server: trusted_proxies – X-Forwarded-For für Rate-Limit hinter Reverse-Proxy

internal/server/auth.go

@@ -103,14 +103,71 @@ func (l *limiter) Reset(key string) {
 	delete(l.hits, key)
 }
 
-func clientIP(r *http.Request) string {
+// clientIP returns the peer address. If the peer is a trusted proxy, the
+// right-most X-Forwarded-For entry that is not itself a trusted proxy is used.
+func (s *Server) clientIP(r *http.Request) string {
 	h, _, err := net.SplitHostPort(r.RemoteAddr)
 	if err != nil {
-		return r.RemoteAddr
+		h = r.RemoteAddr
+	}
+	if !s.isTrustedProxy(h) {
+		return h
+	}
+	var parts []string
+	for _, v := range r.Header.Values("X-Forwarded-For") {
+		for _, p := range strings.Split(v, ",") {
+			if p = strings.TrimSpace(p); p != "" {
+				parts = append(parts, p)
+			}
+		}
+	}
+	for i := len(parts) - 1; i >= 0; i-- {
+		if net.ParseIP(parts[i]) == nil {
+			break
+		}
+		if !s.isTrustedProxy(parts[i]) {
+			return parts[i]
+		}
 	}
 	return h
 }
 
+func (s *Server) isTrustedProxy(ip string) bool {
+	p := net.ParseIP(ip)
+	if p == nil {
+		return false
+	}
+	if len(s.TrustedProxies) == 0 {
+		return p.IsLoopback()
+	}
+	for _, c := range s.TrustedProxies {
+		if c.Contains(p) {
+			return true
+		}
+	}
+	return false
+}
+
+// ParseCIDRs turns IPs or CIDRs into networks (bare IPs become /32 or /128).
+func ParseCIDRs(in []string) ([]*net.IPNet, error) {
+	var out []*net.IPNet
+	for _, v := range in {
+		if !strings.Contains(v, "/") {
+			if ip := net.ParseIP(v); ip != nil && ip.To4() != nil {
+				v += "/32"
+			} else {
+				v += "/128"
+			}
+		}
+		_, n, err := net.ParseCIDR(v)
+		if err != nil {
+			return nil, err
+		}
+		out = append(out, n)
+	}
+	return out, nil
+}
+
 func (s *Server) initAuth() {
 	s.authOnce.Do(func() {
 		if s.limiter == nil {
@@ -194,7 +251,7 @@ func (s *Server) register(w http.ResponseWriter, r *http.Request) {
 		writeErr(w, http.StatusForbidden, "registration disabled")
 		return
 	}
-	ip := clientIP(r)
+	ip := s.clientIP(r)
 	if s.limiter.Blocked(ip) {
 		writeErr(w, http.StatusTooManyRequests, "too many attempts, try again later")
 		return
@@ -232,7 +289,7 @@ func (s *Server) register(w http.ResponseWriter, r *http.Request) {
 
 func (s *Server) login(w http.ResponseWriter, r *http.Request) {
 	s.initAuth()
-	ip := clientIP(r)
+	ip := s.clientIP(r)
 	if s.limiter.Blocked(ip) {
 		writeErr(w, http.StatusTooManyRequests, "too many attempts, try again later")
 		return

internal/server/clientip_test.go

@@ -0,0 +1,26 @@
+package server
+
+import (
+	"net/http/httptest"
+	"testing"
+)
+
+func TestClientIPTrustedProxy(t *testing.T) {
+	s := &Server{}
+	r := httptest.NewRequest("GET", "/", nil)
+	r.RemoteAddr = "127.0.0.1:1234"
+	r.Header.Set("X-Forwarded-For", "6.6.6.6, 1.2.3.4")
+	if got := s.clientIP(r); got != "1.2.3.4" {
+		t.Fatalf("loopback proxy: got %s", got)
+	}
+	r.RemoteAddr = "9.9.9.9:1"
+	if got := s.clientIP(r); got != "9.9.9.9" {
+		t.Fatalf("untrusted peer: got %s", got)
+	}
+	n, _ := ParseCIDRs([]string{"10.0.1.0/24"})
+	s.TrustedProxies = n
+	r.RemoteAddr = "10.0.1.6:1"
+	if got := s.clientIP(r); got != "1.2.3.4" {
+		t.Fatalf("cidr proxy: got %s", got)
+	}
+}

internal/server/server.go

@@ -3,9 +3,10 @@ package server
 
 import (
 	"context"
-		"encoding/json"
+	"encoding/json"
 	"errors"
 	"log/slog"
+	"net"
 	"net/http"
 	"net/url"
 	"strings"
@@ -38,6 +39,8 @@ type Server struct {
 	AllowRegistration bool
 	// TokenTTL is the lifetime of user tokens (default 30 days).
 	TokenTTL time.Duration
+	// TrustedProxies whose X-Forwarded-For is honoured (empty = loopback only).
+	TrustedProxies []*net.IPNet
 
 	authOnce sync.Once
 	limiter  *limiter