标签：接收 发送消息 防火 程序 port bind lte 2.x 通信
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\n", argv[0]); exit(1); } printf("Welcome! This is a UDP server, I can only received message from client and reply with same message\n"); 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\n", 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\n"); 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\n"); 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。
Centos7 多网卡抓包可以抓到UDP但程序recvfrom不到
标签：接收 发送消息 防火 程序 port bind lte 2.x 通信
原文地址：https://www.cnblogs.com/bugutian/p/9075936.html
