Resolving Oracle Enterprise Manager (OEM) on Oracle Cloud Infrastructure (OCI) can sometimes feel like peeling back layers of an onion. You’ve verified your VCN security lists, confirmed your ingress rules are wide open, and even checked that firewalld is inactive. Yet the OEM console persists with a frustrating “No route to hosts” error on port 3872. If you’ve hit this wall, the culprit isn’t your cloud network; it’s hiding right underneath the OS hood. Oracle Linux images in OCI frequently carry pre-configured, low-level kernel packet filters or iptables rule chains that silently drop traffic on non-standard ports by default, bypassing higher-level firewall managers.
The steps below show how to resolve a “No Route to Host” error on the OEM console.
1. Check the ingress rule on the VCN to confirm that the OEM is allowed access to the VM on port 3872.
2. Check Low-Level iptables Rules on the target VM.
Run this command as root to see if a hidden drop or reject rule is intercepting port 3872 traffic:
sudo iptables -L -n -v | grep 3872
If this returns nothing or shows packets being dropped/rejected, iptables is blocking the connection.
3. Test telnet from the OMS to the target VM on port 3872.
Test telnet access from the OMS server to the DR server on port 3872.
The output above shows that the OEM cannot reach the target VM on port 3872.
4. Explicitly Force Open Port 3872 via iptables.
To instantly force the local Linux firewall kernel to accept connections on port 3872, run:
sudo iptables -I INPUT -p tcp --dport 3872 -j ACCEPT
5. Test telnet access again from the OMS server to the DR server on port 3872.
The output shows that the OEM server can now reach the target on port 3872.
6. Add the internal targets and execute the upload agent command.
7. The “No Route to Host” error is resolved, and the target is now up.
The metric evaluation error is resolved, and the target is up on the OEM.
For more information, check out our Oracle Cloud Infrastructure and Oracle Enterprise Manager services, or contact us today, and one of our experts will be in touch.