DNS Anomaly Detection

Dr. Johannes Ullrich of the SANS Internet Storm Center posted a great DNS Anomaly Detection script based on the query logs coming from his DNS server. We can do the same thing with Bro’s dns.log (where Bro captures all the DNS queries it sees on the network).

Please note that this script is only intended for standalone machines and will not work properly on distributed deployments.

This version of the script works on older installations using Bro TSV output:

#!/bin/bash

export PATH=/opt/bro/bin:$PATH
BRO_LOGS="/nsm/bro/logs"
TODAY=`date +%Y-%m-%d`
YESTERDAY=`date -d yesterday +%Y-%m-%d`
OLD_DIRS=`ls $BRO_LOGS |egrep -v "current|stats|$TODAY|$YESTERDAY"`
TMPDIR=/tmp
OLDLOG=$TMPDIR/oldlog
NEWLOG=$TMPDIR/newlog
SUSPECTS=$TMPDIR/suspects

for DIR in $OLD_DIRS; do zcat $BRO_LOGS/$DIR/dns* |bro-cut id.resp_p query; done | grep -v "^5353" | awk '{print $2}' | sort | uniq -c | sort -k2 > $OLDLOG
zcat $BRO_LOGS/$YESTERDAY/dns* |bro-cut id.resp_p query | grep -v "^5353" | awk '{print $2}' | sort | uniq -c | sort -k2 > $NEWLOG
join -1 2 -2 2  -a 2 $OLDLOG $NEWLOG | egrep -v '.* [0-9]+ [0-9]+$' | sort -nr -k2 | head -50 > $SUSPECTS

echo
echo "===================================="
echo "Top 50 First Time Seen DNS queries:"
echo "===================================="
cat $SUSPECTS

We’ve since changed Bro’s default output to json (for faster Logstash parsing), so senatorhotchkiss on our mailing list updated the script, replacing bro-cut with jq as follows:

#!/bin/bash

export PATH=/opt/bro/bin:$PATH
BRO_LOGS="/nsm/bro/logs"
TODAY=`date +%Y-%m-%d`
YESTERDAY=`date -d yesterday +%Y-%m-%d`
OLD_DIRS=`ls $BRO_LOGS |egrep -v "current|stats|$TODAY|$YESTERDAY"`
TMPDIR=/tmp
OLDLOG=$TMPDIR/oldlog
NEWLOG=$TMPDIR/newlog
SUSPECTS=$TMPDIR/suspects

for DIR in $OLD_DIRS; do zcat $BRO_LOGS/$DIR/dns* | jq '{"id.resp_p"},{"query"}' ; done  | grep -v "^5353" | awk '{print $2}' | sort | uniq -c | sort -k2 > $OLDLOG
zcat $BRO_LOGS/$YESTERDAY/dns* | jq '{"id.resp_p"},{"query"}' | grep -v "^5353" | awk '{print $2}' | sort | uniq -c | sort -k2 > $NEWLOG
join -1 2 -2 2  -a 2 $OLDLOG $NEWLOG | egrep -v '.* [0-9]+ [0-9]+$' | sort -nr -k2 | head -50 > $SUSPECTS

echo
echo "===================================="
echo "Top 50 First Time Seen DNS queries:"
echo "===================================="
cat $SUSPECTS