linux - libpcap packet capture results are incomplete?

Using libpcap to capture packets under ubuntu14.04, I want to get a piece of html transmitted using http, but the results obtained are inconsistent with the data obtained by wireshark under the same situation.

The current code is as follows:

#include <pcap.h>
#include <time.h>
#include <stdlib.h>
#include <stdio.h>
#include <linux/if_ether.h>
#include <linux/ip.h>
#include <linux/tcp.h>
#include <string.h>
#include <netinet/in.h>

void getPacket(u_char * arg, const struct pcap_pkthdr * pkthdr, const u_char * packet)
{

    bpf_u_int32 caplen = pkthdr->caplen;
    bpf_u_int32 len = pkthdr->len;

    int * id = (int *)arg;

    struct iphdr *ip_header = (struct iphdr *)(packet + ETH_HLEN);
    struct tcphdr *tcp_header = (struct tcphdr *)(packet + ETH_HLEN + sizeof(struct iphdr));

    const u_char *tcp_data = packet + ETH_HLEN + sizeof(struct iphdr) + sizeof(struct tcphdr);

    printf("%s\n\n", tcp_data);
}

int main()
{
    char errBuf[PCAP_ERRBUF_SIZE];
    pcap_t * device = pcap_open_live("wlan0", 65535, 1, 0, errBuf);

    if(!device)
    {
        printf("错误: pcap_open_live(): %s\n", errBuf);
        exit(1);
    }

    struct bpf_program filter;
    pcap_compile(device, &filter, "tcp port 80 and host 123.206.7.47", 1, 0);
    pcap_setfilter(device, &filter);

    int id = 0;

    pcap_loop(device, -1, getPacket, (u_char*)&id);
    pcap_close(device);

    return 0;
}

The server has a simple html. When I use the browser to access the server http://123.206.7.47/test.html, wireshark (same as bpf) captures 10 packets like this:

What I see when using the debugger on my program is like this. This picture corresponds to the fourth data packet (size 474) in the picture above:

Why does unsigned char * packet appear incomplete sequence? And why is tcp_data so short?

Is there any missing configuration of libpcap in my code or is it due to other reasons?

One more thing to add is that when I visit other websites, I can occasionally capture the complete HTTP request, but not on the web page I visited.

伊谢尔伦2749 days ago660

linux - libpcap packet capture results are incomplete?

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