Manage ssh port forwarding

[daigotanaka]

Some resources are only accessible inside the private network (e.g. AWS Reshift). In such case, a common practice is to SSH to a Bastion server and forward a designated localhost port to the remote address.

handoff could start the port-forwarding service like this at the beginning of the task execution.

Keys and passphrase can be encrypted and stored in the remote parameter store.