LIBPCAP can't parse valid expression from Wireshark

Question

I am wring a custom app for filtering CAP dump files. Before starting its development I tried the expected filtering expression in Wireshark:

(eapol || wlan.fc.type_subtype == 0x08) && wlan.bssid == 00:11:00:11:00:11

and it worked flawlessly. But when I implemented this filtering in the app (see the src code below) I started getting "Bad filter - syntax error" for exactly the same filtering expression. What am I doing wrong with Libpcap and my filter?

#include <stdio.h>  
#include <string.h>
#include <stdlib.h>
#include <unistd.h>   
#include <pcap.h>
#include <time.h>
#include <pcap.h>
#include <netinet/in.h>
#include <netinet/if_ether.h>

    char *inputFileName=NULL;
    char *outputFileName=NULL;
    char *bssId=NULL;
    char *filter_expNew = "(eapol || wlan.fc.type_subtype == 0x08) && wlan.bssid == 00:11:00:11:00:11";

void my_packet_handler(
    u_char *args,
    const struct pcap_pkthdr* header,
    const u_char* packet
) {
    struct ether_header *eth_header;
    /* The packet is larger than the ether_header struct,
    but we just want to look at the first part of the packet
    that contains the header. We force the compiler
    to treat the pointer to the packet as just a pointer
    to the ether_header. The data payload of the packet comes
    after the headers. Different packet types have different header
    lengths though, but the ethernet header is always the same (14 bytes) */
    eth_header = (struct ether_header *) packet;

    if (ntohs(eth_header->ether_type) == ETHERTYPE_IP) {
        printf("IP\n");
    } else  if (ntohs(eth_header->ether_type) == ETHERTYPE_ARP) {
        printf("ARP\n");
    } else  if (ntohs(eth_header->ether_type) == ETHERTYPE_REVARP) {
        printf("Reverse ARP\n");
    }

    FILE *fo = fopen(outputFileName,"wb");
    fclose(fo);
}
void print_packet_info(const u_char *packet, struct pcap_pkthdr packet_header);


// taken from https://www.devdungeon.com/content/using-libpcap-c#load-pcap-file
int main(int argc, char **argv) {


    printf("Filtering expression:%s\n",filter_expNew);

    char dev[] = "any";
    pcap_t *handle;
    char error_buffer[PCAP_ERRBUF_SIZE];
    struct bpf_program filter;
    bpf_u_int32 subnet_mask, ip;

    if (pcap_lookupnet(dev, &ip, &subnet_mask, error_buffer) == -1) {
        printf("Could not get information for device: %s\n", dev);
        ip = 0;
        subnet_mask = 0;
    }
    handle = pcap_open_offline(inputFileName, error_buffer);
    if (handle == NULL) {
        printf("Could not open %s - %s\n", dev, error_buffer);
        return 2;
    }
    if (pcap_compile(handle, &filter, filter_exp, 0, ip) == -1) {
        printf("Bad filter - %s\n", pcap_geterr(handle));
        return 2;
    }
    if (pcap_setfilter(handle, &filter) == -1) {
        printf("Error setting filter - %s\n", pcap_geterr(handle));
        return 2;
    }

    if (pcap_compile(handle, &filter, filter_expNew, 0, ip) == -1) {
        printf("Bad filter - %s\n", pcap_geterr(handle));
        return 2;
    }
    if (pcap_setfilter(handle, &filter) == -1) {
        printf("Error setting filter - %s\n", pcap_geterr(handle));
        return 2;
    }

    pcap_loop(handle, 0, my_packet_handler, NULL);

    pcap_close(handle);

    return 0;
}

Christopher Maynard Christopher Maynard · Accepted Answer · 2019-05-13T13:25:54

I think you are confusing Wireshark display filters with libpcap capture filters. They are incompatible.

LIBPCAP can't parse valid expression from Wireshark

1 Answers