Re: libnet_write

You can capture the packets and modify whatever bytes you want then 
recompute checksums using libnet_do_checksum() and send the packet out to 
the wire using libnet_adv_write_link().

if you want to save the headache of ARPing send the buffer via raw sock 
using this function i used in one of my programs:

int
libnet_adv_write_raw(libnet_t *l, u_int8_t *packet, u_int32_t len)
{
    u_int c;

    if (l == NULL)
    {
        return (-1);
    }

    /* assume error */
    c = -1;

    switch (l->injection_type)
    {
        case LIBNET_RAW4_ADV:
            if (len > LIBNET_MAX_PACKET)
            {
                snprintf(l->err_buf, LIBNET_ERRBUF_SIZE,
                        "%s(): packet is too large %d\n", __func__, len);
                return (c);
            }
            c = libnet_write_raw_ipv4(l, packet, len);
            break;

        case LIBNET_RAW6_ADV:
            c = libnet_write_raw_ipv6(l, packet, len);
            break;

        default:
            snprintf(l->err_buf, LIBNET_ERRBUF_SIZE,
                "%s(): advanced raw mode not enabled\n", __func__);
            return (-1);
    }

    /* do statistics */
    if (c == len)
    {
        l->stats.packets_sent++;
        l->stats.bytes_written += c;
    }
    else
    {
        l->stats.packet_errors++;
        /*
         *  XXX - we probably should have a way to retrieve the number of
         *  bytes actually written (since we might have written something).
         */
        if (c > 0)
        {
            l->stats.bytes_written += c;
        }
    }

    return (c);
}

This function is actually the same like adv_write_link (), it just sends a 
buffer "packet" with length "len" through handle "l"  but you don't have to 
care about the ethernet header.The kernel will take care of this. Note that 
you must initialize the libnet handle to LIBNET_RAW4_ADV.

I hope I could help you a little bit.

Mustaffa Abu Sedira

>From: "Jee J.Z." <[email protected]>
>To: <[email protected]>
>CC: <[email protected]>
>Subject: Re: libnet_write
>Date: Sat, 5 Jun 2004 14:08:54 +0100
>
>Hi Amit,
>
>I'm afraid, yes. At least this is what I'm doing, since I still don't need 
>a large bandwidth at the moment. Maybe in the future I'll get in trouble. 
>If you find a better way to do it, please let me know. Thanks.
>
>Jee
>
> >
> >
> > Hi
> >
> > 1. can libnet_write write packets that were not built in the libnet 
>context
> > ? i dont think so.
> > 2. that means if i capture a TCP packet and inject it back using libnet
> > functions, say even without any changes at all(to headeror payload), I 
>need
> > to reconstruct the tcp header using the call libnet_build_tcp() and then
> > build the ip_header using libnet_build_ip() and only then i can write it
> > using libnet_write() ?? !!, is there no way of passing a pointer to a 
>raw
> > packet(one which we created, or caught and now want to reinject) without
> > going through these build functions, because as you can see calling all
> > these build functions for packets which were caught and are now being
> > reinjected after a delay involves a lot of overhead ??
> >
> > thanks
> > Amit
> >
> >
> > "DISCLAIMER: This message is proprietary to Hughes Software Systems 
>Limited
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