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 1.366 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, 388sk-ssh-ed25519@openssh.com, 389sk-ecdsa-sha2-nistp256@openssh.com, 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 480aes128-gcm@openssh.com 481aes256-gcm@openssh.com 482chacha20-poly1305@openssh.com 483.Ed 484.Pp 485The default is: 486.Bd -literal -offset indent 487chacha20-poly1305@openssh.com, 488aes128-ctr,aes192-ctr,aes256-ctr, 489aes128-gcm@openssh.com,aes256-gcm@openssh.com 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 877ssh-ed25519-cert-v01@openssh.com, 878ecdsa-sha2-nistp256-cert-v01@openssh.com, 879ecdsa-sha2-nistp384-cert-v01@openssh.com, 880ecdsa-sha2-nistp521-cert-v01@openssh.com, 881sk-ssh-ed25519-cert-v01@openssh.com, 882sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 883rsa-sha2-512-cert-v01@openssh.com, 884rsa-sha2-256-cert-v01@openssh.com, 885ssh-ed25519, 886ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 887sk-ssh-ed25519@openssh.com, 888sk-ecdsa-sha2-nistp256@openssh.com, 889rsa-sha2-512,rsa-sha2-256 890.Ed 891.Pp 892The 893.Fl Q 894option of 895.Xr ssh 1 896may be used to list supported signature algorithms. 897This was formerly named HostbasedKeyTypes. 898.It Cm HostbasedAuthentication 899Specifies whether to try rhosts based authentication with public key 900authentication. 901The argument must be 902.Cm yes 903or 904.Cm no 905(the default). 906.It Cm HostKeyAlgorithms 907Specifies the host key signature algorithms 908that the client wants to use in order of preference. 909Alternately if the specified list begins with a 910.Sq + 911character, then the specified signature algorithms will be appended to 912the default set instead of replacing them. 913If the specified list begins with a 914.Sq - 915character, then the specified signature algorithms (including wildcards) 916will be removed from the default set instead of replacing them. 917If the specified list begins with a 918.Sq ^ 919character, then the specified signature algorithms will be placed 920at the head of the default set. 921The default for this option is: 922.Bd -literal -offset 3n 923ssh-ed25519-cert-v01@openssh.com, 924ecdsa-sha2-nistp256-cert-v01@openssh.com, 925ecdsa-sha2-nistp384-cert-v01@openssh.com, 926ecdsa-sha2-nistp521-cert-v01@openssh.com, 927sk-ssh-ed25519-cert-v01@openssh.com, 928sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 929rsa-sha2-512-cert-v01@openssh.com, 930rsa-sha2-256-cert-v01@openssh.com, 931ssh-ed25519, 932ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 933sk-ecdsa-sha2-nistp256@openssh.com, 934sk-ssh-ed25519@openssh.com, 935rsa-sha2-512,rsa-sha2-256 936.Ed 937.Pp 938If hostkeys are known for the destination host then this default is modified 939to prefer their algorithms. 940.Pp 941The list of available signature algorithms may also be obtained using 942.Qq ssh -Q HostKeyAlgorithms . 943.It Cm HostKeyAlias 944Specifies an alias that should be used instead of the 945real host name when looking up or saving the host key 946in the host key database files and when validating host certificates. 947This option is useful for tunneling SSH connections 948or for multiple servers running on a single host. 949.It Cm Hostname 950Specifies the real host name to log into. 951This can be used to specify nicknames or abbreviations for hosts. 952Arguments to 953.Cm Hostname 954accept the tokens described in the 955.Sx TOKENS 956section. 957Numeric IP addresses are also permitted (both on the command line and in 958.Cm Hostname 959specifications). 960The default is the name given on the command line. 961.It Cm IdentitiesOnly 962Specifies that 963.Xr ssh 1 964should only use the configured authentication identity and certificate files 965(either the default files, or those explicitly configured in the 966.Nm 967files 968or passed on the 969.Xr ssh 1 970command-line), 971even if 972.Xr ssh-agent 1 973or a 974.Cm PKCS11Provider 975or 976.Cm SecurityKeyProvider 977offers more identities. 978The argument to this keyword must be 979.Cm yes 980or 981.Cm no 982(the default). 983This option is intended for situations where ssh-agent 984offers many different identities. 985.It Cm IdentityAgent 986Specifies the 987.Ux Ns -domain 988socket used to communicate with the authentication agent. 989.Pp 990This option overrides the 991.Ev SSH_AUTH_SOCK 992environment variable and can be used to select a specific agent. 993Setting the socket name to 994.Cm none 995disables the use of an authentication agent. 996If the string 997.Qq SSH_AUTH_SOCK 998is specified, the location of the socket will be read from the 999.Ev SSH_AUTH_SOCK 1000environment variable. 1001Otherwise if the specified value begins with a 1002.Sq $ 1003character, then it will be treated as an environment variable containing 1004the location of the socket. 1005.Pp 1006Arguments to 1007.Cm IdentityAgent 1008may use the tilde syntax to refer to a user's home directory, 1009the tokens described in the 1010.Sx TOKENS 1011section and environment variables as described in the 1012.Sx ENVIRONMENT VARIABLES 1013section. 1014.It Cm IdentityFile 1015Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA, 1016Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read. 1017The default is 1018.Pa ~/.ssh/id_dsa , 1019.Pa ~/.ssh/id_ecdsa , 1020.Pa ~/.ssh/id_ecdsa_sk , 1021.Pa ~/.ssh/id_ed25519 , 1022.Pa ~/.ssh/id_ed25519_sk 1023and 1024.Pa ~/.ssh/id_rsa . 1025Additionally, any identities represented by the authentication agent 1026will be used for authentication unless 1027.Cm IdentitiesOnly 1028is set. 1029If no certificates have been explicitly specified by 1030.Cm CertificateFile , 1031.Xr ssh 1 1032will try to load certificate information from the filename obtained by 1033appending 1034.Pa -cert.pub 1035to the path of a specified 1036.Cm IdentityFile . 1037.Pp 1038Arguments to 1039.Cm IdentityFile 1040may use the tilde syntax to refer to a user's home directory 1041or the tokens described in the 1042.Sx TOKENS 1043section. 1044.Pp 1045It is possible to have 1046multiple identity files specified in configuration files; all these 1047identities will be tried in sequence. 1048Multiple 1049.Cm IdentityFile 1050directives will add to the list of identities tried (this behaviour 1051differs from that of other configuration directives). 1052.Pp 1053.Cm IdentityFile 1054may be used in conjunction with 1055.Cm IdentitiesOnly 1056to select which identities in an agent are offered during authentication. 1057.Cm IdentityFile 1058may also be used in conjunction with 1059.Cm CertificateFile 1060in order to provide any certificate also needed for authentication with 1061the identity. 1062.It Cm IgnoreUnknown 1063Specifies a pattern-list of unknown options to be ignored if they are 1064encountered in configuration parsing. 1065This may be used to suppress errors if 1066.Nm 1067contains options that are unrecognised by 1068.Xr ssh 1 . 1069It is recommended that 1070.Cm IgnoreUnknown 1071be listed early in the configuration file as it will not be applied 1072to unknown options that appear before it. 1073.It Cm Include 1074Include the specified configuration file(s). 1075Multiple pathnames may be specified and each pathname may contain 1076.Xr glob 7 1077wildcards and, for user configurations, shell-like 1078.Sq ~ 1079references to user home directories. 1080Wildcards will be expanded and processed in lexical order. 1081Files without absolute paths are assumed to be in 1082.Pa ~/.ssh 1083if included in a user configuration file or 1084.Pa /etc/ssh 1085if included from the system configuration file. 1086.Cm Include 1087directive may appear inside a 1088.Cm Match 1089or 1090.Cm Host 1091block 1092to perform conditional inclusion. 1093.It Cm IPQoS 1094Specifies the IPv4 type-of-service or DSCP class for connections. 1095Accepted values are 1096.Cm af11 , 1097.Cm af12 , 1098.Cm af13 , 1099.Cm af21 , 1100.Cm af22 , 1101.Cm af23 , 1102.Cm af31 , 1103.Cm af32 , 1104.Cm af33 , 1105.Cm af41 , 1106.Cm af42 , 1107.Cm af43 , 1108.Cm cs0 , 1109.Cm cs1 , 1110.Cm cs2 , 1111.Cm cs3 , 1112.Cm cs4 , 1113.Cm cs5 , 1114.Cm cs6 , 1115.Cm cs7 , 1116.Cm ef , 1117.Cm le , 1118.Cm lowdelay , 1119.Cm throughput , 1120.Cm reliability , 1121a numeric value, or 1122.Cm none 1123to use the operating system default. 1124This option may take one or two arguments, separated by whitespace. 1125If one argument is specified, it is used as the packet class unconditionally. 1126If two values are specified, the first is automatically selected for 1127interactive sessions and the second for non-interactive sessions. 1128The default is 1129.Cm af21 1130(Low-Latency Data) 1131for interactive sessions and 1132.Cm cs1 1133(Lower Effort) 1134for non-interactive sessions. 1135.It Cm KbdInteractiveAuthentication 1136Specifies whether to use keyboard-interactive authentication. 1137The argument to this keyword must be 1138.Cm yes 1139(the default) 1140or 1141.Cm no . 1142.Cm ChallengeResponseAuthentication 1143is a deprecated alias for this. 1144.It Cm KbdInteractiveDevices 1145Specifies the list of methods to use in keyboard-interactive authentication. 1146Multiple method names must be comma-separated. 1147The default is to use the server specified list. 1148The methods available vary depending on what the server supports. 1149For an OpenSSH server, 1150it may be zero or more of: 1151.Cm bsdauth 1152and 1153.Cm pam . 1154.It Cm KexAlgorithms 1155Specifies the available KEX (Key Exchange) algorithms. 1156Multiple algorithms must be comma-separated. 1157If the specified list begins with a 1158.Sq + 1159character, then the specified algorithms will be appended to the default set 1160instead of replacing them. 1161If the specified list begins with a 1162.Sq - 1163character, then the specified algorithms (including wildcards) will be removed 1164from the default set instead of replacing them. 1165If the specified list begins with a 1166.Sq ^ 1167character, then the specified algorithms will be placed at the head of the 1168default set. 1169The default is: 1170.Bd -literal -offset indent 1171curve25519-sha256,curve25519-sha256@libssh.org, 1172ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1173diffie-hellman-group-exchange-sha256, 1174diffie-hellman-group16-sha512, 1175diffie-hellman-group18-sha512, 1176diffie-hellman-group14-sha256 1177.Ed 1178.Pp 1179The list of available key exchange algorithms may also be obtained using 1180.Qq ssh -Q kex . 1181.It Cm KnownHostsCommand 1182Specifies a command to use to obtain a list of host keys, in addition to 1183those listed in 1184.Cm UserKnownHostsFile 1185and 1186.Cm GlobalKnownHostsFile . 1187This command is executed after the files have been read. 1188It may write host key lines to standard output in identical format to the 1189usual files (described in the 1190.Sx VERIFYING HOST KEYS 1191section in 1192.Xr ssh 1 ) . 1193Arguments to 1194.Cm KnownHostsCommand 1195accept the tokens described in the 1196.Sx TOKENS 1197section. 1198The command may be invoked multiple times per connection: once when preparing 1199the preference list of host key algorithms to use, again to obtain the 1200host key for the requested host name and, if 1201.Cm CheckHostIP 1202is enabled, one more time to obtain the host key matching the server's 1203address. 1204If the command exits abnormally or returns a non-zero exit status then the 1205connection is terminated. 1206.It Cm LocalCommand 1207Specifies a command to execute on the local machine after successfully 1208connecting to the server. 1209The command string extends to the end of the line, and is executed with 1210the user's shell. 1211Arguments to 1212.Cm LocalCommand 1213accept the tokens described in the 1214.Sx TOKENS 1215section. 1216.Pp 1217The command is run synchronously and does not have access to the 1218session of the 1219.Xr ssh 1 1220that spawned it. 1221It should not be used for interactive commands. 1222.Pp 1223This directive is ignored unless 1224.Cm PermitLocalCommand 1225has been enabled. 1226.It Cm LocalForward 1227Specifies that a TCP port on the local machine be forwarded over 1228the secure channel to the specified host and port from the remote machine. 1229The first argument specifies the listener and may be 1230.Sm off 1231.Oo Ar bind_address : Oc Ar port 1232.Sm on 1233or a Unix domain socket path. 1234The second argument is the destination and may be 1235.Ar host : Ns Ar hostport 1236or a Unix domain socket path if the remote host supports it. 1237.Pp 1238IPv6 addresses can be specified by enclosing addresses in square brackets. 1239Multiple forwardings may be specified, and additional forwardings can be 1240given on the command line. 1241Only the superuser can forward privileged ports. 1242By default, the local port is bound in accordance with the 1243.Cm GatewayPorts 1244setting. 1245However, an explicit 1246.Ar bind_address 1247may be used to bind the connection to a specific address. 1248The 1249.Ar bind_address 1250of 1251.Cm localhost 1252indicates that the listening port be bound for local use only, while an 1253empty address or 1254.Sq * 1255indicates that the port should be available from all interfaces. 1256Unix domain socket paths may use the tokens described in the 1257.Sx TOKENS 1258section and environment variables as described in the 1259.Sx ENVIRONMENT VARIABLES 1260section. 1261.It Cm LogLevel 1262Gives the verbosity level that is used when logging messages from 1263.Xr ssh 1 . 1264The possible values are: 1265QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1266The default is INFO. 1267DEBUG and DEBUG1 are equivalent. 1268DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1269.It Cm LogVerbose 1270Specify one or more overrides to LogLevel. 1271An override consists of a pattern lists that matches the source file, function 1272and line number to force detailed logging for. 1273For example, an override pattern of: 1274.Bd -literal -offset indent 1275kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1276.Ed 1277.Pp 1278would enable detailed logging for line 1000 of 1279.Pa kex.c , 1280everything in the 1281.Fn kex_exchange_identification 1282function, and all code in the 1283.Pa packet.c 1284file. 1285This option is intended for debugging and no overrides are enabled by default. 1286.It Cm MACs 1287Specifies the MAC (message authentication code) algorithms 1288in order of preference. 1289The MAC algorithm is used for data integrity protection. 1290Multiple algorithms must be comma-separated. 1291If the specified list begins with a 1292.Sq + 1293character, then the specified algorithms will be appended to the default set 1294instead of replacing them. 1295If the specified list begins with a 1296.Sq - 1297character, then the specified algorithms (including wildcards) will be removed 1298from the default set instead of replacing them. 1299If the specified list begins with a 1300.Sq ^ 1301character, then the specified algorithms will be placed at the head of the 1302default set. 1303.Pp 1304The algorithms that contain 1305.Qq -etm 1306calculate the MAC after encryption (encrypt-then-mac). 1307These are considered safer and their use recommended. 1308.Pp 1309The default is: 1310.Bd -literal -offset indent 1311umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1312hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1313hmac-sha1-etm@openssh.com, 1314umac-64@openssh.com,umac-128@openssh.com, 1315hmac-sha2-256,hmac-sha2-512,hmac-sha1 1316.Ed 1317.Pp 1318The list of available MAC algorithms may also be obtained using 1319.Qq ssh -Q mac . 1320.It Cm NoHostAuthenticationForLocalhost 1321Disable host authentication for localhost (loopback addresses). 1322The argument to this keyword must be 1323.Cm yes 1324or 1325.Cm no 1326(the default). 1327.It Cm NumberOfPasswordPrompts 1328Specifies the number of password prompts before giving up. 1329The argument to this keyword must be an integer. 1330The default is 3. 1331.It Cm PasswordAuthentication 1332Specifies whether to use password authentication. 1333The argument to this keyword must be 1334.Cm yes 1335(the default) 1336or 1337.Cm no . 1338.It Cm PermitLocalCommand 1339Allow local command execution via the 1340.Ic LocalCommand 1341option or using the 1342.Ic !\& Ns Ar command 1343escape sequence in 1344.Xr ssh 1 . 1345The argument must be 1346.Cm yes 1347or 1348.Cm no 1349(the default). 1350.It Cm PermitRemoteOpen 1351Specifies the destinations to which remote TCP port forwarding is permitted when 1352.Cm RemoteForward 1353is used as a SOCKS proxy. 1354The forwarding specification must be one of the following forms: 1355.Pp 1356.Bl -item -offset indent -compact 1357.It 1358.Cm PermitRemoteOpen 1359.Sm off 1360.Ar host : port 1361.Sm on 1362.It 1363.Cm PermitRemoteOpen 1364.Sm off 1365.Ar IPv4_addr : port 1366.Sm on 1367.It 1368.Cm PermitRemoteOpen 1369.Sm off 1370.Ar \&[ IPv6_addr \&] : port 1371.Sm on 1372.El 1373.Pp 1374Multiple forwards may be specified by separating them with whitespace. 1375An argument of 1376.Cm any 1377can be used to remove all restrictions and permit any forwarding requests. 1378An argument of 1379.Cm none 1380can be used to prohibit all forwarding requests. 1381The wildcard 1382.Sq * 1383can be used for host or port to allow all hosts or ports respectively. 1384Otherwise, no pattern matching or address lookups are performed on supplied 1385names. 1386.It Cm PKCS11Provider 1387Specifies which PKCS#11 provider to use or 1388.Cm none 1389to indicate that no provider should be used (the default). 1390The argument to this keyword is a path to the PKCS#11 shared library 1391.Xr ssh 1 1392should use to communicate with a PKCS#11 token providing keys for user 1393authentication. 1394.It Cm Port 1395Specifies the port number to connect on the remote host. 1396The default is 22. 1397.It Cm PreferredAuthentications 1398Specifies the order in which the client should try authentication methods. 1399This allows a client to prefer one method (e.g.\& 1400.Cm keyboard-interactive ) 1401over another method (e.g.\& 1402.Cm password ) . 1403The default is: 1404.Bd -literal -offset indent 1405gssapi-with-mic,hostbased,publickey, 1406keyboard-interactive,password 1407.Ed 1408.It Cm ProxyCommand 1409Specifies the command to use to connect to the server. 1410The command 1411string extends to the end of the line, and is executed 1412using the user's shell 1413.Ql exec 1414directive to avoid a lingering shell process. 1415.Pp 1416Arguments to 1417.Cm ProxyCommand 1418accept the tokens described in the 1419.Sx TOKENS 1420section. 1421The command can be basically anything, 1422and should read from its standard input and write to its standard output. 1423It should eventually connect an 1424.Xr sshd 8 1425server running on some machine, or execute 1426.Ic sshd -i 1427somewhere. 1428Host key management will be done using the 1429.Cm Hostname 1430of the host being connected (defaulting to the name typed by the user). 1431Setting the command to 1432.Cm none 1433disables this option entirely. 1434Note that 1435.Cm CheckHostIP 1436is not available for connects with a proxy command. 1437.Pp 1438This directive is useful in conjunction with 1439.Xr nc 1 1440and its proxy support. 1441For example, the following directive would connect via an HTTP proxy at 1442192.0.2.0: 1443.Bd -literal -offset 3n 1444ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1445.Ed 1446.It Cm ProxyJump 1447Specifies one or more jump proxies as either 1448.Xo 1449.Sm off 1450.Op Ar user No @ 1451.Ar host 1452.Op : Ns Ar port 1453.Sm on 1454or an ssh URI 1455.Xc . 1456Multiple proxies may be separated by comma characters and will be visited 1457sequentially. 1458Setting this option will cause 1459.Xr ssh 1 1460to connect to the target host by first making a 1461.Xr ssh 1 1462connection to the specified 1463.Cm ProxyJump 1464host and then establishing a 1465TCP forwarding to the ultimate target from there. 1466Setting the host to 1467.Cm none 1468disables this option entirely. 1469.Pp 1470Note that this option will compete with the 1471.Cm ProxyCommand 1472option - whichever is specified first will prevent later instances of the 1473other from taking effect. 1474.Pp 1475Note also that the configuration for the destination host (either supplied 1476via the command-line or the configuration file) is not generally applied 1477to jump hosts. 1478.Pa ~/.ssh/config 1479should be used if specific configuration is required for jump hosts. 1480.It Cm ProxyUseFdpass 1481Specifies that 1482.Cm ProxyCommand 1483will pass a connected file descriptor back to 1484.Xr ssh 1 1485instead of continuing to execute and pass data. 1486The default is 1487.Cm no . 1488.It Cm PubkeyAcceptedAlgorithms 1489Specifies the signature algorithms that will be used for public key 1490authentication as a comma-separated list of patterns. 1491If the specified list begins with a 1492.Sq + 1493character, then the algorithms after it will be appended to the default 1494instead of replacing it. 1495If the specified list begins with a 1496.Sq - 1497character, then the specified algorithms (including wildcards) will be removed 1498from the default set instead of replacing them. 1499If the specified list begins with a 1500.Sq ^ 1501character, then the specified algorithms will be placed at the head of the 1502default set. 1503The default for this option is: 1504.Bd -literal -offset 3n 1505ssh-ed25519-cert-v01@openssh.com, 1506ecdsa-sha2-nistp256-cert-v01@openssh.com, 1507ecdsa-sha2-nistp384-cert-v01@openssh.com, 1508ecdsa-sha2-nistp521-cert-v01@openssh.com, 1509sk-ssh-ed25519-cert-v01@openssh.com, 1510sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1511rsa-sha2-512-cert-v01@openssh.com, 1512rsa-sha2-256-cert-v01@openssh.com, 1513ssh-ed25519, 1514ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1515sk-ssh-ed25519@openssh.com, 1516sk-ecdsa-sha2-nistp256@openssh.com, 1517rsa-sha2-512,rsa-sha2-256 1518.Ed 1519.Pp 1520The list of available signature algorithms may also be obtained using 1521.Qq ssh -Q PubkeyAcceptedAlgorithms . 1522.It Cm PubkeyAuthentication 1523Specifies whether to try public key authentication. 1524The argument to this keyword must be 1525.Cm yes 1526(the default) 1527or 1528.Cm no . 1529.It Cm RekeyLimit 1530Specifies the maximum amount of data that may be transmitted before the 1531session key is renegotiated, optionally followed by a maximum amount of 1532time that may pass before the session key is renegotiated. 1533The first argument is specified in bytes and may have a suffix of 1534.Sq K , 1535.Sq M , 1536or 1537.Sq G 1538to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1539The default is between 1540.Sq 1G 1541and 1542.Sq 4G , 1543depending on the cipher. 1544The optional second value is specified in seconds and may use any of the 1545units documented in the TIME FORMATS section of 1546.Xr sshd_config 5 . 1547The default value for 1548.Cm RekeyLimit 1549is 1550.Cm default none , 1551which means that rekeying is performed after the cipher's default amount 1552of data has been sent or received and no time based rekeying is done. 1553.It Cm RemoteCommand 1554Specifies a command to execute on the remote machine after successfully 1555connecting to the server. 1556The command string extends to the end of the line, and is executed with 1557the user's shell. 1558Arguments to 1559.Cm RemoteCommand 1560accept the tokens described in the 1561.Sx TOKENS 1562section. 1563.It Cm RemoteForward 1564Specifies that a TCP port on the remote machine be forwarded over 1565the secure channel. 1566The remote port may either be forwarded to a specified host and port 1567from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote 1568client to connect to arbitrary destinations from the local machine. 1569The first argument is the listening specification and may be 1570.Sm off 1571.Oo Ar bind_address : Oc Ar port 1572.Sm on 1573or, if the remote host supports it, a Unix domain socket path. 1574If forwarding to a specific destination then the second argument must be 1575.Ar host : Ns Ar hostport 1576or a Unix domain socket path, 1577otherwise if no destination argument is specified then the remote forwarding 1578will be established as a SOCKS proxy. 1579When acting as a SOCKS proxy the destination of the connection can be 1580restricted by 1581.Cm PermitRemoteOpen . 1582.Pp 1583IPv6 addresses can be specified by enclosing addresses in square brackets. 1584Multiple forwardings may be specified, and additional 1585forwardings can be given on the command line. 1586Privileged ports can be forwarded only when 1587logging in as root on the remote machine. 1588Unix domain socket paths may use the tokens described in the 1589.Sx TOKENS 1590section and environment variables as described in the 1591.Sx ENVIRONMENT VARIABLES 1592section. 1593.Pp 1594If the 1595.Ar port 1596argument is 0, 1597the listen port will be dynamically allocated on the server and reported 1598to the client at run time. 1599.Pp 1600If the 1601.Ar bind_address 1602is not specified, the default is to only bind to loopback addresses. 1603If the 1604.Ar bind_address 1605is 1606.Ql * 1607or an empty string, then the forwarding is requested to listen on all 1608interfaces. 1609Specifying a remote 1610.Ar bind_address 1611will only succeed if the server's 1612.Cm GatewayPorts 1613option is enabled (see 1614.Xr sshd_config 5 ) . 1615.It Cm RequestTTY 1616Specifies whether to request a pseudo-tty for the session. 1617The argument may be one of: 1618.Cm no 1619(never request a TTY), 1620.Cm yes 1621(always request a TTY when standard input is a TTY), 1622.Cm force 1623(always request a TTY) or 1624.Cm auto 1625(request a TTY when opening a login session). 1626This option mirrors the 1627.Fl t 1628and 1629.Fl T 1630flags for 1631.Xr ssh 1 . 1632.It Cm RevokedHostKeys 1633Specifies revoked host public keys. 1634Keys listed in this file will be refused for host authentication. 1635Note that if this file does not exist or is not readable, 1636then host authentication will be refused for all hosts. 1637Keys may be specified as a text file, listing one public key per line, or as 1638an OpenSSH Key Revocation List (KRL) as generated by 1639.Xr ssh-keygen 1 . 1640For more information on KRLs, see the KEY REVOCATION LISTS section in 1641.Xr ssh-keygen 1 . 1642.It Cm SecurityKeyProvider 1643Specifies a path to a library that will be used when loading any 1644FIDO authenticator-hosted keys, overriding the default of using 1645the built-in USB HID support. 1646.Pp 1647If the specified value begins with a 1648.Sq $ 1649character, then it will be treated as an environment variable containing 1650the path to the library. 1651.It Cm SendEnv 1652Specifies what variables from the local 1653.Xr environ 7 1654should be sent to the server. 1655The server must also support it, and the server must be configured to 1656accept these environment variables. 1657Note that the 1658.Ev TERM 1659environment variable is always sent whenever a 1660pseudo-terminal is requested as it is required by the protocol. 1661Refer to 1662.Cm AcceptEnv 1663in 1664.Xr sshd_config 5 1665for how to configure the server. 1666Variables are specified by name, which may contain wildcard characters. 1667Multiple environment variables may be separated by whitespace or spread 1668across multiple 1669.Cm SendEnv 1670directives. 1671.Pp 1672See 1673.Sx PATTERNS 1674for more information on patterns. 1675.Pp 1676It is possible to clear previously set 1677.Cm SendEnv 1678variable names by prefixing patterns with 1679.Pa - . 1680The default is not to send any environment variables. 1681.It Cm ServerAliveCountMax 1682Sets the number of server alive messages (see below) which may be 1683sent without 1684.Xr ssh 1 1685receiving any messages back from the server. 1686If this threshold is reached while server alive messages are being sent, 1687ssh will disconnect from the server, terminating the session. 1688It is important to note that the use of server alive messages is very 1689different from 1690.Cm TCPKeepAlive 1691(below). 1692The server alive messages are sent through the encrypted channel 1693and therefore will not be spoofable. 1694The TCP keepalive option enabled by 1695.Cm TCPKeepAlive 1696is spoofable. 1697The server alive mechanism is valuable when the client or 1698server depend on knowing when a connection has become unresponsive. 1699.Pp 1700The default value is 3. 1701If, for example, 1702.Cm ServerAliveInterval 1703(see below) is set to 15 and 1704.Cm ServerAliveCountMax 1705is left at the default, if the server becomes unresponsive, 1706ssh will disconnect after approximately 45 seconds. 1707.It Cm ServerAliveInterval 1708Sets a timeout interval in seconds after which if no data has been received 1709from the server, 1710.Xr ssh 1 1711will send a message through the encrypted 1712channel to request a response from the server. 1713The default 1714is 0, indicating that these messages will not be sent to the server. 1715.It Cm SessionType 1716May be used to either request invocation of a subsystem on the remote system, 1717or to prevent the execution of a remote command at all. 1718The latter is useful for just forwarding ports. 1719The argument to this keyword must be 1720.Cm none 1721(same as the 1722.Fl N 1723option), 1724.Cm subsystem 1725(same as the 1726.Fl s 1727option) or 1728.Cm default 1729(shell or command execution). 1730.It Cm SetEnv 1731Directly specify one or more environment variables and their contents to 1732be sent to the server. 1733Similarly to 1734.Cm SendEnv , 1735with the exception of the 1736.Ev TERM 1737variable, the server must be prepared to accept the environment variable. 1738.It Cm StdinNull 1739Redirects stdin from 1740.Pa /dev/null 1741(actually, prevents reading from stdin). 1742Either this or the equivalent 1743.Fl n 1744option must be used when 1745.Nm ssh 1746is run in the background. 1747The argument to this keyword must be 1748.Cm yes 1749(same as the 1750.Fl n 1751option) or 1752.Cm no 1753(the default). 1754.It Cm StreamLocalBindMask 1755Sets the octal file creation mode mask 1756.Pq umask 1757used when creating a Unix-domain socket file for local or remote 1758port forwarding. 1759This option is only used for port forwarding to a Unix-domain socket file. 1760.Pp 1761The default value is 0177, which creates a Unix-domain socket file that is 1762readable and writable only by the owner. 1763Note that not all operating systems honor the file mode on Unix-domain 1764socket files. 1765.It Cm StreamLocalBindUnlink 1766Specifies whether to remove an existing Unix-domain socket file for local 1767or remote port forwarding before creating a new one. 1768If the socket file already exists and 1769.Cm StreamLocalBindUnlink 1770is not enabled, 1771.Nm ssh 1772will be unable to forward the port to the Unix-domain socket file. 1773This option is only used for port forwarding to a Unix-domain socket file. 1774.Pp 1775The argument must be 1776.Cm yes 1777or 1778.Cm no 1779(the default). 1780.It Cm StrictHostKeyChecking 1781If this flag is set to 1782.Cm yes , 1783.Xr ssh 1 1784will never automatically add host keys to the 1785.Pa ~/.ssh/known_hosts 1786file, and refuses to connect to hosts whose host key has changed. 1787This provides maximum protection against man-in-the-middle (MITM) attacks, 1788though it can be annoying when the 1789.Pa /etc/ssh/ssh_known_hosts 1790file is poorly maintained or when connections to new hosts are 1791frequently made. 1792This option forces the user to manually 1793add all new hosts. 1794.Pp 1795If this flag is set to 1796.Cm accept-new 1797then ssh will automatically add new host keys to the user's 1798.Pa known_hosts 1799file, but will not permit connections to hosts with 1800changed host keys. 1801If this flag is set to 1802.Cm no 1803or 1804.Cm off , 1805ssh will automatically add new host keys to the user known hosts files 1806and allow connections to hosts with changed hostkeys to proceed, 1807subject to some restrictions. 1808If this flag is set to 1809.Cm ask 1810(the default), 1811new host keys 1812will be added to the user known host files only after the user 1813has confirmed that is what they really want to do, and 1814ssh will refuse to connect to hosts whose host key has changed. 1815The host keys of 1816known hosts will be verified automatically in all cases. 1817.It Cm SyslogFacility 1818Gives the facility code that is used when logging messages from 1819.Xr ssh 1 . 1820The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1821LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1822The default is USER. 1823.It Cm TCPKeepAlive 1824Specifies whether the system should send TCP keepalive messages to the 1825other side. 1826If they are sent, death of the connection or crash of one 1827of the machines will be properly noticed. 1828However, this means that 1829connections will die if the route is down temporarily, and some people 1830find it annoying. 1831.Pp 1832The default is 1833.Cm yes 1834(to send TCP keepalive messages), and the client will notice 1835if the network goes down or the remote host dies. 1836This is important in scripts, and many users want it too. 1837.Pp 1838To disable TCP keepalive messages, the value should be set to 1839.Cm no . 1840See also 1841.Cm ServerAliveInterval 1842for protocol-level keepalives. 1843.It Cm Tunnel 1844Request 1845.Xr tun 4 1846device forwarding between the client and the server. 1847The argument must be 1848.Cm yes , 1849.Cm point-to-point 1850(layer 3), 1851.Cm ethernet 1852(layer 2), 1853or 1854.Cm no 1855(the default). 1856Specifying 1857.Cm yes 1858requests the default tunnel mode, which is 1859.Cm point-to-point . 1860.It Cm TunnelDevice 1861Specifies the 1862.Xr tun 4 1863devices to open on the client 1864.Pq Ar local_tun 1865and the server 1866.Pq Ar remote_tun . 1867.Pp 1868The argument must be 1869.Sm off 1870.Ar local_tun Op : Ar remote_tun . 1871.Sm on 1872The devices may be specified by numerical ID or the keyword 1873.Cm any , 1874which uses the next available tunnel device. 1875If 1876.Ar remote_tun 1877is not specified, it defaults to 1878.Cm any . 1879The default is 1880.Cm any:any . 1881.It Cm UpdateHostKeys 1882Specifies whether 1883.Xr ssh 1 1884should accept notifications of additional hostkeys from the server sent 1885after authentication has completed and add them to 1886.Cm UserKnownHostsFile . 1887The argument must be 1888.Cm yes , 1889.Cm no 1890or 1891.Cm ask . 1892This option allows learning alternate hostkeys for a server 1893and supports graceful key rotation by allowing a server to send replacement 1894public keys before old ones are removed. 1895.Pp 1896Additional hostkeys are only accepted if the key used to authenticate the 1897host was already trusted or explicitly accepted by the user, the host was 1898authenticated via 1899.Cm UserKnownHostsFile 1900(i.e. not 1901.Cm GlobalKnownHostsFile ) 1902and the host was authenticated using a plain key and not a certificate. 1903.Pp 1904.Cm UpdateHostKeys 1905is enabled by default if the user has not overridden the default 1906.Cm UserKnownHostsFile 1907setting and has not enabled 1908.Cm VerifyHostKeyDNS , 1909otherwise 1910.Cm UpdateHostKeys 1911will be set to 1912.Cm no . 1913.Pp 1914If 1915.Cm UpdateHostKeys 1916is set to 1917.Cm ask , 1918then the user is asked to confirm the modifications to the known_hosts file. 1919Confirmation is currently incompatible with 1920.Cm ControlPersist , 1921and will be disabled if it is enabled. 1922.Pp 1923Presently, only 1924.Xr sshd 8 1925from OpenSSH 6.8 and greater support the 1926.Qq hostkeys@openssh.com 1927protocol extension used to inform the client of all the server's hostkeys. 1928.It Cm User 1929Specifies the user to log in as. 1930This can be useful when a different user name is used on different machines. 1931This saves the trouble of 1932having to remember to give the user name on the command line. 1933.It Cm UserKnownHostsFile 1934Specifies one or more files to use for the user 1935host key database, separated by whitespace. 1936Each filename may use tilde notation to refer to the user's home directory, 1937the tokens described in the 1938.Sx TOKENS 1939section and environment variables as described in the 1940.Sx ENVIRONMENT VARIABLES 1941section. 1942The default is 1943.Pa ~/.ssh/known_hosts , 1944.Pa ~/.ssh/known_hosts2 . 1945.It Cm VerifyHostKeyDNS 1946Specifies whether to verify the remote key using DNS and SSHFP resource 1947records. 1948If this option is set to 1949.Cm yes , 1950the client will implicitly trust keys that match a secure fingerprint 1951from DNS. 1952Insecure fingerprints will be handled as if this option was set to 1953.Cm ask . 1954If this option is set to 1955.Cm ask , 1956information on fingerprint match will be displayed, but the user will still 1957need to confirm new host keys according to the 1958.Cm StrictHostKeyChecking 1959option. 1960The default is 1961.Cm yes 1962if compiled with LDNS and 1963.Cm no 1964otherwise. 1965.Pp 1966See also 1967.Sx VERIFYING HOST KEYS 1968in 1969.Xr ssh 1 . 1970.It Cm VersionAddendum 1971Specifies a string to append to the regular version string to identify 1972OS- or site-specific modifications. 1973The default is 1974.Dq FreeBSD-20211221 . 1975The value 1976.Cm none 1977may be used to disable this. 1978.It Cm VisualHostKey 1979If this flag is set to 1980.Cm yes , 1981an ASCII art representation of the remote host key fingerprint is 1982printed in addition to the fingerprint string at login and 1983for unknown host keys. 1984If this flag is set to 1985.Cm no 1986(the default), 1987no fingerprint strings are printed at login and 1988only the fingerprint string will be printed for unknown host keys. 1989.It Cm XAuthLocation 1990Specifies the full pathname of the 1991.Xr xauth 1 1992program. 1993The default is 1994.Pa /usr/local/bin/xauth . 1995.El 1996.Sh PATTERNS 1997A 1998.Em pattern 1999consists of zero or more non-whitespace characters, 2000.Sq * 2001(a wildcard that matches zero or more characters), 2002or 2003.Sq ?\& 2004(a wildcard that matches exactly one character). 2005For example, to specify a set of declarations for any host in the 2006.Qq .co.uk 2007set of domains, 2008the following pattern could be used: 2009.Pp 2010.Dl Host *.co.uk 2011.Pp 2012The following pattern 2013would match any host in the 192.168.0.[0-9] network range: 2014.Pp 2015.Dl Host 192.168.0.? 2016.Pp 2017A 2018.Em pattern-list 2019is a comma-separated list of patterns. 2020Patterns within pattern-lists may be negated 2021by preceding them with an exclamation mark 2022.Pq Sq !\& . 2023For example, 2024to allow a key to be used from anywhere within an organization 2025except from the 2026.Qq dialup 2027pool, 2028the following entry (in authorized_keys) could be used: 2029.Pp 2030.Dl from=\&"!*.dialup.example.com,*.example.com\&" 2031.Pp 2032Note that a negated match will never produce a positive result by itself. 2033For example, attempting to match 2034.Qq host3 2035against the following pattern-list will fail: 2036.Pp 2037.Dl from=\&"!host1,!host2\&" 2038.Pp 2039The solution here is to include a term that will yield a positive match, 2040such as a wildcard: 2041.Pp 2042.Dl from=\&"!host1,!host2,*\&" 2043.Sh TOKENS 2044Arguments to some keywords can make use of tokens, 2045which are expanded at runtime: 2046.Pp 2047.Bl -tag -width XXXX -offset indent -compact 2048.It %% 2049A literal 2050.Sq % . 2051.It \&%C 2052Hash of %l%h%p%r. 2053.It %d 2054Local user's home directory. 2055.It %f 2056The fingerprint of the server's host key. 2057.It %H 2058The 2059.Pa known_hosts 2060hostname or address that is being searched for. 2061.It %h 2062The remote hostname. 2063.It \%%I 2064A string describing the reason for a 2065.Cm KnownHostsCommand 2066execution: either 2067.Cm ADDRESS 2068when looking up a host by address (only when 2069.Cm CheckHostIP 2070is enabled), 2071.Cm HOSTNAME 2072when searching by hostname, or 2073.Cm ORDER 2074when preparing the host key algorithm preference list to use for the 2075destination host. 2076.It %i 2077The local user ID. 2078.It %K 2079The base64 encoded host key. 2080.It %k 2081The host key alias if specified, otherwise the original remote hostname given 2082on the command line. 2083.It %L 2084The local hostname. 2085.It %l 2086The local hostname, including the domain name. 2087.It %n 2088The original remote hostname, as given on the command line. 2089.It %p 2090The remote port. 2091.It %r 2092The remote username. 2093.It \&%T 2094The local 2095.Xr tun 4 2096or 2097.Xr tap 4 2098network interface assigned if 2099tunnel forwarding was requested, or 2100.Qq NONE 2101otherwise. 2102.It %t 2103The type of the server host key, e.g. 2104.Cm ssh-ed25519 . 2105.It %u 2106The local username. 2107.El 2108.Pp 2109.Cm CertificateFile , 2110.Cm ControlPath , 2111.Cm IdentityAgent , 2112.Cm IdentityFile , 2113.Cm KnownHostsCommand , 2114.Cm LocalForward , 2115.Cm Match exec , 2116.Cm RemoteCommand , 2117.Cm RemoteForward , 2118and 2119.Cm UserKnownHostsFile 2120accept the tokens %%, %C, %d, %h, %i, %k, %L, %l, %n, %p, %r, and %u. 2121.Pp 2122.Cm KnownHostsCommand 2123additionally accepts the tokens %f, %H, %I, %K and %t. 2124.Pp 2125.Cm Hostname 2126accepts the tokens %% and %h. 2127.Pp 2128.Cm LocalCommand 2129accepts all tokens. 2130.Pp 2131.Cm ProxyCommand 2132accepts the tokens %%, %h, %n, %p, and %r. 2133.Sh ENVIRONMENT VARIABLES 2134Arguments to some keywords can be expanded at runtime from environment 2135variables on the client by enclosing them in 2136.Ic ${} , 2137for example 2138.Ic ${HOME}/.ssh 2139would refer to the user's .ssh directory. 2140If a specified environment variable does not exist then an error will be 2141returned and the setting for that keyword will be ignored. 2142.Pp 2143The keywords 2144.Cm CertificateFile , 2145.Cm ControlPath , 2146.Cm IdentityAgent , 2147.Cm IdentityFile , 2148.Cm KnownHostsCommand , 2149and 2150.Cm UserKnownHostsFile 2151support environment variables. 2152The keywords 2153.Cm LocalForward 2154and 2155.Cm RemoteForward 2156support environment variables only for Unix domain socket paths. 2157.Sh FILES 2158.Bl -tag -width Ds 2159.It Pa ~/.ssh/config 2160This is the per-user configuration file. 2161The format of this file is described above. 2162This file is used by the SSH client. 2163Because of the potential for abuse, this file must have strict permissions: 2164read/write for the user, and not writable by others. 2165.It Pa /etc/ssh/ssh_config 2166Systemwide configuration file. 2167This file provides defaults for those 2168values that are not specified in the user's configuration file, and 2169for those users who do not have a configuration file. 2170This file must be world-readable. 2171.El 2172.Sh SEE ALSO 2173.Xr ssh 1 2174.Sh AUTHORS 2175.An -nosplit 2176OpenSSH is a derivative of the original and free 2177ssh 1.2.12 release by 2178.An Tatu Ylonen . 2179.An Aaron Campbell , Bob Beck , Markus Friedl , 2180.An Niels Provos , Theo de Raadt 2181and 2182.An Dug Song 2183removed many bugs, re-added newer features and 2184created OpenSSH. 2185.An Markus Friedl 2186contributed the support for SSH protocol versions 1.5 and 2.0. 2187