User Guide

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 ex-
ists 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
"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 too.
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 the 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 7.6, “Name Resolution” for details
Warning!
The name resolution information is rebuild each time Wireshark is restarted so this
information even might 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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