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 1.392 2018/04/06 03:34:27 djm Exp $ 37.Dd $Mdocdate: April 6 2018 $ 38.Dt SSH 1 39.Os 40.Sh NAME 41.Nm ssh 42.Nd OpenSSH SSH client (remote login program) 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 the authentication agent connection. 114This can also be specified on a per-host basis in a configuration file. 115.Pp 116Agent forwarding should be enabled with caution. 117Users with the ability to bypass file permissions on the remote host 118(for the agent's 119.Ux Ns -domain 120socket) can access the local agent through the forwarded connection. 121An attacker cannot obtain key material from the agent, 122however they can perform operations on the keys that enable them to 123authenticate using the identities loaded into the agent. 124.Pp 125.It Fl a 126Disables forwarding of the authentication agent connection. 127.Pp 128.It Fl B Ar bind_interface 129Bind to the address of 130.Ar bind_interface 131before attempting to connect to the destination host. 132This is only useful on systems with more than one address. 133.Pp 134.It Fl b Ar bind_address 135Use 136.Ar bind_address 137on the local machine as the source address 138of the connection. 139Only useful on systems with more than one address. 140.Pp 141.It Fl C 142Requests compression of all data (including stdin, stdout, stderr, and 143data for forwarded X11, TCP and 144.Ux Ns -domain 145connections). 146The compression algorithm is the same used by 147.Xr gzip 1 . 148Compression is desirable on modem lines and other 149slow connections, but will only slow down things on fast networks. 150The default value can be set on a host-by-host basis in the 151configuration files; see the 152.Cm Compression 153option. 154.Pp 155.It Fl c Ar cipher_spec 156Selects the cipher specification for encrypting the session. 157.Ar cipher_spec 158is a comma-separated list of ciphers 159listed in order of preference. 160See the 161.Cm Ciphers 162keyword in 163.Xr ssh_config 5 164for more information. 165.Pp 166.It Fl D Xo 167.Sm off 168.Oo Ar bind_address : Oc 169.Ar port 170.Sm on 171.Xc 172Specifies a local 173.Dq dynamic 174application-level port forwarding. 175This works by allocating a socket to listen to 176.Ar port 177on the local side, optionally bound to the specified 178.Ar bind_address . 179Whenever a connection is made to this port, the 180connection is forwarded over the secure channel, and the application 181protocol is then used to determine where to connect to from the 182remote machine. 183Currently the SOCKS4 and SOCKS5 protocols are supported, and 184.Nm 185will act as a SOCKS server. 186Only root can forward privileged ports. 187Dynamic port forwardings can also be specified in the configuration file. 188.Pp 189IPv6 addresses can be specified by enclosing the address in square brackets. 190Only the superuser can forward privileged ports. 191By default, the local port is bound in accordance with the 192.Cm GatewayPorts 193setting. 194However, an explicit 195.Ar bind_address 196may be used to bind the connection to a specific address. 197The 198.Ar bind_address 199of 200.Dq localhost 201indicates that the listening port be bound for local use only, while an 202empty address or 203.Sq * 204indicates that the port should be available from all interfaces. 205.Pp 206.It Fl E Ar log_file 207Append debug logs to 208.Ar log_file 209instead of standard error. 210.Pp 211.It Fl e Ar escape_char 212Sets the escape character for sessions with a pty (default: 213.Ql ~ ) . 214The escape character is only recognized at the beginning of a line. 215The escape character followed by a dot 216.Pq Ql \&. 217closes the connection; 218followed by control-Z suspends the connection; 219and followed by itself sends the escape character once. 220Setting the character to 221.Dq none 222disables any escapes and makes the session fully transparent. 223.Pp 224.It Fl F Ar configfile 225Specifies an alternative per-user configuration file. 226If a configuration file is given on the command line, 227the system-wide configuration file 228.Pq Pa /etc/ssh/ssh_config 229will be ignored. 230The default for the per-user configuration file is 231.Pa ~/.ssh/config . 232.Pp 233.It Fl f 234Requests 235.Nm 236to go to background just before command execution. 237This is useful if 238.Nm 239is going to ask for passwords or passphrases, but the user 240wants it in the background. 241This implies 242.Fl n . 243The recommended way to start X11 programs at a remote site is with 244something like 245.Ic ssh -f host xterm . 246.Pp 247If the 248.Cm ExitOnForwardFailure 249configuration option is set to 250.Dq yes , 251then a client started with 252.Fl f 253will wait for all remote port forwards to be successfully established 254before placing itself in the background. 255.Pp 256.It Fl G 257Causes 258.Nm 259to print its configuration after evaluating 260.Cm Host 261and 262.Cm Match 263blocks and exit. 264.Pp 265.It Fl g 266Allows remote hosts to connect to local forwarded ports. 267If used on a multiplexed connection, then this option must be specified 268on the master process. 269.Pp 270.It Fl I Ar pkcs11 271Specify the PKCS#11 shared library 272.Nm 273should use to communicate with a PKCS#11 token providing the user's 274private RSA key. 275.Pp 276.It Fl i Ar identity_file 277Selects a file from which the identity (private key) for 278public key authentication is read. 279The default is 280.Pa ~/.ssh/id_dsa , 281.Pa ~/.ssh/id_ecdsa , 282.Pa ~/.ssh/id_ed25519 283and 284.Pa ~/.ssh/id_rsa . 285Identity files may also be specified on 286a per-host basis in the configuration file. 287It is possible to have multiple 288.Fl i 289options (and multiple identities specified in 290configuration files). 291If no certificates have been explicitly specified by the 292.Cm CertificateFile 293directive, 294.Nm 295will also try to load certificate information from the filename obtained 296by appending 297.Pa -cert.pub 298to identity filenames. 299.Pp 300.It Fl J Ar destination 301Connect to the target host by first making a 302.Nm 303connection to the jump host described by 304.Ar destination 305and then establishing a TCP forwarding to the ultimate destination from 306there. 307Multiple jump hops may be specified separated by comma characters. 308This is a shortcut to specify a 309.Cm ProxyJump 310configuration directive. 311.Pp 312.It Fl K 313Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI 314credentials to the server. 315.Pp 316.It Fl k 317Disables forwarding (delegation) of GSSAPI credentials to the server. 318.Pp 319.It Fl L Xo 320.Sm off 321.Oo Ar bind_address : Oc 322.Ar port : host : hostport 323.Sm on 324.Xc 325.It Fl L Xo 326.Sm off 327.Oo Ar bind_address : Oc 328.Ar port : remote_socket 329.Sm on 330.Xc 331.It Fl L Xo 332.Sm off 333.Ar local_socket : host : hostport 334.Sm on 335.Xc 336.It Fl L Xo 337.Sm off 338.Ar local_socket : remote_socket 339.Sm on 340.Xc 341Specifies that connections to the given TCP port or Unix socket on the local 342(client) host are to be forwarded to the given host and port, or Unix socket, 343on the remote side. 344This works by allocating a socket to listen to either a TCP 345.Ar port 346on the local side, optionally bound to the specified 347.Ar bind_address , 348or to a Unix socket. 349Whenever a connection is made to the local port or socket, the 350connection is forwarded over the secure channel, and a connection is 351made to either 352.Ar host 353port 354.Ar hostport , 355or the Unix socket 356.Ar remote_socket , 357from the remote machine. 358.Pp 359Port forwardings can also be specified in the configuration file. 360Only the superuser can forward privileged ports. 361IPv6 addresses can be specified by enclosing the address in square brackets. 362.Pp 363By default, the local port is bound in accordance with the 364.Cm GatewayPorts 365setting. 366However, an explicit 367.Ar bind_address 368may be used to bind the connection to a specific address. 369The 370.Ar bind_address 371of 372.Dq localhost 373indicates that the listening port be bound for local use only, while an 374empty address or 375.Sq * 376indicates that the port should be available from all interfaces. 377.Pp 378.It Fl l Ar login_name 379Specifies the user to log in as on the remote machine. 380This also may be specified on a per-host basis in the configuration file. 381.Pp 382.It Fl M 383Places the 384.Nm 385client into 386.Dq master 387mode for connection sharing. 388Multiple 389.Fl M 390options places 391.Nm 392into 393.Dq master 394mode with confirmation required before slave connections are accepted. 395Refer to the description of 396.Cm ControlMaster 397in 398.Xr ssh_config 5 399for details. 400.Pp 401.It Fl m Ar mac_spec 402A comma-separated list of MAC (message authentication code) algorithms, 403specified in order of preference. 404See the 405.Cm MACs 406keyword for more information. 407.Pp 408.It Fl N 409Do not execute a remote command. 410This is useful for just forwarding ports. 411.Pp 412.It Fl n 413Redirects stdin from 414.Pa /dev/null 415(actually, prevents reading from stdin). 416This must be used when 417.Nm 418is run in the background. 419A common trick is to use this to run X11 programs on a remote machine. 420For example, 421.Ic ssh -n shadows.cs.hut.fi emacs & 422will start an emacs on shadows.cs.hut.fi, and the X11 423connection will be automatically forwarded over an encrypted channel. 424The 425.Nm 426program will be put in the background. 427(This does not work if 428.Nm 429needs to ask for a password or passphrase; see also the 430.Fl f 431option.) 432.Pp 433.It Fl O Ar ctl_cmd 434Control an active connection multiplexing master process. 435When the 436.Fl O 437option is specified, the 438.Ar ctl_cmd 439argument is interpreted and passed to the master process. 440Valid commands are: 441.Dq check 442(check that the master process is running), 443.Dq forward 444(request forwardings without command execution), 445.Dq cancel 446(cancel forwardings), 447.Dq exit 448(request the master to exit), and 449.Dq stop 450(request the master to stop accepting further multiplexing requests). 451.Pp 452.It Fl o Ar option 453Can be used to give options in the format used in the configuration file. 454This is useful for specifying options for which there is no separate 455command-line flag. 456For full details of the options listed below, and their possible values, see 457.Xr ssh_config 5 . 458.Pp 459.Bl -tag -width Ds -offset indent -compact 460.It AddKeysToAgent 461.It AddressFamily 462.It BatchMode 463.It BindAddress 464.It CanonicalDomains 465.It CanonicalizeFallbackLocal 466.It CanonicalizeHostname 467.It CanonicalizeMaxDots 468.It CanonicalizePermittedCNAMEs 469.It CertificateFile 470.It ChallengeResponseAuthentication 471.It CheckHostIP 472.It Ciphers 473.It ClearAllForwardings 474.It Compression 475.It ConnectionAttempts 476.It ConnectTimeout 477.It ControlMaster 478.It ControlPath 479.It ControlPersist 480.It DynamicForward 481.It EscapeChar 482.It ExitOnForwardFailure 483.It FingerprintHash 484.It ForwardAgent 485.It ForwardX11 486.It ForwardX11Timeout 487.It ForwardX11Trusted 488.It GatewayPorts 489.It GlobalKnownHostsFile 490.It GSSAPIAuthentication 491.It GSSAPIDelegateCredentials 492.It HashKnownHosts 493.It Host 494.It HostbasedAuthentication 495.It HostbasedKeyTypes 496.It HostKeyAlgorithms 497.It HostKeyAlias 498.It HostName 499.It IdentitiesOnly 500.It IdentityAgent 501.It IdentityFile 502.It IPQoS 503.It KbdInteractiveAuthentication 504.It KbdInteractiveDevices 505.It KexAlgorithms 506.It LocalCommand 507.It LocalForward 508.It LogLevel 509.It MACs 510.It Match 511.It NoHostAuthenticationForLocalhost 512.It NumberOfPasswordPrompts 513.It PasswordAuthentication 514.It PermitLocalCommand 515.It PKCS11Provider 516.It Port 517.It PreferredAuthentications 518.It ProxyCommand 519.It ProxyJump 520.It ProxyUseFdpass 521.It PubkeyAcceptedKeyTypes 522.It PubkeyAuthentication 523.It RekeyLimit 524.It RemoteCommand 525.It RemoteForward 526.It RequestTTY 527.It SendEnv 528.It ServerAliveInterval 529.It ServerAliveCountMax 530.It StreamLocalBindMask 531.It StreamLocalBindUnlink 532.It StrictHostKeyChecking 533.It TCPKeepAlive 534.It Tunnel 535.It TunnelDevice 536.It UpdateHostKeys 537.It UsePrivilegedPort 538.It User 539.It UserKnownHostsFile 540.It VerifyHostKeyDNS 541.It VisualHostKey 542.It XAuthLocation 543.El 544.Pp 545.It Fl p Ar port 546Port to connect to on the remote host. 547This can be specified on a 548per-host basis in the configuration file. 549.Pp 550.It Fl Q Ar query_option 551Queries 552.Nm 553for the algorithms supported for the specified version 2. 554The available features are: 555.Ar cipher 556(supported symmetric ciphers), 557.Ar cipher-auth 558(supported symmetric ciphers that support authenticated encryption), 559.Ar mac 560(supported message integrity codes), 561.Ar kex 562(key exchange algorithms), 563.Ar key 564(key types), 565.Ar key-cert 566(certificate key types), 567.Ar key-plain 568(non-certificate key types), and 569.Ar protocol-version 570(supported SSH protocol versions). 571.Pp 572.It Fl q 573Quiet mode. 574Causes most warning and diagnostic messages to be suppressed. 575.Pp 576.It Fl R Xo 577.Sm off 578.Oo Ar bind_address : Oc 579.Ar port : host : hostport 580.Sm on 581.Xc 582.It Fl R Xo 583.Sm off 584.Oo Ar bind_address : Oc 585.Ar port : local_socket 586.Sm on 587.Xc 588.It Fl R Xo 589.Sm off 590.Ar remote_socket : host : hostport 591.Sm on 592.Xc 593.It Fl R Xo 594.Sm off 595.Ar remote_socket : local_socket 596.Sm on 597.Xc 598.It Fl R Xo 599.Sm off 600.Oo Ar bind_address : Oc 601.Ar port 602.Sm on 603.Xc 604Specifies that connections to the given TCP port or Unix socket on the remote 605(server) host are to be forwarded to the local side. 606.Pp 607This works by allocating a socket to listen to either a TCP 608.Ar port 609or to a Unix socket on the remote side. 610Whenever a connection is made to this port or Unix socket, the 611connection is forwarded over the secure channel, and a connection 612is made from the local machine to either an explicit destination specified by 613.Ar host 614port 615.Ar hostport , 616or 617.Ar local_socket , 618or, if no explicit destination was specified, 619.Nm 620will act as a SOCKS 4/5 proxy and forward connections to the destinations 621requested by the remote SOCKS client. 622.Pp 623Port forwardings can also be specified in the configuration file. 624Privileged ports can be forwarded only when 625logging in as root on the remote machine. 626IPv6 addresses can be specified by enclosing the address in square brackets. 627.Pp 628By default, TCP listening sockets on the server will be bound to the loopback 629interface only. 630This may be overridden by specifying a 631.Ar bind_address . 632An empty 633.Ar bind_address , 634or the address 635.Ql * , 636indicates that the remote socket should listen on all interfaces. 637Specifying a remote 638.Ar bind_address 639will only succeed if the server's 640.Cm GatewayPorts 641option is enabled (see 642.Xr sshd_config 5 ) . 643.Pp 644If the 645.Ar port 646argument is 647.Ql 0 , 648the listen port will be dynamically allocated on the server and reported 649to the client at run time. 650When used together with 651.Ic -O forward 652the allocated port will be printed to the standard output. 653.Pp 654.It Fl S Ar ctl_path 655Specifies the location of a control socket for connection sharing, 656or the string 657.Dq none 658to disable connection sharing. 659Refer to the description of 660.Cm ControlPath 661and 662.Cm ControlMaster 663in 664.Xr ssh_config 5 665for details. 666.Pp 667.It Fl s 668May be used to request invocation of a subsystem on the remote system. 669Subsystems facilitate the use of SSH 670as a secure transport for other applications (e.g.\& 671.Xr sftp 1 ) . 672The subsystem is specified as the remote command. 673.Pp 674.It Fl T 675Disable pseudo-terminal allocation. 676.Pp 677.It Fl t 678Force pseudo-terminal allocation. 679This can be used to execute arbitrary 680screen-based programs on a remote machine, which can be very useful, 681e.g. when implementing menu services. 682Multiple 683.Fl t 684options force tty allocation, even if 685.Nm 686has no local tty. 687.Pp 688.It Fl V 689Display the version number and exit. 690.Pp 691.It Fl v 692Verbose mode. 693Causes 694.Nm 695to print debugging messages about its progress. 696This is helpful in 697debugging connection, authentication, and configuration problems. 698Multiple 699.Fl v 700options increase the verbosity. 701The maximum is 3. 702.Pp 703.It Fl W Ar host : Ns Ar port 704Requests that standard input and output on the client be forwarded to 705.Ar host 706on 707.Ar port 708over the secure channel. 709Implies 710.Fl N , 711.Fl T , 712.Cm ExitOnForwardFailure 713and 714.Cm ClearAllForwardings , 715though these can be overridden in the configuration file or using 716.Fl o 717command line options. 718.Pp 719.It Fl w Xo 720.Ar local_tun Ns Op : Ns Ar remote_tun 721.Xc 722Requests 723tunnel 724device forwarding with the specified 725.Xr tun 4 726devices between the client 727.Pq Ar local_tun 728and the server 729.Pq Ar remote_tun . 730.Pp 731The devices may be specified by numerical ID or the keyword 732.Dq any , 733which uses the next available tunnel device. 734If 735.Ar remote_tun 736is not specified, it defaults to 737.Dq any . 738See also the 739.Cm Tunnel 740and 741.Cm TunnelDevice 742directives in 743.Xr ssh_config 5 . 744If the 745.Cm Tunnel 746directive is unset, it is set to the default tunnel mode, which is 747.Dq point-to-point . 748.Pp 749.It Fl X 750Enables X11 forwarding. 751This can also be specified on a per-host basis in a configuration file. 752.Pp 753X11 forwarding should be enabled with caution. 754Users with the ability to bypass file permissions on the remote host 755(for the user's X authorization database) 756can access the local X11 display through the forwarded connection. 757An attacker may then be able to perform activities such as keystroke monitoring. 758.Pp 759For this reason, X11 forwarding is subjected to X11 SECURITY extension 760restrictions by default. 761Please refer to the 762.Nm 763.Fl Y 764option and the 765.Cm ForwardX11Trusted 766directive in 767.Xr ssh_config 5 768for more information. 769.Pp 770.It Fl x 771Disables X11 forwarding. 772.Pp 773.It Fl Y 774Enables trusted X11 forwarding. 775Trusted X11 forwardings are not subjected to the X11 SECURITY extension 776controls. 777.Pp 778.It Fl y 779Send log information using the 780.Xr syslog 3 781system module. 782By default this information is sent to stderr. 783.El 784.Pp 785.Nm 786may additionally obtain configuration data from 787a per-user configuration file and a system-wide configuration file. 788The file format and configuration options are described in 789.Xr ssh_config 5 . 790.Sh AUTHENTICATION 791The OpenSSH SSH client supports SSH protocol 2. 792.Pp 793The methods available for authentication are: 794GSSAPI-based authentication, 795host-based authentication, 796public key authentication, 797challenge-response authentication, 798and password authentication. 799Authentication methods are tried in the order specified above, 800though 801.Cm PreferredAuthentications 802can be used to change the default order. 803.Pp 804Host-based authentication works as follows: 805If the machine the user logs in from is listed in 806.Pa /etc/hosts.equiv 807or 808.Pa /etc/shosts.equiv 809on the remote machine, and the user names are 810the same on both sides, or if the files 811.Pa ~/.rhosts 812or 813.Pa ~/.shosts 814exist in the user's home directory on the 815remote machine and contain a line containing the name of the client 816machine and the name of the user on that machine, the user is 817considered for login. 818Additionally, the server 819.Em must 820be able to verify the client's 821host key (see the description of 822.Pa /etc/ssh/ssh_known_hosts 823and 824.Pa ~/.ssh/known_hosts , 825below) 826for login to be permitted. 827This authentication method closes security holes due to IP 828spoofing, DNS spoofing, and routing spoofing. 829[Note to the administrator: 830.Pa /etc/hosts.equiv , 831.Pa ~/.rhosts , 832and the rlogin/rsh protocol in general, are inherently insecure and should be 833disabled if security is desired.] 834.Pp 835Public key authentication works as follows: 836The scheme is based on public-key cryptography, 837using cryptosystems 838where encryption and decryption are done using separate keys, 839and it is unfeasible to derive the decryption key from the encryption key. 840The idea is that each user creates a public/private 841key pair for authentication purposes. 842The server knows the public key, and only the user knows the private key. 843.Nm 844implements public key authentication protocol automatically, 845using one of the DSA, ECDSA, Ed25519 or RSA algorithms. 846The HISTORY section of 847.Xr ssl 8 848contains a brief discussion of the DSA and RSA algorithms. 849.Pp 850The file 851.Pa ~/.ssh/authorized_keys 852lists the public keys that are permitted for logging in. 853When the user logs in, the 854.Nm 855program tells the server which key pair it would like to use for 856authentication. 857The client proves that it has access to the private key 858and the server checks that the corresponding public key 859is authorized to accept the account. 860.Pp 861The server may inform the client of errors that prevented public key 862authentication from succeeding after authentication completes using a 863different method. 864These may be viewed by increasing the 865.Cm LogLevel 866to 867.Cm DEBUG 868or higher (e.g. by using the 869.Fl v 870flag). 871.Pp 872The user creates his/her key pair by running 873.Xr ssh-keygen 1 . 874This stores the private key in 875.Pa ~/.ssh/id_dsa 876(DSA), 877.Pa ~/.ssh/id_ecdsa 878(ECDSA), 879.Pa ~/.ssh/id_ed25519 880(Ed25519), 881or 882.Pa ~/.ssh/id_rsa 883(RSA) 884and stores the public key in 885.Pa ~/.ssh/id_dsa.pub 886(DSA), 887.Pa ~/.ssh/id_ecdsa.pub 888(ECDSA), 889.Pa ~/.ssh/id_ed25519.pub 890(Ed25519), 891or 892.Pa ~/.ssh/id_rsa.pub 893(RSA) 894in the user's home directory. 895The user should then copy the public key 896to 897.Pa ~/.ssh/authorized_keys 898in his/her home directory on the remote machine. 899The 900.Pa authorized_keys 901file corresponds to the conventional 902.Pa ~/.rhosts 903file, and has one key 904per line, though the lines can be very long. 905After this, the user can log in without giving the password. 906.Pp 907A variation on public key authentication 908is available in the form of certificate authentication: 909instead of a set of public/private keys, 910signed certificates are used. 911This has the advantage that a single trusted certification authority 912can be used in place of many public/private keys. 913See the CERTIFICATES section of 914.Xr ssh-keygen 1 915for more information. 916.Pp 917The most convenient way to use public key or certificate authentication 918may be with an authentication agent. 919See 920.Xr ssh-agent 1 921and (optionally) the 922.Cm AddKeysToAgent 923directive in 924.Xr ssh_config 5 925for more information. 926.Pp 927Challenge-response authentication works as follows: 928The server sends an arbitrary 929.Qq challenge 930text, and prompts for a response. 931Examples of challenge-response authentication include 932.Bx 933Authentication (see 934.Xr login.conf 5 ) 935and PAM (some 936.Pf non- Ox 937systems). 938.Pp 939Finally, if other authentication methods fail, 940.Nm 941prompts the user for a password. 942The password is sent to the remote 943host for checking; however, since all communications are encrypted, 944the password cannot be seen by someone listening on the network. 945.Pp 946.Nm 947automatically maintains and checks a database containing 948identification for all hosts it has ever been used with. 949Host keys are stored in 950.Pa ~/.ssh/known_hosts 951in the user's home directory. 952Additionally, the file 953.Pa /etc/ssh/ssh_known_hosts 954is automatically checked for known hosts. 955Any new hosts are automatically added to the user's file. 956If a host's identification ever changes, 957.Nm 958warns about this and disables password authentication to prevent 959server spoofing or man-in-the-middle attacks, 960which could otherwise be used to circumvent the encryption. 961The 962.Cm StrictHostKeyChecking 963option can be used to control logins to machines whose 964host key is not known or has changed. 965.Pp 966When the user's identity has been accepted by the server, the server 967either executes the given command in a non-interactive session or, 968if no command has been specified, logs into the machine and gives 969the user a normal shell as an interactive session. 970All communication with 971the remote command or shell will be automatically encrypted. 972.Pp 973If an interactive session is requested 974.Nm 975by default will only request a pseudo-terminal (pty) for interactive 976sessions when the client has one. 977The flags 978.Fl T 979and 980.Fl t 981can be used to override this behaviour. 982.Pp 983If a pseudo-terminal has been allocated the 984user may use the escape characters noted below. 985.Pp 986If no pseudo-terminal has been allocated, 987the session is transparent and can be used to reliably transfer binary data. 988On most systems, setting the escape character to 989.Dq none 990will also make the session transparent even if a tty is used. 991.Pp 992The session terminates when the command or shell on the remote 993machine exits and all X11 and TCP connections have been closed. 994.Sh ESCAPE CHARACTERS 995When a pseudo-terminal has been requested, 996.Nm 997supports a number of functions through the use of an escape character. 998.Pp 999A single tilde character can be sent as 1000.Ic ~~ 1001or by following the tilde by a character other than those described below. 1002The escape character must always follow a newline to be interpreted as 1003special. 1004The escape character can be changed in configuration files using the 1005.Cm EscapeChar 1006configuration directive or on the command line by the 1007.Fl e 1008option. 1009.Pp 1010The supported escapes (assuming the default 1011.Ql ~ ) 1012are: 1013.Bl -tag -width Ds 1014.It Cm ~. 1015Disconnect. 1016.It Cm ~^Z 1017Background 1018.Nm . 1019.It Cm ~# 1020List forwarded connections. 1021.It Cm ~& 1022Background 1023.Nm 1024at logout when waiting for forwarded connection / X11 sessions to terminate. 1025.It Cm ~? 1026Display a list of escape characters. 1027.It Cm ~B 1028Send a BREAK to the remote system 1029(only useful if the peer supports it). 1030.It Cm ~C 1031Open command line. 1032Currently this allows the addition of port forwardings using the 1033.Fl L , 1034.Fl R 1035and 1036.Fl D 1037options (see above). 1038It also allows the cancellation of existing port-forwardings 1039with 1040.Sm off 1041.Fl KL Oo Ar bind_address : Oc Ar port 1042.Sm on 1043for local, 1044.Sm off 1045.Fl KR Oo Ar bind_address : Oc Ar port 1046.Sm on 1047for remote and 1048.Sm off 1049.Fl KD Oo Ar bind_address : Oc Ar port 1050.Sm on 1051for dynamic port-forwardings. 1052.Ic !\& Ns Ar command 1053allows the user to execute a local command if the 1054.Ic PermitLocalCommand 1055option is enabled in 1056.Xr ssh_config 5 . 1057Basic help is available, using the 1058.Fl h 1059option. 1060.It Cm ~R 1061Request rekeying of the connection 1062(only useful if the peer supports it). 1063.It Cm ~V 1064Decrease the verbosity 1065.Pq Ic LogLevel 1066when errors are being written to stderr. 1067.It Cm ~v 1068Increase the verbosity 1069.Pq Ic LogLevel 1070when errors are being written to stderr. 1071.El 1072.Sh TCP FORWARDING 1073Forwarding of arbitrary TCP connections over the secure channel can 1074be specified either on the command line or in a configuration file. 1075One possible application of TCP forwarding is a secure connection to a 1076mail server; another is going through firewalls. 1077.Pp 1078In the example below, we look at encrypting communication between 1079an IRC client and server, even though the IRC server does not directly 1080support encrypted communications. 1081This works as follows: 1082the user connects to the remote host using 1083.Nm , 1084specifying a port to be used to forward connections 1085to the remote server. 1086After that it is possible to start the service which is to be encrypted 1087on the client machine, 1088connecting to the same local port, 1089and 1090.Nm 1091will encrypt and forward the connection. 1092.Pp 1093The following example tunnels an IRC session from client machine 1094.Dq 127.0.0.1 1095(localhost) 1096to remote server 1097.Dq server.example.com : 1098.Bd -literal -offset 4n 1099$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 1100$ irc -c '#users' -p 1234 pinky 127.0.0.1 1101.Ed 1102.Pp 1103This tunnels a connection to IRC server 1104.Dq server.example.com , 1105joining channel 1106.Dq #users , 1107nickname 1108.Dq pinky , 1109using port 1234. 1110It doesn't matter which port is used, 1111as long as it's greater than 1023 1112(remember, only root can open sockets on privileged ports) 1113and doesn't conflict with any ports already in use. 1114The connection is forwarded to port 6667 on the remote server, 1115since that's the standard port for IRC services. 1116.Pp 1117The 1118.Fl f 1119option backgrounds 1120.Nm 1121and the remote command 1122.Dq sleep 10 1123is specified to allow an amount of time 1124(10 seconds, in the example) 1125to start the service which is to be tunnelled. 1126If no connections are made within the time specified, 1127.Nm 1128will exit. 1129.Sh X11 FORWARDING 1130If the 1131.Cm ForwardX11 1132variable is set to 1133.Dq yes 1134(or see the description of the 1135.Fl X , 1136.Fl x , 1137and 1138.Fl Y 1139options above) 1140and the user is using X11 (the 1141.Ev DISPLAY 1142environment variable is set), the connection to the X11 display is 1143automatically forwarded to the remote side in such a way that any X11 1144programs started from the shell (or command) will go through the 1145encrypted channel, and the connection to the real X server will be made 1146from the local machine. 1147The user should not manually set 1148.Ev DISPLAY . 1149Forwarding of X11 connections can be 1150configured on the command line or in configuration files. 1151.Pp 1152The 1153.Ev DISPLAY 1154value set by 1155.Nm 1156will point to the server machine, but with a display number greater than zero. 1157This is normal, and happens because 1158.Nm 1159creates a 1160.Dq proxy 1161X server on the server machine for forwarding the 1162connections over the encrypted channel. 1163.Pp 1164.Nm 1165will also automatically set up Xauthority data on the server machine. 1166For this purpose, it will generate a random authorization cookie, 1167store it in Xauthority on the server, and verify that any forwarded 1168connections carry this cookie and replace it by the real cookie when 1169the connection is opened. 1170The real authentication cookie is never 1171sent to the server machine (and no cookies are sent in the plain). 1172.Pp 1173If the 1174.Cm ForwardAgent 1175variable is set to 1176.Dq yes 1177(or see the description of the 1178.Fl A 1179and 1180.Fl a 1181options above) and 1182the user is using an authentication agent, the connection to the agent 1183is automatically forwarded to the remote side. 1184.Sh VERIFYING HOST KEYS 1185When connecting to a server for the first time, 1186a fingerprint of the server's public key is presented to the user 1187(unless the option 1188.Cm StrictHostKeyChecking 1189has been disabled). 1190Fingerprints can be determined using 1191.Xr ssh-keygen 1 : 1192.Pp 1193.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1194.Pp 1195If the fingerprint is already known, it can be matched 1196and the key can be accepted or rejected. 1197If only legacy (MD5) fingerprints for the server are available, the 1198.Xr ssh-keygen 1 1199.Fl E 1200option may be used to downgrade the fingerprint algorithm to match. 1201.Pp 1202Because of the difficulty of comparing host keys 1203just by looking at fingerprint strings, 1204there is also support to compare host keys visually, 1205using 1206.Em random art . 1207By setting the 1208.Cm VisualHostKey 1209option to 1210.Dq yes , 1211a small ASCII graphic gets displayed on every login to a server, no matter 1212if the session itself is interactive or not. 1213By learning the pattern a known server produces, a user can easily 1214find out that the host key has changed when a completely different pattern 1215is displayed. 1216Because these patterns are not unambiguous however, a pattern that looks 1217similar to the pattern remembered only gives a good probability that the 1218host key is the same, not guaranteed proof. 1219.Pp 1220To get a listing of the fingerprints along with their random art for 1221all known hosts, the following command line can be used: 1222.Pp 1223.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1224.Pp 1225If the fingerprint is unknown, 1226an alternative method of verification is available: 1227SSH fingerprints verified by DNS. 1228An additional resource record (RR), 1229SSHFP, 1230is added to a zonefile 1231and the connecting client is able to match the fingerprint 1232with that of the key presented. 1233.Pp 1234In this example, we are connecting a client to a server, 1235.Dq host.example.com . 1236The SSHFP resource records should first be added to the zonefile for 1237host.example.com: 1238.Bd -literal -offset indent 1239$ ssh-keygen -r host.example.com. 1240.Ed 1241.Pp 1242The output lines will have to be added to the zonefile. 1243To check that the zone is answering fingerprint queries: 1244.Pp 1245.Dl $ dig -t SSHFP host.example.com 1246.Pp 1247Finally the client connects: 1248.Bd -literal -offset indent 1249$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1250[...] 1251Matching host key fingerprint found in DNS. 1252Are you sure you want to continue connecting (yes/no)? 1253.Ed 1254.Pp 1255See the 1256.Cm VerifyHostKeyDNS 1257option in 1258.Xr ssh_config 5 1259for more information. 1260.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1261.Nm 1262contains support for Virtual Private Network (VPN) tunnelling 1263using the 1264.Xr tun 4 1265network pseudo-device, 1266allowing two networks to be joined securely. 1267The 1268.Xr sshd_config 5 1269configuration option 1270.Cm PermitTunnel 1271controls whether the server supports this, 1272and at what level (layer 2 or 3 traffic). 1273.Pp 1274The following example would connect client network 10.0.50.0/24 1275with remote network 10.0.99.0/24 using a point-to-point connection 1276from 10.1.1.1 to 10.1.1.2, 1277provided that the SSH server running on the gateway to the remote network, 1278at 192.168.1.15, allows it. 1279.Pp 1280On the client: 1281.Bd -literal -offset indent 1282# ssh -f -w 0:1 192.168.1.15 true 1283# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1284# route add 10.0.99.0/24 10.1.1.2 1285.Ed 1286.Pp 1287On the server: 1288.Bd -literal -offset indent 1289# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1290# route add 10.0.50.0/24 10.1.1.1 1291.Ed 1292.Pp 1293Client access may be more finely tuned via the 1294.Pa /root/.ssh/authorized_keys 1295file (see below) and the 1296.Cm PermitRootLogin 1297server option. 1298The following entry would permit connections on 1299.Xr tun 4 1300device 1 from user 1301.Dq jane 1302and on tun device 2 from user 1303.Dq john , 1304if 1305.Cm PermitRootLogin 1306is set to 1307.Dq forced-commands-only : 1308.Bd -literal -offset 2n 1309tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1310tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1311.Ed 1312.Pp 1313Since an SSH-based setup entails a fair amount of overhead, 1314it may be more suited to temporary setups, 1315such as for wireless VPNs. 1316More permanent VPNs are better provided by tools such as 1317.Xr ipsecctl 8 1318and 1319.Xr isakmpd 8 . 1320.Sh ENVIRONMENT 1321.Nm 1322will normally set the following environment variables: 1323.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1324.It Ev DISPLAY 1325The 1326.Ev DISPLAY 1327variable indicates the location of the X11 server. 1328It is automatically set by 1329.Nm 1330to point to a value of the form 1331.Dq hostname:n , 1332where 1333.Dq hostname 1334indicates the host where the shell runs, and 1335.Sq n 1336is an integer \*(Ge 1. 1337.Nm 1338uses this special value to forward X11 connections over the secure 1339channel. 1340The user should normally not set 1341.Ev DISPLAY 1342explicitly, as that 1343will render the X11 connection insecure (and will require the user to 1344manually copy any required authorization cookies). 1345.It Ev HOME 1346Set to the path of the user's home directory. 1347.It Ev LOGNAME 1348Synonym for 1349.Ev USER ; 1350set for compatibility with systems that use this variable. 1351.It Ev MAIL 1352Set to the path of the user's mailbox. 1353.It Ev PATH 1354Set to the default 1355.Ev PATH , 1356as specified when compiling 1357.Nm . 1358.It Ev SSH_ASKPASS 1359If 1360.Nm 1361needs a passphrase, it will read the passphrase from the current 1362terminal if it was run from a terminal. 1363If 1364.Nm 1365does not have a terminal associated with it but 1366.Ev DISPLAY 1367and 1368.Ev SSH_ASKPASS 1369are set, it will execute the program specified by 1370.Ev SSH_ASKPASS 1371and open an X11 window to read the passphrase. 1372This is particularly useful when calling 1373.Nm 1374from a 1375.Pa .xsession 1376or related script. 1377(Note that on some machines it 1378may be necessary to redirect the input from 1379.Pa /dev/null 1380to make this work.) 1381.It Ev SSH_AUTH_SOCK 1382Identifies the path of a 1383.Ux Ns -domain 1384socket used to communicate with the agent. 1385.It Ev SSH_CONNECTION 1386Identifies the client and server ends of the connection. 1387The variable contains 1388four space-separated values: client IP address, client port number, 1389server IP address, and server port number. 1390.It Ev SSH_ORIGINAL_COMMAND 1391This variable contains the original command line if a forced command 1392is executed. 1393It can be used to extract the original arguments. 1394.It Ev SSH_TTY 1395This is set to the name of the tty (path to the device) associated 1396with the current shell or command. 1397If the current session has no tty, 1398this variable is not set. 1399.It Ev SSH_TUNNEL 1400Optionally set by 1401.Xr sshd 8 1402to contain the interface names assigned if tunnel forwarding was 1403requested by the client. 1404.It Ev SSH_USER_AUTH 1405Optionally set by 1406.Xr sshd 8 , 1407this variable may contain a pathname to a file that lists the authentication 1408methods successfully used when the session was established, including any 1409public keys that were used. 1410.It Ev TZ 1411This variable is set to indicate the present time zone if it 1412was set when the daemon was started (i.e. the daemon passes the value 1413on to new connections). 1414.It Ev USER 1415Set to the name of the user logging in. 1416.El 1417.Pp 1418Additionally, 1419.Nm 1420reads 1421.Pa ~/.ssh/environment , 1422and adds lines of the format 1423.Dq VARNAME=value 1424to the environment if the file exists and users are allowed to 1425change their environment. 1426For more information, see the 1427.Cm PermitUserEnvironment 1428option in 1429.Xr sshd_config 5 . 1430.Sh FILES 1431.Bl -tag -width Ds -compact 1432.It Pa ~/.rhosts 1433This file is used for host-based authentication (see above). 1434On some machines this file may need to be 1435world-readable if the user's home directory is on an NFS partition, 1436because 1437.Xr sshd 8 1438reads it as root. 1439Additionally, this file must be owned by the user, 1440and must not have write permissions for anyone else. 1441The recommended 1442permission for most machines is read/write for the user, and not 1443accessible by others. 1444.Pp 1445.It Pa ~/.shosts 1446This file is used in exactly the same way as 1447.Pa .rhosts , 1448but allows host-based authentication without permitting login with 1449rlogin/rsh. 1450.Pp 1451.It Pa ~/.ssh/ 1452This directory is the default location for all user-specific configuration 1453and authentication information. 1454There is no general requirement to keep the entire contents of this directory 1455secret, but the recommended permissions are read/write/execute for the user, 1456and not accessible by others. 1457.Pp 1458.It Pa ~/.ssh/authorized_keys 1459Lists the public keys (DSA, ECDSA, Ed25519, RSA) 1460that can be used for logging in as this user. 1461The format of this file is described in the 1462.Xr sshd 8 1463manual page. 1464This file is not highly sensitive, but the recommended 1465permissions are read/write for the user, and not accessible by others. 1466.Pp 1467.It Pa ~/.ssh/config 1468This is the per-user configuration file. 1469The file format and configuration options are described in 1470.Xr ssh_config 5 . 1471Because of the potential for abuse, this file must have strict permissions: 1472read/write for the user, and not writable by others. 1473.Pp 1474.It Pa ~/.ssh/environment 1475Contains additional definitions for environment variables; see 1476.Sx ENVIRONMENT , 1477above. 1478.Pp 1479.It Pa ~/.ssh/id_dsa 1480.It Pa ~/.ssh/id_ecdsa 1481.It Pa ~/.ssh/id_ed25519 1482.It Pa ~/.ssh/id_rsa 1483Contains the private key for authentication. 1484These files 1485contain sensitive data and should be readable by the user but not 1486accessible by others (read/write/execute). 1487.Nm 1488will simply ignore a private key file if it is accessible by others. 1489It is possible to specify a passphrase when 1490generating the key which will be used to encrypt the 1491sensitive part of this file using AES-128. 1492.Pp 1493.It Pa ~/.ssh/id_dsa.pub 1494.It Pa ~/.ssh/id_ecdsa.pub 1495.It Pa ~/.ssh/id_ed25519.pub 1496.It Pa ~/.ssh/id_rsa.pub 1497Contains the public key for authentication. 1498These files are not 1499sensitive and can (but need not) be readable by anyone. 1500.Pp 1501.It Pa ~/.ssh/known_hosts 1502Contains a list of host keys for all hosts the user has logged into 1503that are not already in the systemwide list of known host keys. 1504See 1505.Xr sshd 8 1506for further details of the format of this file. 1507.Pp 1508.It Pa ~/.ssh/rc 1509Commands in this file are executed by 1510.Nm 1511when the user logs in, just before the user's shell (or command) is 1512started. 1513See the 1514.Xr sshd 8 1515manual page for more information. 1516.Pp 1517.It Pa /etc/hosts.equiv 1518This file is for host-based authentication (see above). 1519It should only be writable by root. 1520.Pp 1521.It Pa /etc/shosts.equiv 1522This file is used in exactly the same way as 1523.Pa hosts.equiv , 1524but allows host-based authentication without permitting login with 1525rlogin/rsh. 1526.Pp 1527.It Pa /etc/ssh/ssh_config 1528Systemwide configuration file. 1529The file format and configuration options are described in 1530.Xr ssh_config 5 . 1531.Pp 1532.It Pa /etc/ssh/ssh_host_key 1533.It Pa /etc/ssh/ssh_host_dsa_key 1534.It Pa /etc/ssh/ssh_host_ecdsa_key 1535.It Pa /etc/ssh/ssh_host_ed25519_key 1536.It Pa /etc/ssh/ssh_host_rsa_key 1537These files contain the private parts of the host keys 1538and are used for host-based authentication. 1539.Pp 1540.It Pa /etc/ssh/ssh_known_hosts 1541Systemwide list of known host keys. 1542This file should be prepared by the 1543system administrator to contain the public host keys of all machines in the 1544organization. 1545It should be world-readable. 1546See 1547.Xr sshd 8 1548for further details of the format of this file. 1549.Pp 1550.It Pa /etc/ssh/sshrc 1551Commands in this file are executed by 1552.Nm 1553when the user logs in, just before the user's shell (or command) is started. 1554See the 1555.Xr sshd 8 1556manual page for more information. 1557.El 1558.Sh EXIT STATUS 1559.Nm 1560exits with the exit status of the remote command or with 255 1561if an error occurred. 1562.Sh SEE ALSO 1563.Xr scp 1 , 1564.Xr sftp 1 , 1565.Xr ssh-add 1 , 1566.Xr ssh-agent 1 , 1567.Xr ssh-keygen 1 , 1568.Xr ssh-keyscan 1 , 1569.Xr tun 4 , 1570.Xr ssh_config 5 , 1571.Xr ssh-keysign 8 , 1572.Xr sshd 8 1573.Sh STANDARDS 1574.Rs 1575.%A S. Lehtinen 1576.%A C. Lonvick 1577.%D January 2006 1578.%R RFC 4250 1579.%T The Secure Shell (SSH) Protocol Assigned Numbers 1580.Re 1581.Pp 1582.Rs 1583.%A T. Ylonen 1584.%A C. Lonvick 1585.%D January 2006 1586.%R RFC 4251 1587.%T The Secure Shell (SSH) Protocol Architecture 1588.Re 1589.Pp 1590.Rs 1591.%A T. Ylonen 1592.%A C. Lonvick 1593.%D January 2006 1594.%R RFC 4252 1595.%T The Secure Shell (SSH) Authentication Protocol 1596.Re 1597.Pp 1598.Rs 1599.%A T. Ylonen 1600.%A C. Lonvick 1601.%D January 2006 1602.%R RFC 4253 1603.%T The Secure Shell (SSH) Transport Layer Protocol 1604.Re 1605.Pp 1606.Rs 1607.%A T. Ylonen 1608.%A C. Lonvick 1609.%D January 2006 1610.%R RFC 4254 1611.%T The Secure Shell (SSH) Connection Protocol 1612.Re 1613.Pp 1614.Rs 1615.%A J. Schlyter 1616.%A W. Griffin 1617.%D January 2006 1618.%R RFC 4255 1619.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints 1620.Re 1621.Pp 1622.Rs 1623.%A F. Cusack 1624.%A M. Forssen 1625.%D January 2006 1626.%R RFC 4256 1627.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH) 1628.Re 1629.Pp 1630.Rs 1631.%A J. Galbraith 1632.%A P. Remaker 1633.%D January 2006 1634.%R RFC 4335 1635.%T The Secure Shell (SSH) Session Channel Break Extension 1636.Re 1637.Pp 1638.Rs 1639.%A M. Bellare 1640.%A T. Kohno 1641.%A C. Namprempre 1642.%D January 2006 1643.%R RFC 4344 1644.%T The Secure Shell (SSH) Transport Layer Encryption Modes 1645.Re 1646.Pp 1647.Rs 1648.%A B. Harris 1649.%D January 2006 1650.%R RFC 4345 1651.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol 1652.Re 1653.Pp 1654.Rs 1655.%A M. Friedl 1656.%A N. Provos 1657.%A W. Simpson 1658.%D March 2006 1659.%R RFC 4419 1660.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 1661.Re 1662.Pp 1663.Rs 1664.%A J. Galbraith 1665.%A R. Thayer 1666.%D November 2006 1667.%R RFC 4716 1668.%T The Secure Shell (SSH) Public Key File Format 1669.Re 1670.Pp 1671.Rs 1672.%A D. Stebila 1673.%A J. Green 1674.%D December 2009 1675.%R RFC 5656 1676.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer 1677.Re 1678.Pp 1679.Rs 1680.%A A. Perrig 1681.%A D. Song 1682.%D 1999 1683.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99) 1684.%T Hash Visualization: a New Technique to improve Real-World Security 1685.Re 1686.Sh AUTHORS 1687OpenSSH is a derivative of the original and free 1688ssh 1.2.12 release by Tatu Ylonen. 1689Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1690Theo de Raadt and Dug Song 1691removed many bugs, re-added newer features and 1692created OpenSSH. 1693Markus Friedl contributed the support for SSH 1694protocol versions 1.5 and 2.0. 1695