DNS via Linksys ADSL2MUE modem not working properly

[Lea Gris]

Neil Woolford a écrit :
> First of all, I *don't* need suggestions for workarounds, as I already have a
> working fix for this problem, based on editing /etc/dhcp3/dhcp.conf as advised
> at https://wiki.ubuntu.com/StaticDnsWithDhcp.  However I would like to know why
> the system is not working as I would expect, and whether this can be fixed: 
> indeed, if it is a bug, then suggestions as to where to file would be welcome...
>  If I've missed the point about some aspect of networking, then pointers to
> enlightenment would also be welcome...
> 
> I connect to broadband via a Linksys ADSL2MUE modem, at 192.168.1.1.  The modem
> is set to provide DHCP and correctly sets itself to the DNS address settings of
> my ISP.  When connected to my ancient laptop running windows 98SE, the modem
> provides a reliable DNS service under its own address (192.168.1.1).
> 
> Under Ubuntu (Hoary, Breezy and Dapper so far) the DNS Server is automatically
> set  to that address (192.168.1.1).  However, applications such as Firefox and
> Update Manager just hang as they appear not to be able to access any DNS Server.
>  If I run nslookup from a terminal I do get sensible information about numeric
> IP addresses, from the DNS Server at 192.168.1.1, so it does appear to be
> present and active.  As I mentioned before, my Windows machine has no trouble
> using it.

At a glance there is a non conformance of DNS replies from the ADSL2MUE 
modem at IP 192.168.1.1. It may be a DNS proxy forwarder or cache. I 
don't know. What you may be able to achieve is configure the ADSL2MUE 
modem to not act as a DNS proxy service.

Alternatively you can investigate that it may be related to ipv6 
incompatibility of the DNS proxy service of the ADSL2MUE :

To the investigation part.

Install dnsutils package

than dig some names with traces to show what's happening wrong.

Firefox and many other application trys to get ipv6 AAAA records from 
DNS to join servers preferably in ipv6. Since ipv6 is enabled in Ubuntu 
but may not be supported by the DNS forwarder of the ADSL2MUE :

# forcing ipv4 querries
dig -4 example.com @192.168.1.1 +trace

# forcing ipv6 querries
dig -6 example.com @192.168.1.1 +trace

The ADSL2MUE DNS proxy may not be able to reply SERVERFAIL or reply 
SERVERFAIL wrongly whan querried for en AAAA ipv6 DNS record.

If that is the problem then you will have to disable ipv6 in Ubuntu.

To disable ipv6 do:

sudo gedit /etc/modprobe.d/aliases

find a line with alias net-pf-10 ipv6 or alike
comment out the line with a #
add a line with:
alias net-pf-10 off

save the file

At next reboot ipv6 support from the kernel will be disabled.


-- 
Léa Gris