Add article on controlling PURGE method

docs/hypernode-platform/nginx/how-to-control-purge-method.md

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+---
+myst:
+  html_meta:
+    description: The PURGE method is disabled by default, learn how to safely enable
+      it.
+    title: How to Control PURGE Method | Hypernode
+---
+
+# How to Control PURGE Method
+
+Due to security and performance considerations, the `PURGE` method is disabled by default.
+
+It's possible to enable it, but be careful to only open it to clients you trust. We'll show you how in this article.
+
+## What is the PURGE Method?
+
+The `PURGE` method is an HTTP request method used to clear cached content from reverse proxy caches like Varnish. When a `PURGE` request is sent to a URL, it instructs the cache server to remove the cached version of that content, forcing it to fetch a fresh copy from the origin server on the next request.
+
+## Why is PURGE Disabled by Default?
+
+By default, Hypernode blocks all `PURGE` requests through a configuration file located at `nginx/server.block_purge_requests.conf`.
+
+This blocking is in place because:
+
+- **Security Risk**: Without proper access controls, anyone could purge your cache, leading to performance degradation
+- **Performance Impact**: Malicious actors could repeatedly purge your cache, forcing your server to regenerate content constantly
+- **VCL Configuration Issues**: Many Varnish configurations use `client.ip` instead of `X-Real-IP` for ACL checks, which can allow unauthorized purging since requests appear to come from 127.0.0.1
+
+## How to Safely Enable PURGE Method
+
+If you need to enable the `PURGE` method for your application (e.g., for cache invalidation after content updates), you can do so by modifying or removing the blocking configuration. Here are the recommended approaches:
+
+### Allow PURGE from Specific IP Addresses
+
+The safest approach is to allow `PURGE` requests only from trusted IP addresses. Replace the content of `nginx/server.block_purge_requests.conf` with:
+
+```nginx
+# Deny all PURGE requests by default, allow from specific IPs
+if ($request_method = PURGE) {
+    set $purge_allowed 0;
+
+    if ($remote_addr = "10.0.0.10") {
+        set $purge_allowed 1;
+    }
+
+    if ($purge_allowed = 0) {
+        return 401;
+    }
+}
+```
+
+### Allow PURGE with Authentication
+
+You can require authentication for `PURGE` requests by checking for specific headers or parameters:
+
+```nginx
+# Deny all, allow with authentication
+if ($request_method = PURGE) {
+    set $purge_auth 0;
+
+    # Allow with proper authentication header/token
+    if ($http_x_purge_token = "your-secret-token") {
+        set $purge_auth 1;
+    }
+
+    if ($purge_auth = 0) {
+        return 401;
+    }
+}
+```
+
+### Comment Out the Block
+
+For development environments only, you can temporarily disable the blocking by commenting out all lines.
+
+```{caution}
+This approach removes all protection and should only be used in development environments.
+```
+
+## Varnish Configuration
+
+If you're using Varnish, ensure your VCL configuration properly handles purge requests and uses the real client IP:
+
+```vcl
+import std;
+
+acl purge {
+    "127.0.0.1";   # allow localhost
+    "10.0.0.0"/24; # allow your cluster private IP range
+}
+
+sub vcl_recv {
+    if (req.method == "PURGE") {
+        set req.http.X-Purge-IP = std.ip(req.http.X-Real-IP, client.ip);
+        if (std.ip(req.http.X-Purge-IP, "0.0.0.0") !~ purge) {
+            return (synth(405));
+        }
+        return (purge);
+    }
+}
+```