fear(client): extract client's IP and store in database and structured logs

backend/internal/httpapi/client_ip.go

@@ -0,0 +1,63 @@
+package httpapi
+
+import (
+	"net"
+	"net/http"
+	"strings"
+)
+
+func extractClientIP(r *http.Request) *string {
+	if r == nil {
+		return nil
+	}
+
+	// Priority headers commonly set by reverse proxies, load balancers, and CDNs.
+	headers := []string{
+		"CF-Connecting-IP", // Cloudflare
+		"True-Client-IP",   // Cloudflare Enterprise, Akamai
+		"X-Real-IP",        // Nginx, Traefik
+	}
+
+	for _, header := range headers {
+		if val := strings.TrimSpace(r.Header.Get(header)); val != "" {
+			if ip := parseIP(val); ip != "" {
+				return &ip
+			}
+		}
+	}
+
+	// X-Forwarded-For can contain multiple comma-separated IPs: client, proxy1, proxy2. The original client IP is the first entry.
+	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
+		for _, part := range strings.Split(xff, ",") {
+			if ip := parseIP(part); ip != "" {
+				return &ip
+			}
+		}
+	}
+
+	// Fallback to RemoteAddr (typically "host:port" or "[ipv6]:port").
+	if r.RemoteAddr != "" {
+		host, _, err := net.SplitHostPort(r.RemoteAddr)
+		if err != nil {
+			host = r.RemoteAddr
+		}
+		if ip := parseIP(host); ip != "" {
+			return &ip
+		}
+	}
+
+	return nil
+}
+
+func parseIP(raw string) string {
+	raw = strings.TrimSpace(raw)
+	raw = strings.TrimPrefix(raw, "[")
+	raw = strings.TrimSuffix(raw, "]")
+
+	ip := net.ParseIP(raw)
+	if ip == nil {
+		return ""
+	}
+
+	return ip.String()
+}

backend/internal/httpapi/client_ip_test.go

@@ -0,0 +1,143 @@
+package httpapi
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestExtractClientIP(t *testing.T) {
+	tests := []struct {
+		name       string
+		request    *http.Request
+		headers    map[string]string
+		remoteAddr string
+		wantIP     *string
+	}{
+		{
+			name:    "nil request",
+			request: nil,
+			wantIP:  nil,
+		},
+		{
+			name: "CF-Connecting-IP takes precedence",
+			headers: map[string]string{
+				"CF-Connecting-IP": "203.0.113.10",
+				"X-Real-IP":        "198.51.100.20",
+				"X-Forwarded-For":  "192.0.2.30",
+			},
+			remoteAddr: "10.0.0.1:54321",
+			wantIP:     ptr("203.0.113.10"),
+		},
+		{
+			name: "True-Client-IP takes precedence over X-Real-IP",
+			headers: map[string]string{
+				"True-Client-IP":  "203.0.113.11",
+				"X-Real-IP":       "198.51.100.20",
+				"X-Forwarded-For": "192.0.2.30",
+			},
+			remoteAddr: "10.0.0.1:54321",
+			wantIP:     ptr("203.0.113.11"),
+		},
+		{
+			name: "X-Real-IP takes precedence over X-Forwarded-For",
+			headers: map[string]string{
+				"X-Real-IP":       "198.51.100.20",
+				"X-Forwarded-For": "192.0.2.30",
+			},
+			remoteAddr: "10.0.0.1:54321",
+			wantIP:     ptr("198.51.100.20"),
+		},
+		{
+			name: "X-Forwarded-For first entry is extracted",
+			headers: map[string]string{
+				"X-Forwarded-For": "192.0.2.30, 10.0.0.2, 172.16.0.5",
+			},
+			remoteAddr: "10.0.0.1:54321",
+			wantIP:     ptr("192.0.2.30"),
+		},
+		{
+			name: "X-Forwarded-For single entry",
+			headers: map[string]string{
+				"X-Forwarded-For": " 192.0.2.31 ",
+			},
+			remoteAddr: "10.0.0.1:54321",
+			wantIP:     ptr("192.0.2.31"),
+		},
+		{
+			name: "X-Forwarded-For skips invalid first entry to find valid IP",
+			headers: map[string]string{
+				"X-Forwarded-For": "unknown, 192.0.2.32",
+			},
+			remoteAddr: "10.0.0.1:54321",
+			wantIP:     ptr("192.0.2.32"),
+		},
+		{
+			name: "Fallback to RemoteAddr with port",
+			headers: map[string]string{},
+			remoteAddr: "192.168.1.100:12345",
+			wantIP:     ptr("192.168.1.100"),
+		},
+		{
+			name: "Fallback to RemoteAddr without port",
+			headers: map[string]string{},
+			remoteAddr: "192.168.1.101",
+			wantIP:     ptr("192.168.1.101"),
+		},
+		{
+			name: "IPv6 in RemoteAddr with port",
+			headers: map[string]string{},
+			remoteAddr: "[2001:db8::1]:8080",
+			wantIP:     ptr("2001:db8::1"),
+		},
+		{
+			name: "IPv6 in header",
+			headers: map[string]string{
+				"X-Real-IP": "2001:db8::2",
+			},
+			remoteAddr: "10.0.0.1:54321",
+			wantIP:     ptr("2001:db8::2"),
+		},
+		{
+			name: "Invalid headers fall back to RemoteAddr",
+			headers: map[string]string{
+				"X-Real-IP":       "not-an-ip",
+				"X-Forwarded-For": "also-not-an-ip",
+			},
+			remoteAddr: "192.168.1.102:443",
+			wantIP:     ptr("192.168.1.102"),
+		},
+		{
+			name: "No headers and empty RemoteAddr",
+			headers: map[string]string{},
+			remoteAddr: "",
+			wantIP:     nil,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var req *http.Request
+			if tt.name != "nil request" {
+				req = httptest.NewRequest(http.MethodGet, "/", nil)
+				for k, v := range tt.headers {
+					req.Header.Set(k, v)
+				}
+				req.RemoteAddr = tt.remoteAddr
+			}
+
+			got := extractClientIP(req)
+			if (got == nil) != (tt.wantIP == nil) {
+				t.Fatalf("extractClientIP() = %v, want %v", got, tt.wantIP)
+			}
+			if got != nil && tt.wantIP != nil && *got != *tt.wantIP {
+				t.Errorf("extractClientIP() = %q, want %q", *got, *tt.wantIP)
+			}
+		})
+	}
+}
+
+func ptr(s string) *string {
+	return &s
+}
+

backend/internal/httpapi/handler.go

@@ -60,6 +60,7 @@ func NewHandler(
 func (h *Handler) Convert(w http.ResponseWriter, r *http.Request) {
 	startedAt := h.now()
 	requestID := h.newRequestID()
+	clientIP := extractClientIP(r)
 
 	w.Header().Set("X-Request-ID", requestID)
 	w.Header().Set("X-Content-Type-Options", "nosniff")
@@ -89,6 +90,7 @@ func (h *Handler) Convert(w http.ResponseWriter, r *http.Request) {
 		RequestID:  requestID,
 		CreatedAt:  startedAt,
 		Duration:   duration,
+		ClientIP:   clientIP,
 		Status:     outcome.auditStatus,
 		MaskedCard: card.Mask(cardNumber),
 		ResponseJSON: audit.SanitizeResponse(
@@ -102,11 +104,17 @@ func (h *Handler) Convert(w http.ResponseWriter, r *http.Request) {
 		record.UpstreamHTTPStatus = &upstreamStatus
 	}
 
+	var clientIPValue string
+	if clientIP != nil {
+		clientIPValue = *clientIP
+	}
+
 	if err := h.audits.Save(r.Context(), record); err != nil {
 		h.logger.ErrorContext(
 			r.Context(),
 			"request audit failed",
 			"request_id", requestID,
+			"client_ip", clientIPValue,
 			"route", "/api/v1/card-to-iban",
 			"http_status", http.StatusInternalServerError,
 			"audit_status", audit.StatusInternalError,
@@ -128,6 +136,7 @@ func (h *Handler) Convert(w http.ResponseWriter, r *http.Request) {
 		r.Context(),
 		"request completed",
 		"request_id", requestID,
+		"client_ip", clientIPValue,
 		"route", "/api/v1/card-to-iban",
 		"http_status", outcome.httpStatus,
 		"audit_status", outcome.auditStatus,

backend/internal/httpapi/handler_test.go

@@ -248,6 +248,10 @@ func TestHandlerConvert(t *testing.T) {
 				t.Errorf("masked card = %q, want %q", record.MaskedCard, tt.wantMaskedCard)
 			}
 
+			if record.ClientIP == nil || *record.ClientIP != "192.0.2.1" {
+				t.Errorf("client IP = %v, want 192.0.2.1", record.ClientIP)
+			}
+
 			if strings.Contains(string(record.ResponseJSON), "6037997512345678") {
 				t.Error("audit response contains the full card number")
 			}
@@ -291,3 +295,75 @@ func TestHandlerFailsClosedWhenAuditSaveFails(t *testing.T) {
 		t.Errorf("response body = %s, want internal_error", response.Body.String())
 	}
 }
+
+func TestHandlerExtractsClientIP(t *testing.T) {
+	tests := []struct {
+		name       string
+		headerKey  string
+		headerVal  string
+		remoteAddr string
+		wantIP     string
+	}{
+		{
+			name:       "prefers CF-Connecting-IP",
+			headerKey:  "CF-Connecting-IP",
+			headerVal:  "203.0.113.88",
+			remoteAddr: "10.0.0.1:1234",
+			wantIP:     "203.0.113.88",
+		},
+		{
+			name:       "prefers X-Forwarded-For over RemoteAddr",
+			headerKey:  "X-Forwarded-For",
+			headerVal:  "198.51.100.77, 10.0.0.2",
+			remoteAddr: "10.0.0.1:1234",
+			wantIP:     "198.51.100.77",
+		},
+		{
+			name:       "uses RemoteAddr host without port",
+			remoteAddr: "192.168.10.5:9999",
+			wantIP:     "192.168.10.5",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			audits := &fakeAuditSaver{}
+			handler := NewHandler(
+				&fakeConverter{
+					result: service.ConversionResult{
+						IBAN:               "IR820540102680020817909002",
+						UpstreamHTTPStatus: http.StatusOK,
+					},
+				},
+				audits,
+				slog.New(slog.NewTextHandler(io.Discard, nil)),
+			)
+
+			req := httptest.NewRequest(
+				http.MethodPost,
+				"/api/v1/card-to-iban",
+				strings.NewReader(`{"card":"6037997512345678"}`),
+			)
+			if tt.headerKey != "" {
+				req.Header.Set(tt.headerKey, tt.headerVal)
+			}
+			req.RemoteAddr = tt.remoteAddr
+
+			rec := httptest.NewRecorder()
+			handler.Convert(rec, req)
+
+			if rec.Code != http.StatusOK {
+				t.Fatalf("status = %d, want 200", rec.Code)
+			}
+
+			if len(audits.records) != 1 {
+				t.Fatalf("audit records count = %d, want 1", len(audits.records))
+			}
+
+			recIP := audits.records[0].ClientIP
+			if recIP == nil || *recIP != tt.wantIP {
+				t.Errorf("ClientIP = %v, want %q", recIP, tt.wantIP)
+			}
+		})
+	}
+}