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 1.242 2017/02/27 14:30:33 jmc Exp $ 37.Dd $Mdocdate: February 27 2017 $ 38.Dt SSH_CONFIG 5 39.Os 40.Sh NAME 41.Nm ssh_config 42.Nd OpenSSH SSH client configuration files 43.Sh SYNOPSIS 44.Nm ~/.ssh/config 45.Nm /etc/ssh/ssh_config 46.Sh DESCRIPTION 47.Xr ssh 1 48obtains configuration data from the following sources in 49the following order: 50.Pp 51.Bl -enum -offset indent -compact 52.It 53command-line options 54.It 55user's configuration file 56.Pq Pa ~/.ssh/config 57.It 58system-wide configuration file 59.Pq Pa /etc/ssh/ssh_config 60.El 61.Pp 62For each parameter, the first obtained value 63will be used. 64The configuration files contain sections separated by 65.Cm Host 66specifications, and that section is only applied for hosts that 67match one of the patterns given in the specification. 68The matched host name is usually the one given on the command line 69(see the 70.Cm CanonicalizeHostname 71option for exceptions). 72.Pp 73Since the first obtained value for each parameter is used, more 74host-specific declarations should be given near the beginning of the 75file, and general defaults at the end. 76.Pp 77The file contains keyword-argument pairs, one per line. 78Lines starting with 79.Ql # 80and empty lines are interpreted as comments. 81Arguments may optionally be enclosed in double quotes 82.Pq \&" 83in order to represent arguments containing spaces. 84Configuration options may be separated by whitespace or 85optional whitespace and exactly one 86.Ql = ; 87the latter format is useful to avoid the need to quote whitespace 88when specifying configuration options using the 89.Nm ssh , 90.Nm scp , 91and 92.Nm sftp 93.Fl o 94option. 95.Pp 96The possible 97keywords and their meanings are as follows (note that 98keywords are case-insensitive and arguments are case-sensitive): 99.Bl -tag -width Ds 100.It Cm Host 101Restricts the following declarations (up to the next 102.Cm Host 103or 104.Cm Match 105keyword) to be only for those hosts that match one of the patterns 106given after the keyword. 107If more than one pattern is provided, they should be separated by whitespace. 108A single 109.Ql * 110as a pattern can be used to provide global 111defaults for all hosts. 112The host is usually the 113.Ar hostname 114argument given on the command line 115(see the 116.Cm CanonicalizeHostname 117keyword for exceptions). 118.Pp 119A pattern entry may be negated by prefixing it with an exclamation mark 120.Pq Sq !\& . 121If a negated entry is matched, then the 122.Cm Host 123entry is ignored, regardless of whether any other patterns on the line 124match. 125Negated matches are therefore useful to provide exceptions for wildcard 126matches. 127.Pp 128See 129.Sx PATTERNS 130for more information on patterns. 131.It Cm Match 132Restricts the following declarations (up to the next 133.Cm Host 134or 135.Cm Match 136keyword) to be used only when the conditions following the 137.Cm Match 138keyword are satisfied. 139Match conditions are specified using one or more criteria 140or the single token 141.Cm all 142which always matches. 143The available criteria keywords are: 144.Cm canonical , 145.Cm exec , 146.Cm host , 147.Cm originalhost , 148.Cm user , 149and 150.Cm localuser . 151The 152.Cm all 153criteria must appear alone or immediately after 154.Cm canonical . 155Other criteria may be combined arbitrarily. 156All criteria but 157.Cm all 158and 159.Cm canonical 160require an argument. 161Criteria may be negated by prepending an exclamation mark 162.Pq Sq !\& . 163.Pp 164The 165.Cm canonical 166keyword matches only when the configuration file is being re-parsed 167after hostname canonicalization (see the 168.Cm CanonicalizeHostname 169option.) 170This may be useful to specify conditions that work with canonical host 171names only. 172The 173.Cm exec 174keyword executes the specified command under the user's shell. 175If the command returns a zero exit status then the condition is considered true. 176Commands containing whitespace characters must be quoted. 177Arguments to 178.Cm exec 179accept the tokens described in the 180.Sx TOKENS 181section. 182.Pp 183The other keywords' criteria must be single entries or comma-separated 184lists and may use the wildcard and negation operators described in the 185.Sx PATTERNS 186section. 187The criteria for the 188.Cm host 189keyword are matched against the target hostname, after any substitution 190by the 191.Cm Hostname 192or 193.Cm CanonicalizeHostname 194options. 195The 196.Cm originalhost 197keyword matches against the hostname as it was specified on the command-line. 198The 199.Cm user 200keyword matches against the target username on the remote host. 201The 202.Cm localuser 203keyword matches against the name of the local user running 204.Xr ssh 1 205(this keyword may be useful in system-wide 206.Nm 207files). 208.It Cm AddKeysToAgent 209Specifies whether keys should be automatically added to a running 210.Xr ssh-agent 1 . 211If this option is set to 212.Cm yes 213and a key is loaded from a file, the key and its passphrase are added to 214the agent with the default lifetime, as if by 215.Xr ssh-add 1 . 216If this option is set to 217.Cm ask , 218.Xr ssh 1 219will require confirmation using the 220.Ev SSH_ASKPASS 221program before adding a key (see 222.Xr ssh-add 1 223for details). 224If this option is set to 225.Cm confirm , 226each use of the key must be confirmed, as if the 227.Fl c 228option was specified to 229.Xr ssh-add 1 . 230If this option is set to 231.Cm no , 232no keys are added to the agent. 233The argument must be 234.Cm yes , 235.Cm confirm , 236.Cm ask , 237or 238.Cm no 239(the default). 240.It Cm AddressFamily 241Specifies which address family to use when connecting. 242Valid arguments are 243.Cm any 244(the default), 245.Cm inet 246(use IPv4 only), or 247.Cm inet6 248(use IPv6 only). 249.It Cm BatchMode 250If set to 251.Cm yes , 252passphrase/password querying will be disabled. 253This option is useful in scripts and other batch jobs where no user 254is present to supply the password. 255The argument must be 256.Cm yes 257or 258.Cm no 259(the default). 260.It Cm BindAddress 261Use the specified address on the local machine as the source address of 262the connection. 263Only useful on systems with more than one address. 264Note that this option does not work if 265.Cm UsePrivilegedPort 266is set to 267.Cm yes . 268.It Cm CanonicalDomains 269When 270.Cm CanonicalizeHostname 271is enabled, this option specifies the list of domain suffixes in which to 272search for the specified destination host. 273.It Cm CanonicalizeFallbackLocal 274Specifies whether to fail with an error when hostname canonicalization fails. 275The default, 276.Cm yes , 277will attempt to look up the unqualified hostname using the system resolver's 278search rules. 279A value of 280.Cm no 281will cause 282.Xr ssh 1 283to fail instantly if 284.Cm CanonicalizeHostname 285is enabled and the target hostname cannot be found in any of the domains 286specified by 287.Cm CanonicalDomains . 288.It Cm CanonicalizeHostname 289Controls whether explicit hostname canonicalization is performed. 290The default, 291.Cm no , 292is not to perform any name rewriting and let the system resolver handle all 293hostname lookups. 294If set to 295.Cm yes 296then, for connections that do not use a 297.Cm ProxyCommand , 298.Xr ssh 1 299will attempt to canonicalize the hostname specified on the command line 300using the 301.Cm CanonicalDomains 302suffixes and 303.Cm CanonicalizePermittedCNAMEs 304rules. 305If 306.Cm CanonicalizeHostname 307is set to 308.Cm always , 309then canonicalization is applied to proxied connections too. 310.Pp 311If this option is enabled, then the configuration files are processed 312again using the new target name to pick up any new configuration in matching 313.Cm Host 314and 315.Cm Match 316stanzas. 317.It Cm CanonicalizeMaxDots 318Specifies the maximum number of dot characters in a hostname before 319canonicalization is disabled. 320The default, 1, 321allows a single dot (i.e. hostname.subdomain). 322.It Cm CanonicalizePermittedCNAMEs 323Specifies rules to determine whether CNAMEs should be followed when 324canonicalizing hostnames. 325The rules consist of one or more arguments of 326.Ar source_domain_list : Ns Ar target_domain_list , 327where 328.Ar source_domain_list 329is a pattern-list of domains that may follow CNAMEs in canonicalization, 330and 331.Ar target_domain_list 332is a pattern-list of domains that they may resolve to. 333.Pp 334For example, 335.Qq *.a.example.com:*.b.example.com,*.c.example.com 336will allow hostnames matching 337.Qq *.a.example.com 338to be canonicalized to names in the 339.Qq *.b.example.com 340or 341.Qq *.c.example.com 342domains. 343.It Cm CertificateFile 344Specifies a file from which the user's certificate is read. 345A corresponding private key must be provided separately in order 346to use this certificate either 347from an 348.Cm IdentityFile 349directive or 350.Fl i 351flag to 352.Xr ssh 1 , 353via 354.Xr ssh-agent 1 , 355or via a 356.Cm PKCS11Provider . 357.Pp 358Arguments to 359.Cm CertificateFile 360may use the tilde syntax to refer to a user's home directory 361or the tokens described in the 362.Sx TOKENS 363section. 364.Pp 365It is possible to have multiple certificate files specified in 366configuration files; these certificates will be tried in sequence. 367Multiple 368.Cm CertificateFile 369directives will add to the list of certificates used for 370authentication. 371.It Cm ChallengeResponseAuthentication 372Specifies whether to use challenge-response authentication. 373The argument to this keyword must be 374.Cm yes 375(the default) 376or 377.Cm no . 378.It Cm CheckHostIP 379If set to 380.Cm yes 381(the default), 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. 394.It Cm Cipher 395Specifies the cipher to use for encrypting the session 396in protocol version 1. 397Currently, 398.Cm blowfish , 399.Cm 3des 400(the default), 401and 402.Cm des 403are supported, 404though 405.Cm des 406is only supported in the 407.Xr ssh 1 408client for interoperability with legacy protocol 1 implementations; 409its use is strongly discouraged due to cryptographic weaknesses. 410.It Cm Ciphers 411Specifies the ciphers allowed for protocol version 2 412in order of preference. 413Multiple ciphers must be comma-separated. 414If the specified value begins with a 415.Sq + 416character, then the specified ciphers will be appended to the default set 417instead of replacing them. 418If the specified value begins with a 419.Sq - 420character, then the specified ciphers (including wildcards) will be removed 421from the default set instead of replacing them. 422.Pp 423The supported ciphers are: 424.Bd -literal -offset indent 4253des-cbc 426aes128-cbc 427aes192-cbc 428aes256-cbc 429aes128-ctr 430aes192-ctr 431aes256-ctr 432aes128-gcm@openssh.com 433aes256-gcm@openssh.com 434arcfour 435arcfour128 436arcfour256 437blowfish-cbc 438cast128-cbc 439chacha20-poly1305@openssh.com 440.Ed 441.Pp 442The default is: 443.Bd -literal -offset indent 444chacha20-poly1305@openssh.com, 445aes128-ctr,aes192-ctr,aes256-ctr, 446aes128-gcm@openssh.com,aes256-gcm@openssh.com, 447aes128-cbc,aes192-cbc,aes256-cbc 448.Ed 449.Pp 450The list of available ciphers may also be obtained using 451.Qq ssh -Q cipher . 452.It Cm ClearAllForwardings 453Specifies that all local, remote, and dynamic port forwardings 454specified in the configuration files or on the command line be 455cleared. 456This option is primarily useful when used from the 457.Xr ssh 1 458command line to clear port forwardings set in 459configuration files, and is automatically set by 460.Xr scp 1 461and 462.Xr sftp 1 . 463The argument must be 464.Cm yes 465or 466.Cm no 467(the default). 468.It Cm Compression 469Specifies whether to use compression. 470The argument must be 471.Cm yes 472or 473.Cm no 474(the default). 475.It Cm CompressionLevel 476Specifies the compression level to use if compression is enabled. 477The argument must be an integer from 1 (fast) to 9 (slow, best). 478The default level is 6, which is good for most applications. 479The meaning of the values is the same as in 480.Xr gzip 1 . 481Note that this option applies to protocol version 1 only. 482.It Cm ConnectionAttempts 483Specifies the number of tries (one per second) to make before exiting. 484The argument must be an integer. 485This may be useful in scripts if the connection sometimes fails. 486The default is 1. 487.It Cm ConnectTimeout 488Specifies the timeout (in seconds) used when connecting to the 489SSH server, instead of using the default system TCP timeout. 490This value is used only when the target is down or really unreachable, 491not when it refuses the connection. 492.It Cm ControlMaster 493Enables the sharing of multiple sessions over a single network connection. 494When set to 495.Cm yes , 496.Xr ssh 1 497will listen for connections on a control socket specified using the 498.Cm ControlPath 499argument. 500Additional sessions can connect to this socket using the same 501.Cm ControlPath 502with 503.Cm ControlMaster 504set to 505.Cm no 506(the default). 507These sessions will try to reuse the master instance's network connection 508rather than initiating new ones, but will fall back to connecting normally 509if the control socket does not exist, or is not listening. 510.Pp 511Setting this to 512.Cm ask 513will cause 514.Xr ssh 1 515to listen for control connections, but require confirmation using 516.Xr ssh-askpass 1 . 517If the 518.Cm ControlPath 519cannot be opened, 520.Xr ssh 1 521will continue without connecting to a master instance. 522.Pp 523X11 and 524.Xr ssh-agent 1 525forwarding is supported over these multiplexed connections, however the 526display and agent forwarded will be the one belonging to the master 527connection i.e. it is not possible to forward multiple displays or agents. 528.Pp 529Two additional options allow for opportunistic multiplexing: try to use a 530master connection but fall back to creating a new one if one does not already 531exist. 532These options are: 533.Cm auto 534and 535.Cm autoask . 536The latter requires confirmation like the 537.Cm ask 538option. 539.It Cm ControlPath 540Specify the path to the control socket used for connection sharing as described 541in the 542.Cm ControlMaster 543section above or the string 544.Cm none 545to disable connection sharing. 546Arguments to 547.Cm ControlPath 548may use the tilde syntax to refer to a user's home directory 549or the tokens described in the 550.Sx TOKENS 551section. 552It is recommended that any 553.Cm ControlPath 554used for opportunistic connection sharing include 555at least %h, %p, and %r (or alternatively %C) and be placed in a directory 556that is not writable by other users. 557This ensures that shared connections are uniquely identified. 558.It Cm ControlPersist 559When used in conjunction with 560.Cm ControlMaster , 561specifies that the master connection should remain open 562in the background (waiting for future client connections) 563after the initial client connection has been closed. 564If set to 565.Cm no , 566then the master connection will not be placed into the background, 567and will close as soon as the initial client connection is closed. 568If set to 569.Cm yes 570or 0, 571then the master connection will remain in the background indefinitely 572(until killed or closed via a mechanism such as the 573.Qq ssh -O exit ) . 574If set to a time in seconds, or a time in any of the formats documented in 575.Xr sshd_config 5 , 576then the backgrounded master connection will automatically terminate 577after it has remained idle (with no client connections) for the 578specified time. 579.It Cm DynamicForward 580Specifies that a TCP port on the local machine be forwarded 581over the secure channel, and the application 582protocol is then used to determine where to connect to from the 583remote machine. 584.Pp 585The argument must be 586.Sm off 587.Oo Ar bind_address : Oc Ar port . 588.Sm on 589IPv6 addresses can be specified by enclosing addresses in square brackets. 590By default, the local port is bound in accordance with the 591.Cm GatewayPorts 592setting. 593However, an explicit 594.Ar bind_address 595may be used to bind the connection to a specific address. 596The 597.Ar bind_address 598of 599.Cm localhost 600indicates that the listening port be bound for local use only, while an 601empty address or 602.Sq * 603indicates that the port should be available from all interfaces. 604.Pp 605Currently the SOCKS4 and SOCKS5 protocols are supported, and 606.Xr ssh 1 607will act as a SOCKS server. 608Multiple forwardings may be specified, and 609additional forwardings can be given on the command line. 610Only the superuser can forward privileged ports. 611.It Cm EnableSSHKeysign 612Setting this option to 613.Cm yes 614in the global client configuration file 615.Pa /etc/ssh/ssh_config 616enables the use of the helper program 617.Xr ssh-keysign 8 618during 619.Cm HostbasedAuthentication . 620The argument must be 621.Cm yes 622or 623.Cm no 624(the default). 625This option should be placed in the non-hostspecific section. 626See 627.Xr ssh-keysign 8 628for more information. 629.It Cm EscapeChar 630Sets the escape character (default: 631.Ql ~ ) . 632The escape character can also 633be set on the command line. 634The argument should be a single character, 635.Ql ^ 636followed by a letter, or 637.Cm none 638to disable the escape 639character entirely (making the connection transparent for binary 640data). 641.It Cm ExitOnForwardFailure 642Specifies whether 643.Xr ssh 1 644should terminate the connection if it cannot set up all requested 645dynamic, tunnel, local, and remote port forwardings, (e.g.\& 646if either end is unable to bind and listen on a specified port). 647Note that 648.Cm ExitOnForwardFailure 649does not apply to connections made over port forwardings and will not, 650for example, cause 651.Xr ssh 1 652to exit if TCP connections to the ultimate forwarding destination fail. 653The argument must be 654.Cm yes 655or 656.Cm no 657(the default). 658.It Cm FingerprintHash 659Specifies the hash algorithm used when displaying key fingerprints. 660Valid options are: 661.Cm md5 662and 663.Cm sha256 664(the default). 665.It Cm ForwardAgent 666Specifies whether the connection to the authentication agent (if any) 667will be forwarded to the remote machine. 668The argument must be 669.Cm yes 670or 671.Cm no 672(the default). 673.Pp 674Agent forwarding should be enabled with caution. 675Users with the ability to bypass file permissions on the remote host 676(for the agent's Unix-domain socket) 677can access the local agent through the forwarded connection. 678An attacker cannot obtain key material from the agent, 679however they can perform operations on the keys that enable them to 680authenticate using the identities loaded into the agent. 681.It Cm ForwardX11 682Specifies whether X11 connections will be automatically redirected 683over the secure channel and 684.Ev DISPLAY 685set. 686The argument must be 687.Cm yes 688or 689.Cm no 690(the default). 691.Pp 692X11 forwarding should be enabled with caution. 693Users with the ability to bypass file permissions on the remote host 694(for the user's X11 authorization database) 695can access the local X11 display through the forwarded connection. 696An attacker may then be able to perform activities such as keystroke monitoring 697if the 698.Cm ForwardX11Trusted 699option is also enabled. 700.It Cm ForwardX11Timeout 701Specify a timeout for untrusted X11 forwarding 702using the format described in the 703.Sx TIME FORMATS 704section of 705.Xr sshd_config 5 . 706X11 connections received by 707.Xr ssh 1 708after this time will be refused. 709The default is to disable untrusted X11 forwarding after twenty minutes has 710elapsed. 711.It Cm ForwardX11Trusted 712If this option is set to 713.Cm yes , 714remote X11 clients will have full access to the original X11 display. 715.Pp 716If this option is set to 717.Cm no 718(the default), 719remote X11 clients will be considered untrusted and prevented 720from stealing or tampering with data belonging to trusted X11 721clients. 722Furthermore, the 723.Xr xauth 1 724token used for the session will be set to expire after 20 minutes. 725Remote clients will be refused access after this time. 726.Pp 727See the X11 SECURITY extension specification for full details on 728the restrictions imposed on untrusted clients. 729.It Cm GatewayPorts 730Specifies whether remote hosts are allowed to connect to local 731forwarded ports. 732By default, 733.Xr ssh 1 734binds local port forwardings to the loopback address. 735This prevents other remote hosts from connecting to forwarded ports. 736.Cm GatewayPorts 737can be used to specify that ssh 738should bind local port forwardings to the wildcard address, 739thus allowing remote hosts to connect to forwarded ports. 740The argument must be 741.Cm yes 742or 743.Cm no 744(the default). 745.It Cm GlobalKnownHostsFile 746Specifies one or more files to use for the global 747host key database, separated by whitespace. 748The default is 749.Pa /etc/ssh/ssh_known_hosts , 750.Pa /etc/ssh/ssh_known_hosts2 . 751.It Cm GSSAPIAuthentication 752Specifies whether user authentication based on GSSAPI is allowed. 753The default is 754.Cm no . 755.It Cm GSSAPIDelegateCredentials 756Forward (delegate) credentials to the server. 757The default is 758.Cm no . 759.It Cm HashKnownHosts 760Indicates that 761.Xr ssh 1 762should hash host names and addresses when they are added to 763.Pa ~/.ssh/known_hosts . 764These hashed names may be used normally by 765.Xr ssh 1 766and 767.Xr sshd 8 , 768but they do not reveal identifying information should the file's contents 769be disclosed. 770The default is 771.Cm no . 772Note that existing names and addresses in known hosts files 773will not be converted automatically, 774but may be manually hashed using 775.Xr ssh-keygen 1 . 776.It Cm HostbasedAuthentication 777Specifies whether to try rhosts based authentication with public key 778authentication. 779The argument must be 780.Cm yes 781or 782.Cm no 783(the default). 784.It Cm HostbasedKeyTypes 785Specifies the key types that will be used for hostbased authentication 786as a comma-separated pattern list. 787Alternately if the specified value begins with a 788.Sq + 789character, then the specified key types will be appended to the default set 790instead of replacing them. 791If the specified value begins with a 792.Sq - 793character, then the specified key types (including wildcards) will be removed 794from the default set instead of replacing them. 795The default for this option is: 796.Bd -literal -offset 3n 797ecdsa-sha2-nistp256-cert-v01@openssh.com, 798ecdsa-sha2-nistp384-cert-v01@openssh.com, 799ecdsa-sha2-nistp521-cert-v01@openssh.com, 800ssh-ed25519-cert-v01@openssh.com, 801ssh-rsa-cert-v01@openssh.com, 802ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 803ssh-ed25519,ssh-rsa 804.Ed 805.Pp 806The 807.Fl Q 808option of 809.Xr ssh 1 810may be used to list supported key types. 811.It Cm HostKeyAlgorithms 812Specifies the host key algorithms 813that the client wants to use in order of preference. 814Alternately if the specified value begins with a 815.Sq + 816character, then the specified key types will be appended to the default set 817instead of replacing them. 818If the specified value begins with a 819.Sq - 820character, then the specified key types (including wildcards) will be removed 821from the default set instead of replacing them. 822The default for this option is: 823.Bd -literal -offset 3n 824ecdsa-sha2-nistp256-cert-v01@openssh.com, 825ecdsa-sha2-nistp384-cert-v01@openssh.com, 826ecdsa-sha2-nistp521-cert-v01@openssh.com, 827ssh-ed25519-cert-v01@openssh.com, 828ssh-rsa-cert-v01@openssh.com, 829ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 830ssh-ed25519,ssh-rsa 831.Ed 832.Pp 833If hostkeys are known for the destination host then this default is modified 834to prefer their algorithms. 835.Pp 836The list of available key types may also be obtained using 837.Qq ssh -Q key . 838.It Cm HostKeyAlias 839Specifies an alias that should be used instead of the 840real host name when looking up or saving the host key 841in the host key database files. 842This option is useful for tunneling SSH connections 843or for multiple servers running on a single host. 844.It Cm HostName 845Specifies the real host name to log into. 846This can be used to specify nicknames or abbreviations for hosts. 847Arguments to 848.Cm HostName 849accept the tokens described in the 850.Sx TOKENS 851section. 852Numeric IP addresses are also permitted (both on the command line and in 853.Cm HostName 854specifications). 855The default is the name given on the command line. 856.It Cm IdentitiesOnly 857Specifies that 858.Xr ssh 1 859should only use the authentication identity and certificate files explicitly 860configured in the 861.Nm 862files 863or passed on the 864.Xr ssh 1 865command-line, 866even if 867.Xr ssh-agent 1 868or a 869.Cm PKCS11Provider 870offers more identities. 871The argument to this keyword must be 872.Cm yes 873or 874.Cm no 875(the default). 876This option is intended for situations where ssh-agent 877offers many different identities. 878.It Cm IdentityAgent 879Specifies the 880.Ux Ns -domain 881socket used to communicate with the authentication agent. 882.Pp 883This option overrides the 884.Ev SSH_AUTH_SOCK 885environment variable and can be used to select a specific agent. 886Setting the socket name to 887.Cm none 888disables the use of an authentication agent. 889If the string 890.Qq SSH_AUTH_SOCK 891is specified, the location of the socket will be read from the 892.Ev SSH_AUTH_SOCK 893environment variable. 894.Pp 895Arguments to 896.Cm IdentityAgent 897may use the tilde syntax to refer to a user's home directory 898or the tokens described in the 899.Sx TOKENS 900section. 901.It Cm IdentityFile 902Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication 903identity is read. 904The default is 905.Pa ~/.ssh/identity 906for protocol version 1, and 907.Pa ~/.ssh/id_dsa , 908.Pa ~/.ssh/id_ecdsa , 909.Pa ~/.ssh/id_ed25519 910and 911.Pa ~/.ssh/id_rsa 912for protocol version 2. 913Additionally, any identities represented by the authentication agent 914will be used for authentication unless 915.Cm IdentitiesOnly 916is set. 917If no certificates have been explicitly specified by 918.Cm CertificateFile , 919.Xr ssh 1 920will try to load certificate information from the filename obtained by 921appending 922.Pa -cert.pub 923to the path of a specified 924.Cm IdentityFile . 925.Pp 926Arguments to 927.Cm IdentityFile 928may use the tilde syntax to refer to a user's home directory 929or the tokens described in the 930.Sx TOKENS 931section. 932.Pp 933It is possible to have 934multiple identity files specified in configuration files; all these 935identities will be tried in sequence. 936Multiple 937.Cm IdentityFile 938directives will add to the list of identities tried (this behaviour 939differs from that of other configuration directives). 940.Pp 941.Cm IdentityFile 942may be used in conjunction with 943.Cm IdentitiesOnly 944to select which identities in an agent are offered during authentication. 945.Cm IdentityFile 946may also be used in conjunction with 947.Cm CertificateFile 948in order to provide any certificate also needed for authentication with 949the identity. 950.It Cm IgnoreUnknown 951Specifies a pattern-list of unknown options to be ignored if they are 952encountered in configuration parsing. 953This may be used to suppress errors if 954.Nm 955contains options that are unrecognised by 956.Xr ssh 1 . 957It is recommended that 958.Cm IgnoreUnknown 959be listed early in the configuration file as it will not be applied 960to unknown options that appear before it. 961.It Cm Include 962Include the specified configuration file(s). 963Multiple pathnames may be specified and each pathname may contain 964.Xr glob 3 965wildcards and, for user configurations, shell-like 966.Sq ~ 967references to user home directories. 968Files without absolute paths are assumed to be in 969.Pa ~/.ssh 970if included in a user configuration file or 971.Pa /etc/ssh 972if included from the system configuration file. 973.Cm Include 974directive may appear inside a 975.Cm Match 976or 977.Cm Host 978block 979to perform conditional inclusion. 980.It Cm IPQoS 981Specifies the IPv4 type-of-service or DSCP class for connections. 982Accepted values are 983.Cm af11 , 984.Cm af12 , 985.Cm af13 , 986.Cm af21 , 987.Cm af22 , 988.Cm af23 , 989.Cm af31 , 990.Cm af32 , 991.Cm af33 , 992.Cm af41 , 993.Cm af42 , 994.Cm af43 , 995.Cm cs0 , 996.Cm cs1 , 997.Cm cs2 , 998.Cm cs3 , 999.Cm cs4 , 1000.Cm cs5 , 1001.Cm cs6 , 1002.Cm cs7 , 1003.Cm ef , 1004.Cm lowdelay , 1005.Cm throughput , 1006.Cm reliability , 1007or a numeric value. 1008This option may take one or two arguments, separated by whitespace. 1009If one argument is specified, it is used as the packet class unconditionally. 1010If two values are specified, the first is automatically selected for 1011interactive sessions and the second for non-interactive sessions. 1012The default is 1013.Cm lowdelay 1014for interactive sessions and 1015.Cm throughput 1016for non-interactive sessions. 1017.It Cm KbdInteractiveAuthentication 1018Specifies whether to use keyboard-interactive authentication. 1019The argument to this keyword must be 1020.Cm yes 1021(the default) 1022or 1023.Cm no . 1024.It Cm KbdInteractiveDevices 1025Specifies the list of methods to use in keyboard-interactive authentication. 1026Multiple method names must be comma-separated. 1027The default is to use the server specified list. 1028The methods available vary depending on what the server supports. 1029For an OpenSSH server, 1030it may be zero or more of: 1031.Cm bsdauth , 1032.Cm pam , 1033and 1034.Cm skey . 1035.It Cm KexAlgorithms 1036Specifies the available KEX (Key Exchange) algorithms. 1037Multiple algorithms must be comma-separated. 1038Alternately if the specified value begins with a 1039.Sq + 1040character, then the specified methods will be appended to the default set 1041instead of replacing them. 1042If the specified value begins with a 1043.Sq - 1044character, then the specified methods (including wildcards) will be removed 1045from the default set instead of replacing them. 1046The default is: 1047.Bd -literal -offset indent 1048curve25519-sha256,curve25519-sha256@libssh.org, 1049ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1050diffie-hellman-group-exchange-sha256, 1051diffie-hellman-group-exchange-sha1, 1052diffie-hellman-group14-sha1 1053.Ed 1054.Pp 1055The list of available key exchange algorithms may also be obtained using 1056.Qq ssh -Q kex . 1057.It Cm LocalCommand 1058Specifies a command to execute on the local machine after successfully 1059connecting to the server. 1060The command string extends to the end of the line, and is executed with 1061the user's shell. 1062Arguments to 1063.Cm LocalCommand 1064accept the tokens described in the 1065.Sx TOKENS 1066section. 1067.Pp 1068The command is run synchronously and does not have access to the 1069session of the 1070.Xr ssh 1 1071that spawned it. 1072It should not be used for interactive commands. 1073.Pp 1074This directive is ignored unless 1075.Cm PermitLocalCommand 1076has been enabled. 1077.It Cm LocalForward 1078Specifies that a TCP port on the local machine be forwarded over 1079the secure channel to the specified host and port from the remote machine. 1080The first argument must be 1081.Sm off 1082.Oo Ar bind_address : Oc Ar port 1083.Sm on 1084and the second argument must be 1085.Ar host : Ns Ar hostport . 1086IPv6 addresses can be specified by enclosing addresses in square brackets. 1087Multiple forwardings may be specified, and additional forwardings can be 1088given on the command line. 1089Only the superuser can forward privileged ports. 1090By default, the local port is bound in accordance with the 1091.Cm GatewayPorts 1092setting. 1093However, an explicit 1094.Ar bind_address 1095may be used to bind the connection to a specific address. 1096The 1097.Ar bind_address 1098of 1099.Cm localhost 1100indicates that the listening port be bound for local use only, while an 1101empty address or 1102.Sq * 1103indicates that the port should be available from all interfaces. 1104.It Cm LogLevel 1105Gives the verbosity level that is used when logging messages from 1106.Xr ssh 1 . 1107The possible values are: 1108QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1109The default is INFO. 1110DEBUG and DEBUG1 are equivalent. 1111DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1112.It Cm MACs 1113Specifies the MAC (message authentication code) algorithms 1114in order of preference. 1115The MAC algorithm is used for data integrity protection. 1116Multiple algorithms must be comma-separated. 1117If the specified value begins with a 1118.Sq + 1119character, then the specified algorithms will be appended to the default set 1120instead of replacing them. 1121If the specified value begins with a 1122.Sq - 1123character, then the specified algorithms (including wildcards) will be removed 1124from the default set instead of replacing them. 1125.Pp 1126The algorithms that contain 1127.Qq -etm 1128calculate the MAC after encryption (encrypt-then-mac). 1129These are considered safer and their use recommended. 1130.Pp 1131The default is: 1132.Bd -literal -offset indent 1133umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1134hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1135hmac-sha1-etm@openssh.com, 1136umac-64@openssh.com,umac-128@openssh.com, 1137hmac-sha2-256,hmac-sha2-512,hmac-sha1 1138.Ed 1139.Pp 1140The list of available MAC algorithms may also be obtained using 1141.Qq ssh -Q mac . 1142.It Cm NoHostAuthenticationForLocalhost 1143This option can be used if the home directory is shared across machines. 1144In this case localhost will refer to a different machine on each of 1145the machines and the user will get many warnings about changed host keys. 1146However, this option disables host authentication for localhost. 1147The argument to this keyword must be 1148.Cm yes 1149or 1150.Cm no 1151(the default). 1152.It Cm NumberOfPasswordPrompts 1153Specifies the number of password prompts before giving up. 1154The argument to this keyword must be an integer. 1155The default is 3. 1156.It Cm PasswordAuthentication 1157Specifies whether to use password authentication. 1158The argument to this keyword must be 1159.Cm yes 1160(the default) 1161or 1162.Cm no . 1163.It Cm PermitLocalCommand 1164Allow local command execution via the 1165.Ic LocalCommand 1166option or using the 1167.Ic !\& Ns Ar command 1168escape sequence in 1169.Xr ssh 1 . 1170The argument must be 1171.Cm yes 1172or 1173.Cm no 1174(the default). 1175.It Cm PKCS11Provider 1176Specifies which PKCS#11 provider to use. 1177The argument to this keyword is the PKCS#11 shared library 1178.Xr ssh 1 1179should use to communicate with a PKCS#11 token providing the user's 1180private RSA key. 1181.It Cm Port 1182Specifies the port number to connect on the remote host. 1183The default is 22. 1184.It Cm PreferredAuthentications 1185Specifies the order in which the client should try authentication methods. 1186This allows a client to prefer one method (e.g.\& 1187.Cm keyboard-interactive ) 1188over another method (e.g.\& 1189.Cm password ) . 1190The default is: 1191.Bd -literal -offset indent 1192gssapi-with-mic,hostbased,publickey, 1193keyboard-interactive,password 1194.Ed 1195.It Cm Protocol 1196Specifies the protocol versions 1197.Xr ssh 1 1198should support in order of preference. 1199The possible values are 1 and 2. 1200Multiple versions must be comma-separated. 1201When this option is set to 1202.Cm 2,1 1203.Nm ssh 1204will try version 2 and fall back to version 1 1205if version 2 is not available. 1206The default is version 2. 1207Protocol 1 suffers from a number of cryptographic weaknesses and should 1208not be used. 1209It is only offered to support legacy devices. 1210.It Cm ProxyCommand 1211Specifies the command to use to connect to the server. 1212The command 1213string extends to the end of the line, and is executed 1214using the user's shell 1215.Ql exec 1216directive to avoid a lingering shell process. 1217.Pp 1218Arguments to 1219.Cm ProxyCommand 1220accept the tokens described in the 1221.Sx TOKENS 1222section. 1223The command can be basically anything, 1224and should read from its standard input and write to its standard output. 1225It should eventually connect an 1226.Xr sshd 8 1227server running on some machine, or execute 1228.Ic sshd -i 1229somewhere. 1230Host key management will be done using the 1231HostName of the host being connected (defaulting to the name typed by 1232the user). 1233Setting the command to 1234.Cm none 1235disables this option entirely. 1236Note that 1237.Cm CheckHostIP 1238is not available for connects with a proxy command. 1239.Pp 1240This directive is useful in conjunction with 1241.Xr nc 1 1242and its proxy support. 1243For example, the following directive would connect via an HTTP proxy at 1244192.0.2.0: 1245.Bd -literal -offset 3n 1246ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1247.Ed 1248.It Cm ProxyJump 1249Specifies one or more jump proxies as 1250.Xo 1251.Sm off 1252.Op Ar user No @ 1253.Ar host 1254.Op : Ns Ar port 1255.Sm on 1256.Xc . 1257Multiple proxies may be separated by comma characters and will be visited 1258sequentially. 1259Setting this option will cause 1260.Xr ssh 1 1261to connect to the target host by first making a 1262.Xr ssh 1 1263connection to the specified 1264.Cm ProxyJump 1265host and then establishing a 1266TCP forwarding to the ultimate target from there. 1267.Pp 1268Note that this option will compete with the 1269.Cm ProxyCommand 1270option - whichever is specified first will prevent later instances of the 1271other from taking effect. 1272.It Cm ProxyUseFdpass 1273Specifies that 1274.Cm ProxyCommand 1275will pass a connected file descriptor back to 1276.Xr ssh 1 1277instead of continuing to execute and pass data. 1278The default is 1279.Cm no . 1280.It Cm PubkeyAcceptedKeyTypes 1281Specifies the key types that will be used for public key authentication 1282as a comma-separated pattern list. 1283Alternately if the specified value begins with a 1284.Sq + 1285character, then the key types after it will be appended to the default 1286instead of replacing it. 1287If the specified value begins with a 1288.Sq - 1289character, then the specified key types (including wildcards) will be removed 1290from the default set instead of replacing them. 1291The default for this option is: 1292.Bd -literal -offset 3n 1293ecdsa-sha2-nistp256-cert-v01@openssh.com, 1294ecdsa-sha2-nistp384-cert-v01@openssh.com, 1295ecdsa-sha2-nistp521-cert-v01@openssh.com, 1296ssh-ed25519-cert-v01@openssh.com, 1297ssh-rsa-cert-v01@openssh.com, 1298ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1299ssh-ed25519,ssh-rsa 1300.Ed 1301.Pp 1302The list of available key types may also be obtained using 1303.Qq ssh -Q key . 1304.It Cm PubkeyAuthentication 1305Specifies whether to try public key authentication. 1306The argument to this keyword must be 1307.Cm yes 1308(the default) 1309or 1310.Cm no . 1311.It Cm RekeyLimit 1312Specifies the maximum amount of data that may be transmitted before the 1313session key is renegotiated, optionally followed a maximum amount of 1314time that may pass before the session key is renegotiated. 1315The first argument is specified in bytes and may have a suffix of 1316.Sq K , 1317.Sq M , 1318or 1319.Sq G 1320to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1321The default is between 1322.Sq 1G 1323and 1324.Sq 4G , 1325depending on the cipher. 1326The optional second value is specified in seconds and may use any of the 1327units documented in the 1328.Sx TIME FORMATS 1329section of 1330.Xr sshd_config 5 . 1331The default value for 1332.Cm RekeyLimit 1333is 1334.Cm default none , 1335which means that rekeying is performed after the cipher's default amount 1336of data has been sent or received and no time based rekeying is done. 1337.It Cm RemoteForward 1338Specifies that a TCP port on the remote machine be forwarded over 1339the secure channel to the specified host and port from the local machine. 1340The first argument must be 1341.Sm off 1342.Oo Ar bind_address : Oc Ar port 1343.Sm on 1344and the second argument must be 1345.Ar host : Ns Ar hostport . 1346IPv6 addresses can be specified by enclosing addresses in square brackets. 1347Multiple forwardings may be specified, and additional 1348forwardings can be given on the command line. 1349Privileged ports can be forwarded only when 1350logging in as root on the remote machine. 1351.Pp 1352If the 1353.Ar port 1354argument is 0, 1355the listen port will be dynamically allocated on the server and reported 1356to the client at run time. 1357.Pp 1358If the 1359.Ar bind_address 1360is not specified, the default is to only bind to loopback addresses. 1361If the 1362.Ar bind_address 1363is 1364.Ql * 1365or an empty string, then the forwarding is requested to listen on all 1366interfaces. 1367Specifying a remote 1368.Ar bind_address 1369will only succeed if the server's 1370.Cm GatewayPorts 1371option is enabled (see 1372.Xr sshd_config 5 ) . 1373.It Cm RequestTTY 1374Specifies whether to request a pseudo-tty for the session. 1375The argument may be one of: 1376.Cm no 1377(never request a TTY), 1378.Cm yes 1379(always request a TTY when standard input is a TTY), 1380.Cm force 1381(always request a TTY) or 1382.Cm auto 1383(request a TTY when opening a login session). 1384This option mirrors the 1385.Fl t 1386and 1387.Fl T 1388flags for 1389.Xr ssh 1 . 1390.It Cm RevokedHostKeys 1391Specifies revoked host public keys. 1392Keys listed in this file will be refused for host authentication. 1393Note that if this file does not exist or is not readable, 1394then host authentication will be refused for all hosts. 1395Keys may be specified as a text file, listing one public key per line, or as 1396an OpenSSH Key Revocation List (KRL) as generated by 1397.Xr ssh-keygen 1 . 1398For more information on KRLs, see the KEY REVOCATION LISTS section in 1399.Xr ssh-keygen 1 . 1400.It Cm RhostsRSAAuthentication 1401Specifies whether to try rhosts based authentication with RSA host 1402authentication. 1403The argument must be 1404.Cm yes 1405or 1406.Cm no 1407(the default). 1408This option applies to protocol version 1 only and requires 1409.Xr ssh 1 1410to be setuid root. 1411.It Cm RSAAuthentication 1412Specifies whether to try RSA authentication. 1413The argument to this keyword must be 1414.Cm yes 1415(the default) 1416or 1417.Cm no . 1418RSA authentication will only be 1419attempted if the identity file exists, or an authentication agent is 1420running. 1421Note that this option applies to protocol version 1 only. 1422.It Cm SendEnv 1423Specifies what variables from the local 1424.Xr environ 7 1425should be sent to the server. 1426The server must also support it, and the server must be configured to 1427accept these environment variables. 1428Note that the 1429.Ev TERM 1430environment variable is always sent whenever a 1431pseudo-terminal is requested as it is required by the protocol. 1432Refer to 1433.Cm AcceptEnv 1434in 1435.Xr sshd_config 5 1436for how to configure the server. 1437Variables are specified by name, which may contain wildcard characters. 1438Multiple environment variables may be separated by whitespace or spread 1439across multiple 1440.Cm SendEnv 1441directives. 1442The default is not to send any environment variables. 1443.Pp 1444See 1445.Sx PATTERNS 1446for more information on patterns. 1447.It Cm ServerAliveCountMax 1448Sets the number of server alive messages (see below) which may be 1449sent without 1450.Xr ssh 1 1451receiving any messages back from the server. 1452If this threshold is reached while server alive messages are being sent, 1453ssh will disconnect from the server, terminating the session. 1454It is important to note that the use of server alive messages is very 1455different from 1456.Cm TCPKeepAlive 1457(below). 1458The server alive messages are sent through the encrypted channel 1459and therefore will not be spoofable. 1460The TCP keepalive option enabled by 1461.Cm TCPKeepAlive 1462is spoofable. 1463The server alive mechanism is valuable when the client or 1464server depend on knowing when a connection has become inactive. 1465.Pp 1466The default value is 3. 1467If, for example, 1468.Cm ServerAliveInterval 1469(see below) is set to 15 and 1470.Cm ServerAliveCountMax 1471is left at the default, if the server becomes unresponsive, 1472ssh will disconnect after approximately 45 seconds. 1473.It Cm ServerAliveInterval 1474Sets a timeout interval in seconds after which if no data has been received 1475from the server, 1476.Xr ssh 1 1477will send a message through the encrypted 1478channel to request a response from the server. 1479The default 1480is 0, indicating that these messages will not be sent to the server. 1481.It Cm StreamLocalBindMask 1482Sets the octal file creation mode mask 1483.Pq umask 1484used when creating a Unix-domain socket file for local or remote 1485port forwarding. 1486This option is only used for port forwarding to a Unix-domain socket file. 1487.Pp 1488The default value is 0177, which creates a Unix-domain socket file that is 1489readable and writable only by the owner. 1490Note that not all operating systems honor the file mode on Unix-domain 1491socket files. 1492.It Cm StreamLocalBindUnlink 1493Specifies whether to remove an existing Unix-domain socket file for local 1494or remote port forwarding before creating a new one. 1495If the socket file already exists and 1496.Cm StreamLocalBindUnlink 1497is not enabled, 1498.Nm ssh 1499will be unable to forward the port to the Unix-domain socket file. 1500This option is only used for port forwarding to a Unix-domain socket file. 1501.Pp 1502The argument must be 1503.Cm yes 1504or 1505.Cm no 1506(the default). 1507.It Cm StrictHostKeyChecking 1508If this flag is set to 1509.Cm yes , 1510.Xr ssh 1 1511will never automatically add host keys to the 1512.Pa ~/.ssh/known_hosts 1513file, and refuses to connect to hosts whose host key has changed. 1514This provides maximum protection against trojan horse attacks, 1515though it can be annoying when the 1516.Pa /etc/ssh/ssh_known_hosts 1517file is poorly maintained or when connections to new hosts are 1518frequently made. 1519This option forces the user to manually 1520add all new hosts. 1521If this flag is set to 1522.Cm no , 1523ssh will automatically add new host keys to the 1524user known hosts files. 1525If this flag is set to 1526.Cm ask 1527(the default), 1528new host keys 1529will be added to the user known host files only after the user 1530has confirmed that is what they really want to do, and 1531ssh will refuse to connect to hosts whose host key has changed. 1532The host keys of 1533known hosts will be verified automatically in all cases. 1534.It Cm TCPKeepAlive 1535Specifies whether the system should send TCP keepalive messages to the 1536other side. 1537If they are sent, death of the connection or crash of one 1538of the machines will be properly noticed. 1539However, this means that 1540connections will die if the route is down temporarily, and some people 1541find it annoying. 1542.Pp 1543The default is 1544.Cm yes 1545(to send TCP keepalive messages), and the client will notice 1546if the network goes down or the remote host dies. 1547This is important in scripts, and many users want it too. 1548.Pp 1549To disable TCP keepalive messages, the value should be set to 1550.Cm no . 1551.It Cm Tunnel 1552Request 1553.Xr tun 4 1554device forwarding between the client and the server. 1555The argument must be 1556.Cm yes , 1557.Cm point-to-point 1558(layer 3), 1559.Cm ethernet 1560(layer 2), 1561or 1562.Cm no 1563(the default). 1564Specifying 1565.Cm yes 1566requests the default tunnel mode, which is 1567.Cm point-to-point . 1568.It Cm TunnelDevice 1569Specifies the 1570.Xr tun 4 1571devices to open on the client 1572.Pq Ar local_tun 1573and the server 1574.Pq Ar remote_tun . 1575.Pp 1576The argument must be 1577.Sm off 1578.Ar local_tun Op : Ar remote_tun . 1579.Sm on 1580The devices may be specified by numerical ID or the keyword 1581.Cm any , 1582which uses the next available tunnel device. 1583If 1584.Ar remote_tun 1585is not specified, it defaults to 1586.Cm any . 1587The default is 1588.Cm any:any . 1589.It Cm UpdateHostKeys 1590Specifies whether 1591.Xr ssh 1 1592should accept notifications of additional hostkeys from the server sent 1593after authentication has completed and add them to 1594.Cm UserKnownHostsFile . 1595The argument must be 1596.Cm yes , 1597.Cm no 1598(the default) or 1599.Cm ask . 1600Enabling this option allows learning alternate hostkeys for a server 1601and supports graceful key rotation by allowing a server to send replacement 1602public keys before old ones are removed. 1603Additional hostkeys are only accepted if the key used to authenticate the 1604host was already trusted or explicitly accepted by the user. 1605If 1606.Cm UpdateHostKeys 1607is set to 1608.Cm ask , 1609then the user is asked to confirm the modifications to the known_hosts file. 1610Confirmation is currently incompatible with 1611.Cm ControlPersist , 1612and will be disabled if it is enabled. 1613.Pp 1614Presently, only 1615.Xr sshd 8 1616from OpenSSH 6.8 and greater support the 1617.Qq hostkeys@openssh.com 1618protocol extension used to inform the client of all the server's hostkeys. 1619.It Cm UsePrivilegedPort 1620Specifies whether to use a privileged port for outgoing connections. 1621The argument must be 1622.Cm yes 1623or 1624.Cm no 1625(the default). 1626If set to 1627.Cm yes , 1628.Xr ssh 1 1629must be setuid root. 1630Note that this option must be set to 1631.Cm yes 1632for 1633.Cm RhostsRSAAuthentication 1634with older servers. 1635.It Cm User 1636Specifies the user to log in as. 1637This can be useful when a different user name is used on different machines. 1638This saves the trouble of 1639having to remember to give the user name on the command line. 1640.It Cm UserKnownHostsFile 1641Specifies one or more files to use for the user 1642host key database, separated by whitespace. 1643The default is 1644.Pa ~/.ssh/known_hosts , 1645.Pa ~/.ssh/known_hosts2 . 1646.It Cm VerifyHostKeyDNS 1647Specifies whether to verify the remote key using DNS and SSHFP resource 1648records. 1649If this option is set to 1650.Cm yes , 1651the client will implicitly trust keys that match a secure fingerprint 1652from DNS. 1653Insecure fingerprints will be handled as if this option was set to 1654.Cm ask . 1655If this option is set to 1656.Cm ask , 1657information on fingerprint match will be displayed, but the user will still 1658need to confirm new host keys according to the 1659.Cm StrictHostKeyChecking 1660option. 1661The default is 1662.Cm no . 1663.Pp 1664See also 1665.Sx VERIFYING HOST KEYS 1666in 1667.Xr ssh 1 . 1668.It Cm VisualHostKey 1669If this flag is set to 1670.Cm yes , 1671an ASCII art representation of the remote host key fingerprint is 1672printed in addition to the fingerprint string at login and 1673for unknown host keys. 1674If this flag is set to 1675.Cm no 1676(the default), 1677no fingerprint strings are printed at login and 1678only the fingerprint string will be printed for unknown host keys. 1679.It Cm XAuthLocation 1680Specifies the full pathname of the 1681.Xr xauth 1 1682program. 1683The default is 1684.Pa /usr/X11R6/bin/xauth . 1685.El 1686.Sh PATTERNS 1687A 1688.Em pattern 1689consists of zero or more non-whitespace characters, 1690.Sq * 1691(a wildcard that matches zero or more characters), 1692or 1693.Sq ?\& 1694(a wildcard that matches exactly one character). 1695For example, to specify a set of declarations for any host in the 1696.Qq .co.uk 1697set of domains, 1698the following pattern could be used: 1699.Pp 1700.Dl Host *.co.uk 1701.Pp 1702The following pattern 1703would match any host in the 192.168.0.[0-9] network range: 1704.Pp 1705.Dl Host 192.168.0.? 1706.Pp 1707A 1708.Em pattern-list 1709is a comma-separated list of patterns. 1710Patterns within pattern-lists may be negated 1711by preceding them with an exclamation mark 1712.Pq Sq !\& . 1713For example, 1714to allow a key to be used from anywhere within an organization 1715except from the 1716.Qq dialup 1717pool, 1718the following entry (in authorized_keys) could be used: 1719.Pp 1720.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1721.Sh TOKENS 1722Arguments to some keywords can make use of tokens, 1723which are expanded at runtime: 1724.Pp 1725.Bl -tag -width XXXX -offset indent -compact 1726.It %% 1727A literal 1728.Sq % . 1729.It \&%C 1730Shorthand for %l%h%p%r. 1731.It %d 1732Local user's home directory. 1733.It %h 1734The remote hostname. 1735.It %i 1736The local user ID. 1737.It %L 1738The local hostname. 1739.It %l 1740The local hostname, including the domain name. 1741.It %n 1742The original remote hostname, as given on the command line. 1743.It %p 1744The remote port. 1745.It %r 1746The remote username. 1747.It %u 1748The local username. 1749.El 1750.Pp 1751.Cm Match exec 1752accepts the tokens %%, %h, %L, %l, %n, %p, %r, and %u. 1753.Pp 1754.Cm CertificateFile 1755accepts the tokens %%, %d, %h, %l, %r, and %u. 1756.Pp 1757.Cm ControlPath 1758accepts the tokens %%, %C, %h, %i, %L, %l, %n, %p, %r, and %u. 1759.Pp 1760.Cm HostName 1761accepts the tokens %% and %h. 1762.Pp 1763.Cm IdentityAgent 1764and 1765.Cm IdentityFile 1766accept the tokens %%, %d, %h, %l, %r, and %u. 1767.Pp 1768.Cm LocalCommand 1769accepts the tokens %%, %C, %d, %h, %l, %n, %p, %r, and %u. 1770.Pp 1771.Cm ProxyCommand 1772accepts the tokens %%, %h, %p, and %r. 1773.Sh FILES 1774.Bl -tag -width Ds 1775.It Pa ~/.ssh/config 1776This is the per-user configuration file. 1777The format of this file is described above. 1778This file is used by the SSH client. 1779Because of the potential for abuse, this file must have strict permissions: 1780read/write for the user, and not accessible by others. 1781.It Pa /etc/ssh/ssh_config 1782Systemwide configuration file. 1783This file provides defaults for those 1784values that are not specified in the user's configuration file, and 1785for those users who do not have a configuration file. 1786This file must be world-readable. 1787.El 1788.Sh SEE ALSO 1789.Xr ssh 1 1790.Sh AUTHORS 1791.An -nosplit 1792OpenSSH is a derivative of the original and free 1793ssh 1.2.12 release by 1794.An Tatu Ylonen . 1795.An Aaron Campbell , Bob Beck , Markus Friedl , 1796.An Niels Provos , Theo de Raadt 1797and 1798.An Dug Song 1799removed many bugs, re-added newer features and 1800created OpenSSH. 1801.An Markus Friedl 1802contributed the support for SSH protocol versions 1.5 and 2.0. 1803