Document DOWNSTREAM_LOCAL_ADDRESS header variable (#450)

Signed-off-by: Mandar U Jog <mjog@google.com>

docs/root/configuration/access_log.rst

@@ -130,8 +130,14 @@ The following command operators are supported:
     <config_http_conn_man_headers_x-forwarded-for>`.
 
 %DOWNSTREAM_LOCAL_ADDRESS%
-  Remote address of the downstream connection. If the address is an IP address it includes both
+  Local address of the downstream connection. If the address is an IP address it includes both
   address and port.
+  If the original connection was redirected by iptables REDIRECT, this represents
+  the original destination address restored by the
+  :ref:`Original Destination Filter <config_listener_filters_original_dst>` using SO_ORIGINAL_DST socket option.
+
+%DOWNSTREAM_LOCAL_ADDRESS_WITHOUT_PORT%
+    Same as **%DOWNSTREAM_LOCAL_ADDRESS%** excluding port if the address is an IP address.
 
 %REQ(X?Y):Z%
   HTTP

docs/root/configuration/http_conn_man/headers.rst

@@ -318,6 +318,16 @@ Supported variable names are:
       :ref:`proxy proto <envoy_api_field_listener.FilterChain.use_proxy_proto>` or :ref:`x-forwarded-for
       <config_http_conn_man_headers_x-forwarded-for>`.
 
+%DOWNSTREAM_LOCAL_ADDRESS%
+    Local address of the downstream connection. If the address is an IP address it includes both
+    address and port.
+    If the original connection was redirected by iptables REDIRECT, this represents
+    the original destination address restored by the
+    :ref:`Original Destination Filter <config_listener_filters_original_dst>` using SO_ORIGINAL_DST socket option.
+
+%DOWNSTREAM_LOCAL_ADDRESS_WITHOUT_PORT%
+    Same as **%DOWNSTREAM_LOCAL_ADDRESS%** excluding port if the address is an IP address.
+
 %PROTOCOL%
     The original protocol which is already added by Envoy as a
     :ref:`x-forwarded-proto <config_http_conn_man_headers_x-forwarded-proto>` request header.