#!/usr/bin/env bash
#
# JbTecWiz Support Centre -- generated fix script
#
# Fault : "nf_conntrack: table full, dropping packet"
# Fix   : Size the table for the machine
# Source: https://jbtecwiz.com/support/lnx-net-conntrack
#
# Run as     : Root shell
# Expect     : 20 minutes
# Risk       : medium
# Reversible : yes
#
# WHEN THIS IS THE RIGHT FIX
#   The load is genuine.
#
# HOW TO UNDO IT
#   Remove /etc/sysctl.d/99-conntrack.conf and run sysctl --system to
#   return to the defaults.
#
# Walks the fix one step at a time and asks before each. Steps with no
# command are yours to do -- it prints those and waits. DRYRUN=1 prints
# without executing; UNATTENDED=1 does not ask.
#
# --------------------------------------------------------------------
# NO WARRANTY - USE AT YOUR OWN RISK
#
# This script is provided by JbTecWiz as-is and with no warranty of any
# kind, express or implied. You run it entirely at your own risk.
#
# JbTecWiz accepts no liability for any loss or damage arising from its
# use, including but not limited to data loss, downtime, or configuration
# changes that turn out to be wrong for your system.
#
# You are responsible for reading this script before running it, for
# satisfying yourself that it suits the machine in front of you, and for
# having a working backup first. Some steps cannot be undone.
# --------------------------------------------------------------------

set -uo pipefail
DRYRUN="${DRYRUN:-0}"
UNATTENDED="${UNATTENDED:-0}"
failed=0

if [ "$(id -u)" -ne 0 ]; then
  echo "  This fix is documented as needing root. Re-run with sudo." >&2
  exit 3
fi

rule() { printf "\n%s\n" "$(printf '-%.0s' $(seq 1 70))"; if [ $# -gt 0 ]; then echo "$1"; fi; }
prose() { echo "$1" | fold -s -w 74 | sed "s/^/  /"; }

# Returns 0 when the caller should run the command, 1 when it should not.
# A manual step always returns 1 -- there is nothing for the caller to run.
step() {  # step <n> <do> <why> <manual|cmd> [command lines...]
  local n="$1" dotext="$2" why="$3" mode="$4"; shift 4
  rule "  Step $n of 5"
  prose "$dotext"
  if [ -n "$why" ]; then echo; prose "$why"; fi
  if [ "$mode" = "manual" ]; then
    echo; echo "  -> Do this yourself, then press Enter to carry on."
    if [ "$UNATTENDED" = "0" ] && [ "$DRYRUN" = "0" ]; then read -r _; fi
    return 1
  fi
  echo; printf "    %s\n" "$@"; echo
  if [ "$DRYRUN" = "1" ]; then echo "  (dry run -- not executed)"; return 1; fi
  if [ "$UNATTENDED" = "0" ]; then
    read -r -p "  Run this step? [Y]es / [S]kip / [Q]uit " a
    case "$a" in
      [Qq]*) echo "  Stopped at your request."; exit 0 ;;
      [Ss]*) echo "  Skipped."; return 1 ;;
    esac
  fi
  return 0
}

rule
echo "  "nf_conntrack: table full, dropping packet""
echo "  Size the table for the machine"
echo
echo "  Risk: medium   Reversible   20 minutes"
echo
prose 'No warranty. Use at your own risk - JbTecWiz accepts no liability. Read it before you run it, and have a backup.'
rule
echo
if [ "$UNATTENDED" = "0" ] && [ "$DRYRUN" = "0" ]; then
  read -r -p "  Ready? [y/N] " go
  case "$go" in [Yy]*) ;; *) echo "  Nothing was changed."; exit 0;; esac
fi

if step 1 'Raise the maximum. Each entry costs roughly 300 bytes, so a million entries is around 300MB of kernel memory -- budget for it.' '' cmd 'sudo sysctl -w net.netfilter.nf_conntrack_max=524288'; then
  sudo sysctl -w net.netfilter.nf_conntrack_max=524288
  if [ $? -ne 0 ]; then
    failed=$((failed+1))
    echo "  Step 1 failed. The rest of the fix may depend on it." >&2
  fi
fi

if step 2 'Raise the hash table size to match, or lookups slow down as the table grows.' 'The recommended ratio is roughly max divided by four. Raising max alone gives a table that holds more entries and takes longer to search each one, which shows up as latency rather than drops.' cmd 'echo 131072 | sudo tee /sys/module/nf_conntrack/parameters/hashsize'; then
  echo 131072 | sudo tee /sys/module/nf_conntrack/parameters/hashsize
  if [ $? -ne 0 ]; then
    failed=$((failed+1))
    echo "  Step 2 failed. The rest of the fix may depend on it." >&2
  fi
fi

if step 3 'Shorten the timeout for established connections, which is 5 days by default and holds entries long after the connection is gone.' '' cmd 'sudo sysctl -w net.netfilter.nf_conntrack_tcp_timeout_established=86400'; then
  sudo sysctl -w net.netfilter.nf_conntrack_tcp_timeout_established=86400
  if [ $? -ne 0 ]; then
    failed=$((failed+1))
    echo "  Step 3 failed. The rest of the fix may depend on it." >&2
  fi
fi

if step 4 'Make it persistent.' '' cmd 'printf '\''net.netfilter.nf_conntrack_max = 524288\nnet.netfilter.nf_conntrack_tcp_timeout_established = 86400\n'\'' | sudo tee /etc/sysctl.d/99-conntrack.conf' 'sudo sysctl --system'; then
  printf 'net.netfilter.nf_conntrack_max = 524288\nnet.netfilter.nf_conntrack_tcp_timeout_established = 86400\n' | sudo tee /etc/sysctl.d/99-conntrack.conf
  sudo sysctl --system
  if [ $? -ne 0 ]; then
    failed=$((failed+1))
    echo "  Step 4 failed. The rest of the fix may depend on it." >&2
  fi
fi

step 5 'Watch the count under load for a few days before deciding it is enough.' '' manual || true

rule "  Confirm it worked"
prose 'The count stays comfortably below the maximum under peak load and the kernel messages stop.'
if [ "$DRYRUN" = "0" ]; then
  watch -n2 'sysctl net.netfilter.nf_conntrack_count net.netfilter.nf_conntrack_max'
fi

rule
if [ "$failed" -gt 0 ]; then
  echo "  Finished with $failed failed step(s)."
  echo "  Read the full write-up at https://jbtecwiz.com/support/lnx-net-conntrack"
else
  echo "  Finished."
fi
echo
prose 'To undo: Remove /etc/sysctl.d/99-conntrack.conf and run sysctl --system to return to the defaults.'
rule