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 1.437 2023/07/23 20:04:45 naddy Exp $ 37.Dd $Mdocdate: July 23 2023 $ 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 tag 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 an 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 tag 598Specify a tag name that may be used to select configuration in 599.Xr ssh_config 5 . 600Refer to the 601.Cm Tag 602and 603.Cm Match 604keywords in 605.Xr ssh_config 5 606for more information. 607.It Fl p Ar port 608Port to connect to on the remote host. 609This can be specified on a 610per-host basis in the configuration file. 611.Pp 612.It Fl Q Ar query_option 613Queries for the algorithms supported by one of the following features: 614.Ar cipher 615(supported symmetric ciphers), 616.Ar cipher-auth 617(supported symmetric ciphers that support authenticated encryption), 618.Ar help 619(supported query terms for use with the 620.Fl Q 621flag), 622.Ar mac 623(supported message integrity codes), 624.Ar kex 625(key exchange algorithms), 626.Ar key 627(key types), 628.Ar key-ca-sign 629(valid CA signature algorithms for certificates), 630.Ar key-cert 631(certificate key types), 632.Ar key-plain 633(non-certificate key types), 634.Ar key-sig 635(all key types and signature algorithms), 636.Ar protocol-version 637(supported SSH protocol versions), and 638.Ar sig 639(supported signature algorithms). 640Alternatively, any keyword from 641.Xr ssh_config 5 642or 643.Xr sshd_config 5 644that takes an algorithm list may be used as an alias for the corresponding 645query_option. 646.Pp 647.It Fl q 648Quiet mode. 649Causes most warning and diagnostic messages to be suppressed. 650.Pp 651.It Fl R Xo 652.Sm off 653.Oo Ar bind_address : Oc 654.Ar port : host : hostport 655.Sm on 656.Xc 657.It Fl R Xo 658.Sm off 659.Oo Ar bind_address : Oc 660.Ar port : local_socket 661.Sm on 662.Xc 663.It Fl R Xo 664.Sm off 665.Ar remote_socket : host : hostport 666.Sm on 667.Xc 668.It Fl R Xo 669.Sm off 670.Ar remote_socket : local_socket 671.Sm on 672.Xc 673.It Fl R Xo 674.Sm off 675.Oo Ar bind_address : Oc 676.Ar port 677.Sm on 678.Xc 679Specifies that connections to the given TCP port or Unix socket on the remote 680(server) host are to be forwarded to the local side. 681.Pp 682This works by allocating a socket to listen to either a TCP 683.Ar port 684or to a Unix socket on the remote side. 685Whenever a connection is made to this port or Unix socket, the 686connection is forwarded over the secure channel, and a connection 687is made from the local machine to either an explicit destination specified by 688.Ar host 689port 690.Ar hostport , 691or 692.Ar local_socket , 693or, if no explicit destination was specified, 694.Nm 695will act as a SOCKS 4/5 proxy and forward connections to the destinations 696requested by the remote SOCKS client. 697.Pp 698Port forwardings can also be specified in the configuration file. 699Privileged ports can be forwarded only when 700logging in as root on the remote machine. 701IPv6 addresses can be specified by enclosing the address in square brackets. 702.Pp 703By default, TCP listening sockets on the server will be bound to the loopback 704interface only. 705This may be overridden by specifying a 706.Ar bind_address . 707An empty 708.Ar bind_address , 709or the address 710.Ql * , 711indicates that the remote socket should listen on all interfaces. 712Specifying a remote 713.Ar bind_address 714will only succeed if the server's 715.Cm GatewayPorts 716option is enabled (see 717.Xr sshd_config 5 ) . 718.Pp 719If the 720.Ar port 721argument is 722.Ql 0 , 723the listen port will be dynamically allocated on the server and reported 724to the client at run time. 725When used together with 726.Ic -O forward , 727the allocated port will be printed to the standard output. 728.Pp 729.It Fl S Ar ctl_path 730Specifies the location of a control socket for connection sharing, 731or the string 732.Dq none 733to disable connection sharing. 734Refer to the description of 735.Cm ControlPath 736and 737.Cm ControlMaster 738in 739.Xr ssh_config 5 740for details. 741.Pp 742.It Fl s 743May be used to request invocation of a subsystem on the remote system. 744Subsystems facilitate the use of SSH 745as a secure transport for other applications (e.g.\& 746.Xr sftp 1 ) . 747The subsystem is specified as the remote command. 748Refer to the description of 749.Cm SessionType 750in 751.Xr ssh_config 5 752for details. 753.Pp 754.It Fl T 755Disable pseudo-terminal allocation. 756.Pp 757.It Fl t 758Force pseudo-terminal allocation. 759This can be used to execute arbitrary 760screen-based programs on a remote machine, which can be very useful, 761e.g. when implementing menu services. 762Multiple 763.Fl t 764options force tty allocation, even if 765.Nm 766has no local tty. 767.Pp 768.It Fl V 769Display the version number and exit. 770.Pp 771.It Fl v 772Verbose mode. 773Causes 774.Nm 775to print debugging messages about its progress. 776This is helpful in 777debugging connection, authentication, and configuration problems. 778Multiple 779.Fl v 780options increase the verbosity. 781The maximum is 3. 782.Pp 783.It Fl W Ar host : Ns Ar port 784Requests that standard input and output on the client be forwarded to 785.Ar host 786on 787.Ar port 788over the secure channel. 789Implies 790.Fl N , 791.Fl T , 792.Cm ExitOnForwardFailure 793and 794.Cm ClearAllForwardings , 795though these can be overridden in the configuration file or using 796.Fl o 797command line options. 798.Pp 799.It Fl w Xo 800.Ar local_tun Ns Op : Ns Ar remote_tun 801.Xc 802Requests 803tunnel 804device forwarding with the specified 805.Xr tun 4 806devices between the client 807.Pq Ar local_tun 808and the server 809.Pq Ar remote_tun . 810.Pp 811The devices may be specified by numerical ID or the keyword 812.Dq any , 813which uses the next available tunnel device. 814If 815.Ar remote_tun 816is not specified, it defaults to 817.Dq any . 818See also the 819.Cm Tunnel 820and 821.Cm TunnelDevice 822directives in 823.Xr ssh_config 5 . 824.Pp 825If the 826.Cm Tunnel 827directive is unset, it will be set to the default tunnel mode, which is 828.Dq point-to-point . 829If a different 830.Cm Tunnel 831forwarding mode it desired, then it should be specified before 832.Fl w . 833.Pp 834.It Fl X 835Enables X11 forwarding. 836This can also be specified on a per-host basis in a configuration file. 837.Pp 838X11 forwarding should be enabled with caution. 839Users with the ability to bypass file permissions on the remote host 840(for the user's X authorization database) 841can access the local X11 display through the forwarded connection. 842An attacker may then be able to perform activities such as keystroke monitoring. 843.Pp 844For this reason, X11 forwarding is subjected to X11 SECURITY extension 845restrictions by default. 846Refer to the 847.Nm 848.Fl Y 849option and the 850.Cm ForwardX11Trusted 851directive in 852.Xr ssh_config 5 853for more information. 854.Pp 855.It Fl x 856Disables X11 forwarding. 857.Pp 858.It Fl Y 859Enables trusted X11 forwarding. 860Trusted X11 forwardings are not subjected to the X11 SECURITY extension 861controls. 862.Pp 863.It Fl y 864Send log information using the 865.Xr syslog 3 866system module. 867By default this information is sent to stderr. 868.El 869.Pp 870.Nm 871may additionally obtain configuration data from 872a per-user configuration file and a system-wide configuration file. 873The file format and configuration options are described in 874.Xr ssh_config 5 . 875.Sh AUTHENTICATION 876The OpenSSH SSH client supports SSH protocol 2. 877.Pp 878The methods available for authentication are: 879GSSAPI-based authentication, 880host-based authentication, 881public key authentication, 882keyboard-interactive authentication, 883and password authentication. 884Authentication methods are tried in the order specified above, 885though 886.Cm PreferredAuthentications 887can be used to change the default order. 888.Pp 889Host-based authentication works as follows: 890If the machine the user logs in from is listed in 891.Pa /etc/hosts.equiv 892or 893.Pa /etc/shosts.equiv 894on the remote machine, the user is non-root and the user names are 895the same on both sides, or if the files 896.Pa ~/.rhosts 897or 898.Pa ~/.shosts 899exist in the user's home directory on the 900remote machine and contain a line containing the name of the client 901machine and the name of the user on that machine, the user is 902considered for login. 903Additionally, the server 904.Em must 905be able to verify the client's 906host key (see the description of 907.Pa /etc/ssh/ssh_known_hosts 908and 909.Pa ~/.ssh/known_hosts , 910below) 911for login to be permitted. 912This authentication method closes security holes due to IP 913spoofing, DNS spoofing, and routing spoofing. 914[Note to the administrator: 915.Pa /etc/hosts.equiv , 916.Pa ~/.rhosts , 917and the rlogin/rsh protocol in general, are inherently insecure and should be 918disabled if security is desired.] 919.Pp 920Public key authentication works as follows: 921The scheme is based on public-key cryptography, 922using cryptosystems 923where encryption and decryption are done using separate keys, 924and it is unfeasible to derive the decryption key from the encryption key. 925The idea is that each user creates a public/private 926key pair for authentication purposes. 927The server knows the public key, and only the user knows the private key. 928.Nm 929implements public key authentication protocol automatically, 930using one of the DSA, ECDSA, Ed25519 or RSA algorithms. 931The HISTORY section of 932.Xr ssl 8 933contains a brief discussion of the DSA and RSA algorithms. 934.Pp 935The file 936.Pa ~/.ssh/authorized_keys 937lists the public keys that are permitted for logging in. 938When the user logs in, the 939.Nm 940program tells the server which key pair it would like to use for 941authentication. 942The client proves that it has access to the private key 943and the server checks that the corresponding public key 944is authorized to accept the account. 945.Pp 946The server may inform the client of errors that prevented public key 947authentication from succeeding after authentication completes using a 948different method. 949These may be viewed by increasing the 950.Cm LogLevel 951to 952.Cm DEBUG 953or higher (e.g. by using the 954.Fl v 955flag). 956.Pp 957The user creates their key pair by running 958.Xr ssh-keygen 1 . 959This stores the private key in 960.Pa ~/.ssh/id_dsa 961(DSA), 962.Pa ~/.ssh/id_ecdsa 963(ECDSA), 964.Pa ~/.ssh/id_ecdsa_sk 965(authenticator-hosted ECDSA), 966.Pa ~/.ssh/id_ed25519 967(Ed25519), 968.Pa ~/.ssh/id_ed25519_sk 969(authenticator-hosted Ed25519), 970or 971.Pa ~/.ssh/id_rsa 972(RSA) 973and stores the public key in 974.Pa ~/.ssh/id_dsa.pub 975(DSA), 976.Pa ~/.ssh/id_ecdsa.pub 977(ECDSA), 978.Pa ~/.ssh/id_ecdsa_sk.pub 979(authenticator-hosted ECDSA), 980.Pa ~/.ssh/id_ed25519.pub 981(Ed25519), 982.Pa ~/.ssh/id_ed25519_sk.pub 983(authenticator-hosted Ed25519), 984or 985.Pa ~/.ssh/id_rsa.pub 986(RSA) 987in the user's home directory. 988The user should then copy the public key 989to 990.Pa ~/.ssh/authorized_keys 991in their home directory on the remote machine. 992The 993.Pa authorized_keys 994file corresponds to the conventional 995.Pa ~/.rhosts 996file, and has one key 997per line, though the lines can be very long. 998After this, the user can log in without giving the password. 999.Pp 1000A variation on public key authentication 1001is available in the form of certificate authentication: 1002instead of a set of public/private keys, 1003signed certificates are used. 1004This has the advantage that a single trusted certification authority 1005can be used in place of many public/private keys. 1006See the CERTIFICATES section of 1007.Xr ssh-keygen 1 1008for more information. 1009.Pp 1010The most convenient way to use public key or certificate authentication 1011may be with an authentication agent. 1012See 1013.Xr ssh-agent 1 1014and (optionally) the 1015.Cm AddKeysToAgent 1016directive in 1017.Xr ssh_config 5 1018for more information. 1019.Pp 1020Keyboard-interactive authentication works as follows: 1021The server sends an arbitrary 1022.Qq challenge 1023text and prompts for a response, possibly multiple times. 1024Examples of keyboard-interactive authentication include 1025.Bx 1026Authentication (see 1027.Xr login.conf 5 ) 1028and PAM (some 1029.Pf non- Ox 1030systems). 1031.Pp 1032Finally, if other authentication methods fail, 1033.Nm 1034prompts the user for a password. 1035The password is sent to the remote 1036host for checking; however, since all communications are encrypted, 1037the password cannot be seen by someone listening on the network. 1038.Pp 1039.Nm 1040automatically maintains and checks a database containing 1041identification for all hosts it has ever been used with. 1042Host keys are stored in 1043.Pa ~/.ssh/known_hosts 1044in the user's home directory. 1045Additionally, the file 1046.Pa /etc/ssh/ssh_known_hosts 1047is automatically checked for known hosts. 1048Any new hosts are automatically added to the user's file. 1049If a host's identification ever changes, 1050.Nm 1051warns about this and disables password authentication to prevent 1052server spoofing or man-in-the-middle attacks, 1053which could otherwise be used to circumvent the encryption. 1054The 1055.Cm StrictHostKeyChecking 1056option can be used to control logins to machines whose 1057host key is not known or has changed. 1058.Pp 1059When the user's identity has been accepted by the server, the server 1060either executes the given command in a non-interactive session or, 1061if no command has been specified, logs into the machine and gives 1062the user a normal shell as an interactive session. 1063All communication with 1064the remote command or shell will be automatically encrypted. 1065.Pp 1066If an interactive session is requested, 1067.Nm 1068by default will only request a pseudo-terminal (pty) for interactive 1069sessions when the client has one. 1070The flags 1071.Fl T 1072and 1073.Fl t 1074can be used to override this behaviour. 1075.Pp 1076If a pseudo-terminal has been allocated, the 1077user may use the escape characters noted below. 1078.Pp 1079If no pseudo-terminal has been allocated, 1080the session is transparent and can be used to reliably transfer binary data. 1081On most systems, setting the escape character to 1082.Dq none 1083will also make the session transparent even if a tty is used. 1084.Pp 1085The session terminates when the command or shell on the remote 1086machine exits and all X11 and TCP connections have been closed. 1087.Sh ESCAPE CHARACTERS 1088When a pseudo-terminal has been requested, 1089.Nm 1090supports a number of functions through the use of an escape character. 1091.Pp 1092A single tilde character can be sent as 1093.Ic ~~ 1094or by following the tilde by a character other than those described below. 1095The escape character must always follow a newline to be interpreted as 1096special. 1097The escape character can be changed in configuration files using the 1098.Cm EscapeChar 1099configuration directive or on the command line by the 1100.Fl e 1101option. 1102.Pp 1103The supported escapes (assuming the default 1104.Ql ~ ) 1105are: 1106.Bl -tag -width Ds 1107.It Cm ~. 1108Disconnect. 1109.It Cm ~^Z 1110Background 1111.Nm . 1112.It Cm ~# 1113List forwarded connections. 1114.It Cm ~& 1115Background 1116.Nm 1117at logout when waiting for forwarded connection / X11 sessions to terminate. 1118.It Cm ~? 1119Display a list of escape characters. 1120.It Cm ~B 1121Send a BREAK to the remote system 1122(only useful if the peer supports it). 1123.It Cm ~C 1124Open command line. 1125Currently this allows the addition of port forwardings using the 1126.Fl L , 1127.Fl R 1128and 1129.Fl D 1130options (see above). 1131It also allows the cancellation of existing port-forwardings 1132with 1133.Sm off 1134.Fl KL Oo Ar bind_address : Oc Ar port 1135.Sm on 1136for local, 1137.Sm off 1138.Fl KR Oo Ar bind_address : Oc Ar port 1139.Sm on 1140for remote and 1141.Sm off 1142.Fl KD Oo Ar bind_address : Oc Ar port 1143.Sm on 1144for dynamic port-forwardings. 1145.Ic !\& Ns Ar command 1146allows the user to execute a local command if the 1147.Ic PermitLocalCommand 1148option is enabled in 1149.Xr ssh_config 5 . 1150Basic help is available, using the 1151.Fl h 1152option. 1153.It Cm ~R 1154Request rekeying of the connection 1155(only useful if the peer supports it). 1156.It Cm ~V 1157Decrease the verbosity 1158.Pq Ic LogLevel 1159when errors are being written to stderr. 1160.It Cm ~v 1161Increase the verbosity 1162.Pq Ic LogLevel 1163when errors are being written to stderr. 1164.El 1165.Sh TCP FORWARDING 1166Forwarding of arbitrary TCP connections over a secure channel 1167can be specified either on the command line or in a configuration file. 1168One possible application of TCP forwarding is a secure connection to a 1169mail server; another is going through firewalls. 1170.Pp 1171In the example below, we look at encrypting communication for an IRC client, 1172even though the IRC server it connects to does not directly 1173support encrypted communication. 1174This works as follows: 1175the user connects to the remote host using 1176.Nm , 1177specifying the ports to be used to forward the connection. 1178After that it is possible to start the program locally, 1179and 1180.Nm 1181will encrypt and forward the connection to the remote server. 1182.Pp 1183The following example tunnels an IRC session from the client 1184to an IRC server at 1185.Dq server.example.com , 1186joining channel 1187.Dq #users , 1188nickname 1189.Dq pinky , 1190using the standard IRC port, 6667: 1191.Bd -literal -offset 4n 1192$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10 1193$ irc -c '#users' pinky IRC/127.0.0.1 1194.Ed 1195.Pp 1196The 1197.Fl f 1198option backgrounds 1199.Nm 1200and the remote command 1201.Dq sleep 10 1202is specified to allow an amount of time 1203(10 seconds, in the example) 1204to start the program which is going to use the tunnel. 1205If no connections are made within the time specified, 1206.Nm 1207will exit. 1208.Sh X11 FORWARDING 1209If the 1210.Cm ForwardX11 1211variable is set to 1212.Dq yes 1213(or see the description of the 1214.Fl X , 1215.Fl x , 1216and 1217.Fl Y 1218options above) 1219and the user is using X11 (the 1220.Ev DISPLAY 1221environment variable is set), the connection to the X11 display is 1222automatically forwarded to the remote side in such a way that any X11 1223programs started from the shell (or command) will go through the 1224encrypted channel, and the connection to the real X server will be made 1225from the local machine. 1226The user should not manually set 1227.Ev DISPLAY . 1228Forwarding of X11 connections can be 1229configured on the command line or in configuration files. 1230.Pp 1231The 1232.Ev DISPLAY 1233value set by 1234.Nm 1235will point to the server machine, but with a display number greater than zero. 1236This is normal, and happens because 1237.Nm 1238creates a 1239.Dq proxy 1240X server on the server machine for forwarding the 1241connections over the encrypted channel. 1242.Pp 1243.Nm 1244will also automatically set up Xauthority data on the server machine. 1245For this purpose, it will generate a random authorization cookie, 1246store it in Xauthority on the server, and verify that any forwarded 1247connections carry this cookie and replace it by the real cookie when 1248the connection is opened. 1249The real authentication cookie is never 1250sent to the server machine (and no cookies are sent in the plain). 1251.Pp 1252If the 1253.Cm ForwardAgent 1254variable is set to 1255.Dq yes 1256(or see the description of the 1257.Fl A 1258and 1259.Fl a 1260options above) and 1261the user is using an authentication agent, the connection to the agent 1262is automatically forwarded to the remote side. 1263.Sh VERIFYING HOST KEYS 1264When connecting to a server for the first time, 1265a fingerprint of the server's public key is presented to the user 1266(unless the option 1267.Cm StrictHostKeyChecking 1268has been disabled). 1269Fingerprints can be determined using 1270.Xr ssh-keygen 1 : 1271.Pp 1272.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1273.Pp 1274If the fingerprint is already known, it can be matched 1275and the key can be accepted or rejected. 1276If only legacy (MD5) fingerprints for the server are available, the 1277.Xr ssh-keygen 1 1278.Fl E 1279option may be used to downgrade the fingerprint algorithm to match. 1280.Pp 1281Because of the difficulty of comparing host keys 1282just by looking at fingerprint strings, 1283there is also support to compare host keys visually, 1284using 1285.Em random art . 1286By setting the 1287.Cm VisualHostKey 1288option to 1289.Dq yes , 1290a small ASCII graphic gets displayed on every login to a server, no matter 1291if the session itself is interactive or not. 1292By learning the pattern a known server produces, a user can easily 1293find out that the host key has changed when a completely different pattern 1294is displayed. 1295Because these patterns are not unambiguous however, a pattern that looks 1296similar to the pattern remembered only gives a good probability that the 1297host key is the same, not guaranteed proof. 1298.Pp 1299To get a listing of the fingerprints along with their random art for 1300all known hosts, the following command line can be used: 1301.Pp 1302.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1303.Pp 1304If the fingerprint is unknown, 1305an alternative method of verification is available: 1306SSH fingerprints verified by DNS. 1307An additional resource record (RR), 1308SSHFP, 1309is added to a zonefile 1310and the connecting client is able to match the fingerprint 1311with that of the key presented. 1312.Pp 1313In this example, we are connecting a client to a server, 1314.Dq host.example.com . 1315The SSHFP resource records should first be added to the zonefile for 1316host.example.com: 1317.Bd -literal -offset indent 1318$ ssh-keygen -r host.example.com. 1319.Ed 1320.Pp 1321The output lines will have to be added to the zonefile. 1322To check that the zone is answering fingerprint queries: 1323.Pp 1324.Dl $ dig -t SSHFP host.example.com 1325.Pp 1326Finally the client connects: 1327.Bd -literal -offset indent 1328$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1329[...] 1330Matching host key fingerprint found in DNS. 1331Are you sure you want to continue connecting (yes/no)? 1332.Ed 1333.Pp 1334See the 1335.Cm VerifyHostKeyDNS 1336option in 1337.Xr ssh_config 5 1338for more information. 1339.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1340.Nm 1341contains support for Virtual Private Network (VPN) tunnelling 1342using the 1343.Xr tun 4 1344network pseudo-device, 1345allowing two networks to be joined securely. 1346The 1347.Xr sshd_config 5 1348configuration option 1349.Cm PermitTunnel 1350controls whether the server supports this, 1351and at what level (layer 2 or 3 traffic). 1352.Pp 1353The following example would connect client network 10.0.50.0/24 1354with remote network 10.0.99.0/24 using a point-to-point connection 1355from 10.1.1.1 to 10.1.1.2, 1356provided that the SSH server running on the gateway to the remote network, 1357at 192.168.1.15, allows it. 1358.Pp 1359On the client: 1360.Bd -literal -offset indent 1361# ssh -f -w 0:1 192.168.1.15 true 1362# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1363# route add 10.0.99.0/24 10.1.1.2 1364.Ed 1365.Pp 1366On the server: 1367.Bd -literal -offset indent 1368# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1369# route add 10.0.50.0/24 10.1.1.1 1370.Ed 1371.Pp 1372Client access may be more finely tuned via the 1373.Pa /root/.ssh/authorized_keys 1374file (see below) and the 1375.Cm PermitRootLogin 1376server option. 1377The following entry would permit connections on 1378.Xr tun 4 1379device 1 from user 1380.Dq jane 1381and on tun device 2 from user 1382.Dq john , 1383if 1384.Cm PermitRootLogin 1385is set to 1386.Dq forced-commands-only : 1387.Bd -literal -offset 2n 1388tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1389tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1390.Ed 1391.Pp 1392Since an SSH-based setup entails a fair amount of overhead, 1393it may be more suited to temporary setups, 1394such as for wireless VPNs. 1395More permanent VPNs are better provided by tools such as 1396.Xr ipsecctl 8 1397and 1398.Xr isakmpd 8 . 1399.Sh ENVIRONMENT 1400.Nm 1401will normally set the following environment variables: 1402.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1403.It Ev DISPLAY 1404The 1405.Ev DISPLAY 1406variable indicates the location of the X11 server. 1407It is automatically set by 1408.Nm 1409to point to a value of the form 1410.Dq hostname:n , 1411where 1412.Dq hostname 1413indicates the host where the shell runs, and 1414.Sq n 1415is an integer \*(Ge 1. 1416.Nm 1417uses this special value to forward X11 connections over the secure 1418channel. 1419The user should normally not set 1420.Ev DISPLAY 1421explicitly, as that 1422will render the X11 connection insecure (and will require the user to 1423manually copy any required authorization cookies). 1424.It Ev HOME 1425Set to the path of the user's home directory. 1426.It Ev LOGNAME 1427Synonym for 1428.Ev USER ; 1429set for compatibility with systems that use this variable. 1430.It Ev MAIL 1431Set to the path of the user's mailbox. 1432.It Ev PATH 1433Set to the default 1434.Ev PATH , 1435as specified when compiling 1436.Nm . 1437.It Ev SSH_ASKPASS 1438If 1439.Nm 1440needs a passphrase, it will read the passphrase from the current 1441terminal if it was run from a terminal. 1442If 1443.Nm 1444does not have a terminal associated with it but 1445.Ev DISPLAY 1446and 1447.Ev SSH_ASKPASS 1448are set, it will execute the program specified by 1449.Ev SSH_ASKPASS 1450and open an X11 window to read the passphrase. 1451This is particularly useful when calling 1452.Nm 1453from a 1454.Pa .xsession 1455or related script. 1456(Note that on some machines it 1457may be necessary to redirect the input from 1458.Pa /dev/null 1459to make this work.) 1460.It Ev SSH_ASKPASS_REQUIRE 1461Allows further control over the use of an askpass program. 1462If this variable is set to 1463.Dq never 1464then 1465.Nm 1466will never attempt to use one. 1467If it is set to 1468.Dq prefer , 1469then 1470.Nm 1471will prefer to use the askpass program instead of the TTY when requesting 1472passwords. 1473Finally, if the variable is set to 1474.Dq force , 1475then the askpass program will be used for all passphrase input regardless 1476of whether 1477.Ev DISPLAY 1478is set. 1479.It Ev SSH_AUTH_SOCK 1480Identifies the path of a 1481.Ux Ns -domain 1482socket used to communicate with the agent. 1483.It Ev SSH_CONNECTION 1484Identifies the client and server ends of the connection. 1485The variable contains 1486four space-separated values: client IP address, client port number, 1487server IP address, and server port number. 1488.It Ev SSH_ORIGINAL_COMMAND 1489This variable contains the original command line if a forced command 1490is executed. 1491It can be used to extract the original arguments. 1492.It Ev SSH_TTY 1493This is set to the name of the tty (path to the device) associated 1494with the current shell or command. 1495If the current session has no tty, 1496this variable is not set. 1497.It Ev SSH_TUNNEL 1498Optionally set by 1499.Xr sshd 8 1500to contain the interface names assigned if tunnel forwarding was 1501requested by the client. 1502.It Ev SSH_USER_AUTH 1503Optionally set by 1504.Xr sshd 8 , 1505this variable may contain a pathname to a file that lists the authentication 1506methods successfully used when the session was established, including any 1507public keys that were used. 1508.It Ev TZ 1509This variable is set to indicate the present time zone if it 1510was set when the daemon was started (i.e. the daemon passes the value 1511on to new connections). 1512.It Ev USER 1513Set to the name of the user logging in. 1514.El 1515.Pp 1516Additionally, 1517.Nm 1518reads 1519.Pa ~/.ssh/environment , 1520and adds lines of the format 1521.Dq VARNAME=value 1522to the environment if the file exists and users are allowed to 1523change their environment. 1524For more information, see the 1525.Cm PermitUserEnvironment 1526option in 1527.Xr sshd_config 5 . 1528.Sh FILES 1529.Bl -tag -width Ds -compact 1530.It Pa ~/.rhosts 1531This file is used for host-based authentication (see above). 1532On some machines this file may need to be 1533world-readable if the user's home directory is on an NFS partition, 1534because 1535.Xr sshd 8 1536reads it as root. 1537Additionally, this file must be owned by the user, 1538and must not have write permissions for anyone else. 1539The recommended 1540permission for most machines is read/write for the user, and not 1541accessible by others. 1542.Pp 1543.It Pa ~/.shosts 1544This file is used in exactly the same way as 1545.Pa .rhosts , 1546but allows host-based authentication without permitting login with 1547rlogin/rsh. 1548.Pp 1549.It Pa ~/.ssh/ 1550This directory is the default location for all user-specific configuration 1551and authentication information. 1552There is no general requirement to keep the entire contents of this directory 1553secret, but the recommended permissions are read/write/execute for the user, 1554and not accessible by others. 1555.Pp 1556.It Pa ~/.ssh/authorized_keys 1557Lists the public keys (DSA, ECDSA, Ed25519, RSA) 1558that can be used for logging in as this user. 1559The format of this file is described in the 1560.Xr sshd 8 1561manual page. 1562This file is not highly sensitive, but the recommended 1563permissions are read/write for the user, and not accessible by others. 1564.Pp 1565.It Pa ~/.ssh/config 1566This is the per-user configuration file. 1567The file format and configuration options are described in 1568.Xr ssh_config 5 . 1569Because of the potential for abuse, this file must have strict permissions: 1570read/write for the user, and not writable by others. 1571.Pp 1572.It Pa ~/.ssh/environment 1573Contains additional definitions for environment variables; see 1574.Sx ENVIRONMENT , 1575above. 1576.Pp 1577.It Pa ~/.ssh/id_dsa 1578.It Pa ~/.ssh/id_ecdsa 1579.It Pa ~/.ssh/id_ecdsa_sk 1580.It Pa ~/.ssh/id_ed25519 1581.It Pa ~/.ssh/id_ed25519_sk 1582.It Pa ~/.ssh/id_rsa 1583Contains the private key for authentication. 1584These files 1585contain sensitive data and should be readable by the user but not 1586accessible by others (read/write/execute). 1587.Nm 1588will simply ignore a private key file if it is accessible by others. 1589It is possible to specify a passphrase when 1590generating the key which will be used to encrypt the 1591sensitive part of this file using AES-128. 1592.Pp 1593.It Pa ~/.ssh/id_dsa.pub 1594.It Pa ~/.ssh/id_ecdsa.pub 1595.It Pa ~/.ssh/id_ecdsa_sk.pub 1596.It Pa ~/.ssh/id_ed25519.pub 1597.It Pa ~/.ssh/id_ed25519_sk.pub 1598.It Pa ~/.ssh/id_rsa.pub 1599Contains the public key for authentication. 1600These files are not 1601sensitive and can (but need not) be readable by anyone. 1602.Pp 1603.It Pa ~/.ssh/known_hosts 1604Contains a list of host keys for all hosts the user has logged into 1605that are not already in the systemwide list of known host keys. 1606See 1607.Xr sshd 8 1608for further details of the format of this file. 1609.Pp 1610.It Pa ~/.ssh/rc 1611Commands in this file are executed by 1612.Nm 1613when the user logs in, just before the user's shell (or command) is 1614started. 1615See the 1616.Xr sshd 8 1617manual page for more information. 1618.Pp 1619.It Pa /etc/hosts.equiv 1620This file is for host-based authentication (see above). 1621It should only be writable by root. 1622.Pp 1623.It Pa /etc/shosts.equiv 1624This file is used in exactly the same way as 1625.Pa hosts.equiv , 1626but allows host-based authentication without permitting login with 1627rlogin/rsh. 1628.Pp 1629.It Pa /etc/ssh/ssh_config 1630Systemwide configuration file. 1631The file format and configuration options are described in 1632.Xr ssh_config 5 . 1633.Pp 1634.It Pa /etc/ssh/ssh_host_key 1635.It Pa /etc/ssh/ssh_host_dsa_key 1636.It Pa /etc/ssh/ssh_host_ecdsa_key 1637.It Pa /etc/ssh/ssh_host_ed25519_key 1638.It Pa /etc/ssh/ssh_host_rsa_key 1639These files contain the private parts of the host keys 1640and are used for host-based authentication. 1641.Pp 1642.It Pa /etc/ssh/ssh_known_hosts 1643Systemwide list of known host keys. 1644This file should be prepared by the 1645system administrator to contain the public host keys of all machines in the 1646organization. 1647It should be world-readable. 1648See 1649.Xr sshd 8 1650for further details of the format of this file. 1651.Pp 1652.It Pa /etc/ssh/sshrc 1653Commands in this file are executed by 1654.Nm 1655when the user logs in, just before the user's shell (or command) is started. 1656See the 1657.Xr sshd 8 1658manual page for more information. 1659.El 1660.Sh EXIT STATUS 1661.Nm 1662exits with the exit status of the remote command or with 255 1663if an error occurred. 1664.Sh SEE ALSO 1665.Xr scp 1 , 1666.Xr sftp 1 , 1667.Xr ssh-add 1 , 1668.Xr ssh-agent 1 , 1669.Xr ssh-keygen 1 , 1670.Xr ssh-keyscan 1 , 1671.Xr tun 4 , 1672.Xr ssh_config 5 , 1673.Xr ssh-keysign 8 , 1674.Xr sshd 8 1675.Sh STANDARDS 1676.Rs 1677.%A S. Lehtinen 1678.%A C. Lonvick 1679.%D January 2006 1680.%R RFC 4250 1681.%T The Secure Shell (SSH) Protocol Assigned Numbers 1682.Re 1683.Pp 1684.Rs 1685.%A T. Ylonen 1686.%A C. Lonvick 1687.%D January 2006 1688.%R RFC 4251 1689.%T The Secure Shell (SSH) Protocol Architecture 1690.Re 1691.Pp 1692.Rs 1693.%A T. Ylonen 1694.%A C. Lonvick 1695.%D January 2006 1696.%R RFC 4252 1697.%T The Secure Shell (SSH) Authentication Protocol 1698.Re 1699.Pp 1700.Rs 1701.%A T. Ylonen 1702.%A C. Lonvick 1703.%D January 2006 1704.%R RFC 4253 1705.%T The Secure Shell (SSH) Transport Layer Protocol 1706.Re 1707.Pp 1708.Rs 1709.%A T. Ylonen 1710.%A C. Lonvick 1711.%D January 2006 1712.%R RFC 4254 1713.%T The Secure Shell (SSH) Connection Protocol 1714.Re 1715.Pp 1716.Rs 1717.%A J. Schlyter 1718.%A W. Griffin 1719.%D January 2006 1720.%R RFC 4255 1721.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints 1722.Re 1723.Pp 1724.Rs 1725.%A F. Cusack 1726.%A M. Forssen 1727.%D January 2006 1728.%R RFC 4256 1729.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH) 1730.Re 1731.Pp 1732.Rs 1733.%A J. Galbraith 1734.%A P. Remaker 1735.%D January 2006 1736.%R RFC 4335 1737.%T The Secure Shell (SSH) Session Channel Break Extension 1738.Re 1739.Pp 1740.Rs 1741.%A M. Bellare 1742.%A T. Kohno 1743.%A C. Namprempre 1744.%D January 2006 1745.%R RFC 4344 1746.%T The Secure Shell (SSH) Transport Layer Encryption Modes 1747.Re 1748.Pp 1749.Rs 1750.%A B. Harris 1751.%D January 2006 1752.%R RFC 4345 1753.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol 1754.Re 1755.Pp 1756.Rs 1757.%A M. Friedl 1758.%A N. Provos 1759.%A W. Simpson 1760.%D March 2006 1761.%R RFC 4419 1762.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 1763.Re 1764.Pp 1765.Rs 1766.%A J. Galbraith 1767.%A R. Thayer 1768.%D November 2006 1769.%R RFC 4716 1770.%T The Secure Shell (SSH) Public Key File Format 1771.Re 1772.Pp 1773.Rs 1774.%A D. Stebila 1775.%A J. Green 1776.%D December 2009 1777.%R RFC 5656 1778.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer 1779.Re 1780.Pp 1781.Rs 1782.%A A. Perrig 1783.%A D. Song 1784.%D 1999 1785.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99) 1786.%T Hash Visualization: a New Technique to improve Real-World Security 1787.Re 1788.Sh AUTHORS 1789OpenSSH is a derivative of the original and free 1790ssh 1.2.12 release by Tatu Ylonen. 1791Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1792Theo de Raadt and Dug Song 1793removed many bugs, re-added newer features and 1794created OpenSSH. 1795Markus Friedl contributed the support for SSH 1796protocol versions 1.5 and 2.0. 1797