Merge branch 'bridge-mode-doc' of pi31415/libreCMC-cmh into v1.4

docs/Bridge_Mode.md

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+# Bridge Mode
+
+A common feature of COTS Wi-Fi routers is to be able to set the router
+to bridge mode, where the devices stops acting like an IP router
+(layer 3) and starts acting like an ethernet switch (layer
+2). LibreCMC does not actually have a bridge mode, but the same effect
+is achievable with two easy steps.
+
+1. Add the WAN ethernet port to the LAN bridge interface
+2. Set the DHCP server to ignore the LAN interface
+
+## Adjust the LAN bridge interface
+
+Go to the Network >> Interfaces page and press the EDIT button next to
+the LAN Network. Select the Physical Settings tab.
+
+![alt text](images/librecmc-default-interfaces.png "LibreCMC default
+ interfaces view")
+
+Place a checkmark in the box for the "eth0" interface for wan and
+wan6. (I suppose the interface name might vary amongst different
+routers.)
+
+![alt text](images/librecmc-eth0-added-to-lan.png "Adjustments to
+ LibreCMC interface physical settings")
+
+You can also check the box for Enable STP if desired. There doesn't
+seem to be any downside from this, though I suppose it would use a
+little more CPU cycles and memory. STP is a protocol intended to
+prevent infinite communication loops from forming in a network of
+layer 2 switches.
+
+## Adjust DHCP server settings
+
+On the same page, scroll down a little to the DHCP Server section. In
+the General Setup tab, check the "Ignore interface" box.
+
+![alt text](images/librecmc-dhcp-server-ignore-lan.png "Setting
+ LibreCMC DHCP server to ignore LAN interface")
+
+Save and apply your changes.
+
+## Communicating with LibreCMC in "bridge mode"
+
+Now that the DHCP server is turned off, the next time you connect to
+the libreCMC device, you will not be able to communicate with it. You
+must set your connecting device manually to be on the same
+subnet. Since the default libreCMC LAN IP address is 192.168.10.1/24,
+it should work to set your connecting device to IP address
+192.168.10.2/24.On Gnu/Linux systems, the command is usually `ip addr
+add 192.168.10.2/24 dev eth0` or `dev wlan0` if connecting wirelessly.
+
+Alternatively, you may set the libreCMC device to receive an IP
+address from another DHCP server on your network: go to back to the
+Network >> Interfaces page, press the EDIT button again next to the
+LAN Network, select the General Setup tab, and switch the Protocol
+from "Static address" to "DHCP client". (It seems that in LibreCMC,
+bridging the LAN interface to the WAN port disables the operation of
+the DHCP client on the WAN interface.) Of course, if you do this, be
+sure that your DHCP server has reserved a memorable IP address for the
+LibreCMC device's MAC address, so you don't have trouble finding it.
+
+![alt text](images/librecmc-switch-lan-to-dhcp.png)
+
+## Adjusting the firewall...?
+
+To make a more consistent look in the your LibreCMC interface, you
+could go into Network >> Firewall and do things like deleting WAN
+zones or disabling NAT masquerading. But since the ports are bridged,
+it isn't necessary, and it will only make it more work to switch back
+out of "bridge mode" if you want to later.