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 1.286 2018/10/03 06:38:35 djm Exp $ 37.Dd $Mdocdate: October 3 2018 $ 38.Dt SSH_CONFIG 5 39.Os 40.Sh NAME 41.Nm ssh_config 42.Nd OpenSSH SSH client configuration files 43.Sh DESCRIPTION 44.Xr ssh 1 45obtains configuration data from the following sources in 46the following order: 47.Pp 48.Bl -enum -offset indent -compact 49.It 50command-line options 51.It 52user's configuration file 53.Pq Pa ~/.ssh/config 54.It 55system-wide configuration file 56.Pq Pa /etc/ssh/ssh_config 57.El 58.Pp 59For each parameter, the first obtained value 60will be used. 61The configuration files contain sections separated by 62.Cm Host 63specifications, and that section is only applied for hosts that 64match one of the patterns given in the specification. 65The matched host name is usually the one given on the command line 66(see the 67.Cm CanonicalizeHostname 68option for exceptions). 69.Pp 70Since the first obtained value for each parameter is used, more 71host-specific declarations should be given near the beginning of the 72file, and general defaults at the end. 73.Pp 74The file contains keyword-argument pairs, one per line. 75Lines starting with 76.Ql # 77and empty lines are interpreted as comments. 78Arguments may optionally be enclosed in double quotes 79.Pq \&" 80in order to represent arguments containing spaces. 81Configuration options may be separated by whitespace or 82optional whitespace and exactly one 83.Ql = ; 84the latter format is useful to avoid the need to quote whitespace 85when specifying configuration options using the 86.Nm ssh , 87.Nm scp , 88and 89.Nm sftp 90.Fl o 91option. 92.Pp 93The possible 94keywords and their meanings are as follows (note that 95keywords are case-insensitive and arguments are case-sensitive): 96.Bl -tag -width Ds 97.It Cm Host 98Restricts the following declarations (up to the next 99.Cm Host 100or 101.Cm Match 102keyword) to be only for those hosts that match one of the patterns 103given after the keyword. 104If more than one pattern is provided, they should be separated by whitespace. 105A single 106.Ql * 107as a pattern can be used to provide global 108defaults for all hosts. 109The host is usually the 110.Ar hostname 111argument given on the command line 112(see the 113.Cm CanonicalizeHostname 114keyword for exceptions). 115.Pp 116A pattern entry may be negated by prefixing it with an exclamation mark 117.Pq Sq !\& . 118If a negated entry is matched, then the 119.Cm Host 120entry is ignored, regardless of whether any other patterns on the line 121match. 122Negated matches are therefore useful to provide exceptions for wildcard 123matches. 124.Pp 125See 126.Sx PATTERNS 127for more information on patterns. 128.It Cm Match 129Restricts the following declarations (up to the next 130.Cm Host 131or 132.Cm Match 133keyword) to be used only when the conditions following the 134.Cm Match 135keyword are satisfied. 136Match conditions are specified using one or more criteria 137or the single token 138.Cm all 139which always matches. 140The available criteria keywords are: 141.Cm canonical , 142.Cm exec , 143.Cm host , 144.Cm originalhost , 145.Cm user , 146and 147.Cm localuser . 148The 149.Cm all 150criteria must appear alone or immediately after 151.Cm canonical . 152Other criteria may be combined arbitrarily. 153All criteria but 154.Cm all 155and 156.Cm canonical 157require an argument. 158Criteria may be negated by prepending an exclamation mark 159.Pq Sq !\& . 160.Pp 161The 162.Cm canonical 163keyword matches only when the configuration file is being re-parsed 164after hostname canonicalization (see the 165.Cm CanonicalizeHostname 166option.) 167This may be useful to specify conditions that work with canonical host 168names only. 169The 170.Cm exec 171keyword executes the specified command under the user's shell. 172If the command returns a zero exit status then the condition is considered true. 173Commands containing whitespace characters must be quoted. 174Arguments to 175.Cm exec 176accept the tokens described in the 177.Sx TOKENS 178section. 179.Pp 180The other keywords' criteria must be single entries or comma-separated 181lists and may use the wildcard and negation operators described in the 182.Sx PATTERNS 183section. 184The criteria for the 185.Cm host 186keyword are matched against the target hostname, after any substitution 187by the 188.Cm Hostname 189or 190.Cm CanonicalizeHostname 191options. 192The 193.Cm originalhost 194keyword matches against the hostname as it was specified on the command-line. 195The 196.Cm user 197keyword matches against the target username on the remote host. 198The 199.Cm localuser 200keyword matches against the name of the local user running 201.Xr ssh 1 202(this keyword may be useful in system-wide 203.Nm 204files). 205.It Cm AddKeysToAgent 206Specifies whether keys should be automatically added to a running 207.Xr ssh-agent 1 . 208If this option is set to 209.Cm yes 210and a key is loaded from a file, the key and its passphrase are added to 211the agent with the default lifetime, as if by 212.Xr ssh-add 1 . 213If this option is set to 214.Cm ask , 215.Xr ssh 1 216will require confirmation using the 217.Ev SSH_ASKPASS 218program before adding a key (see 219.Xr ssh-add 1 220for details). 221If this option is set to 222.Cm confirm , 223each use of the key must be confirmed, as if the 224.Fl c 225option was specified to 226.Xr ssh-add 1 . 227If this option is set to 228.Cm no , 229no keys are added to the agent. 230The argument must be 231.Cm yes , 232.Cm confirm , 233.Cm ask , 234or 235.Cm no 236(the default). 237.It Cm AddressFamily 238Specifies which address family to use when connecting. 239Valid arguments are 240.Cm any 241(the default), 242.Cm inet 243(use IPv4 only), or 244.Cm inet6 245(use IPv6 only). 246.It Cm BatchMode 247If set to 248.Cm yes , 249passphrase/password querying will be disabled. 250This option is useful in scripts and other batch jobs where no user 251is present to supply the password. 252The argument must be 253.Cm yes 254or 255.Cm no 256(the default). 257.It Cm BindAddress 258Use the specified address on the local machine as the source address of 259the connection. 260Only useful on systems with more than one address. 261.It Cm BindInterface 262Use the address of the specified interface on the local machine as the 263source address of the connection. 264.It Cm CanonicalDomains 265When 266.Cm CanonicalizeHostname 267is enabled, this option specifies the list of domain suffixes in which to 268search for the specified destination host. 269.It Cm CanonicalizeFallbackLocal 270Specifies whether to fail with an error when hostname canonicalization fails. 271The default, 272.Cm yes , 273will attempt to look up the unqualified hostname using the system resolver's 274search rules. 275A value of 276.Cm no 277will cause 278.Xr ssh 1 279to fail instantly if 280.Cm CanonicalizeHostname 281is enabled and the target hostname cannot be found in any of the domains 282specified by 283.Cm CanonicalDomains . 284.It Cm CanonicalizeHostname 285Controls whether explicit hostname canonicalization is performed. 286The default, 287.Cm no , 288is not to perform any name rewriting and let the system resolver handle all 289hostname lookups. 290If set to 291.Cm yes 292then, for connections that do not use a 293.Cm ProxyCommand 294or 295.Cm ProxyJump , 296.Xr ssh 1 297will attempt to canonicalize the hostname specified on the command line 298using the 299.Cm CanonicalDomains 300suffixes and 301.Cm CanonicalizePermittedCNAMEs 302rules. 303If 304.Cm CanonicalizeHostname 305is set to 306.Cm always , 307then canonicalization is applied to proxied connections too. 308.Pp 309If this option is enabled, then the configuration files are processed 310again using the new target name to pick up any new configuration in matching 311.Cm Host 312and 313.Cm Match 314stanzas. 315.It Cm CanonicalizeMaxDots 316Specifies the maximum number of dot characters in a hostname before 317canonicalization is disabled. 318The default, 1, 319allows a single dot (i.e. hostname.subdomain). 320.It Cm CanonicalizePermittedCNAMEs 321Specifies rules to determine whether CNAMEs should be followed when 322canonicalizing hostnames. 323The rules consist of one or more arguments of 324.Ar source_domain_list : Ns Ar target_domain_list , 325where 326.Ar source_domain_list 327is a pattern-list of domains that may follow CNAMEs in canonicalization, 328and 329.Ar target_domain_list 330is a pattern-list of domains that they may resolve to. 331.Pp 332For example, 333.Qq *.a.example.com:*.b.example.com,*.c.example.com 334will allow hostnames matching 335.Qq *.a.example.com 336to be canonicalized to names in the 337.Qq *.b.example.com 338or 339.Qq *.c.example.com 340domains. 341.It Cm CASignatureAlgorithms 342Specifies which algorithms are allowed for signing of certificates 343by certificate authorities (CAs). 344The default is: 345.Bd -literal -offset indent 346ecdsa-sha2-nistp256.ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 347ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 348.Ed 349.Pp 350.Xr ssh 1 351will not accept host certificates signed using algorithms other than those 352specified. 353.It Cm CertificateFile 354Specifies a file from which the user's certificate is read. 355A corresponding private key must be provided separately in order 356to use this certificate either 357from an 358.Cm IdentityFile 359directive or 360.Fl i 361flag to 362.Xr ssh 1 , 363via 364.Xr ssh-agent 1 , 365or via a 366.Cm PKCS11Provider . 367.Pp 368Arguments to 369.Cm CertificateFile 370may use the tilde syntax to refer to a user's home directory 371or the tokens described in the 372.Sx TOKENS 373section. 374.Pp 375It is possible to have multiple certificate files specified in 376configuration files; these certificates will be tried in sequence. 377Multiple 378.Cm CertificateFile 379directives will add to the list of certificates used for 380authentication. 381.It Cm ChallengeResponseAuthentication 382Specifies whether to use challenge-response authentication. 383The argument to this keyword must be 384.Cm yes 385(the default) 386or 387.Cm no . 388.It Cm CheckHostIP 389If set to 390.Cm yes 391(the default), 392.Xr ssh 1 393will additionally check the host IP address in the 394.Pa known_hosts 395file. 396This allows it to detect if a host key changed due to DNS spoofing 397and will add addresses of destination hosts to 398.Pa ~/.ssh/known_hosts 399in the process, regardless of the setting of 400.Cm StrictHostKeyChecking . 401If the option is set to 402.Cm no , 403the check will not be executed. 404.It Cm Ciphers 405Specifies the ciphers allowed and their order of preference. 406Multiple ciphers must be comma-separated. 407If the specified value begins with a 408.Sq + 409character, then the specified ciphers will be appended to the default set 410instead of replacing them. 411If the specified value begins with a 412.Sq - 413character, then the specified ciphers (including wildcards) will be removed 414from the default set instead of replacing them. 415.Pp 416The supported ciphers are: 417.Bd -literal -offset indent 4183des-cbc 419aes128-cbc 420aes192-cbc 421aes256-cbc 422aes128-ctr 423aes192-ctr 424aes256-ctr 425aes128-gcm@openssh.com 426aes256-gcm@openssh.com 427chacha20-poly1305@openssh.com 428.Ed 429.Pp 430The default is: 431.Bd -literal -offset indent 432chacha20-poly1305@openssh.com, 433aes128-ctr,aes192-ctr,aes256-ctr, 434aes128-gcm@openssh.com,aes256-gcm@openssh.com 435.Ed 436.Pp 437The list of available ciphers may also be obtained using 438.Qq ssh -Q cipher . 439.It Cm ClearAllForwardings 440Specifies that all local, remote, and dynamic port forwardings 441specified in the configuration files or on the command line be 442cleared. 443This option is primarily useful when used from the 444.Xr ssh 1 445command line to clear port forwardings set in 446configuration files, and is automatically set by 447.Xr scp 1 448and 449.Xr sftp 1 . 450The argument must be 451.Cm yes 452or 453.Cm no 454(the default). 455.It Cm Compression 456Specifies whether to use compression. 457The argument must be 458.Cm yes 459or 460.Cm no 461(the default). 462.It Cm ConnectionAttempts 463Specifies the number of tries (one per second) to make before exiting. 464The argument must be an integer. 465This may be useful in scripts if the connection sometimes fails. 466The default is 1. 467.It Cm ConnectTimeout 468Specifies the timeout (in seconds) used when connecting to the 469SSH server, instead of using the default system TCP timeout. 470This value is used only when the target is down or really unreachable, 471not when it refuses the connection. 472.It Cm ControlMaster 473Enables the sharing of multiple sessions over a single network connection. 474When set to 475.Cm yes , 476.Xr ssh 1 477will listen for connections on a control socket specified using the 478.Cm ControlPath 479argument. 480Additional sessions can connect to this socket using the same 481.Cm ControlPath 482with 483.Cm ControlMaster 484set to 485.Cm no 486(the default). 487These sessions will try to reuse the master instance's network connection 488rather than initiating new ones, but will fall back to connecting normally 489if the control socket does not exist, or is not listening. 490.Pp 491Setting this to 492.Cm ask 493will cause 494.Xr ssh 1 495to listen for control connections, but require confirmation using 496.Xr ssh-askpass 1 . 497If the 498.Cm ControlPath 499cannot be opened, 500.Xr ssh 1 501will continue without connecting to a master instance. 502.Pp 503X11 and 504.Xr ssh-agent 1 505forwarding is supported over these multiplexed connections, however the 506display and agent forwarded will be the one belonging to the master 507connection i.e. it is not possible to forward multiple displays or agents. 508.Pp 509Two additional options allow for opportunistic multiplexing: try to use a 510master connection but fall back to creating a new one if one does not already 511exist. 512These options are: 513.Cm auto 514and 515.Cm autoask . 516The latter requires confirmation like the 517.Cm ask 518option. 519.It Cm ControlPath 520Specify the path to the control socket used for connection sharing as described 521in the 522.Cm ControlMaster 523section above or the string 524.Cm none 525to disable connection sharing. 526Arguments to 527.Cm ControlPath 528may use the tilde syntax to refer to a user's home directory 529or the tokens described in the 530.Sx TOKENS 531section. 532It is recommended that any 533.Cm ControlPath 534used for opportunistic connection sharing include 535at least %h, %p, and %r (or alternatively %C) and be placed in a directory 536that is not writable by other users. 537This ensures that shared connections are uniquely identified. 538.It Cm ControlPersist 539When used in conjunction with 540.Cm ControlMaster , 541specifies that the master connection should remain open 542in the background (waiting for future client connections) 543after the initial client connection has been closed. 544If set to 545.Cm no , 546then the master connection will not be placed into the background, 547and will close as soon as the initial client connection is closed. 548If set to 549.Cm yes 550or 0, 551then the master connection will remain in the background indefinitely 552(until killed or closed via a mechanism such as the 553.Qq ssh -O exit ) . 554If set to a time in seconds, or a time in any of the formats documented in 555.Xr sshd_config 5 , 556then the backgrounded master connection will automatically terminate 557after it has remained idle (with no client connections) for the 558specified time. 559.It Cm DynamicForward 560Specifies that a TCP port on the local machine be forwarded 561over the secure channel, and the application 562protocol is then used to determine where to connect to from the 563remote machine. 564.Pp 565The argument must be 566.Sm off 567.Oo Ar bind_address : Oc Ar port . 568.Sm on 569IPv6 addresses can be specified by enclosing addresses in square brackets. 570By default, the local port is bound in accordance with the 571.Cm GatewayPorts 572setting. 573However, an explicit 574.Ar bind_address 575may be used to bind the connection to a specific address. 576The 577.Ar bind_address 578of 579.Cm localhost 580indicates that the listening port be bound for local use only, while an 581empty address or 582.Sq * 583indicates that the port should be available from all interfaces. 584.Pp 585Currently the SOCKS4 and SOCKS5 protocols are supported, and 586.Xr ssh 1 587will act as a SOCKS server. 588Multiple forwardings may be specified, and 589additional forwardings can be given on the command line. 590Only the superuser can forward privileged ports. 591.It Cm EnableSSHKeysign 592Setting this option to 593.Cm yes 594in the global client configuration file 595.Pa /etc/ssh/ssh_config 596enables the use of the helper program 597.Xr ssh-keysign 8 598during 599.Cm HostbasedAuthentication . 600The argument must be 601.Cm yes 602or 603.Cm no 604(the default). 605This option should be placed in the non-hostspecific section. 606See 607.Xr ssh-keysign 8 608for more information. 609.It Cm EscapeChar 610Sets the escape character (default: 611.Ql ~ ) . 612The escape character can also 613be set on the command line. 614The argument should be a single character, 615.Ql ^ 616followed by a letter, or 617.Cm none 618to disable the escape 619character entirely (making the connection transparent for binary 620data). 621.It Cm ExitOnForwardFailure 622Specifies whether 623.Xr ssh 1 624should terminate the connection if it cannot set up all requested 625dynamic, tunnel, local, and remote port forwardings, (e.g.\& 626if either end is unable to bind and listen on a specified port). 627Note that 628.Cm ExitOnForwardFailure 629does not apply to connections made over port forwardings and will not, 630for example, cause 631.Xr ssh 1 632to exit if TCP connections to the ultimate forwarding destination fail. 633The argument must be 634.Cm yes 635or 636.Cm no 637(the default). 638.It Cm FingerprintHash 639Specifies the hash algorithm used when displaying key fingerprints. 640Valid options are: 641.Cm md5 642and 643.Cm sha256 644(the default). 645.It Cm ForwardAgent 646Specifies whether the connection to the authentication agent (if any) 647will be forwarded to the remote machine. 648The argument must be 649.Cm yes 650or 651.Cm no 652(the default). 653.Pp 654Agent forwarding should be enabled with caution. 655Users with the ability to bypass file permissions on the remote host 656(for the agent's Unix-domain socket) 657can access the local agent through the forwarded connection. 658An attacker cannot obtain key material from the agent, 659however they can perform operations on the keys that enable them to 660authenticate using the identities loaded into the agent. 661.It Cm ForwardX11 662Specifies whether X11 connections will be automatically redirected 663over the secure channel and 664.Ev DISPLAY 665set. 666The argument must be 667.Cm yes 668or 669.Cm no 670(the default). 671.Pp 672X11 forwarding should be enabled with caution. 673Users with the ability to bypass file permissions on the remote host 674(for the user's X11 authorization database) 675can access the local X11 display through the forwarded connection. 676An attacker may then be able to perform activities such as keystroke monitoring 677if the 678.Cm ForwardX11Trusted 679option is also enabled. 680.It Cm ForwardX11Timeout 681Specify a timeout for untrusted X11 forwarding 682using the format described in the 683.Sx TIME FORMATS 684section of 685.Xr sshd_config 5 . 686X11 connections received by 687.Xr ssh 1 688after this time will be refused. 689Setting 690.Cm ForwardX11Timeout 691to zero will disable the timeout and permit X11 forwarding for the life 692of the connection. 693The default is to disable untrusted X11 forwarding after twenty minutes has 694elapsed. 695.It Cm ForwardX11Trusted 696If this option is set to 697.Cm yes , 698remote X11 clients will have full access to the original X11 display. 699.Pp 700If this option is set to 701.Cm no 702(the default), 703remote X11 clients will be considered untrusted and prevented 704from stealing or tampering with data belonging to trusted X11 705clients. 706Furthermore, the 707.Xr xauth 1 708token used for the session will be set to expire after 20 minutes. 709Remote clients will be refused access after this time. 710.Pp 711See the X11 SECURITY extension specification for full details on 712the restrictions imposed on untrusted clients. 713.It Cm GatewayPorts 714Specifies whether remote hosts are allowed to connect to local 715forwarded ports. 716By default, 717.Xr ssh 1 718binds local port forwardings to the loopback address. 719This prevents other remote hosts from connecting to forwarded ports. 720.Cm GatewayPorts 721can be used to specify that ssh 722should bind local port forwardings to the wildcard address, 723thus allowing remote hosts to connect to forwarded ports. 724The argument must be 725.Cm yes 726or 727.Cm no 728(the default). 729.It Cm GlobalKnownHostsFile 730Specifies one or more files to use for the global 731host key database, separated by whitespace. 732The default is 733.Pa /etc/ssh/ssh_known_hosts , 734.Pa /etc/ssh/ssh_known_hosts2 . 735.It Cm GSSAPIAuthentication 736Specifies whether user authentication based on GSSAPI is allowed. 737The default is 738.Cm no . 739.It Cm GSSAPIDelegateCredentials 740Forward (delegate) credentials to the server. 741The default is 742.Cm no . 743.It Cm HashKnownHosts 744Indicates that 745.Xr ssh 1 746should hash host names and addresses when they are added to 747.Pa ~/.ssh/known_hosts . 748These hashed names may be used normally by 749.Xr ssh 1 750and 751.Xr sshd 8 , 752but they do not reveal identifying information should the file's contents 753be disclosed. 754The default is 755.Cm no . 756Note that existing names and addresses in known hosts files 757will not be converted automatically, 758but may be manually hashed using 759.Xr ssh-keygen 1 . 760.It Cm HostbasedAuthentication 761Specifies whether to try rhosts based authentication with public key 762authentication. 763The argument must be 764.Cm yes 765or 766.Cm no 767(the default). 768.It Cm HostbasedKeyTypes 769Specifies the key types that will be used for hostbased authentication 770as a comma-separated list of patterns. 771Alternately if the specified value begins with a 772.Sq + 773character, then the specified key types will be appended to the default set 774instead of replacing them. 775If the specified value begins with a 776.Sq - 777character, then the specified key types (including wildcards) will be removed 778from the default set instead of replacing them. 779The default for this option is: 780.Bd -literal -offset 3n 781ecdsa-sha2-nistp256-cert-v01@openssh.com, 782ecdsa-sha2-nistp384-cert-v01@openssh.com, 783ecdsa-sha2-nistp521-cert-v01@openssh.com, 784ssh-ed25519-cert-v01@openssh.com, 785rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com, 786ssh-rsa-cert-v01@openssh.com, 787ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 788ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 789.Ed 790.Pp 791The 792.Fl Q 793option of 794.Xr ssh 1 795may be used to list supported key types. 796.It Cm HostKeyAlgorithms 797Specifies the host key algorithms 798that the client wants to use in order of preference. 799Alternately if the specified value begins with a 800.Sq + 801character, then the specified key types will be appended to the default set 802instead of replacing them. 803If the specified value begins with a 804.Sq - 805character, then the specified key types (including wildcards) will be removed 806from the default set instead of replacing them. 807The default for this option is: 808.Bd -literal -offset 3n 809ecdsa-sha2-nistp256-cert-v01@openssh.com, 810ecdsa-sha2-nistp384-cert-v01@openssh.com, 811ecdsa-sha2-nistp521-cert-v01@openssh.com, 812ssh-ed25519-cert-v01@openssh.com, 813rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com, 814ssh-rsa-cert-v01@openssh.com, 815ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 816ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 817.Ed 818.Pp 819If hostkeys are known for the destination host then this default is modified 820to prefer their algorithms. 821.Pp 822The list of available key types may also be obtained using 823.Qq ssh -Q key . 824.It Cm HostKeyAlias 825Specifies an alias that should be used instead of the 826real host name when looking up or saving the host key 827in the host key database files and when validating host certificates. 828This option is useful for tunneling SSH connections 829or for multiple servers running on a single host. 830.It Cm HostName 831Specifies the real host name to log into. 832This can be used to specify nicknames or abbreviations for hosts. 833Arguments to 834.Cm HostName 835accept the tokens described in the 836.Sx TOKENS 837section. 838Numeric IP addresses are also permitted (both on the command line and in 839.Cm HostName 840specifications). 841The default is the name given on the command line. 842.It Cm IdentitiesOnly 843Specifies that 844.Xr ssh 1 845should only use the authentication identity and certificate files explicitly 846configured in the 847.Nm 848files 849or passed on the 850.Xr ssh 1 851command-line, 852even if 853.Xr ssh-agent 1 854or a 855.Cm PKCS11Provider 856offers more identities. 857The argument to this keyword must be 858.Cm yes 859or 860.Cm no 861(the default). 862This option is intended for situations where ssh-agent 863offers many different identities. 864.It Cm IdentityAgent 865Specifies the 866.Ux Ns -domain 867socket used to communicate with the authentication agent. 868.Pp 869This option overrides the 870.Ev SSH_AUTH_SOCK 871environment variable and can be used to select a specific agent. 872Setting the socket name to 873.Cm none 874disables the use of an authentication agent. 875If the string 876.Qq SSH_AUTH_SOCK 877is specified, the location of the socket will be read from the 878.Ev SSH_AUTH_SOCK 879environment variable. 880Otherwise if the specified value begins with a 881.Sq $ 882character, then it will be treated as an environment variable containing 883the location of the socket. 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/id_dsa , 896.Pa ~/.ssh/id_ecdsa , 897.Pa ~/.ssh/id_ed25519 898and 899.Pa ~/.ssh/id_rsa . 900Additionally, any identities represented by the authentication agent 901will be used for authentication unless 902.Cm IdentitiesOnly 903is set. 904If no certificates have been explicitly specified by 905.Cm CertificateFile , 906.Xr ssh 1 907will try to load certificate information from the filename obtained by 908appending 909.Pa -cert.pub 910to the path of a specified 911.Cm IdentityFile . 912.Pp 913Arguments to 914.Cm IdentityFile 915may use the tilde syntax to refer to a user's home directory 916or the tokens described in the 917.Sx TOKENS 918section. 919.Pp 920It is possible to have 921multiple identity files specified in configuration files; all these 922identities will be tried in sequence. 923Multiple 924.Cm IdentityFile 925directives will add to the list of identities tried (this behaviour 926differs from that of other configuration directives). 927.Pp 928.Cm IdentityFile 929may be used in conjunction with 930.Cm IdentitiesOnly 931to select which identities in an agent are offered during authentication. 932.Cm IdentityFile 933may also be used in conjunction with 934.Cm CertificateFile 935in order to provide any certificate also needed for authentication with 936the identity. 937.It Cm IgnoreUnknown 938Specifies a pattern-list of unknown options to be ignored if they are 939encountered in configuration parsing. 940This may be used to suppress errors if 941.Nm 942contains options that are unrecognised by 943.Xr ssh 1 . 944It is recommended that 945.Cm IgnoreUnknown 946be listed early in the configuration file as it will not be applied 947to unknown options that appear before it. 948.It Cm Include 949Include the specified configuration file(s). 950Multiple pathnames may be specified and each pathname may contain 951.Xr glob 7 952wildcards and, for user configurations, shell-like 953.Sq ~ 954references to user home directories. 955Files without absolute paths are assumed to be in 956.Pa ~/.ssh 957if included in a user configuration file or 958.Pa /etc/ssh 959if included from the system configuration file. 960.Cm Include 961directive may appear inside a 962.Cm Match 963or 964.Cm Host 965block 966to perform conditional inclusion. 967.It Cm IPQoS 968Specifies the IPv4 type-of-service or DSCP class for connections. 969Accepted values are 970.Cm af11 , 971.Cm af12 , 972.Cm af13 , 973.Cm af21 , 974.Cm af22 , 975.Cm af23 , 976.Cm af31 , 977.Cm af32 , 978.Cm af33 , 979.Cm af41 , 980.Cm af42 , 981.Cm af43 , 982.Cm cs0 , 983.Cm cs1 , 984.Cm cs2 , 985.Cm cs3 , 986.Cm cs4 , 987.Cm cs5 , 988.Cm cs6 , 989.Cm cs7 , 990.Cm ef , 991.Cm lowdelay , 992.Cm throughput , 993.Cm reliability , 994a numeric value, or 995.Cm none 996to use the operating system default. 997This option may take one or two arguments, separated by whitespace. 998If one argument is specified, it is used as the packet class unconditionally. 999If two values are specified, the first is automatically selected for 1000interactive sessions and the second for non-interactive sessions. 1001The default is 1002.Cm af21 1003(Low-Latency Data) 1004for interactive sessions and 1005.Cm cs1 1006(Lower Effort) 1007for non-interactive sessions. 1008.It Cm KbdInteractiveAuthentication 1009Specifies whether to use keyboard-interactive authentication. 1010The argument to this keyword must be 1011.Cm yes 1012(the default) 1013or 1014.Cm no . 1015.It Cm KbdInteractiveDevices 1016Specifies the list of methods to use in keyboard-interactive authentication. 1017Multiple method names must be comma-separated. 1018The default is to use the server specified list. 1019The methods available vary depending on what the server supports. 1020For an OpenSSH server, 1021it may be zero or more of: 1022.Cm bsdauth , 1023.Cm pam , 1024and 1025.Cm skey . 1026.It Cm KexAlgorithms 1027Specifies the available KEX (Key Exchange) algorithms. 1028Multiple algorithms must be comma-separated. 1029Alternately if the specified value begins with a 1030.Sq + 1031character, then the specified methods will be appended to the default set 1032instead of replacing them. 1033If the specified value begins with a 1034.Sq - 1035character, then the specified methods (including wildcards) will be removed 1036from the default set instead of replacing them. 1037The default is: 1038.Bd -literal -offset indent 1039curve25519-sha256,curve25519-sha256@libssh.org, 1040ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1041diffie-hellman-group-exchange-sha256, 1042diffie-hellman-group16-sha512, 1043diffie-hellman-group18-sha512, 1044diffie-hellman-group-exchange-sha1, 1045diffie-hellman-group14-sha256, 1046diffie-hellman-group14-sha1 1047.Ed 1048.Pp 1049The list of available key exchange algorithms may also be obtained using 1050.Qq ssh -Q kex . 1051.It Cm LocalCommand 1052Specifies a command to execute on the local machine after successfully 1053connecting to the server. 1054The command string extends to the end of the line, and is executed with 1055the user's shell. 1056Arguments to 1057.Cm LocalCommand 1058accept the tokens described in the 1059.Sx TOKENS 1060section. 1061.Pp 1062The command is run synchronously and does not have access to the 1063session of the 1064.Xr ssh 1 1065that spawned it. 1066It should not be used for interactive commands. 1067.Pp 1068This directive is ignored unless 1069.Cm PermitLocalCommand 1070has been enabled. 1071.It Cm LocalForward 1072Specifies that a TCP port on the local machine be forwarded over 1073the secure channel to the specified host and port from the remote machine. 1074The first argument must be 1075.Sm off 1076.Oo Ar bind_address : Oc Ar port 1077.Sm on 1078and the second argument must be 1079.Ar host : Ns Ar hostport . 1080IPv6 addresses can be specified by enclosing addresses in square brackets. 1081Multiple forwardings may be specified, and additional forwardings can be 1082given on the command line. 1083Only the superuser can forward privileged ports. 1084By default, the local port is bound in accordance with the 1085.Cm GatewayPorts 1086setting. 1087However, an explicit 1088.Ar bind_address 1089may be used to bind the connection to a specific address. 1090The 1091.Ar bind_address 1092of 1093.Cm localhost 1094indicates that the listening port be bound for local use only, while an 1095empty address or 1096.Sq * 1097indicates that the port should be available from all interfaces. 1098.It Cm LogLevel 1099Gives the verbosity level that is used when logging messages from 1100.Xr ssh 1 . 1101The possible values are: 1102QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1103The default is INFO. 1104DEBUG and DEBUG1 are equivalent. 1105DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1106.It Cm MACs 1107Specifies the MAC (message authentication code) algorithms 1108in order of preference. 1109The MAC algorithm is used for data integrity protection. 1110Multiple algorithms must be comma-separated. 1111If the specified value begins with a 1112.Sq + 1113character, then the specified algorithms will be appended to the default set 1114instead of replacing them. 1115If the specified value begins with a 1116.Sq - 1117character, then the specified algorithms (including wildcards) will be removed 1118from the default set instead of replacing them. 1119.Pp 1120The algorithms that contain 1121.Qq -etm 1122calculate the MAC after encryption (encrypt-then-mac). 1123These are considered safer and their use recommended. 1124.Pp 1125The default is: 1126.Bd -literal -offset indent 1127umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1128hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1129hmac-sha1-etm@openssh.com, 1130umac-64@openssh.com,umac-128@openssh.com, 1131hmac-sha2-256,hmac-sha2-512,hmac-sha1 1132.Ed 1133.Pp 1134The list of available MAC algorithms may also be obtained using 1135.Qq ssh -Q mac . 1136.It Cm NoHostAuthenticationForLocalhost 1137Disable host authentication for localhost (loopback addresses). 1138The argument to this keyword must be 1139.Cm yes 1140or 1141.Cm no 1142(the default). 1143.It Cm NumberOfPasswordPrompts 1144Specifies the number of password prompts before giving up. 1145The argument to this keyword must be an integer. 1146The default is 3. 1147.It Cm PasswordAuthentication 1148Specifies whether to use password authentication. 1149The argument to this keyword must be 1150.Cm yes 1151(the default) 1152or 1153.Cm no . 1154.It Cm PermitLocalCommand 1155Allow local command execution via the 1156.Ic LocalCommand 1157option or using the 1158.Ic !\& Ns Ar command 1159escape sequence in 1160.Xr ssh 1 . 1161The argument must be 1162.Cm yes 1163or 1164.Cm no 1165(the default). 1166.It Cm PKCS11Provider 1167Specifies which PKCS#11 provider to use. 1168The argument to this keyword is the PKCS#11 shared library 1169.Xr ssh 1 1170should use to communicate with a PKCS#11 token providing the user's 1171private RSA key. 1172.It Cm Port 1173Specifies the port number to connect on the remote host. 1174The default is 22. 1175.It Cm PreferredAuthentications 1176Specifies the order in which the client should try authentication methods. 1177This allows a client to prefer one method (e.g.\& 1178.Cm keyboard-interactive ) 1179over another method (e.g.\& 1180.Cm password ) . 1181The default is: 1182.Bd -literal -offset indent 1183gssapi-with-mic,hostbased,publickey, 1184keyboard-interactive,password 1185.Ed 1186.It Cm ProxyCommand 1187Specifies the command to use to connect to the server. 1188The command 1189string extends to the end of the line, and is executed 1190using the user's shell 1191.Ql exec 1192directive to avoid a lingering shell process. 1193.Pp 1194Arguments to 1195.Cm ProxyCommand 1196accept the tokens described in the 1197.Sx TOKENS 1198section. 1199The command can be basically anything, 1200and should read from its standard input and write to its standard output. 1201It should eventually connect an 1202.Xr sshd 8 1203server running on some machine, or execute 1204.Ic sshd -i 1205somewhere. 1206Host key management will be done using the 1207HostName of the host being connected (defaulting to the name typed by 1208the user). 1209Setting the command to 1210.Cm none 1211disables this option entirely. 1212Note that 1213.Cm CheckHostIP 1214is not available for connects with a proxy command. 1215.Pp 1216This directive is useful in conjunction with 1217.Xr nc 1 1218and its proxy support. 1219For example, the following directive would connect via an HTTP proxy at 1220192.0.2.0: 1221.Bd -literal -offset 3n 1222ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1223.Ed 1224.It Cm ProxyJump 1225Specifies one or more jump proxies as either 1226.Xo 1227.Sm off 1228.Op Ar user No @ 1229.Ar host 1230.Op : Ns Ar port 1231.Sm on 1232or an ssh URI 1233.Xc . 1234Multiple proxies may be separated by comma characters and will be visited 1235sequentially. 1236Setting this option will cause 1237.Xr ssh 1 1238to connect to the target host by first making a 1239.Xr ssh 1 1240connection to the specified 1241.Cm ProxyJump 1242host and then establishing a 1243TCP forwarding to the ultimate target from there. 1244.Pp 1245Note that this option will compete with the 1246.Cm ProxyCommand 1247option - whichever is specified first will prevent later instances of the 1248other from taking effect. 1249.It Cm ProxyUseFdpass 1250Specifies that 1251.Cm ProxyCommand 1252will pass a connected file descriptor back to 1253.Xr ssh 1 1254instead of continuing to execute and pass data. 1255The default is 1256.Cm no . 1257.It Cm PubkeyAcceptedKeyTypes 1258Specifies the key types that will be used for public key authentication 1259as a comma-separated list of patterns. 1260Alternately if the specified value begins with a 1261.Sq + 1262character, then the key types after it will be appended to the default 1263instead of replacing it. 1264If the specified value begins with a 1265.Sq - 1266character, then the specified key types (including wildcards) will be removed 1267from the default set instead of replacing them. 1268The default for this option is: 1269.Bd -literal -offset 3n 1270ecdsa-sha2-nistp256-cert-v01@openssh.com, 1271ecdsa-sha2-nistp384-cert-v01@openssh.com, 1272ecdsa-sha2-nistp521-cert-v01@openssh.com, 1273ssh-ed25519-cert-v01@openssh.com, 1274rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com, 1275ssh-rsa-cert-v01@openssh.com, 1276ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1277ssh-ed25519,rsa-sha2-512,rsa-sha2-256,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 RemoteCommand 1316Specifies a command to execute on the remote machine after successfully 1317connecting to the server. 1318The command string extends to the end of the line, and is executed with 1319the user's shell. 1320Arguments to 1321.Cm RemoteCommand 1322accept the tokens described in the 1323.Sx TOKENS 1324section. 1325.It Cm RemoteForward 1326Specifies that a TCP port on the remote machine be forwarded over 1327the secure channel. 1328The remote port may either be forwarded to a specified host and port 1329from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote 1330client to connect to arbitrary destinations from the local machine. 1331The first argument must be 1332.Sm off 1333.Oo Ar bind_address : Oc Ar port 1334.Sm on 1335If forwarding to a specific destination then the second argument must be 1336.Ar host : Ns Ar hostport , 1337otherwise if no destination argument is specified then the remote forwarding 1338will be established as a SOCKS proxy. 1339.Pp 1340IPv6 addresses can be specified by enclosing addresses in square brackets. 1341Multiple forwardings may be specified, and additional 1342forwardings can be given on the command line. 1343Privileged ports can be forwarded only when 1344logging in as root on the remote machine. 1345.Pp 1346If the 1347.Ar port 1348argument is 0, 1349the listen port will be dynamically allocated on the server and reported 1350to the client at run time. 1351.Pp 1352If the 1353.Ar bind_address 1354is not specified, the default is to only bind to loopback addresses. 1355If the 1356.Ar bind_address 1357is 1358.Ql * 1359or an empty string, then the forwarding is requested to listen on all 1360interfaces. 1361Specifying a remote 1362.Ar bind_address 1363will only succeed if the server's 1364.Cm GatewayPorts 1365option is enabled (see 1366.Xr sshd_config 5 ) . 1367.It Cm RequestTTY 1368Specifies whether to request a pseudo-tty for the session. 1369The argument may be one of: 1370.Cm no 1371(never request a TTY), 1372.Cm yes 1373(always request a TTY when standard input is a TTY), 1374.Cm force 1375(always request a TTY) or 1376.Cm auto 1377(request a TTY when opening a login session). 1378This option mirrors the 1379.Fl t 1380and 1381.Fl T 1382flags for 1383.Xr ssh 1 . 1384.It Cm RevokedHostKeys 1385Specifies revoked host public keys. 1386Keys listed in this file will be refused for host authentication. 1387Note that if this file does not exist or is not readable, 1388then host authentication will be refused for all hosts. 1389Keys may be specified as a text file, listing one public key per line, or as 1390an OpenSSH Key Revocation List (KRL) as generated by 1391.Xr ssh-keygen 1 . 1392For more information on KRLs, see the KEY REVOCATION LISTS section in 1393.Xr ssh-keygen 1 . 1394.It Cm SendEnv 1395Specifies what variables from the local 1396.Xr environ 7 1397should be sent to the server. 1398The server must also support it, and the server must be configured to 1399accept these environment variables. 1400Note that the 1401.Ev TERM 1402environment variable is always sent whenever a 1403pseudo-terminal is requested as it is required by the protocol. 1404Refer to 1405.Cm AcceptEnv 1406in 1407.Xr sshd_config 5 1408for how to configure the server. 1409Variables are specified by name, which may contain wildcard characters. 1410Multiple environment variables may be separated by whitespace or spread 1411across multiple 1412.Cm SendEnv 1413directives. 1414.Pp 1415See 1416.Sx PATTERNS 1417for more information on patterns. 1418.Pp 1419It is possible to clear previously set 1420.Cm SendEnv 1421variable names by prefixing patterns with 1422.Pa - . 1423The default is not to send any environment variables. 1424.It Cm ServerAliveCountMax 1425Sets the number of server alive messages (see below) which may be 1426sent without 1427.Xr ssh 1 1428receiving any messages back from the server. 1429If this threshold is reached while server alive messages are being sent, 1430ssh will disconnect from the server, terminating the session. 1431It is important to note that the use of server alive messages is very 1432different from 1433.Cm TCPKeepAlive 1434(below). 1435The server alive messages are sent through the encrypted channel 1436and therefore will not be spoofable. 1437The TCP keepalive option enabled by 1438.Cm TCPKeepAlive 1439is spoofable. 1440The server alive mechanism is valuable when the client or 1441server depend on knowing when a connection has become inactive. 1442.Pp 1443The default value is 3. 1444If, for example, 1445.Cm ServerAliveInterval 1446(see below) is set to 15 and 1447.Cm ServerAliveCountMax 1448is left at the default, if the server becomes unresponsive, 1449ssh will disconnect after approximately 45 seconds. 1450.It Cm ServerAliveInterval 1451Sets a timeout interval in seconds after which if no data has been received 1452from the server, 1453.Xr ssh 1 1454will send a message through the encrypted 1455channel to request a response from the server. 1456The default 1457is 0, indicating that these messages will not be sent to the server. 1458.It Cm SetEnv 1459Directly specify one or more environment variables and their contents to 1460be sent to the server. 1461Similarly to 1462.Cm SendEnv , 1463the server must be prepared to accept the environment variable. 1464.It Cm StreamLocalBindMask 1465Sets the octal file creation mode mask 1466.Pq umask 1467used when creating a Unix-domain socket file for local or remote 1468port forwarding. 1469This option is only used for port forwarding to a Unix-domain socket file. 1470.Pp 1471The default value is 0177, which creates a Unix-domain socket file that is 1472readable and writable only by the owner. 1473Note that not all operating systems honor the file mode on Unix-domain 1474socket files. 1475.It Cm StreamLocalBindUnlink 1476Specifies whether to remove an existing Unix-domain socket file for local 1477or remote port forwarding before creating a new one. 1478If the socket file already exists and 1479.Cm StreamLocalBindUnlink 1480is not enabled, 1481.Nm ssh 1482will be unable to forward the port to the Unix-domain socket file. 1483This option is only used for port forwarding to a Unix-domain socket file. 1484.Pp 1485The argument must be 1486.Cm yes 1487or 1488.Cm no 1489(the default). 1490.It Cm StrictHostKeyChecking 1491If this flag is set to 1492.Cm yes , 1493.Xr ssh 1 1494will never automatically add host keys to the 1495.Pa ~/.ssh/known_hosts 1496file, and refuses to connect to hosts whose host key has changed. 1497This provides maximum protection against man-in-the-middle (MITM) attacks, 1498though it can be annoying when the 1499.Pa /etc/ssh/ssh_known_hosts 1500file is poorly maintained or when connections to new hosts are 1501frequently made. 1502This option forces the user to manually 1503add all new hosts. 1504.Pp 1505If this flag is set to 1506.Dq accept-new 1507then ssh will automatically add new host keys to the user 1508known hosts files, but will not permit connections to hosts with 1509changed host keys. 1510If this flag is set to 1511.Dq no 1512or 1513.Dq off , 1514ssh will automatically add new host keys to the user known hosts files 1515and allow connections to hosts with changed hostkeys to proceed, 1516subject to some restrictions. 1517If this flag is set to 1518.Cm ask 1519(the default), 1520new host keys 1521will be added to the user known host files only after the user 1522has confirmed that is what they really want to do, and 1523ssh will refuse to connect to hosts whose host key has changed. 1524The host keys of 1525known hosts will be verified automatically in all cases. 1526.It Cm SyslogFacility 1527Gives the facility code that is used when logging messages from 1528.Xr ssh 1 . 1529The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1530LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1531The default is USER. 1532.It Cm TCPKeepAlive 1533Specifies whether the system should send TCP keepalive messages to the 1534other side. 1535If they are sent, death of the connection or crash of one 1536of the machines will be properly noticed. 1537However, this means that 1538connections will die if the route is down temporarily, and some people 1539find it annoying. 1540.Pp 1541The default is 1542.Cm yes 1543(to send TCP keepalive messages), and the client will notice 1544if the network goes down or the remote host dies. 1545This is important in scripts, and many users want it too. 1546.Pp 1547To disable TCP keepalive messages, the value should be set to 1548.Cm no . 1549See also 1550.Cm ServerAliveInterval 1551for protocol-level keepalives. 1552.It Cm Tunnel 1553Request 1554.Xr tun 4 1555device forwarding between the client and the server. 1556The argument must be 1557.Cm yes , 1558.Cm point-to-point 1559(layer 3), 1560.Cm ethernet 1561(layer 2), 1562or 1563.Cm no 1564(the default). 1565Specifying 1566.Cm yes 1567requests the default tunnel mode, which is 1568.Cm point-to-point . 1569.It Cm TunnelDevice 1570Specifies the 1571.Xr tun 4 1572devices to open on the client 1573.Pq Ar local_tun 1574and the server 1575.Pq Ar remote_tun . 1576.Pp 1577The argument must be 1578.Sm off 1579.Ar local_tun Op : Ar remote_tun . 1580.Sm on 1581The devices may be specified by numerical ID or the keyword 1582.Cm any , 1583which uses the next available tunnel device. 1584If 1585.Ar remote_tun 1586is not specified, it defaults to 1587.Cm any . 1588The default is 1589.Cm any:any . 1590.It Cm UpdateHostKeys 1591Specifies whether 1592.Xr ssh 1 1593should accept notifications of additional hostkeys from the server sent 1594after authentication has completed and add them to 1595.Cm UserKnownHostsFile . 1596The argument must be 1597.Cm yes , 1598.Cm no 1599(the default) or 1600.Cm ask . 1601Enabling this option allows learning alternate hostkeys for a server 1602and supports graceful key rotation by allowing a server to send replacement 1603public keys before old ones are removed. 1604Additional hostkeys are only accepted if the key used to authenticate the 1605host was already trusted or explicitly accepted by the user. 1606If 1607.Cm UpdateHostKeys 1608is set to 1609.Cm ask , 1610then the user is asked to confirm the modifications to the known_hosts file. 1611Confirmation is currently incompatible with 1612.Cm ControlPersist , 1613and will be disabled if it is enabled. 1614.Pp 1615Presently, only 1616.Xr sshd 8 1617from OpenSSH 6.8 and greater support the 1618.Qq hostkeys@openssh.com 1619protocol extension used to inform the client of all the server's hostkeys. 1620.It Cm User 1621Specifies the user to log in as. 1622This can be useful when a different user name is used on different machines. 1623This saves the trouble of 1624having to remember to give the user name on the command line. 1625.It Cm UserKnownHostsFile 1626Specifies one or more files to use for the user 1627host key database, separated by whitespace. 1628The default is 1629.Pa ~/.ssh/known_hosts , 1630.Pa ~/.ssh/known_hosts2 . 1631.It Cm VerifyHostKeyDNS 1632Specifies whether to verify the remote key using DNS and SSHFP resource 1633records. 1634If this option is set to 1635.Cm yes , 1636the client will implicitly trust keys that match a secure fingerprint 1637from DNS. 1638Insecure fingerprints will be handled as if this option was set to 1639.Cm ask . 1640If this option is set to 1641.Cm ask , 1642information on fingerprint match will be displayed, but the user will still 1643need to confirm new host keys according to the 1644.Cm StrictHostKeyChecking 1645option. 1646The default is 1647.Cm no . 1648.Pp 1649See also 1650.Sx VERIFYING HOST KEYS 1651in 1652.Xr ssh 1 . 1653.It Cm VisualHostKey 1654If this flag is set to 1655.Cm yes , 1656an ASCII art representation of the remote host key fingerprint is 1657printed in addition to the fingerprint string at login and 1658for unknown host keys. 1659If this flag is set to 1660.Cm no 1661(the default), 1662no fingerprint strings are printed at login and 1663only the fingerprint string will be printed for unknown host keys. 1664.It Cm XAuthLocation 1665Specifies the full pathname of the 1666.Xr xauth 1 1667program. 1668The default is 1669.Pa /usr/X11R6/bin/xauth . 1670.El 1671.Sh PATTERNS 1672A 1673.Em pattern 1674consists of zero or more non-whitespace characters, 1675.Sq * 1676(a wildcard that matches zero or more characters), 1677or 1678.Sq ?\& 1679(a wildcard that matches exactly one character). 1680For example, to specify a set of declarations for any host in the 1681.Qq .co.uk 1682set of domains, 1683the following pattern could be used: 1684.Pp 1685.Dl Host *.co.uk 1686.Pp 1687The following pattern 1688would match any host in the 192.168.0.[0-9] network range: 1689.Pp 1690.Dl Host 192.168.0.? 1691.Pp 1692A 1693.Em pattern-list 1694is a comma-separated list of patterns. 1695Patterns within pattern-lists may be negated 1696by preceding them with an exclamation mark 1697.Pq Sq !\& . 1698For example, 1699to allow a key to be used from anywhere within an organization 1700except from the 1701.Qq dialup 1702pool, 1703the following entry (in authorized_keys) could be used: 1704.Pp 1705.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1706.Pp 1707Note that a negated match will never produce a positive result by itself. 1708For example, attempting to match 1709.Qq host3 1710against the following pattern-list will fail: 1711.Pp 1712.Dl from=\&"!host1,!host2\&" 1713.Pp 1714The solution here is to include a term that will yield a positive match, 1715such as a wildcard: 1716.Pp 1717.Dl from=\&"!host1,!host2,*\&" 1718.Sh TOKENS 1719Arguments to some keywords can make use of tokens, 1720which are expanded at runtime: 1721.Pp 1722.Bl -tag -width XXXX -offset indent -compact 1723.It %% 1724A literal 1725.Sq % . 1726.It \&%C 1727Hash of %l%h%p%r. 1728.It %d 1729Local user's home directory. 1730.It %h 1731The remote hostname. 1732.It %i 1733The local user ID. 1734.It %L 1735The local hostname. 1736.It %l 1737The local hostname, including the domain name. 1738.It %n 1739The original remote hostname, as given on the command line. 1740.It %p 1741The remote port. 1742.It %r 1743The remote username. 1744.It \&%T 1745The local 1746.Xr tun 4 1747or 1748.Xr tap 4 1749network interface assigned if 1750tunnel forwarding was requested, or 1751.Qq NONE 1752otherwise. 1753.It %u 1754The local username. 1755.El 1756.Pp 1757.Cm Match exec 1758accepts the tokens %%, %h, %i, %L, %l, %n, %p, %r, and %u. 1759.Pp 1760.Cm CertificateFile 1761accepts the tokens %%, %d, %h, %i, %l, %r, and %u. 1762.Pp 1763.Cm ControlPath 1764accepts the tokens %%, %C, %h, %i, %L, %l, %n, %p, %r, and %u. 1765.Pp 1766.Cm HostName 1767accepts the tokens %% and %h. 1768.Pp 1769.Cm IdentityAgent 1770and 1771.Cm IdentityFile 1772accept the tokens %%, %d, %h, %i, %l, %r, and %u. 1773.Pp 1774.Cm LocalCommand 1775accepts the tokens %%, %C, %d, %h, %i, %l, %n, %p, %r, %T, and %u. 1776.Pp 1777.Cm ProxyCommand 1778accepts the tokens %%, %h, %p, and %r. 1779.Pp 1780.Cm RemoteCommand 1781accepts the tokens %%, %C, %d, %h, %i, %l, %n, %p, %r, and %u. 1782.Sh FILES 1783.Bl -tag -width Ds 1784.It Pa ~/.ssh/config 1785This is the per-user configuration file. 1786The format of this file is described above. 1787This file is used by the SSH client. 1788Because of the potential for abuse, this file must have strict permissions: 1789read/write for the user, and not accessible by others. 1790.It Pa /etc/ssh/ssh_config 1791Systemwide configuration file. 1792This file provides defaults for those 1793values that are not specified in the user's configuration file, and 1794for those users who do not have a configuration file. 1795This file must be world-readable. 1796.El 1797.Sh SEE ALSO 1798.Xr ssh 1 1799.Sh AUTHORS 1800.An -nosplit 1801OpenSSH is a derivative of the original and free 1802ssh 1.2.12 release by 1803.An Tatu Ylonen . 1804.An Aaron Campbell , Bob Beck , Markus Friedl , 1805.An Niels Provos , Theo de Raadt 1806and 1807.An Dug Song 1808removed many bugs, re-added newer features and 1809created OpenSSH. 1810.An Markus Friedl 1811contributed the support for SSH protocol versions 1.5 and 2.0. 1812