consomme: fix DHCPv4 reply delivery and framing

[alextnewman]

Summary

Correct Consomme’s direct-client DHCPv4 reply behavior.

Consomme previously emitted replies with inconsistent link- and network-layer destinations, incomplete DHCPNAK fields, under-reported checksum state, and BOOTP messages shorter than the required minimum.

Update the DHCPv4 path to:

• deliver OFFER and ACK replies using RFC 2131 ciaddr, broadcast, then yiaddr precedence;
• broadcast DHCPNAKs with zero ciaddr, yiaddr, and siaddr, plus option 54;
• copy and validate ciaddr for renewal and rebinding requests;
• ignore requests selecting another DHCP server;
• ignore relayed requests because Consomme has no relay-facing network path;
• keep siaddr zero unless a real bootstrap server is configured;
• report verified IPv4 and UDP checksums accurately;
• pad every BOOTP/DHCP payload to the RFC 1542 minimum of 300 octets.

This remains a deliberately narrow DHCP server for Consomme’s single-guest NAT topology. It does not add relay support, intercept arbitrary UDP/67 traffic, or change DHCPv6.

Testing

All local and live validation was performed on a macOS ARM64 host.

• Added packet-level coverage for broadcast/unicast OFFER, ACK, renewal, stale-binding NAK, foreign-server silence, unsupported-relay silence, checksum state, and BOOTP padding.
• All 108 consomme tests passed.
• Clippy, docs, and formatting checks passed.
• AZL-3 ARM64 reached SSH and received 10.0.0.2/24 with gateway 10.0.0.1.
• Windows ARM64 reached SSH and received 10.0.0.2/24; gateway, DHCP server, and DNS server were 10.0.0.1.
• Both guests shut down cleanly with OpenVMM exit code 0.

[Copilot]

Pull request overview

This PR corrects Consomme’s DHCPv4 server reply behavior for the direct-client NAT topology, aligning reply delivery/framing and several RFC-required fields/behaviors.

Changes:

• Fixes DHCPv4 reply destination selection (L2/L3) for OFFER/ACK/NAK, including ignoring foreign-server SELECTING requests and relayed requests.
• Improves DHCPv4 reply framing/metadata: BOOTP minimum payload padding (300 bytes) and more accurate checksum state reporting.
• Adds packet-level unit tests covering broadcast/unicast behavior, renewal/NAK cases, relay/foreign-server silence, checksum state, and BOOTP padding.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

vm/devices/net/net_consomme/consomme/src/dhcp.rs:189

• resp_dhcp.emit(...).unwrap() can panic on malformed/unexpected state derived from guest input. Even if it "should never fail" given the current length calculation, avoiding panics in packet paths is important; you can capture the Result inside the closure and propagate it after resp_udp.emit returns.

        resp_udp.emit(
            &mut resp_udp_packet,
            &IpAddress::Ipv4(resp_ipv4.src_addr),
            &IpAddress::Ipv4(resp_ipv4.dst_addr),
            resp_dhcp_len,
            |udp_payload| {
                let mut resp_dhcp_packet = DhcpPacket::new_unchecked(udp_payload);
                resp_dhcp.emit(&mut resp_dhcp_packet).unwrap();
            },

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

[github-actions]

4 Petri tests failed (3 unstable)