Security audit finding NET-02. A discovered `_lanspread._udp` record
was accepted as a QUIC candidate as soon as it carried the current
protocol version and a peer_id TXT entry; the resolved socket address
itself was never inspected. Anyone on the LAN can publish mDNS records,
so a forged advertisement could point every peer's handshake attempt at
a multicast group (224.0.0.251), the broadcast address, 0.0.0.0/::, or
port 0. That wastes a discovery slot per record and sprays QUIC Initial
packets onto addresses no peer can ever answer from.
`validated_candidate_endpoint` now rejects unspecified, multicast and
IPv4 broadcast addresses as well as port 0 before the candidate enters
the negotiation set. Loopback and link-local addresses stay admissible
because single-host and DHCP-less LAN setups legitimately use them.
Test plan: `just test` (new unit tests cover the address filter and the
endpoint validator). Manually: start two peer-cli containers; discovery
still works with real interface addresses.
Claude-Session: https://claude.ai/code/session_017C3Nbgwpdm3YNwZhhFLHwg
ddidderr
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