Update network alias of a gcp instance from metadata

updatealias.go

package main

import (
	"log"
	"os"

	"golang.org/x/net/context"
	"golang.org/x/oauth2/google"
	"google.golang.org/api/compute/v1"
)

func main() {
	ctx := context.Background()

	c, err := google.DefaultClient(ctx, compute.CloudPlatformScope)
	if err != nil {
		log.Fatal(err)
	}

	computeService, err := compute.New(c)
	if err != nil {
		log.Fatal(err)
	}

	arguments := os.Args[1:]
	project := arguments[0]
	zone := arguments[1]
	instance := arguments[2]
	cidrName := arguments[3]
	cidr := arguments[4]

	info, err := computeService.Instances.Get(project, zone, instance).Context(ctx).Do()
	if err != nil {
		log.Fatal(err)
	}

	fingerprint := info.NetworkInterfaces[0].Fingerprint

	var aliasIpRanges []*compute.AliasIpRange
	aliasIpRange := &compute.AliasIpRange{IpCidrRange: cidr, SubnetworkRangeName: cidrName}
	aliasIpRanges = append(aliasIpRanges, aliasIpRange)

	rb := &compute.NetworkInterface{AliasIpRanges: aliasIpRanges, Fingerprint: fingerprint}

	_, err = computeService.Instances.UpdateNetworkInterface(project, zone, instance, "nic0", rb).Context(ctx).Do()
	if err != nil {
		log.Fatal(err)
	}
}

updatealias.sh

#!/usr/bin/env bash

export PATH=/root:$PWD:$PATH
export GOOGLE_APPLICATION_CREDENTIALS="${GOOGLE_APPLICATION_CREDENTIALS:-/root/creds.json}"
PROJECT=$(curl -s -H "Metadata-Flavor: Google" http://169.254.169.254/computeMetadata/v1/project/project-id)
ZONE=$(basename $(curl -s -H "Metadata-Flavor: Google" http://169.254.169.254/computeMetadata/v1/instance/zone))
INSTANCE=$hostname
CIDR_NAME=dual-mynetwork
CIDR=192.168.124.0/25

updatealias $PROJECT $ZONE $INSTANCE $CIDR_NAME $CIDR