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 1.61 2005/07/08 12:53:10 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.Nm ssh 52obtains configuration data from the following sources in 53the following order: 54.Bl -enum -offset indent -compact 55.It 56command-line options 57.It 58user's configuration file 59.Pq Pa ~/.ssh/config 60.It 61system-wide configuration file 62.Pq Pa /etc/ssh/ssh_config 63.El 64.Pp 65For each parameter, the first obtained value 66will be used. 67The configuration files contain sections separated by 68.Dq Host 69specifications, and that section is only applied for hosts that 70match one of the patterns given in the specification. 71The matched host name is the one given on the command line. 72.Pp 73Since the first obtained value for each parameter is used, more 74host-specific declarations should be given near the beginning of the 75file, and general defaults at the end. 76.Pp 77The configuration file has the following format: 78.Pp 79Empty lines and lines starting with 80.Ql # 81are comments. 82.Pp 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. 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. 106.Ql \&* 107and 108.Ql \&? 109can be used as wildcards in the 110patterns. 111A single 112.Ql \&* 113as a pattern can be used to provide global 114defaults for all hosts. 115The host is the 116.Ar hostname 117argument given on the command line (i.e., the name is not converted to 118a canonicalized host name before matching). 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 , 158ssh will additionally check the host IP address in the 159.Pa known_hosts 160file. 161This allows ssh to detect if a host key changed due to DNS spoofing. 162If the option is set to 163.Dq no , 164the check will not be executed. 165The default is 166.Dq no . 167.It Cm Cipher 168Specifies the cipher to use for encrypting the session 169in protocol version 1. 170Currently, 171.Dq blowfish , 172.Dq 3des , 173and 174.Dq des 175are supported. 176.Ar des 177is only supported in the 178.Nm ssh 179client for interoperability with legacy protocol 1 implementations 180that do not support the 181.Ar 3des 182cipher. 183Its use is strongly discouraged due to cryptographic weaknesses. 184The default is 185.Dq 3des . 186.It Cm Ciphers 187Specifies the ciphers allowed for protocol version 2 188in order of preference. 189Multiple ciphers must be comma-separated. 190The supported ciphers are 191.Dq 3des-cbc , 192.Dq aes128-cbc , 193.Dq aes192-cbc , 194.Dq aes256-cbc , 195.Dq aes128-ctr , 196.Dq aes192-ctr , 197.Dq aes256-ctr , 198.Dq arcfour128 , 199.Dq arcfour256 , 200.Dq arcfour , 201.Dq blowfish-cbc , 202and 203.Dq cast128-cbc . 204The default is 205.Bd -literal 206 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, 207 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, 208 aes192-ctr,aes256-ctr'' 209.Ed 210.It Cm ClearAllForwardings 211Specifies that all local, remote and dynamic port forwardings 212specified in the configuration files or on the command line be 213cleared. 214This option is primarily useful when used from the 215.Nm ssh 216command line to clear port forwardings set in 217configuration files, and is automatically set by 218.Xr scp 1 219and 220.Xr sftp 1 . 221The argument must be 222.Dq yes 223or 224.Dq no . 225The default is 226.Dq no . 227.It Cm Compression 228Specifies whether to use compression. 229The argument must be 230.Dq yes 231or 232.Dq no . 233The default is 234.Dq no . 235.It Cm CompressionLevel 236Specifies the compression level to use if compression is enabled. 237The argument must be an integer from 1 (fast) to 9 (slow, best). 238The default level is 6, which is good for most applications. 239The meaning of the values is the same as in 240.Xr gzip 1 . 241Note that this option applies to protocol version 1 only. 242.It Cm ConnectionAttempts 243Specifies the number of tries (one per second) to make before exiting. 244The argument must be an integer. 245This may be useful in scripts if the connection sometimes fails. 246The default is 1. 247.It Cm ConnectTimeout 248Specifies the timeout (in seconds) used when connecting to the ssh 249server, instead of using the default system TCP timeout. 250This value is used only when the target is down or really unreachable, 251not when it refuses the connection. 252.It Cm ControlMaster 253Enables the sharing of multiple sessions over a single network connection. 254When set to 255.Dq yes 256.Nm ssh 257will listen for connections on a control socket specified using the 258.Cm ControlPath 259argument. 260Additional sessions can connect to this socket using the same 261.Cm ControlPath 262with 263.Cm ControlMaster 264set to 265.Dq no 266(the default). 267These sessions will reuse the master instance's network connection rather 268than initiating new ones. 269Setting this to 270.Dq ask 271will cause 272.Nm ssh 273to listen for control connections, but require confirmation using the 274.Ev SSH_ASKPASS 275program before they are accepted (see 276.Xr ssh-add 1 277for details). 278If the 279.Cm ControlPath 280can not be opened, 281.Nm ssh 282will continue without connecting to a master instance. 283.Pp 284X11 and 285.Xr ssh-agent 1 286forwarding is supported over these multiplexed connections, however the 287display and agent fowarded will be the one belonging to the master 288connection i.e. it is not possible to forward multiple displays or agents. 289.Pp 290Two additional options allow for opportunistic multiplexing: try to use a 291master connection but fall back to creating a new one if one does not already 292exist. 293These options are: 294.Dq auto 295and 296.Dq autoask . 297The latter requires confirmation like the 298.Dq ask 299option. 300.It Cm ControlPath 301Specify the path to the control socket used for connection sharing as described 302in the 303.Cm ControlMaster 304section above or the string 305.Dq none 306to disable connection sharing. 307In the path, 308.Ql %h 309will be substituted by the target host name, 310.Ql %p 311the port and 312.Ql %r 313by the remote login username. 314It is recommended that any 315.Cm ControlPath 316used for opportunistic connection sharing include 317all three of these escape sequences. 318This ensures that shared connections are uniquely identified. 319.It Cm DynamicForward 320Specifies that a TCP/IP port on the local machine be forwarded 321over the secure channel, and the application 322protocol is then used to determine where to connect to from the 323remote machine. 324The argument must be a port number. 325Currently the SOCKS4 and SOCKS5 protocols are supported, and 326.Nm ssh 327will act as a SOCKS server. 328Multiple forwardings may be specified, and 329additional forwardings can be given on the command line. 330Only the superuser can forward privileged ports. 331.It Cm EnableSSHKeysign 332Setting this option to 333.Dq yes 334in the global client configuration file 335.Pa /etc/ssh/ssh_config 336enables the use of the helper program 337.Xr ssh-keysign 8 338during 339.Cm HostbasedAuthentication . 340The argument must be 341.Dq yes 342or 343.Dq no . 344The default is 345.Dq no . 346This option should be placed in the non-hostspecific section. 347See 348.Xr ssh-keysign 8 349for more information. 350.It Cm EscapeChar 351Sets the escape character (default: 352.Ql ~ ) . 353The escape character can also 354be set on the command line. 355The argument should be a single character, 356.Ql ^ 357followed by a letter, or 358.Dq none 359to disable the escape 360character entirely (making the connection transparent for binary 361data). 362.It Cm ForwardAgent 363Specifies whether the connection to the authentication agent (if any) 364will be forwarded to the remote machine. 365The argument must be 366.Dq yes 367or 368.Dq no . 369The default is 370.Dq no . 371.Pp 372Agent forwarding should be enabled with caution. 373Users with the ability to bypass file permissions on the remote host 374(for the agent's Unix-domain socket) 375can access the local agent through the forwarded connection. 376An attacker cannot obtain key material from the agent, 377however they can perform operations on the keys that enable them to 378authenticate using the identities loaded into the agent. 379.It Cm ForwardX11 380Specifies whether X11 connections will be automatically redirected 381over the secure channel and 382.Ev DISPLAY 383set. 384The argument must be 385.Dq yes 386or 387.Dq no . 388The default is 389.Dq no . 390.Pp 391X11 forwarding should be enabled with caution. 392Users with the ability to bypass file permissions on the remote host 393(for the user's X11 authorization database) 394can access the local X11 display through the forwarded connection. 395An attacker may then be able to perform activities such as keystroke monitoring 396if the 397.Cm ForwardX11Trusted 398option is also enabled. 399.It Cm ForwardX11Trusted 400If this option is set to 401.Dq yes 402then remote X11 clients will have full access to the original X11 display. 403.Pp 404If this option is set to 405.Dq no 406then remote X11 clients will be considered untrusted and prevented 407from stealing or tampering with data belonging to trusted X11 408clients. 409Furthermore, the 410.Xr xauth 1 411token used for the session will be set to expire after 20 minutes. 412Remote clients will be refused access after this time. 413.Pp 414The default is 415.Dq no . 416.Pp 417See the X11 SECURITY extension specification for full details on 418the restrictions imposed on untrusted clients. 419.It Cm GatewayPorts 420Specifies whether remote hosts are allowed to connect to local 421forwarded ports. 422By default, 423.Nm ssh 424binds local port forwardings to the loopback address. 425This prevents other remote hosts from connecting to forwarded ports. 426.Cm GatewayPorts 427can be used to specify that 428.Nm ssh 429should bind local port forwardings to the wildcard address, 430thus allowing remote hosts to connect to forwarded ports. 431The argument must be 432.Dq yes 433or 434.Dq no . 435The default is 436.Dq no . 437.It Cm GlobalKnownHostsFile 438Specifies a file to use for the global 439host key database instead of 440.Pa /etc/ssh/ssh_known_hosts . 441.It Cm GSSAPIAuthentication 442Specifies whether user authentication based on GSSAPI is allowed. 443The default is 444.Dq no . 445Note that this option applies to protocol version 2 only. 446.It Cm GSSAPIDelegateCredentials 447Forward (delegate) credentials to the server. 448The default is 449.Dq no . 450Note that this option applies to protocol version 2 only. 451.It Cm HashKnownHosts 452Indicates that 453.Nm ssh 454should hash host names and addresses when they are added to 455.Pa ~/.ssh/known_hosts . 456These hashed names may be used normally by 457.Nm ssh 458and 459.Nm sshd , 460but they do not reveal identifying information should the file's contents 461be disclosed. 462The default is 463.Dq no . 464Note that hashing of names and addresses will not be retrospectively applied 465to existing known hosts files, but these may be manually hashed using 466.Xr ssh-keygen 1 . 467.It Cm HostbasedAuthentication 468Specifies whether to try rhosts based authentication with public key 469authentication. 470The argument must be 471.Dq yes 472or 473.Dq no . 474The default is 475.Dq no . 476This option applies to protocol version 2 only and 477is similar to 478.Cm RhostsRSAAuthentication . 479.It Cm HostKeyAlgorithms 480Specifies the protocol version 2 host key algorithms 481that the client wants to use in order of preference. 482The default for this option is: 483.Dq ssh-rsa,ssh-dss . 484.It Cm HostKeyAlias 485Specifies an alias that should be used instead of the 486real host name when looking up or saving the host key 487in the host key database files. 488This option is useful for tunneling ssh connections 489or for multiple servers running on a single host. 490.It Cm HostName 491Specifies the real host name to log into. 492This can be used to specify nicknames or abbreviations for hosts. 493Default is the name given on the command line. 494Numeric IP addresses are also permitted (both on the command line and in 495.Cm HostName 496specifications). 497.It Cm IdentityFile 498Specifies a file from which the user's RSA or DSA authentication identity 499is read. 500The default is 501.Pa ~/.ssh/identity 502for protocol version 1, and 503.Pa ~/.ssh/id_rsa 504and 505.Pa ~/.ssh/id_dsa 506for protocol version 2. 507Additionally, any identities represented by the authentication agent 508will be used for authentication. 509The file name may use the tilde 510syntax to refer to a user's home directory. 511It is possible to have 512multiple identity files specified in configuration files; all these 513identities will be tried in sequence. 514.It Cm IdentitiesOnly 515Specifies that 516.Nm ssh 517should only use the authentication identity files configured in the 518.Nm 519files, 520even if the 521.Nm ssh-agent 522offers more identities. 523The argument to this keyword must be 524.Dq yes 525or 526.Dq no . 527This option is intented for situations where 528.Nm ssh-agent 529offers many different identities. 530The default is 531.Dq no . 532.It Cm KbdInteractiveDevices 533Specifies the list of methods to use in keyboard-interactive authentication. 534Multiple method names must be comma-separated. 535The default is to use the server specified list. 536.It Cm LocalForward 537Specifies that a TCP/IP port on the local machine be forwarded over 538the secure channel to the specified host and port from the remote machine. 539The first argument must be 540.Sm off 541.Oo Ar bind_address : Oc Ar port 542.Sm on 543and the second argument must be 544.Ar host : Ns Ar hostport . 545IPv6 addresses can be specified by enclosing addresses in square brackets or 546by using an alternative syntax: 547.Oo Ar bind_address Ns / Oc Ns Ar port 548and 549.Ar host Ns / Ns Ar hostport . 550Multiple forwardings may be specified, and additional forwardings can be 551given on the command line. 552Only the superuser can forward privileged ports. 553By default, the local port is bound in accordance with the 554.Cm GatewayPorts 555setting. 556However, an explicit 557.Ar bind_address 558may be used to bind the connection to a specific address. 559The 560.Ar bind_address 561of 562.Dq localhost 563indicates that the listening port be bound for local use only, while an 564empty address or 565.Sq * 566indicates that the port should be available from all interfaces. 567.It Cm LogLevel 568Gives the verbosity level that is used when logging messages from 569.Nm ssh . 570The possible values are: 571QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3. 572The default is INFO. 573DEBUG and DEBUG1 are equivalent. 574DEBUG2 and DEBUG3 each specify higher levels of verbose output. 575.It Cm MACs 576Specifies the MAC (message authentication code) algorithms 577in order of preference. 578The MAC algorithm is used in protocol version 2 579for data integrity protection. 580Multiple algorithms must be comma-separated. 581The default is 582.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . 583.It Cm NoHostAuthenticationForLocalhost 584This option can be used if the home directory is shared across machines. 585In this case localhost will refer to a different machine on each of 586the machines and the user will get many warnings about changed host keys. 587However, this option disables host authentication for localhost. 588The argument to this keyword must be 589.Dq yes 590or 591.Dq no . 592The default is to check the host key for localhost. 593.It Cm NumberOfPasswordPrompts 594Specifies the number of password prompts before giving up. 595The argument to this keyword must be an integer. 596Default is 3. 597.It Cm PasswordAuthentication 598Specifies whether to use password authentication. 599The argument to this keyword must be 600.Dq yes 601or 602.Dq no . 603The default is 604.Dq yes . 605.It Cm Port 606Specifies the port number to connect on the remote host. 607Default is 22. 608.It Cm PreferredAuthentications 609Specifies the order in which the client should try protocol 2 610authentication methods. 611This allows a client to prefer one method (e.g.\& 612.Cm keyboard-interactive ) 613over another method (e.g.\& 614.Cm password ) 615The default for this option is: 616.Dq hostbased,publickey,keyboard-interactive,password . 617.It Cm Protocol 618Specifies the protocol versions 619.Nm ssh 620should support in order of preference. 621The possible values are 622.Dq 1 623and 624.Dq 2 . 625Multiple versions must be comma-separated. 626The default is 627.Dq 2,1 . 628This means that 629.Nm ssh 630tries version 2 and falls back to version 1 631if version 2 is not available. 632.It Cm ProxyCommand 633Specifies the command to use to connect to the server. 634The command 635string extends to the end of the line, and is executed with 636.Pa /bin/sh . 637In the command string, 638.Ql %h 639will be substituted by the host name to 640connect and 641.Ql %p 642by the port. 643The command can be basically anything, 644and should read from its standard input and write to its standard output. 645It should eventually connect an 646.Xr sshd 8 647server running on some machine, or execute 648.Ic sshd -i 649somewhere. 650Host key management will be done using the 651HostName of the host being connected (defaulting to the name typed by 652the user). 653Setting the command to 654.Dq none 655disables this option entirely. 656Note that 657.Cm CheckHostIP 658is not available for connects with a proxy command. 659.Pp 660This directive is useful in conjunction with 661.Xr nc 1 662and its proxy support. 663For example, the following directive would connect via an HTTP proxy at 664192.0.2.0: 665.Bd -literal -offset 3n 666ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 667.Ed 668.It Cm PubkeyAuthentication 669Specifies whether to try public key authentication. 670The argument to this keyword must be 671.Dq yes 672or 673.Dq no . 674The default is 675.Dq yes . 676This option applies to protocol version 2 only. 677.It Cm RemoteForward 678Specifies that a TCP/IP port on the remote machine be forwarded over 679the secure channel to the specified host and port from the local machine. 680The first argument must be 681.Sm off 682.Oo Ar bind_address : Oc Ar port 683.Sm on 684and the second argument must be 685.Ar host : Ns Ar hostport . 686IPv6 addresses can be specified by enclosing addresses in square brackets 687or by using an alternative syntax: 688.Oo Ar bind_address Ns / Oc Ns Ar port 689and 690.Ar host Ns / Ns Ar hostport . 691Multiple forwardings may be specified, and additional 692forwardings can be given on the command line. 693Only the superuser can forward privileged ports. 694.Pp 695If the 696.Ar bind_address 697is not specified, the default is to only bind to loopback addresses. 698If the 699.Ar bind_address 700is 701.Ql * 702or an empty string, then the forwarding is requested to listen on all 703interfaces. 704Specifying a remote 705.Ar bind_address 706will only succeed if the server's 707.Cm GatewayPorts 708option is enabled (see 709.Xr sshd_config 5 ) . 710.It Cm RhostsRSAAuthentication 711Specifies whether to try rhosts based authentication with RSA host 712authentication. 713The argument must be 714.Dq yes 715or 716.Dq no . 717The default is 718.Dq no . 719This option applies to protocol version 1 only and requires 720.Nm ssh 721to be setuid root. 722.It Cm RSAAuthentication 723Specifies whether to try RSA authentication. 724The argument to this keyword must be 725.Dq yes 726or 727.Dq no . 728RSA authentication will only be 729attempted if the identity file exists, or an authentication agent is 730running. 731The default is 732.Dq yes . 733Note that this option applies to protocol version 1 only. 734.It Cm SendEnv 735Specifies what variables from the local 736.Xr environ 7 737should be sent to the server. 738Note that environment passing is only supported for protocol 2, the 739server must also support it, and the server must be configured to 740accept these environment variables. 741Refer to 742.Cm AcceptEnv 743in 744.Xr sshd_config 5 745for how to configure the server. 746Variables are specified by name, which may contain the wildcard characters 747.Ql \&* 748and 749.Ql \&? . 750Multiple environment variables may be separated by whitespace or spread 751across multiple 752.Cm SendEnv 753directives. 754The default is not to send any environment variables. 755.It Cm ServerAliveInterval 756Sets a timeout interval in seconds after which if no data has been received 757from the server, 758.Nm ssh 759will send a message through the encrypted 760channel to request a response from the server. 761The default 762is 0, indicating that these messages will not be sent to the server. 763This option applies to protocol version 2 only. 764.It Cm ServerAliveCountMax 765Sets the number of server alive messages (see above) which may be 766sent without 767.Nm ssh 768receiving any messages back from the server. 769If this threshold is reached while server alive messages are being sent, 770.Nm ssh 771will disconnect from the server, terminating the session. 772It is important to note that the use of server alive messages is very 773different from 774.Cm TCPKeepAlive 775(below). 776The server alive messages are sent through the encrypted channel 777and therefore will not be spoofable. 778The TCP keepalive option enabled by 779.Cm TCPKeepAlive 780is spoofable. 781The server alive mechanism is valuable when the client or 782server depend on knowing when a connection has become inactive. 783.Pp 784The default value is 3. 785If, for example, 786.Cm ServerAliveInterval 787(above) is set to 15, and 788.Cm ServerAliveCountMax 789is left at the default, if the server becomes unresponsive ssh 790will disconnect after approximately 45 seconds. 791.It Cm SmartcardDevice 792Specifies which smartcard device to use. 793The argument to this keyword is the device 794.Nm ssh 795should use to communicate with a smartcard used for storing the user's 796private RSA key. 797By default, no device is specified and smartcard support is not activated. 798.It Cm StrictHostKeyChecking 799If this flag is set to 800.Dq yes , 801.Nm ssh 802will never automatically add host keys to the 803.Pa ~/.ssh/known_hosts 804file, and refuses to connect to hosts whose host key has changed. 805This provides maximum protection against trojan horse attacks, 806however, can be annoying when the 807.Pa /etc/ssh/ssh_known_hosts 808file is poorly maintained, or connections to new hosts are 809frequently made. 810This option forces the user to manually 811add all new hosts. 812If this flag is set to 813.Dq no , 814.Nm ssh 815will automatically add new host keys to the 816user known hosts files. 817If this flag is set to 818.Dq ask , 819new host keys 820will be added to the user known host files only after the user 821has confirmed that is what they really want to do, and 822.Nm ssh 823will refuse to connect to hosts whose host key has changed. 824The host keys of 825known hosts will be verified automatically in all cases. 826The argument must be 827.Dq yes , 828.Dq no 829or 830.Dq ask . 831The default is 832.Dq ask . 833.It Cm TCPKeepAlive 834Specifies whether the system should send TCP keepalive messages to the 835other side. 836If they are sent, death of the connection or crash of one 837of the machines will be properly noticed. 838However, this means that 839connections will die if the route is down temporarily, and some people 840find it annoying. 841.Pp 842The default is 843.Dq yes 844(to send TCP keepalive messages), and the client will notice 845if the network goes down or the remote host dies. 846This is important in scripts, and many users want it too. 847.Pp 848To disable TCP keepalive messages, the value should be set to 849.Dq no . 850.It Cm UsePrivilegedPort 851Specifies whether to use a privileged port for outgoing connections. 852The argument must be 853.Dq yes 854or 855.Dq no . 856The default is 857.Dq no . 858If set to 859.Dq yes 860.Nm ssh 861must be setuid root. 862Note that this option must be set to 863.Dq yes 864for 865.Cm RhostsRSAAuthentication 866with older servers. 867.It Cm User 868Specifies the user to log in as. 869This can be useful when a different user name is used on different machines. 870This saves the trouble of 871having to remember to give the user name on the command line. 872.It Cm UserKnownHostsFile 873Specifies a file to use for the user 874host key database instead of 875.Pa ~/.ssh/known_hosts . 876.It Cm VerifyHostKeyDNS 877Specifies whether to verify the remote key using DNS and SSHFP resource 878records. 879If this option is set to 880.Dq yes , 881the client will implicitly trust keys that match a secure fingerprint 882from DNS. 883Insecure fingerprints will be handled as if this option was set to 884.Dq ask . 885If this option is set to 886.Dq ask , 887information on fingerprint match will be displayed, but the user will still 888need to confirm new host keys according to the 889.Cm StrictHostKeyChecking 890option. 891The argument must be 892.Dq yes , 893.Dq no 894or 895.Dq ask . 896The default is 897.Dq no . 898Note that this option applies to protocol version 2 only. 899.It Cm VersionAddendum 900Specifies a string to append to the regular version string to identify 901OS- or site-specific modifications. 902The default is 903.Dq FreeBSD-20050903 . 904.It Cm XAuthLocation 905Specifies the full pathname of the 906.Xr xauth 1 907program. 908The default is 909.Pa /usr/X11R6/bin/xauth . 910.El 911.Sh FILES 912.Bl -tag -width Ds 913.It Pa ~/.ssh/config 914This is the per-user configuration file. 915The format of this file is described above. 916This file is used by the 917.Nm ssh 918client. 919Because of the potential for abuse, this file must have strict permissions: 920read/write for the user, and not accessible by others. 921.It Pa /etc/ssh/ssh_config 922Systemwide configuration file. 923This file provides defaults for those 924values that are not specified in the user's configuration file, and 925for those users who do not have a configuration file. 926This file must be world-readable. 927.El 928.Sh SEE ALSO 929.Xr ssh 1 930.Sh AUTHORS 931OpenSSH is a derivative of the original and free 932ssh 1.2.12 release by Tatu Ylonen. 933Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 934Theo de Raadt and Dug Song 935removed many bugs, re-added newer features and 936created OpenSSH. 937Markus Friedl contributed the support for SSH 938protocol versions 1.5 and 2.0. 939