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CHAPTER
19-1
Cisco ASA 5500 Series Configuration Guide using ASDM
19
Configuring Dynamic DNS
This chapter describes how to configure DDNS update methods and includes the following topics:
• Information About DDNS, page 19-1
• Licensing Requirements for DDNS, page 19-2
• Guidelines and Limitations, page 19-2
• Configuring Dynamic DNS, page 19-2
• DDNS Monitoring, page 19-4
• Feature History for DDNS, page 19-4
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 pre-defined 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 the “Configuring the DNS Server” section on page 17-7. To
configure DHCP, see the “Configuring a DHCP Server” section on page 18-5.
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
• 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 enforces the minimum of the three specified
values.