ethernet: MTU semantic meaning corrections, add FCS logic (#28)

x/xnet/stack-async.go

@@ -325,6 +325,8 @@ func (s *StackAsync) StartLookupIP(host string) error {
 		return err
 	}
 
+	// EDNS0 buffer size: MTU minus overhead for IP+UDP headers and safety margin.
+	// 100 bytes covers IPv4 max header (60) + UDP (8) + 32 byte margin.
 	s.ednsopt.SetEDNS0(uint16(s.link.MTU())-100, 0, 0, nil)
 	rand := s.prand32()
 	err = s.dns.StartResolve(uint16(rand>>1)+1024, uint16(rand), dns.ResolveConfig{