Feat/parametrized postupdown

[dddpaul]

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• I have read the contributing guideline and understand that I have made the correct modifications

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Description:

Add POST_UP and POST_DOWN variables to specify corresponding scripts.

Benefits of this PR and context:

There ara may scenarios for customized postup/postdown scripts can be helpful. For example, if I want to setup my wireguard server as an entrypoint for some services running on other wireguard peers.

For example:
iptables -t nat -A PREROUTING -p tcp --dport 12345 -j DNAT --to-destination 10.13.13.2:12345;
iptables -t nat -A POSTROUTING -p tcp -d 10.13.13.0/24 -j MASQUERADE;

This makes all traffic to external wg server address and port 12345 being NATed and routed to "internal" peer and port 12345.

These rules can be very long and complicated to edit them manually.

Issues

#201

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[drizuid]

@aptalca will be the final decision on this, but the new vars should be optional vars, not mandatory. Personally, I think adding this will be a support nightmare when users start bombing it because they don't understand iptables.

[aptalca]

Thanks for the PR, but I'm not in favor of this for a few reasons:

• It's not crucial for setup, and can be set via templates we provide
• It's not just PostUp and PostDown, there are also PreUp and PreDown
• Every env var we list/advertise gets edited incorrectly and unnecessarily by a sizeable percentage of the users, leading to a substantial increase in support requests

Thses arguments are better left as template config that can be used by advanced users as needed