Truncation Bit

[Mark]

I got 2250 by piping the response to a file and checking the size. I'm sure
that's wrong. How did you get the correct size of the response?

Regards,

Mark




Kevin Darcy <kcd at daimlerchrysler.com> wrote in message
news:3989F73E.DB97C5DE at daimlerchrysler.com...
>
> DNS UDP packets are arbitrarily limited to 512 bytes. The reason you're
seeing
> responses larger than 512 bytes is because smart resolvers (like dig)
will, by
> default, automatically retry truncated queries with TCP. Use a "+i"
(ignore
> truncation errors) on your dig command line to inhibit this behavior.
>
>
> - Kevin
>
> P.S. I'm only getting about 700 bytes in response to that query. How did
you
> get 2250?
>
> Mark wrote:
>
> > 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
>
>
>
>
>
>