Building My Own Home Router, Part 1

This is the first of a series of blog posts on building my own home router from scratch using Debian. My hopes are that by sharing my experiences, it can help others in this endeavor.

I've been kicking around the idea of building my own router for a while now, mostly due to the fact that my trusty WRT54GL is grealy limted by what it can do with its measly 4 MB of flash and weak CPU. After months of casually searching and trying (unsuccessfully) to re-purpose some old hardware, I finally found what I've been looking for: a cheap-ish, low-power, rackmount server with more than one NIC.

The Hardware

I can't believe that I didn't think to check the various Mini-ITX resellers for something like this, because this is almost exactly what I've always been looking for. I got a 2-NIC board since I'm cheap and already have a gigabit switch, but you can easily find boards with more ports if you don't mind shelling out the extra cash.

Once the equipment got to my apartment, I slapped in some old laptop RAM and a spare 2.5" drive and got Debian installed.

802.3 and IPv4

The first order of business was to replicate the core functionality of my old router: IPv4 routing and Ethernet connectivity. The plan was to use eth0 as my public interface (plugged into my cable modem) and eth1 as my internal interface. Before I even plugged in anything, I wrote a basic /etc/network/interfaces file.

auto lo
iface lo inet loopback

# outside
allow-hotplug eth0
iface eth0 inet dhcp
        hwaddress ether AA:BB:CC:DD:EE:FF

# inside
auto eth1
iface eth1 inet static

Note the hwaddress ether line there. Since my ISP (whose name shall not be spoken (not Voldemort, but just as evil)) locks me to a single MAC address, my new router had to spoof my old router's MAC address, which was spoofed from my laptop that I originally set the connection up with. If you seemingly can't get a DHCP lease on your public interface, this is likely the problem.

Now that I had my interfaces configured and rarin' to go, I had to make sure that my ip{,6}tables rules were in order before plugging in.


-A INPUT -i lo -j ACCEPT
-A INPUT -i eth1 -j ACCEPT
-A INPUT -i eth0 -m state --state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -i eth0 -p icmp -m icmp --icmp-type 8 -j ACCEPT
-A FORWARD -i eth0 -o eth1 -m state --state RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -i eth1 -o eth0 -j ACCEPT


-A INPUT -i lo -j ACCEPT
-A INPUT -i eth1 -j ACCEPT

The above rules are my output from iptables-save and ip6tables-save, and are fully compatible with the respective restore programs. Debian even has a nice package called iptables-persistent which will load these rules on boot if you stash them as rules.v4 and rules.v6 in /etc/iptables!

To better understand these rules, it helps to have the Netfilter packet flow diagram in front of you. There are some simple goals with these:

IPv4 Filters

  • Allow traffic coming from loopback or the inside interface
  • Only allow traffic outside->inside if it pertains to an existing in->out connection
  • Secret sauce - that MASQURADE line enables NAT, so my private addresses can hide behind the one public address that my ISP gives me

IPv6 Filters

  • Just drop everything for now unless it comes from the inside and is destined for the router itself

Of course, before I could have a fully-functional internet connection, I had to get DNS set up. And I guess a DHCP server would be nice to have before my leases all expire and everything drops its IP.

Luckily, there's a software package which is geared towards these very tasks: dnsmasq! Getting it running was as easy as running apt-get install dnsmasq and service dnsmasq start, which was enough to get DNS working. To get DHCP working, I created two config files in /etc/dnsmasq.d/:


# My DHCP configs

# options for DNS

# Static DHCP entries


# Only allow DNS/DHCP requests from the inside interface

With all of that, I had a functioning IPv4 router and could do important things again, like idle on IRC and browse Reddit.

This is just the beginning though! You should go check out part 2 of this series where I get 802.11 working.