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Re: [FW-1] DNS Question
Sadir Al-khafaji wrote:
>
> What is incorrect ! AFAIK 53/TCP is used for zone transfers please
> correct me if i am wrong. ???Dear Don please refer to RFC 1035
Please read RFC1035 more carefully,
4.2. Transport
The DNS assumes that messages will be transmitted as datagrams or in a
byte stream carried by a virtual circuit. While virtual circuits can be
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
used for any DNS activity, datagrams are preferred for queries due to
^^^^^^^^^^^^^^^^^^^^^^^^^
their lower overhead and better performance. Zone refresh activities
must use virtual circuits because of the need for reliable transfer.
The Internet supports name server access using TCP [RFC-793] on server
port 53 (decimal) as well as datagram access using UDP [RFC-768] on UDP
port 53 (decimal).
4.2.1. UDP usage
Messages sent using UDP user server port 53 (decimal).
Messages carried by UDP are restricted to 512 bytes (not counting the IP
or UDP headers). Longer messages are truncated and the TC bit is set in
the header.
So, virtual circuits (TCP) can be used for _any_ activity, zone transfers
or queries. Datagrams (UDP) are preferred for queries, but cannot be used
for messages that contain more than 512 bytes. This means that TCP must be
used for queries that involved messages containing more than 512 bytes.
Technically, a DNS server must allow access to 53/udp and 53/tcp to be a
real, compliant Internet nameserver.
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