0

The DNS response against my domain is 1228 bytes long and I need the packet to get fragmented in 3 packets. To achieve this I have set the MTU at name server (bind9 running on the machine) to be 500. TO verify that MTU is set to 500, I ping from the name server with 600 bytes it gives me error that packet is too long (since MTU is 500 bytes). But when I query the name server I get back the complete reply 1228 bytes in one fragment. I need the packet to be fragmented to run some tests. I am using dig from the client side to query to my name server and if use noedns option from the client side I get the reply with DNS truncated bit set and then OS sends the same query over TCP. Any suggestion how could I get IP fragmentation.

Salman Manzoor
  • 1
  • 1
  • You are not showing really the commands you do, nor the output. How do you verify it is one packet? You have indeed both UDP and TCP to handle, as well as EDNS cases, – Patrick Mevzek Apr 24 '19 at 00:09

1 Answers1

0

Are you sure that the response is 1228, that's huge ... check what dig thinks the size of the response is.

dig google.co.uk
...

;; WHEN: Sun Feb 22 11:55:42 2015
;; MSG SIZE  rcvd: 46

Note the last line is the size of the DNS response.

user9517
  • 115,471
  • 20
  • 215
  • 297
  • I have my own domain and for testing purpose I have created large response to get the fragmented response. Yes when I do dig to my domain the packet is 1228 bytes long although it should get fragmented due to 500 bytes MTU size at my own name server but it is not – Salman Manzoor Feb 22 '15 at 12:17
  • Have you run tcpdump on the connection to see what happens ? – user9517 Feb 22 '15 at 12:19
  • My client is on different network than my name server but it should not be a problem since I can access my name server. I used tcpdump at the server and in query part its mentioned that client can handle ends and server then sends 1228 bytes. if I switch off edns then truncated message appears at client and then tpc is used. – Salman Manzoor Feb 22 '15 at 12:23