A Canonical Name (CNAME) record is a type of DNS (Domain Name System) record used to create an alias or alternative name for a domain or subdomain. It is commonly used to point one domain or subdomain to another domain or subdomain.
When a DNS resolver encounters a CNAME record, it will substitute the original domain or subdomain with the target domain or subdomain specified in the CNAME record and then continue the DNS lookup process. This allows multiple domain names to resolve to the same IP address or to point to a different server or hostname.
Here’s an example to illustrate the usage of a CNAME record:
Suppose you have two domains: domain1.com and domain2.com. You want domain2.com to point to the same website as domain1.com. Instead of duplicating the DNS records for domain2.com, you can create a CNAME record for domain2.com with the target set as domain1.com. This way, any DNS lookups for domain2.com will be directed to domain1.com, and both domains will resolve to the same IP address.
CNAME records are particularly useful in scenarios where you want to create subdomains or aliases for existing domains, implement load balancing or failover configurations, or manage multiple domains that need to point to the same content. However, CNAME records cannot be used for the root domain (e.g., example.com) as the main domain record requires an A or AAAA record to directly point to an IP address.
It’s important to note that when using CNAME records, any DNS queries for the original domain or subdomain will be redirected to the target domain or subdomain specified in the CNAME record. Therefore, changes made to the DNS records of the target domain will also affect the domain or subdomain using the CNAME record.
DNSSEC and how CNAME records are used with DNSSEC records like RRSIG and NSEC
DNSSEC (Domain Name System Security Extensions) is a set of security features added to the DNS (Domain Name System) protocol to ensure the authenticity and integrity of DNS data. It provides a way to verify that DNS responses have not been tampered with and helps prevent DNS-based attacks, such as DNS spoofing or DNS cache poisoning.
1. DNSSEC Overview: DNSSEC adds digital signatures to DNS records, creating a chain of trust from the root DNS servers down to the individual domain’s records. It uses public key cryptography to sign and verify DNS data. DNSSEC introduces new resource record types, such as RRSIG (Resource Record Signature) and NSEC (Next Secure).
2. RRSIG: RRSIG records are used in DNSSEC to sign the data records (e.g., A, AAAA, MX, NS, etc.) of a particular domain. Each data record is accompanied by an RRSIG record that contains the digital signature of the data. This allows DNS resolvers to verify the authenticity and integrity of the received data by checking the RRSIG record against the corresponding DNSKEY record.
3. NSEC: NSEC records provide proof of non-existence in DNSSEC. They are used to prove that a particular name does not have any associated records (e.g., when querying for a non-existent subdomain). NSEC records are used to create a chain of authenticated denial, ensuring that the absence of a record is verified and cannot be manipulated.
4. CNAME Records and DNSSEC: CNAME records are used to create aliases or aliases for a domain or subdomain. In the context of DNSSEC, CNAME records can introduce complexities because they impact the way signatures are validated.
– RRSIG and CNAME: When a CNAME record is present, the RRSIG record should cover both the CNAME record and the target record(s) to maintain the integrity of the chain of trust. This means that the RRSIG for the CNAME record should also cover the records pointed to by the CNAME.
– NSEC and CNAME: NSEC records provide proof of non-existence for a domain or subdomain. When a CNAME record is encountered, the NSEC record needs to cover both the CNAME record and the target record(s) to properly demonstrate the non-existence of both.
Proper handling of CNAME records in DNSSEC is crucial to maintain the integrity and security of the DNS data. It is important to ensure that signatures (RRSIG) cover all relevant records, including both the CNAME record and the target records, and that denial proofs (NSEC) cover both the CNAME and target records when dealing with CNAME aliases.
DNSSEC brings enhanced security to the DNS infrastructure by enabling the validation and authentication of DNS data. By incorporating digital signatures and cryptographic keys, DNSSEC helps protect against DNS data manipulation and ensures the trustworthiness of DNS responses.
When NOT to use a CNAME record
MX (Mail Exchanger) and NS (Name Server) records in the DNS (Domain Name System) are used to specify the mail servers and name servers, respectively, responsible for a domain. It is important to note that these records cannot point directly to a CNAME (Canonical Name) record.
A CNAME record will create an alias or nickname for a domain or subdomain. It allows you to associate multiple hostnames with a single IP address or hostname. However, there are specific restrictions on how CNAME records can be used with other DNS record types.
1. MX Records: MX records specify the mail servers that should receive emails for a domain. According to DNS standards, MX records must point to a hostname or an A (Address) record, but they cannot point to a CNAME record. This is because when an email is sent to a domain, the receiving mail server performs a DNS lookup on the MX record to determine where to deliver the email. If the MX record points to a CNAME, it would require an additional DNS lookup to resolve the CNAME to an A record, which can lead to performance and reliability issues.
2. NS Records: NS records specify the authoritative name servers for a domain. These records indicate which servers are responsible for answering DNS queries for a particular domain. Similar to MX records, NS records must point to a hostname or an A record, but they cannot directly point to a CNAME record. This restriction is in place to ensure proper resolution of name server queries without additional DNS lookups.
To work around this limitation, it is recommended to directly specify the target hostname or IP address in the MX and NS records. If you need to associate a CNAME record with an MX or NS record, you should create an A record that points to the desired target and then use that A record in the MX or NS record.
It’s important to ensure DNS record configurations comply with DNS standards and best practices to ensure proper functionality and reliability of the domain’s mail and name server infrastructure.
Can a CNAME record point to another CNAME record?
No, it is not allowed to have a CNAME (Canonical Name) record resolve to another CNAME record in the Domain Name System (DNS). According to the DNS specifications (RFC 1034 and RFC 1912), a CNAME record must always point to a canonical or primary name and cannot coexist with any other record type for the same name.
When a DNS resolver encounters a CNAME record, it follows the alias and replaces the original name with the canonical name. This process allows for the resolution of multiple domain names to a single IP address. However, if a CNAME record points to another CNAME record, it creates a circular reference or loop, which is not permitted.
Using a CNAME chain or loop can lead to various issues, such as DNS resolution failures, increased DNS query latency, and potential infinite loops. It’s essential to ensure that CNAME records are used correctly by pointing them to the actual target hostname or address instead of chaining them together.
Why does google opendns 8.8.8.8 resolve a double cname?
While it is generally not recommended to have a CNAME resolve to another CNAME according to DNS specifications, some DNS resolvers, including Google Public DNS (8.8.8.8), might allow resolution of double CNAMEs as a lenient behavior. This leniency is not universal across all DNS resolvers, and it’s important to note that it deviates from the strict DNS standards.
Google Public DNS is designed to be flexible and provide reliable DNS resolution for a wide range of scenarios. To accommodate various DNS configurations, it may implement certain optimizations or relaxations in its resolver algorithm. One such relaxation could be the resolution of double CNAMEs to provide a better user experience in cases where misconfigurations or unconventional setups are encountered.
However, it’s important to remember that relying on the behavior of a specific DNS resolver, especially when it deviates from the DNS standards, can lead to inconsistent results across different DNS resolvers or future changes in resolver behavior. To ensure maximum compatibility and adherence to DNS standards, it is still recommended to avoid having CNAME records resolve to other CNAME records.