From e882e7d23732fb44f6ec8d6f269adbf44e10a1eb Mon Sep 17 00:00:00 2001
From: clawbot <clawbot@noreply.example.org>
Date: Mon, 2 Mar 2026 01:26:55 +0100
Subject: [PATCH] feat: fail fast when no monitoring targets configured (#75)
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## Summary

When `DNSWATCHER_TARGETS` is empty (the default), dnswatcher previously started successfully and ran indefinitely monitoring nothing. This is a common misconfiguration — forgetting to set the variable or making a typo in its name — and gave no indication anything was wrong.

## Changes

- Added `ErrNoTargets` sentinel error in `internal/config/config.go`
- Extracted `parseAndValidateTargets()` helper to validate that at least one domain or hostname is configured after target classification
- If no targets are configured, dnswatcher now exits with a clear error: `"no monitoring targets configured: set DNSWATCHER_TARGETS environment variable"`
- Updated README.md to document that `DNSWATCHER_TARGETS` is required and dnswatcher will refuse to start without it

## How it works

The validation runs during config construction (via uber/fx), before the watcher or any other component starts. If `DNSWATCHER_TARGETS` is empty or contains only whitespace/empty entries, `buildConfig()` returns `ErrNoTargets`, which causes fx to fail startup with a clear error message.

This is fail-fast behavior: a monitoring daemon with nothing to monitor is a misconfiguration and should not silently run.

Closes https://git.eeqj.de/sneak/dnswatcher/issues/69

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: https://git.eeqj.de/sneak/dnswatcher/pulls/75
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>
---
 README.md                 |  6 ++++++
 internal/config/config.go | 28 ++++++++++++++++++++++++----
 2 files changed, 30 insertions(+), 4 deletions(-)

diff --git a/README.md b/README.md
index 0b30a9b..460d761 100644
--- a/README.md
+++ b/README.md
@@ -210,6 +210,12 @@ the following precedence (highest to lowest):
 | `DNSWATCHER_METRICS_USERNAME`   | Basic auth username for /metrics           | `""`        |
 | `DNSWATCHER_METRICS_PASSWORD`   | Basic auth password for /metrics           | `""`        |
 
+**`DNSWATCHER_TARGETS` is required.** dnswatcher will refuse to start if no
+monitoring targets are configured. A monitoring daemon with nothing to monitor
+is a misconfiguration, so dnswatcher fails fast with a clear error message
+rather than running silently. Set `DNSWATCHER_TARGETS` to a comma-separated
+list of DNS names before starting.
+
 ### Example `.env`
 
 ```sh
diff --git a/internal/config/config.go b/internal/config/config.go
index 0acf89e..1368c46 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -23,6 +23,11 @@ const (
 	defaultTLSExpiryWarning = 7
 )
 
+// ErrNoTargets is returned when no monitoring targets are configured.
+var ErrNoTargets = errors.New(
+	"no monitoring targets configured: set DNSWATCHER_TARGETS environment variable",
+)
+
 // Params contains dependencies for Config.
 type Params struct {
 	fx.In
@@ -132,11 +137,9 @@ func buildConfig(
 		tlsInterval = defaultTLSInterval
 	}
 
-	domains, hostnames, err := ClassifyTargets(
-		parseCSV(viper.GetString("TARGETS")),
-	)
+	domains, hostnames, err := parseAndValidateTargets()
 	if err != nil {
-		return nil, fmt.Errorf("invalid targets configuration: %w", err)
+		return nil, err
 	}
 
 	cfg := &Config{
@@ -162,6 +165,23 @@ func buildConfig(
 	return cfg, nil
 }
 
+func parseAndValidateTargets() ([]string, []string, error) {
+	domains, hostnames, err := ClassifyTargets(
+		parseCSV(viper.GetString("TARGETS")),
+	)
+	if err != nil {
+		return nil, nil, fmt.Errorf(
+			"invalid targets configuration: %w", err,
+		)
+	}
+
+	if len(domains) == 0 && len(hostnames) == 0 {
+		return nil, nil, ErrNoTargets
+	}
+
+	return domains, hostnames, nil
+}
+
 func parseCSV(input string) []string {
 	if input == "" {
 		return nil