feat(dns): init notes

Author: JohnTrunix

general/dns.md

@@ -1 +1,98 @@
-# DNS - Domain Name System
+# DNS - Domain Name System
+
+DNS is a system that translates domain names to IP addresses. This is used to route traffic via an human readable name to an ip address.
+
+- DNS names are only used for human readability
+- Servers use IP addresses to communicate with each other
+
+```mermaid
+sequenceDiagram
+    participant Client
+    participant DNS_Server as DNS Server
+    participant Web_Server as Web Server
+
+    Client->>DNS_Server: DNS Request: www.example.com
+    DNS_Server-->>Client: DNS Response: 200.0.0.0
+    Client->>Web_Server: HTTP GET /index.html
+    Web_Server-->>Client: HTTP Response 200 OK
+
+```
+
+### Zones
+
+A DNS zone is a segment of the domain namespace managed by a specific organization or administrator. It consists of a collection of DNS records maintained by a DNS server. Zones enable different parts of a domain to be managed by separate DNS servers, providing flexibility and delegation in domain administration.
+
+## DNS Records
+
+DNS records are used to store information about a domain in a DNS zone file. General format of a DNS record:
+
+```text
+<name> <ttl> <class> <type> <rdata>
+```
+
+- `<name>`: the domain or subdomain
+- `<ttl>`: time to live (how long the record is cached)
+- `<class>`: the class of the record (IN for internet)
+- `<type>`: the type of record (A for address)
+- `<rdlength>`: length of the rdata field (internaly used)
+- `<rdata>`: the IP address or data associated with the record
+
+> [!INFO]
+>
+> - `<ttl>`, `<class>` and `<type>` are optional fields. If not specified, they will be set to default values.
+> - `<rdlength>` is an internal field
+
+### A and AAAA Records
+
+A record is used to point a domain or subdomain to an IP address. A records are used for IPv4 addresses and AAAA records are used for IPv6 addresses.
+
+```text
+example.com 3600 IN A 200.0.0.0
+example.com 3600 IN AAAA 2001:0db8:85a3:0000:0000:8a2e:0370:7334
+```
+
+### CNAME Record
+
+CNAME record is used to point a domain or subdomain to another domain. The authoritative DNS server will resolve the CNAME record to the IP address of the target domain.
+
+```text
+www.example.com 3600 IN CNAME example.com
+```
+
+> [!WARNING]
+>
+> CNAME records cannot be used for root domains (apex) per [RFC 2181](https://www.rfc-editor.org/rfc/rfc2181#section-10).
+
+### NS Record
+
+NS record is used to indicate the authoritative DNS server for a domain. With NS records, multiple subdomains can be managed with different DNS zones.
+
+```text
+example.com 3600 IN NS ns1.google.com
+example.com 3600 IN NS ns2.google.com
+example.com 3600 IN NS ns3.google.com
+example.com 3600 IN NS ns4.google.com
+```
+
+Often there are multiple NS records for a domain to provide redundancy and load balancing.
+
+### MX Record
+
+MX record is used to specify the mail servers that are responsible for receiving emails for a domain.
+
+```text
+example.com 3600 IN MX 10 mail1.example.com
+example.com 3600 IN MX 20 mail2.example.com
+```
+
+The number (`10`, `20`) in the MX record is the priority of the mail server. Lower numbers have higher priority.
+
+### Other Records
+
+- **SOA Record**: Start of Authority record is used to specify the authoritative DNS server for a domain.
+- **TXT Record**: Text record is used to store arbitrary text data.
+- **PTR Record**: Pointer record is used for reverse DNS lookups.
+- **CAA Record**: Certificate Authority Authorization record is used to specify which certificate authorities are allowed to issue certificates for a domain.
+  ```text
+  example.com CAA 0 issue "letsencrypt.org"
+  ```