The first thing I noticed (because all my web app users screamed) was that PHP sessions suddenly didn't work. Previous to the update, I know that Nginx was running as user "www-user" (in the www-data group), but afterwards it was running as "nginx" (in the nginx group). In addition, I don't know what it was like before, but after the update, the /var/lib/php/session directory's ownership was "root:apache". Weird, since Apache isn't even installed! Even "www-user" was not in an "apache" group, so I know that either the owner or the group (or both) changed. I told Nginx to run as "www-user" again (to hopefully reduce the number of other surprises), and changed that directory to "root:www-data" ("www-data" is the primary group of "www-user") - that works, but I don't know if it's correct or not.
Then I noticed that my one Wordpress website located on that server was no longer successfully concatenating CSS files (the output would abruptly stop after 46,140 bytes regardless of what files were being combined). I turned that feature off as a band-aid, but I assume that something about the server update caused it to stop working, and I'm completely stumped as to what could cause that symptom. I also fear that other PHP processes might have the same problem but I haven't stumbled upon them yet.
Then, in the process of troubleshooting something else, I noticed that the hostname in all the system logs (cron, messages, maillog, etc.) changed from "puphpet2" (the name I gave to this Linode when I created it) to "mail" (which seems like a weird hostname to me - I'm quite sure I didn't change it to that). I confirmed with the "hostname" command, which reported "mail.mydomain.tld". It is true that the same weekend as the update, I was setting up postfix and dovecot, but not only should that not have anything to do with the hostname, but the hostname listed in the logs changed exactly when the server rebooted as part of the update. I'm assuming that an odd hostname is harmless, but it makes me think that the changes caused by that update run pretty deep.
I know this is a broad question, but does anyone know:
- why those types of things, like directory ownership or hostname, would change during an update to the OS and packages?
- what else I should look for (or even how to look - I have tried to look for oddities in logs, but nothing seems obviously errant)?