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>

Dockerfile

@@ -1,8 +1,9 @@
-# run go test inside a docker container for consistency as acceptance testing
-FROM golang:1.21
+# Dockerfile for CI: runs vet and tests inside a consistent Go environment.
+FROM golang:1.25
 
 WORKDIR /app
 COPY go.mod go.sum ./
 RUN go mod download
 COPY . .
-RUN go test -v
+RUN go vet ./...
+RUN go test -v ./...

README.md

@@ -1,88 +1,112 @@
 # synthetic
 
------
+A [CoreDNS](https://coredns.io) plugin that generates DNS records from IP addresses embedded in hostnames.
 
-## Overview
+Inspired by dnsmasq's [`synth-domain`](http://www.thekelleys.org.uk/dnsmasq/docs/dnsmasq-man.html) option, `synthetic` provides automatic alignment between forward and reverse lookups — eliminating a common class of DNS misconfigurations described in [RFC 1912 Section 2.1](https://tools.ietf.org/html/rfc1912#section-2.1).
 
-*synthetic* is a [CoreDNS](http://coredns.io) plugin to synthetically handle DNS records with IP addresses embedded.
-Named after DNSMASQ's "synth-domain" [option](http://www.thekelleys.org.uk/dnsmasq/docs/dnsmasq-man.html).
+## Features
 
-`synthetic` aims to provide an easy mechanism for alignment between forward and reverse lookups.
-This is a common DNS operational and configuration error as noted in [RFC1912](https://tools.ietf.org/html/rfc1912#section-2.1).
+- **Forward lookups** (A/AAAA) — extract an IP from a prefixed hostname and respond directly
+- **Reverse lookups** (PTR) — generate a synthetic hostname from a reverse query
+- **Static record priority** — responses from upstream plugins (e.g., `file`) always take precedence over synthetic records
+- **Dual-stack** — full IPv4 and IPv6 support, including compressed IPv6 notation
+- **Configurable** — custom prefix, TTL, and per-network scoping
 
-This plugin supports works nicely with the file plugin such that records present in the file will take precedence over this plugin.  
+## How It Works
 
-### Forward Lookups
+### Forward Lookups (A / AAAA)
 
-Forward Lookups are hostname -> IP address. 
-`synthetic` supports IPs "embedded" in the DNS hostname. 
-For IP addresses embedded in DNS hostnames the general model is `ip-<address>.example.com`
-(where "address" can be either IPv6 or IPv4, and "example.com" is a domain of your choosing).
-In IPv6 the colons are converted to a dash; in IPv4 the dots are converted to a dash.
+A hostname with an embedded IP address is resolved directly. Dots (IPv4) or colons (IPv6) are replaced with dashes in the hostname label:
 
-The following are all considered valid for AAAA or A queries.
+| Hostname | Record | Address |
+|---|---|---|
+| `ip-192-0-2-1.example.com` | A | `192.0.2.1` |
+| `ip-2001-db8-abcd--1.example.com` | AAAA | `2001:db8:abcd::1` |
+| `ip-2001-0db8-0000-0000-0000-0000-0000-0001.example.com` | AAAA | `2001:db8::1` |
 
- * `ip-2001-0db8-0000-0000-0000-0000-0000-0001.example.com`
- * `ip-2001-db8--1.example.com`
- * `ip-192-0-2-0.example.com`
+Only addresses within a configured `net` CIDR are resolved. All other queries pass through to the next plugin.
 
-### Reverse Lookups
+### Reverse Lookups (PTR)
 
-Reverse Lookups are IP -> hostname, and are known as pointer records (PTR).
-`synthetic` will respond to a PTR query and return a result that is also supported by the forward lookup mechanism.
-Reverse lookups for IPv6 addresses will return a fully compressed IPv6 address (per [RFC5952](https://tools.ietf.org/html/rfc5952#section-2.2)).
+When a PTR query arrives, the plugin first consults the next plugin in the chain. If that plugin provides a successful answer (e.g., from a zone file), it is used as-is. Otherwise, a synthetic PTR record is generated pointing back to the corresponding forward hostname.
 
-## Corefile Configuration Examples
+IPv6 reverse responses use the compressed address form per [RFC 5952](https://tools.ietf.org/html/rfc5952#section-2.2).
 
-Reverse Lookup Example
+## Configuration
+
+### Directives
+
+| Directive | Description | Default |
+|---|---|---|
+| `net` | CIDR network(s) for which synthetic forward responses are generated. May be specified multiple times. | *(none — required for forward lookups)* |
+| `forward` | Domain name appended to synthetic hostnames in PTR responses. | *(none — required for reverse lookups)* |
+| `ttl` | Time-to-live for synthetic responses, in seconds. | `0` |
+| `prefix` | Hostname label prefix identifying synthetic queries. A trailing dash is added automatically. | `ip` |
+
+### Examples
+
+**Forward lookups** for two IPv6 prefixes and one IPv4 prefix:
 
 ```
-2001:db8:abcd::/48 {
+example.com {
+    synthetic {
+        net 2001:db8:abcd::/48
+        net 2001:db8:1234::/48
+        net 192.0.2.0/24
+        prefix ip
+        ttl 300
+    }
+    file db.example.com
+}
+```
+
+**Reverse lookups** for an IPv6 prefix, with zone file records taking priority:
+
+```
+d.c.b.a.8.b.d.0.1.0.0.2.ip6.arpa {
     synthetic {
         forward example.com
+        prefix ip
     }
     file d.c.b.a.8.b.d.0.1.0.0.2.ip6.arpa
 }
 ```
 
-Forward Lookup Example 
-
-```
-example.com {
-    synthetic {
-        net 2001:db8:abcd::/64
-        net 2001:db8:1234::/64
-    }
-    file db.example.com
-```
-
-
 ## Compiling into CoreDNS
 
-To compile this with CoreDNS you can follow the [normal procedure](https://coredns.io/manual/plugins/#plugins) for external plugins. 
-This plugin can be used by adding the following to `plugin.cfg`:
+Follow the standard [external plugin procedure](https://coredns.io/manual/plugins/#plugins). Add the following line to `plugin.cfg` in the CoreDNS source tree:
+
 ```
 synthetic:code.fritzlab.net/dns/synthetic
 ```
 
+Then rebuild CoreDNS:
+
+```sh
+go generate
+go build
+```
+
 ## FAQ
 
-### Why not use templates?
+### Why not use the `template` plugin?
 
-1- It appears that the `template` plugin is the recommended pattern for providing the resolution pattern we're after here.
-However, it's not possible to have the `file` plugin provide the primary source of data and use a `template` at the same time. 
-See [this](https://github.com/coredns/coredns/issues/2977#issuecomment-555938144) GitHub comment. 
-Thus, it's not possible to have a PTR response from a file take priority over a template. 
+Two reasons:
 
-2-  Using regex in a template for IPv4 and IPv6 addresses is very challanging with CIDR notation.
-This plugin provides an easier experience by just providing an IP prefix in CIDR notation.  
+1. The `template` plugin cannot coexist with `file` for the same zone in a way that lets file records take priority. See this [upstream discussion](https://github.com/coredns/coredns/issues/2977#issuecomment-555938144). The `synthetic` plugin is designed to work alongside `file`, deferring to it for static records.
 
-## Development 
+2. Writing regex patterns to match arbitrary IPv4 and IPv6 addresses within CIDR ranges is impractical. This plugin accepts CIDR notation directly.
 
-Standard Go development practices apply.
+## Development
 
-### Tests
+```sh
+# Run tests
+go test -v ./...
 
+# Lint
+go vet ./...
 ```
-$ go test -v
-```
+
+## License
+
+[Apache License 2.0](LICENSE)

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
+}

synthetic.go

@@ -1,183 +1,239 @@
+// Package synthetic implements a CoreDNS plugin that generates DNS records
+// from IP addresses embedded in hostnames, inspired by dnsmasq's "synth-domain"
+// option. It provides automatic alignment between forward (A/AAAA) and reverse
+// (PTR) lookups, eliminating a common class of DNS misconfigurations described
+// in RFC 1912 Section 2.1.
+//
+// Forward lookups extract an IP address from a prefixed hostname
+// (e.g., ip-192-0-2-1.example.com -> 192.0.2.1) and respond only if the
+// address falls within a configured network. Reverse lookups generate a
+// synthetic PTR record, but defer to the next plugin in the chain when it
+// returns a successful answer, allowing static zone entries to take precedence.
 package synthetic
 
 import (
 	"context"
 	"encoding/hex"
+	"net"
+	"strings"
+
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/plugin/pkg/dnstest"
 	clog "github.com/coredns/coredns/plugin/pkg/log"
 	"github.com/coredns/coredns/plugin/test"
 	"github.com/coredns/coredns/request"
 	"github.com/miekg/dns"
-	"net"
-	"strings"
 )
 
 var log = clog.NewWithPlugin("synthetic")
 
-func (s synthetic) Name() string { return "synthetic" }
-
+// synthetic is a CoreDNS plugin that resolves DNS queries for hostnames
+// containing embedded IP addresses.
 type synthetic struct {
 	Next   plugin.Handler
 	Config syntheticConfig
 }
 
-func (s synthetic) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+// Name returns the name of the plugin as registered with CoreDNS.
+func (s synthetic) Name() string { return "synthetic" }
 
-	// Create a new state for this request. This is used to store state and allows us to pass this
+// ServeDNS implements the plugin.Handler interface. It processes A, AAAA, and
+// PTR queries, generating synthetic responses for IP-embedded hostnames while
+// deferring all other query types to the next plugin in the chain.
+func (s synthetic) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
 	state := request.Request{W: w, Req: r}
 	log.Debug("Received request for ", state.QName(), " of type ", state.QType())
 
-	//
-	// FOR FORWARD LOOKUPS
-	// 		we always respond directly (no need to continue the plugin chain)
-	//		need valid networks to check against
-	//
+	// Forward lookups (A/AAAA): respond directly when the hostname embeds
+	// a valid IP within a configured network. No chain continuation needed.
 	if state.QType() == dns.TypeA || state.QType() == dns.TypeAAAA {
-		if strings.HasPrefix(state.Name(), s.Config.prefix) {
-			log.Debug("Possible synthetic response for:", state.QName())
-
-			// pull out the ip address
-			ipStr := strings.TrimPrefix(strings.Split(state.Name(), ".")[0], s.Config.prefix)
-			ip := net.ParseIP(strings.ReplaceAll(ipStr, "-", "."))
-			if ip == nil {
-				ip = net.ParseIP(strings.ReplaceAll(ipStr, "-", ":"))
-			}
-
-			// respond according to the IP type and the request type
-			if ip != nil {
-				log.Debug("Valid IP from hostname:", ip)
-
-				// check if ip is within the synthetic network
-				var found bool
-				for _, n := range s.Config.net {
-					if n.Contains(ip) {
-						found = true
-						break
-					}
-				}
-				// don't continue if the IP is not in the synthetic network (fall back to recorded response)
-				if found {
-					log.Debug("IP ", ip, " is in synthetic network")
-
-					if ip.To4() == nil && state.QType() == dns.TypeAAAA {
-						log.Debug("Responding to AAAA request for ", state.QName())
-						m := new(dns.Msg)
-						m.SetReply(r)
-						hdr := dns.RR_Header{Name: state.QName(), Rrtype: state.QType(), Class: state.QClass(), Ttl: s.Config.ttl}
-						m.Answer = append(m.Answer, &dns.AAAA{Hdr: hdr, AAAA: ip.To16()})
-						w.WriteMsg(m)
-						return dns.RcodeSuccess, nil
-					}
-					if ip.To4() != nil && state.QType() == dns.TypeA {
-						log.Debug("Responding to A request for ", state.QName())
-						m := new(dns.Msg)
-						m.SetReply(r)
-						hdr := dns.RR_Header{Name: state.QName(), Rrtype: state.QType(), Class: state.QClass(), Ttl: s.Config.ttl}
-						m.Answer = append(m.Answer, &dns.A{Hdr: hdr, A: ip.To4()})
-						w.WriteMsg(m)
-						return dns.RcodeSuccess, nil
-					}
-					if ip.To4() == nil && state.QType() == dns.TypeA {
-						log.Debug("Responding to A request for ", state.QName(), " with empty answer")
-						m := new(dns.Msg)
-						m.SetReply(r)
-						w.WriteMsg(m)
-						return dns.RcodeSuccess, nil
-					}
-					if ip.To4() != nil && state.QType() == dns.TypeAAAA {
-						log.Debug("Responding to AAAA request for ", state.QName(), " with empty answer")
-						m := new(dns.Msg)
-						m.SetReply(r)
-						w.WriteMsg(m)
-						return dns.RcodeSuccess, nil
-					}
-					log.Debug("Unexpected code path for: ", state.QName())
-				}
-				log.Debug("IP not in a valid network: ", ip)
-			}
-			log.Debug("Invalid IP from hostname: ", state.QName())
+		if rcode, handled := s.handleForward(state, w, r); handled {
+			return rcode, nil
 		}
 	}
 
-	//
-	// FOR REVERSE LOOKUPS
-	// 		must check next plugin in chain to see if it's a success (static reverse lookups override synthetic)
-	//		no need to check valid networks (guaranteed to be in the synthetic network based on the coredns config)
-	//
-
-	// Continue to the next plugin in the chain, and record the response.
+	// For all other query types (including PTR), invoke the next plugin first.
+	// A successful response from the chain takes priority, ensuring that
+	// static zone entries always win over synthetic records.
 	rec := dnstest.NewRecorder(&test.ResponseWriter{})
 	rc, err := plugin.NextOrFailure(s.Name(), s.Next, ctx, rec, r)
 
-	// If the next plugin in the chain's recorded response is success, we go with that.
 	if rc == dns.RcodeSuccess && len(rec.Msg.Answer) > 0 {
-		log.Debug("Next Plugin's answers are acceptable. no synthetic response")
+		log.Debug("Next plugin returned authoritative answer; skipping synthetic response")
 		w.WriteMsg(rec.Msg)
 		return rc, err
 	}
 
+	// Reverse lookups (PTR): generate a synthetic hostname from the queried
+	// address when the chain did not provide a definitive answer.
 	if state.QType() == dns.TypePTR {
-		log.Debug("Attempting to inject synthetic response for reverse lookup: ", state.QName())
-
-		ip := inArpaToIp(state.QName())
-		log.Debug("Parsed IP: ", ip)
-
-		if ip != nil {
-			forward := ipToDomainName(s.Config.prefix, ip, s.Config.forward)
-			log.Debug("Responding to PTR request for ", state.QName(), " with ", forward)
-			m := new(dns.Msg)
-			m.SetReply(r)
-			hdr := dns.RR_Header{Name: state.QName(), Rrtype: state.QType(), Class: state.QClass(), Ttl: s.Config.ttl}
-			m.Answer = append(m.Answer, &dns.PTR{Hdr: hdr, Ptr: forward})
-			w.WriteMsg(m)
-			return dns.RcodeSuccess, nil
+		if rcode, handled := s.handleReverse(state, w, r); handled {
+			return rcode, nil
 		}
 	}
 
-	// if we got here, we couldn't handle the request (fall back to recorded response)
-	log.Debug("synthetic plugin not needed for ", state.QName(), " of type ", state.QType())
+	// Fall back to whatever the chain produced.
+	log.Debug("Synthetic plugin not applicable for ", state.QName(), " of type ", state.QType())
 	w.WriteMsg(rec.Msg)
 	return rc, err
 }
 
-func inArpaToIp(name string) net.IP {
-	ipv4Suffix := ".in-addr.arpa."
-	ipv6Suffix := ".ip6.arpa."
+// handleForward attempts to resolve an A or AAAA query from an IP-embedded
+// hostname. It returns the DNS rcode and true if the request was handled, or
+// (0, false) if the request should fall through to the plugin chain.
+func (s synthetic) handleForward(state request.Request, w dns.ResponseWriter, r *dns.Msg) (int, bool) {
+	if !strings.HasPrefix(state.Name(), s.Config.prefix) {
+		return 0, false
+	}
 
+	log.Debug("Possible synthetic response for: ", state.QName())
+
+	// Extract the IP address from the hostname label. IPv4 separators are
+	// dots converted to dashes; IPv6 separators are colons converted to dashes.
+	label := strings.Split(state.Name(), ".")[0]
+	ipStr := strings.TrimPrefix(label, s.Config.prefix)
+
+	ip := net.ParseIP(strings.ReplaceAll(ipStr, "-", "."))
+	if ip == nil {
+		ip = net.ParseIP(strings.ReplaceAll(ipStr, "-", ":"))
+	}
+	if ip == nil {
+		log.Debug("Invalid IP from hostname: ", state.QName())
+		return 0, false
+	}
+
+	log.Debug("Valid IP from hostname: ", ip)
+
+	// Verify the parsed IP falls within at least one configured network.
+	if !s.containsIP(ip) {
+		log.Debug("IP not in a configured network: ", ip)
+		return 0, false
+	}
+
+	log.Debug("IP ", ip, " is in synthetic network")
+
+	// Build a response appropriate to the query type and address family.
+	isV4 := ip.To4() != nil
+	switch {
+	case !isV4 && state.QType() == dns.TypeAAAA:
+		return s.writeAnswer(state, w, r, &dns.AAAA{
+			Hdr:  s.rrHeader(state),
+			AAAA: ip.To16(),
+		}), true
+
+	case isV4 && state.QType() == dns.TypeA:
+		return s.writeAnswer(state, w, r, &dns.A{
+			Hdr: s.rrHeader(state),
+			A:   ip.To4(),
+		}), true
+
+	default:
+		// Type mismatch (e.g., AAAA query for an IPv4 address). Return an
+		// empty success response to prevent further lookups.
+		log.Debug("Type mismatch for ", state.QName(), "; returning empty answer")
+		return s.writeEmpty(w, r), true
+	}
+}
+
+// handleReverse attempts to resolve a PTR query by constructing a synthetic
+// forward hostname from the queried reverse address. It returns the DNS rcode
+// and true if the request was handled.
+func (s synthetic) handleReverse(state request.Request, w dns.ResponseWriter, r *dns.Msg) (int, bool) {
+	log.Debug("Attempting synthetic reverse response for: ", state.QName())
+
+	ip := inArpaToIP(state.QName())
+	if ip == nil {
+		return 0, false
+	}
+
+	forward := ipToDomainName(s.Config.prefix, ip, s.Config.forward)
+	log.Debug("Responding to PTR request for ", state.QName(), " with ", forward)
+
+	return s.writeAnswer(state, w, r, &dns.PTR{
+		Hdr: s.rrHeader(state),
+		Ptr: forward,
+	}), true
+}
+
+// containsIP reports whether ip falls within any of the configured networks.
+func (s synthetic) containsIP(ip net.IP) bool {
+	for _, n := range s.Config.net {
+		if n.Contains(ip) {
+			return true
+		}
+	}
+	return false
+}
+
+// rrHeader constructs a dns.RR_Header from the current request state and the
+// plugin's configured TTL.
+func (s synthetic) rrHeader(state request.Request) dns.RR_Header {
+	return dns.RR_Header{
+		Name:   state.QName(),
+		Rrtype: state.QType(),
+		Class:  state.QClass(),
+		Ttl:    s.Config.ttl,
+	}
+}
+
+// writeAnswer writes a single-record DNS response to the client.
+func (s synthetic) writeAnswer(state request.Request, w dns.ResponseWriter, r *dns.Msg, rr dns.RR) int {
+	m := new(dns.Msg)
+	m.SetReply(r)
+	m.Answer = append(m.Answer, rr)
+	w.WriteMsg(m)
+	return dns.RcodeSuccess
+}
+
+// writeEmpty writes an empty success response (NOERROR with no answer section).
+func (s synthetic) writeEmpty(w dns.ResponseWriter, r *dns.Msg) int {
+	m := new(dns.Msg)
+	m.SetReply(r)
+	w.WriteMsg(m)
+	return dns.RcodeSuccess
+}
+
+// inArpaToIP converts a reverse DNS name (in-addr.arpa or ip6.arpa) to the
+// corresponding net.IP. It returns nil if the name cannot be parsed.
+func inArpaToIP(name string) net.IP {
+	const (
+		ipv4Suffix = ".in-addr.arpa."
+		ipv6Suffix = ".ip6.arpa."
+	)
+
+	// IPv4 reverse: four dot-separated octets in reverse order.
 	if idx := strings.Index(name, ipv4Suffix); idx > 6 {
-		name = name[:idx]
-		parts := strings.Split(name, ".")
+		parts := strings.Split(name[:idx], ".")
 		if len(parts) != 4 {
 			return nil
 		}
-
-		name = parts[3] + "." + parts[2] + "." + parts[1] + "." + parts[0]
-		return net.ParseIP(name)
+		reversed := parts[3] + "." + parts[2] + "." + parts[1] + "." + parts[0]
+		return net.ParseIP(reversed)
 	}
 
+	// IPv6 reverse: exactly 73 characters — 32 hex nibbles separated by dots
+	// in reverse order, followed by ".ip6.arpa.".
 	if len(name) == 73 && name[63:] == ipv6Suffix {
-		// we can rely on the fact that v6 reverse hostnames have a fixed length
-
-		// read the characters from the hostname into a buffer in reverse
-		v6chars := make([]byte, 32)
+		nibbles := make([]byte, 32)
 		for i, j := 62, 0; i >= 0; i -= 2 {
-			v6chars[j] = name[i]
+			nibbles[j] = name[i]
 			j++
 		}
 
-		// decode the characters in the buffer into 16 bytes and return it
-		v6bytes := make([]byte, 16)
-		if _, err := hex.Decode(v6bytes, v6chars); err != nil {
+		addr := make([]byte, 16)
+		if _, err := hex.Decode(addr, nibbles); err != nil {
 			return nil
 		}
-
-		return v6bytes
+		return net.IP(addr)
 	}
 
 	return nil
 }
 
+// ipToDomainName converts an IP address to a synthetic forward hostname by
+// joining the address octets/groups with dashes and appending the zone suffix.
+// For example, 192.0.2.1 with prefix "ip-" and zone "example.com" becomes
+// "ip-192-0-2-1.example.com.".
 func ipToDomainName(prefix string, ip net.IP, zone string) string {
 	if ip == nil {
 		return ""
@@ -186,7 +242,6 @@ func ipToDomainName(prefix string, ip net.IP, zone string) string {
 	if !strings.HasPrefix(zone, ".") {
 		zone = "." + zone
 	}
-
 	if !strings.HasSuffix(zone, ".") {
 		zone = zone + "."
 	}
@@ -196,6 +251,5 @@ func ipToDomainName(prefix string, ip net.IP, zone string) string {
 		sep = "."
 	}
 
-	response := strings.Join(strings.Split(ip.String(), sep), "-")
-	return prefix + response + zone
+	return prefix + strings.Join(strings.Split(ip.String(), sep), "-") + zone
 }

synthetic_test.go

@@ -2,378 +2,295 @@ package synthetic
 
 import (
 	"context"
-	"fmt"
+	"net"
+	"testing"
+
 	"github.com/coredns/coredns/plugin/pkg/dnstest"
 	"github.com/coredns/coredns/plugin/test"
 	"github.com/miekg/dns"
-	"net"
-
-	"testing"
 )
 
-func TestServeDNSv4(t *testing.T) {
-	ip, ipNet, err := net.ParseCIDR("192.0.2.0/24")
-	if err != nil {
-		log.Fatal(err)
+// testCase describes a single DNS query and the expected response.
+type testCase struct {
+	name       string
+	qname      string
+	qtype      uint16
+	wantRcode  int
+	wantAnswer []string
+	wantTTL    uint32
+}
+
+// assertResponse validates the DNS response from ServeDNS against tc.
+func assertResponse(t *testing.T, tc testCase, rc int, w *dnstest.Recorder) {
+	t.Helper()
+
+	if rc != tc.wantRcode {
+		t.Errorf("rcode: got %d, want %d", rc, tc.wantRcode)
 	}
 
-	s := synthetic{
-		Next: test.ErrorHandler(),
-		Config: syntheticConfig{
-			net:     []*net.IPNet{{IP: ip, Mask: ipNet.Mask}},
-			forward: "example.com",
-			prefix:  "ip-",
-		},
+	if tc.wantAnswer == nil {
+		if len(w.Msg.Answer) > 0 {
+			t.Errorf("answer: got %v, want none", w.Msg.Answer[0])
+		}
+		return
 	}
 
-	testCases := []struct {
-		qname      string
-		qtype      uint16
-		wantrcode  int
-		wantAnswer []string
-		wantTTL    uint32
-	}{
-		{
-			qname:      "ip-192-0-2-0.example.com",
-			qtype:      dns.TypeA,
-			wantrcode:  dns.RcodeSuccess,
-			wantAnswer: []string{"192.0.2.0"},
-			wantTTL:    0,
-		},
-		{
-			qname:      "ip-192-0-2-255.example.com",
-			qtype:      dns.TypeA,
-			wantrcode:  dns.RcodeSuccess,
-			wantAnswer: []string{"192.0.2.255"},
-			wantTTL:    0,
-		},
-		{
-			qname:      "ip-203-0-113-100.example.com",
-			qtype:      dns.TypeA,
-			wantrcode:  dns.RcodeServerFailure,
-			wantAnswer: nil,
-			wantTTL:    0,
-		},
-		{
-			qname:      "123.2.0.192.in-addr.arpa.",
-			qtype:      dns.TypePTR,
-			wantrcode:  dns.RcodeSuccess,
-			wantAnswer: []string{"ip-192-0-2-123.example.com."},
-			wantTTL:    0,
-		},
-		{
-			qname:      "ip-2001-db8-abcd--.example.com",
-			qtype:      dns.TypeA,
-			wantrcode:  dns.RcodeServerFailure,
-			wantAnswer: nil,
-			wantTTL:    0,
-		},
+	if len(w.Msg.Answer) == 0 {
+		t.Fatal("answer: got none, want an answer")
 	}
 
-	for i, tc := range testCases {
-		errorMsgPrefix := fmt.Sprintf("Test case %v for '%v' failed. Expected", i, tc.qname)
-		ctx := context.TODO()
-		w := dnstest.NewRecorder(&test.ResponseWriter{})
-		r := new(dns.Msg)
-		r.SetQuestion(tc.qname, tc.qtype)
+	hdr := w.Msg.Answer[0].Header()
+	if hdr.Name != tc.qname {
+		t.Errorf("name: got %s, want %s", hdr.Name, tc.qname)
+	}
+	if hdr.Rrtype != tc.qtype {
+		t.Errorf("rrtype: got %d, want %d", hdr.Rrtype, tc.qtype)
+	}
+	if hdr.Ttl != tc.wantTTL {
+		t.Errorf("ttl: got %d, want %d", hdr.Ttl, tc.wantTTL)
+	}
 
-		rc, err := s.ServeDNS(ctx, w, r)
-
-		if err != nil {
-			t.Errorf("%v no error, but got %v", errorMsgPrefix, err)
+	switch rr := w.Msg.Answer[0].(type) {
+	case *dns.A:
+		if rr.A.String() != tc.wantAnswer[0] {
+			t.Errorf("A record: got %s, want %s", rr.A, tc.wantAnswer[0])
 		}
-		if rc != tc.wantrcode {
-			t.Errorf("%v rcode %v, but got %v", errorMsgPrefix, tc.wantrcode, rc)
+	case *dns.AAAA:
+		if rr.AAAA.String() != tc.wantAnswer[0] {
+			t.Errorf("AAAA record: got %s, want %s", rr.AAAA, tc.wantAnswer[0])
 		}
-		if len(w.Msg.Answer) == 0 && tc.wantAnswer != nil {
-			t.Errorf("%v an answer, but got none", errorMsgPrefix)
-			continue
-		}
-		if tc.wantAnswer == nil {
-			if len(w.Msg.Answer) > 0 {
-				t.Errorf("%v no answer, but got %v", errorMsgPrefix, w.Msg.Answer[0])
-			}
-			continue
-		}
-		if w.Msg.Answer[0].Header().Ttl != 0 {
-			t.Errorf("%v TTL to be 0, but got %v", errorMsgPrefix, w.Msg.Answer[0].Header().Ttl)
-		}
-		if w.Msg.Answer[0].Header().Name != tc.qname {
-			t.Errorf("%v Name to be %s, but got %s", errorMsgPrefix, tc.qname, w.Msg.Answer[0].Header().Name)
-		}
-		if w.Msg.Answer[0].Header().Rrtype != tc.qtype {
-			t.Errorf("%v Type to be %d, but got %d", errorMsgPrefix, tc.qtype, w.Msg.Answer[0].Header().Rrtype)
-		}
-		if w.Msg.Answer[0].Header().Ttl != tc.wantTTL {
-			t.Errorf("%v TTL to be %d, but got %d", errorMsgPrefix, tc.wantTTL, w.Msg.Answer[0].Header().Ttl)
-		}
-		switch tc.qtype {
-		case dns.TypeA:
-			if w.Msg.Answer[0].(*dns.A).A.String() != tc.wantAnswer[0] {
-				t.Errorf("%v answer %s, but got %s", errorMsgPrefix, tc.wantAnswer[0], w.Msg.Answer[0].(*dns.A).A.String())
-			}
-		case dns.TypePTR:
-			if w.Msg.Answer[0].(*dns.PTR).Ptr != tc.wantAnswer[0] {
-				t.Errorf("%v answer %s, but got %s", errorMsgPrefix, tc.wantAnswer[0], w.Msg.Answer[0].(*dns.PTR).Ptr)
-			}
+	case *dns.PTR:
+		if rr.Ptr != tc.wantAnswer[0] {
+			t.Errorf("PTR record: got %s, want %s", rr.Ptr, tc.wantAnswer[0])
 		}
 	}
 }
 
-func TestServeDNSv6(t *testing.T) {
-	ip, ipNet, err := net.ParseCIDR("2001:db8:abcd::/48")
+// runTestCases executes a table of test cases against the given synthetic plugin.
+func runTestCases(t *testing.T, s synthetic, cases []testCase) {
+	t.Helper()
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			w := dnstest.NewRecorder(&test.ResponseWriter{})
+			r := new(dns.Msg)
+			r.SetQuestion(tc.qname, tc.qtype)
+
+			rc, err := s.ServeDNS(context.TODO(), w, r)
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+
+			assertResponse(t, tc, rc, w)
+		})
+	}
+}
+
+func TestServeDNSv4(t *testing.T) {
+	_, ipNet, err := net.ParseCIDR("192.0.2.0/24")
 	if err != nil {
-		log.Fatal(err)
+		t.Fatal(err)
+	}
+
+	s := synthetic{
+		Next:   test.ErrorHandler(),
+		Config: syntheticConfig{net: []*net.IPNet{ipNet}, forward: "example.com", prefix: "ip-"},
+	}
+
+	runTestCases(t, s, []testCase{
+		{
+			name:       "v4 first address",
+			qname:      "ip-192-0-2-0.example.com",
+			qtype:      dns.TypeA,
+			wantRcode:  dns.RcodeSuccess,
+			wantAnswer: []string{"192.0.2.0"},
+		},
+		{
+			name:       "v4 last address",
+			qname:      "ip-192-0-2-255.example.com",
+			qtype:      dns.TypeA,
+			wantRcode:  dns.RcodeSuccess,
+			wantAnswer: []string{"192.0.2.255"},
+		},
+		{
+			name:      "v4 out of range",
+			qname:     "ip-203-0-113-100.example.com",
+			qtype:     dns.TypeA,
+			wantRcode: dns.RcodeServerFailure,
+		},
+		{
+			name:       "v4 PTR reverse",
+			qname:      "123.2.0.192.in-addr.arpa.",
+			qtype:      dns.TypePTR,
+			wantRcode:  dns.RcodeSuccess,
+			wantAnswer: []string{"ip-192-0-2-123.example.com."},
+		},
+		{
+			name:      "v6 address with v4 config",
+			qname:     "ip-2001-db8-abcd--.example.com",
+			qtype:     dns.TypeA,
+			wantRcode: dns.RcodeServerFailure,
+		},
+	})
+}
+
+func TestServeDNSv6(t *testing.T) {
+	_, ipNet, err := net.ParseCIDR("2001:db8:abcd::/48")
+	if err != nil {
+		t.Fatal(err)
 	}
 
 	s := synthetic{
 		Next: test.ErrorHandler(),
 		Config: syntheticConfig{
-			net:     []*net.IPNet{{IP: ip, Mask: ipNet.Mask}},
+			net:     []*net.IPNet{ipNet},
 			forward: "example.com.",
 			ttl:     1800,
 			prefix:  "ip6-",
 		},
 	}
 
-	testCases := []struct {
-		qname      string
-		qtype      uint16
-		wantrcode  int
-		wantAnswer []string
-		wantTTL    uint32
-	}{
+	runTestCases(t, s, []testCase{
 		{
+			name:       "v6 zero-padded",
 			qname:      "ip6-2001-db8-abcd--.example.com",
 			qtype:      dns.TypeAAAA,
-			wantrcode:  dns.RcodeSuccess,
+			wantRcode:  dns.RcodeSuccess,
 			wantAnswer: []string{"2001:db8:abcd::"},
 			wantTTL:    1800,
 		},
 		{
+			name:       "v6 fully expanded",
 			qname:      "ip6-2001-db8-abcd-1234-4567-890a-bcde-f123.example.com",
 			qtype:      dns.TypeAAAA,
-			wantrcode:  dns.RcodeSuccess,
+			wantRcode:  dns.RcodeSuccess,
 			wantAnswer: []string{"2001:db8:abcd:1234:4567:890a:bcde:f123"},
 			wantTTL:    1800,
 		},
 		{
-			qname:      "ip6-2001-db8-1234--.example.com",
-			qtype:      dns.TypeAAAA,
-			wantrcode:  dns.RcodeServerFailure,
-			wantAnswer: nil,
-			wantTTL:    1800,
+			name:      "v6 out of range",
+			qname:     "ip6-2001-db8-1234--.example.com",
+			qtype:     dns.TypeAAAA,
+			wantRcode: dns.RcodeServerFailure,
 		},
 		{
+			name:       "v6 PTR reverse",
 			qname:      "3.2.1.f.e.d.c.b.a.0.9.8.7.6.5.4.4.3.2.1.d.c.b.a.8.b.d.0.1.0.0.2.ip6.arpa.",
 			qtype:      dns.TypePTR,
-			wantrcode:  dns.RcodeSuccess,
+			wantRcode:  dns.RcodeSuccess,
 			wantAnswer: []string{"ip6-2001-db8-abcd-1234-4567-890a-bcde-f123.example.com."},
 			wantTTL:    1800,
 		},
 		{
-			qname:      "ip6-192-0-2-0.example.com",
-			qtype:      dns.TypeAAAA,
-			wantrcode:  dns.RcodeServerFailure,
-			wantAnswer: nil,
-			wantTTL:    0,
+			name:      "v4 address with v6 config",
+			qname:     "ip6-192-0-2-0.example.com",
+			qtype:     dns.TypeAAAA,
+			wantRcode: dns.RcodeServerFailure,
 		},
-	}
-
-	for i, tc := range testCases {
-		errorMsgPrefix := fmt.Sprintf("Test case %v for '%v' failed. Expected", i, tc.qname)
-		ctx := context.TODO()
-		w := dnstest.NewRecorder(&test.ResponseWriter{})
-		r := new(dns.Msg)
-		r.SetQuestion(tc.qname, tc.qtype)
-
-		rc, err := s.ServeDNS(ctx, w, r)
-
-		if err != nil {
-			t.Errorf("%v no error, but got %v", errorMsgPrefix, err)
-		}
-		if rc != tc.wantrcode {
-			t.Errorf("%v rcode %v, but got %v", errorMsgPrefix, tc.wantrcode, rc)
-		}
-		if len(w.Msg.Answer) == 0 && tc.wantAnswer != nil {
-			t.Errorf("%v an answer, but got none", errorMsgPrefix)
-			continue
-		}
-		if tc.wantAnswer == nil {
-			if len(w.Msg.Answer) > 0 {
-				t.Errorf("%v no answer, but got %v", errorMsgPrefix, w.Msg.Answer[0])
-			}
-			continue
-		}
-		if w.Msg.Answer[0].Header().Name != tc.qname {
-			t.Errorf("%v Name to be %s, but got %s", errorMsgPrefix, tc.qname, w.Msg.Answer[0].Header().Name)
-		}
-		if w.Msg.Answer[0].Header().Rrtype != tc.qtype {
-			t.Errorf("%v Type to be %d, but got %d", errorMsgPrefix, tc.qtype, w.Msg.Answer[0].Header().Rrtype)
-		}
-		if w.Msg.Answer[0].Header().Ttl != tc.wantTTL {
-			t.Errorf("%v TTL to be %d, but got %d", errorMsgPrefix, tc.wantTTL, w.Msg.Answer[0].Header().Ttl)
-		}
-		switch tc.qtype {
-		case dns.TypeA:
-			if w.Msg.Answer[0].(*dns.A).A.String() != tc.wantAnswer[0] {
-				t.Errorf("%v answer %s, but got %s", errorMsgPrefix, tc.wantAnswer[0], w.Msg.Answer[0].(*dns.A).A.String())
-			}
-		case dns.TypePTR:
-			if w.Msg.Answer[0].(*dns.PTR).Ptr != tc.wantAnswer[0] {
-				t.Errorf("%v answer %s, but got %s", errorMsgPrefix, tc.wantAnswer[0], w.Msg.Answer[0].(*dns.PTR).Ptr)
-			}
-		}
-	}
+	})
 }
 
-// MockSuccessPlugin is a mock plugin that always responds with a successful DNS response.
-type MockSuccessPlugin struct{}
+// mockSuccessPlugin always returns a successful answer, simulating a static
+// zone file or upstream resolver that provides authoritative records.
+type mockSuccessPlugin struct{}
 
-// Name returns the plugin name.
-func (m MockSuccessPlugin) Name() string { return "mock" }
+func (m mockSuccessPlugin) Name() string { return "mock" }
+
+func (m mockSuccessPlugin) ServeDNS(_ context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	msg := new(dns.Msg)
+	msg.SetReply(r)
+	hdr := dns.RR_Header{
+		Name:   r.Question[0].Name,
+		Rrtype: r.Question[0].Qtype,
+		Class:  r.Question[0].Qclass,
+	}
 
-// ServeDNS simulates a successful DNS response.
-func (m MockSuccessPlugin) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
-	m1 := new(dns.Msg)
-	m1.SetReply(r)
-	hdr := dns.RR_Header{Name: r.Question[0].Name, Rrtype: r.Question[0].Qtype, Class: r.Question[0].Qclass, Ttl: 0}
 	switch r.Question[0].Qtype {
 	case dns.TypeA:
-		m1.Answer = append(m1.Answer, &dns.A{Hdr: hdr, A: net.ParseIP("192.0.2.100")})
+		msg.Answer = append(msg.Answer, &dns.A{Hdr: hdr, A: net.ParseIP("192.0.2.100")})
 	case dns.TypeAAAA:
-		m1.Answer = append(m1.Answer, &dns.AAAA{Hdr: hdr, AAAA: net.ParseIP("2001:db8::100")})
+		msg.Answer = append(msg.Answer, &dns.AAAA{Hdr: hdr, AAAA: net.ParseIP("2001:db8::100")})
 	}
-	w.WriteMsg(m1)
+
+	w.WriteMsg(msg)
 	return dns.RcodeSuccess, nil
 }
 
-func TestServeDNSNextPluginRespondsSuccess(t *testing.T) {
-	ip, ipNet, err := net.ParseCIDR("192.0.2.0/24")
+// TestServeDNSNextPluginPrecedence verifies that answers from the next plugin
+// in the chain take priority over synthetic responses for reverse lookups,
+// while forward lookups still produce synthetic answers directly.
+func TestServeDNSNextPluginPrecedence(t *testing.T) {
+	_, ipNet4, err := net.ParseCIDR("192.0.2.0/24")
 	if err != nil {
-		log.Fatal(err)
+		t.Fatal(err)
 	}
-	ip6, ipNet6, err := net.ParseCIDR("2001:db8:abcd::/48")
+	_, ipNet6, err := net.ParseCIDR("2001:db8:abcd::/48")
 	if err != nil {
-		log.Fatal(err)
+		t.Fatal(err)
 	}
 
 	s := synthetic{
-		Next: MockSuccessPlugin{},
+		Next: mockSuccessPlugin{},
 		Config: syntheticConfig{
-			net:     []*net.IPNet{{IP: ip, Mask: ipNet.Mask}, {IP: ip6, Mask: ipNet6.Mask}},
+			net:     []*net.IPNet{ipNet4, ipNet6},
 			forward: "example.com",
 			prefix:  "ip-",
 		},
 	}
 
-	testCases := []struct {
-		qname      string
-		qtype      uint16
-		wantrcode  int
-		wantAnswer []string
-		wantTTL    uint32
-	}{
+	runTestCases(t, s, []testCase{
 		{
+			name:       "forward v4 synthetic wins",
 			qname:      "ip-192-0-2-1.example.com",
 			qtype:      dns.TypeA,
-			wantrcode:  dns.RcodeSuccess,
+			wantRcode:  dns.RcodeSuccess,
 			wantAnswer: []string{"192.0.2.1"},
-			wantTTL:    0,
 		},
 		{
+			name:       "forward non-synthetic uses next plugin",
 			qname:      "foobar.example.com",
 			qtype:      dns.TypeA,
-			wantrcode:  dns.RcodeSuccess,
+			wantRcode:  dns.RcodeSuccess,
 			wantAnswer: []string{"192.0.2.100"},
-			wantTTL:    0,
 		},
 		{
+			name:       "forward v6 synthetic wins",
 			qname:      "ip-2001-db8-abcd--1.example.com",
 			qtype:      dns.TypeAAAA,
-			wantrcode:  dns.RcodeSuccess,
+			wantRcode:  dns.RcodeSuccess,
 			wantAnswer: []string{"2001:db8:abcd::1"},
-			wantTTL:    0,
 		},
 		{
+			name:       "forward non-synthetic AAAA uses next plugin",
 			qname:      "foobar.example.com",
 			qtype:      dns.TypeAAAA,
-			wantrcode:  dns.RcodeSuccess,
-			wantAnswer: []string{"2001:db8:abcd::100"},
-			wantTTL:    0,
+			wantRcode:  dns.RcodeSuccess,
+			wantAnswer: []string{"2001:db8::100"},
 		},
 		{
-			qname:      "ip-2001-db8-abcd--1.example.com",
-			qtype:      dns.TypeA,
-			wantrcode:  dns.RcodeSuccess,
-			wantAnswer: nil,
-			wantTTL:    0,
+			name:      "v6 address queried as A returns empty",
+			qname:     "ip-2001-db8-abcd--1.example.com",
+			qtype:     dns.TypeA,
+			wantRcode: dns.RcodeSuccess,
 		},
 		{
-			qname:      "ip-192-0-2-1.example.com",
-			qtype:      dns.TypeAAAA,
-			wantrcode:  dns.RcodeSuccess,
-			wantAnswer: nil,
-			wantTTL:    0,
+			name:      "v4 address queried as AAAA returns empty",
+			qname:     "ip-192-0-2-1.example.com",
+			qtype:     dns.TypeAAAA,
+			wantRcode: dns.RcodeSuccess,
 		},
 		{
+			name:       "PTR v6 synthetic (mock has no PTR)",
 			qname:      "3.2.1.f.e.d.c.b.a.0.9.8.7.6.5.4.4.3.2.1.d.c.b.a.8.b.d.0.1.0.0.2.ip6.arpa.",
 			qtype:      dns.TypePTR,
-			wantrcode:  dns.RcodeSuccess,
-			wantAnswer: []string{"ip6-2001-db8-abcd-1234-4567-890a-bcde-f123.example.com."},
-			wantTTL:    0,
+			wantRcode:  dns.RcodeSuccess,
+			wantAnswer: []string{"ip-2001-db8-abcd-1234-4567-890a-bcde-f123.example.com."},
 		},
 		{
+			name:       "PTR v4 synthetic (mock has no PTR)",
 			qname:      "123.2.0.192.in-addr.arpa.",
 			qtype:      dns.TypePTR,
-			wantrcode:  dns.RcodeSuccess,
+			wantRcode:  dns.RcodeSuccess,
 			wantAnswer: []string{"ip-192-0-2-123.example.com."},
-			wantTTL:    0,
 		},
-	}
-
-	for i, tc := range testCases {
-		errorMsgPrefix := fmt.Sprintf("Test case %v for '%v' failed. Expected", i, tc.qname)
-		ctx := context.TODO()
-		w := dnstest.NewRecorder(&test.ResponseWriter{})
-		r := new(dns.Msg)
-		r.SetQuestion(tc.qname, tc.qtype)
-
-		rc, err := s.ServeDNS(ctx, w, r)
-
-		if err != nil {
-			t.Errorf("%v no error, but got %v", errorMsgPrefix, err)
-		}
-		if rc != tc.wantrcode {
-			t.Errorf("%v rcode %v, but got %v", errorMsgPrefix, tc.wantrcode, rc)
-		}
-		if len(w.Msg.Answer) == 0 && tc.wantAnswer != nil {
-			t.Errorf("%v an answer, but got none", errorMsgPrefix)
-			continue
-		}
-		if tc.wantAnswer == nil {
-			if len(w.Msg.Answer) > 0 {
-				t.Errorf("%v no answer, but got %v", errorMsgPrefix, w.Msg.Answer[0])
-			}
-			continue
-		}
-		if w.Msg.Answer[0].Header().Ttl != 0 {
-			t.Errorf("%v TTL to be 0, but got %v", errorMsgPrefix, w.Msg.Answer[0].Header().Ttl)
-		}
-		if w.Msg.Answer[0].Header().Name != tc.qname {
-			t.Errorf("%v Name to be %s, but got %s", errorMsgPrefix, tc.qname, w.Msg.Answer[0].Header().Name)
-		}
-		if w.Msg.Answer[0].Header().Rrtype != tc.qtype {
-			t.Errorf("%v Type to be %d, but got %d", errorMsgPrefix, tc.qtype, w.Msg.Answer[0].Header().Rrtype)
-		}
-		if w.Msg.Answer[0].Header().Ttl != tc.wantTTL {
-			t.Errorf("%v TTL to be %d, but got %d", errorMsgPrefix, tc.wantTTL, w.Msg.Answer[0].Header().Ttl)
-		}
-		switch tc.qtype {
-		case dns.TypeA:
-			if w.Msg.Answer[0].(*dns.A).A.String() != tc.wantAnswer[0] {
-				t.Errorf("%v answer %s, but got %s", errorMsgPrefix, tc.wantAnswer[0], w.Msg.Answer[0].(*dns.A).A.String())
-			}
-		}
-	}
+	})
 }