feat(dns): add comprehensive DNS notes

- Documented commands for flushing DNS cache using `resolvectl` and `systemd-resolve`.
- Included instructions for enabling and restarting `systemd-resolved` service.
- Added usage examples for `dig` and `nslookup` to query DNS records.
- Provided contextual explanations and legacy tool considerations.

notes/dns.md

@@ -0,0 +1,64 @@
+# DNS
+
+## Table of Contents
+
+- [DNS](#dns)
+  - [Table of Contents](#table-of-contents)
+  - [Flush DNS Cache](#flush-dns-cache)
+  - [systemctl](#systemctl)
+  - [dig](#dig)
+  - [nslookup](#nslookup)
+
+## Flush DNS Cache
+
+- Clear the DNS cache to ensure that the system resolves domain names with the most up-to-date information.
+
+```bash
+resolvectl flush-caches
+systemd-resolve --flush-caches
+```
+
+- Use `resolvectl` for newer systems or `systemd-resolve` for older systems (pre-2020).
+
+## systemctl
+
+- This command enables the `systemd-resolved` service if it is not already running, ensuring DNS resolution through `systemd`.
+
+```bash
+systemctl enable systemd-resolved.service
+```
+
+**Explanation of the command:**
+
+- `systemd-resolve --flush-caches`: This command clears the DNS cache maintained by `systemd-resolved`, which can help resolve issues with outdated or incorrect DNS entries.
+- After flushing the cache, it may be necessary to restart the `systemd-resolved` service to ensure proper operation.
+
+```bash
+systemctl restart systemd-resolved
+service systemd-resolved restart
+```
+
+- To restart the service, use `systemctl restart systemd-resolved` (preferred). The `service` command is available but is considered legacy.
+
+## dig
+
+```bash
+dig domain.com
+dig +short NS domain.com
+```
+
+## nslookup
+
+- `nslookup` is a legacy tool but still useful for querying DNS. You can also specify custom DNS servers, such as `1.1.1.1` (Cloudflare) or `8.8.8.8` (Google), to query DNS directly without using the system’s default resolver.
+
+```bash
+nslookup domain.com
+nslookup -q=cname domain.com
+nslookup -q=cname domain.com 1.1.1.1
+nslookup -q=cname domain.com 8.8.8.8
+```
+
+```bash
+nslookup -q=mx domain.com
+nslookup -q=txt domain.com
+```