SSH Tunneling Example

08/11/2020 posted in  Commands and Tools

Format

ssh -N -L <local port>:<remote host>:<remote port> <SSH server>

  • -N: Not to execute shell command, only listen and forward.
  • -L: Listen to <local port> on client machine.
  • Any data send to this <local port> is forward to <remote host>:<remote port>. Therefore the application on <remote host> listening to <remote port> is used as if it was a local application.
  • <remote host> is usually the same as <SSH server>, i.e. <remote host> is localhost relative to <SSH server>.
  • <remote host> can be also be not on <SSH server>, i.e. <SSH server> is used as a bridge when data is forwarded from <SSH client>:<local port>and <remote host>:<remote port>

Example

1. Remote Debug a service on a cloud server

Forward client machines's localhost:5050 to 's localhost:5050

ssh -N -L 5050:localhost:5050 <username>@<ssh_server>

2. Remote Debug a service on a cloud server through a bastion host

On client machine, forward client machine's 5022 to <target_host>:22. The goal is to use client machine's 5022 port as if it was <target_host>:22. After this setting, data to local 5022 is forwarded to <target_host>:22 , therefore local becomes a local endpointwe make local 5022 to be the same you can even ssh to <target_host>:22 by ssh -p 5022 localhost.

ssh -A -N -L 5022:<target_host>:22 <username>@<ssh_server>

On client machine, forward client machine's 5050 to localhost:5050 through localhost.

ssh -p 5022 -N -L 5050:localhost:5050 localhost