1.\" -*- nroff -*- 2.\" 3.\" Author: Tatu Ylonen <ylo@cs.hut.fi> 4.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland 5.\" All rights reserved 6.\" 7.\" As far as I am concerned, the code I have written for this software 8.\" can be used freely for any purpose. Any derived versions of this 9.\" software must be clearly marked as such, and if the derived work is 10.\" incompatible with the protocol description in the RFC file, it must be 11.\" called by a name other than "ssh" or "Secure Shell". 12.\" 13.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. 14.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. 15.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. 16.\" 17.\" Redistribution and use in source and binary forms, with or without 18.\" modification, are permitted provided that the following conditions 19.\" are met: 20.\" 1. Redistributions of source code must retain the above copyright 21.\" notice, this list of conditions and the following disclaimer. 22.\" 2. Redistributions in binary form must reproduce the above copyright 23.\" notice, this list of conditions and the following disclaimer in the 24.\" documentation and/or other materials provided with the distribution. 25.\" 26.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 27.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 28.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 29.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 30.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 31.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 32.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 33.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 34.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 35.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 36.\" 37.\" $OpenBSD: ssh.1,v 1.308 2010/08/04 05:37:01 djm Exp $ 38.\" $FreeBSD$ 39.Dd August 4 2010 40.Dt SSH 1 41.Os 42.Sh NAME 43.Nm ssh 44.Nd OpenSSH SSH client (remote login program) 45.Sh SYNOPSIS 46.Nm ssh 47.Bk -words 48.Op Fl 1246AaCfgKkMNnqsTtVvXxYy 49.Op Fl b Ar bind_address 50.Op Fl c Ar cipher_spec 51.Op Fl D Oo Ar bind_address : Oc Ns Ar port 52.Op Fl e Ar escape_char 53.Op Fl F Ar configfile 54.Op Fl I Ar pkcs11 55.Op Fl i Ar identity_file 56.Op Fl L Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport 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 R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport 63.Op Fl S Ar ctl_path 64.Op Fl W Ar host : Ns Ar port 65.Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun 66.Oo Ar user Ns @ Oc Ns Ar hostname 67.Op Ar command 68.Ek 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 replace rlogin and rsh, 74and provide secure encrypted communications between 75two untrusted hosts over an insecure network. 76X11 connections and arbitrary TCP ports 77can also be forwarded over the secure channel. 78.Pp 79.Nm 80connects and logs into the specified 81.Ar hostname 82(with optional 83.Ar user 84name). 85The user must prove 86his/her identity to the remote machine using one of several methods 87depending on the protocol version used (see below). 88.Pp 89If 90.Ar command 91is specified, 92it is executed on the remote host instead of a login shell. 93.Pp 94The options are as follows: 95.Bl -tag -width Ds 96.It Fl 1 97Forces 98.Nm 99to try protocol version 1 only. 100.It Fl 2 101Forces 102.Nm 103to try protocol version 2 only. 104.It Fl 4 105Forces 106.Nm 107to use IPv4 addresses only. 108.It Fl 6 109Forces 110.Nm 111to use IPv6 addresses only. 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.It Fl a 125Disables forwarding of the authentication agent connection. 126.It Fl b Ar bind_address 127Use 128.Ar bind_address 129on the local machine as the source address 130of the connection. 131Only useful on systems with more than one address. 132.It Fl C 133Requests compression of all data (including stdin, stdout, stderr, and 134data for forwarded X11 and TCP connections). 135The compression algorithm is the same used by 136.Xr gzip 1 , 137and the 138.Dq level 139can be controlled by the 140.Cm CompressionLevel 141option for protocol version 1. 142Compression is desirable on modem lines and other 143slow connections, but will only slow down things on fast networks. 144The default value can be set on a host-by-host basis in the 145configuration files; see the 146.Cm Compression 147option. 148.It Fl c Ar cipher_spec 149Selects the cipher specification for encrypting the session. 150.Pp 151Protocol version 1 allows specification of a single cipher. 152The supported values are 153.Dq 3des , 154.Dq blowfish , 155and 156.Dq des . 157.Ar 3des 158(triple-des) is an encrypt-decrypt-encrypt triple with three different keys. 159It is believed to be secure. 160.Ar blowfish 161is a fast block cipher; it appears very secure and is much faster than 162.Ar 3des . 163.Ar des 164is only supported in the 165.Nm 166client for interoperability with legacy protocol 1 implementations 167that do not support the 168.Ar 3des 169cipher. 170Its use is strongly discouraged due to cryptographic weaknesses. 171The default is 172.Dq 3des . 173.Pp 174For protocol version 2, 175.Ar cipher_spec 176is a comma-separated list of ciphers 177listed in order of preference. 178See the 179.Cm Ciphers 180keyword in 181.Xr ssh_config 5 182for more information. 183.It Fl D Xo 184.Sm off 185.Oo Ar bind_address : Oc 186.Ar port 187.Sm on 188.Xc 189Specifies a local 190.Dq dynamic 191application-level port forwarding. 192This works by allocating a socket to listen to 193.Ar port 194on the local side, optionally bound to the specified 195.Ar bind_address . 196Whenever a connection is made to this port, the 197connection is forwarded over the secure channel, and the application 198protocol is then used to determine where to connect to from the 199remote machine. 200Currently the SOCKS4 and SOCKS5 protocols are supported, and 201.Nm 202will act as a SOCKS server. 203Only root can forward privileged ports. 204Dynamic port forwardings can also be specified in the configuration file. 205.Pp 206IPv6 addresses can be specified by enclosing the address in square brackets. 207Only the superuser can forward privileged ports. 208By default, the local port is bound in accordance with the 209.Cm GatewayPorts 210setting. 211However, an explicit 212.Ar bind_address 213may be used to bind the connection to a specific address. 214The 215.Ar bind_address 216of 217.Dq localhost 218indicates that the listening port be bound for local use only, while an 219empty address or 220.Sq * 221indicates that the port should be available from all interfaces. 222.It Fl e Ar escape_char 223Sets the escape character for sessions with a pty (default: 224.Ql ~ ) . 225The escape character is only recognized at the beginning of a line. 226The escape character followed by a dot 227.Pq Ql \&. 228closes the connection; 229followed by control-Z suspends the connection; 230and followed by itself sends the escape character once. 231Setting the character to 232.Dq none 233disables any escapes and makes the session fully transparent. 234.It Fl F Ar configfile 235Specifies an alternative per-user configuration file. 236If a configuration file is given on the command line, 237the system-wide configuration file 238.Pq Pa /etc/ssh/ssh_config 239will be ignored. 240The default for the per-user configuration file is 241.Pa ~/.ssh/config . 242.It Fl f 243Requests 244.Nm 245to go to background just before command execution. 246This is useful if 247.Nm 248is going to ask for passwords or passphrases, but the user 249wants it in the background. 250This implies 251.Fl n . 252The recommended way to start X11 programs at a remote site is with 253something like 254.Ic ssh -f host xterm . 255.Pp 256If the 257.Cm ExitOnForwardFailure 258configuration option is set to 259.Dq yes , 260then a client started with 261.Fl f 262will wait for all remote port forwards to be successfully established 263before placing itself in the background. 264.It Fl g 265Allows remote hosts to connect to local forwarded ports. 266.It Fl I Ar pkcs11 267Specify the PKCS#11 shared library 268.Nm 269should use to communicate with a PKCS#11 token providing the user's 270private RSA key. 271.It Fl i Ar identity_file 272Selects a file from which the identity (private key) for 273RSA or DSA authentication is read. 274The default is 275.Pa ~/.ssh/identity 276for protocol version 1, and 277.Pa ~/.ssh/id_rsa 278and 279.Pa ~/.ssh/id_dsa 280for protocol version 2. 281Identity files may also be specified on 282a per-host basis in the configuration file. 283It is possible to have multiple 284.Fl i 285options (and multiple identities specified in 286configuration files). 287.Nm 288will also try to load certificate information from the filename obtained 289by appending 290.Pa -cert.pub 291to identity filenames. 292.It Fl K 293Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI 294credentials to the server. 295.It Fl k 296Disables forwarding (delegation) of GSSAPI credentials to the server. 297.It Fl L Xo 298.Sm off 299.Oo Ar bind_address : Oc 300.Ar port : host : hostport 301.Sm on 302.Xc 303Specifies that the given port on the local (client) host is to be 304forwarded to the given host and port on the remote side. 305This works by allocating a socket to listen to 306.Ar port 307on the local side, optionally bound to the specified 308.Ar bind_address . 309Whenever a connection is made to this port, the 310connection is forwarded over the secure channel, and a connection is 311made to 312.Ar host 313port 314.Ar hostport 315from the remote machine. 316Port forwardings can also be specified in the configuration file. 317IPv6 addresses can be specified by enclosing the address in square brackets. 318Only the superuser can forward privileged ports. 319By default, the local port is bound in accordance with the 320.Cm GatewayPorts 321setting. 322However, an explicit 323.Ar bind_address 324may be used to bind the connection to a specific address. 325The 326.Ar bind_address 327of 328.Dq localhost 329indicates that the listening port be bound for local use only, while an 330empty address or 331.Sq * 332indicates that the port should be available from all interfaces. 333.It Fl l Ar login_name 334Specifies the user to log in as on the remote machine. 335This also may be specified on a per-host basis in the configuration file. 336.It Fl M 337Places the 338.Nm 339client into 340.Dq master 341mode for connection sharing. 342Multiple 343.Fl M 344options places 345.Nm 346into 347.Dq master 348mode with confirmation required before slave connections are accepted. 349Refer to the description of 350.Cm ControlMaster 351in 352.Xr ssh_config 5 353for details. 354.It Fl m Ar mac_spec 355Additionally, for protocol version 2 a comma-separated list of MAC 356(message authentication code) algorithms can 357be specified in order of preference. 358See the 359.Cm MACs 360keyword for more information. 361.It Fl N 362Do not execute a remote command. 363This is useful for just forwarding ports 364(protocol version 2 only). 365.It Fl n 366Redirects stdin from 367.Pa /dev/null 368(actually, prevents reading from stdin). 369This must be used when 370.Nm 371is run in the background. 372A common trick is to use this to run X11 programs on a remote machine. 373For example, 374.Ic ssh -n shadows.cs.hut.fi emacs & 375will start an emacs on shadows.cs.hut.fi, and the X11 376connection will be automatically forwarded over an encrypted channel. 377The 378.Nm 379program will be put in the background. 380(This does not work if 381.Nm 382needs to ask for a password or passphrase; see also the 383.Fl f 384option.) 385.It Fl O Ar ctl_cmd 386Control an active connection multiplexing master process. 387When the 388.Fl O 389option is specified, the 390.Ar ctl_cmd 391argument is interpreted and passed to the master process. 392Valid commands are: 393.Dq check 394(check that the master process is running), 395.Dq forward 396(request forwardings without command execution) and 397.Dq exit 398(request the master to exit). 399.It Fl o Ar option 400Can be used to give options in the format used in the configuration file. 401This is useful for specifying options for which there is no separate 402command-line flag. 403For full details of the options listed below, and their possible values, see 404.Xr ssh_config 5 . 405.Pp 406.Bl -tag -width Ds -offset indent -compact 407.It AddressFamily 408.It BatchMode 409.It BindAddress 410.It ChallengeResponseAuthentication 411.It CheckHostIP 412.It Cipher 413.It Ciphers 414.It ClearAllForwardings 415.It Compression 416.It CompressionLevel 417.It ConnectionAttempts 418.It ConnectTimeout 419.It ControlMaster 420.It ControlPath 421.It DynamicForward 422.It EscapeChar 423.It ExitOnForwardFailure 424.It ForwardAgent 425.It ForwardX11 426.It ForwardX11Trusted 427.It GatewayPorts 428.It GlobalKnownHostsFile 429.It GSSAPIAuthentication 430.It GSSAPIDelegateCredentials 431.It HashKnownHosts 432.It Host 433.It HostbasedAuthentication 434.It HostKeyAlgorithms 435.It HostKeyAlias 436.It HostName 437.It IdentityFile 438.It IdentitiesOnly 439.It KbdInteractiveDevices 440.It LocalCommand 441.It LocalForward 442.It LogLevel 443.It MACs 444.It NoHostAuthenticationForLocalhost 445.It NumberOfPasswordPrompts 446.It PasswordAuthentication 447.It PermitLocalCommand 448.It PKCS11Provider 449.It Port 450.It PreferredAuthentications 451.It Protocol 452.It ProxyCommand 453.It PubkeyAuthentication 454.It RekeyLimit 455.It RemoteForward 456.It RhostsRSAAuthentication 457.It RSAAuthentication 458.It SendEnv 459.It ServerAliveInterval 460.It ServerAliveCountMax 461.It StrictHostKeyChecking 462.It TCPKeepAlive 463.It Tunnel 464.It TunnelDevice 465.It UsePrivilegedPort 466.It User 467.It UserKnownHostsFile 468.It VerifyHostKeyDNS 469.It VersionAddendum 470.It VisualHostKey 471.It XAuthLocation 472.El 473.It Fl p Ar port 474Port to connect to on the remote host. 475This can be specified on a 476per-host basis in the configuration file. 477.It Fl q 478Quiet mode. 479Causes most warning and diagnostic messages to be suppressed. 480.It Fl R Xo 481.Sm off 482.Oo Ar bind_address : Oc 483.Ar port : host : hostport 484.Sm on 485.Xc 486Specifies that the given port on the remote (server) host is to be 487forwarded to the given host and port on the local side. 488This works by allocating a socket to listen to 489.Ar port 490on the remote side, and whenever a connection is made to this port, the 491connection is forwarded over the secure channel, and a connection is 492made to 493.Ar host 494port 495.Ar hostport 496from the local machine. 497.Pp 498Port forwardings can also be specified in the configuration file. 499Privileged ports can be forwarded only when 500logging in as root on the remote machine. 501IPv6 addresses can be specified by enclosing the address in square braces. 502.Pp 503By default, the listening socket on the server will be bound to the loopback 504interface only. 505This may be overridden by specifying a 506.Ar bind_address . 507An empty 508.Ar bind_address , 509or the address 510.Ql * , 511indicates that the remote socket should listen on all interfaces. 512Specifying a remote 513.Ar bind_address 514will only succeed if the server's 515.Cm GatewayPorts 516option is enabled (see 517.Xr sshd_config 5 ) . 518.Pp 519If the 520.Ar port 521argument is 522.Ql 0 , 523the listen port will be dynamically allocated on the server and reported 524to the client at run time. 525When used together with 526.Ic -O forward 527the allocated port will be printed to the standard output. 528.It Fl S Ar ctl_path 529Specifies the location of a control socket for connection sharing, 530or the string 531.Dq none 532to disable connection sharing. 533Refer to the description of 534.Cm ControlPath 535and 536.Cm ControlMaster 537in 538.Xr ssh_config 5 539for details. 540.It Fl s 541May be used to request invocation of a subsystem on the remote system. 542Subsystems are a feature of the SSH2 protocol which facilitate the use 543of SSH as a secure transport for other applications (eg.\& 544.Xr sftp 1 ) . 545The subsystem is specified as the remote command. 546.It Fl T 547Disable pseudo-tty allocation. 548.It Fl t 549Force pseudo-tty allocation. 550This can be used to execute arbitrary 551screen-based programs on a remote machine, which can be very useful, 552e.g. when implementing menu services. 553Multiple 554.Fl t 555options force tty allocation, even if 556.Nm 557has no local tty. 558.It Fl V 559Display the version number and exit. 560.It Fl v 561Verbose mode. 562Causes 563.Nm 564to print debugging messages about its progress. 565This is helpful in 566debugging connection, authentication, and configuration problems. 567Multiple 568.Fl v 569options increase the verbosity. 570The maximum is 3. 571.It Fl W Ar host : Ns Ar port 572Requests that standard input and output on the client be forwarded to 573.Ar host 574on 575.Ar port 576over the secure channel. 577Implies 578.Fl N , 579.Fl T , 580.Cm ExitOnForwardFailure 581and 582.Cm ClearAllForwardings 583and works with Protocol version 2 only. 584.It Fl w Xo 585.Ar local_tun Ns Op : Ns Ar remote_tun 586.Xc 587Requests 588tunnel 589device forwarding with the specified 590.Xr tun 4 591devices between the client 592.Pq Ar local_tun 593and the server 594.Pq Ar remote_tun . 595.Pp 596The devices may be specified by numerical ID or the keyword 597.Dq any , 598which uses the next available tunnel device. 599If 600.Ar remote_tun 601is not specified, it defaults to 602.Dq any . 603See also the 604.Cm Tunnel 605and 606.Cm TunnelDevice 607directives in 608.Xr ssh_config 5 . 609If the 610.Cm Tunnel 611directive is unset, it is set to the default tunnel mode, which is 612.Dq point-to-point . 613.It Fl X 614Enables X11 forwarding. 615This can also be specified on a per-host basis in a configuration file. 616.Pp 617X11 forwarding should be enabled with caution. 618Users with the ability to bypass file permissions on the remote host 619(for the user's X authorization database) 620can access the local X11 display through the forwarded connection. 621An attacker may then be able to perform activities such as keystroke monitoring. 622.Pp 623For this reason, X11 forwarding is subjected to X11 SECURITY extension 624restrictions by default. 625Please refer to the 626.Nm 627.Fl Y 628option and the 629.Cm ForwardX11Trusted 630directive in 631.Xr ssh_config 5 632for more information. 633.It Fl x 634Disables X11 forwarding. 635.It Fl Y 636Enables trusted X11 forwarding. 637Trusted X11 forwardings are not subjected to the X11 SECURITY extension 638controls. 639.It Fl y 640Send log information using the 641.Xr syslog 3 642system module. 643By default this information is sent to stderr. 644.El 645.Pp 646.Nm 647may additionally obtain configuration data from 648a per-user configuration file and a system-wide configuration file. 649The file format and configuration options are described in 650.Xr ssh_config 5 . 651.Pp 652.Nm 653exits with the exit status of the remote command or with 255 654if an error occurred. 655.Sh AUTHENTICATION 656The OpenSSH SSH client supports SSH protocols 1 and 2. 657The default is to use protocol 2 only, 658though this can be changed via the 659.Cm Protocol 660option in 661.Xr ssh_config 5 662or the 663.Fl 1 664and 665.Fl 2 666options (see above). 667Both protocols support similar authentication methods, 668but protocol 2 is the default since 669it provides additional mechanisms for confidentiality 670(the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) 671and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160). 672Protocol 1 lacks a strong mechanism for ensuring the 673integrity of the connection. 674.Pp 675The methods available for authentication are: 676GSSAPI-based authentication, 677host-based authentication, 678public key authentication, 679challenge-response authentication, 680and password authentication. 681Authentication methods are tried in the order specified above, 682though protocol 2 has a configuration option to change the default order: 683.Cm PreferredAuthentications . 684.Pp 685Host-based authentication works as follows: 686If the machine the user logs in from is listed in 687.Pa /etc/hosts.equiv 688or 689.Pa /etc/shosts.equiv 690on the remote machine, and the user names are 691the same on both sides, or if the files 692.Pa ~/.rhosts 693or 694.Pa ~/.shosts 695exist in the user's home directory on the 696remote machine and contain a line containing the name of the client 697machine and the name of the user on that machine, the user is 698considered for login. 699Additionally, the server 700.Em must 701be able to verify the client's 702host key (see the description of 703.Pa /etc/ssh/ssh_known_hosts 704and 705.Pa ~/.ssh/known_hosts , 706below) 707for login to be permitted. 708This authentication method closes security holes due to IP 709spoofing, DNS spoofing, and routing spoofing. 710[Note to the administrator: 711.Pa /etc/hosts.equiv , 712.Pa ~/.rhosts , 713and the rlogin/rsh protocol in general, are inherently insecure and should be 714disabled if security is desired.] 715.Pp 716Public key authentication works as follows: 717The scheme is based on public-key cryptography, 718using cryptosystems 719where encryption and decryption are done using separate keys, 720and it is unfeasible to derive the decryption key from the encryption key. 721The idea is that each user creates a public/private 722key pair for authentication purposes. 723The server knows the public key, and only the user knows the private key. 724.Nm 725implements public key authentication protocol automatically, 726using either the RSA or DSA algorithms. 727Protocol 1 is restricted to using only RSA keys, 728but protocol 2 may use either. 729The 730.Sx HISTORY 731section of 732.Xr ssl 8 733contains a brief discussion of the two algorithms. 734.Pp 735The file 736.Pa ~/.ssh/authorized_keys 737lists the public keys that are permitted for logging in. 738When the user logs in, the 739.Nm 740program tells the server which key pair it would like to use for 741authentication. 742The client proves that it has access to the private key 743and the server checks that the corresponding public key 744is authorized to accept the account. 745.Pp 746The user creates his/her key pair by running 747.Xr ssh-keygen 1 . 748This stores the private key in 749.Pa ~/.ssh/identity 750(protocol 1), 751.Pa ~/.ssh/id_dsa 752(protocol 2 DSA), 753or 754.Pa ~/.ssh/id_rsa 755(protocol 2 RSA) 756and stores the public key in 757.Pa ~/.ssh/identity.pub 758(protocol 1), 759.Pa ~/.ssh/id_dsa.pub 760(protocol 2 DSA), 761or 762.Pa ~/.ssh/id_rsa.pub 763(protocol 2 RSA) 764in the user's home directory. 765The user should then copy the public key 766to 767.Pa ~/.ssh/authorized_keys 768in his/her home directory on the remote machine. 769The 770.Pa authorized_keys 771file corresponds to the conventional 772.Pa ~/.rhosts 773file, and has one key 774per line, though the lines can be very long. 775After this, the user can log in without giving the password. 776.Pp 777A variation on public key authentication 778is available in the form of certificate authentication: 779instead of a set of public/private keys, 780signed certificates are used. 781This has the advantage that a single trusted certification authority 782can be used in place of many public/private keys. 783See the 784.Sx CERTIFICATES 785section of 786.Xr ssh-keygen 1 787for more information. 788.Pp 789The most convenient way to use public key or certificate authentication 790may be with an authentication agent. 791See 792.Xr ssh-agent 1 793for more information. 794.Pp 795Challenge-response authentication works as follows: 796The server sends an arbitrary 797.Qq challenge 798text, and prompts for a response. 799Protocol 2 allows multiple challenges and responses; 800protocol 1 is restricted to just one challenge/response. 801Examples of challenge-response authentication include 802BSD Authentication (see 803.Xr login.conf 5 ) 804and PAM (some non-OpenBSD systems). 805.Pp 806Finally, if other authentication methods fail, 807.Nm 808prompts the user for a password. 809The password is sent to the remote 810host for checking; however, since all communications are encrypted, 811the password cannot be seen by someone listening on the network. 812.Pp 813.Nm 814automatically maintains and checks a database containing 815identification for all hosts it has ever been used with. 816Host keys are stored in 817.Pa ~/.ssh/known_hosts 818in the user's home directory. 819Additionally, the file 820.Pa /etc/ssh/ssh_known_hosts 821is automatically checked for known hosts. 822Any new hosts are automatically added to the user's file. 823If a host's identification ever changes, 824.Nm 825warns about this and disables password authentication to prevent 826server spoofing or man-in-the-middle attacks, 827which could otherwise be used to circumvent the encryption. 828The 829.Cm StrictHostKeyChecking 830option can be used to control logins to machines whose 831host key is not known or has changed. 832.Pp 833When the user's identity has been accepted by the server, the server 834either executes the given command, or logs into the machine and gives 835the user a normal shell on the remote machine. 836All communication with 837the remote command or shell will be automatically encrypted. 838.Pp 839If a pseudo-terminal has been allocated (normal login session), the 840user may use the escape characters noted below. 841.Pp 842If no pseudo-tty has been allocated, 843the session is transparent and can be used to reliably transfer binary data. 844On most systems, setting the escape character to 845.Dq none 846will also make the session transparent even if a tty is used. 847.Pp 848The session terminates when the command or shell on the remote 849machine exits and all X11 and TCP connections have been closed. 850.Sh ESCAPE CHARACTERS 851When a pseudo-terminal has been requested, 852.Nm 853supports a number of functions through the use of an escape character. 854.Pp 855A single tilde character can be sent as 856.Ic ~~ 857or by following the tilde by a character other than those described below. 858The escape character must always follow a newline to be interpreted as 859special. 860The escape character can be changed in configuration files using the 861.Cm EscapeChar 862configuration directive or on the command line by the 863.Fl e 864option. 865.Pp 866The supported escapes (assuming the default 867.Ql ~ ) 868are: 869.Bl -tag -width Ds 870.It Cm ~. 871Disconnect. 872.It Cm ~^Z 873Background 874.Nm . 875.It Cm ~# 876List forwarded connections. 877.It Cm ~& 878Background 879.Nm 880at logout when waiting for forwarded connection / X11 sessions to terminate. 881.It Cm ~? 882Display a list of escape characters. 883.It Cm ~B 884Send a BREAK to the remote system 885(only useful for SSH protocol version 2 and if the peer supports it). 886.It Cm ~C 887Open command line. 888Currently this allows the addition of port forwardings using the 889.Fl L , 890.Fl R 891and 892.Fl D 893options (see above). 894It also allows the cancellation of existing remote port-forwardings 895using 896.Sm off 897.Fl KR Oo Ar bind_address : Oc Ar port . 898.Sm on 899.Ic !\& Ns Ar command 900allows the user to execute a local command if the 901.Ic PermitLocalCommand 902option is enabled in 903.Xr ssh_config 5 . 904Basic help is available, using the 905.Fl h 906option. 907.It Cm ~R 908Request rekeying of the connection 909(only useful for SSH protocol version 2 and if the peer supports it). 910.El 911.Sh TCP FORWARDING 912Forwarding of arbitrary TCP connections over the secure channel can 913be specified either on the command line or in a configuration file. 914One possible application of TCP forwarding is a secure connection to a 915mail server; another is going through firewalls. 916.Pp 917In the example below, we look at encrypting communication between 918an IRC client and server, even though the IRC server does not directly 919support encrypted communications. 920This works as follows: 921the user connects to the remote host using 922.Nm , 923specifying a port to be used to forward connections 924to the remote server. 925After that it is possible to start the service which is to be encrypted 926on the client machine, 927connecting to the same local port, 928and 929.Nm 930will encrypt and forward the connection. 931.Pp 932The following example tunnels an IRC session from client machine 933.Dq 127.0.0.1 934(localhost) 935to remote server 936.Dq server.example.com : 937.Bd -literal -offset 4n 938$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 939$ irc -c '#users' -p 1234 pinky 127.0.0.1 940.Ed 941.Pp 942This tunnels a connection to IRC server 943.Dq server.example.com , 944joining channel 945.Dq #users , 946nickname 947.Dq pinky , 948using port 1234. 949It doesn't matter which port is used, 950as long as it's greater than 1023 951(remember, only root can open sockets on privileged ports) 952and doesn't conflict with any ports already in use. 953The connection is forwarded to port 6667 on the remote server, 954since that's the standard port for IRC services. 955.Pp 956The 957.Fl f 958option backgrounds 959.Nm 960and the remote command 961.Dq sleep 10 962is specified to allow an amount of time 963(10 seconds, in the example) 964to start the service which is to be tunnelled. 965If no connections are made within the time specified, 966.Nm 967will exit. 968.Sh X11 FORWARDING 969If the 970.Cm ForwardX11 971variable is set to 972.Dq yes 973(or see the description of the 974.Fl X , 975.Fl x , 976and 977.Fl Y 978options above) 979and the user is using X11 (the 980.Ev DISPLAY 981environment variable is set), the connection to the X11 display is 982automatically forwarded to the remote side in such a way that any X11 983programs started from the shell (or command) will go through the 984encrypted channel, and the connection to the real X server will be made 985from the local machine. 986The user should not manually set 987.Ev DISPLAY . 988Forwarding of X11 connections can be 989configured on the command line or in configuration files. 990.Pp 991The 992.Ev DISPLAY 993value set by 994.Nm 995will point to the server machine, but with a display number greater than zero. 996This is normal, and happens because 997.Nm 998creates a 999.Dq proxy 1000X server on the server machine for forwarding the 1001connections over the encrypted channel. 1002.Pp 1003.Nm 1004will also automatically set up Xauthority data on the server machine. 1005For this purpose, it will generate a random authorization cookie, 1006store it in Xauthority on the server, and verify that any forwarded 1007connections carry this cookie and replace it by the real cookie when 1008the connection is opened. 1009The real authentication cookie is never 1010sent to the server machine (and no cookies are sent in the plain). 1011.Pp 1012If the 1013.Cm ForwardAgent 1014variable is set to 1015.Dq yes 1016(or see the description of the 1017.Fl A 1018and 1019.Fl a 1020options above) and 1021the user is using an authentication agent, the connection to the agent 1022is automatically forwarded to the remote side. 1023.Sh VERIFYING HOST KEYS 1024When connecting to a server for the first time, 1025a fingerprint of the server's public key is presented to the user 1026(unless the option 1027.Cm StrictHostKeyChecking 1028has been disabled). 1029Fingerprints can be determined using 1030.Xr ssh-keygen 1 : 1031.Pp 1032.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1033.Pp 1034If the fingerprint is already known, it can be matched 1035and the key can be accepted or rejected. 1036Because of the difficulty of comparing host keys 1037just by looking at hex strings, 1038there is also support to compare host keys visually, 1039using 1040.Em random art . 1041By setting the 1042.Cm VisualHostKey 1043option to 1044.Dq yes , 1045a small ASCII graphic gets displayed on every login to a server, no matter 1046if the session itself is interactive or not. 1047By learning the pattern a known server produces, a user can easily 1048find out that the host key has changed when a completely different pattern 1049is displayed. 1050Because these patterns are not unambiguous however, a pattern that looks 1051similar to the pattern remembered only gives a good probability that the 1052host key is the same, not guaranteed proof. 1053.Pp 1054To get a listing of the fingerprints along with their random art for 1055all known hosts, the following command line can be used: 1056.Pp 1057.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1058.Pp 1059If the fingerprint is unknown, 1060an alternative method of verification is available: 1061SSH fingerprints verified by DNS. 1062An additional resource record (RR), 1063SSHFP, 1064is added to a zonefile 1065and the connecting client is able to match the fingerprint 1066with that of the key presented. 1067.Pp 1068In this example, we are connecting a client to a server, 1069.Dq host.example.com . 1070The SSHFP resource records should first be added to the zonefile for 1071host.example.com: 1072.Bd -literal -offset indent 1073$ ssh-keygen -r host.example.com. 1074.Ed 1075.Pp 1076The output lines will have to be added to the zonefile. 1077To check that the zone is answering fingerprint queries: 1078.Pp 1079.Dl $ dig -t SSHFP host.example.com 1080.Pp 1081Finally the client connects: 1082.Bd -literal -offset indent 1083$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1084[...] 1085Matching host key fingerprint found in DNS. 1086Are you sure you want to continue connecting (yes/no)? 1087.Ed 1088.Pp 1089See the 1090.Cm VerifyHostKeyDNS 1091option in 1092.Xr ssh_config 5 1093for more information. 1094.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1095.Nm 1096contains support for Virtual Private Network (VPN) tunnelling 1097using the 1098.Xr tun 4 1099network pseudo-device, 1100allowing two networks to be joined securely. 1101The 1102.Xr sshd_config 5 1103configuration option 1104.Cm PermitTunnel 1105controls whether the server supports this, 1106and at what level (layer 2 or 3 traffic). 1107.Pp 1108The following example would connect client network 10.0.50.0/24 1109with remote network 10.0.99.0/24 using a point-to-point connection 1110from 10.1.1.1 to 10.1.1.2, 1111provided that the SSH server running on the gateway to the remote network, 1112at 192.168.1.15, allows it. 1113.Pp 1114On the client: 1115.Bd -literal -offset indent 1116# ssh -f -w 0:1 192.168.1.15 true 1117# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1118# route add 10.0.99.0/24 10.1.1.2 1119.Ed 1120.Pp 1121On the server: 1122.Bd -literal -offset indent 1123# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1124# route add 10.0.50.0/24 10.1.1.1 1125.Ed 1126.Pp 1127Client access may be more finely tuned via the 1128.Pa /root/.ssh/authorized_keys 1129file (see below) and the 1130.Cm PermitRootLogin 1131server option. 1132The following entry would permit connections on 1133.Xr tun 4 1134device 1 from user 1135.Dq jane 1136and on tun device 2 from user 1137.Dq john , 1138if 1139.Cm PermitRootLogin 1140is set to 1141.Dq forced-commands-only : 1142.Bd -literal -offset 2n 1143tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1144tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1145.Ed 1146.Pp 1147Since an SSH-based setup entails a fair amount of overhead, 1148it may be more suited to temporary setups, 1149such as for wireless VPNs. 1150More permanent VPNs are better provided by tools such as 1151.Xr ipsecctl 8 1152and 1153.Xr isakmpd 8 . 1154.Sh ENVIRONMENT 1155.Nm 1156will normally set the following environment variables: 1157.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1158.It Ev DISPLAY 1159The 1160.Ev DISPLAY 1161variable indicates the location of the X11 server. 1162It is automatically set by 1163.Nm 1164to point to a value of the form 1165.Dq hostname:n , 1166where 1167.Dq hostname 1168indicates the host where the shell runs, and 1169.Sq n 1170is an integer \*(Ge 1. 1171.Nm 1172uses this special value to forward X11 connections over the secure 1173channel. 1174The user should normally not set 1175.Ev DISPLAY 1176explicitly, as that 1177will render the X11 connection insecure (and will require the user to 1178manually copy any required authorization cookies). 1179.It Ev HOME 1180Set to the path of the user's home directory. 1181.It Ev LOGNAME 1182Synonym for 1183.Ev USER ; 1184set for compatibility with systems that use this variable. 1185.It Ev MAIL 1186Set to the path of the user's mailbox. 1187.It Ev PATH 1188Set to the default 1189.Ev PATH , 1190as specified when compiling 1191.Nm . 1192.It Ev SSH_ASKPASS 1193If 1194.Nm 1195needs a passphrase, it will read the passphrase from the current 1196terminal if it was run from a terminal. 1197If 1198.Nm 1199does not have a terminal associated with it but 1200.Ev DISPLAY 1201and 1202.Ev SSH_ASKPASS 1203are set, it will execute the program specified by 1204.Ev SSH_ASKPASS 1205and open an X11 window to read the passphrase. 1206This is particularly useful when calling 1207.Nm 1208from a 1209.Pa .xsession 1210or related script. 1211(Note that on some machines it 1212may be necessary to redirect the input from 1213.Pa /dev/null 1214to make this work.) 1215.It Ev SSH_AUTH_SOCK 1216Identifies the path of a 1217.Ux Ns -domain 1218socket used to communicate with the agent. 1219.It Ev SSH_CONNECTION 1220Identifies the client and server ends of the connection. 1221The variable contains 1222four space-separated values: client IP address, client port number, 1223server IP address, and server port number. 1224.It Ev SSH_ORIGINAL_COMMAND 1225This variable contains the original command line if a forced command 1226is executed. 1227It can be used to extract the original arguments. 1228.It Ev SSH_TTY 1229This is set to the name of the tty (path to the device) associated 1230with the current shell or command. 1231If the current session has no tty, 1232this variable is not set. 1233.It Ev TZ 1234This variable is set to indicate the present time zone if it 1235was set when the daemon was started (i.e. the daemon passes the value 1236on to new connections). 1237.It Ev USER 1238Set to the name of the user logging in. 1239.El 1240.Pp 1241Additionally, 1242.Nm 1243reads 1244.Pa ~/.ssh/environment , 1245and adds lines of the format 1246.Dq VARNAME=value 1247to the environment if the file exists and users are allowed to 1248change their environment. 1249For more information, see the 1250.Cm PermitUserEnvironment 1251option in 1252.Xr sshd_config 5 . 1253.Sh FILES 1254.Bl -tag -width Ds -compact 1255.It ~/.rhosts 1256This file is used for host-based authentication (see above). 1257On some machines this file may need to be 1258world-readable if the user's home directory is on an NFS partition, 1259because 1260.Xr sshd 8 1261reads it as root. 1262Additionally, this file must be owned by the user, 1263and must not have write permissions for anyone else. 1264The recommended 1265permission for most machines is read/write for the user, and not 1266accessible by others. 1267.Pp 1268.It ~/.shosts 1269This file is used in exactly the same way as 1270.Pa .rhosts , 1271but allows host-based authentication without permitting login with 1272rlogin/rsh. 1273.Pp 1274.It ~/.ssh/ 1275This directory is the default location for all user-specific configuration 1276and authentication information. 1277There is no general requirement to keep the entire contents of this directory 1278secret, but the recommended permissions are read/write/execute for the user, 1279and not accessible by others. 1280.Pp 1281.It ~/.ssh/authorized_keys 1282Lists the public keys (RSA/DSA) that can be used for logging in as this user. 1283The format of this file is described in the 1284.Xr sshd 8 1285manual page. 1286This file is not highly sensitive, but the recommended 1287permissions are read/write for the user, and not accessible by others. 1288.Pp 1289.It ~/.ssh/config 1290This is the per-user configuration file. 1291The file format and configuration options are described in 1292.Xr ssh_config 5 . 1293Because of the potential for abuse, this file must have strict permissions: 1294read/write for the user, and not accessible by others. 1295.Pp 1296.It ~/.ssh/environment 1297Contains additional definitions for environment variables; see 1298.Sx ENVIRONMENT , 1299above. 1300.Pp 1301.It ~/.ssh/identity 1302.It ~/.ssh/id_dsa 1303.It ~/.ssh/id_rsa 1304Contains the private key for authentication. 1305These files 1306contain sensitive data and should be readable by the user but not 1307accessible by others (read/write/execute). 1308.Nm 1309will simply ignore a private key file if it is accessible by others. 1310It is possible to specify a passphrase when 1311generating the key which will be used to encrypt the 1312sensitive part of this file using 3DES. 1313.Pp 1314.It ~/.ssh/identity.pub 1315.It ~/.ssh/id_dsa.pub 1316.It ~/.ssh/id_rsa.pub 1317Contains the public key for authentication. 1318These files are not 1319sensitive and can (but need not) be readable by anyone. 1320.Pp 1321.It ~/.ssh/known_hosts 1322Contains a list of host keys for all hosts the user has logged into 1323that are not already in the systemwide list of known host keys. 1324See 1325.Xr sshd 8 1326for further details of the format of this file. 1327.Pp 1328.It ~/.ssh/rc 1329Commands in this file are executed by 1330.Nm 1331when the user logs in, just before the user's shell (or command) is 1332started. 1333See the 1334.Xr sshd 8 1335manual page for more information. 1336.Pp 1337.It /etc/hosts.equiv 1338This file is for host-based authentication (see above). 1339It should only be writable by root. 1340.Pp 1341.It /etc/shosts.equiv 1342This file is used in exactly the same way as 1343.Pa hosts.equiv , 1344but allows host-based authentication without permitting login with 1345rlogin/rsh. 1346.Pp 1347.It Pa /etc/ssh/ssh_config 1348Systemwide configuration file. 1349The file format and configuration options are described in 1350.Xr ssh_config 5 . 1351.Pp 1352.It /etc/ssh/ssh_host_key 1353.It /etc/ssh/ssh_host_dsa_key 1354.It /etc/ssh/ssh_host_rsa_key 1355These three files contain the private parts of the host keys 1356and are used for host-based authentication. 1357If protocol version 1 is used, 1358.Nm 1359must be setuid root, since the host key is readable only by root. 1360For protocol version 2, 1361.Nm 1362uses 1363.Xr ssh-keysign 8 1364to access the host keys, 1365eliminating the requirement that 1366.Nm 1367be setuid root when host-based authentication is used. 1368By default 1369.Nm 1370is not setuid root. 1371.Pp 1372.It /etc/ssh/ssh_known_hosts 1373Systemwide list of known host keys. 1374This file should be prepared by the 1375system administrator to contain the public host keys of all machines in the 1376organization. 1377It should be world-readable. 1378See 1379.Xr sshd 8 1380for further details of the format of this file. 1381.Pp 1382.It /etc/ssh/sshrc 1383Commands in this file are executed by 1384.Nm 1385when the user logs in, just before the user's shell (or command) is started. 1386See the 1387.Xr sshd 8 1388manual page for more information. 1389.El 1390.Sh SEE ALSO 1391.Xr scp 1 , 1392.Xr sftp 1 , 1393.Xr ssh-add 1 , 1394.Xr ssh-agent 1 , 1395.Xr ssh-keygen 1 , 1396.Xr ssh-keyscan 1 , 1397.Xr tun 4 , 1398.Xr hosts.equiv 5 , 1399.Xr ssh_config 5 , 1400.Xr ssh-keysign 8 , 1401.Xr sshd 8 1402.Rs 1403.%R RFC 4250 1404.%T "The Secure Shell (SSH) Protocol Assigned Numbers" 1405.%D 2006 1406.Re 1407.Rs 1408.%R RFC 4251 1409.%T "The Secure Shell (SSH) Protocol Architecture" 1410.%D 2006 1411.Re 1412.Rs 1413.%R RFC 4252 1414.%T "The Secure Shell (SSH) Authentication Protocol" 1415.%D 2006 1416.Re 1417.Rs 1418.%R RFC 4253 1419.%T "The Secure Shell (SSH) Transport Layer Protocol" 1420.%D 2006 1421.Re 1422.Rs 1423.%R RFC 4254 1424.%T "The Secure Shell (SSH) Connection Protocol" 1425.%D 2006 1426.Re 1427.Rs 1428.%R RFC 4255 1429.%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints" 1430.%D 2006 1431.Re 1432.Rs 1433.%R RFC 4256 1434.%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)" 1435.%D 2006 1436.Re 1437.Rs 1438.%R RFC 4335 1439.%T "The Secure Shell (SSH) Session Channel Break Extension" 1440.%D 2006 1441.Re 1442.Rs 1443.%R RFC 4344 1444.%T "The Secure Shell (SSH) Transport Layer Encryption Modes" 1445.%D 2006 1446.Re 1447.Rs 1448.%R RFC 4345 1449.%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol" 1450.%D 2006 1451.Re 1452.Rs 1453.%R RFC 4419 1454.%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol" 1455.%D 2006 1456.Re 1457.Rs 1458.%R RFC 4716 1459.%T "The Secure Shell (SSH) Public Key File Format" 1460.%D 2006 1461.Re 1462.Rs 1463.%T "Hash Visualization: a New Technique to improve Real-World Security" 1464.%A A. Perrig 1465.%A D. Song 1466.%D 1999 1467.%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)" 1468.Re 1469.Sh AUTHORS 1470OpenSSH is a derivative of the original and free 1471ssh 1.2.12 release by Tatu Ylonen. 1472Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1473Theo de Raadt and Dug Song 1474removed many bugs, re-added newer features and 1475created OpenSSH. 1476Markus Friedl contributed the support for SSH 1477protocol versions 1.5 and 2.0. 1478