---
title: "DNS flag day 2020"
description: "Be prepared for DNS flag day 2020. Check your DNS systems!"
url: "https://mailmum.io/posts/dns-flag-day-2020/"
image: "https://mailmum.io/images/dns-flag-day-2020.png"
---

# DNS flag day 2020

![DNS Flag Day 2020 - all tests are green](https://mailmum.io/images/dns-flag-day-2020.png)

One of the most critical parts of MailMum is to provide DNS service, which
handles [IP blacklisting](https://mailmum.io/glossary/ip-blacklisting/) for our customers over the
[DNS blocklist (DNSBL)](https://mailmum.io/glossary/dns-blocklist-dnsbl/) protocol. They are the primary service our
customer's mail servers are using.

As the [DNS flag day 2020](https://dnsflagday.net/2020/) directed to operational
and security problems in DNS caused by Internet Protocol packet fragmentation,
we've tested all our DNS services to align these rules.

## Why you should test too?

DNS is a distributed system and one of the most critical parts of networks. If
DNS fails; most services stop working as they rely on DNS resolution. Most
companies run at least one DNS server our use a provided DNS server for their
company. Some run thousands to offer their employee and their customers DNS
services.

Every admin and user should take view minutes of their time and test the DNS
servers they are running or DNS services they use for compliance. If the server
does not comply, gently tell your admin. If you are the admin, please take some
time and try to fix the complaints. The [DNS flag days](https://dnsflagday.net/)
page describes the settings for most common software components.

## Building a test suite for DNS

As we already told you, DNS is a critical part of MailMum.  That's why we must
test it in every possible way before production and in production too.

Testing is an essential part of our software development. We've built a test
suite for our server software to comply with global standards based on many
different RFCs. The DNS flag day is one of these days where we recheck the most
common complaints.

The whole test suite runs through CI (Continuous Integration) systems before the
software gets deployed to staging and production. You know these moments when the
test suite works on your development system but fails for some reason on CI or
even worse in the production environment.

We are testing our software with the most widespread mail services like Postfix,
Exim, Sendmail, etc. As every software has a version, we test them based on
standard distribution versions of Debian, Ubuntu, Redhat, and Suse Linux. There
is still a lot of work to do.

To have a second opinion external services for uptime and responsiveness monitor
all our DNS servers. Having a second opinion is an integral part of the
development life cycle to get a second opinion on our data, see outages, analyze
the reasons, fix problems, and see networks and services fail.

## Summary

As we are developers and customers of our services, we use them to provide
services to our mail customers. We've built MailMum for ourselves; that's why we
say: "eat your own dog food."

Thank you for reading and your help in improving small parts of the global DNS
system.

## Links

* [DNS flag days](https://dnsflagday.net)
* [DNS flag day 2020](https://dnsflagday.net/2020/)
* [DNS flag day 2019](https://dnsflagday.net/2019/)
* [RFC 7766 DNS Transport over TCP - Implementation Requirements](https://tools.ietf.org/html/rfc7766)
* [RFC 6891 section 6.2.3.  Requestor's Payload Size](https://tools.ietf.org/html/rfc6891#section-6.2.3)
* [RFC 6891 section 6.2.4.  Responder's Payload Size](https://tools.ietf.org/html/rfc6891#section-6.2.4)