From: Frank (ocsic@web.de)
Date: Wed Dec 06 2006 - 19:55:50 ART
Abu Hamzah schrieb:
Zone transfer is between dns servers to transfer zone informations. This
must be
a reliable process so it uses tcp port 53. udp port 53 is used for
sending out dns information
for clients. Clients send dns queries to servers on port 53 udp. Most
queries are udp based.
Exept queries lager than 512 bytes are send as tcp.
http://www.faqs.org/rfcs/rfc1123.html
6.1.3.2 Transport Protocols
*DNS* resolvers and recursive servers MUST support *UDP*, and
SHOULD support *TCP*, for sending (non-zone-transfer) queries.
Specifically, a *DNS* resolver or server that is sending a
non-zone-transfer query MUST send a *UDP* query first. If the
Answer section of the response is truncated and if the
requester supports *TCP*, it SHOULD try the query again using
*TCP*.
*DNS* servers MUST be able to service *UDP* queries and SHOULD
be able to service *TCP* queries. A name server MAY limit the
resources it devotes to *TCP* queries, but it SHOULD NOT
refuse to service a *TCP* query just because it would have
succeeded with *UDP*.
Frank
> Hi all
>
> Can someone please clarify how UDP and TCP DNS port 53 is used?
>
> I know that when we try to connect to a website we use UDP DNS port 53.
> Also, when a name server is doing Zone transfers it uses TCP port 53 for DNS
> Zone transfer. How about, if we connect to a name server1 for resolving say
> www.abc.com and this server1 can't resolve the name then does server1 use
> TCP port 53 for DNS to and get this from server2?
>
> Any good URL for this background info would be appreciated.
>
> thanks
>
> Abu Hamzah
>
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