Update port, ip choice logic in DNS service (#1514)

Ensure we use WG address instead of loopback addresses for eBPF.
- First try to use 53 port
- Try to use 5053 port on WG interface for eBPF
- Try to use 5053 on WG interface or loopback interface

client/internal/ebpf/ebpf/dns_fwd_linux.go

@@ -13,7 +13,7 @@ const (
 )
 
 func (tf *GeneralManager) LoadDNSFwd(ip string, dnsPort int) error {
-	log.Debugf("load ebpf DNS forwarder: address: %s:%d", ip, dnsPort)
+	log.Debugf("load eBPF DNS forwarder, watching addr: %s:53, redirect to port: %d", ip, dnsPort)
 	tf.lock.Lock()
 	defer tf.lock.Unlock()
 

client/internal/ebpf/ebpf/src/readme.md

@@ -1,5 +1,10 @@
-# Debug
+# DNS forwarder
 
+The agent attach the XDP program to the lo device. We can not use fake address in eBPF because the
+traffic does not appear in the eBPF program. The program capture the traffic on wg_ip:53 and 
+overwrite in it the destination port to 5053.
+
+# Debug
 
 The CONFIG_BPF_EVENTS kernel module is required for bpf_printk.
 Apply this code to use bpf_printk