OpenSSH Client (remote login program)
References
man ssh
ssh user@host # connect to host as user
ssh -p port user@host # connect using port
ssh -D user@host # connect & use bind portssh [-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 port]
[-Q query_option]
[-R address]
[-S ctl_path]
[-W host:port]
[-w local_tun[:remote_tun]]
destination
[command]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 his/her identity to the remote machine using one of several methods (see below).
If a command is specified, it is executed on the remote host instead of a login shell.
-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 per-
form 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_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.
-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 sup-
ported, and ssh will act as a SOCKS server. Only root can forward privileged ports. Dynamic port forwardings can also be specified in the configu-
ration 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
(/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 suc-
cessfully established before placing itself in the background.
-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. The default is ~/.ssh/id_dsa, ~/.ssh/id_ecdsa,
~/.ssh/id_ed25519 and ~/.ssh/id_rsa. 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 file-
names.
-J destination
Connect to the target host by first making a 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. This is a shortcut to specify a ProxyJump con-
figuration 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 connec-
tion 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 for more informa-
tion.
-N Do not execute a remote command. This is useful for just forwarding ports.
-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), ``forward'' (request forwardings without command execu-
tion), ``cancel'' (cancel forwardings), ``exit'' (request the master to exit), and ``stop'' (request the master to stop accepting further multiplex-
ing 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 com-
mand-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
ChallengeResponseAuthentication
CheckHostIP
Ciphers
ClearAllForwardings
Compression
ConnectionAttempts
ConnectTimeout
ControlMaster
ControlPath
ControlPersist
DynamicForward
EscapeChar
ExitOnForwardFailure
FingerprintHash
ForwardAgent
ForwardX11
ForwardX11Timeout
ForwardX11Trusted
GatewayPorts
GlobalKnownHostsFile
GSSAPIAuthentication
GSSAPIDelegateCredentials
HashKnownHosts
Host
HostbasedAuthentication
HostbasedKeyTypes
HostKeyAlgorithms
HostKeyAlias
Hostname
IdentitiesOnly
IdentityAgent
IdentityFile
IPQoS
KbdInteractiveAuthentication
KbdInteractiveDevices
KexAlgorithms
LocalCommand
LocalForward
LogLevel
MACs
Match
NoHostAuthenticationForLocalhost
NumberOfPasswordPrompts
PasswordAuthentication
PermitLocalCommand
PKCS11Provider
Port
PreferredAuthentications
ProxyCommand
ProxyJump
ProxyUseFdpass
PubkeyAcceptedKeyTypes
PubkeyAuthentication
RekeyLimit
RemoteCommand
RemoteForward
RequestTTY
SendEnv
ServerAliveInterval
ServerAliveCountMax
SetEnv
StreamLocalBindMask
StreamLocalBindUnlink
StrictHostKeyChecking
TCPKeepAlive
Tunnel
TunnelDevice
UpdateHostKeys
UseKeychain
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 query_option
Queries ssh for the algorithms supported for the specified version 2. The available features are: 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-cert (certificate key types), key-plain (non-certificate key types),
protocol-version (supported SSH protocol versions), and sig (supported signature algorithms).
-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 destina-
tion 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 appli-
cations (e.g. sftp(1)). The subsystem is specified as the remote command.
-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 configura-
tion 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. Please 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).
The OpenSSH SSH client supports SSH protocol 2.
The methods available for authentication are: GSSAPI-based authentication, host-based authentication, public key authentication, challenge-response authen-
tication, 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 /etc/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 pro-
tocol automatically, using one of the DSA, ECDSA, Ed25519 or RSA algorithms. The HISTORY section of ssl(8) contains a brief discussion of the DSA and 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 his/her key pair by running ssh-keygen(1). This stores the private key in ~/.ssh/id_dsa (DSA), ~/.ssh/id_ecdsa (ECDSA), ~/.ssh/id_ed25519
(Ed25519), or ~/.ssh/id_rsa (RSA) and stores the public key in ~/.ssh/id_dsa.pub (DSA), ~/.ssh/id_ecdsa.pub (ECDSA), ~/.ssh/id_ed25519.pub (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 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 certifi-
cates are used. This has the advantage that a single trusted certification authority can be used in place of many public/private keys. See the CERTIFI-
CATES 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.
Challenge-response authentication works as follows: The server sends an arbitrary "challenge" text, and prompts for a response. Examples of challenge-
response authentication include BSD 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 /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.
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 fol-
low 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 ssh.
~# 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 cancel-
lation 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. !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.
Forwarding of arbitrary TCP connections over a secure channel can be specified either on the command line or in a configuration file. One possible applica-
tion 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 ssh, 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.
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 manu-
ally 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.
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):
$ 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. If only legacy (MD5) fingerprints for the server are avail-
able, 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 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 proba-
bility 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_config(5) for more information.
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).
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.
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-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).
~/.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 (DSA, 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_dsa
~/.ssh/id_ecdsa
~/.ssh/id_ed25519
~/.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 gener-
ating the key which will be used to encrypt the sensitive part of this file using AES-128.
~/.ssh/id_dsa.pub
~/.ssh/id_ecdsa.pub
~/.ssh/id_ed25519.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/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_config(5).
/etc/ssh/ssh_host_key
/etc/ssh/ssh_host_dsa_key
/etc/ssh/ssh_host_ecdsa_key
/etc/ssh/ssh_host_ed25519_key
/etc/ssh/ssh_host_rsa_key
These files contain the private parts of the host keys and are used for host-based authentication.
/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.
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