detect-apt-proxy.sh

#!/bin/bash
#
# Environment:
# APT_PROXY_PORT set proxy port, defaults to 3142
# HOST_IP set the cache IP, defaults to 'apt-proxy'
#
# See https://wiki.debian.org/AptCacherNg#Clients for details on
# return values

APT_PROXY_PORT=${APT_PROXY_PORT:-3142}
HOST_IP=${HOST_IP:-"apt-proxy"}

timeout 1 cat < /dev/null > /dev/tcp/${HOST_IP}/${APT_PROXY_PORT} 2> /dev/null
if [ $? -eq 0 ]; then
    echo "http://${HOST_IP}:${APT_PROXY_PORT}"
else
    echo "DIRECT"
fi