fix(resolver): fix IPv4/IPv6 fallback asymmetry, data race, and caller mutation bugs

Normalize Only/Prefer values (ip4→ipv4, ip6→ipv6) so downstream
comparisons match correctly. Fix asymmetric fallback where caller
network=ip6 fell through to IPv4 but ip4 didn't fall through to IPv6
— both branches now check hard constraints (server.Only || callerPref)
only, allowing soft Prefer to fall back. Fix data race on cache item.ts
by reading it inside RLock. Fix Store mutating caller's dns.Msg by
copying before TTL override. Add singleflight dedup, semaphore-bounded
async refresh, DNS rcode error checking, and bump EDNS0 UDPSize to
4096. Add 25 tests.

go.mod

@@ -119,7 +119,7 @@ require (
 	go.uber.org/multierr v1.11.0 // indirect
 	golang.org/x/arch v0.8.0 // indirect
 	golang.org/x/mod v0.34.0 // indirect
-	golang.org/x/sync v0.20.0 // indirect
+	golang.org/x/sync v0.20.0
 	golang.org/x/term v0.42.0 // indirect
 	golang.org/x/tools v0.43.0 // indirect
 	golang.zx2c4.com/wintun v0.0.0-20230126152724-0fa3db229ce2 // indirect

internal/util/resolver/cache.go

@@ -15,9 +15,11 @@ const (
 	defaultTTL = 60 * time.Second
 )
 
+// CacheKey identifies a cached DNS response by question name, class, and type.
 type CacheKey string
 
-// NewCacheKey generates resolver cache key from question of dns query.
+// NewCacheKey generates a resolver cache key from a DNS question.
+// Returns an empty key if q is nil.
 func NewCacheKey(q *dns.Question) CacheKey {
 	if q == nil {
 		return ""
@@ -32,41 +34,62 @@ type cacheItem struct {
 	ttl time.Duration
 }
 
+// Cache stores DNS responses with TTL-based expiration and optional size bounding.
+// A zero maxSize means unbounded (backward compatible).
+//
+// Load returns:
+//   - (nil, 0) on cache miss
+//   - (msg, ttl > 0) on fresh cache hit
+//   - (msg, ttl <= 0) on expired cache hit (for stale-while-revalidate)
 type Cache struct {
-	m      sync.Map
+	mu      sync.RWMutex
+	entries map[CacheKey]*cacheItem
+	maxSize int // 0 = unlimited
 	logger  logger.Logger
 }
 
+// NewCache creates an unbounded DNS cache.
 func NewCache() *Cache {
-	return &Cache{}
+	return &Cache{entries: make(map[CacheKey]*cacheItem)}
 }
 
+// WithLogger sets the logger for cache debug messages.
 func (c *Cache) WithLogger(logger logger.Logger) *Cache {
 	c.logger = logger
 	return c
 }
 
+// WithMaxSize sets the maximum number of entries. 0 means unlimited.
+// When the limit is reached, expired entries are evicted first, then the oldest.
+func (c *Cache) WithMaxSize(n int) *Cache {
+	c.maxSize = n
+	return c
+}
+
+// Load returns a cached DNS message and its remaining TTL.
+// The returned message is a deep copy safe for caller mutation.
 func (c *Cache) Load(ctx context.Context, key CacheKey) (msg *dns.Msg, ttl time.Duration) {
-	v, ok := c.m.Load(key)
-	if !ok {
-		return
-	}
-
-	item, ok := v.(*cacheItem)
-	if !ok {
-		return
-	}
-
+	var elapsed time.Duration
+	c.mu.RLock()
+	item, ok := c.entries[key]
+	if ok {
 		msg = item.msg.Copy()
+		elapsed = time.Since(item.ts)
+		ttl = item.ttl - elapsed
+	}
+	c.mu.RUnlock()
+	if !ok {
+		return
+	}
+
 	for i := range msg.Answer {
-		d := uint32(time.Since(item.ts).Seconds())
+		d := uint32(elapsed.Seconds())
 		if msg.Answer[i].Header().Ttl > d {
 			msg.Answer[i].Header().Ttl -= d
 		} else {
 			msg.Answer[i].Header().Ttl = 1
 		}
 	}
-	ttl = item.ttl - time.Since(item.ts)
 
 	if log := c.logger; log.IsLevelEnabled(logger.DebugLevel) {
 		if sid := ctxvalue.SidFromContext(ctx); sid != "" {
@@ -80,6 +103,9 @@ func (c *Cache) Load(ctx context.Context, key CacheKey) (msg *dns.Msg, ttl time.
 	return
 }
 
+// Store adds a DNS response to the cache. Negative ttl means "do not cache" and is
+// silently skipped. Zero ttl uses the minimum TTL from the answer records (or
+// defaultTTL if none have a positive TTL). Positive ttl overrides all answer TTLs.
 func (c *Cache) Store(ctx context.Context, key CacheKey, mr *dns.Msg, ttl time.Duration) {
 	if key == "" || mr == nil || ttl < 0 {
 		return
@@ -95,17 +121,38 @@ func (c *Cache) Store(ctx context.Context, key CacheKey, mr *dns.Msg, ttl time.D
 		if ttl == 0 {
 			ttl = defaultTTL
 		}
-	} else {
-		for i := range mr.Answer {
-			mr.Answer[i].Header().Ttl = uint32(ttl.Seconds())
+	}
+
+	cp := mr.Copy()
+	if ttl > 0 {
+		for i := range cp.Answer {
+			cp.Answer[i].Header().Ttl = uint32(ttl.Seconds())
 		}
 	}
 
-	c.m.Store(key, &cacheItem{
-		msg: mr.Copy(),
+	c.mu.Lock()
+	// Evict if over capacity.
+	if c.maxSize > 0 {
+		c.cleanupLocked()
+		if len(c.entries) >= c.maxSize {
+			// Evict the oldest entry.
+			var oldestKey CacheKey
+			var oldestTS time.Time
+			for k, item := range c.entries {
+				if oldestKey == "" || item.ts.Before(oldestTS) {
+					oldestKey = k
+					oldestTS = item.ts
+				}
+			}
+			delete(c.entries, oldestKey)
+		}
+	}
+	c.entries[key] = &cacheItem{
+		msg: cp,
 		ts:  time.Now(),
 		ttl: ttl,
-	})
+	}
+	c.mu.Unlock()
 
 	if log := c.logger; log.IsLevelEnabled(logger.DebugLevel) {
 		if sid := ctxvalue.SidFromContext(ctx); sid != "" {
@@ -117,15 +164,22 @@ func (c *Cache) Store(ctx context.Context, key CacheKey, mr *dns.Msg, ttl time.D
 	}
 }
 
+// RefreshTTL resets the timestamp on a cached entry, extending its TTL.
 func (c *Cache) RefreshTTL(key CacheKey) {
-	v, ok := c.m.Load(key)
-	if !ok {
-		return
-	}
-
-	item, ok := v.(*cacheItem)
-	if !ok {
-		return
-	}
+	c.mu.Lock()
+	item, ok := c.entries[key]
+	if ok {
 		item.ts = time.Now()
+	}
+	c.mu.Unlock()
+}
+
+// cleanupLocked removes expired entries. Caller must hold c.mu.Lock.
+func (c *Cache) cleanupLocked() {
+	now := time.Now()
+	for key, item := range c.entries {
+		if now.Sub(item.ts) > item.ttl {
+			delete(c.entries, key)
+		}
+	}
 }

resolver/exchanger/exchanger.go

@@ -204,7 +204,7 @@ func (ex *exchanger) exchange(ctx context.Context, msg []byte) ([]byte, error) {
 	}
 
 	conn := &dns.Conn{
-		UDPSize: 1024,
+		UDPSize: 4096,
 		Conn:    c,
 	}
 

resolver/resolver.go

@@ -4,6 +4,7 @@ package resolver
 
 import (
 	"context"
+	"fmt"
 	"net"
 	"strings"
 	"time"
@@ -15,6 +16,7 @@ import (
 	resolver_util "github.com/go-gost/x/internal/util/resolver"
 	"github.com/go-gost/x/resolver/exchanger"
 	"github.com/miekg/dns"
+	"golang.org/x/sync/singleflight"
 )
 
 // NameServer describes a DNS nameserver configuration including its address,
@@ -54,11 +56,15 @@ func LoggerOption(logger logger.Logger) Option {
 	}
 }
 
+const maxAsyncRefresh = 32 // max concurrent background refresh goroutines
+
 // localResolver resolves hostnames via DNS using configured nameservers and a cache.
 type localResolver struct {
 	servers    []NameServer
 	cache      *resolver_util.Cache
 	options    options
+	refreshSem chan struct{}   // bounds concurrent async refresh goroutines
+	sfGroup    singleflight.Group
 }
 
 // NewResolver creates a Resolver that queries the given nameservers in order,
@@ -94,8 +100,12 @@ func NewResolver(nameservers []NameServer, opts ...Option) (resolver.Resolver, e
 		server.exchanger = ex
 
 		switch server.Only {
-		case "ip4", "ipv4", "ip6", "ipv6":
-			server.Prefer = server.Only
+		case "ip4", "ipv4":
+			server.Only = "ipv4"
+			server.Prefer = "ipv4"
+		case "ip6", "ipv6":
+			server.Only = "ipv6"
+			server.Prefer = "ipv6"
 		default:
 			server.Only = ""
 		}
@@ -111,9 +121,23 @@ func NewResolver(nameservers []NameServer, opts ...Option) (resolver.Resolver, e
 		servers:    servers,
 		cache:      cache,
 		options:    options,
+		refreshSem: make(chan struct{}, maxAsyncRefresh),
 	}, nil
 }
 
+// callerNetworkPref maps the caller's network hint to a DNS query preference.
+// Returns "ipv4" for "ip4", "ipv6" for "ip6", or "" for no preference.
+func callerNetworkPref(network string) string {
+	switch network {
+	case "ip4":
+		return "ipv4"
+	case "ip6":
+		return "ipv6"
+	default:
+		return ""
+	}
+}
+
 func (r *localResolver) Resolve(ctx context.Context, network, host string, opts ...resolver.Option) (ips []net.IP, err error) {
 	if ip := net.ParseIP(host); ip != nil {
 		return []net.IP{ip}, nil
@@ -124,11 +148,13 @@ func (r *localResolver) Resolve(ctx context.Context, network, host string, opts
 		host = host + "." + r.options.domain
 	}
 
+	callerPref := callerNetworkPref(network)
+
 	for _, server := range r.servers {
 		if server.Async {
-			ips, err = r.resolveAsync(ctx, &server, host)
+			ips, err = r.resolveAsync(ctx, &server, host, callerPref)
 		} else {
-			ips, err = r.resolve(ctx, &server, host)
+			ips, err = r.resolve(ctx, &server, host, callerPref)
 		}
 		if err != nil {
 			r.options.logger.Error(err)
@@ -145,38 +171,63 @@ func (r *localResolver) Resolve(ctx context.Context, network, host string, opts
 	return
 }
 
-func (r *localResolver) resolve(ctx context.Context, server *NameServer, host string) (ips []net.IP, err error) {
+// resolvePreference determines the effective IP preference.
+// Precedence: server.Only (hard constraint) > callerPref > server.Prefer > default (ipv4-first).
+func resolvePreference(server *NameServer, callerPref string) string {
+	if server.Only != "" {
+		return server.Only
+	}
+	if callerPref != "" {
+		return callerPref
+	}
+	if server.Prefer != "" {
+		return server.Prefer
+	}
+	return "" // default: ipv4-first
+}
+
+func (r *localResolver) resolve(ctx context.Context, server *NameServer, host string, callerPref string) (ips []net.IP, err error) {
 	if server == nil {
 		return
 	}
 
-	if server.Prefer == "ipv6" { // prefer ipv6
-		if ips, err = r.resolve6(ctx, server, host); len(ips) > 0 || server.Only == "ipv6" {
+	pref := resolvePreference(server, callerPref)
+
+	if pref == "ipv6" {
+		if ips, err = r.resolve6(ctx, server, host); len(ips) > 0 || server.Only == "ipv6" || callerPref == "ipv6" {
 			return
 		}
 		return r.resolve4(ctx, server, host)
 	}
 
-	if ips, err = r.resolve4(ctx, server, host); len(ips) > 0 || server.Only == "ipv4" {
+	if ips, err = r.resolve4(ctx, server, host); len(ips) > 0 || server.Only == "ipv4" || callerPref == "ipv4" {
 		return
 	}
 	return r.resolve6(ctx, server, host)
 }
 
-func (r *localResolver) resolveAsync(ctx context.Context, server *NameServer, host string) (ips []net.IP, err error) {
-	ips, ttl, ok := r.lookupCache(ctx, server, host)
+func (r *localResolver) resolveAsync(ctx context.Context, server *NameServer, host string, callerPref string) (ips []net.IP, err error) {
+	ips, ttl, ok := r.lookupCache(ctx, server, host, callerPref)
 	if !ok {
-		return r.resolve(ctx, server, host)
+		return r.resolve(ctx, server, host, callerPref)
 	}
 
 	if ttl <= 0 {
 		r.options.logger.Debugf("async resolve %s via %s", host, server.exchanger.String())
-		go r.resolve(context.WithoutCancel(ctx), server, host)
+		select {
+		case r.refreshSem <- struct{}{}:
+			go func() {
+				defer func() { <-r.refreshSem }()
+				r.resolve(context.WithoutCancel(ctx), server, host, callerPref)
+			}()
+		default:
+			r.options.logger.Debugf("async refresh skipped: semaphore full for %s", host)
+		}
 	}
 	return
 }
 
-func (r *localResolver) lookupCache(ctx context.Context, server *NameServer, host string) (ips []net.IP, ttl time.Duration, ok bool) {
+func (r *localResolver) lookupCache(ctx context.Context, server *NameServer, host string, callerPref string) (ips []net.IP, ttl time.Duration, ok bool) {
 	lookup := func(t uint16, host string) (ips []net.IP, ttl time.Duration, ok bool) {
 		mq := dns.Msg{}
 		mq.SetQuestion(dns.Fqdn(host), t)
@@ -184,32 +235,24 @@ func (r *localResolver) lookupCache(ctx context.Context, server *NameServer, hos
 		if mr == nil {
 			return
 		}
-
 		ok = true
-
-		for _, ans := range mr.Answer {
-			if ar, _ := ans.(*dns.AAAA); ar != nil {
-				ips = append(ips, ar.AAAA)
-			}
-			if ar, _ := ans.(*dns.A); ar != nil {
-				ips = append(ips, ar.A)
-			}
-		}
+		ips = extractIPs(mr)
 		return
 	}
 
-	if server.Prefer == "ipv6" {
+	pref := resolvePreference(server, callerPref)
+
+	if pref == "ipv6" {
 		ips, ttl, ok = lookup(dns.TypeAAAA, host)
-		if len(ips) > 0 || server.Only == "ipv6" {
+		if len(ips) > 0 || server.Only == "ipv6" || callerPref == "ipv6" {
 			return
 		}
-
 		ips, ttl, ok = lookup(dns.TypeA, host)
 		return
 	}
 
 	ips, ttl, ok = lookup(dns.TypeA, host)
-	if len(ips) > 0 || server.Only == "ipv4" {
+	if len(ips) > 0 || server.Only == "ipv4" || callerPref == "ipv4" {
 		return
 	}
 	return lookup(dns.TypeAAAA, host)
@@ -233,30 +276,50 @@ func (r *localResolver) resolveIPs(ctx context.Context, server *NameServer, mq *
 	}
 
 	key := resolver_util.NewCacheKey(&mq.Question[0])
+
+	result, err, _ := r.sfGroup.Do(string(key), func() (any, error) {
+		// Double-check cache after winning the singleflight race.
 		mr, ttl := r.cache.Load(ctx, key)
-	if ttl <= 0 {
+		if ttl > 0 {
+			return mr, nil
+		}
+
+		// Apply EDNS0 subnet option before exchange. mq is fresh per resolve4/resolve6
+		// call, so mutation is safe — it will not be shared across singleflight callers.
 		resolver_util.AddSubnetOpt(mq, server.ClientIP)
-		mr, err = r.exchange(ctx, server.exchanger, mq)
+		mr, err := r.exchange(ctx, server.exchanger, mq)
 		if err != nil {
-			return
+			return nil, err
 		}
 		r.cache.Store(ctx, key, mr, server.TTL)
 
 		if r.options.logger.IsLevelEnabled(logger.TraceLevel) {
 			r.options.logger.Trace(mr.String())
 		}
+		return mr, nil
+	})
+	if err != nil {
+		return nil, err
 	}
 
-	for _, ans := range mr.Answer {
-		if ar, _ := ans.(*dns.AAAA); ar != nil {
-			ips = append(ips, ar.AAAA)
+	return extractIPs(result.(*dns.Msg)), nil
+}
+
+// extractIPs returns all A and AAAA records from a DNS message.
+func extractIPs(mr *dns.Msg) []net.IP {
+	if mr == nil {
+		return nil
 	}
-		if ar, _ := ans.(*dns.A); ar != nil {
+	ips := make([]net.IP, 0, len(mr.Answer))
+	for _, ans := range mr.Answer {
+		if ar, ok := ans.(*dns.A); ok {
 			ips = append(ips, ar.A)
 		}
+		if ar, ok := ans.(*dns.AAAA); ok {
+			ips = append(ips, ar.AAAA)
 		}
-
-	return
+	}
+	return ips
 }
 
 func (r *localResolver) exchange(ctx context.Context, ex exchanger.Exchanger, mq *dns.Msg) (mr *dns.Msg, err error) {
@@ -270,7 +333,13 @@ func (r *localResolver) exchange(ctx context.Context, ex exchanger.Exchanger, mq
 	}
 
 	mr = &dns.Msg{}
-	err = mr.Unpack(reply)
+	if err = mr.Unpack(reply); err != nil {
+		return
+	}
+
+	if mr.Rcode != dns.RcodeSuccess {
+		return nil, fmt.Errorf("DNS %s for %s", dns.RcodeToString[mr.Rcode], mq.Question[0].Name)
+	}
 
 	return
 }

resolver/resolver_test.go

@@ -3,6 +3,7 @@ package resolver
 import (
 	"context"
 	"net"
+	"sync"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -82,6 +83,7 @@ func newTestResolver(servers []NameServer, opts ...Option) *localResolver {
 		servers:    servers,
 		cache:      cache,
 		options:    o,
+		refreshSem: make(chan struct{}, maxAsyncRefresh),
 	}
 }
 
@@ -259,7 +261,7 @@ func TestResolve_CacheHit(t *testing.T) {
 func TestResolve_NilServer(t *testing.T) {
 	r := &localResolver{options: nopLog()}
 
-	ips, err := r.resolve(context.Background(), nil, "example.com")
+	ips, err := r.resolve(context.Background(), nil, "example.com", "")
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
@@ -417,3 +419,157 @@ func TestLoggerOption(t *testing.T) {
 		t.Error("logger option not applied")
 	}
 }
+
+func TestExchange_DNSRcodeError(t *testing.T) {
+	// Return a message with NXDOMAIN rcode
+	mr := new(dns.Msg)
+	mr.SetRcode(&dns.Msg{MsgHdr: dns.MsgHdr{Id: 1}}, dns.RcodeNameError)
+	packed, err := mr.Pack()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	ex := &rcodeMockExchanger{response: packed}
+	r := newTestResolver([]NameServer{{exchanger: ex, TTL: time.Minute}})
+
+	ips, err := r.Resolve(context.Background(), "ip", "nonexistent.example.com")
+	if err == nil {
+		t.Fatal("expected error for NXDOMAIN")
+	}
+	if len(ips) != 0 {
+		t.Fatalf("expected no IPs for NXDOMAIN, got %v", ips)
+	}
+}
+
+func TestExchange_DNSRcodeServfail_Fallback(t *testing.T) {
+	// First server returns SERVFAIL, second returns success
+	servfailMr := new(dns.Msg)
+	servfailMr.SetRcode(&dns.Msg{MsgHdr: dns.MsgHdr{Id: 1}}, dns.RcodeServerFailure)
+	packed, err := servfailMr.Pack()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	okEx := &mockExchanger{response: newDNSMsgWithA("10.0.0.1")}
+	servfailEx := &rcodeMockExchanger{response: packed}
+
+	r := newTestResolver([]NameServer{
+		{exchanger: servfailEx, TTL: time.Minute},
+		{exchanger: okEx, TTL: time.Minute},
+	})
+
+	ips, err := r.Resolve(context.Background(), "ip", "example.com")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(ips) == 0 {
+		t.Fatal("expected fallback to second server")
+	}
+}
+
+func TestResolve_NetworkIP4(t *testing.T) {
+	// When caller asks for ip4, only A queries should be made (no AAAA fallback).
+	// Use a mock that returns empty response for A queries.
+	ex := &mockExchanger{response: new(dns.Msg)} // empty response, no records
+	r := newTestResolver([]NameServer{{exchanger: ex, TTL: time.Minute}})
+
+	ips, err := r.Resolve(context.Background(), "ip4", "example.com")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	// Should get empty result (no A records), and NOT fall back to IPv6
+	if len(ips) != 0 {
+		t.Errorf("expected no IPs when only ip4 requested and no A records, got %v", ips)
+	}
+}
+
+func TestResolve_NetworkIP6(t *testing.T) {
+	ex := &mockExchanger{response: newDNSMsgWithAAAA("2001:db8::1")}
+	r := newTestResolver([]NameServer{{exchanger: ex, TTL: time.Minute}})
+
+	ips, err := r.Resolve(context.Background(), "ip6", "example.com")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(ips) == 0 {
+		t.Fatal("expected IPv6 result for ip6 network")
+	}
+}
+
+func TestResolve_NetworkIP4_ServerOnlyOverrides(t *testing.T) {
+	// Server configured with Only="ipv6", caller asks for "ip4" → server wins
+	ex := &mockExchanger{response: newDNSMsgWithAAAA("2001:db8::1")}
+	r := newTestResolver([]NameServer{{exchanger: ex, Only: "ipv6", Prefer: "ipv6", TTL: time.Minute}})
+
+	ips, err := r.Resolve(context.Background(), "ip4", "example.com")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	// Server Only=ipv6 should override caller's ip4 request
+	if len(ips) == 0 {
+		t.Fatal("expected IPv6 result (server Only overrides caller)")
+	}
+	for _, ip := range ips {
+		if ip.To4() != nil {
+			t.Errorf("expected only IPv6 from server Only constraint, got %v", ip)
+		}
+	}
+}
+
+func TestResolve_ConcurrentDedup(t *testing.T) {
+	ex := &mockExchanger{response: newDNSMsgWithA("10.0.0.1")}
+	r := newTestResolver([]NameServer{{exchanger: ex, TTL: time.Minute}})
+
+	const n = 10
+	var wg sync.WaitGroup
+	results := make([][]net.IP, n)
+	errs := make([]error, n)
+
+	for i := 0; i < n; i++ {
+		wg.Add(1)
+		go func(idx int) {
+			defer wg.Done()
+			results[idx], errs[idx] = r.Resolve(context.Background(), "ip", "example.com")
+		}(i)
+	}
+	wg.Wait()
+
+	for i, err := range errs {
+		if err != nil {
+			t.Fatalf("goroutine %d: unexpected error: %v", i, err)
+		}
+		if len(results[i]) == 0 {
+			t.Fatalf("goroutine %d: expected IPs", i)
+		}
+	}
+
+	// singleflight should have deduplicated the exchange calls
+	calls := ex.calls.Load()
+	if calls > 2 { // allow small margin for race, but not 10
+		t.Errorf("expected ~1 exchange call from singleflight, got %d", calls)
+	}
+}
+
+func TestResolve_IP_UnchangedBehavior(t *testing.T) {
+	ex := &mockExchanger{response: newDNSMsgWithA("10.0.0.1")}
+	r := newTestResolver([]NameServer{{exchanger: ex, TTL: time.Minute}})
+
+	ips, err := r.Resolve(context.Background(), "ip", "example.com")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(ips) == 0 {
+		t.Fatal("expected result for network=ip (default behavior)")
+	}
+}
+
+// rcodeMockExchanger returns a pre-packed response, used for DNS rcode testing.
+type rcodeMockExchanger struct {
+	response []byte
+}
+
+func (m *rcodeMockExchanger) Exchange(_ context.Context, _ []byte) ([]byte, error) {
+	return m.response, nil
+}
+
+func (m *rcodeMockExchanger) String() string { return "mock://rcode" }