feat: sync Cloudflare DNS records with Traefik container hosts

Create a record for each Host(...) in running containers' Traefik router
labels, tagged with a configurable comment and optional tags, and delete
owned records once their host has been down for DELETE_AFTER.

config.go

@@ -0,0 +1,163 @@
+package main
+
+import (
+	"fmt"
+	"net"
+	"os"
+	"slices"
+	"strconv"
+	"strings"
+	"time"
+)
+
+const defaultComment = "managed by traefik-cloudflare-dns"
+
+// Config holds every setting, read from environment variables.
+type Config struct {
+	APIToken       string
+	ZoneID         string
+	Domain         string
+	Target         string
+	RecordType     string
+	Proxied        bool
+	TTL            int
+	Comment        string
+	Tags           []string
+	DeleteAfter    time.Duration
+	ResyncInterval time.Duration
+	DryRun         bool
+	DockerHost     string
+	LogLevel       string
+}
+
+// LoadConfig reads the configuration through getenv and validates it.
+func LoadConfig(getenv func(string) string) (Config, error) {
+	c := Config{
+		APIToken:   strings.TrimSpace(getenv("CF_API_TOKEN")),
+		ZoneID:     strings.TrimSpace(getenv("CF_ZONE_ID")),
+		Domain:     strings.ToLower(strings.Trim(strings.TrimSpace(getenv("DOMAIN")), ".")),
+		Target:     strings.TrimSpace(getenv("TARGET")),
+		Comment:    getenv("RECORD_COMMENT"),
+		DockerHost: strings.TrimSpace(getenv("DOCKER_HOST")),
+		LogLevel:   strings.ToLower(strings.TrimSpace(getenv("LOG_LEVEL"))),
+	}
+
+	var missing []string
+	for name, value := range map[string]string{
+		"CF_API_TOKEN": c.APIToken,
+		"CF_ZONE_ID":   c.ZoneID,
+		"DOMAIN":       c.Domain,
+		"TARGET":       c.Target,
+	} {
+		if value == "" {
+			missing = append(missing, name)
+		}
+	}
+	if len(missing) > 0 {
+		slices.Sort(missing)
+		return Config{}, fmt.Errorf("missing required variables: %s", strings.Join(missing, ", "))
+	}
+
+	c.RecordType = recordTypeFor(c.Target)
+	if c.RecordType == "CNAME" {
+		c.Target = strings.ToLower(strings.TrimSuffix(c.Target, "."))
+	}
+
+	if c.Comment == "" {
+		c.Comment = defaultComment
+	}
+	if strings.ContainsAny(c.Comment, "\r\n") {
+		return Config{}, fmt.Errorf("RECORD_COMMENT must not contain line breaks")
+	}
+
+	for _, tag := range strings.Split(getenv("RECORD_TAGS"), ",") {
+		tag = strings.TrimSpace(tag)
+		if tag == "" {
+			continue
+		}
+		if !strings.Contains(tag, ":") {
+			return Config{}, fmt.Errorf("RECORD_TAGS entry %q is not in name:value form", tag)
+		}
+		c.Tags = append(c.Tags, tag)
+	}
+
+	var err error
+	if c.Proxied, err = parseBool(getenv, "PROXIED", false); err != nil {
+		return Config{}, err
+	}
+	if c.DryRun, err = parseBool(getenv, "DRY_RUN", false); err != nil {
+		return Config{}, err
+	}
+	if c.TTL, err = parseInt(getenv, "TTL", 1); err != nil {
+		return Config{}, err
+	}
+	if c.DeleteAfter, err = parseDuration(getenv, "DELETE_AFTER", time.Hour); err != nil {
+		return Config{}, err
+	}
+	if c.ResyncInterval, err = parseDuration(getenv, "RESYNC_INTERVAL", time.Minute); err != nil {
+		return Config{}, err
+	}
+	if c.ResyncInterval <= 0 {
+		return Config{}, fmt.Errorf("RESYNC_INTERVAL must be positive")
+	}
+
+	if c.DockerHost == "" {
+		c.DockerHost = "unix:///var/run/docker.sock"
+	}
+	if c.LogLevel == "" {
+		c.LogLevel = "info"
+	}
+	return c, nil
+}
+
+// recordTypeFor returns A or AAAA for an IP target and CNAME for a hostname.
+func recordTypeFor(target string) string {
+	ip := net.ParseIP(target)
+	switch {
+	case ip == nil:
+		return "CNAME"
+	case ip.To4() != nil:
+		return "A"
+	default:
+		return "AAAA"
+	}
+}
+
+func parseBool(getenv func(string) string, name string, def bool) (bool, error) {
+	v := strings.TrimSpace(getenv(name))
+	if v == "" {
+		return def, nil
+	}
+	b, err := strconv.ParseBool(v)
+	if err != nil {
+		return false, fmt.Errorf("%s: %q is not a boolean", name, v)
+	}
+	return b, nil
+}
+
+func parseInt(getenv func(string) string, name string, def int) (int, error) {
+	v := strings.TrimSpace(getenv(name))
+	if v == "" {
+		return def, nil
+	}
+	n, err := strconv.Atoi(v)
+	if err != nil {
+		return 0, fmt.Errorf("%s: %q is not an integer", name, v)
+	}
+	return n, nil
+}
+
+func parseDuration(getenv func(string) string, name string, def time.Duration) (time.Duration, error) {
+	v := strings.TrimSpace(getenv(name))
+	if v == "" {
+		return def, nil
+	}
+	d, err := time.ParseDuration(v)
+	if err != nil || d < 0 {
+		return 0, fmt.Errorf("%s: %q is not a non-negative duration such as 1h or 30m", name, v)
+	}
+	return d, nil
+}
+
+// osGetenv is the production getenv.
+var osGetenv = os.Getenv