[Pdns-users] DNS answer don't fit UDP packet

Sherwin Daganato sherwin at saturn.emc.com.ph
Fri Oct 10 17:19:49 UTC 2003

On Fri, Oct 10, 2003 at 06:18:58PM +0200, Antoine Levavasseur wrote:
> > For now I reduced my mx list to fit UDP.
> > 
> > But is there any configuration that makes PDNS shrink ADDITIONAL SECTION
> > in order to fit UDP and answer UDP query with UDP.
> Some insightfull comments can be found in RFC1123 

Some additional specifications regarding the UDP 512-byte restriction
and truncation.

FROM RFC1035 (STD13):

4.2. Transport


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.

UDP is not acceptable for zone transfers, but is the recommended method
for standard queries in the Internet.  Queries sent using UDP may be
lost, and hence a retransmission strategy is required.  Queries or their
responses may be reordered by the network, or by processing in name
servers, so resolvers should not depend on them being returned in order.

The optimal UDP retransmission policy will vary with performance of the
Internet and the needs of the client, but the following are recommended:

   - The client should try other servers and server addresses
     before repeating a query to a specific address of a server.

   - The retransmission interval should be based on prior
     statistics if possible.  Too aggressive retransmission can
     easily slow responses for the community at large.  Depending
     on how well connected the client is to its expected servers,
     the minimum retransmission interval should be 2-5 seconds.

More suggestions on server selection and retransmission policy can be
found in the resolver section of this memo.

Hope this helps.

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