D-Link NetDefend DFL-210 User Manual page 129

3.9. DNS
Dynamic DNS
A DNS feature offered by NetDefendOS is the ability to explicitly inform DNS servers when the
external IP address of the NetDefend Firewall has changed. This is sometimes referred to as
Dynamic DNS and is useful where the NetDefend Firewall has an external IP address that can
change.
Dynamic DNS can also be useful in VPN scenarios where both ends of the tunnel have dynamic IP
addresses. If only one side of the tunnel has a dynamic address then the NetDefendOS VPN keep
alive feature solves this problem.
Under System > Misc. Clients in the WebUI, several dynamic DNS services are defined. The
HTTP Poster client is a generic dynamic DNS client with which it is possible to define 3 different
DNS URLs plus an explicit value for Delay in seconds until all URLs are refetched (with a default
of 604800 seconds, equivalent to 7 days).
Everytime NetDefendOS reconfigures, HTTP Poster will send an HTTP GET request to the defined
URLs. It will also repeat this at an interval specified by the refetch delay.
The difference between HTTP Poster and the named DNS servers in the WebUI is that HTTP Poster
can be used to send any URL. The named services are a convenience that make it easy to correctly
format the URL needed for that service. For example, the http:// URL for the dyndns.org service
might be:
myuid:mypwd@members.dyndns.org/nic/update?hostname=mydns.dyndns.org
This could be sent as shown above by using HTTP Poster, or the URL could be automatically
formatted for the administrator by NetDefendOS through choosing the DynDNS menu option and
entering the information required for dyndns.org.
The CLI console command httpposter can be used to troubleshoot problems by seeing what
NetDefendOS is sending and what the servers are returning.
HTTP Poster may be used for other purposes than dynamic DNS. Any need for NetDefendOS to
generate an HTTP GET request can be met by the feature.
Note: A high rate of server queries can cause problems
Dynamic DNS services are often sensitive to repeated logon attempt over short periods
of time and may blacklist IP addresses that are sending excessive requests. It is
therefore not advisable to query these services too often otherwise they may cease to
respond.
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Chapter 3. Fundamentals