Trimming PCAPs

Introduction

PCAPs (as a data type) typically take up the most disk space on a Security Onion sensor, and usually aren't able to be kept for extended periods of time. We can lessen the space consumed by PCAPs and extend our retention time by trimming them, using a special tool called TrimPCAP from NETRESEC. Using this tool, we can trim the flows within PCAPs to a desired size.

Please be aware and cognizant of the fact that it may take a while to process a large amount of PCAPs. With this in mind, you'll want to consider running TrimPCAP at non-peak times (without high PCAP write volume, etc.).

One retention schedule that could be used is as follows:
(http://www.netresec.com/?page=Blog&month=2017-12&post=Don%27t-Delete-PCAP-Files---Trim-Them)

Age	Size (per flow)
Older than 3 days	1MB
Older than 6 days	102KB
Older than 30 days	10KB

Trimming

We can install TrimPCAP using the following commands:

sudo apt-get install python-pip
sudo pip install dpkt
sudo pip install repoze.lru
sudo wget -O /opt/trimpcap.py https://www.netresec.com/?download=trimpcap

Then we can run TrimPCAP, as follows (specifying a size of 102KB per flow, iterating over all PCAPs of all ages, in all <sensor_interface> directories):

sudo /usr/bin/find /nsm/sensor_data/ -name "snort.log.??????????" -type f -exec sudo python /opt/trimpcap.py 102400 {} \;

If we want to this for PCAPs older than 3 days, we can do something like the following:

sudo /usr/bin/find /nsm/sensor_data/ -name "snort.log.??????????" -mmin +$((60*72)) -type f -exec sudo python /opt/trimpcap.py 102400 {} \;

We can then automate this using a cron job, so our PCAPs are checked daily.

#/etc/cron.d/trimpcap
#
#crontab entry for TrimPCAP

TRIMPCAP="/opt/trimpcap.py"
LOG="/var/log/trimpcap.log"
SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

# Trim after 3 days
0 1 * * * root echo $(date) >> $LOG; /usr/bin/find /nsm/sensor_data/ -name "snort.log.??????????" -mmin +$((60*72)) -type f -exec /usr/bin/python
$TRIMPCAP 1000000 {} \; >> $LOG 2>&1;

To automatically configure PCAPs to be trimmed at the above recommended intervals, we can do the following:

sudo wget https://raw.githubusercontent.com/weslambert/misc/master/trimpcap_install && sudo chmod +x ./trimpcap_install && sudo ./trimpcap_install