Hi,
On Sun, Jul 4, 2010 at 2:34 AM, Áki Hermann Barkarson <[email protected]> wrote:
>
> Does anyone know the origin of this 255.255.255.255 default lookup behaviour?
> Is there any particular reason why Cisco are keeping this behaviour around?
> And finally, is there anyone out there using a dns service that replies to
> broadcasts?
I don't know why Cisco choose to keep this as a default but DNS
lookups to multicast and broadcast addresses is mentioned in RFC 1123
-- "Requirements for Internet Hosts -- Application and Support"
>From section 6.1.3.2 ('Transport protocols'):
A server MAY support a UDP query that is delivered using an
IP broadcast or multicast address. However, the Recursion
Desired bit MUST NOT be set in a query that is multicast,
and MUST be ignored by name servers receiving queries via a
broadcast or multicast address. A host that sends broadcast
or multicast DNS queries SHOULD send them only as occasional
probes, caching the IP address(es) it obtains from the
response(s) so it can normally send unicast queries.
DISCUSSION:
Broadcast or (especially) IP multicast can provide a
way to locate nearby name servers without knowing their
IP addresses in advance. However, general broadcasting
of recursive queries can result in excessive and
unnecessary load on both network and servers.
I agree that it's annoying, but I'm sure most (all?) DNS servers do
respond to UDP broadcast queries, and IOS does cache the IP address of
the server for future use. So really, what makes it annoying is when
there is no DNS server on the segment to respond to the query, and
also how frequently IOS attempts to resolve names through broadcasts
-- I reckon it should try once, and if there is no response, just give
up. Or not try at all :-)
Have you found a DNS server that does NOT respond to these queries?
cheers,
Dale
_______________________________________________
For more information regarding industry leading CCIE Lab training, please visit
www.ipexpert.com
