Dynamic DNS

This chapter describes how to configure DDNS update methods and includes the following topics:

Information About DDNS

DDNS update integrates DNS with DHCP. The two protocols are complementary: DHCP centralizes and automates IP address allocation; DDNS update automatically records the association between assigned addresses and hostnames at predefined intervals. DDNS allows frequently changing address-hostname associations to be updated frequently. Mobile hosts, for example, can then move freely on a network without user or administrator intervention. DDNS provides the necessary dynamic update and synchronization of the name-to-address mapping and address-to-name mapping on the DNS server. To configure the DNS server for other uses, see Configuring the DNS Server. To configure DHCP, see Configuring the DHCP Server.

EDNS allows DNS requesters to advertise the size of their UDP packets and facilitates the transfer of packets larger than 512 octets. When a DNS server receives a request over UDP, it identifies the size of the UDP packet from the OPT resource record (RR) and scales its response to contain as many resource records as are allowed in the maximum UDP packet size specified by the requester. The size of the DNS packets can be up to 4096 bytes for BIND or 1280 bytes for the Windows 2003 DNS Server. Several additional message-length maximum commands are available:

  • The existing global limit: message-length maximum 512
  • A client or server specific limit: message-length maximum client 4096 and message-length maximum server 4096
  • The dynamic value specified in the OPT RR field: message-length maximum client auto

If the three commands are present at the same time, the ASA allows the automatically configured length up to the configured client or server maximum. For all other DNS traffic, the message-length maximum is used.

Licensing Requirements for DDNS

The following table shows the licensing requirements for DDNS:

 

Model
License Requirement

ASAv

Standard or Premium License.

All other models

Base License.

Guidelines and Limitations

Failover Guidelines

Supports Active/Active and Active/Standby failover.

Firewall Mode Guidelines

Supported in routed firewall mode.

Context Mode Guidelines

Supported in single and multiple context modes.

Supported in transparent mode for the DNS Client pane.

IPv6 Guidelines

Supports IPv6.

Configuring Dynamic DNS

Command
Purpose

Step 1
ddns update method name
 

ciscoasa(config)# ddns update method ddns-2

Creates a DDNS update method ddns-2 that dynamically updates DNS resource records (RRs).

Step 2
ddns both
 

ciscoasa(DDNS-update-method)# ddns both

Specifies that the client updates both the DNS A and PTR resource records (RRs).

Step 3
interface mapped_name
 

ciscoasa(DDNS-update-method)# interface Ethernet0

Configures an interface Ethernet 0and enters interface configuration mode.

Step 4
ddns update [ method-name | hostname hostname]
 

ciscoasa(config-if)# ddns update ddns-2

ciscoasa(config-if)# ddns update hostname asa.example.com

Associates the the DDNS method ddns-2 with the Ethernet0 interface and an update hostname.

Step 5
dhcp-client update dns [server {both | none}]
 

ciscoasa(config)# dhcp-client update dns server none

Configures the DHCP client to request that the DHCP server perform no updates.

Step 6
ip address dhcp
 

ciscoasa(if-config)# ip address dhcp

Uses DHCP to obtain an IP address for the interface.

Step 7
dhcpd update dns [both] [override] [interface srv_ifc_name]
 

ciscoasa(if-config)# dhcpd update dns both override

Configures DHCP server to override the client update requests.
Command
Purpose

Step 1
interface mapped_name
 

ciscoasa(config)# interface Ethernet0

Configures an interface Ethernet 0.

Step 2
dhcp-client update dns [server {both | none}]
 

ciscoasa(config-if)# dhcp-client update dns both

DHCP client requests that the DHCP server update both the DNS A and PTR resource records.

Step 3
ddns update [ method-name | hostname hostname]
 

ciscoasa(config-if)# ddns update hostname asa

Configures the DHCP client on interface Ethernet 0.

Step 4
dhcpd update dns [both] [override] [interface srv_ifc_name]
 

ciscoasa(config-if)# dhcpd update dns

Configures DHCP server to perform DDNS updates.

Step 5
dhcpd domain domain_name [interface if_name]
 

ciscoasa(config-if)# dhcpd domain example.com

Defines the DNS domain name for DHCP clients.

Command
Purpose

Step 1
ddns update method name
 

ciscoasa(config)# ddns update method ddns-2

Creates a DDNS update method ddns-2 that dynamically updates DNS resource records (RRs).

Step 2
ddns [both]
 

ciscoasa(DDNS-update-method)# ddns

Specifies a dynamic DNS (DDNS) update method.

Step 3
interface mapped_name
 

ciscoasa(DDNS-update-method)# interface Ethernet0

Configures an interface Ethernet 0.

Step 4
dhcp-client update dns [server {both | none}]
 

ciscoasa(config-if)# dhcp-client update dns

Configures the update parameters that the DHCP client passes to the DHCP server.

Step 5
ddns update [ method-name | hostname hostname]
 

ciscoasa(config-if)# ddns update ddns-2

ciscoasa(config-if)# ddns update hostname asa

Associates the the DDNS method ddns-2 with the Ethernet0 interface and an update hostname.

Step 6
dhcpd update dns [both] [override] [interface srv_ifc_name]
 

ciscoasa(if-config)# dhcpd update dns

Configures DHCP server to perform DDNS updates.

Step 7
dhcpd domain domain_name [interface if_name]
 

ciscoasa(config-if)# dhcpd domain example.com

Defines the DNS domain name for DHCP clients.

Dynamic DNS provides address and domain name mapping so that hosts can find each other, even though their DHCP-assigned IP addresses change frequently. The DDNS name and address mapping are stored on the DHCP server in two resource records: the A RR includes the name-to-IP address mapping, while the PTR RR maps addresses to names. Of the two methods for performing DDNS updates—the IETF standard defined by RFC 2136 and a generic HTTP method—the ASA supports the IETF method in this release.

The Dynamic DNS pane shows the configured DDNS update methods and the interfaces that have been configured for DDNS. By automatically recording the association between assigned addresses and hostnames at pre-defined intervals, DDNS allows frequently changing address-hostname associations to be updated regularly. Mobile hosts, for example, can then move freely on a network without user or administrator intervention.

To configure dynamic DNS client settings for updating the DNS server, perform the following steps:

Step 1 In the ASDM main application window, choose Configuration > Device Management > DNS > Dynamic DNS.

Step 2 Click Add to display the Add Dynamic DNS Update Method dialog box.

Step 3 Enter the name for the DDNS update method.

Step 4 Specify the update interval between DNS update attempts configured for the update method in days, hours, minutes, and seconds.

    • Choose the number of days between update attempts from 0 to 364.
    • Choose the number of hours (in whole numbers) between update attempts from 0 to 23.
    • Choose the number of minutes (in whole numbers) between update attempts from 0 to 59.
    • Choose the number of seconds (in whole numbers) between update attempts from 0 to 59.

These units are additive. That is, if you enter 0 days, 0 hours, 5 minutes and 15 seconds, the update method tries an update every 5 minutes and 15 seconds for as long as the method is active.

Step 5 To store server resource record updates that the DNS client updates, choose one of the following options:

    • Both the A resource record and the PTR resource record.
    • The A resource records only.

Step 6 Click OK to close the Add Dynamic DNS Update Method dialog box.

The new dynamic DNS client settings appear.

Step 7 To change these settings, click Edit to display the Edit Dynamic DDNS Update Method dialog box. When you edit an existing method, the Name field is display-only and shows the name of the selected method for editing

Step 8 Make your desired changes, and then click OK to close the Edit Dynamic DDNS Update Method dialog box.

The revised dynamic DNS client settings appear.

Step 9 To remove configured settings, choose the settings from the list, and then click Delete.

Step 10 To add DDNS settings for each interface configured for DDNS, click Add to display the Add Dynamic DNS Interface Settings dialog box.

Step 11 Choose the interface from the drop-down list.

Step 12 Choose the update method assigned to the interface from the drop-down list.

Step 13 Enter the hostname of the DDNS client.

Step 14 To store resource record updates, choose one of the following options:

    • Default (PTR Records) to specify that the client request PTR record updating by the server.
    • Both (PTR Records and A Records) to specify that the client request both the A and PTR DNS resource records by the server.
    • None to specify that the client request no updates by the server.

Note DHCP must be enabled on the selected interface for this action to take effect.

Step 15 Click OK to close the Add Dynamic DNS Interface Settings dialog box.

The new dynamic DNS interface settings appear.

Step 16 To change these settings, click Edit to display the Edit Dynamic DNS Interface Settings dialog box.

Step 17 Make your desired changes, and then click OK to close the Edit Dynamic DNS Interface Settings dialog box.

The revised dynamic DNS interface settings appear.

Step 18 To remove configured settings, choose the settings from the list, and then click Delete.

Step 19 Click Apply to save your changes, or click Reset to discard them and enter new ones.

 

DDNS Monitoring

To monitor DDNS, perform the following steps:

 

Path
Purpose

Tools > Command Line Interface

Enter the show running-config ddns command, then click Send.

Shows the current DDNS configuration.

Tools > Command Line Interface

Enter the show running-config dns server-group command, then click Send.

Shows the current DNS server group status.

Feature History for DDNS

Table 18-1 lists each feature change and the platform release in which it was implemented.

ASDM is backward-compatible with multiple platform releases, so the specific ASDM release in which support was added is not listed.

 

Table 18-1 Feature History for DDNS
Feature Name
Releases
Feature Information

DDNS

7.0(1)

We introduced this feature.

We introduced the following screens:

Configuration > Device Management> DNS > DNS Client.
Configuration > Device Management > DNS > Dynamic DNS.