3d ago

Smallest modem I can put into bridge mode and feed into a router?

There really aren't that many modem/router combos out there you can actually flash with open source firmware.

Router? Sure, no problem.

Modem? Outside of some very niche Fritzbox devices and others listed on OpenWRT Table of Hardware

There really just aren't that many.

So, fine. I will use a proprietary modem.

I will put it into bridge mode, let it decode the DSL analog signals into ethernet and feed that into a router of my choice.

Question is: Which modem?

I don't want two devices heating up my shelf. I just want a tiny modem that converts analog to digital into an RJ45 and then let's the router do its thing.

Does such a magical device exist, because my search on this front has proved largely fruitless

23 comments

You don't say which modem you mean, but from context it seems to be xDSL. I'm running an own cable modem (Vodafone in Germany) which works a lot better than Vodafone's own POS (which insists to reboot itself every day or two despite being in bridge mode). There is a thin client running opnsense behind it.
As to picking the smallest and lowest power, I think it is better to pick one that works best. When I used DSL I bought a cheap hackable DSL modem from an italian ISP and put OpenWRT on it. It also ran in bridge mode. It had an SFP slot which can come handy for galvanic separation, but I didn't bother with it.
Unfortunately modems have the issue that they are made to be provisioned, configured, and owned by the ISP. At least that's how most DOCSIS setups I've seen work.
The only hope I'm seeing (I'm also wanting to do this) is running my own CMTS. I found a service (https://github.com/cablelabs/os-provisioning is the only opensource tool I've seen before) but I am still trying to figure out a reasonable cost way to connect that server to the actual network (so far bladerf sdr is ny best hair brained idea, which is really only cost effective because I want to play with 5g too). The last bit of luck I have is that my ISP isn't actually doing firmware updates on my modems, so if I connected them to my Internet lab network, provisioned my custom OS and firmware, and then connected it back to the ISP I can get their DOCSIS config. In theory at least 😅
Netgear has modems that have their OS and firmware in nice packages with build instructions at least. ARIS's older modem code is out there too but a pain compared to get.
You technically could use that same sdr as the modem on your ISPs network BUT because it's actually RF the FCC regulates it (for good reason I guess lol).
- oh well. One day someone might make an ooen source docsis3 compliant modem....
If your dsl is G.Fast you could try the Zyxel GM4100. That device is only a bridge
Is there any reason you are not just calling your ISP and asking them to put their box into bridge mode and then setting your own router up behind it?
You control everything from the router and into your local network.
If you can't trust your ISPs "modem" in bridge mode, then you also can't trust the infrastructure beyond the modem, and in that case you would need to set up a privacy respecting VPN service directly on your router to encrypt the traffic your ISP sees.
Netgear DM200? Supports openwrt to if you really want. You'll have to buy used now though.
Honestly, I'd just use whatever the ISP provides.
Sure, it's not open source and it kinda sucks... But I mean, if you don't trust the ISP modem, you can't trust the rest of their infrastructure either anyway, so it's kinda moot.
At least that way you have a vague chance of having a modicum of support when shit breaks.
If it can't be put into bridge mode, it probably has some sort of DMZ function where it basically does port forwarding for any/all possible ports.
Double NAT isn't as bad as it sounds these days.
Now to your question...
They exist, they're mostly targeted at ISPs though, so might be a harder find than other things.
They might also be older, as basically all customers also want their ISP to provide Wi-Fi, which a bridge modem won't.
Anyway,
You'll have to know what DSL were talking about, there's... ADSL, ADSL2+, VDSL, VDSL2, etc
One old-ass model we used to use back in the day was.. a Siemens 5200, but that's ADSL2 at best, definitely not VDSL.
- Siemens 5200
  That's a chunky house warmer, let alone shelf warmer!
  I can understand that ISP's need to protect their security (through obscurity) so that no one's going around cloning their neighbors analog signals to score free internet... but surely the actual hardware to do the tone pairing / channel matching analog-to-digital-magic-mumbo-jumbo does not need to be the size of a brick?
  In the spirit of people cutting open Nintendo Wii's shearing down the chip and making compact versions[0], has no one isolated the hardware on a modem, chucked out the routing stuff, and compacted the whole device to be no bigger than a USB stick?
  0: https://www.tomshardware.com/video-games/nintendo/meet-nintendo-kawaii-the-new-smallest-wii-build-that-fits-on-your-keychain
  
  I'm mad you started your citation at 0.
  I'm telling Chicago style!
  
  The fibre modem my ISP uses is pretty small, about the same size as one of those small unifi switches, but it still feels like there's almost nothing in it.
  I have taken apart the fibre modem of a different provider before, which was just a small PCB no larger than a raspberry pi and a fibre extension cable.
I would use OpenWRT
Also, could you get fiber? A DSL connection in 2025 is going to be pretty slow.
Not quite what you're seeking, but maybe relevant.
Have you considered an SFP modem like the Proscend 180T? Might be hard to source these days, but if you have a router with an SFP cage this would be a way to accomplish the single box goal.
- Woah... I had never heard of SFP slots/devices before. It looks like they're great at converting between mediums. If I could get an SFP device that had RJ45 output, that would be the dream
  
  If by RJ45 you mean erhernet over standard 8P8C (i.e. RJ45) your looking for SFP-T modules. Can't connect that to your DSL line though since that's not Ethernet.
  There do seem to exist G.fast sfp modules for dsl, but do note that sadly sfp module and router brand incompatibility is sadly a thing.
  Related note: At our company we also had quite some issues getting some of these more special sfp modules working (e.g. XGS-PON), so ymmv with a dsl one.
  (If you're looking for a device to plug sfp modules into, I can second MikroTik here. They're also great about accepting 3rd party SFPs)
  
  Maybe you could use a Mikrotik RouterBOARD, some feature a single SFP port. They're great and inexpensive.
  
  Not sure if they're still in production. There was also an Italian company selling one (maybe based on the same hardware).
  Like u/floquant I would look at Mikrotik. But you'll need to pay attention to port speed (they still sell 100Mbps gear) and their tested routing throughput numbers to be sure they can hit the numbers you need.
Is this how one prevents viruses?? 😂
- it's how I sanitize all my inputs from unknown devices :-)
I would start with your ISP's list of supported devices.
- I do, they're all TP-Link devices which is a company that makes solid solid hardware, but they have sketchy links to foreign state cyber attacks[0]. I don't mind having a dumb switch from TP-Link that I can just network devices together locally and forbid it from having internet access via the router.
  0: https://www.cnet.com/home/internet/tp-link-is-under-investigation-for-predatory-pricing-should-you-replace-your-router/
  
  The so called investigation is political
  TP-Link isn't perfect but they have decent security practices. (Unlike Cisco, Forinet and many other "premium" vendors)

23 comments