Jon Bentley's questions

I wish to clear my local DNS cache so that I can access a server via a domain name which has recently had its DNS A record changed. For illustration purposes, lets assume the following:

Domain: mydomain.com

Old IP address 111.111.111.111

New IP address 222.222.222.222

Everything seems to be updated at my ISP's DNS and on my router's DNS cache:

$ dig @<ISP DNS> mydomain.com
;; ANSWER SECTION:
mydomain.com. 7186  IN  A   222.222.222.222

$dig @<router>
;; ANSWER SECTION:
mydomain.com. 7186  IN  A   222.222.222.222

The local cache however, is not updated:

$ dig mydomain.com
;; ANSWER SECTION:
mydomain.com. 7186  IN  A   111.111.111.111
;; SERVER: 127.0.0.53#53(127.0.0.53)

Clearing the local cache seems to work, but only temporarily:

$ systemd-resolve --statistics;systemd-resolve --flush-caches;systemd-resolve --statistics
Cache                      
  Current Cache Size: 1    
          Cache Hits: 12222
        Cache Misses: 6507 

<omitted>

Cache                      
  Current Cache Size: 0    
          Cache Hits: 12222
        Cache Misses: 6507 

$ dig mydomain.com
;; ANSWER SECTION:
mydomain.com. 7186  IN  A   222.222.222.222
;; SERVER: 127.0.0.53#53(127.0.0.53)

However, after some time, seemingly at random, I can no longer access the server via the domain name. Running dig again shows that the local cache has reverted back while the router and ISP's entries have remained correct:

$ dig mydomain.com
;; ANSWER SECTION:
mydomain.com. 7186  IN  A   111.111.111.111
;; SERVER: 127.0.0.53#53(127.0.0.53)

$ dig @<ISP DNS> mydomain.com
;; ANSWER SECTION:
mydomain.com. 7186  IN  A   222.222.222.222

$dig @<router>
;; ANSWER SECTION:
mydomain.com. 7186  IN  A   222.222.222.222

I have not figured out how to reliably reproduce the problem. The first time, it reverted overnight. Today it reverted within a few seconds. In between, I have tried accessing the domain name via Firefox which has sometimes correctly loaded the site, and sometimes not (following which a check of the local DNS cache shows it has reverted).

Any suggestions would be most welcome.

On my laptop I have Xubuntu 20.04. I've noticed that running # apt update && apt upgrade -y hardly ever results in updates being applied.

Upon doing a comparison of /etc/apt/sources.list between my laptop and a fresh Xubuntu (installed in a VM using the 20.04.1 point release ISO), I've noticed that the following lines are missing (as opposed to commented out) on my laptop's version of the file:

deb http://gb.archive.ubuntu.com/ubuntu/ focal-updates main restricted
deb http://gb.archive.ubuntu.com/ubuntu/ focal-updates universe
deb http://gb.archive.ubuntu.com/ubuntu/ focal-updates multiverse
deb http://gb.archive.ubuntu.com/ubuntu/ focal-backports main restricted universe multiverse

What could have caused this difference between the two installations and how can I avoid it happening in future?

Edit

1. It has been proposed to close this question on the basis that this is a duplicate. That question asks how to restore a broken sources.list file. I already know how to do that. My question is asking why the lines are missing the in first place. The two questions are not duplicates of each other. Whilst I am grateful for efforts to locate a duplicate, please be careful to make sure that it actually is before submitting a close-vote.
2. It was suggested in the comments that I must have edited the file myself. This is not the case. The file sources.list.save is missing exactly the same lines and has a modified date of 26 April 2020 which is the date that I installed the system (a few days after 20.04 was released in case that is significant).

OS: Xubuntu 20.04

DE: XFCE 4.14

Package: Chromium 81.0.4044.129

How can I disable the Chromium icon in the system tray? It appears the first time I open Chromium, and remains until I shut down the PC, even if I close Chromium. It does not appear to provide any function; clicking on it does nothing with or without Chromium running.

Normally when installing Ubuntu server after selecting the language you can press F4 to access alternative installation modes, and from there you can choose to install a minimal system as per the following example screenshot:

I have recently upgraded my host from 16.04 (with Virtual Machine Manager 1.3.2 and VirtualBox 6.1.2) to 20.04 (with Virtual Machine Manager 2.2.1 and VirtualBox 6.1.6). Now when I try to install Ubuntu Server as a KVM or VirtualBox guest, the option for a minimal install is missing:

I have tried with both 18.04 and 20.04 as the guest OS, using the exact same ISO images that I was previously using in the 16.04 host. The only conclusion I can reach therefore is that the installer is detecting something in the host and removing the minimal install option.

What can I do to bring back the option? Solution preferred with Virtual Machine Manager/KVM as I only used VirtualBox to see if the problem was replicated there.

Ubuntu 18.04 does not come with 32-bit standard ISO files. However, a minimal install ISO is available.

I have some older machines on which I would like to install a server, so 32-bit is a requirement. There are two problems:

1. The minimal install ISO (as of today, 6 May 2018) appears to be a pre-release version. The file has a timestamp of 2018-04-25 21:23, which is a day or two before the release date. This is true of the 64-bit version as well.

2. I want to ensure that I have the exact same installation as I would get if I installed from a full (and post-release) server ISO.

What should I do to obtain a complete and up-to-date 32-bit installation? If I simply install from the pre-release mini.iso and select the server packages during installation, will this achieve my aim?

I have a network printer with a static IP address. Recently I've changed my subnet and had to allocate it a new static IP address.

I've tried to update CUPS with the new IP address by going to Printers -> [Printer name] -> Administration -> Modify printer -> Current connection -> Continue. I'm presented with this screen but the connection string is not an editable box, so I can't change the IP address:

How can I modify the IP address?

EDIT: For the avoidance doubt, this question is not about how to get a list of files that a package installs, as in the suggested duplicate.

Is there a standard way to find out the purpose of a package, that is detailed enough that you can work out what you will get from the package and whether or not it is something that you want?

I'll give a couple of examples to explain what I mean. The first, postgresql-contrib, is what lead me to ask this question. I've come across various installation instructions for PostgreSQL that tell you to install this package in addition to postgresql, without explaining why in any meaningful way (example 1, example 2).

apt-cache show postgresql-contrib only gives me a vague and general description:

additional facilities for PostgreSQL (supported version) This metapackage always depends on the currently supported PostgreSQL database contrib package.

PostgreSQL is a fully featured object-relational database management system. It supports a large part of the SQL standard and is designed to be extensible by users in many aspects. Some of the features are: ACID transactions, foreign keys, views, sequences, subqueries, triggers, user- defined types and functions, outer joins, multiversion concurrency control. Graphical user interfaces and bindings for many programming languages are available as well.

What I'm after is a proper description of what additional facilities I get if I install this package.

Another example is libreoffice-templates, where apt-cache show doesn't provide any useful information about what you'd actually get:

Additional set of templates for LibreOffice
 Additional set of templates for:
  - LibreOffice Calc spreadsheets
  - LibreOffice Writer texts
  - LibreOffice Impress presentations

Is there a standard way of getting more detailed information than apt-cache show gives?

I am trying to create a minimal install of Ubuntu Server 14.04.3. I selected the "minimal" option during installation.

My system now has two meta packages, ubuntu-minimal and ubuntu-standard. The latter contains many packages which go above and beyond what I would consider a "minimal" installation for a server e.g. telnet and ftp clients.

I would like to remove some of these packages however doing so will also remove the metapackage. The description of ubuntu-standard contains the following warning:

It is also used to help ensure proper upgrades, so it is recommended that it not be removed.

I can find little information on what this actually means. The best I've come up with is this answer, which suggests that the removal of a metapackage will cause problems when upgrading to a new version of Ubuntu.

Other than when upgrading to new versions of Ubuntu, are there any other consequences of removing a metapackage? Is it possible that a package could be added to or removed from the metapackage for security reasons, that I would miss out on in the absence of the metapackage? E.g. it is decided that Package X should now be part of ubuntu-standard to fix some newly discovered security issue. Will the update process of any remaining packages on my system be unaffected?