/etc/network/if-up.d/auto-hostname

auto-hostname

#!/bin/sh

set -e

fqdn=$(dig -x `/sbin/ifconfig eth0 | grep 'inet addr:' | cut -d: -f2 | awk '{print $1}'` +noall +answer | grep PTR | awk '{print $5}')
fqdn=${fqdn%.}

if [ -z $fqdn ]; then
    exit 0
fi

HOSTNAME_FILE='/etc/hostname'
echo "$fqdn" > $HOSTNAME_FILE
/bin/hostname -F "$HOSTNAME_FILE"

exit 0