DNS resolver config producer

dns-config.sh

#!/bin/bash

INPUTFILE=${INPUTFILE:-'/etc/resolv.conf'}
DEBUG=${DEBUG:-false}

FALLBACK_NAMESERVERS_IPV4='8.8.8.8 8.8.4.4'
FALLBACK_NAMESERVERS_IPV6='2001:4860:4860::8888 2001:4860:4860::8844'
HOST_ADDRESSES_IPV4=$(ip a | awk '/inet / {print $2}' | grep -v '^127\.' | cut -d/ -f1 | paste -d' ' -s)
HOST_ADDRESSES_IPV6=$(ip a | awk '/inet6 / {print $2}' | grep -v '^::' | cut -d/ -f1 | paste -d' ' -s)
HOST_NAMESERVERS_NOLOCAL_IPV4=$(awk '/^nameserver/ {print $2}' ${INPUTFILE} | grep -v '^127\.\|^::' | grep '\.' | paste -d' ' -s)
HOST_NAMESERVERS_NOLOCAL_IPV6=$(awk '/^nameserver/ {print $2}' ${INPUTFILE} | grep -v '^127\.\|^::' | grep ':' | paste -d' ' -s)

WORKING_NAMESERVER_IPV6=""
WORKING_NAMESERVERS_IPV4=""
WORKING_NAMESERVERS_ALL=""
BROKEN_NAMESERVERS_ALL=""

function test_dns {
  server_list="$@"
  if ${DEBUG}; then
    >&2 echo "--> test_dns server_list: ${server_list}"
  fi
  for server in ${server_list}; do
    if ${DEBUG}; then
      >&2 echo "  |--> test_dns checking server: ${server}"
    fi
    if [[ ${BROKEN_NAMESERVERS_ALL} =~ .*${server}.* ]]; then
      if ${DEBUG}; then
        >&2 echo "     |--> ${server} has already been tested and did not work"
      fi
    elif [[ ${WORKING_NAMESERVERS_ALL} =~ .*${server}.* ]]; then
      if ${DEBUG}; then
        >&2 echo "     |--> ${server} has already been tested and is working"
      fi
    elif $(dig +short +time=1 @${server} google.com > /dev/null); then
      if ${DEBUG}; then
        >&2 echo "     |--> test_dns test to ${server} successful"
      fi
      WORKING_NAMESERVERS_ALL="${WORKING_NAMESERVERS_ALL} ${server}"
      if $(echo ${server} | grep '\.' > /dev/null); then
        WORKING_NAMESERVERS_IPV4="${WORKING_NAMESERVERS_IPV4} ${server}"
      else
        WORKING_NAMESERVERS_IPV6="${WORKING_NAMESERVERS_IPV6} ${server}"
      fi
    else
      if ${DEBUG}; then
        >&2 echo "     |--> test_dns test to ${server} not successful"
      fi
      BROKEN_NAMESERVERS_ALL="${BROKEN_NAMESERVERS_ALL} ${server}"
    fi
  done
}

if ${DEBUG}; then
  >&2 echo -e "\n### Starting Data ###"
  >&2 echo -e "HOST_ADDRESSES_IPV4:\n${HOST_ADDRESSES_IPV4}\n"
  >&2 echo -e "HOST_ADDRESSES_IPV6:\n${HOST_ADDRESSES_IPV6}\n"
  >&2 echo -e "HOST_NAMESERVERS_NOLOCAL_IPV4:\n${HOST_NAMESERVERS_NOLOCAL_IPV4}\n"
  >&2 echo -e "HOST_NAMESERVERS_NOLOCAL_IPV6:\n${HOST_NAMESERVERS_NOLOCAL_IPV6}\n"
fi

# Test to see whether the configured IPV6 resolvers work
if [ ! -z "${HOST_NAMESERVERS_NOLOCAL_IPV6}" ]; then
  test_dns ${HOST_NAMESERVERS_NOLOCAL_IPV6}
fi

# Test to see whether the configured IPV4 resolvers work
if [ ! -z "${HOST_NAMESERVERS_NOLOCAL_IPV4}" ]; then
  test_dns ${HOST_NAMESERVERS_NOLOCAL_IPV4}
fi

if ${DEBUG}; then
  >&2 echo -e "\n### Working Name Servers after testing resolv.conf entries ###"
  >&2 echo -e "WORKING_NAMESERVERS_IPV6:\n${WORKING_NAMESERVERS_IPV6}\n"
  >&2 echo -e "WORKING_NAMESERVERS_IPV4:\n${WORKING_NAMESERVERS_IPV4}\n"
fi

# If none of the resolvers work, the test whether the host has
# a working DNS service listener on one of its IPV6 addresses.
if [ -z "${WORKING_NAMESERVERS_IPV6}" ]; then
  test_dns ${HOST_ADDRESSES_IPV6}
fi

# If none of the resolvers work, the test whether the host has
# a working DNS service listener on one of its IPV4 addresses.
if [ -z "${WORKING_NAMESERVERS_IPV6}" ]; then
  test_dns ${HOST_ADDRESSES_IPV4}
fi

if ${DEBUG}; then
  >&2 echo -e "\n### Working Name Servers after testing the host addresses ###"
  >&2 echo -e "WORKING_NAMESERVERS_IPV6:\n${WORKING_NAMESERVERS_IPV6}\n"
  >&2 echo -e "WORKING_NAMESERVERS_IPV4:\n${WORKING_NAMESERVERS_IPV4}\n"
fi

# We only implement the fallback IPV6 addresses if none of the IPV6 addresses
# in the existing resolv.conf and none of the IPV6 host addresses worked.
if [ -z "${WORKING_NAMESERVERS_IPV6}" ]; then
  test_dns ${FALLBACK_NAMESERVERS_IPV6}
fi

# We only implement the fallback IPV4 addresses if none of the IPV4 addresses
# in the existing resolv.conf and none of the IPV4 host addresses worked.
if [ -z "${WORKING_NAMESERVERS_IPV4}" ]; then
  test_dns ${FALLBACK_NAMESERVERS_IPV4}
fi

if ${DEBUG}; then
  >&2 echo -e "\n### Working Name Servers after testing the fallback addresses ###"
  >&2 echo -e "WORKING_NAMESERVERS_IPV6:\n${WORKING_NAMESERVERS_IPV6}\n"
  >&2 echo -e "WORKING_NAMESERVERS_IPV4:\n${WORKING_NAMESERVERS_IPV4}\n"
fi

WORKING_NAMESERVERS_ALL="${WORKING_NAMESERVERS_IPV6} ${WORKING_NAMESERVERS_IPV4}"

for server in ${WORKING_NAMESERVERS_ALL}; do
  echo "nameserver ${server}"
done