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Ath5k is a new, completely FOSS driver for Atheros cards. Wireless-N-enabled Atheros chipsets should use ath9k instead. Unlike madwifi-ng, this driver is based on the new mac80211 stack, meaning that it requires aircrack-ng v1.0-rc1 or newer.
As of 2.6.27-rc2, ath5k can inject without patches (2.6.26 not tested, ⇐2.6.25 is known to be broken). However, injection support is a bit flakey, and speeds are limited to about 50~100pps. To improve injection speed, apply this patch: http://patches.aircrack-ng.org/ath5k-injection-2.6.27-rc2.patch It is recommended that you also apply the mac80211 fragmentation patch for your kernel version, to enable the fragmentation attack. For the released 2.6.27 and 2.6.28 kernels, the mac80211 patch suffices, no extra patch is needed.
If you have a kernel older than 2.6.26, it's recommended that you install the latest compat-wireless-old package, and apply the 2.6.27 mac80211 fragmentation patch on the package itself. No ath5k-specific patch is needed for either flavor of compat-wireless.
Note that in the latest wireless-testing & linux-2.6 kernels, as well as in compat-wireless-2.6 (but not in compat-wireless-old), there is a bug that prevents the fragmentation attack from working properly when no managed interface is brought up on the same PHY as the monitor interface. As a workaround, bring up wlan0 after creating mon0 with airmon-ng.