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 1.415 2020/12/22 00:15:23 djm Exp $ 37.Dd $Mdocdate: December 22 2020 $ 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 91his/her 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. 262.Pp 263.It Fl G 264Causes 265.Nm 266to print its configuration after evaluating 267.Cm Host 268and 269.Cm Match 270blocks and exit. 271.Pp 272.It Fl g 273Allows remote hosts to connect to local forwarded ports. 274If used on a multiplexed connection, then this option must be specified 275on the master process. 276.Pp 277.It Fl I Ar pkcs11 278Specify the PKCS#11 shared library 279.Nm 280should use to communicate with a PKCS#11 token providing keys for user 281authentication. 282.Pp 283.It Fl i Ar identity_file 284Selects a file from which the identity (private key) for 285public key authentication is read. 286The default is 287.Pa ~/.ssh/id_dsa , 288.Pa ~/.ssh/id_ecdsa , 289.Pa ~/.ssh/id_ecdsa_sk , 290.Pa ~/.ssh/id_ed25519 , 291.Pa ~/.ssh/id_ed25519_sk 292and 293.Pa ~/.ssh/id_rsa . 294Identity files may also be specified on 295a per-host basis in the configuration file. 296It is possible to have multiple 297.Fl i 298options (and multiple identities specified in 299configuration files). 300If no certificates have been explicitly specified by the 301.Cm CertificateFile 302directive, 303.Nm 304will also try to load certificate information from the filename obtained 305by appending 306.Pa -cert.pub 307to identity filenames. 308.Pp 309.It Fl J Ar destination 310Connect to the target host by first making a 311.Nm 312connection to the jump host described by 313.Ar destination 314and then establishing a TCP forwarding to the ultimate destination from 315there. 316Multiple jump hops may be specified separated by comma characters. 317This is a shortcut to specify a 318.Cm ProxyJump 319configuration directive. 320Note that configuration directives supplied on the command-line generally 321apply to the destination host and not any specified jump hosts. 322Use 323.Pa ~/.ssh/config 324to specify configuration for jump hosts. 325.Pp 326.It Fl K 327Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI 328credentials to the server. 329.Pp 330.It Fl k 331Disables forwarding (delegation) of GSSAPI credentials to the server. 332.Pp 333.It Fl L Xo 334.Sm off 335.Oo Ar bind_address : Oc 336.Ar port : host : hostport 337.Sm on 338.Xc 339.It Fl L Xo 340.Sm off 341.Oo Ar bind_address : Oc 342.Ar port : remote_socket 343.Sm on 344.Xc 345.It Fl L Xo 346.Sm off 347.Ar local_socket : host : hostport 348.Sm on 349.Xc 350.It Fl L Xo 351.Sm off 352.Ar local_socket : remote_socket 353.Sm on 354.Xc 355Specifies that connections to the given TCP port or Unix socket on the local 356(client) host are to be forwarded to the given host and port, or Unix socket, 357on the remote side. 358This works by allocating a socket to listen to either a TCP 359.Ar port 360on the local side, optionally bound to the specified 361.Ar bind_address , 362or to a Unix socket. 363Whenever a connection is made to the local port or socket, the 364connection is forwarded over the secure channel, and a connection is 365made to either 366.Ar host 367port 368.Ar hostport , 369or the Unix socket 370.Ar remote_socket , 371from the remote machine. 372.Pp 373Port forwardings can also be specified in the configuration file. 374Only the superuser can forward privileged ports. 375IPv6 addresses can be specified by enclosing the address in square brackets. 376.Pp 377By default, the local port is bound in accordance with the 378.Cm GatewayPorts 379setting. 380However, an explicit 381.Ar bind_address 382may be used to bind the connection to a specific address. 383The 384.Ar bind_address 385of 386.Dq localhost 387indicates that the listening port be bound for local use only, while an 388empty address or 389.Sq * 390indicates that the port should be available from all interfaces. 391.Pp 392.It Fl l Ar login_name 393Specifies the user to log in as on the remote machine. 394This also may be specified on a per-host basis in the configuration file. 395.Pp 396.It Fl M 397Places the 398.Nm 399client into 400.Dq master 401mode for connection sharing. 402Multiple 403.Fl M 404options places 405.Nm 406into 407.Dq master 408mode but with confirmation required using 409.Xr ssh-askpass 1 410before each operation that changes the multiplexing state 411(e.g. opening a new session). 412Refer to the description of 413.Cm ControlMaster 414in 415.Xr ssh_config 5 416for details. 417.Pp 418.It Fl m Ar mac_spec 419A comma-separated list of MAC (message authentication code) algorithms, 420specified in order of preference. 421See the 422.Cm MACs 423keyword for more information. 424.Pp 425.It Fl N 426Do not execute a remote command. 427This is useful for just forwarding ports. 428.Pp 429.It Fl n 430Redirects stdin from 431.Pa /dev/null 432(actually, prevents reading from stdin). 433This must be used when 434.Nm 435is run in the background. 436A common trick is to use this to run X11 programs on a remote machine. 437For example, 438.Ic ssh -n shadows.cs.hut.fi emacs & 439will start an emacs on shadows.cs.hut.fi, and the X11 440connection will be automatically forwarded over an encrypted channel. 441The 442.Nm 443program will be put in the background. 444(This does not work if 445.Nm 446needs to ask for a password or passphrase; see also the 447.Fl f 448option.) 449.Pp 450.It Fl O Ar ctl_cmd 451Control an active connection multiplexing master process. 452When the 453.Fl O 454option is specified, the 455.Ar ctl_cmd 456argument is interpreted and passed to the master process. 457Valid commands are: 458.Dq check 459(check that the master process is running), 460.Dq forward 461(request forwardings without command execution), 462.Dq cancel 463(cancel forwardings), 464.Dq exit 465(request the master to exit), and 466.Dq stop 467(request the master to stop accepting further multiplexing requests). 468.Pp 469.It Fl o Ar option 470Can be used to give options in the format used in the configuration file. 471This is useful for specifying options for which there is no separate 472command-line flag. 473For full details of the options listed below, and their possible values, see 474.Xr ssh_config 5 . 475.Pp 476.Bl -tag -width Ds -offset indent -compact 477.It AddKeysToAgent 478.It AddressFamily 479.It BatchMode 480.It BindAddress 481.It CanonicalDomains 482.It CanonicalizeFallbackLocal 483.It CanonicalizeHostname 484.It CanonicalizeMaxDots 485.It CanonicalizePermittedCNAMEs 486.It CASignatureAlgorithms 487.It CertificateFile 488.It ChallengeResponseAuthentication 489.It CheckHostIP 490.It Ciphers 491.It ClearAllForwardings 492.It Compression 493.It ConnectionAttempts 494.It ConnectTimeout 495.It ControlMaster 496.It ControlPath 497.It ControlPersist 498.It DynamicForward 499.It EscapeChar 500.It ExitOnForwardFailure 501.It FingerprintHash 502.It ForwardAgent 503.It ForwardX11 504.It ForwardX11Timeout 505.It ForwardX11Trusted 506.It GatewayPorts 507.It GlobalKnownHostsFile 508.It GSSAPIAuthentication 509.It GSSAPIDelegateCredentials 510.It HashKnownHosts 511.It Host 512.It HostbasedAuthentication 513.It HostbasedKeyTypes 514.It HostKeyAlgorithms 515.It HostKeyAlias 516.It Hostname 517.It IdentitiesOnly 518.It IdentityAgent 519.It IdentityFile 520.It IPQoS 521.It KbdInteractiveAuthentication 522.It KbdInteractiveDevices 523.It KexAlgorithms 524.It KnownHostsCommand 525.It LocalCommand 526.It LocalForward 527.It LogLevel 528.It MACs 529.It Match 530.It NoHostAuthenticationForLocalhost 531.It NumberOfPasswordPrompts 532.It PasswordAuthentication 533.It PermitLocalCommand 534.It PKCS11Provider 535.It Port 536.It PreferredAuthentications 537.It ProxyCommand 538.It ProxyJump 539.It ProxyUseFdpass 540.It PubkeyAcceptedKeyTypes 541.It PubkeyAuthentication 542.It RekeyLimit 543.It RemoteCommand 544.It RemoteForward 545.It RequestTTY 546.It SendEnv 547.It ServerAliveInterval 548.It ServerAliveCountMax 549.It SetEnv 550.It StreamLocalBindMask 551.It StreamLocalBindUnlink 552.It StrictHostKeyChecking 553.It TCPKeepAlive 554.It Tunnel 555.It TunnelDevice 556.It UpdateHostKeys 557.It User 558.It UserKnownHostsFile 559.It VerifyHostKeyDNS 560.It VisualHostKey 561.It XAuthLocation 562.El 563.Pp 564.It Fl p Ar port 565Port to connect to on the remote host. 566This can be specified on a 567per-host basis in the configuration file. 568.Pp 569.It Fl Q Ar query_option 570Queries 571.Nm 572for the algorithms supported for the specified version 2. 573The available features are: 574.Ar cipher 575(supported symmetric ciphers), 576.Ar cipher-auth 577(supported symmetric ciphers that support authenticated encryption), 578.Ar help 579(supported query terms for use with the 580.Fl Q 581flag), 582.Ar mac 583(supported message integrity codes), 584.Ar kex 585(key exchange algorithms), 586.Ar key 587(key types), 588.Ar key-cert 589(certificate key types), 590.Ar key-plain 591(non-certificate key types), 592.Ar key-sig 593(all key types and signature algorithms), 594.Ar protocol-version 595(supported SSH protocol versions), and 596.Ar sig 597(supported signature algorithms). 598Alternatively, any keyword from 599.Xr ssh_config 5 600or 601.Xr sshd_config 5 602that takes an algorithm list may be used as an alias for the corresponding 603query_option. 604.Pp 605.It Fl q 606Quiet mode. 607Causes most warning and diagnostic messages to be suppressed. 608.Pp 609.It Fl R Xo 610.Sm off 611.Oo Ar bind_address : Oc 612.Ar port : host : hostport 613.Sm on 614.Xc 615.It Fl R Xo 616.Sm off 617.Oo Ar bind_address : Oc 618.Ar port : local_socket 619.Sm on 620.Xc 621.It Fl R Xo 622.Sm off 623.Ar remote_socket : host : hostport 624.Sm on 625.Xc 626.It Fl R Xo 627.Sm off 628.Ar remote_socket : local_socket 629.Sm on 630.Xc 631.It Fl R Xo 632.Sm off 633.Oo Ar bind_address : Oc 634.Ar port 635.Sm on 636.Xc 637Specifies that connections to the given TCP port or Unix socket on the remote 638(server) host are to be forwarded to the local side. 639.Pp 640This works by allocating a socket to listen to either a TCP 641.Ar port 642or to a Unix socket on the remote side. 643Whenever a connection is made to this port or Unix socket, the 644connection is forwarded over the secure channel, and a connection 645is made from the local machine to either an explicit destination specified by 646.Ar host 647port 648.Ar hostport , 649or 650.Ar local_socket , 651or, if no explicit destination was specified, 652.Nm 653will act as a SOCKS 4/5 proxy and forward connections to the destinations 654requested by the remote SOCKS client. 655.Pp 656Port forwardings can also be specified in the configuration file. 657Privileged ports can be forwarded only when 658logging in as root on the remote machine. 659IPv6 addresses can be specified by enclosing the address in square brackets. 660.Pp 661By default, TCP listening sockets on the server will be bound to the loopback 662interface only. 663This may be overridden by specifying a 664.Ar bind_address . 665An empty 666.Ar bind_address , 667or the address 668.Ql * , 669indicates that the remote socket should listen on all interfaces. 670Specifying a remote 671.Ar bind_address 672will only succeed if the server's 673.Cm GatewayPorts 674option is enabled (see 675.Xr sshd_config 5 ) . 676.Pp 677If the 678.Ar port 679argument is 680.Ql 0 , 681the listen port will be dynamically allocated on the server and reported 682to the client at run time. 683When used together with 684.Ic -O forward 685the allocated port will be printed to the standard output. 686.Pp 687.It Fl S Ar ctl_path 688Specifies the location of a control socket for connection sharing, 689or the string 690.Dq none 691to disable connection sharing. 692Refer to the description of 693.Cm ControlPath 694and 695.Cm ControlMaster 696in 697.Xr ssh_config 5 698for details. 699.Pp 700.It Fl s 701May be used to request invocation of a subsystem on the remote system. 702Subsystems facilitate the use of SSH 703as a secure transport for other applications (e.g.\& 704.Xr sftp 1 ) . 705The subsystem is specified as the remote command. 706.Pp 707.It Fl T 708Disable pseudo-terminal allocation. 709.Pp 710.It Fl t 711Force pseudo-terminal allocation. 712This can be used to execute arbitrary 713screen-based programs on a remote machine, which can be very useful, 714e.g. when implementing menu services. 715Multiple 716.Fl t 717options force tty allocation, even if 718.Nm 719has no local tty. 720.Pp 721.It Fl V 722Display the version number and exit. 723.Pp 724.It Fl v 725Verbose mode. 726Causes 727.Nm 728to print debugging messages about its progress. 729This is helpful in 730debugging connection, authentication, and configuration problems. 731Multiple 732.Fl v 733options increase the verbosity. 734The maximum is 3. 735.Pp 736.It Fl W Ar host : Ns Ar port 737Requests that standard input and output on the client be forwarded to 738.Ar host 739on 740.Ar port 741over the secure channel. 742Implies 743.Fl N , 744.Fl T , 745.Cm ExitOnForwardFailure 746and 747.Cm ClearAllForwardings , 748though these can be overridden in the configuration file or using 749.Fl o 750command line options. 751.Pp 752.It Fl w Xo 753.Ar local_tun Ns Op : Ns Ar remote_tun 754.Xc 755Requests 756tunnel 757device forwarding with the specified 758.Xr tun 4 759devices between the client 760.Pq Ar local_tun 761and the server 762.Pq Ar remote_tun . 763.Pp 764The devices may be specified by numerical ID or the keyword 765.Dq any , 766which uses the next available tunnel device. 767If 768.Ar remote_tun 769is not specified, it defaults to 770.Dq any . 771See also the 772.Cm Tunnel 773and 774.Cm TunnelDevice 775directives in 776.Xr ssh_config 5 . 777.Pp 778If the 779.Cm Tunnel 780directive is unset, it will be set to the default tunnel mode, which is 781.Dq point-to-point . 782If a different 783.Cm Tunnel 784forwarding mode it desired, then it should be specified before 785.Fl w . 786.Pp 787.It Fl X 788Enables X11 forwarding. 789This can also be specified on a per-host basis in a configuration file. 790.Pp 791X11 forwarding should be enabled with caution. 792Users with the ability to bypass file permissions on the remote host 793(for the user's X authorization database) 794can access the local X11 display through the forwarded connection. 795An attacker may then be able to perform activities such as keystroke monitoring. 796.Pp 797For this reason, X11 forwarding is subjected to X11 SECURITY extension 798restrictions by default. 799Please refer to the 800.Nm 801.Fl Y 802option and the 803.Cm ForwardX11Trusted 804directive in 805.Xr ssh_config 5 806for more information. 807.Pp 808.It Fl x 809Disables X11 forwarding. 810.Pp 811.It Fl Y 812Enables trusted X11 forwarding. 813Trusted X11 forwardings are not subjected to the X11 SECURITY extension 814controls. 815.Pp 816.It Fl y 817Send log information using the 818.Xr syslog 3 819system module. 820By default this information is sent to stderr. 821.El 822.Pp 823.Nm 824may additionally obtain configuration data from 825a per-user configuration file and a system-wide configuration file. 826The file format and configuration options are described in 827.Xr ssh_config 5 . 828.Sh AUTHENTICATION 829The OpenSSH SSH client supports SSH protocol 2. 830.Pp 831The methods available for authentication are: 832GSSAPI-based authentication, 833host-based authentication, 834public key authentication, 835challenge-response authentication, 836and password authentication. 837Authentication methods are tried in the order specified above, 838though 839.Cm PreferredAuthentications 840can be used to change the default order. 841.Pp 842Host-based authentication works as follows: 843If the machine the user logs in from is listed in 844.Pa /etc/hosts.equiv 845or 846.Pa /etc/shosts.equiv 847on the remote machine, the user is non-root and the user names are 848the same on both sides, or if the files 849.Pa ~/.rhosts 850or 851.Pa ~/.shosts 852exist in the user's home directory on the 853remote machine and contain a line containing the name of the client 854machine and the name of the user on that machine, the user is 855considered for login. 856Additionally, the server 857.Em must 858be able to verify the client's 859host key (see the description of 860.Pa /etc/ssh/ssh_known_hosts 861and 862.Pa ~/.ssh/known_hosts , 863below) 864for login to be permitted. 865This authentication method closes security holes due to IP 866spoofing, DNS spoofing, and routing spoofing. 867[Note to the administrator: 868.Pa /etc/hosts.equiv , 869.Pa ~/.rhosts , 870and the rlogin/rsh protocol in general, are inherently insecure and should be 871disabled if security is desired.] 872.Pp 873Public key authentication works as follows: 874The scheme is based on public-key cryptography, 875using cryptosystems 876where encryption and decryption are done using separate keys, 877and it is unfeasible to derive the decryption key from the encryption key. 878The idea is that each user creates a public/private 879key pair for authentication purposes. 880The server knows the public key, and only the user knows the private key. 881.Nm 882implements public key authentication protocol automatically, 883using one of the DSA, ECDSA, Ed25519 or RSA algorithms. 884The HISTORY section of 885.Xr ssl 8 886contains a brief discussion of the DSA and RSA algorithms. 887.Pp 888The file 889.Pa ~/.ssh/authorized_keys 890lists the public keys that are permitted for logging in. 891When the user logs in, the 892.Nm 893program tells the server which key pair it would like to use for 894authentication. 895The client proves that it has access to the private key 896and the server checks that the corresponding public key 897is authorized to accept the account. 898.Pp 899The server may inform the client of errors that prevented public key 900authentication from succeeding after authentication completes using a 901different method. 902These may be viewed by increasing the 903.Cm LogLevel 904to 905.Cm DEBUG 906or higher (e.g. by using the 907.Fl v 908flag). 909.Pp 910The user creates his/her key pair by running 911.Xr ssh-keygen 1 . 912This stores the private key in 913.Pa ~/.ssh/id_dsa 914(DSA), 915.Pa ~/.ssh/id_ecdsa 916(ECDSA), 917.Pa ~/.ssh/id_ecdsa_sk 918(authenticator-hosted ECDSA), 919.Pa ~/.ssh/id_ed25519 920(Ed25519), 921.Pa ~/.ssh/id_ed25519_sk 922(authenticator-hosted Ed25519), 923or 924.Pa ~/.ssh/id_rsa 925(RSA) 926and stores the public key in 927.Pa ~/.ssh/id_dsa.pub 928(DSA), 929.Pa ~/.ssh/id_ecdsa.pub 930(ECDSA), 931.Pa ~/.ssh/id_ecdsa_sk.pub 932(authenticator-hosted ECDSA), 933.Pa ~/.ssh/id_ed25519.pub 934(Ed25519), 935.Pa ~/.ssh/id_ed25519_sk.pub 936(authenticator-hosted Ed25519), 937or 938.Pa ~/.ssh/id_rsa.pub 939(RSA) 940in the user's home directory. 941The user should then copy the public key 942to 943.Pa ~/.ssh/authorized_keys 944in his/her home directory on the remote machine. 945The 946.Pa authorized_keys 947file corresponds to the conventional 948.Pa ~/.rhosts 949file, and has one key 950per line, though the lines can be very long. 951After this, the user can log in without giving the password. 952.Pp 953A variation on public key authentication 954is available in the form of certificate authentication: 955instead of a set of public/private keys, 956signed certificates are used. 957This has the advantage that a single trusted certification authority 958can be used in place of many public/private keys. 959See the CERTIFICATES section of 960.Xr ssh-keygen 1 961for more information. 962.Pp 963The most convenient way to use public key or certificate authentication 964may be with an authentication agent. 965See 966.Xr ssh-agent 1 967and (optionally) the 968.Cm AddKeysToAgent 969directive in 970.Xr ssh_config 5 971for more information. 972.Pp 973Challenge-response authentication works as follows: 974The server sends an arbitrary 975.Qq challenge 976text, and prompts for a response. 977Examples of challenge-response authentication include 978.Bx 979Authentication (see 980.Xr login.conf 5 ) 981and PAM (some 982.Pf non- Ox 983systems). 984.Pp 985Finally, if other authentication methods fail, 986.Nm 987prompts the user for a password. 988The password is sent to the remote 989host for checking; however, since all communications are encrypted, 990the password cannot be seen by someone listening on the network. 991.Pp 992.Nm 993automatically maintains and checks a database containing 994identification for all hosts it has ever been used with. 995Host keys are stored in 996.Pa ~/.ssh/known_hosts 997in the user's home directory. 998Additionally, the file 999.Pa /etc/ssh/ssh_known_hosts 1000is automatically checked for known hosts. 1001Any new hosts are automatically added to the user's file. 1002If a host's identification ever changes, 1003.Nm 1004warns about this and disables password authentication to prevent 1005server spoofing or man-in-the-middle attacks, 1006which could otherwise be used to circumvent the encryption. 1007The 1008.Cm StrictHostKeyChecking 1009option can be used to control logins to machines whose 1010host key is not known or has changed. 1011.Pp 1012When the user's identity has been accepted by the server, the server 1013either executes the given command in a non-interactive session or, 1014if no command has been specified, logs into the machine and gives 1015the user a normal shell as an interactive session. 1016All communication with 1017the remote command or shell will be automatically encrypted. 1018.Pp 1019If an interactive session is requested 1020.Nm 1021by default will only request a pseudo-terminal (pty) for interactive 1022sessions when the client has one. 1023The flags 1024.Fl T 1025and 1026.Fl t 1027can be used to override this behaviour. 1028.Pp 1029If a pseudo-terminal has been allocated the 1030user may use the escape characters noted below. 1031.Pp 1032If no pseudo-terminal has been allocated, 1033the session is transparent and can be used to reliably transfer binary data. 1034On most systems, setting the escape character to 1035.Dq none 1036will also make the session transparent even if a tty is used. 1037.Pp 1038The session terminates when the command or shell on the remote 1039machine exits and all X11 and TCP connections have been closed. 1040.Sh ESCAPE CHARACTERS 1041When a pseudo-terminal has been requested, 1042.Nm 1043supports a number of functions through the use of an escape character. 1044.Pp 1045A single tilde character can be sent as 1046.Ic ~~ 1047or by following the tilde by a character other than those described below. 1048The escape character must always follow a newline to be interpreted as 1049special. 1050The escape character can be changed in configuration files using the 1051.Cm EscapeChar 1052configuration directive or on the command line by the 1053.Fl e 1054option. 1055.Pp 1056The supported escapes (assuming the default 1057.Ql ~ ) 1058are: 1059.Bl -tag -width Ds 1060.It Cm ~. 1061Disconnect. 1062.It Cm ~^Z 1063Background 1064.Nm . 1065.It Cm ~# 1066List forwarded connections. 1067.It Cm ~& 1068Background 1069.Nm 1070at logout when waiting for forwarded connection / X11 sessions to terminate. 1071.It Cm ~? 1072Display a list of escape characters. 1073.It Cm ~B 1074Send a BREAK to the remote system 1075(only useful if the peer supports it). 1076.It Cm ~C 1077Open command line. 1078Currently this allows the addition of port forwardings using the 1079.Fl L , 1080.Fl R 1081and 1082.Fl D 1083options (see above). 1084It also allows the cancellation of existing port-forwardings 1085with 1086.Sm off 1087.Fl KL Oo Ar bind_address : Oc Ar port 1088.Sm on 1089for local, 1090.Sm off 1091.Fl KR Oo Ar bind_address : Oc Ar port 1092.Sm on 1093for remote and 1094.Sm off 1095.Fl KD Oo Ar bind_address : Oc Ar port 1096.Sm on 1097for dynamic port-forwardings. 1098.Ic !\& Ns Ar command 1099allows the user to execute a local command if the 1100.Ic PermitLocalCommand 1101option is enabled in 1102.Xr ssh_config 5 . 1103Basic help is available, using the 1104.Fl h 1105option. 1106.It Cm ~R 1107Request rekeying of the connection 1108(only useful if the peer supports it). 1109.It Cm ~V 1110Decrease the verbosity 1111.Pq Ic LogLevel 1112when errors are being written to stderr. 1113.It Cm ~v 1114Increase the verbosity 1115.Pq Ic LogLevel 1116when errors are being written to stderr. 1117.El 1118.Sh TCP FORWARDING 1119Forwarding of arbitrary TCP connections over a secure channel 1120can be specified either on the command line or in a configuration file. 1121One possible application of TCP forwarding is a secure connection to a 1122mail server; another is going through firewalls. 1123.Pp 1124In the example below, we look at encrypting communication for an IRC client, 1125even though the IRC server it connects to does not directly 1126support encrypted communication. 1127This works as follows: 1128the user connects to the remote host using 1129.Nm , 1130specifying the ports to be used to forward the connection. 1131After that it is possible to start the program locally, 1132and 1133.Nm 1134will encrypt and forward the connection to the remote server. 1135.Pp 1136The following example tunnels an IRC session from the client 1137to an IRC server at 1138.Dq server.example.com , 1139joining channel 1140.Dq #users , 1141nickname 1142.Dq pinky , 1143using the standard IRC port, 6667: 1144.Bd -literal -offset 4n 1145$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10 1146$ irc -c '#users' pinky IRC/127.0.0.1 1147.Ed 1148.Pp 1149The 1150.Fl f 1151option backgrounds 1152.Nm 1153and the remote command 1154.Dq sleep 10 1155is specified to allow an amount of time 1156(10 seconds, in the example) 1157to start the program which is going to use the tunnel. 1158If no connections are made within the time specified, 1159.Nm 1160will exit. 1161.Sh X11 FORWARDING 1162If the 1163.Cm ForwardX11 1164variable is set to 1165.Dq yes 1166(or see the description of the 1167.Fl X , 1168.Fl x , 1169and 1170.Fl Y 1171options above) 1172and the user is using X11 (the 1173.Ev DISPLAY 1174environment variable is set), the connection to the X11 display is 1175automatically forwarded to the remote side in such a way that any X11 1176programs started from the shell (or command) will go through the 1177encrypted channel, and the connection to the real X server will be made 1178from the local machine. 1179The user should not manually set 1180.Ev DISPLAY . 1181Forwarding of X11 connections can be 1182configured on the command line or in configuration files. 1183.Pp 1184The 1185.Ev DISPLAY 1186value set by 1187.Nm 1188will point to the server machine, but with a display number greater than zero. 1189This is normal, and happens because 1190.Nm 1191creates a 1192.Dq proxy 1193X server on the server machine for forwarding the 1194connections over the encrypted channel. 1195.Pp 1196.Nm 1197will also automatically set up Xauthority data on the server machine. 1198For this purpose, it will generate a random authorization cookie, 1199store it in Xauthority on the server, and verify that any forwarded 1200connections carry this cookie and replace it by the real cookie when 1201the connection is opened. 1202The real authentication cookie is never 1203sent to the server machine (and no cookies are sent in the plain). 1204.Pp 1205If the 1206.Cm ForwardAgent 1207variable is set to 1208.Dq yes 1209(or see the description of the 1210.Fl A 1211and 1212.Fl a 1213options above) and 1214the user is using an authentication agent, the connection to the agent 1215is automatically forwarded to the remote side. 1216.Sh VERIFYING HOST KEYS 1217When connecting to a server for the first time, 1218a fingerprint of the server's public key is presented to the user 1219(unless the option 1220.Cm StrictHostKeyChecking 1221has been disabled). 1222Fingerprints can be determined using 1223.Xr ssh-keygen 1 : 1224.Pp 1225.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1226.Pp 1227If the fingerprint is already known, it can be matched 1228and the key can be accepted or rejected. 1229If only legacy (MD5) fingerprints for the server are available, the 1230.Xr ssh-keygen 1 1231.Fl E 1232option may be used to downgrade the fingerprint algorithm to match. 1233.Pp 1234Because of the difficulty of comparing host keys 1235just by looking at fingerprint strings, 1236there is also support to compare host keys visually, 1237using 1238.Em random art . 1239By setting the 1240.Cm VisualHostKey 1241option to 1242.Dq yes , 1243a small ASCII graphic gets displayed on every login to a server, no matter 1244if the session itself is interactive or not. 1245By learning the pattern a known server produces, a user can easily 1246find out that the host key has changed when a completely different pattern 1247is displayed. 1248Because these patterns are not unambiguous however, a pattern that looks 1249similar to the pattern remembered only gives a good probability that the 1250host key is the same, not guaranteed proof. 1251.Pp 1252To get a listing of the fingerprints along with their random art for 1253all known hosts, the following command line can be used: 1254.Pp 1255.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1256.Pp 1257If the fingerprint is unknown, 1258an alternative method of verification is available: 1259SSH fingerprints verified by DNS. 1260An additional resource record (RR), 1261SSHFP, 1262is added to a zonefile 1263and the connecting client is able to match the fingerprint 1264with that of the key presented. 1265.Pp 1266In this example, we are connecting a client to a server, 1267.Dq host.example.com . 1268The SSHFP resource records should first be added to the zonefile for 1269host.example.com: 1270.Bd -literal -offset indent 1271$ ssh-keygen -r host.example.com. 1272.Ed 1273.Pp 1274The output lines will have to be added to the zonefile. 1275To check that the zone is answering fingerprint queries: 1276.Pp 1277.Dl $ dig -t SSHFP host.example.com 1278.Pp 1279Finally the client connects: 1280.Bd -literal -offset indent 1281$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1282[...] 1283Matching host key fingerprint found in DNS. 1284Are you sure you want to continue connecting (yes/no)? 1285.Ed 1286.Pp 1287See the 1288.Cm VerifyHostKeyDNS 1289option in 1290.Xr ssh_config 5 1291for more information. 1292.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1293.Nm 1294contains support for Virtual Private Network (VPN) tunnelling 1295using the 1296.Xr tun 4 1297network pseudo-device, 1298allowing two networks to be joined securely. 1299The 1300.Xr sshd_config 5 1301configuration option 1302.Cm PermitTunnel 1303controls whether the server supports this, 1304and at what level (layer 2 or 3 traffic). 1305.Pp 1306The following example would connect client network 10.0.50.0/24 1307with remote network 10.0.99.0/24 using a point-to-point connection 1308from 10.1.1.1 to 10.1.1.2, 1309provided that the SSH server running on the gateway to the remote network, 1310at 192.168.1.15, allows it. 1311.Pp 1312On the client: 1313.Bd -literal -offset indent 1314# ssh -f -w 0:1 192.168.1.15 true 1315# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1316# route add 10.0.99.0/24 10.1.1.2 1317.Ed 1318.Pp 1319On the server: 1320.Bd -literal -offset indent 1321# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1322# route add 10.0.50.0/24 10.1.1.1 1323.Ed 1324.Pp 1325Client access may be more finely tuned via the 1326.Pa /root/.ssh/authorized_keys 1327file (see below) and the 1328.Cm PermitRootLogin 1329server option. 1330The following entry would permit connections on 1331.Xr tun 4 1332device 1 from user 1333.Dq jane 1334and on tun device 2 from user 1335.Dq john , 1336if 1337.Cm PermitRootLogin 1338is set to 1339.Dq forced-commands-only : 1340.Bd -literal -offset 2n 1341tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1342tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1343.Ed 1344.Pp 1345Since an SSH-based setup entails a fair amount of overhead, 1346it may be more suited to temporary setups, 1347such as for wireless VPNs. 1348More permanent VPNs are better provided by tools such as 1349.Xr ipsecctl 8 1350and 1351.Xr isakmpd 8 . 1352.Sh ENVIRONMENT 1353.Nm 1354will normally set the following environment variables: 1355.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1356.It Ev DISPLAY 1357The 1358.Ev DISPLAY 1359variable indicates the location of the X11 server. 1360It is automatically set by 1361.Nm 1362to point to a value of the form 1363.Dq hostname:n , 1364where 1365.Dq hostname 1366indicates the host where the shell runs, and 1367.Sq n 1368is an integer \*(Ge 1. 1369.Nm 1370uses this special value to forward X11 connections over the secure 1371channel. 1372The user should normally not set 1373.Ev DISPLAY 1374explicitly, as that 1375will render the X11 connection insecure (and will require the user to 1376manually copy any required authorization cookies). 1377.It Ev HOME 1378Set to the path of the user's home directory. 1379.It Ev LOGNAME 1380Synonym for 1381.Ev USER ; 1382set for compatibility with systems that use this variable. 1383.It Ev MAIL 1384Set to the path of the user's mailbox. 1385.It Ev PATH 1386Set to the default 1387.Ev PATH , 1388as specified when compiling 1389.Nm . 1390.It Ev SSH_ASKPASS 1391If 1392.Nm 1393needs a passphrase, it will read the passphrase from the current 1394terminal if it was run from a terminal. 1395If 1396.Nm 1397does not have a terminal associated with it but 1398.Ev DISPLAY 1399and 1400.Ev SSH_ASKPASS 1401are set, it will execute the program specified by 1402.Ev SSH_ASKPASS 1403and open an X11 window to read the passphrase. 1404This is particularly useful when calling 1405.Nm 1406from a 1407.Pa .xsession 1408or related script. 1409(Note that on some machines it 1410may be necessary to redirect the input from 1411.Pa /dev/null 1412to make this work.) 1413.It Ev SSH_ASKPASS_REQUIRE 1414Allows further control over the use of an askpass program. 1415If this variable is set to 1416.Dq never 1417then 1418.Nm 1419will never attempt to use one. 1420If it is set to 1421.Dq prefer , 1422then 1423.Nm 1424will prefer to use the askpass program instead of the TTY when requesting 1425passwords. 1426Finally, if the variable is set to 1427.Dq force , 1428then the askpass program will be used for all passphrase input regardless 1429of whether 1430.Ev DISPLAY 1431is set. 1432.It Ev SSH_AUTH_SOCK 1433Identifies the path of a 1434.Ux Ns -domain 1435socket used to communicate with the agent. 1436.It Ev SSH_CONNECTION 1437Identifies the client and server ends of the connection. 1438The variable contains 1439four space-separated values: client IP address, client port number, 1440server IP address, and server port number. 1441.It Ev SSH_ORIGINAL_COMMAND 1442This variable contains the original command line if a forced command 1443is executed. 1444It can be used to extract the original arguments. 1445.It Ev SSH_TTY 1446This is set to the name of the tty (path to the device) associated 1447with the current shell or command. 1448If the current session has no tty, 1449this variable is not set. 1450.It Ev SSH_TUNNEL 1451Optionally set by 1452.Xr sshd 8 1453to contain the interface names assigned if tunnel forwarding was 1454requested by the client. 1455.It Ev SSH_USER_AUTH 1456Optionally set by 1457.Xr sshd 8 , 1458this variable may contain a pathname to a file that lists the authentication 1459methods successfully used when the session was established, including any 1460public keys that were used. 1461.It Ev TZ 1462This variable is set to indicate the present time zone if it 1463was set when the daemon was started (i.e. the daemon passes the value 1464on to new connections). 1465.It Ev USER 1466Set to the name of the user logging in. 1467.El 1468.Pp 1469Additionally, 1470.Nm 1471reads 1472.Pa ~/.ssh/environment , 1473and adds lines of the format 1474.Dq VARNAME=value 1475to the environment if the file exists and users are allowed to 1476change their environment. 1477For more information, see the 1478.Cm PermitUserEnvironment 1479option in 1480.Xr sshd_config 5 . 1481.Sh FILES 1482.Bl -tag -width Ds -compact 1483.It Pa ~/.rhosts 1484This file is used for host-based authentication (see above). 1485On some machines this file may need to be 1486world-readable if the user's home directory is on an NFS partition, 1487because 1488.Xr sshd 8 1489reads it as root. 1490Additionally, this file must be owned by the user, 1491and must not have write permissions for anyone else. 1492The recommended 1493permission for most machines is read/write for the user, and not 1494accessible by others. 1495.Pp 1496.It Pa ~/.shosts 1497This file is used in exactly the same way as 1498.Pa .rhosts , 1499but allows host-based authentication without permitting login with 1500rlogin/rsh. 1501.Pp 1502.It Pa ~/.ssh/ 1503This directory is the default location for all user-specific configuration 1504and authentication information. 1505There is no general requirement to keep the entire contents of this directory 1506secret, but the recommended permissions are read/write/execute for the user, 1507and not accessible by others. 1508.Pp 1509.It Pa ~/.ssh/authorized_keys 1510Lists the public keys (DSA, ECDSA, Ed25519, RSA) 1511that can be used for logging in as this user. 1512The format of this file is described in the 1513.Xr sshd 8 1514manual page. 1515This file is not highly sensitive, but the recommended 1516permissions are read/write for the user, and not accessible by others. 1517.Pp 1518.It Pa ~/.ssh/config 1519This is the per-user configuration file. 1520The file format and configuration options are described in 1521.Xr ssh_config 5 . 1522Because of the potential for abuse, this file must have strict permissions: 1523read/write for the user, and not writable by others. 1524.Pp 1525.It Pa ~/.ssh/environment 1526Contains additional definitions for environment variables; see 1527.Sx ENVIRONMENT , 1528above. 1529.Pp 1530.It Pa ~/.ssh/id_dsa 1531.It Pa ~/.ssh/id_ecdsa 1532.It Pa ~/.ssh/id_ecdsa_sk 1533.It Pa ~/.ssh/id_ed25519 1534.It Pa ~/.ssh/id_ed25519_sk 1535.It Pa ~/.ssh/id_rsa 1536Contains the private key for authentication. 1537These files 1538contain sensitive data and should be readable by the user but not 1539accessible by others (read/write/execute). 1540.Nm 1541will simply ignore a private key file if it is accessible by others. 1542It is possible to specify a passphrase when 1543generating the key which will be used to encrypt the 1544sensitive part of this file using AES-128. 1545.Pp 1546.It Pa ~/.ssh/id_dsa.pub 1547.It Pa ~/.ssh/id_ecdsa.pub 1548.It Pa ~/.ssh/id_ecdsa_sk.pub 1549.It Pa ~/.ssh/id_ed25519.pub 1550.It Pa ~/.ssh/id_ed25519_sk.pub 1551.It Pa ~/.ssh/id_rsa.pub 1552Contains the public key for authentication. 1553These files are not 1554sensitive and can (but need not) be readable by anyone. 1555.Pp 1556.It Pa ~/.ssh/known_hosts 1557Contains a list of host keys for all hosts the user has logged into 1558that are not already in the systemwide list of known host keys. 1559See 1560.Xr sshd 8 1561for further details of the format of this file. 1562.Pp 1563.It Pa ~/.ssh/rc 1564Commands in this file are executed by 1565.Nm 1566when the user logs in, just before the user's shell (or command) is 1567started. 1568See the 1569.Xr sshd 8 1570manual page for more information. 1571.Pp 1572.It Pa /etc/hosts.equiv 1573This file is for host-based authentication (see above). 1574It should only be writable by root. 1575.Pp 1576.It Pa /etc/shosts.equiv 1577This file is used in exactly the same way as 1578.Pa hosts.equiv , 1579but allows host-based authentication without permitting login with 1580rlogin/rsh. 1581.Pp 1582.It Pa /etc/ssh/ssh_config 1583Systemwide configuration file. 1584The file format and configuration options are described in 1585.Xr ssh_config 5 . 1586.Pp 1587.It Pa /etc/ssh/ssh_host_key 1588.It Pa /etc/ssh/ssh_host_dsa_key 1589.It Pa /etc/ssh/ssh_host_ecdsa_key 1590.It Pa /etc/ssh/ssh_host_ed25519_key 1591.It Pa /etc/ssh/ssh_host_rsa_key 1592These files contain the private parts of the host keys 1593and are used for host-based authentication. 1594.Pp 1595.It Pa /etc/ssh/ssh_known_hosts 1596Systemwide list of known host keys. 1597This file should be prepared by the 1598system administrator to contain the public host keys of all machines in the 1599organization. 1600It should be world-readable. 1601See 1602.Xr sshd 8 1603for further details of the format of this file. 1604.Pp 1605.It Pa /etc/ssh/sshrc 1606Commands in this file are executed by 1607.Nm 1608when the user logs in, just before the user's shell (or command) is started. 1609See the 1610.Xr sshd 8 1611manual page for more information. 1612.El 1613.Sh EXIT STATUS 1614.Nm 1615exits with the exit status of the remote command or with 255 1616if an error occurred. 1617.Sh SEE ALSO 1618.Xr scp 1 , 1619.Xr sftp 1 , 1620.Xr ssh-add 1 , 1621.Xr ssh-agent 1 , 1622.Xr ssh-keygen 1 , 1623.Xr ssh-keyscan 1 , 1624.Xr tun 4 , 1625.Xr ssh_config 5 , 1626.Xr ssh-keysign 8 , 1627.Xr sshd 8 1628.Sh STANDARDS 1629.Rs 1630.%A S. Lehtinen 1631.%A C. Lonvick 1632.%D January 2006 1633.%R RFC 4250 1634.%T The Secure Shell (SSH) Protocol Assigned Numbers 1635.Re 1636.Pp 1637.Rs 1638.%A T. Ylonen 1639.%A C. Lonvick 1640.%D January 2006 1641.%R RFC 4251 1642.%T The Secure Shell (SSH) Protocol Architecture 1643.Re 1644.Pp 1645.Rs 1646.%A T. Ylonen 1647.%A C. Lonvick 1648.%D January 2006 1649.%R RFC 4252 1650.%T The Secure Shell (SSH) Authentication Protocol 1651.Re 1652.Pp 1653.Rs 1654.%A T. Ylonen 1655.%A C. Lonvick 1656.%D January 2006 1657.%R RFC 4253 1658.%T The Secure Shell (SSH) Transport Layer Protocol 1659.Re 1660.Pp 1661.Rs 1662.%A T. Ylonen 1663.%A C. Lonvick 1664.%D January 2006 1665.%R RFC 4254 1666.%T The Secure Shell (SSH) Connection Protocol 1667.Re 1668.Pp 1669.Rs 1670.%A J. Schlyter 1671.%A W. Griffin 1672.%D January 2006 1673.%R RFC 4255 1674.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints 1675.Re 1676.Pp 1677.Rs 1678.%A F. Cusack 1679.%A M. Forssen 1680.%D January 2006 1681.%R RFC 4256 1682.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH) 1683.Re 1684.Pp 1685.Rs 1686.%A J. Galbraith 1687.%A P. Remaker 1688.%D January 2006 1689.%R RFC 4335 1690.%T The Secure Shell (SSH) Session Channel Break Extension 1691.Re 1692.Pp 1693.Rs 1694.%A M. Bellare 1695.%A T. Kohno 1696.%A C. Namprempre 1697.%D January 2006 1698.%R RFC 4344 1699.%T The Secure Shell (SSH) Transport Layer Encryption Modes 1700.Re 1701.Pp 1702.Rs 1703.%A B. Harris 1704.%D January 2006 1705.%R RFC 4345 1706.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol 1707.Re 1708.Pp 1709.Rs 1710.%A M. Friedl 1711.%A N. Provos 1712.%A W. Simpson 1713.%D March 2006 1714.%R RFC 4419 1715.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 1716.Re 1717.Pp 1718.Rs 1719.%A J. Galbraith 1720.%A R. Thayer 1721.%D November 2006 1722.%R RFC 4716 1723.%T The Secure Shell (SSH) Public Key File Format 1724.Re 1725.Pp 1726.Rs 1727.%A D. Stebila 1728.%A J. Green 1729.%D December 2009 1730.%R RFC 5656 1731.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer 1732.Re 1733.Pp 1734.Rs 1735.%A A. Perrig 1736.%A D. Song 1737.%D 1999 1738.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99) 1739.%T Hash Visualization: a New Technique to improve Real-World Security 1740.Re 1741.Sh AUTHORS 1742OpenSSH is a derivative of the original and free 1743ssh 1.2.12 release by Tatu Ylonen. 1744Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1745Theo de Raadt and Dug Song 1746removed many bugs, re-added newer features and 1747created OpenSSH. 1748Markus Friedl contributed the support for SSH 1749protocol versions 1.5 and 2.0. 1750