dns: add dns.rcode keyword

dns.rcode matches the rcode header field in DNS messages
It's an unsigned integer
valid ranges = [0-15]
Does not support prefilter
Supports matches in both flow directions

Task #6621

doc/userguide/rules/dns-keywords.rst

@@ -57,6 +57,39 @@ Match on DNS requests where the **opcode** is not between 7 and 15:
 
   dns.opcode:!7-15;
 
+dns.rcode
+---------
+
+This keyword matches on the **rcode** field found in the DNS header flags.
+
+dns.rcode uses an :ref:`unsigned 8-bit integer <rules-integer-keywords>`.
+
+Currently, Suricata only supports rcode values in the range [0-15], while
+the current DNS version supports rcode values from [0-23] as specified in
+`RFC 6895 <https://www.iana.org/assignments/dns-parameters/dns-parameters.xhtml#dns-parameters-6>`_.
+
+We plan to extend the rcode values supported by Suricata according to RFC 6895
+as tracked by the ticket: https://redmine.openinfosecfoundation.org/issues/6650
+
+Syntax
+~~~~~~
+
+::
+
+   dns.rcode:[!]<number>
+   dns.rcode:[!]<number1>-<number2>
+
+Examples
+~~~~~~~~
+
+Match on DNS requests and responses with **rcode** 4::
+
+  dns.rcode:4;
+
+Match on DNS requests and responses where the **rcode** is NOT 0::
+
+  dns.rcode:!0;
+
 dns.rrtype
 ----------
 

rust/src/dns/detect.rs

@@ -50,6 +50,33 @@ pub extern "C" fn rs_dns_opcode_match(
     return 0;
 }
 
+/// Perform the DNS rcode match.
+///
+/// 1 will be returned on match, otherwise 0 will be returned.
+#[no_mangle]
+pub extern "C" fn rs_dns_rcode_match(
+    tx: &mut DNSTransaction, detect: &mut DetectUintData<u8>, flags: u8,
+) -> u8 {
+    let header_flags = if flags & Direction::ToServer as u8 != 0 {
+        if let Some(request) = &tx.request {
+            request.header.flags
+        } else {
+            return 0;
+        }
+    } else if let Some(response) = &tx.response {
+        response.header.flags
+    } else {
+        return 0;
+    };
+
+    let rcode = (header_flags & 0xf) as u8;
+
+    if detect_match_uint(detect, rcode) {
+        return 1;
+    }
+    return 0;
+}
+
 /// Perform the DNS rrtype match.
 /// 1 will be returned on match, otherwise 0 will be returned.
 #[no_mangle]
@@ -152,6 +179,85 @@ mod test {
         ));
     }
 
+    #[test]
+    fn parse_rcode_good() {
+        assert_eq!(
+            detect_parse_uint::<u8>("1").unwrap().1,
+            DetectUintData {
+                mode: DetectUintMode::DetectUintModeEqual,
+                arg1: 1,
+                arg2: 0,
+            }
+        );
+        assert_eq!(
+            detect_parse_uint::<u8>("123").unwrap().1,
+            DetectUintData {
+                mode: DetectUintMode::DetectUintModeEqual,
+                arg1: 123,
+                arg2: 0,
+            }
+        );
+        assert_eq!(
+            detect_parse_uint::<u8>("!123").unwrap().1,
+            DetectUintData {
+                mode: DetectUintMode::DetectUintModeNe,
+                arg1: 123,
+                arg2: 0,
+            }
+        );
+        assert_eq!(
+            detect_parse_uint::<u8>("7-15").unwrap().1,
+            DetectUintData {
+                mode: DetectUintMode::DetectUintModeRange,
+                arg1: 7,
+                arg2: 15,
+            }
+        );
+        assert!(detect_parse_uint::<u16>("").is_err());
+        assert!(detect_parse_uint::<u16>("!").is_err());
+        assert!(detect_parse_uint::<u16>("!   ").is_err());
+        assert!(detect_parse_uint::<u16>("!asdf").is_err());
+    }
+
+    #[test]
+    fn test_match_rcode() {
+        assert!(detect_match_uint(
+            &DetectUintData {
+                mode: DetectUintMode::DetectUintModeEqual,
+                arg1: 0,
+                arg2: 0,
+            },
+            0b0000_0000_0000_0000,
+        ));
+
+        assert!(!detect_match_uint(
+            &DetectUintData {
+                mode: DetectUintMode::DetectUintModeNe,
+                arg1: 0,
+                arg2: 0,
+            },
+            0b0000_0000_0000_0000,
+        ));
+
+        assert!(detect_match_uint(
+            &DetectUintData {
+                mode: DetectUintMode::DetectUintModeEqual,
+                arg1: 4,
+                arg2: 0,
+            },
+            4u8,
+        ));
+
+        assert!(!detect_match_uint(
+            &DetectUintData {
+                mode: DetectUintMode::DetectUintModeNe,
+                arg1: 4,
+                arg2: 0,
+            },
+            4u8,
+        ));
+    }
+
     #[test]
     fn parse_rrtype_good() {
         assert_eq!(

src/Makefile.am

@@ -121,6 +121,7 @@ noinst_HEADERS = \
 	detect-dnp3.h \
 	detect-dns-answer-name.h \
 	detect-dns-opcode.h \
+	detect-dns-rcode.h \
 	detect-dns-rrtype.h \
 	detect-dns-query.h \
 	detect-dns-query-name.h \
@@ -744,6 +745,7 @@ libsuricata_c_a_SOURCES = \
 	detect-dnp3.c \
 	detect-dns-answer-name.c \
 	detect-dns-opcode.c \
+	detect-dns-rcode.c \
 	detect-dns-rrtype.c \
 	detect-dns-query.c \
 	detect-dns-query-name.c \

src/detect-dns-rcode.c

@@ -0,0 +1,85 @@
+/* Copyright (C) 2024 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.
+ */
+
+#include "suricata-common.h"
+
+#include "detect-parse.h"
+#include "detect-engine.h"
+#include "detect-dns-rcode.h"
+#include "rust.h"
+#include "detect-engine-uint.h"
+
+static int dns_rcode_list_id = 0;
+
+static void DetectDnsRcodeFree(DetectEngineCtx *, void *ptr);
+
+static int DetectDnsRcodeSetup(DetectEngineCtx *de_ctx, Signature *s, const char *str)
+{
+    SCEnter();
+
+    if (DetectSignatureSetAppProto(s, ALPROTO_DNS) != 0) {
+        SCReturnInt(-1);
+    }
+
+    void *detect = DetectU8Parse(str);
+    if (detect == NULL) {
+        SCLogError("failed to parse dns.rcode: %s", str);
+        SCReturnInt(-1);
+    }
+
+    if (SigMatchAppendSMToList(
+                de_ctx, s, DETECT_AL_DNS_RCODE, (SigMatchCtx *)detect, dns_rcode_list_id) == NULL) {
+        DetectDnsRcodeFree(de_ctx, detect);
+        SCReturnInt(-1);
+    }
+
+    SCReturnInt(0);
+}
+
+static void DetectDnsRcodeFree(DetectEngineCtx *de_ctx, void *ptr)
+{
+    SCEnter();
+    if (ptr != NULL) {
+        rs_detect_u8_free(ptr);
+    }
+    SCReturn;
+}
+
+static int DetectDnsRcodeMatch(DetectEngineThreadCtx *det_ctx, Flow *f, uint8_t flags, void *state,
+        void *txv, const Signature *s, const SigMatchCtx *ctx)
+{
+    return rs_dns_rcode_match(txv, (void *)ctx, flags);
+}
+
+void DetectDnsRcodeRegister(void)
+{
+    sigmatch_table[DETECT_AL_DNS_RCODE].name = "dns.rcode";
+    sigmatch_table[DETECT_AL_DNS_RCODE].desc = "Match the DNS header rcode flag.";
+    sigmatch_table[DETECT_AL_DNS_RCODE].url = "/rules/dns-keywords.html#dns-rcode";
+    sigmatch_table[DETECT_AL_DNS_RCODE].Setup = DetectDnsRcodeSetup;
+    sigmatch_table[DETECT_AL_DNS_RCODE].Free = DetectDnsRcodeFree;
+    sigmatch_table[DETECT_AL_DNS_RCODE].Match = NULL;
+    sigmatch_table[DETECT_AL_DNS_RCODE].AppLayerTxMatch = DetectDnsRcodeMatch;
+
+    DetectAppLayerInspectEngineRegister(
+            "dns.rcode", ALPROTO_DNS, SIG_FLAG_TOSERVER, 0, DetectEngineInspectGenericList, NULL);
+
+    DetectAppLayerInspectEngineRegister(
+            "dns.rcode", ALPROTO_DNS, SIG_FLAG_TOCLIENT, 0, DetectEngineInspectGenericList, NULL);
+
+    dns_rcode_list_id = DetectBufferTypeGetByName("dns.rcode");
+}

src/detect-dns-rcode.h

@@ -0,0 +1,23 @@
+/* Copyright (C) 2024 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.
+ */
+
+#ifndef __DETECT_DNS_RCODE_H__
+#define __DETECT_DNS_RCODE_H__
+
+void DetectDnsRcodeRegister(void);
+
+#endif /* __DETECT_DNS_RCODE_H__ */

src/detect-engine-register.c

@@ -48,6 +48,7 @@
 #include "detect-engine-payload.h"
 #include "detect-engine-dcepayload.h"
 #include "detect-dns-opcode.h"
+#include "detect-dns-rcode.h"
 #include "detect-dns-rrtype.h"
 #include "detect-dns-query.h"
 #include "detect-dns-answer-name.h"
@@ -523,6 +524,7 @@ void SigTableSetup(void)
 
     DetectDnsQueryRegister();
     DetectDnsOpcodeRegister();
+    DetectDnsRcodeRegister();
     DetectDnsRrtypeRegister();
     DetectDnsAnswerNameRegister();
     DetectDnsQueryNameRegister();

src/detect-engine-register.h

@@ -230,6 +230,7 @@ enum DetectKeywordId {
 
     DETECT_AL_DNS_QUERY,
     DETECT_AL_DNS_OPCODE,
+    DETECT_AL_DNS_RCODE,
     DETECT_AL_DNS_RRTYPE,
     DETECT_AL_DNS_ANSWER_NAME,
     DETECT_AL_DNS_QUERY_NAME,