diff --git a/docs/configuration_file_basics.xml b/docs/configuration_file_basics.xml
index 0344e5a30..4590017a3 100644
--- a/docs/configuration_file_basics.xml
+++ b/docs/configuration_file_basics.xml
@@ -1470,6 +1470,46 @@ SHELL cat /etc/shorewall/rules.d/*.rules 2> /dev/null || trueOptions that were not set in shorewall.conf will expand to their
default value.
+
+ Beginning with Shorewall 4.5.11, variables can be altered by
+ compiler directives.
+
+
+
+ ?SET variable value
+
+
+ The variable can be specified
+ either with or without a leading '$' to allow using both Perl and
+ Shell variable representation.
+
+ The value is a Perl-compatible
+ expression.
+
+
+ The Shorewall compiler performs variable expansion within
+ the expression. So variables are expanded even when they appear in
+ single quotes.
+
+
+
+ If a variable within the expression can contain a
+ non-numeric value, it is a good idea to enclose it in quotes.
+ Otherwise, the Shorewall compiler has to guess whether to enclose
+ the variable's value in quotes or not.
+
+
+
+
+
+ ?RESET variable
+
+
+ Removes the named variable from the
+ compiler's variable table.
+
+
+
@@ -1558,7 +1598,9 @@ SHELL cat /etc/shorewall/rules.d/*.rules 2> /dev/null || trueIf it is not found in any of those places, the
variable is assumed to have a value of 0
- (false).
+ (false) in Shorewall versions prior to 4.5.11. In 4.5.11 and later, it is
+ assumed to have the value '' (empty string which also evaluates to
+ false).
The setting in /etc/shorewall/params by be
overridden at runtime, provided the setting in