.Cm IdentityAgent 1112may use the tilde syntax to refer to a user's home directory, 1113the tokens described in the 1114.Sx TOKENS 1115section and environment variables as described in the 1116.Sx ENVIRONMENT VARIABLES 1117section. 1118.It Cm IdentityFile 1119Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA, 1120Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read. 1121You can also specify a public key file to use the corresponding 1122private key that is loaded in 1123.Xr ssh-agent 1 1124when the private key file is not present locally. 1125The default is 1126.Pa ~/.ssh/id_rsa , 1127.Pa ~/.ssh/id_ecdsa , 1128.Pa ~/.ssh/id_ecdsa_sk , 1129.Pa ~/.ssh/id_ed25519 , 1130.Pa ~/.ssh/id_ed25519_sk 1131and 1132.Pa ~/.ssh/id_dsa . 1133Additionally, any identities represented by the authentication agent 1134will be used for authentication unless 1135.Cm IdentitiesOnly 1136is set. 1137If no certificates have been explicitly specified by 1138.Cm CertificateFile , 1139.Xr ssh 1 1140will try to load certificate information from the filename obtained by 1141appending 1142.Pa -cert.pub 1143to the path of a specified 1144.Cm IdentityFile . 1145.Pp 1146Arguments to 1147.Cm IdentityFile 1148may use the tilde syntax to refer to a user's home directory 1149or the tokens described in the 1150.Sx TOKENS 1151section. 1152Alternately an argument of 1153.Cm none 1154may be used to indicate no identity files should be loaded. 1155.Pp 1156It is possible to have 1157multiple identity files specified in configuration files; all these 1158identities will be tried in sequence. 1159Multiple 1160.Cm IdentityFile 1161directives will add to the list of identities tried (this behaviour 1162differs from that of other configuration directives). 1163.Pp 1164.Cm IdentityFile 1165may be used in conjunction with 1166.Cm IdentitiesOnly 1167to select which identities in an agent are offered during authentication. 1168.Cm IdentityFile 1169may also be used in conjunction with 1170.Cm CertificateFile 1171in order to provide any certificate also needed for authentication with 1172the identity. 1173.It Cm IgnoreUnknown 1174Specifies a pattern-list of unknown options to be ignored if they are 1175encountered in configuration parsing. 1176This may be used to suppress errors if 1177.Nm 1178contains options that are unrecognised by 1179.Xr ssh 1 . 1180It is recommended that 1181.Cm IgnoreUnknown 1182be listed early in the configuration file as it will not be applied 1183to unknown options that appear before it. 1184.It Cm Include 1185Include the specified configuration file(s). 1186Multiple pathnames may be specified and each pathname may contain 1187.Xr glob 7 1188wildcards and, for user configurations, shell-like 1189.Sq ~ 1190references to user home directories. 1191Wildcards will be expanded and processed in lexical order. 1192Files without absolute paths are assumed to be in 1193.Pa ~/.ssh 1194if included in a user configuration file or 1195.Pa /etc/ssh 1196if included from the system configuration file. 1197.Cm Include 1198directive may appear inside a 1199.Cm Match 1200or 1201.Cm Host 1202block 1203to perform conditional inclusion. 1204.It Cm IPQoS 1205Specifies the IPv4 type-of-service or DSCP class for connections. 1206Accepted values are 1207.Cm af11 , 1208.Cm af12 , 1209.Cm af13 , 1210.Cm af21 , 1211.Cm af22 , 1212.Cm af23 , 1213.Cm af31 , 1214.Cm af32 , 1215.Cm af33 , 1216.Cm af41 , 1217.Cm af42 , 1218.Cm af43 , 1219.Cm cs0 , 1220.Cm cs1 , 1221.Cm cs2 , 1222.Cm cs3 , 1223.Cm cs4 , 1224.Cm cs5 , 1225.Cm cs6 , 1226.Cm cs7 , 1227.Cm ef , 1228.Cm le , 1229.Cm lowdelay , 1230.Cm throughput , 1231.Cm reliability , 1232a numeric value, or 1233.Cm none 1234to use the operating system default. 1235This option may take one or two arguments, separated by whitespace. 1236If one argument is specified, it is used as the packet class unconditionally. 1237If two values are specified, the first is automatically selected for 1238interactive sessions and the second for non-interactive sessions. 1239The default is 1240.Cm af21 1241(Low-Latency Data) 1242for interactive sessions and 1243.Cm cs1 1244(Lower Effort) 1245for non-interactive sessions. 1246.It Cm KbdInteractiveAuthentication 1247Specifies whether to use keyboard-interactive authentication. 1248The argument to this keyword must be 1249.Cm yes 1250(the default) 1251or 1252.Cm no . 1253.Cm ChallengeResponseAuthentication 1254is a deprecated alias for this. 1255.It Cm KbdInteractiveDevices 1256Specifies the list of methods to use in keyboard-interactive authentication. 1257Multiple method names must be comma-separated. 1258The default is to use the server specified list. 1259The methods available vary depending on what the server supports. 1260For an OpenSSH server, 1261it may be zero or more of: 1262.Cm bsdauth 1263and 1264.Cm pam . 1265.It Cm KexAlgorithms 1266Specifies the available KEX (Key Exchange) algorithms. 1267Multiple algorithms must be comma-separated. 1268If the specified list begins with a 1269.Sq + 1270character, then the specified algorithms will be appended to the default set 1271instead of replacing them. 1272If the specified list begins with a 1273.Sq - 1274character, then the specified algorithms (including wildcards) will be removed 1275from the default set instead of replacing them. 1276If the specified list begins with a 1277.Sq ^ 1278character, then the specified algorithms will be placed at the head of the 1279default set. 1280The default is: 1281.Bd -literal -offset indent 1282sntrup761x25519-sha512@openssh.com, 1283curve25519-sha256,curve25519-sha256@libssh.org, 1284ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1285diffie-hellman-group-exchange-sha256, 1286diffie-hellman-group16-sha512, 1287diffie-hellman-group18-sha512, 1288diffie-hellman-group14-sha256 1289.Ed 1290.Pp 1291The list of available key exchange algorithms may also be obtained using 1292.Qq ssh -Q kex . 1293.It Cm KnownHostsCommand 1294Specifies a command to use to obtain a list of host keys, in addition to 1295those listed in 1296.Cm UserKnownHostsFile 1297and 1298.Cm GlobalKnownHostsFile . 1299This command is executed after the files have been read. 1300It may write host key lines to standard output in identical format to the 1301usual files (described in the 1302.Sx VERIFYING HOST KEYS 1303section in 1304.Xr ssh 1 ) . 1305Arguments to 1306.Cm KnownHostsCommand 1307accept the tokens described in the 1308.Sx TOKENS 1309section. 1310The command may be invoked multiple times per connection: once when preparing 1311the preference list of host key algorithms to use, again to obtain the 1312host key for the requested host name and, if 1313.Cm CheckHostIP 1314is enabled, one more time to obtain the host key matching the server's 1315address. 1316If the command exits abnormally or returns a non-zero exit status then the 1317connection is terminated. 1318.It Cm LocalCommand 1319Specifies a command to execute on the local machine after successfully 1320connecting to the server. 1321The command string extends to the end of the line, and is executed with 1322the user's shell. 1323Arguments to 1324.Cm LocalCommand 1325accept the tokens described in the 1326.Sx TOKENS 1327section. 1328.Pp 1329The command is run synchronously and does not have access to the 1330session of the 1331.Xr ssh 1 1332that spawned it. 1333It should not be used for interactive commands. 1334.Pp 1335This directive is ignored unless 1336.Cm PermitLocalCommand 1337has been enabled. 1338.It Cm LocalForward 1339Specifies that a TCP port on the local machine be forwarded over 1340the secure channel to the specified host and port from the remote machine. 1341The first argument specifies the listener and may be 1342.Sm off 1343.Oo Ar bind_address : Oc Ar port 1344.Sm on 1345or a Unix domain socket path. 1346The second argument is the destination and may be 1347.Ar host : Ns Ar hostport 1348or a Unix domain socket path if the remote host supports it. 1349.Pp 1350IPv6 addresses can be specified by enclosing addresses in square brackets. 1351Multiple forwardings may be specified, and additional forwardings can be 1352given on the command line. 1353Only the superuser can forward privileged ports. 1354By default, the local port is bound in accordance with the 1355.Cm GatewayPorts 1356setting. 1357However, an explicit 1358.Ar bind_address 1359may be used to bind the connection to a specific address. 1360The 1361.Ar bind_address 1362of 1363.Cm localhost 1364indicates that the listening port be bound for local use only, while an 1365empty address or 1366.Sq * 1367indicates that the port should be available from all interfaces. 1368Unix domain socket paths may use the tokens described in the 1369.Sx TOKENS 1370section and environment variables as described in the 1371.Sx ENVIRONMENT VARIABLES 1372section. 1373.It Cm LogLevel 1374Gives the verbosity level that is used when logging messages from 1375.Xr ssh 1 . 1376The possible values are: 1377QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1378The default is INFO. 1379DEBUG and DEBUG1 are equivalent. 1380DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1381.It Cm LogVerbose 1382Specify one or more overrides to LogLevel. 1383An override consists of a pattern lists that matches the source file, function 1384and line number to force detailed logging for. 1385For example, an override pattern of: 1386.Bd -literal -offset indent 1387kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1388.Ed 1389.Pp 1390would enable detailed logging for line 1000 of 1391.Pa kex.c , 1392everything in the 1393.Fn kex_exchange_identification 1394function, and all code in the 1395.Pa packet.c 1396file. 1397This option is intended for debugging and no overrides are enabled by default. 1398.It Cm MACs 1399Specifies the MAC (message authentication code) algorithms 1400in order of preference. 1401The MAC algorithm is used for data integrity protection. 1402Multiple algorithms must be comma-separated. 1403If the specified list begins with a 1404.Sq + 1405character, then the specified algorithms will be appended to the default set 1406instead of replacing them. 1407If the specified list begins with a 1408.Sq - 1409character, then the specified algorithms (including wildcards) will be removed 1410from the default set instead of replacing them. 1411If the specified list begins with a 1412.Sq ^ 1413character, then the specified algorithms will be placed at the head of the 1414default set. 1415.Pp 1416The algorithms that contain 1417.Qq -etm 1418calculate the MAC after encryption (encrypt-then-mac). 1419These are considered safer and their use recommended. 1420.Pp 1421The default is: 1422.Bd -literal -offset indent 1423umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1424hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1425hmac-sha1-etm@openssh.com, 1426umac-64@openssh.com,umac-128@openssh.com, 1427hmac-sha2-256,hmac-sha2-512,hmac-sha1 1428.Ed 1429.Pp 1430The list of available MAC algorithms may also be obtained using 1431.Qq ssh -Q mac . 1432.It Cm NoHostAuthenticationForLocalhost 1433Disable host authentication for localhost (loopback addresses). 1434The argument to this keyword must be 1435.Cm yes 1436or 1437.Cm no 1438(the default). 1439.It Cm NumberOfPasswordPrompts 1440Specifies the number of password prompts before giving up. 1441The argument to this keyword must be an integer. 1442The default is 3. 1443.It Cm ObscureKeystrokeTiming 1444Specifies whether 1445.Xr ssh 1 1446should try to obscure inter-keystroke timings from passive observers of 1447network traffic. 1448If enabled, then for interactive sessions, 1449.Xr ssh 1 1450will send keystrokes at fixed intervals of a few tens of milliseconds 1451and will send fake keystroke packets for some time after typing ceases. 1452The argument to this keyword must be 1453.Cm yes , 1454.Cm no 1455or an interval specifier of the form 1456.Cm interval:milliseconds 1457(e.g.\& 1458.Cm interval:80 1459for 80 milliseconds). 1460The default is to obscure keystrokes using a 20ms packet interval. 1461Note that smaller intervals will result in higher fake keystroke packet rates. 1462.It Cm PasswordAuthentication 1463Specifies whether to use password authentication. 1464The argument to this keyword must be 1465.Cm yes 1466(the default) 1467or 1468.Cm no . 1469.It Cm PermitLocalCommand 1470Allow local command execution via the 1471.Ic LocalCommand 1472option or using the 1473.Ic !\& Ns Ar command 1474escape sequence in 1475.Xr ssh 1 . 1476The argument must be 1477.Cm yes 1478or 1479.Cm no 1480(the default). 1481.It Cm PermitRemoteOpen 1482Specifies the destinations to which remote TCP port forwarding is permitted when 1483.Cm RemoteForward 1484is used as a SOCKS proxy. 1485The forwarding specification must be one of the following forms: 1486.Pp 1487.Bl -item -offset indent -compact 1488.It 1489.Cm PermitRemoteOpen 1490.Sm off 1491.Ar host : port 1492.Sm on 1493.It 1494.Cm PermitRemoteOpen 1495.Sm off 1496.Ar IPv4_addr : port 1497.Sm on 1498.It 1499.Cm PermitRemoteOpen 1500.Sm off 1501.Ar \&[ IPv6_addr \&] : port 1502.Sm on 1503.El 1504.Pp 1505Multiple forwards may be specified by separating them with whitespace. 1506An argument of 1507.Cm any 1508can be used to remove all restrictions and permit any forwarding requests. 1509An argument of 1510.Cm none 1511can be used to prohibit all forwarding requests. 1512The wildcard 1513.Sq * 1514can be used for host or port to allow all hosts or ports respectively. 1515Otherwise, no pattern matching or address lookups are performed on supplied 1516names. 1517.It Cm PKCS11Provider 1518Specifies which PKCS#11 provider to use or 1519.Cm none 1520to indicate that no provider should be used (the default). 1521The argument to this keyword is a path to the PKCS#11 shared library 1522.Xr ssh 1 1523should use to communicate with a PKCS#11 token providing keys for user 1524authentication. 1525.It Cm Port 1526Specifies the port number to connect on the remote host. 1527The default is 22. 1528.It Cm PreferredAuthentications 1529Specifies the order in which the client should try authentication methods. 1530This allows a client to prefer one method (e.g.\& 1531.Cm keyboard-interactive ) 1532over another method (e.g.\& 1533.Cm password ) . 1534The default is: 1535.Bd -literal -offset indent 1536gssapi-with-mic,hostbased,publickey, 1537keyboard-interactive,password 1538.Ed 1539.It Cm ProxyCommand 1540Specifies the command to use to connect to the server. 1541The command 1542string extends to the end of the line, and is executed 1543using the user's shell 1544.Ql exec 1545directive to avoid a lingering shell process. 1546.Pp 1547Arguments to 1548.Cm ProxyCommand 1549accept the tokens described in the 1550.Sx TOKENS 1551section. 1552The command can be basically anything, 1553and should read from its standard input and write to its standard output. 1554It should eventually connect an 1555.Xr sshd 8 1556server running on some machine, or execute 1557.Ic sshd -i 1558somewhere. 1559Host key management will be done using the 1560.Cm Hostname 1561of the host being connected (defaulting to the name typed by the user). 1562Setting the command to 1563.Cm none 1564disables this option entirely. 1565Note that 1566.Cm CheckHostIP 1567is not available for connects with a proxy command. 1568.Pp 1569This directive is useful in conjunction with 1570.Xr nc 1 1571and its proxy support. 1572For example, the following directive would connect via an HTTP proxy at 1573192.0.2.0: 1574.Bd -literal -offset 3n 1575ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1576.Ed 1577.It Cm ProxyJump 1578Specifies one or more jump proxies as either 1579.Xo 1580.Sm off 1581.Op Ar user No @ 1582.Ar host 1583.Op : Ns Ar port 1584.Sm on 1585or an ssh URI 1586.Xc . 1587Multiple proxies may be separated by comma characters and will be visited 1588sequentially. 1589Setting this option will cause 1590.Xr ssh 1 1591to connect to the target host by first making a 1592.Xr ssh 1 1593connection to the specified 1594.Cm ProxyJump 1595host and then establishing a 1596TCP forwarding to the ultimate target from there. 1597Setting the host to 1598.Cm none 1599disables this option entirely. 1600.Pp 1601Note that this option will compete with the 1602.Cm ProxyCommand 1603option - whichever is specified first will prevent later instances of the 1604other from taking effect. 1605.Pp 1606Note also that the configuration for the destination host (either supplied 1607via the command-line or the configuration file) is not generally applied 1608to jump hosts. 1609.Pa ~/.ssh/config 1610should be used if specific configuration is required for jump hosts. 1611.It Cm ProxyUseFdpass 1612Specifies that 1613.Cm ProxyCommand 1614will pass a connected file descriptor back to 1615.Xr ssh 1 1616instead of continuing to execute and pass data. 1617The default is 1618.Cm no . 1619.It Cm PubkeyAcceptedAlgorithms 1620Specifies the signature algorithms that will be used for public key 1621authentication as a comma-separated list of patterns. 1622If the specified list begins with a 1623.Sq + 1624character, then the algorithms after it will be appended to the default 1625instead of replacing it. 1626If the specified list begins with a 1627.Sq - 1628character, then the specified algorithms (including wildcards) will be removed 1629from the default set instead of replacing them. 1630If the specified list begins with a 1631.Sq ^ 1632character, then the specified algorithms will be placed at the head of the 1633default set. 1634The default for this option is: 1635.Bd -literal -offset 3n 1636ssh-ed25519-cert-v01@openssh.com, 1637ecdsa-sha2-nistp256-cert-v01@openssh.com, 1638ecdsa-sha2-nistp384-cert-v01@openssh.com, 1639ecdsa-sha2-nistp521-cert-v01@openssh.com, 1640sk-ssh-ed25519-cert-v01@openssh.com, 1641sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1642rsa-sha2-512-cert-v01@openssh.com, 1643rsa-sha2-256-cert-v01@openssh.com, 1644ssh-ed25519, 1645ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1646sk-ssh-ed25519@openssh.com, 1647sk-ecdsa-sha2-nistp256@openssh.com, 1648rsa-sha2-512,rsa-sha2-256 1649.Ed 1650.Pp 1651The list of available signature algorithms may also be obtained using 1652.Qq ssh -Q PubkeyAcceptedAlgorithms . 1653.It Cm PubkeyAuthentication 1654Specifies whether to try public key authentication. 1655The argument to this keyword must be 1656.Cm yes 1657(the default), 1658.Cm no , 1659.Cm unbound 1660or 1661.Cm host-bound . 1662The final two options enable public key authentication while respectively 1663disabling or enabling the OpenSSH host-bound authentication protocol 1664extension required for restricted 1665.Xr ssh-agent 1 1666forwarding. 1667.It Cm RekeyLimit 1668Specifies the maximum amount of data that may be transmitted or received 1669before the session key is renegotiated, optionally followed by a maximum 1670amount of time that may pass before the session key is renegotiated. 1671The first argument is specified in bytes and may have a suffix of 1672.Sq K , 1673.Sq M , 1674or 1675.Sq G 1676to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1677The default is between 1678.Sq 1G 1679and 1680.Sq 4G , 1681depending on the cipher. 1682The optional second value is specified in seconds and may use any of the 1683units documented in the TIME FORMATS section of 1684.Xr sshd_config 5 . 1685The default value for 1686.Cm RekeyLimit 1687is 1688.Cm default none , 1689which means that rekeying is performed after the cipher's default amount 1690of data has been sent or received and no time based rekeying is done. 1691.It Cm RemoteCommand 1692Specifies a command to execute on the remote machine after successfully 1693connecting to the server. 1694The command string extends to the end of the line, and is executed with 1695the user's shell. 1696Arguments to 1697.Cm RemoteCommand 1698accept the tokens described in the 1699.Sx TOKENS 1700section. 1701.It Cm RemoteForward 1702Specifies that a TCP port on the remote machine be forwarded over 1703the secure channel. 1704The remote port may either be forwarded to a specified host and port 1705from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote 1706client to connect to arbitrary destinations from the local machine. 1707The first argument is the listening specification and may be 1708.Sm off 1709.Oo Ar bind_address : Oc Ar port 1710.Sm on 1711or, if the remote host supports it, a Unix domain socket path. 1712If forwarding to a specific destination then the second argument must be 1713.Ar host : Ns Ar hostport 1714or a Unix domain socket path, 1715otherwise if no destination argument is specified then the remote forwarding 1716will be established as a SOCKS proxy. 1717When acting as a SOCKS proxy, the destination of the connection can be 1718restricted by 1719.Cm PermitRemoteOpen . 1720.Pp 1721IPv6 addresses can be specified by enclosing addresses in square brackets. 1722Multiple forwardings may be specified, and additional 1723forwardings can be given on the command line. 1724Privileged ports can be forwarded only when 1725logging in as root on the remote machine. 1726Unix domain socket paths may use the tokens described in the 1727.Sx TOKENS 1728section and environment variables as described in the 1729.Sx ENVIRONMENT VARIABLES 1730section. 1731.Pp 1732If the 1733.Ar port 1734argument is 0, 1735the listen port will be dynamically allocated on the server and reported 1736to the client at run time. 1737.Pp 1738If the 1739.Ar bind_address 1740is not specified, the default is to only bind to loopback addresses. 1741If the 1742.Ar bind_address 1743is 1744.Ql * 1745or an empty string, then the forwarding is requested to listen on all 1746interfaces. 1747Specifying a remote 1748.Ar bind_address 1749will only succeed if the server's 1750.Cm GatewayPorts 1751option is enabled (see 1752.Xr sshd_config 5 ) . 1753.It Cm RequestTTY 1754Specifies whether to request a pseudo-tty for the session. 1755The argument may be one of: 1756.Cm no 1757(never request a TTY), 1758.Cm yes 1759(always request a TTY when standard input is a TTY), 1760.Cm force 1761(always request a TTY) or 1762.Cm auto 1763(request a TTY when opening a login session). 1764This option mirrors the 1765.Fl t 1766and 1767.Fl T 1768flags for 1769.Xr ssh 1 . 1770.It Cm RequiredRSASize 1771Specifies the minimum RSA key size (in bits) that 1772.Xr ssh 1 1773will accept. 1774User authentication keys smaller than this limit will be ignored. 1775Servers that present host keys smaller than this limit will cause the 1776connection to be terminated. 1777The default is 1778.Cm 1024 1779bits. 1780Note that this limit may only be raised from the default. 1781.It Cm RevokedHostKeys 1782Specifies revoked host public keys. 1783Keys listed in this file will be refused for host authentication. 1784Note that if this file does not exist or is not readable, 1785then host authentication will be refused for all hosts. 1786Keys may be specified as a text file, listing one public key per line, or as 1787an OpenSSH Key Revocation List (KRL) as generated by 1788.Xr ssh-keygen 1 . 1789For more information on KRLs, see the KEY REVOCATION LISTS section in 1790.Xr ssh-keygen 1 . 1791Arguments to 1792.Cm RevokedHostKeys 1793may use the tilde syntax to refer to a user's home directory, 1794the tokens described in the 1795.Sx TOKENS 1796section and environment variables as described in the 1797.Sx ENVIRONMENT VARIABLES 1798section. 1799.It Cm SecurityKeyProvider 1800Specifies a path to a library that will be used when loading any 1801FIDO authenticator-hosted keys, overriding the default of using 1802the built-in USB HID support. 1803.Pp 1804If the specified value begins with a 1805.Sq $ 1806character, then it will be treated as an environment variable containing 1807the path to the library. 1808.It Cm SendEnv 1809Specifies what variables from the local 1810.Xr environ 7 1811should be sent to the server. 1812The server must also support it, and the server must be configured to 1813accept these environment variables. 1814Note that the 1815.Ev TERM 1816environment variable is always sent whenever a 1817pseudo-terminal is requested as it is required by the protocol. 1818Refer to 1819.Cm AcceptEnv 1820in 1821.Xr sshd_config 5 1822for how to configure the server. 1823Variables are specified by name, which may contain wildcard characters. 1824Multiple environment variables may be separated by whitespace or spread 1825across multiple 1826.Cm SendEnv 1827directives. 1828.Pp 1829See 1830.Sx PATTERNS 1831for more information on patterns. 1832.Pp 1833It is possible to clear previously set 1834.Cm SendEnv 1835variable names by prefixing patterns with 1836.Pa - . 1837The default is not to send any environment variables. 1838.It Cm ServerAliveCountMax 1839Sets the number of server alive messages (see below) which may be 1840sent without 1841.Xr ssh 1 1842receiving any messages back from the server. 1843If this threshold is reached while server alive messages are being sent, 1844ssh will disconnect from the server, terminating the session. 1845It is important to note that the use of server alive messages is very 1846different from 1847.Cm TCPKeepAlive 1848(below). 1849The server alive messages are sent through the encrypted channel 1850and therefore will not be spoofable. 1851The TCP keepalive option enabled by 1852.Cm TCPKeepAlive 1853is spoofable. 1854The server alive mechanism is valuable when the client or 1855server depend on knowing when a connection has become unresponsive. 1856.Pp 1857The default value is 3. 1858If, for example, 1859.Cm ServerAliveInterval 1860(see below) is set to 15 and 1861.Cm ServerAliveCountMax 1862is left at the default, if the server becomes unresponsive, 1863ssh will disconnect after approximately 45 seconds. 1864.It Cm ServerAliveInterval 1865Sets a timeout interval in seconds after which if no data has been received 1866from the server, 1867.Xr ssh 1 1868will send a message through the encrypted 1869channel to request a response from the server. 1870The default 1871is 0, indicating that these messages will not be sent to the server. 1872.It Cm SessionType 1873May be used to either request invocation of a subsystem on the remote system, 1874or to prevent the execution of a remote command at all. 1875The latter is useful for just forwarding ports. 1876The argument to this keyword must be 1877.Cm none 1878(same as the 1879.Fl N 1880option), 1881.Cm subsystem 1882(same as the 1883.Fl s 1884option) or 1885.Cm default 1886(shell or command execution). 1887.It Cm SetEnv 1888Directly specify one or more environment variables and their contents to 1889be sent to the server. 1890Similarly to 1891.Cm SendEnv , 1892with the exception of the 1893.Ev TERM 1894variable, the server must be prepared to accept the environment variable. 1895.It Cm StdinNull 1896Redirects stdin from 1897.Pa /dev/null 1898(actually, prevents reading from stdin). 1899Either this or the equivalent 1900.Fl n 1901option must be used when 1902.Nm ssh 1903is run in the background. 1904The argument to this keyword must be 1905.Cm yes 1906(same as the 1907.Fl n 1908option) or 1909.Cm no 1910(the default). 1911.It Cm StreamLocalBindMask 1912Sets the octal file creation mode mask 1913.Pq umask 1914used when creating a Unix-domain socket file for local or remote 1915port forwarding. 1916This option is only used for port forwarding to a Unix-domain socket file. 1917.Pp 1918The default value is 0177, which creates a Unix-domain socket file that is 1919readable and writable only by the owner. 1920Note that not all operating systems honor the file mode on Unix-domain 1921socket files. 1922.It Cm StreamLocalBindUnlink 1923Specifies whether to remove an existing Unix-domain socket file for local 1924or remote port forwarding before creating a new one. 1925If the socket file already exists and 1926.Cm StreamLocalBindUnlink 1927is not enabled, 1928.Nm ssh 1929will be unable to forward the port to the Unix-domain socket file. 1930This option is only used for port forwarding to a Unix-domain socket file. 1931.Pp 1932The argument must be 1933.Cm yes 1934or 1935.Cm no 1936(the default). 1937.It Cm StrictHostKeyChecking 1938If this flag is set to 1939.Cm yes , 1940.Xr ssh 1 1941will never automatically add host keys to the 1942.Pa ~/.ssh/known_hosts 1943file, and refuses to connect to hosts whose host key has changed. 1944This provides maximum protection against man-in-the-middle (MITM) attacks, 1945though it can be annoying when the 1946.Pa /etc/ssh/ssh_known_hosts 1947file is poorly maintained or when connections to new hosts are 1948frequently made. 1949This option forces the user to manually 1950add all new hosts. 1951.Pp 1952If this flag is set to 1953.Cm accept-new 1954then ssh will automatically add new host keys to the user's 1955.Pa known_hosts 1956file, but will not permit connections to hosts with 1957changed host keys. 1958If this flag is set to 1959.Cm no 1960or 1961.Cm off , 1962ssh will automatically add new host keys to the user known hosts files 1963and allow connections to hosts with changed hostkeys to proceed, 1964subject to some restrictions. 1965If this flag is set to 1966.Cm ask 1967(the default), 1968new host keys 1969will be added to the user known host files only after the user 1970has confirmed that is what they really want to do, and 1971ssh will refuse to connect to hosts whose host key has changed. 1972The host keys of 1973known hosts will be verified automatically in all cases. 1974.It Cm SyslogFacility 1975Gives the facility code that is used when logging messages from 1976.Xr ssh 1 . 1977The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1978LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1979The default is USER. 1980.It Cm TCPKeepAlive 1981Specifies whether the system should send TCP keepalive messages to the 1982other side. 1983If they are sent, death of the connection or crash of one 1984of the machines will be properly noticed. 1985However, this means that 1986connections will die if the route is down temporarily, and some people 1987find it annoying. 1988.Pp 1989The default is 1990.Cm yes 1991(to send TCP keepalive messages), and the client will notice 1992if the network goes down or the remote host dies. 1993This is important in scripts, and many users want it too. 1994.Pp 1995To disable TCP keepalive messages, the value should be set to 1996.Cm no . 1997See also 1998.Cm ServerAliveInterval 1999for protocol-level keepalives. 2000.It Cm Tag 2001Specify a configuration tag name that may be later used by a 2002.Cm Match 2003directive to select a block of configuration. 2004.It Cm Tunnel 2005Request 2006.Xr tun 4 2007device forwarding between the client and the server. 2008The argument must be 2009.Cm yes , 2010.Cm point-to-point 2011(layer 3), 2012.Cm ethernet 2013(layer 2), 2014or 2015.Cm no 2016(the default). 2017Specifying 2018.Cm yes 2019requests the default tunnel mode, which is 2020.Cm point-to-point . 2021.It Cm TunnelDevice 2022Specifies the 2023.Xr tun 4 2024devices to open on the client 2025.Pq Ar local_tun 2026and the server 2027.Pq Ar remote_tun . 2028.Pp 2029The argument must be 2030.Sm off 2031.Ar local_tun Op : Ar remote_tun . 2032.Sm on 2033The devices may be specified by numerical ID or the keyword 2034.Cm any , 2035which uses the next available tunnel device. 2036If 2037.Ar remote_tun 2038is not specified, it defaults to 2039.Cm any . 2040The default is 2041.Cm any:any . 2042.It Cm UpdateHostKeys 2043Specifies whether 2044.Xr ssh 1 2045should accept notifications of additional hostkeys from the server sent 2046after authentication has completed and add them to 2047.Cm UserKnownHostsFile . 2048The argument must be 2049.Cm yes , 2050.Cm no 2051or 2052.Cm ask . 2053This option allows learning alternate hostkeys for a server 2054and supports graceful key rotation by allowing a server to send replacement 2055public keys before old ones are removed. 2056.Pp 2057Additional hostkeys are only accepted if the key used to authenticate the 2058host was already trusted or explicitly accepted by the user, the host was 2059authenticated via 2060.Cm UserKnownHostsFile 2061(i.e. not 2062.Cm GlobalKnownHostsFile ) 2063and the host was authenticated using a plain key and not a certificate. 2064.Pp 2065.Cm UpdateHostKeys 2066is enabled by default if the user has not overridden the default 2067.Cm UserKnownHostsFile 2068setting and has not enabled 2069.Cm VerifyHostKeyDNS , 2070otherwise 2071.Cm UpdateHostKeys 2072will be set to 2073.Cm no . 2074.Pp 2075If 2076.Cm UpdateHostKeys 2077is set to 2078.Cm ask , 2079then the user is asked to confirm the modifications to the known_hosts file. 2080Confirmation is currently incompatible with 2081.Cm ControlPersist , 2082and will be disabled if it is enabled. 2083.Pp 2084Presently, only 2085.Xr sshd 8 2086from OpenSSH 6.8 and greater support the 2087.Qq hostkeys@openssh.com 2088protocol extension used to inform the client of all the server's hostkeys. 2089.It Cm User 2090Specifies the user to log in as. 2091This can be useful when a different user name is used on different machines. 2092This saves the trouble of 2093having to remember to give the user name on the command line. 2094.It Cm UserKnownHostsFile 2095Specifies one or more files to use for the user 2096host key database, separated by whitespace. 2097Each filename may use tilde notation to refer to the user's home directory, 2098the tokens described in the 2099.Sx TOKENS 2100section and environment variables as described in the 2101.Sx ENVIRONMENT VARIABLES 2102section. 2103A value of 2104.Cm none 2105causes 2106.Xr ssh 1 2107to ignore any user-specific known hosts files. 2108The default is 2109.Pa ~/.ssh/known_hosts , 2110.Pa ~/.ssh/known_hosts2 . 2111.It Cm VerifyHostKeyDNS 2112Specifies whether to verify the remote key using DNS and SSHFP resource 2113records. 2114If this option is set to 2115.Cm yes , 2116the client will implicitly trust keys that match a secure fingerprint 2117from DNS. 2118Insecure fingerprints will be handled as if this option was set to 2119.Cm ask . 2120If this option is set to 2121.Cm ask , 2122information on fingerprint match will be displayed, but the user will still 2123need to confirm new host keys according to the 2124.Cm StrictHostKeyChecking 2125option. 2126The default is 2127.Cm no . 2128.Pp 2129See also 2130.Sx VERIFYING HOST KEYS 2131in 2132.Xr ssh 1 . 2133.It Cm VisualHostKey 2134If this flag is set to 2135.Cm yes , 2136an ASCII art representation of the remote host key fingerprint is 2137printed in addition to the fingerprint string at login and 2138for unknown host keys. 2139If this flag is set to 2140.Cm no 2141(the default), 2142no fingerprint strings are printed at login and 2143only the fingerprint string will be printed for unknown host keys. 2144.It Cm XAuthLocation 2145Specifies the full pathname of the 2146.Xr xauth 1 2147program. 2148The default is 2149.Pa /usr/local/bin/xauth . 2150.El 2151.Sh PATTERNS 2152A 2153.Em pattern 2154consists of zero or more non-whitespace characters, 2155.Sq * 2156(a wildcard that matches zero or more characters), 2157or 2158.Sq ?\& 2159(a wildcard that matches exactly one character). 2160For example, to specify a set of declarations for any host in the 2161.Qq .co.uk 2162set of domains, 2163the following pattern could be used: 2164.Pp 2165.Dl Host *.co.uk 2166.Pp 2167The following pattern 2168would match any host in the 192.168.0.[0-9] network range: 2169.Pp 2170.Dl Host 192.168.0.? 2171.Pp 2172A 2173.Em pattern-list 2174is a comma-separated list of patterns. 2175Patterns within pattern-lists may be negated 2176by preceding them with an exclamation mark 2177.Pq Sq !\& . 2178For example, 2179to allow a key to be used from anywhere within an organization 2180except from the 2181.Qq dialup 2182pool, 2183the following entry (in authorized_keys) could be used: 2184.Pp 2185.Dl from=\&"!*.dialup.example.com,*.example.com\&" 2186.Pp 2187Note that a negated match will never produce a positive result by itself. 2188For example, attempting to match 2189.Qq host3 2190against the following pattern-list will fail: 2191.Pp 2192.Dl from=\&"!host1,!host2\&" 2193.Pp 2194The solution here is to include a term that will yield a positive match, 2195such as a wildcard: 2196.Pp 2197.Dl from=\&"!host1,!host2,*\&" 2198.Sh TOKENS 2199Arguments to some keywords can make use of tokens, 2200which are expanded at runtime: 2201.Pp 2202.Bl -tag -width XXXX -offset indent -compact 2203.It %% 2204A literal 2205.Sq % . 2206.It \&%C 2207Hash of %l%h%p%r%j. 2208.It %d 2209Local user's home directory. 2210.It %f 2211The fingerprint of the server's host key. 2212.It %H 2213The 2214.Pa known_hosts 2215hostname or address that is being searched for. 2216.It %h 2217The remote hostname. 2218.It \%%I 2219A string describing the reason for a 2220.Cm KnownHostsCommand 2221execution: either 2222.Cm ADDRESS 2223when looking up a host by address (only when 2224.Cm CheckHostIP 2225is enabled), 2226.Cm HOSTNAME 2227when searching by hostname, or 2228.Cm ORDER 2229when preparing the host key algorithm preference list to use for the 2230destination host. 2231.It %i 2232The local user ID. 2233.It %j 2234The contents of the ProxyJump option, or the empty string if this 2235option is unset. 2236.It %K 2237The base64 encoded host key. 2238.It %k 2239The host key alias if specified, otherwise the original remote hostname given 2240on the command line. 2241.It %L 2242The local hostname. 2243.It %l 2244The local hostname, including the domain name. 2245.It %n 2246The original remote hostname, as given on the command line. 2247.It %p 2248The remote port. 2249.It %r 2250The remote username. 2251.It \&%T 2252The local 2253.Xr tun 4 2254or 2255.Xr tap 4 2256network interface assigned if 2257tunnel forwarding was requested, or 2258.Qq NONE 2259otherwise. 2260.It %t 2261The type of the server host key, e.g. 2262.Cm ssh-ed25519 . 2263.It %u 2264The local username. 2265.El 2266.Pp 2267.Cm CertificateFile , 2268.Cm ControlPath , 2269.Cm IdentityAgent , 2270.Cm IdentityFile , 2271.Cm KnownHostsCommand , 2272.Cm LocalForward , 2273.Cm Match exec , 2274.Cm RemoteCommand , 2275.Cm RemoteForward , 2276.Cm RevokedHostKeys , 2277and 2278.Cm UserKnownHostsFile 2279accept the tokens %%, %C, %d, %h, %i, %j, %k, %L, %l, %n, %p, %r, and %u. 2280.Pp 2281.Cm KnownHostsCommand 2282additionally accepts the tokens %f, %H, %I, %K and %t. 2283.Pp 2284.Cm Hostname 2285accepts the tokens %% and %h. 2286.Pp 2287.Cm LocalCommand 2288accepts all tokens. 2289.Pp 2290.Cm ProxyCommand 2291and 2292.Cm ProxyJump 2293accept the tokens %%, %h, %n, %p, and %r. 2294.Pp 2295Note that some of these directives build commands for execution via the shell. 2296Because 2297.Xr ssh 1 2298performs no filtering or escaping of characters that have special meaning in 2299shell commands (e.g. quotes), it is the user's responsibility to ensure that 2300the arguments passed to 2301.Xr ssh 1 2302do not contain such characters and that tokens are appropriately quoted 2303when used. 2304.Sh ENVIRONMENT VARIABLES 2305Arguments to some keywords can be expanded at runtime from environment 2306variables on the client by enclosing them in 2307.Ic ${} , 2308for example 2309.Ic ${HOME}/.ssh 2310would refer to the user's .ssh directory. 2311If a specified environment variable does not exist then an error will be 2312returned and the setting for that keyword will be ignored. 2313.Pp 2314The keywords 2315.Cm CertificateFile , 2316.Cm ControlPath , 2317.Cm IdentityAgent , 2318.Cm IdentityFile , 2319.Cm KnownHostsCommand , 2320and 2321.Cm UserKnownHostsFile 2322support environment variables. 2323The keywords 2324.Cm LocalForward 2325and 2326.Cm RemoteForward 2327support environment variables only for Unix domain socket paths. 2328.Sh FILES 2329.Bl -tag -width Ds 2330.It Pa ~/.ssh/config 2331This is the per-user configuration file. 2332The format of this file is described above. 2333This file is used by the SSH client. 2334Because of the potential for abuse, this file must have strict permissions: 2335read/write for the user, and not writable by others. 2336.It Pa /etc/ssh/ssh_config 2337Systemwide configuration file. 2338This file provides defaults for those 2339values that are not specified in the user's configuration file, and 2340for those users who do not have a configuration file. 2341This file must be world-readable. 2342.El 2343.Sh SEE ALSO 2344.Xr ssh 1 2345.Sh AUTHORS 2346.An -nosplit 2347OpenSSH is a derivative of the original and free 2348ssh 1.2.12 release by 2349.An Tatu Ylonen . 2350.An Aaron Campbell , Bob Beck , Markus Friedl , 2351.An Niels Provos , Theo de Raadt 2352and 2353.An Dug Song 2354removed many bugs, re-added newer features and 2355created OpenSSH. 2356.An Markus Friedl 2357contributed the support for SSH protocol versions 1.5 and 2.0. 2358