#!/usr/bin/env bash
# loss.sh P     drop P percent (may be fractional, e.g. 2.5) of packets in each direction on lo, port 4433, TCP and UDP
# loss.sh P udp  the same, UDP only (100 udp = a firewall that blocks QUIC)
# loss.sh off   remove the rules and restore the loopback MTU
# Loopback MTU is 65536, so TCP would send 64 KiB segments while QUIC sends ~1.2 KB datagrams; a per-packet drop rate
# would then be wildly unfair. While loss is on we set the loopback MTU to 1500 so both protocols use Ethernet-sized packets.
# Kernel-level random drop (iptables -m statistic). This is NOT tc netem (netem was unavailable in the sandbox:
# "Specified qdisc kind is unknown"), so it adds loss but no delay. Needs root / CAP_NET_ADMIN.
set -euo pipefail
CH=H3LOSS
off() { ip link set dev lo mtu 65536; iptables -D INPUT -i lo -j $CH 2>/dev/null || true; iptables -F $CH 2>/dev/null || true; iptables -X $CH 2>/dev/null || true; }
off
[ "${1:?P or off}" = off ] && exit 0
P=$(python3 -c "print(float('$1')/100)")
ip link set dev lo mtu 1500
iptables -N $CH
for proto in ${2:-tcp udp}; do
  for side in --dport --sport; do
    iptables -A $CH -p $proto $side 4433 -m statistic --mode random --probability "$P" -j DROP
  done
done
iptables -A INPUT -i lo -j $CH