Difference between revisions of "Software access point"

Revision as of 21:08, 1 November 2013

Template:Article summary start Template:Article summary text Template:Article summary heading Template:Article summary wiki Template:Article summary wiki Template:Article summary wiki Template:Article summary wiki Template:Article summary end A software access point is used when you want your computer to act as an wifi access point for the local wireless network. It saves you the trouble of getting a separate wireless router.

Requirements

• A nl80211 compatible wireless device (e.g. ath9k)

Overview

Setting up an access point comprises two main parts:

• Setting up the wifi link layer, so that wireless clients can associate to your computer's "software access point" and send/receive IP packets from/to your computer; this is what the hostapd package will do for you.
• Setting up the network configuration on you computer, so that your computer will properly relay IP packets from/to its own Internet connection from/to wireless clients.

Wifi Link Layer

The actual Wifi link is established via the hostapd package (available in the official repositories). The package has WPA2 support.

Adjust the options in hostapd configuration file if necessary. Especially, change the ssid and the wpa_passphrase.

/etc/hostapd/hostapd.conf

ssid=YourWifiName
wpa_passphrase=Somepassphrase
interface=wlan0
bridge=br0
auth_algs=3
channel=7
driver=nl80211
hw_mode=g
logger_stdout=-1
logger_stdout_level=2
max_num_sta=5
rsn_pairwise=CCMP
wpa=2
wpa_key_mgmt=WPA-PSK
wpa_pairwise=TKIP CCMP

For automatically starting hostapd, enable the hostapd.service.

Network configuration

There are two basic ways for implementing this:

1. bridge: create a network bridge on your computer (wireless clients will appear to access the same network interface and the same subnet that's used by your computer)
2. NAT: with IP forwarding/masquerading and DHCP service (wireless clients will use a dedicated subnet, data from/to that subnet is NAT-ted -- similar to a normal WiFi router that's connected to your DSL or cable modem)

The bridge approach is simpler, but it requires that any service that's needed by your wireless clients (like, DHCP) is available on your computers external interface. That means it will not work if you have a dialup connection (e.g., via PPPoE or a 3G modem) or if you're using a cable modem that will supply exactly one IP address to you via DHCP.

The NAT aproach is more versatile, as it clearly separates wifi clients from your computer and it's completely transparent to the outside world. It will work with any kind of network connection, and (if needed) you can introduce traffic policies using the usual iptables approach.

Of course, it is possible to combine both things. For that, studying both articles would be necessary. Example: Like having a bridge that contains both an ethernet device and the wireless device with an static ip, offering DHCP and setting NAT configured to relay the traffic to an additional network device - that can be ppp or eth.

Bridge Setup

You need to create a network bridge and add your network interface (e.g. eth0) to it. You should not add the wireless device (e.g. wlan0) to the bridge; hostapd will add it on its own.

If you use netctl, see Bridge with netctl for details (just do not add tap0 used in that example).

NAT Setup

See Internet Sharing for details.

On that article, the device connected to the lan is net0. That device would be in this case your wireless device (e.g. wlan0).

Scripts

create_ap script combines hostapd, dnsmasq and iptables to create a NATed Access Point.

Troubleshooting

WLAN is very slow

This could be caused by low entropy. Consider installing haveged.

NetworkManager is interfering

hostapd may not work, if the device is managed by NetworkManager. You can mask the device:

/etc/NetworkManager/NetworkManager.conf

[keyfile]
unmanaged-devices=mac:<hwaddr>

See also

• hostapd Linux documentation page
• Router
• Hostapd : The Linux Way to create Virtual Wifi Access Point
• tutorial and script for configuring a subnet with DHCP and DNS