Firewall: Restore the Return Traffic a Default-Drop Box Dropped

After an incident, someone rebuilt this host's nftables ruleset and left the inet filter input chain almost empty: the default policy is still drop (correct — leave it), but nothing lets traffic back in. The console still works and inbound SSH is fine, yet everything the box itself starts — package updates, DNS lookups, HTTPS fetches, time sync — hangs and times out, and the ops team's monitoring can no longer scrape it.

That is the signature of a default-drop firewall with no connection-tracking rule: the box sends its request out, the reply comes back as part of an already-established connection, and there is no rule to accept it, so the reply is dropped.

Repair /etc/nftables.conf so the input chain enforces a correct default-drop baseline: accept loopback traffic, accept the reply traffic of connections the box already established, and accept the monitoring service named in /root/case/assignment.env. Keep the default policy at drop — do not make the symptom disappear by setting the policy to accept or flushing the ruleset, which would simply turn the host firewall off. See /root/case/brief.txt. Grading reads the config file.