Restrict self-advertisements with privacy networks to avoid fingerprinting #316

https://github.com/bitcoin/bitcoin/pull/27411

This would improve privacy for nodes that run on both clearnet (ip v4 or v6) and a privacy network (tor or i2p). BTC and HUSH will tell a clearnet peer it's privacy network address (and vice versa) which leaks metadata, i.e. node X has address Y in IPv4 and address Z on Tor v3, for example. Nodes would protect their privacy better to not do this. This code has recently been merged into BTC (it isn't in a release yet) but their internals have changed a lot so it may be some fun to make this change in our codebase.

Created the p2p_privacy branch to work on this

Functions that need to change to complete this :

GetLocalAddress() in src/netaddress.cpp in BTC, src/net.cpp in HUSH
GetReachabilityFrom() in src/netaddress.cpp
GetExtNetwork() in src/netaddress.cpp
GetLocal() in src/net.cpp

BTC upstream code has not diverged as much as I thought so it should be relatively straight forward to port these changes to our codebase

I have made most of the necessary changes but currently dealing with an issue where BTC's version of GetLocalAddress was already returning a CService before this change, but our version returns a CAddress which means changing code which calls GetLocalAddress to deal with that.

OK, I pushed code to the branch. It has not been tested yet. To test this branch :

Make sure we didn't break nodes that only use IPv4/IPv6
Make sure we didn't break nodes that only use Tor or only i2p
Make sure we didn't break nodes that listen on clearnet and a privacy network (such as IPv4+Tor)
- IPv4+IPv6+Tor
- IPv4+IPv6+i2p

I don't think it's very important to test full syncs, we mostly want to test that a node can stay in sync with peers. So we just need to test partial syncs.

If this code does not work, then nodes may get stuck with no peers or be unable to sync. We want to make sure that doesn't happen.

Looks like I missed some code to port and we don't have a function it wants to call, so I am still working on this.

Current status is that I ported more code but we are starting to run into issues where our internals are out of sync with the BTC code coming in, so lots of updates are required. We didn't have ConnectedThroughNetwork() function which I added, but that relies on the concept of "inbound onions" and a list of "onion binds" which we don't have. Implementing those requires changes to init.cpp and also changing the constructor of the CNode class.