dns-lookup.sh

#!/bin/bash

# Usage: ./dns-lookup.sh example.com

HOST="$1"
DNS_SERVER=${2:-8.8.8.8}
PORT=53

if [[ -z "$HOST" ]]; then
  echo "Usage: $0 <hostname> [dns_server]"
  exit 1
fi

# Build query header
TXID=$(printf "%02x%02x" $((RANDOM%256)) $((RANDOM%256)))
HEADER="${TXID}01000001000000000000"

# Build query question section
QNAME=""
IFS='.' read -ra PARTS <<< "$HOST"
for PART in "${PARTS[@]}"; do
  LEN=$(printf "%02x" ${#PART})
  HEX=$(echo -n "$PART" | xxd -p)
  QNAME="${QNAME}${LEN}${HEX}"
done
QNAME="${QNAME}00" # end of QNAME
QTYPE="0001"       # A record
QCLASS="0001"      # IN

QUERY="${HEADER}${QNAME}${QTYPE}${QCLASS}"

# Convert hex to binary and send over UDP
exec 3<>/dev/udp/${DNS_SERVER}/${PORT}
echo -n -e "$(echo "$QUERY" | sed 's/../\\x&/g')" >&3
RESPONSE=$(dd bs=1 count=512 <&3 2>/dev/null | xxd -p -c 512)

# Extract and print A record IP addresses
echo "$RESPONSE" | grep -oP '(?<=c00c00010001.{8})[0-9a-f]{8}' | while read -r iphex; do
  printf "%d.%d.%d.%d\n" "0x${iphex:0:2}" "0x${iphex:2:2}" "0x${iphex:4:2}" "0x${iphex:6:2}"
done