I have done this any times for customers and i have not found a developed system to make this so i have ever roll my own, the steps that Manwe has given you are more or less what i do when i need them but i will paste here any crude bash scripts i am using (when i have time i want to make this much better in python).
Basically i check if i have internet or not and if i am using the wan backup and make the changes needed
#!/bin/bash
PATH="/bin:/sbin:/usr/bin:/usr/sbin"
primary_gw="192.168.1.1" #for example.
check_one="8.8.8.8"
check_two="8.8.4.4"
#first we check internet connection.
if `ping -c 1 -W 1 $check_one |grep -E '(unreachable|100\%\ packet\ loss)' &> /dev/null` &&\
`ping -c 1 -W 1 $check_two |grep -E '(unreachable|100\%\ packet\ loss)' &>/dev/null`
then #if we don't have internet
if [ -e /tmp/wan_backup ]
#if we are using backup right now we try to change to primary connection
then ./script_change_to_primary.sh && rm /tmp/wan_backup
#else we change to wan backup.
else ./script_change_to_backup.sh && touch /tmp/wan_backup
fi
fi
#if we are using wan backup right now we check if primary connection works.
if [ -e /tmp/wan_backup ]
then
if `ip route add $check_one via $primary_gw; ip route add $check_two via $primary_gw;\
sleep 2; ping -c 1 -W 1 $check_one | grep -E '(unreachable|100\%\ packet\ loss)' &> /dev/null &&\
ping -c 1 -W 1 $check_two | grep -E '(unreachable|100\%\ packet\ loss)' &> /dev/null`
then #don't works we clean the routes and stay using backup
ip route del $check_one via $primary_gw
ip route del $check_two via $primary_gw
else #it works so we change active connection
ip route del $check_one via $primary_gw
ip route del $check_two via $primary_gw
./script_change_to_primary.sh
rm /tmp/wan_backup
fi
fi
Given that you only want your server to use 3g if adsl goes down i would only use iptables snat or masquerade only in the adsl iface and i would block access to squid in ./script_change_to_secondary.sh, your files could be:
script_change_to_secondary.sh
#!/bin/bash
pon 3gIsp #this one it is going to change the default route of server anyway
#drop squid connections, you could disable here the boot snat or masquerade for adsl
#but given your adsl is not active i don't see the need anyway
iptables -I INPUT -s LAN_IP_RANGE -d SERVER_IP -p tcp --dport 3128 -j DROP
script_change_to_primary.sh
#!/bin/bash
poff 3gIsp
iptables -D INPUT -s LAN_IP_RANGE -d SERVER_IP -p tcp --dport 3128 -j DROP
/etc/init.d/openvpn restart
You should have too in /etc/ppp/ip-up.d/ a bash script with "/etc/init.d/openvpn restart", this way every time you connect to a ppp provider your openvpn will restart automatically.
Well like i have said it is a bit ugly and crude but it works :) if you find a integrated clean solution for this make me know please :), one good thing of roll-your-own it is that you have a complete control of the system, this is an oversimplification of what i do in any customers that have two or three connections alive at the same time and do a load balancing and QoS all integrated with scripts that detect connections problems and changes the routes and the QoS.
If you prefer an integrated solution to a roll-your-own you can use a distribution like zentyal, it supports what you want to use but it is a complete distribution tailored to create a SmallBusiness server, i usually prefer to configure my servers at my own but this is a good distribution that can be managed via web.
