Re: [Jnetstream-users] Error in TCP decode
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Re: [Jnetstream-users] Error in TCP decode
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From: Mark B. <voy...@ya...> - 2008-08-05 17:33:11
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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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