all: guarding against DNS forwarding loop

Based on how dnsmasq "--dns-loop-detect" mechanism.

See: https://thekelleys.org.uk/dnsmasq/docs/dnsmasq-man.html

config.go

@@ -2,8 +2,10 @@ package ctrld
 
 import (
 	"context"
+	crand "crypto/rand"
 	"crypto/tls"
 	"crypto/x509"
+	"encoding/hex"
 	"errors"
 	"io"
 	"math/rand"
@@ -217,6 +219,7 @@ type UpstreamConfig struct {
 	http3RoundTripper6 http.RoundTripper
 	certPool           *x509.CertPool
 	u                  *url.URL
+	uid                string
 }
 
 // ListenerConfig specifies the networks configuration that ctrld will run on.
@@ -261,6 +264,7 @@ type Rule map[string][]string
 
 // Init initialized necessary values for an UpstreamConfig.
 func (uc *UpstreamConfig) Init() {
+	uc.uid = upstreamUID()
 	if u, err := url.Parse(uc.Endpoint); err == nil {
 		uc.Domain = u.Host
 		switch uc.Type {
@@ -341,6 +345,11 @@ func (uc *UpstreamConfig) SetupBootstrapIP() {
 	uc.setupBootstrapIP(true)
 }
 
+// UID returns the unique identifier of the upstream.
+func (uc *UpstreamConfig) UID() string {
+	return uc.uid
+}
+
 // SetupBootstrapIP manually find all available IPs of the upstream.
 // The first usable IP will be used as bootstrap IP of the upstream.
 func (uc *UpstreamConfig) setupBootstrapIP(withBootstrapDNS bool) {
@@ -680,3 +689,15 @@ func ResolverTypeFromEndpoint(endpoint string) string {
 func pick(s []string) string {
 	return s[rand.Intn(len(s))]
 }
+
+// upstreamUID generates an unique identifier for an upstream.
+func upstreamUID() string {
+	b := make([]byte, 4)
+	for {
+		if _, err := crand.Read(b); err != nil {
+			ProxyLogger.Load().Warn().Err(err).Msg("could not generate uid for upstream, retrying...")
+			continue
+		}
+		return hex.EncodeToString(b)
+	}
+}