internal/api: resolve IPv6 handling; prevent XFF spoofing & add rate limiting

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: Ibe415a133bbc8bd533a21ed1ccd44cf36a6a6964

internal/api/handler.go

@@ -1,13 +1,15 @@
 package api
 
 import (
-	"log"
+	"math/rand"
+	"net"
 	"net/http"
 	"strings"
 
 	"notashelf.dev/watchdog/internal/aggregate"
 	"notashelf.dev/watchdog/internal/config"
 	"notashelf.dev/watchdog/internal/normalize"
+	"notashelf.dev/watchdog/internal/ratelimit"
 )
 
 // Handles incoming analytics events
@@ -17,6 +19,8 @@ type IngestionHandler struct {
 	pathRegistry *aggregate.PathRegistry
 	refRegistry  *normalize.ReferrerRegistry
 	metricsAgg   *aggregate.MetricsAggregator
+	rateLimiter  *ratelimit.TokenBucket
+	rng          *rand.Rand
 }
 
 // Creates a new ingestion handler
@@ -27,33 +31,72 @@ func NewIngestionHandler(
 	refRegistry *normalize.ReferrerRegistry,
 	metricsAgg *aggregate.MetricsAggregator,
 ) *IngestionHandler {
+	var limiter *ratelimit.TokenBucket
+	if cfg.Limits.MaxEventsPerMinute > 0 {
+		limiter = ratelimit.NewTokenBucket(
+			cfg.Limits.MaxEventsPerMinute,
+			cfg.Limits.MaxEventsPerMinute,
+			60_000_000_000, // 1 minute in nanoseconds
+		)
+	}
+
 	return &IngestionHandler{
 		cfg:          cfg,
 		pathNorm:     pathNorm,
 		pathRegistry: pathRegistry,
 		refRegistry:  refRegistry,
 		metricsAgg:   metricsAgg,
+		rateLimiter:  limiter,
+		rng:          rand.New(rand.NewSource(42)),
 	}
 }
 
 func (h *IngestionHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	// Handle CORS preflight
+	if r.Method == http.MethodOptions {
+		h.handleCORS(w, r)
+		w.WriteHeader(http.StatusNoContent)
+		return
+	}
+
+	// Apply CORS headers to actual request
+	h.handleCORS(w, r)
+
 	// Only accept POST requests
 	if r.Method != http.MethodPost {
 		http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
 		return
 	}
 
+	// Check rate limit
+	if h.rateLimiter != nil && !h.rateLimiter.Allow() {
+		http.Error(w, "Rate limit exceeded", http.StatusTooManyRequests)
+		return
+	}
+
+	// Apply sampling
+	if h.cfg.Site.Sampling < 1.0 {
+		if h.rng.Float64() > h.cfg.Site.Sampling {
+			// Sampled out, return success but don't track
+			w.WriteHeader(http.StatusNoContent)
+			return
+		}
+	}
+
+	// Check context cancellation
+	if r.Context().Err() != nil {
+		return
+	}
+
 	// Parse event from request body
 	event, err := ParseEvent(r.Body)
 	if err != nil {
-		log.Printf("Failed to parse event: %v", err)
 		http.Error(w, "Bad request", http.StatusBadRequest)
 		return
 	}
 
 	// Validate event
 	if err := event.Validate(h.cfg.Site.Domain); err != nil {
-		log.Printf("Event validation failed: %v", err)
 		http.Error(w, "Bad request", http.StatusBadRequest)
 		return
 	}
@@ -63,16 +106,14 @@ func (h *IngestionHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	if !h.pathRegistry.Add(normalizedPath) {
 		// Path was rejected due to cardinality limit
 		h.metricsAgg.RecordPathOverflow()
-		log.Printf("Path overflow: rejected %s", normalizedPath)
 
 		// Still return success to client
 		w.WriteHeader(http.StatusNoContent)
-
 		return
 	}
 
 	// Extract visitor identity for unique tracking
-	ip := extractIP(r)
+	ip := h.extractIP(r)
 	userAgent := r.Header.Get("User-Agent")
 
 	// Track unique visitor if salt rotation is enabled
@@ -88,14 +129,18 @@ func (h *IngestionHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 		// Device classification
 		if h.cfg.Site.Collect.Device {
-			device = classifyDevice(event.Width)
+			device = h.classifyDevice(event.Width)
 		}
 
 		// Referrer classification
 		if h.cfg.Site.Collect.Referrer == "domain" {
 			refDomain := normalize.ExtractReferrerDomain(event.Referrer, h.cfg.Site.Domain)
 			if refDomain != "" {
-				referrer = h.refRegistry.Add(refDomain)
+				accepted := h.refRegistry.Add(refDomain)
+				if accepted == "other" {
+					h.metricsAgg.RecordReferrerOverflow()
+				}
+				referrer = accepted
 			}
 		}
 
@@ -110,15 +155,74 @@ func (h *IngestionHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	w.WriteHeader(http.StatusNoContent)
 }
 
-// extractIP extracts the client IP from the request
-// Checks X-Forwarded-For and X-Real-IP headers for proxied requests
-func extractIP(r *http.Request) string {
-	// Check X-Forwarded-For header (may contain multiple IPs)
+// Adds CORS headers if enabled in config
+func (h *IngestionHandler) handleCORS(w http.ResponseWriter, r *http.Request) {
+	if !h.cfg.Security.CORS.Enabled {
+		return
+	}
+
+	origin := r.Header.Get("Origin")
+	if origin == "" {
+		return
+	}
+
+	// Check if origin is allowed
+	allowed := false
+	for _, allowedOrigin := range h.cfg.Security.CORS.AllowedOrigins {
+		if allowedOrigin == "*" || allowedOrigin == origin {
+			allowed = true
+			break
+		}
+	}
+
+	if allowed {
+		if origin == "*" {
+			w.Header().Set("Access-Control-Allow-Origin", "*")
+		} else {
+			w.Header().Set("Access-Control-Allow-Origin", origin)
+		}
+		w.Header().Set("Access-Control-Allow-Methods", "POST, OPTIONS")
+		w.Header().Set("Access-Control-Allow-Headers", "Content-Type")
+		w.Header().Set("Access-Control-Max-Age", "86400")
+	}
+}
+
+// Extracts the client IP from the requests. Only trusts proxy headers if source
+// IP is in trusted_proxies list
+func (h *IngestionHandler) extractIP(r *http.Request) string {
+	// Get the direct connection IP
+	remoteIP, _, err := net.SplitHostPort(r.RemoteAddr)
+	if err != nil {
+		// RemoteAddr might not have port (shouldn't happen, but handle it anyway)
+		remoteIP = r.RemoteAddr
+	}
+
+	// Check if we should trust proxy headers
+	trustProxy := false
+	if len(h.cfg.Security.TrustedProxies) > 0 {
+		for _, trustedCIDR := range h.cfg.Security.TrustedProxies {
+			if h.ipInCIDR(remoteIP, trustedCIDR) {
+				trustProxy = true
+				break
+			}
+		}
+	}
+
+	// If not trusting proxy, return direct IP
+	if !trustProxy {
+		return remoteIP
+	}
+
+	// Check X-Forwarded-For header
 	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
-		// Take the first IP in the list
+		// Take the rightmost IP that's not from a trusted proxy
 		ips := strings.Split(xff, ",")
-		if len(ips) > 0 {
-			return strings.TrimSpace(ips[0])
+		for i := len(ips) - 1; i >= 0; i-- {
+			ip := strings.TrimSpace(ips[i])
+			if !h.ipInCIDR(ip, "0.0.0.0/0") {
+				continue
+			}
+			return ip
 		}
 	}
 
@@ -128,24 +232,43 @@ func extractIP(r *http.Request) string {
 	}
 
 	// Fall back to RemoteAddr
-	ip := r.RemoteAddr
-	// Strip port if present
-	if idx := strings.LastIndex(ip, ":"); idx != -1 {
-		ip = ip[:idx]
-	}
-	return ip
+	return remoteIP
 }
 
-// Classifies screen width into device categories
-func classifyDevice(width int) string {
-	// FIXME: probably not the best logic for this...
+// Checks if an IP address is within a CIDR range
+func (h *IngestionHandler) ipInCIDR(ip, cidr string) bool {
+	// Parse the IP address
+	testIP := net.ParseIP(ip)
+	if testIP == nil {
+		return false
+	}
+
+	// Parse the CIDR
+	_, network, err := net.ParseCIDR(cidr)
+	if err != nil {
+		// If it's not a CIDR, try as a single IP
+		cidrIP := net.ParseIP(cidr)
+		if cidrIP == nil {
+			return false
+		}
+		return testIP.Equal(cidrIP)
+	}
+
+	return network.Contains(testIP)
+}
+
+// Classifies screen width into device categories using configured breakpoints
+// FIXME: we need a more robust mechanism for classifying devices. Breakpoints
+// are the only ones I can think of *right now* but I'm positive there are better
+// mechanisns. We'll get to this later.
+func (h *IngestionHandler) classifyDevice(width int) string {
 	if width == 0 {
 		return "unknown"
 	}
-	if width < 768 {
+	if width < h.cfg.Limits.DeviceBreakpoints.Mobile {
 		return "mobile"
 	}
-	if width < 1024 {
+	if width < h.cfg.Limits.DeviceBreakpoints.Tablet {
 		return "tablet"
 	}
 	return "desktop"