Focused, hands-on DevOps and server-side problems with hints, feedback, and automated solution checks.
Firewall rules added with the iptables command disappear when the server reboots, and the ports they protect can become exposed again. Learn how to make iptables rules persistent so that they are restored every time the server boots.
Firewall rules added with the nft command disappear when the server reboots, and the ports they protect can become exposed again. Learn how to make nftables rules persistent so that they are restored every time the server boots.
A container port published on a server's internal IP is still reachable from the external network. Learn how to restrict access with firewall rules without disrupting legitimate connections.
The management VPN is down, but you urgently need SSH access to a datacenter server that is still reachable through its application network. Find an alternative path to SSH using a routing trick and Linux's weak end host behavior.
On a dual-homed Linux server, a service bound to one network's IP address may still be reachable from another network. Learn how to prevent this unexpected access without disrupting legitimate connections.
A payment service subscribes to payment requests through NATS. Run the checkout team's test script as a Job in the checkout namespace, with the required settings, and make sure the payment request arrives.
On a dual-homed Linux server, a service bound to the IP address of one interface may still accept connections that arrive through the other interface. Learn how to reach such a service through an interface it doesn't listen on, using a routing trick.
Learn how to access a service bound to localhost of the Docker host from a bridge-networked container, without exposing the service to the local network or disabling network isolation.
Learn how to access services running on the Docker host from a bridge-networked container, without hardcoding host IP addresses or disabling network isolation.
Play a security researcher: identify the hosts a black-box container keeps making encrypted HTTPS calls to, route them through a MITM proxy, fake the certificate, and see what the container sends over the network.
