IP Addressing: DNS Configuration Guide, Cisco IOS XE Release 3S
Configuring DNS
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Configuring DNS

The Domain Name System (DNS) is a distributed database in which you can map host names to IP addresses through the DNS protocol from a DNS server. Each unique IP address can have an associated host name. The Cisco IOS XE software maintains a cache of host name-to-address mappings for use by the connect, telnet, and ping EXEC commands, and related Telnet support operations. This cache speeds the process of converting names to addresses.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

Prerequisites for Configuring DNS

To use DNS, you must have a DNS name server on your network.

Information About DNS

DNS 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. The global naming scheme of the Internet, the DNS, accomplishes this task. This service is enabled by default. The following sections summarize DNS concepts and function:

Host Names for Network Devices

Each unique IP address can have an associated host name. DNS uses a hierarchical scheme for establishing host names for network nodes. This 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.

Domains Names for Groups of Networks

IP defines a naming scheme that allows a device to be identified by its location in the IP. This is a hierarchical naming scheme that provides for domains. 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 IP identifies by a com domain name, so its domain name is cisco.com. A specific device in this domain, the File Transfer Protocol (FTP) system, for example, is identified as ftp.cisco.com.

Name Servers

To keep track of domain names, IP has defined the concept of a name server. Name servers are programs that have complete information about their namespace portion of the domain tree and may also contain pointers to other name servers that can be used to lead to information from any other part of the domain tree. Name servers 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, you must first identify the host names, then specify a name server, and enable the DNS service.

Cache

To speed the process of converting names to addresses, the name server maintains a database, called a cache, of host name-to-address mappings for use by the connect, telnet, and ping EXEC commands, and related Telnet support operations. The cache stores the results from previous responses. Upon receiving a client-issued DNS query, it will check this local storage to see if the answer is available locally.

Name Resolvers

Name resolvers are programs that extract information from name servers in response to client requests. Resolvers must be able to access at least one name server. The resolver either uses that name server's information to answer a query directly or pursues the query using referrals to other names servers. A resolver will typically be a system routine that is directly accessible to user programs. Therefore, no protocol is necessary between the resolver and the user program.

Zones

The domain namespace is divided into areas called zones that are points of delegation in the DNS tree. A zone contains all domains from a certain point downward, except those for which other zones are authoritative.

Authoritative Name Servers

A name server is said to be an authority for the parts of the domain tree for which it has complete information. A zone usually has an authoritative name server, often more than one. An authoritative name server has been configured with host table information or has acquired host table information though a zone transfer (the action that occurs when a secondary DNS server starts up and updates itself from the primary server).

DNS Operation

Within an organization, you can have many name servers, but Internet clients can query only those that the root name servers know. The other name servers answer internal queries only.

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.

How to Configure DNS

Mapping Host Names to IP Addresses

Perform this task to associate host names with IP addresses.

A name server is used to keep track of information associated with domain names. A name server can maintain a database of host name-to-address mappings. Each name can map to one or more IP addresses. In order to use this service to map domain names to IP addresses, you must specify a name server.

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    ip host name [tcp-port-number] address1 [address2 ... address8]

    4.    Do one of the following:

    • ip domain name name
    • ip domain list name

    5.    ip name-server server-address1 [server-address2 ... server-address6]

    6.    ip domain lookup


DETAILED STEPS
      Command or Action Purpose
    Step 1 enable


    Example:
    Router> enable
     

    Enables privileged EXEC mode.

    • Enter your password if prompted.
     
    Step 2 configure terminal


    Example:
    Router# configure terminal
     

    Enters global configuration mode.

     
    Step 3 ip host name [tcp-port-number] address1 [address2 ... address8]


    Example:
    Router(config)# ip host cisco-rtp 192.168.0.148
     

    Defines a static host name-to-address mapping in the host name cache.

    • Typically, it is easier to refer to network devices by symbolic names rather than numerical addresses (services such as Telnet can use host names or addresses). Host names and IP addresses can be associated with one another through static or dynamic means.
    • Manually assigning host names to addresses is useful when dynamic mapping is not available.
     
    Step 4 Do one of the following:
    • ip domain name name
    • ip domain list name


    Example:
    Router(config)# ip domain name cisco.com


    Example:
              


    Example:
    Router(config)# ip domain list cisco1.com
     

    (Optional) Defines a default domain name that the Cisco IOS XE software will use to complete unqualified host names.

    or

    (Optional) Defines a list of default domain names to complete unqualified host names.

    • You can specify a default domain name that the Cisco IOS XE software will use to complete domain name requests. You can specify either a single domain name or a list of domain names. Any host name that does not contain a complete domain name will have the default domain name you specify appended to it before the name is looked up.
    Note   

    If there is no domain list, the domain name that you specified with the ip domain name global configuration command is used. If there is a domain list, the default domain name is not used. The ip domain list command is similar to the ip domain name command, except that with the ip domain list command you can define a list of domains, each to be tried in turn until the system finds a match.

     
    Step 5 ip name-server server-address1 [server-address2 ... server-address6]


    Example:
    Router(config)# ip name-server 172.16.1.111 172.16.1.2
     

    Specifies one or more hosts that supply name information.

    • Specifies one or more hosts (up to six) that can function as a name server to supply name information for DNS.
     
    Step 6 ip domain lookup


    Example:
    Router(config)# ip domain lookup
     

    (Optional) Enables DNS-based address translation.

    • DNS is enabled by default. Use this command if DNS has been disabled.
     
    What to Do Next

    The name lookup system can be statically configured using the commands described in this task. Some other functions in Cisco IOS XE, such as DHCP can dynamically modify the state of the name lookup system. Use the show hosts command to display the cached host names and the DNS configuration.

    Customizing DNS

    Perform this task to customize your DNS configuration.

    In a multiple server configuration without the DNS round-robin functionality, many programs will use the first host server/IP address for the whole time to live (TTL) of the cache while using the second and third host servers/IP addresses only in the event of host failure. This behavior presents a problem when a high volume of users all arrive at the first host during the TTL time. For example, the network access server (NAS) sends out a DNS query; the DNS servers reply with a list of the configured IP addresses to the NAS. The NAS then caches these IP addresses for a given time (for example, five minutes). All users that dial in during the five minute TTL time will land on one host, the first IP address in the list.

    In a multiple server configuration with the DNS round-robin functionality, the DNS server returns the IP address of all hosts to rotate between the cache of host names. During the TTL of the cache, users are distributed among the hosts. This functionality distributes calls across the configured hosts and reduces the amount of DNS queries.

    SUMMARY STEPS

      1.    enable

      2.    configure terminal

      3.    ip domain timeout seconds

      4.    ip domain retry number

      5.    ip domain round-robin


    DETAILED STEPS
        Command or Action Purpose
      Step 1 enable


      Example:
      Router> enable
       

      Enables privileged EXEC mode.

      • Enter your password if prompted.
       
      Step 2 configure terminal


      Example:
      Router# configure terminal
       

      Enters global configuration mode.

       
      Step 3 ip domain timeout seconds


      Example:
      Router(config)# ip domain timeout 17
       

      (Optional) Specifies the amount of time to wait for a response to a DNS query.

      • If the ip domain timeout command is not configured, the Cisco IOS XE software will wait 3 seconds for a response to a DNS query.
       
      Step 4 ip domain retry number


      Example:
      Router(config)# ip domain retry 10
       

      (Optional) Specifies the number of times to retry sending DNS queries.

      • If the ip domain retry command is not configured, the Cisco IOS XE software will retry DNS queries twice.
       
      Step 5 ip domain round-robin


      Example:
      Router(config)# ip domain round-robin
       

      (Optional) Enables round-robin functionality on DNS servers.

       

      Disabling DNS Queries for ISO CLNS Addresses

      Perform this task to disable DNS queries for ISO CLNS addresses.

      If your router has both IP and ISO Connectionless Network Service (ISO CLNS) enabled and you want to use ISO CLNS network service access point (NSAP) addresses, you can use the DNS to query these addresses, as documented in RFC 1348. This feature is enabled by default.

      SUMMARY STEPS

        1.    enable

        2.    configure terminal

        3.    no ip domain lookup nsap


      DETAILED STEPS
          Command or Action Purpose
        Step 1 enable


        Example:
        Router> enable
         

        Enables privileged EXEC mode.

        • Enter your password if prompted.
         
        Step 2 configure terminal


        Example:
        Router# configure terminal
         

        Enters global configuration mode.

         
        Step 3 no ip domain lookup nsap


        Example:
        Router(config)# no ip domain lookup nsap
         

        Disables DNS queries for ISO CLNS addresses.

         

        Verifying DNS

        Perform this task to verify your DNS configuration.

        1. enable
        2. ping hosts
        3. show hosts
        SUMMARY STEPS

          1.    enable

          2.    ping hosts

          3.    show hosts


        DETAILED STEPS
            Command or Action Purpose
          Step 1 enable


          Example:
          Router> enable
           

          Enables privileged EXEC mode.

          • Enter your password if prompted.
           
          Step 2 ping hosts


          Example:
          Router# ping cisco-rtp
           

          Diagnoses basic network connectivity.

          • After the DNS configuration is set, you can verify the DNS server by using a hostname to ping or telnet to a device.
           
          Step 3 show hosts


          Example:
          Router# show hosts
           

          Displays the default domain name, the style of name lookup service, a list of name server hosts, and the cached list of host names and addresses.

          • After a name is resolved using DNS, use the show hosts command to view the cached hostnames and the DNS configuration.
           

          Configuration Examples for DNS

          IP Domains Example

          The following example establishes a domain list with several alternate domain names:

          ip domain list csi.com
          ip domain list telecomprog.edu
          ip domain list merit.edu

          Dynamic Lookup Example

          The following example configures the host name-to-address mapping process. IP DNS-based translation is specified, the addresses of the name servers are specified, and the default domain name is given.

          ! IP DNS-based host name-to-address translation is enabled
          ip domain lookup
          ! Specifies hosts 192.168.1.111 and 192.168.1.2 as name servers
          ip name-server 192.168.1.111 192.168.1.2
          ! Defines cisco.com as the default domain name the router uses to complete
          ! Set the name for unqualified host names
          ip domain name cisco.com

          Customizing DNS Example

          The following example allows a Telnet to company.example.com to connect to each of the three IP addresses specified in the following order: the first time the hostname is referenced, it would connect to 10.0.0.1; the second time the hostname is referenced, it would connect to 10.1.0.1; and the third time the hostname is referenced, it would connect to 10.2.0.1. In each case, the other two addresses would also be tried if the first one failed; this is the normal operation of the Telnet command.

          Router(config)# ip host company.example.com 10.0.0.1 10.1.0.1 10.2.0.1
          Router(config)# ip domain round-robin

          Additional References

          The following sections provide references related to DNS.

          Related Documents

          Related Topic

          Document Title

          DNS commands: complete command syntax, command mode, command history, defaults, usage guidelines, and examples

          Cisco IOS IP Addressing Services Command Reference

          Standards

          Standards

          Title

          No new or modified standards are supported by this functionality.

          --

          MIBs

          MIBs

          MIBs Link

          No new or modified MIBs are supported by this feature, and support for existing MIBs has not been modified by this feature.

          To locate and download MIBs for selected platforms, Cisco IOS XE releases, and feature sets, use Cisco MIB Locator found at the following URL:

          http:/​/​www.cisco.com/​go/​mibs

          RFCs

          RFCs

          Title

          No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature.

          --

          Technical Assistance

          Description

          Link

          The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies.

          To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds.

          Access to most tools on the Cisco Support website requires a Cisco.com user ID and password.

          http:/​/​www.cisco.com/​public/​support/​tac/​home.shtml

          Feature Information for DNS

          The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

          Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

          Table 1 Feature Information for DNS

          Feature Name

          Releases

          Feature Configuration Information

          DNS

          Cisco IOS XE Release 2.1

          The Domain Name System (DNS) is a distributed database in which you can map host names to IP addresses through the DNS protocol from a DNS server. Each unique IP address can have an associated host name. The Cisco IOS XE software maintains a cache of host name-to-address mappings. This cache speeds the process of converting names to addresses.