SSH

NAME
SYNOPSIS
DESCRIPTION
AUTHENTICATION
ESCAPE CHARACTERS
TCP FORWARDING
X11 FORWARDING
VERIFYING HOST KEYS
SSH-BASED VIRTUAL PRIVATE NETWORKS
ENVIRONMENT
FILES
EXIT STATUS
SEE ALSO
STANDARDS
AUTHORS

---

NAME

ssh
− OpenSSH remote login client

SYNOPSIS

ssh
[−46AaCfGgKkMNnqsTtVvXxYy] [−B
bind_interface] [−b bind_address]
[−c cipher_spec]
[−D[bind_address :]port]
[−E log_file] [−e
escape_char] [−F configfile]
[−I pkcs11] [−i
identity_file] [−J destination]
[−L address] [−l
login_name] [−m mac_spec]
[−O ctl_cmd] [−o
option] [−P tag] [−p
port] [−R address]
[−S ctl_path] [−W
host :port] [−w
local_tun[:remote_tun] destination
[command [argument…]
ssh [−Q query_option]

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 provide secure encrypted communications between two
untrusted hosts over an insecure network. X11 connections,
arbitrary TCP ports and UNIX-domain sockets can also be
forwarded over the secure channel.

ssh
connects and logs into the specified destination,
which may be specified as either [user@]hostname or a URI of
the form ssh://[user@]hostname[:port.] The user must prove
their identity to the remote machine using one of several
methods (see below).

If a
command is specified, it will be executed on the
remote host instead of a login shell. A complete command
line may be specified as command, or it may have
additional arguments. If supplied, the arguments will be
appended to the command, separated by spaces, before it is
sent to the server to be executed.

The options are
as follows:

	−4		Forces ssh to use IPv4 addresses only.
	−6		Forces ssh to use IPv6 addresses only.
	−A		Enables forwarding of connections from an authentication agent such as ssh-agent(1). 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 safer
alternative may be to use a jump host (see −J )
.

−a

Disables forwarding of the authentication agent connection.

−B
bind_interface

Bind to the address of
bind_interface before attempting to connect to the
destination host. This is only useful on systems with more
than one address.

−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, TCP and UNIX-domain connections). The compression algorithm is the same used by gzip(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 in ssh_config(5).

−c
cipher_spec

Selects the cipher
specification for encrypting the session. cipher_spec
is a comma-separated list of ciphers listed in order of
preference. See the Ciphers keyword in
ssh_config(5) 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 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
log_file

Append debug logs to
log_file instead of standard error.

−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
(/data/data/com.termux/files/usr/etc/ssh/ssh_config)
will be ignored. The default for the per-user configuration
file is ˜/.ssh/config. If set to
‘‘none’’, no configuration files
will be read.

−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. Refer to the description of
ForkAfterAuthentication in ssh_config(5) for
details.

	−G		Causes ssh to print its configuration after evaluating Host and Match blocks and exit.
	−g		Allows remote hosts to connect to local forwarded ports. If used on a multiplexed connection, then this option must be specified on the master process.

−I
pkcs11

Specify the PKCS#11 shared
library ssh should use to communicate with a PKCS#11
token providing keys for user authentication.

−i
identity_file

Selects a file from which the
identity (private key) for public key authentication is
read. You can also specify a public key file to use the
corresponding private key that is loaded in
ssh-agent(1) when the private key file is not present
locally. The default is ˜/.ssh/id_rsa,
˜/.ssh/id_ecdsa, ˜/.ssh/id_ecdsa_sk,
˜/.ssh/id_ed25519 and
˜/.ssh/id_ed25519_sk. 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). If no
certificates have been explicitly specified by the
CertificateFile directive, ssh will also try
to load certificate information from the filename obtained
by appending -cert.pub to identity filenames.

−J
destination

Connect to the target host by
first making an ssh connection to the jump host
described by destination and then establishing a TCP
forwarding to the ultimate destination from there. Multiple
jump hops may be specified separated by comma characters.
IPv6 addresses can be specified by enclosing the address in
square brackets. This is a shortcut to specify a
ProxyJump configuration directive. Note that
configuration directives supplied on the command-line
generally apply to the destination host and not any
specified jump hosts. Use ˜/.ssh/config to
specify configuration for jump hosts.

	−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
−L [bind_address:]
port:remote_socket
−L local_socket:host:hostport
−L local_socket:remote_socket

Specifies that connections to
the given TCP port or Unix socket on the local (client) host
are to be forwarded to the given host and port, or Unix
socket, on the remote side. This works by allocating a
socket to listen to either a TCP port on the local
side, optionally bound to the specified bind_address,
or to a Unix socket. Whenever a connection is made to the
local port or socket, the connection is forwarded over the
secure channel, and a connection is made to either
host port hostport, or the Unix socket
remote_socket, from the remote machine.

Port
forwardings can also be specified in the configuration file.
Only the superuser can forward privileged ports. IPv6
addresses can be specified by enclosing the address in
square brackets.

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 but with confirmation required using ssh-askpass(1) before each operation that changes the multiplexing state (e.g. opening a new session). Refer to the description of ControlMaster in ssh_config(5) for details.

−m
mac_spec

A comma-separated list of MAC
(message authentication code) algorithms, specified in order
of preference. See the MACs keyword in
ssh_config(5) for more information.

	−N		Do not execute a remote command. This is useful for just forwarding ports. Refer to the description of SessionType in ssh_config(5) for details.
	−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.) Refer to the description of StdinNull in ssh_config(5) for details.

−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), ‘‘forward’’
(request forwardings without command execution),
‘‘cancel’’ (cancel forwardings),
‘‘exit’’ (request the master to
exit), and ‘‘stop’’ (request the
master to stop accepting further multiplexing requests).

−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_config(5).

AddKeysToAgent
AddressFamily
BatchMode
BindAddress
CanonicalDomains
CanonicalizeFallbackLocal
CanonicalizeHostname
CanonicalizeMaxDots
CanonicalizePermittedCNAMEs
CASignatureAlgorithms
CertificateFile
CheckHostIP
Ciphers
ClearAllForwardings
Compression
ConnectionAttempts
ConnectTimeout
ControlMaster
ControlPath
ControlPersist
DynamicForward
EnableEscapeCommandline
EscapeChar
ExitOnForwardFailure
FingerprintHash
ForkAfterAuthentication
ForwardAgent
ForwardX11
ForwardX11Timeout
ForwardX11Trusted
GatewayPorts
GlobalKnownHostsFile
GSSAPIAuthentication
GSSAPIDelegateCredentials
HashKnownHosts

Host

HostbasedAcceptedAlgorithms

HostbasedAuthentication
HostKeyAlgorithms
HostKeyAlias
Hostname
IdentitiesOnly
IdentityAgent
IdentityFile

IPQoS

KbdInteractiveAuthentication

KbdInteractiveDevices
KexAlgorithms
KnownHostsCommand
LocalCommand
LocalForward
LogLevel

	MACs
	Match

NoHostAuthenticationForLocalhost

NumberOfPasswordPrompts
PasswordAuthentication
PermitLocalCommand
PermitRemoteOpen
PKCS11Provider

Port

PreferredAuthentications

ProxyCommand
ProxyJump
ProxyUseFdpass
PubkeyAcceptedAlgorithms
PubkeyAuthentication
RekeyLimit
RemoteCommand
RemoteForward
RequestTTY
RequiredRSASize
SendEnv
ServerAliveInterval
ServerAliveCountMax
SessionType

SetEnv

StdinNull
StreamLocalBindMask
StreamLocalBindUnlink
StrictHostKeyChecking
TCPKeepAlive

Tunnel

TunnelDevice

UpdateHostKeys

User

UserKnownHostsFile

VerifyHostKeyDNS
VisualHostKey
XAuthLocation

−P tag

Specify a tag name that may be used to select configuration in ssh_config(5). Refer to the Tag and Match keywords in ssh_config(5) for more information.

−p port

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

−Q
query_option

Queries for the algorithms
supported by one of the following features: cipher
(supported symmetric ciphers), cipher-auth (supported
symmetric ciphers that support authenticated encryption),
help (supported query terms for use with the
−Q flag), mac (supported message
integrity codes), kex (key exchange algorithms),
key (key types), key-ca-sign (valid CA
signature algorithms for certificates), key-cert
(certificate key types), key-plain (non-certificate
key types), key-sig (all key types and signature
algorithms), protocol-version (supported SSH protocol
versions), and sig (supported signature algorithms).
Alternatively, any keyword from ssh_config(5) or
sshd_config(5) that takes an algorithm list may be
used as an alias for the corresponding query_option.

−q

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

−R
[bind_address:] port:host:hostport
−R [bind_address:] port:local_socket

−R remote_socket:host:hostport
−R remote_socket:local_socket
−R [bind_address:] port

Specifies that connections to
the given TCP port or Unix socket on the remote (server)
host are to be forwarded to the local side.

This works by
allocating a socket to listen to either a TCP port or
to a Unix socket on the remote side. Whenever a connection
is made to this port or Unix socket, the connection is
forwarded over the secure channel, and a connection is made
from the local machine to either an explicit destination
specified by host port hostport, or
local_socket, or, if no explicit destination was
specified, ssh will act as a SOCKS 4/5 proxy and
forward connections to the destinations requested by the
remote SOCKS client.

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 brackets.

By default, TCP
listening sockets 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_config(5)) .

If the
port argument is ‘0’, the listen port
will be dynamically allocated on the server and reported to
the client at run time. When used together with -O
forward, the allocated port will be printed to the
standard output.

−S
ctl_path

Specifies the location of a
control socket for connection sharing, or the string
‘‘none’’ to disable connection
sharing. Refer to the description of ControlPath and
ControlMaster in ssh_config(5) for
details.

	−s		May be used to request invocation of a subsystem on the remote system. Subsystems facilitate the use of SSH as a secure transport for other applications (e.g. sftp(1)) . The subsystem is specified as the remote command. Refer to the description of SessionType in ssh_config(5) for details.
	−T		Disable pseudo-terminal allocation.
	−t		Force pseudo-terminal 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,
though these can be overridden in the configuration file or
using −o command line options.

−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_config(5).

If the
Tunnel directive is unset, it will be set to the
default tunnel mode, which is
‘‘point-to-point’’. If a different
Tunnel forwarding mode it desired, then it should be
specified before −w.

−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. Refer to the ssh
−Y option and the ForwardX11Trusted
directive in ssh_config(5) 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_config(5).

AUTHENTICATION

The OpenSSH SSH
client supports SSH protocol 2.

The methods
available for authentication are: GSSAPI-based
authentication, host-based authentication, public key
authentication, keyboard-interactive authentication, and
password authentication. Authentication methods are tried in
the order specified above, though
PreferredAuthentications can be used to change the
default order.

Host-based
authentication works as follows: If the machine the user
logs in from is listed in /etc/hosts.equiv or
/data/data/com.termux/files/usr/etc/ssh/shosts.equiv
on the remote machine, the user is non-root 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
/data/data/com.termux/files/usr/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 one of the ECDSA, Ed25519 or
RSA 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 server may
inform the client of errors that prevented public key
authentication from succeeding after authentication
completes using a different method. These may be viewed by
increasing the LogLevel to DEBUG or higher
(e.g. by using the −v flag).

The user creates
their key pair by running ssh-keygen(1). This stores
the private key in ˜/.ssh/id_ecdsa (ECDSA),
˜/.ssh/id_ecdsa_sk (authenticator-hosted ECDSA),
˜/.ssh/id_ed25519 (Ed25519),
˜/.ssh/id_ed25519_sk (authenticator-hosted
Ed25519), or ˜/.ssh/id_rsa (RSA) and stores the
public key in ˜/.ssh/id_ecdsa.pub (ECDSA),
˜/.ssh/id_ecdsa_sk.pub (authenticator-hosted
ECDSA), ˜/.ssh/id_ed25519.pub (Ed25519),
˜/.ssh/id_ed25519_sk.pub (authenticator-hosted
Ed25519), or ˜/.ssh/id_rsa.pub (RSA) in the
user’s home directory. The user should then copy the
public key to ˜/.ssh/authorized_keys in their
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
CERTIFICATES section of ssh-keygen(1) for more
information.

The most
convenient way to use public key or certificate
authentication may be with an authentication agent. See
ssh-agent(1) and (optionally) the
AddKeysToAgent directive in ssh_config(5) for
more information.

Keyboard-interactive
authentication works as follows: The server sends an
arbitrary Qq challenge text and prompts for a response,
possibly multiple times. Examples of keyboard-interactive
authentication include Bx Authentication (see
login.conf(5)) 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
/data/data/com.termux/files/usr/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 in a
non-interactive session or, if no command has been
specified, logs into the machine and gives the user a normal
shell as an interactive session. All communication with the
remote command or shell will be automatically encrypted.

If an
interactive session is requested, ssh by default will
only request a pseudo-terminal (pty) for interactive
sessions when the client has one. The flags −T
and −t can be used to override this
behaviour.

If a
pseudo-terminal has been allocated, the user may use the
escape characters noted below.

If no
pseudo-terminal 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.
	˜ˆZ		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 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 port-forwardings with −KL[bind_address:]port for local, −KR[bind_address:]port for remote and −KD[bind_address:]port for dynamic port-forwardings. ! Ns command allows the user to execute a local command if the PermitLocalCommand option is enabled in ssh_config(5). Basic help is available, using the −h option.
	˜R		Request rekeying of the connection (only useful if the peer supports it).
	˜V		Decrease the verbosity (LogLevel) when errors are being written to stderr.
	˜v		Increase the verbosity (LogLevel) when errors are being written to stderr.

TCP FORWARDING

Forwarding of
arbitrary TCP connections over a 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 for an IRC
client, even though the IRC server it connects to does not
directly support encrypted communication. This works as
follows: the user connects to the remote host using ,
specifying the ports to be used to forward the connection.
After that it is possible to start the program locally, and
ssh will encrypt and forward the connection to the
remote server.

The following
example tunnels an IRC session from the client to an IRC
server at ‘‘server.example.com’’,
joining channel ‘‘#users’’, nickname
‘‘pinky’’, using the standard IRC
port, 6667:

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

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 program which is going to use the tunnel. 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-keygen(1):

Dl $ ssh-keygen
-l -f
/data/data/com.termux/files/usr/etc/ssh/ssh_host_rsa_key

If the
fingerprint is already known, it can be matched and the key
can be accepted or rejected. If only legacy (MD5)
fingerprints for the server are available, the
ssh-keygen(1) −E option may be used to
downgrade the fingerprint algorithm to match.

Because of the
difficulty of comparing host keys just by looking at
fingerprint strings, there is also support to compare host
keys visually, using randomart. 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:

Dl $ 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:

Dl $ 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_config(5) 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_config(5) 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 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_ASKPASS_REQUIRE

Allows further control over the
use of an askpass program. If this variable is set to
‘‘never’’ then ssh will never
attempt to use one. If it is set to
‘‘prefer’’, then ssh will
prefer to use the askpass program instead of the TTY when
requesting passwords. Finally, if the variable is set to
‘‘force’’, then the askpass program
will be used for all passphrase input regardless of whether
DISPLAY is set.

SSH_AUTH_SOCK

Identifies the path of a
UNIX-domain 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.

SSH_TUNNEL

Optionally set by
sshd(8) to contain the interface names assigned if
tunnel forwarding was requested by the client.

SSH_USER_AUTH

Optionally set by
sshd(8), this variable may contain a pathname to a
file that lists the authentication methods successfully used
when the session was established, including any public keys
that were used.

	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_config(5).

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 (ECDSA,
Ed25519, RSA) 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_config(5). Because of
the potential for abuse, this file must have strict
permissions: read/write for the user, and not writable by
others.

˜/.ssh/environment

Contains additional definitions
for environment variables; see ENVIRONMENT ,
above.

˜/.ssh/id_ecdsa
˜/.ssh/id_ecdsa_sk
˜/.ssh/id_ed25519
˜/.ssh/id_ed25519_sk
˜/.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 AES-128.

˜/.ssh/id_ecdsa.pub
˜/.ssh/id_ecdsa_sk.pub
˜/.ssh/id_ed25519.pub
˜/.ssh/id_ed25519_sk.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.

/data/data/com.termux/files/usr/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.

/data/data/com.termux/files/usr/etc/ssh/ssh_config

Systemwide configuration file.
The file format and configuration options are described in
ssh_config(5).

/data/data/com.termux/files/usr/etc/ssh/ssh_host_ecdsa_key

/data/data/com.termux/files/usr/etc/ssh/ssh_host_ed25519_key

/data/data/com.termux/files/usr/etc/ssh/ssh_host_rsa_key

These files contain the private
parts of the host keys and are used for host-based
authentication.

/data/data/com.termux/files/usr/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.

/data/data/com.termux/files/usr/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.

EXIT STATUS

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

SEE ALSO

scp(1),
sftp(1), ssh-add(1), ssh-agent(1),
ssh-keygen(1), ssh-keyscan(1), tun(4),
ssh_config(5), ssh-keysign(8),
sshd(8)

STANDARDS

C. Lonvick and
S. Lehtinen, The Secure Shell (SSH) Protocol Assigned
Numbers, RFC 4250, January 2006.

C. Lonvick and
T. Ylonen, The Secure Shell (SSH) Protocol
Architecture, RFC 4251, January 2006.

C. Lonvick and
T. Ylonen, The Secure Shell (SSH) Authentication
Protocol, RFC 4252, January 2006.

C. Lonvick and
T. Ylonen, The Secure Shell (SSH) Transport Layer
Protocol, RFC 4253, January 2006.

C. Lonvick and
T. Ylonen, The Secure Shell (SSH) Connection
Protocol, RFC 4254, January 2006.

W. Griffin and
J. Schlyter, Using DNS to Securely Publish Secure Shell
(SSH) Key Fingerprints, RFC 4255, January 2006.

M. Forssen and
F. Cusack, Generic Message Exchange Authentication for
the Secure Shell Protocol (SSH), RFC 4256, January
2006.

P. Remaker and
J. Galbraith, The Secure Shell (SSH) Session Channel
Break Extension, RFC 4335, January 2006.

C. Namprempre,
T. Kohno and M. Bellare, The Secure Shell (SSH) Transport
Layer Encryption Modes, RFC 4344, January 2006.

B. Harris,
Improved Arcfour Modes for the Secure Shell (SSH)
Transport Layer Protocol, RFC 4345, January 2006.

W. Simpson, N.
Provos and M. Friedl, Diffie-Hellman Group Exchange for
the Secure Shell (SSH) Transport Layer Protocol, RFC
4419, March 2006.

R. Thayer and J.
Galbraith, The Secure Shell (SSH) Public Key File
Format, RFC 4716, November 2006.

J. Green and D.
Stebila, Elliptic Curve Algorithm Integration in the
Secure Shell Transport Layer, RFC 5656, December
2009.

D. Song and A.
Perrig, Hash Visualization: a New Technique to improve
Real-World Security, 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.

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