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When I try to activate wlan0 in CentOS5.5, I get this:

Error for wireless request "Set Mode" (8B06) :
    SET failed on device wlan0 ; Invalid argument.

Determining IP information for wlan0...Internal network device wlan0 is not ready.

EDIT: I have the correct driver now, but it's still not working.

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looks like a driver problem to me....or you have it active in some other mode so it can't set the mode...or you are trying to place it in a mode that isn't supported(monitor/promisc or AP). need more information – RobotHumans Nov 14 '10 at 16:35
All i know is that I'm trying to activate it in system-config-network – Rob Nov 14 '10 at 16:37
try using sudo... – RobotHumans Nov 14 '10 at 16:42
sudo system-config-network – RobotHumans Nov 14 '10 at 16:44
I did. That's how I accessed it. – Rob Nov 14 '10 at 16:47

Try modprobe ath5k, the drivers for your card should be in that module.
It has been part of the Linux kernel since 2.6.25 and Red Hat probably backported it to 2.6.18.

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Done, but I get the same problem still – Rob Nov 26 '10 at 16:13

One possible solution, although coming from Fedora, from this article:
Error for wireless request "Set Mode" (8B06) : SET failed on device wlan0:

The problem you are facing is that there are two methods of controlling a wireless card in Fedora. You are using the one called "Network Configuration". The preferred method of controlling wireless cards is "NetworkManager" which is a different program.

If NetworkManager is running, it automatically stops Network Configuration controlling your card and you get the error you have found.

If you have an entry for your card in Network Configuration where you are now on the "Devices" tab, highlight it and click "Edit". In the new window make sure that "Activate device when computer starts" is NOT ticked. If it is there then "Controlled by NetworkManager" should be ticked. That last will only be there if your Fedora is relatively up to date.

Then look for NetworkManager on the top Gnome Panel in the notification area to the right. When you left click on it, it should present you with available wireless networks. Left click on one to be asked for any encryption key and subsequently a password to hide it behind.

Should NetworkManager not be running, go to the Gnome System Menu - Adminsitration - Services, start it and NetworkManager Dispatcher and save the configuration.

That got mine going.

Another possibility from Set Failed on device wlan0 :

After bringing wlan0 down it accepted the command “sudo iwconfig wlan0 mode ad-hoc” and then I brought it backup again with “sudo ifconfig wlan0 up”. So, if you get this error message, try bringing the interface down first and see if that works.

Since your card is the AR5001, it is known for having problems under Linux.

The article Atheros 802.11g PCMCIA card says :

The Atheros chip-set is supported by the madwifi driver, which is not a part of the standard Linux kernel, but it can be downloaded from

This article then describes how to download and install the driver.

Another useful article might be : Atheros AR5xxx devices (MadWifi), althogh coming from Debian it might be less relevant.

For installation on CentOS, see this : Setup MadWifi on Centos5.
If this doesn't work, maybe you can find a suitable release here : Index of /madwifi (found via google, have no idea who this website is).

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Neither solution was much help, but thanks for trying – Rob Nov 21 '10 at 19:43
What is your card? – harrymc Nov 21 '10 at 19:55
And does this help : – harrymc Nov 21 '10 at 19:58
Atheros Communications Inc. AR5001 Wireless Network Adapter (rev 01) – Rob Nov 21 '10 at 20:17
Found some more stuff that seems useful for the AR5001. – harrymc Nov 21 '10 at 21:19

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