/*
 * Taken from Kernel Documentation/networking/tuntap.txt
 */
int tun_alloc(char *dev)
{
    struct ifreq ifr;
    int fd, err;

    if( (fd = open("/dev/net/tap", O_RDWR)) < 0 ) {
        print_error("Cannot open TUN/TAP dev");
        exit(1);
    }

    CLEAR(ifr);

    /* Flags: IFF_TUN   - TUN device (no Ethernet headers)
     *        IFF_TAP   - TAP device
     *
     *        IFF_NO_PI - Do not provide packet information
     */
    ifr.ifr_flags = IFF_TAP | IFF_NO_PI;
    if( *dev ) {
        strncpy(ifr.ifr_name, dev, IFNAMSIZ);
    }

    if( (err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0 ){
        print_error("ERR: Could not ioctl tun: %s\n", strerror(errno));
        close(fd);
        return err;
    }

    strcpy(dev, ifr.ifr_name);
    return fd;
}

#include <linux/if_ether.h>

struct eth_hdr
{
    unsigned char dmac[6]; // 目标 MAC 地址
    unsigned char smac[6]; // 源 MAC 地址
    uint16_t ethertype; // 有效负载的长度或者类型
    unsigned char payload[];
} __attribute__((packed));

if (tun_read(buf, BUFLEN) < 0) {
    print_error("ERR: Read from tun_fd: %s\n", strerror(errno));
}

struct eth_hdr *hdr = init_eth_hdr(buf);

// 查看 ethertype 字段值，并根据其值判断下一步逻辑
handle_frame(&netdev, hdr);

struct arp_hdr
{
    uint16_t hwtype; // 硬件类型，eg: 以太网(0x0001)
    uint16_t protype; // 协议类型
    unsigned char hwsize;  // 硬件地址长度
    unsigned char prosize; // 协议地址长度
    uint16_t opcode; // 操作码
    unsigned char data[]; // 实际的负载信息
} __attribute__((packed));

// data 中还会包含 IPv4 信息
struct arp_ipv4
{
    unsigned char smac[6]; // 源 MAC
    uint32_t sip; // 源 IP
    unsigned char dmac[6]; // 目标 MAC
    uint32_t dip; // 目标 IP
} __attribute__((packed));

?Do I have the hardware type in ar$hrd?
Yes: (almost definitely)
  [optionally check the hardware length ar$hln]
  ?Do I speak the protocol in ar$pro?
  Yes:
    [optionally check the protocol length ar$pln]
    Merge_flag := false
    If the pair <protocol type, sender protocol address> is
        already in my translation table, update the sender
        hardware address field of the entry with the new
        information in the packet and set Merge_flag to true.
    ?Am I the target protocol address?
    Yes:
      If Merge_flag is false, add the triplet <protocol type,
          sender protocol address, sender hardware address> to
          the translation table.
      ?Is the opcode ares_op$REQUEST?  (NOW look at the opcode!!)
      Yes:
        Swap hardware and protocol fields, putting the local
            hardware and protocol addresses in the sender fields.
        Set the ar$op field to ares_op$REPLY
        Send the packet to the (new) target hardware address on
            the same hardware on which the request was received