Re: [Jnetstream-users] Error in TCP decode

[Frédéric D. <fre...@dr...>]

Hi,

I checkout the code from SVN and I thing there is something odd in the 
"BitBuffer.readBitsOneAtATime()" method. Its probably about the posBits 
which ich not always added to the offset.

I temporary use the following implementation until a fix is commited in 
SVN (This concats the buffer byte per byte instead bit per bit)

Regards,
Frédéric

    private static final int[] RIGHT_BIT_MASK = new int[32];
    static {
        // initialize mask used by readBitsOneAtATime()
        for (int i = 1; i < RIGHT_BIT_MASK.length; i++) {
            RIGHT_BIT_MASK[i] = (RIGHT_BIT_MASK[i - 1] << 1) | 1;
        }
    }

    private int readBitsOneAtATime(int length) {
        // basic checks to avoid unnecessary computation
        if (length == 0) {
            return 0;
        }
        if (length > 32) {
            throw new IndexOutOfBoundsException("0 >= range <= 32bits");
        }
        // value is stored in long since length+posBits
        // could be greater than 32.
        long value = current;
        int acc = 8 - posBits;
        // concatenate buffer byte per byte
        for (; acc < length; acc += 8) {
            // fetch next byte           
            posBits = 8;
            fetch();
            // append new current byte
            value = (value << 8) | (current & 0xFF);
        }
        // right-align the bits and mask high bits
        value = (value >> (acc - length)) & RIGHT_BIT_MASK[length];
        // update posBits value
        posBits = (posBits + length) % 8;
        // return
        return (int) value;
    }








Frédéric Dreier a écrit :
> Hi,
>
> I  have some trouble decoding IP and TCP headers. the header length and 
> the offset are wrongly decoded in most of the packets.
>
> Did I do something wrong?
>
> Regards,
> Frederic Dreier
>
>
>
> Console output -------------------------------
> PACKET DUMP::
> 00 12 3F 19 FC 3F 00 07
> 84 BC A0 C0 08 00 45 00
> 00 28 9D 7A 40 00 2B 06
> 25 94 D0 50 98 03 81 AD
> A2 C0 00 50 D7 59 29 F4
> BC EF 33 C8 F3 C5 50 11
> Vers : 4
> Hlen : 4 <----- should be 5
> TOS  : 0
> Lgt  : 40
> ident: 40314
> flags: 2
> offs : 1    <---- should be 0
> TTL  : 43
> Prot : 6
> Src  : 208.80.152.3
> Dst  : 129.173.162.192
> Console output -------------------------------
>
>
>
> Code snipet ------------------------------
>                            Ip4 ipHd = packet.getHeader(Ip4.class);
>                            Tcp tcpHd = packet.getHeader(Tcp.class);
>                            System.out.println("PACKET DUMP::");
>                            for (int i = 0; i < 6; i++) {
>                                for (int k = i * 8; k < ((i + 1) * 8); 
> k++) {
>                                    System.out.printf("%02X ", 
> packet.getBuffer().getBackingBuffer().get(k) & 0xFF);
>                                }
>                                System.out.println();
>                            }
>                            System.out.printf("Vers : %d\n", ipHd.ver());
>                            System.out.printf("Hlen : %d\n", ipHd.hlen());
>                            System.out.printf("TOS  : %d\n", ipHd.tos());
>                            System.out.printf("Lgt  : %d\n", ipHd.length());
>                            System.out.printf("ident: %d\n", 
> ipHd.id()&0xFFFF);
>                            System.out.printf("flags: %d\n", ipHd.flags());
>                            System.out.printf("offs : %d\n", ipHd.offset());
>                            System.out.printf("TTL  : %d\n", ipHd.ttl());
>                            System.out.printf("Prot : %d\n", 
> ipHd.protocol());
>                            System.out.printf("Src  : %s\n", ipHd.source());
>                            System.out.printf("Dst  : %s\n", 
> ipHd.destination());
> --------------------------------------------
>
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