OCI: Unable to Access TCP Port in Ubuntu Instance

Applies to: 

Oracle Cloud Infrastructure

Linux x86_64

SYMPTOMS

• An application is configured to listen for connections on a TCP port (e.g. 10000) in the instance.
• There are active ingress rules in the instance VCN to allow incoming traffic to port 10000/TCP in the OCI Cloud Console.
• This is a standard Ubuntu 18.04 LTS instance with the ufw disabled and stopped
• The application is active and listening on all IP addresses (0.0.0.0/0) for the port 10000 in the Ubuntu instance.

CAUSE

iptables rules are in place and not allowing incoming connections to the 10000/TCP application port

Solution

Check the existing iptables rules:

# iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT all -- anywhere anywhere state RELATED,ESTABLISHED
ACCEPT icmp -- anywhere anywhere
ACCEPT all -- anywhere anywhere
ACCEPT udp -- anywhere anywhere udp spt:ntp
ACCEPT tcp -- anywhere anywhere state NEW tcp dpt:ssh
REJECT all -- anywhere anywhere reject-with icmp-host-prohibited

Chain FORWARD (policy ACCEPT)
target prot opt source destination
REJECT all -- anywhere anywhere reject-with icmp-host-prohibited

Chain OUTPUT (policy ACCEPT)
target prot opt source destination
InstanceServices all -- anywhere link-local/16 .......