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flowbits snort 3 rule writing guide snort 3 rule writing guide introduction using snort 3 getting started with snort 3 installing snort using snort command line basics reading traffic configuration rules wizard and binder tweaks and scripts trace modules alert logging writing snort rules the basics rule headers rule actions protocols ip addresses port numbers direction operators new rule types service rules file rules file identification rules rule options rule option syntax key general rule options msg reference gid sid rev classtype priority metadata service rem file_meta payload detection rule options content fast_pattern nocase width and endian offset depth distance and within http specific options http_uri and http_raw_uri http_header and http_raw_header http_cookie and http_raw_cookie http_client_body and http_raw_body http_param http_method http_version http_stat_code http_stat_msg http_raw_request and http_raw_status http_trailer and http_raw_trailer http_true_ip http_version_match http_max_header_line http_max_trailer_line http_num_headers http_num_trailers http_num_cookies http_header_test http_trailer_test combining request and response detection bufferlen isdataat dsize pcre regex pkt_data raw_data file_data js_data vba_data base64_decode and base64_data byte_extract byte_test byte_math byte_jump ber_data and ber_skip ssl_state and ssl_version dce specific options sip specific options sd_pattern cvs md5 sha256 and sha512 gtp specific options dnp3 specific options cip specific options iec 104 specific options mms specific options modbus specific options s7commplus specific options non payload detection rule options fragoffset ttl tos id ipopts fragbits ip_proto flags flow flowbits file_type seq ack window itype icode icmp_id icmp_seq rpc stream_reassemble stream_size post detection rule options detection_filter replace tag miscellaneous information shared object rules hexdump lua snortml snort light snort dark snort 3 rule writing guide flowbits the flowbits rule option is used to set and test arbitrary boolean flags to track states throughout the entirety of a transport protocol session udp or tcp there are five flowbit operations all of which are listed below that rule writers can use to track states these are described in the following table argument description set sets the specified states for the current flow unset unsets the specified states for the current flow isset checks if the specified states are set isnotset checks if the specified states are not set noalert cause the rule to not generate an alert regardless of the rest of the detection options setting and checking flowbits the first four operations set unset isset and isnotset are used to track states throughout a transport protocol session these four operations require an additional argument the flowbit flag name which is the name of the flag to be associated with that particular state tracking states is done properly by creating at least two rules 1 a flowbit setter rule that tells snort to set a flag if the other conditions in it are met and 2 a flowbit checker rule to check whether that particular flag has been set or not set previously in the current transport protocol session using that as one of its conditions rule writers can also unset a flag if there s something in a particular packet to warrant such a thing lastly rule writers can also set and evaluate multiple bits at once using the and operators however if setting or unsetting multiple flowbit flags with one flowbit option one must use format setting or unsetting bits flowbits set unset bit bit checking if any bit is set flowbits isset isnotset bit bit checking if all bits are set flowbits isset isnotset bit bit note the names of the flowbit names should be limited to alphanumeric strings and can include periods dashes or underscores examples this example sets a logged_in flag that is used to denote that an imap login has occurred alert tcp any 143 any any msg imap login content ok login flowbits set logged_in flowbits noalert this rule then will only alert if list is found in a packet and the logged_in flag has been set previously during the current transport protocol session alert tcp any any any 143 msg imap list content list flowbits isset logged_in check that flag1 and flag2 have been set previously in the current transport protocol session flowbits isset flag1 flag2 check that flag1 or flag2 have been set previously in the current transport protocol session flowbits isset flag1 flag2 set the flowbits flag1 and flag2 for the current transport protocol session flowbits set flag1 flag2 unset the flowbits flag1 and flag2 for the current transport protocol session flowbits unset flag1 flag2 the noalert flowbit the last flowbit operation is noalert invoking this operation simply tells snort not to generate an alert for that particular rule there is not bit name required for this one this operation is most commonly used in flowbit setter rules since those are usually just a precursor to what one actually wants to detect format flowbits noalert examples flowbits noalert