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Linux Advanced Routing & Traffic Control HOWTO
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4.2. Routing for multiple uplinks/providers
A common configuration is the following, in which there are two
providers that connect a local network (or even a single
machine) to the big Internet.
+------------+ /
| | |
+-------------+ Provider 1 +-------
__ | | | /
___/ \_ +------+-------+ +------------+ |
_/ \__ | if1 | /
/ \ | | |
| Local network -----+ Linux router | | Internet
\_ __/ | | |
\__ __/ | if2 | \
\___/ +------+-------+ +------------+ |
| | | \
+-------------+ Provider 2 +-------
| | |
+------------+ \_____
There are usually two questions given this setup.
4.2.1. Split access
The first is how to route answers to packets coming in over a
particular provider, say Provider 1, back out again over that
same provider.
Let us first set some symbolical names. Let $IF1 be the name of
the first interface (if1 in the picture above) and $IF2 the name
of the second interface. Then let $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.
Now, this is just the very basic setup. It will work for all
processes running on the router itself, and for the local
network, if it is masqueraded. If it is not, then you either have
IP space from both providers or you are going to want to
masquerade to one of the two providers. In both cases you will
want to add rules selecting which provider to route out from
based on the IP address of the machine in the local network.
4.2.2. Load balancing
The second question is how to balance traffic going out over the
two providers. This is actually not hard if you already have set
up split access as above.
Instead of choosing one of the two providers as your default
route, you now set up the default route to be a multipath route.
In the default kernel this will balance routes over
the two providers. It is done as follows (once more building on
the example in the section on split-access):
ip route add default scope global nexthop via $P1 dev $IF1 weight 1 \
nexthop via $P2 dev $IF2 weight 1
This will balance the routes over both providers. The weight
parameters can be tweaked to favor one provider over the other.
Note that balancing will not be perfect, as it is route based,
and routes are cached. This means that routes to often-used
sites will always be over the same provider
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