Add support for client-subnet DNS options

docs/configuration/dns/index.md

@@ -1,3 +1,11 @@
+---
+icon: material/new-box
+---
+
+!!! quote "Changes in sing-box 1.9.0"
+
+    :material-plus: [client_subnet](#client_subnet)
+
 # DNS
 
 ### Structure
@@ -13,6 +21,7 @@
     "disable_expire": false,
     "independent_cache": false,
     "reverse_mapping": false,
+    "client_subnet": "",
     "fakeip": {}
   }
 }
@@ -60,6 +69,10 @@ Stores a reverse mapping of IP addresses after responding to a DNS query in orde
 Since this process relies on the act of resolving domain names by an application before making a request, it can be
 problematic in environments such as macOS, where DNS is proxied and cached by the system.
 
-#### fakeip
+#### client_subnet
 
-[FakeIP](./fakeip/) settings.
+!!! question "Since sing-box 1.9.0"
+
+Append a `edns0-subnet` OPT extra record with the specified IP address to every query by default.
+
+Can be overrides by `servers.[].client_subnet` or `rules.[].client_subnet`.