CODEWITHSUNDEEP
cprotocolschecksumpacketethernet
Kobi Danone
Kobi Danone

Reputation: 119

Ethernet raw data and Frame Check Sequence in C

I'm trying to build a raw data with ethernet frame via C code. I built a packet (included Ethernet->IP->UDP->DHCP protocols) and sent it via the WiFi interface. I followed it via the Wireshark which prints out: Ethernet2 -> Frame Check Sequence -> Incorrect, should be XXX.

I did not build an FCS data in my packet, I left the field blank. Now, I can't find any simple function/code in C which does that. All the codes I found gave me a bad output.

Someone done it before and can share how to implement the FCS in the Ethernet packet?

Thank you in advance

Upvotes: 0

Views: 2076

Answers (2)

Gonzalo Romero
Gonzalo Romero

Reputation: 1

try: 

#define BYTE unsigned char
     int fcs(BYTE* paquete,int n){
        int byte,sum=0;
        n++;
        for(int j=0;j<=n;j++){
                byte=paquete[j];
                for(int i=0;i<8;i++){
                        if(j!=n && i<7){
                                sum = sum+(byte & 0x01);
                                byte = byte >> 1;
                        }
                }
        }
        return sum;
}

Upvotes: 0

Kobi Danone
Kobi Danone

Reputation: 119

FIX: it seems that FCS calculation is only optional, I added the IP checksum calculation instead and that was enough for the DHCP to pass.

Thanks.

Upvotes: 2

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