Truncation Bit

Mark msanp at earthlink.net
Thu Aug 3 22:07:24 UTC 2000


Does anyone know how to get the truncation bit set in a dns response packet?

I am assuming if the response from a nameserver won't fit into a single udp
packet the truncation bit will be set in the Header of the response packet.
Ideally this shouldn't happen too often but I would think you would see it
from time to time. I have looked in the headers a lot of responses and I
have never seen this bit set.

If the maximum size of an IPv4 datagram is 65535 bytes it seems to me my udp
payload can be of size
                  udp payload + udp header + ip header = 65535 bytes or less

digs such as
$ dig  @ns2.altavista.com. ANY altavista.com

produce quite a long response packet but even this is only 2250 bytes. This
doesn't come close to exceeding the size limit.  It seems to me this feature
is unnecessary since the size limit will never reached. Any insights
appreciated.

Regards,

Mark










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