Configuring DNS

This chapter contains the following sections:

DNS Client Overview

If your network devices require connectivity with devices in networks for which you do not control name assignment, you can assign device names that uniquely identify your devices within the entire internetwork using the domain name server (DNS). DNS uses a hierarchical scheme for establishing host names for network nodes, which allows local control of the segments of the network through a client-server scheme. The DNS system can locate a network device by translating the host name of the device into its associated IP address.

On the Internet, a domain is a portion of the naming hierarchy tree that refers to general groupings of networks based on organization type or geography. Domain names are pieced together with periods (.) as the delimiting characters. For example, Cisco is a commercial organization that the Internet identifies by a com domain, so its domain name is cisco.com. A specific host name in this domain, the File Transfer Protocol (FTP) system, for example, is identified as ftp.cisco.com.

Name Servers

Name servers keep track of domain names and know the parts of the domain tree for which they have complete information. A name server may also store information about other parts of the domain tree. To map domain names to IP addresses in Cisco NX-OS, you must first identify the host names, then specify a name server, and enable the DNS service.

Cisco NX-OS allows you to statically map IP addresses to domain names. You can also configure Cisco NX-OS to use one or more domain name servers to find an IP address for a host name.

DNS Operation

A name server handles client-issued queries to the DNS server for locally defined hosts within a particular zone as follows:
  • An authoritative name server responds to DNS user queries for a domain name that is under its zone of authority by using the permanent and cached entries in its own host table. If the query is for a domain name that is under its zone of authority but for which it does not have any configuration information, the authoritative name server simply replies that no such information exists.
  • A name server that is not configured as the authoritative name server responds to DNS user queries by using information that it has cached from previously received query responses. If no router is configured as the authoritative name server for a zone, queries to the DNS server for locally defined hosts will receive nonauthoritative responses.

Name servers answer DNS queries (forward incoming DNS queries or resolve internally generated DNS queries) according to the forwarding and lookup parameters configured for the specific domain.

High Availability

Cisco NX-OS supports stateless restarts for the DNS client. After a reboot or supervisor switchover, Cisco NX-OS applies the running configuration.

Prerequisites for DNS Clients

The DNS client has the following prerequisites:

  • You must have a DNS name server on your network.

Licensing Requirements for DNS Clients

The following table shows the licensing requirements for this feature:

Product Licence Rquirement
Cicco NX-OS DNS requires no license. Any feature not included in a license package is bundled with the Cisco NX-OS system images and is provided at no extra charge to you. For a complete explanation of the Cisco NX-OS licensing scheme, see the Cisco NX-OS Licensing Guide.

Default Settings

The following table shows the default settings for DNS client parameters.

Parameter Default
DNS client Enabled

Configuring DNS Clients

You can configure the DNS client to use a DNS server on your network.

Before You Begin
  • Ensure that you have a domain name server on your network.
Procedure
      Command or Action Purpose
    Step 1 configuration terminal

    Example: 
    switch# configuration terminal
switch(config)#
     

    Enters the configuration terminal mode.

     
    Step 2 vrf context managment

    Example: 
    switch(config)# vrf context management
switch(config)#
     

    Specifies a configurable VRF name.

     
    Step 3 ip host name address1 [address2... address6]

    Example: 
    switch# ip host cisco-rtp 192.0.2.1
switch(config)#
     

    Defines up to six static host name-to-address mappings in the host name cache.

     
    Step 4 ip domain name name [use-vrf vrf-name]

    Example: 
    switch(config)# ip domain-name myserver.com
switch(config)#
     

    (Optional) Defines the default domain name server that Cisco NX-OS uses to complete unqualified host names. You can optionally define a VRF that Cisco NX-OS uses to resolve this domain name server if it cannot be resolved in the VRF that you configured this domain name under.

    Cisco NX-OS appends the default domain name to any host name that does not contain a complete domain name before starting a domain-name lookup.

     
    Step 5 ip domain-list name [use-vrf vrf-name]

    Example: 
    switch(config)# ip domain-list mycompany.com
switch(config)#
     

    (Optional) Defines additional domain name servers that Cisco NX-OS can use to complete unqualified host names. You can optionally define a VRF that Cisco NX-OS uses to resolve this domain name server if it cannot be resolved in the VRF that you configured this domain name under.

    Cisco NX-OS uses each entry in the domain list to append that domain name to any host name that does not contain a complete domain name before starting a domain-name lookup. Cisco NX-OS continues this for each entry in the domain list until it finds a match.

     
    Step 6 ip name-server server-address1 [server-address2... server-address6] [use-vrf vrf-name]

    Example: 
    switch(config)# ip name-server 192.0.2.22
     

    (Optional) Defines up to six name servers. The address can be either an IPv4 address or an IPv6 address.

    You can optionally define a VRF that Cisco NX-OS uses to reach this name server if it cannot be reached in the VRF that you configured this name server under.

     
    Step 7 ip domain-lookup

    Example: 
    switch(config)# ip domain-lookup
     

    (Optional) Enables DNS-based address translation. Enabled by default.

     
    Step 8 show hosts

    Example: 
    switch(config)# show hosts
     

    (Optional) Displays information about DNS.

     
    Step 9 exit

    Example: 
    switch(config)# exit
switch#
     

    Exits configuration mode and returns to EXEC mode.

     
    Step 10 copy running-config startup-config

    Example: 
    switch# copy running-config startup-config
switch#
     

    (Optional) Copies the running configuration to the startup configuration.

     

    This example shows how to configure a default domain name and enable DNS lookup:

    switch# config t
switch(config)# vrf context management
switch(config)# ip domain-name mycompany.com
switch(config)# ip name-server 172.68.0.10     
switch(config)# ip domain-lookup