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AirSnort 是一个无线局域网工具

Last Modified: 17 Aug 02

- AirSnort Installation

*NEW* AirSnort ONLY works with cards that pass their monitor mode
packets up via the PF_PACKET interface (and thus can be read by libpcap)
This includes Prism2 cards running wlan-ng 0.1.14, Orinoco cards with
the orinoco-0.11b-packet patch installed, and Cisco cards.

Assuming you have one of these, and a linux installation, you must have
the following available to build AirSnort:

   * gtk-1.2
   * libpcap 0.7.1 or later. You should patch the stock 0.7.1 to 
     understand the DLT_PRISM_HEADER datalink type OR get one of 
     the "current builds" which incorporate the patch.
   * For Orinoco cards, pcmcia-cs-3.1.34 or kernel orinoco_cs
     drivers v0.11b (so that you can patch it).  The patch is
     available at http://airsnort.shmoo.com/orinocoinfo.html
   * For Prism2 cards, pcmcia-cs-3.1.33 and wlan-ng-0.1.14
   * In order to build pcmcia-cs or wlan-ng, you will need the 
     kernel sources

See the ChangeLog for the latest changes in AirSnort.

All of these are linked from our website (http://airsnort.shmoo.com)

To start, you must configure the kernel source code. This will link
the include directories in the the source tree to the system wide
directory, /usr/include. The kernel source code need not be compiled
or installed, but it must be the same version as the one running. The
kernel version can be determined with the command 'uname -a.' In
addition, the running kernel needs to support loadable modules, PCMCIA,
and PF_PACKET. If the running kernel does not support these, you will
need to compile and install the source code.

The source code for PCMCIA must be installed, and it must be the same
as the version you are running. To determine the version of PCMCIA CS
you are running, consult the output of the command 'cardmgr -V.' If you
have the appropriate version, you need not compile and install the
source code, but you must complete the configuration process. All of
the defaults are okay.  If you plan to use an Orinoco card, you must
patch the pcmcia-cs sources with an appropriate Orinoco monitor mode
patch from the link mentioned above.

The linux-wlan-ng is the kernel module to drive prism2 based wlan cards.
If you are using a prism2 based card, you must make and install the 
linux-wlan-ng driver. You must [re]start the cardmgr, and you can then
insert your wlan card. If anything failed, consult the linux-wlan-ng
documentation.

Install libpcap. Get the tarball from www.tcpdump.org, build and 
install the pcap library.  IMPORTANT NOTE: make sure that you do not
have an old version of libpcap installed that gets located instead.
This may be particularly true for RedHat users that had pcap installed
via rpm.  Make sure you uninstall the old version of pcap, or things 
won't work.

Assuming everything went well, AirSnort can now be built. This makes
the executables 'airsnort', 'gencases', and 'decrypt. If everything 
went well, the section "Running AirSnort" is right for you. If anything
failed, (predictably,) consult the appropriate package's documentation.

- Running AirSnort

The first thing to be done to crack 802.11b is to get unmodified
encrypted packets. This can be done by putting the card in a mode
which gathers all packets indiscriminately. This mode is known as
monitor mode, and it is entered automatically by selecting the
'Start' button.  Choose between "scan" mode and fixed "channel" mode
to scan all 11 802.11b channels or a specific channel respectively. 

With a capture in progress, packets are collected and saved in various
data structures to facilitate cracking.  Packets are not saved to a 
file unless you choose to do so (via the File/Save menu).  You can 
also load the data structures from a previous capture session that did
not result in a successful crack, in order to resume packet collection.
In this version of Airsnort, cracking is attempted in parallel with
packet capture.  In this implementation, the cracker attempts to crack
the captured packets for both a 40 bit and 128 bit key each time it
is instructed to do so by the capture thread.  This happens each time
10 interesting packets are captured.  The breadth of the search used
by the cracker can be controlled via the Settings/Preferences menu
option and defaults to 3 for 40 bit cracks and 2 for 128 bit cracks.

The number of interesting packets needed to perform a successful crack
depends on two things; luck and key length. Assuming that luck is on
your side, the key length is the only important factor. For a key
length of 128 bits, this translates to about 1500 packets. For other
key lengths, assume 115 packets per byte of the key.

For a discussion of the algorithm, and how it affects runtime,
and statistics, see the section "AirSnort Theory."

In any case, if the crack thread believes it has a correct password,
it checks the checksum of a random packet. If this is successful, the
correct password printed in ASCII and Hex, and the successful crack is
indicated by an 'X' in the leftmost column of the display. A successful 
crack causes packet collection for the associated SSID to cease. 

- Capture Details

Capture uses either the pcap library to receive monitor mode packets.
The packets go through two filters. First, non-encrypted packets are
filtered out. Then, if they are encrypted, useless packets are
discarded. All non-data packets are discarded with the exception of
802.11b Beacon and probe response packets which are examined in order
to obtain access point SSID data.

To distinguish encrypted and non-encrypted packets, capture examines
the first two bytes of the output. Since unencrypted packets have a first
pair value of 0xAAAA, all of these packets get dropped. These bytes
comes from the Snap header.

Useful packets are those with the following property of their IV; the
first byte is a number three greater than one of the offsets of the
bytes of the key. For 128 bit encryption, this means a number from
3-16. The second byte must be 255 and the third byte can have any
value. This means that for every byte of the key, there are 256 weak
IVs.  As of version 0.2.0, two additional classes of IVs are 
recognized by airsnort.  See the source code for details.

When every weak IV has been gathered (13 key bytes * 256 = 3315
packets), there is no point to continuing the capture process. In
reality, it takes somewhat fewer packets than this. As of version
0.2.0, there are on the order of 9400 resolving IVs recognized by
airsnort.

- Cracking Details

When executing the cracking operation, crack operates with a partial
key search from the given data. Since it is a probabalistic attack, 
The best guess may not be the right one, so, with limited captured 
data and enough CPU power, you can perform more exaustive searches. 
By setting the breadth parameter, you can specify to search "worse" 
guesses. It is not suggested that you specify a breadth of more than 
three or four.

See the paper linked on our website for a reference to the paper
describing the attack.