How to update Google Cloud DNS using an ephemeral IP address for an instance

Question

I have several instances on GCE for which I really don't need static addresses, but I still need to make them accessible through the dns name. Since the ephemeral external IP addresses change each time the instance is restarted, I thought I could use some kind of script run to update the DNS record for that instance in the cloud-based DNS domain (a bit like dyndns).

Am I missing something, and is there an easier way to map the ephemeral external IPs to the dns entry via gcloud?

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

Answer 1

It is further assumed that you are using the Google Cloud DNS for foo.bar.com (that is, the dns name is “foo.bar.com.”) With the zone name “foo-bar-com” in the same project as your virtual machine and that your virtual machine has a configuration parameter "This instance has full API access for all Google Cloud services." selected. Your virtual machine will be called "my-vm.foo.bar.com" in DNS.

I am sure that this can be changed accordingly to work with DNS in another project and / or more limited permissions.

Perhaps it’s worth noting: it is assumed that you are using the “Google Cloud DNS” and not the (simple) Google Domains registrar, if you use the latter (to host your DNS, and not just as a registrar), then they have direct support for synthetic dynamic IP -addresses with some dyndns, such as the update mechanism (but they are more limited by other methods).

Also note that for a successful transaction there should already be a record with the correct IP and the correct TTL (i.e., at the first start you can delete any record manually through the user interface and run its code with dns_del commented out).

#!/bin/bash ttlify() { local i for i in " $@ "; do [[ "${i}" =~ ^([0-9]+)([az]*)$ ]] || 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 } 

Answer 2

It's been a while since you posted this question, but I will post my answer here for future reference.

I had a similar need and did not want to use the gcloud CLI.

I created a simple python script that is pretty much similar to the previous bash script, but uses Apache Libcloud and Google Cloud API credentials (service account and key).

You can find the code on GitHub .