WinPcap: Capturing the packets without the callback

The callback-based capture mechanism of pcap_loop() is elegant and it could be a good choice in some situations. However, handling a callback is sometimes not practical -- it often makes the program more complex especially in situations with multithreaded applications or C++ classes.

In these cases, pcap_next_ex() retrievs a packet with a direct call -- using pcap_next_ex() packets are received only when the programmer wants them.

The parameters of this function are the same as a capture callback -- it takes an adapter descriptor and a couple of pointers that will be initialized and returned to the user (one to a pcap_pkthdr structure and another to a buffer with the packet data).

In the following program, we recycle the callback code of the previous lesson's example and move it inside main() right after the call to pcap_next_ex().

#include "pcap.h"


main()
{
pcap_if_t *alldevs;
pcap_if_t *d;
int inum;
int i=0;
pcap_t *adhandle;
int res;
char errbuf[PCAP_ERRBUF_SIZE];
struct tm *ltime;
char timestr[16];
struct pcap_pkthdr *header;
const u_char *pkt_data;
time_t local_tv_sec;
    
    
    /* Retrieve the device list on the local machine */
    if (pcap_findalldevs_ex(PCAP_SRC_IF_STRING, NULL, &alldevs, errbuf) == -1)
    {
        fprintf(stderr,"Error in pcap_findalldevs: %s\n", errbuf);
        exit(1);
    }
    
    /* Print the list */
    for(d=alldevs; d; d=d->next)
    {
        printf("%d. %s", ++i, d->name);
        if (d->description)
            printf(" (%s)\n", d->description);
        else
            printf(" (No description available)\n");
    }
    
    if(i==0)
    {
        printf("\nNo interfaces found! Make sure WinPcap is installed.\n");
        return -1;
    }
    
    printf("Enter the interface number (1-%d):",i);
    scanf("%d", &inum);
    
    if(inum < 1 || inum > i)
    {
        printf("\nInterface number out of range.\n");
        /* Free the device list */
        pcap_freealldevs(alldevs);
        return -1;
    }
    
    /* Jump to the selected adapter */
    for(d=alldevs, i=0; i< inum-1 ;d=d->next, i++);
    
    /* Open the device */
    if ( (adhandle= pcap_open(d->name,          // name of the device
                              65536,            // portion of the packet to capture. 
                                                // 65536 guarantees that the whole packet will be captured on all the link layers
                              PCAP_OPENFLAG_PROMISCUOUS,    // promiscuous mode
                              1000,             // read timeout
                              NULL,             // authentication on the remote machine
                              errbuf            // error buffer
                              ) ) == NULL)
    {
        fprintf(stderr,"\nUnable to open the adapter. %s is not supported by WinPcap\n", d->name);
        /* Free the device list */
        pcap_freealldevs(alldevs);
        return -1;
    }
    
    printf("\nlistening on %s...\n", d->description);
    
    /* At this point, we don't need any more the device list. Free it */
    pcap_freealldevs(alldevs);
    
    /* Retrieve the packets */
    while((res = pcap_next_ex( adhandle, &header, &pkt_data)) >= 0){
        
        if(res == 0)
            /* Timeout elapsed */
            continue;
        
        /* convert the timestamp to readable format */
        local_tv_sec = header->ts.tv_sec;
        ltime=localtime(&local_tv_sec);
        strftime( timestr, sizeof timestr, "%H:%M:%S", ltime);
        
        printf("%s,%.6d len:%d\n", timestr, header->ts.tv_usec, header->len);
    }
    
    if(res == -1){
        printf("Error reading the packets: %s\n", pcap_geterr(adhandle));
        return -1;
    }
    
    return 0;
}

Why do we use pcap_next_ex() instead of the old pcap_next()? Because pcap_next() has some drawbacks. First of all, it is inefficient because it hides the callback method but still relies on pcap_dispatch(). Second, it is not able to detect EOF, so it's not very useful when gathering packets from a file.

Notice also that pcap_next_ex() returns different values for success, timeout elapsed, error and EOF conditions.