Watch for DNS changes and reload NGINX (or do something else)

check_dns.sh

#!/usr/bin/env bash
#
## example running it from cron
# MAILTO=""
# SHELL=/bin/bash
# VERBOSE=1
# CMD_ON_FAILURE='/etc/init.d/nginx reload'
# * * * * * root timeout -k 2 5 /tmp/check_dns.sh upstream.server.com 2>>/var/log/check_dns.log
# * * * * * root sleep 10 && sed -e :a -e '$q;N;501,$D;ba' -i /var/log/check_dns.log
#

set -u
set -o pipefail

CHECK_HOSTNAME="${1:?hostname required as single argument}"

: ${CMD_ON_FAILURE:=unset}

TEMP_FILE="/tmp/${CHECK_HOSTNAME//./_}.md5"
DATETIME_NOW="$(date)"
 
# exit codes 
# 0: check successful, dns did not change
# 1: catch-all failure, something went wrong, missing argument?
# 2: check failed, dns changed
# 3: check failed, nothing to compare to, missing previous check file
# 4: dns resolution failed, do nothing
# 5: cannot write to temp file, do nothing
# 6: failed to run CMD_ON_FAILURE

# caveat:
# if CMD_ON_FAILURE is defined, exit code 3 will not fire,
# instead, exit code of CMD_ON_FAILURE will be returned

verbose() {
  [[ ${VERBOSE:-0} == "1" ]] && echo >&2 "[${DATETIME_NOW}]: $@" || true
}

check_record() {
  verbose "Getting DNS resolution of \"${CHECK_HOSTNAME}\".."
  host ${CHECK_HOSTNAME} | grep -v NXDOMAIN | sort
}

record_md5() {
  verbose "Recording MD5 of DNS resolution of \"${CHECK_HOSTNAME}\".."
  if ! touch ${TEMP_FILE} 2>/dev/null; then
    verbose "Cannot write TEMP_FILE (${TEMP_FILE}), exiting.."
    exit 5
  fi
  echo "$@" | md5sum > ${TEMP_FILE}
}

check_md5() {
  verbose "Verifying MD5 of DNS resolution of \"${CHECK_HOSTNAME}\".."
  echo "$@" | md5sum --status -c ${TEMP_FILE}
}

run_cmd_on_failure() {
  if [[ ${CMD_ON_FAILURE} != "unset" ]]; then
    verbose "Executing \"${CMD_ON_FAILURE}\".."
    exec -- ${CMD_ON_FAILURE}
  fi
}

## do stuff
record_data=$(check_record)
if [ $? -ne 0 ]; then
  verbose "DNS resolution failed, bad hostname or timeout? exiting.."
  exit 4
fi

if [ -e ${TEMP_FILE} ]; then
  if ! check_md5 "${record_data}"; then
    verbose "DNS change detected!"
    record_md5 "${record_data}"
    run_cmd_on_failure
    exit 2
  else
    verbose "DNS remains unchanged!"
    exit 0
  fi
else
  verbose "Nothing to compare with, MD5 file for ${CHECK_HOSTNAME} is not present (\"${TEMP_FILE}\")"
  record_md5 "${record_data}"
  run_cmd_on_failure
  exit 3
fi