Inbound Port 53 Blocking (IP53B) Implementation


This page is about the implementation of Inbound Port 53 Blocking (referred to as "IP53B") and about how to request the removal of IP53B.

What Is IP53B?

When a server or other communication device is being used as the platform for DDoS attacks*1 Inbound Port 53 Blocking (IP53B) can be used to block port UDP53 *2.
This blocking can help prevent your IP address being attacked from an external network and therefore also help prevent your server from being used as the springboard for a DDos attack.

*1. DDoS attack: A distributed denial-of-service (DDoS) attack is an attack from a multitude of compromised systems on a single communication server, resulting in the server being flooded with more messages than it can process, essentially shutting it down.
*2. UDP53 port: UDP Port 53 (UDP53) is a port that can be used for communication between computers and DNS servers.


Open DNS Resolver

A DNS resolver is a server that takes domain name inquiries and resolves them into an IP address. An Open DNS Resolver is a server that is publicly accessible and available to resolve DNS inquiries regardless of the source of the inquiry. This makes the server a possible object of DDoS attacks as well as being a possible source of DNS amplification attacks.


Impact On Our Customers

Internet connection and email is not affected by IP53B. Those customers who run their own DNS server and publish it for external access need to be aware that as port 53 is restricted by IP53B it can no longer be used to handle IP address inquiries.


Affected Services

All Asahi Net Internet connection services with the exception of Asahi Net au Hikari.


How To Request IP53B Removal

Customers using Asahi Net's Fixed IP Address option may request the removal of IP53B. Please use the Technical support inquiries form linked below, and when making your request supply your Fixed IP Address and domain name.
Technical support inquiries

We will confirm whether or not your DNS server or router is an open resolver, and should it be so we will not be able to remove IP53B.