How do I fix DNS resolving which doesn't work after upgrading to Ubuntu 13.10 (Saucy)

After upgrade to 13.10 my DNS resolving fails. It seems the DNS servers which I get by DHCP (LAN) are not used.

I could temporary solve the problem by adding nameserver 8.8.8.8 to /etc/resolv.conf. But then the intranet hosts still can not be resolved.

When clicking on the Connection Information menu item on the network indicator, the Primary DNS and the Secondary DNS are set correctly. But my computer seams not to use them.

So my questions:

• What should I put into resolv.conf, if anything?
• How to find out, which name servers my computer is querying?
• Where to look next, to find out, why name servers received by DHCP are not used?

First you need to know a bit about how name resolving works in Ubuntu since Ubuntu 12.04.

Stéphane Graber blogged some information about it last year here. The most important thing to know is that both Ubuntu Server and Ubuntu Desktop use resolvconf to manage the resolv.conf file. That means that you should no longer edit /etc/resolv.conf directly; instead you should configure your network interface configuration utility to provide the right information to resolvconf. For Ubuntu Server the network interface configuration utility is ifup and it is configured by the file /etc/network/interfaces. For Ubuntu Desktop the network interface configuration utility is NetworkManager. This is what you are using.

NetworkManager is configured by means of Network indicator > Edit Connections. However, for network interfaces configured by DHCP it normally isn't necessary to change any settings manually. Normally what happens is that the (remote) DHCP server provides to NetworkManager both an IP address for the local interface and the address of a (remote) DNS nameserver to use. NetworkManager starts an instance of a forwarding nameserver that listens locally at 127.0.1.1. This address, 127.0.1.1, is sent to resolvconf which puts nameserver 127.0.1.1 in /etc/resolv.conf. NetworkManager also gives the (remote) IP address of the DHCP-provided DNS nameserver to the forwarding nameserver. Thus a program running on the local system asks the resolver to translate a host name into an IP address; the resolver queries the local forwarding nameserver at 127.0.1.1; the forwarding nameserver queries the remote nameserver(s) it has been told about, receives an answer and sends it back up the chain.

NetworkManager communicates with the forwarding nameserver process over D-Bus. You can see what NetworkManager told the forwarding nameserver by running the command

nmcli dev list iface eth0 | grep IP4.DNS

Update arising from the comments:
Note that resolvconf actually writes the file /run/resolvconf/resolv.conf to which /etc/resolv.conf is supposed to be a symbolic link. If /etc/resolv.conf is not a symbolic link then you need to recreate it. To do so you can run

sudo dpkg-reconfigure resolvconf

or

sudo ln -sf /run/resolvconf/resolv.conf /etc/resolv.conf        

I made the change suggested on the link below (disabling dnsmasq). Now everything works great! http://www.ubuntugeek.com/how-to-disable-dnsmasq-in-ubuntu-12-04precise.html

Open /etc/NetworkManager/NetworkManager.conf file.

sudo gedit /etc/NetworkManager/NetworkManager.conf

Comment out line as:

#dnsmasq deactivated
#dns=dnsmasq

EDIT 2: Previous post was rightfully deleted by the moderation, I'm posting what I have found to be a solution. Sorry for that.

EDIT: I just found the answer and it's in this very page - sorry for my miopy. I posted my findings below, expanding the correct answer by Richard Lindstedt found in this page. I left my early rumbling for a bit of context. Please upvote Richard's answer, he deserves it.

It's actually really easy.

just open your interfaces conf file --> sudo vi /etc/network/interfaces

That sure did not help the OP and doesn't help me now. We don't want static addresses, we want to use the ones the DHCP server sends us. NetworkManager seems to recognise them, but Ubuntu bluntly ignores them:

# nmcli dev list iface wlan0 | grep IP4.DNS
IP4.DNS[1]:          10.*.*.*
IP4.DNS[2]:          10.*.*.*
IP4.DNS[3]:          8.8.8.8

But...

# dig microsoft.com
; <<>> DiG 9.9.5-4.3-Ubuntu <<>> microsoft.com
;; global options: cmd
;; connection timed out; no servers could be reached

And my /etc/network/interfaces is:

auto lo
iface lo inet loopback

which is a bit odd, I would expect all interfaces being declared here (or am I missing something?).

So, in short:

• I did not mess with any file to start with
• I have already run dpkg-reconfigure resolvconf
• The right symlink is in place
• NetworkManager retrieves the correct DNS servers from DHCP
• Ubuntu DOES NOT use such addresses
• The workaround is to put fixed 8.8.8.8 on /etc/network/interfaces WHICH I DON'T WANT
• I want to use the DHCP-provided DNS servers in any and all situations.

Not opening another thread because it's the exact problem except I'm on 14.10 now (but this have been nagging me since de upgrade from 12.10 to 13.04).

SOLUTION

That last phrase got me on the right track, and only then I noticed Richard's answer.

The problem seems to be related to the conflicting dnsmasq and resolvconf packages. Until 12.10, dnsmasq was used. From 13.04 onwards, Ubuntu seemed to switch to a dnsmasq/resolvconf hybrid, where you have installed the packages dnsmasq-base and resolvconf, but not dnsmasq itself.

I can't say if it's a bug in the upgrade scripts for 13.04 or something else, because when upgrading (as in fresh installs) resolvconf is installed, dnsmasq-base is upgraded and dnsmasq is (correctly) uninstalled.

The catch is, the upgrade script fails to comment out the dns=dnsmasq line in /etc/NetworkManager/NetworkManager.conf. So, even though the dnsmasq daemon is not present anymore on the system, /etc/resolv.conf still expects it to be.