It wasn’t without a bit of a head scratching experience though when I was trying to get application connectivity between two OCI images on the same private 10.0.0.0/24 network I had created.

eg.

curl http://10.0.0.53/
curl: (7) Failed to connect to 10.0.0.53 port 80: No route to host

My first thought was the cloud ingress rules, but i’d added the following as a first desperate attempt to get things working.

Try again, Still no route!

What I discovered is the OCI supplied images (I was using the Ampere Ubuntu 20.04 image in this case) have an interesting set of iptables rules baked into the image.

root@blog:~# cat /etc/iptables/rules.v4
# CLOUD_IMG: This file was created/modified by the Cloud Image build process
# iptables configuration for Oracle Cloud Infrastructure

# See the Oracle-Provided Images section in the Oracle Cloud Infrastructure
# documentation for security impact of modifying or removing these rule

*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [463:49013]
:InstanceServices - [0:0]
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -p udp --sport 123 -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 22 -j ACCEPT
#-A INPUT -j REJECT --reject-with icmp-host-prohibited
-A FORWARD -j REJECT --reject-with icmp-host-prohibited
-A OUTPUT -d 169.254.0.0/16 -j InstanceServices
.
.
.

I’ve commented out the offending line. With OCI supplied images, even though the default filter is ACCEPT, they place a reject-with icmp-host-prohibited at the end of the INPUT table, which effectively rejects everything not specifically allowed (such as the port 22 rule the line before).

My two options were to either put in my specific allows (the right thing to do) or remove the reject and just use the INPUT filter default. I chose the latter as I was experimenting in this case and kept the information at my finger tips for more ‘production-like’ deployments.

The end result, communication between the 2 OCI Ubuntu instances over the private network now works fine.

Caveat: In my case I understood the risks associated with removing the reject for my Use Case. Please perform your own due diligence for your Use Case, you’re probably better off specifically adding the communication rules you want to allow.