Tc bits (#617)

* middleware/erratic: allow TC bit to be set

Add `truncate` as an option.

Fixes #593

middleware/erratic/README.md

@@ -1,8 +1,11 @@
 # erratic
 
 *erratic* is a middleware useful for testing client behavior. It returns a static response to all
-queries, but the responses can be delayed by a random amount of time or dropped all together, i.e.
-no answer at all.
+queries, but the responses can be:
+
+* delayed by some duration
+* dropped all together
+* the truncated bit can be set
 
 The *erratic* middleware will respond to every A or AAAA query. For any other type it will return
 a SERVFAIL response. The reply for A will return 192.0.2.53 (see RFC 5737), for AAAA it returns
@@ -13,11 +16,13 @@ a SERVFAIL response. The reply for A will return 192.0.2.53 (see RFC 5737), for
 ~~~ txt
 erratic {
     drop [AMOUNT]
+    truncate [AMOUNT]
     delay [AMOUNT [DURATION]]
 }
 ~~~
 
-* `drop`: drop 1 per **AMOUNT** of the queries, the default is 2.
+* `drop`: drop 1 per **AMOUNT** of queries, the default is 2.
+* `truncate`: truncate 1 per **AMOUNT** of queries, the default is 2.
 * `delay`: delay 1 per **AMOUNT** of queries for **DURATION**, the default for **AMOUNT** is 2 and
   the default for **DURATION** is 100ms.
 
@@ -39,7 +44,7 @@ Or even shorter if the defaults suits you. Note this only drops queries, it does
 }
 ~~~
 
-Delay 1 in 3 queries for 50ms, but also drop 1 in 2.
+Delay 1 in 3 queries for 50ms
 
 ~~~ txt
 . {
@@ -49,12 +54,24 @@ Delay 1 in 3 queries for 50ms, but also drop 1 in 2.
 }
 ~~~
 
-To stop dropping you'll need to explicitally set that to 0:
+Delay 1 in 3 and truncate 1 in 5.
+
 ~~~ txt
 . {
     erratic {
-        delay 3 50ms
-        drop 0
+        delay 3 5ms
+        truncate 5
+    }
+}
+~~~
+
+Drop every second query.
+
+~~~ txt
+. {
+    erratic {
+        drop 2
+        truncate 2
     }
 }
 ~~~