Изменение пакета

Если пакет был изменен, то надо по меньше мере пересчитать контрольные суммы. Это делается? Если нет, то пакет с неверной контрольной суммой может быть просто отброшен стеком на принимающей стороне.

Примера для C#, к сожалению нет, в следующих версиях соответствующие функции будут добавлены к ndisapi.dll, но ничего не мешает сделать сейчас:

//
// Function recalculates IP checksum
//
void
CNdisApi::RecalculateIPChecksum(
	PINTERMEDIATE_BUFFER pPacket
)
{
	unsigned short word16;
	unsigned int sum = 0;
	unsigned int i = 0;
	PUCHAR buff;

	iphdr_ptr pIpHeader = (iphdr_ptr)&pPacket->m_IBuffer[sizeof(ether_header)];

	// Initialize checksum to zero
	pIpHeader->ip_sum = 0;
	buff = (PUCHAR)pIpHeader;

	// Calculate IP header checksum
	for (i = 0; i < pIpHeader->ip_hl * sizeof(DWORD); i = i + 2)
	{
		word16 = ((buff[i] << 8) & 0xFF00) + (buff[i + 1] & 0xFF);
		sum = sum + word16;
	}

	// keep only the last 16 bits of the 32 bit calculated sum and add the carries
	while (sum >> 16)
		sum = (sum & 0xFFFF) + (sum >> 16);

	// Take the one's complement of sum
	sum = ~sum;

	pIpHeader->ip_sum = htons((unsigned short)sum);
}

//
// Function recalculates ICMP checksum
//
void
CNdisApi::RecalculateICMPChecksum(
	PINTERMEDIATE_BUFFER pPacket
)
{
	unsigned short word16, padd = 0;
	unsigned int i, sum = 0;
	PUCHAR buff;
	DWORD dwIcmpLen;
	icmphdr_ptr pIcmpHeader = NULL;
	iphdr_ptr pIpHeader = (iphdr_ptr)&pPacket->m_IBuffer[sizeof(ether_header)];

	// Sanity check
	if (pIpHeader->ip_p == IPPROTO_ICMP)
	{
		pIcmpHeader = (icmphdr_ptr)(((PUCHAR)pIpHeader) + sizeof(DWORD)*pIpHeader->ip_hl);
	}
	else
		return;

	dwIcmpLen = ntohs(pIpHeader->ip_len) - pIpHeader->ip_hl * 4;

	if ((dwIcmpLen / 2) * 2 != dwIcmpLen)
	{
		padd = 1;
		pPacket->m_IBuffer[dwIcmpLen + pIpHeader->ip_hl * 4 + sizeof(ether_header)] = 0;
	}

	buff = (PUCHAR)pIcmpHeader;
	pIcmpHeader->checksum = 0;

	// make 16 bit words out of every two adjacent 8 bit words and 
	// calculate the sum of all 16 bit words
	for (i = 0; i< dwIcmpLen + padd; i = i + 2) {
		word16 = ((buff[i] << 8) & 0xFF00) + (buff[i + 1] & 0xFF);
		sum = sum + (unsigned long)word16;
	}

	// keep only the last 16 bits of the 32 bit calculated sum and add the carries
	while (sum >> 16)
		sum = (sum & 0xFFFF) + (sum >> 16);

	// Take the one's complement of sum
	sum = ~sum;

	pIcmpHeader->checksum = ntohs((unsigned short)sum);
}

//
// Function recalculates TCP checksum
//
void
CNdisApi::RecalculateTCPChecksum(
	PINTERMEDIATE_BUFFER pPacket
)
{
	tcphdr_ptr pTcpHeader = NULL;
	unsigned short word16, padd = 0;
	unsigned int i, sum = 0;
	PUCHAR buff;
	DWORD dwTcpLen;

	iphdr_ptr pIpHeader = (iphdr_ptr)&pPacket->m_IBuffer[sizeof(ether_header)];

	// Sanity check
	if (pIpHeader->ip_p == IPPROTO_TCP)
	{
		pTcpHeader = (tcphdr_ptr)(((PUCHAR)pIpHeader) + sizeof(DWORD)*pIpHeader->ip_hl);
	}
	else
		return;

	dwTcpLen = ntohs(pIpHeader->ip_len) - pIpHeader->ip_hl * 4;//pPacket->m_Length - ((PUCHAR)(pTcpHeader) - pPacket->m_IBuffer);

	if ((dwTcpLen / 2) * 2 != dwTcpLen)
	{
		padd = 1;
		pPacket->m_IBuffer[dwTcpLen + pIpHeader->ip_hl * 4 + sizeof(ether_header)] = 0;
	}

	buff = (PUCHAR)pTcpHeader;
	pTcpHeader->th_sum = 0;

	// make 16 bit words out of every two adjacent 8 bit words and 
	// calculate the sum of all 16 vit words
	for (i = 0; i< dwTcpLen + padd; i = i + 2) {
		word16 = ((buff[i] << 8) & 0xFF00) + (buff[i + 1] & 0xFF);
		sum = sum + (unsigned long)word16;
	}

	// add the TCP pseudo header which contains:
	// the IP source and destination addresses,

	sum = sum + ntohs(pIpHeader->ip_src.S_un.S_un_w.s_w1) + ntohs(pIpHeader->ip_src.S_un.S_un_w.s_w2);
	sum = sum + ntohs(pIpHeader->ip_dst.S_un.S_un_w.s_w1) + ntohs(pIpHeader->ip_dst.S_un.S_un_w.s_w2);

	// the protocol number and the length of the TCP packet
	sum = sum + IPPROTO_TCP + (unsigned short)dwTcpLen;

	// keep only the last 16 bits of the 32 bit calculated sum and add the carries
	while (sum >> 16)
		sum = (sum & 0xFFFF) + (sum >> 16);

	// Take the one's complement of sum
	sum = ~sum;

	pTcpHeader->th_sum = htons((unsigned short)sum);
}

//
// Function recalculates UDP checksum
//
void
CNdisApi::RecalculateUDPChecksum(
	PINTERMEDIATE_BUFFER pPacket
)
{
	udphdr_ptr pUdpHeader = NULL;
	unsigned short word16, padd = 0;
	unsigned int i, sum = 0;
	PUCHAR buff;
	DWORD dwUdpLen;

	iphdr_ptr pIpHeader = (iphdr_ptr)&pPacket->m_IBuffer[sizeof(ether_header)];

	// Sanity check
	if (pIpHeader->ip_p == IPPROTO_UDP)
	{
		pUdpHeader = (udphdr_ptr)(((PUCHAR)pIpHeader) + sizeof(DWORD)*pIpHeader->ip_hl);
	}
	else
		return;

	dwUdpLen = ntohs(pIpHeader->ip_len) - pIpHeader->ip_hl * 4;//pPacket->m_Length - ((PUCHAR)(pTcpHeader) - pPacket->m_IBuffer);

	if ((dwUdpLen / 2) * 2 != dwUdpLen)
	{
		padd = 1;
		pPacket->m_IBuffer[dwUdpLen + pIpHeader->ip_hl * 4 + sizeof(ether_header)] = 0;
	}

	buff = (PUCHAR)pUdpHeader;
	pUdpHeader->th_sum = 0;

	// make 16 bit words out of every two adjacent 8 bit words and 
	// calculate the sum of all 16 vit words
	for (i = 0; i< dwUdpLen + padd; i = i + 2) {
		word16 = ((buff[i] << 8) & 0xFF00) + (buff[i + 1] & 0xFF);
		sum = sum + (unsigned long)word16;
	}

	// add the UDP pseudo header which contains:
	// the IP source and destination addresses,

	sum = sum + ntohs(pIpHeader->ip_src.S_un.S_un_w.s_w1) + ntohs(pIpHeader->ip_src.S_un.S_un_w.s_w2);
	sum = sum + ntohs(pIpHeader->ip_dst.S_un.S_un_w.s_w1) + ntohs(pIpHeader->ip_dst.S_un.S_un_w.s_w2);

	// the protocol number and the length of the UDP packet
	sum = sum + IPPROTO_UDP + (unsigned short)dwUdpLen;

	// keep only the last 16 bits of the 32 bit calculated sum and add the carries
	while (sum >> 16)
		sum = (sum & 0xFFFF) + (sum >> 16);

	// Take the one's complement of sum
	sum = ~sum;

	pUdpHeader->th_sum = ntohs((unsigned short)sum);
}