Log the client address next to the peer address (closes #270)

Behind a trusted proxy every log line named only the proxy, so abuse could not be traced from webhooker's own logs although the rate limiters already knew the client. The access log, the rate-limit rejection lines, the CSRF warning and the receiver's request line now carry clientIP next to remoteIP. remoteIP still means the connecting peer; clientIP is the address the rate limiters key on, the forwarded client when the peer is inside TRUSTED_PROXIES, worked out once per request by the same code. The README says the field is only as trustworthy as TRUSTED_PROXIES. The access log's 2,560-byte line ceiling holds with the field charged, and a size case with an oversized X-Forwarded-For pins it.

Model: opus-5-5

internal/middleware/accesslog_test.go

@@ -63,6 +63,12 @@ const (
 // capturingMiddleware returns a Middleware whose logger writes JSON
 // lines into the returned buffer, so the access log can be asserted
 // on directly.
+//
+// It trusts 192.0.2.1, the peer address httptest.NewRequestWithContext
+// gives a request, as a proxy, the way a deployment trusts its reverse
+// proxy: a request built that way and carrying X-Forwarded-For is
+// logged with the client that header names as clientIP, and one
+// without it with the peer.
 func capturingMiddleware(t *testing.T) (*middleware.Middleware, *bytes.Buffer) {
 	t.Helper()
 
@@ -72,7 +78,10 @@ func capturingMiddleware(t *testing.T) (*middleware.Middleware, *bytes.Buffer) {
 		&slog.HandlerOptions{Level: slog.LevelInfo},
 	))
 
-	cfg := &config.Config{Environment: config.EnvironmentDev}
+	cfg := &config.Config{
+		Environment:    config.EnvironmentDev,
+		TrustedProxies: trustedProxies("192.0.2.1/32"),
+	}
 
 	return middleware.NewForTest(log, cfg, nil), buf
 }
@@ -81,7 +90,7 @@ func capturingMiddleware(t *testing.T) (*middleware.Middleware, *bytes.Buffer) {
 // internal/logger can select: slog's text handler, which
 // internal/logger/logger.go installs when stderr is a tty. It escapes
 // differently from the JSON one, so the line bound has to be asserted
-// against both.
+// against both. It trusts the same peer.
 func capturingTextMiddleware(
 	t *testing.T,
 ) (*middleware.Middleware, *bytes.Buffer) {
@@ -93,7 +102,10 @@ func capturingTextMiddleware(
 		&slog.HandlerOptions{Level: slog.LevelInfo},
 	))
 
-	cfg := &config.Config{Environment: config.EnvironmentDev}
+	cfg := &config.Config{
+		Environment:    config.EnvironmentDev,
+		TrustedProxies: trustedProxies("192.0.2.1/32"),
+	}
 
 	return middleware.NewForTest(log, cfg, nil), buf
 }
@@ -334,11 +346,12 @@ func oversizedHeaders(value string) map[string]string {
 // sizeCase is one way of pointing 8 KB of client-chosen text at the
 // access log.
 type sizeCase struct {
-	target     string
-	headers    map[string]string
-	wantStatus int
-	wantURL    string
-	bound      int
+	target       string
+	headers      map[string]string
+	wantStatus   int
+	wantURL      string
+	wantClientIP string
+	bound        int
 }
 
 // lineSizeCases enumerates every part of a request that reaches the
@@ -375,8 +388,7 @@ func lineSizeCases() map[string]sizeCase {
 	}
 
 	// The url field on a 5xx keeps the concrete path, so it reaches its
-	// own budget on the same line as the three header fields. That is
-	// the widest access log line the service can be made to write.
+	// own budget on the same line as the three header fields.
 	longPath := "/boom/" + strings.Repeat("x", oversizedSegmentBytes)
 	wantLongURL := longPath[:maxFieldBytes] + truncationSuffix
 
@@ -420,6 +432,29 @@ func lineSizeCases() map[string]sizeCase {
 			}
 	}
 
+	// From a trusted proxy, clientIP is read out of X-Forwarded-For,
+	// which the client writes. What bounds the field is that only one
+	// address from the header is written, and it is written parsed, with
+	// no zone. An IPv6 address with all eight groups at four digits is
+	// the longest such address; here it carries an 8 KB zone, which must
+	// not reach the line. It goes on the 5xx line with all three header
+	// fields at their budget.
+	const longestIPv6 = "ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff"
+
+	forwarded := oversizedHeaders(oversizedValue("h"))
+	forwarded[headerXFF] = oversizedValue("h") + ", " +
+		longestIPv6 + "%" + oversizedValue("h")
+
+	cases["oversized X-Forwarded-For from a trusted proxy "+
+		"with a 5xx concrete url"] = sizeCase{
+		target:       longPath,
+		headers:      forwarded,
+		wantStatus:   http.StatusInternalServerError,
+		wantURL:      wantLongURL,
+		wantClientIP: longestIPv6,
+		bound:        maxCappedLineBytes,
+	}
+
 	return cases
 }
 
@@ -460,6 +495,14 @@ func TestAccessLog_LineSizeDoesNotTrackInputSize(t *testing.T) {
 			require.Len(t, entries, 1)
 			assert.Equal(t, tc.wantURL, entries[0]["url"])
 
+			// Set only by the X-Forwarded-For case, where it proves
+			// the header was read rather than ignored.
+			if tc.wantClientIP != "" {
+				assert.Equal(
+					t, tc.wantClientIP, entries[0]["clientIP"],
+				)
+			}
+
 			// The markers sit at the far end of the client-chosen
 			// text, so their absence is what proves the redaction and
 			// the truncation actually ran.
@@ -648,7 +691,8 @@ func TestAccessLog_RetainsEveryOtherField(t *testing.T) {
 
 	for _, key := range []string{
 		"request_start", "method", "url", "useragent", "request_id",
-		"referer", "proto", "remoteIP", "status", "latency_ms",
+		"referer", "proto", "remoteIP", "clientIP", "status",
+		"latency_ms",
 	} {
 		assert.Contains(t, entries[0], key)
 	}