Reject foreign DHCP replies before policy fallback - #196
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| let Ok(udp_pkt) = UdpPacket::new_checked(ipv4_pkt.payload()) else { | ||
| return false; | ||
| return DhcpResponseDisposition::NotDhcp; |
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When a BOOTP reply is IPv4-fragmented, the first fragment's UDP length describes the complete datagram, so UdpPacket::new_checked rejects the shorter fragment; later fragments contain no UDP header. Returning NotDhcp here sends every fragment through generic policy, which unconditionally permits them when flows is None, allowing the guest to reassemble a foreign DHCP reply and bypass the new client-MAC check. Fragmented gateway UDP traffic must be rejected or reassembled before deciding that it is not DHCP.
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Let's handle this separately in #197, seems out-of-scope because this PR doesn't introduce this issue.
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In #192, additional filtering for DHCP messages coming from the host side was added.
However, foreign DHCP replies were not explicitly rejected and could still fall through to normal host → VM policy and be forwarded to the VM.
Add an enum to properly distinguish between non-DHCP traffic, allowed DHCP replies, and rejected DHCP replies.
/cc @genforAI