When you select Start... from the Capture menu (or use the corresponding item in the "Main" toolbar), Wireshark pops up the "Capture Options" dialog box as shown in Figure 4.3, “The "Capture Options" dialog box”.
![[Tip]](wsug_graphics/tip.png) | Tip! |
|---|---|
If you are unsure which options to choose in this dialog box, just try keeping the defaults as this should work well in many cases. |
You can set the following fields in this dialog box:
- Interface
This field specifies the interface you want to capture on. You can only capture on one interface, and you can only capture on interfaces that Wireshark has found on the system. It is a drop-down list, so simply click on the button on the right hand side and select the interface you want. It defaults to the first non-loopback interface that supports capturing, and if there are none, the first loopback interface. On some systems, loopback interfaces cannot be used for capturing (loopback interfaces are not available on Windows platforms).
This field performs the same function as the -i <interface> command line option.
- IP address
The IP address(es) of the selected interface. If no address could be resolved from the system, "unknown" will be shown.
- Link-layer header type
Unless you are in the rare situation that you need this, just keep the default. For a detailed description, see Section 4.8, “Link-layer header type”
- Buffer size: n megabyte(s)
Enter the buffer size to be used while capturing. This is the size of the kernel buffer which will keep the captured packets, until they are written to disk. If you encounter packet drops, try increasing this value.
![[Note]](wsug_graphics/note.png)
Note This option is only available on Windows platforms.
- Capture packets in promiscuous mode
This checkbox allows you to specify that Wireshark should put the interface in promiscuous mode when capturing. If you do not specify this, Wireshark will only capture the packets going to or from your computer (not all packets on your LAN segment).
Note If some other process has put the interface in promiscuous mode you may be capturing in promiscuous mode even if you turn off this option
Note Even in promiscuous mode you still won't necessarily see all packets on your LAN segment, see http://www.wireshark.org/faq.html#promiscsniff for some more explanations.
- Limit each packet to n bytes
This field allows you to specify the maximum amount of data that will be captured for each packet, and is sometimes referred to as the snaplen. If disabled, the default is 65535, which will be sufficient for most protocols. Some rules of thumb:
If you are unsure, just keep the default value.
If you don't need all of the data in a packet - for example, if you only need the link-layer, IP, and TCP headers - you might want to choose a small snapshot length, as less CPU time is required for copying packets, less buffer space is required for packets, and thus perhaps fewer packets will be dropped if traffic is very heavy.
If you don't capture all of the data in a packet, you might find that the packet data you want is in the part that's dropped, or that reassembly isn't possible as the data required for reassembly is missing.
- Capture Filter
This field allows you to specify a capture filter. Capture filters are discussed in more details in Section 4.9, “Filtering while capturing”. It defaults to empty, or no filter.
You can also click on the button labelled Capture Filter, and Wireshark will bring up the Capture Filters dialog box and allow you to create and/or select a filter. Please see Section 6.6, “Defining and saving filters”
An explanation about capture file usage can be found in Section 4.7, “Capture files and file modes”.
- File
This field allows you to specify the file name that will be used for the capture file. This field is left blank by default. If the field is left blank, the capture data will be stored in a temporary file, see Section 4.7, “Capture files and file modes” for details.
You can also click on the button to the right of this field to browse through the filesystem.
- Use multiple files
Instead of using a single file, Wireshark will automatically switch to a new one, if a specific trigger condition is reached.
- Next file every n megabyte(s)
Multiple files only: Switch to the next file after the given number of byte(s)/kilobyte(s)/megabyte(s)/gigabyte(s) have been captured.
- Next file every n minute(s)
Multiple files only: Switch to the next file after the given number of second(s)/minutes(s)/hours(s)/days(s) have elapsed.
- Ring buffer with n files
Multiple files only: Form a ring buffer of the capture files, with the given number of files.
- Stop capture after n file(s)
Multiple files only: Stop capturing after switching to the next file the given number of times.
- ... after n packet(s)
Stop capturing after the given number of packets have been captured.
- ... after n megabytes(s)
Stop capturing after the given number of byte(s)/kilobyte(s)/megabyte(s)/gigabyte(s) have been captured. This option is greyed out, if "Use multiple files" is selected.
- ... after n minute(s)
Stop capturing after the given number of second(s)/minutes(s)/hours(s)/days(s) have elapsed.
- Update list of packets in real time
This option allows you to specify that Wireshark should update the packet list pane in real time. If you do not specify this, Wireshark does not display any packets until you stop the capture. When you check this, Wireshark captures in a separate process and feeds the captures to the display process.
- Automatic scrolling in live capture
This option allows you to specify that Wireshark should scroll the packet list pane as new packets come in, so you are always looking at the last packet. If you do not specify this, Wireshark simply adds new packets onto the end of the list, but does not scroll the packet list pane. This option is greyed out if "Update list of packets in real time" is disabled.
- Hide capture info dialog
If this option is checked, the capture info dialog described in Section 4.10, “While a Capture is running ...” will be hidden.
- Enable MAC name resolution
This option allows you to control whether or not Wireshark translates MAC addresses into names, see Section 7.7, “Name Resolution”.
- Enable network name resolution
This option allows you to control whether or not Wireshark translates network addresses into names, see Section 7.7, “Name Resolution”.
- Enable transport name resolution
This option allows you to control whether or not Wireshark translates transport addresses into protocols, see Section 7.7, “Name Resolution”.
Once you have set the values you desire and have selected the options you need, simply click on Start to commence the capture, or Cancel to cancel the capture.
If you start a capture, Wireshark allows you to stop capturing when you have enough packets captured, for details see Section 4.10, “While a Capture is running ...”.