slide: added a slide on DNS

src/slides/skynet/4_dns.md

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++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+title   = "Skynet: DNS"
+date    = 2023-10-14
+slides  = true
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+
+
+# DNS
+
+-------------------------------------------------------------------------
+
+## Overview
+
+-------------------------------------------------------------------------
+
+DNS is both simple and complex.
+
+-------------------------------------------------------------------------
+
+DNS translates the human domains into something computers can understand.
+
+-------------------------------------------------------------------------
+
+```dns
+skynet  IN A     193.1.96.165
+*.users IN CNAME skynet
+```
+
+-------------------------------------------------------------------------
+
+It can also provide useful information.
+
+-------------------------------------------------------------------------
+
+```dns
+skynet.ie. IN TXT   "v=spf1 a:mail.skynet.ie -all"
+```
+
+-------------------------------------------------------------------------
+
+## Structure
+
+-------------------------------------------------------------------------
+
+DNS is hierarchical and delegated.
+
+-------------------------------------------------------------------------
+
+Root servers
+
+-------------------------------------------------------------------------
+
+TLD (Top Level Domain) servers (``.ie``, ``.com``, ``.eu``)
+
+-------------------------------------------------------------------------
+
+Domain servers (``skynet.ie``)
+
+-------------------------------------------------------------------------
+
+We run our own DNS servers, Bind9 (``ns1.skynet.ie``, ``ns2.skynet.ie``)
+
+-------------------------------------------------------------------------
+
+We also have control over ``csn.ul.ie`` and ``ulcompsoc.ie``
+
+-------------------------------------------------------------------------
+
+## Viewing DNS records
+ 
+-------------------------------------------------------------------------
+
+On the nixos repo:
+```bash
+colmena build --keep-result --on vendetta
+cat .gcroots/node-vendetta/etc/skynet/dns/skynet.ie
+```
+
+-------------------------------------------------------------------------
+
+On top of that we also manage the reverse DNS (rDNS)
+
+-------------------------------------------------------------------------
+
+Reverse DNS allows you to look up and IP and get the domains.
+
+-------------------------------------------------------------------------
+
+```bash
+cat .gcroots/node-vendetta/etc/skynet/dns/64-64.99.1.193.in-addr.arpa
+```
+
+-------------------------------------------------------------------------
+
+To validate them there is this tool: <<https://bind.jamiewood.io/>>  
+(put ``skynet.ie`` in teh FQDN box)
+
+-------------------------------------------------------------------------
+
+## Commands
+
+-------------------------------------------------------------------------
+
+```bash
+dig skynet.ie
+dig skynet.ie @1.1.1.1
+dig skynet.ie @ns1.skynet.ie
+host 193.1.99.75
+```
+
+-------------------------------------------------------------------------
+
+//Use this time to explore the nixos repo to explain//
+
+-------------------------------------------------------------------------
+
+* <<https://gitlab.skynet.ie/compsoc1/skynet/nixos/-/issues/21>>
+* <<https://gitlab.skynet.ie/compsoc1/skynet/nixos/-/issues/33>>
+