File content as of revision 10:f0854784e960:

#include "mbed.h"
#include  "log.h"
#include  "net.h"
#include  "eth.h"
#include "dhcp.h"

#define REQUEST   1
#define REPLY     2

__packed struct header
{
    int16_t  hardwareType;             //16.bit: (ar$hrd) Hardware address space (e.g., Ethernet, Packet Radio Net). Always 1.
    int16_t  protocolType;             //16.bit: (ar$pro) Protocol address space.  For Ethernet hardware, this is from the set of type fields ether_typ$<protocol>. As held in eth.h, eg IPv4 = 0x8000.
    int8_t   hardwareLength;           // 8.bit: (ar$hln) byte length of each hardware address. Always 6 bytes.
    int8_t   protocolLength;           // 8.bit: (ar$pln) byte length of each protocol address. Always 4 bytes.
    int16_t  opCode;                   //16.bit: (ar$op)  opcode (ares_op$REQUEST = 1 | ares_op$REPLY = 2), high byte transmitted first.
    char     senderHardwareAddress[6]; //nbytes: (ar$sha) Hardware address of sender of this packet, n from the ar$hln field.
    uint32_t senderProtocolAddress;    //mbytes: (ar$spa) Protocol address of sender of this packet, m from the ar$pln field.
    char     targetHardwareAddress[6]; //nbytes: (ar$tha) Hardware address of target of this packet (if known).
    uint32_t targetProtocolAddress;    //mbytes: (ar$tpa) Protocol address of target.
};
int ArpHandleReceivedPacket(char* pSrcMac, void * pPacket, int* pSize, char* pDstMac)
{
    struct header*         pHeader = (header*)pPacket;
    int16_t           hardwareType = NetToHost16(pHeader->hardwareType);
    int16_t           protocolType = NetToHost16(pHeader->protocolType);
    int8_t          hardwareLength =             pHeader->hardwareLength;
    int8_t          protocolLength =             pHeader->protocolLength;
    int16_t                 opCode = NetToHost16(pHeader->opCode);
    uint32_t targetProtocolAddress =             pHeader->targetProtocolAddress;
    
    if (hardwareType          != ETHERNET     ) return DO_NOTHING; //This is not ethernet
    if (protocolType          != IPV4         ) return DO_NOTHING; //This is not IPv4
    if (hardwareLength        != 6            ) return DO_NOTHING; //This is not a MAC hardware address
    if (protocolLength        != 4            ) return DO_NOTHING; //This is not an IPv4 IP address
    if (opCode                != REQUEST      ) return DO_NOTHING; //This is not a request
    if (targetProtocolAddress != DhcpLocalIp  ) return DO_NOTHING; //This packet was not addressed to us
    
    memcpy(pHeader->targetHardwareAddress,  pHeader->senderHardwareAddress, 6);
           pHeader->targetProtocolAddress = pHeader->senderProtocolAddress;
    memcpy(pHeader->senderHardwareAddress,  EthLocalMac,6);
           pHeader->senderProtocolAddress = DhcpLocalIp;
           pHeader->opCode                = NetToHost16(REPLY);
    
    memcpy(pDstMac, pSrcMac, 6);
    
    return UNICAST;
}