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Nick Ciolpan
Nick Ciolpan

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I built a terminal-native Little Snitch alternative for macOS

#go #macos #security #opensource

I wanted to know what my Mac was doing behind my back.

Every app phones home. Electron apps ping telemetry endpoints. Browsers hit trackers. Even system processes make connections you never asked for. Little Snitch shows you all of this beautifully — it's a proper, polished product and well worth the money.

This is not a replacement for Little Snitch. This is what happens when you're curious about network monitoring and want to see how far you can get with Go, lsof, and pfctl in a terminal. Think of it as a learning project that accidentally became useful.

CLI Snitch watches every outbound TCP and UDP connection, prompts you to allow or deny, and enforces your decisions with real macOS firewall rules — all from the terminal.

What it looks like 

$ sudo cli-snitch watch

🚨 New Outbound Connection Detected
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
  📱 Application: Electron Helper
  🌐 Destination: telemetry.example.com:443
  🔌 Protocol:    TCP
  🏷️  Host Info:   Amazon Web Services (AWS)

? What would you like to do?
  ✅ Allow Once
  🔁 Allow Always
  ❌ Deny Once
> 🚫 Deny Always

❌ Electron Helper DENIED -> telemetry.example.com:443
🔥 Firewall rule: block out proto tcp from any to telemetry.example.com port 443
🛡️ Firewall rule applied
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That last line is real. It's not a log message — it's an actual pfctl rule injected into the macOS packet filter. The connection is blocked at the kernel level.

How it works

The architecture is straightforward:

  1. Detect — lsof -i tcp -i udp -n every 2 seconds (adaptive intervals)
  2. Match — Check new connections against saved rules (case-insensitive, supports glob patterns)
  3. Prompt — If no rule matches, ask the user via an interactive terminal prompt
  4. Enforce — Deny decisions create pfctl anchor rules that survive the session
  5. Remember — Decisions are saved as JSON rules and logged to a JSONL history file

The interesting engineering problems were:

Prompt serialization

Multiple connections can arrive within the same 2-second scan. If two prompts hit stdin simultaneously, everything breaks. I solved this with a buffered channel queue:

type promptRequest struct {
    conn     *monitor.Connection
    resultCh chan promptResult
}

func (cp *ConnectionPrompter) QueuePrompt(conn *Connection) (*UserDecision, error) {
    req := &promptRequest{
        conn:     conn,
        resultCh: make(chan promptResult, 1),
    }
    cp.promptQueue <- req
    res := <-req.resultCh
    return res.decision, res.err
}
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A single goroutine reads from the queue and calls the survey prompt — one at a time, no collisions.

pfctl input sanitization

When a user denies a connection, the host and port values end up in a pfctl command. If I naively concatenated those strings, a malformed hostname could inject commands. Every value is validated against regex before it touches pfctl:

var hostPattern = regexp.MustCompile(`^[a-zA-Z0-9._:\-]+$`)
var portPattern = regexp.MustCompile(`^[0-9]+$`)
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Connection deduplication

lsof reports all active connections every scan, not just new ones. The monitor maintains a map of seen connections plus a TTL-based "recent cache" — so connections that get cleaned up from the main map but reappear within 5 minutes don't re-trigger prompts.

What you get

14 commands:

watch              Real-time monitoring (the main event)
list-rules         See all your allow/deny rules
edit-rule          Modify a rule inline
import-rules       Load rules from JSON
export-rules       Back up rules to JSON
history            View connection log with filters
firewall-status    Check pfctl integration
list-firewall      Show active blocking rules
clear-firewall     Remove all pfctl rules
firewall-cleanup   Remove expired temp rules
firewall-monitor   Live firewall status
system-status      Full diagnostics
daemon install     Set up as a launchd service
daemon start/stop  Control the background service
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Some features I'm particularly happy with:

  • Wildcard rules — *.analytics.com blocks all analytics subdomains
  • DNS reverse lookup with caching — so you see hostnames, not just IPs
  • Connection history — every decision logged to JSONL, filterable by process or action
  • Daemon mode — sudo cli-snitch daemon install creates a launchd plist for background monitoring
  • Rule scoping — block a specific connection, all connections to a host, all connections on a port, or everything from a process

Install 

brew tap nickciolpan/tap
brew install cli-snitch
sudo cli-snitch watch
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Or build from source:

git clone https://github.com/nickciolpan/snitcher
cd snitcher
go build -o cli-snitch ./cmd/cli-snitch
sudo ./cli-snitch watch
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What it can't do

Being honest about the limitations:

  • No per-process blocking in pfctl — macOS packet filter doesn't have a concept of process ownership. If two apps connect to the same host:port, a deny rule blocks both. True per-process filtering needs Apple's Network Extension framework (Swift, not Go).
  • No bandwidth tracking — would need BPF packet capture.
  • lsof truncates process names — Google Chrome Helper becomes Google, Slack becomes Slack\x20. Works fine once you know the quirk.

What I learned

  1. lsof is surprisingly good for this use case. It's fast, available everywhere, and the output is parseable. The main gotcha is IPv6 bracket notation and process name encoding.

  2. pfctl anchors are the right abstraction. By isolating all CLI Snitch rules in a named anchor, there's zero risk of clobbering system firewall rules. Cleanup is just "reload an empty anchor file."

  3. Interactive CLI tools need careful goroutine design. The prompt queue pattern — buffered channel + single-reader goroutine — is something I'll reuse in every interactive CLI from now on.

The source is at github.com/nickciolpan/snitcher and the docs are at cli-snitch.ciolpan.com. MIT licensed.

If you've ever wondered what your Mac is doing when you're not looking, give it a try. You might be surprised.

Yes, this was written with the help of an LLM. The code too. Are we still pretending that's not how things get built in 2026? Claude wrote most of the implementation, I steered, tested, broke things, and made the decisions. The architecture is real, the bugs were real, and the pfctl rules definitely blocked my Chrome tabs for real. Tools are tools — what matters is whether the thing works. It does.

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