Sniffer tcp packages on port 7000 using libpcap

sniffer.c

#include <pcap.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>
#include <stdio.h>

#define MAXBYTES2CAPTURE 2048

/* processPacket(): Callback function called by pcap_loop() everytime a packet */
/* arrives to the network card. This function prints the captured raw data in  */
/* hexadecimal.                                                                */
void processPacket(u_char *arg, const struct pcap_pkthdr* pkthdr, const u_char * packet)
{
    int i=0, *counter = (int *)arg;

    printf("Packet Count: %d\n", ++(*counter));
    printf("Received Packet Size: %d\n", pkthdr->len);
    printf("Payload:\n");
    for (i=0; i < (int)pkthdr->len; i++) {

        /* If it is a printable character, print it */
        if ( isprint(packet[i]) ) {
            printf("%c ", packet[i]);
        } else {
            printf(". ");
        }
        if( (i % 32 == 0 && i != 0) || i == (int)(pkthdr->len-1)) {
            printf("\n");
        }
    }
}

/* main(): Main function. Opens network interface and calls pcap_loop() */
int main(int argc, char *argv[])
{
    int count=0;
    pcap_t *descr = NULL;
    char errbuf[PCAP_ERRBUF_SIZE], *device=NULL;
    char filter_exp[] = {"tcp port 7000"};
    bpf_u_int32 mask;		/* Our netmask */
    bpf_u_int32 net;		/* Our IP */
    struct bpf_program fp;	/* The compiled filter */

    memset(errbuf,0,PCAP_ERRBUF_SIZE);

    if(argc > 1) { /* If user supplied interface name, use it. */
        device = argv[1];
    } else if ((device = pcap_lookupdev(errbuf)) == NULL) { /* Get the name of the first device suitable for capture */
        fprintf(stderr, "ERROR: %s\n", errbuf);
        return -1;
    }

    /* Find the properties for the device */
	net = mask = 0;
    if (pcap_lookupnet(device, &net, &mask, errbuf) == -1) {
        fprintf(stderr, "Couldn't get netmask for device %s: %s\n", device, errbuf);
    }

    printf("Opening device %s\n", device);
    printf("Filter expression: %s\n", filter_exp);

    /* Open device in promiscuous mode */
    if ((descr = pcap_open_live(device, MAXBYTES2CAPTURE, 1,  512, errbuf)) == NULL) {
        fprintf(stderr, "ERROR: %s\n", errbuf);
        return -1;
    }

    /* Compile and apply the filter */
    if (pcap_compile(descr, &fp, filter_exp, 0, net) == -1) {
        fprintf(stderr, "Couldn't parse filter %s: %s\n", filter_exp, pcap_geterr(descr));
        return -1;
    }
    if (pcap_setfilter(descr, &fp) == -1) {
        fprintf(stderr, "Couldn't install filter %s: %s\n", filter_exp, pcap_geterr(descr));
        return -1;
    }

    /* Loop forever & call processPacket() for every received packet*/
    if (pcap_loop(descr, -1, processPacket, (u_char *)&count) == -1) {
        fprintf(stderr, "ERROR: %s\n", pcap_geterr(descr) );
        return -1;
    }
    return 0;
}