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.\" $FreeBSD$ 38.Dd $Mdocdate: October 3 2018 $ 39.Dt SSH_CONFIG 5 40.Os 41.Sh NAME 42.Nm ssh_config 43.Nd OpenSSH SSH client configuration files 44.Sh DESCRIPTION 45.Xr ssh 1 46obtains configuration data from the following sources in 47the following order: 48.Pp 49.Bl -enum -offset indent -compact 50.It 51command-line options 52.It 53user's configuration file 54.Pq Pa ~/.ssh/config 55.It 56system-wide configuration file 57.Pq Pa /etc/ssh/ssh_config 58.El 59.Pp 60For each parameter, the first obtained value 61will be used. 62The configuration files contain sections separated by 63.Cm Host 64specifications, and that section is only applied for hosts that 65match one of the patterns given in the specification. 66The matched host name is usually the one given on the command line 67(see the 68.Cm CanonicalizeHostname 69option for exceptions). 70.Pp 71Since the first obtained value for each parameter is used, more 72host-specific declarations should be given near the beginning of the 73file, and general defaults at the end. 74.Pp 75The file contains keyword-argument pairs, one per line. 76Lines starting with 77.Ql # 78and empty lines are interpreted as comments. 79Arguments may optionally be enclosed in double quotes 80.Pq \&" 81in order to represent arguments containing spaces. 82Configuration options may be separated by whitespace or 83optional whitespace and exactly one 84.Ql = ; 85the latter format is useful to avoid the need to quote whitespace 86when specifying configuration options using the 87.Nm ssh , 88.Nm scp , 89and 90.Nm sftp 91.Fl o 92option. 93.Pp 94The possible 95keywords and their meanings are as follows (note that 96keywords are case-insensitive and arguments are case-sensitive): 97.Bl -tag -width Ds 98.It Cm Host 99Restricts the following declarations (up to the next 100.Cm Host 101or 102.Cm Match 103keyword) to be only for those hosts that match one of the patterns 104given after the keyword. 105If more than one pattern is provided, they should be separated by whitespace. 106A single 107.Ql * 108as a pattern can be used to provide global 109defaults for all hosts. 110The host is usually the 111.Ar hostname 112argument given on the command line 113(see the 114.Cm CanonicalizeHostname 115keyword for exceptions). 116.Pp 117A pattern entry may be negated by prefixing it with an exclamation mark 118.Pq Sq !\& . 119If a negated entry is matched, then the 120.Cm Host 121entry is ignored, regardless of whether any other patterns on the line 122match. 123Negated matches are therefore useful to provide exceptions for wildcard 124matches. 125.Pp 126See 127.Sx PATTERNS 128for more information on patterns. 129.It Cm Match 130Restricts the following declarations (up to the next 131.Cm Host 132or 133.Cm Match 134keyword) to be used only when the conditions following the 135.Cm Match 136keyword are satisfied. 137Match conditions are specified using one or more criteria 138or the single token 139.Cm all 140which always matches. 141The available criteria keywords are: 142.Cm canonical , 143.Cm exec , 144.Cm host , 145.Cm originalhost , 146.Cm user , 147and 148.Cm localuser . 149The 150.Cm all 151criteria must appear alone or immediately after 152.Cm canonical . 153Other criteria may be combined arbitrarily. 154All criteria but 155.Cm all 156and 157.Cm canonical 158require an argument. 159Criteria may be negated by prepending an exclamation mark 160.Pq Sq !\& . 161.Pp 162The 163.Cm canonical 164keyword matches only when the configuration file is being re-parsed 165after hostname canonicalization (see the 166.Cm CanonicalizeHostname 167option.) 168This may be useful to specify conditions that work with canonical host 169names only. 170The 171.Cm exec 172keyword executes the specified command under the user's shell. 173If the command returns a zero exit status then the condition is considered true. 174Commands containing whitespace characters must be quoted. 175Arguments to 176.Cm exec 177accept the tokens described in the 178.Sx TOKENS 179section. 180.Pp 181The other keywords' criteria must be single entries or comma-separated 182lists and may use the wildcard and negation operators described in the 183.Sx PATTERNS 184section. 185The criteria for the 186.Cm host 187keyword are matched against the target hostname, after any substitution 188by the 189.Cm Hostname 190or 191.Cm CanonicalizeHostname 192options. 193The 194.Cm originalhost 195keyword matches against the hostname as it was specified on the command-line. 196The 197.Cm user 198keyword matches against the target username on the remote host. 199The 200.Cm localuser 201keyword matches against the name of the local user running 202.Xr ssh 1 203(this keyword may be useful in system-wide 204.Nm 205files). 206.It Cm AddKeysToAgent 207Specifies whether keys should be automatically added to a running 208.Xr ssh-agent 1 . 209If this option is set to 210.Cm yes 211and a key is loaded from a file, the key and its passphrase are added to 212the agent with the default lifetime, as if by 213.Xr ssh-add 1 . 214If this option is set to 215.Cm ask , 216.Xr ssh 1 217will require confirmation using the 218.Ev SSH_ASKPASS 219program before adding a key (see 220.Xr ssh-add 1 221for details). 222If this option is set to 223.Cm confirm , 224each use of the key must be confirmed, as if the 225.Fl c 226option was specified to 227.Xr ssh-add 1 . 228If this option is set to 229.Cm no , 230no keys are added to the agent. 231The argument must be 232.Cm yes , 233.Cm confirm , 234.Cm ask , 235or 236.Cm no 237(the default). 238.It Cm AddressFamily 239Specifies which address family to use when connecting. 240Valid arguments are 241.Cm any 242(the default), 243.Cm inet 244(use IPv4 only), or 245.Cm inet6 246(use IPv6 only). 247.It Cm BatchMode 248If set to 249.Cm yes , 250passphrase/password querying will be disabled. 251This option is useful in scripts and other batch jobs where no user 252is present to supply the password. 253The argument must be 254.Cm yes 255or 256.Cm no 257(the default). 258.It Cm BindAddress 259Use the specified address on the local machine as the source address of 260the connection. 261Only useful on systems with more than one address. 262.It Cm BindInterface 263Use the address of the specified interface on the local machine as the 264source address of the connection. 265.It Cm CanonicalDomains 266When 267.Cm CanonicalizeHostname 268is enabled, this option specifies the list of domain suffixes in which to 269search for the specified destination host. 270.It Cm CanonicalizeFallbackLocal 271Specifies whether to fail with an error when hostname canonicalization fails. 272The default, 273.Cm yes , 274will attempt to look up the unqualified hostname using the system resolver's 275search rules. 276A value of 277.Cm no 278will cause 279.Xr ssh 1 280to fail instantly if 281.Cm CanonicalizeHostname 282is enabled and the target hostname cannot be found in any of the domains 283specified by 284.Cm CanonicalDomains . 285.It Cm CanonicalizeHostname 286Controls whether explicit hostname canonicalization is performed. 287The default, 288.Cm no , 289is not to perform any name rewriting and let the system resolver handle all 290hostname lookups. 291If set to 292.Cm yes 293then, for connections that do not use a 294.Cm ProxyCommand 295or 296.Cm ProxyJump , 297.Xr ssh 1 298will attempt to canonicalize the hostname specified on the command line 299using the 300.Cm CanonicalDomains 301suffixes and 302.Cm CanonicalizePermittedCNAMEs 303rules. 304If 305.Cm CanonicalizeHostname 306is set to 307.Cm always , 308then canonicalization is applied to proxied connections too. 309.Pp 310If this option is enabled, then the configuration files are processed 311again using the new target name to pick up any new configuration in matching 312.Cm Host 313and 314.Cm Match 315stanzas. 316.It Cm CanonicalizeMaxDots 317Specifies the maximum number of dot characters in a hostname before 318canonicalization is disabled. 319The default, 1, 320allows a single dot (i.e. hostname.subdomain). 321.It Cm CanonicalizePermittedCNAMEs 322Specifies rules to determine whether CNAMEs should be followed when 323canonicalizing hostnames. 324The rules consist of one or more arguments of 325.Ar source_domain_list : Ns Ar target_domain_list , 326where 327.Ar source_domain_list 328is a pattern-list of domains that may follow CNAMEs in canonicalization, 329and 330.Ar target_domain_list 331is a pattern-list of domains that they may resolve to. 332.Pp 333For example, 334.Qq *.a.example.com:*.b.example.com,*.c.example.com 335will allow hostnames matching 336.Qq *.a.example.com 337to be canonicalized to names in the 338.Qq *.b.example.com 339or 340.Qq *.c.example.com 341domains. 342.It Cm CASignatureAlgorithms 343Specifies which algorithms are allowed for signing of certificates 344by certificate authorities (CAs). 345The default is: 346.Bd -literal -offset indent 347ecdsa-sha2-nistp256.ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 348ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 349.Ed 350.Pp 351.Xr ssh 1 352will not accept host certificates signed using algorithms other than those 353specified. 354.It Cm CertificateFile 355Specifies a file from which the user's certificate is read. 356A corresponding private key must be provided separately in order 357to use this certificate either 358from an 359.Cm IdentityFile 360directive or 361.Fl i 362flag to 363.Xr ssh 1 , 364via 365.Xr ssh-agent 1 , 366or via a 367.Cm PKCS11Provider . 368.Pp 369Arguments to 370.Cm CertificateFile 371may use the tilde syntax to refer to a user's home directory 372or the tokens described in the 373.Sx TOKENS 374section. 375.Pp 376It is possible to have multiple certificate files specified in 377configuration files; these certificates will be tried in sequence. 378Multiple 379.Cm CertificateFile 380directives will add to the list of certificates used for 381authentication. 382.It Cm ChallengeResponseAuthentication 383Specifies whether to use challenge-response authentication. 384The argument to this keyword must be 385.Cm yes 386(the default) 387or 388.Cm no . 389.It Cm CheckHostIP 390If set to 391.Cm yes , 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. 404The default is 405.Cm no . 406.It Cm Ciphers 407Specifies the ciphers allowed and their order of preference. 408Multiple ciphers must be comma-separated. 409If the specified value begins with a 410.Sq + 411character, then the specified ciphers will be appended to the default set 412instead of replacing them. 413If the specified value begins with a 414.Sq - 415character, then the specified ciphers (including wildcards) will be removed 416from the default set instead of replacing them. 417.Pp 418The supported ciphers are: 419.Bd -literal -offset indent 4203des-cbc 421aes128-cbc 422aes192-cbc 423aes256-cbc 424aes128-ctr 425aes192-ctr 426aes256-ctr 427aes128-gcm@openssh.com 428aes256-gcm@openssh.com 429chacha20-poly1305@openssh.com 430.Ed 431.Pp 432The default is: 433.Bd -literal -offset indent 434chacha20-poly1305@openssh.com, 435aes128-ctr,aes192-ctr,aes256-ctr, 436aes128-gcm@openssh.com,aes256-gcm@openssh.com 437.Ed 438.Pp 439The list of available ciphers may also be obtained using 440.Qq ssh -Q cipher . 441.It Cm ClearAllForwardings 442Specifies that all local, remote, and dynamic port forwardings 443specified in the configuration files or on the command line be 444cleared. 445This option is primarily useful when used from the 446.Xr ssh 1 447command line to clear port forwardings set in 448configuration files, and is automatically set by 449.Xr scp 1 450and 451.Xr sftp 1 . 452The argument must be 453.Cm yes 454or 455.Cm no 456(the default). 457.It Cm Compression 458Specifies whether to use compression. 459The argument must be 460.Cm yes 461or 462.Cm no 463(the default). 464.It Cm ConnectionAttempts 465Specifies the number of tries (one per second) to make before exiting. 466The argument must be an integer. 467This may be useful in scripts if the connection sometimes fails. 468The default is 1. 469.It Cm ConnectTimeout 470Specifies the timeout (in seconds) used when connecting to the 471SSH server, instead of using the default system TCP timeout. 472This value is used only when the target is down or really unreachable, 473not when it refuses the connection. 474.It Cm ControlMaster 475Enables the sharing of multiple sessions over a single network connection. 476When set to 477.Cm yes , 478.Xr ssh 1 479will listen for connections on a control socket specified using the 480.Cm ControlPath 481argument. 482Additional sessions can connect to this socket using the same 483.Cm ControlPath 484with 485.Cm ControlMaster 486set to 487.Cm no 488(the default). 489These sessions will try to reuse the master instance's network connection 490rather than initiating new ones, but will fall back to connecting normally 491if the control socket does not exist, or is not listening. 492.Pp 493Setting this to 494.Cm ask 495will cause 496.Xr ssh 1 497to listen for control connections, but require confirmation using 498.Xr ssh-askpass 1 . 499If the 500.Cm ControlPath 501cannot be opened, 502.Xr ssh 1 503will continue without connecting to a master instance. 504.Pp 505X11 and 506.Xr ssh-agent 1 507forwarding is supported over these multiplexed connections, however the 508display and agent forwarded will be the one belonging to the master 509connection i.e. it is not possible to forward multiple displays or agents. 510.Pp 511Two additional options allow for opportunistic multiplexing: try to use a 512master connection but fall back to creating a new one if one does not already 513exist. 514These options are: 515.Cm auto 516and 517.Cm autoask . 518The latter requires confirmation like the 519.Cm ask 520option. 521.It Cm ControlPath 522Specify the path to the control socket used for connection sharing as described 523in the 524.Cm ControlMaster 525section above or the string 526.Cm none 527to disable connection sharing. 528Arguments to 529.Cm ControlPath 530may use the tilde syntax to refer to a user's home directory 531or the tokens described in the 532.Sx TOKENS 533section. 534It is recommended that any 535.Cm ControlPath 536used for opportunistic connection sharing include 537at least %h, %p, and %r (or alternatively %C) and be placed in a directory 538that is not writable by other users. 539This ensures that shared connections are uniquely identified. 540.It Cm ControlPersist 541When used in conjunction with 542.Cm ControlMaster , 543specifies that the master connection should remain open 544in the background (waiting for future client connections) 545after the initial client connection has been closed. 546If set to 547.Cm no , 548then the master connection will not be placed into the background, 549and will close as soon as the initial client connection is closed. 550If set to 551.Cm yes 552or 0, 553then the master connection will remain in the background indefinitely 554(until killed or closed via a mechanism such as the 555.Qq ssh -O exit ) . 556If set to a time in seconds, or a time in any of the formats documented in 557.Xr sshd_config 5 , 558then the backgrounded master connection will automatically terminate 559after it has remained idle (with no client connections) for the 560specified time. 561.It Cm DynamicForward 562Specifies that a TCP port on the local machine be forwarded 563over the secure channel, and the application 564protocol is then used to determine where to connect to from the 565remote machine. 566.Pp 567The argument must be 568.Sm off 569.Oo Ar bind_address : Oc Ar port . 570.Sm on 571IPv6 addresses can be specified by enclosing addresses in square brackets. 572By default, the local port is bound in accordance with the 573.Cm GatewayPorts 574setting. 575However, an explicit 576.Ar bind_address 577may be used to bind the connection to a specific address. 578The 579.Ar bind_address 580of 581.Cm localhost 582indicates that the listening port be bound for local use only, while an 583empty address or 584.Sq * 585indicates that the port should be available from all interfaces. 586.Pp 587Currently the SOCKS4 and SOCKS5 protocols are supported, and 588.Xr ssh 1 589will act as a SOCKS server. 590Multiple forwardings may be specified, and 591additional forwardings can be given on the command line. 592Only the superuser can forward privileged ports. 593.It Cm EnableSSHKeysign 594Setting this option to 595.Cm yes 596in the global client configuration file 597.Pa /etc/ssh/ssh_config 598enables the use of the helper program 599.Xr ssh-keysign 8 600during 601.Cm HostbasedAuthentication . 602The argument must be 603.Cm yes 604or 605.Cm no 606(the default). 607This option should be placed in the non-hostspecific section. 608See 609.Xr ssh-keysign 8 610for more information. 611.It Cm EscapeChar 612Sets the escape character (default: 613.Ql ~ ) . 614The escape character can also 615be set on the command line. 616The argument should be a single character, 617.Ql ^ 618followed by a letter, or 619.Cm none 620to disable the escape 621character entirely (making the connection transparent for binary 622data). 623.It Cm ExitOnForwardFailure 624Specifies whether 625.Xr ssh 1 626should terminate the connection if it cannot set up all requested 627dynamic, tunnel, local, and remote port forwardings, (e.g.\& 628if either end is unable to bind and listen on a specified port). 629Note that 630.Cm ExitOnForwardFailure 631does not apply to connections made over port forwardings and will not, 632for example, cause 633.Xr ssh 1 634to exit if TCP connections to the ultimate forwarding destination fail. 635The argument must be 636.Cm yes 637or 638.Cm no 639(the default). 640.It Cm FingerprintHash 641Specifies the hash algorithm used when displaying key fingerprints. 642Valid options are: 643.Cm md5 644and 645.Cm sha256 646(the default). 647.It Cm ForwardAgent 648Specifies whether the connection to the authentication agent (if any) 649will be forwarded to the remote machine. 650The argument must be 651.Cm yes 652or 653.Cm no 654(the default). 655.Pp 656Agent forwarding should be enabled with caution. 657Users with the ability to bypass file permissions on the remote host 658(for the agent's Unix-domain socket) 659can access the local agent through the forwarded connection. 660An attacker cannot obtain key material from the agent, 661however they can perform operations on the keys that enable them to 662authenticate using the identities loaded into the agent. 663.It Cm ForwardX11 664Specifies whether X11 connections will be automatically redirected 665over the secure channel and 666.Ev DISPLAY 667set. 668The argument must be 669.Cm yes 670or 671.Cm no 672(the default). 673.Pp 674X11 forwarding should be enabled with caution. 675Users with the ability to bypass file permissions on the remote host 676(for the user's X11 authorization database) 677can access the local X11 display through the forwarded connection. 678An attacker may then be able to perform activities such as keystroke monitoring 679if the 680.Cm ForwardX11Trusted 681option is also enabled. 682.It Cm ForwardX11Timeout 683Specify a timeout for untrusted X11 forwarding 684using the format described in the 685.Sx TIME FORMATS 686section of 687.Xr sshd_config 5 . 688X11 connections received by 689.Xr ssh 1 690after this time will be refused. 691Setting 692.Cm ForwardX11Timeout 693to zero will disable the timeout and permit X11 forwarding for the life 694of the connection. 695The default is to disable untrusted X11 forwarding after twenty minutes has 696elapsed. 697.It Cm ForwardX11Trusted 698If this option is set to 699.Cm yes , 700remote X11 clients will have full access to the original X11 display. 701.Pp 702If this option is set to 703.Cm no 704(the default), 705remote X11 clients will be considered untrusted and prevented 706from stealing or tampering with data belonging to trusted X11 707clients. 708Furthermore, the 709.Xr xauth 1 710token used for the session will be set to expire after 20 minutes. 711Remote clients will be refused access after this time. 712.Pp 713See the X11 SECURITY extension specification for full details on 714the restrictions imposed on untrusted clients. 715.It Cm GatewayPorts 716Specifies whether remote hosts are allowed to connect to local 717forwarded ports. 718By default, 719.Xr ssh 1 720binds local port forwardings to the loopback address. 721This prevents other remote hosts from connecting to forwarded ports. 722.Cm GatewayPorts 723can be used to specify that ssh 724should bind local port forwardings to the wildcard address, 725thus allowing remote hosts to connect to forwarded ports. 726The argument must be 727.Cm yes 728or 729.Cm no 730(the default). 731.It Cm GlobalKnownHostsFile 732Specifies one or more files to use for the global 733host key database, separated by whitespace. 734The default is 735.Pa /etc/ssh/ssh_known_hosts , 736.Pa /etc/ssh/ssh_known_hosts2 . 737.It Cm GSSAPIAuthentication 738Specifies whether user authentication based on GSSAPI is allowed. 739The default is 740.Cm no . 741.It Cm GSSAPIDelegateCredentials 742Forward (delegate) credentials to the server. 743The default is 744.Cm no . 745.It Cm HashKnownHosts 746Indicates that 747.Xr ssh 1 748should hash host names and addresses when they are added to 749.Pa ~/.ssh/known_hosts . 750These hashed names may be used normally by 751.Xr ssh 1 752and 753.Xr sshd 8 , 754but they do not reveal identifying information should the file's contents 755be disclosed. 756The default is 757.Cm no . 758Note that existing names and addresses in known hosts files 759will not be converted automatically, 760but may be manually hashed using 761.Xr ssh-keygen 1 . 762.It Cm HostbasedAuthentication 763Specifies whether to try rhosts based authentication with public key 764authentication. 765The argument must be 766.Cm yes 767or 768.Cm no 769(the default). 770.It Cm HostbasedKeyTypes 771Specifies the key types that will be used for hostbased authentication 772as a comma-separated list of patterns. 773Alternately if the specified value begins with a 774.Sq + 775character, then the specified key types will be appended to the default set 776instead of replacing them. 777If the specified value begins with a 778.Sq - 779character, then the specified key types (including wildcards) will be removed 780from the default set instead of replacing them. 781The default for this option is: 782.Bd -literal -offset 3n 783ecdsa-sha2-nistp256-cert-v01@openssh.com, 784ecdsa-sha2-nistp384-cert-v01@openssh.com, 785ecdsa-sha2-nistp521-cert-v01@openssh.com, 786ssh-ed25519-cert-v01@openssh.com, 787rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com, 788ssh-rsa-cert-v01@openssh.com, 789ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 790ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 791.Ed 792.Pp 793The 794.Fl Q 795option of 796.Xr ssh 1 797may be used to list supported key types. 798.It Cm HostKeyAlgorithms 799Specifies the host key algorithms 800that the client wants to use in order of preference. 801Alternately if the specified value begins with a 802.Sq + 803character, then the specified key types will be appended to the default set 804instead of replacing them. 805If the specified value begins with a 806.Sq - 807character, then the specified key types (including wildcards) will be removed 808from the default set instead of replacing them. 809The default for this option is: 810.Bd -literal -offset 3n 811ecdsa-sha2-nistp256-cert-v01@openssh.com, 812ecdsa-sha2-nistp384-cert-v01@openssh.com, 813ecdsa-sha2-nistp521-cert-v01@openssh.com, 814ssh-ed25519-cert-v01@openssh.com, 815rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com, 816ssh-rsa-cert-v01@openssh.com, 817ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 818ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 819.Ed 820.Pp 821If hostkeys are known for the destination host then this default is modified 822to prefer their algorithms. 823.Pp 824The list of available key types may also be obtained using 825.Qq ssh -Q key . 826.It Cm HostKeyAlias 827Specifies an alias that should be used instead of the 828real host name when looking up or saving the host key 829in the host key database files and when validating host certificates. 830This option is useful for tunneling SSH connections 831or for multiple servers running on a single host. 832.It Cm HostName 833Specifies the real host name to log into. 834This can be used to specify nicknames or abbreviations for hosts. 835Arguments to 836.Cm HostName 837accept the tokens described in the 838.Sx TOKENS 839section. 840Numeric IP addresses are also permitted (both on the command line and in 841.Cm HostName 842specifications). 843The default is the name given on the command line. 844.It Cm IdentitiesOnly 845Specifies that 846.Xr ssh 1 847should only use the authentication identity and certificate files explicitly 848configured in the 849.Nm 850files 851or passed on the 852.Xr ssh 1 853command-line, 854even if 855.Xr ssh-agent 1 856or a 857.Cm PKCS11Provider 858offers more identities. 859The argument to this keyword must be 860.Cm yes 861or 862.Cm no 863(the default). 864This option is intended for situations where ssh-agent 865offers many different identities. 866.It Cm IdentityAgent 867Specifies the 868.Ux Ns -domain 869socket used to communicate with the authentication agent. 870.Pp 871This option overrides the 872.Ev SSH_AUTH_SOCK 873environment variable and can be used to select a specific agent. 874Setting the socket name to 875.Cm none 876disables the use of an authentication agent. 877If the string 878.Qq SSH_AUTH_SOCK 879is specified, the location of the socket will be read from the 880.Ev SSH_AUTH_SOCK 881environment variable. 882Otherwise if the specified value begins with a 883.Sq $ 884character, then it will be treated as an environment variable containing 885the location of the socket. 886.Pp 887Arguments to 888.Cm IdentityAgent 889may use the tilde syntax to refer to a user's home directory 890or the tokens described in the 891.Sx TOKENS 892section. 893.It Cm IdentityFile 894Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication 895identity is read. 896The default is 897.Pa ~/.ssh/id_dsa , 898.Pa ~/.ssh/id_ecdsa , 899.Pa ~/.ssh/id_ed25519 900and 901.Pa ~/.ssh/id_rsa . 902Additionally, any identities represented by the authentication agent 903will be used for authentication unless 904.Cm IdentitiesOnly 905is set. 906If no certificates have been explicitly specified by 907.Cm CertificateFile , 908.Xr ssh 1 909will try to load certificate information from the filename obtained by 910appending 911.Pa -cert.pub 912to the path of a specified 913.Cm IdentityFile . 914.Pp 915Arguments to 916.Cm IdentityFile 917may use the tilde syntax to refer to a user's home directory 918or the tokens described in the 919.Sx TOKENS 920section. 921.Pp 922It is possible to have 923multiple identity files specified in configuration files; all these 924identities will be tried in sequence. 925Multiple 926.Cm IdentityFile 927directives will add to the list of identities tried (this behaviour 928differs from that of other configuration directives). 929.Pp 930.Cm IdentityFile 931may be used in conjunction with 932.Cm IdentitiesOnly 933to select which identities in an agent are offered during authentication. 934.Cm IdentityFile 935may also be used in conjunction with 936.Cm CertificateFile 937in order to provide any certificate also needed for authentication with 938the identity. 939.It Cm IgnoreUnknown 940Specifies a pattern-list of unknown options to be ignored if they are 941encountered in configuration parsing. 942This may be used to suppress errors if 943.Nm 944contains options that are unrecognised by 945.Xr ssh 1 . 946It is recommended that 947.Cm IgnoreUnknown 948be listed early in the configuration file as it will not be applied 949to unknown options that appear before it. 950.It Cm Include 951Include the specified configuration file(s). 952Multiple pathnames may be specified and each pathname may contain 953.Xr glob 7 954wildcards and, for user configurations, shell-like 955.Sq ~ 956references to user home directories. 957Files without absolute paths are assumed to be in 958.Pa ~/.ssh 959if included in a user configuration file or 960.Pa /etc/ssh 961if included from the system configuration file. 962.Cm Include 963directive may appear inside a 964.Cm Match 965or 966.Cm Host 967block 968to perform conditional inclusion. 969.It Cm IPQoS 970Specifies the IPv4 type-of-service or DSCP class for connections. 971Accepted values are 972.Cm af11 , 973.Cm af12 , 974.Cm af13 , 975.Cm af21 , 976.Cm af22 , 977.Cm af23 , 978.Cm af31 , 979.Cm af32 , 980.Cm af33 , 981.Cm af41 , 982.Cm af42 , 983.Cm af43 , 984.Cm cs0 , 985.Cm cs1 , 986.Cm cs2 , 987.Cm cs3 , 988.Cm cs4 , 989.Cm cs5 , 990.Cm cs6 , 991.Cm cs7 , 992.Cm ef , 993.Cm lowdelay , 994.Cm throughput , 995.Cm reliability , 996a numeric value, or 997.Cm none 998to use the operating system default. 999This option may take one or two arguments, separated by whitespace. 1000If one argument is specified, it is used as the packet class unconditionally. 1001If two values are specified, the first is automatically selected for 1002interactive sessions and the second for non-interactive sessions. 1003The default is 1004.Cm af21 1005(Low-Latency Data) 1006for interactive sessions and 1007.Cm cs1 1008(Lower Effort) 1009for non-interactive sessions. 1010.It Cm KbdInteractiveAuthentication 1011Specifies whether to use keyboard-interactive authentication. 1012The argument to this keyword must be 1013.Cm yes 1014(the default) 1015or 1016.Cm no . 1017.It Cm KbdInteractiveDevices 1018Specifies the list of methods to use in keyboard-interactive authentication. 1019Multiple method names must be comma-separated. 1020The default is to use the server specified list. 1021The methods available vary depending on what the server supports. 1022For an OpenSSH server, 1023it may be zero or more of: 1024.Cm bsdauth 1025and 1026.Cm pam . 1027.It Cm KexAlgorithms 1028Specifies the available KEX (Key Exchange) algorithms. 1029Multiple algorithms must be comma-separated. 1030Alternately if the specified value begins with a 1031.Sq + 1032character, then the specified methods will be appended to the default set 1033instead of replacing them. 1034If the specified value begins with a 1035.Sq - 1036character, then the specified methods (including wildcards) will be removed 1037from the default set instead of replacing them. 1038The default is: 1039.Bd -literal -offset indent 1040curve25519-sha256,curve25519-sha256@libssh.org, 1041ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1042diffie-hellman-group-exchange-sha256, 1043diffie-hellman-group16-sha512, 1044diffie-hellman-group18-sha512, 1045diffie-hellman-group-exchange-sha1, 1046diffie-hellman-group14-sha256, 1047diffie-hellman-group14-sha1 1048.Ed 1049.Pp 1050The list of available key exchange algorithms may also be obtained using 1051.Qq ssh -Q kex . 1052.It Cm LocalCommand 1053Specifies a command to execute on the local machine after successfully 1054connecting to the server. 1055The command string extends to the end of the line, and is executed with 1056the user's shell. 1057Arguments to 1058.Cm LocalCommand 1059accept the tokens described in the 1060.Sx TOKENS 1061section. 1062.Pp 1063The command is run synchronously and does not have access to the 1064session of the 1065.Xr ssh 1 1066that spawned it. 1067It should not be used for interactive commands. 1068.Pp 1069This directive is ignored unless 1070.Cm PermitLocalCommand 1071has been enabled. 1072.It Cm LocalForward 1073Specifies that a TCP port on the local machine be forwarded over 1074the secure channel to the specified host and port from the remote machine. 1075The first argument must be 1076.Sm off 1077.Oo Ar bind_address : Oc Ar port 1078.Sm on 1079and the second argument must be 1080.Ar host : Ns Ar hostport . 1081IPv6 addresses can be specified by enclosing addresses in square brackets. 1082Multiple forwardings may be specified, and additional forwardings can be 1083given on the command line. 1084Only the superuser can forward privileged ports. 1085By default, the local port is bound in accordance with the 1086.Cm GatewayPorts 1087setting. 1088However, an explicit 1089.Ar bind_address 1090may be used to bind the connection to a specific address. 1091The 1092.Ar bind_address 1093of 1094.Cm localhost 1095indicates that the listening port be bound for local use only, while an 1096empty address or 1097.Sq * 1098indicates that the port should be available from all interfaces. 1099.It Cm LogLevel 1100Gives the verbosity level that is used when logging messages from 1101.Xr ssh 1 . 1102The possible values are: 1103QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1104The default is INFO. 1105DEBUG and DEBUG1 are equivalent. 1106DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1107.It Cm MACs 1108Specifies the MAC (message authentication code) algorithms 1109in order of preference. 1110The MAC algorithm is used for data integrity protection. 1111Multiple algorithms must be comma-separated. 1112If the specified value begins with a 1113.Sq + 1114character, then the specified algorithms will be appended to the default set 1115instead of replacing them. 1116If the specified value begins with a 1117.Sq - 1118character, then the specified algorithms (including wildcards) will be removed 1119from the default set instead of replacing them. 1120.Pp 1121The algorithms that contain 1122.Qq -etm 1123calculate the MAC after encryption (encrypt-then-mac). 1124These are considered safer and their use recommended. 1125.Pp 1126The default is: 1127.Bd -literal -offset indent 1128umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1129hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1130hmac-sha1-etm@openssh.com, 1131umac-64@openssh.com,umac-128@openssh.com, 1132hmac-sha2-256,hmac-sha2-512,hmac-sha1 1133.Ed 1134.Pp 1135The list of available MAC algorithms may also be obtained using 1136.Qq ssh -Q mac . 1137.It Cm NoHostAuthenticationForLocalhost 1138Disable host authentication for localhost (loopback addresses). 1139The argument to this keyword must be 1140.Cm yes 1141or 1142.Cm no 1143(the default). 1144.It Cm NumberOfPasswordPrompts 1145Specifies the number of password prompts before giving up. 1146The argument to this keyword must be an integer. 1147The default is 3. 1148.It Cm PasswordAuthentication 1149Specifies whether to use password authentication. 1150The argument to this keyword must be 1151.Cm yes 1152(the default) 1153or 1154.Cm no . 1155.It Cm PermitLocalCommand 1156Allow local command execution via the 1157.Ic LocalCommand 1158option or using the 1159.Ic !\& Ns Ar command 1160escape sequence in 1161.Xr ssh 1 . 1162The argument must be 1163.Cm yes 1164or 1165.Cm no 1166(the default). 1167.It Cm PKCS11Provider 1168Specifies which PKCS#11 provider to use. 1169The argument to this keyword is the PKCS#11 shared library 1170.Xr ssh 1 1171should use to communicate with a PKCS#11 token providing the user's 1172private RSA key. 1173.It Cm Port 1174Specifies the port number to connect on the remote host. 1175The default is 22. 1176.It Cm PreferredAuthentications 1177Specifies the order in which the client should try authentication methods. 1178This allows a client to prefer one method (e.g.\& 1179.Cm keyboard-interactive ) 1180over another method (e.g.\& 1181.Cm password ) . 1182The default is: 1183.Bd -literal -offset indent 1184gssapi-with-mic,hostbased,publickey, 1185keyboard-interactive,password 1186.Ed 1187.It Cm ProxyCommand 1188Specifies the command to use to connect to the server. 1189The command 1190string extends to the end of the line, and is executed 1191using the user's shell 1192.Ql exec 1193directive to avoid a lingering shell process. 1194.Pp 1195Arguments to 1196.Cm ProxyCommand 1197accept the tokens described in the 1198.Sx TOKENS 1199section. 1200The command can be basically anything, 1201and should read from its standard input and write to its standard output. 1202It should eventually connect an 1203.Xr sshd 8 1204server running on some machine, or execute 1205.Ic sshd -i 1206somewhere. 1207Host key management will be done using the 1208HostName of the host being connected (defaulting to the name typed by 1209the user). 1210Setting the command to 1211.Cm none 1212disables this option entirely. 1213Note that 1214.Cm CheckHostIP 1215is not available for connects with a proxy command. 1216.Pp 1217This directive is useful in conjunction with 1218.Xr nc 1 1219and its proxy support. 1220For example, the following directive would connect via an HTTP proxy at 1221192.0.2.0: 1222.Bd -literal -offset 3n 1223ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1224.Ed 1225.It Cm ProxyJump 1226Specifies one or more jump proxies as either 1227.Xo 1228.Sm off 1229.Op Ar user No @ 1230.Ar host 1231.Op : Ns Ar port 1232.Sm on 1233or an ssh URI 1234.Xc . 1235Multiple proxies may be separated by comma characters and will be visited 1236sequentially. 1237Setting this option will cause 1238.Xr ssh 1 1239to connect to the target host by first making a 1240.Xr ssh 1 1241connection to the specified 1242.Cm ProxyJump 1243host and then establishing a 1244TCP forwarding to the ultimate target from there. 1245.Pp 1246Note that this option will compete with the 1247.Cm ProxyCommand 1248option - whichever is specified first will prevent later instances of the 1249other from taking effect. 1250.It Cm ProxyUseFdpass 1251Specifies that 1252.Cm ProxyCommand 1253will pass a connected file descriptor back to 1254.Xr ssh 1 1255instead of continuing to execute and pass data. 1256The default is 1257.Cm no . 1258.It Cm PubkeyAcceptedKeyTypes 1259Specifies the key types that will be used for public key authentication 1260as a comma-separated list of patterns. 1261Alternately if the specified value begins with a 1262.Sq + 1263character, then the key types after it will be appended to the default 1264instead of replacing it. 1265If the specified value begins with a 1266.Sq - 1267character, then the specified key types (including wildcards) will be removed 1268from the default set instead of replacing them. 1269The default for this option is: 1270.Bd -literal -offset 3n 1271ecdsa-sha2-nistp256-cert-v01@openssh.com, 1272ecdsa-sha2-nistp384-cert-v01@openssh.com, 1273ecdsa-sha2-nistp521-cert-v01@openssh.com, 1274ssh-ed25519-cert-v01@openssh.com, 1275rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com, 1276ssh-rsa-cert-v01@openssh.com, 1277ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1278ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 1279.Ed 1280.Pp 1281The list of available key types may also be obtained using 1282.Qq ssh -Q key . 1283.It Cm PubkeyAuthentication 1284Specifies whether to try public key authentication. 1285The argument to this keyword must be 1286.Cm yes 1287(the default) 1288or 1289.Cm no . 1290.It Cm RekeyLimit 1291Specifies the maximum amount of data that may be transmitted before the 1292session key is renegotiated, optionally followed a maximum amount of 1293time that may pass before the session key is renegotiated. 1294The first argument is specified in bytes and may have a suffix of 1295.Sq K , 1296.Sq M , 1297or 1298.Sq G 1299to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1300The default is between 1301.Sq 1G 1302and 1303.Sq 4G , 1304depending on the cipher. 1305The optional second value is specified in seconds and may use any of the 1306units documented in the 1307.Sx TIME FORMATS 1308section of 1309.Xr sshd_config 5 . 1310The default value for 1311.Cm RekeyLimit 1312is 1313.Cm default none , 1314which means that rekeying is performed after the cipher's default amount 1315of data has been sent or received and no time based rekeying is done. 1316.It Cm RemoteCommand 1317Specifies a command to execute on the remote machine after successfully 1318connecting to the server. 1319The command string extends to the end of the line, and is executed with 1320the user's shell. 1321Arguments to 1322.Cm RemoteCommand 1323accept the tokens described in the 1324.Sx TOKENS 1325section. 1326.It Cm RemoteForward 1327Specifies that a TCP port on the remote machine be forwarded over 1328the secure channel. 1329The remote port may either be forwarded to a specified host and port 1330from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote 1331client to connect to arbitrary destinations from the local machine. 1332The first argument must be 1333.Sm off 1334.Oo Ar bind_address : Oc Ar port 1335.Sm on 1336If forwarding to a specific destination then the second argument must be 1337.Ar host : Ns Ar hostport , 1338otherwise if no destination argument is specified then the remote forwarding 1339will be established as a SOCKS proxy. 1340.Pp 1341IPv6 addresses can be specified by enclosing addresses in square brackets. 1342Multiple forwardings may be specified, and additional 1343forwardings can be given on the command line. 1344Privileged ports can be forwarded only when 1345logging in as root on the remote machine. 1346.Pp 1347If the 1348.Ar port 1349argument is 0, 1350the listen port will be dynamically allocated on the server and reported 1351to the client at run time. 1352.Pp 1353If the 1354.Ar bind_address 1355is not specified, the default is to only bind to loopback addresses. 1356If the 1357.Ar bind_address 1358is 1359.Ql * 1360or an empty string, then the forwarding is requested to listen on all 1361interfaces. 1362Specifying a remote 1363.Ar bind_address 1364will only succeed if the server's 1365.Cm GatewayPorts 1366option is enabled (see 1367.Xr sshd_config 5 ) . 1368.It Cm RequestTTY 1369Specifies whether to request a pseudo-tty for the session. 1370The argument may be one of: 1371.Cm no 1372(never request a TTY), 1373.Cm yes 1374(always request a TTY when standard input is a TTY), 1375.Cm force 1376(always request a TTY) or 1377.Cm auto 1378(request a TTY when opening a login session). 1379This option mirrors the 1380.Fl t 1381and 1382.Fl T 1383flags for 1384.Xr ssh 1 . 1385.It Cm RevokedHostKeys 1386Specifies revoked host public keys. 1387Keys listed in this file will be refused for host authentication. 1388Note that if this file does not exist or is not readable, 1389then host authentication will be refused for all hosts. 1390Keys may be specified as a text file, listing one public key per line, or as 1391an OpenSSH Key Revocation List (KRL) as generated by 1392.Xr ssh-keygen 1 . 1393For more information on KRLs, see the KEY REVOCATION LISTS section in 1394.Xr ssh-keygen 1 . 1395.It Cm SendEnv 1396Specifies what variables from the local 1397.Xr environ 7 1398should be sent to the server. 1399The server must also support it, and the server must be configured to 1400accept these environment variables. 1401Note that the 1402.Ev TERM 1403environment variable is always sent whenever a 1404pseudo-terminal is requested as it is required by the protocol. 1405Refer to 1406.Cm AcceptEnv 1407in 1408.Xr sshd_config 5 1409for how to configure the server. 1410Variables are specified by name, which may contain wildcard characters. 1411Multiple environment variables may be separated by whitespace or spread 1412across multiple 1413.Cm SendEnv 1414directives. 1415.Pp 1416See 1417.Sx PATTERNS 1418for more information on patterns. 1419.Pp 1420It is possible to clear previously set 1421.Cm SendEnv 1422variable names by prefixing patterns with 1423.Pa - . 1424The default is not to send any environment variables. 1425.It Cm ServerAliveCountMax 1426Sets the number of server alive messages (see below) which may be 1427sent without 1428.Xr ssh 1 1429receiving any messages back from the server. 1430If this threshold is reached while server alive messages are being sent, 1431ssh will disconnect from the server, terminating the session. 1432It is important to note that the use of server alive messages is very 1433different from 1434.Cm TCPKeepAlive 1435(below). 1436The server alive messages are sent through the encrypted channel 1437and therefore will not be spoofable. 1438The TCP keepalive option enabled by 1439.Cm TCPKeepAlive 1440is spoofable. 1441The server alive mechanism is valuable when the client or 1442server depend on knowing when a connection has become inactive. 1443.Pp 1444The default value is 3. 1445If, for example, 1446.Cm ServerAliveInterval 1447(see below) is set to 15 and 1448.Cm ServerAliveCountMax 1449is left at the default, if the server becomes unresponsive, 1450ssh will disconnect after approximately 45 seconds. 1451.It Cm ServerAliveInterval 1452Sets a timeout interval in seconds after which if no data has been received 1453from the server, 1454.Xr ssh 1 1455will send a message through the encrypted 1456channel to request a response from the server. 1457The default 1458is 0, indicating that these messages will not be sent to the server. 1459.It Cm SetEnv 1460Directly specify one or more environment variables and their contents to 1461be sent to the server. 1462Similarly to 1463.Cm SendEnv , 1464the server must be prepared to accept the environment variable. 1465.It Cm StreamLocalBindMask 1466Sets the octal file creation mode mask 1467.Pq umask 1468used when creating a Unix-domain socket file for local or remote 1469port forwarding. 1470This option is only used for port forwarding to a Unix-domain socket file. 1471.Pp 1472The default value is 0177, which creates a Unix-domain socket file that is 1473readable and writable only by the owner. 1474Note that not all operating systems honor the file mode on Unix-domain 1475socket files. 1476.It Cm StreamLocalBindUnlink 1477Specifies whether to remove an existing Unix-domain socket file for local 1478or remote port forwarding before creating a new one. 1479If the socket file already exists and 1480.Cm StreamLocalBindUnlink 1481is not enabled, 1482.Nm ssh 1483will be unable to forward the port to the Unix-domain socket file. 1484This option is only used for port forwarding to a Unix-domain socket file. 1485.Pp 1486The argument must be 1487.Cm yes 1488or 1489.Cm no 1490(the default). 1491.It Cm StrictHostKeyChecking 1492If this flag is set to 1493.Cm yes , 1494.Xr ssh 1 1495will never automatically add host keys to the 1496.Pa ~/.ssh/known_hosts 1497file, and refuses to connect to hosts whose host key has changed. 1498This provides maximum protection against man-in-the-middle (MITM) attacks, 1499though it can be annoying when the 1500.Pa /etc/ssh/ssh_known_hosts 1501file is poorly maintained or when connections to new hosts are 1502frequently made. 1503This option forces the user to manually 1504add all new hosts. 1505.Pp 1506If this flag is set to 1507.Dq accept-new 1508then ssh will automatically add new host keys to the user 1509known hosts files, but will not permit connections to hosts with 1510changed host keys. 1511If this flag is set to 1512.Dq no 1513or 1514.Dq off , 1515ssh will automatically add new host keys to the user known hosts files 1516and allow connections to hosts with changed hostkeys to proceed, 1517subject to some restrictions. 1518If this flag is set to 1519.Cm ask 1520(the default), 1521new host keys 1522will be added to the user known host files only after the user 1523has confirmed that is what they really want to do, and 1524ssh will refuse to connect to hosts whose host key has changed. 1525The host keys of 1526known hosts will be verified automatically in all cases. 1527.It Cm SyslogFacility 1528Gives the facility code that is used when logging messages from 1529.Xr ssh 1 . 1530The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1531LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1532The default is USER. 1533.It Cm TCPKeepAlive 1534Specifies whether the system should send TCP keepalive messages to the 1535other side. 1536If they are sent, death of the connection or crash of one 1537of the machines will be properly noticed. 1538However, this means that 1539connections will die if the route is down temporarily, and some people 1540find it annoying. 1541.Pp 1542The default is 1543.Cm yes 1544(to send TCP keepalive messages), and the client will notice 1545if the network goes down or the remote host dies. 1546This is important in scripts, and many users want it too. 1547.Pp 1548To disable TCP keepalive messages, the value should be set to 1549.Cm no . 1550See also 1551.Cm ServerAliveInterval 1552for protocol-level keepalives. 1553.It Cm Tunnel 1554Request 1555.Xr tun 4 1556device forwarding between the client and the server. 1557The argument must be 1558.Cm yes , 1559.Cm point-to-point 1560(layer 3), 1561.Cm ethernet 1562(layer 2), 1563or 1564.Cm no 1565(the default). 1566Specifying 1567.Cm yes 1568requests the default tunnel mode, which is 1569.Cm point-to-point . 1570.It Cm TunnelDevice 1571Specifies the 1572.Xr tun 4 1573devices to open on the client 1574.Pq Ar local_tun 1575and the server 1576.Pq Ar remote_tun . 1577.Pp 1578The argument must be 1579.Sm off 1580.Ar local_tun Op : Ar remote_tun . 1581.Sm on 1582The devices may be specified by numerical ID or the keyword 1583.Cm any , 1584which uses the next available tunnel device. 1585If 1586.Ar remote_tun 1587is not specified, it defaults to 1588.Cm any . 1589The default is 1590.Cm any:any . 1591.It Cm UpdateHostKeys 1592Specifies whether 1593.Xr ssh 1 1594should accept notifications of additional hostkeys from the server sent 1595after authentication has completed and add them to 1596.Cm UserKnownHostsFile . 1597The argument must be 1598.Cm yes , 1599.Cm no 1600(the default) or 1601.Cm ask . 1602Enabling this option allows learning alternate hostkeys for a server 1603and supports graceful key rotation by allowing a server to send replacement 1604public keys before old ones are removed. 1605Additional hostkeys are only accepted if the key used to authenticate the 1606host was already trusted or explicitly accepted by the user. 1607If 1608.Cm UpdateHostKeys 1609is set to 1610.Cm ask , 1611then the user is asked to confirm the modifications to the known_hosts file. 1612Confirmation is currently incompatible with 1613.Cm ControlPersist , 1614and will be disabled if it is enabled. 1615.Pp 1616Presently, only 1617.Xr sshd 8 1618from OpenSSH 6.8 and greater support the 1619.Qq hostkeys@openssh.com 1620protocol extension used to inform the client of all the server's hostkeys. 1621.It Cm User 1622Specifies the user to log in as. 1623This can be useful when a different user name is used on different machines. 1624This saves the trouble of 1625having to remember to give the user name on the command line. 1626.It Cm UserKnownHostsFile 1627Specifies one or more files to use for the user 1628host key database, separated by whitespace. 1629The default is 1630.Pa ~/.ssh/known_hosts , 1631.Pa ~/.ssh/known_hosts2 . 1632.It Cm VerifyHostKeyDNS 1633Specifies whether to verify the remote key using DNS and SSHFP resource 1634records. 1635If this option is set to 1636.Cm yes , 1637the client will implicitly trust keys that match a secure fingerprint 1638from DNS. 1639Insecure fingerprints will be handled as if this option was set to 1640.Cm ask . 1641If this option is set to 1642.Cm ask , 1643information on fingerprint match will be displayed, but the user will still 1644need to confirm new host keys according to the 1645.Cm StrictHostKeyChecking 1646option. 1647The default is 1648.Cm yes 1649if compiled with LDNS and 1650.Cm no 1651otherwise. 1652.Pp 1653See also 1654.Sx VERIFYING HOST KEYS 1655in 1656.Xr ssh 1 . 1657.It Cm VersionAddendum 1658Specifies a string to append to the regular version string to identify 1659OS- or site-specific modifications. 1660The default is 1661.Dq FreeBSD-20180909 . 1662The value 1663.Cm none 1664may be used to disable this. 1665.It Cm VisualHostKey 1666If this flag is set to 1667.Cm yes , 1668an ASCII art representation of the remote host key fingerprint is 1669printed in addition to the fingerprint string at login and 1670for unknown host keys. 1671If this flag is set to 1672.Cm no 1673(the default), 1674no fingerprint strings are printed at login and 1675only the fingerprint string will be printed for unknown host keys. 1676.It Cm XAuthLocation 1677Specifies the full pathname of the 1678.Xr xauth 1 1679program. 1680The default is 1681.Pa /usr/local/bin/xauth . 1682.El 1683.Sh PATTERNS 1684A 1685.Em pattern 1686consists of zero or more non-whitespace characters, 1687.Sq * 1688(a wildcard that matches zero or more characters), 1689or 1690.Sq ?\& 1691(a wildcard that matches exactly one character). 1692For example, to specify a set of declarations for any host in the 1693.Qq .co.uk 1694set of domains, 1695the following pattern could be used: 1696.Pp 1697.Dl Host *.co.uk 1698.Pp 1699The following pattern 1700would match any host in the 192.168.0.[0-9] network range: 1701.Pp 1702.Dl Host 192.168.0.? 1703.Pp 1704A 1705.Em pattern-list 1706is a comma-separated list of patterns. 1707Patterns within pattern-lists may be negated 1708by preceding them with an exclamation mark 1709.Pq Sq !\& . 1710For example, 1711to allow a key to be used from anywhere within an organization 1712except from the 1713.Qq dialup 1714pool, 1715the following entry (in authorized_keys) could be used: 1716.Pp 1717.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1718.Pp 1719Note that a negated match will never produce a positive result by itself. 1720For example, attempting to match 1721.Qq host3 1722against the following pattern-list will fail: 1723.Pp 1724.Dl from=\&"!host1,!host2\&" 1725.Pp 1726The solution here is to include a term that will yield a positive match, 1727such as a wildcard: 1728.Pp 1729.Dl from=\&"!host1,!host2,*\&" 1730.Sh TOKENS 1731Arguments to some keywords can make use of tokens, 1732which are expanded at runtime: 1733.Pp 1734.Bl -tag -width XXXX -offset indent -compact 1735.It %% 1736A literal 1737.Sq % . 1738.It \&%C 1739Hash of %l%h%p%r. 1740.It %d 1741Local user's home directory. 1742.It %h 1743The remote hostname. 1744.It %i 1745The local user ID. 1746.It %L 1747The local hostname. 1748.It %l 1749The local hostname, including the domain name. 1750.It %n 1751The original remote hostname, as given on the command line. 1752.It %p 1753The remote port. 1754.It %r 1755The remote username. 1756.It \&%T 1757The local 1758.Xr tun 4 1759or 1760.Xr tap 4 1761network interface assigned if 1762tunnel forwarding was requested, or 1763.Qq NONE 1764otherwise. 1765.It %u 1766The local username. 1767.El 1768.Pp 1769.Cm Match exec 1770accepts the tokens %%, %h, %i, %L, %l, %n, %p, %r, and %u. 1771.Pp 1772.Cm CertificateFile 1773accepts the tokens %%, %d, %h, %i, %l, %r, and %u. 1774.Pp 1775.Cm ControlPath 1776accepts the tokens %%, %C, %h, %i, %L, %l, %n, %p, %r, and %u. 1777.Pp 1778.Cm HostName 1779accepts the tokens %% and %h. 1780.Pp 1781.Cm IdentityAgent 1782and 1783.Cm IdentityFile 1784accept the tokens %%, %d, %h, %i, %l, %r, and %u. 1785.Pp 1786.Cm LocalCommand 1787accepts the tokens %%, %C, %d, %h, %i, %l, %n, %p, %r, %T, and %u. 1788.Pp 1789.Cm ProxyCommand 1790accepts the tokens %%, %h, %p, and %r. 1791.Pp 1792.Cm RemoteCommand 1793accepts the tokens %%, %C, %d, %h, %i, %l, %n, %p, %r, and %u. 1794.Sh FILES 1795.Bl -tag -width Ds 1796.It Pa ~/.ssh/config 1797This is the per-user configuration file. 1798The format of this file is described above. 1799This file is used by the SSH client. 1800Because of the potential for abuse, this file must have strict permissions: 1801read/write for the user, and not accessible by others. 1802.It Pa /etc/ssh/ssh_config 1803Systemwide configuration file. 1804This file provides defaults for those 1805values that are not specified in the user's configuration file, and 1806for those users who do not have a configuration file. 1807This file must be world-readable. 1808.El 1809.Sh SEE ALSO 1810.Xr ssh 1 1811.Sh AUTHORS 1812.An -nosplit 1813OpenSSH is a derivative of the original and free 1814ssh 1.2.12 release by 1815.An Tatu Ylonen . 1816.An Aaron Campbell , Bob Beck , Markus Friedl , 1817.An Niels Provos , Theo de Raadt 1818and 1819.An Dug Song 1820removed many bugs, re-added newer features and 1821created OpenSSH. 1822.An Markus Friedl 1823contributed the support for SSH protocol versions 1.5 and 2.0. 1824