1.\" 2.\" Author: Tatu Ylonen <[email protected]> 3.\" Copyright (c) 1995 Tatu Ylonen <[email protected]>, 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 1.366 2021/09/25 09:40:33 kn Exp $ 37.\" $FreeBSD$ 38.Dd $Mdocdate: September 25 2021 $ 39.Dt SSH_CONFIG 5 40.Os 41.Sh NAME 42.Nm ssh_config 43.Nd OpenSSH client configuration file 44.Sh DESCRIPTION 45.Xr ssh 1 46obtains configuration data from the following sources in 47the following order: 48.Pp 49.Bl -enum -offset indent -compact 50.It 51command-line options 52.It 53user's configuration file 54.Pq Pa ~/.ssh/config 55.It 56system-wide configuration file 57.Pq Pa /etc/ssh/ssh_config 58.El 59.Pp 60For each parameter, the first obtained value 61will be used. 62The configuration files contain sections separated by 63.Cm Host 64specifications, and that section is only applied for hosts that 65match one of the patterns given in the specification. 66The matched host name is usually the one given on the command line 67(see the 68.Cm CanonicalizeHostname 69option for exceptions). 70.Pp 71Since the first obtained value for each parameter is used, more 72host-specific declarations should be given near the beginning of the 73file, and general defaults at the end. 74.Pp 75The file contains keyword-argument pairs, one per line. 76Lines starting with 77.Ql # 78and empty lines are interpreted as comments. 79Arguments may optionally be enclosed in double quotes 80.Pq \&" 81in order to represent arguments containing spaces. 82Configuration options may be separated by whitespace or 83optional whitespace and exactly one 84.Ql = ; 85the latter format is useful to avoid the need to quote whitespace 86when specifying configuration options using the 87.Nm ssh , 88.Nm scp , 89and 90.Nm sftp 91.Fl o 92option. 93.Pp 94The possible 95keywords and their meanings are as follows (note that 96keywords are case-insensitive and arguments are case-sensitive): 97.Bl -tag -width Ds 98.It Cm Host 99Restricts the following declarations (up to the next 100.Cm Host 101or 102.Cm Match 103keyword) to be only for those hosts that match one of the patterns 104given after the keyword. 105If more than one pattern is provided, they should be separated by whitespace. 106A single 107.Ql * 108as a pattern can be used to provide global 109defaults for all hosts. 110The host is usually the 111.Ar hostname 112argument given on the command line 113(see the 114.Cm CanonicalizeHostname 115keyword for exceptions). 116.Pp 117A pattern entry may be negated by prefixing it with an exclamation mark 118.Pq Sq !\& . 119If a negated entry is matched, then the 120.Cm Host 121entry is ignored, regardless of whether any other patterns on the line 122match. 123Negated matches are therefore useful to provide exceptions for wildcard 124matches. 125.Pp 126See 127.Sx PATTERNS 128for more information on patterns. 129.It Cm Match 130Restricts the following declarations (up to the next 131.Cm Host 132or 133.Cm Match 134keyword) to be used only when the conditions following the 135.Cm Match 136keyword are satisfied. 137Match conditions are specified using one or more criteria 138or the single token 139.Cm all 140which always matches. 141The available criteria keywords are: 142.Cm canonical , 143.Cm final , 144.Cm exec , 145.Cm host , 146.Cm originalhost , 147.Cm user , 148and 149.Cm localuser . 150The 151.Cm all 152criteria must appear alone or immediately after 153.Cm canonical 154or 155.Cm final . 156Other criteria may be combined arbitrarily. 157All criteria but 158.Cm all , 159.Cm canonical , 160and 161.Cm final 162require an argument. 163Criteria may be negated by prepending an exclamation mark 164.Pq Sq !\& . 165.Pp 166The 167.Cm canonical 168keyword matches only when the configuration file is being re-parsed 169after hostname canonicalization (see the 170.Cm CanonicalizeHostname 171option). 172This may be useful to specify conditions that work with canonical host 173names only. 174.Pp 175The 176.Cm final 177keyword requests that the configuration be re-parsed (regardless of whether 178.Cm CanonicalizeHostname 179is enabled), and matches only during this final pass. 180If 181.Cm CanonicalizeHostname 182is enabled, then 183.Cm canonical 184and 185.Cm final 186match during the same pass. 187.Pp 188The 189.Cm exec 190keyword executes the specified command under the user's shell. 191If the command returns a zero exit status then the condition is considered true. 192Commands containing whitespace characters must be quoted. 193Arguments to 194.Cm exec 195accept the tokens described in the 196.Sx TOKENS 197section. 198.Pp 199The other keywords' criteria must be single entries or comma-separated 200lists and may use the wildcard and negation operators described in the 201.Sx PATTERNS 202section. 203The criteria for the 204.Cm host 205keyword are matched against the target hostname, after any substitution 206by the 207.Cm Hostname 208or 209.Cm CanonicalizeHostname 210options. 211The 212.Cm originalhost 213keyword matches against the hostname as it was specified on the command-line. 214The 215.Cm user 216keyword matches against the target username on the remote host. 217The 218.Cm localuser 219keyword matches against the name of the local user running 220.Xr ssh 1 221(this keyword may be useful in system-wide 222.Nm 223files). 224.It Cm AddKeysToAgent 225Specifies whether keys should be automatically added to a running 226.Xr ssh-agent 1 . 227If this option is set to 228.Cm yes 229and a key is loaded from a file, the key and its passphrase are added to 230the agent with the default lifetime, as if by 231.Xr ssh-add 1 . 232If this option is set to 233.Cm ask , 234.Xr ssh 1 235will require confirmation using the 236.Ev SSH_ASKPASS 237program before adding a key (see 238.Xr ssh-add 1 239for details). 240If this option is set to 241.Cm confirm , 242each use of the key must be confirmed, as if the 243.Fl c 244option was specified to 245.Xr ssh-add 1 . 246If this option is set to 247.Cm no , 248no keys are added to the agent. 249Alternately, this option may be specified as a time interval 250using the format described in the 251.Sx TIME FORMATS 252section of 253.Xr sshd_config 5 254to specify the key's lifetime in 255.Xr ssh-agent 1 , 256after which it will automatically be removed. 257The argument must be 258.Cm no 259(the default), 260.Cm yes , 261.Cm confirm 262(optionally followed by a time interval), 263.Cm ask 264or a time interval. 265.It Cm AddressFamily 266Specifies which address family to use when connecting. 267Valid arguments are 268.Cm any 269(the default), 270.Cm inet 271(use IPv4 only), or 272.Cm inet6 273(use IPv6 only). 274.It Cm BatchMode 275If set to 276.Cm yes , 277user interaction such as password prompts and host key confirmation requests 278will be disabled. 279This option is useful in scripts and other batch jobs where no user 280is present to interact with 281.Xr ssh 1 . 282The argument must be 283.Cm yes 284or 285.Cm no 286(the default). 287.It Cm BindAddress 288Use the specified address on the local machine as the source address of 289the connection. 290Only useful on systems with more than one address. 291.It Cm BindInterface 292Use the address of the specified interface on the local machine as the 293source address of the connection. 294.It Cm CanonicalDomains 295When 296.Cm CanonicalizeHostname 297is enabled, this option specifies the list of domain suffixes in which to 298search for the specified destination host. 299.It Cm CanonicalizeFallbackLocal 300Specifies whether to fail with an error when hostname canonicalization fails. 301The default, 302.Cm yes , 303will attempt to look up the unqualified hostname using the system resolver's 304search rules. 305A value of 306.Cm no 307will cause 308.Xr ssh 1 309to fail instantly if 310.Cm CanonicalizeHostname 311is enabled and the target hostname cannot be found in any of the domains 312specified by 313.Cm CanonicalDomains . 314.It Cm CanonicalizeHostname 315Controls whether explicit hostname canonicalization is performed. 316The default, 317.Cm no , 318is not to perform any name rewriting and let the system resolver handle all 319hostname lookups. 320If set to 321.Cm yes 322then, for connections that do not use a 323.Cm ProxyCommand 324or 325.Cm ProxyJump , 326.Xr ssh 1 327will attempt to canonicalize the hostname specified on the command line 328using the 329.Cm CanonicalDomains 330suffixes and 331.Cm CanonicalizePermittedCNAMEs 332rules. 333If 334.Cm CanonicalizeHostname 335is set to 336.Cm always , 337then canonicalization is applied to proxied connections too. 338.Pp 339If this option is enabled, then the configuration files are processed 340again using the new target name to pick up any new configuration in matching 341.Cm Host 342and 343.Cm Match 344stanzas. 345A value of 346.Cm none 347disables the use of a 348.Cm ProxyJump 349host. 350.It Cm CanonicalizeMaxDots 351Specifies the maximum number of dot characters in a hostname before 352canonicalization is disabled. 353The default, 1, 354allows a single dot (i.e. hostname.subdomain). 355.It Cm CanonicalizePermittedCNAMEs 356Specifies rules to determine whether CNAMEs should be followed when 357canonicalizing hostnames. 358The rules consist of one or more arguments of 359.Ar source_domain_list : Ns Ar target_domain_list , 360where 361.Ar source_domain_list 362is a pattern-list of domains that may follow CNAMEs in canonicalization, 363and 364.Ar target_domain_list 365is a pattern-list of domains that they may resolve to. 366.Pp 367For example, 368.Qq *.a.example.com:*.b.example.com,*.c.example.com 369will allow hostnames matching 370.Qq *.a.example.com 371to be canonicalized to names in the 372.Qq *.b.example.com 373or 374.Qq *.c.example.com 375domains. 376.Pp 377A single argument of 378.Qq none 379causes no CNAMEs to be considered for canonicalization. 380This is the default behaviour. 381.It Cm CASignatureAlgorithms 382Specifies which algorithms are allowed for signing of certificates 383by certificate authorities (CAs). 384The default is: 385.Bd -literal -offset indent 386ssh-ed25519,ecdsa-sha2-nistp256, 387ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 388[email protected], 389[email protected], 390rsa-sha2-512,rsa-sha2-256 391.Ed 392.Pp 393If the specified list begins with a 394.Sq + 395character, then the specified algorithms will be appended to the default set 396instead of replacing them. 397If the specified list begins with a 398.Sq - 399character, then the specified algorithms (including wildcards) will be removed 400from the default set instead of replacing them. 401.Pp 402.Xr ssh 1 403will not accept host certificates signed using algorithms other than those 404specified. 405.It Cm CertificateFile 406Specifies a file from which the user's certificate is read. 407A corresponding private key must be provided separately in order 408to use this certificate either 409from an 410.Cm IdentityFile 411directive or 412.Fl i 413flag to 414.Xr ssh 1 , 415via 416.Xr ssh-agent 1 , 417or via a 418.Cm PKCS11Provider 419or 420.Cm SecurityKeyProvider . 421.Pp 422Arguments to 423.Cm CertificateFile 424may use the tilde syntax to refer to a user's home directory, 425the tokens described in the 426.Sx TOKENS 427section and environment variables as described in the 428.Sx ENVIRONMENT VARIABLES 429section. 430.Pp 431It is possible to have multiple certificate files specified in 432configuration files; these certificates will be tried in sequence. 433Multiple 434.Cm CertificateFile 435directives will add to the list of certificates used for 436authentication. 437.It Cm CheckHostIP 438If set to 439.Cm yes , 440.Xr ssh 1 441will additionally check the host IP address in the 442.Pa known_hosts 443file. 444This allows it to detect if a host key changed due to DNS spoofing 445and will add addresses of destination hosts to 446.Pa ~/.ssh/known_hosts 447in the process, regardless of the setting of 448.Cm StrictHostKeyChecking . 449If the option is set to 450.Cm no 451(the default), 452the check will not be executed. 453The default is 454.Cm no . 455.It Cm Ciphers 456Specifies the ciphers allowed and their order of preference. 457Multiple ciphers must be comma-separated. 458If the specified list begins with a 459.Sq + 460character, then the specified ciphers will be appended to the default set 461instead of replacing them. 462If the specified list begins with a 463.Sq - 464character, then the specified ciphers (including wildcards) will be removed 465from the default set instead of replacing them. 466If the specified list begins with a 467.Sq ^ 468character, then the specified ciphers will be placed at the head of the 469default set. 470.Pp 471The supported ciphers are: 472.Bd -literal -offset indent 4733des-cbc 474aes128-cbc 475aes192-cbc 476aes256-cbc 477aes128-ctr 478aes192-ctr 479aes256-ctr 480[email protected] 481[email protected] 482[email protected] 483.Ed 484.Pp 485The default is: 486.Bd -literal -offset indent 487[email protected], 488aes128-ctr,aes192-ctr,aes256-ctr, 489[email protected],[email protected] 490.Ed 491.Pp 492The list of available ciphers may also be obtained using 493.Qq ssh -Q cipher . 494.It Cm ClearAllForwardings 495Specifies that all local, remote, and dynamic port forwardings 496specified in the configuration files or on the command line be 497cleared. 498This option is primarily useful when used from the 499.Xr ssh 1 500command line to clear port forwardings set in 501configuration files, and is automatically set by 502.Xr scp 1 503and 504.Xr sftp 1 . 505The argument must be 506.Cm yes 507or 508.Cm no 509(the default). 510.It Cm Compression 511Specifies whether to use compression. 512The argument must be 513.Cm yes 514or 515.Cm no 516(the default). 517.It Cm ConnectionAttempts 518Specifies the number of tries (one per second) to make before exiting. 519The argument must be an integer. 520This may be useful in scripts if the connection sometimes fails. 521The default is 1. 522.It Cm ConnectTimeout 523Specifies the timeout (in seconds) used when connecting to the 524SSH server, instead of using the default system TCP timeout. 525This timeout is applied both to establishing the connection and to performing 526the initial SSH protocol handshake and key exchange. 527.It Cm ControlMaster 528Enables the sharing of multiple sessions over a single network connection. 529When set to 530.Cm yes , 531.Xr ssh 1 532will listen for connections on a control socket specified using the 533.Cm ControlPath 534argument. 535Additional sessions can connect to this socket using the same 536.Cm ControlPath 537with 538.Cm ControlMaster 539set to 540.Cm no 541(the default). 542These sessions will try to reuse the master instance's network connection 543rather than initiating new ones, but will fall back to connecting normally 544if the control socket does not exist, or is not listening. 545.Pp 546Setting this to 547.Cm ask 548will cause 549.Xr ssh 1 550to listen for control connections, but require confirmation using 551.Xr ssh-askpass 1 . 552If the 553.Cm ControlPath 554cannot be opened, 555.Xr ssh 1 556will continue without connecting to a master instance. 557.Pp 558X11 and 559.Xr ssh-agent 1 560forwarding is supported over these multiplexed connections, however the 561display and agent forwarded will be the one belonging to the master 562connection i.e. it is not possible to forward multiple displays or agents. 563.Pp 564Two additional options allow for opportunistic multiplexing: try to use a 565master connection but fall back to creating a new one if one does not already 566exist. 567These options are: 568.Cm auto 569and 570.Cm autoask . 571The latter requires confirmation like the 572.Cm ask 573option. 574.It Cm ControlPath 575Specify the path to the control socket used for connection sharing as described 576in the 577.Cm ControlMaster 578section above or the string 579.Cm none 580to disable connection sharing. 581Arguments to 582.Cm ControlPath 583may use the tilde syntax to refer to a user's home directory, 584the tokens described in the 585.Sx TOKENS 586section and environment variables as described in the 587.Sx ENVIRONMENT VARIABLES 588section. 589It is recommended that any 590.Cm ControlPath 591used for opportunistic connection sharing include 592at least %h, %p, and %r (or alternatively %C) and be placed in a directory 593that is not writable by other users. 594This ensures that shared connections are uniquely identified. 595.It Cm ControlPersist 596When used in conjunction with 597.Cm ControlMaster , 598specifies that the master connection should remain open 599in the background (waiting for future client connections) 600after the initial client connection has been closed. 601If set to 602.Cm no 603(the default), 604then the master connection will not be placed into the background, 605and will close as soon as the initial client connection is closed. 606If set to 607.Cm yes 608or 0, 609then the master connection will remain in the background indefinitely 610(until killed or closed via a mechanism such as the 611.Qq ssh -O exit ) . 612If set to a time in seconds, or a time in any of the formats documented in 613.Xr sshd_config 5 , 614then the backgrounded master connection will automatically terminate 615after it has remained idle (with no client connections) for the 616specified time. 617.It Cm DynamicForward 618Specifies that a TCP port on the local machine be forwarded 619over the secure channel, and the application 620protocol is then used to determine where to connect to from the 621remote machine. 622.Pp 623The argument must be 624.Sm off 625.Oo Ar bind_address : Oc Ar port . 626.Sm on 627IPv6 addresses can be specified by enclosing addresses in square brackets. 628By default, the local port is bound in accordance with the 629.Cm GatewayPorts 630setting. 631However, an explicit 632.Ar bind_address 633may be used to bind the connection to a specific address. 634The 635.Ar bind_address 636of 637.Cm localhost 638indicates that the listening port be bound for local use only, while an 639empty address or 640.Sq * 641indicates that the port should be available from all interfaces. 642.Pp 643Currently the SOCKS4 and SOCKS5 protocols are supported, and 644.Xr ssh 1 645will act as a SOCKS server. 646Multiple forwardings may be specified, and 647additional forwardings can be given on the command line. 648Only the superuser can forward privileged ports. 649.It Cm EnableSSHKeysign 650Setting this option to 651.Cm yes 652in the global client configuration file 653.Pa /etc/ssh/ssh_config 654enables the use of the helper program 655.Xr ssh-keysign 8 656during 657.Cm HostbasedAuthentication . 658The argument must be 659.Cm yes 660or 661.Cm no 662(the default). 663This option should be placed in the non-hostspecific section. 664See 665.Xr ssh-keysign 8 666for more information. 667.It Cm EscapeChar 668Sets the escape character (default: 669.Ql ~ ) . 670The escape character can also 671be set on the command line. 672The argument should be a single character, 673.Ql ^ 674followed by a letter, or 675.Cm none 676to disable the escape 677character entirely (making the connection transparent for binary 678data). 679.It Cm ExitOnForwardFailure 680Specifies whether 681.Xr ssh 1 682should terminate the connection if it cannot set up all requested 683dynamic, tunnel, local, and remote port forwardings, (e.g.\& 684if either end is unable to bind and listen on a specified port). 685Note that 686.Cm ExitOnForwardFailure 687does not apply to connections made over port forwardings and will not, 688for example, cause 689.Xr ssh 1 690to exit if TCP connections to the ultimate forwarding destination fail. 691The argument must be 692.Cm yes 693or 694.Cm no 695(the default). 696.It Cm FingerprintHash 697Specifies the hash algorithm used when displaying key fingerprints. 698Valid options are: 699.Cm md5 700and 701.Cm sha256 702(the default). 703.It Cm ForkAfterAuthentication 704Requests 705.Nm ssh 706to go to background just before command execution. 707This is useful if 708.Nm ssh 709is going to ask for passwords or passphrases, but the user 710wants it in the background. 711This implies the 712.Cm StdinNull 713configuration option being set to 714.Dq yes . 715The recommended way to start X11 programs at a remote site is with 716something like 717.Ic ssh -f host xterm , 718which is the same as 719.Ic ssh host xterm 720if the 721.Cm ForkAfterAuthentication 722configuration option is set to 723.Dq yes . 724.Pp 725If the 726.Cm ExitOnForwardFailure 727configuration option is set to 728.Dq yes , 729then a client started with the 730.Cm ForkAfterAuthentication 731configuration option being set to 732.Dq yes 733will wait for all remote port forwards to be successfully established 734before placing itself in the background. 735The argument to this keyword must be 736.Cm yes 737(same as the 738.Fl f 739option) or 740.Cm no 741(the default). 742.It Cm ForwardAgent 743Specifies whether the connection to the authentication agent (if any) 744will be forwarded to the remote machine. 745The argument may be 746.Cm yes , 747.Cm no 748(the default), 749an explicit path to an agent socket or the name of an environment variable 750(beginning with 751.Sq $ ) 752in which to find the path. 753.Pp 754Agent forwarding should be enabled with caution. 755Users with the ability to bypass file permissions on the remote host 756(for the agent's Unix-domain socket) 757can access the local agent through the forwarded connection. 758An attacker cannot obtain key material from the agent, 759however they can perform operations on the keys that enable them to 760authenticate using the identities loaded into the agent. 761.It Cm ForwardX11 762Specifies whether X11 connections will be automatically redirected 763over the secure channel and 764.Ev DISPLAY 765set. 766The argument must be 767.Cm yes 768or 769.Cm no 770(the default). 771.Pp 772X11 forwarding should be enabled with caution. 773Users with the ability to bypass file permissions on the remote host 774(for the user's X11 authorization database) 775can access the local X11 display through the forwarded connection. 776An attacker may then be able to perform activities such as keystroke monitoring 777if the 778.Cm ForwardX11Trusted 779option is also enabled. 780.It Cm ForwardX11Timeout 781Specify a timeout for untrusted X11 forwarding 782using the format described in the 783.Sx TIME FORMATS 784section of 785.Xr sshd_config 5 . 786X11 connections received by 787.Xr ssh 1 788after this time will be refused. 789Setting 790.Cm ForwardX11Timeout 791to zero will disable the timeout and permit X11 forwarding for the life 792of the connection. 793The default is to disable untrusted X11 forwarding after twenty minutes has 794elapsed. 795.It Cm ForwardX11Trusted 796If this option is set to 797.Cm yes , 798remote X11 clients will have full access to the original X11 display. 799.Pp 800If this option is set to 801.Cm no 802(the default), 803remote X11 clients will be considered untrusted and prevented 804from stealing or tampering with data belonging to trusted X11 805clients. 806Furthermore, the 807.Xr xauth 1 808token used for the session will be set to expire after 20 minutes. 809Remote clients will be refused access after this time. 810.Pp 811See the X11 SECURITY extension specification for full details on 812the restrictions imposed on untrusted clients. 813.It Cm GatewayPorts 814Specifies whether remote hosts are allowed to connect to local 815forwarded ports. 816By default, 817.Xr ssh 1 818binds local port forwardings to the loopback address. 819This prevents other remote hosts from connecting to forwarded ports. 820.Cm GatewayPorts 821can be used to specify that ssh 822should bind local port forwardings to the wildcard address, 823thus allowing remote hosts to connect to forwarded ports. 824The argument must be 825.Cm yes 826or 827.Cm no 828(the default). 829.It Cm GlobalKnownHostsFile 830Specifies one or more files to use for the global 831host key database, separated by whitespace. 832The default is 833.Pa /etc/ssh/ssh_known_hosts , 834.Pa /etc/ssh/ssh_known_hosts2 . 835.It Cm GSSAPIAuthentication 836Specifies whether user authentication based on GSSAPI is allowed. 837The default is 838.Cm no . 839.It Cm GSSAPIDelegateCredentials 840Forward (delegate) credentials to the server. 841The default is 842.Cm no . 843.It Cm HashKnownHosts 844Indicates that 845.Xr ssh 1 846should hash host names and addresses when they are added to 847.Pa ~/.ssh/known_hosts . 848These hashed names may be used normally by 849.Xr ssh 1 850and 851.Xr sshd 8 , 852but they do not visually reveal identifying information if the 853file's contents are disclosed. 854The default is 855.Cm no . 856Note that existing names and addresses in known hosts files 857will not be converted automatically, 858but may be manually hashed using 859.Xr ssh-keygen 1 . 860.It Cm HostbasedAcceptedAlgorithms 861Specifies the signature algorithms that will be used for hostbased 862authentication as a comma-separated list of patterns. 863Alternately if the specified list begins with a 864.Sq + 865character, then the specified signature algorithms will be appended 866to the default set instead of replacing them. 867If the specified list begins with a 868.Sq - 869character, then the specified signature algorithms (including wildcards) 870will be removed from the default set instead of replacing them. 871If the specified list begins with a 872.Sq ^ 873character, then the specified signature algorithms will be placed 874at the head of the default set. 875The default for this option is: 876.Bd -literal -offset 3n 877[email protected], 878[email protected], 879[email protected], 880[email protected], 881[email protected], 882[email protected], 883[email protected], 884[email protected], 885[email protected], 886ssh-ed25519, 887ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 888[email protected], 889[email protected], 890rsa-sha2-512,rsa-sha2-256,ssh-rsa 891.Ed 892.Pp 893The 894.Fl Q 895option of 896.Xr ssh 1 897may be used to list supported signature algorithms. 898This was formerly named HostbasedKeyTypes. 899.It Cm HostbasedAuthentication 900Specifies whether to try rhosts based authentication with public key 901authentication. 902The argument must be 903.Cm yes 904or 905.Cm no 906(the default). 907.It Cm HostKeyAlgorithms 908Specifies the host key signature algorithms 909that the client wants to use in order of preference. 910Alternately if the specified list begins with a 911.Sq + 912character, then the specified signature algorithms will be appended to 913the default set instead of replacing them. 914If the specified list begins with a 915.Sq - 916character, then the specified signature algorithms (including wildcards) 917will be removed from the default set instead of replacing them. 918If the specified list begins with a 919.Sq ^ 920character, then the specified signature algorithms will be placed 921at the head of the default set. 922The default for this option is: 923.Bd -literal -offset 3n 924[email protected], 925[email protected], 926[email protected], 927[email protected], 928[email protected], 929[email protected], 930[email protected], 931[email protected], 932[email protected], 933ssh-ed25519, 934ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 935[email protected], 936[email protected], 937rsa-sha2-512,rsa-sha2-256,ssh-rsa 938.Ed 939.Pp 940If hostkeys are known for the destination host then this default is modified 941to prefer their algorithms. 942.Pp 943The list of available signature algorithms may also be obtained using 944.Qq ssh -Q HostKeyAlgorithms . 945.It Cm HostKeyAlias 946Specifies an alias that should be used instead of the 947real host name when looking up or saving the host key 948in the host key database files and when validating host certificates. 949This option is useful for tunneling SSH connections 950or for multiple servers running on a single host. 951.It Cm Hostname 952Specifies the real host name to log into. 953This can be used to specify nicknames or abbreviations for hosts. 954Arguments to 955.Cm Hostname 956accept the tokens described in the 957.Sx TOKENS 958section. 959Numeric IP addresses are also permitted (both on the command line and in 960.Cm Hostname 961specifications). 962The default is the name given on the command line. 963.It Cm IdentitiesOnly 964Specifies that 965.Xr ssh 1 966should only use the configured authentication identity and certificate files 967(either the default files, or those explicitly configured in the 968.Nm 969files 970or passed on the 971.Xr ssh 1 972command-line), 973even if 974.Xr ssh-agent 1 975or a 976.Cm PKCS11Provider 977or 978.Cm SecurityKeyProvider 979offers more identities. 980The argument to this keyword must be 981.Cm yes 982or 983.Cm no 984(the default). 985This option is intended for situations where ssh-agent 986offers many different identities. 987.It Cm IdentityAgent 988Specifies the 989.Ux Ns -domain 990socket used to communicate with the authentication agent. 991.Pp 992This option overrides the 993.Ev SSH_AUTH_SOCK 994environment variable and can be used to select a specific agent. 995Setting the socket name to 996.Cm none 997disables the use of an authentication agent. 998If the string 999.Qq SSH_AUTH_SOCK 1000is specified, the location of the socket will be read from the 1001.Ev SSH_AUTH_SOCK 1002environment variable. 1003Otherwise if the specified value begins with a 1004.Sq $ 1005character, then it will be treated as an environment variable containing 1006the location of the socket. 1007.Pp 1008Arguments to 1009.Cm IdentityAgent 1010may use the tilde syntax to refer to a user's home directory, 1011the tokens described in the 1012.Sx TOKENS 1013section and environment variables as described in the 1014.Sx ENVIRONMENT VARIABLES 1015section. 1016.It Cm IdentityFile 1017Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA, 1018Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read. 1019The default is 1020.Pa ~/.ssh/id_dsa , 1021.Pa ~/.ssh/id_ecdsa , 1022.Pa ~/.ssh/id_ecdsa_sk , 1023.Pa ~/.ssh/id_ed25519 , 1024.Pa ~/.ssh/id_ed25519_sk 1025and 1026.Pa ~/.ssh/id_rsa . 1027Additionally, any identities represented by the authentication agent 1028will be used for authentication unless 1029.Cm IdentitiesOnly 1030is set. 1031If no certificates have been explicitly specified by 1032.Cm CertificateFile , 1033.Xr ssh 1 1034will try to load certificate information from the filename obtained by 1035appending 1036.Pa -cert.pub 1037to the path of a specified 1038.Cm IdentityFile . 1039.Pp 1040Arguments to 1041.Cm IdentityFile 1042may use the tilde syntax to refer to a user's home directory 1043or the tokens described in the 1044.Sx TOKENS 1045section. 1046.Pp 1047It is possible to have 1048multiple identity files specified in configuration files; all these 1049identities will be tried in sequence. 1050Multiple 1051.Cm IdentityFile 1052directives will add to the list of identities tried (this behaviour 1053differs from that of other configuration directives). 1054.Pp 1055.Cm IdentityFile 1056may be used in conjunction with 1057.Cm IdentitiesOnly 1058to select which identities in an agent are offered during authentication. 1059.Cm IdentityFile 1060may also be used in conjunction with 1061.Cm CertificateFile 1062in order to provide any certificate also needed for authentication with 1063the identity. 1064.It Cm IgnoreUnknown 1065Specifies a pattern-list of unknown options to be ignored if they are 1066encountered in configuration parsing. 1067This may be used to suppress errors if 1068.Nm 1069contains options that are unrecognised by 1070.Xr ssh 1 . 1071It is recommended that 1072.Cm IgnoreUnknown 1073be listed early in the configuration file as it will not be applied 1074to unknown options that appear before it. 1075.It Cm Include 1076Include the specified configuration file(s). 1077Multiple pathnames may be specified and each pathname may contain 1078.Xr glob 7 1079wildcards and, for user configurations, shell-like 1080.Sq ~ 1081references to user home directories. 1082Wildcards will be expanded and processed in lexical order. 1083Files without absolute paths are assumed to be in 1084.Pa ~/.ssh 1085if included in a user configuration file or 1086.Pa /etc/ssh 1087if included from the system configuration file. 1088.Cm Include 1089directive may appear inside a 1090.Cm Match 1091or 1092.Cm Host 1093block 1094to perform conditional inclusion. 1095.It Cm IPQoS 1096Specifies the IPv4 type-of-service or DSCP class for connections. 1097Accepted values are 1098.Cm af11 , 1099.Cm af12 , 1100.Cm af13 , 1101.Cm af21 , 1102.Cm af22 , 1103.Cm af23 , 1104.Cm af31 , 1105.Cm af32 , 1106.Cm af33 , 1107.Cm af41 , 1108.Cm af42 , 1109.Cm af43 , 1110.Cm cs0 , 1111.Cm cs1 , 1112.Cm cs2 , 1113.Cm cs3 , 1114.Cm cs4 , 1115.Cm cs5 , 1116.Cm cs6 , 1117.Cm cs7 , 1118.Cm ef , 1119.Cm le , 1120.Cm lowdelay , 1121.Cm throughput , 1122.Cm reliability , 1123a numeric value, or 1124.Cm none 1125to use the operating system default. 1126This option may take one or two arguments, separated by whitespace. 1127If one argument is specified, it is used as the packet class unconditionally. 1128If two values are specified, the first is automatically selected for 1129interactive sessions and the second for non-interactive sessions. 1130The default is 1131.Cm af21 1132(Low-Latency Data) 1133for interactive sessions and 1134.Cm cs1 1135(Lower Effort) 1136for non-interactive sessions. 1137.It Cm KbdInteractiveAuthentication 1138Specifies whether to use keyboard-interactive authentication. 1139The argument to this keyword must be 1140.Cm yes 1141(the default) 1142or 1143.Cm no . 1144.Cm ChallengeResponseAuthentication 1145is a deprecated alias for this. 1146.It Cm KbdInteractiveDevices 1147Specifies the list of methods to use in keyboard-interactive authentication. 1148Multiple method names must be comma-separated. 1149The default is to use the server specified list. 1150The methods available vary depending on what the server supports. 1151For an OpenSSH server, 1152it may be zero or more of: 1153.Cm bsdauth 1154and 1155.Cm pam . 1156.It Cm KexAlgorithms 1157Specifies the available KEX (Key Exchange) algorithms. 1158Multiple algorithms must be comma-separated. 1159If the specified list begins with a 1160.Sq + 1161character, then the specified algorithms will be appended to the default set 1162instead of replacing them. 1163If the specified list begins with a 1164.Sq - 1165character, then the specified algorithms (including wildcards) will be removed 1166from the default set instead of replacing them. 1167If the specified list begins with a 1168.Sq ^ 1169character, then the specified algorithms will be placed at the head of the 1170default set. 1171The default is: 1172.Bd -literal -offset indent 1173curve25519-sha256,[email protected], 1174ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1175diffie-hellman-group-exchange-sha256, 1176diffie-hellman-group16-sha512, 1177diffie-hellman-group18-sha512, 1178diffie-hellman-group14-sha256 1179.Ed 1180.Pp 1181The list of available key exchange algorithms may also be obtained using 1182.Qq ssh -Q kex . 1183.It Cm KnownHostsCommand 1184Specifies a command to use to obtain a list of host keys, in addition to 1185those listed in 1186.Cm UserKnownHostsFile 1187and 1188.Cm GlobalKnownHostsFile . 1189This command is executed after the files have been read. 1190It may write host key lines to standard output in identical format to the 1191usual files (described in the 1192.Sx VERIFYING HOST KEYS 1193section in 1194.Xr ssh 1 ) . 1195Arguments to 1196.Cm KnownHostsCommand 1197accept the tokens described in the 1198.Sx TOKENS 1199section. 1200The command may be invoked multiple times per connection: once when preparing 1201the preference list of host key algorithms to use, again to obtain the 1202host key for the requested host name and, if 1203.Cm CheckHostIP 1204is enabled, one more time to obtain the host key matching the server's 1205address. 1206If the command exits abnormally or returns a non-zero exit status then the 1207connection is terminated. 1208.It Cm LocalCommand 1209Specifies a command to execute on the local machine after successfully 1210connecting to the server. 1211The command string extends to the end of the line, and is executed with 1212the user's shell. 1213Arguments to 1214.Cm LocalCommand 1215accept the tokens described in the 1216.Sx TOKENS 1217section. 1218.Pp 1219The command is run synchronously and does not have access to the 1220session of the 1221.Xr ssh 1 1222that spawned it. 1223It should not be used for interactive commands. 1224.Pp 1225This directive is ignored unless 1226.Cm PermitLocalCommand 1227has been enabled. 1228.It Cm LocalForward 1229Specifies that a TCP port on the local machine be forwarded over 1230the secure channel to the specified host and port from the remote machine. 1231The first argument specifies the listener and may be 1232.Sm off 1233.Oo Ar bind_address : Oc Ar port 1234.Sm on 1235or a Unix domain socket path. 1236The second argument is the destination and may be 1237.Ar host : Ns Ar hostport 1238or a Unix domain socket path if the remote host supports it. 1239.Pp 1240IPv6 addresses can be specified by enclosing addresses in square brackets. 1241Multiple forwardings may be specified, and additional forwardings can be 1242given on the command line. 1243Only the superuser can forward privileged ports. 1244By default, the local port is bound in accordance with the 1245.Cm GatewayPorts 1246setting. 1247However, an explicit 1248.Ar bind_address 1249may be used to bind the connection to a specific address. 1250The 1251.Ar bind_address 1252of 1253.Cm localhost 1254indicates that the listening port be bound for local use only, while an 1255empty address or 1256.Sq * 1257indicates that the port should be available from all interfaces. 1258Unix domain socket paths may use the tokens described in the 1259.Sx TOKENS 1260section and environment variables as described in the 1261.Sx ENVIRONMENT VARIABLES 1262section. 1263.It Cm LogLevel 1264Gives the verbosity level that is used when logging messages from 1265.Xr ssh 1 . 1266The possible values are: 1267QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1268The default is INFO. 1269DEBUG and DEBUG1 are equivalent. 1270DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1271.It Cm LogVerbose 1272Specify one or more overrides to LogLevel. 1273An override consists of a pattern lists that matches the source file, function 1274and line number to force detailed logging for. 1275For example, an override pattern of: 1276.Bd -literal -offset indent 1277kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1278.Ed 1279.Pp 1280would enable detailed logging for line 1000 of 1281.Pa kex.c , 1282everything in the 1283.Fn kex_exchange_identification 1284function, and all code in the 1285.Pa packet.c 1286file. 1287This option is intended for debugging and no overrides are enabled by default. 1288.It Cm MACs 1289Specifies the MAC (message authentication code) algorithms 1290in order of preference. 1291The MAC algorithm is used for data integrity protection. 1292Multiple algorithms must be comma-separated. 1293If the specified list begins with a 1294.Sq + 1295character, then the specified algorithms will be appended to the default set 1296instead of replacing them. 1297If the specified list begins with a 1298.Sq - 1299character, then the specified algorithms (including wildcards) will be removed 1300from the default set instead of replacing them. 1301If the specified list begins with a 1302.Sq ^ 1303character, then the specified algorithms will be placed at the head of the 1304default set. 1305.Pp 1306The algorithms that contain 1307.Qq -etm 1308calculate the MAC after encryption (encrypt-then-mac). 1309These are considered safer and their use recommended. 1310.Pp 1311The default is: 1312.Bd -literal -offset indent 1313[email protected],[email protected], 1314[email protected],[email protected], 1315[email protected], 1316[email protected],[email protected], 1317hmac-sha2-256,hmac-sha2-512,hmac-sha1 1318.Ed 1319.Pp 1320The list of available MAC algorithms may also be obtained using 1321.Qq ssh -Q mac . 1322.It Cm NoHostAuthenticationForLocalhost 1323Disable host authentication for localhost (loopback addresses). 1324The argument to this keyword must be 1325.Cm yes 1326or 1327.Cm no 1328(the default). 1329.It Cm NumberOfPasswordPrompts 1330Specifies the number of password prompts before giving up. 1331The argument to this keyword must be an integer. 1332The default is 3. 1333.It Cm PasswordAuthentication 1334Specifies whether to use password authentication. 1335The argument to this keyword must be 1336.Cm yes 1337(the default) 1338or 1339.Cm no . 1340.It Cm PermitLocalCommand 1341Allow local command execution via the 1342.Ic LocalCommand 1343option or using the 1344.Ic !\& Ns Ar command 1345escape sequence in 1346.Xr ssh 1 . 1347The argument must be 1348.Cm yes 1349or 1350.Cm no 1351(the default). 1352.It Cm PermitRemoteOpen 1353Specifies the destinations to which remote TCP port forwarding is permitted when 1354.Cm RemoteForward 1355is used as a SOCKS proxy. 1356The forwarding specification must be one of the following forms: 1357.Pp 1358.Bl -item -offset indent -compact 1359.It 1360.Cm PermitRemoteOpen 1361.Sm off 1362.Ar host : port 1363.Sm on 1364.It 1365.Cm PermitRemoteOpen 1366.Sm off 1367.Ar IPv4_addr : port 1368.Sm on 1369.It 1370.Cm PermitRemoteOpen 1371.Sm off 1372.Ar \&[ IPv6_addr \&] : port 1373.Sm on 1374.El 1375.Pp 1376Multiple forwards may be specified by separating them with whitespace. 1377An argument of 1378.Cm any 1379can be used to remove all restrictions and permit any forwarding requests. 1380An argument of 1381.Cm none 1382can be used to prohibit all forwarding requests. 1383The wildcard 1384.Sq * 1385can be used for host or port to allow all hosts or ports respectively. 1386Otherwise, no pattern matching or address lookups are performed on supplied 1387names. 1388.It Cm PKCS11Provider 1389Specifies which PKCS#11 provider to use or 1390.Cm none 1391to indicate that no provider should be used (the default). 1392The argument to this keyword is a path to the PKCS#11 shared library 1393.Xr ssh 1 1394should use to communicate with a PKCS#11 token providing keys for user 1395authentication. 1396.It Cm Port 1397Specifies the port number to connect on the remote host. 1398The default is 22. 1399.It Cm PreferredAuthentications 1400Specifies the order in which the client should try authentication methods. 1401This allows a client to prefer one method (e.g.\& 1402.Cm keyboard-interactive ) 1403over another method (e.g.\& 1404.Cm password ) . 1405The default is: 1406.Bd -literal -offset indent 1407gssapi-with-mic,hostbased,publickey, 1408keyboard-interactive,password 1409.Ed 1410.It Cm ProxyCommand 1411Specifies the command to use to connect to the server. 1412The command 1413string extends to the end of the line, and is executed 1414using the user's shell 1415.Ql exec 1416directive to avoid a lingering shell process. 1417.Pp 1418Arguments to 1419.Cm ProxyCommand 1420accept the tokens described in the 1421.Sx TOKENS 1422section. 1423The command can be basically anything, 1424and should read from its standard input and write to its standard output. 1425It should eventually connect an 1426.Xr sshd 8 1427server running on some machine, or execute 1428.Ic sshd -i 1429somewhere. 1430Host key management will be done using the 1431.Cm Hostname 1432of the host being connected (defaulting to the name typed by the user). 1433Setting the command to 1434.Cm none 1435disables this option entirely. 1436Note that 1437.Cm CheckHostIP 1438is not available for connects with a proxy command. 1439.Pp 1440This directive is useful in conjunction with 1441.Xr nc 1 1442and its proxy support. 1443For example, the following directive would connect via an HTTP proxy at 1444192.0.2.0: 1445.Bd -literal -offset 3n 1446ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1447.Ed 1448.It Cm ProxyJump 1449Specifies one or more jump proxies as either 1450.Xo 1451.Sm off 1452.Op Ar user No @ 1453.Ar host 1454.Op : Ns Ar port 1455.Sm on 1456or an ssh URI 1457.Xc . 1458Multiple proxies may be separated by comma characters and will be visited 1459sequentially. 1460Setting this option will cause 1461.Xr ssh 1 1462to connect to the target host by first making a 1463.Xr ssh 1 1464connection to the specified 1465.Cm ProxyJump 1466host and then establishing a 1467TCP forwarding to the ultimate target from there. 1468Setting the host to 1469.Cm none 1470disables this option entirely. 1471.Pp 1472Note that this option will compete with the 1473.Cm ProxyCommand 1474option - whichever is specified first will prevent later instances of the 1475other from taking effect. 1476.Pp 1477Note also that the configuration for the destination host (either supplied 1478via the command-line or the configuration file) is not generally applied 1479to jump hosts. 1480.Pa ~/.ssh/config 1481should be used if specific configuration is required for jump hosts. 1482.It Cm ProxyUseFdpass 1483Specifies that 1484.Cm ProxyCommand 1485will pass a connected file descriptor back to 1486.Xr ssh 1 1487instead of continuing to execute and pass data. 1488The default is 1489.Cm no . 1490.It Cm PubkeyAcceptedAlgorithms 1491Specifies the signature algorithms that will be used for public key 1492authentication as a comma-separated list of patterns. 1493If the specified list begins with a 1494.Sq + 1495character, then the algorithms after it will be appended to the default 1496instead of replacing it. 1497If the specified list begins with a 1498.Sq - 1499character, then the specified algorithms (including wildcards) will be removed 1500from the default set instead of replacing them. 1501If the specified list begins with a 1502.Sq ^ 1503character, then the specified algorithms will be placed at the head of the 1504default set. 1505The default for this option is: 1506.Bd -literal -offset 3n 1507[email protected], 1508[email protected], 1509[email protected], 1510[email protected], 1511[email protected], 1512[email protected], 1513[email protected], 1514[email protected], 1515[email protected], 1516ssh-ed25519, 1517ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1518[email protected], 1519[email protected], 1520rsa-sha2-512,rsa-sha2-256,ssh-rsa 1521.Ed 1522.Pp 1523The list of available signature algorithms may also be obtained using 1524.Qq ssh -Q PubkeyAcceptedAlgorithms . 1525.It Cm PubkeyAuthentication 1526Specifies whether to try public key authentication. 1527The argument to this keyword must be 1528.Cm yes 1529(the default) 1530or 1531.Cm no . 1532.It Cm RekeyLimit 1533Specifies the maximum amount of data that may be transmitted before the 1534session key is renegotiated, optionally followed by a maximum amount of 1535time that may pass before the session key is renegotiated. 1536The first argument is specified in bytes and may have a suffix of 1537.Sq K , 1538.Sq M , 1539or 1540.Sq G 1541to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1542The default is between 1543.Sq 1G 1544and 1545.Sq 4G , 1546depending on the cipher. 1547The optional second value is specified in seconds and may use any of the 1548units documented in the TIME FORMATS section of 1549.Xr sshd_config 5 . 1550The default value for 1551.Cm RekeyLimit 1552is 1553.Cm default none , 1554which means that rekeying is performed after the cipher's default amount 1555of data has been sent or received and no time based rekeying is done. 1556.It Cm RemoteCommand 1557Specifies a command to execute on the remote machine after successfully 1558connecting to the server. 1559The command string extends to the end of the line, and is executed with 1560the user's shell. 1561Arguments to 1562.Cm RemoteCommand 1563accept the tokens described in the 1564.Sx TOKENS 1565section. 1566.It Cm RemoteForward 1567Specifies that a TCP port on the remote machine be forwarded over 1568the secure channel. 1569The remote port may either be forwarded to a specified host and port 1570from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote 1571client to connect to arbitrary destinations from the local machine. 1572The first argument is the listening specification and may be 1573.Sm off 1574.Oo Ar bind_address : Oc Ar port 1575.Sm on 1576or, if the remote host supports it, a Unix domain socket path. 1577If forwarding to a specific destination then the second argument must be 1578.Ar host : Ns Ar hostport 1579or a Unix domain socket path, 1580otherwise if no destination argument is specified then the remote forwarding 1581will be established as a SOCKS proxy. 1582When acting as a SOCKS proxy the destination of the connection can be 1583restricted by 1584.Cm PermitRemoteOpen . 1585.Pp 1586IPv6 addresses can be specified by enclosing addresses in square brackets. 1587Multiple forwardings may be specified, and additional 1588forwardings can be given on the command line. 1589Privileged ports can be forwarded only when 1590logging in as root on the remote machine. 1591Unix domain socket paths may use the tokens described in the 1592.Sx TOKENS 1593section and environment variables as described in the 1594.Sx ENVIRONMENT VARIABLES 1595section. 1596.Pp 1597If the 1598.Ar port 1599argument is 0, 1600the listen port will be dynamically allocated on the server and reported 1601to the client at run time. 1602.Pp 1603If the 1604.Ar bind_address 1605is not specified, the default is to only bind to loopback addresses. 1606If the 1607.Ar bind_address 1608is 1609.Ql * 1610or an empty string, then the forwarding is requested to listen on all 1611interfaces. 1612Specifying a remote 1613.Ar bind_address 1614will only succeed if the server's 1615.Cm GatewayPorts 1616option is enabled (see 1617.Xr sshd_config 5 ) . 1618.It Cm RequestTTY 1619Specifies whether to request a pseudo-tty for the session. 1620The argument may be one of: 1621.Cm no 1622(never request a TTY), 1623.Cm yes 1624(always request a TTY when standard input is a TTY), 1625.Cm force 1626(always request a TTY) or 1627.Cm auto 1628(request a TTY when opening a login session). 1629This option mirrors the 1630.Fl t 1631and 1632.Fl T 1633flags for 1634.Xr ssh 1 . 1635.It Cm RevokedHostKeys 1636Specifies revoked host public keys. 1637Keys listed in this file will be refused for host authentication. 1638Note that if this file does not exist or is not readable, 1639then host authentication will be refused for all hosts. 1640Keys may be specified as a text file, listing one public key per line, or as 1641an OpenSSH Key Revocation List (KRL) as generated by 1642.Xr ssh-keygen 1 . 1643For more information on KRLs, see the KEY REVOCATION LISTS section in 1644.Xr ssh-keygen 1 . 1645.It Cm SecurityKeyProvider 1646Specifies a path to a library that will be used when loading any 1647FIDO authenticator-hosted keys, overriding the default of using 1648the built-in USB HID support. 1649.Pp 1650If the specified value begins with a 1651.Sq $ 1652character, then it will be treated as an environment variable containing 1653the path to the library. 1654.It Cm SendEnv 1655Specifies what variables from the local 1656.Xr environ 7 1657should be sent to the server. 1658The server must also support it, and the server must be configured to 1659accept these environment variables. 1660Note that the 1661.Ev TERM 1662environment variable is always sent whenever a 1663pseudo-terminal is requested as it is required by the protocol. 1664Refer to 1665.Cm AcceptEnv 1666in 1667.Xr sshd_config 5 1668for how to configure the server. 1669Variables are specified by name, which may contain wildcard characters. 1670Multiple environment variables may be separated by whitespace or spread 1671across multiple 1672.Cm SendEnv 1673directives. 1674.Pp 1675See 1676.Sx PATTERNS 1677for more information on patterns. 1678.Pp 1679It is possible to clear previously set 1680.Cm SendEnv 1681variable names by prefixing patterns with 1682.Pa - . 1683The default is not to send any environment variables. 1684.It Cm ServerAliveCountMax 1685Sets the number of server alive messages (see below) which may be 1686sent without 1687.Xr ssh 1 1688receiving any messages back from the server. 1689If this threshold is reached while server alive messages are being sent, 1690ssh will disconnect from the server, terminating the session. 1691It is important to note that the use of server alive messages is very 1692different from 1693.Cm TCPKeepAlive 1694(below). 1695The server alive messages are sent through the encrypted channel 1696and therefore will not be spoofable. 1697The TCP keepalive option enabled by 1698.Cm TCPKeepAlive 1699is spoofable. 1700The server alive mechanism is valuable when the client or 1701server depend on knowing when a connection has become unresponsive. 1702.Pp 1703The default value is 3. 1704If, for example, 1705.Cm ServerAliveInterval 1706(see below) is set to 15 and 1707.Cm ServerAliveCountMax 1708is left at the default, if the server becomes unresponsive, 1709ssh will disconnect after approximately 45 seconds. 1710.It Cm ServerAliveInterval 1711Sets a timeout interval in seconds after which if no data has been received 1712from the server, 1713.Xr ssh 1 1714will send a message through the encrypted 1715channel to request a response from the server. 1716The default 1717is 0, indicating that these messages will not be sent to the server. 1718.It Cm SessionType 1719May be used to either request invocation of a subsystem on the remote system, 1720or to prevent the execution of a remote command at all. 1721The latter is useful for just forwarding ports. 1722The argument to this keyword must be 1723.Cm none 1724(same as the 1725.Fl N 1726option), 1727.Cm subsystem 1728(same as the 1729.Fl s 1730option) or 1731.Cm default 1732(shell or command execution). 1733.It Cm SetEnv 1734Directly specify one or more environment variables and their contents to 1735be sent to the server. 1736Similarly to 1737.Cm SendEnv , 1738with the exception of the 1739.Ev TERM 1740variable, the server must be prepared to accept the environment variable. 1741.It Cm StdinNull 1742Redirects stdin from 1743.Pa /dev/null 1744(actually, prevents reading from stdin). 1745Either this or the equivalent 1746.Fl n 1747option must be used when 1748.Nm ssh 1749is run in the background. 1750The argument to this keyword must be 1751.Cm yes 1752(same as the 1753.Fl n 1754option) or 1755.Cm no 1756(the default). 1757.It Cm StreamLocalBindMask 1758Sets the octal file creation mode mask 1759.Pq umask 1760used when creating a Unix-domain socket file for local or remote 1761port forwarding. 1762This option is only used for port forwarding to a Unix-domain socket file. 1763.Pp 1764The default value is 0177, which creates a Unix-domain socket file that is 1765readable and writable only by the owner. 1766Note that not all operating systems honor the file mode on Unix-domain 1767socket files. 1768.It Cm StreamLocalBindUnlink 1769Specifies whether to remove an existing Unix-domain socket file for local 1770or remote port forwarding before creating a new one. 1771If the socket file already exists and 1772.Cm StreamLocalBindUnlink 1773is not enabled, 1774.Nm ssh 1775will be unable to forward the port to the Unix-domain socket file. 1776This option is only used for port forwarding to a Unix-domain socket file. 1777.Pp 1778The argument must be 1779.Cm yes 1780or 1781.Cm no 1782(the default). 1783.It Cm StrictHostKeyChecking 1784If this flag is set to 1785.Cm yes , 1786.Xr ssh 1 1787will never automatically add host keys to the 1788.Pa ~/.ssh/known_hosts 1789file, and refuses to connect to hosts whose host key has changed. 1790This provides maximum protection against man-in-the-middle (MITM) attacks, 1791though it can be annoying when the 1792.Pa /etc/ssh/ssh_known_hosts 1793file is poorly maintained or when connections to new hosts are 1794frequently made. 1795This option forces the user to manually 1796add all new hosts. 1797.Pp 1798If this flag is set to 1799.Cm accept-new 1800then ssh will automatically add new host keys to the user's 1801.Pa known_hosts 1802file, but will not permit connections to hosts with 1803changed host keys. 1804If this flag is set to 1805.Cm no 1806or 1807.Cm off , 1808ssh will automatically add new host keys to the user known hosts files 1809and allow connections to hosts with changed hostkeys to proceed, 1810subject to some restrictions. 1811If this flag is set to 1812.Cm ask 1813(the default), 1814new host keys 1815will be added to the user known host files only after the user 1816has confirmed that is what they really want to do, and 1817ssh will refuse to connect to hosts whose host key has changed. 1818The host keys of 1819known hosts will be verified automatically in all cases. 1820.It Cm SyslogFacility 1821Gives the facility code that is used when logging messages from 1822.Xr ssh 1 . 1823The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1824LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1825The default is USER. 1826.It Cm TCPKeepAlive 1827Specifies whether the system should send TCP keepalive messages to the 1828other side. 1829If they are sent, death of the connection or crash of one 1830of the machines will be properly noticed. 1831However, this means that 1832connections will die if the route is down temporarily, and some people 1833find it annoying. 1834.Pp 1835The default is 1836.Cm yes 1837(to send TCP keepalive messages), and the client will notice 1838if the network goes down or the remote host dies. 1839This is important in scripts, and many users want it too. 1840.Pp 1841To disable TCP keepalive messages, the value should be set to 1842.Cm no . 1843See also 1844.Cm ServerAliveInterval 1845for protocol-level keepalives. 1846.It Cm Tunnel 1847Request 1848.Xr tun 4 1849device forwarding between the client and the server. 1850The argument must be 1851.Cm yes , 1852.Cm point-to-point 1853(layer 3), 1854.Cm ethernet 1855(layer 2), 1856or 1857.Cm no 1858(the default). 1859Specifying 1860.Cm yes 1861requests the default tunnel mode, which is 1862.Cm point-to-point . 1863.It Cm TunnelDevice 1864Specifies the 1865.Xr tun 4 1866devices to open on the client 1867.Pq Ar local_tun 1868and the server 1869.Pq Ar remote_tun . 1870.Pp 1871The argument must be 1872.Sm off 1873.Ar local_tun Op : Ar remote_tun . 1874.Sm on 1875The devices may be specified by numerical ID or the keyword 1876.Cm any , 1877which uses the next available tunnel device. 1878If 1879.Ar remote_tun 1880is not specified, it defaults to 1881.Cm any . 1882The default is 1883.Cm any:any . 1884.It Cm UpdateHostKeys 1885Specifies whether 1886.Xr ssh 1 1887should accept notifications of additional hostkeys from the server sent 1888after authentication has completed and add them to 1889.Cm UserKnownHostsFile . 1890The argument must be 1891.Cm yes , 1892.Cm no 1893or 1894.Cm ask . 1895This option allows learning alternate hostkeys for a server 1896and supports graceful key rotation by allowing a server to send replacement 1897public keys before old ones are removed. 1898.Pp 1899Additional hostkeys are only accepted if the key used to authenticate the 1900host was already trusted or explicitly accepted by the user, the host was 1901authenticated via 1902.Cm UserKnownHostsFile 1903(i.e. not 1904.Cm GlobalKnownHostsFile ) 1905and the host was authenticated using a plain key and not a certificate. 1906.Pp 1907.Cm UpdateHostKeys 1908is enabled by default if the user has not overridden the default 1909.Cm UserKnownHostsFile 1910setting and has not enabled 1911.Cm VerifyHostKeyDNS , 1912otherwise 1913.Cm UpdateHostKeys 1914will be set to 1915.Cm no . 1916.Pp 1917If 1918.Cm UpdateHostKeys 1919is set to 1920.Cm ask , 1921then the user is asked to confirm the modifications to the known_hosts file. 1922Confirmation is currently incompatible with 1923.Cm ControlPersist , 1924and will be disabled if it is enabled. 1925.Pp 1926Presently, only 1927.Xr sshd 8 1928from OpenSSH 6.8 and greater support the 1929.Qq [email protected] 1930protocol extension used to inform the client of all the server's hostkeys. 1931.It Cm User 1932Specifies the user to log in as. 1933This can be useful when a different user name is used on different machines. 1934This saves the trouble of 1935having to remember to give the user name on the command line. 1936.It Cm UserKnownHostsFile 1937Specifies one or more files to use for the user 1938host key database, separated by whitespace. 1939Each filename may use tilde notation to refer to the user's home directory, 1940the tokens described in the 1941.Sx TOKENS 1942section and environment variables as described in the 1943.Sx ENVIRONMENT VARIABLES 1944section. 1945The default is 1946.Pa ~/.ssh/known_hosts , 1947.Pa ~/.ssh/known_hosts2 . 1948.It Cm VerifyHostKeyDNS 1949Specifies whether to verify the remote key using DNS and SSHFP resource 1950records. 1951If this option is set to 1952.Cm yes , 1953the client will implicitly trust keys that match a secure fingerprint 1954from DNS. 1955Insecure fingerprints will be handled as if this option was set to 1956.Cm ask . 1957If this option is set to 1958.Cm ask , 1959information on fingerprint match will be displayed, but the user will still 1960need to confirm new host keys according to the 1961.Cm StrictHostKeyChecking 1962option. 1963The default is 1964.Cm yes 1965if compiled with LDNS and 1966.Cm no 1967otherwise. 1968.Pp 1969See also 1970.Sx VERIFYING HOST KEYS 1971in 1972.Xr ssh 1 . 1973.It Cm VersionAddendum 1974Specifies a string to append to the regular version string to identify 1975OS- or site-specific modifications. 1976The default is 1977.Dq FreeBSD-20211221 . 1978The value 1979.Cm none 1980may be used to disable this. 1981.It Cm VisualHostKey 1982If this flag is set to 1983.Cm yes , 1984an ASCII art representation of the remote host key fingerprint is 1985printed in addition to the fingerprint string at login and 1986for unknown host keys. 1987If this flag is set to 1988.Cm no 1989(the default), 1990no fingerprint strings are printed at login and 1991only the fingerprint string will be printed for unknown host keys. 1992.It Cm XAuthLocation 1993Specifies the full pathname of the 1994.Xr xauth 1 1995program. 1996The default is 1997.Pa /usr/local/bin/xauth . 1998.El 1999.Sh PATTERNS 2000A 2001.Em pattern 2002consists of zero or more non-whitespace characters, 2003.Sq * 2004(a wildcard that matches zero or more characters), 2005or 2006.Sq ?\& 2007(a wildcard that matches exactly one character). 2008For example, to specify a set of declarations for any host in the 2009.Qq .co.uk 2010set of domains, 2011the following pattern could be used: 2012.Pp 2013.Dl Host *.co.uk 2014.Pp 2015The following pattern 2016would match any host in the 192.168.0.[0-9] network range: 2017.Pp 2018.Dl Host 192.168.0.? 2019.Pp 2020A 2021.Em pattern-list 2022is a comma-separated list of patterns. 2023Patterns within pattern-lists may be negated 2024by preceding them with an exclamation mark 2025.Pq Sq !\& . 2026For example, 2027to allow a key to be used from anywhere within an organization 2028except from the 2029.Qq dialup 2030pool, 2031the following entry (in authorized_keys) could be used: 2032.Pp 2033.Dl from=\&"!*.dialup.example.com,*.example.com\&" 2034.Pp 2035Note that a negated match will never produce a positive result by itself. 2036For example, attempting to match 2037.Qq host3 2038against the following pattern-list will fail: 2039.Pp 2040.Dl from=\&"!host1,!host2\&" 2041.Pp 2042The solution here is to include a term that will yield a positive match, 2043such as a wildcard: 2044.Pp 2045.Dl from=\&"!host1,!host2,*\&" 2046.Sh TOKENS 2047Arguments to some keywords can make use of tokens, 2048which are expanded at runtime: 2049.Pp 2050.Bl -tag -width XXXX -offset indent -compact 2051.It %% 2052A literal 2053.Sq % . 2054.It \&%C 2055Hash of %l%h%p%r. 2056.It %d 2057Local user's home directory. 2058.It %f 2059The fingerprint of the server's host key. 2060.It %H 2061The 2062.Pa known_hosts 2063hostname or address that is being searched for. 2064.It %h 2065The remote hostname. 2066.It \%%I 2067A string describing the reason for a 2068.Cm KnownHostsCommand 2069execution: either 2070.Cm ADDRESS 2071when looking up a host by address (only when 2072.Cm CheckHostIP 2073is enabled), 2074.Cm HOSTNAME 2075when searching by hostname, or 2076.Cm ORDER 2077when preparing the host key algorithm preference list to use for the 2078destination host. 2079.It %i 2080The local user ID. 2081.It %K 2082The base64 encoded host key. 2083.It %k 2084The host key alias if specified, otherwise the original remote hostname given 2085on the command line. 2086.It %L 2087The local hostname. 2088.It %l 2089The local hostname, including the domain name. 2090.It %n 2091The original remote hostname, as given on the command line. 2092.It %p 2093The remote port. 2094.It %r 2095The remote username. 2096.It \&%T 2097The local 2098.Xr tun 4 2099or 2100.Xr tap 4 2101network interface assigned if 2102tunnel forwarding was requested, or 2103.Qq NONE 2104otherwise. 2105.It %t 2106The type of the server host key, e.g. 2107.Cm ssh-ed25519 . 2108.It %u 2109The local username. 2110.El 2111.Pp 2112.Cm CertificateFile , 2113.Cm ControlPath , 2114.Cm IdentityAgent , 2115.Cm IdentityFile , 2116.Cm KnownHostsCommand , 2117.Cm LocalForward , 2118.Cm Match exec , 2119.Cm RemoteCommand , 2120.Cm RemoteForward , 2121and 2122.Cm UserKnownHostsFile 2123accept the tokens %%, %C, %d, %h, %i, %k, %L, %l, %n, %p, %r, and %u. 2124.Pp 2125.Cm KnownHostsCommand 2126additionally accepts the tokens %f, %H, %I, %K and %t. 2127.Pp 2128.Cm Hostname 2129accepts the tokens %% and %h. 2130.Pp 2131.Cm LocalCommand 2132accepts all tokens. 2133.Pp 2134.Cm ProxyCommand 2135accepts the tokens %%, %h, %n, %p, and %r. 2136.Sh ENVIRONMENT VARIABLES 2137Arguments to some keywords can be expanded at runtime from environment 2138variables on the client by enclosing them in 2139.Ic ${} , 2140for example 2141.Ic ${HOME}/.ssh 2142would refer to the user's .ssh directory. 2143If a specified environment variable does not exist then an error will be 2144returned and the setting for that keyword will be ignored. 2145.Pp 2146The keywords 2147.Cm CertificateFile , 2148.Cm ControlPath , 2149.Cm IdentityAgent , 2150.Cm IdentityFile , 2151.Cm KnownHostsCommand , 2152and 2153.Cm UserKnownHostsFile 2154support environment variables. 2155The keywords 2156.Cm LocalForward 2157and 2158.Cm RemoteForward 2159support environment variables only for Unix domain socket paths. 2160.Sh FILES 2161.Bl -tag -width Ds 2162.It Pa ~/.ssh/config 2163This is the per-user configuration file. 2164The format of this file is described above. 2165This file is used by the SSH client. 2166Because of the potential for abuse, this file must have strict permissions: 2167read/write for the user, and not writable by others. 2168.It Pa /etc/ssh/ssh_config 2169Systemwide configuration file. 2170This file provides defaults for those 2171values that are not specified in the user's configuration file, and 2172for those users who do not have a configuration file. 2173This file must be world-readable. 2174.El 2175.Sh SEE ALSO 2176.Xr ssh 1 2177.Sh AUTHORS 2178.An -nosplit 2179OpenSSH is a derivative of the original and free 2180ssh 1.2.12 release by 2181.An Tatu Ylonen . 2182.An Aaron Campbell , Bob Beck , Markus Friedl , 2183.An Niels Provos , Theo de Raadt 2184and 2185.An Dug Song 2186removed many bugs, re-added newer features and 2187created OpenSSH. 2188.An Markus Friedl 2189contributed the support for SSH protocol versions 1.5 and 2.0. 2190