BOOTP_VENDOR_LEN
uint8_t bp_vend[BOOTP_VENDOR_LEN];
#define MAX_RFC1533_VENDLEN (ETH_MAX_MTU - sizeof(struct bootpip_t) + BOOTP_VENDOR_LEN)
sizeof(struct udphdr) + sizeof(struct bootp_t) - BOOTP_VENDOR_LEN);
dhcpack_length = len + sizeof (struct bootp_t) - BOOTP_VENDOR_LEN;