So you set up your Free Oracle Cloud trial, including the always free tier which give you some free VPS virtual machines ostensibly forever. Or you are migrating your business resources to Oracle Cloud, as it is slightly less evil than some other clouds.
You find that you cannot ping, or traceroute, or mtr to your Oracle Cloud VPS virtual machines. Oracle Cloud by default has a limited set of ingress firewall rules, and blocks ICMP. But you can modify those rules!
Unless you have done something special your vm instances will end up in a single subnet which has a common set of firewall filtering rules.
In order to allow ICMP just follow these steps with pretty highlighted screenshots:
From the “hamburger”menu at the top left choose Networking->Virtual Cloud Networks.
Under the Virtual Cloud Networks listing click on your vcn (which will be named differently, but in the same place as the highlighted link above).
Under the Subnets listing click on your subnet (again, it will be named differently, but in the same place as the highlighted link above).
Under the Security Lists display select your Default Security List (in the highlighted location above).
Click Add Ingress Rules to add a new rule.
Set the Ingress Rule up as displayed above with the Source CIDR set to 0.0.0.0/0 (all hosts– or limit to your own subnet), and the IP Protocol set to ICMP (for ping/traceroute/mtr).
Click Add Ingress Rule when done.
That’s it! Now you should be able to ping or traceroute or mtr to your Oracle cloud server. This same Add Ingress Rule can be used to modify your other firewall settings to open or close specific ports, limit ssh to certain source hosts (or change its port), etc. You can adjust all your Oracle Cloud Firewall settings right here. If you want to get advanced you can create different subnets and make different rules for those subnets. I am not sure how much can be done with the free tier, but it is seems fairly powerful.
NOTE on Port 25 SMTP: By default (link and link) outbound port 25 (SMTP email) is now blocked on Oracle Cloud instances. This is a shame. Also, under the free tier I do not believe you can set up reverse DNS (PTR). So it would be difficult to use to send email anyway. You will have to send through a smart host (on, e.g. port 587) instead.