dpdk接收到udp报文后，自己构造一个udp报文，将收到的报文中的源mac，目的mac，源ip，目的ip，源端口和目的端口交换下顺序填充到新的udp报文中，报文中的负载数据和收到的udp保持一致。

注：需要在udp发送端设置静态arp信息（dpdk绑定网卡的ip和mac地址映射），目前该程序还未对arp报文做处理

udp报文格式：

#include <rte_eal.h>
#include <rte_ethdev.h>
#include <rte_mbuf.h>
#include <stdio.h>
#include <arpa/inet.h>


#define NUM_MBUFS (4096-1) // 内存池中 mbuf 的数量
#define BURST_SIZE 32

static uint32_t gSrcIp, gDstIp;
static uint8_t gSrcMac[RTE_ETHER_ADDR_LEN], gDstMac[RTE_ETHER_ADDR_LEN];
static uint16_t gSrcPort, gDstPort;

int gDpdkPortId = 0;

static const struct rte_eth_conf port_conf_default = {
	.rxmode = {.max_rx_pkt_len = RTE_ETHER_MAX_LEN}
};

static void ng_init_port(struct rte_mempool *mbuf_pool) {
	//dpdk绑定的网卡数量
	uint16_t nb_sys_ports = rte_eth_dev_count_avail();
	if (nb_sys_ports == 0) {
		rte_exit(EXIT_FAILURE, "not support eth\n");
	}

	struct rte_eth_dev_info dev_info;
	/*获取以太网设备的配置和状态信息。它通常用于初始化网络设备、
	 *配置网络设备或者获取网络设备的状态信息。
	 *这里的端口号和网卡是一一对应的
	 */
	rte_eth_dev_info_get(gDpdkPortId, &dev_info);

	const int num_rx_queues = 1;	//接收队列个数
	const int num_tx_queues = 1;	//发送队列个数
	struct rte_eth_conf port_conf = port_conf_default;
	
	rte_eth_dev_configure(gDpdkPortId, num_rx_queues, num_tx_queues, &port_conf);
	// 0是0号接收队列
	// 128是队列长度
	if (rte_eth_rx_queue_setup(gDpdkPortId, 0, 128, rte_eth_dev_socket_id(gDpdkPortId), NULL, mbuf_pool) < 0) {
		rte_exit(EXIT_FAILURE, "Could not setup RX queue\n");
	}

	struct rte_eth_txconf txq_conf = dev_info.default_txconf;
	//offloads 成员是一个 64 位无符号整数，每个比特位表示不同的接收功能选项
	txq_conf.offloads = port_conf.rxmode.offloads;
	/* 0是0号发送队列 
	 * 1024是队列长度
	 * 发送队列长度设置太小运行时会报错:Invalid value for nb_tx_desc(=128), should be: <= 4096, >= 512, and a product of 1
	 */
	if (rte_eth_tx_queue_setup(gDpdkPortId, 0, 1024, rte_eth_dev_socket_id(gDpdkPortId), &txq_conf) < 0) {
		rte_exit(EXIT_FAILURE, "Could not setup TX queue\n");
	}

	if (rte_eth_dev_start(gDpdkPortId) < 0) {
		rte_exit(EXIT_FAILURE, "Could not start\n");
	}

}

static void ng_encode_udp_pkt(uint8_t *msg, unsigned char *data, uint16_t total_len) {

	//设置以太网头
	struct rte_ether_hdr *eth = (struct rte_ether_hdr *)msg;
	rte_memcpy(eth->s_addr.addr_bytes, gSrcMac, RTE_ETHER_ADDR_LEN);
	rte_memcpy(eth->d_addr.addr_bytes, gDstMac, RTE_ETHER_ADDR_LEN);
	eth->ether_type = htons(RTE_ETHER_TYPE_IPV4);

	//设置ipv4头
	struct rte_ipv4_hdr *ip = (struct rte_ipv4_hdr *)(eth + 1);
	ip->version_ihl = 0x45;
	ip->type_of_service = 0;
	ip->total_length = htons(total_len - sizeof(struct rte_ether_hdr));
	ip->packet_id = 0;
	ip->fragment_offset = 0;
	ip->time_to_live = 64;
	//dpdk中没有UDP类型的定义，使用内核的协议类型
	ip->next_proto_id = IPPROTO_UDP;
	ip->src_addr = gSrcIp;
	ip->dst_addr = gDstIp;
	//计算ip头部校验和时，先把该字段置为0(ip校验和只包括头部)
	ip->hdr_checksum = 0;
	ip->hdr_checksum = rte_ipv4_cksum(ip);

	//设置udp头
	struct rte_udp_hdr *udp = (struct rte_udp_hdr *)(ip + 1);
	udp->src_port = gSrcPort;
	udp->dst_port = gDstPort;
	uint16_t udp_len = total_len - sizeof(struct rte_ether_hdr) - sizeof(struct rte_ipv4_hdr);
	udp->dgram_len = htons(udp_len);
	memcpy((uint8_t *)(udp + 1), data, udp_len - sizeof(struct rte_udp_hdr));
	//计算udp校验和，udp校验位包括负载数据
	udp->dgram_cksum = 0;
	udp->dgram_cksum = rte_ipv4_udptcp_cksum(ip, udp);
}

static struct rte_mbuf *ng_send(struct rte_mempool *mbuf_pool, unsigned char *data, uint16_t length) {

	const unsigned total_len = sizeof(struct rte_ether_hdr) + sizeof(struct rte_ipv4_hdr) + length;
	
	//从内存中申请一个mbuf
	struct rte_mbuf *mbuf = rte_pktmbuf_alloc(mbuf_pool);
	if (mbuf == NULL) {
		rte_exit(EXIT_FAILURE, "rte_pktmbuf_alloc\n");
	}
	mbuf->pkt_len = total_len;
	mbuf->data_len = total_len;

	//用于将数据包缓冲区（packet buffer）转换为指定类型的数据指针，也就是mbuf存储数据包的首地址
	uint8_t *pktdata = rte_pktmbuf_mtod(mbuf, uint8_t*);
	ng_encode_udp_pkt(pktdata, data, total_len);

	return mbuf;
}


int main(int argc, char *argv[]) {
	/*dpdk初始化资源
	 *用于初始化 Environment Abstraction Layer (EAL)。EAL 是 DPDK 的一个核心组件，
	 *负责抽象和管理硬件和操作系统依赖性，使得上层应用可以在不同的硬件和操作系统上
	 *以统一的方式运行。
	 */
	if (rte_eal_init(argc, argv) < 0) {
		rte_exit(EXIT_FAILURE, "Error with EAL initialization\n");
	}

	//内存池，接收的数据存在该内存池中
	struct rte_mempool *mbuf_pool = rte_pktmbuf_pool_create("mbuf_pool", NUM_MBUFS,
			0, 0, RTE_MBUF_DEFAULT_BUF_SIZE, rte_socket_id());
	if (mbuf_pool == NULL) {
		rte_exit(EXIT_FAILURE, "Could not create mbuf pool\n");
	}

	ng_init_port(mbuf_pool);

	//获取dpdk绑定的网卡源mac
	rte_eth_macaddr_get(gDpdkPortId, (struct rte_ether_addr *)gSrcMac);
	
	while(1) {
		struct rte_mbuf *mbufs[BURST_SIZE] = {0};
		unsigned num_recvd = rte_eth_rx_burst(gDpdkPortId, 0, mbufs, BURST_SIZE);
		if (num_recvd > BURST_SIZE) {
			rte_exit(EXIT_FAILURE, "Error receive from eth\n");
		}

		unsigned int i = 0;
		for (i = 0; i < num_recvd; i++) {
			struct rte_ether_hdr *ehdr = rte_pktmbuf_mtod(mbufs[i], struct rte_ether_hdr*);
			if (ehdr->ether_type != rte_cpu_to_be_16(RTE_ETHER_TYPE_IPV4)) {
				rte_pktmbuf_free(mbufs[i]);
				continue;
			}
			struct rte_ipv4_hdr *iphdr = rte_pktmbuf_mtod_offset(mbufs[i], struct rte_ipv4_hdr*, sizeof(struct rte_ether_hdr));
			if (iphdr->next_proto_id == IPPROTO_UDP) {
				struct rte_udp_hdr *udphdr = (struct rte_udp_hdr *)(iphdr + 1);

				//两个字节以上的变量是需要大小端转换
				uint16_t length = ntohs(udphdr->dgram_len);
				*((char*)udphdr + length) = '\0';


				struct in_addr addr;
				addr.s_addr = iphdr->src_addr;
				printf("src: %s:%d, ", inet_ntoa(addr), ntohs(udphdr->src_port));
				addr.s_addr = iphdr->dst_addr;
				printf("dst: %s:%d, %s\n", inet_ntoa(addr), ntohs(udphdr->dst_port), (char *)(udphdr+1));

				//构造发送数据包
				rte_memcpy(gDstMac, ehdr->s_addr.addr_bytes, RTE_ETHER_ADDR_LEN);
				gSrcIp = iphdr->dst_addr;
				gDstIp = iphdr->src_addr;
				
				gSrcPort = udphdr->dst_port;
				gDstPort = udphdr->src_port;

				struct rte_mbuf *txbuf = ng_send(mbuf_pool, (unsigned char *)(udphdr+1), length);
				//发送报文
				rte_eth_tx_burst(gDpdkPortId, 0, &txbuf, 1);
				rte_pktmbuf_free(txbuf);
				
			}
			rte_pktmbuf_free(mbufs[i]);
		}
		
	}
	
	return 0;
}