IP Passthrough

How do you enable IP Passthrough in OpenWrt. Many commercially available routers from manufacturers such as Cradlepoint, PepWave, Netgear, etc. offer this feature as a user configurable option that is user selectable by simply checking a single checkbox in their webgui/console. I don't see this in OpenWrt. If it exists, can someone please point me to the exact reference page showing the selectable item?

This feature only applies to devices with integrated (xDSL) modems, of which only a selection of lantiq based devices are supported. For these devices, this is documented in detail in https://openwrt.ebilan.co.uk/viewtopic.php?f=7&t=266 - but in short, you bridge the the modem interface with a LAN port.

It also applies to 5G cellular routers where you need the cellular carrier's IP address to be passed through to the LAN device/PC

The same principle applies,

that may not be possible with every 4g/ 5g hardware, details depend on the fine-print.

Pepwave, Netgear, Inseego, and other cellular 5G routers now have the IP Passthrough feature, but they are expensive devices (even my Netgear 341u 4G USB modem had IP Passthrough). It is a valuable feature. I need an opensource alternative.

Afaik no it is not supported.
I had enough bad experiences with using OpenWrt to run a modem card anyway so I would personally not recommend to use OpenWrt at all if you must run a "dumb" modem card.
I bought Mikrotik Chateau LTE12 to run my LTE connection for example. It has IP passthrough like everyone else. I don't use this feature, my main issue was about the modem card(s) dropping connection and OpenWrt was not reacting to this.

That said, the nearest thing you can do from Luci web interface is set up a DMZ aka all-port-forwarding.

If you feel like developing this yourself, you can try to adapt the scripts made by Teltonika vendor (that uses OpenWrt as firmware for their devices) to implement this feature IP passthrough in OpenWrt

Or maybe using trelay? It is basically a brainless packet mirror.

trelay relays ethernet packets between two devices (similar to a bridge), but
without any MAC address checks. This makes it possible to bridge client mode
or ad-hoc mode wifi devices to ethernet VLANs, assuming the remote end uses
the same source MAC address as the device that packets are supposed to exit

you bridge the two LAN and usb0 interfaces of the modem device with trelay

And then you spoof the mac with macchanger or whatever other tool (or UCI as said above), on either the modem wan or the router usb0 so that both ports have the same mac address.

Interesting, there is a full firmware builder for a Teltonika device that has IP passthrough on github https://github.com/sarog/rut9xx
If you search for pridge you find where this IP bridging was added https://github.com/sarog/rut9xx/search?q=pbridge
This is mostly a "development resource", as it requires a developer to port the changes in current OpenWrt before it can be used.

It would be incredibly useful for a developer to port these changes in current OpenWrt. The commercially available products come with all the baggage and expense of a full blown router. My 341u Netgear USB 4G modem for example had just the IP Passthrough feature at very low cost and power, so I know it can be done. The only new 5G modem I see with IP Passthrough is the new Netgear M5200, also very expensive.

Hmm, maybe you are using the wrong name for what you want.

OpenWrt devices with IP passthrough will have the baggage and expense of a full blown router too.
IP passthrough is a feature that allows a modem/router to act as a dumb modem and pipe the network connection straight to another device. But you still paid for a router, even with OpenWrt.
So I do not understand why you need this so much in OpenWrt. You are still paying for the same hardware but you are running OpenWrt on it. Just buy the hardware and use stock firmware.

the "341u Netgear USB 4G modem" technically is a (low end) router, as it has its own web interface and IP and can operate a wifi network on its own (this means it is creating a little LAN). It is similar to the "portable battery powered LTE hotspot devices" but with a USB port and no battery.

If you are just connecting a "dumb LTE/5G modem" device either on USB or on a modem card like this https://www.sierrawireless.com/products-and-solutions/embedded-solutions/products/em7455/ the OpenWrt device will handle the IP of that interface, just like it handles the IP of a ethernet interface when you connect the cable.
There is no "IP passthrough" because the modem dongle or card is not an independent device, it has no IP, it does not exist on the network.
And in this case the OpenWrt device is also the router for your network.

This is what the commercially available devices do for me with the IP Passthrough feature. I need a less expensive alternative, that hopefully can be created using hardware supported with OpenWrt.

Why not just bridge the WAN and a LAN port? Is it because of the "updates its DynDNS" etc? if so you could just do that on the LAN device.

what is the best way to bridge the WAN and a LAN port in OpenWrt

Can anyone answer this simple question about enabling IP Passthrough in OpenWrt?

What is the best way to bridge the WAN and a LAN port in OpenWrt?

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What's the difference between the simplicity of the bridge technique for passthrough that you describe and what @bobafetthotmail describes above? By the way I think the link might not be right since I didn't see any description of this there or perhaps I'm missing something.

I have a Zyxel NR710:

that has an LTE modem (QMI protocol) that runs OpenWrt. My Zyxel is connected via its LAN port to the WAN port of my main router RT3200. I'd quite like my RT3200 to get the WAN address allocated by my 4G ISP.

Would setting up bridge between modem interface and lan in the Zyxel achieve that?