#!/bin/bash -l

set -eE -o pipefail
trap 'echo "$0:$LINENO:error: \"$BASH_COMMAND\" returned $?" >&2' ERR

[[ $EUID == 0 ]] || exec sudo -E "${BASH_SOURCE[0]}" "$@"

# Usage: use when switching from a trusted network to an untrusted one,
# like public wifi.



i() { # install file
  local tmp tmpdir dest="$1"
  local base="${dest##*/}"
  mkdir -p ${dest%/*}
  ir=false # i result
  tmpdir=$(mktemp -d)
  cat >$tmpdir/"$base"
  tmp=$(rsync -ic $tmpdir/"$base" "$dest")
  if [[ $tmp ]]; then
    printf "%s\n" "$tmp"
    ir=true
  fi
  rm -rf $tmpdir
}


read -r _ ver _ < <(systemd-resolve --version)

# removes malware and adult content
servers=(1.1.1.3 1.0.0.3 2606:4700:4700::1113 2606:4700:4700::1003)

servers=(1.1.1.1 1.0.0.1 2606:4700:4700::1111 2606:4700:4700::1001)

## trying out google
#servers=(8.8.8.8 8.8.4.4 2001:4860:4860::8888 2001:4860:4860::8844)


# https://wiki.archlinux.org/index.php/Systemd-resolved#Manually
cat >/etc/systemd/resolved.conf.d/untrusted-network.conf <<EOF
[Resolve]
DNS=${servers[@]}
Domains=b8.nz
DNSOverTLS=yes
EOF

i /etc/NetworkManager/conf.d/dns.conf <<'EOF'
[main]
dns=none
systemd-resolved=false
EOF

if $ir && [[ $(systemctl is-active NetworkManager) == active ]]; then
  m systemctl restart NetworkManager
  m sleep 2
fi

dhclient_restart=false
# man dhclient.conf
if grep -qP '\bdomain-name-servers\b' /etc/dhcp/dhclient.conf; then
  sed -i 's/domain-name-servers,\?//' /etc/dhcp/dhclient.conf
  dhclient_restart=true
fi

read -r _ _ _ _  gateway_if _ < <(ip route get 8.8.8.8)
if [[ $gateway_if ]]; then
  # we could do this, but dhclient is still running and will use its old settings
  # from dependencies of ifupdown,
  # from man dhclient-script
  # from /etc/dhcp/dhclient-enter-hooks.d/resolved
  # rm -f /run/systemd/resolved.conf.d/*$gateway_if*


  if $dhclient_restart && grep -Pq "^ *auto ($gateway_if|.* $gateway_if( |$))" /etc/network/interfaces; then
    m ifdown $gateway_if
    m ifup $gateway_if
  fi

  # at least on systemd 237 ifupdown it sets a global and this is not needed
  systemd-resolve --interface=$gateway_if --revert
fi

reresolv

# just for curiosity i did a
# wrapper around dhclient, then ifdown eth0; ifup eth0:

# Tue Mar  9 18:29:05 EST 2021
# args -4 -v -r -pf /run/dhclient.eth0.pid -lf /var/lib/dhcp/dhclient.eth0.leases -I -df /var/lib/dhcp/dhclient6.eth0.leases eth0
# env
# ADDRFAM=inet
# PHASE=pre-down
# VERBOSITY=0
# PWD=/sbin
# IFACE=eth0
# METHOD=dhcp
# SHLVL=1
# LOGICAL=eth0
# MODE=stop
# PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
# IFUPDOWN_eth0=pre-down
# _=/usr/bin/env
# Tue Mar  9 18:29:07 EST 2021
# args -1 -4 -v -pf /run/dhclient.eth0.pid -lf /var/lib/dhcp/dhclient.eth0.leases -I -df /var/lib/dhcp/dhclient6.eth0.leases eth0
# env
# ADDRFAM=inet
# PHASE=post-up
# VERBOSITY=0
# PWD=/sbin
# IFACE=eth0
# METHOD=dhcp
# SHLVL=1
# LOGICAL=eth0
# MODE=start
# PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
# IFUPDOWN_eth0=post-up
# _=/usr/bin/env