Coyote Point Systems Equalizer GX Series, Administration Manual

Page: 632 / 866

Configuring Server Connections
Direct Server Return (DSR)
In a typical load balancing scenario, server responses to client requests are routed through
Equalizer on their way back to the client. Equalizer examines the headers of each response and
may insert a cookie, before sending the server response on to the client.
In a Direct Server Return (DSR) configuration, the server receiving a client request responds
directly to the client IP, bypassing Equalizer. Because Equalizer only processes incoming
requests, cluster performance is dramatically improved when using DSR in high bandwidth
applications, especially those that deliver a significant amount of streaming content. In such
applications, it is not necessary for Equalizer to receive and examine the server’s responses: the
client makes a request and the server simply streams a large amount of data to the client.
DSR is supported on Layer 4 TCP and UDP clusters only, and is not supported for FTP clusters
(Layer 4 TCP clusters with a start port of 21).
DSR configurations are often configured on a single VLAN or subnet, where the cluster IP and the
server IPs are all on the internal interface. Refer to
"Configuring Direct Server Return"
on page 382 for
details.
DSR can also be used in multiple VLAN configurations, although this is less common. Cluster IP
addresses are on one VLAN/subnet, while server IP addresses are on another VLAN/subnet.
In any DSR configuration, note that the incoming client traffic is assumed to originate on the other
side of the gateway device for the subnets on which Equalizer and the servers reside. The servers
will usually have their default gateway set to something other than Equalizer so that they can
respond directly to client requests.
In DSR configurations where a client device resides on the same side of the gateway as the DSR
servers, there is the possibility that the servers will receive the ARP (Address Resolution Protocol)
request for the virtual cluster IP address. Since the cluster IP address is configured on the
loopback interface of each server (See
"Configuring Direct Server Return"
on page 382 ), one or more
may respond to the ARP request. The client, and possibly even the gateway, will then route
requests for the cluster IP to servers directly without going through Equalizer. If this occurs, you
need to reconfigure the servers so that they do not respond to ARP requests for the cluster IP
addresses configured on the loopback interface. The procedure to follow to do this is specific to
the operating system running on the servers, so please consult the documentation for your server
operating system.
632 Copyright © 2014 Coyote Point Systems, A Subsidiary of Fortinet, Inc.