A Debian Wifi Router: THE REBOOT

Just like a Hollywood movie, it’s time to do a reboot. Fucking around with the networking stack after every reboot got old real fast. But this time around it looks like the remake is actually better than the original.

The router is still my trusty Thinkpad T60. Other than a couple of scares because of its age it still works! Just need to replace the CPU fan sooner rather than later.

Sources for this were:

Yes, this writing assumes you know what you’re doing on the command line.

Install Operating System

Whatever your version of Debian is, make sure that it’s up to date and that you install the stuff you’ll need:

# aptitude install hostapd rfkill dnsmasq fail2ban

Switched away from BIND and ISC-DHCP because they’re a pain in the ass to deal with. Maybe in the future I’ll use them again, but for the time being… this will do the trick.

Something I keep doing and forgetting to document is changing sshd port from 22 to something else and switching from password authentication to key-based authentication. Fail2ban is useful in blocking people trying to force their way in.

Networking

I had to add both wlan0 and wlan1 manually. Why? Dunno. I suppose they only get added automatically when you’re installing Xorg, which I’m not. Here’s my /etc/network/interfaces:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet dhcp

# Intel PRO/Wireless 3945ABG (Golan) Rev 02
#auto wlan0
#allow-hotplug wlan0
#iface wlan0 inet dhcp

# Qualcomm Atheros AR922X Wireless Network Adapter (rev 01)
auto wlan1
allow-hotplug wlan1
iface wlan1 inet static
    address 192.168.1.1
    netmask 255.255.255.0

iptables

Edit /etc/sysctl.conf and uncomment the line for ipv4 packet forwarding:

net.ipv4.ip_forward=1

Create /etc/network/if-pre-up.d/iptables with the following contents:

#!/bin/sh
/sbin/iptables-restore < /etc/network/iptables

Change permissions and make executable:

# sudo chown root /etc/network/if-pre-up.d/iptables
# chmod 755 /etc/network/if-pre-up.d/iptables

Create /etc/network/iptables and put the following in it:

*nat
:PREROUTING ACCEPT [0:0]
:INPUT ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
# eth0 is WAN interface
-A POSTROUTING -o eth0 -j MASQUERADE

COMMIT

*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]

# Forward traffic from wlan0 (LAN) to eth0(WAN)
-A FORWARD -i wlan0 -o eth0 -j ACCEPT

# Service rules
-A INPUT -j ACCEPT

# Forwarding rules
-A FORWARD -j ACCEPT

COMMIT

This part remains unchanged from my the initial post. If you want it to do a more thorough job of filtering packets the Ars post is a good place to start.

dnsmasq

Edit /etc/dnsmasq.d/local.conf:

# If you don't want dnsmasq to read /etc/resolv.conf or any other
# file, getting its servers from this file instead (see below), then
# uncomment this.
no-resolv

# Never forward addresses in the non-routed address spaces.
bogus-priv

# If you want dnsmasq to listen for DHCP and DNS requests only on
# specified interfaces (and the loopback) give the name of the
# interface (eg eth0) here.
# Repeat the line for more than one interface.
interface=wlan1

# Uncomment this to enable the integrated DHCP server, you need
# to supply the range of addresses available for lease and optionally
# a lease time. If you have more than one network, you will need to
# repeat this for each network on which you want to supply DHCP
# service.
dhcp-range=192.168.1.1,192.168.1.100,24h

# Set nameservers to use
server=192.168.1.1
server=8.8.8.8
server=8.8.4.4
server=208.67.220.220

Then restart the service with # service dnsmasq restart.

Much, much simpler than trying to configure both BIND and ISC-DHCP just for local networking. The options I included are pretty self-explanatory. If you're going to be assigning IP addresses on more interfaces, make sure to add them.

hostapd

The great big bad. If your card doesn't support Master mode, just stop trying now and go get yourself a plain old router.

For me these are the settings that work on my Atheros card. You will need to figure out what settings work on your own based on your hardware; you can query its capabilities with iw.

Edit /etc/hostapd/hostapd.conf:

# the interface used by the AP
interface=wlan1
# a means 5GHz, g means 2.4GHz
hw_mode=g
# the channel to use. Using 0 will make the AP use ACS to search for the channel with least interference
channel=0
# limit the frequencies used to those allowed in the country
ieee80211d=1
# the country code
country_code=US
# ieee80211n: Whether IEEE 802.11n (HT) is enabled
ieee80211n=1
# QoS support
wmm_enabled=1

# SSID (name of the AP)
ssid=SSID
# 1=wpa, 2=wep, 3=both
auth_algs=1
# WPA2 only
wpa=2
wpa_key_mgmt=WPA-PSK
wpa_pairwise=CCMP TKIP
rsn_pairwise=CCMP
wpa_passphrase=KEY

I tried using hw_mode=a but then OSX and iOS devices have a hard time connecting and staying connected to the network, so I switched bands to 2.4GHz. It's slower but far more stable. You'll want to configure the ht_capab command to better make use of your NIC.

After this is done, edit /etc/default/hostapd, and change the DAEMON_CONF line to match the location of the hostapd config file:

DAEMON_CONF="/etc/hostapd/hostapd.conf"

Make sure to test out the configuration file by actually running hostapd and making sure it runs:

# hostapd /etc/hostapd/hostapd.conf

If you run into issues here you'll have to determine where the issue is coming from:

  • Network card driver
  • Hostapd configuration

In that case run hostapd with one of the debug switches (hostapd -d /etc/hostapd/hostapd.conf) to figure this out. For maximum driver support the best option is to use Atheros-based cards using the ath5k, ath9k and ath10k drivers. Various cards from other suppliers will work, but will require fiddling with the configuration. They're worth a shot if you need to share Internet access with others around you quickly.

Reboot

Yes, you'll actually want to reboot at this point. Why? Because the point of a router is that when you turn it on everything works automatically, without you having to manually start a service. So reboot, and check that

  • All services come up by themselves
  • You're able to have the system act as a router
  • You're not having latency/DNS/throughput anomalies

That's pretty much it. It's a better setup than the first one and it's much easier to control with much less software installed.

A Debian wifi router

The search of a living space has been over for over a month now and I’m more or less settled in. I had a bad experience with my previous landlord keeping my wireless router (an Asus AC-RT68U) cos he’s a fucking asshole so I decided to give this a shot. I’m actively trying to solve money now so it really wasn’t an option to just buy another one as my living costs are now much, much higher.

I’ve hoarded a bit of equipment so I did have a couple of computers available to me. With a bit of looking I found about hostapd and I got it to work more or less reliably on a Thinkpad X220 with an Intel Centrino chipset using the iwlwifi driver. The real problem started when I tried using my older Thinkpad T60 laptop for the same task. Mine has an Intel PRO/Wireless 3945ABG [Golan] chipset also using the iwlwifi driver but the crucial difference was that this chipset doesn’t support Master mode, so you can’t use it with hostapd to enable an AP. Looked around for a bit and tried using this Archer T2UH from Amazon but the support on Linux is a fucking joke, basically; they actually want you to compile the drivers from scratch to get everything going. Ended up with an Ubiquiti Networks SR71-C card using the ath9k chipset. I tried setting it up using the already-installed environment but I couldn’t get it work so I basically reinstalled everything from scratch to start fresh.

I pieced this together from instructions found on the Debian Wiki, this guide from Ars Technica on building your own router, hostapd documentation, and a whole host of blog posts detailing how to pull this off, most of them with information easy to mess up.

As it is right now I’m typing this up on a Windows 10 system connected to the laptop and the signal and throughput are good enough for extended sessions playing Unreal Tournament pre-alpha.

Install Operating System

Installed Debian Stable fresh on the system with no desktop environment. This computer will now be a router so we don’t need any kind of desktop environment. Most desktop environments now require NetworkManager or wicd to manage network connectivity and I wanted to avoid all that crap. It just makes things more complicated for something that will be complicated enough already. I used the netinst install media with the laptop connected to ISP through Ethernet and getting an IP address through DHCP, so I didn’t have to fiddle with any PPPoE clients to get online and install the stuff required.

# aptitude update
# aptitude install hostapd bridge-utils isc-dhcp-server bind9 rfkill vbetool vim-runtime vim-goodies tmux

Enable Wifi Adapter

For some weird and lame reason Debian didn’t enable the adapter after installation, so it has to be setup manually. These instructions are based on the Debian Wiki. Atheros card is wlan0 while built-in Intel adapter is wlan1.

# ip a
# rfkill unblock all
# iwconfig
# ip link set wlan0 up
# iwlist scan

The card scanned for networks around so it was working.

Edit /etc/network/interfaces so the WLAN interface doesn’t wait for an IP address. If you don’t do this it turns out the OS stops booting until it gets bored of waiting for an IP address.

auto wlan0
iface wlan0 inet static
    wireless-mode Master
    address 192.168.1.1
    netmask 255.255.255.0

You can use whatever IP address (like 10.0.0.1) but make sure the same subnet is used throughout the entire configuration. I went with 192.168.1.1 cos that’s what I like my networks to be on.

Configure iptables

Edit /etc/sysctl.conf and uncomment the line for IPV4 packet forwarding:

net.ipv4.ip_forward=1

Create /etc/network/if-pre-up.d/iptables with the following contents

#!/bin/sh
/sbin/iptables-restore < /etc/network/iptables

Then

# sudo chown root /etc/network/if-pre-up.d/iptables ; chmod 755 /etc/network/if-pre-up.d/iptables

Create /etc/network/iptables and put the following in it:

*nat
:PREROUTING ACCEPT [0:0]
:INPUT ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
# eth0 is WAN interface
-A POSTROUTING -o eth0 -j MASQUERADE

COMMIT

*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]

# Forward traffic from wlan0 (LAN) to eth0(WAN)
-A FORWARD -i wlan0 -o eth0 -j ACCEPT

# Service rules
-A INPUT -j ACCEPT

# Forwarding rules
-A FORWARD -j ACCEPT

COMMIT

At this point iptables will simply pass along all the traffic that his the computer. I’ve since changed my configuration but this is a starting point as I just wanted to get online at this point. iptables rules are a pain in the ass to type.

Configure DHCP

Edit /etc/dhcp/dhcpd.conf and add the following at the end of the file:

subnet 192.168.1.1 netmask 255.255.255.0 {
range 192.168.1.100 192.168.99.199;
option routers 192.168.1.1;
option domain-name-servers 192.168.1.1;
option broadcast-address 192.168.1.255;
}

Configure DNS

BIND works out of the box, thankfully enough.

Configure hostapd

Create and edit /etc/hostapd/hostapd.conf and put the following in it:

interface=wlan0
driver=nl80211
# SSID on 5GHz band
ssid="MY WIFI NETWORK"
hw_mode=a
channel=44

# Radar an' stuff
country_pre=US
ieee80211d=1
ieee80211h=0

# 802.11n goodness
wmm_enabled=1
ieee80211n=1

# wpa
auth_algs=1
wpa=3
wpa_passphrase=MyWifiPassword
wpa_key_mgmt=WPA-PSK
wpa_pairwise=TKIP
rsn_pairwise=CCMP

ht_capab=[HT20][HT40+][SHORT-GI-40][TX-STBC][RX-STBC1][DSSS_CCK-40]

The ht_capab are the ones I’ve found work with the SR71-C card. I’ll keep playing with them.

At this point everything looked good and rebooted. I actually rebooted a whole bunch of times as I kept prodding and poking stuff every step of the way but I guess it all could be done in one go and just reboot at the end.

Actually be a Wifi Router

After rebooting do

$ sudo hostapd /etc/hostapd/hostapd.conf

Runs with the SSID, picks a channel, and starts the laptop as an AP. iptables is routing traffic from one interface to another, ISC DHCP gives out IP addresses and BIND does DNS. I did run into an issue where wlan0 kept sending out DHCP requests to which dhcpd would successfully assign a new IP address for wlan0. I got lazy and just set a static IP for the mac address of the interface on dhcp.conf. I’ll fix it… eventually.

There’s also the issue of tuning the kernel, firewall and a lot of other stuff since the computer is a single-purpose device rather than a multi-user computer. Most of the documentation I can find is for systems handling traffic at internet scale that would be overkill, so a lot of documentation will be read and a ton of adjustments made.

There will be also a lot of adjustments to hostapd itself to maximize throughput and signal stability. The SRC71C card only does 802.11n but the bitrate on this standard maxes out depending on the blue of the sky, the stage of the moon, how hot the dumpster fires are on twitter and how pissed off the boss is at work.

From what I’m getting this is relatively new stuff when it comes to DIY. People were doing this in the past but was way more costly (like this sweet setup right here). I’m doing this on the cheap and it seems to be working well enough.