Add NTP and DHCP L7 dissectors

NTP decodes the fixed header's version/mode/stratum - the timestamp
fields need NTP era/fraction fixed-point math to render meaningfully
and add nothing a one-line summary needs, so they're left alone.

DHCP decodes RFC 2131's BOOTP fixed header + magic cookie, then walks
the TLV options bounds-safely for option 53 (message type), plus
yiaddr when the server has assigned one. It's the first dissector
needing two well-known ports (67 server, 68 client) rather than one;
registering at just 67 still matches both directions since
l7_summarize() already falls back from dst_port to src_port.

Verified live: NTP against a real pool.ntp.org query on wlp1s0 (a
genuine stratum-2 reply came back). DHCP over loopback with a
synthetic-but-wire-format-real DISCOVER/OFFER exchange rather than a
real lease renewal, to avoid disrupting this machine's actual network
state - caught a test-setup mistake in the process (both packets
sent from the same ephemeral port instead of the OFFER actually
originating from port 67), not a dissector bug.

3 files changed, 190 insertions, 1 deletions

diff --git a/include/packeteer/l7/dhcp.hpp b/include/packeteer/l7/dhcp.hpp
new file mode 100644
index 0000000..ac2662b
--- /dev/null
+++ b/include/packeteer/l7/dhcp.hpp
@@ -0,0 +1,116 @@
+#pragma once
+
+#include <algorithm>
+#include <cstdint>
+#include <cstdio>
+#include <optional>
+#include <span>
+#include <string>
+
+#include "packeteer/l7/dissector.hpp"
+#include "packeteer/net/ipv4.hpp"
+
+// RFC 2131 DHCP: BOOTP's fixed 236-byte header, a 4-byte magic cookie
+// (99.130.83.99), then variable-length TLV options. Only option 53
+// (DHCP Message Type) is decoded - the one field that actually says
+// what kind of message this is (DISCOVER/OFFER/REQUEST/ACK/...);
+// every other option is walked over (to find option 53, and to stay
+// bounds-safe scanning past it) but not decoded itself.
+//
+// DHCP is unusual among this project's L7 dissectors in genuinely
+// using two well-known ports - 67 (server) and 68 (client) - rather
+// than one. Registering at just 67 still matches traffic in both
+// directions: l7_summarize() already falls back from dst_port to
+// src_port (built for exactly this: a client's request and a server's
+// reply use the well-known port on opposite sides), and one side of a
+// DHCP exchange is always 67.
+namespace packeteer::net {
+
+inline constexpr std::uint16_t kDhcpServerPort = 67;
+
+inline constexpr std::uint8_t kDhcpDiscover = 1;
+inline constexpr std::uint8_t kDhcpOffer = 2;
+inline constexpr std::uint8_t kDhcpRequest = 3;
+inline constexpr std::uint8_t kDhcpDecline = 4;
+inline constexpr std::uint8_t kDhcpAck = 5;
+inline constexpr std::uint8_t kDhcpNak = 6;
+inline constexpr std::uint8_t kDhcpRelease = 7;
+inline constexpr std::uint8_t kDhcpInform = 8;
+
+struct DhcpMessage {
+    std::uint8_t op;     // 1 = BOOTREQUEST, 2 = BOOTREPLY
+    Ipv4Address yiaddr;  // "your" (client) IP; all-zero if not yet assigned
+    std::optional<std::uint8_t> message_type;  // option 53, if present
+};
+
+inline std::string dhcp_message_type_name(std::uint8_t type) {
+    switch (type) {
+        case kDhcpDiscover: return "DISCOVER";
+        case kDhcpOffer: return "OFFER";
+        case kDhcpRequest: return "REQUEST";
+        case kDhcpDecline: return "DECLINE";
+        case kDhcpAck: return "ACK";
+        case kDhcpNak: return "NAK";
+        case kDhcpRelease: return "RELEASE";
+        case kDhcpInform: return "INFORM";
+        default: return "type=" + std::to_string(type);
+    }
+}
+
+inline std::optional<DhcpMessage> parse_dhcp(std::span<const unsigned char> bytes) {
+    if (bytes.size() < 240) return std::nullopt;  // 236-byte fixed header + 4-byte magic cookie
+    if (bytes[236] != 0x63 || bytes[237] != 0x82 || bytes[238] != 0x53 || bytes[239] != 0x63) {
+        return std::nullopt;  // not the DHCP magic cookie - bare BOOTP, or not this protocol
+    }
+
+    DhcpMessage msg{};
+    msg.op = bytes[0];
+    std::copy_n(bytes.begin() + 16, 4, msg.yiaddr.bytes.begin());
+
+    std::size_t pos = 240;
+    while (pos < bytes.size()) {
+        std::uint8_t opt_type = bytes[pos];
+        if (opt_type == 0xFF) break;         // End option
+        if (opt_type == 0x00) { ++pos; continue; }  // Pad option: no length byte follows
+        if (pos + 1 >= bytes.size()) break;  // truncated: no room for a length byte
+        std::uint8_t opt_len = bytes[pos + 1];
+        if (pos + 2 + opt_len > bytes.size()) break;  // truncated option data
+        if (opt_type == 53 && opt_len >= 1) {
+            msg.message_type = bytes[pos + 2];
+        }
+        pos += 2 + opt_len;
+    }
+
+    return msg;
+}
+
+class DhcpDissector : public L7Dissector {
+public:
+    std::uint16_t port() const override { return kDhcpServerPort; }
+
+    std::optional<std::string> summarize(std::span<const unsigned char> payload) const override {
+        auto msg = parse_dhcp(payload);
+        if (!msg) return std::nullopt;
+
+        std::string out = "DHCP " + (msg->message_type ? dhcp_message_type_name(*msg->message_type)
+                                                         : std::string(msg->op == 1 ? "request"
+                                                                                     : "reply"));
+
+        // yiaddr is genuinely new information here, not a repeat of
+        // the IPv4 line's src/dst above it - it's the address being
+        // offered/assigned, the actual point of DISCOVER/OFFER/
+        // REQUEST/ACK. A tiny local formatter rather than reusing
+        // summarize.hpp's ipv4_to_string: summarize.hpp already
+        // depends on this header, so the reverse include would be
+        // circular.
+        const auto& b = msg->yiaddr.bytes;
+        if (b[0] || b[1] || b[2] || b[3]) {
+            char buf[24];
+            std::snprintf(buf, sizeof(buf), " yiaddr=%u.%u.%u.%u", b[0], b[1], b[2], b[3]);
+            out += buf;
+        }
+        return out;
+    }
+};
+
+}  // namespace packeteer::net
diff --git a/include/packeteer/l7/ntp.hpp b/include/packeteer/l7/ntp.hpp
new file mode 100644
index 0000000..a62566a
--- /dev/null
+++ b/include/packeteer/l7/ntp.hpp
@@ -0,0 +1,64 @@
+#pragma once
+
+#include <cstdint>
+#include <optional>
+#include <span>
+#include <string>
+
+#include "packeteer/l7/dissector.hpp"
+
+// RFC 5905 NTP. Only the first two bytes matter for a one-line summary
+// - version, mode (client/server/broadcast/...), and stratum - not
+// the timestamp fields, which need 64-bit fixed-point NTP era/fraction
+// math to render meaningfully and add nothing a one-line summary needs.
+namespace packeteer::net {
+
+inline constexpr std::uint16_t kNtpPort = 123;
+
+struct NtpHeader {
+    std::uint8_t version;  // 3 bits
+    std::uint8_t mode;     // 3 bits
+    std::uint8_t stratum;
+};
+
+inline std::string ntp_mode_name(std::uint8_t mode) {
+    switch (mode) {
+        case 1: return "symmetric-active";
+        case 2: return "symmetric-passive";
+        case 3: return "client";
+        case 4: return "server";
+        case 5: return "broadcast";
+        case 6: return "control";
+        default: return "mode=" + std::to_string(mode);
+    }
+}
+
+inline std::optional<NtpHeader> parse_ntp(std::span<const unsigned char> bytes) {
+    if (bytes.size() < 48) return std::nullopt;  // the fixed header, no extension fields/MAC
+
+    NtpHeader header{};
+    header.version = static_cast<std::uint8_t>((bytes[0] >> 3) & 0x07);
+    header.mode = static_cast<std::uint8_t>(bytes[0] & 0x07);
+    header.stratum = bytes[1];
+    return header;
+}
+
+class NtpDissector : public L7Dissector {
+public:
+    std::uint16_t port() const override { return kNtpPort; }
+
+    std::optional<std::string> summarize(std::span<const unsigned char> payload) const override {
+        auto ntp = parse_ntp(payload);
+        if (!ntp) return std::nullopt;
+
+        std::string out = "NTP v" + std::to_string(ntp->version) + " " + ntp_mode_name(ntp->mode);
+        // Stratum only means something once a role is established --
+        // 0 on a client request is just "not applicable yet".
+        if (ntp->mode == 3 || ntp->mode == 4) {
+            out += " stratum=" + std::to_string(ntp->stratum);
+        }
+        return out;
+    }
+};
+
+}  // namespace packeteer::net
diff --git a/include/packeteer/summarize.hpp b/include/packeteer/summarize.hpp
index 7ff06a4..4143e44 100644
--- a/include/packeteer/summarize.hpp
+++ b/include/packeteer/summarize.hpp
@@ -8,10 +8,12 @@
 
 #include <pcap.h>
 
+#include "packeteer/l7/dhcp.hpp"
 #include "packeteer/l7/dissector.hpp"
 #include "packeteer/l7/dns.hpp"
 #include "packeteer/l7/http.hpp"
 #include "packeteer/l7/mdns.hpp"
+#include "packeteer/l7/ntp.hpp"
 #include "packeteer/l7/ssh.hpp"
 #include "packeteer/l7/tls.hpp"
 #include "packeteer/net/arp.hpp"
@@ -87,13 +89,18 @@ inline std::string tcp_flags_to_string(std::uint8_t flags) {
 // DNS's own parser (same wire format, different port/label) at
 // essentially no extra cost. SSH is the first dissector whose *entire*
 // protocol is one cleartext line before everything else encrypts --
-// unlike TLS's SNI, there's nothing further to ever add here.
+// unlike TLS's SNI, there's nothing further to ever add here. NTP is
+// as simple as DNS/mDNS - a fixed, small header, no TLVs. DHCP is the
+// first dissector to genuinely need two well-known ports (67 and 68);
+// see dhcp.hpp for why registering at just one still works.
 inline const net::L7Registry& l7_registry() {
     static const net::DnsDissector dns_dissector;
     static const net::HttpDissector http_dissector;
     static const net::TlsSniDissector tls_dissector;
     static const net::MdnsDissector mdns_dissector;
     static const net::SshDissector ssh_dissector;
+    static const net::NtpDissector ntp_dissector;
+    static const net::DhcpDissector dhcp_dissector;
     static const net::L7Registry registry = [] {
         net::L7Registry r;
         r.add(&dns_dissector);
@@ -101,6 +108,8 @@ inline const net::L7Registry& l7_registry() {
         r.add(&tls_dissector);
         r.add(&mdns_dissector);
         r.add(&ssh_dissector);
+        r.add(&ntp_dissector);
+        r.add(&dhcp_dissector);
         return r;
     }();
     return registry;