Re: [Jnetstream-users] Error in TCP decode
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Re: [Jnetstream-users] Error in TCP decode
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From: Frédéric D. <fre...@dr...> - 2008-08-05 18:19:39
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Actually I will fligh back in europe tomorrow and also have to visit my
chiro :-)
Actually the "readBitsOneAtATime()" I posted correct something but break
the TCP decoding (start offset is wrong). so I will look at it again (I
find the exercise quite interessting) and I could wait some days. No
problem, really. I will share my thoughts if I find something pertinent.
Regards,
Frederic
Mark Bednarczyk a écrit :
> I made changes in there recently. Its possible I screwed up.
>
> I hurt my back coming back from Europe yesterday and I can barely sit at the
> computer right now. Hopefully tomorrow I'll feel better after my visit to
> the chiropractor today and be able to take a look at the problem quickly. I
> do have some jUnit test cases checked in that test various BitBuffer
> conditions in the CommonUtils module.
>
> Cheers,
> mark...
>
>
>> -----Original Message-----
>> From: jne...@li...
>> [mailto:jne...@li...] On
>> Behalf Of Frédéric Dreier
>> Sent: Tuesday, August 05, 2008 1:10 PM
>> To: Jne...@li...
>> Subject: Re: [Jnetstream-users] Error in TCP decode
>>
>> 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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>
>
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