User manual

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
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.
TCP Connection
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.
10.4.6. SLB_SAT Rules
The key component in setting up SLB is the SLB_SAT rule in the IP rule set. The steps that should
be followed are:
1. Define an IP address object for each server for which SLB is to 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
Note: FwdFast rules should not be used with SLB
In order to function, SLB requires that the NetDefendOS state engine keeps track of
connections. FwdFast IP rules should not be used with SLB since packets that are
forwarded by these rules are under state engine control.
The table below shows the rules that would be defined for a typical scenario of a set of webservers
behind the NetDefend Firewall for which the load is being balanced. The Allow rule allows external
clients to access the webservers.
Rule Name Rule Type Src. Interface Src. Network Dest. Interface Dest. Network
WEB_SLB SLB_SAT any all-nets core ip_ext
WEB_SLB_ALW Allow any all-nets core ip_ext
If there are clients on the same network as the webservers that also need access to those webservers
then an NAT rule would also be used:
Rule Name Rule Type Src. Interface Src. Network Dest. Interface Dest. Network
WEB_SLB SLB_SAT any all-nets core ip_ext
WEB_SLB_NAT NAT lan lannet core ip_ext
10.4.6. SLB_SAT Rules Chapter 10. Traffic Management
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