Monday, November 09, 2009

Cannot connect to some servers

If you have been unable to connect to a small portion of servers on the Internet for a few days, you may want to read the rest of this message.

I couldn't get to,,,,, and pretty much everything in those domains, including the Mathematica's WeatherData functions and similar stuff. Of course, a proxy was enough to circumvent most of these problems.

While the pages were not loaded, Google Chrome sometimes used to complain about the following error:
Error 101 (net::ERR_CONNECTION_RESET): Unknown error
My Internet provider was able to provide me with the right solution, within a few hours. Something has changed about the format of the packets. So the Maximum transmission unit (MTU) had to be lowered from 1492 bytes to 1460 bytes. I suspect that another possible subtlety was allowed in the packets that can occupy up to 32 bytes.

Click the screenshot to zoom in.

To change the value, connect to your modem, e.g. via or whatever is the right counterpart for your modem. It is a web page "stored" directly in your ADSL modem. Unless you know otherwise, the username and password is going to be admin - both of them. You find the menu SETUP / Data, or something like that, and change MTU: 1492 bytes to MTU: 1460 bytes.

In the case of my modem, the critical place is highlighted by red ink in the screenshot above.

Click Apply, and/or Save. Disconnect from the Internet and reconnect to the Internet, in a "software" fashion. You don't even need to reboot - but rebooting may be OK if you don't know how to disconnect more peacefully. And all the servers should be instantly working.


  1. I just tried to connect to and got there fine without changing mtu settings. I haven't connected to them for a while so it's possible that they fixed something after you ran into the problem.

    This is very strange, as it seems like it would most likely be a problem with a network provider somewhere in transit between you and them rather than at the endpoints. Why would anyone change the max transmission unit allowed? Makes no sense.

    I wonder if there was some other problem that they eventually fixed but that your mtu change got around in the meantime?

  2. Dear George, yes, I had known that pretty much everyone could have gotten to So I was nearly the only person in the world affected by the bug.

    There can be many other reasons - people can have "soft MTU" setting that gets redefined whenever needed, or whatever. Also, I have seen that PPPoE should have 1492 but PPPT shuold have 1460 - but I needed to combine PPPoE with 1460. Maybe I effectively have PPPT, whatever it means.

    The people at my ISP apparently knew for sure what was the reason - so something had to change somewhere and they were informed. Strange but I will leave the investigation to the experts.