Bridging wlan0 to eth0

Question

On Arch Linux, I would like to have eth0 (connected to bridged router) share the connection received from wlan0, I've read tutorials but I'm not command savvy as other users are and don't completely understand.

Answer 1

UPDATE

It is not possible to bridge between wireless (client a.k.a. station mode) and wired interfaces according to this thread on linux-ath5k-devel.

Setup NAT

One should set up NAT instead:

echo 1 > /proc/sys/net/ipv4/ip_forward
iptables -t nat -A POSTROUTING -o wlan0 -j MASQUERADE

Assigning an IP

Then you have to assign IP addresses to yourself:

ifconfig eth0 10.0.0.1 netmask 255.255.255.0 up

Install dhcp daemon

Install a dhcp server and add the following text to its config file (in /etc/dhcpd.conf or something similar)

subnet 10.0.0.0 netmask 255.255.255.0 {
    range 10.0.0.100 10.0.0.120;
    option routers 10.0.0.1;
    option domain-name-servers the-ip-address-you-have-in-etc-resolv.conf;
}

Start dhcpd

Then start it /etc/init.d/dhcpd start

And that's it!

Only read below if you are interested in the non-working bridging setup

---

brctl addbr mybridge
brctl addif mybridge eth0
brctl addif mybridge wlan0

First you create a bridge interface I choose an arbitrary name mybridge then add intefaces to it.

You should request a new ip address (This is needed only if you want to get a valid IP for the bridging device itself):

dhclient -d mybridge

Answer 2

To bridge wifi interface you may use iw tool to enable 4addr likewise:

# iw dev <wifiInterface> set 4addr on

ie:

# brctl addif <bridgename> <wifiInterface>
can't add <wifiInterface> to bridge <bridgename>: Operation not supported

# iw dev <wifiInterface> set 4addr on
# brctl addif <bridgename> <wifiInterface>

Now it should work. You can show bridges using:

# brctl show

Answer 3

Depends on how mean the AP is to you:

1) It might only want to see packets coming from you, with your known link layer address (and hence not of bridged packets) 2) It might actually be even smarter, and know which IP address should belong to which link layer address (cause it knows DHCP and inspects it)

If 1+2 are both true, you need indeed something like IP NAT, DHCP, ..

But if only 1) is the case, you can fake the link-layer address, and reverse map it onto the right one in the other direction as described here:

https://wiki.debian.org/BridgeNetworkConnections#Bridging_with_a_wireless_NIC

Answer 4

To bridge wifi interface you may use iw tool to enable 4addr likewise:

# iw dev <wifiInterface> set 4addr on

ie:

# brctl addif <bridgename> <wifiInterface>
# can't add <wifiInterface> to bridge <bridgename>: Operation not supported

# iw dev <wifiInterface> set 4addr on
# brctl addif <bridgename> <wifiInterface>

Now it should work. You can check with:

# brctl show

It might not work if iw itself reports an error, like "command failed: Operation not supported (-95)" (seen on Raspbian). Not all drivers implement the feature, apparently.

Answer 5

4addr as described in other answers is certainly the best way when supported by the adapter/driver, but not all of them does. NAT might work for some things, but getting proper communication both ways on the lan will become problematic (ex. connecting a printer or accessing other IoT devices on the other side of the NAT). Anything relying on broadcast/multicast (ex. auto-discovery, bonjour) will fail through the NAT.

The alternative is using an ARP Proxy (parprouted) as described in https://wiki.debian.org/BridgeNetworkConnectionsProxyArp. I've set this up on a Raspberry Pi for a printer and it works like a charm (I have added a 10 second sleep in the post-up commands to let it get an IP address first, it might have to do with the slowness of my old RPi...)