~ubuntu-branches/ubuntu/lucid/openssh/lucid

.\"  -*- 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_config.5,v 1.119 2009/02/22 23:50:57 djm Exp $

.Dd $Mdocdate: February 22 2009 $

.Dt SSH_CONFIG 5

.Os

.Sh NAME

.Nm ssh_config

.Nd OpenSSH SSH client configuration files

.Sh SYNOPSIS

.Nm ~/.ssh/config

.Nm /etc/ssh/ssh_config

.Sh DESCRIPTION

.Xr ssh 1

obtains configuration data from the following sources in

the following order:

.Pp

.Bl -enum -offset indent -compact

.It

command-line options

.It

user's configuration file

.Pq Pa ~/.ssh/config

.It

system-wide configuration file

.Pq Pa /etc/ssh/ssh_config

.El

.Pp

For each parameter, the first obtained value

will be used.

The configuration files contain sections separated 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

Note that the Debian

.Ic openssh-client

package sets several options as standard in

.Pa /etc/ssh/ssh_config

which are not the default in

.Xr ssh 1 :

.Pp

.Bl -bullet -offset indent -compact

.It

.Cm SendEnv No LANG LC_*

.It

.Cm HashKnownHosts No yes

.It

.Cm GSSAPIAuthentication No yes

.El

.Pp

The configuration file has the following format:

.Pp

Empty lines and lines starting with

.Ql #

are comments.

Otherwise a line is of the format

.Dq keyword arguments .

Configuration options may be separated by whitespace or

optional whitespace and exactly one

.Ql = ;

the latter format is useful to avoid the need to quote whitespace

when specifying configuration options using the

.Nm ssh ,

.Nm scp ,

and

.Nm sftp

.Fl o

option.

Arguments may optionally be enclosed in double quotes

.Pq \&"

in order to represent arguments containing spaces.

.Pp

The possible

keywords and their meanings are as follows (note that

keywords are case-insensitive and arguments 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.

If more than one pattern is provided, they should be separated by whitespace.

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).

.Pp

See

.Sx PATTERNS

for more information on patterns.

.It Cm AddressFamily

Specifies which address family to use when connecting.

Valid arguments are

.Dq any ,

.Dq inet

(use IPv4 only), or

.Dq inet6

(use IPv6 only).

.It Cm BatchMode

If set to

.Dq yes ,

passphrase/password querying will be disabled.

In addition, the 

.Cm ServerAliveInterval 

option will be set to 300 seconds by default.

This option is useful in scripts and other batch jobs where no user

is present to supply the password,

and where it is desirable to detect a broken network swiftly.

The argument must be

.Dq yes

or

.Dq no .

The default is

.Dq no .

.It Cm BindAddress

Use the specified address on the local machine as the source address of

the connection.

Only useful on systems with more than one address.

Note that this option does not work if

.Cm UsePrivilegedPort

is set to

.Dq yes .

.It Cm ChallengeResponseAuthentication

Specifies whether to use challenge-response authentication.

The argument to this keyword must be

.Dq yes

or

.Dq no .

The default is

.Dq yes .

.It Cm CheckHostIP

If this flag is set to

.Dq yes ,

.Xr ssh 1

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 ,

.Dq 3des ,

and

.Dq des

are supported.

.Ar des

is only supported in the

.Xr ssh 1

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 .

.It Cm Ciphers

Specifies the ciphers allowed for protocol version 2

in order of preference.

Multiple ciphers must be comma-separated.

The supported ciphers are

.Dq 3des-cbc ,

.Dq aes128-cbc ,

.Dq aes192-cbc ,

.Dq aes256-cbc ,

.Dq aes128-ctr ,

.Dq aes192-ctr ,

.Dq aes256-ctr ,

.Dq arcfour128 ,

.Dq arcfour256 ,

.Dq arcfour ,

.Dq blowfish-cbc ,

and

.Dq cast128-cbc .

The default is:

.Bd -literal -offset 3n

aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,

aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,

aes256-cbc,arcfour

.Ed

.It Cm ClearAllForwardings

Specifies that all local, remote, and dynamic port forwardings

specified in the configuration files or on the command line be

cleared.

This option is primarily useful when used from the

.Xr ssh 1

command line to clear port forwardings set in

configuration files, and is automatically set by

.Xr scp 1

and

.Xr sftp 1 .

The argument must be

.Dq yes

or

.Dq no .

The default is

.Dq no .

.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 exiting.

The argument must be an integer.

This may be useful in scripts if the connection sometimes fails.

The default is 1.

.It Cm ConnectTimeout

Specifies the timeout (in seconds) used when connecting to the

SSH server, instead of using the default system TCP timeout.

This value is used only when the target is down or really unreachable,

not when it refuses the connection.

.It Cm ControlMaster

Enables the sharing of multiple sessions over a single network connection.

When set to

.Dq yes ,

.Xr ssh 1

will listen for connections on a control socket specified using the

.Cm ControlPath

argument.

Additional sessions can connect to this socket using the same

.Cm ControlPath

with

.Cm ControlMaster

set to

.Dq no

(the default).

These sessions will try to reuse the master instance's network connection

rather than initiating new ones, but will fall back to connecting normally

if the control socket does not exist, or is not listening.

.Pp

Setting this to

.Dq ask

will cause ssh

to listen for control connections, but require confirmation using the

.Ev SSH_ASKPASS

program before they are accepted (see

.Xr ssh-add 1

for details).

If the

.Cm ControlPath

cannot be opened,

ssh will continue without connecting to a master instance.

.Pp

X11 and

.Xr ssh-agent 1

forwarding is supported over these multiplexed connections, however the

display and agent forwarded will be the one belonging to the master

connection i.e. it is not possible to forward multiple displays or agents.

.Pp

Two additional options allow for opportunistic multiplexing: try to use a

master connection but fall back to creating a new one if one does not already

exist.

These options are:

.Dq auto

and

.Dq autoask .

The latter requires confirmation like the

.Dq ask

option.

.It Cm ControlPath

Specify the path to the control socket used for connection sharing as described

in the

.Cm ControlMaster

section above or the string

.Dq none

to disable connection sharing.

In the path,

.Ql %l

will be substituted by the local host name,

.Ql %h

will be substituted by the target host name,

.Ql %p

the port, and

.Ql %r

by the remote login username.

It is recommended that any

.Cm ControlPath

used for opportunistic connection sharing include

at least %h, %p, and %r.

This ensures that shared connections are uniquely identified.

.It Cm DynamicForward

Specifies that a TCP port on the local machine be forwarded

over the secure channel, and the application

protocol is then used to determine where to connect to from the

remote machine.

.Pp

The argument must be

.Sm off

.Oo Ar bind_address : Oc Ar port .

.Sm on

IPv6 addresses can be specified by enclosing addresses in square brackets or

by using an alternative syntax:

.Oo Ar bind_address Ns / Oc Ns Ar port .

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.

.Pp

Currently the SOCKS4 and SOCKS5 protocols are supported, and

.Xr ssh 1

will act as a SOCKS server.

Multiple forwardings may be specified, and

additional forwardings can be given on the command line.

Only the superuser can forward privileged ports.

.It Cm EnableSSHKeysign

Setting this option to

.Dq yes

in the global client configuration file

.Pa /etc/ssh/ssh_config

enables the use of the helper program

.Xr ssh-keysign 8

during

.Cm HostbasedAuthentication .

The argument must be

.Dq yes

or

.Dq no .

The default is

.Dq no .

This option should be placed in the non-hostspecific section.

See

.Xr ssh-keysign 8

for more information.

.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 ExitOnForwardFailure

Specifies whether

.Xr ssh 1

should terminate the connection if it cannot set up all requested

dynamic, tunnel, local, and remote port forwardings.

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 .

.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 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 .

.Pp

X11 forwarding should be enabled with caution.

Users with the ability to bypass file permissions on the remote host

(for the user's X11 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

if the

.Cm ForwardX11Trusted

option is also enabled.

.It Cm ForwardX11Trusted

If this option is set to

.Dq yes ,

remote X11 clients will have full access to the original X11 display.

.Pp

If this option is set to

.Dq no ,

remote X11 clients will be considered untrusted and prevented

from stealing or tampering with data belonging to trusted X11

clients.

Furthermore, the

.Xr xauth 1

token used for the session will be set to expire after 20 minutes.

Remote clients will be refused access after this time.

.Pp

The default is

.Dq yes

(Debian-specific).

.Pp

See the X11 SECURITY extension specification for full details on

the restrictions imposed on untrusted clients.

.It Cm GatewayPorts

Specifies whether remote hosts are allowed to connect to local

forwarded ports.

By default,

.Xr ssh 1

binds local port forwardings to the loopback address.

This prevents other remote hosts from connecting to forwarded ports.

.Cm GatewayPorts

can be used to specify that ssh

should bind local port forwardings to the wildcard address,

thus allowing remote hosts to connect to 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 global

host key database instead of

.Pa /etc/ssh/ssh_known_hosts .

.It Cm GSSAPIAuthentication

Specifies whether user authentication based on GSSAPI is allowed.

The default is

.Dq no .

Note that this option applies to protocol version 2 only.

.It Cm GSSAPIKeyExchange

Specifies whether key exchange based on GSSAPI may be used. When using

GSSAPI key exchange the server need not have a host key.

The default is

.Dq no .

Note that this option applies to protocol version 2 only.

.It Cm GSSAPIClientIdentity

If set, specifies the GSSAPI client identity that ssh should use when 

connecting to the server. The default is unset, which means that the default 

identity will be used.

.It Cm GSSAPIDelegateCredentials

Forward (delegate) credentials to the server.

The default is

.Dq no .

Note that this option applies to protocol version 2 connections using GSSAPI.

.It Cm GSSAPIRenewalForcesRekey

If set to 

.Dq yes

then renewal of the client's GSSAPI credentials will force the rekeying of the

ssh connection. With a compatible server, this can delegate the renewed 

credentials to a session on the server.

The default is

.Dq no .

.It Cm GSSAPITrustDns

Set to 

.Dq yes to indicate that the DNS is trusted to securely canonicalize

the name of the host being connected to. If 

.Dq no, the hostname entered on the

command line will be passed untouched to the GSSAPI library.

The default is

.Dq no .

This option only applies to protocol version 2 connections using GSSAPI.

.It Cm HashKnownHosts

Indicates that

.Xr ssh 1

should hash host names and addresses when they are added to

.Pa ~/.ssh/known_hosts .

These hashed names may be used normally by

.Xr ssh 1

and

.Xr sshd 8 ,

but they do not reveal identifying information should the file's contents

be disclosed.

The default is

.Dq no .

Note that existing names and addresses in known hosts files

will not be converted automatically,

but may be manually hashed using

.Xr ssh-keygen 1 .

Use of this option may break facilities such as tab-completion that rely

on being able to read unhashed host names from

.Pa ~/.ssh/known_hosts .

.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 for 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.

The 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 IdentitiesOnly

Specifies that

.Xr ssh 1

should only use the authentication identity files configured in the

.Nm

files,

even if

.Xr ssh-agent 1

offers more identities.

The argument to this keyword must be

.Dq yes

or

.Dq no .

This option is intended for situations where ssh-agent

offers many different identities.

The default is

.Dq no .

.It Cm IdentityFile

Specifies a file from which the user's RSA or DSA authentication identity

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.

Additionally, any identities represented by the authentication agent

will be used for authentication.

.Pp

The file name may use the tilde

syntax to refer to a user's home directory or one of the following

escape characters:

.Ql %d

(local user's home directory),

.Ql %u

(local user name),

.Ql %l

(local host name),

.Ql %h

(remote host name) or

.Ql %r

(remote user name).

.Pp

It is possible to have

multiple identity files specified in configuration files; all these

identities will be tried in sequence.

.It Cm KbdInteractiveAuthentication

Specifies whether to use keyboard-interactive authentication.

The argument to this keyword must be

.Dq yes

or

.Dq no .

The default is

.Dq yes .

.It Cm KbdInteractiveDevices

Specifies the list of methods to use in keyboard-interactive authentication.

Multiple method names must be comma-separated.

The default is to use the server specified list.

The methods available vary depending on what the server supports.

For an OpenSSH server,

it may be zero or more of:

.Dq bsdauth ,

.Dq pam ,

and

.Dq skey .

.It Cm LocalCommand

Specifies a command to execute on the local machine after successfully

connecting to the server.

The command string extends to the end of the line, and is executed with

the user's shell.

The following escape character substitutions will be performed:

.Ql %d

(local user's home directory),

.Ql %h

(remote host name),

.Ql %l

(local host name),

.Ql %n

(host name as provided on the command line),

.Ql %p

(remote port),

.Ql %r

(remote user name) or

.Ql %u

(local user name).

This directive is ignored unless

.Cm PermitLocalCommand

has been enabled.

.It Cm LocalForward

Specifies that a TCP port on the local machine be forwarded over

the secure channel to the specified host and port from the remote machine.

The first argument must be

.Sm off

.Oo Ar bind_address : Oc Ar port

.Sm on

and the second argument must be

.Ar host : Ns Ar hostport .

IPv6 addresses can be specified by enclosing addresses in square brackets or

by using an alternative syntax:

.Oo Ar bind_address Ns / Oc Ns Ar port

and

.Ar host Ns / Ns Ar hostport .

Multiple forwardings may be specified, and additional forwardings can be

given on the command line.

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 Cm LogLevel

Gives the verbosity level that is used when logging messages from

.Xr ssh 1 .

The possible values are:

SILENT, QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.

The default is INFO.

DEBUG and DEBUG1 are equivalent.

DEBUG2 and DEBUG3 each specify higher levels of verbose output.

.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:

.Bd -literal -offset indent

hmac-md5,hmac-sha1,umac-64@openssh.com,

hmac-ripemd160,hmac-sha1-96,hmac-md5-96

.Ed

.It Cm NoHostAuthenticationForLocalhost

This option can be used if the home directory is shared across machines.

In this case localhost will refer to a different machine on each of

the machines and the user will get many warnings about changed host keys.

However, this option disables host authentication for localhost.

The argument to this keyword must be

.Dq yes

or

.Dq no .

The default is to check the host key for localhost.

.It Cm NumberOfPasswordPrompts

Specifies the number of password prompts before giving up.

The argument to this keyword must be an integer.

The 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 PermitLocalCommand

Allow local command execution via the

.Ic LocalCommand

option or using the

.Ic !\& Ns Ar command

escape sequence in

.Xr ssh 1 .

The argument must be

.Dq yes

or

.Dq no .

The default is

.Dq no .

.It Cm Port

Specifies the port number to connect on the remote host.

The 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:

.Do gssapi-with-mic ,

hostbased,

publickey,

keyboard-interactive,

password

.Dc .

.It Cm Protocol

Specifies the protocol versions

.Xr ssh 1

should support in order of preference.

The possible values are

.Sq 1

and

.Sq 2 .

Multiple versions must be comma-separated.

The default is

.Dq 2,1 .

This means that ssh

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

the user's shell.

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).

Setting the command to

.Dq none

disables this option entirely.

Note that

.Cm CheckHostIP

is not available for connects with a proxy command.

.Pp

This directive is useful in conjunction with

.Xr nc 1

and its proxy support.

For example, the following directive would connect via an HTTP proxy at

192.0.2.0:

.Bd -literal -offset 3n

ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p

.Ed

.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 RekeyLimit

Specifies the maximum amount of data that may be transmitted before the

session key is renegotiated.

The argument is the number of bytes, with an optional suffix of

.Sq K ,

.Sq M ,

or

.Sq G

to indicate Kilobytes, Megabytes, or Gigabytes, respectively.

The default is between

.Sq 1G

and

.Sq 4G ,

depending on the cipher.

This option applies to protocol version 2 only.

.It Cm RemoteForward

Specifies that a TCP port on the remote machine be forwarded over

the secure channel to the specified host and port from the local machine.

The first argument must be

.Sm off

.Oo Ar bind_address : Oc Ar port

.Sm on

and the second argument must be

.Ar host : Ns Ar hostport .

IPv6 addresses can be specified by enclosing addresses in square brackets

or by using an alternative syntax:

.Oo Ar bind_address Ns / Oc Ns Ar port

and

.Ar host Ns / Ns Ar hostport .

Multiple forwardings may be specified, and additional

forwardings can be given on the command line.

Privileged ports can be forwarded only when

logging in as root on the remote machine.

.Pp

If the

.Ar port

argument is

.Ql 0 ,

the listen port will be dynamically allocated on the server and reported

to the client at run time.

.Pp

If the

.Ar bind_address

is not specified, the default is to only bind to loopback addresses.

If the

.Ar bind_address

is

.Ql *

or an empty string, then the forwarding is requested to 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 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 no .

This option applies to protocol version 1 only and requires

.Xr ssh 1

to be setuid root.

.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 SendEnv

Specifies what variables from the local

.Xr environ 7

should be sent to the server.

Note that environment passing is only supported for protocol 2.

The server must also support it, and the server must be configured to

accept these environment variables.

Refer to

.Cm AcceptEnv

in

.Xr sshd_config 5

for how to configure the server.

Variables are specified by name, which may contain wildcard characters.

Multiple environment variables may be separated by whitespace or spread

across multiple

.Cm SendEnv

directives.

The default is not to send any environment variables.

.Pp

See

.Sx PATTERNS

for more information on patterns.

.It Cm ServerAliveCountMax

Sets the number of server alive messages (see below) which may be

sent without

.Xr ssh 1

receiving any messages back from the server.

If this threshold is reached while server alive messages are being sent,

ssh will disconnect from the server, terminating the session.

It is important to note that the use of server alive messages is very

different from

.Cm TCPKeepAlive

(below).

The server alive messages are sent through the encrypted channel

and therefore will not be spoofable.

The TCP keepalive option enabled by

.Cm TCPKeepAlive

is spoofable.

The server alive mechanism is valuable when the client or

server depend on knowing when a connection has become inactive.

.Pp

The default value is 3.

If, for example,

.Cm ServerAliveInterval

(see below) is set to 15 and

.Cm ServerAliveCountMax