When setting up a subdomain, what A record should I use, and when should I use a CNAME?

Key findings

• Direct mapping: An A record directly links a subdomain to a specific IPv4 address. This means if the IP address changes, the A record must be manually updated.
• Alias function: A CNAME record (Canonical Name) serves as an alias, pointing a subdomain to another domain name rather than directly to an IP address.
• CNAME restriction: A critical DNS rule dictates that if a CNAME record exists for a subdomain, no other DNS records (such as A, MX, or TXT records like SPF or DMARC) can exist for that same subdomain name at the same level.
• Email sending: For email sending and tracking, many Email Service Providers (ESPs) recommend or require CNAME records for subdomains, as it allows them to manage the underlying IP addresses dynamically without requiring you to update your DNS records manually.

Key considerations

• DNS interface requirements: Your DNS management interface may impose specific requirements or default behaviors, influencing whether you initially set up an A record or a CNAME for a subdomain. Some interfaces may require an A record even if it will eventually be aliased.
• Purpose of the subdomain: The choice heavily depends on the subdomain's primary function. If it's for a website with a stable IP, an A record might suffice. For email sending, tracking, or other services managed by a third-party (like an ESP), a CNAME is often more appropriate. For more on setting up email subdomains, see how to set up email subdomains.
• Future flexibility: CNAMEs offer greater flexibility because if the underlying IP address of the target domain changes, you do not need to update your DNS record; the alias automatically resolves to the new IP. This is especially beneficial for services that frequently update their infrastructure.
• DNS coexistence: Remember that a subdomain cannot have both a CNAME record and other record types (like A or MX records) simultaneously for the same hostname. This means if your ESP requires a CNAME for email.yourdomain.com, you cannot also have an A record or an MX record directly on email.yourdomain.com. Consider configuring CNAME records for subdomains to avoid conflicts. For more on DNS requirements, see what DNS records are needed for email sending subdomains.

Key opinions

• ESP guidance: Many marketers primarily rely on their ESP's instructions for setting up DNS records for subdomains, as ESPs typically provide exact CNAMEs or other records needed for proper functionality.
• Tracking CNAMEs: CNAME records are widely used for email tracking subdomains (e.g., t.yourdomain.com) as they allow ESPs to manage redirects without needing direct IP configuration.
• A record purpose: If an A record is required by a DNS management interface, marketers suggest pointing it to a web server you control, even if the subdomain's primary purpose is email sending or tracking via a CNAME on a sub-subdomain.
• Subdomain purpose: The choice between an A record and a CNAME often comes down to the intended use of the subdomain; A records for direct IP connections, CNAMEs for aliasing to other domains or services. For more on this, check out why use email subdomains.

Key considerations

• Coexistence rule: Marketers must be aware that a CNAME record cannot coexist with any other record types (A, MX, TXT) for the same hostname. This necessitates careful planning of subdomain structure if both web hosting and email functionalities are desired.
• DNS setup complexity: While ESPs simplify the process, understanding the underlying DNS principles helps in troubleshooting and advanced configurations, such as when dealing with SPF authentication issues with multiple ESPs or subdomains.
• Redirection strategy: If an A record is set for a subdomain that is not intended for web content, it is good practice to configure the web server to redirect that subdomain to the primary domain (e.g., from investors.yourdomain.com to www.yourdomain.com). This ensures a consistent user experience.
• CNAME for email tracking: The use of CNAME records for email tracking offers an advantage by simplifying the management of multiple subdomains without needing to create A records for each. This is discussed in a guide to DNS CNAME records.

Key opinions

• CNAME rule: Experts consistently point out the critical DNS rule: you cannot have a CNAME record for a subdomain if there are any other records (like A or MX records) at that same level for the same subdomain name.
• ESP requirements: ESPs typically guide users to set up specific CNAMEs for email sending and tracking domains, making these the preferred record type for integrating with their services.
• A record use: If an A record is required by your DNS system, experts recommend pointing it to an IP address that you own or have a clear contractual relationship with, such as one of your web servers.
• Ownership importance: Never point your domain or subdomain to an IP address you do not own or control, as this can lead to security vulnerabilities or loss of control over the domain.

Key considerations

• DNS hierarchy: If you need a CNAME for email tracking on investors.domain.com, but your DNS provider also requires an A record for investors.domain.com, you might need to use a sub-subdomain for the CNAME (e.g., t.investors.domain.com) to avoid conflicts. This helps maintain your DNS records like SPF, DKIM, and DMARC.
• Deliverability impact: Incorrect or conflicting DNS records, whether A records or CNAMEs, can severely impact email deliverability by causing authentication failures or resolution issues. Regular auditing of DNS records is essential.
• Postmaster tools: When setting up subdomains for email, ensure that DNS records align with the requirements of major mailbox providers. Using tools like SpamResource can provide insights into deliverability issues related to DNS.
• DMARC and subdomains: Pay attention to how DMARC policies apply to subdomains, especially when CNAMEs are in play, as this can affect authentication. Consider the best practice for DMARC record placement for subdomains.

Key findings

• A record definition: Documentation defines an A record as a DNS record type that maps a domain name or subdomain to an IPv4 address, which is a numerical address used to identify a device on the internet.
• CNAME definition: A CNAME record, or Canonical Name record, is used to create an alias, pointing one domain or subdomain to another domain name. This is particularly useful for services that may change their underlying IP addresses frequently.
• The CNAME rule: A core principle in DNS is that if a CNAME record is configured for a specific name, no other resource records (e.g., A, MX, TXT) can exist for that exact same name. This prevents ambiguous resolution.
• Common use cases: CNAMEs are often used for subdomains to point to web hosts or other services that require or provide a canonical name, streamlining DNS management. For more on DNS lookups, see best practices for DNS lookups.

Key considerations

• RFC adherence: DNS records, including A and CNAMEs, must adhere to the specifications outlined in relevant RFCs (Request for Comments) to ensure proper resolution across the internet.
• Service integration: When integrating a subdomain with a third-party service, always consult their specific documentation regarding DNS requirements. They will specify whether an A record, CNAME, or other record types are needed.
• Avoiding conflicts: To prevent DNS resolution errors or conflicts, avoid placing an A record on a subdomain that already has a CNAME record. This is a common source of DNS troubleshooting, similar to issues causing SPF DNS timeouts with Microsoft.
• Subdomain creation: Platforms like Cloudflare allow users to choose between A, AAAA, or CNAME record types when creating subdomain records, depending on whether they need to point to an IPv4 address, IPv6 address, or another domain. For detailed instructions, refer to Cloudflare's guide on creating subdomain records.