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Jochen Arndt - Professional Profile


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Jochen started programming in the 80s using Assembler (Z80, 8080, x86), BASIC, C, and TurboPascal. Actually he uses mainly C++. He is a graduate engineer in communications engineering (University of Applied Sciences Kiel, 1991).
31 Dec 2018 CodeProject MVP 2019
31 Dec 2017 MVP: CodeProject MVP 2018
31 Dec 2016 MVP: CodeProject MVP 2017

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RantA rant on T-Com Speedport routers Pin
Jochen Arndt3-Mar-16 0:16
professionalJochen Arndt3-Mar-16 0:16 
So my boss brought this router to the company provided by German T-Com because he bought his own one. I should install it because it provides WLAN which was actually not present. Using it for WLAN only makes no sense so I decided to use it also as replacement for the existing DSL modem and router (which was a good idea because I learned later that using the WLAN part only is nearly impossible).

First step was connecting it to my PC to perform some pre-configurations like changing IP adress and disable DHCP. While this requires some work (shutdown system, connect router, boot, perform the setup, shutdown) all went well.

Then I prepared the connection and had to wait for a moment of no internet traffic (that is the only one at work). Switched to the new router, configured the dial-in parameters and checked the internet connection. Activated the WLAN and connected with my smart phone. No problems.

Before leaving I decided to check for new mails. Starting Thunderbird (I had let my system down after using it to pre-configure the router and started it after the new router has been activated) and tried to login to the local IMAP server:
Unknown server.
Tried to open an intranet page using the browser. Same result: Name can't be resolved.

OK. Lets open a Putty session to the local Linux server providing all these services (DHCP, DNS, IMAP, SMTP). Same problem but using the server's IP I was able to login. All services are running. No warning or error messages.

Next try is a nslookup for local names on my Windows system. The answering DNS server is those of the router! But why?

Let's check the IP setting. 'ipconfig /all' shows that there are two DNS servers:
The IPv4 address of the local server and an IPv6 address for the router. IPv6 is higher priorised so that every request will ask the router's DNS server which can not resolve names from the local net.

But how did the Windows client get that address? DHCP is disabled on the router. Time for a web research. A few minutes later I know the answer. The router is advertising itself as IPv6 DNS server sending router advertisements (Neighbor Discovery Protocol - Wikipedia, the free encyclopedia[^]) in regular intervals. So the idea I had in mind already to implement IPv6 for DHCP and DNS on the server will not work.

The web research brought me also to some posts in the T-Com forums. There were a lot of people with the same problem with different router models. And there will be no solution by a firmware update because "the router is a low cost SOHO device that did not require advanced configuration options". Please T-Com: There are (power) user's and small companies that have their own DNS server for the local net.

There were also two solutions mentioned for the problem:

  • Buy a router without restricted firmware. Most routers will have an option to configure this.
  • Disable IPv6 on all Windows clients

The second option isn't one so we will go for the first. But I thought there must be another solution. And I found it using the search term "windows disable router advertisements" and changing it to "windows disable router discovery" after reading some hits.

So here it is for those who have similar problems:

Open a command prompt as administrator and execute
netsh interface ipv6 show interface

to get the index (Idx) or name of the LAN interface.
Then execute (passing the index or name of the interface)
netsh interface ipv6 set interface [Idx|"name"] routerdiscovery=disabled

Reboot to get rid of the still present IPv6 DNS server entry.

However, this can't be used when local servers use router advertisement too (e.g. using the radvd (Router Advertisement Daemon) on Linux servers or the netsh command with the advertise=enabled option on Windows servers).

To complete the rant there is another annoying "feature":
Each time openening the router configuration in the browser, the Cancel button must be pressed because the router want's the phone configuration to be completed. There is no option to tell him that there are no telephones.

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