DNS Part 4 – Forwarders, Conditional Forwarders, Recursive & Iterative Query, Round Robin
In this segment of the MCSA certification course, we will dive deep into the essential DNS concepts that are crucial for both DNS functionality and system administration. By understanding DNS forwarders, conditional forwarders, round robin, and more, you will be better prepared for your MCSA exam and enhance your overall DNS knowledge. Let’s explore these topics in detail.
DNS Forwarders
What are DNS Forwarders?
DNS forwarders are DNS servers configured to forward requests to other DNS servers instead of resolving them locally. When a DNS server receives a query that it cannot resolve from its cache or zone files, it will forward that query to a specified forwarder server. This is especially useful for DNS queries to external domains or when the local DNS server does not have enough resources to resolve every query independently.
Why are DNS Forwarders Essential?
DNS forwarders help to:
- Improve query resolution efficiency by leveraging external DNS servers.
- Reduce the load on local DNS servers.
- Allow for easier management of DNS traffic between different networks.
- Enable centralized DNS query handling when multiple DNS servers are part of a large infrastructure.
Step-by-Step Configuration Guide
- Open the DNS Manager on the server.
- Right-click on the Server name and choose Properties.
- Go to the Forwarders tab.
- Click Edit, and in the pop-up window, add the IP addresses of the forwarder DNS servers.
- Click OK to save the changes.
Now your DNS server will forward unresolved queries to the specified DNS servers.
2. Conditional Forwarders
What are Conditional Forwarders?
Conditional forwarders are similar to regular DNS forwarders, but they only forward DNS queries for specific domains. This means that instead of forwarding all unresolved queries, a DNS server will only forward queries for domains that match a particular condition.
How Do Conditional Forwarders Work?
With conditional forwarders, you can set DNS servers to forward queries for certain domains to specific DNS servers. For example, if your organization uses internal domains that are not publicly available, you can configure a DNS server to forward all internal domain queries to an internal DNS server.
Example Use Case
If a company has both internal domains (e.g., corp.local) and external domains (e.g., example.com), you can set up a conditional forwarder to send all queries for corp.local to an internal DNS server and forward external domain queries to an external DNS server.
Configuration Steps
- Open the DNS Manager.
- Right-click on the server name and select Properties.
- Go to the Forwarders tab.
- Click New Conditional Forwarder and enter the domain name you wish to forward.
- Add the IP addresses of the DNS servers for that domain.
- Click OK to save the changes.
3. Root Hints Server
What are Root Hint Servers?
The root hint servers are a list of authoritative DNS servers that help a DNS server start the process of resolving domain names. These servers are the first point of contact for DNS servers when they need to resolve a domain name but don’t have a cached record. The root hint servers direct DNS queries to the correct Top-Level Domain (TLD) servers, which then point to the authoritative servers for the requested domain.
The 13 Root Hint Servers
There are 13 root hint servers globally, denoted by letters A through M. These servers are critical for the initial steps of domain resolution. Their role is fundamental in ensuring the global DNS system functions seamlessly.
Role in DNS Resolution
When a DNS server receives a query for a domain name it cannot resolve, it will reference its root hints to contact one of the 13 root servers. The root server will then direct it to the relevant TLD DNS server, which finally resolves the domain.
4. Round Robin in DNS
What is Round Robin DNS?
Round Robin DNS is a load-balancing technique that distributes DNS requests across a group of servers. When multiple servers are available to handle a specific domain, Round Robin DNS will resolve the same domain name to different IP addresses in a rotating manner.
How Does It Work?
Each time a client requests a domain name, the DNS server responds with one of the IP addresses in the list in a rotating order. This helps to spread the load evenly among the servers, improving performance and availability.
Example
If a website has three servers handling traffic, Round Robin DNS would return:
- First request: IP1
- Second request: IP2
- Third request: IP3
- Fourth request: IP1 again, and so on.
This process helps to distribute traffic among available resources.
5. Netmask Ordering in DNS
What is Netmask Ordering?
Netmask ordering is a DNS feature that helps direct traffic based on the client’s network. This technique ensures that DNS responses are tailored to the client’s geographic or network-based location. By considering the client’s subnet mask, DNS servers can return the most relevant server address.
How Does Netmask Ordering Work?
When a DNS server receives a query, it checks the client’s IP address and compares it with available subnets. It then returns the IP address of the server that is geographically or topologically closest to the client.
This feature is particularly useful in large organizations with multiple locations or data centers, ensuring that users connect to the nearest server, minimizing latency and improving the user experience.
6. Secure Cache Against Pollution
What is DNS Cache Poisoning?
DNS cache pollution (or poisoning) occurs when a DNS server’s cache is tampered with to provide malicious or incorrect answers. This can lead to users being redirected to malicious websites or servers.
How to Secure the DNS Cache?
To secure the DNS cache against pollution, you should:
- Regularly update and patch DNS servers to prevent vulnerabilities.
- Implement DNSSEC (Domain Name System Security Extensions) to ensure the authenticity of DNS responses.
- Monitor DNS logs for any unusual activities or attempts at tampering.
7. Recursive vs. Iterative Queries
Recursive Queries
In a recursive query, the DNS server takes full responsibility for resolving the query. The server will continue querying other DNS servers on behalf of the client until it either resolves the query or returns an error. This is the type of query most users encounter.
Iterative Queries
In contrast, an iterative query only involves a DNS server providing the best possible answer it can. If the server cannot fully resolve the query, it will refer the client to another server that might have more information.
Example of Recursive Query: A client asks for example.com. The DNS server will handle the entire process of contacting root servers, TLD servers, and authoritative servers to resolve the query.
Example of Iterative Query: A client asks for example.com. The DNS server may refer the client to another server if it does not have the full answer.
Importance in DNS Resolution
Recursive queries are important for providing users with quick, complete results. Iterative queries, on the other hand, help optimize resources by involving multiple servers and reducing the load on any one server.
Conclusion
In this post, we’ve covered critical topics related to DNS, including DNS forwarders, conditional forwarders, round robin DNS, recursive vs. iterative queries, and more. Mastering these concepts is essential for your MCSA certification journey. For a more detailed, step-by-step guide with visual explanations, check out our YouTube video below.
Watch the full DNS guide on YouTube
For a detailed, step-by-step installation guide, check out my video where I explain each step in detail. If you’re still facing any issues, feel free to leave a comment below, and I’ll be happy to help you resolve them.