39.Dt SSH 1 40.Os 41.Sh NAME 42.Nm ssh 43.Nd OpenSSH SSH client (remote login program) 44.Sh SYNOPSIS 45.Nm ssh 46.Bk -words 47.Op Fl 1246AaCfgKkMNnqsTtVvXxYy 48.Op Fl b Ar bind_address 49.Op Fl c Ar cipher_spec 50.Op Fl D Oo Ar bind_address : Oc Ns Ar port 51.Op Fl E Ar log_file 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 Q Cm cipher | cipher-auth | mac | kex | key 63.Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport 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.Oo Ar user Ns @ Oc Ns Ar hostname 68.Op Ar command 69.Ek 70.Sh DESCRIPTION 71.Nm 72(SSH client) is a program for logging into a remote machine and for 73executing commands on a remote machine. 74It is intended to replace rlogin and rsh, 75and provide secure encrypted communications between 76two untrusted hosts over an insecure network. 77X11 connections and arbitrary TCP ports 78can also be forwarded over the secure channel. 79.Pp 80.Nm 81connects and logs into the specified 82.Ar hostname 83(with optional 84.Ar user 85name). 86The user must prove 87his/her identity to the remote machine using one of several methods 88depending on the protocol version used (see below). 89.Pp 90If 91.Ar command 92is specified, 93it is executed on the remote host instead of a login shell. 94.Pp 95The options are as follows: 96.Bl -tag -width Ds 97.It Fl 1 98Forces 99.Nm 100to try protocol version 1 only. 101.It Fl 2 102Forces 103.Nm 104to try protocol version 2 only. 105.It Fl 4 106Forces 107.Nm 108to use IPv4 addresses only. 109.It Fl 6 110Forces 111.Nm 112to use IPv6 addresses only. 113.It Fl A 114Enables forwarding of the authentication agent connection. 115This can also be specified on a per-host basis in a configuration file. 116.Pp 117Agent forwarding should be enabled with caution. 118Users with the ability to bypass file permissions on the remote host 119(for the agent's 120.Ux Ns -domain 121socket) can access the local agent through the forwarded connection. 122An attacker cannot obtain key material from the agent, 123however they can perform operations on the keys that enable them to 124authenticate using the identities loaded into the agent. 125.It Fl a 126Disables forwarding of the authentication agent connection. 127.It Fl b Ar bind_address 128Use 129.Ar bind_address 130on the local machine as the source address 131of the connection. 132Only useful on systems with more than one address. 133.It Fl C 134Requests compression of all data (including stdin, stdout, stderr, and 135data for forwarded X11 and TCP connections). 136The compression algorithm is the same used by 137.Xr gzip 1 , 138and the 139.Dq level 140can be controlled by the 141.Cm CompressionLevel 142option for protocol version 1. 143Compression is desirable on modem lines and other 144slow connections, but will only slow down things on fast networks. 145The default value can be set on a host-by-host basis in the 146configuration files; see the 147.Cm Compression 148option. 149.It Fl c Ar cipher_spec 150Selects the cipher specification for encrypting the session. 151.Pp 152Protocol version 1 allows specification of a single cipher. 153The supported values are 154.Dq 3des , 155.Dq blowfish , 156and 157.Dq des . 158.Ar 3des 159(triple-des) is an encrypt-decrypt-encrypt triple with three different keys. 160It is believed to be secure. 161.Ar blowfish 162is a fast block cipher; it appears very secure and is much faster than 163.Ar 3des . 164.Ar des 165is only supported in the 166.Nm 167client for interoperability with legacy protocol 1 implementations 168that do not support the 169.Ar 3des 170cipher. 171Its use is strongly discouraged due to cryptographic weaknesses. 172The default is 173.Dq 3des . 174.Pp 175For protocol version 2, 176.Ar cipher_spec 177is a comma-separated list of ciphers 178listed in order of preference. 179See the 180.Cm Ciphers 181keyword in 182.Xr ssh_config 5 183for more information. 184.It Fl D Xo 185.Sm off 186.Oo Ar bind_address : Oc 187.Ar port 188.Sm on 189.Xc 190Specifies a local 191.Dq dynamic 192application-level port forwarding. 193This works by allocating a socket to listen to 194.Ar port 195on the local side, optionally bound to the specified 196.Ar bind_address . 197Whenever a connection is made to this port, the 198connection is forwarded over the secure channel, and the application 199protocol is then used to determine where to connect to from the 200remote machine. 201Currently the SOCKS4 and SOCKS5 protocols are supported, and 202.Nm 203will act as a SOCKS server. 204Only root can forward privileged ports. 205Dynamic port forwardings can also be specified in the configuration file. 206.Pp 207IPv6 addresses can be specified by enclosing the address in square brackets. 208Only the superuser can forward privileged ports. 209By default, the local port is bound in accordance with the 210.Cm GatewayPorts 211setting. 212However, an explicit 213.Ar bind_address 214may be used to bind the connection to a specific address. 215The 216.Ar bind_address 217of 218.Dq localhost 219indicates that the listening port be bound for local use only, while an 220empty address or 221.Sq * 222indicates that the port should be available from all interfaces. 223.It Fl E Ar log_file 224Append debug logs to 225.Ar log_file 226instead of standard error. 227.It Fl e Ar escape_char 228Sets the escape character for sessions with a pty (default: 229.Ql ~ ) . 230The escape character is only recognized at the beginning of a line. 231The escape character followed by a dot 232.Pq Ql \&. 233closes the connection; 234followed by control-Z suspends the connection; 235and followed by itself sends the escape character once. 236Setting the character to 237.Dq none 238disables any escapes and makes the session fully transparent. 239.It Fl F Ar configfile 240Specifies an alternative per-user configuration file. 241If a configuration file is given on the command line, 242the system-wide configuration file 243.Pq Pa /etc/ssh/ssh_config 244will be ignored. 245The default for the per-user configuration file is 246.Pa ~/.ssh/config . 247.It Fl f 248Requests 249.Nm 250to go to background just before command execution. 251This is useful if 252.Nm 253is going to ask for passwords or passphrases, but the user 254wants it in the background. 255This implies 256.Fl n . 257The recommended way to start X11 programs at a remote site is with 258something like 259.Ic ssh -f host xterm . 260.Pp 261If the 262.Cm ExitOnForwardFailure 263configuration option is set to 264.Dq yes , 265then a client started with 266.Fl f 267will wait for all remote port forwards to be successfully established 268before placing itself in the background. 269.It Fl g 270Allows remote hosts to connect to local forwarded ports. 271.It Fl I Ar pkcs11 272Specify the PKCS#11 shared library 273.Nm 274should use to communicate with a PKCS#11 token providing the user's 275private RSA key. 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/identity 281for protocol version 1, and 282.Pa ~/.ssh/id_dsa , 283.Pa ~/.ssh/id_ecdsa , 284.Pa ~/.ssh/id_ed25519 285and 286.Pa ~/.ssh/id_rsa 287for protocol version 2. 288Identity files may also be specified on 289a per-host basis in the configuration file. 290It is possible to have multiple 291.Fl i 292options (and multiple identities specified in 293configuration files). 294.Nm 295will also try to load certificate information from the filename obtained 296by appending 297.Pa -cert.pub 298to identity filenames. 299.It Fl K 300Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI 301credentials to the server. 302.It Fl k 303Disables forwarding (delegation) of GSSAPI credentials to the server. 304.It Fl L Xo 305.Sm off 306.Oo Ar bind_address : Oc 307.Ar port : host : hostport 308.Sm on 309.Xc 310Specifies that the given port on the local (client) host is to be 311forwarded to the given host and port on the remote side. 312This works by allocating a socket to listen to 313.Ar port 314on the local side, optionally bound to the specified 315.Ar bind_address . 316Whenever a connection is made to this port, the 317connection is forwarded over the secure channel, and a connection is 318made to 319.Ar host 320port 321.Ar hostport 322from the remote machine. 323Port forwardings can also be specified in the configuration file. 324IPv6 addresses can be specified by enclosing the address in square brackets. 325Only the superuser can forward privileged ports. 326By default, the local port is bound in accordance with the 327.Cm GatewayPorts 328setting. 329However, an explicit 330.Ar bind_address 331may be used to bind the connection to a specific address. 332The 333.Ar bind_address 334of 335.Dq localhost 336indicates that the listening port be bound for local use only, while an 337empty address or 338.Sq * 339indicates that the port should be available from all interfaces. 340.It Fl l Ar login_name 341Specifies the user to log in as on the remote machine. 342This also may be specified on a per-host basis in the configuration file. 343.It Fl M 344Places the 345.Nm 346client into 347.Dq master 348mode for connection sharing. 349Multiple 350.Fl M 351options places 352.Nm 353into 354.Dq master 355mode with confirmation required before slave connections are accepted. 356Refer to the description of 357.Cm ControlMaster 358in 359.Xr ssh_config 5 360for details. 361.It Fl m Ar mac_spec 362Additionally, for protocol version 2 a comma-separated list of MAC 363(message authentication code) algorithms can 364be specified in order of preference. 365See the 366.Cm MACs 367keyword for more information. 368.It Fl N 369Do not execute a remote command. 370This is useful for just forwarding ports 371(protocol version 2 only). 372.It Fl n 373Redirects stdin from 374.Pa /dev/null 375(actually, prevents reading from stdin). 376This must be used when 377.Nm 378is run in the background. 379A common trick is to use this to run X11 programs on a remote machine. 380For example, 381.Ic ssh -n shadows.cs.hut.fi emacs & 382will start an emacs on shadows.cs.hut.fi, and the X11 383connection will be automatically forwarded over an encrypted channel. 384The 385.Nm 386program will be put in the background. 387(This does not work if 388.Nm 389needs to ask for a password or passphrase; see also the 390.Fl f 391option.) 392.It Fl O Ar ctl_cmd 393Control an active connection multiplexing master process. 394When the 395.Fl O 396option is specified, the 397.Ar ctl_cmd 398argument is interpreted and passed to the master process. 399Valid commands are: 400.Dq check 401(check that the master process is running), 402.Dq forward 403(request forwardings without command execution), 404.Dq cancel 405(cancel forwardings), 406.Dq exit 407(request the master to exit), and 408.Dq stop 409(request the master to stop accepting further multiplexing requests). 410.It Fl o Ar option 411Can be used to give options in the format used in the configuration file. 412This is useful for specifying options for which there is no separate 413command-line flag. 414For full details of the options listed below, and their possible values, see 415.Xr ssh_config 5 . 416.Pp 417.Bl -tag -width Ds -offset indent -compact 418.It AddressFamily 419.It BatchMode 420.It BindAddress 421.It CanonicalDomains 422.It CanonicalizeFallbackLocal 423.It CanonicalizeHostname 424.It CanonicalizeMaxDots 425.It CanonicalizePermittedCNAMEs 426.It ChallengeResponseAuthentication 427.It CheckHostIP 428.It Cipher 429.It Ciphers 430.It ClearAllForwardings 431.It Compression 432.It CompressionLevel 433.It ConnectionAttempts 434.It ConnectTimeout 435.It ControlMaster 436.It ControlPath 437.It ControlPersist 438.It DynamicForward 439.It EscapeChar 440.It ExitOnForwardFailure 441.It ForwardAgent 442.It ForwardX11 443.It ForwardX11Timeout 444.It ForwardX11Trusted 445.It GatewayPorts 446.It GlobalKnownHostsFile 447.It GSSAPIAuthentication 448.It GSSAPIDelegateCredentials 449.It HashKnownHosts 450.It Host 451.It HostbasedAuthentication 452.It HostKeyAlgorithms 453.It HostKeyAlias 454.It HostName 455.It IdentityFile 456.It IdentitiesOnly 457.It IPQoS 458.It KbdInteractiveAuthentication 459.It KbdInteractiveDevices 460.It KexAlgorithms 461.It LocalCommand 462.It LocalForward 463.It LogLevel 464.It MACs 465.It Match 466.It NoHostAuthenticationForLocalhost 467.It NumberOfPasswordPrompts 468.It PasswordAuthentication 469.It PermitLocalCommand 470.It PKCS11Provider 471.It Port 472.It PreferredAuthentications 473.It Protocol 474.It ProxyCommand 475.It ProxyUseFdpass 476.It PubkeyAuthentication 477.It RekeyLimit 478.It RemoteForward 479.It RequestTTY 480.It RhostsRSAAuthentication 481.It RSAAuthentication 482.It SendEnv 483.It ServerAliveInterval 484.It ServerAliveCountMax 485.It StrictHostKeyChecking 486.It TCPKeepAlive 487.It Tunnel 488.It TunnelDevice 489.It UsePrivilegedPort 490.It User 491.It UserKnownHostsFile 492.It VerifyHostKeyDNS 493.It VersionAddendum 494.It VisualHostKey 495.It XAuthLocation 496.El 497.It Fl p Ar port 498Port to connect to on the remote host. 499This can be specified on a 500per-host basis in the configuration file. 501.It Fl Q Cm cipher | cipher-auth | mac | kex | key 502Queries 503.Nm 504for the algorithms supported for the specified version 2. 505The available features are: 506.Ar cipher 507(supported symmetric ciphers), 508.Ar cipher-auth 509(supported symmetric ciphers that support authenticated encryption), 510.Ar mac 511(supported message integrity codes), 512.Ar kex 513(key exchange algorithms), 514.Ar key 515(key types). 516.It Fl q 517Quiet mode. 518Causes most warning and diagnostic messages to be suppressed. 519.It Fl R Xo 520.Sm off 521.Oo Ar bind_address : Oc 522.Ar port : host : hostport 523.Sm on 524.Xc 525Specifies that the given port on the remote (server) host is to be 526forwarded to the given host and port on the local side. 527This works by allocating a socket to listen to 528.Ar port 529on the remote side, and whenever a connection is made to this port, the 530connection is forwarded over the secure channel, and a connection is 531made to 532.Ar host 533port 534.Ar hostport 535from the local machine. 536.Pp 537Port forwardings can also be specified in the configuration file. 538Privileged ports can be forwarded only when 539logging in as root on the remote machine. 540IPv6 addresses can be specified by enclosing the address in square brackets. 541.Pp 542By default, the listening socket on the server will be bound to the loopback 543interface only. 544This may be overridden by specifying a 545.Ar bind_address . 546An empty 547.Ar bind_address , 548or the address 549.Ql * , 550indicates that the remote socket should listen on all interfaces. 551Specifying a remote 552.Ar bind_address 553will only succeed if the server's 554.Cm GatewayPorts 555option is enabled (see 556.Xr sshd_config 5 ) . 557.Pp 558If the 559.Ar port 560argument is 561.Ql 0 , 562the listen port will be dynamically allocated on the server and reported 563to the client at run time. 564When used together with 565.Ic -O forward 566the allocated port will be printed to the standard output. 567.It Fl S Ar ctl_path 568Specifies the location of a control socket for connection sharing, 569or the string 570.Dq none 571to disable connection sharing. 572Refer to the description of 573.Cm ControlPath 574and 575.Cm ControlMaster 576in 577.Xr ssh_config 5 578for details. 579.It Fl s 580May be used to request invocation of a subsystem on the remote system. 581Subsystems are a feature of the SSH2 protocol which facilitate the use 582of SSH as a secure transport for other applications (eg.\& 583.Xr sftp 1 ) . 584The subsystem is specified as the remote command. 585.It Fl T 586Disable pseudo-tty allocation. 587.It Fl t 588Force pseudo-tty allocation. 589This can be used to execute arbitrary 590screen-based programs on a remote machine, which can be very useful, 591e.g. when implementing menu services. 592Multiple 593.Fl t 594options force tty allocation, even if 595.Nm 596has no local tty. 597.It Fl V 598Display the version number and exit. 599.It Fl v 600Verbose mode. 601Causes 602.Nm 603to print debugging messages about its progress. 604This is helpful in 605debugging connection, authentication, and configuration problems. 606Multiple 607.Fl v 608options increase the verbosity. 609The maximum is 3. 610.It Fl W Ar host : Ns Ar port 611Requests that standard input and output on the client be forwarded to 612.Ar host 613on 614.Ar port 615over the secure channel. 616Implies 617.Fl N , 618.Fl T , 619.Cm ExitOnForwardFailure 620and 621.Cm ClearAllForwardings . 622Works with Protocol version 2 only. 623.It Fl w Xo 624.Ar local_tun Ns Op : Ns Ar remote_tun 625.Xc 626Requests 627tunnel 628device forwarding with the specified 629.Xr tun 4 630devices between the client 631.Pq Ar local_tun 632and the server 633.Pq Ar remote_tun . 634.Pp 635The devices may be specified by numerical ID or the keyword 636.Dq any , 637which uses the next available tunnel device. 638If 639.Ar remote_tun 640is not specified, it defaults to 641.Dq any . 642See also the 643.Cm Tunnel 644and 645.Cm TunnelDevice 646directives in 647.Xr ssh_config 5 . 648If the 649.Cm Tunnel 650directive is unset, it is set to the default tunnel mode, which is 651.Dq point-to-point . 652.It Fl X 653Enables X11 forwarding. 654This can also be specified on a per-host basis in a configuration file. 655.Pp 656X11 forwarding should be enabled with caution. 657Users with the ability to bypass file permissions on the remote host 658(for the user's X authorization database) 659can access the local X11 display through the forwarded connection. 660An attacker may then be able to perform activities such as keystroke monitoring. 661.Pp 662For this reason, X11 forwarding is subjected to X11 SECURITY extension 663restrictions by default. 664Please refer to the 665.Nm 666.Fl Y 667option and the 668.Cm ForwardX11Trusted 669directive in 670.Xr ssh_config 5 671for more information. 672.It Fl x 673Disables X11 forwarding. 674.It Fl Y 675Enables trusted X11 forwarding. 676Trusted X11 forwardings are not subjected to the X11 SECURITY extension 677controls. 678.It Fl y 679Send log information using the 680.Xr syslog 3 681system module. 682By default this information is sent to stderr. 683.El 684.Pp 685.Nm 686may additionally obtain configuration data from 687a per-user configuration file and a system-wide configuration file. 688The file format and configuration options are described in 689.Xr ssh_config 5 . 690.Sh AUTHENTICATION 691The OpenSSH SSH client supports SSH protocols 1 and 2. 692The default is to use protocol 2 only, 693though this can be changed via the 694.Cm Protocol 695option in 696.Xr ssh_config 5 697or the 698.Fl 1 699and 700.Fl 2 701options (see above). 702Both protocols support similar authentication methods, 703but protocol 2 is the default since 704it provides additional mechanisms for confidentiality 705(the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) 706and integrity (hmac-md5, hmac-sha1, 707hmac-sha2-256, hmac-sha2-512, 708umac-64, umac-128, hmac-ripemd160). 709Protocol 1 lacks a strong mechanism for ensuring the 710integrity of the connection. 711.Pp 712The methods available for authentication are: 713GSSAPI-based authentication, 714host-based authentication, 715public key authentication, 716challenge-response authentication, 717and password authentication. 718Authentication methods are tried in the order specified above, 719though protocol 2 has a configuration option to change the default order: 720.Cm PreferredAuthentications . 721.Pp 722Host-based authentication works as follows: 723If the machine the user logs in from is listed in 724.Pa /etc/hosts.equiv 725or 726.Pa /etc/shosts.equiv 727on the remote machine, and the user names are 728the same on both sides, or if the files 729.Pa ~/.rhosts 730or 731.Pa ~/.shosts 732exist in the user's home directory on the 733remote machine and contain a line containing the name of the client 734machine and the name of the user on that machine, the user is 735considered for login. 736Additionally, the server 737.Em must 738be able to verify the client's 739host key (see the description of 740.Pa /etc/ssh/ssh_known_hosts 741and 742.Pa ~/.ssh/known_hosts , 743below) 744for login to be permitted. 745This authentication method closes security holes due to IP 746spoofing, DNS spoofing, and routing spoofing. 747[Note to the administrator: 748.Pa /etc/hosts.equiv , 749.Pa ~/.rhosts , 750and the rlogin/rsh protocol in general, are inherently insecure and should be 751disabled if security is desired.] 752.Pp 753Public key authentication works as follows: 754The scheme is based on public-key cryptography, 755using cryptosystems 756where encryption and decryption are done using separate keys, 757and it is unfeasible to derive the decryption key from the encryption key. 758The idea is that each user creates a public/private 759key pair for authentication purposes. 760The server knows the public key, and only the user knows the private key. 761.Nm 762implements public key authentication protocol automatically, 763using one of the DSA, ECDSA, ED25519 or RSA algorithms. 764Protocol 1 is restricted to using only RSA keys, 765but protocol 2 may use any. 766The HISTORY section of 767.Xr ssl 8 768contains a brief discussion of the DSA and RSA algorithms. 769.Pp 770The file 771.Pa ~/.ssh/authorized_keys 772lists the public keys that are permitted for logging in. 773When the user logs in, the 774.Nm 775program tells the server which key pair it would like to use for 776authentication. 777The client proves that it has access to the private key 778and the server checks that the corresponding public key 779is authorized to accept the account. 780.Pp 781The user creates his/her key pair by running 782.Xr ssh-keygen 1 . 783This stores the private key in 784.Pa ~/.ssh/identity 785(protocol 1), 786.Pa ~/.ssh/id_dsa 787(protocol 2 DSA), 788.Pa ~/.ssh/id_ecdsa 789(protocol 2 ECDSA), 790.Pa ~/.ssh/id_ed25519 791(protocol 2 ED25519), 792or 793.Pa ~/.ssh/id_rsa 794(protocol 2 RSA) 795and stores the public key in 796.Pa ~/.ssh/identity.pub 797(protocol 1), 798.Pa ~/.ssh/id_dsa.pub 799(protocol 2 DSA), 800.Pa ~/.ssh/id_ecdsa.pub 801(protocol 2 ECDSA), 802.Pa ~/.ssh/id_ed25519.pub 803(protocol 2 ED25519), 804or 805.Pa ~/.ssh/id_rsa.pub 806(protocol 2 RSA) 807in the user's home directory. 808The user should then copy the public key 809to 810.Pa ~/.ssh/authorized_keys 811in his/her home directory on the remote machine. 812The 813.Pa authorized_keys 814file corresponds to the conventional 815.Pa ~/.rhosts 816file, and has one key 817per line, though the lines can be very long. 818After this, the user can log in without giving the password. 819.Pp 820A variation on public key authentication 821is available in the form of certificate authentication: 822instead of a set of public/private keys, 823signed certificates are used. 824This has the advantage that a single trusted certification authority 825can be used in place of many public/private keys. 826See the CERTIFICATES section of 827.Xr ssh-keygen 1 828for more information. 829.Pp 830The most convenient way to use public key or certificate authentication 831may be with an authentication agent. 832See 833.Xr ssh-agent 1 834for more information. 835.Pp 836Challenge-response authentication works as follows: 837The server sends an arbitrary 838.Qq challenge 839text, and prompts for a response. 840Protocol 2 allows multiple challenges and responses; 841protocol 1 is restricted to just one challenge/response. 842Examples of challenge-response authentication include 843.Bx 844Authentication (see 845.Xr login.conf 5 ) 846and PAM (some 847.Pf non- Ox 848systems). 849.Pp 850Finally, if other authentication methods fail, 851.Nm 852prompts the user for a password. 853The password is sent to the remote 854host for checking; however, since all communications are encrypted, 855the password cannot be seen by someone listening on the network. 856.Pp 857.Nm 858automatically maintains and checks a database containing 859identification for all hosts it has ever been used with. 860Host keys are stored in 861.Pa ~/.ssh/known_hosts 862in the user's home directory. 863Additionally, the file 864.Pa /etc/ssh/ssh_known_hosts 865is automatically checked for known hosts. 866Any new hosts are automatically added to the user's file. 867If a host's identification ever changes, 868.Nm 869warns about this and disables password authentication to prevent 870server spoofing or man-in-the-middle attacks, 871which could otherwise be used to circumvent the encryption. 872The 873.Cm StrictHostKeyChecking 874option can be used to control logins to machines whose 875host key is not known or has changed. 876.Pp 877When the user's identity has been accepted by the server, the server 878either executes the given command, or logs into the machine and gives 879the user a normal shell on the remote machine. 880All communication with 881the remote command or shell will be automatically encrypted. 882.Pp 883If a pseudo-terminal has been allocated (normal login session), the 884user may use the escape characters noted below. 885.Pp 886If no pseudo-tty has been allocated, 887the session is transparent and can be used to reliably transfer binary data. 888On most systems, setting the escape character to 889.Dq none 890will also make the session transparent even if a tty is used. 891.Pp 892The session terminates when the command or shell on the remote 893machine exits and all X11 and TCP connections have been closed. 894.Sh ESCAPE CHARACTERS 895When a pseudo-terminal has been requested, 896.Nm 897supports a number of functions through the use of an escape character. 898.Pp 899A single tilde character can be sent as 900.Ic ~~ 901or by following the tilde by a character other than those described below. 902The escape character must always follow a newline to be interpreted as 903special. 904The escape character can be changed in configuration files using the 905.Cm EscapeChar 906configuration directive or on the command line by the 907.Fl e 908option. 909.Pp 910The supported escapes (assuming the default 911.Ql ~ ) 912are: 913.Bl -tag -width Ds 914.It Cm ~. 915Disconnect. 916.It Cm ~^Z 917Background 918.Nm . 919.It Cm ~# 920List forwarded connections. 921.It Cm ~& 922Background 923.Nm 924at logout when waiting for forwarded connection / X11 sessions to terminate. 925.It Cm ~? 926Display a list of escape characters. 927.It Cm ~B 928Send a BREAK to the remote system 929(only useful for SSH protocol version 2 and if the peer supports it). 930.It Cm ~C 931Open command line. 932Currently this allows the addition of port forwardings using the 933.Fl L , 934.Fl R 935and 936.Fl D 937options (see above). 938It also allows the cancellation of existing port-forwardings 939with 940.Sm off 941.Fl KL Oo Ar bind_address : Oc Ar port 942.Sm on 943for local, 944.Sm off 945.Fl KR Oo Ar bind_address : Oc Ar port 946.Sm on 947for remote and 948.Sm off 949.Fl KD Oo Ar bind_address : Oc Ar port 950.Sm on 951for dynamic port-forwardings. 952.Ic !\& Ns Ar command 953allows the user to execute a local command if the 954.Ic PermitLocalCommand 955option is enabled in 956.Xr ssh_config 5 . 957Basic help is available, using the 958.Fl h 959option. 960.It Cm ~R 961Request rekeying of the connection 962(only useful for SSH protocol version 2 and if the peer supports it). 963.It Cm ~V 964Decrease the verbosity 965.Pq Ic LogLevel 966when errors are being written to stderr. 967.It Cm ~v 968Increase the verbosity 969.Pq Ic LogLevel 970when errors are being written to stderr. 971.El 972.Sh TCP FORWARDING 973Forwarding of arbitrary TCP connections over the secure channel can 974be specified either on the command line or in a configuration file. 975One possible application of TCP forwarding is a secure connection to a 976mail server; another is going through firewalls. 977.Pp 978In the example below, we look at encrypting communication between 979an IRC client and server, even though the IRC server does not directly 980support encrypted communications. 981This works as follows: 982the user connects to the remote host using 983.Nm , 984specifying a port to be used to forward connections 985to the remote server. 986After that it is possible to start the service which is to be encrypted 987on the client machine, 988connecting to the same local port, 989and 990.Nm 991will encrypt and forward the connection. 992.Pp 993The following example tunnels an IRC session from client machine 994.Dq 127.0.0.1 995(localhost) 996to remote server 997.Dq server.example.com : 998.Bd -literal -offset 4n 999$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 1000$ irc -c '#users' -p 1234 pinky 127.0.0.1 1001.Ed 1002.Pp 1003This tunnels a connection to IRC server 1004.Dq server.example.com , 1005joining channel 1006.Dq #users , 1007nickname 1008.Dq pinky , 1009using port 1234. 1010It doesn't matter which port is used, 1011as long as it's greater than 1023 1012(remember, only root can open sockets on privileged ports) 1013and doesn't conflict with any ports already in use. 1014The connection is forwarded to port 6667 on the remote server, 1015since that's the standard port for IRC services. 1016.Pp 1017The 1018.Fl f 1019option backgrounds 1020.Nm 1021and the remote command 1022.Dq sleep 10 1023is specified to allow an amount of time 1024(10 seconds, in the example) 1025to start the service which is to be tunnelled. 1026If no connections are made within the time specified, 1027.Nm 1028will exit. 1029.Sh X11 FORWARDING 1030If the 1031.Cm ForwardX11 1032variable is set to 1033.Dq yes 1034(or see the description of the 1035.Fl X , 1036.Fl x , 1037and 1038.Fl Y 1039options above) 1040and the user is using X11 (the 1041.Ev DISPLAY 1042environment variable is set), the connection to the X11 display is 1043automatically forwarded to the remote side in such a way that any X11 1044programs started from the shell (or command) will go through the 1045encrypted channel, and the connection to the real X server will be made 1046from the local machine. 1047The user should not manually set 1048.Ev DISPLAY . 1049Forwarding of X11 connections can be 1050configured on the command line or in configuration files. 1051.Pp 1052The 1053.Ev DISPLAY 1054value set by 1055.Nm 1056will point to the server machine, but with a display number greater than zero. 1057This is normal, and happens because 1058.Nm 1059creates a 1060.Dq proxy 1061X server on the server machine for forwarding the 1062connections over the encrypted channel. 1063.Pp 1064.Nm 1065will also automatically set up Xauthority data on the server machine. 1066For this purpose, it will generate a random authorization cookie, 1067store it in Xauthority on the server, and verify that any forwarded 1068connections carry this cookie and replace it by the real cookie when 1069the connection is opened. 1070The real authentication cookie is never 1071sent to the server machine (and no cookies are sent in the plain). 1072.Pp 1073If the 1074.Cm ForwardAgent 1075variable is set to 1076.Dq yes 1077(or see the description of the 1078.Fl A 1079and 1080.Fl a 1081options above) and 1082the user is using an authentication agent, the connection to the agent 1083is automatically forwarded to the remote side. 1084.Sh VERIFYING HOST KEYS 1085When connecting to a server for the first time, 1086a fingerprint of the server's public key is presented to the user 1087(unless the option 1088.Cm StrictHostKeyChecking 1089has been disabled). 1090Fingerprints can be determined using 1091.Xr ssh-keygen 1 : 1092.Pp 1093.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1094.Pp 1095If the fingerprint is already known, it can be matched 1096and the key can be accepted or rejected. 1097Because of the difficulty of comparing host keys 1098just by looking at hex strings, 1099there is also support to compare host keys visually, 1100using 1101.Em random art . 1102By setting the 1103.Cm VisualHostKey 1104option to 1105.Dq yes , 1106a small ASCII graphic gets displayed on every login to a server, no matter 1107if the session itself is interactive or not. 1108By learning the pattern a known server produces, a user can easily 1109find out that the host key has changed when a completely different pattern 1110is displayed. 1111Because these patterns are not unambiguous however, a pattern that looks 1112similar to the pattern remembered only gives a good probability that the 1113host key is the same, not guaranteed proof. 1114.Pp 1115To get a listing of the fingerprints along with their random art for 1116all known hosts, the following command line can be used: 1117.Pp 1118.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1119.Pp 1120If the fingerprint is unknown, 1121an alternative method of verification is available: 1122SSH fingerprints verified by DNS. 1123An additional resource record (RR), 1124SSHFP, 1125is added to a zonefile 1126and the connecting client is able to match the fingerprint 1127with that of the key presented. 1128.Pp 1129In this example, we are connecting a client to a server, 1130.Dq host.example.com . 1131The SSHFP resource records should first be added to the zonefile for 1132host.example.com: 1133.Bd -literal -offset indent 1134$ ssh-keygen -r host.example.com. 1135.Ed 1136.Pp 1137The output lines will have to be added to the zonefile. 1138To check that the zone is answering fingerprint queries: 1139.Pp 1140.Dl $ dig -t SSHFP host.example.com 1141.Pp 1142Finally the client connects: 1143.Bd -literal -offset indent 1144$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1145[...] 1146Matching host key fingerprint found in DNS. 1147Are you sure you want to continue connecting (yes/no)? 1148.Ed 1149.Pp 1150See the 1151.Cm VerifyHostKeyDNS 1152option in 1153.Xr ssh_config 5 1154for more information. 1155.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1156.Nm 1157contains support for Virtual Private Network (VPN) tunnelling 1158using the 1159.Xr tun 4 1160network pseudo-device, 1161allowing two networks to be joined securely. 1162The 1163.Xr sshd_config 5 1164configuration option 1165.Cm PermitTunnel 1166controls whether the server supports this, 1167and at what level (layer 2 or 3 traffic). 1168.Pp 1169The following example would connect client network 10.0.50.0/24 1170with remote network 10.0.99.0/24 using a point-to-point connection 1171from 10.1.1.1 to 10.1.1.2, 1172provided that the SSH server running on the gateway to the remote network, 1173at 192.168.1.15, allows it. 1174.Pp 1175On the client: 1176.Bd -literal -offset indent 1177# ssh -f -w 0:1 192.168.1.15 true 1178# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1179# route add 10.0.99.0/24 10.1.1.2 1180.Ed 1181.Pp 1182On the server: 1183.Bd -literal -offset indent 1184# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1185# route add 10.0.50.0/24 10.1.1.1 1186.Ed 1187.Pp 1188Client access may be more finely tuned via the 1189.Pa /root/.ssh/authorized_keys 1190file (see below) and the 1191.Cm PermitRootLogin 1192server option. 1193The following entry would permit connections on 1194.Xr tun 4 1195device 1 from user 1196.Dq jane 1197and on tun device 2 from user 1198.Dq john , 1199if 1200.Cm PermitRootLogin 1201is set to 1202.Dq forced-commands-only : 1203.Bd -literal -offset 2n 1204tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1205tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1206.Ed 1207.Pp 1208Since an SSH-based setup entails a fair amount of overhead, 1209it may be more suited to temporary setups, 1210such as for wireless VPNs. 1211More permanent VPNs are better provided by tools such as 1212.Xr ipsecctl 8 1213and 1214.Xr isakmpd 8 . 1215.Sh ENVIRONMENT 1216.Nm 1217will normally set the following environment variables: 1218.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1219.It Ev DISPLAY 1220The 1221.Ev DISPLAY 1222variable indicates the location of the X11 server. 1223It is automatically set by 1224.Nm 1225to point to a value of the form 1226.Dq hostname:n , 1227where 1228.Dq hostname 1229indicates the host where the shell runs, and 1230.Sq n 1231is an integer \*(Ge 1. 1232.Nm 1233uses this special value to forward X11 connections over the secure 1234channel. 1235The user should normally not set 1236.Ev DISPLAY 1237explicitly, as that 1238will render the X11 connection insecure (and will require the user to 1239manually copy any required authorization cookies). 1240.It Ev HOME 1241Set to the path of the user's home directory. 1242.It Ev LOGNAME 1243Synonym for 1244.Ev USER ; 1245set for compatibility with systems that use this variable. 1246.It Ev MAIL 1247Set to the path of the user's mailbox. 1248.It Ev PATH 1249Set to the default 1250.Ev PATH , 1251as specified when compiling 1252.Nm . 1253.It Ev SSH_ASKPASS 1254If 1255.Nm 1256needs a passphrase, it will read the passphrase from the current 1257terminal if it was run from a terminal. 1258If 1259.Nm 1260does not have a terminal associated with it but 1261.Ev DISPLAY 1262and 1263.Ev SSH_ASKPASS 1264are set, it will execute the program specified by 1265.Ev SSH_ASKPASS 1266and open an X11 window to read the passphrase. 1267This is particularly useful when calling 1268.Nm 1269from a 1270.Pa .xsession 1271or related script. 1272(Note that on some machines it 1273may be necessary to redirect the input from 1274.Pa /dev/null 1275to make this work.) 1276.It Ev SSH_AUTH_SOCK 1277Identifies the path of a 1278.Ux Ns -domain 1279socket used to communicate with the agent. 1280.It Ev SSH_CONNECTION 1281Identifies the client and server ends of the connection. 1282The variable contains 1283four space-separated values: client IP address, client port number, 1284server IP address, and server port number. 1285.It Ev SSH_ORIGINAL_COMMAND 1286This variable contains the original command line if a forced command 1287is executed. 1288It can be used to extract the original arguments. 1289.It Ev SSH_TTY 1290This is set to the name of the tty (path to the device) associated 1291with the current shell or command. 1292If the current session has no tty, 1293this variable is not set. 1294.It Ev TZ 1295This variable is set to indicate the present time zone if it 1296was set when the daemon was started (i.e. the daemon passes the value 1297on to new connections). 1298.It Ev USER 1299Set to the name of the user logging in. 1300.El 1301.Pp 1302Additionally, 1303.Nm 1304reads 1305.Pa ~/.ssh/environment , 1306and adds lines of the format 1307.Dq VARNAME=value 1308to the environment if the file exists and users are allowed to 1309change their environment. 1310For more information, see the 1311.Cm PermitUserEnvironment 1312option in 1313.Xr sshd_config 5 . 1314.Sh FILES 1315.Bl -tag -width Ds -compact 1316.It Pa ~/.rhosts 1317This file is used for host-based authentication (see above). 1318On some machines this file may need to be 1319world-readable if the user's home directory is on an NFS partition, 1320because 1321.Xr sshd 8 1322reads it as root. 1323Additionally, this file must be owned by the user, 1324and must not have write permissions for anyone else. 1325The recommended 1326permission for most machines is read/write for the user, and not 1327accessible by others. 1328.Pp 1329.It Pa ~/.shosts 1330This file is used in exactly the same way as 1331.Pa .rhosts , 1332but allows host-based authentication without permitting login with 1333rlogin/rsh. 1334.Pp 1335.It Pa ~/.ssh/ 1336This directory is the default location for all user-specific configuration 1337and authentication information. 1338There is no general requirement to keep the entire contents of this directory 1339secret, but the recommended permissions are read/write/execute for the user, 1340and not accessible by others. 1341.Pp 1342.It Pa ~/.ssh/authorized_keys 1343Lists the public keys (DSA, ECDSA, ED25519, RSA) 1344that can be used for logging in as this user. 1345The format of this file is described in the 1346.Xr sshd 8 1347manual page. 1348This file is not highly sensitive, but the recommended 1349permissions are read/write for the user, and not accessible by others. 1350.Pp 1351.It Pa ~/.ssh/config 1352This is the per-user configuration file. 1353The file format and configuration options are described in 1354.Xr ssh_config 5 . 1355Because of the potential for abuse, this file must have strict permissions: 1356read/write for the user, and not writable by others. 1357.Pp 1358.It Pa ~/.ssh/environment 1359Contains additional definitions for environment variables; see 1360.Sx ENVIRONMENT , 1361above. 1362.Pp 1363.It Pa ~/.ssh/identity 1364.It Pa ~/.ssh/id_dsa 1365.It Pa ~/.ssh/id_ecdsa 1366.It Pa ~/.ssh/id_ed25519 1367.It Pa ~/.ssh/id_rsa 1368Contains the private key for authentication. 1369These files 1370contain sensitive data and should be readable by the user but not 1371accessible by others (read/write/execute). 1372.Nm 1373will simply ignore a private key file if it is accessible by others. 1374It is possible to specify a passphrase when 1375generating the key which will be used to encrypt the 1376sensitive part of this file using 3DES. 1377.Pp 1378.It Pa ~/.ssh/identity.pub 1379.It Pa ~/.ssh/id_dsa.pub 1380.It Pa ~/.ssh/id_ecdsa.pub 1381.It Pa ~/.ssh/id_ed25519.pub 1382.It Pa ~/.ssh/id_rsa.pub 1383Contains the public key for authentication. 1384These files are not 1385sensitive and can (but need not) be readable by anyone. 1386.Pp 1387.It Pa ~/.ssh/known_hosts 1388Contains a list of host keys for all hosts the user has logged into 1389that are not already in the systemwide list of known host keys. 1390See 1391.Xr sshd 8 1392for further details of the format of this file. 1393.Pp 1394.It Pa ~/.ssh/rc 1395Commands in this file are executed by 1396.Nm 1397when the user logs in, just before the user's shell (or command) is 1398started. 1399See the 1400.Xr sshd 8 1401manual page for more information. 1402.Pp 1403.It Pa /etc/hosts.equiv 1404This file is for host-based authentication (see above). 1405It should only be writable by root. 1406.Pp 1407.It Pa /etc/shosts.equiv 1408This file is used in exactly the same way as 1409.Pa hosts.equiv , 1410but allows host-based authentication without permitting login with 1411rlogin/rsh. 1412.Pp 1413.It Pa /etc/ssh/ssh_config 1414Systemwide configuration file. 1415The file format and configuration options are described in 1416.Xr ssh_config 5 . 1417.Pp 1418.It Pa /etc/ssh/ssh_host_key 1419.It Pa /etc/ssh/ssh_host_dsa_key 1420.It Pa /etc/ssh/ssh_host_ecdsa_key 1421.It Pa /etc/ssh/ssh_host_ed25519_key 1422.It Pa /etc/ssh/ssh_host_rsa_key 1423These files contain the private parts of the host keys 1424and are used for host-based authentication. 1425If protocol version 1 is used, 1426.Nm 1427must be setuid root, since the host key is readable only by root. 1428For protocol version 2, 1429.Nm 1430uses 1431.Xr ssh-keysign 8 1432to access the host keys, 1433eliminating the requirement that 1434.Nm 1435be setuid root when host-based authentication is used. 1436By default 1437.Nm 1438is not setuid root. 1439.Pp 1440.It Pa /etc/ssh/ssh_known_hosts 1441Systemwide list of known host keys. 1442This file should be prepared by the 1443system administrator to contain the public host keys of all machines in the 1444organization. 1445It should be world-readable. 1446See 1447.Xr sshd 8 1448for further details of the format of this file. 1449.Pp 1450.It Pa /etc/ssh/sshrc 1451Commands in this file are executed by 1452.Nm 1453when the user logs in, just before the user's shell (or command) is started. 1454See the 1455.Xr sshd 8 1456manual page for more information. 1457.El 1458.Sh EXIT STATUS 1459.Nm 1460exits with the exit status of the remote command or with 255 1461if an error occurred. 1462.Sh SEE ALSO 1463.Xr scp 1 , 1464.Xr sftp 1 , 1465.Xr ssh-add 1 , 1466.Xr ssh-agent 1 , 1467.Xr ssh-keygen 1 , 1468.Xr ssh-keyscan 1 , 1469.Xr tun 4 , 1470.Xr hosts.equiv 5 , 1471.Xr ssh_config 5 , 1472.Xr ssh-keysign 8 , 1473.Xr sshd 8 1474.Sh STANDARDS 1475.Rs 1476.%A S. Lehtinen 1477.%A C. Lonvick 1478.%D January 2006 1479.%R RFC 4250 1480.%T The Secure Shell (SSH) Protocol Assigned Numbers 1481.Re 1482.Pp 1483.Rs 1484.%A T. Ylonen 1485.%A C. Lonvick 1486.%D January 2006 1487.%R RFC 4251 1488.%T The Secure Shell (SSH) Protocol Architecture 1489.Re 1490.Pp 1491.Rs 1492.%A T. Ylonen 1493.%A C. Lonvick 1494.%D January 2006 1495.%R RFC 4252 1496.%T The Secure Shell (SSH) Authentication Protocol 1497.Re 1498.Pp 1499.Rs 1500.%A T. Ylonen 1501.%A C. Lonvick 1502.%D January 2006 1503.%R RFC 4253 1504.%T The Secure Shell (SSH) Transport Layer Protocol 1505.Re 1506.Pp 1507.Rs 1508.%A T. Ylonen 1509.%A C. Lonvick 1510.%D January 2006 1511.%R RFC 4254 1512.%T The Secure Shell (SSH) Connection Protocol 1513.Re 1514.Pp 1515.Rs 1516.%A J. Schlyter 1517.%A W. Griffin 1518.%D January 2006 1519.%R RFC 4255 1520.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints 1521.Re 1522.Pp 1523.Rs 1524.%A F. Cusack 1525.%A M. Forssen 1526.%D January 2006 1527.%R RFC 4256 1528.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH) 1529.Re 1530.Pp 1531.Rs 1532.%A J. Galbraith 1533.%A P. Remaker 1534.%D January 2006 1535.%R RFC 4335 1536.%T The Secure Shell (SSH) Session Channel Break Extension 1537.Re 1538.Pp 1539.Rs 1540.%A M. Bellare 1541.%A T. Kohno 1542.%A C. Namprempre 1543.%D January 2006 1544.%R RFC 4344 1545.%T The Secure Shell (SSH) Transport Layer Encryption Modes 1546.Re 1547.Pp 1548.Rs 1549.%A B. Harris 1550.%D January 2006 1551.%R RFC 4345 1552.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol 1553.Re 1554.Pp 1555.Rs 1556.%A M. Friedl 1557.%A N. Provos 1558.%A W. Simpson 1559.%D March 2006 1560.%R RFC 4419 1561.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 1562.Re 1563.Pp 1564.Rs 1565.%A J. Galbraith 1566.%A R. Thayer 1567.%D November 2006 1568.%R RFC 4716 1569.%T The Secure Shell (SSH) Public Key File Format 1570.Re 1571.Pp 1572.Rs 1573.%A D. Stebila 1574.%A J. Green 1575.%D December 2009 1576.%R RFC 5656 1577.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer 1578.Re 1579.Pp 1580.Rs 1581.%A A. Perrig 1582.%A D. Song 1583.%D 1999 1584.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99) 1585.%T Hash Visualization: a New Technique to improve Real-World Security 1586.Re 1587.Sh AUTHORS 1588OpenSSH is a derivative of the original and free 1589ssh 1.2.12 release by Tatu Ylonen. 1590Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1591Theo de Raadt and Dug Song 1592removed many bugs, re-added newer features and 1593created OpenSSH. 1594Markus Friedl contributed the support for SSH 1595protocol versions 1.5 and 2.0.
| 39.Dt SSH 1 40.Os 41.Sh NAME 42.Nm ssh 43.Nd OpenSSH SSH client (remote login program) 44.Sh SYNOPSIS 45.Nm ssh 46.Bk -words 47.Op Fl 1246AaCfgKkMNnqsTtVvXxYy 48.Op Fl b Ar bind_address 49.Op Fl c Ar cipher_spec 50.Op Fl D Oo Ar bind_address : Oc Ns Ar port 51.Op Fl E Ar log_file 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 Q Cm cipher | cipher-auth | mac | kex | key 63.Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport 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.Oo Ar user Ns @ Oc Ns Ar hostname 68.Op Ar command 69.Ek 70.Sh DESCRIPTION 71.Nm 72(SSH client) is a program for logging into a remote machine and for 73executing commands on a remote machine. 74It is intended to replace rlogin and rsh, 75and provide secure encrypted communications between 76two untrusted hosts over an insecure network. 77X11 connections and arbitrary TCP ports 78can also be forwarded over the secure channel. 79.Pp 80.Nm 81connects and logs into the specified 82.Ar hostname 83(with optional 84.Ar user 85name). 86The user must prove 87his/her identity to the remote machine using one of several methods 88depending on the protocol version used (see below). 89.Pp 90If 91.Ar command 92is specified, 93it is executed on the remote host instead of a login shell. 94.Pp 95The options are as follows: 96.Bl -tag -width Ds 97.It Fl 1 98Forces 99.Nm 100to try protocol version 1 only. 101.It Fl 2 102Forces 103.Nm 104to try protocol version 2 only. 105.It Fl 4 106Forces 107.Nm 108to use IPv4 addresses only. 109.It Fl 6 110Forces 111.Nm 112to use IPv6 addresses only. 113.It Fl A 114Enables forwarding of the authentication agent connection. 115This can also be specified on a per-host basis in a configuration file. 116.Pp 117Agent forwarding should be enabled with caution. 118Users with the ability to bypass file permissions on the remote host 119(for the agent's 120.Ux Ns -domain 121socket) can access the local agent through the forwarded connection. 122An attacker cannot obtain key material from the agent, 123however they can perform operations on the keys that enable them to 124authenticate using the identities loaded into the agent. 125.It Fl a 126Disables forwarding of the authentication agent connection. 127.It Fl b Ar bind_address 128Use 129.Ar bind_address 130on the local machine as the source address 131of the connection. 132Only useful on systems with more than one address. 133.It Fl C 134Requests compression of all data (including stdin, stdout, stderr, and 135data for forwarded X11 and TCP connections). 136The compression algorithm is the same used by 137.Xr gzip 1 , 138and the 139.Dq level 140can be controlled by the 141.Cm CompressionLevel 142option for protocol version 1. 143Compression is desirable on modem lines and other 144slow connections, but will only slow down things on fast networks. 145The default value can be set on a host-by-host basis in the 146configuration files; see the 147.Cm Compression 148option. 149.It Fl c Ar cipher_spec 150Selects the cipher specification for encrypting the session. 151.Pp 152Protocol version 1 allows specification of a single cipher. 153The supported values are 154.Dq 3des , 155.Dq blowfish , 156and 157.Dq des . 158.Ar 3des 159(triple-des) is an encrypt-decrypt-encrypt triple with three different keys. 160It is believed to be secure. 161.Ar blowfish 162is a fast block cipher; it appears very secure and is much faster than 163.Ar 3des . 164.Ar des 165is only supported in the 166.Nm 167client for interoperability with legacy protocol 1 implementations 168that do not support the 169.Ar 3des 170cipher. 171Its use is strongly discouraged due to cryptographic weaknesses. 172The default is 173.Dq 3des . 174.Pp 175For protocol version 2, 176.Ar cipher_spec 177is a comma-separated list of ciphers 178listed in order of preference. 179See the 180.Cm Ciphers 181keyword in 182.Xr ssh_config 5 183for more information. 184.It Fl D Xo 185.Sm off 186.Oo Ar bind_address : Oc 187.Ar port 188.Sm on 189.Xc 190Specifies a local 191.Dq dynamic 192application-level port forwarding. 193This works by allocating a socket to listen to 194.Ar port 195on the local side, optionally bound to the specified 196.Ar bind_address . 197Whenever a connection is made to this port, the 198connection is forwarded over the secure channel, and the application 199protocol is then used to determine where to connect to from the 200remote machine. 201Currently the SOCKS4 and SOCKS5 protocols are supported, and 202.Nm 203will act as a SOCKS server. 204Only root can forward privileged ports. 205Dynamic port forwardings can also be specified in the configuration file. 206.Pp 207IPv6 addresses can be specified by enclosing the address in square brackets. 208Only the superuser can forward privileged ports. 209By default, the local port is bound in accordance with the 210.Cm GatewayPorts 211setting. 212However, an explicit 213.Ar bind_address 214may be used to bind the connection to a specific address. 215The 216.Ar bind_address 217of 218.Dq localhost 219indicates that the listening port be bound for local use only, while an 220empty address or 221.Sq * 222indicates that the port should be available from all interfaces. 223.It Fl E Ar log_file 224Append debug logs to 225.Ar log_file 226instead of standard error. 227.It Fl e Ar escape_char 228Sets the escape character for sessions with a pty (default: 229.Ql ~ ) . 230The escape character is only recognized at the beginning of a line. 231The escape character followed by a dot 232.Pq Ql \&. 233closes the connection; 234followed by control-Z suspends the connection; 235and followed by itself sends the escape character once. 236Setting the character to 237.Dq none 238disables any escapes and makes the session fully transparent. 239.It Fl F Ar configfile 240Specifies an alternative per-user configuration file. 241If a configuration file is given on the command line, 242the system-wide configuration file 243.Pq Pa /etc/ssh/ssh_config 244will be ignored. 245The default for the per-user configuration file is 246.Pa ~/.ssh/config . 247.It Fl f 248Requests 249.Nm 250to go to background just before command execution. 251This is useful if 252.Nm 253is going to ask for passwords or passphrases, but the user 254wants it in the background. 255This implies 256.Fl n . 257The recommended way to start X11 programs at a remote site is with 258something like 259.Ic ssh -f host xterm . 260.Pp 261If the 262.Cm ExitOnForwardFailure 263configuration option is set to 264.Dq yes , 265then a client started with 266.Fl f 267will wait for all remote port forwards to be successfully established 268before placing itself in the background. 269.It Fl g 270Allows remote hosts to connect to local forwarded ports. 271.It Fl I Ar pkcs11 272Specify the PKCS#11 shared library 273.Nm 274should use to communicate with a PKCS#11 token providing the user's 275private RSA key. 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/identity 281for protocol version 1, and 282.Pa ~/.ssh/id_dsa , 283.Pa ~/.ssh/id_ecdsa , 284.Pa ~/.ssh/id_ed25519 285and 286.Pa ~/.ssh/id_rsa 287for protocol version 2. 288Identity files may also be specified on 289a per-host basis in the configuration file. 290It is possible to have multiple 291.Fl i 292options (and multiple identities specified in 293configuration files). 294.Nm 295will also try to load certificate information from the filename obtained 296by appending 297.Pa -cert.pub 298to identity filenames. 299.It Fl K 300Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI 301credentials to the server. 302.It Fl k 303Disables forwarding (delegation) of GSSAPI credentials to the server. 304.It Fl L Xo 305.Sm off 306.Oo Ar bind_address : Oc 307.Ar port : host : hostport 308.Sm on 309.Xc 310Specifies that the given port on the local (client) host is to be 311forwarded to the given host and port on the remote side. 312This works by allocating a socket to listen to 313.Ar port 314on the local side, optionally bound to the specified 315.Ar bind_address . 316Whenever a connection is made to this port, the 317connection is forwarded over the secure channel, and a connection is 318made to 319.Ar host 320port 321.Ar hostport 322from the remote machine. 323Port forwardings can also be specified in the configuration file. 324IPv6 addresses can be specified by enclosing the address in square brackets. 325Only the superuser can forward privileged ports. 326By default, the local port is bound in accordance with the 327.Cm GatewayPorts 328setting. 329However, an explicit 330.Ar bind_address 331may be used to bind the connection to a specific address. 332The 333.Ar bind_address 334of 335.Dq localhost 336indicates that the listening port be bound for local use only, while an 337empty address or 338.Sq * 339indicates that the port should be available from all interfaces. 340.It Fl l Ar login_name 341Specifies the user to log in as on the remote machine. 342This also may be specified on a per-host basis in the configuration file. 343.It Fl M 344Places the 345.Nm 346client into 347.Dq master 348mode for connection sharing. 349Multiple 350.Fl M 351options places 352.Nm 353into 354.Dq master 355mode with confirmation required before slave connections are accepted. 356Refer to the description of 357.Cm ControlMaster 358in 359.Xr ssh_config 5 360for details. 361.It Fl m Ar mac_spec 362Additionally, for protocol version 2 a comma-separated list of MAC 363(message authentication code) algorithms can 364be specified in order of preference. 365See the 366.Cm MACs 367keyword for more information. 368.It Fl N 369Do not execute a remote command. 370This is useful for just forwarding ports 371(protocol version 2 only). 372.It Fl n 373Redirects stdin from 374.Pa /dev/null 375(actually, prevents reading from stdin). 376This must be used when 377.Nm 378is run in the background. 379A common trick is to use this to run X11 programs on a remote machine. 380For example, 381.Ic ssh -n shadows.cs.hut.fi emacs & 382will start an emacs on shadows.cs.hut.fi, and the X11 383connection will be automatically forwarded over an encrypted channel. 384The 385.Nm 386program will be put in the background. 387(This does not work if 388.Nm 389needs to ask for a password or passphrase; see also the 390.Fl f 391option.) 392.It Fl O Ar ctl_cmd 393Control an active connection multiplexing master process. 394When the 395.Fl O 396option is specified, the 397.Ar ctl_cmd 398argument is interpreted and passed to the master process. 399Valid commands are: 400.Dq check 401(check that the master process is running), 402.Dq forward 403(request forwardings without command execution), 404.Dq cancel 405(cancel forwardings), 406.Dq exit 407(request the master to exit), and 408.Dq stop 409(request the master to stop accepting further multiplexing requests). 410.It Fl o Ar option 411Can be used to give options in the format used in the configuration file. 412This is useful for specifying options for which there is no separate 413command-line flag. 414For full details of the options listed below, and their possible values, see 415.Xr ssh_config 5 . 416.Pp 417.Bl -tag -width Ds -offset indent -compact 418.It AddressFamily 419.It BatchMode 420.It BindAddress 421.It CanonicalDomains 422.It CanonicalizeFallbackLocal 423.It CanonicalizeHostname 424.It CanonicalizeMaxDots 425.It CanonicalizePermittedCNAMEs 426.It ChallengeResponseAuthentication 427.It CheckHostIP 428.It Cipher 429.It Ciphers 430.It ClearAllForwardings 431.It Compression 432.It CompressionLevel 433.It ConnectionAttempts 434.It ConnectTimeout 435.It ControlMaster 436.It ControlPath 437.It ControlPersist 438.It DynamicForward 439.It EscapeChar 440.It ExitOnForwardFailure 441.It ForwardAgent 442.It ForwardX11 443.It ForwardX11Timeout 444.It ForwardX11Trusted 445.It GatewayPorts 446.It GlobalKnownHostsFile 447.It GSSAPIAuthentication 448.It GSSAPIDelegateCredentials 449.It HashKnownHosts 450.It Host 451.It HostbasedAuthentication 452.It HostKeyAlgorithms 453.It HostKeyAlias 454.It HostName 455.It IdentityFile 456.It IdentitiesOnly 457.It IPQoS 458.It KbdInteractiveAuthentication 459.It KbdInteractiveDevices 460.It KexAlgorithms 461.It LocalCommand 462.It LocalForward 463.It LogLevel 464.It MACs 465.It Match 466.It NoHostAuthenticationForLocalhost 467.It NumberOfPasswordPrompts 468.It PasswordAuthentication 469.It PermitLocalCommand 470.It PKCS11Provider 471.It Port 472.It PreferredAuthentications 473.It Protocol 474.It ProxyCommand 475.It ProxyUseFdpass 476.It PubkeyAuthentication 477.It RekeyLimit 478.It RemoteForward 479.It RequestTTY 480.It RhostsRSAAuthentication 481.It RSAAuthentication 482.It SendEnv 483.It ServerAliveInterval 484.It ServerAliveCountMax 485.It StrictHostKeyChecking 486.It TCPKeepAlive 487.It Tunnel 488.It TunnelDevice 489.It UsePrivilegedPort 490.It User 491.It UserKnownHostsFile 492.It VerifyHostKeyDNS 493.It VersionAddendum 494.It VisualHostKey 495.It XAuthLocation 496.El 497.It Fl p Ar port 498Port to connect to on the remote host. 499This can be specified on a 500per-host basis in the configuration file. 501.It Fl Q Cm cipher | cipher-auth | mac | kex | key 502Queries 503.Nm 504for the algorithms supported for the specified version 2. 505The available features are: 506.Ar cipher 507(supported symmetric ciphers), 508.Ar cipher-auth 509(supported symmetric ciphers that support authenticated encryption), 510.Ar mac 511(supported message integrity codes), 512.Ar kex 513(key exchange algorithms), 514.Ar key 515(key types). 516.It Fl q 517Quiet mode. 518Causes most warning and diagnostic messages to be suppressed. 519.It Fl R Xo 520.Sm off 521.Oo Ar bind_address : Oc 522.Ar port : host : hostport 523.Sm on 524.Xc 525Specifies that the given port on the remote (server) host is to be 526forwarded to the given host and port on the local side. 527This works by allocating a socket to listen to 528.Ar port 529on the remote side, and whenever a connection is made to this port, the 530connection is forwarded over the secure channel, and a connection is 531made to 532.Ar host 533port 534.Ar hostport 535from the local machine. 536.Pp 537Port forwardings can also be specified in the configuration file. 538Privileged ports can be forwarded only when 539logging in as root on the remote machine. 540IPv6 addresses can be specified by enclosing the address in square brackets. 541.Pp 542By default, the listening socket on the server will be bound to the loopback 543interface only. 544This may be overridden by specifying a 545.Ar bind_address . 546An empty 547.Ar bind_address , 548or the address 549.Ql * , 550indicates that the remote socket should listen on all interfaces. 551Specifying a remote 552.Ar bind_address 553will only succeed if the server's 554.Cm GatewayPorts 555option is enabled (see 556.Xr sshd_config 5 ) . 557.Pp 558If the 559.Ar port 560argument is 561.Ql 0 , 562the listen port will be dynamically allocated on the server and reported 563to the client at run time. 564When used together with 565.Ic -O forward 566the allocated port will be printed to the standard output. 567.It Fl S Ar ctl_path 568Specifies the location of a control socket for connection sharing, 569or the string 570.Dq none 571to disable connection sharing. 572Refer to the description of 573.Cm ControlPath 574and 575.Cm ControlMaster 576in 577.Xr ssh_config 5 578for details. 579.It Fl s 580May be used to request invocation of a subsystem on the remote system. 581Subsystems are a feature of the SSH2 protocol which facilitate the use 582of SSH as a secure transport for other applications (eg.\& 583.Xr sftp 1 ) . 584The subsystem is specified as the remote command. 585.It Fl T 586Disable pseudo-tty allocation. 587.It Fl t 588Force pseudo-tty allocation. 589This can be used to execute arbitrary 590screen-based programs on a remote machine, which can be very useful, 591e.g. when implementing menu services. 592Multiple 593.Fl t 594options force tty allocation, even if 595.Nm 596has no local tty. 597.It Fl V 598Display the version number and exit. 599.It Fl v 600Verbose mode. 601Causes 602.Nm 603to print debugging messages about its progress. 604This is helpful in 605debugging connection, authentication, and configuration problems. 606Multiple 607.Fl v 608options increase the verbosity. 609The maximum is 3. 610.It Fl W Ar host : Ns Ar port 611Requests that standard input and output on the client be forwarded to 612.Ar host 613on 614.Ar port 615over the secure channel. 616Implies 617.Fl N , 618.Fl T , 619.Cm ExitOnForwardFailure 620and 621.Cm ClearAllForwardings . 622Works with Protocol version 2 only. 623.It Fl w Xo 624.Ar local_tun Ns Op : Ns Ar remote_tun 625.Xc 626Requests 627tunnel 628device forwarding with the specified 629.Xr tun 4 630devices between the client 631.Pq Ar local_tun 632and the server 633.Pq Ar remote_tun . 634.Pp 635The devices may be specified by numerical ID or the keyword 636.Dq any , 637which uses the next available tunnel device. 638If 639.Ar remote_tun 640is not specified, it defaults to 641.Dq any . 642See also the 643.Cm Tunnel 644and 645.Cm TunnelDevice 646directives in 647.Xr ssh_config 5 . 648If the 649.Cm Tunnel 650directive is unset, it is set to the default tunnel mode, which is 651.Dq point-to-point . 652.It Fl X 653Enables X11 forwarding. 654This can also be specified on a per-host basis in a configuration file. 655.Pp 656X11 forwarding should be enabled with caution. 657Users with the ability to bypass file permissions on the remote host 658(for the user's X authorization database) 659can access the local X11 display through the forwarded connection. 660An attacker may then be able to perform activities such as keystroke monitoring. 661.Pp 662For this reason, X11 forwarding is subjected to X11 SECURITY extension 663restrictions by default. 664Please refer to the 665.Nm 666.Fl Y 667option and the 668.Cm ForwardX11Trusted 669directive in 670.Xr ssh_config 5 671for more information. 672.It Fl x 673Disables X11 forwarding. 674.It Fl Y 675Enables trusted X11 forwarding. 676Trusted X11 forwardings are not subjected to the X11 SECURITY extension 677controls. 678.It Fl y 679Send log information using the 680.Xr syslog 3 681system module. 682By default this information is sent to stderr. 683.El 684.Pp 685.Nm 686may additionally obtain configuration data from 687a per-user configuration file and a system-wide configuration file. 688The file format and configuration options are described in 689.Xr ssh_config 5 . 690.Sh AUTHENTICATION 691The OpenSSH SSH client supports SSH protocols 1 and 2. 692The default is to use protocol 2 only, 693though this can be changed via the 694.Cm Protocol 695option in 696.Xr ssh_config 5 697or the 698.Fl 1 699and 700.Fl 2 701options (see above). 702Both protocols support similar authentication methods, 703but protocol 2 is the default since 704it provides additional mechanisms for confidentiality 705(the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) 706and integrity (hmac-md5, hmac-sha1, 707hmac-sha2-256, hmac-sha2-512, 708umac-64, umac-128, hmac-ripemd160). 709Protocol 1 lacks a strong mechanism for ensuring the 710integrity of the connection. 711.Pp 712The methods available for authentication are: 713GSSAPI-based authentication, 714host-based authentication, 715public key authentication, 716challenge-response authentication, 717and password authentication. 718Authentication methods are tried in the order specified above, 719though protocol 2 has a configuration option to change the default order: 720.Cm PreferredAuthentications . 721.Pp 722Host-based authentication works as follows: 723If the machine the user logs in from is listed in 724.Pa /etc/hosts.equiv 725or 726.Pa /etc/shosts.equiv 727on the remote machine, and the user names are 728the same on both sides, or if the files 729.Pa ~/.rhosts 730or 731.Pa ~/.shosts 732exist in the user's home directory on the 733remote machine and contain a line containing the name of the client 734machine and the name of the user on that machine, the user is 735considered for login. 736Additionally, the server 737.Em must 738be able to verify the client's 739host key (see the description of 740.Pa /etc/ssh/ssh_known_hosts 741and 742.Pa ~/.ssh/known_hosts , 743below) 744for login to be permitted. 745This authentication method closes security holes due to IP 746spoofing, DNS spoofing, and routing spoofing. 747[Note to the administrator: 748.Pa /etc/hosts.equiv , 749.Pa ~/.rhosts , 750and the rlogin/rsh protocol in general, are inherently insecure and should be 751disabled if security is desired.] 752.Pp 753Public key authentication works as follows: 754The scheme is based on public-key cryptography, 755using cryptosystems 756where encryption and decryption are done using separate keys, 757and it is unfeasible to derive the decryption key from the encryption key. 758The idea is that each user creates a public/private 759key pair for authentication purposes. 760The server knows the public key, and only the user knows the private key. 761.Nm 762implements public key authentication protocol automatically, 763using one of the DSA, ECDSA, ED25519 or RSA algorithms. 764Protocol 1 is restricted to using only RSA keys, 765but protocol 2 may use any. 766The HISTORY section of 767.Xr ssl 8 768contains a brief discussion of the DSA and RSA algorithms. 769.Pp 770The file 771.Pa ~/.ssh/authorized_keys 772lists the public keys that are permitted for logging in. 773When the user logs in, the 774.Nm 775program tells the server which key pair it would like to use for 776authentication. 777The client proves that it has access to the private key 778and the server checks that the corresponding public key 779is authorized to accept the account. 780.Pp 781The user creates his/her key pair by running 782.Xr ssh-keygen 1 . 783This stores the private key in 784.Pa ~/.ssh/identity 785(protocol 1), 786.Pa ~/.ssh/id_dsa 787(protocol 2 DSA), 788.Pa ~/.ssh/id_ecdsa 789(protocol 2 ECDSA), 790.Pa ~/.ssh/id_ed25519 791(protocol 2 ED25519), 792or 793.Pa ~/.ssh/id_rsa 794(protocol 2 RSA) 795and stores the public key in 796.Pa ~/.ssh/identity.pub 797(protocol 1), 798.Pa ~/.ssh/id_dsa.pub 799(protocol 2 DSA), 800.Pa ~/.ssh/id_ecdsa.pub 801(protocol 2 ECDSA), 802.Pa ~/.ssh/id_ed25519.pub 803(protocol 2 ED25519), 804or 805.Pa ~/.ssh/id_rsa.pub 806(protocol 2 RSA) 807in the user's home directory. 808The user should then copy the public key 809to 810.Pa ~/.ssh/authorized_keys 811in his/her home directory on the remote machine. 812The 813.Pa authorized_keys 814file corresponds to the conventional 815.Pa ~/.rhosts 816file, and has one key 817per line, though the lines can be very long. 818After this, the user can log in without giving the password. 819.Pp 820A variation on public key authentication 821is available in the form of certificate authentication: 822instead of a set of public/private keys, 823signed certificates are used. 824This has the advantage that a single trusted certification authority 825can be used in place of many public/private keys. 826See the CERTIFICATES section of 827.Xr ssh-keygen 1 828for more information. 829.Pp 830The most convenient way to use public key or certificate authentication 831may be with an authentication agent. 832See 833.Xr ssh-agent 1 834for more information. 835.Pp 836Challenge-response authentication works as follows: 837The server sends an arbitrary 838.Qq challenge 839text, and prompts for a response. 840Protocol 2 allows multiple challenges and responses; 841protocol 1 is restricted to just one challenge/response. 842Examples of challenge-response authentication include 843.Bx 844Authentication (see 845.Xr login.conf 5 ) 846and PAM (some 847.Pf non- Ox 848systems). 849.Pp 850Finally, if other authentication methods fail, 851.Nm 852prompts the user for a password. 853The password is sent to the remote 854host for checking; however, since all communications are encrypted, 855the password cannot be seen by someone listening on the network. 856.Pp 857.Nm 858automatically maintains and checks a database containing 859identification for all hosts it has ever been used with. 860Host keys are stored in 861.Pa ~/.ssh/known_hosts 862in the user's home directory. 863Additionally, the file 864.Pa /etc/ssh/ssh_known_hosts 865is automatically checked for known hosts. 866Any new hosts are automatically added to the user's file. 867If a host's identification ever changes, 868.Nm 869warns about this and disables password authentication to prevent 870server spoofing or man-in-the-middle attacks, 871which could otherwise be used to circumvent the encryption. 872The 873.Cm StrictHostKeyChecking 874option can be used to control logins to machines whose 875host key is not known or has changed. 876.Pp 877When the user's identity has been accepted by the server, the server 878either executes the given command, or logs into the machine and gives 879the user a normal shell on the remote machine. 880All communication with 881the remote command or shell will be automatically encrypted. 882.Pp 883If a pseudo-terminal has been allocated (normal login session), the 884user may use the escape characters noted below. 885.Pp 886If no pseudo-tty has been allocated, 887the session is transparent and can be used to reliably transfer binary data. 888On most systems, setting the escape character to 889.Dq none 890will also make the session transparent even if a tty is used. 891.Pp 892The session terminates when the command or shell on the remote 893machine exits and all X11 and TCP connections have been closed. 894.Sh ESCAPE CHARACTERS 895When a pseudo-terminal has been requested, 896.Nm 897supports a number of functions through the use of an escape character. 898.Pp 899A single tilde character can be sent as 900.Ic ~~ 901or by following the tilde by a character other than those described below. 902The escape character must always follow a newline to be interpreted as 903special. 904The escape character can be changed in configuration files using the 905.Cm EscapeChar 906configuration directive or on the command line by the 907.Fl e 908option. 909.Pp 910The supported escapes (assuming the default 911.Ql ~ ) 912are: 913.Bl -tag -width Ds 914.It Cm ~. 915Disconnect. 916.It Cm ~^Z 917Background 918.Nm . 919.It Cm ~# 920List forwarded connections. 921.It Cm ~& 922Background 923.Nm 924at logout when waiting for forwarded connection / X11 sessions to terminate. 925.It Cm ~? 926Display a list of escape characters. 927.It Cm ~B 928Send a BREAK to the remote system 929(only useful for SSH protocol version 2 and if the peer supports it). 930.It Cm ~C 931Open command line. 932Currently this allows the addition of port forwardings using the 933.Fl L , 934.Fl R 935and 936.Fl D 937options (see above). 938It also allows the cancellation of existing port-forwardings 939with 940.Sm off 941.Fl KL Oo Ar bind_address : Oc Ar port 942.Sm on 943for local, 944.Sm off 945.Fl KR Oo Ar bind_address : Oc Ar port 946.Sm on 947for remote and 948.Sm off 949.Fl KD Oo Ar bind_address : Oc Ar port 950.Sm on 951for dynamic port-forwardings. 952.Ic !\& Ns Ar command 953allows the user to execute a local command if the 954.Ic PermitLocalCommand 955option is enabled in 956.Xr ssh_config 5 . 957Basic help is available, using the 958.Fl h 959option. 960.It Cm ~R 961Request rekeying of the connection 962(only useful for SSH protocol version 2 and if the peer supports it). 963.It Cm ~V 964Decrease the verbosity 965.Pq Ic LogLevel 966when errors are being written to stderr. 967.It Cm ~v 968Increase the verbosity 969.Pq Ic LogLevel 970when errors are being written to stderr. 971.El 972.Sh TCP FORWARDING 973Forwarding of arbitrary TCP connections over the secure channel can 974be specified either on the command line or in a configuration file. 975One possible application of TCP forwarding is a secure connection to a 976mail server; another is going through firewalls. 977.Pp 978In the example below, we look at encrypting communication between 979an IRC client and server, even though the IRC server does not directly 980support encrypted communications. 981This works as follows: 982the user connects to the remote host using 983.Nm , 984specifying a port to be used to forward connections 985to the remote server. 986After that it is possible to start the service which is to be encrypted 987on the client machine, 988connecting to the same local port, 989and 990.Nm 991will encrypt and forward the connection. 992.Pp 993The following example tunnels an IRC session from client machine 994.Dq 127.0.0.1 995(localhost) 996to remote server 997.Dq server.example.com : 998.Bd -literal -offset 4n 999$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 1000$ irc -c '#users' -p 1234 pinky 127.0.0.1 1001.Ed 1002.Pp 1003This tunnels a connection to IRC server 1004.Dq server.example.com , 1005joining channel 1006.Dq #users , 1007nickname 1008.Dq pinky , 1009using port 1234. 1010It doesn't matter which port is used, 1011as long as it's greater than 1023 1012(remember, only root can open sockets on privileged ports) 1013and doesn't conflict with any ports already in use. 1014The connection is forwarded to port 6667 on the remote server, 1015since that's the standard port for IRC services. 1016.Pp 1017The 1018.Fl f 1019option backgrounds 1020.Nm 1021and the remote command 1022.Dq sleep 10 1023is specified to allow an amount of time 1024(10 seconds, in the example) 1025to start the service which is to be tunnelled. 1026If no connections are made within the time specified, 1027.Nm 1028will exit. 1029.Sh X11 FORWARDING 1030If the 1031.Cm ForwardX11 1032variable is set to 1033.Dq yes 1034(or see the description of the 1035.Fl X , 1036.Fl x , 1037and 1038.Fl Y 1039options above) 1040and the user is using X11 (the 1041.Ev DISPLAY 1042environment variable is set), the connection to the X11 display is 1043automatically forwarded to the remote side in such a way that any X11 1044programs started from the shell (or command) will go through the 1045encrypted channel, and the connection to the real X server will be made 1046from the local machine. 1047The user should not manually set 1048.Ev DISPLAY . 1049Forwarding of X11 connections can be 1050configured on the command line or in configuration files. 1051.Pp 1052The 1053.Ev DISPLAY 1054value set by 1055.Nm 1056will point to the server machine, but with a display number greater than zero. 1057This is normal, and happens because 1058.Nm 1059creates a 1060.Dq proxy 1061X server on the server machine for forwarding the 1062connections over the encrypted channel. 1063.Pp 1064.Nm 1065will also automatically set up Xauthority data on the server machine. 1066For this purpose, it will generate a random authorization cookie, 1067store it in Xauthority on the server, and verify that any forwarded 1068connections carry this cookie and replace it by the real cookie when 1069the connection is opened. 1070The real authentication cookie is never 1071sent to the server machine (and no cookies are sent in the plain). 1072.Pp 1073If the 1074.Cm ForwardAgent 1075variable is set to 1076.Dq yes 1077(or see the description of the 1078.Fl A 1079and 1080.Fl a 1081options above) and 1082the user is using an authentication agent, the connection to the agent 1083is automatically forwarded to the remote side. 1084.Sh VERIFYING HOST KEYS 1085When connecting to a server for the first time, 1086a fingerprint of the server's public key is presented to the user 1087(unless the option 1088.Cm StrictHostKeyChecking 1089has been disabled). 1090Fingerprints can be determined using 1091.Xr ssh-keygen 1 : 1092.Pp 1093.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1094.Pp 1095If the fingerprint is already known, it can be matched 1096and the key can be accepted or rejected. 1097Because of the difficulty of comparing host keys 1098just by looking at hex strings, 1099there is also support to compare host keys visually, 1100using 1101.Em random art . 1102By setting the 1103.Cm VisualHostKey 1104option to 1105.Dq yes , 1106a small ASCII graphic gets displayed on every login to a server, no matter 1107if the session itself is interactive or not. 1108By learning the pattern a known server produces, a user can easily 1109find out that the host key has changed when a completely different pattern 1110is displayed. 1111Because these patterns are not unambiguous however, a pattern that looks 1112similar to the pattern remembered only gives a good probability that the 1113host key is the same, not guaranteed proof. 1114.Pp 1115To get a listing of the fingerprints along with their random art for 1116all known hosts, the following command line can be used: 1117.Pp 1118.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1119.Pp 1120If the fingerprint is unknown, 1121an alternative method of verification is available: 1122SSH fingerprints verified by DNS. 1123An additional resource record (RR), 1124SSHFP, 1125is added to a zonefile 1126and the connecting client is able to match the fingerprint 1127with that of the key presented. 1128.Pp 1129In this example, we are connecting a client to a server, 1130.Dq host.example.com . 1131The SSHFP resource records should first be added to the zonefile for 1132host.example.com: 1133.Bd -literal -offset indent 1134$ ssh-keygen -r host.example.com. 1135.Ed 1136.Pp 1137The output lines will have to be added to the zonefile. 1138To check that the zone is answering fingerprint queries: 1139.Pp 1140.Dl $ dig -t SSHFP host.example.com 1141.Pp 1142Finally the client connects: 1143.Bd -literal -offset indent 1144$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1145[...] 1146Matching host key fingerprint found in DNS. 1147Are you sure you want to continue connecting (yes/no)? 1148.Ed 1149.Pp 1150See the 1151.Cm VerifyHostKeyDNS 1152option in 1153.Xr ssh_config 5 1154for more information. 1155.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1156.Nm 1157contains support for Virtual Private Network (VPN) tunnelling 1158using the 1159.Xr tun 4 1160network pseudo-device, 1161allowing two networks to be joined securely. 1162The 1163.Xr sshd_config 5 1164configuration option 1165.Cm PermitTunnel 1166controls whether the server supports this, 1167and at what level (layer 2 or 3 traffic). 1168.Pp 1169The following example would connect client network 10.0.50.0/24 1170with remote network 10.0.99.0/24 using a point-to-point connection 1171from 10.1.1.1 to 10.1.1.2, 1172provided that the SSH server running on the gateway to the remote network, 1173at 192.168.1.15, allows it. 1174.Pp 1175On the client: 1176.Bd -literal -offset indent 1177# ssh -f -w 0:1 192.168.1.15 true 1178# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1179# route add 10.0.99.0/24 10.1.1.2 1180.Ed 1181.Pp 1182On the server: 1183.Bd -literal -offset indent 1184# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1185# route add 10.0.50.0/24 10.1.1.1 1186.Ed 1187.Pp 1188Client access may be more finely tuned via the 1189.Pa /root/.ssh/authorized_keys 1190file (see below) and the 1191.Cm PermitRootLogin 1192server option. 1193The following entry would permit connections on 1194.Xr tun 4 1195device 1 from user 1196.Dq jane 1197and on tun device 2 from user 1198.Dq john , 1199if 1200.Cm PermitRootLogin 1201is set to 1202.Dq forced-commands-only : 1203.Bd -literal -offset 2n 1204tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1205tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1206.Ed 1207.Pp 1208Since an SSH-based setup entails a fair amount of overhead, 1209it may be more suited to temporary setups, 1210such as for wireless VPNs. 1211More permanent VPNs are better provided by tools such as 1212.Xr ipsecctl 8 1213and 1214.Xr isakmpd 8 . 1215.Sh ENVIRONMENT 1216.Nm 1217will normally set the following environment variables: 1218.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1219.It Ev DISPLAY 1220The 1221.Ev DISPLAY 1222variable indicates the location of the X11 server. 1223It is automatically set by 1224.Nm 1225to point to a value of the form 1226.Dq hostname:n , 1227where 1228.Dq hostname 1229indicates the host where the shell runs, and 1230.Sq n 1231is an integer \*(Ge 1. 1232.Nm 1233uses this special value to forward X11 connections over the secure 1234channel. 1235The user should normally not set 1236.Ev DISPLAY 1237explicitly, as that 1238will render the X11 connection insecure (and will require the user to 1239manually copy any required authorization cookies). 1240.It Ev HOME 1241Set to the path of the user's home directory. 1242.It Ev LOGNAME 1243Synonym for 1244.Ev USER ; 1245set for compatibility with systems that use this variable. 1246.It Ev MAIL 1247Set to the path of the user's mailbox. 1248.It Ev PATH 1249Set to the default 1250.Ev PATH , 1251as specified when compiling 1252.Nm . 1253.It Ev SSH_ASKPASS 1254If 1255.Nm 1256needs a passphrase, it will read the passphrase from the current 1257terminal if it was run from a terminal. 1258If 1259.Nm 1260does not have a terminal associated with it but 1261.Ev DISPLAY 1262and 1263.Ev SSH_ASKPASS 1264are set, it will execute the program specified by 1265.Ev SSH_ASKPASS 1266and open an X11 window to read the passphrase. 1267This is particularly useful when calling 1268.Nm 1269from a 1270.Pa .xsession 1271or related script. 1272(Note that on some machines it 1273may be necessary to redirect the input from 1274.Pa /dev/null 1275to make this work.) 1276.It Ev SSH_AUTH_SOCK 1277Identifies the path of a 1278.Ux Ns -domain 1279socket used to communicate with the agent. 1280.It Ev SSH_CONNECTION 1281Identifies the client and server ends of the connection. 1282The variable contains 1283four space-separated values: client IP address, client port number, 1284server IP address, and server port number. 1285.It Ev SSH_ORIGINAL_COMMAND 1286This variable contains the original command line if a forced command 1287is executed. 1288It can be used to extract the original arguments. 1289.It Ev SSH_TTY 1290This is set to the name of the tty (path to the device) associated 1291with the current shell or command. 1292If the current session has no tty, 1293this variable is not set. 1294.It Ev TZ 1295This variable is set to indicate the present time zone if it 1296was set when the daemon was started (i.e. the daemon passes the value 1297on to new connections). 1298.It Ev USER 1299Set to the name of the user logging in. 1300.El 1301.Pp 1302Additionally, 1303.Nm 1304reads 1305.Pa ~/.ssh/environment , 1306and adds lines of the format 1307.Dq VARNAME=value 1308to the environment if the file exists and users are allowed to 1309change their environment. 1310For more information, see the 1311.Cm PermitUserEnvironment 1312option in 1313.Xr sshd_config 5 . 1314.Sh FILES 1315.Bl -tag -width Ds -compact 1316.It Pa ~/.rhosts 1317This file is used for host-based authentication (see above). 1318On some machines this file may need to be 1319world-readable if the user's home directory is on an NFS partition, 1320because 1321.Xr sshd 8 1322reads it as root. 1323Additionally, this file must be owned by the user, 1324and must not have write permissions for anyone else. 1325The recommended 1326permission for most machines is read/write for the user, and not 1327accessible by others. 1328.Pp 1329.It Pa ~/.shosts 1330This file is used in exactly the same way as 1331.Pa .rhosts , 1332but allows host-based authentication without permitting login with 1333rlogin/rsh. 1334.Pp 1335.It Pa ~/.ssh/ 1336This directory is the default location for all user-specific configuration 1337and authentication information. 1338There is no general requirement to keep the entire contents of this directory 1339secret, but the recommended permissions are read/write/execute for the user, 1340and not accessible by others. 1341.Pp 1342.It Pa ~/.ssh/authorized_keys 1343Lists the public keys (DSA, ECDSA, ED25519, RSA) 1344that can be used for logging in as this user. 1345The format of this file is described in the 1346.Xr sshd 8 1347manual page. 1348This file is not highly sensitive, but the recommended 1349permissions are read/write for the user, and not accessible by others. 1350.Pp 1351.It Pa ~/.ssh/config 1352This is the per-user configuration file. 1353The file format and configuration options are described in 1354.Xr ssh_config 5 . 1355Because of the potential for abuse, this file must have strict permissions: 1356read/write for the user, and not writable by others. 1357.Pp 1358.It Pa ~/.ssh/environment 1359Contains additional definitions for environment variables; see 1360.Sx ENVIRONMENT , 1361above. 1362.Pp 1363.It Pa ~/.ssh/identity 1364.It Pa ~/.ssh/id_dsa 1365.It Pa ~/.ssh/id_ecdsa 1366.It Pa ~/.ssh/id_ed25519 1367.It Pa ~/.ssh/id_rsa 1368Contains the private key for authentication. 1369These files 1370contain sensitive data and should be readable by the user but not 1371accessible by others (read/write/execute). 1372.Nm 1373will simply ignore a private key file if it is accessible by others. 1374It is possible to specify a passphrase when 1375generating the key which will be used to encrypt the 1376sensitive part of this file using 3DES. 1377.Pp 1378.It Pa ~/.ssh/identity.pub 1379.It Pa ~/.ssh/id_dsa.pub 1380.It Pa ~/.ssh/id_ecdsa.pub 1381.It Pa ~/.ssh/id_ed25519.pub 1382.It Pa ~/.ssh/id_rsa.pub 1383Contains the public key for authentication. 1384These files are not 1385sensitive and can (but need not) be readable by anyone. 1386.Pp 1387.It Pa ~/.ssh/known_hosts 1388Contains a list of host keys for all hosts the user has logged into 1389that are not already in the systemwide list of known host keys. 1390See 1391.Xr sshd 8 1392for further details of the format of this file. 1393.Pp 1394.It Pa ~/.ssh/rc 1395Commands in this file are executed by 1396.Nm 1397when the user logs in, just before the user's shell (or command) is 1398started. 1399See the 1400.Xr sshd 8 1401manual page for more information. 1402.Pp 1403.It Pa /etc/hosts.equiv 1404This file is for host-based authentication (see above). 1405It should only be writable by root. 1406.Pp 1407.It Pa /etc/shosts.equiv 1408This file is used in exactly the same way as 1409.Pa hosts.equiv , 1410but allows host-based authentication without permitting login with 1411rlogin/rsh. 1412.Pp 1413.It Pa /etc/ssh/ssh_config 1414Systemwide configuration file. 1415The file format and configuration options are described in 1416.Xr ssh_config 5 . 1417.Pp 1418.It Pa /etc/ssh/ssh_host_key 1419.It Pa /etc/ssh/ssh_host_dsa_key 1420.It Pa /etc/ssh/ssh_host_ecdsa_key 1421.It Pa /etc/ssh/ssh_host_ed25519_key 1422.It Pa /etc/ssh/ssh_host_rsa_key 1423These files contain the private parts of the host keys 1424and are used for host-based authentication. 1425If protocol version 1 is used, 1426.Nm 1427must be setuid root, since the host key is readable only by root. 1428For protocol version 2, 1429.Nm 1430uses 1431.Xr ssh-keysign 8 1432to access the host keys, 1433eliminating the requirement that 1434.Nm 1435be setuid root when host-based authentication is used. 1436By default 1437.Nm 1438is not setuid root. 1439.Pp 1440.It Pa /etc/ssh/ssh_known_hosts 1441Systemwide list of known host keys. 1442This file should be prepared by the 1443system administrator to contain the public host keys of all machines in the 1444organization. 1445It should be world-readable. 1446See 1447.Xr sshd 8 1448for further details of the format of this file. 1449.Pp 1450.It Pa /etc/ssh/sshrc 1451Commands in this file are executed by 1452.Nm 1453when the user logs in, just before the user's shell (or command) is started. 1454See the 1455.Xr sshd 8 1456manual page for more information. 1457.El 1458.Sh EXIT STATUS 1459.Nm 1460exits with the exit status of the remote command or with 255 1461if an error occurred. 1462.Sh SEE ALSO 1463.Xr scp 1 , 1464.Xr sftp 1 , 1465.Xr ssh-add 1 , 1466.Xr ssh-agent 1 , 1467.Xr ssh-keygen 1 , 1468.Xr ssh-keyscan 1 , 1469.Xr tun 4 , 1470.Xr hosts.equiv 5 , 1471.Xr ssh_config 5 , 1472.Xr ssh-keysign 8 , 1473.Xr sshd 8 1474.Sh STANDARDS 1475.Rs 1476.%A S. Lehtinen 1477.%A C. Lonvick 1478.%D January 2006 1479.%R RFC 4250 1480.%T The Secure Shell (SSH) Protocol Assigned Numbers 1481.Re 1482.Pp 1483.Rs 1484.%A T. Ylonen 1485.%A C. Lonvick 1486.%D January 2006 1487.%R RFC 4251 1488.%T The Secure Shell (SSH) Protocol Architecture 1489.Re 1490.Pp 1491.Rs 1492.%A T. Ylonen 1493.%A C. Lonvick 1494.%D January 2006 1495.%R RFC 4252 1496.%T The Secure Shell (SSH) Authentication Protocol 1497.Re 1498.Pp 1499.Rs 1500.%A T. Ylonen 1501.%A C. Lonvick 1502.%D January 2006 1503.%R RFC 4253 1504.%T The Secure Shell (SSH) Transport Layer Protocol 1505.Re 1506.Pp 1507.Rs 1508.%A T. Ylonen 1509.%A C. Lonvick 1510.%D January 2006 1511.%R RFC 4254 1512.%T The Secure Shell (SSH) Connection Protocol 1513.Re 1514.Pp 1515.Rs 1516.%A J. Schlyter 1517.%A W. Griffin 1518.%D January 2006 1519.%R RFC 4255 1520.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints 1521.Re 1522.Pp 1523.Rs 1524.%A F. Cusack 1525.%A M. Forssen 1526.%D January 2006 1527.%R RFC 4256 1528.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH) 1529.Re 1530.Pp 1531.Rs 1532.%A J. Galbraith 1533.%A P. Remaker 1534.%D January 2006 1535.%R RFC 4335 1536.%T The Secure Shell (SSH) Session Channel Break Extension 1537.Re 1538.Pp 1539.Rs 1540.%A M. Bellare 1541.%A T. Kohno 1542.%A C. Namprempre 1543.%D January 2006 1544.%R RFC 4344 1545.%T The Secure Shell (SSH) Transport Layer Encryption Modes 1546.Re 1547.Pp 1548.Rs 1549.%A B. Harris 1550.%D January 2006 1551.%R RFC 4345 1552.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol 1553.Re 1554.Pp 1555.Rs 1556.%A M. Friedl 1557.%A N. Provos 1558.%A W. Simpson 1559.%D March 2006 1560.%R RFC 4419 1561.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 1562.Re 1563.Pp 1564.Rs 1565.%A J. Galbraith 1566.%A R. Thayer 1567.%D November 2006 1568.%R RFC 4716 1569.%T The Secure Shell (SSH) Public Key File Format 1570.Re 1571.Pp 1572.Rs 1573.%A D. Stebila 1574.%A J. Green 1575.%D December 2009 1576.%R RFC 5656 1577.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer 1578.Re 1579.Pp 1580.Rs 1581.%A A. Perrig 1582.%A D. Song 1583.%D 1999 1584.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99) 1585.%T Hash Visualization: a New Technique to improve Real-World Security 1586.Re 1587.Sh AUTHORS 1588OpenSSH is a derivative of the original and free 1589ssh 1.2.12 release by Tatu Ylonen. 1590Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1591Theo de Raadt and Dug Song 1592removed many bugs, re-added newer features and 1593created OpenSSH. 1594Markus Friedl contributed the support for SSH 1595protocol versions 1.5 and 2.0.
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