Désactiver le réseau sur le Live-CD Ubuntu 13.04 remasterisé sauf pour localhost

Je l'ai résolu en modifiant /etc/init/failsafe.conf qui contenait les messages et les délais :

# failsafe

description "Failsafe Boot Delay"
author "Clint Byrum <clint@ubuntu.com>"

start on filesystem and net-device-up IFACE=lo
stop on static-network-up or starting rc-sysinit

emits failsafe-boot

console output

script
    # Determine if plymouth is available
    if [ -x /bin/plymouth ] && /bin/plymouth --ping ; then
        PLYMOUTH=/bin/plymouth
    else
        PLYMOUTH=":"
    fi

    # The point here is to wait for 2 minutes before forcibly booting 
    # the system. Anything that is in an "or" condition with 'started 
    # failsafe' in rc-sysinit deserves consideration for mentioning in
    # these messages. currently only static-network-up counts for that.
    sleep 20

    # Plymouth errors should not stop the script because we *must* reach
    # the end of this script to avoid letting the system spin forever
    # waiting on it to start.
    $PLYMOUTH message --text="Waiting for network configuration..." || :
    sleep 40

    $PLYMOUTH message --text="Waiting up to 60 more seconds for network configuration..." || :
    sleep 59

    $PLYMOUTH message --text="Booting system without full network configuration..." || :

    # give user 1 second to see this message since plymouth will go
    # away as soon as failsafe starts.
    sleep 1
    exec initctl emit --no-wait failsafe-boot
end script

post-start exec logger -t 'failsafe' -p daemon.warning "Failsafe of 120 seconds reached."

Il suffit de remplacer le xx en sleep xx avec une petite valeur. Il serait également possible de supprimer les commandes de sommeil. Cela ne désactive pas nécessairement les interfaces réseau, mais réduit le temps pendant lequel le système essaie de trouver une connexion.