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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.
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.
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To use DNS, you must have a DNS name server on your network.
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:
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.
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.
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.
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 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.
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.
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).
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:
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.
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.
1. enable
2. configure terminal
3. ip host name [tcp-port-number] address1 [address2 ... address8]
5. ip name-server server-address1 [server-address2 ... server-address6]
6. ip domain lookup
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.
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.
1. enable
2. configure terminal
3. ip domain timeout seconds
4. ip domain retry number
5. ip domain round-robin
Command or Action | Purpose | |
---|---|---|
Step 1 | enable Example: Router> enable |
Enables privileged EXEC mode.
|
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.
|
Step 4 | ip domain retry number Example: Router(config)# ip domain retry 10 |
(Optional) Specifies the number of times to retry sending DNS queries.
|
Step 5 | ip domain round-robin Example: Router(config)# ip domain round-robin |
(Optional) Enables round-robin functionality on DNS servers. |
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.
1. enable
2. configure terminal
3. no ip domain lookup nsap
Command or Action | Purpose | |
---|---|---|
Step 1 | enable Example: Router> enable |
Enables privileged EXEC mode.
|
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. |
Perform this task to verify your DNS configuration.
1. enable
2. ping hosts
3. show hosts
Command or Action | Purpose | |
---|---|---|
Step 1 | enable Example: Router> enable |
Enables privileged EXEC mode.
|
Step 2 | ping hosts Example: Router# ping cisco-rtp |
Diagnoses basic network connectivity.
|
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.
|
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
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
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
The following sections provide references related to DNS.
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 |
Title |
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No new or modified standards are supported by this functionality. |
-- |
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: |
RFCs |
Title |
---|---|
No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature. |
-- |
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. |
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.
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. |