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I have built an image based on the corporate JDK image for our Java application, which needs to connect to Oracle DB.

I deployed it using docker stack on TWO other servers in another availability zones (lets say Z1). Now I am banging my head on a new server (all 3 are based on Ubuntu 18), which is in another zone (lets call it Z2).

Images are based on RHEL 7 apparently, I managed to install sqlplus to test outside the application - on both Host and Docker containers based on the same image, in both Z1 and Z2.

Results show that sqlplus is able to connect instantly on the Z1 docker container (based on same image), but is not connecting on the Z2 container! The sqlplus on Z2 host is also connecting without an issue! So it is not firewalls or external issues, it is docker specific issue on Z2 machine!

For reference, sqlplus connect string I used:

sqlplus64 USER/PASS@//INTERNAL_HOSTNAME:1521/DB_NAME

I tested pinging the hostname from docker container on the problematic Z2, and it works there.. so not corporate DNS issue..

I tried to docker network inspect STACKNAME_default on both Z1 and Z2, and no differences in structure, except subnet, gateway and container IPs, as far as I can tell.

I am not sure what my next step should be... I am sad, Docker containers supposed to work the same way across infrastructure, and in this instance it does not

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