Table of Contents
To understand which information will remain available after the captured packets are saved to a capture 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 development, which will fix these drawbacks. However, it still might take a while until the new file format is ready and Wireshark can use it.
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/Development/LibpcapFileFormat
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]](../wsug_graphics/warning.png)
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 machine later!
the number of packets dropped while capturing
packet marks set with "Edit/Mark Packet"
time references set with "Edit/Time Reference"
the current display filter
...