Changing the hostname permanently

[vladsmail]

Changing the hostname permanently

Changing the hostname usually is a trivial task on most Linux OSes. Normally you just change /etc/hostname file to reflect the hostname you want to have. In BackTrack this unfortunately doesn’t work as expected. First of all BackTrack keeps its hostname not in /etc/hostname but in /etc/HOSTNAME. Now because BackTrack is build upon modules any changes made to the filesystem aren’t saved from boot to boot (unless you have specified a changes folder). So obviously you have to create a module that at boot time will replace the original /etc/HOSTNAME with your customized one.

Usually this would work, but it doesn’t, because BackTrack has a little bug. Well it’s not actually a bug, but it simply has some crud left over from when it was built that is interfering with /etc/HOSTNAME. During a normal bootup of the system, before the hostname is set, the system initializes you’re network cards, and once that is done it tries to set up these cards using DHCP. This DHCP information usually contains useful information like what IP the card should have, what subnet it should use, etc… One of the information you can get through DHCP is the domain of the computer. That’s why when the DHCP requests for you cards have finished, the next part of the bootup tries to append the domain from those DHCP requests to the hostname(but it appends it to bt hostname located in rc.M and not the one in /etc/HOSTNAME). Now because BackTrack has some crud left over, previous request are still in /etc/dhcpc/ directory, the HOSTNAME file is allways ignored. So basically even if you overwrite the /etc/HOSTNAME file, it will never be used, because it’s always overwritten with the information provided by the files located in /etc/dhcpc/.

Here are the steps that need to be taken in order to restore BackTrack to it’s intended working state. We are going to have to extract the BT3/base/etc.lzm module to a temporary directory, and remove all files from the etc/dhcpc/ directory. We are then going to have to remake the module from the directory contents, and replace the original etc.lzm file. Here are the commands used:

mkdir /tmp/etc_tmp
lzm2dir /mnt/sda1/BT3/base/etc.lzm /tmp/etc_tmp
rm –rf /tmp/etc_tmp/etc/dhcpc/*
dir2lzm /tmp/etc_tmp /tmp/etc.lzm
mv /tmp/etc.lzm /mnt/sda1/BT3/base/etc.lzm

Please note that /mnt/sda1 is the location of boot media! If yours differs please change accordingly, also this guide assumes that you are using BackTrack on read-writable medium.

If you have done everything correctly you should now be able to create a module with the your hostname settings, that will be loaded correctly at boot time. Here is a example of creating such a module:

mkdir /tmp/hostnameSettings
mkdir /tmp/hostnameSettings/etc
echo “MyHostname.MyDomain” > /tmp/hostnameSettings/etc/HOSTNAME
dir2lzm /tmp/hostnameSettings/ /tmp/CustomHostname.lzm
mv /tmp/CustomHostname.lzm /mnt/sda1/BT3/modules/CustomHostname.lzm

Hope this helps someone out. Any comments or suggestions are welcome.

[purehate]

We very much appreciate the tutorials so this is in no way meant to discourage you however there is a much easier way to do this. Its not a bug its just that there is a start up script that runs at boot which overwrites the host name file.

The script is located at /etc/rc.d/rc.M and the relevant line is line 98 where is says
echo "bt." $DOMAIN > /etc/HOSTNAME

All you have to do is change the "bt" to whatever you want however make sure you leave the "." after the hostname or else it will be all messed up.

[vladsmail]

Actually I'm aware of that I just wanted to show how it should have been from the start. The files in /etc/dhcpc/ directory shouldn't have been there in the first place. But you are right about my method not being the most simple. It's a fix and tutorial combo

The best way to fix this would be to change the script so that it takes the hostname from the /etc/HOSTNAME file, and if a domain is found by dhcp it would only append the domain to the existing hostname in the /etc/HOSTNAME.
I would change the following:

# Set hostname by using DHCP response
ls -aA1b /etc/dhcpc | egrep ".info\$" | while read INFOFILE; do
   # the next line won't affect rc.M variables, because it's in >while read< loop
   . /etc/dhcpc/$INFOFILE
   echo "bt."$DOMAIN >/etc/HOSTNAME
   break
done

Into the following:

# Set hostname by using DHCP response
ls -aA1b /etc/dhcpc | egrep ".info\$" | while read INFOFILE; do
   # the next line won't affect rc.M variables, because it's in >while read< loop
   . /etc/dhcpc/$INFOFILE
   HOSTNAME=$(cat /etc/HOSTNAME | head -n 1 | cut -d . -f 1)
   [ "$HOSTNAME" == "" ] && HOSTNAME="bt"
   echo $HOSTNAME"."$DOMAIN >/etc/HOSTNAME
   break
done

This solution is more universal I think...