#!/bin/bash
# I, Ian Kelling, follow the GNU license recommendations at
# https://www.gnu.org/licenses/license-recommendations.en.html. They
# recommend that small programs, < 300 lines, be licensed under the
# Apache License 2.0. This file contains or is part of one or more small
# programs. If a small program grows beyond 300 lines, I plan to switch
# its license to GPL.

# Copyright 2024 Ian Kelling

# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at

#     http://www.apache.org/licenses/LICENSE-2.0

# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.


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

#set -x
#exec &> >(logger)

source /a/bin/bash-bear-trap/bash-bear

usage() {
  cat <<'EOF'
Usage: mail-route up|down|show

Marks tcp packets on port 25, 143 and 587 to be routed through
a vpn ip. If called from --up/--down in openvpn, (we have multiple args) $1 is the
tun_dev, and action is from $script_type env variable, openvpn sets this.

It is idempotent.

I'm not using this script currently:
The problem with this approach is that if we dont have a non-vpn global
ipv6 address and route, we need to send all ipv6 traffic through the
vpn. Using a network namespace for the vpn fixes that, so I've switched
over to that. We could still use this script inside the network
namespace, and I have tested it, but it is just unneeded
complexity. Simpler to just have a default route to the vpn. This script
does separate out lan ip ranges for ipv4, but we could also do that with
a simple route.

EOF
  exit $1
}

if (( $# < 1 )); then
  usage 1
fi

up() {
  start=true
  stop=false
  iptables_op=-A
  ip_op=add
  timeout_secs=20
  if [[ ! $tun_dev ]]; then
    # delays because I was running this outside of openvpn before
    found=false
    for ((i=1; i<=timeout_secs; i++)); do
      tun_dev=$(ip a show to 10.8.0.4/24 | sed -rn '1s/^\S+\s+([^:]+).*/\1/p')
      if [[ $tun_dev == tun* ]]; then
        found=true
        break
      fi
      sleep 1
    done
    if ! $found; then
      echo "$0: error: timeout after $timeout_secs waiting for valid tun_dev, currently:$tun_dev"
      exit 1
    fi
  fi
  e() { echo "$0: $*"; "$@"; }
  err-cleanup() { stop; }
  modify
  # we leave it as is even when stopping, because we would like it to be default, but the only way
  # to change the default is for every device, and I want to avoid that, even though I wouldn't mind, others users of this script might.
  val=$(sysctl -n net.ipv4.conf.$tun_dev.rp_filter)
  if [[ $val != 2 ]]; then
    echo "net.ipv4.conf.$tun_dev.rp_filter = $val"
    e sysctl net.ipv4.conf.$tun_dev.rp_filter=2
  fi

}
down() {
  start=false
  stop=true
  iptables_op=-D
  ip_op=del
  # note, this is not going to work if the interface has been deleted.
  # we could also check for an iptable rule that on some tun interface like the one
  # we use, but meh, the way I'm using the script now, tun_dev is supplied by openvpn
  if [[ ! $tun_dev ]]; then
    tun_dev=$(ip a show to 10.8.0.4/24 | sed -rn '1s/^\S+\s+([^:]+).*/\1/p')
  fi
  e() { echo "$0: $*"; "$@" || printf "maybe ok failure: %s\n" "$*"; }
  modify
}

show() {
  e() { printf "=================================\n# %s\n\n" "$*"; "$@"; }
  e iptables -t mangle -S
  e ip6tables -t mangle -S
  e iptables -t nat -S
  e ip rule
  e ip -6 rule
  e ip route show table 1
  e ip -6 route show table 1
  e ip -6 route show default

  tun_dev=$(ip a show to 10.8.0.4/24 | sed -rn '1s/^\S+\s+([^:]+).*/\1/p')
  if [[ $tun_dev == tun* ]]; then
    e sysctl net.ipv4.conf.$tun_dev.rp_filter
  else
    echo "$0: note, no tun device found"
  fi
  exit 0
}

runtest() {
  # debugging:
  #echo start=$start stop=$stop exists=$exists
  { $start && ! $exists; } || { $stop && $exists; }
}

iptmod() { #iptables modify
  local cmd="$*"
  local exists=true
  ${cmd/-[AD]/-C} &>/dev/null || exists=false
  if runtest; then e $cmd; fi
}

# code common to start and stop.
modify() {
  # match source or dest port. note, when we send to a port, it picks a random high port as
  # the source.

  if [[ $HOSTNAME == bk ]]; then
    ports=25
    ipsuf=5
  else
    ports=25,143,587
    ipsuf=4
  fi
  iptcommon="OUTPUT -m tcp -p tcp -m multiport --ports $ports -j MARK --set-mark"
  iptmod iptables -t mangle $iptables_op $iptcommon 0x1
  iptmod iptables -t mangle $iptables_op $iptcommon 0x0 -d 10.0.0.0/8,172.16.0.0/12,192.168.0.0/16,127.0.0.0/8
  # note, we could have used a custom chain and returned instead of setting the mark again.
  # in case anyone was ever curious, the inverse of private ips is:   #0.0.0.0/5,8.0.0.0/7,11.0.0.0/8,12.0.0.0/6,16.0.0.0/4,32.0.0.0/3,64.0.0.0/2,128.0.0.0/3,160.0.0.0/5,168.0.0.0/6,172.0.0.0/12,172.32.0.0/11,172.64.0.0/10,172.128.0.0/9,173.0.0.0/8,174.0.0.0/7,176.0.0.0/4,192.0.0.0/9,192.128.0.0/11,192.160.0.0/13,192.169.0.0/16,192.170.0.0/15,192.172.0.0/14,192.176.0.0/12,192.192.0.0/10,193.0.0.0/8,194.0.0.0/7,196.0.0.0/6,200.0.0.0/5,208.0.0.0/4,224.0.0.0/3
  iptmod ip6tables -t mangle $iptables_op $iptcommon 0x1 -d 2000::/3

  if [[ $tun_dev ]]; then
    # when $tun_dev goes away, so does this rule.
    iptmod iptables -t nat $iptables_op POSTROUTING -o $tun_dev -m mark --mark 0x1 -j SNAT --to-source 10.8.0.$ipsuf
    iptmod ip6tables -t nat $iptables_op POSTROUTING -o $tun_dev -m mark --mark 0x1 -j SNAT --to-source 2600:3c00:e002:3800::$ipsuf
  fi


  iprulecmd="fwmark 1 table 1"
  for v in -4 -6; do
    exists=true; ip $v rule show $iprulecmd | grep . &>/dev/null || exists=false
    if runtest; then e ip $v rule $ip_op $iprulecmd; fi
  done

  iproutecmd="default via 10.8.0.1 table 1"
  exists=true; ip route show $iproutecmd 2>/dev/null | grep . &>/dev/null || exists=false
  if runtest; then e ip route $ip_op $iproutecmd; fi
  if [[ $tun_dev ]]; then
    # when $tun_dev goes away, so does this route.
    iproutecmd="default dev $tun_dev table 1"
    exists=true; ip -6 route show $iproutecmd | grep . &>/dev/null || exists=false
    if runtest; then e ip -6 route $ip_op $iproutecmd; fi

    # We have to ensure we have a normal default ipv6 route, or else linux will
    # refuse to do send ipv6 packets at all. When running in a network namespace,
    # having the table 1 route is totally redundant since I dont setup a
    # normal default ipv6 route. But maybe I will someday.
    #
    # We could only do this if we dont have a default route with [[ ! $(ip -6 r show default) ]] but
    # metric seems to be perfectly good. 6000 because on my home comp,
    # its about 6 times slower to ping google than the default 1024 metric.
    # See commit 572dd5847440e58481ee2d7b9080 for using an ipv6 test.
    iproutecmd="default dev $tun_dev"
    exists=true; ip -6 route show $iproutecmd | grep . &>/dev/null || exists=false
    if runtest; then e ip -6 route $ip_op $iproutecmd metric 6000; fi
  fi
  # on debian this is 0 (no filter), on ubuntu it\'s 1, which is no good. 0 or 2 both work fine.
  # 2 drops it if the packet is not routable, martian address, or my default route is screwed up,
  # so, eh, might as well. some rhel docs recommend using it.


}

if (( $# > 1 )); then
  tun_dev=$1
  # environment var from openvpn
  # shellcheck disable=SC2154
  $script_type
else
  case $1 in
    up|down|show) $1 ;;
    *) usage 1 ;;
  esac
fi

exit 0


# background: something like this does not work for packets which
# exim is replying to. I don't know why.
#iptables -t mangle -A OUTPUT -m owner --uid-owner Debian-exim -j MARK --set-mark 0x1
#
# note: exim will misreport the I= interface for remote hosts that would
# not use the default route. It still goes through the vpn, you can
# verify with tcpdump.