How to force a local DNS resolver to be used using resolvconf

I know it has been a while, but after reading a blog post by Anand Kumria over at planet.debian.org I decided to have a quick look at one of the problems he described.

Basically, Anand wants to force the local resolver to be used for each and every network connection, may that connection be established manually or via NetworkManager. He wrote that fixing this configuration for every new connection manually is tedious, and I fully agree on that. So here is a solution to do this all automatically, using resolvconf:

After installing the resolvconf package every time /etc/resolv.conf is to be updated resolvconf takes care of that. Using the files in /etc/resolvconf this process can be controlled and the resulting file modified to fit one own's needs.

So at first we would like the local resolver to be used for every connection. This works by simply adding the "nameserver" directive to the /etc/resolvconf/resolv.conf.d/head file. Simple as that. Every time /etc/resolv.conf gets generated the contents of the head file are actually used as /etc/resolv.conf's header.

Using this method the local resolver is used for every connection. But Anand wanted to use only the local resolver and discard any resolvers possibly obtained via DHCP for example. Guess what, this is also possible using resolvconf.

Adding TRUNCATE_NAMESERVER_LIST_AFTER_127="yes" to /etc/default/resolvconf does exactly that. Now every nameserver directive after the one is ignored and will not make it into /etc/resolv.conf. You can of course add more nameservers to the head file above the directive.

Problem fixed I guess.
Don't forget to re-connect to the network or manually force re-creation of /etc/resolv.conf so the changes you made get populated. I really hope this is of use to some of you facing similar problems.


ISC dhcpd and IP assignments from a pool to specific hosts only

Assigning an IP address statically to a host with a given MAC address using ISC dhcpd is quite trivial, one host entry, a hardware ethernet entry and a fixed-address entry and you are up and running.
But what if you want to assign IP addresses from a pool to only a few hosts with specific MAC addresses?

Before you ask yourself why someone might want to do that, have a look at my (very real) use-case.
I am currently working on setting up an installation server for my employer, ANEXIA Internetdienstleistungs GmbH. The server itself uses PXE, TFTP and FAI for installing systems. To be able to do PXE booting one has to set up an DHCP server to provide configuration details, like the TFTP Server Address and the boot filename.

Now what one should consider is that this system is designed to provide automatic installations for internet-facing hosts, namely ones in public IP networks. Running a DHCP server in such a network is not a good idea. We neither want to dish out configurations to each and every hosts that asks for them, neither do not want to do a PXE boot each and every time one of our systems is restarted. Now the combination of FAI and pxelinux allows for default configurations which force local booting, but this still causes the (re-)boot time for those systems to increase and potentially also increases the load on the TFTP server. Also, let's not even consider thinking about whether this setup is "clean" or not. I personally believe that dishing out IP addresses in a public IP network is a bad thing(tm) and I guess a lot of people will be nodding when reading these lines.

What I was asking myself is how to get something like that set up in a cleaner way, and guess what, I found a solution.
The basic idea behind this is only providing IP configuration via DHCP to a specific set of hosts (with a specific set of MAC addresses) and not providing any information to all other hosts. The specific set of hosts are those that we want to do an install run on. This is a no-brainer and I guess the right way to do that, but implementing this approach is not as straight-forward as I initially thought.

Actually the implementation of that idea caused me a bit of a headache and cost me a few work-hours to get right, that's why I'd like to share the configuration details with you.