| .\" -*- nroff -*- |
| .\" |
| .\" Author: Tatu Ylonen <ylo@cs.hut.fi> |
| .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland |
| .\" All rights reserved |
| .\" |
| .\" As far as I am concerned, the code I have written for this software |
| .\" can be used freely for any purpose. Any derived versions of this |
| .\" software must be clearly marked as such, and if the derived work is |
| .\" incompatible with the protocol description in the RFC file, it must be |
| .\" called by a name other than "ssh" or "Secure Shell". |
| .\" |
| .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. |
| .\" Copyright (c) 1999 Aaron Campbell. All rights reserved. |
| .\" Copyright (c) 1999 Theo de Raadt. All rights reserved. |
| .\" |
| .\" Redistribution and use in source and binary forms, with or without |
| .\" modification, are permitted provided that the following conditions |
| .\" are met: |
| .\" 1. Redistributions of source code must retain the above copyright |
| .\" notice, this list of conditions and the following disclaimer. |
| .\" 2. Redistributions in binary form must reproduce the above copyright |
| .\" notice, this list of conditions and the following disclaimer in the |
| .\" documentation and/or other materials provided with the distribution. |
| .\" |
| .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR |
| .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES |
| .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. |
| .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, |
| .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT |
| .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, |
| .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY |
| .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT |
| .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF |
| .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. |
| .\" |
| .\" $OpenBSD: ssh.1,v 1.271 2008/01/19 19:13:28 djm Exp $ |
| .Dd $Mdocdate: January 19 2008 $ |
| .Dt SSH 1 |
| .Os |
| .Sh NAME |
| .Nm ssh |
| .Nd OpenSSH SSH client (remote login program) |
| .Sh SYNOPSIS |
| .Nm ssh |
| .Op Fl 1246AaCfgKkMNnqsTtVvXxY |
| .Op Fl b Ar bind_address |
| .Op Fl c Ar cipher_spec |
| .Oo Fl D\ \& |
| .Sm off |
| .Oo Ar bind_address : Oc |
| .Ar port |
| .Sm on |
| .Oc |
| .Op Fl e Ar escape_char |
| .Op Fl F Ar configfile |
| .Bk -words |
| .Op Fl i Ar identity_file |
| .Ek |
| .Oo Fl L\ \& |
| .Sm off |
| .Oo Ar bind_address : Oc |
| .Ar port : host : hostport |
| .Sm on |
| .Oc |
| .Bk -words |
| .Op Fl l Ar login_name |
| .Ek |
| .Op Fl m Ar mac_spec |
| .Op Fl O Ar ctl_cmd |
| .Op Fl o Ar option |
| .Op Fl p Ar port |
| .Oo Fl R\ \& |
| .Sm off |
| .Oo Ar bind_address : Oc |
| .Ar port : host : hostport |
| .Sm on |
| .Oc |
| .Op Fl S Ar ctl_path |
| .Bk -words |
| .Oo Fl w Ar local_tun Ns |
| .Op : Ns Ar remote_tun Oc |
| .Oo Ar user Ns @ Oc Ns Ar hostname |
| .Op Ar command |
| .Ek |
| .Sh DESCRIPTION |
| .Nm |
| (SSH client) is a program for logging into a remote machine and for |
| executing commands on a remote machine. |
| It is intended to replace rlogin and rsh, |
| and provide secure encrypted communications between |
| two untrusted hosts over an insecure network. |
| X11 connections and arbitrary TCP ports |
| can also be forwarded over the secure channel. |
| .Pp |
| .Nm |
| connects and logs into the specified |
| .Ar hostname |
| (with optional |
| .Ar user |
| name). |
| The user must prove |
| his/her identity to the remote machine using one of several methods |
| depending on the protocol version used (see below). |
| .Pp |
| If |
| .Ar command |
| is specified, |
| it is executed on the remote host instead of a login shell. |
| .Pp |
| The options are as follows: |
| .Bl -tag -width Ds |
| .It Fl 1 |
| Forces |
| .Nm |
| to try protocol version 1 only. |
| .It Fl 2 |
| Forces |
| .Nm |
| to try protocol version 2 only. |
| .It Fl 4 |
| Forces |
| .Nm |
| to use IPv4 addresses only. |
| .It Fl 6 |
| Forces |
| .Nm |
| to use IPv6 addresses only. |
| .It Fl A |
| Enables forwarding of the authentication agent connection. |
| This can also be specified on a per-host basis in a configuration file. |
| .Pp |
| Agent forwarding should be enabled with caution. |
| Users with the ability to bypass file permissions on the remote host |
| (for the agent's Unix-domain socket) |
| can access the local agent through the forwarded connection. |
| An attacker cannot obtain key material from the agent, |
| however they can perform operations on the keys that enable them to |
| authenticate using the identities loaded into the agent. |
| .It Fl a |
| Disables forwarding of the authentication agent connection. |
| .It Fl b Ar bind_address |
| Use |
| .Ar bind_address |
| on the local machine as the source address |
| of the connection. |
| Only useful on systems with more than one address. |
| .It Fl C |
| Requests compression of all data (including stdin, stdout, stderr, and |
| data for forwarded X11 and TCP connections). |
| The compression algorithm is the same used by |
| .Xr gzip 1 , |
| and the |
| .Dq level |
| can be controlled by the |
| .Cm CompressionLevel |
| option for protocol version 1. |
| Compression is desirable on modem lines and other |
| slow connections, but will only slow down things on fast networks. |
| The default value can be set on a host-by-host basis in the |
| configuration files; see the |
| .Cm Compression |
| option. |
| .It Fl c Ar cipher_spec |
| Selects the cipher specification for encrypting the session. |
| .Pp |
| Protocol version 1 allows specification of a single cipher. |
| The supported values are |
| .Dq 3des , |
| .Dq blowfish , |
| and |
| .Dq des . |
| .Ar 3des |
| (triple-des) is an encrypt-decrypt-encrypt triple with three different keys. |
| It is believed to be secure. |
| .Ar blowfish |
| is a fast block cipher; it appears very secure and is much faster than |
| .Ar 3des . |
| .Ar des |
| is only supported in the |
| .Nm |
| client for interoperability with legacy protocol 1 implementations |
| that do not support the |
| .Ar 3des |
| cipher. |
| Its use is strongly discouraged due to cryptographic weaknesses. |
| The default is |
| .Dq 3des . |
| .Pp |
| For protocol version 2, |
| .Ar cipher_spec |
| is a comma-separated list of ciphers |
| listed in order of preference. |
| The supported ciphers are: |
| 3des-cbc, |
| aes128-cbc, |
| aes192-cbc, |
| aes256-cbc, |
| aes128-ctr, |
| aes192-ctr, |
| aes256-ctr, |
| arcfour128, |
| arcfour256, |
| arcfour, |
| blowfish-cbc, |
| and |
| cast128-cbc. |
| The default is: |
| .Bd -literal -offset indent |
| aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, |
| arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, |
| aes192-ctr,aes256-ctr |
| .Ed |
| .It Fl D Xo |
| .Sm off |
| .Oo Ar bind_address : Oc |
| .Ar port |
| .Sm on |
| .Xc |
| Specifies a local |
| .Dq dynamic |
| application-level port forwarding. |
| This works by allocating a socket to listen to |
| .Ar port |
| on the local side, optionally bound to the specified |
| .Ar bind_address . |
| Whenever a connection is made to this port, the |
| connection is forwarded over the secure channel, and the application |
| protocol is then used to determine where to connect to from the |
| remote machine. |
| Currently the SOCKS4 and SOCKS5 protocols are supported, and |
| .Nm |
| will act as a SOCKS server. |
| Only root can forward privileged ports. |
| Dynamic port forwardings can also be specified in the configuration file. |
| .Pp |
| IPv6 addresses can be specified with an alternative syntax: |
| .Sm off |
| .Xo |
| .Op Ar bind_address No / |
| .Ar port |
| .Xc |
| .Sm on |
| or by enclosing the address in square brackets. |
| Only the superuser can forward privileged ports. |
| By default, the local port is bound in accordance with the |
| .Cm GatewayPorts |
| setting. |
| However, an explicit |
| .Ar bind_address |
| may be used to bind the connection to a specific address. |
| The |
| .Ar bind_address |
| of |
| .Dq localhost |
| indicates that the listening port be bound for local use only, while an |
| empty address or |
| .Sq * |
| indicates that the port should be available from all interfaces. |
| .It Fl e Ar escape_char |
| Sets the escape character for sessions with a pty (default: |
| .Ql ~ ) . |
| The escape character is only recognized at the beginning of a line. |
| The escape character followed by a dot |
| .Pq Ql \&. |
| closes the connection; |
| followed by control-Z suspends the connection; |
| and followed by itself sends the escape character once. |
| Setting the character to |
| .Dq none |
| disables any escapes and makes the session fully transparent. |
| .It Fl F Ar configfile |
| Specifies an alternative per-user configuration file. |
| If a configuration file is given on the command line, |
| the system-wide configuration file |
| .Pq Pa /etc/ssh/ssh_config |
| will be ignored. |
| The default for the per-user configuration file is |
| .Pa ~/.ssh/config . |
| .It Fl f |
| Requests |
| .Nm |
| to go to background just before command execution. |
| This is useful if |
| .Nm |
| is going to ask for passwords or passphrases, but the user |
| wants it in the background. |
| This implies |
| .Fl n . |
| The recommended way to start X11 programs at a remote site is with |
| something like |
| .Ic ssh -f host xterm . |
| .It Fl g |
| Allows remote hosts to connect to local forwarded ports. |
| .It Fl I Ar smartcard_device |
| Specify the device |
| .Nm |
| should use to communicate with a smartcard used for storing the user's |
| private RSA key. |
| This option is only available if support for smartcard devices |
| is compiled in (default is no support). |
| .It Fl i Ar identity_file |
| Selects a file from which the identity (private key) for |
| RSA or DSA authentication is read. |
| The default is |
| .Pa ~/.ssh/identity |
| for protocol version 1, and |
| .Pa ~/.ssh/id_rsa |
| and |
| .Pa ~/.ssh/id_dsa |
| for protocol version 2. |
| Identity files may also be specified on |
| a per-host basis in the configuration file. |
| It is possible to have multiple |
| .Fl i |
| options (and multiple identities specified in |
| configuration files). |
| .It Fl K |
| Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI |
| credentials to the server. |
| .It Fl k |
| Disables forwarding (delegation) of GSSAPI credentials to the server. |
| .It Fl L Xo |
| .Sm off |
| .Oo Ar bind_address : Oc |
| .Ar port : host : hostport |
| .Sm on |
| .Xc |
| Specifies that the given port on the local (client) host is to be |
| forwarded to the given host and port on the remote side. |
| This works by allocating a socket to listen to |
| .Ar port |
| on the local side, optionally bound to the specified |
| .Ar bind_address . |
| Whenever a connection is made to this port, the |
| connection is forwarded over the secure channel, and a connection is |
| made to |
| .Ar host |
| port |
| .Ar hostport |
| from the remote machine. |
| Port forwardings can also be specified in the configuration file. |
| IPv6 addresses can be specified with an alternative syntax: |
| .Sm off |
| .Xo |
| .Op Ar bind_address No / |
| .Ar port No / Ar host No / |
| .Ar hostport |
| .Xc |
| .Sm on |
| or by enclosing the address in square brackets. |
| Only the superuser can forward privileged ports. |
| By default, the local port is bound in accordance with the |
| .Cm GatewayPorts |
| setting. |
| However, an explicit |
| .Ar bind_address |
| may be used to bind the connection to a specific address. |
| The |
| .Ar bind_address |
| of |
| .Dq localhost |
| indicates that the listening port be bound for local use only, while an |
| empty address or |
| .Sq * |
| indicates that the port should be available from all interfaces. |
| .It Fl l Ar login_name |
| Specifies the user to log in as on the remote machine. |
| This also may be specified on a per-host basis in the configuration file. |
| .It Fl M |
| Places the |
| .Nm |
| client into |
| .Dq master |
| mode for connection sharing. |
| Multiple |
| .Fl M |
| options places |
| .Nm |
| into |
| .Dq master |
| mode with confirmation required before slave connections are accepted. |
| Refer to the description of |
| .Cm ControlMaster |
| in |
| .Xr ssh_config 5 |
| for details. |
| .It Fl m Ar mac_spec |
| Additionally, for protocol version 2 a comma-separated list of MAC |
| (message authentication code) algorithms can |
| be specified in order of preference. |
| See the |
| .Cm MACs |
| keyword for more information. |
| .It Fl N |
| Do not execute a remote command. |
| This is useful for just forwarding ports |
| (protocol version 2 only). |
| .It Fl n |
| Redirects stdin from |
| .Pa /dev/null |
| (actually, prevents reading from stdin). |
| This must be used when |
| .Nm |
| is run in the background. |
| A common trick is to use this to run X11 programs on a remote machine. |
| For example, |
| .Ic ssh -n shadows.cs.hut.fi emacs & |
| will start an emacs on shadows.cs.hut.fi, and the X11 |
| connection will be automatically forwarded over an encrypted channel. |
| The |
| .Nm |
| program will be put in the background. |
| (This does not work if |
| .Nm |
| needs to ask for a password or passphrase; see also the |
| .Fl f |
| option.) |
| .It Fl O Ar ctl_cmd |
| Control an active connection multiplexing master process. |
| When the |
| .Fl O |
| option is specified, the |
| .Ar ctl_cmd |
| argument is interpreted and passed to the master process. |
| Valid commands are: |
| .Dq check |
| (check that the master process is running) and |
| .Dq exit |
| (request the master to exit). |
| .It Fl o Ar option |
| Can be used to give options in the format used in the configuration file. |
| This is useful for specifying options for which there is no separate |
| command-line flag. |
| For full details of the options listed below, and their possible values, see |
| .Xr ssh_config 5 . |
| .Pp |
| .Bl -tag -width Ds -offset indent -compact |
| .It AddressFamily |
| .It BatchMode |
| .It BindAddress |
| .It ChallengeResponseAuthentication |
| .It CheckHostIP |
| .It Cipher |
| .It Ciphers |
| .It ClearAllForwardings |
| .It Compression |
| .It CompressionLevel |
| .It ConnectionAttempts |
| .It ConnectTimeout |
| .It ControlMaster |
| .It ControlPath |
| .It DynamicForward |
| .It EscapeChar |
| .It ExitOnForwardFailure |
| .It ForwardAgent |
| .It ForwardX11 |
| .It ForwardX11Trusted |
| .It GatewayPorts |
| .It GlobalKnownHostsFile |
| .It GSSAPIAuthentication |
| .It GSSAPIDelegateCredentials |
| .It HashKnownHosts |
| .It Host |
| .It HostbasedAuthentication |
| .It HostKeyAlgorithms |
| .It HostKeyAlias |
| .It HostName |
| .It IdentityFile |
| .It IdentitiesOnly |
| .It KbdInteractiveDevices |
| .It LocalCommand |
| .It LocalForward |
| .It LogLevel |
| .It MACs |
| .It NoHostAuthenticationForLocalhost |
| .It NumberOfPasswordPrompts |
| .It PasswordAuthentication |
| .It PermitLocalCommand |
| .It Port |
| .It PreferredAuthentications |
| .It Protocol |
| .It ProxyCommand |
| .It PubkeyAuthentication |
| .It RekeyLimit |
| .It RemoteForward |
| .It RhostsRSAAuthentication |
| .It RSAAuthentication |
| .It SendEnv |
| .It ServerAliveInterval |
| .It ServerAliveCountMax |
| .It SmartcardDevice |
| .It StrictHostKeyChecking |
| .It TCPKeepAlive |
| .It Tunnel |
| .It TunnelDevice |
| .It UsePrivilegedPort |
| .It User |
| .It UserKnownHostsFile |
| .It VerifyHostKeyDNS |
| .It XAuthLocation |
| .El |
| .It Fl p Ar port |
| Port to connect to on the remote host. |
| This can be specified on a |
| per-host basis in the configuration file. |
| .It Fl q |
| Quiet mode. |
| Causes most warning and diagnostic messages to be suppressed. |
| .It Fl R Xo |
| .Sm off |
| .Oo Ar bind_address : Oc |
| .Ar port : host : hostport |
| .Sm on |
| .Xc |
| Specifies that the given port on the remote (server) host is to be |
| forwarded to the given host and port on the local side. |
| This works by allocating a socket to listen to |
| .Ar port |
| on the remote side, and whenever a connection is made to this port, the |
| connection is forwarded over the secure channel, and a connection is |
| made to |
| .Ar host |
| port |
| .Ar hostport |
| from the local machine. |
| .Pp |
| Port forwardings can also be specified in the configuration file. |
| Privileged ports can be forwarded only when |
| logging in as root on the remote machine. |
| IPv6 addresses can be specified by enclosing the address in square braces or |
| using an alternative syntax: |
| .Sm off |
| .Xo |
| .Op Ar bind_address No / |
| .Ar host No / Ar port No / |
| .Ar hostport |
| .Xc . |
| .Sm on |
| .Pp |
| By default, the listening socket on the server will be bound to the loopback |
| interface only. |
| This may be overriden by specifying a |
| .Ar bind_address . |
| An empty |
| .Ar bind_address , |
| or the address |
| .Ql * , |
| indicates that the remote socket should listen on all interfaces. |
| Specifying a remote |
| .Ar bind_address |
| will only succeed if the server's |
| .Cm GatewayPorts |
| option is enabled (see |
| .Xr sshd_config 5 ) . |
| .It Fl S Ar ctl_path |
| Specifies the location of a control socket for connection sharing. |
| Refer to the description of |
| .Cm ControlPath |
| and |
| .Cm ControlMaster |
| in |
| .Xr ssh_config 5 |
| for details. |
| .It Fl s |
| May be used to request invocation of a subsystem on the remote system. |
| Subsystems are a feature of the SSH2 protocol which facilitate the use |
| of SSH as a secure transport for other applications (eg.\& |
| .Xr sftp 1 ) . |
| The subsystem is specified as the remote command. |
| .It Fl T |
| Disable pseudo-tty allocation. |
| .It Fl t |
| Force pseudo-tty allocation. |
| This can be used to execute arbitrary |
| screen-based programs on a remote machine, which can be very useful, |
| e.g. when implementing menu services. |
| Multiple |
| .Fl t |
| options force tty allocation, even if |
| .Nm |
| has no local tty. |
| .It Fl V |
| Display the version number and exit. |
| .It Fl v |
| Verbose mode. |
| Causes |
| .Nm |
| to print debugging messages about its progress. |
| This is helpful in |
| debugging connection, authentication, and configuration problems. |
| Multiple |
| .Fl v |
| options increase the verbosity. |
| The maximum is 3. |
| .It Fl w Xo |
| .Ar local_tun Ns Op : Ns Ar remote_tun |
| .Xc |
| Requests |
| tunnel |
| device forwarding with the specified |
| .Xr tun 4 |
| devices between the client |
| .Pq Ar local_tun |
| and the server |
| .Pq Ar remote_tun . |
| .Pp |
| The devices may be specified by numerical ID or the keyword |
| .Dq any , |
| which uses the next available tunnel device. |
| If |
| .Ar remote_tun |
| is not specified, it defaults to |
| .Dq any . |
| See also the |
| .Cm Tunnel |
| and |
| .Cm TunnelDevice |
| directives in |
| .Xr ssh_config 5 . |
| If the |
| .Cm Tunnel |
| directive is unset, it is set to the default tunnel mode, which is |
| .Dq point-to-point . |
| .It Fl X |
| Enables X11 forwarding. |
| This can also be specified on a per-host basis in a configuration file. |
| .Pp |
| X11 forwarding should be enabled with caution. |
| Users with the ability to bypass file permissions on the remote host |
| (for the user's X authorization database) |
| can access the local X11 display through the forwarded connection. |
| An attacker may then be able to perform activities such as keystroke monitoring. |
| .Pp |
| For this reason, X11 forwarding is subjected to X11 SECURITY extension |
| restrictions by default. |
| Please refer to the |
| .Nm |
| .Fl Y |
| option and the |
| .Cm ForwardX11Trusted |
| directive in |
| .Xr ssh_config 5 |
| for more information. |
| .It Fl x |
| Disables X11 forwarding. |
| .It Fl Y |
| Enables trusted X11 forwarding. |
| Trusted X11 forwardings are not subjected to the X11 SECURITY extension |
| controls. |
| .El |
| .Pp |
| .Nm |
| may additionally obtain configuration data from |
| a per-user configuration file and a system-wide configuration file. |
| The file format and configuration options are described in |
| .Xr ssh_config 5 . |
| .Pp |
| .Nm |
| exits with the exit status of the remote command or with 255 |
| if an error occurred. |
| .Sh AUTHENTICATION |
| The OpenSSH SSH client supports SSH protocols 1 and 2. |
| Protocol 2 is the default, with |
| .Nm |
| falling back to protocol 1 if it detects protocol 2 is unsupported. |
| These settings may be altered using the |
| .Cm Protocol |
| option in |
| .Xr ssh_config 5 , |
| or enforced using the |
| .Fl 1 |
| and |
| .Fl 2 |
| options (see above). |
| Both protocols support similar authentication methods, |
| but protocol 2 is preferred since |
| it provides additional mechanisms for confidentiality |
| (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) |
| and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160). |
| Protocol 1 lacks a strong mechanism for ensuring the |
| integrity of the connection. |
| .Pp |
| The methods available for authentication are: |
| GSSAPI-based authentication, |
| host-based authentication, |
| public key authentication, |
| challenge-response authentication, |
| and password authentication. |
| Authentication methods are tried in the order specified above, |
| though protocol 2 has a configuration option to change the default order: |
| .Cm PreferredAuthentications . |
| .Pp |
| Host-based authentication works as follows: |
| If the machine the user logs in from is listed in |
| .Pa /etc/hosts.equiv |
| or |
| .Pa /etc/shosts.equiv |
| on the remote machine, and the user names are |
| the same on both sides, or if the files |
| .Pa ~/.rhosts |
| or |
| .Pa ~/.shosts |
| exist in the user's home directory on the |
| remote machine and contain a line containing the name of the client |
| machine and the name of the user on that machine, the user is |
| considered for login. |
| Additionally, the server |
| .Em must |
| be able to verify the client's |
| host key (see the description of |
| .Pa /etc/ssh/ssh_known_hosts |
| and |
| .Pa ~/.ssh/known_hosts , |
| below) |
| for login to be permitted. |
| This authentication method closes security holes due to IP |
| spoofing, DNS spoofing, and routing spoofing. |
| [Note to the administrator: |
| .Pa /etc/hosts.equiv , |
| .Pa ~/.rhosts , |
| and the rlogin/rsh protocol in general, are inherently insecure and should be |
| disabled if security is desired.] |
| .Pp |
| Public key authentication works as follows: |
| The scheme is based on public-key cryptography, |
| using cryptosystems |
| where encryption and decryption are done using separate keys, |
| and it is unfeasible to derive the decryption key from the encryption key. |
| The idea is that each user creates a public/private |
| key pair for authentication purposes. |
| The server knows the public key, and only the user knows the private key. |
| .Nm |
| implements public key authentication protocol automatically, |
| using either the RSA or DSA algorithms. |
| Protocol 1 is restricted to using only RSA keys, |
| but protocol 2 may use either. |
| The |
| .Sx HISTORY |
| section of |
| .Xr ssl 8 |
| contains a brief discussion of the two algorithms. |
| .Pp |
| The file |
| .Pa ~/.ssh/authorized_keys |
| lists the public keys that are permitted for logging in. |
| When the user logs in, the |
| .Nm |
| program tells the server which key pair it would like to use for |
| authentication. |
| The client proves that it has access to the private key |
| and the server checks that the corresponding public key |
| is authorized to accept the account. |
| .Pp |
| The user creates his/her key pair by running |
| .Xr ssh-keygen 1 . |
| This stores the private key in |
| .Pa ~/.ssh/identity |
| (protocol 1), |
| .Pa ~/.ssh/id_dsa |
| (protocol 2 DSA), |
| or |
| .Pa ~/.ssh/id_rsa |
| (protocol 2 RSA) |
| and stores the public key in |
| .Pa ~/.ssh/identity.pub |
| (protocol 1), |
| .Pa ~/.ssh/id_dsa.pub |
| (protocol 2 DSA), |
| or |
| .Pa ~/.ssh/id_rsa.pub |
| (protocol 2 RSA) |
| in the user's home directory. |
| The user should then copy the public key |
| to |
| .Pa ~/.ssh/authorized_keys |
| in his/her home directory on the remote machine. |
| The |
| .Pa authorized_keys |
| file corresponds to the conventional |
| .Pa ~/.rhosts |
| file, and has one key |
| per line, though the lines can be very long. |
| After this, the user can log in without giving the password. |
| .Pp |
| The most convenient way to use public key authentication may be with an |
| authentication agent. |
| See |
| .Xr ssh-agent 1 |
| for more information. |
| .Pp |
| Challenge-response authentication works as follows: |
| The server sends an arbitrary |
| .Qq challenge |
| text, and prompts for a response. |
| Protocol 2 allows multiple challenges and responses; |
| protocol 1 is restricted to just one challenge/response. |
| Examples of challenge-response authentication include |
| BSD Authentication (see |
| .Xr login.conf 5 ) |
| and PAM (some non-OpenBSD systems). |
| .Pp |
| Finally, if other authentication methods fail, |
| .Nm |
| prompts the user for a password. |
| The password is sent to the remote |
| host for checking; however, since all communications are encrypted, |
| the password cannot be seen by someone listening on the network. |
| .Pp |
| .Nm |
| automatically maintains and checks a database containing |
| identification for all hosts it has ever been used with. |
| Host keys are stored in |
| .Pa ~/.ssh/known_hosts |
| in the user's home directory. |
| Additionally, the file |
| .Pa /etc/ssh/ssh_known_hosts |
| is automatically checked for known hosts. |
| Any new hosts are automatically added to the user's file. |
| If a host's identification ever changes, |
| .Nm |
| warns about this and disables password authentication to prevent |
| server spoofing or man-in-the-middle attacks, |
| which could otherwise be used to circumvent the encryption. |
| The |
| .Cm StrictHostKeyChecking |
| option can be used to control logins to machines whose |
| host key is not known or has changed. |
| .Pp |
| When the user's identity has been accepted by the server, the server |
| either executes the given command, or logs into the machine and gives |
| the user a normal shell on the remote machine. |
| All communication with |
| the remote command or shell will be automatically encrypted. |
| .Pp |
| If a pseudo-terminal has been allocated (normal login session), the |
| user may use the escape characters noted below. |
| .Pp |
| If no pseudo-tty has been allocated, |
| the session is transparent and can be used to reliably transfer binary data. |
| On most systems, setting the escape character to |
| .Dq none |
| will also make the session transparent even if a tty is used. |
| .Pp |
| The session terminates when the command or shell on the remote |
| machine exits and all X11 and TCP connections have been closed. |
| .Sh ESCAPE CHARACTERS |
| When a pseudo-terminal has been requested, |
| .Nm |
| supports a number of functions through the use of an escape character. |
| .Pp |
| A single tilde character can be sent as |
| .Ic ~~ |
| or by following the tilde by a character other than those described below. |
| The escape character must always follow a newline to be interpreted as |
| special. |
| The escape character can be changed in configuration files using the |
| .Cm EscapeChar |
| configuration directive or on the command line by the |
| .Fl e |
| option. |
| .Pp |
| The supported escapes (assuming the default |
| .Ql ~ ) |
| are: |
| .Bl -tag -width Ds |
| .It Cm ~. |
| Disconnect. |
| .It Cm ~^Z |
| Background |
| .Nm . |
| .It Cm ~# |
| List forwarded connections. |
| .It Cm ~& |
| Background |
| .Nm |
| at logout when waiting for forwarded connection / X11 sessions to terminate. |
| .It Cm ~? |
| Display a list of escape characters. |
| .It Cm ~B |
| Send a BREAK to the remote system |
| (only useful for SSH protocol version 2 and if the peer supports it). |
| .It Cm ~C |
| Open command line. |
| Currently this allows the addition of port forwardings using the |
| .Fl L |
| and |
| .Fl R |
| options (see above). |
| It also allows the cancellation of existing remote port-forwardings |
| using |
| .Sm off |
| .Fl KR Oo Ar bind_address : Oc Ar port . |
| .Sm on |
| .Ic !\& Ns Ar command |
| allows the user to execute a local command if the |
| .Ic PermitLocalCommand |
| option is enabled in |
| .Xr ssh_config 5 . |
| Basic help is available, using the |
| .Fl h |
| option. |
| .It Cm ~R |
| Request rekeying of the connection |
| (only useful for SSH protocol version 2 and if the peer supports it). |
| .El |
| .Sh TCP FORWARDING |
| Forwarding of arbitrary TCP connections over the secure channel can |
| be specified either on the command line or in a configuration file. |
| One possible application of TCP forwarding is a secure connection to a |
| mail server; another is going through firewalls. |
| .Pp |
| In the example below, we look at encrypting communication between |
| an IRC client and server, even though the IRC server does not directly |
| support encrypted communications. |
| This works as follows: |
| the user connects to the remote host using |
| .Nm , |
| specifying a port to be used to forward connections |
| to the remote server. |
| After that it is possible to start the service which is to be encrypted |
| on the client machine, |
| connecting to the same local port, |
| and |
| .Nm |
| will encrypt and forward the connection. |
| .Pp |
| The following example tunnels an IRC session from client machine |
| .Dq 127.0.0.1 |
| (localhost) |
| to remote server |
| .Dq server.example.com : |
| .Bd -literal -offset 4n |
| $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 |
| $ irc -c '#users' -p 1234 pinky 127.0.0.1 |
| .Ed |
| .Pp |
| This tunnels a connection to IRC server |
| .Dq server.example.com , |
| joining channel |
| .Dq #users , |
| nickname |
| .Dq pinky , |
| using port 1234. |
| It doesn't matter which port is used, |
| as long as it's greater than 1023 |
| (remember, only root can open sockets on privileged ports) |
| and doesn't conflict with any ports already in use. |
| The connection is forwarded to port 6667 on the remote server, |
| since that's the standard port for IRC services. |
| .Pp |
| The |
| .Fl f |
| option backgrounds |
| .Nm |
| and the remote command |
| .Dq sleep 10 |
| is specified to allow an amount of time |
| (10 seconds, in the example) |
| to start the service which is to be tunnelled. |
| If no connections are made within the time specified, |
| .Nm |
| will exit. |
| .Sh X11 FORWARDING |
| If the |
| .Cm ForwardX11 |
| variable is set to |
| .Dq yes |
| (or see the description of the |
| .Fl X , |
| .Fl x , |
| and |
| .Fl Y |
| options above) |
| and the user is using X11 (the |
| .Ev DISPLAY |
| environment variable is set), the connection to the X11 display is |
| automatically forwarded to the remote side in such a way that any X11 |
| programs started from the shell (or command) will go through the |
| encrypted channel, and the connection to the real X server will be made |
| from the local machine. |
| The user should not manually set |
| .Ev DISPLAY . |
| Forwarding of X11 connections can be |
| configured on the command line or in configuration files. |
| .Pp |
| The |
| .Ev DISPLAY |
| value set by |
| .Nm |
| will point to the server machine, but with a display number greater than zero. |
| This is normal, and happens because |
| .Nm |
| creates a |
| .Dq proxy |
| X server on the server machine for forwarding the |
| connections over the encrypted channel. |
| .Pp |
| .Nm |
| will also automatically set up Xauthority data on the server machine. |
| For this purpose, it will generate a random authorization cookie, |
| store it in Xauthority on the server, and verify that any forwarded |
| connections carry this cookie and replace it by the real cookie when |
| the connection is opened. |
| The real authentication cookie is never |
| sent to the server machine (and no cookies are sent in the plain). |
| .Pp |
| If the |
| .Cm ForwardAgent |
| variable is set to |
| .Dq yes |
| (or see the description of the |
| .Fl A |
| and |
| .Fl a |
| options above) and |
| the user is using an authentication agent, the connection to the agent |
| is automatically forwarded to the remote side. |
| .Sh VERIFYING HOST KEYS |
| When connecting to a server for the first time, |
| a fingerprint of the server's public key is presented to the user |
| (unless the option |
| .Cm StrictHostKeyChecking |
| has been disabled). |
| Fingerprints can be determined using |
| .Xr ssh-keygen 1 : |
| .Pp |
| .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key |
| .Pp |
| If the fingerprint is already known, |
| it can be matched and verified, |
| and the key can be accepted. |
| If the fingerprint is unknown, |
| an alternative method of verification is available: |
| SSH fingerprints verified by DNS. |
| An additional resource record (RR), |
| SSHFP, |
| is added to a zonefile |
| and the connecting client is able to match the fingerprint |
| with that of the key presented. |
| .Pp |
| In this example, we are connecting a client to a server, |
| .Dq host.example.com . |
| The SSHFP resource records should first be added to the zonefile for |
| host.example.com: |
| .Bd -literal -offset indent |
| $ ssh-keygen -r host.example.com. |
| .Ed |
| .Pp |
| The output lines will have to be added to the zonefile. |
| To check that the zone is answering fingerprint queries: |
| .Pp |
| .Dl $ dig -t SSHFP host.example.com |
| .Pp |
| Finally the client connects: |
| .Bd -literal -offset indent |
| $ ssh -o "VerifyHostKeyDNS ask" host.example.com |
| [...] |
| Matching host key fingerprint found in DNS. |
| Are you sure you want to continue connecting (yes/no)? |
| .Ed |
| .Pp |
| See the |
| .Cm VerifyHostKeyDNS |
| option in |
| .Xr ssh_config 5 |
| for more information. |
| .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS |
| .Nm |
| contains support for Virtual Private Network (VPN) tunnelling |
| using the |
| .Xr tun 4 |
| network pseudo-device, |
| allowing two networks to be joined securely. |
| The |
| .Xr sshd_config 5 |
| configuration option |
| .Cm PermitTunnel |
| controls whether the server supports this, |
| and at what level (layer 2 or 3 traffic). |
| .Pp |
| The following example would connect client network 10.0.50.0/24 |
| with remote network 10.0.99.0/24 using a point-to-point connection |
| from 10.1.1.1 to 10.1.1.2, |
| provided that the SSH server running on the gateway to the remote network, |
| at 192.168.1.15, allows it. |
| .Pp |
| On the client: |
| .Bd -literal -offset indent |
| # ssh -f -w 0:1 192.168.1.15 true |
| # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 |
| # route add 10.0.99.0/24 10.1.1.2 |
| .Ed |
| .Pp |
| On the server: |
| .Bd -literal -offset indent |
| # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 |
| # route add 10.0.50.0/24 10.1.1.1 |
| .Ed |
| .Pp |
| Client access may be more finely tuned via the |
| .Pa /root/.ssh/authorized_keys |
| file (see below) and the |
| .Cm PermitRootLogin |
| server option. |
| The following entry would permit connections on |
| .Xr tun 4 |
| device 1 from user |
| .Dq jane |
| and on tun device 2 from user |
| .Dq john , |
| if |
| .Cm PermitRootLogin |
| is set to |
| .Dq forced-commands-only : |
| .Bd -literal -offset 2n |
| tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane |
| tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john |
| .Ed |
| .Pp |
| Since an SSH-based setup entails a fair amount of overhead, |
| it may be more suited to temporary setups, |
| such as for wireless VPNs. |
| More permanent VPNs are better provided by tools such as |
| .Xr ipsecctl 8 |
| and |
| .Xr isakmpd 8 . |
| .Sh ENVIRONMENT |
| .Nm |
| will normally set the following environment variables: |
| .Bl -tag -width "SSH_ORIGINAL_COMMAND" |
| .It Ev DISPLAY |
| The |
| .Ev DISPLAY |
| variable indicates the location of the X11 server. |
| It is automatically set by |
| .Nm |
| to point to a value of the form |
| .Dq hostname:n , |
| where |
| .Dq hostname |
| indicates the host where the shell runs, and |
| .Sq n |
| is an integer \*(Ge 1. |
| .Nm |
| uses this special value to forward X11 connections over the secure |
| channel. |
| The user should normally not set |
| .Ev DISPLAY |
| explicitly, as that |
| will render the X11 connection insecure (and will require the user to |
| manually copy any required authorization cookies). |
| .It Ev HOME |
| Set to the path of the user's home directory. |
| .It Ev LOGNAME |
| Synonym for |
| .Ev USER ; |
| set for compatibility with systems that use this variable. |
| .It Ev MAIL |
| Set to the path of the user's mailbox. |
| .It Ev PATH |
| Set to the default |
| .Ev PATH , |
| as specified when compiling |
| .Nm . |
| .It Ev SSH_ASKPASS |
| If |
| .Nm |
| needs a passphrase, it will read the passphrase from the current |
| terminal if it was run from a terminal. |
| If |
| .Nm |
| does not have a terminal associated with it but |
| .Ev DISPLAY |
| and |
| .Ev SSH_ASKPASS |
| are set, it will execute the program specified by |
| .Ev SSH_ASKPASS |
| and open an X11 window to read the passphrase. |
| This is particularly useful when calling |
| .Nm |
| from a |
| .Pa .xsession |
| or related script. |
| (Note that on some machines it |
| may be necessary to redirect the input from |
| .Pa /dev/null |
| to make this work.) |
| .It Ev SSH_AUTH_SOCK |
| Identifies the path of a |
| .Ux Ns -domain |
| socket used to communicate with the agent. |
| .It Ev SSH_CONNECTION |
| Identifies the client and server ends of the connection. |
| The variable contains |
| four space-separated values: client IP address, client port number, |
| server IP address, and server port number. |
| .It Ev SSH_ORIGINAL_COMMAND |
| This variable contains the original command line if a forced command |
| is executed. |
| It can be used to extract the original arguments. |
| .It Ev SSH_TTY |
| This is set to the name of the tty (path to the device) associated |
| with the current shell or command. |
| If the current session has no tty, |
| this variable is not set. |
| .It Ev TZ |
| This variable is set to indicate the present time zone if it |
| was set when the daemon was started (i.e. the daemon passes the value |
| on to new connections). |
| .It Ev USER |
| Set to the name of the user logging in. |
| .El |
| .Pp |
| Additionally, |
| .Nm |
| reads |
| .Pa ~/.ssh/environment , |
| and adds lines of the format |
| .Dq VARNAME=value |
| to the environment if the file exists and users are allowed to |
| change their environment. |
| For more information, see the |
| .Cm PermitUserEnvironment |
| option in |
| .Xr sshd_config 5 . |
| .Sh FILES |
| .Bl -tag -width Ds -compact |
| .It ~/.rhosts |
| This file is used for host-based authentication (see above). |
| On some machines this file may need to be |
| world-readable if the user's home directory is on an NFS partition, |
| because |
| .Xr sshd 8 |
| reads it as root. |
| Additionally, this file must be owned by the user, |
| and must not have write permissions for anyone else. |
| The recommended |
| permission for most machines is read/write for the user, and not |
| accessible by others. |
| .Pp |
| .It ~/.shosts |
| This file is used in exactly the same way as |
| .Pa .rhosts , |
| but allows host-based authentication without permitting login with |
| rlogin/rsh. |
| .Pp |
| .It ~/.ssh/authorized_keys |
| Lists the public keys (RSA/DSA) that can be used for logging in as this user. |
| The format of this file is described in the |
| .Xr sshd 8 |
| manual page. |
| This file is not highly sensitive, but the recommended |
| permissions are read/write for the user, and not accessible by others. |
| .Pp |
| .It ~/.ssh/config |
| This is the per-user configuration file. |
| The file format and configuration options are described in |
| .Xr ssh_config 5 . |
| Because of the potential for abuse, this file must have strict permissions: |
| read/write for the user, and not accessible by others. |
| .Pp |
| .It ~/.ssh/environment |
| Contains additional definitions for environment variables; see |
| .Sx ENVIRONMENT , |
| above. |
| .Pp |
| .It ~/.ssh/identity |
| .It ~/.ssh/id_dsa |
| .It ~/.ssh/id_rsa |
| Contains the private key for authentication. |
| These files |
| contain sensitive data and should be readable by the user but not |
| accessible by others (read/write/execute). |
| .Nm |
| will simply ignore a private key file if it is accessible by others. |
| It is possible to specify a passphrase when |
| generating the key which will be used to encrypt the |
| sensitive part of this file using 3DES. |
| .Pp |
| .It ~/.ssh/identity.pub |
| .It ~/.ssh/id_dsa.pub |
| .It ~/.ssh/id_rsa.pub |
| Contains the public key for authentication. |
| These files are not |
| sensitive and can (but need not) be readable by anyone. |
| .Pp |
| .It ~/.ssh/known_hosts |
| Contains a list of host keys for all hosts the user has logged into |
| that are not already in the systemwide list of known host keys. |
| See |
| .Xr sshd 8 |
| for further details of the format of this file. |
| .Pp |
| .It ~/.ssh/rc |
| Commands in this file are executed by |
| .Nm |
| when the user logs in, just before the user's shell (or command) is |
| started. |
| See the |
| .Xr sshd 8 |
| manual page for more information. |
| .Pp |
| .It /etc/hosts.equiv |
| This file is for host-based authentication (see above). |
| It should only be writable by root. |
| .Pp |
| .It /etc/shosts.equiv |
| This file is used in exactly the same way as |
| .Pa hosts.equiv , |
| but allows host-based authentication without permitting login with |
| rlogin/rsh. |
| .Pp |
| .It Pa /etc/ssh/ssh_config |
| Systemwide configuration file. |
| The file format and configuration options are described in |
| .Xr ssh_config 5 . |
| .Pp |
| .It /etc/ssh/ssh_host_key |
| .It /etc/ssh/ssh_host_dsa_key |
| .It /etc/ssh/ssh_host_rsa_key |
| These three files contain the private parts of the host keys |
| and are used for host-based authentication. |
| If protocol version 1 is used, |
| .Nm |
| must be setuid root, since the host key is readable only by root. |
| For protocol version 2, |
| .Nm |
| uses |
| .Xr ssh-keysign 8 |
| to access the host keys, |
| eliminating the requirement that |
| .Nm |
| be setuid root when host-based authentication is used. |
| By default |
| .Nm |
| is not setuid root. |
| .Pp |
| .It /etc/ssh/ssh_known_hosts |
| Systemwide list of known host keys. |
| This file should be prepared by the |
| system administrator to contain the public host keys of all machines in the |
| organization. |
| It should be world-readable. |
| See |
| .Xr sshd 8 |
| for further details of the format of this file. |
| .Pp |
| .It /etc/ssh/sshrc |
| Commands in this file are executed by |
| .Nm |
| when the user logs in, just before the user's shell (or command) is started. |
| See the |
| .Xr sshd 8 |
| manual page for more information. |
| .El |
| .Sh SEE ALSO |
| .Xr scp 1 , |
| .Xr sftp 1 , |
| .Xr ssh-add 1 , |
| .Xr ssh-agent 1 , |
| .Xr ssh-keygen 1 , |
| .Xr ssh-keyscan 1 , |
| .Xr tun 4 , |
| .Xr hosts.equiv 5 , |
| .Xr ssh_config 5 , |
| .Xr ssh-keysign 8 , |
| .Xr sshd 8 |
| .Rs |
| .%R RFC 4250 |
| .%T "The Secure Shell (SSH) Protocol Assigned Numbers" |
| .%D 2006 |
| .Re |
| .Rs |
| .%R RFC 4251 |
| .%T "The Secure Shell (SSH) Protocol Architecture" |
| .%D 2006 |
| .Re |
| .Rs |
| .%R RFC 4252 |
| .%T "The Secure Shell (SSH) Authentication Protocol" |
| .%D 2006 |
| .Re |
| .Rs |
| .%R RFC 4253 |
| .%T "The Secure Shell (SSH) Transport Layer Protocol" |
| .%D 2006 |
| .Re |
| .Rs |
| .%R RFC 4254 |
| .%T "The Secure Shell (SSH) Connection Protocol" |
| .%D 2006 |
| .Re |
| .Rs |
| .%R RFC 4255 |
| .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints" |
| .%D 2006 |
| .Re |
| .Rs |
| .%R RFC 4256 |
| .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)" |
| .%D 2006 |
| .Re |
| .Rs |
| .%R RFC 4335 |
| .%T "The Secure Shell (SSH) Session Channel Break Extension" |
| .%D 2006 |
| .Re |
| .Rs |
| .%R RFC 4344 |
| .%T "The Secure Shell (SSH) Transport Layer Encryption Modes" |
| .%D 2006 |
| .Re |
| .Rs |
| .%R RFC 4345 |
| .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol" |
| .%D 2006 |
| .Re |
| .Rs |
| .%R RFC 4419 |
| .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol" |
| .%D 2006 |
| .Re |
| .Rs |
| .%R RFC 4716 |
| .%T "The Secure Shell (SSH) Public Key File Format" |
| .%D 2006 |
| .Re |
| .Sh AUTHORS |
| OpenSSH is a derivative of the original and free |
| ssh 1.2.12 release by Tatu Ylonen. |
| Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, |
| Theo de Raadt and Dug Song |
| removed many bugs, re-added newer features and |
| created OpenSSH. |
| Markus Friedl contributed the support for SSH |
| protocol versions 1.5 and 2.0. |