Setting up dual band concurrent AP with hostapd

Question

I have an MT7615 based wireless network adapter which apparently supports dual band concurrent mode. So how would I go about setting up two access points one on 2.4GHz frequency and another one on 5GHz using hostapd?

My current 2.4GHz hostapd config looks something like this:

ssid=XXX
interface=wlan0
hw_mode=g
channel=9
bridge=br0
driver=nl80211

logger_syslog=0
logger_syslog_level=0
wmm_enabled=1
wpa=2
preamble=1

wpa_psk=XXX
wpa_passphrase=XXX
wpa_key_mgmt=WPA-PSK
wpa_pairwise=TKIP
rsn_pairwise=CCMP
auth_algs=1
macaddr_acl=0

ieee80211n=1
ht_capab=[SHORT-GI-20][SHORT-GI-40][HT40+]
country_code=XX
ieee80211d=1

ctrl_interface=/var/run/hostapd
ctrl_interface_group=0

Answer 1

As far as I can tell, if your adapter really is dual band then it will have multiple radios in it, each of which will present as a different network interface (e.g. wlan0 and wlan1).

Most consumer devices aren't meant for use as access points, so they will only have one radio, which means only one network interface, and consequently can only run an AP on a single frequency (e.g. 2.4 GHz or 5 GHz but not both at the same time).

If yours does have multiple radios, you just run multiple versions of hostapd and specify a different config file for each. Each config file will be set up for a different frequency band. Use the interface= option in the config file to tell hostapd which radio you are setting up. For example in one file you might have interface=wlan0 and set a 2.4 GHz channel, and in a second file you might have interface=wlan1 and set a 5 GHz channel. You can use iw list to figure out which interfaces support which frequency bands.

If you connect to a wireless router running OpenWRT or similar, you'll see it has two or more network interfaces, one for each frequency band that can be used concurrently. In this case each interface will only support a single band (e.g. wlan0 only works on 2.4 GHz, wlan1 only works at 5 GHz, etc.)