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Hack 57 Getting Started with Host AP

Use a Prism II radio card with Linux as if it
were a hardware access point.

The Host AP driver will allow a
Prism-based radio card to operate as a BSS Master or Slave, and can
also do IBSS mode. PCMCIA, PCI, and mini-PCI radios are all
supported. It is highly recommended that you use a 2.4 Linux or later
kernel if you use Host AP. To get started with Host AP, first
download the driver at http://hostap.epitest.fi/. If you need to run
a Hermes-based card (such as the Lucent/Orinoco/Avaya/Proxim), take a
look at [Hack #61].

Once the driver is unpacked,
you simply run a make with the name of the
driver you want to build: make pccard builds the
PCMCIA
driver, make plx builds the non-PCMCIA
(plx-based) PCI PC Card driver, and
make pci builds the standard
PCI
driver. The hardware-independent driver code is automatically built
regardless of the driver you choose. It doesn't hurt
to build all of the drivers, unless space is a critical consideration
on your system. To install the drivers, run make
install_pccard, make install_plx, or
make install_pci respectively.

PCMCIA

If you are installing the PCMCIA
driver, the make process automatically copies
hostap_cs.conf to your
/etc/pcmcia/ directory, so that your cards will
be properly detected when they are inserted. It
doesn't hurt to stop and start PCMCIA services once
you have installed the Host AP driver. Once installed, the wireless
device will be called wlan0 (and the second is
called wlan1, etc.).

Setting up radio parameters is very straightforward. If you are using
the PCMCIA driver, all of the wireless parameters are set in
/etc/pcmcia/wireless.opts.

Here's an example wireless.opts
for BSS Master (i.e., Host AP) mode:

#
# wireless.opts
#
case "$ADDRESS" in
*,*,*,*)
INFO="A card in Host AP Master mode"
ESSID="NoCat"
MODE="Master"
CHANNEL="6"
RATE="auto"
;;
esac

You may be thinking, "My God, it's
full of stars...." But if you have ever worked with
network.opts, the syntax is exactly the same. If
you haven't, those asterisks allow for tremendous
flexibility.

The script is passed a string in $ADDRESS that
gives details about the card that was inserted, so you can have
different entries for different cards. The address matching syntax
is:

scheme, socket, instance, MAC address)

The scheme allows for setting up as many arbitrary
profiles as you like. The most common use for schemes is on a client
laptop, where you may have different network settings for your office
wireless network than for your home network. You can display the
current scheme by issuing the
cardctl scheme
command as root, and change it by using a command like
cardctl scheme home or
cardctl scheme office. Both
wireless.opts and
network.opts are
scheme-aware, allowing you to change your network and wireless
settings quickly with a single command.

The second parameter, socket, is the socket number
that the PCMCIA card was inserted into. Usually, they start with 0
and go up to the number of PCMCIA slots you have available. To find
out which is which, insert a card in one slot and issue the
cardctl status command.

The third parameter, instance, is used for exotic
network cards that have more than one interface. I
haven't come across one of these, but if you have a
network card that has more than one network device in it, use this to
set different parameters for each device, starting with 0.

I find the last parameter very useful, as you can match the setting
to a specific MAC address. You can even include wildcards, to match a
partial MAC address, like this:

*,*,*,00:02:6F:*)

This would match a Senao/EnGenius card inserted in any slot, in any
scheme. Keep in mind that the wireless.opts is
only called to set radio parameters. Network settings (such as IP
address, default gateway, and whether to use DHCP) are set in
network.opts, just like any other PCMCIA network
device.

One further caveat when using the
PCMCIA Host AP driver: some machines
(notably the Stylistic 1000) have a problem loading the Host AP
driver with some Prism II cards. The symptom is that the card is
detected on insert, but mysteriously fails to initialize, reporting
an obscure error to the effect of "GetNextTuple: No
more items". If you are having trouble with the
driver, try adding this line to your hostap_cs.conf
(replacing any existing module
"hostap_cs" line):

module "hostap_cs" opts "ignore_cis_vcc=1"

Normally, the driver attempts to verify that one entry on the
card's acceptable voltage table matches the voltage
reported by your PCMCIA slot. In some cases, this voltage can be
incorrectly reported, causing the driver to fail to initialize. This
option causes the driver to ignore the reported voltage and load
anyway. It works like a charm on my Stylistic 1000 with a Senao 200mW
card.

PCI

Configuration of a
PCI or mini-PCI Prism II card is
much the same as any other network device. At some stage, you need to
configure the radio parameters manually with calls to
iwconfig. This is best done in the boot-up process
after the kernel modules are all loaded, but before your device
configures its IP stack. Simply create an initialization script with
a line like this:

iwconfig wlan0 essid "NoCat" mode "Master" channel 6 rate "Auto"

Copy this script to
/etc/init.d/, and symlink it to an appropriate
place in /etc/rc2.d/, or wherever the
appropriate place is for your default run level. Unfortunately, every
Linux distribution has a slightly different mechanism for processing
the boot up rc
scripts, so if in doubt, consult your distribution's
documentation.

Once your radio parameters are configured, treat the wlan0 device as
if it were any other Ethernet interface. Give it an IP address, set
up your routing, and bind processes to its IP as you normally would.
The Host AP driver takes care of all of the details of managing your
wireless clients for you. If you ever need to change the radio
parameters of the card (whether using
PCMCIA or PCI), you can always
change parameters and view the current status with
iwconfig:

root@pebble:~# iwconfig wlan0
wlan0     IEEE 802.11b  ESSID:"NoCat"  
Mode:Master  Frequency:2.437GHz  Access Point: 00:02:6F:01:85:74  
Bit Rate:11Mb/s   Tx-Power=24 dBm   Sensitivity=1/3  
Retry min limit:8   RTS thr:off   Fragment thr:off
Encryption key:off
Power Management:off
Link Quality:0  Signal level:0  Noise level:0
Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
Tx excessive retries:0  Invalid misc:0   Missed beacon:0

That is all you need to do to bring up a simple Host AP
configuration. If you are curious about how to tweak your Host AP to
perform more elaborate functions, or about how to monitor its status
and that of all of your connected clients, see the other hacks in
this section.