transport/internet/splithttp: per-request path randomization (pathPool)

infra/conf/transport_internet.go

@@ -223,6 +223,7 @@ type SplitHTTPConfig struct {
 	SessionKey           string            `json:"sessionKey"`
 	SeqPlacement         string            `json:"seqPlacement"`
 	SeqKey               string            `json:"seqKey"`
+	PathPool             []string          `json:"pathPool"`
 	UplinkDataPlacement  string            `json:"uplinkDataPlacement"`
 	UplinkDataKey        string            `json:"uplinkDataKey"`
 	UplinkChunkSize      Int32Range        `json:"uplinkChunkSize"`
@@ -406,6 +407,7 @@ func (c *SplitHTTPConfig) Build() (proto.Message, error) {
 		SeqPlacement:         c.SeqPlacement,
 		SessionKey:           c.SessionKey,
 		SeqKey:               c.SeqKey,
+		PathPool:             c.PathPool,
 		UplinkDataPlacement:  c.UplinkDataPlacement,
 		UplinkDataKey:        c.UplinkDataKey,
 		UplinkChunkSize:      newRangeConfig(c.UplinkChunkSize),

transport/internet/splithttp/config.go

@@ -316,6 +316,9 @@ func (c *Config) FillStreamRequest(request *http.Request, sessionId string, seqS
 
 	c.ApplyXPaddingToRequest(request, config)
 	c.ApplyMetaToRequest(request, sessionId, "")
+	if c.pathMetaIsOffPath() {
+		c.DecorateRequestPath(request)
+	}
 
 	if request.Body != nil && !c.NoGRPCHeader { // stream-up/one
 		request.Header.Set("Content-Type", "application/grpc")
@@ -366,6 +369,9 @@ func (c *Config) FillPacketRequest(request *http.Request, sessionId string, seqS
 
 	c.ApplyXPaddingToRequest(request, config)
 	c.ApplyMetaToRequest(request, sessionId, seqStr)
+	if c.pathMetaIsOffPath() {
+		c.DecorateRequestPath(request)
+	}
 
 	return nil
 }

transport/internet/splithttp/config.proto

@@ -50,4 +50,5 @@ message Config {
   string uplinkDataKey = 25;
   RangeConfig uplinkChunkSize = 26;
   int32 serverMaxHeaderBytes = 27;
+  repeated string pathPool = 28;
 }

transport/internet/splithttp/obfs_path.go

@@ -0,0 +1,62 @@
+package splithttp
+
+import (
+	"net/http"
+	"strconv"
+	"strings"
+)
+
+// Per-request path randomization (operator supplied).
+//
+// When the session and seq are carried off-path (cookie / header / query), the
+// URL path is free decoration: the server only checks
+// strings.HasPrefix(reqPath, configPath) and ignores everything after the
+// configured base path. If the operator sets pathPool, each request appends a
+// randomly chosen entry to the base path, so the transport stops sending every
+// request to one fixed path. A "*" inside an entry is replaced with a random
+// decimal number, so a short list can cover id / cursor style paths
+// (e.g. "items/*", "users/*/feed").
+//
+// No pool is shipped by default. A fixed list baked into the binary would just
+// become a new shared signature; the operator picks entries that match the
+// traffic they want to blend into.
+
+func randIntn(n int) int {
+	if n <= 0 {
+		return 0
+	}
+	s, ok := randStringFromCharset(6, "0123456789")
+	if !ok {
+		return 0
+	}
+	v, err := strconv.Atoi(s)
+	if err != nil {
+		return 0
+	}
+	return v % n
+}
+
+// pathMetaIsOffPath reports whether both session and seq are carried somewhere
+// other than the path, which leaves the path free to decorate.
+func (c *Config) pathMetaIsOffPath() bool {
+	return c.GetNormalizedSessionPlacement() != PlacementPath &&
+		c.GetNormalizedSeqPlacement() != PlacementPath
+}
+
+// DecorateRequestPath appends a random entry from PathPool to req.URL.Path.
+// It is a no-op when no pool is configured.
+func (c *Config) DecorateRequestPath(req *http.Request) {
+	if len(c.PathPool) == 0 {
+		return
+	}
+	seg := c.PathPool[randIntn(len(c.PathPool))]
+	for strings.Contains(seg, "*") {
+		seg = strings.Replace(seg, "*", strconv.Itoa(1000+randIntn(900000)), 1)
+	}
+	seg = strings.TrimPrefix(seg, "/")
+	base := req.URL.Path
+	if !strings.HasSuffix(base, "/") {
+		base += "/"
+	}
+	req.URL.Path = base + seg
+}

transport/internet/splithttp/obfs_path_test.go

@@ -0,0 +1,34 @@
+package splithttp
+
+import (
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+func TestDecorateRequestPath(t *testing.T) {
+	// No pool: the path is left untouched.
+	c := &Config{}
+	req := httptest.NewRequest("GET", "https://example.com/api/v1/", nil)
+	c.DecorateRequestPath(req)
+	if req.URL.Path != "/api/v1/" {
+		t.Fatalf("empty pool changed the path: %q", req.URL.Path)
+	}
+
+	// With a pool: append one entry, keep the base prefix, expand "*".
+	c = &Config{PathPool: []string{"timeline", "items/*"}}
+	for i := 0; i < 50; i++ {
+		req := httptest.NewRequest("GET", "https://example.com/api/v1/", nil)
+		c.DecorateRequestPath(req)
+		rest, ok := strings.CutPrefix(req.URL.Path, "/api/v1/")
+		if !ok {
+			t.Fatalf("lost the base prefix: %q", req.URL.Path)
+		}
+		if strings.Contains(rest, "*") {
+			t.Fatalf("star was not expanded: %q", rest)
+		}
+		if rest != "timeline" && !strings.HasPrefix(rest, "items/") {
+			t.Fatalf("unexpected segment: %q", rest)
+		}
+	}
+}