mcns/internal/dns/forwarder.go

package dns

import (
	"fmt"
	"time"

	"github.com/miekg/dns"
)

// Forwarder handles forwarding DNS queries to upstream resolvers.
type Forwarder struct {
	upstreams []string
	client    *dns.Client
	cache     *Cache
}

// NewForwarder creates a Forwarder with the given upstream addresses.
func NewForwarder(upstreams []string) *Forwarder {
	return &Forwarder{
		upstreams: upstreams,
		client: &dns.Client{
			Timeout: 2 * time.Second,
		},
		cache: NewCache(),
	}
}

// Forward sends a query to upstream resolvers and returns the response.
// Responses are cached by (qname, qtype, qclass) with TTL-based expiry.
func (f *Forwarder) Forward(r *dns.Msg) (*dns.Msg, error) {
	if len(r.Question) == 0 {
		return nil, fmt.Errorf("empty question")
	}

	q := r.Question[0]

	// Check cache.
	if cached := f.cache.Get(q.Name, q.Qtype, q.Qclass); cached != nil {
		return cached.Copy(), nil
	}

	// Try each upstream in order.
	var lastErr error
	for _, upstream := range f.upstreams {
		resp, _, err := f.client.Exchange(r, upstream)
		if err != nil {
			lastErr = err
			continue
		}

		// Don't cache SERVFAIL or REFUSED.
		if resp.Rcode != dns.RcodeServerFailure && resp.Rcode != dns.RcodeRefused {
			ttl := minTTL(resp)
			if ttl > 300 {
				ttl = 300
			}
			if ttl > 0 {
				f.cache.Set(q.Name, q.Qtype, q.Qclass, resp, time.Duration(ttl)*time.Second)
			}
		}

		return resp, nil
	}

	return nil, fmt.Errorf("all upstreams failed: %w", lastErr)
}

// minTTL returns the minimum TTL from all resource records in a response.
func minTTL(msg *dns.Msg) uint32 {
	var min uint32
	first := true

	for _, sections := range [][]dns.RR{msg.Answer, msg.Ns, msg.Extra} {
		for _, rr := range sections {
			ttl := rr.Header().Ttl
			if first || ttl < min {
				min = ttl
				first = false
			}
		}
	}

	if first {
		return 60 // No records; default to 60s.
	}
	return min
}