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 1.119 2009/02/22 23:50:57 djm Exp $ 38.Dd $Mdocdate: February 22 2009 $ 39.Dt SSH_CONFIG 5 40.Os 41.Sh NAME 42.Nm ssh_config 43.Nd OpenSSH SSH client configuration files 44.Sh SYNOPSIS 45.Nm ~/.ssh/config 46.Nm /etc/ssh/ssh_config 47.Sh DESCRIPTION 48.Xr ssh 1 49obtains configuration data from the following sources in 50the following order: 51.Pp 52.Bl -enum -offset indent -compact 53.It 54command-line options 55.It 56user's configuration file 57.Pq Pa ~/.ssh/config 58.It 59system-wide configuration file 60.Pq Pa /etc/ssh/ssh_config 61.El 62.Pp 63For each parameter, the first obtained value 64will be used. 65The configuration files contain sections separated by 66.Dq Host 67specifications, and that section is only applied for hosts that 68match one of the patterns given in the specification. 69The matched host name is the one given on the command line. 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 configuration file has the following format: 76.Pp 77Empty lines and lines starting with 78.Ql # 79are comments. 80Otherwise a line is of the format 81.Dq keyword arguments . 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. 93Arguments may optionally be enclosed in double quotes 94.Pq \&" 95in order to represent arguments containing spaces. 96.Pp 97The possible 98keywords and their meanings are as follows (note that 99keywords are case-insensitive and arguments are case-sensitive): 100.Bl -tag -width Ds 101.It Cm Host 102Restricts the following declarations (up to the next 103.Cm Host 104keyword) to be only for those hosts that match one of the patterns 105given after the keyword. 106If more than one pattern is provided, they should be separated by whitespace. 107A single 108.Ql * 109as a pattern can be used to provide global 110defaults for all hosts. 111The host is the 112.Ar hostname 113argument given on the command line (i.e. the name is not converted to 114a canonicalized host name before matching). 115.Pp 116See 117.Sx PATTERNS 118for more information on patterns. 119.It Cm AddressFamily 120Specifies which address family to use when connecting. 121Valid arguments are 122.Dq any , 123.Dq inet 124(use IPv4 only), or 125.Dq inet6 126(use IPv6 only). 127.It Cm BatchMode 128If set to 129.Dq yes , 130passphrase/password querying will be disabled. 131This option is useful in scripts and other batch jobs where no user 132is present to supply the password. 133The argument must be 134.Dq yes 135or 136.Dq no . 137The default is 138.Dq no . 139.It Cm BindAddress 140Use the specified address on the local machine as the source address of 141the connection. 142Only useful on systems with more than one address. 143Note that this option does not work if 144.Cm UsePrivilegedPort 145is set to 146.Dq yes . 147.It Cm ChallengeResponseAuthentication 148Specifies whether to use challenge-response authentication. 149The argument to this keyword must be 150.Dq yes 151or 152.Dq no . 153The default is 154.Dq yes . 155.It Cm CheckHostIP 156If this flag is set to 157.Dq yes , 158.Xr ssh 1 159will additionally check the host IP address in the 160.Pa known_hosts 161file. 162This allows ssh to detect if a host key changed due to DNS spoofing. 163If the option is set to 164.Dq no , 165the check will not be executed. 166The default is 167.Dq yes . 168.It Cm Cipher 169Specifies the cipher to use for encrypting the session 170in protocol version 1. 171Currently, 172.Dq blowfish , 173.Dq 3des , 174and 175.Dq des 176are supported. 177.Ar des 178is only supported in the 179.Xr ssh 1 180client for interoperability with legacy protocol 1 implementations 181that do not support the 182.Ar 3des 183cipher. 184Its use is strongly discouraged due to cryptographic weaknesses. 185The default is 186.Dq 3des . 187.It Cm Ciphers 188Specifies the ciphers allowed for protocol version 2 189in order of preference. 190Multiple ciphers must be comma-separated. 191The supported ciphers are 192.Dq 3des-cbc , 193.Dq aes128-cbc , 194.Dq aes192-cbc , 195.Dq aes256-cbc , 196.Dq aes128-ctr , 197.Dq aes192-ctr , 198.Dq aes256-ctr , 199.Dq arcfour128 , 200.Dq arcfour256 , 201.Dq arcfour , 202.Dq blowfish-cbc , 203and 204.Dq cast128-cbc . 205The default is: 206.Bd -literal -offset 3n 207aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, 208aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, 209aes256-cbc,arcfour 210.Ed 211.It Cm ClearAllForwardings 212Specifies that all local, remote, and dynamic port forwardings 213specified in the configuration files or on the command line be 214cleared. 215This option is primarily useful when used from the 216.Xr ssh 1 217command line to clear port forwardings set in 218configuration files, and is automatically set by 219.Xr scp 1 220and 221.Xr sftp 1 . 222The argument must be 223.Dq yes 224or 225.Dq no . 226The default is 227.Dq no . 228.It Cm Compression 229Specifies whether to use compression. 230The argument must be 231.Dq yes 232or 233.Dq no . 234The default is 235.Dq no . 236.It Cm CompressionLevel 237Specifies the compression level to use if compression is enabled. 238The argument must be an integer from 1 (fast) to 9 (slow, best). 239The default level is 6, which is good for most applications. 240The meaning of the values is the same as in 241.Xr gzip 1 . 242Note that this option applies to protocol version 1 only. 243.It Cm ConnectionAttempts 244Specifies the number of tries (one per second) to make before exiting. 245The argument must be an integer. 246This may be useful in scripts if the connection sometimes fails. 247The default is 1. 248.It Cm ConnectTimeout 249Specifies the timeout (in seconds) used when connecting to the 250SSH server, instead of using the default system TCP timeout. 251This value is used only when the target is down or really unreachable, 252not when it refuses the connection. 253.It Cm ControlMaster 254Enables the sharing of multiple sessions over a single network connection. 255When set to 256.Dq yes , 257.Xr ssh 1 258will listen for connections on a control socket specified using the 259.Cm ControlPath 260argument. 261Additional sessions can connect to this socket using the same 262.Cm ControlPath 263with 264.Cm ControlMaster 265set to 266.Dq no 267(the default). 268These sessions will try to reuse the master instance's network connection 269rather than initiating new ones, but will fall back to connecting normally 270if the control socket does not exist, or is not listening. 271.Pp 272Setting this to 273.Dq ask 274will cause ssh 275to listen for control connections, but require confirmation using the 276.Ev SSH_ASKPASS 277program before they are accepted (see 278.Xr ssh-add 1 279for details). 280If the 281.Cm ControlPath 282cannot be opened, 283ssh will continue without connecting to a master instance. 284.Pp 285X11 and 286.Xr ssh-agent 1 287forwarding is supported over these multiplexed connections, however the 288display and agent forwarded will be the one belonging to the master 289connection i.e. it is not possible to forward multiple displays or agents. 290.Pp 291Two additional options allow for opportunistic multiplexing: try to use a 292master connection but fall back to creating a new one if one does not already 293exist. 294These options are: 295.Dq auto 296and 297.Dq autoask . 298The latter requires confirmation like the 299.Dq ask 300option. 301.It Cm ControlPath 302Specify the path to the control socket used for connection sharing as described 303in the 304.Cm ControlMaster 305section above or the string 306.Dq none 307to disable connection sharing. 308In the path, 309.Ql %l 310will be substituted by the local host name, 311.Ql %h 312will be substituted by the target host name, 313.Ql %p 314the port, and 315.Ql %r 316by the remote login username. 317It is recommended that any 318.Cm ControlPath 319used for opportunistic connection sharing include 320at least %h, %p, and %r. 321This ensures that shared connections are uniquely identified. 322.It Cm DynamicForward 323Specifies that a TCP port on the local machine be forwarded 324over the secure channel, and the application 325protocol is then used to determine where to connect to from the 326remote machine. 327.Pp 328The argument must be 329.Sm off 330.Oo Ar bind_address : Oc Ar port . 331.Sm on 332IPv6 addresses can be specified by enclosing addresses in square brackets or 333by using an alternative syntax: 334.Oo Ar bind_address Ns / Oc Ns Ar port . 335By default, the local port is bound in accordance with the 336.Cm GatewayPorts 337setting. 338However, an explicit 339.Ar bind_address 340may be used to bind the connection to a specific address. 341The 342.Ar bind_address 343of 344.Dq localhost 345indicates that the listening port be bound for local use only, while an 346empty address or 347.Sq * 348indicates that the port should be available from all interfaces. 349.Pp 350Currently the SOCKS4 and SOCKS5 protocols are supported, and 351.Xr ssh 1 352will act as a SOCKS server. 353Multiple forwardings may be specified, and 354additional forwardings can be given on the command line. 355Only the superuser can forward privileged ports. 356.It Cm EnableSSHKeysign 357Setting this option to 358.Dq yes 359in the global client configuration file 360.Pa /etc/ssh/ssh_config 361enables the use of the helper program 362.Xr ssh-keysign 8 363during 364.Cm HostbasedAuthentication . 365The argument must be 366.Dq yes 367or 368.Dq no . 369The default is 370.Dq no . 371This option should be placed in the non-hostspecific section. 372See 373.Xr ssh-keysign 8 374for more information. 375.It Cm EscapeChar 376Sets the escape character (default: 377.Ql ~ ) . 378The escape character can also 379be set on the command line. 380The argument should be a single character, 381.Ql ^ 382followed by a letter, or 383.Dq none 384to disable the escape 385character entirely (making the connection transparent for binary 386data). 387.It Cm ExitOnForwardFailure 388Specifies whether 389.Xr ssh 1 390should terminate the connection if it cannot set up all requested 391dynamic, tunnel, local, and remote port forwardings. 392The argument must be 393.Dq yes 394or 395.Dq no . 396The default is 397.Dq no . 398.It Cm ForwardAgent 399Specifies whether the connection to the authentication agent (if any) 400will be forwarded to the remote machine. 401The argument must be 402.Dq yes 403or 404.Dq no . 405The default is 406.Dq no . 407.Pp 408Agent forwarding should be enabled with caution. 409Users with the ability to bypass file permissions on the remote host 410(for the agent's Unix-domain socket) 411can access the local agent through the forwarded connection. 412An attacker cannot obtain key material from the agent, 413however they can perform operations on the keys that enable them to 414authenticate using the identities loaded into the agent. 415.It Cm ForwardX11 416Specifies whether X11 connections will be automatically redirected 417over the secure channel and 418.Ev DISPLAY 419set. 420The argument must be 421.Dq yes 422or 423.Dq no . 424The default is 425.Dq no . 426.Pp 427X11 forwarding should be enabled with caution. 428Users with the ability to bypass file permissions on the remote host 429(for the user's X11 authorization database) 430can access the local X11 display through the forwarded connection. 431An attacker may then be able to perform activities such as keystroke monitoring 432if the 433.Cm ForwardX11Trusted 434option is also enabled. 435.It Cm ForwardX11Trusted 436If this option is set to 437.Dq yes , 438remote X11 clients will have full access to the original X11 display. 439.Pp 440If this option is set to 441.Dq no , 442remote X11 clients will be considered untrusted and prevented 443from stealing or tampering with data belonging to trusted X11 444clients. 445Furthermore, the 446.Xr xauth 1 447token used for the session will be set to expire after 20 minutes. 448Remote clients will be refused access after this time. 449.Pp 450The default is 451.Dq no . 452.Pp 453See the X11 SECURITY extension specification for full details on 454the restrictions imposed on untrusted clients. 455.It Cm GatewayPorts 456Specifies whether remote hosts are allowed to connect to local 457forwarded ports. 458By default, 459.Xr ssh 1 460binds local port forwardings to the loopback address. 461This prevents other remote hosts from connecting to forwarded ports. 462.Cm GatewayPorts 463can be used to specify that ssh 464should bind local port forwardings to the wildcard address, 465thus allowing remote hosts to connect to forwarded ports. 466The argument must be 467.Dq yes 468or 469.Dq no . 470The default is 471.Dq no . 472.It Cm GlobalKnownHostsFile 473Specifies a file to use for the global 474host key database instead of 475.Pa /etc/ssh/ssh_known_hosts . 476.It Cm GSSAPIAuthentication 477Specifies whether user authentication based on GSSAPI is allowed. 478The default is 479.Dq no . 480Note that this option applies to protocol version 2 only. 481.It Cm GSSAPIDelegateCredentials 482Forward (delegate) credentials to the server. 483The default is 484.Dq no . 485Note that this option applies to protocol version 2 only. 486.It Cm HashKnownHosts 487Indicates that 488.Xr ssh 1 489should hash host names and addresses when they are added to 490.Pa ~/.ssh/known_hosts . 491These hashed names may be used normally by 492.Xr ssh 1 493and 494.Xr sshd 8 , 495but they do not reveal identifying information should the file's contents 496be disclosed. 497The default is 498.Dq no . 499Note that existing names and addresses in known hosts files 500will not be converted automatically, 501but may be manually hashed using 502.Xr ssh-keygen 1 . 503.It Cm HostbasedAuthentication 504Specifies whether to try rhosts based authentication with public key 505authentication. 506The argument must be 507.Dq yes 508or 509.Dq no . 510The default is 511.Dq no . 512This option applies to protocol version 2 only and 513is similar to 514.Cm RhostsRSAAuthentication . 515.It Cm HostKeyAlgorithms 516Specifies the protocol version 2 host key algorithms 517that the client wants to use in order of preference. 518The default for this option is: 519.Dq ssh-rsa,ssh-dss . 520.It Cm HostKeyAlias 521Specifies an alias that should be used instead of the 522real host name when looking up or saving the host key 523in the host key database files. 524This option is useful for tunneling SSH connections 525or for multiple servers running on a single host. 526.It Cm HostName 527Specifies the real host name to log into. 528This can be used to specify nicknames or abbreviations for hosts. 529The default is the name given on the command line. 530Numeric IP addresses are also permitted (both on the command line and in 531.Cm HostName 532specifications). 533.It Cm IdentitiesOnly 534Specifies that 535.Xr ssh 1 536should only use the authentication identity files configured in the 537.Nm 538files, 539even if 540.Xr ssh-agent 1 541offers more identities. 542The argument to this keyword must be 543.Dq yes 544or 545.Dq no . 546This option is intended for situations where ssh-agent 547offers many different identities. 548The default is 549.Dq no . 550.It Cm IdentityFile 551Specifies a file from which the user's RSA or DSA authentication identity 552is read. 553The default is 554.Pa ~/.ssh/identity 555for protocol version 1, and 556.Pa ~/.ssh/id_rsa 557and 558.Pa ~/.ssh/id_dsa 559for protocol version 2. 560Additionally, any identities represented by the authentication agent 561will be used for authentication. 562.Pp 563The file name may use the tilde 564syntax to refer to a user's home directory or one of the following 565escape characters: 566.Ql %d 567(local user's home directory), 568.Ql %u 569(local user name), 570.Ql %l 571(local host name), 572.Ql %h 573(remote host name) or 574.Ql %r 575(remote user name). 576.Pp 577It is possible to have 578multiple identity files specified in configuration files; all these 579identities will be tried in sequence. 580.It Cm KbdInteractiveAuthentication 581Specifies whether to use keyboard-interactive authentication. 582The argument to this keyword must be 583.Dq yes 584or 585.Dq no . 586The default is 587.Dq yes . 588.It Cm KbdInteractiveDevices 589Specifies the list of methods to use in keyboard-interactive authentication. 590Multiple method names must be comma-separated. 591The default is to use the server specified list. 592The methods available vary depending on what the server supports. 593For an OpenSSH server, 594it may be zero or more of: 595.Dq bsdauth , 596.Dq pam , 597and 598.Dq skey . 599.It Cm LocalCommand 600Specifies a command to execute on the local machine after successfully 601connecting to the server. 602The command string extends to the end of the line, and is executed with 603the user's shell. 604The following escape character substitutions will be performed: 605.Ql %d 606(local user's home directory), 607.Ql %h 608(remote host name), 609.Ql %l 610(local host name), 611.Ql %n 612(host name as provided on the command line), 613.Ql %p 614(remote port), 615.Ql %r 616(remote user name) or 617.Ql %u 618(local user name). 619This directive is ignored unless 620.Cm PermitLocalCommand 621has been enabled. 622.It Cm LocalForward 623Specifies that a TCP port on the local machine be forwarded over 624the secure channel to the specified host and port from the remote machine. 625The first argument must be 626.Sm off 627.Oo Ar bind_address : Oc Ar port 628.Sm on 629and the second argument must be 630.Ar host : Ns Ar hostport . 631IPv6 addresses can be specified by enclosing addresses in square brackets or 632by using an alternative syntax: 633.Oo Ar bind_address Ns / Oc Ns Ar port 634and 635.Ar host Ns / Ns Ar hostport . 636Multiple forwardings may be specified, and additional forwardings can be 637given on the command line. 638Only the superuser can forward privileged ports. 639By default, the local port is bound in accordance with the 640.Cm GatewayPorts 641setting. 642However, an explicit 643.Ar bind_address 644may be used to bind the connection to a specific address. 645The 646.Ar bind_address 647of 648.Dq localhost 649indicates that the listening port be bound for local use only, while an 650empty address or 651.Sq * 652indicates that the port should be available from all interfaces. 653.It Cm LogLevel 654Gives the verbosity level that is used when logging messages from 655.Xr ssh 1 . 656The possible values are: 657QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 658The default is INFO. 659DEBUG and DEBUG1 are equivalent. 660DEBUG2 and DEBUG3 each specify higher levels of verbose output. 661.It Cm MACs 662Specifies the MAC (message authentication code) algorithms 663in order of preference. 664The MAC algorithm is used in protocol version 2 665for data integrity protection. 666Multiple algorithms must be comma-separated. 667The default is: 668.Bd -literal -offset indent 669hmac-md5,hmac-sha1,umac-64@openssh.com, 670hmac-ripemd160,hmac-sha1-96,hmac-md5-96 671.Ed 672.It Cm NoHostAuthenticationForLocalhost 673This option can be used if the home directory is shared across machines. 674In this case localhost will refer to a different machine on each of 675the machines and the user will get many warnings about changed host keys. 676However, this option disables host authentication for localhost. 677The argument to this keyword must be 678.Dq yes 679or 680.Dq no . 681The default is to check the host key for localhost. 682.It Cm NumberOfPasswordPrompts 683Specifies the number of password prompts before giving up. 684The argument to this keyword must be an integer. 685The default is 3. 686.It Cm PasswordAuthentication 687Specifies whether to use password authentication. 688The argument to this keyword must be 689.Dq yes 690or 691.Dq no . 692The default is 693.Dq yes . 694.It Cm PermitLocalCommand 695Allow local command execution via the 696.Ic LocalCommand 697option or using the 698.Ic !\& Ns Ar command 699escape sequence in 700.Xr ssh 1 . 701The argument must be 702.Dq yes 703or 704.Dq no . 705The default is 706.Dq no . 707.It Cm Port 708Specifies the port number to connect on the remote host. 709The default is 22. 710.It Cm PreferredAuthentications 711Specifies the order in which the client should try protocol 2 712authentication methods. 713This allows a client to prefer one method (e.g.\& 714.Cm keyboard-interactive ) 715over another method (e.g.\& 716.Cm password ) 717The default for this option is: 718.Do gssapi-with-mic , 719hostbased, 720publickey, 721keyboard-interactive, 722password 723.Dc . 724.It Cm Protocol 725Specifies the protocol versions 726.Xr ssh 1 727should support in order of preference. 728The possible values are 729.Sq 1 730and 731.Sq 2 . 732Multiple versions must be comma-separated. 733The default is 734.Dq 2,1 . 735This means that ssh 736tries version 2 and falls back to version 1 737if version 2 is not available. 738.It Cm ProxyCommand 739Specifies the command to use to connect to the server. 740The command 741string extends to the end of the line, and is executed with 742the user's shell. 743In the command string, 744.Ql %h 745will be substituted by the host name to 746connect and 747.Ql %p 748by the port. 749The command can be basically anything, 750and should read from its standard input and write to its standard output. 751It should eventually connect an 752.Xr sshd 8 753server running on some machine, or execute 754.Ic sshd -i 755somewhere. 756Host key management will be done using the 757HostName of the host being connected (defaulting to the name typed by 758the user). 759Setting the command to 760.Dq none 761disables this option entirely. 762Note that 763.Cm CheckHostIP 764is not available for connects with a proxy command. 765.Pp 766This directive is useful in conjunction with 767.Xr nc 1 768and its proxy support. 769For example, the following directive would connect via an HTTP proxy at 770192.0.2.0: 771.Bd -literal -offset 3n 772ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 773.Ed 774.It Cm PubkeyAuthentication 775Specifies whether to try public key authentication. 776The argument to this keyword must be 777.Dq yes 778or 779.Dq no . 780The default is 781.Dq yes . 782This option applies to protocol version 2 only. 783.It Cm RekeyLimit 784Specifies the maximum amount of data that may be transmitted before the 785session key is renegotiated. 786The argument is the number of bytes, with an optional suffix of 787.Sq K , 788.Sq M , 789or 790.Sq G 791to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 792The default is between 793.Sq 1G 794and 795.Sq 4G , 796depending on the cipher. 797This option applies to protocol version 2 only. 798.It Cm RemoteForward 799Specifies that a TCP port on the remote machine be forwarded over 800the secure channel to the specified host and port from the local machine. 801The first argument must be 802.Sm off 803.Oo Ar bind_address : Oc Ar port 804.Sm on 805and the second argument must be 806.Ar host : Ns Ar hostport . 807IPv6 addresses can be specified by enclosing addresses in square brackets 808or by using an alternative syntax: 809.Oo Ar bind_address Ns / Oc Ns Ar port 810and 811.Ar host Ns / Ns Ar hostport . 812Multiple forwardings may be specified, and additional 813forwardings can be given on the command line. 814Privileged ports can be forwarded only when 815logging in as root on the remote machine. 816.Pp 817If the 818.Ar port 819argument is 820.Ql 0 , 821the listen port will be dynamically allocated on the server and reported 822to the client at run time. 823.Pp 824If the 825.Ar bind_address 826is not specified, the default is to only bind to loopback addresses. 827If the 828.Ar bind_address 829is 830.Ql * 831or an empty string, then the forwarding is requested to listen on all 832interfaces. 833Specifying a remote 834.Ar bind_address 835will only succeed if the server's 836.Cm GatewayPorts 837option is enabled (see 838.Xr sshd_config 5 ) . 839.It Cm RhostsRSAAuthentication 840Specifies whether to try rhosts based authentication with RSA host 841authentication. 842The argument must be 843.Dq yes 844or 845.Dq no . 846The default is 847.Dq no . 848This option applies to protocol version 1 only and requires 849.Xr ssh 1 850to be setuid root. 851.It Cm RSAAuthentication 852Specifies whether to try RSA authentication. 853The argument to this keyword must be 854.Dq yes 855or 856.Dq no . 857RSA authentication will only be 858attempted if the identity file exists, or an authentication agent is 859running. 860The default is 861.Dq yes . 862Note that this option applies to protocol version 1 only. 863.It Cm SendEnv 864Specifies what variables from the local 865.Xr environ 7 866should be sent to the server. 867Note that environment passing is only supported for protocol 2. 868The server must also support it, and the server must be configured to 869accept these environment variables. 870Refer to 871.Cm AcceptEnv 872in 873.Xr sshd_config 5 874for how to configure the server. 875Variables are specified by name, which may contain wildcard characters. 876Multiple environment variables may be separated by whitespace or spread 877across multiple 878.Cm SendEnv 879directives. 880The default is not to send any environment variables. 881.Pp 882See 883.Sx PATTERNS 884for more information on patterns. 885.It Cm ServerAliveCountMax 886Sets the number of server alive messages (see below) which may be 887sent without 888.Xr ssh 1 889receiving any messages back from the server. 890If this threshold is reached while server alive messages are being sent, 891ssh will disconnect from the server, terminating the session. 892It is important to note that the use of server alive messages is very 893different from 894.Cm TCPKeepAlive 895(below). 896The server alive messages are sent through the encrypted channel 897and therefore will not be spoofable. 898The TCP keepalive option enabled by 899.Cm TCPKeepAlive 900is spoofable. 901The server alive mechanism is valuable when the client or 902server depend on knowing when a connection has become inactive. 903.Pp 904The default value is 3. 905If, for example, 906.Cm ServerAliveInterval 907(see below) is set to 15 and 908.Cm ServerAliveCountMax 909is left at the default, if the server becomes unresponsive, 910ssh will disconnect after approximately 45 seconds. 911This option applies to protocol version 2 only. 912.It Cm ServerAliveInterval 913Sets a timeout interval in seconds after which if no data has been received 914from the server, 915.Xr ssh 1 916will send a message through the encrypted 917channel to request a response from the server. 918The default 919is 0, indicating that these messages will not be sent to the server. 920This option applies to protocol version 2 only. 921.It Cm SmartcardDevice 922Specifies which smartcard device to use. 923The argument to this keyword is the device 924.Xr ssh 1 925should use to communicate with a smartcard used for storing the user's 926private RSA key. 927By default, no device is specified and smartcard support is not activated. 928.It Cm StrictHostKeyChecking 929If this flag is set to 930.Dq yes , 931.Xr ssh 1 932will never automatically add host keys to the 933.Pa ~/.ssh/known_hosts 934file, and refuses to connect to hosts whose host key has changed. 935This provides maximum protection against trojan horse attacks, 936though it can be annoying when the 937.Pa /etc/ssh/ssh_known_hosts 938file is poorly maintained or when connections to new hosts are 939frequently made. 940This option forces the user to manually 941add all new hosts. 942If this flag is set to 943.Dq no , 944ssh will automatically add new host keys to the 945user known hosts files. 946If this flag is set to 947.Dq ask , 948new host keys 949will be added to the user known host files only after the user 950has confirmed that is what they really want to do, and 951ssh will refuse to connect to hosts whose host key has changed. 952The host keys of 953known hosts will be verified automatically in all cases. 954The argument must be 955.Dq yes , 956.Dq no , 957or 958.Dq ask . 959The default is 960.Dq ask . 961.It Cm TCPKeepAlive 962Specifies whether the system should send TCP keepalive messages to the 963other side. 964If they are sent, death of the connection or crash of one 965of the machines will be properly noticed. 966However, this means that 967connections will die if the route is down temporarily, and some people 968find it annoying. 969.Pp 970The default is 971.Dq yes 972(to send TCP keepalive messages), and the client will notice 973if the network goes down or the remote host dies. 974This is important in scripts, and many users want it too. 975.Pp 976To disable TCP keepalive messages, the value should be set to 977.Dq no . 978.It Cm Tunnel 979Request 980.Xr tun 4 981device forwarding between the client and the server. 982The argument must be 983.Dq yes , 984.Dq point-to-point 985(layer 3), 986.Dq ethernet 987(layer 2), 988or 989.Dq no . 990Specifying 991.Dq yes 992requests the default tunnel mode, which is 993.Dq point-to-point . 994The default is 995.Dq no . 996.It Cm TunnelDevice 997Specifies the 998.Xr tun 4 999devices to open on the client 1000.Pq Ar local_tun 1001and the server 1002.Pq Ar remote_tun . 1003.Pp 1004The argument must be 1005.Sm off 1006.Ar local_tun Op : Ar remote_tun . 1007.Sm on 1008The devices may be specified by numerical ID or the keyword 1009.Dq any , 1010which uses the next available tunnel device. 1011If 1012.Ar remote_tun 1013is not specified, it defaults to 1014.Dq any . 1015The default is 1016.Dq any:any . 1017.It Cm UsePrivilegedPort 1018Specifies whether to use a privileged port for outgoing connections. 1019The argument must be 1020.Dq yes 1021or 1022.Dq no . 1023The default is 1024.Dq no . 1025If set to 1026.Dq yes , 1027.Xr ssh 1 1028must be setuid root. 1029Note that this option must be set to 1030.Dq yes 1031for 1032.Cm RhostsRSAAuthentication 1033with older servers. 1034.It Cm User 1035Specifies the user to log in as. 1036This can be useful when a different user name is used on different machines. 1037This saves the trouble of 1038having to remember to give the user name on the command line. 1039.It Cm UserKnownHostsFile 1040Specifies a file to use for the user 1041host key database instead of 1042.Pa ~/.ssh/known_hosts . 1043.It Cm VerifyHostKeyDNS 1044Specifies whether to verify the remote key using DNS and SSHFP resource 1045records. 1046If this option is set to 1047.Dq yes , 1048the client will implicitly trust keys that match a secure fingerprint 1049from DNS. 1050Insecure fingerprints will be handled as if this option was set to 1051.Dq ask . 1052If this option is set to 1053.Dq ask , 1054information on fingerprint match will be displayed, but the user will still 1055need to confirm new host keys according to the 1056.Cm StrictHostKeyChecking 1057option. 1058The argument must be 1059.Dq yes , 1060.Dq no , 1061or 1062.Dq ask . 1063The default is 1064.Dq no . 1065Note that this option applies to protocol version 2 only. 1066.Pp 1067See also 1068.Sx VERIFYING HOST KEYS 1069in 1070.Xr ssh 1 . 1071.It Cm VisualHostKey 1072If this flag is set to 1073.Dq yes , 1074an ASCII art representation of the remote host key fingerprint is 1075printed in addition to the hex fingerprint string at login and 1076for unknown host keys. 1077If this flag is set to 1078.Dq no , 1079no fingerprint strings are printed at login and 1080only the hex fingerprint string will be printed for unknown host keys. 1081The default is 1082.Dq no . 1083.It Cm XAuthLocation 1084Specifies the full pathname of the 1085.Xr xauth 1 1086program. 1087The default is 1088.Pa /usr/X11R6/bin/xauth . 1089.El 1090.Sh PATTERNS 1091A 1092.Em pattern 1093consists of zero or more non-whitespace characters, 1094.Sq * 1095(a wildcard that matches zero or more characters), 1096or 1097.Sq ?\& 1098(a wildcard that matches exactly one character). 1099For example, to specify a set of declarations for any host in the 1100.Dq .co.uk 1101set of domains, 1102the following pattern could be used: 1103.Pp 1104.Dl Host *.co.uk 1105.Pp 1106The following pattern 1107would match any host in the 192.168.0.[0-9] network range: 1108.Pp 1109.Dl Host 192.168.0.? 1110.Pp 1111A 1112.Em pattern-list 1113is a comma-separated list of patterns. 1114Patterns within pattern-lists may be negated 1115by preceding them with an exclamation mark 1116.Pq Sq !\& . 1117For example, 1118to allow a key to be used from anywhere within an organisation 1119except from the 1120.Dq dialup 1121pool, 1122the following entry (in authorized_keys) could be used: 1123.Pp 1124.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1125.Sh FILES 1126.Bl -tag -width Ds 1127.It Pa ~/.ssh/config 1128This is the per-user configuration file. 1129The format of this file is described above. 1130This file is used by the SSH client. 1131Because of the potential for abuse, this file must have strict permissions: 1132read/write for the user, and not accessible by others. 1133.It Pa /etc/ssh/ssh_config 1134Systemwide configuration file. 1135This file provides defaults for those 1136values that are not specified in the user's configuration file, and 1137for those users who do not have a configuration file. 1138This file must be world-readable. 1139.El 1140.Sh SEE ALSO 1141.Xr ssh 1 1142.Sh AUTHORS 1143OpenSSH is a derivative of the original and free 1144ssh 1.2.12 release by Tatu Ylonen. 1145Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1146Theo de Raadt and Dug Song 1147removed many bugs, re-added newer features and 1148created OpenSSH. 1149Markus Friedl contributed the support for SSH 1150protocol versions 1.5 and 2.0. 1151