Checks whether your website and mail server resolutions works fine around the globe

README.md

Download DNS list from https://public-dns.info/, which will be a file like below:

ip_address,name,as_number,as_org,country_code,city,version,error,dnssec,reliability,checked_at,created_at
8.8.8.8,dns.google.,15169,GOOGLE,US,,,,true,1.00,2022-04-17T08:03:50Z,2020-07-16T14:19:04Z

then, run the command below to run tests

./test_dns.sh -f us.csv -d gordarg.com -n 5 -r MX

check_dns_resolution.sh

#!/usr/bin/env bash
set -uo pipefail

usage() {
  cat <<EOF
Usage: $0 -f servers.csv -d domain [-r record] [-n tries] [-p parallelism]
  -f  CSV file (with header, including ip_address, as_org, city, country_code)
  -d  Domain to query
  -r  DNS record type to query (A, NS, MX, TXT, etc.; default: MX)
  -n  Number of attempts per server (default: 3)
  -p  Max parallel jobs (default: 10)
EOF
  exit 1
}

# defaults
tries=3
parallelism=10
record="MX"

# parse options
while getopts ":f:d:r:n:p:" opt; do
  case $opt in
    f) file="$OPTARG" ;;
    d) domain="$OPTARG" ;;
    r) record="${OPTARG^^}" ;;  # uppercase record type
    n) tries="$OPTARG" ;;
    p) parallelism="$OPTARG" ;;
    *) usage ;;
  esac
done
[[ -z "${file:-}" || -z "${domain:-}" ]] && usage

# prepare workspace
mapfile -t lines < <(tail -n +2 "$file" | sed 's/\r$//')
workdir=$(mktemp -d)
lockfile="$workdir/.lock"
touch "$lockfile"
trap 'rm -rf "$workdir"' EXIT

# header
printf "\nQuerying %-4s records for %-25s\n\n" "$record" "$domain"
printf "%-15s | %-20s | %-15s | %-10s | %-2s | %8s | %9s | %11s | %15s\n" \
  "IP Address" "Org" "City" "ASN" "CC" "Attempts" "Successes" "Reliability" "Avg Delay(ms)"
printf -- "-----------------+----------------------+-----------------+------------+----+----------+-----------+-------------+----------------\n"

# per-server worker
process_server(){
  local line=$1
  IFS=',' read -r ip _ asn org cc city _ _ _ _ <<<"$line"
  ip="${ip//[[:space:]]/}" || return
  [[ -z "$ip" ]] && return

  local success=0 total_delay=0
  for ((i=1; i<=tries; i++)); do
    if out=$(dig +time=5 +tries=1 +stats "$record" "$domain" @"$ip" 2>&1); then
      if grep -qE "^[^;].+${record}[[:space:]]" <<<"$out"; then
        (( success++ ))
        delay=$(awk '/^;; Query time:/ {print $4}' <<<"$out")
        total_delay=$(( total_delay + delay ))
      fi
    fi
  done

  local avg_delay=0
  (( success > 0 )) && avg_delay=$(( total_delay / success ))
  local reliability
  reliability=$(awk "BEGIN{printf \"%.1f\", ($success/$tries)*100}")

  # flag any server that worked
  if (( success > 0 )); then
    touch "$workdir/${ip//./-}.ok"
  fi

  # save raw for summary
  printf "%s|%s|%s|%s|%s|%d|%d|%s|%d\n" \
    "$ip" "${org:-—}" "${city:-—}" "${asn:-—}" "${cc:-—}" \
    "$tries" "$success" "$reliability" "$avg_delay" \
    > "$workdir/${ip//./-}.out"

  # real-time row (locked)
  local disp_avg="—"
  (( success > 0 )) && disp_avg="$avg_delay"
  flock -x "$lockfile" -c \
    "printf \"%-15s | %-20s | %-15s | %-10s | %-2s | %8d | %9d | %10s%% | %15s\n\" \
      \"$ip\" \"${org:-—}\" \"${city:-—}\" \"${asn:-—}\" \"${cc:-—}\" \
      \"$tries\" \"$success\" \"$reliability\" \"$disp_avg\""
}

# semaphore
wait_for_slot(){
  while (( $(jobs -rp | wc -l) >= parallelism )); do
    sleep 0.05
  done
}

# dispatch
for line in "${lines[@]}"; do
  wait_for_slot
  process_server "$line" &
done

wait

# final aggregation
total_servers=0
total_successes=0
total_delay_sum=0

for f in "$workdir"/*.out; do
  IFS='|' read -r ip org city asn cc att succ reli avg <<<"$(<"$f")"
  (( total_servers++ ))
  total_successes=$(( total_successes + succ ))
  total_delay_sum=$(( total_delay_sum + avg ))
done

# count servers that worked
total_ok=$(ls "$workdir"/*.ok 2>/dev/null | wc -l)

# summary
echo -e "\n📊 Overall Summary:"
echo "---------------------------"
echo "Record Type              : $record"
echo "Domain                   : $domain"
echo "Total DNS Servers Tested : $total_servers"
echo "Total Successful Queries : $total_successes"
echo "Servers that worked      : $total_ok / $total_servers"

if (( total_servers > 0 )); then
  avg_reliability=$(awk "BEGIN{printf \"%.1f\", ($total_successes/($total_servers*tries))*100}")
  avg_response_delay=$(( total_delay_sum / total_servers ))
  echo "Average Reliability      : $avg_reliability%"
  echo "Average Response Delay   : ${avg_response_delay} ms"
else
  echo "Average Reliability      : —"
  echo "Average Response Delay   : —"
fi