DNS protocol (for example, DNS records, TTL, DNSSEC, DNS delegation, zones)

What is the purpose of DNS delegation and how is it implemented in a DNS infrastructure?

DNS delegation is used to split the responsibility for managing DNS name space among different DNS servers. It is implemented by creating NS (Name Server) records in the parent domain’s DNS zone file to direct traffic to the child domain’s DNS servers, which are authoritative for their respective subdomains. This allows efficient management and distribution of DNS hierarchies.

What types of DNS records are commonly used and what are their purposes?

Common DNS records include A (Address) records that map hostnames to IPv4 addresses, AAAA records for IPv6 addresses, CNAME (Canonical Name) records for aliasing one name to another, MX (Mail Exchange) records to specify mail servers, NS (Name Server) records to delegate a DNS zone to specific DNS servers, PTR (Pointer) records for reverse DNS lookups, and TXT records for arbitrary text information.

Explain how TTL (Time to Live) values in DNS affect domain name resolution.

TTL values determine how long a DNS record is cached by a resolver or DNS client before it is discarded and a new request is sent to an authoritative DNS server for updated information. A shorter TTL means more frequent updates, while a longer TTL can reduce the load on DNS servers but may delay propagation of changes to DNS records.

What is DNSSEC and why is it important for DNS security?

DNSSEC (Domain Name System Security Extensions) is a suite of extensions to DNS that provides authentication of DNS data, data integrity, and adds a layer of trust to the DNS hierarchy. It’s important because it helps to prevent attacks such as cache poisoning and man-in-the-middle by ensuring end-users are communicating with the correct servers for a domain.

How do DNS zones differ from DNS domains?

A DNS zone is a distinct part of the DNS namespace that is managed by a specific organization or administrator, usually corresponding to a domain or subdomain. A DNS domain is a part of the DNS namespace, which is a broader concept under which there can be multiple zones managed by different entities.

Describe a scenario where a CNAME record might be preferable over an A record.

A CNAME record might be preferable when you need to alias a domain or subdomain to another domain name. This is often used when hosting on platforms where IP addresses may change dynamically (e.g., cloud services) or when consolidating multiple domain names to a single target domain, such as with a branded marketing campaign.

Can you explain what an SOA (Start of Authority) record is and what it does?

The SOA record is a type of DNS record that contains administrative information about a zone, such as the primary name server, the email of the domain administrator, the domain serial number, and parameters such as refresh rate, retry rate, expire time, and minimum TTL. It is essential for zone transfer and maintenance of DNS databases.

In what scenario might you prefer to use an ALIAS record instead of a CNAME record?

An ALIAS record, also known as a DNS ANAME, is used to map the root of a domain to another domain but it allows for domain resolution similarly to A and AAAA records. Unlike CNAME records, ALIAS records can coexist with other records on the same domain level. They are helpful when you want to point the root domain (example.com) to an external hostname that changes IP addresses without violating DNS rules, which would otherwise require an A record with a static IP address.

How might DNS round-robin be used in a network infrastructure?

DNS round-robin is a simple method of load distributing among multiple servers by configuring multiple A or AAAA records for the same host name within a DNS zone. Resolvers rotate through the list of IP addresses provided in response, effectively spreading the load across several servers or endpoints.

What is the main difference between a forward and a reverse DNS lookup?

A forward DNS lookup translates a domain name into an IP address, which is the common way most DNS queries are made to resolve a website’s IP. In contrast, a reverse DNS lookup converts an IP address into a domain name, which is used for tasks like logging or authentication by network services to determine if the IP matches the expected domain.

What considerations should be taken into account when setting the TTL for a DNS record?

When setting the TTL for a DNS record, you should consider the balance between network traffic, server load, and the need for timely updates. A shorter TTL can be useful for frequent changes, ensuring that updates propagate quickly, while a longer TTL can reduce DNS traffic and load, but it may cause delays in updates reaching users.

Explain the difference between a primary DNS server and a secondary DNS server in a zone.

A primary DNS server holds the original read-write version of the zone file and is the authoritative source for information about that zone. Secondary DNS servers hold read-only copies of the zone file that are obtained through zone transfer from the primary server. Secondary servers provide redundancy and load distribution, serving as backup servers that respond to queries if the primary server fails or is overwhelmed.