The relay now looks through ordinary IPv6 extension headers to catch remote
DHCPv6 server replies and Router Advertisements. IPv6 fragments are still an
evasion risk because later fragments may not contain the upper-layer ports or
ICMPv6 type that the relay safety policy checks.
For the MVP, make that boundary conservative: remote-client IPv6 fragments are
filtered before they can reach the physical LAN. LAN-origin fragments are still
allowed to flow back to remote clients, so this does not block ordinary LAN
traffic returning through the gateway. Add a dedicated diagnostics drop reason
so logs explain the policy clearly.
Test Plan:
- cargo fmt --check
- cargo test -p lanparty-relay -p lanparty-obs
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check
Refs: MVP relay L2 safety filters
ddidderr
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