Suspecious DNS traffic

WBrown at WBrown at
Mon Mar 25 16:48:57 UTC 2013

babu dheen wrote on 03/25/2013 12:21:30 PM:

> Still not convinced because if i need to allow >1024 port from  our 
> DNS server to external world(internet).. where is the security?

Total security requires total isolation.  It is a matter of accepting some 
risks to perform the needed task.
> I beleive we just need to allow TCP and UDP 53 from our DNS server 
> to internet(any) which is already done. Not sure why we have to open
> non standard port from our DNS server to internet?
> Kindly provide some details.

You send request via UDP from random high port to an authoritative server. 
 Answer is too large to fit in UDP packet, so it responds via TCP to the 
source port of the request (random high port from above).  If you block 
that TCP connection, you cannot receive answer to your query.

Another reason for TCP replies is DNS Response Rate Limiting (RRL). 

Some "modern" stateful firewalls understand DNS and if there is a UDP 
packet sent to port 53, it will accept TCP connections back from the 
destination address on port 53 to the source address/port.

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