Search code examples
c++network-programmingethernetarp

How to Extract the Payload from Ethernet Packet (vector<unsigned char>)?

For my assignment I have been given the task of making a router. The ethernet packet datatype is vector <unsigned char> and the ethernet header is as follows: enter image description here

Now I do not have much experience working with vectors, but my question is how can I extract the payload part? For example if the ethernet packet is an ARP packet I would like to be able to use the following structure to check the opcode and essentially be able to do something like 

arp_hdr *arphdr = (arp_hdr *) packet_arp;

struct arp_hdr
{
  unsigned short  arp_hrd;                 /* format of hardware address   */
  unsigned short  arp_pro;                 /* format of protocol address   */
  unsigned char   arp_hln;                 /* length of hardware address   */
  unsigned char   arp_pln;                 /* length of protocol address   */
  unsigned short  arp_op;                  /* ARP opcode (command)         */
  unsigned char   arp_sha[ETHER_ADDR_LEN]; /* sender hardware address      */
  uint32_t        arp_sip;                 /* sender IP address            */
  unsigned char   arp_tha[ETHER_ADDR_LEN]; /* target hardware address      */
  uint32_t        arp_tip;                 /* target IP address            */
} __attribute__ ((packed)) ;

And the ethernet header is also given:

struct ethernet_hdr
{
#ifndef ETHER_ADDR_LEN
#define ETHER_ADDR_LEN 6
#endif
  uint8_t  ether_dhost[ETHER_ADDR_LEN]; /* destination ethernet address */
  uint8_t  ether_shost[ETHER_ADDR_LEN]; /* source ethernet address */
  uint16_t ether_type;                  /* packet type ID */
} __attribute__ ((packed)) ;
Solution

  • To copy a vector into another vector, you can use the 5th std::vector constructor here https://en.cppreference.com/w/cpp/container/vector/vector

    template< class InputIt > vector( InputIt first, InputIt last, const Allocator& alloc = Allocator() );

    So to extract the payload (assuming the payload is everything in the dataframe that is not in the header) you'd do:

    ...assuming the existence of a std::vector<unsigned char> dataframe;
std::vector<unsigned char> payload(dataframe.begin() + sizeof(ethernet_hdr), dataframe.end());