GSLB (Global Server Load Balance) is also known as Smart DNS (SDNS). This function allows you to distribute Web traffic among a collection of servers deployed in multiple geographic locations. We will cover introduction of GSLB and the examples of GSLB configuration in this chapter.
In GSLB solution, the FortiBalancer appliance works as a complementary DNS server which is able to resolve a set of defined domain names based on load balancing methods. When DNS queries (typically forwarded by corporate DNS server or ISP DNS server) for the domain name are received, GSLB function will resolve the domain name with IP addresses selected from its Domain Name and IP Service Database with configured load balancing method.
SDNS maintains a local Domain Name and IP Service Database by continuously exchanging their local load (Hello message) and domain name/IP address information (Report message) with other members (also FortiBalancer appliances) in the GSLB network. For example, when an FortiBalancer appliance joins the SDNS network, the FortiBalancer appliance will continuously send its local domain name/IP address information to all other participating members (see LLB configuration). For each message transmitted, a confirmation message is expected in return. If a confirmation message is missed or a message is not updated for a period of time (3 tries), GSLB will mark the non-responsive member as down and all the domain name/IP addresses that are hosted by that FortiBalancer appliance will be removed from its local Domain Name and IP Service Database.
The SDNS process works as follows:
Figure 14-1 SDNS Working Mechanism
As shown in the above figure, the SDNS module will process a normal DNS request from the client as follows:
- The client’s browser generates a DNS request for the domain name of the Web site he wants to visit, and sends the request to its local DNS server.
- The local DNS server receives the request and searches in its local cache. If no cache entry hits, it will forward the request to the upper-level SDNS device. In the above example figure, the request is sent to an SDNS server at Beijing according to configurations on the local DNS server.
- The SDNS server at Beijing continuously collects the status information of all the application servers in its local Domain Name and IP Service Database, and then forwards the request to a proper application server based on pre-configured load balancing algorithms. In the above example, the application server at New York is selected.
- The SDNS server at Beijing returns back the IP addresses of the application server at New York to the local application server of the client.
- Upon receiving the response, the local application server forwards IP address to the client directly.
- The client’s browser uses the IP address in the response to open an HTTP connection with the corresponding FortiBalancer appliance and proceeds to download the Web page.
In this process, the response is cached on both the client’s local DNS server and the client’s browser.
Note: In this chapter, we will use the term “member” or “SDNS member” frequently. Either
“member” or “SDNS member” is an FortiBalancer appliance which participates in the GSLB management.
All SDNS members can be divided into two groups: SDNS server and HTTP proxy cache server. They are all FortiBalancer appliances, while HTTP proxy cache servers serve as the “reporter” and SDNS servers serve as the “receiver”.
Figure 14-2 SDNS Reporter-Receiver Hierarchy
SDNS servers are responsible for DNS resolving. Every HTTP proxy cache server will report its status information to SDNS servers. The status information includes:
- The domain name configured on proxy cache servers
- The IPs which are configured for a domain name and their status (“UP” or “DOWN”)
- The domain name traffic on proxy servers, IP traffic and proxy traffic
- The status of proxy cache servers (“UP” or “DOWN”)
HTTP Proxy Cache Servers
HTTP proxy cache servers are responsible for HTTP services. All kinds of HTTP requests will be directed to HTTP proxy cache servers, mostly by the SDNS servers. The HTTP proxy cache servers will collect the local status information and send it to SDNS servers at specified frequency. If an FortiBalancer appliance is a DNS server and a proxy cache server at the same time, it will report its local status information to all the SDNS servers (including itself) and collect the status information from all the proxy cache servers.
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