[ubuntu-uk] Using a 3G modem as a fallback

[Andrew Oakley]

I'm moving out of The Sticks, slightly to the west of nowhere in the
Cotswolds - which nevertheless already has 2.5Mbit ADSL - to the urban
metropolis that is a satellite village near Tewkesbury. With this comes
wonders of the modern age, the likes of which I have never experienced
near my home, such as street lighting, paved footpaths, gas that comes
in pipes instead of bottles, mains sewerage and 3G coverage.
 
Given that I already have my Ubuntu 8.04 LTS server set up as a NAT
router for ADSL broadband, how difficult would it be to set up a cheapo
3G modem as a fallback?
 
The idea being:
* Some kind of heartbeat ping (eg. once every 5 secs to my ISP's
gateway?)
* On consistent ping fail (eg. 20 secs / 4 pings, no reply) run a script
to
* Fire up the 3G modem and log in
* Change the routing table to prioritise the 3G gateway
* (and here's the interesting bit) Re-establish the ADSL priority when
it comes back
 
The last bit is probably the bit that I'm having the most trouble with.
Once the 3G is up, it is entirely possible that I could ping my ADSL
ISP's gateway from the 3G connection. So simply getting a reply from the
ping test is no guarantee that ADSL is back online. Maybe some kind of
traceroute test? Or force a ping over a particular ETH, how?
 
Of course, all this assumes that I will actually need a fallback in the
new house. In my current rural cottage, ADSL drops out for about five
minutes every day or so, probably due to the fact that although the
village telephone exchange is only 500 metres from the house, my
phoneline nevertheless takes a 4.5 kilometre detour through neighbouring
villages to get there. The new house will be on a relatively new-build
estate - is it likely that the ADSL connection will be so reliable as to
be not worth having a fallback at all?
 
Andrew Oakley
Head of Software Development
Higher Education Statistics Agency (HESA)
95 Promenade, Cheltenham, Gloucestershire GL50 1HZ
T 01242 211460  F 01242 211122  W www.hesa.ac.uk
<http://www.hesa.ac.uk/> 
 

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