GDPR.txt feedback

Your case is a bit specific, I'm not totally sure of what I suggest here. What I understand is that there are two kind of process:

1. account name and DNS query are recorded via syslog (purpose: "debug potential errors happening on the service / legitimate interest" I guess?)
2. DNS query is performed against a nameserver (purpose: "process the query / legitimate interest")

Note that vital lawfulness should only be used when data processing is done in order to protect the vital interests of the person.

I'm not sure what you meant by "information about the use of the service". A good way to find purposes is to ask the question: why do I need these data?

About the visibility, I would recommend to consider the point of view of the application. Since there are no "roles" in your software, I would simply say data are "private".

Last comment: my proposition was that comments are on a dedicated line. I think the information you give in comments can be moved in the description.

A slightly modified version:

purpose: debug potential errors happening on the service
lawfulness: legitimate interest

purpose: process the query
lawfulness: legitimate interest

data: fediverse account name
required: yes
visibility: private
description: by default, the program uses syslog to record every
  request, with this data. It is required because the fediverse
  works that way.
mitigation: use a dedicated account

data: DNS query name and type
required: yes
visibility: private
description: by default, the program uses syslog to record every
  request, with this data. It is required because we could not reply
  if we don't have the question 
mitigation: none