p2p sends local-scope addresses when IPv4 is behind NAT

[luke-jr]

Wireshark tells me I am sending my local-scope IPv6 address, when connecting over IPv4 (behind NAT)... :/

Context: #8594

[laanwj]

Added privacy tag. Leaking internal addresses behind a NAT is a serious privacy issue, see #8594 (comment) .
Aren't local-scope addresses based on the MAC address? Ouch.

[sipa]

@luke-jr Is this an RFC4862 address (fe80::/64) ?

[luke-jr]

Hmm, no. I don't seem to actually have the address on any of my interfaces.

Begins with fd87. Second-guessing if this is actually IPv6 local, or perhaps some way of encoding a Tor hidden service?

[sipa]

Yes, that's an onioncat address - the way Bitcoin P2P encodes onion addresses in IPv6.

[sipa]

Closing; feel free to reopen if you see other local addresses being relayed.

[laanwj]

Sending an onion address as from-address is a privacy breach, too.

[sipa]

Hmm, I believe the logic is that we send the most compatible known local address to peers. That means that you'd send your onion address to non-tor peers if you don't know of any other local reachable non-onion address.

Perhaps that logic should be revisited, and we should never send an address out except to peers that are already connected to through that interface. That is complicated, as it means we need a way to identify incoming onion connections as belonging to tor.

[laanwj]

I think it should leave the addrFrom address blank in version messages, full stop. There's no need to fill it in at all it if it's no longer needed by peers connected to (e.g. #8594).

[sipa]

@laanwj Perhaps, but that's orthogonal. We also announce our own address using ADDR messages.