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.
Reader Rod Roark notes: '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 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.
Furthermore, if you really want to do this, you probably
also want to look at Julian Anastasov's patches at
http://www.ssi.bg/~ja/#routes , Julian's route patch page.
http://lartc.org/howto/lartc.rpdb.multi...
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