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Author Topic: Fallback to 3G dongle using Firebrick FB2700  (Read 2117 times)

Weaver

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Fallback to 3G dongle using Firebrick FB2700
« on: April 23, 2016, 02:33:15 AM »

The Firebrick FB2700 router has a USB port on it into which you can plug a 3G NIC and this can be  used as an ultimate do fallback connection should your main, wired Internet connection go down. (Do Draytek offer something similar ?)

I have a few questions about how this would work and the implications of this. Does anyone have an FB2700?

* A link over 3G using the Andrews and Arnold 3G data SIM (over the Three network, hopefully 4G too soon?) won't support native IPv6 (yet), so I'm told. So I suppose I should try setting up 6 over 4 Proto 41 to keep IPv6 working in the event of a switchover?

Q1 Would AA over Three's 3G allow IPv4 Proto 41 traffic? (Hopefully it's a clean, transparent IPv4 pipe.)

Q2 What happens to IPv4 (and IPv6) my-end addresses after a switchover to 3G? Does the address change?

Q3 sanity check - that's going to break all known existing TCP connections surely. Am I right? And some UDP too? This is not good at all. Roll on SCTP!

Q4 What if anything should I do about this (Q2)? Is there any way to avoid that problem?

Q5 If I implement Q1 then there's going to be a dynamic reduction in MRU/MTU because of the added protocol 41 overhead. What problems is this going to cause?

Q6 Is there any tricky way to maintain the MTU? (which is at the full 1500 currently)

Q7 Can I include the 3G link as a 'line' and just make it part of the bonded set of lines? Thus avoiding the problem of switching source addresses?

* I wouldn't want to include the 3G link all the time as an active part of the set. Even though it would give me an enormous performance boost, it would bankrupt me, and it would break everything because of the lack of IPv6 (or mismatched MRU/MTU if I did 6 over 4 on that one line while the other lines are also up, unless I reduced MTU everywhere to keep things matched).

Q8 Could I make this work somehow though while keeping the 3G 'line' in the down state until a failure is detected on all the other (DSL) lines? Stuck then with the disappearing IPv6 problem.

Q9 I loathe NAT, but could IPv6->IPv6 NAT and/or IPv4->IPv4 NAT help me out in any of the above problems?

Q10 Aside from all the problems NAT brings to applications (those which mention their own source addresses in the data), there would then be the problem of changing from no-NAT to NAT on the fly, with no warning. Presumably apps that detect NAT would then break as they would have detected its absence at the start and then stick with that later?

Q11 Is the whole 3G fallback thing just to horrible to use then? What if they get IPv6 sorted out?

Q12 Am I worrying over nothing regarding the change of IPv4 address? Can the address be kept the same at my end (like the case where it is a member of a bonded set) and the changeover be handled by re-routing at Andrews and Arnold? If not, then wish-list, AA, please. I was assuming that the switchover has to be handled by the FB2700, I realise that I don't actually know how it works, which end does the good thing?

[Sorry this has grown to be such a long post. :-) ]
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aesmith

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Re: Fallback to 3G dongle using Firebrick FB2700
« Reply #1 on: April 23, 2016, 09:09:16 AM »

Just one quick point, some 3G/4G services use carrier NAT so the dongle's "external IP" will be an RFC1918 address, so I think that would be the first thing you should check with the provider.   Just about all your other questions hinge on this.   
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Weaver

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Re: Fallback to 3G dongle using Firebrick FB2700
« Reply #2 on: April 23, 2016, 11:39:12 AM »

I would use an AA 3G SIM, which has a single global 'proper' IPv4, and hope that instead I could just get my existing IPv4 block and eventually IPv6 /64 mapped onto the 3G NIC, including it like 'another line' if at all possible,NAS mentioned in the earlier post.

I don't know what the IP packets are encapsulated in when you get them over a service like the AA-over-"Three" service.

[ Btw : What are Three going to do next, rename themselves to "Four" when the name becomes an embarrassment rather than cool? Skip straight to "Five"? ]
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Weaver

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Re: Fallback to 3G dongle using Firebrick FB2700
« Reply #3 on: April 23, 2016, 11:41:12 AM »

Actually, didn't you say AA are not giving out those SIMs any more? Still on their website. Really annoying if they're no longer available.

VPN to AA, anyone?
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