TUN-10525: Add prechecks kill switch

Instead of having the  --precheck flag in cloudflared, we allow controlling prechecks via a DNS flag, so we can short-circuit this behavior in case anything goes wrong. Although we don't expect pre-checks to add that much traffic, we should still guarantee that we can stop pre-checks in case something goes wrong.

features/features.go

@@ -42,6 +42,10 @@ type FeatureSnapshot struct {
 	// We provide the list of features since we need it to send in the ConnectionOptions during connection
 	// registrations.
 	FeaturesList []string
+
+	// SkipPrechecks indicates when to skip connectivity pre-checks at startup.
+	// Controlled via DNS TXT record to allow remote kill-switch in case of issues.
+	SkipPrechecks bool
 }
 
 type PostQuantumMode uint8

features/selector.go

@@ -24,6 +24,7 @@ const (
 
 type featuresRecord struct {
 	DatagramV3Percentage uint32 `json:"dv3_2"`
+	SkipPrechecks        bool   `json:"skip_prechecks"`
 
 	// DatagramV3Percentage int32 `json:"dv3"` // Removed in TUN-9291
 	// DatagramV3Percentage uint32 `json:"dv3_1"` // Removed in TUN-9883
@@ -89,6 +90,7 @@ func (fs *featureSelector) Snapshot() FeatureSnapshot {
 		PostQuantum:     fs.postQuantumMode(),
 		DatagramVersion: fs.datagramVersion(),
 		FeaturesList:    fs.clientFeatures(),
+		SkipPrechecks:   fs.prechecksSkip(),
 	}
 }
 
@@ -121,6 +123,12 @@ func (fs *featureSelector) datagramVersion() DatagramVersion {
 	return DatagramV2
 }
 
+// prechecksSkip returns whether prechecks are enabled via DNS flag.
+// Defaults to false if not set in the DNS TXT record.
+func (fs *featureSelector) prechecksSkip() bool {
+	return fs.remoteFeatures.SkipPrechecks
+}
+
 // clientFeatures will return the list of currently available features that cloudflared should provide to the edge.
 func (fs *featureSelector) clientFeatures() []string {
 	// Evaluate any remote features along with static feature list to construct the list of features
@@ -186,7 +194,7 @@ func (dr *dnsResolver) lookupRecord(ctx context.Context) ([]byte, error) {
 	}
 
 	if len(records) == 0 {
-		return nil, fmt.Errorf("No TXT record found for %s to determine which features to opt-in", featureSelectorHostname)
+		return nil, fmt.Errorf("no TXT record found for %s to determine which features to opt-in", featureSelectorHostname)
 	}
 
 	return []byte(records[0]), nil