How to use packet injection with mac80211
=========================================

mac80211 now allows arbitrary packets to be injected down any Monitor Mode
interface from userland.  The packet you inject needs to be composed in the
following format:

 [ radiotap header  ]
 [ ieee80211 header ]
 [ payload ]

The radiotap format is discussed in
./Documentation/networking/radiotap-headers.txt.

Despite 13 radiotap argument types are currently defined, most only make sense
to appear on received packets.  The following information is parsed from the
radiotap headers and used to control injection:

 * IEEE80211_RADIOTAP_RATE

   rate in 500kbps units, automatic if invalid or not present


 * IEEE80211_RADIOTAP_ANTENNA

   antenna to use, automatic if not present


 * IEEE80211_RADIOTAP_DBM_TX_POWER

   transmit power in dBm, automatic if not present


 * IEEE80211_RADIOTAP_FLAGS

   IEEE80211_RADIOTAP_F_FCS: FCS will be removed and recalculated
   IEEE80211_RADIOTAP_F_WEP: frame will be encrypted if key available
   IEEE80211_RADIOTAP_F_FRAG: frame will be fragmented if longer than the
			      current fragmentation threshold. Note that
			      this flag is only reliable when software
			      fragmentation is enabled)

The injection code can also skip all other currently defined radiotap fields
facilitating replay of captured radiotap headers directly.

Here is an example valid radiotap header defining these three parameters

	0x00, 0x00, // <-- radiotap version
	0x0b, 0x00, // <- radiotap header length
	0x04, 0x0c, 0x00, 0x00, // <-- bitmap
	0x6c, // <-- rate
	0x0c, //<-- tx power
	0x01 //<-- antenna

The ieee80211 header follows immediately afterwards, looking for example like
this:

	0x08, 0x01, 0x00, 0x00,
	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
	0x13, 0x22, 0x33, 0x44, 0x55, 0x66,
	0x13, 0x22, 0x33, 0x44, 0x55, 0x66,
	0x10, 0x86

Then lastly there is the payload.

After composing the packet contents, it is sent by send()-ing it to a logical
mac80211 interface that is in Monitor mode.  Libpcap can also be used,
(which is easier than doing the work to bind the socket to the right
interface), along the following lines:

	ppcap = pcap_open_live(szInterfaceName, 800, 1, 20, szErrbuf);
...
	r = pcap_inject(ppcap, u8aSendBuffer, nLength);

You can also find sources for a complete inject test applet here:

http://penumbra.warmcat.com/_twk/tiki-index.php?page=packetspammer

Andy Green <andy@warmcat.com>