Linux WireLess

hostapd Linux documentation page

About hostapd

Homepage: http://w1.fi/hostapd/

hostapd is an IEEE 802.11 AP and IEEE 802.1X/WPA/WPA2/EAP/RADIUS Authenticator. This page is dedicated to the Linux documentation of it's implementation and use. Please refer to the hostapd home page for information for other Operating Systems.

As far a Linux is concerned, out of the old drivers you can only use these 3 drivers with hostapd:

• HostAP
• madwifi
• prism54

All new mac80211 based drivers that implement AP functionality are supported with hostapd's nl80211 driver.

The mac80211 subsystem moves all aspects of master mode into user space. It depends on hostapd to handle authenticating clients, setting encryption keys, establishing key rotation policy, and other aspects of the wireless infrastructure. Due to this, the old method of issuing 'iwconfig mode master' no longer works. Userspace programs like hostapd now use netlink (the nl80211 driver) to create a master mode interface for your traffic and a monitor mode interface for receiving and transmitting management frames.

Getting hostapd

Using your distributions hostapd

It is advisable to try your distributions version of hostapd before taking the time to compile and install your own copy. This will make future maintenance easier as you'll be able to use the init scripts shipped by the distro and hostapd will be updated by it as well. If your distribution ships 0.6.8 or later, you can test with this bare minimum config by creating the file hostapd-minimal.conf:

#change wlan0 to your wireless device
interface=wlan0
driver=nl80211
ssid=test
channel=1

If that config errors out with something like:

hostapd $ sudo hostapd ./hostapd-minimal.conf
Configuration file: ./hostapd-minimal.conf
Line 2: invalid/unknown driver 'nl80211'
1 errors found in configuration file './hostapd-minimal.conf'

that means that your distro is not shipping hostapd with nl80211 driver support and you'll need to follow the building instructions that follow. If it works, you can skip down to the configuring hostapd section. If not, continue on.

Download and compile hostapd

Using hostapd with nl80211 requires you to have at least libnl-1.0 pre8 as this release introduced genl, Generic Netlink, which nl80211 relies on. Most distributions are shipping this or a later release by now. To compile on fedora or other distibutions that separate out the headers from the binaries, you need the libnl-devel package.

Throughout this section, versions will be referred to by: x.y.z

ex: hostapd-0.6.8.tar.gz would be referred to as hostapd-x.y.z.tar.gz

You can get the latest development version of hostapd from the git repository with:

git clone git://w1.fi/srv/git/hostap.git
cd hostap/hostapd

Or you can get a stable release (0.6.8 or later reccomended) by downloading the tarball from http://w1.fi/hostapd/.

wget http://w1.fi/releases/hostapd-x.y.z.tar.gz
tar xzvf hostapd-x.y.z.tar.gz
cd hostapd-x.y.z/hostapd

Next, we need to configure the hostapd build to enable nl80211 driver support. Copy defconfig to .config, and open it in your preferred text editor. Also, there are other options that you may want to enable, like 802.11n support if your hardware can do it. Most of the other encryption types and features aren't needed for most applications, so if you're questioning if you need to enable it, you probably don't need to.

cp defconfig .config
vi .config

Now find this line:

#CONFIG_DRIVER_NL80211=y

and uncomment it by removing the '#' sign. Repeat for other settings that you may be interested in. The basic configuration, with only this line uncommented is enough to get hostapd up and running with WPA/WPA2 authentication and encryption.

Next, compile hostapd:

make

if this fails with errors like:

driver_nl80211.c:21:31: warning: netlink/genl/genl.h: No such file or directory
driver_nl80211.c:22:33: warning: netlink/genl/family.h: No such file or directory
driver_nl80211.c:23:31: warning: netlink/genl/ctrl.h: No such file or directory
driver_nl80211.c:24:25: warning: netlink/msg.h: No such file or directory
driver_nl80211.c:25:26: warning: netlink/attr.h: No such file or directory

you need to install/update libnl-1.0pre8 (or later). If all goes well and the compilation finishes, try the minimal hostapd again, see the section Using your distributions hostapd above for that.

hostapd # ./hostapd ./hostapd-minimal.conf
Configuration file: ./hostapd-minimal.conf
Using interface wlan1 with hwaddr 00:0d:0b:cf:04:40 and ssid 'test'

If that starts as the example here shows, you can move on to configuring hostapd. If it fails to start with errors about the driver not being found, review the steps listed above for compiling hostapd again. If it gets the error messages:

Hardware does not support configured mode
wlan0: IEEE 802.11 Hardware does not support configured mode (2)
Could not select hw_mode and channel. (-2)
wlan0: Unable to setup interface.
rmdir[ctrl_interface]: No such file or directory

then it means the hw_mode (a, b or g) in the config file is set to a value not supported by the hardware.

Configuring hostapd

Establishing Baseline for Configuration

Before configuring hostapd, you need to know the capabilities of the clients that will be using it. Not all clients will support all of the methods you may want to implement, so a baseline configuration needs to be established. You will also want to do a survey of your area to find the channel that has the fewest other APs on it. When choosing which channel to use, it is important to remember that the channels overlap with any channels that are within 20MHz.

Examples of the baseline you might establish:

Encryption: wpa-psk + tkip
Wireless Mode: g
Normal for an environment that has to support semi legacy devices, that don't support ccmp or wpa2

Encryption: wpa2-psk + ccmp
Wireless Mode: g+n
Normal for an environment that has only up to date hardware and software

Encryption: wep
Wireless Mode: b
This is the works case scenario, as wep is broken and can be trivially cracked.  Don't consider this as anything more than keeping casual free loaders out.

Once you've found your baseline, it's time to edit hostapd.conf. The configuration options will be broken into 3 sections:

Common Options: options that you will probably want to set
Additional Options: options that are likely useful to at least know you have
Extra Options: options that you aren't likely to need for most setups

Common Options

The most basic set of options for using hostapd with the nl80211 driver have already been provided as the hostapd-minimal.conf. That is all you need if you don't care about consistantly being on the same channel, don't need/want encryption, and don't need a flashy name. However, that is not a realistic idea in the real world.

First, we'll setup the wireless interface settings, then the wireless environment settings, and finally the authentication and encryption.

Wireless Interface

Setting Summary:

• interface: Tells hostapd what wireless interface to use
• bridge: Set to a bridge if the wireless interface in use is part of a network bridge interface
• driver: For our purposes, always nl80211

If you only have 1 wireless interface, and it's going to be bridged with a wired interface, a good example setup would be:

interface=wlan0
bridge=br0
driver=nl80211

Wireless Environment

Setting Summary:

• ssid: Sets the name (SSID = service set identifier) of the network, wireless extensions/iwconfig incorrectly calls this "essid".
• hw_mode: Sets the operating mode of the interface, and the allowed channels. Valid values depend on hardware, but are always a subset of a, b, g
• channel: Sets the channel for hostapd to operate on. Must be a channel supported by the mode set in hw_mode, as well as allowed by your countries Wireless Regulatory rules.

The ssid is just for ease of configuration. It is what shows up in scan results, and can help in configuring your clients. Check the scan results for your area and choose a name.

hw_mode needs to be something that all of your hardware supports. Setting this to 'g' is probably the most common setup, and also enables backwards compatability with 802.11b devices. Note, this is not where you enable 802.11n support, as 802.11n operates on top of 802.11a or 802.11g's functionality.

channel should be chosen so that it has the minimum overlap with other APs or other networks in your area. 802.11 channels are 20mhz (4 channels) wide in total, or 10mhz (2 channels) wide on each side. This means that an access point on channel 3 will interfere with an acces point on channel 1 or channel 5. Use this to pick a channel. Most consumer APs default to channel 6, so you can use channel 1 or channel 11 in most cases for the best results. Also note that the channels available to you depends heavily entirerly on the local regulatory rules.

An example of a good normal setup is:

ssid=MyNetwork
hw_mode=g
channel=1

802.11n Setting Summary

802.11n builds on the settings above, and adds additional functionality. If your hardware doesn't support 802.11n, or you don't plan on using it, you can ignore these.

• ieee80211n: Set to 1 to enable 802.11n support, 0 to disable it
• ht_capab: A list of the 802.11n features supported by your device

The explanation of these settings in the sample config file are quite helpful, so I'll suggest reading those. You can use the command 'iw list' to find a short list of the capabilities of your device.

Example settings:

wme_enabled=1
ieee80211n=1
ht_capab=[HT40+][SHORT-GI-40][DSSS_CCK-40]

Authentication and Encryption

There is a lot to the authentication and encryption options in hostapd. This section will cover the basics as far as wep/wpa/wpa2 goes, as well as some of the other commonly used options.

Settings Summary:

• macaddr_acl: This controls mac address filtering. Mac addresses are easily spoofed, so only consider the use of this to be augmenting other security measures you have in place.
• auth_algs: This is a bit field where the first bit (1) is for open auth, the second bit (2) is for Shared key auth (wep) and both (3) is both.
• ignore_broadcast_ssid: This enables/disables broadcasting the ssid.
• wpa: This is a bitfield like auth_algs. The first bit enables wpa1 (1), the second bit enables wpa2 (2), and bothe enables both (3)
• wpa_psk/wpa_passphrase: These establish what the pre-shared key will be for wpa authentication.
• wpa_key_mgmt: This controls what key management algorithms a client can authenticate with.
• wpa_pairwise: This controls wpa's data encryption
• rsn_pairwise: This controls wpa2's data encryption

First, scratch macaddr_acl and ignore_broadcast_ssid from your priorities as they only enhance security (and even then, only slightly). Also, WEP has been effectively broken now, so unless you HAVE to support wep, scratch that from your list. This just leaves wpa/wpa2. Per the draft standard, wpa2 is required for 802.11n, and as there are known attacks on wpa now, wpa2 is the recommended authentication and encryption suite to use. Fortunately, you can have both enabled at once. If Windows clients are going to be connecting, you should leave ccmp encryption out of the wpa_pairwise option, as some windows drivers have problems with systems that enable it.

A good starting point for a wpa & wpa2 enabled access point is:

macaddr_acl=0
auth_algs=1
ignore_broadcast_ssid=0
wpa=3
wpa_passphrase=YourPassPhrase
wpa_key_mgmt=WPA-PSK
wpa_pairwise=TKIP
rsn_pairwise=CCMP

If, alternately, you just want to support wpa2, you could use something like:

macaddr_acl=0
auth_algs=1
ignore_broadcast_ssid=0
wpa=2
wpa_passphrase=YourPassPhrase
wpa_key_mgmt=WPA-PSK
wpa_pairwise=TKIP
rsn_pairwise=CCMP

That should be all of the settings that you'll need to change for a basic, secure, access point using hostapd with an AP enabled mac80211 driver.

Additional Options

Extra Options

Dynamic VLAN tagging

hostapd can be configured to move STAs into separate VLANs based on RADIUS tunnel attributes (as specified in RFC3580, http://tools.ietf.org/html/rfc3580#section-3.31):

Tunnel-Type=VLAN (13)
Tunnel-Medium-Type=802
Tunnel-Private-Group-ID=VLANID

To enable dynamic VLAN tagging the following options in hostapd.conf need to be set:

dynamic_vlan=1
vlan_file=/etc/hostapd.vlan

A value of 0 disables dynamic VLAN tagging, a value of 1 allows dynamic VLAN tagging and a value of 2 will reject the authentication if the RADIUS server does not provide the appropriate tunnel attributes.

Furthermore, hostapd needs to know how the VLAN interfaces should be named, this is done through an additional config file as specified in vlan_file.

Example /etc/hostapd.vlan:

1       wlan0.1
*       wlan0.#

This will create a wlan0.1 interface on top of wlan0 and move all STAs with the RADIUS supplied vlantag 1 to that interface. The second entry is used to dynamically create VLAN interfaces on top of wlan0, hostapd will create an interface wlan0.vlantag for each different vlantag as supplied by the RADIUS server. For example, if a STA associates and the RADIUS server attributes contain the vlantag 100 hostapd will create a wlan0.100 interface and map the STA to this new interface. 

Read more