Wireless AP with udhcpd and NAT

Setting up a wireless AP with udhcpd and NAT

(based largely on the raspberry pi wireless router howto)

Dependencies

Install the hostapd access point software, busybox-extras (for the udhcpd DHCP server), and the iptables firewall manager.

apk add hostapd busybox-extras iptables

If you want to connect clients to the internet, you need to provide some way of redirecting traffic from the AP to the internet. There are two main possibilities:

• A bridge
• Network Address Translation

If you use a bridge and get your IP via DHCP, you may have a hard time configuring it such that the bridge gets an IP address without screwing up your local internet connection.

This guide only covers NAT. See Bridge for more info.

Configure hostapd

You need to write a configuration file. Alpine ships with a sample file in /etc/hostapd/hostapd.conf, but it didn't work for me (possibly because I used a pre-wireless-N card, supported by ath5k?).

Here's a sample based on something that did work for me (I've changed ssid & wpa_passphrase):

ctrl_interface=/var/run/hostapd
ctrl_interface_group=0
interface=wlan0
driver=nl80211
logger_syslog=-1
logger_syslog_level=2
logger_stdout=-1
logger_stdout_level=2
ssid=alpine-test
hw_mode=g
channel=6
max_num_sta=32
rts_threshold=2347
fragm_threshold=2346
macaddr_acl=0
auth_algs=3
ignore_broadcast_ssid=0
wpa=2
wpa_passphrase=supertopsecret
wpa_key_mgmt=WPA-PSK WPA-PSK-SHA256
wpa_pairwise=TKIP CCMP

Change "interface" to match your wireless interface. Change "ssid" and "wpa_passphrase" as necessary. Set "wpa" to 3 if you want plain wpa and wpa2. or 1 for plain WPA1 only.

The example in the package uses wpa_psk_file (needed for WPS) instead of a static passphrase. That does not enable WPS.

You may want to change the channel to avoid collisions with other local APs. Unfortunately, the automatic channnel selection (channel=0) is *not* currently enabled at compile time, so we can't use it. Scan for channels

in use with

iwlist wlan0 scanning

or equivalent, before setup.

max_num_sta sets a limit on the number of clients that can connect to your AP. Set it higher than you think you might have, but not much higher.

If you don't put this in /etc/hostapd/hostapd.conf, you will need to change the CONFIGS line in /etc/conf.d/hostapd to point at it. I prefer doing that, so that the default is available for reference.

Configure udhcpd

Edit /etc/udhcpd.conf. The default is very well-commented, but not really ready to use. Here's a skeleton, loosely based on mine:

start		192.168.2.2
end		192.168.2.254
max_leases	64
interface	wlan0
static_lease	00:1b:de:ad:be:ef	192.168.2.100
opt	dns	192.168.0.1 8.8.8.8
opt	subnet	255.255.255.0
opt	router	192.168.2.1
opt	lease	864000

Note the following:

• max_leases should be set to at least as many clients as you might have in the lifetime of a lease. If you have any clients connecting via bridges, note that the bridge itself gets a DHCP address.
• interface is the interface clients will be connecting to (wlan0 or your wireless interface in this example)
• router should be the static IP address you assign to your wireless interface.
• start and end should be within the same subnet as the IP address you configure wlan0 with, but the address for wlan0 should be outside the range.

(e.g. 192.168.2.1 and 192.168.2.255 are both suitable for the router address)

• set the DNS option to point to any nameservers you want. You can repeat it, but the limit is 3 nameservers.
• static_lease takes two arguments: a MAC address designating a specific network adaptor, and the IP address that should be assigned to it.

It can be repeated multiple times, to assign different addresses to different users. This comes in handy for printers, if you can trust network users to not do MAC spoofing.

Configure iptables

I used raw iptables, configuring it thus:

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE iptables -A FORWARD -i eth0 -o wlan0 -m state --state RELATED,ESTABLISHED -j ACCEPT iptables -A FORWARD -i wlan0 -o eth0 -j ACCEPT

1. this saves the state in a location the service can restore it from

service iptables save

Test

/etc/init.d/hostapd start

/etc/init.d/udhcpd start

sysctl net.ipv4.ip_forward=1 and try connecting from another computer.

Make the changes permanent

rc-update add hostapd rc-update add udhcpd rc-update add sysctl

Configuring ifup

Now, the odd parts: iptables tries to set net.ipv4.ip_forward to 1 when it's started, but in my experience, this is not reliable. You do *not* want to enable the "iptables" service. It starts before networking, and may result in your wireless interface not getting configured. (Apparently, ifup thinks that wlan0 is up and skips it. This was not something I expected, but it's the only explanation I have for how things worked...)

Rather, modify /etc/network/interfaces, commenting out any configuration for your wireless interface. Then add:

auto wlan0
iface wlan0 inet static
	address 192.168.2.1
	netmask 255.255.255.0
	up /etc/init.d/iptables start
	up sysctl net.ipv4.ip_forward=1
	down /etc/init.d/iptables stop

(It's possible to set up everything so that hostapd and udhcpd get started and stopped from the wlan0 stanza. I didn't bother doing that.)

Finishing touches

(See Setting up a SSH server for alternatives and more information) Add dropbear SSH server, configure it to run on only the wireless interface:

setup-sshd -c dropbear

edit /etc/conf.d/dropbear to add

DROPBEAR_OPTS="-p 192.168.2.1:22"

(assuming the wireless interface has the IP address 192.168.2.1 and you want SSH on port 22). This is optional, but if you're using a wireless router it helps to be able to administer it, and listening on all addresses is rather risky.

Things this doesn't cover but it would be nice to

• Some way to get more entropy (see Entropy_and_randomness)
• DNS server, publishing device names (TinyDNS_Format looks most useful)
• use awall instead of raw iptables (and/or switch to nftables)
• Setup-acf to manage the router

This would require:

• • acf-core, acf-alpine-conf, acf-apk-tools
  • acf-iptables, or acf-awall + rewrite
  • acf-ssh + switch to openssh, or new acf-dropbear
  • acf-dhcp + switch to dhcp, or new acf-udhcpd
  • new acf-hostapd (probably hardest part!)
  • acf-tinydns after adding tinydns