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 1.419 2021/02/15 20:43:15 markus Exp $ 37.Dd $Mdocdate: February 15 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 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 HostbasedAcceptedAlgorithms 513.It HostbasedAuthentication 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 PermitRemoteOpen 535.It PKCS11Provider 536.It Port 537.It PreferredAuthentications 538.It ProxyCommand 539.It ProxyJump 540.It ProxyUseFdpass 541.It PubkeyAcceptedAlgorithms 542.It PubkeyAuthentication 543.It RekeyLimit 544.It RemoteCommand 545.It RemoteForward 546.It RequestTTY 547.It SendEnv 548.It ServerAliveInterval 549.It ServerAliveCountMax 550.It SetEnv 551.It StreamLocalBindMask 552.It StreamLocalBindUnlink 553.It StrictHostKeyChecking 554.It TCPKeepAlive 555.It Tunnel 556.It TunnelDevice 557.It UpdateHostKeys 558.It User 559.It UserKnownHostsFile 560.It VerifyHostKeyDNS 561.It VisualHostKey 562.It XAuthLocation 563.El 564.Pp 565.It Fl p Ar port 566Port to connect to on the remote host. 567This can be specified on a 568per-host basis in the configuration file. 569.Pp 570.It Fl Q Ar query_option 571Queries 572.Nm 573for the algorithms supported for the specified version 2. 574The available features are: 575.Ar cipher 576(supported symmetric ciphers), 577.Ar cipher-auth 578(supported symmetric ciphers that support authenticated encryption), 579.Ar help 580(supported query terms for use with the 581.Fl Q 582flag), 583.Ar mac 584(supported message integrity codes), 585.Ar kex 586(key exchange algorithms), 587.Ar key 588(key types), 589.Ar key-cert 590(certificate key types), 591.Ar key-plain 592(non-certificate key types), 593.Ar key-sig 594(all key types and signature algorithms), 595.Ar protocol-version 596(supported SSH protocol versions), and 597.Ar sig 598(supported signature algorithms). 599Alternatively, any keyword from 600.Xr ssh_config 5 601or 602.Xr sshd_config 5 603that takes an algorithm list may be used as an alias for the corresponding 604query_option. 605.Pp 606.It Fl q 607Quiet mode. 608Causes most warning and diagnostic messages to be suppressed. 609.Pp 610.It Fl R Xo 611.Sm off 612.Oo Ar bind_address : Oc 613.Ar port : host : hostport 614.Sm on 615.Xc 616.It Fl R Xo 617.Sm off 618.Oo Ar bind_address : Oc 619.Ar port : local_socket 620.Sm on 621.Xc 622.It Fl R Xo 623.Sm off 624.Ar remote_socket : host : hostport 625.Sm on 626.Xc 627.It Fl R Xo 628.Sm off 629.Ar remote_socket : local_socket 630.Sm on 631.Xc 632.It Fl R Xo 633.Sm off 634.Oo Ar bind_address : Oc 635.Ar port 636.Sm on 637.Xc 638Specifies that connections to the given TCP port or Unix socket on the remote 639(server) host are to be forwarded to the local side. 640.Pp 641This works by allocating a socket to listen to either a TCP 642.Ar port 643or to a Unix socket on the remote side. 644Whenever a connection is made to this port or Unix socket, the 645connection is forwarded over the secure channel, and a connection 646is made from the local machine to either an explicit destination specified by 647.Ar host 648port 649.Ar hostport , 650or 651.Ar local_socket , 652or, if no explicit destination was specified, 653.Nm 654will act as a SOCKS 4/5 proxy and forward connections to the destinations 655requested by the remote SOCKS client. 656.Pp 657Port forwardings can also be specified in the configuration file. 658Privileged ports can be forwarded only when 659logging in as root on the remote machine. 660IPv6 addresses can be specified by enclosing the address in square brackets. 661.Pp 662By default, TCP listening sockets on the server will be bound to the loopback 663interface only. 664This may be overridden by specifying a 665.Ar bind_address . 666An empty 667.Ar bind_address , 668or the address 669.Ql * , 670indicates that the remote socket should listen on all interfaces. 671Specifying a remote 672.Ar bind_address 673will only succeed if the server's 674.Cm GatewayPorts 675option is enabled (see 676.Xr sshd_config 5 ) . 677.Pp 678If the 679.Ar port 680argument is 681.Ql 0 , 682the listen port will be dynamically allocated on the server and reported 683to the client at run time. 684When used together with 685.Ic -O forward 686the allocated port will be printed to the standard output. 687.Pp 688.It Fl S Ar ctl_path 689Specifies the location of a control socket for connection sharing, 690or the string 691.Dq none 692to disable connection sharing. 693Refer to the description of 694.Cm ControlPath 695and 696.Cm ControlMaster 697in 698.Xr ssh_config 5 699for details. 700.Pp 701.It Fl s 702May be used to request invocation of a subsystem on the remote system. 703Subsystems facilitate the use of SSH 704as a secure transport for other applications (e.g.\& 705.Xr sftp 1 ) . 706The subsystem is specified as the remote command. 707.Pp 708.It Fl T 709Disable pseudo-terminal allocation. 710.Pp 711.It Fl t 712Force pseudo-terminal allocation. 713This can be used to execute arbitrary 714screen-based programs on a remote machine, which can be very useful, 715e.g. when implementing menu services. 716Multiple 717.Fl t 718options force tty allocation, even if 719.Nm 720has no local tty. 721.Pp 722.It Fl V 723Display the version number and exit. 724.Pp 725.It Fl v 726Verbose mode. 727Causes 728.Nm 729to print debugging messages about its progress. 730This is helpful in 731debugging connection, authentication, and configuration problems. 732Multiple 733.Fl v 734options increase the verbosity. 735The maximum is 3. 736.Pp 737.It Fl W Ar host : Ns Ar port 738Requests that standard input and output on the client be forwarded to 739.Ar host 740on 741.Ar port 742over the secure channel. 743Implies 744.Fl N , 745.Fl T , 746.Cm ExitOnForwardFailure 747and 748.Cm ClearAllForwardings , 749though these can be overridden in the configuration file or using 750.Fl o 751command line options. 752.Pp 753.It Fl w Xo 754.Ar local_tun Ns Op : Ns Ar remote_tun 755.Xc 756Requests 757tunnel 758device forwarding with the specified 759.Xr tun 4 760devices between the client 761.Pq Ar local_tun 762and the server 763.Pq Ar remote_tun . 764.Pp 765The devices may be specified by numerical ID or the keyword 766.Dq any , 767which uses the next available tunnel device. 768If 769.Ar remote_tun 770is not specified, it defaults to 771.Dq any . 772See also the 773.Cm Tunnel 774and 775.Cm TunnelDevice 776directives in 777.Xr ssh_config 5 . 778.Pp 779If the 780.Cm Tunnel 781directive is unset, it will be set to the default tunnel mode, which is 782.Dq point-to-point . 783If a different 784.Cm Tunnel 785forwarding mode it desired, then it should be specified before 786.Fl w . 787.Pp 788.It Fl X 789Enables X11 forwarding. 790This can also be specified on a per-host basis in a configuration file. 791.Pp 792X11 forwarding should be enabled with caution. 793Users with the ability to bypass file permissions on the remote host 794(for the user's X authorization database) 795can access the local X11 display through the forwarded connection. 796An attacker may then be able to perform activities such as keystroke monitoring. 797.Pp 798For this reason, X11 forwarding is subjected to X11 SECURITY extension 799restrictions by default. 800Please refer to the 801.Nm 802.Fl Y 803option and the 804.Cm ForwardX11Trusted 805directive in 806.Xr ssh_config 5 807for more information. 808.Pp 809.It Fl x 810Disables X11 forwarding. 811.Pp 812.It Fl Y 813Enables trusted X11 forwarding. 814Trusted X11 forwardings are not subjected to the X11 SECURITY extension 815controls. 816.Pp 817.It Fl y 818Send log information using the 819.Xr syslog 3 820system module. 821By default this information is sent to stderr. 822.El 823.Pp 824.Nm 825may additionally obtain configuration data from 826a per-user configuration file and a system-wide configuration file. 827The file format and configuration options are described in 828.Xr ssh_config 5 . 829.Sh AUTHENTICATION 830The OpenSSH SSH client supports SSH protocol 2. 831.Pp 832The methods available for authentication are: 833GSSAPI-based authentication, 834host-based authentication, 835public key authentication, 836challenge-response authentication, 837and password authentication. 838Authentication methods are tried in the order specified above, 839though 840.Cm PreferredAuthentications 841can be used to change the default order. 842.Pp 843Host-based authentication works as follows: 844If the machine the user logs in from is listed in 845.Pa /etc/hosts.equiv 846or 847.Pa /etc/shosts.equiv 848on the remote machine, the user is non-root and the user names are 849the same on both sides, or if the files 850.Pa ~/.rhosts 851or 852.Pa ~/.shosts 853exist in the user's home directory on the 854remote machine and contain a line containing the name of the client 855machine and the name of the user on that machine, the user is 856considered for login. 857Additionally, the server 858.Em must 859be able to verify the client's 860host key (see the description of 861.Pa /etc/ssh/ssh_known_hosts 862and 863.Pa ~/.ssh/known_hosts , 864below) 865for login to be permitted. 866This authentication method closes security holes due to IP 867spoofing, DNS spoofing, and routing spoofing. 868[Note to the administrator: 869.Pa /etc/hosts.equiv , 870.Pa ~/.rhosts , 871and the rlogin/rsh protocol in general, are inherently insecure and should be 872disabled if security is desired.] 873.Pp 874Public key authentication works as follows: 875The scheme is based on public-key cryptography, 876using cryptosystems 877where encryption and decryption are done using separate keys, 878and it is unfeasible to derive the decryption key from the encryption key. 879The idea is that each user creates a public/private 880key pair for authentication purposes. 881The server knows the public key, and only the user knows the private key. 882.Nm 883implements public key authentication protocol automatically, 884using one of the DSA, ECDSA, Ed25519 or RSA algorithms. 885The HISTORY section of 886.Xr ssl 8 887contains a brief discussion of the DSA and RSA algorithms. 888.Pp 889The file 890.Pa ~/.ssh/authorized_keys 891lists the public keys that are permitted for logging in. 892When the user logs in, the 893.Nm 894program tells the server which key pair it would like to use for 895authentication. 896The client proves that it has access to the private key 897and the server checks that the corresponding public key 898is authorized to accept the account. 899.Pp 900The server may inform the client of errors that prevented public key 901authentication from succeeding after authentication completes using a 902different method. 903These may be viewed by increasing the 904.Cm LogLevel 905to 906.Cm DEBUG 907or higher (e.g. by using the 908.Fl v 909flag). 910.Pp 911The user creates his/her key pair by running 912.Xr ssh-keygen 1 . 913This stores the private key in 914.Pa ~/.ssh/id_dsa 915(DSA), 916.Pa ~/.ssh/id_ecdsa 917(ECDSA), 918.Pa ~/.ssh/id_ecdsa_sk 919(authenticator-hosted ECDSA), 920.Pa ~/.ssh/id_ed25519 921(Ed25519), 922.Pa ~/.ssh/id_ed25519_sk 923(authenticator-hosted Ed25519), 924or 925.Pa ~/.ssh/id_rsa 926(RSA) 927and stores the public key in 928.Pa ~/.ssh/id_dsa.pub 929(DSA), 930.Pa ~/.ssh/id_ecdsa.pub 931(ECDSA), 932.Pa ~/.ssh/id_ecdsa_sk.pub 933(authenticator-hosted ECDSA), 934.Pa ~/.ssh/id_ed25519.pub 935(Ed25519), 936.Pa ~/.ssh/id_ed25519_sk.pub 937(authenticator-hosted Ed25519), 938or 939.Pa ~/.ssh/id_rsa.pub 940(RSA) 941in the user's home directory. 942The user should then copy the public key 943to 944.Pa ~/.ssh/authorized_keys 945in his/her home directory on the remote machine. 946The 947.Pa authorized_keys 948file corresponds to the conventional 949.Pa ~/.rhosts 950file, and has one key 951per line, though the lines can be very long. 952After this, the user can log in without giving the password. 953.Pp 954A variation on public key authentication 955is available in the form of certificate authentication: 956instead of a set of public/private keys, 957signed certificates are used. 958This has the advantage that a single trusted certification authority 959can be used in place of many public/private keys. 960See the CERTIFICATES section of 961.Xr ssh-keygen 1 962for more information. 963.Pp 964The most convenient way to use public key or certificate authentication 965may be with an authentication agent. 966See 967.Xr ssh-agent 1 968and (optionally) the 969.Cm AddKeysToAgent 970directive in 971.Xr ssh_config 5 972for more information. 973.Pp 974Challenge-response authentication works as follows: 975The server sends an arbitrary 976.Qq challenge 977text, and prompts for a response. 978Examples of challenge-response authentication include 979.Bx 980Authentication (see 981.Xr login.conf 5 ) 982and PAM (some 983.Pf non- Ox 984systems). 985.Pp 986Finally, if other authentication methods fail, 987.Nm 988prompts the user for a password. 989The password is sent to the remote 990host for checking; however, since all communications are encrypted, 991the password cannot be seen by someone listening on the network. 992.Pp 993.Nm 994automatically maintains and checks a database containing 995identification for all hosts it has ever been used with. 996Host keys are stored in 997.Pa ~/.ssh/known_hosts 998in the user's home directory. 999Additionally, the file 1000.Pa /etc/ssh/ssh_known_hosts 1001is automatically checked for known hosts. 1002Any new hosts are automatically added to the user's file. 1003If a host's identification ever changes, 1004.Nm 1005warns about this and disables password authentication to prevent 1006server spoofing or man-in-the-middle attacks, 1007which could otherwise be used to circumvent the encryption. 1008The 1009.Cm StrictHostKeyChecking 1010option can be used to control logins to machines whose 1011host key is not known or has changed. 1012.Pp 1013When the user's identity has been accepted by the server, the server 1014either executes the given command in a non-interactive session or, 1015if no command has been specified, logs into the machine and gives 1016the user a normal shell as an interactive session. 1017All communication with 1018the remote command or shell will be automatically encrypted. 1019.Pp 1020If an interactive session is requested 1021.Nm 1022by default will only request a pseudo-terminal (pty) for interactive 1023sessions when the client has one. 1024The flags 1025.Fl T 1026and 1027.Fl t 1028can be used to override this behaviour. 1029.Pp 1030If a pseudo-terminal has been allocated the 1031user may use the escape characters noted below. 1032.Pp 1033If no pseudo-terminal has been allocated, 1034the session is transparent and can be used to reliably transfer binary data. 1035On most systems, setting the escape character to 1036.Dq none 1037will also make the session transparent even if a tty is used. 1038.Pp 1039The session terminates when the command or shell on the remote 1040machine exits and all X11 and TCP connections have been closed. 1041.Sh ESCAPE CHARACTERS 1042When a pseudo-terminal has been requested, 1043.Nm 1044supports a number of functions through the use of an escape character. 1045.Pp 1046A single tilde character can be sent as 1047.Ic ~~ 1048or by following the tilde by a character other than those described below. 1049The escape character must always follow a newline to be interpreted as 1050special. 1051The escape character can be changed in configuration files using the 1052.Cm EscapeChar 1053configuration directive or on the command line by the 1054.Fl e 1055option. 1056.Pp 1057The supported escapes (assuming the default 1058.Ql ~ ) 1059are: 1060.Bl -tag -width Ds 1061.It Cm ~. 1062Disconnect. 1063.It Cm ~^Z 1064Background 1065.Nm . 1066.It Cm ~# 1067List forwarded connections. 1068.It Cm ~& 1069Background 1070.Nm 1071at logout when waiting for forwarded connection / X11 sessions to terminate. 1072.It Cm ~? 1073Display a list of escape characters. 1074.It Cm ~B 1075Send a BREAK to the remote system 1076(only useful if the peer supports it). 1077.It Cm ~C 1078Open command line. 1079Currently this allows the addition of port forwardings using the 1080.Fl L , 1081.Fl R 1082and 1083.Fl D 1084options (see above). 1085It also allows the cancellation of existing port-forwardings 1086with 1087.Sm off 1088.Fl KL Oo Ar bind_address : Oc Ar port 1089.Sm on 1090for local, 1091.Sm off 1092.Fl KR Oo Ar bind_address : Oc Ar port 1093.Sm on 1094for remote and 1095.Sm off 1096.Fl KD Oo Ar bind_address : Oc Ar port 1097.Sm on 1098for dynamic port-forwardings. 1099.Ic !\& Ns Ar command 1100allows the user to execute a local command if the 1101.Ic PermitLocalCommand 1102option is enabled in 1103.Xr ssh_config 5 . 1104Basic help is available, using the 1105.Fl h 1106option. 1107.It Cm ~R 1108Request rekeying of the connection 1109(only useful if the peer supports it). 1110.It Cm ~V 1111Decrease the verbosity 1112.Pq Ic LogLevel 1113when errors are being written to stderr. 1114.It Cm ~v 1115Increase the verbosity 1116.Pq Ic LogLevel 1117when errors are being written to stderr. 1118.El 1119.Sh TCP FORWARDING 1120Forwarding of arbitrary TCP connections over a secure channel 1121can be specified either on the command line or in a configuration file. 1122One possible application of TCP forwarding is a secure connection to a 1123mail server; another is going through firewalls. 1124.Pp 1125In the example below, we look at encrypting communication for an IRC client, 1126even though the IRC server it connects to does not directly 1127support encrypted communication. 1128This works as follows: 1129the user connects to the remote host using 1130.Nm , 1131specifying the ports to be used to forward the connection. 1132After that it is possible to start the program locally, 1133and 1134.Nm 1135will encrypt and forward the connection to the remote server. 1136.Pp 1137The following example tunnels an IRC session from the client 1138to an IRC server at 1139.Dq server.example.com , 1140joining channel 1141.Dq #users , 1142nickname 1143.Dq pinky , 1144using the standard IRC port, 6667: 1145.Bd -literal -offset 4n 1146$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10 1147$ irc -c '#users' pinky IRC/127.0.0.1 1148.Ed 1149.Pp 1150The 1151.Fl f 1152option backgrounds 1153.Nm 1154and the remote command 1155.Dq sleep 10 1156is specified to allow an amount of time 1157(10 seconds, in the example) 1158to start the program which is going to use the tunnel. 1159If no connections are made within the time specified, 1160.Nm 1161will exit. 1162.Sh X11 FORWARDING 1163If the 1164.Cm ForwardX11 1165variable is set to 1166.Dq yes 1167(or see the description of the 1168.Fl X , 1169.Fl x , 1170and 1171.Fl Y 1172options above) 1173and the user is using X11 (the 1174.Ev DISPLAY 1175environment variable is set), the connection to the X11 display is 1176automatically forwarded to the remote side in such a way that any X11 1177programs started from the shell (or command) will go through the 1178encrypted channel, and the connection to the real X server will be made 1179from the local machine. 1180The user should not manually set 1181.Ev DISPLAY . 1182Forwarding of X11 connections can be 1183configured on the command line or in configuration files. 1184.Pp 1185The 1186.Ev DISPLAY 1187value set by 1188.Nm 1189will point to the server machine, but with a display number greater than zero. 1190This is normal, and happens because 1191.Nm 1192creates a 1193.Dq proxy 1194X server on the server machine for forwarding the 1195connections over the encrypted channel. 1196.Pp 1197.Nm 1198will also automatically set up Xauthority data on the server machine. 1199For this purpose, it will generate a random authorization cookie, 1200store it in Xauthority on the server, and verify that any forwarded 1201connections carry this cookie and replace it by the real cookie when 1202the connection is opened. 1203The real authentication cookie is never 1204sent to the server machine (and no cookies are sent in the plain). 1205.Pp 1206If the 1207.Cm ForwardAgent 1208variable is set to 1209.Dq yes 1210(or see the description of the 1211.Fl A 1212and 1213.Fl a 1214options above) and 1215the user is using an authentication agent, the connection to the agent 1216is automatically forwarded to the remote side. 1217.Sh VERIFYING HOST KEYS 1218When connecting to a server for the first time, 1219a fingerprint of the server's public key is presented to the user 1220(unless the option 1221.Cm StrictHostKeyChecking 1222has been disabled). 1223Fingerprints can be determined using 1224.Xr ssh-keygen 1 : 1225.Pp 1226.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1227.Pp 1228If the fingerprint is already known, it can be matched 1229and the key can be accepted or rejected. 1230If only legacy (MD5) fingerprints for the server are available, the 1231.Xr ssh-keygen 1 1232.Fl E 1233option may be used to downgrade the fingerprint algorithm to match. 1234.Pp 1235Because of the difficulty of comparing host keys 1236just by looking at fingerprint strings, 1237there is also support to compare host keys visually, 1238using 1239.Em random art . 1240By setting the 1241.Cm VisualHostKey 1242option to 1243.Dq yes , 1244a small ASCII graphic gets displayed on every login to a server, no matter 1245if the session itself is interactive or not. 1246By learning the pattern a known server produces, a user can easily 1247find out that the host key has changed when a completely different pattern 1248is displayed. 1249Because these patterns are not unambiguous however, a pattern that looks 1250similar to the pattern remembered only gives a good probability that the 1251host key is the same, not guaranteed proof. 1252.Pp 1253To get a listing of the fingerprints along with their random art for 1254all known hosts, the following command line can be used: 1255.Pp 1256.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1257.Pp 1258If the fingerprint is unknown, 1259an alternative method of verification is available: 1260SSH fingerprints verified by DNS. 1261An additional resource record (RR), 1262SSHFP, 1263is added to a zonefile 1264and the connecting client is able to match the fingerprint 1265with that of the key presented. 1266.Pp 1267In this example, we are connecting a client to a server, 1268.Dq host.example.com . 1269The SSHFP resource records should first be added to the zonefile for 1270host.example.com: 1271.Bd -literal -offset indent 1272$ ssh-keygen -r host.example.com. 1273.Ed 1274.Pp 1275The output lines will have to be added to the zonefile. 1276To check that the zone is answering fingerprint queries: 1277.Pp 1278.Dl $ dig -t SSHFP host.example.com 1279.Pp 1280Finally the client connects: 1281.Bd -literal -offset indent 1282$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1283[...] 1284Matching host key fingerprint found in DNS. 1285Are you sure you want to continue connecting (yes/no)? 1286.Ed 1287.Pp 1288See the 1289.Cm VerifyHostKeyDNS 1290option in 1291.Xr ssh_config 5 1292for more information. 1293.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1294.Nm 1295contains support for Virtual Private Network (VPN) tunnelling 1296using the 1297.Xr tun 4 1298network pseudo-device, 1299allowing two networks to be joined securely. 1300The 1301.Xr sshd_config 5 1302configuration option 1303.Cm PermitTunnel 1304controls whether the server supports this, 1305and at what level (layer 2 or 3 traffic). 1306.Pp 1307The following example would connect client network 10.0.50.0/24 1308with remote network 10.0.99.0/24 using a point-to-point connection 1309from 10.1.1.1 to 10.1.1.2, 1310provided that the SSH server running on the gateway to the remote network, 1311at 192.168.1.15, allows it. 1312.Pp 1313On the client: 1314.Bd -literal -offset indent 1315# ssh -f -w 0:1 192.168.1.15 true 1316# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1317# route add 10.0.99.0/24 10.1.1.2 1318.Ed 1319.Pp 1320On the server: 1321.Bd -literal -offset indent 1322# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1323# route add 10.0.50.0/24 10.1.1.1 1324.Ed 1325.Pp 1326Client access may be more finely tuned via the 1327.Pa /root/.ssh/authorized_keys 1328file (see below) and the 1329.Cm PermitRootLogin 1330server option. 1331The following entry would permit connections on 1332.Xr tun 4 1333device 1 from user 1334.Dq jane 1335and on tun device 2 from user 1336.Dq john , 1337if 1338.Cm PermitRootLogin 1339is set to 1340.Dq forced-commands-only : 1341.Bd -literal -offset 2n 1342tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1343tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1344.Ed 1345.Pp 1346Since an SSH-based setup entails a fair amount of overhead, 1347it may be more suited to temporary setups, 1348such as for wireless VPNs. 1349More permanent VPNs are better provided by tools such as 1350.Xr ipsecctl 8 1351and 1352.Xr isakmpd 8 . 1353.Sh ENVIRONMENT 1354.Nm 1355will normally set the following environment variables: 1356.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1357.It Ev DISPLAY 1358The 1359.Ev DISPLAY 1360variable indicates the location of the X11 server. 1361It is automatically set by 1362.Nm 1363to point to a value of the form 1364.Dq hostname:n , 1365where 1366.Dq hostname 1367indicates the host where the shell runs, and 1368.Sq n 1369is an integer \*(Ge 1. 1370.Nm 1371uses this special value to forward X11 connections over the secure 1372channel. 1373The user should normally not set 1374.Ev DISPLAY 1375explicitly, as that 1376will render the X11 connection insecure (and will require the user to 1377manually copy any required authorization cookies). 1378.It Ev HOME 1379Set to the path of the user's home directory. 1380.It Ev LOGNAME 1381Synonym for 1382.Ev USER ; 1383set for compatibility with systems that use this variable. 1384.It Ev MAIL 1385Set to the path of the user's mailbox. 1386.It Ev PATH 1387Set to the default 1388.Ev PATH , 1389as specified when compiling 1390.Nm . 1391.It Ev SSH_ASKPASS 1392If 1393.Nm 1394needs a passphrase, it will read the passphrase from the current 1395terminal if it was run from a terminal. 1396If 1397.Nm 1398does not have a terminal associated with it but 1399.Ev DISPLAY 1400and 1401.Ev SSH_ASKPASS 1402are set, it will execute the program specified by 1403.Ev SSH_ASKPASS 1404and open an X11 window to read the passphrase. 1405This is particularly useful when calling 1406.Nm 1407from a 1408.Pa .xsession 1409or related script. 1410(Note that on some machines it 1411may be necessary to redirect the input from 1412.Pa /dev/null 1413to make this work.) 1414.It Ev SSH_ASKPASS_REQUIRE 1415Allows further control over the use of an askpass program. 1416If this variable is set to 1417.Dq never 1418then 1419.Nm 1420will never attempt to use one. 1421If it is set to 1422.Dq prefer , 1423then 1424.Nm 1425will prefer to use the askpass program instead of the TTY when requesting 1426passwords. 1427Finally, if the variable is set to 1428.Dq force , 1429then the askpass program will be used for all passphrase input regardless 1430of whether 1431.Ev DISPLAY 1432is set. 1433.It Ev SSH_AUTH_SOCK 1434Identifies the path of a 1435.Ux Ns -domain 1436socket used to communicate with the agent. 1437.It Ev SSH_CONNECTION 1438Identifies the client and server ends of the connection. 1439The variable contains 1440four space-separated values: client IP address, client port number, 1441server IP address, and server port number. 1442.It Ev SSH_ORIGINAL_COMMAND 1443This variable contains the original command line if a forced command 1444is executed. 1445It can be used to extract the original arguments. 1446.It Ev SSH_TTY 1447This is set to the name of the tty (path to the device) associated 1448with the current shell or command. 1449If the current session has no tty, 1450this variable is not set. 1451.It Ev SSH_TUNNEL 1452Optionally set by 1453.Xr sshd 8 1454to contain the interface names assigned if tunnel forwarding was 1455requested by the client. 1456.It Ev SSH_USER_AUTH 1457Optionally set by 1458.Xr sshd 8 , 1459this variable may contain a pathname to a file that lists the authentication 1460methods successfully used when the session was established, including any 1461public keys that were used. 1462.It Ev TZ 1463This variable is set to indicate the present time zone if it 1464was set when the daemon was started (i.e. the daemon passes the value 1465on to new connections). 1466.It Ev USER 1467Set to the name of the user logging in. 1468.El 1469.Pp 1470Additionally, 1471.Nm 1472reads 1473.Pa ~/.ssh/environment , 1474and adds lines of the format 1475.Dq VARNAME=value 1476to the environment if the file exists and users are allowed to 1477change their environment. 1478For more information, see the 1479.Cm PermitUserEnvironment 1480option in 1481.Xr sshd_config 5 . 1482.Sh FILES 1483.Bl -tag -width Ds -compact 1484.It Pa ~/.rhosts 1485This file is used for host-based authentication (see above). 1486On some machines this file may need to be 1487world-readable if the user's home directory is on an NFS partition, 1488because 1489.Xr sshd 8 1490reads it as root. 1491Additionally, this file must be owned by the user, 1492and must not have write permissions for anyone else. 1493The recommended 1494permission for most machines is read/write for the user, and not 1495accessible by others. 1496.Pp 1497.It Pa ~/.shosts 1498This file is used in exactly the same way as 1499.Pa .rhosts , 1500but allows host-based authentication without permitting login with 1501rlogin/rsh. 1502.Pp 1503.It Pa ~/.ssh/ 1504This directory is the default location for all user-specific configuration 1505and authentication information. 1506There is no general requirement to keep the entire contents of this directory 1507secret, but the recommended permissions are read/write/execute for the user, 1508and not accessible by others. 1509.Pp 1510.It Pa ~/.ssh/authorized_keys 1511Lists the public keys (DSA, ECDSA, Ed25519, RSA) 1512that can be used for logging in as this user. 1513The format of this file is described in the 1514.Xr sshd 8 1515manual page. 1516This file is not highly sensitive, but the recommended 1517permissions are read/write for the user, and not accessible by others. 1518.Pp 1519.It Pa ~/.ssh/config 1520This is the per-user configuration file. 1521The file format and configuration options are described in 1522.Xr ssh_config 5 . 1523Because of the potential for abuse, this file must have strict permissions: 1524read/write for the user, and not writable by others. 1525.Pp 1526.It Pa ~/.ssh/environment 1527Contains additional definitions for environment variables; see 1528.Sx ENVIRONMENT , 1529above. 1530.Pp 1531.It Pa ~/.ssh/id_dsa 1532.It Pa ~/.ssh/id_ecdsa 1533.It Pa ~/.ssh/id_ecdsa_sk 1534.It Pa ~/.ssh/id_ed25519 1535.It Pa ~/.ssh/id_ed25519_sk 1536.It Pa ~/.ssh/id_rsa 1537Contains the private key for authentication. 1538These files 1539contain sensitive data and should be readable by the user but not 1540accessible by others (read/write/execute). 1541.Nm 1542will simply ignore a private key file if it is accessible by others. 1543It is possible to specify a passphrase when 1544generating the key which will be used to encrypt the 1545sensitive part of this file using AES-128. 1546.Pp 1547.It Pa ~/.ssh/id_dsa.pub 1548.It Pa ~/.ssh/id_ecdsa.pub 1549.It Pa ~/.ssh/id_ecdsa_sk.pub 1550.It Pa ~/.ssh/id_ed25519.pub 1551.It Pa ~/.ssh/id_ed25519_sk.pub 1552.It Pa ~/.ssh/id_rsa.pub 1553Contains the public key for authentication. 1554These files are not 1555sensitive and can (but need not) be readable by anyone. 1556.Pp 1557.It Pa ~/.ssh/known_hosts 1558Contains a list of host keys for all hosts the user has logged into 1559that are not already in the systemwide list of known host keys. 1560See 1561.Xr sshd 8 1562for further details of the format of this file. 1563.Pp 1564.It Pa ~/.ssh/rc 1565Commands in this file are executed by 1566.Nm 1567when the user logs in, just before the user's shell (or command) is 1568started. 1569See the 1570.Xr sshd 8 1571manual page for more information. 1572.Pp 1573.It Pa /etc/hosts.equiv 1574This file is for host-based authentication (see above). 1575It should only be writable by root. 1576.Pp 1577.It Pa /etc/shosts.equiv 1578This file is used in exactly the same way as 1579.Pa hosts.equiv , 1580but allows host-based authentication without permitting login with 1581rlogin/rsh. 1582.Pp 1583.It Pa /etc/ssh/ssh_config 1584Systemwide configuration file. 1585The file format and configuration options are described in 1586.Xr ssh_config 5 . 1587.Pp 1588.It Pa /etc/ssh/ssh_host_key 1589.It Pa /etc/ssh/ssh_host_dsa_key 1590.It Pa /etc/ssh/ssh_host_ecdsa_key 1591.It Pa /etc/ssh/ssh_host_ed25519_key 1592.It Pa /etc/ssh/ssh_host_rsa_key 1593These files contain the private parts of the host keys 1594and are used for host-based authentication. 1595.Pp 1596.It Pa /etc/ssh/ssh_known_hosts 1597Systemwide list of known host keys. 1598This file should be prepared by the 1599system administrator to contain the public host keys of all machines in the 1600organization. 1601It should be world-readable. 1602See 1603.Xr sshd 8 1604for further details of the format of this file. 1605.Pp 1606.It Pa /etc/ssh/sshrc 1607Commands in this file are executed by 1608.Nm 1609when the user logs in, just before the user's shell (or command) is started. 1610See the 1611.Xr sshd 8 1612manual page for more information. 1613.El 1614.Sh EXIT STATUS 1615.Nm 1616exits with the exit status of the remote command or with 255 1617if an error occurred. 1618.Sh SEE ALSO 1619.Xr scp 1 , 1620.Xr sftp 1 , 1621.Xr ssh-add 1 , 1622.Xr ssh-agent 1 , 1623.Xr ssh-keygen 1 , 1624.Xr ssh-keyscan 1 , 1625.Xr tun 4 , 1626.Xr ssh_config 5 , 1627.Xr ssh-keysign 8 , 1628.Xr sshd 8 1629.Sh STANDARDS 1630.Rs 1631.%A S. Lehtinen 1632.%A C. Lonvick 1633.%D January 2006 1634.%R RFC 4250 1635.%T The Secure Shell (SSH) Protocol Assigned Numbers 1636.Re 1637.Pp 1638.Rs 1639.%A T. Ylonen 1640.%A C. Lonvick 1641.%D January 2006 1642.%R RFC 4251 1643.%T The Secure Shell (SSH) Protocol Architecture 1644.Re 1645.Pp 1646.Rs 1647.%A T. Ylonen 1648.%A C. Lonvick 1649.%D January 2006 1650.%R RFC 4252 1651.%T The Secure Shell (SSH) Authentication Protocol 1652.Re 1653.Pp 1654.Rs 1655.%A T. Ylonen 1656.%A C. Lonvick 1657.%D January 2006 1658.%R RFC 4253 1659.%T The Secure Shell (SSH) Transport Layer Protocol 1660.Re 1661.Pp 1662.Rs 1663.%A T. Ylonen 1664.%A C. Lonvick 1665.%D January 2006 1666.%R RFC 4254 1667.%T The Secure Shell (SSH) Connection Protocol 1668.Re 1669.Pp 1670.Rs 1671.%A J. Schlyter 1672.%A W. Griffin 1673.%D January 2006 1674.%R RFC 4255 1675.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints 1676.Re 1677.Pp 1678.Rs 1679.%A F. Cusack 1680.%A M. Forssen 1681.%D January 2006 1682.%R RFC 4256 1683.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH) 1684.Re 1685.Pp 1686.Rs 1687.%A J. Galbraith 1688.%A P. Remaker 1689.%D January 2006 1690.%R RFC 4335 1691.%T The Secure Shell (SSH) Session Channel Break Extension 1692.Re 1693.Pp 1694.Rs 1695.%A M. Bellare 1696.%A T. Kohno 1697.%A C. Namprempre 1698.%D January 2006 1699.%R RFC 4344 1700.%T The Secure Shell (SSH) Transport Layer Encryption Modes 1701.Re 1702.Pp 1703.Rs 1704.%A B. Harris 1705.%D January 2006 1706.%R RFC 4345 1707.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol 1708.Re 1709.Pp 1710.Rs 1711.%A M. Friedl 1712.%A N. Provos 1713.%A W. Simpson 1714.%D March 2006 1715.%R RFC 4419 1716.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 1717.Re 1718.Pp 1719.Rs 1720.%A J. Galbraith 1721.%A R. Thayer 1722.%D November 2006 1723.%R RFC 4716 1724.%T The Secure Shell (SSH) Public Key File Format 1725.Re 1726.Pp 1727.Rs 1728.%A D. Stebila 1729.%A J. Green 1730.%D December 2009 1731.%R RFC 5656 1732.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer 1733.Re 1734.Pp 1735.Rs 1736.%A A. Perrig 1737.%A D. Song 1738.%D 1999 1739.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99) 1740.%T Hash Visualization: a New Technique to improve Real-World Security 1741.Re 1742.Sh AUTHORS 1743OpenSSH is a derivative of the original and free 1744ssh 1.2.12 release by Tatu Ylonen. 1745Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1746Theo de Raadt and Dug Song 1747removed many bugs, re-added newer features and 1748created OpenSSH. 1749Markus Friedl contributed the support for SSH 1750protocol versions 1.5 and 2.0. 1751