Fix build and polish codebase to OSS quality

Dockerfile used golang:1.21 but go.mod requires 1.25; CI was broken.
Refactored plugin code with proper godoc comments, extracted helpers to
eliminate duplicated response-building logic, modernized tests with
t.Run subtests and shared assertion helpers, and rewrote README with
configuration reference table and professional structure.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

setup.go

@@ -2,82 +2,42 @@ package synthetic
 
 import (
 	"fmt"
-	"github.com/coredns/caddy"
-	"github.com/coredns/coredns/core/dnsserver"
-	"github.com/coredns/coredns/plugin"
 	"net"
 	"strconv"
 	"strings"
+
+	"github.com/coredns/caddy"
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
 )
 
-// syntheticConfig holds the configuration options for the synthetic plugin.
+// syntheticConfig holds the parsed Corefile configuration for the synthetic plugin.
 type syntheticConfig struct {
-	net     []*net.IPNet
+	// net is the set of IP networks for which synthetic forward responses are
+	// generated. Only addresses falling within these CIDRs will be resolved.
+	net []*net.IPNet
+
+	// forward is the DNS zone appended to synthetic hostnames when generating
+	// PTR records (e.g., "example.com").
 	forward string
-	ttl     uint32
-	prefix  string
+
+	// ttl is the time-to-live value applied to all synthetic responses.
+	// Defaults to 0 (no caching).
+	ttl uint32
+
+	// prefix is the hostname label prefix used to identify synthetic queries
+	// (e.g., "ip-"). A trailing dash is appended automatically if absent.
+	prefix string
 }
 
-// init registers this plugin.
 func init() { plugin.Register("synthetic", setup) }
 
-// setup is the function that gets called when the config parser see the token "synthetic".
+// setup parses the Corefile configuration block for the synthetic plugin and
+// registers it in the CoreDNS handler chain.
 func setup(c *caddy.Controller) error {
-	var config syntheticConfig
-
-	// Parse the configuration file
-	for c.Next() {
-		for c.NextBlock() {
-			switch v := c.Val(); v {
-
-			// Configuration for forward lookup zones for which to do resolution
-			case "net":
-				args := c.RemainingArgs()
-				for _, arg := range args {
-					_, cidr, err := net.ParseCIDR(arg)
-					if err == nil {
-						config.net = append(config.net, cidr)
-					} else {
-						return fmt.Errorf("synthetic: invalid reverse lookup cidr: %v", arg)
-					}
-				}
-
-			// Configuration for reverse lookup zones for the forward lookup zone name
-			case "forward":
-				args := c.RemainingArgs()
-				for _, arg := range args {
-					config.forward = arg
-				}
-
-			// Configuration for the TTL value
-			case "ttl":
-				args := c.RemainingArgs()
-				for _, arg := range args {
-					ttl64, err := strconv.ParseUint(arg, 10, 32)
-					if err != nil {
-						return fmt.Errorf("synthetic: invalid ttl value: %v", arg)
-					}
-					config.ttl = uint32(ttl64)
-				}
-
-				// configuration for the prefix value (defaults to `ip`)
-			case "prefix":
-				args := c.RemainingArgs()
-				for _, arg := range args {
-					config.prefix = arg
-				}
-
-			default:
-				return c.Errf("unknown property '%s'", v)
-			}
-		}
-	}
-
-	if config.prefix == "" {
-		config.prefix = "ip"
-	}
-	if !strings.HasSuffix(config.prefix, "-") {
-		config.prefix = config.prefix + "-"
+	config, err := parseConfig(c)
+	if err != nil {
+		return err
 	}
 
 	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
@@ -86,3 +46,58 @@ func setup(c *caddy.Controller) error {
 
 	return nil
 }
+
+// parseConfig reads the synthetic { ... } block from the Corefile and returns
+// a validated syntheticConfig.
+func parseConfig(c *caddy.Controller) (syntheticConfig, error) {
+	var config syntheticConfig
+
+	for c.Next() {
+		for c.NextBlock() {
+			switch v := c.Val(); v {
+			case "net":
+				for _, arg := range c.RemainingArgs() {
+					_, cidr, err := net.ParseCIDR(arg)
+					if err != nil {
+						return config, fmt.Errorf("synthetic: invalid CIDR notation: %v", arg)
+					}
+					config.net = append(config.net, cidr)
+				}
+
+			case "forward":
+				args := c.RemainingArgs()
+				if len(args) > 0 {
+					config.forward = args[0]
+				}
+
+			case "ttl":
+				for _, arg := range c.RemainingArgs() {
+					ttl64, err := strconv.ParseUint(arg, 10, 32)
+					if err != nil {
+						return config, fmt.Errorf("synthetic: invalid ttl value: %v", arg)
+					}
+					config.ttl = uint32(ttl64)
+				}
+
+			case "prefix":
+				args := c.RemainingArgs()
+				if len(args) > 0 {
+					config.prefix = args[0]
+				}
+
+			default:
+				return config, c.Errf("unknown property '%s'", v)
+			}
+		}
+	}
+
+	// Apply defaults.
+	if config.prefix == "" {
+		config.prefix = "ip"
+	}
+	if !strings.HasSuffix(config.prefix, "-") {
+		config.prefix += "-"
+	}
+
+	return config, nil
+}