Router - Linux upload bonding using policy routing

I'm using PPP on the Linux box - in my case via a Solos ADSL card, but this will also work using PPPoE.

First, for convenience, name your extra routing tables by editing /etc/iproute2/rt_tables:

1       ppp0
2       ppp1

Add a script in /etc/ppp/ip-up.d and in /etc/ppp/ipv6-up.d to put the routes in the per-interface tables:

#!/bin/bash
# ip-up.d/set-routes

ip route replace default dev ${PPP_IFACE} table ${PPP_IFACE}

#!/bin/bash
# ipv6-up.d/set-routes

ip -6 route replace default dev ${PPP_IFACE} table ${PPP_IFACE}

Add a script in /etc/ppp/ip-down.d and in /etc/ppp/ipv6-down.d to remove the per-interface routes when the PPP link goes down:

#!/bin/bash
# ip-down.d/remove-routes

ip route flush table ${PPP_IFACE}

#!/bin/bash
# ipv6-down.d/remove-routes

ip -6 route flush table ${PPP_IFACE}

Run the following script on boot:

#!/bin/bash

# fwmark 1 is line 1
ip rule add fwmark 1 table ppp0 prio 40000
ip -6 rule add fwmark 1 table ppp0 prio 40000

# fwmark 2 is line 2
ip rule add fwmark 2 table ppp1 prio 40001
ip -6 rule add fwmark 2 table ppp1 prio 40001

# unmarked prefers line 1, then does line 2 if line 1 is not possible
ip rule add from all table ppp0 prio 50000
ip -6 rule add from all table ppp0 prio 50000
ip rule add from all table ppp1 prio 50001
ip -6 rule add from all table ppp1 prio 50001

This uses iproute2's ip command to set up policy routing rules; the first set give you a firewall mark per line. (rules at prio 40000 and 40001). The last block (rules at 50000 and 50001) serves two purposes:

It ensures that packets that are marked for routing via a dead line are passed onto the other line
It provides a fallback if you forget to add firewall marks in PREROUTING for some packets.

Finally, apply firewall marks in PREROUTING to choose your load balancing policy. For example:

for IPT in iptables ip6tables
do
    $IPT -A PREROUTING ! -i ppp+ -m statistic --mode nth --every 2 --packet 0 -j MARK --set-mark 1
    $IPT -A PREROUTING ! -i ppp+ -m statistic --mode nth --every 2 --packet 1 -j MARK --set-mark 2
done

simply load balances by alternating packets on each line.