Family of Wireless Capture
Adapters
User’s Guide
Contents and Figures
Contents
The AirPcap Product Family........................................................................3
A Brief Introduction to 802.11.....................................................................4
Terminology............................................................................................4
802.11 Standards.....................................................................................4
Channels..................................................................................................5
Types of Frames......................................................................................6
How AirPcap Adapters Operate...................................................................7
Multiple Channel Capture (applies to USB adapters only)....................8
Configuring the Adapters: the AirPcap Control Panel ................................9
Identifying the AirPcap Adapters ...........................................................9
Settings ..................................................................................................10
WEP Keys .............................................................................................11
The Multi-Channel Aggregator (applies to USB AirPcap
adapters only) ........................................................................................12
AirPcap and Wireshark...............................................................................14
Identifying the AirPcap Adapters in Wireshark ...................................14
The Wireless Toolbar............................................................................14
The Wireless Settings Dialog................................................................16
The Decryption Keys Management Dialog..........................................18
The Multi-Channel Aggregator (applies to USB AirPcap
adapters only) ........................................................................................19
Transmit Raw 802.11 Frames on Your Network.......................................20
Where to Learn More .................................................................................21
Appendix A: 802.11 Frequencies...............................................................22
2.4GHz Band.........................................................................................22
5GHz Band............................................................................................22
Channels Supported by the AirPcap Product Family...........................22
i
Figures
Figure 1: The AirPcap Control Panel. Settings Tab.....................................9
Figure 2: AirPcap N and Extension ChannelSetting................................. 10
Figure 3: The AirPcap Control Panel. Keys Tab. ..................................... 12
Figure 4: Multi-Channel Aggregator......................................................... 13
Figure 5: The Wireshark Adapters List..................................................... 14
Figure 6: The Wireshark Wireless Toolbar............................................... 15
Figure 7: Wireless Settings Dialog in Wireshark...................................... 17
Figure 8: Decryption Keys Management Dialog in Wireshark................ 18
Tables
Table 1. Feature Comparison for the AirPcap Product Family ...................3
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AirPcap User’s Guide
The AirPcap Product Family
The AirPcap offerings are the first open, affordable and easy-to-deploy
packet capture solution for Windows. All of the AirPcap offerings will
capture full 802.11 data, management, and control frames that can be
viewed in Wireshark thereby providing in-depth protocol dissection and
analysis capabilities. Below we provide a feature matrix that gives a high-
level overview of the feature sets of the adapters in the AirPcap Product
Family.
More detailed information regarding each the member of the AirPcap
Product Family can be found on the CACE Technologies Website
AirPcap
Classic
AirPcap Tx
AirPcap Ex
AirPcap N
Captures Full 802.11 Frames
Fully Integrated with Wireshark
Open API
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No
Yes
Yes
Yes
Multi-Channel Monitoring
(with 2 or more adapters)
Yes
Yes
Yes
Packet Transmission
External Antenna Connector
Form Factors
No
Yes
Yes
No
No
No
Yes
No
1
USB Dongle
USB Dongle
USB Dongle
Cardbus (32 bits)
Frequency Bands
2.4GHz (b/g)
2.4GHz (b/g)
2.4 and 5GHz
(a/b/g)
2.4 and 5GHz
(a/b/g/n)
Table 1. Feature Comparison for the AirPcap Product Family
1
Other form factors available by special order are: mini-PCI and mini-PCI Express
3
A Brief Introduction to 802.11
Terminology
The terms Wireless LAN or WLAN are used to indicate a wireless local
area network, e.g. a network between two or more “stations” that uses
radio frequencies instead of wires for the communication.
All components that can “connect” to a WLAN are referred to as stations.
Stations fall into one of two categories: access points or wireless clients.
Access points transmit and receive information to/from stations using
radio frequencies. As we shall see later, the particular choice of a radio
frequency determines a wireless “channel.” An access point usually acts as
a “gateway” between a wired network and a wireless network.
Wireless clients can be mobile devices such as laptops, personal digital
assistants (PDAs), IP phones or fixed devices such as desktops and
workstations that are equipped with a wireless network interface card.
In some configurations, wireless devices can communicate directly with
each other, without the intermediation of an access point. This kind of
network configuration is called peer-to-peer or ad-hoc.
A Basic Service Set (BSS) is the basic building block of a WLAN. The
“coverage” of one access point is called a BSS. The access point acts as
the master to control the stations within that BSS. A BSS can be thought
of as the wireless version of an IP subnet. Every BSS has an id called the
BSSID, which is the MAC address of the access point servicing the BSS,
and a text identifier called the SSID.
802.11 Standards
802.11 is a standard that defines the physical layer and the data-link layer
for communication among wireless devices. The original 802.11
specification was ratified in 1997, uses the 2.4 GHz frequency band, and
allows transmission rates of 1 or 2 Mbps.
802.11a, ratified in 1999, is an extension of 802.11 that operates at 5 GHz.
It supports 8 additional transmission rates: 6, 9, 12, 18, 24, 36, 48 and 54
Mbps.
802.11b, ratified in 1999, is an extension of 802.11 that uses the same 2.4
GHz frequency band, and supports two additional transmission rates: 5.5
and 11 Mbps.
802.11g, ratified in 2003, is backward compatible with 802.11b, and
supports the same additional transmission rates found in 802.11a: 6, 9, 12,
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AirPcap User’s Guide
18, 24, 36, 48 and 54 Mbps.
802.11i, ratified in 2004, defines an enhanced security mechanism based
on AES.
802.11n, expected to be ratified in 2009, is backward compatible with
802.11a, b, and g, and will operate at 2.4 GHz and optionally 5 GHz. It
can potentially support data rates up to 600 Mbps.
Channels
802.11b and 802.11g divide the 2.4 GHz spectrum into 13 channels,
beginning with channel 1 and ending with channel 13. The center
frequency of channel 1 is 2,412MHz, channel 2 is 2,417MHz, etc. The
center frequencies of adjacent channels are 5 MHz apart. The bandwidth
of each channel is 20 MHz which means that channels may “overlap.”
The commonly-used non-overlapping channels are channels 1, 6, and 13.
There is a 14th channel whose center frequency is 12MHz above channel
13. These frequency bands are referred to as channels and stations
communicate using a particular channel.
802.11a and 802.11n operate in the 5 GHz range which is divided into a
large number of channels. The center frequency of channel 0 is 5,000
MHz, the center frequency of channel 1 is 5,005 MHz. The formula for
relating channels (n) to center frequencies in the 5 GHz range is:
Center frequency (MHz) = 5,000 + 5*n, where n = 0, …, 199,
Center frequency (MHz) = 5,000 – 5*(256 – n), where n = 240, …, 255.
Note that channels 240 to 255 range from 4,920 MHz to 4,995 MHz. As
with the 2.4 GHz band, each channel is 20 MHz wide. 802.11n allows for
“wide” channels – that is, two adjacent 20 Mhz bands (note that the
channel numbers of the two adjancent 20 MHz bands are not adjacent)
can be used “side-by-side” in order to be backward-compatible with
802.11a, b, and g, or they can be combined into a single 40 MHz channel
in “Greenfield” mode.
The actual use of the channels, however, depends on the country. For
example,in the USA, the FCC allows channels 1 through 11 in the 2.4
GHz band, whereas most of Europe can use channels 1 through 13. No
matter where you are, you can use AirPcap to listen on any supported
channel. The regulations for the 5GHz band are much more complex.
Each BSS operates on a particular channel, i.e., the access point and all of
the wireless clients within a BSS communicate over a common channel.
The same channel may be used by more than one BSS. When this
happens, and if the BSSs are within communication range of each other,
the different BSSs compete for the bandwidth of the channel, and this can
reduce the overall throughput of the interfering BSSs. On the other hand,
selecting different channels for nearby access points will mitigate channel
AirPcap User’s Guide
5
interference and accommodate good wireless coverage using multiple
BSSs.
A BSS is formed by wireless clients “associating” themselves with a
particular access point. Naturally, a wireless client will have to “discover”
whether there is an access point within range and its corresponding
channel. For this purpose, access points advertise themselves with
“beacon” frames and wireless clients can (passively) listen for these
frames. Another discovery approach is for the wireless client to send out
“probe” requests to see if certain access points are within range.
Following the discovery process, wireless clients will send requests to be
associated with a particular BSS.
Types of Frames
The 802.11 link layer is much more complicated than the Ethernet one.
The main reason is that wireless links have lower reliability compared to
the reliability of wired links, and therefore the 802.11 link layer has
features to reduce the effects of frame loss. For example, every data frame
is acknowledged with an ACK frame. Moreover, the protocol needs to
support access point discovery, association and disassociation,
authentication, wired/wireless bridging, and many other features that are
not necessarily needed in a wired link layer.
When capturing on a wireless channel, you will see three main kinds of
frames:
•
•
Data frames
Control frames
o
o
o
Acknowledgement
Request to Send
Clear to Send
•
Management frames
o
o
o
o
o
o
Beacons
Probe Requests / Probe Responses
Association Requests / Association Responses
Reassociation Requests / Reassociation Responses
Disassociations
Authentications / Deauthentications
Additionally, frame headers may contain Quality of Service (QoS) and
High Throughput (+HTC) information.
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AirPcap User’s Guide
The Control frames are used to improve the reliability characteristics of
the link. The establishment of a BSS through the process of discovery and
association is supported by the Management frames, including possible
authentication steps in the process.
It is beyond the scope of this brief introduction to describe the details of
these frames and their usage in the 802.11 protocol. If you are interested in
additional details, you can consult the following websites:
4d5a-8b1e-4289db0bcafd1033.mspx?mfr=true
Another good source is the book 802.11® Wireless Networks: The
Definitive Guide (2nd Edition) by Matthew Gast (ISBN-10 0-596-10052-
3).
How AirPcap Adapters Operate
The AirPcap adapter captures the traffic on a single channel at a time; the
channel setting for the AirPcap adapter can be changed using the AirPcap
Control Panel, or from the “Advanced Wireless Settings” dialog in
Wireshark. Depending on the capabilities of your AirPcap adapter, it can
be set to any valid 802.11a/b/g/n channel for packet capture.
All of the AirPcap adapters can operate in a completely passive mode.
This means that they can capture the traffic on a channel without
associating with an access point, or interacting with any other wireless
device. Unless you are transmitting with either AirPcapTx or AirPcap Ex,
none of the adapters is detectable by any other wireless station.
The AirPcap adapters can work in, so called, Monitor Mode. In this mode,
the AirPcap adapter will capture all of the frames that are transferred on a
channel, not just frames that are addressed to it. This includes data frames,
control frames and management frames.
When more than one BSS shares the same channel, the AirPcap adapter
will capture the data, control and management frames from all of the BSSs
that are sharing the channel and that are within range of the AirPcap
adapter.
The AirPcap software can optionally be configured to decrypt WEP-
encrypted frames. An arbitrary number of keys can be configured in the
driver at the same time, so that the driver can decrypt the traffic of more
than one access point at the same time. WPA and WPA2 support is
handled by applications such as Wireshark and Aircrack-ng. See the
AirPcap User’s Guide
7
section WEP Keys on page 11 and The Decryption Keys Management
Dialog on page 18 for more information.
Multiple Channel Capture (applies to USB adapters only)
This section applies to all members of the AirPcap Product family except
AirPcap N. When listening on a single channel is not enough, multiple
AirPcap adapters can be plugged in a PC and used at the same time to
capture traffic simultaneously from different channels.
The AirPcap driver provides support for this operation through to the
Multi-Channel Aggregator technology, that exports capture streams from
multiple AirPcap adapters as a single capture stream.
The Multi-Channel Aggregator consists of a virtual interface that can be
used from Wireshark or any other AirPcap-based application. Using this
interface, the application will receive the traffic from all the installed
AirPcap adapters, as if it was coming from a single device.
The Multi-Channel Aggregator can be configured like any real AirPcap
device, and therefore can have its own decryption, FCS checking and
packet filtering settings.
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AirPcap User’s Guide
Configuring the Adapters: the AirPcap Control Panel
The AirPcap control panel (Figure 1) provides a convenient and intuitive
way to configure the parameters of currently-connected AirPcap adapters.
The changes made to an adapter using the AirPcap control panel will be
reflected in all of the applications using that adapter.
To start the AirPcap control panel, click on
START→PROGRAMS→AirPcap→AirPcap Control Panel
Select an Adapter from the
Drop-down List of Installed
AirPcap Adapters
Blink the Led of the
Selected Adapter
Select a Channel
from the
Drop-down List
of Available Channels
Figure 1: The AirPcap Control Panel. Settings Tab.
The drop-down list in the Interface box at the top of the panel presents a
list of currently-installed adapters. Selecting one of the adapters in the list
allows you to view/edit its configuration.
Identifying the AirPcap Adapters
The drop-down list identifies the USB AirPcap adapters using adapter
numbers (e.g. 00, 01, …) and does not distinguish between AirPcap
Classic, AirPcap Tx, and AirPcap Ex. Fortunately, the AirPcap adapters
have an Led that can be caused to blink by first selecting the adapter from
the drop-down list and clicking on the Blink Led button. This feature is
useful in distinguishing among the USB AirPcap adapters when multiple
adapters are plugged into your system and an easy way to associate the
physical adapters with the adapter numbers assigned by the system.
AirPcap N appears as “AirPcap N Wireless Capture Device” in the drop-
down list, making it easy to identify if it is present (see Figure 2).
AirPcap User’s Guide
9
Settings
The AirPcap N adapter
Is Selected.
Extension Channel
Drop-down List for
AirPcap N
Figure 2: AirPcap N and Extension ChannelSetting
The Basic Configuration box contains the following settings:
•
Channel: The channels available in the Channel list box depend upon
the selected adapter. Since channel numbers 1, …, 14 in the 2.4GHz
and 5GHz bands overlap and there are center frequncies (channels)
that do not have channels numbers, each available channel is given by
its center frequency. Where applicable, the BG or A channel numbers
are also given. All of the channel center-frequencies supported by the
selected adapter will be made available in the Channel list. The
bandwidth of each channel is 20MHz.
•
Extension Channel: For 802.11n adapters, one can use the Extension
Channel list create a “wide” channel. The choices are -1 (the
preceding 20MHz frequency band), 0 (no extension channel), or +1
(the succeeding 20MHz frequency band). The channel of the
additional frequency band is called the extension channel. The
Extension Channel list box lets you choose a valid extension channel
(above or below) for a given channel (See Figure 2). Not all channels
have above and below extension channels. For example, BG channels
1, 2, 3, and 4 do not have a -1 (below) extension channel. The reason
is that the center frequencies of the primay and extension channels
need to be separated by 20MHz. So if 4 were the primary channel,
channel 1 (which is the lowest BG center frequency) is only 15 MHz
below channel 4.
•
Capture Type: 802.11 frames only, 802.11 frames plus radio
information (See Radiotap), or 802.11 frames plus the Per-Packet
Information (PPI) header (See Downloads for the current PPI
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AirPcap User’s Guide
specification). PPI and radio information includes additional
information not contained in the 802.11 frame: transmit rate, signal
power, signal quality, channel, and (for PPI) multiple antenna
information.
•
Include 802.11 FCS in Frames: if checked the captured frames will
include the 802.11 4-bytes Frame Check Sequence. This option can be
disabled if an application has difficulty decoding the packets that have
the Frame Check Sequence.
•
FCS Filter: this drop-down list allows you to configure the kind of
Frame Check Sequence filtering that the selected adapter will
perform:
o
o
o
All Frames: the adapter will capture all the frames
regardless of whether the FCS is valid or not.
Valid Frames: the adapter will only capture frames that
have a valid FCS.
Invalid Frames: the adapter will only capture frames that
have an invalid FCS.
Note:
AirPcap stores the configuration information on a per-adapter
basis. This means that changing the configuration of an adapter
does not affect the settings of any of the other adapters.
WEP Keys
The AirPcap driver is able to use a set of WEP keys to decrypt traffic that
is WEP encrypted. If a frame is WEP encrypted, the driver will attempt to
decrypt the frame using the user-supplied set of WEP keys – the driver
will try all of the WEP keys for each frame until it finds one that decrypts
the frame. If the decryption is successful, the unencrypted frame is passed
to the user application, otherwise the original frame is passed along. By
configuring the AirPcap driver with multiple WEP keys, it is possible to
decrypt traffic coming from multiple access points that are using different
WEP keys, but transmitting on the same channel.
The list of keys can be edited by selecting the Keys tab in the AirPcap
control panel (Figure 3).
To add or remove a key, use the “Add New Key” or “Remove Key”
buttons, respectively. “Edit Key” allows you to change the value of an
existing key. “Move Key Up” and “Move Key Down” can be used to
change the order of the keys. This may be an important performance
AirPcap User’s Guide
11
consideration, since the driver uses the keys in the order they appear in
this list.
The currently configured keys are shown in the “Keys” list.
It is possible to turn WEP decryption on and off at any time by using the
“Enable WEP Decryption” check box.
Figure 3: The AirPcap Control Panel. Keys Tab.
The keys are applied to the packets in the same order they appear in the
keys list. Therefore, putting frequently used keys at the beginning of the
list improves performance.
Note:
The keys are stored by the AirPcap Control Panel globally. This
means that any keys specified in the list will be used by all AirPcap
adapters (Including AirPcap N).
The Multi-Channel Aggregator (applies to USB AirPcap adapters
only)
When more than one USB AirPcap adapter is plugged in, the AirPcap
Control Panel will show one additional interface: the Multi-Channel
Aggregator.
As explained in the Multiple Channel Capture (applies to USB adapters
only) section of this manual, the Multi-Channel Aggregator is a virtual
capture interface that can be used from Wireshark or any other AirPcap-
based application. Using this capture interface, the application will receive
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AirPcap User’s Guide
|