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flow_meter module - README


This NEMEA module creates flows from input PCAP file / network interface and exports them to output interface.


  • To compile this module you will need libpcap development library installed.
  • Root priviliges are needed when capturing from network interface.



  • PCAP file
  • Network interface


  • Unirec containing <COLLECTOR_FLOW> + fields added by active plugins
  • IPFIX RFC 5101


Module specific parameters

  • -p STRING Activate specified parsing plugins. Output interface for each plugin correspond the order which you specify items in -i and -p param. For example: '-i u:a,u:b,u:c -p http,basic,dns' http traffic will be send to interface u:a, basic flow to u:b etc. If you don't specify -p parameter, flow meter will require one output interface for basic flow by default. Format: plugin_name[,...] Supported plugins: http,https,dns,sip,ntp,smtp,basic,arp,passivedns
  • -c NUMBER Quit after NUMBER of packets are captured.
  • -I STRING Capture from given network interface. Parameter require interface name (eth0 for example).
  • -r STRING Pcap file to read. - to read from stdin.
  • -n Don't send NULL record when flow_meter exits.
  • -l NUMBER Snapshot length when reading packets. Set value between 120-65535.
  • -t NUM:NUM Active and inactive timeout in seconds. Format: DOUBLE:DOUBLE. Value default means use default value 300.0:30.0.
  • -s STRING Size of flow cache. Parameter is used as an exponent to the power of two. Valid numbers are in range 4-30. default is 17 (131072 records).
  • -S NUMBER Print flow cache statistics. NUMBER specifies interval between prints.
  • -P Print pcap statistics every 5 seconds. The statistics do not behave the same way on all platforms.
  • -L NUMBER Link bit field value.
  • -D NUMBER Direction bit field value.
  • -F STRING String containing filter expression to filter traffic. See man pcap-filter.
  • -O Send ODID field instead of LINK_BIT_FIELD.
  • -x STRING Export to IPFIX collector. Format: HOST:PORT or [HOST]:PORT.
  • -u Use UDP when exporting to IPFIX collector.

Common TRAP parameters

  • -h [trap,1] Print help message for this module / for libtrap specific parameters.
  • -i IFC_SPEC Specification of interface types and their parameters.
  • -v Be verbose.
  • -vv Be more verbose.
  • -vvv Be even more verbose.


Stores packets from input PCAP file / network interface in flow cache to create flows. After whole PCAP file is processed, flows from flow cache are exported to output interface. When capturing from network interface, flows are continuously send to output interfaces until N (or unlimited number of packets if the -c option is not specified) packets are captured and exported.


flow_meter can be extended by new plugins for exporting various new information from flow. There are already some existing plugins that export e.g. DNS, HTTP, SIP, NTP, PassiveDNS.

Adding new plugin

To create new plugin use script. This interactive script will generate .cpp and .h file template and will also print TODO guide what needs to be done.

Exporting packets

It is possible to export single packet with additional information using plugins (ARP).

Possible issues

Flows are not send to output interface when reading small pcap file

Turn off message buffering using buffer=off option on output interfaces.

./flow_meter -i u:abc:buffer=off -r traffic.pcap

Output data


Basic unirec fields exported on interface with basic (pseudo) plugin. These fields are also exported on interfaces where HTTP, DNS, SIP and NTP plugins are active.

Unirec field Type Description
DST_MAC macaddr destination MAC address
SRC_MAC macaddr source MAC address
DST_IP ipaddr destination IP address
SRC_IP ipaddr source IP address
BYTES uint64 number of bytes in data flow
LINK_BIT_FIELD or ODID uint64 or uint32 exporter identification
TIME_FIRST time first time stamp
TIME_LAST time last time stamp
PACKETS uint32 number of packet in data flow
DST_PORT uint16 transport layer destination port
SRC_PORT uint16 transport layer source port
DIR_BIT_FIELD uint8 bit field for determining outgoing/incoming traffic
PROTOCOL uint8 transport protocol
TCP_FLAGS uint8 TCP protocol flags
TOS uint8 IP type of service
TTL uint8 IP time to live


List of unirec fields exported together with basic flow fields on interface by HTTP plugin.

Unirec field Type Description
HTTP_REQUEST_METHOD string HTTP request method
HTTP_REQUEST_HOST string HTTP request host
HTTP_REQUEST_URL string HTTP request url
HTTP_REQUEST_AGENT string HTTP request user agent
HTTP_REQUEST_REFERER string HTTP request referer
HTTP_RESPONSE_CONTENT_TYPE string HTTP response content type


List of unirec fields exported together with basic flow fields on interface by HTTPS plugin.

Unirec field Type Description
HTTPS_SNI string HTTPS server name indication


List of unirec fields exported together with basic flow fields on interface by DNS plugin.

Unirec field Type Description
DNS_ID uint16 transaction ID
DNS_ANSWERS uint16 number of DNS answer records
DNS_RCODE uint8 response code field
DNS_NAME string question domain name
DNS_QTYPE uint16 question type field
DNS_CLASS uint16 class field of DNS question
DNS_RR_TTL uint32 resource record TTL field
DNS_RLENGTH uint16 length of DNS_RDATA
DNS_RDATA bytes resource record specific data
DNS_PSIZE uint16 requestor's payload size
DNS_DO uint8 DNSSEC OK bit

DNS_RDATA format

DNS_RDATA formatting is implemented for some base DNS RR Types in human-readable output. Same as here:

Record Format
A <IPv4 in dotted decimal representation>
AAAA <IPv6 represented as groups separated by semicolons>
NS <parsed hostname>
CNAME <parsed hostname>
PTR <parsed hostname>
DNAME <parsed hostname>
SOA <mname> <rname> <serial> <refresh> <retry> <expire> <min ttl>
SRV <service> <protocol> <name> <target> <priority> <weight> <port>
MX <priority> <mx hostname>
TXT <txt string>
MINFO <rmailbx> <emailbx>
HINFO <txt string>
ISDN <txt string>
DS <keytag> <algorithm> <digest> <publickey>*
RRSIG <type_covered> <algorithm> <labels> <original_ttl> <signature_exp> <signature_inc> <keytag> <signer_signature>*
DNSKEY <flags> <protocol> <algorithm> <publickey>*
other <not impl>*

* binary data are skipped and not printed


List of unirec fields exported together with basic flow fields on interface by PassiveDNS plugin.

Unirec field Type Description
DNS_ID uint16 transaction ID
DNS_ATYPE uint8 response record type
DNS_NAME string question domain name
DNS_RR_TTL uint32 resource record TTL field
DNS_IP ipaddr IP address from PTR, A or AAAA record


List of unirec fields exported together with basic flow fields on interface by SIP plugin.

Unirec field Type Description
SIP_MSG_TYPE uint16 SIP message code
SIP_STATUS_CODE uint16 status of the SIP request
SIP_CSEQ string CSeq field of SIP packet
SIP_CALLING_PARTY string calling party (from) URI
SIP_CALLED_PARTY string called party (to) URI
SIP_CALL_ID string call ID
SIP_USER_AGENT string user agent field of SIP packet
SIP_REQUEST_URI string SIP request URI
SIP_VIA string via field of SIP packet


List of unirec fields exported together with basic flow fields on interface by NTP plugin.

Unirec field Type Description
NTP_LEAP uint8 NTP leap field
NTP_VERSION uint8 NTP message version
NTP_MODE uint8 NTP mode field
NTP_STRATUM uint8 NTP stratum field
NTP_POLL uint8 NTP poll interval
NTP_PRECISION uint8 NTP precision field
NTP_DELAY uint32 NTP root delay
NTP_DISPERSION uint32 NTP root dispersion
NTP_REF_ID string NTP reference ID
NTP_REF string NTP reference timestamp
NTP_ORIG string NTP origin timestamp
NTP_RECV string NTP receive timestamp
NTP_SENT string NTP transmit timestamp


List of unirec fields exported on interface by SMTP plugin

Unirec field Type Description
SMTP_2XX_STAT_CODE_COUNT uint32 number of 2XX status codes
SMTP_3XX_STAT_CODE_COUNT uint32 number of 3XX status codes
SMTP_4XX_STAT_CODE_COUNT uint32 number of 4XX status codes
SMTP_5XX_STAT_CODE_COUNT uint32 number of 5XX status codes
SMTP_COMMAND_FLAGS uint32 bit array of commands present
SMTP_MAIL_CMD_COUNT uint32 number of MAIL commands
SMTP_RCPT_CMD_COUNT uint32 number of RCPT commands
SMTP_STAT_CODE_FLAGS uint32 bit array of status codes present
SMTP_DOMAIN string domain name of the SMTP client
SMTP_FIRST_SENDER string first sender in MAIL command
SMTP_FIRST_RECIPIENT string first recipient in RCPT command


The following table shows bit values of SMTP\_COMMAND\_FLAGS for each SMTP command present in communication.

Command Value
EHLO 0x0001
HELO 0x0002
MAIL 0x0004
RCPT 0x0008
DATA 0x0010
RSET 0x0020
VRFY 0x0040
EXPN 0x0080
HELP 0x0100
NOOP 0x0200
QUIT 0x0400
UNKNOWN 0x8000


The following table shows bit values of SMTP\_STAT_CODE\_FLAGS for each present in communication.

Status code Value
211 0x00000001
214 0x00000002
220 0x00000004
221 0x00000008
250 0x00000010
251 0x00000020
252 0x00000040
354 0x00000080
421 0x00000100
450 0x00000200
451 0x00000400
452 0x00000800
455 0x00001000
500 0x00002000
501 0x00004000
502 0x00008000
503 0x00010000
504 0x00020000
550 0x00040000
551 0x00080000
552 0x00100000
553 0x00200000
554 0x00400000
555 0x00800000
* 0x40000000
UNKNOWN 0x80000000
  • Bit is set if answer contains SPAM keyword.


List of unirec fields exported on interface by ARP plugin.

Unirec field Type Description
SRC_MAC macaddr source MAC address
DST_MAC macaddr destinaton MAC address
ETHERTYPE uint16 protocol encapsulated in L2 frame
TIME time time packet was received
ARP_HA_FORMAT uint16 hardware address format
ARP_PA_FORMAT uint16 protocol address format
ARP_OPCODE uint16 type of ARP message
ARP_SRC_HA bytes source hardware address
ARP_SRC_PA bytes source protocol address
ARP_DST_HA bytes destination hardware address
ARP_DST_PA bytes destination protocol address

Simplified function diagram

Diagram below shows how flow_meter works.

  1. Packet is read from pcap file or network interface
  2. Packet is processed by PcapReader and is about to put to flow cache
  3. Flow cache create or update flow and call pre_create, post_create, pre_update, post_update and pre_export functions for each active plugin at appropriate time
  4. Flow is put into exporter when considered as expired, flow cache is full or is forced to by a plugin
  5. Exporter fills unirec record, which is then send it to output libtrap interface
       | pcap file or network interface |
          1. |
             |                                  +-----+
    +--------v---------+                              |
    |                  |             +-----------+    |
    |    PcapReader    |      +------>  Plugin1  |    |
    |                  |      |      +-----------+    |
    +--------+---------+      |                       |
             |                |      +-----------+    |
          2. |                +------>  Plugin2  |    |
             |                |      +-----------+    |
    +--------v---------+      |                       |
    |                  |  3.  |      +-----------+    +----+ active plugins
    |   NHTFlowCache   +------------->  Plugin3  |    |
    |                  |      |      +-----------+    |
    +--------+---------+      |                       |
             |                |            .          |
          4. |                |            .          |
             |                |            .          |
    +--------v---------+      |                       |
    |                  |      |      +-----------+    |
    |  UnirecExporter  |      +------>  PluginN  |    |
    |                  |             +-----------+    |
    +--------+---------+                              |
             |                                  +-----+
          5. |
       |    libtrap output interface    |