dhclient hook for setting the hostname, updating /etc/hosts, and re-generating ssh keys on Ubuntu. Useful for creating CloudStack templates. See https://issues.apache.org/jira/browse/CLOUDSTACK-4556

cloudstack-sethostname.sh

#!/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"