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 1.426 2021/09/03 07:27:03 dtucker Exp $ 37.Dd $Mdocdate: September 3 2021 $ 38.Dt SSH 1 39.Os 40.Sh NAME 41.Nm ssh 42.Nd OpenSSH remote login client 43.Sh SYNOPSIS 44.Nm ssh 45.Op Fl 46AaCfGgKkMNnqsTtVvXxYy 46.Op Fl B Ar bind_interface 47.Op Fl b Ar bind_address 48.Op Fl c Ar cipher_spec 49.Op Fl D Oo Ar bind_address : Oc Ns Ar port 50.Op Fl E Ar log_file 51.Op Fl e Ar escape_char 52.Op Fl F Ar configfile 53.Op Fl I Ar pkcs11 54.Op Fl i Ar identity_file 55.Op Fl J Ar destination 56.Op Fl L Ar address 57.Op Fl l Ar login_name 58.Op Fl m Ar mac_spec 59.Op Fl O Ar ctl_cmd 60.Op Fl o Ar option 61.Op Fl p Ar port 62.Op Fl Q Ar query_option 63.Op Fl R Ar address 64.Op Fl S Ar ctl_path 65.Op Fl W Ar host : Ns Ar port 66.Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun 67.Ar destination 68.Op Ar command 69.Sh DESCRIPTION 70.Nm 71(SSH client) is a program for logging into a remote machine and for 72executing commands on a remote machine. 73It is intended to provide secure encrypted communications between 74two untrusted hosts over an insecure network. 75X11 connections, arbitrary TCP ports and 76.Ux Ns -domain 77sockets can also be forwarded over the secure channel. 78.Pp 79.Nm 80connects and logs into the specified 81.Ar destination , 82which may be specified as either 83.Sm off 84.Oo user @ Oc hostname 85.Sm on 86or a URI of the form 87.Sm off 88.No ssh:// Oo user @ Oc hostname Op : port . 89.Sm on 90The user must prove 91their identity to the remote machine using one of several methods 92(see below). 93.Pp 94If a 95.Ar command 96is specified, 97it is executed on the remote host instead of a login shell. 98.Pp 99The options are as follows: 100.Pp 101.Bl -tag -width Ds -compact 102.It Fl 4 103Forces 104.Nm 105to use IPv4 addresses only. 106.Pp 107.It Fl 6 108Forces 109.Nm 110to use IPv6 addresses only. 111.Pp 112.It Fl A 113Enables forwarding of connections from an authentication agent such as 114.Xr ssh-agent 1 . 115This can also be specified on a per-host basis in a configuration file. 116.Pp 117Agent forwarding should be enabled with caution. 118Users with the ability to bypass file permissions on the remote host 119(for the agent's 120.Ux Ns -domain 121socket) can access the local agent through the forwarded connection. 122An attacker cannot obtain key material from the agent, 123however they can perform operations on the keys that enable them to 124authenticate using the identities loaded into the agent. 125A safer alternative may be to use a jump host 126(see 127.Fl J ) . 128.Pp 129.It Fl a 130Disables forwarding of the authentication agent connection. 131.Pp 132.It Fl B Ar bind_interface 133Bind to the address of 134.Ar bind_interface 135before attempting to connect to the destination host. 136This is only useful on systems with more than one address. 137.Pp 138.It Fl b Ar bind_address 139Use 140.Ar bind_address 141on the local machine as the source address 142of the connection. 143Only useful on systems with more than one address. 144.Pp 145.It Fl C 146Requests compression of all data (including stdin, stdout, stderr, and 147data for forwarded X11, TCP and 148.Ux Ns -domain 149connections). 150The compression algorithm is the same used by 151.Xr gzip 1 . 152Compression is desirable on modem lines and other 153slow connections, but will only slow down things on fast networks. 154The default value can be set on a host-by-host basis in the 155configuration files; see the 156.Cm Compression 157option. 158.Pp 159.It Fl c Ar cipher_spec 160Selects the cipher specification for encrypting the session. 161.Ar cipher_spec 162is a comma-separated list of ciphers 163listed in order of preference. 164See the 165.Cm Ciphers 166keyword in 167.Xr ssh_config 5 168for more information. 169.Pp 170.It Fl D Xo 171.Sm off 172.Oo Ar bind_address : Oc 173.Ar port 174.Sm on 175.Xc 176Specifies a local 177.Dq dynamic 178application-level port forwarding. 179This works by allocating a socket to listen to 180.Ar port 181on the local side, optionally bound to the specified 182.Ar bind_address . 183Whenever a connection is made to this port, the 184connection is forwarded over the secure channel, and the application 185protocol is then used to determine where to connect to from the 186remote machine. 187Currently the SOCKS4 and SOCKS5 protocols are supported, and 188.Nm 189will act as a SOCKS server. 190Only root can forward privileged ports. 191Dynamic port forwardings can also be specified in the configuration file. 192.Pp 193IPv6 addresses can be specified by enclosing the address in square brackets. 194Only the superuser can forward privileged ports. 195By default, the local port is bound in accordance with the 196.Cm GatewayPorts 197setting. 198However, an explicit 199.Ar bind_address 200may be used to bind the connection to a specific address. 201The 202.Ar bind_address 203of 204.Dq localhost 205indicates that the listening port be bound for local use only, while an 206empty address or 207.Sq * 208indicates that the port should be available from all interfaces. 209.Pp 210.It Fl E Ar log_file 211Append debug logs to 212.Ar log_file 213instead of standard error. 214.Pp 215.It Fl e Ar escape_char 216Sets the escape character for sessions with a pty (default: 217.Ql ~ ) . 218The escape character is only recognized at the beginning of a line. 219The escape character followed by a dot 220.Pq Ql \&. 221closes the connection; 222followed by control-Z suspends the connection; 223and followed by itself sends the escape character once. 224Setting the character to 225.Dq none 226disables any escapes and makes the session fully transparent. 227.Pp 228.It Fl F Ar configfile 229Specifies an alternative per-user configuration file. 230If a configuration file is given on the command line, 231the system-wide configuration file 232.Pq Pa /etc/ssh/ssh_config 233will be ignored. 234The default for the per-user configuration file is 235.Pa ~/.ssh/config . 236If set to 237.Dq none , 238no configuration files will be read. 239.Pp 240.It Fl f 241Requests 242.Nm 243to go to background just before command execution. 244This is useful if 245.Nm 246is going to ask for passwords or passphrases, but the user 247wants it in the background. 248This implies 249.Fl n . 250The recommended way to start X11 programs at a remote site is with 251something like 252.Ic ssh -f host xterm . 253.Pp 254If the 255.Cm ExitOnForwardFailure 256configuration option is set to 257.Dq yes , 258then a client started with 259.Fl f 260will wait for all remote port forwards to be successfully established 261before placing itself in the background. 262Refer to the description of 263.Cm ForkAfterAuthentication 264in 265.Xr ssh_config 5 266for details. 267.Pp 268.It Fl G 269Causes 270.Nm 271to print its configuration after evaluating 272.Cm Host 273and 274.Cm Match 275blocks and exit. 276.Pp 277.It Fl g 278Allows remote hosts to connect to local forwarded ports. 279If used on a multiplexed connection, then this option must be specified 280on the master process. 281.Pp 282.It Fl I Ar pkcs11 283Specify the PKCS#11 shared library 284.Nm 285should use to communicate with a PKCS#11 token providing keys for user 286authentication. 287.Pp 288.It Fl i Ar identity_file 289Selects a file from which the identity (private key) for 290public key authentication is read. 291You can also specify a public key file to use the corresponding 292private key that is loaded in 293.Xr ssh-agent 1 294when the private key file is not present locally. 295The default is 296.Pa ~/.ssh/id_dsa , 297.Pa ~/.ssh/id_ecdsa , 298.Pa ~/.ssh/id_ecdsa_sk , 299.Pa ~/.ssh/id_ed25519 , 300.Pa ~/.ssh/id_ed25519_sk 301and 302.Pa ~/.ssh/id_rsa . 303Identity files may also be specified on 304a per-host basis in the configuration file. 305It is possible to have multiple 306.Fl i 307options (and multiple identities specified in 308configuration files). 309If no certificates have been explicitly specified by the 310.Cm CertificateFile 311directive, 312.Nm 313will also try to load certificate information from the filename obtained 314by appending 315.Pa -cert.pub 316to identity filenames. 317.Pp 318.It Fl J Ar destination 319Connect to the target host by first making a 320.Nm 321connection to the jump host described by 322.Ar destination 323and then establishing a TCP forwarding to the ultimate destination from 324there. 325Multiple jump hops may be specified separated by comma characters. 326This is a shortcut to specify a 327.Cm ProxyJump 328configuration directive. 329Note that configuration directives supplied on the command-line generally 330apply to the destination host and not any specified jump hosts. 331Use 332.Pa ~/.ssh/config 333to specify configuration for jump hosts. 334.Pp 335.It Fl K 336Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI 337credentials to the server. 338.Pp 339.It Fl k 340Disables forwarding (delegation) of GSSAPI credentials to the server. 341.Pp 342.It Fl L Xo 343.Sm off 344.Oo Ar bind_address : Oc 345.Ar port : host : hostport 346.Sm on 347.Xc 348.It Fl L Xo 349.Sm off 350.Oo Ar bind_address : Oc 351.Ar port : remote_socket 352.Sm on 353.Xc 354.It Fl L Xo 355.Sm off 356.Ar local_socket : host : hostport 357.Sm on 358.Xc 359.It Fl L Xo 360.Sm off 361.Ar local_socket : remote_socket 362.Sm on 363.Xc 364Specifies that connections to the given TCP port or Unix socket on the local 365(client) host are to be forwarded to the given host and port, or Unix socket, 366on the remote side. 367This works by allocating a socket to listen to either a TCP 368.Ar port 369on the local side, optionally bound to the specified 370.Ar bind_address , 371or to a Unix socket. 372Whenever a connection is made to the local port or socket, the 373connection is forwarded over the secure channel, and a connection is 374made to either 375.Ar host 376port 377.Ar hostport , 378or the Unix socket 379.Ar remote_socket , 380from the remote machine. 381.Pp 382Port forwardings can also be specified in the configuration file. 383Only the superuser can forward privileged ports. 384IPv6 addresses can be specified by enclosing the address in square brackets. 385.Pp 386By default, the local port is bound in accordance with the 387.Cm GatewayPorts 388setting. 389However, an explicit 390.Ar bind_address 391may be used to bind the connection to a specific address. 392The 393.Ar bind_address 394of 395.Dq localhost 396indicates that the listening port be bound for local use only, while an 397empty address or 398.Sq * 399indicates that the port should be available from all interfaces. 400.Pp 401.It Fl l Ar login_name 402Specifies the user to log in as on the remote machine. 403This also may be specified on a per-host basis in the configuration file. 404.Pp 405.It Fl M 406Places the 407.Nm 408client into 409.Dq master 410mode for connection sharing. 411Multiple 412.Fl M 413options places 414.Nm 415into 416.Dq master 417mode but with confirmation required using 418.Xr ssh-askpass 1 419before each operation that changes the multiplexing state 420(e.g. opening a new session). 421Refer to the description of 422.Cm ControlMaster 423in 424.Xr ssh_config 5 425for details. 426.Pp 427.It Fl m Ar mac_spec 428A comma-separated list of MAC (message authentication code) algorithms, 429specified in order of preference. 430See the 431.Cm MACs 432keyword for more information. 433.Pp 434.It Fl N 435Do not execute a remote command. 436This is useful for just forwarding ports. 437Refer to the description of 438.Cm SessionType 439in 440.Xr ssh_config 5 441for details. 442.Pp 443.It Fl n 444Redirects stdin from 445.Pa /dev/null 446(actually, prevents reading from stdin). 447This must be used when 448.Nm 449is run in the background. 450A common trick is to use this to run X11 programs on a remote machine. 451For example, 452.Ic ssh -n shadows.cs.hut.fi emacs & 453will start an emacs on shadows.cs.hut.fi, and the X11 454connection will be automatically forwarded over an encrypted channel. 455The 456.Nm 457program will be put in the background. 458(This does not work if 459.Nm 460needs to ask for a password or passphrase; see also the 461.Fl f 462option.) 463Refer to the description of 464.Cm StdinNull 465in 466.Xr ssh_config 5 467for details. 468.Pp 469.It Fl O Ar ctl_cmd 470Control an active connection multiplexing master process. 471When the 472.Fl O 473option is specified, the 474.Ar ctl_cmd 475argument is interpreted and passed to the master process. 476Valid commands are: 477.Dq check 478(check that the master process is running), 479.Dq forward 480(request forwardings without command execution), 481.Dq cancel 482(cancel forwardings), 483.Dq exit 484(request the master to exit), and 485.Dq stop 486(request the master to stop accepting further multiplexing requests). 487.Pp 488.It Fl o Ar option 489Can be used to give options in the format used in the configuration file. 490This is useful for specifying options for which there is no separate 491command-line flag. 492For full details of the options listed below, and their possible values, see 493.Xr ssh_config 5 . 494.Pp 495.Bl -tag -width Ds -offset indent -compact 496.It AddKeysToAgent 497.It AddressFamily 498.It BatchMode 499.It BindAddress 500.It CanonicalDomains 501.It CanonicalizeFallbackLocal 502.It CanonicalizeHostname 503.It CanonicalizeMaxDots 504.It CanonicalizePermittedCNAMEs 505.It CASignatureAlgorithms 506.It CertificateFile 507.It CheckHostIP 508.It Ciphers 509.It ClearAllForwardings 510.It Compression 511.It ConnectionAttempts 512.It ConnectTimeout 513.It ControlMaster 514.It ControlPath 515.It ControlPersist 516.It DynamicForward 517.It EscapeChar 518.It ExitOnForwardFailure 519.It FingerprintHash 520.It ForkAfterAuthentication 521.It ForwardAgent 522.It ForwardX11 523.It ForwardX11Timeout 524.It ForwardX11Trusted 525.It GatewayPorts 526.It GlobalKnownHostsFile 527.It GSSAPIAuthentication 528.It GSSAPIDelegateCredentials 529.It HashKnownHosts 530.It Host 531.It HostbasedAcceptedAlgorithms 532.It HostbasedAuthentication 533.It HostKeyAlgorithms 534.It HostKeyAlias 535.It Hostname 536.It IdentitiesOnly 537.It IdentityAgent 538.It IdentityFile 539.It IPQoS 540.It KbdInteractiveAuthentication 541.It KbdInteractiveDevices 542.It KexAlgorithms 543.It KnownHostsCommand 544.It LocalCommand 545.It LocalForward 546.It LogLevel 547.It MACs 548.It Match 549.It NoHostAuthenticationForLocalhost 550.It NumberOfPasswordPrompts 551.It PasswordAuthentication 552.It PermitLocalCommand 553.It PermitRemoteOpen 554.It PKCS11Provider 555.It Port 556.It PreferredAuthentications 557.It ProxyCommand 558.It ProxyJump 559.It ProxyUseFdpass 560.It PubkeyAcceptedAlgorithms 561.It PubkeyAuthentication 562.It RekeyLimit 563.It RemoteCommand 564.It RemoteForward 565.It RequestTTY 566.It SendEnv 567.It ServerAliveInterval 568.It ServerAliveCountMax 569.It SessionType 570.It SetEnv 571.It StdinNull 572.It StreamLocalBindMask 573.It StreamLocalBindUnlink 574.It StrictHostKeyChecking 575.It TCPKeepAlive 576.It Tunnel 577.It TunnelDevice 578.It UpdateHostKeys 579.It User 580.It UserKnownHostsFile 581.It VerifyHostKeyDNS 582.It VisualHostKey 583.It XAuthLocation 584.El 585.Pp 586.It Fl p Ar port 587Port to connect to on the remote host. 588This can be specified on a 589per-host basis in the configuration file. 590.Pp 591.It Fl Q Ar query_option 592Queries for the algorithms supported by one of the following features: 593.Ar cipher 594(supported symmetric ciphers), 595.Ar cipher-auth 596(supported symmetric ciphers that support authenticated encryption), 597.Ar help 598(supported query terms for use with the 599.Fl Q 600flag), 601.Ar mac 602(supported message integrity codes), 603.Ar kex 604(key exchange algorithms), 605.Ar key 606(key types), 607.Ar key-cert 608(certificate key types), 609.Ar key-plain 610(non-certificate key types), 611.Ar key-sig 612(all key types and signature algorithms), 613.Ar protocol-version 614(supported SSH protocol versions), and 615.Ar sig 616(supported signature algorithms). 617Alternatively, any keyword from 618.Xr ssh_config 5 619or 620.Xr sshd_config 5 621that takes an algorithm list may be used as an alias for the corresponding 622query_option. 623.Pp 624.It Fl q 625Quiet mode. 626Causes most warning and diagnostic messages to be suppressed. 627.Pp 628.It Fl R Xo 629.Sm off 630.Oo Ar bind_address : Oc 631.Ar port : host : hostport 632.Sm on 633.Xc 634.It Fl R Xo 635.Sm off 636.Oo Ar bind_address : Oc 637.Ar port : local_socket 638.Sm on 639.Xc 640.It Fl R Xo 641.Sm off 642.Ar remote_socket : host : hostport 643.Sm on 644.Xc 645.It Fl R Xo 646.Sm off 647.Ar remote_socket : local_socket 648.Sm on 649.Xc 650.It Fl R Xo 651.Sm off 652.Oo Ar bind_address : Oc 653.Ar port 654.Sm on 655.Xc 656Specifies that connections to the given TCP port or Unix socket on the remote 657(server) host are to be forwarded to the local side. 658.Pp 659This works by allocating a socket to listen to either a TCP 660.Ar port 661or to a Unix socket on the remote side. 662Whenever a connection is made to this port or Unix socket, the 663connection is forwarded over the secure channel, and a connection 664is made from the local machine to either an explicit destination specified by 665.Ar host 666port 667.Ar hostport , 668or 669.Ar local_socket , 670or, if no explicit destination was specified, 671.Nm 672will act as a SOCKS 4/5 proxy and forward connections to the destinations 673requested by the remote SOCKS client. 674.Pp 675Port forwardings can also be specified in the configuration file. 676Privileged ports can be forwarded only when 677logging in as root on the remote machine. 678IPv6 addresses can be specified by enclosing the address in square brackets. 679.Pp 680By default, TCP listening sockets on the server will be bound to the loopback 681interface only. 682This may be overridden by specifying a 683.Ar bind_address . 684An empty 685.Ar bind_address , 686or the address 687.Ql * , 688indicates that the remote socket should listen on all interfaces. 689Specifying a remote 690.Ar bind_address 691will only succeed if the server's 692.Cm GatewayPorts 693option is enabled (see 694.Xr sshd_config 5 ) . 695.Pp 696If the 697.Ar port 698argument is 699.Ql 0 , 700the listen port will be dynamically allocated on the server and reported 701to the client at run time. 702When used together with 703.Ic -O forward 704the allocated port will be printed to the standard output. 705.Pp 706.It Fl S Ar ctl_path 707Specifies the location of a control socket for connection sharing, 708or the string 709.Dq none 710to disable connection sharing. 711Refer to the description of 712.Cm ControlPath 713and 714.Cm ControlMaster 715in 716.Xr ssh_config 5 717for details. 718.Pp 719.It Fl s 720May be used to request invocation of a subsystem on the remote system. 721Subsystems facilitate the use of SSH 722as a secure transport for other applications (e.g.\& 723.Xr sftp 1 ) . 724The subsystem is specified as the remote command. 725Refer to the description of 726.Cm SessionType 727in 728.Xr ssh_config 5 729for details. 730.Pp 731.It Fl T 732Disable pseudo-terminal allocation. 733.Pp 734.It Fl t 735Force pseudo-terminal allocation. 736This can be used to execute arbitrary 737screen-based programs on a remote machine, which can be very useful, 738e.g. when implementing menu services. 739Multiple 740.Fl t 741options force tty allocation, even if 742.Nm 743has no local tty. 744.Pp 745.It Fl V 746Display the version number and exit. 747.Pp 748.It Fl v 749Verbose mode. 750Causes 751.Nm 752to print debugging messages about its progress. 753This is helpful in 754debugging connection, authentication, and configuration problems. 755Multiple 756.Fl v 757options increase the verbosity. 758The maximum is 3. 759.Pp 760.It Fl W Ar host : Ns Ar port 761Requests that standard input and output on the client be forwarded to 762.Ar host 763on 764.Ar port 765over the secure channel. 766Implies 767.Fl N , 768.Fl T , 769.Cm ExitOnForwardFailure 770and 771.Cm ClearAllForwardings , 772though these can be overridden in the configuration file or using 773.Fl o 774command line options. 775.Pp 776.It Fl w Xo 777.Ar local_tun Ns Op : Ns Ar remote_tun 778.Xc 779Requests 780tunnel 781device forwarding with the specified 782.Xr tun 4 783devices between the client 784.Pq Ar local_tun 785and the server 786.Pq Ar remote_tun . 787.Pp 788The devices may be specified by numerical ID or the keyword 789.Dq any , 790which uses the next available tunnel device. 791If 792.Ar remote_tun 793is not specified, it defaults to 794.Dq any . 795See also the 796.Cm Tunnel 797and 798.Cm TunnelDevice 799directives in 800.Xr ssh_config 5 . 801.Pp 802If the 803.Cm Tunnel 804directive is unset, it will be set to the default tunnel mode, which is 805.Dq point-to-point . 806If a different 807.Cm Tunnel 808forwarding mode it desired, then it should be specified before 809.Fl w . 810.Pp 811.It Fl X 812Enables X11 forwarding. 813This can also be specified on a per-host basis in a configuration file. 814.Pp 815X11 forwarding should be enabled with caution. 816Users with the ability to bypass file permissions on the remote host 817(for the user's X authorization database) 818can access the local X11 display through the forwarded connection. 819An attacker may then be able to perform activities such as keystroke monitoring. 820.Pp 821For this reason, X11 forwarding is subjected to X11 SECURITY extension 822restrictions by default. 823Please refer to the 824.Nm 825.Fl Y 826option and the 827.Cm ForwardX11Trusted 828directive in 829.Xr ssh_config 5 830for more information. 831.Pp 832.It Fl x 833Disables X11 forwarding. 834.Pp 835.It Fl Y 836Enables trusted X11 forwarding. 837Trusted X11 forwardings are not subjected to the X11 SECURITY extension 838controls. 839.Pp 840.It Fl y 841Send log information using the 842.Xr syslog 3 843system module. 844By default this information is sent to stderr. 845.El 846.Pp 847.Nm 848may additionally obtain configuration data from 849a per-user configuration file and a system-wide configuration file. 850The file format and configuration options are described in 851.Xr ssh_config 5 . 852.Sh AUTHENTICATION 853The OpenSSH SSH client supports SSH protocol 2. 854.Pp 855The methods available for authentication are: 856GSSAPI-based authentication, 857host-based authentication, 858public key authentication, 859keyboard-interactive authentication, 860and password authentication. 861Authentication methods are tried in the order specified above, 862though 863.Cm PreferredAuthentications 864can be used to change the default order. 865.Pp 866Host-based authentication works as follows: 867If the machine the user logs in from is listed in 868.Pa /etc/hosts.equiv 869or 870.Pa /etc/shosts.equiv 871on the remote machine, the user is non-root and the user names are 872the same on both sides, or if the files 873.Pa ~/.rhosts 874or 875.Pa ~/.shosts 876exist in the user's home directory on the 877remote machine and contain a line containing the name of the client 878machine and the name of the user on that machine, the user is 879considered for login. 880Additionally, the server 881.Em must 882be able to verify the client's 883host key (see the description of 884.Pa /etc/ssh/ssh_known_hosts 885and 886.Pa ~/.ssh/known_hosts , 887below) 888for login to be permitted. 889This authentication method closes security holes due to IP 890spoofing, DNS spoofing, and routing spoofing. 891[Note to the administrator: 892.Pa /etc/hosts.equiv , 893.Pa ~/.rhosts , 894and the rlogin/rsh protocol in general, are inherently insecure and should be 895disabled if security is desired.] 896.Pp 897Public key authentication works as follows: 898The scheme is based on public-key cryptography, 899using cryptosystems 900where encryption and decryption are done using separate keys, 901and it is unfeasible to derive the decryption key from the encryption key. 902The idea is that each user creates a public/private 903key pair for authentication purposes. 904The server knows the public key, and only the user knows the private key. 905.Nm 906implements public key authentication protocol automatically, 907using one of the DSA, ECDSA, Ed25519 or RSA algorithms. 908The HISTORY section of 909.Xr ssl 8 910contains a brief discussion of the DSA and RSA algorithms. 911.Pp 912The file 913.Pa ~/.ssh/authorized_keys 914lists the public keys that are permitted for logging in. 915When the user logs in, the 916.Nm 917program tells the server which key pair it would like to use for 918authentication. 919The client proves that it has access to the private key 920and the server checks that the corresponding public key 921is authorized to accept the account. 922.Pp 923The server may inform the client of errors that prevented public key 924authentication from succeeding after authentication completes using a 925different method. 926These may be viewed by increasing the 927.Cm LogLevel 928to 929.Cm DEBUG 930or higher (e.g. by using the 931.Fl v 932flag). 933.Pp 934The user creates their key pair by running 935.Xr ssh-keygen 1 . 936This stores the private key in 937.Pa ~/.ssh/id_dsa 938(DSA), 939.Pa ~/.ssh/id_ecdsa 940(ECDSA), 941.Pa ~/.ssh/id_ecdsa_sk 942(authenticator-hosted ECDSA), 943.Pa ~/.ssh/id_ed25519 944(Ed25519), 945.Pa ~/.ssh/id_ed25519_sk 946(authenticator-hosted Ed25519), 947or 948.Pa ~/.ssh/id_rsa 949(RSA) 950and stores the public key in 951.Pa ~/.ssh/id_dsa.pub 952(DSA), 953.Pa ~/.ssh/id_ecdsa.pub 954(ECDSA), 955.Pa ~/.ssh/id_ecdsa_sk.pub 956(authenticator-hosted ECDSA), 957.Pa ~/.ssh/id_ed25519.pub 958(Ed25519), 959.Pa ~/.ssh/id_ed25519_sk.pub 960(authenticator-hosted Ed25519), 961or 962.Pa ~/.ssh/id_rsa.pub 963(RSA) 964in the user's home directory. 965The user should then copy the public key 966to 967.Pa ~/.ssh/authorized_keys 968in their home directory on the remote machine. 969The 970.Pa authorized_keys 971file corresponds to the conventional 972.Pa ~/.rhosts 973file, and has one key 974per line, though the lines can be very long. 975After this, the user can log in without giving the password. 976.Pp 977A variation on public key authentication 978is available in the form of certificate authentication: 979instead of a set of public/private keys, 980signed certificates are used. 981This has the advantage that a single trusted certification authority 982can be used in place of many public/private keys. 983See the CERTIFICATES section of 984.Xr ssh-keygen 1 985for more information. 986.Pp 987The most convenient way to use public key or certificate authentication 988may be with an authentication agent. 989See 990.Xr ssh-agent 1 991and (optionally) the 992.Cm AddKeysToAgent 993directive in 994.Xr ssh_config 5 995for more information. 996.Pp 997Keyboard-interactive authentication works as follows: 998The server sends an arbitrary 999.Qq challenge 1000text and prompts for a response, possibly multiple times. 1001Examples of keyboard-interactive authentication include 1002.Bx 1003Authentication (see 1004.Xr login.conf 5 ) 1005and PAM (some 1006.Pf non- Ox 1007systems). 1008.Pp 1009Finally, if other authentication methods fail, 1010.Nm 1011prompts the user for a password. 1012The password is sent to the remote 1013host for checking; however, since all communications are encrypted, 1014the password cannot be seen by someone listening on the network. 1015.Pp 1016.Nm 1017automatically maintains and checks a database containing 1018identification for all hosts it has ever been used with. 1019Host keys are stored in 1020.Pa ~/.ssh/known_hosts 1021in the user's home directory. 1022Additionally, the file 1023.Pa /etc/ssh/ssh_known_hosts 1024is automatically checked for known hosts. 1025Any new hosts are automatically added to the user's file. 1026If a host's identification ever changes, 1027.Nm 1028warns about this and disables password authentication to prevent 1029server spoofing or man-in-the-middle attacks, 1030which could otherwise be used to circumvent the encryption. 1031The 1032.Cm StrictHostKeyChecking 1033option can be used to control logins to machines whose 1034host key is not known or has changed. 1035.Pp 1036When the user's identity has been accepted by the server, the server 1037either executes the given command in a non-interactive session or, 1038if no command has been specified, logs into the machine and gives 1039the user a normal shell as an interactive session. 1040All communication with 1041the remote command or shell will be automatically encrypted. 1042.Pp 1043If an interactive session is requested 1044.Nm 1045by default will only request a pseudo-terminal (pty) for interactive 1046sessions when the client has one. 1047The flags 1048.Fl T 1049and 1050.Fl t 1051can be used to override this behaviour. 1052.Pp 1053If a pseudo-terminal has been allocated the 1054user may use the escape characters noted below. 1055.Pp 1056If no pseudo-terminal has been allocated, 1057the session is transparent and can be used to reliably transfer binary data. 1058On most systems, setting the escape character to 1059.Dq none 1060will also make the session transparent even if a tty is used. 1061.Pp 1062The session terminates when the command or shell on the remote 1063machine exits and all X11 and TCP connections have been closed. 1064.Sh ESCAPE CHARACTERS 1065When a pseudo-terminal has been requested, 1066.Nm 1067supports a number of functions through the use of an escape character. 1068.Pp 1069A single tilde character can be sent as 1070.Ic ~~ 1071or by following the tilde by a character other than those described below. 1072The escape character must always follow a newline to be interpreted as 1073special. 1074The escape character can be changed in configuration files using the 1075.Cm EscapeChar 1076configuration directive or on the command line by the 1077.Fl e 1078option. 1079.Pp 1080The supported escapes (assuming the default 1081.Ql ~ ) 1082are: 1083.Bl -tag -width Ds 1084.It Cm ~. 1085Disconnect. 1086.It Cm ~^Z 1087Background 1088.Nm . 1089.It Cm ~# 1090List forwarded connections. 1091.It Cm ~& 1092Background 1093.Nm 1094at logout when waiting for forwarded connection / X11 sessions to terminate. 1095.It Cm ~? 1096Display a list of escape characters. 1097.It Cm ~B 1098Send a BREAK to the remote system 1099(only useful if the peer supports it). 1100.It Cm ~C 1101Open command line. 1102Currently this allows the addition of port forwardings using the 1103.Fl L , 1104.Fl R 1105and 1106.Fl D 1107options (see above). 1108It also allows the cancellation of existing port-forwardings 1109with 1110.Sm off 1111.Fl KL Oo Ar bind_address : Oc Ar port 1112.Sm on 1113for local, 1114.Sm off 1115.Fl KR Oo Ar bind_address : Oc Ar port 1116.Sm on 1117for remote and 1118.Sm off 1119.Fl KD Oo Ar bind_address : Oc Ar port 1120.Sm on 1121for dynamic port-forwardings. 1122.Ic !\& Ns Ar command 1123allows the user to execute a local command if the 1124.Ic PermitLocalCommand 1125option is enabled in 1126.Xr ssh_config 5 . 1127Basic help is available, using the 1128.Fl h 1129option. 1130.It Cm ~R 1131Request rekeying of the connection 1132(only useful if the peer supports it). 1133.It Cm ~V 1134Decrease the verbosity 1135.Pq Ic LogLevel 1136when errors are being written to stderr. 1137.It Cm ~v 1138Increase the verbosity 1139.Pq Ic LogLevel 1140when errors are being written to stderr. 1141.El 1142.Sh TCP FORWARDING 1143Forwarding of arbitrary TCP connections over a secure channel 1144can be specified either on the command line or in a configuration file. 1145One possible application of TCP forwarding is a secure connection to a 1146mail server; another is going through firewalls. 1147.Pp 1148In the example below, we look at encrypting communication for an IRC client, 1149even though the IRC server it connects to does not directly 1150support encrypted communication. 1151This works as follows: 1152the user connects to the remote host using 1153.Nm , 1154specifying the ports to be used to forward the connection. 1155After that it is possible to start the program locally, 1156and 1157.Nm 1158will encrypt and forward the connection to the remote server. 1159.Pp 1160The following example tunnels an IRC session from the client 1161to an IRC server at 1162.Dq server.example.com , 1163joining channel 1164.Dq #users , 1165nickname 1166.Dq pinky , 1167using the standard IRC port, 6667: 1168.Bd -literal -offset 4n 1169$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10 1170$ irc -c '#users' pinky IRC/127.0.0.1 1171.Ed 1172.Pp 1173The 1174.Fl f 1175option backgrounds 1176.Nm 1177and the remote command 1178.Dq sleep 10 1179is specified to allow an amount of time 1180(10 seconds, in the example) 1181to start the program which is going to use the tunnel. 1182If no connections are made within the time specified, 1183.Nm 1184will exit. 1185.Sh X11 FORWARDING 1186If the 1187.Cm ForwardX11 1188variable is set to 1189.Dq yes 1190(or see the description of the 1191.Fl X , 1192.Fl x , 1193and 1194.Fl Y 1195options above) 1196and the user is using X11 (the 1197.Ev DISPLAY 1198environment variable is set), the connection to the X11 display is 1199automatically forwarded to the remote side in such a way that any X11 1200programs started from the shell (or command) will go through the 1201encrypted channel, and the connection to the real X server will be made 1202from the local machine. 1203The user should not manually set 1204.Ev DISPLAY . 1205Forwarding of X11 connections can be 1206configured on the command line or in configuration files. 1207.Pp 1208The 1209.Ev DISPLAY 1210value set by 1211.Nm 1212will point to the server machine, but with a display number greater than zero. 1213This is normal, and happens because 1214.Nm 1215creates a 1216.Dq proxy 1217X server on the server machine for forwarding the 1218connections over the encrypted channel. 1219.Pp 1220.Nm 1221will also automatically set up Xauthority data on the server machine. 1222For this purpose, it will generate a random authorization cookie, 1223store it in Xauthority on the server, and verify that any forwarded 1224connections carry this cookie and replace it by the real cookie when 1225the connection is opened. 1226The real authentication cookie is never 1227sent to the server machine (and no cookies are sent in the plain). 1228.Pp 1229If the 1230.Cm ForwardAgent 1231variable is set to 1232.Dq yes 1233(or see the description of the 1234.Fl A 1235and 1236.Fl a 1237options above) and 1238the user is using an authentication agent, the connection to the agent 1239is automatically forwarded to the remote side. 1240.Sh VERIFYING HOST KEYS 1241When connecting to a server for the first time, 1242a fingerprint of the server's public key is presented to the user 1243(unless the option 1244.Cm StrictHostKeyChecking 1245has been disabled). 1246Fingerprints can be determined using 1247.Xr ssh-keygen 1 : 1248.Pp 1249.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1250.Pp 1251If the fingerprint is already known, it can be matched 1252and the key can be accepted or rejected. 1253If only legacy (MD5) fingerprints for the server are available, the 1254.Xr ssh-keygen 1 1255.Fl E 1256option may be used to downgrade the fingerprint algorithm to match. 1257.Pp 1258Because of the difficulty of comparing host keys 1259just by looking at fingerprint strings, 1260there is also support to compare host keys visually, 1261using 1262.Em random art . 1263By setting the 1264.Cm VisualHostKey 1265option to 1266.Dq yes , 1267a small ASCII graphic gets displayed on every login to a server, no matter 1268if the session itself is interactive or not. 1269By learning the pattern a known server produces, a user can easily 1270find out that the host key has changed when a completely different pattern 1271is displayed. 1272Because these patterns are not unambiguous however, a pattern that looks 1273similar to the pattern remembered only gives a good probability that the 1274host key is the same, not guaranteed proof. 1275.Pp 1276To get a listing of the fingerprints along with their random art for 1277all known hosts, the following command line can be used: 1278.Pp 1279.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1280.Pp 1281If the fingerprint is unknown, 1282an alternative method of verification is available: 1283SSH fingerprints verified by DNS. 1284An additional resource record (RR), 1285SSHFP, 1286is added to a zonefile 1287and the connecting client is able to match the fingerprint 1288with that of the key presented. 1289.Pp 1290In this example, we are connecting a client to a server, 1291.Dq host.example.com . 1292The SSHFP resource records should first be added to the zonefile for 1293host.example.com: 1294.Bd -literal -offset indent 1295$ ssh-keygen -r host.example.com. 1296.Ed 1297.Pp 1298The output lines will have to be added to the zonefile. 1299To check that the zone is answering fingerprint queries: 1300.Pp 1301.Dl $ dig -t SSHFP host.example.com 1302.Pp 1303Finally the client connects: 1304.Bd -literal -offset indent 1305$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1306[...] 1307Matching host key fingerprint found in DNS. 1308Are you sure you want to continue connecting (yes/no)? 1309.Ed 1310.Pp 1311See the 1312.Cm VerifyHostKeyDNS 1313option in 1314.Xr ssh_config 5 1315for more information. 1316.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1317.Nm 1318contains support for Virtual Private Network (VPN) tunnelling 1319using the 1320.Xr tun 4 1321network pseudo-device, 1322allowing two networks to be joined securely. 1323The 1324.Xr sshd_config 5 1325configuration option 1326.Cm PermitTunnel 1327controls whether the server supports this, 1328and at what level (layer 2 or 3 traffic). 1329.Pp 1330The following example would connect client network 10.0.50.0/24 1331with remote network 10.0.99.0/24 using a point-to-point connection 1332from 10.1.1.1 to 10.1.1.2, 1333provided that the SSH server running on the gateway to the remote network, 1334at 192.168.1.15, allows it. 1335.Pp 1336On the client: 1337.Bd -literal -offset indent 1338# ssh -f -w 0:1 192.168.1.15 true 1339# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1340# route add 10.0.99.0/24 10.1.1.2 1341.Ed 1342.Pp 1343On the server: 1344.Bd -literal -offset indent 1345# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1346# route add 10.0.50.0/24 10.1.1.1 1347.Ed 1348.Pp 1349Client access may be more finely tuned via the 1350.Pa /root/.ssh/authorized_keys 1351file (see below) and the 1352.Cm PermitRootLogin 1353server option. 1354The following entry would permit connections on 1355.Xr tun 4 1356device 1 from user 1357.Dq jane 1358and on tun device 2 from user 1359.Dq john , 1360if 1361.Cm PermitRootLogin 1362is set to 1363.Dq forced-commands-only : 1364.Bd -literal -offset 2n 1365tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1366tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1367.Ed 1368.Pp 1369Since an SSH-based setup entails a fair amount of overhead, 1370it may be more suited to temporary setups, 1371such as for wireless VPNs. 1372More permanent VPNs are better provided by tools such as 1373.Xr ipsecctl 8 1374and 1375.Xr isakmpd 8 . 1376.Sh ENVIRONMENT 1377.Nm 1378will normally set the following environment variables: 1379.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1380.It Ev DISPLAY 1381The 1382.Ev DISPLAY 1383variable indicates the location of the X11 server. 1384It is automatically set by 1385.Nm 1386to point to a value of the form 1387.Dq hostname:n , 1388where 1389.Dq hostname 1390indicates the host where the shell runs, and 1391.Sq n 1392is an integer \*(Ge 1. 1393.Nm 1394uses this special value to forward X11 connections over the secure 1395channel. 1396The user should normally not set 1397.Ev DISPLAY 1398explicitly, as that 1399will render the X11 connection insecure (and will require the user to 1400manually copy any required authorization cookies). 1401.It Ev HOME 1402Set to the path of the user's home directory. 1403.It Ev LOGNAME 1404Synonym for 1405.Ev USER ; 1406set for compatibility with systems that use this variable. 1407.It Ev MAIL 1408Set to the path of the user's mailbox. 1409.It Ev PATH 1410Set to the default 1411.Ev PATH , 1412as specified when compiling 1413.Nm . 1414.It Ev SSH_ASKPASS 1415If 1416.Nm 1417needs a passphrase, it will read the passphrase from the current 1418terminal if it was run from a terminal. 1419If 1420.Nm 1421does not have a terminal associated with it but 1422.Ev DISPLAY 1423and 1424.Ev SSH_ASKPASS 1425are set, it will execute the program specified by 1426.Ev SSH_ASKPASS 1427and open an X11 window to read the passphrase. 1428This is particularly useful when calling 1429.Nm 1430from a 1431.Pa .xsession 1432or related script. 1433(Note that on some machines it 1434may be necessary to redirect the input from 1435.Pa /dev/null 1436to make this work.) 1437.It Ev SSH_ASKPASS_REQUIRE 1438Allows further control over the use of an askpass program. 1439If this variable is set to 1440.Dq never 1441then 1442.Nm 1443will never attempt to use one. 1444If it is set to 1445.Dq prefer , 1446then 1447.Nm 1448will prefer to use the askpass program instead of the TTY when requesting 1449passwords. 1450Finally, if the variable is set to 1451.Dq force , 1452then the askpass program will be used for all passphrase input regardless 1453of whether 1454.Ev DISPLAY 1455is set. 1456.It Ev SSH_AUTH_SOCK 1457Identifies the path of a 1458.Ux Ns -domain 1459socket used to communicate with the agent. 1460.It Ev SSH_CONNECTION 1461Identifies the client and server ends of the connection. 1462The variable contains 1463four space-separated values: client IP address, client port number, 1464server IP address, and server port number. 1465.It Ev SSH_ORIGINAL_COMMAND 1466This variable contains the original command line if a forced command 1467is executed. 1468It can be used to extract the original arguments. 1469.It Ev SSH_TTY 1470This is set to the name of the tty (path to the device) associated 1471with the current shell or command. 1472If the current session has no tty, 1473this variable is not set. 1474.It Ev SSH_TUNNEL 1475Optionally set by 1476.Xr sshd 8 1477to contain the interface names assigned if tunnel forwarding was 1478requested by the client. 1479.It Ev SSH_USER_AUTH 1480Optionally set by 1481.Xr sshd 8 , 1482this variable may contain a pathname to a file that lists the authentication 1483methods successfully used when the session was established, including any 1484public keys that were used. 1485.It Ev TZ 1486This variable is set to indicate the present time zone if it 1487was set when the daemon was started (i.e. the daemon passes the value 1488on to new connections). 1489.It Ev USER 1490Set to the name of the user logging in. 1491.El 1492.Pp 1493Additionally, 1494.Nm 1495reads 1496.Pa ~/.ssh/environment , 1497and adds lines of the format 1498.Dq VARNAME=value 1499to the environment if the file exists and users are allowed to 1500change their environment. 1501For more information, see the 1502.Cm PermitUserEnvironment 1503option in 1504.Xr sshd_config 5 . 1505.Sh FILES 1506.Bl -tag -width Ds -compact 1507.It Pa ~/.rhosts 1508This file is used for host-based authentication (see above). 1509On some machines this file may need to be 1510world-readable if the user's home directory is on an NFS partition, 1511because 1512.Xr sshd 8 1513reads it as root. 1514Additionally, this file must be owned by the user, 1515and must not have write permissions for anyone else. 1516The recommended 1517permission for most machines is read/write for the user, and not 1518accessible by others. 1519.Pp 1520.It Pa ~/.shosts 1521This file is used in exactly the same way as 1522.Pa .rhosts , 1523but allows host-based authentication without permitting login with 1524rlogin/rsh. 1525.Pp 1526.It Pa ~/.ssh/ 1527This directory is the default location for all user-specific configuration 1528and authentication information. 1529There is no general requirement to keep the entire contents of this directory 1530secret, but the recommended permissions are read/write/execute for the user, 1531and not accessible by others. 1532.Pp 1533.It Pa ~/.ssh/authorized_keys 1534Lists the public keys (DSA, ECDSA, Ed25519, RSA) 1535that can be used for logging in as this user. 1536The format of this file is described in the 1537.Xr sshd 8 1538manual page. 1539This file is not highly sensitive, but the recommended 1540permissions are read/write for the user, and not accessible by others. 1541.Pp 1542.It Pa ~/.ssh/config 1543This is the per-user configuration file. 1544The file format and configuration options are described in 1545.Xr ssh_config 5 . 1546Because of the potential for abuse, this file must have strict permissions: 1547read/write for the user, and not writable by others. 1548.Pp 1549.It Pa ~/.ssh/environment 1550Contains additional definitions for environment variables; see 1551.Sx ENVIRONMENT , 1552above. 1553.Pp 1554.It Pa ~/.ssh/id_dsa 1555.It Pa ~/.ssh/id_ecdsa 1556.It Pa ~/.ssh/id_ecdsa_sk 1557.It Pa ~/.ssh/id_ed25519 1558.It Pa ~/.ssh/id_ed25519_sk 1559.It Pa ~/.ssh/id_rsa 1560Contains the private key for authentication. 1561These files 1562contain sensitive data and should be readable by the user but not 1563accessible by others (read/write/execute). 1564.Nm 1565will simply ignore a private key file if it is accessible by others. 1566It is possible to specify a passphrase when 1567generating the key which will be used to encrypt the 1568sensitive part of this file using AES-128. 1569.Pp 1570.It Pa ~/.ssh/id_dsa.pub 1571.It Pa ~/.ssh/id_ecdsa.pub 1572.It Pa ~/.ssh/id_ecdsa_sk.pub 1573.It Pa ~/.ssh/id_ed25519.pub 1574.It Pa ~/.ssh/id_ed25519_sk.pub 1575.It Pa ~/.ssh/id_rsa.pub 1576Contains the public key for authentication. 1577These files are not 1578sensitive and can (but need not) be readable by anyone. 1579.Pp 1580.It Pa ~/.ssh/known_hosts 1581Contains a list of host keys for all hosts the user has logged into 1582that are not already in the systemwide list of known host keys. 1583See 1584.Xr sshd 8 1585for further details of the format of this file. 1586.Pp 1587.It Pa ~/.ssh/rc 1588Commands in this file are executed by 1589.Nm 1590when the user logs in, just before the user's shell (or command) is 1591started. 1592See the 1593.Xr sshd 8 1594manual page for more information. 1595.Pp 1596.It Pa /etc/hosts.equiv 1597This file is for host-based authentication (see above). 1598It should only be writable by root. 1599.Pp 1600.It Pa /etc/shosts.equiv 1601This file is used in exactly the same way as 1602.Pa hosts.equiv , 1603but allows host-based authentication without permitting login with 1604rlogin/rsh. 1605.Pp 1606.It Pa /etc/ssh/ssh_config 1607Systemwide configuration file. 1608The file format and configuration options are described in 1609.Xr ssh_config 5 . 1610.Pp 1611.It Pa /etc/ssh/ssh_host_key 1612.It Pa /etc/ssh/ssh_host_dsa_key 1613.It Pa /etc/ssh/ssh_host_ecdsa_key 1614.It Pa /etc/ssh/ssh_host_ed25519_key 1615.It Pa /etc/ssh/ssh_host_rsa_key 1616These files contain the private parts of the host keys 1617and are used for host-based authentication. 1618.Pp 1619.It Pa /etc/ssh/ssh_known_hosts 1620Systemwide list of known host keys. 1621This file should be prepared by the 1622system administrator to contain the public host keys of all machines in the 1623organization. 1624It should be world-readable. 1625See 1626.Xr sshd 8 1627for further details of the format of this file. 1628.Pp 1629.It Pa /etc/ssh/sshrc 1630Commands in this file are executed by 1631.Nm 1632when the user logs in, just before the user's shell (or command) is started. 1633See the 1634.Xr sshd 8 1635manual page for more information. 1636.El 1637.Sh EXIT STATUS 1638.Nm 1639exits with the exit status of the remote command or with 255 1640if an error occurred. 1641.Sh SEE ALSO 1642.Xr scp 1 , 1643.Xr sftp 1 , 1644.Xr ssh-add 1 , 1645.Xr ssh-agent 1 , 1646.Xr ssh-keygen 1 , 1647.Xr ssh-keyscan 1 , 1648.Xr tun 4 , 1649.Xr ssh_config 5 , 1650.Xr ssh-keysign 8 , 1651.Xr sshd 8 1652.Sh STANDARDS 1653.Rs 1654.%A S. Lehtinen 1655.%A C. Lonvick 1656.%D January 2006 1657.%R RFC 4250 1658.%T The Secure Shell (SSH) Protocol Assigned Numbers 1659.Re 1660.Pp 1661.Rs 1662.%A T. Ylonen 1663.%A C. Lonvick 1664.%D January 2006 1665.%R RFC 4251 1666.%T The Secure Shell (SSH) Protocol Architecture 1667.Re 1668.Pp 1669.Rs 1670.%A T. Ylonen 1671.%A C. Lonvick 1672.%D January 2006 1673.%R RFC 4252 1674.%T The Secure Shell (SSH) Authentication Protocol 1675.Re 1676.Pp 1677.Rs 1678.%A T. Ylonen 1679.%A C. Lonvick 1680.%D January 2006 1681.%R RFC 4253 1682.%T The Secure Shell (SSH) Transport Layer Protocol 1683.Re 1684.Pp 1685.Rs 1686.%A T. Ylonen 1687.%A C. Lonvick 1688.%D January 2006 1689.%R RFC 4254 1690.%T The Secure Shell (SSH) Connection Protocol 1691.Re 1692.Pp 1693.Rs 1694.%A J. Schlyter 1695.%A W. Griffin 1696.%D January 2006 1697.%R RFC 4255 1698.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints 1699.Re 1700.Pp 1701.Rs 1702.%A F. Cusack 1703.%A M. Forssen 1704.%D January 2006 1705.%R RFC 4256 1706.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH) 1707.Re 1708.Pp 1709.Rs 1710.%A J. Galbraith 1711.%A P. Remaker 1712.%D January 2006 1713.%R RFC 4335 1714.%T The Secure Shell (SSH) Session Channel Break Extension 1715.Re 1716.Pp 1717.Rs 1718.%A M. Bellare 1719.%A T. Kohno 1720.%A C. Namprempre 1721.%D January 2006 1722.%R RFC 4344 1723.%T The Secure Shell (SSH) Transport Layer Encryption Modes 1724.Re 1725.Pp 1726.Rs 1727.%A B. Harris 1728.%D January 2006 1729.%R RFC 4345 1730.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol 1731.Re 1732.Pp 1733.Rs 1734.%A M. Friedl 1735.%A N. Provos 1736.%A W. Simpson 1737.%D March 2006 1738.%R RFC 4419 1739.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 1740.Re 1741.Pp 1742.Rs 1743.%A J. Galbraith 1744.%A R. Thayer 1745.%D November 2006 1746.%R RFC 4716 1747.%T The Secure Shell (SSH) Public Key File Format 1748.Re 1749.Pp 1750.Rs 1751.%A D. Stebila 1752.%A J. Green 1753.%D December 2009 1754.%R RFC 5656 1755.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer 1756.Re 1757.Pp 1758.Rs 1759.%A A. Perrig 1760.%A D. Song 1761.%D 1999 1762.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99) 1763.%T Hash Visualization: a New Technique to improve Real-World Security 1764.Re 1765.Sh AUTHORS 1766OpenSSH is a derivative of the original and free 1767ssh 1.2.12 release by Tatu Ylonen. 1768Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1769Theo de Raadt and Dug Song 1770removed many bugs, re-added newer features and 1771created OpenSSH. 1772Markus Friedl contributed the support for SSH 1773protocol versions 1.5 and 2.0. 1774