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I'm unsure what this is called, but the situation is the following:
I want to have a Raspberry Pi + WiFi USB Module connect to a WiFi network.. then share that connection with another router that is connected via Ethernet.. (A spare DIR-655 to be exact).
Is this a gateway? Do I plug the ethernet into my router's "Internet" Port?
You're describing a wireless bridge between the RPi's Ethernet and wireless interfaces. Simply using normal Ethernet Layer 2 bridging (e.g. bridge-utils) won't work due to wifi design limitations, however you can achive the same effect via Proxy ARP. Proxy ARP is a routed (Layer 3) solution, but works for all IP traffic: see https://wiki.debian.org/BridgeNetworkConnectionsProxyArp for details, but it is as simple as:
$ sudo apt-get install parprouted dhcp-helper avahi-daemon
Edit /etc/network/interfaces to configure the interfaces:
auto lo
iface lo inet loopback
auto eth0
allow-hotplug eth0
iface eth0 inet manual
auto wlan0
allow-hotplug wlan0
iface wlan0 inet dhcp
wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf
post-up /usr/sbin/parprouted eth0 wlan0
post-down /usr/bin/killall /usr/sbin/parprouted
# clone the dhcp-allocated IP to eth0 so dhcp-relay will relay for the correct subnet
post-up /sbin/ip addr add $(/sbin/ip addr show wlan0 | perl -wne 'm|^\s+inet (.*)/| && print $1')/32 dev eth0
post-down /sbin/ifdown eth0
Edit /etc/sysctl.d/local.conf to enable IP forwarding:
net.ipv4.ip_forward=1
Enable DHCP relay: /etc/default/dhcp-helper
# relay dhcp requests as broadcast to wlan0
DHCPHELPER_OPTS="-b wlan0"
Edit /etc/avahi/avahi-daemon.conf to enable mDNS relaying:
[reflector]
enable-reflector=yes
Reboot, and hosts connected to the bridge's ethernet should acquire a DHCP address and have full IP connectivity!
You can connect another router to the Pi's Ethernet, in which case you'd very likely want to use that router's WAN port connected to the Pi (dont use the LAN ports - you're likely to end up with duelling DHCP servers at the least). If all you're doing is connecting multiple hosts via the Pi, just use a switch.
wpa-ssid and wpa-psk entries somewhere in there as well? right now they were underneath iface wlan0 inet dhcp
wpa-* items should remain under iface wlan .... In my example I'm using the separate conf file rather than individual wpa-* entries. Once the wifi interface is working, just add the post-up and post-down bits (though the post-down items aren't strictly required to "just make it work"), make the other listed changes, reboot and it should be good to go.
Yo can use OpenWRT. Connect the raspi to the Wi-Fi network and then, configure eth to give dhcp to the router. You will need to allow "forwarding" between networks in firewall config or just try to "bridge" Wi-Fi and eth if you're getting dhcp from the main wireless router.
Wi-FI Router > Usb Pi > Router Wan. Don't expect to get more than 10Mbps.
Let me know if you need any help.
If understand correctly you're asking for using the raspberry pi a wireless access point, in that case there are many tutorials for you to choose from, e.g:
