Monitoring Netflow with NfSen

Introduction

Goals

Learn how to export flows from your rtr1-gY router

Notes

Commands preceded with "$" imply that you should execute the command as a general user - not as root.
Commands preceded with "#" imply that you should be working as root.
Commands with more specific command lines (e.g. "rtr1-gY>" or "mysql>") imply that you are executing commands on remote equipment, or within another program.

Export flows from rtr1-gY

You will configure your router to export flows to the database (srv1-gY) server in your group.

lab@srv1-gY:~$ ssh lab@rtr1-gY.lab.workalaya.net
rtr1-gY> enable

Now do the following:

rtr1-gY# configure terminal
rtr1-gY(config)#

flow exporter EXPORTER-1
 description Export to srv1-gY
 destination 100.68.Y.254
 transport udp 9996
 template data timeout 60

flow monitor FLOW-MONITOR-V4
 exporter EXPORTER-1
 record netflow ipv4 original-input
 cache timeout active 300

interface FastEthernet 0/0
 ip flow monitor FLOW-MONITOR-V4 input
 ip flow monitor FLOW-MONITOR-V4 output

snmp-server ifindex persist

Since you have not specified a protocol version for the exported flow records, you get the default which is Netflow v9.

Netflow v9 packets cannot be decoded by the receiver until it has received a template packet. The command template data timeout 60 tells the router to send it every 60 seconds, to make the lab exercises work more quickly. (In production a value of 300 is fine).

The cache timeout active 300 command breaks up long-lived flows into 5-minute fragments. If you leave it at the default of 30 minutes your traffic graphs will have spikes.

Aside: if you want to monitor IPv6 flows you would have to create a new flow monitor for IPv6 and attach it to the interface and the existing exporters.

flow monitor FLOW-MONITOR-V6
  exporter EXPORTER-1
  record netflow ipv6 original-input
  cache timeout active 300
 interface FastEthernet 0/0
  ipv6 flow monitor FLOW-MONITOR-V6 input
  ipv6 flow monitor FLOW-MONITOR-V6 output

The command snmp-server ifindex persist enables ifIndex persistence globally. This ensures that the ifIndex values are retained during router reboots - also if you add or remove interface modules to your network devices.

Now we'll verify what we've done.

First exit from the configuration session:

rtr1-gY(config)# end

rtr1-gY# show flow exporter EXPORTER-1
rtr1-gY# show flow monitor FLOW-MONITOR-V4

It's possible to see the individual flows that are active in the router:

rtr1-gY# show flow monitor FLOW-MONITOR-V4 cache

But on a busy router there will be thousands of individual flows, so that's not useful. Press 'q' to escape from the screen output if necessary.

Instead, group the flows so you can see your "top talkers" (traffic destinations and sources). This is one very long command line:

rtr1-gY# show flow monitor FLOW-MONITOR-V4 cache aggregate ipv4 source address ipv4 destination address sort counter bytes top 20

If it all looks good then write your running-config to non-volatile RAM (i.e. the startup-config):

rtr1-gY# wr mem

You can exit from the router now:

rtr1-gY# exit

To check flow packets are arriving at your VM you can use tcpdump: Have one person in your group connect to your srv1-gY server and then do:

lab@srv1-gY:~$ sudo apt install tcpdump
lab@srv1-gY:~$ sudo tcpdump -i eth0 -nn udp port 9996

Wait a few seconds and you should see packets arriving. These are the UDP packets containing individual flow records. After seeing some packets you can press ctrl-c to exit from tcpdump. You should see something like:

lab@srv1-gY:~$ sudo tcpdump -i eth0 -nn -c 5 udp port 9996
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
14:59:59.281887 IP 100.68.12.1.54457 > 100.68.12.254.9996: UDP, length 104
15:00:21.155926 IP 100.68.12.1.54457 > 100.68.12.254.9996: UDP, length 236
15:00:25.217719 IP 100.68.12.1.54457 > 100.68.12.254.9996: UDP, length 130
15:00:48.282375 IP 100.68.12.1.54457 > 100.68.12.254.9996: UDP, length 130
15:00:49.286459 IP 100.68.12.1.54457 > 100.68.12.254.9996: UDP, length 236
5 packets captured
5 packets received by filter
0 packets dropped by kernel

lab@srv1-gY:~$ sudo tcpdump -i eth0 -nn -c 5 -Tcnfp port 9996
[sudo] password for lab:
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
14:58:28.284436 IP 100.68.12.1.54457 > 100.68.12.254.9996: NetFlow v9
14:58:29.289145 IP 100.68.12.1.54457 > 100.68.12.254.9996: NetFlow v9
14:58:38.292437 IP 100.68.12.1.54457 > 100.68.12.254.9996: NetFlow v9
14:58:48.261794 IP 100.68.12.1.54457 > 100.68.12.254.9996: NetFlow v9
14:58:59.279264 IP 100.68.12.1.54457 > 100.68.12.254.9996: NetFlow v9
5 packets captured
5 packets received by filter
0 packets dropped by kernel

Once you see that records are arriving you can log off machines by doing:

lab@srv1-gY:~$ exit

OPTIONAL: you can use tshark (the text version of wireshark), which is able to fully decode Netflow v9 packets:

lab@srv1-gY:~$ sudo apt install tshark
lab@srv1-gY:~$ sudo tshark -i eth0 -nnV -s0 -d udp.port==9996,cflow udp port 9996

You are now done with this lab.