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, 89 insertions, 0 deletions

diff --git a/cmd/dnsupdate/ts.go b/cmd/dnsupdate/ts.go
new file mode 100644
index 0000000..4d3ebb3
--- /dev/null
+++ b/cmd/dnsupdate/ts.go
@@ -0,0 +1,89 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"net/http"
+	"os"
+
+	"inet.af/netaddr"
+)
+
+type device struct {
+	Hostname  string   `json:"hostname"`
+	ID        string   `json:"id"`
+	Addresses []string `json:"addresses"`
+}
+
+const (
+	TS_NAME       = "franck.cuny@gmail.com"
+	TS_API_DOMAIN = "api.tailscale.com"
+)
+
+func getTsDevice(ctx context.Context, deviceName string) (*device, error) {
+	apiKey, found := os.LookupEnv("TS_API_KEY")
+	if !found {
+		return nil, errors.New("the environment variable TS_API_KEY is not set")
+	}
+
+	url := fmt.Sprintf("https://%s/api/v2/tailnet/%s/devices", TS_API_DOMAIN, TS_NAME)
+	req, err := http.NewRequestWithContext(ctx, "GET", url, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	req.SetBasicAuth(apiKey, "")
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return nil, err
+	}
+
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("non-ok status code %d returned from tailscale api: %s", resp.StatusCode, resp.Status)
+	}
+	var buf struct {
+		Devices []device `json:"devices"`
+	}
+	if err := json.NewDecoder(resp.Body).Decode(&buf); err != nil {
+		return nil, err
+	}
+
+	for _, d := range buf.Devices {
+		if d.Hostname == deviceName {
+			return &d, nil
+		}
+	}
+	return nil, fmt.Errorf("could not find the tailscale device named %s", deviceName)
+}
+
+// Get the Tailscale IPv4 and IPv6 addresses associated with the given device.
+func getTsIpsDevice(ctx context.Context, device string) ([]string, []string, error) {
+	ts_device, err := getTsDevice(ctx, device)
+	if err != nil {
+		return nil, nil, fmt.Errorf("failed to get Tailscale device information: %v", err)
+	}
+
+	var (
+		tsIpV4Addresses = []string{}
+		tsIpV6Addresses = []string{}
+	)
+	for _, ipString := range ts_device.Addresses {
+		// we convert the string to a netaddr.IP so we can check if
+		// it's an IP v4 or v6. We need to know what's the version in
+		// order to use it properly when creating/updating the
+		// record. Then we convert it back as a string, since this is
+		// what the DNS API expect.
+		ip := netaddr.MustParseIP(ipString)
+		if ip.Is4() {
+			tsIpV4Addresses = append(tsIpV4Addresses, ip.String())
+		} else {
+			tsIpV6Addresses = append(tsIpV6Addresses, ip.String())
+		}
+	}
+
+	return tsIpV4Addresses, tsIpV6Addresses, nil
+}