document.diff

diff --git a/Documentation/lkl.txt b/Documentation/lkl.txt
index 221ea8905f02..ca199e5e6e63 100644
--- a/Documentation/lkl.txt
+++ b/Documentation/lkl.txt
@@ -134,16 +134,46 @@ LKL hijack library
 
 LKL hijack library (liblkl-hijack.so) is used to replace system calls used by an
 application on the fly so that the application can use LKL instead of the kernel
-of host operating system. LD_PRELOAD is used to dynamically override system calls
-with this library when you execute a program.
+of host operating system. LD_PRELOAD is used to dynamically override system
+calls with this library when you execute a program.
 
 You can usually use this library via a wrapper script.
 
     $ cd tools/lkl
     $ ./bin/lkl-hijack.sh ip address show
 
-There are environmental variables to configure the behavior of LKL. The followings
-are the list of those variable for your environment.
+In order to configure the behavior of LKL, a json file can be used.  If there is
+nothing specified, LKL tries to find with the name 'lkl-hijack.json' for the
+configuration file.  You can also use the old-style configuration with
+environmental variables (e.g., LKL_HIJACK_NET_IFTYPE) but those are overridden
+if a json file is specified
+
+The followings are the list of keys to describe a JSON file.
+
+* gateway
+
+  the gateway IP address of LKL network stack.
+```
+     "gateway":"202.214.86.129"
+```
+
+(snip)
+
+* interfaces
+
+This key takes a set of sub-keys to configure a single interface. Each key is
+defined as follows.
+
+** mac
+
+  the MAC address of the interface.
+
+```
+     "mac":"4e:ed:db:a0:67:ad"
+```
+
+(snip)
+
 
 * LKL_HIJACK_NET_IFTYPE