Lucent Technologies Ethereal manual Text2pcap Converting Ascii hexdumps to network captures

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C.7. text2pcap: Converting ASCII hexdumps to network captures

There may be some occasions when you wish to convert a hex dump of some network traffic into a libpcap file.

Text2pcap is a program that reads in an ASCII hex dump and writes the data described into a libp- cap-style capture file. text2pcap can read hexdumps with multiple packets in them, and build a cap- ture file of multiple packets. text2pcap is also capable of generating dummy Ethernet, IP and UDP headers, in order to build fully processable packet dumps from hexdumps of application-level data only.

Text2pcap understands a hexdump of the form generated by od -t x1. In other words, each byte is in- dividually displayed and surrounded with a space. Each line begins with an offset describing the po- sition in the file. The offset is a hex number (can also be octal - see -o), of more than two hex digits. Here is a sample dump that text2pcap can recognize:

000000	00	e0	1e	a7	05	6f	00	10
000008	5a	a0	b9	12	08	00	46	00
000010	03	68	00	00	00	00	0a	2e
000018 ee 33 0f 19					08 7f		0f	19
000020	03	80	94	04	00	00	10	01
000028	16	a2	0a	00	03	50	00	0c
000030	01	01	0f	19	03	80	11	01

There is no limit on the width or number of bytes per line. Also the text dump at the end of the line is ignored. Bytes/hex numbers can be uppercase or lowercase. Any text before the offset is ignored, including email forwarding characters '>'. Any lines of text between the bytestring lines is ignored. The offsets are used to track the bytes, so offsets must be correct. Any line which has only bytes without a leading offset is ignored. An offset is recognized as being a hex number longer than two characters. Any text after the bytes is ignored (e.g. the character dump). Any hex numbers in this text are also ignored. An offset of zero is indicative of starting a new packet, so a single text file with a series of hexdumps can be converted into a packet capture with multiple packets. Multiple packets are read in with timestamps differing by one second each. In general, short of these restric- tions, text2pcap is pretty liberal about reading in hexdumps and has been tested with a variety of mangled outputs (including being forwarded through email multiple times, with limited line wrap etc.)

There are a couple of other special features to note. Any line where the first non-whitespace charac- ter is '#' will be ignored as a comment. Any line beginning with #TEXT2PCAP is a directive and options can be inserted after this command to be processed by text2pcap. Currently there are no dir- ectives implemented; in the future, these may be used to give more fine grained control on the dump and the way it should be processed e.g. timestamps, encapsulation type etc.

Text2pcap also allows the user to read in dumps of application-level data, by inserting dummy L2, L3 and L4 headers before each packet. The user can elect to insert Ethernet headers, Ethernet and IP, or Ethernet, IP and UDP headers before each packet. This allows Ethereal or any other full- packet decoder to handle these dumps.

Example C.5. Help information available for text2pcap

$ text2pcap.exe -h

Usage: text2pcap.exe [-h] [-d] [-q] [-o ho] [-l typenum] [-e l3pid] [-i proto] [-m max-packet] [-u srcp,destp] [-T srcp,destp] [-s srcp,destp,tag] [-S srcp,destp,tag] [-t timefmt] <input-filename> <output-filename>

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