Docs: add proper README

README.md

@@ -1,3 +1,68 @@
-# dns-keeper
+# DNS Keeper
 
-A security-hardened dynamic DNS updater with self-verification and multi-API IP consensus.
+A small but robust program to keep DNS records updated with your public _(external)_ IPv4 address.
+
+Mostly like DDNS but using API driven updates.  
+Currently only gandi's liveDNS API is supported.
+
+## How it works
+
+Every `POLL_INTERVAL` seconds, DNS Keeper will check if the IPv4 in the domain(s) is still  
+valid by seeing if `DNS_KEEPER_RECORD`.`DNS_KEEPER_DOMAIN` still points to itself.
+
+> [!NOTE]
+> On startup DNS Keeper starts a detached thread with a UDP socket on which it answers with
+> a random set of challenge bytes.  
+> DNS Keeper will send an empty data-frame to its own socket using
+> the DNS resolved IPv4 address to see if it gets a response with the correct challenge bytes.  
+> The challenge bytes are changed before every self-check.
+
+If DNS Keeper fails to get a response from itself for [3](./src/selfcheck.h#6)
+times it will check it's cached IP against the response of multiple external
+[services](./src/ipcheck.c#8) that return your public IPv4.
+
+If the returned public IP is the same as the cached one,
+DNS Keepr will see that as drift.  
+If this happens too much it will revalidate its own cache against the self-record.
+
+If the returned public IP is different from the cached one,
+DNS Keeper will check the new IP using its own socket again,
+this time without DNS.  
+If it proves to be correct it will update all A record of the specified `DOMAINS`
+who's value match the old IP to the new IP.  
+The chached IP is only updated after the self-record is updated correctly.
+
+> [!NOTE]
+> DNS Keeper enforces a strict majority amongst the services.  
+> If a strict majority cannot be reached or not enough services
+> were able to respond, it will take no action and reset the failed counter.  
+> This is to prevent any issues should external services be compromised
+> and to avoid spamming these services in case something else is wrong.
+
+## Environment Variables
+
+| Field               | Descript                                                                                 | Default |
+| ------------------- | ---------------------------------------------------------------------------------------- | ------- |
+| `GANDI_API_TOKEN`   | Gandi API Token with permission to "See and renew domain names".                         |         |
+| `DOMAINS`           | The domains _(eg. "example.noreal,demo.notreal")_ to keep the A records up to date of.   |         |
+| `DNS_KEEPER_PORT`   | The port on which DNS Keepr runs its self-check UDP Socket.                              | 65535   |
+| `DNS_KEEPER_DOMAIN` | The domain on which the record pointing to dns-keeper lives. (eg. example.notreal).      |         |
+| `DNS_KEEPER_RECORD` | DNS record that points to your public IP/DNS-Keeper (eg. _dnsk).                         |         |
+| `POLL_INTERVAL`     | The interval (in seconds) at which DNS Keeper should check if it can still reach itself. | 5       |
+
+## Setup
+
+You can the binary using the dynamically linked or static using make.
+
+```sh
+# Dynamically linked
+make
+
+# Static:
+make STATIC=1
+```
+
+The dynamically linked binary is smaller but requires dependencies:
+libcurl, libcjson and pthread to be on the target machine.
+
+The static binary is slightly bigger but is fully self contained.