Dear all. I'd successfully setup the wireless card for
Ap mode , and channel 1~11 is O.K.
But I found it difficultly to setup channel 13. how
should I setup on my hostapd.conf or on other file?

This could be a regulatory domain issue with the wireless driver. What make/model wireless card are you using?

dnxa-92

I'm not familiar with that card, but I believe it's Atheros-based.

I came across a thread on the Gentoo forums about regulatory domain issues with the Atheros driver, and it's been marked as solved. You could try making the same change in regd.c and see if it solves the problem.

I use this command and got nothing , why?

I can't find the file regd.c in my linux??

You'll need to download the kernel source. The fix as described in the forum thread involves editing a source code file for the Atheros driver and then recompile the kernel. It's not all that hard, but if you haven't done anything like that before, the learning curve may feel a bit steep.

Check which kernel you're currently running with uname -r, and either install the kernel source package for your distribution or download the unmodified source code from kernel.org. You should then be able to locate the regd.c file in the /usr/src/<kernel_version>/drivers/net/wireless/ath directory.

Do I need to compile all the kernel source code?

Theoretically, if the current Atheros driver is compiled as a module, you only have to recompile that particular module. In any case you could compile the entire kernel, but then just copy the module to the relevant /lib/modules/<kernel_version> subdirectory.

If the Atheros driver is compiled into the kernel (unlikely), you'll have to recompile the entire kernel, install it and reboot.

Do I need to modify the regd.c? For what content?

The second last post in the thread on the Gentoo forum suggests adding a #define statement. Earlier, the OP seems to have worked around his issue (which was related to the 5 GHz band) by extracting and using EEPROM code from the Windows driver.

You haven't stated exactly what error you got when you tried using channel 13, so it's hard to say for sure that it is indeed a regulatory domain issue, but the fact that channel 11 works is a strong indicator.

I suggest you try one or both of the methods used by the OP in that thread, and return here if you're still experiencing problems (preferably with details about what you've tried, the contents of your config files, and the exact error messages you're getting, if any.)