eth0 (modem usb) and wlan0 (wifi module) interconnection

I’m trying to either make a bridge between eth0 (Usb modem) and wlan0 (Wifi Hotspot) or route all traffic from wlan0 to eth0.

Procedure:

Bridge won’t be successful if the interfaces already have an ip.

ip addr flush dev eth0
ip addr flush dev wlan0

Create the bridge and add the network.

brctl addbr br0
brctl addif br0 eth0

Start the wifi hotspot. At this point I can see the network with my phone.

hostapd /etc/hostapd.conf -B &

Assign the bridge an ip.

ifconfig br0 192.168.0.1

Start dhcp-server so my hotspot can automatically assign an ip address to the connected devices.

/etc/init.d/S80dhcp-server start

Reset my bridge.

ifdown br0
ifup br0

Configuration files

hostapd.conf:

interface=wlan0 
bridge=br0 
driver=nl80211 
ctrl_interface=/var/run/hostapd 
ssid=My_Network 
dtim_period=2 
beacon_int=100 
channel=7 
hw_mode=g 
max_num_sta=8 
ap_max_inactivity=300

/etc/network/interfaces:

auto br0
iface br0 inet manual
bridge_ports eth0 wlan0

/etc/dhcp/dhcp.conf

ddns-update-style none;
option domain-name "google.com";
option domain-name-servers 8.8.8.8, 8.8.4.4;
default-lease-time 600;
max-lease-time 7200;
authoritative;
log-facility local7;
subnet 192.168.0.0 netmask 255.255.255.0 {
  range 192.168.0.100 192.168.0.110;
  option broadcast-address 192.168.0.255;
  option routers 192.168.0.1;
}

The dhcp-server returned OK but when I try to connect to the wifi hotspot I can’t get an ip. Any ideas on how to proceed?

Answer

I finally managed to get it done. I didn’t use bridge but iptables.

Just these two commands were enough:

echo 1 > /proc/sys/net/ipv4/ip_forward

iptables -t nat -A POSTROUTING -o eth0 -s 192.168.0.1/24 -j MASQUERADE

Here is the source of the answer with the explanation