DNS Lookup Check Shell Script

After DNS or CDN cutovers, the top check is a dns lookup shell script that fails when dig returns the wrong A record — reusable in cron until TTLs settle.

What this script does

Takes hostname and optional expected IPv4
Uses dig +short A with resolver override
Compares answer to EXPECTED_IP env or arg
Logs failures for ticket attachment
Exit 0 only when resolution matches

Prerequisites

bind-utils or dnsutils (dig)
Network DNS egress
IPv4 focus (extend for AAAA separately)

Step 1: Save the script

sudo nano /usr/local/bin/dns-check.sh
sudo chmod +x /usr/local/bin/dns-check.sh

Step 2: Full script (scroll to read)

dns-check.sh

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

HOST="${1:?usage: dns-check.sh HOST [EXPECTED_IPV4]}"
EXPECTED="${2:-${EXPECTED_IP:-}}"
RESOLVER="${DNS_SERVER:-}"
LOG="/var/log/dns-check.log"

log(){ echo "[$(date '+%F %T')] $*" | tee -a "$LOG"; }

dig_cmd=(dig +short A "$HOST")
[[ -n "$RESOLVER" ]] && dig_cmd=(dig +short A "$HOST" "@$RESOLVER")

answer=$("${dig_cmd[@]}" 2>/dev/null | head -n1 || true)
if [[ -z "$answer" ]]; then
  log "FAIL no A record for $HOST"
  exit 1
fi

log "OK $HOST -> $answer"
if [[ -n "$EXPECTED" && "$answer" != "$EXPECTED" ]]; then
  log "FAIL expected $EXPECTED got $answer"
  exit 1
fi
exit 0

Scroll inside the box to read the full script.

Step 3: Configure settings

CLI DNS: nslookup guide
Network path: mtr
Pair with ping monitor

Step 4: Test manually

./dns-check.sh www.example.com

EXPECTED_IP=93.184.216.34 ./dns-check.sh example.com

Schedule with cron

sudo crontab -e

Add:

*/15 * * * * /usr/local/bin/dns-check.sh api.example.com 203.0.113.10 >> /var/log/dns-check.log 2>&1

DNS Lookup Check Shell Script Linux (dig, Alerts on Failure)