wtclientrpc: prevent watchtower connections to local addresses

[anibilthare]

Change Description

Fixes 5522

Adds validation to prevent watchtower client connections to local addresses by:

• Implementing IsLocalAddress() to detect localhost/local network addresses
• Adding check in AddTower RPC to reject local tower connections
• Including unit tests for address validation

This helps prevent security issues from misconfigured watchtower setups that accidentally expose local addresses.

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[anibilthare]

@ellemouton Please review.

[ellemouton]

@anibilthare - I think the issue was more about connecting to your own watchtower on the same LND node. ie, connecting the wtclient to the watchtower server on the same LND node

[anibilthare]

Thanks for the quick response @ellemouton !!

I chose to check for all local addresses (rather than just same-node) for these reasons:

1. Security: Running multiple LND nodes on the same machine means they share the same failure domain - if the machine is compromised, all nodes including their watchtowers are compromised
2. Redundancy: The purpose of watchtowers is to provide backup/monitoring from an independent system. Having the watchtower on the same physical machine defeats this purpose since any hardware/system failure would affect both the node and its watchtower
3. Practical Implementation: Detecting "same LND node" would require comparing node identities, while checking local addresses provides a robust way to prevent all potentially problematic local configurations
4. Future-proofing: This approach also prevents users from accidentally creating other insecure local watchtower setups as they experiment with different configurations

Would you prefer we narrow the scope to only prevent same-node connections? I'm open to adjusting the approach if you think the current implementation is too broad.

[anibilthare]

@ellemouton Any feedback ?

[ellemouton]

@anibilthare - my concern is that this makes testing hard/impossible. We could just log a warning instead? Your points are all valid but i think the user should have the flexibility to run things how they want to & they themselves should be aware of the risks of certain run configurations

imo, the original issue isnt really an issue anyways... maybe others disagree.