| .\" -*- 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 1.115 2001/06/22 21:55:49 markus Exp $ |
| .Dd September 25, 1999 |
| .Dt SSH 1 |
| .Os |
| .Sh NAME |
| .Nm ssh |
| .Nd OpenSSH SSH client (remote login program) |
| .Sh SYNOPSIS |
| .Nm ssh |
| .Op Fl l Ar login_name |
| .Op Ar hostname | user@hostname |
| .Op Ar command |
| .Pp |
| .Nm ssh |
| .Op Fl afgknqstvxACNPTX1246 |
| .Op Fl b Ar bind_address |
| .Op Fl c Ar cipher_spec |
| .Op Fl e Ar escape_char |
| .Op Fl i Ar identity_file |
| .Op Fl l Ar login_name |
| .Op Fl m Ar mac_spec |
| .Op Fl o Ar option |
| .Op Fl p Ar port |
| .Oo Fl L Xo |
| .Sm off |
| .Ar port : |
| .Ar host : |
| .Ar hostport |
| .Sm on |
| .Xc |
| .Oc |
| .Oo Fl R Xo |
| .Sm off |
| .Ar port : |
| .Ar host : |
| .Ar hostport |
| .Sm on |
| .Xc |
| .Oc |
| .Op Ar hostname | user@hostname |
| .Op Ar command |
| .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/IP ports can also be forwarded over the secure channel. |
| .Pp |
| .Nm |
| connects and logs into the specified |
| .Ar hostname . |
| The user must prove |
| his/her identity to the remote machine using one of several methods |
| depending on the protocol version used: |
| .Pp |
| .Ss SSH protocol version 1 |
| .Pp |
| First, 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, the user is immediately permitted to log in. |
| Second, if |
| .Pa \&.rhosts |
| or |
| .Pa \&.shosts |
| exists in the user's home directory on the |
| remote machine and contains a line containing the name of the client |
| machine and the name of the user on that machine, the user is |
| permitted to log in. |
| This form of authentication alone is normally not |
| allowed by the server because it is not secure. |
| .Pp |
| The second authentication method is the |
| .Pa rhosts |
| or |
| .Pa hosts.equiv |
| method combined with RSA-based host authentication. |
| It means that if the login would be permitted by |
| .Pa $HOME/.rhosts , |
| .Pa $HOME/.shosts , |
| .Pa /etc/hosts.equiv , |
| or |
| .Pa /etc/shosts.equiv , |
| and if additionally the server can verify the client's |
| host key (see |
| .Pa /etc/ssh_known_hosts |
| and |
| .Pa $HOME/.ssh/known_hosts |
| in the |
| .Sx FILES |
| section), only then login is 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 $HOME/.rhosts , |
| and the rlogin/rsh protocol in general, are inherently insecure and should be |
| disabled if security is desired.] |
| .Pp |
| As a third authentication method, |
| .Nm |
| supports RSA based authentication. |
| The scheme is based on public-key cryptography: there are cryptosystems |
| where encryption and decryption are done using separate keys, and it |
| is not possible to derive the decryption key from the encryption key. |
| RSA is one such system. |
| 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. |
| The file |
| .Pa $HOME/.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 server checks if this key is permitted, and if |
| so, sends the user (actually the |
| .Nm |
| program running on behalf of the user) a challenge, a random number, |
| encrypted by the user's public key. |
| The challenge can only be |
| decrypted using the proper private key. |
| The user's client then decrypts the |
| challenge using the private key, proving that he/she knows the private |
| key but without disclosing it to the server. |
| .Pp |
| .Nm |
| implements the RSA authentication protocol automatically. |
| The user creates his/her RSA key pair by running |
| .Xr ssh-keygen 1 . |
| This stores the private key in |
| .Pa $HOME/.ssh/identity |
| and the public key in |
| .Pa $HOME/.ssh/identity.pub |
| in the user's home directory. |
| The user should then copy the |
| .Pa identity.pub |
| to |
| .Pa $HOME/.ssh/authorized_keys |
| in his/her home directory on the remote machine (the |
| .Pa authorized_keys |
| file corresponds to the conventional |
| .Pa $HOME/.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. |
| RSA authentication is much |
| more secure than rhosts authentication. |
| .Pp |
| The most convenient way to use RSA authentication may be with an |
| authentication agent. |
| See |
| .Xr ssh-agent 1 |
| for more information. |
| .Pp |
| 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 |
| .Ss SSH protocol version 2 |
| .Pp |
| When a user connects using the protocol version 2 |
| different authentication methods are available. |
| Using the default values for |
| .Cm PreferredAuthentications , |
| the client will try to authenticate first using the public key method; |
| if this method fails password authentication is attempted, |
| and finally if this method fails keyboard-interactive authentication |
| is attempted. |
| If this method fails password authentication is |
| tried. |
| .Pp |
| The public key method is similar to RSA authentication described |
| in the previous section and allows the RSA or DSA algorithm to be used: |
| The client uses his private key, |
| .Pa $HOME/.ssh/id_dsa |
| or |
| .Pa $HOME/.ssh/id_rsa , |
| to sign the session identifier and sends the result to the server. |
| The server checks whether the matching public key is listed in |
| .Pa $HOME/.ssh/authorized_keys |
| and grants access if both the key is found and the signature is correct. |
| The session identifier is derived from a shared Diffie-Hellman value |
| and is only known to the client and the server. |
| .Pp |
| If public key authentication fails or is not available a password |
| can be sent encrypted to the remote host for proving the user's identity. |
| .Pp |
| Additionally, |
| .Nm |
| supports hostbased or challenge response authentication. |
| .Pp |
| Protocol 2 provides additional mechanisms for confidentiality |
| (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour) |
| and integrity (hmac-md5, hmac-sha1). |
| Note that protocol 1 lacks a strong mechanism for ensuring the |
| integrity of the connection. |
| .Pp |
| .Ss Login session and remote execution |
| .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/IP connections have been closed. |
| The exit status of the remote program is returned as the exit status |
| of |
| .Nm ssh . |
| .Pp |
| .Ss Escape Characters |
| .Pp |
| When a pseudo terminal has been requested, ssh 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 above). |
| 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 ssh |
| .It Cm ~# |
| List forwarded connections |
| .It Cm ~& |
| Background ssh at logout when waiting for forwarded connection / X11 sessions |
| to terminate (protocol version 1 only) |
| .It Cm ~? |
| Display a list of escape characters |
| .It Cm ~R |
| Request rekeying of the connection (only useful for SSH protocol version 2 |
| and if the peer supports it) |
| .El |
| .Pp |
| .Ss X11 and TCP forwarding |
| .Pp |
| If the |
| .Cm ForwardX11 |
| variable is set to |
| .Dq yes |
| (or, see the description of the |
| .Fl X |
| and |
| .Fl x |
| options described later) |
| 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 user is using an authentication agent, the connection to the agent |
| is automatically forwarded to the remote side unless disabled on |
| command line or in a configuration file. |
| .Pp |
| Forwarding of arbitrary TCP/IP connections over the secure channel can |
| be specified either on command line or in a configuration file. |
| One possible application of TCP/IP forwarding is a secure connection to an |
| electronic purse; another is going through firewalls. |
| .Pp |
| .Ss Server authentication |
| .Pp |
| .Nm |
| automatically maintains and checks a database containing |
| identifications for all hosts it has ever been used with. |
| RSA host keys are stored in |
| .Pa $HOME/.ssh/known_hosts |
| and |
| host keys used in the protocol version 2 are stored in |
| .Pa $HOME/.ssh/known_hosts2 |
| in the user's home directory. |
| Additionally, the files |
| .Pa /etc/ssh_known_hosts |
| and |
| .Pa /etc/ssh_known_hosts2 |
| are 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 a |
| trojan horse from getting the user's password. |
| Another purpose of |
| this mechanism is to prevent man-in-the-middle attacks which could |
| otherwise be used to circumvent the encryption. |
| The |
| .Cm StrictHostKeyChecking |
| option (see below) can be used to prevent logins to machines whose |
| host key is not known or has changed. |
| .Pp |
| The options are as follows: |
| .Bl -tag -width Ds |
| .It Fl a |
| Disables forwarding of the authentication agent connection. |
| .It Fl A |
| Enables forwarding of the authentication agent connection. |
| This can also be specified on a per-host basis in a configuration file. |
| .It Fl b Ar bind_address |
| Specify the interface to transmit from on machines with multiple |
| interfaces or aliased addresses. |
| .It Fl c Ar blowfish|3des |
| Selects the cipher to use for encrypting the session. |
| .Ar 3des |
| is used by default. |
| It is believed to be secure. |
| .Ar 3des |
| (triple-des) is an encrypt-decrypt-encrypt triple with three different keys. |
| It is presumably more secure than the |
| .Ar des |
| cipher which is no longer fully supported in |
| .Nm ssh . |
| .Ar blowfish |
| is a fast block cipher, it appears very secure and is much faster than |
| .Ar 3des . |
| .It Fl c Ar cipher_spec |
| Additionally, for protocol version 2 a comma-separated list of ciphers can |
| be specified in order of preference. |
| See |
| .Cm Ciphers |
| for more information. |
| .It Fl e Ar ch|^ch|none |
| 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 |
| 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 identity_file |
| Selects the file from which the identity (private key) for |
| RSA or DSA authentication is read. |
| Default is |
| .Pa $HOME/.ssh/identity |
| in the user's home directory. |
| 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 |
| Disables forwarding of Kerberos tickets and AFS tokens. |
| This may also be specified on a per-host basis in the configuration file. |
| .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 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 |
| 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 N |
| Do not execute a remote command. |
| This is useful if you just want to forward ports |
| (protocol version 2 only). |
| .It Fl o Ar option |
| Can be used to give options in the format used in the config file. |
| This is useful for specifying options for which there is no separate |
| command-line flag. |
| The option has the same format as a line in the configuration file. |
| .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 P |
| Use a non-privileged port for outgoing connections. |
| This can be used if your firewall does |
| not permit connections from privileged ports. |
| Note that this option turns off |
| .Cm RhostsAuthentication |
| and |
| .Cm RhostsRSAAuthentication |
| for older servers. |
| .It Fl q |
| Quiet mode. |
| Causes all warning and diagnostic messages to be suppressed. |
| Only fatal errors are displayed. |
| .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. sftp). The |
| subsystem is specified as the remote command. |
| .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 T |
| Disable pseudo-tty allocation. |
| .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 increases the verbosity. |
| Maximum is 3. |
| .It Fl x |
| Disables X11 forwarding. |
| .It Fl X |
| Enables X11 forwarding. |
| This can also be specified on a per-host basis in a configuration file. |
| .It Fl C |
| Requests compression of all data (including stdin, stdout, stderr, and |
| data for forwarded X11 and TCP/IP connections). |
| The compression algorithm is the same used by |
| .Xr gzip 1 , |
| and the |
| .Dq level |
| can be controlled by the |
| .Cm CompressionLevel |
| option (see below). |
| 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 Compress |
| option below. |
| .It Fl L Ar port:host:hostport |
| 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, 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 remote machine. |
| Port forwardings can also be specified in the configuration file. |
| Only root can forward privileged ports. |
| IPv6 addresses can be specified with an alternative syntax: |
| .Ar port/host/hostport |
| .It Fl R Ar port:host:hostport |
| 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. |
| 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 with an alternative syntax: |
| .Ar port/host/hostport |
| .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. |
| .El |
| .Sh CONFIGURATION FILES |
| .Nm |
| obtains configuration data from the following sources (in this order): |
| command line options, user's configuration file |
| .Pq Pa $HOME/.ssh/config , |
| and system-wide configuration file |
| .Pq Pa /etc/ssh_config . |
| For each parameter, the first obtained value |
| will be used. |
| The configuration files contain sections bracketed by |
| .Dq Host |
| specifications, and that section is only applied for hosts that |
| match one of the patterns given in the specification. |
| The matched host name is the one given on the command line. |
| .Pp |
| Since the first obtained value for each parameter is used, more |
| host-specific declarations should be given near the beginning of the |
| file, and general defaults at the end. |
| .Pp |
| The configuration file has the following format: |
| .Pp |
| Empty lines and lines starting with |
| .Ql # |
| are comments. |
| .Pp |
| Otherwise a line is of the format |
| .Dq keyword arguments . |
| The possible |
| keywords and their meanings are as follows (note that the |
| configuration files are case-sensitive): |
| .Bl -tag -width Ds |
| .It Cm Host |
| Restricts the following declarations (up to the next |
| .Cm Host |
| keyword) to be only for those hosts that match one of the patterns |
| given after the keyword. |
| .Ql \&* |
| and |
| .Ql ? |
| can be used as wildcards in the |
| patterns. |
| A single |
| .Ql \&* |
| as a pattern can be used to provide global |
| defaults for all hosts. |
| The host is the |
| .Ar hostname |
| argument given on the command line (i.e., the name is not converted to |
| a canonicalized host name before matching). |
| .It Cm AFSTokenPassing |
| Specifies whether to pass AFS tokens to remote host. |
| The argument to this keyword must be |
| .Dq yes |
| or |
| .Dq no . |
| This option applies to protocol version 1 only. |
| .It Cm BatchMode |
| If set to |
| .Dq yes , |
| passphrase/password querying will be disabled. |
| This option is useful in scripts and other batch jobs where you have no |
| user to supply the password. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .It Cm BindAddress |
| Specify the interface to transmit from on machines with multiple |
| interfaces or aliased addresses. |
| Note that this option does not work if |
| .Cm UsePrivilegedPort |
| is set to |
| .Dq yes . |
| .It Cm CheckHostIP |
| If this flag is set to |
| .Dq yes , |
| ssh will additionally check the host IP address in the |
| .Pa known_hosts |
| file. |
| This allows ssh to detect if a host key changed due to DNS spoofing. |
| If the option is set to |
| .Dq no , |
| the check will not be executed. |
| The default is |
| .Dq yes . |
| .It Cm Cipher |
| Specifies the cipher to use for encrypting the session |
| in protocol version 1. |
| Currently, |
| .Dq blowfish |
| and |
| .Dq 3des |
| are supported. |
| The default is |
| .Dq 3des . |
| .It Cm Ciphers |
| Specifies the ciphers allowed for protocol version 2 |
| in order of preference. |
| Multiple ciphers must be comma-separated. |
| The default is |
| .Pp |
| .Bd -literal |
| ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour, |
| aes192-cbc,aes256-cbc'' |
| .Ed |
| .It Cm Compression |
| Specifies whether to use compression. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .It Cm CompressionLevel |
| Specifies the compression level to use if compression is enabled. |
| The argument must be an integer from 1 (fast) to 9 (slow, best). |
| The default level is 6, which is good for most applications. |
| The meaning of the values is the same as in |
| .Xr gzip 1 . |
| Note that this option applies to protocol version 1 only. |
| .It Cm ConnectionAttempts |
| Specifies the number of tries (one per second) to make before falling |
| back to rsh or exiting. |
| The argument must be an integer. |
| This may be useful in scripts if the connection sometimes fails. |
| The default is 4. |
| .It Cm EscapeChar |
| Sets the escape character (default: |
| .Ql ~ ) . |
| The escape character can also |
| be set on the command line. |
| The argument should be a single character, |
| .Ql ^ |
| followed by a letter, or |
| .Dq none |
| to disable the escape |
| character entirely (making the connection transparent for binary |
| data). |
| .It Cm FallBackToRsh |
| Specifies that if connecting via |
| .Nm |
| fails due to a connection refused error (there is no |
| .Xr sshd 8 |
| listening on the remote host), |
| .Xr rsh 1 |
| should automatically be used instead (after a suitable warning about |
| the session being unencrypted). |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .It Cm ForwardAgent |
| Specifies whether the connection to the authentication agent (if any) |
| will be forwarded to the remote machine. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .It Cm ForwardX11 |
| Specifies whether X11 connections will be automatically redirected |
| over the secure channel and |
| .Ev DISPLAY |
| set. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .It Cm GatewayPorts |
| Specifies whether remote hosts are allowed to connect to local |
| forwarded ports. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .It Cm GlobalKnownHostsFile |
| Specifies a file to use for the protocol version 1 global |
| host key database instead of |
| .Pa /etc/ssh_known_hosts . |
| .It Cm GlobalKnownHostsFile2 |
| Specifies a file to use for the protocol version 2 global |
| host key database instead of |
| .Pa /etc/ssh_known_hosts2 . |
| .It Cm HostbasedAuthentication |
| Specifies whether to try rhosts based authentication with public key |
| authentication. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| This option applies to protocol version 2 only and |
| is similar to |
| .Cm RhostsRSAAuthentication . |
| .It Cm HostKeyAlgorithms |
| Specifies the protocol version 2 host key algorithms |
| that the client wants to use in order of preference. |
| The default for this option is: |
| .Dq ssh-rsa,ssh-dss |
| .It Cm HostKeyAlias |
| Specifies an alias that should be used instead of the |
| real host name when looking up or saving the host key |
| in the host key database files. |
| This option is useful for tunneling ssh connections |
| or if you have multiple servers running on a single host. |
| .It Cm HostName |
| Specifies the real host name to log into. |
| This can be used to specify nicknames or abbreviations for hosts. |
| Default is the name given on the command line. |
| Numeric IP addresses are also permitted (both on the command line and in |
| .Cm HostName |
| specifications). |
| .It Cm IdentityFile |
| Specifies the file from which the user's RSA or DSA authentication identity |
| is read (default |
| .Pa $HOME/.ssh/identity |
| in the user's home directory). |
| Additionally, any identities represented by the authentication agent |
| will be used for authentication. |
| The file name may use the tilde |
| syntax to refer to a user's home directory. |
| It is possible to have |
| multiple identity files specified in configuration files; all these |
| identities will be tried in sequence. |
| .It Cm KeepAlive |
| Specifies whether the system should send keepalive messages to the |
| other side. |
| If they are sent, death of the connection or crash of one |
| of the machines will be properly noticed. |
| However, this means that |
| connections will die if the route is down temporarily, and some people |
| find it annoying. |
| .Pp |
| The default is |
| .Dq yes |
| (to send keepalives), and the client will notice |
| if the network goes down or the remote host dies. |
| This is important in scripts, and many users want it too. |
| .Pp |
| To disable keepalives, the value should be set to |
| .Dq no |
| in both the server and the client configuration files. |
| .It Cm KerberosAuthentication |
| Specifies whether Kerberos authentication will be used. |
| The argument to this keyword must be |
| .Dq yes |
| or |
| .Dq no . |
| .It Cm KerberosTgtPassing |
| Specifies whether a Kerberos TGT will be forwarded to the server. |
| This will only work if the Kerberos server is actually an AFS kaserver. |
| The argument to this keyword must be |
| .Dq yes |
| or |
| .Dq no . |
| .It Cm LocalForward |
| Specifies that a TCP/IP port on the local machine be forwarded over |
| the secure channel to given host:port from the remote machine. |
| The first argument must be a port number, and the second must be |
| host:port. |
| Multiple forwardings may be specified, and additional |
| forwardings can be given on the command line. |
| Only the superuser can forward privileged ports. |
| .It Cm LogLevel |
| Gives the verbosity level that is used when logging messages from |
| .Nm ssh . |
| The possible values are: |
| QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG. |
| The default is INFO. |
| .It Cm MACs |
| Specifies the MAC (message authentication code) algorithms |
| in order of preference. |
| The MAC algorithm is used in protocol version 2 |
| for data integrity protection. |
| Multiple algorithms must be comma-separated. |
| The default is |
| .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . |
| .It Cm NumberOfPasswordPrompts |
| Specifies the number of password prompts before giving up. |
| The argument to this keyword must be an integer. |
| Default is 3. |
| .It Cm PasswordAuthentication |
| Specifies whether to use password authentication. |
| The argument to this keyword must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq yes . |
| .It Cm Port |
| Specifies the port number to connect on the remote host. |
| Default is 22. |
| .It Cm PreferredAuthentications |
| Specifies the order in which the client should try protocol 2 |
| authentication methods. This allows a client to prefer one method (e.g. |
| .Cm keyboard-interactive ) |
| over another method (e.g. |
| .Cm password ) |
| The default for this option is: |
| .Dq publickey,hostbased,password,keyboard-interactive |
| .It Cm Protocol |
| Specifies the protocol versions |
| .Nm |
| should support in order of preference. |
| The possible values are |
| .Dq 1 |
| and |
| .Dq 2 . |
| Multiple versions must be comma-separated. |
| The default is |
| .Dq 2,1 . |
| This means that |
| .Nm |
| tries version 2 and falls back to version 1 |
| if version 2 is not available. |
| .It Cm ProxyCommand |
| Specifies the command to use to connect to the server. |
| The command |
| string extends to the end of the line, and is executed with |
| .Pa /bin/sh . |
| In the command string, |
| .Ql %h |
| will be substituted by the host name to |
| connect and |
| .Ql %p |
| by the port. |
| The command can be basically anything, |
| and should read from its standard input and write to its standard output. |
| It should eventually connect an |
| .Xr sshd 8 |
| server running on some machine, or execute |
| .Ic sshd -i |
| somewhere. |
| Host key management will be done using the |
| HostName of the host being connected (defaulting to the name typed by |
| the user). |
| Note that |
| .Cm CheckHostIP |
| is not available for connects with a proxy command. |
| .Pp |
| .It Cm PubkeyAuthentication |
| Specifies whether to try public key authentication. |
| The argument to this keyword must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq yes . |
| This option applies to protocol version 2 only. |
| .It Cm RemoteForward |
| Specifies that a TCP/IP port on the remote machine be forwarded over |
| the secure channel to given host:port from the local machine. |
| The first argument must be a port number, and the second must be |
| host:port. |
| Multiple forwardings may be specified, and additional |
| forwardings can be given on the command line. |
| Only the superuser can forward privileged ports. |
| .It Cm RhostsAuthentication |
| Specifies whether to try rhosts based authentication. |
| Note that this |
| declaration only affects the client side and has no effect whatsoever |
| on security. |
| Disabling rhosts authentication may reduce |
| authentication time on slow connections when rhosts authentication is |
| not used. |
| Most servers do not permit RhostsAuthentication because it |
| is not secure (see |
| .Cm RhostsRSAAuthentication ) . |
| The argument to this keyword must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq yes . |
| This option applies to protocol version 1 only. |
| .It Cm RhostsRSAAuthentication |
| Specifies whether to try rhosts based authentication with RSA host |
| authentication. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq yes . |
| This option applies to protocol version 1 only. |
| .It Cm RSAAuthentication |
| Specifies whether to try RSA authentication. |
| The argument to this keyword must be |
| .Dq yes |
| or |
| .Dq no . |
| RSA authentication will only be |
| attempted if the identity file exists, or an authentication agent is |
| running. |
| The default is |
| .Dq yes . |
| Note that this option applies to protocol version 1 only. |
| .It Cm ChallengeResponseAuthentication |
| Specifies whether to use challenge response authentication. |
| Currently there is only support for |
| .Xr skey 1 |
| authentication. |
| The argument to this keyword must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| .It Cm StrictHostKeyChecking |
| If this flag is set to |
| .Dq yes , |
| .Nm |
| will never automatically add host keys to the |
| .Pa $HOME/.ssh/known_hosts |
| and |
| .Pa $HOME/.ssh/known_hosts2 |
| files, and refuses to connect to hosts whose host key has changed. |
| This provides maximum protection against trojan horse attacks. |
| However, it can be somewhat annoying if you don't have good |
| .Pa /etc/ssh_known_hosts |
| and |
| .Pa /etc/ssh_known_hosts2 |
| files installed and frequently |
| connect to new hosts. |
| This option forces the user to manually |
| add all new hosts. |
| If this flag is set to |
| .Dq no , |
| .Nm |
| will automatically add new host keys to the |
| user known hosts files. |
| If this flag is set to |
| .Dq ask , |
| new host keys |
| will be added to the user known host files only after the user |
| has confirmed that is what they really want to do, and |
| .Nm |
| will refuse to connect to hosts whose host key has changed. |
| The host keys of |
| known hosts will be verified automatically in all cases. |
| The argument must be |
| .Dq yes , |
| .Dq no |
| or |
| .Dq ask . |
| The default is |
| .Dq ask . |
| .It Cm UsePrivilegedPort |
| Specifies whether to use a privileged port for outgoing connections. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| The default is |
| .Dq no . |
| Note that you need to set this option to |
| .Dq yes |
| if you want to use |
| .Cm RhostsAuthentication |
| and |
| .Cm RhostsRSAAuthentication |
| with older servers. |
| .It Cm User |
| Specifies the user to log in as. |
| This can be useful if you have a different user name on different machines. |
| This saves the trouble of |
| having to remember to give the user name on the command line. |
| .It Cm UserKnownHostsFile |
| Specifies a file to use for the protocol version 1 user |
| host key database instead of |
| .Pa $HOME/.ssh/known_hosts . |
| .It Cm UserKnownHostsFile2 |
| Specifies a file to use for the protocol version 2 user |
| host key database instead of |
| .Pa $HOME/.ssh/known_hosts2 . |
| .It Cm UseRsh |
| Specifies that rlogin/rsh should be used for this host. |
| It is possible that the host does not at all support the |
| .Nm |
| protocol. |
| This causes |
| .Nm |
| to immediately execute |
| .Xr rsh 1 . |
| All other options (except |
| .Cm HostName ) |
| are ignored if this has been specified. |
| The argument must be |
| .Dq yes |
| or |
| .Dq no . |
| .It Cm XAuthLocation |
| Specifies the location of the |
| .Xr xauth 1 |
| program. |
| The default is |
| .Pa /usr/X11R6/bin/xauth . |
| .El |
| .Sh ENVIRONMENT |
| .Nm |
| will normally set the following environment variables: |
| .Bl -tag -width Ds |
| .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 hostname indicates |
| the host where the shell runs, and n is an integer >= 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 point the user's mailbox. |
| .It Ev PATH |
| Set to the default |
| .Ev PATH , |
| as specified when compiling |
| .Nm ssh . |
| .It Ev SSH_AUTH_SOCK |
| indicates the path of a unix-domain socket used to communicate with the |
| agent. |
| .It Ev SSH_CLIENT |
| Identifies the client end of the connection. |
| The variable contains |
| three space-separated values: client ip-address, client port number, |
| and server port number. |
| .It Ev SSH_ORIGINAL_COMMAND |
| The 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 |
| The timezone variable is set to indicate the present timezone 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 $HOME/.ssh/environment , |
| and adds lines of the format |
| .Dq VARNAME=value |
| to the environment. |
| .Sh FILES |
| .Bl -tag -width Ds |
| .It Pa $HOME/.ssh/known_hosts, $HOME/.ssh/known_hosts2 |
| Records host keys for all hosts the user has logged into (that are not |
| in |
| .Pa /etc/ssh_known_hosts |
| for protocol version 1 or |
| .Pa /etc/ssh_known_hosts2 |
| for protocol version 2). |
| See |
| .Xr sshd 8 . |
| .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa |
| Contains the authentication identity of the user. |
| They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively. |
| These files |
| contain sensitive data and should be readable by the user but not |
| accessible by others (read/write/execute). |
| Note that |
| .Nm |
| ignores a private key file if it is accessible by others. |
| It is possible to specify a passphrase when |
| generating the key; the passphrase will be used to encrypt the |
| sensitive part of this file using 3DES. |
| .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub |
| Contains the public key for authentication (public part of the |
| identity file in human-readable form). |
| The contents of the |
| .Pa $HOME/.ssh/identity.pub |
| file should be added to |
| .Pa $HOME/.ssh/authorized_keys |
| on all machines |
| where you wish to log in using protocol version 1 RSA authentication. |
| The contents of the |
| .Pa $HOME/.ssh/id_dsa.pub |
| and |
| .Pa $HOME/.ssh/id_rsa.pub |
| file should be added to |
| .Pa $HOME/.ssh/authorized_keys |
| on all machines |
| where you wish to log in using protocol version 2 DSA/RSA authentication. |
| These files are not |
| sensitive and can (but need not) be readable by anyone. |
| These files are |
| never used automatically and are not necessary; they are only provided for |
| the convenience of the user. |
| .It Pa $HOME/.ssh/config |
| This is the per-user configuration file. |
| The format of this file is described above. |
| This file is used by the |
| .Nm |
| client. |
| This file does not usually contain any sensitive information, |
| but the recommended permissions are read/write for the user, and not |
| accessible by others. |
| .It Pa $HOME/.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. |
| .It Pa /etc/ssh_known_hosts, /etc/ssh_known_hosts2 |
| Systemwide list of known host keys. |
| .Pa /etc/ssh_known_hosts |
| contains RSA and |
| .Pa /etc/ssh_known_hosts2 |
| contains RSA or DSA keys for protocol version 2. |
| These files should be prepared by the |
| system administrator to contain the public host keys of all machines in the |
| organization. |
| This file should be world-readable. |
| This file contains |
| public keys, one per line, in the following format (fields separated |
| by spaces): system name, number of bits in modulus, public exponent, |
| modulus, and optional comment field. |
| When different names are used |
| for the same machine, all such names should be listed, separated by |
| commas. |
| The format is described on the |
| .Xr sshd 8 |
| manual page. |
| .Pp |
| The canonical system name (as returned by name servers) is used by |
| .Xr sshd 8 |
| to verify the client host when logging in; other names are needed because |
| .Nm |
| does not convert the user-supplied name to a canonical name before |
| checking the key, because someone with access to the name servers |
| would then be able to fool host authentication. |
| .It Pa /etc/ssh_config |
| Systemwide configuration file. |
| This file provides defaults for those |
| values that are not specified in the user's configuration file, and |
| for those users who do not have a configuration file. |
| This file must be world-readable. |
| .It Pa $HOME/.rhosts |
| This file is used in |
| .Pa \&.rhosts |
| authentication to list the |
| host/user pairs that are permitted to log in. |
| (Note that this file is |
| also used by rlogin and rsh, which makes using this file insecure.) |
| Each line of the file contains a host name (in the canonical form |
| returned by name servers), and then a user name on that host, |
| separated by a space. |
| On some machines this file may need to be |
| world-readable if the user's home directory is on a 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 |
| Note that by default |
| .Xr sshd 8 |
| will be installed so that it requires successful RSA host |
| authentication before permitting \s+2.\s0rhosts authentication. |
| If your server machine does not have the client's host key in |
| .Pa /etc/ssh_known_hosts , |
| you can store it in |
| .Pa $HOME/.ssh/known_hosts . |
| The easiest way to do this is to |
| connect back to the client from the server machine using ssh; this |
| will automatically add the host key to |
| .Pa $HOME/.ssh/known_hosts . |
| .It Pa $HOME/.shosts |
| This file is used exactly the same way as |
| .Pa \&.rhosts . |
| The purpose for |
| having this file is to be able to use rhosts authentication with |
| .Nm |
| without permitting login with |
| .Xr rlogin 1 |
| or |
| .Xr rsh 1 . |
| .It Pa /etc/hosts.equiv |
| This file is used during |
| .Pa \&.rhosts authentication. |
| It contains |
| canonical hosts names, one per line (the full format is described on |
| the |
| .Xr sshd 8 |
| manual page). |
| If the client host is found in this file, login is |
| automatically permitted provided client and server user names are the |
| same. |
| Additionally, successful RSA host authentication is normally |
| required. |
| This file should only be writable by root. |
| .It Pa /etc/shosts.equiv |
| This file is processed exactly as |
| .Pa /etc/hosts.equiv . |
| This file may be useful to permit logins using |
| .Nm |
| but not using rsh/rlogin. |
| .It Pa /etc/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. |
| .It Pa $HOME/.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. |
| .It Pa $HOME/.ssh/environment |
| Contains additional definitions for environment variables, see section |
| .Sx ENVIRONMENT |
| above. |
| .El |
| .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. |
| .Sh SEE ALSO |
| .Xr rlogin 1 , |
| .Xr rsh 1 , |
| .Xr scp 1 , |
| .Xr sftp 1 , |
| .Xr ssh-add 1 , |
| .Xr ssh-agent 1 , |
| .Xr ssh-keygen 1 , |
| .Xr telnet 1 , |
| .Xr sshd 8 |
| .Rs |
| .%A T. Ylonen |
| .%A T. Kivinen |
| .%A M. Saarinen |
| .%A T. Rinne |
| .%A S. Lehtinen |
| .%T "SSH Protocol Architecture" |
| .%N draft-ietf-secsh-architecture-07.txt |
| .%D January 2001 |
| .%O work in progress material |
| .Re |