Specifications

Chapter 3: Configuring the Barracuda Link Balancer 31
disrupted. A short TTL value for this record ensures that the cached address for the failed link times
out quickly.
Specifying a short TTL for A records also assists in link balancing. Because the address for a host that
is returned varies among the available links, the short TTL guarantees that the link used for incoming
traffic directed to that host also varies frequently.
Recommended Deployment
We recommend that you take advantage of this feature if you are hosting services such as Web
servers, VPNs and email that are name based. This increases the availability of your services and
provides a way to do inbound link balancing.
Split DNS
The Barracuda Link Balancer supports a split DNS infrastructure. If the same hostname is used for a
resource that is accessible both internally and externally, internal network clients receive the internal
IP address and external clients receive the external IP address when they ask for the address of that
hostname. Specifically, the A record for the hostname includes two views, one with the internal IP
address and one with the external IP address. In this way, clients only see the address that they should
use.
Details of how to make this work can be found in Step 3 - Set up DNS for Internal Clients on page 33.
The split DNS infrastructure handles accessing resources using a hostname. What about accessing
externally accessible resources using an IP address? If local clients use external IP addresses to access
internal servers, the Barracuda Link Balancer translates the address and properly forwards those
requests back to internal servers.
DNS Zone Transfer Blocking
The Barracuda Link Balancer can be configured to block zone transfers on some or all of the domains
that it hosts. An AXFR/IXFR query that is sent from another DNS server to the Barracuda Link
Balancer (to request a copy of the DNS records) is rejected if zone transfers are disabled for that
domain. By default, zone transfers are enabled for all domains created.
Becoming an Authoritative DNS Host
Table 3.1 provides an overview of the steps required to make the Barracuda Link Balancer an
Authoritative DNS host.
Table 3.1: Configuration Steps
Step Explanation
Enable authoritative DNS on the Barracuda Link
Balancer.
Identify which WAN links are to be used as Name
Servers.