1.\" -*- nroff -*- 2.\" 3.\" Author: Tatu Ylonen <ylo@cs.hut.fi> 4.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland 5.\" All rights reserved 6.\" 7.\" As far as I am concerned, the code I have written for this software 8.\" can be used freely for any purpose. Any derived versions of this 9.\" software must be clearly marked as such, and if the derived work is 10.\" incompatible with the protocol description in the RFC file, it must be 11.\" called by a name other than "ssh" or "Secure Shell". 12.\" 13.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. 14.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. 15.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. 16.\" 17.\" Redistribution and use in source and binary forms, with or without 18.\" modification, are permitted provided that the following conditions 19.\" are met: 20.\" 1. Redistributions of source code must retain the above copyright 21.\" notice, this list of conditions and the following disclaimer. 22.\" 2. Redistributions in binary form must reproduce the above copyright 23.\" notice, this list of conditions and the following disclaimer in the 24.\" documentation and/or other materials provided with the distribution. 25.\" 26.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 27.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 28.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 29.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 30.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 31.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 32.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 33.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 34.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 35.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 36.\" 37.\" $OpenBSD: ssh_config.5,v 1.111 2008/06/26 11:46:31 grunk Exp $ 38.\" $FreeBSD$ 39.Dd June 26 2008 40.Dt SSH_CONFIG 5 41.Os 42.Sh NAME 43.Nm ssh_config 44.Nd OpenSSH SSH client configuration files 45.Sh SYNOPSIS 46.Nm ~/.ssh/config 47.Nm /etc/ssh/ssh_config 48.Sh DESCRIPTION 49.Xr ssh 1 50obtains configuration data from the following sources in 51the following order: 52.Pp 53.Bl -enum -offset indent -compact 54.It 55command-line options 56.It 57user's configuration file 58.Pq Pa ~/.ssh/config 59.It 60system-wide configuration file 61.Pq Pa /etc/ssh/ssh_config 62.El 63.Pp 64For each parameter, the first obtained value 65will be used. 66The configuration files contain sections separated by 67.Dq Host 68specifications, and that section is only applied for hosts that 69match one of the patterns given in the specification. 70The matched host name is the one given on the command line. 71.Pp 72Since the first obtained value for each parameter is used, more 73host-specific declarations should be given near the beginning of the 74file, and general defaults at the end. 75.Pp 76The configuration file has the following format: 77.Pp 78Empty lines and lines starting with 79.Ql # 80are comments. 81Otherwise a line is of the format 82.Dq keyword arguments . 83Configuration options may be separated by whitespace or 84optional whitespace and exactly one 85.Ql = ; 86the latter format is useful to avoid the need to quote whitespace 87when specifying configuration options using the 88.Nm ssh , 89.Nm scp , 90and 91.Nm sftp 92.Fl o 93option. 94Arguments may optionally be enclosed in double quotes 95.Pq \&" 96in order to represent arguments containing spaces. 97.Pp 98The possible 99keywords and their meanings are as follows (note that 100keywords are case-insensitive and arguments are case-sensitive): 101.Bl -tag -width Ds 102.It Cm Host 103Restricts the following declarations (up to the next 104.Cm Host 105keyword) to be only for those hosts that match one of the patterns 106given after the keyword. 107If more than one pattern is provided, they should be separated by whitepsace. 108A single 109.Ql * 110as a pattern can be used to provide global 111defaults for all hosts. 112The host is the 113.Ar hostname 114argument given on the command line (i.e. the name is not converted to 115a canonicalized host name before matching). 116.Pp 117See 118.Sx PATTERNS 119for more information on patterns. 120.It Cm AddressFamily 121Specifies which address family to use when connecting. 122Valid arguments are 123.Dq any , 124.Dq inet 125(use IPv4 only), or 126.Dq inet6 127(use IPv6 only). 128.It Cm BatchMode 129If set to 130.Dq yes , 131passphrase/password querying will be disabled. 132This option is useful in scripts and other batch jobs where no user 133is present to supply the password. 134The argument must be 135.Dq yes 136or 137.Dq no . 138The default is 139.Dq no . 140.It Cm BindAddress 141Use the specified address on the local machine as the source address of 142the connection. 143Only useful on systems with more than one address. 144Note that this option does not work if 145.Cm UsePrivilegedPort 146is set to 147.Dq yes . 148.It Cm ChallengeResponseAuthentication 149Specifies whether to use challenge-response authentication. 150The argument to this keyword must be 151.Dq yes 152or 153.Dq no . 154The default is 155.Dq yes . 156.It Cm CheckHostIP 157If this flag is set to 158.Dq yes , 159.Xr ssh 1 160will additionally check the host IP address in the 161.Pa known_hosts 162file. 163This allows ssh to detect if a host key changed due to DNS spoofing. 164If the option is set to 165.Dq no , 166the check will not be executed. 167The default is 168.Dq no . 169.It Cm Cipher 170Specifies the cipher to use for encrypting the session 171in protocol version 1. 172Currently, 173.Dq blowfish , 174.Dq 3des , 175and 176.Dq des 177are supported. 178.Ar des 179is only supported in the 180.Xr ssh 1 181client for interoperability with legacy protocol 1 implementations 182that do not support the 183.Ar 3des 184cipher. 185Its use is strongly discouraged due to cryptographic weaknesses. 186The default is 187.Dq 3des . 188.It Cm Ciphers 189Specifies the ciphers allowed for protocol version 2 190in order of preference. 191Multiple ciphers must be comma-separated. 192The supported ciphers are 193.Dq 3des-cbc , 194.Dq aes128-cbc , 195.Dq aes192-cbc , 196.Dq aes256-cbc , 197.Dq aes128-ctr , 198.Dq aes192-ctr , 199.Dq aes256-ctr , 200.Dq arcfour128 , 201.Dq arcfour256 , 202.Dq arcfour , 203.Dq blowfish-cbc , 204and 205.Dq cast128-cbc . 206The default is: 207.Bd -literal -offset 3n 208aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, 209arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, 210aes192-ctr,aes256-ctr 211.Ed 212.It Cm ClearAllForwardings 213Specifies that all local, remote, and dynamic port forwardings 214specified in the configuration files or on the command line be 215cleared. 216This option is primarily useful when used from the 217.Xr ssh 1 218command line to clear port forwardings set in 219configuration files, and is automatically set by 220.Xr scp 1 221and 222.Xr sftp 1 . 223The argument must be 224.Dq yes 225or 226.Dq no . 227The default is 228.Dq no . 229.It Cm Compression 230Specifies whether to use compression. 231The argument must be 232.Dq yes 233or 234.Dq no . 235The default is 236.Dq no . 237.It Cm CompressionLevel 238Specifies the compression level to use if compression is enabled. 239The argument must be an integer from 1 (fast) to 9 (slow, best). 240The default level is 6, which is good for most applications. 241The meaning of the values is the same as in 242.Xr gzip 1 . 243Note that this option applies to protocol version 1 only. 244.It Cm ConnectionAttempts 245Specifies the number of tries (one per second) to make before exiting. 246The argument must be an integer. 247This may be useful in scripts if the connection sometimes fails. 248The default is 1. 249.It Cm ConnectTimeout 250Specifies the timeout (in seconds) used when connecting to the 251SSH server, instead of using the default system TCP timeout. 252This value is used only when the target is down or really unreachable, 253not when it refuses the connection. 254.It Cm ControlMaster 255Enables the sharing of multiple sessions over a single network connection. 256When set to 257.Dq yes , 258.Xr ssh 1 259will listen for connections on a control socket specified using the 260.Cm ControlPath 261argument. 262Additional sessions can connect to this socket using the same 263.Cm ControlPath 264with 265.Cm ControlMaster 266set to 267.Dq no 268(the default). 269These sessions will try to reuse the master instance's network connection 270rather than initiating new ones, but will fall back to connecting normally 271if the control socket does not exist, or is not listening. 272.Pp 273Setting this to 274.Dq ask 275will cause ssh 276to listen for control connections, but require confirmation using the 277.Ev SSH_ASKPASS 278program before they are accepted (see 279.Xr ssh-add 1 280for details). 281If the 282.Cm ControlPath 283cannot be opened, 284ssh will continue without connecting to a master instance. 285.Pp 286X11 and 287.Xr ssh-agent 1 288forwarding is supported over these multiplexed connections, however the 289display and agent forwarded will be the one belonging to the master 290connection i.e. it is not possible to forward multiple displays or agents. 291.Pp 292Two additional options allow for opportunistic multiplexing: try to use a 293master connection but fall back to creating a new one if one does not already 294exist. 295These options are: 296.Dq auto 297and 298.Dq autoask . 299The latter requires confirmation like the 300.Dq ask 301option. 302.It Cm ControlPath 303Specify the path to the control socket used for connection sharing as described 304in the 305.Cm ControlMaster 306section above or the string 307.Dq none 308to disable connection sharing. 309In the path, 310.Ql %l 311will be substituted by the local host name, 312.Ql %h 313will be substituted by the target host name, 314.Ql %p 315the port, and 316.Ql %r 317by the remote login username. 318It is recommended that any 319.Cm ControlPath 320used for opportunistic connection sharing include 321at least %h, %p, and %r. 322This ensures that shared connections are uniquely identified. 323.It Cm DynamicForward 324Specifies that a TCP port on the local machine be forwarded 325over the secure channel, and the application 326protocol is then used to determine where to connect to from the 327remote machine. 328.Pp 329The argument must be 330.Sm off 331.Oo Ar bind_address : Oc Ar port . 332.Sm on 333IPv6 addresses can be specified by enclosing addresses in square brackets or 334by using an alternative syntax: 335.Oo Ar bind_address Ns / Oc Ns Ar port . 336By default, the local port is bound in accordance with the 337.Cm GatewayPorts 338setting. 339However, an explicit 340.Ar bind_address 341may be used to bind the connection to a specific address. 342The 343.Ar bind_address 344of 345.Dq localhost 346indicates that the listening port be bound for local use only, while an 347empty address or 348.Sq * 349indicates that the port should be available from all interfaces. 350.Pp 351Currently the SOCKS4 and SOCKS5 protocols are supported, and 352.Xr ssh 1 353will act as a SOCKS server. 354Multiple forwardings may be specified, and 355additional forwardings can be given on the command line. 356Only the superuser can forward privileged ports. 357.It Cm EnableSSHKeysign 358Setting this option to 359.Dq yes 360in the global client configuration file 361.Pa /etc/ssh/ssh_config 362enables the use of the helper program 363.Xr ssh-keysign 8 364during 365.Cm HostbasedAuthentication . 366The argument must be 367.Dq yes 368or 369.Dq no . 370The default is 371.Dq no . 372This option should be placed in the non-hostspecific section. 373See 374.Xr ssh-keysign 8 375for more information. 376.It Cm EscapeChar 377Sets the escape character (default: 378.Ql ~ ) . 379The escape character can also 380be set on the command line. 381The argument should be a single character, 382.Ql ^ 383followed by a letter, or 384.Dq none 385to disable the escape 386character entirely (making the connection transparent for binary 387data). 388.It Cm ExitOnForwardFailure 389Specifies whether 390.Xr ssh 1 391should terminate the connection if it cannot set up all requested 392dynamic, tunnel, local, and remote port forwardings. 393The argument must be 394.Dq yes 395or 396.Dq no . 397The default is 398.Dq no . 399.It Cm ForwardAgent 400Specifies whether the connection to the authentication agent (if any) 401will be forwarded to the remote machine. 402The argument must be 403.Dq yes 404or 405.Dq no . 406The default is 407.Dq no . 408.Pp 409Agent forwarding should be enabled with caution. 410Users with the ability to bypass file permissions on the remote host 411(for the agent's Unix-domain socket) 412can access the local agent through the forwarded connection. 413An attacker cannot obtain key material from the agent, 414however they can perform operations on the keys that enable them to 415authenticate using the identities loaded into the agent. 416.It Cm ForwardX11 417Specifies whether X11 connections will be automatically redirected 418over the secure channel and 419.Ev DISPLAY 420set. 421The argument must be 422.Dq yes 423or 424.Dq no . 425The default is 426.Dq no . 427.Pp 428X11 forwarding should be enabled with caution. 429Users with the ability to bypass file permissions on the remote host 430(for the user's X11 authorization database) 431can access the local X11 display through the forwarded connection. 432An attacker may then be able to perform activities such as keystroke monitoring 433if the 434.Cm ForwardX11Trusted 435option is also enabled. 436.It Cm ForwardX11Trusted 437If this option is set to 438.Dq yes , 439remote X11 clients will have full access to the original X11 display. 440.Pp 441If this option is set to 442.Dq no , 443remote X11 clients will be considered untrusted and prevented 444from stealing or tampering with data belonging to trusted X11 445clients. 446Furthermore, the 447.Xr xauth 1 448token used for the session will be set to expire after 20 minutes. 449Remote clients will be refused access after this time. 450.Pp 451The default is 452.Dq no . 453.Pp 454See the X11 SECURITY extension specification for full details on 455the restrictions imposed on untrusted clients. 456.It Cm GatewayPorts 457Specifies whether remote hosts are allowed to connect to local 458forwarded ports. 459By default, 460.Xr ssh 1 461binds local port forwardings to the loopback address. 462This prevents other remote hosts from connecting to forwarded ports. 463.Cm GatewayPorts 464can be used to specify that ssh 465should bind local port forwardings to the wildcard address, 466thus allowing remote hosts to connect to forwarded ports. 467The argument must be 468.Dq yes 469or 470.Dq no . 471The default is 472.Dq no . 473.It Cm GlobalKnownHostsFile 474Specifies a file to use for the global 475host key database instead of 476.Pa /etc/ssh/ssh_known_hosts . 477.It Cm GSSAPIAuthentication 478Specifies whether user authentication based on GSSAPI is allowed. 479The default is 480.Dq no . 481Note that this option applies to protocol version 2 only. 482.It Cm GSSAPIDelegateCredentials 483Forward (delegate) credentials to the server. 484The default is 485.Dq no . 486Note that this option applies to protocol version 2 only. 487.It Cm HashKnownHosts 488Indicates that 489.Xr ssh 1 490should hash host names and addresses when they are added to 491.Pa ~/.ssh/known_hosts . 492These hashed names may be used normally by 493.Xr ssh 1 494and 495.Xr sshd 8 , 496but they do not reveal identifying information should the file's contents 497be disclosed. 498The default is 499.Dq no . 500Note that existing names and addresses in known hosts files 501will not be converted automatically, 502but may be manually hashed using 503.Xr ssh-keygen 1 . 504.It Cm HostbasedAuthentication 505Specifies whether to try rhosts based authentication with public key 506authentication. 507The argument must be 508.Dq yes 509or 510.Dq no . 511The default is 512.Dq no . 513This option applies to protocol version 2 only and 514is similar to 515.Cm RhostsRSAAuthentication . 516.It Cm HostKeyAlgorithms 517Specifies the protocol version 2 host key algorithms 518that the client wants to use in order of preference. 519The default for this option is: 520.Dq ssh-rsa,ssh-dss . 521.It Cm HostKeyAlias 522Specifies an alias that should be used instead of the 523real host name when looking up or saving the host key 524in the host key database files. 525This option is useful for tunneling SSH connections 526or for multiple servers running on a single host. 527.It Cm HostName 528Specifies the real host name to log into. 529This can be used to specify nicknames or abbreviations for hosts. 530The default is the name given on the command line. 531Numeric IP addresses are also permitted (both on the command line and in 532.Cm HostName 533specifications). 534.It Cm IdentitiesOnly 535Specifies that 536.Xr ssh 1 537should only use the authentication identity files configured in the 538.Nm 539files, 540even if 541.Xr ssh-agent 1 542offers more identities. 543The argument to this keyword must be 544.Dq yes 545or 546.Dq no . 547This option is intended for situations where ssh-agent 548offers many different identities. 549The default is 550.Dq no . 551.It Cm IdentityFile 552Specifies a file from which the user's RSA or DSA authentication identity 553is read. 554The default is 555.Pa ~/.ssh/identity 556for protocol version 1, and 557.Pa ~/.ssh/id_rsa 558and 559.Pa ~/.ssh/id_dsa 560for protocol version 2. 561Additionally, any identities represented by the authentication agent 562will be used for authentication. 563.Pp 564The file name may use the tilde 565syntax to refer to a user's home directory or one of the following 566escape characters: 567.Ql %d 568(local user's home directory), 569.Ql %u 570(local user name), 571.Ql %l 572(local host name), 573.Ql %h 574(remote host name) or 575.Ql %r 576(remote user name). 577.Pp 578It is possible to have 579multiple identity files specified in configuration files; all these 580identities will be tried in sequence. 581.It Cm KbdInteractiveAuthentication 582Specifies whether to use keyboard-interactive authentication. 583The argument to this keyword must be 584.Dq yes 585or 586.Dq no . 587The default is 588.Dq yes . 589.It Cm KbdInteractiveDevices 590Specifies the list of methods to use in keyboard-interactive authentication. 591Multiple method names must be comma-separated. 592The default is to use the server specified list. 593The methods available vary depending on what the server supports. 594For an OpenSSH server, 595it may be zero or more of: 596.Dq bsdauth , 597.Dq pam , 598and 599.Dq skey . 600.It Cm LocalCommand 601Specifies a command to execute on the local machine after successfully 602connecting to the server. 603The command string extends to the end of the line, and is executed with 604the user's shell. 605The following escape character substitutions will be performed: 606.Ql %d 607(local user's home directory), 608.Ql %h 609(remote host name), 610.Ql %l 611(local host name), 612.Ql %n 613(host name as provided on the command line), 614.Ql %p 615(remote port), 616.Ql %r 617(remote user name) or 618.Ql %u 619(local user name). 620This directive is ignored unless 621.Cm PermitLocalCommand 622has been enabled. 623.It Cm LocalForward 624Specifies that a TCP port on the local machine be forwarded over 625the secure channel to the specified host and port from the remote machine. 626The first argument must be 627.Sm off 628.Oo Ar bind_address : Oc Ar port 629.Sm on 630and the second argument must be 631.Ar host : Ns Ar hostport . 632IPv6 addresses can be specified by enclosing addresses in square brackets or 633by using an alternative syntax: 634.Oo Ar bind_address Ns / Oc Ns Ar port 635and 636.Ar host Ns / Ns Ar hostport . 637Multiple forwardings may be specified, and additional forwardings can be 638given on the command line. 639Only the superuser can forward privileged ports. 640By default, the local port is bound in accordance with the 641.Cm GatewayPorts 642setting. 643However, an explicit 644.Ar bind_address 645may be used to bind the connection to a specific address. 646The 647.Ar bind_address 648of 649.Dq localhost 650indicates that the listening port be bound for local use only, while an 651empty address or 652.Sq * 653indicates that the port should be available from all interfaces. 654.It Cm LogLevel 655Gives the verbosity level that is used when logging messages from 656.Xr ssh 1 . 657The possible values are: 658QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 659The default is INFO. 660DEBUG and DEBUG1 are equivalent. 661DEBUG2 and DEBUG3 each specify higher levels of verbose output. 662.It Cm MACs 663Specifies the MAC (message authentication code) algorithms 664in order of preference. 665The MAC algorithm is used in protocol version 2 666for data integrity protection. 667Multiple algorithms must be comma-separated. 668The default is: 669.Bd -literal -offset indent 670hmac-md5,hmac-sha1,umac-64@openssh.com, 671hmac-ripemd160,hmac-sha1-96,hmac-md5-96 672.Ed 673.It Cm NoHostAuthenticationForLocalhost 674This option can be used if the home directory is shared across machines. 675In this case localhost will refer to a different machine on each of 676the machines and the user will get many warnings about changed host keys. 677However, this option disables host authentication for localhost. 678The argument to this keyword must be 679.Dq yes 680or 681.Dq no . 682The default is to check the host key for localhost. 683.It Cm NumberOfPasswordPrompts 684Specifies the number of password prompts before giving up. 685The argument to this keyword must be an integer. 686The default is 3. 687.It Cm PasswordAuthentication 688Specifies whether to use password authentication. 689The argument to this keyword must be 690.Dq yes 691or 692.Dq no . 693The default is 694.Dq yes . 695.It Cm PermitLocalCommand 696Allow local command execution via the 697.Ic LocalCommand 698option or using the 699.Ic !\& Ns Ar command 700escape sequence in 701.Xr ssh 1 . 702The argument must be 703.Dq yes 704or 705.Dq no . 706The default is 707.Dq no . 708.It Cm Port 709Specifies the port number to connect on the remote host. 710The default is 22. 711.It Cm PreferredAuthentications 712Specifies the order in which the client should try protocol 2 713authentication methods. 714This allows a client to prefer one method (e.g.\& 715.Cm keyboard-interactive ) 716over another method (e.g.\& 717.Cm password ) 718The default for this option is: 719.Do gssapi-with-mic , 720hostbased, 721publickey, 722keyboard-interactive, 723password 724.Dc . 725.It Cm Protocol 726Specifies the protocol versions 727.Xr ssh 1 728should support in order of preference. 729The possible values are 730.Sq 1 731and 732.Sq 2 . 733Multiple versions must be comma-separated. 734The default is 735.Dq 2,1 . 736This means that ssh 737tries version 2 and falls back to version 1 738if version 2 is not available. 739.It Cm ProxyCommand 740Specifies the command to use to connect to the server. 741The command 742string extends to the end of the line, and is executed with 743the user's shell. 744In the command string, 745.Ql %h 746will be substituted by the host name to 747connect and 748.Ql %p 749by the port. 750The command can be basically anything, 751and should read from its standard input and write to its standard output. 752It should eventually connect an 753.Xr sshd 8 754server running on some machine, or execute 755.Ic sshd -i 756somewhere. 757Host key management will be done using the 758HostName of the host being connected (defaulting to the name typed by 759the user). 760Setting the command to 761.Dq none 762disables this option entirely. 763Note that 764.Cm CheckHostIP 765is not available for connects with a proxy command. 766.Pp 767This directive is useful in conjunction with 768.Xr nc 1 769and its proxy support. 770For example, the following directive would connect via an HTTP proxy at 771192.0.2.0: 772.Bd -literal -offset 3n 773ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 774.Ed 775.It Cm PubkeyAuthentication 776Specifies whether to try public key authentication. 777The argument to this keyword must be 778.Dq yes 779or 780.Dq no . 781The default is 782.Dq yes . 783This option applies to protocol version 2 only. 784.It Cm RekeyLimit 785Specifies the maximum amount of data that may be transmitted before the 786session key is renegotiated. 787The argument is the number of bytes, with an optional suffix of 788.Sq K , 789.Sq M , 790or 791.Sq G 792to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 793The default is between 794.Sq 1G 795and 796.Sq 4G , 797depending on the cipher. 798This option applies to protocol version 2 only. 799.It Cm RemoteForward 800Specifies that a TCP port on the remote machine be forwarded over 801the secure channel to the specified host and port from the local machine. 802The first argument must be 803.Sm off 804.Oo Ar bind_address : Oc Ar port 805.Sm on 806and the second argument must be 807.Ar host : Ns Ar hostport . 808IPv6 addresses can be specified by enclosing addresses in square brackets 809or by using an alternative syntax: 810.Oo Ar bind_address Ns / Oc Ns Ar port 811and 812.Ar host Ns / Ns Ar hostport . 813Multiple forwardings may be specified, and additional 814forwardings can be given on the command line. 815Only the superuser can forward privileged ports. 816.Pp 817If the 818.Ar bind_address 819is not specified, the default is to only bind to loopback addresses. 820If the 821.Ar bind_address 822is 823.Ql * 824or an empty string, then the forwarding is requested to listen on all 825interfaces. 826Specifying a remote 827.Ar bind_address 828will only succeed if the server's 829.Cm GatewayPorts 830option is enabled (see 831.Xr sshd_config 5 ) . 832.It Cm RhostsRSAAuthentication 833Specifies whether to try rhosts based authentication with RSA host 834authentication. 835The argument must be 836.Dq yes 837or 838.Dq no . 839The default is 840.Dq no . 841This option applies to protocol version 1 only and requires 842.Xr ssh 1 843to be setuid root. 844.It Cm RSAAuthentication 845Specifies whether to try RSA authentication. 846The argument to this keyword must be 847.Dq yes 848or 849.Dq no . 850RSA authentication will only be 851attempted if the identity file exists, or an authentication agent is 852running. 853The default is 854.Dq yes . 855Note that this option applies to protocol version 1 only. 856.It Cm SendEnv 857Specifies what variables from the local 858.Xr environ 7 859should be sent to the server. 860Note that environment passing is only supported for protocol 2. 861The server must also support it, and the server must be configured to 862accept these environment variables. 863Refer to 864.Cm AcceptEnv 865in 866.Xr sshd_config 5 867for how to configure the server. 868Variables are specified by name, which may contain wildcard characters. 869Multiple environment variables may be separated by whitespace or spread 870across multiple 871.Cm SendEnv 872directives. 873The default is not to send any environment variables. 874.Pp 875See 876.Sx PATTERNS 877for more information on patterns. 878.It Cm ServerAliveCountMax 879Sets the number of server alive messages (see below) which may be 880sent without 881.Xr ssh 1 882receiving any messages back from the server. 883If this threshold is reached while server alive messages are being sent, 884ssh will disconnect from the server, terminating the session. 885It is important to note that the use of server alive messages is very 886different from 887.Cm TCPKeepAlive 888(below). 889The server alive messages are sent through the encrypted channel 890and therefore will not be spoofable. 891The TCP keepalive option enabled by 892.Cm TCPKeepAlive 893is spoofable. 894The server alive mechanism is valuable when the client or 895server depend on knowing when a connection has become inactive. 896.Pp 897The default value is 3. 898If, for example, 899.Cm ServerAliveInterval 900(see below) is set to 15 and 901.Cm ServerAliveCountMax 902is left at the default, if the server becomes unresponsive, 903ssh will disconnect after approximately 45 seconds. 904This option applies to protocol version 2 only. 905.It Cm ServerAliveInterval 906Sets a timeout interval in seconds after which if no data has been received 907from the server, 908.Xr ssh 1 909will send a message through the encrypted 910channel to request a response from the server. 911The default 912is 0, indicating that these messages will not be sent to the server. 913This option applies to protocol version 2 only. 914.It Cm SmartcardDevice 915Specifies which smartcard device to use. 916The argument to this keyword is the device 917.Xr ssh 1 918should use to communicate with a smartcard used for storing the user's 919private RSA key. 920By default, no device is specified and smartcard support is not activated. 921.It Cm StrictHostKeyChecking 922If this flag is set to 923.Dq yes , 924.Xr ssh 1 925will never automatically add host keys to the 926.Pa ~/.ssh/known_hosts 927file, and refuses to connect to hosts whose host key has changed. 928This provides maximum protection against trojan horse attacks, 929though it can be annoying when the 930.Pa /etc/ssh/ssh_known_hosts 931file is poorly maintained or when connections to new hosts are 932frequently made. 933This option forces the user to manually 934add all new hosts. 935If this flag is set to 936.Dq no , 937ssh will automatically add new host keys to the 938user known hosts files. 939If this flag is set to 940.Dq ask , 941new host keys 942will be added to the user known host files only after the user 943has confirmed that is what they really want to do, and 944ssh will refuse to connect to hosts whose host key has changed. 945The host keys of 946known hosts will be verified automatically in all cases. 947The argument must be 948.Dq yes , 949.Dq no , 950or 951.Dq ask . 952The default is 953.Dq ask . 954.It Cm TCPKeepAlive 955Specifies whether the system should send TCP keepalive messages to the 956other side. 957If they are sent, death of the connection or crash of one 958of the machines will be properly noticed. 959However, this means that 960connections will die if the route is down temporarily, and some people 961find it annoying. 962.Pp 963The default is 964.Dq yes 965(to send TCP keepalive messages), and the client will notice 966if the network goes down or the remote host dies. 967This is important in scripts, and many users want it too. 968.Pp 969To disable TCP keepalive messages, the value should be set to 970.Dq no . 971.It Cm Tunnel 972Request 973.Xr tun 4 974device forwarding between the client and the server. 975The argument must be 976.Dq yes , 977.Dq point-to-point 978(layer 3), 979.Dq ethernet 980(layer 2), 981or 982.Dq no . 983Specifying 984.Dq yes 985requests the default tunnel mode, which is 986.Dq point-to-point . 987The default is 988.Dq no . 989.It Cm TunnelDevice 990Specifies the 991.Xr tun 4 992devices to open on the client 993.Pq Ar local_tun 994and the server 995.Pq Ar remote_tun . 996.Pp 997The argument must be 998.Sm off 999.Ar local_tun Op : Ar remote_tun . 1000.Sm on 1001The devices may be specified by numerical ID or the keyword 1002.Dq any , 1003which uses the next available tunnel device. 1004If 1005.Ar remote_tun 1006is not specified, it defaults to 1007.Dq any . 1008The default is 1009.Dq any:any . 1010.It Cm UsePrivilegedPort 1011Specifies whether to use a privileged port for outgoing connections. 1012The argument must be 1013.Dq yes 1014or 1015.Dq no . 1016The default is 1017.Dq no . 1018If set to 1019.Dq yes , 1020.Xr ssh 1 1021must be setuid root. 1022Note that this option must be set to 1023.Dq yes 1024for 1025.Cm RhostsRSAAuthentication 1026with older servers. 1027.It Cm User 1028Specifies the user to log in as. 1029This can be useful when a different user name is used on different machines. 1030This saves the trouble of 1031having to remember to give the user name on the command line. 1032.It Cm UserKnownHostsFile 1033Specifies a file to use for the user 1034host key database instead of 1035.Pa ~/.ssh/known_hosts . 1036.It Cm VerifyHostKeyDNS 1037Specifies whether to verify the remote key using DNS and SSHFP resource 1038records. 1039If this option is set to 1040.Dq yes , 1041the client will implicitly trust keys that match a secure fingerprint 1042from DNS. 1043Insecure fingerprints will be handled as if this option was set to 1044.Dq ask . 1045If this option is set to 1046.Dq ask , 1047information on fingerprint match will be displayed, but the user will still 1048need to confirm new host keys according to the 1049.Cm StrictHostKeyChecking 1050option. 1051The argument must be 1052.Dq yes , 1053.Dq no , 1054or 1055.Dq ask . 1056The default is 1057.Dq no . 1058Note that this option applies to protocol version 2 only. 1059.Pp 1060See also 1061.Sx VERIFYING HOST KEYS 1062in 1063.Xr ssh 1 . 1064.It Cm VersionAddendum 1065Specifies a string to append to the regular version string to identify 1066OS- or site-specific modifications. 1067The default is 1068.Dq FreeBSD-20080801 . 1069.It Cm VisualHostKey 1070If this flag is set to 1071.Dq yes , 1072an ASCII art representation of the remote host key fingerprint is 1073printed additionally to the hex fingerprint string. 1074If this flag is set to 1075.Dq no , 1076only the hex fingerprint string will be printed. 1077The default is 1078.Dq no . 1079.It Cm XAuthLocation 1080Specifies the full pathname of the 1081.Xr xauth 1 1082program. 1083The default is 1084.Pa /usr/local/bin/xauth . 1085.El 1086.Sh PATTERNS 1087A 1088.Em pattern 1089consists of zero or more non-whitespace characters, 1090.Sq * 1091(a wildcard that matches zero or more characters), 1092or 1093.Sq ?\& 1094(a wildcard that matches exactly one character). 1095For example, to specify a set of declarations for any host in the 1096.Dq .co.uk 1097set of domains, 1098the following pattern could be used: 1099.Pp 1100.Dl Host *.co.uk 1101.Pp 1102The following pattern 1103would match any host in the 192.168.0.[0-9] network range: 1104.Pp 1105.Dl Host 192.168.0.? 1106.Pp 1107A 1108.Em pattern-list 1109is a comma-separated list of patterns. 1110Patterns within pattern-lists may be negated 1111by preceding them with an exclamation mark 1112.Pq Sq !\& . 1113For example, 1114to allow a key to be used from anywhere within an organisation 1115except from the 1116.Dq dialup 1117pool, 1118the following entry (in authorized_keys) could be used: 1119.Pp 1120.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1121.Sh FILES 1122.Bl -tag -width Ds 1123.It Pa ~/.ssh/config 1124This is the per-user configuration file. 1125The format of this file is described above. 1126This file is used by the SSH client. 1127Because of the potential for abuse, this file must have strict permissions: 1128read/write for the user, and not accessible by others. 1129.It Pa /etc/ssh/ssh_config 1130Systemwide configuration file. 1131This file provides defaults for those 1132values that are not specified in the user's configuration file, and 1133for those users who do not have a configuration file. 1134This file must be world-readable. 1135.El 1136.Sh SEE ALSO 1137.Xr ssh 1 1138.Sh AUTHORS 1139OpenSSH is a derivative of the original and free 1140ssh 1.2.12 release by Tatu Ylonen. 1141Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1142Theo de Raadt and Dug Song 1143removed many bugs, re-added newer features and 1144created OpenSSH. 1145Markus Friedl contributed the support for SSH 1146protocol versions 1.5 and 2.0. 1147