ubuntu hostname update based on dhcp

edit /etc/dhcp/dhclient-exit-hooks.d/hostname

#!/bin/sh
# dhclient change hostname script for Ubuntu
# /etc/dhcp/dhclient-exit-hooks.d/sethostname
# logs in /var/log/upstart/network-interface-eth0.log
 
# for debugging:
echo "sethostname BEGIN"
export
set -x
 
if [ $reason = "BOUND" ]; then
    echo new_ip_address=$new_ip_address
    echo new_host_name=$new_host_name
    echo new_domain_name=$new_domain_name
 
    oldhostname=$(hostname -s)
    if [ $oldhostname != $new_host_name ]; then
 
        # Rename Host
        echo $new_host_name > /etc/hostname
        hostname -F /etc/hostname
 
        # Update /etc/hosts if needed
        TMPHOSTS=/etc/hosts.dhcp.new
        if ! grep "$new_ip_address $new_host_name.$new_domain_name $new_host_name" /etc/hosts; then
            # Remove the 127.0.1.1 put there by the debian installer
            grep -v '127\.0\.1\.1 ' < /etc/hosts > $TMPHOSTS
            # Add the our new ip address and name
            echo "$new_ip_address $new_host_name.$new_domain_name $new_host_name" >> $TMPHOSTS
            mv $TMPHOSTS /etc/hosts
        fi
 
        # Recreate SSH2 keys
        export DEBIAN_FRONTEND=noninteractive 
        dpkg-reconfigure openssh-server
    fi
fi
echo "sethostname END"

If not working,

#!/bin/sh

# Filename:     /etc/dhcp/dhclient-exit-hooks.d/hostname

# Purpose:      Used by dhclient-script to set the hostname of the system

#               to match the DNS information for the host as provided by

#               DHCP.

#

# Do not update hostname for virtual machine IP assignments

if [ "$interface" != "eth0" ]

then

    return

fi

if [ "$reason" != BOUND ] && [ "$reason" != RENEW ] \

   && [ "$reason" != REBIND ] && [ "$reason" != REBOOT ]

then

        return

fi

echo dhclient-exit-hooks.d/hostname: Dynamic IP address = $new_ip_address

hostname=$(host $new_ip_address | cut -d ' ' -f 5)

echo $hostname > /etc/hostname

hostname $hostname

echo dhclient-exit-hooks.d/hostname: Dynamic Hostname = $hostname

Make sure it is readable...

chmod a+r /etc/dhcp/dhclient-exit-hooks.d/hostname

That's all. On the next dhcp response your hostname will update automatically.