一台机器为物理机，装的ubuntu,另一台是虚拟机，装的redhat。收发程序如果运行在同一个系统里面，则通信正常，分开放在2个系统里面则不通。
        收的程序:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <sys/types.h>          /* See NOTES */
#include <sys/socket.h>
#include <unistd.h>
#include <sys/select.h>
#include <sys/time.h>
#include <netinet/in.h>

#define SERPORT     65530
#define CLIPORT     65531
#define BUFLEN      1500

#define IPADDR      "192.168.1.108"

int main ( int argc, char *argv[] )
{
    int serverfd;
    int nready,rn = 0,wn = 0;
    fd_set rset,wset;
    struct sockaddr_in server_addr, client_addr;
    socklen_t peer_addr_size = sizeof(client_addr);
    struct timeval tv;
    char buf[BUFLEN];

    serverfd = socket(AF_INET, SOCK_DGRAM, 0);

    memset(&server_addr, 0, sizeof(struct sockaddr_in));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(SERPORT);
    //server_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    inet_pton(AF_INET, IPADDR, &server_addr.sin_addr);
    
    memset(&client_addr, 0, sizeof(struct sockaddr_in));
    client_addr.sin_family = AF_INET;
    client_addr.sin_port = htons(CLIPORT);
    //server_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    inet_pton(AF_INET, IPADDR, &client_addr.sin_addr);
    
    if (bind(serverfd, (struct sockaddr *) &server_addr, sizeof(struct sockaddr_in)) == -1)
    {
        perror("bind");
        close(serverfd);
        return -1;
    }
    
    while ( 1 )
    {
        FD_ZERO(&rset);
        FD_ZERO(&wset);
        FD_SET(serverfd, &rset);
        tv.tv_sec = 1;
        tv.tv_usec = 0;
        
        nready = select(serverfd+1, &rset, NULL, NULL, &tv);
        if ( nready < 0 )
        {
            perror("select");
            continue;
        }
        else if ( nready == 0 )
        {
            continue;
        }
        else
        {
            if (FD_ISSET(serverfd, &rset))
            {
                rn = recvfrom(serverfd, buf, BUFLEN, 0, NULL, NULL);
                if ( rn < 0 )
                {
                    perror("recvfrom");
                    continue;
                }
                printf("rn:%d>>%s\n",rn, buf);
            }
        }
    }
    
    return 0;
}
发的程序
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <sys/types.h>          /* See NOTES */
#include <sys/socket.h>
#include <unistd.h>
#include <sys/select.h>
#include <sys/time.h>
#include <netinet/in.h>

#define SERPORT     65530
#define CLIPORT     65531
#define BUFLEN      1500

#define IPADDR      "192.168.1.103"

int main ( int argc, char *argv[] )
{
    int sockfd;
    int nready,rn = 0,wn = 0;
    fd_set rset,wset;
    struct sockaddr_in server_addr, client_addr;
    socklen_t peer_addr_size = sizeof(client_addr);
    struct timeval tv;
    char buf[BUFLEN];

    sockfd = socket(AF_INET, SOCK_DGRAM, 0);

    memset(&server_addr, 0, sizeof(struct sockaddr_in));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(SERPORT);
    //server_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    inet_pton(AF_INET, IPADDR, &server_addr.sin_addr);
    
    memset(&client_addr, 0, sizeof(struct sockaddr_in));
    client_addr.sin_family = AF_INET;
    client_addr.sin_port = htons(CLIPORT);
    //client_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    inet_pton(AF_INET, IPADDR, &client_addr.sin_addr);
    if (bind(sockfd, (struct sockaddr *) &client_addr, sizeof(struct sockaddr_in)) == -1)
    {
        perror("bind");
        close(sockfd);
        return -1;
    }
    
    while ( 1 )
    {
        FD_ZERO(&wset);
        FD_SET(sockfd, &wset);
        tv.tv_sec = 1;
        tv.tv_usec = 0;
        
        nready = select(sockfd+1, NULL, &wset, NULL, &tv);
        if ( nready < 0 )
        {
            perror("select");
            continue;
        }
        else if ( nready == 0 )
        {
            continue;
        }
        else
        {
            if (FD_ISSET(sockfd, &wset))
            {
                wn = sendto(sockfd, "abc", 4, 0, (struct sockaddr *)&server_addr, peer_addr_size);
                sleep(1);
            }
        }
    }

    close(sockfd);
    return 0;
}


```
 
```

已关闭双方防火墙，selinux。
使用nmap分别在2个系统上扫描对方的udp端口，状态均为
PORT      STATE         SERVICE
65530/udp open|filtered unknown

PORT      STATE         SERVICE
65531/udp open|filtered unknown

网络是自家局域网，没发现路由做了端口禁用。如何分析？