Feature request: Event handlers

[almarzn]

What's the feature 🧐

It would be nice to have event handlers, special commands that are run by gluetun. It should be possible to hack the port forward command for such purpose but it would be cleaner to do centralize everything

EVENT_ON_TUNNEL_UP
EVENT_ON_TUNNEL_DOWN
EVENT_ON_IP_CHANGE
EVENT_ON_FORWARD_PORT_CHANGE

Etc

What do you think? I could work on it I think we just need to agree on the variables names and which event to support

Extra information and references

No response

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[qdm12]

We already have port forwarding up and down commands https://github.com/qdm12/gluetun-wiki/blob/main/setup/advanced/vpn-port-forwarding.md#custom-port-forwarding-updown-command

We could have a change detector event, but I would rather keep it as it is and let other programs/scripts detect if the port(s) change, at this time at least.

For the other variables:

• Let's have it as COMMAND_ON_XYZ to precise this is a command to be run. EVENT might suggest it does something else
• I let you propose some variables (i.e. {{PORTS}} for port forwarding) for each of these 3 events.

Code-wise, I don't think these are really centralizable, they each should run in their own package/sub-loop.

[qdm12]

Let's continue on #2348

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