Maintain ssh tunnel

Based on this: https://github.com/daradib/sidedoor

1. installation

sudo apt install sidedoor

2. create ssh public key and add to remote user ~/.ssh/authorized_keys

sudo ssh-keygen -t rsa -N '' -f /home/myaccount/id_rsa
cat /home/myaccount/id_rsa.pub | ssh myaccount@my.remote.server 'cat >> .ssh/authorized_keys'

3. modify /etc/sidedoor/config

Host my.remote.server
  # SSH private key to access the remote server.
  IdentityFile /home/myname/id_rsa

  # because I am using ddns, my IP might change, so I set it to no
  StrictHostKeyChecking no

  # SSH host key of the remote server.
  UserKnownHostsFile /dev/null

  # If tunneled traffic is encrypted (e.g., SSH),
  # it probably does not compress well.
  Compression no

  # Disable password authentication.
  BatchMode yes

  # Terminate if unable to set up port forwarding.
  ExitOnForwardFailure yes

  # Enable SSH keepalives.
  ServerAliveInterval 30

  # Disconnect after unresponsive SSH keepalives.
  ServerAliveCountMax 3

4. modify /etc/default/sidedoor

# Configuration for sidedoor service

# Arguments to pass to sidedoor, which are in turn passed to ssh.
# See man ssh for possible options.
# For example, uncomment the line below to create a remote port forward
# to the local SSH server (running on port 22, exposed on port 8022).
OPTIONS='-R 12345:localhost:9999'

# Remote SSH server to connect to, i.e., [user@]hostname.
REMOTE_SERVER=myaccount@my.remote.server

5. start up sidedoor service and check sidedoor status

sudo systemctl enable sidedoor.service
sudo systemctl restart sidedoor.service
systemctl status sidedoor.service

Ssh port forwarding

操作之前記得先修改 sshd_config 加上
PermitTunnel yes # 允許做 ssh tunnel
GatewayPorts yes # 允許 remote port forwarding

Local Port forwarding (Local to Remote)

ssh -L 1234:192.168.1.1:80 user@example.com 

當使用帳號 user ssh 到 example.com 後，本地端的 port 1234 會對應到遠端 192.168.1.1 的 port 80。
當這個連線保持開啟時，開啟瀏覽器連線至 127.00.1:1234 則會變成連線到 192.168.1.1:80。

Remote Port Forwarding (Remote to Local)

ssh -R 1234:192.168.1.1:80 user@example.com 

當使用帳號 user ssh 到 example.com 後，遠端的 port 1234 會對應到 192.168.1.1 的 port 80。
(192.168.1.1 為本地端的 IP) 這表示當此連線保持開啟時，連線至 example.com:1234 都會直接導至本地端的 port 80。