#!/bin/bash

IFNAME=enp1s0

user=$SUDO_USER

ip netns add ns1
ip link add dev veth0 type veth peer name veth1
ip link set veth1 netns ns1

ip addr add 10.20.30.0/31 dev veth0
ip -n ns1 addr add 10.20.30.1/31 dev veth1

ip link set veth0 up
ip -n ns1 link set veth1 up
gateway=$(ip -4 addr show dev $IFNAME | grep -oP '(?<=inet\s)\d+(\.\d+){3}' | sed 's/\.[0-9]\+$/\.1/')

ip -n ns1 r add default via 10.20.30.0 dev veth1 
ip rule add iif veth0 lookup 100
ip r add default via $gateway dev $IFNAME table 100

iptables -t nat -I POSTROUTING -o $IFNAME -j MASQUERADE
echo 1 > /proc/sys/net/ipv4/ip_forward

mkdir -p /etc/netns/ns1
echo "nameserver 1.1.1.1" > /etc/netns/ns1/resolv.conf 2>/dev/null
IP=$(ip netns exec ns1 curl -s -4 ifconfig.me)
ip netns exec ns1 sudo -u $user PS1="($IP) $PS1" bash --norc