What is DNS Zone Transfer?

DNS zone transfer is essential for ensuring domain information’s accuracy and consistency. This blog post will explore the key steps to configure a successful transfer and ensure a secure, efficient, and reliable data transfer.

What is DNS Zone Transfer?

DNS Zone Transfer is moving DNS data from a master source, known as a primary domain name server (DNS), to one or more secondary DNS servers. This data is in the form of DNS records, which define the ‘zones’ of your domain. Transferring this data ensures that your domain name and associated services are consistently accessible and updated everywhere. When the resource record changes are made in the primary zone, the secondary server’s job is to complete the DNS zone transfer in a specific order. This process is usually automatic but can be manually triggered by administrators.

Why is it needed?

DNS is a critical service. If a DNS server for a zone is not working and cached information has expired, the domain is inaccessible to all services (web, mail, and more). Therefore, each zone should have at least two DNS servers. For more critical zones, there may be even more.

However, a zone may be large and may require frequent changes. If you manually edit zone data on each server separately, it takes a lot of time and there is a lot of potential for a mistake. This is why DNS zone transfer is needed.

You can use different mechanisms for DNS zone transfer but the simplest one is AXFR (technically speaking, AXFR refers to the protocol used during a DNS zone transfer). It is a client-initiated request. Therefore, you can edit information on the primary DNS server and then use AXFR from the secondary DNS server to download the entire zone.

Types of Zone Transfers

There are two types of zone transfers: full zone transfers (referred to as AXFR) and incremental zone transfers (referred to as IXFR). A full DNS zone transfer is exactly what it sounds like — a full copy of the DNS zone file. Incremental DNS zone transfers are also aptly named as they are a copy of the most recent changes to a zone file.

Returning to our subway map analogy, imagine two stations were recently renamed. You could get a whole new map (AXFR) or write in the new station names on your old map. Either method successfully updates your map.

A full DNS zone transfer is a more time-consuming and bandwidth-intensive process. Unless most of the DNS records have changed, an incremental zone transfer is a more efficient option. You might also use a full zone transfer when adding additional DNS servers.

An incremental zone transfer ensures you have the most up-to-date information faster, using less network bandwidth. Incremental zone transfers can also be used more frequently since they only update changed information.

Practical Application of DNS Zone Transfer

A practical application of DNS zone transfer can be seen in the management of large-scale websites and online services. For instance, a global e-commerce website might have multiple DNS servers located in different parts of the world. By employing DNS zone transfer, the website ensures that all these servers have up-to-date DNS information. This synchronization allows users from any location to access the website quickly and reliably, as their DNS queries can be resolved by the nearest server.

Common reasons for a failed zone transfer

A DNS zone transfer can be a useful way to get information about different domains and networks, however, it is not a guarantee that the transfer will always be successful. Common reasons why a DNS zone transfer may fail include unsupported file formats, incorrect information in the DNS records, and an improperly configured firewall.

One of the most frequent reasons for a failed DNS zone transfer is unsupported file formats. DNSSEC is an emerging protocol that is not supported by all DNS servers and can therefore lead to a failed transfer. An incorrect entry in the DNS records, such as a missing domain name server, can also cause the transfer to fail. Other issues, such as an improperly configured firewall, can also be to blame. To ensure successful transfers, it is important to use supported file formats, double-check DNS records regularly, and audit firewall settings to make sure they are not blocking certain types of traffic.

Troubleshooting Zone Transfer Issues

As with any technology, DNS servers sometimes require troubleshooting. Some of the more common issues will center around one server being unable to communicate with another due to authoritative or network issues.

Check Server Logs
If a zone transfer fails, check the server logs for any error messages which may indicate the issue. If you cannot determine the cause using logs, move on to additional troubleshooting steps.

Verify Zone Transfer Settings
Check zone transfer settings on your primary DNS server. Is the IP address of the secondary server authorized to request zone transfers?

Review TTL Settings 
Did the TTL (time to live) time out on one of the servers? TTL is the amount of time a request is valid and able to elicit a response, so if the TTL expired, the subsequent information would never be sent… If the TTL did expire, check for network latency issues between devices.

Conclusion

DNS zone transfers are crucial to maintaining multiple DNS servers across a network, ensuring accuracy, data consistency, and time and cost savings. With the right strategy and processes in place, DNS zone transfers make it easy to keep your DNS up-to-date and secure.