dnsforward: imp code

internal/dnsforward/dns64.go

@@ -2,7 +2,6 @@ package dnsforward
 
 import (
 	"fmt"
-	"math"
 	"net"
 	"net/netip"
 
@@ -181,11 +180,10 @@ func (s *Server) synthDNS64(origReq, origResp, resp *dns.Msg) (ok bool) {
 	}
 
 	// The Time to Live (TTL) field is set to the minimum of the TTL of the
-	// original A RR and the SOA RR for the queried domain.
-	//
-	// Set initially to [math.MaxUint32], so that if there is no SOA record, the
-	// TTL will still be set according to the rules.
-	soaTTL := uint32(math.MaxUint32)
+	// original A RR and the SOA RR for the queried domain.  If the original
+	// response contains no SOA records, the minimum of the TTL of the original
+	// A RR and [maxDNS64SynTTL] should be used.  See [maxDNS64SynTTL].
+	soaTTL := maxDNS64SynTTL
 	for _, rr := range origResp.Ns {
 		if hdr := rr.Header(); hdr.Rrtype == dns.TypeSOA && hdr.Name == origReq.Question[0].Name {
 			soaTTL = hdr.Ttl
@@ -277,9 +275,7 @@ func (s *Server) performDNS64(prx *proxy.Proxy, dctx *dnsContext) (rc resultCode
 // synthRR synthesizes a DNS64 resource record in compliance with RFC 6147.  If
 // rr is not an A record, it's returned as is.  A records are modified to become
 // a DNS64-synthesized AAAA records, and the TTL is set according to the
-// original TTL of a record and soaTTL.  If the original response contains no
-// SOA records, soaTTL is expected to be [mathMaxUint32].  It returns nil on
-// invalid A records.
+// original TTL of a record and soaTTL.  It returns nil on invalid A records.
 func (s *Server) synthRR(rr dns.RR, soaTTL uint32) (result dns.RR) {
 	aResp, ok := rr.(*dns.A)
 	if !ok {
@@ -302,13 +298,6 @@ func (s *Server) synthRR(rr dns.RR, soaTTL uint32) (result dns.RR) {
 		AAAA: s.mapDNS64(addr),
 	}
 
-	// Use [maxDNS64SynTTL] as value of SOA's TTL in case the SOA record is
-	// absent since the resulting TTL still should be picked as the minimum of
-	// the original TTL and the compared value.
-	if soaTTL == math.MaxUint32 {
-		soaTTL = maxDNS64SynTTL
-	}
-
 	if rrTTL := aResp.Hdr.Ttl; rrTTL < soaTTL {
 		aaaa.Hdr.Ttl = rrTTL
 	} else {