用c++在Linux下读取ICMP应答

我正在使用c++和原始套接字向路由器发送ICMP请求，之后我想读取ICMP应答。我的问题是，select()没有一直接收重放和超时。我没有得到任何错误(errno返回成功)。路由器正在发送ICMP应答，因为我可以通过Wireshark看到响应。

https://i.stack.imgur.com/exdAI.png wireshark截图

为了测试我的程序，我使用Ubuntu 12.10在VirtualBox 4.2.6和GN3上运行虚拟网络。

源代码:

char buffer[IP_MAXPACKET]; // for the received ICMP reply
struct iphdr *ipRec; // ICMP header
timeval tv; // timeout
fd_set mySet; // descriptor set
...
tv.tv_sec = 3; // default time-out 3s
tv.tv_usec = 0;
int retval; // select
...
do {
        FD_ZERO(&mySet);
        FD_SET(mysocket, &mySet);
        retval = select(mysocket+1, &mySet, NULL, NULL, &tv);
        cout << "Errno after select:" << strerror(errno) << endl;
        if(retval == -1) {
            cerr << "select error" << endl;
            break;
        }
        else if (retval) {
            if((length = recvfrom(mysocket, buffer, MAX, 0, result->ai_addr, &(result->ai_addrlen))) == -1) {
                cerr << "Error: while receiving data." << endl;
            }
            else {
                cout << "good" << endl;
                ipRec = (struct iphdr*) buffer;
                icmpRec = (struct icmphdr*) (buffer + ipRec->ihl * 4);
                cout << "the packet." << " PID: " << ntohs(icmpRec->un.echo.id) << " Seq: " << ntohs(icmpRec->un.echo.sequence) << endl;
                if ((icmpRec->type == ICMP_ECHOREPLY) && (ntohs(icmpRec->un.echo.id) == pid) && (ntohs(icmpRec->un.echo.sequence) == (seq - 1))) {
                    minBuff = lengthBuff;
                }
            }
        } else {
            // getting here all the time = select times-out and reads no data
            cout << "mysocket:" << mysocket << endl;
            cout << "retval:" << retval << endl;
            maxBuff = lengthBuff;
            break;
        }
    } while (!((icmpRec->type == ICMP_ECHOREPLY) && (ntohs(icmpRec->un.echo.id) == pid) && (ntohs(icmpRec->un.echo.sequence) == (seq - 1))));
    if (packet)
        delete(packet);
    ...

谢谢你的帮助。