How to update Google Cloud DNS with ephemeral IP for an instance

I have couple of instances on GCE that I don't really need static addresses for, but I still need to make them accessible via dns name. Since ephemeral external ip addresses change every time an instance is restarted, I thought that I could use some sort of startup script to update dns entry for that instance in Google Cloud DNS (a bit like dyndns).

Did I miss something and there is an easier way to map ephemeral external ip addresses to a dns record via gcloud?

If not, any pointers on how to write such script would be highly appreciated!

The following assumes that you are using Google Cloud DNS for foo.bar.com (ie. dns name "foo.bar.com.") with zone name "foo-bar-com" in the same project as your VM and that your VM has configuration option "This instance has full API access to all Google Cloud services." selected. Your VM will be called "my-vm.foo.bar.com" in DNS.

I'm sure this could be appropriately modified to work with DNS in a different project and/or more limited permissions.

Probably worth noting: this assumes you are using 'Google Cloud DNS' and not (just) 'Google Domains' registrar, if you're using the latter (to host your DNS, and not just as a registrar) then they have direct support for synthetic dynamic ip address with some dyndns like update mechanism (but they're more limited in a bunch of other ways).

Also note that for transaction to succeed there already has to be a record with the right IP and the right TTL (ie. the first time you run this you may want to delete any entry by hand via the UI, and run this code with dns_del commented out).

#!/bin/bash

ttlify() {
  local i
  for i in "$@"; do
    [[ "${i}" =~ ^([0-9]+)([a-z]*)$ ]] || continue
    local num="${BASH_REMATCH[1]}"
    local unit="${BASH_REMATCH[2]}"
    case "${unit}" in
                     weeks|week|wee|we|w) unit=''; num=$[num*60*60*24*7];;
                           days|day|da|d) unit=''; num=$[num*60*60*24];;
                     hours|hour|hou|ho|h) unit=''; num=$[num*60*60];;
      minutes|minute|minut|minu|min|mi|m) unit=''; num=$[num*60];;
      seconds|second|secon|seco|sec|se|s) unit=''; num=$[num];;
    esac
    echo "${num}${unit}"
  done
}

dns_start() {
  gcloud dns record-sets transaction start    -z "${ZONENAME}"
}

dns_info() {
  gcloud dns record-sets transaction describe -z "${ZONENAME}"
}

dns_abort() {
  gcloud dns record-sets transaction abort    -z "${ZONENAME}"
}

dns_commit() {
  gcloud dns record-sets transaction execute  -z "${ZONENAME}"
}

dns_add() {
  if [[ -n "$1" && "$1" != '@' ]]; then
    local -r name="$1.${ZONE}."
  else
    local -r name="${ZONE}."
  fi
  local -r ttl="$(ttlify "$2")"
  local -r type="$3"
  shift 3
  gcloud dns record-sets transaction add      -z "${ZONENAME}" --name "${name}" --ttl "${ttl}" --type "${type}" "$@"
}

dns_del() {
  if [[ -n "$1" && "$1" != '@' ]]; then
    local -r name="$1.${ZONE}."
  else
    local -r name="${ZONE}."
  fi
  local -r ttl="$(ttlify "$2")"
  local -r type="$3"
  shift 3
  gcloud dns record-sets transaction remove   -z "${ZONENAME}" --name "${name}" --ttl "${ttl}" --type "${type}" "$@"
}

lookup_dns_ip() {
  host "$1" | sed -rn 's@^.* has address @@p'
}

my_ip() {
  ip -4 addr show dev eth0 | sed -rn 's@^    inet ([0-9.]+).*@\1@p'
}

doit() {
  ZONE=foo.bar.com
  ZONENAME=foo-bar-com
  dns_start
  dns_del my-vm 5min A `lookup_dns_ip "my-vm.${ZONE}."`
  dns_add my-vm 5min A `my_ip`
  dns_commit
}