all: allowing config defined discover ptr endpoints

The default gateway is usually the DNS server in normal home network
setup for most users. However, there's case that it is not, causing
discover ptr failed.

This commit add discover_ptr_endpoints config parameter, so users can
define what DNS nameservers will be used.

internal/clientinfo/client_info.go

@@ -3,7 +3,9 @@ package clientinfo
 import (
 	"context"
 	"fmt"
+	"net"
 	"net/netip"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -183,6 +185,25 @@ func (t *Table) init() {
 	// PTR lookup.
 	if t.discoverPTR() {
 		t.ptr = &ptrDiscover{resolver: ctrld.NewPrivateResolver()}
+		if len(t.svcCfg.DiscoverPtrEndpoints) > 0 {
+			nss := make([]string, 0, len(t.svcCfg.DiscoverPtrEndpoints))
+			for _, ns := range t.svcCfg.DiscoverPtrEndpoints {
+				host, port := ns, "53"
+				if h, p, err := net.SplitHostPort(ns); err == nil {
+					host, port = h, p
+				}
+				// Only use valid ip:port pair.
+				if _, portErr := strconv.Atoi(port); portErr == nil && port != "0" && net.ParseIP(host) != nil {
+					nss = append(nss, net.JoinHostPort(host, port))
+				} else {
+					ctrld.ProxyLogger.Load().Warn().Msgf("ignoring invalid nameserver for ptr discover: %q", ns)
+				}
+			}
+			if len(nss) > 0 {
+				t.ptr.resolver = ctrld.NewResolverWithNameserver(nss)
+				ctrld.ProxyLogger.Load().Debug().Msgf("using nameservers %v for ptr discovery", nss)
+			}
+		}
 		ctrld.ProxyLogger.Load().Debug().Msg("start ptr discovery")
 		if err := t.ptr.refresh(); err != nil {
 			ctrld.ProxyLogger.Load().Error().Err(err).Msg("could not init PTR discover")