Server is accessible through domain name from Outside, but not from inside the Intranet

idplanter asked:

I have an apache webserver / mailserver (running on Ubuntu) setup like following:

  • Speedport Router with NAT for all required ports
  • DynDNS to get the domain name by using a non-static IP address

The problem I face is, that the website can be accessed ok from outside of the Intranet but not anymore from the inside.

The speedport router does not allow to make any changes regarding to the domain name routing

This is my hosts file:           localhost localhost.localdomain           localhost
# marvin
#          marvin
# marvin

::1     localhost ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

This used to work ok for a year and suddenly stopped working, which puzzles me.
It seems like in the Intranet the domainname is not published / routed correctly.

My answer:

This is a side effect of using NAT with IPv4. Your intranet clients get the “outside” IP address, but that address is only accessible from outside the intranet.

There are two solutions: The first is split-horizon DNS. The second (and probably much better) solution is to deploy IPv6, which does not suffer from this problem.

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