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 1.359 2021/07/23 04:04:52 djm Exp $ 37.Dd $Mdocdate: July 23 2021 $ 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 59For each parameter, the first obtained value 60will be used. 61The configuration files contain sections separated by 62.Cm Host 63specifications, and that section is only applied for hosts that 64match one of the patterns given in the specification. 65The matched host name is usually the one given on the command line 66(see the 67.Cm CanonicalizeHostname 68option for exceptions). 69.Pp 70Since the first obtained value for each parameter is used, more 71host-specific declarations should be given near the beginning of the 72file, and general defaults at the end. 73.Pp 74The file contains keyword-argument pairs, one per line. 75Lines starting with 76.Ql # 77and empty lines are interpreted as comments. 78Arguments may optionally be enclosed in double quotes 79.Pq \&" 80in order to represent arguments containing spaces. 81Configuration options may be separated by whitespace or 82optional whitespace and exactly one 83.Ql = ; 84the latter format is useful to avoid the need to quote whitespace 85when specifying configuration options using the 86.Nm ssh , 87.Nm scp , 88and 89.Nm sftp 90.Fl o 91option. 92.Pp 93The possible 94keywords and their meanings are as follows (note that 95keywords are case-insensitive and arguments are case-sensitive): 96.Bl -tag -width Ds 97.It Cm Host 98Restricts the following declarations (up to the next 99.Cm Host 100or 101.Cm Match 102keyword) to be only for those hosts that match one of the patterns 103given after the keyword. 104If more than one pattern is provided, they should be separated by whitespace. 105A single 106.Ql * 107as a pattern can be used to provide global 108defaults for all hosts. 109The host is usually the 110.Ar hostname 111argument given on the command line 112(see the 113.Cm CanonicalizeHostname 114keyword for exceptions). 115.Pp 116A pattern entry may be negated by prefixing it with an exclamation mark 117.Pq Sq !\& . 118If a negated entry is matched, then the 119.Cm Host 120entry is ignored, regardless of whether any other patterns on the line 121match. 122Negated matches are therefore useful to provide exceptions for wildcard 123matches. 124.Pp 125See 126.Sx PATTERNS 127for more information on patterns. 128.It Cm Match 129Restricts the following declarations (up to the next 130.Cm Host 131or 132.Cm Match 133keyword) to be used only when the conditions following the 134.Cm Match 135keyword are satisfied. 136Match conditions are specified using one or more criteria 137or the single token 138.Cm all 139which always matches. 140The available criteria keywords are: 141.Cm canonical , 142.Cm final , 143.Cm exec , 144.Cm host , 145.Cm originalhost , 146.Cm user , 147and 148.Cm localuser . 149The 150.Cm all 151criteria must appear alone or immediately after 152.Cm canonical 153or 154.Cm final . 155Other criteria may be combined arbitrarily. 156All criteria but 157.Cm all , 158.Cm canonical , 159and 160.Cm final 161require an argument. 162Criteria may be negated by prepending an exclamation mark 163.Pq Sq !\& . 164.Pp 165The 166.Cm canonical 167keyword matches only when the configuration file is being re-parsed 168after hostname canonicalization (see the 169.Cm CanonicalizeHostname 170option). 171This may be useful to specify conditions that work with canonical host 172names only. 173.Pp 174The 175.Cm final 176keyword requests that the configuration be re-parsed (regardless of whether 177.Cm CanonicalizeHostname 178is enabled), and matches only during this final pass. 179If 180.Cm CanonicalizeHostname 181is enabled, then 182.Cm canonical 183and 184.Cm final 185match during the same pass. 186.Pp 187The 188.Cm exec 189keyword executes the specified command under the user's shell. 190If the command returns a zero exit status then the condition is considered true. 191Commands containing whitespace characters must be quoted. 192Arguments to 193.Cm exec 194accept the tokens described in the 195.Sx TOKENS 196section. 197.Pp 198The other keywords' criteria must be single entries or comma-separated 199lists and may use the wildcard and negation operators described in the 200.Sx PATTERNS 201section. 202The criteria for the 203.Cm host 204keyword are matched against the target hostname, after any substitution 205by the 206.Cm Hostname 207or 208.Cm CanonicalizeHostname 209options. 210The 211.Cm originalhost 212keyword matches against the hostname as it was specified on the command-line. 213The 214.Cm user 215keyword matches against the target username on the remote host. 216The 217.Cm localuser 218keyword matches against the name of the local user running 219.Xr ssh 1 220(this keyword may be useful in system-wide 221.Nm 222files). 223.It Cm AddKeysToAgent 224Specifies whether keys should be automatically added to a running 225.Xr ssh-agent 1 . 226If this option is set to 227.Cm yes 228and a key is loaded from a file, the key and its passphrase are added to 229the agent with the default lifetime, as if by 230.Xr ssh-add 1 . 231If this option is set to 232.Cm ask , 233.Xr ssh 1 234will require confirmation using the 235.Ev SSH_ASKPASS 236program before adding a key (see 237.Xr ssh-add 1 238for details). 239If this option is set to 240.Cm confirm , 241each use of the key must be confirmed, as if the 242.Fl c 243option was specified to 244.Xr ssh-add 1 . 245If this option is set to 246.Cm no , 247no keys are added to the agent. 248Alternately, this option may be specified as a time interval 249using the format described in the 250.Sx TIME FORMATS 251section of 252.Xr sshd_config 5 253to specify the key's lifetime in 254.Xr ssh-agent 1 , 255after which it will automatically be removed. 256The argument must be 257.Cm no 258(the default), 259.Cm yes , 260.Cm confirm 261(optionally followed by a time interval), 262.Cm ask 263or a time interval. 264.It Cm AddressFamily 265Specifies which address family to use when connecting. 266Valid arguments are 267.Cm any 268(the default), 269.Cm inet 270(use IPv4 only), or 271.Cm inet6 272(use IPv6 only). 273.It Cm BatchMode 274If set to 275.Cm yes , 276user interaction such as password prompts and host key confirmation requests 277will be disabled. 278This option is useful in scripts and other batch jobs where no user 279is present to interact with 280.Xr ssh 1 . 281The argument must be 282.Cm yes 283or 284.Cm no 285(the default). 286.It Cm BindAddress 287Use the specified address on the local machine as the source address of 288the connection. 289Only useful on systems with more than one address. 290.It Cm BindInterface 291Use the address of the specified interface on the local machine as the 292source address of the connection. 293.It Cm CanonicalDomains 294When 295.Cm CanonicalizeHostname 296is enabled, this option specifies the list of domain suffixes in which to 297search for the specified destination host. 298.It Cm CanonicalizeFallbackLocal 299Specifies whether to fail with an error when hostname canonicalization fails. 300The default, 301.Cm yes , 302will attempt to look up the unqualified hostname using the system resolver's 303search rules. 304A value of 305.Cm no 306will cause 307.Xr ssh 1 308to fail instantly if 309.Cm CanonicalizeHostname 310is enabled and the target hostname cannot be found in any of the domains 311specified by 312.Cm CanonicalDomains . 313.It Cm CanonicalizeHostname 314Controls whether explicit hostname canonicalization is performed. 315The default, 316.Cm no , 317is not to perform any name rewriting and let the system resolver handle all 318hostname lookups. 319If set to 320.Cm yes 321then, for connections that do not use a 322.Cm ProxyCommand 323or 324.Cm ProxyJump , 325.Xr ssh 1 326will attempt to canonicalize the hostname specified on the command line 327using the 328.Cm CanonicalDomains 329suffixes and 330.Cm CanonicalizePermittedCNAMEs 331rules. 332If 333.Cm CanonicalizeHostname 334is set to 335.Cm always , 336then canonicalization is applied to proxied connections too. 337.Pp 338If this option is enabled, then the configuration files are processed 339again using the new target name to pick up any new configuration in matching 340.Cm Host 341and 342.Cm Match 343stanzas. 344.It Cm CanonicalizeMaxDots 345Specifies the maximum number of dot characters in a hostname before 346canonicalization is disabled. 347The default, 1, 348allows a single dot (i.e. hostname.subdomain). 349.It Cm CanonicalizePermittedCNAMEs 350Specifies rules to determine whether CNAMEs should be followed when 351canonicalizing hostnames. 352The rules consist of one or more arguments of 353.Ar source_domain_list : Ns Ar target_domain_list , 354where 355.Ar source_domain_list 356is a pattern-list of domains that may follow CNAMEs in canonicalization, 357and 358.Ar target_domain_list 359is a pattern-list of domains that they may resolve to. 360.Pp 361For example, 362.Qq *.a.example.com:*.b.example.com,*.c.example.com 363will allow hostnames matching 364.Qq *.a.example.com 365to be canonicalized to names in the 366.Qq *.b.example.com 367or 368.Qq *.c.example.com 369domains. 370.It Cm CASignatureAlgorithms 371Specifies which algorithms are allowed for signing of certificates 372by certificate authorities (CAs). 373The default is: 374.Bd -literal -offset indent 375ssh-ed25519,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 376sk-ssh-ed25519@openssh.com,sk-ecdsa-sha2-nistp256@openssh.com, 377rsa-sha2-512,rsa-sha2-256 378.Ed 379.Pp 380.Xr ssh 1 381will not accept host certificates signed using algorithms other than those 382specified. 383.It Cm CertificateFile 384Specifies a file from which the user's certificate is read. 385A corresponding private key must be provided separately in order 386to use this certificate either 387from an 388.Cm IdentityFile 389directive or 390.Fl i 391flag to 392.Xr ssh 1 , 393via 394.Xr ssh-agent 1 , 395or via a 396.Cm PKCS11Provider 397or 398.Cm SecurityKeyProvider . 399.Pp 400Arguments to 401.Cm CertificateFile 402may use the tilde syntax to refer to a user's home directory, 403the tokens described in the 404.Sx TOKENS 405section and environment variables as described in the 406.Sx ENVIRONMENT VARIABLES 407section. 408.Pp 409It is possible to have multiple certificate files specified in 410configuration files; these certificates will be tried in sequence. 411Multiple 412.Cm CertificateFile 413directives will add to the list of certificates used for 414authentication. 415.It Cm CheckHostIP 416If set to 417.Cm yes 418.Xr ssh 1 419will additionally check the host IP address in the 420.Pa known_hosts 421file. 422This allows it to detect if a host key changed due to DNS spoofing 423and will add addresses of destination hosts to 424.Pa ~/.ssh/known_hosts 425in the process, regardless of the setting of 426.Cm StrictHostKeyChecking . 427If the option is set to 428.Cm no 429(the default), 430the check will not be executed. 431.It Cm Ciphers 432Specifies the ciphers allowed and their order of preference. 433Multiple ciphers must be comma-separated. 434If the specified list begins with a 435.Sq + 436character, then the specified ciphers will be appended to the default set 437instead of replacing them. 438If the specified list begins with a 439.Sq - 440character, then the specified ciphers (including wildcards) will be removed 441from the default set instead of replacing them. 442If the specified list begins with a 443.Sq ^ 444character, then the specified ciphers will be placed at the head of the 445default set. 446.Pp 447The supported ciphers are: 448.Bd -literal -offset indent 4493des-cbc 450aes128-cbc 451aes192-cbc 452aes256-cbc 453aes128-ctr 454aes192-ctr 455aes256-ctr 456aes128-gcm@openssh.com 457aes256-gcm@openssh.com 458chacha20-poly1305@openssh.com 459.Ed 460.Pp 461The default is: 462.Bd -literal -offset indent 463chacha20-poly1305@openssh.com, 464aes128-ctr,aes192-ctr,aes256-ctr, 465aes128-gcm@openssh.com,aes256-gcm@openssh.com 466.Ed 467.Pp 468The list of available ciphers may also be obtained using 469.Qq ssh -Q cipher . 470.It Cm ClearAllForwardings 471Specifies that all local, remote, and dynamic port forwardings 472specified in the configuration files or on the command line be 473cleared. 474This option is primarily useful when used from the 475.Xr ssh 1 476command line to clear port forwardings set in 477configuration files, and is automatically set by 478.Xr scp 1 479and 480.Xr sftp 1 . 481The argument must be 482.Cm yes 483or 484.Cm no 485(the default). 486.It Cm Compression 487Specifies whether to use compression. 488The argument must be 489.Cm yes 490or 491.Cm no 492(the default). 493.It Cm ConnectionAttempts 494Specifies the number of tries (one per second) to make before exiting. 495The argument must be an integer. 496This may be useful in scripts if the connection sometimes fails. 497The default is 1. 498.It Cm ConnectTimeout 499Specifies the timeout (in seconds) used when connecting to the 500SSH server, instead of using the default system TCP timeout. 501This timeout is applied both to establishing the connection and to performing 502the initial SSH protocol handshake and key exchange. 503.It Cm ControlMaster 504Enables the sharing of multiple sessions over a single network connection. 505When set to 506.Cm yes , 507.Xr ssh 1 508will listen for connections on a control socket specified using the 509.Cm ControlPath 510argument. 511Additional sessions can connect to this socket using the same 512.Cm ControlPath 513with 514.Cm ControlMaster 515set to 516.Cm no 517(the default). 518These sessions will try to reuse the master instance's network connection 519rather than initiating new ones, but will fall back to connecting normally 520if the control socket does not exist, or is not listening. 521.Pp 522Setting this to 523.Cm ask 524will cause 525.Xr ssh 1 526to listen for control connections, but require confirmation using 527.Xr ssh-askpass 1 . 528If the 529.Cm ControlPath 530cannot be opened, 531.Xr ssh 1 532will continue without connecting to a master instance. 533.Pp 534X11 and 535.Xr ssh-agent 1 536forwarding is supported over these multiplexed connections, however the 537display and agent forwarded will be the one belonging to the master 538connection i.e. it is not possible to forward multiple displays or agents. 539.Pp 540Two additional options allow for opportunistic multiplexing: try to use a 541master connection but fall back to creating a new one if one does not already 542exist. 543These options are: 544.Cm auto 545and 546.Cm autoask . 547The latter requires confirmation like the 548.Cm ask 549option. 550.It Cm ControlPath 551Specify the path to the control socket used for connection sharing as described 552in the 553.Cm ControlMaster 554section above or the string 555.Cm none 556to disable connection sharing. 557Arguments to 558.Cm ControlPath 559may use the tilde syntax to refer to a user's home directory, 560the tokens described in the 561.Sx TOKENS 562section and environment variables as described in the 563.Sx ENVIRONMENT VARIABLES 564section. 565It is recommended that any 566.Cm ControlPath 567used for opportunistic connection sharing include 568at least %h, %p, and %r (or alternatively %C) and be placed in a directory 569that is not writable by other users. 570This ensures that shared connections are uniquely identified. 571.It Cm ControlPersist 572When used in conjunction with 573.Cm ControlMaster , 574specifies that the master connection should remain open 575in the background (waiting for future client connections) 576after the initial client connection has been closed. 577If set to 578.Cm no 579(the default), 580then the master connection will not be placed into the background, 581and will close as soon as the initial client connection is closed. 582If set to 583.Cm yes 584or 0, 585then the master connection will remain in the background indefinitely 586(until killed or closed via a mechanism such as the 587.Qq ssh -O exit ) . 588If set to a time in seconds, or a time in any of the formats documented in 589.Xr sshd_config 5 , 590then the backgrounded master connection will automatically terminate 591after it has remained idle (with no client connections) for the 592specified time. 593.It Cm DynamicForward 594Specifies that a TCP port on the local machine be forwarded 595over the secure channel, and the application 596protocol is then used to determine where to connect to from the 597remote machine. 598.Pp 599The argument must be 600.Sm off 601.Oo Ar bind_address : Oc Ar port . 602.Sm on 603IPv6 addresses can be specified by enclosing addresses in square brackets. 604By default, the local port is bound in accordance with the 605.Cm GatewayPorts 606setting. 607However, an explicit 608.Ar bind_address 609may be used to bind the connection to a specific address. 610The 611.Ar bind_address 612of 613.Cm localhost 614indicates that the listening port be bound for local use only, while an 615empty address or 616.Sq * 617indicates that the port should be available from all interfaces. 618.Pp 619Currently the SOCKS4 and SOCKS5 protocols are supported, and 620.Xr ssh 1 621will act as a SOCKS server. 622Multiple forwardings may be specified, and 623additional forwardings can be given on the command line. 624Only the superuser can forward privileged ports. 625.It Cm EnableSSHKeysign 626Setting this option to 627.Cm yes 628in the global client configuration file 629.Pa /etc/ssh/ssh_config 630enables the use of the helper program 631.Xr ssh-keysign 8 632during 633.Cm HostbasedAuthentication . 634The argument must be 635.Cm yes 636or 637.Cm no 638(the default). 639This option should be placed in the non-hostspecific section. 640See 641.Xr ssh-keysign 8 642for more information. 643.It Cm EscapeChar 644Sets the escape character (default: 645.Ql ~ ) . 646The escape character can also 647be set on the command line. 648The argument should be a single character, 649.Ql ^ 650followed by a letter, or 651.Cm none 652to disable the escape 653character entirely (making the connection transparent for binary 654data). 655.It Cm ExitOnForwardFailure 656Specifies whether 657.Xr ssh 1 658should terminate the connection if it cannot set up all requested 659dynamic, tunnel, local, and remote port forwardings, (e.g.\& 660if either end is unable to bind and listen on a specified port). 661Note that 662.Cm ExitOnForwardFailure 663does not apply to connections made over port forwardings and will not, 664for example, cause 665.Xr ssh 1 666to exit if TCP connections to the ultimate forwarding destination fail. 667The argument must be 668.Cm yes 669or 670.Cm no 671(the default). 672.It Cm FingerprintHash 673Specifies the hash algorithm used when displaying key fingerprints. 674Valid options are: 675.Cm md5 676and 677.Cm sha256 678(the default). 679.It Cm ForkAfterAuthentication 680Requests 681.Nm ssh 682to go to background just before command execution. 683This is useful if 684.Nm ssh 685is going to ask for passwords or passphrases, but the user 686wants it in the background. 687This implies the 688.Cm StdinNull 689configuration option being set to 690.Dq yes . 691The recommended way to start X11 programs at a remote site is with 692something like 693.Ic ssh -f host xterm , 694which is the same as 695.Ic ssh host xterm 696if the 697.Cm ForkAfterAuthentication 698configuration option is set to 699.Dq yes . 700.Pp 701If the 702.Cm ExitOnForwardFailure 703configuration option is set to 704.Dq yes , 705then a client started with the 706.Cm ForkAfterAuthentication 707configuration option being set to 708.Dq yes 709will wait for all remote port forwards to be successfully established 710before placing itself in the background. 711The argument to this keyword must be 712.Cm yes 713(same as the 714.Fl f 715option) or 716.Cm no 717(the default). 718.It Cm ForwardAgent 719Specifies whether the connection to the authentication agent (if any) 720will be forwarded to the remote machine. 721The argument may be 722.Cm yes , 723.Cm no 724(the default), 725an explicit path to an agent socket or the name of an environment variable 726(beginning with 727.Sq $ ) 728in which to find the path. 729.Pp 730Agent forwarding should be enabled with caution. 731Users with the ability to bypass file permissions on the remote host 732(for the agent's Unix-domain socket) 733can access the local agent through the forwarded connection. 734An attacker cannot obtain key material from the agent, 735however they can perform operations on the keys that enable them to 736authenticate using the identities loaded into the agent. 737.It Cm ForwardX11 738Specifies whether X11 connections will be automatically redirected 739over the secure channel and 740.Ev DISPLAY 741set. 742The argument must be 743.Cm yes 744or 745.Cm no 746(the default). 747.Pp 748X11 forwarding should be enabled with caution. 749Users with the ability to bypass file permissions on the remote host 750(for the user's X11 authorization database) 751can access the local X11 display through the forwarded connection. 752An attacker may then be able to perform activities such as keystroke monitoring 753if the 754.Cm ForwardX11Trusted 755option is also enabled. 756.It Cm ForwardX11Timeout 757Specify a timeout for untrusted X11 forwarding 758using the format described in the 759.Sx TIME FORMATS 760section of 761.Xr sshd_config 5 . 762X11 connections received by 763.Xr ssh 1 764after this time will be refused. 765Setting 766.Cm ForwardX11Timeout 767to zero will disable the timeout and permit X11 forwarding for the life 768of the connection. 769The default is to disable untrusted X11 forwarding after twenty minutes has 770elapsed. 771.It Cm ForwardX11Trusted 772If this option is set to 773.Cm yes , 774remote X11 clients will have full access to the original X11 display. 775.Pp 776If this option is set to 777.Cm no 778(the default), 779remote X11 clients will be considered untrusted and prevented 780from stealing or tampering with data belonging to trusted X11 781clients. 782Furthermore, the 783.Xr xauth 1 784token used for the session will be set to expire after 20 minutes. 785Remote clients will be refused access after this time. 786.Pp 787See the X11 SECURITY extension specification for full details on 788the restrictions imposed on untrusted clients. 789.It Cm GatewayPorts 790Specifies whether remote hosts are allowed to connect to local 791forwarded ports. 792By default, 793.Xr ssh 1 794binds local port forwardings to the loopback address. 795This prevents other remote hosts from connecting to forwarded ports. 796.Cm GatewayPorts 797can be used to specify that ssh 798should bind local port forwardings to the wildcard address, 799thus allowing remote hosts to connect to forwarded ports. 800The argument must be 801.Cm yes 802or 803.Cm no 804(the default). 805.It Cm GlobalKnownHostsFile 806Specifies one or more files to use for the global 807host key database, separated by whitespace. 808The default is 809.Pa /etc/ssh/ssh_known_hosts , 810.Pa /etc/ssh/ssh_known_hosts2 . 811.It Cm GSSAPIAuthentication 812Specifies whether user authentication based on GSSAPI is allowed. 813The default is 814.Cm no . 815.It Cm GSSAPIDelegateCredentials 816Forward (delegate) credentials to the server. 817The default is 818.Cm no . 819.It Cm HashKnownHosts 820Indicates that 821.Xr ssh 1 822should hash host names and addresses when they are added to 823.Pa ~/.ssh/known_hosts . 824These hashed names may be used normally by 825.Xr ssh 1 826and 827.Xr sshd 8 , 828but they do not visually reveal identifying information if the 829file's contents are disclosed. 830The default is 831.Cm no . 832Note that existing names and addresses in known hosts files 833will not be converted automatically, 834but may be manually hashed using 835.Xr ssh-keygen 1 . 836.It Cm HostbasedAcceptedAlgorithms 837Specifies the signature algorithms that will be used for hostbased 838authentication as a comma-separated list of patterns. 839Alternately if the specified list begins with a 840.Sq + 841character, then the specified signature algorithms will be appended 842to the default set instead of replacing them. 843If the specified list begins with a 844.Sq - 845character, then the specified signature algorithms (including wildcards) 846will be removed from the default set instead of replacing them. 847If the specified list begins with a 848.Sq ^ 849character, then the specified signature algorithms will be placed 850at the head of the default set. 851The default for this option is: 852.Bd -literal -offset 3n 853ssh-ed25519-cert-v01@openssh.com, 854ecdsa-sha2-nistp256-cert-v01@openssh.com, 855ecdsa-sha2-nistp384-cert-v01@openssh.com, 856ecdsa-sha2-nistp521-cert-v01@openssh.com, 857sk-ssh-ed25519-cert-v01@openssh.com, 858sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 859rsa-sha2-512-cert-v01@openssh.com, 860rsa-sha2-256-cert-v01@openssh.com, 861ssh-rsa-cert-v01@openssh.com, 862ssh-ed25519, 863ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 864sk-ssh-ed25519@openssh.com, 865sk-ecdsa-sha2-nistp256@openssh.com, 866rsa-sha2-512,rsa-sha2-256,ssh-rsa 867.Ed 868.Pp 869The 870.Fl Q 871option of 872.Xr ssh 1 873may be used to list supported signature algorithms. 874This was formerly named HostbasedKeyTypes. 875.It Cm HostbasedAuthentication 876Specifies whether to try rhosts based authentication with public key 877authentication. 878The argument must be 879.Cm yes 880or 881.Cm no 882(the default). 883.It Cm HostKeyAlgorithms 884Specifies the host key signature algorithms 885that the client wants to use in order of preference. 886Alternately if the specified list begins with a 887.Sq + 888character, then the specified signature algorithms will be appended to 889the default set instead of replacing them. 890If the specified list begins with a 891.Sq - 892character, then the specified signature algorithms (including wildcards) 893will be removed from the default set instead of replacing them. 894If the specified list begins with a 895.Sq ^ 896character, then the specified signature algorithms will be placed 897at the head of the default set. 898The default for this option is: 899.Bd -literal -offset 3n 900ssh-ed25519-cert-v01@openssh.com, 901ecdsa-sha2-nistp256-cert-v01@openssh.com, 902ecdsa-sha2-nistp384-cert-v01@openssh.com, 903ecdsa-sha2-nistp521-cert-v01@openssh.com, 904sk-ssh-ed25519-cert-v01@openssh.com, 905sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 906rsa-sha2-512-cert-v01@openssh.com, 907rsa-sha2-256-cert-v01@openssh.com, 908ssh-rsa-cert-v01@openssh.com, 909ssh-ed25519, 910ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 911sk-ecdsa-sha2-nistp256@openssh.com, 912sk-ssh-ed25519@openssh.com, 913rsa-sha2-512,rsa-sha2-256,ssh-rsa 914.Ed 915.Pp 916If hostkeys are known for the destination host then this default is modified 917to prefer their algorithms. 918.Pp 919The list of available signature algorithms may also be obtained using 920.Qq ssh -Q HostKeyAlgorithms . 921.It Cm HostKeyAlias 922Specifies an alias that should be used instead of the 923real host name when looking up or saving the host key 924in the host key database files and when validating host certificates. 925This option is useful for tunneling SSH connections 926or for multiple servers running on a single host. 927.It Cm Hostname 928Specifies the real host name to log into. 929This can be used to specify nicknames or abbreviations for hosts. 930Arguments to 931.Cm Hostname 932accept the tokens described in the 933.Sx TOKENS 934section. 935Numeric IP addresses are also permitted (both on the command line and in 936.Cm Hostname 937specifications). 938The default is the name given on the command line. 939.It Cm IdentitiesOnly 940Specifies that 941.Xr ssh 1 942should only use the configured authentication identity and certificate files 943(either the default files, or those explicitly configured in the 944.Nm 945files 946or passed on the 947.Xr ssh 1 948command-line), 949even if 950.Xr ssh-agent 1 951or a 952.Cm PKCS11Provider 953or 954.Cm SecurityKeyProvider 955offers more identities. 956The argument to this keyword must be 957.Cm yes 958or 959.Cm no 960(the default). 961This option is intended for situations where ssh-agent 962offers many different identities. 963.It Cm IdentityAgent 964Specifies the 965.Ux Ns -domain 966socket used to communicate with the authentication agent. 967.Pp 968This option overrides the 969.Ev SSH_AUTH_SOCK 970environment variable and can be used to select a specific agent. 971Setting the socket name to 972.Cm none 973disables the use of an authentication agent. 974If the string 975.Qq SSH_AUTH_SOCK 976is specified, the location of the socket will be read from the 977.Ev SSH_AUTH_SOCK 978environment variable. 979Otherwise if the specified value begins with a 980.Sq $ 981character, then it will be treated as an environment variable containing 982the location of the socket. 983.Pp 984Arguments to 985.Cm IdentityAgent 986may use the tilde syntax to refer to a user's home directory, 987the tokens described in the 988.Sx TOKENS 989section and environment variables as described in the 990.Sx ENVIRONMENT VARIABLES 991section. 992.It Cm IdentityFile 993Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA, 994Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read. 995The default is 996.Pa ~/.ssh/id_dsa , 997.Pa ~/.ssh/id_ecdsa , 998.Pa ~/.ssh/id_ecdsa_sk , 999.Pa ~/.ssh/id_ed25519 , 1000.Pa ~/.ssh/id_ed25519_sk 1001and 1002.Pa ~/.ssh/id_rsa . 1003Additionally, any identities represented by the authentication agent 1004will be used for authentication unless 1005.Cm IdentitiesOnly 1006is set. 1007If no certificates have been explicitly specified by 1008.Cm CertificateFile , 1009.Xr ssh 1 1010will try to load certificate information from the filename obtained by 1011appending 1012.Pa -cert.pub 1013to the path of a specified 1014.Cm IdentityFile . 1015.Pp 1016Arguments to 1017.Cm IdentityFile 1018may use the tilde syntax to refer to a user's home directory 1019or the tokens described in the 1020.Sx TOKENS 1021section. 1022.Pp 1023It is possible to have 1024multiple identity files specified in configuration files; all these 1025identities will be tried in sequence. 1026Multiple 1027.Cm IdentityFile 1028directives will add to the list of identities tried (this behaviour 1029differs from that of other configuration directives). 1030.Pp 1031.Cm IdentityFile 1032may be used in conjunction with 1033.Cm IdentitiesOnly 1034to select which identities in an agent are offered during authentication. 1035.Cm IdentityFile 1036may also be used in conjunction with 1037.Cm CertificateFile 1038in order to provide any certificate also needed for authentication with 1039the identity. 1040.It Cm IgnoreUnknown 1041Specifies a pattern-list of unknown options to be ignored if they are 1042encountered in configuration parsing. 1043This may be used to suppress errors if 1044.Nm 1045contains options that are unrecognised by 1046.Xr ssh 1 . 1047It is recommended that 1048.Cm IgnoreUnknown 1049be listed early in the configuration file as it will not be applied 1050to unknown options that appear before it. 1051.It Cm Include 1052Include the specified configuration file(s). 1053Multiple pathnames may be specified and each pathname may contain 1054.Xr glob 7 1055wildcards and, for user configurations, shell-like 1056.Sq ~ 1057references to user home directories. 1058Wildcards will be expanded and processed in lexical order. 1059Files without absolute paths are assumed to be in 1060.Pa ~/.ssh 1061if included in a user configuration file or 1062.Pa /etc/ssh 1063if included from the system configuration file. 1064.Cm Include 1065directive may appear inside a 1066.Cm Match 1067or 1068.Cm Host 1069block 1070to perform conditional inclusion. 1071.It Cm IPQoS 1072Specifies the IPv4 type-of-service or DSCP class for connections. 1073Accepted values are 1074.Cm af11 , 1075.Cm af12 , 1076.Cm af13 , 1077.Cm af21 , 1078.Cm af22 , 1079.Cm af23 , 1080.Cm af31 , 1081.Cm af32 , 1082.Cm af33 , 1083.Cm af41 , 1084.Cm af42 , 1085.Cm af43 , 1086.Cm cs0 , 1087.Cm cs1 , 1088.Cm cs2 , 1089.Cm cs3 , 1090.Cm cs4 , 1091.Cm cs5 , 1092.Cm cs6 , 1093.Cm cs7 , 1094.Cm ef , 1095.Cm le , 1096.Cm lowdelay , 1097.Cm throughput , 1098.Cm reliability , 1099a numeric value, or 1100.Cm none 1101to use the operating system default. 1102This option may take one or two arguments, separated by whitespace. 1103If one argument is specified, it is used as the packet class unconditionally. 1104If two values are specified, the first is automatically selected for 1105interactive sessions and the second for non-interactive sessions. 1106The default is 1107.Cm af21 1108(Low-Latency Data) 1109for interactive sessions and 1110.Cm cs1 1111(Lower Effort) 1112for non-interactive sessions. 1113.It Cm KbdInteractiveAuthentication 1114Specifies whether to use keyboard-interactive authentication. 1115The argument to this keyword must be 1116.Cm yes 1117(the default) 1118or 1119.Cm no . 1120.Cm ChallengeResponseAuthentication 1121is a deprecated alias for this. 1122.It Cm KbdInteractiveDevices 1123Specifies the list of methods to use in keyboard-interactive authentication. 1124Multiple method names must be comma-separated. 1125The default is to use the server specified list. 1126The methods available vary depending on what the server supports. 1127For an OpenSSH server, 1128it may be zero or more of: 1129.Cm bsdauth , 1130.Cm pam , 1131and 1132.Cm skey . 1133.It Cm KexAlgorithms 1134Specifies the available KEX (Key Exchange) algorithms. 1135Multiple algorithms must be comma-separated. 1136If the specified list begins with a 1137.Sq + 1138character, then the specified methods will be appended to the default set 1139instead of replacing them. 1140If the specified list begins with a 1141.Sq - 1142character, then the specified methods (including wildcards) will be removed 1143from the default set instead of replacing them. 1144If the specified list begins with a 1145.Sq ^ 1146character, then the specified methods will be placed at the head of the 1147default set. 1148The default is: 1149.Bd -literal -offset indent 1150curve25519-sha256,curve25519-sha256@libssh.org, 1151ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1152diffie-hellman-group-exchange-sha256, 1153diffie-hellman-group16-sha512, 1154diffie-hellman-group18-sha512, 1155diffie-hellman-group14-sha256 1156.Ed 1157.Pp 1158The list of available key exchange algorithms may also be obtained using 1159.Qq ssh -Q kex . 1160.It Cm KnownHostsCommand 1161Specifies a command to use to obtain a list of host keys, in addition to 1162those listed in 1163.Cm UserKnownHostsFile 1164and 1165.Cm GlobalKnownHostsFile . 1166This command is executed after the files have been read. 1167It may write host key lines to standard output in identical format to the 1168usual files (described in the 1169.Sx VERIFYING HOST KEYS 1170section in 1171.Xr ssh 1 ) . 1172Arguments to 1173.Cm KnownHostsCommand 1174accept the tokens described in the 1175.Sx TOKENS 1176section. 1177The command may be invoked multiple times per connection: once when preparing 1178the preference list of host key algorithms to use, again to obtain the 1179host key for the requested host name and, if 1180.Cm CheckHostIP 1181is enabled, one more time to obtain the host key matching the server's 1182address. 1183If the command exits abnormally or returns a non-zero exit status then the 1184connection is terminated. 1185.It Cm LocalCommand 1186Specifies a command to execute on the local machine after successfully 1187connecting to the server. 1188The command string extends to the end of the line, and is executed with 1189the user's shell. 1190Arguments to 1191.Cm LocalCommand 1192accept the tokens described in the 1193.Sx TOKENS 1194section. 1195.Pp 1196The command is run synchronously and does not have access to the 1197session of the 1198.Xr ssh 1 1199that spawned it. 1200It should not be used for interactive commands. 1201.Pp 1202This directive is ignored unless 1203.Cm PermitLocalCommand 1204has been enabled. 1205.It Cm LocalForward 1206Specifies that a TCP port on the local machine be forwarded over 1207the secure channel to the specified host and port from the remote machine. 1208The first argument specifies the listener and may be 1209.Sm off 1210.Oo Ar bind_address : Oc Ar port 1211.Sm on 1212or a Unix domain socket path. 1213The second argument is the destination and may be 1214.Ar host : Ns Ar hostport 1215or a Unix domain socket path if the remote host supports it. 1216.Pp 1217IPv6 addresses can be specified by enclosing addresses in square brackets. 1218Multiple forwardings may be specified, and additional forwardings can be 1219given on the command line. 1220Only the superuser can forward privileged ports. 1221By default, the local port is bound in accordance with the 1222.Cm GatewayPorts 1223setting. 1224However, an explicit 1225.Ar bind_address 1226may be used to bind the connection to a specific address. 1227The 1228.Ar bind_address 1229of 1230.Cm localhost 1231indicates that the listening port be bound for local use only, while an 1232empty address or 1233.Sq * 1234indicates that the port should be available from all interfaces. 1235Unix domain socket paths may use the tokens described in the 1236.Sx TOKENS 1237section and environment variables as described in the 1238.Sx ENVIRONMENT VARIABLES 1239section. 1240.It Cm LogLevel 1241Gives the verbosity level that is used when logging messages from 1242.Xr ssh 1 . 1243The possible values are: 1244QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1245The default is INFO. 1246DEBUG and DEBUG1 are equivalent. 1247DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1248.It Cm LogVerbose 1249Specify one or more overrides to LogLevel. 1250An override consists of a pattern lists that matches the source file, function 1251and line number to force detailed logging for. 1252For example, an override pattern of: 1253.Bd -literal -offset indent 1254kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1255.Ed 1256.Pp 1257would enable detailed logging for line 1000 of 1258.Pa kex.c , 1259everything in the 1260.Fn kex_exchange_identification 1261function, and all code in the 1262.Pa packet.c 1263file. 1264This option is intended for debugging and no overrides are enabled by default. 1265.It Cm MACs 1266Specifies the MAC (message authentication code) algorithms 1267in order of preference. 1268The MAC algorithm is used for data integrity protection. 1269Multiple algorithms must be comma-separated. 1270If the specified list begins with a 1271.Sq + 1272character, then the specified algorithms will be appended to the default set 1273instead of replacing them. 1274If the specified list begins with a 1275.Sq - 1276character, then the specified algorithms (including wildcards) will be removed 1277from the default set instead of replacing them. 1278If the specified list begins with a 1279.Sq ^ 1280character, then the specified algorithms will be placed at the head of the 1281default set. 1282.Pp 1283The algorithms that contain 1284.Qq -etm 1285calculate the MAC after encryption (encrypt-then-mac). 1286These are considered safer and their use recommended. 1287.Pp 1288The default is: 1289.Bd -literal -offset indent 1290umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1291hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1292hmac-sha1-etm@openssh.com, 1293umac-64@openssh.com,umac-128@openssh.com, 1294hmac-sha2-256,hmac-sha2-512,hmac-sha1 1295.Ed 1296.Pp 1297The list of available MAC algorithms may also be obtained using 1298.Qq ssh -Q mac . 1299.It Cm NoHostAuthenticationForLocalhost 1300Disable host authentication for localhost (loopback addresses). 1301The argument to this keyword must be 1302.Cm yes 1303or 1304.Cm no 1305(the default). 1306.It Cm NumberOfPasswordPrompts 1307Specifies the number of password prompts before giving up. 1308The argument to this keyword must be an integer. 1309The default is 3. 1310.It Cm PasswordAuthentication 1311Specifies whether to use password authentication. 1312The argument to this keyword must be 1313.Cm yes 1314(the default) 1315or 1316.Cm no . 1317.It Cm PermitLocalCommand 1318Allow local command execution via the 1319.Ic LocalCommand 1320option or using the 1321.Ic !\& Ns Ar command 1322escape sequence in 1323.Xr ssh 1 . 1324The argument must be 1325.Cm yes 1326or 1327.Cm no 1328(the default). 1329.It Cm PermitRemoteOpen 1330Specifies the destinations to which remote TCP port forwarding is permitted when 1331.Cm RemoteForward 1332is used as a SOCKS proxy. 1333The forwarding specification must be one of the following forms: 1334.Pp 1335.Bl -item -offset indent -compact 1336.It 1337.Cm PermitRemoteOpen 1338.Sm off 1339.Ar host : port 1340.Sm on 1341.It 1342.Cm PermitRemoteOpen 1343.Sm off 1344.Ar IPv4_addr : port 1345.Sm on 1346.It 1347.Cm PermitRemoteOpen 1348.Sm off 1349.Ar \&[ IPv6_addr \&] : port 1350.Sm on 1351.El 1352.Pp 1353Multiple forwards may be specified by separating them with whitespace. 1354An argument of 1355.Cm any 1356can be used to remove all restrictions and permit any forwarding requests. 1357An argument of 1358.Cm none 1359can be used to prohibit all forwarding requests. 1360The wildcard 1361.Sq * 1362can be used for host or port to allow all hosts or ports respectively. 1363Otherwise, no pattern matching or address lookups are performed on supplied 1364names. 1365.It Cm PKCS11Provider 1366Specifies which PKCS#11 provider to use or 1367.Cm none 1368to indicate that no provider should be used (the default). 1369The argument to this keyword is a path to the PKCS#11 shared library 1370.Xr ssh 1 1371should use to communicate with a PKCS#11 token providing keys for user 1372authentication. 1373.It Cm Port 1374Specifies the port number to connect on the remote host. 1375The default is 22. 1376.It Cm PreferredAuthentications 1377Specifies the order in which the client should try authentication methods. 1378This allows a client to prefer one method (e.g.\& 1379.Cm keyboard-interactive ) 1380over another method (e.g.\& 1381.Cm password ) . 1382The default is: 1383.Bd -literal -offset indent 1384gssapi-with-mic,hostbased,publickey, 1385keyboard-interactive,password 1386.Ed 1387.It Cm ProxyCommand 1388Specifies the command to use to connect to the server. 1389The command 1390string extends to the end of the line, and is executed 1391using the user's shell 1392.Ql exec 1393directive to avoid a lingering shell process. 1394.Pp 1395Arguments to 1396.Cm ProxyCommand 1397accept the tokens described in the 1398.Sx TOKENS 1399section. 1400The command can be basically anything, 1401and should read from its standard input and write to its standard output. 1402It should eventually connect an 1403.Xr sshd 8 1404server running on some machine, or execute 1405.Ic sshd -i 1406somewhere. 1407Host key management will be done using the 1408.Cm Hostname 1409of the host being connected (defaulting to the name typed by the user). 1410Setting the command to 1411.Cm none 1412disables this option entirely. 1413Note that 1414.Cm CheckHostIP 1415is not available for connects with a proxy command. 1416.Pp 1417This directive is useful in conjunction with 1418.Xr nc 1 1419and its proxy support. 1420For example, the following directive would connect via an HTTP proxy at 1421192.0.2.0: 1422.Bd -literal -offset 3n 1423ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1424.Ed 1425.It Cm ProxyJump 1426Specifies one or more jump proxies as either 1427.Xo 1428.Sm off 1429.Op Ar user No @ 1430.Ar host 1431.Op : Ns Ar port 1432.Sm on 1433or an ssh URI 1434.Xc . 1435Multiple proxies may be separated by comma characters and will be visited 1436sequentially. 1437Setting this option will cause 1438.Xr ssh 1 1439to connect to the target host by first making a 1440.Xr ssh 1 1441connection to the specified 1442.Cm ProxyJump 1443host and then establishing a 1444TCP forwarding to the ultimate target from there. 1445Setting the host to 1446.Cm none 1447disables this option entirely. 1448.Pp 1449Note that this option will compete with the 1450.Cm ProxyCommand 1451option - whichever is specified first will prevent later instances of the 1452other from taking effect. 1453.Pp 1454Note also that the configuration for the destination host (either supplied 1455via the command-line or the configuration file) is not generally applied 1456to jump hosts. 1457.Pa ~/.ssh/config 1458should be used if specific configuration is required for jump hosts. 1459.It Cm ProxyUseFdpass 1460Specifies that 1461.Cm ProxyCommand 1462will pass a connected file descriptor back to 1463.Xr ssh 1 1464instead of continuing to execute and pass data. 1465The default is 1466.Cm no . 1467.It Cm PubkeyAcceptedAlgorithms 1468Specifies the signature algorithms that will be used for public key 1469authentication as a comma-separated list of patterns. 1470If the specified list begins with a 1471.Sq + 1472character, then the algorithms after it will be appended to the default 1473instead of replacing it. 1474If the specified list begins with a 1475.Sq - 1476character, then the specified algorithms (including wildcards) will be removed 1477from the default set instead of replacing them. 1478If the specified list begins with a 1479.Sq ^ 1480character, then the specified algorithms will be placed at the head of the 1481default set. 1482The default for this option is: 1483.Bd -literal -offset 3n 1484ssh-ed25519-cert-v01@openssh.com, 1485ecdsa-sha2-nistp256-cert-v01@openssh.com, 1486ecdsa-sha2-nistp384-cert-v01@openssh.com, 1487ecdsa-sha2-nistp521-cert-v01@openssh.com, 1488sk-ssh-ed25519-cert-v01@openssh.com, 1489sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1490rsa-sha2-512-cert-v01@openssh.com, 1491rsa-sha2-256-cert-v01@openssh.com, 1492ssh-rsa-cert-v01@openssh.com, 1493ssh-ed25519, 1494ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1495sk-ssh-ed25519@openssh.com, 1496sk-ecdsa-sha2-nistp256@openssh.com, 1497rsa-sha2-512,rsa-sha2-256,ssh-rsa 1498.Ed 1499.Pp 1500The list of available signature algorithms may also be obtained using 1501.Qq ssh -Q PubkeyAcceptedAlgorithms . 1502.It Cm PubkeyAuthentication 1503Specifies whether to try public key authentication. 1504The argument to this keyword must be 1505.Cm yes 1506(the default) 1507or 1508.Cm no . 1509.It Cm RekeyLimit 1510Specifies the maximum amount of data that may be transmitted before the 1511session key is renegotiated, optionally followed by a maximum amount of 1512time that may pass before the session key is renegotiated. 1513The first argument is specified in bytes and may have a suffix of 1514.Sq K , 1515.Sq M , 1516or 1517.Sq G 1518to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1519The default is between 1520.Sq 1G 1521and 1522.Sq 4G , 1523depending on the cipher. 1524The optional second value is specified in seconds and may use any of the 1525units documented in the TIME FORMATS section of 1526.Xr sshd_config 5 . 1527The default value for 1528.Cm RekeyLimit 1529is 1530.Cm default none , 1531which means that rekeying is performed after the cipher's default amount 1532of data has been sent or received and no time based rekeying is done. 1533.It Cm RemoteCommand 1534Specifies a command to execute on the remote machine after successfully 1535connecting to the server. 1536The command string extends to the end of the line, and is executed with 1537the user's shell. 1538Arguments to 1539.Cm RemoteCommand 1540accept the tokens described in the 1541.Sx TOKENS 1542section. 1543.It Cm RemoteForward 1544Specifies that a TCP port on the remote machine be forwarded over 1545the secure channel. 1546The remote port may either be forwarded to a specified host and port 1547from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote 1548client to connect to arbitrary destinations from the local machine. 1549The first argument is the listening specification and may be 1550.Sm off 1551.Oo Ar bind_address : Oc Ar port 1552.Sm on 1553or, if the remote host supports it, a Unix domain socket path. 1554If forwarding to a specific destination then the second argument must be 1555.Ar host : Ns Ar hostport 1556or a Unix domain socket path, 1557otherwise if no destination argument is specified then the remote forwarding 1558will be established as a SOCKS proxy. 1559When acting as a SOCKS proxy the destination of the connection can be 1560restricted by 1561.Cm PermitRemoteOpen . 1562.Pp 1563IPv6 addresses can be specified by enclosing addresses in square brackets. 1564Multiple forwardings may be specified, and additional 1565forwardings can be given on the command line. 1566Privileged ports can be forwarded only when 1567logging in as root on the remote machine. 1568Unix domain socket paths may use the tokens described in the 1569.Sx TOKENS 1570section and environment variables as described in the 1571.Sx ENVIRONMENT VARIABLES 1572section. 1573.Pp 1574If the 1575.Ar port 1576argument is 0, 1577the listen port will be dynamically allocated on the server and reported 1578to the client at run time. 1579.Pp 1580If the 1581.Ar bind_address 1582is not specified, the default is to only bind to loopback addresses. 1583If the 1584.Ar bind_address 1585is 1586.Ql * 1587or an empty string, then the forwarding is requested to listen on all 1588interfaces. 1589Specifying a remote 1590.Ar bind_address 1591will only succeed if the server's 1592.Cm GatewayPorts 1593option is enabled (see 1594.Xr sshd_config 5 ) . 1595.It Cm RequestTTY 1596Specifies whether to request a pseudo-tty for the session. 1597The argument may be one of: 1598.Cm no 1599(never request a TTY), 1600.Cm yes 1601(always request a TTY when standard input is a TTY), 1602.Cm force 1603(always request a TTY) or 1604.Cm auto 1605(request a TTY when opening a login session). 1606This option mirrors the 1607.Fl t 1608and 1609.Fl T 1610flags for 1611.Xr ssh 1 . 1612.It Cm RevokedHostKeys 1613Specifies revoked host public keys. 1614Keys listed in this file will be refused for host authentication. 1615Note that if this file does not exist or is not readable, 1616then host authentication will be refused for all hosts. 1617Keys may be specified as a text file, listing one public key per line, or as 1618an OpenSSH Key Revocation List (KRL) as generated by 1619.Xr ssh-keygen 1 . 1620For more information on KRLs, see the KEY REVOCATION LISTS section in 1621.Xr ssh-keygen 1 . 1622.It Cm SecurityKeyProvider 1623Specifies a path to a library that will be used when loading any 1624FIDO authenticator-hosted keys, overriding the default of using 1625the built-in USB HID support. 1626.Pp 1627If the specified value begins with a 1628.Sq $ 1629character, then it will be treated as an environment variable containing 1630the path to the library. 1631.It Cm SendEnv 1632Specifies what variables from the local 1633.Xr environ 7 1634should be sent to the server. 1635The server must also support it, and the server must be configured to 1636accept these environment variables. 1637Note that the 1638.Ev TERM 1639environment variable is always sent whenever a 1640pseudo-terminal is requested as it is required by the protocol. 1641Refer to 1642.Cm AcceptEnv 1643in 1644.Xr sshd_config 5 1645for how to configure the server. 1646Variables are specified by name, which may contain wildcard characters. 1647Multiple environment variables may be separated by whitespace or spread 1648across multiple 1649.Cm SendEnv 1650directives. 1651.Pp 1652See 1653.Sx PATTERNS 1654for more information on patterns. 1655.Pp 1656It is possible to clear previously set 1657.Cm SendEnv 1658variable names by prefixing patterns with 1659.Pa - . 1660The default is not to send any environment variables. 1661.It Cm ServerAliveCountMax 1662Sets the number of server alive messages (see below) which may be 1663sent without 1664.Xr ssh 1 1665receiving any messages back from the server. 1666If this threshold is reached while server alive messages are being sent, 1667ssh will disconnect from the server, terminating the session. 1668It is important to note that the use of server alive messages is very 1669different from 1670.Cm TCPKeepAlive 1671(below). 1672The server alive messages are sent through the encrypted channel 1673and therefore will not be spoofable. 1674The TCP keepalive option enabled by 1675.Cm TCPKeepAlive 1676is spoofable. 1677The server alive mechanism is valuable when the client or 1678server depend on knowing when a connection has become unresponsive. 1679.Pp 1680The default value is 3. 1681If, for example, 1682.Cm ServerAliveInterval 1683(see below) is set to 15 and 1684.Cm ServerAliveCountMax 1685is left at the default, if the server becomes unresponsive, 1686ssh will disconnect after approximately 45 seconds. 1687.It Cm ServerAliveInterval 1688Sets a timeout interval in seconds after which if no data has been received 1689from the server, 1690.Xr ssh 1 1691will send a message through the encrypted 1692channel to request a response from the server. 1693The default 1694is 0, indicating that these messages will not be sent to the server. 1695.It Cm SessionType 1696May be used to either request invocation of a subsystem on the remote system, 1697or to prevent the execution of a remote command at all. 1698The latter is useful for just forwarding ports. 1699The argument to this keyword must be 1700.Cm none 1701(same as the 1702.Fl N 1703option), 1704.Cm subsystem 1705(same as the 1706.Fl s 1707option) or 1708.Cm default 1709(shell or command execution). 1710.It Cm SetEnv 1711Directly specify one or more environment variables and their contents to 1712be sent to the server. 1713Similarly to 1714.Cm SendEnv , 1715with the exception of the 1716.Ev TERM 1717variable, the server must be prepared to accept the environment variable. 1718.It Cm StdinNull 1719Redirects stdin from 1720.Pa /dev/null 1721(actually, prevents reading from stdin). 1722Either this or the equivalent 1723.Fl n 1724option must be used when 1725.Nm ssh 1726is run in the background. 1727The argument to this keyword must be 1728.Cm yes 1729(same as the 1730.Fl n 1731option) or 1732.Cm no 1733(the default). 1734.It Cm StreamLocalBindMask 1735Sets the octal file creation mode mask 1736.Pq umask 1737used when creating a Unix-domain socket file for local or remote 1738port forwarding. 1739This option is only used for port forwarding to a Unix-domain socket file. 1740.Pp 1741The default value is 0177, which creates a Unix-domain socket file that is 1742readable and writable only by the owner. 1743Note that not all operating systems honor the file mode on Unix-domain 1744socket files. 1745.It Cm StreamLocalBindUnlink 1746Specifies whether to remove an existing Unix-domain socket file for local 1747or remote port forwarding before creating a new one. 1748If the socket file already exists and 1749.Cm StreamLocalBindUnlink 1750is not enabled, 1751.Nm ssh 1752will be unable to forward the port to the Unix-domain socket file. 1753This option is only used for port forwarding to a Unix-domain socket file. 1754.Pp 1755The argument must be 1756.Cm yes 1757or 1758.Cm no 1759(the default). 1760.It Cm StrictHostKeyChecking 1761If this flag is set to 1762.Cm yes , 1763.Xr ssh 1 1764will never automatically add host keys to the 1765.Pa ~/.ssh/known_hosts 1766file, and refuses to connect to hosts whose host key has changed. 1767This provides maximum protection against man-in-the-middle (MITM) attacks, 1768though it can be annoying when the 1769.Pa /etc/ssh/ssh_known_hosts 1770file is poorly maintained or when connections to new hosts are 1771frequently made. 1772This option forces the user to manually 1773add all new hosts. 1774.Pp 1775If this flag is set to 1776.Dq accept-new 1777then ssh will automatically add new host keys to the user 1778known hosts files, but will not permit connections to hosts with 1779changed host keys. 1780If this flag is set to 1781.Dq no 1782or 1783.Dq off , 1784ssh will automatically add new host keys to the user known hosts files 1785and allow connections to hosts with changed hostkeys to proceed, 1786subject to some restrictions. 1787If this flag is set to 1788.Cm ask 1789(the default), 1790new host keys 1791will be added to the user known host files only after the user 1792has confirmed that is what they really want to do, and 1793ssh will refuse to connect to hosts whose host key has changed. 1794The host keys of 1795known hosts will be verified automatically in all cases. 1796.It Cm SyslogFacility 1797Gives the facility code that is used when logging messages from 1798.Xr ssh 1 . 1799The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1800LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1801The default is USER. 1802.It Cm TCPKeepAlive 1803Specifies whether the system should send TCP keepalive messages to the 1804other side. 1805If they are sent, death of the connection or crash of one 1806of the machines will be properly noticed. 1807However, this means that 1808connections will die if the route is down temporarily, and some people 1809find it annoying. 1810.Pp 1811The default is 1812.Cm yes 1813(to send TCP keepalive messages), and the client will notice 1814if the network goes down or the remote host dies. 1815This is important in scripts, and many users want it too. 1816.Pp 1817To disable TCP keepalive messages, the value should be set to 1818.Cm no . 1819See also 1820.Cm ServerAliveInterval 1821for protocol-level keepalives. 1822.It Cm Tunnel 1823Request 1824.Xr tun 4 1825device forwarding between the client and the server. 1826The argument must be 1827.Cm yes , 1828.Cm point-to-point 1829(layer 3), 1830.Cm ethernet 1831(layer 2), 1832or 1833.Cm no 1834(the default). 1835Specifying 1836.Cm yes 1837requests the default tunnel mode, which is 1838.Cm point-to-point . 1839.It Cm TunnelDevice 1840Specifies the 1841.Xr tun 4 1842devices to open on the client 1843.Pq Ar local_tun 1844and the server 1845.Pq Ar remote_tun . 1846.Pp 1847The argument must be 1848.Sm off 1849.Ar local_tun Op : Ar remote_tun . 1850.Sm on 1851The devices may be specified by numerical ID or the keyword 1852.Cm any , 1853which uses the next available tunnel device. 1854If 1855.Ar remote_tun 1856is not specified, it defaults to 1857.Cm any . 1858The default is 1859.Cm any:any . 1860.It Cm UpdateHostKeys 1861Specifies whether 1862.Xr ssh 1 1863should accept notifications of additional hostkeys from the server sent 1864after authentication has completed and add them to 1865.Cm UserKnownHostsFile . 1866The argument must be 1867.Cm yes , 1868.Cm no 1869or 1870.Cm ask . 1871This option allows learning alternate hostkeys for a server 1872and supports graceful key rotation by allowing a server to send replacement 1873public keys before old ones are removed. 1874.Pp 1875Additional hostkeys are only accepted if the key used to authenticate the 1876host was already trusted or explicitly accepted by the user, the host was 1877authenticated via 1878.Cm UserKnownHostsFile 1879(i.e. not 1880.Cm GlobalKnownHostsFile ) 1881and the host was authenticated using a plain key and not a certificate. 1882.Pp 1883.Cm UpdateHostKeys 1884is enabled by default if the user has not overridden the default 1885.Cm UserKnownHostsFile 1886setting and has not enabled 1887.Cm VerifyHostKeyDNS , 1888otherwise 1889.Cm UpdateHostKeys 1890will be set to 1891.Cm no . 1892.Pp 1893If 1894.Cm UpdateHostKeys 1895is set to 1896.Cm ask , 1897then the user is asked to confirm the modifications to the known_hosts file. 1898Confirmation is currently incompatible with 1899.Cm ControlPersist , 1900and will be disabled if it is enabled. 1901.Pp 1902Presently, only 1903.Xr sshd 8 1904from OpenSSH 6.8 and greater support the 1905.Qq hostkeys@openssh.com 1906protocol extension used to inform the client of all the server's hostkeys. 1907.It Cm User 1908Specifies the user to log in as. 1909This can be useful when a different user name is used on different machines. 1910This saves the trouble of 1911having to remember to give the user name on the command line. 1912.It Cm UserKnownHostsFile 1913Specifies one or more files to use for the user 1914host key database, separated by whitespace. 1915Each filename may use tilde notation to refer to the user's home directory, 1916the tokens described in the 1917.Sx TOKENS 1918section and environment variables as described in the 1919.Sx ENVIRONMENT VARIABLES 1920section. 1921The default is 1922.Pa ~/.ssh/known_hosts , 1923.Pa ~/.ssh/known_hosts2 . 1924.It Cm VerifyHostKeyDNS 1925Specifies whether to verify the remote key using DNS and SSHFP resource 1926records. 1927If this option is set to 1928.Cm yes , 1929the client will implicitly trust keys that match a secure fingerprint 1930from DNS. 1931Insecure fingerprints will be handled as if this option was set to 1932.Cm ask . 1933If this option is set to 1934.Cm ask , 1935information on fingerprint match will be displayed, but the user will still 1936need to confirm new host keys according to the 1937.Cm StrictHostKeyChecking 1938option. 1939The default is 1940.Cm no . 1941.Pp 1942See also 1943.Sx VERIFYING HOST KEYS 1944in 1945.Xr ssh 1 . 1946.It Cm VisualHostKey 1947If this flag is set to 1948.Cm yes , 1949an ASCII art representation of the remote host key fingerprint is 1950printed in addition to the fingerprint string at login and 1951for unknown host keys. 1952If this flag is set to 1953.Cm no 1954(the default), 1955no fingerprint strings are printed at login and 1956only the fingerprint string will be printed for unknown host keys. 1957.It Cm XAuthLocation 1958Specifies the full pathname of the 1959.Xr xauth 1 1960program. 1961The default is 1962.Pa /usr/X11R6/bin/xauth . 1963.El 1964.Sh PATTERNS 1965A 1966.Em pattern 1967consists of zero or more non-whitespace characters, 1968.Sq * 1969(a wildcard that matches zero or more characters), 1970or 1971.Sq ?\& 1972(a wildcard that matches exactly one character). 1973For example, to specify a set of declarations for any host in the 1974.Qq .co.uk 1975set of domains, 1976the following pattern could be used: 1977.Pp 1978.Dl Host *.co.uk 1979.Pp 1980The following pattern 1981would match any host in the 192.168.0.[0-9] network range: 1982.Pp 1983.Dl Host 192.168.0.? 1984.Pp 1985A 1986.Em pattern-list 1987is a comma-separated list of patterns. 1988Patterns within pattern-lists may be negated 1989by preceding them with an exclamation mark 1990.Pq Sq !\& . 1991For example, 1992to allow a key to be used from anywhere within an organization 1993except from the 1994.Qq dialup 1995pool, 1996the following entry (in authorized_keys) could be used: 1997.Pp 1998.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1999.Pp 2000Note that a negated match will never produce a positive result by itself. 2001For example, attempting to match 2002.Qq host3 2003against the following pattern-list will fail: 2004.Pp 2005.Dl from=\&"!host1,!host2\&" 2006.Pp 2007The solution here is to include a term that will yield a positive match, 2008such as a wildcard: 2009.Pp 2010.Dl from=\&"!host1,!host2,*\&" 2011.Sh TOKENS 2012Arguments to some keywords can make use of tokens, 2013which are expanded at runtime: 2014.Pp 2015.Bl -tag -width XXXX -offset indent -compact 2016.It %% 2017A literal 2018.Sq % . 2019.It \&%C 2020Hash of %l%h%p%r. 2021.It %d 2022Local user's home directory. 2023.It %f 2024The fingerprint of the server's host key. 2025.It %H 2026The 2027.Pa known_hosts 2028hostname or address that is being searched for. 2029.It %h 2030The remote hostname. 2031.It \%%I 2032A string describing the reason for a 2033.Cm KnownHostsCommand 2034execution: either 2035.Cm ADDRESS 2036when looking up a host by address (only when 2037.Cm CheckHostIP 2038is enabled), 2039.Cm HOSTNAME 2040when searching by hostname, or 2041.Cm ORDER 2042when preparing the host key algorithm preference list to use for the 2043destination host. 2044.It %i 2045The local user ID. 2046.It %K 2047The base64 encoded host key. 2048.It %k 2049The host key alias if specified, otherwise the original remote hostname given 2050on the command line. 2051.It %L 2052The local hostname. 2053.It %l 2054The local hostname, including the domain name. 2055.It %n 2056The original remote hostname, as given on the command line. 2057.It %p 2058The remote port. 2059.It %r 2060The remote username. 2061.It \&%T 2062The local 2063.Xr tun 4 2064or 2065.Xr tap 4 2066network interface assigned if 2067tunnel forwarding was requested, or 2068.Qq NONE 2069otherwise. 2070.It %t 2071The type of the server host key, e.g. 2072.Cm ssh-ed25519 2073.It %u 2074The local username. 2075.El 2076.Pp 2077.Cm CertificateFile , 2078.Cm ControlPath , 2079.Cm IdentityAgent , 2080.Cm IdentityFile , 2081.Cm KnownHostsCommand , 2082.Cm LocalForward , 2083.Cm Match exec , 2084.Cm RemoteCommand , 2085.Cm RemoteForward , 2086and 2087.Cm UserKnownHostsFile 2088accept the tokens %%, %C, %d, %h, %i, %k, %L, %l, %n, %p, %r, and %u. 2089.Pp 2090.Cm KnownHostsCommand 2091additionally accepts the tokens %f, %H, %I, %K and %t. 2092.Pp 2093.Cm Hostname 2094accepts the tokens %% and %h. 2095.Pp 2096.Cm LocalCommand 2097accepts all tokens. 2098.Pp 2099.Cm ProxyCommand 2100accepts the tokens %%, %h, %n, %p, and %r. 2101.Sh ENVIRONMENT VARIABLES 2102Arguments to some keywords can be expanded at runtime from environment 2103variables on the client by enclosing them in 2104.Ic ${} , 2105for example 2106.Ic ${HOME}/.ssh 2107would refer to the user's .ssh directory. 2108If a specified environment variable does not exist then an error will be 2109returned and the setting for that keyword will be ignored. 2110.Pp 2111The keywords 2112.Cm CertificateFile , 2113.Cm ControlPath , 2114.Cm IdentityAgent , 2115.Cm IdentityFile , 2116.Cm KnownHostsCommand , 2117and 2118.Cm UserKnownHostsFile 2119support environment variables. 2120The keywords 2121.Cm LocalForward 2122and 2123.Cm RemoteForward 2124support environment variables only for Unix domain socket paths. 2125.Sh FILES 2126.Bl -tag -width Ds 2127.It Pa ~/.ssh/config 2128This is the per-user configuration file. 2129The format of this file is described above. 2130This file is used by the SSH client. 2131Because of the potential for abuse, this file must have strict permissions: 2132read/write for the user, and not writable by others. 2133.It Pa /etc/ssh/ssh_config 2134Systemwide configuration file. 2135This file provides defaults for those 2136values that are not specified in the user's configuration file, and 2137for those users who do not have a configuration file. 2138This file must be world-readable. 2139.El 2140.Sh SEE ALSO 2141.Xr ssh 1 2142.Sh AUTHORS 2143.An -nosplit 2144OpenSSH is a derivative of the original and free 2145ssh 1.2.12 release by 2146.An Tatu Ylonen . 2147.An Aaron Campbell , Bob Beck , Markus Friedl , 2148.An Niels Provos , Theo de Raadt 2149and 2150.An Dug Song 2151removed many bugs, re-added newer features and 2152created OpenSSH. 2153.An Markus Friedl 2154contributed the support for SSH protocol versions 1.5 and 2.0. 2155