Field Kit

A free desktop app with the small network and security tools you keep opening sketchy websites for — subnet math, CVSS scoring, JWT decoding, defanging IOCs, cert checks — in one window, running entirely on your machine.

Built by NetOps Field Notes and SecOps Field Notes. Windows, macOS and Linux — portable, no installer, no signup, no telemetry.

The tools

Network

• Subnet calculator / VLSM splitter — carve a block into named subnets and get the full allocation table
• CIDR summarizer — paste a messy prefix list, get the minimal aggregated route set back
• Wildcard / ACL helper — CIDR ↔ Cisco wildcard, with a ready ACL line, and it flags discontiguous masks
• MTU / MSS calculator — stack VLAN/GRE/IPsec/VXLAN/WireGuard overheads, get the MTU and the MSS clamp
• Transfer time — how long that copy actually takes at that rate
• Config diff — compare two configs without pasting them into someone's website; can skip comments and whitespace
• Ping, TCP port check, DNS lookup — the quick liveness trio; DNS shows your resolver next to 1.1.1.1 so split-horizon surprises jump out
• TLS certificate inspector — what cert a host really serves, expiry countdown, and whether your machine trusts the chain
• Local interfaces / public IP — both ends of your connectivity at a glance

Security

• IOC defang / refang — hxxps[://], [.], [at], both directions
• CVSS v3.1 calculator — metric buttons ↔ vector string ↔ score
• JWT decoder — claims, expiry status, and a loud warning on alg:none. Decode-only, local
• Hashes — MD5/SHA-1/SHA-256/SHA-512 of text or files, compare against a published hash, identify unknown hash formats
• Timestamp converter — epoch in s/ms/µs, Windows FILETIME, ISO 8601
• Decoder workbench — Base64/URL/hex/HTML entities/ROT13, with output-to-input chaining for peeling layered payloads

Download

Grab the latest build from the releases page. SHA256SUMS is published with every release if you want to verify the download.

• Windows / macOS — extract the zip anywhere and run. No installer, no admin rights, nothing written to the system. Delete the folder and it's gone.
• Linux — sudo apt install ./fieldkit_<version>_amd64.deb on Debian/Ubuntu-family (desktop entry and icon included), or the tarball's ./install.sh for everything else — it stays inside ~/.local, no root.

The one-time unsigned-app prompt

These builds aren't code-signed yet, so the first launch asks for a click:

• Windows — SmartScreen will interject. Click More info → Run anyway. That's it, once.
• macOS — right-click the app → Open (macOS 14 and earlier), or go to System Settings → Privacy & Security and hit Open Anyway (macOS 15+). Terminal people: xattr -dr com.apple.quarantine "Field Kit.app" does the same thing.

What talks to the network

Almost nothing. Every converter, calculator, decoder and hash runs locally — paste a config or a token into this app and it does not leave your machine. The exceptions are the tools whose whole job is the network: ping, port check and the TLS inspector talk to the host you point them at, DNS lookup also queries 1.1.1.1 over HTTPS, and the public-IP check calls whatismynetip.com. No analytics, no update phone-home, nothing else.

Building from source

It's a standard Flutter desktop app:

flutter pub get
flutter test          # 53 tests, including the CVSS/RFC vectors
flutter build windows # or: macos / linux

License

MIT. Do whatever you want with it. If it saves you a bad paste into a random website, tell a colleague.