Making sure that data from two modems gets routed via the same channel it came through even when its split over two simultaneously connected modem

Ok so since i have shown a method to get access to the internet with multiple modems , someone asked what about if i need the same data to pass though the same modem (why? incase of a download that shouldnt be stopped/doesnt allow resume support) so... heres a method...

so we come up with names. Let $IF1 be the name of the first interface and $IF2 the name of the second interface. Then  $IP1 be the IP address associated with $IF1 and $IP2 the IP address associated with $IF2. Next, let $P1 be the IP address of the gateway at Provider 1, and $P2 the IP address of the gateway at provider 2. Finally, let $P1_NET be the IP network $P1 is in, and $P2_NET the IP network $P2 is in.

One creates two additional routing tables, say T1 and T2. These are added in /etc/iproute2/rt_tables. Then you set up routing in these tables as follows:

   ip route add $P1_NET dev $IF1 src $IP1 table T1
   ip route add default via $P1 table T1
   ip route add $P2_NET dev $IF2 src $IP2 table T2
   ip route add default via $P2 table T2
 

Nothing spectacular, just build a route to the gateway and build a default route via that gateway, as you would do in the case of a single upstream provider, but put the routes in a separate table per provider. Note that the network route suffices, as it tells you how to find any host in that network, which includes the gateway, as specified above.

Next you set up the main routing table. It is a good idea to route things to the direct neighbour through the interface connected to that neighbour. Note the `src' arguments, they make sure the right outgoing IP address is chosen.

     ip route add $P1_NET dev $IF1 src $IP1
     ip route add $P2_NET dev $IF2 src $IP2
   

Then, your preference for default route:

     ip route add default via $P1
   

Next, you set up the routing rules. These actually choose what routing table to route with. You want to make sure that you route out a given interface if you already have the corresponding source address:

     ip rule add from $IP1 table T1
     ip rule add from $IP2 table T2
   

This set of commands makes sure all answers to traffic coming in on a particular interface get answered from that interface.

NB

: 'If $P0_NET is the local network and $IF0 is its interface, the following additional entries are desirable: ip route add $P0_NET dev $IF0 table T1 ip route add $P2_NET dev $IF2 table T1 ip route add 127.0.0.0/8 dev lo table T1 ip route add $P0_NET dev $IF0 table T2 ip route add $P1_NET dev $IF1 table T2 ip route add 127.0.0.0/8 dev lo table T2 '

Now, this is just the very basic setup. It will work for all processes running on the route itself, and for the local network, if it is masqueraded. If it is not, then you either have IP space from both modems or you are going to want to masquerade to one of the two modems. In both cases you will want to add rules selecting which modems to route out from based on the IP address of the machine in the local network.