1.) ssh -Nf -L 10000:target_server:80 ssh_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 con‐
nection 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 spec‐
ified 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. How‐
ever, 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.
登录ssh_server并由其将从客户端端口10000来的数据，转发到 target_server:80.-Nf是为了让ssh tunnel挂到后台执行.
-N Do not execute a remote command. This is useful for just for‐
warding ports (protocol version 2 only).
-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 for‐
wards to be successfully established before placing itself in the
background.