Using apt-cacher proxies with fallback to direct

02proxy

## /etc/apt/apt.conf.d/02proxy

# You can add this line for faster failover 
Acquire::Retries 0;

# Make sure you use the full path 
Acquire::http::Proxy-Auto-Detect "/usr/bin/apt-proxy-detect.sh";

apt-proxy-detect.sh

#!/bin/bash
## Tells  Proxy-Auto-Config if a specified proxy is reachable or if
## it should just fallback to a direct connection.
##
## This file should be executable and referenced by its full path
## in the /etc/apt/apt.conf.d/02proxy file we create too.

ip="192.168.88.1"
port=3142

## This will install netcat automatically if it's missing if uncommented
#if [[ $(which nc >/dev/null; echo $?) -ne 0 ]]; then
#	apt install -y netcat
#fi

if [[ $(nc -w1 -z $ip $port &>/dev/null; echo $?) -eq 0 ]]; then
    echo -n "http://${ip}:${port}/"
else 
    echo -n "DIRECT"
fi