UDP DNS Response

[Burak Özalp]

Assume that i am a firewall between client and server. If client send an UDP DNS query, I should response this query with set the truncated flag=1. As a result, client will be forced to use TCP instead of UDP.

Is it possible to generate such UDP responses with DNS.python?

[Anand Buddhdev]

On 20/08/15 08:50, Burak Özalp wrote:

Hi Burak,

I don't like your idea of intercepting DNS packets and messing around
with them. But... it should be possible.

Assuming you have already received a query and it is stored in the
variable q, then you can do:

r = dns.message.make_response(q, our_payload=512)
r.flags += dns.flags.TC

This will give you an empty response message with the TC flag set. Now
you can send it to the client.

Regards,
Anand

[Burak Özalp]

I wrote the following code( http://codepad.org/Y7R8R5dw)  to listen o UDP port like a server, and response the client with changed the Truncated Bit=1. But i have two problems;

1) The program doesn't terminate and stuck on the line "  response=dns.query.udp(r,UDP_IP,port=UDP_PORT)"
2) I manage the response the request query but i couldn't manage to change destination and source ports, the destination port still same both on request and response.

Could anyone has a solution ?

Thanks,
Best Regards
Burak Ozalp

[Anand Buddhdev]

On 21/08/15 13:27, Burak Özalp wrote:

Hi Burak,

> I wrote the following code( http://codepad.org/Y7R8R5dw) to listen o UDP
> port like a server, and response the client with changed the Truncated
> Bit=1. But i have two problems;
>
>
> 1) The program doesn't terminate and stuck on the line "
> response=dns.query.udp(r,UDP_IP,port=UDP_PORT)"

The dns.query.udp function is ONLY for SENDING queries, not for
receiving anything from the network.

In order to write a server for listening to the network, and receiving
DNS queries, you'll have to use something like the SocketServer module.
The dnspython module does not contain any server code.

Regards,
Anand

[Burak Özalp]

Dear Anand,

As you can see from the code, i use socket module to receive from network, and i use pythondns only for sending response.

Best Regards
Burak Ozalp



 Hi Burak, 

[Bob Halley]

On Friday, August 21, 2015 at 5:30:33 AM UTC-7, Burak Özalp wrote:

As you can see from the code, i use socket module to receive from network, and i use pythondns only for sending response.

dns.query.udp() is used for making queries and waiting for a response.  You're sending a response, so it's not appropriate.

You want to do

wire = r.to_wire()

sock.sendto(wire, addr)