Upgrade legacy 16.04 LTS to 18.04 LTS --- 'do-release-upgrade' shows many packages not found. Should I go through with it?

Question

I'm new to programming and running software/OS updates, so I'm trying to be cautious, though my files are backed up just in case. I've included the full context of this upgrade process, so if you want to skip to the question at hand feel free to scroll to the end.

I'm upgrading our legacy server. After removing the old PPA references using:

sudo add-apt-repository --remove ppa:<username>/<ppaname>

..and updating our release url references to old-releases using:

sudo sed -i -re 's/([a-z]{2}\.)?archive.ubuntu.com|security.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list

..and installing manager-core:

apt-get install update-manager-core

I ran the update:

sudo apt-get update

..and was relieved to see that most of the packages were updated, excepting these 5 packages:

N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: The repository 'http://old-releases.ubuntu.com/ubuntu xenial-updates Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: The repository 'http://old-releases.ubuntu.com/ubuntu xenial-backports Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: The repository 'http://old-releases.ubuntu.com/ubuntu xenial-security Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch http://old-releases.ubuntu.com/ubuntu/dists/xenial/main/binary-amd64/Packages  404  Not Found [IP: 91.189.91.123 80]
E: Failed to fetch http://old-releases.ubuntu.com/ubuntu/dists/xenial-updates/main/binary-amd64/Packages  404  Not Found [IP: 91.189.91.123 80]
E: Failed to fetch http://old-releases.ubuntu.com/ubuntu/dists/xenial-backports/main/binary-amd64/Packages  404  Not Found [IP: 91.189.91.123 80]
E: Failed to fetch http://old-releases.ubuntu.com/ubuntu/dists/xenial-security/main/binary-amd64/Packages  404  Not Found [IP: 91.189.91.123 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.

At this point wasn't sure if this was a deal-breaker on the upgrade or not (difficulty find clear answers to this online) so I went ahead and ran the upgrade:

sudo apt-get upgrade -y

This seemed to go well, though nothing happened:

The following packages have been kept back:
  certbot python-acme python-certbot-apache python-parsedatetime python3-requests python3-urllib3
0 upgraded, 0 newly installed, 0 to remove and 6 not upgraded.

..so I went to the next step and ran:

sudo apt-get dist-upgrade

This ran fine, threw no errors, so I went to the next step and ran:

sudo do-release-upgrade

..and THAT's when I got perhaps a hundred of these 404 errors (all from the same IP address):

Err http://old-releases.ubuntu.com/ubuntu xenial/main all Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial/main Translation-en
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial/restricted amd64 Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial/restricted i386 Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial/restricted all Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial/restricted Translation-en
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial/universe amd64 Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial/universe i386 Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial/universe all Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial/universe Translation-en
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial/multiverse amd64 Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial/multiverse i386 Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial/multiverse all Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial/multiverse Translation-en
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial-updates/main i386 Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial-updates/main all Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial-updates/main Translation-en
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial-updates/restricted amd64 Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial-updates/restricted i386 Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial-updates/restricted all Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial-updates/restricted Translation-en
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial-updates/universe amd64 Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial-updates/universe i386 Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial-updates/universe all Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial-updates/universe Translation-en
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial-updates/multiverse amd64 Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial-updates/multiverse i386 Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial-updates/multiverse all Packages
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial-updates/multiverse Translation-en
  404  Not Found [IP: 185.125.190.37 80]
Err http://old-releases.ubuntu.com/ubuntu xenial-backports/main amd64 Packages
  404  Not Found [IP: 185.125.190.37 80]

also still need to research these snaps:

No snaps are installed yet. Try 'snap install hello-world'.
Checking for installed snaps
Calculating snap size requirements
No candidate ver:  liblockfile1
No candidate ver:  python-pbr
No candidate ver:  sendmail-base
No candidate ver:  sendmail-bin
No candidate ver:  sendmail-cf
No candidate ver:  ssmtp
Updating repository information
Third party sources disabled
Some third party entries in your sources.list were disabled. You can
re-enable them after the upgrade with the 'software-properties' tool
or your package manager.
To continue please press [ENTER]
^C
=== Command terminated with signal 2 (Wed Aug  3 13:41:27 2022) ===

I pinged 185.125.190.37 just fine, so it seems those packages just don't exist at this IP address... I was afraid to run the upgrade without making sure it was okay that the majority of those packages were not retrieved. Which leads to my question--

Question: Is it still okay to run the do-release-upgrade with so many missing packages? If not, how can I get these packages? Is it too late for Ubuntu 16.04?

Thank you so much for your help!

Answer 1

As already stated here, 16.04 ESM is still supported thus you don't need to use old-releases.ubuntu.com, however standard support has ended and as such there are now complications to the upgrade, which may depend on where you are in the world. I'll provide some thoughts

• To have good CA certificates for your location, you may encounter EOL certificates unless you've applied ESM fixes & thus enabled ESM & thus fully-upgraded using the packages only available via ESM.

• As standard support has ended, many PPA & 3rd party mirrors may have dropped support for the release, so if you're using one of those you may have packages that will now create problems for you that you'll only discover when you hit the problems. If you use only Ubuntu repository software though; I'd not expect an issue here.

• Return your source to the main archive and give the do-release-upgrade a try. I did this merely weeks ago to leave a comment on a xenial system I discovered I had, after a few (5-8) mins of fixing issues & upgrading packages - the do-release-upgrade started operating normally and downloading bionic packages... In my case I aborted the upgrade & returned my box to xenial (I'll upgrade it via re-install when I need it) but it had reached a point where I was happy it was working & I could leave my comment.

In your case I'd revert the old-releases.ubuntu.com to archive.ubuntu.com & ensure you have all packages, and try again. What issues you'll have as stated may depend on where you are in the world (certificates; you may need to apply ESM so as to get upgraded certificates or be lucky like me & not need to) or have complications with 3rd party packages (no issue for me; I had none).

If you want to avoid this; just maintain the system & upgrade before it reaches end of standard support. Ubuntu 16.04 LTS tells you it was the 2016-April release in it's name, it comes with 5 years of standard support; so adding 5 (years) to 16 (2016) gives 2021-April for end of standard support (EOSS) or when you need to have done the release-upgrade done before additional work maybe required.

Answer 2

It looks like releases that are still in ESM (and not EOL yet) are located in https://us.archive.ubuntu.com (or your own two-letter country code), not https://old-releases.ubuntu.com. To confirm this, you can try visiting both of them at the url + /ubuntu/dists, and see if your release is located there. In this case, I checked, and at the moment, xenial is not in old-releases at all. You can try switching your sources.list to point to that site, and see if the updates go through.

Answer 3

I think that "upgrading" from one End Of Life unsupported release to another one which is EOL also is a bad idea. I'd suggest you upgrade (maybe by a clean install?) to 22.04 or to 20.04 at least.