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alert logging 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 alert logging when a snort rule matches some traffic what s called an event is generated and snort provides numerous ways to output the details of those events there are seven alert logger plugins in total and each one provides a unique way of presenting event information snort list plugins grep alert_ logger alert_csv v0 static logger alert_fast v0 static logger alert_full v0 static logger alert_json v0 static logger alert_syslog v0 static logger alert_talos v0 static logger alert_unixsock v0 static the following sections showcase each of these loggers detailing what information from the event can be captured and the different ways in which that information can be formatted these sections will also show a few ways to configure the different plugins and to see the full configuration options available for each plugin users can run snort help module plugin_name at the command line alert_json the alert_json logging plugin outputs event data in json format by default the following fields are included in the json output timestamp pkt_num proto pkt_gen pkt_len dir src_ap dst_ap rule action for example snort q r 3 rules r log4j exploit pcap a alert_json timestamp 07 16 09 23 39 153899 pkt_num 5 proto tcp pkt_gen stream_tcp pkt_len 97 dir c2s src_ap 192 168 1 2 50284 dst_ap 192 168 2 3 80 rule 1 1000000 0 action would_drop a lot more information about the event and the alerting rules can be logged by modifying the alert_json fields field in their snort configuration the value of this field is a string containing a list of desired fields where each field is separated by a space character for example to log the rule message and classification in addition to the default fields one would set their alert_json config object as follows snort q r 3 rules r log4j exploit pcap a alert_json lua alert_json fields timestamp pkt_num proto pkt_gen pkt_len dir src_ap dst_ap rule action msg class timestamp 07 16 09 23 39 153899 pkt_num 5 proto tcp pkt_gen stream_tcp pkt_len 97 dir c2s src_ap 192 168 1 2 50284 dst_ap 192 168 2 3 80 rule 1 1000000 0 action would_drop msg server webapp apache log4j arbitrary code execution attempt class attempted user privilege gain note that the json key value pairs will be outputted in the order they appear in one s config and a list of all possible fields is available at the end of this page in the informational fields section as seen above this alert mode outputs to stdout by default but this behavior can be changed by setting the file boolean to true and this will write the json logs to a file named alert_json txt furthermore specifying the l flag will allow users to write the log files to a particular directory for example the following command will have snort write the json event data to alert_json txt in the logs directory snort q r 3 rules r log4j exploit pcap a alert_json l logs lua alert_json file true alert_csv the alert_csv logger is nearly identical to alert_json but will output event information in csv format instead of json snort q r 3 rules r log4j exploit pcap a alert_csv lua alert_csv fields timestamp pkt_num proto pkt_gen pkt_len dir src_ap dst_ap rule action msg class 07 16 09 23 39 153899 5 tcp stream_tcp 97 c2s 192 168 1 2 50284 192 168 2 3 80 1 1000000 0 would_drop server webapp apache log4j arbitrary code execution attempt attempted user privilege gain the same fields that can be displayed in the json output can also be displayed in csv format alert_fast the alert_fast logger outputs event information in a brief text format but it can be configured to also print packet and buffer dumps as well without any configuration this logger produces a single line detailing when the event occurred the details of the alerting snort rule as well as basic traffic information snort q r 3 rules r log4j exploit pcap a alert_fast 07 16 09 23 39 153899 1 1000000 0 server webapp apache log4j arbitrary code execution attempt classification attempted user privilege gain priority 1 appid http tcp 192 168 1 2 50284 192 168 2 3 80 alert_fast can also be configured to print a packet dump as well as the traffic as seen by snort commonly referred to as buffers snort help module alert_fast alert_fast help output event with brief text format type logger usage global configuration bool alert_fast file false output to alert_fast txt instead of stdout bool alert_fast packet false output packet dump with alert enum alert_fast buffers none output ips buffer dump evaluated by ips rule or an inspector none rule inspector both int alert_fast buffers_depth 0 number of ips buffer bytes to dump per buffer 0 is unlimited 0 maxsz int alert_fast limit 0 set maximum size in mb before rollover 0 is unlimited 0 maxsz for example to also dump the rule buffers we could set the alert_fast object like so snort q r 3 rules r log4j exploit pcap a alert_fast lua alert_fast packet true buffers rule 07 16 09 23 39 153899 would_drop 1 1000000 0 server webapp apache log4j arbitrary code execution attempt classification attempted user privilege gain priority 1 tcp 192 168 1 2 50284 192 168 2 3 80 http_inspect http_method 3 47 45 54 get http_inspect http_version 8 48 54 54 50 2f 31 2e 31 http 1 1 http_inspect http_uri 10 2f 69 6e 64 65 78 2e 70 68 70 index p hp http_inspect http_header 97 48 6f 73 74 3a 20 65 78 61 6d 70 6c 65 2e 63 6f host ex ample co 6d 0d 0a 55 73 65 72 2d 41 67 65 6e 74 3a 20 24 m user agent 7b 6a 6e 64 69 3a 6c 64 61 70 3a 2f 2f 62 61 64 jndi ld ap bad 2d 64 6f 6d 61 69 6e 2e 63 6f 6d 7d 0d 0a 41 63 domain com ac 63 65 70 74 2d 4c 61 6e 67 75 61 67 65 3a 20 65 cept lan guage e 6e 2d 75 73 0d 0a 41 63 63 65 70 74 3a 20 2a 2f n us ac cept 2a note that specifying a cmg also produces output like this one alert_full the alert_full logging module provides basic event information but it can also print out header details of each layer snort q r 3 rules r rce pcap a alert_full 1 1000001 0 server other multiple products remote code execution classification attempted user privilege gain priority 1 07 16 09 23 39 153973 192 168 1 2 50284 192 168 2 3 80 tcp ttl 64 tos 0x0 id 5 iplen 20 dgmlen 40 a seq 0x81 ack 0xa4 win 0x2000 tcplen 20 alert_syslog the alert_syslog logger outputs event information to the host s system logger for example on arch linux hosts using systemd snort events can be seen in the journal and will look very similar to the alert_fast output snort q r 3 rules r log4j exploit pcap a alert_syslog journalctl grep i snort jul 16 11 28 05 archlinux snort 150566 1 1000000 0 server webapp apache log4j arbitrary code execution attempt classification attempted user privilege gain priority 1 tcp 192 168 1 2 50284 192 168 2 3 80 alert_talos alert_talos is a special kind of logger that outputs events in the format that many talos analysts use when creating snort detection this format is relatively simple and shows two main things the pcaps tested and any rules that fire on those pcaps for example snort q r 3 rules r log4j exploit pcap a alert_talos log4j exploit pcap 1 1000000 0 server webapp apache log4j arbitrary code execution attempt alerts 1 alert_unixsock the alert_unixsock logger sets up and uses a unix domain socket for sending event data and logging information this plugin expects a unix socket file named snort_alert that will be used for communication between the server and clients informational fields action the action taken e g allowed blocked etc class the classification type of the event b64_data the packet data of the event encoded in base64 client_bytes number of bytes sent by the client client_pkts number of packets sent by the client dir direction of the traffic e g s2c for server to client c2s for client to server dst_addr destination ip address dst_ap destination ip address and port e g 192 168 213 149 45676 dst_port destination tcp or udp port eth_dst destination ethernet address eth_len length of an ethernet packet eth_src source ethernet address eth_type type of ethernet packet flowstart_time epoch in seconds of when the flow began geneve_vni geneve virtual network identifier value gid generator id of the rule that generated the event icmp_code icmp message code icmp_id icmp identifier number icmp_seq icmp sequence number icmp_type icmp message type iface where the traffic was received by the daq ip_id ip header identification value ip_len ip header length value msg message of the rule that generated the event mpls mpls label pkt_gen snort inspector the traffic was passed to pkt_len length of the event generating packet pkt_num where the packet sits in the flow priority priority associated with the snort rule proto protocol detected e g tcp udp rev revision number of the rule that generated the event rule rule s full identifier in gid sid rev format seconds timestamp in seconds of the packet that generated an event server_bytes number of bytes sent by the server server_pkts number of packets sent by the server service service detected in the traffic that generated the event if one was detected sgt the security group tag sid snort id of the rule that generated the event src_addr source ip address src_ap source ip address and port src_port source tcp or udp port target target ip address tcp_ack acknowledgment value from the tcp header tcp_flags the tcp flags in a stringified form tcp_len length of the packet if tcp tcp_seq sequence value from the tcp header tcp_win window value from the tcp header timestamp timestamp formatted as mm dd hh mm ss of the packet that generated an event tos tos value from the ip header ttl ttl value from the ip header udp_len length of the packet if udp vlan vlan flow id
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