Dynamic DNS is a method of automatically updating a name server in the Domain Name System, often
in real time, with the active DNS configuration of its configured hostnames, addresses or
other information.
The term is used to describe two different concepts.
At the administration levels of the Internet, "dynamic DNS updating" refers to systems that
are used to update traditional DNS records without manual editing.
These mechanisms are explained in RFC 2136, and use the TSIG mechanism to provide security.
Another kind of dynamic DNS permits lightweight and immediate updates to its local database,
often using a web-based mechanism.
It is used to resolve a well-known domain name to an IP address that may change frequently.
It provides a persistent addressing method for devices that change their location or
configuration.
Background In the initial stages of the Internet addressing
of hosts on the network was achieved by static translation tables that mapped hostnames to
IP addresses.
The tables were maintained manually in form of the host file.
The Domain Name System brought a method of distributing the same address information
automatically online through recursive queries to remote databases configured for each network,
or domain.
Even this DNS facility still used static lookup tables at each participating node.
IP addresses, once assigned to a particular host, rarely changed and the mechanism was
initially sufficient.
However, the rapid growth of the Internet and the proliferation of personal computers
in the workplace and in homes created the substantial burden for administrators of keeping
track of assigned IP addresses and managing their address space.
The Dynamic Host Configuration Protocol allowed enterprises and Internet service providers
to assign addresses to computers automatically as they powered up.
In addition, this helped conserve the address space available, since not all devices might
be actively used at all times and addresses could be assigned as needed.
This feature required that DNS servers be kept current automatically as well.
The first implementations of dynamic DNS fulfilled this purpose: Host computers gained the feature
to notify their respective DNS server of the address they had received from a DHCP server
or through self-configuration.
This protocol-based DNS update method was documented and standardized in IETF publication
RFC 2136 in 1997 and has become a standard part of the DNS protocol.
The explosive growth and proliferation of the Internet into homes brought a growing
shortage of available IP addresses.
DHCP became an important tool for ISPs as well to manage their address spaces for connecting
home and small-business end-users with a single IP address each by implementing network address
translation at the customer premise router.
The private network behind these routers uses address space set aside for these purposes,
masqueraded by the NAT device.
This, however, broke the end-to-end principle of Internet architecture and methods were
required to allow private networks, with frequently changing external IP addresses, to discover
their public address and insert it into the Domain Name System in order to participate
in Internet communications properly.
Today, numerous providers, called Dynamic DNS service providers, offer such technology
and services on the Internet.
Domain Name System DNS is based on a distributed database that
takes some time to update globally.
When DNS was first introduced, the database was small and could be easily maintained by
hand.
As the system grew this task became difficult for any one site to handle, and a new management
structure was introduced to spread out the updates among many domain name registrars.
Registrars today offer end-user updating to their account information, typically using
a web-based form, and the registrar then pushes out update information to other DNS servers.
Due to the distributed nature of the DNS systems and its registrars, updates to the global
DNS system may take hours to distribute.
Thus DNS is only suitable for services that do not change their IP address very often,
as is the case for most large services like Wikipedia or Google.
Smaller services, however, are generally much more likely to move from host to host over
shorter periods of time.
Servers being run on certain types of Internet service provider, cable modems in particular,
are likely to change their IP address over very short periods of time, on the order of
days or hours.
DDNS Dynamic DNS is a system that addresses the
problem of rapid updates.
The term is used in two contexts which, while technically similar, have very different purposes
and user populations.
End users of Internet access receive an allocation of IP addresses, often only a single address,
by their Internet service provider.
The assigned addresses may either be fixed, or may change from time to time, a situation
called dynamic.
Dynamic addresses are generally given only to residential customers and small businesses,
as most enterprises specifically require static addresses.
Dynamic IP addresses present a problem if the customer wants to provide a service to
other users on the Internet, such as a web service.
As the IP address may change frequently, corresponding domain names must be quickly re-mapped in
the DNS, to maintain accessibility using a well-known URL.
Many providers offer commercial or free Dynamic DNS service for this scenario.
The automatic reconfiguration is generally implemented in the user's router or computer,
which runs software to update the DDNS service.
The communication between the user's equipment and the provider is not standardized, although
a few standard web-based methods of updating have emerged over time.
Standards-based dynamic DNS update The standardized method of dynamically updating
domain name server records is prescribed by RFC 2136, commonly known as dynamic DNS update.
The method described by RFC 2136 is a network protocol for use with managed DNS servers,
and it includes a security mechanism.
RFC 2136 supports all DNS record types, but often it is used only as an extension of the
DHCP system, and in which the authorized DHCP servers register the client records in the
DNS.
This form of support for RFC 2136 is provided by a plethora of client and server software,
including those that are components of most current operating systems.
Support for RFC 2136 is also an integral part of many directory services, including LDAP
and Windows' Active Directory domains.
Function In Microsoft Windows networks, dynamic DNS
is an integral part of Active Directory, because domain controllers register their network
service types in DNS so that other computers in the Domain can access them.
Increasing efforts to secure Internet communications today involve encryption of all dynamic updates
via the public Internet, as these public dynamic DNS services have been abused increasingly
to design security breaches.
Standards-based methods within the DNSSEC protocol suite, such as TSIG, have been developed
to secure DNS updates, but are not widely in use.
Microsoft developed alternative technology based on Kerberos authentication.
Some freeware DNS server software systems, such as dnsmasq, support a dynamic update
procedure that directly involves a built-in DHCP server.
This server automatically updates or adds the DNS records as it assigns addresses, relieving
the administrator of the task of specifically configuring dynamic updates.
DDNS for Internet access devices Dynamic DNS providers offer a software client
program that automates the discovery and registration of the client system's public IP addresses.
The client program is executed on a computer or device in the private network.
It connects to the DDNS provider's systems with a unique login name; the provider uses
the name to link the discovered public IP address of the home network with a hostname
in the domain name system.
Depending on the provider, the hostname is registered within a domain owned by the provider,
or within the customer's own domain name.
These services can function by a number of mechanisms.
Often they use an HTTP service request since even restrictive environments usually allow
HTTP service.
The provider might use RFC 2136 to update the DNS servers.
Many home networking modem/routers include client applications in their firmware, compatible
with a variety of DDNS providers.
DDNS for security appliance manufacturers Dynamic DNS is an expected feature or even
requirement for IP based security appliances like DVRs and IP cameras.
Many options are available for today's manufacturer, and these include the use of existing DDNS
services like dyn.com or no-ip.com or the use of custom services hosted by the manufacturer
themselves.
A manufacturer may choose to develop their own DDNS software or use a readymade software
product like MintDNS or GNUDIP.
In almost all cases, a simple HTTP based update API is used as it allows for easy integration
of a DDNS client into a device's firmware.
There are many pre-made tools that exist that can help to ease the burden of client development
even further like cURL or Inadyn.
Most web-based DDNS services use a standard user name and password security schema.
This requires that a user first create an account at the DDNS server website and then
configure their device to send updates to the DDNS server whenever an IP address change
is detected.
This is the method used by almost all web based DDNS services.
Some device manufacturers go a step further by only allowing their DDNS Service to be
used by the devices they manufacture, and also eliminate the need for user names and
passwords altogether.
Generally this is accomplished by encrypting the device's MAC address using an encryption
algorithm kept secret on both the DDNS server and within the device's firmware.
The resulting decryption or decryption failure is used to secure or deny updates.
Resources for the development of custom DDNS services are generally limited and involve
a full software development cycle to design and field a secure and robust DDNS server.
Some pre-made software products are available, though, and a few companies that specialize
in the development of secure custom DDNS services do exist.
See also DNS hosting service
Fast-flux DNS Comparison of DNS server software
External links Dynamic DNS Services at DMOZ