limen/src/arp.cpp

#include "arp.h"
#include "net.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& ns = net::get_state();
    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 != ns.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 = ns.mac;
    reply->spa = ns.ip;
    reply->tha = arp_hdr->sha;
    reply->tpa = arp_hdr->spa;
    eth::prepend(buf, arp_hdr->sha, ns.mac, eth::ETH_ARP);

    net::send_raw(buf.span());
}

__attribute__((constructor))
static void register_ethertype() {
    net::register_ethertype(eth::ETH_ARP, handle);
}

} // namespace arp