typedef struct tuntap_dev {
  int           fd;
  u_int8_t      mac_addr[6];//MAC地址
  u_int32_t     ip_addr, device_mask;//IP地址与子网掩码
  u_int         mtu;//mtu值
} tuntap_dev;//定义虚拟网卡的数据结构

enum packet_type {
  packet_unreliable_data = 0,  /* no ACK needed */
  packet_reliable_data,    /* needs ACK     */
  packet_ping,
  packet_pong
};//定义数据包的类型

struct peer_addr {
  u_int8_t family;
  u_int16_t port;
  union {
    u_int8_t  v6_addr[16];
    u_int32_t v4_addr;
  } addr_type;
};//定义节点地址端口信息数据结构，预留了IPv6

struct n2n_packet_header {
  u_int8_t version, msg_type, ttl, sent_by_supernode;
  //版本号、消息类型、(ttl还有待查明)、服务器节点转发标示位
  char community_name[COMMUNITY_LEN], src_mac[6], dst_mac[6];
  //客户所处子网社区名称、源MAC、目的MAC
  struct peer_addr public_ip, private_ip;
  //节点公网地址端口、私网地址端口信息
  enum packet_type pkt_type;//数据包类型
  u_int32_t sequence_id;//序列号
  u_int32_t crc; // 校验位,用来区分伪造数据包
};//n2n数据包头信息

struct peer_info {
  char community_name[16], mac_addr[6];//子网社区名、MAC地址
  struct peer_addr public_ip, private_ip;//公网地址端口、私网地址端口
  time_t last_seen;//时间信息
  struct peer_info *next;//下一个节点
  /* socket */
  n2n_sock_info_t sinfo;//sock信息
};//节点信息

struct n2n_edge
{
  u_char              re_resolve_supernode_ip;
  struct peer_addr    supernode;//服务器地址端口信息
  char                supernode_ip[48];//服务器IP地址
  char *              community_name;//子网社区名，默认为NULL
  
  n2n_sock_info_t     sinfo;//sock信息
  u_int               pkt_sent;//标示位，具体含义待查.默认为0
  tuntap_dev          device;//虚拟网卡设备
  int                 allow_routing;//路由转发标示位，默认为0
  int                 drop_ipv6_ndp;//标示位，具体含义待查.默认为0
  char *              encrypt_key;//加密密钥
  TWOFISH *           tf;
  struct peer_info *  known_peers /* = NULL*/;
  struct peer_info *  pending_peers /* = NULL*/;
  time_t              last_register /* = 0*/;
};//客户节点数据结构，最重要的数据结构

使用：
static void send_grat_arps(n2n_edge_t * eee,) {
  char buffer[48];
  size_t len;

  traceEvent(TRACE_NORMAL, "Sending gratuitous ARP...");
  len = build_gratuitous_arp(buffer, sizeof(buffer));
  send_packet2net(eee, buffer, len);
  send_packet2net(eee, buffer, len); /* Two is better than one :-) */
}
----包体定义-------
static char gratuitous_arp[] = {
  0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, /* Dest mac */
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, /* Src mac idx:6*/
  0x08, 0x06, /* ARP */
  0x00, 0x01, /* Ethernet */
  0x08, 0x00, /* IP */
  0x06, /* Hw Size */
  0x04, /* Protocol Size */
  0x00, 0x01, /* ARP Request */
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, /* Src mac idx:22*/
  0x00, 0x00, 0x00, 0x00, /* Src IP idx:28*/
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, /* Target mac */
    0x00, 0x00, 0x00, 0x00 /* Target IP idx:38*/
};

static int build_gratuitous_arp(char *buffer, u_short buffer_len) {
  if(buffer_len < sizeof(gratuitous_arp)) return(-1);

  memcpy(buffer, gratuitous_arp, sizeof(gratuitous_arp));
  memcpy(&buffer[6], device.mac_addr, 6);
  memcpy(&buffer[22], device.mac_addr, 6);
  memcpy(&buffer[28], &device.ip_addr, 4);

  /* REVISIT: BbMaj7 - use a real netmask here. This is valid only by accident
   * for /24 IPv4 networks. */
  buffer[31] = 0xFF; /* Use a faked broadcast address */
  memcpy(&buffer[38], &device.ip_addr, 4);
  return(sizeof(gratuitous_arp));
}

char* intoa(u_int32_t /* host order */ addr, char* buf, u_short buf_len) {
    char *cp, *retStr;
    u_int byte;
    int n;
    /*
     addr=268435456
     >>>268435456&255=0
     //右移8位
     >>>1048576&255=0
     //右移8位
     >>>4096&255=0
     //右移8位
     >>>16&255|16
     */
    printf(">>>%d|%d\n",addr,addr&255);
    addr >>= 8;
    printf(">>>%d|%d\n",addr,addr&255);
    addr >>= 8;
    printf(">>>%d|%d\n",addr,addr&255);
    addr >>= 8;
    printf(">>>%d|%d\n",addr,addr&255);
    cp = &buf[buf_len];
    *--cp = '\0';
    n = 4;
    do {
        //0xff=255
        byte = addr & 0xff;
        *--cp = byte % 10 + '0';
        byte /= 10;
        if (byte > 0) {
            *--cp = byte % 10 + '0';
            byte /= 10;
            if (byte > 0)
                *--cp = byte + '0';
        }
        *--cp = '.';
        addr >>= 8;
    } while (--n > 0);
    
    /* Convert the string to lowercase */
    retStr = (char*)(cp+1);
    
    return(retStr);
}