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 1.240 2016/10/15 19:56:25 jmc Exp $ 37.\" $FreeBSD$ 38.Dd $Mdocdate: October 15 2016 $ 39.Dt SSH_CONFIG 5 40.Os 41.Sh NAME 42.Nm ssh_config 43.Nd OpenSSH SSH client configuration files 44.Sh SYNOPSIS 45.Nm ~/.ssh/config 46.Nm /etc/ssh/ssh_config 47.Sh DESCRIPTION 48.Xr ssh 1 49obtains configuration data from the following sources in 50the following order: 51.Pp 52.Bl -enum -offset indent -compact 53.It 54command-line options 55.It 56user's configuration file 57.Pq Pa ~/.ssh/config 58.It 59system-wide configuration file 60.Pq Pa /etc/ssh/ssh_config 61.El 62.Pp 63For each parameter, the first obtained value 64will be used. 65The configuration files contain sections separated by 66.Cm Host 67specifications, and that section is only applied for hosts that 68match one of the patterns given in the specification. 69The matched host name is usually the one given on the command line 70(see the 71.Cm CanonicalizeHostname 72option for exceptions). 73.Pp 74Since the first obtained value for each parameter is used, more 75host-specific declarations should be given near the beginning of the 76file, and general defaults at the end. 77.Pp 78The file contains keyword-argument pairs, one per line. 79Lines starting with 80.Ql # 81and empty lines are interpreted as comments. 82Arguments may optionally be enclosed in double quotes 83.Pq \&" 84in order to represent arguments containing spaces. 85Configuration options may be separated by whitespace or 86optional whitespace and exactly one 87.Ql = ; 88the latter format is useful to avoid the need to quote whitespace 89when specifying configuration options using the 90.Nm ssh , 91.Nm scp , 92and 93.Nm sftp 94.Fl o 95option. 96.Pp 97The possible 98keywords and their meanings are as follows (note that 99keywords are case-insensitive and arguments are case-sensitive): 100.Bl -tag -width Ds 101.It Cm Host 102Restricts the following declarations (up to the next 103.Cm Host 104or 105.Cm Match 106keyword) to be only for those hosts that match one of the patterns 107given after the keyword. 108If more than one pattern is provided, they should be separated by whitespace. 109A single 110.Ql * 111as a pattern can be used to provide global 112defaults for all hosts. 113The host is usually the 114.Ar hostname 115argument given on the command line 116(see the 117.Cm CanonicalizeHostname 118keyword for exceptions). 119.Pp 120A pattern entry may be negated by prefixing it with an exclamation mark 121.Pq Sq !\& . 122If a negated entry is matched, then the 123.Cm Host 124entry is ignored, regardless of whether any other patterns on the line 125match. 126Negated matches are therefore useful to provide exceptions for wildcard 127matches. 128.Pp 129See 130.Sx PATTERNS 131for more information on patterns. 132.It Cm Match 133Restricts the following declarations (up to the next 134.Cm Host 135or 136.Cm Match 137keyword) to be used only when the conditions following the 138.Cm Match 139keyword are satisfied. 140Match conditions are specified using one or more criteria 141or the single token 142.Cm all 143which always matches. 144The available criteria keywords are: 145.Cm canonical , 146.Cm exec , 147.Cm host , 148.Cm originalhost , 149.Cm user , 150and 151.Cm localuser . 152The 153.Cm all 154criteria must appear alone or immediately after 155.Cm canonical . 156Other criteria may be combined arbitrarily. 157All criteria but 158.Cm all 159and 160.Cm canonical 161require an argument. 162Criteria may be negated by prepending an exclamation mark 163.Pq Sq !\& . 164.Pp 165The 166.Cm canonical 167keyword matches only when the configuration file is being re-parsed 168after hostname canonicalization (see the 169.Cm CanonicalizeHostname 170option.) 171This may be useful to specify conditions that work with canonical host 172names only. 173The 174.Cm exec 175keyword executes the specified command under the user's shell. 176If the command returns a zero exit status then the condition is considered true. 177Commands containing whitespace characters must be quoted. 178Arguments to 179.Cm exec 180accept the tokens described in the 181.Sx TOKENS 182section. 183.Pp 184The other keywords' criteria must be single entries or comma-separated 185lists and may use the wildcard and negation operators described in the 186.Sx PATTERNS 187section. 188The criteria for the 189.Cm host 190keyword are matched against the target hostname, after any substitution 191by the 192.Cm Hostname 193or 194.Cm CanonicalizeHostname 195options. 196The 197.Cm originalhost 198keyword matches against the hostname as it was specified on the command-line. 199The 200.Cm user 201keyword matches against the target username on the remote host. 202The 203.Cm localuser 204keyword matches against the name of the local user running 205.Xr ssh 1 206(this keyword may be useful in system-wide 207.Nm 208files). 209.It Cm AddKeysToAgent 210Specifies whether keys should be automatically added to a running 211.Xr ssh-agent 1 . 212If this option is set to 213.Cm yes 214and a key is loaded from a file, the key and its passphrase are added to 215the agent with the default lifetime, as if by 216.Xr ssh-add 1 . 217If this option is set to 218.Cm ask , 219.Xr ssh 1 220will require confirmation using the 221.Ev SSH_ASKPASS 222program before adding a key (see 223.Xr ssh-add 1 224for details). 225If this option is set to 226.Cm confirm , 227each use of the key must be confirmed, as if the 228.Fl c 229option was specified to 230.Xr ssh-add 1 . 231If this option is set to 232.Cm no , 233no keys are added to the agent. 234The argument must be 235.Cm yes , 236.Cm confirm , 237.Cm ask , 238or 239.Cm no 240(the default). 241.It Cm AddressFamily 242Specifies which address family to use when connecting. 243Valid arguments are 244.Cm any 245(the default), 246.Cm inet 247(use IPv4 only), or 248.Cm inet6 249(use IPv6 only). 250.It Cm BatchMode 251If set to 252.Cm yes , 253passphrase/password querying will be disabled. 254This option is useful in scripts and other batch jobs where no user 255is present to supply the password. 256The argument must be 257.Cm yes 258or 259.Cm no 260(the default). 261.It Cm BindAddress 262Use the specified address on the local machine as the source address of 263the connection. 264Only useful on systems with more than one address. 265Note that this option does not work if 266.Cm UsePrivilegedPort 267is set to 268.Cm yes . 269.It Cm CanonicalDomains 270When 271.Cm CanonicalizeHostname 272is enabled, this option specifies the list of domain suffixes in which to 273search for the specified destination host. 274.It Cm CanonicalizeFallbackLocal 275Specifies whether to fail with an error when hostname canonicalization fails. 276The default, 277.Cm yes , 278will attempt to look up the unqualified hostname using the system resolver's 279search rules. 280A value of 281.Cm no 282will cause 283.Xr ssh 1 284to fail instantly if 285.Cm CanonicalizeHostname 286is enabled and the target hostname cannot be found in any of the domains 287specified by 288.Cm CanonicalDomains . 289.It Cm CanonicalizeHostname 290Controls whether explicit hostname canonicalization is performed. 291The default, 292.Cm no , 293is not to perform any name rewriting and let the system resolver handle all 294hostname lookups. 295If set to 296.Cm yes 297then, for connections that do not use a 298.Cm ProxyCommand , 299.Xr ssh 1 300will attempt to canonicalize the hostname specified on the command line 301using the 302.Cm CanonicalDomains 303suffixes and 304.Cm CanonicalizePermittedCNAMEs 305rules. 306If 307.Cm CanonicalizeHostname 308is set to 309.Cm always , 310then canonicalization is applied to proxied connections too. 311.Pp 312If this option is enabled, then the configuration files are processed 313again using the new target name to pick up any new configuration in matching 314.Cm Host 315and 316.Cm Match 317stanzas. 318.It Cm CanonicalizeMaxDots 319Specifies the maximum number of dot characters in a hostname before 320canonicalization is disabled. 321The default, 1, 322allows a single dot (i.e. hostname.subdomain). 323.It Cm CanonicalizePermittedCNAMEs 324Specifies rules to determine whether CNAMEs should be followed when 325canonicalizing hostnames. 326The rules consist of one or more arguments of 327.Ar source_domain_list : Ns Ar target_domain_list , 328where 329.Ar source_domain_list 330is a pattern-list of domains that may follow CNAMEs in canonicalization, 331and 332.Ar target_domain_list 333is a pattern-list of domains that they may resolve to. 334.Pp 335For example, 336.Qq *.a.example.com:*.b.example.com,*.c.example.com 337will allow hostnames matching 338.Qq *.a.example.com 339to be canonicalized to names in the 340.Qq *.b.example.com 341or 342.Qq *.c.example.com 343domains. 344.It Cm CertificateFile 345Specifies a file from which the user's certificate is read. 346A corresponding private key must be provided separately in order 347to use this certificate either 348from an 349.Cm IdentityFile 350directive or 351.Fl i 352flag to 353.Xr ssh 1 , 354via 355.Xr ssh-agent 1 , 356or via a 357.Cm PKCS11Provider . 358.Pp 359Arguments to 360.Cm CertificateFile 361may use the tilde syntax to refer to a user's home directory 362or the tokens described in the 363.Sx TOKENS 364section. 365.Pp 366It is possible to have multiple certificate files specified in 367configuration files; these certificates will be tried in sequence. 368Multiple 369.Cm CertificateFile 370directives will add to the list of certificates used for 371authentication. 372.It Cm ChallengeResponseAuthentication 373Specifies whether to use challenge-response authentication. 374The argument to this keyword must be 375.Cm yes 376(the default) 377or 378.Cm no . 379.It Cm CheckHostIP 380If set to 381.Cm yes , 382.Xr ssh 1 383will additionally check the host IP address in the 384.Pa known_hosts 385file. 386This allows it to detect if a host key changed due to DNS spoofing 387and will add addresses of destination hosts to 388.Pa ~/.ssh/known_hosts 389in the process, regardless of the setting of 390.Cm StrictHostKeyChecking . 391If the option is set to 392.Cm no , 393the check will not be executed. 394The default is 395.Cm no . 396.It Cm Cipher 397Specifies the cipher to use for encrypting the session 398in protocol version 1. 399Currently, 400.Cm blowfish , 401.Cm 3des 402(the default), 403and 404.Cm des 405are supported, 406though 407.Cm des 408is only supported in the 409.Xr ssh 1 410client for interoperability with legacy protocol 1 implementations; 411its use is strongly discouraged due to cryptographic weaknesses. 412.It Cm Ciphers 413Specifies the ciphers allowed for protocol version 2 414in order of preference. 415Multiple ciphers must be comma-separated. 416If the specified value begins with a 417.Sq + 418character, then the specified ciphers will be appended to the default set 419instead of replacing them. 420.Pp 421The supported ciphers are: 422.Bd -literal -offset indent 4233des-cbc 424aes128-cbc 425aes192-cbc 426aes256-cbc 427aes128-ctr 428aes192-ctr 429aes256-ctr 430aes128-gcm@openssh.com 431aes256-gcm@openssh.com 432arcfour 433arcfour128 434arcfour256 435blowfish-cbc 436cast128-cbc 437chacha20-poly1305@openssh.com 438.Ed 439.Pp 440The default is: 441.Bd -literal -offset indent 442chacha20-poly1305@openssh.com, 443aes128-ctr,aes192-ctr,aes256-ctr, 444aes128-gcm@openssh.com,aes256-gcm@openssh.com, 445aes128-cbc,aes192-cbc,aes256-cbc 446.Ed 447.Pp 448The list of available ciphers may also be obtained using 449.Qq ssh -Q cipher . 450.It Cm ClearAllForwardings 451Specifies that all local, remote, and dynamic port forwardings 452specified in the configuration files or on the command line be 453cleared. 454This option is primarily useful when used from the 455.Xr ssh 1 456command line to clear port forwardings set in 457configuration files, and is automatically set by 458.Xr scp 1 459and 460.Xr sftp 1 . 461The argument must be 462.Cm yes 463or 464.Cm no 465(the default). 466.It Cm Compression 467Specifies whether to use compression. 468The argument must be 469.Cm yes 470or 471.Cm no 472(the default). 473.It Cm CompressionLevel 474Specifies the compression level to use if compression is enabled. 475The argument must be an integer from 1 (fast) to 9 (slow, best). 476The default level is 6, which is good for most applications. 477The meaning of the values is the same as in 478.Xr gzip 1 . 479Note that this option applies to protocol version 1 only. 480.It Cm ConnectionAttempts 481Specifies the number of tries (one per second) to make before exiting. 482The argument must be an integer. 483This may be useful in scripts if the connection sometimes fails. 484The default is 1. 485.It Cm ConnectTimeout 486Specifies the timeout (in seconds) used when connecting to the 487SSH server, instead of using the default system TCP timeout. 488This value is used only when the target is down or really unreachable, 489not when it refuses the connection. 490.It Cm ControlMaster 491Enables the sharing of multiple sessions over a single network connection. 492When set to 493.Cm yes , 494.Xr ssh 1 495will listen for connections on a control socket specified using the 496.Cm ControlPath 497argument. 498Additional sessions can connect to this socket using the same 499.Cm ControlPath 500with 501.Cm ControlMaster 502set to 503.Cm no 504(the default). 505These sessions will try to reuse the master instance's network connection 506rather than initiating new ones, but will fall back to connecting normally 507if the control socket does not exist, or is not listening. 508.Pp 509Setting this to 510.Cm ask 511will cause 512.Xr ssh 1 513to listen for control connections, but require confirmation using 514.Xr ssh-askpass 1 . 515If the 516.Cm ControlPath 517cannot be opened, 518.Xr ssh 1 519will continue without connecting to a master instance. 520.Pp 521X11 and 522.Xr ssh-agent 1 523forwarding is supported over these multiplexed connections, however the 524display and agent forwarded will be the one belonging to the master 525connection i.e. it is not possible to forward multiple displays or agents. 526.Pp 527Two additional options allow for opportunistic multiplexing: try to use a 528master connection but fall back to creating a new one if one does not already 529exist. 530These options are: 531.Cm auto 532and 533.Cm autoask . 534The latter requires confirmation like the 535.Cm ask 536option. 537.It Cm ControlPath 538Specify the path to the control socket used for connection sharing as described 539in the 540.Cm ControlMaster 541section above or the string 542.Cm none 543to disable connection sharing. 544Arguments to 545.Cm ControlPath 546may use the tilde syntax to refer to a user's home directory 547or the tokens described in the 548.Sx TOKENS 549section. 550It is recommended that any 551.Cm ControlPath 552used for opportunistic connection sharing include 553at least %h, %p, and %r (or alternatively %C) and be placed in a directory 554that is not writable by other users. 555This ensures that shared connections are uniquely identified. 556.It Cm ControlPersist 557When used in conjunction with 558.Cm ControlMaster , 559specifies that the master connection should remain open 560in the background (waiting for future client connections) 561after the initial client connection has been closed. 562If set to 563.Cm no , 564then the master connection will not be placed into the background, 565and will close as soon as the initial client connection is closed. 566If set to 567.Cm yes 568or 0, 569then the master connection will remain in the background indefinitely 570(until killed or closed via a mechanism such as the 571.Qq ssh -O exit ) . 572If set to a time in seconds, or a time in any of the formats documented in 573.Xr sshd_config 5 , 574then the backgrounded master connection will automatically terminate 575after it has remained idle (with no client connections) for the 576specified time. 577.It Cm DynamicForward 578Specifies that a TCP port on the local machine be forwarded 579over the secure channel, and the application 580protocol is then used to determine where to connect to from the 581remote machine. 582.Pp 583The argument must be 584.Sm off 585.Oo Ar bind_address : Oc Ar port . 586.Sm on 587IPv6 addresses can be specified by enclosing addresses in square brackets. 588By default, the local port is bound in accordance with the 589.Cm GatewayPorts 590setting. 591However, an explicit 592.Ar bind_address 593may be used to bind the connection to a specific address. 594The 595.Ar bind_address 596of 597.Cm localhost 598indicates that the listening port be bound for local use only, while an 599empty address or 600.Sq * 601indicates that the port should be available from all interfaces. 602.Pp 603Currently the SOCKS4 and SOCKS5 protocols are supported, and 604.Xr ssh 1 605will act as a SOCKS server. 606Multiple forwardings may be specified, and 607additional forwardings can be given on the command line. 608Only the superuser can forward privileged ports. 609.It Cm EnableSSHKeysign 610Setting this option to 611.Cm yes 612in the global client configuration file 613.Pa /etc/ssh/ssh_config 614enables the use of the helper program 615.Xr ssh-keysign 8 616during 617.Cm HostbasedAuthentication . 618The argument must be 619.Cm yes 620or 621.Cm no 622(the default). 623This option should be placed in the non-hostspecific section. 624See 625.Xr ssh-keysign 8 626for more information. 627.It Cm EscapeChar 628Sets the escape character (default: 629.Ql ~ ) . 630The escape character can also 631be set on the command line. 632The argument should be a single character, 633.Ql ^ 634followed by a letter, or 635.Cm none 636to disable the escape 637character entirely (making the connection transparent for binary 638data). 639.It Cm ExitOnForwardFailure 640Specifies whether 641.Xr ssh 1 642should terminate the connection if it cannot set up all requested 643dynamic, tunnel, local, and remote port forwardings, (e.g.\& 644if either end is unable to bind and listen on a specified port). 645Note that 646.Cm ExitOnForwardFailure 647does not apply to connections made over port forwardings and will not, 648for example, cause 649.Xr ssh 1 650to exit if TCP connections to the ultimate forwarding destination fail. 651The argument must be 652.Cm yes 653or 654.Cm no 655(the default). 656.It Cm FingerprintHash 657Specifies the hash algorithm used when displaying key fingerprints. 658Valid options are: 659.Cm md5 660and 661.Cm sha256 662(the default). 663.It Cm ForwardAgent 664Specifies whether the connection to the authentication agent (if any) 665will be forwarded to the remote machine. 666The argument must be 667.Cm yes 668or 669.Cm no 670(the default). 671.Pp 672Agent forwarding should be enabled with caution. 673Users with the ability to bypass file permissions on the remote host 674(for the agent's Unix-domain socket) 675can access the local agent through the forwarded connection. 676An attacker cannot obtain key material from the agent, 677however they can perform operations on the keys that enable them to 678authenticate using the identities loaded into the agent. 679.It Cm ForwardX11 680Specifies whether X11 connections will be automatically redirected 681over the secure channel and 682.Ev DISPLAY 683set. 684The argument must be 685.Cm yes 686or 687.Cm no 688(the default). 689.Pp 690X11 forwarding should be enabled with caution. 691Users with the ability to bypass file permissions on the remote host 692(for the user's X11 authorization database) 693can access the local X11 display through the forwarded connection. 694An attacker may then be able to perform activities such as keystroke monitoring 695if the 696.Cm ForwardX11Trusted 697option is also enabled. 698.It Cm ForwardX11Timeout 699Specify a timeout for untrusted X11 forwarding 700using the format described in the 701.Sx TIME FORMATS 702section of 703.Xr sshd_config 5 . 704X11 connections received by 705.Xr ssh 1 706after this time will be refused. 707The default is to disable untrusted X11 forwarding after twenty minutes has 708elapsed. 709.It Cm ForwardX11Trusted 710If this option is set to 711.Cm yes , 712remote X11 clients will have full access to the original X11 display. 713.Pp 714If this option is set to 715.Cm no 716(the default), 717remote X11 clients will be considered untrusted and prevented 718from stealing or tampering with data belonging to trusted X11 719clients. 720Furthermore, the 721.Xr xauth 1 722token used for the session will be set to expire after 20 minutes. 723Remote clients will be refused access after this time. 724.Pp 725See the X11 SECURITY extension specification for full details on 726the restrictions imposed on untrusted clients. 727.It Cm GatewayPorts 728Specifies whether remote hosts are allowed to connect to local 729forwarded ports. 730By default, 731.Xr ssh 1 732binds local port forwardings to the loopback address. 733This prevents other remote hosts from connecting to forwarded ports. 734.Cm GatewayPorts 735can be used to specify that ssh 736should bind local port forwardings to the wildcard address, 737thus allowing remote hosts to connect to forwarded ports. 738The argument must be 739.Cm yes 740or 741.Cm no 742(the default). 743.It Cm GlobalKnownHostsFile 744Specifies one or more files to use for the global 745host key database, separated by whitespace. 746The default is 747.Pa /etc/ssh/ssh_known_hosts , 748.Pa /etc/ssh/ssh_known_hosts2 . 749.It Cm GSSAPIAuthentication 750Specifies whether user authentication based on GSSAPI is allowed. 751The default is 752.Cm no . 753.It Cm GSSAPIDelegateCredentials 754Forward (delegate) credentials to the server. 755The default is 756.Cm no . 757.It Cm HashKnownHosts 758Indicates that 759.Xr ssh 1 760should hash host names and addresses when they are added to 761.Pa ~/.ssh/known_hosts . 762These hashed names may be used normally by 763.Xr ssh 1 764and 765.Xr sshd 8 , 766but they do not reveal identifying information should the file's contents 767be disclosed. 768The default is 769.Cm no . 770Note that existing names and addresses in known hosts files 771will not be converted automatically, 772but may be manually hashed using 773.Xr ssh-keygen 1 . 774.It Cm HostbasedAuthentication 775Specifies whether to try rhosts based authentication with public key 776authentication. 777The argument must be 778.Cm yes 779or 780.Cm no 781(the default). 782.It Cm HostbasedKeyTypes 783Specifies the key types that will be used for hostbased authentication 784as a comma-separated pattern list. 785Alternately if the specified value begins with a 786.Sq + 787character, then the specified key types will be appended to the default set 788instead of replacing them. 789The default for this option is: 790.Bd -literal -offset 3n 791ecdsa-sha2-nistp256-cert-v01@openssh.com, 792ecdsa-sha2-nistp384-cert-v01@openssh.com, 793ecdsa-sha2-nistp521-cert-v01@openssh.com, 794ssh-ed25519-cert-v01@openssh.com, 795ssh-rsa-cert-v01@openssh.com, 796ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 797ssh-ed25519,ssh-rsa 798.Ed 799.Pp 800The 801.Fl Q 802option of 803.Xr ssh 1 804may be used to list supported key types. 805.It Cm HostKeyAlgorithms 806Specifies the host key algorithms 807that the client wants to use in order of preference. 808Alternately if the specified value begins with a 809.Sq + 810character, then the specified key types will be appended to the default set 811instead of replacing them. 812The default for this option is: 813.Bd -literal -offset 3n 814ecdsa-sha2-nistp256-cert-v01@openssh.com, 815ecdsa-sha2-nistp384-cert-v01@openssh.com, 816ecdsa-sha2-nistp521-cert-v01@openssh.com, 817ssh-ed25519-cert-v01@openssh.com, 818ssh-rsa-cert-v01@openssh.com, 819ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 820ssh-ed25519,ssh-rsa 821.Ed 822.Pp 823If hostkeys are known for the destination host then this default is modified 824to prefer their algorithms. 825.Pp 826The list of available key types may also be obtained using 827.Qq ssh -Q key . 828.It Cm HostKeyAlias 829Specifies an alias that should be used instead of the 830real host name when looking up or saving the host key 831in the host key database files. 832This option is useful for tunneling SSH connections 833or for multiple servers running on a single host. 834.It Cm HostName 835Specifies the real host name to log into. 836This can be used to specify nicknames or abbreviations for hosts. 837Arguments to 838.Cm HostName 839accept the tokens described in the 840.Sx TOKENS 841section. 842Numeric IP addresses are also permitted (both on the command line and in 843.Cm HostName 844specifications). 845The default is the name given on the command line. 846.It Cm IdentitiesOnly 847Specifies that 848.Xr ssh 1 849should only use the authentication identity and certificate files explicitly 850configured in the 851.Nm 852files 853or passed on the 854.Xr ssh 1 855command-line, 856even if 857.Xr ssh-agent 1 858or a 859.Cm PKCS11Provider 860offers more identities. 861The argument to this keyword must be 862.Cm yes 863or 864.Cm no 865(the default). 866This option is intended for situations where ssh-agent 867offers many different identities. 868.It Cm IdentityAgent 869Specifies the 870.Ux Ns -domain 871socket used to communicate with the authentication agent. 872.Pp 873This option overrides the 874.Ev SSH_AUTH_SOCK 875environment variable and can be used to select a specific agent. 876Setting the socket name to 877.Cm none 878disables the use of an authentication agent. 879If the string 880.Qq SSH_AUTH_SOCK 881is specified, the location of the socket will be read from the 882.Ev SSH_AUTH_SOCK 883environment variable. 884.Pp 885Arguments to 886.Cm IdentityAgent 887may use the tilde syntax to refer to a user's home directory 888or the tokens described in the 889.Sx TOKENS 890section. 891.It Cm IdentityFile 892Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication 893identity is read. 894The default is 895.Pa ~/.ssh/identity 896for protocol version 1, and 897.Pa ~/.ssh/id_dsa , 898.Pa ~/.ssh/id_ecdsa , 899.Pa ~/.ssh/id_ed25519 900and 901.Pa ~/.ssh/id_rsa 902for protocol version 2. 903Additionally, any identities represented by the authentication agent 904will be used for authentication unless 905.Cm IdentitiesOnly 906is set. 907If no certificates have been explicitly specified by 908.Cm CertificateFile , 909.Xr ssh 1 910will try to load certificate information from the filename obtained by 911appending 912.Pa -cert.pub 913to the path of a specified 914.Cm IdentityFile . 915.Pp 916Arguments to 917.Cm IdentityFile 918may use the tilde syntax to refer to a user's home directory 919or the tokens described in the 920.Sx TOKENS 921section. 922.Pp 923It is possible to have 924multiple identity files specified in configuration files; all these 925identities will be tried in sequence. 926Multiple 927.Cm IdentityFile 928directives will add to the list of identities tried (this behaviour 929differs from that of other configuration directives). 930.Pp 931.Cm IdentityFile 932may be used in conjunction with 933.Cm IdentitiesOnly 934to select which identities in an agent are offered during authentication. 935.Cm IdentityFile 936may also be used in conjunction with 937.Cm CertificateFile 938in order to provide any certificate also needed for authentication with 939the identity. 940.It Cm IgnoreUnknown 941Specifies a pattern-list of unknown options to be ignored if they are 942encountered in configuration parsing. 943This may be used to suppress errors if 944.Nm 945contains options that are unrecognised by 946.Xr ssh 1 . 947It is recommended that 948.Cm IgnoreUnknown 949be listed early in the configuration file as it will not be applied 950to unknown options that appear before it. 951.It Cm Include 952Include the specified configuration file(s). 953Multiple pathnames may be specified and each pathname may contain 954.Xr glob 3 955wildcards and, for user configurations, shell-like 956.Sq ~ 957references to user home directories. 958Files without absolute paths are assumed to be in 959.Pa ~/.ssh 960if included in a user configuration file or 961.Pa /etc/ssh 962if included from the system configuration file. 963.Cm Include 964directive may appear inside a 965.Cm Match 966or 967.Cm Host 968block 969to perform conditional inclusion. 970.It Cm IPQoS 971Specifies the IPv4 type-of-service or DSCP class for connections. 972Accepted values are 973.Cm af11 , 974.Cm af12 , 975.Cm af13 , 976.Cm af21 , 977.Cm af22 , 978.Cm af23 , 979.Cm af31 , 980.Cm af32 , 981.Cm af33 , 982.Cm af41 , 983.Cm af42 , 984.Cm af43 , 985.Cm cs0 , 986.Cm cs1 , 987.Cm cs2 , 988.Cm cs3 , 989.Cm cs4 , 990.Cm cs5 , 991.Cm cs6 , 992.Cm cs7 , 993.Cm ef , 994.Cm lowdelay , 995.Cm throughput , 996.Cm reliability , 997or a numeric value. 998This option may take one or two arguments, separated by whitespace. 999If one argument is specified, it is used as the packet class unconditionally. 1000If two values are specified, the first is automatically selected for 1001interactive sessions and the second for non-interactive sessions. 1002The default is 1003.Cm lowdelay 1004for interactive sessions and 1005.Cm throughput 1006for non-interactive sessions. 1007.It Cm KbdInteractiveAuthentication 1008Specifies whether to use keyboard-interactive authentication. 1009The argument to this keyword must be 1010.Cm yes 1011(the default) 1012or 1013.Cm no . 1014.It Cm KbdInteractiveDevices 1015Specifies the list of methods to use in keyboard-interactive authentication. 1016Multiple method names must be comma-separated. 1017The default is to use the server specified list. 1018The methods available vary depending on what the server supports. 1019For an OpenSSH server, 1020it may be zero or more of: 1021.Cm bsdauth , 1022.Cm pam , 1023and 1024.Cm skey . 1025.It Cm KexAlgorithms 1026Specifies the available KEX (Key Exchange) algorithms. 1027Multiple algorithms must be comma-separated. 1028Alternately if the specified value begins with a 1029.Sq + 1030character, then the specified methods will be appended to the default set 1031instead of replacing them. 1032The default is: 1033.Bd -literal -offset indent 1034curve25519-sha256,curve25519-sha256@libssh.org, 1035ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1036diffie-hellman-group-exchange-sha256, 1037diffie-hellman-group-exchange-sha1, 1038diffie-hellman-group14-sha1 1039.Ed 1040.Pp 1041The list of available key exchange algorithms may also be obtained using 1042.Qq ssh -Q kex . 1043.It Cm LocalCommand 1044Specifies a command to execute on the local machine after successfully 1045connecting to the server. 1046The command string extends to the end of the line, and is executed with 1047the user's shell. 1048Arguments to 1049.Cm LocalCommand 1050accept the tokens described in the 1051.Sx TOKENS 1052section. 1053.Pp 1054The command is run synchronously and does not have access to the 1055session of the 1056.Xr ssh 1 1057that spawned it. 1058It should not be used for interactive commands. 1059.Pp 1060This directive is ignored unless 1061.Cm PermitLocalCommand 1062has been enabled. 1063.It Cm LocalForward 1064Specifies that a TCP port on the local machine be forwarded over 1065the secure channel to the specified host and port from the remote machine. 1066The first argument must be 1067.Sm off 1068.Oo Ar bind_address : Oc Ar port 1069.Sm on 1070and the second argument must be 1071.Ar host : Ns Ar hostport . 1072IPv6 addresses can be specified by enclosing addresses in square brackets. 1073Multiple forwardings may be specified, and additional forwardings can be 1074given on the command line. 1075Only the superuser can forward privileged ports. 1076By default, the local port is bound in accordance with the 1077.Cm GatewayPorts 1078setting. 1079However, an explicit 1080.Ar bind_address 1081may be used to bind the connection to a specific address. 1082The 1083.Ar bind_address 1084of 1085.Cm localhost 1086indicates that the listening port be bound for local use only, while an 1087empty address or 1088.Sq * 1089indicates that the port should be available from all interfaces. 1090.It Cm LogLevel 1091Gives the verbosity level that is used when logging messages from 1092.Xr ssh 1 . 1093The possible values are: 1094QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1095The default is INFO. 1096DEBUG and DEBUG1 are equivalent. 1097DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1098.It Cm MACs 1099Specifies the MAC (message authentication code) algorithms 1100in order of preference. 1101The MAC algorithm is used for data integrity protection. 1102Multiple algorithms must be comma-separated. 1103If the specified value begins with a 1104.Sq + 1105character, then the specified algorithms will be appended to the default set 1106instead of replacing them. 1107.Pp 1108The algorithms that contain 1109.Qq -etm 1110calculate the MAC after encryption (encrypt-then-mac). 1111These are considered safer and their use recommended. 1112.Pp 1113The default is: 1114.Bd -literal -offset indent 1115umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1116hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1117hmac-sha1-etm@openssh.com, 1118umac-64@openssh.com,umac-128@openssh.com, 1119hmac-sha2-256,hmac-sha2-512,hmac-sha1 1120.Ed 1121.Pp 1122The list of available MAC algorithms may also be obtained using 1123.Qq ssh -Q mac . 1124.It Cm NoHostAuthenticationForLocalhost 1125This option can be used if the home directory is shared across machines. 1126In this case localhost will refer to a different machine on each of 1127the machines and the user will get many warnings about changed host keys. 1128However, this option disables host authentication for localhost. 1129The argument to this keyword must be 1130.Cm yes 1131or 1132.Cm no . 1133(the default). 1134.It Cm NumberOfPasswordPrompts 1135Specifies the number of password prompts before giving up. 1136The argument to this keyword must be an integer. 1137The default is 3. 1138.It Cm PasswordAuthentication 1139Specifies whether to use password authentication. 1140The argument to this keyword must be 1141.Cm yes 1142(the default) 1143or 1144.Cm no . 1145.It Cm PermitLocalCommand 1146Allow local command execution via the 1147.Ic LocalCommand 1148option or using the 1149.Ic !\& Ns Ar command 1150escape sequence in 1151.Xr ssh 1 . 1152The argument must be 1153.Cm yes 1154or 1155.Cm no 1156(the default). 1157.It Cm PKCS11Provider 1158Specifies which PKCS#11 provider to use. 1159The argument to this keyword is the PKCS#11 shared library 1160.Xr ssh 1 1161should use to communicate with a PKCS#11 token providing the user's 1162private RSA key. 1163.It Cm Port 1164Specifies the port number to connect on the remote host. 1165The default is 22. 1166.It Cm PreferredAuthentications 1167Specifies the order in which the client should try authentication methods. 1168This allows a client to prefer one method (e.g.\& 1169.Cm keyboard-interactive ) 1170over another method (e.g.\& 1171.Cm password ) . 1172The default is: 1173.Bd -literal -offset indent 1174gssapi-with-mic,hostbased,publickey, 1175keyboard-interactive,password 1176.Ed 1177.It Cm Protocol 1178Specifies the protocol versions 1179.Xr ssh 1 1180should support in order of preference. 1181The possible values are 1 and 2. 1182Multiple versions must be comma-separated. 1183When this option is set to 1184.Cm 2,1 1185.Nm ssh 1186will try version 2 and fall back to version 1 1187if version 2 is not available. 1188The default is version 2. 1189Protocol 1 suffers from a number of cryptographic weaknesses and should 1190not be used. 1191It is only offered to support legacy devices. 1192.It Cm ProxyCommand 1193Specifies the command to use to connect to the server. 1194The command 1195string extends to the end of the line, and is executed 1196using the user's shell 1197.Ql exec 1198directive to avoid a lingering shell process. 1199.Pp 1200Arguments to 1201.Cm ProxyCommand 1202accept the tokens described in the 1203.Sx TOKENS 1204section. 1205The command can be basically anything, 1206and should read from its standard input and write to its standard output. 1207It should eventually connect an 1208.Xr sshd 8 1209server running on some machine, or execute 1210.Ic sshd -i 1211somewhere. 1212Host key management will be done using the 1213HostName of the host being connected (defaulting to the name typed by 1214the user). 1215Setting the command to 1216.Cm none 1217disables this option entirely. 1218Note that 1219.Cm CheckHostIP 1220is not available for connects with a proxy command. 1221.Pp 1222This directive is useful in conjunction with 1223.Xr nc 1 1224and its proxy support. 1225For example, the following directive would connect via an HTTP proxy at 1226192.0.2.0: 1227.Bd -literal -offset 3n 1228ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1229.Ed 1230.It Cm ProxyJump 1231Specifies one or more jump proxies as 1232.Xo 1233.Sm off 1234.Op Ar user No @ 1235.Ar host 1236.Op : Ns Ar port 1237.Sm on 1238.Xc . 1239Multiple proxies may be separated by comma characters and will be visited 1240sequentially. 1241Setting this option will cause 1242.Xr ssh 1 1243to connect to the target host by first making a 1244.Xr ssh 1 1245connection to the specified 1246.Cm ProxyJump 1247host and then establishing a 1248TCP forwarding to the ultimate target from there. 1249.Pp 1250Note that this option will compete with the 1251.Cm ProxyCommand 1252option - whichever is specified first will prevent later instances of the 1253other from taking effect. 1254.It Cm ProxyUseFdpass 1255Specifies that 1256.Cm ProxyCommand 1257will pass a connected file descriptor back to 1258.Xr ssh 1 1259instead of continuing to execute and pass data. 1260The default is 1261.Cm no . 1262.It Cm PubkeyAcceptedKeyTypes 1263Specifies the key types that will be used for public key authentication 1264as a comma-separated pattern list. 1265Alternately if the specified value begins with a 1266.Sq + 1267character, then the key types after it will be appended to the default 1268instead of replacing it. 1269The default for this option is: 1270.Bd -literal -offset 3n 1271ecdsa-sha2-nistp256-cert-v01@openssh.com, 1272ecdsa-sha2-nistp384-cert-v01@openssh.com, 1273ecdsa-sha2-nistp521-cert-v01@openssh.com, 1274ssh-ed25519-cert-v01@openssh.com, 1275ssh-rsa-cert-v01@openssh.com, 1276ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1277ssh-ed25519,ssh-rsa 1278.Ed 1279.Pp 1280The list of available key types may also be obtained using 1281.Qq ssh -Q key . 1282.It Cm PubkeyAuthentication 1283Specifies whether to try public key authentication. 1284The argument to this keyword must be 1285.Cm yes 1286(the default) 1287or 1288.Cm no . 1289.It Cm RekeyLimit 1290Specifies the maximum amount of data that may be transmitted before the 1291session key is renegotiated, optionally followed a maximum amount of 1292time that may pass before the session key is renegotiated. 1293The first argument is specified in bytes and may have a suffix of 1294.Sq K , 1295.Sq M , 1296or 1297.Sq G 1298to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1299The default is between 1300.Sq 1G 1301and 1302.Sq 4G , 1303depending on the cipher. 1304The optional second value is specified in seconds and may use any of the 1305units documented in the 1306.Sx TIME FORMATS 1307section of 1308.Xr sshd_config 5 . 1309The default value for 1310.Cm RekeyLimit 1311is 1312.Cm default none , 1313which means that rekeying is performed after the cipher's default amount 1314of data has been sent or received and no time based rekeying is done. 1315.It Cm RemoteForward 1316Specifies that a TCP port on the remote machine be forwarded over 1317the secure channel to the specified host and port from the local machine. 1318The first argument must be 1319.Sm off 1320.Oo Ar bind_address : Oc Ar port 1321.Sm on 1322and the second argument must be 1323.Ar host : Ns Ar hostport . 1324IPv6 addresses can be specified by enclosing addresses in square brackets. 1325Multiple forwardings may be specified, and additional 1326forwardings can be given on the command line. 1327Privileged ports can be forwarded only when 1328logging in as root on the remote machine. 1329.Pp 1330If the 1331.Ar port 1332argument is 0, 1333the listen port will be dynamically allocated on the server and reported 1334to the client at run time. 1335.Pp 1336If the 1337.Ar bind_address 1338is not specified, the default is to only bind to loopback addresses. 1339If the 1340.Ar bind_address 1341is 1342.Ql * 1343or an empty string, then the forwarding is requested to listen on all 1344interfaces. 1345Specifying a remote 1346.Ar bind_address 1347will only succeed if the server's 1348.Cm GatewayPorts 1349option is enabled (see 1350.Xr sshd_config 5 ) . 1351.It Cm RequestTTY 1352Specifies whether to request a pseudo-tty for the session. 1353The argument may be one of: 1354.Cm no 1355(never request a TTY), 1356.Cm yes 1357(always request a TTY when standard input is a TTY), 1358.Cm force 1359(always request a TTY) or 1360.Cm auto 1361(request a TTY when opening a login session). 1362This option mirrors the 1363.Fl t 1364and 1365.Fl T 1366flags for 1367.Xr ssh 1 . 1368.It Cm RevokedHostKeys 1369Specifies revoked host public keys. 1370Keys listed in this file will be refused for host authentication. 1371Note that if this file does not exist or is not readable, 1372then host authentication will be refused for all hosts. 1373Keys may be specified as a text file, listing one public key per line, or as 1374an OpenSSH Key Revocation List (KRL) as generated by 1375.Xr ssh-keygen 1 . 1376For more information on KRLs, see the KEY REVOCATION LISTS section in 1377.Xr ssh-keygen 1 . 1378.It Cm RhostsRSAAuthentication 1379Specifies whether to try rhosts based authentication with RSA host 1380authentication. 1381The argument must be 1382.Cm yes 1383or 1384.Cm no 1385(the default). 1386This option applies to protocol version 1 only and requires 1387.Xr ssh 1 1388to be setuid root. 1389.It Cm RSAAuthentication 1390Specifies whether to try RSA authentication. 1391The argument to this keyword must be 1392.Cm yes 1393(the default) 1394or 1395.Cm no . 1396RSA authentication will only be 1397attempted if the identity file exists, or an authentication agent is 1398running. 1399Note that this option applies to protocol version 1 only. 1400.It Cm SendEnv 1401Specifies what variables from the local 1402.Xr environ 7 1403should be sent to the server. 1404The server must also support it, and the server must be configured to 1405accept these environment variables. 1406Note that the 1407.Ev TERM 1408environment variable is always sent whenever a 1409pseudo-terminal is requested as it is required by the protocol. 1410Refer to 1411.Cm AcceptEnv 1412in 1413.Xr sshd_config 5 1414for how to configure the server. 1415Variables are specified by name, which may contain wildcard characters. 1416Multiple environment variables may be separated by whitespace or spread 1417across multiple 1418.Cm SendEnv 1419directives. 1420The default is not to send any environment variables. 1421.Pp 1422See 1423.Sx PATTERNS 1424for more information on patterns. 1425.It Cm ServerAliveCountMax 1426Sets the number of server alive messages (see below) which may be 1427sent without 1428.Xr ssh 1 1429receiving any messages back from the server. 1430If this threshold is reached while server alive messages are being sent, 1431ssh will disconnect from the server, terminating the session. 1432It is important to note that the use of server alive messages is very 1433different from 1434.Cm TCPKeepAlive 1435(below). 1436The server alive messages are sent through the encrypted channel 1437and therefore will not be spoofable. 1438The TCP keepalive option enabled by 1439.Cm TCPKeepAlive 1440is spoofable. 1441The server alive mechanism is valuable when the client or 1442server depend on knowing when a connection has become inactive. 1443.Pp 1444The default value is 3. 1445If, for example, 1446.Cm ServerAliveInterval 1447(see below) is set to 15 and 1448.Cm ServerAliveCountMax 1449is left at the default, if the server becomes unresponsive, 1450ssh will disconnect after approximately 45 seconds. 1451.It Cm ServerAliveInterval 1452Sets a timeout interval in seconds after which if no data has been received 1453from the server, 1454.Xr ssh 1 1455will send a message through the encrypted 1456channel to request a response from the server. 1457The default 1458is 0, indicating that these messages will not be sent to the server. 1459.It Cm StreamLocalBindMask 1460Sets the octal file creation mode mask 1461.Pq umask 1462used when creating a Unix-domain socket file for local or remote 1463port forwarding. 1464This option is only used for port forwarding to a Unix-domain socket file. 1465.Pp 1466The default value is 0177, which creates a Unix-domain socket file that is 1467readable and writable only by the owner. 1468Note that not all operating systems honor the file mode on Unix-domain 1469socket files. 1470.It Cm StreamLocalBindUnlink 1471Specifies whether to remove an existing Unix-domain socket file for local 1472or remote port forwarding before creating a new one. 1473If the socket file already exists and 1474.Cm StreamLocalBindUnlink 1475is not enabled, 1476.Nm ssh 1477will be unable to forward the port to the Unix-domain socket file. 1478This option is only used for port forwarding to a Unix-domain socket file. 1479.Pp 1480The argument must be 1481.Cm yes 1482or 1483.Cm no 1484(the default). 1485.It Cm StrictHostKeyChecking 1486If this flag is set to 1487.Cm yes , 1488.Xr ssh 1 1489will never automatically add host keys to the 1490.Pa ~/.ssh/known_hosts 1491file, and refuses to connect to hosts whose host key has changed. 1492This provides maximum protection against trojan horse attacks, 1493though it can be annoying when the 1494.Pa /etc/ssh/ssh_known_hosts 1495file is poorly maintained or when connections to new hosts are 1496frequently made. 1497This option forces the user to manually 1498add all new hosts. 1499If this flag is set to 1500.Cm no , 1501ssh will automatically add new host keys to the 1502user known hosts files. 1503If this flag is set to 1504.Cm ask 1505(the default), 1506new host keys 1507will be added to the user known host files only after the user 1508has confirmed that is what they really want to do, and 1509ssh will refuse to connect to hosts whose host key has changed. 1510The host keys of 1511known hosts will be verified automatically in all cases. 1512.It Cm TCPKeepAlive 1513Specifies whether the system should send TCP keepalive messages to the 1514other side. 1515If they are sent, death of the connection or crash of one 1516of the machines will be properly noticed. 1517However, this means that 1518connections will die if the route is down temporarily, and some people 1519find it annoying. 1520.Pp 1521The default is 1522.Cm yes 1523(to send TCP keepalive messages), and the client will notice 1524if the network goes down or the remote host dies. 1525This is important in scripts, and many users want it too. 1526.Pp 1527To disable TCP keepalive messages, the value should be set to 1528.Cm no . 1529.It Cm Tunnel 1530Request 1531.Xr tun 4 1532device forwarding between the client and the server. 1533The argument must be 1534.Cm yes , 1535.Cm point-to-point 1536(layer 3), 1537.Cm ethernet 1538(layer 2), 1539or 1540.Cm no 1541(the default). 1542Specifying 1543.Cm yes 1544requests the default tunnel mode, which is 1545.Cm point-to-point . 1546.It Cm TunnelDevice 1547Specifies the 1548.Xr tun 4 1549devices to open on the client 1550.Pq Ar local_tun 1551and the server 1552.Pq Ar remote_tun . 1553.Pp 1554The argument must be 1555.Sm off 1556.Ar local_tun Op : Ar remote_tun . 1557.Sm on 1558The devices may be specified by numerical ID or the keyword 1559.Cm any , 1560which uses the next available tunnel device. 1561If 1562.Ar remote_tun 1563is not specified, it defaults to 1564.Cm any . 1565The default is 1566.Cm any:any . 1567.It Cm UpdateHostKeys 1568Specifies whether 1569.Xr ssh 1 1570should accept notifications of additional hostkeys from the server sent 1571after authentication has completed and add them to 1572.Cm UserKnownHostsFile . 1573The argument must be 1574.Cm yes , 1575.Cm no 1576(the default) or 1577.Cm ask . 1578Enabling this option allows learning alternate hostkeys for a server 1579and supports graceful key rotation by allowing a server to send replacement 1580public keys before old ones are removed. 1581Additional hostkeys are only accepted if the key used to authenticate the 1582host was already trusted or explicitly accepted by the user. 1583If 1584.Cm UpdateHostKeys 1585is set to 1586.Cm ask , 1587then the user is asked to confirm the modifications to the known_hosts file. 1588Confirmation is currently incompatible with 1589.Cm ControlPersist , 1590and will be disabled if it is enabled. 1591.Pp 1592Presently, only 1593.Xr sshd 8 1594from OpenSSH 6.8 and greater support the 1595.Qq hostkeys@openssh.com 1596protocol extension used to inform the client of all the server's hostkeys. 1597.It Cm UsePrivilegedPort 1598Specifies whether to use a privileged port for outgoing connections. 1599The argument must be 1600.Cm yes 1601or 1602.Cm no 1603(the default). 1604If set to 1605.Cm yes , 1606.Xr ssh 1 1607must be setuid root. 1608Note that this option must be set to 1609.Cm yes 1610for 1611.Cm RhostsRSAAuthentication 1612with older servers. 1613.It Cm User 1614Specifies the user to log in as. 1615This can be useful when a different user name is used on different machines. 1616This saves the trouble of 1617having to remember to give the user name on the command line. 1618.It Cm UserKnownHostsFile 1619Specifies one or more files to use for the user 1620host key database, separated by whitespace. 1621The default is 1622.Pa ~/.ssh/known_hosts , 1623.Pa ~/.ssh/known_hosts2 . 1624.It Cm VerifyHostKeyDNS 1625Specifies whether to verify the remote key using DNS and SSHFP resource 1626records. 1627If this option is set to 1628.Cm yes , 1629the client will implicitly trust keys that match a secure fingerprint 1630from DNS. 1631Insecure fingerprints will be handled as if this option was set to 1632.Cm ask . 1633If this option is set to 1634.Cm ask , 1635information on fingerprint match will be displayed, but the user will still 1636need to confirm new host keys according to the 1637.Cm StrictHostKeyChecking 1638option. 1639The default is 1640.Cm yes 1641if compiled with LDNS and 1642.Cm no 1643otherwise. 1644.Pp 1645See also 1646.Sx VERIFYING HOST KEYS 1647in 1648.Xr ssh 1 . 1649.It Cm VersionAddendum 1650Specifies a string to append to the regular version string to identify 1651OS- or site-specific modifications. 1652The default is 1653.Dq FreeBSD-20170305 . 1654The value 1655.Cm none 1656may be used to disable this. 1657.It Cm VisualHostKey 1658If this flag is set to 1659.Cm yes , 1660an ASCII art representation of the remote host key fingerprint is 1661printed in addition to the fingerprint string at login and 1662for unknown host keys. 1663If this flag is set to 1664.Cm no 1665(the default), 1666no fingerprint strings are printed at login and 1667only the fingerprint string will be printed for unknown host keys. 1668.It Cm XAuthLocation 1669Specifies the full pathname of the 1670.Xr xauth 1 1671program. 1672The default is 1673.Pa /usr/local/bin/xauth . 1674.El 1675.Sh PATTERNS 1676A 1677.Em pattern 1678consists of zero or more non-whitespace characters, 1679.Sq * 1680(a wildcard that matches zero or more characters), 1681or 1682.Sq ?\& 1683(a wildcard that matches exactly one character). 1684For example, to specify a set of declarations for any host in the 1685.Qq .co.uk 1686set of domains, 1687the following pattern could be used: 1688.Pp 1689.Dl Host *.co.uk 1690.Pp 1691The following pattern 1692would match any host in the 192.168.0.[0-9] network range: 1693.Pp 1694.Dl Host 192.168.0.? 1695.Pp 1696A 1697.Em pattern-list 1698is a comma-separated list of patterns. 1699Patterns within pattern-lists may be negated 1700by preceding them with an exclamation mark 1701.Pq Sq !\& . 1702For example, 1703to allow a key to be used from anywhere within an organization 1704except from the 1705.Qq dialup 1706pool, 1707the following entry (in authorized_keys) could be used: 1708.Pp 1709.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1710.Sh TOKENS 1711Arguments to some keywords can make use of tokens, 1712which are expanded at runtime: 1713.Pp 1714.Bl -tag -width XXXX -offset indent -compact 1715.It %% 1716A literal 1717.Sq % . 1718.It \&%C 1719Shorthand for %l%h%p%r. 1720.It %d 1721Local user's home directory. 1722.It %h 1723The remote hostname. 1724.It %i 1725The local user ID. 1726.It %L 1727The local hostname. 1728.It %l 1729The local hostname, including the domain name. 1730.It %n 1731The original remote hostname, as given on the command line. 1732.It %p 1733The remote port. 1734.It %r 1735The remote username. 1736.It %u 1737The local username. 1738.El 1739.Pp 1740.Cm Match exec 1741accepts the tokens %%, %h, %L, %l, %n, %p, %r, and %u. 1742.Pp 1743.Cm CertificateFile 1744accepts the tokens %%, %d, %h, %l, %r, and %u. 1745.Pp 1746.Cm ControlPath 1747accepts the tokens %%, %C, %h, %i, %L, %l, %n, %p, %r, and %u. 1748.Pp 1749.Cm HostName 1750accepts the tokens %% and %h. 1751.Pp 1752.Cm IdentityAgent 1753and 1754.Cm IdentityFile 1755accept the tokens %%, %d, %h, %l, %r, and %u. 1756.Pp 1757.Cm LocalCommand 1758accepts the tokens %%, %C, %d, %h, %l, %n, %p, %r, and %u. 1759.Pp 1760.Cm ProxyCommand 1761accepts the tokens %%, %h, %p, and %r. 1762.Sh FILES 1763.Bl -tag -width Ds 1764.It Pa ~/.ssh/config 1765This is the per-user configuration file. 1766The format of this file is described above. 1767This file is used by the SSH client. 1768Because of the potential for abuse, this file must have strict permissions: 1769read/write for the user, and not accessible by others. 1770.It Pa /etc/ssh/ssh_config 1771Systemwide configuration file. 1772This file provides defaults for those 1773values that are not specified in the user's configuration file, and 1774for those users who do not have a configuration file. 1775This file must be world-readable. 1776.El 1777.Sh SEE ALSO 1778.Xr ssh 1 1779.Sh AUTHORS 1780.An -nosplit 1781OpenSSH is a derivative of the original and free 1782ssh 1.2.12 release by 1783.An Tatu Ylonen . 1784.An Aaron Campbell , Bob Beck , Markus Friedl , 1785.An Niels Provos , Theo de Raadt 1786and 1787.An Dug Song 1788removed many bugs, re-added newer features and 1789created OpenSSH. 1790.An Markus Friedl 1791contributed the support for SSH protocol versions 1.5 and 2.0. 1792