Thread: recvfrom result differs from ip_len field (IPv4)

I'am writing a simple network sniffer on windows xp.
Here is the code:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <windows.h>

#define SIO_RCVALL _WSAIOW(IOC_VENDOR,1)

void die(const char* msg) {
    printf("Error: %s\n", msg);
    exit(-1);
}

int Init(WSADATA wsa) {
    return WSAStartup(0x0101, &wsa);
}

int main(int argc, char** argv) {
    WSADATA wsa;
    if(Init(wsa)) die("init");

    SOCKET s;
    long flag = 1;
    struct sockaddr_in local;
    
    local.sin_family = AF_INET;
    local.sin_port = 0;
    local.sin_addr.S_un.S_addr = inet_addr("192.168.1.2");

    if((s = socket(AF_INET, SOCK_RAW, IPPROTO_IP)) == INVALID_SOCKET) die("invalid socket");
    if(bind(s, (SOCKADDR*) &local, sizeof(local)) == SOCKET_ERROR) die("bind");
    if(ioctlsocket(s, SIO_RCVALL, &flag)) die("ioctl");
    
    struct sockaddr_in remote;
    int rsize = sizeof(remote);

    int rcvd = 0;
    char buff[0xFFFF]; //64k

    int ip_len = 0;

    int i = 0;
    while((rcvd = recvfrom(s, buff, sizeof(buff), 0, (SOCKADDR*) &remote, &rsize)) != SOCKET_ERROR) {
        ip_len = (buff[2] & 0xFF << 8) | (buff[3] & 0xFF);
        if(ip_len != rcvd) {
            printf("\nIPLEN: %6i, RECVFROM: %6i", ip_len, rcvd);
        } else {
            printf(".");
        }
    }

    printf("Error: %i\n", WSAGetLastError());

    return (EXIT_SUCCESS);
}

Packets are caught, but i have a problem: value, returned by recvfrom (number of bytes read) differs from IPv4 packet's ip_len field.
Here is the example of output:

Code:

IPLEN:     39, RECVFROM:    551...
IPLEN:    187, RECVFROM:    955..
IPLEN:    112, RECVFROM:    880..
IPLEN:    250, RECVFROM:    762
IPLEN:    122, RECVFROM:    890.
IPLEN:     39, RECVFROM:    295...
IPLEN:     96, RECVFROM:    608........
IPLEN:    122, RECVFROM:    890....
IPLEN:     22, RECVFROM:    534.....
IPLEN:      9, RECVFROM:    521..
IPLEN:     79, RECVFROM:    847..

(dots means packets with equal values)
So, how to fix this?

thanks. no troubles at all now.