Documentation / networking / mac80211-injection.rst


Based on kernel version 5.14. Page generated on 2021-08-31 10:40 EST.

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.. SPDX-License-Identifier: GPL-2.0

=========================================
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.rst.

Despite many radiotap parameters being 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_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.
   =========================  ===========================================

 * IEEE80211_RADIOTAP_TX_FLAGS

   =============================  ========================================
   IEEE80211_RADIOTAP_F_TX_NOACK  frame should be sent without waiting for
				  an ACK even if it is a unicast frame
   =============================  ========================================

 * IEEE80211_RADIOTAP_RATE

   legacy rate for the transmission (only for devices without own rate control)

 * IEEE80211_RADIOTAP_MCS

   HT rate for the transmission (only for devices without own rate control).
   Also some flags are parsed

   ============================  ========================
   IEEE80211_RADIOTAP_MCS_SGI    use short guard interval
   IEEE80211_RADIOTAP_MCS_BW_40  send in HT40 mode
   ============================  ========================

 * IEEE80211_RADIOTAP_DATA_RETRIES

   number of retries when either IEEE80211_RADIOTAP_RATE or
   IEEE80211_RADIOTAP_MCS was used

 * IEEE80211_RADIOTAP_VHT

   VHT mcs and number of streams used in the transmission (only for devices
   without own rate control). Also other fields are parsed

   flags field
	IEEE80211_RADIOTAP_VHT_FLAG_SGI: use short guard interval

   bandwidth field
	* 1: send using 40MHz channel width
	* 4: send using 80MHz channel width
	* 11: send using 160MHz channel width

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 some 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 a link to a complete inject application here:

https://wireless.wiki.kernel.org/en/users/Documentation/packetspammer

Andy Green <andy@warmcat.com>