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NAME

ssh

– OpenSSH SSH client (remote login program)

 

SYNOPSIS

ssh

[-1246AaCfgKkMNnqsTtVvXxYy [-b bind_address

]

]

[-c cipher_spec

]

[-

[bind_address : ]

port

]
[-e escape_char

]

[-F configfile

]

[-I pkcs11

]

-words

[-i identity_file

]

[-

[bind_address : ]

port : host : hostport

]
-words

[-l login_name

]

[-m mac_spec

]

[-O ctl_cmd

]

[-o option

]

[-p port

]

[-

[bind_address : ]

port : host : hostport

]
[-S ctl_path

]

[-W host : port

]

[-w local_tun [: remote_tun ]

]

[user @ hostname

]

[command

]

 

DESCRIPTION

ssh

(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.

ssh

connects and logs into the specified
hostname

(with optional
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).

If
command

is specified,
it is executed on the remote host instead of a login shell.

The options are as follows:


-1


Forces
ssh

to try protocol version 1 only.

-2


Forces
ssh

to try protocol version 2 only.

-4


Forces
ssh

to use IPv4 addresses only.

-6


Forces
ssh

to use IPv6 addresses only.

-A


Enables forwarding of the authentication agent connection.
This can also be specified on a per-host basis in a configuration file.

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.

-a


Disables forwarding of the authentication agent connection.
-b bind_address


Use
bind_address

on the local machine as the source address
of the connection.
Only useful on systems with more than one address.

-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
gzip(1),

and the
“level”

can be controlled by the
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
Compression

option.

-c cipher_spec


Selects the cipher specification for encrypting the session.

Protocol version 1 allows specification of a single cipher.
The supported values are
“3des”

“blowfish”

and
“des”

3des

(triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
It is believed to be secure.
blowfish

is a fast block cipher; it appears very secure and is much faster than
3des

des

is only supported in the
ssh

client for interoperability with legacy protocol 1 implementations
that do not support the
3des

cipher.
Its use is strongly discouraged due to cryptographic weaknesses.
The default is
“3des”

For protocol version 2,
cipher_spec

is a comma-separated list of ciphers
listed in order of preference.
See the
Ciphers

keyword for more information.

-D

[bind_address : ]

port

Specifies a local
“dynamic”

application-level port forwarding.
This works by allocating a socket to listen to
port

on the local side, optionally bound to the specified
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
ssh

will act as a SOCKS server.
Only root can forward privileged ports.
Dynamic port forwardings can also be specified in the configuration file.

IPv6 addresses can be specified with an alternative syntax:

[bind_address /

]

port

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
GatewayPorts

setting.
However, an explicit
bind_address

may be used to bind the connection to a specific address.
The
bind_address

of
“localhost”

indicates that the listening port be bound for local use only, while an
empty address or
`*’

indicates that the port should be available from all interfaces.

-e escape_char


Sets the escape character for sessions with a pty (default:
`~’

) .
The escape character is only recognized at the beginning of a line.
The escape character followed by a dot
(`.’

)

closes the connection;
followed by control-Z suspends the connection;
and followed by itself sends the escape character once.
Setting the character to
“none”

disables any escapes and makes the session fully transparent.

-F configfile


Specifies an alternative per-user configuration file.
If a configuration file is given on the command line,
the system-wide configuration file
(/etc/ssh/ssh_config

)

will be ignored.
The default for the per-user configuration file is
~/.ssh/config

-f


Requests
ssh

to go to background just before command execution.
This is useful if
ssh

is going to ask for passwords or passphrases, but the user
wants it in the background.
This implies
-n

The recommended way to start X11 programs at a remote site is with
something like
ssh -f host xterm

If the
ExitOnForwardFailure

configuration option is set to
“yes”

then a client started with
-f

will wait for all remote port forwards to be successfully established
before placing itself in the background.

-g


Allows remote hosts to connect to local forwarded ports.
-I pkcs11


Specify the PKCS#11 shared libarary
ssh

should use to communicate with a PKCS#11 token used for storing the user’s
private RSA key.
This option is only available if support for PKCS#11
is compiled in (default is no support).

-i identity_file


Selects a file from which the identity (private key) for
RSA or DSA authentication is read.
The default is
~/.ssh/identity

for protocol version 1, and
~/.ssh/id_rsa

and
~/.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
-i

options (and multiple identities specified in
configuration files).
ssh

will also try to load certificate information from the filename obtained
by appending
-cert.pub

to identity filenames.

-K


Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
credentials to the server.
-k


Disables forwarding (delegation) of GSSAPI credentials to the server.
-L

[bind_address : ]

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
port

on the local side, optionally bound to the specified
bind_address

Whenever a connection is made to this port, the
connection is forwarded over the secure channel, and a connection is
made to
host

port
hostport

from the remote machine.
Port forwardings can also be specified in the configuration file.
IPv6 addresses can be specified with an alternative syntax:

[bind_address /

]

port / host /

hostport

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
GatewayPorts

setting.
However, an explicit
bind_address

may be used to bind the connection to a specific address.
The
bind_address

of
“localhost”

indicates that the listening port be bound for local use only, while an
empty address or
`*’

indicates that the port should be available from all interfaces.

-l 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.
-M


Places the
ssh

client into
“master”

mode for connection sharing.
Multiple
-M

options places
ssh

into
“master”

mode with confirmation required before slave connections are accepted.
Refer to the description of
ControlMaster

in
ssh_config5

for details.

-m 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
MACs keyword for more information.

-N


Do not execute a remote command.
This is useful for just forwarding ports
(protocol version 2 only).
-n


Redirects stdin from
/dev/null

(actually, prevents reading from stdin).
This must be used when
ssh

is run in the background.
A common trick is to use this to run X11 programs on a remote machine.
For example,
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
ssh

program will be put in the background.
(This does not work if
ssh

needs to ask for a password or passphrase; see also the
-f

option.)

-O ctl_cmd


Control an active connection multiplexing master process.
When the
-O

option is specified, the
ctl_cmd

argument is interpreted and passed to the master process.
Valid commands are:
“check”

(check that the master process is running) and
“exit”

(request the master to exit).

-o 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
ssh_config5.


AddressFamily

BatchMode

BindAddress

ChallengeResponseAuthentication

CheckHostIP

Cipher

Ciphers

ClearAllForwardings

Compression

CompressionLevel

ConnectionAttempts

ConnectTimeout

ControlMaster

ControlPath

DynamicForward

EscapeChar

ExitOnForwardFailure

ForwardAgent

ForwardX11

ForwardX11Trusted

GatewayPorts

GlobalKnownHostsFile

GSSAPIAuthentication

GSSAPIDelegateCredentials

HashKnownHosts

Host

HostbasedAuthentication

HostKeyAlgorithms

HostKeyAlias

HostName

IdentityFile

IdentitiesOnly

KbdInteractiveDevices

LocalCommand

LocalForward

LogLevel

MACs
NoHostAuthenticationForLocalhost


NumberOfPasswordPrompts

PasswordAuthentication

PermitLocalCommand

PKCS11Provider

Port

PreferredAuthentications

Protocol

ProxyCommand

PubkeyAuthentication

RekeyLimit

RemoteForward

RhostsRSAAuthentication

RSAAuthentication

SendEnv

ServerAliveInterval

ServerAliveCountMax

StrictHostKeyChecking

TCPKeepAlive

Tunnel

TunnelDevice

UsePrivilegedPort

User

UserKnownHostsFile

VerifyHostKeyDNS

VisualHostKey

XAuthLocation


-p port


Port to connect to on the remote host.
This can be specified on a
per-host basis in the configuration file.
-q


Quiet mode.
Causes most warning and diagnostic messages to be suppressed.
-R

[bind_address : ]

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
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
host

port
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 by enclosing the address in square braces or
using an alternative syntax:

[bind_address /

]

host / port /

hostport

.

By default, the listening socket on the server will be bound to the loopback
interface only.
This may be overridden by specifying a
bind_address

An empty
bind_address

or the address
`*’

,
indicates that the remote socket should listen on all interfaces.
Specifying a remote
bind_address

will only succeed if the server’s
GatewayPorts

option is enabled (see
sshd_config5).

If the
port

argument is
`0′

,
the listen port will be dynamically allocated on the server and reported
to the client at run time.

-S ctl_path


Specifies the location of a control socket for connection sharing.
Refer to the description of
ControlPath

and
ControlMaster

in
ssh_config5

for details.

-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(1)).

The subsystem is specified as the remote command.

-T


Disable pseudo-tty allocation.
-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
-t

options force tty allocation, even if
ssh

has no local tty.

-V


Display the version number and exit.
-v


Verbose mode.
Causes
ssh

to print debugging messages about its progress.
This is helpful in
debugging connection, authentication, and configuration problems.
Multiple
-v

options increase the verbosity.
The maximum is 3.

-W host : port


Requests that standard input and output on the client be forwarded to
host

on
port

over the secure channel.
Implies
-N

-T

ExitOnForwardFailure

and
ClearAllForwardings

and works with Protocol version 2 only.

-w

local_tun [: remote_tun

]

Requests
tunnel
device forwarding with the specified
tun(4)

devices between the client
(local_tun

)

and the server
(remote_tun

)

The devices may be specified by numerical ID or the keyword
“any”

which uses the next available tunnel device.
If
remote_tun

is not specified, it defaults to
“any”

See also the
Tunnel

and
TunnelDevice

directives in
ssh_config5.

If the
Tunnel

directive is unset, it is set to the default tunnel mode, which is
“point-to-point”

-X


Enables X11 forwarding.
This can also be specified on a per-host basis in a configuration file.

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.

For this reason, X11 forwarding is subjected to X11 SECURITY extension
restrictions by default.
Please refer to the
ssh

-Y

option and the
ForwardX11Trusted

directive in
ssh_config5

for more information.

-x


Disables X11 forwarding.
-Y


Enables trusted X11 forwarding.
Trusted X11 forwardings are not subjected to the X11 SECURITY extension
controls.
-y


Send log information using the
syslog(3)

system module.
By default this information is sent to stderr.


ssh

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
ssh_config5.

ssh

exits with the exit status of the remote command or with 255
if an error occurred.
 

AUTHENTICATION

The OpenSSH SSH client supports SSH protocols 1 and 2.
Protocol 2 is the default, with
ssh

falling back to protocol 1 if it detects protocol 2 is unsupported.
These settings may be altered using the
Protocol

option in
ssh_config5,

or enforced using the
-1

and
-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,
hmac-sha2-256, hmac-sha2-512,
umac-64, hmac-ripemd160).
Protocol 1 lacks a strong mechanism for ensuring the
integrity of the connection.

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:
PreferredAuthentications

Host-based authentication works as follows:
If the machine the user logs in from is listed in
/etc/hosts.equiv

or
/etc/ssh/shosts.equiv

on the remote machine, and the user names are
the same on both sides, or if the files
~/.rhosts

or
~/.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
must

be able to verify the client’s
host key (see the description of
/etc/ssh/ssh_known_hosts

and
~/.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:
/etc/hosts.equiv

~/.rhosts

and the rlogin/rsh protocol in general, are inherently insecure and should be
disabled if security is desired.]

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.
ssh

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
ssl(8)

contains a brief discussion of the two algorithms.

The file
~/.ssh/authorized_keys

lists the public keys that are permitted for logging in.
When the user logs in, the
ssh

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.

The user creates his/her key pair by running
ssh-keygen1.

This stores the private key in
~/.ssh/identity

(protocol 1),
~/.ssh/id_dsa

(protocol 2 DSA),
or
~/.ssh/id_rsa

(protocol 2 RSA)
and stores the public key in
~/.ssh/identity.pub

(protocol 1),
~/.ssh/id_dsa.pub

(protocol 2 DSA),
or
~/.ssh/id_rsa.pub

(protocol 2 RSA)
in the user’s home directory.
The user should then copy the public key
to
~/.ssh/authorized_keys

in his/her home directory on the remote machine.
The
authorized_keys

file corresponds to the conventional
~/.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.

A variation on public key authentication
is available in the form of certificate authentication:
instead of a set of public/private keys,
signed certificates are used.
This has the advantage that a single trusted certification authority
can be used in place of many public/private keys.
See the
Sx CERTIFICATES

section of
ssh-keygen1

for more information.

The most convenient way to use public key or certificate authentication
may be with an authentication agent.
See
ssh-agent1

for more information.

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
login.conf5)

and PAM (some non-OpenBSD systems).

Finally, if other authentication methods fail,
ssh

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.

ssh

automatically maintains and checks a database containing
identification for all hosts it has ever been used with.
Host keys are stored in
~/.ssh/known_hosts

in the user’s home directory.
Additionally, the file
/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,
ssh

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
StrictHostKeyChecking

option can be used to control logins to machines whose
host key is not known or has changed.

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.

If a pseudo-terminal has been allocated (normal login session), the
user may use the escape characters noted below.

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
“none”

will also make the session transparent even if a tty is used.

The session terminates when the command or shell on the remote
machine exits and all X11 and TCP connections have been closed.
 

ESCAPE CHARACTERS

When a pseudo-terminal has been requested,
ssh

supports a number of functions through the use of an escape character.

A single tilde character can be sent as
~~

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
EscapeChar

configuration directive or on the command line by the
-e

option.

The supported escapes (assuming the default
`~’

)
are:


~.


Disconnect.
~^Z


Background
.

~#


List forwarded connections.
~&


Background
ssh

at logout when waiting for forwarded connection / X11 sessions to terminate.

~?


Display a list of escape characters.
~B


Send a BREAK to the remote system
(only useful for SSH protocol version 2 and if the peer supports it).
~C


Open command line.
Currently this allows the addition of port forwardings using the
-L

-R

and
-D

options (see above).
It also allows the cancellation of existing remote port-forwardings
using

-KR [bind_address : port

]

! command

allows the user to execute a local command if the
PermitLocalCommand

option is enabled in
ssh_config5.

Basic help is available, using the
-h

option.

~R


Request rekeying of the connection
(only useful for SSH protocol version 2 and if the peer supports it).

 

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.

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
,

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
ssh

will encrypt and forward the connection.

The following example tunnels an IRC session from client machine
“127.0.0.1”

(localhost)
to remote server
“server.example.com”


$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
$ irc -c ‘#users’ -p 1234 pinky 127.0.0.1

This tunnels a connection to IRC server
“server.example.com”

joining channel
“#users”

nickname
“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.

The
-f

option backgrounds
ssh

and the remote command
“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,
ssh

will exit.
 

X11 FORWARDING

If the
ForwardX11

variable is set to
“yes”

(or see the description of the
-X

-x

and
-Y

options above)
and the user is using X11 (the
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
DISPLAY

Forwarding of X11 connections can be
configured on the command line or in configuration files.

The
DISPLAY

value set by
ssh

will point to the server machine, but with a display number greater than zero.
This is normal, and happens because
ssh

creates a
“proxy”

X server on the server machine for forwarding the
connections over the encrypted channel.

ssh

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).

If the
ForwardAgent

variable is set to
“yes”

(or see the description of the
-A

and
-a

options above) and
the user is using an authentication agent, the connection to the agent
is automatically forwarded to the remote side.
 

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
StrictHostKeyChecking

has been disabled).
Fingerprints can be determined using
ssh-keygen1:

$ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key


If the fingerprint is already known, it can be matched
and the key can be accepted or rejected.
Because of the difficulty of comparing host keys
just by looking at hex strings,
there is also support to compare host keys visually,
using
random art

By setting the
VisualHostKey

option to
“yes”

a small ASCII graphic gets displayed on every login to a server, no matter
if the session itself is interactive or not.
By learning the pattern a known server produces, a user can easily
find out that the host key has changed when a completely different pattern
is displayed.
Because these patterns are not unambiguous however, a pattern that looks
similar to the pattern remembered only gives a good probability that the
host key is the same, not guaranteed proof.

To get a listing of the fingerprints along with their random art for
all known hosts, the following command line can be used:

$ ssh-keygen -lv -f ~/.ssh/known_hosts


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.

In this example, we are connecting a client to a server,
“host.example.com”

The SSHFP resource records should first be added to the zonefile for
host.example.com:


$ ssh-keygen -r host.example.com.

The output lines will have to be added to the zonefile.
To check that the zone is answering fingerprint queries:

$ dig -t SSHFP host.example.com


Finally the client connects:


$ ssh -o "VerifyHostKeyDNS ask" host.example.com
[...]
Matching host key fingerprint found in DNS.
Are you sure you want to continue connecting (yes/no)?

See the
VerifyHostKeyDNS

option in
ssh_config5

for more information.
 

SSH-BASED VIRTUAL PRIVATE NETWORKS

ssh

contains support for Virtual Private Network (VPN) tunnelling
using the
tun(4)

network pseudo-device,
allowing two networks to be joined securely.
The
sshd_config5

configuration option
PermitTunnel

controls whether the server supports this,
and at what level (layer 2 or 3 traffic).

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.

On the client:


# 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

On the server:


# 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

Client access may be more finely tuned via the
/root/.ssh/authorized_keys

file (see below) and the
PermitRootLogin

server option.
The following entry would permit connections on
tun(4)

device 1 from user
“jane”

and on tun device 2 from user
“john”

if
PermitRootLogin

is set to
“forced-commands-only”


tunnel="1",command="sh /etc/netstart tun1" ssh-rsa … jane
tunnel="2",command="sh /etc/netstart tun2" ssh-rsa … john

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
ipsecctl(8)

and
isakmpd(8).

 

ENVIRONMENT

ssh

will normally set the following environment variables:


DISPLAY


The
DISPLAY

variable indicates the location of the X11 server.
It is automatically set by
ssh

to point to a value of the form
“hostname:n”

where
“hostname”

indicates the host where the shell runs, and
`n’

is an integer &gt=; 1.
ssh

uses this special value to forward X11 connections over the secure
channel.
The user should normally not set
DISPLAY

explicitly, as that
will render the X11 connection insecure (and will require the user to
manually copy any required authorization cookies).

HOME


Set to the path of the user’s home directory.
LOGNAME


Synonym for
USER

set for compatibility with systems that use this variable.

MAIL


Set to the path of the user’s mailbox.
PATH


Set to the default
PATH

as specified when compiling
.

SSH_ASKPASS


If
ssh

needs a passphrase, it will read the passphrase from the current
terminal if it was run from a terminal.
If
ssh

does not have a terminal associated with it but
DISPLAY

and
SSH_ASKPASS

are set, it will execute the program specified by
SSH_ASKPASS

and open an X11 window to read the passphrase.
This is particularly useful when calling
ssh

from a
.xsession

or related script.
(Note that on some machines it
may be necessary to redirect the input from
/dev/null

to make this work.)

SSH_AUTH_SOCK


Identifies the path of a
UNIX
socket used to communicate with the agent.
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.
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.
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.
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).
USER


Set to the name of the user logging in.

Additionally,
ssh

reads
~/.ssh/environment

and adds lines of the format
“VARNAME=value”

to the environment if the file exists and users are allowed to
change their environment.
For more information, see the
PermitUserEnvironment

option in
sshd_config5.

 

ENVIRONMENT


SSH_USE_STRONG_RNG


The reseeding of the OpenSSL random generator is usually done from
/dev/urandom

If the
SSH_USE_STRONG_RNG

environment variable is set to value other than
0

the OpenSSL random generator is reseeded from
/dev/random

The number of bytes read is defined by the SSH_USE_STRONG_RNG value.
Minimum is 14 bytes.
This setting is not recommended on the computers without the hardware
random generator because insufficient entropy causes the connection to
be blocked until enough entropy is available.


 

FILES


~/.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
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.

~/.shosts

This file is used in exactly the same way as
.rhosts

but allows host-based authentication without permitting login with
rlogin/rsh.

~/.ssh/

This directory is the default location for all user-specific configuration
and authentication information.
There is no general requirement to keep the entire contents of this directory
secret, but the recommended permissions are read/write/execute for the user,
and not accessible by others.

~/.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
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.

~/.ssh/config

This is the per-user configuration file.
The file format and configuration options are described in
ssh_config5.

Because of the potential for abuse, this file must have strict permissions:
read/write for the user, and not accessible by others.

~/.ssh/environment

Contains additional definitions for environment variables; see
Sx ENVIRONMENT ,

above.

~/.ssh/identity

~/.ssh/id_dsa

~/.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).
ssh

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.

~/.ssh/identity.pub

~/.ssh/id_dsa.pub

~/.ssh/id_rsa.pub

Contains the public key for authentication.
These files are not
sensitive and can (but need not) be readable by anyone.

~/.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
sshd(8)

for further details of the format of this file.

~/.ssh/rc

Commands in this file are executed by
ssh

when the user logs in, just before the user’s shell (or command) is
started.
See the
sshd(8)

manual page for more information.

/etc/hosts.equiv

This file is for host-based authentication (see above).
It should only be writable by root.

/etc/ssh/shosts.equiv

This file is used in exactly the same way as
hosts.equiv

but allows host-based authentication without permitting login with
rlogin/rsh.

/etc/ssh/ssh_config



Systemwide configuration file.
The file format and configuration options are described in
ssh_config5.

/etc/ssh/ssh_host_key

/etc/ssh/ssh_host_dsa_key

/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,
ssh

must be setuid root, since the host key is readable only by root.
For protocol version 2,
ssh

uses
ssh-keysign8

to access the host keys,
eliminating the requirement that
ssh

be setuid root when host-based authentication is used.
By default
ssh

is not setuid root.

/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
sshd(8)

for further details of the format of this file.

/etc/ssh/sshrc

Commands in this file are executed by
ssh

when the user logs in, just before the user’s shell (or command) is started.
See the
sshd(8)

manual page for more information.


 

IPV6

IPv6 address can be used everywhere where IPv4 address. In all entries must be the IPv6 address enclosed in square brackets. Note: The square brackets are metacharacters for the shell and must be escaped in shell.
 

SEE ALSO

scp(1),

sftp(1),

ssh-add1,

ssh-agent1,

ssh-keygen1,

ssh-keyscan1,

tun(4),

hosts.equiv5,

ssh_config5,

ssh-keysign8,

sshd(8)


RFC 4250
"The Secure Shell (SSH) Protocol Assigned Numbers"
2006


RFC 4251
"The Secure Shell (SSH) Protocol Architecture"
2006


RFC 4252
"The Secure Shell (SSH) Authentication Protocol"
2006


RFC 4253
"The Secure Shell (SSH) Transport Layer Protocol"
2006


RFC 4254
"The Secure Shell (SSH) Connection Protocol"
2006


RFC 4255
"Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
2006


RFC 4256
"Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
2006


RFC 4335
"The Secure Shell (SSH) Session Channel Break Extension"
2006


RFC 4344
"The Secure Shell (SSH) Transport Layer Encryption Modes"
2006


RFC 4345
"Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
2006


RFC 4419
"Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
2006


RFC 4716
"The Secure Shell (SSH) Public Key File Format"
2006


"Hash Visualization: a New Technique to improve Real-World Security"
A. Perrig
D. Song
1999
"International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC ’99)"

 

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.



 

Index



NAME

SYNOPSIS

DESCRIPTION

AUTHENTICATION

ESCAPE CHARACTERS

TCP FORWARDING

X11 FORWARDING

VERIFYING HOST KEYS

SSH-BASED VIRTUAL PRIVATE NETWORKS

ENVIRONMENT

ENVIRONMENT

FILES

IPV6

SEE ALSO

AUTHORS



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