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The classifier in proxy::host reads a gateway DHCP reply to find the VM MAC. It looked at the first fragment only. An attacker on the gateway link could split the reply so the first fragment holds no complete BOOTP header. The classifier then read past it and matched the wrong bytes. That let traffic through the gateway check. Check the fragment offset before the parse. A reply that arrives in more than one fragment is now rejected, since a real DHCP reply from the gateway is a single datagram. Add tests for the fragmented and unfragmented cases.
RainieLLM
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Fixes #197
Summary
The DHCP reply classifier parsed the UDP header before it checked the IPv4 fragment fields. A fragmented reply could then be treated as
NotDhcp. In stateless mode, that path allowed the packet without the VM MAC check, so a foreign DHCP reply could reach the guest.This change rejects gateway fragments before the UDP parse. Complete replies keep the existing VM MAC check. Fragments from other sources remain
NotDhcp.Tests
The new tests cover first and later gateway fragments, valid and foreign complete replies, and fragments from other sources.
cargo fmt --checkcargo clippy --all-targets --all-features -- -D warningscargo test --lib proxy::host::tests