// write an ICMP echo request
public static int WriteIcmpEchoRequest(byte[] !pkt, int offset, ref IcmpEchoRequest request)
{
int o = offset;
// write the common header
o = WriteIcmpHeader(pkt, offset, ref request.header);
// dump the data (endian doesn't matter, since it is a user data)
Array.Copy(request.data, 0, pkt, o, request.data.Length);
// calc the checksum
ushort chk = IPFormat.Checksum(pkt, offset,
request.GetSize());
pkt[offset + 2] = (byte)(((ushort)chk) >> 8);
pkt[offset + 3] = (byte)(((ushort)chk) & 0xff);
request.header.chksum = chk;
return(o);
}
// a fast method that uses that modified the ECHO request
// packet to an ECHO reply.
// return true if success
public static void CreateFastEchoReply(byte[] !pkt, int icmpPacketSize)
{
// 1. switch the source IP and destination IP
// 2. change ICMP code to IcmpFormat.IcmpType.ECHO_REPLY
// 3. should use ARP table to find out the mac address of the target
// (if it is on the local network)
// CURRENTLY: just switch MAC addresses as well.
// 4. recompute ICMP and IP checksums
int ipStart = EthernetFormat.Size;
int icmpStart = ipStart + IPFormat.Size;
pkt[icmpStart] = (byte)IcmpFormat.IcmpType.ECHO_REPLY;
// zero checksum
pkt[icmpStart + 2] = 0;
pkt[icmpStart + 3] = 0;
ushort chk = IPFormat.Checksum(pkt, icmpStart, icmpPacketSize);
pkt[icmpStart + 2] = (byte)(chk >> 8);
pkt[icmpStart + 3] = (byte)chk;
// switch IPs
byte[] tmp = new byte[6];
Array.Copy(pkt, ipStart + 16, tmp, 0, 4); // dest->tmp
Array.Copy(pkt, ipStart + 12, pkt, ipStart + 16, 4); // src->dest
Array.Copy(tmp, 0, pkt, ipStart + 12, 4); // tmp->src
// zero the checksum
pkt[ipStart + 10] = 0;
pkt[ipStart + 11] = 0;
// calculate checksum
chk = IPFormat.Checksum(pkt, ipStart, IPFormat.Size);
pkt[ipStart + 10] = (byte)(((ushort)chk) >> 8);
pkt[ipStart + 11] = (byte)(((ushort)chk) & 0xff);
// TBC: currently just switch mac addresses
Array.Copy(pkt, 6, tmp, 0, 6); // src -> temp
Array.Copy(pkt, 0, pkt, 6, 6); // dest -> src
Array.Copy(tmp, 0, pkt, 0, 6); // tmp -> dest
}
