如何检查以太网标头是否为 IEEE 802.1Q 类型？

Question

I use pcap_open_offline for parsing packets. I what to check if ethernet header is type IEEE 802.1Q. I know I need to check if first 16 bits in 802.1Q tag are equal to 8100 but I do not know how to do it. Or if you know another way I can try it.

Answer 1

Assume you want a solution in C, here is a simple implementation:

struct ether_header {
    /* destination MAC */
    uint8_t dst_mac[6];
    /* source MAC */
    uint8_t src_mac[6];
    /* EtherType */
    uint16_t ether_type;
};

#define ETHERTYPE_VLAN 0x8100

/* this method gets the packet data read from pcap file and returns 1 if ether type is 802.1Q, 0 otherwise */
int is_IEEE_802_1Q(const uint8_t* packet_data) {
    /* cast ethernet header */
    ether_header* eth_header = (ether_header*)packet_data;

    /* assuming big endian as most pcap files are in big endian */
    if (eth_header->ether_type == ETHERTYPE_VLAN) {
        return 1;
    }

    return 0;
}