Libnet TCP checksum error

Question

I am trying to build a TCP packet using libnet library. I use '0' for autocomputation of checksum value in the libnet_build_tcp function. However, it seems the checksum ignores the pseudo-header when being computed resulting in a useless packet because of checksum error. Does anyone know how to solve this?

Answer 1

As far as I can see in the code, as long as you haven't used libnet_toggle_checksum(l, ptag, 1); your 0 in the sum parameter of libnet_build_tcp() should be causing it to autocompute a checksum for you by calling libnet_pblock_setflags(p, LIBNET_PBLOCK_DO_CHECKSUM).

I couldn't really tell you how to solve it but since you are the one building the packet maybe you could opt for creating your own checksum?

The TCP pseudoheader is a 12 byte field containing the source and destination IP addresses which are both 4 bytes each, a reserved field which is 2 bytes and always set to zeros, and a 4 byte TCP segment length which is equal to the size of the TCP header + the payload length.

You could create something like this:

First create your variables:

int bytesRead;
int pseudoHeaderLength;
int payloadLength;
int tcpHdrLength;
int tcpSegmentLength;
int ipHdrLength;
int headerLengths;
int ethIPHdrLengths;

struct ethhdr *ethHdr;
struct iphdr *ipHdr;
struct tcphdr *tcpHdr;

u_int8_t *pkt_data;
u_int8_t pseudoHeader[12];
u_int8_t packetBuffer[1600];
u_int16_t newChecksum;

Then create the actual function where b->len is the total size of the packet (just add all the headers + the payload and get the total size) and you would just have to memcpy your header and data to pkt_data:

ethHdr = (struct ethhdr*)pkt_data;
ipHdr = (struct iphdr*)(pkt_data + ETH_HLEN);

ipHdrLength = ipHdr->ihl * 4;

ethIPHdrLengths = ETH_HLEN + ipHdrLength;

tcpHdr = (struct tcphdr*)(pkt_data + ethIPHdrLengths);

tcpHdrLength = tcpHdr->doff * 4;

headerLengths = ethIPHdrLengths + tcpHdrLength;

payloadLength = b->len - headerLengths;

tcpSegmentLength = tcpHdrLength + payloadLength;

pseudoHeaderLength = tcpSegmentLength + 12;

pseudoHeader[0] = pkt_data[30];
pseudoHeader[1] = pkt_data[31];
pseudoHeader[2] = pkt_data[32];
pseudoHeader[3] = pkt_data[33];
pseudoHeader[4] = pkt_data[26];
pseudoHeader[5] = pkt_data[27];
pseudoHeader[6] = pkt_data[28];
pseudoHeader[7] = pkt_data[29];
pseudoHeader[8] = 0x00;
pseudoHeader[9] = 0x06;
pseudoHeader[10] = (tcpSegmentLength >> 8) & 0xFF;
pseudoHeader[11] = tcpSegmentLength & 0xFF;

bytesRead = 0;

for(i = 0; i < 12; i++) {
    packetBuffer[bytesRead] = pseudoHeader[i];
    bytesRead++;
}

for(i = ethIPHdrLengths; i < headerLengths; i++) {
    packetBuffer[bytesRead] = pkt_data[i];
    bytesRead++;
}

for(i = b->len - payloadLength; i < b->len; i++) {
    packetBuffer[bytesRead] = pkt_data[i];
    bytesRead++;
}

newChecksum = checksum((uint16_t *)packetBuffer, pseudoHeaderLength);

And just use the checksum function provided by https://tools.ietf.org/html/rfc1071 to calculate the checksum over the buffer:

uint16_t checksum(uint16_t *addr, int len)
{

int count = len;
register uint32_t sum = 0;
uint16_t answer = 0;

while (count > 1) {
    sum += *(addr++);
    count -= 2;
}

if (count > 0) {
    sum += *(uint8_t *) addr;
}

while (sum >> 16) {
    sum = (sum & 0xffff) + (sum >> 16);
}

answer = ~sum;

return (answer);
}

I use this in a realistic environment to calculate checksums at 5 million PPS.