watcher: save state when it stops and wait for that save (closes #114)

The final save at shutdown came from the state's own stop hook, while
the watcher's stop hook only cancelled its run loop, so a check under
way could change state after that save or be cut off at exit. Run now
saves state as it returns, and the watcher's stop hook waits for Run,
bounded by the shutdown deadline. The state's own save stays; Save
holds the state lock for the whole write, so the two cannot overlap.
The start hook derives the watcher's context with WithoutCancel, so
the linter needs no exception. A new test stops a watcher built by New
and reads the change back from the state file, with no DNS.

Model: opus-5-5

README.md

@@ -347,9 +347,9 @@ minute, failed logins included; beyond that it answers `429 Too Many Requests`
 without checking the password. A Prometheus server scraping every 15 seconds
 sends 4 a minute. IPv6 addresses in one /64 count as one client. When the
 request comes from a private or loopback address, such as a reverse proxy's,
-the client address is taken from the `X-Real-IP` or `X-Forwarded-For` header
+the client address is taken from the `X-Real-IP` header the proxy sets, or else
-the proxy sets; a proxy that sets neither makes all its clients share one
+from `X-Forwarded-For`, as the last address in it that is not private or
-allowance.
+loopback. A proxy that sets neither makes all its clients share one allowance.
 
 **`DNSWATCHER_DNS_INTERVAL` and `DNSWATCHER_TLS_INTERVAL`** take a positive
 duration: a number followed by a unit such as `s`, `m` or `h`, for example

TODO.md

@@ -21,6 +21,8 @@ nameserver IP address changes: https://git.eeqj.de/sneak/dnswatcher/issues/105
 
 - 2026-10-01: the watcher saves state when it stops, and shutdown waits for that
   save, so it no longer relies on the state's own stop hook (closes #114).
+- 2026-10-01: the client address from `X-Forwarded-For` is the last entry that
+  is not a trusted proxy, not the first, which the client sets (closes #181).
 - 2026-10-01: a nameserver that does not answer is saved as `error` with the
   reason, and NS failure and NS recovery are notified (closes #104).
 - 2026-10-01: a `DNSWATCHER_DNS_INTERVAL` or `DNSWATCHER_TLS_INTERVAL` that is

internal/middleware/export_test.go

@@ -1,6 +1,9 @@
 package middleware
 
-import "time"
+import (
+	"net/http"
+	"time"
+)
 
 // The /metrics rate limit, exported so the tests can count requests
 // against it.
@@ -8,3 +11,9 @@ const (
 	MetricsRequestLimit                = metricsRequestLimit
 	MetricsRequestWindow time.Duration = metricsRequestWindow
 )
+
+// RealIP is realIP, exported so the tests can check which address it
+// takes as the client's.
+func RealIP(r *http.Request) string {
+	return realIP(r)
+}

internal/middleware/middleware.go

@@ -209,6 +209,12 @@ func isTrustedProxy(ip net.IP) bool {
 
 // realIP extracts the client's real IP address from the request.
 // Proxy headers are only trusted from RFC1918/loopback addresses.
+//
+// Each proxy adds to the end of X-Forwarded-For the address it got the
+// request from, so the client can write every entry before the one the
+// first trusted proxy added. The client address is therefore the
+// rightmost entry that is not a trusted proxy, or the leftmost entry
+// when they all are.
 func realIP(r *http.Request) string {
 	addr := ipFromHostPort(r.RemoteAddr)
 	remoteIP := net.ParseIP(addr)
@@ -223,16 +229,26 @@ func realIP(r *http.Request) string {
 		return ip
 	}
 
-	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
+	// A proxy may add its entry as a header line of its own instead of
-		if parts := strings.SplitN(
+	// appending to the line the client sent, so all lines form one list.
-			xff, ",", 2, //nolint:mnd
+	entries := strings.Split(
-		); len(parts) > 0 {
+		strings.Join(r.Header.Values("X-Forwarded-For"), ","), ",",
-			if ip := strings.TrimSpace(parts[0]); ip != "" {
+	)
-				return ip
+	client := strings.TrimSpace(entries[0])
-			}
+
+	for i := len(entries) - 1; i > 0; i-- {
+		entry := strings.TrimSpace(entries[i])
+		if !isTrustedProxy(net.ParseIP(entry)) {
+			client = entry
+
+			break
 		}
 	}
 
+	if client != "" {
+		return client
+	}
+
 	return addr
 }
 

internal/middleware/middleware_test.go

@@ -342,9 +342,9 @@ func TestDashboardRendersWithSecurityHeaders(t *testing.T) {
 	}
 }
 
-// Addresses for the rate limit tests: a client connecting directly, a
+// Addresses for the rate limit and realIP tests: a client connecting
-// trusted proxy, and a client behind that proxy as its X-Real-IP
+// directly, a trusted proxy, and a client behind that proxy as the
-// header names it.
+// proxy's X-Real-IP or X-Forwarded-For header names it.
 const (
 	directClient  = "198.51.100.1:4000"
 	trustedProxy  = "10.0.0.1:4000"
@@ -482,3 +482,84 @@ func TestMetricsRateLimitKeysOnClientAddress(t *testing.T) {
 		})
 	}
 }
+
+// TestRealIP checks which address realIP takes as the client's. Each
+// element of forwardedFor is sent as an X-Forwarded-For header line of
+// its own, and 198.51.100.9 is always an entry the client wrote itself.
+func TestRealIP(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name         string
+		remoteAddr   string
+		xRealIP      string
+		forwardedFor []string
+		want         string
+	}{
+		{
+			"untrusted peer, both headers ignored",
+			directClient, proxiedClient, []string{"198.51.100.9"},
+			"198.51.100.1",
+		},
+		{
+			"X-Real-IP from a trusted proxy wins",
+			trustedProxy, proxiedClient, []string{"203.0.113.8"},
+			proxiedClient,
+		},
+		{
+			"client's own entry, then the one the proxy added",
+			trustedProxy, "", []string{"198.51.100.9, 203.0.113.1"},
+			proxiedClient,
+		},
+		{
+			"several trusted proxies",
+			trustedProxy, "",
+			[]string{"198.51.100.9, 203.0.113.1, 10.0.0.3, 10.0.0.2"},
+			proxiedClient,
+		},
+		{
+			"proxy adds a header line of its own",
+			trustedProxy, "", []string{"198.51.100.9", proxiedClient},
+			proxiedClient,
+		},
+		{
+			"every entry a trusted proxy",
+			trustedProxy, "", []string{"10.0.0.3, 10.0.0.2"},
+			"10.0.0.3",
+		},
+		{
+			"empty where the client address belongs",
+			trustedProxy, "", []string{"203.0.113.1, , 10.0.0.2"},
+			"10.0.0.1",
+		},
+		{
+			"no headers from a trusted proxy",
+			trustedProxy, "", nil,
+			"10.0.0.1",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			req := httptest.NewRequestWithContext(
+				t.Context(), http.MethodGet, "/", nil,
+			)
+			req.RemoteAddr = tt.remoteAddr
+
+			if tt.xRealIP != "" {
+				req.Header.Set("X-Real-IP", tt.xRealIP)
+			}
+
+			for _, line := range tt.forwardedFor {
+				req.Header.Add("X-Forwarded-For", line)
+			}
+
+			got := middleware.RealIP(req)
+			if got != tt.want {
+				t.Errorf("realIP = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}