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POHow to pack with variable bit order and length in python
 

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  1. POHow to pack with variable bit order and length in python
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    <p>I need to create a IP Packet in Python, but the length of the fields are different. Every time I have a field bigger than 1 byte I need to change the byte order to big endian. So what I did was to pack each variable separately and save everything in a file. I'm saving in a file because I need to do the checksum after packing and then pack all over again with the checksum (If someone has a better idea is more than welcome).</p> <p>I'm having two problems: </p> <p>1) The packet that I generate is bigger than the original one from were I parsed the information (the IP packet I generate only change in one field that is the TTL, but it should remain the same size)</p> <p>2) When I parse my packet (stream of data in the file), some information has change (I didn't change it).</p> <p>This is the original packet information:</p> <pre><code>Ver: 4 HL: 12 TOS: 0 LEN: 50 ID= 456 OFF: 16384 TTL: 5 PROT: 6 CHSUM: 30512 SRC: 16885952 DST: 167880896 rem_head: ['Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q'] data: ['A', 'A'] </code></pre> <p>This result is from parsing the packet. Then I verify the check sum and decrease the TTL by one and reassemble the packet like this:</p> <pre><code>pk_cksum=0 arch= open('packet.out', 'w') arch.write( struct.pack('B',frstb) ) arch.write( struct.pack('&gt;B', pk_tos) ) arch.write( struct.pack('&gt;H', pk_len) ) arch.write( struct.pack('&gt;H', pk_id) ) arch.write( struct.pack('&gt;H',pk_off) ) arch.write( struct.pack('B', pk_ttl) ) arch.write( struct.pack('B', pk_p) ) arch.write( struct.pack('&gt;H', pk_cksum) ) arch.write( struct.pack('&gt;I', pk_src) ) arch.write( struct.pack('&gt;I', pk_dst) ) if (pk_hl&gt;5): for i in range(len(pk_head)): arch.write(struct.pack('c', pk_head[i])[0]) if (pk_len&gt;(pk_hl*4)): for j in range(len(pk_data)): arch.write(struct.pack('c', pk_data[j])[0]) arch.close() </code></pre> <p>To verify if the packing was successful I use my same code to parse this last packet and then I get this information:</p> <pre><code>Ver: 4 HL: 0 TOS: 0 LEN: 50 ID= 456 OFF: 16384 TTL: 4 PROT: 6 CHSUM: 0 SRC: 3232235777 DST: 2818640397 data: ['@', '\x00', '\x00', '2', '\x01', '\xc8', '@', '\x00', '\x04', '\x06', '\x00', '\x00', '\x01', '\x01', '\xa8', '\xc0', '\r', '\n', '\x01', '\xa8', '\xc0', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'Q', 'A'] </code></pre> <p>As you see, some of the variables have another value and I don't know why.</p> <p>Can someone tell me what I'm doing wrong?</p> <p>Thanks</p>
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    1. USrocco
    1. USrocco
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    1. PO
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    1. CO*the compiler says its wrong* isn't a very good description of your problem. What did you expect it to do? What is it currently doing? (Also not that python is not *compiled*)
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      1. POHow to pack with variable bit order and length in python
      1. USHunter McMillen
    2. CODo you mean bit-order or byte-order. "endianess" is usually used to describe the latter. If the field lengths vary, you can just use a different format string calculated on-the-fly. An optional first letter of the format string can be used to specify the byte endianess desired, the default is native (see the [docs](http://docs.python.org/2/library/struct.html#byte-order-size-and-alignment)).
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      1. POHow to pack with variable bit order and length in python
      1. USmartineau
 

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