This is another post that started after several toots on mastodon. Most of the things presented here were already tooted by other people, but I think this is a good chance to write a mini tutorial about what to look at. We’ll use DNS, whois, BGP and your favourite search engine.

What is DNS4EU?

To quote the web page:

Supported by the European Union Agency for Cybersecurity (ENISA), the European Union’s DNS4EU secure-infrastructure project provides a protective, privacy-compliant, and resilient DNS service to strengthen digital sovereignty and security for EU citizens, governments, and critical infrastructure.

Just so I find it again when I run across the same problem in the future:

I wanted to log in to one of my servers and couldn’t access it, because I couldn’t resolve the name from one specific domain. All other domains worked fine.

So let’s head over to the DNS server and check what we can do. I’m running BIND, and it ships with two tools, named-checkconf and named-checkzone. As the name implies named-checkconf checks the overall configuration, named-checkzone single zone. There is also an option -z for named-checkconf that checks all the primary zones found in the configuration.

Another troubleshooting tale.

When you look at the following output, what is your first guess?

root@mail:~# postqueue -p
-Queue ID-  --Size-- ----Arrival Time---- -Sender/Recipient-------
D94947FBE83    1331 Thu Apr  3 15:18:58  jens@example.net
(Host or domain name not found. Name service error for name=example.com type=MX: Host not found, try again)
                                         foobar@example.com

DNS right? Nope! Well kind off.

In reality it was another domain and not example.com, and it was of many mails I wrote that day and the only one that couldn’t be delivered. What looked like a DNS problem at first glance was accutaly an IPv4 routing problem inside the provider network.

Some shameless advertising:

If you need some help with IPv6, DNS, Linux, Automation and related stuff feel free to contact me. I’m available for about two days a week, remote preferred.

• CLAT in Linux Networkmanager
• DNS delegation
• jc - Convert command output to json
• Stratoshark - Analyse syscalls
• curl from start to end
• Security Bingo
• Linux Routing Fundamentals
• Packet where are you
• nftable-tracer