30.7. Creating a New Slave Zone
A slave or secondary zone is one for which your DNS server gets the list of records from a master server for the zone. Generally, slave servers are used to reduce the load on the primary server or act as a backup in case it goes down. For important zones (such as a company's Internet domain), you should always have at least one slave server so that your website is still accessible and email can still be delivered even if the primary goes down.
The secondary DNS server for a domain should not usually be located on the same network as the master, so that the failure of that network cannot take them both down. Many ISPs and hosting companies will host secondary zones for their customer's domains for free, on their own DNS servers. If your ISP provides this service and you want to set up a secondary server for an Internet domain, you should take advantage of it. If so, most of this section can be skipped. However, if you want to add a slave server for an internal domain or have a large company network with many connections to the Internet, then the instructions below explain how to set it up:
1. | On the main page of the BIND DNS Server module, click on the Create a new slave zone link above or below the list of existing zones. This will bring up the form shown in Figure 30.5, for entering the details of the new domain. Figure 30.5. The slave zone creation form.
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2. | For a forward zone like example.com, set the Zone type field to Forward and enter the zone name into the Domain name / Network field. For a reverse zone that maps IP addresses to hostnames for a network, choose the Reverse option and enter the network address (like 192.168.1) into the Domain name / Network text field. |
3. | The Records file field determines if BIND keeps a cache of the records in this zone in a file and, if so, where that file is located. If the option None is chosen, records that the DNS server transfers from the master will be kept in memory only, and lost when the server is re-started. This should only be chosen if there is a good network connect between the master and slave servers, as it will increase the number of zone transfers that your server must perform. If you choose Automatic, Webmin will generate a filename in the zone files directory specified in the named.conf file (usually /var/named). Whenever your server does a zone transfer, all records will be written to this file in the standard format. If the final option is selected, you can enter the full path to a file in which records should be stored into the field next to. This can be useful if you want to separate the records files for master and slave zones. |
4. | In the Master servers field, enter the IP addresses of the master DNS server and any other secondary servers for the zone. BIND will try these servers in order when doing a zone transfer, so the master should be first on the list. You must enter at least one address, so that your server knows where to get records from. |
5. | Click the Create button to have the new slave zone added to your server's configuration. Your browser will be re-directed to a page for editing options for the zone. |
6. | Return to the module's main page and click the Apply Changes button on the main page to make the addition active. |
7. | On the master server, add a new Name Server (NS) record for the zone with the IP address of the secondary server. This can be easily done in Webmin by following the instructions in Section 30.4 “Adding and Editing Records”. |
8. | Configure the master DNS server to notify this slave of any changes to records in the zone. The steps in Section 30.6 “Editing a Master Zone” explain how. |
9. | If this is an Internet domain, notify the registrar for the parent zone of the new secondary server. Most provide online forms for editing the list of nameservers for a domain, to which you can add the secondary's IP. This is necessary so that other hosts on the Internet know to use the slave server is the master is down. |
Another type of zone that is closely related to the slave zone is the stub. They are like slave zones, but only contain Name Server records that have been transferred from a master server, instead of all the records. Stub zones are rarely used, but can be useful for ensuring that the Name Server records in a zone for its sub-domains are the same as those use in the sub-domain itself. The steps for creating one are almost identical to those above, but in Step 1 you must use the Create a new stub zone link on the main page instead.