Using time interval for probing ipv6

A backoff with small max time will flood requests to Control D server,
causing false positive for abuse mitiation system. While a big max time
will cause ctrld not realize network change as fast as possible.

While at it, also sync DoH3 code with DoH code, ensuring no others place
can trigger requests flooding for ipv6 probing.

config_quic.go

@@ -10,13 +10,10 @@ import (
 	"net/http"
 	"runtime"
 	"sync"
-	"time"
 
 	"github.com/miekg/dns"
 	"github.com/quic-go/quic-go"
 	"github.com/quic-go/quic-go/http3"
-
-	ctrldnet "github.com/Control-D-Inc/ctrld/internal/net"
 )
 
 func (uc *UpstreamConfig) setupDOH3Transport() {
@@ -29,9 +26,7 @@ func (uc *UpstreamConfig) setupDOH3Transport() {
 		uc.http3RoundTripper = uc.newDOH3Transport(uc.bootstrapIPs6)
 	case IpStackSplit:
 		uc.http3RoundTripper4 = uc.newDOH3Transport(uc.bootstrapIPs4)
-		ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
-		defer cancel()
-		if ctrldnet.IPv6Available(ctx) {
+		if hasIPv6() {
 			uc.http3RoundTripper6 = uc.newDOH3Transport(uc.bootstrapIPs6)
 		} else {
 			uc.http3RoundTripper6 = uc.http3RoundTripper4

net.go

@@ -2,13 +2,10 @@ package ctrld
 
 import (
 	"context"
-	"errors"
 	"sync"
 	"sync/atomic"
 	"time"
 
-	"tailscale.com/logtail/backoff"
-
 	ctrldnet "github.com/Control-D-Inc/ctrld/internal/net"
 )
 
@@ -17,30 +14,36 @@ var (
 	ipv6Available atomic.Bool
 )
 
+const ipv6ProbingInterval = 10 * time.Second
+
 func hasIPv6() bool {
 	hasIPv6Once.Do(func() {
 		ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
 		defer cancel()
 		val := ctrldnet.IPv6Available(ctx)
 		ipv6Available.Store(val)
-		go probingIPv6(val)
+		go probingIPv6(context.TODO(), val)
 	})
 	return ipv6Available.Load()
 }
 
 // TODO(cuonglm): doing poll check natively for supported platforms.
-func probingIPv6(old bool) {
-	b := backoff.NewBackoff("probingIPv6", func(format string, args ...any) {}, 30*time.Second)
-	bCtx := context.Background()
+func probingIPv6(ctx context.Context, old bool) {
+	ticker := time.NewTicker(ipv6ProbingInterval)
+	defer ticker.Stop()
 	for {
-		func() {
-			ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
-			defer cancel()
-			cur := ctrldnet.IPv6Available(ctx)
-			if ipv6Available.CompareAndSwap(old, cur) {
-				old = cur
-			}
-		}()
-		b.BackOff(bCtx, errors.New("no change"))
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+			func() {
+				ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+				defer cancel()
+				cur := ctrldnet.IPv6Available(ctx)
+				if ipv6Available.CompareAndSwap(old, cur) {
+					old = cur
+				}
+			}()
+		}
 	}
 }