extract arp into its own static library; add empty arp::init() called from dispatch_init to anchor the .o so the self-registration constructor links in

arp/CMakeLists.txt

@@ -0,0 +1,11 @@
+add_library(arp STATIC arp.cpp)
+
+target_include_directories(arp PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
+
+target_compile_options(arp PRIVATE -Wall -Wextra -Wno-unused-parameter)
+
+target_link_libraries(arp PUBLIC
+    util
+    eth
+    ipv4
+)

arp/arp.cpp

@@ -0,0 +1,51 @@
+#include "arp.h"
+#include "eth.h"
+#include "ipv4.h"
+#include "parse_buffer.h"
+#include "prepend_buffer.h"
+
+namespace arp {
+
+static constexpr uint16_t ARP_HTYPE_ETH = __builtin_bswap16(1);
+static constexpr uint16_t ARP_PTYPE_IPV4 = __builtin_bswap16(0x0800);
+static constexpr uint16_t ARP_OP_REQUEST = __builtin_bswap16(1);
+static constexpr uint16_t ARP_OP_REPLY = __builtin_bswap16(2);
+
+void handle(std::span<const uint8_t> frame, span_writer& tx) {
+    const auto& mac = eth::get_mac();
+    const auto& ip = ipv4::get_ip();
+    parse_buffer pb(frame);
+    pb.consume<eth::header>();
+    auto* arp_hdr = pb.consume<header>();
+    if (!arp_hdr) return;
+
+    if (arp_hdr->htype != ARP_HTYPE_ETH) return;
+    if (arp_hdr->ptype != ARP_PTYPE_IPV4) return;
+    if (arp_hdr->hlen != 6 || arp_hdr->plen != 4) return;
+    if (arp_hdr->oper != ARP_OP_REQUEST) return;
+    if (arp_hdr->tpa != ip) return;
+
+    prepend_buffer<4096> buf;
+    auto* reply = buf.template prepend<header>();
+    reply->htype = ARP_HTYPE_ETH;
+    reply->ptype = ARP_PTYPE_IPV4;
+    reply->hlen = 6;
+    reply->plen = 4;
+    reply->oper = ARP_OP_REPLY;
+    reply->sha = mac;
+    reply->spa = ip;
+    reply->tha = arp_hdr->sha;
+    reply->tpa = arp_hdr->spa;
+    eth::prepend(buf, arp_hdr->sha, mac, eth::ETH_ARP);
+
+    eth::send_raw(buf.span());
+}
+
+void init() {}
+
+__attribute__((constructor))
+static void register_ethertype() {
+    eth::register_ethertype(eth::ETH_ARP, handle);
+}
+
+} // namespace arp

arp/arp.h

@@ -0,0 +1,25 @@
+#pragma once
+#include <span>
+#include "eth.h"
+#include "ipv4.h"
+#include "span_writer.h"
+
+namespace arp {
+
+struct __attribute__((packed)) header {
+    uint16_t htype;
+    uint16_t ptype;
+    uint8_t hlen;
+    uint8_t plen;
+    uint16_t oper;
+    eth::mac_addr sha;
+    ipv4::ip4_addr spa;
+    eth::mac_addr tha;
+    ipv4::ip4_addr tpa;
+};
+static_assert(sizeof(header) == 28);
+
+void init();
+void handle(std::span<const uint8_t> frame, span_writer& tx);
+
+} // namespace arp