New cable modem kills DNS

English Network/Internet
1

So I got a new cable modem so I could get rid of the one I’ve been leasing from the cable company. It is a Motorola sb6120. Everything is fine with the ISP, it’s approved for operation on their network, the only hitch was I had to use my Windows laptop to run their activation software.

At any rate, with the new modem installed, I hop on my Linux box and go to work. I’m seeing some terrible delays in looking up names. When I look with firebug, all the dns lookups for new names are taking 15+ seconds. First I just think maybe the names aren’t cached or something is weird for a minute and it will get better, but no. After several restarts and then a couple days persevering with the new cable modem, all DNS queries are still terribly slow (including zypper, mail client, web client).

Suddenly I remember an issue I was having with, I believe version 11.1 when I first installed it. For just the computer on my network with a fixed IP address, DNS resolution was slow like this and if I disabled IPv6 then all was well. I believe there was even a patch issued back then for gethostbyname() that was supposed to fix the issue? So, when I disable IPv6, DNS lookup is back to normal. The two Linux computers on my network had been working fine with IPv6 and the old modem, though, and the Windows box is still working fine even with the new modem.

I can disable IPv6 and make it fast again, but really why should I have to? Any ideas here?

thanks,
–Jason

2
  • poorboywilly wrote, On 11/13/2010 09:06 PM:
    > I can disable IPv6 and make it fast again, but really why should I have
    > to? Any ideas here?

Because your provider doesn’t support IPv6, DNS queries run into timeouts. Diabling it is totally fine if you don’t need it.

Uwe

3

But they do. IPv6 worked fine before the new modem.

4
  • poorboywilly wrote, On 11/15/2010 06:06 PM:
    > But they do. IPv6 worked fine before the new modem.

Can you try a different OS to rule out the new modem as the problem?

Uwe

5

Yes. Windows XP works just fine both before the switch and after.

6
  • poorboywilly wrote, On 11/16/2010 07:36 PM:
    >
    > Yes. Windows XP works just fine both before the switch and after.

But does it look up IPv6 addresses? Check with nslookup.

Uwe

7

Honestly this isn’t even necessary because even with IPv6 disabled, DNS lookups are still ridiculous. At the very least firefox, thunderbird, and openssh are terribly slow still. I can disable IPv6 in firefox and thunderbird (disabling something in individual programs which you have disabled system-wide is real fun) but I have no idea how this could be done for ssh. If I do a “dig -6 google.com” it takes just over 15 seconds to fail (let me guess what the timeout is on DNS lookups). Why in the world is this taking so long to fail when IPv6 is disabled?

When I get time I can look at Windows and see about IPv6.