Direct Server Return Example (Single Legged)

This simple "Direct Server Return" example should give you a starting point for your own test setup. You should change the example network "172.16.1.0/24" to match your own network setup (and the server address too). This example is fully operational with the free BalanceNG Basic Trial License.

1. The initial network

Consider this to be the simple initial network configuration before inserting the BalanceNG node (the network 172.16.1.0/24 should represent a public allocated address space):

Figure 1: Network configuration without BalanceNG

What should be working:

• You should be able to ping the default gateway 172.16.1.254 from the Web Server.
• Clients from the Internet are able to initiate TCP connection to port 80 on the Web Server 172.16.1.10.

2. Load balancing to two targets in DSR-Mode (Direct Server Return)

The key idea for BalanceNG in DSR mode is to configure the IP address of the Virtual Server (172.16.1.10 in this example) as an alias on the Loopback interface inside the Targets. This makes the Targets addressable on Layer 2 leaving the Virtual Server Address unchanged. The machine must not answer any ARP requests for the virtual server address.

Figure 2: Network configuration BalanceNG single legged balancing to two targets

Establishing the Loopback Aliases

Use the following commands to establish the Loopback-alias on the Targets:

Linux:      
  ifconfig lo:0 <IP-address> netmask 255.255.255.255 -arp up
Solaris: 
  ifconfig lo0:1 plumb
  ifconfig lo0:1 <IP-address> netmask 255.255.255.255 up

In our example this would be the following command line (assuming a Linux operating system) executed on both Target 1 and Target 2:

# ifconfig lo:0 172.16.1.10 netmask 255.255.255.255 -arp up 

Additionally it is necessary to disable invalid ARP replies (Linux 2.6 kernel, take a look at the FAQ question Q2.09):

# echo 1 > /proc/sys/net/ipv4/conf/all/arp_ignore
# echo 2 > /proc/sys/net/ipv4/conf/all/arp_announce

The Packet flow in this setup is as follows:

Figure 3: Packet Flow in a DSR (Direct Server Return) setup

The BalanceNG config file

This is the BalanceNG configuration for this DSR example. The BalanceNG box is connected to the switch with one interface (eth0) only. The "real" address in the network section is being used as source address for ARP resolving and health checks.

//        configuration taken Fri Sep 28 21:38:37 2007
//        BalanceNG 1.897 (created 2007/09/18)
interface eth0
network   1 {
          name "local network"
          addr 172.16.1.0
          mask 255.255.255.0
          real 172.16.1.252
          interface eth0
}
register  network 1
enable    network 1
server    1 {
          ipaddr 172.16.1.10 
          port 80
          protocol tcp
          targets 1,2
}
register  server 1
enable    server 1
target    1 {
          ipaddr 172.16.1.100
          port 80
          protocol tcp
          tcpopen 80,3,10
          dsr enable
}
target    2 {
          ipaddr 172.16.1.101 
          port 80
          protocol tcp
          tcpopen 80,3,10
          dsr enable
}
register  targets 1,2
enable    targets 1,2
//        end of configuration

What should be working:

• You should be able to ping the default gateway 172.16.1.254 from the targets (as before).
• You should be able to ping the virtual web server 172.16.1.10 from the router.
• Clients from the Internet are able to initiate TCP connection to port 80 on the virtual Web Server 172.16.1.10 (now being represented by BalanceNG).
• "show servers" and "show targets" in BalanceNG interactive mode should show everything up and operating.

Please contact us in case of further questions, errors or suggestions at info@inlab.de.