ipswap

A tray-resident Windows utility for switching a network adapter between saved static-IP configurations. Built for field and support work, where a laptop hops between customer subnets — broadcast control networks, media networks, management VLANs — many times a day.

Switching a preset is two clicks and no browser. Editing fifty presets happens in a real UI. Those are different problems, so they use different interfaces.

How it works

Fast path (native, no browser). The tray icon holds a menu of presets grouped into submenus. Clicking one reads the adapter's live configuration, shows a Win32 message box with a before/after diff, and on Yes runs the netsh sequence. The tray tooltip then names the preset that is actually applied.

Slow path (browser). "Manage presets…" starts a local HTTP server on a random loopback port and opens your default browser to it. Full CRUD, adapter picker, import/export, settings, and switching. The server shuts itself down five minutes after the browser stops sending heartbeats, and on exit.

Switching works from either side. The tray confirms with a native MessageBox; the browser shows the identical before/after text in a modal. Both read the adapter live at prompt time and both go through the same code — see internal/switcher, which exists so that path is written once.

Launching

Opening ipswap — from the desktop shortcut, the Start menu, or by running the .exe — opens the editor in your browser. If ipswap is not already running that also starts the tray; if it is, the running copy opens the browser and the second process exits. You never get two tray icons.

That is done with a loopback control listener owned by the running instance, whose port and token live in %APPDATA%\ipswap\session.json (mode 600). A second launch reads that file, calls the listener and exits. On Windows a session-local named mutex settles who is primary when two launches race.

The start-with-Windows entry passes --background, which starts the tray without opening a browser. Nobody wants a browser tab at login.

Building

Pure Go, no cgo anywhere, so a Linux or macOS host cross-compiles the Windows binary directly — no Wine, no mingw, no fyne-cross.

make build      # -> bin/ipswap.exe
make check      # tests + vet + typecheck for both targets
make dist       # bin/ipswap.exe plus dist/SHA256SUMS
make installer  # -> Output/ipswap-<version>-setup.exe (needs Inno Setup)

make build regenerates cmd/ipswap/resource_windows.syso first. That object carries the icon, the version resource and the manifest — the manifest is the part that matters, because it is what asks for asInvoker and per-monitor DPI.

GOOS=windows go vet ./... is worth running on its own during development: the Windows-only files (netsh, user32, the registry) are most of the risk and a plain Linux build never looks at them. make vet does both targets.

go run ./cmd/devserver starts the editor alone against a throwaway preset file, so the web UI can be opened on a non-Windows box. It is a development aid; the shipped binary is cmd/ipswap only.

Installer

Inno-compile.iss builds a per-user installer: it installs to %APPDATA%\ipswap with PrivilegesRequired=lowest, so it never shows a UAC prompt. That is deliberate and matches the app — ipswap ships an asInvoker manifest, and an installer demanding elevation would undo the point. It also means the in-app updater can replace the .exe without a privilege prompt, which a Program Files install would not allow.

It offers a desktop shortcut and an optional start-with-Windows entry, and it uses the app's own single-instance mutex as AppMutex so setup notices a running copy — a running .exe on Windows cannot be overwritten.

Uninstalling leaves presets.json, config.json and the log alone. Inno only removes what it installed, and preset packs represent real work.

Inno Setup is a Windows tool, so this is the one build step that is not cross-platform; it runs fine under wine if you would rather not use Windows. The .exe itself still cross-compiles natively, so CI never needs wine.

Layout

cmd/ipswap/          entrypoint, manifest, version resource
cmd/devserver/       runs the editor alone for development
internal/preset/     data model, store, prefix parsing, import/export
internal/netcfg/     adapter reads (Win32) and the netsh command plan
internal/switcher/   the one place a preset is applied; both front ends use it
internal/instance/   single-instance lock and the launch control channel
internal/tray/       tray menu, confirm-then-apply, icon
internal/server/     the local editor server and its embedded web app
internal/updater/    Gitea release check, download, verify, swap
internal/dialog/     Win32 MessageBoxW wrappers
internal/elevate/    token elevation check, relaunch via ShellExecuteW runas
internal/desktop/    open-in-browser, HKCU Run key
internal/config/     paths and settings
internal/applog/     rotating log

internal/server/web/apointless.css is vendored verbatim from bsncubed/css — re-pull it to update, do not edit it here. ipswap.css beside it adds only what the design system does not ship (page chrome, tables, modals, stat tiles) and is built strictly on its tokens, so a refreshed apointless.css restyles the app too.

Two rules shape the split. Anything portable lives in portable code and is tested on any host — most usefully netcfg.Plan, which builds the exact netsh sequence and is covered without a Windows box. Anything Windows-specific has a _windows.go and an _other.go, so the whole tree stays buildable and vettable during development on Linux.

Elevation

The manifest asks for asInvoker, not requireAdministrator. On a machine with UAC relaxed, or when the parent process is already elevated, that means zero prompts — including at login, if start-with-Windows is on.

Membership of the Administrators group is not the same as holding an elevated token: with UAC on, a member's process gets a filtered token and netsh interface ipv4 set address fails with "The requested operation requires elevation." So ipswap checks the token at startup, and if it is not elevated it adds a "Relaunch as administrator" item to the tray. A failed apply offers the same thing. Nothing blocks startup either way.

Data

%APPDATA%\ipswap\presets.json    presets
%APPDATA%\ipswap\config.json     settings
%APPDATA%\ipswap\ipswap.log      rotating, 5 × 1 MB

presets.json is the export format too, so a preset pack is just this file and stays hand-editable. See examples/preset-pack.json.

Masks are stored as an integer prefix. Both /24 and 255.255.255.0 are accepted on input, in the editor and in a hand-written file; the display style is a single global setting.

Why applies are destructive

After an apply, the adapter has exactly the addresses in the preset and nothing else.

netsh interface ipv4 set address … static replaces the primary address but leaves previously-added secondary addresses attached. A naive implementation therefore leaks addresses across switches — after visiting three presets the adapter is still carrying secondaries from the first two. So every apply enumerates the live addresses, deletes each one, then sets the primary and adds the preset's secondaries. internal/netcfg/plan_test.go pins that ordering.

Reads use GetAdaptersAddresses, not netsh show config: netsh's output is localised and parsing it breaks on a non-English Windows.

Updates

ipswap checks https://gitea.apointless.space/bsncubed/ipswap by default. The check runs at startup in a goroutine, behind a short timeout, and fails silently to the log — a laptop on a customer site usually cannot reach the host, and that is not an error worth showing.

Point it elsewhere, or clear it to switch the check off entirely, in Settings (update_repo in config.json). Forks should change it: otherwise they will offer their users an upstream binary.

A newer tag adds "Update available — vX.Y.Z" to the top of the tray menu. Clicking it downloads the .exe, verifies its SHA256 against the release, and hands off to a small batch helper that waits for ipswap to exit, swaps the binary and relaunches. A release that publishes no checksum is refused — an unverified binary that is about to be run is not worth the convenience. It never installs on its own.

Not in v1

Revert / restore-previous / timed auto-revert; post-apply connectivity checks (the confirmation diff is the check); global hotkeys; IPv6; adapter matching by MAC address; per-preset scripts.