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文章目录

1. `flowbits`1.1 `flowbits`安装1.2 `flowbits`匹配1.2.1 `flowbits: set, name`1.2.2 `flowbits: isset, name`1.2.3 `flowbits: toggle, name`1.2.4 `flowbits: unset, name`1.2.5 `flowbits: isnotset, name`1.2.6 `flowbits: noalert`

1. flowbits

flowbits由两部分组成：

第一部分描述将要执行的动作，第二部分是flowbits的名称。

有多个数据包属于一个流。Suricata将这些流保存在内存中。flowbits可以确保在两个不同的数据包匹配时才将生成警报。仅当两个数据包匹配时才会生成警报。

因此，当第二个数据包匹配时，Suricata必须知道第一个数据包是否也匹配。如果数据包匹配，则flowbits会标记流，因此当Suricata“知道”第二个数据包也匹配时，它将生成警报。

示例： 当你看第一条规则时，你会注意到如果它匹配，如果不是在规则末尾的flowbits:noalert，它就会生成一个警报。该规则的目的是检查userlogin上的匹配项，并在流中进行标记。因此，无需生成警报。

如果没有第一条规则，第二条规则将无效。如果第一个规则匹配，flowbits将设置流中存在的特定条件。现在，使用第二条规则，可以检查前一个数据包是否满足第一个条件。如果此时第二条规则匹配，将生成警报。可以在规则中多次使用流位，并组合不同的功能。

void DetectFlowbitsRegister (void) { sigmatch_table[DETECT_FLOWBITS].name = "flowbits"; sigmatch_table[DETECT_FLOWBITS].desc = "operate on flow flag"; sigmatch_table[DETECT_FLOWBITS].url = DOC_URL DOC_VERSION "/rules/flow-keywords.html#flowbits"; sigmatch_table[DETECT_FLOWBITS].Match = DetectFlowbitMatch; sigmatch_table[DETECT_FLOWBITS].Setup = DetectFlowbitSetup; sigmatch_table[DETECT_FLOWBITS].Free = DetectFlowbitFree; sigmatch_table[DETECT_FLOWBITS].RegisterTests = NULL; /* this is compatible to ip-only signatures */ sigmatch_table[DETECT_FLOWBITS].flags |= SIGMATCH_IPONLY_COMPAT; DetectSetupParseRegexes(PARSE_REGEX, &parse_regex, &parse_regex_study); }

1.1 flowbits安装

解析flowbits具有的6种不同的动作： noalertissetisnotsetsetunsettoggle int DetectFlowbitSetup (DetectEngineCtx *de_ctx, Signature *s, const char *rawstr) { DetectFlowbitsData *cd = NULL; SigMatch *sm = NULL; uint8_t fb_cmd = 0; char fb_cmd_str[16] = "", fb_name[256] = ""; if (!DetectFlowbitParse(rawstr, fb_cmd_str, sizeof(fb_cmd_str), fb_name, sizeof(fb_name))) { return -1; } if (strcmp(fb_cmd_str,"noalert") == 0) { fb_cmd = DETECT_FLOWBITS_CMD_NOALERT; } else if (strcmp(fb_cmd_str,"isset") == 0) { fb_cmd = DETECT_FLOWBITS_CMD_ISSET; } else if (strcmp(fb_cmd_str,"isnotset") == 0) { fb_cmd = DETECT_FLOWBITS_CMD_ISNOTSET; } else if (strcmp(fb_cmd_str,"set") == 0) { fb_cmd = DETECT_FLOWBITS_CMD_SET; } else if (strcmp(fb_cmd_str,"unset") == 0) { fb_cmd = DETECT_FLOWBITS_CMD_UNSET; } else if (strcmp(fb_cmd_str,"toggle") == 0) { fb_cmd = DETECT_FLOWBITS_CMD_TOGGLE; } else { SCLogError(SC_ERR_UNKNOWN_VALUE, "ERROR: flowbits action \"%s\" is not supported.", fb_cmd_str); goto error; } 对noalert设置SIG_FLAG_NOALERT选项。 switch (fb_cmd) { case DETECT_FLOWBITS_CMD_NOALERT: if (strlen(fb_name) != 0) goto error; s->flags |= SIG_FLAG_NOALERT; return 0; case DETECT_FLOWBITS_CMD_ISNOTSET: case DETECT_FLOWBITS_CMD_ISSET: case DETECT_FLOWBITS_CMD_SET: case DETECT_FLOWBITS_CMD_UNSET: case DETECT_FLOWBITS_CMD_TOGGLE: default: if (strlen(fb_name) == 0) goto error; break; } 分配flowbits id，并加入到签名的匹配条件中。 isset和isnotset加入DETECT_SM_LIST_MATCH组。set、unset及toggle加入DETECT_SM_LIST_POSTMATCH组。 ... cd = SCMalloc(sizeof(DetectFlowbitsData)); if (unlikely(cd == NULL)) goto error; cd->idx = VarNameStoreSetupAdd(fb_name, VAR_TYPE_FLOW_BIT); de_ctx->max_fb_id = MAX(cd->idx, de_ctx->max_fb_id); cd->cmd = fb_cmd; SCLogDebug("idx %" PRIu32 ", cmd %s, name %s", cd->idx, fb_cmd_str, strlen(fb_name) ? fb_name : "(none)"); /* Okay so far so good, lets get this into a SigMatch * and put it in the Signature. */ sm = SigMatchAlloc(); if (sm == NULL) goto error; sm->type = DETECT_FLOWBITS; sm->ctx = (SigMatchCtx *)cd; switch (fb_cmd) { /* case DETECT_FLOWBITS_CMD_NOALERT can't happen here */ case DETECT_FLOWBITS_CMD_ISNOTSET: case DETECT_FLOWBITS_CMD_ISSET: /* checks, so packet list */ SigMatchAppendSMToList(s, sm, DETECT_SM_LIST_MATCH); break; case DETECT_FLOWBITS_CMD_SET: case DETECT_FLOWBITS_CMD_UNSET: case DETECT_FLOWBITS_CMD_TOGGLE: /* modifiers, only run when entire sig has matched */ SigMatchAppendSMToList(s, sm, DETECT_SM_LIST_POSTMATCH); break; // suppress coverity warning as scan-build-7 warns w/o this. // coverity[deadcode : FALSE] default: goto error; } return 0; error: if (cd != NULL) SCFree(cd); if (sm != NULL) SCFree(sm); return -1; }

1.2 flowbits匹配

如果DETECT_SM_LIST_MATCH有匹配项，进行注册Match函数注册。 int DetectEnginePktInspectionSetup(Signature *s) { ... if (s->sm_arrays[DETECT_SM_LIST_MATCH]) { if (DetectEnginePktInspectionAppend(s, DetectEngineInspectRulePacketMatches, NULL) < 0) ... } 遍历DETECT_SM_LIST_MATCH中所有选项进行匹配。 static int DetectEngineInspectRulePacketMatches( DetectEngineThreadCtx *det_ctx, const DetectEnginePktInspectionEngine *engine, const Signature *s, Packet *p, uint8_t *_alert_flags) { ... if (sigmatch_table[smd->type].Match(det_ctx, p, s, smd->ctx) <= 0) { ... } 对不同类型的flowbits进行匹配。 int DetectFlowbitMatch (DetectEngineThreadCtx *det_ctx, Packet *p, const Signature *s, const SigMatchCtx *ctx) { const DetectFlowbitsData *fd = (const DetectFlowbitsData *)ctx; if (fd == NULL) return 0; switch (fd->cmd) { case DETECT_FLOWBITS_CMD_ISSET: return DetectFlowbitMatchIsset(p,fd); case DETECT_FLOWBITS_CMD_ISNOTSET: return DetectFlowbitMatchIsnotset(p,fd); case DETECT_FLOWBITS_CMD_SET: return DetectFlowbitMatchSet(p,fd); case DETECT_FLOWBITS_CMD_UNSET: return DetectFlowbitMatchUnset(p,fd); case DETECT_FLOWBITS_CMD_TOGGLE: return DetectFlowbitMatchToggle(p,fd); default: SCLogError(SC_ERR_UNKNOWN_VALUE, "unknown cmd %" PRIu32 "", fd->cmd); return 0; } return 0; }

1.2.1 flowbits: set, name

在流中设置条件/名称（如果存在）。

static int DetectFlowbitMatchSet (Packet *p, const DetectFlowbitsData *fd) { if (p->flow == NULL) return 0; FlowBitSet(p->flow,fd->idx); return 1; }

1.2.2 flowbits: isset, name

可在规则中使用，以确保在规则匹配且条件在流中设置时生成告警。

static int DetectFlowbitMatchIsset (Packet *p, const DetectFlowbitsData *fd) { if (p->flow == NULL) return 0; return FlowBitIsset(p->flow,fd->idx); }

1.2.3 flowbits: toggle, name

反转当前设置。因此，如果设置了条件，则将不设置条件，反之亦然。

void FlowBitToggle(Flow *f, uint32_t idx) { FlowBit *fb = FlowBitGet(f, idx); if (fb != NULL) { FlowBitRemove(f, idx); } else { FlowBitAdd(f, idx); } } static int DetectFlowbitMatchToggle (Packet *p, const DetectFlowbitsData *fd) { if (p->flow == NULL) return 0; FlowBitToggle(p->flow,fd->idx); return 1; }

1.2.4 flowbits: unset, name

可用于取消设置流中的条件。

static int DetectFlowbitMatchUnset (Packet *p, const DetectFlowbitsData *fd) { if (p->flow == NULL) return 0; FlowBitUnset(p->flow,fd->idx); return 1; }

1.2.5 flowbits: isnotset, name

可在规则中使用，以确保在规则匹配且条件未在流中设置时生成告警。

static int DetectFlowbitMatchIsnotset (Packet *p, const DetectFlowbitsData *fd) { if (p->flow == NULL) return 0; return FlowBitIsnotset(p->flow,fd->idx); }

1.2.6 flowbits: noalert

此规则将不会生成告警。

static inline void DetectRulePacketRules( ThreadVars * const tv, DetectEngineCtx * const de_ctx, DetectEngineThreadCtx * const det_ctx, Packet * const p, Flow * const pflow, const DetectRunScratchpad *scratch ) { ... if (!(sflags & SIG_FLAG_NOALERT)) { /* stateful sigs call PacketAlertAppend from DeStateDetectStartDetection */ if (!state_alert) PacketAlertAppend(det_ctx, s, p, 0, alert_flags); } else { /* apply actions even if not alerting */ DetectSignatureApplyActions(p, s, alert_flags); } ... }

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未完待续。。。