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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.
If you have a kernel older than 2.6.26, it's recommended that you install the latest compat-wireless-2.6-old package, and apply the above patch (as well as the mac80211 fragmentation patch) on the package itself.