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Using 9front as a home router

When people discuss Plan 9 it is usually mentioned that Plan 9 is a real file orientated design but I think its is sometimes hard to conceptulize the benefits of this from an outside perspective. To assist with this I wanted to discuss how I have my 9front box at home configured to act as my home router, and show how it's done.

My hardware setup is an old DELL OEM that I've jammed a 4 port ethernet pcie card in to. I use this for both the LAN and uplink since the on board ethernet port is not gigabit. Nothing fancy, just some hardware I had lying around.


Before we get in to the weeds here we're going to need to know our tools. In Plan 9 the kernel exposes a number of functionality through kernel filesystems, think stuff like Linux's /sys/. However unlike Linux Plan 9 exposes multiple roots for different subsections that the user may bind(1) in to their namespace at will. These kernel devices can be access by accessing a path that starts with '#' and is followed be a single rune identifier. For example the IP stack is accessed via '#I' and disks (/dev/sd*) are exposed via '#S'.

Much like 9p filesystems a "mount argument" may also be supplied when accessing these devices, this is usually used to acces a specific instance of a device. For example '#l0' refers to ethernet card 0, '#l1' refers to ethernet card 1 and so on. The ip device ('#I') also allows for a integer argument to specify which IP stack you would like to access, which can be used to setup multiple disjoint IP stacks.


For making this router happen we'll create a /cfg/$sysname/cpurc script, which will run at startup.

# Place IP stack 1 on /net.alt, this will be our "outside" IP stack
bind '#I1' /net.alt

# Place ethernet card 0 within that outside IP stack
# This is just organizational, not binding it to the IP stack yet
bind -a '#l0' /net.alt

# Create a ethernet bridge and add it to our "inside" IP stack
bind -a '#B' /net
# Add all of our internal ports to our "inside" IP stack
bind -a '#l2' /net
bind -a '#l3' /net
bind -a '#l4' /net
# Bind the interfaces to the bridge
echo 'bind ether port1 0 /net/ether2' >/net/bridge0/ctl
echo 'bind ether port2 0 /net/ether3' >/net/bridge0/ctl
echo 'bind ether port3 0 /net/ether4' >/net/bridge0/ctl

# Create a virtual IP interface for both the outside IP stack
# We open the clone file, and the kernel will then give us the
# id of the new created interface.
<$x/ipifc/clone {
	# Read the new interface number

	# Write in to the ctl file of the newly created interface to configure it
	>$xi/ctl {
		# This is a packet interface
		echo bind pkt

		# Our ip is and we only allow remote connections from
		echo add

		# Route packets to others
		echo iprouting 1

		# Now create a new interface on the inside IP stack
		<$o/ipifc/clone {
			>$oi/ctl {
				# Hook up this device to the outside IP stack device
				echo bind netdev $xi/data

				# Our ip is and we only allow remote connections from
				echo add
				echo iprouting 1

# Configure our route table for both the inside and outside IP stacks
# Arguments are: target mask nexthop interface(addressed by IP)
echo add > $x/iproute
echo add /96 > $o/iproute

# Do DHCP on the external interface. -x tells us which
# IP stack to use. -t tells us that we are doing NAT
# and to configure the route table as such. NAT is implemented
# as just a route table flag.
ip/ipconfig -x /net.alt -t ether /net.alt/ether0

# Configure a static IP on our internal interface, which will
# act as a gateway for our internal network.
ip/ipconfig ether /net/ether2

# Start dhcpd on our internal network, our DHCP range is
/bin/ip/dhcpd 100

It is worth noting that ip/ipconfig is mostly a convenience and has no magic under the hood, it itself is just talking to files in /net. This allows us to pass different /net's via -x as we like.

That's all folks. I've been using this for about a year now and haven't had any problems with it.

Further Reading

You can read more about the specific kernel devices within section 3 of the 9front manual. Some of the ones we used today: ether bridge ip.

9front also has a network database (NDB) that is used to infer systems and their ip addresses (among other things) but was omitted to keep a focus here if you want to read more about it, look at ndb(6) and the wiki