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|
package resolver
import (
"context"
"errors"
"fmt"
"net"
"time"
"github.com/miekg/dns"
)
var (
ErrMaxDelegations = errors.New("max delegations exceeded")
ErrNoServers = errors.New("no nameservers available")
)
type Resolver struct {
roots []string
maxDelegations int
timeout time.Duration
retries int
client *dns.Client
}
type Option func(*Resolver)
func New(opts ...Option) *Resolver {
r := &Resolver{
roots: loadRootServers(),
maxDelegations: 30,
timeout: 2 * time.Second,
retries: 2,
client: &dns.Client{
Net: "udp",
UDPSize: 4096,
Timeout: 2 * time.Second,
},
}
for _, opt := range opts {
opt(r)
}
return r
}
func WithRootAddresses(addrs []string) Option {
return func(r *Resolver) {
r.roots = addrs
}
}
func WithTimeout(d time.Duration) Option {
return func(r *Resolver) {
r.timeout = d
}
}
func (r *Resolver) Lookup(ctx context.Context, qname string, qtype uint16) (*dns.Msg, error) {
if ctx == nil {
ctx = context.Background()
}
return r.resolve(ctx, qname, qtype)
}
func (r *Resolver) resolve(ctx context.Context, qname string, qtype uint16) (*dns.Msg, error) {
servers := make([]string, len(r.roots))
copy(servers, r.roots)
for depth := 0; depth < r.maxDelegations; depth++ {
select {
case <-ctx.Done():
return nil, ctx.Err()
default:
}
if len(servers) == 0 {
return nil, ErrNoServers
}
reply, err := r.exchangeWithRetries(ctx, servers, qname, qtype)
if err != nil {
return nil, fmt.Errorf("resolve %s %s: %w",
qname, dns.TypeToString[qtype], err)
}
switch {
case reply.Rcode == dns.RcodeSuccess && len(reply.Answer) > 0:
return reply, nil
case reply.Rcode == dns.RcodeNameError:
return reply, nil
case reply.Rcode == dns.RcodeSuccess && isReferral(reply):
next, err := r.nextServers(ctx, reply)
if err != nil {
return nil, err
}
servers = next
case reply.Rcode == dns.RcodeSuccess:
return reply, nil
default:
if len(servers) > 1 {
servers = servers[1:]
} else {
return reply, nil
}
}
}
return nil, ErrMaxDelegations
}
func isReferral(msg *dns.Msg) bool {
return !msg.Authoritative && len(msg.Ns) > 0
}
func (r *Resolver) nextServers(ctx context.Context, msg *dns.Msg) ([]string, error) {
var targets []string
glue := make(map[string]string)
for _, rr := range msg.Ns {
ns, ok := rr.(*dns.NS)
if !ok {
continue
}
targets = append(targets, ns.Ns)
}
for _, rr := range msg.Extra {
switch v := rr.(type) {
case *dns.A:
if _, exists := glue[v.Hdr.Name]; !exists {
glue[v.Hdr.Name] = v.A.String()
}
case *dns.AAAA:
if _, exists := glue[v.Hdr.Name]; !exists {
glue[v.Hdr.Name] = v.AAAA.String()
}
}
}
var addrs []string
var unresolved []string
for _, t := range targets {
if ip, ok := glue[t]; ok {
addrs = append(addrs, ip)
} else {
unresolved = append(unresolved, t)
}
}
if len(addrs) > 0 {
return addrs, nil
}
for _, name := range unresolved {
reply, err := r.resolve(ctx, name, dns.TypeA)
if err != nil {
continue
}
if reply.Rcode != dns.RcodeSuccess {
continue
}
for _, rr := range reply.Answer {
if a, ok := rr.(*dns.A); ok {
addrs = append(addrs, a.A.String())
break
}
}
}
if len(addrs) == 0 {
return nil, ErrNoServers
}
return addrs, nil
}
func (r *Resolver) exchangeWithRetries(ctx context.Context, servers []string,
qname string, qtype uint16) (*dns.Msg, error) {
msg := new(dns.Msg)
msg.SetQuestion(qname, qtype)
msg.SetEdns0(4096, false)
msg.RecursionDesired = false
type result struct {
reply *dns.Msg
err error
}
ch := make(chan result, len(servers))
for _, srv := range servers {
addr := srv
if _, _, err := net.SplitHostPort(srv); err != nil {
addr = net.JoinHostPort(srv, "53")
}
go func(addr string) {
for attempt := 0; attempt < r.retries; attempt++ {
reply, _, err := r.client.ExchangeContext(ctx, msg, addr)
if err == nil {
if reply.Truncated {
tcpClient := &dns.Client{Net: "tcp", Timeout: r.timeout}
reply, _, err = tcpClient.ExchangeContext(ctx, msg, addr)
if err == nil {
ch <- result{reply: reply, err: nil}
return
}
}
ch <- result{reply: reply, err: nil}
return
}
delay := time.Duration(1<<attempt) * 100 * time.Millisecond
select {
case <-time.After(delay):
case <-ctx.Done():
ch <- result{nil, ctx.Err()}
return
}
}
ch <- result{nil, fmt.Errorf("all retries failed for %s", addr)}
}(addr)
}
var lastErr error
for i := 0; i < len(servers); i++ {
res := <-ch
if res.err == nil {
return res.reply, nil
}
lastErr = res.err
}
return nil, lastErr
}
|
