17. Components » Flow Sensor

Most routers and many enterprise switches are able to collect IP traffic statistics and export them as flow records to Flow Sensor. Since the flow protocol performs pre-aggregation of traffic data, the flows sent to Flow Sensor are much smaller than the monitored traffic. This makes Flow Sensor an excellent option for monitoring remote or high-traffic networks. The advantages and disadvantages of flow-based monitoring versus packet-based monitoring are listed in the Choosing a Method of Traffic Monitoring chapter.

Appendix 2 shows how to enable NetFlow, sFlow, or IPFIX on a few devices, but the best and most up-to-date instructions can only be found in the vendor’s documentation.

To add a Flow Sensor, click the [+] button from the title bar of the Configuration » Components panel. To modify an existing Flow Sensor, go to Configuration » Components and click its name.


Flow Sensor Configuration parameters:

Sensor Name – A short name to help you identify the Flow Sensor
Reports Visibility – Toggles the listing inside the Reports » Devices panel
Device Group – Optional description used to group up components (e.g., by location or role). You can use it to restrict the access of Guest accounts
Sensor Server – Select a server that fulfills the minimum system requirements for running the Flow Sensor. If this is not the Console server, follow the NFS configuration steps to make the flow data visible in the web interface
Listener IP:Port – The IP address (IPv4 or IPv6) of the network interface that receives the flow packets, and the destination port set on the flow exporter. The destination port can be used only by a single flow exporter, so when multiple flow exporters are used each of them must be configured to send flows to a different port
Flow Exporter IP – IP address of the flow exporter (router, switch, probe). Usually, it is the loopback address of the router. For sFlow exporters, enter the IP that sends the flow packets, not the Agent IP
SNMP Settings – Click the button on the right-hand side of the Flow Exporter IP field. You must enable SNMP on the flow exporter to allow Console to extract interface information automatically. When SNMP is not configured, you must enter manually for each interface the SNMP index, speed, etc.
Flow Exporter TZ – Set the time offset between the time zone (TZ) of the Flow Sensor server and the time zone of the flow exporter. Running NTP on both devices to keep their clocks synchronized is a critical requirement
Flow Protocol – Flow protocol used by the flow exporter: NetFlow, IPFIX or sFlow
Flows Timeout (s) – Juniper MX and a few other flow exporters maintain the start time of previously exported flows. If this is the case, you need to set here the same flow active/inactive timeout value (in seconds) as the one defined in the flow exporter’s configuration. The most common value is 60 seconds
Sensor License – The license used by the Flow Sensor. Wanguard provides all features; Wansight does not provide traffic anomaly detection and reaction
Flow Collector – When enabled, the flow data is saved on disk. You can query flow records in Reports » Tools » Flows
Flow Collector Options - Flow Sensor can compress the flow data using several algorithms. LZO offers the fastest compression. BZ2 offers the best compression rate, but it’s around 30 times slower than LZO. LZ4 offers a balance between speed and efficiency. When the Autofill Src./Dst. ASN option is enabled, Flow Sensor replaces the ASN data from flow records with GeoIP-based ASNs. The Flow Filtering Expression parameter can be used to restrict the flows that are analyzed by Flow Sensor
IP Zone – Flow Sensor needs an IP Zone from which to learn the network’s boundaries and to extract per-subnet settings
Repeater IP:Port – Flow Sensor can retransmit flow packets to another flow collector. This feature is enabled if the field contains the IP of the other flow collector and a destination port
IP Validation – This parameter is frequently used for distinguishing the traffic’s direction (relative to the monitored network):
Off – Flow Sensor examines all flows. The traffic direction must be set manually in the configuration of each monitored interface
On – Flow Sensor examines only the flows that have the source IP and/or destination IP inside the selected IP Zone. When a flow has the destination IP inside the IP Zone, that traffic is considered inbound. When the source IP is inside the IP Zone, that traffic is considered outbound. This option simplifies the configuration of interfaces because the direction of each interface can remain set to Auto, but it doesn’t show inbound/outbound traffic as traffic entering/exiting the interface (like SNMP Sensor and other SNMP-based tools), but traffic entering/exiting the network
Strict – Flow Sensor examines only the flows that have either the source IP or the destination IP inside the IP Zone
Exclusive – Flow Sensor examines only the flows that have the destination IP inside the IP Zone
IP Validation Options – Useful for troubleshooting. If the Log Invalidated Flows parameter is set to Periodically, the event log will show the percentage of invalidated flows and ten flows failing IP validation, once every ten ticks
AS Validation – BGP-enabled routers can export flows that contain the source and destination ASN (Autonomous System Number). In most cases, if the AS number is set to 0, then the IP address belongs to the local ASN. This option is rarely-used for establishing traffic direction. AS Validation provides three choices:
Off – Disables AS validation
On – Flow Sensor examines only the flows that have the source ASN and/or the destination ASN inside the local AS list (defined below)
Strict – Flow Sensor examines only the flows that have either the source ASN or the destination ASN inside the local AS list (defined below)
AS Validation Options – When AS Validation is enabled, enter all your AS numbers, separated by space, into the Local AS List field. Set the Log Invalidated Flows field to Periodically if you want to see in the event log the percentage of invalidated flows and ten flows failing AS validation, once every ten ticks
Granularity – Low values increase the accuracy of Sensor graphs at the expense of RAM usage. The default value of 20 seconds is recommended in most cases
Sampling (1/N) – Enter the sampling rate configured on the flow exporter, or leave the default value unchanged when no sampling rate is configured. For NetFlow v9 and sFlow the value entered here is ignored because the flow protocol automatically adjusts the sampling rate. To force a particular sampling value when the flow exporter is not reporting the sampling correctly, enter it as a negative value
Monitored Interfaces – This grid contains the interfaces that will be monitored. To avoid producing duplicate flow entries, add only upstream interfaces. To add interfaces one by one, click the Add Interface button. To add interfaces in bulk, click the Manage Interfaces button. The following parameters define each monitored interface:
SNMP Index – Each interface is internally identifiable by its SNMP index. You can configure SNMP Settings and have this number auto-filled, or you must extract it from the flow exporter and enter it manually
Interface Name – A short description that identifies the monitored interface. Descriptions longer than ten characters might clutter some reports
Interface Color – The color used in graphs for the interface. The default color is a random one. You can change it by clicking the drop-down menu
Traffic Direction – Direction of the traffic entering the interface, relative to your network:
Auto – When selected, the direction of traffic is established exclusively by IP and/or AS Validation. This is the recommended setting in most cases
Upstream – Set for upstream interfaces (e.g., peering interfaces, interfaces connected to the Internet)
Downstream – Set for downstream interfaces (e.g., customer interfaces, interfaces connected to your network)
Null – Traffic entering Null interfaces is discarded by the router and will be ignored
Stats Engine – Collects various traffic tops and AS (Autonomous System) data:
Basic – Enables tops for internal IPs (IPs included in the IP Zone), IP protocols, TCP/UDP ports, and IP versions
Extended – Enables all tops from Basic as well as tops and graphs for Upstream ASNs and countries. It adds a minimal performance penalty. When the router is not exporting AS information in flows (e.g., non-BGP router), Flow Sensor uses an internal GeoIP database to obtain AS data, makeing live stats for autonomous systems inaccurate when the GeoIP database contains obsolete information. This is the recommended value
Full – Enables all tops from Extended as well as tops for external IPs (IPs not included in the IP Zone) but increases the RAM usage several times over, especially during spoofed attacks from randomized sources. Live stats for autonomous systems and countries are very accurate. Only this option permits the detection of threshold violations for external IPs
Stats Engine Options – When Stats Engine is set to Extended or Full, you can click the button next to it. To enable tops and graphs for Transit, Peering and Downstream ASNs, enter the path to an existing BGP Dump File exported by BGPd in MTR format, and the IPv4 and optionally IPv6 address of the BGP router. MTR files can also be downloaded from RIPE
Link Speed In & Link Speed Out – Enter the interface’s speed (bandwidth, capacity). The values are used for percentage-based reports and percentage-based bits/s thresholds
Comments – Comments about the Flow Sensor can be saved here. These observations are not visible elsewhere
To start the Flow Sensor, click the small button displayed next to its name in Configuration » Components. Make sure that the Flow Sensor starts correctly by watching the event log. If after 5 minutes you can’t see the correct traffic values in Reports » Devices » Overview, follow the troubleshooting steps listed below.


To obtain detailed information about the sources of the attacks detected by Flow Sensor, add a Flow Filter (leave the default parameters unchanged), and then activate it in the “When an anomaly is detected…” panel of the Response. If the network supports BGP Flowspec, you could also perform DDoS mitigation directly on the router by configuring an ExaBGP Connector.

17.1. Flow Sensor Troubleshooting

✔ Look for warnings or errors produced by the Flow Sensor in the event log
✔ Check if you have correctly configured the Flow Sensor. Each configuration parameter is described in the previous section
✔ Check if the server is receiving flow packets on the configured Listener IP:Port by executing the following command which shows the first 100 packets received from the flow exporter:
[root@localhost ~]# tcpdump -i <interface_eth0_p1p1_etc> -n -c 100 host <flow_exporter_ip> and udp and port <destination_port>
✔ Check if the local firewall permits the Flow Sensor to receive flow packets:
[root@localhost ~]# ufw status || firewall-cmd --list-all || iptables -L -n -v && iptables -t raw -L -n -v
✔ Check if the reverse path filtering mechanism is not dropping the packets:
[root@localhost ~]# netstat -s | grep Filter
✔ Make sure that the clocks of the server and of the flow exporter are synchronized, preferably to the same NTP server. When both devices don’t reside in the same time zone, adjust the Flow Exporter TZ parameter accordingly. To verify that the server is synchronized by NTP, execute:
[root@localhost ~]# ntpq -p || chronyc tracking || timedatectl status
✔ If the event log receives the warning “Received flow <starting/ending> <X> seconds ago”, check the following:
▪ When the warning refers to the starting time, make sure that the server clock and flow exporter clock are synchronized and that the time zone is set correctly. For some routers, such as Juniper MX, it is necessary to set the Flow Timeout parameter to the same value (usually 30 seconds) as configured on the router. These routers maintain the start time of exported flows
▪ When the warning refers to the ending time, make sure that the clocks are synchronized, the time zone is set correctly, the flow exporter is properly configured, and the PFC PIC is not overloaded (on Juniper in particular). If the number of seconds is around a multiple of 8600 seconds (1 hour), then the time zones are not matched
▪ In some JunOS versions, there is a flow export rate limit with a default of 1k pps, which leads to flow aging errors. To raise the limit to 40k pps you need to execute:
set forwarding-options sampling instance NETFLOW family inet output inline-jflow flow-export-rate 40
▪ Some Cisco IOS XE devices do not export flows using NetFlow version 5, in under 5 minutes, even when configured to do so. In this case, switch to using Flexible NetFlow
▪ In order to provide fast and up-to-date traffic statistics, Flow Sensor accepts only flows describing traffic that started and ended in the last 5 minutes. All flows aged and exported with a delay exceeding 300 seconds are ignored. This behavior cannot be changed
▪ Flow Sensor does not misinterpret the start/end time of flows. Some flow exporters are known to have bugs, limitations, or inconsistencies regarding flow aging and stamping flow packets with the correct time. In this case, contact your vendor to ensure that the flow exporter is correctly configured, runs the latest firmware, and can expire flows in under 5 minutes. In some cases, a router reboot fixes the issue
▪ You can double-check whether the Flow Sensor’s time and the flow’s start/end time differ by more than 300 seconds. In Reports » Tools » Flows » Flow Records, select any interface of the Flow Sensor, set Display to Extended, and generate a listing for the last 5 minutes:
◦ Column “Received Time” indicates the time when the Flow Sensor received the flow packet, according to the clock of the server
◦ Column “Start Time” indicates the time when the flow started, according to the clock of the flow exporter
◦ Column “Stop Time” indicates the time when the flow ended, according to the clock of the flow exporter
✔ The event log warning “Sensor frozen for X seconds. Restarting the collector” can have several causes. It is generated when the flow packets are too scarce (1 every few seconds) or when flow packets are not received for tens of seconds (e.g., due to a network outage or router reload). Another cause indicates a performance issue, with the Flow Sensor not having enough CPU and I/O resources to analyze traffic and send data to the SQL server in a timely manner. In this case, use a physical server instead of a virtual machine, and decrease from the IP Zone the IP graphs and IP accounting data that need to be collected
✔ If you don’t see traffic on some/all of your monitored interfaces, but you see in Reports » Devices » Overview that the Flow Sensor is receiving flows, you need to check if you have correctly configured the flow exporter to send flows to the server for each of the monitored interfaces. To list the interfaces that are actually sending flows, go to Reports » Tools » Flows » Flow Tops, select any Flow Sensor interface, set Top Type to Any Interface, check Include Unmonitored Ifs in the Display Options selector, and generate a top for the last 10 minutes. The column “In/Out If” lists the SNMP index of every interface that exports flows, even if it wasn’t configured as a monitored interface in the Flow Sensor configuration
✔ When you add interfaces with the Traffic Direction parameter set to Auto, make sure that the IP Zone you have selected contains all your IP blocks because IP Validation and/or AS Validation will be used to establish traffic direction. To capture a sample of flows failing validation in the event log, set the Log Invalidated Flows parameter to Periodically
✔ The traffic readings of the Flow Sensor may differ from other SNMP-based monitoring tools. If IP Validation is enabled, Flow Sensor counts In/Out traffic as traffic entering/exiting the IP Zone, unlike SNMP-based tools, which show In/Out traffic as traffic entering/exiting the interface. To see if the traffic readings differ, add an SNMP Sensor and configure it to monitor the same flow exporter and the same interfaces (the Interface Discovery parameter from the Components » SNMP Sensor section will make this very easy)
✔ If the Flow Sensor does not show the correct statistics after upgrading the router’s firmware, the SNMP index of each interface probably changed. In this case, adjust the SNMP indexes of the monitored interfaces manually, or redefine them
✔ If you only see statistics for a single traffic direction, either inbound or outbound, go to Reports » Tools » Flows » Flow Records and generate a listing for the last 10 minutes. If all your IPs are listed in a single column, check the flow exporter’s configuration and feature list. Not all devices can export flows in both directions or with the same SNMP index. Some Brocade routers are known to generate only inbound sFlow
✔ Flow Sensor could crash during spoofed attacks for not having enough RAM if a monitored interface has the Stats Engine parameter set to Full. It is highly recommended to set this parameter to Extended, especially on systems that don’t have enough RAM
✔ If the traffic is too low after upgrading to JunOS 15.1F2 or 16.1R1, execute:
set chassis fpc inline-services flow-table-size ipv4-flow-table-size 15
✔ To troubleshoot Sensor graph or IP graph issues, follow the Graphs Troubleshooting guide
✔ The event log error “License key not compatible with the existing server” indicates that the server is unregistered and you need to send the string from Configuration » Servers » [Server] » Hardware Key to sales@andrisoft.com
✔ Go to Help » Software Updates to make sure that you are running the latest version of the software