Wireless AP with udhcpd and NAT: Difference between revisions
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Setting up a wireless AP with udhcpd and NAT | Setting up a wireless AP with udhcpd and NAT | ||
(based largely on the [ | (based largely on the [https://elinux.org/RPI-Wireless-Hotspot raspberry pi wireless router howto]) | ||
= Dependencies = | = Dependencies = | ||
Line 12: | Line 12: | ||
If you want to connect clients to the internet, you need to provide some way | If you want to connect clients to the internet, you need to provide some way | ||
of redirecting traffic from the AP to | of redirecting traffic from the AP to the internet. | ||
There are two main possibilities: | 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 | If you use a bridge and get your IP via DHCP, you may have a hard time | ||
configuring it | configuring it such that the bridge gets an IP address without screwing up your | ||
local internet connection. | local internet connection. | ||
This guide only covers NAT | |||
This guide only covers NAT. See [[Bridge]] for more info. | |||
= Configure hostapd = | = Configure hostapd = | ||
You need to write a configuration file | 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 | /etc/hostapd/hostapd.conf, but it didn't work for me (possibly because I | ||
used a pre-wireless-N card, supported by ath5k?). | used a pre-wireless-N card, supported by ath5k?). | ||
Here's a sample | Here's a sample based on something that did work for me | ||
(I've changed ssid & wpa_passphrase): | (I've changed ssid & wpa_passphrase): | ||
ctrl_interface=/var/run/hostapd | ctrl_interface=/var/run/hostapd | ||
Line 52: | Line 53: | ||
Change "interface" to match your wireless interface. | Change "interface" to match your wireless interface. | ||
Change "ssid" and "wpa_passphrase" | Change "ssid" and "wpa_passphrase" as necessary. | ||
Set "wpa" to 3 if you want plain wpa and wpa2. or 1 for plain WPA1 only. | 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 | The example in the package uses wpa_psk_file (needed for WPS) instead of a | ||
static passphrase | static passphrase. That does not enable WPS. | ||
You may want to change the channel to avoid | You may want to change the channel to avoid collisions with other local APs. | ||
Unfortunately, the automatic channnel selection (channel=0) is *not* | Unfortunately, the automatic channnel selection (channel=0) is *not* | ||
currently enabled at compile time, so we can't use it | currently enabled at compile time, so we can't use it. Scan for channels | ||
in use with {{Cmd|iwlist wlan0 scanning}} or equivalent before setup. | in use with {{Cmd|iwlist wlan0 scanning}} or equivalent, before setup. | ||
max_num_sta | max_num_sta sets a limit on the number of clients that can connect to your AP. | ||
Set it higher than you think you | 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 | 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. | the CONFIGS line in /etc/conf.d/hostapd to point at it. | ||
I prefer doing that, so that the default is available for reference. | I prefer doing that, so that the default is available for reference. | ||
= Configure udhcpd = | = Configure udhcpd = | ||
Edit /etc/udhcpd.conf. | Edit /etc/udhcpd.conf. | ||
The default is very well-commented, but not | The default is very well-commented, but not really ready to use. | ||
Here's a skeleton, loosely based on mine: | Here's a skeleton, loosely based on mine: | ||
start 192.168.2.2 | start 192.168.2.2 | ||
Line 87: | Line 87: | ||
Note the following: | Note the following: | ||
*max_leases should be set to at least as many clients as you might have in | *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. | ||
the lifetime of a lease | *interface is the interface clients will be connecting to (wlan0 or your wireless interface in this example) | ||
note that the bridge itself gets a | *router should be the static IP address you assign to your wireless interface. | ||
*interface is the interface clients will be connecting to (wlan0 or your | *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. | ||
wireless interface in | (e.g. 192.168.2.1 and 192.168.2.255 are both suitable for the router address) | ||
*router should be the static IP address you | *set the DNS option to point to any nameservers you want. You can repeat it, but the limit is 3 nameservers. | ||
*start and end should be within the same subnet as the IP you configure | *static_lease takes two arguments: a MAC address designating a specific network adaptor, and the IP address that should be assigned to it. | ||
wlan0 with, but the | 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. | ||
*set the | |||
but | |||
*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 | |||
users. | |||
This comes in handy for printers, if you can trust | |||
= Configure iptables = | = Configure iptables = | ||
Line 111: | Line 102: | ||
{{Cmd|iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE | {{Cmd|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 eth0 -o wlan0 -m state --state RELATED,ESTABLISHED -j ACCEPT | ||
iptables -A FORWARD -i wlan0 -o eth0 -j ACCEPT | iptables -A FORWARD -i wlan0 -o eth0 -j ACCEPT}} | ||
# this saves the state | # this saves the state in a location the service can restore it from | ||
service iptables save | <code> | ||
service iptables save | |||
</code> | |||
= Test = | = Test = | ||
<code> | <code> | ||
rc-service hostapd start | |||
rc-service udhcpd start | |||
sysctl net.ipv4.ip_forward=1 | sysctl net.ipv4.ip_forward=1 | ||
Line 125: | Line 118: | ||
and try connecting from another computer. | and try connecting from another computer. | ||
= Make changes permanent = | = Make the changes permanent = | ||
{{Cmd|rc-update add hostapd | {{Cmd|rc-update add hostapd | ||
Line 134: | Line 127: | ||
Now, the odd parts: | Now, the odd parts: | ||
iptables tries to set net.ipv4.ip_forward to 1 when it's started, but in | iptables tries to set net.ipv4.ip_forward to 1 when it's started, but in | ||
my experience, this | my experience, this is not reliable. | ||
You do *not* want to enable the "iptables" service | You do *not* want to enable the "iptables" service. It starts before | ||
networking, and may result in your wireless interface not getting configured. | 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...) | (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 | Rather, modify /etc/network/interfaces, commenting out any configuration for | ||
your wireless interface. Then add | your wireless interface. Then add: | ||
<pre> | <pre> | ||
Line 147: | Line 140: | ||
address 192.168.2.1 | address 192.168.2.1 | ||
netmask 255.255.255.0 | netmask 255.255.255.0 | ||
up | up rc-service iptables start | ||
up sysctl net.ipv4.ip_forward=1 | up sysctl net.ipv4.ip_forward=1 | ||
down | down rc-service iptables stop | ||
</pre> | </pre> | ||
(It | (It's possible to set up everything so that hostapd and udhcpd get | ||
started and stopped from the wlan0 stanza | started and stopped from the wlan0 stanza. I didn't bother doing that.) | ||
= Finishing touches = | = Finishing touches = | ||
(See [[ | (See [[Setting up a SSH server]] for alternatives and more information) | ||
Add dropbear SSH server, configure it to run on only the wireless interface: | Add dropbear SSH server, configure it to run on only the wireless interface: | ||
{{Cmd|setup-sshd -c dropbear}} | {{Cmd|setup-sshd -c dropbear}} | ||
Line 165: | Line 157: | ||
DROPBEAR_OPTS="-p 192.168.2.1:22" | DROPBEAR_OPTS="-p 192.168.2.1:22" | ||
(assuming | (assuming the wireless interface has the IP address 192.168.2.1 and you | ||
want SSH on port 22). | want SSH on port 22). | ||
This is optional, but if you're using a wireless router it helps to be able | 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. | to administer it, and listening on all addresses is rather risky. | ||
= Things this doesn't cover but it would be nice to = | = Things this doesn't cover but it would be nice to = |
Latest revision as of 10:31, 17 November 2023
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
- this saves the state in a location the service can restore it from
service iptables save
Test
rc-service hostapd start
rc-service 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 rc-service iptables start up sysctl net.ipv4.ip_forward=1 down rc-service 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