#!/bin/bash

if [ -z "$sip" ] || [ -z "$dip" ] || [ -z "$dprefix" ] || [ -z "$sport" ] || [ -z "$dport" ]; then
    >&2 echo 'missing required env var'
    exit 1
fi

NAME="F$(echo "$sip:$sport -> $dip/$dprefix:$dport ${src_subnet:-any} ${excluded_src:-none}" | sha256sum | head -c 15)"

for kind in INPUT FORWARD ACCEPT; do
    if ! iptables -C $kind -j "${NAME}_${kind}" 2> /dev/null; then
        iptables -N "${NAME}_${kind}" 2> /dev/null
        iptables -A $kind -j "${NAME}_${kind}"
    fi
done
for kind in PREROUTING INPUT OUTPUT POSTROUTING; do
    if ! iptables -t nat -C $kind -j "${NAME}_${kind}" 2> /dev/null; then
        iptables -t nat -N "${NAME}_${kind}" 2> /dev/null
        iptables -t nat -A $kind -j "${NAME}_${kind}"
    fi
done

err=0
trap 'err=1' ERR

for kind in INPUT FORWARD ACCEPT; do
    iptables -F "${NAME}_${kind}" 2> /dev/null
done
for kind in PREROUTING INPUT OUTPUT POSTROUTING; do
    iptables -t nat -F "${NAME}_${kind}" 2> /dev/null
done
if [ "$UNDO" = 1 ]; then
    conntrack -D -p tcp -d $sip --dport $sport || true # conntrack returns exit 1 if no connections are active
    conntrack -D -p udp -d $sip --dport $sport || true # conntrack returns exit 1 if no connections are active
    exit $err
fi

# DNAT: rewrite destination for incoming packets (external traffic)
# When src_subnet is set, only forward traffic from that subnet (private forwards)
# excluded_src: comma-separated gateway/router IPs to reject (they may masquerade internet traffic)
if [ -n "$src_subnet" ]; then
    if [ -n "$excluded_src" ]; then
        IFS=',' read -ra EXCLUDED <<< "$excluded_src"
        for excl in "${EXCLUDED[@]}"; do
            iptables -t nat -A ${NAME}_PREROUTING -s "$excl" -d "$sip" -p tcp --dport "$sport" -j RETURN
            iptables -t nat -A ${NAME}_PREROUTING -s "$excl" -d "$sip" -p udp --dport "$sport" -j RETURN
        done
    fi
    iptables -t nat -A ${NAME}_PREROUTING -s "$src_subnet" -d "$sip" -p tcp --dport "$sport" -j DNAT --to-destination "$dip:$dport"
    iptables -t nat -A ${NAME}_PREROUTING -s "$src_subnet" -d "$sip" -p udp --dport "$sport" -j DNAT --to-destination "$dip:$dport"
else
    iptables -t nat -A ${NAME}_PREROUTING -d "$sip" -p tcp --dport "$sport" -j DNAT --to-destination "$dip:$dport"
    iptables -t nat -A ${NAME}_PREROUTING -d "$sip" -p udp --dport "$sport" -j DNAT --to-destination "$dip:$dport"
fi

# DNAT: rewrite destination for locally-originated packets (hairpin from host itself)
iptables -t nat -A ${NAME}_OUTPUT -d "$sip" -p tcp --dport "$sport" -j DNAT --to-destination "$dip:$dport"
iptables -t nat -A ${NAME}_OUTPUT -d "$sip" -p udp --dport "$sport" -j DNAT --to-destination "$dip:$dport"

# MASQUERADE: rewrite source for all forwarded traffic to the destination
# This ensures responses are routed back through the host regardless of source IP
iptables -t nat -A ${NAME}_POSTROUTING -d "$dip" -p tcp --dport "$dport" -j MASQUERADE
iptables -t nat -A ${NAME}_POSTROUTING -d "$dip" -p udp --dport "$dport" -j MASQUERADE

# Allow new connections to be forwarded to the destination
iptables -A ${NAME}_FORWARD -d $dip -p tcp --dport $dport -m state --state NEW -j ACCEPT
iptables -A ${NAME}_FORWARD -d $dip -p udp --dport $dport -m state --state NEW -j ACCEPT

exit $err