A simple script that overrides /etc/resolv.conf to custom one on per-app basis (Linux only)

odns

#!/bin/sh
set -e

trap clean TERM INT EXIT

tmp_resolv_conf=""

clean() {
  rm -f -- "$tmp_resolv_conf"
}

split() {
  local IFS="$1"
  read array <<EOF
$2
EOF
  echo $array
}

generate_entries() {
  printf "nameserver %s\n" "$@"
}

script_name=$(basename "$0")

if [ -z "$*" ]; then
  echo "Usage: [ODNS_SERVERS=] [ODNS_OPTIONS=] $script_name <command>"
  echo
  echo "Overrides /etc/resolv.conf on per-app basis."
  echo "Check it using \`$script_name cat /etc/resolv.conf\`."
  echo
  echo "Options:"
  echo " - ODNS_SERVERS=        environment variable, a comma-separated list of DNS"
  echo "                        servers; defaults to \"9.9.9.9,149.112.112.112\""
  echo " - ODNS_OPTIONS=        environment variable, /etc/resolv.conf options;"
  echo "                        defaults to \"edns0 single-request-reopen\""
  exit 1
fi

tmp_resolv_conf=$(mktemp --suffix ".odns")
nameservers=$(split , "${ODNS_SERVERS:-9.9.9.9,149.112.112.112}")

cat >| "$tmp_resolv_conf" <<EOF
# Generated by odns
$(generate_entries $nameservers)\
$([ -z "${ODNS_OPTIONS+.}" ] && echo "\\noptions ${ODNS_OPTIONS:-edns0 single-request-reopen}")
EOF

unshare -rm sh -c "mount --bind \"$tmp_resolv_conf\" /etc/resolv.conf; \
exec unshare --map-user=$(id -ru) --map-group=$(id -rg) -- $*"