Add bufsize plugin for preparing the DNS Flag Day and avoiding IP fragmentation (#3401)

* add bufsize plugin

Signed-off-by: ykhr53 <yukihira.lab@gmail.com>

* add docstring and comment

Signed-off-by: ykhr53 <yukihira.lab@gmail.com>

* delete stdout messages when get an error

Signed-off-by: ykhr53 <yukihira.lab@gmail.com>

* change to context.Background from TODO

Signed-off-by: ykhr53 <yukihira.lab@gmail.com>

* define default bufsize as defaultBufSize constant

Signed-off-by: ykhr53 <yukihira.lab@gmail.com>

* fix some comments

Signed-off-by: ykhr53 <yukihira.lab@gmail.com>

* function name change: parse

Signed-off-by: ykhr53 <yukihira.lab@gmail.com>

* function name change: parse

Signed-off-by: ykhr53 <yukihira.lab@gmail.com>

plugin/bufsize/README.md

@@ -0,0 +1,30 @@
+# bufsize
+## Name
+*bufsize* - sizes EDNS0 buffer size to prevent IP fragmentation.
+
+## Description
+*bufsize* limits a requester's UDP payload size.  
+It prevents IP fragmentation so that to deal with DNS vulnerability.
+
+## Syntax
+```txt
+bufsize [SIZE]
+```
+
+**[SIZE]** is an int value for setting the buffer size.  
+The default value is 512, and the value must be within 512 - 4096.  
+Only one argument is acceptable, and it covers both IPv4 and IPv6.
+
+## Examples
+```corefile
+. {
+    bufsize 512
+    forward . 172.31.0.10
+    log
+}
+```
+
+If you run a resolver on 172.31.0.10, the buffer size of incoming query on the resolver will be set to 512 bytes.
+
+## Considerations
+For now, if a client does not use EDNS, this plugin adds OPT RR.