[plug] Safely using an untrusted router
dean.bergin at gmail.com
Wed Oct 21 23:12:02 UTC 2015
This is probably not going to help solve your particular issue, but one
thing I recently did, was install OpenWRT on a Rpi2 and set up PPPoE over
one of two subinterfaces (VLAN) to a cheap netgear modem (with the help of
a Cisco Catalyst switch). I also put the Rpi2 OpenWRT effectively into it's
own routed subnet/DMZ (part of the design) so that even if there where to
be some kind of funny business, things like uPNP theoretically should not
work since my experience has taught me that most consumer-grade
modems/routers do not route/NAT anything other than their resident subnet,
therefore I believe that not only are uPNP implementations (and many other
services on consumer-grade routers) usually bound to the subnet to which
they are running on, but should be disabled in cases where the device is in
>Does anyone know whether 4G modems (and smart phones, for that matter) are
assigned a publicly-routable IP address or are they typically NAT'd behind
a small number of IP addresses of the mobile service provider's servers? I
can't imagine billions(?) of mobile phones all having unique
publicly-routable IP addresses (on top of all the servers and so on, around
I had the opportunity to test this, as I was able to tether my phone to a
Rpi2 running OpenWRT as part of the labs I did for my now current nework
design, but I did not think to test this specific scenario.
Shouldn't be too difficult to create a lab to test this, if someone has a
spare raspberry pi (mine is currently in 'prod' now)?
On Wed, Oct 21, 2015 at 6:27 PM Dirk <justanothergreenguy at gmail.com> wrote:
> Thanks Andrew. Will follow up on those ideas too, thanks.
> However, I have another idea, a bit left field, but it may just do the
> Does anyone know whether 4G modems (and smart phones, for that matter) are
> assigned a publicly-routable IP address or are they typically NAT'd behind
> a small number of IP addresses of the mobile service provider's servers? I
> can't imagine billions(?) of mobile phones all having unique
> publicly-routable IP addresses (on top of all the servers and so on, around
> the world).
> If they're NAT'd, then maybe a pre-paid 4G USB modem dongle would be the
> way to go for low MB critical online work, eg. fetching package lists,
> logging in to ASIC, ATO, webmail, our utilities, etc. Should block
> all scanners on the net that are looking for routers to exploit, by virtue
> of sitting behind the Svc providers routers. (...and then use an unsecured
> computer and ADSL router pair for general web browsing, content streaming,
> Does anyone know if this would work?
> (Of course, if a 4G dongle is not NAT'd then I don't really gain anything).
> On Wednesday, 21 October 2015, Andrew Cooks <acooks at gmail.com> wrote:
>> On Wed, Oct 21, 2015 at 9:43 AM, Dirk <justanothergreenguy at gmail.com>
>>> Cheers for that Pavel. And thanks again Brad for your input. You've
>>> both given me some ideas, although I was hoping for an easy OpenVPN option
>>> If anyone else has any thoughts or suggestions, please let me know!
>>> My internet access is slow enough, so I'm not really excited about
>> pushing everything through a VPN.
>> I trust my router. I have a TP-Link TD-8817 modem in bridge mode,
>> connected to a fit-pc (http://www.fit-pc.com/web/solutions/multilan/)
>> running IPFire (http://www.ipfire.org/). IPFire tells me I can trust my
>> DNS. IPFire packages are kept up to date. The modem could conceivably
>> modify the PPPoE frames in transit, except that it's a dirt cheap consumer
>> product with little functionality that could be exploitable and it's
>> unlikely to have enough processing power to do that kind of thing.
>> There is nowhere safe, only acceptable risks.
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