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.1,v 1.430 2022/03/31 17:27:27 naddy Exp $ 37.Dd $Mdocdate: March 31 2022 $ 38.Dt SSH 1 39.Os 40.Sh NAME 41.Nm ssh 42.Nd OpenSSH remote login client 43.Sh SYNOPSIS 44.Nm ssh 45.Op Fl 46AaCfGgKkMNnqsTtVvXxYy 46.Op Fl B Ar bind_interface 47.Op Fl b Ar bind_address 48.Op Fl c Ar cipher_spec 49.Op Fl D Oo Ar bind_address : Oc Ns Ar port 50.Op Fl E Ar log_file 51.Op Fl e Ar escape_char 52.Op Fl F Ar configfile 53.Op Fl I Ar pkcs11 54.Op Fl i Ar identity_file 55.Op Fl J Ar destination 56.Op Fl L Ar address 57.Op Fl l Ar login_name 58.Op Fl m Ar mac_spec 59.Op Fl O Ar ctl_cmd 60.Op Fl o Ar option 61.Op Fl p Ar port 62.Op Fl Q Ar query_option 63.Op Fl R Ar address 64.Op Fl S Ar ctl_path 65.Op Fl W Ar host : Ns Ar port 66.Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun 67.Ar destination 68.Op Ar command Op Ar argument ... 69.Sh DESCRIPTION 70.Nm 71(SSH client) is a program for logging into a remote machine and for 72executing commands on a remote machine. 73It is intended to provide secure encrypted communications between 74two untrusted hosts over an insecure network. 75X11 connections, arbitrary TCP ports and 76.Ux Ns -domain 77sockets can also be forwarded over the secure channel. 78.Pp 79.Nm 80connects and logs into the specified 81.Ar destination , 82which may be specified as either 83.Sm off 84.Oo user @ Oc hostname 85.Sm on 86or a URI of the form 87.Sm off 88.No ssh:// Oo user @ Oc hostname Op : port . 89.Sm on 90The user must prove 91their identity to the remote machine using one of several methods 92(see below). 93.Pp 94If a 95.Ar command 96is specified, 97it will be executed on the remote host instead of a login shell. 98A complete command line may be specified as 99.Ar command , 100or it may have additional arguments. 101If supplied, the arguments will be appended to the command, separated by 102spaces, before it is sent to the server to be executed. 103.Pp 104The options are as follows: 105.Pp 106.Bl -tag -width Ds -compact 107.It Fl 4 108Forces 109.Nm 110to use IPv4 addresses only. 111.Pp 112.It Fl 6 113Forces 114.Nm 115to use IPv6 addresses only. 116.Pp 117.It Fl A 118Enables forwarding of connections from an authentication agent such as 119.Xr ssh-agent 1 . 120This can also be specified on a per-host basis in a configuration file. 121.Pp 122Agent forwarding should be enabled with caution. 123Users with the ability to bypass file permissions on the remote host 124(for the agent's 125.Ux Ns -domain 126socket) can access the local agent through the forwarded connection. 127An attacker cannot obtain key material from the agent, 128however they can perform operations on the keys that enable them to 129authenticate using the identities loaded into the agent. 130A safer alternative may be to use a jump host 131(see 132.Fl J ) . 133.Pp 134.It Fl a 135Disables forwarding of the authentication agent connection. 136.Pp 137.It Fl B Ar bind_interface 138Bind to the address of 139.Ar bind_interface 140before attempting to connect to the destination host. 141This is only useful on systems with more than one address. 142.Pp 143.It Fl b Ar bind_address 144Use 145.Ar bind_address 146on the local machine as the source address 147of the connection. 148Only useful on systems with more than one address. 149.Pp 150.It Fl C 151Requests compression of all data (including stdin, stdout, stderr, and 152data for forwarded X11, TCP and 153.Ux Ns -domain 154connections). 155The compression algorithm is the same used by 156.Xr gzip 1 . 157Compression is desirable on modem lines and other 158slow connections, but will only slow down things on fast networks. 159The default value can be set on a host-by-host basis in the 160configuration files; see the 161.Cm Compression 162option. 163.Pp 164.It Fl c Ar cipher_spec 165Selects the cipher specification for encrypting the session. 166.Ar cipher_spec 167is a comma-separated list of ciphers 168listed in order of preference. 169See the 170.Cm Ciphers 171keyword in 172.Xr ssh_config 5 173for more information. 174.Pp 175.It Fl D Xo 176.Sm off 177.Oo Ar bind_address : Oc 178.Ar port 179.Sm on 180.Xc 181Specifies a local 182.Dq dynamic 183application-level port forwarding. 184This works by allocating a socket to listen to 185.Ar port 186on the local side, optionally bound to the specified 187.Ar bind_address . 188Whenever a connection is made to this port, the 189connection is forwarded over the secure channel, and the application 190protocol is then used to determine where to connect to from the 191remote machine. 192Currently the SOCKS4 and SOCKS5 protocols are supported, and 193.Nm 194will act as a SOCKS server. 195Only root can forward privileged ports. 196Dynamic port forwardings can also be specified in the configuration file. 197.Pp 198IPv6 addresses can be specified by enclosing the address in square brackets. 199Only the superuser can forward privileged ports. 200By default, the local port is bound in accordance with the 201.Cm GatewayPorts 202setting. 203However, an explicit 204.Ar bind_address 205may be used to bind the connection to a specific address. 206The 207.Ar bind_address 208of 209.Dq localhost 210indicates that the listening port be bound for local use only, while an 211empty address or 212.Sq * 213indicates that the port should be available from all interfaces. 214.Pp 215.It Fl E Ar log_file 216Append debug logs to 217.Ar log_file 218instead of standard error. 219.Pp 220.It Fl e Ar escape_char 221Sets the escape character for sessions with a pty (default: 222.Ql ~ ) . 223The escape character is only recognized at the beginning of a line. 224The escape character followed by a dot 225.Pq Ql \&. 226closes the connection; 227followed by control-Z suspends the connection; 228and followed by itself sends the escape character once. 229Setting the character to 230.Dq none 231disables any escapes and makes the session fully transparent. 232.Pp 233.It Fl F Ar configfile 234Specifies an alternative per-user configuration file. 235If a configuration file is given on the command line, 236the system-wide configuration file 237.Pq Pa /etc/ssh/ssh_config 238will be ignored. 239The default for the per-user configuration file is 240.Pa ~/.ssh/config . 241If set to 242.Dq none , 243no configuration files will be read. 244.Pp 245.It Fl f 246Requests 247.Nm 248to go to background just before command execution. 249This is useful if 250.Nm 251is going to ask for passwords or passphrases, but the user 252wants it in the background. 253This implies 254.Fl n . 255The recommended way to start X11 programs at a remote site is with 256something like 257.Ic ssh -f host xterm . 258.Pp 259If the 260.Cm ExitOnForwardFailure 261configuration option is set to 262.Dq yes , 263then a client started with 264.Fl f 265will wait for all remote port forwards to be successfully established 266before placing itself in the background. 267Refer to the description of 268.Cm ForkAfterAuthentication 269in 270.Xr ssh_config 5 271for details. 272.Pp 273.It Fl G 274Causes 275.Nm 276to print its configuration after evaluating 277.Cm Host 278and 279.Cm Match 280blocks and exit. 281.Pp 282.It Fl g 283Allows remote hosts to connect to local forwarded ports. 284If used on a multiplexed connection, then this option must be specified 285on the master process. 286.Pp 287.It Fl I Ar pkcs11 288Specify the PKCS#11 shared library 289.Nm 290should use to communicate with a PKCS#11 token providing keys for user 291authentication. 292.Pp 293.It Fl i Ar identity_file 294Selects a file from which the identity (private key) for 295public key authentication is read. 296You can also specify a public key file to use the corresponding 297private key that is loaded in 298.Xr ssh-agent 1 299when the private key file is not present locally. 300The default is 301.Pa ~/.ssh/id_rsa , 302.Pa ~/.ssh/id_ecdsa , 303.Pa ~/.ssh/id_ecdsa_sk , 304.Pa ~/.ssh/id_ed25519 , 305.Pa ~/.ssh/id_ed25519_sk 306and 307.Pa ~/.ssh/id_dsa . 308Identity files may also be specified on 309a per-host basis in the configuration file. 310It is possible to have multiple 311.Fl i 312options (and multiple identities specified in 313configuration files). 314If no certificates have been explicitly specified by the 315.Cm CertificateFile 316directive, 317.Nm 318will also try to load certificate information from the filename obtained 319by appending 320.Pa -cert.pub 321to identity filenames. 322.Pp 323.It Fl J Ar destination 324Connect to the target host by first making a 325.Nm 326connection to the jump host described by 327.Ar destination 328and then establishing a TCP forwarding to the ultimate destination from 329there. 330Multiple jump hops may be specified separated by comma characters. 331This is a shortcut to specify a 332.Cm ProxyJump 333configuration directive. 334Note that configuration directives supplied on the command-line generally 335apply to the destination host and not any specified jump hosts. 336Use 337.Pa ~/.ssh/config 338to specify configuration for jump hosts. 339.Pp 340.It Fl K 341Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI 342credentials to the server. 343.Pp 344.It Fl k 345Disables forwarding (delegation) of GSSAPI credentials to the server. 346.Pp 347.It Fl L Xo 348.Sm off 349.Oo Ar bind_address : Oc 350.Ar port : host : hostport 351.Sm on 352.Xc 353.It Fl L Xo 354.Sm off 355.Oo Ar bind_address : Oc 356.Ar port : remote_socket 357.Sm on 358.Xc 359.It Fl L Xo 360.Sm off 361.Ar local_socket : host : hostport 362.Sm on 363.Xc 364.It Fl L Xo 365.Sm off 366.Ar local_socket : remote_socket 367.Sm on 368.Xc 369Specifies that connections to the given TCP port or Unix socket on the local 370(client) host are to be forwarded to the given host and port, or Unix socket, 371on the remote side. 372This works by allocating a socket to listen to either a TCP 373.Ar port 374on the local side, optionally bound to the specified 375.Ar bind_address , 376or to a Unix socket. 377Whenever a connection is made to the local port or socket, the 378connection is forwarded over the secure channel, and a connection is 379made to either 380.Ar host 381port 382.Ar hostport , 383or the Unix socket 384.Ar remote_socket , 385from the remote machine. 386.Pp 387Port forwardings can also be specified in the configuration file. 388Only the superuser can forward privileged ports. 389IPv6 addresses can be specified by enclosing the address in square brackets. 390.Pp 391By default, the local port is bound in accordance with the 392.Cm GatewayPorts 393setting. 394However, an explicit 395.Ar bind_address 396may be used to bind the connection to a specific address. 397The 398.Ar bind_address 399of 400.Dq localhost 401indicates that the listening port be bound for local use only, while an 402empty address or 403.Sq * 404indicates that the port should be available from all interfaces. 405.Pp 406.It Fl l Ar login_name 407Specifies the user to log in as on the remote machine. 408This also may be specified on a per-host basis in the configuration file. 409.Pp 410.It Fl M 411Places the 412.Nm 413client into 414.Dq master 415mode for connection sharing. 416Multiple 417.Fl M 418options places 419.Nm 420into 421.Dq master 422mode but with confirmation required using 423.Xr ssh-askpass 1 424before each operation that changes the multiplexing state 425(e.g. opening a new session). 426Refer to the description of 427.Cm ControlMaster 428in 429.Xr ssh_config 5 430for details. 431.Pp 432.It Fl m Ar mac_spec 433A comma-separated list of MAC (message authentication code) algorithms, 434specified in order of preference. 435See the 436.Cm MACs 437keyword for more information. 438.Pp 439.It Fl N 440Do not execute a remote command. 441This is useful for just forwarding ports. 442Refer to the description of 443.Cm SessionType 444in 445.Xr ssh_config 5 446for details. 447.Pp 448.It Fl n 449Redirects stdin from 450.Pa /dev/null 451(actually, prevents reading from stdin). 452This must be used when 453.Nm 454is run in the background. 455A common trick is to use this to run X11 programs on a remote machine. 456For example, 457.Ic ssh -n shadows.cs.hut.fi emacs & 458will start an emacs on shadows.cs.hut.fi, and the X11 459connection will be automatically forwarded over an encrypted channel. 460The 461.Nm 462program will be put in the background. 463(This does not work if 464.Nm 465needs to ask for a password or passphrase; see also the 466.Fl f 467option.) 468Refer to the description of 469.Cm StdinNull 470in 471.Xr ssh_config 5 472for details. 473.Pp 474.It Fl O Ar ctl_cmd 475Control an active connection multiplexing master process. 476When the 477.Fl O 478option is specified, the 479.Ar ctl_cmd 480argument is interpreted and passed to the master process. 481Valid commands are: 482.Dq check 483(check that the master process is running), 484.Dq forward 485(request forwardings without command execution), 486.Dq cancel 487(cancel forwardings), 488.Dq exit 489(request the master to exit), and 490.Dq stop 491(request the master to stop accepting further multiplexing requests). 492.Pp 493.It Fl o Ar option 494Can be used to give options in the format used in the configuration file. 495This is useful for specifying options for which there is no separate 496command-line flag. 497For full details of the options listed below, and their possible values, see 498.Xr ssh_config 5 . 499.Pp 500.Bl -tag -width Ds -offset indent -compact 501.It AddKeysToAgent 502.It AddressFamily 503.It BatchMode 504.It BindAddress 505.It CanonicalDomains 506.It CanonicalizeFallbackLocal 507.It CanonicalizeHostname 508.It CanonicalizeMaxDots 509.It CanonicalizePermittedCNAMEs 510.It CASignatureAlgorithms 511.It CertificateFile 512.It CheckHostIP 513.It Ciphers 514.It ClearAllForwardings 515.It Compression 516.It ConnectionAttempts 517.It ConnectTimeout 518.It ControlMaster 519.It ControlPath 520.It ControlPersist 521.It DynamicForward 522.It EscapeChar 523.It ExitOnForwardFailure 524.It FingerprintHash 525.It ForkAfterAuthentication 526.It ForwardAgent 527.It ForwardX11 528.It ForwardX11Timeout 529.It ForwardX11Trusted 530.It GatewayPorts 531.It GlobalKnownHostsFile 532.It GSSAPIAuthentication 533.It GSSAPIDelegateCredentials 534.It HashKnownHosts 535.It Host 536.It HostbasedAcceptedAlgorithms 537.It HostbasedAuthentication 538.It HostKeyAlgorithms 539.It HostKeyAlias 540.It Hostname 541.It IdentitiesOnly 542.It IdentityAgent 543.It IdentityFile 544.It IPQoS 545.It KbdInteractiveAuthentication 546.It KbdInteractiveDevices 547.It KexAlgorithms 548.It KnownHostsCommand 549.It LocalCommand 550.It LocalForward 551.It LogLevel 552.It MACs 553.It Match 554.It NoHostAuthenticationForLocalhost 555.It NumberOfPasswordPrompts 556.It PasswordAuthentication 557.It PermitLocalCommand 558.It PermitRemoteOpen 559.It PKCS11Provider 560.It Port 561.It PreferredAuthentications 562.It ProxyCommand 563.It ProxyJump 564.It ProxyUseFdpass 565.It PubkeyAcceptedAlgorithms 566.It PubkeyAuthentication 567.It RekeyLimit 568.It RemoteCommand 569.It RemoteForward 570.It RequestTTY 571.It SendEnv 572.It ServerAliveInterval 573.It ServerAliveCountMax 574.It SessionType 575.It SetEnv 576.It StdinNull 577.It StreamLocalBindMask 578.It StreamLocalBindUnlink 579.It StrictHostKeyChecking 580.It TCPKeepAlive 581.It Tunnel 582.It TunnelDevice 583.It UpdateHostKeys 584.It User 585.It UserKnownHostsFile 586.It VerifyHostKeyDNS 587.It VersionAddendum 588.It VisualHostKey 589.It XAuthLocation 590.El 591.Pp 592.It Fl p Ar port 593Port to connect to on the remote host. 594This can be specified on a 595per-host basis in the configuration file. 596.Pp 597.It Fl Q Ar query_option 598Queries for the algorithms supported by one of the following features: 599.Ar cipher 600(supported symmetric ciphers), 601.Ar cipher-auth 602(supported symmetric ciphers that support authenticated encryption), 603.Ar help 604(supported query terms for use with the 605.Fl Q 606flag), 607.Ar mac 608(supported message integrity codes), 609.Ar kex 610(key exchange algorithms), 611.Ar key 612(key types), 613.Ar key-cert 614(certificate key types), 615.Ar key-plain 616(non-certificate key types), 617.Ar key-sig 618(all key types and signature algorithms), 619.Ar protocol-version 620(supported SSH protocol versions), and 621.Ar sig 622(supported signature algorithms). 623Alternatively, any keyword from 624.Xr ssh_config 5 625or 626.Xr sshd_config 5 627that takes an algorithm list may be used as an alias for the corresponding 628query_option. 629.Pp 630.It Fl q 631Quiet mode. 632Causes most warning and diagnostic messages to be suppressed. 633.Pp 634.It Fl R Xo 635.Sm off 636.Oo Ar bind_address : Oc 637.Ar port : host : hostport 638.Sm on 639.Xc 640.It Fl R Xo 641.Sm off 642.Oo Ar bind_address : Oc 643.Ar port : local_socket 644.Sm on 645.Xc 646.It Fl R Xo 647.Sm off 648.Ar remote_socket : host : hostport 649.Sm on 650.Xc 651.It Fl R Xo 652.Sm off 653.Ar remote_socket : local_socket 654.Sm on 655.Xc 656.It Fl R Xo 657.Sm off 658.Oo Ar bind_address : Oc 659.Ar port 660.Sm on 661.Xc 662Specifies that connections to the given TCP port or Unix socket on the remote 663(server) host are to be forwarded to the local side. 664.Pp 665This works by allocating a socket to listen to either a TCP 666.Ar port 667or to a Unix socket on the remote side. 668Whenever a connection is made to this port or Unix socket, the 669connection is forwarded over the secure channel, and a connection 670is made from the local machine to either an explicit destination specified by 671.Ar host 672port 673.Ar hostport , 674or 675.Ar local_socket , 676or, if no explicit destination was specified, 677.Nm 678will act as a SOCKS 4/5 proxy and forward connections to the destinations 679requested by the remote SOCKS client. 680.Pp 681Port forwardings can also be specified in the configuration file. 682Privileged ports can be forwarded only when 683logging in as root on the remote machine. 684IPv6 addresses can be specified by enclosing the address in square brackets. 685.Pp 686By default, TCP listening sockets on the server will be bound to the loopback 687interface only. 688This may be overridden by specifying a 689.Ar bind_address . 690An empty 691.Ar bind_address , 692or the address 693.Ql * , 694indicates that the remote socket should listen on all interfaces. 695Specifying a remote 696.Ar bind_address 697will only succeed if the server's 698.Cm GatewayPorts 699option is enabled (see 700.Xr sshd_config 5 ) . 701.Pp 702If the 703.Ar port 704argument is 705.Ql 0 , 706the listen port will be dynamically allocated on the server and reported 707to the client at run time. 708When used together with 709.Ic -O forward , 710the allocated port will be printed to the standard output. 711.Pp 712.It Fl S Ar ctl_path 713Specifies the location of a control socket for connection sharing, 714or the string 715.Dq none 716to disable connection sharing. 717Refer to the description of 718.Cm ControlPath 719and 720.Cm ControlMaster 721in 722.Xr ssh_config 5 723for details. 724.Pp 725.It Fl s 726May be used to request invocation of a subsystem on the remote system. 727Subsystems facilitate the use of SSH 728as a secure transport for other applications (e.g.\& 729.Xr sftp 1 ) . 730The subsystem is specified as the remote command. 731Refer to the description of 732.Cm SessionType 733in 734.Xr ssh_config 5 735for details. 736.Pp 737.It Fl T 738Disable pseudo-terminal allocation. 739.Pp 740.It Fl t 741Force pseudo-terminal allocation. 742This can be used to execute arbitrary 743screen-based programs on a remote machine, which can be very useful, 744e.g. when implementing menu services. 745Multiple 746.Fl t 747options force tty allocation, even if 748.Nm 749has no local tty. 750.Pp 751.It Fl V 752Display the version number and exit. 753.Pp 754.It Fl v 755Verbose mode. 756Causes 757.Nm 758to print debugging messages about its progress. 759This is helpful in 760debugging connection, authentication, and configuration problems. 761Multiple 762.Fl v 763options increase the verbosity. 764The maximum is 3. 765.Pp 766.It Fl W Ar host : Ns Ar port 767Requests that standard input and output on the client be forwarded to 768.Ar host 769on 770.Ar port 771over the secure channel. 772Implies 773.Fl N , 774.Fl T , 775.Cm ExitOnForwardFailure 776and 777.Cm ClearAllForwardings , 778though these can be overridden in the configuration file or using 779.Fl o 780command line options. 781.Pp 782.It Fl w Xo 783.Ar local_tun Ns Op : Ns Ar remote_tun 784.Xc 785Requests 786tunnel 787device forwarding with the specified 788.Xr tun 4 789devices between the client 790.Pq Ar local_tun 791and the server 792.Pq Ar remote_tun . 793.Pp 794The devices may be specified by numerical ID or the keyword 795.Dq any , 796which uses the next available tunnel device. 797If 798.Ar remote_tun 799is not specified, it defaults to 800.Dq any . 801See also the 802.Cm Tunnel 803and 804.Cm TunnelDevice 805directives in 806.Xr ssh_config 5 . 807.Pp 808If the 809.Cm Tunnel 810directive is unset, it will be set to the default tunnel mode, which is 811.Dq point-to-point . 812If a different 813.Cm Tunnel 814forwarding mode it desired, then it should be specified before 815.Fl w . 816.Pp 817.It Fl X 818Enables X11 forwarding. 819This can also be specified on a per-host basis in a configuration file. 820.Pp 821X11 forwarding should be enabled with caution. 822Users with the ability to bypass file permissions on the remote host 823(for the user's X authorization database) 824can access the local X11 display through the forwarded connection. 825An attacker may then be able to perform activities such as keystroke monitoring. 826.Pp 827For this reason, X11 forwarding is subjected to X11 SECURITY extension 828restrictions by default. 829Refer to the 830.Nm 831.Fl Y 832option and the 833.Cm ForwardX11Trusted 834directive in 835.Xr ssh_config 5 836for more information. 837.Pp 838.It Fl x 839Disables X11 forwarding. 840.Pp 841.It Fl Y 842Enables trusted X11 forwarding. 843Trusted X11 forwardings are not subjected to the X11 SECURITY extension 844controls. 845.Pp 846.It Fl y 847Send log information using the 848.Xr syslog 3 849system module. 850By default this information is sent to stderr. 851.El 852.Pp 853.Nm 854may additionally obtain configuration data from 855a per-user configuration file and a system-wide configuration file. 856The file format and configuration options are described in 857.Xr ssh_config 5 . 858.Sh AUTHENTICATION 859The OpenSSH SSH client supports SSH protocol 2. 860.Pp 861The methods available for authentication are: 862GSSAPI-based authentication, 863host-based authentication, 864public key authentication, 865keyboard-interactive authentication, 866and password authentication. 867Authentication methods are tried in the order specified above, 868though 869.Cm PreferredAuthentications 870can be used to change the default order. 871.Pp 872Host-based authentication works as follows: 873If the machine the user logs in from is listed in 874.Pa /etc/hosts.equiv 875or 876.Pa /etc/shosts.equiv 877on the remote machine, the user is non-root and the user names are 878the same on both sides, or if the files 879.Pa ~/.rhosts 880or 881.Pa ~/.shosts 882exist in the user's home directory on the 883remote machine and contain a line containing the name of the client 884machine and the name of the user on that machine, the user is 885considered for login. 886Additionally, the server 887.Em must 888be able to verify the client's 889host key (see the description of 890.Pa /etc/ssh/ssh_known_hosts 891and 892.Pa ~/.ssh/known_hosts , 893below) 894for login to be permitted. 895This authentication method closes security holes due to IP 896spoofing, DNS spoofing, and routing spoofing. 897[Note to the administrator: 898.Pa /etc/hosts.equiv , 899.Pa ~/.rhosts , 900and the rlogin/rsh protocol in general, are inherently insecure and should be 901disabled if security is desired.] 902.Pp 903Public key authentication works as follows: 904The scheme is based on public-key cryptography, 905using cryptosystems 906where encryption and decryption are done using separate keys, 907and it is unfeasible to derive the decryption key from the encryption key. 908The idea is that each user creates a public/private 909key pair for authentication purposes. 910The server knows the public key, and only the user knows the private key. 911.Nm 912implements public key authentication protocol automatically, 913using one of the DSA, ECDSA, Ed25519 or RSA algorithms. 914The HISTORY section of 915.Xr ssl 8 916contains a brief discussion of the DSA and RSA algorithms. 917.Pp 918The file 919.Pa ~/.ssh/authorized_keys 920lists the public keys that are permitted for logging in. 921When the user logs in, the 922.Nm 923program tells the server which key pair it would like to use for 924authentication. 925The client proves that it has access to the private key 926and the server checks that the corresponding public key 927is authorized to accept the account. 928.Pp 929The server may inform the client of errors that prevented public key 930authentication from succeeding after authentication completes using a 931different method. 932These may be viewed by increasing the 933.Cm LogLevel 934to 935.Cm DEBUG 936or higher (e.g. by using the 937.Fl v 938flag). 939.Pp 940The user creates their key pair by running 941.Xr ssh-keygen 1 . 942This stores the private key in 943.Pa ~/.ssh/id_dsa 944(DSA), 945.Pa ~/.ssh/id_ecdsa 946(ECDSA), 947.Pa ~/.ssh/id_ecdsa_sk 948(authenticator-hosted ECDSA), 949.Pa ~/.ssh/id_ed25519 950(Ed25519), 951.Pa ~/.ssh/id_ed25519_sk 952(authenticator-hosted Ed25519), 953or 954.Pa ~/.ssh/id_rsa 955(RSA) 956and stores the public key in 957.Pa ~/.ssh/id_dsa.pub 958(DSA), 959.Pa ~/.ssh/id_ecdsa.pub 960(ECDSA), 961.Pa ~/.ssh/id_ecdsa_sk.pub 962(authenticator-hosted ECDSA), 963.Pa ~/.ssh/id_ed25519.pub 964(Ed25519), 965.Pa ~/.ssh/id_ed25519_sk.pub 966(authenticator-hosted Ed25519), 967or 968.Pa ~/.ssh/id_rsa.pub 969(RSA) 970in the user's home directory. 971The user should then copy the public key 972to 973.Pa ~/.ssh/authorized_keys 974in their home directory on the remote machine. 975The 976.Pa authorized_keys 977file corresponds to the conventional 978.Pa ~/.rhosts 979file, and has one key 980per line, though the lines can be very long. 981After this, the user can log in without giving the password. 982.Pp 983A variation on public key authentication 984is available in the form of certificate authentication: 985instead of a set of public/private keys, 986signed certificates are used. 987This has the advantage that a single trusted certification authority 988can be used in place of many public/private keys. 989See the CERTIFICATES section of 990.Xr ssh-keygen 1 991for more information. 992.Pp 993The most convenient way to use public key or certificate authentication 994may be with an authentication agent. 995See 996.Xr ssh-agent 1 997and (optionally) the 998.Cm AddKeysToAgent 999directive in 1000.Xr ssh_config 5 1001for more information. 1002.Pp 1003Keyboard-interactive authentication works as follows: 1004The server sends an arbitrary 1005.Qq challenge 1006text and prompts for a response, possibly multiple times. 1007Examples of keyboard-interactive authentication include 1008.Bx 1009Authentication (see 1010.Xr login.conf 5 ) 1011and PAM (some 1012.Pf non- Ox 1013systems). 1014.Pp 1015Finally, if other authentication methods fail, 1016.Nm 1017prompts the user for a password. 1018The password is sent to the remote 1019host for checking; however, since all communications are encrypted, 1020the password cannot be seen by someone listening on the network. 1021.Pp 1022.Nm 1023automatically maintains and checks a database containing 1024identification for all hosts it has ever been used with. 1025Host keys are stored in 1026.Pa ~/.ssh/known_hosts 1027in the user's home directory. 1028Additionally, the file 1029.Pa /etc/ssh/ssh_known_hosts 1030is automatically checked for known hosts. 1031Any new hosts are automatically added to the user's file. 1032If a host's identification ever changes, 1033.Nm 1034warns about this and disables password authentication to prevent 1035server spoofing or man-in-the-middle attacks, 1036which could otherwise be used to circumvent the encryption. 1037The 1038.Cm StrictHostKeyChecking 1039option can be used to control logins to machines whose 1040host key is not known or has changed. 1041.Pp 1042When the user's identity has been accepted by the server, the server 1043either executes the given command in a non-interactive session or, 1044if no command has been specified, logs into the machine and gives 1045the user a normal shell as an interactive session. 1046All communication with 1047the remote command or shell will be automatically encrypted. 1048.Pp 1049If an interactive session is requested, 1050.Nm 1051by default will only request a pseudo-terminal (pty) for interactive 1052sessions when the client has one. 1053The flags 1054.Fl T 1055and 1056.Fl t 1057can be used to override this behaviour. 1058.Pp 1059If a pseudo-terminal has been allocated, the 1060user may use the escape characters noted below. 1061.Pp 1062If no pseudo-terminal has been allocated, 1063the session is transparent and can be used to reliably transfer binary data. 1064On most systems, setting the escape character to 1065.Dq none 1066will also make the session transparent even if a tty is used. 1067.Pp 1068The session terminates when the command or shell on the remote 1069machine exits and all X11 and TCP connections have been closed. 1070.Sh ESCAPE CHARACTERS 1071When a pseudo-terminal has been requested, 1072.Nm 1073supports a number of functions through the use of an escape character. 1074.Pp 1075A single tilde character can be sent as 1076.Ic ~~ 1077or by following the tilde by a character other than those described below. 1078The escape character must always follow a newline to be interpreted as 1079special. 1080The escape character can be changed in configuration files using the 1081.Cm EscapeChar 1082configuration directive or on the command line by the 1083.Fl e 1084option. 1085.Pp 1086The supported escapes (assuming the default 1087.Ql ~ ) 1088are: 1089.Bl -tag -width Ds 1090.It Cm ~. 1091Disconnect. 1092.It Cm ~^Z 1093Background 1094.Nm . 1095.It Cm ~# 1096List forwarded connections. 1097.It Cm ~& 1098Background 1099.Nm 1100at logout when waiting for forwarded connection / X11 sessions to terminate. 1101.It Cm ~? 1102Display a list of escape characters. 1103.It Cm ~B 1104Send a BREAK to the remote system 1105(only useful if the peer supports it). 1106.It Cm ~C 1107Open command line. 1108Currently this allows the addition of port forwardings using the 1109.Fl L , 1110.Fl R 1111and 1112.Fl D 1113options (see above). 1114It also allows the cancellation of existing port-forwardings 1115with 1116.Sm off 1117.Fl KL Oo Ar bind_address : Oc Ar port 1118.Sm on 1119for local, 1120.Sm off 1121.Fl KR Oo Ar bind_address : Oc Ar port 1122.Sm on 1123for remote and 1124.Sm off 1125.Fl KD Oo Ar bind_address : Oc Ar port 1126.Sm on 1127for dynamic port-forwardings. 1128.Ic !\& Ns Ar command 1129allows the user to execute a local command if the 1130.Ic PermitLocalCommand 1131option is enabled in 1132.Xr ssh_config 5 . 1133Basic help is available, using the 1134.Fl h 1135option. 1136.It Cm ~R 1137Request rekeying of the connection 1138(only useful if the peer supports it). 1139.It Cm ~V 1140Decrease the verbosity 1141.Pq Ic LogLevel 1142when errors are being written to stderr. 1143.It Cm ~v 1144Increase the verbosity 1145.Pq Ic LogLevel 1146when errors are being written to stderr. 1147.El 1148.Sh TCP FORWARDING 1149Forwarding of arbitrary TCP connections over a secure channel 1150can be specified either on the command line or in a configuration file. 1151One possible application of TCP forwarding is a secure connection to a 1152mail server; another is going through firewalls. 1153.Pp 1154In the example below, we look at encrypting communication for an IRC client, 1155even though the IRC server it connects to does not directly 1156support encrypted communication. 1157This works as follows: 1158the user connects to the remote host using 1159.Nm , 1160specifying the ports to be used to forward the connection. 1161After that it is possible to start the program locally, 1162and 1163.Nm 1164will encrypt and forward the connection to the remote server. 1165.Pp 1166The following example tunnels an IRC session from the client 1167to an IRC server at 1168.Dq server.example.com , 1169joining channel 1170.Dq #users , 1171nickname 1172.Dq pinky , 1173using the standard IRC port, 6667: 1174.Bd -literal -offset 4n 1175$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10 1176$ irc -c '#users' pinky IRC/127.0.0.1 1177.Ed 1178.Pp 1179The 1180.Fl f 1181option backgrounds 1182.Nm 1183and the remote command 1184.Dq sleep 10 1185is specified to allow an amount of time 1186(10 seconds, in the example) 1187to start the program which is going to use the tunnel. 1188If no connections are made within the time specified, 1189.Nm 1190will exit. 1191.Sh X11 FORWARDING 1192If the 1193.Cm ForwardX11 1194variable is set to 1195.Dq yes 1196(or see the description of the 1197.Fl X , 1198.Fl x , 1199and 1200.Fl Y 1201options above) 1202and the user is using X11 (the 1203.Ev DISPLAY 1204environment variable is set), the connection to the X11 display is 1205automatically forwarded to the remote side in such a way that any X11 1206programs started from the shell (or command) will go through the 1207encrypted channel, and the connection to the real X server will be made 1208from the local machine. 1209The user should not manually set 1210.Ev DISPLAY . 1211Forwarding of X11 connections can be 1212configured on the command line or in configuration files. 1213.Pp 1214The 1215.Ev DISPLAY 1216value set by 1217.Nm 1218will point to the server machine, but with a display number greater than zero. 1219This is normal, and happens because 1220.Nm 1221creates a 1222.Dq proxy 1223X server on the server machine for forwarding the 1224connections over the encrypted channel. 1225.Pp 1226.Nm 1227will also automatically set up Xauthority data on the server machine. 1228For this purpose, it will generate a random authorization cookie, 1229store it in Xauthority on the server, and verify that any forwarded 1230connections carry this cookie and replace it by the real cookie when 1231the connection is opened. 1232The real authentication cookie is never 1233sent to the server machine (and no cookies are sent in the plain). 1234.Pp 1235If the 1236.Cm ForwardAgent 1237variable is set to 1238.Dq yes 1239(or see the description of the 1240.Fl A 1241and 1242.Fl a 1243options above) and 1244the user is using an authentication agent, the connection to the agent 1245is automatically forwarded to the remote side. 1246.Sh VERIFYING HOST KEYS 1247When connecting to a server for the first time, 1248a fingerprint of the server's public key is presented to the user 1249(unless the option 1250.Cm StrictHostKeyChecking 1251has been disabled). 1252Fingerprints can be determined using 1253.Xr ssh-keygen 1 : 1254.Pp 1255.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1256.Pp 1257If the fingerprint is already known, it can be matched 1258and the key can be accepted or rejected. 1259If only legacy (MD5) fingerprints for the server are available, the 1260.Xr ssh-keygen 1 1261.Fl E 1262option may be used to downgrade the fingerprint algorithm to match. 1263.Pp 1264Because of the difficulty of comparing host keys 1265just by looking at fingerprint strings, 1266there is also support to compare host keys visually, 1267using 1268.Em random art . 1269By setting the 1270.Cm VisualHostKey 1271option to 1272.Dq yes , 1273a small ASCII graphic gets displayed on every login to a server, no matter 1274if the session itself is interactive or not. 1275By learning the pattern a known server produces, a user can easily 1276find out that the host key has changed when a completely different pattern 1277is displayed. 1278Because these patterns are not unambiguous however, a pattern that looks 1279similar to the pattern remembered only gives a good probability that the 1280host key is the same, not guaranteed proof. 1281.Pp 1282To get a listing of the fingerprints along with their random art for 1283all known hosts, the following command line can be used: 1284.Pp 1285.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1286.Pp 1287If the fingerprint is unknown, 1288an alternative method of verification is available: 1289SSH fingerprints verified by DNS. 1290An additional resource record (RR), 1291SSHFP, 1292is added to a zonefile 1293and the connecting client is able to match the fingerprint 1294with that of the key presented. 1295.Pp 1296In this example, we are connecting a client to a server, 1297.Dq host.example.com . 1298The SSHFP resource records should first be added to the zonefile for 1299host.example.com: 1300.Bd -literal -offset indent 1301$ ssh-keygen -r host.example.com. 1302.Ed 1303.Pp 1304The output lines will have to be added to the zonefile. 1305To check that the zone is answering fingerprint queries: 1306.Pp 1307.Dl $ dig -t SSHFP host.example.com 1308.Pp 1309Finally the client connects: 1310.Bd -literal -offset indent 1311$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1312[...] 1313Matching host key fingerprint found in DNS. 1314Are you sure you want to continue connecting (yes/no)? 1315.Ed 1316.Pp 1317See the 1318.Cm VerifyHostKeyDNS 1319option in 1320.Xr ssh_config 5 1321for more information. 1322.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1323.Nm 1324contains support for Virtual Private Network (VPN) tunnelling 1325using the 1326.Xr tun 4 1327network pseudo-device, 1328allowing two networks to be joined securely. 1329The 1330.Xr sshd_config 5 1331configuration option 1332.Cm PermitTunnel 1333controls whether the server supports this, 1334and at what level (layer 2 or 3 traffic). 1335.Pp 1336The following example would connect client network 10.0.50.0/24 1337with remote network 10.0.99.0/24 using a point-to-point connection 1338from 10.1.1.1 to 10.1.1.2, 1339provided that the SSH server running on the gateway to the remote network, 1340at 192.168.1.15, allows it. 1341.Pp 1342On the client: 1343.Bd -literal -offset indent 1344# ssh -f -w 0:1 192.168.1.15 true 1345# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1346# route add 10.0.99.0/24 10.1.1.2 1347.Ed 1348.Pp 1349On the server: 1350.Bd -literal -offset indent 1351# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1352# route add 10.0.50.0/24 10.1.1.1 1353.Ed 1354.Pp 1355Client access may be more finely tuned via the 1356.Pa /root/.ssh/authorized_keys 1357file (see below) and the 1358.Cm PermitRootLogin 1359server option. 1360The following entry would permit connections on 1361.Xr tun 4 1362device 1 from user 1363.Dq jane 1364and on tun device 2 from user 1365.Dq john , 1366if 1367.Cm PermitRootLogin 1368is set to 1369.Dq forced-commands-only : 1370.Bd -literal -offset 2n 1371tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1372tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1373.Ed 1374.Pp 1375Since an SSH-based setup entails a fair amount of overhead, 1376it may be more suited to temporary setups, 1377such as for wireless VPNs. 1378More permanent VPNs are better provided by tools such as 1379.Xr ipsecctl 8 1380and 1381.Xr isakmpd 8 . 1382.Sh ENVIRONMENT 1383.Nm 1384will normally set the following environment variables: 1385.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1386.It Ev DISPLAY 1387The 1388.Ev DISPLAY 1389variable indicates the location of the X11 server. 1390It is automatically set by 1391.Nm 1392to point to a value of the form 1393.Dq hostname:n , 1394where 1395.Dq hostname 1396indicates the host where the shell runs, and 1397.Sq n 1398is an integer \*(Ge 1. 1399.Nm 1400uses this special value to forward X11 connections over the secure 1401channel. 1402The user should normally not set 1403.Ev DISPLAY 1404explicitly, as that 1405will render the X11 connection insecure (and will require the user to 1406manually copy any required authorization cookies). 1407.It Ev HOME 1408Set to the path of the user's home directory. 1409.It Ev LOGNAME 1410Synonym for 1411.Ev USER ; 1412set for compatibility with systems that use this variable. 1413.It Ev MAIL 1414Set to the path of the user's mailbox. 1415.It Ev PATH 1416Set to the default 1417.Ev PATH , 1418as specified when compiling 1419.Nm . 1420.It Ev SSH_ASKPASS 1421If 1422.Nm 1423needs a passphrase, it will read the passphrase from the current 1424terminal if it was run from a terminal. 1425If 1426.Nm 1427does not have a terminal associated with it but 1428.Ev DISPLAY 1429and 1430.Ev SSH_ASKPASS 1431are set, it will execute the program specified by 1432.Ev SSH_ASKPASS 1433and open an X11 window to read the passphrase. 1434This is particularly useful when calling 1435.Nm 1436from a 1437.Pa .xsession 1438or related script. 1439(Note that on some machines it 1440may be necessary to redirect the input from 1441.Pa /dev/null 1442to make this work.) 1443.It Ev SSH_ASKPASS_REQUIRE 1444Allows further control over the use of an askpass program. 1445If this variable is set to 1446.Dq never 1447then 1448.Nm 1449will never attempt to use one. 1450If it is set to 1451.Dq prefer , 1452then 1453.Nm 1454will prefer to use the askpass program instead of the TTY when requesting 1455passwords. 1456Finally, if the variable is set to 1457.Dq force , 1458then the askpass program will be used for all passphrase input regardless 1459of whether 1460.Ev DISPLAY 1461is set. 1462.It Ev SSH_AUTH_SOCK 1463Identifies the path of a 1464.Ux Ns -domain 1465socket used to communicate with the agent. 1466.It Ev SSH_CONNECTION 1467Identifies the client and server ends of the connection. 1468The variable contains 1469four space-separated values: client IP address, client port number, 1470server IP address, and server port number. 1471.It Ev SSH_ORIGINAL_COMMAND 1472This variable contains the original command line if a forced command 1473is executed. 1474It can be used to extract the original arguments. 1475.It Ev SSH_TTY 1476This is set to the name of the tty (path to the device) associated 1477with the current shell or command. 1478If the current session has no tty, 1479this variable is not set. 1480.It Ev SSH_TUNNEL 1481Optionally set by 1482.Xr sshd 8 1483to contain the interface names assigned if tunnel forwarding was 1484requested by the client. 1485.It Ev SSH_USER_AUTH 1486Optionally set by 1487.Xr sshd 8 , 1488this variable may contain a pathname to a file that lists the authentication 1489methods successfully used when the session was established, including any 1490public keys that were used. 1491.It Ev TZ 1492This variable is set to indicate the present time zone if it 1493was set when the daemon was started (i.e. the daemon passes the value 1494on to new connections). 1495.It Ev USER 1496Set to the name of the user logging in. 1497.El 1498.Pp 1499Additionally, 1500.Nm 1501reads 1502.Pa ~/.ssh/environment , 1503and adds lines of the format 1504.Dq VARNAME=value 1505to the environment if the file exists and users are allowed to 1506change their environment. 1507For more information, see the 1508.Cm PermitUserEnvironment 1509option in 1510.Xr sshd_config 5 . 1511.Sh FILES 1512.Bl -tag -width Ds -compact 1513.It Pa ~/.rhosts 1514This file is used for host-based authentication (see above). 1515On some machines this file may need to be 1516world-readable if the user's home directory is on an NFS partition, 1517because 1518.Xr sshd 8 1519reads it as root. 1520Additionally, this file must be owned by the user, 1521and must not have write permissions for anyone else. 1522The recommended 1523permission for most machines is read/write for the user, and not 1524accessible by others. 1525.Pp 1526.It Pa ~/.shosts 1527This file is used in exactly the same way as 1528.Pa .rhosts , 1529but allows host-based authentication without permitting login with 1530rlogin/rsh. 1531.Pp 1532.It Pa ~/.ssh/ 1533This directory is the default location for all user-specific configuration 1534and authentication information. 1535There is no general requirement to keep the entire contents of this directory 1536secret, but the recommended permissions are read/write/execute for the user, 1537and not accessible by others. 1538.Pp 1539.It Pa ~/.ssh/authorized_keys 1540Lists the public keys (DSA, ECDSA, Ed25519, RSA) 1541that can be used for logging in as this user. 1542The format of this file is described in the 1543.Xr sshd 8 1544manual page. 1545This file is not highly sensitive, but the recommended 1546permissions are read/write for the user, and not accessible by others. 1547.Pp 1548.It Pa ~/.ssh/config 1549This is the per-user configuration file. 1550The file format and configuration options are described in 1551.Xr ssh_config 5 . 1552Because of the potential for abuse, this file must have strict permissions: 1553read/write for the user, and not writable by others. 1554.Pp 1555.It Pa ~/.ssh/environment 1556Contains additional definitions for environment variables; see 1557.Sx ENVIRONMENT , 1558above. 1559.Pp 1560.It Pa ~/.ssh/id_dsa 1561.It Pa ~/.ssh/id_ecdsa 1562.It Pa ~/.ssh/id_ecdsa_sk 1563.It Pa ~/.ssh/id_ed25519 1564.It Pa ~/.ssh/id_ed25519_sk 1565.It Pa ~/.ssh/id_rsa 1566Contains the private key for authentication. 1567These files 1568contain sensitive data and should be readable by the user but not 1569accessible by others (read/write/execute). 1570.Nm 1571will simply ignore a private key file if it is accessible by others. 1572It is possible to specify a passphrase when 1573generating the key which will be used to encrypt the 1574sensitive part of this file using AES-128. 1575.Pp 1576.It Pa ~/.ssh/id_dsa.pub 1577.It Pa ~/.ssh/id_ecdsa.pub 1578.It Pa ~/.ssh/id_ecdsa_sk.pub 1579.It Pa ~/.ssh/id_ed25519.pub 1580.It Pa ~/.ssh/id_ed25519_sk.pub 1581.It Pa ~/.ssh/id_rsa.pub 1582Contains the public key for authentication. 1583These files are not 1584sensitive and can (but need not) be readable by anyone. 1585.Pp 1586.It Pa ~/.ssh/known_hosts 1587Contains a list of host keys for all hosts the user has logged into 1588that are not already in the systemwide list of known host keys. 1589See 1590.Xr sshd 8 1591for further details of the format of this file. 1592.Pp 1593.It Pa ~/.ssh/rc 1594Commands in this file are executed by 1595.Nm 1596when the user logs in, just before the user's shell (or command) is 1597started. 1598See the 1599.Xr sshd 8 1600manual page for more information. 1601.Pp 1602.It Pa /etc/hosts.equiv 1603This file is for host-based authentication (see above). 1604It should only be writable by root. 1605.Pp 1606.It Pa /etc/shosts.equiv 1607This file is used in exactly the same way as 1608.Pa hosts.equiv , 1609but allows host-based authentication without permitting login with 1610rlogin/rsh. 1611.Pp 1612.It Pa /etc/ssh/ssh_config 1613Systemwide configuration file. 1614The file format and configuration options are described in 1615.Xr ssh_config 5 . 1616.Pp 1617.It Pa /etc/ssh/ssh_host_key 1618.It Pa /etc/ssh/ssh_host_dsa_key 1619.It Pa /etc/ssh/ssh_host_ecdsa_key 1620.It Pa /etc/ssh/ssh_host_ed25519_key 1621.It Pa /etc/ssh/ssh_host_rsa_key 1622These files contain the private parts of the host keys 1623and are used for host-based authentication. 1624.Pp 1625.It Pa /etc/ssh/ssh_known_hosts 1626Systemwide list of known host keys. 1627This file should be prepared by the 1628system administrator to contain the public host keys of all machines in the 1629organization. 1630It should be world-readable. 1631See 1632.Xr sshd 8 1633for further details of the format of this file. 1634.Pp 1635.It Pa /etc/ssh/sshrc 1636Commands in this file are executed by 1637.Nm 1638when the user logs in, just before the user's shell (or command) is started. 1639See the 1640.Xr sshd 8 1641manual page for more information. 1642.El 1643.Sh EXIT STATUS 1644.Nm 1645exits with the exit status of the remote command or with 255 1646if an error occurred. 1647.Sh SEE ALSO 1648.Xr scp 1 , 1649.Xr sftp 1 , 1650.Xr ssh-add 1 , 1651.Xr ssh-agent 1 , 1652.Xr ssh-keygen 1 , 1653.Xr ssh-keyscan 1 , 1654.Xr tun 4 , 1655.Xr ssh_config 5 , 1656.Xr ssh-keysign 8 , 1657.Xr sshd 8 1658.Sh STANDARDS 1659.Rs 1660.%A S. Lehtinen 1661.%A C. Lonvick 1662.%D January 2006 1663.%R RFC 4250 1664.%T The Secure Shell (SSH) Protocol Assigned Numbers 1665.Re 1666.Pp 1667.Rs 1668.%A T. Ylonen 1669.%A C. Lonvick 1670.%D January 2006 1671.%R RFC 4251 1672.%T The Secure Shell (SSH) Protocol Architecture 1673.Re 1674.Pp 1675.Rs 1676.%A T. Ylonen 1677.%A C. Lonvick 1678.%D January 2006 1679.%R RFC 4252 1680.%T The Secure Shell (SSH) Authentication Protocol 1681.Re 1682.Pp 1683.Rs 1684.%A T. Ylonen 1685.%A C. Lonvick 1686.%D January 2006 1687.%R RFC 4253 1688.%T The Secure Shell (SSH) Transport Layer Protocol 1689.Re 1690.Pp 1691.Rs 1692.%A T. Ylonen 1693.%A C. Lonvick 1694.%D January 2006 1695.%R RFC 4254 1696.%T The Secure Shell (SSH) Connection Protocol 1697.Re 1698.Pp 1699.Rs 1700.%A J. Schlyter 1701.%A W. Griffin 1702.%D January 2006 1703.%R RFC 4255 1704.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints 1705.Re 1706.Pp 1707.Rs 1708.%A F. Cusack 1709.%A M. Forssen 1710.%D January 2006 1711.%R RFC 4256 1712.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH) 1713.Re 1714.Pp 1715.Rs 1716.%A J. Galbraith 1717.%A P. Remaker 1718.%D January 2006 1719.%R RFC 4335 1720.%T The Secure Shell (SSH) Session Channel Break Extension 1721.Re 1722.Pp 1723.Rs 1724.%A M. Bellare 1725.%A T. Kohno 1726.%A C. Namprempre 1727.%D January 2006 1728.%R RFC 4344 1729.%T The Secure Shell (SSH) Transport Layer Encryption Modes 1730.Re 1731.Pp 1732.Rs 1733.%A B. Harris 1734.%D January 2006 1735.%R RFC 4345 1736.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol 1737.Re 1738.Pp 1739.Rs 1740.%A M. Friedl 1741.%A N. Provos 1742.%A W. Simpson 1743.%D March 2006 1744.%R RFC 4419 1745.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 1746.Re 1747.Pp 1748.Rs 1749.%A J. Galbraith 1750.%A R. Thayer 1751.%D November 2006 1752.%R RFC 4716 1753.%T The Secure Shell (SSH) Public Key File Format 1754.Re 1755.Pp 1756.Rs 1757.%A D. Stebila 1758.%A J. Green 1759.%D December 2009 1760.%R RFC 5656 1761.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer 1762.Re 1763.Pp 1764.Rs 1765.%A A. Perrig 1766.%A D. Song 1767.%D 1999 1768.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99) 1769.%T Hash Visualization: a New Technique to improve Real-World Security 1770.Re 1771.Sh AUTHORS 1772OpenSSH is a derivative of the original and free 1773ssh 1.2.12 release by Tatu Ylonen. 1774Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1775Theo de Raadt and Dug Song 1776removed many bugs, re-added newer features and 1777created OpenSSH. 1778Markus Friedl contributed the support for SSH 1779protocol versions 1.5 and 2.0. 1780