Wireshark - 1.0 Betriebsanweisung PDF herunterladen (Seite 248)

Appendix A. Files and Folders

A.1. Capture Files

To understand which information will remain available after the captured packets are saved to a cap-

ture file, it's helpful to know a bit about the capture file contents.

Wireshark uses the libpcap file format as the default format to save captured packets; this format has

existed for a long time and it's pretty simple. However, it has some drawbacks: it's not extensible

and lacks some information that would be really helpful (e.g. being able to add a comment to a

packet such as "the problems start here" would be really nice).

In addition to the libpcap format, Wireshark supports several different capture file formats.

However, the problems described above also applies for these formats.

A new capture file format "PCAP Next Generation Dump File Format" is currently under develop-

ment, which will fix these drawbacks. However, it still might take a while until the new file format

is ready and Wireshark can use it.

A.1.1. Libpcap File Contents

At the start of each libpcap capture file some basic information is stored like a magic number to

identify the libpcap file format. The most interesting information of this file start is the link layer

type (Ethernet, Token Ring, ...).

The following data is saved for each packet:

• the timestamp with millisecond resolution

• the packet length as it was "on the wire"

• the packet length as it's saved in the file

• the packet's raw bytes

A detailed description of the libpcap file format can be found at: http://wiki.wireshark.org/Develop-

ment/LibpcapFileFormat

A.1.2. Not Saved in the Capture File

Probably even more interesting for everyday Wireshark usage is to know the things that are not

saved in the capture file:

• current selections (selected packet, ...)

• name resolution information, see Section 7.7, “Name Resolution” for details

Warning!

The name resolution information is rebuilt each time Wireshark is restarted so this

information might even change when the capture file is reopened on the same ma-

chine later!

• the number of packets dropped while capturing

• packet marks set with "Edit/Mark Packet"

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Wireshark - 1.0 Betriebsanweisung Seite 248