Re: [lwip-users] pinging

[Jonathan Larmour]

Marcin Prusinski wrote:
> Hey !
> I was looking for example that will help me send some icmp echo 
> requests. I only found post of Larry Piggins who wrote exactly something 
> i need, but i couldn't find the code.

You need to make a raw socket, i.e. NETCONN_RAW, ensuring LWIP_RAW is on in 
your configuration.

> I'm using standalone version of LWIP.
> I'm just a begginer and I can't figure out how to compute ICMP ECHO 
> checksum.

Here's some code obtained from elsewhere:

static int inet_cksum(u_short *addr, int len)
{
    register int nleft = len;
    register u_short *w = addr;
    register u_short answer;
    register u_int sum = 0;
    u_short odd_byte = 0;

    /*
     *  Our algorithm is simple, using a 32 bit accumulator (sum),
     *  we add sequential 16 bit words to it, and at the end, fold
     *  back all the carry bits from the top 16 bits into the lower
     *  16 bits.
     */
    while( nleft > 1 )  {
        sum += *w++;
        nleft -= 2;
    }

    /* mop up an odd byte, if necessary */
    if( nleft == 1 ) {
        *(u_char *)(&odd_byte) = *(u_char *)w;
        sum += odd_byte;
    }

    /*
     * add back carry outs from top 16 bits to low 16 bits
     */
    sum = (sum >> 16) + (sum & 0x0000ffff); /* add hi 16 to low 16 */
    sum += (sum >> 16);                     /* add carry */
    answer = ~sum;                          /* truncate to 16 bits */
    return (answer);
}

Hope this helps,

Jifl
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