Lock Down the Network Stack

Defense & IR · medium · solved in-browser (no VM needed) · 3 tiered hints

Write /etc/sysctl.d/99-net-hardening.conf to harden IPv4. Enable TCP SYN cookies (net.ipv4.tcp_syncookies = 1), disable acceptance of ICMP redirects (net.ipv4.conf.all.accept_redirects = 0), disable sending redirects (net.ipv4.conf.all.send_redirects = 0), enable reverse-path filtering (net.ipv4.conf.all.rp_filter = 1), and disable IP forwarding (net.ipv4.ip_forward = 0). (Authored config, graded structurally — the engine isn't run here.)

▶ Start this challenge

Nobody has solved this one yet — be the first clear on the leaderboard.

How it works

Open the challenge and an in-page editor appears — write your answer and tmpfs.tech grades it server-side against the same checks a real box would run. No VM, no install, no signup needed to try.

More Defense & IR challenges

Incident response: evict rogue accounts, eradicate persistence, attribute a breach.

Audit the User Databaseeasy · in-browser
Biggest log fileeasy
Block the Attacker at TCP Wrapperseasy · in-browser
Count failed SSH loginseasy
Disable Unused Kernel Moduleseasy · in-browser
Evict a hidden root accounteasy
Find the hidden stasheasy
Harden the SSH Daemoneasy · in-browser

See all Defense & IR challenges →

FAQ

Is it free? Yes — play as a guest, no signup required to start.

Do I need to install anything? No. Everything runs in your browser.

How is it graded? Automatically and deterministically — your work is checked against the exact rules the live box would apply.