Tool ·
Paste iptables-save or nft list ruleset output and a packet, and step it through the netfilter hooks in the kernel's order — every chain, every rule tested with the reason it matched or didn't, jumps, returns, policies, DNAT and MASQUERADE — to the final verdict; with Docker's and kube-proxy's chains explained.
Tool ·
Paste the output of uptime, dmesg, vmstat, mpstat, pidstat, iostat, free, sar, top, /proc/meminfo, /proc/pressure, ss and cgroup files — several at once, in any order — and get a reading in the style of Brendan Gregg's 60-second checklist and the USE method: what each column means, what is abnormal, what to run next. Plus exit codes and signals (137 OOM-kill vs SIGKILL, 143) for Docker and Kubernetes.
Tool ·
Paste strace output — with or without -f, -tt/-ttt/-r, -T, -y/-yy, unfinished/resumed pairs, signals, -c tables — or macOS dtruss output, and get a per-thread timeline, a summary that matches strace -c, the slowest calls, errors grouped (path probing, EAGAIN loops), files with bytes read and written, connections and DNS lookups decoded from the buffers, the process tree with argv, and hints.
Tool ·
A real Linux booted in the page (a small kernel and BusyBox in the v86 x86 emulator) with a terminal, sample logs, CSVs and a config tree in your home directory, and 26 exercises — text pipelines, find and xargs, redirection, permissions, processes and signals, environment variables, loops — that check your answer.
Tool ·
Build and explain nginx, Caddy, systemd and SSH configs — reverse proxies with Mozilla TLS settings, WebSockets and rate limits (with a limit_req simulation), static sites, hardened systemd services and timers with next run times and an exposure estimate, and ssh_config hosts and tunnels with a diagram — or paste your own and have each line explained and checked.
Article ·
How I built CryptoFS, a stacked Linux filesystem that encrypts every file with its own AES-256-GCM key at the VFS layer — authorised processes see plaintext, everyone else sees ciphertext — and what it taught me about nonces.
Tool ·
Convert between the permission checkboxes, octal (755), and symbolic (rwxr-xr-x) modes — with setuid, setgid, sticky, umask, ls -l parsing, and ready-to-run chmod commands.
Project ·
A Linux kernel module that encrypts files transparently at the VFS layer with per-file AES-256-GCM keys — authorised processes see plaintext, everyone else sees ciphertext.