path: root/main.go

package main

import (
	"context"
	"flag"
	"fmt"
	"log"
	"os"
	"strings"

	"google.golang.org/api/dns/v1"
)

const (
	GCP_PROJECT_NAME = "fcuny-homelab"
	GCP_MANAGED_ZONE = "fcuny-xyz"
	TS_DEVICE_NAME   = "tahoe"
	TTL              = 300
)

var desiredRecords = []string{
	"bt",
	"dash",
	"music",
	"unifi",
}

const usage = `Usage:
    dns-updater [OPTIONS] <COMMAND>

Options:
    -ts-api-key    API key for tailscale

Examples:
    $ dns-updater -ts-api-key $(passage credentials/tailscale)
    $ dns-updater -ts-api-key $(passage credentials/tailscale) update
`

var l = log.New(os.Stderr, "", 0)

func main() {
	var (
		tsAPIKeyFlag string
		command      string
	)

	flag.Usage = func() { fmt.Fprintf(os.Stderr, "%s\n", usage) }
	flag.StringVar(&tsAPIKeyFlag, "ts-api-key", "", "the API key for tailscale")
	flag.Parse()

	if flag.NArg() > 1 {
		flag.Usage()
		os.Exit(1)
	} else {
		command = flag.Arg(0)
		if command != "" && command != "update" {
			flag.Usage()
			os.Exit(1)
		}
	}

	ctx := context.Background()

	// we only care about IPv4 for now
	tsIpV4Addresses, _, err := getTsIpsDevice(ctx, tsAPIKeyFlag, TS_DEVICE_NAME)
	if err != nil {
		l.Fatalf("failed to get the IP addresses for %s: %v", TS_DEVICE_NAME, err)
	}

	svc, err := dns.NewService(ctx)
	if err != nil {
		l.Fatalf("failed to create the client for Google Cloud DNS: %v", err)
	}

	zone, err := svc.ManagedZones.Get(GCP_PROJECT_NAME, GCP_MANAGED_ZONE).Context(ctx).Do()
	if err != nil {
		l.Fatalf("failed to get information about the managed zone %s: %+v", GCP_MANAGED_ZONE, err)
	}

	recordSets, err := svc.ResourceRecordSets.List(GCP_PROJECT_NAME, GCP_MANAGED_ZONE).Context(ctx).Do()
	if err != nil {
		l.Fatalf("failed to get the list of records: %+v", err)
	}

	switch command {
	case "update":
		updateRecords(ctx, svc, recordSets, zone, tsIpV4Addresses)
	default:
		listRecords(recordSets)
	}
}

func listRecords(recordSets *dns.ResourceRecordSetsListResponse) {
	for _, record := range recordSets.Rrsets {
		switch record.Type {
		case "A":
			ips := strings.Join(record.Rrdatas[:], ",")
			fmt.Printf("%-25s %4d %s\n", record.Name, record.Ttl, ips)
		}
	}
}

func updateRecords(ctx context.Context, svc *dns.Service, recordSets *dns.ResourceRecordSetsListResponse, zone *dns.ManagedZone, tsIpV4Addresses []string) {
	var (
		existingRecordSets = []*dns.ResourceRecordSet{}
		recordSetsToAdd    = []*dns.ResourceRecordSet{}
		recordSetsToDelete = []*dns.ResourceRecordSet{}
	)

	for _, record := range recordSets.Rrsets {
		if record.Type == "A" {
			existingRecordSets = append(existingRecordSets, record)
		}
	}

	// first pass: create what's missing
	for _, subdomain := range desiredRecords {
		found := false
		subdomain = fmt.Sprintf("%s.%s", subdomain, zone.DnsName)
		for _, r := range existingRecordSets {
			if subdomain == r.Name && r.Type == "A" {
				// check that the IP addresses are correct
				ipsFound := 0
				for _, rr := range r.Rrdatas {
					for _, ip := range tsIpV4Addresses {
						if rr == ip {
							ipsFound += 1
							continue
						}
					}
				}
				// while we found the subdomain with the correct type,
				// we also need to make sure the list of IPs is
				// correct. If they are not, we delete the record and
				// add it again with the correct values.
				if ipsFound == len(tsIpV4Addresses) {
					found = true
					continue
				} else {
					l.Printf("will delete %s (incorrect IPv4 addresses)\n", subdomain)
					recordSetsToDelete = append(recordSetsToDelete, r)
				}
			}
		}
		if !found {
			l.Printf("will add %s\n", subdomain)
			r := &dns.ResourceRecordSet{
				Name:    subdomain,
				Type:    "A",
				Ttl:     TTL,
				Rrdatas: tsIpV4Addresses,
			}
			recordSetsToAdd = append(recordSetsToAdd, r)
		}
	}

	// second pass: delete what's not needed
	for _, r := range existingRecordSets {
		found := false
		for _, subdomain := range desiredRecords {
			subdomain = fmt.Sprintf("%s.%s", subdomain, zone.DnsName)
			if subdomain == r.Name && r.Type == "A" {
				found = true
				continue
			}
		}
		if !found {
			l.Printf("will delete %s\n", r.Name)
			recordSetsToDelete = append(recordSetsToDelete, r)
		}
	}

	if len(recordSetsToAdd) > 0 || len(recordSetsToDelete) > 0 {
		change := &dns.Change{Additions: recordSetsToAdd, Deletions: recordSetsToDelete}
		if _, err := svc.Changes.Create(GCP_PROJECT_NAME, GCP_MANAGED_ZONE, change).Context(ctx).Do(); err != nil {
			l.Fatalf("failed to apply the change: %+v", err)
		}
	}
}