Enslaving a bind DNS server on CentOS

One thing I have been trying to accomplish ever since I commissioned our server in Manchester almost 9 months ago now is getting it set up as a secondary DNS server, this I have finally accomplished and it is an experience worth sharing. There are many howto's on the Internet which tell you how to set up a DNS server on windows or Linux but what I was after was to setup BIND on Linux as a fail over system for our main Microsoft DNS server and this is a far less well documented scenario. Also contrary to what you might read its actually really easy just don't step off the path!

For those of you who don't know, DNS is the system which marshals the traffic around the Internet, for example in the absence of DNS you would have to type in 72.21.206.5 instead of amazon.com to get to your favourite e commerce merchant :o). So DNS is important for the Internet but also as local networks are now very much modeled upon the Internet scheme, without DNS it becomes very difficult to manage your network in a user friendly manner. Which as usual is great until it breaks!

So very quickly as your office network becomes more central to the workings of your business it is natural to want a secondary system in case the first one breaks, especially when you are using a VPN as we are as the loss of our central DNS server would render our remote systems unusable as well. So that's the background, here is the solution to setting up a secondary DNS server using BIND on Linux as a slave to an Active Directory DNS server. Bear in mind this is for CentOS 4.5 (RHEL 4 equivalent) using the command line, if you are using a GUI just use the GUI tool!

1. On the Microsoft box open DNS and right click on the forward lookup zone you wish to replicate, eg, somebiz.local. Under 'Name Servers' add the IP address of your Linux box.
2. If you have already been playing, completely remove your existing BIND installation (yum remove bind), and trash any files in /var/named/chroot/var/named.
3. Run yum install bind to install a fresh one.
4. Paste the following into /var/named/chroot/var/etc/named.conf


// Red Hat BIND Configuration Tool
// Default initial "Caching Only" name server configuration

options { directory "/var/named"; };

zone "mydomain.local" IN {
type slave;
file "slaves/mydomain.local";
masters { xxx.xxx.xxx.xxx port 53;};
};

include "/etc/rndc.key";

5. substitute your domain for mydomain.local and your active directory server's IP address in the xxx.xxx.xxx.xxx space.

6. Run service named start and make a cuppa cos your done!

Obviously this is not a comprehensive look at this subject, there is an awful lot more to play with in bind but that really is all you need to do to get going. Hope it helps..

Finding extra space on a VMWare Virtual Machine

Now our Redgate SQL Server backup system is running nicely it has exposed a slight deficiency in my server setup over in Manchester, the problem now is that when I created virtual machines for all our little applications running on the VMware system I only gave then 4GB disks :o)

For the Nagios system, the Intranet applications, the source code repository and the knowledgebase these disks are perfectly adequate but for database backup obviously a bit more space is required. Having utilised a spare folder on the knowledgebase virtual server for the SQL Backup it would have been very inconvenient to erase the machine and create a new one and also given the fact that the host server is not overburdened with RAM I didn't feel it would be wise to put more than 4 machines on the system.

So we come to a less well documented feature of a VMWare virtual machine, when you set the hard disk size on creating the machine you cannot increase it in the future. So you have 2 choices, the Lego approach - smash it up and start again, or the clever approach, add on a new virtual disk. I chose to be clever and in a nice twist of fate got away with it :o)

So I think a little how-to is in order.

Begin by adding a new disk in VMWare, it was recommended that I choose a SCSI disk so I complied, bear in mind this has to be done with the virtual machine powered down. When you have added the disk just power up again and you are there, all you have to do now is to get your Linux install to make use of it.

As I have blogged before our flavour of choice when it comes to Linux is CentOS but RHEL4 or Fedora would probably work in exactly the same way because its pretty basic stuff really.

First use Fdisk to create a new partition, as this is our second disk its SDB (SCSI Disk B)
so:
fdisk sbd
and then just follow the instructions to create a new partition.

Next you need to format the new disk so:
mkfs.ext2 /dev/sdb

At this point you need to check the space on the disk so:
df
and you should see your new disk listed

Finally you need to mount the disk so:
mount -t ext2 /dev/sdb /home/samba/sql

As I was using samba to share the /home/samba folder all I needed to do was update the permissions on the folder and restart the service - job done.

Now I have a bit more space on the share the backup has worked a treat, it is one of the real strengths of the Red Gate system that you can see immediately how your backup routine is performing using the timeline GUI. As you can see from the picture here the first couple of databases backed up nicely but given that they are larger databases they are a bit snug so I might just ease them apart a little so we don't get a clash as they grow.

Nagios on Centos - a grudging union

Centos is one of the great network operating systems, it was developed by a group of people who saw Red Hat Enterprise version 4 had become super reliable but slightly bloaty, they exercised their rights under the GNU public license got the source of RHEL4, put it on a stairmaster and gave it to the people.

Likewise Nagios is a great open source network monitoring system, if you are a Linux user and run a network chances are you will have come across Nagios, short of forking out about £1000 it is in fact pretty much your only option. About 12 months ago I installed Nagios on Fedora and it was a breeze, even though Nagios is a very comprehensive system requiring lots of fiddly configuration, on Fedora if you follow the instructions you will succeed in getting going in about an hour.

Unfortunately given that Centos and Fedora have a common ancestry and are very similar, trying to install Nagios on Centos will drive you up the wall. Unless I have done something stupid without realising it, installing the system from RHEL4 RPM's seems to scatter the files from one end of the disk to the other and it takes lots of patience to track them all down and link everything up. My advice would be follow the instructions to the letter but if you don't find the files you are looking for don't be surprised. Click here for the main Nagios site, this post is not a guide to installing Nagios on Centos just an amendment to the install guide based upon my rather frustrating experience.

Just in case I forget or anyone else trips over this, the locations are as follows:

Config CFG files - /etc/nagios
Web interface files - /usr/share/nagios
Log files - /var/log/
CGI files - /usr/lib/nagios/cgi

A guy called Dag (??) has done some Centos RPM's but I couldn't subscribe to his repository, if you can it is quite possible that he has reworked the install to follow the instructions. Couldn't resist doing my 'Google Images' thing for Dag, turns out this Swedish guy is also comfortable going my the name Dag. There are some great translations for Dag on wikipedia, in Swedish it means 'Day' and in Turkish it refers to a 'Mountain'.




So now the dust has settled after our mammoth network rewire last week and Nagios is running sweetly I feel quite satisfied with everything. As expected we have had a few static routes crawl out of the woodwork and we have renewed our efforts to use DNS rather that IP addresses for routing around the network. It turns out reversing the VPN connections was not all that it promised and we have moved them all back again, it also seems that having Nagios running is actually very good for the stability of the VPN as the frequent pinging seems to keep the routers awake and the tunnels in good repair.

One job left to complete is to define a custom status map for Nagios, as we have over a hundred nodes on the network being monitored the auto generated map is a bit of a mess so I have to define the map by hand which is a bit of a pain. That said it will look very nice as we have purchased an icon library for our software development and their networking set is very sweet. See left for a sneak peek, note however that our main managed switches are not down it is just that Netgear have issued a firmware upgrade they are short of. One day I would love to do a more comprehensive Flash front-end to Nagios but frankly right now I have better things to do.

Another job I think would pay dividends would be to set up a secondary DNS server at Manchester, it is probably quite straight forward but I think I will let the dust settle before attempting this one.