1.\" 2.\" Author: Tatu Ylonen <ylo@cs.hut.fi> 3.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland 4.\" All rights reserved 5.\" 6.\" As far as I am concerned, the code I have written for this software 7.\" can be used freely for any purpose. Any derived versions of this 8.\" software must be clearly marked as such, and if the derived work is 9.\" incompatible with the protocol description in the RFC file, it must be 10.\" called by a name other than "ssh" or "Secure Shell". 11.\" 12.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. 13.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. 14.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. 15.\" 16.\" Redistribution and use in source and binary forms, with or without 17.\" modification, are permitted provided that the following conditions 18.\" are met: 19.\" 1. Redistributions of source code must retain the above copyright 20.\" notice, this list of conditions and the following disclaimer. 21.\" 2. Redistributions in binary form must reproduce the above copyright 22.\" notice, this list of conditions and the following disclaimer in the 23.\" documentation and/or other materials provided with the distribution. 24.\" 25.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 26.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 27.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 28.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 29.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 30.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 31.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 32.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 33.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 34.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 35.\" 36.\" $OpenBSD: ssh_config.5,v 1.157 2012/06/29 13:57:25 naddy Exp $ 37.\" $FreeBSD$ 38.Dd June 29 2012 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 116A pattern entry may be negated by prefixing it with an exclamation mark 117.Pq Sq !\& . 118If a negated entry is matched, then the 119.Cm Host 120entry is ignored, regardless of whether any other patterns on the line 121match. 122Negated matches are therefore useful to provide exceptions for wildcard 123matches. 124.Pp 125See 126.Sx PATTERNS 127for more information on patterns. 128.It Cm AddressFamily 129Specifies which address family to use when connecting. 130Valid arguments are 131.Dq any , 132.Dq inet 133(use IPv4 only), or 134.Dq inet6 135(use IPv6 only). 136.It Cm BatchMode 137If set to 138.Dq yes , 139passphrase/password querying will be disabled. 140This option is useful in scripts and other batch jobs where no user 141is present to supply the password. 142The argument must be 143.Dq yes 144or 145.Dq no . 146The default is 147.Dq no . 148.It Cm BindAddress 149Use the specified address on the local machine as the source address of 150the connection. 151Only useful on systems with more than one address. 152Note that this option does not work if 153.Cm UsePrivilegedPort 154is set to 155.Dq yes . 156.It Cm ChallengeResponseAuthentication 157Specifies whether to use challenge-response authentication. 158The argument to this keyword must be 159.Dq yes 160or 161.Dq no . 162The default is 163.Dq yes . 164.It Cm CheckHostIP 165If this flag is set to 166.Dq yes , 167.Xr ssh 1 168will additionally check the host IP address in the 169.Pa known_hosts 170file. 171This allows ssh to detect if a host key changed due to DNS spoofing. 172If the option is set to 173.Dq no , 174the check will not be executed. 175The default is 176.Dq no . 177.It Cm Cipher 178Specifies the cipher to use for encrypting the session 179in protocol version 1. 180Currently, 181.Dq blowfish , 182.Dq 3des , 183and 184.Dq des 185are supported. 186.Ar des 187is only supported in the 188.Xr ssh 1 189client for interoperability with legacy protocol 1 implementations 190that do not support the 191.Ar 3des 192cipher. 193Its use is strongly discouraged due to cryptographic weaknesses. 194The default is 195.Dq 3des . 196.It Cm Ciphers 197Specifies the ciphers allowed for protocol version 2 198in order of preference. 199Multiple ciphers must be comma-separated. 200The supported ciphers are 201.Dq 3des-cbc , 202.Dq aes128-cbc , 203.Dq aes192-cbc , 204.Dq aes256-cbc , 205.Dq aes128-ctr , 206.Dq aes192-ctr , 207.Dq aes256-ctr , 208.Dq arcfour128 , 209.Dq arcfour256 , 210.Dq arcfour , 211.Dq blowfish-cbc , 212and 213.Dq cast128-cbc . 214The default is: 215.Bd -literal -offset 3n 216aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, 217aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, 218aes256-cbc,arcfour 219.Ed 220.It Cm ClearAllForwardings 221Specifies that all local, remote, and dynamic port forwardings 222specified in the configuration files or on the command line be 223cleared. 224This option is primarily useful when used from the 225.Xr ssh 1 226command line to clear port forwardings set in 227configuration files, and is automatically set by 228.Xr scp 1 229and 230.Xr sftp 1 . 231The argument must be 232.Dq yes 233or 234.Dq no . 235The default is 236.Dq no . 237.It Cm Compression 238Specifies whether to use compression. 239The argument must be 240.Dq yes 241or 242.Dq no . 243The default is 244.Dq no . 245.It Cm CompressionLevel 246Specifies the compression level to use if compression is enabled. 247The argument must be an integer from 1 (fast) to 9 (slow, best). 248The default level is 6, which is good for most applications. 249The meaning of the values is the same as in 250.Xr gzip 1 . 251Note that this option applies to protocol version 1 only. 252.It Cm ConnectionAttempts 253Specifies the number of tries (one per second) to make before exiting. 254The argument must be an integer. 255This may be useful in scripts if the connection sometimes fails. 256The default is 1. 257.It Cm ConnectTimeout 258Specifies the timeout (in seconds) used when connecting to the 259SSH server, instead of using the default system TCP timeout. 260This value is used only when the target is down or really unreachable, 261not when it refuses the connection. 262.It Cm ControlMaster 263Enables the sharing of multiple sessions over a single network connection. 264When set to 265.Dq yes , 266.Xr ssh 1 267will listen for connections on a control socket specified using the 268.Cm ControlPath 269argument. 270Additional sessions can connect to this socket using the same 271.Cm ControlPath 272with 273.Cm ControlMaster 274set to 275.Dq no 276(the default). 277These sessions will try to reuse the master instance's network connection 278rather than initiating new ones, but will fall back to connecting normally 279if the control socket does not exist, or is not listening. 280.Pp 281Setting this to 282.Dq ask 283will cause ssh 284to listen for control connections, but require confirmation using the 285.Ev SSH_ASKPASS 286program before they are accepted (see 287.Xr ssh-add 1 288for details). 289If the 290.Cm ControlPath 291cannot be opened, 292ssh will continue without connecting to a master instance. 293.Pp 294X11 and 295.Xr ssh-agent 1 296forwarding is supported over these multiplexed connections, however the 297display and agent forwarded will be the one belonging to the master 298connection i.e. it is not possible to forward multiple displays or agents. 299.Pp 300Two additional options allow for opportunistic multiplexing: try to use a 301master connection but fall back to creating a new one if one does not already 302exist. 303These options are: 304.Dq auto 305and 306.Dq autoask . 307The latter requires confirmation like the 308.Dq ask 309option. 310.It Cm ControlPath 311Specify the path to the control socket used for connection sharing as described 312in the 313.Cm ControlMaster 314section above or the string 315.Dq none 316to disable connection sharing. 317In the path, 318.Ql %L 319will be substituted by the first component of the local host name, 320.Ql %l 321will be substituted by the local host name (including any domain name), 322.Ql %h 323will be substituted by the target host name, 324.Ql %n 325will be substituted by the original target host name 326specified on the command line, 327.Ql %p 328the port, 329.Ql %r 330by the remote login username, and 331.Ql %u 332by the username of the user running 333.Xr ssh 1 . 334It is recommended that any 335.Cm ControlPath 336used for opportunistic connection sharing include 337at least %h, %p, and %r. 338This ensures that shared connections are uniquely identified. 339.It Cm ControlPersist 340When used in conjunction with 341.Cm ControlMaster , 342specifies that the master connection should remain open 343in the background (waiting for future client connections) 344after the initial client connection has been closed. 345If set to 346.Dq no , 347then the master connection will not be placed into the background, 348and will close as soon as the initial client connection is closed. 349If set to 350.Dq yes , 351then the master connection will remain in the background indefinitely 352(until killed or closed via a mechanism such as the 353.Xr ssh 1 354.Dq Fl O No exit 355option). 356If set to a time in seconds, or a time in any of the formats documented in 357.Xr sshd_config 5 , 358then the backgrounded master connection will automatically terminate 359after it has remained idle (with no client connections) for the 360specified time. 361.It Cm DynamicForward 362Specifies that a TCP port on the local machine be forwarded 363over the secure channel, and the application 364protocol is then used to determine where to connect to from the 365remote machine. 366.Pp 367The argument must be 368.Sm off 369.Oo Ar bind_address : Oc Ar port . 370.Sm on 371IPv6 addresses can be specified by enclosing addresses in square brackets. 372By default, the local port is bound in accordance with the 373.Cm GatewayPorts 374setting. 375However, an explicit 376.Ar bind_address 377may be used to bind the connection to a specific address. 378The 379.Ar bind_address 380of 381.Dq localhost 382indicates that the listening port be bound for local use only, while an 383empty address or 384.Sq * 385indicates that the port should be available from all interfaces. 386.Pp 387Currently the SOCKS4 and SOCKS5 protocols are supported, and 388.Xr ssh 1 389will act as a SOCKS server. 390Multiple forwardings may be specified, and 391additional forwardings can be given on the command line. 392Only the superuser can forward privileged ports. 393.It Cm EnableSSHKeysign 394Setting this option to 395.Dq yes 396in the global client configuration file 397.Pa /etc/ssh/ssh_config 398enables the use of the helper program 399.Xr ssh-keysign 8 400during 401.Cm HostbasedAuthentication . 402The argument must be 403.Dq yes 404or 405.Dq no . 406The default is 407.Dq no . 408This option should be placed in the non-hostspecific section. 409See 410.Xr ssh-keysign 8 411for more information. 412.It Cm EscapeChar 413Sets the escape character (default: 414.Ql ~ ) . 415The escape character can also 416be set on the command line. 417The argument should be a single character, 418.Ql ^ 419followed by a letter, or 420.Dq none 421to disable the escape 422character entirely (making the connection transparent for binary 423data). 424.It Cm ExitOnForwardFailure 425Specifies whether 426.Xr ssh 1 427should terminate the connection if it cannot set up all requested 428dynamic, tunnel, local, and remote port forwardings. 429The argument must be 430.Dq yes 431or 432.Dq no . 433The default is 434.Dq no . 435.It Cm ForwardAgent 436Specifies whether the connection to the authentication agent (if any) 437will be forwarded to the remote machine. 438The argument must be 439.Dq yes 440or 441.Dq no . 442The default is 443.Dq no . 444.Pp 445Agent forwarding should be enabled with caution. 446Users with the ability to bypass file permissions on the remote host 447(for the agent's Unix-domain socket) 448can access the local agent through the forwarded connection. 449An attacker cannot obtain key material from the agent, 450however they can perform operations on the keys that enable them to 451authenticate using the identities loaded into the agent. 452.It Cm ForwardX11 453Specifies whether X11 connections will be automatically redirected 454over the secure channel and 455.Ev DISPLAY 456set. 457The argument must be 458.Dq yes 459or 460.Dq no . 461The default is 462.Dq no . 463.Pp 464X11 forwarding should be enabled with caution. 465Users with the ability to bypass file permissions on the remote host 466(for the user's X11 authorization database) 467can access the local X11 display through the forwarded connection. 468An attacker may then be able to perform activities such as keystroke monitoring 469if the 470.Cm ForwardX11Trusted 471option is also enabled. 472.It Cm ForwardX11Timeout 473Specify a timeout for untrusted X11 forwarding 474using the format described in the 475.Sx TIME FORMATS 476section of 477.Xr sshd_config 5 . 478X11 connections received by 479.Xr ssh 1 480after this time will be refused. 481The default is to disable untrusted X11 forwarding after twenty minutes has 482elapsed. 483.It Cm ForwardX11Trusted 484If this option is set to 485.Dq yes , 486remote X11 clients will have full access to the original X11 display. 487.Pp 488If this option is set to 489.Dq no , 490remote X11 clients will be considered untrusted and prevented 491from stealing or tampering with data belonging to trusted X11 492clients. 493Furthermore, the 494.Xr xauth 1 495token used for the session will be set to expire after 20 minutes. 496Remote clients will be refused access after this time. 497.Pp 498The default is 499.Dq no . 500.Pp 501See the X11 SECURITY extension specification for full details on 502the restrictions imposed on untrusted clients. 503.It Cm GatewayPorts 504Specifies whether remote hosts are allowed to connect to local 505forwarded ports. 506By default, 507.Xr ssh 1 508binds local port forwardings to the loopback address. 509This prevents other remote hosts from connecting to forwarded ports. 510.Cm GatewayPorts 511can be used to specify that ssh 512should bind local port forwardings to the wildcard address, 513thus allowing remote hosts to connect to forwarded ports. 514The argument must be 515.Dq yes 516or 517.Dq no . 518The default is 519.Dq no . 520.It Cm GlobalKnownHostsFile 521Specifies one or more files to use for the global 522host key database, separated by whitespace. 523The default is 524.Pa /etc/ssh/ssh_known_hosts , 525.Pa /etc/ssh/ssh_known_hosts2 . 526.It Cm GSSAPIAuthentication 527Specifies whether user authentication based on GSSAPI is allowed. 528The default is 529.Dq no . 530Note that this option applies to protocol version 2 only. 531.It Cm GSSAPIDelegateCredentials 532Forward (delegate) credentials to the server. 533The default is 534.Dq no . 535Note that this option applies to protocol version 2 only. 536.It Cm HashKnownHosts 537Indicates that 538.Xr ssh 1 539should hash host names and addresses when they are added to 540.Pa ~/.ssh/known_hosts . 541These hashed names may be used normally by 542.Xr ssh 1 543and 544.Xr sshd 8 , 545but they do not reveal identifying information should the file's contents 546be disclosed. 547The default is 548.Dq no . 549Note that existing names and addresses in known hosts files 550will not be converted automatically, 551but may be manually hashed using 552.Xr ssh-keygen 1 . 553.It Cm HostbasedAuthentication 554Specifies whether to try rhosts based authentication with public key 555authentication. 556The argument must be 557.Dq yes 558or 559.Dq no . 560The default is 561.Dq no . 562This option applies to protocol version 2 only and 563is similar to 564.Cm RhostsRSAAuthentication . 565.It Cm HostKeyAlgorithms 566Specifies the protocol version 2 host key algorithms 567that the client wants to use in order of preference. 568The default for this option is: 569.Bd -literal -offset 3n 570ecdsa-sha2-nistp256-cert-v01@openssh.com, 571ecdsa-sha2-nistp384-cert-v01@openssh.com, 572ecdsa-sha2-nistp521-cert-v01@openssh.com, 573ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com, 574ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com, 575ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 576ssh-rsa,ssh-dss 577.Ed 578.Pp 579If hostkeys are known for the destination host then this default is modified 580to prefer their algorithms. 581.It Cm HostKeyAlias 582Specifies an alias that should be used instead of the 583real host name when looking up or saving the host key 584in the host key database files. 585This option is useful for tunneling SSH connections 586or for multiple servers running on a single host. 587.It Cm HostName 588Specifies the real host name to log into. 589This can be used to specify nicknames or abbreviations for hosts. 590If the hostname contains the character sequence 591.Ql %h , 592then this will be replaced with the host name specified on the command line 593(this is useful for manipulating unqualified names). 594The default is the name given on the command line. 595Numeric IP addresses are also permitted (both on the command line and in 596.Cm HostName 597specifications). 598.It Cm IdentitiesOnly 599Specifies that 600.Xr ssh 1 601should only use the authentication identity files configured in the 602.Nm 603files, 604even if 605.Xr ssh-agent 1 606offers more identities. 607The argument to this keyword must be 608.Dq yes 609or 610.Dq no . 611This option is intended for situations where ssh-agent 612offers many different identities. 613The default is 614.Dq no . 615.It Cm IdentityFile 616Specifies a file from which the user's DSA, ECDSA or RSA authentication 617identity is read. 618The default is 619.Pa ~/.ssh/identity 620for protocol version 1, and 621.Pa ~/.ssh/id_dsa , 622.Pa ~/.ssh/id_ecdsa 623and 624.Pa ~/.ssh/id_rsa 625for protocol version 2. 626Additionally, any identities represented by the authentication agent 627will be used for authentication. 628.Xr ssh 1 629will try to load certificate information from the filename obtained by 630appending 631.Pa -cert.pub 632to the path of a specified 633.Cm IdentityFile . 634.Pp 635The file name may use the tilde 636syntax to refer to a user's home directory or one of the following 637escape characters: 638.Ql %d 639(local user's home directory), 640.Ql %u 641(local user name), 642.Ql %l 643(local host name), 644.Ql %h 645(remote host name) or 646.Ql %r 647(remote user name). 648.Pp 649It is possible to have 650multiple identity files specified in configuration files; all these 651identities will be tried in sequence. 652Multiple 653.Cm IdentityFile 654directives will add to the list of identities tried (this behaviour 655differs from that of other configuration directives). 656.It Cm IPQoS 657Specifies the IPv4 type-of-service or DSCP class for connections. 658Accepted values are 659.Dq af11 , 660.Dq af12 , 661.Dq af13 , 662.Dq af21 , 663.Dq af22 , 664.Dq af23 , 665.Dq af31 , 666.Dq af32 , 667.Dq af33 , 668.Dq af41 , 669.Dq af42 , 670.Dq af43 , 671.Dq cs0 , 672.Dq cs1 , 673.Dq cs2 , 674.Dq cs3 , 675.Dq cs4 , 676.Dq cs5 , 677.Dq cs6 , 678.Dq cs7 , 679.Dq ef , 680.Dq lowdelay , 681.Dq throughput , 682.Dq reliability , 683or a numeric value. 684This option may take one or two arguments, separated by whitespace. 685If one argument is specified, it is used as the packet class unconditionally. 686If two values are specified, the first is automatically selected for 687interactive sessions and the second for non-interactive sessions. 688The default is 689.Dq lowdelay 690for interactive sessions and 691.Dq throughput 692for non-interactive sessions. 693.It Cm KbdInteractiveAuthentication 694Specifies whether to use keyboard-interactive authentication. 695The argument to this keyword must be 696.Dq yes 697or 698.Dq no . 699The default is 700.Dq yes . 701.It Cm KbdInteractiveDevices 702Specifies the list of methods to use in keyboard-interactive authentication. 703Multiple method names must be comma-separated. 704The default is to use the server specified list. 705The methods available vary depending on what the server supports. 706For an OpenSSH server, 707it may be zero or more of: 708.Dq bsdauth , 709.Dq pam , 710and 711.Dq skey . 712.It Cm KexAlgorithms 713Specifies the available KEX (Key Exchange) algorithms. 714Multiple algorithms must be comma-separated. 715The default is: 716.Bd -literal -offset indent 717ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 718diffie-hellman-group-exchange-sha256, 719diffie-hellman-group-exchange-sha1, 720diffie-hellman-group14-sha1, 721diffie-hellman-group1-sha1 722.Ed 723.It Cm LocalCommand 724Specifies a command to execute on the local machine after successfully 725connecting to the server. 726The command string extends to the end of the line, and is executed with 727the user's shell. 728The following escape character substitutions will be performed: 729.Ql %d 730(local user's home directory), 731.Ql %h 732(remote host name), 733.Ql %l 734(local host name), 735.Ql %n 736(host name as provided on the command line), 737.Ql %p 738(remote port), 739.Ql %r 740(remote user name) or 741.Ql %u 742(local user name). 743.Pp 744The command is run synchronously and does not have access to the 745session of the 746.Xr ssh 1 747that spawned it. 748It should not be used for interactive commands. 749.Pp 750This directive is ignored unless 751.Cm PermitLocalCommand 752has been enabled. 753.It Cm LocalForward 754Specifies that a TCP port on the local machine be forwarded over 755the secure channel to the specified host and port from the remote machine. 756The first argument must be 757.Sm off 758.Oo Ar bind_address : Oc Ar port 759.Sm on 760and the second argument must be 761.Ar host : Ns Ar hostport . 762IPv6 addresses can be specified by enclosing addresses in square brackets. 763Multiple forwardings may be specified, and additional forwardings can be 764given on the command line. 765Only the superuser can forward privileged ports. 766By default, the local port is bound in accordance with the 767.Cm GatewayPorts 768setting. 769However, an explicit 770.Ar bind_address 771may be used to bind the connection to a specific address. 772The 773.Ar bind_address 774of 775.Dq localhost 776indicates that the listening port be bound for local use only, while an 777empty address or 778.Sq * 779indicates that the port should be available from all interfaces. 780.It Cm LogLevel 781Gives the verbosity level that is used when logging messages from 782.Xr ssh 1 . 783The possible values are: 784QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 785The default is INFO. 786DEBUG and DEBUG1 are equivalent. 787DEBUG2 and DEBUG3 each specify higher levels of verbose output. 788.It Cm MACs 789Specifies the MAC (message authentication code) algorithms 790in order of preference. 791The MAC algorithm is used in protocol version 2 792for data integrity protection. 793Multiple algorithms must be comma-separated. 794The default is: 795.Bd -literal -offset indent 796hmac-md5,hmac-sha1,umac-64@openssh.com, 797hmac-sha2-256,hmac-sha2-512,hmac-ripemd160, 798hmac-sha1-96,hmac-md5-96 799.Ed 800.It Cm NoHostAuthenticationForLocalhost 801This option can be used if the home directory is shared across machines. 802In this case localhost will refer to a different machine on each of 803the machines and the user will get many warnings about changed host keys. 804However, this option disables host authentication for localhost. 805The argument to this keyword must be 806.Dq yes 807or 808.Dq no . 809The default is to check the host key for localhost. 810.It Cm NumberOfPasswordPrompts 811Specifies the number of password prompts before giving up. 812The argument to this keyword must be an integer. 813The default is 3. 814.It Cm PasswordAuthentication 815Specifies whether to use password authentication. 816The argument to this keyword must be 817.Dq yes 818or 819.Dq no . 820The default is 821.Dq yes . 822.It Cm PermitLocalCommand 823Allow local command execution via the 824.Ic LocalCommand 825option or using the 826.Ic !\& Ns Ar command 827escape sequence in 828.Xr ssh 1 . 829The argument must be 830.Dq yes 831or 832.Dq no . 833The default is 834.Dq no . 835.It Cm PKCS11Provider 836Specifies which PKCS#11 provider to use. 837The argument to this keyword is the PKCS#11 shared library 838.Xr ssh 1 839should use to communicate with a PKCS#11 token providing the user's 840private RSA key. 841.It Cm Port 842Specifies the port number to connect on the remote host. 843The default is 22. 844.It Cm PreferredAuthentications 845Specifies the order in which the client should try protocol 2 846authentication methods. 847This allows a client to prefer one method (e.g.\& 848.Cm keyboard-interactive ) 849over another method (e.g.\& 850.Cm password ) . 851The default is: 852.Bd -literal -offset indent 853gssapi-with-mic,hostbased,publickey, 854keyboard-interactive,password 855.Ed 856.It Cm Protocol 857Specifies the protocol versions 858.Xr ssh 1 859should support in order of preference. 860The possible values are 861.Sq 1 862and 863.Sq 2 . 864Multiple versions must be comma-separated. 865When this option is set to 866.Dq 2,1 867.Nm ssh 868will try version 2 and fall back to version 1 869if version 2 is not available. 870The default is 871.Sq 2 . 872.It Cm ProxyCommand 873Specifies the command to use to connect to the server. 874The command 875string extends to the end of the line, and is executed with 876the user's shell. 877In the command string, any occurrence of 878.Ql %h 879will be substituted by the host name to 880connect, 881.Ql %p 882by the port, and 883.Ql %r 884by the remote user name. 885The command can be basically anything, 886and should read from its standard input and write to its standard output. 887It should eventually connect an 888.Xr sshd 8 889server running on some machine, or execute 890.Ic sshd -i 891somewhere. 892Host key management will be done using the 893HostName of the host being connected (defaulting to the name typed by 894the user). 895Setting the command to 896.Dq none 897disables this option entirely. 898Note that 899.Cm CheckHostIP 900is not available for connects with a proxy command. 901.Pp 902This directive is useful in conjunction with 903.Xr nc 1 904and its proxy support. 905For example, the following directive would connect via an HTTP proxy at 906192.0.2.0: 907.Bd -literal -offset 3n 908ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 909.Ed 910.It Cm PubkeyAuthentication 911Specifies whether to try public key authentication. 912The argument to this keyword must be 913.Dq yes 914or 915.Dq no . 916The default is 917.Dq yes . 918This option applies to protocol version 2 only. 919.It Cm RekeyLimit 920Specifies the maximum amount of data that may be transmitted before the 921session key is renegotiated. 922The argument is the number of bytes, with an optional suffix of 923.Sq K , 924.Sq M , 925or 926.Sq G 927to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 928The default is between 929.Sq 1G 930and 931.Sq 4G , 932depending on the cipher. 933This option applies to protocol version 2 only. 934.It Cm RemoteForward 935Specifies that a TCP port on the remote machine be forwarded over 936the secure channel to the specified host and port from the local machine. 937The first argument must be 938.Sm off 939.Oo Ar bind_address : Oc Ar port 940.Sm on 941and the second argument must be 942.Ar host : Ns Ar hostport . 943IPv6 addresses can be specified by enclosing addresses in square brackets. 944Multiple forwardings may be specified, and additional 945forwardings can be given on the command line. 946Privileged ports can be forwarded only when 947logging in as root on the remote machine. 948.Pp 949If the 950.Ar port 951argument is 952.Ql 0 , 953the listen port will be dynamically allocated on the server and reported 954to the client at run time. 955.Pp 956If the 957.Ar bind_address 958is not specified, the default is to only bind to loopback addresses. 959If the 960.Ar bind_address 961is 962.Ql * 963or an empty string, then the forwarding is requested to listen on all 964interfaces. 965Specifying a remote 966.Ar bind_address 967will only succeed if the server's 968.Cm GatewayPorts 969option is enabled (see 970.Xr sshd_config 5 ) . 971.It Cm RequestTTY 972Specifies whether to request a pseudo-tty for the session. 973The argument may be one of: 974.Dq no 975(never request a TTY), 976.Dq yes 977(always request a TTY when standard input is a TTY), 978.Dq force 979(always request a TTY) or 980.Dq auto 981(request a TTY when opening a login session). 982This option mirrors the 983.Fl t 984and 985.Fl T 986flags for 987.Xr ssh 1 . 988.It Cm RhostsRSAAuthentication 989Specifies whether to try rhosts based authentication with RSA host 990authentication. 991The argument must be 992.Dq yes 993or 994.Dq no . 995The default is 996.Dq no . 997This option applies to protocol version 1 only and requires 998.Xr ssh 1 999to be setuid root. 1000.It Cm RSAAuthentication 1001Specifies whether to try RSA authentication. 1002The argument to this keyword must be 1003.Dq yes 1004or 1005.Dq no . 1006RSA authentication will only be 1007attempted if the identity file exists, or an authentication agent is 1008running. 1009The default is 1010.Dq yes . 1011Note that this option applies to protocol version 1 only. 1012.It Cm SendEnv 1013Specifies what variables from the local 1014.Xr environ 7 1015should be sent to the server. 1016Note that environment passing is only supported for protocol 2. 1017The server must also support it, and the server must be configured to 1018accept these environment variables. 1019Refer to 1020.Cm AcceptEnv 1021in 1022.Xr sshd_config 5 1023for how to configure the server. 1024Variables are specified by name, which may contain wildcard characters. 1025Multiple environment variables may be separated by whitespace or spread 1026across multiple 1027.Cm SendEnv 1028directives. 1029The default is not to send any environment variables. 1030.Pp 1031See 1032.Sx PATTERNS 1033for more information on patterns. 1034.It Cm ServerAliveCountMax 1035Sets the number of server alive messages (see below) which may be 1036sent without 1037.Xr ssh 1 1038receiving any messages back from the server. 1039If this threshold is reached while server alive messages are being sent, 1040ssh will disconnect from the server, terminating the session. 1041It is important to note that the use of server alive messages is very 1042different from 1043.Cm TCPKeepAlive 1044(below). 1045The server alive messages are sent through the encrypted channel 1046and therefore will not be spoofable. 1047The TCP keepalive option enabled by 1048.Cm TCPKeepAlive 1049is spoofable. 1050The server alive mechanism is valuable when the client or 1051server depend on knowing when a connection has become inactive. 1052.Pp 1053The default value is 3. 1054If, for example, 1055.Cm ServerAliveInterval 1056(see below) is set to 15 and 1057.Cm ServerAliveCountMax 1058is left at the default, if the server becomes unresponsive, 1059ssh will disconnect after approximately 45 seconds. 1060This option applies to protocol version 2 only. 1061.It Cm ServerAliveInterval 1062Sets a timeout interval in seconds after which if no data has been received 1063from the server, 1064.Xr ssh 1 1065will send a message through the encrypted 1066channel to request a response from the server. 1067The default 1068is 0, indicating that these messages will not be sent to the server. 1069This option applies to protocol version 2 only. 1070.It Cm StrictHostKeyChecking 1071If this flag is set to 1072.Dq yes , 1073.Xr ssh 1 1074will never automatically add host keys to the 1075.Pa ~/.ssh/known_hosts 1076file, and refuses to connect to hosts whose host key has changed. 1077This provides maximum protection against trojan horse attacks, 1078though it can be annoying when the 1079.Pa /etc/ssh/ssh_known_hosts 1080file is poorly maintained or when connections to new hosts are 1081frequently made. 1082This option forces the user to manually 1083add all new hosts. 1084If this flag is set to 1085.Dq no , 1086ssh will automatically add new host keys to the 1087user known hosts files. 1088If this flag is set to 1089.Dq ask , 1090new host keys 1091will be added to the user known host files only after the user 1092has confirmed that is what they really want to do, and 1093ssh will refuse to connect to hosts whose host key has changed. 1094The host keys of 1095known hosts will be verified automatically in all cases. 1096The argument must be 1097.Dq yes , 1098.Dq no , 1099or 1100.Dq ask . 1101The default is 1102.Dq ask . 1103.It Cm TCPKeepAlive 1104Specifies whether the system should send TCP keepalive messages to the 1105other side. 1106If they are sent, death of the connection or crash of one 1107of the machines will be properly noticed. 1108However, this means that 1109connections will die if the route is down temporarily, and some people 1110find it annoying. 1111.Pp 1112The default is 1113.Dq yes 1114(to send TCP keepalive messages), and the client will notice 1115if the network goes down or the remote host dies. 1116This is important in scripts, and many users want it too. 1117.Pp 1118To disable TCP keepalive messages, the value should be set to 1119.Dq no . 1120.It Cm Tunnel 1121Request 1122.Xr tun 4 1123device forwarding between the client and the server. 1124The argument must be 1125.Dq yes , 1126.Dq point-to-point 1127(layer 3), 1128.Dq ethernet 1129(layer 2), 1130or 1131.Dq no . 1132Specifying 1133.Dq yes 1134requests the default tunnel mode, which is 1135.Dq point-to-point . 1136The default is 1137.Dq no . 1138.It Cm TunnelDevice 1139Specifies the 1140.Xr tun 4 1141devices to open on the client 1142.Pq Ar local_tun 1143and the server 1144.Pq Ar remote_tun . 1145.Pp 1146The argument must be 1147.Sm off 1148.Ar local_tun Op : Ar remote_tun . 1149.Sm on 1150The devices may be specified by numerical ID or the keyword 1151.Dq any , 1152which uses the next available tunnel device. 1153If 1154.Ar remote_tun 1155is not specified, it defaults to 1156.Dq any . 1157The default is 1158.Dq any:any . 1159.It Cm UsePrivilegedPort 1160Specifies whether to use a privileged port for outgoing connections. 1161The argument must be 1162.Dq yes 1163or 1164.Dq no . 1165The default is 1166.Dq no . 1167If set to 1168.Dq yes , 1169.Xr ssh 1 1170must be setuid root. 1171Note that this option must be set to 1172.Dq yes 1173for 1174.Cm RhostsRSAAuthentication 1175with older servers. 1176.It Cm User 1177Specifies the user to log in as. 1178This can be useful when a different user name is used on different machines. 1179This saves the trouble of 1180having to remember to give the user name on the command line. 1181.It Cm UserKnownHostsFile 1182Specifies one or more files to use for the user 1183host key database, separated by whitespace. 1184The default is 1185.Pa ~/.ssh/known_hosts , 1186.Pa ~/.ssh/known_hosts2 . 1187.It Cm VerifyHostKeyDNS 1188Specifies whether to verify the remote key using DNS and SSHFP resource 1189records. 1190If this option is set to 1191.Dq yes , 1192the client will implicitly trust keys that match a secure fingerprint 1193from DNS. 1194Insecure fingerprints will be handled as if this option was set to 1195.Dq ask . 1196If this option is set to 1197.Dq ask , 1198information on fingerprint match will be displayed, but the user will still 1199need to confirm new host keys according to the 1200.Cm StrictHostKeyChecking 1201option. 1202The argument must be 1203.Dq yes , 1204.Dq no , 1205or 1206.Dq ask . 1207The default is 1208.Dq no . 1209Note that this option applies to protocol version 2 only. 1210.Pp 1211See also 1212.Sx VERIFYING HOST KEYS 1213in 1214.Xr ssh 1 . 1215.It Cm VersionAddendum 1216Specifies a string to append to the regular version string to identify 1217OS- or site-specific modifications. 1218The default is 1219.Dq FreeBSD-20120901 . 1220.It Cm VisualHostKey 1221If this flag is set to 1222.Dq yes , 1223an ASCII art representation of the remote host key fingerprint is 1224printed in addition to the hex fingerprint string at login and 1225for unknown host keys. 1226If this flag is set to 1227.Dq no , 1228no fingerprint strings are printed at login and 1229only the hex fingerprint string will be printed for unknown host keys. 1230The default is 1231.Dq no . 1232.It Cm XAuthLocation 1233Specifies the full pathname of the 1234.Xr xauth 1 1235program. 1236The default is 1237.Pa /usr/local/bin/xauth . 1238.El 1239.Sh PATTERNS 1240A 1241.Em pattern 1242consists of zero or more non-whitespace characters, 1243.Sq * 1244(a wildcard that matches zero or more characters), 1245or 1246.Sq ?\& 1247(a wildcard that matches exactly one character). 1248For example, to specify a set of declarations for any host in the 1249.Dq .co.uk 1250set of domains, 1251the following pattern could be used: 1252.Pp 1253.Dl Host *.co.uk 1254.Pp 1255The following pattern 1256would match any host in the 192.168.0.[0-9] network range: 1257.Pp 1258.Dl Host 192.168.0.? 1259.Pp 1260A 1261.Em pattern-list 1262is a comma-separated list of patterns. 1263Patterns within pattern-lists may be negated 1264by preceding them with an exclamation mark 1265.Pq Sq !\& . 1266For example, 1267to allow a key to be used from anywhere within an organisation 1268except from the 1269.Dq dialup 1270pool, 1271the following entry (in authorized_keys) could be used: 1272.Pp 1273.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1274.Sh FILES 1275.Bl -tag -width Ds 1276.It Pa ~/.ssh/config 1277This is the per-user configuration file. 1278The format of this file is described above. 1279This file is used by the SSH client. 1280Because of the potential for abuse, this file must have strict permissions: 1281read/write for the user, and not accessible by others. 1282.It Pa /etc/ssh/ssh_config 1283Systemwide configuration file. 1284This file provides defaults for those 1285values that are not specified in the user's configuration file, and 1286for those users who do not have a configuration file. 1287This file must be world-readable. 1288.El 1289.Sh SEE ALSO 1290.Xr ssh 1 1291.Sh AUTHORS 1292OpenSSH is a derivative of the original and free 1293ssh 1.2.12 release by Tatu Ylonen. 1294Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1295Theo de Raadt and Dug Song 1296removed many bugs, re-added newer features and 1297created OpenSSH. 1298Markus Friedl contributed the support for SSH 1299protocol versions 1.5 and 2.0. 1300