fix: proper iterative resolution for A/NS lookups (closes #24)

Reviewed-on: #38

.gitea/workflows/check.yml

@@ -1,26 +1,9 @@
-name: Check
-
-on:
-  push:
-    branches: [main]
-  pull_request:
-    branches: [main]
-
+name: check
+on: [push]
 jobs:
     check:
         runs-on: ubuntu-latest
         steps:
-      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
-
-      - uses: actions/setup-go@40f1582b2485089dde7abd97c1529aa768e1baff # v5
-        with:
-          go-version-file: go.mod
-
-      - name: Install golangci-lint
-        run: go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@5d1e709b7be35cb2025444e19de266b056b7b7ee # v2.10.1
-
-      - name: Install goimports
-        run: go install golang.org/x/tools/cmd/goimports@009367f5c17a8d4c45a961a3a509277190a9a6f0 # v0.42.0
-
-      - name: Run make check
-        run: make check
+            # actions/checkout v4.2.2, 2026-02-28
+            - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
+            - run: docker build .

Dockerfile

@@ -1,11 +1,13 @@
 # Build stage
-FROM golang:1.25-alpine AS builder
+# golang 1.25-alpine, 2026-02-28
+FROM golang@sha256:f6751d823c26342f9506c03797d2527668d095b0a15f1862cddb4d927a7a4ced AS builder
 
-RUN apk add --no-cache git make gcc musl-dev
+RUN apk add --no-cache git make gcc musl-dev binutils-gold
 
-# Install golangci-lint v2
-RUN go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@latest
-RUN go install golang.org/x/tools/cmd/goimports@latest
+# golangci-lint v2.10.1
+RUN go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@5d1e709b7be35cb2025444e19de266b056b7b7ee
+# goimports v0.42.0
+RUN go install golang.org/x/tools/cmd/goimports@009367f5c17a8d4c45a961a3a509277190a9a6f0
 
 WORKDIR /src
 COPY go.mod go.sum ./
@@ -20,7 +22,8 @@ RUN make check
 RUN make build
 
 # Runtime stage
-FROM alpine:3.21
+# alpine 3.21, 2026-02-28
+FROM alpine@sha256:c3f8e73fdb79deaebaa2037150150191b9dcbfba68b4a46d70103204c53f4709
 
 RUN apk add --no-cache ca-certificates tzdata
 

internal/resolver/errors.go

@@ -4,11 +4,6 @@ import "errors"
 
 // Sentinel errors returned by the resolver.
 var (
-	// ErrNotImplemented indicates a method is stubbed out.
-	ErrNotImplemented = errors.New(
-		"resolver not yet implemented",
-	)
-
 	// ErrNoNameservers is returned when no authoritative NS
 	// could be discovered for a domain.
 	ErrNoNameservers = errors.New(
@@ -24,8 +19,4 @@ var (
 	// ErrContextCanceled wraps context cancellation for the
 	// resolver's iterative queries.
 	ErrContextCanceled = errors.New("context canceled")
-
-	// ErrSERVFAIL is returned when a DNS server responds with
-	// SERVFAIL after all retries are exhausted.
-	ErrSERVFAIL = errors.New("SERVFAIL from server")
 )

internal/resolver/iterative.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"math/rand"
 	"net"
 	"sort"
 	"strings"
@@ -42,22 +41,6 @@ func rootServerList() []string {
 	}
 }
 
-const maxRootServers = 3
-
-// randomRootServers returns a shuffled subset of root servers.
-func randomRootServers() []string {
-	all := rootServerList()
-	rand.Shuffle(len(all), func(i, j int) {
-		all[i], all[j] = all[j], all[i]
-	})
-
-	if len(all) > maxRootServers {
-		return all[:maxRootServers]
-	}
-
-	return all
-}
-
 func checkCtx(ctx context.Context) error {
 	err := ctx.Err()
 	if err != nil {
@@ -244,7 +227,7 @@ func (r *Resolver) followDelegation(
 
 		authNS := extractNSSet(resp.Ns)
 		if len(authNS) == 0 {
-			return r.resolveNSRecursive(ctx, domain)
+			return r.resolveNSIterative(ctx, domain)
 		}
 
 		glue := extractGlue(resp.Extra)
@@ -308,60 +291,84 @@ func (r *Resolver) resolveNSIPs(
 	return ips
 }
 
-// resolveNSRecursive queries for NS records using recursive
-// resolution as a fallback for intercepted environments.
-func (r *Resolver) resolveNSRecursive(
+// resolveNSIterative queries for NS records using iterative
+// resolution as a fallback when followDelegation finds no
+// authoritative answer in the delegation chain.
+func (r *Resolver) resolveNSIterative(
 	ctx context.Context,
 	domain string,
 ) ([]string, error) {
-	domain = dns.Fqdn(domain)
-	msg := new(dns.Msg)
-	msg.SetQuestion(domain, dns.TypeNS)
-	msg.RecursionDesired = true
-
-	for _, ip := range randomRootServers() {
 	if checkCtx(ctx) != nil {
 		return nil, ErrContextCanceled
 	}
 
-		addr := net.JoinHostPort(ip, "53")
+	domain = dns.Fqdn(domain)
+	servers := rootServerList()
 
-		resp, _, err := r.client.ExchangeContext(ctx, msg, addr)
+	for range maxDelegation {
+		if checkCtx(ctx) != nil {
+			return nil, ErrContextCanceled
+		}
+
+		resp, err := r.queryServers(
+			ctx, servers, domain, dns.TypeNS,
+		)
 		if err != nil {
-			continue
+			return nil, err
 		}
 
 		nsNames := extractNSSet(resp.Answer)
 		if len(nsNames) > 0 {
 			return nsNames, nil
 		}
+
+		// Follow delegation.
+		authNS := extractNSSet(resp.Ns)
+		if len(authNS) == 0 {
+			break
+		}
+
+		glue := extractGlue(resp.Extra)
+		nextServers := glueIPs(authNS, glue)
+
+		if len(nextServers) == 0 {
+			break
+		}
+
+		servers = nextServers
 	}
 
 	return nil, ErrNoNameservers
 }
 
-// resolveARecord resolves a hostname to IPv4 addresses.
+// resolveARecord resolves a hostname to IPv4 addresses using
+// iterative resolution through the delegation chain.
 func (r *Resolver) resolveARecord(
 	ctx context.Context,
 	hostname string,
 ) ([]string, error) {
-	hostname = dns.Fqdn(hostname)
-	msg := new(dns.Msg)
-	msg.SetQuestion(hostname, dns.TypeA)
-	msg.RecursionDesired = true
-
-	for _, ip := range randomRootServers() {
 	if checkCtx(ctx) != nil {
 		return nil, ErrContextCanceled
 	}
 
-		addr := net.JoinHostPort(ip, "53")
+	hostname = dns.Fqdn(hostname)
+	servers := rootServerList()
 
-		resp, _, err := r.client.ExchangeContext(ctx, msg, addr)
-		if err != nil {
-			continue
+	for range maxDelegation {
+		if checkCtx(ctx) != nil {
+			return nil, ErrContextCanceled
 		}
 
+		resp, err := r.queryServers(
+			ctx, servers, hostname, dns.TypeA,
+		)
+		if err != nil {
+			return nil, fmt.Errorf(
+				"resolving %s: %w", hostname, err,
+			)
+		}
+
+		// Check for A records in the answer section.
 		var ips []string
 
 		for _, rr := range resp.Answer {
@@ -373,6 +380,24 @@ func (r *Resolver) resolveARecord(
 		if len(ips) > 0 {
 			return ips, nil
 		}
+
+		// Follow delegation if present.
+		authNS := extractNSSet(resp.Ns)
+		if len(authNS) == 0 {
+			break
+		}
+
+		glue := extractGlue(resp.Extra)
+		nextServers := glueIPs(authNS, glue)
+
+		if len(nextServers) == 0 {
+			// Resolve NS IPs iteratively — but guard
+			// against infinite recursion by using only
+			// already-resolved servers.
+			break
+		}
+
+		servers = nextServers
 	}
 
 	return nil, fmt.Errorf(
@@ -402,7 +427,7 @@ func (r *Resolver) FindAuthoritativeNameservers(
 		candidate := strings.Join(labels[i:], ".") + "."
 
 		nsNames, err := r.followDelegation(
-			ctx, candidate, randomRootServers(),
+			ctx, candidate, rootServerList(),
 		)
 		if err == nil && len(nsNames) > 0 {
 			sort.Strings(nsNames)
@@ -435,6 +460,23 @@ func (r *Resolver) QueryNameserver(
 	return r.queryAllTypes(ctx, nsHostname, nsIPs[0], hostname)
 }
 
+// QueryNameserverIP queries a nameserver by its IP address directly,
+// bypassing NS hostname resolution.
+func (r *Resolver) QueryNameserverIP(
+	ctx context.Context,
+	nsHostname string,
+	nsIP string,
+	hostname string,
+) (*NameserverResponse, error) {
+	if checkCtx(ctx) != nil {
+		return nil, ErrContextCanceled
+	}
+
+	hostname = dns.Fqdn(hostname)
+
+	return r.queryAllTypes(ctx, nsHostname, nsIP, hostname)
+}
+
 func (r *Resolver) queryAllTypes(
 	ctx context.Context,
 	nsHostname string,
@@ -459,11 +501,6 @@ func (r *Resolver) queryAllTypes(
 	return resp, nil
 }
 
-const (
-	singleTypeMaxRetries     = 3
-	singleTypeInitialBackoff = 100 * time.Millisecond
-)
-
 type queryState struct {
 	gotNXDomain bool
 	gotSERVFAIL bool
@@ -495,21 +532,6 @@ func (r *Resolver) queryEachType(
 	return state
 }
 
-// isTimeout checks whether an error represents a DNS timeout.
-func isTimeout(err error) bool {
-	if err == nil {
-		return false
-	}
-
-	var netErr net.Error
-	if errors.As(err, &netErr) && netErr.Timeout() {
-		return true
-	}
-
-	// Also catch i/o timeout strings from the dns library.
-	return strings.Contains(err.Error(), "i/o timeout")
-}
-
 func (r *Resolver) querySingleType(
 	ctx context.Context,
 	nsIP string,
@@ -518,95 +540,23 @@ func (r *Resolver) querySingleType(
 	resp *NameserverResponse,
 	state *queryState,
 ) {
-	msg, lastErr := r.querySingleTypeWithRetry(
-		ctx, nsIP, hostname, qtype,
-	)
-	if msg == nil {
-		r.recordRetryFailure(lastErr, state)
+	msg, err := r.queryDNS(ctx, nsIP, hostname, qtype)
+	if err != nil {
+		if isTimeout(err) {
+			state.gotTimeout = true
+		}
 
 		return
 	}
 
-	r.handleDNSResponse(msg, resp, state)
-}
+	if msg.Rcode == dns.RcodeNameError {
+		state.gotNXDomain = true
 
-func (r *Resolver) querySingleTypeWithRetry(
-	ctx context.Context,
-	nsIP string,
-	hostname string,
-	qtype uint16,
-) (*dns.Msg, error) {
-	var lastErr error
-
-	backoff := singleTypeInitialBackoff
-
-	for attempt := range singleTypeMaxRetries {
-		if checkCtx(ctx) != nil {
-			return nil, ErrContextCanceled
-		}
-
-		if attempt > 0 {
-			if !waitBackoff(ctx, backoff) {
-				return nil, ErrContextCanceled
-			}
-
-			backoff *= timeoutMultiplier
-		}
-
-		msg, err := r.queryDNS(ctx, nsIP, hostname, qtype)
-		if err != nil {
-			lastErr = err
-
-			if !isTimeout(err) {
-				return nil, err
-			}
-
-			continue
+		return
 	}
 
 	if msg.Rcode == dns.RcodeServerFailure {
-			lastErr = ErrSERVFAIL
-
-			continue
-		}
-
-		return msg, nil
-	}
-
-	return nil, lastErr
-}
-
-func waitBackoff(ctx context.Context, d time.Duration) bool {
-	select {
-	case <-ctx.Done():
-		return false
-	case <-time.After(d):
-		return true
-	}
-}
-
-func (r *Resolver) recordRetryFailure(
-	lastErr error,
-	state *queryState,
-) {
-	if lastErr == nil {
-		return
-	}
-
-	if isTimeout(lastErr) {
-		state.gotTimeout = true
-	} else if errors.Is(lastErr, ErrSERVFAIL) {
 		state.gotSERVFAIL = true
-	}
-}
-
-func (r *Resolver) handleDNSResponse(
-	msg *dns.Msg,
-	resp *NameserverResponse,
-	state *queryState,
-) {
-	if msg.Rcode == dns.RcodeNameError {
-		state.gotNXDomain = true
 
 		return
 	}
@@ -633,16 +583,26 @@ func collectAnswerRecords(
 	}
 }
 
+// isTimeout checks whether an error is a network timeout.
+func isTimeout(err error) bool {
+	var netErr net.Error
+	if errors.As(err, &netErr) {
+		return netErr.Timeout()
+	}
+
+	return false
+}
+
 func classifyResponse(resp *NameserverResponse, state queryState) {
 	switch {
 	case state.gotNXDomain && !state.hasRecords:
 		resp.Status = StatusNXDomain
 	case state.gotTimeout && !state.hasRecords:
 		resp.Status = StatusTimeout
-		resp.Error = "all queries timed out after retries"
+		resp.Error = "all queries timed out"
 	case state.gotSERVFAIL && !state.hasRecords:
 		resp.Status = StatusError
-		resp.Error = "server failure (SERVFAIL) after retries"
+		resp.Error = "server returned SERVFAIL"
 	case !state.hasRecords && !state.gotNXDomain:
 		resp.Status = StatusNoData
 	}

internal/resolver/resolver_test.go

@@ -10,6 +10,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/miekg/dns"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
@@ -622,6 +623,59 @@ func TestQueryAllNameservers_ContextCanceled(t *testing.T) {
 	assert.Error(t, err)
 }
 
+// ----------------------------------------------------------------
+// Timeout tests
+// ----------------------------------------------------------------
+
+func TestQueryNameserverIP_Timeout(t *testing.T) {
+	t.Parallel()
+
+	log := slog.New(slog.NewTextHandler(
+		os.Stderr,
+		&slog.HandlerOptions{Level: slog.LevelDebug},
+	))
+
+	r := resolver.NewFromLoggerWithClient(
+		log, &timeoutClient{},
+	)
+
+	ctx, cancel := context.WithTimeout(
+		context.Background(), 10*time.Second,
+	)
+	t.Cleanup(cancel)
+
+	// Query any IP — the client always returns a timeout error.
+	resp, err := r.QueryNameserverIP(
+		ctx, "unreachable.test.", "192.0.2.1",
+		"example.com",
+	)
+	require.NoError(t, err)
+
+	assert.Equal(t, resolver.StatusTimeout, resp.Status)
+	assert.NotEmpty(t, resp.Error)
+}
+
+// timeoutClient simulates DNS timeout errors for testing.
+type timeoutClient struct{}
+
+func (c *timeoutClient) ExchangeContext(
+	_ context.Context,
+	_ *dns.Msg,
+	_ string,
+) (*dns.Msg, time.Duration, error) {
+	return nil, 0, &net.OpError{
+		Op:  "read",
+		Net: "udp",
+		Err: &timeoutError{},
+	}
+}
+
+type timeoutError struct{}
+
+func (e *timeoutError) Error() string   { return "i/o timeout" }
+func (e *timeoutError) Timeout() bool   { return true }
+func (e *timeoutError) Temporary() bool { return true }
+
 func TestResolveIPAddresses_ContextCanceled(t *testing.T) {
 	t.Parallel()