Load Balancing over Multiple Gateways

The typical situation where you got one router and want to connect to two ISPs:

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Of course, you want to do load balancing. There are several ways how to do it. Depending on the particular situation, you may find one best suited for you.

Policy Routing based on Client IP Address

If you have a number of hosts, you may group them by IP addresses. Then, send the traffic out through Gateway 1 or 2 depending on the source IP address.

Let us assume we use for our workstations IP addresses from network 192.168.100.0/24. The IP addresses are assigned as follows:

• 192.168.100.1-127 are used for Group A workstations
• 192.168.100.128-253 are used for Group B workstations
• 192.168.100.254 is ised for the router.

All workstations have IP configuration with the IP address from the relevant group, they all have network mask 255.255.255.0, and the default gateway 192.168.100.254

We will talk about DNS servers later.

Next, we need to configure policy routing. We have workstations devided into groups, where we can reffer to them using subnet addressing:

• Group A is 192.168.100.0/25, i.e., addresses 192.168.100.0-127
• Group B is 192.168.100.128/25, i.e., addresses 192.18.100.128-255

If you do not understand this, take the TCP/IP Basics course, or, look for some resources about subnetting on the Internet!

Adding Mangle Rules

We need to add two IP Firewall Mangle rules to mark the packets originated from Group A or B workstations.

For Group A, start with specifying Chain forward and Src. Address 192.168.100.0/25:

Next, specify the Action mark routing and add New Routing Mark GroupA.

It is a good practice to add a comment as well. Your mangle rules might be interesting for someone else and for yourself as well after some time.

For Group B, specify

• Chain forward and Src. Address 192.168.100.128/25
• Action mark routing and New Routing Mark GroupB

All IP traffic coming from workstations is marked with the routing marks GroupA or GroupB. We can use these marks in the routing table.

Next, we should specify two default routes (destination 0.0.0.0/0) with appropriate routing marks and gateways:

This thing is not going to work, unless you do masquerading for your LAN! The simpliest way to it is by adding one NAT rule for Src. Address 192.168.100.0/24 and Action masquerade:

Test the setup by using traceroute to some IP address on the Internet!

From a workstation of Group A, it should go like this:

C:\>tracert -d 8.8.8.8

Tracing route to 8.8.8.8 over a maximum of 30 hops

  1     2 ms     2 ms     2 ms  192.168.100.254
  2    10 ms     4 ms     3 ms  10.1.0.1
  ...

From a workstation of Group B, it should go like this:

C:\>tracert -d 8.8.8.8

Tracing route to 8.8.8.8 over a maximum of 30 hops

  1     2 ms     2 ms     2 ms  192.168.100.254
  2    10 ms     4 ms     3 ms  10.5.8.1
  ...

You can specify the the DNS server for workstations quite freely, just make it can be reached (test it by using traceroute to the IP address of DNS server you are using)!