dnstest/backend/internal/resolver/resolver.go

package resolver

import (
	"fmt"
	"strings"
	"time"

	"github.com/miekg/dns"
)

// Resolver wraps miekg/dns with timeout and retry logic.
type Resolver struct {
	Timeout time.Duration
	Retries int
}

// NewResolver creates a Resolver with sensible defaults.
func NewResolver() *Resolver {
	return &Resolver{
		Timeout: 3 * time.Second,
		Retries: 1,
	}
}

// Query sends a UDP DNS query. If the response is truncated it automatically
// retries over TCP.
func (r *Resolver) Query(name string, server string, qtype uint16) (*dns.Msg, error) {
	m := new(dns.Msg)
	m.SetQuestion(dns.Fqdn(name), qtype)
	m.RecursionDesired = true

	c := new(dns.Client)
	c.Timeout = r.Timeout
	c.Net = "udp"

	var resp *dns.Msg
	var err error

	for attempt := 0; attempt <= r.Retries; attempt++ {
		resp, _, err = c.Exchange(m, ensurePort(server))
		if err == nil {
			break
		}
	}
	if err != nil {
		return nil, fmt.Errorf("udp query %s @%s: %w", dns.TypeToString[qtype], server, err)
	}

	// Fall back to TCP on truncation.
	if resp.Truncated {
		return r.QueryTCP(name, server, qtype)
	}
	return resp, nil
}

// QueryTCP sends a DNS query over TCP.
func (r *Resolver) QueryTCP(name string, server string, qtype uint16) (*dns.Msg, error) {
	m := new(dns.Msg)
	m.SetQuestion(dns.Fqdn(name), qtype)
	m.RecursionDesired = true

	c := new(dns.Client)
	c.Timeout = r.Timeout
	c.Net = "tcp"

	var resp *dns.Msg
	var err error

	for attempt := 0; attempt <= r.Retries; attempt++ {
		resp, _, err = c.Exchange(m, ensurePort(server))
		if err == nil {
			break
		}
	}
	if err != nil {
		return nil, fmt.Errorf("tcp query %s @%s: %w", dns.TypeToString[qtype], server, err)
	}
	return resp, nil
}

// QueryNoRecurse sends a UDP query with RD=0 (non-recursive). Falls back to
// TCP on truncation.
func (r *Resolver) QueryNoRecurse(name string, server string, qtype uint16) (*dns.Msg, error) {
	m := new(dns.Msg)
	m.SetQuestion(dns.Fqdn(name), qtype)
	m.RecursionDesired = false

	c := new(dns.Client)
	c.Timeout = r.Timeout
	c.Net = "udp"

	var resp *dns.Msg
	var err error

	for attempt := 0; attempt <= r.Retries; attempt++ {
		resp, _, err = c.Exchange(m, ensurePort(server))
		if err == nil {
			break
		}
	}
	if err != nil {
		return nil, fmt.Errorf("udp query (no recurse) %s @%s: %w", dns.TypeToString[qtype], server, err)
	}
	if resp.Truncated {
		m.RecursionDesired = false
		c2 := new(dns.Client)
		c2.Timeout = r.Timeout
		c2.Net = "tcp"
		for attempt := 0; attempt <= r.Retries; attempt++ {
			resp, _, err = c2.Exchange(m, ensurePort(server))
			if err == nil {
				break
			}
		}
		if err != nil {
			return nil, fmt.Errorf("tcp query (no recurse) %s @%s: %w", dns.TypeToString[qtype], server, err)
		}
	}
	return resp, nil
}

// QueryEDNS sends a UDP query with EDNS0 buffer size set.
func (r *Resolver) QueryEDNS(name string, server string, qtype uint16, bufsize uint16) (*dns.Msg, error) {
	m := new(dns.Msg)
	m.SetQuestion(dns.Fqdn(name), qtype)
	m.RecursionDesired = false
	m.SetEdns0(bufsize, false)

	c := new(dns.Client)
	c.Timeout = r.Timeout
	c.Net = "udp"

	var resp *dns.Msg
	var err error

	for attempt := 0; attempt <= r.Retries; attempt++ {
		resp, _, err = c.Exchange(m, ensurePort(server))
		if err == nil {
			break
		}
	}
	if err != nil {
		return nil, fmt.Errorf("edns query %s @%s: %w", dns.TypeToString[qtype], server, err)
	}
	return resp, nil
}

// QueryVersionBind asks for version.bind TXT in the CH class.
func (r *Resolver) QueryVersionBind(server string) (string, error) {
	m := new(dns.Msg)
	m.SetQuestion("version.bind.", dns.TypeTXT)
	m.Question[0].Qclass = dns.ClassCHAOS
	m.RecursionDesired = false

	c := new(dns.Client)
	c.Timeout = r.Timeout
	c.Net = "udp"

	var resp *dns.Msg
	var err error

	for attempt := 0; attempt <= r.Retries; attempt++ {
		resp, _, err = c.Exchange(m, ensurePort(server))
		if err == nil {
			break
		}
	}
	if err != nil {
		return "", err
	}
	for _, rr := range resp.Answer {
		if txt, ok := rr.(*dns.TXT); ok {
			if len(txt.Txt) > 0 {
				return txt.Txt[0], nil
			}
		}
	}
	return "", nil
}

// QueryAXFR attempts a zone transfer. Returns true if the server allows it.
// Uses a short timeout to avoid blocking on unresponsive servers.
func (r *Resolver) QueryAXFR(name string, server string) (bool, error) {
	type axfrResult struct {
		allowed bool
		err     error
	}
	ch := make(chan axfrResult, 1)

	go func() {
		m := new(dns.Msg)
		m.SetQuestion(dns.Fqdn(name), dns.TypeAXFR)

		tr := new(dns.Transfer)
		tr.DialTimeout = 3 * time.Second
		tr.ReadTimeout = 3 * time.Second

		env, err := tr.In(m, ensurePort(server))
		if err != nil {
			ch <- axfrResult{false, nil}
			return
		}
		for e := range env {
			if e.Error != nil {
				ch <- axfrResult{false, nil}
				return
			}
			if len(e.RR) > 0 {
				ch <- axfrResult{true, nil}
				return
			}
		}
		ch <- axfrResult{false, nil}
	}()

	select {
	case res := <-ch:
		return res.allowed, res.err
	case <-time.After(5 * time.Second):
		return false, nil
	}
}

func ensurePort(server string) string {
	// Already has port (IPv4:port or [IPv6]:port).
	if strings.Contains(server, "]:") || (!strings.Contains(server, "[") && strings.Count(server, ":") == 1) {
		return server
	}
	// IPv6 without brackets/port.
	if strings.Contains(server, ":") && !strings.Contains(server, "[") {
		return "[" + server + "]:53"
	}
	return server + ":53"
}