数据包捕获c代码不会终止，显示捕获的数据包数量

Question

我正在学习用c编写pcap代码。下面，我编写了一个简单的C代码，用于自动检测设备进行加密，获取IP和子网掩码，获取链路层标头并过滤流量，然后打印数据包大小。

代码成功遵从，但被卡在

发现网络设备：wlo1

运行时。卸下过滤器部件确实会打印数据包大小。并删除包的主要部分；该程序符合要求并成功运行。

我认为我对过滤部分缺乏了解。

我使用linux编译：gcc程序名-lpcap

代码输出为：找到网络设备：wlo1

wlo1是无线设备

#include <stdio.h>
#include <pcap.h>

int main(int argc, char *argv[]){

    char *dev; //device automatically detected for sniffing
    char errbuf[PCAP_ERRBUF_SIZE]; //error string
    pcap_t *handle; //session hnadle
    struct bpf_program fp;  //The compiled filter expression
    char filter_exp[] = "port 23";  //The filter expression
    bpf_u_int32 mask;   //The netmask of our sniffing device
    bpf_u_int32 net;    //The IP of our sniffing device
    struct pcap_pkthdr header;
    const unsigned char *packet;

    //device detection block
    dev = pcap_lookupdev(errbuf);
    if (dev == NULL){
            printf("Error finding device: %s\n", errbuf);
            return 1;
        }
    printf("Network device found: %s\n", dev);

    //opening device for sniffing
    handle = pcap_open_live(dev, BUFSIZ, 1, 1000, errbuf);
    if(handle == NULL){
        fprintf(stderr,"Couldn't open device %s : %s\n",dev,errbuf);
        return 1;
    }

    // //check for link-layer header of the device
    if(pcap_datalink(handle) != DLT_EN10MB){ //for ethernet data link layer
        if(pcap_datalink(handle) != DLT_IEEE802_11){ //for wlan data link layer
            fprintf(stderr, "Device %s doesn't provide WLAN headers - not supported\n", dev);
            return 1;
        }
        else{
            fprintf(stderr, "Device %s doesn't provide Ethernet headers - not supported\n", dev);
            return 1;
        }
    }

    //block to get device ip and subnet mask
    if(pcap_lookupnet(dev, &net, &mask, errbuf) == -1){
        fprintf(stderr, "Can't get netmask for device %s\n", dev);
        net = 0;
        mask = 0;
    }

    //block for filtering traffic we want to sniff
    if(pcap_compile(handle, &fp, filter_exp, 0, net) == -1) {
        fprintf(stderr, "Couldn't parse filter %s: %s\n", filter_exp, pcap_geterr(handle));
        return 1;
    }
    if(pcap_setfilter(handle, &fp) == -1) {
        fprintf(stderr, "Couldn't install filter %s: %s\n", filter_exp, pcap_geterr(handle));
        return 1;
    }
    /* Grab a packet */
    packet = pcap_next(handle, &header);
    /* Print its length */
    printf("Jacked a packet with length of [%d]\n", header.len);
    /* And close the session */
    pcap_close(handle);
    return 0;
}

Answer 1

如果wlo1在“受保护”网络（使用WEP或WPA / WPA2 / WPA3在链路层上加密流量的网络）中以监视模式捕获，则任何在链路层上工作的过滤器-例如一个TCP / UDP层过滤器（“端口80”是-）将不起作用，因为传递到过滤代码的数据包将加密802.11有效负载，因此过滤器将无法对其使用。

因此，没有数据包将通过过滤器。