Bridging Wired and Wireless Networks, Gentoo-style

I want my wired and wireless networks to share a single 192.168.1.x address space (instead of separate 192.168.0.x and 192.168.1.x addresses).

In order to do that, we need to set up a bridge to merge disparate networks into a single space.

Part 1: The Basic Configuration

ADMtek NC100 (uses tulip driver)
Ralink RT61 PCI (uses rt61pci driver)
hostapd
linux 4.1.15-gentoo-r1
net-misc/bridge-utils 1.5
net-wireless/iw 3.17

Part 2: Making It Work

I started out creating a basic bridge, using the Gentoo Wiki as a guide:

cd /etc/init.d
ln -s net.lo net.br0

/etc/init.d/net.br0 start

There’s no need to change how hostapd starts; it still talks to wlan0 (not br0).

# /etc/conf.d/net

modules_wlan0="!iwconfig !wpa_supplicant"
config_wlan0="null"
config_eth0="null"
config_br0="192.168.1.1/24"
brctl_br0="setfd 0
sethello 10
stp off"
bridge_br0="eth0 wlan0"

The Problem

The above config is naive and doesn’t work right.  I got this error:

Can't add wlan0 to bridge br0: Operation not supported

Huh.  There’s nothing indicative in dmesg about the error, the last entry shows the bridge being created on the wired card and then being taken down.  Just to be sure, I created a bridge with just eth0 and it worked:

$ brctl show
bridge name   bridge id           STP enabled   interfaces
br0           8000.00045a42a698   no            eth0

After casting about a bit, I found a serverfault.com page that pointed to this fix:

$ iw dev wlan0 set 4addr on
$ brctl addif br0 wlan0

That works, but that won’t do me much good as a long-term solution.  I would need to pay a visit to the basement after every planned reboot and unplanned power outage, or else nobody can get onto the network.

( More about the 4addr option here. )

You can’t just add the option to modules_wlan0, it doesn’t work that way.  A quick visit back to the wiki suggested the solution, though, which is to define a preup function where we can execute arbitrary commands.

The Working Config

These statements are in addition to the WAN interface config:

# /etc/conf.d/net
modules_wlan0="!iwconfig !wpa_supplicant"
config_wlan0="null"
config_eth0="null"
config_br0="192.168.1.1/24"
brctl_br0="setfd 0
sethello 10
stp off"
bridge_br0="eth0 wlan0"

preup() {
    # br0 uses wlan0, and wlan0 needs to set the
    # 4addr option before being used on a bridge
    if echo "${IFACE}" | grep -q 'br0' ; then
        /usr/sbin/iw dev wlan0 set 4addr on
    fi

    return 0
}

Then do all the accounting to clean up:

rc-update add net.br0 default
rc-update del net.eth0 default
rc-update del net.wlan0 default

I also had to update my iptables config to refer to br0 instead of eth0 and wlan0.

Finally, a reboot to test that everything starts properly.

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