htp: randomization of htp inspection sizes

This is an implementation of #940. It randomize libhtp request
and response size if the same way this has been done for stream
inspection.

src/app-layer-htp.c

@@ -2112,6 +2112,8 @@ static void HTPConfigSetDefaultsPhase1(HTPCfgRec *cfg_prec)
     cfg_prec->request_inspect_window = HTP_CONFIG_DEFAULT_REQUEST_INSPECT_WINDOW;
     cfg_prec->response_inspect_min_size = HTP_CONFIG_DEFAULT_RESPONSE_INSPECT_MIN_SIZE;
     cfg_prec->response_inspect_window = HTP_CONFIG_DEFAULT_RESPONSE_INSPECT_WINDOW;
+    cfg_prec->randomize = HTP_CONFIG_DEFAULT_RANDOMIZE;
+    cfg_prec->randomize_range = HTP_CONFIG_DEFAULT_RANDOMIZE_RANGE;
 
     htp_config_register_request_header_data(cfg_prec->cfg, HTPCallbackRequestHeaderData);
     htp_config_register_request_trailer_data(cfg_prec->cfg, HTPCallbackRequestHeaderData);
@@ -2150,6 +2152,25 @@ static void HTPConfigSetDefaultsPhase1(HTPCfgRec *cfg_prec)
  * the query and path. */
 static void HTPConfigSetDefaultsPhase2(HTPCfgRec *cfg_prec)
 {
+    /* randomize inspection size if needed */
+    if (cfg_prec->randomize) {
+        int rdrange = cfg_prec->randomize_range;
+
+        cfg_prec->request_inspect_min_size +=
+            (int) (cfg_prec->request_inspect_min_size *
+                   (random() * 1.0 / RAND_MAX - 0.5) * rdrange / 100);
+        cfg_prec->request_inspect_window +=
+            (int) (cfg_prec->request_inspect_window *
+                   (random() * 1.0 / RAND_MAX - 0.5) * rdrange / 100);
+
+        cfg_prec->response_inspect_min_size +=
+            (int) (cfg_prec->response_inspect_min_size *
+                   (random() * 1.0 / RAND_MAX - 0.5) * rdrange / 100);
+        cfg_prec->response_inspect_window +=
+            (int) (cfg_prec->response_inspect_window *
+                   (random() * 1.0 / RAND_MAX - 0.5) * rdrange / 100);
+    }
+
     htp_config_register_request_line(cfg_prec->cfg, HTPCallbackRequestLine);
 
     return;
@@ -2355,6 +2376,19 @@ static void HTPConfigParseParameters(HTPCfgRec *cfg_prec, ConfNode *s,
             htp_config_set_field_limits(cfg_prec->cfg,
                     (size_t)HTP_CONFIG_DEFAULT_FIELD_LIMIT_SOFT,
                     (size_t)limit);
+        } else if (strcasecmp("randomize-inspection-sizes", p->name) == 0) {
+            cfg_prec->randomize = ConfValIsTrue(p->val);
+        } else if (strcasecmp("randomize-inspection-range", p->name) == 0) {
+            uint32_t range = atoi(p->val);
+            if (range > 100) {
+                SCLogError(SC_ERR_SIZE_PARSE, "Invalid value for randomize"
+                           " inspection range setting from conf file - %s."
+                           " It should be inferior to 100."
+                           " Killing engine",
+                           p->val);
+                exit(EXIT_FAILURE);
+            }
+            cfg_prec->randomize_range = range;
         } else {
             SCLogWarning(SC_ERR_UNKNOWN_VALUE, "LIBHTP Ignoring unknown "
                          "default config: %s", p->name);

src/app-layer-htp.h

@@ -48,6 +48,9 @@
 #define HTP_CONFIG_DEFAULT_FIELD_LIMIT_SOFT             9000U
 #define HTP_CONFIG_DEFAULT_FIELD_LIMIT_HARD             18000U
 
+#define HTP_CONFIG_DEFAULT_RANDOMIZE                    1
+#define HTP_CONFIG_DEFAULT_RANDOMIZE_RANGE              10
+
 /** a boundary should be smaller in size */
 #define HTP_BOUNDARY_MAX                            200U
 
@@ -147,6 +150,8 @@ typedef struct HTPCfgRec_ {
 
     uint32_t            response_inspect_min_size;
     uint32_t            response_inspect_window;
+    int                 randomize;
+    int                 randomize_range;
 } HTPCfgRec;
 
 /** Struct used to hold chunks of a body on a request */

suricata.yaml.in

@@ -1123,6 +1123,15 @@ app-layer:
            request-body-inspect-window: 4kb
            response-body-minimal-inspect-size: 32kb
            response-body-inspect-window: 4kb
+           # Take a random value for inspection sizes around the specified value.
+           # This lower the risk of some evasion technics but could lead
+           # detection change between runs. It is set to 'yes' by default.
+           #randomize-inspection-sizes: yes
+           # If randomize-inspection-sizes is active, the value of various
+           # inspection size will be choosen in the [1 - range%, 1 + range%]
+           # range
+           # Default value of randomize-inspection-range is 10.
+           #randomize-inspection-range: 10
 
            # decoding
            double-decode-path: no