Inbound Load Balancing and Failover (Multihoming)
Load Balancing & Fault Tolerance
Inbound Load Balancing and Failover (Multihoming)
Multihoming
Multihoming is a technique when external users request any server’s IP address; Multihoming promptly returns DNS
response according to the link quality. This provides unmatched availability of bandwidth and load-balances incoming
traffic across the multiple ISP lines.
Simultaneously using multiple IP address provided by the ISP connections can result in problems with inbound traffic.
For example, if the network is currently using an IP provided by ISP1, and a problem occurs with this ISP, then the
inbound query will not be received because the external traffic only knows the IP address provided by ISP1. Also, by
using the IP provided ISP1, ISP2 cannot manage the inbound traffic of ISP1. Therefore the concern with multiple ISP
links is how to effectively display IP address to the external environment.
Multihoming uses DNS fault-tolerance technique to resolve this problems with the simultaneous use of multiple ISP
connections. For example, if the web server for external traffic uses a single ISP connection, then any problems with
that connection will affect the network. However, if the DNS periodically assigns different IP addresses provided by
different ISP connections, then the external traffic will always have a valid IP to connect to. The actual implementation
is assigning a name of different IP, and any query to this name will receive an IP address. As a result, different users
can access the web server through different IPs, which is the purpose of Multihoming.
Assuming, there are three WAN links (therefore three different IPs) for the web site of www.example.com, the DNS
record has three entries:
www IN A 211.21.10.3
www IN A 63.98.110.123
www IN A 192.136.1.243
All DNS requests to www.example.com will be sent to FortiWAN. Multihoming will constantly measure the health
conditions as well as the state of each WAN link and compute the optimal return answer to the DNS queries, defined
as the SwiftDNS technology. The SwiftDNS technology will not only ensure fault tolerance for inbound traffic, it also
supports powerful and flexible load balancing algorithms as in the Auto Routing mechanism to enable users with heavy
web presence to maximize the reliability and efficiency of their web services.
The SwiftDNS Multihoming mechanism requires network administrators to understand the details of the system
behavior. The fundamental concept of the DNS mechanism is shown in the next section. A step by step deployment
tutorial will also be provided.
Introduction to DNS
DNS server differs from the host file based on name resolution. Host file contains information of IP address mapping
information. It is only useful for intranet where the information of host machines is relatively static. Name resolution by
DNS server is dynamic because it can adapt to changes easily. The way it works is based on DNS server hierarchy on
the Internet. If a DNS server cannot resolve a name (the information is not in its cache), it will ask other DNS servers.
There is a protocol on how and where to ask other DNS servers.
A name resolution request may go through a number of DNS servers. When an answer is found, it will be saved in
cache so that the same request can be answered immediately without asking other DNS servers again. Each name
resolution result saved in cache has a TTL (Time To Live). After the period of TTL, it will be discarded in order to avoid
stale information.
The whole internet has a large DNS hierarchy. The top of the hierarchy is called Root. It consists of a set of Root DNS
servers coordinated by ICANN. The next level below Root is Top Level Domain (TLD). TLD registration database
FortiWAN Handbook
Fortinet Technologies Inc.
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