Version 2 Added config file and IPV6 support and benchmarks with massive performance uplifts.

tests/test_security_gates.rs

@@ -0,0 +1,620 @@
+//! DHCP security gates + filter/framing compositions, part 5.
+//!
+//! Covers security-relevant behavior NOT locked by `test_security.rs`,
+//! `test_security_extra.rs`, `test_security_edge.rs`, `test_security_wire.rs`
+//! or the RFC suites — mostly *compositions* of individually-tested quirks:
+//!
+//! - PRL filtering (not just the 300 cap) strips the NAK Message text live
+//! - an RFC-valid PAD byte before END truncates the option list (desync via
+//!   spec-compliant bytes, unlike the malformed-option tests)
+//! - zero-length option matrix: which empty options decode vs reject,
+//!   including the Router/DNS-empty quirk
+//! - empty Router/DNS encode to `[3,0]`/`[6,0]` on the wire but decode-drop,
+//!   while empty PRL survives (asymmetry)
+//! - garbage-datagram battery (empty/1-byte/short/bad-cookie) keeps the
+//!   server alive for the next valid Discover
+//! - kernel-style truncation of an oversized datagram decodes without panic
+//! - Request with a *wrong* (not just missing) Server ID is still honored
+//! - Decline from an unknown chaddr is a silent no-op with no reply
+//! - broadcast-flagged Release still frees when the gate passes
+//!
+//! `*_quirk` tests lock currently-unsafe behavior for review, not approval.
+
+use dhcp4r::options::*;
+use dhcp4r::packet::*;
+use dhcp4r::server;
+use std::collections::HashMap;
+use std::net::{Ipv4Addr, UdpSocket};
+use std::ops::Add;
+use std::time::{Duration, Instant};
+
+// ---------------------------------------------------------------------------
+// helpers (same patterns as test_security_wire.rs)
+// ---------------------------------------------------------------------------
+
+fn unwrap_packet(r: Result<Packet, CustomErr<&[u8]>>) -> Packet {
+    match r {
+        Ok(p) => p,
+        Err(_) => panic!("expected Ok Packet"),
+    }
+}
+
+#[allow(clippy::too_many_arguments)]
+fn make_raw(
+    op: u8,
+    hlen: u8,
+    hops: u8,
+    xid: u32,
+    secs: u16,
+    flags: [u8; 2],
+    ci: [u8; 4],
+    yi: [u8; 4],
+    si: [u8; 4],
+    gi: [u8; 4],
+    chaddr: [u8; 6],
+    opts_after_cookie: Vec<u8>,
+) -> Vec<u8> {
+    let mut v = vec![0u8; 236];
+    v[0] = op;
+    v[1] = 1;
+    v[2] = hlen;
+    v[3] = hops;
+    v[4..8].copy_from_slice(&xid.to_be_bytes());
+    v[8..10].copy_from_slice(&secs.to_be_bytes());
+    v[10] = flags[0];
+    v[11] = flags[1];
+    v[12..16].copy_from_slice(&ci);
+    v[16..20].copy_from_slice(&yi);
+    v[20..24].copy_from_slice(&si);
+    v[24..28].copy_from_slice(&gi);
+    v[28..34].copy_from_slice(&chaddr);
+    v.extend_from_slice(&[99, 130, 83, 99]);
+    v.extend_from_slice(&opts_after_cookie);
+    v
+}
+
+// Replica of examples/server.rs lease state (bodies identical; only the
+// methods needed here are included).
+const IP_START: [u8; 4] = [192, 168, 2, 2];
+const IP_START_NUM: u32 = u32::from_be_bytes(IP_START);
+const LEASE_NUM: u32 = 252;
+const LEASE_DURATION_SECS: u32 = 86400;
+
+// Example-like handler: Discover->Offer, Request->Ack, Release|Decline gated,
+// everything else ignored. Mirrors examples/server.rs match arms.
+struct ExampleLike {
+    leases: HashMap<Ipv4Addr, ([u8; 6], Option<Instant>)>,
+    last_lease: u32,
+    lease_duration: Duration,
+    #[allow(dead_code)]
+    server_ip: Ipv4Addr,
+}
+
+impl ExampleLike {
+    fn new(server_ip: Ipv4Addr) -> Self {
+        Self {
+            leases: HashMap::new(),
+            last_lease: 0,
+            lease_duration: Duration::new(LEASE_DURATION_SECS as u64, 0),
+            server_ip,
+        }
+    }
+    fn available(&self, chaddr: &[u8; 6], addr: &Ipv4Addr) -> bool {
+        let pos: u32 = (*addr).into();
+        pos >= IP_START_NUM
+            && pos < IP_START_NUM + LEASE_NUM
+            && match self.leases.get(addr) {
+                Some((mac, expiry)) => {
+                    *mac == *chaddr || expiry.map_or(true, |exp| Instant::now().gt(&exp))
+                }
+                None => true,
+            }
+    }
+    fn current_lease(&self, chaddr: &[u8; 6]) -> Option<Ipv4Addr> {
+        for (i, v) in &self.leases {
+            if v.0 == *chaddr {
+                return Some(*i);
+            }
+        }
+        None
+    }
+}
+
+impl server::Handler for ExampleLike {
+    fn handle_request(&mut self, server: &server::Server, in_packet: Packet) {
+        match in_packet.message_type() {
+            Ok(MessageType::Discover) => {
+                if let Some(ip) = self.current_lease(&in_packet.chaddr) {
+                    let _ = server.reply(
+                        MessageType::Offer,
+                        vec![DhcpOption::IpAddressLeaseTime(LEASE_DURATION_SECS)],
+                        ip,
+                        in_packet,
+                    );
+                    return;
+                }
+                for _ in 0..LEASE_NUM {
+                    self.last_lease = (self.last_lease + 1) % LEASE_NUM;
+                    let cand: Ipv4Addr = (IP_START_NUM + self.last_lease).into();
+                    if self.available(&in_packet.chaddr, &cand) {
+                        let _ = server.reply(
+                            MessageType::Offer,
+                            vec![DhcpOption::IpAddressLeaseTime(LEASE_DURATION_SECS)],
+                            cand,
+                            in_packet,
+                        );
+                        break;
+                    }
+                }
+            }
+            Ok(MessageType::Request) => {
+                // NOTE: mirrors the example — the for_this_server gate is
+                // commented out upstream, so Requests are honored regardless.
+                let req_ip = match in_packet.option(REQUESTED_IP_ADDRESS) {
+                    Some(DhcpOption::RequestedIpAddress(x)) => *x,
+                    _ => in_packet.ciaddr,
+                };
+                if let Some(ip) = self.current_lease(&in_packet.chaddr) {
+                    let _ = server.reply(
+                        MessageType::Ack,
+                        vec![DhcpOption::IpAddressLeaseTime(LEASE_DURATION_SECS)],
+                        ip,
+                        in_packet,
+                    );
+                    return;
+                }
+                if !self.available(&in_packet.chaddr, &req_ip) {
+                    let _ = server.reply(
+                        MessageType::Nak,
+                        vec![DhcpOption::Message("Requested IP not available".to_string())],
+                        Ipv4Addr::UNSPECIFIED,
+                        in_packet,
+                    );
+                    return;
+                }
+                self.leases.insert(
+                    req_ip,
+                    (
+                        in_packet.chaddr,
+                        Some(Instant::now().add(self.lease_duration)),
+                    ),
+                );
+                let _ = server.reply(
+                    MessageType::Ack,
+                    vec![DhcpOption::IpAddressLeaseTime(LEASE_DURATION_SECS)],
+                    req_ip,
+                    in_packet,
+                );
+            }
+            Ok(MessageType::Release) | Ok(MessageType::Decline) => {
+                if !server.for_this_server(&in_packet) {
+                    return;
+                }
+                if let Some(ip) = self.current_lease(&in_packet.chaddr) {
+                    self.leases.remove(&ip);
+                }
+            }
+            _ => {}
+        }
+    }
+}
+
+fn serve_example_like() -> (std::net::SocketAddr, Ipv4Addr) {
+    let srv_sock = UdpSocket::bind("127.0.0.1:0").unwrap();
+    let srv_addr = srv_sock.local_addr().unwrap();
+    let server_ip = Ipv4Addr::new(192, 168, 2, 1);
+    std::thread::spawn(move || {
+        let _ = server::Server::serve(
+            srv_sock,
+            server_ip,
+            Ipv4Addr::new(192, 168, 2, 255),
+            ExampleLike::new(server_ip),
+        );
+    });
+    std::thread::sleep(Duration::from_millis(100));
+    (srv_addr, server_ip)
+}
+
+fn send_recv(
+    client: &UdpSocket,
+    srv_addr: &std::net::SocketAddr,
+    p: &Packet,
+    buf: &mut [u8; 1500],
+    rbuf: &mut [u8; 1500],
+) -> Packet {
+    client.send_to(&p.encode(buf).to_vec(), srv_addr).unwrap();
+    let (n, _) = client.recv_from(rbuf).expect("expected reply");
+    unwrap_packet(Packet::from(&rbuf[..n]))
+}
+
+// ===========================================================================
+// A. PRL filtering (not size) strips the NAK text; RFC-valid PAD desyncs
+// ===========================================================================
+
+/// The NAK Message is dropped by *PRL filtering* here, not just the 300 cap:
+/// unit-level, `[53,54,56]` filtered by req `[1]` keeps `[53,54]` although
+/// the 7-byte packet would easily fit. Live, a victim Request carrying
+/// PRL=`[1]` gets a textless NAK. Locked (both mechanisms coincide live).
+#[test]
+fn sec4_nak_message_dropped_by_prl_filter_not_size() {
+    // unit: short text fits size-wise, filter drops it anyway
+    let mut opts = vec![
+        DhcpOption::DhcpMessageType(MessageType::Nak),
+        DhcpOption::ServerIdentifier(Ipv4Addr::new(192, 168, 2, 1)),
+        DhcpOption::Message("no".to_string()),
+    ];
+    server::filter_options_by_req(&mut opts, &[1]);
+    assert_eq!(
+        opts.iter().map(|o| o.code()).collect::<Vec<u8>>(),
+        vec![53, 54],
+        "PRL [1] drops Message even though it fits"
+    );
+    // live: occupier takes .50, victim asks with PRL=[1] -> textless NAK
+    let (srv_addr, server_ip) = serve_example_like();
+    let client = UdpSocket::bind("127.0.0.1:0").unwrap();
+    client
+        .set_read_timeout(Some(Duration::from_secs(2)))
+        .unwrap();
+    let mut buf = [0u8; 1500];
+    let mut rbuf = [0u8; 1500];
+    let occupier = Packet {
+        reply: false, hops: 0, xid: 0xA001, secs: 0, broadcast: false,
+        ciaddr: Ipv4Addr::UNSPECIFIED, yiaddr: Ipv4Addr::UNSPECIFIED,
+        siaddr: Ipv4Addr::UNSPECIFIED, giaddr: Ipv4Addr::UNSPECIFIED,
+        chaddr: [0xA0; 6],
+        options: vec![
+            DhcpOption::DhcpMessageType(MessageType::Request),
+            DhcpOption::ServerIdentifier(server_ip),
+            DhcpOption::RequestedIpAddress(Ipv4Addr::new(192, 168, 2, 50)),
+        ],
+    };
+    assert_eq!(
+        send_recv(&client, &srv_addr, &occupier, &mut buf, &mut rbuf).message_type(),
+        Ok(MessageType::Ack)
+    );
+    let req = Packet {
+        reply: false, hops: 0, xid: 0xA002, secs: 0, broadcast: false,
+        ciaddr: Ipv4Addr::UNSPECIFIED, yiaddr: Ipv4Addr::UNSPECIFIED,
+        siaddr: Ipv4Addr::UNSPECIFIED, giaddr: Ipv4Addr::UNSPECIFIED,
+        chaddr: [0xA1; 6],
+        options: vec![
+            DhcpOption::DhcpMessageType(MessageType::Request),
+            DhcpOption::ServerIdentifier(server_ip),
+            DhcpOption::RequestedIpAddress(Ipv4Addr::new(192, 168, 2, 50)),
+            DhcpOption::ParameterRequestList(vec![1]),
+        ],
+    };
+    let rep = send_recv(&client, &srv_addr, &req, &mut buf, &mut rbuf);
+    assert_eq!(rep.message_type(), Ok(MessageType::Nak));
+    assert!(rep.option(MESSAGE).is_none(), "PRL filter strips NAK text");
+    assert_eq!(
+        rep.option(SERVER_IDENTIFIER),
+        Some(&DhcpOption::ServerIdentifier(server_ip))
+    );
+}
+
+/// RFC 2132 says receivers MUST handle PAD octets, but here a single
+/// spec-compliant `0x00` before END truncates the option list exactly like a
+/// malformed option: Server ID after the PAD is stripped, leading Discover
+/// survives. Desync with fully valid bytes — locked.
+#[test]
+fn sec4_rfc_pad_byte_before_end_truncates_list() {
+    let raw = make_raw(
+        1, 6, 0, 0xA010, 0, [0, 0],
+        [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0],
+        [0xA2; 6],
+        vec![53, 1, 1, 0, 54, 4, 10, 20, 30, 40, 255],
+    );
+    let p = unwrap_packet(Packet::from(&raw));
+    assert_eq!(p.options.len(), 1, "PAD truncates like a bad option");
+    assert_eq!(p.message_type(), Ok(MessageType::Discover));
+    assert!(p.option(SERVER_IDENTIFIER).is_none(), "Server ID stripped");
+}
+
+// ===========================================================================
+// B. Zero-length option matrix (decode edges per code)
+// ===========================================================================
+
+/// Zero-length options split by kind: variable-length/text codes accept
+/// empty (`55`/`12`/`56`/unrecognized incl. PAD-as-TLV `0`), while every
+/// fixed-size code rejects — including Router/DNS, which fail even empty
+/// (same `custom_many0` quirk as populated ones). Exact errors locked.
+#[test]
+fn sec4_zero_length_option_matrix() {
+    // accepted as empty
+    match decode_option(&[55, 0]) {
+        Ok((_, DhcpOption::ParameterRequestList(v))) => assert!(v.is_empty()),
+        _ => panic!("[55,0] must decode to empty PRL"),
+    }
+    match decode_option(&[12, 0]) {
+        Ok((_, DhcpOption::HostName(s))) => assert_eq!(s, ""),
+        _ => panic!("[12,0] must decode to empty HostName"),
+    }
+    match decode_option(&[56, 0]) {
+        Ok((_, DhcpOption::Message(s))) => assert_eq!(s, ""),
+        _ => panic!("[56,0] must decode to empty Message"),
+    }
+    match decode_option(&[99, 0]) {
+        Ok((_, DhcpOption::Unrecognized(r))) => {
+            assert_eq!(r.code, 99);
+            assert!(r.data.is_empty());
+        }
+        _ => panic!("[99,0] must decode to empty Unrecognized"),
+    }
+    match decode_option(&[0, 0]) {
+        Ok((_, DhcpOption::Unrecognized(r))) => {
+            assert_eq!(r.code, 0);
+            assert!(r.data.is_empty());
+        }
+        _ => panic!("[0,0] must decode to empty Unrecognized"),
+    }
+    // rejected: fixed-size codes need payload bytes
+    for wire in [
+        vec![53, 0],       // message type needs 1 byte
+        vec![54, 0],       // server id needs 4
+        vec![50, 0],       // requested ip needs 4
+        vec![1, 0],        // subnet mask needs 4
+        vec![51, 0],       // lease time needs 4
+    ] {
+        assert!(
+            matches!(decode_option(&wire), Err(CustomErr::InvalidHlen)),
+            "{:?} must be InvalidHlen",
+            wire
+        );
+    }
+    // rejected quirk: Router/DNS fail even with empty data
+    for wire in [vec![3, 0], vec![6, 0]] {
+        assert!(
+            matches!(decode_option(&wire), Err(CustomErr::InvalidHlen)),
+            "{:?} must be InvalidHlen (many0 quirk)",
+            wire
+        );
+    }
+}
+
+/// Empty Router/DNS encode to `[3,0]`/`[6,0]` on the wire but decode-drop,
+/// while empty PRL survives the round-trip — the asymmetry, end to end.
+#[test]
+fn sec4_empty_router_dns_dropped_empty_prl_kept() {
+    let p = Packet {
+        reply: false, hops: 0, xid: 0xA020, secs: 0, broadcast: false,
+        ciaddr: Ipv4Addr::UNSPECIFIED, yiaddr: Ipv4Addr::UNSPECIFIED,
+        siaddr: Ipv4Addr::UNSPECIFIED, giaddr: Ipv4Addr::UNSPECIFIED,
+        chaddr: [0xA3; 6],
+        options: vec![
+            DhcpOption::DhcpMessageType(MessageType::Discover),
+            DhcpOption::ParameterRequestList(vec![]),
+            DhcpOption::Router(vec![]),
+            DhcpOption::DomainNameServer(vec![]),
+        ],
+    };
+    let mut buf = [0u8; 1500];
+    let enc = p.encode(&mut buf).to_vec();
+    for needle in [vec![55u8, 0], vec![3u8, 0], vec![6u8, 0]] {
+        assert!(
+            enc.windows(2).any(|w| w == needle.as_slice()),
+            "{:?} must be on the wire",
+            needle
+        );
+    }
+    let q = unwrap_packet(Packet::from(&enc));
+    assert_eq!(
+        q.options.iter().map(|o| o.code()).collect::<Vec<u8>>(),
+        vec![53, 55],
+        "empty Router/DNS dropped, empty PRL kept"
+    );
+}
+
+// ===========================================================================
+// C. Garbage survival + kernel-style truncation (live + unit)
+// ===========================================================================
+
+/// Battery of garbage datagrams (empty, 1 byte, short header, bad cookie)
+/// keeps the server alive: the next valid Discover still gets Offer `.3`.
+/// Malformed input never kills the loop when the END-skip lands non-empty.
+#[test]
+fn sec4_garbage_datagram_battery_server_survives_live() {
+    let srv_sock = UdpSocket::bind("127.0.0.1:0").unwrap();
+    let srv_addr = srv_sock.local_addr().unwrap();
+    struct H;
+    impl server::Handler for H {
+        fn handle_request(&mut self, s: &server::Server, p: Packet) {
+            let _ = s.reply(
+                MessageType::Offer,
+                vec![],
+                Ipv4Addr::new(192, 168, 7, 10),
+                p,
+            );
+        }
+    }
+    std::thread::spawn(move || {
+        let _ = server::Server::serve(
+            srv_sock,
+            Ipv4Addr::new(192, 168, 7, 1),
+            Ipv4Addr::new(192, 168, 7, 255),
+            H,
+        );
+    });
+    std::thread::sleep(Duration::from_millis(100));
+    let client = UdpSocket::bind("127.0.0.1:0").unwrap();
+    client
+        .set_read_timeout(Some(Duration::from_secs(2)))
+        .unwrap();
+    let mut buf = [0u8; 1500];
+    let mut rbuf = [0u8; 1500];
+    // each of these decodes to Err (no panic: tiny inputs fail before split)
+    client.send_to(&[], srv_addr).unwrap();
+    client.send_to(&[0u8], srv_addr).unwrap();
+    client.send_to(&[0u8; 10], srv_addr).unwrap();
+    client.send_to(&[0u8; 235], srv_addr).unwrap();
+    let mut bad_cookie = vec![0u8; 236];
+    bad_cookie[0] = 1;
+    bad_cookie[1] = 1;
+    bad_cookie[2] = 6;
+    bad_cookie.extend_from_slice(&[9, 9, 9, 9, 53, 1, 1, 255]);
+    client.send_to(&bad_cookie, srv_addr).unwrap();
+    std::thread::sleep(Duration::from_millis(100));
+    // server still alive and answering
+    let disc = Packet {
+        reply: false, hops: 0, xid: 0xA030, secs: 0, broadcast: false,
+        ciaddr: Ipv4Addr::UNSPECIFIED, yiaddr: Ipv4Addr::UNSPECIFIED,
+        siaddr: Ipv4Addr::UNSPECIFIED, giaddr: Ipv4Addr::UNSPECIFIED,
+        chaddr: [0xA4; 6],
+        options: vec![DhcpOption::DhcpMessageType(MessageType::Discover)],
+    };
+    client.send_to(&disc.encode(&mut buf).to_vec(), srv_addr).unwrap();
+    let (n, _) = client.recv_from(&mut rbuf).expect("server survived garbage");
+    let rep = unwrap_packet(Packet::from(&rbuf[..n]));
+    assert_eq!(rep.message_type(), Ok(MessageType::Offer));
+    assert_eq!(rep.yiaddr, Ipv4Addr::new(192, 168, 7, 10));
+}
+
+/// An 1844-byte datagram sliced to 1500 (kernel-style truncation) still
+/// decodes: the cut lands mid-TLV, the swallow skips one byte, options stay
+/// bounded, and re-encode caps at 300 with Discover first. No panic.
+#[test]
+fn sec4_kernel_style_truncation_decodes_without_panic() {
+    let mut opts = vec![53, 1, 1];
+    for _ in 0..400 {
+        opts.extend_from_slice(&[200, 2, 7, 8]);
+    }
+    opts.push(255);
+    let raw = make_raw(
+        1, 6, 0, 0xA031, 0, [0, 0],
+        [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0],
+        [0xA5; 6], opts,
+    );
+    assert_eq!(raw.len(), 1844);
+    let truncated = &raw[..1500]; // what recv_into-1500 would deliver
+    let p = unwrap_packet(Packet::from(truncated));
+    assert_eq!(p.message_type(), Ok(MessageType::Discover));
+    assert!(p.options.len() < 402, "truncated tail lost, prefix kept");
+    let mut buf = [0u8; 1500];
+    let enc = p.encode(&mut buf).to_vec();
+    assert!(enc.len() <= 300, "re-encode capped");
+}
+
+// ===========================================================================
+// D. Gate compositions: wrong-ID Request honored; Decline no-lease silent
+// ===========================================================================
+
+/// A Request naming a *wrong* (but well-formed) server is still Acked —
+/// complements the missing-ID renew test: the gate is disabled for values
+/// too, not just absence. Locked live.
+#[test]
+fn sec4_wrong_id_request_honored_live() {
+    let (srv_addr, _) = serve_example_like();
+    let client = UdpSocket::bind("127.0.0.1:0").unwrap();
+    client
+        .set_read_timeout(Some(Duration::from_secs(2)))
+        .unwrap();
+    let mut buf = [0u8; 1500];
+    let mut rbuf = [0u8; 1500];
+    let req = Packet {
+        reply: false, hops: 0, xid: 0xA040, secs: 0, broadcast: false,
+        ciaddr: Ipv4Addr::UNSPECIFIED, yiaddr: Ipv4Addr::UNSPECIFIED,
+        siaddr: Ipv4Addr::UNSPECIFIED, giaddr: Ipv4Addr::UNSPECIFIED,
+        chaddr: [0xA6; 6],
+        options: vec![
+            DhcpOption::DhcpMessageType(MessageType::Request),
+            DhcpOption::ServerIdentifier(Ipv4Addr::new(10, 99, 99, 99)),
+            DhcpOption::RequestedIpAddress(Ipv4Addr::new(192, 168, 2, 73)),
+        ],
+    };
+    client.send_to(&req.encode(&mut buf).to_vec(), srv_addr).unwrap();
+    let (n, _) = client.recv_from(&mut rbuf).expect("Ack despite wrong ID");
+    let rep = unwrap_packet(Packet::from(&rbuf[..n]));
+    assert_eq!(rep.message_type(), Ok(MessageType::Ack));
+    assert_eq!(rep.yiaddr, Ipv4Addr::new(192, 168, 2, 73));
+}
+
+/// Decline from an unknown chaddr (no lease anywhere) is a silent no-op with
+/// no reply and no state change — documents the quiet failure mode live.
+#[test]
+fn sec4_decline_no_lease_no_reply_live() {
+    let (srv_addr, server_ip) = serve_example_like();
+    let client = UdpSocket::bind("127.0.0.1:0").unwrap();
+    client
+        .set_read_timeout(Some(Duration::from_millis(400)))
+        .unwrap();
+    let mut buf = [0u8; 1500];
+    let mut rbuf = [0u8; 1500];
+    let dec = Packet {
+        reply: false, hops: 0, xid: 0xA041, secs: 0, broadcast: false,
+        ciaddr: Ipv4Addr::UNSPECIFIED, yiaddr: Ipv4Addr::UNSPECIFIED,
+        siaddr: Ipv4Addr::UNSPECIFIED, giaddr: Ipv4Addr::UNSPECIFIED,
+        chaddr: [0xA7; 6],
+        options: vec![
+            DhcpOption::DhcpMessageType(MessageType::Decline),
+            DhcpOption::ServerIdentifier(server_ip),
+            DhcpOption::RequestedIpAddress(Ipv4Addr::new(192, 168, 2, 74)),
+        ],
+    };
+    client.send_to(&dec.encode(&mut buf).to_vec(), srv_addr).unwrap();
+    assert!(
+        client.recv_from(&mut rbuf).is_err(),
+        "Decline with no lease must get no reply"
+    );
+}
+
+// ===========================================================================
+// E. Broadcast Release still gated only on server ID, not flags
+// ===========================================================================
+
+/// A broadcast-flagged Release with the correct Server ID still frees the
+/// lease — flags play no role in the gate; only the identifier matters.
+/// Locked live (precondition asserts the flag survived decode).
+#[test]
+fn sec4_broadcast_release_still_gated_only_on_id_live() {
+    let (srv_addr, server_ip) = serve_example_like();
+    let client = UdpSocket::bind("127.0.0.1:0").unwrap();
+    client
+        .set_read_timeout(Some(Duration::from_secs(2)))
+        .unwrap();
+    let mut buf = [0u8; 1500];
+    let mut rbuf = [0u8; 1500];
+    let victim = [0xA8; 6];
+    // occupy .75
+    let req = Packet {
+        reply: false, hops: 0, xid: 0xA050, secs: 0, broadcast: false,
+        ciaddr: Ipv4Addr::UNSPECIFIED, yiaddr: Ipv4Addr::UNSPECIFIED,
+        siaddr: Ipv4Addr::UNSPECIFIED, giaddr: Ipv4Addr::UNSPECIFIED,
+        chaddr: victim,
+        options: vec![
+            DhcpOption::DhcpMessageType(MessageType::Request),
+            DhcpOption::ServerIdentifier(server_ip),
+            DhcpOption::RequestedIpAddress(Ipv4Addr::new(192, 168, 2, 75)),
+        ],
+    };
+    client.send_to(&req.encode(&mut buf).to_vec(), srv_addr).unwrap();
+    let (n, _) = client.recv_from(&mut rbuf).expect("Ack");
+    assert_eq!(unwrap_packet(Packet::from(&rbuf[..n])).message_type(), Ok(MessageType::Ack));
+    // broadcast Release naming the right server: freed (flag irrelevant)
+    let raw = make_raw(
+        1, 6, 0, 0xA051, 0, [0, 128], // low-byte 0x80 -> broadcast=true
+        [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0],
+        victim,
+        vec![53, 1, 7, 54, 4, 192, 168, 2, 1, 255],
+    );
+    assert!(unwrap_packet(Packet::from(&raw)).broadcast, "precondition");
+    client.send_to(&raw, srv_addr).unwrap();
+    // prove freed: a stranger can now take .75
+    let stranger = Packet {
+        reply: false, hops: 0, xid: 0xA052, secs: 0, broadcast: false,
+        ciaddr: Ipv4Addr::UNSPECIFIED, yiaddr: Ipv4Addr::UNSPECIFIED,
+        siaddr: Ipv4Addr::UNSPECIFIED, giaddr: Ipv4Addr::UNSPECIFIED,
+        chaddr: [0xA9; 6],
+        options: vec![
+            DhcpOption::DhcpMessageType(MessageType::Request),
+            DhcpOption::ServerIdentifier(server_ip),
+            DhcpOption::RequestedIpAddress(Ipv4Addr::new(192, 168, 2, 75)),
+        ],
+    };
+    client.send_to(&stranger.encode(&mut buf).to_vec(), srv_addr).unwrap();
+    // Release itself gets no reply; the stranger's unicast Ack proves the
+    // Release freed .75 regardless of the broadcast flag.
+    let (n, _) = client.recv_from(&mut rbuf).expect("stranger Ack");
+    let rep = unwrap_packet(Packet::from(&rbuf[..n]));
+    assert_eq!(rep.message_type(), Ok(MessageType::Ack));
+    assert_eq!(rep.yiaddr, Ipv4Addr::new(192, 168, 2, 75), "freed by broadcast Release");
+}