source: libpacketdump/ip_47.c @ 3996329

#include <sys/types.h>
#include <netinet/in.h>
#include <stdio.h>
#include "libpacketdump.h"

typedef struct gre_t {
        uint16_t flags;
        uint16_t ethertype;
        uint16_t checksum;
        uint16_t reserved1;
} gre_t;

void decode(int link_type,char *packet,unsigned len)
{
        // GRE
        if (len<2) {
                printf(" GRE: [Truncated]");
                return;
        }
        printf(" GRE: %s\n",
                ntohs(((gre_t*)packet)->flags) & 0x8000 
                        ? "Checksum present"
                        : "Checksum absent");
        printf(" GRE: Version: %d\n", ntohs(((gre_t*)packet)->flags) & 0x0007);
        printf(" GRE: Protocol: %04x\n", ntohs(((gre_t*)packet)->ethertype));

        if (ntohs(((gre_t*)packet)->flags) & 0x8000) {
                decode_next(packet+4,len-4,"link",
                                ntohs(((gre_t*)packet)->ethertype));
        }
        else {
                decode_next(packet+8,len-8,"link",
                                ntohs(((gre_t*)packet)->ethertype));
        }
        return;
}