Centos7 多网卡抓包可以抓到UDP但程序recvfrom不到

问题：

     Centos7多网卡，抓包时发现某网卡上有UDP包，但是用程序recvfrom无法接收到消息。

解决步骤:

1.确认防火墙是否关闭；

已关闭

2.确认网卡是否开启过滤：cat /proc/sys/net/ipv4/conf/ethxxx/rp_filter    (ethxxx是网卡名称)

结果：已关闭(当然此配置项只影响组播，这是病急乱投医)

3.随便找一个服务端和客户端的代码，测试是否能正常进行udp通信;  

结果发现完全正常。

server.cpp

#include <sys/types.h>
#include <sys/socket.h>
#include <pthread.h>
#include <netinet/in.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <arpa/inet.h>

int main(int argc, char **argv)
{
    if (argc != 2)
    {
        printf("Usage: %s port
", argv[0]);
        exit(1);
    }
    printf("Welcome! This is a UDP server, I can only received message from client and reply with same message
");
    
    struct sockaddr_in addr;
    addr.sin_family = AF_INET;
    addr.sin_port = htons(atoi(argv[1]));
    addr.sin_addr.s_addr = htonl(INADDR_ANY);

    int sock;
    if ( (sock = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
    {
        perror("socket");
        exit(1);
    }
    if (bind(sock, (struct sockaddr *)&addr, sizeof(addr)) < 0)
    {
        perror("bind");
        exit(1);
    }
    char buff[409600];
    struct sockaddr_in clientAddr;
    int n;
    unsigned int len = sizeof(clientAddr);
    while (1)
    {
        n = recvfrom(sock, buff, 409600, 0, (struct sockaddr*)&clientAddr, &len);
        if (n>0)
        {
            buff[n] = 0;
            printf("%s %u says: %s
", inet_ntoa(clientAddr.sin_addr), ntohs(clientAddr.sin_port), buff);
            n = sendto(sock, buff, n, 0, (struct sockaddr *)&clientAddr, sizeof(clientAddr));
            if (n < 0)
            {
                perror("sendto");
                break;
            }
        }
        else
        {
            perror("recv");
            break;
        }
    }
    return 0;
}

client.cpp

#include <sys/types.h>
#include <sys/socket.h>
#include <pthread.h>
#include <netinet/in.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <arpa/inet.h>


int main(int argc, char **argv)
{
    if (argc != 3)
    {
        printf("Usage: %s ip port", argv[0]);
        exit(1);
    }
    printf("This is a UDP client
");
    struct sockaddr_in addr;
    int sock;

    if ( (sock=socket(AF_INET, SOCK_DGRAM, 0)) <0)
    {
        perror("socket");
        exit(1);
    }
    addr.sin_family = AF_INET;
    addr.sin_port = htons(atoi(argv[2]));
    addr.sin_addr.s_addr = inet_addr(argv[1]);
    if (addr.sin_addr.s_addr == INADDR_NONE)
    {
        printf("Incorrect ip address!");
        close(sock);
        exit(1);
    }

    char buff[512]="aaaaaaaaaaaaaaa";
    unsigned int len = sizeof(addr);
    while (1)
    {
        gets(buff);
        int n;
        n = sendto(sock, buff, strlen(buff), 0, (struct sockaddr *)&addr, sizeof(addr));
        if (n < 0)
        {
            perror("sendto");
            close(sock);
            break;
        }
        n = recvfrom(sock, buff, 512, 0, (struct sockaddr *)&addr, &len);
        if (n>0)
        {
            buff[n] = 0;
            printf("received:");
            puts(buff);
        }
        else if (n==0)
        {
            printf("server closed
");
            close(sock);
            break;
        }
        else if (n == -1)
        {
            perror("recvfrom");
            close(sock);
            break;
        }
    }
    
    return 0;
}

4.再次抓包查看

此次抓包发现，双网口的Android设备用两根独立的网线连到服务器不同的网卡，设置两个网段：192.168.1.xx和192.168.2.xx，其中192.168.1.xx是Android设备的默认路由网段。从Android设备发送UDP消息到192.168.2.xx网段，结果服务器端对应的网口抓包显示的源地址为192.168.1.xx，将UDP发送消息改为192.168.1.xx后，在服务器的相应网卡地址接收，问题解决。

问题原因：

Android设备的多网卡不共用同一路由表，因此造成路由失效；当路由生效时，又把默认的网卡地址组装到UDP中，造成此问题。