Spin BIRD up or down depending on the state of DNS

should-we-anycast.sh

#!/bin/sh
## should we anycast?
## - WHAT? a brief script that will spin bird up or down, based on
##        results from a dns test.
##
## - WHY? because if our dns server falls down, we don't want a dns
##        blackhole for our querying users
##
## omachonu ogali // oogali@gmail.com // @oogali

## define our constants
## - RR is the resource record we wish to lookup
## - NAMESERVER is the server we want to test
## - BIRDCTL is the path to the bird initscript
## - DIG_ARGS are optional arguments to pass to dig
RR=localhost
NAMESERVER=127.0.0.1
BIRDCTL=/etc/init.d/bird
DIG_ARGS="+short"

## make sure we have pgrep and dig installed
PGREP=`which pgrep 2>/dev/null`
if [ -z "${PGREP}" ]; then
  echo "$0: could not find pgrep!"
  exit 1
fi

DIG=`which dig 2>/dev/null`
if [ -z "${DIG}" ]; then
  echo "$0: could not find dig!"
  exit 1
fi

## get pid of bird instance that's running
bird_pid=`${PGREP} bird`

## do a dns lookup for our defined resource record
${DIG} ${DIG_ARGS} ${RR} @${NAMESERVER} >/dev/null 2>&1
if [ $? -eq 0 ]; then
  ## if dns lookup succeeds, but bird isn't running, start it.
  if [ -z "${bird_pid}" ]; then
    ${BIRDCTL} start
  fi
else
  ## if dns lookup fails, but bird IS running, stop it.
  if [ ! -z "${bird_pid}" ]; then
    ${BIRDCTL} stop
  fi
fi