// set the checksum field, totalSize covers all the fields for
// which the checksum is calculated
// offset is points to the beginning of the IP header (!!!)
// Should be called after the UDP header + data have been written
public static void SetUdpChecksum(byte[] !packet,
int ipOffset,
ref UdpHeader udpHeader)
{
// sum IP pseudo
ushort ipPayloadSize = 0;
ushort headerSum = IPFormat.SumPseudoHeader(packet, ipOffset,
ref ipPayloadSize);
Debug.Assert(((ushort)udpHeader.length) == ipPayloadSize);
// now add it to the udp header + data
int ipHeaderSize = (packet[ipOffset] & 0xf) * 4;
int udpOffset = ipOffset + ipHeaderSize;
Debug.Assert(packet[udpOffset + 6] == 0);
Debug.Assert(packet[udpOffset + 7] == 0);
ushort payloadSum = IPFormat.SumShortValues(packet, udpOffset,
ipPayloadSize);
udpHeader.checksum = IPFormat.ComplementAndFixZeroChecksum(
IPFormat.SumShortValues(headerSum, payloadSum));
packet[udpOffset + 6] = (byte)(udpHeader.checksum >> 8);
packet[udpOffset + 7] = (byte)(udpHeader.checksum & 0xff);
}
// set the checksum field
// offset points to the beginning of the IP header (!!!)
// Should be called after the TCP header + data have been written
public static void SetTcpChecksum(byte[] !packet,
int ipOffset,
ref TcpHeader tcpHeader)
{
// sum IP pseudo
ushort ipPayloadLength = 0;
ushort headerSum =
IPFormat.SumPseudoHeader(packet, ipOffset,
ref ipPayloadLength);
int ipHeaderSize = (packet[ipOffset] & 0xf) * 4;
int tcpOffset = ipOffset + ipHeaderSize;
Debug.Assert(packet[tcpOffset + 16] == 0);
Debug.Assert(packet[tcpOffset + 17] == 0);
// now add it to the TCP header + data
ushort payloadSum = IPFormat.SumShortValues(packet, tcpOffset,
ipPayloadLength);
tcpHeader.checksum = (ushort)
~(IPFormat.SumShortValues(headerSum, payloadSum));
// Complement and change +0 to -0 if needed.
ushort tcpChecksum = IPFormat.ComplementAndFixZeroChecksum(
(IPFormat.SumShortValues(headerSum, payloadSum)));
tcpHeader.checksum = tcpChecksum;
unchecked {
packet[tcpOffset + 16] = (byte)(tcpChecksum >> 8);
packet[tcpOffset + 17] = (byte)(tcpChecksum >> 0);
}
}
public static bool IsChecksumValid(IPFormat.IPHeader !ipHeader,
TcpHeader tcpHeader,
NetPacket !payload)
{
// Compute partial checksums of headers
ushort checksum = IPFormat.SumPseudoHeader(ipHeader);
checksum = IPFormat.SumShortValues(checksum,
SumHeader(tcpHeader));
// Checksum payload (without potential odd byte)
int length = payload.Available;
int end = length & ~1;
int i = 0;
uint payloadChecksum = 0;
while (i != end)
{
byte b0 = payload.PeekAvailable(i++);
byte b1 = payload.PeekAvailable(i++);
payloadChecksum += ((((uint)b0) << 8) + (uint)b1);
}
// Handle odd byte.
if (i != length)
{
payloadChecksum += (((uint)payload.PeekAvailable(i++)) << 8);
}
// Merge bits from payload checksum
checksum = IPFormat.SumUInt16AndUInt32Values(checksum, payloadChecksum);
// Complement and change +0 to -0 if needed.
checksum = IPFormat.ComplementAndFixZeroChecksum(checksum);
// Check for match.
bool checksumMatch = (tcpHeader.checksum == checksum);
// If checksum error, unconditionally output message to debugger.
if (checksumMatch == false)
{
DebugStub.WriteLine("Bad TCP checksum {0:x4} != {1:x4}: SEQ {2:x8} ACK {3:x8}",
__arglist(tcpHeader.checksum, checksum,
tcpHeader.seq,
tcpHeader.ackSeq
));
}
// IsValid is a Match.
return(checksumMatch);
}
/// <summary>
/// Update checksum when a 16-bit value that composed the original
/// checksum changes. Useful for recomputing checksum when
/// TTL changes when acting as a router.
/// </summary>
public static ushort UpdateChecksum(ushort checksum,
ushort oldValue,
ushort newValue)
{
// From RFC1624
ushort newChecksum;
unchecked {
newChecksum = (ushort)~checksum;
newChecksum += (ushort)~oldValue;
newChecksum += newValue;
}
return(IPFormat.ComplementAndFixZeroChecksum(newChecksum));
}
/// <summary>
/// Write IP and UDP headers and payload data into a
/// byte array.
/// </summary>
/// <param name="pkt">Array of bytes representing
/// packet to be sent.</param>
/// <param name="offset">Offset of IP Header within
/// packet.</param>
/// <param name="ipHeader">IP header to be written
/// to packet.</param>
/// <param name="udpHeader">UDP header to be written
/// to packet.</param>
/// <param name="payload">Payload of UDP Packet.</param>
/// <param name="payloadOffset">The offset of start
/// of the payload data within the payload
/// array.</param>
/// <param name="payloadLength">The size of the payload data.</param>
public static void WriteUdpPacket(byte[] !pkt,
int offset,
IPFormat.IPHeader !ipHeader,
ref UdpHeader udpHeader,
byte[] payload,
int payloadOffset,
int payloadLength)
{
int udpStart = IPFormat.WriteIPHeader(pkt, offset, ipHeader);
int udpEnd = WriteUdpHeader(pkt, udpStart, ref udpHeader);
if (pkt != payload || udpEnd != payloadOffset)
{
Array.Copy(payload, payloadOffset,
pkt, udpEnd, payloadLength);
}
SetUdpChecksum(pkt, offset, ref udpHeader);
}
// 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
}
// writes an IP header to a packet
// return the next place to write to, fixes the checksum field
public static int WriteIPHeader(byte[] !pkt, int offset, IPHeader !hdr)
{
// check we have enough packet space
if (pkt.Length - offset < Size)
{
return(offset);
}
int o = offset;
pkt[o++] = hdr.verLen;
pkt[o++] = hdr.tos;
pkt[o++] = (byte)(((ushort)hdr.totalLength) >> 8);
pkt[o++] = (byte)(((ushort)hdr.totalLength) & 0xff);
pkt[o++] = (byte)(((ushort)hdr.id) >> 8);
pkt[o++] = (byte)(((ushort)hdr.id) & 0xff);
pkt[o++] = (byte)(((ushort)hdr.offset) >> 8);
pkt[o++] = (byte)(((ushort)hdr.offset) & 0xff);
pkt[o++] = hdr.ttl;
pkt[o++] = hdr.protocol;
pkt[o++] = 0;
pkt[o++] = 0;
// set the ip addresses
hdr.srcAddr.CopyOut(pkt, o);
o += IPv4.Length;
hdr.destAddr.CopyOut(pkt, o);
o += IPv4.Length;
// calculate checksum
ushort chk = IPFormat.FixZeroChecksum(Checksum(pkt, offset, Size));
pkt[offset + 10] = (byte)(((ushort)chk) >> 8);
pkt[offset + 11] = (byte)(((ushort)chk) & 0xff);
// save the header checksum
hdr.checksum = chk;
return(o);
}
public static bool IsChecksumValid(IPFormat.IPHeader !ipHeader,
UdpHeader udpHeader,
NetPacket !payload)
{
// Compute partial checksums of headers
ushort checksum = IPFormat.SumPseudoHeader(ipHeader);
checksum = IPFormat.SumShortValues(checksum,
UdpFormat.SumHeader(udpHeader));
// Checksum payload
int length = payload.Available;
int end = length & ~1;
int i = 0;
while (i != end)
{
int x = ((((int)payload.PeekAvailable(i++)) << 8) +
(int)payload.PeekAvailable(i++));
checksum = IPFormat.SumShortValues(checksum, (ushort)x);
}
if (i != length)
{
int x = (((int)payload.PeekAvailable(i++)) << 8);
checksum = IPFormat.SumShortValues(checksum, (ushort)x);
}
checksum = IPFormat.ComplementAndFixZeroChecksum(checksum);
if (udpHeader.checksum != checksum)
{
DebugStub.WriteLine("Bad UDP checksum {0:x4} != {1:x4}",
__arglist(udpHeader.checksum, checksum));
}
return(udpHeader.checksum == checksum);
}
public static bool IsCheckSumOK(IPHeader !hdr)
{
ushort sum = (ushort)((((int)hdr.verLen) << 8) + ((int)hdr.tos));
sum = SumShortValues(sum, hdr.totalLength);
sum = SumShortValues(sum, hdr.id);
sum = SumShortValues(sum, hdr.offset);
sum = SumShortValues(sum, (ushort)((((int)hdr.ttl) << 8) + ((int)hdr.protocol)));
sum = SumShortValues(sum, (ushort)(((uint)hdr.srcAddr) >> 16));
sum = SumShortValues(sum, (ushort)(((uint)hdr.srcAddr) & 0xFFFFU));
sum = SumShortValues(sum, (ushort)(((uint)hdr.destAddr) >> 16));
sum = SumShortValues(sum, (ushort)(((uint)hdr.destAddr) & 0xFFFFU));
sum = IPFormat.ComplementAndFixZeroChecksum(sum);
if (sum != hdr.checksum)
{
DebugStub.WriteLine("Bad IP Checksum {0:x4} != {1:x4}",
__arglist(hdr.checksum, sum));
}
return(sum == hdr.checksum);
}
// a helper method to write the TCP segment (without data)
public static void WriteTcpSegment(byte[] !pktData,
ushort localPort,
ushort destPort,
uint ackSeq,
uint seq,
ushort wnd,
IPv4 sIP,
IPv4 dIP,
ushort dataLen,
bool isAck,
bool isSyn,
bool isRst,
bool isFin,
bool withOptions)
{
// now create the IP,TCP headers!
int start = EthernetFormat.Size;
// prepare it...
TcpFormat.TcpHeader tcpHeader = new TcpFormat.TcpHeader();
tcpHeader.sourcePort = localPort;
tcpHeader.destPort = destPort;
if (withOptions)
{
tcpHeader.off_res1 = 0x60;
}
else
{
tcpHeader.off_res1 = 0x50;
}
tcpHeader.ackSeq = ackSeq;
tcpHeader.seq = seq;
tcpHeader.window = wnd;
if (isFin)
{
TcpFormat.SetFIN(ref tcpHeader);
}
if (isSyn)
{
TcpFormat.SetSYN(ref tcpHeader);
}
if (isAck)
{
TcpFormat.SetACK(ref tcpHeader);
}
if (isRst)
{
TcpFormat.SetReset(ref tcpHeader);
}
IPFormat.IPHeader ipHeader = new IPFormat.IPHeader();
ipHeader.SetDefaults(IPFormat.Protocol.TCP);
ipHeader.totalLength = (ushort)(IPFormat.Size + TcpFormat.Size + dataLen);
ipHeader.srcAddr = sIP;
ipHeader.destAddr = dIP;
IPFormat.SetDontFragBit(ipHeader);
// write the ip header + ip checksum
start = IPFormat.WriteIPHeader(pktData, start, ipHeader);
// write TCP header, no checksum
start = TcpFormat.WriteTcpHeader(pktData, start, ref tcpHeader);
// calc the checksum...
TcpFormat.SetTcpChecksum(pktData, EthernetFormat.Size, ref tcpHeader);
}
