detect/sctp: add sctp.chunk_cnt keyword

Add a U8 numeric keyword to match the number of SCTP chunks
parsed in a packet with prefilter support.

Ticket #4251

src/Makefile.am

@@ -229,6 +229,7 @@ noinst_HEADERS = \
 	detect-igmphdr.h \
 	detect-igmp-type.h \
 	detect-sctphdr.h \
+	detect-sctp-chunk-cnt.h \
 	detect-sctp-chunk-type.h \
 	detect-ipaddr.h \
 	detect-ipopts.h \
@@ -807,6 +808,7 @@ libsuricata_c_a_SOURCES = \
 	detect-igmphdr.c \
 	detect-igmp-type.c \
 	detect-sctphdr.c \
+	detect-sctp-chunk-cnt.c \
 	detect-sctp-chunk-type.c \
 	detect-ipaddr.c \
 	detect-ipopts.c \

src/detect-engine-register.c

@@ -157,6 +157,7 @@
 #include "detect-igmp-type.h"
 #include "detect-sctphdr.h"
 #include "detect-sctp-chunk-type.h"
+#include "detect-sctp-chunk-cnt.h"
 #include "detect-urilen.h"
 #include "detect-bsize.h"
 #include "detect-detection-filter.h"
@@ -680,6 +681,7 @@ void SigTableSetup(void)
     DetectIGMPTypeRegister();
     DetectSCTPHdrRegister();
     DetectSCTPChunkTypeRegister();
+    DetectSCTPChunkCntRegister();
     DetectTlsRegister();
     DetectTlsValidityRegister();
     DetectTlsVersionRegister();

src/detect-engine-register.h

@@ -53,6 +53,7 @@ enum DetectKeywordId {
     DETECT_IGMP_TYPE,
     DETECT_SCTPHDR,
     DETECT_SCTP_CHUNK_TYPE,
+    DETECT_SCTP_CHUNK_CNT,
     DETECT_DSIZE,
 
     DETECT_FLOW,

src/detect-sctp-chunk-cnt.c

@@ -0,0 +1,122 @@
+/* Copyright (C) 2026 Open Information Security Foundation
+ *
+ * You can copy, redistribute or modify this Program under the terms of
+ * the GNU General Public License version 2 as published by the Free
+ * Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * version 2 along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ */
+
+/**
+ * \file
+ *
+ * Implements sctp.chunk_cnt keyword
+ *
+ * Author: Giuseppe Longo <[email protected]>
+ */
+
+#include "suricata-common.h"
+#include "decode.h"
+
+#include "detect.h"
+#include "detect-parse.h"
+#include "detect-engine-prefilter-common.h"
+#include "detect-engine-build.h"
+
+#include "detect-sctp-chunk-cnt.h"
+#include "detect-engine-uint.h"
+
+#include "util-byte.h"
+#include "util-debug.h"
+
+static int DetectSCTPChunkCntMatch(
+        DetectEngineThreadCtx *, Packet *, const Signature *, const SigMatchCtx *);
+static int DetectSCTPChunkCntSetup(DetectEngineCtx *, Signature *, const char *);
+void DetectSCTPChunkCntFree(DetectEngineCtx *, void *);
+
+static int PrefilterSetupSCTPChunkCnt(DetectEngineCtx *de_ctx, SigGroupHead *sgh);
+static bool PrefilterSCTPChunkCntIsPrefilterable(const Signature *s);
+
+void DetectSCTPChunkCntRegister(void)
+{
+    sigmatch_table[DETECT_SCTP_CHUNK_CNT].name = "sctp.chunk_cnt";
+    sigmatch_table[DETECT_SCTP_CHUNK_CNT].desc = "match on the SCTP chunk count";
+    sigmatch_table[DETECT_SCTP_CHUNK_CNT].url = "/rules/sctp-keywords.html#sctp-chunk-cnt";
+    sigmatch_table[DETECT_SCTP_CHUNK_CNT].Match = DetectSCTPChunkCntMatch;
+    sigmatch_table[DETECT_SCTP_CHUNK_CNT].Setup = DetectSCTPChunkCntSetup;
+    sigmatch_table[DETECT_SCTP_CHUNK_CNT].Free = DetectSCTPChunkCntFree;
+    sigmatch_table[DETECT_SCTP_CHUNK_CNT].flags = SIGMATCH_INFO_UINT8;
+    sigmatch_table[DETECT_SCTP_CHUNK_CNT].SupportsPrefilter = PrefilterSCTPChunkCntIsPrefilterable;
+    sigmatch_table[DETECT_SCTP_CHUNK_CNT].SetupPrefilter = PrefilterSetupSCTPChunkCnt;
+}
+
+static int DetectSCTPChunkCntMatch(
+        DetectEngineThreadCtx *det_ctx, Packet *p, const Signature *s, const SigMatchCtx *ctx)
+{
+    DEBUG_VALIDATE_BUG_ON(PKT_IS_PSEUDOPKT(p));
+
+    if (!PacketIsSCTP(p)) {
+        return 0;
+    }
+
+    uint8_t val = p->l4.vars.sctp.chunk_cnt;
+    const DetectU8Data *data = (const DetectU8Data *)ctx;
+    return DetectU8Match(val, data);
+}
+
+static int DetectSCTPChunkCntSetup(DetectEngineCtx *de_ctx, Signature *s, const char *str)
+{
+    DetectU8Data *data = DetectU8Parse(str);
+    if (data == NULL)
+        return -1;
+
+    if (SCSigMatchAppendSMToList(de_ctx, s, DETECT_SCTP_CHUNK_CNT, (SigMatchCtx *)data,
+                DETECT_SM_LIST_MATCH) == NULL) {
+        DetectSCTPChunkCntFree(de_ctx, data);
+        return -1;
+    }
+    s->flags |= SIG_FLAG_REQUIRE_PACKET;
+
+    return 0;
+}
+
+void DetectSCTPChunkCntFree(DetectEngineCtx *de_ctx, void *ptr)
+{
+    DetectU8Data *data = (DetectU8Data *)ptr;
+    SCDetectU8Free(data);
+}
+
+static void PrefilterPacketSCTPChunkCntMatch(
+        DetectEngineThreadCtx *det_ctx, Packet *p, const void *pectx)
+{
+    DEBUG_VALIDATE_BUG_ON(PKT_IS_PSEUDOPKT(p));
+
+    if (PacketIsSCTP(p)) {
+        uint8_t val = p->l4.vars.sctp.chunk_cnt;
+        const PrefilterPacketU8HashCtx *h = pectx;
+        const SigsArray *sa = h->array[val];
+        if (sa) {
+            PrefilterAddSids(&det_ctx->pmq, sa->sigs, sa->cnt);
+        }
+    }
+}
+
+static int PrefilterSetupSCTPChunkCnt(DetectEngineCtx *de_ctx, SigGroupHead *sgh)
+{
+    return PrefilterSetupPacketHeaderU8Hash(de_ctx, sgh, DETECT_SCTP_CHUNK_CNT,
+            SIG_MASK_REQUIRE_REAL_PKT, PrefilterPacketU8Set, PrefilterPacketU8Compare,
+            PrefilterPacketSCTPChunkCntMatch);
+}
+
+static bool PrefilterSCTPChunkCntIsPrefilterable(const Signature *s)
+{
+    return PrefilterIsPrefilterableById(s, DETECT_SCTP_CHUNK_CNT);
+}

src/detect-sctp-chunk-cnt.h

@@ -0,0 +1,27 @@
+/* Copyright (C) 2026 Open Information Security Foundation
+ *
+ * You can copy, redistribute or modify this Program under the terms of
+ * the GNU General Public License version 2 as published by the Free
+ * Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * version 2 along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ */
+
+/**
+ * \file
+ */
+
+#ifndef SURICATA_DETECT_SCTP_CHUNK_CNT_H
+#define SURICATA_DETECT_SCTP_CHUNK_CNT_H
+
+void DetectSCTPChunkCntRegister(void);
+
+#endif /* SURICATA_DETECT_SCTP_CHUNK_CNT_H */