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.Dd $Mdocdate: June 29 2012 $ 38.Dt SSH_CONFIG 5 39.Os 40.Sh NAME 41.Nm ssh_config 42.Nd OpenSSH SSH client configuration files 43.Sh SYNOPSIS 44.Nm ~/.ssh/config 45.Nm /etc/ssh/ssh_config 46.Sh DESCRIPTION 47.Xr ssh 1 48obtains configuration data from the following sources in 49the following order: 50.Pp 51.Bl -enum -offset indent -compact 52.It 53command-line options 54.It 55user's configuration file 56.Pq Pa ~/.ssh/config 57.It 58system-wide configuration file 59.Pq Pa /etc/ssh/ssh_config 60.El 61.Pp 62For each parameter, the first obtained value 63will be used. 64The configuration files contain sections separated by 65.Dq Host 66specifications, and that section is only applied for hosts that 67match one of the patterns given in the specification. 68The matched host name is the one given on the command line. 69.Pp 70Since the first obtained value for each parameter is used, more 71host-specific declarations should be given near the beginning of the 72file, and general defaults at the end. 73.Pp 74The configuration file has the following format: 75.Pp 76Empty lines and lines starting with 77.Ql # 78are comments. 79Otherwise a line is of the format 80.Dq keyword arguments . 81Configuration options may be separated by whitespace or 82optional whitespace and exactly one 83.Ql = ; 84the latter format is useful to avoid the need to quote whitespace 85when specifying configuration options using the 86.Nm ssh , 87.Nm scp , 88and 89.Nm sftp 90.Fl o 91option. 92Arguments may optionally be enclosed in double quotes 93.Pq \&" 94in order to represent arguments containing spaces. 95.Pp 96The possible 97keywords and their meanings are as follows (note that 98keywords are case-insensitive and arguments are case-sensitive): 99.Bl -tag -width Ds 100.It Cm Host 101Restricts the following declarations (up to the next 102.Cm Host 103keyword) to be only for those hosts that match one of the patterns 104given after the keyword. 105If more than one pattern is provided, they should be separated by whitespace. 106A single 107.Ql * 108as a pattern can be used to provide global 109defaults for all hosts. 110The host is the 111.Ar hostname 112argument given on the command line (i.e. the name is not converted to 113a canonicalized host name before matching). 114.Pp 115A pattern entry may be negated by prefixing it with an exclamation mark 116.Pq Sq !\& . 117If a negated entry is matched, then the 118.Cm Host 119entry is ignored, regardless of whether any other patterns on the line 120match. 121Negated matches are therefore useful to provide exceptions for wildcard 122matches. 123.Pp 124See 125.Sx PATTERNS 126for more information on patterns. 127.It Cm AddressFamily 128Specifies which address family to use when connecting. 129Valid arguments are 130.Dq any , 131.Dq inet 132(use IPv4 only), or 133.Dq inet6 134(use IPv6 only). 135.It Cm BatchMode 136If set to 137.Dq yes , 138passphrase/password querying will be disabled. 139This option is useful in scripts and other batch jobs where no user 140is present to supply the password. 141The argument must be 142.Dq yes 143or 144.Dq no . 145The default is 146.Dq no . 147.It Cm BindAddress 148Use the specified address on the local machine as the source address of 149the connection. 150Only useful on systems with more than one address. 151Note that this option does not work if 152.Cm UsePrivilegedPort 153is set to 154.Dq yes . 155.It Cm ChallengeResponseAuthentication 156Specifies whether to use challenge-response authentication. 157The argument to this keyword must be 158.Dq yes 159or 160.Dq no . 161The default is 162.Dq yes . 163.It Cm CheckHostIP 164If this flag is set to 165.Dq yes , 166.Xr ssh 1 167will additionally check the host IP address in the 168.Pa known_hosts 169file. 170This allows ssh to detect if a host key changed due to DNS spoofing. 171If the option is set to 172.Dq no , 173the check will not be executed. 174The default is 175.Dq no . 176.It Cm Cipher 177Specifies the cipher to use for encrypting the session 178in protocol version 1. 179Currently, 180.Dq blowfish , 181.Dq 3des , 182and 183.Dq des 184are supported. 185.Ar des 186is only supported in the 187.Xr ssh 1 188client for interoperability with legacy protocol 1 implementations 189that do not support the 190.Ar 3des 191cipher. 192Its use is strongly discouraged due to cryptographic weaknesses. 193The default is 194.Dq 3des . 195.It Cm Ciphers 196Specifies the ciphers allowed for protocol version 2 197in order of preference. 198Multiple ciphers must be comma-separated. 199The supported ciphers are 200.Dq 3des-cbc , 201.Dq aes128-cbc , 202.Dq aes192-cbc , 203.Dq aes256-cbc , 204.Dq aes128-ctr , 205.Dq aes192-ctr , 206.Dq aes256-ctr , 207.Dq arcfour128 , 208.Dq arcfour256 , 209.Dq arcfour , 210.Dq blowfish-cbc , 211and 212.Dq cast128-cbc . 213The default is: 214.Bd -literal -offset 3n 215aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, 216aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, 217aes256-cbc,arcfour 218.Ed 219.It Cm ClearAllForwardings 220Specifies that all local, remote, and dynamic port forwardings 221specified in the configuration files or on the command line be 222cleared. 223This option is primarily useful when used from the 224.Xr ssh 1 225command line to clear port forwardings set in 226configuration files, and is automatically set by 227.Xr scp 1 228and 229.Xr sftp 1 . 230The argument must be 231.Dq yes 232or 233.Dq no . 234The default is 235.Dq no . 236.It Cm Compression 237Specifies whether to use compression. 238The argument must be 239.Dq yes 240or 241.Dq no . 242The default is 243.Dq no . 244.It Cm CompressionLevel 245Specifies the compression level to use if compression is enabled. 246The argument must be an integer from 1 (fast) to 9 (slow, best). 247The default level is 6, which is good for most applications. 248The meaning of the values is the same as in 249.Xr gzip 1 . 250Note that this option applies to protocol version 1 only. 251.It Cm ConnectionAttempts 252Specifies the number of tries (one per second) to make before exiting. 253The argument must be an integer. 254This may be useful in scripts if the connection sometimes fails. 255The default is 1. 256.It Cm ConnectTimeout 257Specifies the timeout (in seconds) used when connecting to the 258SSH server, instead of using the default system TCP timeout. 259This value is used only when the target is down or really unreachable, 260not when it refuses the connection. 261.It Cm ControlMaster 262Enables the sharing of multiple sessions over a single network connection. 263When set to 264.Dq yes , 265.Xr ssh 1 266will listen for connections on a control socket specified using the 267.Cm ControlPath 268argument. 269Additional sessions can connect to this socket using the same 270.Cm ControlPath 271with 272.Cm ControlMaster 273set to 274.Dq no 275(the default). 276These sessions will try to reuse the master instance's network connection 277rather than initiating new ones, but will fall back to connecting normally 278if the control socket does not exist, or is not listening. 279.Pp 280Setting this to 281.Dq ask 282will cause ssh 283to listen for control connections, but require confirmation using the 284.Ev SSH_ASKPASS 285program before they are accepted (see 286.Xr ssh-add 1 287for details). 288If the 289.Cm ControlPath 290cannot be opened, 291ssh will continue without connecting to a master instance. 292.Pp 293X11 and 294.Xr ssh-agent 1 295forwarding is supported over these multiplexed connections, however the 296display and agent forwarded will be the one belonging to the master 297connection i.e. it is not possible to forward multiple displays or agents. 298.Pp 299Two additional options allow for opportunistic multiplexing: try to use a 300master connection but fall back to creating a new one if one does not already 301exist. 302These options are: 303.Dq auto 304and 305.Dq autoask . 306The latter requires confirmation like the 307.Dq ask 308option. 309.It Cm ControlPath 310Specify the path to the control socket used for connection sharing as described 311in the 312.Cm ControlMaster 313section above or the string 314.Dq none 315to disable connection sharing. 316In the path, 317.Ql %L 318will be substituted by the first component of the local host name, 319.Ql %l 320will be substituted by the local host name (including any domain name), 321.Ql %h 322will be substituted by the target host name, 323.Ql %n 324will be substituted by the original target host name 325specified on the command line, 326.Ql %p 327the port, 328.Ql %r 329by the remote login username, and 330.Ql %u 331by the username of the user running 332.Xr ssh 1 . 333It is recommended that any 334.Cm ControlPath 335used for opportunistic connection sharing include 336at least %h, %p, and %r. 337This ensures that shared connections are uniquely identified. 338.It Cm ControlPersist 339When used in conjunction with 340.Cm ControlMaster , 341specifies that the master connection should remain open 342in the background (waiting for future client connections) 343after the initial client connection has been closed. 344If set to 345.Dq no , 346then the master connection will not be placed into the background, 347and will close as soon as the initial client connection is closed. 348If set to 349.Dq yes , 350then the master connection will remain in the background indefinitely 351(until killed or closed via a mechanism such as the 352.Xr ssh 1 353.Dq Fl O No exit 354option). 355If set to a time in seconds, or a time in any of the formats documented in 356.Xr sshd_config 5 , 357then the backgrounded master connection will automatically terminate 358after it has remained idle (with no client connections) for the 359specified time. 360.It Cm DynamicForward 361Specifies that a TCP port on the local machine be forwarded 362over the secure channel, and the application 363protocol is then used to determine where to connect to from the 364remote machine. 365.Pp 366The argument must be 367.Sm off 368.Oo Ar bind_address : Oc Ar port . 369.Sm on 370IPv6 addresses can be specified by enclosing addresses in square brackets. 371By default, the local port is bound in accordance with the 372.Cm GatewayPorts 373setting. 374However, an explicit 375.Ar bind_address 376may be used to bind the connection to a specific address. 377The 378.Ar bind_address 379of 380.Dq localhost 381indicates that the listening port be bound for local use only, while an 382empty address or 383.Sq * 384indicates that the port should be available from all interfaces. 385.Pp 386Currently the SOCKS4 and SOCKS5 protocols are supported, and 387.Xr ssh 1 388will act as a SOCKS server. 389Multiple forwardings may be specified, and 390additional forwardings can be given on the command line. 391Only the superuser can forward privileged ports. 392.It Cm EnableSSHKeysign 393Setting this option to 394.Dq yes 395in the global client configuration file 396.Pa /etc/ssh/ssh_config 397enables the use of the helper program 398.Xr ssh-keysign 8 399during 400.Cm HostbasedAuthentication . 401The argument must be 402.Dq yes 403or 404.Dq no . 405The default is 406.Dq no . 407This option should be placed in the non-hostspecific section. 408See 409.Xr ssh-keysign 8 410for more information. 411.It Cm EscapeChar 412Sets the escape character (default: 413.Ql ~ ) . 414The escape character can also 415be set on the command line. 416The argument should be a single character, 417.Ql ^ 418followed by a letter, or 419.Dq none 420to disable the escape 421character entirely (making the connection transparent for binary 422data). 423.It Cm ExitOnForwardFailure 424Specifies whether 425.Xr ssh 1 426should terminate the connection if it cannot set up all requested 427dynamic, tunnel, local, and remote port forwardings. 428The argument must be 429.Dq yes 430or 431.Dq no . 432The default is 433.Dq no . 434.It Cm ForwardAgent 435Specifies whether the connection to the authentication agent (if any) 436will be forwarded to the remote machine. 437The argument must be 438.Dq yes 439or 440.Dq no . 441The default is 442.Dq no . 443.Pp 444Agent forwarding should be enabled with caution. 445Users with the ability to bypass file permissions on the remote host 446(for the agent's Unix-domain socket) 447can access the local agent through the forwarded connection. 448An attacker cannot obtain key material from the agent, 449however they can perform operations on the keys that enable them to 450authenticate using the identities loaded into the agent. 451.It Cm ForwardX11 452Specifies whether X11 connections will be automatically redirected 453over the secure channel and 454.Ev DISPLAY 455set. 456The argument must be 457.Dq yes 458or 459.Dq no . 460The default is 461.Dq no . 462.Pp 463X11 forwarding should be enabled with caution. 464Users with the ability to bypass file permissions on the remote host 465(for the user's X11 authorization database) 466can access the local X11 display through the forwarded connection. 467An attacker may then be able to perform activities such as keystroke monitoring 468if the 469.Cm ForwardX11Trusted 470option is also enabled. 471.It Cm ForwardX11Timeout 472Specify a timeout for untrusted X11 forwarding 473using the format described in the 474.Sx TIME FORMATS 475section of 476.Xr sshd_config 5 . 477X11 connections received by 478.Xr ssh 1 479after this time will be refused. 480The default is to disable untrusted X11 forwarding after twenty minutes has 481elapsed. 482.It Cm ForwardX11Trusted 483If this option is set to 484.Dq yes , 485remote X11 clients will have full access to the original X11 display. 486.Pp 487If this option is set to 488.Dq no , 489remote X11 clients will be considered untrusted and prevented 490from stealing or tampering with data belonging to trusted X11 491clients. 492Furthermore, the 493.Xr xauth 1 494token used for the session will be set to expire after 20 minutes. 495Remote clients will be refused access after this time. 496.Pp 497The default is 498.Dq no . 499.Pp 500See the X11 SECURITY extension specification for full details on 501the restrictions imposed on untrusted clients. 502.It Cm GatewayPorts 503Specifies whether remote hosts are allowed to connect to local 504forwarded ports. 505By default, 506.Xr ssh 1 507binds local port forwardings to the loopback address. 508This prevents other remote hosts from connecting to forwarded ports. 509.Cm GatewayPorts 510can be used to specify that ssh 511should bind local port forwardings to the wildcard address, 512thus allowing remote hosts to connect to forwarded ports. 513The argument must be 514.Dq yes 515or 516.Dq no . 517The default is 518.Dq no . 519.It Cm GlobalKnownHostsFile 520Specifies one or more files to use for the global 521host key database, separated by whitespace. 522The default is 523.Pa /etc/ssh/ssh_known_hosts , 524.Pa /etc/ssh/ssh_known_hosts2 . 525.It Cm GSSAPIAuthentication 526Specifies whether user authentication based on GSSAPI is allowed. 527The default is 528.Dq no . 529Note that this option applies to protocol version 2 only. 530.It Cm GSSAPIDelegateCredentials 531Forward (delegate) credentials to the server. 532The default is 533.Dq no . 534Note that this option applies to protocol version 2 only. 535.It Cm HashKnownHosts 536Indicates that 537.Xr ssh 1 538should hash host names and addresses when they are added to 539.Pa ~/.ssh/known_hosts . 540These hashed names may be used normally by 541.Xr ssh 1 542and 543.Xr sshd 8 , 544but they do not reveal identifying information should the file's contents 545be disclosed. 546The default is 547.Dq no . 548Note that existing names and addresses in known hosts files 549will not be converted automatically, 550but may be manually hashed using 551.Xr ssh-keygen 1 . 552.It Cm HostbasedAuthentication 553Specifies whether to try rhosts based authentication with public key 554authentication. 555The argument must be 556.Dq yes 557or 558.Dq no . 559The default is 560.Dq no . 561This option applies to protocol version 2 only and 562is similar to 563.Cm RhostsRSAAuthentication . 564.It Cm HostKeyAlgorithms 565Specifies the protocol version 2 host key algorithms 566that the client wants to use in order of preference. 567The default for this option is: 568.Bd -literal -offset 3n 569ecdsa-sha2-nistp256-cert-v01@openssh.com, 570ecdsa-sha2-nistp384-cert-v01@openssh.com, 571ecdsa-sha2-nistp521-cert-v01@openssh.com, 572ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com, 573ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com, 574ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 575ssh-rsa,ssh-dss 576.Ed 577.Pp 578If hostkeys are known for the destination host then this default is modified 579to prefer their algorithms. 580.It Cm HostKeyAlias 581Specifies an alias that should be used instead of the 582real host name when looking up or saving the host key 583in the host key database files. 584This option is useful for tunneling SSH connections 585or for multiple servers running on a single host. 586.It Cm HostName 587Specifies the real host name to log into. 588This can be used to specify nicknames or abbreviations for hosts. 589If the hostname contains the character sequence 590.Ql %h , 591then this will be replaced with the host name specified on the command line 592(this is useful for manipulating unqualified names). 593The default is the name given on the command line. 594Numeric IP addresses are also permitted (both on the command line and in 595.Cm HostName 596specifications). 597.It Cm IdentitiesOnly 598Specifies that 599.Xr ssh 1 600should only use the authentication identity files configured in the 601.Nm 602files, 603even if 604.Xr ssh-agent 1 605offers more identities. 606The argument to this keyword must be 607.Dq yes 608or 609.Dq no . 610This option is intended for situations where ssh-agent 611offers many different identities. 612The default is 613.Dq no . 614.It Cm IdentityFile 615Specifies a file from which the user's DSA, ECDSA or RSA authentication 616identity is read. 617The default is 618.Pa ~/.ssh/identity 619for protocol version 1, and 620.Pa ~/.ssh/id_dsa , 621.Pa ~/.ssh/id_ecdsa 622and 623.Pa ~/.ssh/id_rsa 624for protocol version 2. 625Additionally, any identities represented by the authentication agent 626will be used for authentication. 627.Xr ssh 1 628will try to load certificate information from the filename obtained by 629appending 630.Pa -cert.pub 631to the path of a specified 632.Cm IdentityFile . 633.Pp 634The file name may use the tilde 635syntax to refer to a user's home directory or one of the following 636escape characters: 637.Ql %d 638(local user's home directory), 639.Ql %u 640(local user name), 641.Ql %l 642(local host name), 643.Ql %h 644(remote host name) or 645.Ql %r 646(remote user name). 647.Pp 648It is possible to have 649multiple identity files specified in configuration files; all these 650identities will be tried in sequence. 651Multiple 652.Cm IdentityFile 653directives will add to the list of identities tried (this behaviour 654differs from that of other configuration directives). 655.It Cm IPQoS 656Specifies the IPv4 type-of-service or DSCP class for connections. 657Accepted values are 658.Dq af11 , 659.Dq af12 , 660.Dq af13 , 661.Dq af21 , 662.Dq af22 , 663.Dq af23 , 664.Dq af31 , 665.Dq af32 , 666.Dq af33 , 667.Dq af41 , 668.Dq af42 , 669.Dq af43 , 670.Dq cs0 , 671.Dq cs1 , 672.Dq cs2 , 673.Dq cs3 , 674.Dq cs4 , 675.Dq cs5 , 676.Dq cs6 , 677.Dq cs7 , 678.Dq ef , 679.Dq lowdelay , 680.Dq throughput , 681.Dq reliability , 682or a numeric value. 683This option may take one or two arguments, separated by whitespace. 684If one argument is specified, it is used as the packet class unconditionally. 685If two values are specified, the first is automatically selected for 686interactive sessions and the second for non-interactive sessions. 687The default is 688.Dq lowdelay 689for interactive sessions and 690.Dq throughput 691for non-interactive sessions. 692.It Cm KbdInteractiveAuthentication 693Specifies whether to use keyboard-interactive authentication. 694The argument to this keyword must be 695.Dq yes 696or 697.Dq no . 698The default is 699.Dq yes . 700.It Cm KbdInteractiveDevices 701Specifies the list of methods to use in keyboard-interactive authentication. 702Multiple method names must be comma-separated. 703The default is to use the server specified list. 704The methods available vary depending on what the server supports. 705For an OpenSSH server, 706it may be zero or more of: 707.Dq bsdauth , 708.Dq pam , 709and 710.Dq skey . 711.It Cm KexAlgorithms 712Specifies the available KEX (Key Exchange) algorithms. 713Multiple algorithms must be comma-separated. 714The default is: 715.Bd -literal -offset indent 716ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 717diffie-hellman-group-exchange-sha256, 718diffie-hellman-group-exchange-sha1, 719diffie-hellman-group14-sha1, 720diffie-hellman-group1-sha1 721.Ed 722.It Cm LocalCommand 723Specifies a command to execute on the local machine after successfully 724connecting to the server. 725The command string extends to the end of the line, and is executed with 726the user's shell. 727The following escape character substitutions will be performed: 728.Ql %d 729(local user's home directory), 730.Ql %h 731(remote host name), 732.Ql %l 733(local host name), 734.Ql %n 735(host name as provided on the command line), 736.Ql %p 737(remote port), 738.Ql %r 739(remote user name) or 740.Ql %u 741(local user name). 742.Pp 743The command is run synchronously and does not have access to the 744session of the 745.Xr ssh 1 746that spawned it. 747It should not be used for interactive commands. 748.Pp 749This directive is ignored unless 750.Cm PermitLocalCommand 751has been enabled. 752.It Cm LocalForward 753Specifies that a TCP port on the local machine be forwarded over 754the secure channel to the specified host and port from the remote machine. 755The first argument must be 756.Sm off 757.Oo Ar bind_address : Oc Ar port 758.Sm on 759and the second argument must be 760.Ar host : Ns Ar hostport . 761IPv6 addresses can be specified by enclosing addresses in square brackets. 762Multiple forwardings may be specified, and additional forwardings can be 763given on the command line. 764Only the superuser can forward privileged ports. 765By default, the local port is bound in accordance with the 766.Cm GatewayPorts 767setting. 768However, an explicit 769.Ar bind_address 770may be used to bind the connection to a specific address. 771The 772.Ar bind_address 773of 774.Dq localhost 775indicates that the listening port be bound for local use only, while an 776empty address or 777.Sq * 778indicates that the port should be available from all interfaces. 779.It Cm LogLevel 780Gives the verbosity level that is used when logging messages from 781.Xr ssh 1 . 782The possible values are: 783QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 784The default is INFO. 785DEBUG and DEBUG1 are equivalent. 786DEBUG2 and DEBUG3 each specify higher levels of verbose output. 787.It Cm MACs 788Specifies the MAC (message authentication code) algorithms 789in order of preference. 790The MAC algorithm is used in protocol version 2 791for data integrity protection. 792Multiple algorithms must be comma-separated. 793The default is: 794.Bd -literal -offset indent 795hmac-md5,hmac-sha1,umac-64@openssh.com, 796hmac-sha2-256,hmac-sha2-512,hmac-ripemd160, 797hmac-sha1-96,hmac-md5-96 798.Ed 799.It Cm NoHostAuthenticationForLocalhost 800This option can be used if the home directory is shared across machines. 801In this case localhost will refer to a different machine on each of 802the machines and the user will get many warnings about changed host keys. 803However, this option disables host authentication for localhost. 804The argument to this keyword must be 805.Dq yes 806or 807.Dq no . 808The default is to check the host key for localhost. 809.It Cm NumberOfPasswordPrompts 810Specifies the number of password prompts before giving up. 811The argument to this keyword must be an integer. 812The default is 3. 813.It Cm PasswordAuthentication 814Specifies whether to use password authentication. 815The argument to this keyword must be 816.Dq yes 817or 818.Dq no . 819The default is 820.Dq yes . 821.It Cm PermitLocalCommand 822Allow local command execution via the 823.Ic LocalCommand 824option or using the 825.Ic !\& Ns Ar command 826escape sequence in 827.Xr ssh 1 . 828The argument must be 829.Dq yes 830or 831.Dq no . 832The default is 833.Dq no . 834.It Cm PKCS11Provider 835Specifies which PKCS#11 provider to use. 836The argument to this keyword is the PKCS#11 shared library 837.Xr ssh 1 838should use to communicate with a PKCS#11 token providing the user's 839private RSA key. 840.It Cm Port 841Specifies the port number to connect on the remote host. 842The default is 22. 843.It Cm PreferredAuthentications 844Specifies the order in which the client should try protocol 2 845authentication methods. 846This allows a client to prefer one method (e.g.\& 847.Cm keyboard-interactive ) 848over another method (e.g.\& 849.Cm password ) . 850The default is: 851.Bd -literal -offset indent 852gssapi-with-mic,hostbased,publickey, 853keyboard-interactive,password 854.Ed 855.It Cm Protocol 856Specifies the protocol versions 857.Xr ssh 1 858should support in order of preference. 859The possible values are 860.Sq 1 861and 862.Sq 2 . 863Multiple versions must be comma-separated. 864When this option is set to 865.Dq 2,1 866.Nm ssh 867will try version 2 and fall back to version 1 868if version 2 is not available. 869The default is 870.Sq 2 . 871.It Cm ProxyCommand 872Specifies the command to use to connect to the server. 873The command 874string extends to the end of the line, and is executed with 875the user's shell. 876In the command string, any occurrence of 877.Ql %h 878will be substituted by the host name to 879connect, 880.Ql %p 881by the port, and 882.Ql %r 883by the remote user name. 884The command can be basically anything, 885and should read from its standard input and write to its standard output. 886It should eventually connect an 887.Xr sshd 8 888server running on some machine, or execute 889.Ic sshd -i 890somewhere. 891Host key management will be done using the 892HostName of the host being connected (defaulting to the name typed by 893the user). 894Setting the command to 895.Dq none 896disables this option entirely. 897Note that 898.Cm CheckHostIP 899is not available for connects with a proxy command. 900.Pp 901This directive is useful in conjunction with 902.Xr nc 1 903and its proxy support. 904For example, the following directive would connect via an HTTP proxy at 905192.0.2.0: 906.Bd -literal -offset 3n 907ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 908.Ed 909.It Cm PubkeyAuthentication 910Specifies whether to try public key authentication. 911The argument to this keyword must be 912.Dq yes 913or 914.Dq no . 915The default is 916.Dq yes . 917This option applies to protocol version 2 only. 918.It Cm RekeyLimit 919Specifies the maximum amount of data that may be transmitted before the 920session key is renegotiated. 921The argument is the number of bytes, with an optional suffix of 922.Sq K , 923.Sq M , 924or 925.Sq G 926to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 927The default is between 928.Sq 1G 929and 930.Sq 4G , 931depending on the cipher. 932This option applies to protocol version 2 only. 933.It Cm RemoteForward 934Specifies that a TCP port on the remote machine be forwarded over 935the secure channel to the specified host and port from the local machine. 936The first argument must be 937.Sm off 938.Oo Ar bind_address : Oc Ar port 939.Sm on 940and the second argument must be 941.Ar host : Ns Ar hostport . 942IPv6 addresses can be specified by enclosing addresses in square brackets. 943Multiple forwardings may be specified, and additional 944forwardings can be given on the command line. 945Privileged ports can be forwarded only when 946logging in as root on the remote machine. 947.Pp 948If the 949.Ar port 950argument is 951.Ql 0 , 952the listen port will be dynamically allocated on the server and reported 953to the client at run time. 954.Pp 955If the 956.Ar bind_address 957is not specified, the default is to only bind to loopback addresses. 958If the 959.Ar bind_address 960is 961.Ql * 962or an empty string, then the forwarding is requested to listen on all 963interfaces. 964Specifying a remote 965.Ar bind_address 966will only succeed if the server's 967.Cm GatewayPorts 968option is enabled (see 969.Xr sshd_config 5 ) . 970.It Cm RequestTTY 971Specifies whether to request a pseudo-tty for the session. 972The argument may be one of: 973.Dq no 974(never request a TTY), 975.Dq yes 976(always request a TTY when standard input is a TTY), 977.Dq force 978(always request a TTY) or 979.Dq auto 980(request a TTY when opening a login session). 981This option mirrors the 982.Fl t 983and 984.Fl T 985flags for 986.Xr ssh 1 . 987.It Cm RhostsRSAAuthentication 988Specifies whether to try rhosts based authentication with RSA host 989authentication. 990The argument must be 991.Dq yes 992or 993.Dq no . 994The default is 995.Dq no . 996This option applies to protocol version 1 only and requires 997.Xr ssh 1 998to be setuid root. 999.It Cm RSAAuthentication 1000Specifies whether to try RSA authentication. 1001The argument to this keyword must be 1002.Dq yes 1003or 1004.Dq no . 1005RSA authentication will only be 1006attempted if the identity file exists, or an authentication agent is 1007running. 1008The default is 1009.Dq yes . 1010Note that this option applies to protocol version 1 only. 1011.It Cm SendEnv 1012Specifies what variables from the local 1013.Xr environ 7 1014should be sent to the server. 1015Note that environment passing is only supported for protocol 2. 1016The server must also support it, and the server must be configured to 1017accept these environment variables. 1018Refer to 1019.Cm AcceptEnv 1020in 1021.Xr sshd_config 5 1022for how to configure the server. 1023Variables are specified by name, which may contain wildcard characters. 1024Multiple environment variables may be separated by whitespace or spread 1025across multiple 1026.Cm SendEnv 1027directives. 1028The default is not to send any environment variables. 1029.Pp 1030See 1031.Sx PATTERNS 1032for more information on patterns. 1033.It Cm ServerAliveCountMax 1034Sets the number of server alive messages (see below) which may be 1035sent without 1036.Xr ssh 1 1037receiving any messages back from the server. 1038If this threshold is reached while server alive messages are being sent, 1039ssh will disconnect from the server, terminating the session. 1040It is important to note that the use of server alive messages is very 1041different from 1042.Cm TCPKeepAlive 1043(below). 1044The server alive messages are sent through the encrypted channel 1045and therefore will not be spoofable. 1046The TCP keepalive option enabled by 1047.Cm TCPKeepAlive 1048is spoofable. 1049The server alive mechanism is valuable when the client or 1050server depend on knowing when a connection has become inactive. 1051.Pp 1052The default value is 3. 1053If, for example, 1054.Cm ServerAliveInterval 1055(see below) is set to 15 and 1056.Cm ServerAliveCountMax 1057is left at the default, if the server becomes unresponsive, 1058ssh will disconnect after approximately 45 seconds. 1059This option applies to protocol version 2 only. 1060.It Cm ServerAliveInterval 1061Sets a timeout interval in seconds after which if no data has been received 1062from the server, 1063.Xr ssh 1 1064will send a message through the encrypted 1065channel to request a response from the server. 1066The default 1067is 0, indicating that these messages will not be sent to the server. 1068This option applies to protocol version 2 only. 1069.It Cm StrictHostKeyChecking 1070If this flag is set to 1071.Dq yes , 1072.Xr ssh 1 1073will never automatically add host keys to the 1074.Pa ~/.ssh/known_hosts 1075file, and refuses to connect to hosts whose host key has changed. 1076This provides maximum protection against trojan horse attacks, 1077though it can be annoying when the 1078.Pa /etc/ssh/ssh_known_hosts 1079file is poorly maintained or when connections to new hosts are 1080frequently made. 1081This option forces the user to manually 1082add all new hosts. 1083If this flag is set to 1084.Dq no , 1085ssh will automatically add new host keys to the 1086user known hosts files. 1087If this flag is set to 1088.Dq ask , 1089new host keys 1090will be added to the user known host files only after the user 1091has confirmed that is what they really want to do, and 1092ssh will refuse to connect to hosts whose host key has changed. 1093The host keys of 1094known hosts will be verified automatically in all cases. 1095The argument must be 1096.Dq yes , 1097.Dq no , 1098or 1099.Dq ask . 1100The default is 1101.Dq ask . 1102.It Cm TCPKeepAlive 1103Specifies whether the system should send TCP keepalive messages to the 1104other side. 1105If they are sent, death of the connection or crash of one 1106of the machines will be properly noticed. 1107However, this means that 1108connections will die if the route is down temporarily, and some people 1109find it annoying. 1110.Pp 1111The default is 1112.Dq yes 1113(to send TCP keepalive messages), and the client will notice 1114if the network goes down or the remote host dies. 1115This is important in scripts, and many users want it too. 1116.Pp 1117To disable TCP keepalive messages, the value should be set to 1118.Dq no . 1119.It Cm Tunnel 1120Request 1121.Xr tun 4 1122device forwarding between the client and the server. 1123The argument must be 1124.Dq yes , 1125.Dq point-to-point 1126(layer 3), 1127.Dq ethernet 1128(layer 2), 1129or 1130.Dq no . 1131Specifying 1132.Dq yes 1133requests the default tunnel mode, which is 1134.Dq point-to-point . 1135The default is 1136.Dq no . 1137.It Cm TunnelDevice 1138Specifies the 1139.Xr tun 4 1140devices to open on the client 1141.Pq Ar local_tun 1142and the server 1143.Pq Ar remote_tun . 1144.Pp 1145The argument must be 1146.Sm off 1147.Ar local_tun Op : Ar remote_tun . 1148.Sm on 1149The devices may be specified by numerical ID or the keyword 1150.Dq any , 1151which uses the next available tunnel device. 1152If 1153.Ar remote_tun 1154is not specified, it defaults to 1155.Dq any . 1156The default is 1157.Dq any:any . 1158.It Cm UsePrivilegedPort 1159Specifies whether to use a privileged port for outgoing connections. 1160The argument must be 1161.Dq yes 1162or 1163.Dq no . 1164The default is 1165.Dq no . 1166If set to 1167.Dq yes , 1168.Xr ssh 1 1169must be setuid root. 1170Note that this option must be set to 1171.Dq yes 1172for 1173.Cm RhostsRSAAuthentication 1174with older servers. 1175.It Cm User 1176Specifies the user to log in as. 1177This can be useful when a different user name is used on different machines. 1178This saves the trouble of 1179having to remember to give the user name on the command line. 1180.It Cm UserKnownHostsFile 1181Specifies one or more files to use for the user 1182host key database, separated by whitespace. 1183The default is 1184.Pa ~/.ssh/known_hosts , 1185.Pa ~/.ssh/known_hosts2 . 1186.It Cm VerifyHostKeyDNS 1187Specifies whether to verify the remote key using DNS and SSHFP resource 1188records. 1189If this option is set to 1190.Dq yes , 1191the client will implicitly trust keys that match a secure fingerprint 1192from DNS. 1193Insecure fingerprints will be handled as if this option was set to 1194.Dq ask . 1195If this option is set to 1196.Dq ask , 1197information on fingerprint match will be displayed, but the user will still 1198need to confirm new host keys according to the 1199.Cm StrictHostKeyChecking 1200option. 1201The argument must be 1202.Dq yes , 1203.Dq no , 1204or 1205.Dq ask . 1206The default is 1207.Dq no . 1208Note that this option applies to protocol version 2 only. 1209.Pp 1210See also 1211.Sx VERIFYING HOST KEYS 1212in 1213.Xr ssh 1 . 1214.It Cm VersionAddendum 1215Specifies a string to append to the regular version string to identify 1216OS- or site-specific modifications. 1217The default is 1218.Dq DragonFly-20121028 . 1219.It Cm VisualHostKey 1220If this flag is set to 1221.Dq yes , 1222an ASCII art representation of the remote host key fingerprint is 1223printed in addition to the hex fingerprint string at login and 1224for unknown host keys. 1225If this flag is set to 1226.Dq no , 1227no fingerprint strings are printed at login and 1228only the hex fingerprint string will be printed for unknown host keys. 1229The default is 1230.Dq no . 1231.It Cm XAuthLocation 1232Specifies the full pathname of the 1233.Xr xauth 1 1234program. 1235The default is 1236.Pa /usr/X11R6/bin/xauth . 1237.El 1238.Sh PATTERNS 1239A 1240.Em pattern 1241consists of zero or more non-whitespace characters, 1242.Sq * 1243(a wildcard that matches zero or more characters), 1244or 1245.Sq ?\& 1246(a wildcard that matches exactly one character). 1247For example, to specify a set of declarations for any host in the 1248.Dq .co.uk 1249set of domains, 1250the following pattern could be used: 1251.Pp 1252.Dl Host *.co.uk 1253.Pp 1254The following pattern 1255would match any host in the 192.168.0.[0-9] network range: 1256.Pp 1257.Dl Host 192.168.0.? 1258.Pp 1259A 1260.Em pattern-list 1261is a comma-separated list of patterns. 1262Patterns within pattern-lists may be negated 1263by preceding them with an exclamation mark 1264.Pq Sq !\& . 1265For example, 1266to allow a key to be used from anywhere within an organisation 1267except from the 1268.Dq dialup 1269pool, 1270the following entry (in authorized_keys) could be used: 1271.Pp 1272.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1273.Sh FILES 1274.Bl -tag -width Ds 1275.It Pa ~/.ssh/config 1276This is the per-user configuration file. 1277The format of this file is described above. 1278This file is used by the SSH client. 1279Because of the potential for abuse, this file must have strict permissions: 1280read/write for the user, and not accessible by others. 1281.It Pa /etc/ssh/ssh_config 1282Systemwide configuration file. 1283This file provides defaults for those 1284values that are not specified in the user's configuration file, and 1285for those users who do not have a configuration file. 1286This file must be world-readable. 1287.El 1288.Sh SEE ALSO 1289.Xr ssh 1 1290.Sh AUTHORS 1291OpenSSH is a derivative of the original and free 1292ssh 1.2.12 release by Tatu Ylonen. 1293Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1294Theo de Raadt and Dug Song 1295removed many bugs, re-added newer features and 1296created OpenSSH. 1297Markus Friedl contributed the support for SSH 1298protocol versions 1.5 and 2.0. 1299