1.\" $NetBSD: ssh.1,v 1.36 2023/07/28 05:06:44 rin Exp $ 2.\" 3.\" Author: Tatu Ylonen <ylo@cs.hut.fi> 4.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland 5.\" All rights reserved 6.\" 7.\" As far as I am concerned, the code I have written for this software 8.\" can be used freely for any purpose. Any derived versions of this 9.\" software must be clearly marked as such, and if the derived work is 10.\" incompatible with the protocol description in the RFC file, it must be 11.\" called by a name other than "ssh" or "Secure Shell". 12.\" 13.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. 14.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. 15.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. 16.\" 17.\" Redistribution and use in source and binary forms, with or without 18.\" modification, are permitted provided that the following conditions 19.\" are met: 20.\" 1. Redistributions of source code must retain the above copyright 21.\" notice, this list of conditions and the following disclaimer. 22.\" 2. Redistributions in binary form must reproduce the above copyright 23.\" notice, this list of conditions and the following disclaimer in the 24.\" documentation and/or other materials provided with the distribution. 25.\" 26.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 27.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 28.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 29.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 30.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 31.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 32.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 33.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 34.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 35.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 36.\" 37.\" $OpenBSD: ssh.1,v 1.433 2022/11/28 01:37:36 djm Exp $ 38.Dd November 28 2022 39.Dt SSH 1 40.Os 41.Sh NAME 42.Nm ssh 43.Nd OpenSSH remote login client 44.Sh SYNOPSIS 45.Nm ssh 46.Op Fl 46AaCfGgKkMNnqsTtVvXxYy 47.Op Fl B Ar bind_interface 48.Op Fl b Ar bind_address 49.Op Fl c Ar cipher_spec 50.Op Fl D Oo Ar bind_address : Oc Ns Ar port 51.Op Fl E Ar log_file 52.Op Fl e Ar escape_char 53.Op Fl F Ar configfile 54.Op Fl I Ar pkcs11 55.Op Fl i Ar identity_file 56.Op Fl J Ar destination 57.Op Fl L Ar address 58.Op Fl l Ar login_name 59.Op Fl m Ar mac_spec 60.Op Fl O Ar ctl_cmd 61.Op Fl o Ar option 62.Op Fl p Ar port 63.Op Fl Q Ar query_option 64.Op Fl R Ar address 65.Op Fl S Ar ctl_path 66.Op Fl W Ar host : Ns Ar port 67.Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun 68.Ar destination 69.Op Ar command Op Ar argument ... 70.Sh DESCRIPTION 71.Nm 72(SSH client) is a program for logging into a remote machine and for 73executing commands on a remote machine. 74It is intended to provide secure encrypted communications between 75two untrusted hosts over an insecure network. 76X11 connections, arbitrary TCP ports and 77.Ux Ns -domain 78sockets can also be forwarded over the secure channel. 79.Pp 80.Nm 81connects and logs into the specified 82.Ar destination , 83which may be specified as either 84.Sm off 85.Oo user @ Oc hostname 86.Sm on 87or a URI of the form 88.Sm off 89.No ssh:// Oo user @ Oc hostname Op : port . 90.Sm on 91The user must prove 92their identity to the remote machine using one of several methods 93(see below). 94.Pp 95If a 96.Ar command 97is specified, 98it will be executed on the remote host instead of a login shell. 99A complete command line may be specified as 100.Ar command , 101or it may have additional arguments. 102If supplied, the arguments will be appended to the command, separated by 103spaces, before it is sent to the server to be executed. 104.Pp 105The options are as follows: 106.Pp 107.Bl -tag -width Ds -compact 108.It Fl 4 109Forces 110.Nm 111to use IPv4 addresses only. 112.Pp 113.It Fl 6 114Forces 115.Nm 116to use IPv6 addresses only. 117.Pp 118.It Fl A 119Enables forwarding of connections from an authentication agent such as 120.Xr ssh-agent 1 . 121This can also be specified on a per-host basis in a configuration file. 122.Pp 123Agent forwarding should be enabled with caution. 124Users with the ability to bypass file permissions on the remote host 125(for the agent's 126.Ux Ns -domain 127socket) can access the local agent through the forwarded connection. 128An attacker cannot obtain key material from the agent, 129however they can perform operations on the keys that enable them to 130authenticate using the identities loaded into the agent. 131A safer alternative may be to use a jump host 132(see 133.Fl J ) . 134.Pp 135.It Fl a 136Disables forwarding of the authentication agent connection. 137.Pp 138.It Fl B Ar bind_interface 139Bind to the address of 140.Ar bind_interface 141before attempting to connect to the destination host. 142This is only useful on systems with more than one address. 143.Pp 144.It Fl b Ar bind_address 145Use 146.Ar bind_address 147on the local machine as the source address 148of the connection. 149Only useful on systems with more than one address. 150.Pp 151.It Fl C 152Requests compression of all data (including stdin, stdout, stderr, and 153data for forwarded X11, TCP and 154.Ux Ns -domain 155connections). 156The compression algorithm is the same used by 157.Xr gzip 1 . 158Compression is desirable on modem lines and other 159slow connections, but will only slow down things on fast networks. 160The default value can be set on a host-by-host basis in the 161configuration files; see the 162.Cm Compression 163option in 164.Xr ssh_config 5 . 165.Pp 166.It Fl c Ar cipher_spec 167Selects the cipher specification for encrypting the session. 168.Ar cipher_spec 169is a comma-separated list of ciphers 170listed in order of preference. 171See the 172.Cm Ciphers 173keyword in 174.Xr ssh_config 5 175for more information. 176.Pp 177.It Fl D Xo 178.Sm off 179.Oo Ar bind_address : Oc 180.Ar port 181.Sm on 182.Xc 183Specifies a local 184.Dq dynamic 185application-level port forwarding. 186This works by allocating a socket to listen to 187.Ar port 188on the local side, optionally bound to the specified 189.Ar bind_address . 190Whenever a connection is made to this port, the 191connection is forwarded over the secure channel, and the application 192protocol is then used to determine where to connect to from the 193remote machine. 194Currently the SOCKS4 and SOCKS5 protocols are supported, and 195.Nm 196will act as a SOCKS server. 197Only root can forward privileged ports. 198Dynamic port forwardings can also be specified in the configuration file. 199.Pp 200IPv6 addresses can be specified by enclosing the address in square brackets. 201Only the superuser can forward privileged ports. 202By default, the local port is bound in accordance with the 203.Cm GatewayPorts 204setting. 205However, an explicit 206.Ar bind_address 207may be used to bind the connection to a specific address. 208The 209.Ar bind_address 210of 211.Dq localhost 212indicates that the listening port be bound for local use only, while an 213empty address or 214.Sq * 215indicates that the port should be available from all interfaces. 216.Pp 217.It Fl E Ar log_file 218Append debug logs to 219.Ar log_file 220instead of standard error. 221.Pp 222.It Fl e Ar escape_char 223Sets the escape character for sessions with a pty (default: 224.Ql ~ ) . 225The escape character is only recognized at the beginning of a line. 226The escape character followed by a dot 227.Pq Ql \&. 228closes the connection; 229followed by control-Z suspends the connection; 230and followed by itself sends the escape character once. 231Setting the character to 232.Dq none 233disables any escapes and makes the session fully transparent. 234.Pp 235.It Fl F Ar configfile 236Specifies an alternative per-user configuration file. 237If a configuration file is given on the command line, 238the system-wide configuration file 239.Pq Pa /etc/ssh/ssh_config 240will be ignored. 241The default for the per-user configuration file is 242.Pa ~/.ssh/config . 243If set to 244.Dq none , 245no configuration files will be read. 246.Pp 247.It Fl f 248Requests 249.Nm 250to go to background just before command execution. 251This is useful if 252.Nm 253is going to ask for passwords or passphrases, but the user 254wants it in the background. 255This implies 256.Fl n . 257The recommended way to start X11 programs at a remote site is with 258something like 259.Ic ssh -f host xterm . 260.Pp 261If the 262.Cm ExitOnForwardFailure 263configuration option is set to 264.Dq yes , 265then a client started with 266.Fl f 267will wait for all remote port forwards to be successfully established 268before placing itself in the background. 269Refer to the description of 270.Cm ForkAfterAuthentication 271in 272.Xr ssh_config 5 273for details. 274.Pp 275.It Fl G 276Causes 277.Nm 278to print its configuration after evaluating 279.Cm Host 280and 281.Cm Match 282blocks and exit. 283.Pp 284.It Fl g 285Allows remote hosts to connect to local forwarded ports. 286If used on a multiplexed connection, then this option must be specified 287on the master process. 288.Pp 289.It Fl I Ar pkcs11 290Specify the PKCS#11 shared library 291.Nm 292should use to communicate with a PKCS#11 token providing keys for user 293authentication. 294.Pp 295.It Fl i Ar identity_file 296Selects a file from which the identity (private key) for 297public key authentication is read. 298You can also specify a public key file to use the corresponding 299private key that is loaded in 300.Xr ssh-agent 1 301when the private key file is not present locally. 302The default is 303.Pa ~/.ssh/id_rsa , 304.Pa ~/.ssh/id_ecdsa , 305.Pa ~/.ssh/id_ecdsa_sk , 306.Pa ~/.ssh/id_ed25519 , 307.Pa ~/.ssh/id_ed25519_sk 308and 309.Pa ~/.ssh/id_dsa . 310Identity files may also be specified on 311a per-host basis in the configuration file. 312It is possible to have multiple 313.Fl i 314options (and multiple identities specified in 315configuration files). 316If no certificates have been explicitly specified by the 317.Cm CertificateFile 318directive, 319.Nm 320will also try to load certificate information from the filename obtained 321by appending 322.Pa -cert.pub 323to identity filenames. 324.Pp 325.It Fl J Ar destination 326Connect to the target host by first making a 327.Nm 328connection to the jump host described by 329.Ar destination 330and then establishing a TCP forwarding to the ultimate destination from 331there. 332Multiple jump hops may be specified separated by comma characters. 333This is a shortcut to specify a 334.Cm ProxyJump 335configuration directive. 336Note that configuration directives supplied on the command-line generally 337apply to the destination host and not any specified jump hosts. 338Use 339.Pa ~/.ssh/config 340to specify configuration for jump hosts. 341.Pp 342.It Fl K 343Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI 344credentials to the server. 345.Pp 346.It Fl k 347Disables forwarding (delegation) of GSSAPI credentials to the server. 348.Pp 349.It Fl L Xo 350.Sm off 351.Oo Ar bind_address : Oc 352.Ar port : host : hostport 353.Sm on 354.Xc 355.It Fl L Xo 356.Sm off 357.Oo Ar bind_address : Oc 358.Ar port : remote_socket 359.Sm on 360.Xc 361.It Fl L Xo 362.Sm off 363.Ar local_socket : host : hostport 364.Sm on 365.Xc 366.It Fl L Xo 367.Sm off 368.Ar local_socket : remote_socket 369.Sm on 370.Xc 371Specifies that connections to the given TCP port or Unix socket on the local 372(client) host are to be forwarded to the given host and port, or Unix socket, 373on the remote side. 374This works by allocating a socket to listen to either a TCP 375.Ar port 376on the local side, optionally bound to the specified 377.Ar bind_address , 378or to a Unix socket. 379Whenever a connection is made to the local port or socket, the 380connection is forwarded over the secure channel, and a connection is 381made to either 382.Ar host 383port 384.Ar hostport , 385or the Unix socket 386.Ar remote_socket , 387from the remote machine. 388.Pp 389Port forwardings can also be specified in the configuration file. 390Only the superuser can forward privileged ports. 391IPv6 addresses can be specified by enclosing the address in square brackets. 392.Pp 393By default, the local port is bound in accordance with the 394.Cm GatewayPorts 395setting. 396However, an explicit 397.Ar bind_address 398may be used to bind the connection to a specific address. 399The 400.Ar bind_address 401of 402.Dq localhost 403indicates that the listening port be bound for local use only, while an 404empty address or 405.Sq * 406indicates that the port should be available from all interfaces. 407.Pp 408.It Fl l Ar login_name 409Specifies the user to log in as on the remote machine. 410This also may be specified on a per-host basis in the configuration file. 411.Pp 412.It Fl M 413Places the 414.Nm 415client into 416.Dq master 417mode for connection sharing. 418Multiple 419.Fl M 420options places 421.Nm 422into 423.Dq master 424mode but with confirmation required using 425.Xr ssh-askpass 1 426before each operation that changes the multiplexing state 427(e.g. opening a new session). 428Refer to the description of 429.Cm ControlMaster 430in 431.Xr ssh_config 5 432for details. 433.Pp 434.It Fl m Ar mac_spec 435A comma-separated list of MAC (message authentication code) algorithms, 436specified in order of preference. 437See the 438.Cm MACs 439keyword in 440.Xr ssh_config 5 441for more information. 442.Pp 443.It Fl N 444Do not execute a remote command. 445This is useful for just forwarding ports. 446Refer to the description of 447.Cm SessionType 448in 449.Xr ssh_config 5 450for details. 451.Pp 452.It Fl n 453Redirects stdin from 454.Pa /dev/null 455(actually, prevents reading from stdin). 456This must be used when 457.Nm 458is run in the background. 459A common trick is to use this to run X11 programs on a remote machine. 460For example, 461.Ic ssh -n shadows.cs.hut.fi emacs & 462will start an emacs on shadows.cs.hut.fi, and the X11 463connection will be automatically forwarded over an encrypted channel. 464The 465.Nm 466program will be put in the background. 467(This does not work if 468.Nm 469needs to ask for a password or passphrase; see also the 470.Fl f 471option.) 472Refer to the description of 473.Cm StdinNull 474in 475.Xr ssh_config 5 476for details. 477.Pp 478.It Fl O Ar ctl_cmd 479Control an active connection multiplexing master process. 480When the 481.Fl O 482option is specified, the 483.Ar ctl_cmd 484argument is interpreted and passed to the master process. 485Valid commands are: 486.Dq check 487(check that the master process is running), 488.Dq forward 489(request forwardings without command execution), 490.Dq cancel 491(cancel forwardings), 492.Dq exit 493(request the master to exit), and 494.Dq stop 495(request the master to stop accepting further multiplexing requests). 496.Pp 497.It Fl o Ar option 498Can be used to give options in the format used in the configuration file. 499This is useful for specifying options for which there is no separate 500command-line flag. 501For full details of the options listed below, and their possible values, see 502.Xr ssh_config 5 . 503.Pp 504.Bl -tag -width Ds -offset indent -compact 505.It AddKeysToAgent 506.It AddressFamily 507.It BatchMode 508.It BindAddress 509.It CanonicalDomains 510.It CanonicalizeFallbackLocal 511.It CanonicalizeHostname 512.It CanonicalizeMaxDots 513.It CanonicalizePermittedCNAMEs 514.It CASignatureAlgorithms 515.It CertificateFile 516.It CheckHostIP 517.It Ciphers 518.It ClearAllForwardings 519.It Compression 520.It ConnectionAttempts 521.It ConnectTimeout 522.It ControlMaster 523.It ControlPath 524.It ControlPersist 525.It DynamicForward 526.It EnableEscapeCommandline 527.It EscapeChar 528.It ExitOnForwardFailure 529.It FingerprintHash 530.It ForkAfterAuthentication 531.It ForwardAgent 532.It ForwardX11 533.It ForwardX11Timeout 534.It ForwardX11Trusted 535.It GatewayPorts 536.It GlobalKnownHostsFile 537.It GSSAPIAuthentication 538.It GSSAPIDelegateCredentials 539.It HashKnownHosts 540.It Host 541.It HostbasedAcceptedAlgorithms 542.It HostbasedAuthentication 543.It HostKeyAlgorithms 544.It HostKeyAlias 545.It Hostname 546.It IdentitiesOnly 547.It IdentityAgent 548.It IdentityFile 549.It IPQoS 550.It KbdInteractiveAuthentication 551.It KbdInteractiveDevices 552.It KexAlgorithms 553.It KnownHostsCommand 554.It LocalCommand 555.It LocalForward 556.It LogLevel 557.It MACs 558.It Match 559.It NoHostAuthenticationForLocalhost 560.It NumberOfPasswordPrompts 561.It PasswordAuthentication 562.It PermitLocalCommand 563.It PermitRemoteOpen 564.It PKCS11Provider 565.It Port 566.It PreferredAuthentications 567.It ProxyCommand 568.It ProxyJump 569.It ProxyUseFdpass 570.It PubkeyAcceptedAlgorithms 571.It PubkeyAuthentication 572.It RekeyLimit 573.It RemoteCommand 574.It RemoteForward 575.It RequestTTY 576.It RequiredRSASize 577.It SendEnv 578.It ServerAliveInterval 579.It ServerAliveCountMax 580.It SessionType 581.It SetEnv 582.It StdinNull 583.It StreamLocalBindMask 584.It StreamLocalBindUnlink 585.It StrictHostKeyChecking 586.It TCPKeepAlive 587.It Tunnel 588.It TunnelDevice 589.It UpdateHostKeys 590.It User 591.It UserKnownHostsFile 592.It VerifyHostKeyDNS 593.It VisualHostKey 594.It XAuthLocation 595.El 596.Pp 597.It Fl p Ar port 598Port to connect to on the remote host. 599This can be specified on a 600per-host basis in the configuration file. 601.Pp 602.It Fl Q Ar query_option 603Queries for the algorithms supported by one of the following features: 604.Ar cipher 605(supported symmetric ciphers), 606.Ar cipher-auth 607(supported symmetric ciphers that support authenticated encryption), 608.Ar help 609(supported query terms for use with the 610.Fl Q 611flag), 612.Ar mac 613(supported message integrity codes), 614.Ar kex 615(key exchange algorithms), 616.Ar key 617(key types), 618.Ar key-cert 619(certificate key types), 620.Ar key-plain 621(non-certificate key types), 622.Ar key-sig 623(all key types and signature algorithms), 624.Ar protocol-version 625(supported SSH protocol versions), and 626.Ar sig 627(supported signature algorithms). 628Alternatively, any keyword from 629.Xr ssh_config 5 630or 631.Xr sshd_config 5 632that takes an algorithm list may be used as an alias for the corresponding 633query_option. 634.Pp 635.It Fl q 636Quiet mode. 637Causes most warning and diagnostic messages to be suppressed. 638.Pp 639.It Fl R Xo 640.Sm off 641.Oo Ar bind_address : Oc 642.Ar port : host : hostport 643.Sm on 644.Xc 645.It Fl R Xo 646.Sm off 647.Oo Ar bind_address : Oc 648.Ar port : local_socket 649.Sm on 650.Xc 651.It Fl R Xo 652.Sm off 653.Ar remote_socket : host : hostport 654.Sm on 655.Xc 656.It Fl R Xo 657.Sm off 658.Ar remote_socket : local_socket 659.Sm on 660.Xc 661.It Fl R Xo 662.Sm off 663.Oo Ar bind_address : Oc 664.Ar port 665.Sm on 666.Xc 667Specifies that connections to the given TCP port or Unix socket on the remote 668(server) host are to be forwarded to the local side. 669.Pp 670This works by allocating a socket to listen to either a TCP 671.Ar port 672or to a Unix socket on the remote side. 673Whenever a connection is made to this port or Unix socket, the 674connection is forwarded over the secure channel, and a connection 675is made from the local machine to either an explicit destination specified by 676.Ar host 677port 678.Ar hostport , 679or 680.Ar local_socket , 681or, if no explicit destination was specified, 682.Nm 683will act as a SOCKS 4/5 proxy and forward connections to the destinations 684requested by the remote SOCKS client. 685.Pp 686Port forwardings can also be specified in the configuration file. 687Privileged ports can be forwarded only when 688logging in as root on the remote machine. 689IPv6 addresses can be specified by enclosing the address in square brackets. 690.Pp 691By default, TCP listening sockets on the server will be bound to the loopback 692interface only. 693This may be overridden by specifying a 694.Ar bind_address . 695An empty 696.Ar bind_address , 697or the address 698.Ql * , 699indicates that the remote socket should listen on all interfaces. 700Specifying a remote 701.Ar bind_address 702will only succeed if the server's 703.Cm GatewayPorts 704option is enabled (see 705.Xr sshd_config 5 ) . 706.Pp 707If the 708.Ar port 709argument is 710.Ql 0 , 711the listen port will be dynamically allocated on the server and reported 712to the client at run time. 713When used together with 714.Ic -O forward , 715the allocated port will be printed to the standard output. 716.Pp 717.It Fl S Ar ctl_path 718Specifies the location of a control socket for connection sharing, 719or the string 720.Dq none 721to disable connection sharing. 722Refer to the description of 723.Cm ControlPath 724and 725.Cm ControlMaster 726in 727.Xr ssh_config 5 728for details. 729.Pp 730.It Fl s 731May be used to request invocation of a subsystem on the remote system. 732Subsystems facilitate the use of SSH 733as a secure transport for other applications (e.g.\& 734.Xr sftp 1 ) . 735The subsystem is specified as the remote command. 736Refer to the description of 737.Cm SessionType 738in 739.Xr ssh_config 5 740for details. 741.Pp 742.It Fl T 743Disable pseudo-terminal allocation. 744.Pp 745.It Fl t 746Force pseudo-terminal allocation. 747This can be used to execute arbitrary 748screen-based programs on a remote machine, which can be very useful, 749e.g. when implementing menu services. 750Multiple 751.Fl t 752options force tty allocation, even if 753.Nm 754has no local tty. 755.Pp 756.It Fl V 757Display the version number and exit. 758.Pp 759.It Fl v 760Verbose mode. 761Causes 762.Nm 763to print debugging messages about its progress. 764This is helpful in 765debugging connection, authentication, and configuration problems. 766Multiple 767.Fl v 768options increase the verbosity. 769The maximum is 3. 770.Pp 771.It Fl W Ar host : Ns Ar port 772Requests that standard input and output on the client be forwarded to 773.Ar host 774on 775.Ar port 776over the secure channel. 777Implies 778.Fl N , 779.Fl T , 780.Cm ExitOnForwardFailure 781and 782.Cm ClearAllForwardings , 783though these can be overridden in the configuration file or using 784.Fl o 785command line options. 786.Pp 787.It Fl w Xo 788.Ar local_tun Ns Op : Ns Ar remote_tun 789.Xc 790Requests 791tunnel 792device forwarding with the specified 793.Xr tun 4 794devices between the client 795.Pq Ar local_tun 796and the server 797.Pq Ar remote_tun . 798.Pp 799The devices may be specified by numerical ID or the keyword 800.Dq any , 801which uses the next available tunnel device. 802If 803.Ar remote_tun 804is not specified, it defaults to 805.Dq any . 806See also the 807.Cm Tunnel 808and 809.Cm TunnelDevice 810directives in 811.Xr ssh_config 5 . 812.Pp 813If the 814.Cm Tunnel 815directive is unset, it will be set to the default tunnel mode, which is 816.Dq point-to-point . 817If a different 818.Cm Tunnel 819forwarding mode it desired, then it should be specified before 820.Fl w . 821.Pp 822.It Fl X 823Enables X11 forwarding. 824This can also be specified on a per-host basis in a configuration file. 825.Pp 826X11 forwarding should be enabled with caution. 827Users with the ability to bypass file permissions on the remote host 828(for the user's X authorization database) 829can access the local X11 display through the forwarded connection. 830An attacker may then be able to perform activities such as keystroke monitoring. 831.Pp 832For this reason, X11 forwarding is subjected to X11 SECURITY extension 833restrictions by default. 834Refer to the 835.Nm 836.Fl Y 837option and the 838.Cm ForwardX11Trusted 839directive in 840.Xr ssh_config 5 841for more information. 842.Pp 843.It Fl x 844Disables X11 forwarding. 845.Pp 846.It Fl Y 847Enables trusted X11 forwarding. 848Trusted X11 forwardings are not subjected to the X11 SECURITY extension 849controls. 850.Pp 851.It Fl y 852Send log information using the 853.Xr syslog 3 854system module. 855By default this information is sent to stderr. 856.El 857.Pp 858.Nm 859may additionally obtain configuration data from 860a per-user configuration file and a system-wide configuration file. 861The file format and configuration options are described in 862.Xr ssh_config 5 . 863.Sh AUTHENTICATION 864The OpenSSH SSH client supports SSH protocol 2. 865.Pp 866The methods available for authentication are: 867GSSAPI-based authentication, 868host-based authentication, 869public key authentication, 870keyboard-interactive authentication, 871and password authentication. 872Authentication methods are tried in the order specified above, 873though 874.Cm PreferredAuthentications 875can be used to change the default order. 876.Pp 877Host-based authentication works as follows: 878If the machine the user logs in from is listed in 879.Pa /etc/hosts.equiv 880or 881.Pa /etc/shosts.equiv 882on the remote machine, the user is non-root and the user names are 883the same on both sides, or if the files 884.Pa ~/.rhosts 885or 886.Pa ~/.shosts 887exist in the user's home directory on the 888remote machine and contain a line containing the name of the client 889machine and the name of the user on that machine, the user is 890considered for login. 891Additionally, the server 892.Em must 893be able to verify the client's 894host key (see the description of 895.Pa /etc/ssh/ssh_known_hosts 896and 897.Pa ~/.ssh/known_hosts , 898below) 899for login to be permitted. 900This authentication method closes security holes due to IP 901spoofing, DNS spoofing, and routing spoofing. 902[Note to the administrator: 903.Pa /etc/hosts.equiv , 904.Pa ~/.rhosts , 905and the rlogin/rsh protocol in general, are inherently insecure and should be 906disabled if security is desired.] 907.Pp 908Public key authentication works as follows: 909The scheme is based on public-key cryptography, 910using cryptosystems 911where encryption and decryption are done using separate keys, 912and it is unfeasible to derive the decryption key from the encryption key. 913The idea is that each user creates a public/private 914key pair for authentication purposes. 915The server knows the public key, and only the user knows the private key. 916.Nm 917implements public key authentication protocol automatically, 918using one of the DSA, ECDSA, Ed25519 or RSA algorithms. 919The HISTORY section of 920.Xr ssl 8 921contains a brief discussion of the DSA and RSA algorithms. 922.Pp 923The file 924.Pa ~/.ssh/authorized_keys 925lists the public keys that are permitted for logging in. 926When the user logs in, the 927.Nm 928program tells the server which key pair it would like to use for 929authentication. 930The client proves that it has access to the private key 931and the server checks that the corresponding public key 932is authorized to accept the account. 933.Pp 934The server may inform the client of errors that prevented public key 935authentication from succeeding after authentication completes using a 936different method. 937These may be viewed by increasing the 938.Cm LogLevel 939to 940.Cm DEBUG 941or higher (e.g. by using the 942.Fl v 943flag). 944.Pp 945The user creates their key pair by running 946.Xr ssh-keygen 1 . 947This stores the private key in 948.Pa ~/.ssh/id_dsa 949(DSA), 950.Pa ~/.ssh/id_ecdsa 951(ECDSA), 952.Pa ~/.ssh/id_ecdsa_sk 953(authenticator-hosted ECDSA), 954.Pa ~/.ssh/id_ed25519 955(Ed25519), 956.Pa ~/.ssh/id_ed25519_sk 957(authenticator-hosted Ed25519), 958or 959.Pa ~/.ssh/id_rsa 960(RSA) 961and stores the public key in 962.Pa ~/.ssh/id_dsa.pub 963(DSA), 964.Pa ~/.ssh/id_ecdsa.pub 965(ECDSA), 966.Pa ~/.ssh/id_ecdsa_sk.pub 967(authenticator-hosted ECDSA), 968.Pa ~/.ssh/id_ed25519.pub 969(Ed25519), 970.Pa ~/.ssh/id_ed25519_sk.pub 971(authenticator-hosted Ed25519), 972or 973.Pa ~/.ssh/id_rsa.pub 974(RSA) 975in the user's home directory. 976The user should then copy the public key 977to 978.Pa ~/.ssh/authorized_keys 979in their home directory on the remote machine. 980The 981.Pa authorized_keys 982file corresponds to the conventional 983.Pa ~/.rhosts 984file, and has one key 985per line, though the lines can be very long. 986After this, the user can log in without giving the password. 987.Pp 988A variation on public key authentication 989is available in the form of certificate authentication: 990instead of a set of public/private keys, 991signed certificates are used. 992This has the advantage that a single trusted certification authority 993can be used in place of many public/private keys. 994See the CERTIFICATES section of 995.Xr ssh-keygen 1 996for more information. 997.Pp 998The most convenient way to use public key or certificate authentication 999may be with an authentication agent. 1000See 1001.Xr ssh-agent 1 1002and (optionally) the 1003.Cm AddKeysToAgent 1004directive in 1005.Xr ssh_config 5 1006for more information. 1007.Pp 1008Keyboard-interactive authentication works as follows: 1009The server sends an arbitrary 1010.Qq challenge 1011text and prompts for a response, possibly multiple times. 1012Examples of keyboard-interactive authentication include 1013.Bx 1014Authentication (see 1015.Xr login.conf 5 ) 1016and PAM (some 1017.Pf non- Ox 1018systems). 1019.Pp 1020Finally, if other authentication methods fail, 1021.Nm 1022prompts the user for a password. 1023The password is sent to the remote 1024host for checking; however, since all communications are encrypted, 1025the password cannot be seen by someone listening on the network. 1026.Pp 1027.Nm 1028automatically maintains and checks a database containing 1029identification for all hosts it has ever been used with. 1030Host keys are stored in 1031.Pa ~/.ssh/known_hosts 1032in the user's home directory. 1033Additionally, the file 1034.Pa /etc/ssh/ssh_known_hosts 1035is automatically checked for known hosts. 1036Any new hosts are automatically added to the user's file. 1037If a host's identification ever changes, 1038.Nm 1039warns about this and disables password authentication to prevent 1040server spoofing or man-in-the-middle attacks, 1041which could otherwise be used to circumvent the encryption. 1042The 1043.Cm StrictHostKeyChecking 1044option can be used to control logins to machines whose 1045host key is not known or has changed. 1046.Pp 1047When the user's identity has been accepted by the server, the server 1048either executes the given command in a non-interactive session or, 1049if no command has been specified, logs into the machine and gives 1050the user a normal shell as an interactive session. 1051All communication with 1052the remote command or shell will be automatically encrypted. 1053.Pp 1054If an interactive session is requested, 1055.Nm 1056by default will only request a pseudo-terminal (pty) for interactive 1057sessions when the client has one. 1058The flags 1059.Fl T 1060and 1061.Fl t 1062can be used to override this behaviour. 1063.Pp 1064If a pseudo-terminal has been allocated, the 1065user may use the escape characters noted below. 1066.Pp 1067If no pseudo-terminal has been allocated, 1068the session is transparent and can be used to reliably transfer binary data. 1069On most systems, setting the escape character to 1070.Dq none 1071will also make the session transparent even if a tty is used. 1072.Pp 1073The session terminates when the command or shell on the remote 1074machine exits and all X11 and TCP connections have been closed. 1075.Sh ESCAPE CHARACTERS 1076When a pseudo-terminal has been requested, 1077.Nm 1078supports a number of functions through the use of an escape character. 1079.Pp 1080A single tilde character can be sent as 1081.Ic ~~ 1082or by following the tilde by a character other than those described below. 1083The escape character must always follow a newline to be interpreted as 1084special. 1085The escape character can be changed in configuration files using the 1086.Cm EscapeChar 1087configuration directive or on the command line by the 1088.Fl e 1089option. 1090.Pp 1091The supported escapes (assuming the default 1092.Ql ~ ) 1093are: 1094.Bl -tag -width Ds 1095.It Cm ~. 1096Disconnect. 1097.It Cm ~^Z 1098Background 1099.Nm . 1100.It Cm ~# 1101List forwarded connections. 1102.It Cm ~& 1103Background 1104.Nm 1105at logout when waiting for forwarded connection / X11 sessions to terminate. 1106.It Cm ~? 1107Display a list of escape characters. 1108.It Cm ~B 1109Send a BREAK to the remote system 1110(only useful if the peer supports it). 1111.It Cm ~C 1112Open command line. 1113Currently this allows the addition of port forwardings using the 1114.Fl L , 1115.Fl R 1116and 1117.Fl D 1118options (see above). 1119It also allows the cancellation of existing port-forwardings 1120with 1121.Sm off 1122.Fl KL Oo Ar bind_address : Oc Ar port 1123.Sm on 1124for local, 1125.Sm off 1126.Fl KR Oo Ar bind_address : Oc Ar port 1127.Sm on 1128for remote and 1129.Sm off 1130.Fl KD Oo Ar bind_address : Oc Ar port 1131.Sm on 1132for dynamic port-forwardings. 1133.Ic !\& Ns Ar command 1134allows the user to execute a local command if the 1135.Ic PermitLocalCommand 1136option is enabled in 1137.Xr ssh_config 5 . 1138Basic help is available, using the 1139.Fl h 1140option. 1141.It Cm ~R 1142Request rekeying of the connection 1143(only useful if the peer supports it). 1144.It Cm ~V 1145Decrease the verbosity 1146.Pq Ic LogLevel 1147when errors are being written to stderr. 1148.It Cm ~v 1149Increase the verbosity 1150.Pq Ic LogLevel 1151when errors are being written to stderr. 1152.El 1153.Sh TCP FORWARDING 1154Forwarding of arbitrary TCP connections over a secure channel 1155can be specified either on the command line or in a configuration file. 1156One possible application of TCP forwarding is a secure connection to a 1157mail server; another is going through firewalls. 1158.Pp 1159In the example below, we look at encrypting communication for an IRC client, 1160even though the IRC server it connects to does not directly 1161support encrypted communication. 1162This works as follows: 1163the user connects to the remote host using 1164.Nm , 1165specifying the ports to be used to forward the connection. 1166After that it is possible to start the program locally, 1167and 1168.Nm 1169will encrypt and forward the connection to the remote server. 1170.Pp 1171The following example tunnels an IRC session from the client 1172to an IRC server at 1173.Dq server.example.com , 1174joining channel 1175.Dq #users , 1176nickname 1177.Dq pinky , 1178using the standard IRC port, 6667: 1179.Bd -literal -offset 4n 1180$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10 1181$ irc -c '#users' pinky IRC/127.0.0.1 1182.Ed 1183.Pp 1184The 1185.Fl f 1186option backgrounds 1187.Nm 1188and the remote command 1189.Dq sleep 10 1190is specified to allow an amount of time 1191(10 seconds, in the example) 1192to start the program which is going to use the tunnel. 1193If no connections are made within the time specified, 1194.Nm 1195will exit. 1196.Sh X11 FORWARDING 1197If the 1198.Cm ForwardX11 1199variable is set to 1200.Dq yes 1201(or see the description of the 1202.Fl X , 1203.Fl x , 1204and 1205.Fl Y 1206options above) 1207and the user is using X11 (the 1208.Ev DISPLAY 1209environment variable is set), the connection to the X11 display is 1210automatically forwarded to the remote side in such a way that any X11 1211programs started from the shell (or command) will go through the 1212encrypted channel, and the connection to the real X server will be made 1213from the local machine. 1214The user should not manually set 1215.Ev DISPLAY . 1216Forwarding of X11 connections can be 1217configured on the command line or in configuration files. 1218.Pp 1219The 1220.Ev DISPLAY 1221value set by 1222.Nm 1223will point to the server machine, but with a display number greater than zero. 1224This is normal, and happens because 1225.Nm 1226creates a 1227.Dq proxy 1228X server on the server machine for forwarding the 1229connections over the encrypted channel. 1230.Pp 1231.Nm 1232will also automatically set up Xauthority data on the server machine. 1233For this purpose, it will generate a random authorization cookie, 1234store it in Xauthority on the server, and verify that any forwarded 1235connections carry this cookie and replace it by the real cookie when 1236the connection is opened. 1237The real authentication cookie is never 1238sent to the server machine (and no cookies are sent in the plain). 1239.Pp 1240If the 1241.Cm ForwardAgent 1242variable is set to 1243.Dq yes 1244(or see the description of the 1245.Fl A 1246and 1247.Fl a 1248options above) and 1249the user is using an authentication agent, the connection to the agent 1250is automatically forwarded to the remote side. 1251.Sh VERIFYING HOST KEYS 1252When connecting to a server for the first time, 1253a fingerprint of the server's public key is presented to the user 1254(unless the option 1255.Cm StrictHostKeyChecking 1256has been disabled). 1257Fingerprints can be determined using 1258.Xr ssh-keygen 1 : 1259.Pp 1260.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1261.Pp 1262If the fingerprint is already known, it can be matched 1263and the key can be accepted or rejected. 1264If only legacy (MD5) fingerprints for the server are available, the 1265.Xr ssh-keygen 1 1266.Fl E 1267option may be used to downgrade the fingerprint algorithm to match. 1268.Pp 1269Because of the difficulty of comparing host keys 1270just by looking at fingerprint strings, 1271there is also support to compare host keys visually, 1272using 1273.Em random art . 1274By setting the 1275.Cm VisualHostKey 1276option to 1277.Dq yes , 1278a small ASCII graphic gets displayed on every login to a server, no matter 1279if the session itself is interactive or not. 1280By learning the pattern a known server produces, a user can easily 1281find out that the host key has changed when a completely different pattern 1282is displayed. 1283Because these patterns are not unambiguous however, a pattern that looks 1284similar to the pattern remembered only gives a good probability that the 1285host key is the same, not guaranteed proof. 1286.Pp 1287To get a listing of the fingerprints along with their random art for 1288all known hosts, the following command line can be used: 1289.Pp 1290.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1291.Pp 1292If the fingerprint is unknown, 1293an alternative method of verification is available: 1294SSH fingerprints verified by DNS. 1295An additional resource record (RR), 1296SSHFP, 1297is added to a zonefile 1298and the connecting client is able to match the fingerprint 1299with that of the key presented. 1300.Pp 1301In this example, we are connecting a client to a server, 1302.Dq host.example.com . 1303The SSHFP resource records should first be added to the zonefile for 1304host.example.com: 1305.Bd -literal -offset indent 1306$ ssh-keygen -r host.example.com. 1307.Ed 1308.Pp 1309The output lines will have to be added to the zonefile. 1310To check that the zone is answering fingerprint queries: 1311.Pp 1312.Dl $ dig -t SSHFP host.example.com 1313.Pp 1314Finally the client connects: 1315.Bd -literal -offset indent 1316$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1317[...] 1318Matching host key fingerprint found in DNS. 1319Are you sure you want to continue connecting (yes/no)? 1320.Ed 1321.Pp 1322See the 1323.Cm VerifyHostKeyDNS 1324option in 1325.Xr ssh_config 5 1326for more information. 1327.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1328.Nm 1329contains support for Virtual Private Network (VPN) tunnelling 1330using the 1331.Xr tun 4 1332network pseudo-device, 1333allowing two networks to be joined securely. 1334The 1335.Xr sshd_config 5 1336configuration option 1337.Cm PermitTunnel 1338controls whether the server supports this, 1339and at what level (layer 2 or 3 traffic). 1340.Pp 1341The following example would connect client network 10.0.50.0/24 1342with remote network 10.0.99.0/24 using a point-to-point connection 1343from 10.1.1.1 to 10.1.1.2, 1344provided that the SSH server running on the gateway to the remote network, 1345at 192.168.1.15, allows it. 1346.Pp 1347On the client: 1348.Bd -literal -offset indent 1349# ssh -f -w 0:1 192.168.1.15 true 1350# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1351# route add 10.0.99.0/24 10.1.1.2 1352.Ed 1353.Pp 1354On the server: 1355.Bd -literal -offset indent 1356# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1357# route add 10.0.50.0/24 10.1.1.1 1358.Ed 1359.Pp 1360Client access may be more finely tuned via the 1361.Pa /root/.ssh/authorized_keys 1362file (see below) and the 1363.Cm PermitRootLogin 1364server option. 1365The following entry would permit connections on 1366.Xr tun 4 1367device 1 from user 1368.Dq jane 1369and on tun device 2 from user 1370.Dq john , 1371if 1372.Cm PermitRootLogin 1373is set to 1374.Dq forced-commands-only : 1375.Bd -literal -offset 2n 1376tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1377tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1378.Ed 1379.Pp 1380Since an SSH-based setup entails a fair amount of overhead, 1381it may be more suited to temporary setups, 1382such as for wireless VPNs. 1383More permanent VPNs are better provided by tools such as 1384.Xr ipsecctl 8 1385and 1386.Xr isakmpd 8 . 1387.Sh ENVIRONMENT 1388.Nm 1389will normally set the following environment variables: 1390.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1391.It Ev DISPLAY 1392The 1393.Ev DISPLAY 1394variable indicates the location of the X11 server. 1395It is automatically set by 1396.Nm 1397to point to a value of the form 1398.Dq hostname:n , 1399where 1400.Dq hostname 1401indicates the host where the shell runs, and 1402.Sq n 1403is an integer \*(Ge 1. 1404.Nm 1405uses this special value to forward X11 connections over the secure 1406channel. 1407The user should normally not set 1408.Ev DISPLAY 1409explicitly, as that 1410will render the X11 connection insecure (and will require the user to 1411manually copy any required authorization cookies). 1412.It Ev HOME 1413Set to the path of the user's home directory. 1414.It Ev LOGNAME 1415Synonym for 1416.Ev USER ; 1417set for compatibility with systems that use this variable. 1418.It Ev MAIL 1419Set to the path of the user's mailbox. 1420.It Ev PATH 1421Set to the default 1422.Ev PATH , 1423as specified when compiling 1424.Nm . 1425.It Ev SSH_ASKPASS 1426If 1427.Nm 1428needs a passphrase, it will read the passphrase from the current 1429terminal if it was run from a terminal. 1430If 1431.Nm 1432does not have a terminal associated with it but 1433.Ev DISPLAY 1434and 1435.Ev SSH_ASKPASS 1436are set, it will execute the program specified by 1437.Ev SSH_ASKPASS 1438and open an X11 window to read the passphrase. 1439This is particularly useful when calling 1440.Nm 1441from a 1442.Pa .xsession 1443or related script. 1444(Note that on some machines it 1445may be necessary to redirect the input from 1446.Pa /dev/null 1447to make this work.) 1448.It Ev SSH_ASKPASS_REQUIRE 1449Allows further control over the use of an askpass program. 1450If this variable is set to 1451.Dq never 1452then 1453.Nm 1454will never attempt to use one. 1455If it is set to 1456.Dq prefer , 1457then 1458.Nm 1459will prefer to use the askpass program instead of the TTY when requesting 1460passwords. 1461Finally, if the variable is set to 1462.Dq force , 1463then the askpass program will be used for all passphrase input regardless 1464of whether 1465.Ev DISPLAY 1466is set. 1467.It Ev SSH_AUTH_SOCK 1468Identifies the path of a 1469.Ux Ns -domain 1470socket used to communicate with the agent. 1471.It Ev SSH_CONNECTION 1472Identifies the client and server ends of the connection. 1473The variable contains 1474four space-separated values: client IP address, client port number, 1475server IP address, and server port number. 1476.It Ev SSH_ORIGINAL_COMMAND 1477This variable contains the original command line if a forced command 1478is executed. 1479It can be used to extract the original arguments. 1480.It Ev SSH_TTY 1481This is set to the name of the tty (path to the device) associated 1482with the current shell or command. 1483If the current session has no tty, 1484this variable is not set. 1485.It Ev SSH_TUNNEL 1486Optionally set by 1487.Xr sshd 8 1488to contain the interface names assigned if tunnel forwarding was 1489requested by the client. 1490.It Ev SSH_USER_AUTH 1491Optionally set by 1492.Xr sshd 8 , 1493this variable may contain a pathname to a file that lists the authentication 1494methods successfully used when the session was established, including any 1495public keys that were used. 1496.It Ev TZ 1497This variable is set to indicate the present time zone if it 1498was set when the daemon was started (i.e. the daemon passes the value 1499on to new connections). 1500.It Ev USER 1501Set to the name of the user logging in. 1502.El 1503.Pp 1504Additionally, 1505.Nm 1506reads 1507.Pa ~/.ssh/environment , 1508and adds lines of the format 1509.Dq VARNAME=value 1510to the environment if the file exists and users are allowed to 1511change their environment. 1512For more information, see the 1513.Cm PermitUserEnvironment 1514option in 1515.Xr sshd_config 5 . 1516.Sh FILES 1517.Bl -tag -width Ds -compact 1518.It Pa ~/.rhosts 1519This file is used for host-based authentication (see above). 1520On some machines this file may need to be 1521world-readable if the user's home directory is on an NFS partition, 1522because 1523.Xr sshd 8 1524reads it as root. 1525Additionally, this file must be owned by the user, 1526and must not have write permissions for anyone else. 1527The recommended 1528permission for most machines is read/write for the user, and not 1529accessible by others. 1530.Pp 1531.It Pa ~/.shosts 1532This file is used in exactly the same way as 1533.Pa .rhosts , 1534but allows host-based authentication without permitting login with 1535rlogin/rsh. 1536.Pp 1537.It Pa ~/.ssh/ 1538This directory is the default location for all user-specific configuration 1539and authentication information. 1540There is no general requirement to keep the entire contents of this directory 1541secret, but the recommended permissions are read/write/execute for the user, 1542and not accessible by others. 1543.Pp 1544.It Pa ~/.ssh/authorized_keys 1545Lists the public keys (DSA, ECDSA, Ed25519, RSA) 1546that can be used for logging in as this user. 1547The format of this file is described in the 1548.Xr sshd 8 1549manual page. 1550This file is not highly sensitive, but the recommended 1551permissions are read/write for the user, and not accessible by others. 1552.Pp 1553.It Pa ~/.ssh/config 1554This is the per-user configuration file. 1555The file format and configuration options are described in 1556.Xr ssh_config 5 . 1557Because of the potential for abuse, this file must have strict permissions: 1558read/write for the user, and not writable by others. 1559.Pp 1560.It Pa ~/.ssh/environment 1561Contains additional definitions for environment variables; see 1562.Sx ENVIRONMENT , 1563above. 1564.Pp 1565.It Pa ~/.ssh/id_dsa 1566.It Pa ~/.ssh/id_ecdsa 1567.It Pa ~/.ssh/id_ecdsa_sk 1568.It Pa ~/.ssh/id_ed25519 1569.It Pa ~/.ssh/id_ed25519_sk 1570.It Pa ~/.ssh/id_rsa 1571Contains the private key for authentication. 1572These files 1573contain sensitive data and should be readable by the user but not 1574accessible by others (read/write/execute). 1575.Nm 1576will simply ignore a private key file if it is accessible by others. 1577It is possible to specify a passphrase when 1578generating the key which will be used to encrypt the 1579sensitive part of this file using AES-128. 1580.Pp 1581.It Pa ~/.ssh/id_dsa.pub 1582.It Pa ~/.ssh/id_ecdsa.pub 1583.It Pa ~/.ssh/id_ecdsa_sk.pub 1584.It Pa ~/.ssh/id_ed25519.pub 1585.It Pa ~/.ssh/id_ed25519_sk.pub 1586.It Pa ~/.ssh/id_rsa.pub 1587Contains the public key for authentication. 1588These files are not 1589sensitive and can (but need not) be readable by anyone. 1590.Pp 1591.It Pa ~/.ssh/known_hosts 1592Contains a list of host keys for all hosts the user has logged into 1593that are not already in the systemwide list of known host keys. 1594See 1595.Xr sshd 8 1596for further details of the format of this file. 1597.Pp 1598.It Pa ~/.ssh/rc 1599Commands in this file are executed by 1600.Nm 1601when the user logs in, just before the user's shell (or command) is 1602started. 1603See the 1604.Xr sshd 8 1605manual page for more information. 1606.Pp 1607.It Pa /etc/hosts.equiv 1608This file is for host-based authentication (see above). 1609It should only be writable by root. 1610.Pp 1611.It Pa /etc/shosts.equiv 1612This file is used in exactly the same way as 1613.Pa hosts.equiv , 1614but allows host-based authentication without permitting login with 1615rlogin/rsh. 1616.Pp 1617.It Pa /etc/ssh/ssh_config 1618Systemwide configuration file. 1619The file format and configuration options are described in 1620.Xr ssh_config 5 . 1621.Pp 1622.It Pa /etc/ssh/ssh_host_key 1623.It Pa /etc/ssh/ssh_host_dsa_key 1624.It Pa /etc/ssh/ssh_host_ecdsa_key 1625.It Pa /etc/ssh/ssh_host_ed25519_key 1626.It Pa /etc/ssh/ssh_host_rsa_key 1627These files contain the private parts of the host keys 1628and are used for host-based authentication. 1629.Pp 1630.It Pa /etc/ssh/ssh_known_hosts 1631Systemwide list of known host keys. 1632This file should be prepared by the 1633system administrator to contain the public host keys of all machines in the 1634organization. 1635It should be world-readable. 1636See 1637.Xr sshd 8 1638for further details of the format of this file. 1639.Pp 1640.It Pa /etc/ssh/sshrc 1641Commands in this file are executed by 1642.Nm 1643when the user logs in, just before the user's shell (or command) is started. 1644See the 1645.Xr sshd 8 1646manual page for more information. 1647.El 1648.Sh EXIT STATUS 1649.Nm 1650exits with the exit status of the remote command or with 255 1651if an error occurred. 1652.Sh SEE ALSO 1653.Xr scp 1 , 1654.Xr sftp 1 , 1655.Xr ssh-add 1 , 1656.Xr ssh-agent 1 , 1657.Xr ssh-keygen 1 , 1658.Xr ssh-keyscan 1 , 1659.Xr tun 4 , 1660.Xr ssh_config 5 , 1661.Xr ssh-keysign 8 , 1662.Xr sshd 8 1663.Sh STANDARDS 1664.Rs 1665.%A S. Lehtinen 1666.%A C. Lonvick 1667.%D January 2006 1668.%R RFC 4250 1669.%T The Secure Shell (SSH) Protocol Assigned Numbers 1670.Re 1671.Pp 1672.Rs 1673.%A T. Ylonen 1674.%A C. Lonvick 1675.%D January 2006 1676.%R RFC 4251 1677.%T The Secure Shell (SSH) Protocol Architecture 1678.Re 1679.Pp 1680.Rs 1681.%A T. Ylonen 1682.%A C. Lonvick 1683.%D January 2006 1684.%R RFC 4252 1685.%T The Secure Shell (SSH) Authentication Protocol 1686.Re 1687.Pp 1688.Rs 1689.%A T. Ylonen 1690.%A C. Lonvick 1691.%D January 2006 1692.%R RFC 4253 1693.%T The Secure Shell (SSH) Transport Layer Protocol 1694.Re 1695.Pp 1696.Rs 1697.%A T. Ylonen 1698.%A C. Lonvick 1699.%D January 2006 1700.%R RFC 4254 1701.%T The Secure Shell (SSH) Connection Protocol 1702.Re 1703.Pp 1704.Rs 1705.%A J. Schlyter 1706.%A W. Griffin 1707.%D January 2006 1708.%R RFC 4255 1709.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints 1710.Re 1711.Pp 1712.Rs 1713.%A F. Cusack 1714.%A M. Forssen 1715.%D January 2006 1716.%R RFC 4256 1717.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH) 1718.Re 1719.Pp 1720.Rs 1721.%A J. Galbraith 1722.%A P. Remaker 1723.%D January 2006 1724.%R RFC 4335 1725.%T The Secure Shell (SSH) Session Channel Break Extension 1726.Re 1727.Pp 1728.Rs 1729.%A M. Bellare 1730.%A T. Kohno 1731.%A C. Namprempre 1732.%D January 2006 1733.%R RFC 4344 1734.%T The Secure Shell (SSH) Transport Layer Encryption Modes 1735.Re 1736.Pp 1737.Rs 1738.%A B. Harris 1739.%D January 2006 1740.%R RFC 4345 1741.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol 1742.Re 1743.Pp 1744.Rs 1745.%A M. Friedl 1746.%A N. Provos 1747.%A W. Simpson 1748.%D March 2006 1749.%R RFC 4419 1750.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 1751.Re 1752.Pp 1753.Rs 1754.%A J. Galbraith 1755.%A R. Thayer 1756.%D November 2006 1757.%R RFC 4716 1758.%T The Secure Shell (SSH) Public Key File Format 1759.Re 1760.Pp 1761.Rs 1762.%A D. Stebila 1763.%A J. Green 1764.%D December 2009 1765.%R RFC 5656 1766.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer 1767.Re 1768.Pp 1769.Rs 1770.%A A. Perrig 1771.%A D. Song 1772.%D 1999 1773.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99) 1774.%T Hash Visualization: a New Technique to improve Real-World Security 1775.Re 1776.Sh AUTHORS 1777OpenSSH is a derivative of the original and free 1778ssh 1.2.12 release by Tatu Ylonen. 1779Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1780Theo de Raadt and Dug Song 1781removed many bugs, re-added newer features and 1782created OpenSSH. 1783Markus Friedl contributed the support for SSH 1784protocol versions 1.5 and 2.0. 1785