Code größtenteils von diesem Tutorial abgeschrieben: http://www.tcpdump.org/pcap.html. Hinzu kommt, dass einige Codepassagen nicht problemlos mit meiner Version von libpcap (1.5.X) funktionieren. Ich sollte probieren eine aktuellere Version der Bibliothek zu verwenden.

main.c

#include <stdio.h>
#include <pcap.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <string.h>
 
/* Ethernet addresses are 6 bytes */
#define ETHER_ADDR_LEN	6
#define SIZE_ETHERNET 14
 
	/* Ethernet header */
	struct sniff_ethernet {
		u_char ether_dhost[ETHER_ADDR_LEN]; /* Destination host address */
		u_char ether_shost[ETHER_ADDR_LEN]; /* Source host address */
		u_short ether_type; /* IP? ARP? RARP? etc */
	};
 
	/* IP header */
	struct sniff_ip {
		u_char ip_vhl;		/* version << 4 | header length >> 2 */
		u_char ip_tos;		/* type of service */
		u_short ip_len;		/* total length */
		u_short ip_id;		/* identification */
		u_short ip_off;		/* fragment offset field */
	#define IP_RF 0x8000		/* reserved fragment flag */
	#define IP_DF 0x4000		/* dont fragment flag */
	#define IP_MF 0x2000		/* more fragments flag */
	#define IP_OFFMASK 0x1fff	/* mask for fragmenting bits */
		u_char ip_ttl;		/* time to live */
		u_char ip_p;		/* protocol */
		u_short ip_sum;		/* checksum */
		struct in_addr ip_src,ip_dst; /* source and dest address */
	};
	#define IP_HL(ip)		(((ip)->ip_vhl) & 0x0f)
	#define IP_V(ip)		(((ip)->ip_vhl) >> 4)
 
	/* TCP header */
	typedef u_int tcp_seq;
 
	struct sniff_tcp {
		u_short th_sport;	/* source port */
		u_short th_dport;	/* destination port */
		tcp_seq th_seq;		/* sequence number */
		tcp_seq th_ack;		/* acknowledgement number */
		u_char th_offx2;	/* data offset, rsvd */
	#define TH_OFF(th)	(((th)->th_offx2 & 0xf0) >> 4)
		u_char th_flags;
	#define TH_FIN 0x01
	#define TH_SYN 0x02
	#define TH_RST 0x04
	#define TH_PUSH 0x08
	#define TH_ACK 0x10
	#define TH_URG 0x20
	#define TH_ECE 0x40
	#define TH_CWR 0x80
	#define TH_FLAGS (TH_FIN|TH_SYN|TH_RST|TH_ACK|TH_URG|TH_ECE|TH_CWR)
		u_short th_win;		/* window */
		u_short th_sum;		/* checksum */
		u_short th_urp;		/* urgent pointer */
};
 
void got_packet ( u_char* usr, const struct pcap_pkthdr *h, const u_char *packet )
{
	const struct sniff_ethernet *ethernet; /* The ethernet header */
	const struct sniff_ip *ip; /* The IP header */
	const struct sniff_tcp *tcp; /* The TCP header */
	const char *payload; /* Packet payload */
 
	u_int size_ip;
	u_int size_tcp;
 
	/*ethernet = (struct sniff_ethernet*)(packet);
	ip = (struct sniff_ip*)(packet + SIZE_ETHERNET);
	size_ip = IP_HL(ip)*4;
	if (size_ip < 20) {
		printf("   * Invalid IP header length: %u bytes\n", size_ip);
		return;
	}
	tcp = (struct sniff_tcp*)(packet + SIZE_ETHERNET + size_ip);
	size_tcp = TH_OFF(tcp)*4;
	if (size_tcp < 20) {
		printf("   * Invalid TCP header length: %u bytes\n", size_tcp);
		return;
	}
	payload = (u_char *)(packet + SIZE_ETHERNET + size_ip + size_tcp);*/
 
	printf ("Got Packet of size: %d\n\n %s \n", h->len, packet);
}
 
void list_devices ()
{
    pcap_if_t * alldev;
    pcap_if_t * dev;
    char* errbuf[PCAP_ERRBUF_SIZE];
    pcap_findalldevs(&alldev, errbuf);
    if (alldev==NULL)
        return;
    dev = alldev;
    while (dev != NULL)
    {
        printf ("DEVICE: %s", dev->name);
        /*switch (dev->flags)
        {
            case PCAP_IF_LOOPBACK:
                printf ("(LOOPBACK)");
                break;
            case PCAP_IF_UP:
                printf ("(UP)");
                break;
            case PCAP_IF_RUNNING:
                printf ("(RUNNING)");
                break;
        }*/
        if (dev->description != NULL)
            printf ("\n >> %s", dev->description);
        printf ("\n\n");
        dev = dev->next;
    }
    pcap_freealldevs(alldev);
}
 
int main ( int argc, char* argv[] )
{
    char *dev, errbuf[PCAP_ERRBUF_SIZE];
    if (argc<2 || strcmp (argv[1], "-h") == 0 )
    {
        list_devices ();
    }
 
    //const u_char* packet;
    //struct pcap_pkthdr header;		
    //struct bpf_program fp;		/* The compiled filter */
	//char filter_exp[] = "port 23";	/* The filter expression */
	//bpf_u_int32 mask;		/* Our netmask */
	//bpf_u_int32 net;		/* Our IP */
    pcap_t *handle;
    dev = "wlan0";
 
    handle = pcap_open_live ( dev, BUFSIZ, 1, 1000, errbuf );
    if (handle==NULL)
    {
       printf ("Could not open device %s : %s\n", dev, errbuf );
       return (2);
    }
    if (pcap_datalink(handle) != DLT_EN10MB) {
		fprintf(stderr, "Device %s doesn't provide Ethernet headers - not supported\n", dev);
		return(2);
	}
	/* Find the properties for the device */
	/*if (pcap_lookupnet(dev, &net, &mask, errbuf) == -1) {
		fprintf(stderr, "Couldn't get netmask for device %s: %s\n", dev, errbuf);
		net = 0;
		mask = 0;
	}*/
	/* Compile and apply the filter */
	/*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(2);
	}
	if (pcap_setfilter(handle, &fp) == -1) {
		fprintf(stderr, "Couldn't install filter %s: %s\n", filter_exp, pcap_geterr(handle));
		return(2);
	}*/
    //packet = pcap_next(handle, &header);
 
    pcap_loop(handle, 10, got_packet, NULL);
 
    /* Print its length */
    printf("STOP\n");
    /* And close the session */
    pcap_close(handle);
    return(0);
}

Makefile

all:
	gcc main.c -lpcap -pedantic -Wall -g