Add flatten external plugin

Signed-off-by: Thomas Dang <14917550+litobro@users.noreply.github.com>

content/explugins/flatten.md

@@ -0,0 +1,84 @@
++++
+title = "flatten"
+description = "*<plugin name>* provides minimal CNAME flattening mirroring the Cloudflare RFC 1034 compliant implementation."
+weight = 10
+tags = [  "plugin" , "<plugin name>" ]
+categories = [ "plugin", "external" ]
+date = "2017-07-22T12:37:19+01:00"
+repo = "https://github.com/litobro/flatten"
+home = "https://github.com/litobro/flatten/blob/main/README.md"
++++
+
+## Description
+
+The *<plugin name>* plugin is an attempt to provide a minimal CNAME flattening solution that is RFC compliant in accordance with the Cloudflare implementation.
+
+## Syntax
+
+~~~ txt
+flatten [FROM] [TO] [DNSIP:PORT]
+~~~
+
+- `FROM`: Original requested NAME
+- `TO`: Name to overwrite the A and AAAA records from
+- `DNSIP:PORT`: The DNS server and port to resolve the `TO` records from
+
+## Example Corefile
+```
+example.org:53 {
+    log
+    flatten example.org google.ca 1.1.1.1:53
+
+    forward . 1.1.1.1
+}
+```
+
+### Example output
+Run the server
+```
+$ ./coredns -conf Corefile
+
+example.org.:53
+CoreDNS-1.11.2
+linux/amd64, go1.21.8, 8de4531d-dirty
+[INFO] plugin/flatten: 127.0.0.1:37237 - [example.org.] flattened to [google.ca.] via 1.1.1.1:53
+[INFO] plugin/flatten: 127.0.0.1:36670 - [example.org.] flattened to [google.ca.] via 1.1.1.1:53
+```
+
+Make a DNS request to the server
+```
+$ nslookup example.org 127.0.0.1
+
+Server:         127.0.0.1
+Address:        127.0.0.1#53
+
+Name:   example.org
+Address: 142.250.217.67
+Name:   example.org
+Address: 2607:f8b0:400a:80b::2003
+Name:   example.org
+Address: 142.250.217.67
+Name:   example.org
+Address: 2607:f8b0:400a:804::2003
+
+$ dig TXT example.org @127.0.0.1
+
+; <<>> DiG 9.18.18-0ubuntu0.22.04.1-Ubuntu <<>> TXT example.org @127.00.0.1
+;; global options: +cmd
+;; Got answer:
+;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 63388
+;; flags: qr rd ra ad; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1
+
+;; OPT PSEUDOSECTION:
+; EDNS: version: 0, flags:; udp: 1232
+;; QUESTION SECTION:
+;example.org.                   IN      TXT
+
+;; ANSWER SECTION:
+example.org.            86400   IN      TXT     "v=spf1 -all"
+example.org.            86400   IN      TXT     "6r4wtj10lt2hw0zhyhk7cgzzffhjp7fl"
+
+;; Query time: 99 msec
+;; SERVER: 127.0.0.1#53(127.00.0.1) (UDP)
+;; WHEN: Wed Apr 17 09:42:46 MDT 2024
+;; MSG SIZE  rcvd: 131