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# 4. Set Up The Head Node

So let's get "wolfing." Choose the most powerful box to be the head node. Install Linux there and choose every package you want. The only requirement is that you choose "Network Servers" [in Red Hat terminology] because you need to have NFS and ssh. That's all you need. In my case, I was going to do development of the Beowulf application, so I added X and C development.

It is my experience that you do not actually need NFS, but I found it invaluable for copying files between nodes, and for automating the install process. Later in this document I will describe how you can run a simple Beowulf application without the use of NFS, but a more complex application may use NFS or actually depend upon it.

Those of you researching Beowulf systems will also know how you can have a second network card on the head node so you can access it from the outside world. This is not required for the operation of a cluster.

I learned the hard way: use a password that obeys the strong password constraints for your Linux distribution. I used an easily typed password like "a" for my user, and the whole thing did not work. When I changed my password to a legal password, with mixed numbers, characters, upper and lower case, it worked.

If you use lam as your message passing interface, you will read in the manual to turn OFF the firewalls, because they use random port numbers to communicate between nodes. Here is a rule: If the manual tells you to do something, DO IT! The lam manual also tells you to run as a non-root user. Make the same user for every box. Build every box on the cluster with that same user and password. I named that non root user "wolf".

## 4.1. Hosts

First we modify /etc/hosts. In it, you will see the comments telling you to leave the "localhost" line alone. Ignore that advice and change it to not include the name of your box in the loopback address.

Modify the line that says:

    127.0.0.1 wolf00 localhost.localdomain localhost

...to now say:

    127.0.0.1 localhost.localdomain localhost

Then add all the boxes you want on your cluster. Note: This is not required for the operation of a Beowulf cluster; only convenient, so that you may type a simple "wolf01" when you refer to a box on your cluster instead of the more tedious 192.168.0.101:

    192.168.0.100 wolf00
    192.168.0.101 wolf01
    192.168.0.102 wolf02
    192.168.0.103 wolf03
    192.168.0.104 wolf04

## 4.2. Groups

In order to responsibly set up your cluster, especially if you are a "user" of your boxes [see Definitions], you should have some measure of security.

After you create your user, create a group, and add the user to the group. Then, you may modify your files and directories to only be accessible by the users within that group:

    groupadd beowulf
    usermod -g beowulf wolf

...and add the following to /home/wolf/.bash_profile:

    umask 007

Now any files created by the user "wolf" [or any user within the group] will be automatically only writeable by the group "beowulf".

## 4.3. NFS

Refer to the following web site: http://www.ibiblio.org/mdw/HOWTO/NFS-HOWTO/server.html

Print that up, and have it at your side. I will be directing you how to modify your system in order to create an NFS server, but I have found this site invaluable, as you may also.

Make a directory for everybody to share:

    mkdir /mnt/wolf
    chmod 770 /mnt/wolf
    chown wolf:beowulf /mnt/wolf -R

Go to the /etc directory, and add your "shared" directory to the exports file:

    cd /etc
    cat >> exports
    /mnt/wolf 192.168.0.100/192.168.0.255 (rw)
    <control d>

## 4.4. IP Addresses

My network is 192.168.0.nnn because it is one of the "private" IP ranges. Thomas Sterling talks about it on page 106 of his book. It is inside my firewall, and works just fine.

My head node, which I call "wolf00" is 192.168.0.100, and every other node is named "wolfnn", with an ip of 192.168.0.100 + nn. I am following the sage advice of many of the web pages out there, and setting myself up for an easier task of scaling up my cluster.

## 4.5. Services

Make sure that services we want are up:

    chkconfig -add sshd
    chkconfig -add nfs
    chkconfig -add rexec
    chkconfig -add rlogin
    chkconfig -level 3 rsh on
    chkconfig -level 3 nfs on
    chkconfig -level 3 rexec on
    chkconfig -level 3 rlogin on

...And, during startup, I saw some services that I know I don't want, and in my opinion, could be removed. You may add or remove others that suit your needs; just include the ones shown above.

    chkconfig -del atd
    chkconfig -del rsh
    chkconfig -del sendmail

## 4.6. SSH

To be responsible, we make ssh work. While logged in as root, you must modify the /etc/ssh/sshd_config file. The lines:

    #RSAAuthentication yes
    #AuthorizedKeysFile .ssh/authorized_keys

...are commented out, so uncomment them [remove the #].

Reboot, and log back in as wolf, because the operation of your cluster will always be done from the user "wolf". Also, the hosts file modifications done earlier must take effect. Logging out and back in will not do this. To be sure, reboot the box, and make sure your prompt shows hostname "wolf00".

To generate your public and private SSH keys, do this:

    ssh-keygen -b 1024 -f ~/.ssh/id_rsa -t rsa -N ""

...and it will display a few messages, and tell you that it created the public / private key pair. You will see these files, id_rsa and id_rsa.pub, in the /home/wolf/.ssh directory.

Copy the id_rsa.pub file into a file called "authorized_keys" right there in the .ssh directory. We will be using this file later. Verify that the contents of this file show the hostname [the reason we rebooted the box]. Modify the security on the files, and the directory:

    chmod 644 ~/.ssh/auth*
    chmod 755 ~/.ssh

According to the LAM user group, only the head node needs to log on to the slave nodes; not the other way around. Therefore when we copy the public key files, we only copy the head node's key file to each slave node, and set up the agent on the head node. This is MUCH easier than copying all authorized_keys files to all nodes. I will describe this in more detail later.

Note: I only am documenting what the LAM distribution of the message passing interface requires; if you chose another message passing interface to build your cluster, your requirements may differ.

At the end of /home/wolf/.bash_profile, add the following statements [again this is lam-specific; your requirements may vary]:

    export LAMRSH='ssh -x'
    ssh-agent sh -c 'ssh-add && bash'

## 4.7. MPI

Lastly, put your message passing interface on the box. As stated in 1.2 Requirements, I used lam. You can get lam from here:

http://www.lam-mpi.org/

...but you can use any other message passing interface or parallel virtual machine software you want. Again, I am showing you what worked for me.

You can either build LAM from the supplied source, or use their precompiled RPM package. It is not in the scope of this document to describe that; I just got the source and followed the directions, and in another experiment I installed their rpm. Both of them worked fine. Remember the whole reason we are doing this is to learn; go forth and learn.

You may also read more documentation regarding LAM and other message passing interface software here.

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