dns server

• I used this guide (written in rust) to learn how to create a dns server.
• Here’s my own implementation of a dns resolver (written in typescript).

dns → domain name system

Send and receive UDP packets

Create a UDP socket that listens to port 2053.

const udpSocket = dgram.createSocket("udp4");
udpSocket.bind(2053, '127.0.0.1')

Then you can listen to events like error, message, close, etc.

udpSocket.on("message", (msg, rinfo) => {
	// listen to recieved messages
})

rinfo is remote info which contains information about the server sending the message

You can also messages using the send() method.

udpSocket.send(...)

Parts of a dns server

All communications in the DNS protocol are carried in a single format called a “message”. Each message consists of 5 sections:

1. header → information about query / response
2. question → request for information about query name (domain) and record type
3. answer → relevant records of the requested type
4. authority → list of name servers (for recursive resolve), and
5. an additional space

Header section

The header section of a DNS message contains the following fields:

Field	Size	Description
Packet Identifier (ID)	16 bits	A random ID assigned to query packets. Response packets must reply with the same ID.
Query/Response Indicator (QR)	1 bit	1 for a reply packet, 0 for a question packet.
Operation Code (OPCODE)	4 bits	Specifies the kind of query in a message.
Authoritative Answer (AA)	1 bit	1 if the responding server “owns” the domain queried, i.e., it’s authoritative.
Truncation (TC)	1 bit	1 if the message is larger than 512 bytes. Always 0 in UDP responses.
Recursion Desired (RD)	1 bit	Sender sets this to 1 if the server should recursively resolve this query, 0 otherwise.
Recursion Available (RA)	1 bit	Server sets this to 1 to indicate that recursion is available.
Reserved (Z)	3 bits	Used by DNSSEC queries. At inception, it was reserved for future use.
Response Code (RCODE)	4 bits	Response code indicating the status of the response.
Question Count (QDCOUNT)	16 bits	Number of questions in the Question section.
Answer Record Count (ANCOUNT)	16 bits	Number of records in the Answer section.
Authority Record Count (NSCOUNT)	16 bits	Number of records in the Authority section.
Additional Record Count (ARCOUNT)	16 bits	Number of records in the Additional section.
The header section is always 12 bytes long. Integers are encoded in big-endian format.

Storing flags in a bit mask

const flags = 
	(qr << 15) | 
	(opcode << 11) |
	(aa << 10) |
	(tc << 9) |
	(rd << 8) |
	(ra << 7) |
	(z << 4) |
	rcode;

Retrieving flags

const readflags = (flags: number): HeaderFlagParams => {
  const qr = (flags >> 15) & 0b1;
  const opcode = (flags >> 11) & 0b1111;
  const aa = (flags >> 10) & 0b1;
  const tc = (flags >> 9) & 0b1;
  const rd = (flags >> 8) & 0b1;
  const ra = (flags >> 7) & 0b1;
  const z = (flags >> 4) & 0b111;
  const rcode = flags & 0b1111;
  return { qr, opcode, aa, tc, rd, ra, z, rcode };
};

Question section

The question section contains a list of questions (usually just 1) that the sender wants to ask the receiver. This section is present in both query and reply packets.

Each question has the following structure:

• Name: A domain name, represented as a sequence of “labels” (more on this below)
• Type: 2-byte int; the type of record (1 for an A record, 5 for a CNAME record etc., full list here)
• Class: 2-byte int; usually set to 1 (full list here)

Answer section

The answer section contains a list of RRs (Resource Records), which are answers to the questions asked in the question section.

Each RR has the following structure:

Field	Type	Description
Name	Label Sequence	The domain name encoded as a sequence of labels.
Type	2-byte Integer	1 for an A record, 5 for a CNAME record etc., full list here
Class	2-byte Integer	Usually set to 1 (full list here)
TTL (Time-To-Live)	4-byte Integer	The duration in seconds a record can be cached before requerying.
Length (RDLENGTH)	2-byte Integer	Length of the RDATA field in bytes.
Data (RDATA)	Variable	Data specific to the record type.

Section 3.2.1 of the RFC covers the answer section format in detail.

In this stage, we’ll only deal with the “A” record type, which maps a domain name to an IPv4 address. The RDATA field for an “A” record type is a 4-byte integer representing the IPv4 address.

Header compression

Dns Server

A dns server is generally either of these two types:

Authoritative server

A dns server hosting one or more zones. Its the final holder of the IP address of the domain you are looking for.

For example, the authoritative servers for the zone google.com are ns1.google.com, ns2.google.com, ns3.google.com and ns4.google.com.

Caching Server

Caching Server - A DNS server that services DNS lookups by first checking its cache to see if it already knows of the record being requested, and if not performing a recursive lookup to figure it out.

Recursive Resolve