feat(proxyproto): add proxy protocol support (#7738)

Signed-off-by: Adphi <philippe.adrien.nousse@gmail.com>

plugin/proxyproto/README.md

@@ -0,0 +1,63 @@
+# proxyproto
+
+## Name
+
+*proxyproto* - add [PROXY protocol](https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt) support.
+
+## Description
+
+This plugin adds support for the PROXY protocol version 1 and 2. It allows CoreDNS to receive
+connections from a load balancer or proxy that uses the PROXY protocol to forward the original
+client's IP address and port information.
+
+## Syntax
+
+~~~ txt
+proxyproto {
+    allow <CIDR...>
+    default <use|ignore|reject|skip>
+}
+~~~
+
+If `allow` is unspecified, PROXY protocol headers are accepted from all IP addresses.
+The `default` option controls how connections from sources not listed in `allow` are handled.
+If `default` is unspecified, it defaults to `ignore`.
+The possible values are:
+- `use`: accept and use PROXY protocol headers from these sources
+- `ignore`: accept and ignore PROXY protocol headers from other sources
+- `reject`: reject connections with PROXY protocol headers from other sources
+- `skip`: skip PROXY protocol processing for connections from other sources, treating them as normal connections preserving the PROXY protocol headers.
+
+
+## Examples
+
+In this configuration, we allow PROXY protocol connections from all IP addresses:
+~~~ corefile
+. {
+    proxyproto
+    forward . /etc/resolv.conf
+}
+~~~
+
+In this configuration, we only allow PROXY protocol connections from the specified CIDR ranges
+and ignore proxy protocol headers from other sources:
+~~~ corefile
+. {
+    proxyproto {
+        allow 192.168.1.1/32 192.168.0.1/32
+    }
+    forward . /etc/resolv.conf
+}
+~~~
+
+In this configuration, we only allow PROXY protocol headers from the specified CIDR ranges and reject
+connections without valid PROXY protocol headers from those sources:
+~~~ corefile
+. {
+    proxyproto {
+        allow 192.168.1.1/32
+        default reject
+    }
+    forward . /etc/resolv.conf
+}
+~~~