Chapter 24
| Domain Name Service Commands
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Default Setting
Enabled
Command Mode
Global Configuration
Command Usage
◆ mDNS allows a network device to choose a domain name in the local DNS
name space and announce it using a special multicast IP address. This allows
any user to give their computers a link-local mDNS host name of the form
“single-dns-label.local.” Any name ending in “.local.” is therefore link-local, and
names within this domain are meaningful only on the link where they originate.
◆ When looking for the given host’s IP address, the client sends a single-shot
mDNS IP multicast query message to all the hosts sharing its local network. Any
DNS query for a name ending with “.local.” is sent to the mDNS multicast
address 224.0.0.251 (or its IPv6 equivalent FF02::FB).
The corresponding host replies with a multicast message announcing itself. All
machines in the subnet can then update their mDNS cache with the host’s
information sent in the reply message.
◆ To maintain an on-going cache of host names requires a process of continuous
multicast DNS querying. This is done in several phases:
■
Probing – The DNS responder sends a probe message to the local network
in order to verify that each entry its local cache is unique.
■
Announcing – The responder sends an unsolicited mDNS Response
containing all of its newly registered resource records (both shared records,
and unique records that have completed the probing step).
■
Updating – The responder repeats the Announcing step to update
neighbor caches when the data for any local mDNS record changes.
Example
Console(config)#ip mdns
Console#
ip name-server This command specifies the address of one or more domain name servers to use for
name-to-address resolution. Use the no form to remove a name server from this
list.
Syntax
[no] ip name-server server-address1 [server-address2 … server-address6]
server-address1 - IPv4 or IPv6 address of domain-name server.
server-address2 … server-address6 - IPv4 or IPv6 address of additional
domain-name servers.
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