disable iptables
systemctl disable iptables
systemctl stop iptables
I figured it out. The connectivity issue was due to Oracle’s default use of iptables on all Oracle-provided images. Literally the very first thing I did when spinning up this instance was check ufw, presuming there were a few firewall restrictions in place. The ufw status was inactive, so I concluded the firewall was locally wide open. Because to my understanding both ufw and iptables look at the netfilter kernel firewall, and because ufw is the de facto (standard?) firewall solution on Ubuntu, I’ve no idea why they concluded it made sense to use iptables in this fashion. Maybe just to standardize across all images?
I learned about the rules by running:
$ sudo iptables -L
Then I saved the rules to a file so I could add the relevant ones back later:
$ sudo iptables-save > ~/iptables-rules
Then I ran these rules to effectively disable iptables by allowing all traffic through:
$ iptables -P INPUT ACCEPT
$ iptables -P OUTPUT ACCEPT
$ iptables -P FORWARD ACCEPT
$ iptables -F
To clear all iptables rules at once, run this command:
$ iptables --flush
Anyway, hope this helps somebody else out because documentation on the matter is non-existent.