cmd: add a command to update fcuny.xyz

I'm using fcuny.xyz as a domain to run a number of services on a host
using the IP provided by Tailscale.

Instead of manually updating the DNS configuration in the console every
time I create a new subdomain, let's do this with a small program.

The program query Tailscale API to get the IP address of the host
`tahoe`, and then query the DNS API to see if anything is missing or is
mis-configured. If it's the case, it will resolve the problems.

1 files changed, 125 insertions, 0 deletions

diff --git a/cmd/dnsupdate/main.go b/cmd/dnsupdate/main.go
new file mode 100644
index 0000000..59ed67a
--- /dev/null
+++ b/cmd/dnsupdate/main.go
@@ -0,0 +1,125 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+
+	dns "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",
+	"drone",
+	"music",
+	"unifi",
+}
+
+func main() {
+	ctx := context.Background()
+
+	// we only care about IPv4 for now
+	tsIpV4Addresses, _, err := getTsIpsDevice(ctx, TS_DEVICE_NAME)
+	if err != nil {
+		log.Fatalf("failed to get the IP addresses for %s: %v", TS_DEVICE_NAME, err)
+	}
+
+	svc, err := dns.NewService(ctx)
+	if err != nil {
+		log.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 {
+		log.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 {
+		log.Fatalf("failed to get the list of records: %+v", err)
+	}
+
+	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 {
+					log.Printf("will delete %s (incorrect IPv4 addresses)\n", subdomain)
+					recordSetsToDelete = append(recordSetsToDelete, r)
+				}
+			}
+		}
+		if !found {
+			log.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 {
+			log.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 {
+			log.Fatalf("failed to apply the change: %+v", err)
+		}
+	}
+}