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Manual Chapter: Replacing a DNS Server with BIG-IP GTM
Manual Chapter
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Overview: Replacing a DNS server with BIG-IP GTM

BIG-IP Global Traffic Manager (GTM) load balances incoming wide IP traffic to your network resources. BIG-IP GTM can also replace a local DNS server as the authoritative nameserver for wide IPs, zones, and all other DNS-related traffic. You can configure BIG-IP GTM to replace the DNS server that currently manages www.siterequest.com. BIG-IP GTM becomes the authoritative nameserver for www.siterequest.com and load balances traffic across the web-based applications store.siterequest.com and checkout.siterequest.com.
Traffic flow when BIG-IP GTM replaces DNS server

About listeners

A listener is a specialized virtual server that uses port 53 and to which you assign a specific IP address. When traffic is sent to that IP address, the listener alerts BIG-IP GTM and the system either handles the traffic locally or forwards the traffic to the appropriate resource.

You control how BIG-IP GTM responds to network traffic on a per-listener basis. The number of listeners you create depends on your network configuration and the destinations to which you want to send specific DNS requests. For example, a single BIG-IP GTM can be the primary authoritative server for one domain, while forwarding other DNS requests to a different DNS server. Regardless of how many listeners you configure, BIG-IP GTM always manages and responds to requests for the wide IPs that you have configured on the system.

Task summary

Perform these tasks to replace a DNS server with BIG-IP GTM.

Configuring the legacy DNS server to allow zone file transfers

If you are unfamiliar with how to modify DNS server files, review the fifth edition of DNS and BIND, available from O’Reilly Media.
To configure the legacy DNS server to allow zone file transfers to BIG-IP system, add to the DNS server an allow-transfer statement that specifies the IP address of the new BIG-IP system.
You can modify the following allow-transfer statement to use the IP address of your BIG-IP system: allow-transfer { localhost; <IP address of BIG-IP system>; };

Acquiring zone files from the legacy DNS server

Ensure that you have configured the legacy DNS server with an allow-transfer statement that authorizes zone transfers to BIG-IP GTM.
For BIG-IP GTM to acquire zone files from the legacy DNS server, create a new zone.
  1. On the Main tab, click Global Traffic > ZoneRunner > Zone List. The Zone List screen opens.
  2. Click Create. The New Zone screen opens.
  3. From the View Name list, select the view that you want this zone to be a member of. The default view is external.
  4. In the Zone Name field, type a name for the zone file in this format, including the trailing dot: db.[viewname].[zonename]. For example, db.external.siterequest.com.
  5. From the Zone Type list, select Master.
  6. From the Records Creation Method list, select Transfer from Server.
  7. In the Source Server field, type the IP address of the DNS server (the server from which you want BIG-IP GTM to acquire zone files).
  8. Click Finished.

Creating a self IP address using the IP address of the legacy DNS server

To avoid a conflict on your network, unplug BIG-IP GTM from the network.
When you want BIG-IP GTM to handle DNS traffic previously handled by a DNS server, create a self IP address on BIG-IP GTM using the IP address of the legacy DNS server.
  1. On the Main tab, click Network > Self IPs. The Self IPs screen opens.
  2. Click Create. The New Self IP screen opens.
  3. In the IP Address field, type the IP address of the legacy DNS server. The system accepts IP addresses in both the IPv4 and IPv6 formats.
  4. In the Netmask field, type the network mask for the specified IP address.
  5. Click Finished. The screen refreshes, and displays the new self IP address in the list.

Designating GTM as the primary server for the zone

Ensure that you have created a self IP address on BIG-IP GTM using the IP address of the legacy DNS server.
Add this self IP address to the BIG-IP GTM server object. Then modify the DNS server based on your network configuration.
  1. Log on to BIG-IP GTM.
  2. On the Main tab, click Global Traffic > Servers. The Server List screen opens.
  3. Click the name of the BIG-IP GTM system that you want to modify. The server settings and values display.
  4. In the Address List area, add the new self IP address.
  5. Click Update.
  6. Do one of the following based on your network configuration:
    • Modify the IP address of the legacy DNS server so that it becomes a secondary DNS server to BIG-IP GTM. Ensure that the IP address of the DNS server does not conflict with the self IP address that you added to the BIG-IP GTM server object.
      Note: If you are using BIND servers, and you are unfamiliar with how to change a DNS server from a primary to a secondary, refer to the fifth edition of DNS and BIND, available from O’Reilly Media.
    • Remove the legacy DNS server from your network.
BIG-IP GTM is now the primary authoritative name server for the zone. The servers for the zone do not need to be updated, because the IP address of the legacy DNS server was assigned to BIG-IP GTM.

Creating listeners to alert GTM to DNS traffic destined for the system

To alert the BIG-IP GTM system to DNS traffic (previously handled by the DNS server), create two listeners: one that uses the UDP protocol, and one that uses the TCP protocol.
Note: DNS zone transfers use TCP port 53. If you do not configure a listener for TCP the client may receive the error: connection refused or TCP RSTs.
  1. On the Main tab, click Global Traffic > Listeners. The Listeners List screen opens.
  2. Click Create. The new Listeners screen opens.
  3. In the Destination field, type the IP address previously used by the legacy DNS server.
  4. From the VLAN Traffic list, select All VLANs.
  5. From the Protocol list, select UDP.
  6. Click Finished.
Create another listener with the same IP address, but select TCP from the Protocol list.

Creating a wide IP

Ensure that at least one load balancing pool exists in the configuration before you start creating a wide IP.
Create a wide IP to map a FQDN to one or more pools of virtual servers that host the content of the domain.
  1. On the Main tab, click Global Traffic > Wide IPs The Wide IPs List screen opens.
  2. Click Create. The New Wide IP screen opens.
  3. Type a name for the wide IP.
    Tip: You can use two different wildcard characters in the wide IP name: asterisk (*) to represent several characters and question mark (?) to represent a single character. This reduces the number of aliases you have to add to the configuration.
  4. Specify the pools for this wide IP to use for load balancing. The system evaluates the pools in the order in which they are listed, until it finds a matching pool.
    1. From the Poollist, select a pool. A pool can belong to more than one wide IP.
    2. Click Add.
  5. Click Finished.

Implementation result

BIG-IP GTM replaces the legacy DNS server as the primary authoritative nameserver for the zone. BIG-IP GTM handles all incoming DNS traffic, whether destined for a wide IP or handled by the BIND instance on the system.

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