Server Health Monitoring; Setting Up Slb_Sat Rules - D-Link NetDefendOS User Manual

Regardless which algorithm is chosen, if a server goes down, traffic will be sent to other servers.
And when the server comes back online, it can automatically be placed back into the server farm
and start getting requests again.

10.4.5. Server Health Monitoring

SLB uses Server Health Monitoring to continuously check the condition of the servers in an SLB
configuration. SLB can monitor different OSI layers to check the condition of each server.
Regardless of the algorithms used, if a server is deemed to have failed, SLB will not open any
more connections to it until the server is restored to full functionality.
D-Link Server Load Balancing provides the following monitoring modes:
ICMP Ping
TCP Connection

10.4.6. Setting Up SLB_SAT Rules

The key component in setting up SLB are IP rules that have SLB_SAT as the action. The steps that
should be followed for setting up such rules are:
1. Define an IP address object for each server for which SLB is to be enabled.
2. Define an IP address group object which includes all these individual objects.
3. Define an SLB_SAT rule in the IP rule set which refers to this IP address group and where all
other SLB parameters are defined.
4. Define a further rule that duplicates the source/destination interface/network of the
SLB_SAT rule that permits the traffic through. This could be one rule or a combination of
rules using the actions:
• Allow
• NAT
Figure 10.12. Stickiness and Connection-rate
This works at OSI layer 3. SLB will ping the IP address of each individual
server in the server farm. This will detect any failed servers.
This works at OSI layer 4. SLB attempts to connect to a specified port on
each server. For example, if a server is specified as running web services
on port 80, the SLB will send a TCP SYN request to that port. If SLB does
not receive a TCP SYN/ACK back, it will mark port 80 on that server as
down. SLB recognizes the conditions no response, normal response or
closed port response from servers.
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Chapter 10: Traffic Management