dnshelper/resolver/fallback.go

package resolver

import (
	"context"
	"fmt"
	"io"
	"math/rand"
	"net"
	"strings"
	"time"

	"golang.org/x/net/dns/dnsmessage"
)

// ParallelAFallback sends A queries for hostname to all resolvers simultaneously.
// Returns the first successful result (first response with at least one A record).
// The buffered channel ensures goroutines never block on write even after the
// collector returns early.
// w receives Stage 3 diagnostic lines (pass io.Discard to suppress).
func ParallelAFallback(ctx context.Context, w io.Writer, resolvers []string, hostname string, timeout time.Duration) ([]string, error) {
	if len(resolvers) == 0 {
		return nil, fmt.Errorf("no resolvers available for fallback A lookup of %s", hostname)
	}

	type aResult struct {
		Resolver string
		IPs      []string
		Err      error
	}
	ch := make(chan aResult, len(resolvers))

	fmt.Fprintf(w, "[dns] Stage 3: Parallel A fallback for %s (%d resolvers)\n", hostname, len(resolvers))

	for _, r := range resolvers {
		go func(resolver string) {
			queryCtx, cancel := context.WithTimeout(ctx, timeout)
			defer cancel()
			ips, err := queryA(queryCtx, resolver, hostname, timeout)
			ch <- aResult{Resolver: resolver, IPs: ips, Err: err}
		}(r)
	}

	// Collect all results to avoid goroutine leaks (channel is buffered).
	var lastErr error
	for i := 0; i < len(resolvers); i++ {
		result := <-ch
		if result.Err == nil && len(result.IPs) > 0 {
			fmt.Fprintf(w, "[dns]   %s \u2192 %s\n", result.Resolver, strings.Join(result.IPs, ", "))
			// First success wins; remaining goroutines write to the buffered channel
			// and exit cleanly even though we return early here.
			return result.IPs, nil
		}
		if result.Err != nil {
			errLabel := "error"
			if strings.Contains(result.Err.Error(), "NXDOMAIN") {
				errLabel = "NXDOMAIN"
			}
			fmt.Fprintf(w, "[dns]   %s \u2192 %s\n", result.Resolver, errLabel)
			lastErr = result.Err
		}
	}

	if lastErr != nil {
		return nil, fmt.Errorf("all resolvers failed for %s: %w", hostname, lastErr)
	}
	return nil, fmt.Errorf("no resolver returned A records for %s", hostname)
}

// queryA sends a single recursive A query for hostname to resolver and returns
// the IP addresses from the answer section.
// NXDOMAIN is returned as an error.
func queryA(ctx context.Context, resolver, hostname string, timeout time.Duration) ([]string, error) {
	fqdn := hostname
	if !strings.HasSuffix(fqdn, ".") {
		fqdn += "."
	}
	name, err := dnsmessage.NewName(fqdn)
	if err != nil {
		return nil, fmt.Errorf("invalid hostname %q: %w", hostname, err)
	}

	id := uint16(rand.Uint32()) //nolint:gosec
	msg := dnsmessage.Message{
		Header: dnsmessage.Header{
			ID:               id,
			RecursionDesired: true,
		},
		Questions: []dnsmessage.Question{{
			Name:  name,
			Type:  dnsmessage.TypeA,
			Class: dnsmessage.ClassINET,
		}},
	}
	packed, err := msg.Pack()
	if err != nil {
		return nil, fmt.Errorf("packing A query: %w", err)
	}

	resp, err := UDPQuery(ctx, resolver, packed, timeout)
	if err != nil {
		return nil, err
	}

	var parser dnsmessage.Parser
	respHeader, err := parser.Start(resp)
	if err != nil {
		return nil, fmt.Errorf("parsing A response: %w", err)
	}
	if respHeader.ID != id {
		return nil, fmt.Errorf("A response ID mismatch (expected %d, got %d)", id, respHeader.ID)
	}
	if respHeader.RCode == dnsmessage.RCodeNameError {
		return nil, fmt.Errorf("NXDOMAIN for %s", hostname)
	}
	if respHeader.RCode != dnsmessage.RCodeSuccess {
		return nil, fmt.Errorf("A query for %s returned %s", hostname, respHeader.RCode)
	}

	if err := parser.SkipAllQuestions(); err != nil {
		return nil, fmt.Errorf("skipping questions in A response: %w", err)
	}

	var ips []string
	for {
		hdr, err := parser.AnswerHeader()
		if err == dnsmessage.ErrSectionDone {
			break
		}
		if err != nil {
			return nil, fmt.Errorf("parsing A answer header: %w", err)
		}
		switch hdr.Type {
		case dnsmessage.TypeA:
			aRec, err := parser.AResource()
			if err != nil {
				return nil, fmt.Errorf("parsing A record: %w", err)
			}
			ips = append(ips, net.IP(aRec.A[:]).String())
		default:
			if err := parser.SkipAnswer(); err != nil {
				return nil, fmt.Errorf("skipping A answer: %w", err)
			}
		}
	}

	if len(ips) == 0 {
		return nil, fmt.Errorf("no A records returned for %s", hostname)
	}
	return ips, nil
}