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 1.97 2006/07/27 08:00:50 jmc Exp $ 38.\" $FreeBSD$ 39.Dd September 25, 1999 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.Bl -tag -width Ds -compact 47.It Pa ~/.ssh/config 48.It Pa /etc/ssh/ssh_config 49.El 50.Sh DESCRIPTION 51.Xr ssh 1 52obtains configuration data from the following sources in 53the following order: 54.Pp 55.Bl -enum -offset indent -compact 56.It 57command-line options 58.It 59user's configuration file 60.Pq Pa ~/.ssh/config 61.It 62system-wide configuration file 63.Pq Pa /etc/ssh/ssh_config 64.El 65.Pp 66For each parameter, the first obtained value 67will be used. 68The configuration files contain sections separated by 69.Dq Host 70specifications, and that section is only applied for hosts that 71match one of the patterns given in the specification. 72The matched host name is the one given on the command line. 73.Pp 74Since the first obtained value for each parameter is used, more 75host-specific declarations should be given near the beginning of the 76file, and general defaults at the end. 77.Pp 78The configuration file has the following format: 79.Pp 80Empty lines and lines starting with 81.Ql # 82are comments. 83Otherwise a line is of the format 84.Dq keyword arguments . 85Configuration options may be separated by whitespace or 86optional whitespace and exactly one 87.Ql = ; 88the latter format is useful to avoid the need to quote whitespace 89when specifying configuration options using the 90.Nm ssh , 91.Nm scp , 92and 93.Nm sftp 94.Fl o 95option. 96Arguments may optionally be enclosed in double quotes 97.Pq \&" 98in order to represent arguments containing spaces. 99.Pp 100The possible 101keywords and their meanings are as follows (note that 102keywords are case-insensitive and arguments are case-sensitive): 103.Bl -tag -width Ds 104.It Cm Host 105Restricts the following declarations (up to the next 106.Cm Host 107keyword) to be only for those hosts that match one of the patterns 108given after the keyword. 109A single 110.Ql * 111as a pattern can be used to provide global 112defaults for all hosts. 113The host is the 114.Ar hostname 115argument given on the command line (i.e. the name is not converted to 116a canonicalized host name before matching). 117.Pp 118See 119.Sx PATTERNS 120for more information on patterns. 121.It Cm AddressFamily 122Specifies which address family to use when connecting. 123Valid arguments are 124.Dq any , 125.Dq inet 126(use IPv4 only), or 127.Dq inet6 128(use IPv6 only). 129.It Cm BatchMode 130If set to 131.Dq yes , 132passphrase/password querying will be disabled. 133This option is useful in scripts and other batch jobs where no user 134is present to supply the password. 135The argument must be 136.Dq yes 137or 138.Dq no . 139The default is 140.Dq no . 141.It Cm BindAddress 142Use the specified address on the local machine as the source address of 143the connection. 144Only useful on systems with more than one address. 145Note that this option does not work if 146.Cm UsePrivilegedPort 147is set to 148.Dq yes . 149.It Cm ChallengeResponseAuthentication 150Specifies whether to use challenge-response authentication. 151The argument to this keyword must be 152.Dq yes 153or 154.Dq no . 155The default is 156.Dq yes . 157.It Cm CheckHostIP 158If this flag is set to 159.Dq yes , 160.Xr ssh 1 161will additionally check the host IP address in the 162.Pa known_hosts 163file. 164This allows ssh to detect if a host key changed due to DNS spoofing. 165If the option is set to 166.Dq no , 167the check will not be executed. 168The default is 169.Dq no . 170.It Cm Cipher 171Specifies the cipher to use for encrypting the session 172in protocol version 1. 173Currently, 174.Dq blowfish , 175.Dq 3des , 176and 177.Dq des 178are supported. 179.Ar des 180is only supported in the 181.Xr ssh 1 182client for interoperability with legacy protocol 1 implementations 183that do not support the 184.Ar 3des 185cipher. 186Its use is strongly discouraged due to cryptographic weaknesses. 187The default is 188.Dq 3des . 189.It Cm Ciphers 190Specifies the ciphers allowed for protocol version 2 191in order of preference. 192Multiple ciphers must be comma-separated. 193The supported ciphers are 194.Dq 3des-cbc , 195.Dq aes128-cbc , 196.Dq aes192-cbc , 197.Dq aes256-cbc , 198.Dq aes128-ctr , 199.Dq aes192-ctr , 200.Dq aes256-ctr , 201.Dq arcfour128 , 202.Dq arcfour256 , 203.Dq arcfour , 204.Dq blowfish-cbc , 205and 206.Dq cast128-cbc . 207The default is: 208.Bd -literal -offset 3n 209aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, 210arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, 211aes192-ctr,aes256-ctr 212.Ed 213.It Cm ClearAllForwardings 214Specifies that all local, remote, and dynamic port forwardings 215specified in the configuration files or on the command line be 216cleared. 217This option is primarily useful when used from the 218.Xr ssh 1 219command line to clear port forwardings set in 220configuration files, and is automatically set by 221.Xr scp 1 222and 223.Xr sftp 1 . 224The argument must be 225.Dq yes 226or 227.Dq no . 228The default is 229.Dq no . 230.It Cm Compression 231Specifies whether to use compression. 232The argument must be 233.Dq yes 234or 235.Dq no . 236The default is 237.Dq no . 238.It Cm CompressionLevel 239Specifies the compression level to use if compression is enabled. 240The argument must be an integer from 1 (fast) to 9 (slow, best). 241The default level is 6, which is good for most applications. 242The meaning of the values is the same as in 243.Xr gzip 1 . 244Note that this option applies to protocol version 1 only. 245.It Cm ConnectionAttempts 246Specifies the number of tries (one per second) to make before exiting. 247The argument must be an integer. 248This may be useful in scripts if the connection sometimes fails. 249The default is 1. 250.It Cm ConnectTimeout 251Specifies the timeout (in seconds) used when connecting to the 252SSH server, instead of using the default system TCP timeout. 253This value is used only when the target is down or really unreachable, 254not when it refuses the connection. 255.It Cm ControlMaster 256Enables the sharing of multiple sessions over a single network connection. 257When set to 258.Dq yes , 259.Xr ssh 1 260will listen for connections on a control socket specified using the 261.Cm ControlPath 262argument. 263Additional sessions can connect to this socket using the same 264.Cm ControlPath 265with 266.Cm ControlMaster 267set to 268.Dq no 269(the default). 270These sessions will try to reuse the master instance's network connection 271rather than initiating new ones, but will fall back to connecting normally 272if the control socket does not exist, or is not listening. 273.Pp 274Setting this to 275.Dq ask 276will cause ssh 277to listen for control connections, but require confirmation using the 278.Ev SSH_ASKPASS 279program before they are accepted (see 280.Xr ssh-add 1 281for details). 282If the 283.Cm ControlPath 284cannot be opened, 285ssh will continue without connecting to a master instance. 286.Pp 287X11 and 288.Xr ssh-agent 1 289forwarding is supported over these multiplexed connections, however the 290display and agent forwarded will be the one belonging to the master 291connection i.e. it is not possible to forward multiple displays or agents. 292.Pp 293Two additional options allow for opportunistic multiplexing: try to use a 294master connection but fall back to creating a new one if one does not already 295exist. 296These options are: 297.Dq auto 298and 299.Dq autoask . 300The latter requires confirmation like the 301.Dq ask 302option. 303.It Cm ControlPath 304Specify the path to the control socket used for connection sharing as described 305in the 306.Cm ControlMaster 307section above or the string 308.Dq none 309to disable connection sharing. 310In the path, 311.Ql %l 312will be substituted by the local host name, 313.Ql %h 314will be substituted by the target host name, 315.Ql %p 316the port, and 317.Ql %r 318by the remote login username. 319It is recommended that any 320.Cm ControlPath 321used for opportunistic connection sharing include 322at least %h, %p, and %r. 323This ensures that shared connections are uniquely identified. 324.It Cm DynamicForward 325Specifies that a TCP port on the local machine be forwarded 326over the secure channel, and the application 327protocol is then used to determine where to connect to from the 328remote machine. 329.Pp 330The argument must be 331.Sm off 332.Oo Ar bind_address : Oc Ar port . 333.Sm on 334IPv6 addresses can be specified by enclosing addresses in square brackets or 335by using an alternative syntax: 336.Oo Ar bind_address Ns / Oc Ns Ar port . 337By default, the local port is bound in accordance with the 338.Cm GatewayPorts 339setting. 340However, an explicit 341.Ar bind_address 342may be used to bind the connection to a specific address. 343The 344.Ar bind_address 345of 346.Dq localhost 347indicates that the listening port be bound for local use only, while an 348empty address or 349.Sq * 350indicates that the port should be available from all interfaces. 351.Pp 352Currently the SOCKS4 and SOCKS5 protocols are supported, and 353.Xr ssh 1 354will act as a SOCKS server. 355Multiple forwardings may be specified, and 356additional forwardings can be given on the command line. 357Only the superuser can forward privileged ports. 358.It Cm EnableSSHKeysign 359Setting this option to 360.Dq yes 361in the global client configuration file 362.Pa /etc/ssh/ssh_config 363enables the use of the helper program 364.Xr ssh-keysign 8 365during 366.Cm HostbasedAuthentication . 367The argument must be 368.Dq yes 369or 370.Dq no . 371The default is 372.Dq no . 373This option should be placed in the non-hostspecific section. 374See 375.Xr ssh-keysign 8 376for more information. 377.It Cm EscapeChar 378Sets the escape character (default: 379.Ql ~ ) . 380The escape character can also 381be set on the command line. 382The argument should be a single character, 383.Ql ^ 384followed by a letter, or 385.Dq none 386to disable the escape 387character entirely (making the connection transparent for binary 388data). 389.It Cm ExitOnForwardFailure 390Specifies whether 391.Xr ssh 1 392should terminate the connection if it cannot set up all requested 393dynamic, local, and remote port forwardings. 394The argument must be 395.Dq yes 396or 397.Dq no . 398The default is 399.Dq no . 400.It Cm ForwardAgent 401Specifies whether the connection to the authentication agent (if any) 402will be forwarded to the remote machine. 403The argument must be 404.Dq yes 405or 406.Dq no . 407The default is 408.Dq no . 409.Pp 410Agent forwarding should be enabled with caution. 411Users with the ability to bypass file permissions on the remote host 412(for the agent's Unix-domain socket) 413can access the local agent through the forwarded connection. 414An attacker cannot obtain key material from the agent, 415however they can perform operations on the keys that enable them to 416authenticate using the identities loaded into the agent. 417.It Cm ForwardX11 418Specifies whether X11 connections will be automatically redirected 419over the secure channel and 420.Ev DISPLAY 421set. 422The argument must be 423.Dq yes 424or 425.Dq no . 426The default is 427.Dq no . 428.Pp 429X11 forwarding should be enabled with caution. 430Users with the ability to bypass file permissions on the remote host 431(for the user's X11 authorization database) 432can access the local X11 display through the forwarded connection. 433An attacker may then be able to perform activities such as keystroke monitoring 434if the 435.Cm ForwardX11Trusted 436option is also enabled. 437.It Cm ForwardX11Trusted 438If this option is set to 439.Dq yes , 440remote X11 clients will have full access to the original X11 display. 441.Pp 442If this option is set to 443.Dq no , 444remote X11 clients will be considered untrusted and prevented 445from stealing or tampering with data belonging to trusted X11 446clients. 447Furthermore, the 448.Xr xauth 1 449token used for the session will be set to expire after 20 minutes. 450Remote clients will be refused access after this time. 451.Pp 452The default is 453.Dq no . 454.Pp 455See the X11 SECURITY extension specification for full details on 456the restrictions imposed on untrusted clients. 457.It Cm GatewayPorts 458Specifies whether remote hosts are allowed to connect to local 459forwarded ports. 460By default, 461.Xr ssh 1 462binds local port forwardings to the loopback address. 463This prevents other remote hosts from connecting to forwarded ports. 464.Cm GatewayPorts 465can be used to specify that ssh 466should bind local port forwardings to the wildcard address, 467thus allowing remote hosts to connect to forwarded ports. 468The argument must be 469.Dq yes 470or 471.Dq no . 472The default is 473.Dq no . 474.It Cm GlobalKnownHostsFile 475Specifies a file to use for the global 476host key database instead of 477.Pa /etc/ssh/ssh_known_hosts . 478.It Cm GSSAPIAuthentication 479Specifies whether user authentication based on GSSAPI is allowed. 480The default is 481.Dq no . 482Note that this option applies to protocol version 2 only. 483.It Cm GSSAPIDelegateCredentials 484Forward (delegate) credentials to the server. 485The default is 486.Dq no . 487Note that this option applies to protocol version 2 only. 488.It Cm HashKnownHosts 489Indicates that 490.Xr ssh 1 491should hash host names and addresses when they are added to 492.Pa ~/.ssh/known_hosts . 493These hashed names may be used normally by 494.Xr ssh 1 495and 496.Xr sshd 8 , 497but they do not reveal identifying information should the file's contents 498be disclosed. 499The default is 500.Dq no . 501Note that existing names and addresses in known hosts files 502will not be converted automatically, 503but may be manually hashed using 504.Xr ssh-keygen 1 . 505.It Cm HostbasedAuthentication 506Specifies whether to try rhosts based authentication with public key 507authentication. 508The argument must be 509.Dq yes 510or 511.Dq no . 512The default is 513.Dq no . 514This option applies to protocol version 2 only and 515is similar to 516.Cm RhostsRSAAuthentication . 517.It Cm HostKeyAlgorithms 518Specifies the protocol version 2 host key algorithms 519that the client wants to use in order of preference. 520The default for this option is: 521.Dq ssh-rsa,ssh-dss . 522.It Cm HostKeyAlias 523Specifies an alias that should be used instead of the 524real host name when looking up or saving the host key 525in the host key database files. 526This option is useful for tunneling SSH connections 527or for multiple servers running on a single host. 528.It Cm HostName 529Specifies the real host name to log into. 530This can be used to specify nicknames or abbreviations for hosts. 531The default is the name given on the command line. 532Numeric IP addresses are also permitted (both on the command line and in 533.Cm HostName 534specifications). 535.It Cm IdentitiesOnly 536Specifies that 537.Xr ssh 1 538should only use the authentication identity files configured in the 539.Nm 540files, 541even if 542.Xr ssh-agent 1 543offers more identities. 544The argument to this keyword must be 545.Dq yes 546or 547.Dq no . 548This option is intended for situations where ssh-agent 549offers many different identities. 550The default is 551.Dq no . 552.It Cm IdentityFile 553Specifies a file from which the user's RSA or DSA authentication identity 554is read. 555The default is 556.Pa ~/.ssh/identity 557for protocol version 1, and 558.Pa ~/.ssh/id_rsa 559and 560.Pa ~/.ssh/id_dsa 561for protocol version 2. 562Additionally, any identities represented by the authentication agent 563will be used for authentication. 564.Pp 565The file name may use the tilde 566syntax to refer to a user's home directory or one of the following 567escape characters: 568.Ql %d 569(local user's home directory), 570.Ql %u 571(local user name), 572.Ql %l 573(local host name), 574.Ql %h 575(remote host name) or 576.Ql %r 577(remote user name). 578.Pp 579It is possible to have 580multiple identity files specified in configuration files; all these 581identities will be tried in sequence. 582.It Cm KbdInteractiveDevices 583Specifies the list of methods to use in keyboard-interactive authentication. 584Multiple method names must be comma-separated. 585The default is to use the server specified list. 586The methods available vary depending on what the server supports. 587For an OpenSSH server, 588it may be zero or more of: 589.Dq bsdauth , 590.Dq pam , 591and 592.Dq skey . 593.It Cm LocalCommand 594Specifies a command to execute on the local machine after successfully 595connecting to the server. 596The command string extends to the end of the line, and is executed with 597.Pa /bin/sh . 598This directive is ignored unless 599.Cm PermitLocalCommand 600has been enabled. 601.It Cm LocalForward 602Specifies that a TCP port on the local machine be forwarded over 603the secure channel to the specified host and port from the remote machine. 604The first argument must be 605.Sm off 606.Oo Ar bind_address : Oc Ar port 607.Sm on 608and the second argument must be 609.Ar host : Ns Ar hostport . 610IPv6 addresses can be specified by enclosing addresses in square brackets or 611by using an alternative syntax: 612.Oo Ar bind_address Ns / Oc Ns Ar port 613and 614.Ar host Ns / Ns Ar hostport . 615Multiple forwardings may be specified, and additional forwardings can be 616given on the command line. 617Only the superuser can forward privileged ports. 618By default, the local port is bound in accordance with the 619.Cm GatewayPorts 620setting. 621However, an explicit 622.Ar bind_address 623may be used to bind the connection to a specific address. 624The 625.Ar bind_address 626of 627.Dq localhost 628indicates that the listening port be bound for local use only, while an 629empty address or 630.Sq * 631indicates that the port should be available from all interfaces. 632.It Cm LogLevel 633Gives the verbosity level that is used when logging messages from 634.Xr ssh 1 . 635The possible values are: 636QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 637The default is INFO. 638DEBUG and DEBUG1 are equivalent. 639DEBUG2 and DEBUG3 each specify higher levels of verbose output. 640.It Cm MACs 641Specifies the MAC (message authentication code) algorithms 642in order of preference. 643The MAC algorithm is used in protocol version 2 644for data integrity protection. 645Multiple algorithms must be comma-separated. 646The default is: 647.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . 648.It Cm NoHostAuthenticationForLocalhost 649This option can be used if the home directory is shared across machines. 650In this case localhost will refer to a different machine on each of 651the machines and the user will get many warnings about changed host keys. 652However, this option disables host authentication for localhost. 653The argument to this keyword must be 654.Dq yes 655or 656.Dq no . 657The default is to check the host key for localhost. 658.It Cm NumberOfPasswordPrompts 659Specifies the number of password prompts before giving up. 660The argument to this keyword must be an integer. 661The default is 3. 662.It Cm PasswordAuthentication 663Specifies whether to use password authentication. 664The argument to this keyword must be 665.Dq yes 666or 667.Dq no . 668The default is 669.Dq yes . 670.It Cm PermitLocalCommand 671Allow local command execution via the 672.Ic LocalCommand 673option or using the 674.Ic !\& Ns Ar command 675escape sequence in 676.Xr ssh 1 . 677The argument must be 678.Dq yes 679or 680.Dq no . 681The default is 682.Dq no . 683.It Cm Port 684Specifies the port number to connect on the remote host. 685The default is 22. 686.It Cm PreferredAuthentications 687Specifies the order in which the client should try protocol 2 688authentication methods. 689This allows a client to prefer one method (e.g.\& 690.Cm keyboard-interactive ) 691over another method (e.g.\& 692.Cm password ) 693The default for this option is: 694.Do gssapi-with-mic , 695hostbased, 696publickey, 697keyboard-interactive, 698password 699.Dc . 700.It Cm Protocol 701Specifies the protocol versions 702.Xr ssh 1 703should support in order of preference. 704The possible values are 705.Sq 1 706and 707.Sq 2 . 708Multiple versions must be comma-separated. 709The default is 710.Dq 2,1 . 711This means that ssh 712tries version 2 and falls back to version 1 713if version 2 is not available. 714.It Cm ProxyCommand 715Specifies the command to use to connect to the server. 716The command 717string extends to the end of the line, and is executed with 718.Pa /bin/sh . 719In the command string, 720.Ql %h 721will be substituted by the host name to 722connect and 723.Ql %p 724by the port. 725The command can be basically anything, 726and should read from its standard input and write to its standard output. 727It should eventually connect an 728.Xr sshd 8 729server running on some machine, or execute 730.Ic sshd -i 731somewhere. 732Host key management will be done using the 733HostName of the host being connected (defaulting to the name typed by 734the user). 735Setting the command to 736.Dq none 737disables this option entirely. 738Note that 739.Cm CheckHostIP 740is not available for connects with a proxy command. 741.Pp 742This directive is useful in conjunction with 743.Xr nc 1 744and its proxy support. 745For example, the following directive would connect via an HTTP proxy at 746192.0.2.0: 747.Bd -literal -offset 3n 748ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 749.Ed 750.It Cm PubkeyAuthentication 751Specifies whether to try public key authentication. 752The argument to this keyword must be 753.Dq yes 754or 755.Dq no . 756The default is 757.Dq yes . 758This option applies to protocol version 2 only. 759.It Cm RekeyLimit 760Specifies the maximum amount of data that may be transmitted before the 761session key is renegotiated. 762The argument is the number of bytes, with an optional suffix of 763.Sq K , 764.Sq M , 765or 766.Sq G 767to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 768The default is between 769.Sq 1G 770and 771.Sq 4G , 772depending on the cipher. 773This option applies to protocol version 2 only. 774.It Cm RemoteForward 775Specifies that a TCP port on the remote machine be forwarded over 776the secure channel to the specified host and port from the local machine. 777The first argument must be 778.Sm off 779.Oo Ar bind_address : Oc Ar port 780.Sm on 781and the second argument must be 782.Ar host : Ns Ar hostport . 783IPv6 addresses can be specified by enclosing addresses in square brackets 784or by using an alternative syntax: 785.Oo Ar bind_address Ns / Oc Ns Ar port 786and 787.Ar host Ns / Ns Ar hostport . 788Multiple forwardings may be specified, and additional 789forwardings can be given on the command line. 790Only the superuser can forward privileged ports. 791.Pp 792If the 793.Ar bind_address 794is not specified, the default is to only bind to loopback addresses. 795If the 796.Ar bind_address 797is 798.Ql * 799or an empty string, then the forwarding is requested to listen on all 800interfaces. 801Specifying a remote 802.Ar bind_address 803will only succeed if the server's 804.Cm GatewayPorts 805option is enabled (see 806.Xr sshd_config 5 ) . 807.It Cm RhostsRSAAuthentication 808Specifies whether to try rhosts based authentication with RSA host 809authentication. 810The argument must be 811.Dq yes 812or 813.Dq no . 814The default is 815.Dq no . 816This option applies to protocol version 1 only and requires 817.Xr ssh 1 818to be setuid root. 819.It Cm RSAAuthentication 820Specifies whether to try RSA authentication. 821The argument to this keyword must be 822.Dq yes 823or 824.Dq no . 825RSA authentication will only be 826attempted if the identity file exists, or an authentication agent is 827running. 828The default is 829.Dq yes . 830Note that this option applies to protocol version 1 only. 831.It Cm SendEnv 832Specifies what variables from the local 833.Xr environ 7 834should be sent to the server. 835Note that environment passing is only supported for protocol 2. 836The server must also support it, and the server must be configured to 837accept these environment variables. 838Refer to 839.Cm AcceptEnv 840in 841.Xr sshd_config 5 842for how to configure the server. 843Variables are specified by name, which may contain wildcard characters. 844Multiple environment variables may be separated by whitespace or spread 845across multiple 846.Cm SendEnv 847directives. 848The default is not to send any environment variables. 849.Pp 850See 851.Sx PATTERNS 852for more information on patterns. 853.It Cm ServerAliveCountMax 854Sets the number of server alive messages (see below) which may be 855sent without 856.Xr ssh 1 857receiving any messages back from the server. 858If this threshold is reached while server alive messages are being sent, 859ssh will disconnect from the server, terminating the session. 860It is important to note that the use of server alive messages is very 861different from 862.Cm TCPKeepAlive 863(below). 864The server alive messages are sent through the encrypted channel 865and therefore will not be spoofable. 866The TCP keepalive option enabled by 867.Cm TCPKeepAlive 868is spoofable. 869The server alive mechanism is valuable when the client or 870server depend on knowing when a connection has become inactive. 871.Pp 872The default value is 3. 873If, for example, 874.Cm ServerAliveInterval 875(see below) is set to 15 and 876.Cm ServerAliveCountMax 877is left at the default, if the server becomes unresponsive, 878ssh will disconnect after approximately 45 seconds. 879This option applies to protocol version 2 only. 880.It Cm ServerAliveInterval 881Sets a timeout interval in seconds after which if no data has been received 882from the server, 883.Xr ssh 1 884will send a message through the encrypted 885channel to request a response from the server. 886The default 887is 0, indicating that these messages will not be sent to the server. 888This option applies to protocol version 2 only. 889.It Cm SmartcardDevice 890Specifies which smartcard device to use. 891The argument to this keyword is the device 892.Xr ssh 1 893should use to communicate with a smartcard used for storing the user's 894private RSA key. 895By default, no device is specified and smartcard support is not activated. 896.It Cm StrictHostKeyChecking 897If this flag is set to 898.Dq yes , 899.Xr ssh 1 900will never automatically add host keys to the 901.Pa ~/.ssh/known_hosts 902file, and refuses to connect to hosts whose host key has changed. 903This provides maximum protection against trojan horse attacks, 904though it can be annoying when the 905.Pa /etc/ssh/ssh_known_hosts 906file is poorly maintained or when connections to new hosts are 907frequently made. 908This option forces the user to manually 909add all new hosts. 910If this flag is set to 911.Dq no , 912ssh will automatically add new host keys to the 913user known hosts files. 914If this flag is set to 915.Dq ask , 916new host keys 917will be added to the user known host files only after the user 918has confirmed that is what they really want to do, and 919ssh will refuse to connect to hosts whose host key has changed. 920The host keys of 921known hosts will be verified automatically in all cases. 922The argument must be 923.Dq yes , 924.Dq no , 925or 926.Dq ask . 927The default is 928.Dq ask . 929.It Cm TCPKeepAlive 930Specifies whether the system should send TCP keepalive messages to the 931other side. 932If they are sent, death of the connection or crash of one 933of the machines will be properly noticed. 934However, this means that 935connections will die if the route is down temporarily, and some people 936find it annoying. 937.Pp 938The default is 939.Dq yes 940(to send TCP keepalive messages), and the client will notice 941if the network goes down or the remote host dies. 942This is important in scripts, and many users want it too. 943.Pp 944To disable TCP keepalive messages, the value should be set to 945.Dq no . 946.It Cm Tunnel 947Request 948.Xr tun 4 949device forwarding between the client and the server. 950The argument must be 951.Dq yes , 952.Dq point-to-point 953(layer 3), 954.Dq ethernet 955(layer 2), 956or 957.Dq no . 958Specifying 959.Dq yes 960requests the default tunnel mode, which is 961.Dq point-to-point . 962The default is 963.Dq no . 964.It Cm TunnelDevice 965Specifies the 966.Xr tun 4 967devices to open on the client 968.Pq Ar local_tun 969and the server 970.Pq Ar remote_tun . 971.Pp 972The argument must be 973.Sm off 974.Ar local_tun Op : Ar remote_tun . 975.Sm on 976The devices may be specified by numerical ID or the keyword 977.Dq any , 978which uses the next available tunnel device. 979If 980.Ar remote_tun 981is not specified, it defaults to 982.Dq any . 983The default is 984.Dq any:any . 985.It Cm UsePrivilegedPort 986Specifies whether to use a privileged port for outgoing connections. 987The argument must be 988.Dq yes 989or 990.Dq no . 991The default is 992.Dq no . 993If set to 994.Dq yes , 995.Xr ssh 1 996must be setuid root. 997Note that this option must be set to 998.Dq yes 999for 1000.Cm RhostsRSAAuthentication 1001with older servers. 1002.It Cm User 1003Specifies the user to log in as. 1004This can be useful when a different user name is used on different machines. 1005This saves the trouble of 1006having to remember to give the user name on the command line. 1007.It Cm UserKnownHostsFile 1008Specifies a file to use for the user 1009host key database instead of 1010.Pa ~/.ssh/known_hosts . 1011.It Cm VerifyHostKeyDNS 1012Specifies whether to verify the remote key using DNS and SSHFP resource 1013records. 1014If this option is set to 1015.Dq yes , 1016the client will implicitly trust keys that match a secure fingerprint 1017from DNS. 1018Insecure fingerprints will be handled as if this option was set to 1019.Dq ask . 1020If this option is set to 1021.Dq ask , 1022information on fingerprint match will be displayed, but the user will still 1023need to confirm new host keys according to the 1024.Cm StrictHostKeyChecking 1025option. 1026The argument must be 1027.Dq yes , 1028.Dq no , 1029or 1030.Dq ask . 1031The default is 1032.Dq no . 1033Note that this option applies to protocol version 2 only. 1034.Pp 1035See also 1036.Sx VERIFYING HOST KEYS 1037in 1038.Xr ssh 1 . 1039.It Cm VersionAddendum 1040Specifies a string to append to the regular version string to identify 1041OS- or site-specific modifications. 1042The default is 1043.Dq FreeBSD-20061110 . 1044.It Cm XAuthLocation 1045Specifies the full pathname of the 1046.Xr xauth 1 1047program. 1048The default is 1049.Pa /usr/local/bin/xauth . 1050.El 1051.Sh PATTERNS 1052A 1053.Em pattern 1054consists of zero or more non-whitespace characters, 1055.Sq * 1056(a wildcard that matches zero or more characters), 1057or 1058.Sq ?\& 1059(a wildcard that matches exactly one character). 1060For example, to specify a set of declarations for any host in the 1061.Dq .co.uk 1062set of domains, 1063the following pattern could be used: 1064.Pp 1065.Dl Host *.co.uk 1066.Pp 1067The following pattern 1068would match any host in the 192.168.0.[0-9] network range: 1069.Pp 1070.Dl Host 192.168.0.? 1071.Pp 1072A 1073.Em pattern-list 1074is a comma-separated list of patterns. 1075Patterns within pattern-lists may be negated 1076by preceding them with an exclamation mark 1077.Pq Sq !\& . 1078For example, 1079to allow a key to be used from anywhere within an organisation 1080except from the 1081.Dq dialup 1082pool, 1083the following entry (in authorized_keys) could be used: 1084.Pp 1085.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1086.Sh FILES 1087.Bl -tag -width Ds 1088.It Pa ~/.ssh/config 1089This is the per-user configuration file. 1090The format of this file is described above. 1091This file is used by the SSH client. 1092Because of the potential for abuse, this file must have strict permissions: 1093read/write for the user, and not accessible by others. 1094.It Pa /etc/ssh/ssh_config 1095Systemwide configuration file. 1096This file provides defaults for those 1097values that are not specified in the user's configuration file, and 1098for those users who do not have a configuration file. 1099This file must be world-readable. 1100.El 1101.Sh SEE ALSO 1102.Xr ssh 1 1103.Sh AUTHORS 1104OpenSSH is a derivative of the original and free 1105ssh 1.2.12 release by Tatu Ylonen. 1106Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1107Theo de Raadt and Dug Song 1108removed many bugs, re-added newer features and 1109created OpenSSH. 1110Markus Friedl contributed the support for SSH 1111protocol versions 1.5 and 2.0. 1112