This script lets you connect Windows 3.11 to the internet with PPP and Trumpet WinSock in DOSBox

dosbox_ppp.sh

#!/bin/bash
#
# [------------------------------------------------------------------------------------------------]
# [ modified from (legend!) Soldier of FORTRAN's script to work in my testing on Ubuntu 20.04      ]
# [     intended, if it be the will of the demo gads, for a my talk entitled,                      ]
# [         "the root password was klondike" at kernelcon 2022, https://kernelcon.org              ]
# [------------------------------------------------------------------------------------------------]
# [ stolen de:                                                                                     ]
# [   https://gist.github.com/mainframed/2300903d9cc259a2a2ab431ca152dffc                          ]
# [ also see:                                                                                      ]
# [   https://mainframed767.tumblr.com/post/164524161692/dosbox-and-trump-winsock-together-at-last ]
# [------------------------------------------------------------------------------------------------]
# [ @anthonykava                      https://forensic.coffee                           2022-03-26 ]
# [------------------------------------------------------------------------------------------------]
#
# Heavily Modified from: https://www.dosbox.com/wiki/PPP_configuration_on_linux_host
#
# Usage:
# sudo ./isp.sh
#
# This script makes it so you can browse the net with DOSBox and Trumpet Winsock in
# Windows 3.11
#
# LINUX:
#   To use this script simply change the IP addresses below to two unused IP addresses on your network
#   then run with root (needed for port 23/proxyarp)
#
# WINDOWS 3.11:
#   Install Trumpet Winsock
#   Click on 'Dialer->Manual Login'
#   Type: AT (if you see 'ERROR' type AT again)
#   ATDT <LINUX IP ADDRESS>
#   e.g. ATDT 127.0.0.1
#   You should see 'CONNECT'
#   Hit the Escape button and your good to go!
#
# DOSBox Config:
#   Add this to the bottom of your config
#   [serial]
#   serial1=modem listenport:2323
#   serial2=dummy
#   serial3=disabled
#   serial4=disabled

# ----------------------------
# ,.-='{ sOmE vAriAbLeS }`=-.,
#
ifMasq="enp3s0"
ipHost="10.10.0.200"
ipGuest="10.10.0.201"
mockTTY="/tmp/trumpet"
portTTY="23"
#
# ----------------------------

# check for rootage [ needed to listen on port 23 and work with ppp0 ]
if [ $(id -u) -ne 0 ]; then
    echo -e "\nwe will need to be root for this sort of business, try again mit sudo\n"
    exit 1
fi

# beware: enables kernel IPv4 forwarding
grep -q 1 /proc/sys/net/ipv4/ip_forward || \
    ( echo 1 >/proc/sys/net/ipv4/ip_forward )

# trap ctrl-c and call ctrl_c()
trap ctrl_c INT

function ctrl_c() {
    echo "** Trapped CTRL-C"
    echo "** Shutting down ISP"
    echo "** Goodbye 1995"
    pkill -9 pppd
    [ -e "$Serial" ] && rm "$Serial"
    iptables -D FORWARD -s "${ipGuest}" -m comment \
        --comment "forward dosbox traffic" -j ACCEPT >/dev/null 2>&1
    iptables -D FORWARD -d "${ipGuest}" -m conntrack --ctstate ESTABLISHED,RELATED -m comment \
        --comment "forward dosbox related traffic" -j ACCEPT >/dev/null 2>&1
    iptables -t nat -D POSTROUTING -s "${ipGuest}" -o "${ifMasq}" -m comment \
        --comment "masquerade dosbox traffic" -j MASQUERADE >/dev/null 2>&1
    exit 0
}

echo "SoF Presents:"
echo "  1995 ISP Simulator"
echo "Starting Simulator:"
echo "   ____   _______   _____   _______     -------
  / __ \ |__   __| /   _ \ |__   __|  -====------
 | (__) |   | |    \  \ \_\   | |    -======------
 |  __  |   | |    /   \ __   | |    --====-------
 | |  | |   | |   |  (\ / /   | |     -----------
 |_|  |_|   |_|    \_____/    |_|       -------"
echo "** Creating fake ISP"
echo "** Using Serial ${mockTTY}"

while true
do
    if sleep 0.1 && pgrep socat > /dev/null 2>&1; then
        sleep 0.1
    else
        echo "** Starting socat listener on port ${portTTY}"
        socat TCP4-LISTEN:${portTTY} PTY,link="${mockTTY}" &
    fi
    sleep 0.5
    if pgrep pppd > /dev/null 2>&1; then
        sleep 1
    elif [ -e "${mockTTY}" ]; then
        #pppd "/tmp/trumpet" defaultroute mtu 576 10.10.0.200:10.10.0.201 login proxyarp #> /dev/null 2>&1
        iptables -D FORWARD -s "${ipGuest}" -m comment \
            --comment "forward dosbox traffic" -j ACCEPT >/dev/null 2>&1
        iptables -I FORWARD -s "${ipGuest}" -m comment \
            --comment "forward dosbox traffic" -j ACCEPT
        iptables -D FORWARD -d "${ipGuest}" -m conntrack --ctstate ESTABLISHED,RELATED -m comment \
            --comment "forward dosbox related traffic" -j ACCEPT >/dev/null 2>&1
        iptables -I FORWARD -d "${ipGuest}" -m conntrack --ctstate ESTABLISHED,RELATED -m comment \
            --comment "forward dosbox related traffic" -j ACCEPT
        iptables -t nat -D POSTROUTING -s "${ipGuest}" -o "${ifMasq}" -m comment \
            --comment "masquerade dosbox traffic" -j MASQUERADE >/dev/null 2>&1
        iptables -t nat -I POSTROUTING -s "${ipGuest}" -o "${ifMasq}" -m comment \
            --comment "masquerade dosbox traffic" -j MASQUERADE
        pppd "${mockTTY}" 57600 ${ipHost}:${ipGuest} \
            defaultroute mtu 576 login proxyarp noauth ms-dns 1.1.1.1 >/dev/null 2>&1
        # ^-- worked for me on Ubuntu 20.04, stock packages as of 2022-03-26,    (@anthonykava)
        #  ^- had to set DNS in Trumpet Winsock (v3.0D) manually despite ms-dns and best wishes
    fi
done

#vim: set ts=4 sw=4 sts=4 noet