DNS Ports

[Joseph S D Yao]

On Fri, Jul 18, 2003 at 05:56:15PM -0700, Dave Harman wrote:
> Hi, NG
> 
> I am using BIND 9.2.1-16, the one that comes with RedHat 9
> I am setting up Iptables and I am having difficulty in finding out
> exactly what ports Bind uses to send and receive queries and data.
> 
> I know DNS uses UDP to send and get data.=20

Did you know that it also uses TCP?  For more than just zone transfers.

> Does DNS always use the unprivileged ports to send queries ?

Unless you define the port used.

> Does DNS always receive answers and quesies on port 53 ?

Yes, both TCP and UDP.

> Does the resolver use different ports than the DNS server ?

The server contains a resolver, since it has to query other servers.
The above referred to the built-in resolver.  The resolver that
programs use is called a "stub" resolver.  All it does is send queries
and return answers.  And the port on which it sends queries is system
dependent, but I believe that BIND's stub resolver also uses
unprivileged ports to send queries, and this cannot be re-defined.

OBTW, some programs cache their own responses, which should not be
confused with the stub resolver doing it.

-- 
Joe Yao				jsdy at center.osis.gov - Joseph S. D. Yao
OSIS Center Systems Support					EMT-B
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