DNS forwarder and common ebpf loader (#1083)

In case the 53 UDP port is not an option to bind then we hijack the DNS traffic with eBPF, and we forward the traffic to the listener on a custom port. With this implementation, we should be able to listen to DNS queries on any address and still set the local host system to send queries to the custom address on port 53.

Because we tried to attach multiple XDP programs to the same interface, I did a refactor in the WG traffic forward code also.

client/internal/ebpf/ebpf/dns_fwd_linux.go

@@ -0,0 +1,51 @@
+package ebpf
+
+import (
+	"encoding/binary"
+	"net"
+
+	log "github.com/sirupsen/logrus"
+)
+
+const (
+	mapKeyDNSIP   uint32 = 0
+	mapKeyDNSPort uint32 = 1
+)
+
+func (tf *GeneralManager) LoadDNSFwd(ip string, dnsPort int) error {
+	log.Debugf("load ebpf DNS forwarder: address: %s:%d", ip, dnsPort)
+	tf.lock.Lock()
+	defer tf.lock.Unlock()
+
+	err := tf.loadXdp()
+	if err != nil {
+		return err
+	}
+
+	err = tf.bpfObjs.NbMapDnsIp.Put(mapKeyDNSIP, ip2int(ip))
+	if err != nil {
+		return err
+	}
+
+	err = tf.bpfObjs.NbMapDnsPort.Put(mapKeyDNSPort, uint16(dnsPort))
+	if err != nil {
+		return err
+	}
+
+	tf.setFeatureFlag(featureFlagDnsForwarder)
+	err = tf.bpfObjs.NbFeatures.Put(mapKeyFeatures, tf.featureFlags)
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+func (tf *GeneralManager) FreeDNSFwd() error {
+	log.Debugf("free ebpf DNS forwarder")
+	return tf.unsetFeatureFlag(featureFlagDnsForwarder)
+}
+
+func ip2int(ipString string) uint32 {
+	ip := net.ParseIP(ipString)
+	return binary.BigEndian.Uint32(ip.To4())
+}