clan: add temporary patched coredns service

Needed for IPv6 support, and to set the host names
in the auth zone.

clanServices/coredns/README.md

@@ -0,0 +1,73 @@
+!!! Danger "Experimental"
+    This service is experimental and will change in the future.
+
+This module enables hosting clan-internal services easily, which can be resolved
+inside your VPN. This allows defining a custom top-level domain (e.g. `.clan`)
+and exposing endpoints from a machine to others, which will be
+accessible under `http://<service>.clan` in your browser.
+
+The service consists of two roles:
+
+- A `server` role: This is the DNS-server that will be queried when trying to
+  resolve clan-internal services. It defines the top-level domain.
+- A `default` role: This does two things. First, it sets up the nameservers so
+  that clan-internal queries are resolved via the `server` machine, while
+  external queries are resolved as normal via DHCP. Second, it allows exposing
+  services (see example below).
+
+## Example Usage
+
+Here the machine `dnsserver` is designated as internal DNS-server for the TLD
+`.foo`. `server01` will host an application that shall be reachable at
+`http://one.foo` and `server02` is going to be reachable at `http://two.foo`.
+`client` is any other machine that is part of the clan but does not host any
+services.
+
+When `client` tries to resolve `http://one.foo`, the DNS query will be
+routed to `dnsserver`, which will answer with `192.168.1.3`. If it tries to
+resolve some external domain (e.g. `https://clan.lol`), the query will not be
+routed to `dnsserver` but resolved as before, via the nameservers advertised by
+DHCP.
+
+```nix
+inventory = {
+
+  machines = {
+    dnsserver = { }; # 192.168.1.2
+    server01 = { };  # 192.168.1.3
+    server02 = { };  # 192.168.1.4
+    client = { };    # 192.168.1.5
+  };
+
+  instances = {
+    coredns = {
+
+      module.name = "@clan/coredns";
+      module.input = "self";
+
+      # Add the default role to all machines, including `client`
+      roles.default.tags.all = { };
+
+      # DNS server queries to http://<name>.foo are resolved here
+      roles.server.machines."dnsserver".settings = {
+        ip = "192.168.1.2";
+        tld = "foo";
+      };
+
+      # First service
+      # Registers http://one.foo will resolve to 192.168.1.3
+      # underlying service runs on server01
+      roles.default.machines."server01".settings = {
+        ip = "192.168.1.3";
+        services = [ "one" ];
+      };
+
+      # Second service
+      roles.default.machines."server02".settings = {
+        ip = "192.168.1.4";
+        services = [ "two" ];
+      };
+    };
+  };
+};
+```