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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>