The Domain Name System

We have already explained the main principles that underlie the utilization of names on the Internet and their mapping to addresses in section Naming and addressing.

The last component of the Domain Name System is the DNS protocol. The original DNS protocol runs above both the datagram and the bytestream services. In practice, the datagram service is used when short queries and responses are exchanged, and the bytestream service is used when longer responses are expected. In this section, we first focus on the utilization of the DNS protocol above the datagram service. We will discuss later other recently proposed protocols to carry DNS information.

DNS messages are composed of five parts that are named sections in RFC 1035. The first three sections are mandatory and the last two sections are optional. The first section of a DNS message is its Header. It contains information about the message type and the content of the other sections. The second section contains the Question sent to the nameserver or resolver. The third section contains the Answer to the Question. When a client sends a DNS query, the Answer section is empty. The fourth section, named Authority, contains information about the servers that can provide an authoritative answer if required. The last section contains additional information that is supplied by the resolver or nameserver but was not requested in the question.

The header of DNS messages is composed of 12 bytes. The figure below presents its structure.

../_images/dnsheader.png

The DNS header

The Transaction ID (transaction identifier) is a 16-bits random value chosen by the client. When a client sends a question to a DNS server, it remembers the question and its identifier. When a server returns an answer, it returns in the Transaction ID field the identifier chosen by the client. Thanks to this identifier, the client can match the received answer with the question that it sent.

The DNS header contains a series of flags. The QR flag is used to distinguish between queries and responses. It is set to 0 in DNS queries and 1 in DNS answers. The Opcode is used to specify the query type. For instance, a standard query is used when a client sends a name and the server returns the corresponding data. An update request is used when the client sends a name and new data and the server then updates its database.

The AA bit is set when the server that sent the response has authority for the domain name found in the question section. In the original DNS deployments, two types of servers were considered : authoritative servers and non-authoritative servers. The authoritative servers are managed by the system administrators responsible for a given domain. They always store the most recent information about a domain. Non-authoritative servers are servers or resolvers that store DNS information about external domains without being managed by the owners of a domain. They may thus provide answers that are out of date. From a security point of view, the authoritative bit is not an absolute indication about the validity of an answer. Securing the Domain Name System is a complex problem that was only addressed satisfactorily recently by the utilization of cryptographic signatures in the DNSSEC extensions to DNS described in RFC 4033.

The RD (recursion desired) bit is set by a client when it sends a query to a resolver. Such a query is said to be recursive because the resolver will recursively traverse the DNS hierarchy to retrieve the answer on behalf of the client. In the past, all resolvers were configured to perform recursive queries on behalf of any Internet host. However, this exposes the resolvers to several security risks. The simplest one is that the resolver could become overloaded by having too many recursive queries to process. Most resolvers 1 only allow recursive queries from clients belonging to their company or network and discard all other recursive queries. The RA bit indicates whether the server supports recursion. The RCODE is used to distinguish between different types of errors. See RFC 1035 for additional details. The last four fields indicate the size of the Question, Answer, Authority and Additional sections of the DNS message.

The last four sections of the DNS message contain Resource Records (RR). All RRs have the same top level format shown in the figure below.

../_images/dnsrr.png

DNS Resource Records

In a Resource Record (RR), the Name indicates the name of the node to which this resource record pertains. The two bytes Type field indicates the type of resource record. The Class field was used to support the utilization of the DNS in other environments than the Internet. The IN Class refers to Internet names.

The TTL field indicates the lifetime of the Resource Record in seconds. This field is set by the server that returns an answer and indicates for how long a client or a resolver can store the Resource Record inside its cache. A long TTL indicates a stable RR. Some companies use short TTL values for mobile hosts and also for popular servers. For example, a web hosting company that wants to spread the load over a pool of hundred servers can configure its nameservers to return different answers to different clients. If each answer has a small TTL, the clients will be forced to send DNS queries regularly. The nameserver will reply to these queries by supplying the address of the less loaded server.

The RDLength field is the length of the RData field that contains the information of the type specified in the Type field.

Several types of DNS RR are used in practice. The A type encodes the IPv4 address that corresponds to the specified name. The AAAA type is encodes the IPv6 address that corresponds to the specified name. A NS record contains the name of the DNS server that is responsible for a given domain. For example, a query for the AAAA record associated to the www.ietf.org name returned the following answer.

../_images/dns6-www-ietf-org.png

Query for the AAAA record of www.ietf.org

This answer contains several pieces of information. First, the name www.ietf.org is associated to IP address 2001:1890:123a::1:1e. Second, the ietf.org domain is managed by six different nameservers. Five of these nameservers are reachable via IPv4 and IPv6.

CNAME (or canonical names) are used to define aliases. For example www.example.com could be a CNAME for pc12.example.com that is the actual name of the server on which the web server for www.example.com runs.

Note

Reverse DNS

The DNS is mainly used to find the address that corresponds to a given name. However, it is sometimes useful to obtain the name that corresponds to an IP address. This done by using the PTR (pointer) RR. The RData part of a PTR RR contains the name while the Name part of the RR contains the IP address encoded in the in-addr.arpa domain. IPv4 addresses are encoded in the in-addr.arpa by reversing the four digits that compose the dotted decimal representation of the address. For example, consider IPv4 address 192.0.2.11. The hostname associated to this address can be found by requesting the PTR RR that corresponds to 11.2.0.192.in-addr.arpa. A similar solution is used to support IPv6 addresses RFC 3596, but slightly more complex given the length of the IPv6 addresses. For example, consider IPv6 address 2001:1890:123a::1:1e. To obtain the name that corresponds to this address, we need first to convert it in a reverse dotted decimal notation : e.1.0.0.1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.a.3.2.1.0.9.8.1.1.0.0.2. In this notation, each character between dots corresponds to one nibble, i.e. four bits. The low-order byte (e) appears first and the high order (2) last. To obtain the name that corresponds to this address, one needs to append the ip6.arpa domain name and query for e.1.0.0.1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.a.3.2.1.0.9.8.1.1.0.0.2.ip6.arpa. In practice, tools and libraries do the conversion automatically and the user does not need to worry about it.

An important point to note regarding the Domain Name System it that it is extensible. Thanks to the Type and RDLength fields, the format of the Resource Records can easily be extended. Furthermore, a DNS implementation that receives a new Resource Record that it does not understand can ignore the record while still being able to process the other parts of the message. This allows, for example, a DNS server that only supports IPv6 can safely ignore the IPv4 addresses listed in the DNS reply for www.ietf.org while still being able to correctly parse the Resource Records that it understands. This allowed the Domain Name System to evolve over the years while still preserving the backward compatibility with already deployed DNS implementations.

Footnotes

1

Some DNS resolvers allow any host to send queries. Google operates a public DNS resolver at addresses 2001:4860:4860::8888 and 2001:4860:4860::8844. Other companies provide similar services.