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Minimal example of extracting the IPv6 Flow Label from IPv6 packets
#include <stdio.h>
#include <pcap.h>
#include <arpa/inet.h>
#include <linux/if_ether.h>
#include <netinet/ip6.h>
#define SNAP_LEN 1500
int main(int argc, char *argv[]) {
char *eth_if = argv[1];
char *errbuf;
pcap_t *p = pcap_open_live(eth_if, SNAP_LEN, 1, 0, errbuf);
if (p == NULL) {
printf("Yikes, something went wrong:\n%s\n", errbuf);
return 1; // exit
}
unsigned char *packet;
struct pcap_pkthdr header;
while (packet = (unsigned char *)pcap_next(p, &header)){
// assuming it's ethernet, we check whether it carries IPv6
struct ethhdr *eth_h = (struct ethhdr*) packet;
if (ntohs(eth_h->h_proto) == ETH_P_IPV6) {
// now, fill the ip6_hdr struct
struct ip6_hdr *ipv6_h = (struct ip6_hdr*)(packet + sizeof(struct ethhdr));
// and extract the flow label. We're only interested in the last 20 bits, so 0xfffff
uint32_t ipv6_fl = ntohl(ipv6_h->ip6_flow) & 0xfffff;
printf("Flow label for this packet: 0x%x\n", ipv6_fl);
}
}
return(0);
}
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